Title of the invention: Impact absorbing member, manufacturing method of impact absorbing member, and manufacturing method of steel sheet for cold plastic working.
Technical field
[0001]
　The present invention relates to a shock absorbing member, a method for manufacturing a shock absorbing member, and a method for manufacturing a steel sheet for cold plastic working.
Background technology
[0002]
　In recent years, the application of high-strength steel sheets as vehicle body members has been expanding from the viewpoint of weight reduction that contributes to improving fuel efficiency of automobiles (see, for example, Patent Documents 1 to 7).
Prior art literature
Patent documents
[0003]
Patent Document 1: Japanese Patent Application Laid-Open No. 2008-144233
Patent Document 2: WO2018 / 174082
Patent Document 3: Japanese Patent Application Laid-Open No. 2015-58810
Patent Document 4: WO2014 / 077294
Patent Document 5: Japanese Patent Application Laid-Open No. 6418363
Patent Document 6: WO2020 / 022481
Patent Document 7: WO2020 / 071523A.
Outline of the invention
Problems to be solved by the invention
[0004]
　For the purpose of ensuring the safety of occupants, it is desired to improve the collision performance, and a material having high strength and capable of increasing the impact absorption energy at the time of a collision of an automobile is required.
[0005]
　In view of the above background, one of the objects of the present invention is a shock absorbing member capable of increasing the shock absorbing energy and thinning the steel sheet as a material, a method for manufacturing the shock absorbing member, and a steel sheet for cold plastic working. Is to provide a manufacturing method for.
Means to solve problems
[0006]
　The gist of the present invention is the following shock absorbing member, a method for manufacturing a shock absorbing member, and a method for manufacturing a steel sheet for cold plastic working.
[0007]
　(1) A shock absorbing member including a ridge portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridge portion, and
　the elongation strain in the tensile test is 5 in the wall portion. The ratio of the tensile stress σ 5 at% to the shear stress τ 5 when the shear stress is 5√3% is 1.70 or less, or the elongation strain in the tensile test is 1.70 or less. The ratio σ 10 / τ 10 of the tensile stress σ 10 at 10% and the shear stress τ 10 when the shear stress in the shear test is 10√3% is 1.70 or less, as 　viewed from the longitudinal direction. A shock absorbing member having a Vickers hardness Hvc of 250 or more in the central portion of the wall portion.
[0008]
　(2) The shock absorbing member according to (1) above, wherein the ratio Hvr / Hvc of the Vickers hardness Hvr of the ridgeline portion to the Vickers hardness Hvc of the central portion of the wall portion is 1.05 or more.
[0009]
　(3) The shock absorbing member according to (2) above, wherein the Vickers hardness ratio Hvr / Hvc is 1.10 or more.
[0010]
　(4) The shock absorbing member according to any one of (1) to (3) above, wherein the shock absorbing member is a pillar, a side sill, or a roof side rail.
[0011]
　(5) The shock absorbing member is a member having a closed cross section,
　and the ratio Hvr / Hvc of the Vickers hardness Hvr of the ridgeline portion to the Vickers hardness Hvc of the central portion of the wall portion is less than 1.10. The shock absorbing member according to (1).
[0012]
　(6) The shock absorbing member according to (1) or (5) above, wherein the shock absorbing member is a front side member, a crash box, a rear side member, a subframe, or a floor cross member.
[0013]
　(7) The impact absorbing member according to any one of (1) to (6) above, wherein the impact absorbing member has a tensile strength of 780 MPa or more.
[0014]
　(8) A first heat treatment step in which the steel plate is heat-treated to be held at a temperature T1 satisfying the following formula (1) for 60 to 900 seconds, and the
　steel plate after the heat treatment is subjected to cold plastic processing to be steel. A cold plastic working step as a member and a
　second heat treatment step of heat-treating the steel member at a temperature T2 of 80 to 200 ° C. for 300 to 1800 seconds are provided, and the
　steel plate is made of martensite. The area fraction is 5% or more and less than 95%, the tensile strength is 780 MPa or more, and the ratio Cmax / Cmin of the maximum value Cmax and the minimum value Cmin of the Si content in the cross section in the thickness direction is 1.25. The following is a
　method for manufacturing a shock absorbing member.
　80 × Si + 100 ≦ T1 ≦ 125 × Si + 250 ... (1)
　However, Si in the above formula (1) means the Si content (mass%) in the steel sheet.
[0015]
　(9) The shock absorbing member includes a ridge line portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridge line portion, and the
　cold plastic working is foam molding. ). The method for manufacturing a shock absorbing member.
[0016]
　(10) The shock absorbing member includes a ridgeline portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridgeline portion, and the
　cold plastic working is draw forming. ). The method for manufacturing a shock absorbing member.
[0017]
　(11) The method for manufacturing a shock absorbing member according to any one of (8) to (10) above, wherein the steel sheet is a Dual-Phase steel sheet.
[0018]
　(12) The method for manufacturing a shock absorbing member according to any one of (8) to (11) above, wherein the first heat treatment step is carried out on the steel sheet after final annealing and before winding on a coil. ..
[0019]
　(13) The first heat treatment step is carried out on the steel sheet wound and rewound by a coil after final annealing, or the steel sheet rewound and bent and straightened. The method for manufacturing a shock absorbing member according to any one of (11).
[0020]
　(14) Any of the above (8) to (11), wherein the first heat treatment step is carried out on the steel sheet that has been wound up by a coil, rewound, bent and straightened, and blanked after the final annealing. The method for manufacturing a shock absorbing member according to item 1.
[0021]
　(15) The area fraction of martensite is 5% or more and less than 95%, the tensile strength is 780 MPa or more, and the ratio of the maximum value Cmax and the minimum value Cmin of the Si content in the cross section in the thickness direction is Cmax /. A method for producing a steel sheet for cold plastic working, wherein a steel sheet having a Cmin of 1.25 or less is heat-treated to be held at a temperature T1 satisfying the following formula (1) for 60 to 900 seconds.
　80 × Si + 100 ≦ T1 ≦ 125 × Si + 250 ... (1)
　However, Si in the above formula (1) means the Si content (mass%) in the steel sheet.
The invention's effect
[0022]
　According to the present invention, the impact absorption energy can be increased and the steel plate as the material can be thinned.
A brief description of the drawing
[0023]
FIG. 1 is a schematic perspective view of a main part of an automobile body having a shock absorbing member according to an embodiment of the present invention.
FIG. 2A is a cross-sectional view of a front side member, showing a state in which the front side member is viewed in the length direction. FIG. 2B is a cross-sectional view of the center pillar base, showing a state in which the center pillar base is viewed in the height direction. FIG. 2C is a cross-sectional view of the center pillar main body, showing a state in which the center pillar main body is viewed in the height direction.
FIG. 3 is an image diagram showing a precipitation state of precipitates in a high-strength steel plate as a material of a shock absorbing member according to the present embodiment.
FIG. 4 is a schematic view showing a process of manufacturing a shock absorbing member from a high-strength steel plate, and shows a case where a first heat treatment step is performed at a steel mill that manufactures a high-strength steel plate.
FIG. 5 is a schematic view showing a process of manufacturing a shock absorbing member from a high-strength steel plate, and shows a case where a first heat treatment step is performed at a coil center.
FIG. 6 is a schematic view showing a process of manufacturing a shock absorbing member from a high-strength steel plate, and shows a case where a first heat treatment step is performed at a parts factory (molding factory) of the shock absorbing member. ..
7 (A) is a side view of a bent component viewed from a direction orthogonal to the longitudinal direction, and FIG. 7 (B) is a view of the bent component viewed from a longitudinal direction.
FIG. 8A is a schematic diagram of a shear test. FIG. 8B is an explanatory diagram relating to the shear test.
FIG. 9A is a graph showing the relationship between tensile strength and bending load. FIG. 9B is a graph showing the relationship between the tensile strength and the impact absorption energy at the time of bending deformation.
FIG. 10A is a graph showing an example of the relationship between the amount of deformation (stroke) of the bending member in the direction perpendicular to the length of the bending member and the load generated on the bending member. FIG. 10B is a graph showing an example of the relationship between the amount of deformation (stroke) of the bending member in the direction perpendicular to the length of the bending member and the absorbed energy of the bending member.
11 (A) is a side view of a shaft crushing component, and FIG. 11 (B) is a cross-sectional view taken along the line XIB-XIB of FIG. 11 (A).
FIG. 12 is a graph showing the relationship between tensile strength and impact absorption energy at the time of axial crush deformation.
Embodiment for carrying out the invention
[0024]
　Hereinafter, the circumstances leading to the idea of ​​the present invention will be described first, and then the embodiments will be described in detail.
[0025]
In
　order to reduce the weight of an automobile body, the thickness of the body members constituting the automobile body is being reduced. The vehicle body member includes a shock absorbing member. The shock absorbing member is a member that absorbs a shock when an automobile collides with an object. Examples of the shock absorbing member include a shaft crushing part and a bending part.
[0026]
　The shaft crushed part absorbs an impact by being crushed while being compressed in the axial direction of the shaft crushed part. Examples of the axial crushing component include a front side member, a crash box, a rear side member, a subframe, and a floor cross member.
[0027]
　The bent part absorbs the impact by bending and deforming the bent part while receiving the bending force. Examples of the bending component include an A pillar, a center pillar (B pillar), a C pillar, a side sill, and a roof side rail.
[0028]
　Such a shock absorbing member is made by cutting a cold-rolled steel sheet (hereinafter, this steel sheet is also referred to as a "high-strength steel sheet" or a "cold plastic working steel sheet"), press-molding, painting, and baking-painting after painting. It may be molded by performing (baking and hardening treatment). In the steel sheet which is the material of such a shock absorbing member, a baking treatment is usually performed as a heat treatment after the final annealing. Then, it was generally considered that if the tempering treatment is performed after the final annealing, the amount of baking hardening is rather lowered. However, as a result of diligent research, the inventor of the present application has found that the amount of curing by annealing can be further increased by performing a tempering treatment under certain conditions after the final annealing and then performing a baking hardening treatment. This led to the idea of ​​the present invention.
[0029]

　Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0030]

　FIG. 1 is a schematic perspective view of a main part of an automobile body 100 (hereinafter, also simply referred to as “vehicle body 100”) having a shock absorbing member according to an embodiment of the present invention. With reference to FIG. 1, the vehicle body 100 is, for example, a passenger car. Examples of the passenger car include a sedan type passenger car, a coupe type passenger car, a hatchback type passenger car, a minivan type passenger car, an SUV (Sport Utility Vehicle) type passenger car, and the like.
[0031]
　In the present embodiment, the vehicle length direction, the vehicle width direction, and the vehicle height direction of the vehicle body 100 are referred to as a length direction X, a width direction Y, and a height direction Z, respectively.
[0032]
　The vehicle body 100 has a front side member 1 which is a shock absorbing member and is a shaft crushing component, a crash box 2, a rear side member 3, a floor cross member 4, and a subframe 5. These shaft crushed parts absorb impact by plastically deforming in the axial direction of the shaft crushed parts and shrinking while being crushed at the time of a collision of an automobile.
[0033]
　Further, the vehicle body 100 has an A pillar 11 as a shock absorbing member and a bending component, a center pillar 12, a C pillar 13, a side sill 14, and a roof side rail 15. These bent parts absorb impact by bending deformation (plastic deformation) at the time of a collision of an automobile.
[0034]
　It should be noted that the heat treatments applied to the shaft crushed parts from the steel plate as the raw material to the forming into the shaft crushed parts are the same as each other. Therefore, in the following, the front side member 1 will be described as an example of the shaft crushing component, and the detailed description of the other shaft crushing component will be omitted. Similarly, the heat treatments applied to the bent parts from the steel plate as the raw material to the formed into the bent parts are the same as each other. Therefore, in the following, the center pillar 12 will be described as an example of the bending component, and the detailed description of the other bending component will be omitted.
[0035]

　FIG. 2A is a cross-sectional view of the front side member 1 and shows a state in which the front side member 1 is viewed in the length direction X. The front side member 1 shown in FIGS. 1 and 2A is configured to collapse along the length direction X when an impact load is applied from the front of the automobile. That is, the front side member 1 is used as a shock absorbing member at the time of an automobile collision, particularly at the time of a frontal collision. A pair of left and right floor side members 1 are arranged on the front portion of the vehicle body 100.
[0036]
　The front side member 1 is a hollow member having a closed cross section formed by welding a plurality of steel plates, and is formed in an elongated beam shape with the length direction X as the longitudinal direction. In at least a part of the front side member 1 in the length direction X, a cross section orthogonal to the length direction X constitutes a closed cross section. The "closed cross section" means that the member is formed in an endless annular shape in a cross section orthogonal to the longitudinal direction at least in a part of the longitudinal direction of the member. This closed cross-sectional shape is, for example, a rectangular shape.
[0037]
　The front side member 1 has a first half portion 20 constituting one side portion of the front side member 1 in the width direction Y and a second half portion 30 constituting the other side portion of the front side member 1 in the width direction Y. And have.
[0038]
　The first half portion 20 and the second half portion 30 are each formed into a thin plate shape by pressing a steel plate. This steel sheet is preferably a high-strength steel sheet, and the tensile strength of this steel sheet is preferably 780 MPa or more. This steel sheet is more preferably an ultra-high strength steel sheet, and the tensile strength in this case is preferably 980 MPa or more, more preferably 1180 MPa, still more preferably 1470 MPa or more. By forming the front side member 1 as a shock absorbing member from the high-strength steel plate in this way, it is possible to generate sufficient shock absorbing energy while reducing the plate thickness of the front side member 1.
[0039]
　The first half portion 20 and the second half portion 30 are each formed in a hat shape when viewed from the length direction X. The first half portion 20 and the second half portion 30 are integrated by flange coupling to form a front side member 1. It should be noted that any one of the first half portion 20 and the second half portion 30 may be formed in a flat plate shape which is not bent when viewed from the length direction X. Further, the first half portion 20 and the second half portion 30 are integrally molded by a tubular member formed in an annular shape (for example, a polygonal annular shape such as a rectangle, an annular shape, or an elliptical shape) when viewed from the length direction X. (For example, see FIG. 11 (B) described later). In the following, unless otherwise specified, the front side member 1 will be described with reference to a state seen from the length direction X.
[0040]
　The first half portion 20 includes a first wall portion 21, a pair of first ridge line portions 22, 22, a pair of second wall portions 23, 23, a pair of second ridge line portions 24, 24, and a pair of flanges. It has 25, 25, and so on.
[0041]
　The first wall portion 21 is a vertical wall in the present embodiment, is a ceiling wall in the hat shape of the first half portion 20, and extends straight in the height direction Z. A pair of first ridge line portions 22, 22 are continuous with a pair of end portions of the first wall portion 21 in the height direction Z. In the present embodiment, the first ridge line portion 22 is formed in a bent shape when viewed from the length direction X, and has a predetermined radius of curvature. The first ridge line portion 22 advances toward the inside of the width direction Y from the first wall portion 21 toward the second wall portion 23. A pair of second wall portions 23, 23 are continuous with the pair of first ridge line portions 22, 22. In the present embodiment, the second wall portion 23 is a horizontal wall, and in the hat shape of the first half portion 20, it is a vertical wall connecting the ceiling wall (first wall portion 21) and the flange 25, and is X in the length direction. Seen in, it extends straight in the width direction Y. The pair of second wall portions 23, 23 may extend in a tapered shape so that the distance between the pair of second wall portions 23, 23 increases toward the center side of the front side member 1 in the width direction Y.
[0042]
　A pair of second ridge line portions 24, 24 are continuous with the vehicle inner end portion of the pair of second wall portions 23 in the width direction Y. In the present embodiment, the second ridge line portion 24 is formed in a bent shape when viewed from the length direction X, and has a predetermined radius of curvature. On the other hand, the second ridge line portion 24 advances upward from the corresponding second wall portion 23. The other second ridge line portion 24 advances downward from the corresponding second wall portion 23.
[0043]
　In the above configuration, the radius of curvature of the boundary between the first ridge line portion 22 and the first wall portion 21 and the boundary between the first ridge line portion 22 and the second wall portion 23 changes in the length direction X. It is a part. Similarly, the boundary between the second ridge line portion 24 and the second wall portion 23 and the boundary between the second ridge line portion 24 and the flange 25 are portions where the radius of curvature changes in the length direction X.
[0044]
　With the above configuration, the first wall portion 21 and the second wall portion 23 extend from the first ridge line portion 22, and the second wall portion 23 and the flange 25 extend from the second ridge line portion 24.
[0045]
　The second half portion 30 includes a first wall portion 31, a pair of first ridge line portions 32, 32, a pair of second wall portions 33, 33, a pair of second ridge line portions 34, 34, and a pair of flanges. It has 35, 35 and so on.
[0046]
　In the present embodiment, the second half portion 30 is formed in a shape symmetrical to the first half portion 20 in the width direction Y. Therefore, the description of each part of the second half part 30 will be omitted. The first half portion 20 and the second half portion 30 are fixed to each other by joining a pair of flanges 25 and 25 and a pair of flanges 35 and 35. The method of joining the pair of flanges 25, 25 and the pair of flanges 35, 35 includes welding of spots, lasers, arcs, etc., mechanical joining such as rivets, caulking, bolt fastening, and bonding with an adhesive or the like.
[0047]
　With respect to the front side member 1 of the present embodiment, the shear strain in the tensile test is applied to the wall portion of the front side member 1, that is, the first wall portion 21, the pair of second wall portions 23, 23, or the pair of flanges 25, 25. The ratio of the tensile stress σ 10 when is 10 % and the shear stress τ 10 when the shear strain in the shear test is 10√3% (about 17.32%) is 1.70 or less. Become. When the first half portion 20 is formed by using a steel plate having excellent seizure curability, the above ratio σ 10 / τ 10 is 1.70 or less. As a result, collision resistance (for example, shaft crush resistance) is improved. The reason why the ratio σ 10 / τ 10 when the shear strain is √3 times the elongation strain is adopted is that the nominal strain (elongation strain) in the tensile test is set according to the yield condition of von Misses (tensile test). This is because the conversion coefficient is √3 when converted to the shear strain in the shear test (of the same equivalent strain as). As used herein, unless otherwise specified, "stress" refers to the nominal stress, i.e., (load / initial test piece cross-sectional area).
[0048]
　In the present embodiment, the first half portion 20 constituting the front side member 1 is manufactured by using a cold-rolled steel plate having high seizure curability. Therefore, the ratio σ 10 / τ 10 in the wall portion whose hardness or strength is further increased by the baking hardening heat treatment is 1.70 or less. A steel sheet having such characteristics can be manufactured by optimizing the manufacturing process, as will be described in detail later. If necessary, the upper limit of this ratio σ 10 / τ 10 may be 1.65, 1.60, 1.56 or 1.53. It is not necessary to set the lower limit of this ratio σ 10 / τ 10 in particular, but it may be 1.10, 1.20 or 1.30.
　Since the σ / τ ratio decreases as the strain increases , the ratio σ 10 / τ 10 also becomes 1.70 or less when the ratio σ 5 / τ 5 is 1.70 or less, which will be described later. Therefore, setting the ratio σ 10 / τ 10 to 1.70 or less means that the ratio σ 5 / τ 5May be changed to 1.70 or less.
[0049]
　Further, in the front side member 1, the Vickers hardness Hvc250 or more in the central portion 27 of the first wall portion 21, that is, Hvc ≧ 250. The Vickers hardness in the present specification means the hardness according to the JIS Z 2244: 2009 Vickers hardness test. The Vickers hardness here is HV0.5, which is the Vickers hardness when the test force is 4.903 N (0.5 kgf), and 5 points are measured and the average value is used as the test result. The central portion 27 is a central portion of the first wall portion 21, which is a wall portion between the pair of first ridge line portions 22, 22. By setting the tensile strength of the steel plate constituting the front side member 1 to 780 MPa, Hvc ≧ 250 can be realized. In other words, since Hvc ≧ 250, it can be estimated that the tensile strength of the steel plate constituting the front side member 1 is 780 MPa. If necessary, the lower limit of Hvc may be 270, 290, 310 or 340. It is not necessary to set the upper limit of Hvc in particular, but it may be 500, 450, 410 or 370.
[0050]
　Further, in the front side member 1, the ratio Hvr / Hvc between the Vickers hardness Hvr in the first ridge line portion 22 and the Vickers hardness Hvc in the central portion 27 of the first wall portion 21 continuous with the first ridge line portion 22 is determined. It is less than 1.10. By setting this ratio Hvr / Hvc to less than 1.10, it is possible to secure a material that contributes to shaft crushing resistance, and the collision performance is improved. This ratio Hvr / Hvc is more preferably less than 1.09, less than 1.07 or less than 1.06. The ratio Hvr / Hvc is preferably 1.00 or more. When this ratio Hvr / Hvc is less than 1.00, there is a remarkable difference in strength between the first ridge line portion 22 and the central portion 27, so that it is easy to break at the boundary. Therefore, it is desirable that this ratio is 1.00 or more. If necessary, the lower limit of this ratio may be 1.02 or 1.04.
[0051]
The relationship of the ratio σ 10 / τ 10 between the tensile stress σ 10 and the shear stress τ 10 　at the wall portion of the first half portion 20 is the first wall portion 31 of the second half portion 30 and the pair of second parts. The same applies to the relationship of the ratio σ 10 / τ 10 of the tensile stress σ 10 and the shear stress τ 10 at the wall portions 33, 33 or the pair of flanges 35, 35 . Further, the Vickers hardness ratio Hvr / Hvc in the central portion 27 of the first ridge line portion 22 and the first wall portion 21 is the central portion 37 of the first ridge line portion 32 and the first wall portion 31 in the second half portion 30. Similarly, the relationship of the Vickers hardness ratio Hvr / Hvc in.
[0052]
As described above, the front side member 1 has a ratio of σ 10 / τ 10 　in the (a1) wall portion (first wall portion 21, pair of second wall portions 23, 23, pair of flanges 25, 25, etc.). Satisfying the conditions of ≦ 1.70 and (a2) Vickers hardness Hvc ≧ 250 in the central portion 27 of the first wall portion 21, as a more preferable configuration, (a3) ​​the first ridge portion 22 The ratio of the Vickers hardness Hvr to the Vickers hardness of the central portion 27 of the first wall portion 21 is Hvr / Hvc <1.10. All three conditions are satisfied. According to this configuration, when the front side member 1 is manufactured using a cold-rolled steel sheet having excellent seizure curability, which is suitable as a steel sheet for automobiles having a tensile strength of 780 MPa or more (including baking heat treatment), the above-mentioned σ The / τ ratio characteristic (ratio σ 10 / τ 10 ≦ 1.70) is satisfied, and the collision resistance performance (shaft crush resistance characteristic) of the front side member 1 is improved. This effect is further enhanced by the fact that the front side member 1 is formed by using the second half portion 30 and the first half portion 20 having the same configuration as the first half portion 20.
[0053]
　The above is the schematic configuration of the front side member 1. Next, the configuration of the center pillar 12 will be described.
[0054]

　The center pillar 12 is configured to be deformed toward the vehicle center side in the width direction Y when an impact load is received from the side of the automobile. That is, the center pillar 12 is used as a shock absorbing member at the time of a collision of an automobile, particularly at the time of a side collision. The center pillars 12 are arranged in pairs on the vehicle body 100 at substantially the center of the cabin of the automobile in the length direction X. The center pillar 12 is joined to the side sill 14 and the roof side rail 15.
[0055]
　The center pillar 12 is a hollow member having a closed cross section formed by welding a plurality of steel plates, and is formed in an elongated columnar shape with the height direction Z as the longitudinal direction. In at least a part of the center pillar 12 in the height direction Z, the cross section orthogonal to the height direction Z constitutes a closed cross section. This closed cross-sectional shape is, for example, a rectangular shape.
[0056]
　The center pillar 12 has a center pillar base 16 joined to the side sill 14 and a center pillar main body 17 extending upward from the center pillar base 16 and joined to the roof side rail 15.
[0057]
　FIG. 2B is a cross-sectional view of the center pillar base 16 and shows a state in which the center pillar base 16 is viewed in the height direction Z. In FIG. 2 (B), the portion on the back side of the cross section of the center pillar base 16 in FIG. 2 (B) is not shown. With reference to FIGS. 1 and 2B, the center pillar base 16 is formed in a portion of the center pillar 12 near the bottom, and in the height direction Z, for example, from the upper position of the side sill 14 to the floor. It extends toward the height position of the upper end of the cross member 4. The center pillar base 16 is joined and fixed to each of the side sill 14 and the center pillar main body 17 by the same method as the above-mentioned joining method in the front side member 1.
[0058]
　The center pillar base 16 is supported at both ends by the side sill 14 and the center pillar main body 17, and the support span SP1 between the side sill 14 and the center pillar main body 17 is relatively shortened. Therefore, the center pillar base 16 is deformed so that the closed cross-sectional shape in the cross section orthogonal to the height direction Z is crushed at the time of a side collision.
[0059]
　The center pillar base 16 includes a third half portion 40 that constitutes an outer portion of the center pillar base 16 in the width direction Y, and a fourth half portion 50 that constitutes an inner portion of the center pillar base 16 in the width direction Y. have.
[0060]
　The third half portion 40 and the fourth half portion 50 are each formed into a thin plate shape by pressing a steel plate. This steel sheet is preferably a high-strength steel sheet. The tensile strength of this steel sheet is set in the same manner as the tensile strength of the front side member 1 described above, and is preferably 780 MPa or more, more preferably 1470 MPa or more. By forming the center pillar 12 as a shock absorbing member from the high-strength steel plate in this way, it is possible to generate sufficient shock absorbing energy while reducing the plate thickness of the center pillar 12.
[0061]
　The third half portion 40 and the fourth half portion 50 are each formed in a hat shape when viewed from the height direction Z. The third half portion 40 and the fourth half portion 50 are integrated by flange coupling to form a center pillar base 16. In addition, any of the third half portion 40 and the fourth half portion 50 may be formed in a flat plate shape which is not bent when viewed from the height direction Z. Further, the third half portion 40 and the fourth half portion 50 are integrally molded by a tubular member formed in an annular shape (for example, a polygonal annular shape such as a rectangle, an annular shape, or an elliptical shape) when viewed from the height direction Z. (For example, see FIG. 11 (B) described later). In the following, unless otherwise specified, the center pillar 12 will be described with reference to a state viewed from the height direction Z.
[0062]
　The third half portion 40 includes a first wall portion 41, a pair of first ridge line portions 42, 42, a pair of second wall portions 43, 43, a pair of second ridge line portions 44, 44, and a pair of flanges. It has 45, 45 and.
[0063]
　The first wall portion 41 is the outer wall of the center pillar base 16 in the present embodiment. A pair of first ridge line portions 42, 42 are continuous with a pair of end portions of the first wall portion 41 in the length direction X. In the present embodiment, the first ridge line portion 42 is formed in a bent shape when viewed from the height direction Z direction X, and has a predetermined radius of curvature. The first ridge line portion 42 advances toward the inside of the width direction Y from the first wall portion 41 toward the second wall portion 43. A pair of second wall portions 43, 43 are continuous with the pair of first ridge line portions 42, 42. In the present embodiment, the pair of second wall portions 43 are a front wall and a rear wall, and extend in the width direction Y. The pair of second wall portions 23, 23 may extend in a tapered shape so that the distance between the second wall portions 23, 23 increases toward the center side of the automobile in the width direction Y.
[0064]
　A pair of second ridges 44, 44 are continuous with the vehicle inner end of the pair of second wall portions 43 in the width direction Y. In the present embodiment, the second ridge line portion 44 is formed in a bent shape when viewed from the height direction Z, and has a predetermined radius of curvature. On the other hand, the second ridge line portion 44 advances forward from the corresponding second wall portion 43. The other second ridge line portion 24 advances rearward from the corresponding second wall portion 23.
[0065]
　In the above configuration, the radius of curvature of the boundary between the first ridge line portion 42 and the first wall portion 41 and the boundary between the first ridge line portion 42 and the second wall portion 43 changes in the height direction Z. It is a part. Similarly, the boundary between the second ridge line portion 44 and the second wall portion 43 and the boundary between the second ridge line portion 44 and the flange 45 are portions where the radius of curvature changes in the height direction Z.
[0066]
　With the above configuration, the first wall portion 41 and the second wall portion 43 extend from the first ridge line portion 42, and the second wall portion 43 and the flange 45 extend from the second ridge line portion 44.
[0067]
　The fourth half portion 50 includes a first wall portion 51, a pair of first ridge line portions 52, 52, a pair of second wall portions 53, 53, a pair of second ridge line portions 54, 54, and a pair of flanges. It has 55, 55 and.
[0068]
　In the present embodiment, the fourth half portion 50 is formed in a shape substantially symmetrical to the third half portion 40 in the width direction Y. Therefore, the description of each part of the fourth half part 50 will be omitted. The third half portion 40 and the fourth half portion 50 are fixed to each other by joining a pair of flanges 45 and 45 and a pair of flanges 55 and 55. The method of joining the pair of flanges 45, 45 and the pair of flanges 55, 55 is the same as the joining method described above for the front side member 1.
[0069]
　In the center pillar base 16, the elongation strain in the tensile test is 5 at the wall portion of the third half portion 40, that is, the first wall portion 41, the pair of second wall portions 43, 43, or the pair of flanges 45, 45. The ratio σ 5 / τ 5 between the tensile stress σ 5 at% and the shear stress τ 5 when the shear strain in the shear test is 5√3% (about 8.66%) is 1.70 or less. Since the σ / τ ratio decreases as the strain increases, if the ratio σ 5 / τ 5 is 1.70 or less, the ratio σ 10 / τ 10 at the wall of the third half 40 is also 1.70 or less . Will be. When the third half portion 40 is manufactured by using a cold-rolled steel sheet having low seizure curability, this ratio σ 5 / τ 5 also exceeds 1.70. In this case, the material does not undergo plastic deformation and breaks immediately, and sufficient collision resistance (for example, bending resistance) as a member cannot be obtained. Therefore, this ratio σ 5 / τ 5 is set to 1.70 or less.
[0070]
　In the present embodiment, the third half portion 40 constituting the center pillar base 16 is manufactured by using a cold-rolled steel plate having high seizure curability. Therefore, the ratio σ 5 / τ 5 in the wall portion whose hardness or strength is further increased by the baking hardening heat treatment is 1.70 or less. Cold-rolled steel sheets having such characteristics can be manufactured by optimizing the manufacturing process, as will be described in detail later. If necessary, the upper limit of this ratio σ 5 / τ 5 may be 1.65, 1.60, 1.56 or 1.53. It is not necessary to set the lower limit of this ratio σ 5 / τ 5 in particular, but it may be 1.10, 1.20 or 1.30.
[0071]
　Further, in the third half portion 40 of the center pillar base 16, the Vickers hardness Hvc in the central portion 47 of the second wall portion 43 is 250 or more, that is, Hvc ≧ 250. The central portion 47 is a central portion of the second wall portion 43 which is a wall portion between the first ridge line portion 42 and the second ridge line portion 44 continuous with the flange 45. By setting the tensile strength of the steel plate constituting the third half portion 40 to 780 MPa, Hvc ≧ 250 can be realized. In other words, since Hvc ≧ 250, it can be estimated that the tensile strength of the steel sheet constituting the third half portion 40 is 780 MPa. If necessary, the lower limit of Hvc may be 270, 290, 310 or 340. It is not necessary to set the upper limit of Hvc in particular, but it may be 500, 450, 410 or 370.
[0072]
　Further, in the center pillar base 16, the Vickers hardness Hvr at the first ridge line portion 42 of the third half portion 40 and the Vickers hardness Hvc at the central portion 47 of the second wall portion 43 continuous with the first ridge line portion 42. The ratio of Hvr / Hvc is 1.05 or more. By setting this ratio Hvr / Hvc to 1.05 or more, work hardening can be enhanced, the strength of the material contributing to bending deformation can be ensured, and the collision performance is enhanced. By setting this ratio Hvr / Hvc to 1.07 or more, 1.09 or more, or 1.10 or more, the collision performance can be further improved. It is more preferable that the ratio Hvr / Hvc is 1.12 or more, 1.14 or more, or 1.15 or more. When this ratio Hvr / Hvc exceeds 1.30, there is a remarkable difference in strength between the first ridge line portion 42 and the central portion 47, so that it is easy to break at the boundary. Therefore, it is desirable that this ratio Hvr / Hvc is 1.30 or less. If necessary, the upper limit of this ratio may be 1.25 or 1.20.
[0073]
The relationship of the ratio σ 5 / τ 5 between the tensile stress σ 5 and the shear stress τ 5 　at the wall portion of the third half portion 40 is the wall portion of the fourth half portion 50, that is, the first wall portion 51. , The relationship of the ratio σ 5 / τ 5 of the tensile stress σ 5 and the shear stress τ 5 in the pair of second wall portions 53, 53 or the pair of flanges 55, 55 may be similarly established. It does not have to be established. Similarly, the Vickers hardness ratio Hvr / Hvc at the central portion 47 of the first ridge line portion 42 and the second wall portion 43 in the third half portion 40 is the first ridge line portion 52 and the second wall in the fourth half portion 50. The relationship of the Vickers hardness ratio Hvr / Hvc in the central portion 57 of the portion 53 may or may not be established. This is because, in the center pillar base 16, of the third half portion 40 and the fourth half portion 50, the fourth half portion 50 is mainly subjected to an impact load to cause bending deformation. This is because the high impact absorption performance of the third half 40 is not required. That is, in the present embodiment, at least one half portion of the center pillar base 16 (specifically, the third half portion 40 constituting the outer portion of the center pillar base 16 in the width direction Y) has the above-mentioned characteristics (described later). (B1) and (b2), etc.) may be satisfied.
[0074]
As described above, the ratio σ 5 / τ 5 ≦ 　in the (b1) wall portion (first wall portion 41, pair of second wall portions 43, 43, or pair of flanges 45, 45, etc.) of the center pillar base 16. Satisfying the conditions of 1.70 and (b2) Vickers hardness Hvc ≧ 250 in the central portion 47 of the second wall portion 43, as a more preferable configuration, (b3) Vickers of the first ridge line portion 42. All three conditions that the ratio of the hardness Hvr to the Vickers hardness Hvc of the central portion 47 of the second wall portion 43 is Hvr / Hvc ≧ 1.05 are satisfied. According to this configuration, when the center pillar base 16 is manufactured using a cold-rolled steel sheet having excellent seizure curability, which is suitable as a steel sheet for automobiles having a tensile strength of 780 MPa or more (including a seizure heat treatment), the above-mentioned σ The / τ ratio characteristic (ratio σ 5 / τ 5 ≦ 1.70) is satisfied, and the collision resistance (bending resistance) of the center pillar base 16 is improved.
[0075]
　The above is the schematic configuration of the center pillar base 16. Next, the configuration of the center pillar main body 17 will be described.
[0076]
　FIG. 2C is a cross-sectional view of the center pillar main body 17, showing a state in which the center pillar main body 17 is viewed in the height direction Z. In FIG. 2C, the portion on the back side of the cross section of the center pillar main body 17 in FIG. 2C is not shown. With reference to FIGS. 1 and 2 (C), the center pillar body 17 is provided as an intermediate portion and an upper portion of the center pillar 12, and in the height direction Z, for example, the upper end of the floor cross member 4. It extends from the height position to the roof side rail 15. The center pillar main body 17 is joined and fixed to each of the center pillar base 16 and the roof side rail 15 by the same method as the above-mentioned joining method in the front side member 1.
[0077]
　The center pillar main body 17 is supported at both ends by the center pillar base 16 and the roof side rail 15, and the support span SP2 between the center pillar main body 17 and the roof side rail 15 is relatively long. Therefore, the center pillar main body 17 bends and deforms into a bow shape that is convex inward in the width direction Y at the time of a side collision.
[0078]
　At the time of a side collision, the impact absorption energy in the center pillar body 17 is larger due to bending deformation than due to collapse deformation of the closed cross-sectional shape. On the other hand, the impact absorption energy in the center pillar base 16 is larger due to the collapse deformation of the closed cross-sectional shape than due to the bending deformation. As described above, the aspect of impact absorption at the time of a side collision is different between the center pillar base 16 and the center pillar main body 17. This difference in aspect is due to the difference between the length of the center pillar base 16 and the length of the center pillar main body 17 in the height direction Z. The length of the center pillar body 17 is longer than the length of the center pillar base 16.
[0079]
　The center pillar main body 17 includes a fifth half portion 60 that constitutes an outer portion of the center pillar main body 17 in the width direction Y, and a sixth half portion 70 that constitutes an inner portion of the center pillar main body 17 in the width direction Y. have.
[0080] [0080]
　The fifth half portion 60 and the sixth half portion 70 are each formed into a thin plate shape by pressing a steel plate. This steel sheet is preferably a high-strength steel sheet. The tensile strength of this steel sheet is set in the same manner as the tensile strength of the front side member 1 described above, and is preferably 780 MPa or more, more preferably 1470 MPa or more. The tensile strength of the steel material constituting the center pillar main body 17 is preferably larger than the tensile strength of the steel material constituting the center pillar base 16.
[0081]
　The fifth half portion 60 and the sixth half portion 70 are each formed in a hat shape when viewed from the height direction Z. The fifth half portion 60 and the sixth half portion 70 are integrated by flange coupling to form the center pillar main body 17. In addition, any of the fifth half portion 60 and the sixth half portion 70 may be formed in a flat plate shape which is not bent when viewed from the height direction Z. Further, the fifth half portion 60 and the sixth half portion 70 are integrally molded by a tubular member formed in an annular shape (for example, a polygonal annular shape such as a rectangle, an annular shape, or an elliptical shape) when viewed from the height direction Z. (For example, see FIG. 11 (B) described later).
[0082]
　The fifth half portion 60 includes a first wall portion 61, a pair of first ridge line portions 62, 62, a pair of second wall portions 63, 63, a pair of second ridge line portions 64, 64, and a pair of flanges. It has 65, 65 and.
[0083]
　The first wall portion 61 is the outer wall of the center pillar main body 17 in the present embodiment. A pair of first ridge line portions 62, 62 are continuous with a pair of end portions of the first wall portion 61 in the length direction X. In the present embodiment, the first ridge line portion 62 is formed in a bent shape when viewed from the height direction Z, and has a predetermined radius of curvature. The first ridge line portion 62 advances toward the inside of the width direction Y from the first wall portion 61 toward the second wall portion 63. A pair of second wall portions 63, 63 are continuous with the pair of first ridge line portions 62, 62. In the present embodiment, the pair of second wall portions 63, 63 are a front wall and a rear wall, and extend in the width direction Y. The pair of second wall portions 63, 63 may extend in a tapered shape so that the distance between the second wall portions 63 and 63 increases toward the center side of the automobile in the width direction Y.
[0084]
　A pair of second ridge line portions 64, 64 are continuous with the vehicle inner end portion of the pair of second wall portions 63 in the width direction Y. In the present embodiment, the second ridge line portion 64 is formed in a bent shape when viewed from the height direction Z, and has a predetermined radius of curvature. On the other hand, the second ridge line portion 64 advances forward from the corresponding second wall portion 63. The other second ridge line portion 64 advances rearward from the corresponding second wall portion 63.
[0085]
　In the above configuration, the radius of curvature of the boundary between the first ridge line portion 62 and the first wall portion 61 and the boundary between the first ridge line portion 62 and the second wall portion 63 changes in the height direction Z. It is a part. Similarly, the boundary between the second ridge line portion 64 and the second wall portion 63 and the boundary between the second ridge line portion 64 and the flange 65 are portions where the radius of curvature changes in the height direction Z.
[0086]
　With the above configuration, the first wall portion 61 and the second wall portion 63 extend from the first ridge line portion 62, and the second wall portion 63 and the flange 65 extend from the second ridge line portion 64.
[0087]
　The sixth half portion 70 includes a first wall portion 71, a pair of first ridge line portions 72, 72, a pair of second wall portions 73, 73, a pair of second ridge line portions 74, 74, and a pair of flanges. It has 75, 75 and.
[0088]
　In the present embodiment, the sixth half portion 70 is formed in a shape substantially symmetrical to the first half portion 60 in the width direction Y. Therefore, the description of each part of the second half part 70 will be omitted. The first half portion 60 and the second half portion 70 are fixed to each other by joining a pair of flanges 65 and 65 and a pair of flanges 75 and 75. The method of joining the pair of flanges 65, 65 and the pair of flanges 75, 75 is the same as the joining method described above for the front side member 1.
[0089]
　In the center pillar main body 17, the elongation strain in the tensile test is 5 in the wall portion of the fifth half portion 60, that is, the first wall portion 61, the pair of second wall portions 63, 63, or the pair of flanges 65, 65. The ratio of the tensile stress σ 5 at% to the shear stress τ 5 when the shear strain in the shear test is 5√3% (about 8.66%) is 1.70 or less. Since the σ / τ ratio decreases as the strain increases, when the ratio σ 5 / τ 5 is 1.70 or less, the ratio σ 10 / τ 10 in the fifth half 60 is also 1.70 or less. When the fifth half portion 60 is manufactured using a steel plate having low seizure curability, the ratio σ 5 / τ 5 also exceeds 1.70. In this case, the material does not undergo plastic deformation and breaks immediately, and sufficient impact resistance (for example, bending resistance) as a member cannot be obtained. Therefore, this ratio ratio σ 5 / τ 5 is set to 1.70 or less.
[0090]
　In the present embodiment, the fifth half portion 60 constituting the center pillar main body 17 is manufactured by using a cold-rolled steel plate having a high degree of baking hardening. Therefore, the ratio σ 5 / τ 5 ≦ 1.70 is obtained in the wall portion whose hardness or strength is further increased by the baking hardening heat treatment. A steel sheet having such characteristics can be manufactured by optimizing the manufacturing process, as will be described in detail later. If necessary, the upper limit of this ratio σ 5 / τ 5 may be 1.65, 1.60, 1.56 or 1.53. It is not necessary to set the lower limit of this ratio σ 5 / τ 5 in particular, but it may be 1.10, 1.20 or 1.30.
[0091]
　Further, in the fifth half portion 60 of the center pillar main body 17, the Vickers hardness Hvc in the central portion 67 of the second wall portion 63 is 250 or more, that is, Hvc ≧ 250. The central portion 67 is a central portion of the second wall portion 63, which is a wall portion between the first ridge line portion 62 and the second ridge line portion 64 continuous with the flange 65. By setting the tensile strength of the steel plate constituting the fifth half portion 60 to 780 MPa, Hvc ≧ 250 can be realized. In other words, since Hvc ≧ 250, it can be estimated that the tensile strength of the steel sheet constituting the fifth half portion 60 is 780 MPa. If necessary, the lower limit of Hvc may be 270, 290, 310 or 340. It is not necessary to set the upper limit of Hvc in particular, but it may be 500, 450, 410 or 370.
[0092]
　Further, in the center pillar main body 17, the Vickers hardness Hvr at the first ridge line portion 62 of the fifth half portion 60 and the Vickers hardness Hvc at the central portion 67 of the second wall portion 63 continuous with the first ridge line portion 62. The ratio of Hvr / Hvc is 1.05 or more. By setting this ratio Hvr / Hvc to 1.05 or more, work hardening can be enhanced, the strength of the material contributing to bending deformation can be ensured, and the collision performance is enhanced. By setting this ratio Hvr / Hvc to 1.07 or more, 1.09 or more, or 1.10 or more, the collision performance can be further improved. In the center pillar 12, since the center pillar main body 17 is a long span member having a long support span in the height direction Z, the amount of deflection becomes large, and as a result, the characteristics of the first ridge line portion 62 have an impact absorption performance. Especially affects. Therefore, the ratio Hvr / Hvc in the center pillar main body 17 is more preferably 1.10 or more. It is even more preferable that the ratio Hvr / Hvc is 1.12 or more, 1.14 or more, or 1.15 or more. The ratio Hvr / Hvc is preferably 1.30 or less. When this ratio Hvr / Hvc exceeds 1.3, there is a remarkable difference in strength between the first ridge line portion 61 and the central portion 67, so that it is easy to break at the boundary. Therefore, it is desirable that this ratio Hvr / Hvc is 1.30 or less. If necessary, the upper limit of this ratio may be 1.25 or 1.20.
[0093]
The relationship of the ratio σ 5 / τ 5 between 　the tensile stress and the shear stress at the wall portion of the fifth half portion 60 is the wall portion of the sixth half portion 70, that is, the first wall portion 71, a pair of first walls. Similarly, the relationship of the ratio σ 5 / τ 5 between the tensile strong stress and the shear stress at the two wall portions 73, 73 or the pair of flanges 75, 75 may or may not be established. Similarly, the Vickers hardness ratio Hvr / Hvc at the central portion 67 of the first ridge line portion 62 and the second wall portion 63 in the fifth half portion 60 is the first ridge line portion 72 and the second wall in the sixth half portion 70. The relationship of the Vickers hardness ratio Hvr / Hvc in the central portion 77 of the portion 73 may or may not be established. This is because, in the center pillar main body 17, of the first half portion 60 and the second half portion 70, mainly the first half portion 60 receives an impact load and undergoes bending deformation. This is because the high impact absorption performance of the first half 60 is not required. That is, in the present embodiment, at least one half portion of the center pillar main body 17 (specifically, the fifth half portion 60 constituting the outer portion of the center pillar main body 17 in the width direction Y) has the above-mentioned characteristics (described later). (C1) and (c2), etc.) may be satisfied.
[0094]
　As described above, the fifth half portion 60 of the center pillar main body 17 is formed on the (c1) wall portion (first wall portion 61, pair of second wall portions 63, 63, pair of flanges 65, 65, etc.). As a more preferable configuration, (c3) satisfies the conditions that the ratio σ 5 / τ 5 ≦ 1.70 and (c2) the Vickers hardness Hvc ≧ 250 in the central portion 67 of the second wall portion 63. All three conditions that the ratio of the Vickers hardness Hvr of the first ridge line portion 62 to the Vickers hardness Hvc of the central portion 67 of the second wall portion 63 is Hvr / Hvc ≧ 1.05 are satisfied. According to this configuration, when the center pillar main body 17 is manufactured using a cold-rolled steel sheet having excellent seizure curability, which is suitable as a steel sheet for automobiles having a tensile strength of 780 MPa or more (including a baking heat treatment), the above-mentioned σ The / τ ratio characteristic (ratio σ 5 / τ 5 ≦ 1.70) is satisfied, and the collision resistance (bending resistance) of the center pillar main body 17 is improved.
[0095]
　The above is the schematic configuration of the center pillar 12. Next, a method of manufacturing the front side member 1 as a shaft crushing part of the shock absorbing member and a method of manufacturing the center pillar 12 as a bending part of the shock absorbing member will be described.
[0096]

　High-strength steel sheets (steel sheets for cold plastic processing) as materials for shock-absorbing members such as the front side member 1 and center pillar 12 of the present embodiment and front sides as products. The member 1 and the center pillar 12 have a tensile strength of 780 MPa or more, and the ratio Cmax / of the maximum value Cmax (unit: mass%) and the minimum value Cmin (unit: mass%) of the Si content in the thickness direction cross section of the steel sheet. Cmin is 1.25 or less.
[0097]
(Tensile strength: 780 MPa or more)
　According to the high-strength steel plate of the present embodiment having the above composition and structure, high tensile strength, specifically, 780 MPa or more can be achieved. Here, the reason why the tensile strength is set to 780 MPa or more is to satisfy the demand for weight reduction of the automobile body. The tensile strength is preferably 1300 MPa or more, more preferably 1470 MPa or more.
[0098]
(The ratio Cmax / Cmin of the Si content is 1.25 or less) The ratio Cmax / Cmin
　of the maximum value Cmax and the minimum value Cmin of the Si content in the cross section in the thickness direction of the high-strength steel sheet is 1.25 or less. More preferably, Cmax / Cmin is 1.22 or less, 1.18 or less, or 1.15 or less. When Cmax / Cmin is 1.25 or less, segregation of Si can be controlled, the structure becomes uniform, and precipitates such as iron carbides shown below can be uniformly deposited. It can enhance the sex.
[0099]
　The degree of segregation of Si represented by Cmax / Cmin is measured as follows. After adjusting the steel sheet so that the surface whose rolling direction is normal (that is, the cross section in the thickness direction of the steel sheet) can be observed, mirror polishing is performed, and the thickness direction of the steel sheet is measured by an EPMA (Electronic Probe Microanalyzer) device. About the range of 100 μm × 100 μm at the point where the thickness of the steel sheet is 1/4 minute advanced from the surface of the steel sheet in the cross section, 200 points are spaced from one side to the other side along the thickness direction of the steel plate at intervals of 0.5 μm. Measure the Si content. Similar measurements were made on different 4 lines to cover almost the entire area within the same 100 μm × 100 μm range, and the highest value was obtained among the 1000 Si contents measured on all 5 lines. The ratio Cmax / Cmin is calculated by setting the maximum value Cmax (mass%) of the Si content and the minimum value Cmin (mass%) of the Si content. The lower limit of Cmax / Cmin is 1.00.
[0100]
　Since the high-strength steel plate used as the material of the shock absorbing member has a ratio Cmax / Cmin of 1.25 or less, the shock absorbing member manufactured from the material also has a ratio Cmax / Cmin of 1.25 or less.
[0101]
　The high-strength steel sheet is a steel sheet containing martensite. As an example of the steel sheet, there is a Dual-Phase steel sheet (composite structure steel sheet, DP steel sheet). In the DP steel, martensite and ferrite are distributed in a mosaic pattern, and a hard part that has been transformed and strengthened and a soft part that has not been transformed coexist. When DP steel is used as a high-strength steel plate, deformation due to cold plastic working (press forming) mainly occurs in ferrite, which is a soft structure. In the present embodiment, the DP steel sheet has a metal structure having an area ratio of martensite of 5% or more and less than 95%, ferrite of 5% or more and less than 95%, and the total of martensite and ferrite is It is 85 to 100% and means a steel sheet in which the residual structure is bainite, retained austenite, or pearlite. If desired, the lower limit of martensite may be 10%, 20%, 30%, 35% or 40%, and the upper limit of martensite may be 90%, 80%, 70%, 65% or 60%. The lower limit of ferrite may be 10%, 20%, 30%, 35% or 40%, and the upper limit of ferrite may be 90%, 80%, 70%, 65% or 60%, of martensite and ferrite. The lower limit of the total may be 90% or 95%, the retained austenite in the remnant tissue may be 0%, and the pearlite may be 0%. The high-strength steel plate may contain at least martensite, and steel other than DP steel may be used.
[0102]
　Next, the preferable chemical composition of the high-strength steel sheet will be described. In the following description, "%", which is a unit of the content of each element contained in the high-strength steel plate, means "mass%" unless otherwise specified.
[0103]
(C: 0.02% to 0.40%)
　C has an effect of increasing the amount of solid solution carbon and enhancing the baking curability. In addition, it has the effect of enhancing the quenchability and increasing the strength by containing it in the martensite structure. If the C content is less than 0.02%, a sufficient solid solution carbon amount cannot be secured when carbides such as iron carbides are precipitated, and the baking hardening amount decreases. Therefore, the C content is preferably 0.02% or more, preferably 0.10% or more or 0.20% or more. On the other hand, when the C content exceeds 0.40%, incomplete martensitic transformation occurs in cooling after annealing, and the retained austenite fraction becomes high. In addition, the strength is too high to guarantee formability. Therefore, the C content is preferably 0.40% or less, preferably 0.35% or less or 0.30% or less.
[0104]
(Si: 0.01% to 3.00%)
　Si is an element necessary for finely and a large amount of precipitates such as iron carbides for suppressing dislocation cells. If the Si content is less than 0.50%, even if the segregation has a uniform structure, sufficient action and effect cannot be obtained, coarse precipitates are generated, and the formation of dislocation cells cannot be suppressed. Therefore, the Si content is preferably 0.01% or more, more preferably 0.05% or more, 0.40% or more, or 0.80% or more. On the other hand, if the Si content is more than 3.00%, the effect of precipitating a large amount of precipitates in a fine amount is saturated, which increases the cost and deteriorates the surface texture. Therefore, the Si content is preferably 3.00% or less, preferably 2.50% or 2.00% or less.
[0105]
(Mn: 0.50% to 5.00%)
　Mn is an element for improving hardenability, and is an element necessary for forming a martensite structure without limiting the cooling rate. In order to effectively exert this effect, the Mn content is preferably 0.50% or more, preferably 1.00% or more or 1.80% or more. However, the content of excess Mn is preferably 5.00% or less, preferably 4.50% or less or 3.50% or less, because the low temperature toughness is lowered due to the precipitation of MnS.
[0106]
(P: 0.100% or less)
　P is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the P content, the better. In particular, when the P content is more than 0.100%, the weldability is significantly deteriorated. Therefore, the P content is preferably 0.100% or less, preferably 0.030% or less. Reducing the P content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the lower limit of the P content is 0%, but the P content may be 0.0001% or more. Further, since P contributes to the improvement of strength, the P content may be 0.0001% or more from such a viewpoint.
[0107]
(S: 0.010% or less)
　S is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the S content, the better. The higher the S content, the higher the precipitation amount of MnS and the lower the low temperature toughness. In particular, when the S content is more than 0.010%, the weldability and the low temperature toughness are significantly reduced. Therefore, the S content is preferably 0.010% or less, preferably 0.003% or less. Reducing the S content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the lower limit of the S content is 0%, but the S content may be 0.0001% or more.
[0108]
(Al: 0.001% to 2.000%)
　Al has an effect on deoxidation. In order to effectively exert the above effects, the Al content is preferably 0.001% or more, preferably 0.010% or more. On the other hand, when the Al content exceeds 2.000%, the weldability is lowered, the oxide-based inclusions are increased, and the surface texture is deteriorated. Therefore, the Al content is preferably 2.000% or less, preferably 1.000% or less, 0.100% or less, or 0.050% or less.
[0109]
(N: 0.010% or less)
　N is not an essential element and is contained as an impurity in steel, for example. From the viewpoint of weldability, the lower the N content, the better. In particular, when the N content is more than 0.010%, the weldability is significantly deteriorated. Therefore, the N content is preferably 0.010% or less, preferably 0.006% or less or 0.005% or less. Reducing the N content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the lower limit of the N content is 0%, but the N content may be 0.0001% or more.
[0110]
　The basic composition of the high-strength steel sheet of the present embodiment is as described above. Further, the high-strength steel plate of the present embodiment may contain the following optional elements, if necessary. These elements are arbitrary elements, and the lower limit thereof is 0%.
[0111]
(Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, one or more selected from Ti, Nb and V: 0 to 0.100% in total)
　Ti , Nb and V contribute to the improvement of strength. Therefore, Ti, Nb or V or any combination thereof may be contained. In order to sufficiently obtain this effect, the content of Ti, Nb or V, or the total content of any combination of two or more thereof is preferably 0.003% or more. On the other hand, if the content of Ti, Nb or V, or the total content of any combination of two or more of these is more than 0.100%, hot rolling and cold rolling become difficult. Therefore, the total content of Ti content, Nb content or V content, or any combination of two or more of these is preferably 0.100% or less, and the upper limit of these is 0.060% or 0. It may be .040%. That is, the limiting range in the case of each component alone is Ti: 0.003% to 0.100%, Nb: 0.003% to 0.100%, and V: 0.003% to 0.100%. In addition, the total content when these are arbitrarily combined is preferably 0.003 to 0.100%.
[0112]
(Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 1.00%, one or more selected from Cu, Ni, Mo and Cr. : 0 to 1.00% in total)
　Cu, Ni, Mo and Cr contribute to the improvement of strength. Therefore, Cu, Ni, Mo, or Cr or any combination thereof may be contained. In particular, Cr and Mo are hardenable elements and have the effect of forming a martensite structure when they cannot be supplemented by Mn alone. In order to sufficiently obtain this effect, the content of Cu, Ni, Mo and Cr is preferably in the range of 0.005 to 1.00% in the case of each component alone, and two or more of these are arbitrarily combined. It is preferable that the total content is 0.005% or more and 1.00% or less. On the other hand, if the content of Cu, Ni, Mo and Cr, or the total content of any combination of two or more of these is more than 1.00%, the effect of the above action is saturated and the cost is unnecessarily high. It gets higher. Therefore, the upper limit of the content of Cu, Ni, Mo and Cr, or the total content when two or more of these are arbitrarily combined is preferably 1.00%, and the upper limit of these is 0.50%. Alternatively, it may be 0.30%. That is, Cu: 0.005% to 1.00%, Ni: 0.005% to 1.00%, Mo: 0.005% to 1.00%, and Cr: 0.005% to 1.00%. In addition, the total content when these are arbitrarily combined is preferably 0.005 to 1.00%.
[0113]
(W: 0 to 0.005%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, REM: 0 to 0.010%, total content of W, Ca, Mg: 0 in total ~ 0.010%)
　W, Ca, Mg and REM contribute to the fine dispersion of inclusions and enhance toughness. Therefore, W, Ca, Mg or REM or any combination thereof may be contained. In order to sufficiently obtain this effect, the total content of W, Ca, Mg and REM, or any combination of two or more thereof, is preferably 0.0003% or more. On the other hand, when the total content of W, Ca, Mg and REM exceeds 0.010%, the surface texture deteriorates. Therefore, the total content of W, Ca, Mg and REM is preferably 0.010% or less. That is, W: 0.005% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.010% or less, and the total content of any two or more of these is 0. It is preferably 0.003 to 0.010%.
[0114]
　REM (rare earth metal) refers to a total of 17 elements of Sc, Y and lanthanoids, and "REM content" means the total content of these 17 elements. Lanthanoids are industrially added, for example in the form of misch metal.
[0115]
(B: 0 to 0.0300%)
　B is an element for improving hardenability and is a useful element for forming a martensite structure. B may be contained in an amount of 0.0003% (3 ppm) or 0.0010% (10 ppm) or more. However, if B is contained in excess of 0.0300% (300 ppm), the above effect is saturated and it is economically wasteful. Therefore, the B content is preferably 0.0300% or less, preferably 0. .0250% or less or 0.0050% or less.
[0116]
　In the high-strength steel plate according to the present embodiment, the balance other than the above components is composed of Fe and impurities. Here, the impurities are components that are mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when the high-strength steel plate is industrially manufactured, and are related to the present embodiment. It means a component that is not intentionally added to a high-strength steel plate.
[0117]
　Next, the structure of the high-strength steel plate, which is the material of the shock absorbing member according to the present embodiment, will be described. Hereinafter, the organizational requirements will be described, but% related to the organizational fraction means "area ratio". In the present embodiment, the metallographic structure of the high-strength steel plate has martensite of 5% or more and less than 95% in terms of area ratio. In the metal structure other than martensite, the area ratio of ferrite is 5% or more and less than 95%, the total of martensite and ferrite is 85 to 100%, and the remaining structure is bainite, retained austenite, and pearlite. Is preferable. If desired, the lower limit of martensite may be 10%, 20%, 30%, 35% or 40%, and the upper limit of martensite may be 90%, 80%, 70%, 65% or 60%. The lower limit of ferrite may be 10%, 20%, 30%, 35% or 40%, and the upper limit of ferrite may be 90%, 80%, 70%, 65% or 60%, of martensite and ferrite. The lower limit of the total may be 90% or 95%, the retained austenite in the remnant tissue may be 0%, and the pearlite may be 0%.
[0118]
(Martensite: 5% or more and less than 95%)
　In the present embodiment, it is preferable that martensite is secured in an area ratio of 5% or more. As a result, sufficient solid solution carbon can be secured, and as a result, the seizure curability can be enhanced. In order to further enhance such effects, it is recommended that martensite be secured at an area ratio of 10% or more.
[0119]
　In the present invention, the area ratio of martensite is measured as follows. First, a sample having a cross section parallel to the rolling direction and the plate thickness direction of the high-strength steel plate or the shock absorbing member (however, the rolling direction of the shock absorbing member is unknown, and the cross section may be parallel to the plate thickness direction). Is collected, and the surface whose cross section is mirror-finished by polishing is used as the observation surface. Of this observation surface, the region of 100 μm × 100 μm centered on the position of 1/4 of the thickness of the steel plate (hereinafter, “thickness of steel plate” is referred to as “plate thickness”) from the surface of the steel plate is defined as the observation area. .. This observation area is corroded with Nital reagent. The corroded observation area is observed with a secondary electron image of a field emission scanning electron microscope (FE-SEM: Field Emission-Scanning Electron Microscope) at a magnification of 1000 to 5000 times. In this observation region, martensite is identified from the position of cementite contained in the tissue and the sequence of cementite as follows.
[0120]
　In the present embodiment, as will be described later, in the high-strength steel plate, or in the shock absorbing member to which the high-strength steel plate is further subjected to cold plastic working, the first heat treatment step and the second heat treatment step, the martensite is used. Precipitates are deposited inside the truss. For this reason, in the high-strength steel plate, there is almost no "as-quenched martensite" in which no precipitate is deposited inside the martensite, and in many martensites, the precipitate is present inside the martensite. Here, since there are two or more types of crystal orientation relationships between the martensitic truss and the precipitate, the produced precipitate has a plurality of variants. By detecting the characteristics of these precipitates, each tissue is identified and the area ratio is calculated. When it is necessary to confirm that the above-mentioned precipitates are cementite, several precipitates are analyzed by SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy) or the like, if necessary. However, it may be confirmed that the precipitate is cementite. Furthermore, in the analysis of variants, TEM-EBSD (Electron Backscatter Diffraction Electron Microscopy: Electron Backscattered Diffraction Pattern) or SEM-EBSD (Electron Backscatter Diffraction Scanning) or SEM-EBSD (Electron Backscatter Diffraction Scanning) or SEM-EBSD (Electron Backscatter Diffraction Scanning), depending on the size of the precipitate. The crystal orientation may be measured by a scanning electron microscope (Electron Backscattered Diffraction Pattern).
[0121]
　When it is desired to strictly measure the area ratio of martensite including "as-quenched martensite" in a high-strength steel plate, an observation surface similar to the above observation surface is etched with a repera solution. Corrosion by the repera solution does not corrode as-quenched martensite and retained austenite. Therefore, the observation area corroded by the repera solution is observed by FE-SEM, and the uncorroded area is defined as as-quenched martensite and retained austenite. Then, the total area ratio of the "quenched martensite" and the retained austenite identified in this way is calculated.
[0122]
　Next, the area ratio of retained austenite is measured by X-ray diffraction method. Specifically, the position surface of 1/4 of the plate thickness is exposed from the surface of the high-strength steel plate. Then, the exposed surface is irradiated with MoKα rays, and the bcc phase (200) plane, (211) plane, and fcc phase (200) plane, (220) plane, and (311) plane are diffracted. Find the integrated intensity ratio of the peaks. The volume fraction of retained austenite can be calculated from the integrated intensity ratio of this diffraction peak. As this calculation method, a general 5-peak method can be used. The volume fraction of retained austenite thus obtained is regarded as the area fraction of retained austenite. The area ratio of "as-quenched martensite" can be calculated by subtracting the area ratio of retained austenite measured in this way from the total area ratio of "as-quenched martensite" and retained austenite. The "as-quenched martensite" calculated in this way may be summed with the above-mentioned area ratio of martensite (precipitate is deposited on the martensite truss) to obtain the area ratio of martensite.
[0123]
　However, in the shock absorbing member of the present invention, the high-strength steel plate is subjected to the first heat treatment step and the second heat treatment step. Therefore, the metal structure of the shock absorbing member does not include "as-quenched martensite" in which no precipitate is deposited inside the martensite truss. Therefore, when measuring the area ratio of martensite of the shock absorbing member, it is not necessary to perform corrosion and X-ray diffraction by the repera liquid as described above.
[0124]
　In this embodiment, the martensite structure may be contained, and any or a plurality of bainite, ferrite, pearlite, and retained austenite may be contained. That is, in the present embodiment, it is essential that at least the martensite fraction is present among the fractions of each tissue. The method for measuring the area ratio of retained austenite in a high-strength steel plate or a shock absorbing member is as described above. When measuring the area ratio of bainite, ferrite and pearlite, the following method is used.
[0125]
(Bainite)
　When measuring the area ratio of bainite, the FE-SEM of the observation region corroded by the nital reagent is the same as the above-mentioned method for measuring the area ratio of martensite (precipitate is deposited on martensite). Bainite is identified and its area ratio is measured from the position of the precipitate contained in the tissue and the arrangement of the precipitate in the observation of 1000 to 5000 times by the secondary electron image of.
[0126]
　Bainite is a collection of lath-shaped crystal grains, and contains no iron-based carbide or retained austenite with a major axis of 20 nm or more inside, or iron-based carbide or retained austenite with a major axis of 20 nm or more inside, and this carbide is , A single variant, i.e., belongs to a group of iron-based carbides extending in the same direction. Here, the iron-based carbide group elongated in the same direction means that the difference in the elongation direction of the iron-based carbide group is within 5 °. Bainite counts bainite surrounded by grain boundaries with an orientation difference of 15 ° or more as one bainite grain. If necessary, the crystal orientation of the iron carbide and bcc structure in bainite is measured by TEM-EBSD or SEM-EBSD, and the elemental analysis of the precipitate is performed by a method such as SEM-EDS, and the precipitate is cementite. You may confirm that it is.
[0127]
(Ferite and pearlite)
　When measuring the area ratio of ferrite and pearlite, the following method is used. First, as in the measurement of the area ratio of martensite described above, a sample having a cross section parallel to the rolling direction and the plate thickness direction of the high-strength steel plate or the shock absorbing member was collected, and the cross section was polished to a mirror surface. Observe. Of this observation surface, a region of 100 μm × 100 μm centered on a position of 1/4 of the thickness of the steel plate from the surface of the steel plate is defined as an observation region. This observation area is observed by FE-SEM at a magnification of 1000 to 5000, and an electron channeling contrast image is obtained. The electronic channeling contrast image is an image that displays the difference in crystal orientation of crystal grains as a difference in contrast. In this electron channeling contrast image, the region of uniform contrast is ferrite, and the area ratio thereof is obtained.
[0128]
　Subsequently, the observation surface is corroded with a nital reagent. Of the corroded observation surface, the region of 100 μm × 100 μm centered on the position of 1/4 of the thickness from the surface of the steel sheet is defined as the observation region. This observation area is observed with a secondary electron image of FE-SEM at a magnification of 1000 to 5000, and the area where cementite observed in the observation image with white contrast is arranged in a row or layer is pearlite, and the area ratio is determined. Ask.
[0129]
　In addition, in the first heat treatment step and the second heat treatment step, the "martensite as hardened" which may be present in a small amount on the high-strength steel plate is the above-mentioned martensite (precipitate is deposited on the martensite lath). There is no change in the area ratio of each metal structure except that it changes to the site. Therefore, the area ratios of ferrite, pearlite, bainite and retained austenite of the high-strength steel plate can be regarded as the same as the area ratio of each metal structure of the shock absorbing member. In addition, as described above, the area ratio of "as-quenched martensite" is 0% or extremely small, so the measurement results of the area ratio of martensite on high-strength steel sheets can be obtained from the measurement results of martensite on the shock absorbing member. You can think of it as the area ratio of the site.
[0130]

　Next, a preferable manufacturing method of a high-strength steel sheet as a material of the impact absorbing member of the present embodiment will be described.
[0131]
　The following description is intended to illustrate a characteristic method for manufacturing the high-strength steel plate of the present embodiment, and is limited to those in which the high-strength steel plate is manufactured by the manufacturing method as described below. It is not intended to be done.
[0132]
　A preferred method for producing the high-strength steel plate of the present embodiment is a step of casting a molten steel having the chemical composition described above to form a slab,
　and rough rolling the slab in a temperature range of 1050 ° C or higher and 1250 ° C or lower. In the rolling step, the rough rolling includes reverse rolling with a rolling reduction rate of 30% or less per pass performed an even number of times in 2 passes or more and 16 passes or less, and the rolling reduction rate between 2 passes in one round trip. A rough-rolled step in which the difference is 20% or less, the reduction rate of even times in one round trip is 5% or more higher than the reduction rate of odd times, and the rough-
　rolled steel plate is held for 5 seconds or more after the rough rolling. A finish rolling process in which finish rolling is performed in a temperature range of 850 ° C. or higher and 1050 ° C. or lower. A finish rolling process in which the rolled steel sheet is wound in a temperature range of 400 ° C. or less,
　a cold rolling process in which the obtained hot-rolled steel sheet is cold-rolled at a rolling reduction of 15% or more and 45% or less, and
　obtained. The cold-rolled steel sheet is heated at an average heating rate of 10 ° C./sec or higher and held in a temperature range of more than Ac 1 and less than Ac 3 for 10 to 1000 seconds, and then 70 ° C. or lower at an average cooling rate of 10 ° C./sec or higher. Includes a rolling step, which cools down to
. Hereinafter, each step will be described.
[0133]
(Slab forming step)
　First, a molten steel having the chemical composition of the high-strength steel sheet described above is cast to form a slab to be subjected to rough rolling. As the casting method, a normal casting method may be used, and a continuous casting method, an ingot forming method, or the like can be adopted, but the continuous casting method is preferable from the viewpoint of productivity.
[0134]
(Rough Rolling Step)
　It is preferable to heat the slab to a solution temperature range of 1000 ° C. or higher and 1300 ° C. or lower before rough rolling. Although the heating holding time is not particularly specified, it is preferable to hold the heating temperature for 30 minutes or more in order to bring the slab to a predetermined temperature. The heating holding time is preferably 10 hours or less, more preferably 5 hours or less, in order to suppress excessive scale loss. If the temperature of the slab after casting is 1050 ° C. or higher and 1250 ° C. or lower, the slab may be subjected to rough rolling as it is without being heated and held in the temperature range, and may be directly fed or rolled directly.
[0135]
　Next, by roughly rolling the slab by reverse rolling, the Si segregated portion in the slab formed during solidification in the slab forming step is made into a uniform structure without forming a plate-shaped segregated portion extending in one direction. be able to. To explain the formation of the Si content distribution having such a uniform structure in more detail, first, in the slab before the start of rough rolling, the portion where the alloying element such as Si is concentrated is formed from both surfaces of the slab. It is in a state where multiple pieces are lined up almost vertically in the form of a comb toward the inside.
[0136]
　On the other hand, in rough rolling, the surface of the slab is stretched in the rolling traveling direction for each rolling pass. The rolling traveling direction is the direction in which the slab advances with respect to the rolling roll. Then, as the surface of the slab is stretched in the traveling direction of rolling in this way, the Si segregated portion growing inward from the surface of the slab is in a state of being inclined in the traveling direction of the slab for each rolling pass. Be made.
[0137]
　Here, in the case of so-called one-way rolling in which the traveling direction of the slab in each pass of rough rolling is always the same direction, the Si segregated portion gradually moves in the same direction for each pass while maintaining a slightly straight state. The slope becomes stronger. Then, at the end of rough rolling, the Si segregation portion is in a posture substantially parallel to the surface of the slab while maintaining a substantially straight state, and a flat microsegregation is formed.
[0138]
　On the other hand, in the case of reverse rolling in which the traveling directions of the slabs in each pass of rough rolling are alternately opposite to each other, the Si segregated portion inclined in the immediately preceding pass is inclined in the opposite direction in the next pass. As a result, the Si segregated portion has a bent shape. For this reason, in reverse rolling, the Si segregated portions are alternately bent in a zigzag shape by repeatedly performing each pass in the opposite direction alternately.
[0139]
　When a plurality of zigzag shapes that are alternately bent in this way are lined up, the plate-like microsegregation disappears and the Si content distribution becomes uniformly intricate. By adopting such a structure, Si can be more easily diffused by the heat treatment in the subsequent step, and a hot-rolled steel sheet having a more uniform Si content can be obtained. In addition, since the above-mentioned reverse rolling results in a uniformly intricate Si content distribution over the entire steel sheet, such a uniform structure has not only a plate thickness cross section parallel to the rolling direction but also a plate whose normal is the rolling direction. It is similarly formed in a thick cross section.
[0140]
　If the rough rolling temperature range is less than 1050 ° C., it becomes difficult to complete finish rolling at 850 ° C. or higher in the final pass of rough rolling, resulting in poor shape. Therefore, the rough rolling temperature range is preferably 1050 ° C. or higher. More preferably, it is 1100 ° C. or higher. If the rough rolling temperature range exceeds 1250 ° C., scale loss increases and there is a concern that slab cracking may occur. Therefore, the rough rolling temperature range is preferably 1250 ° C. or lower.
[0141]
　If the rolling reduction rate per pass in rough rolling exceeds 30%, the shear stress during rolling becomes large, the Si segregated portion becomes non-uniform, and a uniform structure cannot be obtained. Therefore, the rolling reduction rate per pass in rough rolling is set to 30% or less. The smaller the rolling reduction, the smaller the shear strain during rolling, and a uniform structure can be obtained. Therefore, the lower limit of the rolling ratio is not particularly set, but 10% or more is preferable from the viewpoint of productivity.
[0142]
　In order to make the Si content distribution a uniform structure, reverse rolling is preferably 2 passes or more, more preferably 4 passes or more. However, if it is applied in excess of 16 passes, it becomes difficult to secure a sufficient finish rolling temperature, so the number is 16 passes or less. Further, it is desirable that each pass in which the traveling directions are opposite to each other is performed the same number of times, that is, the total number of passes is an even number. However, in a general rough rolling line, the entry side and the exit side of rough rolling are located on opposite sides of the roll. For this reason, the number of passes (rolling) in the direction from the entry side to the exit side of rough rolling increases once. Then, in the final pass (rolling), the Si segregated portion has a flat shape, and it becomes difficult to form a uniform structure. In the case of rough rolling on such a hot rolling line, it is preferable to leave a space between rolls in the final pass and omit rolling.
[0143]
　In reverse rolling, if there is a difference in rolling reduction between two passes included in one reciprocating rolling, shape defects are likely to occur, and the Si segregated portion becomes non-uniform, making it impossible to form a uniform structure. Therefore, during rough rolling, the rolling reduction difference between the two passes included in one round trip of reverse rolling is set to 9 to 20% or less. It is preferably 10% or more.
[0144]
　As will be described later, in order to miniaturize the recrystallized structure, tandem multi-stage rolling in finish rolling is effective, but tandem rolling tends to form flat microsegregation. In order to utilize tandem multi-step rolling, the even-numbered rolling reduction in reverse rolling must be larger than the odd-numbered rolling, and the microsegregation formed in the subsequent tandem rolling must be controlled. The effect becomes remarkable when the reduction rate of even-numbered times (return path) is 5% or more higher than the reduction rate of odd-numbered times (outward path) in one round trip of reverse rolling. Therefore, it is preferable that the even-numbered rolling reduction rate is 5% or more higher than the odd-numbered rolling reduction rate in one round trip of reverse rolling.
[0145]
　In order to make the complex structure of Si generated by reverse rolling in rough rolling uniform by austenite grain boundary movement, it is preferable to hold it for 5 seconds or more from rough rolling to finish rolling.
[0146]
(Finish rolling process)
　After reverse rolling in rough rolling, in order to narrow the spacing of Si segregation zones caused by the dendrite secondary arm by increasing the rolling reduction in tandem rolling in finish rolling, finish rolling is performed in 4 steps. It is preferably done on one or more consecutive rolling stands. If the finish rolling temperature is less than 850 ° C, recrystallization does not occur sufficiently and the structure is stretched in the rolling direction, and a plate-like structure due to the stretched structure is generated in the subsequent step. Therefore, the finish rolling temperature is 850 ° C. The above is preferable. More preferably, it is 900 ° C. or higher. On the other hand, when the finish rolling temperature exceeds 1050 ° C., it becomes difficult to generate fine recrystallized grains of austenite, it becomes difficult to segregate Si at the grain boundaries, and the Si segregation zone tends to become flat. Therefore, the finish rolling temperature is preferably 1050 ° C. or lower. If the temperature is appropriate, the rough-rolled steel sheet may be heated after the rough-rolling step and before the finish-rolling step, if necessary. Further, when the reduction ratio of the first stand for finish rolling is set to 15% or more, a large amount of recrystallized grains are generated, and Si is easily dispersed uniformly by the subsequent grain boundary movement. As described above, by limiting not only the rough rolling process but also the finish rolling process, microsegregation of flat Si can be suppressed.
[0147]
　If the take-up temperature exceeds 400 ° C., the surface texture deteriorates due to internal oxidation, so the take-up temperature is preferably 400 ° C. or lower. When the steel sheet structure has a homogeneous structure of martensite or bainite, it is easy to form a homogeneous structure by annealing, so that the winding temperature is more preferably 300 ° C. or lower.
[0148]
(Cold Rolling Step)
　The hot-rolled steel sheet obtained in the finish rolling step is pickled and then subjected to cold rolling to obtain a cold-rolled steel sheet. In order to maintain the martensite lath, the reduction rate is preferably 15% or more and 45% or less. The pickling may be a normal pickling.
[0149]
(Annealing step)
　The steel sheet obtained through the above cold rolling step is annealed. For heating at the annealing temperature, the temperature is raised at an average heating rate of 10 ° C./sec or more, and heating is maintained for 10 to 1000 seconds in a temperature range of more than Ac 1 and less than Ac 3 ° C. This temperature range and annealing time are intended to give the steel sheet a desired austenite fraction. When the holding temperature is Ac 3 ° C. or higher or the annealing time is more than 1000 seconds, the austenite particle size becomes coarse and martensite having a large lath width is formed, resulting in a decrease in toughness. Therefore, the annealing temperature is more than Ac 1 and less than Ac 3 , and the annealing time is 10 to 1000 seconds.
[0150]
　In addition, Ac 1 point and Ac 3 point are calculated by the following formula. The mass% of the element is substituted for the element symbol in the following formula. Substitute 0% by mass for elements that do not contain.
　　Ac 1 = 751-16 x C + 35 x Si-28 x Mn-16 x Ni + 13 x Cr-6 x Cu + 3 x Mo
　　Ac 3 = 881-335 x C + 22 x Si-24 x Mn-17 x Ni-1 x Cr-27 × Cu + 41 × Mo
[0151]
　After maintaining the annealing temperature, cooling is performed at an average cooling rate of 5 ° C./sec or higher. In order to freeze the tissue and efficiently induce martensitic transformation, the cooling rate should be high. However, if the temperature is lower than 5 ° C./sec, martensite is not sufficiently generated and the desired tissue cannot be controlled. Therefore, the temperature is set to 5 ° C./sec or higher. If the above cooling rate can be maintained after annealing and holding, a plating step may be added during cooling.
[0152]
　The cooling shutdown temperature is 250 to 600 ° C, preferably 400 to 600 ° C, and more preferably 430 to 470 ° C. This is because cooling produces martensite of 5% or more and less than 95%. When the cooling shutdown temperature is 400 to 600 ° C., the martensite fraction can be 90% or less. On the other hand, if cooling is stopped at a temperature higher than 600 ° C., it becomes impossible to secure martensite having an area ratio of 5% or more. Further, when the cooling stop temperature is set to a temperature lower than 250 ° C., the martensite fraction becomes 95% or more, which is outside the scope of the present application. Therefore, the cooling shutdown temperature is set to 250 to 600 ° C, preferably 400 to 600 ° C, and more preferably 430 to 470 ° C so that the martensite fraction can be set to the range of the present application of 5% or more and less than 95%. .. By setting such a cooling shutdown temperature, martensitic transformation is performed after the start of ferrite transformation. After cooling to 250 to 600 ° C. at an average cooling rate of 5 ° C./sec or higher, it is cooled to room temperature by normal air cooling (for example, cooling rate of 1 ° C./sec or lower). However, when the first heat treatment is performed at the high-strength steel plate manufacturing plant described later, after cooling to 250 to 600 ° C. at an average cooling rate of 5 ° C./sec or higher, after being held for 10 seconds or longer and 1000 seconds or lower. , The first heat treatment described later is performed.
[0153]
　The cooling stop temperature is different between the steel sheet described in WO2020 / 022481A cited as Patent Document 6 and the steel sheet of the present application. The cooling shutdown temperature (250 to 600 ° C.) of the steel sheet of the present application is higher than the cooling shutdown temperature (100 ° C. or lower) of the steel sheet described in the above document. According to Patent Document 6, in order to make martensite 95% or more in area ratio, the cooling shutdown temperature needs to be 100 ° C. or less, but martensite is made less than 95% in area ratio as in the present application. Therefore, it is necessary to set a high cooling shutdown temperature of 250 to 600 ° C.
[0154]
　In this way, the high-strength steel plate according to the embodiment of the present invention can be manufactured. The high-strength steel sheet according to the embodiment of the present invention has been described as a steel sheet for cold plastic working. As long as the above-mentioned production conditions are satisfied, the steel sheet for cold plastic working may be subjected to hot-dip plating (including alloyed hot-dip plating). However, the steel sheet subjected to hot-dip plating after cold rolling, that is, the hot-dip plated steel sheet or the alloyed hot-dip plated steel sheet may be excluded, and only the cold-plasticized steel sheet may be used.
[0155]

　Next, a method of manufacturing a shock absorbing member (front side member 1 and center pillar 12) from a high-strength steel plate completed through the above annealing step (final annealing). An example will be described. In the following, when the front side member 1 and the center pillar 12 are collectively referred to as the shock absorbing member, they are also referred to as “shock absorbing members 1, 12”.
[0156]
　In the present embodiment, a preferable method for manufacturing the shock absorbing members 1 and 12 from the high-strength steel sheet is a method of cold-plastic working the high-strength steel sheet after final annealing to manufacture the shock absorbing members 1 and 12. ..
　In this method, the
　steel sheet is subjected to a first heat treatment step in which the steel sheet is heat-treated to be held at a temperature T1 satisfying the following formula (1) for 60 to 900 seconds, and the
　steel sheet after the heat treatment is subjected to cold plastic processing. The steel 　plate comprises a cold plastic working step of forming a steel member and a
　second heat treatment step of heat-treating the steel member at a temperature T2 of 80 to 200 ° C. for 300 to 1800 seconds.
It contains martensite of 5% or more and less than 95% in fraction, has a tensile strength of 780 MPa or more, and has a maximum value Cmax (unit: mass%) and a minimum value Cmin (unit: mass%) of Si content in the thickness direction cross section of the plate.
　A method for manufacturing a shock absorbing member, wherein the ratio Cmax / Cmin (% by mass) is 1.25 or less .
　80 × Si + 100 ≦ T1 ≦ 125 × Si + 250 ... (1)
　However, Si in the above formula (1) means the Si content (mass%) in the steel sheet. In the present embodiment, the first heat-treated steel sheet may be a Dual-Phase steel sheet.
[0157]
　Hereinafter, each step will be described along with the flow of manufacturing the shock absorbing members 1 and 12 from the high-strength steel plate 110 manufactured by the above-mentioned manufacturing method.
[0158]
　4 to 6 are schematic views showing a process of manufacturing the shock absorbing members 1 and 12 from the high-strength steel plate 110, respectively. FIG. 4 shows a case where the first heat treatment step is performed at the steelworks 101 that manufactures the high-strength steel plate 110. FIG. 5 shows a case where the first heat treatment step is performed at the coil center 102. FIG. 6 shows a case where the first heat treatment step is performed at the parts factory 103 (molding factory) of the shock absorbing members 1 and 12.
[0159]
With
　reference to FIG. 4, the steelworks 101 of the high-strength steel plate 110 has high strength from molten steel produced using, for example, a blast furnace or an electric furnace. It is a steel mill that manufactures steel plate 110. The high-strength steel sheet 110 that has been finally annealed is subjected to a first heat treatment step in a heating furnace 112 installed in a steel mill 101 before being wound into a coil 111. The first heat treatment step is, for example, a tempering step.
[0160]
　The temperature T1 of the high-strength steel plate 110 in the heating furnace 112 is set within the range of the above-mentioned formula (1). When the temperature T1 in the first heat treatment step is at least the above lower limit, the effect that the major axis of the precipitate is 0.05 μm or more can be obtained. Further, when the temperature T1 is not more than the above upper limit, the effect that the number density is high and the major axis of the precipitate is 0.60 μm or less can be obtained.
[0161]
　In the first heat treatment step, the high-strength steel plate 110 is held at a constant temperature T1 within the range of the above formula (1) for 60 to 900 seconds. When the holding time of the temperature T1 in the first heat treatment step is equal to or longer than the above lower limit, the effect of stably precipitating iron carbide can be obtained. Further, when the holding time of the temperature T1 is not more than the above upper limit, the effect that the number density is high and the major axis of the precipitate is 0.60 μm or less can be obtained. Next, the properties of the high-strength steel plate 110 developed after the first heat treatment will be described.
[0162]
(The number of precipitates having a major axis of 0.05 to 0.60 μm and an aspect ratio of 1: 3 or more is 30 / μm 2 or more) In the
　present embodiment, the major axis is 0.05 μm or more by the first heat treatment. 30 precipitates with an aspect ratio of 1: 3 or more at 60 μm or less / μm 2It is possible to obtain a high-strength steel plate 110 having the above number density. In the present embodiment, the aspect ratio means the ratio between the longest diameter (major diameter) of the precipitate and the longest diameter (minor diameter) of the precipitates orthogonal to the longest diameter (major diameter). The precipitate is not particularly limited as long as it satisfies the above requirements for major axis and aspect ratio, and examples thereof include carbides and the like. In particular, in this embodiment, the precipitate may contain or consist of iron carbide. According to the present embodiment, by containing a relatively large amount of such precipitates in the structure, for example, the cell formation of dislocations caused by the entanglement of dislocations is suppressed, which is caused by carbon diffused during baking and curing. The amount of dislocations that are fixed can be increased, and as a result, the amount of baking hardening can be significantly increased. Such findings have not been known in the past and are extremely surprising and surprising. The size of the dislocation cells generated in martensite is about several tens of nm or more and several hundreds of nm or less. Therefore, in order to suppress the formation of dislocation cells, the size of the precipitate is required to be the same. If the major axis is less than 0.05 μm, the formation of dislocation cell formation cannot be suppressed. Therefore, the major axis of the precipitate is preferably 0.05 μm or more. More preferably, it is 0.10 μm or more. On the other hand, if the major axis is larger than 0.60 μm, the precipitate becomes coarse and the amount of solid solution carbon is greatly reduced, and the amount of baking hardening is reduced. Therefore, the major axis of the precipitate is preferably 0.60 μm or less. More preferably, it is 0.50 μm or less.
[0163]
　The shape of the precipitate is preferably needle-shaped rather than spherical, and the aspect ratio is preferably 1: 3 or more. If the aspect ratio is less than 1: 3, the shape of the precipitate is considered to be spherical, and the formation of dislocation cells cannot be suppressed. Therefore, the aspect ratio is set to 1: 3 or more. More preferably, it is 1: 5 or more.
[0164]
　The precipitation location of the precipitate is preferably in the lath. This is because the place where the dislocation cells are most easily formed is in the lath, and the dislocation cells are hardly seen between the laths. Here, the lath refers to a structure formed in the former austenite grain boundaries by martensitic transformation. For ease of understanding, FIG. 3 is provided with an image diagram showing the precipitation state of the precipitate in the high-strength steel plate as the material of the impact absorbing member according to the present embodiment. Referring to FIG. 3, in the lath structure 83 generated in the former austenite grain boundaries 82 during microsegregation of Si having a uniform structure 81, needle-like precipitates are uniformly formed on the entire surface of the lath 84, not between the lath 84. It can be seen that 85 is precipitated.
[0165]
　The number density of the precipitates 85 is preferably 30 pieces / μm 2 or more. If the number density of the precipitate 85 is less than 30 pieces / μm 2 , when dislocations are introduced and moved by prestrain, the dislocations interact with other dislocations before they meet the precipitates, and dislocation cells are formed. .. Therefore, the number density of the precipitates 85 is preferably 30 pieces / μm 2 or more. More preferably, it is 40 pieces / μm 2 or more.
[0166]
　In the present embodiment, the morphology and number density of the precipitate 85 are determined by observation with an electron microscope, and are measured by, for example, TEM (transmission electron microscope) observation. Specifically, a thin film sample is cut out from the surface of the steel sheet from the region from the 3/4 position to the 1/4 position of the thickness of the steel sheet and observed in a bright field. 1 μm 2 is cut out at an appropriate magnification of 10,000 to 100,000 times, and precipitates 85 having a major axis of 0.05 μm or more and 0.60 μm or less and an aspect ratio of 1: 3 or more are counted and obtained. At this time, the number of thin film samples per unit area converted with the thickness of the thin film sample being 30 nm is counted. This work is performed in five or more continuous fields of view, and the average thereof is taken as the number density.
　The high-strength steel plate 110 of the present embodiment satisfies the requirement that the major axis is 0.05 to 0.60 μm and the number of precipitates having an aspect ratio of 1: 3 or more is 30 pieces / μm 2 or more. Precipitates of shock absorbing members manufactured using the high-strength steel plate 110 also satisfy this requirement.
[0167]
　Hereinafter, the continuation of the flow of manufacturing the shock absorbing members 1 and 12 from the high-strength steel plate 110 will be described.
[0168]
　The high-strength steel plate 110 subjected to the first heat treatment step is wound up at the steel mill 101 to become a coil 111. The coil 111 is shipped from the steelworks 101 to the coil center 102. The coil center 102 stores the coil 111 and ships the ordered coil 111 to the parts factory 103.
[0169]
　The parts factory 103 is a factory for molding the shock absorbing members 1 and 12 from the coil 111. Each process performed in the parts factory 103 may be performed using one factory as the parts factory 103, or a different factory may be used for each one or a plurality of processes, and the different factories as a whole may be used as a parts factory. It may be used as 103.
[0170]
　At the parts factory 103, first, the received coil 111 is rewound, bent and straightened, and then returned to the flat high-strength steel plate 110. Next, the high-strength steel plate 110 is blanked. Blank 115 for shock absorbing members 1 and 12 is formed by blanking. For example, the blank 115 is formed by a blanking press 114. Actually, the first half 20 and the second half 30 of the front side member 1, the third half 40 and the fourth half 50 of the center pillar base 16, and the fifth half of the center pillar main body 17. Blanks corresponding to the respective shapes of the portion 60 and the sixth half portion 70 are formed. In the present embodiment, the blanks corresponding to each of the half portions 20, 30, 40, 50, 60, and 70 are collectively referred to as blank 115.
[0171]
　Next, by cold plastic working the blank 115, the half portions 20, 30, 40, 50, 60, 70 before baking coating are formed. Specifically, the blank 115 is subjected to draw forming (drawing forming) or foam forming as cold plastic working to form steel as half portions 20, 30, 40, 50, 60, 70 before baking finish. The member 117 is molded. Draw molding is performed using, for example, a draw molding machine 116.
[0172]
　The draw forming machine 116 has a punch 116a, a die 116b, and a blank holder 116c that cooperates with the die 116b to hold the end portion of the blank 115. In draw forming, the blank 115 is formed with the ends of the blank 115 restrained by the die 116b and the blank holder 116c. Therefore, in the steel member 117 after forming the blank 115, the bent portion 117a and the wall portion 117b are formed. Prestrain is applied to both.
[0173]
　The foam forming machine 118 has a punch 118a, a die 118b, and a pad 118c that cooperates with the punch 118a to sandwich an intermediate portion of the blank 115. In foam forming, the end portion of the blank 115 is not constrained by the die 118b, so that in the steel member 117 after forming the blank 115, prestrain is applied to the bent portion 117a in the vicinity of the portion sandwiched between the punch 118a and the die 118b. On the other hand, no prestrain is substantially applied to the portions other than the bent portion 117a.
[0174]
　In the present embodiment, in the front side member 1 as a shaft crushing part, both the first half portion 20 before baking finish and the second half portion 30 before baking finish are made by drawing the blank 115 with the steel member 117. It is molded by doing. Regarding the center pillar 12 as a bending part, the center pillar base 16 as a short span member has at least the third half 40 (in the width direction Y) of the third half 40 and the fourth half 50 before baking finish. The outer member) is formed by forming the blank 115 into a steel member 117 by draw forming.
[0175]
　On the other hand, among the center pillars 12, the center pillar main body 17 as a long span member has at least the fifth half portion 60 (outer member in the width direction Y) of the fifth half portion 60 and the sixth half portion 70 before baking coating. However, the blank 115 is formed into a steel member 117 by foam forming.
[0176]
　With reference to FIG. 2 (A), the first half portion 20, the second half portion 30, and the third half portion 40 (more accurately, the corresponding first half portion 20) thus drawn-molded. , The first ridge line portions 22, 32, 42 as the portions marked with circles and triangles in the plurality of molded products 117) constituting the second half portion 30 and the third half portion 40. Prestrains are applied to the central portions 27, 37, 47 of the wall portions 21, 31, 43 and the second ridge line portions 24, 34, 44, respectively. The prestrain is at least 2%. By controlling the forming conditions such as the operating amount of the punch 116a in the draw forming and applying a prestrain of 2% or more, the baking hardening amount can be sufficiently increased. In seizure hardening, intrusive elements (mainly carbon) move and adhere to dislocations (line defects that are the elementary process of plastic deformation) that enter the steel sheet due to cold plastic working (pre-strain), thereby inhibiting their motion. , A phenomenon in which the strength increases, and is also called strain aging.
[0177]
　In FIG. 2, the portion where the pre-strain is applied in the ridge line portion is indicated by a circle, and the portion where the pre-strain is applied in the wall portion is indicated by a Δ mark. However, the second ridge line portion 44 of the third half portion 40 does not have to be prestrained because it does not have a significant effect on the impact absorption energy at the time of a side collision.
[0178]
　On the other hand, in the foam-molded fifth half portion 60, prestrain is applied to the first ridge line portion 62 as the portion marked with a circle. Also in foam forming, as in the case of draw forming, prestrain is applied by controlling forming conditions such as the amount of movement of the punch 118a. On the other hand, no prestrain is substantially applied to each of the central portion 67 of the second wall portion 63 and the second ridge line portion 64.
[0179]
　The fourth half portion 50 and the sixth half portion 70 (the inner half portion in the width direction Y in the center pillar 12) may be formed by draw forming or may be formed by foam forming.
[0180]
　With reference to FIG. 4 again, after cold plastic working (press forming), the steel member 117 is used to assemble the vehicle body 100 including the shock absorbing members 1 and 12. Specifically, the steel member 117 serving as the first half portion 20 and the steel member 117 serving as the second half portion 30 are flanged to each other to form a unit of the steel member 117 for the front side member 1. To. That is, the intermediate body to be the front side member 1 is formed by being coated and baked and cured. Further, by joining the steel members 117 to be the third to sixth halves 30, 40, 50, 60, 70 to each other, the unit of the steel member 117 for the center pillar 12 is formed. That is, the intermediate body to be the center pillar 12 is molded by being painted and baked and cured. Then, the vehicle body 100 is assembled by combining the units of the steel members 117 for the shock absorbing members 1 and 12 with other vehicle body members.
[0181]
　Next, the assembled vehicle body 100 is painted. This coating includes, for example, three types of coating: electrodeposition coating, intermediate coating, and top coating (base and clear coating). Water-based paint or solvent paint is used for painting. In the electrodeposition coating step, electrodeposition coating is applied to the entire surface of the vehicle body 100 in a state where the vehicle body 100 is submerged in an electrodeposition tank in which paint is stored. Further, in the intermediate coating step, the coating is manually applied to the entire surface of the vehicle body 100 by spraying the paint from the spray nozzle onto the vehicle body 100 by a painting robot or a worker. Further, in the top coat coating step, the paint is sprayed onto the vehicle body 100 from the spray nozzle by manual work by a coating robot or a worker, so that the top coat is applied to the entire surface of the vehicle body 100. As a result, the surface of the vehicle body 100 is composed of a coating film having a thickness of about 100 μm.
[0182]
　The above-mentioned painting step includes a second heat treatment step. The second heat treatment is a baking and drying process for baking the coating film on the base material (steel plate) of the vehicle body 100, and is a process for baking and hardening the steel plate. The second heat treatment step may be performed after the electrodeposition coating and before the intermediate coating in the coating process, or may be performed between the intermediate coating and the intermediate coating which are performed a plurality of times. It may be performed after the intermediate coating and before the top coating, between the top coating and the top coating performed multiple times, or after the top coating. ..
[0183]
　In the second heat treatment step, the vehicle body 100 is carried into the drying furnace 119. As described above, the temperature T2 of the vehicle body 100 in the drying furnace 119 is set in the range of 80 ° C. to 200 ° C. When the temperature T2 in the second heat treatment step is equal to or higher than the above lower limit, the paint can be reliably baked onto the steel plate of the vehicle body 100, and the steel plate constituting the vehicle body 100 can be more reliably cured. Can be done. Further, if the temperature T2 exceeds the above upper limit, the cost of the manufacturing process of the automobile is increased. Therefore, the upper limit of the holding temperature is set to 200 ° C. or lower.
[0184]
　The holding time of the vehicle body 100 in the drying furnace 119 is set in the range of 300 to 1800 seconds as described above. When the holding time in the second heat treatment step is equal to or longer than the above lower limit, the paint can be reliably baked onto the steel plate of the vehicle body 100, and the steel plate constituting the vehicle body 100 can be more reliably cured. Can be done. Further, if the holding time exceeds 1800 seconds, the cost of the manufacturing process of the automobile is increased. Therefore, the holding time is set to 1800 seconds or less.
[0185]
　In the second heat treatment step, the steel plate of the vehicle body 100 is continuously held for 300 to 1800 seconds at a constant temperature T2 within the above temperature range. When the holding time of the temperature T2 in the second heat treatment step is equal to or longer than the above lower limit, the effect that the paint is baked can be obtained. Further, when the holding time of the temperature T2 exceeds the above upper limit, the manufacturing cost of the automobile increases. Therefore, the holding time of T2 is set to 1800 seconds or less.
[0186]
　By going through the painting process including the above second heat treatment step, the vehicle body 100 including the shock absorbing members 1 and 12 is completed.
[0187]

　With reference to FIG. 5, the high-strength steel sheet 110 finally annealed at the steel mill 101 is wound up at the steel mill 101 to become the coil 111. The coil 111 is shipped from the steelworks 101 to the coil center 102. At the coil center 102, the coil 111 is rewound. Then, the rewound high-strength steel sheet 110 is heated in the heating furnace 112 installed in the coil center 102 to perform the first heat treatment. The first heat treatment step at this time is the same as the above-mentioned contents (first heat treatment at the steelworks 101) with reference to FIG. When the first heat treatment step is performed at the coil center 102, the first heat treatment step may be performed on the high-strength steel plate 110 which has been rewound from the coil 111 and then further bent and straightened to become flat. .. The high-strength steel plate 110 subjected to the first heat treatment is wound again to form a coil 111, and is shipped to the parts factory 103.
[0188]
　Since the process at the parts factory 103 in which the vehicle body 100 is manufactured by using the high-strength steel plate 110 received from the coil center 102 is the same as the above-mentioned contents with reference to FIG. 4, detailed description thereof will be omitted. At the parts factory 103, the vehicle body 100 is completed by undergoing a painting process including the above-mentioned second heat treatment step.
[0189]

　With reference to FIG. 6, the high-strength steel plate 110 completed at the steelworks 101 is wound up at the steelworks 101 to become a coil 111. The coil 111 is shipped from the steelworks 101 to the coil center 102, and then to the parts factory 103.
[0190]
　At the parts factory 103, the coil 111 is rewound, bent and straightened to be flattened, so that the coil 111 is returned to the flat sheet-shaped high-strength steel plate 110. Next, the blank 115 for the shock absorbing members 1 and 12 is formed by blanking the sheet-shaped high-strength steel plate 110.
[0191]
　Next, the blank 115 is subjected to the first heat treatment. In this case, the blank 115 is subjected to the first heat treatment in the heating furnace 112 installed in the parts factory 103. The first heat treatment step at this time is the same as the above-mentioned contents with reference to FIG. When the first heat treatment step is performed at the parts factory 103, the first heat treatment may be performed on the sheet-shaped high-strength steel plate 110 before being formed into the blank 115. Since the blanking process and the process step after the first heat treatment (process after the cold plastic working process) are the same as those described with reference to FIG. 4, the description thereof will be omitted.
[0192]
　The vehicle body 100 is completed by going through the painting process including the above second heat treatment step.
[0193]
　As described above, according to the present embodiment, after the final annealing, the first heat treatment (tempering treatment) is performed, and then the second heat treatment (baking hardening treatment) is performed, so that the second heat treatment is performed. The amount of shrinkage hardening of the steel plate (steel member 117) at that time can be further increased. As a result, it is relatively soft and easy to mold during molding (cold plastic working, etc.), the amount of baking hardening during coating baking after molding is large, and the impact absorption energy can be increased in the state after baking hardening. It is possible to manufacture shock absorbing members 1 and 12 that can thin the steel plate that is the material.
[0194]
　Further, according to the present embodiment, the fifth half portion 60 of the center pillar main body 17 as a long span member in the center pillar 12 is formed by applying foam forming to the steel member 117. With this configuration, pre-strain can be applied to the first ridge line portion 62 of the fifth half portion 60, which generates impact absorption energy by causing bending deformation, in the center pillar main body 17 at the time of a side collision of an automobile. As a result, the amount of baking hardening of the first ridge line portion 62 can be sufficiently increased by the baking hardening treatment (second heat treatment) of the first ridge line portion 62. As a result, the impact absorption energy at the first ridge line portion 62, which is substantially plastically deformed when the center pillar main body 17 is bent and deformed due to a side collision of the automobile, can be further increased. Moreover, in the case of foam forming, the blank 115 is formed on the steel member 117 without restraining the outer peripheral edge portion of the blank 115. Therefore, it is not necessary to apply an extra load to a portion other than the portion to which the prestrain is desired (first ridge line portion 62).
[0195]
　Further, according to the present embodiment, the third half portion 40 of the center pillar base 16 as a short span member in the center pillar 12 is formed by applying draw forming to the steel member 117. With this configuration, in the case of a side collision of an automobile, the third half of the center pillar base 16 generates shock absorption energy by causing deformation such that the closed cross section in the cross section orthogonal to the height direction Z is crushed. In 40, prestrain can be applied to both the ridge line portion and the wall portion, particularly both the first ridge line portion 42 and the second wall portion 43. As a result, the amount of baking hardening of the first ridge line portion 42 and the second wall portion 43 can be sufficiently increased by the baking hardening treatment (second heat treatment) of the first ridge line portion 42 and the second wall portion 43. As a result, the shock absorption energy at the first ridge line portion 42 and the second wall portion 43, which are substantially plastically deformed when the center pillar base 16 is crushed and deformed due to a side collision of the automobile, can be further increased.
[0196]
　Further, according to the present embodiment, the first half portion 20 and the second half portion 30 of the front side member 1 as a shaft crushing part are formed by applying draw forming to the steel member 117. With this configuration, in the case of a frontal collision of an automobile, the ridgeline portion and the wall portion, particularly the first portion, in each of the first half portion 20 and the second half portion 30 that generate impact absorption energy by collapsing in the length direction X. Prestrain can be applied to all of the ridge line portions 22, 32, the second wall portions 23, 33, and the second ridge line portions 24, 34. As a result, by the baking hardening treatment (second heat treatment) of the first ridge line portions 22, 32, the second wall portions 23, 33, and the second ridge line portions 24, 34, the first ridge line portions 22, 32, first. The amount of baking hardening of the two wall portions 23, 33 and the second ridge line portions 24, 34 can be sufficiently increased. As a result, the impact absorption energy when the front side member 1 is crushed and deformed due to a side collision of the automobile can be further increased.
[0197]
　Further, according to the present embodiment, the first heat treatment (tempering treatment) may be performed on the high-strength steel plate 110 after the final annealing and before winding on the coil 111 at the steelworks 101. In this case, the first heat treatment can also be collectively performed at the steelworks 101 at the time of manufacturing the high-strength steel plate 110.
[0198]
　Further, according to the present embodiment, the first heat treatment (tempering treatment) may be performed on the high-strength steel plate 110 after being rewound from the coil 111 at the coil center 102. In this case, in the distribution process between the steel mill 101 and the parts factory 103, the first heat treatment can be performed at the coil center 102.
[0199]
　Further, according to the present embodiment, the first heat treatment (tempering treatment) may be performed on the blank 115 (steel plate) after blanking at the parts factory 103. In this case, when processing the sheet-shaped high-strength steel plate 110, the first heat treatment can also be performed collectively at the parts factory 103.
[0200]
　Further, according to the present embodiment, the first heat treatment is performed on the high-strength steel sheet 110, which is a dual-phase steel sheet. In this case, the first heat treatment as a pre-heat treatment is applied to the steel sheet whose segregation is reduced by 780 MPa or more. As a result, the formability and collision performance of the shock absorbing members 1 and 2 formed by using the high-strength steel plate 110 can be further improved.
[0201]
　Further, the high-strength steel plate 110 constituting the center pillar 12 (bent part) is subjected to the first heat treatment as a preheat treatment, and the center pillar 12 is manufactured by selectively using draw forming and foam forming. Performance is improved. This mechanism is considered to be the following two superimposition effects (d1) and (d2).
　(D1) The seizure curability was enhanced by the first heat treatment (pre-heat treatment).
　High-tensile steels containing martensite have higher seizure curability depending on the tempering temperature determined by the Si content. When the seizure curability is increased, for convenience of the test, the stress in the tensile test shows the maximum and then decreases immediately, leading to rupture. On the other hand, in the shear test, immediate fracture does not occur and measurement is possible. Then, in order to improve the collision performance, the tensile stress σ 5 at the elongation strain 5% in the tensile test and the shear strain 5√3 in the shear test are obtained for each of the center pillar base 16 and the center pillar main body 17 of the center pillar 12. The ratio σ 5 / τ 5 to the shear stress τ 5 at% must be 1.70 or less. 　(D2) In the center pillar 12, the most effective molding method for collision due to bending is adopted so that the strength differs between the first ridge line portions 42, 62 and the central portions 47, 67 of the corresponding wall portions 42, 62. ..

　In the center pillar base 16 and the center pillar main body 17, strains are applied to the first ridges 42 and 62 by draw molding or foam molding to increase the strength after work hardening, and the first ridges 42 and 62 and the wall portion are increased. At the central portions 47 and 67 of 42 and 62, there is a difference in strength and work hardening ability. For that purpose, the Vickers hardness ratio Hvr / Hvc between the first ridge line portions 42, 62 and the central portions 47, 67 of the second wall portions 43, 63 needs to be 1.05 or more, respectively. Due to the superposition effect of (d1) and (d2) described above, the collision performance due to the bending of the center pillar 12 is improved.
[0202]
　Further, the collision performance is enhanced by subjecting the high-strength steel plate 110 constituting the front side member 1 (shaft crushing component) to the first heat treatment as a preheat treatment and manufacturing the front side member 1 by draw forming. This mechanism is considered to be the following two superimposing effects (e1) and (e2).
　(E1) The seizure curability was enhanced by the first heat treatment (pre-heat treatment).
　High-tensile steels containing martensite have higher seizure curability depending on the tempering temperature determined by the Si content. When the seizure curability is increased, for convenience of the test, the stress in the tensile test shows the maximum and then decreases immediately, leading to rupture. On the other hand, in the shear test, immediate fracture does not occur and measurement is possible. Further, when the seizure curability is increased, the stress of high strain is increased, so that the collision performance due to axial crushing can be improved. In order to improve the collision performance in this way, the ratio of the tensile stress σ 10 when the elongation strain is 10% in the tensile test and the shear stress τ 10 when the shear strain is 10√3% in the shear test σ 10 / τ 10 Must be 1.70 or less.
　(E2) The most effective molding method for collision due to axial crushing was adopted so that the same strength could be obtained at the central portion 27 of the first ridge line portion 22 and the first wall portion 21.
　Strain due to draw molding is applied to the first ridge line portion 22 and the second wall portion 23 to the same extent to increase the strength after baking and curing to improve the collision performance. For that purpose, the ratio Hvr / Hvc of the Vickers hardness between the first ridge line portion 22 and the central portion 27 of the first wall portion 21 needs to be less than 1.10. Due to the superposition effect of (e1) and (e2) described above, the collision performance due to the axial crushing of the front side member 1 is improved.
Example
[0203]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0204]

　A slab having the chemical composition shown in Table 1-1 is manufactured, the slab is heated to 1300 ° C. for 1 hour, and then rough rolling and finish rolling are performed under the conditions shown in Table 1-2. A hot-rolled steel sheet was obtained. Then, the hot-rolled steel sheet was pickled and cold-rolled at the reduction ratio shown in Table 1-2 to obtain a cold-rolled steel sheet. Subsequently, annealing was performed under the conditions shown in Table 1-2. Each temperature shown in Table 1-2 is the surface temperature of the steel sheet.
[0205]
　In each table, the parts shown in bold and underline indicate that they are out of the preferable range.
[0206]
[Table 1-1]

[0207]
[Table 1-2]

[0208]
[Table 1-3]

[0209]
Ac 1 and Ac 3 　in Table 1-2 were calculated by the following formulas. The mass% of the element is substituted for the element symbol in the following formula. For elements not contained, 0% by mass was substituted.
　　Ac 1 = 751-16 x C + 35 x Si-28 x Mn-16 x Ni + 13 x Cr-6 x Cu + 3 x Mo
　　Ac 3 = 881-335 x C + 22 x Si-24 x Mn-17 x Ni-1 x Cr-27 × Cu + 41 × Mo
[0210]
　The structure and mechanical property values ​​of the high-strength steel sheet after the above annealing (final annealing) are shown in Table 1-3. As for the tissue, the martensite area ratio, the residual tissue, and Cmax / Cmin are shown.
[0211]
　The measurement of the area ratio of martensite is performed as follows. First, a sample having a cross section parallel to the rolling direction and the plate thickness direction of the high-strength steel sheet was collected. The area ratio of martensite was measured by measuring this sample by the method described above.
[0212]
　As for the remaining structure, ferrite is indicated by F and bainite is indicated by B. The ferrite and bainite were measured by the above-mentioned method using the above-mentioned sample for measuring the area ratio of martensite in a high-strength steel plate. In the present application, the area ratio of martensite is important for the seizure curing performance, and the area ratio of ferrite and the area ratio of bainite do not necessarily have a great influence on the seizure curing performance. Therefore, the measurement of the area ratio of ferrite and the measurement of the area ratio of bainite are omitted.
[0213]
　The segregation degree of Si represented by Cmax / Cmin was measured as follows. After adjusting the high-strength steel sheet so that the surface whose rolling direction is normal (that is, the cross section in the thickness direction of the steel sheet) can be observed, mirror polishing is performed, and the EPMA device is used in the thickness direction cross section of the high-strength steel sheet. The Si content of 200 points was measured at intervals of 0.5 μm from one side to the other along the thickness direction of the steel sheet in the range of 100 μm × 100 μm in the center of the steel sheet. Similar measurements were made on different 4 lines to cover almost the entire area within the same 100 μm × 100 μm range, and the highest value was obtained among the 1000 Si contents measured on all 5 lines. The ratio Cmax / Cmin was calculated by setting the maximum value Cmax (mass%) of the Si content and the minimum value Cmin (mass%) of the Si content.
[0214]
　As the mechanical property values, the tensile strength TS and the breaking elongation EL were measured. This measurement was carried out in accordance with the JIS (Japanese Industrial Standards) Z2241: 2011 metal material tensile test method by collecting JIS No. 5 tensile test pieces whose longitudinal direction is perpendicular to the rolling direction.
[0215]
　The above-mentioned structure of the high-strength steel sheet (martensite area ratio, residual structure, and Cmax / Cmin) can be obtained by subjecting the high-strength steel sheet to the first heat treatment, cold plastic working, and second heat treatment described later. It is maintained almost unchanged in the parts after it is done. Therefore, it can be said that the contents shown in Table 1-3 also apply to the bending parts and the shaft crushing parts described later.
[0216]
The
　high-strength steel sheets (cold-rolled steel sheets) shown in Table 1-3 were subjected to the first heat treatment (tempering). The temperature of the high-strength steel plate in the first heat treatment and the holding time of this temperature are shown in Table 2-1. Then, the high-strength steel sheet subjected to the first heat treatment was subjected to cold plastic working to form the cold-rolled steel sheet into the shape of a bent part. The cold plastic working method at this time is shown in Table 2-1. Next, a second heat treatment (baking and curing) was performed on the part molded into the shape of the bent part to produce a bent part. The temperature of the parts in the second heat treatment and the holding time of this temperature are shown in Table 2-1.
[0217]
[Table 2-1]

[0218]
[Table 2-2]

[0219]
　7 (A) and 7 (B) show a bending component 121 illustrating the above-mentioned bending component. FIG. 7A is a side view of the bent component 121 viewed from a direction orthogonal to the longitudinal direction, and FIG. 7B is a view of the bent component 121 viewed from the longitudinal direction.
[0220]
　With reference to FIGS. 7 (A) and 7 (B), the bent component 121 is a hollow shaft-shaped member having a total length of 800 mm and a plate thickness of t1 = 1.2 mm. The bent part 121 has a first half portion 130 having a hat-shaped cross section and a flat plate-shaped second half portion 140. The first half portion 130 includes a first wall portion 131, a pair of first ridge line portions 132, 132, a pair of second wall portions 133, 133, a pair of second ridge line portions 134, 134, and a pair of flanges. It has 135, 135, and so on.
[0221]
　The width of the first wall portion 131 side of the first half portion 130 is 80 mm, and the height of the hat portion is 60 mm. The radius of curvature R of the first ridge line portion 132 of the first half portion 30 is 5 mm. The inclination angle of the second wall portion 133 with respect to the direction orthogonal to the second half portion 140 is 5 °. The width of the second half 140 is 130 mm. A pair of flanges 135 and 135 are joined to the second half 140 by spot welding. The nugget diameter of this spot welding ≧ 5√ plate thickness t1. Further, the arrangement interval of the joint points in the longitudinal direction of the bent component 121 is 40 mm.
[0222]

　As shown in Table 2-2, for the bending parts 121, the tensile stress σ 5 when the elongation strain in the tensile test is 5% and the shear stress 5√3 in the shear test on the wall part. Ratio with shear stress τ 5 at% σ 5 / τ 5 (5% ratio), Vickers hardness Hvc at the center of the wall 137, Vickers hardness Hvr at the first ridge 132 and 137 at the center of the wall. The ratio Hvr / Hvc to the Vickers hardness Hvc, the density of the number of precipitates, the maximum load generated in the bent part, and the shock absorption energy of the bent part were measured.
[0223]
　In order to calculate the 5% ratio, a tensile test and a shear test were performed on the flat surface portion 136 (wall portion) in the vicinity of the first ridge line portion 132 among the bending parts 121. The tensile test at this time was performed by cutting out the flat surface portion 136 in the vicinity of the first ridge line portion 132 from the bending component 121 and attaching it to a tensile tester (not shown). The test piece for the tensile test at this time may have a size of about 10 × 100 mm. This tensile test was carried out in accordance with JIS (Japanese Industrial Standards) Z2241: 2011 Metallic Material Tensile Test Method. Further, in the shear test, the flat surface portion 136 in the vicinity of the first ridge line portion 132 is cut out from the bending component 121 to prepare a rectangular test piece 201, and the rectangular test piece 201 is used in the shear tester 200 shown in FIG. 8 (A). It was installed and done. The test piece for the shear test at this time may have a size of about 30 mm × 30 mm. FIG. 8A shows a schematic diagram of the shear test 200. FIG. 8B is an explanatory diagram relating to the shear test. With reference to FIGS. 8 (A) and 8 (B), the shear test chucks a pair of long sides of the rectangular test piece 201, with one long side vertically relative to the other long side. Simple shear deformation was applied by moving in the (shear direction), and the load generated by this shear deformation was measured. The cross-sectional area for calculating the shear stress was the length of the rectangular test piece 201 in the shear direction × the plate thickness. The shear strain was obtained at any time from the inclination of the straight line 202 drawn in the central portion of the test piece 201 along the direction orthogonal to the shear direction. Then, the tensile stress σ 5 when the elongation strain in the tensile test is 5% and the shear stress τ 5 when the shear strain in the shear test is 5√3% are measured, and the ratio σ 5 / τ 5 is measured.(5% ratio) was calculated. The results are shown in Table 2-2.
[0224]
　Further, each of the first ridge line portion 132 of the bending component 121 and the central portion 137 of the second wall portion 133 was cut out, and Vickers hardness Hvr and Hvc were measured at locations corresponding to 1/4 of the plate thickness. The Vickers hardness measurement test was carried out according to JIS Z 2244: 2009 Vickers hardness test. The Vickers hardness was HV 0.5, 5 points were measured, and the average was used as the test result. Then, the ratio Hvr / Hvc of the Vickers hardness Hvr of the first ridge line portion 132 and the Vickers hardness Hvc of the central portion 137 of the second wall portion 133 was calculated. The results are shown in Table 2-2.
[0225]
　The number of precipitates density refers to the density of precipitates having a major axis of 0.05 μm or more and 0.60 μm or less and an aspect ratio of 1: 3 or more obtained by the first heat treatment. The morphology and number density of the precipitates were determined by observation with an electron microscope, and in this example, they were measured by TEM (Transmission Electron Microscope) observation. Specifically, the thin film is formed from the region from the 3/4 position to the 1/4 position of the thickness of the steel plate constituting the first half portion 130 of the bent component 121 with the surface of the steel plate constituting the bent component 121 as a reference. A sample was cut out. Then, this thin film sample is observed in a bright field, and 1 μm 2 is cut out at an appropriate magnification of 10,000 to 100,000 times, and a precipitate having a major axis of 0.05 μm or more and 0.60 μm or less and an aspect ratio of 1: 3 or more. Was counted and asked. At this time, the number of thin film samples per unit area converted with the thickness of the thin film sample being 30 nm was counted. This work was performed in five or more continuous fields of view, and the average was taken as the number density. The results are shown in Table 2-2.
[0226]
　Then, a three-point bending deformation test was performed to measure the maximum load and impact absorption energy of the bending component 121. Specifically, with reference to FIGS. 7 (A) and 7 (B), a pair of hemispherical first indenters 123 are arranged symmetrically in the longitudinal direction of the bending component 121 at intervals of 700 mm. The radius of curvature of the first indenter 123 is 30 mm. Further, the second indenter 124 is arranged at the center of the bending component 121 in the longitudinal direction. The radius of curvature of the second indenter 124 is 50 mm. The apex of each first indenter 123 faces the second half 140. The apex of the second indenter 124 faces the first half 130. Then, the first indenters 123 and 123 and the bending component 121 were moved toward the second indenter 124 at 7.2 km / h (2 m / s), and the first half portion 140 was made to collide with the second indenter 124. The maximum load generated at this time and the impact absorption energy were measured. The results are shown in Table 2-2.
[0227]
　Next, a more specific evaluation of the bent component 121 will be described.
[0228]
　The evaluation of the bending component 121 is based on the case where the maximum load and the impact absorption energy satisfy both of the following two equations (2) and (3), and satisfy at least one of the equations (2) and (3). The case where there was no such case was used as a comparative example.
　Maximum load> 0.11 x TS ... (2)
　Impact absorption energy> 0.00055 x TS + 0.27 ... (3)
　The TS in the above equation is the tension shown in Table 1-3. Shows strength.
[0229]
　FIG. 9A is a graph showing the relationship between the tensile strength TS and the maximum bending load. FIG. 9B is a graph showing the relationship between the tensile strength TS and the impact absorption energy at the time of bending deformation. In FIG. 9A, the straight line represented by the bending load = 0.11 × TS described in the equation (2) is shown as a critical line. Further, in FIG. 9B, the impact absorption energy = 0.00055 × TS + 0.27 described in the formula (3) is shown as a critical line. Further, in FIGS. 9A and 9B, trend lines of Examples and Comparative Examples are shown. The trend line of the embodiment of FIG. 9A is 0.0123 × TS-0.6182. The tendency line of the comparative example of FIG. 9A is 0.0115 × TS-0.3599. The trend line of the embodiment of FIG. 9B is 0.0005 × TS + 0.3025. The tendency line of the comparative example of FIG. 9B is 0.0005 × TS + 0.2631.
[0230]
　With reference to Table 2-2, FIGS. 9 (A) and 9 (B), the examples show bending loads and impact absorption energies beyond the critical line, respectively, in equations (2) and (3). It can be seen that both of) are satisfied. On the other hand, the comparative example shows a tendency to fall below the critical line especially for the shock absorption energy. Table 2-2 shows the calculated values ​​of 0.11 × TS represented by the formula (2) and 0.00055 × TS + 0.27 represented by the formula (3). In each of the examples, the 5% ratio (σ / τ) is 1.70 or less because the baking curability is good. On the other hand, in all of the comparative examples, the 5% ratio (σ / τ) exceeds 1.70 due to poor seizure curability.
[0231]
　When an example of the bending load and the impact absorption energy is shown in a graph, the effects shown in FIGS. 10 (A) and 10 (B) can be obtained. FIG. 10A is a graph showing an example of the relationship between the amount of deformation (stroke) of the bending member in the direction perpendicular to the length of the bending member (direction perpendicular to the length direction) and the load generated on the bending member. be. FIG. 10B is a graph showing an example of the relationship between the amount of deformation (stroke) of the bending member in the direction perpendicular to the length of the bending member and the absorbed energy of the bending member.
[0232]
　In FIG. 10 (A), No. 1 is an example. No. 2 and No. 2 which is a comparative example. 14, No. Each of the 15 properties is shown. The horizontal axis of the graph of FIG. 10A is the stroke amount of the bending member, and the vertical axis shows the load acting on the bending member. As is clear from FIG. 10 (A), No. The peak value P1 of the load in No. 2 is No. 2 which is a comparative example. It is clearly higher than the peak value P2 of the load at 15. In addition, No. Regarding the peak value P3 of the load at No. 14, No. 14 is a comparative example. It is substantially the same as the peak value P2 of the load of 15.
[0233]
　In FIG. 10B, No. 2. No. 14, No. Each of the 15 properties is shown. The horizontal axis of the graph of FIG. 10B is the stroke amount of the bending member, and the vertical axis shows the absorbed energy acting on the bending member. As is clear from FIG. 10 (B), No. The peak value PE1 of the absorption energy in No. 2 is No. 2 which is a comparative example. 14, No. Clearly higher than the absorbed energy at 15.
[0234]
　Based on the above, it has become clear that the examples of the bent parts have an improved maximum load and are less likely to crack as compared with the bent parts of the comparative example, so that the impact absorption energy tends to be improved. In particular, it can be seen that the impact absorption energy is improved in the examples as compared with the comparative example when the tensile strength is the same.
[0235]
　As for the manufacturing method of the bent part 121, as shown in Tables 1-3 and 2-1 of the example, No. 1 to No. 13 is within the scope of the present invention. On the other hand, No. 14-No. 26 has items outside the scope of the present invention. More specifically, No. As shown in Table 2-1 of 14, the time of the second heat treatment step is out of the scope of the present invention. No. Reference numeral 15 is out of the scope of the present invention because the first heat treatment step has not been performed as shown in Table 2-1. No. 16, No. In 17, as shown in Table 1-3, Cmax / Cmin is out of the scope of the present invention. No. As shown in Table 2-1 in No. 18, the temperature of the first heat treatment step is out of the scope of the present invention. No. No. 19 has a martensite area ratio of zero and does not contain martensite, as shown in Table 1-3. Further, the tensile strength does not reach 780 MPa, which is outside the scope of the present invention. No. As shown in Table 2-1 of No. 20, the temperature in the second heat treatment step is out of the scope of the present invention. No. 21 is out of the scope of the present invention because the tensile strength does not reach 780 MPa. No. As shown in Table 2-1 of 22, the temperature in the first heat treatment step is out of the scope of the present invention. No. 23, No. In 24, as shown in Table 1-3, Cmax / Cmin is out of the scope of the present invention. No. As shown in Table 2-1 of 25, the time in the first heat treatment step is out of the scope of the present invention. No. 26 is out of the scope of the present invention because the second heat treatment step has not been performed as shown in Table 2-1.
[0236]
　Further, in terms of the characteristics of the bending component 121 itself, as is well shown in Table 2-2, No. 1 to No. 13 is within the range recommended in the present specification. On the other hand, No. No. 14-No. In each of 26, the 5% ratio is out of the range recommended in the present specification. In particular, No. 15, No. 18, No. 19, No. 21, No. 22, No. 25 is a precipitate number density below the range recommended herein.
[0237]
The
　high-strength steel sheets (cold-rolled steel sheets) shown in Table 1-3 were subjected to the first heat treatment (tempering). The temperature of the high-strength steel plate in the first heat treatment and the holding time of this temperature are shown in Table 3-1. Then, the high-strength steel sheet subjected to the first heat treatment was subjected to cold plastic working to form the cold-rolled steel sheet into the shape of a shaft crushed part. The cold plastic working method at this time is shown in Table 3-1. Next, a second heat treatment (baking and curing) was performed on the part molded into the shape of the shaft crushed part to produce a shaft crushed part. The temperature of the parts in the second heat treatment and the holding time of this temperature are shown in Table 3-1.
[0238]
[Table 3-1]

[0239]
[Table 3-2]

[0240]
　11 (A) and 11 (B) show an axial crushing component 122 showing the above-mentioned axial crushing component. 11 (A) is a side view of the axial crushing component 122, and FIG. 11 (B) is a cross-sectional view taken along the line XIB-XIB of FIG. 11 (A).
[0241]
　With reference to FIGS. 11A and 11B, the axial crushing component 122 is a hollow square cylinder member having a total length of 300 mm and a plate thickness of t2 = 1.2 mm. The axial crushing component 122 has four ridge line portions 142 and four wall portions 143, and is formed in a substantially square shape as a whole when viewed in the longitudinal direction of the axial crushing component 122. The length of one side of the shaft crushing component 122 is 50 mm. The radius of curvature R of each ridge portion 142 is 5 mm. Further, rectangular steel plates 150 are joined to both ends of the axial crushing component 122 by arc joining. The rectangular steel plate has a side of 200 mm and a plate thickness of 9 mm.
[0242]

　As shown in Table 3-2, for the shaft crushed parts 122, the tensile stress when the elongation strain in the tensile test is 10% in the flat surface portion 146 (wall portion) near the ridge line portion 142. The ratio of σ 10 to the shear stress τ 10 when the shear strain in the shear test is 10√3% σ 10 / τ 10 (10% ratio), the Vickers hardness Hvc at the center of the wall 147, and the ridgeline. The ratio Hvr / Hvc of the Vickers hardness Hvc of 142 to the Vickers hardness Hvr of the central portion 147 of the wall portion, the number density of precipitates, and the shock absorption energy were measured.
[0243]
　In order to calculate the 10% ratio, a tensile test and a shear test were performed on the flat surface portion 146 in which the strain from the ridgeline portion 142 is transmitted in the vicinity of the ridgeline portion 142 among the axial crushing parts 122. The tensile test at this time was performed by cutting the flat surface portion 146 in the vicinity of the ridge line portion 142 from the axial crushing component 122 and attaching it to a tensile tester (not shown). The test piece for the tensile test at this time may have a size of about 10 × 100 mm square. This tensile test was performed according to the JIS (Japanese Industrial Standards) Z2241: 2011 metal material tensile test method. Further, in the shear test, the flat surface portion 146 in the vicinity of the ridge line portion 142 is cut out from the axial crushing component 122 to prepare a rectangular test piece 201, and the rectangular test piece 201 is shown in FIGS. 8 (A) and 8 (B). It was carried out by attaching to a shear tester 200. The test piece for the shear test at this time may have a size of about 30 mm × 30 mm. The test content in this shear tester 200 is the same as the test in the rectangular test piece 201 produced by cutting from the bending component 121. Then, the tensile stress σ 10 when the elongation strain in the tensile test is 10% and the shear stress τ 10 when the shear stress in the shear test is 10√3% are measured, respectively, and the tensile stress σ 10 and the shear stress τ are measured. The ratio σ 10 / τ 10 to 10 was calculated. The results are shown in Table 3-2.
[0244]
　Further, each of the ridge line portion 142 of the shaft crushing component 122 and the central portion 147 of the wall portion 143 was cut out, and the Vickers hardness was measured at a portion corresponding to 1/4 of the plate thickness. The Vickers hardness measurement test was carried out according to JIS Z 2244: 2009 Vickers hardness test. The Vickers hardness was HV 0.5, 5 points were measured, and the average was used as the test result. Then, the ratio Hvr / Hvc of the Vickers hardness Hvr of the ridge line portion 142 and the Vickers hardness Hvc of the central portion 147 of the wall portion 143 was calculated. The results are shown in Table 3-2.
[0245]
　The number of precipitates density refers to the density of precipitates having a major axis of 0.05 μm or more and 0.60 μm or less and an aspect ratio of 1: 3 or more obtained by the first heat treatment. The morphology and the number density of the precipitates were determined by observation with an electron microscope, and in this example, they were measured by TEM observation in the same manner as the measurement of the number density of precipitates in the bent component 121. Specifically, a thin film sample was cut out from a region from the 3/4 position to the 1/4 position of the thickness of the steel plate constituting the axial crushing component 122 with reference to the surface of the steel plate constituting the axial crushing component 122. .. Then, this thin film sample is observed in a bright field, and 1 μm 2 is cut out at an appropriate magnification of 10,000 to 100,000 times, and a precipitate having a major axis of 0.05 μm or more and 0.60 μm or less and an aspect ratio of 1: 3 or more. Was counted and asked. At this time, the number of thin film samples per unit area converted with the thickness of the thin film sample being 30 nm was counted. This work was performed in five or more continuous fields of view, and the average was taken as the number density. The results are shown in Table 3-2.
[0246]
　Then, a shaft crushing test was performed to measure the impact absorption energy of the shaft crushing component 122. Specifically, the axial crushing component 122 was installed vertically, and a weight 151 having a mass of 890 kg was made to collide with the upper end of the axial crushing component 122 at a speed of 22.5 km / h. The shock absorption energy generated at this time was measured. The results are shown in Table 3-2.
[0247]
　Next, a more specific evaluation of the axial crushing component 122 will be described.
[0248]
　In the evaluation of the shaft crushing component 122, the case where the impact absorption energy satisfies the following formula (4) was taken as an example, and the case where the shock absorbing energy did not satisfy the formula (4) was taken as a comparative example.
　Impact absorption energy> 0.00185 × TS + 2.35 ... (4)
　The TS in the above formula indicates the tensile strength shown in Table 1-3.
[0249]
　FIG. 12 is a graph showing the relationship between the tensile strength TS and the impact absorption energy at the time of axial crush deformation. In FIG. 12, the impact absorption energy = 0.00185 × TS + 2.35 described in the equation (4) is shown as a critical line. Further, in FIG. 12, the tendency lines of the examples and the comparative examples are shown. The trend line of the embodiment of FIG. 12 is 0.0022 × TS + 2.1044. The tendency line of the comparative example of FIG. 12 is 0.0017 × TS + 2.3666.
[0250]
　With reference to Table 3-2 and FIG. 12, it can be seen that the examples show the shock absorption energy beyond the critical line and satisfy the equation (4). On the other hand, all of the comparative examples only show the impact absorption energy below the critical line. Table 3-2 shows the calculated values ​​of 0.00185 × TS + 2.35 shown in the formula (4). In each of the examples, the 10% ratio (σ 10 / τ 10 ) is less than 1.70 due to the good seizure curability. On the other hand, in all of the comparative examples, the 10% ratio (σ 10 / τ 10 ) exceeds 1.70 due to poor seizure curability .
[0251]
　Based on the above, it became clear that the examples of the shaft crushing parts have a tendency to clearly improve the impact absorption energy as compared with the shaft crushing parts of the comparative example. In particular, it can be seen that the impact absorption energy is improved in the examples as compared with the comparative example when the tensile strength is the same.
[0252]
　As for the manufacturing method of the shaft crushing component 122, as shown in Tables 1-3 and 3-1 of the Examples, No. 1 to No. 13 is within the scope of the present invention. On the other hand, No. 14-No. 27 has items outside the scope of the present invention. More specifically, No. As shown in Table 3-1 of 14, the time of the second heat treatment step is out of the scope of the present invention. No. Reference numeral 15 is out of the scope of the present invention because the first heat treatment step has not been performed as shown in Table 3-1. No. 16, No. In 17, as shown in Table 1-3, Cmax / Cmin is out of the scope of the present invention. No. In No. 18, as shown in Table 3-1 the temperature of the first heat treatment step is out of the scope of the present invention. No. No. 19 has a martensite area ratio of zero and does not contain martensite, as shown in Table 1-3. Further, the tensile strength does not reach 780 MPa, which is outside the scope of the present invention. No. As shown in Table 3-1 of 20, the temperature in the second heat treatment step is out of the scope of the present invention. No. As shown in Table 1-3, No. 21 is out of the scope of the present invention because the tensile strength does not reach 780 MPa. No. As shown in Table 3-1 of 22, the temperature in the first heat treatment step is out of the scope of the present invention. No. 23, No. In 24, as shown in Table 1-3, Cmax / Cmin is out of the scope of the present invention. No. As shown in Table 3-1 25, the time in the first heat treatment step is out of the scope of the present invention. No. 26 is out of the scope of the present invention because the second heat treatment step has not been performed as shown in Table 3-1.
[0253]
　Further, in terms of the characteristics of the shaft crushing component 122 itself, as is well shown in Table 3-2, No. 1 to No. 13 is within the range recommended in the present specification. On the other hand, No. 14-No. In each of 27, the 10% ratio is out of the range recommended in the present specification. In particular, No. 14, No. 15, No. 18, No. 19, No. 21, No. No. 22 has a precipitate number density below the range recommended herein.
[0254]
　In the examples, the cold-rolled steel sheet has been described. However, even if it is a plated steel sheet, the same tendency as that of the cold-rolled steel sheet can be read, and even if it is a plated steel sheet, the same result as that of the above-mentioned cold-rolled steel sheet can be obtained.
Industrial applicability
[0255]
　The present invention can be widely applied as a method for manufacturing a shock absorbing member.
Code description
[0256]
1 Front side member (shock absorbing member)
2 Crash box (shock absorbing member)
3 Rear side member (shock absorbing member)
4 Floor cross member (shock absorbing member)
5 Subframe (shock absorbing member)
11 A pillar (shock absorbing member)
12 Center pillar (shock absorbing member)
13 C pillar (shock absorbing member)
14 Side sill (shock absorbing member)
15 Roof side rail (shock absorbing member)
22, 32, 42, 62, 132, 142 Ridge line 21, 23,
25 , 31, 33, 35, 41, 43, 45, 61, 63, 65, 131, 133, 135, 143 Wall part
136,146 Flat part (wall part)
27,47,67,137,147 Wall part center part
110 High-strength steel plate (steel plate)
111 Coil
X Length direction X (longitudinal direction)
Y Length direction (longitudinal direction)
The scope of the claims
[Claim 1]
　A shock absorbing member including a ridge portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridge portion, and when
　the elongation strain in the tensile test is 5% in the wall portion. The ratio of the tensile stress σ 5 to the shear stress τ 5 when the shear stress in the shear test is 5√3% is 1.70 or less, or the elongation strain in the tensile test is 10 %. The ratio σ 10 / τ 10 between the tensile stress σ 10 at the time and the shear stress τ 10 when the shear stress in the shear test is 10√3% is 1.70 or less, and the 　wall portion seen from the longitudinal direction. A shock absorbing member having a Vickers hardness Hvc of 250 or more in the central portion.
[Claim 2]
　The shock absorbing member according to claim 1, wherein the ratio Hvr / Hvc of the Vickers hardness Hvr of the ridgeline portion and the Vickers hardness Hvc of the central portion of the wall portion is 1.05 or more.
[Claim 3]
　The shock absorbing member according to claim 2, wherein the Vickers hardness ratio Hvr / Hvc is 1.10 or more.
[Claim 4]
　The shock absorbing member according to any one of claims 1 to 3, wherein the shock absorbing member is a pillar, a side sill, or a roof side rail.
[Claim 5]
　The shock absorbing member is a member having a closed cross section,
　and the ratio Hvr / Hvc of the Vickers hardness Hvr of the ridgeline portion and the Vickers hardness Hvc of the central portion of the wall portion is less than 1.10. The shock absorbing member described.
[Claim 6]
　The shock absorbing member according to claim 1 or 5, wherein the shock absorbing member is a front side member, a crash box, a rear side member, a subframe, or a floor cross member.
[Claim 7]
　The shock absorbing member according to any one of claims 1 to 6, wherein the impact absorbing member has a tensile strength of 780 MPa or more.
[Claim 8]
　The steel plate is subjected to a first heat treatment step in which the steel plate is heat-treated to be held at a temperature T1 satisfying the following formula (1) for 60 to 900 seconds, and the
　steel plate after the heat treatment is subjected to cold plastic processing to obtain a steel member.
　The steel member is provided with a second heat treatment step of performing a cold plastic working step and a heat treatment for holding the steel member at a temperature T2 of 80 to 200 ° C. for 300 to 1800 seconds
　. The ratio is 5% or more and less than 95%, the tensile strength is 780 MPa or more, and the ratio Cmax / Cmin of the maximum value Cmax and the minimum value Cmin of the Si content in the cross section in the thickness direction is 1.25 or less. ,
　Manufacturing method of shock absorbing member.
　80 × Si + 100 ≦ T1 ≦ 125 × Si + 250 ... (1)
　However, Si in the above formula (1) means the Si content (mass%) in the steel sheet.
[Claim 9]
　The eighth aspect of the present invention, wherein the shock absorbing member includes a ridge line portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridge line portion, and the
　cold plastic working is foam molding. Manufacturing method of shock absorbing member.
[Claim 10]
　The eighth aspect of the present invention, wherein the shock absorbing member includes a ridge line portion formed in a bent shape when viewed from the longitudinal direction and a wall portion extending from the ridge line portion, and the
　cold plastic working is draw molding. Manufacturing method of shock absorbing member.
[Claim 11]
　The method for manufacturing a shock absorbing member according to any one of claims 8 to 10, wherein the steel sheet is a Dual-Phase steel sheet.
[Claim 12]
　The method for manufacturing a shock absorbing member according to any one of claims 8 to 11, wherein the first heat treatment step is carried out on the steel sheet after final annealing and before winding on a coil.
[Claim 13]
　10. The method for manufacturing a shock absorbing member according to item 1.
[Claim 14]
　17. Manufacturing method of shock absorbing member.
[Claim 15]
　The area fraction of martensite is 5% or more and less than 95%, the tensile strength is 780 MPa or more, and the ratio Cmax / Cmin of the maximum value Cmax and the minimum value Cmin of the Si content in the cross section in the thickness direction is 1. A method for producing a steel sheet for cold plastic working, in which a steel sheet having a temperature of .25 or less is heat-treated to be held at a temperature T1 satisfying the following formula (1) for 60 to 900 seconds.
　80 × Si + 100 ≦ T1 ≦ 125 × Si + 250 ... (1)
　However, Si in the above formula (1) means the Si content (mass%) in the steel sheet.