Title of invention: Automotive skeleton member
Technical field
[0001]
　The present disclosure relates to automobile frame members.
Background technology
[0002]
　In recent years, in order to reduce the emission of carbon dioxide contained in the exhaust gas of automobiles and improve fuel efficiency, it has been required to reduce the weight of the vehicle body. In order to further reduce the weight of the vehicle body, for example, the plate thickness of the portion where high strength is required is increased, and the plate thickness of the portion where high strength is not required is reduced to optimize the member shape. Is desirable.
[0003]
　Further, in the skeleton member of an automobile, further improvement in strength is required, and further improvement in shock absorption characteristics necessary for protecting the inside of the vehicle body skeleton in the event of a collision is also required.
[0004]
　Patent Document 1 below describes a technique of welding a main body member of an automobile skeleton member in a partially polymerized state to achieve both high strength and light weight. Further, Patent Document 2 below describes a technique of hot press molding after welding a first blank and a second blank.
Prior art literature
Patent documents
[0005]
Patent Document 1: Japanese Patent Application Laid-Open No. 2013-71532
Patent Document 2: Japanese Patent Application Laid-Open No. 2013-189173
Outline of the invention
Problems to be solved by the invention
[0006]
　However, when members are welded to each other, it is known that in the peripheral region of the weld metal portion of the welded portion, the characteristics and structure of the base metal change due to heat input during welding. The peripheral region is called a heat-affected zone (HAZ).
　In the technique described in Patent Document 1, the decrease in strength of the main body member and the reinforcement member in HAZ is not taken into consideration, and there is a problem that there is room for improvement from the viewpoint of improving the strength. In addition, there is a problem that there is room for improvement in the shock absorption characteristics required for the skeleton member.
　Further, in the technique described in Patent Document 2, although the strength in HAZ can be expected to be improved by hot press molding, there is room for further improvement in the joint strength between members.
[0007]
　Therefore, the present disclosure has been made in view of the above problems, and an object of the present invention is to achieve both improvement of the strength of the entire member including the welded portion and improvement of the shock absorption characteristics at a higher level. Further, it is an object of the present invention to provide a new and improved skeletal member capable of improving the joint strength between the members.
Means to solve problems
[0008]
　In order to solve the above problems, according to the present disclosure, a
first steel plate and a second steel plate, and a first weld metal portion that joins the interface between the first steel plate and the second steel plate are formed. provided,
wherein the tensile strength of the first steel sheet is less than 1.6GPa than 1.0 GPa,
the tensile strength of the second steel plate is less than 2.5GPa than 1.8 GPa,
the first steel plate A groove portion is provided,
the second steel plate is superposed on the groove portion, and
the minimum Vickers hardness of a region within 4 mm around the first weld metal portion of the second steel plate is the region of the second steel plate. An
automobile frame member having an outer hardness of 80% or more is provided.
[0009]
The Vickers hardness of the first weld metal portion may be 400 or more and 540 or less.
[0010]
The first steel plate includes a flange portion on the outside of the groove portion, and includes a
third steel plate and a second weld metal portion that joins the interface between the third steel plate and the flange portion
. The tensile strength of the steel sheet of No. 1 may be 0.45 GPa or more and 1.6 GPa or less.
The invention's effect
[0011]
　According to the present disclosure, there is provided an automobile skeleton member capable of both improving the strength of the entire skeleton member and improving the shock absorption characteristics at a higher level, and further improving the joint strength between the members.
A brief description of the drawing
[0012]
FIG. 1 is a perspective view showing an example of an automobile skeleton member according to the first embodiment of the present disclosure.
FIG. 2 is a graph showing the relationship between the strength ratio of the automobile skeleton member according to the same embodiment and the tensile strength of the second steel plate.
FIG. 3 is an XZ plan sectional view showing an example of an automobile skeleton member according to the same embodiment.
FIG. 4 is an enlarged view showing an example of a welded portion according to the same embodiment.
FIG. 5A is a graph showing a change in hardness of a welded portion according to a conventional example.
FIG. 5B is a graph showing an example of a change in hardness of a welded portion according to the same embodiment.
FIG. 6 is an explanatory diagram of a test for measuring the tensile shear strength of the first weld metal part.
FIG. 7 is a graph showing the results of a test in which the tensile shear strength of the first weld metal portion was measured.
FIG. 8 is a perspective view showing another example of an automobile skeleton member according to the same embodiment.
FIG. 9 is a diagram showing an example of a method for manufacturing an automobile skeleton member according to the same embodiment.
FIG. 10 is a diagram showing automobile skeleton parts to which the automobile skeleton member according to the same embodiment may be applied.
FIG. 11A is an external perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a B pillar.
FIG. 11B is a cross-sectional view taken along the line II-II'in FIG. 11A.
FIG. 12A is an external perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a B pillar.
FIG. 12B is a cross-sectional view taken along the line III-III'in FIG. 12A.
FIG. 13A is an external perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a roof rail.
FIG. 13B is an external perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a roof rail.
FIG. 14A is an external perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a side sill.
FIG. 14B is a sectional view taken along line VV'in FIG. 14A.
FIG. 15A is an exploded perspective view showing an example in which an automobile skeleton member according to the present embodiment is applied as a rear side member.
FIG. 15B is a sectional view taken along line VI-VI'in FIG. 15A.
FIG. 15C is a cross-sectional view showing an example in which an automobile skeleton member according to the present embodiment is applied as a floor member.
Mode for carrying out the invention
[0013]
　Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.
[0014]
　<1. First Embodiment>
　[Example of Appearance of Skeleton Member]
　First, a schematic configuration of an automobile skeleton member 1 according to the first embodiment of the present disclosure will be described with reference to FIG. In the following, the "automobile skeleton member" may be abbreviated as the "skeleton member".
　FIG. 1 is a perspective view showing an example of the skeleton member 1 according to the present embodiment. As shown in FIG. 1, as an example, the skeleton member 1 extends in the Y direction shown in FIG. 1 as the longitudinal direction, and is opened in the Z direction in a cross-sectional (XX plane) view along the lateral direction. It is a member having a rectangular shape. In particular, the skeleton member 1 has a substantially hat shape in a cross-sectional view (XX plane) along the lateral direction. In the skeleton member 1, the first steel plate 10 and the second steel plate 20 are overlapped with each other. The first steel plate 10 and the second steel plate 20 are welded and integrated via a plurality of first weld metal portions 40.
[0015]
　The first steel plate 10 is a member formed in a substantially hat shape in the cross-sectional view of the XX plane in FIG. 1, and forms the outer shape of the skeleton member 1. The first steel plate 10 includes a first top wall portion 11, a first vertical wall portion 15 bent from the first top wall portion 11 via a first bent portion 13, and a first vertical wall. It has a flange portion 17 that is bent outward from an end portion of the portion 15 opposite to the first top wall portion 11 side. The first steel plate 10 includes a groove portion 18 formed by having a first vertical wall portion 15 bent via a first bent portion 13 on both sides of the first top wall portion 11. Further, the first steel plate 10 includes flange portions 17 arranged on both outer sides of the groove portion 18.
　The skeleton member 1 can have a closed cross-sectional shape by welding the flange portion 17 to a plate-shaped member (not shown, which corresponds to the plate-shaped member 30 as a third steel plate described later). Here, the XX plane cross section in FIG. 1 is a plane perpendicular to the ridgeline of the first bent portion 13. The ridgeline of the first bent portion 13 is a line of intersection between the virtual surface extending the outer surface of the first top wall portion 11 and the virtual surface extending the outer surface of the first vertical wall portion 15. Is the ridgeline.
[0016]
　The second steel plate 20 is formed in a rectangular shape with one side opened in the cross-sectional view of the XX plane in FIG. The second steel plate 20 has a second top wall portion 21 and a second vertical wall portion 25 extending from the second top wall portion 21 via the second bent portion 23. The second steel plate 20 is attached so as to be superposed on the substantially hat-shaped inner wall surface of the first steel plate 10, and functions as a reinforcing member. That is, since the second steel plate 20 is superposed on the inside of the substantially hat shape of the first steel plate 10, the plate thickness of the skeleton member 1 can be increased, and the strength of the skeleton member 1 as a whole can be increased. improves.
[0017]
　The second steel plate 20 may be partially provided in the longitudinal direction of the first steel plate 10. Further, the second steel plate 20 may be provided so as to extend in the longitudinal direction (Y direction in FIG. 1) of the first steel plate 10. Further, the second steel plate 20 may be provided on the outer wall surface of the first steel plate 10 in a substantially hat shape.
[0018]
　The second steel plate 20 is not limited to a rectangular shape in which one side is opened in the cross-sectional view of the XX plane in FIG. For example, the member may have an L-shaped cross section. In this case, the second steel plate 20 is attached to the inside or outside of the bending of the first bent portion 13 of the first steel plate 10.
The second steel plate 20 may be overlapped with the first steel plate 10 in the groove portion 18, and may be in any of the following cases. When the second steel plate 20 is arranged inside or outside the first top wall portion 11 of the first steel plate 10 and the first vertical wall portions 15 on both sides. When the second steel plate 20 is arranged inside or outside the first top wall portion 11 of the first steel plate 10 and the first vertical wall portion 15 on only one side. When the second steel plate 20 is arranged inside or outside any of the first top wall portion 11 and the first vertical wall portion 15 of the first steel plate 10.
[0019]
　Examples of the material constituting the first steel sheet 10 include steel sheets having a tensile strength of 1.0 GPa or more and 1.6 GPa or less. Further, as the material constituting the first steel sheet 10, a steel sheet having a tensile strength of 1.5 GPa or less is desirable. Further, as a material constituting the first steel sheet 10, a steel sheet having a tensile strength of 1.35 GPa or less is desirable. The tensile strength of the first steel sheet 10 referred to in the present disclosure is the tensile strength after the hardness is controlled by the hot stamping method described later. The thickness of the steel plate used for the first steel plate 10 is about 0.9 to 2.3 mm. Further, the carbon component in the steel sheet used for the first steel sheet 10 can be 0.23% by mass or less. Further, the carbon component in the steel sheet used for the first steel sheet 10 can be 0.16% by mass or less. By keeping the carbon component in the steel sheet low, it is possible to suppress a decrease in toughness of the first weld metal portion 40.
[0020]
　Examples of the material constituting the second steel sheet 20 include steel sheets having a tensile strength of 1.8 GPa or more and 2.5 GPa or less. From the viewpoint of weldability, the tensile strength is more preferably 2.15 GPa or less. The tensile strength of the second steel sheet 20 referred to in the present disclosure is the tensile strength after the hardness is controlled by the hot stamping method described later. If a tensile test sample cannot be collected, the tensile strength may be used by converting the Vickers hardness. For the hardness conversion, the JIS hardness conversion table (SAE J 417 revised in 1983) may be used. Tensile strength 2.15 GPa not shown in the hardness conversion table is regarded as Hv618, and 2.5 GPa is regarded as Hv720. The thickness of the steel plate used for the second steel plate 20 is about 0.9 to 2.6 mm. The carbon component in the steel material used for the second steel sheet 20 can be, for example, 0.27% or more and 0.38% or less in order to secure the strength as a reinforcing material.
[0021]
　The first steel plate 10 and the second steel plate 20 may have the same plate thickness or different plate thicknesses. In the case of different plate thicknesses, the first steel plate 10 forming the outer shape of the skeleton member 1 is made a thin plate thickness, while the second steel plate 20 as a reinforcing member is made relatively thick to ensure the strength of the skeleton. The weight of the member 1 as a whole can be reduced.
[0022]
[Hardness of the peripheral region of the first weld metal portion] With
　reference to FIG. 2, the effect of improving the strength of the skeleton member 1 by the second steel plate 20 will be specifically described. FIG. 2 is a graph showing the relationship between the bending strength ratio of the skeleton member 1 according to the present embodiment and the tensile strength of the second steel plate 20. In FIG. 2, when the steel plate of the second steel plate 20 is a steel plate having a tensile strength of 1.6 GPa, which is the upper limit of the strength class of the steel plate used for the first steel plate 10, the bending strength of the skeleton member 1 Is set as 1 for comparison (white circle in FIG. 2). At this time, when a steel plate having a tensile strength of 1.8 GPa is used as the second steel plate 20, the bending strength ratio of the skeleton member 1 shows a value of about 1.15. That is, by setting the tensile strength of the steel plate of the second steel plate 20 to 1.8 GPa or more, the strength of the skeleton member 1 as a whole is improved. Further, when a steel plate having a tensile strength of 2.0 GPa is used as the second steel plate 20, the bending strength ratio of the skeleton member 1 shows a value of about 1.23. Further, when a steel plate having a tensile strength of 2.5 GPa is used as the second steel plate 20, the bending strength ratio of the skeleton member 1 shows a value of about 1.45.
[0023]
　As described above, by using a steel plate having a tensile strength of 1.8 GPa or more as the second steel plate 20, even if a steel plate of 1.6 GPa or less is used as the first steel plate 10, the entire skeleton member 1 is used. The strength of the steel can be improved.
　On the other hand, if the tensile strength of the second steel plate 20 becomes too high, the hardness of the first weld metal portion 40 becomes too high, as will be described later, and the first steel plate 10 and the second steel plate 20 are joined together. There is a risk that the strength will decrease. Therefore, the tensile strength of the second steel plate 20 is set to 2.5 GPa or less.
[0024]
　Next, with reference to FIG. 3, the cross-sectional shape of the XX plane of the skeleton member 1 formed in the closed cross section will be described. FIG. 3 is a cross-sectional view of an XX plane showing an example of the skeleton member 1 according to the present embodiment, and is a cross-sectional view taken along the line II'in FIG. However, as shown in FIG. 3, the skeleton member 1 according to the present embodiment has a closed cross section due to the flange portion 17 of the first steel plate 10 being welded to the plate-shaped member 30 as the third steel plate. It is formed. A second steel plate 20 is provided inside the closed cross section of the skeleton member 1. The second steel plate 20 is welded to the first steel plate 10 via the first weld metal portion 40. The interface between the plate-shaped member 30 (third steel plate) and the flange portion 17 is joined by the second weld metal portion 41.
[0025]
　In the present embodiment, the first weld metal portion 40 is formed at the interface between the first top wall portion 11 of the first steel plate 10 and the second top wall portion 21 of the second steel plate 20. .. Further, the first weld metal portion 40 is formed at the interface between the first vertical wall portion 15 of the first steel plate 10 and the second vertical wall portion 25 of the second steel plate 20. The first weld metal portion 40 is at the interface between the first top wall portion 11 and the second top wall portion 21 or the interface between the first vertical wall portion 15 and the second vertical wall portion 25. It suffices if it is formed in at least one of them.
[0026]
　The first weld metal portion 40 is the interface between the first top wall portion 11 and the second top wall portion 21 or the first vertical wall portion 15 and the second vertical wall portion 25 in the XX plane. It may be formed in a plurality of places along the interface with. Further, as shown in FIG. 1, the first weld metal portions 40 may be formed at a plurality of locations along the longitudinal direction of the skeleton member 1 (Y direction in FIG. 1). Further, the first weld metal portion 40 may be provided not only in a dot shape but also in a C shape, a U shape, an ellipse shape, a linear shape having a predetermined length, and a zigzag shape.
　When the skeleton member 1 is formed in a closed cross section, the second weld metal portion 41 can be at an arbitrary position at the interface between the flange portion 17 and the plate-shaped member 30, and can be point-shaped, C-shaped, or C-shaped. It can be U-shaped, oval, linear with a predetermined length, zigzag, or the like.
[0027]
　The first weld metal portion 40 and the second weld metal portion 41 can be formed by applying various joining techniques which are known techniques. As an example of the method of forming the first weld metal portion 40 and the second weld metal portion 41, spot welding, laser welding, and combined use of spot welding and laser welding can be mentioned.
[0028]
　Next, the first weld metal portion 40 and the heat-affected zone around the first weld metal portion 40 according to the present embodiment will be described with reference to FIGS. 4, 5A, and 5B. FIG. 4 is an enlarged view showing an example of the first weld metal portion 40 according to the present embodiment. As shown in FIG. 4, the first weld metal portion 40 is formed by the base material of the first steel plate 10 and the second steel plate 20 at the interface where the first steel plate 10 and the second steel plate 20 are overlapped. The base metal is a portion formed by melting and solidifying each other. That is, the first steel plate 10 and the second steel plate 20 are welded by the first weld metal portion 40 at the interface where they are overlapped with each other.
[0029]
　Here, conventionally, it is known that in the peripheral region of a region where base materials are melted and solidified by welding (so-called nugget portion), the characteristics and structure of the base metal change due to heat input during welding. .. The peripheral region is called a heat-affected zone (HAZ). When the member to be welded is a steel plate containing a martensite structure, tempering and softening occur partially due to the temperature rise due to heat input at the heat-affected zone. As a result, the hardness of the heat-affected zone may decrease with respect to the base material. Such a decrease in hardness at the heat-affected zone becomes a starting point of fracture when the member after welding receives a load, and may greatly affect the strength of the entire member.
[0030]
　The present inventors have found that, in particular, when the member to be welded is a high-strength steel plate having different tensile strengths, the effect of the hardness difference in the heat-affected zone becomes large due to the difference in strength. That is, in a steel sheet having a relatively high strength, the hardness of the original base material is sufficiently high, so that the hardness reduction width of the heat-affected zone tends to be large. On the other hand, in the steel sheet having low strength, the hardness reduction width in the heat-affected zone is suppressed to be lower than that in the steel sheet having high strength. Therefore, when members having different strengths are welded to each other, the decrease in hardness at the heat-affected zone on the steel plate side having higher strength becomes relatively remarkable. Furthermore, when members with different strengths are welded together for the purpose of reinforcing the members, it is expected that they will be used under conditions where a larger load is generated in anticipation of the reinforcement. In such a case, the decrease in hardness at the heat-affected zone may have a greater effect on the strength of the entire member.
[0031]
　The change in hardness at the actual welded portion will be described with reference to FIG. 5A. FIG. 5A is a graph showing an example of changes in hardness of the welded portion and its surroundings according to the conventional example. The conditions for measuring the hardness of the welded portion are as follows.
[0032]
　A cross section perpendicular to the plate surface of the sample including the vicinity of the first weld metal portion of the skeleton member is taken, a sample of the measurement surface is prepared, and the sample is subjected to a hardness test. The method for preparing the measurement surface is carried out according to JIS Z 2244. After polishing the measurement surface with # 600 to # 1500 silicon carbide paper, finish it to a mirror surface using a diluted solution such as alcohol or a liquid dispersed in pure water with diamond powder having a particle size of 1 μm to 6 μm. Corrodes with and makes a nugget appear. The hardness test is carried out by the method described in JIS Z 2244. The hardness of the sample whose measurement surface has been prepared is measured using a Vickers hardness tester. Measured at a position 0.2 mm from the overlapping surface of the first steel plate 10 and the second steel plate 20 in the sample cross section from the entire area of ​​the first weld metal portion to the base metal side with a load of 1 kgf. Hardness is measured at a pitch of 0.25 mm. The measurement points are schematically shown by the continuous dots 50 in FIG.
[0033]
　The graph of FIG. 5A plots the hardness at each measurement point according to the distance when the center position of the first weld metal portion 40 (corresponding to the nugget) is used as the origin.
[0034]
　As shown in FIG. 5A, the hardness of the first weld metal portion 40 is a substantially average value of the hardness of the first steel plate 10 and the hardness of the second steel plate 20, and is, for example, about 500 Hv. Further, the hardness of the first steel plate 10 on the base material side is, for example, about 400 Hv. The hardness of the second steel plate 20 on the base material side is, for example, about 600 Hv.
[0035]
　Here, the hardness on the base metal side is the hardness in a region (a position sufficiently distant from the first weld metal portion 40) in which the characteristics do not change due to the heat effect from the first weld metal portion 40, which is determined by the welding conditions. It is an average value. The hardness on the base metal side is about the same as the hardness of each member before welding. The hardness of the first weld metal portion refers to the average value of the hardness of the first weld metal portion 40 (nugget).
[0036]
　As shown in FIG. 5A, as described above, the hardness of both the first steel plate 10 and the second steel plate 20 is partially reduced in the heat-affected zone 61 existing around the first weld metal portion 40. doing. The range of the heat-affected zone 61 is within a range of 4 mm outside the end of the first weld metal portion 40.
[0037]
　Here, the portion where the hardness is partially reduced is a portion which shows a peak due to the hardness decrease in the measurement result of the hardness change, and refers to a significant decrease in hardness excluding the change such as a measurement error. Specifically, for example, it refers to a decrease in hardness of 25 Hv or more in absolute value.
[0038]
　The hardness of the heat-affected zone 61 of the first steel plate 10 is partially significantly lower than the hardness of the base material of the first steel plate 10 (X = about 3 mm to 4 mm, X = about -4 mm to -3. 5 mm position). The absolute value of the hardness difference is about 80 Hv at the maximum.
[0039]
　The hardness of the heat-affected zone 61 of the second steel plate 20 is partially significantly lower than the hardness of the base material of the second steel plate 20 (X = about 3 mm to 5 mm, X = about -5.5 mm to-. 3.5 mm). The absolute value of the hardness difference is about 200 Hv at the maximum.
[0040]
　The decrease in hardness of the first steel plate 10 and the second steel plate 20 may have a great influence on the strength of the entire member. In particular, the decrease in hardness of the second steel plate 20 may have a great influence on the strength of the entire member.
[0041]
　Therefore, as a result of diligent studies, the present inventors have come up with the idea of ​​suppressing a decrease in hardness around the first weld metal portion 40. In particular, the present inventors perform a predetermined treatment described later to reduce the influence of heat input of the first weld metal portion 40, and perform the peripheral region of the first weld metal portion 40 according to the present embodiment. Hardness was controlled.
[0042]
　Hereinafter, the hardness change of the welded portion (first weld metal portion 40 and its periphery) according to the present embodiment will be described with reference to FIG. 5B. FIG. 5B is a graph showing an example of changes in hardness of the first weld metal portion 40 and its surroundings according to the present embodiment. The conditions for measuring hardness are the same as in the case of FIG. 5A.
[0043]
　As shown in FIG. 5B, the hardness of the first weld metal portion 40, the hardness of the first steel plate 10 on the base metal side, and the hardness of the second steel plate 20 on the base metal side are all the same as in FIG. 5A. It is about the same.
[0044]
　Here, as described above, in the vicinity of the first weld metal portion 40 according to the present embodiment, a process for reducing the influence of hardness reduction due to heat input during welding is performed. As a result, there is a peripheral region 62 corresponding to the heat-affected zone 61 in place of the conventional heat-affected zone 61 around the first weld metal portion 40. The range of the peripheral region 62 is a range from the end portion of the first weld metal portion 40 to 4 mm outside along the peripheral edge of the first weld metal portion 40.
[0045]
　The end portion of the first weld metal portion 40 refers to the boundary line of the first weld metal portion 40 that can be visually recognized by the corrosion treatment under the hardness measurement conditions of the first weld metal portion 40 described above. Specifically, when the welding method is spot welding, it is the boundary between the first weld metal portion 40 and the base metal. Further, in the case of laser welding, it is the boundary of the end portion in the width direction of the first weld metal portion 40.
[0046]
　As shown in FIG. 5B, the hardness of the peripheral region 62 of the first steel plate 10 according to the present embodiment is not partially significantly reduced with respect to the hardness of the base material of the first steel plate 10. In other words, the lower limit of the hardness in the peripheral region 62 of the first steel plate 10 according to the present embodiment is equal to or higher than the hardness of the base material of the first steel plate 10.
[0047]
　Further, as shown in FIG. 5B, the hardness in the peripheral region 62 of the second steel plate 20 according to the present embodiment is not partially significantly reduced with respect to the hardness of the base material of the second steel plate 20. That is, the hardness of the peripheral region 62 of the second steel plate 20 is about 600 Hv, which is equivalent to the hardness of the base material of the second steel plate 20. Specifically, as shown in FIG. 5B, the difference in hardness between the lower limit of hardness in the peripheral region 62 of the second steel plate 20 according to the present embodiment and the base material of the second steel plate 20 is 100 Hv in absolute value. It fits below.
[0048]
　Further, in order to understand the relationship between the lower limit of the hardness of the peripheral region 62 of the second steel plate 20 and the hardness of the base material of the second steel plate 20, the lower limit of the hardness of the peripheral region 62 and the mother of the second steel plate 20 The effect of the ratio to the hardness of the material on the occurrence of fracture in the peripheral region 62 was investigated.
[0049]
　The experimental conditions for the survey are as follows. From a steel plate with a size of 1.3t x 25mm x 25mm and a tensile strength of 1.8 GPa at the center of the first test piece made of a steel plate with a tensile strength of 1.3 GPa and a size of 1.6t x 25 mm x 200 mm. The second test piece was superposed and spot welded to prepare a plurality of tensile test pieces. After spot welding, each test piece was heat-treated while changing the conditions as appropriate to obtain test pieces having different lower limit values ​​of hardness / base metal hardness (%) in the peripheral region 62. A tensile test was carried out on these test pieces at a speed of 10 mm / min, and the fracture surface was observed after fracture to evaluate the fracture mode. The results are summarized in Table 1 below.
[0050]
[table 1]

[0051]
　As shown in Table 1, when the lower limit value (minimum Vickers hardness) of the hardness in the peripheral region 62 of the second steel plate 20 is less than 80% of the hardness of the base material of the second steel plate 20. When an impact load was applied to the skeleton member 1, a break occurred in the peripheral region 62. On the other hand, when the lower limit of the hardness in the peripheral region 62 of the second steel plate 20 is 80% or more with respect to the hardness of the second steel plate 20, even if an impact load is applied to the skeleton member 1, the peripheral region No break occurred at 62.
[0052]
　Therefore, the lower limit value (minimum Vickers hardness) of the hardness in the peripheral region 62 of the second steel plate 20 according to the present embodiment is set to 80% or more with respect to the hardness of the base material of the second steel plate 20. It was found that the strength of the skeleton member 1 as a whole was improved. In particular, the lower limit of the hardness (minimum Vickers hardness) of the peripheral region 62 of the second steel plate 20 may be 90% or more with respect to the hardness of the base material of the second steel plate 20. The effect of the ratio of the lower limit of the hardness of the peripheral region 62 to the hardness of the second steel plate 20 on the occurrence of fracture in the peripheral region 62 has been described above.
[0053]
　Various known techniques such as surface treatment, surface treatment, and heat treatment are applied to control the hardness of the heat-affected zone 61 and the peripheral region 62 between the first weld metal portion 40 and the base metal according to the present embodiment. It is done by doing. As an example of the hardness control method, there is a hardness control by a hot stamping method after welding, which will be described later.
[0054]
[Hardness of the first weld metal part] Further, in
　addition to controlling the lower limit value (minimum Vickers hardness) of the hardness in the peripheral region 62 of the second steel plate 20, the Vickers hardness of the first weld metal part 40 is adjusted. It is also important that it is within a predetermined range. That is, in the first weld metal portion 40, the first steel plate 10 and the second steel plate 20 are melted and solidified, so that the hardness of the first weld metal portion 40 is the hardness of the first steel plate 10 and the first. It can be estimated to be an approximately average value with the hardness of the second steel plate 20.
　As described above, in order to improve the overall strength of the skeleton member 1, the higher the tensile strength of the second steel plate 20 used as the reinforcing member, the more effective it is. However, since the hardness of the first weld metal portion 40 is almost an average value of the hardness of the first steel plate 10 and the hardness of the second steel plate 20, when the tensile strength of the second steel plate 20 becomes high, The hardness of the first weld metal portion 40 is proportionally increased. As a result, the hardness of the first weld metal portion 40 becomes too high and the toughness deteriorates, and there is a concern that the first weld metal portion 40 may break when an external force is applied to the skeleton member 1.
[0055]
　Therefore, in the present disclosure, by setting the tensile strength of the second steel sheet 20 to 2.5 GPa or less, deterioration of toughness due to the hardness of the first weld metal portion 40 becoming too high can be avoided.
　As shown in FIG. 6, the flat plate-shaped first steel plate 10 and the second steel plate 20 were joined by the first weld metal portion 40. The diameter (nugget diameter) of the first weld metal portion 40 is 6.3 mm. Then, the first steel plate 10 and the second steel plate 20 were pulled from each other, and the tensile shear strength (kN) was measured. The results are shown in Table 2 and FIG.
[0056]
[Table 2]

[0057]
　As an index of deterioration of toughness in the first weld metal portion 40, a tensile shear strength of 20.0 kN was set as a passing line. At the marks D, E, F, and G in which the hardness of the first weld metal portion 40 is in the range of 400 to 540, the tensile shear stress is 20.0 kN or more, which satisfies the passing line. On the other hand, at marks A, B, and C in which the hardness of the first weld metal portion 40 is less than 400, and in marks H and I where the hardness of the first weld metal portion 40 exceeds 540, the tensile shear stress is 20. It was less than 0 kN.
[0058]
(Effect of Action)
　According to the present embodiment, the first steel plate 10 made of a steel plate having a relatively low strength and the second steel plate 20 made of a steel plate having a relatively high strength are welded to obtain strength as a skeleton member 1. While improving, the hardness of the first welded metal portion 40 is prevented from being lowered, so that the reinforcing effect of the high-strength steel plate can be sufficiently exhibited. Further, in the present embodiment, instead of the conventional heat-affected zone, the change in hardness in the region from the end portion of the first weld metal portion 40 to 4 mm outside is controlled. As a result, since the strength reduction region of the heat-affected zone does not occur around the first weld metal portion 40, the first steel plate 10 and the second steel plate do not break from the strength reduction region at the time of collision. The effect of improving the strength by welding with 20 can be maximized.
[0059]
　According to the present embodiment, the outer shape portion of the skeleton member 1 is formed of the first steel plate 10 having a relatively low strength. As a result, when an impact load is input to the skeleton member 1, the skeleton member 1 is greatly deformed without breaking, so that the impact absorption energy can be increased.
[0060]
　According to the present embodiment, when the tensile strength of the second steel plate 20 is 2.5 GPa or less, the hardness of the first weld metal portion 40 is in the range of 400 to 540, and the toughness is improved. Therefore, the joint strength between the first steel plate 10 and the second steel plate 20 is increased, and even when an impact load is input to the skeleton member 1, the joint state between the first steel plate 10 and the second steel plate 20 is increased. Can be maintained. Therefore, since the skeleton member 1 can be greatly deformed without breaking, the shock absorption energy can be further increased.
[0061]
　[Modification Example]
　Next, a modification of the skeleton member 1 according to the present embodiment will be described with reference to FIG. FIG. 8 is a perspective view showing another example of the skeleton member 1 according to the present embodiment. This modification differs from the above-described embodiment in the shape of the edge of the second vertical wall portion 25 of the second steel plate 20. Since other configurations of this modification are common to the above-described embodiment, the description thereof will be omitted.
[0062]
　As shown in FIG. 8, in the present modification, the first steel plate 10 has a substantially hat shape in the cross-sectional view of the XX plane in FIG. The second steel plate 20 has a rectangular shape with one side opened in a cross-sectional view taken along the line XX. The second steel plate 20 is attached to the inner wall surface of the first steel plate 10.
[0063]
　The second steel plate 20 has a corrugated shape in which the edge of the second vertical wall portion 25 alternately repeats unevenness along the longitudinal direction (Y direction) of the skeleton member 1. That is, in the second steel plate 20, the extending direction (Z direction) length of the second vertical wall portion 25 changes periodically. A first weld metal portion 40 is formed on the convex portion of the second vertical wall portion 25, and the first steel plate 10 and the second steel plate 20 are welded via the first weld metal portion 40. ing.
[0064]
　In this modification, since the edge of the second vertical wall portion 25 of the second steel plate 20 has irregularities, the weight can be reduced by the amount of the concave portions. Further, since the first weld metal portion 40 is formed on the convex portion of the second vertical wall portion 25 of the second steel plate 20, the stress transmission from the longitudinal direction is divided by the concave portion, and the first Stress concentration on the weld metal portion 40 is reduced. The modification of the skeleton member 1 according to the present embodiment has been described above.
[0065]
[Hot Stamping Method]
　Next, an example of a method for manufacturing the skeleton member 1 according to the present embodiment will be described with reference to FIG. FIG. 9 is a diagram showing an example of a method for manufacturing a skeleton member according to the present embodiment. As shown in FIG. 9, first, the first steel plate 10 and the second steel plate 20 are prepared as blank materials (flat plate members). Subsequently, the first steel plate 10 and the second steel plate 20 are welded to each other via the first weld metal portion 40. The blank material composed of the first steel plate 10 and the second steel plate 20 integrated via the first weld metal portion 40 is heated to an austenite region at about 900 ° C. in a heating furnace. After that, the blank material is formed into a predetermined shape by a hot stamping method and is quenched to form the skeleton member 1. At this time, the hardness of the heat-affected zone of the first weld metal portion 40 is controlled within a predetermined range by the heating / quenching process. Subsequently, shot blasting is performed to remove the scale on the surface of the steel sheet. If the steel sheet is plated, such as aluminum-based plating or zinc-based plating, the shot blasting step is not required.
[0066]
　In general, steel sheets having a tensile strength of 1.6 GPa or less after quenching are often not used as steel sheets for hot stamping materials (steel sheets used in the hot stamping method). This is because the method of cold-pressing a cold high-strength steel sheet in a region where the tensile strength is 1.6 GPa or less is economically advantageous. If the tensile strength is 1.5 GPa or less, and further, 1.35 GPa or less, the steel sheet for hot press material is further not adopted.
In order to partially increase the rigidity of a skeleton member of an automobile or the like, a second steel plate is superposed on a first steel plate at a portion where the rigidity is increased. At this time, when the hardened (hot pressed) steel sheets are overlapped and welded, the periphery of the weld metal portion is softened. That is, a HAZ softened zone appears around the weld metal portion. If there is a HAZ softened portion, the skeleton member is easily broken from the HAZ softened portion when a load is applied to the skeleton member of an automobile or the like. In order to avoid this, in the present disclosure, a steel plate for hot stamping prepared in advance by superimposing and welding the first steel plate 10 and the second steel plate 20 is prepared in advance, and hot stamping (hot stamping) is performed. ). As a result, the HAZ softened zone generated during welding can be eliminated by quenching during hot stamping.
[0067]
However, even if the HAZ softened portion disappears, if the hardness of the first weld metal portion 40 is too high after quenching, when a load is applied to the skeleton member 1 of an automobile or the like, the first weld metal portion 40 breaks, and the skeleton member 1 becomes fragile. In order to avoid this, in the present disclosure, the tensile strength of the first steel plate 10 is relatively low and the second steel plate is made so that the hardness does not become too high even if the first weld metal portion 40 is hardened. An upper limit was set for a tensile strength of 20. The tensile strength of the first steel sheet 10 after quenching (after hot stamping) is 1.0 GPa to 1.6 GPa, and the tensile strength of the second steel sheet 20 after quenching (after hot stamping) is 1.0 GPa to 1.6 GPa. If the combination is 1.8 GPa to 2.6 GPa, it is possible to prevent the hardness of the first weld metal portion 40 from becoming too high. That is, if a steel plate having a tensile strength of 1.6 GPa or less, which is not normally used for hot stamping, is used for the first weld metal portion 40, the hardness of the first weld metal portion 40 becomes too high. Can be avoided. When the tensile strength is 1.5 GPa or less, and further, 1.35 GPa or less, a higher effect is exhibited.
It is desirable that the hardness of the first weld metal portion 40 after quenching (after hot stamping) is 400 to 540 Hv in Vickers hardness.
[0068]
[Relationship with Third Steel Plate]
　The skeleton member 1 of the present disclosure may be formed in a closed cross section as described above in FIG. When the cross section is closed, the plate-shaped member 30 as the third steel plate is welded to the flange portion 17 of the first steel plate 10. Therefore, the second weld metal portion 41 exists at the interface between the flange portion 17 of the first steel plate 10 and the plate-shaped member 30.
[0069]
　Usually, the welding of the flange portion 17 of the first steel plate 10 and the plate-shaped member 30 is performed after hot stamping (hot stamping). Therefore, in the flange portion 17 and the plate-shaped member 30 of the first steel plate 10, a heat-affected zone (HAZ) appears in the vicinity of the second weld metal portion 41.
　However, as previously shown in FIG. 5A, the tensile strength of the first steel sheet 10 is a relatively low value of 1.0 GPa to 1.6 GPa, so that the strength of the first steel sheet 10 due to HAZ The effect of the decline can be reduced. Similarly, by setting the tensile strength of the plate-shaped member 30 as the third steel plate to a relatively low value of 0.45 to 1.6 GPa, the effect of the strength reduction due to HAZ on the plate-shaped member 30 is also affected. Can be reduced. More preferably, the tensile strength of the plate-shaped member 30 is 0.6 to 1.35 GPa, and optimally 0.6 to 1.25 GPa.
　The plate-shaped member 30 is also generally referred to as a closing plate. Even if the plate-shaped member 30 (closing plate) has a low tensile strength, the performance (initial load, impact energy absorption performance) of the entire skeleton member 1 does not easily decrease. Further, by using a steel plate having a relatively low tensile strength and a low carbon content for the first steel plate 10 and the plate-shaped member 30, welding of the flange portion 17 of the first steel plate 10 and the plate-shaped member 30 is performed. Is also good, and the joint strength between the two is improved.
[0070]
　According to the present embodiment, the outer shape portion of the skeleton member 1 is formed of the first steel plate 10 having a relatively low strength, and is welded to the plate-shaped member 30 by the flange portion 17 of the first steel plate 10, and the skeleton member 1 Is formed as a closed cross section. Since the strength of the first steel plate 10 is set to be relatively low, it is possible to reduce the decrease in hardness at the heat-affected zone in the welding of the flange portion 17. As a result, the welding strength between the first steel plate 10 and the plate-shaped member 30 can be increased. That is, in the skeleton member 1, the occurrence of fracture starting from the welded portion between the first steel plate 10 and the plate-shaped member 30 can be suppressed, and the strength of the skeleton member 1 can be improved.
[0071]
　[Application Example of Skeletal Member According to the Embodiment of the Present
　Disclosure ] The preferred embodiment of the present disclosure has been described in detail above. From here, an application example of the skeleton member according to the embodiment of the present disclosure will be described with reference to FIGS. 10 to 15C. FIG. 10 is a diagram showing an automobile skeleton 100 as an example to which the skeleton member 1 according to the present disclosure embodiment is applied. The skeleton member 1 may constitute the automobile skeleton 100 as a cabin skeleton or a shock absorbing skeleton. Examples of application of the skeleton member 1 as a cabin skeleton include roof center reinforcement 201, roof rail 203, B pillar 207, side sill 209, tunnel 211, A pillar lower 213, A pillar upper 215, kick clean force 227, floor cross member 229, and so on. Examples include the under-lean force 231 and the front header 233.
[0072]
　Further, examples of application of the skeleton member 1 as the shock absorbing skeleton include a rear side member 205, an apron upper member 217, a bumperin force 219, a crash box 221 and a front side member 223.
[0073]
　FIG. 11A is an external perspective view showing an example in which the skeleton member 1 according to the present embodiment is applied as the B pillar 207a. 11B is a cross-sectional view taken along the line II-II'in FIG. 11A. As shown in FIGS. 10 and 11A, the B-pillar 207a is a columnar member that connects the floor and the roof between the front seats and the rear seats on the side surface of the vehicle. In the B pillar 207a, the skeleton member 1 according to the present embodiment is used for a portion connecting the floor and the roof.
[0074]
　As shown in FIG. 11B, the B pillar 207a is formed into a closed cross section by welding the flange portion of the first steel plate 10 having a substantially hat shape in cross section to the mating member 70 as the third steel plate. There is. Inside the closed cross section of the B pillar 207, a rectangular second steel plate 20 having one side opened in cross section is provided. The second steel plate 20 is welded to the inner wall surface of the first steel plate 10 via the first weld metal portion 40. As an example, the mating member 70 has flange portions at both ends in a cross-sectional view along the plate width direction, and two projecting portions adjacent to each flange portion and protruding outward in a closed cross section. Further, in the mating member 70, a flat plate portion is connected between the two protruding portions.
[0075]
　The B-pillar 207a is arranged so that the first steel plate 10 side is on the outside of the vehicle body and the mating member 70 side is on the inside of the vehicle body. A cover member as a fourth member that covers the first steel plate 10 from the outside may be further provided on the outside of the vehicle body of the B pillar 207a.
[0076]
　Subsequently, FIG. 12A is an external perspective view showing another example in which the skeleton member 1 according to the present embodiment is applied as the B pillar 207b. FIG. 12B is a sectional view taken along line III-III'in FIG. 12A. As shown in FIGS. 12A and 12B, the B-pillar 207b has a closed cross section by welding the flange portion of the first steel plate 10 having a substantially hat shape in cross section to the mating member 70 as the third steel plate. It is formed. On the outside of the closed cross section of the B pillar 207b, a rectangular second steel plate 20 having one side opened in cross section is provided. The second steel plate 20 is welded to the outer wall surface of the first steel plate 10 via the first weld metal portion 40. In the form shown in FIG. 12A, the lower portion of the first steel plate 10 has a structure in which the fourth steel plate 71, which has a lower tensile strength than the first steel plate 10, is butt-welded 72 by a laser. As a result, the fourth steel plate 71, which is the lower part of the B pillar 207b, is deformed at the time of a side collision, so that the collision energy is efficiently absorbed. The first steel plate 10 and the fourth steel plate 71 are, for example, press-formed from a tailored blank material (TWB). The first steel plate 10 and the fourth steel plate may be partially overlapped and spot welded.
Other configurations are the same as those of the B pillar 207a shown in FIGS. 11A and 11B, and thus the description thereof will be omitted. Regarding the B-pillar 207a shown in FIG. 11A, the lower portion of the first steel plate 10 may be welded to the fourth steel plate having a lower tensile strength than the first steel plate 10 as in FIG. 12A.
[0077]
　Subsequently, FIG. 13A is an external perspective view showing an example in which the skeleton member 1 according to the present embodiment is applied as the roof rail 203. 13B is a sectional view taken along line IV-IV'in FIG. 13A. As shown in FIGS. 10 and 13A, the roof rail 203 is a columnar member extending in the front-rear direction of the vehicle body and forming a side portion of the roof in the vehicle body width direction. The skeleton member 1 according to the present embodiment is applied to the roof rail 203.
[0078]
　As shown in FIG. 13B, the roof rail 203 is formed into a closed cross section by welding the end portion of the first steel plate 10 having a substantially C shape in cross section to the mating member 70 as the third steel plate. There is. Inside the closed cross section of the roof rail 203, a rectangular second steel plate 20 having one side opened in cross section is provided. The second steel plate 20 is welded to the inner wall surface of the first steel plate 10 via the first weld metal portion 40.
[0079]
　The roof rail 203 is arranged so that the first steel plate 10 side is on the outside of the vehicle body and the mating member 70 side is on the inside of the vehicle body. As an example, the mating member 70 is bent at a plurality of positions in the plate width direction, and has a curved shape that is convex outward in the closed cross section in a cross-sectional view along the plate width direction. Further, one end of the mating member 70 in the width direction is bent to form a flange portion. A cover member 80 as a fourth member may be further provided on the outside of the vehicle body to cover the first steel plate 10 from the outside.
[0080]
　Subsequently, FIG. 14A is an external perspective view showing an example in which the skeleton member 1 according to the present embodiment is applied as the side sill 209. 14B is a sectional view taken along line VV'in FIG. 14A. As shown in FIGS. 10 and 14A, the side sill 209 is a columnar member located in the lower part of the side surface of the vehicle body and extending in the front-rear direction of the vehicle body. The skeleton member 1 according to the present embodiment is applied to the side sill 209.
[0081]
　As shown in FIG. 14B, the side sill 209 is formed to have a closed cross section by welding the flange portion of the first steel plate 10 having a substantially hat shape in cross section to the mating member 70 as the third steel plate. .. Inside the closed cross section of the side sill 209, a second steel plate 20 formed in an L shape in cross section is provided. The second steel plate 20 is welded to the inner wall surface of the bent portion of the first steel plate 10 via the first weld metal portion 40.
[0082]
　The side sill 209 is arranged so that the first steel plate 10 side is on the outside of the vehicle body and the mating member 70 side is on the inside of the vehicle body. The mating member 70 is formed in a substantially hat shape in cross-sectional view. A cover member 80 as a fourth member may be further provided on the outside of the vehicle body to cover the first steel plate 10 from the outside.
[0083]
　Subsequently, FIG. 15A is an exploded perspective view showing an example in which the skeleton member 1 according to the present embodiment is applied as the rear side member 205. 15B is a sectional view taken along line VI-VI'in FIG. 15A. Further, FIG. 15C is a cross-sectional view showing an example in which the skeleton member according to the present embodiment is applied as the floor member 237. The floor member 237 is a columnar member extending in the front-rear direction or the width direction of the vehicle body on the lower surface of the vehicle body to form the floor. The rear side member 205 is a columnar member that forms a floor, especially at the rear of the vehicle body. The skeleton member 1 according to the present embodiment is applied to the rear side member 205 or the floor member 237.
[0084]
　As shown in FIGS. 15A and 15B, a rectangular second steel plate 20 having a substantially hat shape in cross section is provided inside the first steel plate 10 having a substantially hat shape in cross section. The second steel plate 20 is welded to the inner wall surface of the first steel plate 10 via the first weld metal portion 40.
[0085]
　Further, as shown in FIG. 15C, the floor member 237 is formed in a closed cross section by welding the flange portion of the first steel plate 10 to the mating member 70 as the third steel plate. A second steel plate 20 (a member connected from the front side member kick portion) is provided inside the closed cross section of the floor member 237. The second steel plate 20 is welded to the inner wall surface of the first steel plate 10 via the first weld metal portion 40. Further, the mating member 70 is formed in a substantially hat shape in cross-sectional view. Further, a plate-shaped member 30 (floor panel) is further provided between the first steel plate 10 and the mating member 70. A cover member as a fourth member that covers the mating member 70 from the outside may be further provided.
[0086]
　As described above, since the skeleton member 1 is used as a cabin skeleton or a shock absorbing skeleton, the skeleton member 1 has a sufficient load capacity, so that deformation at the time of collision can be reduced. Further, the skeleton member 1 has improved deformability, and even when an input such as a side collision is input to the automobile skeleton 100, the skeleton member 1 can absorb an impact by sufficient deformation and protect the inside of the skeleton. The application example of the skeleton member according to the embodiment of the present disclosure has been described above.
Example
[0087]
　In order to evaluate the characteristics of the skeleton member 1 according to the present embodiment, the skeleton member 1 according to the present embodiment was actually prepared and the characteristics were evaluated from various viewpoints. The cross-sectional structure of the skeleton member 1 is as shown in FIG. 3, the height of the skeleton member 1 (Z direction in FIG. 3) is 60 mm, the width of the skeleton member 1 (X direction in FIG. 3) is 80 mm, and the length of the skeleton member 1. (Y direction in 3 in the figure) was set to 800 mm. In the embodiment, a 1.2 GPa steel plate is used as the first steel plate 10, a 1.8 GPa steel plate is used as the second steel plate 20, and the first steel plate 10 and the second steel plate 20 are subjected to predetermined conditions. The blank material spot-welded in (1) was formed into a substantially hat shape by a hot stamping method. Further, the substantially hat-shaped flange portion 17 was welded to the plate-shaped member 30 to form a skeleton member 1 having a closed cross section.
[0088]
　In Comparative Example 1, a member forming the outer shape of the skeleton member 1 and a steel plate of 1.8 GPa were used as the reinforcing member. First, each member was formed into a substantially hat shape by a hot stamping method, and then spot welded and integrated. Further, a substantially hat-shaped flange portion was welded to a plate-shaped member to form a skeleton member having a closed cross section.
[0089]
　In Comparative Example 2, a single 1.8 GPa steel plate was formed into a substantially hat shape, and partial tempering was performed at the flange portion to reduce the hardness. Further, the hat-shaped member was welded to the plate-shaped member at the flange portion to obtain a skeleton member having a closed cross section. A 780 MPa class steel plate was used as the plate member.
[0090]
　The hardness around the weld was measured for these skeleton members. In addition, a crushing test was conducted in which bending moments were applied to both ends of the skeleton member. The evaluation results are summarized in Table 3.
[0091]
[Table 3]

[0092]
　As shown in Table 3, in Comparative Example 1, the rigidity as a skeleton member was sufficient by welding the two members, and the evaluation was OK. On the other hand, in Comparative Example 1, from the viewpoint of the joint strength between the two members, the heat input during spot welding causes the hardness of the heat-affected zone around the first weld metal portion 40 to decrease, and the joint strength is sufficient. No good value was obtained, and the evaluation was NG. Further, in Comparative Example 1, when the substantially hat-shaped flange portion and the plate-shaped member were welded, the hardness decreased in the heat-affected zone around the first weld metal portion 40 and the strength of the first weld metal portion 40. The decrease was remarkable, and the evaluation was NG. This is because the substantially hat-shaped member in Comparative Example 1 is made of a steel plate of 1.8 GPa, the heat-affected zone is remarkably softened, and the carbon component in the steel plate is relatively high, so that the first weld metal portion 40 is brittle. It is thought that the cause is that the embrittlement is large. Further, in Comparative Example 1, the vertical wall portion of the substantially hat-shaped member is a steel plate of 1.8 GPa and has high strength, but since the deformability is small, cracks occur in the vertical wall portion without significant deformation. , NG evaluation.
[0093]
　Further, in Comparative Example 2, since a single member was formed into a substantially hat shape, the rigidity was insufficient, and the evaluation was NG. Further, since Comparative Example 2 is a single member, the joint strength between the members could not be evaluated. In Comparative Example 2, when the substantially hat-shaped flange portion and the plate-shaped member were welded, the flange portion was partially tempered, so that a local decrease in hardness could be avoided around the weld metal portion. , OK evaluation. In Comparative Example 2, the substantially hat-shaped vertical wall portion is a steel plate of 1.8 GPa and has high strength, but since the deformability is small, cracks occur in the vertical wall portion without significant deformation, and the evaluation is NG. It became.
[0094]
　In the example, by welding the two members, the rigidity of the skeleton member 1 as a whole was sufficient, and the evaluation was OK. Further, in the embodiment, from the viewpoint of the joint strength between the two members, the hardness change of the peripheral region 62 of the first weld metal portion 40 is set within a predetermined range, so that the hardness does not decrease. The joint strength was a sufficient value, and the evaluation was OK. Further, in the embodiment, when the first steel plate 10 and the plate-shaped member 30 are welded, the flange portion 17 is a steel plate of 1.2 GPa, and the flange portion 17 surrounds the second weld metal portion 41. It was possible to reduce the decrease in hardness at the heat-affected zone. Further, since the first steel plate 10 has a relatively small amount of carbon component, it was possible to suppress a decrease in toughness of the second weld metal portion 41 at the flange portion 17. Therefore, the joint strength with the plate-shaped member 30 did not decrease, and the evaluation was OK. In the example, the first vertical wall portion 15 is also a steel plate of 1.2 GPa and has a large deformability, so that it is greatly deformed by an impact load and the impact absorption energy is large, so that the evaluation was OK. .. As described above, it was shown that the skeleton member 1 according to the present embodiment has high performance from various viewpoints.
[0095]
　Although the preferred embodiments of the present disclosure have been described in detail with reference to the accompanying drawings, the present disclosure is not limited to such examples. It is clear that anyone with ordinary knowledge in the field of technology to which this disclosure belongs can come up with various modifications or modifications within the scope of the technical ideas set forth in the claims. , These are also naturally understood to belong to the technical scope of the present disclosure.
[0096]
　For example, in the above embodiment, the flange portion 17 is welded to the plate-shaped member 30, but the present disclosure is not limited to such an example. For example, the end portion of the first vertical wall portion 15 may be directly welded without passing through the flange portion 17. Further, for example, the flange portion 17 of the skeleton member 1 according to the present embodiment is welded to the flange portion of the mating member, which is not a plate-shaped member 30 but a member having a cross-section hat shape in which the mating member has a flange portion. May be good. As an example, the plate-shaped member 30 includes one or more molded steel plates having a plate thickness of 0.6 mm to 2.6 mm and a tensile strength of 270 MPa to 1600 MPa. The surface of the steel plate of the plate-shaped member 30 may be unplated, or may be plated with zinc-based plating, aluminum-based plating, or the like. The welding methods include spot welding, laser welding, arc welding, spot welding and laser welding, spot welding and arc welding, spot welding and mechanical joining such as bolts, screws, and rivets, and spot welding. The combined use of a sealer or an adhesive can be mentioned. Further, the second weld metal portion 41 at this time may be provided not only in a dot shape but also in a C shape, a U shape, an ellipse shape, a linear shape having a predetermined length, or a zigzag shape.
Description of the sign
[0097]
　1 Skeleton member
　10 First steel plate
　11 First top wall part
　13 First bent part
　15 First vertical wall part
　17 Flange part
　18 Groove part
　20 Second steel plate
　21 Second top wall part
　23 Second bending Part
　25 Second vertical wall part
　30 Plate-shaped member (third steel plate)
　40 First weld metal part
　41 Second weld metal part
　62 Peripheral area (area)
　70 Mating member (third steel plate)
The scope of the claims
[Claim 1]
It is provided with a first steel plate, a second steel plate, and a first weld metal portion that joins the interface between the first steel plate and the second steel plate, and
the tensile strength of the first steel plate is 1. and at 0GPa least 1.6GPa or less,
tensile strength of the second steel plate is less than 2.5GPa least 1.8 GPa,
with the first steel plate groove,
said second steel sheet overlaid on the groove is,
the lowest Vickers hardness of the region within the surrounding 4mm of the first weld metal of the second steel plate is 80% or more outer hardness of said region of said second steel plate,
an automobile frame member.
[Claim 2]
The automobile skeleton member according to claim 1, wherein the Vickers hardness of the first weld metal portion is 400 or more and 540 or less.
[Claim 3]
The first steel plate includes a flange portion on the outside of the groove portion, and includes a
third steel plate and a second weld metal portion that joins the interface between the third steel plate and the flange portion
. The
automobile skeleton member according to claim 1 or 2 , wherein the tensile strength of the steel sheet is 0.45 GPa or more and 1.6 GPa or less .