The present invention relates to a hot stamped molded article having excellent bending deformability, which is used for structural members and reinforcing members of automobiles and structures that require strength.
Background technology
[0002]
　In recent years, there has been a demand for weight reduction of automobile bodies from the viewpoint of environmental protection and resource saving, and therefore, the application of high-strength steel plates to automobile members is accelerating. However, since the formability deteriorates as the strength of the steel sheet increases, the formability of a high-strength steel sheet into a member having a complicated shape becomes an issue.
[0003]
　In order to solve such a problem, the application of hot stamping in which a steel sheet is heated to a high temperature in the austenite region and then press-formed is being promoted. Since hot stamping is hardened in a mold at the same time as press working, it is attracting attention as a technology that achieves both molding into automobile members and ensuring strength.
[0004]
　On the other hand, a molded body formed by hot stamping a high-strength steel plate needs to have the ability to absorb impact at the time of collision (collision deformation site), and for that purpose, high impact absorption ability (bending deformation ability) is required. NS.
[0005]
　According to Patent Document 1, as a technique for meeting this requirement, a steel plate for hot stamping is annealed to concentrate Mn and Cr in the carbide to make it difficult to dissolve, so that austenite is produced by these carbides during hot stamping. A technique for suppressing the growth of manganese and making it finer is disclosed.
[0006]
　Patent Document 2 discloses a technique for finely granulating austenite by raising the temperature at a heating rate of 90 ° C./s or less during hot stamp heating.
[0007]
　Patent Document 3, Patent Document 4, and Patent Document 5 also disclose a technique for finely granulating austenite to improve toughness.
Prior art literature
Patent documents
[0008]
Patent Document 1: International Publication No. 2015/147216
Patent Document 2: Patent No. 5369714
Patent Document 3: Patent Document 5114691
Patent Document 4: Japanese Patent Application Laid-Open No. 2014-15638
Patent Document 5: Japanese Patent Application Laid-Open No. 2002-309345 Gazette
Outline of the invention
Problems to be solved by the invention
[0009]
　However, with the techniques disclosed in Patent Documents 1 to 5, it is difficult to obtain finer-grained austenite, and it is not expected to obtain higher strength or bending deformability than before.
[0010]
　In view of the problems of the prior art, it is an object of the present invention to secure a better bending deformability in a hot stamped body of a high-strength steel plate, and an object of the present invention is to provide a hot stamped body that solves the problem. And.
Means to solve problems
[0011]
　The present inventors have diligently studied a method for solving the above problems. As a result, in the hot stamp molded body, the rotation angle is 5 ° or more and 75 ° or less with respect to the <011> direction of the crystal grains of the lower bainite, martensite, and tempered martensite. It has been found that excellent bending deformability can be obtained by generating 80% or more of grain boundaries having a grain boundary of 15 ° or more.
[0012]
　The invention of the present application has been further studied based on the above findings, and the gist thereof is as follows.
[0013]
　(1) Ingredient composition is C: 0.35% or more, 0.75% or less, Si: 0.005% or more, 0.25% or less, Mn: 0.5% or more, 3.0 in mass%. % Or less, sol.Al: 0.0002% or more, 3.0% or less, Cr: 0.05% or more, 1.00% or less, B: 0.0005% or more, 0.010% or less, Nb: 0 0.01% or more, 0.15% or less, Mo: 0.005% or more, 1.00% or less, Ti: 0% or more, 0.15% or less, Ni: 0% or more, 3.00% or less, P : 0.10% or less, S: 0.10% or less, and N: 0.010% or less, the balance is Fe and unavoidable impurities, and the microstructure is lower bainite, martensite, and tempered martensite. It contains at least one type of site in terms of area ratio of 90% or more, and the rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of the lower bainite, the martensite, and the tempered martensite as the axis of rotation. A hot stamp molded body characterized in that the ratio of the length of the grain boundary having a rotation angle of 15 ° or more to the length of the grain boundary to be becomes 80% or more.
[0014]
　(2) The hot stamped molded article according to (1) above, which has a plating layer.
The invention's effect
[0015]
　According to the present invention, it is possible to provide a hot stamped molded product having excellent bending deformability.
Forms for carrying out the invention
[0016]
　The feature of the present invention is that in the hot stamped product, among the grain boundaries in which the rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of lower bainite or martensite and tempered martensite as the rotation axis. Excellent bending deformability can be obtained by generating 80% or more of grain boundaries having a rotation angle of 15 ° or more. By making the structure of the hot stamped body such a structure, the excellent bending deformability is improved because the large tilt angle grain boundaries of 15 ° or more have more cracks than the small tilt angle grain boundaries of less than 15 °. This is because the effect of suppressing propagation is high. As a result of diligent studies, the present inventors have found that the above-mentioned structure can be obtained by the following method.
[0017]
　As a first step, the casting amount of molten steel per unit time is controlled. This suppresses the precipitation of Mo and Nb and increases the solid solution amount of Mo and Nb in the steel.
[0018]
　When the amount of molten steel cast per unit time is controlled to suppress the precipitation of Mo and Nb, the microsegregation of Mn is also suppressed at the same time, so that the trap site of P disappears and P becomes the old austenite grain boundary during finish rolling. Segregate. Then, since the embrittlement strength of the grain boundaries is lowered, the bending deformability cannot be sufficiently obtained even if the crystal orientation is controlled. This is because the segregation of Mn functions as a trap site for P because the affinity between Mn and P is high, and P diffuses into the old austenite grain boundaries by eliminating the segregation of Mn. In the present invention, this problem is solved by controlling the rolling conditions.
[0019]
　As the second step, the concentration of Mn and Cr in the carbide is suppressed by controlling the rolling reduction, temperature, and cooling conditions after rolling in hot finish rolling. In order to make the grain boundaries of lower bainite, martensite, and tempered martensite the preferential reverse transformation sites of austenite, it is desirable that carbides are easily dissolved. Therefore, it is important not to concentrate elements such as Mn and Cr that inhibit the dissolution of carbides in the carbides.
[0020]
　Further, by suppressing the precipitation of Mo and Nb and dissolving Nb and Mo in the grain boundaries of the former austenite, the segregation site of P is occupied by Nb and Mo, thereby eliminating the segregation of P into the former austenite. do. As a result, not only the improvement of the grain boundary strength due to Mo or Nb but also the reduction of the embrittlement strength of the grain boundary can be suppressed.
[0021]
　Furthermore, by controlling the coil winding conditions, the strength of austenite can be increased by the effect of the solid solution Mo and Nb. In addition, upon phase transformation from austenite to lower bainite, martensite, and tempered martensite, an advantageous crystal orientation that relieves the stress generated by the transformation is preferentially generated. Thereby, in the steel plate for hot stamping, the X-ray random intensity ratio of {112} <111> of the crystal grains of lower bainite, martensite and tempered martensite can be controlled.
[0022]
　By subjecting the steel plate for hot stamping having such characteristics to the hot stamping process, the texture memory effect of austenite and martensite causes the grain grains of lower bainite, martensite, and tempered martensite to be present in the hot stamping compact. Of the grain boundaries having a rotation angle of 5 ° or more and 75 ° or less with the <011> direction as the rotation axis, 80% or more of the grain boundaries having a rotation angle of 15 ° or more are generated.
[0023]
　In the present invention, in the hot stamping process, the grain boundaries of lower bainite, martensite, and tempered martensite are utilized as reverse transformation sites of austenite to control the crystal orientation expressed in the steel plate for hot stamping. It can be handed over to the molded body.
[0024]
　Hereinafter, the hot stamped molded article of the present invention and a method for producing the same will be described.
[0025]
　First, the reason for limiting the composition of the components constituting the hot stamped molded article of the present invention will be described. Hereinafter,% related to the component composition means mass%.
[0026]
　"C: 0.35% or more, 0.75% or less"
　C is an important element for obtaining a tensile strength of 2000 MPa or more. If it is less than 0.35%, martensite is soft and it is difficult to secure a tensile strength of 2000 MPa or more. Therefore, C is set to 0.35% or more. It is preferably 0.37% or more. The upper limit is not particularly set, but the upper limit is set to 0.75% in consideration of the balance between the required strength and the suppression of early fracture.
[0027]
　"Si: 0.005% or more, 0.25% or less"
　Si is an element that enhances bending deformation ability and contributes to improvement of shock absorption ability. If it is less than 0.005%, the bending deformation ability is poor and the shock absorbing ability is deteriorated. Therefore, 0.005% or more is added. It is preferably 0.01% or more. On the other hand, if it exceeds 0.25%, the amount of solid solution to the carbide increases and the carbide becomes difficult to dissolve, and the undissolved carbide becomes a reverse transformation site of austenite, and lower bainite or martensite or tempered martensite. Of the grain boundaries whose rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of the site as the rotation axis, the grain boundaries whose rotation angle is 15 ° or more cannot be controlled to 80% or more. Is 0.25%. It is preferably 0.22% or less.
[0028]
　"Mn: 0.5% or more, 3.0% or less"
　Mn is an element that contributes to the improvement of strength by strengthening solid solution. If it is less than 0.5%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 2000 MPa or more. Therefore, 0.5% or more is added. It is preferably 0.7% or more. On the other hand, if it is added in excess of 3.0%, the amount of solid solution in the carbide increases and the carbide becomes difficult to dissolve, and the undissolved carbide becomes a reverse transformation site of austenite, and lower bainite or martensite or Of the grain boundaries whose rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the tempered martensite crystal grains as the rotation axis, the grain boundaries whose rotation angle is 15 ° or more cannot be controlled to 80% or more. , 3.0% is the upper limit. Preferably, it is 2.5% or less.
[0029]
　"Sol.
　Al : 0.0002% or more, 3.0% or less" Al is an element that deoxidizes molten steel and makes the steel sound. If it is less than 0.0002%, deoxidation is sufficient and coarse oxides are formed, causing premature fracture. Al is 0.0002% or more. It is preferably 0.0010% or more. On the other hand, if it is added in excess of 3.0%, coarse oxides are generated and premature fracture is caused, so the content should be 3.0% or less. It is preferably 2.5% or less, more preferably 0.5% or less.
[0030]
　"Cr: 0.05% or more, 1.00% or less"
　Cr is an element that contributes to the improvement of strength by strengthening solid solution. If it is less than 0.05%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 2000 MPa or more. Therefore, 0.05% or more is added. It is preferably 0.1% or more. On the other hand, if it is added in excess of 1.00%, the amount of solid solution in the carbide increases and the carbide becomes difficult to dissolve, and the undissolved carbide becomes a reverse transformation site of austenite, and lower bainite or martensite or Of the grain boundaries whose rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the tempered martensite crystal grains as the rotation axis, the grain boundaries whose rotation angle is 15 ° or more cannot be controlled to 80% or more. , 1.00% is the upper limit. Preferably, it is 0.8% or less.
[0031]
　"B: 0.0005% or more, 0.010% or less"
　B is an element that contributes to the improvement of strength by strengthening the solid solution. If it is less than 0.0005%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 2000 MPa or more. Therefore, 0.0005% or more is added. It is preferably 0.0008% or more. On the other hand, if it is added in excess of 0.010%, the amount of solid solution in the carbide increases and the carbide becomes difficult to dissolve, and the undissolved carbide becomes a reverse transformation site of austenite, and lower bainite or martensite or Of the grain boundaries whose rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the tempered martensite crystal grains as the rotation axis, the grain boundaries whose rotation angle is 15 ° or more cannot be controlled to 80% or more. , 0.010% is the upper limit. Preferably, it is 0.007% or less.
[0032]
　"Nb: 0.01% or more, 0.15% or less"
　Nb is an element that dissolves in the grain boundaries of former austenite and increases the strength of the grain boundaries. Further, since Nb inhibits the segregation of P at the grain boundary by being dissolved in the grain boundary, the embrittlement strength of the grain boundary is improved. Therefore, 0.01% or more is added. It is preferably 0.030% or more. On the other hand, if it is added in excess of 0.15%, it tends to precipitate as charcoal, and in the steel plate for hot stamping, the X-ray random intensity of {112} <111> of the crystal grains of lower bainite or martensite or tempered martensite. The ratio cannot be 2.8 or more, and as a result, the grain boundaries where the rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of lower bainite or martensite or tempered martensite as the rotation axis. Of these, the grain boundary at which the rotation angle is 15 ° or more cannot be controlled to 80% or more, so the grain boundary is set to 0.15% or less. It is preferably 0.12% or less.
[0033]
　"Mo: 0.005% or more, 1.00% or less"
　Mo is an element that dissolves in the grain boundaries of former austenite and increases the strength of the grain boundaries. Further, since Mo inhibits the segregation of P at the grain boundary by being dissolved in the grain boundary, the embrittlement strength of the grain boundary is improved. Therefore, 0.005 or more is added. It is preferably 0.030% or more. On the other hand, if it is added in excess of 1.00%, it is likely to be precipitated as charcoal, and is likely to be precipitated as charcoal. The X-ray random intensity ratio of 111> could not be 2.8 or more, and as a result, the rotation angle was 5 ° or more with the <011> direction of the crystal grains of lower bainite or martensite or tempered martensite as the rotation axis. Of the grain boundaries having a rotation angle of 75 ° or less, the grain boundaries having a rotation angle of 15 ° or more cannot be controlled to 80% or more, so the value is set to 1.00% or less. It is preferably 0.80% or less.
[0034]
　"Ti: 0% or more, 0.15% or less"
　Ti is not an essential element, but it may be added as necessary because it is an element that contributes to the improvement of strength by strengthening the solid solution. When Ti is added, it is preferably 0.01% or more in order to obtain the effect of the addition. It is preferably 0.02% or more. On the other hand, if it is added in excess of 0.15%, coarse carbides and nitrides are formed and premature fracture is caused, so the content is 0.15% or less. It is preferably 0.12% or less.
[0035]
　"Ni: 0% or more, 3.00% or less"
　Ni is not an essential element, but it may be added as necessary because it is an element that contributes to the improvement of strength by strengthening the solid solution. When Ni is added, it is preferably 0.01% or more in order to obtain the effect of the addition. It is preferably 0.02% or more. On the other hand, if it is added in excess of 3.00%, the steel becomes brittle and causes early fracture, so the content should be 3.00% or less. It is preferably 2.00% or less.
[0036]
　"P: 0.10% or less"
　P is an impurity element, which easily segregates at the grain boundaries and lowers the embrittlement strength of the grain boundaries. If it exceeds 0.10%, the embrittlement strength of the grain boundaries is remarkably lowered and early fracture is caused. Therefore, P is set to 0.10% or less. It is preferably 0.050% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0001%, the de-P cost will increase significantly and it will be economically disadvantageous. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.
[0037]
　"S: 0.10% or less"
　S is an impurity element and is an element that forms inclusions. If it exceeds 0.10%, inclusions are formed and cause premature fracture, so S is set to 0.10% or less. It is preferably 0.0050% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0015%, the cost of removing S will increase significantly and it will be economically disadvantageous. Therefore, 0.0015% is a practical lower limit on the practical steel sheet.
[0038]
　"N: 0.010% or less"
　N is an impurity element and forms a nitride to cause premature fracture. Therefore, it is set to 0.010% or less. It is preferably 0.0075% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0001%, the N removal cost will increase significantly and it will be economically disadvantageous. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.
[0039]
　The rest of the component composition is Fe and impurities. Examples of impurities include elements that are unavoidably mixed from steel raw materials or scrap and / or in the steelmaking process and are allowed as long as they do not impair the characteristics of the hot stamped product of the present invention.
[0040]
　Next, the reason for limiting the microstructure of the hot stamped molded article of the present invention will be described.
[0041]
　"Of the grain boundaries whose rotation angles are 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of lower bainite, martensite, and tempered martensite as the rotation axis, the grain boundaries whose rotation angle is 15 ° or more. 80% or more "
[0042]
　Grain orientation control of lower bainite, martensite, and tempered martensite is an important tissue factor to ensure excellent bending deformability. According to the studies by the present inventors, in order to obtain the shock absorbing ability required for the hot stamped molded product, the rotation axis is the <011> direction of the crystal grains of the lower bainite, martensite, and tempered martensite. Of the grain boundaries having a rotation angle of 5 ° or more and 75 ° or less, it is preferable to increase the grain boundaries having a rotation angle of 15 ° or more, and it is necessary to control the ratio to 80% or more. More preferably, it is 85% or more.
[0043]
　Of the grain boundaries whose rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of lower bainite, martensite, or tempered martensite as the rotation axis, the ratio of the grain boundaries whose rotation angle is 15 ° or more is , Measure as follows.
[0044]
　A sample is cut out from the central portion of the hot stamped body so that a cross section (thickness cross section) perpendicular to the plate surface can be observed. After polishing the measurement surface with # 600 to # 1500 silicon carbide paper, a mirror surface is finished using a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 μm to 6 μm is dispersed in pure water.
[0045]
　Next, a standard colloidal silica suspension (particle size 0.04 μm) is used for finish polishing for 8 to 20 minutes.
[0046]
　The polished sample is washed with acetone or ethyl alcohol, dried, and set in a scanning electron microscope. As the scanning electron microscope to be used, a model equipped with an EBSD detector (DVC5 type detector manufactured by TSL) is used.
[0047]
　Crystal orientation information is obtained by EBSD measurement at a measurement interval of 0.1 μm in a range of 50 μm in the plate thickness direction and 50 μm in the rolling direction at the 3/8 to 5/8 position of the sample plate thickness. The measurement conditions are a vacuum level of 9.6 × 10-5 or less, an acceleration voltage of 15 kV, an irradiation current of 13 nA, a binning size of 4 × 4, and an exposure time of 42 seconds.
[0048]
　Among the grain boundaries of crystal grains with a body-centered cubic structure, the measurement data is used with the "Inverse Pole Figure Map" and "Axis Angle" functions installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. , The length of the grain boundary whose rotation angle is 5 ° or more and 75 ° or less is calculated with the <011> direction as the rotation axis.
[0049]
　Next, the length of the grain boundary whose rotation angle is 15 ° or more and 75 ° or less is calculated with the <011> direction as the rotation axis, and the rotation angle is 5 ° or more and 75 ° or less with the <011> direction as the rotation axis. Calculate the value divided by the length of the grain boundary.
[0050]
　The above measurements were carried out at at least 5 locations, and the average value was taken as the grain boundary with a rotation angle of 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of lower bainite, martensite, or tempered martensite as the axis of rotation. Of these, the ratio of grain boundaries having a rotation angle of 15 ° or more is used.
[0051]
　"More than 90% of the microstructure area ratio is one or more of lower bainite, martensite and tempered martensite."
[0052]
　In order for the hot stamped article to obtain a tensile strength of 1500 MPa or more, the microstructure needs to contain martensite or tempered martensite having an area ratio of 90% or more. It is preferably 94% or more. From the viewpoint of ensuring tensile strength, the microstructure may be lower bainite. The structure having an area ratio of 90% or more may be one of lower bainite, martensite and tempered martensite, or a mixed structure thereof.
[0053]
　The remainder of the microstructure is not particularly specified, and examples thereof include upper bainite, retained austenite, and pearlite.
[0054]
　The area ratio of lower bainite, martensite, and tempered martensite is measured as follows.
[0055]
　A cross section perpendicular to the plate surface is cut out from the center of the hot stamped product, and the measurement surface is polished using # 600 to # 1500 silicon carbide paper. Finish with a mirror surface using a liquid dispersed in pure water.
[0056]
　Immerse in a 1.5-3% nitric acid-alcohol solution for 5-10 seconds to reveal high-inclined grain boundaries. At this time, the corrosion work is carried out in the exhaust treatment device, and the temperature of the work atmosphere is room temperature.
[0057]
　The corroded sample is washed with acetone or ethyl alcohol, dried, and subjected to scanning electron microscopy. The scanning electron microscope used shall be equipped with a two-electron detector. In a vacuum of 9.6 × 10-5 or less, the sample is irradiated with an electron beam at an acceleration voltage of 10 kV and an irradiation current level of 8, and the sample is placed at the 1/8 to 3/8 position centered on the 1/4 position of the sample thickness. Take a secondary electron image of the range. The shooting magnification is 10000 times based on a screen having a width of 386 mm and a height of 290 mm, and the number of shooting fields of view is 10 or more.
[0058]
　In the photographed secondary electron image, the crystal grain boundaries and the carbides are imaged as a bright contrast, so that the structure can be easily determined by the positions of the crystal grain boundaries and the carbides. When carbides are formed inside the crystal grains, it is tempered martensite or lower bainite, and the structure in which no carbides are observed inside the crystal grains is martensite.
[0059]
　On the other hand, the structure in which carbides are formed at the grain boundaries is upper bainite or pearlite.
[0060]
　Since the crystal structure of the retained austenite is different from that of the microstructure, the same field of view as the position where the secondary electron image is imaged is measured by the electron backscatter diffraction method. The scanning electron microscope used shall be equipped with a camera capable of electron backscatter diffraction. In a vacuum of 9.6 × 10-5 or less, the sample is irradiated with an electron beam at an acceleration voltage of 25 kV and an irradiation current level of 16, and measurement is performed, and a face-centered cubic lattice map is created from the obtained measurement data.
[0061]
　A mesh with an interval of 2 μm is created on a photograph taken at 10000 times with a screen of 386 mm in width × 290 mm in height as a reference, and microstructures located at the intersection of the meshes are selected. The value obtained by dividing the number of intersections of each tissue by all the intersections is defined as the surface integral of the microstructure. This operation is performed in 10 fields of view, the average value is calculated, and the area ratio of the microstructure is used.
[0062]
　"Manufacturing Method of Steel Plate for Hot Stamping"
　Next, a form of a manufacturing method for obtaining a hot stamped molded product according to the present invention and a steel plate for hot stamping used for manufacturing the hot stamped molded product will be described. Is not limited to the forms described below.
[0063]
　
[0064]
　(1) Continuous casting process
　A molten steel having the above-mentioned chemical composition is made into a steel piece (slab) by a continuous casting method. In this continuous casting step, the amount of molten steel cast per unit time is preferably 6 tons / minute or less. If the casting amount (casting speed) of molten steel per unit time exceeds 6 tons / minute during continuous casting, the microsegregation of Mn increases and the nucleation amount of precipitates mainly composed of Mo and Nb increases. .. It is more preferable that the casting amount is 5 ton / min or less. The lower limit of the casting amount is not particularly limited, but is preferably 0.1 ton / min or more from the viewpoint of operating cost.
[0065]
　(2) Hot rolling process The
　above-mentioned steel pieces are hot-rolled to obtain a steel plate. At that time, hot rolling is completed in a temperature range of A3 transformation temperature + 10 ° C. or higher and A3 transformation temperature + 200 ° C. or lower defined by the formula (2), and the final stage reduction ratio at that time is set to 12% or higher, and finish rolling is performed. Cooling is started within 1 second after the completion, the temperature range from the finish rolling end temperature to 550 ° C. is cooled at a cooling rate of 100 ° C./sec or more, and the mixture is wound at a temperature of less than 500 ° C.
[0066]
　A3 transformation temperature = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo ・ ・ ・ ・ Equation (2)
[0067]
　By setting the finish rolling temperature to A3 transformation temperature + 10 ° C. or higher, recrystallization of austenite is promoted. As a result, the formation of small tilt angle grain boundaries in the crystal grains is suppressed, and the precipitation sites of Nb and Mo can be reduced. Further, by reducing the precipitation sites of Nb and Mo, the consumption of C can be suppressed, so that the number density of carbides can be increased in a later step. Preferably, the A3 transformation temperature is + 30 ° C. or higher.
[0068]
　By setting the finish rolling temperature to A3 transformation temperature + 200 ° C. or lower, excessive grain growth of austenite is suppressed. By finish rolling in a temperature range of A3 transformation temperature + 200 ° C. or lower, recrystallization of austenite is promoted, and excessive grain growth does not occur, so that fine carbides can be obtained in the winding step. Preferably, the A3 transformation temperature is + 150 ° C. or lower.
[0069]
　Recrystallization of austenite is promoted by setting the rolling reduction ratio of finish rolling to 12% or more. As a result, the formation of small tilt angle grain boundaries in the crystal grains is suppressed, and the precipitation sites of Nb and Mo can be reduced. Preferably, it is 15% or more.
[0070]
　Cooling is started within 1 second, preferably within 0.8 seconds after the finish rolling, and the temperature range from the finish rolling end temperature to 550 ° C. is cooled at a cooling rate of 100 ° C./sec or more to obtain Nb and The residence time in the temperature range where the precipitation of Mn is promoted can be reduced. As a result, the precipitation of Nb and Mo in austenite can be suppressed, and the solid solution amount of Nb and Mo in the austenite grain boundary increases.
[0071]
　By setting the winding temperature to less than 500 ° C., the above effect can be enhanced, and the X-ray random intensity ratio of {112} <111> of crystal grains can be controlled in the hot stamping steel plate. Immediately after finish rolling, Nb and Mo are solid-solved in austenite, and Nb and Mo are transformed from austenite in which Nb and Mo are solid-solved to lower bainite, martensite, or tempered martensite. In order to relieve the stress generated by the transformation, advantageous crystal orientation is preferentially generated, so that the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled. It is preferably less than 480 ° C. The lower limit is not specified, but since it is difficult to wind up below room temperature in actual operation, room temperature is the lower limit.
[0072]
　(3) Formation of Plating Layer A
　plating layer may be formed on the surface of the softened layer for the purpose of improving corrosion resistance and the like. The plating layer may be either an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include an electrogalvanizing layer and an electric Zn—Ni alloy plating layer. The hot-dip plating layer includes a hot-dip zinc plating layer, an alloyed hot-dip zinc plating layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si alloy. A plating layer and the like are exemplified. The amount of adhesion of the plating layer is not particularly limited and may be a general amount of adhesion.
[0073]
　(4) Other Steps In
　the production of the hot stamping steel sheet, known production methods such as pickling, cold rolling, and temper rolling may be included.
[0074]
　
[0075]
　In the hot stamping compact of the present invention, a steel plate for hot stamping is heated and held in a temperature range of 500 ° C. or higher and A3 point or lower at an average heating rate of less than 100 ° C./s, and then hot stamped and molded. , The molded product is produced by cooling to room temperature.
[0076]
　Further, in order to adjust the strength, a part or all of the hot stamped molded product may be tempered at a temperature of 200 ° C. or higher and 500 ° C. or lower.
[0077]
　By heating a temperature range of 500 ° C. or higher and A3 point or lower at an average heating rate of less than 100 ° C./s, the grain boundaries of lower bainite, martensite, and tempered martensite formed on the steel plate for hot stamping are the reverse of austenite. It functions as a transformation site, and due to the texture memory effect of austenite and martensite, the rotation angle is 5 ° or more and 75 ° or more with the <011> direction of the lower baynite or martensite or tempered martensite crystal grains as the axis of rotation in the hot stamp molded product. Of the grain boundaries having a temperature of ° or less, 80% or more of the grain boundaries having a rotation angle of 15 ° or more can be generated.
[0078]
　When the average heating rate is 100 ° C./s or more, the fine carbides become reverse transformation sites of austenite, so that the texture memory effect of austenite and martensite cannot be obtained. It is preferably 90 ° C./s or less. The lower limit is not particularly specified, but if it is less than 0.01 ° C./s, the manufacturing cost becomes disadvantageous, so 0.01 ° C./s or more is preferable. More preferably, it is 1 ° C./s or higher.
[0079]
　The holding temperature at the time of hot stamping is preferably A3 point + 10 ° C. or higher and A3 point + 150 ° C. or lower in order to refine the old austenite grains. Further, the cooling rate after hot stamping is preferably 10 ° C./s or more from the viewpoint of improving the strength.
Example
[0080]
　Next, an example 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 described in this one condition example. It is not limited. 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.
[0081]
　Steel pieces produced by casting molten steel having the composition shown in Tables 1-1 to 1-3 are subjected to hot rolling and cold rolling shown in Tables 2-1 to 2-3 to obtain steel sheets for hot stamping. The steel sheet for hot stamping was subjected to the heat treatments shown in Tables 3-1 to 3-3 to perform hot stamping to produce a molded product.
[0082]
　Tables 3-1 to 3-3 show the microstructure and mechanical properties of the hot stamped product.
[0083]
[Table 1-1]

[0084]
[Table 1-2]

[0085]
[Table 1-3]

[0086]
[Table 2-1]

[0087]
[Table 2-2]

[0088]
[Table 2-3]

[0089]
[Table 3-1]

[0090]
[Table 3-2]

[0091]
[Table 3-3]

[0092]
　In the hot stamped product, the rotation angle is 5 with the area ratio of lower bainite, martensite, and tempered martensite, and the <011> direction of the crystal grains of lower bainite or martensite or tempered martensite as the rotation axis by the method described above. The ratio of the grain boundaries having a rotation angle of 15 ° or more to the grain boundaries having a temperature of ° or more and 75 ° or less was measured.
[0093]
　The strength of the hot stamped product was evaluated by performing a tensile test. The tensile test was carried out according to the test method described in JIS Z 2241 after preparing the No. 5 test piece described in JIS Z 2201, and the maximum strength was 2000 MPa or more.
[0094]
　The bending deformability was evaluated under the following measurement conditions based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry. In the present invention, the displacement at the maximum load obtained in the bending test is converted into an angle based on the VDA standard, the maximum bending angle is obtained, and the material having the maximum bending angle of 50 ° or more is accepted.
[0095]
　　Specimen dimensions: 60 mm (rolling direction) x 30 mm (direction perpendicular to rolling), plate thickness 1.0 mm
　　Bending ridge: direction perpendicular to rolling
　　Test method: roll support, punch pushing
　　roll diameter: φ30 mm
　　Punch shape: tip R = 0 .4mm
　　Distance between rolls: 2.0 × 1.0 (mm) + 0.5mm
　　Pushing speed: 20mm / min
　　Testing machine: SHIMADZU AUTOGRAPH 20kN
[0096]
　It was confirmed that the hot stamped molded product of the present invention has a tensile strength of 2000 MPa or more and has excellent bending deformability. On the other hand, in the case where the chemical composition and the production method were not appropriate, the target characteristics could not be obtained.
The scope of the claims
[Claim 1]
　Ingredient composition is mass%,
　　C: 0.35% or more, 0.75% or less,
　　Si: 0.005% or more, 0.25% or less,
　　Mn: 0.5% or more, 3.0% or less,
　　sol. Al: 0.0002% or more, 3.0% or less,
　　Cr: 0.05% or more, 1.00% or less,
　　B: 0.0005% or more, 0.010% or less,
　　Nb: 0.01% or more, 0.15% or less,
　　Mo: 0.005% or more, 1.00% or less,
　　Ti: 0% or more, 0.15% or less,
　　Ni: 0% or more, 3.00% or less,
　　P: 0.10% Hereinafter,
　　S: 0.10% or less and
　　N: 0.010% or less are contained, the balance is Fe and unavoidable impurities, and the
　microstructure is at least one of lower bainite, martensite, and tempered martensite. 90% or more in area ratio
　, and the rotation angle is 5 ° or more and 75 ° or less with the <011> direction of the crystal grains of the lower bainite, the martensite, and the tempered martensite as the axis of rotation.
A hot stamp molded body characterized in that the ratio of the length of the grain boundary having a rotation angle of 15 ° or more to the length is 80% or more .
[Claim 2]
　The hot stamped molded article according to claim 1, further comprising a plating layer.