Technical field
[0001]The present invention, high-strength steel sheet used for structural members and reinforcement members intensity automobiles and structures required, in particular, relates to good hot stamping body crashworthiness properties and hydrogen embrittlement resistance.
BACKGROUND
[0002]Recently, weight reduction of automobile bodies from the viewpoint of environmental protection and resource saving has been required, therefore, the application of high-strength steel sheets to automobile members is accelerating. However, since the formability due to the high strength of the steel sheet deteriorates in the high-strength steel sheet, it becomes a problem formability into complex shaped members.
[0003]To solve such problems, the application of hot stamping to implement press molding after heating the steel to a high temperature of the austenite region is underway. Hot stamping, pressing and simultaneously, since carrying out the hardening process in the mold, it is possible to obtain a strength corresponding to the C content of the steel sheet, it is attracting attention as a technology to achieve both shaping and strength secured to automotive parts ing.
[0004]However, conventional hot pressed parts manufactured by press hardening, to the plate thickness throughout is formed of a hard tissue (mainly martensite), the bending deformation at the time of vehicle collision occurs, part of the buckling portion easy greatest distortion enters, progresses cracks starting from the vicinity of the surface layer of the steel sheet, and finally lead to breakage. Also, penetration of hydrogen is promoted due to the high density of the surface layer of the lattice defects of the steel sheet, it is a problem that becomes poor hydrogen embrittlement resistance of the member. For this reason, hot pressed parts manufactured by press hardening, application site in automotive parts has been limited.
[0005]
　To the above problem, a technology for suppressing cracks by increasing the deformability of the hot pressed part is proposed. In Patent Document 1, hot pressed parts while the hardness of the plate thickness center and more than 400Hv of, by forming the surface layer has a thickness of 20μm or more 200μm or less hardness 300Hv following soft layer, the tensile strength of 1300MPa or higher while ensuring the strength of, it is disclosed that suppress the cracking during car crash. Furthermore, Patent Document 1, above the soft layer is disclosed to have a tempered structure.
[0006]
　In Patent Document 2, by controlling the carbon concentration in the surface layer of high strength automobile member carbon concentration of the inner layer steel 1/5 below, disclosed that improve the resistance to hydrogen embrittlement by reducing the density of the surface layer of lattice defects It is.
[0007]
　In Patent Document 3, a steel structure and multi-phase structure of ferrite and martensite by increasing than the area ratio of ferrite in the surface layer portion in the inner layer portion, high tensile strength, hot having excellent ductility and bendability to obtain a press-parts is disclosed.
[0008]
　However, the members described in Patent Documents 1 and 2, the surface layer portion of the sheet thickness and soft tissue, the central portion of the plate thickness by forming a hard tissue, the gradient of the sudden hardness occurs in the thickness direction and will. Therefore, when subjected to bending deformation, cracking near the boundary of the soft slope of sudden hardness is occurring tissue and the hard tissue is a problem that tends to occur. Furthermore, the slope in the member, the surface portion of the sheet thickness and soft tissue, the central portion of the plate thickness by the complex structure of the hard tissue and soft tissue, the abrupt stiffness in the thickness direction described in Patent Document 3 thereby reducing the. However, since the central portion of the plate thickness and the complex structure, the upper limit of the tensile strength becomes about 1300 MPa, it is difficult to ensure a higher tensile strength 1500MPa required for the hot pressed part.
CITATION
Patent Document
[0009]
Patent Document 1: JP 2015-30890 JP
Patent Document 2: JP 2006-104546 Patent Publication
Patent Document 3: WO 2015/097882
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　In view of the problems of the prior art, and an object thereof is to provide an excellent hot stamping body crashworthiness properties and hydrogen embrittlement resistance.
Means for Solving the Problems
[0011]
　The present inventors have intensively studied how to solve the above problems. First, in order to improve the hydrogen embrittlement resistance, it is effective to reduce the density of lattice defects in the sheet thickness of the surface layer. For this purpose, it is necessary to form a soft tissue surface. Meanwhile, in order to ensure a tensile strength of not less than 1500MPa, it is necessary to configure the central portion of the thickness only hard tissue. Accordingly, the present inventors, the thickness of the surface layer and the soft tissue, in the case where the central portion of the sheet thickness of a hard tissue, rapid in the thickness direction generated near the boundary of the hard tissue and soft tissue Hardness if it is possible to reduce the gradient of while securing the above tensile strength and good resistance to hydrogen embrittlement 1500 MPa, it was considered good bending properties are obtained. More specifically, the boundary of the hard tissue and soft tissue, by forming tissue (intermediate layer) having an intermediate hardness of these tissues, to reduce the gradient of the thickness direction stiffness, bending during deformation the stress concentration was alleviated. As a result, it is possible to bend suppress the occurrence of cracking during deformation, while ensuring the above tensile strength and good resistance to hydrogen embrittlement 1500 MPa, it is possible to obtain a good bending resistance, crashworthiness and anti We were able to obtain an excellent hot stamping body to hydrogen embrittlement characteristics.
[0012]
　Further, the present inventors have found that a relatively high value the amount of Mn in the central part of the plate thickness, more specifically 1.50% or more, by controlling to less than 3.00%, increasing the hardenability possible to reduce variations in hardness in the molding body Te, i.e. found that it is possible to ensure a stable high strength. As a result, while securing the above tensile strength and good resistance to hydrogen embrittlement 1500 MPa, the flexural properties just not strength stability (hardness variation) hot stamping molded article having excellent crashworthiness in terms of We were able to obtain.
[0013]
　Furthermore, the present inventors found that the contribution relatively high value the amount of Si in the central portion of the plate thickness, more specifically 0.50% is greater than the improvement of the control to deformability less than 3.00% It found that it is possible to improve the ductility by securing the tissue. As a result, while securing the above tensile strength and good resistance to hydrogen embrittlement 1500 MPa, it was also able to obtain an excellent hot stamping body crashworthiness characteristics in terms of ductility as well bendability.
[0014]
　In addition, the present inventors have found that a relatively high value the amount of Mn and Si in the central portion of the plate thickness, more specifically each 1.50% or more to less than 3.00% and 0.50 percent by controlling to less than 3.00%, improves the ductility, it enhances the hardenability to reduce variations in hardness in the molded body, i.e. it found that it is possible to ensure a stable high strength . As a result, while securing the above tensile strength and good resistance to hydrogen embrittlement 1500 MPa, not only bending resistance, strength stability (hardness variation) and excellent hot stamp crashworthiness characteristics in terms of ductility I was able to obtain a molded product.
[0015]
　The present invention has been completed based on the above findings and has as its gist is as follows.
　(1) and the plate thickness center part, a hot stamp shaped body comprising a surface layer disposed on both sides or one side of the plate thickness center part,
　the hot stamping molded body, and the plate thickness central portion and the surface layer further comprising an intermediate layer formed adjacent to them between,
　the plate thickness central portion, in
　mass%, C: 0.20% to less than 0.70%
　Si: less than
　3.00%, Mn : 0.20% to less than 3.00%
　P: 0.10% or
　less, S: 0.10% or
　less, sol. Al: 0.0002% or more, 3.0000% or less,
　N: contains 0.01% or less,
　the balance being Fe and unavoidable impurities,
　the hardness of the plate thickness center portion than 500 Hv, below 800Hv There,
　the thickness direction of the hardness of change [Delta] H in the surface layer 1 is more 10HV, less than 200 Hv,
　the change in the hardness of the plate thickness direction of the intermediate layer [Delta] H 2 is more than 50 Hv, and wherein less than 200 Hv to, hot stamping body.
　(2) Si content in the thickness center portion is not more than 0.50% Mn content is 0.20% or more in the thickness center portion, and less than 1.50%, hot stamping molded article according to (1).
　(3) the plate is a Si content of 0.50% or less in the thickness center portion, Mn content is 1.50% or more in the thickness center portion, and less than 3.00%, hot stamping molded article according to (1).
　(4) the thickness central Si content 0.50% in unit exceeds, less than 3.00%, Mn content in the plate thickness central portion of 0.20% or more, be less than 1.50% the thickness central portion, an area fraction of 1.0% or more, characterized in that it comprises a residual austenite of less than 5.0%, hot stamping molded article according to the above (1).
　(5) the thickness central Si content 0.50% in unit exceeds, less than 3.00%, Mn content in the plate thickness central portion of 1.50% or more, be less than 3.00% the thickness central portion, an area fraction of 1.0% or more, characterized in that it comprises a residual austenite of less than 5.0%, hot stamping molded article according to the above (1).
　(6) the thickness center portion, further, by mass% Ni: 0.01% or more, characterized in that it contains less 3.00%, any of the above (1) to (5) 1 hot stamping molded article according to claim.
　(7) the thickness center portion, further, by mass%, Nb: 0.010% or more 0.150% or less, Ti: 0.010% or more 0.150% or less, Mo: 0.005% or 1.000% or less, and B: 0.0005% or more, characterized in that it contains one or more 0.0100% or less, any of the above (1) to (6) 1 hot stamping molded article according to claim.
　(8), further comprising a plating layer on each surface of the surface layer, the (1) hot stamping molded article according to any one of (1) to (7).
Effect of the invention
[0016]
　According to the present invention, it is possible to be realized excellent bendability can provide an excellent hot stamping body crashworthiness properties and hydrogen embrittlement resistance. Further, according to the present invention, by controlling the addition amount of Mn in the central part of the plate thickness to a relatively high value, the crashworthiness characteristics in terms of bending resistance just not strength stability (hardness variation) it can be further improved. Furthermore, according to the present invention, by controlling the addition amount of Si in the thickness of the central portion to a relatively high value, it is possible to further improve the crashworthiness characteristics in terms of ductility as well bendability. In addition, according to the present invention, by controlling the addition amount of Mn and Si in the central portion of the plate thickness to a relatively high value, the strength stability (hardness variation) as well bendability and from the viewpoint of ductility it can also further improve crash-resisting properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
FIG. 1 is a schematic view for explaining the diffusion of C atoms in the production of high-strength steel sheet of the present invention.
It is a graph showing the dislocation density change after rolling pass about rough rolling used in the method for producing a high-strength steel sheet of the present invention; FIG.
DESCRIPTION OF THE INVENTION
[0018]
　Hereinafter will be described a hot stamping molded article of the present invention and its manufacturing method.
[0019]
　First, a description will be given reasons for limiting the chemical composition of the sheet thickness central part constituting the hot stamping molding of the present invention. Hereinafter,% of the component composition means mass%.
[0020]
"C: 0.20% to less than 0.70%"
　C is, 500 Hv or more at mid-thickness portion, is an important element to obtain the following hardness 800 Hv. If it is less than 0.20%, since it is difficult to ensure a more 500Hv at mid-thickness portion, C is 0.20% or more. Preferably 0.30% or more. On the other hand, 0.70% or more, the hardness of the plate thickness central portion exceeds the 800 Hv, since bending resistance is reduced, C is set to less than 0.70%. Preferably not more than 0.50%.
[0021]
"Si: less than 3.00%"
　Si are the elements contributing to the improvement of strength in the solid solution strengthening, it may be added 0.50% as the upper limit from the viewpoint of improving the strength. On the other hand, the effect of the strength improved if added over 0.50% is the upper limit of 0.50% to saturation. Preferably not more than 0.30%. Si also without compromising the hydrogen embrittlement resistance and bending resistance expressed by a surface layer of tissue control, is also an element having an effect of enhancing the ductility. In particular, when bending deformation at the time of vehicle collision occurs, localized deformation by hat member buckles, the load bearing of the member decreases. That member and the maximum load as well as members strength also affected the likelihood of buckling. Ductile and deformable region of the steel sheet is less likely localized in member states. That is hard to buckle. Therefore it is important ductility in hot stamping member, generally ductility of martensite is low. From this point of view, by adding more than 0.50% of Si, it is possible to secure more than 1.0% of residual austenite in area fraction, to improve the ductility, Si 0.50 % it is preferable to adding over. More preferably not less than 1.00%. On the other hand, the addition of more than 3.00%, residual austenite becomes an area fraction of 5.0% or more, because it causes bending of the deterioration, the upper limit is less than 3.00%. Preferably less than 2.00%.
[0022]
"Mn: 0.20% or more and less than 3.00%"
　Mn is an element contributing to the improvement of the strength of the in the solid-solution strengthening. If it is less than 0.20% from the viewpoint of improving the strength the effect is not obtained, adding 0.20% or more. Preferably 0.70% or more. Meanwhile, the effect of the strength improvement by the addition over 1.50% to saturation, to a maximum of less than 1.50%. Mn also without compromising the hydrogen embrittlement resistance and bending resistance expressed by a surface layer of tissue control, is also an element having an effect of enhancing the hardenability. In hot stamping member is not how to contact with the mold is necessarily uniform, for example, the cooling rate tends to decrease at such vertical wall portion of the hat member. May be locally hardness lower region is formed in the order steel. Local softening unit deformation is concentrated at the time of a collision, to become a factor of cracking, reducing the variation in hardness in the molded body by increasing the hardenability, that is, to secure stable strength, resistance it is important in ensuring the collision characteristics. From this point of view, the addition of Mn 1.50% or more, it is possible to obtain a stable high strength by increasing hardenability, Mn is preferably added more than 1.50%. More preferably 1.70% or more. On the other hand, the strength stability effect even when added more than 3.00% in order to saturate, the upper limit should be less than 3.00%. Preferably, less than 2.00%.
[0023]
"P: 0.10% or less"
　P is segregated at the grain boundaries, is an element that inhibits the strength of the grain boundary. Exceeds 0.10%, significantly reduced the strength of the grain boundary, so hydrogen embrittlement resistance and bending resistance is lowered, P is at most 0.10%. Preferably 0.05% or less. The lower limit is not particularly limited, when reduced to less than 0.0001%, de P cost increases greatly, since economically disadvantageous, practically steel, 0.0001%, which is substantially the lower.
[0024]
"S: 0.10% or less"
　S is an element which forms inclusions. It exceeds 0.10%, since inclusions generated hydrogen embrittlement resistance and bending resistance is lowered, S is at most 0.10%. Preferably not more than 0.005%. The lower limit is not particularly limited, when reduced to less than 0.0015%, de S cost increases greatly, since economically disadvantageous, practically steel 0.0015% are substantially the lower.
[0025]
"Sol.Al:0.0002% above, 3.0000% or less,"
　Al is an element which forms an action of sound of the steel by deoxidation of molten steel. If less than 0.0002%, the deoxidation is insufficient, sol. Al is 0.0002% or more. Preferably is 0.0010% or more. On the other hand, even if added over 3.0000% since the effect is saturated, and less 3.0000%.
[0026]
"N: 0.01% or less"
　N is an impurity element, an element that inhibits bendability to form a nitride. Exceeds 0.01%, the bendability generates coarse nitrides is significantly reduced, N is the set to 0.01% or less. Preferably is less than or equal to 0.0075%. The lower limit is not particularly limited, when reduced to less than 0.0001%, de-N cost increases greatly, since economically disadvantageous, practically steel, 0.0001%, which is substantially the lower.
[0027]
"Ni: 0.01% or more, 3.00% or less"
　Ni are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect is obtained is less than 0.01%, adding 0.01% or more. Preferably 0.50% or more. On the other hand, even if added over 3.00% since the effect is saturated, and less 3.00%. Preferably not more than 2.50%.
[0028]
"Nb: 0.010% or more 0.150% or less"
　Nb are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.010%, the addition of 0.010% or more. Preferably 0.035% or more. On the other hand, even if added over 0.150% of its effect will be saturated, 0.150% or less. Preferably is less than or equal to 0.120%.
[0029]
"Ti: 0.010% or more 0.150% or less"
　Ti are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.010%, 0.010% or more. Preferably 0.020%. On the other hand, even if added over 0.150% of its effect will be saturated, 0.150% or less. Preferably is less than or equal to 0.120%.
[0030]
"Mo: 0.005% or more 1.000% or less"
　Mo are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.005%, 0.005% or more. Preferably 0.010% or more. On the other hand, even if added over 1.000%, the effect is to saturate, is 1.000% or less. Preferably is less than or equal to 0.800%.
[0031]
"B: 0.0005% or more, 0.0100% or less,"
　B is an element that improves segregation and strength of grain boundaries in a grain boundary, may be added as necessary. Since the addition effect is not sufficiently obtained is less than 0.0005% is added 0.0005% or more. Preferably is 0.0010% or more. On the other hand, its effect if added over 0.0100% in order to saturate, and 0.0100% or less. Preferably is less than or equal to 0.0075%.
[0032]
　Balance of composition of the mid-thickness portion is Fe and unavoidable impurities. Unavoidable impurities inevitably mixed in steel raw material and / or steel manufacturing process, an element that is allowed in a range that does not inhibit the characteristics of the hot stamping molding of the present invention.
[0033]
　Next, a description will be given surface component composition constituting the hot stamping molding of the present invention.
[0034]
　The surface layer of the component, C content, Si content, and any one of the Mn content or two or more, not more than 0.6 times the corresponding element content of the mid-thickness portion preferably, the preferred range of each component of such a case is as follows.
[0035]
"C: 0.05% to less than 0.42%"
　C is added in order to increase the strength. Since no effect can be obtained with less than 0.05%, the addition of 0.05% or more. In the viewpoint of improving the impact properties by increasing the load bearing as members, preferably 0.10% or more. On the other hand, since the hardness of the surface layer lower than the hardness of the plate thickness center part, it is preferable to be less than the thickness center portion. The content of the surface layer of the preferred C for this is less than 0.42%, preferably not more than 0.35%.
[0036]
"Si: less than 2.00%"
　Si are the elements contributing to the improvement of strength in the solid solution strengthening, it is added to improve the strength. To the hardness of the surface layer lower than the hardness of the plate thickness center part, it is preferable to be less than the thickness center portion. Content less than 2.00% of the surface layer of the preferred Si for this, preferably 1.50% or less, more preferably 0.30% or less, still more preferably 0.20% or less.
[0037]
"Mn: 0.01% or more, 1.80% less than"
　is Mn, since an element which contributes to improving the strength in the solid solution strengthening, are added to improve the strength. To the hardness of the surface layer lower than the hardness of the plate thickness center part, it is preferable to be less than the thickness center portion. Content less than 1.80% of the surface layer of the preferred Mn for this, preferably 1.40% or less, more preferably less than 0.90%, still more preferably 0.70%.
[0038]
　There is no particular limitation on the other components of the surface layer. In general, the surface layer, C, in addition to Si and Mn, may optionally include one or more of the following components.
[0039]
"P: 0.10% or less"
　P is segregated at the grain boundaries, is an element that inhibits the strength of the grain boundary. Exceeds 0.10%, significantly reduced the strength of the grain boundary, so hydrogen embrittlement resistance and bending resistance is lowered, P is at most 0.10%. Preferably 0.05% or less. The lower limit is not particularly limited, when reduced to less than 0.0001%, de P cost increases greatly, since economically disadvantageous, practically steel, 0.0001%, which is substantially the lower.
[0040]
"S: 0.10% or less"
　S is an element which forms inclusions. It exceeds 0.10%, since inclusions generated hydrogen embrittlement resistance and bending resistance is lowered, S is at most 0.10%. Preferably not more than 0.005%. The lower limit is not particularly limited, when reduced to less than 0.0015%, de S cost increases greatly, since economically disadvantageous, practically steel 0.0015% are substantially the lower.
[0041]
"Sol.Al:0.0002% above, 3.0000% or less,"
　Al is an element which forms an action of sound of the steel by deoxidation of molten steel. If less than 0.0002%, the deoxidation is insufficient, sol. Al is 0.0002% or more. Preferably is 0.0010% or more. On the other hand, even if added over 3.0000% since the effect is saturated, and less 3.0000%.
[0042]
"N: 0.01% or less"
　N is an impurity element, an element that inhibits bendability to form a nitride. Exceeds 0.01%, the bendability generates coarse nitrides is significantly reduced, N is the set to 0.01% or less. Preferably is less than or equal to 0.0075%. The lower limit is not particularly limited, when reduced to less than 0.0001%, de-N cost increases greatly, since economically disadvantageous, practically steel, 0.0001%, which is substantially the lower.
[0043]
"Ni: 0.01% or more, 3.00% or less"
　Ni are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect is obtained is less than 0.01%, adding 0.01% or more. Preferably 0.50% or more. On the other hand, even if added over 3.00% since the effect is saturated, and less 3.00%. Preferably not more than 2.50%.
[0044]
"Nb: 0.010% or more 0.150% or less"
　Nb are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.010%, the addition of 0.010% or more. Preferably 0.035% or more. On the other hand, even if added over 0.150% of its effect will be saturated, 0.150% or less. Preferably is less than or equal to 0.120%.
[0045]
"Ti: 0.010% or more 0.150% or less"
　Ti are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.010%, 0.010% or more. Preferably 0.020%. On the other hand, even if added over 0.150% of its effect will be saturated, 0.150% or less. Preferably is less than or equal to 0.120%.
[0046]
"Mo: 0.005% or more 1.000% or less"
　Mo are the elements contributing to the improvement of strength in solid solution strengthening may be added as necessary. Since no effect can be obtained with less than 0.005%, 0.005% or more. Preferably 0.010% or more. On the other hand, even if added over 1.000%, the effect is to saturate, is 1.000% or less. Preferably is less than or equal to 0.800%.
[0047]
"B: 0.0005% or more, 0.0100% or less,"
　B is an element that improves segregation and strength of grain boundaries in a grain boundary, may be added as necessary. Since the addition effect is not sufficiently obtained is less than 0.0005% is added 0.0005% or more. Preferably is 0.0010% or more. On the other hand, its effect if added over 0.0100% in order to saturate, and 0.0100% or less. Preferably is less than or equal to 0.0075%.
[0048]
　The remainder of the surface layer of the component composition is Fe and unavoidable impurities. Unavoidable impurities inevitably mixed in steel raw material and / or steel manufacturing process, an element that is allowed in a range that does not inhibit the characteristics of the hot stamping molding of the present invention.
[0049]
　It will now be described microstructure of the hot stamping molding of the present invention.
[0050]
"Hardness of the plate thickness central portion than 500 Hv, 800 Hv or less"
　hardness of the plate thickness central portion If it is more than 500 Hv, can be secured over 1500MPa as the tensile strength of the hot stamping molding. Preferably, it is greater than or equal to 600Hv. On the other hand, the hardness of the plate thickness central portion is more than 800 Hv, the difference in hardness between the surface layer and the intermediate layer is too large, because it causes bending of the deterioration, the upper limit 800 Hv. Preferably is less than or equal to 720Hv.
[0051]
"Mid-thickness portion, at an area fraction of 1.0% or more, including retained austenite of less than 5.0%"
　The Si content in the sheet thickness center portion 0.50% greater, to less than 3.00% controlled and, the thickness center portion is a metal structure as a residual austenite an area fraction of 1.0% or more, by to contain less than 5.0%, to improve the ductility of the resulting hot stamping member can. Preferably 2.0% or more. On the other hand, when the area fraction of retained austenite is 5.0% or more, because it causes bending of the degradation, and the upper limit of less than 5.0%. Preferably less than 4.5%.
[0052]
　In the present invention, an area fraction of retained austenite is measured by the following method. A member after the hot stamping, samples were taken and scalped from the rolling surface normal direction to 1/4 depth of thickness, subjected to X-ray diffraction measurement. From an image obtained by Mo X-ray diffraction using the Kα line of area fraction Vγ of retained austenite is determined using the following equation.
　Vγ = (2/3) {100 / (0.7 × α (211) / γ (220) +1)} + (1/3) {100 / (0.78 × α (211) / γ (311) +1)}
　where, alpha (211) reflection plane intensity at (211) plane of the ferrite, gamma (220) reflection plane intensity at (220) plane of the austenite, gamma (311) is in (311) plane of austenite a reflective surface strength.
[0053]
"Thickness direction of the hardness of the change ΔH in the surface layer 1 is more 10HV, less than 200 Hv, the change ΔH in the plate thickness direction of the intermediate layer hardness 2 or more 50 Hv, less than 200 Hv."
　In the present invention, a surface layer is intended to refer to the region from both sides or one side of the hot stamping material up to 8% of the thickness of the hot stamping material, i.e. the surface layer has a thickness of 8% of the thickness of the hot stamping member . Similarly, in the present invention, an intermediate layer, which refers to the portion excluding the surface layer of the of the region from both sides or one side of the hot stamping material up to 20% of the plate thickness of the hot stamping material, i.e. each intermediate layer has 12% of the thickness of the plate thickness of the hot stamping molding. In the present invention, the plate thickness center part, which from the hot stamping material refers to a moiety excluding the surface layer and the intermediate layer described above, i.e. mid-thickness portion, the surface layer on both sides of the mid-thickness portion and, in the case of hot stamping molded intermediate layer is arranged, has a 60% of the thickness of the plate thickness of the hot stamping material, the surface layer and the intermediate layer is disposed only on one side of the mid-thickness portion and in the case of hot stamping body has 80% of the thickness of the plate thickness of the hot stamping molding. Here, [Delta] H 1 represents the change in the hardness of the plate thickness direction in the surface layer, [Delta] H 2 shows a change in hardness of the plate thickness direction of the intermediate layer. The present inventors have studied intensively, in view of the effect of bending resistance, hardness change in this region ([Delta] H 1 , [Delta] H 2 ) is important, [Delta] H 1But above 10HV, if it is less than 200 Hv, good bending properties and resistance to hydrogen embrittlement was found to be obtained. By having such a good bending property, it is possible to suppress the bending deformation relaxation to breakage or cracking stress due like at the time of collision, to achieve the excellent crashworthiness in hot stamping member possible it is. Meanwhile, [Delta] H 1 is less than 10HV, such bending stress is not obtained the effect of mitigating at deformation, since the crack from the surface layer is likely to progress, the lower limit and 10HV. Preferably 20Hv, and more preferably not less than 30 HV. Further, [Delta] H 1 is the case where less than 200 Hv, is enhanced effect to relax the stress concentration at the time of bending deformation, for good bending resistance is obtained, the upper limit is less than 200 Hv. Preferably less than 150 Hv, and more preferably less 100Hv or less than 95Hv, and most preferably not more than 90Hv.
[0054]
　Similarly, [Delta] H 2 is more than 50 Hv, if it is less than 200 Hv, good bending property was found to be obtained. [Delta] H 2 in the above 200 Hv, the gradient of the hardness becomes sharp in the intermediate layer, bending stress concentrations it becomes difficult to relieve upon deformation, because the bending property is deteriorated, the upper limit of less than 200 Hv. Preferably 190Hv or less, and more preferably not more than 180 Hv. The lower limit is preferably at least 60 HV, more preferably 70Hv or more.
[0055]
　Method of measuring the hardness of the plate thickness central portion is as follows. Collected a cross section perpendicular to the plate surface of the hot stamping material, performs sample preparation of the measurement surface, subjected to hardness testing. Process for the preparation of the measurement surface may be carried out according to JIS Z 2244, for example, after polishing the measurement surface using silicon carbide paper # 1500 and # 600, alcohols such as diamond powder 6μm from the particle size 1μm it Shiagere mirror using a liquid dispersed in a diluent or pure water. Hardness test may be carried out in the method described in JIS Z 2244, using a micro Vickers hardness tester, 1/2 position of the plate thickness of the hot stamping material, a load 1 kgf, 3 times the indentation It measured 10 points by the above intervals, and the average value and the hardness of the plate thickness center part.
[0056]
　Next, a description will be given of the measurement method of the hardness of the surface layer and the intermediate layer. Collected a cross section perpendicular to the plate surface of the hot stamping body perform sample preparation measurement surface subjected to the hardness test. Preparation of the measurement surface, in order to accurately measure the hardness of the vicinity of the surface of the hot stamping material, as much as possible uneven small is carried out as everyone does not occur in the vicinity of the surface. For example, Japan using electron-made cross-section polisher, sputtering measurement surface with argon ion beam. In this case, for the purpose of suppressing the streak-like unevenness occurs in the measurement plane, using a sample rotation holder JEOL, it may be irradiated with an argon ion beam to the measurement surface from the direction of 360 degrees.
[0057]
　For hot stamp molded surface layer and the intermediate layer is disposed on either side of the mid-thickness portion, with respect to specimens prepared measurement surface, by using a micro Vickers hardness tester, carrying out two measurements. First time, the region from the first surface of the hot stamping material up to 20% of the plate thickness of the hot stamping material, a plate surface perpendicular direction (thickness direction), a load 1 kgf, the indentation 3 It is measured at more than double the interval. In this case, although the total of the measurement points depending on the plate thickness of the hot stamping body is different, [Delta] H below 1 and [Delta] H 2 in order to calculate the may be performed to measure the at least two points. Measuring position in the most surface side of the hot stamping molded article, the plate surface (if the plating layer is present, immediately below the alloy layer between immediately below the plating layer or plated layer and the base material) in the region from to within 20μm to be carried out. Second measurement is carried out from the first surface of the opposite side of the hot stamping molding. That is, the region from the second surface of the hot stamping material up to 20% of the plate thickness, the plate surface direction perpendicular (thickness direction), a load 1 kgf, measured at least three times the interval of the indentation. Measuring position in the most surface side of the hot stamping molded article, the plate surface (if the plating layer is present, immediately below the alloy layer between immediately below the plating layer or plated layer and the base material) in the region from to within 20μm to be carried out.
[0058]
　For mid-thickness portion of one side only is disposed surface and intermediate layer hot stamping member, to the sample prepared with the measuring surface, using a micro Vickers hardness tester, the surface layer of the hot stamping member an area of up to 20% of the plate thickness of the hot stamping material, a plate surface perpendicular direction (thickness direction), a load 1 kgf, measured at least three times the interval of the indentation. In this case, although the total of the measurement points depending on the plate thickness of the hot stamping body is different, [Delta] H below 1 and [Delta] H 2 in order to calculate the may be performed to measure the at least two points. Measuring position in the most surface side of the hot stamping molded article, the plate surface (if the plating layer is present, immediately below the alloy layer between immediately below the plating layer or plated layer and the base material) in the region from to within 20μm to be carried out.
[0059]
　Next, [Delta] H in the case of a plate thickness central portion hot stamping molded surface layer and the intermediate layer is disposed on both sides of 1 illustrating the method for calculating the. First, all of the measurement points included in the region from the first surface of the hot stamping material up to 8% the thickness to calculate the first surface side surface hardness gradient Δa by equation (1). Here, a i is the distance from the first surface of the i-th measurement point ([mu] m), c i is a i of Vickers hardness in (Hv), n is from the first surface to 8% the thickness is the sum of all measurement points included in the region. Then, the equation (2) using all of the measurement points included in the region from the second surface of the hot stamping material up to 8% the thickness, calculating a gradient Δb hardness of the second surface side surface to. Here, b i is the distance from the second surface in the i-th measurement point ([mu] m), d i is b i Vickers hardness in (Hv), m is up to 8% the thickness from the second surface is the sum of all measurement points included in the region. After calculating the Δa and [Delta] b, using the equation (3-1), the change ΔH in the thickness direction of the hardness in the surface layer 1 is calculated. Here, t is the thickness of the hot stamping molded article ([mu] m).
[0060]
　On the other hand, in the case of a plate thickness central portion of one side only is disposed surface and intermediate layer hot stamping molded article, using the equation (3-2), the change ΔH in the thickness direction of the hardness in the surface layer 1 calculated to.
[0061]
　Next, [Delta] H in the case of hot stamping molded surface layer and the intermediate layer is disposed on either side of the mid-thickness portion 2 will be described the method of calculating the. First, the first thickness of 8% of the positions on the surface side of the hot stamping material, from all measurement points included in the region of up to 20% thickness, of the first surface-side intermediate layer according to the equations (4) calculating a hardness gradient .DELTA.A. Here, A i is the distance from the first surface of the i-th measurement point ([mu] m), C i is A i Vickers hardness in (Hv), N is the thickness 8% in the first surface from the position, which is the sum of all measured points included in the region of up to 20% thickness. Then, from a thickness of 8% of the positions in the second surface side of the hot stamping material, from all measurement points included in the region of up to 20% thickness, the second surface side intermediate layer by formula (5) calculating a hardness gradient .DELTA.B. Here, B i is the distance from the second surface in the i-th measurement point ([mu] m), D i is B i Vickers hardness in (Hv), M is 8% of the thickness of the second surface is the sum of all measurement points included in the region from up to 20%. After calculating the ΔA and .DELTA.B, using Equation (6-1), the change ΔH in the plate thickness direction of the intermediate layer hardness 2 is calculated.
[0062]
　On the other hand, in the case of a plate thickness central portion of one side only is disposed surface and intermediate layer hot stamping molded article, using the equation (6-2), the change ΔH in the thickness direction of the hardness in the surface layer 2 calculated to.
[0063]
[Equation 1]

Here,
[Delta] H 1 : plate thickness direction of the stiffness change in the surface layer (Hv)
.DELTA.a: gradient of hardness of the first surface side surface (Hv / [mu] m) a
i at point: the i-th
c i : a i Vickers hardness in (Hv)
sum of all measurement points included in the first surface side surface layer: n
hardness of the second surface side surface: [Delta] b gradient (Hv / [mu]
m) b i : distance from the second surface in the i-th measurement point ([mu]
m) d i : b i Vickers hardness in
(Hv) m: all included in the second surface side surface the sum of the measurement points
[Delta] H 2 : plate thickness direction of the stiffness change in the intermediate layer (Hv)
.DELTA.A: gradient of hardness of the first surface-side intermediate layer (Hv / [mu] m)
a i: Distance from the first surface of the i-th measurement point ([mu] m)
C i : A i Vickers hardness in (Hv) N:
a first sum of all the measurement points included in the surface-side intermediate layer
.DELTA.B: No. gradient of the hardness of the second surface-side intermediate layer (Hv / [mu]
m) B i : distance from the second surface in the i-th measurement point ([mu]
m) D i : B i Vickers hardness in
(Hv) M: No. the sum of all the measurement points included in the second surface side intermediate layer
t: plate thickness ([mu] m)
is.
[0064]
　Each surface of the surface layer of the hot stamping material, for the purpose of improvement of corrosion resistance may be formed a plating layer. Plating layer may be either a electroplating layer and the molten plating layer. Electroplating layer includes, for example, electro-galvanized layer, the electrical Zn-Ni alloy plating layer and the like.
[0065]
　Molten plating layer, for example, galvanized layer, galvannealed layer, molten aluminum plating layer, melt Zn-Al alloy plating layer, hot-dip Zn-Al-Mg alloy plating layer, hot-dip Zn-Al-Mg-Si including the alloy plating layer and the like. Coating weight of the plating layer may be a common adhesion amount is not particularly limited.
[0066]
　Next, the form of preparation for obtaining the hot stamp molding of the present invention. The following description, there intended for purposes of illustration of the process for obtaining the hot stamp molding of the present invention, multiple layers of hot stamp molding of the present invention by laminating two steel plates as described below It not intended to be limited to those obtained from the steel sheet. For example, to obtain a high strength steel sheet consisting of the surface layer and mid-thickness portion by softening the surface layer of the single layer steel sheet with decarburization, is prepared by heat treatment or the like as in the case it multilayered steel it is also possible.
[0067]
　After melting the base material steel plate satisfying the component of the plate thickness center part of the by grinding the both surfaces or one surface to remove surface oxides, on both sides or one side, to adhere the surface layer steel plate in arc welding. Incidentally, C content, Si content, and one or more one of the Mn content be laminated surface layer steel plate is less than 0.6 times the corresponding element content of the base material steel plate preferable. Reason is not clear, the results of the examination of a hot stamping molded article exhibits excellent bendability, C content of the surface layer steel plate, Si content, and any one of the Mn content or two or more It was less than 0.6 times the corresponding element content of the base material steel plate.
[0068]
　The above laminate (multilayered steel) to hot rolling, cold rolling, hot stamping, by performing such continuous hot dipping, high-strength steel sheet according to the present invention, more specifically to obtain a hot stamping member it can.
[0069]
　For example, the case of obtaining a hot-rolled steel sheet, a multilayer steel sheet produced by the above method, 1100 ° C. or higher, it is preferable to hold less than 60 minutes over 20 minutes at 1350 ° C. or lower. Such heat treatment by the applied change ΔH in the thickness direction of the hardness in the surface layer after the hot press 1 to 10Hv or more, less than 200 Hv, in particular can be controlled to be less than 100 Hv. Further, the above heat treatment, by diffusing elements between the base steel sheet and the surface layer steel plate to form an intermediate layer therebetween, and further changes in the thickness direction of the hardness at the middle layer after hot pressing [Delta] H 2 to 50Hv or more can be controlled to less than 200 Hv. In contrast, the heating temperature is less than 1100 ° C., the change [Delta] H in the thickness direction of the hardness in the surface layer after the hot press 1 exceeds the 200 Hv, the change in hardness of the plate thickness direction of the intermediate layer after hot pressing [Delta] H 2 There is less than 10Hv. In this case, the promoted infiltration of hydrogen from the hot stamping member surface, leading to deterioration of hydrogen embrittlement resistance, to further can not be obtained good bending resistance, the lower limit and 1100 ° C.. On the other hand, if the heating temperature exceeds 1350 ° C., [Delta] H 1 is less than 10HV, further, [Delta] H 2 for may exceed the 200 Hv, it is impossible to obtain good bending properties, the upper limit is 1350 ° C.. Heating and holding is preferably performed less than 60 minutes over 20 minutes. The present inventors have studied intensively, if the retention time is 20 minutes to less than 60 minutes, it is possible to obtain a bending resistance and good resistance to hydrogen embrittlement, microstructure obtained at that time is, [Delta] H 2But more than 50Hv, was found to be less than 200Hv. Therefore, the holding time is less than 60 minutes than 20 minutes.
[0070]
　Further, in order to further promote the formation of the intermediate layer in the present invention, the hot rolling after the heat treatment of the multilayer steel plate, includes a rough rolling and finish rolling, the rough rolling is rough rolling temperature is 1100 ° C. or higher, less than the plate thickness reduction rate per pass of 5% to 50%, and interpass time is 2 times or more performed are preferably under the conditions of more than 3 seconds.
[0071]
　More specifically, in order to further promote the formation of the intermediate layer in the present invention, it is necessary to control the alloying elements, in particular the concentration of C atoms distributed loosely. Distribution of C concentration obtained by the diffusion of C atoms, the diffusion frequency of C atoms is increased as the temperature rises. Therefore, in order to control the C concentration is controlled in the rough rolling it is important from the hot rolling heating. The hot rolling heating, in order to promote the diffusion of C atoms, it is necessary to high temperature heating temperature, preferably 1350 ° C. 1100 ° C. or higher, more preferably at most 1150 ° C. Ultra 1350 ° C.. The hot rolling heating, a change shown in FIG. 1 (i) and (ii) occurs. (I) is the diffusion of C atoms from a thickness center portion to the surface layer, (ii) is a decarburization of C desorbed to the outside from the surface layer. Distribution C concentration caused by balance of the diffusion and elimination reaction of the C atoms of the (i) and (ii). Is less than 1100 ° C., since the reaction of (i) is insufficient, not obtained preferably C concentration distribution. On the other hand, in the 1350 ° C. greater than the reactivity of (ii) it is excessively generated, can not be obtained likewise preferred concentration distribution.
[0072]
　After having controlled the preferred C concentration distribution by adjusting the hot rolling heating temperature, in order to obtain a more optimal C concentration distribution, the path control in the rough rolling is very important. Rough rolling the rough rolling temperature is 1100 ° C. or more, the thickness reduction rate per pass is 5% or more and less than 50%, and interpass time is performed twice or more under the conditions of more than 3 seconds. This is because the distortion introduced by the rough rolling, in order to promote the diffusion of C atoms of (i) in FIG. Assuming that a slab with a controlled C concentration in a preferable state in hot rolling heating rough rolling and finish rolling in the usual manner, so that the C atom plate thickness without being able to sufficiently diffuse within the surface layer is reduced. Accordingly, the slab having a thickness of more than 200 mm, when manufacturing a hot rolled steel sheet having a thickness of several mm by hot rolling of a conventional method, C concentration in the surface layer becomes rapidly changing steel, gradual change in hardness is obtained no. Found methods to solve this is the path control of the rough rolling. Diffusion of C atoms is greatly influenced by the strain (dislocation density) as well temperature. In particular, in comparison with the lattice diffusion, since the increasing spread frequency more than 10 times in the dislocation diffusion, while leaving the dislocation density, it is necessary to devise to reduce the sheet thickness by rolling. Curve 1 in Figure 2 is of the plate thickness reduction ratio per one pass of the rough rolling is small, it shows the dislocation density change after rolling pass, it can be seen that the strain remains for a long time. By thus leaving the strain on the surface layer for a long time, occur diffusion of C atoms in the surface layer is sufficient, it is possible to obtain an optimal C concentration distribution. Meanwhile, the curve 2 is the change of the dislocation density when the sheet thickness reduction rate large, the amount of strain introduced by rolling is increased, the recovery is likely to be promoted, the dislocation density is rapidly lowered. Therefore, in order to obtain an optimal concentration distribution of C, it is necessary not to cause a change of the dislocation density, such as curve 2. From this viewpoint, the upper limit of the sheet thickness reduction rate per one pass is less than 50%. In order to promote the diffusion of C atoms in the surface layer, because the secure dislocation density and the retention time of a quantity required, next to the lower limit of the sheet thickness reduction rate of 5%, secured over three seconds as the path between the time Is required.
[0073]
　Finish rolling may be a finish rolling carried out under normal conditions. For example, the finishing temperature may be carried out in a temperature range of not lower than 810 ° C., cooling conditions followed is not particularly necessary to define, implement the winding in a temperature range of 750 ° C. or less. Further, it is also possible to implement the re-heat treatment for softening the hot-rolled steel sheet.
[0074]
　Heating during hot stamping, molding, cooling process also may be carried out at normal conditions. For example, hot rolled steel sheet rewind wound hot rolled steel sheet in the hot rolling step, or cold-rolled steel sheet subjected to cold rolling rewind the hot-rolled steel sheet was wound, or a plated cold rolled steel sheet subjected to, 0.1 ° C. / s or higher, at 200 ° C. / s or less in heating rate, 810 ° C. or higher, then heated to a temperature of 1000 ° C. or less, conventional hot stamping the steel sheet was maintained at this temperature, the required shape in molding. Retention time, so may be set according to the molding aspect is not particularly limited, 30 seconds or more, cooling the green compact after it hot stamp equal to or less than 600 seconds to room temperature. Cooling speed may be set to a normal condition, for example, the average cooling rate in the temperature range from the heating temperature to 400 ° C. can fall within 50 ° C. / s or higher. Mid-thickness Si content 0.50% in unit exceeds, less than 3.00%, Mn content in the sheet thickness center portion is 0.20% or more, steel plate is less than 1.50% and, mid-thickness Si content 0.50% in unit exceeds, less than 3.00%, Mn content in the sheet thickness center portion is 1.50% or more, when the steel plate is less than 3.00% is , in order to improve the ductility by increasing the production amount of residual austenite, the cooling after heating and holding, 200 ° C. or higher, controlling the average cooling rate in a temperature range of 400 ° C. or less to less than 50 ° C. / s It is preferred. Further, for the purpose of adjustment of the intensity it may be subjected to tempering in the range of 0.99 ° C. ~ 600 ° C. in cooling the molded body to room temperature.
[0075]
　Cold rolling the usual reduction ratio, for example, it is a cold rolling conducted at 30 to 90%. The hot-rolled steel sheet and cold-rolled steel sheet, other than those that remain hot rolling and cold rolling are also steel sheets and subjected to recrystallization annealing under normal conditions hot-rolled steel sheet or cold-rolled steel sheet, the usual condition steel sheet was subjected to temper rolling in is also included. Conditions of the plating is not particularly limited, and may be ordinary conditions. Hot-rolled steel sheet, cold rolled steel, or the recrystallization annealing and / or temper rolling the steel sheet was subjected to cold-rolled steel sheet, if necessary, plated with ordinary plating conditions.
Example
[0076]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0077]
　In this embodiment, the hardness of the steel sheet after the hot stamping, as measured by the foregoing method, the hardness of the plate thickness center part, the plate thickness direction in the surface hardness of the changes [Delta] H 1 , the plate thickness direction of the intermediate layer change ΔH in hardness 2 was calculated.
[0078]
　In addition, a tensile test was carried out of the steel sheet after hot stamp. Tensile test, to prepare a No. 5 test piece described in JIS Z 2201, was carried out according to the test method described in JIS Z 2241.
[0079]
　Hydrogen embrittlement resistance of a hot stamping molded article was evaluated using a test piece cut out from the molded body. Generally hot stamping body is other joining parts using bonding techniques, such as spot welding, depending part shape accuracy stress applied torsion hot stamping body is added. Stress depends on the location of the parts, it is difficult to accurately calculate this, it is believed that there is no practical problem to be delayed fracture in yield stress. Therefore, from the shaped body, cut out a test specimen having a thickness of 1.2 mm × width 6 mm × length 68mm, after applying the distortion of considerable yield stress at four-point bending test, and 100h immersed in pH3 hydrochloric acid, cracks It was to evaluate the resistance to hydrogen embrittlement in the occurrence or non-occurrence. Pass the case without a break (○), in the case of rupture there was judged as failure (×).
[0080]
　Crashworthiness properties of hot stamping molded article, was performed by evaluating the bending of the hot stamping material the following measurement condition based on the German Automobile Industry Association at defined VDA standard (VDA238-100). The displacement at maximum load obtained by bending test in the present invention is converted into an angle in VDA standard to determine the maximum bending angle.
　Specimen dimensions: 60 mm (rolling direction) × 60 mm (rolling direction perpendicular), or, 30 mm (rolling direction) × 60 mm (rolling direction perpendicular)
　Bending ridgeline: rolling direction perpendicular
　Test method: roll support, punches push
　roll diameter: .phi.30 mm
　punch shape: front end R = 0.4 mm
　distance between the rolls: 2.0 × thickness (mm) + 0.5 mm
　indentation speed: 20 mm / min
　tester: Shimadzu AUTOGRAPH 20 kN
[0081]
[Example A]
　After removing the surface oxide by grinding the surface of the base material steel plate having a chemical composition shown in Table 1, the surface layer steel sheet having a chemical composition shown in Table 2 on both surfaces or one surface in arc welding It was laminated. Incidentally, the thickness of the sum of the surface layer for steel sheet and the base material steel plate after arc welding and 200 mm ~ 300 mm, the thickness of the surface layer for steel sheet, about 1/3 of the thickness of the base steel sheet (in the case of one-sided 1 / about 4) to. Production No. 1-36 and 38-40 are steels with a welded surface steel plate on both sides, production No. 37 is a steel with a welded surface steel plate only on one side. The laminated steel plate subjected to hot rolling and / or cold rolling shown in Table 3, the resulting steel sheet is subjected to a heat treatment shown in Table 3, it performs hot stamping, to produce a molded body. Table 4 shows the microstructure and mechanical properties of the steel sheet after hot stamping (hot stamping molded article). The position of the sheet thickness 1/2 of the samples taken from the steel sheet after the hot stamping, and the component composition of the analysis of the position of 20 [mu] m (position in the surface layer) from the surface, the base steel sheet and shown in Tables 1 and 2 It was equivalent to composition of the surface layer steel plate.
[0082]
[Table 1-1]

[0083]
[Table 1-2]

[0084]
[table 2-1]

[0085]
[Table 2-2]

[0086]
[Table 3-1]

[0087]
[Table 3-2]

[0088]
[Table 4-1]

[0089]
[Table 4-2]

[0090]
　A tensile strength of not less than 1500 MPa, the maximum bending angle (°) is not less 70 (°) or more and a case where hydrogen embrittlement resistance becomes accepted, excellent crashworthiness properties and hydrogen embrittlement resistance It was evaluated as a hot stamping body (examples in Table 4). On the other hand, among the three performance if that does not satisfy one or a sample of Comparative Example.
[0091]
Example B (Mn: 1.50% or more and less than 3.00%)]
　After removing the surface oxide by grinding the surface of the base material steel plate having a chemical composition shown in Table 5, on both surfaces or one surface the surface layer steel sheet having a chemical composition shown in Table 6 was laminated by arc welding. Incidentally, the thickness of the sum of the surface layer for steel sheet and the base material steel plate after arc welding and 200 mm ~ 300 mm, the thickness of the surface layer for steel sheet, about 1/3 of the thickness of the base steel sheet (in the case of one-sided 1 / about 4) to. Production No. 101-135 and 137-139 are steel with a welded surface steel plate on both sides, production No. 136 is a steel with a welded surface steel plate only on one side. The laminated steel plate subjected to hot rolling and / or cold rolling are shown in Table 7, the resulting steel sheet is subjected to a heat treatment shown in Table 7, it performs hot stamping, to produce a molded body. Table 8 shows the microstructure and mechanical properties of the steel sheet after hot stamping (hot stamping molded article). The position of the sheet thickness 1/2 of the samples taken from the steel sheet after the hot stamping, and the component composition of the analysis of the position of 20μm from the surface (the position in the surface layer) is the base material steel plate and shown in Tables 5 and 6 It was equivalent to composition of the surface layer steel plate.
[0092]
[Table 5-1]

[0093]
[Table 5-2]

[0094]
[Table 6-1]

[0095]
[Table 6-2]

[0096]
[Table 7-1]

[0097]
[Table 7-2]

[0098]
[Table 8-1]

[0099]
[Table 8-2]

[0100]
　Local softening unit deformation is concentrated at the time of a collision, to become a factor of cracking, variation in the hardness in the molding body is small, that is, to secure stable strength to ensure the anti-collision properties in is important. Therefore, in this embodiment, it was also evaluated in terms of hardness variation crash-resisting properties of the hot stamping molding. The cross section perpendicular to the longitudinal direction of the elongated hot stamping molded article, taken at an arbitrary position in the longitudinal direction, to measure the hardness of the plate thickness center position of the entire cross-section area including the vertical wall. Using Vickers tester for measuring, measuring load is 1 kgf, measurement interval was set to 1 mm. Hardness variation is less the case where there is no measurement point below the 100Hv from the average value of all the measurement points, i.e. excellent strength stability, the result was a pass as excellent crashworthiness characteristics (○) as there is a measurement point below 100Hv case was judged as failure (×). More specifically, the difference in hardness mean value of all the measurement points (average cross-sectional hardness in Table 8) and the minimum hardness value of all the measurement points is regarded as acceptable if: 100 Hv, the case of 100 Hv than It was rejected.
[0101]
　As in Example A, a tensile strength of not less than 1500 MPa, the maximum bending angle (°) is not less 70 (°) or more and a case where hydrogen embrittlement resistance becomes accepted, crashworthiness characteristics and it was evaluated as an excellent hot stamp moldings hydrogen embrittlement resistance (examples in Table 8). Furthermore the average cross-sectional hardness - where minimum hardness is less than 100 Hv, crashworthiness characteristics in terms of strength stability in addition to the bending resistance was evaluated as improved hot stamping member (except Example 111 in Table 8 example of). On the other hand, "tensile strength", of the requirements of the "maximum bending angle" and "hydrogen embrittlement resistance", if that does not satisfy one or the sample of Comparative Example.
[0102]
Example C (Si: 0.50%, greater than 3.00% less)
　after removing the surface oxide by grinding the surface of the base material steel plate having a chemical composition shown in Table 9, on both surfaces or one surface the surface layer steel sheet having a chemical composition shown in Table 10 were laminated in arc welding. Incidentally, the thickness of the sum of the surface layer for steel sheet and the base material steel plate after arc welding and 200 mm ~ 300 mm, the thickness of the surface layer for steel sheet, about 1/3 of the thickness of the base steel sheet (in the case of one-sided 1 / about 4) to. Production No. 201-236 and 238-240 are steel with a welded surface steel plate on both sides, production No. 237 is a steel with a welded surface layer steel plate only on one side. The laminated steel plate subjected to hot rolling and / or cold rolling are shown in Table 11, the resulting steel sheet is subjected to a heat treatment shown in Table 11, it performs hot stamping, to produce a molded body. Table 12 shows the microstructure and mechanical properties of the steel sheet after hot stamping (hot stamping molded article). The position of the sheet thickness 1/2 of the samples taken from the steel sheet after the hot stamping, and the component composition of the analysis of the position of 20 [mu] m (position in the surface layer) from the surface, the base steel sheet and shown in Tables 9 and 10 It was equivalent to composition of the surface layer steel plate.
[0103]
[Table 9-1]

[0104]
[Table 9-2]

[0105]
[Table 10-1]

[0106]
[Table 10-2]

[0107]
[Table 11-1]

[0108]
[Table 11-2]

[0109]
[Table 12-1]

[0110]
[Table 12-2]

[0111]
　In this embodiment, the anti-collision properties of a hot stamping molded article was evaluated in terms of ductility. Specifically, to evaluate the crashworthiness characteristics seeking uniform elongation of the steel sheet by a tensile test of the steel sheet after the hot stamping. Tensile test, to prepare a No. 5 test piece described in JIS Z 2201, was performed according to the test method described in JIS Z 2241, and the uniform elongation of the elongation maximum tensile load was obtained.
[0112]
　As in Example A, a tensile strength of not less than 1500 MPa, the maximum bending angle (°) is not less 70 (°) or more and a case where hydrogen embrittlement resistance becomes accepted, crashworthiness characteristics and it was evaluated as an excellent hot stamp moldings hydrogen embrittlement resistance (examples in Table 12). Further where uniform elongation is 5% or more, crashworthiness characteristics in terms of ductility in addition to bending resistance was evaluated as improved hot stamping member (embodiments other than 210 and 211 in Table 12) . On the other hand, "tensile strength", of the requirements of the "maximum bending angle" and "hydrogen embrittlement resistance", if that does not satisfy one or the sample of Comparative Example.
[0113]
EXAMPLE D (Mn: 1.50% to less than 3.00%, and Si: 0.50% greater, less than 3.00%)
　grinding the surface of the base material steel plate having a chemical composition shown in Table 13 after removal of the surface oxide and, by laminating a surface layer for steel sheet having a chemical composition shown in Table 14 on both surfaces or one surface in arc welding. Incidentally, the thickness of the sum of the surface layer for steel sheet and the base material steel plate after arc welding and 200 mm ~ 300 mm, the thickness of the surface layer for steel sheet, about 1/3 of the thickness of the base steel sheet (in the case of one-sided 1 / about 4) to. Production No. 301-339 and 341-343 are steel with a welded surface steel plate on both sides, production No. 340 is a steel with a welded surface layer steel plate only on one side. The laminated steel plate subjected to hot rolling and / or cold rolling are shown in Table 15, the resulting steel sheet is subjected to a heat treatment shown in Table 15, it performs hot stamping, to produce a molded body. Table 16 shows the microstructure and mechanical properties of the steel sheet after hot stamping (hot stamping molded article). The position of the sheet thickness 1/2 of the samples taken from the steel sheet after the hot stamping, and the component composition of the analysis of the position of 20μm from the surface (the position in the surface layer) is the base material steel plate and respectively shown in Tables 13 and 14 It was equivalent to composition of the surface layer steel plate.
[0114]
[Table 13-1]

[0115]
[Table 13-2]

[0116]
[Table 14-1]

[0117]
[Table 14-2]

[0118]
[Table 15-1]

[0119]
[Table 15-2]

[0120]
[Table 16-1]

[0121]
[Table 16-2]

[0122]
　In this embodiment, as in the case of Example B, it was also evaluated in terms of hardness variation crash-resisting properties of the hot stamping molding. The cross section perpendicular to the longitudinal direction of the elongated hot stamping molded article, taken at an arbitrary position in the longitudinal direction, to measure the hardness of the plate thickness center position of the entire cross-section area including the vertical wall. Using Vickers tester for measuring, measuring load is 1 kgf, measurement interval was set to 1 mm. Hardness variation is less the case where there is no measurement point below the 100Hv from the average value of all the measurement points, i.e. excellent strength stability, the result was a pass as excellent crashworthiness characteristics (○) as there is a measurement point below 100Hv case was judged as failure (×). More specifically, the difference in hardness mean value of all the measurement points (average cross-sectional hardness in Table 16) and the minimum hardness value of all the measurement points is regarded as acceptable if: 100 Hv, the case of 100 Hv than It was rejected.
[0123]
　Further, in this embodiment, as in Example C, and the crashworthiness properties of hot stamping member was evaluated in terms of ductility. Specifically, to evaluate the crashworthiness characteristics seeking uniform elongation of the steel sheet by a tensile test of the steel sheet after the hot stamping. Tensile test, to prepare a No. 5 test piece described in JIS Z 2201, was performed according to the test method described in JIS Z 2241, and the uniform elongation of the elongation maximum tensile load was obtained.
[0124]
　As in Example A, a tensile strength of not less than 1500 MPa, the maximum bending angle (°) is not less 70 (°) or more and a case where hydrogen embrittlement resistance becomes accepted, crashworthiness characteristics and it was evaluated as an excellent hot stamp moldings hydrogen embrittlement resistance (examples in Table 16). Further uniform elongation is not less than 5% and an average cross-sectional hardness - where minimum hardness is less than 100 Hv, as a hot stamping molded article crashworthiness characteristics in terms of ductility and strength stability is improved in addition to the bending resistance evaluated (embodiments other than 310, 311 and 313-315 in Table 16). On the other hand, "tensile strength", of the requirements of the "maximum bending angle" and "hydrogen embrittlement resistance", if that does not satisfy one or the sample of Comparative Example.

The scope of the claims
[Requested item 1]And the plate thickness center part, a hot stamp shaped body comprising a surface layer disposed on both sides or one side of the plate thickness center part,
the hot stamping molded body, between said plate thickness central portion and the surface layer include an intermediate layer formed adjacent thereto further
　the thickness center portion, in
　mass%, C: 0.20% to less than 0.70%
　Si: less than
　3.00%, Mn: 0. more than 20%, less than 3.00%
　P: 0.10% or
　less, S: 0.10% or
　less, sol. Al: 0.0002% or more, 3.0000% or
　less, N: contains 0.01% or less,
　the balance being Fe and unavoidable impurities,
　the hardness of the plate thickness center portion than 500 Hv, below 800Hv There,
　the thickness direction of the hardness of change [Delta] H in the surface layer 1 is more 10HV, less than 200 Hv,
the change in the hardness of the plate thickness direction of the intermediate layer [Delta] H 2 is more than 50 Hv, and wherein less than 200 Hv to, hot stamping body.
[Requested item 2]And the Si content is 0.50% or less in the thickness center portion, and wherein the Mn content in the plate thickness central portion of 0.20% or more and less than 1.50%, according to claim 1 hot stamping body according to.
[Requested item 3]The plate is the Si content is 0.50% or less in the thickness center portion, Mn content in the plate thickness central portion of 1.50% or more, and less than 3.00%, according to claim 1 hot stamping body according to.
[Requested item 4]The plate Si content in the thickness center portion 0.50 percent, less than 3.00% Mn content is 0.20% or more in the thickness center portion is less than 1.50%, the plate thick central portion, an area fraction of 1.0% or more, characterized in that it comprises a residual austenite of less than 5.0%, hot stamping molded article according to claim 1.
[Requested item 5]The plate Si content in the thickness center portion 0.50 percent, less than 3.00%, Mn content is 1.50% or more in the thickness center portion is less than 3.00%, the plate thick central portion, an area fraction of 1.0% or more, characterized in that it comprises a residual austenite of less than 5.0%, hot stamping molded article according to claim 1.
[Requested item 6]The thickness center portion, further, by mass% Ni: 0.01% or more, characterized in that it contains less 3.00%, hot stamping according to any one of claims 1 to 5 molded body.
[Requested item 7]The thickness center portion, further, by mass%, Nb: 0.010% or more 0.150% or less, Ti: 0.010% or more 0.150% or less, Mo: 0.005% or more, 1 .000% or less, and B: 0.0005% or more, characterized in that it contains one or more 0.0100%, hot stamping according to any one of claims 1 to 6 molded body.
[Requested item 8]
And further comprising a plating layer on each surface of the surface layer, hot stamping molded article according to any one of claims 1 to 7.