[0001]The present invention relates to a hot stamped article. The present application claims priority based on Japanese Patent Application No. 2019-101988 filed in Japan on May 31, 2019, the contents of which are incorporated herein by reference.
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 the application of high-strength steel sheets to automobile parts is accelerating. Automotive members are manufactured by press forming, but as the strength of steel sheets increases, not only the forming load increases, but also the formability decreases. Therefore, in high-strength steel sheets, the formability into members with complicated shapes Is an issue. In order to solve such problems, the application of hot stamping technology in which press forming is performed after heating to a high temperature in the austenite region where the steel sheet softens is being promoted. Hot stamping is attracting attention as a technology that achieves both formability into automobile members and ensuring the strength of automobile members by performing quenching in the mold at the same time as press working.
[0003]
　However, the conventional hot stamping compact manufactured by hot stamping has a high dislocation density because the entire area in the plate thickness direction is formed of a hard structure (mainly martensite). When the dislocation density is high, the hydrogen embrittlement sensitivity is also high, so that hydrogen embrittlement cracking occurs even with a small amount of hydrogen. Therefore, improvement of hydrogen embrittlement resistance may be an issue.
[0004]
　In Patent Document 1, by controlling the cooling rate from finish rolling to winding in the hot rolling process, the crystal orientation difference in bainite is controlled to 5 to 14 °, and the deformability such as elongation flangeability is defined. Techniques for improvement are disclosed.
[0005]
　Patent Document 2 describes a technique for improving local deformability by controlling the strength of a specific crystal orientation group among ferrite crystal grains by controlling the manufacturing conditions from finish rolling to winding in the hot rolling process. Is disclosed.
[0006]
　Patent Document 3 describes voids formed at the interface between ZnO and the steel sheet and the interface between ZnO and the Zn-based plating layer during heating before hot pressing by heat-treating the hot stamping steel sheet to form ferrite on the surface layer. There is disclosed a technique for improving perforated corrosion resistance and the like by reducing the amount of zinc oxide.
[0007]
　However, the above techniques may not provide sufficient strength and hydrogen embrittlement resistance.
Prior art literature
Patent documents
[0008]
Patent Document 1: International Publication No. 2016/132545
Patent Document 2: Japanese Patent Application Laid-Open No. 2012-172203
Patent Document 3: Japanese Patent No. 5861766
Non-patent literature
[0009]
Non-Patent Document 1: T.I. Ungar, 3 outsiders, Journal of Applied Crystallography, 1999, Vol. 32, pp. 992-1002
Outline of the invention
Problems to be solved by the invention
[0010]
　It is an object of the present invention to provide a hot stamped body having excellent strength and hydrogen embrittlement resistance in view of the problems of the prior art.
Means to solve problems
[0011]
　As a result of diligent studies on a method for solving the above problems, the present inventors have obtained the following findings.
[0012]
　The present inventors investigated the hydrogen embrittlement resistance property of the hot stamp molded product. As a result, the present inventors have found that the metal structure of the surface layer region of the base steel sheet constituting the hot stamped body contains martensite in an area% of 90.0% or more, and Ni in the former austenite grain boundaries of the surface layer region. It was found that when the concentration is 5.5% by mass or more, the hydrogen embrittlement resistance property of the hot stamped compact is improved.
[0013]
　Further, in order to obtain the above-mentioned metal structure in the surface layer region of the base steel sheet constituting the hot stamped body, the present inventors set the average dislocation density to 4 × in the surface layer region of the hot stamping steel sheet before hot stamping. 10 15 m / m 3 or more, the ratio of one or more crystal grains of martensite and lower bainite not subjected to autotempering is 15.0% or more in area%, and hot stamping under predetermined conditions. It was found that it is necessary to apply.
[0014]
　The present invention has been further studied based on the above findings, and the gist thereof is as follows.
[1] The hot stamp molded product according to one aspect of the present invention has, as a chemical component,
C: 0.15% or more, less than 0.70%,
Si: 0.005% or more, 0.250% in mass%. Hereinafter,
Mn: 0.30% or more, 3.00% or less,
sol. Al: 0.0002% or more, 0.500% or less,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% or more, 0.150% or less,
Ti: 0% or more, 0.150% or less,
Mo: 0% or more, 1.000% or less,
Cr: 0% or more, 1.000% or less,
B: 0% or more, 0.0100% or less,
Ca: Amount of adhesion per side to the base steel sheet containing 0 % or more, 0.0100% or less and
REM: 0% or more, 0.30% or less
, and the balance being Fe and impurities, and
　the surface of the base steel sheet. Is 10 g / m 2 or more, 90 g / m 2It is a region having a Ni content of 10% by mass or more and 25% by mass or less, a plating layer composed of Zn and impurities as a balance, and a
　depth of 50 μm from the surface of the base steel sheet. The metallographic structure of the surface layer region contains martensite of 90.0% or more in area%, and
　the Ni concentration of the former austenite grain boundaries in the surface layer region is 5.5% by mass or more. [2] The hot stamp molded body according to the above [1] has Nb: 0.010% or more, 0.150% or less, Ti: 0.010%
in mass% as a chemical component on the base steel sheet. Above, 0.150% or less, Mo: 0.005% or more, 1.000% or less, Cr: 0.005% or more, 1.000% or less, B: 0.0005% or more, 0.0100% or less, It may contain one or more selected from the group consisting of Ca: 0.0005% or more and 0.0100% or less and REM: 0.0005% or more and 0.30% or less.

The invention's effect
[0015]
　According to the above aspect according to the present invention, it is possible to provide a hot stamp molded product having excellent strength and hydrogen embrittlement resistance.
A brief description of the drawing
[0016]
[Fig. 1] Fig. 1 is a diagram showing a test piece used for measuring the Ni concentration of the old austenite grain boundaries.
[Fig. 2] Fig. 2 is a diagram showing a test piece used for evaluation of hydrogen embrittlement resistance of Examples.
Embodiment for carrying out the invention
[0017]
　In the hot stamped body according to the present embodiment, the metal structure of the surface layer region, which is a region from the surface of the base steel sheet constituting the hot stamped body to a depth of 50 μm, has martensite in an area% of 90.0% or more. It is characterized by having a Ni concentration of the former austenite grain boundaries in the surface layer region of 5.5% by mass or more. By having this feature, it is possible to obtain a hot stamped compact having excellent strength and hydrogen embrittlement resistance. In addition, in this embodiment, "having excellent strength" means that the tensile (maximum) strength is 1500 MPa or more.
[0018]
　As a result of diligent studies, the present inventors have found that a hot stamped molded product having the above metal structure can be obtained by the following method.
[0019]
　As a first step, in the hot rolling process, cooling is started so that the average cooling rate on the surface of the base steel sheet becomes 80 ° C./s or more within 5 seconds after the finish rolling is completed, and the temperature range is less than 500 ° C. Cool to and wind up. Even after winding, continue water cooling until it reaches room temperature (about 40 ° C or less). As described above, by increasing the average cooling rate and lowering the winding temperature as compared with the prior art, it is possible to suppress the formation of carbides, the ferrite transformation and the bainite transformation. As a result, in the metal structure of the surface layer region of the hot stamping steel sheet, the ratio of one or two types of crystal grains of martensite and lower bainite that have not been auto-tempered is 15.0% or more in area%, and the surface layer. The average dislocation density of the region can be 4 × 10 15 m / m 3 or more.
[0020]
　As the second step, a Zn-based plating layer containing 10 to 25% by mass of Ni is formed on the surface of the base steel sheet so that the adhesion amount per side is 10 to 90 g / m 2 , and the steel sheet for hot stamping is formed. do.
[0021]
　As a third step, by controlling the average heating rate of heating before hot stamping, Ni in the plating layer arranged on the surface of the base steel sheet is diffused into the surface layer region of the base steel sheet.
[0022]
　In general, hot-rolled steel sheets containing 0.15% by mass or more of C, having a metal structure containing martensite, and having a high dislocation density that is not tempered, have significantly deteriorated ductility, toughness, and hydrogen embrittlement resistance. do. In addition, when cold rolling is performed after winding, the hot-rolled steel sheet as described above is not excellent in ductility, so that cracks are likely to occur. Therefore, the hot-rolled steel sheet as described above is generally tempered after hot rolling and before going to a subsequent process. In order to improve the bendability and hydrogen embrittlement resistance of hot-rolled steel sheets, it is important to improve the ductility of the surface layer region. Therefore, the above-mentioned steel sheets are treated to soften the surface layer region (for example, surface decarburization). Processing, etc.) may be applied.
[0023]
　Further, in general, when a steel sheet containing 0.15% by mass or more of C is hot-stamped, the hydrogen embrittlement resistance of the hot-stamped compact may not be excellent.
[0024]
　However, in the present embodiment, the metal structure of the surface layer region of the hot stamping steel sheet is in a preferable state, and Ni in the plating layer arranged on the surface of the base steel sheet is diffused into the surface layer region of the base steel sheet by heating before hot stamping. By doing so, the hydrogen embrittlement resistance of the hot stamped compact can be improved without tempering after hot stamping.
[0025]
　The metal structure of the surface layer region of the hot stamping steel sheet applied to the hot stamping compact according to the present embodiment is composed of one or two crystal grains of martensite and lower bainite that have not been autotempered in an area% of 15. Includes 0.0% or more. The old austenite grain boundaries of martensite and lower bainite that have not been auto-tempered have few grain boundary segregating elements such as C and precipitates, so Ni is likely to diffuse. Therefore, when the average heating rate of heating before hot stamping is increased, Ni can be preferentially diffused to the old austenite grain boundaries. The present inventors set the average heating rate before hot stamping to 100 ° C./s or more and less than 200 ° C./s, and preferentially diffuse Ni to the old austenite grain boundaries in the surface layer region of the base steel plate. It was found that the old austenite grain boundaries hinder hydrogen intrusion and can improve the hydrogen embrittlement resistance of the hot stamped product.
[0026]
　Hereinafter, the hot stamp molded body and the manufacturing method thereof according to the present embodiment will be described in detail. First, the reason for limiting the chemical composition of the base steel sheet constituting the hot stamped body according to the present embodiment will be described.
　The numerical limit range described below includes the lower limit value and the upper limit value. Numerical values ​​indicated as "less than" and "greater than" do not include the value in the numerical range. All% of the chemical composition indicate mass%.
[0027]
　The base steel sheet constituting the hot stamped body according to the present embodiment has, as a chemical component, C: 0.15% or more, less than 0.70%, Si: 0.005% or more, 0.250 in mass%. % Or less, Mn: 0.30% or more, 3.00% or less, sol. Al: 0.0002% or more, 0.500% or less, P: 0.100% or less, S: 0.1000% or less and N: 0.0100% or less, balance: Fe and impurities are contained.
[0028]
"C: 0.15% or more and less than 0.70%"
　C is an important element for obtaining a tensile strength of 1500 MPa or more in a hot stamp molded product. If the C content is less than 0.15%, the martensite becomes soft and it is difficult to obtain a tensile strength of 1500 MPa or more. Further, when the C content is less than 0.15%, the martensite and the lower portion that have not been auto-tempered in the metal structure of the surface layer region of the hot stamping steel sheet applied to the hot stamping compact according to the present embodiment. The bainite area ratio becomes smaller. Therefore, the C content is set to 0.15% or more. The C content is preferably 0.20% or more, and more preferably 0.30% or more. On the other hand, when the C content is 0.70% or more, coarse carbides are generated and fracture is likely to occur, and the bendability and hydrogen embrittlement resistance of the hot stamped compact are deteriorated. Therefore, the C content is set to less than 0.70%. The C content is preferably 0.50% or less, more preferably 0.45% or less.
[0029]
"Si: 0.005% or more, 0.250% or less"
　Si is an element contained in order to ensure hardenability. If the Si content is less than 0.005%, the above effect cannot be obtained, and in the hot stamping steel sheet, the dislocation density may decrease or martensite and lower bainite that have not been autotempered may not be obtained. The desired metal structure cannot be obtained in the stamped body. Therefore, the Si content is set to 0.005% or more. Since the above effect is saturated even if Si of more than 0.250% is contained, the Si content is set to 0.250% or less. The Si content is preferably 0.210% or less.
[0030]
"Mn: 0.30% or more, 3.00% or less"
　Mn is an element that contributes to the improvement of the strength of the hot stamp molded product by strengthening the solid solution. When the Mn content is less than 0.30%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to obtain a tensile strength of 1500 MPa or more in the hot stamped compact. Therefore, the Mn content is set to 0.30% or more. The Mn content is preferably 0.50% or more, and 0.70% or more. On the other hand, when the Mn content is more than 3.00%, coarse inclusions are generated in the steel and fracture is likely to occur, and the bendability and hydrogen embrittlement resistance of the hot stamped body are deteriorated. The upper limit is .00%. The Mn content is preferably 2.50% or less, or 2.00% or less.
[0031]
"Sol.Al (acid-soluble Al): 0.0002% or more, 0.500% or less"
　Al deoxidizes molten steel to make the steel sound (suppresses the occurrence of defects such as blow holes in the steel). ) It is an element that has an action. sol. If the Al content is less than 0.0002%, deoxidation is not sufficiently performed and the above effect cannot be obtained. The Al content is 0.0002% or more. sol. The Al content is preferably 0.0010% or more, or 0.0020% or more. On the other hand, sol. When the Al content exceeds 0.500%, coarse oxides are formed in the steel, and the bendability and hydrogen embrittlement resistance of the hot stamped body are deteriorated. Therefore, sol. The Al content is 0.500% or less. sol. The Al content is preferably 0.400% or less, or 0.300% or less.
[0032]
"P: 0.100% or less"
　P is an element that segregates at the grain boundaries and reduces the strength of the grain boundaries. When the P content exceeds 0.100%, the strength of the grain boundaries is remarkably lowered, and the bendability and hydrogen embrittlement resistance of the hot stamped compact are lowered. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. The lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the cost of removing P is significantly increased, which is economically unfavorable. Therefore, 0.0001% may be set as the lower limit in actual operation.
[0033]
"S: 0.1000% or less"
　S is an element that forms inclusions in steel. When the S content exceeds 0.1000%, a large amount of inclusions are formed in the steel, and the bendability and hydrogen embrittlement resistance of the hot stamped body are deteriorated. Therefore, the S content is set to 0.1000% or less. The S content is preferably 0.0050% or less. The lower limit of the S content is not particularly limited, but if it is reduced to less than 0.00015%, the cost of removing S is significantly increased, which is economically unfavorable. Therefore, 0.00015% may be set as the lower limit in actual operation.
[0034]
"N: 0.0100% or less"
　N is an impurity element, which is an element that forms a nitride in steel and deteriorates the toughness and hydrogen embrittlement resistance of the hot stamped product. When the N content exceeds 0.0100%, coarse nitrides are formed in the steel, and the bendability and hydrogen embrittlement resistance of the hot stamped body are significantly deteriorated. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0075% or less. The lower limit of the N content is not particularly limited, but if it is reduced to less than 0.0001%, the N removal cost is significantly increased, which is economically unfavorable. Therefore, 0.0001% may be set as the lower limit in actual operation.
[0035]
　The balance of the chemical composition of the base steel sheet constituting the hot stamped body according to the present embodiment 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 compact according to the present embodiment.
[0036]
　Further, the Ni content of the base steel sheet constituting the hot stamp molded body according to the present embodiment is less than 0.005%. Since Ni is an expensive element, in the present embodiment, the cost can be kept low as compared with the case where Ni is intentionally contained and the Ni content is 0.005% or more.
[0037]
　The base steel sheet constituting the hot stamp molded body according to the present embodiment may contain the following elements as optional elements. When the following optional elements are not contained, the content is 0%.
[0038]
"Nb: 0% or more, 0.150% or less" Since
　Nb is an element that contributes to the improvement of the strength of the hot stamp molded product by solid solution strengthening, it may be contained as necessary. When Nb is contained, the Nb content is preferably 0.010% or more in order to surely exert the above effect. The Nb content is more preferably 0.035% or more. On the other hand, even if Nb is contained in an amount of more than 0.150%, the above effect is saturated, so that the Nb content is preferably 0.150% or less. The Nb content is more preferably 0.120% or less.
[0039]
"Ti: 0% or more, 0.150% or less" Since
　Ti is an element that contributes to the improvement of the strength of the hot stamp molded product by strengthening the solid solution, it may be contained if necessary. When Ti is contained, the Ti content is preferably 0.010% or more in order to surely exert the above effect. The Ti content is preferably 0.020% or more. On the other hand, since the above effect is saturated even if the content exceeds 0.150%, the Ti content is preferably 0.150% or less. The Ti content is more preferably 0.120% or less.
[0040]
"Mo: 0% or more, 1.000% or less"
　Mo is an element that contributes to the improvement of the strength of the hot stamp molded product by strengthening the solid solution, and may be contained as necessary. When Mo is contained, the Mo content is preferably 0.005% or more in order to surely exert the above effect. The Mo content is more preferably 0.010% or more. On the other hand, since the above effect is saturated even if the content exceeds 1.000%, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.800% or less.
[0041]
"Cr: 0% or more, 1.000% or less"
　Cr is an element that contributes to the improvement of the strength of the hot stamped molded product by strengthening the solid solution, and may be contained as necessary. When Cr is contained, the Cr content is preferably 0.005% or more in order to surely exert the above effect. The Cr content is more preferably 0.100% or more. On the other hand, since the above effect is saturated even if the content exceeds 1.000%, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less.
[0042]
"B: 0% or more, 0.0100% or less"
　Since B is an element that segregates at the grain boundaries and improves the strength of the grain boundaries, it may be contained as necessary. When B is contained, the B content is preferably 0.0005% or more in order to surely exert the above effect. The B content is preferably 0.0010% or more. On the other hand, since the above effect is saturated even if the content exceeds 0.0100%, the B content is preferably 0.0100% or less. The B content is more preferably 0.0075% or less.
[0043]
"Ca: 0% or more, 0.0100% or less"
　Ca is an element having an action of deoxidizing molten steel to make the steel sound. In order to ensure this effect, the Ca content is preferably 0.0005% or more. On the other hand, since the above effect is saturated even if the content exceeds 0.0100%, the Ca content is preferably 0.0100% or less.
[0044]
"REM: 0% or more, 0.30% or less"
　REM is an element having an action of deoxidizing molten steel to make the steel sound. In order to ensure this effect, the REM content is preferably 0.0005% or more. On the other hand, since the above effect is saturated even if the content exceeds 0.30%, the REM content is preferably 0.30% or less.
　In the present embodiment, REM refers to a total of 17 elements composed of Sc, Y and lanthanoids, and the content of REM refers to the total content of these elements.
[0045]
　The chemical composition of the hot stamped product described above may be measured by a general analytical method. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrum) may be used for measurement. In addition, C and S may be measured by using the combustion-infrared absorption method, and N may be measured by using the inert gas melting-thermal conductivity method. The plating layer on the surface may be removed by mechanical grinding and then the chemical composition may be analyzed.
[0046]
　Next, the metal structure and the plating layer of the steel sheet constituting the hot stamping steel sheet applied to the hot stamping body according to the present embodiment will be described.
[0047]

"The metal structure of the surface layer region, which is the region from the surface of the base steel to a depth of 50 μm, covers the area of ​​one or two types of crystal grains of martensite and lower bainite that have not been autotempered. 15.0% or more in%, and the average dislocation density of the surface layer region is 4 × 10 15 m / m 3 or more. ”
　In the metal structure of the surface layer region, which is the region from the surface of the base steel to a depth of 50 μm, the autotemper The ratio of one or two crystal grains of untreated martensite and lower bainite shall be 15.0% or more in area%, and the average dislocation density in the surface layer region shall be 4 × 10 15 m / m 3 or more. As a result, Ni in the plating layer can be diffused into the surface layer region of the steel sheet by heating before hot stamping. The upper limit of the average dislocation density in the surface layer region is not particularly limited, and may be, for example, 5 × 10 17 m / m 3 or less, or 1 × 10 18 m / m 3 or less.
[0048]
　When the average heating rate of heating before hot stamping is controlled to 100 ° C./s or more and less than 200 ° C./s, the Ni in the plating layer is the old austenite grain boundary in the surface layer region of the base steel sheet constituting the hot stamped body. Priority is given to spreading. The old austenite grain boundaries in which Ni is diffused become an obstacle to hydrogen intrusion, and the hydrogen embrittlement resistance of the hot stamped compact can be improved.
[0049]
　In order to obtain the above effects, the ratio of one or two crystal grains of martensite and lower bainite that have not undergone autotempering in the surface layer region is set to 15.0% or more in area%. The ratio of these crystal grains is area%, preferably 20.0% or more. From the viewpoint of suppressing the occurrence of cracks during cold rolling in the subsequent step, the ratio of these crystal grains may be 30.0% or more in terms of area%. The upper limit of the ratio of one or two crystal grains of untempered martensite and lower bainite in the metal structure of the surface layer region is not particularly limited, and may be, for example, 50% or less in terms of area%. However, it may be 90% or less. In addition, the metallographic structure in the surface layer region includes one or more of ferrite, upper bainite, retained austenite, and autotempered martensite as the residual structure other than martensite and lower bainite that have not been autotempered. It may be included.
[0050]
　The metal structure of the central portion of the base steel sheet is not particularly limited, but is usually one or more of ferrite, upper bainite, lower bainite, martensite, retained austenite, iron carbide and alloy carbide. Here, the central portion of the base steel plate is from a position 0.2 mm in the center direction of the plate thickness from one surface of the base steel plate to a position 0.2 mm in the center direction of the plate thickness from the other surface of the base steel plate. Say the part of.
[0051]
"Measurement of Area Fractions of Crystal Grains of Martensite and Lower Bainite Not Subjected to Autotemperation" In
　the surface layer region of the base steel sheet constituting the hot stamping steel sheet applied to the hot stamping compact according to the present embodiment. A method for measuring the area fraction of martensite and lower bainite crystal grains that have not been auto-tempered will be described.
[0052]
　First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot stamping steel plate so that a cross section in the rolling direction (plate thickness cross section) perpendicular to the surface can be observed. The size of the sample depends on the measuring device, but is set to a size that can be observed by about 10 mm in the rolling direction. After polishing the measurement surface of the sample corresponding to the rolling direction cross section using silicon carbide paper of # 600 to # 1500, the measurement surface is subjected to diamond powder having a particle size of 1 to 6 μm in a diluted solution such as alcohol or pure water. Use the dispersed liquid to finish the mirror surface. Next, polishing at room temperature with colloidal silica containing no alkaline solution for 8 minutes to remove the strain present on the surface layer of the sample. Then, using a cross section polisher manufactured by JEOL Ltd., the measurement surface is sputtered with an argon ion beam. At this time, for the purpose of suppressing the occurrence of streaky irregularities on the measurement surface, an argon ion beam is irradiated to the measurement surface from the direction of 360 degrees using a sample rotation holder manufactured by JEOL Ltd.
[0053]
　A region with a length of 50 μm and a depth of 50 μm from the interface between the plating layer and the surface of the base steel sheet is measured at an arbitrary position in the rolling direction of the measurement surface by the electron backscattering diffraction method at a measurement interval of 0.1 μm. Obtain crystal orientation information. For the measurement, a device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the apparatus is 9.6 × 10 -5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation time of the electron beam is 0.5 seconds / point. The obtained crystal orientation information is analyzed using the "Grain Average Image Quality" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. With this function, the sharpness of the crystal orientation information can be quantified as an IQ value, and it is possible to discriminate a structure that has not been auto-tempered. Martensite and lower bainite that have not been auto-tempered have low IQ values ​​due to poor crystallinity. The area where the IQ value is calculated to be 60,000 or less by the "Grain Average Image Quality" function is defined as martensite or lower bainite which has not been auto-tempered, and its surface integral is calculated. By the method described above, the area% of the crystal grains of martensite and lower bainite that have not undergone autotempering in the surface layer region is obtained.
[0054]
"Measurement of average dislocation density"
　Next, a method for measuring the average dislocation density in the surface layer region will be described. The average dislocation density can be measured by X-ray diffraction method or transmission electron microscope observation, but in this embodiment, it is measured by using X-ray diffraction method.
[0055]
　First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the base steel sheet. The size of the sample depends on the measuring device, but is about 20 mm square. Using a mixed solution of 48% distilled water, 48% hydrogen peroxide solution, and 4% hydrofluoric acid, the front surface and the back surface of the sample are each reduced by 25 μm, and the total thickness is reduced by 50 μm. This exposes a region of 25 μm from the surface of the sample before thickness reduction. X-ray diffraction measurements are performed on this exposed surface to identify multiple diffraction peaks in the body-centered cubic lattice. By analyzing the average dislocation density from the half width of these diffraction peaks, the average dislocation density in the surface layer region is obtained. As the analysis method, the modified Williamson-Hall method described in Non-Patent Document 1 is used.
[0056]
"Plating layer having an adhesion amount of 10 g / m 2 or more and 90 g / m 2 or less per surface, a Ni content of 10% by mass or more and 25% by mass or less, and a balance of Zn and impurities"
　This embodiment . The hot stamping steel sheet applied to the hot stamping molded body according to the above has an adhesion amount of 10 g / m 2 or more and 90 g / m 2 or less per side to the surface of the base steel sheet constituting the hot stamping steel sheet. The Ni content is 10% by mass or more and 25% by mass or less, and the balance has a plating layer composed of Zn and impurities. This makes it possible to diffuse Ni into the surface layer region during heating before hot stamping.
[0057]
　When the adhesion amount per one side of the plating layer is less than 10 g / m 2 , or the Ni content in the plating layer is less than 10% by mass, the amount of Ni concentrated in the surface layer region of the base steel sheet is reduced, and hot stamping is performed. The desired metallographic structure cannot be obtained in the later surface layer region. On the other hand, when the adhesion amount per side exceeds 90 g / m 2 , or when the Ni content in the plating layer exceeds 25% by mass, Ni is excessively concentrated at the interface between the plating layer and the base steel sheet, and plating is performed. The adhesion between the layer and the base steel sheet is lowered, Ni in the plating layer is difficult to diffuse into the surface layer region of the base steel sheet, and a desired metal structure cannot be obtained in the hot stamped molded body after hot stamping.
　The amount of adhesion of the plating layer per one side is preferably 30 g / m 2 or more, and more preferably 40 g / m 2 or more. The amount of the plating layer adhered to one side is preferably 80 g / m 2 or less, and more preferably 60 g / m 2 or less.
[0058]
　The plating adhesion amount of the hot stamping steel sheet and the Ni content in the plating layer are measured by the following methods.
　The amount of plating adhesion is measured by collecting a test piece from an arbitrary position on the hot stamping steel plate according to the test method described in JIS H 0401: 2013. The Ni content in the plating layer is determined by collecting a test piece from an arbitrary position on the hot stamping steel sheet according to the test method described in JIS K 0150: 2005, and containing Ni at 1/2 of the total thickness of the plating layer. By measuring the amount, the Ni content of the plating layer in the hot stamping steel sheet is obtained.
[0059]
　The thickness of the hot stamping steel plate applied to the hot stamping compact according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.
[0060]
　Next, the hot stamp molded body according to this embodiment will be described.
[0061]
"The metal structure of the surface layer region, which is the region from the surface of the base steel sheet to a 　depth of
50 μm, contains martensite in an area% of 90.0% or more."
When the ratio of martensite is 90.0% or more in the area% in the metal structure of the surface layer region, which is the region up to the above, excellent strength and hydrogen embrittlement resistance can be obtained in the hot stamped compact. The higher the percentage of martensite, the better.
As the residual structure other than martensite in the surface layer region, one or more of ferrite, upper bainite, lower bainite, and retained austenite may be contained.
[0062]
"Measuring method of metal structure" A method of measuring
　the metal structure of the surface layer region, which is a region from the surface of the steel sheet to a depth of 50 μm, will be described.
　First, a sample is cut out so that a rolling direction cross section (plate thickness cross section) perpendicular to the surface can be observed from an arbitrary position 50 mm or more away from the end face of the hot stamp molded body. The size of the sample depends on the measuring device, but is set to a size that can be observed by about 10 mm in the rolling direction.
[0063]
　After polishing the measurement surface corresponding to the rolling direction cross section of the sample using silicon carbide paper of # 600 to # 1500, the measurement surface is subjected to diamond powder having a particle size of 1 to 6 μm in a diluted solution such as alcohol or pure water. The dispersed liquid is used to finish the mirror surface and perform nightal etching. Next, observation is performed using a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) with a region within 50 μm from the end portion on the surface side of the base metal steel plate on the observation surface as an observation field of view. The surface integral of martensite can be calculated by summing the surface integrals of tempered martensite and fresh martensite. Tempering martensite is a collection of lath-shaped crystal grains, and is distinguished as a structure in which iron carbides have two or more elongation directions. Fresh martensite is not sufficiently etched by night game etching, so it can be distinguished from other structures to be etched. However, since retained austenite is not sufficiently etched like fresh martensite, the area% of fresh martensite is calculated by the difference between the surface integral of the structure not etched by night game and the surface integral of retained austenite calculated above. .. By calculating the total area% of the tempered martensite and the fresh martensite obtained by the above method, the surface integral ratio of the martensite in the surface layer region is obtained.
[0064]
　After polishing the measurement surface of the above sample (a sample different from the one used for measuring the area fraction of martensite) with silicon carbide paper of # 600 to # 1500, the measurement surface has a particle size of 1 to 6 μm. Finish the diamond powder to a mirror surface using a diluted solution such as alcohol or a liquid dispersed in pure water. Next, polishing at room temperature with colloidal silica containing no alkaline solution for 8 minutes to remove the strain present on the surface layer of the sample. Crystal orientation information is measured by electron backscatter diffraction at an arbitrary position in the rolling direction of the measurement surface of the sample, measuring a region from the surface of the base steel sheet to a depth of 50 μm at a measurement interval of 0.1 μm. To get. For the measurement, a device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the apparatus is 9.6 × 10 -5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation time of the electron beam is 0.01 seconds / point. The obtained crystal orientation information is used in the surface integral region of the retained austenite by using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Obtain the surface integral of retained austenite. If the crystal structure is an fcc structure, it is determined to be retained austenite.
[0065]
"Ni concentration at the
　former austenite grain boundaries in the surface layer region is 5.5% by mass or more" When the Ni concentration at the former austenite grain boundaries in the surface layer region is 5.5% by mass, the hot stamped body has excellent strength and hydrogen resistance. Embrittlement properties can be obtained. The Ni concentration is preferably 7.0% by mass or more. The higher the Ni concentration, the more preferable, but since it is difficult to make it 12.0% by mass or more in normal actual operation, 12.0% by mass is a practical upper limit.
[0066]
"Method for measuring Ni concentration at the
　former austenite grain boundaries in the surface layer region" A method for measuring the Ni concentration at the former austenite grain boundaries in the surface layer region will be described.
　A test piece having the dimensions shown in FIG. 1 is produced from the central portion of the hot stamped molded product after the heat treatment. A wire cutter is inserted into the notch at the center of the test piece, and the joint at the bottom of the notch is controlled from 100 μm to 200 μm. Next, the test piece is immersed in a 40% -ammonium thiocyanate solution for 24-48 hours. Within 0.5 hr after the immersion is completed, the front and back surfaces of the test piece are galvanized. After galvanizing, it is subjected to Auger electron emission spectroscopic analysis within 1.5 hours. The type of device for performing Auger electron emission spectroscopic analysis is not particularly limited. The test piece is set in the analyzer, and in a vacuum of 9.6 × 10-5 Pa or less, it is broken from the notch portion of the test piece to expose the old austenite grain boundaries. The former austenite grain boundaries exposed in the region of the surface layer 50 μm in the plate thickness direction are irradiated with an electron beam at an acceleration voltage of 1 to 30 kV, and the Ni concentration (mass%) at the grain boundaries is measured. Measurements are performed at 10 or more former austenite grain boundaries. Measurements are completed within 30 minutes of destruction to prevent grain boundary contamination. By calculating the average value of the obtained Ni concentration (mass%), the Ni concentration of the former austenite grain boundaries in the surface layer region is obtained.
[0067]
"Plating layer in which the adhesion amount per side is 10 g / m 2 or more and 90 g / m 2 or less, the Ni content is 10% by mass or more and 25% by mass or less, and the balance is Zn and impurities"
　This embodiment . The hot stamping steel sheet applied to the hot stamping body according to the above has an adhesion amount of 10 g / m 2 or more and 90 g / m 2 or less per side to the surface of the base steel sheet constituting the hot stamped body. The Ni content is 10% by mass or more and 25% by mass or less, and the balance has a plating layer composed of Zn and impurities.
[0068]
　When the adhesion amount per one side is less than 10 g / m 2 or the Ni content in the plating layer is less than 10% by mass, the amount of Ni concentrated in the surface layer region of the base steel sheet is small, and the surface layer after hot stamping is reduced. The desired metallographic structure cannot be obtained in the region. On the other hand, when the adhesion amount per side exceeds 90 g / m 2 , or when the Ni content in the plating layer exceeds 25% by mass, Ni is excessively concentrated at the interface between the plating layer and the base steel sheet, and plating is performed. The adhesion between the layer and the base steel sheet is lowered, Ni in the plating layer is difficult to diffuse into the surface layer region of the base steel sheet, and a desired metal structure cannot be obtained in the hot stamped body.
　The amount of adhesion of the plating layer per one side is preferably 30 g / m 2 or more, and more preferably 40 g / m 2 or more. The amount of the plating layer adhered to one side is preferably 80 g / m 2 or less, and more preferably 60 g / m 2 or less.
[0069]
　The plating adhesion amount and the Ni content in the plating layer of the hot stamp molded product are measured by the following methods.
　The amount of plating adhered is measured by collecting a test piece from an arbitrary position of the hot stamped molded product according to the test method described in JIS H 0401: 2013. The Ni content in the plating layer is determined by collecting a test piece from an arbitrary position of the hot stamped body according to the test method described in JIS K 0150: 2005 and containing Ni at 1/2 position of the total thickness of the plating layer. By measuring the amount, the Ni content of the plating layer in the hot stamp molded body is obtained.
[0070]
　Next, a preferable manufacturing method of a steel sheet for hot stamping applied to the hot stamping compact according to the present embodiment will be described.
[0071]
The
　steel piece (steel material) to be used for hot rolling may be any steel piece manufactured by a conventional method, and is manufactured by a general method such as a continuous casting slab or a thin slab caster. Any steel piece may be used. Rough rolling may also be performed by a general method and is not particularly limited.
[0072]
"Finish rolling"
　In the final rolling (final pass) of finish rolling, it is necessary to perform finish rolling at a rolling reduction of less than 20% in the temperature range of A3 points or more . Under the final rolling of finish rolling, when rolling at a temperature of less than A3 point or rolling with a rolling reduction ratio of 20 % or more, ferrite is generated in the surface area, and martensite and lower bainite that are not auto-tempered are 1 The ratio of seeds or two kinds of crystal grains cannot be 15.0% or more in area%. The A3 points are represented by the following equation (1).
[0073]
　A 3 points = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo ... (1) In
the above formula (1), C, N, Mn, Nb, Ti, B, Cr and Mo are , The content (mass%) of each element.
[0074]
"Cooling"
　Within 5 seconds after finishing rolling, cooling with an average cooling rate of 80 ° C./s or higher is started, and the product is cooled to a temperature range of less than 500 ° C. and wound up. Also, even after winding, continue water cooling until it reaches room temperature. When the cooling start time exceeds 5 seconds, the average cooling rate is less than 80 ° C / s, or the winding start temperature is more than 500 ° C, ferrite, pearlite, and upper bainite are likely to be formed, and in the surface layer region, The ratio of one or two crystal grains of martensite and lower bainite that have not been auto-tempered cannot be 15.0% or more in area%. The average cooling rate at this time is calculated from the temperature change on the surface of the steel sheet, and indicates the average cooling rate from the finish rolling temperature to the winding start temperature.
[0075]
"Plating"
　The above-mentioned hot-rolled steel sheet is used as it is or after being cold-rolled, the adhesion amount per side is 10 g / m 2 or more and 90 g / m 2 or less, and the Ni content is 10% by mass or more and 25. A plating layer having a mass of% or less and containing Zn and impurities as a balance is formed to obtain a steel sheet for hot stamping. When cold rolling is performed before plating is applied, the rolling reduction in cold rolling is not particularly limited, but is preferably 40 to 60% from the viewpoint of shape stability of the steel sheet. In the production of the steel sheet for hot stamping, a known production method such as pickling, tempering and rolling may be included before plating is applied. However, when tempering is performed at a temperature of Ms point -15 ° C or higher, the ratio of one or two types of crystal grains of martensite and lower bainite that have not undergone autotempering in the surface layer region is 15.0% in area%. The above cannot be achieved, and the average dislocation density cannot be 4 × 10 15 m / m 3 or more, and as a result, a hot stamped compact having a desired metal structure cannot be obtained. Therefore, if it is necessary to perform tempering before plating due to a high C content or the like, tempering is performed at a temperature of less than the Ms point −15 ° C. The Ms point is represented by the following equation (2).
[0076]
　Ms = 493-300 × C-33.3 × Mn-11.1 × Si-22.2 × Cr-16.7 × Ni-11.1 × Mo ... (2) In
the above formula (2), C, Mn, Si, Cr, Ni and Mo are the contents (mass%) of each element.
[0077]
　Next, a method for manufacturing a hot stamping molded product according to the present embodiment using the hot stamping steel sheet manufactured by the above method will be described.
[0078]
In the
　hot stamp molded product, the hot stamping steel plate obtained above is used in a temperature range of 500 ° C. or higher and A3 points or lower at 100 ° C./s or higher and less than 200 ° C./s. After heating at the average heating rate of, keep at a temperature of A 3 points or more and A 3 points + 150 ° C or less, hot stamp so that the elapsed time from the start of heating to the start of molding is within a predetermined time, and reach room temperature. Manufactured by cooling.
　Further, in order to adjust the strength of the hot stamped product, a softened region may be formed by baking a part or all of the region of the hot stamped product at a temperature lower than the Ms point −15 ° C. ..
[0079]
The surface layer is heated at an average heating rate of 100 ° C./s or more and less than 200 ° C./s in a temperature range of 　500 ° C. or higher and A3 points or lower , and the elapsed time from the start of heating to the start of molding is less than 240 seconds. The desired metallographic structure can be obtained in the region. As a result, excellent hydrogen embrittlement resistance can be obtained in the hot stamp molded product. The average heating rate is preferably 120 ° C./s or higher. The average heating rate is less than 200 ° C / s because the transformation of carbides contained in the hot stamping steel sheet to austenite is promoted while the dissolution is not completed, and the hydrogen embrittlement resistance of the hot stamping compact is deteriorated. do. The average heating rate is preferably less than 180 ° C./s. The elapsed time from the start of heating to the start of molding is preferably 120 seconds or more, and preferably 180 seconds or less.
[0080] [0080]
　The holding temperature at the time of hot stamping is preferably A 3 points + 10 ° C or higher and A 3 points + 150 ° C or lower. The average cooling rate after hot stamping is preferably 10 ° C./s or higher.
Example
[0081]
　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 based on this one condition example. 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.
[0082]
　Steel pieces manufactured by casting molten steel having the chemical compositions shown in Tables 1 and 2 are hot-rolled, cold-rolled, and plated under the conditions shown in Tables 3 and 4, and are used for hot stamping shown in Tables 3 and 4. Obtained a steel plate. The obtained steel sheets for hot stamping were subjected to the heat treatments shown in Tables 5 and 6 and hot stamped to obtain hot stamped bodies shown in Tables 5 and 6. The partially softened region was formed by irradiating a part of the hot stamp molded product with a laser to bring the irradiated portion to less than Ms-15 ° C. and baking it.
　The underline in the table indicates that the product is out of the scope of the present invention, that the manufacturing conditions are not preferable, or that the characteristic value is not preferable.
[0083]
[table 1]

[0084]
[Table 2]

[0085]
[Table 3]

[0086]
[Table 4]

[0087]
[Table 5]

[0088]
[Table 6]

[0089]
　The metallographic structure, average dislocation density and Ni concentration (content) of the hot stamping steel plate and the hot stamping compact were measured by the above-mentioned measuring method. In addition, the mechanical properties of the hot stamped body were evaluated by the following method.
[0090]
"Tensile strength"
　The tensile strength of the hot stamped body is determined by preparing the No. 5 test piece described in JIS Z 2201: 2011 from an arbitrary position of the hot stamped body and following the test method described in JIS Z 2241: 2011. rice field. If the tensile strength was less than 1500 MPa, it was judged to be unacceptable, and the test described later was not performed.
[0091]
"Hydrogen embrittlement resistance" The hydrogen embrittlement resistance
　of the hot stamped product was evaluated by the following method. FIG. 2 shows the shape of the test piece used for evaluating the hydrogen embrittlement resistance. After applying 1100 MPa with the nominal stress calculated by dividing the load by the cross-sectional area of ​​the notch bottom to the test piece of FIG. 2 having a V notch, 0.1 mA / cm in a 3 mass% NaCl aqueous solution at room temperature. A constant load test was carried out in which electrolytic hydrogen was charged at a current density of 2 for 48 hours, and the determination was made based on the presence or absence of breakage. In the table, the case without breakage is described as pass (OK), and the case with breakage is described as fail (NG). Note that R10 shown in FIG. 2 indicates that the radius of curvature is 10 mm.
[0092]
　In Tables 5 and 6, when the tensile strength is 1500 MPa or more and the hydrogen embrittlement resistance is acceptable (OK), it is judged to be the invention steel as being excellent in strength and hydrogen embrittlement resistance. If any one of the above two performances was not satisfied, it was judged to be comparative steel.
[0093]
　Looking at Tables 5 and 6, it can be seen that the hot stamped product having a chemical composition, a plating composition and a metal structure within the scope of the present invention has excellent strength and hydrogen embrittlement resistance.
　On the other hand, it can be seen that the hot stamped body in which any one or more of the chemical composition and the metal structure deviates from the present invention is inferior in one or more of the strength and the hydrogen embrittlement resistance.
Industrial applicability
[0094]
　According to the present invention, it is possible to provide a hot stamp molded product having excellent strength and hydrogen embrittlement resistance.

WE CLAIMS

As chemical components, in mass%,
C: 0.15% or more, less than 0.70%,
Si: 0.005% or more, 0.250% or less,
Mn: 0.30% or more, 3.00% or less,
sol. Al: 0.0002% or more, 0.500% or less,
P: 0.100% or less,
S: 0.1000% or less,
N: 0.0100% or less,
Nb: 0% or more, 0.150% or less,
Ti: 0% or more, 0.150% or less,
Mo: 0% or more, 1.000% or less,
Cr: 0% or more, 1.000% or less,
B: 0% or more, 0.0100% or less,
Ca: Amount of adhesion per side to the base steel sheet containing 0 % or more, 0.0100% or less and
REM: 0% or more, 0.30% or less
, and the balance being Fe and impurities, and
　the surface of the base steel sheet. Is 10 g / m 2 or more and 90 g / m 2 or less, the Ni content is 10% by mass or more and 25% by mass or less, and the balance is a plating layer made of Zn and impurities.
　The metal structure of the surface layer region, which is a region from the surface of the base steel sheet to a depth of 50 μm, contains martensite in an area% of 90.0% or more, and
　the Ni concentration of the former austenite grain boundaries in the surface layer region is 5.
A hot stamp molded body characterized by having a content of 5% by mass or more .
[Claim 2]
　The base steel sheet has, as a chemical component,
Nb: 0.010% or more, 0.150% or less,
Ti: 0.010% or more, 0.150% or less,
Mo: 0.005% or more, by mass%. 1,000% or less,
Cr: 0.005% or more, 1.000% or less,
B: 0.0005% or more, 0.0100% or less,
Ca: 0.0005% or more, 0.0100% or less and
REM:
The hot stamp molded product according to claim 1, wherein one or more selected from the group consisting of 0.0005% or more and 0.30% or less is contained .