HOT STAMPED BODYWORK

Abstract

A hot-stamped body with high strength, good flexural capacity, and resistance to crack propagation, composed of: in % by mass, C: 0.06% or more to less than 0.20%, Si: 0.010-1.00%, Mn: 0.80-2.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.010-0.500%, N: 0.010% or less, Nb: more than 0.020% to 0.10% or less, Ti: 0-0.10%, V: 0-0.10%, Cr: 0-0.50%, Mo: 0-1.00%, B: 0-0.0100%, Ni: 0-0.50%, REM: 0-0.0100%, Mg: 0-0.010%, Ca: 0-0.0100%, and Co: 0-2.0%, with the remainder: Fe and impurities, wherein a microstructure includes, in area fraction, ferrite 5-50%, and martensite: 50-95%, a proportion of regions in the martensite in which the GAIQ values ​​are 35000 or more to less than 45000 is 30% of area or more, and a maximum bending angle α (degrees) is 90 or more.

Description

HOT-Stamped Bodywork FIELD OF INVENTION

[001] The present invention relates to a hot-stamped body. STATE OF THE ART

[002] Collision safety standards for automobiles have become increasingly strict, and members or elements of the automobile are required to have improved collision performance. For the improvement of collision performance, there is a deformation-suppressing member that does not deform and maintains the shape of the member even when it receives a collision, and an impact-absorbing member that absorbs the energy of a collision by bending deformation. The former must be made of a material that has high toughness. This is because it is important that it does not deform but maintains the shape of the member even when it receives a collision. Conversely, the latter must be made of a material that has high bending ability. This is because it is important to absorb the energy of a collision by bending deformation. In recent years, a component that combines these functions has been applied to a component such as the center pillar.Specifically, a tailored member has been applied, in which a material having deformation-suppressing performance is used on its upper side of a component to keep an occupant's space stable, and a material having shock-absorbing performance on its lower side to make the member actively deform.

[003] Patent Document 1 describes an invention relating to a hot-stamped car body having a predetermined chemical composition and including a microstructure including at least one of the following: primary austenite with an average grain diameter of 3 pm or less, lower bainite, martensite, and tempered martensite, in an area fraction of 90%. According to the invention, by fixing the average grain diameter of the primary austenite at 3 pm or less and dissolving one or two of Nb and Mo in grain boundaries of the primary austenite to increase the resistance to grain boundary brittleness, a more excellent shock absorption performance than conventional ones is obtained.

[004] Patent Document 2 describes an invention relating to a pressure-hardened steel component having a predetermined chemical composition and including a steel microstructure including: less than 40% bainite, less than 5% austenite and less than 5% ferrite, the remainder being martensite, wherein the martensite contains self-tempered martensite. Patent Document 2 describes that by controlling a cooling rate of 750 to 450 °C after hot pressing at 40 to 360 °C / s and by controlling a cooling rate of 450 to 250 °C at 15 to 150 °C / s, the microstructure of the steel can be formed into a mixed structure of bainite and self-tempered martensite, and as a result, a strength of 950 to 1200 MPa at TS and a bending capacity, determined in accordance with the VDA-238 bending standard, of more than 75 degrees are obtained, which allows the energy absorbed by the impact to be improved.

[005] Patent Document 3 describes an invention relating to a steel component having a predetermined chemical composition and having a steel microstructure including at least one of 75% equiaxed ferrite, martensite in an amount of 5% or more to 20% or less, and bainite in an amount of 10% or less. LIST OF STATE OF THE ART DOCUMENTS PATENT DOCUMENT

[006] Patent document l:WO2019-186931A Patent Document 2: JP2018-527457A Patent Document 3: JP2010-521584A SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[007] According to the invention of Patent Document 1, the average grain diameter of primary austenite is controlled to 3 pm or less by controlling the conditions for hot finish rolling and a heating rate during hot stamping heating, but Patent Document 1 does not mention self-tempering of martensite.

[008] According to the invention of Patent Document 2, setting a surface proportion of self-tempered martensite of 5% or more is disclosed, but Patent Document 2 only discloses that the measurement is performed by inspecting its cross section under an optical microscope or a scanning electron microscope and performing image analysis using a known method, which is not clear. Furthermore, according to the invention of Patent Document 2, an amount of ferrite less than 5% is set to obtain a desired strength. On the contrary, according to the invention of Patent Document 3, the presence of island martensite in a ferrite matrix is ​​caused by setting an amount of ferrite to 75% or more, so that the tensile strength is improved without decreasing the ductility. However, a degree of the ductility remains at 23.5% at most.

[009] The present invention has been made to solve the problems of the state of the art and aims to provide a hot-stamped car body having a tensile strength TS of 590 MPa or more to less than 980 MPa and having excellent ductility and excellent impact energy absorption performance. ηοβ / ηη / ζζηζ / Ε / γίΛΐ

[0010] Conventionally, it has been considered that the absorption energy obtained until cracking occurs (i.e., until the bending angle is maximized) is important for improving the ductility and collision energy absorption performance of the hot-stamped body. However, according to the studies of the present inventors, it was found that it is important to increase the crack propagation resistance in addition to improving the bending capacity in order to further improve the collision energy absorption performance of the hot-stamped body.

[0011] It was found that, in order to improve ductility, it is important to make the microstructure have ferrite with an area ratio of 5 to 50%, and further, in order to increase the crack propagation resistance, it is important to increase the proportion of self-tempered martensite crystal grains (hereinafter also referred to as self-tempering amount) more than the conventional one among the martensite in the microstructure of the hot-stamped body.

[0012] Since self-annealing is a phenomenon in which crystal grains that have completed martensitic transformation are tempered in order, martensitic crystal grains that have been transformed at low temperature are difficult to temper. In addition, since martensite produced at low temperature is hard and brittle, sufficient tempering will increase the improvement scope of mechanical properties.

[0013] In order to increase the amount of self-tempering more than that of the conventional one, it was found that it is important to reduce the difference (Ms-M80) between a temperature at which martensitic transformation begins (Ms) and a temperature that brings about 80% completion of martensitic transformation (M80). In order to reduce Ms-M80, it is important to keep the interfacial pressure applied to the blank at the time of hot stamping higher than usual. Although the exact reason is unknown, it is presumed that austenite becomes unstable, and martensitic transformation tends to proceed earlier by setting the interfacial pressure at the time of hot stamping within a predetermined range. Therefore, if the interfacial pressure is applied higher than usual, most of the martensite is transformed at a relatively high temperature, and the amount of self-tempering also increases.By applying interfacial pressure in this manner, it is possible to increase the amount of self-tempered crystal grains compared to the prior art, and it is possible to improve the energy absorbed by the impact. Typically, in hot stamping, molding is performed in a state where the steel material is heated and softened. Molding at a high load increases manufacturing costs, and therefore, in the conventional method, molding has been performed with the lowest possible interfacial pressure. The present inventors have found the aforementioned novel findings contrary to such general knowledge.

[0014] The present invention has been made on the basis of the above findings, and the essential idea of ​​the present invention is the following hot stamping body. ηοβ / ηη / ζζηζ / Ε / γίΛΐ (1) A hot-stamped body comprising a chemical composition consisting of: in % by mass, C: 0.06% or more to less than 0.20%, Yes: 0.010 to 1.00%, Mn: 0.80 to 2.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.500%, N: 0.010% or less, Nb: more than 0.020% to 0.10% or less, Ti: 0 to 0.10%, V: 0 to 0.10%, Cr: 0 to 0.50%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ni: 0 to 0.50%, REM: 0 to 0.0100%, Mg: 0 to 0.010%, Ca: 0 to 0.0100%, and Co: 0 to 2.0%, and the remainder: Fe and impurities, where a microstructure includes, in area fraction, ferrite: 5 to 50%, and martensite: 50 to 95%, a proportion of regions in the martensite in which GAIQ values ​​are 35000 or more to less than 45000 is 30% by area or more, and a maximum bending angle a (degrees) in accordance with VDA238-100 of the German Automotive Industry Association is 90 or more. (2) The hot-stamped body according to the above (1), wherein the chemical composition contains one or more elements selected from, in % by mass: Ti: 0.001 to 0.10%, V: 0.001 to 0.100%, Cr: 0.010 to 0.50%, Mo: 0.010 to 1.000%, ηοβ / ηη / ζζηζ / Ε / γίΛΐ Β: 0.0001 to 0.010%, Ni: 0.001 to 0.50%, REM: 0.001 to 0.010%, Mg: 0.001 to 0.010%, Ca: 0.001 to 0.010%, and Co: 0.01 to 2.0%. (3) The hot-stamped body as described above (1) or (2), wherein a tensile strength is from 590 MPa or more to less than 980 MPa. (4) The hot stamping body according to any one of (1) to (3), wherein the hot stamping body is not heated to 350°C or more after hot stamping forming. (5) The hot-stamped body according to any one of (1) to (4), which comprises a metal coating layer on an outer layer of the hot-stamped body. ADVANTAGEOUS EFFECT OF THE INVENTION

[0015] According to the present invention, a hot-stamped car body is obtained which combines a strength of TS: 590 MPa or more to less than 980 MPa, a ductility as excellent as an elongation at break (total elongation) of 25% or more, a bending ability as excellent as a maximum bending angle determined on the basis of VDA 238-100 (April 2017 Edition) of the German Association of the Automotive Industry (hereinafter simply referred to as the maximum bending angle), denoted by a, of 90 (degrees) or more, and an excellent resistance to crack propagation is obtained. BRIEF DESCRIPTION OF THE FIGURES

[0016] Figure 1 illustrates a distribution (histogram) of GAIQ values ​​of a test sample in which self-tempered and non-self-tempered grains are intermixed. Figure 2 illustrates a GAIQ map created for a hot-stamped body from EXAMPLE Test No. 9 by temarization with GAIQ values ​​of 35000 and 45000 taken as limit values. Figure 3 illustrates a schematic diagram of the impulsive force-displacement curve. DESCRIPTION OF THE MODALITIES

[0017] A hot-stamped body according to the present embodiment and a method for producing the hot-stamped body will be described in detail below. ηοβ / ηη / ζζηζ / Ε / γίΛΐ

[0018] <Composición química de la carrocería estampada en caliente> First, the reasons for limiting the chemical composition of a steel sheet from which the hot-stamped body is formed in accordance with the present embodiment will be described. Hereinafter, the symbols % used for chemical composition all mean % by mass.

[0019] C: 0.06% or more to less than 0.20% C is an important element in obtaining a tensile strength of 590 MPa or more to less than 980 MPa for hot-stamped car bodies. If the C content is less than 0.06%, the martensite becomes soft, making it difficult to obtain sufficient tensile strength, so the C content is set at 0.06% or more. On the other hand, if the C content is 0.20% or more, self-tempering does not progress, and therefore the martensite becomes hard, decreasing the bending ability of the hot-stamped car body, and therefore the C content is set at less than 0.20%. A preferable lower limit of C content is 0.07%, 0.08% or 0.09%, and a preferable upper limit of C content is 0.17%, 0.15%, 0.13% or 0.11%.

[0020] Yes: 0.010 to 1.00% Si has resistance to softening by annealing and therefore has an effect on limiting the decrease in strength due to self-tempering during hot stamping quenching. If the Si content is less than 0.010%, this effect is not achieved, and it may result in the failure to achieve tensile strength or the deterioration of flexural strength, so the Si content is set at 0.010% or more. When the Si content is greater than 1.00%, the Aqui point increases, and it may result in the failure to develop a single austenite phase during hot stamping heating, in which case the microstructure of the hot stamped body becomes uneven, resulting in a deterioration in flexural strength. Therefore, the Si content is set at 1.00% or less. A preferable lower limit of Si content is 0.02%, 0.10%, 0.20% or 0.30%, and a preferable upper limit of Si content is 0.90%, 0.80%, 0.70% or 0.60%.

[0021] “Mn: 0.80 to 2.00%. Mn is an element that increases the hardenability of steel and is useful for ensuring a stable tensile strength of 590 MPa or more. If the Mn content is less than 0.80%, the hardenability becomes insufficient, making it difficult to guarantee a tensile strength of 590 MPa or more for hot-stamped body parts. Therefore, the Mn content is set at 0.80% or more. On the other hand, if the Mn content is set at more than 2.00%, microsegregation is encouraged, causing the steel's microstructure to become uneven. Fracture is likely to occur, resulting in a decrease in the bending capacity of hot-stamped body parts; therefore, 2.00% is set as the upper limit. A preferable lower limit of Mn content is 0.90%, 1.00%, 1.15%, or 1.30%, and a preferable upper limit of Mn content is 1.90%, 1.80%, or 1.60%. ηοβ i nn / zznz / E / YiAi

[0022] P: 0.100% or less P is an element that segregates at grain boundaries, decreasing grain boundary strength. If the P content exceeds 0.100%, the grain boundary strength decreases significantly, which reduces the toughness and flexural ability of the hot-stamped body. Therefore, the P content is set at 0.100% or less. An upper limit of the P content is preferably 0.050%, 0.030%, 0.020%, or 0.015%. A lower limit of the P content is not limited to a particular limit; however, if the P content is reduced to less than 0.0001%, the cost of dephosphorization increases greatly, which is economically undesirable. In actual operation, the P content may be set at 0.0001% or more.

[0023] S: 0.010% or less S is an element that forms inclusions in steel. If the S content exceeds 0.010%, a large number of inclusions form in the steel, which are the source of bending cracks, thereby decreasing the flexural ability of the hot-stamped body. Therefore, the S content is set at 0.010% or less. An upper limit of the S content is preferably 0.0060%, 0.0040%, or 0.0030%. A lower limit of the S content is not limited to a particular limit; however, if the S content is reduced to less than 0.00015%, the cost of desulfurization increases greatly, which is economically undesirable. In actual operation, the S content may be set at 0.00015% or more.

[0024] Al: 0.010 to 0.500% Al is an element that deoxidizes molten steel to make it sound (suppressing the appearance of defects such as blow holes in the steel). If the Al content is less than 0.010%, deoxidation is insufficient, so the Al content is set at 0.010% or more. A lower limit for the Al content is preferably 0.010%, 0.020%, or 0.030%. On the other hand, if the Al content is higher than 0.500%, coarse oxides form in the steel, which cause bending cracks and reduce the bending capacity of the hot-stamped body. Therefore, the Al content is set at 0.500% or less. A preferable upper limit of Al content is 0.400%, 0.300%, 0.100% or 0.080%.

[0025] N: 0.010% or less N is an impurity element and forms nitrides, which cause flexural cracking in steel, decreasing the flexural capacity of hot-stamped dies. If the N content exceeds 0.010%, coarse nitrides form in the steel, significantly decreasing the flexural capacity of hot-stamped dies. Therefore, the N content is set at 0.010% or less. An upper limit of the N content is preferably 0.0075%, 0.0060%, or 0.0050%. A lower limit of the N content is not limited to a particular limit; however, if the N content is reduced to less than 0.0001%, the cost of denitrogenation increases greatly, making it economically undesirable. In actual operation, the N content can be set at 0.0001% or more.

[0026] Nb: more than 0.020 to 0.10% or less Nb is an element that improves the strength of hot-stamped body by solid solution hardening and forms its carbonitride to contribute to the refinement of primary austenite grains, improving the flexural ability. To exert the effects, an Nb content of more than 0.020% is set. The Nb content is more preferably 0.025%, 0.030%, 0.035% or 0.040%. On the other hand, if the Nb content is more than 10%, coarse Nb carbide is produced in the steel, and the flexural ability of the hot-stamped body may be reduced; therefore, the Nb content is set to 0.10% or less. The Nb content is more preferably 0.080%, 0.070% or 0.060%.

[0027] Ti: 0 to 0.10% Ti consumes dissolved nitride to form its carbonitride, restricting the formation of BN, to ensure a quantity of dissolved B necessary to ensure hardenability; therefore, Ti can be contained when necessary. A lower limit of the Ti content is 0%. To obtain the effect, the Ti content is preferably set at 0.001% or more. The Ti content is more preferably 0.002% or more. On the other hand, if the Ti content is more than 0.10%, coarse TiN is produced, which serves as a source of flexural cracking, resulting in a deterioration of flexural ability. The Ti content is preferably 0.10% or less. An upper limit of the Ti content is more preferably 0.08%, 0.05%, or 0.03%.

[0028] V: 0 to 0.10% V is an element that improves the strength of hot-stamped body parts through solid solution hardening. Furthermore, V forms carbonitride, contributing to grain refinement of primary or pre-austenite, improving flexural strength. Therefore, V can be contained when necessary. The lower limit of the V content is 0%. To achieve these effects, the V content is preferably set at 0.001% or more. The V content is preferably 0.005% or more. If the V content exceeds 0.100%, the austenitic grain refinement progresses excessively, decreasing hardenability, and ferrite may form, decreasing the flexural strength of the hot-stamped body parts. Therefore, the V content is set at 0.100% or less. The upper limit of the V content is preferably 0.08%, 0.05%, or 0.02%.

[0029] Cr: 0 to 0.50% ηοβ / nn / zznz / E / YiAi Cr is an element that increases hardenability and inhibits the formation of ferrite, which degrades bending ability; therefore, Cr may be contained when necessary. The lower limit of the Cr content is 0%. To obtain the effects, the Cr content is preferably 0.010% or more. A more preferable lower limit of the Cr content is 0.02%. However, Cr lowers the Ms point and is therefore an element that restricts self-tempering in a quenching process in hot stamping forming. The Cr content is preferably set at 0.50% or less. An upper limit of the Cr content is more preferably 0.40%, 0.20%, 0.10%, 0.05%, or 0.02%.

[0030] Mo: 0 to 1.00% Molecular weight is an element that improves the strength of hot-stamped bodies through solid solution hardening, increases the hardenability of steel, and inhibits the formation of ferrite, which impairs bending ability. Therefore, Molecular weight can be contained when necessary. The lower limit of Molecular weight is 0%. To achieve the desired effect, the Molecular weight is preferably 0.010% or more. A preferable lower limit of Molecular weight is 0.015%. On the other hand, a Molecular weight above 1,000% not only results in a stagnation of the effect but also leads to an increase in alloy costs. Therefore, the Molecular weight is set at 1,000% or less. An upper limit of Molecular weight is more preferably 0.80%, 0.40%, 0.10%, 0.06%, or 0.03%.

[0031] B: 0 to 0.0100% B is an element that segregates at grain boundaries to increase the hardenability of steel, and therefore B can be contained when necessary. The lower limit of B content is 0%. To obtain the effects, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more. On the other hand, if the B content is greater than 0.0100%, coarse BN is formed, which is the origin of bending cracks, resulting in a deterioration of flexural ability. Therefore, the B content is set at 0.0100% or less. An upper limit of the B content is more preferably 0.0075%, 0.0040%, 0.0020%, 0.0015%, 0.0010%, or 0.0003%.

[0032] Ni: 0 to 0.50% Ni is an element that dissolves in austenite, increases the hardenability of steel, and is useful for ensuring a strength of 590 MPa or more with stability; therefore, Ni can be contained when necessary. The lower limit of Ni content is 0%. To obtain the effects, the Ni content is preferably set at 0.001% or more. On the other hand, a Ni content greater than 0.50% causes a stagnation of the effect and leads to an increase in alloy cost; therefore, the Ni content is preferably set at 0.50% or less. A preferable lower limit of Ni content is 0.01%, and a preferable upper limit of Ni content is 0.40%, 0.20%, 0.10%, 0.07%, or 0.03%. ηοβ / ηη / ζζηζ / Ε / γίΛΐ

[0033] REM: 0 to 0.0100% REMs are elements that deoxidize molten steel, making it sound and improving bendability. Therefore, REMs can be contained when necessary. The lower limit for REM content is 0%. However, if the REM content exceeds 0.010%, the effect stagnates and costs increase; therefore, the REM content is preferably set at 0.010% or less. A lower limit for REM content is preferably 0.0002%, and more preferably 0.0005%. A preferable upper limit for REM content is 0.0080%, 0.0050%, 0.0030%, or 0.0020%. Note that, in this embodiment, REM refers to a total of 17 elements, including Se, Y, and lanthanoids. In this embodiment, REM content refers to the total content of these elements.

[0034] Mg: 0 to 0.010% Mg is an element that has a deoxidizing effect on molten steel to make the steel sound and improve bendability; therefore, Mg can be contained when necessary. A lower limit of Mg content is 0%. However, containing more than 0.010% Mg only results in a stagnation of the effect and leads to an increase in cost; therefore, the Mg content is preferably set at 0.010% or less. The lower limit of Mg content is preferably 0.0001% and more preferably 0.0005%. A preferable upper limit of Mg is 0.008%, 0.005%, or 0.003%.

[0035] Ca: 0 to 0.010% Ca is an element that has a deoxidizing effect on molten steel to make the steel sound and improve bending ability; therefore, Ca can be contained when necessary. A lower limit of Ca content is 0%. However, if the Ca content is higher than 0.010%, the effect stagnates and the cost increases; therefore, the Ca content is preferably set at 0.010% or less. The lower limit of Ca content is preferably 0.001% and more preferably 0.005%. A preferable upper limit of Ca content is 0.0080%, 0.0050%, 0.0030% or 0.0020%.

[0036] Co:0a2.0% Co is an element that has the effect of raising the Ms point and is an element that improves flexural ability; therefore, Co can be contained when necessary. A lower limit of the Co content is 0%. To exert the effects, the Co content is preferably set at 0.01% or more. The Co content is more preferably 0.02% or more. On the other hand, if the Co content is more than 2.0%, the hardenability of the steel decreases, making it difficult to obtain a strength of 590 MPa or more; therefore, the Co content is preferably 2.0% or less. An upper limit of the Co content is more preferably 1.5%, 0.8%, 0.3%, or 0.1%.

[0037] The remainder of the chemical composition of the hot-stamped body according to the present embodiment consists of Fe and impurities. Examples of the impurities include elements that are unavoidably contained from steel raw materials or scrap and / or unavoidably contained in a steelmaking process and which are allowed to be contained within the ranges within which the characteristics of the hot-stamped body according to the present embodiment are not affected.

[0038] <Microestructura de la carrocería estampada en caliente> Next, a microstructure of the hot-stamped body according to this embodiment will be described.

[0039] The ratio of regions in martensite in which GAIQ values ​​are 35000 or more to less than 45000 is 30% area or more The most striking feature of the present invention is that, in a quenching process in hot stamping forming, the microstructure transforms into martensite, thereafter, the martensite grains having a relatively high dislocation density are self-tempered to form into grains having a relatively low dislocation density, so as to improve the flexural ability. Therefore, it is important to determine a proportion of the self-tempered martensite grains. Accordingly, the present inventors conducted studies on a method for measuring the proportion, carried out intensive studies on a method for determining the proportion of the self-tempered martensite grains, and as a result, established the following method.

[0040] First, the steel microstructure of a test sample is measured by an electron backscatter diffraction method, and from the obtained measurement data, a steel microstructure with a bcc structure is analyzed in terms of a grain mean image quality (GAIQ) parameter.

[0041] Figure 1 illustrates a distribution (histogram) of GAIQ values ​​of a test sample in which self-tempered and non-self-tempered grains are intermixed. When a GAIQ value is high, the dislocation density is low, and when a GAIQ value is low, the dislocation density is high; therefore, the GAIQ value is a parameter reflecting the dislocation density of the grains. As illustrated in Figure 1, it is understood that the histogram on the test sample has two peaks, including the highest peak and the second highest peak (near the GAIQs of 34500 and 37500). That is, from the histogram of GAIQ values, it is possible to separate the grains that have been made to have low dislocation densities by self-tempering and the grains that have not been self-tempered and still have high dislocation densities from each other.The tendency for a histogram over a sample to have two peaks was confirmed in various types of materials. From this fact, for a hot-stamped body having a mechanical strength and a microstructure of steel provided in the present invention, the present inventors determined that regions in which GAIQ values ​​are from 35000 or more to less than 45000 should be classified as self-tempered martensite grains.

[0042] Note that, according to a metallographic investigation, it is confirmed that a region in which the GAIQ values ​​are 45,000 or more is mainly composed of ferrite. Furthermore, according to a similar investigation, it is confirmed that the GAIQ values ​​of bainite (upper bainite and lower bainite) are 35,000 or more to less than 45,000.

[0043] Figure 2 illustrates a GAIQ map created by temarization with GAIQ values ​​of 35000 and 45000 taken as limiting values. With the GAIQ map illustrated in Figure 2, it is possible to simply visualize grains that are made to have low dislocation densities by self-tempering, consider regions in martensite where GAIQ values ​​are 35000 or more to less than 45000 to be regions where self-tempered martensite grains are present, and calculate a ratio (area fraction) of the regions. In the present invention, a ratio of an area of ​​regions in which there are self-tempered martensite grains to an area of ​​martensite is calculated.

[0044] In the present invention, when the proportion of regions in martensite in which GAIQ values ​​are from 35000 or more to less than 45000 is 30 area % or more, the number of self-tempered martensite grains in the martensite can be sufficiently increased, and thus it is possible to improve a flexural ability of the hot-stamped body. Note that the proportion is preferably 40 area % or more. On the other hand, although there is no specific restriction on an upper limit of the proportion, if the number of self-tempered regions is excessive, a problem arises in that the strength of 590 MPa or more cannot be guaranteed; therefore, the upper limit of the proportion of the regions in martensite in which GAIQ values ​​are from 35000 or more to less than 45000 is preferably set to 95 area %, more preferably 90 area %.

[0045] The microstructure of the hot-stamped body according to the present invention mainly contains, in area fraction, 5 to 50% of ferrite and 50 to 95% of martensite. In particular, the microstructure preferably contains, in area fraction, 60% or more of martensite, preferably 70% or more of martensite. That is, a lower limit of the total area fraction of martensite and ferrite is 65%. The lower limit is preferably 75%, 85% or 90%, more preferably 95%, 98% or 100%. Although there is no particular restriction on the microstructure of the steel other than ferrite and martensite in the microstructure, the microstructure of steel other than ferrite and martensite includes upper bainite, lower bainite, retained austenite and the like. In addition, they may contain iron carbide and the like.An upper limit of the remaining microstructure, other than martensite and ferrite, is 35%, preferably 25%, 15% or 10%, and more preferably 5%, 2% or 0%, in area fraction. ηοβ / ηη / ζζηζ / Β / γίΛΐ The remaining microstructure can be defined, for example, as one or more microstructure classes selected from upper bainite, lower bainite, retained austenite, and iron carbide. The area fractions of the structures can be measured using the following method.

[0046] (Method for measuring the area fraction and GAIQ value of martensite) (Method for identifying martensite) A sample is taken from a position sufficiently far from an end face of the hot-stamped body (usually a position 50 mm or more away) so that a cross-section through the sheet thickness can be observed. This cross-section through the sheet thickness is taken as the observation surface. The observation surface of the sample is mirror-polished, and then 10 indentations are made with a penetration load of 100 gf using a Vickers micro durometer in a region centered on a sheet thickness position t / 4 of the sample (note that the region is limited to a region between a depth of 1 / 8 sheet thickness from a surface and a depth of 3 / 8 sheet thickness from the surface) to specify the capture positions under a scanning electron microscope. One surface of the sample is then immersed in an acetylacetone electrolyte to subject it to electrolytic etching.This makes it possible to clarify the morphology of ferrous carbides in the microstructure of steel and can reveal the contrast of grain boundaries.

[0047] A field emission scanning electron microscope (FE-SEM) equipped with a secondary electron detector is then used to capture secondary electron images at a collection magnification of x5000 in each of the 10 fields of view in which the indentations have been previously made (note that an area of ​​each field of view is set to 0.0001 mm2 or more). In the capture images obtained by this method, ferrite and hard phases (martensite, bainite and retained austenite) are distinguished.

[0048] Next, to discriminate martensite, upper bainite, and lower bainite from each other, secondary electron images are captured in the same fields of view at a capture magnification of xlOOOO. Upper bainite, lower bainite, and martensite can be distinguished from each other based on the presence of iron carbides in the lath grains and based on the stretching directions of the iron carbides. Retained austenite is not sufficiently attacked; therefore, the amount of retained austenite is measured by the method described below. Upper bainite is a microstructure of steel formed by the aggregation of lath grains with precipitation of carbides between the laths. Lower bainite and tempered martensite are also steel microstructures formed by aggregation of lath grains, but they are steel microstructures containing carbides within the laths.Lower bainite and tempered martensite are distinguished from each other based on the elongation directions of the carbides. Lower bainite carbides have a single variant; the ηοβ / ηη / ζζηζ / Ε / γίΛA carbides present in a block have angular differences within 5o and therefore have substantially the same direction. In contrast, tempered martensite carbides have a plurality of variants; the carbides in a block are elongated in a plurality of directions. Based on the difference, lower bainite and tempered martensite are distinguished from each other.

[0049] (Method for measuring the area fraction of retained austenite) In the same observation regions where the captured images are obtained, an area fraction of the retained austenite is measured. The observation surface is polished again with #600 to #1500 silicon carbide papers and mirror-polished. The observation surface is then polished with alkali-free colloidal silica at room temperature for eight minutes, thereby relieving the stress introduced into an outer layer of the observation surface. The observation surface is measured using the electron backscatter diffraction method at measurement intervals of 0.1 pm, thereby obtaining information on crystal orientation. The measurement is carried out using an apparatus that includes a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 detector manufactured by TSL Solutions). At this time, the vacuum degree in the apparatus is set to 9.6xlO'5Pa or less, an accelerating voltage is set to 15 kV, an irradiation current level is set to 13, and an electron beam irradiation level is set to 62. From the obtained crystal orientation information, an area fraction of retained austenite, which has an fcc structure, is calculated using the Phase Map function built into the OIM Analysis (R) software provided with an EBSD analyzer, whereby the area fraction of retained austenite is obtained.

[0050] From the above, it is possible to distinguish, in the hard phase, martensite, tempered martensite, upper bainite, lower bainite and retained austenite from each other: consequently, an area fraction of the total martensite can be determined by subtracting the area fractions of upper bainite, lower bainite and retained austenite from the area fraction of the hard phase. [0051 ] (Method for measuring GAIQ value)In each of the 10 fields of view in which indentations have been previously made (note that an area of ​​each field of view is set to 0.0001 mm2 or more), a grain average misorientation image quality map (GAIQ map) is obtained using the Grain Average Misorientation function built into the software that comes with the EBSD analyzer. In the obtained GAIQ map, regions where GAIQ values ​​are 35000 or more to less than 45000 are identified, while the targets for GAIQ value analysis are limited to steel microstructures that have bcc structures (ferrite, martensite, and bainite). That is, the targets for GAIQ value analysis do not include steel microstructures other than those with bcc structures, for example, retained austenite that has an fcc structure.As described above, the GAIQ values ​​of bainite are from 35000 ηοβ / ηη / ζζηζ / Ε / γίΛA or more to less than 45000, and the GAIQ values ​​of ferrite are 45000 or more. Therefore, by then removing the bainite identified in the secondary electron images (capture magnification: x 10000) using the Highlight function, the area fraction of martensite having GAIQ values ​​from 35000 or more to less than 45000 is obtained.

[0052] That is, first, the bainite images identified in the secondary electron images are recorded. Then, a GAIQ map is created from the same visual fields by EBSD, and the hard steel microstructures corresponding to GAIQ of 35000 or more to less than 45000 are extracted, while the targets for GAIQ value analysis are limited to the steel microstructures having bcc structures (ferrite, martensite, and bainite). At this time, ferrite having a GAIQ of 45000 or more, and martensite and retained austenite having a GAIQ of less than 35000 are excluded, and thus martensite and bainite having a GAIQ of 35000 or more are extracted to less than 45000. In addition, from these steel microstructures, the Highlight function of OIM analysis is used to identify martensite having the predetermined GAIQ values.The Highlight function extracts and displays data about the grains specified in the created map. Specifically, a secondary electron image and a GAIQ map are superimposed, and regions determined to be bainite in the secondary electron image are excluded by the Highlight function. Using the procedure described above, the remaining hard steel microstructures are identified as martensite with a GAIQ of 35,000 or more to less than 45,000.

[0053] The hot-stamped body according to the present invention may include a metal coating layer on its surface. This is to suppress scale formation in a hot-stamping step, improve corrosion resistance of a hot-stamped member, and the like.

[0054] Examples of hot-dip include hot-dip galvanizing, annealing galvanizing, hot-dip aluminizing, and furthermore, aluminizing and hot-dip galvanizing. If a hot-dipped layer is hard, a crack may occur in the hot-stamped conformation, degrading a corrosion resistance of the hot-stamped member. For this reason, hot-dip is preferably hot-dip galvanizing and annealing galvanizing, which result in a soft layer of metal coating.

[0055] In a case where hot-dip galvanizing is hot-dip galvanizing or annealing galvanizing, an adhered amount of metal coating on a surface of a steel sheet is preferably 3 to 800 g / m2 per side. If the adhered amount of metal coating is less than 3 g / m2 per side, it is difficult to obtain the advantageous effect of improving corrosion resistance reliably. On the other hand, if the adhered amount of metal coating is more than 800 g / m2 per ηοβ / ηη / ζζηζ / Ε / γίΛA side, a defect such as blow holes is likely to occur during welding. From the standpoint of improving corrosion resistance and suppressing an increase in cost, the adhered amount of metal coating is more preferably 10 to 200 g / m2.

[0056] In particular, in order to prevent or reduce vaporization of a metal coating before hot stamping forming to improve the corrosion resistance of the hot-pressed member, the metal coating is preferably annealed galvanized. As a degree of annealed galvanized metal coating, an Fe content in the electroplating coating (or metal coating) is preferably 3% or more to 25% or less. If the Fe content in the electroplating coating is less than 3%, vaporization of the electroplating coating during hot stamping forming cannot be prevented or sufficiently reduced; on the other hand, if the Fe content in the electroplating coating or metal coating is more than 25%, the spraying properties of the hot-pressed member deteriorate.

[0057] From the viewpoint of preventing or reducing vaporization of the electroplating coating and ensuring the sputtering properties, the Fe content in the electroplating coating is more preferably 7 to 18%. Note that an organic or inorganic coating may be additionally formed on a surface of the zinc plating layer or the annealed galvanizing layer.

[0058] <Resistencia a la tracción, etc. de la carrocería estampada en caliente> A tensile strength TS of the hot-stamped body according to the present embodiment is 590 MPa or more to less than 980 MPa. When necessary, a lower limit of the tensile strength TS may be set at 610 MPa, 640 MPa, 680 MPa, or 720 MPa, and an upper limit of the tensile strength TS may be set at 960 MPa, 920 MPa, 880 MPa, or 840 MPa. A maximum bending angle α (degrees) of the hot-stamped body according to the present embodiment is set at 90 or more. When necessary, the maximum bending angle α may be set at 95 or more, 98 or more, 101 or more, or 105 or more. Although there is no particular need to specify an upper limit for the maximum bending angle α, the upper limit may be set to 180 or less, 150 or less, 130 or less, or 120 or less. A thickness of the hot-stamped body according to the present embodiment may be set between 0.3 and 6.0 mm approximately, but there is no particular need to limit the thickness. Where necessary, a lower thickness limit may be set at 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm or 1.2 mm, and an upper thickness limit may be set at 5.0 mm, 4.5 mm, 4.0 mm, 3.6 mm, 3.2 mm or 2.8 mm.

[0059] <Método para producir una carrocería estampada en caliente> ηοβ / ηη / ζζηζ / Ε / γίΛΐ Next, a preferred method for producing the hot-stamped body according to the present embodiment will be described. First, a method for producing a hot-stamped steel sheet applicable to the hot-stamped body according to the present embodiment will be described.

[0060] (Method for producing a hot stamping steel sheet) A plate having the chemical composition described above is prepared, and the hot stamping steel sheet is produced, for example, by the following production method.

[0061] Heating step The plate to be hot-rolled is only required to be produced by a conventional method; for example, the plate is only required to be produced by typical methods such as continuous casting and thin slab casting. The steel material with the chemical composition described above is hot-rolled, heated to 1200°C or more in a single hot-rolling step, and subjected to a holding process for 20 minutes or more. If the heating temperature is lower than 1200°C or if the holding continues for less than 20 minutes, the remelting of coarse inclusions such as Ti does not progress, and the coarse inclusions remain to serve as a source of fracture; as a result, the bending capacity may deteriorate. Preferably, the heating temperature is 1250°C or more, and the holding duration is 25 minutes or more.A preferable upper limit of the heating temperature is 13-50 °C, and a preferable upper limit of the holding time is 120 minutes.

[0062] Finishing rolling stage It is preferable to subsequently perform finish rolling at a temperature range not lower than the An point. If the finish rolling is completed at a temperature lower than the Ar3 point, the finish rolling is performed as intercritical rolling, and the shape of the sheet resulting from the rolling may deteriorate. Therefore, a finish rolling temperature is preferably the Ara point or higher, and more preferably Ara + 30 °C or higher. A preferable upper limit of the finish rolling temperature is 1050 °C. The Ara point is expressed by formula (1) shown below. In formula (1), each element symbol indicates a content of the element (% by mass). Point Ar3(°C) = 850 + 10 x (C + N) x Mn + 350 x Nb + 250 x Ti + 40 x B + 10 x Cr + x Mo Formula (1) ηοβ / ηη / ζζηζ / Ε / γίΛΐ

[0063] Winding stage The steel sheet subjected to finish rolling is wound into a coil at 750°C or less. If the coiling temperature is higher than 750°C, scale is produced in large quantities, making it difficult to remove scale in the subsequent pickling step; therefore, the coiling temperature is set at 750°C or less. The coiling temperature is preferably 600°C or less. A preferable lower limit for the coiling temperature is 350°C.

[0064] The hot-rolled steel sheet may be subjected to a reheating treatment to soften it when necessary. In addition, the hot-rolled steel sheet may be subjected to steps of cold rolling, continuous annealing, and continuous hot-dip galvanizing.

[0065] As for cold rolling, it will be sufficient to carry out cold rolling with a normal rolling reduction, for example, from 30 to 90%.

[0066] In a case where a metal coating layer is to be formed on a surface of the cold-rolled sheet, various types of known hot-dipping or electroplating (metal electrodeposition) may be performed according to a purpose such as preventing or reducing scale production in the hot-stamping step and improving corrosion resistance of the hot-stamped member.

[0067] (Method for producing a hot-stamped body) From the hot stamping steel sheet obtained in the manner described above, the hot stamped body is produced, for example, by the following production method.

[0068] Warm-up step In the hot stamping stage, heating is carried out at an average heating rate of 150 °C / s or less. If the average heating rate exceeds 150 °C / s, the remelting of carbides does not progress, and the carbon concentration in the austenite becomes locally uneven, resulting in an uneven amount of self-tempering, forming an uneven steel microstructure; as a result, the bending ability may deteriorate. Heating is preferably carried out at 100 °C / s or less. Although the lower limit of the heating rate is not limited to a specific heating rate, the lower limit is preferably 1 °C / s or more, and more preferably 2 °C / s or more from the perspective of productivity.The heating temperature is set at a point no lower than AC3, and the steel sheet is held within the temperature range for 10 to 300 seconds and then subjected to hot forming. If the heating temperature is lower than AC3, the heating is carried out as intercritical heating, which causes ferrite precipitation, producing an uneven steel microstructure. In addition, the problem arises that the carbide remelting does not progress, resulting in a deterioration of the ηοβ / ηη / ζζηζ / Ε / γίΛA flexural capacity. For this reason, the lower limit of the heating temperature is set at AC3 or higher. The lower limit is preferably AC3 + 20 °C.Although the upper limit of the heating temperature is not limited to a specific temperature, setting a higher temperature increases the heating cost; therefore, the upper limit of the heating temperature is set at the Ac3+100 °C point or below from the production cost perspective. The upper limit is preferably the Ac3+80 °C point or below. The Ac3 point is expressed by formula (2) shown below. In formula (2), each element symbol indicates an element content (% by mass). Point Ac3(°C) = 910 - 203 x C05+ 66 x Si - 25 x Mn + 700 x P - 11 x Cr + 109 x Al + 400 x Ti - 15.2 x Ni + 104 x V + 31.5 x Mo Formula (2)

[0069] Shaping step The forming step is carried out at a temperature of 650 to 800 °C such that an interfacial pressure P (MPa) satisfying a condition expressed by Formula (3) shown below is applied to the hot stamping steel sheet. The interfacial pressure P is a pressing force per unit area applied to the hot stamping steel sheet and is determined by Pressing Force / Area of ​​Hot Stamping Steel Sheet. -0.65Ms + 400 < P < 200 Formula (3) Ms in the above formula (3) can be determined by formula (4) shown below. Ms = 539 - 423(%C) - 30(%Mn) - 12(%Cr) - 17(%Ni) - 7.5(%Mo) Formula (4)

[0070] In this case, when a sufficiently high interfacial pressure is applied to the hot-stamping steel sheet that is heated to an austenite zone to cause shear deformation, stress concentration occurs at the grain boundaries of the austenite, causing martensitic transformation to proceed easily. As a result, a temperature involving 80% completion of martensitic transformation (Meo) can be increased, and accordingly it is possible to reduce a difference (Ms - Mso). When forming is carried out under such conditions, martensitic transformation proceeds easily, and a temperature at which martensite grains self-anneal is increased; therefore, it is possible to increase a proportion of the tempered martensite grains in the hot-stamped body.

[0071] Accordingly, it is necessary to apply the interfacial pressure P of -0.65Ms + 400 or more to the steel sheet for hot stamping. On the other hand, a substantial upper limit of the interfacial pressure P is 200 MPa in view of the capacity of a press machine.

[0072] As the Ms point increases, the temperature at which martensitic transformation begins increases, whereby the number of self-tempered martensite grains also increases. For this reason, ηοβ / nn / zznz / E / YiAi the Ms point is preferably 250 °C or more and more preferably 290 °C or more. The upper limit of the Ms point is preferably set at 550 °C in order to prevent the bending ability from deteriorating due to coarsening of carbides with excessive promotion of self-tempering. The upper limit of the Ms point is preferably set at 500 °C.

[0073] In this case, for hot pressing, a relatively small pressing machine has been used. The reasons for this include that it is not easy to load a heated steel sheet taken out from a heating apparatus into a large press machine that has a very high pressing force while the steel sheet is at a high temperature, to perform pressing, that processing using a large press machine greatly increases a production cost, that there is no need to use a large press machine because a steel sheet for hot stamping that is heated to the austenite zone is easily deformed to begin with, and the like. For these reasons, an interfacial pressure during conventional hot press working is very low, which is an interfacial pressure lower than the lower limit of the range expressed by Formula (3) shown above.

[0074] Cooling step A cooling rate (average cooling rate) for a temperature range from a temperature after hot stamping forming to 250 °C is preferably set at 20 °C / s or more to 500 °C / s or less. By controlling the cooling rate for the temperature range from the temperature after hot stamping forming to 250 °C to 20 °C / s or more to 500 °C / s or less, it is possible to form the microstructure of the hot stamped body into martensite (tempered martensite). If the cooling rate is less than 20 °C / s, quenching is not performed, soft phases such as ferrite are formed in the microstructure, and the tensile strength of the hot stamped body may be less than 590 MPa. For this reason, the cooling rate is preferably set at 20 °C / s or more. Preferably, the cooling rate is 30 °C / s or more.On the other hand, if the cooling rate exceeds 500°C / s, the self-tempering of martensite does not progress, and its flexural strength may deteriorate. For this reason, the cooling rate is set at 500°C / s or less. Preferably, the cooling rate is 300°C / s or less.

[0075] On the contrary, it is important to reduce a cooling rate for a temperature range of 250 °C or less as much as possible to increase a proportion of self-tempered martensite grains. That is, within a temperature range of 250 °C to 100 °C, the hot-stamped body is cooled at an average cooling rate of 1 °C / s or more to 50 °C / s or less.

[0076] After hot stamping forming, tempering may be performed in a temperature range of 100°C to 350°C to adjust a strength. In order to increase the tensile strength of the hot stamped body, it is preferable not to heat the hot stamped body to 350°C or more after hot stamping forming. A heating temperature after hot stamping forming may be set to 300°C or less, 250°C or less, or 200°C or less when necessary. In addition, after hot stamping forming, the body may be made to partially include a softened region to improve the deformability of the hot stamped body. The softened region here means, for example, a softened region provided on a body part by partially tempering the body part (e.g., a flange part).Furthermore, even when a sufficiently high interfacial pressure is applied during forming, a hot-stamped body having a certain shape may have a region where a value in Formula (3) above falls below the value on the left side of Formula (3). Such a region is also considered a softened region.

[0077] The present invention has an object to separate from each other grains which are made to have low dislocation densities by self-tempering and grains which have not been self-tempered and still have high dislocation densities, by utilizing the fact that a grain having a lower dislocation density gives rise to a high GAIQ value. However, the dislocation densities of the martensite grains decrease upon performing tempering. For example, even in a case where the interfacial pressure during hot stamping forming is low, and the grains of the resulting hot stamped body have not been self-tempered, tempering is performed thereafter in some cases.If the annealing temperature is relatively low (about 200°C), the GAIQ values ​​become less than 35000; however, if the annealing temperature is relatively high (350°C or more), the tensile strength TS of the hot-stamped body may be reduced, or the GAIQ values ​​may become from 35000 or more to less than 45000. Between a body tempered at a relatively high temperature and a self-tempered body according to the present application, there is no difference in the microstructure, and it has not been possible to detect the difference. However, it was found that the hot-stamped body tempered at a high temperature in this manner deteriorates in mechanical properties, particularly bendability, and does not have the performances required in the present invention, specifically, performances such as a maximum bending angle (degrees) of 90 or more. EXAMPLE

[0078] Next, an EXAMPLE of the present invention will be described; however, the conditions described in the EXAMPLE are merely an example of conditions that were adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. In the present invention, various conditions may be adopted as long as the conditions make it possible to achieve the objective of the present invention without departing from the essence thereof. ηοβ i nn / zznz / E / YiAi

[0079] Steels having chemical compositions shown in Table 1 were melted and subjected to continuous casting, the obtained castings were held at 1200 °C for 30 minutes and then subjected to hot rolling under a finishing temperature condition of 970 °C; and the resulting hot-rolled steel strips were coiled at 550 °C. These hot-rolled steel strips were subjected to cold rolling with a total rolling reduction of 50%, whereby hot-stamping steel sheets having a thickness of 1.6 mm were obtained. Some of the hot-stamping steel sheets were subjected to a hot-dip galvanizing process, whereby metallic-coated hot-stamping steel sheets were obtained.Each hot-stamping steel sheet and each hot-stamping metal-coated steel sheet (hereinafter collectively referred to as hot-stamping steel sheet) underwent stamping or hot-stamping forming under the conditions shown in Table 2, resulting in hot-stamped bodies. Some of the hot-stamped bodies were annealed.

[0080] The hot-stamped bodies were subjected to microstructure measurement by the measurement method described above. In addition, the mechanical properties of the hot-stamped bodies were measured. The measurement results are shown in Table 3. Figure 2 illustrates a GAIQ map that was created for Test No. 9, which is an inventive steel, by ternarization with GAIQ values ​​of 35000 and 45000 taken as limit values. The mechanical properties of the hot-stamped bodies were measured and evaluated by the following method. [0081 ] Tensile strength and ductility For the tensile strength and ductility of the hot-stamped body, a test coupon No. 5 described in JIS Z 2201:2011 was manufactured from a certain position of each hot-stamped body, and the tensile strength TS (MPa) and total elongation T.EL (%) of the hot-stamped body were measured in accordance with the test method described in JIS Z 2241:2011.

[0082] Flexural capacity The bending capacity was evaluated based on the VDA238-100 standard (April 2017 edition) of the German Association of the Automotive Industry under the following measurement conditions. In the present invention, the maximum bending angle α was determined by converting a displacement at a maximum load obtained by a bending test into an angle in accordance with the VDA standard. A case in which α is 90 (degrees) or more was determined to be good in the bending test. (Measurement conditions) Test specimen dimensions: 60 mm (rolling direction) x 60 mm (direction perpendicular to the rolling direction) ηοβ / nn / zznz / E / YiAi Flex line: A punch was pressed so that a flex line extended in a direction perpendicular to the rolling direction. Test method: Roller supported, punch press Roller diameter: φ30 mm Punch shape: R tip = 0.4 mm Roller distance: 2.0 Sheet thickness (mm) + 0.5 mm Test head speed: 20 mm / min Testing machine: SIMADZU AUTOGRAPH 20kN

[0083] Resistance to crack propagation Crack propagation resistance was evaluated using the following method. Charpy specimens with a sheet thickness of 1.2 mm, a length of 55 mm, and a width of 10 mm were taken from hot-stamped steel sheets (hot-stamped car bodies). The longitudinal direction of each specimen was set as the rolling direction, and a V-notch with a length of 2 mm was processed in a direction perpendicular to the rolling direction. Three of the manufactured specimens were stacked, secured with screws, and subjected to instrumental impact testing. The instrumental impact test was carried out at room temperature, and a time from the start to the end of the test and an impulsive force were measured. The displacements were calculated from the products of the test velocities and the times measured in the instrumental impact test.The fracture surface lengths of the Charpy specimens were 8 mm, so the background was taken as an average of the observed impulsive forces in regions where displacements were 8 mm or greater. After subtracting the background impulsive forces at all measurement points, an impulsive force versus displacement curve was created.

[0084] Figure 3 illustrates a schematic diagram of the impulsive force versus displacement curve. From the resulting impulsive force versus displacement curve, the area under the curve was calculated from a displacement of 0 mm to a displacement of 8 mm, and the resulting value was used as the total impact energy. An impulsive force was then found at which a steep decline in the impulsive force versus displacement curve began (at the onset of a crack in Figure 3), and a corresponding displacement (a crack onset offset) was determined. The area under the curve was calculated for a displacement of 0 mm up to the crack onset offset and used as the crack onset energy. A value obtained by subtracting the crack onset energy from the total absorbed energy was used as the crack propagation energy.The ratio of crack propagation energy to total impact energy was used as an index of crack propagation resistance. A case where this ratio of crack propagation energy to total impact energy ηοβ / nn / zznz / E / YiAi is 10% or more was determined as excellent in crack propagation resistance and was rated as good (O), and a case where the ratio is less than 10% was rated as poor (x).

[0085] Evaluation method A case in which the tensile strength was 590 MPa or more to less than 980 MPa, and the total elongation T.EL (%) was 25% or more, and which was good in a test of flexural capacity and crack propagation resistance was determined to be excellent in strength, ductility, flexural capacity, and crack propagation resistance. A case that did not satisfy any of the four performances described above was determined to be poor in strength, ductility, flexural capacity, and crack propagation resistance.

[0086] [Table 1] ηοβ / ηη / ζζηζ / Ε / γίΛA Table 1 > your NCNNCC > « ac

[0087] [Table 2] Table 2 ηοβ / ηη / ζζηζ / Ε / γίΛΐ Test No. Steel No. Metallic Coating HOT STAMPING CONDITION Comments Ac3 (°C Heating Rate (°C / s) Heating Temp. (°C) Holding Time (s) Forming Temp. (°C) Interfacial Pressure (MPa) Ms (°C -0.65Ms • 400 (MPa) Cooling Rate between Temp. after Forming at 250 BC (°C / s) Cooling Rate 25(1 and 100 'C (°C / s) Annealing Hot Stamped Body (°C) 1 [ No 870 4 940 180 724 100 473 92.3 100 35 No Comparative Steel 2 2 No 861 6 920 180 710 135 469 95.2 100 45 No Invention Steel 3 3 No 826 1 1 900 180 689 185 424 124.4 60 25 No Invention steel 4 4 No 826 4 880 300 655 150 417 128.8 30 20 No Invention steel 5 5 No 830 7 920 60 733 145 426 123.2 300 15 No Comparative steel 6 6 No 824 6 840 120 690 110 460 100.9 100 35 No Comparative steel 7 7 Yes 824 8 860 240 662 125 452 106.0 200 35 No Invention steel 8 8 No 853 6 920 240 683 120 446 110.1 30 10 No Invention Steel 9 9 No 877 7 960 240 710 135 450 107.6 100 15 No Invention Steel 10 10 No 904 6 960 120 676 165 452 105.9 60 15 No Comparative Steel 1 1 11 Yes 872 4 960 120 660 130 480 88.0 5 45 No Comparative Steel 12 12 No 850 1 1 900 240 726 195 403 138.0 450 25 No Invention Steel 13 13 Yes 874 8 900 60 713 130 480 88.0 30 45 No. Invention steel 14 14 No. 874 144 880 240 732 145 480 87.8 100 15 No. Invention steel 15 15 No. 833 4 900 60 722 170 431 120.0 300 45 No. Comparative steel 16 16 No. 939 7 1000 180 722 115 469 95.4 60 45 No. Comparative steel 17 17 No. 836 10 900 60 683 135 441 113.2 450 20 No. Comparative steel 18 18 No. 855 5 920 240 717 170 463 99.1 100 40 No Comparative steel 19 19 No 898 5 980 240 746 160 442 112.9 60 10 No Comparative steel 20 20 No 846 7 900 180 690 165 451 107.1 100 45 No Comparative steel 21 21 No 861 5 960 180 719 140 473 92.9 60 20 No Comparative steel 22 22 Yes 848 4 880 180 740 180 465 97.6 30 45 No Comparative Steel 23 23 Yes 847 107 860 240 658 140 443 111.8 200 25 No Invention Steel 24 24 No 867 5 960 240 699 115 468 95.6 60 45 No Invention Steel 25 25 No 857 9 920 240 698 150 462 99.6 60 40 No Invention Steel 26 26 No 858 6 900 240 675 180 457 103.0 60 10 No Invention Steel 27 27 No 875 11 920 300 670 155 468 95.8 30 40 200 Invention steel 28 28 No 874 12 920 60 663 105 481 87.4 100 20 No Invention steel 29 29 No 863 11 960 240 664 165 465 97.6 450 30 No Invention steel 30 30 No 863 7 940 120 697 140 463 98.8 30 20 200 Invention steel 31 31 No 899 12 920 60 654 140 479 88.4 60 25 No Invention steel 32 32 No 858 33 880 300 744 120 463 99.1 200 20 No Invention steel 33 23 Yes 847 12 820 120 658 190 443 111.8 450 20 No Comparative steel 34 23 Yes 847 5 880 60 692 100 443 111.8 30 30 No Comparative steel 35 23 Yes 847 52 880 240 663 210 443 111.8 ..... No Comparative steel 36 23 Yes 847 5 920 60 650 130 443 111.8 15 40 No Comparative steel 37 23 Yes 847 10 880 300 733 130 443 111.8 210 20 No Comparative steel 38 23 Yes 847 5 940 180 675 190 443 111.8 100 60 No Comparative steel 39 27 No 875 10 850 240 703 125 468 95.8 100 10 No Comparative steel 40 27 No 875 10 980 120 666 80 468 95.8 30 45 No Comparative steel 41 27 No 875 4 940 180 731 215 468 95.8 — No Comparative steel 42 27 No 875 4 960 180 688 155 468 95.8 10 30 No Comparative steel 43 27 No 875 7 920 60 690 115 468 95.8 550 35 No Comparative steel 44 27 No 875 5 900 300 741 105 468 95.8 200 70 No Comparative steel 45 23 Yes 847 5 880 60 695 25 443 111.8 30 30 400 Comparative steel 46 27 No 875 5 980 120 670 40 468 95.8 35 30 500 Comparative steel 47 4 No 826 10 880 300 650 25 417 128.8 30 20 200 Comparative steel 48 9 No 877 7 970 260 700 135 450 107.6 50 15 No Invention steel 49 12 No 850 10 910 250 725 195 403 138.0 50 25 No Invention steel 50 12 No 850 10 910 250 724 195 403 138.0 50 25 200 Invention steel. Underlining means that it is outside the scope specified in the present invention or the recommended manufacturing conditions.

[0088] [Table 3] Table 3 ηοβ / ηη / ζζηζ / Ε / γίΛΐ Test No. Steel No. Hot-stamped body Mechanical Properties Comments Microstructure (area%) TS (MPa) Elongation (%) Flexural capacity at (deg) Crack propagation properties Friction Martensite Bainilla + Retained auscultation ® 1 1 28 72 0 69 569 37 120 O Comparative steel 2 2 12 88 0 83 641 34 III O Invention steel 3 3 21 79 0 62 755 27 104 O Invention steel 4 4 17 83 0 45 745 29 100 O Invention steel 5 5 6 94 0 28 1030 18 84 X Comparative steel 6 6 11 89 0 90 578 35 120 or Comparative steel 7 7 24 76 0 68 776 31 112 or Invention steel 8 8 11 89 0 73 871 32 108 or Invention steel 9 9 25 75 0 65 751 29 114 or Invention steel 10 10 80 20 0 15 532 33 69 X Comparative steel 1 1 11 60 40 0 ​​28 621 31 81 X Comparative steel 12 12 6 94 0 65 838 27 95 or Invention steel 13 13 41 59 0 87 715 28 121 or Invention steel 14 14 42 58 0 67 897 27 99 or Invention steel 15 15 49 51 0 58 719 29 95 X Comparative steel 16 16 29 71 0 89 825 29 103 X SteelComparative 17 17 38 62 0 70 676 31 99 X Comparative steel 18 18 45 55 0 83 922 26 103 X Comparative steel 19 19 6 94 0 60 874 31 103 X Comparative steel 20 20 34 66 0 76 691 34 99 X Comparative steel 21 21 3 97 0 73 693 23 105 or Comparative steel 22 22 55 45 0 25 793 27 85 X Comparative steel 23 23 24 76 0 84 899 27 96 or Invention steel 24 24 7 93 0 90 677 32 118 or Invention steel 25 25 27 73 0 75 663 30 116 or Invention steel 26 26 12 88 0 76 643 31 117 or Invention steel 27 27 28 72 0 75 732 31 107 or Invention steel 28 28 16 84 0 73 917 26 118 or Invention steel 29 29 43 57 0 74 776 28 110 or Invention steel 30 30 9 91 0 76 600 30 III or Invention steel 31 31 15 85 0 72 871 28 113 or Invention steel 32 32 10 90 0 79 768 31 124 or Invention steel 33 23 56 44 0 26 972 26 82 X Comparative steel 34 23 33 67 0 27 695 31 108 X Comparative steel 35 23 ..... ..... Comparative steel 36 23 62 38 0 90 570 37 105 X Comparative steel 32 23 16 84 0 25 914 28 95 X SteelComparative 38 23 31 69 0 29 790 29 107 X Comparative Steel 39 27 66 34 0 20 799 33 79 X Comparative Steel 40 27 38 62 0 28 936 26 102 X Comparative Steel 41 27 ..... ..... Comparative Steel 42 27 70 30 0 90 540 35 131 or Comparative Steel 43 27 9 91 0 23 751 28 100 X Comparative Steel 44 27 27 73 0 26 917 26 101 X Comparative Steel 45 23 32 68 0 85 620 33 88 X Comparative Steel 46 27 25 75 0 90 611 31 85 X Comparative steel 47 4 19 81 0 29 769 26 80 X Comparative steel 48 9 23 67 10 70 715 31 118 or Invention steel 49 12 5 70 25 67 789 28 115 or Invention steel 50 12 5 71 24 75 774 28 117 or Invention steel Underline means that it is outside the scope or evaluation criteria specified in the present invention (T) means that the ratio of the GAIQ value in martensite is equal to or greater than 35,000 and less than 45,000.

[0089] As shown in Tables 1 to 3, the examples satisfying the conditions defined in the present invention all had excellent mechanical properties. The examples not satisfying the conditions defined in the present invention all had poor mechanical properties.

[0090] In Test No. 1, its C content fell below the lower limit of C content; therefore, its martensite became soft, and the tensile strength of 590 MPa or more could not be obtained. In Test No. 5, its C content exceeded the upper limit of C content; therefore, the self-tempering amount decreased, a tensile strength of 980 MPa or more was exerted, and its flexural ability and crack propagation resistance deteriorated. In Test No. 6, its Si content fell below the lower limit of Si content; therefore, the tempering softening resistance was not obtained, and the tensile strength of 590 MPa or more could not be obtained. In Test No.10, its Si content exceeded the upper limit of Si content; therefore, a single austenite phase did not develop during hot stamping heating, ferrite was produced excessively, and as a result, its area fraction from the martensite regions where GAIQ was 35,000 or more to less than 45,000 was less than 30%, failing to ensure a sufficient amount of self-tempering. In Test No. 11, its Mn content fell below the lower limit of Mn content; therefore, its hardenability deteriorated, and excessive ferrite was produced. As a result, ensuring a sufficient amount of self-tempering failed, and its resistance to crack propagation deteriorated. [0091 ] In Test No. 15, its Mn content exceeded the upper limit of Mn content; therefore, its crack propagation resistance deteriorated due to microsegregation. In Test No. 16, its P content exceeded the upper limit of P content; therefore, its grain boundary strength decreased due to grain boundary segregation, resulting in a deterioration of crack propagation resistance. In Test No. 17, its S content exceeded the upper limit of S content; therefore, inclusions occurred in large amounts, resulting in a deterioration of crack propagation resistance. In Test No. 18, its Al content fell below the lower limit of Al content; therefore, blowholes formed in its steel, resulting in a deterioration of crack propagation resistance. In Test No.In Test 19, its Al content exceeded the upper limit of Al content; therefore, coarse Al oxides were produced, resulting in a deterioration of crack propagation resistance. In Test No. 20, its N content exceeded the upper limit of N content; therefore, coarse nitrides were produced, resulting in a deterioration of crack propagation resistance. In Test No. 21, its Nb content fell below the lower limit of Nb content; therefore, the steel microstructures of the primary or previous austenite grains became coarse, increasing their hardenability, and producing little ferrite, resulting in a decrease in elongation. ηοβ / ηη / ζζηζ / Ε / γίΛΐ

[0092] In Test No. 22, its Nb content exceeded the upper limit of Nb content; therefore, excessive ferrite was produced, and ensuring a sufficient amount of self-tempering failed, resulting in a deterioration of crack propagation resistance. In Test No. 33, its hot stamping heating temperature was excessively low; therefore, austenitization did not progress sufficiently, and excessive ferrite was produced, resulting in a deterioration of crack propagation resistance. In Test No. 34, its interfacial pressure fell below the lower limit of interfacial pressure; therefore, the amount of self-tempering was insufficient, resulting in a decrease in crack propagation resistance. In Test No.35, its load exceeded the pressing capacity, making it impossible to form; therefore, its microstructure and mechanical properties could not be evaluated. In Test No. 36, its cooling rate for forming at 250 °C fell below the lower limit of the cooling rate; therefore, quenching was not performed, and a tensile strength of 590 MPa or more could not be achieved.

[0093] In Test No. 37, its cooling rate for forming at 250 °C exceeded the upper limit of the cooling rate; therefore, the amount of self-tempering was insufficient, resulting in a deterioration of crack propagation resistance. In Test No. 38, its cooling rate from 250 to 100 °C exceeded the upper limit of the cooling rate; therefore, the amount of self-tempering was insufficient, resulting in a deterioration of crack propagation resistance. In Test No. 39, the heating temperature of hot stamping was excessively low, so that austenitization did not progress sufficiently, excessive ferrite was produced, and as a result, the amount of self-tempering was insufficient, resulting in a deterioration of crack propagation resistance. In Test No.40, its interfacial pressure fell below the lower limit of the interfacial pressure; therefore, the amount of self-tempering was insufficient, resulting in a deterioration of crack propagation resistance. In Test No. 41, its load exceeded the pressing capacity, making it impossible to form; therefore, its microstructure and mechanical properties could not be evaluated. In Test No. 42, its cooling rate for forming at 250 °C fell below the lower limit of the cooling rate; therefore, tempering was not performed, and a tensile strength of 590 MPa or more could not be obtained.

[0094] In Test No. 43, its cooling rate for forming at 250 °C exceeded the upper limit of the cooling rate; therefore, the amount of self-tempering was insufficient, resulting in a deterioration of the crack propagation resistance. In Test No. 44, its cooling rate from 250 to 100 °C exceeded the upper limit of the cooling rate; therefore, the amount of self-tempering was insufficient, resulting in a deterioration of the crack propagation resistance. In Tests No. 45 and 46, the condition that the regions where GAIQ of 35000 or more to less than 45000 account for 30% or more in the area fraction was satisfied by high temperature tempering after hot stamping forming.However, in these examples, their interfacial pressures fell below the lower limit of the interfacial pressure; therefore, the amount of self-tempering was insufficient, and the carbides coarsened to serve as a source of flexural cracking, promoting crack propagation, resulting in a deterioration of flexural ability. In Test No. 47, their interfacial pressure fell below the lower limit of the interfacial pressure; therefore, although tempering was performed after hot-stamping, the amount of self-tempering was insufficient, resulting in a deterioration of flexural ability.

[0095] Furthermore, it is understood that, as illustrated in Figure 2, in test No. 9, which is an inventive steel, the number of self-tempered martensite grains, i.e., the regions with low dislocation density, is large (area fraction of 65%) compared to the number of non-self-tempered grains. INDUSTRIAL APPLICATION

[0096] In accordance with the present invention, there is obtained a hot-stamped body combining a strength of TS: 590 MPa or more to less than 980 MPa, a ductility as excellent as an elongation at break (total elongation) of 25% or more, a bending ability as excellent as a: 90 degrees or more, and an excellent resistance to crack propagation.

Claims

1. A hot-stamped car body comprising a chemical composition consisting of: in % by mass, C: 0.06% or more or less than 0.20%, Si: 0.010 to 1.00%, Mn: 0.80 to 2.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.500%, N: 0.010% or less, Nb: not more than 0.020 to 0.10% or less, Ti: 0 to 0.10%, V: 0 to 0.10%, Cr: 0 to 0.50%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ni: 0 to 0.50%, REM: 0 to 0.0100%, Mg: 0 to 0.010%, Ca: 0 to 0.0100%, and Co: 0 to 2.0%, the remainder: Fe and impurities, where a microstructure includes, in area fraction, ferrite: 5 to 50%, and martensite: 50 to 95% a proportion of regions in the martensite in which the GAIQ values ​​are 35000 or more to less than 45000 is 30% of area or more, and a maximum bending angle ct (degrees) according to VDA238-100 of the German Association of the Automotive Industry is 90 or more.

2. The hot-stamped car body according to claim 1, wherein the chemical composition contains one or more elements selected from, in % by mass: Ti: 0.001 to 0.10%, V: 0.001 to 0.100%, Cr: 0.010 to 0.50%, Mo: 0.010 to 1.000%, B: 0.0001 to 0.010%, Ni: 0.001 to 0.50%, REM: 0.001 to 0.010%, Mg: 0.001 to 0.010%, Ca: 0.001 to 0.010%, and Co: 0.01 to 2.0%.

3. The hot-stamped body according to claim 1, wherein the tensile strength is from 590 MPa or more to less than 980 MPa.

4. The hot-stamped body according to claim 1, wherein the hot-stamped body is not heated to 350 °C or more after hot-stamping.

5. The hot-stamped body according to claim 1, comprising a metallic coating layer on an outer layer of the hot-stamped body.

6. The hot-stamped bodywork according to claim 2, wherein the tensile strength is from 590 MPa or more to less than 980 MPa.

7. The hot-stamped body according to claim 2, wherein the hot-stamped body is not heated to 350 °C or more after the hot-stamping process.

8. The hot-stamped body according to claim 3, wherein the hot-stamped body is not heated to 350 °C or more after the hot-stamping process.

9. The hot-stamped body according to claim 6, wherein the hot-stamped body is not heated to 350 °C or more after the hot-stamping process.

10. The hot-stamped body according to claim 2, comprising a metallic coating layer on an outer layer of the hot-stamped body.

11. The hot-stamped body according to claim 3, comprising a metallic coating layer on an outer layer of the hot-stamped body.

12. The hot-stamped body according to claim 4, comprising a metallic coating layer on an outer layer of the hot-stamped body.

13. The hot-stamped body according to claim 6, comprising a metallic coating layer on an outer layer of the hot-stamped body.

14. The hot-stamped body according to claim 7, comprising a metallic coating layer on an outer layer of the hot-stamped body.

15. The hot-stamped bodywork according to claim 8, comprising a metallic coating layer or an outer layer of the hot-stamped bodywork.

16. The hot-stamped body according to claim 9, comprising a metallic coating layer on an outer layer of the hot-stamped body.

17. The hot-stamped body comprising: a chemical composition comprising: in % by mass, C: 0.06% or more to less than 0.20%, Si: 0.010 to 1.00%, Mn: 0.80 to 2.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.500%, N: 0.010% or less, Nb: more than 0.020 to 0.10% or less, Ti: 0 to 0.10%, V: 0 to 0.10%, Cr: 0 to 0.50%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ni: 0 to 0.50%, REM: 0 to 0.0100%, Mg: 0 to 0.010%, Ca: 0 to 0.0100%, and Co: 0 to 2.0%, with the remainder being: Fe and impurities, wherein a microstructure includes, in area fraction, Ferrite: 5 to 50%, and Martensite: 50 to 95%, a proportion of regions in the martensite in which the GAIQ values ​​are 35000 or more to less than 45000 is 30% of area or more, and a maximum bending angle (degrees) in accordance with VDA238-100 of the German Association of the Automotive Industry is 90 or more.