Hot stamped compact

Abstract

To provide a hot stamp molded body excellent in strength, extensibility and crack propagation resistance.SOLUTION: A hot stamp molded body has a specific chemical composition and includes a metallographic structure containing: inside a crystal grain surrounded by a grain boundary with a mean crystal orientation difference of 5° or more, by area%, 80% or more of ferrite with a mean crystal orientation difference of less than 0.4°; less than 20% of ferrite with a mean crystal orientation difference of 0.4° or more; and 1-5% of a balance structure. The total of the ferrite with a mean crystal orientation difference of less than 0.4° and the ferrite with a mean crystal orientation difference of 0.4° or more by area% is 95-99%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-stamped steel sheet, particularly to a hot-stamped steel sheet excellent in strength, ductility, and crack propagation resistance, which contributes to reducing the weight of vehicle bodies and improving collision safety. [Background technology]

[0002] In recent years, high-strength steel sheets have been applied to automobile body parts due to the demand for lighter vehicle weight and improved collision safety. Since body parts are formed by press forming, improving press formability, especially shape fixability, is a challenge. Therefore, hot stamping has attracted attention as a method for manufacturing high-strength body parts with excellent shape precision.

[0003] In recent years, the application of tailored blanks to hot stamping has been investigated. A tailored blank is a steel sheet made by welding together steel sheets with different thicknesses, chemical compositions, metal structures, etc. In a tailored blank, the properties of the joined steel sheet can be partially changed. For example, by imparting high strength to a certain portion, deformation in that portion can be suppressed, and by imparting low strength to another portion, deformation in that portion can be suppressed, thereby absorbing impact.

[0004] One technique for applying a tailored blank to a hot stamping method is to use a tailored blank in which a steel sheet that has low strength after hot stamping and a steel sheet that has high strength after hot stamping are joined by welding. As a steel sheet that has high strength after hot stamping, for example, a steel sheet as disclosed in Patent Document 1 can be used. As a steel sheet that has low strength after hot stamping, the chemical composition of the steel may be adjusted so that the strength is low after the die is cooled during hot stamping.

[0005] Low-carbon steel is one type of steel that can be used for tailored blanks. Low-carbon steel has a low carbon content, making it difficult to increase its strength even when rapidly cooled after heating. Patent Document 2 discloses the use of ultra-low carbon steel as a low-strength material for hot stamping. Patent Document 2 also discloses a technology for improving local deformability by heating a steel plate to a temperature equal to or higher than the Ac3 point and then hot stamping it to create a metallographic structure primarily composed of bainite and bainitic ferrite. Patent Document 2 also discloses that this technology reduces the likelihood of fracture when a vehicle body part is deformed in a bending mode during a collision, resulting in excellent impact absorption capabilities due to plastic deformation.

[0006] When low-carbon steel is used as the low-strength material, deformation is concentrated during a collision, and when the steel is subjected to large tensile deformation rather than bending mode, fracture may occur, significantly reducing the energy absorption capacity of the part. Therefore, low-carbon steel used as the low-strength material for tailored blanks is required to have excellent ductility after hot stamping and to suppress crack propagation in order to ensure the amount of energy absorption during a collision. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-197213 [Patent Document 2] International Publication No. 2012 / 157581 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a hot-stamped steel sheet that is excellent in ductility and crack propagation resistance. Another object of the present invention is to provide a hot-stamped steel sheet that has the above-mentioned properties and also has high strength, which is a property required of a general hot-stamped steel sheet. [Means for solving the problem]

[0009] As a result of extensive research by the present inventors into the ductility and crack propagation resistance of hot-stamped steel sheets, they have discovered that the ductility and crack propagation resistance of hot-stamped steel sheets are affected by the metallurgical structure after hot stamping. When austenite is transformed by heating during hot stamping, and then hot stamped from the austenite state, rapid cooling by the die causes the transformation from austenite to martensite to begin before the ferrite transformation has been sufficiently promoted. In this case, the fraction of ferrite is reduced, making it impossible to ensure ductility.

[0010] After heating in hot stamping, if the material is cooled by air cooling, which has a slow cooling rate, rather than rapid cooling using a die, and hot stamping is performed after some of the austenite has transformed into ferrite, the metal structure after hot stamping will be a composite structure of ferrite and martensite. With ferrite providing ductility and martensite providing strength, a hot stamped product with excellent strength and ductility can be obtained.

[0011] Furthermore, in order to increase the amount of energy absorbed during a collision, it is effective to suppress crack propagation in the hot-stamped product. When large deformation occurs during a collision, cracks occur, but suppressing their propagation can prevent a decrease in energy absorption. Crack propagation can be delayed by increasing the mobility of dislocations within the crystal. Dislocations that have migrated within a crystal grain are stopped by the grain boundaries of adjacent crystal grains, and the larger the angle of the crystal boundary, the more suppressed the movement of dislocations. Therefore, dislocation mobility can be increased by minimizing the crystal orientation misorientation within the crystal grain. As a result of extensive research, the present inventors have found that the effect of suppressing crack propagation is enhanced by reducing the average crystal orientation misorientation within ferrite crystal grains.

[0012] Whether or not the transformation from austenite to ferrite with a small average crystal orientation mismatch within the grains has started can be determined by measuring the temperature of the steel sheet during air cooling after removal from the heating furnace and observing the transformation heat. By starting hot stamping after the transformation heat has occurred, a metal structure containing ferrite with a small average crystal orientation mismatch can be obtained in the hot-stamped steel. The inventors have discovered that by using a steel sheet with an optimized chemical composition and hot stamping it using the method described above, a hot-stamped steel with excellent strength, ductility, and crack propagation resistance can be produced.

[0013] The present invention was made based on the above findings, and the gist of the present invention is as follows. (1) A hot-stamped steel according to one aspect of the present invention has a chemical composition, in mass%, of C: 0.010~0.100%, Mn: 0.50 to 1.50% Al: 0.10 to 1.00%, Nb: 0.005 to 0.050%, Ti: 0.010 to 0.080%, Cr: 0.05 to 0.20%, P: 0.500% or less, S: 0.1000% or less, N: 0.0100% or less, Si: 0 to 0.50% V: 0~0.10%, Zr: 0 to 0.10% Mo: 0-1.0% B: 0~0.005%, The sum of Ni and Cu: 0 to 2.0%, and Sn, Sb and As total: 0 to 0.10% Contains the balance being Fe and impurities, crystal Surrounded by grain boundaries with a misorientation of 5° or more The above is defined as a crystal grain. Inside the grain When the average crystal orientation misorientation is calculated, , area % is 80% or more The aforementioned Ferrite with an average crystal misorientation of less than 0.4° and less than 20% The aforementionedThe alloy is composed of ferrite with an average crystal orientation difference of 0.4° or more and 1 to 5% of the remaining structure. the remaining structure is composed of at least one of martensite and bainite, The metal structure has a total area percentage of ferrite having an average crystal misorientation of less than 0.4° and ferrite having an average crystal misorientation of 0.4° or more of 95 to 99%. (2) The hot stamped steel according to (1) above, wherein the chemical composition is, in mass%, V: 0.005~0.10%, Zr: 0.005 to 0.10%, Mo: 0.005 to 1.0%, B: 0.0002 to 0.005%, and Sum of Ni and Cu: 0.005 to 2.0% The compound may contain one or more of the group consisting of: (3) The hot stamped product according to (1) or (2) above may have a plating layer on the surface. [Effects of the Invention]

[0014] According to the above aspect of the present invention, it is possible to provide a hot-stamped steel sheet excellent in strength, ductility, and crack propagation resistance. [Brief explanation of the drawings]

[0015] [Figure 1A] FIG. 1 is a graph showing the relationship between the temperature of a steel sheet and the time elapsed after the steel sheet is heated to 940° C. and carried out of a heating furnace. [Figure 1B] FIG. 1B is a graph showing the temperature of the steel plate in FIG. 1A obtained by second-order differentiation with respect to elapsed time. [Figure 2A] FIG. 1 is a graph showing the relationship between the temperature of a steel sheet and the time elapsed after the steel sheet is heated to 980° C. and carried out of a heating furnace. [Figure 2B] FIG. 2B is a graph showing the temperature of the steel plate in FIG. 2A obtained by second-order differentiation with respect to elapsed time. [Figure 3] FIG. 1 is a schematic diagram of an impact force-displacement curve obtained by an instrumented impact test. DETAILED DESCRIPTION OF THE INVENTION

[0016] The hot-stamped steel according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the hot-stamped steel according to this embodiment will be described. Note that the numerical limit ranges described below include the lower limit and upper limit. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. Furthermore, all % in the chemical composition means % by mass.

[0017] The hot-stamped steel according to this embodiment has a chemical composition, in mass%, of 0.010 to 0.100% C, 0.50 to 1.50% Mn, 0.10 to 1.00% Al, 0.005 to 0.050% Nb, 0.010 to 0.080% Ti, 0.05 to 0.20% Cr, 0.500% or less P, 0.1000% or less S, 0.0100% or less N, and the balance: Fe and impurities. Each element will be described below.

[0018] C: 0.010 to 0.100% C is an element that significantly affects the strength and ductility of hot-stamped steel sheets. If the C content is too high, the transformation from austenite to ferrite is inhibited, making it impossible to obtain the desired amount of ferrite, and the ductility of the hot-stamped steel sheets is reduced. Therefore, the C content is set to 0.100% or less, and preferably 0.080% or less, or 0.040% or less. If the C content is too low, the martensitic transformation is not promoted, the strength of the hot-stamped steel sheets is reduced, and fracture due to insufficient strength is more likely to occur. Therefore, the C content is set to 0.010% or more.

[0019] Mn: 0.50 to 1.50% Mn is an alloying element with solid solution strengthening ability and is added to improve the strength of the hot-stamped body. However, if the Mn content exceeds 1.50%, the hardenability of the steel sheet increases, and the formation of ferrite during air cooling after heating in hot stamping is suppressed, resulting in a decrease in the ductility of the hot-stamped body. Therefore, the Mn content is set to 1.50% or less, preferably 1.20% or less. Furthermore, if the Mn content is too low, the ferrite transformation proceeds too quickly, making it difficult to form martensite, and the desired strength of the hot-stamped body cannot be obtained. Therefore, the Mn content is set to 0.50% or more, preferably 0.60% or more.

[0020] Al: 0.10 to 1.00% Al is an important element for promoting ferrite transformation. If the Al content is less than 0.10%, the ferrite transformation will be difficult to proceed, and the area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° will not be able to be 80% or more. Therefore, the Al content is set to 0.10% or more, and preferably 0.20% or more. On the other hand, if the Al content exceeds 1.00%, the transformation to ferrite will proceed excessively, and the desired amount of martensite will not be obtained, resulting in a decrease in strength. Therefore, the Al content is set to 1.00% or less, and preferably 0.50% or less.

[0021] Nb: 0.005 to 0.050% Nb is an element that suppresses austenite grain growth, refines austenite grains, and promotes transformation to ferrite. To obtain a desired amount of ferrite, the Nb content is set to 0.005% or more, preferably 0.010% or more, or 0.015% or more. However, if the Nb content is too high, austenite becomes hard, and excessive dislocations are introduced into ferrite during the transformation from austenite to ferrite, resulting in the formation of excessive ferrite with a large crystal orientation misorientation. As a result, the crack propagation resistance of the hot-stamped steel sheet decreases. Therefore, the Nb content is set to 0.050% or less, preferably 0.040% or less.

[0022] Ti: 0.010 to 0.080% Ti is also an element that suppresses austenite grain growth, refines austenite grains, and promotes transformation to ferrite. To obtain the desired amount of ferrite, the Ti content is set to 0.010% or more, preferably 0.025% or more. However, if the Ti content is too high, the austenite becomes hard, and excessive dislocations are introduced into the ferrite during the transformation from austenite to ferrite, resulting in the formation of too much ferrite with a large crystal orientation misorientation. As a result, the crack propagation resistance of the hot-stamped steel sheet decreases. Therefore, the Ti content is set to 0.080% or less, preferably 0.070% or less, or 0.050% or less.

[0023] Cr: 0.05 to 0.20% Cr is also an element that suppresses austenite grain growth, refines austenite grains, and promotes transformation to ferrite. To obtain the desired amount of ferrite, the Cr content is set to 0.05% or more, preferably 0.10% or more. However, if the Cr content is too high, the austenite becomes hard, and excessive dislocations are introduced into the ferrite during the transformation from austenite to ferrite, resulting in the formation of too much ferrite with a large crystal orientation misorientation. As a result, the crack propagation resistance of the hot-stamped steel sheet decreases. Therefore, the Cr content is set to 0.20% or less, preferably 0.15% or less.

[0024] P:0.500% or less P has solid solution strengthening ability and is an effective element for obtaining the desired strength of the hot-stamped body. However, if the P content exceeds 0.500%, the ductility of the hot-stamped body deteriorates, so the P content is set to 0.500% or less. Preferably, it is 0.300% or less, 0.100% or less, or 0.050% or less. There is no particular lower limit, but from the viewpoint of ensuring strength by P, the P content may be set to 0.003% or more, or 0.010% or more.

[0025] S: 0.1000% or less S affects nonmetallic inclusions in steel and reduces the crack propagation resistance of hot-stamped steel. Therefore, the S content is set to 0.1000% or less. Preferably, it is set to 0.0300% or less, 0.0100% or less, or 0.0050% or less. Although there is no particular lower limit, if the S content is reduced too much, the manufacturing cost in the desulfurization process increases, so the S content may be set to 0.0010% or more.

[0026] N: 0.0100% or less N is an impurity element that forms nitrides in steel and deteriorates the ductility of hot-stamped steel. If the N content exceeds 0.0100%, the nitrides in the steel become coarse, deteriorating the ductility of the hot-stamped steel. Therefore, the N content is set to 0.0100% or less. Although there is no particular lower limit, if the N content is reduced too much, the steelmaking cost in the steelmaking process increases, so the N content may be set to 0.0010% or more.

[0027] The hot stamped steel according to this embodiment may contain the above elements, with the remainder being Fe and impurities. However, in order to improve various properties, the following elements (optional elements) may be contained in place of part of the Fe. In order to reduce alloy costs, it is not necessary to intentionally include these optional elements in the steel, and therefore the lower limit of the content of each of these optional elements is 0%. Note that examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process and are permissible within a range that does not impair the properties of the hot stamped steel according to this embodiment.

[0028] Si: 0 to 0.50% Si is an alloying element with solid solution strengthening ability and may be added as needed to obtain the strength of the hot stamped body. However, if the Si content exceeds 0.50%, the ferrite transformation proceeds excessively, making it impossible to obtain martensite. Therefore, the Si content is set to 0.50% or less. Furthermore, to ensure the above effects, the Si content is preferably set to 0.005% or more.

[0029] V: 0-0.10%, Zr: 0-0.10%, and Mo: 0-1.0% V, Zr, and Mo may be added as needed because they form carbides in the steel and improve the strength of the hot-stamped body through precipitation strengthening. To ensure the above effects, it is preferable that the content of at least one of V, Zr, and Mo be 0.005% or more. On the other hand, if the V content or Zr content exceeds 0.10%, or if the Mo content exceeds 1.0%, the strength of the hot-stamped body increases excessively and its ductility decreases. Therefore, the V content and Zr content are set to 0.10% or less, and the Mo content is set to 1.0% or less.

[0030] B: 0 to 0.005% B has the effect of suppressing grain growth and is an element that contributes to improving the strength of the hot-stamped body, so it may be added as needed. To ensure this effect, the B content is preferably 0.0002% or more. If the B content is too high, the ductility of the hot-stamped body will decrease, so the B content is set to 0.005% or less.

[0031] Sum of Ni and Cu: 0 to 2.0% Ni and Cu are elements that contribute to improving the strength of the hot stamped body, so they may be added as needed. To ensure this effect, the total content of Ni and Cu is preferably 0.005% or more. If the total content of Ni and Cu is too high, castability may be reduced, so the total content of Ni and Cu is set to 2.0% or less.

[0032] Sn, Sb and As total: 0 to 0.10% In addition to the above-mentioned optional elements, Sn, Sb, and As may be contained. Since excessive content of unavoidably contained impurity elements such as Sn, Sb, and As may deteriorate the ductility of the hot stamped steel sheet, the total content of these elements may be 0.10% or less.

[0033] The chemical composition of the hot-stamped body described above can be measured by a common analytical method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using a combustion-infrared absorption method, and N can be measured using an inert gas fusion-thermal conductivity method. If the hot-stamped body has a plating layer on its surface, the plating layer can be removed by mechanical grinding before analyzing the chemical composition.

[0034] Next, the metal structure of the hot stamped steel according to this embodiment will be described. The hot stamped body according to this embodiment has a metal structure that, within crystal grains surrounded by grain boundaries with an average crystal orientation misorientation of 5° or more, contains, by area percentage, 80% or more of ferrite with an average crystal orientation misorientation of less than 0.4°, less than 20% of ferrite with an average crystal orientation misorientation of 0.4° or more, and 1 to 5% of the remaining structure, with the total area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° and ferrite with an average crystal orientation misorientation of 0.4° or more being 95 to 99%.

[0035] Area fraction of ferrite with an average crystal orientation misorientation of less than 0.4°: 80% or more Ferrite with an average crystal orientation misorientation of less than 0.4° is an important structure for increasing the crack propagation resistance of hot-stamped steel sheets. Ferrite with an average crystal orientation misorientation of less than 0.4° is a structure that forms during air cooling from the time a steel sheet is heated to a temperature of Ac3-30°C or higher in a heating furnace until the steel sheet is pressed out of the heating furnace. The average cooling rate during air cooling from 900°C to 800°C is approximately 10 to 30°C / s, which is slower than the cooling rate achieved by rapid cooling using a die. Therefore, sufficient diffusion of Fe and C atoms during air cooling causes a transformation from austenite to ferrite with an average crystal orientation misorientation of less than 0.4°. To increase the crack propagation resistance of hot-stamped steel sheets, the area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° is set to 80% or more, preferably 85% or more.

[0036] Area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more: Less than 20% The area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more is a structure that is formed by transformation from austenite due to rapid cooling caused by contact heat transfer through a die. If the area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more becomes too high, the ductility of the hot-stamped body decreases. Therefore, the area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more is set to less than 20%, preferably 15% or less, or 10% or less. From the viewpoint of ensuring strength, the area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more may be set to 5% or more.

[0037] The total area percentage of ferrite with an average crystal misorientation of less than 0.4° and ferrite with an average crystal misorientation of 0.4° or more is 95 to 99% In order to ensure the desired ductility of the hot-stamped steel, the total area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° and ferrite with an average crystal orientation misorientation of 0.4° or more must be 95% or more. It is preferably 97% or more. On the other hand, if the total area fraction of these ferrites exceeds 99%, the strength of the hot-stamped steel cannot be ensured. Therefore, the total area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° and ferrite with an average crystal orientation misorientation of 0.4° or more must be 99% or less. It is preferably 98% or less.

[0038] Area fraction of residual tissue: 1~5% In this embodiment, the remaining structure is a hard phase consisting of at least one of martensite and bainite. The remaining structure (hard phase) is a structure necessary for obtaining the strength of the hot-stamped body. To obtain the desired strength of the hot-stamped body, the area fraction of the remaining structure is set to 1% or more, preferably 2% or more. On the other hand, if the remaining structure is too much, the ductility of the hot-stamped body deteriorates, so the area fraction of the remaining structure is set to 5% or less, preferably 4% or less, or 3% or less.

[0039] Method for measuring area fraction of metal structure The area fraction of the metal structure is measured by the following method. First, a sample is taken from a position at least 10 mm away from the end face of the hot-stamped compact so that the cross section perpendicular to the surface (thickness cross section) serves as the observation surface. The sample should be large enough to allow observation of approximately 10 mm in the rolling direction, depending on the measurement device. The cross section of the cut sample is polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol and pure water. Next, the sample is polished for 8 minutes at room temperature using colloidal silica without alkaline solution to remove any strain introduced into the surface layer of the sample. If the shape of the hot-stamped product makes it impossible to take a sample from a position 10 mm or more away from the end face of the hot-stamped product, a sample may be taken from a location other than the end that has not been sufficiently heat-treated.

[0040] At a position 1 / 4 of the plate thickness from the surface of the sample cross section, an area of ​​50 μm in the rolling direction and 50 μm in the plate thickness direction is measured at a measurement interval of 0.1 μm using electron backscatter diffraction to obtain crystal orientation information. For the measurement, a device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector) is used. The degree of vacuum inside the device is 9.6 × 10 -5The electron beam irradiation time is set to 0.01 seconds per point, with a scanning current of 13 Pa or less, an acceleration voltage of 15 kV, a probe current level of 13, and a scanning electron beam irradiation time of 0.01 seconds per point. From the obtained crystal orientation information, the "Image Quality" function of the "OIM Analysis (registered trademark)" software attached to the EBSD analyzer is used to identify grain boundaries with a misorientation (GAM value: Grain Average Misorientation) of 5° or more, grains surrounded by grain boundaries with a misorientation of 5° or more with an average misorientation of less than 0.4°, and grains surrounded by grain boundaries with a misorientation of 5° or more with an average misorientation of 0.4° or more. The above-mentioned operation is performed in at least five regions. The area fraction of ferrite with an average misorientation of less than 0.4° is obtained by calculating the average area fraction of grains surrounded by grain boundaries with a misorientation of 5° or more with an average misorientation of less than 0.4°. In addition, the area fraction of ferrite with an average crystal orientation misorientation of 0.4° or more is obtained by calculating the average area fraction of crystal grains with an average crystal orientation misorientation of 0.4° or more within crystal grains surrounded by crystal grain boundaries with an orientation misorientation of 5° or more.

[0041] Ferrite transformed by rapid cooling introduces a lot of strain into the crystal grains, resulting in large fluctuations in the crystal orientation within the crystal grains and an average crystal misorientation of 0.4° or more.On the other hand, ferrite transformed at a slow cooling rate due to the diffusion of Fe and C atoms introduces only a small amount of strain into the crystal grains, resulting in an average crystal misorientation of less than 0.4°.

[0042] The area fraction of the remaining structure is obtained by subtracting from 100% the sum of the area fraction of ferrite having an average crystal orientation misorientation of less than 0.4° obtained by the above-mentioned method and the area fraction of ferrite having an average crystal orientation misorientation of 0.4° or more.

[0043] A plating layer may be formed on the surface of the hot stamped body according to this embodiment. Having a plating layer on the surface is preferable because it improves the corrosion resistance of the hot stamped body. Examples of applicable plating include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrogalvanizing, and alloyed hot-dip galvanizing.

[0044] Next, a method for manufacturing the hot-stamped steel sheet according to this embodiment will be described. First, a method for manufacturing the steel sheet to be used for the hot-stamped steel sheet according to this embodiment will be described.

[0045] The method for producing the steel sheet to be used in the hot-stamped steel according to this embodiment is not particularly limited, and may be a general production method. For example, a steel slab produced by a general method such as continuous casting or thin slab caster is heated and hot-rolled, then cooled, pickled as necessary, and cold-rolled to obtain a steel sheet.

[0046] The steel sheet may be plated as required. By hot stamping the plated steel sheet, a hot-stamped product having a plated layer on its surface can be obtained. Examples of plating methods include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, and zinc plating. Conventional plating methods can be used. For example, a bath concentration of 5-12% Si is suitable for aluminum plating, a bath concentration of 40-50% Zn is suitable for aluminum-zinc plating, and a bath concentration of 50-60% Zn is suitable for zinc plating. Steel sheets with similar properties can be produced without any particular problems, regardless of whether Mg or Zn is mixed in the aluminum plating layer or the aluminum-zinc plating layer. The plating can be applied in a continuous plating facility with or without a non-oxidizing furnace, or under conventional atmospheric conditions. Furthermore, zinc plating can be applied using methods such as hot-dip galvanizing, electrogalvanizing, and galvannealed hot-dip galvanizing.

[0047] There is no problem with applying Ni pre-plating, Fe pre-plating, or other metal pre-plating that improves plating properties. There is no problem with applying a different type of metal plating or a film of an inorganic compound or an organic compound to the surface of the plating layer.

[0048] Next, a method for producing a hot-stamped steel according to this embodiment using the steel sheet obtained by the above-described method will be described. The method for producing a hot-stamped product according to this embodiment includes the steps of: a heating step of heating the steel sheet to a temperature range of Ac3-30 to 1100°C; a holding step of holding the temperature in the temperature range for 10 to 2400 seconds; a hot forming step of cooling the steel sheet to a temperature range of transformation heat generation start temperature −20°C to transformation heat generation start temperature −150°C at an average cooling rate of 5 to 30°C / s, and starting forming in that temperature range; The method includes a rapid cooling step of cooling the hot-formed product to a temperature range of 400°C or lower at an average cooling rate of 20 to 1000°C / s. Each step will be described below.

[0049] [Heating process] When hot stamping a tailored blank made by joining a low-strength material (a material according to this embodiment) and a high-strength material (a material not according to this embodiment), the high-strength material must be sufficiently austenitized. The low-strength material does not necessarily need to be completely austenitized; it can be heated to Ac3-30°C or higher. The Ac3 transformation temperature can be calculated from the composition of the low-strength material using the following formula. When the steel sheet is heated to Ac3-30°C or higher, ferrite transforms to austenite, and strain introduced by shearing or welding is released. The heating temperature is set to 1100°C or lower to prevent the formation of a large amount of scale if the steel sheet does not have a coating layer on its surface, and to prevent excessive alloying if the steel sheet has a coating layer on its surface. Preferably, the heating temperature is set to 1000°C or lower.

[0050] Ac3=912-262×C+59×Si-23×Mn-18.1×Ni-14.8×Cr+16.8×Mo-39.8×Cu Here, the contents of C, Si, Mn, Cu, Ni, Mo, and Cu in the steel are expressed in mass%, and if the element is not contained, enter 0. The unit of Ac3 is °C.

[0051] [Holding process] After heating the steel sheet to the above heating temperature, the holding time is set to 10 to 2400 seconds to transform ferrite to austenite. If the holding time exceeds 2400 seconds, a large amount of scale will be generated if there is no plating layer on the surface, and excessive alloying will occur if there is a plating layer on the surface. Therefore, the holding time is set to 2400 seconds or less. Preferably, it is set to 2000 seconds or less, 1000 seconds or less, or 500 seconds or less. On the other hand, if the holding time is less than 10 seconds, ferrite will not sufficiently transform to austenite. The area fraction of crystal grains with an average crystal orientation misorientation of 0.4° or more will increase, and the ductility of the hot-stamped steel will decrease. Therefore, the holding time is set to 10 seconds or more. Preferably, it is set to 50 seconds or more, 100 seconds or more, or 200 seconds or more.

[0052] [Hot forming process] Next, the steel sheet after holding is cooled to a temperature range of transformation heat generation start temperature -20°C to transformation heat generation start temperature -150°C at an average cooling rate of 5 to 30°C / s, and forming is initiated in this temperature range. If the average cooling rate is less than 5°C / s, the transformation from austenite to ferrite proceeds excessively, making it impossible to ensure a residual structure of 1% or more in area percentage. If the average cooling rate exceeds 30°C / s, the cooling rate is too fast to allow sufficient diffusion of Fe atoms and C atoms, making it impossible to ensure an area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° of 80% or more. Cooling at an average cooling rate of 5 to 30°C / s is preferably performed by air cooling.

[0053] Forming is initiated in a temperature range below the transformation heat generation start temperature -20°C in order to obtain the desired amount of ferrite in the hot stamped body with an average crystal orientation mismatch of less than 0.4°. Transformation to ferrite with an average crystal orientation mismatch of less than 0.4° proceeds in the high temperature range. The transformation heat generation slows the temperature drop, allowing the transformation of ferrite with an average crystal orientation mismatch of less than 0.4° to proceed. The temperature at which the temperature rises due to the transformation heat generation is the transformation heat generation start temperature.

[0054] At temperatures below the transformation heat generation start temperature -150°C, the ferrite transformation has progressed sufficiently, and forming at temperatures below this temperature leaves residual strain due to forming, resulting in reduced elongation. Therefore, the lower limit of the forming temperature is set to the transformation heat generation start temperature -150°C. The preferred lower limit is the transformation heat generation start temperature -100°C or the transformation heat generation start temperature -50°C.

[0055] The transformation exothermic start temperature can be determined by measuring the change in temperature of a steel sheet removed from a heating furnace and then taking the second derivative of the steel sheet temperature with respect to elapsed time. A steel sheet having the chemical composition shown in Table 1 was heated to 940°C and removed from the heating furnace. Figure 1A shows the relationship between the time elapsed after the steel sheet was removed from the heating furnace and its temperature, and Figure 1B shows the second derivative of the steel sheet temperature with respect to elapsed time. A steel sheet having the chemical composition shown in Table 1 was heated to 980°C and removed from the heating furnace. Figure 2A shows the relationship between the time elapsed after the steel sheet was removed from the heating furnace and its temperature, and Figure 2B shows the second derivative of the steel sheet temperature with respect to elapsed time. The steel sheet temperatures shown in Figures 1A and 2A were measured using a thermocouple attached to the steel sheet at a sampling interval of 0.5 s. The values ​​in Table 1 are the contents in mass%, with the remainder consisting of Fe and impurities.

[0056] [Table 1]

[0057] The time when transformation heat generation starts can be determined by taking the second derivative of the steel sheet temperature with respect to elapsed time. In Figures 1B and 2B, the time when the second derivative of the temperature with respect to elapsed time changes from a positive to a negative value is the time when transformation heat generation reaches a maximum. The inflection point immediately before this maximum transformation heat generation is the time when transformation heat generation starts.

[0058] The transformation heat generation start temperature can be obtained by determining the time when transformation heat generation starts from a graph obtained by quadratically differentiating the steel sheet temperature with respect to elapsed time and then calculating the steel sheet temperature at that time. That is, the transformation heat generation start temperature can be obtained by determining the time when transformation heat generation starts from Figures 1B and 2B and then calculating the steel sheet temperature at that time from Figures 1A and 2A.

[0059] [Rapid cooling process] After forming, the hot-stamped steel is held in a die and cooled to a temperature range of 400°C or less at an average cooling rate of 20 to 1000°C / s. This allows for the production of a hot-stamped steel with an excellent metal structure. If the average cooling rate is less than 20°C / s, it is impossible to ensure 1% or more of martensite. If the average cooling rate exceeds 1000°C / s, the total area fraction of ferrite with an average crystal orientation misorientation of less than 0.4° and ferrite with an average crystal orientation misorientation of 0.4° or more cannot be made 95% or more, resulting in reduced ductility of the hot-stamped steel. Furthermore, if the cooling termination temperature exceeds 400°C, the transformation to martensite does not proceed, making it impossible to obtain the desired amount of martensite and ensuring strength. The cooling rate can be adjusted by lowering the coolant temperature, using a die with high thermal conductivity, increasing the pressure to increase thermal conductivity, or spraying water on the hot-stamped steel after hot stamping. The average cooling rate is preferably 100°C / s or more, or 200°C / s or more. The average cooling rate is preferably 1000° C. / s or less, or 500° C. / s or less.

[0060] The hot-stamped steel according to this embodiment can be obtained by the method described above. The steel sheet used for the hot-stamped steel according to this embodiment has a low C content and low strength, so it is joined with a steel sheet that becomes high-strength after hot stamping to form a tailored blank, which is then hot stamped to form into a vehicle body part. This vehicle body part has low-strength portions and high-strength portions because it is manufactured by hot stamping a tailored blank made of low-strength material and high-strength material.

[0061] The welding method used to manufacture the tailored blank may be any of a variety of methods, including laser welding, seam welding, arc welding, and plasma welding, but is not particularly limited thereto. Furthermore, the high-strength material (steel plate that becomes high-strength after hot stamping) used together with the low-strength material applied to the hot-stamped steel according to this embodiment is also not particularly limited. Appropriate materials may be selected for each part to be manufactured.

[0062] There is no problem in manufacturing vehicle body parts and the like using only a steel sheet having the chemical composition of the hot-stamped product according to this embodiment without applying the steel sheet to a tailored blank. There is no problem in manufacturing a blank by joining steel sheets by spot welding, such as for patchwork, and then hot stamping the blank. [Example]

[0063] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0064] Cold-rolled steel sheets and plated steel sheets having the chemical compositions shown in Tables 2A and 2B were hot-stamped under the conditions shown in Tables 3A to 3C to obtain the hot-stamped products shown in Tables 4A to 4C. Prior to hot stamping, a thermocouple was attached to the center of the steel sheets, and the steel sheets were removed from the heating furnace and allowed to cool naturally in the air. The temperature change over time was measured to determine the transformation exotherm start temperature. In the coating columns in Tables 4A to 4C, CR indicates no coating, GI indicates hot-dip galvanized, GA indicates alloyed hot-dip galvanized, and AlSi indicates aluminum-silicon coating.

[0065] Hot stamping was performed by clamping the steel sheet between flat water molds and applying pressure to facilitate the preparation of tensile test specimens and test specimens for metallographic observation. JIS No. 5 test specimens were taken from the hot-stamped steel sheets (hot-stamped products), and tensile tests were performed in accordance with JIS Z 2241:2011 to determine the tensile (maximum) strength (MPa) and total elongation (%). Samples for metallographic observation were also taken, and the area fraction of the metallographic structure was determined using the method described above. A tensile strength of 400 MPa or more was judged to be high strength and passed, while a tensile strength of less than 400 MPa was judged to be unsatisfactory. A total elongation of 25% or more was judged to be excellent in ductility and passed, while a total elongation of less than 25% was judged to be unsatisfactory. Furthermore, an elongation of 30% or more was judged to be even more excellent in ductility.

[0066] Crack propagation properties were evaluated using the following method. Charpy test specimens measuring 1.2 mm thick, 55 mm long, and 10 mm wide were cut from the hot-stamped steel sheets (hot-stamped compacts). The longitudinal axis of the test specimen was aligned with the rolling direction, and a 2 mm V-notch was formed perpendicular to the rolling direction. Three of the prepared specimens were stacked and secured with screws for instrumented impact testing. The instrumented impact test was conducted at room temperature, and the time and impact force were measured from the start to the end of the test. The displacement was calculated from the product of the test speed and the measured time. Since the fracture surface of the Charpy test specimen was 8 mm long, the average impact force observed in the region with a displacement of 8 mm or more was used as the background. After subtracting the background from the impact force at all measurement points, an impact force-displacement curve was created. Figure 3 shows a schematic diagram of the impact force-displacement curve. The area under the curve from 0 mm to 8 mm was calculated, and the resulting value was used as the total impact energy. Next, the impact force at which the impact force-displacement curve begins to drop sharply (the impact force at crack initiation in Figure 3) was found, and the corresponding displacement (displacement at crack initiation) was determined. The area under the curve from 0 mm displacement to the displacement at crack initiation was calculated as the energy at crack initiation (crack initiation energy). The crack propagation energy was determined by subtracting the crack initiation energy from the total absorbed energy. The ratio of the crack propagation energy to the total impact energy was used as an index of crack propagation resistance. If the ratio of the crack propagation energy to the total impact energy was 20% or more, the specimen was deemed to have excellent crack propagation resistance and was judged to have passed (OK), and if it was less than 20%, it was judged to have failed (NG).

[0067] The test results are shown in Tables 4A to 4C.

[0068] [Table 2A]

[0069] [Table 2B]

[0070] [Table 3A]

[0071] [Table 3B]

[0072] [Table 3C]

[0073] [Table 4A]

[0074] [Table 4B]

[0075] [Table 4C]

[0076] According to Tables 2A to 4C, the invention examples, whose chemical compositions and metal structures were within the ranges of the present invention, were excellent in ductility and crack propagation resistance, and had a tensile strength of 400 MPa or more. On the other hand, the comparative examples, whose chemical compositions and / or metal structures were outside the scope of the present invention, exhibited poor tensile strength, elongation or crack propagation resistance. [Industrial Applicability]

[0077] According to the above aspect of the present invention, a hot-stamped steel sheet having excellent strength, ductility, and crack propagation resistance can be provided.

Claims

1. The chemical composition, in mass%, is C: 0.010-0.100%, Mn: 0.50 to 1.50%, Al: 0.10-1.00%, Nb: 0.005-0.050%, Ti: 0.010 to 0.080%, Cr: 0.05-0.20%, P: 0.500% or less, S: 0.1000% or less, N: 0.0100% or less, Si: 0 to 0.50%, V: 0-0.10%, Zr: 0 to 0.10%, Mo: 0-1.0%, B: 0 to 0.005%, The sum of Ni and Cu: 0 to 2.0%, and Sn, Sb and As total: 0 to 0.10% Contains the balance being Fe and impurities; When a crystal grain is defined as an object surrounded by a grain boundary having a crystal orientation misorientation of 5° or more, and the average crystal orientation misorientation within the crystal grain is calculated, the crystal grain is composed of, in area percentage, 80% or more of ferrite having an average crystal orientation misorientation of less than 0.4°, less than 20% of ferrite having an average crystal orientation misorientation of 0.4° or more, and 1 to 5% of a remaining structure, the remaining structure being composed of at least one of martensite and bainite, The metal structure has a total area percentage of ferrite having an average crystal orientation misorientation of less than 0.4° and ferrite having an average crystal orientation misorientation of 0.4° or more of 95 to 99%. A hot stamped product characterized by:

2. The chemical composition is, in mass %, V: 0.005-0.10%, Zr: 0.005 to 0.10%, Mo: 0.005-1.0%, B: 0.0002 to 0.005%, and Sum of Ni and Cu: 0.005 to 2.0% The hot-stamped product according to claim 1, characterized in that it contains one or more members selected from the group consisting of

3. 3. The hot-stamped product according to claim 1, further comprising a plating layer on the surface thereof.

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