Steel sheet and member, and method for producing same

Abstract

Provided is a steel sheet that has TS of 780 MPa or more and is excellent in press moldability and hydrogen embrittlement resistance after processing. In this invention, the component composition is appropriately controlled, the area ratio of residual austenite is controlled to 2%-40%, the amount of diffusible hydrogen in steel is controlled to 0.50 ppm by mass or less, and the amount of solid solution hydrogen in residual austenite is controlled to 0.30 ppm by mass or less.

Description

Steel plates and components, and their manufacturing methods

[0001] The present invention relates to a steel plate, a member made from the steel plate, and a method for manufacturing the same.

[0002] In recent years, from the perspective of global environmental conservation, ２ There is a strong demand for improved fuel efficiency to reduce emissions. Accordingly, there has been an active movement to reduce the weight of automobile bodies by thinning the thickness of their body parts. This has led to an increasing need for higher strength steel sheets used as materials for automobile body parts (hereinafter referred to as automotive steel sheets). In particular, there is a high demand for higher strength steel sheets used around the cabin from the perspective of ensuring automobile collision safety.

[0003] For example, Patent Documents 1 to 3 disclose techniques relating to the above-mentioned steel sheets for automobiles.

[0004] JP-A-2-101117 Patent No. 5463685 Patent No. 4894863

[0005] Meanwhile, automotive steel sheets are also required to have excellent press formability so as not to impair the degree of freedom in the shape of parts. Furthermore, automotive steel sheets are also required to have excellent hydrogen embrittlement resistance (hereinafter also referred to as hydrogen embrittlement resistance), particularly after processing such as press forming to make parts (hereinafter also referred to as post-processing hydrogen embrittlement resistance). Hydrogen embrittlement is a phenomenon in which hydrogen atoms are absorbed into steel sheets, reducing the ductility and toughness of the steel sheets and making them brittle.

[0006] However, none of the techniques disclosed in Patent Documents 1 to 3 can be said to simultaneously achieve a TS of 780 MPa or more, excellent press formability, and excellent resistance to hydrogen embrittlement after working. Therefore, there is a demand for the development of a steel sheet that has a TS of 780 MPa or more and is also excellent in press formability and resistance to hydrogen embrittlement after working.

[0007] The present invention has been developed to meet the above-mentioned demands, and aims to provide a steel sheet having a TS of 780 MPa or more and excellent press formability and post-forming hydrogen embrittlement resistance, together with an advantageous manufacturing method thereof. Another aim of the present invention is to provide a member made from the above-mentioned steel sheet, and a manufacturing method thereof. In this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0008] The inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have made the following discoveries. (1) In order to achieve both TS: 780 MPa or more and excellent press formability, it is effective to appropriately control the chemical composition and to include a predetermined amount of retained austenite in the steel structure. (2) In order to improve hydrogen embrittlement resistance while achieving TS: 780 MPa or more and excellent press formability, it is effective to reduce the amount of diffusible hydrogen in the steel. (3) However, in a steel sheet having a steel structure containing retained austenite (hereinafter also referred to as a steel sheet containing retained austenite), simply reducing the amount of diffusible hydrogen in the steel measured in the as-manufactured state does not sufficiently improve hydrogen embrittlement resistance after working.

[0009] (4) To improve the post-processing hydrogen embrittlement resistance of such steel sheets containing retained austenite, it is extremely important to reduce the amount of dissolved hydrogen in the retained austenite in addition to reducing the amount of diffusible hydrogen in the steel. That is, retained austenite has an FCC structure. Therefore, a larger amount of hydrogen can be dissolved in retained austenite than in ferrite with a BCC structure or martensite with a BCT structure. In other words, hydrogen is more likely to remain in retained austenite than in other phases. However, the diffusion rate of dissolved hydrogen in retained austenite is low and it appears to be trapped. Therefore, dissolved hydrogen in retained austenite is usually not counted as diffusible hydrogen in the steel. It is also thought that dissolved hydrogen in retained austenite does not adversely affect hydrogen embrittlement, unlike diffusible hydrogen in the steel. However, when steel sheets are processed, for example, by press forming to form a part, the retained austenite undergoes a deformation-induced transformation to martensite in the processed portion of the steel sheet. As a result, dissolved hydrogen in the retained austenite is converted into diffusible hydrogen in the steel, and the amount of diffusible hydrogen in the steel in the processed portion increases. As a result, hydrogen embrittlement resistance after processing deteriorates. Therefore, in order to improve the hydrogen embrittlement resistance after processing in steel sheets containing retained austenite, it is extremely important to reduce the amount of dissolved hydrogen in the retained austenite. The present invention was completed based on the above findings and further investigations.

[0010] That is, the gist of the present invention is as follows: 1. A steel sheet having a chemical composition, in mass%, of C: 0.050% to 0.350%, Si: 0.01% to 3.00%, Mn: 1.00% to 6.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.01% to 2.00%, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and a steel structure in which an area fraction of retained austenite is 2% to 40%, wherein the amount of diffusible hydrogen in the steel is 0.50 ppm by mass or less, and the amount of dissolved hydrogen in the retained austenite is 0.30 ppm by mass or less, and wherein the tensile strength is 780 MPa or more.

[0011] 2. The chemical composition further includes, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Sb: 0.300% or less, Sn: 0.300% or less, V: 0.100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, 2. The steel sheet according to 1 above, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM (excluding Ce): 0.0200% or less.

[0012] 3. The steel sheet according to 1 or 2 above, which has a plating layer on the surface.

[0013] 4. A member made using the steel plate according to any one of 1 to 3 above.

[0014] 5. a rolling step in which a steel slab having the chemical composition described in 1 or 2 is hot-rolled or further cold-rolled under the conditions of a steel slab heating temperature of 1100 to 1300°C, a finish rolling temperature of 800 to 1000°C, and a coiling temperature of 700°C or less to obtain a rolled steel sheet; a first annealing step in which the rolled steel sheet is held under the conditions of a holding temperature of 650 to 950°C, a holding time of 10 seconds or more, and an atmospheric hydrogen concentration of 0.2 to 30% by volume; a first cooling step in which the rolled steel sheet is cooled according to the following first cooling pattern, second cooling pattern, or third cooling pattern; and a second cooling step in which the rolled steel sheet is held under the conditions of an atmospheric hydrogen concentration of 20% by volume or less, a reheating temperature of 100 to 450°C, and a cooling end temperature of the first cooling step or higher, and a reheating time of 5 to 600 seconds. and a second cooling step of cooling the rolled steel sheet under the conditions of an atmospheric hydrogen concentration of 0.2% by volume or less, a residence time in a temperature range of 100 to 300°C of 2 seconds or more, and a cooling end temperature of 50°C or less. [First cooling pattern] Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C [Second cooling pattern] (First cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 600°C (Second cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C [Third cooling pattern] (First cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 550°C (Intermediate holding) Atmospheric hydrogen concentration: 0.2 to 20% by volume, holding temperature: 350 to 550°C, holding time: 5 to 300 s (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C

[0015] 6. The method for producing a steel sheet according to 5 above, further comprising a second annealing step between the first annealing step and the first cooling step, in which the rolled steel sheet is held under the following conditions: a holding temperature of 650 to 950°C, a holding time of 10 seconds or more, and an atmospheric hydrogen concentration of 0.2 to 8% by volume.

[0016] 7. The method for producing a steel sheet according to 5 or 6 above, wherein the cooling pattern in the first cooling step is the second cooling pattern or the third cooling pattern, the atmospheric hydrogen concentration in the pre-stage cooling is 0.2 to 15% by volume, and the average cooling rate in the pre-stage cooling is 2 to 40°C / s.

[0017] 8. The method for producing a steel sheet according to any one of items 5 to 7, further comprising a plating step of plating the rolled steel sheet during the first cooling step, between the reheating step and the second cooling step, or after the second cooling step.

[0018] 9. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of 1 to 3 above to at least one of forming and joining to form a component.

[0019] According to the present invention, a steel sheet having a TS of 780 MPa or more and excellent press formability and hydrogen embrittlement resistance after working can be obtained. Furthermore, the steel sheet of the present invention can be suitably used as a steel sheet for automobiles, for example, and therefore can be used for reducing the CO2 emissions of automobiles. ２ This effectively contributes to reducing emissions and is extremely advantageous for industry.

[0020] The present invention will be described based on the following embodiments.

[0021] [1] Steel Plate First, the chemical composition of a steel plate according to one embodiment of the present invention will be described. Note that the units for the chemical composition are all "mass%", and hereinafter, unless otherwise specified, they will be simply referred to as "%".

[0022] C: 0.050% or more and 0.350% or less C is an element that stabilizes austenite and improves hardenability. C is also an effective element for obtaining desired retained austenite and strength. Here, if the C content is less than 0.050%, desired retained austenite and strength cannot be obtained. Therefore, the C content is 0.050% or more, preferably 0.070% or more, and more preferably 0.080% or more. On the other hand, if the C content exceeds 0.350%, the strength of martensite becomes excessively high, which causes deterioration of press formability and post-processing hydrogen embrittlement resistance. Therefore, the C content is 0.350% or less, preferably 0.330% or less, and more preferably 0.320% or less.

[0023] Si: 0.01% or more and 3.00% or less Si is an element that suppresses the formation of carbides such as cementite and promotes the concentration of C in austenite. In other words, Si is an element that is effective in obtaining stable retained austenite. To significantly obtain this effect, the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 3.00%, a large amount of Si oxide is generated on the surface of the steel sheet, deteriorating the surface quality. This causes deterioration in plating processability, resulting in defects such as non-plating, and uneven appearance due to poor plating alloying, which leads to deterioration in appearance quality. It also leads to deterioration in weldability. Therefore, the Si content is set to 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0024] Mn: 1.00% or more and 6.00% or less Mn improves hardenability and suppresses excessive ferrite transformation and pearlite transformation during cooling after annealing. That is, Mn is an effective element for obtaining the required strength. Mn is also an important element for stabilizing austenite and obtaining the required amount of retained austenite. To achieve these effects, the Mn content is 1.00% or more, preferably 1.20% or more, and more preferably 1.40% or more. On the other hand, a Mn content exceeding 6.00% increases costs. It also deteriorates plating processability, causing defects such as unplated areas, which can lead to deterioration in appearance quality. Therefore, the Mn content is 6.00% or less, preferably 5.80% or less, and more preferably 5.50% or less.

[0025] P: 0.100% or less P is an element effective in strengthening steel. On the other hand, if P is contained in excess, P segregates at prior austenite grain boundaries, embrittling the grain boundaries and causing deterioration of press formability. Therefore, the P content is 0.100% or less, preferably 0.050% or less. The lower limit of the P content is not particularly limited and may be 0%. However, since excessive dephosphorization increases costs, the P content is preferably 0.001% or more.

[0026] S: 0.0200% or less S segregates at grain boundaries and embrittles steel during hot working. S also generates sulfides such as MnS, which deteriorates press formability. Therefore, the lower the S content, the better. Therefore, the S content is 0.0200% or less, preferably 0.0150% or less, and more preferably 0.0100% or less. The lower limit of the S content is not particularly limited and may be 0%. However, since excessive desulfurization increases costs, the S content is preferably 0.0001% or more.

[0027] Al: 0.01% or more and 2.00% or less Al is an element that suppresses the formation of carbides such as cementite and promotes the concentration of C in austenite. That is, Al is an element effective in obtaining stable retained austenite. Furthermore, Al acts as a deoxidizer and is also an element effective in purifying steel. To obtain these effects significantly, the Al content is set to 0.01% or more. On the other hand, an Al content exceeding 2.00% increases the risk of cracking of the steel billet during continuous casting, reducing productivity. Furthermore, it can cause deterioration in plating processability, resulting in defects such as non-plating, and uneven appearance due to poor plating alloying, leading to deterioration in appearance quality. Therefore, the Al content is set to 2.00% or less, preferably 1.80% or less, and more preferably 1.60% or less.

[0028] N: 0.0100% or less N is an element that deteriorates the aging resistance of steel. In particular, if the N content exceeds 0.0100%, the deterioration of the aging resistance of steel becomes significant. Therefore, the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less. The lower limit of the N content is not particularly limited and may be 0%. However, since excessive denitrification increases costs, the N content is preferably 0.0005% or more.

[0029] The basic composition of a steel sheet according to one embodiment of the present invention has been described above. The steel sheet according to one embodiment of the present invention contains the elements according to the above basic composition (hereinafter also referred to as basic element), with the balance other than the basic element including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic element, with the balance consisting of Fe and unavoidable impurities. The steel sheet according to one embodiment of the present invention may contain, in addition to the above basic element, at least one element selected from the following, either alone or in combination, as an optional additional element:

[0030] B: 0.0100% or less B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries, and is useful for controlling the structure. To achieve this effect, the B content is preferably 0.0001% or more, and more preferably 0.0003% or more. On the other hand, excessive B content may deteriorate press formability. Therefore, the B content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0031] Ti: 0.200% or less Ti has the effect of precipitating fine carbides and increasing the strength of steel. To achieve this effect, the Ti content is preferably 0.001% or more. On the other hand, excessive Ti content may cause the carbides to become coarse, resulting in deterioration of press formability. Therefore, the Ti content is preferably 0.200% or less, and more preferably 0.100% or less.

[0032] Nb: 0.200% or less Nb has the effect of precipitating fine carbides and increasing the strength of steel. To achieve this effect, the Nb content is preferably 0.001% or more. On the other hand, excessive Nb content may cause the carbides to become coarse, resulting in deterioration of press formability. Therefore, the Nb content is preferably 0.200% or less, and more preferably 0.100% or less.

[0033] Sb: 0.300% or less Sb is an element that is effective in suppressing excessive decarburization of the steel sheet surface and preventing a decrease in the amount of martensite formed. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, excessive Sb content leads to a deterioration in toughness. Therefore, the Sb content is preferably 0.300% or less, and more preferably 0.100% or less.

[0034] Sn: 0.300% or less Sn is an element that is effective in suppressing excessive decarburization of the steel sheet surface and preventing a decrease in the amount of martensite formed. To achieve this effect, the Sn content is preferably 0.001% or more. On the other hand, excessive Sn content leads to a deterioration in toughness. Therefore, the Sn content is preferably 0.300% or less, and more preferably 0.100% or less.

[0035] V: 0.100% or less V precipitates fine carbides, contributing to an increase in the strength of steel. To achieve this effect, the V content is preferably 0.001% or more. On the other hand, excessive V content may cause the carbides to become coarse, resulting in a deterioration in press formability. Therefore, the V content is preferably 0.100% or less, and more preferably 0.060% or less.

[0036] Cu: 1.000% or less Cu is an element that improves the hardenability of steel. In other words, Cu is an element that is effective in controlling the area ratio of hard phases such as martensite within a suitable range. To achieve this effect, the Cu content is preferably 0.005% or more, and more preferably 0.020% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Cu content is preferably 1.000% or less, and more preferably 0.800% or less.

[0037] Cr: 1.000% or less By adding Cr, the balance between strength and ductility can be improved. To achieve this effect, the Cr content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Cr content is preferably 1.000% or less, and more preferably 0.800% or less.

[0038] Ni: 1.000% or less By adding Ni, the balance between strength and ductility can be improved. To achieve this effect, the Ni content is preferably 0.005% or more. On the other hand, from the viewpoint of suppressing excessive cost increase, the Ni content is preferably 1.000% or less, and more preferably 0.800% or less.

[0039] Mo: 1.000% or less The addition of Mo can improve the balance between strength and ductility. To achieve this effect, the Mo content is preferably 0.005% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Mo content is preferably 1.000% or less, more preferably 0.500% or less, and even more preferably 0.450% or less.

[0040] Ta: 0.100% or less By including Ta, it is possible to obtain the effect of improving strength. To obtain this effect, the Ta content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing excessive cost increase, the Ta content is preferably 0.100% or less, and more preferably 0.050% or less.

[0041] W: 0.500% or less The inclusion of W can provide an effect of improving strength. To achieve this effect, the W content is preferably 0.001% or more, and more preferably 0.003% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the W content is preferably 0.500% or less, and more preferably 0.450% or less.

[0042] Zr: 0.0200% or less Zr increases the ultimate deformability of the steel sheet and improves press formability. To achieve this effect, the Zr content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Zr content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0043] Ca: 0.0200% or less Ca is effective in controlling the morphology of sulfides and improves ductility and toughness. To achieve this effect, the Ca content is preferably 0.0005% or more. On the other hand, excessive Ca content may actually deteriorate ductility. Therefore, the Ca content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0044] Mg: 0.0200% or less Mg is effective in controlling the morphology of sulfides and improves ductility and toughness. To achieve this effect, the Mg content is preferably 0.0005% or more. On the other hand, excessive Mg content can actually lead to a deterioration in ductility. Therefore, the Mg content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0045] Zn: 0.0200% or less Zn increases the ultimate deformability of the steel sheet and improves press formability. To achieve this effect, the Zn content is preferably 0.0005% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Zn content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0046] Co: 0.0200% or less Co increases the ultimate deformability of the steel sheet and improves press formability. To achieve this effect, the Co content is preferably 0.0010% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the Co content is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0047] Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM (excluding Ce): 0.0200% or less. These elements increase the ultimate deformability of steel sheets and improve press formability. To achieve this effect, it is preferable to contain at least one of these elements in an amount of 0.0001% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, the content of each of these elements is preferably 0.0200% or less, more preferably 0.0100% or less. Note that REM as used here is a collective term for 16 elements, including 14 lanthanoid elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, excluding Ce, Sc (scandium) with atomic number 21, and Y (yttrium) with atomic number 39. These 16 elements can be contained alone or in combination. Note that the REM content refers to the total content of these 16 elements. Among the REMs, La is particularly preferable.

[0048] The balance other than the above elements is Fe and inevitable impurities. Note that any of the above optional elements may be 0%. Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are allowed to be contained within a range that does not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include O (oxygen). Furthermore, when the content of each of the above optional elements is less than the preferred lower limit, it can be said that the element is contained as an inevitable impurity.

[0049] Next, the reasons for limiting the steel structure of the steel plate according to one embodiment of the present invention will be explained. The area ratio of each phase is the area ratio that each phase occupies with respect to the entire structure.

[0050] Area fraction of retained austenite: 2% or more and 40% or less To ensure sufficient ductility and obtain excellent press formability, the area fraction of retained austenite is 2% or more, preferably 5% or more. On the other hand, if the amount of retained austenite is excessive, the retained austenite undergoes work-induced transformation to hard martensite during press forming, promoting crack generation. Furthermore, the amount of hydrogen that converts to diffusible hydrogen in the steel during press forming may increase excessively, potentially deteriorating hydrogen embrittlement resistance after processing. Therefore, the area fraction of retained austenite is 40% or less, preferably 30% or less.

[0051] The steel structure of a steel plate according to one embodiment of the present invention may have an area ratio of retained austenite of 2% to 40%. However, the remaining structure other than the retained austenite may contain ferrite, martensite, bainite, and other structures. The preferred area ratios of each phase are as follows:

[0052] Ferrite area ratio: 70% or less (including 0%). If ferrite is excessive, it may be necessary to include a large amount of alloying elements to achieve a TS of 780 MPa or more. Therefore, the ferrite area ratio is preferably 70% or less, more preferably 65% ​​or less, and even more preferably 60% or less. The lower limit of the ferrite area ratio is not particularly limited and may be 0%. Furthermore, the ferrite area ratio is preferably 2% or more, more preferably 5% or more.

[0053] Area fraction of martensite: 10% or more and 98% or less Martensite is generated from austenite in a temperature range below the martensitic transformation start point (hereinafter also referred to as the Ms point). Note that the martensite referred to here includes so-called as-quenched martensite (hereinafter also referred to as fresh martensite) as well as tempered martensite obtained by tempering fresh martensite. By utilizing martensite, it becomes easy to achieve a TS of 780 MPa or more. Therefore, the area fraction of martensite is preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more. The upper limit of the area fraction of martensite may be, for example, 98% or less. Furthermore, the area fraction of martensite is preferably 95% or less, more preferably 90% or less.

[0054] Area Fraction of Bainite: 50% or Less (Including 0%) Bainite is formed from austenite at relatively low temperatures above the Ms point. Bainite consists of fine carbides dispersed in acicular or plate-like ferrite. Bainite also has a hardness intermediate between that of ferrite and martensite. Therefore, adding a certain amount of bainite to the steel structure helps to balance strength and ductility. The lower limit of the area fraction of bainite is not particularly limited, and may be 0%. To achieve the above effects, the area fraction of bainite is preferably 5% or more, more preferably 10% or more. Furthermore, excessive bainite in the steel structure may make it difficult to control the area fractions of retained austenite and martensite in the steel structure within the specified ranges. Therefore, the area fraction of bainite is preferably 50% or less, more preferably 45% or less, and even more preferably 40% or less.

[0055] Area ratio of other structures: 10% or less (including 0%) The other structures are phases other than retained austenite, ferrite, martensite, and bainite. The other structures may include, for example, pearlite and carbides such as cementite (excluding cementite in pearlite). From the viewpoint of more advantageously obtaining the predetermined effects, the area ratio of other structures is preferably 10% or less, and more preferably 5% or less. There is no particular lower limit for the area ratio of other structures, and it may be 0%.

[0056] The area ratio of each phase can be measured according to a conventional method. The area ratio of each phase can be measured, for example, as follows. A sample is cut out from a steel sheet so that the observation surface is a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet. The observation surface of the sample is then polished and etched with nital. An area of ​​60 μm × 80 μm of the observation surface of the sample is then observed and photographed using a scanning electron microscope (SEM) at a magnification of 1500x to obtain image data. In the image data, retained austenite and fresh martensite are observed as white regions. Ferrite is observed as black regions (excluding island-shaped black regions contained in bainite). Bainite is observed as island-shaped black regions or gray regions containing uniformly oriented carbides. Tempered martensite is observed as light gray regions containing fine, randomly oriented carbides. Then, in the image data, the areas occupied by retained austenite, martensite (fresh martensite + tempered martensite), ferrite, and bainite are each determined. Next, the areas of the areas occupied by retained austenite, martensite, ferrite, and bainite are each divided by the area of ​​the entire image data, and the result is multiplied by 100 to obtain the area ratios of retained austenite, martensite, ferrite, and bainite. The area ratios of other structures are determined by subtracting the area ratios of retained austenite, martensite, ferrite, and bainite determined as above from 100%.

[0057] In addition, when it is difficult to distinguish between retained austenite and fresh martensite, the area fractions of retained austenite and fresh martensite may be measured as follows. That is, with an observation position at 1 / 4 of the steel plate thickness, an X-ray diffractometer is used with CoKα radiation to determine the ratio of the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks of the {200}, {211}, and {220} planes of bcc iron. Next, the volume fraction of retained austenite is calculated from the ratio of the integrated intensities of each plane. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area fraction of retained austenite. In addition, the area of ​​the regions observed as white (retained austenite and fresh martensite) in the above image data is divided by the area of ​​the entire image data, multiplied by 100, and the area ratio of retained austenite measured by X-ray diffraction is subtracted from this value to determine the area ratio of fresh martensite.

[0058] Amount of diffusible hydrogen in steel: 0.50 mass ppm or less From the viewpoint of obtaining excellent hydrogen embrittlement resistance and further excellent hydrogen embrittlement resistance after processing, the amount of diffusible hydrogen in steel is 0.50 mass ppm or less, preferably 0.45 mass ppm or less, and more preferably 0.40 mass ppm or less. The lower limit of the amount of diffusible hydrogen in steel is not particularly limited, and may be 0 mass ppm.

[0059] Here, the amount of diffusible hydrogen in steel refers to the amount of hydrogen H (ppm by mass) released from a steel sheet when the steel sheet is heated from room temperature (for example, 0 to 40°C, typically 25°C) to 200°C at a heating rate of 200°C / h by thermal desorption analysis.

[0060] The amount of diffusible hydrogen in steel can be measured, for example, as follows. Specifically, a 5 × 30 mm sample is collected from a steel sheet. If the steel sheet has a plating layer on its surface, the plating layer is removed using a precision grinder or the like. The sample is then loaded into a quartz tube whose atmosphere is replaced with Ar gas. The sample is then heated from room temperature to 200°C at a heating rate of 200°C / h, and the amount of hydrogen released from the sample during the heating period is measured using a gas chromatograph. The integrated value of the amount of hydrogen released in the temperature range from room temperature to 200°C is defined as the amount of diffusible hydrogen (ppm by mass). The timing of measuring the amount of diffusible hydrogen in steel is not particularly limited. For example, it is preferable to measure the amount of diffusible hydrogen in steel within 72 hours after the steel sheet first reaches room temperature (40°C or less) after the second cooling step in the manufacturing method described below. The diffusible hydrogen in steel is primarily present in ferrite with a BCC structure and martensite with a BCT structure.

[0061] Amount of dissolved hydrogen in retained austenite: 0.30 mass ppm or less As described above, when a steel sheet is processed, for example, by press forming to form a part, the retained austenite undergoes a work-induced transformation to martensite in the processed portion of the steel sheet. This converts the dissolved hydrogen in the retained austenite into diffusible hydrogen in the steel, increasing the amount of diffusible hydrogen in the steel in the processed portion. As a result, the post-processing hydrogen embrittlement resistance deteriorates. Therefore, reducing the amount of dissolved hydrogen in the retained austenite is important to improve the post-processing hydrogen embrittlement resistance of a steel sheet containing retained austenite. In particular, by setting the amount of dissolved hydrogen in the retained austenite to 0.30 mass ppm or less, the post-processing hydrogen embrittlement resistance can be significantly improved. Therefore, the amount of dissolved hydrogen in the retained austenite is 0.30 mass ppm or less, preferably 0.25 mass ppm or less, and more preferably 0.20 mass ppm or less. Since there is no advantage to leaving dissolved hydrogen in the retained austenite, the lower limit of the amount of dissolved hydrogen in the retained austenite is not particularly limited, and may be 0 ppm by mass.

[0062] Here, the amount of dissolved hydrogen in retained austenite is measured as follows. That is, a sample taken from a steel plate is subjected to the following [A] or [B]. Next, the sample is heated from room temperature to 200°C at a heating rate of 200°C / h using thermal desorption analysis, and the amount of hydrogen H1 (ppm by mass) released from the sample is measured. For example, this can be measured in the same manner as the amount of diffusible hydrogen in steel (H2) described above. The value obtained by subtracting H2 from H1 (H1 - H2) is then taken as the amount of dissolved hydrogen in retained austenite. [A] A tensile test is conducted in accordance with JIS Z2241 (2011), and a tensile load (maximum tensile load) that results in uniform elongation is applied to the sample. [B] The sample is held at a temperature range of -180°C or lower for 24 hours or more. In addition, if the area fraction of retained austenite in the sample after performing the above [A] or [B] is 2% or more, a tensile load exceeding the uniform elongation may be applied in [A], or the sample may be held at a lower temperature range for a longer period of time in [B].

[0063] TS: 780 MPa or more The TS of a steel sheet according to one embodiment of the present invention is 780 MPa or more, preferably 880 MPa or more, and more preferably 980 MPa or more. There is no particular upper limit for the TS of a steel sheet according to one embodiment of the present invention. For example, the TS is preferably 2000 MPa or less.

[0064] Furthermore, the steel sheet according to one embodiment of the present invention may have a plating layer on its surface. The plating layer may be provided on only one surface of the steel sheet, or on both surfaces. The type of plating layer is not particularly limited. Examples of the type of plating layer include a zinc plating layer containing Zn as the main component (Zn content of 50.0% by mass or more) and an aluminum plating layer containing Al as the main component (Al content of 50.0% by mass or more). From the viewpoint of use as an automotive steel sheet, a zinc plating layer is preferred as the plating layer.

[0065] Examples of the zinc-plated layer include a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer, and a vapor-deposited zinc-plated layer. A steel sheet having a zinc-plated layer can also be referred to as a galvanized steel sheet. Furthermore, the steel sheets having the above-mentioned hot-dip galvanized layer, galvannealed layer, and electrogalvanized layer can also be referred to as a hot-dip galvanized steel sheet (GI), a galvannealed steel sheet (GA), an electrogalvanized steel sheet (EG), and a vapor-deposited zinc-plated steel sheet, respectively.

[0066] The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder of the hot-dip galvanized layer other than the above elements is unavoidable impurities.

[0067] The galvannealed layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the galvannealed layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. The Fe content of the galvannealed layer is more preferably 15.0 mass% or less, and even more preferably 12.0 mass% or less. The remainder of the galvannealed layer other than the above elements is unavoidable impurities.

[0068] The electrogalvanized layer and vapor-deposited zinc-plated layer preferably have a Zn content of 70.0% by mass or more. The electrogalvanized layer may optionally contain one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Cr, Mg, Si, Al, Zr, V, Cu, Pb, Sb, Sn, Ca, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more but less than 30.0% by mass. The remainder of the electrogalvanized layer and vapor-deposited zinc-plated layer other than the above elements is unavoidable impurities.

[0069] The amount of zinc coating per side is not particularly limited, but is preferably 20 g / m ２ 80g / m or more ２ The following is preferred:

[0070] An example of the aluminum plating layer is a hot-dip aluminum plating layer. Note that a steel sheet having an aluminum plating layer can also be referred to as an aluminum-plated steel sheet. Furthermore, a steel sheet having the above-described hot-dip aluminum plating layer can also be referred to as a hot-dip aluminum-plated steel sheet.

[0071] The hot-dip aluminum plating layer is preferably composed of, for example, Al and less than 50.0 mass% Fe. The hot-dip aluminum plating layer may optionally contain one or more elements selected from the group consisting of Zn, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 25.0 mass%. The Fe content of the hot-dip aluminum plating layer is more preferably less than 25.0 mass%. The remainder of the hot-dip aluminum plating layer other than the above elements is unavoidable impurities.

[0072] The plating weight of the aluminum plating layer per side is not particularly limited, but is preferably 20 g / m ２ 120g / m or more ２ The following is preferred:

[0073] The coating weights of the zinc plating layer and the aluminum plating layer are measured, for example, as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the plating layer. The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the portion that was covered with plating) to determine the coating weight (g / m ２ ) is calculated.

[0074] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.2 mm or more and 3.2 mm or less.

[0075] Furthermore, "excellent press formability" means that TS × El is 11,500 MPa·% or more. "Excellent resistance to hydrogen embrittlement after processing" means that the critical stress / YS is 0.85 or more, preferably 1.00 or more. Here, the critical stress is the maximum stress that can be applied to a test piece punched with a hole under the conditions described in the examples below in a constant-load uniaxial tensile test according to the procedures described in the examples below without cracking. Furthermore, the procedures for measuring TS, El, YS, and critical stress are as described in the examples below.

[0076] [2] Member Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above-mentioned steel plate has a TS of 780 MPa or more, and is also excellent in press formability and hydrogen embrittlement resistance after processing. Therefore, the member according to one embodiment of the present invention is particularly suitable as, for example, an automobile body part. This makes it possible to improve fuel efficiency by reducing the weight of the automobile body, and CO ２ This will make a significant contribution to reducing emissions.

[0077] [3] Manufacturing Method of Steel Plate Next, a manufacturing method of a steel plate according to one embodiment of the present invention will be described. Unless otherwise specified, the temperatures in each step are the surface temperatures of the steel slab and the steel plate (rolled steel plate). Furthermore, unless otherwise specified, the average cooling rate is also based on the surface temperatures of the steel slab and the steel plate (rolled steel plate). The average cooling rate can be calculated using the following formula: [Average cooling rate (°C / s)] = ([Cooling start temperature (°C)] - [Cooling end temperature (°C)]) / Cooling time (s). The cooling start temperature may be, for example, the temperature at the end of the immediately preceding step. For example, the cooling start temperature of the first cooling step may be the holding temperature of the first annealing step or the holding temperature of the second annealing step.

[0078] First, a steel having the above-described chemical composition is melted in a conventional manner and then formed into a steel slab by a continuous casting method, an ingot casting method, a thin slab casting method, etc. Next, the steel slab is hot-rolled under the following conditions, and optionally further cold-rolled to form a rolled steel sheet.

[0079] Heating temperature of steel slab: 1100 to 1300°C. Precipitates such as carbides present at the start of heating of the steel slab remain as coarse precipitates in the final steel sheet (hereinafter also referred to as the final product), causing insufficient strength and deterioration of press formability. Therefore, it is necessary to sufficiently dissolve these precipitates. Here, if the heating temperature of the steel slab is less than 1100°C, the dissolution of the precipitates will be insufficient. Furthermore, problems such as an increased risk of trouble during hot rolling due to increased rolling load will arise. Furthermore, by heating the steel slab to 1100°C or higher, defects such as bubbles and segregations in the surface layer of the steel slab are scaled off. As a result, cracks and irregularities on the surface of the steel sheet are reduced, making it possible to obtain a smooth surface. Therefore, the heating temperature of the steel slab is 1100°C or higher, preferably 1150°C or higher. On the other hand, if the heating temperature of the steel slab exceeds 1300°C, the amount of oxidation increases and scale loss increases. Therefore, the heating temperature of the steel slab is 1300°C or less, preferably 1250°C or less.

[0080] Finish rolling temperature: 800 to 1000°C If the finish rolling temperature is less than 800°C, the rolling load increases, resulting in a large rolling load. In addition, the rolling reduction rate increases when the austenite is in an unrecrystallized state. This causes an abnormal texture to develop, resulting in significant in-plane anisotropy in the final product. As a result, the uniformity of the material is impaired. Furthermore, ductility also decreases. Therefore, the finish rolling temperature is 800°C or higher, preferably 850°C or higher. On the other hand, if the finish rolling temperature exceeds 1000°C, the crystal grains become coarse, which may cause insufficient strength and deterioration of bendability. Therefore, the finish rolling temperature is 1000°C or lower, preferably 950°C or lower.

[0081] Coiling temperature: 700°C or less If the coiling temperature exceeds 700°C, the carbides in the steel sheet will coarsen. As a result, the carbides will not dissolve before the subsequent annealing step, which may cause deterioration in ductility in the final product. Therefore, the coiling temperature is 700°C or less, and preferably 650°C or less. There is no particular restriction on the lower limit of the coiling temperature. Furthermore, from the viewpoint of suppressing the occurrence of shape defects in the steel sheet and preventing the steel sheet from becoming excessively hardened, the coiling temperature is preferably 400°C or more.

[0082] After hot rolling, cold rolling may be further performed. The cold rolling conditions are not particularly limited and may be performed according to conventional methods. For example, a rolled steel sheet (hereinafter also referred to as a hot-rolled steel sheet) obtained by hot rolling may be subjected to pretreatment such as pickling and degreasing according to conventional methods, and then cold-rolled to obtain a cold-rolled steel sheet. Furthermore, in order to reduce the rolling load in cold rolling, the hot-rolled steel sheet may be subjected to heat treatment such as batch annealing, and then cold-rolled. The cold-rolling reduction is not particularly limited. However, if the cold-rolling reduction is less than 20%, the surface flatness may be poor and the structure may become non-uniform. Therefore, the cold-rolling reduction is preferably 20% or more. The upper limit of the cold-rolling reduction is not particularly limited, and for example, the cold-rolling reduction is preferably 80% or less. Note that cold rolling may be omitted.

[0083] First Annealing Step Next, the rolled steel sheet is held (annealed) under the following conditions.

[0084] Holding temperature in the first annealing step: 650 to 950°C. In the first annealing step, annealing is performed in a temperature range of the austenite single phase region or the two-phase region of austenite and ferrite. If the holding temperature in the first annealing step (hereinafter also referred to as the first annealing temperature) is less than 650°C, ferrite recrystallization and reverse transformation to austenite will be insufficient, resulting in insufficient strength and deterioration of press formability. Therefore, the first annealing temperature is 650°C or higher, preferably 700°C or higher. On the other hand, if the first annealing temperature exceeds 950°C, energy consumption will be significant, resulting in increased costs. Therefore, the first annealing temperature is 950°C or lower, preferably 920°C or lower. Furthermore, the first annealing temperature does not have to be constant during holding, as long as it is in the temperature range of 650 to 950°C and the temperature fluctuation is within ±15°C of the set temperature. The same applies to the holding temperature in the second annealing step, the holding temperature for intermediate holding in the first cooling step, and the reheating temperature in the reheating step, which will be described later.

[0085] Holding time of the first annealing step: 10 seconds or more If the holding time of the first annealing step (hereinafter also referred to as the first annealing time) is less than 10 seconds, the recrystallization of ferrite and the reverse transformation to austenite will be insufficient, resulting in insufficient strength and deterioration of press formability. Therefore, the first annealing time is 10 seconds or more, preferably 20 seconds or more. On the other hand, an excessively long first annealing time will result in increased costs. Therefore, the first annealing time is preferably 2000 seconds or less, more preferably 1000 seconds or less. In particular, when the second annealing step described below is performed, the first annealing time is preferably 600 seconds or less, more preferably 300 seconds or less. Note that the first annealing time here refers to the holding time at the first annealing temperature. The same applies to the holding time of the second annealing step, the holding time of the intermediate holding in the first cooling step, and the reheating time in the reheating step described below.

[0086] Atmospheric hydrogen concentration in the first annealing step: 0.2 to 30 vol%. If the atmospheric hydrogen concentration in the first annealing step (hereinafter also referred to as the first annealing hydrogen concentration) is less than 0.2 vol%, the reduction of the Fe oxide film present on the surface of the rolled steel sheet may be insufficient. Furthermore, the first annealing step is performed in a high-temperature range of 650°C or higher. Therefore, if the first annealing hydrogen concentration is less than 0.2 vol%, there is a risk of significant reoxidation of the surface of the rolled steel sheet. Therefore, the first annealing hydrogen concentration is 0.2 vol% or higher, preferably 0.5 vol% or higher, more preferably 1.0 vol% or higher, and even more preferably 2.0 vol% or higher. In particular, when intermediate holding is performed in the second annealing step or cooling step described below, the first annealing hydrogen concentration is preferably 2.0 vol% or higher, more preferably 5.0 vol% or higher. On the other hand, if the first annealing hydrogen concentration exceeds 30 vol%, the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite may not be sufficiently reduced in the final product. Therefore, the hydrogen concentration in the first annealing is 30% by volume or less, and preferably 25% by volume or less.

[0087] The remaining gases other than hydrogen in the atmosphere are, for example, inert gases and gases that are inevitably mixed in. Examples of inert gases include nitrogen, argon, and carbon dioxide. Examples of gases that are inevitably mixed in include water (water vapor) and oxygen. The same applies to the remaining gases other than hydrogen in the atmospheres of the second annealing step, first cooling step, and reheating step described below.

[0088] Second Annealing Step Next, the rolled steel sheet may be further held under the following conditions: Holding temperature: 650 to 950°C Holding time: 10 seconds or more Atmospheric hydrogen concentration: 0.2 to 8% by volume Note that the holding temperature in the second annealing step (hereinafter also referred to as the second annealing temperature) may be controlled within the same range as the above-mentioned first annealing temperature, and therefore a description thereof will be omitted here.

[0089] Holding time of the second annealing step: 10 seconds or more From the viewpoint of obtaining the desired TS and press formability, the holding time of the second annealing step (hereinafter also referred to as the second annealing time) is preferably 10 seconds or more, more preferably 20 seconds or more. On the other hand, an excessively long second annealing time leads to an increase in costs. Therefore, the second annealing time is preferably 2000 seconds or less, more preferably 1000 seconds or less. The sum of the first annealing time and the second annealing time is preferably 20 seconds or more, more preferably 40 seconds or more. Furthermore, the sum of the first annealing time and the second annealing time is preferably 2000 seconds or less, more preferably 1500 seconds or less.

[0090] Atmospheric hydrogen concentration in the second annealing step: 0.2 to 8 vol%. The first annealing step described above requires the reduction of the Fe oxide film present on the surface of the rolled steel sheet. Therefore, the first annealing step is generally performed in an Fe-reducing atmosphere, particularly a hydrogen-containing atmosphere. However, prolonged exposure to a hydrogen-containing atmosphere is undesirable from the viewpoint of reducing the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite in the final product. Therefore, it is preferable to perform the second annealing step after reducing the Fe oxide film in the first annealing step. Here, if the atmospheric hydrogen concentration in the second annealing step (hereinafter also referred to as the second annealing hydrogen concentration) is less than 0.2 vol%, there is a risk of significant reoxidation of the surface of the rolled steel sheet. Therefore, the second annealing hydrogen concentration is preferably 0.2 vol% or more, more preferably 0.5 vol% or more. On the other hand, from the viewpoint of reducing the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite in the final product, the hydrogen concentration in the second annealing is preferably 8% by volume or less, more preferably 7% by volume or less, and even more preferably 5% by volume or less. The hydrogen concentration in the second annealing is preferably lower than the hydrogen concentration in the first annealing.

[0091] First Cooling Step: Next, the rolled steel sheet is cooled according to the following first, second, or third cooling pattern. Among the following cooling patterns, the second and third cooling patterns are preferred, and the third cooling pattern is more preferred.

[0092] [First cooling pattern] Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C

[0093] Atmospheric hydrogen concentration: 30 vol% or less If the atmospheric hydrogen concentration exceeds 30 vol%, the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite in the final product may not be sufficiently reduced. Therefore, the atmospheric hydrogen concentration is 30 vol% or less, preferably 25 vol% or less. There is no particular lower limit for the atmospheric hydrogen concentration. However, from the viewpoint of reducing the risk of reoxidation of the rolled steel sheet surface, the atmospheric hydrogen concentration is preferably 0.5 vol% or more, more preferably 1.0 vol% or more.

[0094] Average cooling rate: 5 to 50°C / s In order to suppress excessive ferrite transformation in the first cooling step, the average cooling rate is 5°C / s or more, preferably 10°C / s or more. However, excessively increasing the cooling rate will result in increased costs. Therefore, the average cooling rate is 50°C / s or less, preferably 40°C / s or less.

[0095] Cooling end temperature: 20 to 420°C If the cooling end temperature is less than 20°C, most of the austenite produced during annealing will transform into martensite, which may result in an insufficient amount of retained austenite in the final product. Strength may also be insufficient. Therefore, the cooling end temperature is 20°C or higher, and preferably 50°C or higher. On the other hand, if the cooling end temperature exceeds 420°C, the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite in the final product cannot be sufficiently reduced. Therefore, the cooling end temperature is 420°C or lower, preferably 400°C or lower, more preferably 380°C or lower, and even more preferably 360°C or lower.

[0096] [Second Cooling Pattern] (Pre-cooling) Atmospheric hydrogen concentration: 30 vol% or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 600°C; (Later-cooling) Atmospheric hydrogen concentration: 30 vol% or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C. The atmospheric hydrogen concentration, average cooling rate, and cooling end temperature in the latter-stage cooling (hereinafter also referred to as the latter-stage hydrogen concentration, the latter-stage cooling rate, and the latter-stage cooling end temperature) can be controlled within the same ranges as those in the first cooling pattern, and therefore further explanation is omitted here. Note that if a plating process (described later) is performed during the first cooling step, for example, between the former-stage cooling (or intermediate holding) and the latter-stage cooling, a plating layer is formed on the surface of the rolled steel sheet, thereby suppressing reoxidation of the rolled steel sheet. In this case, the latter-stage hydrogen concentration is preferably 0.2 vol% or less, more preferably 0.1 vol% or less. The lower limit of the latter-stage hydrogen concentration is not particularly limited and may be 0 vol%. In this case, the atmosphere in the latter stage of cooling may be, for example, air or 100% by volume N ２ An example of such an inert atmosphere is an inert gas atmosphere. The same applies to the atmosphere in the latter-stage cooling and the reheating step of the third cooling pattern described below. The cooling end temperature in the latter-stage cooling is preferably lower than the cooling end temperature in the former-stage cooling. The same applies to the third cooling pattern described below.

[0097] Atmospheric hydrogen concentration in pre-cooling (hereinafter also referred to as pre-cooling hydrogen concentration): 30 vol% or less The pre-cooling hydrogen concentration is 30 vol% or less, similar to the atmospheric hydrogen concentration in the first cooling pattern. Furthermore, in the temperature range from the first annealing temperature (or second annealing temperature) to 600°C where pre-cooling is performed, the cooling efficiency of atmospheric cooling is higher than in the temperature range below 600°C. Therefore, the pre-cooling hydrogen concentration may be relatively low. Furthermore, if the pre-cooling hydrogen concentration is high, the pre-cooling rate may increase excessively, which may result in an increase in the amount of diffusible hydrogen in the steel in the final product. Therefore, the pre-cooling hydrogen concentration is preferably 15 vol% or less, more preferably 12 vol% or less. However, from the viewpoint of reducing the risk of reoxidation of the rolled steel sheet surface, the pre-cooling hydrogen concentration is preferably 0.2 vol% or more, more preferably 0.5 vol% or more.

[0098] Average cooling rate in the initial cooling stage (hereinafter also referred to as the initial cooling rate): 2 to 50°C / s In order to appropriately promote the ferrite transformation of austenite produced in the annealing step, it is effective to control the cooling rate in the temperature range from the first annealing temperature (or the second annealing temperature) to 600°C within an appropriate range. If the initial cooling rate exceeds 50°C / s, the cooling becomes too fast, and ferrite transformation hardly occurs. Therefore, the initial cooling rate is 50°C / s or less, preferably 40°C / s or less, and more preferably 30°C / s or less. On the other hand, if the initial cooling rate is less than 2°C / s, excessive bainite transformation may occur, making it impossible to obtain the desired structure. Therefore, the initial cooling rate is 2°C / s or more, preferably 5°C / s or more.

[0099] Cooling end temperature in the first-stage cooling (hereinafter also referred to as the first-stage cooling end temperature): 350 to 600°C. The first-stage cooling end temperature is 350°C or higher, preferably 400°C or higher, and more preferably 450°C or higher, from the viewpoint of performing a plating treatment such as hot-dip galvanizing, which will be described later, between the first-stage cooling and the second-stage cooling. From the same viewpoint, the first-stage cooling end temperature is 600°C or lower, and preferably 550°C or lower.

[0100] [Third Cooling Pattern] (Preliminary Cooling) Atmospheric Hydrogen Concentration: 30% by volume or less, Average Cooling Rate: 2 to 50°C / s, Cooling End Temperature: 350 to 550°C (Intermediate Hold) Atmospheric Hydrogen Concentration: 0.2 to 20% by volume, Hold Temperature: 350 to 550°C, Hold Time: 5 to 300 s (Later Cooling) Atmospheric Hydrogen Concentration: 30% by volume or less, Average Cooling Rate: 5 to 50°C / s, Cooling End Temperature: 20 to 420°C. Note that the third cooling pattern basically performs an intermediate hold between the preliminary cooling and rear cooling stages of the second cooling pattern, and therefore a description of the preliminary and rear cooling stages of the third cooling pattern will be omitted. However, the preliminary cooling end temperature of the third cooling pattern is set to 350 to 550°C in view of the intermediate hold.

[0101] Atmospheric hydrogen concentration during intermediate holding (hereinafter also referred to as intermediate hydrogen concentration): 0.2 to 20 vol%. During intermediate holding, a portion of the austenite produced by annealing is transformed into bainite with a BCC structure. Furthermore, by holding the rolled steel sheet in an atmosphere with a lower hydrogen concentration than that in the first annealing step, it is possible to further reduce the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite in the final product. Furthermore, carbon enrichment in untransformed austenite is promoted, making it easier to obtain retained austenite in the final product. To effectively achieve these effects, the intermediate hydrogen concentration is 20 vol% or less, preferably 15 vol% or less, more preferably 13 vol% or less, and even more preferably 10 vol% or less. The intermediate hydrogen concentration is preferably lower than the hydrogen concentration during the first annealing. On the other hand, from the viewpoint of reducing the risk of reoxidation of the rolled steel sheet surface, the intermediate hydrogen concentration is 0.2 vol% or more, preferably 0.5 vol% or more.

[0102] Holding temperature for intermediate holding (hereinafter also referred to as intermediate holding temperature): 350 to 550°C If the intermediate holding temperature is less than 350°C or more than 550°C, bainite transformation hardly occurs, and the effectiveness of intermediate holding becomes low. Therefore, the intermediate holding temperature is 350°C or higher, and preferably 380°C or higher. In addition, the intermediate holding temperature is 550°C or lower, and preferably 520°C or lower.

[0103] Holding time of intermediate holding (hereinafter also referred to as intermediate holding time): 5 to 300 seconds If the intermediate holding time is less than 5 seconds, bainite transformation hardly occurs, and the effectiveness of the intermediate holding is reduced. Therefore, the intermediate holding time is 5 seconds or more, preferably 10 seconds or more. On the other hand, if the intermediate holding time exceeds 300 seconds, productivity will decrease. Furthermore, excessive bainite transformation may occur, making it impossible to obtain the desired structure. Therefore, the intermediate holding time is 300 seconds or less, preferably 200 seconds or less.

[0104] Reheating Step Next, the rolled steel plate is reheated under the following conditions.

[0105] Atmospheric hydrogen concentration during the reheating process (hereinafter also referred to as reheating hydrogen concentration): 20 vol% or less. The reheating process aims to reduce dissolved hydrogen in martensite and other constituent phases partially formed in the rolled steel sheet. Furthermore, holding during the reheating process further induces bainite transformation, promoting carbon enrichment in untransformed austenite. This makes it possible to secure the necessary amount of retained austenite. Here, if the reheating hydrogen concentration exceeds 20 vol%, the amount of dissolved hydrogen in the retained austenite in the final product will increase. Therefore, the reheating hydrogen concentration is 20 vol% or less, preferably 18 vol% or less, and more preferably 15 vol% or less. On the other hand, from the viewpoint of reducing the risk of reoxidation of the rolled steel sheet surface, the reheating hydrogen concentration is preferably 0.2 vol% or more, more preferably 0.5 vol% or more.

[0106] Reheating temperature: equal to or higher than the cooling end temperature of the first cooling step and 100 to 450°C From the viewpoint of sufficiently reducing the amount of dissolved hydrogen in the retained austenite in the final product, the reheating temperature is equal to or higher than the cooling end temperature of the first cooling step and 100°C or higher. The reheating temperature is preferably 120°C or higher, and more preferably 150°C or higher. Furthermore, from the viewpoint of obtaining desired press formability, the reheating temperature is 450°C or lower, preferably 430°C or lower, and more preferably 400°C or lower. Note that when pre-stage cooling and post-stage cooling are performed in the first cooling step (in the case of the second cooling pattern or the third cooling pattern), the cooling end temperature of the first cooling step means the post-stage cooling end temperature.

[0107] Reheating time: 5 to 600 seconds To ensure sufficient effectiveness of reheating, the reheating time is set to 5 to 600 seconds. The reheating time is preferably 10 seconds or more, more preferably 20 seconds or more. The reheating time is preferably 450 seconds or less, more preferably 300 seconds or less.

[0108] Second Cooling Step Next, the rolled steel plate is cooled under the following conditions.

[0109] Atmospheric hydrogen concentration in the second cooling step (hereinafter also referred to as the second cooling hydrogen concentration): 0.2 vol% or less. In the second cooling step, which is the final cooling step, a portion of the untransformed austenite present in the rolled steel sheet at the start of the step transforms to martensite, and a portion of the remaining untransformed austenite becomes retained austenite. The cooling conditions for the second cooling step are then appropriately controlled, particularly by reducing the second cooling hydrogen concentration. This promotes the desorption of dissolved hydrogen from martensite and untransformed austenite, thereby sufficiently reducing the amount of dissolved hydrogen in the retained austenite in the final product. If the second cooling hydrogen concentration exceeds 0.2 vol%, it becomes difficult to sufficiently reduce the amount of dissolved hydrogen in each phase, particularly the retained austenite, in the final product. Therefore, the second cooling hydrogen concentration is 0.2 vol% or less, preferably 0.1 vol% or less. The lower limit of the second cooling hydrogen concentration is not particularly limited and may be 0 vol%. The atmosphere for the second cooling step may be, for example, air or 100 vol% N. ２ An example of the inert atmosphere is a gas atmosphere.

[0110] Residence time in the 100 to 300°C temperature range in the second cooling step (hereinafter also referred to as the second cooling residence time): 2 seconds or more. The untransformed austenite present in the rolled steel sheet at the start of the second cooling step has a lowered Ms point due to the influence of carbon distribution that occurred in the steps preceding the second cooling step. To promote the reduction of dissolved hydrogen in the retained austenite (hereinafter also referred to as the hydrogen reduction effect) in this step, it is important to ensure a certain period of residence time at high temperatures in a state in which a portion of the untransformed austenite (FCC) has been transformed to martensite (BCC). In other words, to fully achieve the hydrogen reduction effect, it is important to control the temperature range in which the untransformed austenite with a lowered Ms point is likely to begin transforming to martensite, specifically, the residence time in the 100 to 300°C temperature range (second cooling residence time). Note that at temperatures below 100°C, the hydrogen diffusion distance is reduced, and the above-mentioned hydrogen reduction effect is not fully achieved. Here, if the second cooling residence time is less than 2 seconds, the diffusion distance of hydrogen is small, and the effect of reducing the amount of dissolved hydrogen in each phase, particularly in retained austenite, is not sufficiently obtained. Therefore, the second cooling residence time is 2 seconds or more, preferably 5 seconds or more, and more preferably 10 seconds or more. There is no particular upper limit to the second cooling residence time. For example, from the viewpoint of productivity, the second cooling residence time is preferably 300 seconds or less.

[0111] Cooling end temperature of the second cooling step (hereinafter also referred to as second cooling end temperature): 50°C or less The second cooling end temperature is 50°C or less. There is no particular limitation on the lower limit of the second cooling end temperature. For example, the second cooling end temperature may be 0°C or more.

[0112] Plating process The rolled steel sheet may also be plated. The plating process may be performed, for example, during the first cooling process, between the reheating process and the second cooling process, or after the second cooling process. When the plating process is performed during the first cooling process, in the case of the second cooling pattern, it is preferable to perform the plating process, for example, between the first cooling process and the second cooling process. In addition, in the case of the third cooling pattern, it is preferable to perform the plating process, for example, between the intermediate holding process and the second cooling process.

[0113] The type of plating process is not particularly limited. Examples of the type of plating process include zinc plating processes such as hot-dip galvanizing, galvannealed hot-dip galvanizing, electrogalvanizing, and vapor-deposition galvanizing. Examples of plating processes other than zinc plating include aluminum plating processes such as hot-dip aluminum plating. Among these, hot-dip galvanizing is preferred. Hot-dip galvanizing, galvannealed hot-dip galvanizing, and hot-dip aluminum plating are preferably performed during the first cooling step. Electrical galvanizing and vapor-deposition galvanizing are preferably performed after the second cooling step.

[0114] The plating conditions are not particularly limited and may be conventional. For example, in the case of hot-dip galvanizing, the hot-dip galvanizing bath contains Zn, Al, and unavoidable impurities. In one example, the Al concentration of the hot-dip galvanizing bath (hereinafter also referred to as the bath Al concentration) may be 0.05 mass% or more and 0.190 mass% or less. When the bath Al concentration is 0.05 mass% or more, the occurrence of bottom dross can be more effectively prevented. Furthermore, when the bath Al concentration is 0.190 mass% or less, the occurrence of top dross can be more effectively prevented. From a cost perspective, it is preferable that the bath Al concentration be 0.190 mass% or less. The temperature of the plating bath (hereinafter also referred to as the plating bath temperature) is also not particularly limited. In one example, the plating bath temperature may be 440°C or more and 500°C or less. The coating weight per side is also not particularly limited. In one example, the coating weight per side is 20 g / m ２ or more, preferably 25 g / m ２ and may be 120 g / m or more. ２ and preferably 100 g / m ２ The plating coverage per side can be 20 g / m or less. ２ If the coating weight per side is 120 g / m or more, the corrosion resistance is particularly good and the coating weight is particularly easy to control. ２The coating adhesion is particularly good if the coating weight is below 1000 kJ / s. The method for adjusting the coating weight is not particularly limited. For example, gas wiping can be used, and the coating weight can be adjusted by changing the gas pressure and the distance between the wiping nozzle and the steel sheet.

[0115] Furthermore, after the hot-dip galvanizing treatment, an alloying treatment may be performed, i.e., a hot-dip galvannealing treatment. This can further promote the desorption of hydrogen from the steel during the reheating process. The alloying conditions are not particularly limited, and may be performed according to conventional methods. For example, the alloying treatment may be performed at an alloying temperature of 440°C or higher and 600°C or lower for an alloying time of 5 seconds or higher and 60 seconds or lower. The alloying time is the holding time at the alloying temperature. Furthermore, when performing the alloying treatment, it is preferable that the Fe content in the coating layer (hereinafter also referred to as the degree of alloying) be 7% by mass or higher and 15% by mass or lower. By setting the degree of alloying to 7% by mass or higher, the remaining η-Zn phase in the coating layer can be prevented, and hydrogen in the steel can be more effectively reduced during the reheating process. Furthermore, if the degree of alloying exceeds 15% by mass, the formation of the Γ phase at the interface between the coating layer and the steel sheet may be promoted, resulting in reduced coating adhesion. Therefore, the degree of alloying is preferably 15% by mass or lower.

[0116] In the case of electrolytic zinc plating, for example, a plating solution with a divalent zinc ion concentration of 80 g / L is used, and the current density is set to 10 to 80 A / dm ２ The plating amount is adjusted within the range of 20 g / m per side by adjusting the electrolysis time. ２ 80g / m or more ２ The zinc ions can be controlled as follows: The divalent zinc ions in the plating solution may be added, for example, as sulfate. The treatment conditions for the vapor deposition zinc plating treatment are not particularly limited, and may be those of a conventional method.

[0117] In the case of hot-dip aluminum plating, for example, a rolled steel sheet is immersed in an aluminum plating bath at 660 to 730°C. Then, the coating weight is adjusted by gas wiping or the like. The coating weight is set to 20 g / m per side. ２ 120g / m or more ２The following is preferred: The hot-dip aluminum plating bath is not particularly limited as long as it has the above-mentioned composition of the hot-dip aluminum plating layer, and may be any of the usual methods.

[0118] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0119] [4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described. The manufacturing method of a member according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press forming can be used. Furthermore, the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, and caulking joining can be used. Note that the forming conditions and joining conditions are not particularly limited, and may be in accordance with conventional methods.

[0120] Steel sheets (thickness: 1.4 mm) were produced under the conditions shown in Table 2 using steel slabs having the chemical compositions shown in Table 1 (the balance being Fe and unavoidable impurities). When cooling was performed using the first cooling pattern in the first cooling step, the initial hydrogen concentration, initial cooling rate, and initial cooling end temperature in Table 2 refer to the atmospheric hydrogen concentration, average cooling rate, and cooling end temperature of the first cooling pattern, respectively. Some of the steel sheets were also plated to form a coating layer on their surfaces. The plating conditions are as shown below. Conditions not specified were those according to conventional methods. The identification of the steel structure, the measurement of the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite, and the evaluation of each characteristic were all performed during the second cooling step or within 24 hours after the steel sheet first reached room temperature (below 40°C) after the second cooling step. For the evaluation of hydrogen embrittlement resistance after processing, 24 hours elapsed during some of the tests. [Hot-dip galvanizing treatment (GI)] Plating bath composition: Zn, Al, and inevitable impurities (Al concentration in the bath: 0.13 mass%, balance: Zn and inevitable impurities) Plating bath temperature: 460°C Plating weight per side: 30 to 60 g / m ２[Galvannealed hot-dip galvanizing treatment (GA) (hot-dip galvanizing treatment is the same as above)] Alloying temperature: 500 to 560°C Alloying degree: 8.0 to 14.0 mass% [Electrogalvanizing treatment (EG)] Divalent zinc ion concentration in plating solution: 80 g / L (divalent zinc ions were added as sulfate.) pH of plating solution: 2.0 (pH of plating solution was adjusted with sulfuric acid.) Temperature of plating solution: 55°C Current density: 50 A / dm ２ Electrolysis time: 30 seconds Plating weight per side: 50 g / m ２ [Hot-dip aluminum plating treatment (Al)] Plating bath composition: Al and inevitable impurities Plating bath temperature: 700°C Plating weight per side: 60 g / m ２

[0121] From the steel sheet thus obtained, a sample for microstructure observation was cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel sheet served as the observation surface. The observation surface of the sample was then polished and etched with nital. Next, using an SEM (scanning electron microscope), images of three fields of view were taken at a magnification of 1500x for the region from the steel sheet surface to the 1 / 8 plate thickness position and the region from the 1 / 8 plate thickness position to the 3 / 8 plate thickness position on the observation surface of the sample, to obtain image data. From the obtained image data, the area ratio of each constituent phase was calculated as described above, and the average value over the entire field of view was taken as the area ratio of each constituent phase. The area ratio of retained austenite was measured by X-ray diffraction as described above. The results are shown in Table 3.

[0122] The steel sheets were then measured for the amount of diffusible hydrogen in the steel and the amount of dissolved hydrogen in the retained austenite using the above-described method. The results are also shown in Table 3. In Table 3, "A" indicates that the hydrogen content measurement method was performed on a sample taken from the steel sheet, after which the above-described [A] was performed, and then the amount of dissolved hydrogen in the retained austenite was measured. Similarly, "B" indicates that the sample taken from the steel sheet was subjected to the above-described [B], and then the amount of dissolved hydrogen in the retained austenite was measured. In [A], a tensile load was applied to the sample taken from the steel sheet in the same manner as in the tensile test described below, resulting in uniform elongation. Here, the uniform elongation was defined as the elongation at the maximum stress point (maximum tensile load point) in the stress-strain curve obtained in the tensile test described below. In [B], the sample taken from the steel sheet was immersed in liquid nitrogen (below -180°C) without processing and held there for 48 hours. The amounts of dissolved hydrogen in the retained austenite measured using both the above-described [A] and [B] methods were comparable in range.

[0123] Furthermore, the TS, press formability, and hydrogen embrittlement resistance after working were evaluated in the following manner. The evaluation results are also shown in Table 3.

[0124] <Evaluation of TS and press formability> JIS No. 5 tensile test pieces (JIS Z2201 (1988)) were taken from the obtained steel sheets so that the direction perpendicular to the rolling direction was the longitudinal direction. Using the taken test pieces, a tensile test was performed in accordance with the provisions of JIS Z2241 (2011) to determine the yield strength (hereinafter also referred to as YS), TS, and elongation (hereinafter also referred to as El). The strain rate was 10 －３ / s. TS and press formability were evaluated according to the following criteria. (TS) Pass: TS is 780 MPa or more Fail: TS is less than 780 MPa (Press formability) Pass (excellent): The product of TS and El, TS × El, is 11,500 MPa·% or more Fail (poor): TS × El is less than 11,500 MPa·%

[0125] Evaluation of Hydrogen Embrittlement Resistance after Processing: Rectangular test specimens with a major axis length of 100 mm and a minor axis length of 20 mm were taken from the obtained steel plates. The major axis of the test specimen was perpendicular to the rolling direction (perpendicular to the rolling direction and the plate thickness direction). Next, a 10 mm diameter hole was punched at the center of the major and minor axes of the test specimen with a clearance of 14% to form a processed portion in the test specimen, in other words, a region where retained austenite had undergone processing-induced transformation to martensite. A constant-load uniaxial tensile test was then performed on the test specimen to measure the critical stress. The test time was 100 hours. The presence or absence of cracks on the end faces of the hole and in the vicinity of the hole was visually confirmed, and the maximum stress that could be applied to the test specimen without cracking was defined as the critical stress. To prevent changes in the amount of diffusible hydrogen in the steel, the constant-load uniaxial tensile test was started within 10 minutes after the rectangular plate was taken. Then, the hydrogen embrittlement resistance after working was evaluated according to the following criteria based on the ratio of critical stress to YS, critical stress / YS: Pass A (particularly excellent): Critical stress / YS is 1.00 or more Pass B (excellent): Critical stress / YS is 0.85 or more and less than 1.00 Fail (poor): Critical stress / YS is less than 0.85

[0126]

[0127]

[0128]

[0129] As shown in Table 3, all of the inventive examples had a TS of 780 MPa or more, and were also excellent in press formability and post-processing hydrogen embrittlement resistance.

[0130] On the other hand, in the comparative examples, at least one of TS, press formability, and post-working hydrogen embrittlement resistance was insufficient.

[0131] According to the present invention, a steel sheet having a TS of 780 MPa or more and excellent press formability and hydrogen embrittlement resistance after working can be obtained. Furthermore, the steel sheet of the present invention can be suitably used as a steel sheet for automobiles, for example, and therefore can be used for reducing the CO2 emissions of automobiles. ２ This contributes to reducing emissions and is extremely advantageous for industry.

Claims

1. A steel plate having a component composition, in mass%, of C: 0.050% or more and 0.350% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.00% or more and 6.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.01% or more and 2.00% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities; and a steel structure in which the area fraction of retained austenite is 2% or more and 40% or less, wherein the amount of diffusible hydrogen in the steel is 0.50 ppm by mass or less, and the amount of dissolved hydrogen in the retained austenite is 0.30 ppm by mass or less, and the tensile strength is 780 MPa or more.

2. The chemical composition further includes, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Sb: 0.300% or less, Sn: 0.300% or less, V: 0.100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, 2. The steel sheet according to claim 1, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM (excluding Ce): 0.0200% or less.

3. The steel sheet according to claim 1 or 2, which has a plating layer on its surface.

4. A member made using the steel plate according to any one of claims 1 to 3.

5. A rolling process in which a steel slab having the chemical composition defined in claim 1 or 2 is hot-rolled or further cold-rolled under the conditions of a steel slab heating temperature of 1100 to 1300°C, a finish rolling temperature of 800 to 1000°C, and a coiling temperature of 700°C or less to obtain a rolled steel sheet; a first annealing process in which the rolled steel sheet is held under the conditions of a holding temperature of 650 to 950°C, a holding time of 10 seconds or more, and an atmospheric hydrogen concentration of 0.2 to 30% by volume; a first cooling process in which the rolled steel sheet is cooled according to the following first, second, or third cooling pattern; and a second cooling process in which the rolled steel sheet is cooled under the conditions of an atmospheric hydrogen concentration of 20% by volume or less, a reheating temperature of 100 to 450°C and above the cooling end temperature of the first cooling process. A method for producing a steel sheet, comprising: a reheating step of reheating the rolled steel sheet under the conditions of a reheating time of 5 to 600 seconds; and a second cooling step of cooling the rolled steel sheet under the conditions of an atmospheric hydrogen concentration of 0.2% by volume or less, a residence time in a temperature range of 100 to 300°C of 2 seconds or more, and a cooling end temperature of 50°C or less. [First cooling pattern] Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C [Second cooling pattern] (First cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 600°C (Second cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C [Third cooling pattern] (First cooling stage) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 550°C (Intermediate holding) Atmospheric hydrogen concentration: 0.2 to 20% by volume, holding temperature: 350 to 550°C, holding time: 5 to 300 s (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5 to 50°C / s, cooling end temperature: 20 to 420°C 6. The method for producing a steel sheet according to claim 5, further comprising a second annealing step between the first annealing step and the first cooling step, in which the rolled steel sheet is held under the following conditions: a holding temperature of 650 to 950°C, a holding time of 10 seconds or more, and an atmospheric hydrogen concentration of 0.2 to 8% by volume.

7. A method for manufacturing a steel plate according to claim 5 or 6, wherein the cooling pattern of the first cooling step is the second cooling pattern or the third cooling pattern, the atmospheric hydrogen concentration of the pre-cooling is 0.2 to 15% by volume, and the average cooling rate of the pre-cooling is 2 to 40°C / s.

8. A method for producing a steel sheet according to any one of claims 5 to 7, further comprising a plating step of plating the rolled steel sheet during the first cooling step, between the reheating step and the second cooling step, or after the second cooling step.

9. A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 3 to at least one of forming and joining to form a component.

Images