Steel plates and components, and their manufacturing methods

Abstract

The steel sheet has a TS of 780 MPa or more and is excellent in press formability and post-forming hydrogen embrittlement resistance. The chemical composition is appropriately controlled, and the area fraction of retained austenite is controlled to 2% to 40%, the amount of diffusible hydrogen in the steel is controlled to 0.50 ppm by mass or less, and the amount of dissolved hydrogen in the retained austenite is controlled to 0.30 ppm by mass or less.

Description

[Technical Field]

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

[0002] In recent years, from the perspective of protecting the global environment, there has been a strong demand for improved fuel efficiency to reduce CO2 emissions from automobiles. Accordingly, there has been an active movement to reduce the weight of automobile bodies by thinning the thickness of automobile body parts. As a result, there is a growing need for higher strength steel sheets used as materials for automobile body parts (hereinafter also 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2-101117 [Patent Document 2] Patent No. 5463685 [Patent Document 3] Patent No. 4894863 Summary of the Invention [Problem to be solved by the invention]

[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 post-forming hydrogen embrittlement resistance. 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 post-forming hydrogen embrittlement resistance.

[0007] The present invention was 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. [Means for solving the problem]

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

[0009] (4) To improve the post-processing hydrogen embrittlement resistance of such steel sheets containing retained austenite, it is crucial to reduce the amount of dissolved hydrogen in the retained austenite in addition to reducing the amount of diffusible hydrogen in the steel. Specifically, retained austenite has an FCC structure. Therefore, retained austenite can dissolve a larger amount of hydrogen than ferrite with a BCC structure or martensite with a BCT structure. This means that 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 and configuration of the present invention are as follows. 1. By mass%, 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; N: 0.0100% or less and the remainder being Fe and unavoidable impurities; A steel structure in which the area fraction of retained austenite is 2% or more and 40% or less, The amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, the amount of dissolved hydrogen in the retained austenite is 0.30 ppm by mass or less, A steel plate having a tensile strength of 780 MPa or more.

[0011] 2. The component composition is further expressed in mass% as follows: 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, 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 2. The steel sheet according to 1 above, containing at least one selected from the following:

[0012] 3. The steel sheet according to 1 or 2 above, having 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 steel slab having the chemical composition described in 1 or 2 above, Steel slab heating temperature: 1100~1300℃, Finishing rolling temperature: 800~1000℃ and Winding temperature: 700℃ or less a rolling step of hot rolling the steel sheet under the conditions of (a) above or further cold rolling the steel sheet to obtain a rolled steel sheet; Next, the rolled steel sheet is Holding temperature: 650~950℃, Holding time: 10 seconds or more Atmospheric hydrogen concentration: 0.2 to 30% by volume a first annealing step in which the steel sheet is maintained under the following conditions; Next, a first cooling step is performed in which the rolled steel sheet is cooled according to the following first cooling pattern, second cooling pattern, or third cooling pattern. Next, the rolled steel sheet is Atmospheric hydrogen concentration: 20% by volume or less Reheating temperature: 100 to 450°C or higher than the cooling end temperature of the first cooling step; Reheating time: 5~600s A reheating step of reheating under the conditions of Next, the rolled steel sheet is Atmospheric hydrogen concentration: 0.2% by volume or less Residence time in the temperature range of 100 to 300°C: 2 seconds or more Cooling end temperature: 50℃ or less and a second cooling step of cooling the steel sheet under the conditions: [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] (Pre-cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2-50°C / s, cooling end temperature: 350-600°C (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5-50°C / s, cooling end temperature: 20-420°C [Third cooling pattern] (Pre-cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2-50°C / s, cooling end temperature: 350-550°C (Intermediate hold) Atmospheric hydrogen concentration: 0.2 to 20% by volume, hold temperature: 350 to 550°C, hold time: 5 to 300 seconds (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5-50°C / s, cooling end temperature: 20-420°C

[0015] 6. Between the first annealing step and the first cooling step, The rolled steel plate, Holding temperature: 650~950℃, Holding time: 10 seconds or more Atmospheric hydrogen concentration: 0.2 to 8% by volume 6. The method for producing a steel sheet according to item 5 above, further comprising a second annealing step of maintaining the steel sheet under the conditions of

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

[0017] 8. The method for producing a steel sheet according to any one of 5 to 7 above, 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 producing a component, comprising the 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. [Effects of the Invention]

[0019] According to the present invention, a steel sheet having a TS of 780 MPa or more and excellent press formability and post-forming hydrogen embrittlement resistance can be obtained. Furthermore, the steel sheet of the present invention can be suitably used, for example, as a steel sheet for automobiles, and therefore effectively contributes to reducing CO2 emissions from automobiles, and is extremely advantageous industrially. DETAILED DESCRIPTION OF THE INVENTION

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

[0021] [1] Steel plate First, the chemical composition of a steel sheet according to an 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 represented as "%".

[0022] C: 0.050% or more and 0.350% or less C is an element that stabilizes austenite and improves hardenability. Furthermore, C is 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 effective element for obtaining stable retained austenite. To obtain these effects significantly, 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 leads to deterioration in the plating processability, resulting in defects such as non-plating, and uneven appearance due to poor plating alloying, which causes 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 obtain 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% leads to increased 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. However, excessive P segregates at prior austenite grain boundaries, embrittling the grain boundaries and deteriorating press formability. Therefore, the P content is 0.100% or less, and preferably 0.050% or less. There is no particular lower limit for the P content, and it 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 forms 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, because 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. In other words, Al is an effective element for obtaining stable retained austenite. Furthermore, Al acts as a deoxidizer and is also effective in purifying steel. To obtain these effects significantly, the Al content is set to 0.01% or more. On the other hand, if the Al content exceeds 2.00%, the risk of cracking of the steel billet during continuous casting increases, 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 chemical 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 chemical composition (hereinafter also referred to as basic chemical elements), with the balance other than the basic chemical elements including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above basic chemical elements, 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 chemical elements, 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 can cause the carbides to become coarse, which can deteriorate 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 can cause the carbides to become coarse, which can deteriorate 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 effectively suppresses excessive decarburization of the steel sheet surface and prevents a decrease in the amount of martensite formed. To achieve this effect, the Sb content is preferably 0.001% or more. However, excessive Sb content can lead 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 increased strength of steel. To achieve this effect, the V content is preferably 0.001% or more. However, excessive V content can cause the carbides to become coarse, resulting in poor 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 The addition of Cr can improve the balance between strength and ductility. 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 The addition of Ni can improve the balance between strength and ductility. To achieve this effect, the Ni content is preferably 0.005% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, 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 The inclusion of Ta can provide the effect of improving strength. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing excessive cost increases, 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 the effect of improving strength. To achieve this effect, the W content is preferably 0.001% or more, 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, more preferably 0.450% or less.

[0042] Zr: 0.0200% or less Zr increases the ultimate deformability of steel sheets 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 can actually lead to a deterioration in 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. However, 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 steel sheets 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 herein 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, as well as 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 REM elements, 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 ratio of retained austenite: 2% to 40% 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 into 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 area ratio of ferrite is preferably 70% or less, more preferably 65% ​​or less, and even more preferably 60% or less. There is no particular restriction on the lower limit of the area ratio of ferrite, and it may be 0%. Moreover, the area ratio of ferrite is preferably 2% or more, more preferably 5% or more.

[0053] Martensite area ratio: 10% to 98% Martensite is generated from austenite in a temperature range below the martensitic transformation start point (hereinafter also referred to as the Ms point). 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] Bainite area ratio: 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 needle-like or plate-like ferrite. Bainite also has a hardness intermediate between that of ferrite and martensite. Therefore, adding a certain amount of bainite to a steel structure helps to balance strength and ductility. The lower limit of the bainite area fraction is not particularly limited and may be 0%. To achieve the above effects, the bainite area fraction is preferably 5% or more, more preferably 10% or more. 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 a predetermined range. Therefore, the bainite area fraction is preferably 50% or less, more preferably 45% or less, and even more preferably 40% or less.

[0055] Area ratio of other tissues: 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 the other structures is preferably 10% or less, more preferably 5% or less. The lower limit of the area ratio of the other structures is not particularly limited and may be 0%.

[0056] The area ratio of each phase can be measured using standard methods. The area ratio of each phase can be measured, for example, as follows: A sample is cut from a steel plate so that the observation surface is a thickness cross section (L cross section) parallel to the rolling direction of the steel plate. The observation surface of the sample is then polished and etched with nital. A 60 μm × 80 μm area 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. Ferrite is observed as black regions (excluding island-like black regions contained in bainite). Bainite is observed as island-like 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] If 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. Specifically, the observation position is set at 1 / 4 of the steel plate thickness using an X-ray diffractometer with CoKα radiation, and 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 is calculated. The volume fraction of retained austenite is then calculated from the ratio of the integrated intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and 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] Diffusible hydrogen content in steel: 0.50 mass ppm or less From the viewpoint of obtaining excellent hydrogen embrittlement resistance and further excellent hydrogen embrittlement resistance after working, the amount of diffusible hydrogen in the steel is 0.50 ppm by mass or less, preferably 0.45 ppm by mass or less, and more preferably 0.40 ppm by mass or less. The lower limit of the amount of diffusible hydrogen in the steel is not particularly limited, and may be 0 ppm by mass.

[0059] Here, the amount of diffusible hydrogen in steel is the amount of hydrogen H2 (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 taken 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) following the second cooling step in the manufacturing method described below. The diffusible hydrogen in steel is mainly 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, retained austenite undergoes a work-induced transformation to martensite in the processed portion of the steel sheet. This converts 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, post-processing hydrogen embrittlement resistance deteriorates. Therefore, reducing the amount of dissolved hydrogen in the retained austenite is important for improving post-processing hydrogen embrittlement resistance in a steel sheet containing retained austenite. In particular, setting the amount of dissolved hydrogen in the retained austenite to 0.30 ppm by mass or less can significantly improve post-processing hydrogen embrittlement resistance. Therefore, the amount of dissolved hydrogen in the retained austenite is 0.30 ppm by mass or less, preferably 0.25 ppm by mass or less, and more preferably 0.20 ppm by mass or less. Because there is no benefit 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 sheet 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 by thermal desorption analysis, and the amount of hydrogen H1 (mass ppm) 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 (H1 - H2) obtained by subtracting H2 from H1 is then taken as the amount of dissolved hydrogen in retained austenite. [A] Conduct a tensile test in accordance with JIS Z2241 (2011) and apply a tensile load (maximum tensile load) that results in uniform elongation to the sample. [B] The sample is kept at a temperature of -180°C or lower for 24 hours or more. 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 for a longer period of time in [B].

[0063] TS:780MPa or more The TS of a steel sheet according to an 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 to the TS of a steel sheet according to an 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 types 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 called a galvanized steel sheet. The steel sheets having the above-mentioned hot-dip galvanized layer, galvannealed layer, and electrogalvanized layer can also be called hot-dip galvanized steel sheet (GI), galvannealed steel sheet (GA), electrogalvanized steel sheet (EG), and 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 Fe, and 0.001 mass% to 1.0 mass% 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 Fe, and 0.001 mass% to 1.0 mass% 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 galvanized layer preferably contain, for example, 70.0% or more by mass of Zn. The electrogalvanized layer may also 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% or more but less than 30.0% by mass. The remainder of the electrogalvanized layer and vapor-deposited galvanized 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 More than 80g / m 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 More than 120g / m 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 part 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 without cracking, in a constant-load uniaxial tensile test according to the procedures described in the Examples below. Furthermore, the procedures for measuring TS, El, YS, and critical stress are as described in the Examples below.

[0076] [2] Materials 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 allows for improved fuel efficiency by reducing the weight of the automobile body, greatly contributing to reducing CO2 emissions.

[0077] [3] Steel plate manufacturing method Next, a method for manufacturing 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 (℃ / s)] = ([Cooling start temperature (℃)] - [Cooling end temperature (℃)]) / 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] Rolling process First, a steel having the above-mentioned 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. The steel slab is then hot-rolled under the following conditions, and optionally further cold-rolled to form a rolled steel sheet.

[0079] Steel slab heating temperature: 1100~1300℃ Precipitates such as carbides present at the start of heating a 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 an increased rolling load will arise. Furthermore, by setting the heating temperature of 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, resulting in increased scale loss. Therefore, the heating temperature of the steel slab is 1300°C or lower, preferably 1250°C or lower.

[0080] Finishing rolling temperature: 800~1000℃ If the finish rolling temperature is less than 800°C, the rolling load increases, resulting in a large rolling load. Furthermore, the rolling reduction rate increases when the austenite is not recrystallized. This leads to the development of an abnormal texture, resulting in significant in-plane anisotropy in the final product. As a result, the material uniformity 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] Winding temperature: 700℃ 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 process, which may cause deterioration in ductility in the final product. Therefore, the coiling temperature is 700°C or lower, and preferably 650°C or lower. 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 higher.

[0082] After the 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 obtained by hot rolling (hereinafter also referred to as a hot-rolled steel sheet) is subjected to pretreatment such as pickling and degreasing according to conventional methods, and then cold-rolled to obtain a cold-rolled steel sheet. In addition, for the purpose of reducing 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 reduction ratio of the cold rolling is not particularly limited. However, if the reduction ratio of the cold rolling is less than 20%, the surface flatness may be poor and the structure may become non-uniform. Therefore, the reduction ratio of the cold rolling is preferably 20% or more. In addition, the upper limit of the reduction ratio of the cold rolling is not particularly limited, and for example, the reduction ratio of the cold rolling is preferably 80% or less. Note that the cold rolling may be omitted.

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

[0084] Holding temperature for the first annealing process: 650 to 950°C In the first annealing step, annealing is performed in a temperature range of the austenite single phase region or the austenite-ferrite two-phase region. If the holding temperature in the first annealing step (hereinafter also referred to as the first annealing temperature) is less than 650°C, the recrystallization of ferrite and the reverse transformation to austenite become 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 becomes excessive, 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 need 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 the 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 process: 10 seconds or more If the holding time in 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 an increase in 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 referred to here is the holding time for holding at the first annealing temperature. The same applies to the holding time in the second annealing step, the holding time for the intermediate holding in the first cooling step, and the reheating time in the reheating step described below.

[0086] Hydrogen concentration in the atmosphere during the first annealing step: 0.2 to 30% by volume 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 atmosphere of the second annealing step, first cooling step, and reheating step described below.

[0088] Second annealing process The rolled steel sheet may then be further held under the following conditions: Holding temperature: 650~950℃ Holding time: 10s or more Atmospheric hydrogen concentration: 0.2 to 8% by volume 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 first annealing temperature, and therefore a detailed description thereof will be omitted here.

[0089] Holding time for the second annealing process: 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. 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] Hydrogen concentration in the atmosphere during the second annealing step: 0.2 to 8% by volume 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 second annealing hydrogen concentration is preferably 8 vol% or less, more preferably 7 vol% or less, and even more preferably 5 vol% or less. The hydrogen concentration in the second annealing is preferably lower than the hydrogen concentration in the first annealing.

[0091] First cooling process The rolled steel sheet is then cooled according to the following cooling pattern 1, 2, or 3. Of 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% by volume or less If the atmospheric hydrogen concentration exceeds 30% by volume, 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% by volume or less, and preferably 25% by volume 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 surface of the rolled steel sheet, the atmospheric hydrogen concentration is preferably 0.5% by volume or more, and more preferably 1.0% by volume or more.

[0094] Average cooling rate: 5~50℃ / 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 increase 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. Furthermore, the strength may also be insufficient. Therefore, the cooling end temperature is 20°C or higher, 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% by volume or less, average cooling rate: 2-50°C / s, cooling end temperature: 350-600°C (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5-50°C / s, cooling end temperature: 20-420°C Here, 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) may be controlled within the same ranges as in the first cooling pattern, and therefore further description thereof will be omitted. Note that if a plating treatment (described later) is performed during the first cooling step, for example, between the initial cooling (or intermediate holding described later) and the latter-stage cooling, a plating layer is formed on the surface of the rolled steel sheet, thereby preventing reoxidation of the rolled steel sheet. In this case, the hydrogen concentration in the latter-stage is preferably 0.2% by volume or less, more preferably 0.1% by volume or less. The lower limit of the hydrogen concentration in the latter-stage is not particularly limited and may be 0% by volume. In this case, the atmosphere in the latter-stage cooling can be, for example, an air atmosphere or an inert atmosphere such as 100% by volume N2 gas. The same applies to the atmosphere in the latter-stage cooling and the reheating step in the third cooling pattern described later. The cooling end temperature in the latter-stage cooling is preferably lower than the cooling end temperature in the initial cooling. This also applies to the third cooling pattern described later.

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

[0098] Average cooling rate in the first stage (hereinafter referred to as the first stage 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 within an appropriate range in the temperature range from the first annealing temperature (or second annealing temperature) to 600°C. If the pre-cooling rate exceeds 50°C / s, the cooling becomes too fast, and ferrite transformation hardly occurs. Therefore, the pre-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 pre-cooling rate is less than 2°C / s, excessive bainite transformation may occur, making it impossible to obtain the desired structure. Therefore, the pre-cooling rate is 2°C / s or more, and preferably 5°C / s or more.

[0099] Cooling end temperature in the first stage (hereinafter referred to as the first stage cooling end temperature): 350 to 600°C The finish temperature of the first-stage cooling is, for example, 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 finish temperature of the first-stage cooling is 600°C or lower, and preferably 550°C or lower.

[0100] [Third cooling pattern] (Pre-cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2-50°C / s, cooling end temperature: 350-550°C (Intermediate hold) Atmospheric hydrogen concentration: 0.2 to 20% by volume, hold temperature: 350 to 550°C, hold time: 5 to 300 seconds (Later cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 5-50°C / s, cooling end temperature: 20-420°C The third cooling pattern basically involves intermediate holding between the first and second cooling stages of the second cooling pattern, and therefore explanations of the first and second cooling stages of the third cooling pattern will be omitted. However, the end temperature of the first cooling stage of the third cooling pattern is set to 350 to 550°C in consideration of the intermediate holding.

[0101] Atmospheric hydrogen concentration during intermediate storage (hereinafter referred to as intermediate hydrogen concentration): 0.2 to 20% by volume During the intermediate holding step, 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% by volume or less, preferably 15% by volume or less, more preferably 13% by volume or less, and even more preferably 10% by volume or less. It is preferable that the intermediate hydrogen concentration be lower than the hydrogen concentration during the first annealing step. On the other hand, from the viewpoint of reducing the risk of reoxidation of the surface of the rolled steel sheet, the intermediate hydrogen concentration is 0.2% by volume or more, preferably 0.5% by volume or more.

[0102] Intermediate holding temperature (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 the intermediate holding becomes low. Therefore, the intermediate holding temperature is 350°C or higher, and preferably 380°C or higher. Also, the intermediate holding temperature is 550°C or lower, and preferably 520°C or lower.

[0103] Intermediate hold time (hereinafter referred to as intermediate hold 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 process The rolled steel plate is then reheated under the following conditions.

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

[0106] Reheating temperature: 100 to 450°C, and above the cooling end temperature of the first cooling step From the viewpoint of sufficiently reducing the amount of dissolved hydrogen in the retained austenite in the final product, the reheating temperature is set to be equal to or higher than the cooling end temperature of the first cooling step and equal to or higher than 100°C. The reheating temperature is preferably 120°C or higher, and more preferably 150°C or higher. Furthermore, from the viewpoint of obtaining the 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-cooling and post-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-cooling end temperature.

[0107] Reheating time: 5~600s To ensure sufficient effectiveness of the 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 process The rolled steel plate is then cooled under the following conditions.

[0109] Atmospheric hydrogen concentration in the second cooling step (hereinafter also referred to as second cooling hydrogen concentration): 0.2% by volume or less In the second cooling step, which corresponds to the final cooling step, a portion of the untransformed austenite present in the rolled steel sheet at the start of the second cooling step transforms into 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% by volume, it becomes difficult to sufficiently reduce the amount of dissolved hydrogen in each phase, particularly in the retained austenite, in the final product. Therefore, the second cooling hydrogen concentration is 0.2% by volume or less, preferably 0.1% by volume or less. The lower limit of the second cooling hydrogen concentration is not particularly limited and may be 0% by volume. The atmosphere for the second cooling step can be, for example, air or an inert atmosphere such as 100% by volume N2 gas.

[0110] Residence time in the temperature range of 100 to 300 ° C. in the second cooling step (hereinafter 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 during the process preceding the second cooling step. To promote the reduction of dissolved hydrogen in retained austenite (hereinafter referred to as the hydrogen reduction effect) in addition to diffusible hydrogen in the steel during this process, it is important to ensure a certain period of time for the steel to remain at high temperature while partially transforming untransformed austenite (FCC) to martensite (BCC). In other words, to fully achieve the hydrogen reduction effect, it is important to control the residence time (second cooling residence time) within a temperature range where the untransformed austenite with a lowered Ms point is likely to begin transforming to martensite, specifically, within the 100–300°C temperature range. Note that at temperatures below 100°C, the hydrogen diffusion distance is reduced, and the hydrogen reduction effect is not fully achieved. Here, a second cooling residence time of less than 2 s results in a small hydrogen diffusion distance, and the effect of reducing the amount of dissolved hydrogen in each phase, especially in retained austenite, is not fully achieved. 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 the second cooling end temperature): 50°C or less The second cooling end temperature is 50° C. or lower. 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 higher.

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

[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. For 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. If the bath Al concentration is 0.05 mass% or more, the occurrence of bottom dross can be more effectively prevented. Furthermore, if 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. For 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. For example, the coating weight per side is 20 g / m 2 or more, preferably 25 g / m 2 It can be more than 120 g / m 2 may be less than or equal to 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. 2If the coating weight is less than 10 ...

[0115] Furthermore, after the hot-dip galvanizing treatment, an alloying treatment may be performed, which is a hot-dip galvannealing treatment. This can further promote the desorption of hydrogen from the steel during the reheating process. The conditions for the alloying treatment 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 to 600°C for a time period of 5 seconds to 60 seconds. The alloying time is the holding time at the alloying temperature. When the alloying treatment is performed, the Fe content in the coating layer (hereinafter also referred to as the degree of alloying) is preferably 7% by mass to 15% by mass. By setting the degree of alloying to 7% by mass or more, the η-Zn phase is prevented from remaining in the coating layer, 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 is promoted, which may result in a decrease in coating adhesion. Therefore, the degree of alloying is preferably 15% by mass or less.

[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 weight is adjusted within this range and the electrolysis time is adjusted. 2 More than 80g / m 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 More than 120g / m 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 components Next, a method for manufacturing a member according to one embodiment of the present invention will be described. The method for manufacturing 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. [Example]

[0120] Steel sheets (thickness: 1.4 mm) were manufactured under the conditions shown in Table 2 using steel slabs with the chemical composition shown in Table 1 (the remainder 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 based on conventional methods. Furthermore, 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 property 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 unavoidable impurities (Al concentration in the bath: 0.13 mass%, balance: Zn and unavoidable impurities) Plating bath temperature: 460℃ Plating weight per side: 30-60g / m 2 [Galvannealed (GA) treatment (hot-dip galvanizing treatment is the same as above)] ·Alloying temperature: 500~560℃ ·Alloying degree: 8.0~14.0% by mass [Electrogalvanized (EG)] Divalent zinc ion concentration in the plating solution: 80 g / L (divalent zinc ions were added as sulfate.) pH of plating solution: 2.0 (The pH of the plating solution was adjusted with sulfuric acid.) Plating solution temperature: 55℃ ·Current density: 50A / dm 2 ·Electrolysis time: 30s Plating weight per side: 50g / m 2 [Hot-dip aluminum plating (Al)] Plating bath composition: Al and unavoidable impurities Plating bath temperature: 700℃ Plating weight per side: 60g / m 2

[0121] From the steel sheet thus obtained, a specimen 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 specimen was then polished and etched with nital. Next, using an SEM (scanning electron microscope), images were taken of three fields of view 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 specimen, 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] Also, in accordance with the above-mentioned procedures, the amount of diffusible hydrogen in the steel of the obtained steel sheet and the amount of dissolved hydrogen in the retained austenite were measured. The results are also shown in Table 3. Note that "A" regarding the measurement method of the hydrogen amount in Table 3 is the measurement of the amount of dissolved hydrogen in the retained austenite after subjecting the sample taken from the steel sheet to the above [A]. Similarly, "B" is the measurement of the amount of dissolved hydrogen in the retained austenite after subjecting the sample taken from the steel sheet to the above [B]. In [A], a tensile load that results in uniform elongation was applied to the sample taken from the steel sheet in the same manner as the tensile test described later. Here, the uniform elongation was defined as the elongation amount at the maximum stress point (maximum tensile load point) in the stress-strain diagram obtained from the tensile test described later. In [B], the sample taken from the steel sheet was immersed in liquid nitrogen (-180 °C or lower) without processing and held for 48 h. Also, for both the above measurement methods [A] and [B], the amount of dissolved hydrogen in the measured retained austenite was within a range where it could be said to be equivalent.

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

[0124] <Evaluation of TS and Press Formability> From the obtained steel sheet, a JIS No. 5 tensile test piece (JIS Z2201 (1988)) was taken such that the direction perpendicular to the rolling direction was the longitudinal direction. Using the taken test piece, a tensile test was conducted in accordance with the provisions of JIS Z2241 (2011) to obtain 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. Then, 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): TS × El, which is the product of TS and El, is 11500 MPa·% or more Fail (poor): TS × El is less than 11500 MPa·%

[0125] <Evaluation of hydrogen embrittlement resistance after processing> From the resulting steel sheets, rectangular test specimens with a major axis length of 100 mm and a minor axis length of 20 mm were cut. 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, i.e., a region where retained austenite had been transformed to martensite through stress-induced transformation. The test specimens were then subjected to a constant-load uniaxial tensile test to measure the critical stress. The test time was 100 hours. The presence or absence of cracks on the edge 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 of cutting the rectangular plates. Then, the hydrogen embrittlement resistance after processing was evaluated according to the following criteria using the ratio of the critical stress to the 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 Rejection (poor) Critical stress / YS is less than 0.85

[0126] [Table 1]

[0127] [Table 2] TIFF0007747247000003.tif233123

[0128] [Table 3] TIFF0007747247000005.tif233159

[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. [Industrial Applicability]

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

Claims

1. In mass%, 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; N: 0.0100% or less and 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, The amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, the amount of dissolved hydrogen in the retained austenite is 0.30 ppm by mass or less, A steel plate having a tensile strength of 780 MPa or more.

2. The component composition further comprises, 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, 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 The steel sheet according to claim 1, comprising at least one selected from the following:

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

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

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

6. A method for producing the steel sheet according to any one of claims 1 to 4, comprising: The method comprises: A steel slab having the chemical composition according to claim 1 or 2, Heating temperature of steel slab: 1100-1300°C, Finishing rolling temperature: 800 to 1000°C and Winding temperature: 700℃ or less a rolling step of hot rolling the steel sheet under the conditions of (a) above or further cold rolling the steel sheet to obtain a rolled steel sheet; Next, the rolled steel sheet is Holding temperature: 650-950℃, Holding time: 10 seconds or more Atmospheric hydrogen concentration: 0.2 to 30% by volume a first annealing step in which the steel sheet is maintained under the following conditions; Next, a first cooling step is performed in which the rolled steel sheet is cooled according to the following first cooling pattern, second cooling pattern, or third cooling pattern. Next, the rolled steel sheet is Atmospheric hydrogen concentration: 20% by volume or less, Reheating temperature: the cooling end temperature of the first cooling step or higher and 100 to 450°C; Reheating time: 5-600s A reheating step of reheating under the conditions of Next, the rolled steel sheet is Atmospheric hydrogen concentration: 0.2% by volume or less, Residence time in the temperature range of 100 to 300 ° C: 2 seconds or more; Cooling end temperature: 50°C or less and a second cooling step of cooling the steel sheet under the condition of [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] (Pre-cooling) Atmospheric hydrogen concentration: 30% by volume or less, average cooling rate: 2 to 50°C / s, cooling end temperature: 350 to 600°C (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 [Third Cooling Pattern] (Pre-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 holding) Atmospheric hydrogen concentration: 0.2 to 20% by volume, holding temperature: 350 to 550°C, holding time: 5 to 300 seconds (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

7. Between the first annealing step and the first cooling step, The rolled steel plate, Holding temperature: 650-950℃, Holding time: 10 seconds or more Atmospheric hydrogen concentration: 0.2 to 8% by volume The method for producing a steel sheet according to claim 6, further comprising a second annealing step of holding the steel sheet under the condition of

8. the cooling pattern of the first cooling step is the second cooling pattern or the third cooling pattern, The atmospheric hydrogen concentration in the pre-cooling is 0.2 to 15% by volume, The method for producing a steel sheet according to claim 6, wherein an average cooling rate of the pre-stage cooling is 2 to 40°C / s.

9. the cooling pattern of the first cooling step is the second cooling pattern or the third cooling pattern, The atmospheric hydrogen concentration in the pre-cooling is 0.2 to 15% by volume, The method for producing a steel sheet according to claim 7, wherein an average cooling rate of the pre-stage cooling is 2 to 40°C / s.

10. 7. The method for producing a steel sheet according to claim 6, 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.

11. 8. The method for producing a steel sheet according to claim 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.

12. 9. The method for producing a steel sheet according to claim 8, 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.

13. 10. The method for producing a steel sheet according to claim 9, 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.

14. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming processing and joining processing to form a component.