Steel plates and components, and the manufacturing methods for them.

Abstract

Invention details;

Description

Steel plates and components, and their manufacturing methods

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

[0002] In recent years, from the perspective of global environmental conservation, the automobile industry has ２ Attempts are being made to reduce exhaust gases such as those mentioned above. Specifically, by increasing the strength and thinning of steel sheets, which are the raw material for automobile parts, the weight of the vehicle body can be reduced and fuel efficiency can be improved. In this way, attempts are being made to reduce exhaust gases.

[0003] For example, Patent Document 1 describes a steel sheet that can be used as a material for such automotive components as follows: "In mass%, C: 0.05% or more and 0.35% or less, Si: 0.05% or more and 2.5% or less, Mn: 0.6% or more and 3.0% or less, P: 0.001% or more and 0.1% or less, S: 0.0002% or more and 0.05% or less, N: 0.0010% or more and 0.020% or less, Al: 0.001% or more and 2.0% or less, The steel sheet has a steel composition containing ferrite, bainite, and tempered martensite, with the balance being iron and unavoidable impurities, and the metal structure contains one or more of ferrite, bainite, and tempered martensite, and contains 3% or more retained austenite, wherein the average grain size of the austenite is 1 μm or more and 8 μm or less, and at the interface where the austenite grains contact the ferrite, bainite, and tempered martensite, 50% or more austenite grains have a central austenite grain concentration Cgc and a grain boundary austenite grain concentration Cgb in the range that satisfies (Equation 1). Cgb / Cgc ≧ 1.1 (Equation 1) is disclosed.

[0004] Patent Document 2 discloses "a galvannealed steel sheet having excellent ductility and corrosion resistance, characterized in that it contains, by mass%, C: 0.10 to 0.50%, Mn: 1.0 to 3.0%, Si: 0.005 to 2.5%, Al: 0.005 to 2.5%, P: 0.05% or less, S: 0.02% or less, and N: 0.006% or less, the total of Si and Al being Si + Al ≥ 0.8%, and its microstructure contains, by area ratio, 10 to 75% ferrite and 2 to 30% retained austenite, and the amount of C in the retained austenite is 0.8 to 1.0%."

[0005] Patent Document 3 states, "In mass%, C: 0.05% or more and 0.35% or less, Si: 0.05% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.0010% or more and 0.1% or less, S: 0.0005% or more and 0.05% or less, N: 0.0010% or more and 0.010% or less, Al: 0.01% or more and 2.0% or less." The steel sheet disclosed has a steel composition containing the above-mentioned elements, with the balance being iron and unavoidable impurities, and has a microstructure in which, in terms of area fraction, the total of the ferrite phase and the bainite phase is 10% or more and 93% or less, the area fraction of the retained austenite phase is 5% or more and 30% or less, and the area fraction of the martensite phase is 5% or more and 20% or less, the retained austenite phase is in a lath-like and island-like form, and the area fraction γi of the island-like retained austenite phase and the area fraction γ of the total retained austenite phase satisfy the following formula (1): 0.7≧γi / γ≧0.3 ... formula (1)

[0006] JP 2012-031505 A JP 2011-168816 A JP 2012-041573 A

[0007] Generally, increasing the strength of a steel sheet reduces its ductility. However, steel sheets used as raw materials for automotive components are required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter simply referred to as El) in a tensile test.

[0008] Furthermore, among automobile components, steel sheets used in automobile frame structural components, in particular, are required to have high component strength when press-formed. To improve the strength of automobile components, for example, it is effective to increase the yield stress (hereinafter also simply referred to as YS) of the steel sheet.

[0009] Furthermore, steel sheets used in automobile frame structural members and the like are formed into complex shapes, and therefore require excellent formability, particularly excellent bendability.

[0010] However, the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to satisfy all of the above-mentioned required properties. Furthermore, the technology of Patent Document 2 requires long-term holding after annealing in order to stabilize the retained austenite. This requires a larger annealing facility, which raises concerns about increased facility costs.

[0011] The present invention has been developed to meet the above-mentioned demands, and aims to provide a steel sheet that combines high strength, high YS, excellent ductility, and excellent bendability, 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.

[0012] Here, high strength and high YS mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test in accordance with JIS Z 2241 satisfy the following formulas, respectively: TS 780 MPa≦TS YS 420 MPa≦YS when 780 MPa≦TS<980 MPa 550 MPa≦YS when 980 MPa≦TS

[0013] Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula: 19% ≤ El when 780 MPa ≤ TS < 980 MPa, and 10% ≤ El when 980 MPa ≤ TS.

[0014] Excellent bendability means that R / t measured in a V-bend test in accordance with JIS Z 2248 satisfies the following formula: 2.0≧R / t when 780 MPa≦TS<980 MPa, 4.0≧R / t when 980 MPa≦TS, where R: limit bending radius (mm), t: thickness of steel plate (mm).

[0015] To achieve the above object, the inventors have conducted extensive research and have discovered the following: (1) After adjusting the component composition to a predetermined range, the total area fraction of ferrite and bainite and the area fraction of martensite are controlled to 10% or more and 87% or less, respectively. This makes it possible to achieve both high strength and excellent ductility. (2) The area fraction of retained austenite is controlled to 3% or more, and the number density of particles constituting the retained austenite is controlled to 0.05 particles / μm ２ The above control is performed. This improves both ductility and bendability. (3) Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less are controlled to satisfy the following (A), (B), (C), and (D). This improves the stability of the retained austenite. As a result, stress-induced transformation from the retained austenite to hard fresh martensite is suppressed. Here, stress-induced transformation occurs during primary processing such as press forming. Furthermore, by controlling the distribution morphology of the retained austenite, it is possible to suppress stress concentration even if hard fresh martensite is formed. This further improves both ductility and bendability. (A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in area ratio. (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more. (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less. (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less. (4) The concentration distribution of Mn in the steel is appropriately controlled, i.e., [Mn] Ｃ / [Mn] is controlled to be 1.05 or more and 2.00 or less. This further improves both ductility and bendability. Ｃis the average Mn concentration (mass%) in the Mn-enriched region in the steel, and [Mn] is the average Mn concentration (mass%) in the steel. The present invention has been completed based on the above findings and further investigations.

[0016] That is, the gist and configuration of the present invention are as follows: 1. an elemental composition, in mass%, of C: 0.05% or more and 0.20% or less, Si: 0.1% or more and 1.8% or less, Mn: 1.5% or more and 3.0% or less, P: 0.001% or more and 0.100% or less, S: 0.0500% or less, Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, and one or both of Nb and Ti: 0.005% or more and 0.200% or less in total, with the balance being Fe and unavoidable impurities; an area fraction of one or both of ferrite and bainite: 10% or more and 87% or less in total, an area fraction of martensite: 10% or more and 87% or less, and an area fraction of retained austenite: 3% or more, and a number density of particles constituting the retained austenite is 0.05 particles / μm ２ or more, among the particles constituting the retained austenite, particles with an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D), [Mn] Ｃ / [Mn] is 1.05 or more and 2.00 or less, and [Mn] Ｃ A steel sheet having a steel structure in which [Mn] is the average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is the average Mn concentration (mass%) in the steel, and having a tensile strength of 780 MPa or more. (A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting retained austenite: 60% or more in area ratio, (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3% or more by mass, (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less, and (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less.

[0017] 2. The chemical composition further includes, in mass %, V: 0.45% or less, B: 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, 2. The steel plate according to 1 above, containing at least one selected from Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less.

[0018] 3. The steel sheet according to 1 or 2 above, having a soft layer having a thickness of 1 μm to 50 μm, wherein the soft layer is a region having a hardness of 65% or less of the hardness at a position 1 / 4 of the sheet thickness of the steel sheet.

[0019] 4. The steel sheet according to any one of 1 to 3 above, which has a zinc plating layer on the surface.

[0020] 5. The steel sheet according to 4 above, wherein the zinc plating layer is a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer.

[0021] 6. A member made using the steel plate according to any one of 1 to 5 above.

[0022] 7. A slab heating step of heating a steel slab having the chemical composition described in 1 or 2 above under the conditions of a slab heating temperature of 1220°C or higher and a slab heating time of 1.0 hour or longer, a hot rolling step of hot rolling the steel slab under the conditions of a finish rolling end temperature of 840°C or higher and 1000°C or lower, an average cooling rate in the temperature range from the finish rolling end temperature to 700°C of 10°C / second or higher, and a coiling temperature of 620°C or lower to obtain a hot rolled steel sheet, a cold rolling step of cold rolling the hot rolled steel sheet under the conditions of a rolling reduction of 20% or higher and 80% or lower to obtain a cold rolled steel sheet, and a heating step of heating the cold rolled steel sheet under the conditions of an average heating rate of 1°C / second or higher and 15°C / second or lower in the temperature range from 600°C to 750°C, the cold-rolled steel sheet is then annealed at an annealing temperature of 750°C or higher and 920°C or lower for an annealing time of 1 second or higher and 30 seconds or lower; the cold-rolled steel sheet is then cooled at an average cooling rate of 5°C / second or higher and 100°C / second or lower in a temperature range from -30°C to 600°C for an annealing temperature and a cooling stop temperature of 400°C to 600°C; and the cold-rolled steel sheet is allowed to remain in a temperature range of 400°C to 600°C for a residence time of 1 second or higher and 90 seconds or lower.

[0023] 8. The method for producing a steel sheet according to 7 above, wherein the dew points of the atmosphere in the heating step and the annealing step are both −35° C. or higher, and the dew point of the atmosphere in the cooling step is −35° C. or lower.

[0024] 9. The method for producing a steel sheet according to 7 or 8 above, further comprising a galvanizing treatment step of subjecting the cold-rolled steel sheet to a galvanizing treatment after the retention step.

[0025] 10. The method for producing a steel sheet according to 9 above, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0026] 11. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of 1 to 5 above to at least one of forming and joining to form a component.

[0027] According to the present invention, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained. Furthermore, since the steel sheet of the present invention has high strength, high YS, excellent ductility, and excellent bendability, it can be extremely advantageously used as a material for automotive frame structural members having complex shapes.

[0028] 1 is an example of an observation image used to measure the particle number density of retained austenite, etc.

[0029] The present invention will be described based on the following embodiments. [1] Steel Sheet First, the chemical composition of a steel sheet according to one embodiment of the present invention will be described. Note that although the unit of chemical composition is always "mass %", hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0030] C: 0.05% or more and 0.20% or less. C has the effect of increasing the strength of martensite and bainite and generating appropriate amounts of these phases. This effect makes it possible to ensure a predetermined strength. Here, if the C content is less than 0.05%, the strength of martensite decreases. Furthermore, the area fraction of ferrite increases excessively, making it difficult to obtain a predetermined strength. On the other hand, if the C content exceeds 0.20%, TS becomes excessively high and El decreases. Furthermore, the strength of martensite increases excessively, reducing bendability. Therefore, the C content is set to 0.05% or more and 0.20% or less. The C content is preferably 0.07% or more, more preferably 0.09% or more. The C content is preferably 0.18% or less, more preferably 0.17% or less.

[0031] Si: 0.1% or more and 1.8% or less Si is an element that improves the strength of steel sheet through solid solution strengthening. Si also increases the strength of ferrite, thereby improving ductility while suppressing strength degradation. Furthermore, Si promotes ferrite transformation during the annealing process and the subsequent cooling process. That is, Si is an element that affects the ferrite area ratio. Here, if the Si content is less than 0.1%, the ferrite area ratio decreases and ductility decreases. On the other hand, if the Si content is excessive, particularly if it exceeds 1.8%, it results in a significant increase in the rolling load during hot rolling and cold rolling. It also results in a decrease in toughness. Therefore, the Si content is set to 0.1% or more and 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. The Si content is preferably 1.5% or less, more preferably 1.0% or less.

[0032] Mn: 1.5% or more and 3.0% or less. Mn is added to improve the hardenability of the steel and ensure the required area ratios of martensite and bainite. Here, if the Mn content is less than 1.5%, the hardenability is insufficient and ferrite is excessively formed. This makes it difficult to achieve a TS of 780 MPa or more. On the other hand, if Mn is added excessively, the bainite transformation is delayed, making it difficult to obtain the required amount of retained austenite. This results in a decrease in ductility. Therefore, the Mn content is set to 1.5% or more and 3.0% or less. The Mn content is preferably 1.65% or more, more preferably 1.8% or more. The Mn content is preferably 2.85% or less, more preferably 2.7% or less.

[0033] P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS of steel sheet. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. As a result, when bending stress is applied to the steel sheet, voids are generated and cracks propagate along the prior austenite grain boundaries, preventing the desired bendability from being achieved. Therefore, the P content is set to 0.100% or less. Due to production technology constraints, the P content is preferably 0.002% or more. The P content is also preferably 0.050% or less, more preferably 0.030% or less.

[0034] S: 0.0500% or less S forms MnS and the like, which reduces ductility. Furthermore, when Ti is contained together with S, TiS, Ti(C,S), and the like are formed, which may reduce bendability. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. The lower limit of the S content is not particularly limited. The S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.

[0035] Al: 0.010% or more and 1.000% or less Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite. Here, if the Al content is less than 0.010%, the area ratio of ferrite decreases, and ductility decreases. On the other hand, if the Al content exceeds 1.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. Therefore, the Al content is set to 0.010% or more and 1.000% or less. The Al content is preferably 0.015% or more, more preferably 0.030% or more. Moreover, the Al content is preferably 0.500% or less, more preferably 0.100% or less.

[0036] N: 0.0100% or less N is an element that generates nitride-based precipitates such as AlN that pin grain boundaries, and can be added to improve elongation. However, if the N content exceeds 0.0100%, the nitride-based precipitates such as AlN become coarse, resulting in a decrease in elongation. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0070% or less, more preferably 0.0050% or less. There is no particular lower limit for the N content. Due to constraints in production technology, the N content is preferably 0.0006% or more.

[0037] One or both of Nb and Ti: 0.005% or more and 0.200% or less in total Nb and Ti are elements that contribute to improving TS, ductility, and even bendability through, for example, refining prior austenite grains. That is, Nb and Ti contribute to increasing TS by refining the internal structure of martensite and bainite due to the refinement of prior austenite grains. Furthermore, Nb and Ti also contribute to increasing TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling and annealing processes. In addition, by refining prior austenite grains and forming fine precipitates, Nb and Ti increase the number of nucleation sites for ferrite and bainite during the cooling and retention processes, thereby promoting ferrite transformation and bainite transformation. Furthermore, the increase in the number of nucleation sites for ferrite and bainite makes it possible to appropriately control the distribution of particles constituting retained austenite. In particular, for particles with an aspect ratio of 3 or less, the above-mentioned (C) and (D) can be appropriately controlled. As a result, Nb and Ti contribute to improving ductility and even bendability. To achieve these effects, the total content of Nb and Ti is set to 0.005% or more. On the other hand, excessive Nb and Ti contents result in the formation of large amounts of coarse precipitates and inclusions, which in turn leads to a decrease in ductility and bendability. In particular, when Nb and Ti are added in combination, precipitates tend to stabilize and remain as coarse inclusions. Therefore, the total content of Nb and Ti is set to 0.200% or less. The total content of Nb and Ti is preferably 0.008% or more, more preferably 0.010% or more, even more preferably 0.011% or more, and even more preferably 0.015% or more. The total content of Nb and Ti is preferably 0.150% or less, more preferably 0.080% or less.

[0038] Note that the individual contents of Nb and Ti are not particularly limited as long as the total content of Nb and Ti is 0.005% or more and 0.200% or less. For example, the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Nb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less. The Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.

[0039] The basic component composition of a steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a component composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a component composition containing the above basic components, 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 components, at least one optional element selected from the following: V: 0.45% or less, B: 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less and REM: 0.02% or less. Note that the effects of the present invention can be obtained as long as the optional elements are contained in amounts not exceeding the upper limits, so no lower limits are particularly set. Note that when the optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

[0040] V: 0.45% or less Like Nb and Ti, V increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling and annealing processes. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.45%, a large amount of coarse precipitates and inclusions may be formed, which may result in a decrease in ductility. Therefore, when V is contained, the V content is preferably 0.45% or less. The V content is more preferably 0.060% or less.

[0041] B: 0.010% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B is an element that controls the formation and grain growth of ferrite in the cooling process after the annealing process. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.010%, the amount of nitride-based precipitates such as BN becomes excessive, which may result in a decrease in ductility. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0042] Cr: 1.0% or less Cr is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.0%, the area ratio of martensite increases, which may result in a decrease in ductility. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. The Cr content is more preferably 0.60% or less, and even more preferably 0.30% or less.

[0043] Ni: 1.0% or less Ni is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.0%, the area ratio of martensite increases, which may result in a decrease in ductility. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The Ni content is more preferably 0.5% or less.

[0044] Mo: 1.0% or less Mo is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.0%, the area ratio of martensite increases, and the desired ductility may not be obtained. Therefore, when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.5% or less, and even more preferably 0.3% or less.

[0045] Sb: 0.1% or less Sb is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 780 MPa or more. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.1%, castability decreases. Therefore, when Sb is contained, the Sb content is preferably 0.1% or less. The Sb content is more preferably 0.06% or less, and even more preferably 0.04% or less.

[0046] Sn: 0.1% or less Sn suppresses oxidation and nitriding near the steel sheet surface, thereby suppressing the resulting decrease in the C and B contents near the steel sheet surface. This suppresses excessive ferrite formation near the steel sheet surface, contributing to achieving a TS of 780 MPa or more. From this perspective, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.1%, castability decreases. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less. The Sn content is more preferably 0.04% or less, and even more preferably 0.02% or less.

[0047] Cu: 1.0% or less Cu is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.0%, the area ratio of martensite may increase excessively, which may reduce ductility. In addition, large amounts of coarse precipitates and inclusions may be formed, which may reduce ductility. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.2% or less.

[0048] Ta: 0.1% or less Like Ti, Nb, and V, Ta increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.1%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may reduce ductility and even bendability. Therefore, when Ta is contained, the Ta content is preferably 0.1% or less. The Ta content is more preferably 0.05% or less.

[0049] W: 0.2% or less Like Ti, Nb, and V, W increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling and annealing processes. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, if the W content exceeds 0.2%, a large amount of coarse precipitates and inclusions is formed, resulting in a decrease in ductility. Therefore, when W is contained, the W content is preferably 0.2% or less. The W content is more preferably 0.060% or less.

[0050] Mg: 0.01% or less Mg is an element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides to improve the hole expandability and bendability of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. However, if the Mg content exceeds 0.01%, the surface quality deteriorates. In some cases, this may even result in a deterioration in bendability. Therefore, when Mg is contained, the Mg content is preferably 0.01% or less. The Mg content is more preferably 0.005% or less, and even more preferably 0.001% or less.

[0051] Zn: 0.02% or less Zn is an element that is effective in spheroidizing the shape of inclusions and improving the bendability of steel sheets. To achieve this effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 0.02%, a large amount of coarse precipitates and inclusions may be formed, which may actually result in a decrease in bendability. Therefore, when Zn is contained, the Zn content is preferably 0.02% or less.

[0052] Co: 0.02% or less Like Zn, Co is an element that is effective in spheroidizing the shape of inclusions and improving the bendability of steel sheets. To achieve this effect, the Co content is preferably 0.001% or more. On the other hand, if the Co content exceeds 0.02%, a large amount of coarse precipitates and inclusions may be formed, which may actually result in a decrease in bendability. Therefore, when Co is contained, the Co content is preferably 0.02% or less.

[0053] Zr: 0.2% or less Zr contributes to high strength by refining prior austenite grains. Zr also contributes to high strength by reducing the block size and vein grain size, which are the internal structural units of martensite and bainite, due to the refinement of prior austenite grains. Furthermore, Zr improves castability. To achieve these effects, the Zr content is preferably 0.001% or more. However, adding a large amount of Zr increases the amount of coarse ZrN-based and ZrS-based precipitates that remain in an undissolved state during the slab heating process, thereby reducing ductility. Therefore, when Zr is contained, the Zr content is preferably 0.2% or less. The Zr content is more preferably 0.05% or less, and even more preferably 0.01% or less.

[0054] Ca: 0.02% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.02%, a large amount of coarse inclusions may be generated, which may reduce ductility and bendability. Furthermore, surface quality may also deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.02% or less. The lower limit of the Ca content is not particularly limited. For example, the Ca content is preferably 0.0005% or more. Furthermore, due to constraints in production technology, the Ca content is more preferably 0.0010% or more.

[0055] Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the bendability of steel sheet. To obtain such effects, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.02% each, or if the content of As exceeds 0.05%, large amounts of coarse precipitates and inclusions may be formed, which may actually reduce bendability. Therefore, when these elements are contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are each 0.02% or less, and the content of As is 0.05% or less. Note that Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM may be contained alone or in combination.

[0056] The elements other than those mentioned above are Fe and unavoidable impurities.

[0057] Next, the steel structure of a steel plate according to one embodiment of the present invention will be described. The steel structure of a steel plate according to one embodiment of the present invention has an area ratio of one or both of ferrite and bainite: 10% to 87% in total, an area ratio of martensite: 10% to 87%, and an area ratio of retained austenite: 3% or more, and the number density of particles constituting the retained austenite is 0.05 particles / μm ２ or more, among the particles constituting the retained austenite, particles with an aspect ratio of 3 or less satisfy the above (A), (B), (C), and (D), and [Mn] Ｃ / [Mn] is 1.05 or more and 2.00 or less, and [Mn] Ｃis the average Mn concentration (mass%) of Mn-enriched regions in the steel, [Mn] is the average Mn concentration (mass%) in the steel. The reasons for each limitation will be explained below. The area ratio of each phase is the ratio of the area occupied by each phase to the area of ​​the entire steel structure.

[0058] Total area ratio of one or both of ferrite and bainite (hereinafter also referred to as the total area ratio of ferrite and bainite): 10% or more and 87% or less. Ferrite and bainite are soft, and therefore effective in obtaining excellent ductility. In order to obtain the desired ductility, the total area ratio of ferrite and bainite is set to 10% or more. On the other hand, if the area ratios of ferrite and bainite are excessive, it becomes difficult to achieve a TS of 780 MPa or more. Therefore, the total area ratio of ferrite and bainite is set to 87% or less. The total area ratio of ferrite and bainite is preferably 20% or more, more preferably 30% or more. Furthermore, the total area ratio of ferrite and bainite is preferably 75% or less, more preferably 65% ​​or less. Ferrite and bainite may be contained alone, or both may be contained.

[0059] Area fraction of martensite: 10% or more and 87% or less Martensite is hard and is a structure necessary for increasing the strength of steel sheets. Here, if the area fraction of martensite is less than 10%, the desired TS cannot be obtained. On the other hand, an excessive increase in the area fraction of martensite causes a decrease in ductility. Therefore, the area fraction of martensite is set to 10% or more and 87% or less. The area fraction of martensite is preferably 20% or more, more preferably 30% or more. The area fraction of martensite is preferably 75% or less, more preferably 65% ​​or less. Note that martensite is a hard structure formed by transformation from austenite below the martensite transformation point (also simply referred to as the Ms point). Martensite includes both as-quenched so-called fresh martensite and tempered fresh martensite.

[0060] Area Fraction of Retained Austenite: 3% or More Retained austenite is a structure necessary for achieving both strength and ductility. Here, if the area fraction of retained austenite is less than 3%, both strength and ductility cannot be achieved. Therefore, the area fraction of retained austenite is set to 3% or more. The area fraction of retained austenite is preferably 5% or more, more preferably 7% or more. There is no particular upper limit to the area fraction of retained austenite. However, if the retained austenite is excessive, for example, when the steel sheet is formed into a part, the retained austenite may transform into martensite, increasing the number of initiation points for bending cracks. Therefore, the area fraction of retained austenite is preferably 20% or less, more preferably 15% or less. Note that retained austenite refers to austenite that remains without transforming from austenite to ferrite, martensite, bainite, or other metallic phases. Furthermore, retained austenite is generated, for example, when elements such as C are concentrated in austenite, causing the martensite transformation point to fall below room temperature (austenite remains without transforming).

[0061] The area ratio of the remaining structure other than the above is preferably 15% or less. The area ratio of the remaining structure is more preferably 10% or less, and even more preferably 5% or less. The area ratio of the remaining structure may be 0%. The remaining structure is not particularly limited, and examples thereof include pearlite and carbides such as cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM). Pearlite is formed from austenite at relatively high temperatures and is a structure consisting of lamellar ferrite and cementite.

[0062] Here, the total area ratio of ferrite and bainite, and the area ratio of martensite are measured at a quarter-thickness position of the steel plate as follows. That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate serves as the observation surface. Next, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Next, the observation surface of the sample is etched with nital to reveal the structure. Then, five fields of view of the observation surface of the sample are observed using an SEM (Scanning Electron Microscope) at a magnification of 1500x. Next, the following regions are color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems. Then, the total area ratio of ferrite and bainite, and the area ratio of martensite are calculated using the point counting method. Specifically, 16 × 15 lattice points were placed at 4.8 μm intervals in an area of ​​82 μm × 57 μm in actual length in each SEM image. The number of lattice points on ferrite, bainite, and martensite was then counted. The number of lattice points on ferrite, bainite, and martensite was then divided by the total number of lattice points, and multiplied by 100 to calculate the total area ratio of ferrite and bainite, as well as the area ratio of martensite. Ferrite: Black region, morphology: massive. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite at relatively high temperatures. Bainite: Black to dark gray region, morphology: massive, amorphous, etc. As described above, bainite is a hard structure in which fine carbides are dispersed in acicular or plate-like ferrite. Bainite is formed from austenite at relatively low temperatures (above the Ms point). Bainite contains a relatively small number of carbides. Martensite: A region that is white to light gray. As mentioned above, martensite is a hard structure that is formed by transformation from austenite below the Ms point.Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.

[0063] The area fraction of retained austenite is measured at a quarter-thickness position of the steel plate as follows. Specifically, the steel plate is mechanically ground in the thickness direction (depth direction) to a quarter-thickness position, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated. The volume fraction of retained austenite is then calculated from the ratio of the diffraction 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.

[0064] Furthermore, the area ratio of the remaining structure is calculated by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite calculated as described above from 100%: [Area ratio of remaining structure (%)] = 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)]

[0065] Number density of particles constituting retained austenite (hereinafter also referred to as particle number density of retained austenite): 0.05 particles / μm ２ In the steel sheet according to one embodiment of the present invention, the particle number density of the retained austenite is set to 0.05 particles / μm ２ Hard martensite (hereinafter also referred to as deformation-induced martensite) that is generated by deformation-induced transformation of retained austenite during processing (hereinafter also referred to as processing) to form steel sheets into parts is a structure that promotes void generation and crack propagation during processing. However, when the particle number density of retained austenite is 0.05 particles / μm ２By setting the grain size to 0.05, the stress on the deformation-induced martensite is dispersed, suppressing stress concentration and suppressing the generation of voids and the propagation of cracks. ２ The particle number density of the retained austenite is preferably 0.15 particles / μm ２ More preferably, 0.25 particles / μm ２ The upper limit of the particle number density of the retained austenite is not particularly limited. However, if the particle number density of the retained austenite becomes excessive, it will lead to an increase in the number of initiation points of bending cracks. Therefore, the particle number density of the retained austenite is preferably 100 particles / μm ２ Less than or equal to 10 particles / μm, more preferably 10 particles / μm ２ The following is the result.

[0066] (A) The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite (hereinafter also referred to as the ratio of particles with an aspect ratio of 3 or less): 60% or more in area ratio. When the aspect ratio of the retained austenite is large, stress concentration tends to occur around the processing-induced martensite generated by processing. This promotes void generation and crack propagation, and reduces ductility and bendability. Therefore, the ratio of particles with an aspect ratio of 3 or less is set to 60% or more in area ratio. The ratio of particles with an aspect ratio of 3 or less is preferably 65% ​​or more, more preferably 70% or more in area ratio. The upper limit of the ratio of particles with an aspect ratio of 3 or less is not particularly limited, and may be 100% in area ratio.

[0067] (B) Average C Concentration of Particles with an Aspect Ratio of 3 or Less: 0.3 Mass% or More In a steel sheet according to one embodiment of the present invention, it is important that the average C concentration of particles with an aspect ratio of 3 or less be 0.3 mass% or more. That is, the higher the average C concentration of particles with an aspect ratio of 3 or less, the higher the stability of retained austenite, resulting in an excellent balance between strength and ductility. If the average C concentration of particles with an aspect ratio of 3 or less is less than 0.3 mass%, a good balance between strength and ductility cannot be achieved. Furthermore, due to the low stability of retained austenite, for example, the amount of retained austenite that becomes deformation-induced martensite upon processing increases, resulting in reduced bendability. Therefore, the average C concentration of particles with an aspect ratio of 3 or less is set to 0.3 mass% or more. The average C concentration of particles with an aspect ratio of 3 or less is preferably 0.5 mass% or more, more preferably 0.7 mass% or more. The upper limit of the average C concentration of particles with an aspect ratio of 3 or less is not particularly limited. However, if the C concentration in particles with an aspect ratio of 3 or less is excessively high, the progress of the stress-induced transformation from retained austenite to martensite may be excessively suppressed, and sufficient work hardening ability may not be obtained. Therefore, the average C concentration in particles with an aspect ratio of 3 or less is preferably 2.0 mass% or less.

[0068] (C) Average value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the average interparticle distance value): 5 μm or less. In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse retained austenite particles. By uniformly dispersing retained austenite particles, particularly particles with an aspect ratio of 3 or less, stress concentration on deformation-induced transformed martensite caused by processing is suppressed, improving ductility and bendability. To achieve this, it is important to set the average interparticle distance to 5 μm or less. The average interparticle distance is preferably 4 μm or less, more preferably 3 μm or less. The lower limit of the average interparticle distance is not particularly limited. However, from the viewpoints of cost and productivity, the average interparticle distance is preferably 0.8 μm or more.

[0069] (D) Maximum value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the maximum interparticle distance value): 15 μm or less. In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse retained austenite. By uniformly dispersing retained austenite particles, particularly particles with an aspect ratio of 3 or less, stress concentration on the deformation-induced transformed martensite caused by processing is suppressed, thereby improving ductility and bendability. Here, if the maximum interparticle distance value is large, particularly exceeding 15 μm, retained austenite particles will be present locally, resulting in a decrease in ductility and bendability. Therefore, the maximum interparticle distance value is set to 15 μm or less. The maximum interparticle distance value is preferably 10 μm or less, more preferably 7 μm or less. Note that there is no particular limitation on the lower limit of the maximum interparticle distance value. For example, the maximum interparticle distance value is preferably 1 μm or more.

[0070] Here, the particle number density of retained austenite, the ratio of particles with an aspect ratio of 3 or less, the average carbon concentration of particles with an aspect ratio of 3 or less, the average interparticle distance, and the maximum interparticle distance are each determined using electron backscatter diffraction (EBSD) attached to an FE-SEM as follows. Specifically, a sample is cut out from the steel sheet so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the steel sheet. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, a 50 μm × 50 μm region is observed using EBSD, with the observation position being a quarter of the steel sheet thickness. Then, using phase map, image analysis is performed using ImageJ, and the aspect ratio and center of gravity position of each observed retained austenite particle are determined using the particle analysis function. For reference, an example of an observation image is shown in Figure 1. In the observation image, the white areas represent retained austenite particles. The particles constituting the retained austenite are those with an equivalent circle diameter of 0.8 μm or more. The number of particles constituting the retained austenite is counted, and the particle number density of the retained austenite is calculated by dividing the number of particles by the area of ​​the observation region. The area occupied by particles with an aspect ratio of 3 or less among all the particles constituting the retained austenite counted above is divided by the area occupied by all the particles constituting the retained austenite, and the result is multiplied by 100 to calculate the ratio of particles with an aspect ratio of 3 or less. Furthermore, for each particle with an aspect ratio of 3 or less, the distance to the nearest particle with an aspect ratio of 3 or less (the distance between the centers of gravity of the particles) is calculated. The average and maximum of the calculated distances are defined as the average interparticle distance and the maximum interparticle distance, respectively.

[0071] The average C concentration of particles with an aspect ratio of 3 or less is determined as follows. That is, for the sample used in the above EBSD observation, the observation position is set at a position 1 / 4 of the steel plate thickness, as in the case of EBSD, and the C concentration is measured in a grid pattern in a 23 μm square area with a measurement interval of 0.1 μm. Next, a region of particles with an aspect ratio of 3 or less is extracted from the EBSD phase map, and the average value of the C concentration at each measurement point in that region is taken as the average C concentration of particles with an aspect ratio of 3 or less.

[0072] [Mn] Ｃ / [Mn]: 1.05 or more and 2.00 or less [Mn] is the ratio of the average Mn concentration in the Mn-enriched region in the steel to the average Mn concentration in the steel. Ｃ A large value of [Mn] / [Mn] means that Mn enrichment in austenite has progressed during the annealing process. The Mn concentration in austenite in a steel sheet immediately after the annealing process is one of the factors that determine whether the phase transformed from austenite during the cooling and retention processes after the annealing process is ferrite and bainite, or martensite. Excessive enrichment of Mn in austenite during the annealing process delays the ferrite and bainite transformation during the cooling process. This can lead to failure to obtain the desired area ratios of ferrite and bainite, resulting in reduced ductility and bendability. Furthermore, the delay in the ferrite and bainite transformation suppresses the enrichment of C in untransformed austenite. Therefore, sufficient retained austenite, which contributes to improved ductility, cannot be obtained. Therefore, [Mn] Ｃ / [Mn] is 2.00 or less. [Mn] Ｃ / [Mn] is preferably 1.80 or less, more preferably 1.60 or less. Ｃ When [Mn] is 1.05 or more, that is, when the Mn concentration distribution is moderately non-uniform, ferrite transformation and bainite transformation are promoted in austenite with a low Mn concentration. As a result, C enrichment in untransformed austenite progresses. This results in good ductility and bendability. Therefore, [Mn] Ｃ / [Mn] is 1.05 or more. [Mn] Ｃ / [Mn] is preferably 1.10 or more, more preferably 1.15 or more.

[0073] Here, [Mn] Ｃ / [Mn] is determined as follows. That is, a sample is cut out from the steel sheet so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel sheet serves as the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1 / 4 of the plate thickness of the steel sheet, and the Mn concentration is measured in a grid pattern in a 23 μm square area with a measurement interval of 0.1 μm using EPMA. Then, the average value of the Mn concentrations at all measurement points is taken as the average Mn concentration [Mn] (mass%) in the steel. Furthermore, the top 10% of the regions with the highest Mn concentrations among all measurement points are taken as Mn-enriched regions. Then, the average value of the Mn concentrations measured in the Mn-enriched regions is taken as the average Mn concentration [Mn] of the Mn-enriched regions in the steel. Ｃ (mass%). And, [Mn] Ｃ By dividing by [Mn], [Mn] Ｃ / [Mn] is calculated.

[0074] Furthermore, the steel sheet according to one embodiment of the present invention preferably has a soft layer with a thickness of 1 μm or more and 50 μm or less. In particular, by having a soft layer with a thickness of 1 μm or more and 50 μm or less from the surface of the steel sheet in the thickness direction, better bendability can be obtained. Therefore, it is preferable to have a soft layer from the surface of the steel sheet in the thickness direction, and it is also preferable that the thickness be 1 μm or more. However, if an excessive soft layer is formed, it becomes difficult to obtain the desired TS. Therefore, when a soft layer is present, its thickness is preferably 50 μm or less. The thickness of the soft layer is more preferably 40 μm or less.

[0075] Here, the soft layer refers to a region where the hardness is 65% or less of the hardness at the 1 / 4 position of the steel plate thickness. The thickness of the soft layer is measured as follows. Specifically, the surface of a thickness cross section (L cross section) parallel to the rolling direction of the steel plate is smoothed by wet polishing. Next, using a Vickers hardness tester, hardness measurements are performed at 1 μm intervals in the thickness (depth) direction from a position 1 μm deep from the surface of the steel plate to a position 100 μm deep under a load of 10 gf. Furthermore, under the same conditions, hardness measurements are performed at 20 μm intervals in the thickness (depth) direction from a position 100 μm deep from the surface of the steel plate to the center position of the thickness. The hardness measured at the 1 / 4 position of the steel plate thickness is defined as the reference hardness, and a depth position at which the hardness is 65% or less of the reference hardness is identified on the surface side of the 1 / 4 position of the steel plate thickness. The distance from the surface of the steel plate to the deepest depth position where the hardness is 65% or less of the standard hardness (hereinafter also referred to as the depth of the region where the hardness is 65% or less of the standard hardness) is measured. This measurement is performed at five locations at intervals of 3 mm or more in the rolling direction, and the average value of the measured depths of the regions where the hardness is 65% or less of the standard hardness is taken as the thickness of the soft layer. Note that, since the steel structure of a steel plate is usually approximately symmetrical from top to bottom in the thickness direction, any one of the surfaces (front and back) of the steel plate is used as a representative for measuring the thickness of the soft layer. For example, any one of the surfaces (front and back) of the steel plate may be used as the starting point of the thickness position (0 thickness position), such as the 1 / 4 thickness position. Note that, when the soft layer is present on only one side of the steel plate, the surface where the soft layer is present is taken as the starting point of the thickness position (0 thickness position). The thickness of the soft layer is the thickness per side.

[0076] Furthermore, the smaller the fluctuation range of the soft phase thickness in the longitudinal direction (rolling direction) of the steel sheet, the better. In particular, if the fluctuation range of the soft phase thickness exceeds 20 μm, variations in bendability occur in the longitudinal direction of the steel sheet, and there is a risk of localized deterioration in bendability. Therefore, it is preferable that the fluctuation range of the soft phase thickness is 20 μm or less. The fluctuation range of the soft phase thickness is preferably 15 μm or less, more preferably 10 μm or less. There is no particular restriction on the lower limit of the fluctuation range of the soft phase thickness. The fluctuation range of the soft phase thickness may be 0 μm. Here, the fluctuation range of the soft phase thickness is the difference (maximum value - minimum value) between the maximum and minimum values ​​of the depth of the region where the hardness is 65% or less of the reference hardness measured in the soft phase thickness measurement described above.

[0077] Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described.

[0078] Tensile strength (TS): 780 MPa or more The tensile strength of the steel plate according to one embodiment of the present invention is 780 MPa or more. There is no particular upper limit to the tensile strength of the steel plate according to one embodiment of the present invention. However, for example, the tensile strength of the steel plate according to one embodiment of the present invention is preferably less than 1180 MPa.

[0079] The yield stress (YS), total elongation (El), and R / t of the steel sheet according to one embodiment of the present invention are as described above. The tensile strength (TS), yield stress (YS), total elongation (El), and R / t are measured as described later in the examples.

[0080] Furthermore, the steel sheet according to one embodiment of the present invention may have a galvanized layer on its surface. The galvanized layer may be provided on only one surface of the steel sheet, or on both surfaces. The galvanized layer refers to a plating layer containing Zn as a main component (Zn content of 50.0 mass% or more). Examples of the galvanized layer include a hot-dip galvanized layer and a galvannealed hot-dip galvanized layer. A steel sheet having a galvanized layer can also be referred to as a galvanized steel sheet. The above-mentioned steel sheets having a hot-dip galvanized layer and a galvannealed hot-dip galvanized layer can also be referred to as a hot-dip galvanized steel sheet (GI) and a galvannealed hot-dip galvanized steel sheet (GA), respectively.

[0081] Here, 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 other than the above elements is unavoidable impurities.

[0082] The galvannealed layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass to 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0083] In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g / m ２ 80g / m or more ２ It is preferable to do the following:

[0084] The coating weight of the zinc plating layer is measured 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 zinc 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 steel sheet (the surface area of ​​the part that was covered with plating) to determine the coating weight (g / m２ ) is calculated.

[0085] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.

[0086] [2] Member Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned steel plate. For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above-mentioned steel plate has a TS of 780 MPa or more, a high YS, excellent ductility, and excellent bendability. Therefore, the member according to one embodiment of the present invention has high strength and is particularly suitable for application to complex-shaped members used in the automotive field.

[0087] [3] Method for Manufacturing Steel Sheet Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

[0088] A method for producing a steel sheet according to one embodiment of the present invention includes: a slab heating step of heating a steel slab having the above-described chemical composition at a slab heating temperature of 1220°C or higher and a slab heating time of 1.0 hour or longer; a hot rolling step of hot rolling the steel slab at a finish rolling end temperature of 840°C or higher and 1000°C or lower, an average cooling rate in the temperature range from the finish rolling end temperature to 700°C of 10°C / second or higher, and a coiling temperature of 620°C or lower to obtain a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet at a rolling reduction of 20% or higher and 80% or lower to obtain a cold-rolled steel sheet; and a heating step of heating the cold-rolled steel sheet at an average heating rate of 1°C / second or higher and 15°C / second or lower in the temperature range from 600°C to 750°C. Next, the method includes an annealing step of annealing the cold-rolled steel sheet under the conditions of an annealing temperature of 750°C to 920°C and an annealing time of 1 second to 30 seconds, a cooling step of cooling the cold-rolled steel sheet under the conditions of an annealing temperature of -30°C to 600°C at an average cooling rate of 5°C / second to 100°C / second and a cooling stop temperature of 400°C to 600°C, and a retention step of retaining the cold-rolled steel sheet under the conditions of a retention time of 1 second to 90 seconds in a temperature range of 400°C to 600°C. Note that the above temperatures refer to the surface temperatures of the steel slab and the steel sheet unless otherwise specified.

[0089] First, a steel slab having the above-described chemical composition is prepared. For example, a steel material is melted to produce molten steel having the above-described chemical composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The resulting molten steel is then solidified to produce a steel slab. The method for obtaining a steel slab from the molten steel is not particularly limited. For example, continuous casting, ingot casting, or thin slab casting can be used. From the viewpoint of preventing macrosegregation, continuous casting is preferred. Furthermore, a conventional method in which the steel slab is cooled to room temperature after production and then reheated can be applied. Furthermore, energy-saving processes such as direct rolling (a method in which the steel slab is not cooled to room temperature, but is charged as a hot slab into a heating furnace and hot-rolled) and direct rolling (a method in which the steel slab is briefly held at room temperature and then immediately rolled) can also be applied without any problems.

[0090] [Slab Heating Step] Next, the steel slab is heated. At this time, in the method for producing a steel plate according to one embodiment of the present invention, it is important to satisfy the following conditions.

[0091] Slab heating temperature: 1220°C or higher When the slab heating temperature is 1220°C or higher, coarse Nb- and Ti-based precipitates formed during casting are fully dissolved. This reduces the amount of coarse precipitates. Furthermore, it is possible to uniformly disperse retained austenite particles, particularly to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the final steel sheet product. Therefore, the slab heating temperature is set to 1220°C or higher. The slab heating temperature is preferably 1230°C or higher, more preferably 1240°C or higher. There is no particular upper limit to the slab heating temperature. For example, the slab heating temperature is preferably 1400°C or lower. The slab heating temperature is the maximum temperature reached by the steel slab during the slab heating process.

[0092] Slab heating time: 1.0 hour or more. When the slab heating temperature is 1220°C or more and the slab heating time is 1.0 hour or more, coarse Nb- and Ti-based precipitates formed during casting are sufficiently solid-dissolved. This reduces the amount of coarse precipitates. Furthermore, it is possible to uniformly disperse retained austenite particles, particularly to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the final steel sheet product. Therefore, the slab heating time is 1.0 hour or more. The slab heating time is preferably 1.1 hours or more, more preferably 1.2 hours or more. The upper limit of the slab heating time is not particularly limited. For example, the slab heating time is preferably 3.0 hours or less. The slab heating time is the holding time in the temperature range of 1220°C or more.

[0093] [Hot Rolling Step] Next, the steel slab is hot rolled to obtain a hot rolled steel sheet. In this hot rolling step, it is important to satisfy the following conditions.

[0094] Finish rolling end temperature: 840°C or higher and 1000°C or lower From the viewpoint of suppressing Mn enrichment in the steel structure of the steel sheet that will become the final product, it is important to suppress Mn enrichment (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the finish rolling end temperature is less than 840°C, ferrite formation is promoted, and excessive ferrite is formed before the hot-rolled steel sheet is coiled. As a result, C is enriched in untransformed austenite. Excessive C enrichment in untransformed austenite promotes pearlite transformation. In other words, excessive pearlite is formed in the steel structure of the hot-rolled steel sheet obtained after hot rolling. Pearlite is a lamellar structure of ferrite and cementite, and Mn is enriched in cementite. As a result, variation in Mn concentration occurs. Therefore, the finish rolling end temperature is set to 840°C or higher. The finish rolling end temperature is preferably 850°C or higher. On the other hand, if the finish rolling end temperature is excessively high, it may become difficult to cool the steel sheet to the coiling temperature described below. Therefore, the finish rolling end temperature is set to 1000°C or lower. The finish rolling end temperature is preferably 950°C or lower, and more preferably 920°C or lower.

[0095] Average cooling rate in the temperature range from the finish rolling end temperature to 700°C (hereinafter also referred to as the first cooling rate): 10°C / sec or more. As described above, from the viewpoint of suppressing Mn enrichment in the steel structure of the final product, it is important to suppress Mn enrichment (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the first cooling rate is slow, the amount of ferrite produced during cooling becomes excessive, leading to the enrichment of C in untransformed austenite. Excessive C enrichment in untransformed austenite promotes pearlite transformation. That is, excessive pearlite is produced in the steel structure of the hot-rolled steel sheet obtained after hot rolling. As described above, pearlite is a lamellar structure of ferrite and cementite, and Mn enriches in cementite. As a result, variation in Mn concentration occurs. Furthermore, if the first cooling rate is slow, precipitates such as carbides and nitrides of Nb and Ti become coarse. Therefore, the effects of increasing TS and improving ductility due to refinement of prior austenite grains and fine precipitates cannot be obtained. Therefore, the first cooling rate is set to 10°C / sec or more. The first cooling rate is preferably 15°C / sec or more. There is no particular upper limit to the first cooling rate. However, from the viewpoint of energy saving in cooling equipment, the first cooling rate is preferably set to 1000°C / sec or less.

[0096] Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, pearlite will be excessively increased during coiling, promoting Mn concentration. The lower the coiling temperature, the less pearlite is produced, so a lower coiling temperature is preferable. Furthermore, from the viewpoint of finely precipitating Nb and Ti carbides and nitrides, a lower coiling temperature is also preferable. Therefore, the coiling temperature is set to 620°C or less. The coiling temperature is preferably 600°C or less, more preferably 580°C or less. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may become excessively hardened, which may cause fracture during cold rolling. Therefore, the coiling temperature is preferably 400°C or more. The coiling temperature is more preferably 450°C or more.

[0097] In addition, descaling may be performed as appropriate to remove primary and secondary scales formed on the surface of the hot-rolled steel sheet. Before cold rolling, the hot-rolled steel sheet is preferably thoroughly pickled to reduce remaining scale. Furthermore, from the viewpoint of reducing the load during cold rolling, the hot-rolled steel sheet may optionally be subjected to hot-rolled sheet annealing.

[0098] [Cold Rolling Step] Next, the hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet.

[0099] Reduction ratio: 20% or more and 80% or less The reduction ratio in cold rolling is set to 20% or more. That is, if the reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, resulting in reduced TS and bendability in the final product. Therefore, the reduction ratio is set to 20% or more. On the other hand, if the reduction ratio exceeds 80%, the steel sheet is likely to have a defective shape. Furthermore, there is a risk that non-uniformity in the steel structure and non-uniformity in the amount of zinc coating may occur due to temperature unevenness during the annealing process. Therefore, the reduction ratio is set to 80% or less. The reduction ratio is preferably 30% or more. Furthermore, the reduction ratio is preferably 70% or less.

[0100] [Heating Step] Next, the cold-rolled steel sheet is heated to the annealing temperature. At this time, it is important to appropriately control the average heating rate in the temperature range from 600°C to 750°C.

[0101] Average heating rate in the temperature range from 600°C to 750°C (hereinafter also referred to as heating rate): 1°C / sec or more and 15°C / sec or less. When the residence time of the cold-rolled steel sheet in the temperature range from 600°C to 750°C (hereinafter also referred to as heating temperature range) during the heating process is reduced, Mn diffuses and the enrichment of Mn in austenite is suppressed. That is, the longer the residence time in the heating temperature range, the more the enrichment of Mn in austenite is promoted. Therefore, it is effective to shorten the residence time in the heating temperature range, in other words, to increase the heating rate. Furthermore, shortening the residence time in the heating temperature range suppresses the coarsening of prior austenite grains. As a result, it is possible to ensure a predetermined ratio of particles with an aspect ratio of 3 or less that constitute the retained austenite. Furthermore, a faster heating rate is preferable from the viewpoint of promoting ferrite transformation and bainite transformation and uniformly dispersing the retained austenite. Therefore, the heating rate is set to 1°C / sec or more. The temperature rise rate is preferably 2°C / sec or more, more preferably 3°C / sec or more. On the other hand, if the temperature rise rate exceeds 15°C / sec, Mn enrichment in austenite during the temperature rise step is excessively suppressed. Therefore, the temperature rise rate is set to 15°C / sec or less. The temperature rise rate is preferably 12°C / sec or less, more preferably 9°C / sec or less.

[0102] Atmospheric dew point: -35°C or higher From the viewpoint of forming a soft layer of the desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable that the atmospheric dew point in the temperature-raising step is -35°C or higher. If the atmospheric dew point is less than -35°C, it becomes difficult to form a soft phase of the desired thickness. Therefore, it is preferable that the atmospheric dew point in the temperature-raising step is -35°C or higher. The atmospheric dew point in the temperature-raising step is more preferably -20°C or higher, and even more preferably -10°C or higher. Note that the upper limit of the atmospheric dew point in the temperature-raising step is not particularly limited. In order to set the TS within a suitable range, the atmospheric dew point in the temperature-raising step is preferably 15°C or lower, more preferably 5°C or lower.

[0103] [Annealing Step] Next, the cold-rolled steel sheet is annealed under the conditions of an annealing temperature of 750° C. to 920° C. and an annealing time of 1 second to 30 seconds.

[0104] Annealing temperature: 750°C or higher and 920°C or lower If the annealing temperature is lower than 750°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, and the desired TS cannot be obtained. On the other hand, if the annealing temperature exceeds 920°C, the desired area ratios of ferrite and bainite cannot be obtained, and ductility decreases. Therefore, the annealing temperature is set to 750°C or higher and 920°C or lower. The annealing temperature is preferably 880°C or lower. The annealing temperature is the maximum temperature reached in the annealing process.

[0105] Annealing Time: 1 Second or More and 30 Seconds or Less In a steel sheet manufacturing method according to an embodiment of the present invention, the annealing time is important for controlling the aspect ratio of particles constituting the retained austenite. That is, the shorter the annealing time, the better from the following viewpoints: - Suppressing grain growth during annealing and suppressing Mn enrichment in austenite. - Promoting ferrite transformation and bainite transformation and reducing the aspect ratio of particles constituting the retained austenite. - Promoting C enrichment in particles with an aspect ratio of 3 or less. - Suppressing coarsening of austenite (particles) during annealing and uniformly dispersing the retained austenite. Therefore, the annealing time is set to 30 seconds or less. The annealing time is preferably 25 seconds or less, more preferably 20 seconds or less. On the other hand, if the annealing time is less than 1 second, coarse Fe-based precipitates do not dissolve, resulting in reduced elongation. Therefore, the annealing time is set to 1 second or more. The annealing time is preferably 3 seconds or more, more preferably 5 seconds or more. The annealing time refers to the holding time at the annealing temperature.

[0106] Atmosphere dew point: -35°C or higher From the viewpoint of forming a soft layer of a desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable that the atmosphere dew point be -35°C or higher in the annealing step following the above-mentioned heating step. If the atmosphere dew point is less than -35°C, it becomes difficult to form a soft phase of a desired thickness. Therefore, it is preferable that the atmosphere dew point in the annealing step be -35°C or higher. The atmosphere dew point in the annealing step is more preferably -20°C or higher, and even more preferably -10°C or higher. Note that the upper limit of the atmosphere dew point in the annealing step is not particularly limited. In order to set TS within a suitable range, the atmosphere dew point in the annealing step is preferably 15°C or lower, more preferably 5°C or lower.

[0107] [Cooling Step] Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.

[0108] Average cooling rate in the annealing temperature range from -30°C to 600°C: 5°C / sec or more and 100°C / sec or less. In this cooling process, ferrite and bainite are formed. Therefore, it is necessary to appropriately control the cooling rate, particularly the average cooling rate in the annealing temperature range from -30°C to 600°C (hereinafter also referred to as the second cooling rate). If the second cooling rate is slow, excessive ferrite is formed. In addition, excessive pearlite is formed, resulting in a decrease in TS. Furthermore, an appropriate amount of retained austenite is not obtained. Therefore, the second cooling rate is set to 5°C / sec or more. The second cooling rate is preferably 9°C / sec or more, more preferably 12°C / sec or more. On the other hand, if the second cooling rate exceeds 100°C / sec, ferrite transformation and bainite transformation may be excessively suppressed, resulting in a decrease in ductility. Therefore, the second cooling rate is set to 100°C / sec or less. The second cooling rate is preferably 75°C / sec or less, more preferably 50°C / sec or less.

[0109] Cooling stop temperature: 400°C or higher and 600°C or lower If the cooling stop temperature is lower than 400°C, the aspect ratio of bainite increases, and accordingly, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the retained austenite. Therefore, the cooling stop temperature is set to 400°C or higher. The cooling stop temperature is preferably 430°C or higher, more preferably 460°C or higher. On the other hand, if the cooling stop temperature exceeds 600°C, excessive pearlite may be generated, making it difficult to obtain the desired TS. Therefore, the cooling stop temperature is set to 600°C or lower. The cooling stop temperature is preferably 570°C or lower, more preferably 540°C or lower.

[0110] Atmospheric dew point: -35°C or less From the viewpoint of homogenizing the soft phase formed from the steel sheet surface in the sheet thickness direction, it is preferable to set the atmospheric dew point in the cooling step to -35°C or less. In other words, if the atmospheric dew point during cooling exceeds -35°C, the soft phase may not be homogenized and unevenness may occur. Therefore, it is preferable to set the atmospheric dew point in the cooling step to -35°C or less. The atmospheric dew point in the cooling step is more preferably -40°C or less. Note that the lower limit of the atmospheric dew point in the cooling step is not limited. From the viewpoint of controllability, the atmospheric dew point in the cooling step is preferably -60°C or more, more preferably -55°C or more.

[0111] [Dwelling Step] Next, the cold-rolled steel sheet cooled as described above is allowed to dwell in a temperature range of 400°C or higher and 600°C or lower for 1 second or higher and 90 seconds or lower.

[0112] Residence temperature range: 400°C or higher and 600°C or lower The residence temperature range is set to 400°C or higher and 600°C or lower from the viewpoint of ensuring an appropriate amount of bainite and retained austenite. If the residence temperature range is lower than 400°C, the number of particles with an aspect ratio of more than 3 will increase among all particles constituting the retained austenite. On the other hand, if the residence temperature range exceeds 600°C, ferrite and bainite may be excessively generated, making it difficult to obtain the desired TS. Therefore, the residence temperature range is set to 400°C or higher and 600°C or lower. The residence temperature range is preferably 420°C or higher, more preferably 440°C or higher. The residence temperature range is also preferably 560°C or lower, more preferably 520°C or lower. Note that when performing a galvanizing process, particularly a hot-dip galvanizing process or a galvannealed hot-dip galvanizing process, described below, it is preferable to reheat the cold-rolled steel sheet immediately before the galvanizing process so that the sheet temperature at entry into the galvanizing bath is higher than the galvanizing bath temperature.

[0113] Residence time: 1 second or more and 90 seconds or less In order to ensure an appropriate amount of retained austenite, it is necessary to appropriately control the residence time in the above residence temperature range (hereinafter simply referred to as residence time). Here, the longer the residence time, the more retained austenite there is. Therefore, the residence time is 1 second or more. The residence time is preferably 7 seconds or more, more preferably 15 seconds or more. On the other hand, if the residence time is too long, the amount of bainite becomes excessive, and martensite necessary to ensure strength cannot be obtained. Therefore, the residence time is 90 seconds or less. The residence time is preferably 80 seconds or less, more preferably 70 seconds or less. Note that the residence time here does not include the residence time in the temperature range of 400°C or more and 600°C or less (before cooling is stopped) in the above cooling step.

[0114] After the above-mentioned retention step, the cold-rolled steel sheet may be further subjected to a surface treatment such as a chemical conversion treatment or an organic coating treatment.

[0115] [Zinc plating process] Next, the cold-rolled steel sheet may be optionally subjected to a zinc plating process. Examples of the zinc plating process include hot-dip galvanizing and hot-dip galvannealing. The treatment conditions may be those of a conventional method.

[0116] For example, in the case of hot-dip galvanizing, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at 440°C or higher and 500°C or lower, and then adjust the coating weight by gas wiping or the like. The galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use, for example, a plating bath having an Al content of 0.10 mass% or higher and 0.23 mass% or lower, with the balance consisting of Zn and unavoidable impurities. Furthermore, when hot-dip galvanizing and alloyed hot-dip galvanizing, which will be described later, are performed, it is preferable to reheat the cold-rolled steel sheet immediately before the step so that the temperature of the sheet at entry into the plating bath is higher than the temperature of the plating bath.

[0117] Furthermore, in the case of alloying hot-dip galvanizing treatment, after performing the hot-dip galvanizing treatment as described above, it is preferable to perform an alloying treatment in a temperature range of 450°C or higher and 600°C or lower. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate may be excessively slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform to pearlite, resulting in a decrease in TS and ductility. Therefore, the alloying temperature in the alloying treatment is preferably 450°C or higher and 600°C or lower. The alloying temperature in the alloying treatment is more preferably 460°C or higher, and even more preferably 470°C or higher. The alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.

[0118] The plating weight is 20 g / m per side. ２ 80g / m or more ２ It is preferable that the plating thickness is set to the following: The plating thickness can be adjusted by gas wiping or the like.

[0119] The steel sheet obtained as described above may further be subjected to temper rolling. If the elongation rate in temper rolling exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the elongation rate in temper rolling is preferably 2.00% or less. The lower limit of the elongation rate in temper rolling is not particularly limited. From the viewpoint of productivity, the elongation rate in temper rolling is preferably 0.05% or more. Temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned processes (offline). The number of times temper rolling may be one, two, or more. As long as an elongation rate equivalent to that of temper rolling can be imparted, rolling using a leveler or the like may also be used.

[0120] From the viewpoint of productivity, it is preferable that a series of processes such as the annealing process and the galvanizing process be carried out in a continuous annealing line (CAL) or a hot-dip galvanizing line (CGL). After the hot-dip galvanizing process, wiping can be performed to adjust the coating weight of the coating.

[0121] According to the method for producing a steel sheet according to one embodiment of the present invention described above, a steel sheet having high strength, a high YS, excellent ductility, and excellent bendability can be obtained, and the steel sheet can be suitably used for, for example, automobile parts.

[0122] [4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described. The manufacturing method of a member according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press forming can be used. Furthermore, the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. can be used. Note that the forming conditions and joining conditions are not particularly limited, and may be in accordance with conventional methods.

[0123] A steel material having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and continuously cast into a steel slab. The steel slab was then heated under the conditions shown in Table 2, and hot-rolled, consisting of rough rolling and finish rolling, to obtain a hot-rolled steel sheet. The slab heating time in No. 13 refers to the holding time at the slab heating temperature. The resulting hot-rolled steel sheet was then pickled and cold-rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet. The resulting cold-rolled steel sheet was then subjected to a heating process, an annealing process, a cooling process, and a galvanizing process under the conditions shown in Table 2 to obtain a steel sheet as a final product. Conditions not specified were those according to conventional methods.

[0124] Here, in the galvanizing treatment step, a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of galvanizing treatment step is also indicated as "GI" or "GA". In addition, "CR" as the type of galvanizing treatment step in Table 2 means that the galvanizing treatment step was not performed and the steel sheet remains as a cold-rolled steel sheet.

[0125] In the hot dip galvanizing treatment, the plating bath contained 0.20 mass% Al, with the remainder consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The coating weight was 45 to 72 g / m per side. ２The zinc plating layer of the finally obtained GI had a composition of Fe: 0.1 to 1.0 mass %, Al: 0.2 to 1.0 mass %, and the remainder consisting of Zn and unavoidable impurities.

[0126] In the galvannealed hot-dip galvanizing treatment, the plating bath contained 0.14 mass% Al, with the remainder consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The coating weight was 45 g / m per side. ２ The alloying temperature was 520°C. The composition of the finally obtained zinc-plated layer of GA was Fe: 7 to 15% by mass, Al: 0.1 to 1.0% by mass, and the balance was Zn and unavoidable impurities.

[0127] Using the steel sheet thus obtained, the steel structure of the steel sheet was identified in the manner described above, and the particle number density of retained austenite, (A) particle ratio (area %) of aspect ratios of 3 or less, (B) average C concentration (mass %) of aspect ratios of 3 or less, (C) average interparticle distance (μm), (D) maximum interparticle distance (μm), [Mn] Ｃ The tensile strength of the steel sheets was measured. The results of the measurements are shown in Table 3. In the steel sheets having a soft layer, the soft layer was formed on both sides of the steel sheet, and the soft layer had the same thickness on both sides. In addition, in No. 25, the soft layer was not confirmed (the soft layer thickness was less than 1 μm), so the soft layer thickness column in Table 2 is entered as "0."

[0128] Further, a tensile test and a V-bend test were carried out in the following manner, and the tensile strength (TS), yield stress (YS), total elongation (El) and R / t were evaluated according to the following criteria. TS Pass: 780MPa≦TS Fail: TS<780MPa YS Pass: 420MPa≦YS if 780MPa≦TS<980MPa 550MPa≦YS if 980MPa≦TS Fail: YS<420MPa if 780MPa≦TS<980MPa YS<550MPa if 980MPa≦TS El Pass: 19%≦El if 780MPa≦TS<980MPa 10%≦El if 980MPa≦TS Fail: El<19% if 780MPa≦TS<980MPa El<10% if 980MPa≦TS R / t Pass: If 780MPa≦TS<980MPa, then 2.0≧R / t If 980MPa≦TS, then 4.0≧R / t Fail: If 780MPa≦TS<980MPa, then R / t>2.0 If 980MPa≦TS, then R / t>4.0

[0129] The tensile test was carried out in accordance with JIS Z 2241. That is, JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheets. Using the taken test pieces, a tensile test was carried out at a crosshead speed of 10 mm / min, and TS, YS, and El were measured. The results are also shown in Table 3.

[0130] The V (90°) bending test was performed in accordance with JIS Z 2248. Specifically, 100 mm x 35 mm test specimens were obtained from the steel plate by shearing and edge grinding. The 100 mm side was obtained so that it was parallel to the direction perpendicular to the rolling direction (C) (width direction). The obtained test specimens were then subjected to a V (90°) bending test under the following conditions: Bending radius R: Varying at 0.5 mm intervals; Test method: Die support, punch indentation; Forming load: 10 ton; Test speed: 30 mm / min; Hold time: 5 seconds; Bending direction: Perpendicular to the rolling direction (C). The test was performed three times, and the minimum bending radius R at which no cracks occurred in any of the three tests was defined as R. R / t was calculated by dividing R by the plate thickness t. The test specimens were observed at a magnification of 25x using a Leica stereomicroscope, and cracks were deemed to have occurred if cracks of 200 μm or more in length were observed. The results are shown in Table 3.

[0131]

[0132]

[0133]

[0134] As shown in Table 3, all of the inventive examples passed the TS, YS, El, and R / t criteria. Furthermore, all of the components obtained by forming or joining the steel sheets of the inventive examples had the desired shapes without cracking, and met the pass criteria for TS, YS, El, and R / t. On the other hand, the comparative examples failed in at least one of TS, YS, El, and R / t.

[0135] According to the present invention, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained. Furthermore, the steel sheet can be extremely advantageously used as a material for automotive frame structural members having complex shapes. This allows for the reduction of vehicle body weight and the improvement of fuel efficiency, and therefore has great industrial utility value.

Images