Steel sheet, member and production methods for those

Abstract

Provided are: a steel sheet which exhibits excellent ductility, bore expanding properties and chemical conversion properties, while having a tensile strength of 780 MPa or more; a member; and a production method for the steel sheet and a production method for the member.　The steel sheet has a component composition which contains, in mass%, C, Si, Mn, P, S, sol. Al and N in predetermined ranges, and satisfies formula (1), and a steel structure in which the area ratio or the like of polygonal ferrite is within a predetermined range. With respect to the steel sheet, the maximum concentration [Pm] of P in a portion within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more, and formula (2) is satisfied. Formula (1): [Si] / [Mn] ≤ 0.35 Formula (2): [Pm] / [Pi] ≥ 1.5

Description

Steel plates, components, and their manufacturing methods

[0001] The present invention relates to a steel sheet, a member, and a method for manufacturing the same, which are suitable for use in press-formed products having complex shapes that are used in automobiles, home appliances, and the like through a press-forming process and which have excellent chemical conversion treatability.

[0002] Global CO ２ In response to the backdrop of stricter emission regulations, there is a growing demand for automobiles to use high-strength steel sheets to reduce vehicle weight, and the use of high-strength steel sheets of 590 MPa or higher has been promoted for body and seat components, replacing the existing 440 MPa-class cold-rolled steel sheets. Generally, increasing the strength of steel sheets reduces press formability, such as ductility and stretch flangeability, making them more susceptible to cracking during press forming and reducing the degree of freedom in shape, limiting their application to parts with simple shapes. Therefore, in order to apply high-strength steel sheets to parts with complex shapes, it is important to increase the strength of steel sheets while maintaining or improving formability.

[0003] In light of this background, TRIP steel, in which retained austenite (residual γ) is dispersed in the microstructure of the steel sheet, has been developed as a technology for improving the ductility of steel sheet. TRIP steel is manufactured using two heat treatment processes: austempering, which involves isothermal holding in the bainite transformation temperature range after soaking, and Q&P (Quenching & Partitioning), which involves cooling to a temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point) during the cooling process, followed by reheating and holding to stabilize the retained austenite. In both heat treatment processes, a large amount of Si, which can suppress carbide precipitation, is added to form retained γ in the microstructure. In addition, Q&P is a heat treatment process in which a portion of the structure is transformed into martensite during the cooling process, and the martensite structure is subsequently tempered by reheating, thereby reducing the hardness difference between different phases in the structure and improving not only ductility but also hole expandability. For example, Patent Document 1 discloses a steel sheet and its manufacturing method in which a cold-rolled steel sheet containing 0.6 to 2.5% Si is held at a first soaking temperature of 750°C or higher, cooled to a cooling stop temperature in the temperature range of 150 to 350°C, and then reheated to a temperature range of 350 to 500°C, thereby ensuring a retained austenite volume fraction of 5 to 15%, achieving both a TS of 980 MPa or higher and ductility with an elongation of 17%, and having excellent hole expandability with a hole expandability of 50%.

[0004] On the other hand, it is known that as the Si content increases, Si concentrates on the surface of the steel sheet after annealing, forming Si-based oxides, which deteriorates chemical conversion treatability. To address this issue, for example, Patent Document 2 discloses a method of improving chemical conversion treatability by adding Ni to prevent Si from concentrating on the steel sheet surface. Patent Document 3 discloses a method of improving chemical conversion treatability by appropriately controlling the content of Mn, which concentrates on the surface together with Si, so that the Si / Mn ratio is 0.40 or less, thereby forming Mn-Si composite oxides on the surface. Patent Document 4 discloses a method of improving chemical conversion treatability by directly removing Si-based oxides by pickling or brushing after annealing.

[0005] Japanese Patent No. 5821911 Japanese Patent No. 2951480 Japanese Patent No. 3889768 Japanese Patent Laid-Open No. 2003-201538

[0006] As described above, although the addition of Si is effective in improving the ductility of high-strength steel sheets, when actively utilizing Si to ensure high workability, there is a trade-off between the Si content and the phosphatability of the steel sheet. The methods disclosed in Patent Documents 2 and 4 are effective methods for improving the phosphatability of steels with high Si contents, but there has been a need to establish other techniques that adjust the alloying elements to be added, annealing conditions, etc. Furthermore, the inventors' studies have revealed that even the method disclosed in Patent Document 3 does not necessarily ensure good phosphatability. Thus, there has been a need to establish a new technology for high-strength steel sheets that have excellent ductility and phosphatability, as well as hole expandability.

[0007] The present invention has been made in consideration of the above circumstances, and its object is to provide a steel plate, a member, and a method for manufacturing the same that have excellent ductility, hole expandability, and chemical conversion treatability and a tensile strength of 780 MPa or more.

[0008] Here, the tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).

[0009] Furthermore, excellent ductility means that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies any one of the following (A) to (C): (A) When TS is 780 MPa or more and less than 980 MPa, EL is 16.0% or more; (B) When TS is 980 MPa or more and less than 1180 MPa, EL is 14.0% or more; (C) When TS is 1180 MPa or more, EL is 12.0% or more.

[0010] Furthermore, excellent hole expandability means that the hole expanding ratio λ (%) (= {(d - d ０ ) / d ０}×100) is 45% or more at any TS level.

[0011] Furthermore, excellent chemical conversion treatability refers to a steel sheet that has been subjected to degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreaser: FC-E2011 manufactured by Nihon Parkerizing Co., Ltd.) and surface conditioning (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nihon Parkerizing Co., Ltd.), followed by chemical conversion treatment using a zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature: 35°C, treatment time: 120 seconds, chemical conversion treatment solution: Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd.), with the area in which the base steel is exposed accounting for less than 10% of the total area.

[0012] In order to solve the above problems, the present inventors have conducted extensive research into the steel components, heat treatment conditions, and microstructures that affect the ductility and chemical conversion treatability of various steel sheets having a tensile strength of 780 MPa or more. As a result, they have found that the steel sheets contain, in mass%, C: 0.05 to 0.25%, Si: 0.30 to 1.50%, Mn: 1.5 to 4.5%, P: 0.005 to 0.050%, S: 0.01% or less, sol. The steel sheet has a component composition containing Al: less than 1.0% and N: less than 0.015%, which satisfies the following formula (1), with the balance consisting of iron and unavoidable impurities, the area ratio of polygonal ferrite is 10% to 70%, the total area ratio of upper bainite, tempered martensite, and lower bainite is 20% to 80%, the volume fraction of retained austenite (residual γ) is 5% to 20%, the area ratio of quenched martensite is 13% or less (including 0%), and further consists of a steel structure consisting of a balance structure. When the phosphorus emission intensity measured by glow discharge analysis from the steel sheet surface in the thickness direction is analyzed, the maximum concentration of phosphorus within 1 μm in the thickness direction from the steel sheet surface is 0.025 mass% or more, and the following formula (2) is satisfied. It has been found that a high-strength cold-rolled steel sheet having excellent ductility, hole expandability, and chemical conversion treatability can be obtained by using a steel structure in which phosphorus is locally concentrated. [Si] / [Mn]≦0.35 (formula (1)) [Pm] / [P]≧1.5 (formula (2)) In formula (1), [Si] is the Si content (mass%), [Mn] is the Mn content (mass%), and in formula (2), [P] is the P content (mass%).

[0013] The present invention has been made based on the above findings, and its gist is as follows: [1] In mass %, C: 0.05 to 0.25%, Si: 0.30 to 1.50%, Mn: 1.5 to 4.5%, P: 0.005 to 0.050%, S: 0.01% or less, sol. 1. A steel sheet having a component composition containing Al: less than 1.0% and N: less than 0.015%, satisfying the following formula (1), with the balance consisting of iron and unavoidable impurities; and a steel structure having an area ratio of polygonal ferrite: 10% or more and 70% or less, a total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less, a volume ratio of retained austenite: 5% or more and 20% or less, and an area ratio of quenched martensite: 13% or less (including 0%), wherein the maximum concentration of P [Pm] within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more, and the steel sheet satisfies the following formula (2). [Si] / [Mn]≦0.35 (formula (1)) [Pm] / [P]≧1.5 (formula (2)) In formula (1), [Si] is the Si content (mass%), [Mn] is the Mn content (mass%), and in formula (2), [P] is the P content (mass%). [2] The steel sheet according to [1], wherein the chemical composition further contains, in mass%, one or more elements selected from the group consisting of Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less. [3] A member made using the steel sheet according to [1] or [2]. [4] A method for producing a steel sheet, comprising: hot rolling, pickling, and cold rolling a steel slab having the chemical composition according to [1] or [2]; and then annealing the resulting cold-rolled steel sheet; wherein the annealing comprises: subjecting the cold-rolled steel sheet to a furnace atmosphere having a dew point of −40° C. or less; ｃ１ Point +20℃ or more A ｃ３a soaking holding step of heating a steel sheet to a soaking temperature that is equal to or lower than the soaking point and equal to or higher than Tc calculated by formula (3), and holding the steel sheet at the soaking temperature for 30 to 500 seconds; a first cooling step of cooling the steel sheet to a first cooling stop temperature of 350 to 550°C at a first average cooling rate of 2 to 50°C / s; a second cooling step of stopping cooling at the first cooling stop temperature, allowing the steel sheet to remain in a temperature range of 350 to 550°C for 10 to 60 seconds, and then cooling the steel sheet to a second cooling stop temperature of 100 to 300°C at a second average cooling rate of 2 to 50°C / s; and a reheating holding step of heating the steel sheet from the second cooling stop temperature at an average heating rate of 2.0°C / s or higher to a reheating temperature of second cooling stop temperature + 50°C or higher and 450°C or lower, and holding the steel sheet for 60 seconds or more and 3000 seconds or less. Tc (°C) = 663 - 1.2 × exp (20 / t) × Tdp (3) where t is the holding time (s) at the soaking temperature, and Tdp is the dew point (°C). [5] A method for producing a steel sheet, comprising: hot rolling, pickling, and cold rolling a steel slab having the chemical composition according to [1] or [2] above, and then annealing the resulting cold-rolled steel sheet; and the annealing comprises: subjecting the cold-rolled steel sheet to A in a furnace atmosphere having a dew point of -40°C or less. ｃ１ Point +20℃ or more A ｃ３ a soaking temperature holding step of heating a steel sheet to a soaking temperature equal to or lower than the soaking point Tc calculated by equation (3) and holding the steel sheet at the soaking temperature for 30 to 500 seconds; a cooling step of cooling from the soaking temperature to a cooling stop temperature of 100 to 300°C at an average cooling rate of 2 to 50°C / s; and a reheating holding step of heating from the cooling stop temperature to a reheating temperature of at least 50°C above the cooling stop temperature and equal to or lower than 450°C at an average heating rate of 2.0°C / s, and holding the temperature for 60 to 3000 seconds. Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp (3), where t is the holding time (s) at the soaking temperature, and Tdp is the dew point (°C). [6] A method for manufacturing a component, comprising the step of forming or joining the steel sheet according to item [1] or [2] above to form a component.

[0014] According to the present invention, a steel sheet and a component can be obtained that have a high tensile strength (TS) of 780 MPa or more and excellent ductility, hole expandability, and chemical conversion treatability. When the steel sheet of the present invention is applied to the frame components of an automobile body, it can be used to manufacture difficult-to-form components with complex shapes by cold press working, which can greatly contribute to reducing the weight of the automobile body. There is no need for expensive alloying elements or post-annealing treatments to improve chemical conversion treatability, making it possible to reduce material costs.

[0015] FIG. 1 is a graph for explaining the maximum concentration of P [Pm] according to the present invention.

[0016] The present invention will be specifically described below, but the present invention is not limited to the following embodiments.

[0017] (Steel Plate) The steel plate of the present invention contains, by mass, C: 0.05 to 0.25%, Si: 0.30 to 1.50%, Mn: 1.5 to 4.5%, P: 0.005 to 0.050%, S: 0.01% or less, sol. Al: less than 1.0%, and N: less than 0.015%, and satisfies the following formula (1), with the balance being iron and unavoidable impurities. The steel plate also has a component composition in which the area ratio of polygonal ferrite is 10% or more and 70% or less, and the total area ratio of upper bainite, tempered martensite, and lower bainite is 20% or more and 80% or less. and a steel structure in which the volume fraction of retained austenite is 5% or more and 20% or less, and the area fraction of quenched martensite is 13% or less (including 0%), and when the phosphorus emission intensity is analyzed from the surface in the thickness direction by glow discharge analysis, the maximum concentration of phosphorus [Pm] within 1 μm in the thickness direction from the steel sheet surface is 0.025 mass% or more, and the steel sheet satisfies the following formula (2), and has a high tensile strength TS of 780 MPa or more, and is excellent in ductility, hole expandability, and chemical conversion treatability: [Si] / [Mn]≦0.35 (formula (1)) [Pm] / [P]≧1.5 (formula (2)) where, in formula (1), [Si] is the Si content (mass%), [Mn] is the Mn content (mass%), and in formula (2), [P] is the P content (mass%).

[0018] The steel sheet of the present invention will be described below in the order of chemical composition and steel structure. First, the reasons for limiting the chemical composition of the present invention will be described. In the following description, all percentages indicating the steel composition are by mass % unless otherwise specified.

[0019] <C: 0.05 to 0.25%> C is included to ensure a predetermined strength through transformation strengthening and to secure a predetermined amount of retained austenite (hereinafter also referred to as retained γ) to improve ductility. If the C content is less than 0.05%, these effects cannot be fully ensured. On the other hand, the upper limit of the C content is set to 0.25% due to concerns about hole expandability, which is important in press formability, and weldability, which is important during spot welding or laser welding when assembling into a vehicle body after forming into an automotive component. For this reason, the C content is set to 0.05 to 0.25%. The C content is preferably 0.08% or more, more preferably 0.10% or more. The C content is preferably 0.22% or less, more preferably 0.20% or less.

[0020] <Si: 0.30 to 1.50%> Si is included from the viewpoint of strengthening ferrite to increase strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite, ensuring a predetermined amount of retained γ, and improving ductility. If the Si content is less than 0.30%, these effects cannot be sufficiently ensured. On the other hand, if the Si content exceeds 1.50%, good chemical conversion treatability cannot be ensured even with the manufacturing method specified in the present invention. Therefore, the Si content is set to 0.30 to 1.50%. The Si content is preferably 0.35% or more, more preferably 0.40% or more. Furthermore, the Si content is preferably 1.20% or less, more preferably 1.00% or less.

[0021] <Mn: 1.5 to 4.5%> Mn is included to improve the hardenability of steel sheets, promote high strength through transformation strengthening, and, like Si, inhibit the formation of carbides in bainite and promote the formation of retained austenite, which contributes to ductility, thereby improving ductility. To achieve these effects, a Mn content of 1.5% or more is required. On the other hand, if the Mn content exceeds 4.5%, the bainite transformation is significantly delayed, making it impossible to secure a predetermined amount of retained austenite, and ductility is reduced. Furthermore, if the Mn content exceeds 4.5%, the martensitic transformation start temperature is lowered, making it difficult to suppress the formation of coarse quenched martensite, and stretch flangeability (hole expandability) is degraded. Therefore, the Mn content is set to 1.5% or more and 4.5% or less. The Mn content is preferably 1.8% or more, more preferably 2.0% or more. The Mn content is preferably 3.5% or less, more preferably 3.0% or less.

[0022] <P: 0.005 to 0.050%> P is an element that strengthens steel. Furthermore, by appropriately controlling the P content, P surface-enriched areas are formed on the surface of the steel sheet after annealing, thereby improving chemical conversion treatability. From this perspective, the P content is set to 0.005% or more. On the other hand, a high P content deteriorates spot weldability. From this perspective, the P content is set to 0.050% or less. Therefore, the P content is set to 0.005 to 0.050%. The P content is preferably 0.007% or more, more preferably 0.009% or more. The P content is preferably 0.040% or less, more preferably 0.030% or less.

[0023] <S: 0.01% or less> S has the effect of improving scale peelability during hot rolling and suppressing nitriding during annealing, but is an element that has a negative effect on spot weldability, bendability, and hole expandability. In order to reduce these negative effects, the S content is at least 0.01% or less, and preferably 0.0050% or less. Note that S may not be contained, but reducing it to less than 0.0001% requires a great deal of cost, so the S content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The S content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0024] <Sol. Al: Less than 1.0%> Al is contained for the purpose of deoxidation or obtaining residual γ. Although the lower limit of sol. Al is not particularly specified, the sol. Al content is preferably 0.005% or more to ensure stable deoxidation. On the other hand, if the sol. Al content is 1.0% or more, the amount of coarse Al-based inclusions increases significantly, resulting in a decrease in stretch flangeability (hole expandability). Furthermore, Al is an element that deteriorates the chemical conversion treatability of steel sheets, and if the sol. Al content is 1.0% or more, good chemical conversion treatability cannot be ensured even in the present invention. For this reason, the sol. Al content is set to less than 1.0%. The sol. Al content is preferably 0.80% or less, more preferably 0.06% or less.

[0025] <N: Less than 0.015%> N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flangeability (hole expandability), so its content needs to be limited. Therefore, the N content is less than 0.015%. The N content is preferably 0.010% or less, and more preferably 0.006% or less. Note that N does not need to be contained, but reducing it to less than 0.0001% requires a great deal of cost, so the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0026] <[Si] / [Mn]≦0.35...Equation (1)> In equation (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%). [Si] / [Mn] (Si / Mn ratio) determines the Si to Mn component ratio of the surface oxide formed during annealing. Within the range of manufacturing conditions specified in the present invention, if [Si] / [Mn] exceeds 0.35, good chemical conversion treatability cannot be ensured. Therefore, [Si] / [Mn] is set to 0.35 or less. [Si] / [Mn] is preferably 0.32 or less, more preferably 0.30 or less. Furthermore, although there is no particular lower limit, [Si] / [Mn] is preferably 0.10 or more, more preferably 0.15 or more.

[0027] The steel sheet according to the present invention preferably has a composition containing the above-mentioned elemental elements as the basic components, with the balance being iron (Fe) and unavoidable impurities.

[0028] In addition to the above-mentioned components, the steel sheet of the present invention may contain one or more optional elements (selective elements) selected from the following: Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less.

[0029] <Ti: 0.1% or less> Ti fixes N in steel as TiN, thereby improving hot ductility and improving the hardenability of B. It also has the effect of refining the structure by precipitation of TiC. To achieve these effects, the Ti content is desirably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is more preferably 0.010% or more. On the other hand, a Ti content exceeding 0.1% increases the rolling load and decreases ductility due to an increase in the amount of precipitation strengthening. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. Preferably, the Ti content is 0.05% or less, more preferably 0.03% or less.

[0030] <B: 0.001% or less> B is an element that improves the hardenability of steel and has the advantage of easily forming tempered martensite and / or bainite with a predetermined area ratio. Therefore, the B content is preferably 0.0005% or more. On the other hand, if the B content exceeds 0.001%, concentration of B in oxides occurs during annealing, promoting coarsening of the oxides and deteriorating chemical conversion treatability. Therefore, when B is contained, the B content is set to 0.001% or less.

[0031] <Cu: 1% or Less> Cu improves corrosion resistance in an automotive usage environment. Furthermore, corrosion products of Cu coat the surface of the steel sheet, suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is utilized as a raw material. Allowing the inclusion of Cu allows recycled materials to be utilized as raw materials, thereby reducing manufacturing costs. From this perspective, the Cu content is preferably 0.005% or more. Furthermore, from the perspective of improving delayed fracture resistance, the Cu content is more preferably 0.05% or more. The Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, and even more preferably, 0.50% or more. However, since an excessively high Cu content can cause surface defects, when Cu is contained, the Cu content is set to 1% or less.

[0032] <Ni: 1% or less> Like Cu, Ni is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects, which tend to occur when Cu is contained. Therefore, it is desirable to contain 0.01% or more of Ni. The Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more. However, if the Ni content is too high, scale generation in the heating furnace becomes non-uniform, which may actually cause surface defects. It also results in increased costs. Therefore, when Ni is contained, the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, and more preferably 0.3% or less.

[0033] <Cr: 1% or less> Cr can be added to improve the hardenability of the steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and even more preferably 0.06% or more. However, an excessive Cr content deteriorates pitting corrosion resistance, so when Cr is added, the Cr content is set to 1% or less. The Cr content is preferably 0.3% or less, and more preferably 0.1% or less.

[0034] <Mo: 0.5% or less> Mo can be added to improve the hardenability of steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.03% or more, and even more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, and even more preferably 0.2% or more. However, since Mo significantly deteriorates the chemical conversion treatability of cold-rolled steel sheets, when Mo is contained, the Mo content is set to 0.5% or less.

[0035] <V: 0.5% or Less> V can be added to improve the hardenability of steel, suppress the formation of carbides in martensite and upper / lower bainite, refine the structure, and precipitate carbides to improve delayed fracture resistance. To achieve these effects, the V content is preferably 0.003% or more. The V content is more preferably 0.005% or more, and even more preferably 0.010% or more. The V content is still more preferably 0.020% or more, and even more preferably 0.050% or more. However, since a large amount of V significantly deteriorates castability, when V is contained, the V content is set to 0.5% or less. Preferably, the V content is 0.3% or less, and more preferably 0.2% or less. The V content is preferably 0.2% or less, and even more preferably 0.1% or less.

[0036] <Nb: 0.1% or less> Nb can be added to refine the steel structure and increase strength, promote bainite transformation through grain refinement, improve bendability, and enhance delayed fracture resistance. To achieve these effects, the Nb content is preferably 0.010% or more. The Nb content is preferably 0.015% or more, and more preferably 0.020% or more. However, a large amount of Nb added results in excessive precipitation strengthening and reduced ductility. It also increases the rolling load and deteriorates castability. Therefore, when Nb is added, the Nb content is set to 0.1% or less. The Nb content is preferably 0.08% or less, and more preferably 0.05% or less.

[0037] <Mg: 0.0050% or less> Mg fixes O as MgO and contributes to improving formability such as bendability. For this reason, the Mg content is preferably 0.0002% or more. The Mg content is preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, adding a large amount of Mg deteriorates the surface quality and bendability, so when Mg is contained, the Mg content is set to 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

[0038] <Ca: 0.0050% or less> Ca fixes S as CaS, contributing to improvements in bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, adding a large amount of Ca deteriorates surface quality and bendability, so when Ca is contained, the Ca content is set to 0.0050% or less. Preferably, the Ca content is 0.0040% or less.

[0039] <Sn: 0.1% or less> Sn suppresses oxidation and nitriding of the surface layer of the steel sheet, thereby suppressing the resulting reduction in the C and B contents in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing strength and improving fatigue resistance. From this perspective, the Sn content is preferably 0.003% or more. The Sn content is more preferably 0.010% or more, and even more preferably 0.015% or more. The Sn content is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, if the Sn content exceeds 0.1%, castability deteriorates. In addition, Sn segregates at prior γ grain boundaries, deteriorating delayed fracture resistance. Therefore, when Sn is contained, the Sn content is set to 0.1% or less.

[0040] <Sb: 0.1% or less> Sb suppresses oxidation and nitriding of the surface layer of the steel sheet, thereby suppressing the resulting reduction in the C and B contents in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing strength and improving fatigue resistance. From this perspective, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, and even more preferably 0.006% or more. More preferably, the Sb content is 0.008% or more, and even more preferably 0.010% or more. The Sb content is preferably 0.015% or more, and more preferably 0.030% or more. On the other hand, if the Sb content exceeds 0.1%, castability deteriorates and, further, Sb segregates at prior γ grain boundaries, deteriorating delayed fracture resistance. Therefore, when Sb is contained, the Sb content is set to 0.1% or less.

[0041] <REM: 0.0050% or less> REM is an element that spheroidizes the shape of sulfides, thereby suppressing the adverse effects of sulfides on stretch flangeability and improving stretch flangeability. To achieve these effects, the REM content is preferably 0.0005% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. On the other hand, if the REM content exceeds 0.0050%, the effect of improving stretch flangeability saturates. Therefore, when REM is contained, the REM content is set to 0.0050% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.

[0042] When the optional components are contained in an amount less than the lower limit, the optional elements contained in an amount less than the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained in an amount less than the lower limit, the optional elements are considered to be contained as inevitable impurities.

[0043] Next, the mechanical properties of the steel sheet (cold-rolled steel sheet excellent in material stability) to which the present invention is directed will be described.

[0044] The steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more. There is no particular upper limit to the tensile strength, but from the viewpoint of compatibility with other properties, the tensile strength is preferably 1300 MPa or less.

[0045] In the steel sheet of the present invention, excellent ductility is ensured by a total elongation EL of 16.0% or more when TS is 780 MPa or more but less than 980 MPa, 14.0% or more when TS is 980 MPa or more but less than 1180 MPa, and 12.0% or more when TS is 1180 MPa or more. Furthermore, hole expandability is ensured by a hole expansion ratio λ of 45% or more. This significantly improves the stability of press forming.

[0046] To evaluate the tensile properties, a JIS No. 5 tensile test piece is taken from the center position of the sheet width, and a tensile test (in accordance with JIS Z2241 (2011)) is performed with N=3. Each evaluation is based on the average value of the three points. A steel sheet having a tensile strength of 780 MPa or more is considered a high-strength steel sheet. A steel sheet having a total elongation EL of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and 12.0% or more when TS is 1180 MPa or more is considered to have excellent ductility. In addition, the hole expandability is measured using the hole expansion ratio λ (%) (= {(d - d ０ ) / d ０}×100) is 45% or more as an essential condition of the present invention.

[0047] Next, the steel structure of the steel plate of the present invention will be described.

[0048] <Area Fraction of Polygonal Ferrite: 10% or More and 70% or Less> From the viewpoint of ensuring high ductility, the area fraction of polygonal ferrite is set to 10% or more, and to obtain even higher ductility, it is preferably set to 20% or more. On the other hand, if the area fraction of polygonal ferrite exceeds 70%, the desired strength may not be obtained, so the area fraction of polygonal ferrite is set to 70% or less, preferably 65% ​​or less, and more preferably 60%.

[0049] <Total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less> To obtain the desired strength, the total area ratio of upper bainite, tempered martensite, and lower bainite is set to 20% or more, and to obtain even higher strength, it is preferably set to 25% or more. On the other hand, if the total area ratio of upper bainite, tempered martensite, and lower bainite exceeds 80%, excessive strength is obtained, resulting in a decrease in ductility, so the area ratio is set to 80% or less. More preferably, it is set to 75% or less, and even more preferably, it is set to 70% or less.

[0050] <Volume Fraction of Retained Austenite (Residual γ): 5% or More and 20% or Less> If the volume fraction of retained austenite is less than 5%, the desired ductility may not be ensured. Furthermore, if the volume fraction of retained austenite is less than 5%, the desired strength may not be ensured. Furthermore, if the volume fraction of retained austenite is less than 5%, the desired hole expandability may not be ensured. From the viewpoint of ductility, the volume fraction of retained austenite is set to 5% or more, preferably 7% or more. On the other hand, if the retained austenite exceeds 20%, stretch flangeability (hole expandability) decreases, so the retained austenite is set to 20% or less. Preferably, it is 15% or less, more preferably 13% or less.

[0051] <Area Fraction of Quenched Martensite: 13% or Less (Including 0%)> Because the hard quenched martensite structure reduces λ, its area fraction needs to be suppressed. To obtain the practically necessary λ, the area fraction of quenched martensite is set to 13% or less. To obtain λ more stably, the area fraction of quenched martensite is preferably 11% or less, more preferably 9% or less. The area fraction of quenched martensite may be 0% or 5% or more.

[0052] <Remaining structure> The steel structure is composed of the remaining structure except for the above. The area ratio of the remaining structure is preferably 5% or less. The remaining structure may be unrecrystallized ferrite, carbide, or pearlite. These structures may be determined by SEM observation as described below.

[0053] <Maximum P concentration [Pm] within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more and formula (2) is satisfied> [Pm] / [P]≧1.5 (2) In formula (2), [P] (which can also be expressed as [Pi]) is the P content (mass%). As a result of extensive investigation into various elements that affect phosphatability, their surface concentration, and the oxide species formed during annealing, it has become clear that sufficient phosphatability cannot be ensured even under manufacturing conditions in which oxide formation is not observed. For steel sheets with ensured phosphatability, the maximum P concentration near the surface was quantitatively evaluated using the method described below. When the P emission intensity measured by GDS (glow discharge spectroscopy) from the surface in the sheet thickness direction was analyzed, it was found that good phosphatability can be ensured by forming a steel structure in which the maximum P concentration [Pm] within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more and formula (2) is satisfied. Although the detailed mechanism is unknown, it is important that the maximum P concentration in the surface layer is locally high relative to the steel components. Furthermore, if this maximum P concentration is insufficient, the shape of the chemical crystals after chemical conversion treatment is scaly. Therefore, it is believed that local surface concentration of P has the effect of suppressing the formation of surface Si-based oxides and Si-Mn-based oxides, which adversely affect chemical conversion treatability. [Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more. Although there is no particular upper limit, [Pm] is preferably 0.100 mass% or less, more preferably 0.090 mass% or less. [Pm] / [Pi] is preferably 1.7 or more, more preferably 1.9 or more. Although there is no particular upper limit, [Pm] / [Pi] is preferably 10.0 or less, more preferably 9.0 or less.

[0054] Next, a method for measuring the steel structure will be described. To measure the area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite), a cross section of the plate parallel to the rolling direction is cut out, mirror-polished, and etched with 1 vol% nital. Ten fields of view (25 μm × 20 μm) are observed at 1 / 4 thickness using a SEM at 5000x magnification, and the resulting structure photographs are quantified using image analysis. Polygonal ferrite refers to relatively equiaxed ferrite with almost no carbides inside. This is the region that appears the blackest under SEM. Upper bainite is a ferrite structure with carbides or retained austenite inside, which appears white under SEM. If it is difficult to distinguish between upper bainite and polygonal ferrite, the ferrite region with an aspect ratio of ≦2.0 is classified as polygonal ferrite, and the region with an aspect ratio >2.0 is classified as upper bainite, and the area ratios are calculated accordingly. Here, the aspect ratio is determined by determining the major axis length a at which the particle length is longest, and the minor axis length b at which the particle is cut perpendicular to the major axis a, where a / b is the aspect ratio. Tempered martensite and lower bainite are regions with lath-like substructures and carbide precipitations within them, as seen under SEM. Quenched martensite (fresh martensite) is a massive region that appears white under SEM, with no visible substructures. The residual structure is a structure containing at least one of unrecrystallized ferrite, carbides, and pearlite. Unrecrystallized ferrite is a structure that can be confirmed by SEM as ferrite with a black contrast, including deformed structures introduced by rolling, while carbides and pearlite are confirmed as white contrasts. Carbides are distinguishable by their particle diameters of 1 μm or less, and pearlite by its lamellar (layer) structure.

[0055] The volume fraction of retained austenite is determined by chemically polishing the surface layer at a position 1 / 4 the thickness away from the surface and then performing X-ray diffraction. A Co-Kα source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction can be taken as the area fraction of retained austenite.

[0056] The amount of surface enrichment of P on the steel sheet surface was measured using a GDS (Shimadzu Corporation) under the following conditions: Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, measurement time: 150 s. Sputtering analysis was performed in the depth direction (sheet thickness direction). The amount of surface enrichment of P was measured, and the maximum P concentration [Pm] within 1 μm in the sheet thickness direction from the steel sheet surface was determined using a calibration curve obtained in advance. Note that under these measurement conditions, the measurement position d (μm) from the surface was obtained using the sputtering time ts by the formula d = ts / 1.7 (μm). In the present invention, as shown in Figure 1, the highest P intensity value during the 150 s measurement time was converted to mass% using the calibration curve, and the maximum concentration ([Pm]) was determined. The method for converting to mass% involves determining the correlation between the intensity of P element obtained by GDS and the amount of P in data obtained by measuring a standard material having a known amount of P under the same conditions, and converting the measured P intensity of the example into a concentration. In Figure 1, Pi is the P content (mass%) in the steel sheet.

[0057] (Method for manufacturing steel sheet) Next, a method for manufacturing a steel sheet of the present invention will be described. <First embodiment> The method for manufacturing a steel sheet of the first embodiment of the present invention is a method for manufacturing a steel sheet in which a steel slab having the above-described composition is hot-rolled, pickled, and cold-rolled, and then the obtained cold-rolled steel sheet is annealed. The annealing is performed by subjecting the cold-rolled steel sheet to A in a furnace atmosphere having a dew point of −40° C. or less. ｃ１ Point +20℃ or more A ｃ３a first cooling step of cooling the steel sheet to a first cooling stop temperature of 350 to 550°C at a first average cooling rate of 2 to 50°C / s in a temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550°C; a second cooling step of stopping the cooling at the first cooling stop temperature, allowing the steel sheet to remain in a temperature range of 350 to 550°C for 10 to 60 seconds, and then cooling the steel sheet to a second cooling stop temperature of 100 to 300°C at a second average cooling rate of 2 to 50°C / s; and a reheating step of heating the steel sheet from the second cooling stop temperature at an average heating rate of 2.0°C / s or more to a reheating temperature of at least second cooling stop temperature + 50°C but not more than 450°C, and holding the steel sheet for 60 to 3000 seconds. Tc (° C.)=663−1.2×exp(20 / t)×Tdp (3) Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).

[0058] <Hot rolling> Methods for hot rolling a steel slab include a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting after subjecting it to a short-term heat treatment. Hot rolling may be carried out according to a conventional method, and for example, the slab heating temperature may be 1100°C or higher. The slab heating temperature may be 1300°C or lower. The soaking temperature may be 20 min or higher. The soaking temperature may be 300 min or lower. The finish rolling temperature may be A ｒ３ The temperature should be above the transformation point. ｒ３ The coiling temperature may be set to transformation point +200°C or lower. The coiling temperature may be set to 400°C or higher. The coiling temperature may be set to 720°C or lower. The coiling temperature is preferably controlled from the viewpoint of suppressing thickness fluctuation and stably ensuring high strength. Specifically, the coiling temperature is preferably set to 430°C or higher. The coiling temperature is preferably set to 530°C or lower. ｒ３ The transformation point can be calculated from the composition of the steel sheet and the following empirical formula (A): ｒ３Point (°C) = 910 - 310 x [C] - 80 x [Mn] - 20 x [Cu] - 15 x [Cr] - 55 x [Ni] - 80 x [Mo] Formula (A) (In the above formula, [M] is the content (mass%) of element M in the steel slab, and the value of an element that is not contained is zero (0).)

[0059] <Pickling> Pickling may be carried out in accordance with a conventional method.

[0060] <Cold Rolling> Cold rolling may be carried out according to a conventional method, with the rolling ratio (cumulative rolling ratio) being 30% or more. The rolling ratio (cumulative rolling ratio) may be 85% or less. It is preferable to control the rolling ratio from the viewpoint of stably ensuring high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more. When the rolling load is high, softening annealing can be performed at 450 to 730°C in a CAL (continuous annealing line) or BAF (box annealing furnace).

[0061] <Annealing> A cold-rolled steel sheet manufactured according to a conventional method is annealed under the following conditions. The annealing equipment is not particularly limited, but from the viewpoints of productivity and ensuring the desired heating rate and cooling rate, it is preferable to carry out the annealing in a continuous annealing line (CAL).

[0062] [Soaking process: In a furnace atmosphere with a dew point of −40° C. or less, ｃ１ Point +20℃ or more A ｃ３ The steel sheet is heated to a soaking temperature that is equal to or lower than the Tc point and equal to or higher than the Tc point, and is held at the soaking temperature for 30 to 500 seconds. The dew point affects the formation of oxides on the steel sheet surface during annealing, and if the dew point exceeds -40°C, the amount of oxides formed on the steel sheet surface increases excessively, deteriorating chemical conversion treatability. For this reason, the dew point is set to -40°C or lower. There is no particular lower limit, but the dew point is preferably -70°C or higher, and more preferably -60°C or higher.

[0063] The steel sheet obtained by the present invention contains a soft ferrite structure, which improves ductility. ｃ１ Point +20℃ or more A ｃ３ points or less.

[0064] Furthermore, by setting the soaking temperature to Tc (°C) or higher, it is possible to ensure that the amount of P surface segregation in the P surface segregation portion formed on the steel sheet surface is the amount specified in the present invention. Tc is calculated from the dew point and soaking holding time in formula (3): Tc (°C) = 663 - 1.2 x exp (20 / t) x Tdp ... formula (3) where t is the holding time (s) at the soaking temperature, and Tdp is the dew point (°C). If the soaking temperature is lower than Tc, the predetermined amount of P surface segregation cannot be ensured, and the chemical conversion treatability deteriorates. For this reason, in a furnace atmosphere with a dew point of -40°C or lower, the soaking temperature is set to A ｃ１ Point +20℃ or more A ｃ３ The temperature is set to be equal to or lower than the temperature point and equal to or higher than Tc (°C).

[0065] Furthermore, if the holding time at the soaking temperature is less than 30 seconds, austenite formation at the soaking temperature is insufficient, resulting in an increase in polygonal ferrite, which may prevent the desired total area ratio of upper bainite, tempered martensite, and lower bainite from being obtained, resulting in the failure to obtain the desired strength. Furthermore, sufficient retained austenite may not be obtained, resulting in the failure to ensure the desired ductility. On the other hand, if the holding time at the soaking temperature is more than 500 seconds, significant coarsening of the structure occurs, making it impossible to ensure the desired strength. Therefore, the holding time at the annealing temperature (soaking time) is set to 30 to 500 seconds. The holding time at the soaking temperature (soaking time) is preferably 60 seconds or more, more preferably 100 seconds or more. The holding time at the soaking temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.

[0066] In addition, the above A ｃ１ and A ｃ３ is obtained from the empirical formulas (4) and (5) below. ｃ１ and A ｃ３ It is sufficient to use the following. ｃ１ =723+22×[C]-18×[Si]+17×[Cr]+4.5×[Mo]+16×[V] ...Formula (4) A ｃ３ =910-203×([C]) １／２+ 44.7 × [Si] − 30 × [Mn] + 700 × [P] + 400 × [sol. Al] − 20 × [Cu] + 31.5 × [Mo] + 104 × [V] + 400 × [Ti] ... Formula (5) Here, [M] is the mass% of each element.

[0067] [First cooling step: cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50°C / s in the temperature range from the soaking temperature to the first cooling stop temperature of 350 to 550°C] A ｃ１ Point +20℃ or more A ｃ３ After being held at a soaking temperature that is below the soaking point and above Tc (after the soaking holding step), the material is cooled at a first average cooling rate of 2 to 50°C / s in the temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550°C. If the cooling rate is below 2°C / s, ferrite transformation during cooling will proceed excessively, and the desired amount of polygonal ferrite will not be obtained, so the first average cooling rate is set to 2°C / s or more. The first average cooling rate is preferably 5°C / s or more. On the other hand, if the first average cooling rate is too high, the sheet shape will deteriorate, so the first average cooling rate is set to 50°C / s or less. The first average cooling rate is preferably 40°C / s or less, and more preferably less than 30°C / s. Here, the first average cooling rate is "(soaking temperature (°C) - first cooling stop temperature (°C)) / cooling time (seconds) from the soaking temperature to the first cooling stop temperature."

[0068] [Second Cooling Step (1): After Stopping Cooling at the First Cooling Stop Temperature, Retention at a Dwell Temperature of 350 to 550°C for 10 to 60 Seconds] By forming upper bainite at a temperature below the first cooling stop temperature and in the temperature range (retention temperature) of 350 to 550°C, a larger amount of retained γ can be obtained compared to a manufacturing method that does not involve retention at that temperature for 10 to 60 seconds, thereby improving ductility. In the present disclosure, whether or not to perform the second cooling step (1): retention at a temperature of 350 to 550°C for 10 to 60 seconds can be determined based on the desired properties. Bainite transformation has an incubation period, and retention at that temperature is required for a certain period of time to obtain the desired amount of bainite. If the residence temperature range, including the residence start temperature (= first cooling stop temperature) and residence end temperature, is outside the range of 350 to 550°C, and / or if the residence time (hereinafter also referred to as residence time) is less than 10 seconds, the desired amount of bainite may not be obtained, the formation of retained austenite may be suppressed, and the desired ductility may not be obtained. On the other hand, if the residence time exceeds 60 seconds, C concentration from bainite to massive untransformed γ progresses, leading to an increase in the amount of remaining massive structure and a concern about a decrease in λ. Therefore, the residence time is set to 10 seconds or more and 60 seconds or less. This residence time is preferably 20 seconds or more. Furthermore, this residence time is preferably 50 seconds or less.

[0069] [Second Cooling Step (2): Cooling to a Second Cooling Stop Temperature of 100 to 300°C at a Second Average Cooling Rate of 2 to 50°C / s] After the above-mentioned retention, rapid cooling is required to prevent excessive bainite transformation. If the average cooling rate (second average cooling rate) in the temperature range from the retention end temperature to the second cooling stop temperature of 100 to 300°C is less than 2°C / s, excessive bainite transformation may occur, resulting in an excessive increase in retained austenite and an insufficient amount of quenched martensite, which may result in a decrease in strength. If the second average cooling rate is less than 2°C / s, the desired ductility and hole expandability may not be achieved. Therefore, the second average cooling rate in the temperature range from the retention end temperature to the second cooling stop temperature of 100 to 300°C is set to 2°C / s or more. The second average cooling rate is preferably 5°C / s or more, and more preferably 8°C / s or more. If the cooling rate in this temperature range is too high, the plate shape will deteriorate, so the cooling rate in this temperature range (second average cooling rate) is set to 50°C / s or less. It is preferably set to 40°C / s or less. If the second cooling stop temperature exceeds 300°C, the tempered martensite or lower bainite will not reach the desired area ratio, and the area ratio of quenched martensite after annealing will increase, resulting in poor hole expandability. For this reason, the second cooling stop temperature is set to 300°C or less. The second cooling stop temperature is preferably set to 280°C or less. On the other hand, if the second cooling stop temperature is less than 100°C, excessive martensitic transformation will occur, which may make it impossible to obtain the desired amount of retained γ, thereby degrading ductility. For this reason, the second cooling stop temperature is set to 100°C or higher. The second cooling stop temperature is preferably set to 220°C or higher. Here, the second average cooling rate is defined as "dwell end temperature (°C) - second cooling stop temperature (°C) / cooling time (seconds) from the dwell end temperature to the second cooling stop temperature."

[0070] [Reheating and Holding Process: Heating from the Second Cooling Stop Temperature to a reheating temperature of not less than 50°C and not more than 450°C above the Second Cooling Stop Temperature at an average heating rate of not less than 2.0°C / s, and holding for not less than 60 s and not more than 3000 s] After the second cooling process, in order to promote carbon distribution from martensite to austenite, the steel sheet is heated from the second cooling stop temperature to a reheating temperature of not less than 50°C and not more than 450°C above the Second Cooling Stop Temperature. If the reheating temperature is less than the Second Cooling Stop Temperature + 50°C, the effect of carbon distribution from martensite to austenite is not obtained, and the desired volume fraction of retained austenite is not obtained. Furthermore, if the reheating temperature exceeds 450°C, excessive tempering of martensite may occur, making it impossible to obtain the desired TS. Furthermore, the decomposition reaction of austenite occurs, making it impossible to obtain the desired volume fraction of retained austenite. Therefore, the reheating temperature is set to not less than 50°C and not more than 450°C above the Second Cooling Stop Temperature. Furthermore, if the average heating rate is less than 2.0°C / s, carbide precipitation is promoted rather than carbon partitioning, and as a result, the desired volume fraction of retained austenite cannot be obtained. Therefore, the average heating rate is set to 2.0°C / s or more. The average heating rate is preferably 4.0°C / s or more, and more preferably 6.0°C / s or more. The average heating rate is preferably 50.0°C / s or less, and more preferably 35.0°C / s or less.

[0071] Holding at a reheating temperature between 50°C and 450°C above the second cooling stop temperature is performed to adjust the strength of the formed martensite through tempering and promote C enrichment in the residual γ. Holding the reheating temperature for less than 60 seconds results in insufficient tempering, resulting in the formation of high-strength martensite. Furthermore, bainite transformation is insufficient, suppressing C enrichment in the residual γ. This reduces the residual γ and increases the amount of quenched martensite, potentially resulting in the desired ductility, hole expandability, or both. On the other hand, holding the reheating temperature for more than 3000 seconds causes a decomposition reaction of the retained austenite, preventing the desired volume fraction of retained austenite from being obtained and ensuring ductility. Therefore, the holding time at the reheating temperature is set to 60 seconds or more and 3000 seconds or less. The holding time at the reheating temperature is preferably 100 seconds or more, more preferably 150 seconds or more. The holding time at the reheating temperature is preferably 2500 seconds or less, more preferably 2000 seconds or less.

[0072] Second Embodiment A method for producing a steel sheet according to a second embodiment of the present invention is a method for producing a steel sheet, in which a steel slab having the above-described component composition is hot-rolled, pickled, and cold-rolled, and then the obtained cold-rolled steel sheet is annealed. The annealing is performed by subjecting the cold-rolled steel sheet to a furnace atmosphere having a dew point of −40° C. or less, ｃ１ Point +20℃ or more A ｃ３ a soaking temperature holding step of heating a steel sheet to a soaking temperature equal to or lower than the soaking point Tc calculated by Equation (3) and holding the steel sheet at the soaking temperature for 30 to 500 seconds, a cooling step of cooling the steel sheet to a cooling stop temperature of 100 to 300°C at an average cooling rate of 2 to 50°C / s, and a reheating step of heating the steel sheet from the cooling stop temperature to a reheating temperature equal to or higher than the cooling stop temperature + 50°C and 450°C at an average heating rate of 2.0°C / s or higher and holding the steel sheet at the soaking temperature for 60 seconds to 3000 seconds. Tc (°C) = 663 - 1.2 × exp (20 / t) × Tdp Equation (3) where t is the holding time (s) at the soaking temperature and Tdp is the dew point (°C).

[0073] In the second embodiment, the treatments in the hot rolling, pickling, cold rolling, and annealing soaking steps can be performed under the same conditions as in the first embodiment. Furthermore, in the second embodiment, the treatment in the first cooling step in the annealing of the first embodiment can be omitted. Furthermore, in the second embodiment, the cooling step in the annealing corresponds to the second cooling step in the annealing of the first embodiment, but the cooling step in this embodiment can omit the retention treatment in the second cooling step in the first embodiment (retention for 10 to 60 seconds in a temperature range of 350 to 550°C). Furthermore, the reheating and holding step in the annealing of the second embodiment can be performed under substantially the same conditions as the reheating and holding step in the annealing of the first embodiment, except that the second cooling stop temperature is set to the cooling stop temperature. Hereinafter, in this embodiment, the cooling step in the annealing will be mainly described.

[0074] [Cooling Process: Cooling from the Soaking Temperature to a Cooling Stop Temperature of 100 to 300°C at an Average Cooling Rate of 2 to 50°C / s] After the soaking process, rapid cooling is required to prevent excessive bainite transformation. If the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100 to 300°C is less than 2°C / s, ferrite transformation may proceed excessively, resulting in the desired amount of ferrite and a decrease in strength. Furthermore, if the average cooling rate is less than 2°C / s, excessive ferrite transformation may not ensure the desired residual γ, and ductility may not be achieved. Therefore, the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100 to 300°C is set to 2°C / s or more. The average cooling rate is preferably 5°C / s or more, and more preferably 8°C / s or more. If the cooling rate in this temperature range is too high, the sheet shape may deteriorate, so the cooling rate (average cooling rate) in this temperature range is set to 50°C / s or less. The cooling stop temperature is preferably 40°C / s or less. If the cooling stop temperature exceeds 300°C, the tempered martensite or lower bainite will not reach the desired area ratio, and the area ratio of quenched martensite after annealing will increase, making it impossible to secure retained γ and resulting in deterioration of ductility. Furthermore, if the cooling stop temperature exceeds 300°C, the desired hole expandability may not be obtained. For this reason, the cooling stop temperature is set to 300°C or less. The cooling stop temperature is preferably 280°C or less. On the other hand, if the cooling stop temperature is less than 100°C, excessive martensitic transformation will occur, making it impossible to obtain the desired amount of retained austenite, and ductility may be deteriorated. For this reason, the cooling stop temperature is set to 100°C or more. The cooling stop temperature is preferably 120°C or more. Here, the average cooling rate is defined as "soaking temperature (°C) - cooling stop temperature (°C) / cooling time (seconds) from soaking temperature to cooling stop temperature".

[0075] [Thickness] The steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more, and more preferably 3.0 mm or less.

[0076] (Member and manufacturing method of the member) Next, the member of the present invention and its manufacturing method will be described.

[0077] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0078] The steel sheet of the present invention has a tensile strength of 780 MPa or more and has excellent ductility, hole expandability, and chemical conversion treatability. Therefore, a member obtained using the steel sheet of the present invention also has a tensile strength of 780 MPa or more and has excellent ductility, hole expandability, and chemical conversion treatability. Furthermore, the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.

[0079] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions.

[0080] Example 1 A slab produced by continuous casting and having the chemical composition shown in Table 1 was heated to 1200°C, subjected to a hot rolling process in which the soaking time was 200 min, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. The slab was then cold rolled at a rolling reduction of 50% to produce a cold-rolled steel sheet having a thickness of 1.4 mm. The cold-rolled steel sheet was then treated under the annealing conditions shown in Table 2 to produce a steel sheet according to the present invention and a steel sheet as a comparative example.

[0081]

[0082] The steel structure was measured using the following method. The measurement results are shown in Table 3. The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting a cross section of the plate parallel to the rolling direction, mirror-polishing it, and etching it with 1 vol% nital. Ten fields of view (25 μm × 20 μm) were observed at 5000x magnification using an SEM at the quarter-thickness position, and the photographed structure was quantified using image analysis. Polygonal ferrite refers to relatively equiaxed ferrite with almost no carbides inside. This is the region that appears the blackest under SEM. Upper bainite is a ferrite structure with carbides or retained austenite inside, which appears white under SEM. When it was difficult to distinguish between upper bainite and polygonal ferrite, the ferrite region with an aspect ratio of ≦2.0 was classified as polygonal ferrite, and the region with an aspect ratio >2.0 was classified as upper bainite, and the area ratio was calculated. Here, the aspect ratio was calculated by determining the major axis length a at which the particle length was longest, and the minor axis length b at which the particle was longest in the direction perpendicular to the major axis length a, and a / b was defined as the aspect ratio. Tempered martensite and lower bainite are regions with lath-like substructures and carbide precipitations within them under SEM. Quenched martensite (fresh martensite) is a massive region that appears white under SEM, with no substructure visible. The residual structure is a carbide and / or pearlite structure, which can be confirmed by a white contrast under SEM. Carbide has a particle diameter of 1 μm or less, and pearlite can be distinguished from carbide because of its lamellar (layer) structure.

[0083] The volume fraction of retained austenite was determined by chemically polishing the surface layer at a position 1 / 4 the thickness away from the surface and then performing X-ray diffraction analysis. A Co-Kα source was used as the incident X-ray source, and the volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0084] JIS No. 5 tensile test pieces were taken from the obtained steel sheets, and a tensile test (in accordance with JIS Z2241 (2011)) was carried out with N=3. Each evaluation was based on the average value of three points. Steel sheets with a tensile strength of 780 MPa or more were judged to have excellent strength. With regard to total elongation EL, when TS was 780 MPa or more and less than 980 MPa, EL was 16.0% or more, when TS was 980 MPa or more and less than 1180 MPa, EL was 14.0% or more, and when TS was 1180 MPa or more, EL was 12.0% or more, it was judged to have excellent ductility. In addition, a hole expansion test in accordance with the provisions of JFST1001 was carried out with N=3, and the hole expansion ratio λ (%) (= {(d - d ０ ) / d ０}×100) was calculated, and a hole expandability of 45% or more was judged to be excellent. The measurement results are shown in Table 3.

[0085] For the annealed steel sheet, the amount of surface enrichment of the P surface enriched portion on the steel sheet surface was measured by depth sputtering analysis using a GDS (manufactured by Shimadzu Corporation) under conditions of Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s, and the maximum P concentration near the surface layer (within 1 μm in the sheet thickness direction from the steel sheet surface) was measured. In this measurement, a P calibration curve was obtained using standard materials having various P contents from 0.005 to 0.020 mass%.

[0086] The annealed steel sheet was degreased and surface-conditioned, and then chemically treated using a zinc phosphate chemical conversion treatment solution. Specifically, the chemical conversion treatment was carried out as follows: degreasing step: treatment temperature; 40°C, treatment time; 120 seconds, spray degreasing; surface conditioning step: pH 9.5, treatment temperature; room temperature, treatment time; 20 seconds; chemical conversion treatment step: chemical conversion treatment solution temperature; 35°C, treatment time; 120 seconds. Note that the treatment agents used in the degreasing step, surface conditioning step, and chemical conversion treatment step, respectively, were degreaser FC-E2011, surface conditioner PL-X, and chemical conversion treatment solution Palbond PB-L3065, all manufactured by Nihon Parkerizing Co., Ltd. Magnification: 1000x, 5 fields of view (50,000 μm ２ The surface chemical conversion structure was observed by SEM observation of the area (area above 10%), and the area where the base steel was exposed was evaluated as ◯ when it was less than 10% of the total area, and x when it was 10% or more. The results are shown in Table 3.

[0087]

[0088]

[0089] The examples of the present invention shown in Tables 2 and 3 were excellent in strength, ductility, hole expandability and chemical conversion treatability, whereas the comparative examples were inferior in any of these properties.

[0090] Example 2 A slab produced by continuous casting and having the chemical composition shown in Table 1 was heated to 1200°C, subjected to a hot rolling process in which the soaking time was 200 min, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. The hot rolling was then performed by cold rolling at a rolling reduction of 50%, to produce a cold-rolled steel sheet having a thickness of 1.4 mm. The cold-rolled steel sheet was then treated under the annealing conditions shown in Table 4, to produce a steel sheet of the present invention and a steel sheet of a comparative example. Evaluations similar to those in Example 1 were carried out. The results are shown in Table 5.

[0091]

[0092]

[0093] The examples of the present invention shown in Tables 4 and 5 were excellent in strength, ductility, hole expandability and chemical conversion treatability, whereas the comparative examples were inferior in any of these properties.

[0094] Furthermore, it was found that the components obtained by forming and joining the steel plates of the present invention have excellent strength, ductility, hole expandability and chemical treatment properties, similar to the steel plates of the present invention, because the steel plates of the present invention have excellent strength, ductility, hole expandability and chemical treatment properties.

Claims

1. In mass percent, C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol. Al: less than 1.0%; N: less than 0.015%; The following formula (1) is satisfied: The balance being iron and unavoidable impurities; Area ratio of polygonal ferrite: 10% or more and 70% or less, Total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less, Volume fraction of retained austenite: 5% or more and 20% or less, A steel structure having an area ratio of quenched martensite of 13% or less (including 0%); having A steel sheet having a maximum P concentration [Pm] of 0.025 mass% or more within 1 μm from the steel sheet surface in the sheet thickness direction, and satisfying the following formula (2): [Si] / [Mn]≦0.35...Formula (1) [Pm] / [P]≧1.5...Formula (2) In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass), In formula (2), [P] is the P content (mass%).

2. The component composition is sol. Al: 0.332 mass% or less, Further, in mass%, Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less The steel sheet according to claim 1, containing one or more selected from the following:

3. A member made using the steel sheet according to claim 1 or 2.

4. In mass %, C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol. Al: less than 1.0%; N: less than 0.015%; The following formula (1) is satisfied: Further optionally, Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less Contains one or more selected from the following: A method for producing a steel sheet, comprising the steps of hot rolling, pickling and cold rolling a steel slab having a composition with the balance being iron and unavoidable impurities, and then annealing the resulting cold-rolled steel sheet, The annealing is The cold-rolled steel sheet is subjected to a furnace atmosphere having a dew point of −40° C. or less, ｃ１ Point +20℃ or more A ｃ３ a soaking temperature holding step of heating the material to a soaking temperature that is equal to or lower than the Tc point and equal to or higher than the Tc calculated by formula (3) and holding the material at the soaking temperature for 30 to 500 s; A first cooling step of cooling the temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550 ° C. at a first average cooling rate of 2 to 50 ° C. / s to the first cooling stop temperature; After stopping the cooling at the first cooling stop temperature, the material is allowed to stay in a temperature range of 350 to 550 ° C. for 10 to 60 s, A second cooling step of cooling to a second cooling stop temperature of 100 to 300 ° C. at a second average cooling rate of 2 to 50 ° C. / s; A reheating and holding step of heating from the second cooling stop temperature to a reheating temperature of 50° C. to 450° C. at an average heating rate of 2.0° C. / s or more from the second cooling stop temperature and holding the temperature for 60 s to 3000 s; A method for manufacturing a steel sheet, comprising: [Si] / [Mn]≦0.35...Formula (1) Here, in formula (1), [Si] is the Si content (% by mass), and [Mn] is the Mn content (% by mass). Tc (°C) = 663-1.2 x exp (20 / t) x Tdp (3) Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).

5. In mass %, C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol. Al: less than 1.0%; N: less than 0.015%; The following formula (1) is satisfied: Further optionally, Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less Contains one or more selected from the following: A method for producing a steel sheet, comprising the steps of hot rolling, pickling and cold rolling a steel slab having a composition with the balance being iron and unavoidable impurities, and then annealing the resulting cold-rolled steel sheet, The annealing is The cold-rolled steel sheet is subjected to a furnace atmosphere having a dew point of −40° C. or less, ｃ１ Point +20℃ or more A ｃ３ a soaking temperature holding step of heating the material to a soaking temperature that is equal to or lower than the Tc point and equal to or higher than the Tc calculated by formula (3) and holding the material at the soaking temperature for 30 to 500 s; a cooling step of cooling from the soaking temperature to a cooling stop temperature of 100 to 300 ° C. at an average cooling rate of 2 to 50 ° C. / s; A reheating and holding step of heating from the cooling stop temperature to a reheating temperature of 50°C to 450°C at an average heating rate of 2.0°C / s or more from the cooling stop temperature and holding the temperature for 60 s to 3000 s; A method for manufacturing a steel sheet, comprising: [Si] / [Mn]≦0.35...Formula (1) Here, in formula (1), [Si] is the Si content (% by mass), and [Mn] is the Mn content (% by mass). Tc (°C) = 663-1.2 x exp (20 / t) x Tdp (3) Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).

6. In mass %, C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol. Al: less than 1.0%; N: less than 0.015%; The following formula (1) is satisfied: Further optionally, Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less Contains one or more selected from the following: The balance being iron and unavoidable impurities; Area ratio of polygonal ferrite: 10% or more and 70% or less, Total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less, Volume fraction of retained austenite: 5% or more and 20% or less, A steel structure having an area ratio of quenched martensite of 13% or less (including 0%); having A method for manufacturing a member, comprising the steps of: subjecting a steel plate having a maximum P concentration [Pm] of 0.025 mass% or more within 1 μm in the plate thickness direction from the steel plate surface to at least one of forming and joining to obtain a member; [Si] / [Mn]≦0.35...Formula (1) [Pm] / [P]≧1.5...Formula (2) In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass), In formula (2), [P] is the P content (mass%).