Steel plates, members, and methods for manufacturing the same

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

Technical Field

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

Background Art

[0002] Against the backdrop of the increasing global CO2 emission regulations, there is an even greater demand for weight reduction of vehicle bodies by increasing the strength of automotive steel sheets. For body and seat parts, the application of high-strength steel sheets of 590 MPa or higher has been progressing from the existing cold-rolled steel sheets of 440 MPa grade. Generally, when the strength of a steel sheet is increased, the press formability such as ductility and stretch flangeability decreases, cracks are likely to occur during press forming, and the degree of freedom of shape decreases, so it is limited to the application to parts with a simple shape. Therefore, in order to apply high-strength steel sheets to complex-shaped parts, it is important to increase the strength of the steel sheet while maintaining or improving the formability.

[0003] Against this background, as a technology for improving the ductility of steel sheets, TRIP steel in which retained austenite (retained γ) is dispersed in the microstructure of the steel sheet has been developed. For the production of TRIP steel, after soaking and holding, austempering treatment in which isothermal holding is performed in the bainite transformation temperature range, and cooling is performed once in the temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point) during the cooling process, and then reheating and holding are performed to stabilize the retained austenite. The so-called Q&P; Quenching & Partitioning (quenching and carbon partitioning from martensite to austenite) heat treatment process is used. In any heat treatment process, a large amount of Si that can suppress carbide precipitation is added in order to form retained γ in the microstructure. In addition, in Q&P, a part of the structure is transformed into martensite during the cooling process, and the hardness difference between different phases in the structure is reduced by tempering the martensite structure by subsequent reheating and holding, which is a heat treatment process that improves not only ductility but also hole expansion property. For example, in Patent Document 1, after holding a cold-rolled steel sheet containing 0.6 to 2.5% Si at a first soaking temperature of 750°C or higher, cooling it to a cooling stop temperature in the temperature range of 150 to 350°C, and then reheating it to the temperature range of 350 to 500°C, a retained austenite with a volume fraction of 5 to 15% is ensured, achieving both a TS of 980 MPa or higher and an elongation of 17%, and having excellent hole expansion properties with a hole expansion rate of 50%. A steel sheet and a manufacturing method thereof are disclosed.

[0004] On the other hand, it is known that with an increase in the Si content, Si is concentrated on the surface of the steel sheet after annealing, and the formation of Si-based oxides deteriorates the phosphatability. To address this issue, for example, Patent Document 2 discloses a method of adding Ni so that Si is not concentrated on the steel sheet surface to improve the phosphatability. In addition, Patent Document 3 discloses a method of appropriately controlling the content of Mn that concentrates on the surface together with Si so that the Si / Mn ratio is 0.40 or less, thereby forming a Mn-Si composite oxide on the surface to improve the phosphatability. Also, Patent Document 4 discloses a method of improving the phosphatability by directly removing Si-based oxides by pickling or brushing after annealing.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] As described above, although adding Si is effective in improving the ductility of high-strength steel sheets, when actively using Si to ensure high workability, there is a trade-off relationship between the Si content and the chemical conversion treatability of the steel sheets. The methods disclosed in Patent Document 2 and Patent Document 4 are effective as methods for improving the chemical conversion treatability in steels with a high Si content, but the establishment of other technologies in which alloying elements to be contained, annealing conditions, etc. are adjusted has also been desired. Also, in the method disclosed in Patent Document 3, it has been clarified by the inventors' studies that good chemical conversion treatability is not necessarily ensured. Thus, as a technology for high-strength steel sheets having excellent ductility, chemical conversion treatability, and furthermore, hole expansion property, the establishment of new technologies has been demanded.

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet, a member, and a method for manufacturing them, which have excellent ductility, hole expansion property, and chemical conversion treatability and have 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] Also, being excellent in 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: 780 MPa or more and less than 980 MPa, EL: 16.0% or more, (B) When TS: 980 MPa or more and less than 1180 MPa, EL: 14.0% or more, (C) When TS: 1180 MPa or more, EL: 12.0% or more

[0010] Also, being excellent in hole expansion property means that in order to ensure the hole expansion property required in practice, the hole expansion rate λ (%) ({(d - d0) / d0} × 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 is 45% or more at any TS level.

[0011] In addition, excellent phosphatizing processability means that degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreasing agent: FC-E2011 manufactured by Nippon Parkerizing Co., Ltd.), surface conditioning (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nippon Parkerizing Co., Ltd.) are carried out, and then phosphatizing treatment is performed using a zinc phosphate phosphatizing treatment solution (temperature of the phosphatizing treatment solution: 35°C, treatment time: 120 seconds, phosphatizing treatment solution: Parbond PB-L3065 manufactured by Nippon Parkerizing Co., Ltd.), and it means that the area where the base metal is exposed is less than 10% of the total area.

Means for Solving the Problems

[0012] In order to solve the above problems, the inventors of the present invention have intensively studied the steel components, heat treatment conditions, and microstructure with respect to ductility and phosphatizing processability for various thin steel sheets having a tensile strength of 780 MPa or more. As a result, 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.Al: less than 1.0%, N: less than 0.015%, satisfying the following formula (1), with the balance being iron and inevitable impurities as the component composition, the area ratio of polygonal ferrite being 10% or more and 70% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite being 20% or more and 80% or less, the volume ratio of retained austenite (retained γ) being 5% or more and 20% or less, the area ratio of quenched martensite being 13% or less (including 0%), and further with the steel structure composed of the remaining structure, when analyzing the emission intensity of P measured by glow discharge analysis in the plate thickness direction from the steel plate surface, the maximum concentration of P within 1 μm in the plate thickness direction from the steel plate surface is 0.025 mass% or more, and by making the steel structure in which P is locally concentrated so as to satisfy the following formula (2), it has been found that a high-strength cold-rolled steel sheet having excellent ductility, hole expansion property, and phosphatizing processability can be obtained. [Si] / [Mn] ≦ 0.35 ··· Formula (1) [Pm] / [P] ≧ 1.5 ··· Formula (2) Here, in formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%). In formula (2), [P] represents the P content (mass%).

[0013] The present invention is made based on the above findings, and its gist is as follows. [1] 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%, N: less than 0.015% is contained, satisfies the following formula (1), a component composition in which the balance consists of iron and inevitable impurities, the area ratio of polygonal ferrite: 10% or more and 70% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less, the volume ratio of retained austenite: 5% or more and 20% or less, the area ratio of quenched martensite: 13% or less (including 0%) steel structure, and a steel sheet in which the maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel sheet surface is 0.025 mass% or more and satisfies the following formula (2). [Si] / [Mn] ≦ 0.35 ··· Formula (1) [Pm] / [P] ≧ 1.5 ··· Formula (2) Here, in formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%), in formula (2), [P] is the P content (mass%). [2] As the component composition, further, by 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: less than 0.5%, Nb: less than 0.1%, Mg: less than 0.0050%, Ca: less than 0.0050%, Sn: less than 0.1%, Sb: less than 0.1%, REM: less than 0.0050%, The steel sheet according to [1], containing one or more selected from among them. [3] A member made using the steel sheet according to [1] or [2]. [4] A method for manufacturing a steel sheet, in which hot rolling, pickling, and cold rolling are performed on a steel slab having the component composition according to [1] or [2], and then annealing is performed on the obtained cold-rolled steel sheet, The annealing is as follows: For the cold-rolled steel sheet, in a furnace atmosphere with a dew point of -40°C or lower, at a temperature of A c1 point + 20°C or higher and A c3 point or lower, and heating to a soaking temperature of Tc or higher calculated by formula (3), and holding at the soaking temperature for 30 to 500 s; a soaking holding step, A first cooling step of cooling from the soaking temperature to a first cooling stop temperature in the temperature range of 350 to 550°C at a first average cooling rate of 2 to 50°C / s, After stopping cooling at the first cooling stop temperature, retaining for 10 to 60 s in the temperature range of 350 to 550°C, and then A second cooling step of cooling to a second cooling stop temperature in the range of 100 to 300°C at a second average cooling rate of 2 to 50°C / s, A reheating holding step of heating from the second cooling stop temperature to a reheating temperature in the range of the second cooling stop temperature + 50°C or higher and 450°C or lower at an average heating rate of 2.0°C / s or higher, and holding for 60 s or more and 3000 s or less, The method for manufacturing a steel sheet includes the above steps. Tc (°C) = 663 - 1.2×exp(20 / t)×Tdp ···(3) Here, t represents the holding time (s) at the soaking temperature, and Tdp represents the dew point (°C). [5] A method for manufacturing a steel sheet, which comprises hot rolling, pickling, and cold rolling a steel slab having the component composition described in [1] or [2], and then annealing the obtained cold-rolled steel sheet. The annealing is For the cold-rolled steel sheet, in a furnace atmosphere with a dew point of -40°C or lower, at a temperature from A c1 point + 20°C or higher to A c3 point or lower, and heating to a soaking temperature equal to or higher than Tc calculated by formula (3), and holding at the soaking temperature for 30 to 500 s; a soaking holding step. 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 holding step of heating from the cooling stop temperature to a reheating temperature of 50°C or higher and 450°C or lower than the cooling stop temperature at an average heating rate of 2.0°C / s or higher, and holding for 60 s or longer and 3000 s or shorter. A method for manufacturing a steel sheet, including the above steps. Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp ···(3) Here, t represents the holding time (s) at the soaking temperature, and Tdp represents the dew point (°C). [6] A method for manufacturing a member, which includes a step of subjecting the steel sheet described in [1] or [2] to at least one of forming and joining to form a member.

Advantages of the Invention

[0014] According to the present invention, a steel sheet and a member having a high strength with a tensile strength TS of 780 MPa or higher, excellent ductility, hole expansion property, and chemical conversion treatment property can be obtained. When the steel sheet of the present invention is applied to the skeletal members of an automobile body, a member with a complex shape and difficult formability can be manufactured by cold press working, thus greatly contributing to the weight reduction of the automobile body. There is no need for expensive alloying elements or post-treatment after annealing to improve the chemical conversion treatment property, and it is possible to reduce the material cost.

Brief Description of the Drawings

[0015]

Figure 1

DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, the present invention will be specifically described. Note that 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%, N: less than 0.015%, satisfies the following formula (1), and the balance is composed of iron and unavoidable impurities, with an area ratio of polygonal ferrite: 10% or more and 70% or less, and 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%). When analyzing the emission intensity of P measured by glow discharge analysis in the plate thickness direction from the surface, the maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel plate surface is 0.025 mass% or more, and it satisfies the following formula (2). It is a high-strength steel plate with a tensile strength TS of 780 MPa or more, excellent ductility, hole expansion property, and chemical conversion treatment property. [Si] / [Mn] ≦ 0.35 ··· Formula (1) [Pm] / [P] ≧ 1.5 ··· Formula (2) Here, in formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%). In formula (2), [P] is the P content (mass%).

[0018] Hereinafter, the steel plate of the present invention will be described in the order of the component composition and the steel structure. First, the reasons for limiting the component composition of the present invention will be described. In the following description, % indicating the components of the steel is all mass% unless otherwise specified.

[0019] <C: 0.05 to 0.25%> C is contained from the viewpoint of ensuring a predetermined strength by transformation strengthening and ensuring 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 sufficiently ensured. On the other hand, the upper limit of the C content is set to 0.25% due to concerns such as hole expansion property important in press formability and weldability important during spot welding or laser welding when incorporated into the vehicle body after forming into automotive members. Therefore, the C content is set to 0.05 - 0.25%. The C content is preferably 0.08% or more, more preferably 0.10% or more. Also, the C content is preferably 0.22% or less, more preferably 0.20% or less.

[0020] <Si: 0.30 - 1.50%> Si is contained from the viewpoints of strengthening ferrite to increase strength and suppressing carbide formation in martensite and bainite to ensure a predetermined amount of retained γ and improve ductility. If the Si content is less than 0.30%, these effects cannot be sufficiently ensured. On the other hand, when the Si content exceeds 1.50%, good chemical conversion treatability cannot be ensured even with the manufacturing method defined in the present invention. Therefore, the Si content is set to 0.30 - 1.50%. The Si content is preferably 0.35% or more, more preferably 0.40% or more. Also, the Si content is preferably 1.20% or less, more preferably 1.00% or less.

[0021] <Mn: 1.5 - 4.5%> Mn is contained from the viewpoints of improving the hardenability of the steel sheet and promoting high strength by transformation strengthening, and suppressing the formation of carbides in bainite and promoting the formation of retained austenite contributing to ductility to improve ductility. To obtain these effects, the Mn content needs to be 1.5% or more. On the one hand, when the Mn content exceeds 4.5%, the bainite transformation is significantly delayed, a predetermined amount of retained austenite cannot be ensured, and the ductility decreases. Also, when the Mn content exceeds 4.5%, it becomes difficult to suppress the formation of coarse martensite by the low-temperature shift of the martensite transformation start temperature, and the stretch flange formability (hole expansion property) deteriorates. Therefore, the Mn content is set to be 1.5% or more and 4.5% or less. The Mn content is preferably 1.8% or more, more preferably 2.0% or more. Also, the Mn content is preferably 3.5% or less, more preferably 3.0% or less.

[0022] <P: 0.005~0.050%> P is an element that strengthens steel. Also, by appropriately controlling the P content, a surface enrichment part of P can be formed on the surface of the steel sheet after annealing, and thus it is an element that can improve the chemical conversion treatment property. From this viewpoint, the P content is set to be 0.005% or more. On the one hand, when the P content is high, the spot weldability deteriorates. From this viewpoint, the P content is set to be 0.050% or less. Therefore, the P content is set to be 0.005~0.050%. The P content is preferably 0.007% or more, more preferably 0.009% or more. Also, 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 the scale peelability during hot rolling and the effect of suppressing nitriding during annealing, but it is an element that has an adverse effect on spot weldability, bendability, and hole expansion property. In order to reduce these adverse effects, at least the S content is set to be 0.01% or less, preferably 0.0050% or less. Note that S may not be contained, but it is costly to reduce it to less than 0.0001%, so from the viewpoint of manufacturing cost, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0024] <Less than 1.0% soluble Al> Al is contained for the purpose of deoxidation or obtaining residual γ. The lower limit of soluble Al is not particularly specified, but in order to perform deoxidation stably, the soluble Al content is preferably 0.005% or more. On the other hand, when the soluble Al content becomes 1.0% or more, a large amount of Al-based coarse inclusions increases, and the elongation flange formability (hole expansion property) deteriorates. Also, Al is an element that deteriorates the chemical conversion treatment property of the steel sheet. When the soluble Al content becomes 1.0% or more, good chemical conversion treatment property cannot be ensured even in the present invention. Therefore, the soluble Al content is less than 1.0%. The soluble Al content is preferably 0.80% or less, and more preferably 0.06% or less.

[0025] <Less than 0.015% N> N is an element that forms nitrides such as BN, AlN, and TiN in steel and deteriorates the elongation flange formability (hole expansion property), so it is necessary to limit its content. 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 it may not contain N, but it takes a great cost to reduce it to less than 0.0001%. Therefore, from the viewpoint of manufacturing cost, the N content is preferably 0.0001% or more. The N content is more preferably 0.0005% or more, and still more preferably 0.001% or more.

[0026] <[Si] / [Mn] ≤ 0.35 ··· Formula (1)> In Formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%). [Si] / [Mn] (Si / Mn ratio) determines the component ratio of Si and Mn in the surface oxide formed during annealing. When [Si] / [Mn] exceeds 0.35 within the range of the manufacturing conditions defined in the present invention, good chemical conversion treatability cannot be ensured. Therefore, [Si] / [Mn] should be 0.35 or less. [Si] / [Mn] is preferably 0.32 or less, and more preferably 0.30 or less. Also, although the lower limit is not particularly limited, [Si] / [Mn] is preferably 0.10 or more, and more preferably 0.15 or more.

[0027] The component composition of the steel sheet in the present invention contains the above component elements as basic components, and the balance contains iron (Fe) and inevitable impurities. Note that the component composition of the steel sheet in the present invention preferably has a component composition in which the balance consists of Fe and inevitable impurities.

[0028] In addition to the above components, the component composition of the steel sheet of the present invention can appropriately contain one or more selected from the following as optional elements (selected elements). 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

[0029] <Ti: 0.1% or less> Ti has the effect of fixing N in the steel as TiN, improving hot workability, and enhancing the hardenability improvement effect of B. Also, it has the effect of refining the structure by the precipitation of TiC. To obtain these effects, it is desirable to set the Ti content to 0.002% or more. From the viewpoint of sufficiently fixing N, it is more preferable to set the Ti content to 0.008% or more. The Ti content is more preferably 0.010% or more. On the one hand, when the Ti content exceeds 0.1%, it will lead to an increase in rolling load and a decrease in ductility due to an increase in the amount of precipitation strengthening. Therefore, when Ti is contained, the Ti content should be 0.1% or less. Preferably, the Ti content is 0.05% or less, and 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 generating tempered martensite and / or bainite with a predetermined area ratio. Therefore, it is preferable to set the B content to 0.0005% or more. On the other hand, when the B content exceeds 0.001%, enrichment to oxides occurs during annealing, promoting coarsening of the oxides and deteriorating the chemical conversion treatability. Therefore, when B is contained, the B content should be 0.001% or less.

[0031] <Cu: 1% or less> Cu improves the corrosion resistance in the usage environment of automobiles. In addition, the corrosion products of Cu have the effect of coating the surface of the steel sheet and suppressing hydrogen intrusion into the steel sheet. Cu is an element that is mixed in when using scrap as a raw material. By allowing the mixing of Cu, recycled materials can be utilized as raw materials, and the manufacturing cost can be reduced. From such a perspective, it is preferable to contain Cu at 0.005% or more. Further, from the perspective of improving the stress corrosion cracking resistance characteristics, it is more preferable to contain Cu at 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, if the Cu content becomes too high, it will cause the occurrence of surface defects. Therefore, when Cu is contained, the Cu content should be 1% or less.

[0032] <Ni: 1% or less> Ni, like Cu, is an element that can improve corrosion resistance. In addition, Ni has the effect of suppressing the generation of surface defects, which is likely to occur when Cu is contained. Therefore, it is desirable to contain Ni at 0.01% or more. 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, the scale formation in the heating furnace becomes non-uniform, which instead causes surface defects. In addition, it also increases costs. Therefore, when Ni is contained, the Ni content should be 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 contained due to its effect of improving the hardenability of steel and suppressing carbide formation in martensite and upper / lower bainite. To obtain such 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, if Cr is contained in excess, the pitting corrosion resistance deteriorates. Therefore, when Cr is contained, the Cr content should be 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 contained due to its effect of improving the hardenability of steel and suppressing carbide formation in martensite and upper / lower bainite. To obtain such 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, Mo significantly deteriorates the chemical conversion treatment property of cold-rolled steel sheets. Therefore, when Mo is contained, the Mo content should be 0.5% or less.

[0035] <V: 0.5% or less> V can be contained for the effects of improving the hardenability of steel, suppressing carbide formation in martensite and upper / lower bainite, refining the structure, and depositing carbides to improve the stress corrosion cracking resistance. To obtain 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. Even more preferably, the V content is 0.020% or more, and even more preferably 0.050% or more. However, when a large amount of V is contained, the castability deteriorates significantly. Therefore, when V is contained, the V content is 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 more preferably 0.1% or less.

[0036] <Nb: 0.1% or less> Nb can be contained for the effects of refining the steel structure and increasing the strength, promoting bainite transformation through grain refinement, improving the bendability, and improving the stress corrosion cracking resistance. To obtain 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, when a large amount of Nb is contained, the precipitation strengthening becomes too strong and the ductility decreases. Also, it causes an increase in the rolling load and deterioration of the castability. Therefore, when Nb is contained, the Nb content is 0.1% or less. Preferably, the Nb content is 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 the improvement of formability such as bendability. Therefore, 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 one hand, when a large amount of Mg is added, the surface quality and bendability deteriorate. Therefore, when Mg is contained, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

[0038] <Ca: 0.0050% or less> Ca fixes S as CaS and contributes to the improvement of bendability and the improvement of stress relaxation fracture resistance characteristics. For this reason, 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 one hand, when a large amount of Ca is added, the surface quality and bendability deteriorate. Therefore, when Ca is contained, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0040% or less.

[0039] <Sn: 0.1% or less> Sn suppresses the oxidation and nitridation of the steel sheet surface layer, thereby suppressing the reduction of the content of C and B in the surface layer. Due to this effect, the formation of ferrite in the steel sheet surface layer is suppressed, the strength is increased, and the fatigue resistance characteristics are improved. From such a 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 one hand, when the Sn content exceeds 0.1%, the castability deteriorates. Also, Sn segregates at the old γ grain boundaries, and the stress relaxation fracture resistance characteristics deteriorate. Therefore, when Sn is contained, the Sn content should be 0.1% or less.

[0040] <Sb: 0.1% or less> Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, thereby suppressing reduction in the content of C and B in the surface layer. Due to this effect, ferrite formation in the surface layer of the steel sheet is suppressed, the strength is increased, and the fatigue resistance characteristics are improved. From such a viewpoint, 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, when the Sb content exceeds 0.1%, the castability deteriorates, and segregation occurs at the prior γ grain boundaries, resulting in deterioration of the stress corrosion cracking resistance characteristics. Therefore, when containing Sb, the Sb content should be 0.1% or less.

[0041] <REM:0.0050% or less> REM is an element that suppresses the adverse effect of sulfides on the elongation flange formability by spheroidizing the shape of sulfides and improves the elongation flange formability. In order to obtain 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, when the REM content exceeds 0.0050%, the improvement effect of the elongation flange formability saturates. Therefore, when containing REM, the REM content should be 0.0050% or less. In addition, REM as referred to in the present invention 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 is the total content of one or more elements selected from the above-mentioned REM.

[0042] When containing the above optional component less than the lower limit value, the optional element contained less than the lower limit value does not impair the effects of the present invention. Therefore, when containing the above optional element less than the lower limit value, it is assumed that the above optional element is contained as an inevitable impurity.

[0043] Next, the mechanical properties of the steel sheet (cold-rolled steel sheet with excellent material stability) targeted by the present invention will be described.

[0044] For the steel sheet of the present invention, the tensile strength (TS) is set to 780 MPa or more. The upper limit of the tensile strength is not particularly limited, 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, as excellent ductility, the total elongation EL is ensured to be EL: 16.0% or more when TS is 780 MPa or more and less than 980 MPa, EL: 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and EL: 12.0% or more when TS is 1180 MPa or more. Also, as the hole expansion property, the hole expansion rate λ is ensured to be 45% or more. Thereby, the stability of press forming is remarkably improved.

[0046] The evaluation of the tensile properties is carried out by taking a JIS No. 5 tensile test piece from the center position of the plate width and conducting a tensile test (in accordance with JIS Z2241 (2011)) with N = 3. Each evaluation is performed based on the average value of three points. A steel sheet with a tensile strength of 780 MPa or more is defined as a high-strength steel sheet. The total elongation EL is considered to be a steel sheet with excellent ductility, being 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. Also, for the hole expansion property, it is an essential condition of the present invention that the hole expansion rate λ (%) ({(d - d0) / d0} × 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 is 45% or more.

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

[0048] <Area ratio of polygonal ferrite: 10% or more and 70% or less> From the viewpoint of ensuring high ductility, the polygonal ferrite has an area ratio of 10% or more, and preferably 20% or more in order to obtain higher ductility. On the other hand, when the polygonal ferrite exceeds 70%, the desired strength may not be obtained. Therefore, the polygonal ferrite should be 70% or less in area ratio, 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> In order to obtain the desired strength, the total area ratio of upper bainite, tempered martensite and lower bainite should be 20% or more. To obtain higher strength, it is preferably 25% or more. On the other hand, when the total area ratio of upper bainite, tempered martensite and lower bainite exceeds 80%, the ductility decreases due to excessive high strength. Therefore, the area ratio should be 80% or less. More preferably, it is 75% or less, and even more preferably 70% or less.

[0050] <Volume ratio of retained austenite (retained γ): 5% or more and 20% or less> When the volume ratio of retained austenite is less than 5%, the desired ductility may not be ensured. Also, when the volume ratio of retained austenite is less than 5%, the desired strength may not be ensured. Further, when the volume ratio of retained austenite is less than 5%, the desired hole expansion property may not be ensured. From the perspective of ductility, the volume ratio of retained austenite should be 5% or more, preferably 7% or more. On the other hand, when the retained austenite exceeds 20%, the elongation flange formability (hole expansion property) decreases. Therefore, the retained austenite should be 20% or less. Preferably, it is 15% or less, and more preferably 13% or less.

[0051] <Area ratio of quenched martensite: 13% or less (including 0%)> Since the hard martensite structure obtained by quenching reduces λ, it is necessary to suppress its area ratio. In order to obtain the required λ in practical use, the area ratio of the quenched martensite is set to 13% or less. In order to obtain λ more stably, the area ratio of the quenched martensite is preferably 11% or less, more preferably 9% or less. The area ratio of the quenched martensite may be 0% or may be 5% or more.

[0052] <The remaining structure> Regarding the steel structure, except for the above, it consists of the remaining structure. 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 later.

[0053] <The maximum concentration of P [Pm] within 1 μm from the steel plate surface in the plate thickness direction is 0.025 mass% or more and satisfies Equation (2)> [Pm] / [P] ≧ 1.5 ··· Equation (2) In Equation (2), [P] (which can also be denoted as [Pi]) is the P content (mass%). As a result of intensively studying various elements affecting the chemical conversion treatment properties, their surface enrichment amounts, and the types of oxides formed during annealing, it was revealed that the chemical conversion treatment properties could not be sufficiently ensured even under manufacturing conditions where no oxide formation was observed. Regarding the steel plate with ensured chemical conversion treatment properties, as a result of quantitatively evaluating the maximum concentration of P in the vicinity of the surface layer by the method described later, when analyzing the emission intensity of P measured by GDS (glow discharge analysis method) in the plate thickness direction from the steel plate surface, the maximum concentration of P [Pm] within 1 μm from the steel plate surface in the plate thickness direction is 0.025 mass% or more, and it was found that good chemical conversion treatment properties are ensured by having a steel structure that satisfies Equation (2). Although the detailed mechanism is unknown, it is important that the maximum concentration of P in the surface layer becomes locally high with respect to the steel components. Also, since the shape of the chemical conversion crystals after the chemical conversion treatment was scaly when the maximum concentration of this P was insufficient, it is considered that the local surface enrichment of P has the effect of suppressing the formation of Si-based oxides and Si-Mn-based oxides on the surface that have an adverse effect on the chemical conversion treatment properties. [Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more. Also, although the upper limit is not particularly limited, [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. Also, although the upper limit is not particularly limited, [Pm] / [Pi] is preferably 10.0 or less, more preferably 9.0 or less.

[0054] Next, the method for measuring the steel structure will be described. For the measurement of the area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite), a plate thickness cross-section parallel to the rolling direction is cut out, mirror-polished, then etched with 1 vol% nital, and at the 1 / 4 thickness position, 10 fields of view in the range of 25 μm × 20 μm are observed at 5000 times magnification with SEM, and the area ratios are quantified by image analysis of the taken microstructure photographs. Polygonal ferrite has almost no carbides inside and targets relatively equiaxed ferrite. It is the region that appears darkest in SEM. Upper bainite is a ferrite structure accompanied by the formation of carbides or retained austenite that appears white in SEM. When it is difficult to distinguish upper bainite from polygonal ferrite, the region of ferrite with an aspect ratio ≦ 2.0 is regarded as polygonal ferrite, and the region with an aspect ratio > 2.0 is classified as upper bainite to calculate the area ratio. Here, the aspect ratio is obtained by determining the major axis length a when the particle length is the longest, and the particle length when cutting across the particle longest in the direction perpendicular to it is taken as the minor axis length b, and a / b is defined as the aspect ratio. Tempered martensite and lower bainite are regions accompanied by a lath-like lower structure and carbide precipitation inside in SEM. Quenched martensite (fresh martensite) is a massive region that appears white inside in SEM with no visible lower structure. The remaining structure refers to a structure containing at least one of unrecrystallized ferrite, carbide, and pearlite. By SEM, the unrecrystallized ferrite is a ferrite with a black contrast containing the deformed structure introduced by rolling, and the carbide and pearlite are structures that can be confirmed with a white contrast. The carbide is a structure with a particle size of 1 μm or less, and since the pearlite is a lamellar (layered) structure, it can be distinguished.

[0055] The volume fraction of retained austenite is determined by chemical polishing the position at 1 / 4 thickness from the surface layer and using X-ray diffraction. For the incident X-ray, a Co-Kα radiation source is used, and the volume fraction of retained austenite is calculated from the intensity ratios of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite obtained by X-ray diffraction can be taken as the area fraction of retained austenite.

[0056] The surface enrichment amount of P in the surface enrichment part of the steel plate surface is measured by using GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure: 600 Pa, high-frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s. A sputtering analysis in the depth direction (plate thickness direction) is performed to measure the surface enrichment amount of P, and from the calibration curve obtained in advance, the maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel plate surface is determined. In this measurement condition, the measurement position d (μm) from the surface is obtained by the formula d = ts / 1.7 (μm) using the sputtering time ts. In the present invention, as shown in FIG. 1, during the above measurement time of 150 s, the value obtained by converting the highest intensity value of P to mass% by the calibration curve is taken as the maximum concentration ([Pm]). As a method for converting to this mass%, a standard material having a known amount of P is used, and in the data obtained by measuring under the same conditions, the correlation between the intensity (Intensity) of the P element obtained by GDS and the amount of P is determined, and thereby the intensity of P in the measured example is converted to concentration. In FIG. 1, Pi is the P content (mass%) in the steel plate.

[0057] (Method for manufacturing steel sheet) Next, the method for manufacturing the steel sheet of the present invention will be described. <First Embodiment> The method for manufacturing a steel sheet according to the first embodiment of the present invention is a method for manufacturing a steel sheet in which a steel slab having the above-described component composition is subjected to hot rolling, pickling, and cold rolling, and then annealing is performed on the obtained cold-rolled steel sheet. The annealing is performed on the cold-rolled steel sheet in a furnace atmosphere with a dew point of -40°C or lower, at a temperature of A c1 point + 20°C or higher and A c3 point or lower, and heated to a soaking temperature of Tc or higher calculated by formula (3), and held at the soaking temperature for 30 to 500 s. A soaking holding step, a first cooling step of cooling from the soaking temperature to a first cooling stop temperature in the temperature range of 350 to 550°C at a first average cooling rate of 2 to 50°C / s, and after stopping the cooling at the first cooling stop temperature, staying in the temperature range of 350 to 550°C for 10 to 60 s, and then 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 second cooling step, and a reheating holding step of heating from the second cooling stop temperature to a reheating temperature of the second cooling stop temperature + 50°C or higher and 450°C or lower at an average heating rate of 2.0°C / s or higher and holding for 60 to 3000 s. It is a method for manufacturing a steel sheet. Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp ··· formula (3) Here, t represents the holding time (soaking holding time) at the soaking temperature (s), and Tdp represents the dew point (°C).

[0058] <Hot Rolling> Methods for hot rolling a steel slab include a method of heating and then rolling the slab, a method of directly rolling the slab after continuous casting without heating, a method of performing a short-time heat treatment on the slab after continuous casting and then rolling, etc. Hot rolling may be carried out according to a conventional method. For example, the slab heating temperature may be 1100°C or higher. Also, the slab heating temperature may be 1300°C or lower. Also, the soaking temperature may be 20 min or longer. Also, the soaking temperature may be 300 min or shorter. Also, the finish rolling temperature may be A r3 above the transformation point. Also, the finish rolling temperature may be Ar3 The transformation point may be set to 200°C or lower. Also, the coiling temperature may be set to 400°C or higher. Further, the coiling temperature may be set to 720°C or lower. The coiling temperature is preferably controlled from the viewpoint of suppressing plate thickness variation and stably ensuring high strength. Specifically, the coiling temperature is preferably 430°C or higher. Also, the coiling temperature is preferably 530°C or lower. In addition, A r3 The transformation point can be calculated from the components of the steel plate and the following empirical formula (A). A r3 Point (°C) = 910 - 310×[C] - 80×[Mn] - 20×[Cu] - 15×[Cr] - 55×[Ni] - 80×[Mo] ··· Formula (A) (In the above formula, [M] is the content (mass%) of element M in the steel slab, and the value of the element not contained is zero (0).)

[0059] <Pickling> Pickling may be carried out according to a conventional method.

[0060] <Cold rolling> Cold rolling may be carried out according to a conventional method, and the rolling ratio (cumulative rolling ratio) may be 30% or higher. Also, the rolling ratio (cumulative rolling ratio) may be 85% or lower. The rolling ratio is preferably controlled from the viewpoint of stably ensuring high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or higher. In addition, when the rolling load is high, it is possible to perform a softening annealing treatment at 450 - 730°C in a CAL (Continuous Annealing Line) or a BAF (Box Annealing Furnace).

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

[0062] [Soaking holding process: In a furnace atmosphere with a dew point of -40°C or lower, A c1 Point + 20°C or higher A c3Heat to a soaking temperature below the point and above Tc, and hold at the soaking temperature for 30 to 500 s The dew point affects the formation of oxides on the surface of the steel sheet during annealing. When the dew point exceeds -40°C, the amount of oxides formed on the steel sheet surface increases excessively, deteriorating the chemical conversion treatability. Therefore, the dew point should be -40°C or lower. The lower limit is not particularly limited, but the dew point is preferably -70°C or higher, more preferably -60°C or higher.

[0063] The steel sheet obtained in the present invention contains a soft ferrite structure, thereby improving ductility. Therefore, the soaking temperature is above the A c1 point + 20°C and below the A c3 point.

[0064] Furthermore, by setting the soaking temperature to Tc (°C) or higher, the surface enrichment amount of P in the surface enrichment portion of P formed on the steel sheet surface can be ensured to be the amount specified in the present invention. Tc is calculated from the dew point and the soaking holding time in Equation (3) Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp ··· Equation (3) Here, t represents the holding time (s) at the soaking temperature, and Tdp represents the dew point (°C). If the soaking temperature is less than Tc, a predetermined surface enrichment amount of P cannot be ensured, and the chemical conversion treatability deteriorates. Therefore, in a furnace atmosphere with a dew point of -40°C or lower, the soaking temperature is above the A c1 point + 20°C and below the A c3 point, and is Tc (°C) or higher.

[0065] Also, if the holding time (soaking holding time) at the above soaking temperature is less than 30 seconds, the formation of austenite at the above soaking temperature is not sufficiently carried out, the amount of polygonal ferrite increases, and the total area ratio of the desired upper bainite, tempered martensite, and lower bainite cannot be obtained, resulting in a situation where the desired strength may not be obtained. Also, sufficient retained austenite may not be obtained, and the desired ductility may not be ensured. On the other hand, if the holding time at the soaking temperature (soaking holding time) exceeds 500 seconds, significant coarsening of the structure occurs, and thus the desired strength cannot be ensured. Therefore, the holding time (soaking time) at the annealing temperature is set to 30 to 500 seconds. The holding time (soaking time) at the soaking temperature is preferably 60 seconds or more, more preferably 100 seconds or more. Also, the holding time (soaking time) at the soaking temperature is preferably 400 seconds or less, more preferably 300 seconds or less.

[0066] In addition, the above A c1 and A c3 may be obtained from the empirical formulas of the following formulas (4) and (5). c1 and A c3 can be used. A 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 from the soaking temperature to the first cooling stop temperature of 350 to 550 °C with the first average cooling rate of 2 to 50 °C / s until the first cooling stop temperature] A c1 point + 20 °C or more and A c3 point or less, and after holding at the soaking temperature that is Tc or higher (after the above soaking holding step), cool the temperature range from the soaking temperature to the first cooling stop temperature of 350 to 550 °C at the first average cooling rate of 2 to 50 °C / s. If it is less than 2 °C / s, the ferrite transformation during cooling progresses excessively, and the desired amount of polygonal ferrite cannot be obtained. Therefore, the first average cooling rate is 2 °C / s or more. The first average cooling rate is preferably 5 °C / s or more. On the one hand, if the first average cooling rate becomes too high, the plate shape deteriorates, so it 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, hold at a holding temperature of 350 to 550 °C for 10 s or more and 60 s or less] By forming upper bainite in the temperature range (holding temperature) below the above first cooling stop temperature and from 350 °C to 550 °C, a larger amount of retained γ can be obtained compared to a manufacturing method in which holding is not performed at this temperature for 10 s or more and 60 s or less, and thereby ductility can be improved. In the disclosure of the present invention, in the second cooling step (1): holding at a holding temperature of 350 to 550 °C for 10 s or more and 60 s or less may be determined according to the presence or absence of implementation in consideration of desired characteristics. Bainite transformation has a latent period, and in order to obtain a desired amount of bainite, it must be held at that temperature for a certain period of time. When the holding temperature range including the holding start temperature (= first cooling stop temperature) and the holding end temperature is outside the range of 350 to 550 °C, and / or when the holding time (hereinafter also referred to as the holding time) is less than 10 s, a desired amount of bainite cannot be obtained, the formation of retained austenite is suppressed, and desired ductility may not be obtained. On the other hand, when the holding time exceeds 60 s, the enrichment of C from bainite to massive untransformed γ progresses, leading to an increase in the remaining amount of the massive structure, and there is concern about a decrease in λ. Therefore, the holding time is set to 10 s or more and 60 s or less. This holding time is preferably 20 s or more. Also, this holding time is preferably 50 s or less.

[0069] [Second cooling step (2): Cool 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 holding, it is necessary to cool rapidly so that the bainite transformation does not proceed excessively. When the average cooling rate (second average cooling rate) in the temperature range from the above-mentioned holding end temperature to the second cooling stop temperature of 100°C or more and 300°C or less is less than 2°C / s, the bainite transformation proceeds excessively, resulting in an excessive increase in retained austenite. Also, the desired amount of quenched martensite cannot be ensured, which may lead to a decrease in strength. When the second average cooling rate is less than 2°C / s, the desired ductility and hole expansion property may not be obtained. Therefore, the second average cooling rate in the temperature range from the holding end temperature to the second cooling stop temperature of 100°C or more and 300°C or less 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 becomes too large, the plate shape deteriorates. Therefore, the cooling rate (second average cooling rate) in this temperature range is set to 50°C / s or less. Preferably it is 40°C / s or less. When the second cooling stop temperature exceeds 300°C, the tempering martensite or lower bainite does not reach the predetermined area ratio, and the area ratio of the quenched martensite after annealing increases, resulting in deterioration of the hole expansion property. For this reason, the second cooling stop temperature is set to 300°C or less. The second cooling stop temperature is preferably 280°C or less. On the other hand, when the second cooling stop temperature is less than 100°C, martensite transformation occurs excessively, and thus the desired amount of residual γ may not be obtained, etc., deteriorating the ductility. For this reason, the second cooling stop temperature is set to 100°C or more. The second cooling stop temperature is preferably 220°C or more. Here, the second average cooling rate is "(holding end temperature (°C) - second cooling stop temperature (°C) / cooling time (seconds) from the holding end temperature to the second cooling stop temperature)".

[0070] [Reheating holding process: Heat from the second cooling stop temperature at an average heating rate of 2.0°C / s or more to a reheating temperature of the second cooling stop temperature + 50°C or more and 450°C or less, and hold for 60s or more and 3000s or less] After the second cooling step, in order to promote the C distribution from martensite to austenite, the steel sheet is heated from the second cooling stop temperature to a reheating temperature of not less than the second cooling stop temperature + 50°C and not more than 450°C. When the reheating temperature is less than the second cooling stop temperature + 50°C, the effect of C distribution from martensite to austenite cannot be obtained, and a desired volume fraction of retained austenite cannot be obtained. Further, when the reheating temperature exceeds 450°C, excessive tempering of martensite may occur, and a desired TS may not be obtained. Further, due to the occurrence of the decomposition reaction of austenite, a desired volume fraction of retained austenite cannot be obtained. Therefore, the reheating temperature is set to be not less than the second cooling stop temperature + 50°C and not more than 450°C. Also, when the average heating rate is less than 2.0°C / s, carbide precipitation is promoted rather than carbon distribution, and as a result, a desired volume fraction of retained austenite cannot be obtained. Therefore, the average heating rate is set to be 2.0°C / s or more. The average heating rate is preferably 4.0°C / s or more, more preferably 6.0°C / s or more. Also, the average heating rate is preferably 50.0°C / s or less, more preferably 35.0°C / s or less.

[0071] The holding at the reheating temperature of not less than the second cooling stop temperature + 50°C and not more than 450°C is carried out from the viewpoint of promoting the strength adjustment by the tempering treatment of the formed martensite and the C enrichment to the retained γ. When the holding time at the above reheating temperature is less than 60 s, tempering is insufficient and martensite with high strength is formed, and the bainite transformation does not occur sufficiently, suppressing the C enrichment to the retained γ. As a result, the retained γ decreases and the quenched martensite increases, and thus desired ductility, hole expansion property, or any of them may not be ensured. On the other hand, when the holding time at the above reheating temperature exceeds 3000 s, the decomposition reaction of the retained austenite occurs, and a desired volume fraction of retained austenite cannot be obtained, and ductility cannot be ensured. Therefore, the holding time at the above re-heating temperature shall be 60 s or more and 3,000 s or less. The holding time at the above re-heating temperature is preferably 100 s or more, more preferably 150 s or more. The holding time at the above re-heating temperature is preferably 2,500 s or less, more preferably 2,000 s or less.

[0072] <Second Embodiment> The method for manufacturing a steel sheet according to the second embodiment of the present invention is a method for manufacturing a steel sheet in which hot rolling, pickling, and cold rolling are performed on a steel slab having the above-described component composition, and then annealing is performed on the obtained cold-rolled steel sheet. The above annealing is performed on the above cold-rolled steel sheet in a furnace atmosphere with a dew point of -40°C or lower, at a temperature of A c1 point + 20°C or higher and A c3 point or lower, and heating to a soaking temperature of Tc or higher calculated by formula (3), and holding at the soaking temperature for 30 to 500 s; a cooling step of cooling from the soaking temperature to a cooling stop temperature in the temperature range of 100 to 300°C at an average cooling rate of 2 to 50°C / s; and a re-heating holding step of heating from the cooling stop temperature to a re-heating temperature of cooling stop temperature + 50°C or higher and 450°C or lower at an average heating rate of 2.0°C / s or higher and holding for 60 s or more and 3,000 s or less. Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp ··· formula (3) Here, t represents the holding time (s) at the soaking temperature, and Tdp represents the dew point (°C).

[0073] In the second embodiment, the treatments in the soaking holding step of hot rolling, pickling, cold rolling, and annealing can be performed under the same conditions as in the first embodiment. Also, in the second embodiment, the treatment in the first cooling step in the annealing of the first embodiment can be omitted. Also, in the second embodiment, the cooling step in the annealing corresponds to the second cooling step in the annealing of the first embodiment, but in the cooling step of this embodiment, the residence treatment (residence for 10 to 60 s in the temperature range of 350 to 550°C) in the second cooling step of the first embodiment can be omitted. In addition, the reheating and holding step in the annealing of the second embodiment can be carried out 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 used as the cooling stop temperature. Hereinafter, in this embodiment, the cooling step in annealing will be mainly described.

[0074] [Cooling step: Cooling from the soaking temperature to the cooling stop temperature of 100 to 300°C at an average cooling rate of 2 to 50°C / s] After the treatment in the above soaking and holding step, it is necessary to cool rapidly so that excessive bainite transformation does not proceed. When the average cooling rate in the temperature range from the above soaking temperature to the cooling stop temperature of 100°C or higher and 300°C or lower is less than 2°C / s, ferrite transformation may proceed excessively, and the desired amount of ferrite may not be ensured, leading to a decrease in strength. Also, when the average cooling rate is less than 2°C / s, the desired retained γ may not be ensured due to excessive ferrite transformation, and ductility may not be obtained. Therefore, the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100°C or higher and 300°C or lower is set to 2°C / s or higher. The average cooling rate is preferably 5°C / s or higher, and more preferably 8°C / s or higher. If the cooling rate in this temperature range becomes too high, the plate shape deteriorates. Therefore, the cooling rate (average cooling rate) in this temperature range is set to 50°C / s or lower. Preferably, it is 40°C / s or lower. When the cooling stop temperature exceeds 300°C, tempered martensite or lower bainite does not reach the predetermined area ratio, and the area ratio of martensite after quenching increases after annealing, so that the retained γ cannot be ensured and the ductility may deteriorate. Also, when the cooling stop temperature exceeds 300°C, the desired hole expansion property may not be obtained. For this reason, the cooling stop temperature is set to 300°C or lower. The cooling stop temperature is preferably 280°C or lower. On the other hand, when the cooling stop temperature is less than 100°C, martensite transformation occurs excessively, so that a predetermined amount of retained austenite cannot be obtained, and the ductility may deteriorate. For this reason, the cooling stop temperature is set to 100°C or higher. The cooling stop temperature is preferably 120°C or higher. Here, the average cooling rate is defined as "soaking temperature (°C) - cooling stop temperature (°C) / cooling time (seconds) from the soaking temperature to the cooling stop temperature".

[0075] [Plate thickness] The steel plate of the present invention obtained as described above preferably has a plate thickness of 0.5 mm or more. Also, the plate thickness is preferably 3.0 mm or less.

[0076] (Member and method for manufacturing 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 processing and joining processing. 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 processing and joining processing to obtain a member.

[0078] The steel plate of the present invention has a tensile strength of 780 MPa or more and has excellent ductility, hole expansion property, and chemical conversion treatment property. Therefore, the member obtained using the steel plate of the present invention also has a tensile strength of 780 MPa or more and has excellent ductility, hole expansion property, and chemical conversion treatment property. Also, by using the member of the present invention, weight reduction is possible. Therefore, the member of the present invention can be suitably used, for example, for vehicle body frame parts.

[0079] For the forming processing, general processing methods such as press processing can be used without limitation. Also, for the joining processing, general welding such as spot welding and arc welding, as well as rivet joining, caulking joining, etc. can be used without limitation.

Example

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

[0081] [Table 1]

[0082] The steel structure was measured by the following method. The measurement results are shown in Table 3. The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite) were measured by cutting out a cross section of the plate thickness parallel to the rolling direction, mirror polishing it, and then etching it with 1 vol% nital. At the 1 / 4 thickness position, an area of ​​25 μm x 20 μm was observed in 10 fields of view at 5,000x magnification using an SEM, and the photographed structure was quantified using image analysis. Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest under SEM. Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM. When it was difficult to distinguish between upper bainite and polygonal ferrite, the area of ​​ferrite with an aspect ratio of ≦2.0 was classified as polygonal ferrite, and the area with an aspect ratio of >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 is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to that, and a / b was defined as the aspect ratio. Tempered martensite and lower bainite are regions with lath-shaped substructure and carbide precipitation inside when viewed under SEM. Hardened martensite (fresh martensite) is a blocky region that appears white under an SEM, with no internal substructure visible. The remaining structure is carbide and / or pearlite structure, which can be confirmed with white contrast in SEM. Carbide has a particle size of 1 μm or less, and pearlite can be distinguished because it is a lamellar (layered) structure.

[0083] The volume fraction of retained austenite is determined by chemical polishing the 1 / 4 thickness position from the surface layer and using X-ray diffraction. For the incident X-ray, a Co-Kα ray source is used, and the volume fraction of retained austenite is calculated from the intensity ratios of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite.

[0084] From the obtained steel plate, JIS No. 5 tensile test pieces were sampled, and a tensile test (in accordance with JIS Z2241 (2011)) was carried out with N = 3. Each evaluation was performed based on the average value of three points. Steel plates with a tensile strength of 780 MPa or more were judged to be excellent in strength. For the total elongation EL, when TS is 780 MPa or more and less than 980 MPa, EL is 16.0% or more; when TS is 980 MPa or more and less than 1180 MPa, EL is 14.0% or more; when TS is 1180 MPa or more, EL is 12.0% or more, and it was judged to be excellent in ductility. Also, a hole expansion test in accordance with the provisions of JFST1001 was carried out with N = 3, the average of the hole expansion rate λ (%) ({(d - d0) / d0} × 100) was calculated, and 45% or more was judged to be excellent in hole expandability. The measurement results are shown in Table 3.

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

[0086] The annealed steel sheet was degreased and surface-conditioned, and then chemical conversion treatment was performed using a zinc phosphate chemical conversion treatment solution. Specifically, in the degreasing process: treatment temperature; 40°C, treatment time; 120 seconds, spray degreasing, in the surface conditioning process: pH 9.5, treatment temperature; room temperature, treatment time; 20 seconds, and in the chemical conversion treatment process: the temperature of the chemical conversion treatment solution; 35°C, treatment time; 120 seconds. In addition, as the treatment agents in the degreasing process, surface conditioning process, and chemical conversion treatment process respectively, a degreasing agent: FC-E2011, a surface conditioner: PL-X, and a chemical conversion treatment solution: Parbond PB-L3065, all manufactured by Nippon Parkerizing Co., Ltd., were used. Magnification: Observation was carried out at a magnification of 1000 times in 5 fields of view (50000μm 2 2 The surface chemical conversion structure was observed by SEM in the above regions), and those in which the area where the base metal was exposed was less than 10% of the total area were evaluated as ○, and those in which it was 10% or more were evaluated as ×. The results are shown in Table 3.

[0087]

Table 2

[0088]

Table 3

[0089] The examples of the present invention shown in Tables 2 and 3 were excellent in strength, ductility, hole expansion property, and chemical conversion treatability, while any of the comparative examples was inferior.

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

[0091]

Table 4

[0092]

Table 5

[0093] The examples of the present invention shown in Tables 4 and 5 are excellent in strength, ductility, hole expansion property, and chemical conversion treatment property, whereas any of the comparative examples was inferior.

[0094] In addition, with respect to the members obtained by performing forming on the steel sheet of the example of the present invention and the members obtained by performing joining, since the steel sheet of the example of the present invention is excellent in strength, ductility, hole expansion property, and chemical conversion treatment property, it was found that they are excellent in strength, ductility, hole expansion property, and chemical conversion treatment property, similarly to the steel sheet of the example of the present invention.

Claims

1. By mass percentage, 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: 0.332% or less, N: less than 0.015% is contained, satisfies the following formula (1), a component composition in which the balance consists of iron and inevitable 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 ratio of retained austenite: 5% or more and 20% or less, area ratio of quenched martensite: 13% or less (including 0%), a steel structure in which the area ratio of the remaining structure is 5% or less, and has, a steel sheet in which the maximum concentration [Pm] of P within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more and satisfies the following formula (2). [Si] / [Mn] ≤ 0.35... Formula (1) [Pm] / [P] ≥ 1.5... Formula (2) Here, in Formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%). In Formula (2), [P] is the P content (mass%).

2. As the component composition, further, by mass percentage, 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 among them, the steel sheet according to Claim 1.

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

4. A method for manufacturing a steel sheet, which comprises subjecting a steel slab having the component composition according to Claim 1 or 2 to hot rolling, pickling, and cold rolling, and then annealing the obtained cold-rolled steel sheet, wherein the annealing is For the cold-rolled steel sheet, in a furnace atmosphere with a dew point of -40°C or lower, A ｃ１ point + 20°C or higher and A ｃ３ point or lower, and heating to a soaking temperature of Tc or higher calculated by formula (3), and holding at the soaking temperature for 30 to 500 s; a soaking holding step a first cooling step of cooling to the first cooling stop temperature of 350 to 550 °C from the soaking temperature at a first average cooling rate of 2 to 50 °C / s, after stopping cooling at the first cooling stop temperature, holding for 10 to 60 s in the temperature range of 350 to 550 °C, and then a second cooling step of cooling to the second cooling stop temperature of 100 to 300 °C at a second average cooling rate of 2 to 50 °C / s. Heating is performed from the second cooling stop temperature at an average heating rate of 2.0 °C / s or more to a reheating temperature of 50 °C or more and 450 °C or less above the second cooling stop temperature, and holding is performed for 60 s or more and 3000 s or less, the reheating and holding step; A method for manufacturing a steel sheet, including: a steel structure having an area ratio of polygonal ferrite of 10% or more and 70% or less, a total area ratio of upper bainite, tempered martensite, and lower bainite of 20% or more and 80% or less, a volume ratio of retained austenite of 5% or more and 20% or less, an area ratio of quenched martensite of 13% or less (including 0%), and an area ratio of the remaining structure of 5% or less; and having a maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel sheet surface of 0.025 mass% or more and satisfying the following formula (2). [Pm] / [P] ≥ 1.5... Formula (2) Here, in Formula (2), [P] is the P content (mass%). Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp... (3) Here, in Formula (3), t is the holding time (s) at the soaking temperature, and Tdp indicates the dew point (°C).

5. A method for manufacturing a steel sheet, in which hot rolling, pickling, and cold rolling are performed on a steel slab having the component composition according to Claim 1 or 2, and then annealing is performed on the obtained cold-rolled steel sheet, The annealing includes: For the cold-rolled steel sheet, in a furnace atmosphere with a dew point of -40°C or lower, at a temperature of A ｃ１ point + 20°C or higher and A ｃ３ point or lower, and heating to a soaking temperature of Tc or higher calculated by formula (3), and holding at the soaking temperature for 30 to 500 s, a soaking holding step; 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 until the cooling stop temperature; A reheating and holding step of heating from the cooling stop temperature at an average heating rate of 2.0 °C / s or more to a reheating temperature of 50 °C or more and 450 °C or less above the cooling stop temperature, and holding for 60 s or more and 3000 s or less; A method for manufacturing a steel sheet, including: a steel structure having an area ratio of polygonal ferrite of 10% or more and 70% or less, a total area ratio of upper bainite, tempered martensite, and lower bainite of 20% or more and 80% or less, a volume ratio of retained austenite of 5% or more and 20% or less, an area ratio of quenched martensite of 13% or less (including 0%), and an area ratio of the remaining structure of 5% or less; and having a maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel sheet surface of 0.025 mass% or more and satisfying the following formula (2). [Pm] / [P] ≥ 1.5... Formula (2) Here, in Formula (2), [P] is the P content (mass%). Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp... (3) Here, in formula (3), t represents the holding time (s) at the soaking temperature, and Tdp represents the dew point (°C).

6. A method for manufacturing a member, comprising a step of subjecting the steel sheet according to claim 1 or 2 to at least one of forming and joining to form a member.

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