Steel plates, members, and methods for manufacturing them

Abstract

Provided are: a steel sheet having excellent ductility, hole expandability, and chemical conversion treatability and having a tensile strength of 780 MPa; a member; and a method for producing same.　The steel sheet has a component composition containing, by mass%, C, Si, Mn, P, S, sol. Al, and N in predetermined ranges and satisfying expression (1), and has a steel structure in which the area ratio of polygonal ferrite or the like is within a predetermined range. [Pm], which is the maximum concentration of P within 1 μm from the surface of the steel sheet in the thickness direction, is at least 0.025 mass% and satisfies expression (2). Expression (1): [Si] / [Mn] ≤ 0.35 Expression (2): [Pm] / [P] ≥ 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 increasing global CO2 emission regulations, there is an increasing demand for weight reduction of vehicle bodies through the use of high-strength steel sheets for automobiles. The application of high-strength steel sheets of 590 MPa or higher has been promoted for body and seat parts, replacing the existing cold-rolled steel sheets of 440 MPa grade. Generally, when the strength of a steel sheet is increased, its press formability, such as ductility and stretch flange formability, 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 simple shapes. 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 its formability.

[0003] Against this background, as a technology for improving the ductility of steel sheets, transformation-induced plasticity (TRIP) steel in which retained austenite (retained γ) is dispersed in the microstructure of the steel sheet has been developed. Since TRIP steel forms retained γ in its microstructure, a large amount of Si is added. For example, in Patent Document 1, a steel containing C: 0.04 to 0.12%, Si: 0.8 to 2.5%, and Mn: 0.5 to 2.0% is annealed and then held at 300 to 500 °C for 10 to 900 seconds by austempering (carbon partitioning associated with bainite transformation) to generate 2 to 10% of retained γ, thereby obtaining a steel sheet having a high ductility of TS×El≧21000 MPa·% and a high stretch flange formability of 70% or more.

[0004] On the other hand, it is known that as the Si content increases, Si is concentrated on the surface of the steel sheet after annealing, and the chemical conversion treatment property deteriorates due to the formation of Si-based oxides. To address this issue, for example, Patent Document 2 discloses a method of adding Ni so that Si is not concentrated on the surface of the steel sheet to improve the chemical conversion treatment property. In Patent Document 3, regarding the content of Mn concentrated on the surface together with Si, a method of forming a Mn-Si composite oxide on the surface to improve the chemical conversion treatment property by appropriately controlling Si / Mn to be 0.40 or less is disclosed. In addition, Patent Document 4 discloses a method of improving the chemical conversion treatment property 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 for improving the ductility of high-strength steel sheets, when actively utilizing Si to ensure high workability, there is a trade-off relationship between the Si content and the chemical conversion treatment property of the steel sheet. The methods disclosed in Patent Document 2 and Patent Document 4 are effective as methods for improving the chemical conversion treatment property in steels with a high Si content, but the establishment of other technologies that adjust alloying elements to be contained, annealing conditions, etc. has also been desired. Also, in the method disclosed in Patent Document 3, it has been clarified by the inventors' study that good chemical conversion treatment property is not always ensured. Thus, as a technology for high-strength steel sheets having excellent ductility and chemical conversion treatment property, and further having 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 the same, which have excellent ductility, hole expansion property, and chemical conversion treatment property, 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] Further, 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: 16.0% or more, (B) When TS is 980 MPa or more and less than 1180 MPa, EL: 14.0% or more, (C) When TS is 1180 MPa or more, EL: 12.0% or more

[0010] Further, excellent hole expansion property means that the hole expansion rate λ (%) ({(d - d0) / d0} × 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 is 30% or more in order to ensure the hole expansion property required in practical use. The hole expansion rate λ (%) may be less than 45%.

[0011] Further, excellent chemical conversion treatment property 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 performed, and then chemical conversion treatment (chemical conversion treatment solution temperature: 35°C, treatment time: 120 seconds, chemical conversion treatment solution: Parbond PB-L3065 manufactured by Nippon Parkerizing Co., Ltd.) is performed using a zinc phosphate chemical conversion treatment solution, and the area where the base metal is exposed is less than 10% of the entire area.

Means for Solving the Problems

[0012] In order to solve the above problems, the inventors have intensively studied the steel components, heat treatment conditions, and microstructure that affect ductility and chemical conversion treatment properties for various thin steel sheets having a tensile strength of 780 MPa or more. As a result, in terms of 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 consisting of iron and unavoidable impurities, the area ratio of polygonal ferrite being 10% or more and 80% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite being 10% or more and 70% or less, the volume ratio of retained austenite (retained γ) being 3% or more and 15% or less, the area ratio of quenched martensite being 15% or less (including 0%), and with the remaining microstructure being a steel microstructure, 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 microstructure 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 chemical conversion treatment property 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] is the P content (mass%).

[0013] The present invention has been made based on the above findings, and the gist thereof is as follows. [1] In terms of 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 has a component composition in which the balance consists of iron and inevitable impurities, the area ratio of polygonal ferrite: 10% or more and 80% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less, the volume ratio of retained austenite: 3% or more and 15% or less, and a steel structure in which the area ratio of quenched martensite is 15% or less (including 0%), and a steel sheet in which the maximum concentration of P [Pm] 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%), and in Formula (2), [P] is the P content (mass%). [2] As the component composition, 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, and contains one or more selected from the above, the steel sheet according to [1]. [3] A member made using the steel sheet according to [1] or [2]. [4] A method for manufacturing a steel sheet, comprising subjecting a steel slab having the component composition described in [1] or [2] to hot rolling, pickling, and cold rolling, 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, heat it to a soaking temperature that is 20°C or higher and lower than the A c1 point and is equal to or higher than Tc calculated by formula (3), and hold it at the soaking temperature for 30 to 500 s in a soaking holding step; c3 In a first cooling step, cool it from the soaking temperature to a first cooling stop temperature in the range of 350 to 550°C at a first average cooling rate of 2 to 50°C / s; After stopping the cooling at the first cooling stop temperature, hold it in the temperature range of 350 to 550°C for 10 to 60 s, and then In a second cooling step, cool it to a second cooling stop temperature in the range of 200 to 420°C at a second average cooling rate of 2 to 50°C / s; In an isothermal holding step, hold it at the second cooling stop temperature for 60 to 3000 s. 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, comprising subjecting a steel slab having the component composition described in [1] or [2] to hot rolling, pickling, and cold rolling, 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, heat it to a soaking temperature that is 20°C or higher and lower than the A c1 point and is equal to or higher than Tc calculated by formula (3), and hold it at the soaking temperature for 30 to 500 s in a soaking holding step; c3 In a cooling step, cool it from the soaking temperature to a cooling stop temperature in the range of 200 to 420°C at an average cooling rate of 2 to 50°C / s; In an isothermal holding step, hold it at the cooling stop temperature for 60 to 3000 s. ​A method for manufacturing a steel sheet, including 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, including a step of subjecting the steel sheet according to [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 more, excellent ductility, hole expansion property, and chemical conversion treatment property can be obtained. When the steel sheet of the present invention is applied to a skeletal member of an automobile body, a member with a complex shape and difficult formability can be manufactured by cold press working, which can greatly contribute to the weight reduction of the automobile body. It is not necessary to use expensive alloying elements or improve the chemical conversion treatment property by post-treatment after annealing, and the material cost can be reduced. [Brief Description of the Drawings]

[0015]

Figure 1

[0016] Hereinafter, the present invention will be specifically described. Note that the present invention is not limited to the following embodiments.

[0017] (Steel Sheet) The steel sheet 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 consists of iron and inevitable impurities. The area ratio of polygonal ferrite is 10% or more and 80% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more and 70% or less, the volume ratio of retained austenite is 3% or more and 15% or less, and the area ratio of quenched martensite is 15% or less (including 0%). When analyzing the emission intensity of P measured by glow discharge analysis in the thickness direction from the surface of the steel sheet, the maximum concentration [Pm] of P within 1 μm in the thickness direction from the surface of the steel sheet is 0.025 mass% or more, and it satisfies the following formula (2). It is a high-strength steel sheet 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 sheet 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 explained. 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 the hole expansion property important in press formability and the 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 to 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 to 1.50%> Si is contained from the viewpoints of strengthening ferrite to increase strength and suppressing carbide formation in martensite and bainite to secure a predetermined amount of retained γ and improve ductility. If the Si content is less than 0.30%, these effects cannot be sufficiently secured. On the other hand, when the Si content exceeds 1.50%, good chemical conversion treatability cannot be secured even with the manufacturing method defined 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. Also, the Si content is preferably 1.20% or less, more preferably 1.00% or less.

[0021] <Mn: 1.5 to 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 that contributes to ductility, similar to Si, to improve ductility. To obtain these effects, the Mn content needs to be 1.5% or more. On the other hand, when the Mn content exceeds 4.5%, bainite transformation is significantly delayed, and a predetermined amount of retained austenite may not be secured, resulting in a decrease in ductility. Also, when the Mn content exceeds 4.5%, it becomes difficult to suppress the formation of coarse martensite due to the lowering of the martensite transformation start temperature, and the flange formability (hole expansion property) of elongation deteriorates. Therefore, the Mn content is set to 1.5 to 4.5%. 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 to 0.050%> P is an element that strengthens steel. Also, by appropriately controlling the P content, a P surface enrichment part can be produced 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 perspective, the P content should be 0.005% or more. On the other hand, when the content of P is high, it deteriorates the spot weldability. From this perspective, the P content should be 0.050% or less. Therefore, the P content is 0.005 to 0.050%. The P content is preferably 0.007% or more, and more preferably 0.009% or more. Also, the P content is preferably 0.035% or less, and more preferably 0.020% or less.

[0023] <S: 0.01% or less> S has the effect of improving the scale peelability during hot rolling and 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 should be 0.01% or less, and preferably 0.0050% or less. Note that S may not be contained, but it costs a great deal to reduce it to less than 0.0001%. Therefore, from the perspective 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] <sol.Al: less than 1.0%> Al is contained for the purpose of deoxidation or obtaining retained γ. The lower limit of sol.Al is not particularly specified, but in order to perform deoxidation stably, the sol.Al content is preferably 0.005% or more. On the one hand, when the sol.Al content is 1.0% or more, a large amount of Al-based coarse inclusions increases, and the elongation flange formability (hole expansion property) decreases. In addition, Al is an element that deteriorates the chemical conversion treatment property of the steel sheet. When the sol.Al content is 1.0% or more, good chemical conversion treatment property cannot be ensured even in the present invention. Therefore, the sol.Al content is less than 1.0%. The sol.Al content is preferably 0.80% or less, and 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 the elongation flange formability (hole expansion property). Therefore, it is necessary to limit its content. Thus, 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 is not necessary to contain N, but it is very costly 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 treatment property cannot be ensured. Therefore, [Si] / [Mn] is 0.35 or less. [Si] / [Mn] is preferably 0.32 or less, and more preferably 0.30 or less. Also, the lower limit is not particularly limited, but [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 fixes N in the steel as TiN, has the effect of improving hot ductility and the effect of improving the hardenability of B. In addition, there is an effect of refining the structure due to the precipitation of TiC. In order to obtain 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, when the Ti content exceeds 0.1%, it causes an increase in rolling load and a decrease in ductility due to an increase in precipitation strengthening amount. Therefore, when Ti is contained, the Ti content is 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 one hand, when the B content exceeds 0.001%, enrichment to oxides occurs during annealing, promoting coarsening of the oxides and deteriorating the chemical conversion treatment performance. Therefore, when containing B, the B content should be 0.001% or less. Preferably, the B content is less than 0.0010%.

[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 steel plate surface and suppressing hydrogen intrusion into the steel plate. Cu is an element 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 0.005% or more of Cu. Further, from the perspective of improving the stress corrosion cracking resistance characteristics, it is more preferable to contain 0.05% or more of Cu. More preferably, the Cu content is 0.10% or more. Even more preferably, the Cu content is 0.25% or more, and even more preferably, it is 0.50% or more. However, if the Cu content becomes too high, it will cause the generation of surface defects. Therefore, when containing Cu, the Cu content should be 1% or less.

[0032] <Ni: 1% or less> Ni is also an element that has the effect of improving corrosion resistance, similar to Cu. In addition, Ni has the effect of suppressing the generation of surface defects that tend to occur when containing Cu. For this reason, it is desirable to contain 0.01% or more of Ni. More preferably, the Ni content is 0.04% or more, and even more preferably, it is 0.06% or more. However, if the Ni content becomes too high, the scale formation in the heating furnace becomes non-uniform, which instead causes the generation of surface defects. It also leads to an increase in cost. Therefore, when containing Ni, the Ni content should be 1% or less. Preferably, the Ni content is 0.5% or less, and more preferably, it is 0.3% or less.

[0033] <Cr: 1% or less> Cr can be contained due to its effect of improving the hardenability of steel and its effect of 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 still more preferably 0.06% or more. However, when the Cr content is excessive, 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 its effect of 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 still 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 treatment property of the cold-rolled steel sheet, when Mo is contained, the Mo content should be 0.5% or less.

[0035] <V: 0.5% or less> V can be contained due to its effect of improving the hardenability of steel, its effect of suppressing carbide formation in martensite and upper / lower bainite, its effect of refining the structure, and its effect of precipitating 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 still more preferably 0.010% or more. Even more preferably, the V content is 0.020% or more, and 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 should be 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 the bainite transformation through grain refinement, improving the bendability, and enhancing 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 should be 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 other 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 stress corrosion cracking 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, 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 oxidation and nitridation of the steel sheet surface layer, thereby suppressing a decrease in the content of C and B in the surface layer. Due to this effect, ferrite formation in the steel sheet surface layer is suppressed, the strength is increased, and the fatigue resistance characteristics are improved. From such a viewpoint, 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, when the Sn content exceeds 0.1%, the castability deteriorates. Also, Sn segregates at the prior γ grain boundaries, and the stress corrosion cracking resistance deteriorates. 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 steel sheet surface layer, thereby suppressing a decrease in the content of C and B in the surface layer. Due to this effect, ferrite formation in the steel sheet surface layer 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 Sb segregates at the prior γ grain boundaries, and the stress corrosion cracking resistance deteriorates. Therefore, when Sb is contained, the Sb content should be 0.1% or less.

[0041] <REM: 0.0050% or less> REM is an element that suppresses the adverse effects of sulfides on the stretch flange formability by spheroidizing the shape of sulfides and improves the stretch flange formability. In order to obtain these effects, it is preferable that the REM content is 0.0005% or more. The REM content is more preferably 0.0010% or more, and still more preferably 0.0020% or more. On the other hand, when the REM content exceeds 0.0050%, the improvement effect of the stretch 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 the above optional component is contained 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 the above optional element is contained 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] The steel sheet of the present invention has a tensile strength (TS) of 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 30% or more. Thereby, the stability of press forming is remarkably improved.

[0046] For the evaluation of tensile properties, JIS No. 5 tensile test pieces are sampled from the center position of the plate width, and a tensile test (in accordance with JIS Z2241 (2011)) is carried out with N = 3. Each evaluation is based on the average value of three points. A steel plate with a tensile strength of 780 MPa or more is defined as a high-strength steel plate. For the total elongation EL, when the TS is 780 MPa or more and less than 980 MPa, a steel plate with 16.0% or more, when the TS is 980 MPa or more and less than 1180 MPa, a steel plate with 14.0% or more, and when the TS is 1180 MPa or more, a steel plate with 12.0% or more is defined as a steel plate with excellent ductility. Also, in order to ensure the necessary hole expansion property in practical use, it is an essential condition of the present invention that the hole expansion rate λ (%) obtained by the hole expansion test in accordance with the provisions of JFST1001 ({(d - d0) / d0}×100) is 30% or more. The hole expansion rate λ (%) may be less than 45%.

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

[0048] <Area ratio of polygonal ferrite: 10% or more and 80% or less> From the viewpoint of ensuring high ductility, the area ratio of polygonal ferrite is 10% or more, and preferably 20% or more in order to obtain higher ductility. On the other hand, when the polygonal ferrite exceeds 80%, the desired strength cannot be obtained. Therefore, the area ratio of polygonal ferrite is 80% or less, preferably 75% or less, and more preferably 70% or less.

[0049] <Total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less> In order to obtain the desired strength, the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more, and preferably 15% or more in order to obtain higher strength. On the other hand, when the total area ratio of upper bainite, tempered martensite, and lower bainite exceeds 70%, the ductility decreases due to excessive high strength. Therefore, the area ratio is 70% or less. More preferably 65% or less, and even more preferably 60% or less.

[0050] <Volume ratio of retained austenite (retained γ): 3% or more and 15% or less> When the volume ratio of retained austenite is less than 3%, it may not be possible to ensure the desired ductility. From the viewpoint of ductility, the volume ratio of retained austenite is 3% or more, preferably 5% or more. On the other hand, when the volume ratio of retained austenite exceeds 15%, the stretch flange formability (hole expansion property) decreases. Therefore, the volume ratio of retained austenite is 15% or less. The volume ratio of retained austenite is preferably 13% or less.

[0051] <Area ratio of quenched martensite: 15% or less (including 0%)> Since the hard quenched martensite structure reduces λ, it is necessary to suppress its area ratio. In order to obtain the required λ in practical use, the area ratio of quenched martensite is 15% or less. In order to obtain λ more stably, the area ratio of quenched martensite is preferably 12% or less, more preferably 10% or less. The area ratio of quenched martensite may be 0% or may be 3% or more.

[0052] <Remaining structure> Regarding the steel structure, except for the above, it is preferably composed 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 below.

[0053] <The maximum concentration of P [Pm] within 1 μm in the plate thickness direction from the steel plate surface 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 intensive studies on various elements affecting the phosphatability, their surface enrichment amounts, and the types of oxides formed during annealing, it was revealed that even under production conditions where no oxide formation was observed, sufficient phosphatability could not be ensured. For the steel sheet with ensured phosphatability, as a result of quantitatively evaluating the maximum concentration of P in the vicinity of the surface layer by the method described below, when analyzing the emission intensity of P measured by GDS (glow discharge analysis method) in the thickness direction from the surface of the steel sheet, the maximum concentration of P [Pm] within 1 μm from the steel sheet surface in the thickness direction is 0.025 mass% or more, and it was found that good phosphatability can be ensured by having a steel structure that satisfies formula (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 phosphate crystals after the phosphating 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 phosphatability. [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] / [P] is preferably 1.7 or more, more preferably 1.9 or more. Also, although the upper limit is not particularly limited, [Pm] / [P] 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 in a range of 25 μm × 20 μm are observed at 5000 times magnification with SEM, and the taken tissue photographs are quantified by image analysis. Polygonal ferrite is targeted at relatively equiaxed ferrite with almost no carbides inside. It is the region that appears darkest in SEM. Upper bainite is a ferrite structure that involves the formation of carbides or retained austenite that appears white under SEM inside. When it is difficult to distinguish upper bainite from polygonal ferrite, the ferrite regions with an aspect ratio ≤ 2.0 are regarded as polygonal ferrite, and the regions with an aspect ratio > 2.0 are classified as upper bainite, and the area ratio is calculated. Here, for the aspect ratio, the major axis length a when the particle length is the longest is determined, the particle length when crossing 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 that involve a lath-like lower structure and carbide precipitation inside under SEM. Quenched martensite (fresh martensite) is a massive region that appears white with no visible lower structure inside under SEM. The remaining structure is a structure containing at least one of unrecrystallized ferrite, carbide, and pearlite. By SEM respectively, unrecrystallized ferrite is ferrite with a black contrast containing the deformed structure introduced by rolling processing, and carbide and pearlite are structures that can be confirmed with a white contrast. Carbide is a structure with a particle diameter of 1 μm or less, and since pearlite is a lamellar (layered) structure, it can be distinguished.

[0055] The volume ratio of retained austenite is obtained by chemical polishing at the 1 / 4 thickness position from the surface layer and using X-ray diffraction. A Co-Kα ray source is used for the incident X-ray, and the volume ratio 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 retained austenite is randomly distributed, the volume ratio of retained austenite obtained by X-ray diffraction can be regarded as the area ratio of retained austenite.

[0056] The surface enrichment amount of P in the surface enrichment part on the steel plate surface is measured by sputtering analysis in the depth direction (plate thickness 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). Then, the surface enrichment amount of P is measured, and the maximum concentration [Pm] of P within 1 μm in the plate thickness direction from the steel plate surface is obtained using a previously obtained calibration curve. Here, under these measurement conditions, 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-mentioned measurement time of 150 s, the value obtained by converting the highest intensity value of P into mass% using the calibration curve is defined as the maximum concentration ([Pm]). As a method for converting to this mass%, a standard material having a known P amount 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 P amount is determined, and thereby the intensity of P in the measured example is converted into concentration. In FIG. 1, Pi is the P content (mass%) in the steel plate.

[0057] (Manufacturing method of steel plate) Next, the manufacturing method of the steel plate of the present invention will be described. <First Embodiment> The manufacturing method of the steel plate according to the first embodiment of the present invention is a manufacturing method of a steel plate 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 plate. The above annealing is performed on the above cold-rolled steel plate in a furnace atmosphere with a dew point of -40°C or lower at A c1 point + 20°C or higher A c3It is below the specified point, heated to a soaking temperature equal to or higher than Tc calculated by formula (3), and held at the soaking temperature for 30 to 500 s in a soaking holding step; cooled to the first cooling stop temperature in the temperature range from the soaking temperature to 350 to 550 °C at a first average cooling rate of 2 to 50 °C / s in a first cooling step; after stopping the cooling at the first cooling stop temperature, held in the temperature range of 350 to 550 °C for 10 to 60 s, and then cooled to the second cooling stop temperature of 200 to 420 °C at a second average cooling rate of 2 to 50 °C / s in a second cooling step; and held at the second cooling stop temperature for 60 to 3000 s in an isothermal holding step. This is a method for manufacturing a steel plate. Tc (°C) = 663 - 1.2 × exp(20 / t) × Tdp ··· Formula (3) Here, t represents the holding time (soaking holding time) (s) at the soaking temperature, and Tdp represents the dew point (°C).

[0058] <Hot rolling> Methods for hot rolling a steel slab include a method of heating the slab and then rolling it, a method of directly rolling the slab after continuous casting without heating it, a method of subjecting the slab after continuous casting to a short-time heat treatment and then rolling it, 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 is A r3 should be above the transformation point. Also, the finish rolling temperature is A r3 should be at or below the transformation point + 200 °C. Also, the coiling temperature should be 400 °C or higher. Also, the coiling temperature should be 720 °C or lower. The coiling temperature is preferably controlled from the perspective of suppressing sheet thickness fluctuations 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. Note that A r3 The transformation point can be calculated from the components of the steel plate and the following empirical formula (A). A r3Point (°C) = 910 - 310×[C] - 80×[Mn] - 20×[Cu] - 15×[Cr] - 55×[Ni] - 80×[Mo] ··· Formula (A) (In the above formula, [M] is the content (% by mass) of element M in the steel slab, and the value of an element not contained is zero (0).)

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

[0060] <Cold rolling> Cold rolling may be performed according to a conventional method, and the rolling ratio (cumulative rolling ratio) may be 30% or more. Also, the rolling ratio (cumulative rolling ratio) may be 85% or less. 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 more. When the rolling load is high, it is possible to perform a softening annealing treatment at 450 to 730 °C in a CAL (Continuous Annealing Line) or a BAF (Box Annealing Furnace).

[0061] <Annealing> For cold-rolled steel sheets (cold-rolled steel sheets) manufactured according to a conventional method, annealing is performed 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 step: In a furnace atmosphere with a dew point of -40 °C or lower, at a temperature above Ac1 + 20 °C and below Ac3, and heated to a soaking temperature of Tc or higher, and held at the soaking temperature for 30 to 500 s] c1 Point + 20 °C or higher and below Ac3 c3 Point, and heated to a soaking temperature of Tc or higher, and held 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, and the chemical conversion treatability cannot be satisfied by the technology of the present disclosure. Therefore, the dew point is -40 °C or lower. The lower limit is not particularly limited, but the dew point is preferably -70 °C or higher, and 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 A c1 point + 20°C or higher and A c3 point or lower.

[0064] Furthermore, by setting the soaking temperature to Tc (°C) or higher, the surface enrichment amount of P in the surface enrichment part 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 Formula (3). 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). When the soaking temperature is less than Tc (°C), a predetermined surface enrichment amount of P cannot be ensured, and the chemical conversion treatability deteriorates. Therefore, in a furnace atmosphere where the dew point is -40°C or lower, the soaking temperature is A c1 point + 20°C or higher and A c3 point or lower, and Tc (°C) or higher.

[0065] In addition, when the time for holding at the above soaking temperature (soaking holding time) is less than 30 seconds, austenite formation at the above soaking temperature is not sufficiently performed, polygonal ferrite increases, and the total area ratio of the desired upper bainite, tempered martensite, and lower bainite cannot be obtained, and the desired strength may not be obtained, or retained austenite cannot be sufficiently obtained, and the desired ductility may not be ensured. On the other hand, when the time for holding at the above soaking temperature (soaking holding time) exceeds 500 seconds, significant coarsening of the structure occurs, and thus the desired strength may not be ensured. Therefore, the time for holding at the above annealing temperature (soaking time) is 30 to 500 seconds. The time for holding at the soaking temperature (soaking time) is preferably 60 seconds or longer, more preferably 100 seconds or longer. Also, the time for holding at the soaking temperature (soaking time) is preferably 400 seconds or shorter, more preferably 300 seconds or shorter.

[0066] Note that the above Ac1 and A c3 A is 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: Cool from the soaking temperature to the first cooling stop temperature in the temperature range of 350 to 550°C at 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 below the A c3 point, and after holding at the soaking temperature above Tc (after the above soaking holding step), cool in the temperature range from the soaking temperature to the first cooling stop temperature in the range 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 should be 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 becomes too large, the plate shape deteriorates, so it should be 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, stay at a residence temperature of 350 to 550°C for 10 s or more and 60 s or less] When the temperature is below the above-mentioned first cooling stop temperature and within the temperature range (holding temperature) from 350°C to 550°C, upper bainite can be formed, a predetermined amount of retained austenite can be obtained, and desired ductility can be obtained. Bainite transformation has a latency period, and in order to obtain a desired amount of bainite, it must be held at this 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 deviates from 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 cannot be obtained. On the other hand, when the holding time exceeds 60 s, the enrichment of C from bainite to massive untransformed γ proceeds, leading to an increase in the remaining amount of massive quenched martensite 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] Note that according to the desired properties, the second cooling step (1) can be omitted. In that case, heat to a soaking temperature of A + 20°C or higher and A or lower and at a temperature not lower than Tc, and after holding at the soaking temperature for 30 to 500 s in a soaking holding step, the treatment in the second cooling step (2) may be performed. The manufacturing method in which the second cooling step (1) is omitted will be described in the second embodiment described later. c1 +20°C or higher A c3 or lower, and heat to a soaking temperature not lower than Tc, and after holding at the soaking temperature for 30 to 500 s in a soaking holding step, the treatment in the second cooling step (2) may be performed. The manufacturing method in which the second cooling step (1) is omitted will be described in the second embodiment described later.

[0070] [Second cooling step (2): Cool to a second cooling stop temperature of 200 to 420°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 a second cooling stop temperature of 200°C or higher and 420°C or lower is less than 2°C / s, the bainite transformation proceeds excessively, resulting in an excessive increase in retained austenite, and the desired amount of 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 residence end temperature to the second cooling stop temperature of 200°C or higher and 420°C or lower is set to 2°C / s or higher. The second 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 (second average cooling rate) in this temperature range is set to 50°C / s or lower. Preferably, it is 40°C / s or lower. If the second cooling stop temperature exceeds 420°C, the tempered martensite or lower bainite does not reach the predetermined area ratio, and the area ratio of the quenched martensite after annealing increases, resulting in deteriorated hole expansion properties. For this reason, the second cooling stop temperature is set to 420°C or lower. The second cooling stop temperature is preferably 400°C or lower. On the other hand, if the second cooling stop temperature is less than 200°C, the tempering effect of martensite cannot be sufficiently obtained, not only does the quenched martensite increase, but also the C enrichment in the retained γ is suppressed, deteriorating the ductility. For this reason, the second cooling stop temperature is set to 200°C or higher. The second cooling stop temperature is preferably 220°C or higher. Here, the second average cooling rate is "(residence end temperature (°C) - second cooling stop temperature (°C)) / cooling time (seconds) from the residence end temperature to the second cooling stop temperature".

[0071] [Isothermal holding process: Hold at the second cooling stop temperature for 60 to 3000 s] The holding at the second cooling stop temperature is carried out from the viewpoint of promoting the strength adjustment by the tempering treatment of the formed martensite and the C enrichment in the retained γ. If it is less than 60 s, the tempering is insufficient and martensite with high strength is formed, and since the C enrichment in the retained γ is suppressed, the desired strength and ductility may not be ensured. On the other hand, if the holding time at the second cooling stop temperature exceeds 3000 s, excessive tempering of martensite may occur, and the desired strength may not be ensured. Also, if the holding time at the second cooling stop temperature exceeds 3000 s, the decomposition reaction of the retained austenite is promoted, so the desired volume ratio of the retained austenite cannot be obtained, and the desired ductility cannot be obtained. Therefore, the holding time at the second cooling stop temperature shall be 60 s or more and 3000 s or less. The holding time at the second cooling stop temperature is preferably 100 s or more, more preferably 150 s or more. Also, the holding time at the second cooling stop temperature is preferably 2500 s or less, more preferably 2000 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 is performed to a soaking temperature of Tc or higher calculated by formula (3), and a soaking holding step of holding at the soaking temperature for 30 to 500 s, a cooling step of cooling from the soaking temperature to a cooling stop temperature of 200 to 420°C at an average cooling rate of 2 to 50°C / s, and an isothermal holding step of holding at the cooling stop temperature for 60 to 3000 s. 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 hot rolling, pickling, cold rolling, and soaking holding steps of 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. However, 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 isothermal holding step in the annealing of the second embodiment can be set under substantially the same conditions as the isothermal 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: Cool at an average cooling rate of 2 to 50 °C / s to a cooling stop temperature of 200 to 420 °C] After the treatment in the soaking holding step, it is necessary to cool rapidly so that the bainite transformation does not proceed excessively. When the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 200 °C or higher and 420 °C or lower is less than 2 °C / s, the bainite transformation proceeds excessively, and the desired amount of quenched martensite cannot be ensured, which may lead to a decrease in strength. Also, when the average cooling rate is less than 2 °C / s, the C enrichment associated with the bainite transformation proceeds excessively, increasing the amount of hard fresh martensite and possibly preventing the desired hole expansion property from being obtained. Therefore, the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 200 °C or higher and 420 °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 420 °C, carbide formation occurs significantly, residual γ cannot be ensured, and the ductility deteriorates. For this reason, the cooling stop temperature is set to 420 °C or lower. On the other hand, when the cooling stop temperature is less than 200 °C, the tempering effect of martensite cannot be sufficiently obtained, not only does the quenched martensite increase, but also the C enrichment in the residual γ is suppressed, deteriorating the ductility. For this reason, the cooling stop temperature is set to 200 °C or higher. Here, the average cooling rate is "(soaking temperature (°C) - cooling stop temperature (°C) / cooling time (seconds) from the soaking temperature to the cooling stop temperature)".

[0075] [Plate thickness] The steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more. Further, the 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 formed by performing at least one of forming and joining on the steel sheet of the present invention. Further, the method for manufacturing the member of the present invention includes a step of forming a member by performing at least one of forming and joining on the steel sheet of the present invention.

[0078] The steel sheet 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 sheet 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. Further, weight reduction is possible by using the member of the present invention. Therefore, the member of the present invention can be suitably used for, for example, vehicle body frame parts.

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

Example

[0080] <Example 1> A slab manufactured 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, cold rolling was performed at a rolling reduction of 50% to manufacture a cold-rolled steel sheet with a thickness of 1.4 mm. This was treated under the annealing conditions shown in Table 2 to manufacture the steel sheet of the present invention and the steel sheet of the comparative example.

[0081]

Table 1

[0082] The measurement of the steel structure was carried out by the following method. The measurement results are shown in Table 3. 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 was cut out, mirror-polished, then corroded with 1 vol% nital, and at the 1 / 4 thickness position, 10 fields of view in the range of 25 μm × 20 μm were observed at 5000 times magnification with SEM, and the tissue photos taken were quantified by image analysis. Polygonal ferrite is targeted at relatively equiaxed ferrite with almost no carbides inside. It is the region that appears darkest in SEM. Upper bainite is a ferrite structure with the formation of carbides or retained austenite that appears white in SEM inside. When it is difficult to distinguish upper bainite from polygonal ferrite, the region of ferrite with an aspect ratio ≦ 2.0 was regarded 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 determined by obtaining the major axis length a where the particle length is the longest, and taking the particle length when cutting across the particle longest in the direction perpendicular to it as the minor axis length b, and a / b was taken as the aspect ratio. Tempered martensite and lower bainite are regions with a lath-like lower structure and carbide precipitation inside in SEM. Quenched martensite (fresh martensite) is a massive region that appears white with no visible lower structure inside in SEM. The remaining structure refers to a structure containing at least one of unrecrystallized ferrite, carbide, and pearlite. In SEM, unrecrystallized ferrite can be confirmed as ferrite with a black contrast containing the deformed structure introduced by rolling. Also, carbide and pearlite are structures that can be confirmed with a white contrast. Carbide is a structure with a particle diameter of 1 μm or less, and pearlite is a lamellar (layered) structure, so they can be distinguished.

[0083] The volume fraction of retained austenite is determined by chemical polishing the position at 1 / 4 thickness from the surface layer and measuring it by 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 taken, 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, it is judged to be excellent in ductility if it is 16.0% or more, when TS is 980 MPa or more, it is 14.0% or more, and when TS is 1180 MPa or more, it is 12.0% or more. 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 a value of 30% or more was judged to be excellent in hole expandability. The measurement results are shown in Table 3.

[0085] For the steel plate after annealing, 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 having 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 carried out 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, in the chemical conversion treatment process: the temperature of the chemical conversion treatment solution; 35°C, treatment time; 120 seconds for chemical conversion treatment. Note that 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

[0087]

Table 2

[0088]

Table 3

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

[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, cold rolling was carried out at a rolling reduction of 50% to produce a cold-rolled steel sheet with a thickness of 1.4 mm, which 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 formation 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 formation treatment property, it has been found that they are excellent in strength, ductility, hole expansion property, and formation 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: 0.331% 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 80% or less, total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less, volume ratio of retained austenite: 3% or more and 15% or less, area ratio of quenched martensite: 15% 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 in the plate thickness direction from the steel sheet surface is 0.025 mass% or more and satisfies 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 The steel sheet according to claim 1, containing one or more selected from among them.

3. A member made using the steel sheet according to claim 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 claim 1 or 2, and then annealing is performed on the obtained cold-rolled steel sheet. The annealing is as follows. With respect to 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 in the temperature range from the soaking temperature to 350 to 550 °C at a first average cooling rate of 2 to 50 °C / s. After stopping the cooling at the first cooling stop temperature, after retaining for 10 to 60 s in the temperature range of 350 to 550 °C, a second cooling step of cooling to the second cooling stop temperature of 200 to 420 °C at a second average cooling rate of 2 to 50 °C / s. An isothermal holding step of holding for 60 to 3000 s at the second cooling stop temperature. The area ratio of polygonal ferrite is 10% or more and 80% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more and 70% or less, the volume ratio of retained austenite is 3% or more and 15% or less, the area ratio of quenched martensite is 15% or less (including 0%), and the area ratio of the remaining structure is 5% or less. The steel sheet has a maximum concentration of P [Pm] within 1 μm in the plate thickness direction from the steel sheet surface of 0.025 mass% or more and satisfies formula (2). A method for manufacturing a steel sheet. [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... Formula (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, which comprises performing hot rolling, pickling, and cold rolling on a steel slab having the component composition described in claim 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 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 at an average cooling rate of 2 to 50 °C / s from the soaking temperature to a cooling stop temperature of 200 to 420 °C, An isothermal holding step of holding for 60 to 3000 s at the cooling stop temperature. The area ratio of polygonal ferrite is 10% or more and 80% or less, the total area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more and 70% or less, the volume ratio of retained austenite is 3% or more and 15% or less, the area ratio of quenched martensite is 15% or less (including 0%), and the area ratio of the remaining structure is 5% or less. The steel sheet has a maximum concentration of P [Pm] within 1 μm in the plate thickness direction from the steel sheet surface of 0.025 mass% or more and satisfies formula (2). A method for manufacturing a steel sheet. [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... Formula (3) Here, in formula (3), t is the holding time (s) at the soaking temperature, and Tdp indicates the dew point (°C).

6. A method for manufacturing a member, which includes a step of subjecting the steel sheet described in claim 1 or 2 to at least one of forming processing and joining processing to form a member.

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