Steel sheet, member and production methods for those

Abstract

Provided are a steel sheet, a member, and methods for manufacturing them, the steel sheet having excellent ductility, flangeability, and phosphatability, and a tensile strength of 780 MPa or more. A steel sheet has a chemical composition that contains, in mass%, C, Si, Mn, P, S, sol. Al, and N in predetermined ranges and that satisfies formula (1), and has a steel microstructure having area fractions of polygonal ferrite and so forth within predetermined ranges, in which the maximum concentration of P [Pm] within 1 µm from the surface of the steel sheet in the thickness direction is 0.025 mass% or more, formula (2) is satisfied, and a cumulative amount of Si enrichment within 1 µm from the surface of the steel sheet in the thickness direction is 120 or less, Si / Mn≤0.35 1,000×B / Mn≤0.70 Pm / P≥1.5

Description

Technical Field

[0001] The present invention relates to a steel sheet, a member, and methods for manufacturing them, the steel sheet being suitable for press-formed products having complex shapes, which are formed through a press-forming process and used for, for example, automobiles and home appliances, and having excellent phosphatability.Background Art

[0002] In response to the tightening of global CO 2 emission regulations, there is an increasing demand for weight reduction of automobile bodies through the use of higher-strength steel sheets for automobiles. Thus, 590 MPa-grade or higher high strength steel sheets are increasingly used for bodies and seat parts in place of the existing 440 MPa-grade cold rolled steel sheets. Typically, higher strength of steel sheets leads to lower press formability, such as ductility and stretch flangeability. This increases the likelihood of cracking during press forming and limits the flexibility in shape design. As a result, such steel sheets are limited to use in parts with simple shapes. It is thus important to enhance the strength of the steel sheets while maintaining or improving their formability in order to use high strength steel sheets for complex-shaped parts.

[0003] In light of this background, transformation-induced plasticity (TRIP) steels, in which retained austenite (retained γ) is dispersed in microstructures of steel sheets, have been developed as one technique to improve the ductility of steel sheets. In TRIP steels, large amounts of Si are added to form retained γ in the microstructures. For example, Patent Literature 1 discloses that a steel sheet having a high ductility of TS × El ≥ 21,000 MPa·% and a high stretch flange formability of 70% or more is obtained by subjecting an annealed steel containing C: 0.04% to 0.12%, Si: 0.8% to 2.5%, and Mn: 0.5% to 2.0% to austempering (carbon partitioning associated with bainite transformation) in which the steel is held at 300°C to 500°C for 10 to 900 seconds to form 2% to 10% retained γ.

[0004] It is known that an increase in Si content leads to the enrichment of Si on the surface of a steel sheet after annealing to result in the formation of Si-based oxides, thereby leading to a deterioration in phosphatability. To address this issue, for example, Patent Literature 2 discloses a method for improving phosphatability by adding Ni to inhibit the enrichment of Si on the surface of a steel sheet.

[0005] Patent Literature 3 discloses a method for improving phosphatability by appropriately controlling the amount of Mn, which is enriched on the surface together with Si, in such a manner that the Si / Mn ratio is 0.40 or less, thereby forming a Mn-Si complex oxide on the surface.

[0006] Patent Literature 4 discloses a method for improving phosphatability by directly removing Si-based oxides through pickling or brushing after annealing.Citation ListPatent Literature

[0007] PTL 1: Japanese Patent No. 5515623 PTL 2: Japanese Patent No. 2951480 PTL 3: Japanese Patent No. 3889768 PTL 4: Japanese Unexamined Patent Application Publication No. 2003-201538 Summary of InventionTechnical Problem

[0008] As described above, the addition of Si is effective in improving the ductility of high strength steel sheets. However, when Si is actively used to ensure high workability, there is a trade-off relationship between the Si content and the phosphatability of the steel sheet. The methods disclosed in Patent Literatures 2 and 4 are effective as methods for improving the phosphatability of steels with a high Si content. There has also been a demand for the establishment of other techniques in which the alloying elements to be incorporated, the annealing conditions, and so forth are adjusted.

[0009] Furthermore, the inventors' investigations revealed that even in the method disclosed in Patent Literature 3, good phosphatability is not necessarily ensured, and that when the steel contains a certain amount of B, the enrichment of Si on the surface of the steel sheet is promoted, resulting in a deterioration in phosphatability.

[0010] As described above, the above-mentioned techniques are not yet sufficient as techniques for manufacturing high strength steel sheets having excellent ductility, phosphatability, and flangeability.

[0011] The present invention has been accomplished in consideration of the above circumstances. It is an object of the present invention to provide a steel sheet, a member, and methods for manufacturing them, the steel sheet having excellent ductility, flangeability, and phosphatability, and having excellent ductility at a tensile strength of 780 MPa or more.

[0012] Here, the tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).

[0013] Excellent ductility indicates that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies any one of the following (A) to (C). (A) For TS: 780 MPa or more and less than 980 MPa, EL: 16.0% or more. (B) For TS: 980 MPa or more and less than 1,180 MPa, EL: 14.0% or more. (C) For TS: 1,180 MPa or more, EL: 12.0% or more.

[0014] Excellent flangeability indicates that the hole expansion ratio λ (%) (= {(d - d 0 ) / d 0 } x 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 in order to ensure the flangeability required for practical use is 30% or more.

[0015] Excellent phosphatability indicates that after degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreasing agent: FC-E2011 manufactured by Nihon Parkerizing Co., Ltd.), surface conditioning (pH: 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nihon Parkerizing Co., Ltd.), and then zinc phosphate treatment (temperature of zinc phosphate treatment solution: 35°C, treatment time: 120 seconds, zinc phosphate treatment solution: Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd.) using a zinc phosphate treatment solution, the area where a steel substrate is exposed is less than 10% of the total area. Solution to Problem

[0016] To achieve the above object, the inventors have conducted intensive studies on the influence of steel components, heat treatment conditions, and microstructures on ductility and phosphatability for various steel sheets having a tensile strength of 780 MPa or more. As a result, it has been found that a high-strength cold rolled steel sheet having excellent ductility, flangeability, and phosphatability can be obtained by having a chemical composition containing, 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%, Ti: 0.005% to 1.000%, and B: 0.0010% to 0.0030%, formula (1) and formula (2) below being satisfied, and the balance being iron and incidental impurities, and by including a steel microstructure having an area fraction of polygonal ferrite of 10% or more and 80% or less, a total area fraction of upper bainite, tempered martensite, and lower bainite of 10% or more and 70% or less, a volume fraction of retained austenite (retained γ) of 3% or more and 15% or less, an area fraction of quenched martensite of 15% or less (including 0%), and a remaining microstructure, in which the steel microstructure is such that when the emission intensity of P measured from the surface of the steel sheet in the thickness direction by glow discharge spectrometry is analyzed, the maximum concentration of P [Pm] within 1 µm from the surface of the steel sheet in the thickness direction is 0.025 mass% or more, such that formula (3) is satisfied, and such that a cumulative amount of Si enrichment within 1 µm from the surface of the steel sheet in the thickness direction is 120 or less, Si / Mn ≤ 0.35 1 , 000 × B / Mn ≤ 0.70 Pm / P ≥ 1.5 where in formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%), in formula (2), [B] is the B content (mass%), and [Mn] is the Mn content (mass%), and in formula (3), [P] is the P content (mass%).

[0017] The present invention has been made on the basis of these findings, and the gist thereof is described below. [1] A steel sheet has: a chemical composition containing, 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%, Ti: 0.005% to 1.000%, and B: 0.0010% to 0.0030%, formula (1) and formula (2) below being satisfied, and the balance being iron and incidental impurities; and a steel microstructure having: an area fraction of polygonal ferrite: 10% or more and 80% or less, a total area fraction of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less, a volume fraction of retained austenite: 3% or more and 15% or less, and an area fraction of quenched martensite: 15% or less (including 0%), in which the maximum concentration of P [Pm] within 1 µm from the surface of the steel sheet in the thickness direction is 0.025 mass% or more, formula (3) is satisfied, and a cumulative amount of Si enrichment within 1 µm from a surface layer in the thickness direction is 120 or less, Si / Mn ≤ 0.35 1 , 000 × B / Mn ≤ 0.70 Pm / P ≥ 1.5 where in formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%), in formula (2), [B] is the B content (mass%), [Mn] is the Mn content (mass%), and in formula (3), [P] is the P content (mass%). [2] In the steel sheet described in [1], the chemical composition further contains, in mass%, one or two or more selected from: Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less. [3] A member made using the steel sheet described in [1] or [2]. [4] A method for manufacturing a steel sheet includes subjecting a steel slab having the chemical composition described in [1] or [2] to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, in which the annealing includes: a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a first cooling step of performing cooling to a first cooling stop temperature of 350°C to 550°C at a first average cooling rate: 2 to 50 °C / s in a temperature range from the soaking temperature to the first cooling stop temperature, a second cooling step of, after stopping cooling at the first cooling stop temperature, performing holding in a temperature range of 350°C to 550°C for 10 to 60 seconds and then performing cooling to a second cooling stop temperature of 200°C to 420°C at a second average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the second cooling stop temperature for 60 to 3,000 seconds, Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (s) at the soaking temperature, and Tdp is the dew point (°C). [5] A method for manufacturing a steel sheet includes subjecting a steel slab having the chemical composition described in [1] or [2] to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, in which the annealing includes: a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a cooling step of performing cooling from the soaking temperature to a cooling stop temperature of 200°C to 420°C at an average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the cooling stop temperature for 60 to 3,000 seconds, Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (s) at the soaking temperature, and Tdp is the dew point (°C). [6] A method for manufacturing a member includes a step of subjecting the steel sheet described in [1] or [2] to at least one of forming or joining to provide a member. Advantageous Effects of Invention

[0018] According to the present invention, it is possible to provide a steel sheet and a member, having a high tensile strength TS of 780 MPa or more, excellent ductility, flangeability, and phosphatability.

[0019] When the steel sheet of the present invention is used for frame members of automobile bodies, difficult-to-form members having complex shapes can be manufactured by cold pressing. This can greatly contribute to reducing the weight of the automobile bodies. There is no need for expensive alloying elements or post-treatment after annealing to improve phosphatability, making it possible to reduce material costs.Brief Description of Drawings

[0020] [Fig. 1] Fig. 1 is a graph for explaining the maximum concentration of P [Pm] of the present invention. [Fig. 2] Fig. 2 is a graph for explaining a cumulative amount of Si enrichment according to the present invention. Description of Embodiments

[0021] The present invention will be specifically described below. The present invention is not limited to the following embodiments.(Steel Sheet)

[0022] A steel sheet of the present invention has a high tensile strength TS of 780 MPa or more, having excellent ductility, flangeability, and phosphatability, has a chemical composition containing, 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%, Ti: 0.005% to 1.000%, and B: 0.0010% to 0.0030%, formula (1) and formula (2) below being satisfied, and the balance being iron and incidental impurities, and includes a steel microstructure having an area fraction of polygonal ferrite: 10% or more and 80% or less, a total area fraction of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less, a volume fraction of retained austenite: 3% or more and 15% or less, and an area fraction of quenched martensite: 15% or less (including 0%), in which when the emission intensity of P measured from a surface in the thickness direction by glow discharge spectrometry is analyzed, the maximum concentration of P [Pm] within 1 µm from the surface of the steel sheet in the thickness direction is 0.025 mass% or more, formula (3) below is satisfied, and a cumulative amount of Si enrichment within 1 µm from the surface of the steel sheet in the thickness direction is 120 or less, Si / Mn ≤ 0.35 1 , 000 × B / Mn ≤ 0.70 Pm / P ≥ 1.5 where in formula (1), [Si] is a Si content (mass%), and [Mn] is a Mn content (mass%), in formula (2), [B] is a B content (mass%), [Mn] is a Mn content (mass%), and in formula (3), [P] is a P content (mass%).

[0023] Hereinafter, the steel sheet of the present invention will be described in the order of the chemical composition and the steel microstructure. First, the reasons for the limitation of the chemical composition in the present invention will be described. In the following description, all percentages (%) indicating the components of steel are expressed in mass% unless otherwise specified.<C: 0.05% to 0.25%>

[0024] C is contained from the viewpoints of ensuring a predetermined strength through transformation strengthening and also ensuring a predetermined amount of retained austenite (retained γ) to improve ductility. When the C content is less than 0.05%, these effects cannot be sufficiently provided.

[0025] The upper limit of the C content is set to 0.25% in consideration of flangeability, which is important for press formability, and weldability, which is important for spot welding or laser welding when formed automobile members are assembled into automobile bodies.

[0026] For this reason, the C content is set to 0.05% to 0.25%. The C content is preferably 0.08% or more, more preferably 0.10% or more. The C content is preferably 0.22% or less, more preferably 0.20% or less.<Si: 0.30% to 1.50%>

[0027] Si is contained from the viewpoint of strengthening ferrite to increase strength, and of inhibiting the formation of carbides in martensite and bainite and ensuring a predetermined amount of retained γ to improve ductility. When the Si content is less than 0.30%, these effects cannot be sufficiently provided.

[0028] When the Si content is more than 1.50%, good phosphatability cannot be ensured even by the manufacturing method specified in the present invention.

[0029] For this reason, 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. The Si content is preferably 1.20% or less, more preferably 1.00% or less.<Mn: 1.5% to 4.5%>

[0030] Mn is contained from the viewpoint of improving the hardenability of a steel sheet to promote an increase in strength through transformation strengthening, and from the viewpoint of inhibiting the formation of carbides in bainite, similar to Si, to promote the formation of retained austenite that contributes to ductility, thereby improving ductility. To provide these effects, the Mn content needs to be 1.5% or more.

[0031] When the Mn content is more than 4.5%, the bainite transformation is significantly delayed, a predetermined amount of retained austenite cannot be ensured, thereby reducing ductility. Furthermore, when the Mn content is more than 4.5%, it is difficult to inhibit the formation of coarse quenched martensite due to the lowering of the martensite start temperature, thereby resulting in a deterioration in stretch flange formability (flangeability).

[0032] For this reason, 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. The Mn content is preferably 3.5% or less, more preferably 3.0% or less.<P: 0.005% to 0.050%>

[0033] P is an element that strengthens steel. P is an element that can form a P-rich surface portion on the surface of the steel sheet after annealing by appropriately controlling the P content, thereby improving the phosphatability. From this point of view, the P content is set to 0.005% or more.

[0034] When the P content is high, the spot weldability is deteriorated. From this point of view, the P content is set to 0.050% or less.

[0035] Therefore, the P content is set to 0.005% to 0.050%. The P content is preferably 0.007% or more, more preferably 0.009% or more. The P content is preferably 0.040% or less, more preferably 0.030% or less.<S: 0.01% or Less>

[0036] S has the effect of improving descalability in hot rolling and the effect of inhibiting nitriding during annealing, but is an element that adversely affects spot weldability, bendability, and flangeability. To reduce these adverse effects, the S content is set to 0.01% or less at most, and preferably 0.0050% or less.

[0037] S need not be contained. However, reducing the S content to less than 0.0001% entails high costs. Thus, from the viewpoint of manufacturing costs, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more, even more preferably 0.0010% or more.<sol. Al: Less than 1.0%>

[0038] Al is contained for the purpose of deoxidization or obtaining retained γ. Although the lower limit of the sol. Al is not particularly specified, in order to perform stable deoxidation, the sol. Al content is preferably 0.005% or more.

[0039] When the sol. Al content is 1.0% or more, the number of coarse Al-based inclusions increases significantly, thereby leading to a deterioration in stretch flange formability (flangeability). Al is an element that degrades the phosphatability of steel sheets. When the sol. Al content is 1.0% or more, good phosphatability cannot be ensured even in the present invention. For this reason, the sol. Al content is set to less than 1.0%. The sol. Al content is preferably 0.80% or less, more preferably 0.06% or less.<N: Less than 0.015%>

[0040] N is an element that forms nitrides, such as BN, AlN, and TiN, in steel and reduces stretch flange formability (flangeability), and the N content needs to be limited. Thus, the N content is set to less than 0.015%. The N content is preferably 0.010% or less, more preferably 0.006% or less.

[0041] N need not be contained. However, reducing the N content to less than 0.0001% entails high costs. Thus, the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and even more preferably 0.001% or more.<Ti: 0.005% to 1.000%>

[0042] Ti has the effect of fixing N in steel in the form of TiN to improve hot ductility and provides the effect of improving the hardenability effect of B. The precipitation of TiC has the effect of refining the microstructure. To provide these effects, the Ti content is set to 0.005% or more. The Ti content is more preferably 0.010% or more. The Ti content is more preferably 0.020% or more.

[0043] A Ti content of more than 1.000% leads to an increase in rolling load and a decrease in ductility due to the increased amount of precipitation strengthening. Thus, the Ti content is set to 1.000% or less. The Ti content is preferably 0.080% or less, more preferably 0.050% or less.<B: 0.0010% to 0.0030%>

[0044] B is an element that improves the hardenability of steel, and has the advantage of facilitating the formation of tempered martensite and / or bainite with a specified area fraction. To provide this effect, the B content is set to 0.0010% or more.

[0045] A B content of more than 0.0030% results in the enrichment of B on the surface of the steel sheet during soaking and the coarsening of Mn-based oxides, leading to a deterioration in phosphatability. Thus, the B content is set to 0.0030% or less. The B content is preferably 0.0020% or less. Si / Mn ≤ 0.35

[0046] In formula (1), [Si] is the Si content (mass%), and [Mn] is the Mn content (mass%).

[0047] [Si] / [Mn] (Si / Mn ratio) determines the component ratio of Si to Mn in the surface oxide formed during annealing. Within the range of production conditions specified in the present invention, when the [Si] / [Mn] ratio is more than 0.35, good phosphatability cannot be ensured. For this reason, [Si] / [Mn] is set to 0.35 or less. [Si] / [Mn] is preferably 0.32 or less, more preferably 0.30 or less. The lower limit is not specifically limited; however, [Si] / [Mn] is preferably 0.10 or more, more preferably 0.15 or more. 1 , 000 × B / Mn ≤ 0.70

[0048] In formula (2), [B] is the B content (mass%), [Mn] is the Mn content (mass%).

[0049] B is an element that is enriched on the surface of the steel sheet during soaking. It has been newly found that B promotes the formation of coarse Mn-based oxides on the surface of the steel sheet, causing Mn-deficient regions around them, and promotes the formation of Si-based oxides on the surface of the steel sheet, thereby deteriorating phosphatability, even when [Si] / [Mn] ≤ 0.35 is satisfied. As a result of thorough investigation into this issue, it has been found that, with regard to the B content and the Mn content, satisfying the condition 1,000 × [B] / [Mn] ≤ 0.70 can inhibit the formation of Mn-deficient regions during soaking and control the formation of Si-based oxides to a level at which the phosphatability is not degraded. Thus, 1,000 × [B] / [Mn] is set to 0.70 or less. Preferably, 1,000 × [B] / [Mn] is 0.68 or less, more preferably 0.65 or less. The lower limit is not particularly limited. However, 1,000 × [B] / [Mn] is preferably 0.30 or more, more preferably 0.35 or more.

[0050] The chemical composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, with the balance containing iron (Fe) and incidental impurities. The steel sheet in the present invention preferably has a chemical composition with the balance being Fe and incidental impurities.

[0051] The chemical composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or two or more elements selected from the following as optional elements (selected elements).

[0052] Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less<Cu: 1% or Less>

[0053] Cu improves the corrosion resistance in the environment in which the automobile is used. The corrosion products of Cu have the effect of covering the surface of the steel sheet to inhibit hydrogen ingress into the steel sheet. Cu is an element that is mixed in when scrap is utilized as a raw material. When Cu is allowed to be mixed in, recycled materials can be used as raw materials to reduce manufacturing costs. From this point of view, the Cu content is preferably 0.005% or more. Furthermore, from the viewpoint of improving delayed fracture resistance, the Cu content is more preferably 0.05% or more. The Cu content is even more preferably 0.10% or more. The Cu content is even more preferably 0.25% or more, even further more preferably 0.50% or more.

[0054] However, an excessively high Cu content leads to surface defects. Thus, when Cu is included, the Cu content is set to 1% or less.<Ni: 1% or Less>

[0055] Ni, like Cu, is an element that acts to improve corrosion resistance. Ni also inhibits the formation of surface defects that are likely to occur when Cu is contained. For this reason, Ni is desirably contained in an amount of 0.01% or more. The Ni content is more preferably 0.04% or more, even more preferably 0.06% or more.

[0056] However, an excessively high Ni content leads to nonuniform scale formation in the heating furnace, causing surface defects and leading to higher costs. For this reason, when Ni is contained, the Ni content is set to 1% or less. The Ni content is preferably 0.5% or less, more preferably 0.3% or less.<Cr: 1% or Less>

[0057] Cr can be added because of its effect of improving the hardenability of steel and its effect of inhibiting the formation of carbides in martensite, upper bainite, and lower bainite. To provide the effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, even more preferably 0.06% or more.

[0058] However, when an excessive amount of Cr is contained, the pitting corrosion resistance is degraded. Thus, when Cr is contained, the Cr content is set to 1% or less. The Cr content is preferably 0.75% or less, more preferably 0.50% or less. The Cr content is even more preferably 0.30% or less, even further more preferably 0.10% or less.<Mo: 0.5% or Less>

[0059] Mo can be added because of its effect of improving the hardenability of steel and its effect of inhibiting the formation of carbides in martensite, upper bainite, and lower bainite. To provide the effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.03% or more, even more preferably 0.06% or more. The Mo content is even more preferably 0.1% or more, even further more preferably 0.2% or more.

[0060] However, Mo significantly degrades the phosphatability of the cold rolled steel sheet. Thus, when Mo is contained, the Mo content is set to 0.5% or less. More preferably, the Mo content is 0.4% or less.<V: 0.5% or Less>

[0061] V can be added because of its effects of improving the hardenability of steel, inhibiting the formation of carbides in martensite, upper bainite, and lower bainite, refining the microstructure, and precipitating carbides to improve delayed fracture resistance. To provide these effects, the V content is preferably 0.003% or more. The V content is more preferably 0.005% or more, even more preferably 0.010% or more. The V content is even more preferably 0.020% or more, even further more preferably 0.050% or more.

[0062] However, when a large amount of V is contained, castability is significantly degraded. Thus, when V is contained, the V content is set to 0.5% or less. The V content is preferably 0.3% or less, more preferably 0.2% or less.<Nb: 0.1% or Less>

[0063] Nb can be added because it has the effects of refining the steel microstructure to increase strength, promoting bainite transformation through grain refinement, improving bendability, and improving delayed fracture resistance. To provide these effects, the Nb content is preferably 0.002% or more. The Nb content is preferably 0.004% or more, more preferably 0.010% or more.

[0064] However, when a large amount of Nb is contained, precipitation strengthening is too strong, thereby reducing ductility. This also leads to an increase in rolling load and a deterioration in castability. Thus, when Nb is contained, the Nb content is set to 0.1% or less. The Nb content is preferably 0.05% or less, more preferably 0.03% or less.<Mg: 0.0050% or Less>

[0065] Mg fixes O in the form of MgO to contribute to improving formability, such as bendability. For this reason, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, even more preferably 0.0006% or more. The Mg content is preferably 0.0010% or more, more preferably 0.0015% or more.

[0066] When a large amount of Mg is added, the surface quality and bendability deteriorate. Thus, when Mg is contained, the Mg content is set to 0.0050% or less. The Mg content is preferably 0.0040% or less.<Ca: 0.0050% or Less>

[0067] Ca fixes S in the form of CaS to contribute to improving bendability and delayed fracture resistance. For this reason, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, even more preferably 0.0010% or more. The Ca content is preferably 0.0015% or more, more preferably 0.0020% or more.

[0068] When a large amount of Ca is added, the surface quality and bendability deteriorate. Thus, when Ca is contained, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0040% or less.<Sn: 0.1% or Less>

[0069] Sn inhibits the oxidation and nitriding of a surface layer portion of the steel sheet and inhibits reductions in the C content and the B content of the surface layer caused thereby. The effects inhibit the formation of ferrite in the surface layer portion of the steel sheet to increase strength and improve the fatigue resistance. From the points of view, the Sn content is preferably 0.003% or more. The Sn content is more preferably 0.010% or more, even more preferably 0.015% or more. The Sn content is preferably 0.020% or more, more preferably 0.030% or more.

[0070] When the Sn content is more than 0.1%, the castability deteriorates. Sn segregates at the prior γ grain boundaries to degrade the delayed fracture resistance. Thus, when Sn is contained, the Sn content is set to 0.1% or less.<Sb: 0.1% or Less>

[0071] Sb inhibits the oxidation and nitriding of a surface layer portion of the steel sheet, and inhibits reductions in the C content and the B content of the surface layer caused thereby. The effects inhibit the formation of ferrite in the surface layer portion of the steel sheet to increase strength and improve fatigue resistance. From the points of view, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, even more preferably 0.006% or more. The Sb content is more preferably 0.008% or more, even more preferably, 0.010% or more. The Sb content is preferably 0.015% or more, more preferably 0.030% or more.

[0072] When the Sb content is more than 0.1%, the castability deteriorates. Furthermore, Sb segregates at prior γ grain boundaries to degrade the delayed fracture resistance. Thus, when Sb is contained, the Sb content is set to 0.1% or less.<REM: 0.0050% or Less>

[0073] REM is an element that makes sulfides spherical in shape to inhibit the adverse effects of sulfides on stretch flange formability, thereby improving the stretch flange formability. To provide these effects, the REM content is preferably 0.0005% or more. The REM content is more preferably 0.0010% or more, even more preferably 0.0020% or more.

[0074] A REM content of more than 0.0050% results in the saturation of the effect of improving the stretch flange formability. Thus, when REM is contained, the REM content is set to 0.0050% or less.

[0075] In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the 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 two or more elements selected from the above-mentioned REM elements.

[0076] When the optional components are contained in amounts less than the lower limits, the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Thus, when the optional elements are contained in amounts less than the lower limits, the optional elements are considered to be contained as incidental impurities.

[0077] The mechanical properties of the steel sheet (cold rolled steel sheet excellent in material stability) targeted by the present invention will be described below.

[0078] 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. However, from the viewpoint of compatibility with other properties, the tensile strength is preferably 1,300 MPa or less.

[0079] In the steel sheet of the present invention, the total elongation EL is ensured to be 16.0% or more for TS: 780 MPa or more and less than 980 MPa, 14.0% or more for TS: 980 MPa or more and less than 1,180 MPa, or 12.0% or more for TS: 1,180 MPa or more. This significantly improves the stability of press forming.

[0080] To evaluate the tensile properties, JIS No. 5 test pieces for a tensile test are taken from the center position of the sheet width, and the tensile test (in accordance with JIS Z2241 (2011)) is performed with N = 3. Each evaluation is performed on the basis of the average value of three test pieces. A steel sheet with a tensile strength of 780 MPa or more is defined as a high strength steel sheet. A steel sheet having a total elongation EL of 16.0% or more for TS: 780 MPa or more and less than 980 MPa, 14.0% or more for TS: 980 MPa or more and less than 1,180 MPa, or 12.0% or more for TS: 1,180 MPa or more is defined as a steel sheet having excellent ductility. To ensure the flangeability required for practical use, it is an essential condition of the present invention that the hole expansion ratio λ (%) (= {(d - d 0 ) / d 0 } × 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 is 30% or more.

[0081] The steel microstructure of the steel sheet of the present invention will be described below.<Area Fraction of Polygonal Ferrite: 10% or More and 80% or Less>

[0082] From the viewpoint of ensuring high ductility, the area fraction of polygonal ferrite is set to 10% or more. To obtain even higher ductility, the area fraction of polygonal ferrite is preferably set to 20% or more.

[0083] A polygonal ferrite fraction of more than 80% can lead to failure to provide desired strength. Thus, the area fraction of polygonal ferrite is set to 80% or less, preferably 75% or less, and more preferably 70%.<Total Area Fraction of Upper Bainite, Tempered Martensite, and Lower Bainite: 10% or More and 70% or Less>

[0084] To obtain desired strength, the total area fraction of upper bainite, tempered martensite, and lower bainite is set to 10% or more. To obtain even higher strength, the total area fraction is preferably set to 15% or more.

[0085] A total area fraction of upper bainite, tempered martensite, and lower bainite of more than 70% results in a reduction in ductility due to excessively high strength. Thus, the area fraction is set to 70% or less. The area fraction is more preferably 65% or less, even more preferably 60% or less.<Volume Fraction of Retained austenite (Retained γ): 3% or More and 15% or Less>

[0086] A volume fraction of retained austenite of less than 3% can result in failure to ensure the desired ductility. From the viewpoint of ductility, the volume fraction of retained austenite is set to 3% or more, preferably 5% or more.

[0087] A volume fraction of retained austenite of more than 15% can result in a deterioration in stretch flange formability (flangeability). Thus, the volume fraction of retained austenite is set to 15% or less. The volume fraction of retained austenite is preferably 13% or less.<Area Fraction of Quenched Martensite: 15% or Less (Including 0%)>

[0088] The hard quenched martensite microstructure reduces λ. Thus, the area fraction of quenched martensite needs to be controlled. To obtain the practically necessary λ, the area fraction of quenched martensite is set to 15% or less. To obtain λ more stably, the area fraction of quenched martensite is preferably 13% or less, more preferably 11% or less. The area fraction of quenched martensite may be 0% or may be 3% or more.<Remaining Microstructure>

[0089] The steel microstructure other than the above includes the remaining microstructure. The area fraction of the remaining microstructure is preferably 5% or less. The remaining microstructure may be non-recrystallized ferrite, carbide, and pearlite. These microstructures may be determined by SEM observation as described below.<Maximum Concentration of P [Pm] Within 1 µm from Surface of Steel Sheet in Thickness Direction Is 0.025 Mass% or More and Satisfies Formula (3)>

[0090] Pm / P ≥ 1.5

[0091] In formula (3), [P] (which can also be referred to as [Pi]) is the P content (mass%).

[0092] The results of our intensive studies on the effects of various elements on phosphatability, the amounts of these elements concentrated on a surface, and the types of oxides formed during annealing have revealed that even under manufacturing conditions where no oxides are formed, sufficient phosphatability cannot be ensured. For a steel sheet with ensured phosphatability, the maximum concentration of P near a surface layer was quantitatively evaluated by a method described below. The results revealed that good phosphatability was ensured when the steel microstructure was such that upon analysis of the emission intensity of P measured from the surface of the steel sheet in the thickness direction by glow discharge spectrometry (GDS), the maximum concentration of P [Pm] within 1 µm from the surface of the steel sheet in the thickness direction was 0.025 mass% or more and such that formula (3) was satisfied.

[0093] Although the detailed mechanism is unclear, it is important that the maximum concentration of P in the surface layer is locally high relative to the steel components. When this maximum concentration of P is insufficient, the shape of zinc phosphate crystals after zinc phosphate treatment is scaly. From this, the local enrichment of P on the surface is considered to provide the effect of inhibiting the formation of Si-based oxides on the surface, which have an adverse effect on phosphatability.

[0094] [Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more. The upper limit is not particularly limited. However, [Pm] is preferably 0.100 mass% or less, more preferably 0.090 mass% or less.

[0095] [Pm] / [P] is preferably 1.7 or more, more preferably 1.9 or more. The upper limit is not particularly limited. However, [Pm] / [P] is preferably 10.0 or less, more preferably 9.0 or less.<Cumulative Amount of Si Enrichment Within 1 µm from Surface of Steel Sheet in Thickness Direction Is 120 or Less>

[0096] In the present invention, in order to inhibit the formation of Si-based oxides during soaking, 1,000 × [B] / [Mn] is set to 0.70 or less. To obtain this effect more reliably, in the present invention, the cumulative amount of Si enrichment within 1 µm from the surface of the produced steel sheet is set to 120 or less. This ensures good phosphatability. As described above, in the present invention, the cumulative amount of Si enrichment within 1 µm from the steel sheet surface is set to 120 or less, preferably 100 or less. The lower limit is not particularly limited, but is preferably 80 or more, more preferably 90 or more.

[0097] A method for measuring a steel microstructure will be described below.

[0098] To measure the area fractions of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite), a section in the thickness direction parallel to the rolling direction is cut, mirror-polished, and etched with 1 vol% nital. At a 1 / 4 thickness position, 10 fields of view are observed with an SEM at 5,000× in an area of 25 µm × 20 um. The resulting microstructure images are quantified using image analysis.

[0099] Polygonal ferrite refers to a relatively equiaxed ferrite with almost no carbides inside. It appears as the darkest region under the SEM.

[0100] Upper bainite is a ferrite microstructure inside of which carbides or retained austenite, which appear white under the SEM, are formed. When it is difficult to distinguish between upper bainite and polygonal ferrite, the area fractions are calculated by classifying the ferrite region having an aspect ratio ≤ 2.0 as polygonal ferrite and the region having an aspect ratio > 2.0 as upper bainite. The aspect ratio is determined as follows: A grain length with the greatest grain length is defined as a long-axis length a. A grain length with the greatest grain length in a direction perpendicular to the long axis is defined as a short-axis length, b. The aspect ratio is defined as a / b.

[0101] Tempered martensite and lower bainite are regions that contain a lath-shaped submicrostructure and carbide precipitates therein under the SEM.

[0102] Quenched martensite (fresh martensite) is a massive region that appears white under the SEM with no internal submicrostructure visible.

[0103] The remaining microstructure refers to a microstructure containing at least one of non-recrystallized ferrite, carbides, and pearlite. Each can be identified with the SEM, where non-recrystallized ferrite can be observed as ferrite with dark contrast, the ferrite containing deformed microstructures introduced by a rolling process, while carbides and pearlite can be observed as microstructures with bright contrast. Carbides have microstructure with a grain size of 1 µm or less. Pearlite has a lamellar (layer) microstructure. Thus, they can be distinguished from each other.

[0104] The volume fraction of retained austenite is determined by chemically polishing a portion from a surface layer to the 1 / 4 thickness position, and then subjecting the portion to X-ray diffraction. A Co-Kα radiation source is used for the incident X-rays. The volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. The retained austenite is randomly distributed. Thus, the volume fraction of the retained austenite determined by X-ray diffraction can be taken as the area fraction of the retained austenite.

[0105] With regard to the amounts of surface enrichment of P and Si on and near the surface of the steel sheet, sputtering analysis in the depth direction (thickness direction) is performed with a GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure: 600 Pa, high-frequency output: 35 W, measurement time interval: 0.1 seconds, and measurement time: 150 seconds. The maximum concentration of P [Pm] and the cumulative amount of Si enrichment within 1 µm from the surface of the steel sheet in the thickness direction are determined from calibration curves previously established using standard samples with known amounts of P and standard samples with known amounts of Si.

[0106] The cumulative amount of Si enrichment is determined by calculating the difference between the Si amount obtained every 0.1 seconds within 1 µm from the surface of the steel sheet in the thickness direction and the average Si amount obtained over a measurement time of 140 to 150 seconds, and then integrating the measurement data within 1 um. Under these measurement conditions, the measurement position d (um) from the surface can be obtained using the sputtering time ts by the formula d = ts / 1.7 (µm).

[0107] In the present invention, as illustrated in Fig. 1, the highest P intensity value measured during the measurement time of 150 seconds is converted into a value in units of mass% using the calibration curve, and the resulting value is defined as the maximum concentration ([Pm]).

[0108] The conversion method to mass% is as follows: In the data obtained by measuring under the same conditions using standard materials with known amounts of P, the correlation between the intensity of the P element obtained with the GDS and the amount of P is determined. The intensity of P measured in the example is then converted to a concentration. In Fig. 1, Pi is the P content (mass%) in the steel sheet.

[0109] The cumulative amount of Si enrichment is determined by dividing the measurement time (s) illustrated in Fig. 2(a) by 1.7 to convert it to a depth position (um), and then integrating the difference between the average bulk concentration (see the dashed line in Fig. 2(b)) and the Si concentration (mass%) at each depth position (see the shaded area in Fig. 2(b)).(Method for Manufacturing Steel Sheet)

[0110] A method for manufacturing a steel sheet according to the present invention will be described below.<First Embodiment>

[0111] A method for manufacturing a steel sheet according to a first embodiment of the present invention includes subjecting a steel slab having the chemical composition described above to hot rolling, pickling, and cold rolling, and then subjecting the resulting cold rolled steel sheet to annealing, in which the annealing includes a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a first cooling step of performing cooling to a first cooling stop temperature of 350°C to 550°C at a first average cooling rate: 2 to 50 °C / s in a temperature range from the soaking temperature to the first cooling stop temperature, a second cooling step of, after stopping cooling at the first cooling stop temperature, performing holding in a temperature range of 350°C to 550°C for 10 to 60 seconds and then performing cooling to a second cooling stop temperature of 200°C to 420°C at a second average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the second cooling stop temperature for 60 to 3,000 seconds, Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (soaking time) (s) at the soaking temperature, and Tdp is the dew point (°C).<Hot Rolling>

[0112] Examples of a method for hot-rolling a steel slab include a method in which a slab is heated and then rolled, a method in which a slab is rolled immediately after continuous casting without being heated, and a method in which a slab is subjected to a short-term heat treatment after continuous casting and then rolled. The hot rolling may be performed in the usual manner. For example, the slab heating temperature may be 1,100°C or higher. The slab heating temperature may be set to 1,300°C or lower. The soaking temperature may be set to 20 minutes or more. The soaking temperature may be set to 300 minutes or less. The finish rolling temperature may be set to an A r3 transformation point or higher. The finish rolling temperature may be set to the A r3 transformation point + 200°C or lower. The coiling temperature may be set to 400°C or higher. The coiling temperature may be set to 720°C or lower. The coiling temperature is preferably controlled from the viewpoints of inhibiting thickness fluctuation to stably ensure high strength. Specifically, the coiling temperature is preferably 430°C or higher. The coiling temperature is preferably 530°C or lower.

[0113] The A r3 transformation point can be calculated from the components of the steel sheet and the following empirical formula (A): (where in the above formula, each [M] is the amount of element M contained (mass%) in the steel slab, and the value of an element that is not contained is zero (0)).<Pickling>

[0114] The pickling may be performed in the usual manner.<Cold Rolling>

[0115] Cold rolling may be performed in the usual manner. The rolling reduction ratio (cumulative rolling reduction ratio) may be 30% or more. The rolling reduction ratio (cumulative rolling reduction ratio) may be 85% or less. The rolling reduction ratio is preferably controlled from the viewpoints of stably ensuring high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more. When the rolling load is high, softening annealing treatment can be performed at 450°C to 730°C in a continuous annealing line (CAL) or box annealing furnace (BAF).<Annealing>

[0116] A cold rolled steel sheet (steel sheet subjected to cold rolling) produced in the usual manner is annealed under the following conditions. The annealing facility is not particularly limited. However, from the viewpoints of productivity and ensuring desired heating and cooling rates, annealing is preferably performed in a continuous annealing line (CAL).[Soaking Step: in Furnace Atmosphere with Dew Point of -40°C or Lower, Heating Is Performed to Soaking Temperature that Is A c1 Point + 20°C or Higher and A c3 Point or Lower and that Is Tc or Higher, and Holding Is Performed at Soaking Temperature for 30 to 500 Seconds]

[0117] The dew point affects the formation of oxides on the surface of the steel sheet during annealing. A dew point higher than -40°C results in an excessive increase in the amount of oxides formed on the surface of the steel sheet, thereby leading to a deterioration in phosphatability. For this reason, the dew point is set to -40°C or lower.

[0118] The lower limit is not particularly limited. However, the dew point is preferably -70°C or higher, more preferably -60°C or higher.

[0119] The steel sheet obtained according to the present invention contains a soft ferrite microstructure, thereby improving ductility. Thus, the soaking temperature is the A c1 point + 20°C or higher and the A c3 point or lower, at which ferrite is formed.

[0120] Furthermore, by setting the soaking temperature to Tc (°C) or higher, the amount of surface enrichment of P in the P-rich surface portion formed on the surface of the steel sheet can be ensured to be the amount specified in the present invention. Tc is calculated from the dew point and the soaking time in formula (4): Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is the holding time (s) at the soaking temperature, and Tdp is the dew point (°C).

[0121] When the soaking temperature is lower than Tc, a predetermined amount of surface enrichment of P cannot be ensured, leading to a deterioration in phosphatability. Thus, in the furnace atmosphere with a dew point of -40°C or lower, the soaking temperature is the A c1 point + 20°C or higher and the A c3 point or lower, and is Tc (°C) or higher.

[0122] When the holding time (soaking time) at the soaking temperature is less than 30 seconds, the formation of austenite at the soaking temperature is insufficient, polygonal ferrite increases, and the desired total area fraction of upper bainite, tempered martensite, and lower bainite cannot be obtained; thus, the desired strength may fail to be obtained. In addition, retained austenite may fail to be sufficiently obtained; thus, the desired ductility may fail to be ensured.

[0123] When the holding time (soaking time) at the above-mentioned soaking temperature is more than 500 seconds, the microstructure coarsens significantly, thereby possibly failing to ensure the desired strength. The desired ductility may fail to be provided.

[0124] Thus, 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. The holding time (soaking time) at the soaking temperature is preferably 400 seconds or less, more preferably 300 seconds or less.

[0125] A c1 and A c3 described above may be obtained from empirical formulae (5) and (6) below: where [M] is the mass percentage of each element.[First Cooling Step: Cooling Is Performed to First Cooling Stop Temperature of 350°C to 550°C at First Average Cooling Rate: 2 to 50 °C / s in Temperature Range of Soaking Temperature to First Cooling Stop Temperature]

[0126] After holding at the soaking temperature that is the A c1 point + 20°C or higher and A c3 point or lower and that is Tc or higher (after the soaking step), cooling is performed at the first average cooling rate of 2 to 50 °C / s in the temperature range from the soaking temperature to the first cooling stop temperature of 350°C to 550°C.

[0127] When the first average cooling rate is less than 2 °C / s, the ferrite transformation during cooling proceeds excessively, thereby leading to failure to obtain the desired amount of polygonal ferrite. For this reason, the first average cooling rate is set to 2 °C / s or more. The first average cooling rate is preferably 5 °C / s or more.

[0128] When the first average cooling rate is too high, the sheet shape deteriorates. For this reason, the first average cooling rate is set to 50 °C / s or less. The first average cooling rate is preferably 40 °C / s or less, more preferably less than 30 °C / s.

[0129] 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".[Second Cooling Step (1): Holding Is Performed at Residence Temperature of 350°C to 550°C for 10 Seconds to 60 Seconds]

[0130] Upper bainite is formed at the first cooling stop temperature or lower and in the temperature range (residence temperature) of 350°C to 550°C, and a predetermined amount of retained austenite can be obtained, thereby providing the desired ductility. The bainite transformation has an incubation period. To obtain a desired amount of bainite, holding must be performed at the temperature for a certain period of time. When the residence temperature range, including the residence start temperature (= first cooling stop temperature) and the residence end temperature, is outside the range of 350°C to 550°C and / or the time of residence (hereinafter, also referred to as the residence time) is less than 10 seconds, the desired amount of bainite is not obtained, and the formation of retained austenite is inhibited; thereby, the desired ductility is not obtained.

[0131] When the residence time is more than 60 seconds, the enrichment of C from bainite to the massive non-transformed γ proceeds to lead to an increase in the amount of remaining massive quenched martensite microstructure or the excessive increase of retained austenite, possibly resulting in a decrease in λ. Thus, the residence time is set to 10 seconds or more and 60 seconds or less. The residence time is preferably 20 seconds or more. The residence time is preferably 50 seconds or less.

[0132] Depending on the desired properties, the second cooling step (1) can be omitted. In that case, after the soaking step of performing heating to the soaking temperature that is the A c1 point + 20°C or higher and the A c3 point or lower and that is Tc or higher and performing holding at the soaking temperature for 30 to 500 seconds, and the treatment in the second cooling step (2) is performed. A manufacturing method in which the second cooling step (1) is omitted will be described in a second embodiment below.[Second Cooling Step (2): Cooling Is Performed to Second Cooling Stop Temperature of 200°C to 420°C at Second Average Cooling Rate: 2 to 50 °C / s]

[0133] After the above residence, cooling needs to be performed rapidly so as not to cause bainite transformation to proceed excessively. When the average cooling rate (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 less than 2 °C / s, there is a possibility that the bainite transformation can proceed excessively, leading to an increase in retained austenite, or that the desired amount of martensite can fail to be ensured, leading to a reduction in strength. When the second average cooling rate is less than 2 °C / s, the desired ductility and flangeability may fail to be obtained.

[0134] Thus, 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 more. The second average cooling rate is preferably 5 °C / s or more, more preferably 8 °C / s or more.

[0135] An excessively high cooling rate in this temperature range results in the deterioration of the sheet shape. Thus, the cooling rate (second average cooling rate) in this temperature range is set to 50 °C / s or less, preferably 40 °C / s or less.

[0136] When the second cooling stop temperature is higher than 420°C, the predetermined area fraction of tempered martensite or lower bainite is not obtained, thereby increasing the area fraction of quenched martensite after annealing. This can result in failure to ensure retained γ, leading to a deterioration in ductility. In addition, the flangeability may deteriorate. 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.

[0137] When the second cooling stop temperature is lower than 200°C, the tempering effect of martensite cannot be sufficiently obtained. In addition, the formation of lower bainite is inhibited. This results in the increase of quenched martensite and the inhibition of the enrichment of C in retained γ, leading to a deterioration in ductility. For this reason, the second cooling stop temperature is set to 200°C or higher.

[0138] Here, the second average cooling rate is "(residence end temperature (°C) - second cooling stop temperature (°C)) / cooling time (seconds) from residence end temperature to second cooling stop temperature".[Isothermal Holding Step: Holding at Second Cooling Stop Temperature for 60 to 3,000 Seconds]

[0139] The holding at the second cooling stop temperature is performed from the viewpoints of promoting the enrichment of C to retained γ and adjusting strength by tempering the formed martensite. At less than 60 seconds, tempering is insufficient, resulting in the formation of high-strength martensite. Furthermore, the enrichment of C in retained γ is inhibited. Thus, the desired ductility and flangeability are not ensured.

[0140] When the holding time at the second cooling stop temperature is more than 3,000 seconds, martensite is excessively tempered, and the desired strength may fail to be ensured. Furthermore, when the holding time at the second cooling stop temperature is more than 3,000 seconds, the desired ductility may fail to be obtained.

[0141] Thus, the holding time at the second cooling stop temperature is set to 60 seconds or more and 3,000 seconds or less. The holding time at the second cooling stop temperature is preferably 100 seconds or more, more preferably 150 seconds or more. The holding time at the second cooling stop temperature is preferably 2,500 seconds or less, more preferably 2,000 seconds or less.<Second Embodiment>

[0142] A method for manufacturing a steel sheet according to a second embodiment of the present invention includes subjecting a steel slab having the chemical composition described above to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, in which the annealing includes a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a cooling step of performing cooling from the soaking temperature to a cooling stop temperature of 200°C to 420°C at an average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the cooling stop temperature for 60 to 3,000 seconds, Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (s) at the soaking temperature, and Tdp is the dew point (°C).

[0143] In the second embodiment, the treatments in the hot rolling, the pickling, the cold rolling, and the soaking step of the annealing can be performed under the same conditions as in the first embodiment.

[0144] In the second embodiment, the treatment in the first cooling step in the annealing in the first embodiment can be omitted.

[0145] In the second embodiment, the cooling step in the annealing corresponds to the second cooling step in the annealing in the first embodiment. In the cooling step of the present embodiment, residence treatment (holding for 10 to 60 seconds in a temperature range of 350°C to 550°C) in the second cooling step in the first embodiment can be omitted.

[0146] The isothermal holding step in the annealing of the second embodiment can be performed 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 the cooling stop temperature.

[0147] In the present embodiment, the cooling step in the annealing will be mainly described below.[Cooling Step: Cooling to Cooling Stop Temperature of 200°C to 420°C at Average Cooling Rate of 2 to 50 °C / s]

[0148] After treatment in the above-mentioned soaking step, cooling needs to be performed rapidly so as not to cause bainite transformation to proceed excessively. When the average cooling rate in the temperature range from the soaking temperature to the second cooling stop temperature that is 200°C or higher and 420°C or lower is less than 2 °C / s, bainite transformation may proceed excessively, and the desired amount of quenched martensite may fail to be ensured, thereby possibly resulting in a reduction in strength. Furthermore, when the average cooling rate is less than 2 °C / s, the desired flangeability may fail to be obtained.

[0149] Thus, 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 more. The average cooling rate is preferably 5 °C / s or more, more preferably 8 °C / s or more.

[0150] An excessively high cooling rate in this temperature range results in the deterioration of the sheet shape. Thus, the cooling rate (second average cooling rate) in this temperature range is set to 50 °C / s or less, preferably 40 °C / s or less.

[0151] When the cooling stop temperature is higher than 420°C, the predetermined area fraction of tempered martensite or lower bainite may fail to be obtained to thereby increase the area fraction of quenched martensite after annealing, leading to a deterioration in flangeability. For this reason, the cooling stop temperature is set to 420°C or lower.

[0152] When the cooling stop temperature is lower than 200°C, the tempering effect of martensite may fail to be sufficiently obtained. This may increase quenched martensite and inhibit the enrichment of C in retained γ, possibly resulting in a deterioration in ductility. For this reason, the cooling stop temperature is set to 200°C or higher.

[0153] Here, the average cooling rate is "(soaking temperature (°C) - cooling stop temperature (°C)) / cooling time (seconds) from soaking temperature to cooling stop temperature".[Thickness]

[0154] The steel sheet of the present invention preferably has a thickness of 0.5 mm or more. The thickness is preferably 3.0 mm or less.(Member and Method for Manufacturing Member)

[0155] A member of the present invention and a method for manufacturing the member will be described below.

[0156] The member of the present invention is obtained by subjecting the steel sheet of the present invention to at least one of forming or joining. The method for manufacturing a member of the present invention includes a step of subjecting the steel sheet of the present invention to at least one of forming or joining to provide a member.

[0157] The steel sheet of the present invention has a tensile strength of 780 MPa or more, excellent ductility, flangeability, and phosphatability. Thus, the member obtained using the steel sheet of the present invention also has a tensile strength of 780 MPa or more, excellent ductility, flangeability, and phosphatability. Furthermore, the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used for, for example, automobile body frame parts.

[0158] The forming can be performed by any common processing method, such as press forming without limitation. The joining can be performed by common welding, such as spot welding or arc welding, or, for example, riveting or caulking without limitation.EXAMPLES<Example 1>

[0159] Slabs having chemical compositions given in Table 1 were produced by continuous casting. Each of the slabs was subjected to a hot rolling process in which the slab was heated to 1,200°C, the soaking time was 200 minutes, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. Then cold rolling was performed at a rolling reduction ratio of 50% to produce cold rolled steel sheet having a thickness of 1.4 mm. The cold rolled steel sheet was treated under the annealing conditions given in Table 2 to provide steel sheets of the present invention and steel sheets of comparative examples. [Table 1]Steel gradeChemical composition (mass%)[Si] / [Mn][B] / [Mn] ×1,000RemarksCSiMnPSsol. AlNTiBothersA0.0820.442.110.0220.00290.0330.00490.0300.0012-0.210.57Compliant steelB0.0620.493.020.0370.00330.0290.00330.0140.0014-0.160.46Compliant steelC0.1510.722.590.0100.00520.0370.00290.0260.0012Nb: 0.0240.280.46Compliant steelD0.1111.384.290.0210.00380.0240.01320.0340.0027Ca: 0.0037, Sb: 0.0700.320.63Compliant steelE0.2100.882.590.0270.00220.0220.00810.0230.0018Ni: 0.08, Cr: 0.060.340.69Compliant steelF0.0990.522.690.0080.00350.1620.00230.0810.0011V: 0.070, Mo: 0.33, Mg: 0.00380.190.41Compliant steelG0.1321.093.680.0150.00310.0310.00880.0160.0013Sn: 0.080, REM: 0.00290.300.35Compliant steelH0.2360.463.110.0220.00440.3320.00390.0210.0016Cu: 0.900.150.51Compliant steelI0.1451.003.120.0120.00290.0300.00290.0120.0020-0.320.64Compliant steelJ0.1360.391.670.0110.00310.0360.00710.0290.0011-0.230.66Compliant steelK0.1110.562.320.0150.00380.0390.00650.0320.0014-0.240.60Compliant steelL0.0390.772.660.0080.00590.0330.00220.0320.0017-0.290.64Comparative steelM0.2560.923.090.0070.00290.0260.00230.0700.0014-0.300.45Comparative steelN0.1310.212.180.0210.00330.0320.00300.0230.0011-0.100.50Comparative steelO0.1571.534.460.0110.00510.0290.00390.0210.0022-0.340.49Comparative steelP0.1290.421.420.0150.00300.0220.00290.0220.0010-0.300.70Comparative steelQ0.1110.854.660.0080.00270.1320.00710.0190.0018-0.180.39Comparative steelR0.1391.112.620.0070.00440.0280.00810.0180.0011-0.420.42Comparative steelS0.1000.392.880.0030.00460.0380.01110.0240.0019-0.140.66Comparative steelT0.1310.622.910.0190.00330.0220.00350.0210.0021-0.210.72Comparative steelU0.1050.542.850.0200.00221.0500.00290.0100.0014-0.190.49Comparative steelV0.1220.612.560.0130.00360.0220.01800.0160.0016-0.240.63Comparative steelW0.1420.774.430.0110.00410.0220.00390.0160.0031-0.170.70Comparative steel· The balance other than the above components is Fe and incidental impurities. Note: Underlined items are outside the scope of the present invention.

[0160] The steel microstructures were measured by the following method. The measurement results are presented in Table 3.

[0161] To measure the area fractions of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite), a section in the thickness direction parallel to the rolling direction was cut, mirror-polished, and etched with 1 vol% nital. At a 1 / 4 thickness position, 10 fields of view were observed with an SEM at 5,000× in an area of 25 µm × 20 µm. The resulting microstructure images were quantified using image analysis.

[0162] Polygonal ferrite refers to a relatively equiaxed ferrite with almost no carbides inside. It appears as the darkest region under the SEM.

[0163] Upper bainite is a ferrite microstructure inside of which carbides or retained austenite, which appear white under the SEM, are formed. When it was difficult to distinguish between upper bainite and polygonal ferrite, the area fractions were calculated by classifying the ferrite region having an aspect ratio ≤ 2.0 as polygonal ferrite and the region having an aspect ratio > 2.0 as upper bainite. The aspect ratio is determined as follows: A grain length with the greatest grain length is defined as a long-axis length a. A grain length with the greatest grain length in a direction perpendicular to the long axis is defined as a short-axis length, b. Then a / b was defined as the aspect ratio.

[0164] Tempered martensite and lower bainite are regions that contain a lath-shaped submicrostructure and carbide precipitates therein under the SEM.

[0165] Quenched martensite (fresh martensite) is a massive region that appears white under the SEM with no internal submicrostructure visible.

[0166] The remaining microstructure refers to a microstructure including at least one of non-recrystallized ferrite, carbides, and pearlite. Under the SEM, the non-recrystallized ferrite can be observed as dark contrast ferrite containing a deformed microstructure introduced by a rolling process. Carbides and pearlite can be observed as microstructures with bright contrast. Carbides have a microstructure with a grain size of 1 µm or less. Pearlite has a lamellar (layer) microstructure. Thus, they can be distinguished from each other.

[0167] The volume fraction of retained austenite is determined by chemically polishing a portion from a surface layer to the 1 / 4 thickness position, and then subjecting the portion to X-ray diffraction. A Co-Kα radiation source was used for the incident X-rays. The volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0168] From each obtained steel sheet, JIS No. 5 test pieces for a tensile test were taken, and the tensile test (in accordance with JIS Z 2241 (2011)) was performed with N = 3. Each evaluation was conducted on the basis of the average value of three test pieces. A steel sheet having a tensile strength of 780 MPa or more was determined to have excellent strength.

[0169] A total elongation EL of 16.0% or more for TS: 780 MPa or more, 14.0% or more for TS: 980 MPa or more, or 12.0% or more for TS: 1,180 MPa or more was determined to provide excellent ductility.

[0170] A hole expansion test in accordance with the provisions of JFST1001 was performed with N = 3. The acceptance criterion was that the average of three hole expansion ratios λ (%) (= {(d - d 0 ) / d 0 } × 100) was 30% or more. A hole expansion ratio of 30% or more was determined to indicate excellent flangeability.

[0171] The measurement results are presented in Table 3.

[0172] For the steel sheet after annealing, the amounts of surface enrichment of P and Si on the surface of the steel sheet were measured using a GDS (manufactured by Shimadzu Corporation) by performing 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 seconds, and measurement time: 150 seconds, to measure the maximum concentration of P [Pm] and the cumulative amount of Si enrichment near the surface layer (within 1 µm from the surface of the steel sheet in the thickness direction). In this measurement, calibration curves for P and Si were obtained using standard samples having various P contents ranging from 0.005 to 0.020 mass% and standard samples having various Si contents ranging from 1.0 to 3.0 mass%.

[0173] The annealed steel sheets were subjected to degreasing and surface conditioning, and then subjected to zinc phosphate treatment using a zinc phosphate treatment solution. Specifically, the zinc phosphate treatment was performed under the following conditions: degreasing step: treatment temperature: 40°C, treatment time: 120 seconds, and spray degreasing; surface conditioning step: pH: 9.5, treatment temperature: room temperature, and treatment time: 20 seconds; and zinc phosphate treatment step: zinc phosphate treatment solution temperature: 35°C, and treatment time: 120 seconds. The treatment agents used in the degreasing step, the surface conditioning step, and the zinc phosphate treatment step were an FC-E2011 degreasing agent, a PL-X surface conditioning agent, and a Palbond PB-L3065 zinc phosphate treatment solution, respectively, all manufactured by Nihon Parkerizing Co., Ltd. The surface zinc phosphate treatment microstructure was observed by SEM observation in five fields of view (area of 50,000 µm 2< or more) at a magnification of 1,000×. When the area where the steel substrate was exposed was less than 10% of the total area, the surface zinc phosphate treatment microstructure was evaluated as ∘, and when the area was 10% or more, the surface zinc phosphate treatment microstructure was evaluated as ×. The results are presented in Table 3.

[0174] In the examples of the present invention presented in Tables 2 and 3, strength, ductility, flangeability, and phosphatability were excellent, whereas in the comparative examples, at least one of these properties was inferior.<Example 2>

[0175] Slabs having chemical compositions given in Table 1 were produced by continuous casting. Each of the slabs was subjected to a hot rolling process in which the slab was heated to 1,200°C, the soaking time was 200 minutes, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. Then cold rolling was performed at a rolling reduction ratio of 50% to produce cold rolled steel sheet having a thickness of 1.4 mm. The cold rolled steel sheet was treated under the annealing conditions given in Table 4 to provide steel sheets of the present invention and steel sheets of comparative examples. The same evaluations as in Example 1 were performed. The results are presented in Table 5.

[0176] In the examples of the present invention presented in Tables 4 and 5, ductility and phosphatability were excellent, whereas in the comparative examples, at least one of these properties was inferior.

[0177] It was found that members obtained by forming and members obtained by joining, using the steel sheets of the present invention, had excellent strength, ductility, flangeability, and phosphatability, similar to the steel sheets of the present invention, because the steel sheets of the present invention had excellent strength, ductility, flangeability, and phosphatability .

Claims

1. A steel sheet, comprising: a chemical composition containing, 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%, Ti: 0.005% to 1.000%, and B: 0.0010% to 0.0030%, formula (1) and formula (2) below being satisfied, and the balance being iron and incidental impurities; and a steel microstructure having: an area fraction of polygonal ferrite: 10% or more and 80% or less, a total area fraction of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less, a volume fraction of retained austenite: 3% or more and 15% or less, and an area fraction of quenched martensite: 15% or less (including 0%), wherein a maximum concentration of P [Pm] within 1 µm from a surface of the steel sheet in a thickness direction is 0.025 mass% or more, formula (3) below is satisfied, and a cumulative amount of Si enrichment within 1 µm from the surface of the steel sheet in the thickness direction is 120 or less, Si / Mn ≤ 0.35 1 , 000 × B / Mn ≤ 0.70 Pm / P ≥ 1.5 where in formula (1), [Si] is a Si content (mass%), and [Mn] is a Mn content (mass%), in formula (2), [B] is a B content (mass%), [Mn] is a Mn content (mass%), and in formula (3), [P] is a P content (mass%).

2. The steel sheet according to Claim 1, wherein the chemical composition further contains, in mass%, one or two or more selected from: Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less.

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

4. A method for manufacturing a steel sheet, comprising subjecting a steel slab having the chemical composition according to Claim 1 or 2 to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, wherein the annealing includes: a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an Ac1 point + 20°C or higher and an Ac3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a first cooling step of performing cooling to a first cooling stop temperature of 350°C to 550°C at a first average cooling rate: 2 to 50 °C / s in a temperature range from the soaking temperature to the first cooling stop temperature, a second cooling step of, after stopping cooling at the first cooling stop temperature, performing holding in a temperature range of 350°C to 550°C for 10 to 60 seconds and then performing cooling to a second cooling stop temperature of 200°C to 420°C at a second average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the second cooling stop temperature for 60 to 3,000 seconds, Tc ° C = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (s) at the soaking temperature, and Tdp is the dew point (°C).

5. A method for manufacturing a steel sheet, comprising subjecting a steel slab having the chemical composition according to Claim 1 or 2 to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, wherein the annealing includes: a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an Ac1 point + 20°C or higher and an Ac3 point or lower and that is higher than or equal to Tc calculated from formula (4) and holding at the soaking temperature for 30 to 500 seconds, a cooling step of performing cooling from the soaking temperature to a cooling stop temperature of 200°C to 420°C at an average cooling rate: 2 to 50 °C / s, and an isothermal holding step of performing holding at the cooling stop temperature for 60 to 3,000 seconds, Tc = 663 − 1.2 × exp 20 / t × Tdp where t is a holding time (s) at the soaking temperature, and Tdp is 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 or joining to produce a member.

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