Steel sheet, member, and method for producing same

Abstract

Provided are: a steel sheet having excellent ductility, hole expandability, and chemical conversion treatability, and having a tensile strength of 780 MPa or more; a member; and methods for producing the steel sheet and the member. A steel sheet which has a component composition that contains C, Si, Mn, P, S, sol.Al and N in prescribed ranges in terms of mass% and satisfies formula (1), and a steel structure in which the area ratio of polygonal ferrite or the like is in prescribed ranges, and which satisfies formula (2) and in which the maximum concentration [Pm] of P within 1 [mu] m in the sheet thickness direction from the surface of the steel sheet is 0.025 mass% or more. The integral concentration of Si within 1 [mu] m in the sheet thickness direction from the surface of the steel sheet is 120 or less. [Si] / [Mn] < = 0.35... Equation (1) 1000 * [B] / [Mn] < = 0.70... Equation (2) [Pm] / [P] > = 1.5... Equation (3)

Description

Technical Field

[0001] This invention relates to steel sheets, components, and methods for manufacturing the same, which are suitable for use in stamped products with complex shapes after stamping processes in automobiles, home appliances, etc. Background Technology

[0002] Against the backdrop of increasingly stringent global CO2 emission restrictions, there is a growing demand for lighter vehicle weights through the use of higher-strength steel sheets in automobiles. For body and seat components, there is a growing trend towards using high-strength steel sheets with a strength of 590MPa or higher, moving away from the existing 440MPa cold-rolled steel sheets. However, increasing the strength of steel sheets typically reduces their ductility, flangeability, and other stamping formability properties, making them more prone to cracking during stamping and reducing the freedom of shape creation. Therefore, their application is limited to components with simple shapes. Thus, to apply high-strength steel sheets to complex-shaped components, it is crucial to increase the strength of the steel sheets while maintaining or improving formability.

[0003] Against this backdrop, as a technology to improve the ductility of steel sheets, TRIP steel, in which retained austenite (retained γ) is dispersed in the microstructure of the steel sheet, has been developed. TRIP steel forms retained γ in its microstructure, thus adding a large amount of Si. For example, Patent Document 1 discloses that by annealing steel containing C: 0.04–0.12%, Si: 0.8–2.5%, and Mn: 0.5–2.0%, followed by austenitic tempering at 300–500°C for 10–900 seconds (carbon distribution associated with bainitic transformation), 2–10% retained γ is generated, thereby obtaining a steel sheet with high ductility (TS×El ≥ 21000 MPa·%) and high elongation flange formability of over 70%.

[0004] On the other hand, it is known that as the Si content increases, Si accumulates on the surface of the annealed steel sheet, forming Si-based oxides, thereby deteriorating its chemical conversion treatability. To address this issue, for example, Patent Document 2 discloses a method for improving chemical conversion treatability by adding Ni in a manner that does not cause Si to accumulate on the surface of the steel sheet.

[0005] In addition, Patent Document 3 discloses a method in which the content of Mn enriched on the surface together with Si is appropriately controlled such that the Si / Mn ratio is 0.40 or less, thereby forming Mn-Si composite oxide on the surface and improving chemical conversion treatability.

[0006] In addition, Patent Document 4 discloses the following method: Si-based oxides are directly removed by pickling or brushing after annealing, thereby improving chemical conversion processability.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 5515623

[0010] Patent Document 2: Japanese Patent No. 2951480

[0011] Patent Document 3: Japanese Patent No. 3889768

[0012] Patent Document 4: Japanese Patent Application Publication No. 2003-201538 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] As mentioned above, adding Si is effective in improving the ductility of high-strength steel sheets. However, when actively utilizing Si to ensure high workability, the Si content and the chemical conversion treatability of the steel sheet become a trade-off. The methods disclosed in Patent Documents 2 and 4 are effective in improving the chemical conversion treatability of steels with high Si content, but it is also desirable to establish other techniques that adjust the alloying elements, annealing conditions, etc.

[0015] Furthermore, through the inventors' research, it was discovered that the method disclosed in Patent Document 3 may not necessarily ensure good chemical conversion treatment. When the steel contains a certain amount of B, the enrichment of Si on the surface of the steel plate is promoted, thus deteriorating the chemical conversion treatment.

[0016] Thus, the aforementioned technology, as a high-strength steel sheet with excellent ductility, chemical conversion properties, and pore-expanding properties, cannot be considered sufficient.

[0017] The present invention was made in view of the above circumstances, and its object is to provide steel plates, components and methods thereof with excellent ductility, hole expansion and chemical conversion treatment properties, and tensile strength of 780 MPa or more.

[0018] Here, tensile strength refers to the tensile strength (TS) obtained according to JIS Z2241 (2011).

[0019] In addition, excellent ductility means that the total elongation EL obtained according to JIS Z2241 (2011) satisfies any one of (A) to (C) below.

[0020] (A) When TS is 780MPa or higher and less than 980MPa, EL is 16.0% or higher;

[0021] (B) When TS is above 980MPa and below 1180MPa, EL is above 14.0%;

[0022] (C) When TS is above 1180MPa, EL is above 12.0%.

[0023] In addition, excellent porosity means that, in order to ensure the porosity required for practical use, the porosity λ (%) (={(d-d0) / d0}×100) obtained by the porosity test according to JFST1001 is 30% or more.

[0024] In addition, excellent chemical conversion treatment means that after degreasing (treatment temperature: 40℃, treatment time: 120 seconds, spray degreasing, degreasing agent: FC-E2011 manufactured by PALBOND Co., Ltd., Japan), surface conditioning (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by PALBOND Co., Ltd., Japan), and then chemical conversion treatment with zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature: 35℃, treatment time: 120 seconds, chemical conversion treatment solution: PALBOND PB-L3065 manufactured by PALBOND Co., Ltd., Japan), the exposed area of ​​the steel substrate is less than 10% relative to the total area.

[0025] Methods for solving problems

[0026] To address the aforementioned problems, the inventors conducted in-depth research on the steel composition, heat treatment conditions, and microstructure affecting the ductility and chemical conversion properties of various thin steel plates with tensile strengths of 780 MPa or higher. The results showed that by forming a composition containing, by mass%, C: 0.05–0.25%, Si: 0.30–1.50%, Mn: 1.5–4.5%, P: 0.005–0.050%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, Ti: 0.005–1.000%, B: 0.0010–0.0030%, satisfying the following formulas (1) and (2), with the balance consisting of iron and unavoidable impurities, and forming a composition in which the area ratio of polygonal ferrite is 10% or more and less than 80%, and the combined area ratio of upper bainite, tempered martensite, and lower bainite is 10% or more and less than 70%, the desired ductility and chemical conversion properties can be achieved. A steel structure consisting of a residual austenite (residual γ) of 3% to 15% or less and a quenched martensite area of ​​15% or less (including 0%) is formed. Based on this, a steel structure is formed in which the maximum concentration of P [Pm] within 1 μm from the surface of the steel plate in the thickness direction is 0.025% by mass or more when analyzing the luminescence intensity of P measured by glow discharge analysis from the surface of the steel plate in the thickness direction, and satisfies Equation (3), and the integral enrichment of Si within 1 μm from the surface of the steel plate in the thickness direction is 120 or less. Thus, a high-strength cold-rolled steel plate with excellent ductility, porosity and chemical conversion treatment properties is obtained.

[0027] [Si] / [Mn]≤0.35 …Equation (1)

[0028] 1000×[B] / [Mn]≤0.70 …Equation (2)

[0029] [Pm] / [P]≥1.5 …Equation (3)

[0030] In equation (1), [Si] represents the Si content (mass %), and [Mn] represents the Mn content (mass %).

[0031] In formula (2), [B] represents the content of B (mass%), and [Mn] represents the content of Mn (mass%).

[0032] In equation (3), [P] represents the P content (mass %).

[0033] This invention is based on the above insights, and its main points are as follows.

[0034] [1] A steel plate having:

[0035] Containing, by mass%, C: 0.05–0.25%, Si: 0.30–1.50%, Mn: 1.5–4.5%, P: 0.005–0.050%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, Ti: 0.005–1.000%, B: 0.0010–0.0030%, satisfying the following formulas (1) and (2), with the balance consisting of iron and unavoidable impurities; and

[0036] Steel microstructures with an area ratio of polygonal ferrite of 10% or more and less than 80%, a combined area ratio of upper bainite, tempered martensite, and lower bainite of 10% or more and less than 70%, a volume ratio of retained austenite of 3% or more and less than 15%, and an area ratio of quenched martensite of 15% or less (including 0%).

[0037] The maximum concentration of P [Pm] within 1 μm of the steel plate surface along the plate thickness is 0.025% by mass or more, and satisfies equation (3).

[0038] Furthermore, the integral enrichment of Si within 1 μm of the thickness direction from the surface layer is less than 120.

[0039] [Si] / [Mn]≤0.35 …Equation (1)

[0040] 1000×[B] / [Mn]≤0.70 …Equation (2)

[0041] [Pm] / [P]≥1.5 …Equation (3)

[0042] In equation (1), [Si] represents the Si content (mass %), and [Mn] represents the Mn content (mass %).

[0043] In formula (2), [B] represents the content of B (mass%), and [Mn] represents the content of Mn (mass%).

[0044] In equation (3), [P] represents the P content (mass %).

[0045] [2] The steel plate according to [1], wherein, as part of the above-mentioned composition, it further contains one or more of the following components selected by mass percentage: Cu: less than 1%, Ni: less than 1%, Cr: less than 1%, Mo: less than 0.5%, V: less than 0.5%, Nb: less than 0.1%, Mg: less than 0.0050%, Ca: less than 0.0050%, Sn: less than 0.1%, Sb: less than 0.1%, REM: less than 0.0050%.

[0046] [3] A component made of steel plate as described in [1] or [2] above.

[0047] [4] A method for manufacturing a steel plate, comprising a method for manufacturing a cold-rolled steel plate obtained by hot rolling, pickling and cold rolling of a steel billet having the composition described in [1] or [2] above, wherein,

[0048] The above annealing includes:

[0049] The homogenization and holding process, wherein, for the aforementioned cold-rolled steel sheet, is heated to A in a furnace atmosphere with a dew point below -40°C. c1 Point +20℃ and A c3 The homogenization temperature below the point and above Tc calculated by equation (4) is maintained at the above homogenization temperature for 30 to 500 s;

[0050] The first cooling process involves cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50°C / s within a temperature range from the above-mentioned heat homogenization temperature to a first cooling stop temperature of 350 to 550°C.

[0051] The second cooling process includes, after stopping cooling at the first cooling stop temperature, maintaining a temperature range of 350–550°C for 10–60 seconds, and then cooling at a second average cooling rate of 2–50°C / s to a second cooling stop temperature of 200–420°C; and

[0052] The isothermal holding process involves holding the temperature at the second cooling stop temperature for 60 to 3000 seconds.

[0053] Tc(℃)=663-1.2×exp(20 / t)×Tdp…(4)

[0054] Here, t represents the holding time (s) at the above homogenization temperature, and Tdp represents the dew point (°C).

[0055] [5] A method for manufacturing a steel plate, comprising a method for manufacturing a cold-rolled steel plate obtained by hot rolling, pickling and cold rolling of a steel billet having the composition described in [1] or [2] above, wherein,

[0056] The above annealing includes:

[0057] The homogenization and holding process, wherein, for the aforementioned cold-rolled steel sheet, is heated to A in a furnace atmosphere with a dew point below -40°C. c1 Point +20℃ and A c3 The homogenization temperature below the point and above Tc calculated by equation (4) is maintained at the above homogenization temperature for 30 to 500 s;

[0058] The cooling process includes cooling from the aforementioned heat spreader temperature to a cooling stop temperature of 200–420°C at an average cooling rate of 2–50°C / s; and

[0059] The isothermal holding process involves holding the temperature at the aforementioned cooling stop temperature for 60–3000 seconds.

[0060] Tc(℃)=663-1.2×exp(20 / t)×Tdp…(4)

[0061] Here, t represents the holding time (s) at the above homogenization temperature, and Tdp represents the above dew point (°C).

[0062] [6] A method for manufacturing a component, comprising a step of forming or joining a steel plate as described in [1] or [2] to produce the component.

[0063] Invention Effects

[0064] According to the present invention, steel plates and components with high strength (TS) of 780 MPa or higher, and excellent ductility, hole expansion and chemical conversion properties can be obtained.

[0065] When the steel sheet of this invention is applied to the skeleton components of an automobile body, complex and difficult-to-form components can be manufactured by cold stamping, thus greatly contributing to the lightweighting of the automobile body. It eliminates the need for expensive alloying elements and post-annealing treatments to improve chemical conversion processability, thereby reducing material costs. Attached Figure Description

[0066] Figure 1 This is a graph illustrating the maximum concentration of P [Pm] used in this invention.

[0067] Figure 2 This is a graph illustrating the Si integral enrichment amount of the present invention. Detailed Implementation

[0068] The present invention will now be described in detail. It should be noted that the present invention is not limited to the embodiments described below.

[0069] (steel plate)

[0070] The steel plate of the present invention is a high-strength steel plate with excellent ductility, hole expansion and chemical conversion treatment properties, having a tensile strength (TS) of 780 MPa or higher. It comprises, by mass%, C: 0.05–0.25%, Si: 0.30%–1.50%, Mn: 1.5–4.5%, P: 0.005–0.050%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, Ti: 0.005–1.000%, B: 0.0010–0.0030%, satisfying the following formulas (1) and (2), with the balance consisting of iron and unavoidable impurities; and a polygonal shape. For a steel microstructure with an area fraction of ferrite of 10% or more and 80% or less, a combined area fraction of upper bainite, tempered martensite, and lower bainite of 10% or more and 70% or less, a volume fraction of retained austenite of 3% or more and 15% or less, and an area fraction of quenched martensite of 15% or less (including 0%), when analyzing the luminescence intensity of P measured by glow discharge analysis in the thickness direction from the surface, the maximum concentration of P [Pm] within 1 μm in the thickness direction from the surface of the steel plate is 0.025% by mass or more, and satisfies the following equation (3), and furthermore, the integral enrichment of Si within 1 μm in the thickness direction from the surface of the steel plate is 120 or less.

[0071] [Si] / [Mn]≤0.35 …Equation (1)

[0072] 1000×[B] / [Mn]≤0.70 …Equation (2)

[0073] [Pm] / [P]≥1.5 …Equation (3)

[0074] In equation (1), [Si] represents the Si content (mass %), and [Mn] represents the Mn content (mass %).

[0075] In formula (2), [B] represents the content of B (mass%), and [Mn] represents the content of Mn (mass%).

[0076] In equation (3), [P] represents the P content (mass %).

[0077] The steel sheet of the present invention will now be described in the order of composition and steel structure. First, the rationale for defining the composition of the present invention will be explained. It should be noted that, unless otherwise specified, all percentages of steel components in the following description are by mass.

[0078] <C: 0.05~0.25%>

[0079] C is included from the viewpoint of ensuring the specified strength through phase transformation strengthening and improving ductility by ensuring a specified amount of retained austenite (retained γ). When the C content is less than 0.05%, these effects cannot be adequately ensured.

[0080] On the other hand, considering concerns about the hole expansion property, which is important in stamping formability, and the weldability, which is important when assembling automotive components onto the car body for spot welding or laser welding, the upper limit of C content is set at 0.25%.

[0081] 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. In addition, the C content is preferably 0.22% or less, more preferably 0.20% or less.

[0082] <Si: 0.30~1.50%>

[0083] Si is included from the perspective of increasing strength by strengthening ferrite and improving ductility by suppressing the formation of carbides in martensite and bainite and ensuring a specified amount of residual γ. When the Si content is less than 0.30%, these effects cannot be adequately ensured.

[0084] On the other hand, when the Si content exceeds 1.50%, even the manufacturing method specified in this invention cannot ensure good chemical conversion treatment.

[0085] 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. In addition, the Si content is preferably 1.20% or less, more preferably 1.00% or less.

[0086] <Mn: 1.5~4.5%>

[0087] Mn is included from the perspective of improving the hardenability of steel plates, promoting high strength through phase transformation strengthening, and, like Si, inhibiting the formation of carbides in bainite and promoting the formation of retained austenite that contributes to ductility, thereby improving ductility.

[0088] To achieve these effects, the Mn content needs to be above 1.5%.

[0089] On the other hand, when the Mn content exceeds 4.5%, the bainitic transformation is significantly delayed, and the required amount of residual austenite cannot be guaranteed, resulting in reduced ductility. In addition, when the Mn content exceeds 4.5%, the martensitic transformation initiation temperature is lowered, making it difficult to suppress the formation of coarse quenched martensite, which deteriorates the elongation flange formability (pore expansion).

[0090] 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. In addition, the Mn content is preferably 3.5% or less, more preferably 3.0% or less.

[0091] <P: 0.005~0.050%>

[0092] P is an element that strengthens steel. In addition, by appropriately controlling the P content, a surface enrichment of P is generated on the surface of the annealed steel sheet. Therefore, P is an element that can improve chemical conversion treatment. From this point of view, the P content is set to 0.005% or more.

[0093] On the other hand, a high phosphorus content degrades spot weldability. From this perspective, the phosphorus content is set to be below 0.050%.

[0094] Therefore, the phosphorus (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. Furthermore, the P content is preferably 0.040% or less, more preferably 0.030% or less.

[0095] <S: less than 0.01%>

[0096] S is an element that improves the peeling properties of oxide scale during hot rolling and inhibits nitriding during annealing, but it also has adverse effects on spot weldability, bendability, and hole expansion. To reduce these adverse effects, the S content is set to at least 0.01% or less, and preferably to 0.0050% or less.

[0097] It should be noted that it is also possible to eliminate sulfur, but reducing it to less than 0.0001% would incur significant costs. Therefore, from a manufacturing cost perspective, the sulfur content is preferably set to 0.0001% or more. More preferably, the sulfur content is 0.0005% or more, and even more preferably 0.0010% or more.

[0098] <sol.Al: less than 1.0%>

[0099] Al is included for deoxygenation or to obtain residual γ. There is no specific lower limit for sol.Al, but for stable deoxygenation, the sol.Al content is preferably set to 0.005% or higher.

[0100] On the other hand, when the sol.Al content is 1.0% or more, the amount of coarse Al inclusions increases significantly, reducing the formability (hole expansion) of the extended flange. Furthermore, Al is an element that degrades the chemical conversion treatability of the steel sheet; also, when the sol.Al content is 1.0% or more, good chemical conversion treatability cannot be guaranteed in this invention. Therefore, the sol.Al content is set to be less than 1.0%. The sol.Al content is preferably 0.80% or less, more preferably 0.06% or less.

[0101] <N: less than 0.015%>

[0102] Nitrogen (N) is an element that forms nitrides such as BN, AlN, and TiN in steel. Because it reduces the formability (hole expansion) of extended flanges, its content needs to be limited. Therefore, the N content is set to be less than 0.015%. The N content is preferably 0.010% or less, more preferably 0.006% or less.

[0103] It should be noted that it is also possible to eliminate N, but reducing it to less than 0.0001% would incur significant costs. Therefore, from a manufacturing cost perspective, the N content is preferably 0.0001% or more. More preferably, the N content is 0.0005% or more, and even more preferably 0.001% or more.

[0104] <Ti: 0.005~1.000%>

[0105] Ti has the effect of fixing nitrogen in steel in the form of TiN, improving hot ductility, and improving hardenability of boron. In addition, it has the effect of refining the microstructure through the precipitation of TiC. To obtain these effects, the Ti content is set to 0.005% or more. The Ti content is further preferably set to 0.010% or more. The Ti content is more preferably 0.020% or more.

[0106] On the other hand, when the Ti content exceeds 1.000%, it leads to an increase in rolling load and a decrease in ductility due to the increase in precipitation strengthening. Therefore, the Ti content is set to 1.000% or less. Preferably, the Ti content is 0.080% or less, and more preferably 0.050% or less.

[0107] <B: 0.0010~0.0030%>

[0108] Boron (B) is an element that improves the hardenability of steel, and has the advantage of easily forming tempered martensite and / or bainite with a specified area ratio. To achieve this effect, the B content is set to 0.0010% or higher.

[0109] On the other hand, when the boron content exceeds 0.0030%, it accumulates on the surface of the steel plate during homogenization, causing the Mn-based oxides to coarsen and thus deteriorating the chemical conversion treatment properties. Therefore, the boron content is set to 0.0030% or less. The boron content is preferably 0.0020% or less.

[0110] <[Si] / [Mn]≤0.35…Formula (1)>

[0111] In equation (1), [Si] represents the Si content (mass%) and [Mn] represents the Mn content (mass%).

[0112] The [Si] / [Mn] ratio determines the Si to Mn composition ratio of the surface oxide formed during annealing. Within the manufacturing conditions specified in this invention, a Si / Mn ratio exceeding 0.35 cannot ensure good chemical conversion processability. Therefore, the [Si] / [Mn] ratio is set to 0.35 or less. The [Si] / [Mn] ratio is preferably 0.32 or less, more preferably 0.30 or less. Furthermore, while there is no particular limitation on the lower limit, the [Si] / [Mn] ratio is preferably 0.10 or more, more preferably 0.15 or more.

[0113] <1000×[B] / [Mn]≤0.70 …Equation (2)>

[0114] In equation (2), [B] represents the content of B (mass%), and [Mn] represents the content of Mn (mass%).

[0115] B is an element that accumulates on the surface of the steel sheet during homogenization. A new discovery: B promotes the formation of coarse Mn-based oxides on the steel sheet surface, creating Mn-deficient regions around them. Even when [Si] / [Mn] ≤ 0.35, it also promotes the formation of Si-based oxides on the steel sheet surface, deteriorating the chemical conversion treatability. In-depth research on this issue revealed that by satisfying 1000 × [B] / [Mn] ≤ 0.70, the formation of Mn-deficient regions generated during homogenization can be suppressed, and the formation of Si-based oxides can be suppressed to a level that does not deteriorate the chemical conversion treatability. Therefore, 1000 × [B] / [Mn] is set to 0.70 or less. 1000 × [B] / [Mn] is preferably 0.68 or less, more preferably 0.65 or less. Furthermore, the lower limit is not particularly limited, but 1000 × [B] / [Mn] is preferably 0.30 or more, more preferably 0.35 or more.

[0116] The steel plate of this invention contains the aforementioned constituent elements as basic components, with the balance being iron (Fe) and unavoidable impurities. It should be noted that the steel plate of this invention preferably has a composition in which the balance consists of Fe and unavoidable impurities.

[0117] In addition to the above-mentioned components, the steel plate of the present invention may also contain one or more optional elements selected from the following.

[0118] Cu: less than 1%, Ni: less than 1%, Cr: less than 1%, Mo: less than 0.5%, V: less than 0.5%, Nb: less than 0.1%, Mg: less than 0.0050%, Ca: less than 0.0050%, Sn: less than 0.1%, Sb: less than 0.1%, and REM: less than 0.0050%.

[0119] <Cu: less than 1%>

[0120] Cu improves the corrosion resistance of automobiles in the operating environment. Furthermore, the corrosion products containing Cu coat the surface of the steel sheet, inhibiting hydrogen penetration. Cu is an element incorporated when utilizing waste materials as raw materials; by allowing the inclusion of Cu, recycled materials can be utilized as raw materials, reducing manufacturing costs. From this perspective, a Cu content of 0.005% or more is preferred, and from the viewpoint of improving resistance to delayed fracture, a Cu content of 0.05% or more is more preferred. A Cu content of 0.10% or more is even more preferred. A Cu content of 0.25% or more is even more preferred, and a Cu content of 0.50% or more is even more preferred.

[0121] However, excessive Cu content leads to surface defects. Therefore, in cases where Cu is present, the Cu content is set to below 1%.

[0122] <Ni: less than 1%>

[0123] Like Cu, Ni is an element that improves corrosion resistance. Furthermore, Ni helps suppress the formation of surface defects that are prone to occur when Cu is present. Therefore, it is preferable that Ni contains 0.01% or more. More preferably, the Ni content is 0.04% or more, and even more preferably 0.06% or more.

[0124] However, when the Ni content becomes too high, the oxide scale formation in the furnace becomes uneven, which in turn causes surface defects. Furthermore, it increases costs. Therefore, in the case of Ni, the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, more preferably 0.3% or less.

[0125] <Cr: less than 1%>

[0126] Cr can be included to improve the hardenability of steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve such effects, the Cr content is preferably set to 0.01% or more. More preferably, the Cr content is 0.03% or more, and even more preferably 0.06% or more.

[0127] However, excessive Cr content deteriorates pitting corrosion resistance; therefore, when Cr is present, 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 further preferably 0.30% or less, and even more preferably 0.10% or less.

[0128] <Mo: 0.5% or less>

[0129] Mo can be included to improve the hardenability of steel and to suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Mo content is preferably set to 0.01% or more. More preferably, the Mo content is 0.03% or more, and even more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, and even more preferably 0.2% or more.

[0130] However, Mo significantly degrades the chemical conversion processability of cold-rolled steel sheets; therefore, in cases where Mo is present, the Mo content is set to 0.5% or less. More preferably, the Mo content is 0.4% or less.

[0131] <V: below 0.5%>

[0132] V can be included to improve the hardenability of steel, suppress carbide formation in martensite, upper bainite / lower bainite, refine the microstructure, and improve resistance to delayed fracture by precipitating carbides. To achieve these effects, the V content is preferably set to 0.003% or more. More preferably, the V content is 0.005% or more, and even more preferably 0.010% or more. Even more preferably, the V content is 0.020% or more, and even more preferably 0.050% or more.

[0133] However, when a large amount of V is present, the castability deteriorates significantly. Therefore, when V is present, the V content is set to 0.5% or less. Preferably, the V content is 0.3% or less, and more preferably 0.2% or less.

[0134] <Nb: below 0.1%>

[0135] Nitrogen (Nb) can be included to achieve the effects of refining the steel microstructure, increasing its strength, promoting bainitic phase transformation through grain refinement, improving bending properties, and enhancing resistance to delayed fracture. To obtain these effects, the Nb content is preferably set to 0.002% or more. More preferably, the Nb content is 0.004% or more, and even more preferably 0.010% or more.

[0136] However, when Nb is present in large quantities, precipitation strengthening becomes excessive, reducing ductility. Furthermore, it leads to increased rolling load and deterioration of castability. Therefore, when Nb is present, the Nb content is set to 0.1% or less. Preferably, the Nb content is 0.05% or less, more preferably 0.03% or less.

[0137] <Mg: less than 0.0050%>

[0138] Mg fixes O in the form of MgO, which helps improve formability such as flexibility. Therefore, the Mg content is preferably set to 0.0002% or more. The Mg content is more preferably 0.0004% or more, and even more preferably 0.0006% or more. The Mg content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0139] On the other hand, adding a large amount of Mg deteriorates the surface quality and flexibility. Therefore, when Mg is present, the Mg content is set to 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

[0140] <Ca: below 0.0050%>

[0141] Ca fixes S in the form of CaS, which helps improve flexural properties and resistance to delayed fracture. Therefore, the Ca content is preferably set to 0.0002% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. The Ca content is preferably 0.0015% or more, and more preferably 0.0020% or more.

[0142] On the other hand, adding a large amount of Ca deteriorates surface quality and flexibility. Therefore, when Ca is present, the Ca content is set to 0.0050% or less. Preferably, the Ca content is 0.0040% or less.

[0143] <Sn: less than 0.1%>

[0144] Sn inhibits oxidation and nitriding on the surface of the steel plate, thus suppressing the resulting decrease in the content of carbon (C) and boron (B) in the surface layer. This effect suppresses ferrite formation on the surface of the steel plate, resulting in increased strength and improved fatigue resistance. From this perspective, the Sn content is preferably set to 0.003% or more. More preferably, the Sn content is 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.

[0145] On the other hand, when the Sn content exceeds 0.1%, castability deteriorates. Furthermore, Sn segregates at the original γ grain boundaries, worsening resistance to delayed fracture. Therefore, in cases where Sn is present, the Sn content is set to 0.1% or less.

[0146] <Sb: below 0.1%>

[0147] Sb inhibits oxidation and nitriding of the steel plate surface layer, thereby suppressing the resulting decrease in C and B content in the surface layer. This effect suppresses ferrite formation in the steel plate surface layer, resulting in increased strength and improved fatigue resistance. From this perspective, the Sb content is preferably set to 0.002% or more. More preferably, it is 0.004% or more, and even more preferably 0.006% or more. More preferably, it is 0.008% or more, and even more preferably 0.010% or more. Preferably, it is 0.015% or more, and even more preferably 0.030% or more.

[0148] On the other hand, when the Sb content exceeds 0.1%, castability deteriorates, and segregation at the original γ grain boundaries worsens resistance to delayed fracture. Therefore, in the case of Sb, the Sb content is set to 0.1% or less.

[0149] <REM: below 0.0050%>

[0150] REM (Resin M) is an element that suppresses the adverse effects of sulfides on the formability of extended flanges by spherizing the shape of sulfides, thereby improving the formability of extended flanges. To achieve these effects, it is preferable to set the REM content to 0.0005% or more. More preferably, the REM content is 0.0010% or more, and even more preferably 0.0020% or more.

[0151] On the other hand, when the REM content exceeds 0.0050%, the improvement effect on the formability of the extended flange becomes saturated. Therefore, in the case of REM, the REM content is set to 0.0050% or less.

[0152] It should be noted that, in this invention, REM refers to the lanthanide elements, namely scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in this invention refers to the total content of one or more elements selected from the aforementioned REM elements.

[0153] When the optional components described above are contained in amounts below the lower limit, the presence of optional elements in amounts below the lower limit does not impair the effects of the present invention. Therefore, when the optional elements described above are contained in amounts below the lower limit, these optional elements are contained as unavoidable impurities.

[0154] Next, the mechanical properties of the steel sheet (cold-rolled steel sheet with excellent material stability) to which this invention is applied will be described.

[0155] The tensile strength (TS) of the steel plate of the present invention is set to be 780 MPa or higher. There is no particular upper limit to the tensile strength, but from the viewpoint of balancing other properties, the tensile strength is preferably 1300 MPa or lower.

[0156] For the steel sheet of the present invention, a total elongation EL of 16.0% or more is ensured when TS is 780 MPa or more and less than 980 MPa, a total elongation EL of 14.0% or more is ensured when TS is 980 MPa or more and less than 1180 MPa, and a total elongation EL of 12.0% or more is ensured when TS is 1180 MPa or more. As a result, the stability of stamping is significantly improved.

[0157] In the evaluation of tensile properties, JIS No. 5 tensile test pieces were cut from the center of the plate width, and tensile tests were conducted with N=3 (according to JIS Z2241(2011)). Each evaluation was based on the average value of three points. Steel plates with a tensile strength of 780 MPa or higher were considered high-strength steel plates. Steel plates with a total elongation EL of 16.0% or higher when TS is 780 MPa or higher but less than 980 MPa, a total elongation EL of 14.0% or higher when TS is 980 MPa or higher but less than 1180 MPa, and a total elongation EL of 12.0% or higher when TS is 1180 MPa or higher were considered steel plates with excellent ductility. Furthermore, to ensure the required hole-expanding properties for practical use, a hole-expanding ratio λ(%) (={(d-d0) / d0}×100) obtained through the hole-expanding test according to JFST1001 was required to be 30% or higher as a necessary condition for this invention.

[0158] Next, the steel structure of the steel plate of the present invention will be described.

[0159] <Area ratio of polygonal ferrite: 10% to 80%>

[0160] From the perspective of ensuring high ductility, the polygonal ferrite is set to 10% or more in terms of area ratio, and preferably 20% or more in order to obtain even higher ductility.

[0161] On the other hand, when the polygonal ferrite content exceeds 80%, the desired strength is sometimes not obtained. Therefore, the polygonal ferrite content is set to 80% or less in terms of area ratio, preferably 75% or less, and more preferably 70%.

[0162] <The combined area ratio of upper bainite, tempered martensite, and lower bainite: ≥10% and ≤70%>

[0163] To obtain the desired strength, the total area ratio of upper bainite, tempered martensite, and lower bainite is set to 10% or more, and preferably 15% or more to obtain even higher strength.

[0164] On the other hand, when the combined area ratio of upper bainite, tempered martensite, and lower bainite exceeds 70%, the ductility decreases due to excessive strength enhancement; therefore, the area ratio is set to 70% or less. More preferably, it is set to 65% or less, and even more preferably, it is set to 60% or less.

[0165] <Volume fraction of retained austenite (retained γ): 3% or more and 15% or less>

[0166] When the volume fraction of retained austenite is less than 3%, the desired ductility cannot always be guaranteed. From a ductility point of view, the volume fraction of retained austenite is set to 3% or more, preferably 5% or more.

[0167] On the other hand, when the volume fraction of retained austenite exceeds 15%, the formability (cavity expansion) of the extended flange sometimes decreases. Therefore, the volume fraction of retained austenite is set to 15% or less. Preferably, the volume fraction of retained austenite is 13% or less.

[0168] <Area ratio of quenched martensite: 15% or less (including 0%)>

[0169] The hard, quenched martensite structure reduces λ, therefore its area fraction needs to be suppressed. To obtain the practically desired 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 can be 0% or more, or 3% or more.

[0170] <Residual Organization>

[0171] Regarding the steel microstructure, the components other than those described above constitute the remaining microstructure. The area fraction of the remaining microstructure is preferably set to 5% or less. The remaining microstructure can be unrecrystallized ferrite, carbides, or pearlite. These microstructures can be determined by SEM observation, as described later.

[0172] <The maximum concentration of P [Pm] within 1 μm of the steel plate surface along the thickness direction is 0.025% by mass or more, and satisfies equation (3)>

[0173] [Pm] / [P]≥1.5 …Equation (3)

[0174] In equation (3), [P] (which can also be expressed as [Pi]) is the P content (mass %).

[0175] A thorough study was conducted on various elements affecting chemical conversion treatability, their surface enrichment, and the types of oxides formed during annealing. The results showed that even under manufacturing conditions where oxide formation was not observed, chemical conversion treatability could not be adequately ensured. For steel plates that had ensured chemical conversion treatability, the maximum concentration of P near the surface was quantitatively evaluated using the method described later. The results showed that good chemical conversion treatability could be ensured by forming a steel microstructure in which the maximum concentration of P [Pm] within 1 μm of the thickness from the surface of the steel plate was 0.025% by mass or more, and which satisfied Equation (3), when analyzing the luminescence intensity of P measured by GDS (glow discharge analysis) in the thickness direction from the surface.

[0176] The detailed mechanism is not yet clear, but it is believed that the maximum concentration of P on the surface is locally higher than that of the steel composition. In addition, if the maximum concentration of P is insufficient, the shape of the chemically converted crystals after chemical conversion treatment is scaly. Therefore, the local surface enrichment of P has the effect of suppressing the formation of Si-based oxides on the surface that have an adverse effect on the chemical conversion treatment.

[0177] [Pm] is preferably 0.030% by mass or more, more preferably 0.035% by mass or more. Furthermore, there is no particular upper limit, but [Pm] is preferably 0.100% by mass or less, more preferably 0.090% by mass or less.

[0178] The ratio of [Pm] to [P] is preferably 1.7 or higher, and more preferably 1.9 or higher. Furthermore, there is no particular upper limit, but the ratio of [Pm] to [P] is preferably 10.0 or lower, and more preferably 9.0 or lower.

[0179] <The integral enrichment of Si within 1 μm of the steel plate surface along the thickness direction is less than 120>

[0180] In this invention, to suppress the formation of Si-based oxides during homogenization, 1000 × [B] / [Mn] is set to 0.70 or less. To more reliably achieve this effect, in this invention, the integral enrichment of Si within 1 μm from the surface of the manufactured steel sheet is set to 120 or less. This ensures good chemical conversion processability. Thus, in this invention, the integral enrichment of Si within 1 μm from the surface of the steel sheet is set to 120 or less. More preferably, it is 100 or less. The lower limit is not particularly limited, but preferably 80 or more, more preferably 90 or more.

[0181] Next, the method for measuring steel microstructure will be explained.

[0182] The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were determined as follows: A plate section parallel to the rolling direction was cut out, mirror-polished, and etched with 1% nitric acid ethanol solution. At 1 / 4 of the thickness, a 25μm × 20μm area in 10 fields of view was observed using SEM at 5000x magnification and photographed. The obtained microstructure images were quantified using image analysis.

[0183] Polygonal ferrite is defined as ferrite that is relatively equiaxial and contains almost no carbides internally. In SEM, it appears as the darkest region.

[0184] Upper bainite is a ferrite microstructure containing carbides or retained austenite that appear white under SEM. It should be noted that when it is difficult to distinguish between upper bainite and polygonal ferrite, regions of ferrite with an aspect ratio ≤ 2.0 are classified as polygonal ferrite, and regions with an aspect ratio > 2.0 are classified as upper bainite, and their area ratios are calculated. Here, regarding the aspect ratio, the longest major axis length *a* of the particle is determined, and the longest length of the particle that traverses the particle in the direction perpendicular to it is defined as the minor axis length *b*. *a / b* is then used as the aspect ratio.

[0185] Tempered martensite and lower bainite are regions in which lath-like substructures and carbide precipitation are present inside under SEM.

[0186] Quenched martensite (fresh martensite) is a whitish, blocky region that appears to have no internal substructure under SEM.

[0187] The remaining microstructure contains at least one of unrecrystallized ferrite, carbides, and pearlite. These were identified by SEM. The unrecrystallized ferrite was identified as ferrite with a black contrast, indicating deformation introduced through rolling. The carbides and pearlite were identified as microstructures with a white contrast. The carbides had a grain size of less than 1 μm, and the pearlite was a lamellar structure, thus allowing for differentiation.

[0188] The volume fraction of retained austenite was determined by X-ray diffraction after chemical grinding from the surface to a position representing 1 / 4 of the thickness. An incident X-ray source (Co-Kα) was used, and the volume fraction of retained austenite was calculated by the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction can be used as the area fraction of retained austenite.

[0189] Regarding the surface enrichment of P and Si on and near the surface of the steel plate, sputtering analysis was performed in the depth direction (thickness direction) using a GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure of 600 Pa, high-frequency output of 35 W, measurement interval of 0.1 s, and measurement time of 150 s. Then, using a standard curve obtained in advance with known P and Si contents from standard samples, the maximum concentration of P [Pm] and the integral enrichment of Si within 1 μm of the steel plate surface in the thickness direction were determined.

[0190] Regarding the integral enrichment of Si, the difference between the Si amount obtained every 0.1 seconds within 1 μm along the thickness direction from the steel plate surface and the average Si amount obtained over a measurement time of 140–150 seconds is calculated. The measurement data within 1 μm are then integrated to determine the integral enrichment of Si. It should be noted that under these measurement conditions, the measurement position d (μm) from the surface is obtained using the sputtering time ts from the formula d = ts / 1.7 (μm).

[0191] In this invention, such as Figure 1 As shown, during the above 150s measurement time, the maximum concentration ([Pm]) is obtained by converting the highest P intensity value to mass% using the standard curve.

[0192] As a method for converting to this mass %, the correlation between the intensity of the P element obtained by GDS and the amount of P is determined in the data obtained by measuring a standard material with a known amount of P under the same conditions, and the intensity of P in the example determined therefrom is converted into concentration. Figure 1 In this context, Pi represents the phosphorus content (mass %) in the steel plate.

[0193] Furthermore, regarding the integral enrichment of Si, by using... Figure 2 The measurement time (s) shown in (a) is divided by 1.7 to convert to depth position (μm), relative to the average concentration of the bulk matrix (refer to). Figure 2 (b) The integral enrichment of Si is obtained by integrating the difference between the dashed area and the Si concentration (mass%) at each depth location. Figure 2 (b) The diagonal part.

[0194] (Methods for manufacturing steel plates)

[0195] Next, the method for manufacturing the steel plate of the present invention will be described.

[0196] <First Implementation>

[0197] The steel sheet manufacturing method of the first embodiment of the present invention is a method for manufacturing a steel sheet by hot rolling, pickling and cold rolling a steel billet having the above-mentioned composition, followed by annealing the resulting cold-rolled steel sheet, wherein the annealing includes a homogenization and holding process, wherein the cold-rolled steel sheet is heated to A in a furnace atmosphere with a dew point of -40°C or lower. c1 Point +20℃ and A c3 The process includes: a homogenization temperature below the point and above Tc calculated by equation (4), held at the homogenization temperature for 30 to 500 s; a first cooling process, wherein the temperature is cooled to the first cooling stop temperature at a first average cooling rate of 2 to 50 °C / s within a temperature range from the homogenization temperature to a first cooling stop temperature of 350 to 550 °C; a second cooling process, wherein after cooling is stopped at the first cooling stop temperature, the temperature is held for 10 to 60 s within a temperature range of 350 to 550 °C, and then cooled to a second cooling stop temperature of 200 to 420 °C at a second average cooling rate of 2 to 50 °C / s; and an isothermal holding process, wherein the temperature is held at the second cooling stop temperature for 60 to 3000 s.

[0198] Tc(℃) = 663 - 1.2 × exp(20 / t) × Tdp … Equation (4)

[0199] Here, t represents the holding time at the above homogenization temperature (homogenization holding time) (s), and Tdp represents the dew point (°C).

[0200] <Hot Rolled>

[0201] Methods for hot rolling steel billets include heating the billet before rolling, rolling continuously cast billets directly without heating, and subjecting continuously cast billets to short-time heat treatment before rolling. Hot rolling can be carried out using conventional methods; for example, the billet heating temperature can be set above 1100℃. Alternatively, the billet heating temperature can be set below 1300℃. Furthermore, the soaking temperature can be set at 20 minutes or more. Alternatively, the soaking temperature can be set below 300 minutes. Additionally, the finishing rolling temperature can be set to A... r3 Above the phase transformation point. Additionally, the finishing rolling temperature can be set to A. r3 The phase transition point is below +200°C. Additionally, the winding temperature can be set to 400°C or higher. Alternatively, the winding temperature can be set to 720°C or lower. The winding temperature is preferably controlled from the viewpoint of suppressing sheet thickness variation and stably ensuring high strength. Specifically, the winding temperature is preferably set to 430°C or higher. Furthermore, the winding temperature is preferably set to 530°C or lower.

[0202] It should be noted that A r3 The phase transformation point can be calculated based on the composition of the steel plate and the following empirical formula (A).

[0203] A r3 Point (℃)=910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]…Formula (A)

[0204] (In the above formula, [M] represents the content (mass%) of element M in the steel billet, and the value of elements that are not present is set to zero (0).)

[0205] <Pickling>

[0206] Pickling can be performed using standard methods.

[0207] <Cold Rolling>

[0208] Cold rolling can be performed using conventional methods, with the rolling rate (cumulative rolling rate) set at 30% or higher. Alternatively, the rolling rate (cumulative rolling rate) can be set to 85% or lower. The rolling rate is preferably controlled from the perspective of consistently ensuring high strength and minimizing anisotropy. Specifically, a rolling rate of 35% or higher is preferred. It should be noted that under high rolling loads, softening annealing treatment can be performed at 450–730°C using a CAL (continuous annealing line) or BAF (box annealing furnace).

[0209] Annealing

[0210] For cold-rolled steel sheets manufactured according to conventional methods, annealing is performed under the following conditions. There are no particular limitations on the annealing equipment, but from the viewpoint of ensuring productivity and the desired heating and cooling rates, it is preferable to carry out the annealing using a continuous annealing line (CAL).

[0211] [Heating and holding process: In a furnace atmosphere with a dew point below -40℃, heat to A] c1 Point +20℃ and A c3 [The temperature is below the point and above Tc, and the temperature is maintained at the homogenization temperature for 30 to 500 seconds.]

[0212] Dew point affects oxide formation on the surface of steel plates during annealing. When the dew point exceeds -40°C, the amount of oxides formed on the steel plate surface increases excessively, thus deteriorating the chemical conversion treatment properties. Therefore, the dew point is set below -40°C.

[0213] There is no particular limitation on the lower limit, but the dew point is preferably set to -70°C or higher, and more preferably to -60°C or higher.

[0214] The steel sheet obtained in this invention contains a soft ferrite structure, thus improving ductility. Therefore, the soaking temperature is set at A, where ferrite is formed. c1 Point +20℃ and A c3Click below.

[0215] Furthermore, by setting the homogenization temperature to Tc (°C) or higher, it is possible to ensure that the surface enrichment of P in the surface enrichment portion of P formed on the steel plate surface is the amount specified in this invention. Tc is calculated from the dew point and the homogenization holding time in equation (4).

[0216] Tc(℃) = 663 - 1.2 × exp(20 / t) × Tdp … Equation (4)

[0217] Here, t represents the holding time (s) at the homogenization temperature, and Tdp represents the dew point (°C).

[0218] When the soaking temperature is below Tc (°C), the specified surface enrichment of P cannot be guaranteed, leading to deterioration of the chemical conversion treatment. Therefore, in a furnace atmosphere with a dew point below -40°C, the soaking temperature is set to A. c1 Point +20℃ and A c3 Below the point and above Tc (°C).

[0219] In addition, when the holding time at the above-mentioned homogenization temperature is less than 30 seconds, the formation of austenite at the above-mentioned homogenization temperature may not be sufficient, resulting in more polygonal ferrite and failing to obtain the desired total area ratio of upper bainite, tempered martensite and lower bainite, thus failing to obtain the desired strength; sometimes, the retained austenite cannot be sufficiently obtained, and the desired ductility cannot be ensured.

[0220] On the other hand, when the holding time at the aforementioned homogenization temperature exceeds 500 seconds, significant coarsening of the tissue occurs, thus sometimes failing to ensure the desired strength. Additionally, the desired ductility is sometimes not achieved.

[0221] Therefore, the holding time (soaking time) at the above annealing temperature is set to 30 to 500 seconds.

[0222] The time spent at the homogenization temperature (homogenization time) is preferably 60 seconds or more, more preferably 100 seconds or more. Furthermore, the time spent at the homogenization temperature (homogenization time) is preferably 400 seconds or less, more preferably 300 seconds or less.

[0223] It should be noted that the above A c1 and A c3 A can be obtained using empirical formulas from the following equations (5) and (6). c1 and A c3 .

[0224] A c1 =723+22×[C]-18×[Si]+17×[Cr]+4.5×[Mo]+16×[V]…Equation (5)

[0225] 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]…Equation (6)

[0226] Here, [M] represents the mass of each element.

[0227] [First cooling process: Cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50°C / s within a temperature range from the homogenization temperature to the first cooling stop temperature of 350 to 550°C]

[0228] In A c1 Point +20℃ and A c3 After holding at a homogenization temperature below the point and above Tc (after the above-described homogenization holding process), cooling is performed at a first average cooling rate of 2 to 50°C / s within a temperature range from the above-described homogenization temperature to a first cooling stop temperature of 350 to 550°C. If the rate is less than 2°C / s, excessive ferrite phase transformation occurs during cooling, and the desired amount of polygonal ferrite is not obtained; therefore, the first average cooling rate is set to 2°C / s or more. Preferably, the first average cooling rate is 5°C / s or more.

[0229] On the other hand, if the first average cooling rate becomes too high, the plate shape deteriorates, so it is set to 50°C / s or less. The first average cooling rate is preferably 40°C / s or less, and more preferably less than 30°C / s.

[0230] Here, the first average cooling rate is defined as "(soaking temperature (°C) - first cooling stop temperature (°C)) / cooling time (seconds) from the soaking temperature to the first cooling stop temperature".

[0231] [Second cooling process (1): Hold at a temperature of 350-550°C for more than 10 seconds and less than 60 seconds]

[0232] Within the temperature range of 350°C to 550°C (the dwell temperature) below the first cooling stop temperature, upper bainite can be formed, yielding the specified retained austenite and achieving the desired ductility. The bainitic phase transformation has a latency period; to obtain the desired amount of bainite, the dwell temperature must be maintained for a certain period. When the dwell temperature range, including the dwell start temperature (= the first cooling stop temperature) and the dwell end temperature, deviates from the range of 350–550°C and / or the dwell time (hereinafter also referred to as dwell time) is less than 10 seconds, the desired amount of bainite cannot be obtained, the formation of retained austenite is suppressed, and the desired ductility is not achieved.

[0233] On the other hand, when the residence time exceeds 60 s, carbon enriches from bainite to blocky untransformed γ, leading to an increase in the amount of residual blocky quenched martensite or an excessive increase in retained austenite, which raises concerns about a decrease in λ. Therefore, the residence time is set to be 10 s or more and 60 s or less. This residence time is preferably 20 s or more. Furthermore, this residence time is preferably 50 s or less.

[0234] It should be noted that, depending on the desired characteristics, the second cooling step (1) can be omitted. In this case, the cooling process can be completed by heating to A. c1 +20℃ and A c3 The process involves maintaining a homogenization temperature of Tc or higher at that temperature for 30 to 500 seconds, followed by a second cooling step (2). A manufacturing method that omits the second cooling step (1) will be described in the second embodiment described later.

[0235] [Second cooling process (2): Cooling to a second cooling stop temperature of 200-420°C at a second average cooling rate of 2-50°C / s]

[0236] After the aforementioned cooling period, rapid cooling is necessary to prevent excessive bainitic transformation. If the average cooling rate (second average cooling rate) over the temperature range from the end of the cooling period to a second cooling stop temperature above 200°C and below 420°C is less than 2°C / s, excessive bainitic transformation may occur, leading to an excessive increase in retained austenite, or the desired amount of martensite may not be ensured, resulting in reduced strength. Furthermore, when the second average cooling rate is less than 2°C / s, the desired ductility and porosity may not be achieved.

[0237] Therefore, the second average cooling rate for the temperature range from the end of the residence period to the second cooling stop temperature of 200°C or higher and 420°C or lower is set to 2°C / s or higher. Preferably, the second average cooling rate is set to 5°C / s or higher, and more preferably to 8°C / s or higher.

[0238] When 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 less. Preferably, it is 40°C / s or less.

[0239] When the second cooling stop temperature exceeds 420°C, the tempered martensite or lower bainite will not reach the specified area ratio, and the area ratio of the quenched martensite after annealing will increase, thus failing to ensure residual γ and sometimes deteriorating ductility. Furthermore, porosity may also deteriorate. Therefore, the second cooling stop temperature is set to below 420°C. Preferably, the second cooling stop temperature is below 400°C.

[0240] On the other hand, when the second cooling stop temperature is below 200°C, the tempering effect of martensite cannot be fully obtained. In addition, the formation of lower bainite is suppressed. Therefore, not only does the amount of quenched martensite increase, but the enrichment of residual γ in the C direction is also suppressed, resulting in deterioration of ductility. Therefore, the second cooling stop temperature is set to above 200°C.

[0241] Here, the second average cooling rate refers to "the end temperature of the residence (°C) - the second cooling stop temperature (°C) / the cooling time (seconds) from the end temperature of the residence to the second cooling stop temperature".

[0242] [Isothermal holding process: Hold at the second cooling stop temperature for 60–3000 seconds]

[0243] During the holding period at the second cooling stop temperature, the process is implemented from the perspective of strength adjustment resulting from tempering treatment that promotes the formation of martensite and enrichment of C-oriented residual γ. If the tempering time is less than 60 s, the tempering is insufficient, resulting in the formation of high-strength martensite. In addition, the enrichment of C-oriented residual γ is suppressed, thus failing to ensure the desired ductility and porosity.

[0244] On the other hand, when the holding time at the second cooling stop temperature exceeds 3000 s, excessive tempering of martensite sometimes occurs, failing to ensure the desired strength. Furthermore, when the holding time at the second cooling stop temperature exceeds 3000 s, the desired ductility is sometimes not obtained.

[0245] Therefore, the holding time at the second cooling stop temperature is set to be 60 seconds or more and 3000 seconds or less. The holding time at the second cooling stop temperature is preferably 100 seconds or more, more preferably 150 seconds or more. Furthermore, the holding time at the second cooling stop temperature is preferably 2500 seconds or less, more preferably 2000 seconds or less.

[0246] <Second Implementation>

[0247] The steel sheet manufacturing method of the second embodiment of the present invention is a method for manufacturing a steel sheet by hot rolling, pickling and cold rolling a steel billet having the above-mentioned composition, and then annealing the resulting cold-rolled steel sheet, wherein the annealing includes a homogenization and holding process, wherein the cold-rolled steel sheet is heated to A in a furnace atmosphere with a dew point of -40°C or lower. c1 Point +20℃ and A c3 The homogenization temperature is below the point and above Tc calculated by equation (4), and is maintained at the homogenization temperature for 30 to 500 s; a cooling process, wherein the homogenization temperature is cooled 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 process, wherein the temperature is maintained at the cooling stop temperature for 60 to 3000 s.

[0248] Tc(℃) = 663 - 1.2 × exp(20 / t) × Tdp … Equation (4)

[0249] Here, t represents the holding time (s) at the above homogenization temperature, and Tdp represents the dew point (°C).

[0250] In the second embodiment, the heat treatment in the hot rolling, pickling, cold rolling, and annealing homogenization and holding processes can be carried out under the same conditions as in the first embodiment.

[0251] In addition, in the second embodiment, the first cooling process in the annealing of the first embodiment can be omitted.

[0252] In addition, in the second embodiment, the cooling process in annealing corresponds to the second cooling process in annealing in the first embodiment. However, in the cooling process of this embodiment, the dwell treatment (dwelling for 10 to 60 seconds in the temperature range of 350 to 550°C) in the second cooling process of the first embodiment can be omitted.

[0253] Furthermore, the isothermal holding process in the annealing of the second embodiment can be set to substantially the same conditions as the isothermal holding process in the annealing of the first embodiment, except that the second cooling stop temperature is used as the cooling stop temperature.

[0254] In this embodiment, the cooling process during annealing will be described in detail below.

[0255] [Cooling process: Cooling to a stopping temperature of 200-420°C at an average cooling rate of 2-50°C / s]

[0256] Following the above-mentioned soaking and holding process, rapid cooling is necessary to prevent excessive bainitic transformation. When the average cooling rate from the soaking temperature to the second cooling stop temperature (between 200°C and 420°C) is less than 2°C / s, excessive bainitic transformation may occur, failing to ensure the desired amount of quenched martensite and resulting in reduced strength. Furthermore, an average cooling rate of less than 2°C / s may sometimes result in undesirable porosity.

[0257] Therefore, the average cooling rate for the temperature range from the homogenization temperature to the cooling stop temperature of 200°C or higher and 420°C or lower is set to 2°C / s or higher. Preferably, the average cooling rate is set to 5°C / s or higher, and more preferably to 8°C / s or higher.

[0258] If the cooling rate in this temperature range becomes too high, the plate shape will deteriorate. Therefore, the cooling rate (second average cooling rate) in this temperature range is set to 50°C / s or less. Preferably, it is 40°C / s or less.

[0259] When the cooling stop temperature exceeds 420°C, the tempered martensite or lower bainite will not reach the specified area ratio, and the area ratio of the quenched martensite after annealing will increase, thus sometimes deteriorating the porosity. Therefore, the cooling stop temperature is set below 420°C.

[0260] On the other hand, when the cooling stop temperature is below 200°C, the tempering effect of martensite cannot be fully obtained. Not only does the amount of quenched martensite increase, but the enrichment of residual γ in the carbon direction is also suppressed, sometimes leading to a deterioration in ductility. Therefore, the cooling stop temperature is set above 200°C.

[0261] Here, the average cooling rate is defined as "average temperature (°C) - cooling stop temperature (°C) / cooling time (seconds) from the average temperature to the cooling stop temperature".

[0262] [Plate thickness]

[0263] The thickness of the steel plate of the present invention is preferably set to 0.5 mm or more. Alternatively, the thickness is preferably set to 3.0 mm or less.

[0264] (Components and their manufacturing methods)

[0265] Next, the components of the present invention and their manufacturing method will be described.

[0266] The component of the present invention is formed by performing at least one of forming and joining processes on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention includes a step of forming and joining processes on the steel plate of the present invention to form the component.

[0267] The steel sheet of the present invention has a tensile strength of 780 MPa or higher and exhibits excellent ductility, porosity, and chemical conversion properties. Therefore, components obtained using the steel sheet of the present invention also have a tensile strength of 780 MPa or higher and exhibit excellent ductility, porosity, and chemical conversion properties. Furthermore, using components of the present invention enables weight reduction. Therefore, components of the present invention are suitable, for example, for use in vehicle body frame components.

[0268] Forming processes can utilize general processing methods such as stamping without limitation. Furthermore, joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and rivet joining without limitation.

[0269] Example

[0270] <Example 1>

[0271] A steel billet with the composition shown in Table 1, manufactured by continuous casting, is heated to 1200°C. After a hot rolling process with a soaking time of 200 minutes, a finishing rolling temperature of 860°C or higher, and a coiling temperature of 550°C, it is cold rolled at a rolling rate of 50%. The resulting cold-rolled steel sheet with a thickness of 1.4 mm is then processed 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.

[0272]

[0273] The steel microstructure was determined using the following method. The results are shown in Table 3.

[0274] The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were determined as follows: A plate section parallel to the rolling direction was cut out, mirror-polished, and etched with 1% nitric acid ethanol solution. At 1 / 4 of the thickness, a 25μm × 20μm area in 10 fields of view was observed using SEM at 5000x magnification and photographed. The obtained microstructure images were quantified using image analysis.

[0275] Polygonal ferrite is defined as ferrite that is relatively equiaxial and contains almost no carbides internally. In SEM, it appears as the darkest region.

[0276] Upper bainite is a ferrite microstructure containing carbides or retained austenite that appear white under SEM. It should be noted that when it is difficult to distinguish between upper bainite and polygonal ferrite, regions of ferrite with an aspect ratio ≤ 2.0 are classified as polygonal ferrite, and regions with an aspect ratio > 2.0 are classified as upper bainite, and their area ratios are calculated. Here, regarding the aspect ratio, the longest major axis length *a* of the particle is determined, and the longest length of the particle that traverses the particle in the direction perpendicular to it is defined as the minor axis length *b*. *a / b* is then used as the aspect ratio.

[0277] Tempered martensite and lower bainite are regions in which lath-like substructures and carbide precipitation are present inside under SEM.

[0278] Quenched martensite (fresh martensite) is a whitish, blocky region that appears to have no internal substructure under SEM.

[0279] The remaining microstructure contains at least one of unrecrystallized ferrite, carbides, and pearlite. By SEM, the unrecrystallized ferrite can be identified as ferrite with a black contrast, exhibiting deformation characteristics introduced through rolling. Furthermore, the carbides and pearlite are microstructures that can be identified with a white contrast. The carbides have a grain size of 1 μm or less, and the pearlite has a lamellar (layered) structure, thus allowing for differentiation.

[0280] The volume fraction of retained austenite was determined by X-ray diffraction after chemical grinding from the surface to a position of 1 / 4 thickness. The incident X-rays were obtained using a Co-Kα source, and the volume fraction of retained austenite was calculated by the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0281] JIS 5 tensile test specimens were cut from the obtained steel plate and tensile tests were performed with N=3 (according to JIS Z2241(2011)). Evaluations were based on the average of the three points. Steel plates with a tensile strength of 780 MPa or higher were judged to have excellent strength.

[0282] Excellent ductility is defined as having a total elongation (EL) of 16.0% or more when the total elongation (TS) is 780 MPa or more, a total elongation (EL) of 14.0% or more when the TS is 980 MPa or more, and a total elongation (EL) of 12.0% or more when the TS is 1180 MPa or more.

[0283] In addition, the hole expansion test according to JFST1001 is carried out with N=3. The average of the three points of hole expansion rate λ(%) (={(d-d0) / d0}×100) is 30% or more as the qualification standard, and 30% or more is judged as excellent hole expansion performance.

[0284] The measurement results are shown in Table 3.

[0285] For annealed steel sheets, the surface enrichment of P and Si on the steel sheet surface was determined using a GDS (manufactured by Shimadzu Corporation). Sputtering analysis was performed in the depth direction under the conditions of Ar gas pressure of 600 Pa, high-frequency output of 35 W, measurement interval of 0.1 s, and measurement time of 150 s. The maximum concentration of P [Pm] and the integral enrichment of Si in the vicinity of the surface layer (within 1 μm of the steel sheet thickness from the surface) were measured. In this determination, standard curves for P and Si were obtained using standard samples with various P contents ranging from 0.005% to 0.020% by mass and standard samples with various Si contents ranging from 1.0% to 3.0% by mass.

[0286] After annealing, the steel sheet undergoes degreasing and surface conditioning, followed by chemical conversion treatment using a zinc phosphate chemical conversion solution. Specifically, the degreasing process is carried out at a temperature of 40°C for 120 seconds; the spray degreasing and surface conditioning processes are performed at pH 9.5, room temperature, and a treatment time of 20 seconds; and the chemical conversion treatment process is conducted at a solution temperature of 35°C for 120 seconds. It should be noted that the degreasing agent FC-E2011, the surface conditioning agent PL-X, and the chemical conversion treatment solution PALBOND PB-L3065, manufactured by PALBOND Seiko Co., Ltd., are used sequentially in the degreasing, surface conditioning, and chemical conversion treatment processes. The results are obtained at 1000x magnification in 5 fields of view (50000μm). 2 SEM observations were performed on the above-mentioned areas to observe the surface chemical transformation structure. Areas where the exposed steel substrate was less than 10% of the total area were rated as 0, while areas where the exposed steel substrate was more than 10% of the total area were rated as ×. The results are shown in Table 3.

[0287]

[0288]

[0289] The examples of the present invention shown in Tables 2 and 3 exhibit excellent strength, ductility, porosity, and chemical conversion treatability, while the comparative examples are all inferior.

[0290] <Example 2>

[0291] A steel billet with the composition shown in Table 1, manufactured by continuous casting, was heated to 1200°C and hot-rolled at a soaking time of 200 minutes, a finishing rolling temperature of 860°C or higher, and a coiling temperature of 550°C. It was then cold-rolled at a rolling rate of 50%, and the resulting cold-rolled steel sheet with a thickness of 1.4 mm was processed under the annealing conditions shown in Table 4 to manufacture the steel sheet of the present invention and the steel sheet of the comparative example. The same evaluation as in Example 1 was performed. The results are shown in Table 5.

[0292]

[0293]

[0294] The examples of the present invention shown in Tables 4 and 5 have excellent ductility and chemical conversion treatment properties, while the comparative examples are all poor.

[0295] Furthermore, it is understood that since the steel plate of the present invention has excellent strength, ductility, hole-expanding properties and chemical conversion properties, the components obtained by forming and joining the steel plate of the present invention have the same excellent strength, ductility, hole-expanding properties and chemical conversion properties as the steel plate of the present invention.

Claims

1. A steel plate having: Containing, by mass%, C: 0.05–0.25%, Si: 0.30–1.50%, Mn: 1.5–4.5%, P: 0.005–0.050%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, Ti: 0.005–1.000%, B: 0.0010–0.0030%, satisfying the following formulas (1) and (2), with the balance consisting of iron and unavoidable impurities; and Steel microstructures with an area ratio of polygonal ferrite of 10% or more and less than 80%, a combined area ratio of upper bainite, tempered martensite, and lower bainite of 10% or more and less than 70%, a volume ratio of retained austenite of 3% or more and less than 15%, and an area ratio of quenched martensite of 15% or less (including 0%). The maximum concentration of P [Pm] within 1 μm of the steel plate surface along the thickness direction is 0.025% by mass or more, and satisfies the following equation (3). Furthermore, the integral enrichment of Si within 1 μm along the thickness direction from the surface of the steel plate is less than 120. [Si] / [Mn]≤0.35 …Equation (1) 1000×[B] / [Mn]≤0.70 …Equation (2) [Pm] / [P]≥1.5 …Equation (3) In equation (1), [Si] represents the Si content (mass %), and [Mn] represents the Mn content (mass %). In formula (2), [B] represents the content of B (mass%), and [Mn] represents the content of Mn (mass%). In equation (3), [P] represents the P content (mass %).

2. The steel plate according to claim 1, wherein, As part of the composition, it also contains one or more of the following components selected by mass percentage: Cu: less than 1%, Ni: less than 1%, Cr: less than 1%, Mo: less than 0.5%, V: less than 0.5%, Nb: less than 0.1%, Mg: less than 0.0050%, Ca: less than 0.0050%, Sn: less than 0.1%, Sb: less than 0.1%, and REM: less than 0.0050%.

3. A component made of the steel plate as described in claim 1 or 2.

4. A method for manufacturing a steel plate, comprising a method for manufacturing a cold-rolled steel plate obtained by hot rolling, pickling, and cold rolling a steel billet having the composition described in claim 1 or 2, wherein... The annealing includes: The homogenization and holding process, wherein the cold-rolled steel sheet is heated to A in a furnace atmosphere with a dew point below -40°C. c1 Point +20℃ and A c3 The homogenization temperature is below the point and above Tc calculated by equation (4), and is maintained at the homogenization temperature for 30 to 500 s; The first cooling process involves cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50°C / s within a temperature range from the homogenization temperature to a first cooling stop temperature of 350 to 550°C. The second cooling process includes, after stopping cooling at the first cooling stop temperature, maintaining a temperature range of 350–550°C for 10–60 seconds, and then cooling at a second average cooling rate of 2–50°C / s to a second cooling stop temperature of 200–420°C; and The isothermal holding process involves holding the temperature at the second cooling stop temperature for 60–3000 seconds. Tc(℃)=663-1.2×exp(20 / t)×Tdp…(4) Here, t represents the holding time (s) at the homogenization temperature, and Tdp represents the dew point (°C).

5. A method for manufacturing a steel plate, comprising a method for manufacturing a cold-rolled steel plate obtained by hot rolling, pickling, and cold rolling a steel billet having the composition described in claim 1 or 2, wherein... The annealing includes: The homogenization and holding process, wherein the cold-rolled steel sheet is heated to A in a furnace atmosphere with a dew point below -40°C. c1 Point +20℃ and A c3 The homogenization temperature is below the point and above Tc calculated by equation (4), and is maintained at the homogenization temperature for 30 to 500 s; The cooling process includes cooling from the homogenization temperature to a cooling stop temperature of 200–420°C at an average cooling rate of 2–50°C / s; and The isothermal holding process involves holding the temperature at the cooling stop temperature for 60–3000 seconds. Tc=663-1.2×exp(20 / t)×Tdp (3) Here, t represents the holding time (s) at the homogenization temperature, and Tdp represents the dew point (°C).

6. A method for manufacturing a component, comprising the steps of performing at least one of forming or joining processes on the steel plate as described in claim 1 or 2 to produce the component.

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