High strength steel plate and method for manufacturing the same

Abstract

The objective of this study is to provide high-strength steel sheets that maintain a TS of 980 MPa or more while achieving a YR of 70% or more, high ductility, and hydrogen embrittlement resistance in bent sections, and a manufacturing method for these sheets. A high-strength steel sheet having a predetermined chemical composition, wherein the steel structure at the 1 / 4 coil width position and the 1 / 4 plate thickness position has an area fraction of ferrite of 0 to 5%, a volume fraction of retained austenite of 3 to 20%, an area fraction of tempered martensite of 70% or more, and an area fraction of fresh martensite of 0 to 20%, wherein the steel structure at the 1 / 4 coil width position and the outermost plate thickness position has an area fraction of ferrite of 0 to 5%, and the proportion of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite is 50% or less, TS≧980MPa, 100×YR≧70, and in the hardness distribution in the plate thickness direction, the relationship between the hardness value H025 at the 1 / 4 plate thickness position and the hardness value H005 at the 1 / 20 plate thickness position is 2.5≦100×(H025-H005) / H025≦20.0.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet having a tensile strength (TS) of 980 MPa or more, which is suitable for use mainly in structural parts of automobile bodies and electrical parts, and to a method for producing the same. [Background technology]

[0002] In recent years, growing concern about global environmental issues has led to stricter CO2 emission regulations. In the automotive sector, there is a need to improve fuel efficiency by reducing the weight of vehicle bodies, and to ensure the crashworthiness of large batteries installed in vehicles as they become more electrified. To address this need, the use of high-strength steel materials that can maintain high body rigidity and crashworthiness even when thinner and lighter is increasingly being applied to automobiles. Many steel manufacturers are expanding their lineup of high-strength steel sheets for automobiles, with tensile strengths of 980 MPa and even higher. To ensure the crashworthiness of automotive parts, it is important that the steel sheet not only have high tensile strength, but also high yield strength and ductility.

[0003] Generally, solid solution strengthening, precipitation strengthening, and dislocation strengthening (transformation strengthening) are known as methods for strengthening steel. To increase the strength of steel sheets with a tensile strength of over 980 MPa, it is necessary to increase the proportion of hard structures such as martensite and bainite through transformation strengthening. However, if the steel sheet contains more of these hard phases, the workability of the steel sheet decreases. As a result, it cannot be used in parts that require complex processing, and the range of applicable parts is limited. In recent years, a major challenge in the development of steel materials for automobiles has been achieving both high strength and high workability while retaining hard phases.

[0004] A method of utilizing tempered martensite is known as a method for improving the mechanical properties of high-strength steel sheets that utilize such hard phases. For example, Patent Documents 1 and 2 disclose a technique in which, in the final annealing step, the steel sheet is cooled to a temperature below the Ms point, then reheated, and the martensite formed during cooling is tempered to obtain high stretch flangeability. Patent Document 3 discloses a method of tempering a steel sheet after plating. Furthermore, Patent Document 4 discloses a method for producing a steel sheet with good bending properties and bendability, in which the steel sheet is plated, cooled to 200°C or below, and then further tempered in a temperature range of 100°C to 600°C.

[0005] Furthermore, by further improving these techniques, a technique has been established in which a steel sheet has both high strength and high ductility by using tempered martensite as the main component and also utilizing the austenite phase, which is responsible for ductility. This technique is shown, for example, in Patent Document 5. In this technique, in the final annealing process, a martensite and austenite structure is formed by cooling to a temperature below the martensite transformation start temperature and above the martensite transformation completion temperature. Thereafter, the steel is reheated and held to stabilize the austenite and temper the martensite.

[0006] Furthermore, Patent Document 7 discloses a technique for controlling the yield ratio by controlling the structure of quenched / tempered martensite, and in its examples, it shows high yield ratios of over 85%. However, there are problems with the area fraction of tempered martensite shown in this patent document being low, at around 40%, and the total elongation value being low, at 9.9%. Similarly, Patent Document 8 also has the problem of a yield ratio of 85% in its examples, but a low total elongation value of 9%. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5463685 [Patent Document 2] International Publication No. 2009 / 054539 [Patent Document 3] Japanese Patent Application Publication No. 06-108152 [Patent Document 4] Japanese Patent Application Laid-Open No. 2017-48412 [Patent Document 5] Patent No. 6787525 [Patent Document 6] Patent No. 6705561 [Patent Document 7] Patent No. 6414246 [Patent Document 8] Patent No. 6631760 [Patent Document 9] Patent No. 6787526 [Patent Document 10] International Publication No. 2017 / 179372 Summary of the Invention [Problem to be solved by the invention]

[0008] Furthermore, high-strength steel sheets with a tensile strength (TS) of 980 MPa or higher not only exhibit poor bendability, but also exhibit cracks on the surface of the steel sheet during bending. These microcracks then initiate hydrogen embrittlement, leading to fracture. The inventors have discovered that bending fractures become particularly pronounced under severe bending conditions, particularly when the bending angle is less than 90°. To address this issue, methods can be used to modify the chemical composition or microstructure fraction of the steel sheet. For example, Patent Document 4 discloses an invention that achieves good elongation properties by forming a surface decarburized layer, and explains that soft ferrite in the surface layer improves bending formability. However, the formation of soft ferrite in the surface layer not only reduces the strength level of the base material, but also increases the number of microstructure interfaces between ferrite and martensite, which have different mechanical properties, thereby promoting the formation of microcracks and voids during processing. Materials with a tensile strength exceeding 980 MPa are particularly susceptible to hydrogen embrittlement and fracture.

[0009] The present inventors have investigated a manufacturing method for improving the strength-ductility balance of a high-strength steel plate that contains 70% or more of tempered martensite as a main component, with the remainder consisting mainly of austenite, and that has a tensile strength (TS) of 980 MPa or more.

[0010] First, we attempted to adjust annealing parameters in the final annealing process, such as annealing temperature / time, cooling rate, cooling stop temperature / time, and reheating temperature / time, according to known literature techniques. However, we found a trade-off between the strength (TS in tensile tests) and elongation (El in tensile tests) of the steel sheet, and no clear improvement in the strength-ductility balance was observed. Furthermore, we found that the conventional annealing pattern, which involves rapid cooling followed by reheating, results in uneven temperature distribution within the coil, resulting in non-uniform mechanical properties.

[0011] Subsequently, the annealing process was drastically reviewed, and improvements in properties were investigated by performing an additional heat treatment at Ac1 point to Ac1 point + 130°C after cold rolling, as described in Patent Document 6. However, when applying the publicly known method, it was found that the additional heat treatment coarsened the structure of the steel, reducing its strength and, in fact, tending to result in a poor balance between strength and ductility.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a high-strength steel sheet that has a TS of 980 MPa or more, a yield ratio (YR) of 70% or more, an El of 10,000 / TS (MPa)% or more, i.e., El × TS ≥ 10,000 MPa·%, and hydrogen embrittlement resistance in the bent portion, as well as a manufacturing method thereof. [Means for solving the problem]

[0013] The present inventors have conducted extensive research to solve the above problems.

[0014] The inventors focused on the upstream manufacturing process, including the chemical composition of the steel, and primarily made improvements to the heat treatment process beyond simple adjustments of temperature and time.

[0015] As a result, it was possible to form a different distribution of properties in the sheet thickness direction without utilizing a soft ferrite phase, i.e., to form a softer structure on the surface layer side of the steel sheet and a harder structure on the center side of the sheet thickness. It was found that this makes it possible to achieve both a high yield ratio, high ductility, and hydrogen embrittlement resistance in the bent portion while maintaining a tensile strength of 980 MPa. The present disclosure is based on the above findings. Specifically, the gist of the present disclosure is as follows: [1] In mass%, C: 0.090% to 0.300%, Si: 0.50% to 2.50%, Mn: 1.8% to 4.0%, P: 0.100% to 0.100%, S: 0.0200% to 0.0200%, Al: 0.200% to 0.200%, N: 0.0200% to 0.0200%, and O: 0.0100% to 0.0100%. Cu: 0.005% to 0.5%, Sn: 0.005% to 0.5%, and Sb: 0.001% to 0.07%. and below, in a total amount such that 0.005≦α≦0.07, where α is a dimensionless quantity expressed as [Sb]+[Cu] / 10+[Sn] / 10, and [Sb], [Cu], and [Sn] represent the respective contents (mass%), and are zero if not contained, with the remainder consisting of Fe and unavoidable impurities. The steel structure at the 1 / 4 coil width position and 1 / 4 plate thickness position satisfies the following: ferrite area fraction 0% to 5%, retained austenite volume fraction 3% to 20%, tempered martensite area fraction 70% or more, and fresh martensite area fraction 0% to 20%. Furthermore, the steel structure at the 1 / 4 coil width position and the outermost plate thickness layer satisfies the following: ferrite area fraction 0% to 5%, retained austenite volume fraction 3% to 20%, tempered martensite area fraction 70% or more, and fresh martensite area fraction 0% to 20%. A high-strength steel plate in which the proportion of retained austenite with a tonnage ratio of 2.0 or more to the total retained austenite is 50% or less, the tensile strength TS and yield strength YS are TS ≥ 980 MPa and 100 × YS / TS ≥ 70, and in the hardness distribution in the plate thickness direction, the relationship between the hardness value H025 at 1 / 4 of the plate thickness position and the hardness value H005 at 1 / 20 of the plate thickness position is 2.5 ≤ 100 × (H025 - H005) / H025 ≤ 20.0. [2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, in mass%, at least one element selected from Ti: 0.200% or less, B: 0.0100% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [3] A high-strength steel sheet according to [1] or [2], wherein an electrogalvanized layer is formed on the surface. [4] A high-strength steel sheet according to [1] or [2], wherein a hot-dip galvanized layer is formed on the surface. [5] A high-strength steel sheet according to [1] or [2], wherein a galvannealed layer is formed on the surface. [6] A method for producing a high-strength steel plate according to [1] or [2], wherein a steel slab having the above-mentioned composition is heated to a temperature range of 1080°C or more and 1300°C or less, and then hot-rolled at a finishing temperature of 850°C or more and 1000°C or less, and then coiled at a temperature range of Ms+50°C or more and Bs°C or less, and soft-annealed at Ac3 point or more with the hot-rolled scale remaining on the surface, and then soft-annealed at 400°C or more and Ac1 point or less and in a temperature range T, T≧5000√α a method for producing a high-strength steel sheet, the method comprising: removing scale from the steel sheet surface by pickling, followed by annealing the cold-rolled sheet obtained with a cold rolling reduction of 30% or more at a temperature of from Ac3+10°C to 950°C, cooling the cold-rolled sheet at an average cooling rate of from 10°C / s to 100°C / s to 400°C, cooling the cold-rolled sheet at an average cooling rate of from 5°C / s to 20°C / s from 400°C to a cooling stop temperature range of from 50°C to 300°C, heating the cold-rolled sheet to a temperature range of from 250°C to 530°C, holding the sheet in the temperature range for 10 seconds or more, cooling the sheet, and then skin-pass rolling the sheet to an elongation of 0.02% or more; [7] The method for producing a high-strength steel plate according to [6], wherein the steel is cooled from 400°C to a cooling stop temperature range of 50°C to 300°C at an average cooling rate of 5°C / s to 20°C / s, and then held in the cooling stop temperature range for 0.5 seconds or more. [8] The method for producing a high-strength steel sheet according to [6] or [7], further comprising electrogalvanizing the steel sheet. [9] The method for producing a high-strength steel sheet according to [6] or [7], further comprising the step of performing hot-dip galvanizing.

[10] The method for producing a high-strength steel sheet according to [6] or [7], further comprising the steps of: performing hot-dip galvanizing; and then performing a galvanizing alloying treatment in a temperature range of 470°C or higher and 530°C or lower. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a higher strength-ductility balance than conventional materials while maintaining a high yield strength ratio. As a result, the range of applications for automobile body parts is expanded, and it can also be applied to parts that require complex press working. In particular, it can significantly contribute to reducing the weight of automobile bodies, and it can also improve the hydrogen embrittlement resistance of processed parts, especially bent parts that are formed under particularly strict conditions, which is a problem specific to high-strength materials. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure is not limited to the following embodiments. First, the appropriate range of the chemical composition of the steel slab used as the raw material for the high-strength steel plate of the present invention and the reasons for limiting it will be described. In the following description, "%" representing the content of the component elements of the steel means "mass %" unless otherwise specified.

[0018] [C: 0.090% or more and 0.300% or less] C is one of the important basic components of steel, and in the present invention, it is a particularly important element that affects the fractions of martensite, ferrite, and retained austenite. If the C content is less than 0.090%, the fraction of martensite decreases, making it difficult to achieve the desired tensile strength. On the other hand, if the C content exceeds 0.300%, the martensite becomes embrittled, making it difficult to achieve the desired elongation. Therefore, the C content is set to 0.090% or more and 0.300% or less. The preferred lower limit is 0.120% or more, and more preferably 0.150% or more. The preferred upper limit is 0.280% or less, and even more preferably 0.240% or less.

[0019] [Si:0.50% or more and 2.50% or less] Silicon (Si) is one of the important basic components of steel. In particular, in the present invention, silicon inhibits carbide formation during continuous annealing and promotes the formation of retained austenite, thereby affecting the hardness of martensite and the fraction of retained austenite. If the Si content is less than 0.50%, the fraction of retained austenite decreases, making it difficult to achieve the desired elongation. On the other hand, if the Si content exceeds 2.50%, the carbon concentration in the retained austenite increases excessively, resulting in reduced local ductility. For example, cracks are more likely to form on the surface layer side of the bent portion during bending tests. Therefore, the Si content is set to 0.50% or more and 2.50% or less. The preferred lower limit is 0.60% or more, more preferably 0.80% or more. The preferred upper limit is 2.00% or less, more preferably 1.80% or less.

[0020] [Mn:1.8% or more and 4.0% or less] Mn is one of the important basic components of steel, and in the present invention, it is a particularly important element that affects the martensite fraction. If the Mn content is less than 1.8%, the martensite fraction decreases, making it difficult to achieve a TS of 980 MPa or more. On the other hand, if the Mn content exceeds 4.0%, the tempered martensite fraction decreases, reducing local ductility. Bendability also decreases. Therefore, the Mn content is set to 1.8% or more and 4.0% or less. The preferred lower limit is 2.0% or more, more preferably 2.2% or more. The preferred upper limit is 3.8% or less, more preferably 3.6% or less.

[0021] [P:0.100% or less] P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the local ductility and elongation of the steel sheet. It also reduces bendability. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to set it to 0.001% or more. Therefore, the P content is set to 0.100% or less, preferably 0.070% or less.

[0022] [S:0.0200% or less] S exists as sulfides and reduces the local ductility of steel sheets, thereby reducing elongation. It also reduces bendability. Therefore, the S content must be 0.0200% or less. While there is no specific lower limit for the S content, it is preferable to set it to 0.0001% or more due to production technology constraints. Therefore, the S content is set to 0.0200% or less, preferably 0.0050% or less.

[0023] [Al:0.200% or less] When added in large amounts, Al raises the A3 transformation point and causes the microstructure to contain a large amount of ferrite, preventing the use of martensite to achieve high strength. Therefore, the Al content must be 0.200% or less. Although there is no particular lower limit for the Al content, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. A more preferred range is 0.150% or less.

[0024] [N:0.0200% or less] N exists as nitrides and reduces the local ductility of steel sheets, thereby reducing elongation. It also reduces bendability. Therefore, the N content must be 0.0200% or less. There is no particular lower limit for the N content, but due to production technology constraints, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0200% or less, preferably 0.0050% or less.

[0025] [O:0.0100% or less] O exists as an oxide and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the O content must be 0.0100% or less. There is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is 0.0100% or less, more preferably 0.0050% or less.

[0026] [Cu: 0.005% or more and 0.5% or less, Sn: 0.005% or more and 0.5% or less, Sb: 0.001% or more and 0.07% or less] If Cu and Sn are 0.5% or less, the amount of coarse precipitates and inclusions will not increase, and the local ductility of the steel sheet will not be reduced, so elongation will not be reduced. Furthermore, bendability will not be reduced. Therefore, the Cu and Sn contents are preferably 0.5% or less. While there is no particular lower limit for the contents, because these elements improve hardenability, the contents are more preferably 0.005% or more. A more preferable lower limit for Cu and Sn is 0.01% or more. An even more preferable upper limit for Cu and Sn is 0.30% or less.

[0027] If Sb is 0.07% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, the Sb content is preferably 0.07% or less. While there is no particular lower limit for the Sb content, the content is more preferably 0.001% or more in order to obtain the effect of surface decarburization. More preferably, it is 0.004% or more.

[0028] [α: 0.005 or more and 0.07 or less] As described below, Cu, Sn, and Sb are features of the present invention. At least two of these elements must be present, and α must be 0.005 or greater. Here, α is a dimensionless quantity expressed as [Sb] + [Cu] / 10 + [Sn] / 10, where [Sb], [Cu], and [Sn] represent their respective contents (mass%) and are zero when not present. These elements are known as surface segregation elements, and adding certain amounts can suppress surface decarburization during the annealing process and ultimately suppress through-thickness microstructural heterogeneity. More preferably, α is 0.010 or greater, and even more preferably, 0.015 or greater. The upper limit of α must be 0.07 or less. If α exceeds 0.07, bendability deteriorates. The upper limit of α is preferably 0.05 or less, and even more preferably, 0.03 or less.

[0029] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities. Examples of unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. A total of 0.100% or less of these impurities is permitted.

[0030] In addition to the above-described chemical composition, the high-strength steel sheet of the present invention may further contain, in mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.

[0031] If V is 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and the local ductility of the steel sheet is not reduced, so elongation is not reduced. Furthermore, bendability is not reduced. Therefore, the V content is preferably 0.200% or less. While there is no particular lower limit for the V content, since V increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, the V content is more preferably 0.001% or more. Therefore, when V is contained, its content is 0.200% or less. A more preferable lower limit is 0.001% or more. A more preferable upper limit is 0.100% or less.

[0032] If Ta and W are contained in an amount of 0.10% or less, large amounts of coarse precipitates and inclusions are not formed, and the local ductility of the steel sheet is not reduced, resulting in a decrease in elongation. Furthermore, bendability is not reduced. Therefore, the Ta and W contents are preferably 0.10% or less. While there are no specific lower limits for the Ta and W contents, the Ta and W contents are more preferably 0.01% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ta and W are contained, their contents are each 0.10% or less. More preferably, the Ta and W contents are 0.01% or more. Even more preferably, the Ta and W contents are 0.08% or less.

[0033] If Cr and Mo are each 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the local ductility of the steel sheet will not be reduced, resulting in a decrease in elongation. Furthermore, bendability will not be reduced. Therefore, the Cr and Mo contents are preferably 1.00% or less. While there are no particular lower limits for the Cr and Mo contents, because these elements improve hardenability, it is more preferable that the Cr and Mo contents be 0.01% or more. Therefore, when Cr and Mo are contained, their contents should each be 1.00% or less. More preferably, the Cr and Mo contents should be 0.01% or more. Even more preferably, the Cr and Mo contents should be 0.80% or less.

[0034] If Co is 0.010% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, the Co content is preferably 0.010% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. Therefore, when Co is contained, its content is 0.010% or less. A more preferable Co content is 0.001% or more. An even more preferable Co content is 0.008% or less.

[0035] If Ni is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the local ductility of the steel sheet will not be reduced, so elongation will not be reduced. Furthermore, bendability will not be reduced. Therefore, the Ni content is preferably 0.200% or less. While there is no particular lower limit for the Ni content, since Ni is an element that improves hardenability (generally an element that improves corrosion resistance), it is more preferable that the Ni content be 0.001% or more. Therefore, if Ni is contained, its content should be 0.200% or less, more preferably 0.001% or more. An even more preferable Ni content is 0.150% or less.

[0036] If the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, resulting in a decrease in elongation. Furthermore, bendability is not reduced. Therefore, the Ca, Mg, and REM contents are preferably 0.0100% or less. While there are no specific lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet, so the Ca, Mg, and REM contents are more preferably 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, their contents should each be 0.0100% or less. More preferably, the Ca, Mg, and REM contents are 0.0005% or more. Even more preferably, the Ca, Mg, and REM contents are 0.0050% or less. REM (rare earth elements) is a general term for 15 elements ranging from Sc, Y, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0037] If Zr and Te are contained in an amount of 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and the local ductility of the steel sheet will not be reduced, resulting in a decrease in elongation. Furthermore, bendability will not be reduced. Therefore, the Zr and Te contents are preferably 0.100% or less. While there are no specific lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet, it is more preferable that the Zr and Te contents be 0.001% or more. Therefore, when Zr and Te are contained, their contents should be 0.100% or less. More preferably, the Zr and Te contents should be 0.001% or more. Even more preferably, the Zr and Te contents should be 0.080% or less.

[0038] If Hf is 0.10% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, the Hf content is preferably 0.10% or less. While there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the local ductility of the steel sheet, the Hf content is more preferably 0.01% or more. Therefore, if Hf is contained, its content should be 0.10% or less. A more preferred Hf content is 0.01% or more. An even more preferred Hf content is 0.08% or less.

[0039] If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the local ductility of the steel sheet is not reduced, so elongation does not decrease. Furthermore, bendability does not decrease. Therefore, the Bi content is preferably 0.200% or less. While there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, its content is 0.200% or less. A more preferred Bi content is 0.001% or more. An even more preferred Bi content is 0.100% or less.

[0040] If Ti and Nb are contained in an amount of 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and the local ductility of the steel sheet is not reduced, so that elongation is not reduced. Furthermore, bendability is not reduced. Therefore, the Ti and Nb contents are preferably each 0.200% or less. While there are no particular lower limits for the Ti and Nb contents, the Ti and Nb contents are more preferably 0.001% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when Ti and Nb are contained, their contents are each 0.200% or less. More preferably, the Ti and Nb contents are 0.001% or more. Even more preferably, the Ti and Nb contents are 0.100% or less.

[0041] If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the local ductility of the steel sheet will not be reduced, so elongation will not be reduced. Furthermore, bendability will not be reduced. Therefore, the B content is preferably 0.0100% or less. While there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, if B is contained, its content should be 0.0100% or less. A more preferred B content is 0.0003% or more. An even more preferred B content is 0.0080% or less.

[0042] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Sn, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0043] Next, we will explain the microstructure. Unless otherwise specified, the location for defining the structure is the 1 / 4 coil width and 1 / 4 plate thickness position. This is because the 1 / 4 coil width and 1 / 4 plate thickness positions are generally considered to be the locations that have the average properties and structure of the steel plate within the coil. Note that the coil width is the same as the plate width of the steel plate, and 1 / 4 coil width is equivalent to 1 / 4 plate width of the steel plate.

[0044] [Ferrite: 0% to 5%. Electron microscope observation field area ratio] Ferrite is a soft structure, and therefore is effective in improving workability. However, in the case of the 980 MPa-class high-strength steel plate that is the subject of the present invention, it has a strong effect of reducing the strength of the steel plate. Here, ferrite may be polygonal ferrite, pseudo-polygonal ferrite, or granular bainitic ferrite. In order to achieve high strength, the ferrite content at the 1 / 4 position of the plate thickness is set to 5% or less. More preferably, it is set to 3% or less. Furthermore, an even more preferable condition is that the ferrite content is less than 1%. Note that ferrite does not necessarily need to be present, so the lower limit is set to 0% or more.

[0045] The ferrite area ratio is determined by the following method. First, a thickness cross section (L cross section) parallel to the rolling direction of the steel plate is polished and then corroded with 3 vol.% nital. Ten fields of view are observed at 1 / 4 of the plate thickness position (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) at 2000x magnification using a scanning electron microscope (SEM). Next, using the obtained structural images, the area ratio of each structure for the 10 fields of view is calculated using Image-Pro from Media Cybernetics. The average of the area ratios for these 10 fields of view is defined as the "ferrite area ratio." In the structural image above, ferrite appears as a gray structure (base structure). Retained austenite and martensite each appear as a white structure.

[0046] [Retained austenite: 3% to 20%. Volume fraction determined by X-ray diffraction] Retained austenite contributes to the ductility of steel sheets through the TRIP effect and is an essential structure in the present invention. To obtain a sufficient effect, the lower limit is set to 3% or more. Furthermore, if retained austenite is contained in excess, it may transform into a hard martensite structure when applied to and formed into automotive parts, etc., and may impair processing characteristics. For this reason, the upper limit is set to 20% or less. A more preferable lower limit is 6% or more. A more preferable upper limit is less than 16%. Note that this volume fraction of retained austenite can be converted into an area fraction.

[0047] The volume fraction of retained austenite is measured as follows. A steel sheet is mechanically ground in the thickness direction (depth direction) to one-quarter of its thickness, and then chemically polished with oxalic acid to obtain an observation surface. This observation surface is observed by X-ray diffraction. A Co Kα radiation source is used as the incident X-ray, and the diffraction peak intensities of the {200}, {220}, and {311} planes of fcc iron (austenite) are measured relative to the diffraction intensities of the {200}, {211}, and {220} planes of bcc iron. Next, the volume fraction of retained austenite is calculated by multiplying the diffraction peaks of martensite and austenite by a correction factor and averaging them to eliminate the influence of the preferred orientation. Rγ(200) = 33.0, Rγ(220) = 17.6, Rγ(311) = 25.1, Rα(200) = 13.2, Rα(211) = 27.3, and Rα(220) = 12.5.

[0048]

number

[0049] [Tempered martensite: 70% or more. Electron microscope observation area ratio] Tempered martensite is characterized by an aggregate of lath-shaped crystal grains containing iron-based carbides. It has higher ductility than fresh martensite and is also characterized by its tendency to easily obtain the high yield strength required to ensure the crashworthiness of automotive components. To achieve these effects, the area fraction of tempered martensite must be 70% or more. More preferably, the area fraction of tempered martensite is 75% or more, and even more preferably, 80% or more. While there is no particular upper limit to the area fraction of tempered martensite, in order to ensure the area fraction of retained austenite, the area fraction of tempered martensite is preferably 94.0% or less. The area fraction of tempered martensite can be measured by the method described in the Examples below.

[0050] [Fresh martensite: 0% to 20%. Electron microscope observation field area ratio] Fresh martensite is martensite that does not contain iron-based carbides. Steel sheets containing fresh martensite have high strength but poor processing characteristics, so the area fraction of fresh martensite in the steel sheet according to this embodiment is limited to 20% or less. On the other hand, even if the area fraction of fresh martensite is 0%, the steel sheet according to this embodiment has sufficient strength. Therefore, the steel sheet according to this embodiment does not need to contain fresh martensite, so the lower limit of the area fraction of fresh martensite is 0% or more. A more preferable range is 10% or less.

[0051] Furthermore, in the steel structure according to the present invention, in addition to the above-mentioned ferrite, tempered martensite, fresh martensite, and retained austenite, the effects of the present invention are not impaired even if bainite is contained in an area ratio of 20% or less. Furthermore, carbides such as pearlite (P) and cementite (θ) and other known steel sheet structures may also be contained. Even if these are contained, the effects of the present invention are not impaired as long as their total area ratio is within a range of 10% or less. The other steel sheet structures (remaining structures) can be confirmed and determined, for example, by SEM observation.

[0052] The area fractions of tempered martensite, fresh martensite, pearlite, and bainite were measured as follows. After annealing, a sample of the steel sheet was cut, and the cross-section of the sheet thickness parallel to the rolling direction was polished and etched with 3 vol.% nital. Three fields of view were photographed at 1 / 4 of the sheet thickness using a scanning electron microscope (SEM) at 1500x magnification. The "1 / 4 of the sheet thickness" refers to the position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the steel sheet. Observation at a higher magnification may be performed to confirm carbides in detail. In the case of a galvanized steel sheet, the "surface" refers to the interface between the galvanized layer and the steel sheet. The obtained image data was used to calculate the area fractions of each structure using Image-Pro (Media Cybernetics), and the average area fraction of each structure in the field of view was defined as the area fraction of each structure. In the image data, fresh martensite was distinguished as a white or light gray region, tempered martensite as a gray or dark gray region containing misoriented carbides, and pearlite as a black and white lamellar structure. Furthermore, gray or dark gray regions containing oriented carbides are bainite. However, in actual observation, the above classification can be difficult. For example, even tempered martensite may be observed as having oriented carbides. In such cases, gray or dark gray regions containing oriented carbides and in which the structure interfaces extend linearly are classified as bainite, while gray or dark gray regions containing other carbides are classified as tempered martensite. Furthermore, fresh martensite is difficult to distinguish from retained austenite, which also appears as white or light gray regions. Therefore, after determining the area fraction of the white or light gray regions, the volume fraction of retained austenite determined by the above method is considered to be the area fraction and is subtracted to determine the area fraction.

[0053] Next, the structure of the outermost layer in the plate thickness will be described. The structure of the outermost layer in the plate thickness here includes the structure of the region from the outermost surface of the steel plate to about 20 μm into the plate thickness. This is because, when evaluating the structure fraction by SEM observation, it is necessary to observe the region from the outermost layer to several μm into the plate thickness. In addition, the very outermost layer within about 5 μm from the outermost surface of the steel plate has scale and is unsteady, so it may have a structure that is essentially different from this surface layer structure.

[0054] Since the present invention is characterized by not using a soft ferrite structure in the surface layer, the structure at the 1 / 4 coil width position and the outermost layer in the plate thickness has an area ratio of ferrite of 0% to 5%. As with the structure further inside the plate thickness, the other structures are mainly composed of tempered martensite, and specifically, it is preferable that they are as follows. Retained austenite volume fraction: 3% to 20% Tempered martensite is 70% or more in area ratio Fresh martensite is 0% to 20% in area ratio Here, the structure at the outermost layer in the plate thickness direction can be determined by a measurement method similar to the method for measuring the steel structure at the 1 / 4 position in the coil width direction and the 1 / 4 position in the plate thickness direction.

[0055] Furthermore, the steel structure at the 1 / 4 position of the coil width and the outermost layer of the plate thickness is assumed to be as follows.

[0056] It is known that retained austenite transforms to martensite through stress-induced transformation. During bending as described in the present invention, many microcracks and voids are formed near the martensite generated by stress-induced transformation, and these are thought to become the initiation points for cracks due to hydrogen embrittlement. The inventors have found that microcracks are particularly likely to form when the aspect ratio (≥ 1) is large, exceeding 2.0. Note that an aspect ratio ≥ 1 refers to a value expressed as the major axis diameter / minor axis diameter of the crystal. While the detailed mechanism is unclear, it is thought that most of the microcracks formed after stress-induced transformation of prior austenite grains are linear along the prior austenite grain boundaries, facilitating crack propagation. Based on the above, it is preferable that the proportion of retained austenite with an aspect ratio of 2.0 or more in the total retained austenite is low, and should be 50% or less. The proportion of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite refers to the proportion of the area of ​​retained austenite having an aspect ratio of 2.0 or more to the area of ​​the total retained austenite. The percentage of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite (%) = 100 × (area ratio of retained austenite with an aspect ratio of 2.0 or more) / (area ratio of total retained austenite) Because retained austenite with a large aspect ratio is likely to form when the carbon concentration distribution is unevenly formed by intercritical annealing or other methods, it is preferable to avoid intercritical annealing as much as possible. Retained austenite with an aspect ratio of 2.0 or more is evaluated using the SEM-EBSD method. For example, the EBSD pattern can be obtained by acquiring and analyzing approximately three fields of view in a 20 μm × 50 μm region at the outermost layer position at 0.05 μm intervals. Other conditions can be determined using the method described in Patent Document 9, for example.

[0057] [Tensile strength of 980 MPa or more] The tensile strength level of the steel sheet targeted by this invention is 980 MPa or higher. The strength level is not limited to 980 MPa, but includes high-strength steel sheets up to approximately 2.0 GPa, such as 1180 MPa, 1300 MPa, and 1470 MPa. The manufacturing method described in this invention can be applied to various materials regardless of the strength level of the steel sheet. TS and YS are values ​​evaluated using a tensile test piece in the sheet width direction at a position 1 / 4 of the coil width. The tensile test is performed in accordance with the method described in JIS Z 2241, using a JIS No. 5 test piece with the longitudinal direction perpendicular to the rolling direction.

[0058] [Yield ratio (YR) 100 x YS / TS is 70% or more] The steel sheet to which the present invention is directed has a high yield ratio of 70% or more, more preferably 75% or more, and even more preferably 80% or more. YS here refers to yield strength, and if an upper yield point clearly occurs, it is referred to as upper yield point strength, and if not, it is referred to as 0.2% proof stress strength.

[0059] [Hardness distribution in the thickness direction: 2.5≦100×(H025-H005) / H025≦20.0] The soft layer on the surface side of the plate thickness, a fundamental element of the present invention, is defined by data obtained from the micro Vickers hardness test shown below. That is, the hardness distribution in the plate thickness direction must satisfy 2.5≦(H025−H005) / H025≦20.0. Here, H025 is the hardness value at the 1 / 4 position of the plate thickness, and H005 is the hardness value at the 1 / 20 position of the plate thickness. The 1 / 20 position and the aforementioned outermost layer of the plate thickness do not necessarily overlap, but they may. Softening the surface layer without utilizing a ferrite structure can improve the ductility of the surface side, particularly the hydrogen embrittlement resistance of bent portions. Hardness evaluation tests can be performed in accordance with the method described in JIS Z 2244. The surface side of the plate thickness must be at least 2.5 times softer than the 1 / 4 position. More preferably, it is 3.0 or more. On the other hand, if the steel sheet is softened excessively, it becomes difficult to obtain the desired YS, and the difference between the thickness surface layer structure and the inner structure increases, impairing bendability, etc. Therefore, the upper limit of 100 × (H025 - H005) / H025 is set to 20.0 or less, and more preferably to 15.0 or less.

[0060] Next, a method for producing a high-strength steel sheet according to the present invention will be described. [Heated to a temperature range of 1080℃ to 1300℃] A steel material having the above-described chemical composition is melted using a conventional refining process and then formed into a steel slab using a conventional ingot-blooming or continuous casting method. Alternatively, a thin steel slab with a thickness of 100 mm or less can be produced using a direct casting method. The steel slab is heated and held at a temperature of 1080°C to 1300°C, then subjected to hot rolling, followed by rough rolling and finish rolling to produce a hot-rolled sheet, which is then coiled. Here, if the heating temperature is below 1080°C, the alloy elements will be unevenly distributed in the steel, reducing the residual γ fraction and impairing the properties of the final product sheet. Furthermore, heating above 1300°C will excessively soften the steel slab, causing surface defects such as slab sagging and scabs, which will impair quality. The thickness of the hot-rolled sheet is preferably 0.8 mm to 4.0 mm.

[0061] [Hot rolling at a finishing temperature between 850℃ and 1000℃] Here, the rolling end temperature (finish hot rolling temperature or finish rolling end temperature) is set to 850°C or higher and 1000°C or lower. Hot rolling must be completed in the austenite single phase region in order to improve the strength and ductility balance after annealing by homogenizing the structure within the steel sheet and reducing the anisotropy of the material, so the finish rolling end temperature is set to 850°C or higher. On the other hand, if the finish rolling end temperature exceeds 1000°C, the hot rolled structure becomes coarse and the properties after annealing deteriorate. A more preferable range of the rolling end temperature is 875°C or higher and 950°C or lower.

[0062] [Winding at a temperature between Ms+50℃ and Bs℃] After the finish rolling, the steel is cooled and then coiled at a coiling temperature of Ms+50°C or higher and Bs°C or lower, thereby completing the hot rolling process. If the coiling temperature is Ms+50°C or lower, the hot-rolled steel sheet becomes excessively hard, making coiling of the hot-rolled steel sheet difficult and also suppressing the formation of scale on the steel sheet surface, which is useful in the present invention. Here, Ms and Bs are calculated using the following formulas, respectively. Ms=561-474[C]-33[Mn]-17[Ni]-17[Cr]-21[Mo] Bs=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo] On the other hand, if the coiling temperature is excessively high, the fraction of ferrite phase generated by phase transformation during the coiling process increases, causing the structure to become non-uniform to an extent that cannot be resolved during the subsequent annealing of the hot-rolled sheet. As a result, gauge fluctuations during cold rolling, variations in the quality of the final product sheet, and an increase in the fraction of austenite with a large aspect ratio are caused, so the upper limit is set to Bs. In the prior art, it is known to promote surface decarburization during coiling of the hot-rolled sheet, but in this case, a relatively high coiling temperature is required, which causes the above-mentioned problems. In the present invention, decarburization is not actively performed during coiling, and surface decarburization occurs in the subsequent process.

[0063] The above-mentioned finish hot rolling temperature and coiling temperature are values ​​measured at the center of the coil width.

[0064] [Skin pass rolling] Subsequently, skin pass rolling may be performed after the end of finish rolling and before annealing the hot-rolled sheet. Skin pass rolling can further flatten the shape of the steel sheet and also destroy scale formed on the hot-rolled sheet, thereby increasing the decarburization ability in the subsequent heat treatment step before cold rolling.

[0065] [Softening annealing (1st and 2nd stages)] Next, in order to decarburize the surface layer of the steel sheet and to soften the steel sheet to reduce the cold rolling load, the steel sheet is annealed at the Ac3 point or higher (referred to as the first stage) without removing the hot-rolled scale, followed by annealing at 400°C or higher and the Ac1 point or lower in the temperature range T (referred to as the second stage). This step is essential for forming the soft surface layer that is a feature of the present invention. Although steps of annealing hot-rolled sheets have been used in the past, as described in, for example, Patent Document 10, most of these were performed after the hot-rolled sheet was pickled. In the present invention, unlike the past, the hot-rolled sheet is annealed in a state in which the hot-rolled scale formed on the surface of the hot-rolled sheet is left without being removed by pickling. By annealing the hot-rolled sheet with the hot-rolled scale left on the surface, it is possible to achieve optimal decarburization of the steel sheet surface.

[0066] If the first-stage annealing temperature is below the Ac3 point, solute carbon is difficult to diffuse, and the non-uniform carbon concentration distribution in the thickness direction of the hot-rolled sheet is not resolved. Furthermore, if annealing occurs in the two-phase region, an excessive carbon-depleted layer is formed in the surface layer. Therefore, the first-stage annealing temperature is set to the Ac3 point or higher. Furthermore, if the first-stage annealing time is short, solute carbon may not be able to diffuse sufficiently, so annealing for 5 seconds or longer is preferred. There is no particular upper limit.

[0067] Subsequently, it is preferable to cool to a temperature range of 400° C. or higher. The cooling rate is not particularly limited, but it is preferable that the cooling rate be 1° C. / s or higher in the temperature range of 400° C. or higher in order to suppress diffusion of the carbon concentration during cooling and maintain the uniform carbon concentration distribution formed in the first annealing stage.

[0068] Subsequently, after cooling, the temperature is maintained or increased to 400°C or higher and the temperature range T is maintained between the Ac1 point and the temperature range T. When increasing the temperature, there is no problem even if this is performed in a separate process (separate line) from the first annealing stage. This second annealing stage homogenizes the structure, thereby suppressing the formation of retained austenite with an aspect ratio of 2.0 or higher in the steel sheet surface. It also softens the hard structure formed by transformation from austenite, reducing the load of the subsequent cold rolling. If the second annealing temperature exceeds the Ac1 point, an austenite phase is formed, which causes the structure to become non-uniform during subsequent cooling, resulting in a decrease in the fraction of retained γ with the desired aspect ratio. In addition, if precipitation strengthening elements such as Ti and Nb are added, carbides and nitrides composed of these elements precipitate coarsely and do not contribute to a high yield ratio. On the other hand, if the second annealing temperature is lower than 400°C, the effects of surface decarburization and softening cannot be obtained. Furthermore, the second annealing time is preferably 60 seconds or longer. This is because if the second-stage annealing time is less than 60 seconds, decarburization may not proceed smoothly. While there is no upper limit on the second-stage annealing time, it is preferable to set it to 48 hours or less from the standpoint of productivity and cost. The temperature must be between 400°C and Ac1, and the temperature range T must also be satisfied. T is 5000√α - 600°C or greater. Here, α = [Sb] + [Cu] / 10 + [Sn] / 10. This parameter corresponds to the amount of surface-segregated elements Sb, Cu, and Sn. The larger the value, the more effective it is in suppressing decarburization during annealing.

[0069] The present inventors have found that the annealing temperature T at which sufficient decarburization progresses can be determined depending on the value of α. If the above temperature conditions are not met, decarburization of the surface layer does not progress sufficiently.

[0070] [Pickling and cold rolling] Next, the steel sheet is pickled and then cold-rolled to the desired thickness at a reduction of 30% or more. Cold rolling can be performed by either tandem rolling (unidirectional rolling) or reverse rolling, or by using known warm rolling techniques. If the reduction is too low, the driving force for recrystallization is low, preventing sufficient recrystallization in the subsequent annealing process. This leads to a decrease in the strength-ductility balance and to a non-uniform structure, making it difficult to achieve the desired residual γ fraction. While there is no particular upper limit, a reduction of 80% or less is preferable from the viewpoint of the rolling load.

[0071] Next, in the annealing process, the obtained steel sheet is subjected to a heat treatment and, if necessary, a plating treatment. It is heated to a temperature 10°C above the Ac3 point and up to 950°C, and then cooled at an average cooling rate of 10°C / s to 100°C / s to at least 400°C, and then cooled at an average cooling rate of 5°C / s to 20°C / s from 400°C to a cooling stop temperature range of 50°C to 300°C. It is preferably held in this cooling stop temperature range for 0.5 seconds or more. Next, it is heated to a temperature range of 250°C to 530°C, and then held in this temperature range for 10 seconds or more, followed by cooling.

[0072] [Annealing temperature: Ac3+10℃ or higher, 950℃ or lower] In the annealing process, the steel sheet is first heated to a temperature of Ac3+10°C or higher and 950°C or lower. While the heating rate is not particularly limited, an excessively low rate can lead to coarsening of grains and loss of toughness, so an average heating rate of 5°C / s or higher is preferred. There is no particular upper limit, and there is no problem even if a heating rate exceeding 100°C / s is applied using an induction heating method or the like. In the temperature range of Ac3+10°C or higher and 950°C, the structure is austenitized, softening the strong carbon concentration distribution formed by surface decarburization during annealing before cold rolling. However, a key point of the present invention is not to achieve complete homogenization. Complete homogenization would prevent the desired hardness distribution from being formed. In the subsequent cooling process, hard martensite is formed. If the heating rate is lower than Ac3+10°C, ferrite remains during annealing, reducing the fraction of tempered martensite and significantly reducing strength. Furthermore, if the annealing temperature exceeds 950°C, the austenite grains grow excessively, and the martensite structure after cooling also becomes coarse, resulting in a low YS. Furthermore, decarburization from the surface layer progresses during annealing, increasing the ferrite fraction in the surface layer of the steel sheet, resulting in a low YS. A more preferable range is Ac3+20°C or higher and 920°C or lower.

[0073] The Ac1 and Ac3 values ​​shown in this specification can be calculated from the following formulas (A) and (B) described in "Leslie Steel Science" (Maruzen Co., Ltd., translated and supervised by Shigeyasu Koda, published May 31, 1985, page 273). In the formulas, [ ] indicates the content (mass%) of each element, and the content of elements not contained in the steel sheet or elements below the lower detection limit of analysis may be calculated as 0 mass%. Ac1(℃)=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]...(A) Ac3(℃)=910-203×√[C]-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W ]-30×[Mn]-11×[Cr]-20×[Cu]+700×[P]+400×[Al]+120×[As]+400×[Ti]...(B) Furthermore, the present inventors have found that when thick scales like those formed on hot-rolled sheets are not formed on the surface layer of a steel sheet and when α corresponding to the elemental amounts of Sb, Sn, and Cu satisfies the specified conditions, surface decarburization hardly occurs even when annealing is performed up to the above-mentioned high temperature range.

[0074] [Average cooling rate up to 400°C: 10°C / s to 100°C / s] The cooling rate after annealing is set to 10°C / s or more. This is because, particularly when the carbon concentration in the surface layer side is reduced by annealing before cold rolling, an insufficient cooling rate will cause a high-temperature precipitation phase to form in the surface layer side, softening the steel sheet. A cooling rate of 15°C / s or more is more preferred, and 20°C / s or more is even more preferred. On the other hand, an excessively high cooling rate will cause uneven cooling in the coil width direction, impairing the flatness of the sheet, so the cooling rate is set to 100°C / s or less.

[0075] [From 400°C to the cooling stop temperature range of 50°C to 300°C, average cooling rate is 5°C / s to 20°C / s] Next, the cooling is performed from 400°C to the cooling stop temperature at an average cooling rate of 5°C / s or more and 20°C / s or less. This cooling transforms a portion of the austenite produced during annealing into martensite. To suppress the formation of high-temperature precipitates, a higher cooling rate after annealing is preferable, at 5°C / s or more. On the other hand, an excessively fast cooling rate makes it difficult to ensure a uniform cooling stop temperature across the coil width, so the upper limit is set to 20°C / s. The cooling stop temperature is set to 50°C or more and 300°C or less. If the cooling rate is lower than 50°C, the fraction of austenite, which contributes to ductility, is excessively reduced. On the other hand, if the cooling rate is higher than 300°C, the fraction of austenite becomes excessive, and in the subsequent cooling process, fresh martensite is formed while tempered martensite is reduced, resulting in reduced ductility. A more preferable range is 80°C or more and 250°C or less. As the cooling means of the present invention, any known method may be used, such as gas cooling, oil cooling, mist cooling, and low-melting-point liquid metal cooling.

[0076] [Holding time in the cooling stop temperature range for 0.5 seconds or more] After cooling to the above temperature, the steel sheet may be held at that temperature for 0.5 seconds or more without being immediately heated. Experiments by the inventors have shown that due to the impossibility of completely uniform cooling, a temperature difference of several tens of degrees may occur in the steel sheet width direction immediately after cooling is stopped. The temperature difference formed after cooling is stopped may affect the temperature difference in the width direction after the subsequent heating and cooling process. This may result in non-uniform material properties in the coil width direction. Setting a holding time of 0.5 seconds or more in the cooling stop temperature range is preferable because it equalizes the temperature difference in the coil width direction during holding, uniforms the material properties in the coil width direction after final cooling, and also enables uniform mechanical properties. A more preferable time is 1.0 seconds or more, and even more preferably 3 seconds or more.

[0077] [Reheating: Heat to a temperature range above the cooling stop temperature and above 250°C to 530°C, then hold at that temperature range for 10 seconds or more] After the cooling stop, the steel is reheated to temper the martensite formed and to stabilize the retained austenite. Reheating may be performed immediately after the cooling stop, or after maintaining a temperature within a range in which carbides do not significantly precipitate. If the reheating temperature is excessively low, carbon diffusion to obtain the above-mentioned structure is not sufficiently promoted, and the desired strength-ductility balance cannot be ensured. Furthermore, if the reheating temperature is excessively high, coarse carbides precipitate, resulting in softening. A more preferred range is above the cooling stop temperature and between 280°C and 450°C. Furthermore, if the holding time is less than 10 seconds, carbon diffusion necessary for austenite stabilization is insufficient, resulting in an excessively high fresh martensite fraction and insufficient improvement in ductility. A more preferred holding time is 60 seconds or more, and even more preferably 180 seconds or more.

[0078] In the annealing step, the steel sheet may be subjected to hot-dip galvanization within the above-mentioned temperature range, i.e., 250°C or higher and 530°C or lower, to obtain a hot-dip galvanized steel sheet, or may be subjected to alloying treatment in a temperature range of 470°C or higher and 530°C or lower after hot-dip galvanization to obtain a galvannealed steel sheet. Furthermore, the steel sheet of the present invention may be electroplated to obtain an electroplated steel sheet.

[0079] [Skin pass rolling with elongation of 0.02% or more] After the annealing process, the steel sheet must be cooled and then skin-pass rolled at an elongation rate of 0.02% or more to achieve flattening and increase the yield ratio. The upper limit of the elongation rate is preferably 2.0% or less, and more preferably 0.5% or less. The lower limit of the elongation rate is preferably 0.05% or more.

[0080] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method. [Example]

[0081] Examples of the present invention are shown below, but the present invention is not limited to these examples and can be implemented with appropriate modifications within the scope of the spirit of the present invention, and all such modifications are considered to be included in the technical scope of the present invention.

[0082] The above steel material having the chemical composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted and produced into thin steel sheets under the conditions shown in Table 2. In the hot rolling process, the steel material was reheated, subjected to rough rolling and finish rolling, then cooled and coiled. The thickness of the hot-rolled sheet was adjusted to a range of 1.8 to 4.0 mm. Then, heat treatment was performed with the hot-rolled scale remaining on the surface, and the scale was removed by pickling. Then, cold rolling was performed. The thickness of the cold-rolled sheet was adjusted to a range of 0.8 to 2.4 mm. Then, heat treatment (CR) was performed. Some of the sheets were subjected to hot-dip galvanizing (GI), galvannealed hot-dip galvanizing (GA), or electrogalvanizing (EG) during or after the heat treatment.

[0083] [Tensile test] JIS No. 5 test pieces (gauge length 50 mm, parallel section width 25 mm) were cut from the resulting steel plate at a position 1 / 4 of the plate width in the direction perpendicular to the rolling direction, and tensile tests were performed in accordance with JIS Z 2241 (2022). The test pieces were butted together to measure El (total elongation), and a value of 10,000 / TS (MPa)% or greater was considered acceptable. Furthermore, the test pieces were embedded in carbon resin so that the rolling direction and the plate thickness direction were the observation surfaces, and hardness tests were performed in accordance with JIS Z 2244 to evaluate the microstructure. Furthermore, 50L x 100C samples were punched out from the 1 / 4 plate width position with a clearance of 15%, and then bent into a V-shape at an angle of 30° to obtain a bending radius (R / t) of 4.

[0084] [Hydrogen embrittlement property test for bent parts] Next, the bent portion of the test piece was restrained by changing the opening angle, applying a residual stress of up to 1.4 × TS to the surface layer of the test piece. The test piece in this restrained state was then placed in hydrochloric acid at a liquid temperature of 25°C and a pH of 1.7 for 200 hours. The presence or absence of cracks in the center of the bend was determined by visual observation. Those in which no cracks were visually observed were judged to have excellent hydrogen embrittlement resistance in the bent portion. The results are summarized in Table 3.

[0085] [Table 1]

[0086] [Table 2]

[0087] [Table 3]

Claims

1. In mass%, C: 0.090% or more and 0.300% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.8% or more and 4.0% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.200% or less, N: 0.0200% or less, and O: Contains 0.0100% or less, Cu: 0.005% or more and 0.5% or less, Sn: 0.005% or more and 0.5% or less, and Sb: 0.001% or more and 0.07% or less, At least two of the above are contained in total so that α is 0.005≦α≦0.07, Here, α is a dimensionless quantity expressed as [Sb] + [Cu] / 10 + [Sn] / 10, and [Sb], [Cu], and [Sn] represent the respective contents (mass%) and are zero when not contained. The balance has a composition consisting of Fe and unavoidable impurities, The steel structure at the 1 / 4 coil width position and the 1 / 4 plate thickness position is Ferrite area ratio is 0% or more and 5% or less, The volume fraction of retained austenite is 3% or more and 20% or less, Tempered martensite has an area ratio of 70% or more, and The area ratio of fresh martensite is 0% or more and 20% or less, moreover, The steel structure at the 1 / 4 coil width position and the outermost layer of the plate thickness is The area ratio of ferrite is 0% or more and 5% or less, and further, In the steel structure at the 1 / 4 position of the coil width and the outermost layer of the plate thickness, The proportion of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite satisfies the requirement of 50% or less, Regarding tensile strength TS and yield strength YS, TS≧980MPa 100 x YS / TS≧70 Satisfied, In the hardness distribution in the thickness direction, The relationship between the Vickers hardness value H025 at the 1 / 4 position of the plate thickness and the Vickers hardness value H005 at the 1 / 20 position of the plate thickness is as follows: 2.5≦100×(H025-H005) / H025≦20.0 High-strength steel plate.

2. The component composition further includes, in mass %, Ti: 0.200% or less, B: 0.0100% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, The high strength steel plate according to claim 1, further comprising at least one element selected from the group consisting of:

3. The high-strength steel sheet according to claim 1 or 2, wherein an electrogalvanized layer is formed on the surface.

4. The high-strength steel sheet according to claim 1 or 2, wherein a hot-dip galvanized layer is formed on the surface thereof.

5. The high-strength steel sheet according to claim 1 or 2, wherein a galvannealed layer is formed on the surface thereof.

6. 3. A method for producing a high-strength steel plate according to claim 1 or 2, comprising the steps of: After heating to a temperature range of 1080°C or higher and 1300°C or lower, Hot rolling is performed at a finishing temperature in the temperature range of 850°C or more and 1000°C or less, Next, the coil is wound in a temperature range of Ms+50°C or more and Bs°C or less. With the hot rolling scale remaining on the surface, Softening annealing After annealing at Ac3 point or higher, At 400°C or higher and Ac1 point or lower and in the temperature range T, T≧5000√α −600℃ where α=[Sb]+[Cu] / 10+[Sn] / 10, and [Sb], [Cu], and [Sn] represent the respective contents (mass%), and are zero when not contained; After removing the scale from the steel plate surface by pickling, A cold-rolled sheet obtained at a cold rolling reduction rate of 30% or more, Heating to Ac3+10°C or higher and 950°C or lower, Furthermore, the average cooling rate to 400°C is 10°C / s or more and 100°C / s or less; From 400°C to the cooling stop temperature range of 50°C to 300°C, Cooling at an average cooling rate of 5°C / s or more and 20°C / s or less, Next, the mixture is heated to a temperature range of 250°C or higher and 530°C or lower, Maintaining the temperature range for 10 seconds or more, A method for manufacturing high-strength steel sheets, in which after cooling, skin-pass rolling is performed to an elongation of 0.02% or more.

7. 7. The method for producing a high-strength steel plate according to claim 6, wherein the steel is cooled from 400°C to a cooling stop temperature range of 50°C to 300°C at an average cooling rate of 5°C / s to 20°C / s, and then held in the cooling stop temperature range for 0.5 seconds or more.

8. The method for producing a high-strength steel sheet according to claim 6, further comprising electrogalvanizing the steel sheet.

9. The method for producing a high-strength steel sheet according to claim 7, further comprising electrogalvanizing the steel sheet.

10. The method for producing a high-strength steel sheet according to claim 6, further comprising the step of hot-dip galvanizing the steel sheet.

11. The method for producing a high-strength steel sheet according to claim 7, further comprising the step of hot-dip galvanizing the steel sheet.

12. The method for producing a high-strength steel sheet according to claim 6, further comprising the steps of: hot-dip galvanizing; and then performing alloying treatment of galvanizing in a temperature range of 470°C or higher and 530°C or lower.

13. The method for producing a high-strength steel sheet according to claim 7, further comprising the steps of: hot-dip galvanizing; and then performing alloying treatment of galvanizing in a temperature range of 470°C or higher and 530°C or lower.

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