High strength steel plate and method for manufacturing the same

Abstract

Regarding the steel structure at the 1 / 4 position in the plate thickness and the outermost layer, the area fraction of (ferrite + bainite) is 0 to 20%, the volume fraction of retained γ is 4 to 20%, the area fraction of tempered martensite is 70% or more, and the area fraction of fresh martensite is 0 to 20%. The texture of the tempered martensite evaluated by the SEM-EBSD method on the plate thickness cross section in the rolling direction is shown in the Euler space of 0≦φ1≦90°, φ2=45°, 0≦Φ≦90° expressed by the Bunge method. A high-strength steel plate having a tensile strength of 980 MPa or more and excellent hydrogen embrittlement resistance in bent sections, in which the maximum ODF of {111} grains, which have the maximum strength, is 2.0 or more at φ1=5×n°, φ2=45°, Φ=55° (n is an integer from 0 to 18) at the 1 / 4 position in the plate thickness, and the maximum ODF of {100} grains, which have the maximum strength, is 1.0 or more at φ1=5×n°, φ2=45°, Φ=0° (n is an integer from 0 to 18) at the plate thickness surface layer, and a manufacturing method thereof.

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 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, and in the automotive sector, there is a demand for improved fuel efficiency through lighter vehicle bodies and for crashworthiness of the large batteries installed in vehicles as they become more electrified. To this end, there has been an increase in the use of high-strength steel materials for automobiles, which can achieve high body rigidity and crashworthiness even when made thinner and lighter. Many steel manufacturers are expanding their lineups of high-strength steel sheets for automobiles, with tensile strengths of 980 MPa and even higher.

[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] In high-strength steel sheets utilizing such hard phases, a method of utilizing tempered martensite is known as a method for improving the mechanical properties. 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 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 a hard tempered martensite as the main phase and also utilizing the austenite phase, which provides 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. [Prior art documents] [Patent documents]

[0006] [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 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, improving ductility, which decreases with increasing material strength, has long been a challenge in the technical field. Furthermore, increasing material strength not only leads to a deterioration in total elongation, but also to poor formability, such as bendability. Furthermore, cracks may occur in the surface layer of the steel sheet during bending, and these microcracks may initiate hydrogen embrittlement and lead to fracture. The inventors have discovered that bending fracture is particularly prevalent under severe bending conditions, such as bending angles of less than 90° and severe environmental conditions for hydrogen penetration, such as pH levels below 1.0. To address these issues, Patent Document 4, for example, discloses an invention that achieves good elongation by forming a surface decarburized layer, and explains the effect of soft ferrite in the surface layer on improving bendability. However, the formation of soft ferrite in the surface layer not only reduces the strength level of the base material, but also promotes the formation of microcracks and voids during processing by increasing the interface between ferrite and martensite, which have different mechanical properties. In particular, materials with a tensile strength exceeding 980 MPa are susceptible to hydrogen embrittlement, leading to fracture.

[0008] The inventors have investigated a manufacturing method for improving the strength-ductility balance and hydrogen embrittlement resistance of high-strength steel plate that contains 70% or more of tempered martensite as the main component, with the remainder consisting mainly of austenite, and has a tensile strength (TS) of 980 MPa or more.

[0009] First, according to known literature techniques, the annealing temperature / time, cooling rate, We attempted to adjust annealing parameters such as cooling stop temperature / time and reheating temperature / time. However, we found that there was a trade-off between the strength (TS in tensile tests) and ductility (total elongation) of steel sheets, and no clear improvement in the strength-ductility balance was observed. Next, we conducted a fundamental review of the annealing process and investigated improving properties by performing an additional heat treatment at Ac1 to Ac1 + 130°C after cold rolling, as described in Patent Document 6. However, when applying the publicly known method, we found that the additional heat treatment coarsened the steel structure, reducing strength and actually tending to result in a poor strength-ductility balance.

[0010] The present invention has been made in view of the above-mentioned circumstances. Specifically, it is an object of the present invention to provide a high-strength steel sheet that has excellent hydrogen embrittlement resistance in a bent portion and an improved strength-ductility balance without utilizing a softened surface layer while maintaining a TS of 980 MPa or more, and a manufacturing method thereof. Here, the bent portion is a portion that has been subjected to particularly severe forming conditions, for example, a bending process with a bending angle exceeding 30 degrees. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above problems.

[0012] The present inventors also focused on the upstream manufacturing process and mainly reviewed the heat treatment method, which was not limited to simple adjustment of temperature and time.

[0013] First, the inventors conducted the following experiment, which was the impetus for developing the present invention. The experiment was conducted to verify whether it was possible to improve the strength-ductility balance and hydrogen embrittlement resistance by drastically changing the heat treatment pattern.

[0014] <Experiment 1> The above steel material, having the chemical composition indicated by the symbol AA in Table 1, with the balance consisting of Fe and unavoidable impurities, was melted and then bloomed into a steel slab. It was then heated to 1200°C and hot-rolled for a total of five passes at an exit rolling temperature of 900°C, then air-cooled to 400°C, and coiled. The front and back surfaces of the 3.4 mm thick steel were then uniformly ground to a finished thickness of 3.0 mm. It was then soaked at 500°C for 3600 minutes in a nitrogen atmosphere and cold-rolled to 1.4 mm. It then underwent additional post-cold-rolling heat treatment in a nitrogen atmosphere between 500°C and 850°C for 60 minutes, cooled, and pickled to remove any minor surface scale. It was then soaked at 880°C for 60 seconds in a nitrogen atmosphere, gas-cooled to 150°C, reheated and held at 300°C for 45 seconds, and gas-cooled to room temperature. The obtained steel sheets were pickled, and then JIS No. 5 test pieces (gauge length 50 mm, parallel portion width 25 mm) were cut out in the direction perpendicular to the rolling direction, and tensile tests were carried out in accordance with JIS Z2241.

[0015] [Table 1]

[0016] The results are shown in Table 2. Under the condition of no additional heat treatment after cold rolling (symbol 1), the elongation was 9.3%, whereas under the condition of additional heat treatment after cold rolling, a maximum elongation of 10.3% was obtained. It was also revealed that the heat treatment temperature had a strong effect, with increased elongation only being achieved in the temperature range of 600°C or higher but lower than 800°C (below point A1). Although strength tended to be slightly lower under the 850°C condition, no significant differences were observed under other conditions.

[0017] [Table 2]

[0018] In addition, 50L × 100C samples were punched out from the resulting steel plates with a clearance of 15% and bent at 60°C into a V-shape with a bending radius of 4. Test specimens were then prepared. The bent portion of the test specimen was then restrained by varying the opening angle, applying a residual stress of up to 1.2 × TS to the surface layer. The restrained test specimens were then placed in hydrochloric acid at 25°C and pH 0.8 for 24 hours. The presence or absence of cracks in the center of the bent portion was then visually observed. As a result, cracks were observed in all steel plates used in Experiment 1.

[0019] The present inventors further conducted the following experiment.

[0020] <Experiment 2> The above steel material, having the chemical composition indicated by the symbol BB in Table 1, with the balance consisting of Fe and unavoidable impurities, was melted and then bloomed into a steel slab. It was then heated to 1250°C and hot-rolled for a total of six passes at an exit rolling temperature of 850°C, followed by air cooling to 400°C and coiling to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then uniformly ground on both sides from a thickness of 3.6 mm to a finished thickness of 3.2 mm. As an additional heat treatment before cold rolling, it was soaked at each temperature for 3600 minutes in a nitrogen atmosphere and allowed to cool in the air. It was then pickled and cold-rolled to a thickness of 1.0 mm to obtain a cold-rolled steel sheet. The cold-rolled steel sheet was then heat-treated in a nitrogen atmosphere at 675°C for 60 minutes, cooled, and pickled to remove any minor scale that had formed on the surface. The cold-rolled steel sheet was then soaked at 880°C for 60 seconds in a nitrogen atmosphere, gas-cooled to 150°C, reheated and held at 300°C for 45 seconds, and gas-cooled to room temperature. The resulting steel sheet was pickled, and then JIS No. 5 test pieces (gauge length 50 mm, parallel portion width 25 mm) were cut out in the direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241 (2022).

[0021] The results are shown in Table 3. When no additional heat treatment was performed before cold rolling (symbol 1), the elongation of the steel sheet was 9.2%, whereas additional heat treatment before cold rolling achieved a maximum elongation of 10.1%. It was also revealed that the heat treatment temperature had a strong effect, with the increase in elongation only being achieved in the temperature range of 400°C or higher and lower than 800°C. When annealed at 800°C, the material hardened, making it impossible to roll, and the tensile properties could not be evaluated. This is thought to be because the austenite formed during annealing at 800°C transformed into a hard structure during cooling. Although strength tended to decrease by 10-20 MPa at temperatures above 400°C, the difference was not significant.

[0022] [Table 3]

[0023] In addition, 50L × 100C samples were punched out from the resulting steel sheets with a clearance of 15% and bent at 75°C into a V-shape with a bending radius of 5°C to prepare test specimens. The opening angle of the bent portion of the test specimen was changed to constrain the surface of the test specimen to a residual stress of up to 1.2 × TS. The test specimens in this constrained state were then placed in hydrochloric acid at a liquid temperature of 25°C and a pH of 0.8 for 200 hours. The presence or absence of cracks in the center of the bent portion was then visually observed. As a result, cracks were observed in all steel sheets used in Experiment 2.

[0024] Based on the above preliminary test results, the inventors have found that the strength-ductility balance can be improved by performing a two-stage heat treatment, that is, a heat treatment before cold rolling and a heat treatment after cold rolling. As will be described in detail below, they have also found that hydrogen embrittlement resistance can be improved by combining this with an unprecedented optimization of the addition of Cu, Sb, and Sn, and have completed the present invention.

[0025] As a result, the inventors have found that by controlling the structure before cold rolling, applying ultra-strong rolling during cold rolling, and performing slow heating or two-stage annealing (heat treatment) in the final annealing, it is possible to highly control the crystal orientation of the steel sheet without utilizing the soft ferrite phase. This has led to the discovery that it is possible to improve the strength-ductility balance while maintaining excellent hydrogen embrittlement resistance and a tensile strength of 980 MPa class. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.090% to 0.300%, Si: 0.40% to 2.50%, Mn: 1.8% to 4.0%, P: 0.100% to 0.100%, S: 0.0200% to 0.200%, Al: 0.200% to 0.200%, N: 0.0200% to 0.0100%, Cu: 0.005% to 0.5%, Sn: 0.005% to 0.5%, and Sb: 0.00 The steel has a chemical composition consisting of at least two of the following: 1% or more and 0.07% or less, with a total of 0.01% or more and 1.0% or less; Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less, with a total of 0.01% or more; and the balance consisting of Fe and unavoidable impurities. The steel structure at the 1 / 4 position of the plate thickness has an area ratio of 0.01% or more and 0.07% or less of ferrite and bainite. % or more and 20% or less, retained austenite volume fraction of 4% or more and 20% or less, tempered martensite area fraction of 70% or more, and fresh martensite area fraction of 0% or more and 20% or less, and the steel structure at the outermost layer position satisfies the following: total area fraction of ferrite and bainite of 0% or more and 20% or less, retained austenite volume fraction of 4% or more and 20% or less, tempered martensite area fraction of 70% or more, and fresh martensite area fraction of 0% or more and 20% or less, and the rolling direction thickness cross section was evaluated by SEM-EBSD method, and the texture of the tempered martensite was found to be φ1=5×n°, φ2=45°, Φ=55° at the 1 / 4 position of the sheet thickness in the orientation distribution function (ODF) shown in the Euler space of 0°≦φ1≦90°, φ2=45°, 0°≦Φ≦90° expressed by the Bunge method. A high-strength steel plate having a tensile strength of 980 MPa or more, in which the maximum ODF of {111} grains, represented by the maximum strength at (n=0, 1, 2, . . . , 18), is 2.0 or more, and the maximum ODF of {100} grains, represented by the maximum strength at φ1=5×n°, φ2=45°, Φ=0° (n=0, 1, 2, . . . , 18), is 1.0 or more at the outermost layer position. [2] The high-strength steel plate according to [1], wherein the chemical composition further contains, in mass%, at least one element selected from 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: 1.00% 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] The high-strength steel plate according to [2], wherein the chemical composition further contains, in mass%, Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.200% or less, and B: 0.0005% or more and 0.0100% or less. [4] A high-strength steel sheet according to any one of [1] to [3], wherein an electrogalvanized layer is formed on the surface. [5] A high-strength steel sheet according to any one of [1] to [3], wherein a hot-dip galvanized layer is formed on the surface. [6] A high-strength steel sheet according to any one of [1] to [3], which has a galvannealed layer formed on the surface. [7] A method for producing a high strength steel plate according to any one of [1] to [3], wherein a steel slab having the above-mentioned chemical composition is heated to a temperature range of 1080°C or more and 1300°C or less, and then finish-rolled at a finish rolling end temperature in a temperature range of 850°C or more and 1000°C or less, and after finish-rolling, is cooled to 720°C or less within 2 seconds at a cooling rate of 70°C or more per second, and is coiled in a temperature range of Ms+100°C or less, and then softening annealing is performed for 48 hours or more in a temperature range of 400°C or more and Ac1 point or less, with or without holding the temperature, before being cold-rolled, and then, in the first pass or the second pass, a cold-rolled sheet obtained by a process of cold-rolling to a total reduction of 50% or more through at least one rolling pass of a steel sheet, the cold-rolled sheet being annealed for 60 seconds or more in a temperature range of 600°C or more and less than the Ac1 point, with or without holding the temperature, and then heated to a temperature range of 50°C or more and 350°C or less at an average cooling rate of 10°C / s or more to at least 500°C, and then heated to a temperature range of the cooling stop temperature or more and higher than 250°C and 600°C or less, and then held in that temperature range for 10 seconds or more. [8] The method for producing a high-strength steel sheet according to [7], further comprising electrogalvanizing the steel sheet. [9] The method for producing a high-strength steel sheet according to [7], further comprising the step of performing hot-dip galvanizing.

[10] The method for producing a high-strength steel sheet according to [7], further comprising the steps of: hot-dip galvanizing; and then performing a galvanizing alloying treatment in a temperature range of 470°C or higher and 600°C or lower. [Effects of the Invention]

[0026] According to the present invention, a higher strength-ductility balance than conventional materials can be obtained. As a result, the range of applications for automobile body parts can be expanded, and the material can be applied to parts that require complex press working, which can greatly contribute to reducing the weight of automobile bodies. In addition, the hydrogen embrittlement resistance of processed parts, particularly bent parts that are subjected to particularly strict forming conditions, which is a problem specific to high-strength materials, can be improved. DETAILED DESCRIPTION OF THE INVENTION

[0027] 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.

[0028] [C: 0.090% or more and 0.300% or less] Carbon (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. Furthermore, the reduced amount of carbides reduces the number of {100} grains, resulting in significant cracking during bending tests. On the other hand, if the C content exceeds 0.300%, 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, more preferably 0.150% or more. The preferred upper limit is 0.280% or less, and even more preferably 0.240% or less.

[0029] [Si:0.40% or more and 2.50% or less] Silicon is one of the important basic components of steel. In particular, in the present invention, silicon inhibits the formation of carbides 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 0.40%, the fraction of retained austenite decreases, making it difficult to achieve the desired El. On the other hand, if the Si content exceeds 2.50%, the carbon concentration in the retained austenite increases excessively, reducing local ductility and bending properties. Therefore, the Si content is set to 0.40% 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.

[0030] [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.

[0031] [P:0.100% or less] P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the local ductility of the steel sheet and decreasing elongation. It also reduces bendability. Therefore, the P content must be 0.100% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is set to 0.100% or less, and preferably 0.070% or less.

[0032] [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. There is no particular lower limit for the S content, but due to production technology constraints, it is preferable to set it to 0.0001% or more. Therefore, the S content is set to 0.0200% or less, and preferably 0.0050% or less.

[0033] [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. The preferred Al content is 0.150% or less.

[0034] [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, and preferably 0.0100% or less.

[0035] [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, and preferably 0.0050% or less.

[0036] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, At least one of these is 0.01% or more Ti, Nb, and V combine with carbon and nitrogen to refine the crystal structure, which in turn has the effect of increasing the strength of the material. Therefore, it is necessary to contain at least one of them in an amount of 0.01% or more. More preferably, it is 0.015% or more, and even more preferably, it is 0.04% or more. When two or more elements are contained, the total amount must be 0.01% or more.

[0037] Furthermore, if Ti and Nb are each 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 Ti and Nb contents are preferably 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 each 0.001% or more. Even more preferably, the Ti and Nb contents are each 0.100% or less.

[0038] 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, the V content is more preferably 0.001% or more because V increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, if V is contained, its content is 0.200% or less. A more preferred V content is 0.001% or more. A more preferred V content is 0.100% or less.

[0039] [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 present in an amount of 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, resulting in a decrease in elongation. Furthermore, the bendability will not be reduced. Therefore, the Cu and Sn contents are preferably set to 0.5% or less. While there is no particular lower limit for the content, because these elements improve hardenability, the content is more preferably set to 0.005% or more. More preferably, it is set to 0.01% or more. Furthermore, it is more preferably set to 0.30% or less.

[0040] 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 to obtain the effect of suppressing surface decarburization. More preferably, it is 0.004% or more.

[0041] [At least two of Cu, Sn, and Sb, total content between 0.01% and 1.0%] As will be described later, Cu, Sn, and Sb are features of the present invention, and at least two of these elements must be included, with a total content of 0.01% or more. These elements are known as surface segregation elements, and adding a certain amount can suppress surface decarburization during the annealing process, thereby suppressing the decrease in strength of the base material due to the uneven structure in the thickness direction and the softening of the surface layer through the thickness. More preferably, at least two of Cu, Sn, and Sb should be included in a total content of 0.015% or more.

[0042] 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.

[0043] 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 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: 1.00% 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.

[0044] Ta and W, when present in an amount of 0.10% or less, do not form large amounts of coarse precipitates or inclusions, thereby preventing a decrease in the local ductility of the steel sheet and thus preventing a decrease in elongation. Furthermore, bendability is not impaired. Therefore, the Ta and W contents are preferably each 0.10% or less. While there are no specific lower limits for the Ta and W contents, they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, the Ta and W contents are more preferably 0.01% or more. Therefore, when Ta and W are contained, their contents should be 0.10% or less. More preferably, the Ta and W contents should be 0.01% or more. Even more preferably, the Ta and W contents should be 0.08% or less.

[0045] 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.

[0046] 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.

[0047] If Ni is 1.00% 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 1.00% 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 1.00% or less, more preferably 0.001% or more. An even more preferable Ni content is 0.500% or less.

[0048] 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 each 0.0100% or less. While there are no specific lower limits for the Ca, Mg, and REM contents, since these elements spheroidize the shape of nitrides and sulfides and improve the local ductility of the steel sheet, 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 should be 0.0005% or more. Even more preferably, the Ca, Mg, and REM contents should be 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] Among the elements mentioned above, Ti, Nb, and B are actively used in high-strength steel sheets because they are relatively inexpensive and readily available. A common feature of these elements is their ability to delay the recrystallization of the steel structure. This effect is particularly pronounced when Ti, Nb, and B are added simultaneously. As will be described later, the present invention is a technology for suppressing the austenite transformation from the elongated, non-recrystallized structure during final annealing, and therefore exhibits particularly favorable effects in steel types to which Ti, Nb, and B are added simultaneously. The ranges of Ti, Nb, and B are Ti: 0.010% to 0.200%, Nb: 0.005% to 0.200%, and B: 0.0005% to 0.0100%.

[0054] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0055] Next, the microstructure will be explained. The locations for defining the structure are the 1 / 4 position in the plate thickness direction and the outermost layer position. First, the structure at the 1 / 4 position in the plate thickness direction, which is generally considered to be the location having the average properties and structure of a steel plate, will be explained.

[0056] [The sum of ferrite and bainite is 0% to 20%. Electron microscope observation field area ratio] Although ferrite is a soft structure and is effective in improving workability, it has a significant effect of reducing the strength of the 980 MPa-class high-strength steel sheet targeted by the present invention. Here, ferrite may be polygonal ferrite, pseudo-polygonal ferrite, or granular bainitic ferrite. Bainite, like martensite, is an aggregate of lath-shaped crystal grains, but is softer than martensite. Here, bainite includes upper bainite and lower bainite. To achieve high strength, the total content of ferrite and bainite is set to 20% or less. More preferably, the total content of ferrite and bainite is set to 15% or less. Even more preferably, the total content of ferrite and bainite is less than 2% and 10% or less. Since ferrite and bainite are not necessarily present, the lower limit is set to 0% or more.

[0057] The ferrite area ratio is determined by the following method. First, a thickness cross section (L cross section) of the steel sheet parallel to the rolling direction is polished and then etched with 3 vol.% nital. Ten observations at a 1 / 4 thickness position are performed using a scanning electron microscope (SEM) at 2000x magnification. The 1 / 4 thickness position refers to the position corresponding to the 1 / 4 position of the thickness in the depth direction from the steel sheet surface. Furthermore, in the case of a zinc-plated steel sheet, the steel sheet surface refers to the interface between the zinc-plated layer and the base steel sheet. Next, using the obtained structural images, the structural (ferrite) area ratios for the 10 observations are calculated using Media Cybernetics' Image-Pro. The average of the area ratios for these 10 observations is defined as the "ferrite area ratio." In the structural images above, ferrite appears as a gray structure (base structure). Retained austenite and martensite appear as white structures.

[0058] [Retained austenite 4% to 20%. Volume fraction measured 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 4% 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 preferred lower limit is 6% or more. A preferred upper limit is less than 16%. Note that this volume fraction of retained austenite can be converted into an area fraction.

[0059] 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.

[0060]

number

[0061] [Tempered martensite is 70% or more. Electron microscope observation area ratio] Tempered martensite is characterized by an aggregate of lath-shaped crystal grains and the inclusion of iron-based carbides therein. 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. A more preferred area fraction of tempered martensite is 75% or more, and even more preferably 80% or more. While there is no particular upper limit for the area fraction of tempered martensite, a tempered martensite area fraction of 94% or less is preferred to ensure the area fraction of retained austenite. The area fraction of tempered martensite can be measured by the method described below.

[0062] [Fresh martensite: 0% to 20%. Electron microscope observation field area ratio] Fresh martensite is martensite that does not contain iron-based carbides. Steel plates containing fresh martensite have high strength but poor processing characteristics, so the area fraction of fresh martensite in the steel plate 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 plate according to this embodiment has sufficient strength. Therefore, the steel plate according to this embodiment does not need to contain fresh martensite, so the lower limit of the area fraction of fresh martensite is 0%. A more preferable range is 10% or less.

[0063] Furthermore, in the steel structure according to the present invention, in addition to the above-mentioned ferrite, tempered martensite, bainite, fresh martensite, and retained austenite, carbides such as pearlite and cementite and other known structures of steel sheets may be contained in an area ratio of 8% or less in total without impairing the effects of the present invention. Note that other structures of the steel sheet (remaining structures) may be confirmed and determined, for example, by SEM observation.

[0064] 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 (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) using a scanning electron microscope (SEM) at 1500x magnification. Observation at higher magnifications may be performed to confirm carbides in detail. Note that the steel sheet surface in the case of a zinc-plated steel sheet refers to the interface between the zinc-plated 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. Therefore, gray or dark gray regions containing oriented carbides and in which the structure interfaces extend linearly were classified as bainite, and gray or dark gray regions containing other carbides were classified as tempered martensite. Furthermore, since fresh martensite is difficult to distinguish from retained austenite, which also appears in white or light gray regions, the area fraction of the white or light gray regions was determined, and the volume fraction of retained austenite determined by the above method was then subtracted from the area fraction to determine the area fraction.

[0065] Next, the structure at the outermost layer position will be described. The outermost layer position was selected as a region that does not include unsteady parts such as scale in the very outermost layer. The outermost layer position refers to a position 20 μm into the sheet thickness from the steel sheet surface. As with the 1 / 4 sheet thickness position, if a zinc-plated layer is present, the interface between the zinc-plated layer and the base steel sheet is defined as the steel sheet surface. The structure at the outermost layer position also includes a structure fraction within the range specified for the 1 / 4 sheet thickness. However, since the structure is more preferably uniform throughout the entire sheet thickness, the following is used.

[0066] A feature of the present invention is that the structure at the outermost layer is not significantly different from the structure at the 1 / 4 thickness position. Therefore, it is preferable that each of the above structures is within a range of ±5% of the value at the 1 / 4 thickness position. Because finite-field observation involves a certain degree of variation, the structure will not be exactly the same as the value at the 1 / 4 thickness position, but more preferably, it is within a range of ±3% of the value at the 1 / 4 thickness position. For example, Patent Document 4 discloses an invention that obtains good elongation properties by forming a surface decarburized layer. The present invention is characterized by suppressing such a non-steady surface structure that was previously formed intentionally or unavoidably during annealing, and is therefore distinct from such inventions.

[0067] [Tensile strength of 980 MPa or more] The tensile strength level of the steel sheet targeted by the present invention is 980 MPa or higher. The strength level is not limited to 980 MPa, but includes high strengths up to about 1.5 GPa, such as 1180 MPa, 1300 MPa, and 1470 MPa. The manufacturing method shown in the present invention can be applied to various materials regardless of the strength level of the steel sheet.

[0068] [The rolling direction thickness cross section was evaluated by SEM-EBSD. The texture of the tempered martensite was evaluated by the orientation distribution function (ODF) expressed by the Bunge method in the Euler space of 0°≦φ1≦90°, φ2=45°, 0°≦Φ≦90°. In the orientation distribution function (ODF) of the {111} grains, expressed as the maximum intensity at φ1=5×n°, φ2=45°, Φ=55° (n=0, 1, 2, . . . , 18) at 1 / 4 of the thickness, the maximum ODF of the {111} grains was 2.0 or more, and in the thickness surface, the maximum ODF of the {100} grains, expressed as the maximum intensity at φ1=5×n°, φ2=45°, Φ=0° (n=0, 1, 2, . . . , 18) was 1.0 or more.] The texture of the bcc phase grains is characterized by satisfying the above-mentioned orientation distribution function (ODF) in the Euler space of 0°≦φ1≦90°, φ2=45°, and 0°≦Φ≦90°, expressed by the Bunge method. The crystal orientation grains indicated by φ1=5×n°, φ2=45°, and Φ=55° (referred to here as {111} grains) are known to increase local elongation. While this crystal orientation is used to ensure ductility in mild steel sheets, its effectiveness in high-strength steel sheets has not been fully recognized due to the lack of established crystal orientation control technology.

[0069] The ODF was measured using the SEM-EBSD method described above, using an OIM Analysis 8.0 manufactured by TSL. The measurement range for the surface layer was from the surface to a position 20 μm in the sheet thickness direction. While the EBSD method was used to measure the ODF, other known methods, such as XRD, may also be used. In Experiment 2 above, the inventors investigated the reason why heat treatment before cold rolling improved ductility and examined the texture of the resulting steel sheet. As a result, it was found that high ductility is exhibited when the maximum ODF of the {111} grains, represented by the maximum strength at φ1=5×n°, φ2=45°, and Φ=55° (n=0, 1, 2, . . . , 18) at the 1 / 4 sheet thickness position, is 2.0 or more. More preferably, it is 2.5 or more.

[0070] Furthermore, the inventors have determined that the crystal orientation indicated by φ1=5×n°, φ2=45°, and Φ=0° It was discovered that hydrogen embrittlement resistance can be improved by forming more grains (here referred to as {100} grains) in the surface layer of the steel sheet. Although the detailed mechanism is unknown, it is thought that the ND / / {100} grains, which have the above Euler angles, make it difficult for iron to dissolve during the corrosion reaction of hydrochloric acid immersion, and therefore also suppress the penetration of hydrogen into the steel. The above-mentioned high hydrogen embrittlement resistance can be achieved by forming more grains (here referred to as {100} grains) in the surface layer of the steel sheet. = 0° (n = 0, 1, 2, . . . , 18) is shown when the maximum ODF of the {100} grains, expressed as the maximum intensity at 0° (n = 0, 1, 2, . . . , 18), is 1.0 or more. More preferably, it is 1.5 or more.

[0071] The high-strength steel sheet may be coated with an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer by a conventional method.

[0072] Next, a method for producing a high strength steel sheet according to the present invention will be described.

[0073] A steel material having the aforementioned 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 may be produced using a direct casting method. The steel slab is heated and held at a temperature of 1080°C to 1300°C, and then subjected to hot rolling. A hot-rolled sheet is produced by rough rolling and finish rolling, and then wound into a coil. After finish rolling, the sheet is cooled to 720°C or less within 2 seconds at a cooling rate of 70°C per second or more. The thickness of the hot-rolled sheet is preferably 0.8 mm to 4.0 mm. The reasons for limiting the above hot-rolling conditions are as follows.

[0074] [Heating temperature: 1080℃ or higher and 1300℃ or lower] If the temperature is lower than 1080°C, precipitates in the steel do not dissolve sufficiently, and a fine structure cannot be obtained in the subsequent heat treatment process, which is disadvantageous for increasing strength. Therefore, the lower limit is 1080°C or higher, and more preferably 1120°C or higher. Furthermore, if the temperature is higher than 1300°C, the slab becomes more likely to deform, increasing the frequency of defects such as scabs. Therefore, the upper limit is set to 1300°C or lower, and more preferably 1280°C or lower.

[0075] [Finishing temperature: 850℃ to 1000℃] Hot rolling must be completed in the austenite single-phase region to improve the strength and ductility balance of the material by homogenizing the structure within the steel sheet, so the finish rolling temperature is set to 850°C or higher. Furthermore, if the finish rolling temperature exceeds 1000°C, the hot-rolled structure becomes coarse, which in turn causes the structure after annealing to become coarse, resulting in a decrease in the strength of the steel sheet. The preferred lower limit of the finish rolling temperature is 875°C or higher. The preferred upper limit of the finish rolling temperature is 950°C or lower.

[0076] [Cooling after finish hot rolling: Cool to 720°C or below within 2 seconds at an average cooling rate of 70°C or more per second] If the steel sheet is held at temperatures above 720°C for a long time, precipitates such as Ti coarsen, losing their strength-enhancing effect. As a result, even if strength is increased by adjusting other manufacturing conditions, the desired ductility cannot be achieved. Furthermore, the increase in the grain size before cold rolling reduces the number of {111} grains formed in the steel sheet after final annealing. Therefore, cooling is initiated within 2 seconds after finish rolling to 720°C or below, more preferably within 1 second. During cooling, the sheet temperature must be kept as low as possible to prevent precipitate coarsening. Therefore, the average cooling rate to 720°C is set to 70°C or higher per second, more preferably 90°C or higher. Here, "cooling" refers to water cooling, oil cooling, or gas cooling performed to lower the steel sheet temperature. The average cooling rate to 720°C refers to the average cooling rate from the start of cooling until the steel sheet temperature reaches 720°C.

[0077] [Wind-up temperature: Ms+100℃ or less] After finish rolling, the steel sheet is cooled and then coiled at a coiling temperature of Ms+100°C or lower, completing the hot rolling process. If the coiling temperature exceeds Ms+100°C, a large amount of scale and grain boundary oxidation will form on the steel sheet, deteriorating pickling properties, and the hot-rolled structure will become coarse, making it difficult to form {111} grains, which are advantageous for improving local ductility after cold rolling annealing. Here, Ms is the temperature expressed by the following formula. Ms=561-474[C]-33[Mn]-17[Ni]-17[Cr]-21[Mo] Here, [C], [Mn], [Ni], [Cr], and [Mo] refer to the content (mass%) of each element, and if no element is contained, it is set to zero. If the winding temperature is too low, an excessively hard structure is formed, impairing winding properties, so the lower limit of the temperature is preferably 350°C or higher.

[0078] Subsequently, after the finish rolling is completed, skin pass rolling may be performed before the hot-rolled sheet is annealed. By the skin pass rolling, the shape of the steel sheet can be corrected.

[0079] [Softening annealing] The hot-rolled sheet is then annealed before cold rolling to coarsen the carbides in the steel while suppressing surface decarburization. This step is important for sharpening the crystal orientation after final annealing, i.e., for obtaining the ODF described above. When coiled under the coiling temperature conditions of the present invention, a large amount of hard structure is formed, which results in a high cold rolling load. Therefore, this step is also useful from the viewpoint of reducing the rolling load.

[0080] [Hot-rolled sheet annealing: 48 hours or more at 400°C or higher and below Ac1 point] If the annealing temperature exceeds the Ac1 point, an austenite phase will be formed, and carbides that are detrimental to the formation of {111} grains in the final annealed sheet will be formed during the cooling process. Therefore, the annealing temperature should be below the Ac1 point. On the other hand, if the annealing temperature is too low, the effect of coarsening the carbides will be reduced, so the lower limit is set to 400°C, and more preferably, it should be 450°C or higher.

[0081] The annealing time is set to 48 hours or more. The reason for this limitation is that a long annealing time is necessary to sufficiently spheroidize the carbides that precipitated relatively finely during coiling. If the annealing time is less than 48 hours, the spheroidization of the carbides does not progress sufficiently, making it difficult to obtain {111} grains, which are advantageous for local ductility, in the subsequent rolling annealing process. A more preferable annealing time is 60 hours or more. Furthermore, decarburization from the steel sheet surface during annealing is generally promoted at higher temperatures. However, for the above-mentioned reasons, the annealing temperature must be set to Ac1 or less, and the promotion of decarburization due to high temperatures is suppressed. Even so, although decarburization tends to progress with long annealing, decarburization is suppressed due to the addition of specified amounts of Cu, Sb, and Sn. Although there is no particular upper limit to the annealing time, it is desirable that it be 300 hours or less from the viewpoint of productivity and cost. In this case, as long as the annealing temperature is within the range of 400°C or higher and Ac1 point or lower, the effect obtained will be the same whether or not the annealing temperature is maintained at a constant temperature. As an example of not maintaining a constant temperature, it may take 48 hours or more to raise the temperature from 400°C or higher to Ac1 point or lower.

[0082] [Cold rolling] The steel sheet is then preferably pickled and then cold rolled to a desired thickness. The cold rolling may be performed by either tandem rolling (unidirectional rolling) or reverse rolling, or by using a known warm rolling technique or interpass aging technique. The reasons for limiting the cold rolling conditions are as follows.

[0083] [Total reduction: 50% or more] The total reduction is also important for obtaining the above-mentioned ODF. If the total reduction is too low, the driving force for recrystallization is low, and sufficient recrystallization does not occur in the subsequent annealing process, making it impossible to achieve both high strength and high ductility. Furthermore, since {111} grains, which contribute to improving local ductility, are more likely to be obtained when the cold rolling rate is high, the total reduction is set to 50% or more, and more preferably 60% or more. There is no particular upper limit, but from the viewpoint of the rolling load, it is preferable to set it to 95% or less.

[0084] [Rolling with a first-pass reduction ratio of at least 30% in the first or second pass] In the present invention, this is a necessary step for obtaining the above-mentioned ODF after final annealing. If the single-pass rolling ratio in the first or second pass is low, strain concentrates in the surface layer, making it difficult to form {111} grains, which contribute to improving local ductility, within the sheet thickness. To form {111} grains, a higher single-pass rolling ratio is preferable, and 30% or more is required. A more preferable single-pass rolling ratio is 35% or more. There is no particular upper limit, but if it is too high, the steel sheet shape will be inferior, so it is less than 50%, and in this case, 49% or less. The number of rolling passes may be at least one.

[0085] [Two-step annealing process] Next, in the annealing process, the obtained steel sheet is subjected to heat treatment and, if necessary, plating treatment. First, the cold-rolled steel sheet is heated to 600°C or higher and lower than the Ac1 point. The average heating rate is not particularly limited, but if it is too low, the crystal grains may become coarse and the strength may be impaired. Therefore, it is recommended to set the heating rate at 5°C / There is no specific upper limit, and the heating rate may exceed 100°C / s by induction heating or other methods. There is no problem in applying such conditions.

[0086] The heat treatment involves first heating to a temperature range of 600°C or higher but lower than the Ac1 point (first stage), holding that temperature range for 60 seconds or longer, and then heating to a temperature range of 50°C or higher but lower than the Ac1 point (second stage). The material is then cooled to a cooling stop temperature range of 50°C or higher but lower than 350°C at an average cooling rate of 10°C / s or higher up to at least 500°C. The material is then heated to a temperature range of above the cooling stop temperature but higher than 250°C but lower than 600°C, and held at that temperature range for 10 seconds or longer.

[0087] First, the first stage annealing conditions will be explained below.

[0088] [Annealing temperature: 600°C or higher and lower than Ac1 point, temperature maintained / not maintained, annealing time: 60 seconds or more] The first annealing stage plays an important role in the manufacturing method of the present invention and is a process for actively recrystallizing steel sheets at temperatures below the Ac1 point. It is known that steel sheets subjected to large strain during cold rolling form many {111} grains after recrystallization. Therefore, if recrystallization can be performed prior to the subsequent transformation of the structure by heating to the Ac1 point or higher, it is possible to form many {111} grains. Annealing temperatures below 600°C result in the failure of the rolled structure to recrystallize, leaving a coarse structure elongated in the rolling direction. The presence of a coarse unrecrystallized structure reduces the proportion of high-angle grain boundaries where austenite grains preferentially form in the structure prior to austenite transformation, resulting in a reduced number of austenite grains. On the other hand, temperatures above the Ac1 point result in the formation of austenite grains during annealing, which promotes element partitioning between ferrite and austenite grains, resulting in a non-uniform structure and impaired ductility in the final annealed steel sheet. That is, in order to fully induce recrystallization, the temperature is set to 600°C or higher, more preferably 650°C or higher. As long as the first-stage annealing temperature is within the range of 600°C or higher and lower than the Ac1 point, the effect obtained will be the same whether or not the temperature is held at a constant value. As an example of not holding the temperature at a constant value, it may take 60 seconds or more to raise the temperature from 600°C or higher to lower than the Ac1 point.

[0089] In addition, if the annealing time is short, recrystallization does not occur, so the annealing time is set to 60 seconds or more. There is no particular upper limit.

[0090] Furthermore, the effect of suppressing surface decarburization by adding the specified amounts of Cu, Sb, and Sn is also exhibited during the main annealing process. Conventionally, when steel sheets without the specified amounts of Cu, Sb, and Sn were annealed for a long time before cold rolling, a soft decarburized layer formed in the surface layer and a structure with relatively few carbides was formed. When such a structure was subsequently processed by cold rolling, most of the strain accumulated near the ferrite grain boundaries. As a result, in the subsequent first annealing stage, excessive {111} grains were formed near the ferrite grain boundaries. On the other hand, few {100} grains were recrystallized near the coarse carbides. Therefore, as long as the specified amounts of Cu, Sb, and Sn were added, it was possible to ensure a consistent {100} grain density in the surface layer during the main annealing process, even when annealing before cold rolling was performed.

[0091] Furthermore, there is no problem if the first annealing step and the subsequent second annealing step are performed in different production lines. For example, when performing annealing for a long period of time, the first annealing step may be performed in a batch system, followed by the second annealing step by continuous annealing.

[0092] Next, the second stage annealing conditions will be explained below.

[0093] [Annealing temperature: Ac1 point or higher, 950°C or lower] If the annealing temperature is lower than the Ac1 point, a large amount of ferrite remains in the structure after annealing, and sufficient strength cannot be obtained. To obtain high strength, it is better to have as little ferrite as possible, and therefore it is more preferable to set the annealing temperature at the Ac3 point or higher. On the other hand, if the annealing temperature is higher than 950°C, the structure becomes excessively large, making it difficult to achieve both high strength and high ductility, and the {111} grains decrease, so it is more preferable to set the annealing temperature at 920°C or lower.

[0094] The Ac1 and Ac3 points 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 point (℃)=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]...(A) Ac3 points (℃)=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) [Average cooling rate to at least 500°C: 10°C / s or more] In the cooling process after annealing, the average cooling rate to at least 500°C is preferably high, and is set to 10°C / s or more. This is also to suppress the grain growth of the already existing ferrite and to avoid excessive softening of the material. More preferably, it is 15°C / s or more. On the other hand, there is no particular upper limit. The reason for specifying the average cooling rate to at least 500°C is that, within the range of the composition of the present invention, if the cooling rate is slow in the vicinity of this temperature range, structures such as pearlite and bainite that are not favorable for improving the balance between strength and ductility are likely to form.

[0095] [Cooling stop temperature: 50℃ or higher and 350℃ or lower] The cooling following the annealing is performed to a temperature of 50°C or higher and 350°C or lower. By cooling to 350°C or lower, a portion of the austenite produced during annealing is transformed into martensite. Below 50°C, the austenite necessary for improving ductility is completely transformed into martensite or other materials and lost. Above 350°C, martensite transformation is insufficient, resulting in a decrease in the fraction of tempered martensite necessary for high strength. On the other hand, the fraction of fresh martensite, which is useful for high strength but has poor ductility, increases, making it difficult to achieve both high strength and high ductility even when the subsequent heat treatment conditions are adjusted. The preferred lower limit is 75°C or higher. The preferred upper limit is 300°C or lower. The cooling method used in the present invention may be any known method, such as gas cooling, oil cooling, mist cooling, or low-melting-point liquid metal cooling.

[0096] [Reheating: Heating to a temperature range above the cooling stop temperature and above 250°C to 600°C, and then holding at that temperature range for 10 seconds or more] After the cooling is stopped, the steel is reheated to temper the martensite that has formed and to stabilize the retained austenite. After the cooling is stopped, the steel may be reheated immediately, or may be reheated after maintaining a temperature within a range in which carbides do not precipitate significantly. Here, the temperature range in which carbides do not precipitate significantly is preferably 250°C or lower, and more preferably 200°C or lower.

[0097] If the reheating temperature is too low, carbon diffusion required to obtain the above-mentioned structure is not sufficiently promoted, making it impossible to achieve both high strength and high ductility. Furthermore, if the reheating temperature is too high, carbides precipitate coarsely, resulting in softening. Therefore, after reheating to a temperature range equal to or higher than the cooling stop temperature and greater than 250°C and less than or equal to 600°C, the steel must be held in that temperature range for 10 seconds or longer. A more preferred range is equal to or higher than the cooling stop temperature and greater than 280°C and less than or equal to 400°C. Furthermore, if the holding time is less than 10 seconds, carbon diffusion required for austenite stabilization is insufficient, resulting in insufficient improvement in ductility. A holding time of 30 seconds or longer is more preferred, and 60 seconds or longer is even more preferred.

[0098] In the annealing step, the steel sheet may be subjected to hot-dip galvanization within the above-mentioned range, i.e., within a temperature range of more than 250°C and not more than 600°C, to obtain a hot-dip galvanized steel sheet, or may be subjected to alloying treatment in a temperature range of 470°C or more and 600°C or less after hot-dip galvanization to obtain an alloyed hot-dip galvanized steel sheet. Furthermore, the steel sheet of the present invention may be electroplated to obtain an electroplated steel sheet.

[0099] After the annealing step, temper rolling may be carried out. The preferred range of elongation is 0.02% to 2.0%.

[0100] The manufacturing conditions other than those mentioned above can be determined by ordinary methods.

[0101] The techniques disclosed in this disclosure make it possible to manufacture thin steel sheets that are high in strength and excellent in ductility. [Example]

[0102] The following examples of the present invention are provided. However, the present invention is not limited to the following examples, and can be practiced with appropriate modifications within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0103] The above steel material having the chemical composition shown in Table 4, with the balance being Fe and unavoidable impurities, was melted and bloomed to form a steel slab, which was then hot-rolled under the conditions shown in Table 5, cooled, and coiled. The slab was then pickled and cold-rolled. It was then subjected to one or two heat treatments. The number of heat treatments was one if the first-stage heating temperature was a continuous temperature rise, and two if the first-stage heating temperature was not a continuous temperature rise. Some of the slabs were subjected to hot-dip galvanizing, galvannealing, and electrogalvanizing after the heat treatment.

[0104] [Table 4]

[0105] [Table 5]

[0106] [Table 6]

[0107] [Tensile test] JIS No. 5 test pieces (gauge length 50 mm, parallel width 25 mm) were cut from the obtained steel sheets in the direction perpendicular to the rolling direction and subjected to tensile tests in accordance with JIS Z 2241. Strengths of 980 MPa or higher were considered to be high strength. The elongation value was calculated by measuring the gauge length when the test pieces were butted together after tensile fracture and calculating the change before and after the tensile test. Here, elongation values ​​of 10% or higher were considered to be high elongation values.

[0108] [Structural observation] Furthermore, the test pieces were embedded in carbon resin so that the rolling direction and the thickness direction were the observation surfaces, and the structure was evaluated.

[0109] [Hydrogen embrittlement property test for bent parts] Additionally, a 50L x 100C sample was punched from the 1 / 4 width position with a 15% clearance and bent at 50°C into a V-shape with a bending radius of 5. The specimen was then restrained by varying the opening angle of the bent portion, applying a residual stress of up to 1.5 x TS to the surface of the specimen. The restrained specimen was then placed in a hydrogen penetration environment of 25°C, pH 0.9 hydrochloric acid for 24 hours, and the presence or absence of cracks in the center of the bend was determined by visual observation. Specimens with no visible cracks were deemed to have excellent hydrogen embrittlement resistance in the bent portion.

Claims

1. In mass%, C: 0.090% or more and 0.300% or less, Si: 0.40% 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, Contains at least two of the following in a total amount of 0.01% or more and 1.0% or less, Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less, Contains 0.01% or more of at least one of the following: The balance has a composition consisting of Fe and unavoidable impurities, The steel structure at the 1 / 4 position of the plate thickness is The total area ratio of ferrite and bainite is 0% or more and 20% or less, The volume fraction of retained austenite is 4% or more and 20% or less, Tempered martensite is 70% or more in area ratio, Fresh martensite area ratio is 0% to 20% Satisfying the above, and The steel structure at the outermost layer is The total area ratio of ferrite and bainite is 0% or more and 20% or less, The volume fraction of retained austenite is 4% or more and 20% or less, Tempered martensite is 70% or more in area ratio, The area ratio of fresh martensite is 0% or more and 20% or less, The tempered martensite specimen was evaluated by SEM-EBSD in the thickness direction of the rolling direction. The texture of the above is expressed by the Bunge method in the Euler space of 0°≦φ1≦90°, φ2=45°, 0°≦Φ≦90°, and the orientation distribution function (ODF) is as follows: At the quarter-thickness position, the maximum ODF of the {111} grains, represented by the maximum value of the strength at φ1 = 5 × n °, φ2 = 45 °, Φ = 55 ° (n = 0, 1, 2, ..., 18), is 2.0 or more; A high-strength steel plate having a tensile strength of 980 MPa or more, in which the maximum ODF of {100} grains, expressed by the maximum value of strength at φ1=5×n°, φ2=45°, Φ=0° (n=0, 1, 2, ..., 18), is 1.0 or more at the outermost layer position.

2. The component composition further includes, in mass %, 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: 1.00% 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 component composition further includes, in mass %, Ti: 0.010% or more and 0.200% or less, Nb: 0.005% or more and 0.200% or less, and B: 0.0005% or more and 0.0100% or less The high-strength steel plate according to claim 2, further comprising:

4. The high-strength steel sheet according to any one of claims 1 to 3, wherein an electrogalvanized layer is formed on the surface.

5. The high-strength steel sheet according to any one of claims 1 to 3, wherein a hot-dip galvanized layer is formed on the surface.

6. The high-strength steel sheet according to any one of claims 1 to 3, wherein a galvannealed layer is formed on the surface.

7. The method for producing a high-strength steel plate according to any one of claims 1 to 3, comprising the steps of: After heating to a temperature range of 1080°C or higher and 1300°C or lower, Finish rolling is performed at a finish rolling end temperature in the temperature range of 850°C or higher and 1000°C or lower, After finish rolling, the material is cooled to 720°C or less within 2 seconds at an average cooling rate of 70°C or more per second. After winding in a temperature range of Ms+100°C or less, Before cold rolling, softening annealing is performed in a temperature range of 400 ° C. or higher and Ac point or lower, with or without holding the temperature, for 48 hours or more, Thereafter, the cold-rolled sheet obtained through the process of cold rolling to a total reduction rate of 50% or more by rolling to a rolling rate of at least 30% or more in the first or second pass is kept at a temperature range of 600 ° C. or more and less than the Ac1 point, Alternatively, after annealing for 60 seconds or more without holding, Heating to a temperature of from the Ac1 point to 950°C, Furthermore, the average cooling rate to at least 500°C is set to 10°C / s or more, Cooling to a cooling stop temperature range of 50°C or higher and 350°C or lower, Next, the steel sheet is heated to a temperature range that is equal to or higher than the cooling stop temperature and higher than 250°C but not higher than 600°C, and then held at that temperature range for 10 seconds or more.

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

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

10. 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 600°C or lower.

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