High-strength steel sheet and method for manufacturing same

Abstract

Provided are a high-strength steel sheet and a method for manufacturing same, the high-strength steel sheet having a tensile strength of 980MPa or more and excellent hydrogen embrittlement resistance of a bent part, wherein: a steel structure at a 1 / 4 plate thickness position and in an outermost layer contains 0%-20% by area ratio of (ferrite + bainite), 4%-20% by volume ratio of residual γ, 70% or more by area ratio of tempered martensite, and 0%-20% by area ratio of fresh martensite; and when the texture of the tempered martensite in a plate thickness cross section in the rolling direction is evaluated by a SEM-EBSD method, in an orientation distribution function represented in the Euler space of 0 ≤ φ1 ≤ 90°, φ2 = 45°, and 0 ≤ Φ ≤ 90° expressed by the Bunge's method, the maximum ODF of {111} grains represented by the maximum intensity value with φ1 = 5 × n°, φ2 = 45°, and Φ = 55° (n is an integer of 0-18) at the 1 / 4 plate thickness position is 2.0 or more, and the maximum ODF of {100} grains represented by the maximum intensity value with φ1 = 5 × n°, φ2 = 45°, and Φ = 0° (n is an integer of 0-18) in the plate thickness surface layer is 1.0 or more.

Description

High strength steel plate and method for manufacturing the same

[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.

[0002] In recent years, with growing interest in global environmental issues, CO ２ As emissions regulations become stricter, the automotive industry is seeking 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 this end, the use of high-strength steel materials that can maintain high body rigidity and crashworthiness even with thinner and lighter steel materials is becoming more prevalent. 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 fraction 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 applied to parts that require complex processing, and the applicable parts are 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 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 by plating the steel sheet, cooling it to 200°C or below, and then further tempering it 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 is responsible for ductility. This technique is shown, for example, in Patent Document 5. In this technique, in the final annealing step, 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 sheet is reheated and held to stabilize the austenite and temper the martensite.

[0006] Japanese Patent No. 5463685 International Publication No. 2009 / 054539 Japanese Patent Application Laid-Open No. 06-108152 Japanese Patent Application Laid-Open No. 2017-48412 Japanese Patent No. 6787525 Japanese Patent No. 6705561

[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 a deterioration in formability, such as bendability. Furthermore, cracks occur in the surface layer of the steel sheet during bending, and the microcracks that form can initiate hydrogen embrittlement and lead to fracture. The inventors have discovered that bending fractures become more pronounced, particularly under severe bending conditions, such as bending angles of less than 90°, or under severe environmental conditions for hydrogen penetration, such as a pH of less than 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 structures with different mechanical properties, such as ferrite and martensite. In particular, materials with a tensile strength exceeding 980 MPa have the problem of being subject to hydrogen embrittlement and leading to fracture.

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

[0009] 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 that there was a trade-off between the strength (TS in tensile tests) and ductility (total elongation) of the steel sheet, and no clear improvement in the strength-ductility balance was observed. Next, we thoroughly reviewed the annealing process and investigated improving properties by performing additional heat treatment at Ac1 to Ac1 + 130°C after cold rolling, as described in Patent Document 6. However, when applying the known method, we found that the additional heat treatment coarsened the steel's structure, reducing its strength and, in fact, tending to result in a poorer strength-ductility balance.

[0010] The present invention has been made in view of the above-mentioned circumstances. Specifically, an object of the present invention is 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 layer on the surface 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 in which the bending angle exceeds 30 degrees.

[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. Next, the material was heated to 1200°C and hot-rolled for a total of five passes at an outlet rolling temperature of 900°C, followed by air cooling to 400°C and coiling. The front and back surfaces of the 3.4 mm plate were then uniformly ground to a 3.0 mm thickness. The plate was then soaked at 500°C for 3600 minutes in a nitrogen atmosphere and cold-rolled to 1.4 mm. The cold-rolled plate was then subjected to additional heat treatment in a nitrogen atmosphere between 500°C and 850°C for 60 minutes, cooled, and pickled to remove any minor surface scale. The plate 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]

[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 an increase in elongation only being achieved in the temperature range of 600°C or higher and lower than 800°C (below point A1). Regarding strength, although it tended to be slightly lower under the 850°C condition, no significant differences were observed under other conditions.

[0017]

[0018] In addition, 50L x 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 R / t of 4 to prepare test specimens. Next, the opening angle of the bent portion of the test specimen was changed to constrain the surface layer of the test specimen to a state in which a residual stress of up to 1.2 x TS was applied, and the test specimen in this constrained state was placed in hydrochloric acid at a liquid temperature of 25°C and a pH of 0.8 for 24 hours. The presence or absence of cracks in the center of the bent portion was then determined by visual observation. As a result, cracks were observed in all steel plates used in this experiment 1.

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

[0020] <Experiment 2> A steel material having the chemical composition shown 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. Next, the steel was heated to 1250°C and hot-rolled for a total of six passes at an outlet 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 pre-cold rolling heat treatment, the steel sheet was soaked at each temperature for 3600 minutes in a nitrogen atmosphere and allowed to cool in the air. The steel sheet 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 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 obtained steel sheet was pickled, and then a JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) was cut out in a 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. Under the condition of no additional heat treatment before cold rolling (symbol 1), the elongation of the steel sheet was 9.2%, whereas a maximum elongation of 10.1% was obtained by additional heat treatment before cold rolling. It was also revealed that the heat treatment temperature had a strong effect, and the increase in elongation was only obtained in the temperature range of 400°C or higher and lower than 800°C. When annealed at 800°C, the material hardened and could not be rolled, making it impossible to evaluate the tensile properties. This is thought to be because the austenite formed during annealing at 800°C transformed into a hard structure during cooling. Regarding strength, a tendency for a decrease of about 10-20 MPa was observed at temperatures above 400°C, but the difference was not significant.

[0022]

[0023] In addition, 50L x 100C samples were punched out from the resulting thin steel plates with a clearance of 15% and bent at 75°C into a V-shape with a bending R / t of 5 to prepare test specimens. The opening angle of the bent portion of the test specimen was changed to constrain the surface layer of the test specimen to a residual stress of up to 1.2 x 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 plates 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, performing ultra-strong rolling in cold rolling, and performing slow heating or two-stage annealing (heat treatment) in 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 achieve excellent hydrogen embrittlement resistance and an improved balance of strength and ductility while maintaining a tensile strength of 980 MPa. That is, the gist of the present disclosure is as follows.[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: 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: 0.00 The steel has a chemical composition comprising at least two of the following in a total amount: 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 in a total amount 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 of ferrite and bainite in total. % or more and 20% or less, retained austenite at a volume fraction of 4% or more and 20% or less, tempered martensite at an area fraction of 70% or more, and fresh martensite at an area fraction of 0% or more and 20% or less, and the steel structure at the outermost layer position satisfies the following: a total of ferrite and bainite at an area fraction of 0% or more and 20% or less, retained austenite at a volume fraction of 4% or more and 20% or less, tempered martensite at an area fraction of 70% or more, and fresh martensite at an area fraction of 0% or more and 20% or less, and the rolling direction sheet thickness cross section was evaluated by SEM-EBSD method, and the texture of the tempered martensite was evaluated by SEM-EBSD method in an orientation distribution function (ODF) shown in an Euler space of 0°≦φ1≦90°, φ2=45°, and 0°≦Φ≦90° expressed by the Bunge method, and 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 value of strength at (n = 0, 1, 2, ..., 18), is 2.0 or more, and the maximum ODF of {100} grains, represented 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 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 sheet 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] The high-strength steel sheet according to any one of [1] to [3], wherein an electrolytic galvanized layer is formed on the surface. [5] The high-strength steel sheet according to any one of [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 [1] to [3], wherein an alloyed hot-dip galvanized layer is 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, cooled to 720°C or less within 2 seconds at a cooling rate of 70°C or more per second, and coiled in a temperature range of Ms+100°C or less, and then softening annealed in a temperature range of 400°C or more and Ac1 point or less for 48 hours or more, with or without holding the temperature, before cold rolling, and then in the first pass or the second pass The cold-rolled sheet obtained through a process of cold-rolling to a one-pass reduction of 30% or more and a total reduction of 50% or more is 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 up to at least 500°C, and then heated to a temperature range of the cooling stop temperature or more and more 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. [9] The method for producing a high-strength steel sheet according to [7], further comprising hot-dip galvanizing.

[10] The method for producing a high-strength steel sheet according to [7], further comprising hot-dip galvanizing, followed by alloying treatment of galvanizing in a temperature range of 470°C or higher and 600°C or lower.

[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.

[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] C is one of the important basic components of steel, and in the present invention, it is an 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] Si is one of the important basic components of steel. In particular, in the present invention, Si suppresses 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 0.40%, 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 strength. 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 an important element that affects the fraction of martensite. If the Mn content is less than 1.8%, the fraction of martensite 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 fraction of tempered martensite 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, thereby reducing the local ductility of the steel sheet and reducing elongation. It also reduces bendability. Therefore, the P content must be 0.100% or less. While there is no particular lower limit for the P content, it is preferable that the P content be 0.001% or more because P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is 0.100% or less. Preferably, it is 0.070% or less.

[0032] [S: 0.0200% or less] S exists as sulfides and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the S content needs to be 0.0200% or less. There is no particular lower limit for the S content, but due to constraints on production technology, it is preferably 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 a large amount of ferrite to be contained in the microstructure, preventing high strength achieved by utilizing the martensite structure. 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 a nitride and reduces the local ductility of the steel sheet, thereby reducing elongation. It also reduces bendability. Therefore, the N content needs to be 0.0200% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is 0.0200% or less, 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. While there is no particular lower limit for the O content, 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] [0.01% or more of at least one of Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less] Ti, Nb, and V combine with carbon and nitrogen to refine the crystal structure, thereby increasing the strength of the material. Therefore, at least one of them must be contained 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, their 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. Although 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 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] Cu and Sn, if present in an amount of 0.5% or less, do not increase coarse precipitates or inclusions, and do not reduce the local ductility of the steel sheet, thereby preventing a decrease in elongation. Furthermore, bendability is not reduced. Therefore, the Cu and Sn contents are preferably set to 0.5% or less. While there is no particular lower limit for the Cu and Sn contents, because these elements improve hardenability, the contents are preferably set to 0.005% or more. More preferably, they are set to 0.01% or more. Furthermore, they are 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 is not reduced. Furthermore, bendability is not reduced. 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 suppressing surface decarburization. More preferably, it is 0.004% or more.

[0041] [0.01% to 1.0% in total of at least two of Cu, Sn, and Sb] As described below, Cu, Sn, and Sb are features of the present invention, and at least two of these elements are essential and must be added in a total amount 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 reduction in strength of the base material due to structural non-uniformity in the thickness direction and softening of the surface layer through the thickness. More preferably, at least two of Cu, Sn, and Sb are added in a total amount 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 content of 0.100% or less of these impurities is permitted.

[0043] In addition to the above-described chemical composition, the high-strength steel plate 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, if 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 also not reduced. Therefore, the Ta and W contents are preferably each 0.10% or less. While there are no particular lower limits for the Ta and W contents, these elements 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 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.

[0045] If Cr and Mo are each 1.00% 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 Cr and Mo contents are preferably 1.00% or less. While there are no particular lower limits for the Cr and Mo contents, since 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 are each 1.00% or less. More preferably, the Cr and Mo contents are 0.01% or more. Furthermore, the Cr and Mo contents are 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 is not reduced. Furthermore, bendability is not reduced. Therefore, the Co content is preferably 0.010% or less. While 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 preferred Co content is 0.001% or more. An even more preferred 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, when 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, so elongation does not decrease. Furthermore, bendability does not decrease. 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 are 0.0005% or more. Even more preferably, the Ca, Mg, and REM contents are 0.0050% or less. Note that REM (rare earth elements) is a collective term for Sc, Y, and 15 elements ranging from 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 particular 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, when Hf is contained, its content is 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 is not reduced. Furthermore, bendability is not reduced. 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, when B is contained, its content is 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 inexpensively available. A common feature of these elements is their ability to delay 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 austenite transformation from an elongated, unrecrystallized 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 preferred lower limit, the effect of the present invention is not impaired, and therefore, these elements are included as unavoidable impurities.

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

[0056] [The total of ferrite and bainite is 0% or more and 20% or less. Electron microscope observation field area ratio] Although ferrite is a soft structure and is effective in improving workability, it has a strong 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 of ferrite and bainite is set to 20% or less. More preferably, the total of ferrite and bainite is set to 15% or less. Even more preferably, the total of ferrite and bainite is less than 2% and the total of ferrite and bainite is 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) parallel to the rolling direction of the steel sheet is polished and then corroded 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 a position corresponding to a 1 / 4 position in the thickness 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 10 visual fields are calculated using Image-Pro from Media Cybernetics. The average of the area ratios for these 10 visual fields is defined as the "ferrite area ratio." In the structural images, ferrite appears as a gray structure (base structure). Retained austenite and martensite appear as white structures.

[0058] [Retained austenite: 4% or more and 20% or less. Volume fraction by X-ray diffraction] Retained austenite contributes to the ductility of the steel sheet 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 the sheet thickness, and then chemically polished with oxalic acid to obtain an observation surface. The 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 direction. Rγ(200) = 33.0, Rγ(220) = 17.6, Rγ(311) = 25.1, Rα(200) = 13.2, Rα(211) = 27.3, Rα(220) = 12.5.

[0060]

[0061] [Tempered martensite is 70% or more. Electron microscope observation field 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 collision safety of automotive components. To achieve these effects, the area ratio of tempered martensite must be 70% or more. A more preferred area ratio of tempered martensite is 75% or more, and even more preferably 80% or more. While there is no particular upper limit for the area ratio of tempered martensite, a tempered martensite area ratio of 94% or less is preferred to ensure the area ratio of retained austenite. The area ratio 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 sheets containing fresh martensite have high strength but poor processing characteristics, so the area ratio 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 ratio 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 ratio 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 not more than 8% 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 ratios of tempered martensite, fresh martensite, pearlite, and bainite were measured as follows. A sample of annealed steel sheet was cut out, and the cross section of the sheet thickness parallel to the rolling direction was polished and then corroded 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. For the purpose of confirming carbides in detail, observation at a higher magnification may be performed. Note that, in the case of a galvanized steel sheet, the steel sheet surface refers to the interface between the galvanized layer and the steel sheet. The area ratios of each structure were determined using the obtained image data using Image-Pro manufactured by Media Cybernetics, and the average area ratio of each structure in the field of view was taken as the area ratio of each structure. In the image data, fresh martensite is 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. The gray or dark gray region containing aligned carbides is bainite. However, in actual observation, the above classification can be difficult. For example, even tempered martensite may be observed to have aligned carbide orientation. Therefore, gray or dark gray regions containing aligned carbides and in which the structure interfaces extend linearly are classified as bainite, and gray or dark gray regions containing other carbides are classified as tempered martensite. Furthermore, because 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 region is determined, and the volume fraction of retained austenite determined by the above method is considered to be the area fraction and 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 surface of the steel sheet, and similar to 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 at 1 / 4 sheet thickness above. 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 quarter-thickness position. Therefore, it is preferable that each of the above structures is within a range of plus or minus 5% of the value at the quarter-thickness position. Because finite-field observation involves a certain degree of variation, the structure will not be exactly equivalent to the value at the quarter-thickness position, but more preferably, it is within a range of plus or minus 3% of the value at the quarter-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 surface non-steady 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 more. The strength level is not limited to the 980 MPa class, and high strengths up to about 1.5 GPa class, such as 1180 MPa class, 1300 MPa class, and 1470 MPa class, are targeted. Furthermore, 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, and the texture of the tempered martensite was evaluated. 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, the maximum ODF of {111} grains, represented by the maximum intensity at φ1=5×n°, φ2=45°, Φ=55° (n=0, 1, 2, ..., 18), for 1 / 4 of the thickness was 2.0 or more, and the maximum ODF of {100} grains, represented by the maximum intensity at φ1=5×n°, φ2=45°, Φ=0° (n=0, 1, 2, ..., 18) for the thickness surface layer was 1.0 or more.] The texture of the bcc phase crystal grains is characterized by satisfying the above in the orientation distribution function (ODF) shown in the Euler space of 0°≦φ1≦90°, φ2=45°, and 0°≦Φ≦90° expressed by the Bunge method. It is known that the crystal orientation grains shown by φ1=5×n°, φ2=45°, and Φ=55° (referred to here as {111} grains) are oriented grains that increase local elongation. This is a crystal orientation used to ensure ductility in mild steel sheets, but its effectiveness has not been fully recognized in high-strength steel sheets because crystal orientation control technology has not been fully established.

[0069] The ODF was measured by 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. Here, the EBSD method was used to measure the ODF, but other known methods such as XRD may also be used. In the above-mentioned <Experiment 2>, 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 {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 found that hydrogen embrittlement resistance can be improved by increasing the number of grains with a crystal orientation indicated by φ1 = 5 × n°, φ2 = 45°, and Φ = 0° (referred to here as {100} grains) in the surface layer of the steel sheet. While the detailed mechanism is unclear, it is believed that the ND / / {100} grains indicated by the above Euler angles are less susceptible to iron elution during the corrosion reaction of hydrochloric acid immersion, and therefore also suppress hydrogen penetration into the steel. The above-mentioned high hydrogen embrittlement resistance is exhibited when the maximum ODF of the {100} grains, expressed as the maximum strength at φ1 = 5 × n°, φ2 = 45°, and Φ = 0° (n = 0, 1, 2, ..., 18), is 1.0 or more at the outermost surface. 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 and rolling method or continuous casting method. Alternatively, a thin steel slab having a thickness of 100 mm or less may be produced by a direct casting method. The steel slab is heated and held at a temperature of 1080°C or higher and 1300°C or lower, and then subjected to hot rolling. A hot-rolled sheet is produced by rough rolling and finish rolling, and then wound into a coil. Here, after finish rolling, the sheet is cooled to 720°C or lower within 2 seconds at a cooling rate of 70°C or higher per second. The thickness of the hot-rolled sheet is preferably 0.8 mm or higher and 4.0 mm or lower. The reasons for limiting the above hot rolling conditions are as follows.

[0074] [Heating temperature: 1080°C or higher and 1300°C 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 step, 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] [Finish rolling end temperature: 850°C or higher and 1000°C or lower] Hot rolling needs to be completed in the austenite single phase region in order to improve the strength and ductility balance of the material by homogenizing the structure within the steel sheet, so the finish rolling end temperature is set to 850°C or higher. Furthermore, if the finish rolling end temperature exceeds 1000°C, the hot-rolled structure becomes coarse, and accordingly, the structure after annealing also becomes coarse, resulting in a decrease in the strength of the steel sheet. A preferable lower limit of the finish rolling end temperature is 875°C or higher. A preferable upper limit of the finish rolling end temperature is 950°C or lower.

[0076] [Cooling after finish hot rolling: cooling to 720°C or less within 2 seconds at an average cooling rate of 70°C or more per second] If the steel sheet is held in a temperature range above 720°C for a long time, precipitates such as Ti coarsen, losing the effect of increasing strength. As a result, even if strength is increased by adjusting other manufacturing conditions, the desired ductility cannot be obtained. 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 started within 2 seconds after finish rolling to 720°C or less. More preferably, it is within 1 second. During cooling, the sheet temperature must be kept as low as possible to prevent coarsening of precipitates, so the average cooling rate to 720°C is set to 70°C or more per second. More preferably, it is set to 90°C or more. Cooling here refers to water cooling, oil cooling, or gas cooling that is performed for the purpose of lowering the steel sheet temperature, and the average cooling rate up to 720°C refers to the average cooling rate from the time cooling starts until the steel sheet temperature reaches 720°C.

[0077] [Coiling temperature: Ms + 100°C or less] After finish rolling, the steel sheet is cooled and then coiled at a coiling temperature of Ms + 100°C or less, completing the hot rolling process. If the coiling temperature exceeds Ms + 100°C, a large amount of scale and grain boundary oxidation will occur on the steel sheet, deteriorating pickling properties. In addition, 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 represented by the following formula: Ms = 561 - 474 [C] - 33 [Mn] - 17 [Ni] - 17 [Cr] - 21 [Mo] Here, [C], [Mn], [Ni], [Cr], and [Mo] represent the content (mass%) of each element, and if no element is contained, it will be set to zero. If the winding temperature is too low, an excessively hard structure is generated, 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] Subsequently, the obtained hot-rolled sheet is annealed before cold rolling to coarsen the carbides in the steel while suppressing surface decarburization. This step is an important process for sharpening the crystal orientation after final annealing, i.e., for obtaining the above-mentioned ODF. When coiling is performed under the coiling temperature conditions of the present invention, a large amount of hard structure is formed, and as such, the cold rolling load is high, so 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 Ac1 point or lower with / without temperature holding] If the annealing temperature exceeds the Ac1 point, an austenite phase is generated, and carbides that are detrimental to the formation of {111} grains in the final annealed sheet are formed during the cooling process. Therefore, the annealing temperature is set to Ac1 point or lower. On the other hand, if the annealing temperature is too low, the effect of coarsening carbides is reduced, so the lower limit is set to 400°C, and more preferably, the annealing temperature is set to 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 layer during annealing is generally promoted as the temperature increases. However, for the above-mentioned reasons, the annealing temperature must be set to Ac1 or less, and 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 predetermined amounts of Cu, Sb, and Sn. There is no particular upper limit on the annealing time, but it is preferably 300 hours or less from the standpoint of productivity and cost. In this case, the annealing temperature is within the range of 400° C. or higher and Ac1 point or lower, and the obtained effect is the same whether or not the annealing temperature is maintained at a constant temperature. As an example of not maintaining the temperature at a constant temperature, it may take 48 hours or more to raise the temperature from 400° C. 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 may utilize 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 low, the recrystallization driving force is low, so 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, more preferably 60% or more. Although there is no particular upper limit, it is preferably set to 95% or less from the viewpoint of the rolling load.

[0084] [Rolling with a one-pass rolling ratio of at least 30% or more 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 one-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. For the formation of {111} grains, a higher one-pass rolling ratio is preferable, and 30% or more is required. A more preferable one-pass rolling ratio is 35% or more. Although there is no particular upper limit, if it is excessively high, the steel sheet shape becomes inferior, so it is less than 50%, and here it is set to 49% or less. The number of rolling passes may be one or more.

[0085] [Two-stage annealing treatment] Next, in the annealing step, the obtained steel sheet is subjected to a heat treatment and, if necessary, a 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, so it is set to 5°C / s or higher. There is no particular upper limit, and there is no problem even if conditions exceeding 100°C / s are applied using an induction heating method or the like.

[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 the 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 250°C or higher but lower than 600°C and then held in the 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, with / without temperature holding, and annealing time: 60 seconds or longer] This first annealing stage plays an important role in the manufacturing method of the present invention and is a process for actively recrystallizing in a temperature range lower than 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. When the annealing temperature is lower than 600°C, the rolled structure is not recrystallized, and a coarse structure elongated in the rolling direction remains. If a coarse unrecrystallized structure remains, the proportion of high-angle grain boundaries where austenite grains preferentially form is reduced in the structure prior to austenite transformation, resulting in a decrease in the number of austenite grains formed. On the other hand, if the temperature is higher than the Ac1 point, austenite grains are formed during annealing, element distribution between ferrite and austenite grains progresses, the structure becomes non-uniform, and the ductility of the final annealed sheet is impaired. That is, in order to fully express recrystallization, the temperature is 600 ° C. or higher, more preferably 650 ° C. or higher. Note that 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 remains the same whether or not it is held at a constant temperature. As an example of not holding at a constant temperature, it may take 60 seconds or more to raise the temperature from 600 ° C. or higher to lower than the Ac1 point.

[0089] If the annealing time is too 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 predetermined amounts of Cu, Sb, and Sn is also exhibited during the main annealing. Conventionally, when a steel sheet not containing the predetermined amounts of Cu, Sb, and Sn is subjected to long-term annealing before cold rolling, a soft decarburized layer is formed in the surface layer, and a structure with relatively few carbides is formed. When such a structure is subsequently processed by cold rolling, most of the strain accumulates near the ferrite grain boundaries. As a result, in the subsequent first-stage annealing, excessive {111} grains are generated near the ferrite grain boundaries. On the other hand, few {100} grains are recrystallized near the coarse carbides. Therefore, as long as the predetermined amounts of Cu, Sb, and Sn are added, it is possible to ensure a certain amount of {100} grains in the surface layer during the main annealing process, even if annealing before cold rolling is performed.

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

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

[0093] [Annealing temperature: Ac1 point or higher and 950°C or lower] If the annealing temperature is lower than the Ac1 point, a large amount of ferrite phase remains in the structure after annealing, and sufficient strength cannot be obtained. In order to obtain high strength, it is better to have as little ferrite phase as possible, and for this purpose, it is more preferable that the annealing temperature be 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 also reducing the {111} grains, so the annealing temperature is more preferably 920°C or lower.

[0094] The Ac1 point and Ac3 point 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 (°C) = 723 - 10.7 x [Mn] - 16.9 x [Ni] + 29.1 x [Si] + 16.9 x [Cr] + 290 x [As] + 6.38 x [W] (A) Ac3 point (°C) = 910 - 203 x √[C] - 15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W] - 30 x [Mn] - 11 x [Cr] - 20 x [Cu] + 700 x [P] + 400 x [Al] + 120 x [As] + 400 x [Ti] (B) [Average cooling rate to at least 500°C: 10°C / s or more] In the cooling step after annealing, the average cooling rate to at least 500°C is preferably high, and is set to 10°C / s or higher. This is to suppress the amount of ferrite newly generated by ferrite transformation, and also to suppress the grain growth of existing ferrite, thereby avoiding excessive softening of the material. A more preferable rate is 15°C / s or higher. Meanwhile, no particular upper limit is specified. 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 near this temperature range, structures that are undesirable for improving the strength-ductility balance, such as pearlite and bainite, are likely to form.

[0095] [Cooling Stop Temperature: 50°C or More and 350°C or Less] The cooling following the annealing is performed to a temperature of 50°C or more and 350°C or less. By cooling to 350°C or less, a portion of the austenite generated by annealing is transformed into martensite. If the temperature is below 50°C, all of the austenite necessary for improving ductility is transformed into martensite or the like and disappears. Furthermore, if the temperature is above 350°C, martensite transformation does not occur sufficiently, and the fraction of tempered martensite structure necessary for high strength decreases. 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 if the subsequent heat treatment conditions are adjusted. A preferred lower limit is 75°C or more. A preferred upper limit is 300°C or less. As the cooling means of the present invention, known methods may be used, such as gas cooling, oil cooling, mist cooling, and low-melting-point liquid metal cooling.

[0096] [Reheating: Heating to a temperature range equal to or higher than the cooling stop temperature and higher than 250°C but lower than 600°C, and then holding at that temperature range for 10 seconds or longer] After the cooling stop, reheating is performed to temper the martensite that has formed and to stabilize the retained austenite. After the cooling stop, reheating may be performed immediately, or may be performed after holding the temperature within a range in which carbides do not significantly precipitate. Here, the temperature range in which carbides do not significantly precipitate 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 equal to or lower than 600°C, it is necessary to hold the steel in this 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 equal to or lower than 400°C. Furthermore, if the holding time is less than 10 seconds, carbon diffusion required for austenite stabilization is not sufficiently achieved, and ductility is not sufficiently improved. 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 the same as those in the ordinary method.

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

[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. Thereafter, the slab was subjected to one or two heat treatments. The number of heat treatments was one if the first-stage heating temperature was a continuous temperature increase, and two if the first-stage heating temperature was not a continuous temperature increase. Some of the slabs were subjected to hot-dip galvanizing, galvannealing, and electrogalvanizing after the heat treatment.

[0104]

[0105]

[0106]

[0107] [Tensile test] JIS No. 5 test pieces (gauge length 50 mm, parallel portion width 25 mm) were cut from the obtained steel sheets in the direction perpendicular to the rolling direction, and tensile tests were performed in accordance with JIS Z2241. Test pieces with a strength of 980 MPa or more were considered to have 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 in elongation value before and after the tensile test. Here, a value of 10% or more was considered to be a high elongation value.

[0108] [Observation of Structure] Furthermore, the test piece was embedded in carbon resin so that the rolling direction and the sheet thickness direction were the observation surfaces, and the structure was evaluated.

[0109] [Hydrogen embrittlement property test of bent portion] A 50L x 100C sample was punched from the 1 / 4 position of the plate width with a clearance of 15% and bent into a V-shape at 50°C so that bending R / t = 5. The bent portion of the test piece was then restrained by changing the opening angle, applying a residual stress of up to 1.5 x TS to the surface layer of the test piece. The restrained test piece 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 bent portion was visually observed. Test pieces with no visible cracks were judged to have excellent hydrogen embrittlement resistance in the bent portion.

Claims

1. Containing, by 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: 0.0100% or less, and containing at least two of the following in a total amount of 0.01% or more and 1.0% 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, Ti: 0.200% or less, Nb: 0.200% or less, and V: 0.200% or less, and the balance is Fe and unavoidable impurities, and the steel structure at a 1 / 4 position in the plate thickness satisfies the following: a total area fraction of ferrite and bainite of 0% to 20%, a volume fraction of retained austenite of 4% to 20%, an area fraction of tempered martensite of 70% or more, and an area fraction of fresh martensite of 0% to 20%, and the steel structure at a surface layer position satisfies the following: a total area fraction of ferrite and bainite of 0% to 20%, a volume fraction of retained austenite of 4% to 20%, an area fraction of tempered martensite of 70% or more, and an area fraction of fresh martensite of 0% to 20%, A high-strength steel sheet having a tensile strength of 980 MPa or more, wherein the texture of the tempered martensite is evaluated by SEM-EBSD on a rolling direction sheet thickness cross section, and the orientation distribution function (ODF) is shown in an Euler space of 0°≦φ1≦90°, φ2=45°, 0°≦Φ≦90° expressed by the Bunge method, and wherein the maximum ODF of {111} grains, represented by the maximum value of strength at φ1=5×n°, φ2=45°, Φ=55° (n=0, 1, 2, ..., 18), at a quarter-thickness position is 2.0 or more, and the maximum ODF of {100} grains, represented by the maximum value of strength at φ1=5×n°, φ2=45°, Φ=0° (n=0, 1, 2, ..., 18), at a surface layer position is 1.0 or more.

2. A high-strength steel plate according to claim 1, wherein the chemical composition further contains, in mass%, at least one element selected from the following: 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.

3. A high-strength steel plate according to claim 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 claims 1 to 3, having an electrogalvanized layer formed on the surface.

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

6. A high-strength steel sheet according to any one of claims 1 to 3, which has a galvannealed layer formed on the surface.

7. A method for producing a high-strength steel sheet according to any one of claims 1 to 3, comprising the steps of: heating a steel slab having the above-mentioned chemical composition to a temperature range of 1080°C or higher and 1300°C or lower, finish-rolling it at a finish-rolling end temperature in the temperature range of 850°C or higher and 1000°C or lower, cooling it to 720°C or lower within 2 seconds at an average cooling rate of 70°C or higher per second, coiling it in a temperature range of Ms+100°C or lower, and then softening annealing it in a temperature range of 400°C or higher and Ac1 point or lower for 48 hours or more before cold rolling, followed by rolling it to a first or second pass with a one-pass rolling reduction of 30% or higher and cold rolling it to a total reduction of 50% or higher, and annealing the cold-rolled sheet obtained by this step in a temperature range of 600°C or higher and lower than Ac1 point for 60 seconds or more, with or without holding the temperature, A method for producing a high-strength steel plate, comprising heating the steel plate to a temperature range of not less than the Ac1 point and not more than 950°C, further cooling the steel plate to a cooling stop temperature range of not less than 50°C and not more than 350°C at an average cooling rate of not less than 10°C / s to at least 500°C, and then heating the steel plate to a temperature range of not less than the cooling stop temperature and more than 250°C but not more than 600°C, and then holding the steel plate in the temperature range for not less than 10 seconds.

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.

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 a galvanizing alloying treatment in the temperature range of 470°C to 600°C.

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