HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH MEMBER, AND METHODS FOR MANUFACTURING THEM

Abstract

The subjects of the present invention are to provide a high-strength steel sheet and a high-strength member having a high performance ratio and excellent material uniformity, and methods for manufacturing the same.The high-strength steel sheet of the present invention has a specific chemical composition, and contains, in terms of area fraction relative to a complete steel microstructure, 30% or more and 100% or less of ferrite, 0% or more and 70% or less of martensite, and less than 20% in total of pearlite, bainite, and retained austenite, a total Nb and Ti content contained in a precipitate with a particle size less than 20 nm is equal to or greater than 25 ppm by mass and equal to or less than 220 ppm by mass, and the difference between the maximum and minimum value of the total Nb and Ti content contained in the precipitate with a particle size less than 20 nm, in the longitudinal direction of the steel sheet, is less than 20 ppm by mass.

Description

HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH MEMBER, AND METHODS FOR MANUFACTURING THEM rn? Lnn / zznz / E / YiAi Technical field The present invention relates to a high-strength steel sheet and a high-strength member used for automotive parts and so forth, and methods for manufacturing the same. More specifically, the present invention relates to a high-strength steel sheet and a high-strength member having a high performance ratio and excellent material uniformity, and methods for manufacturing the same. Background In recent years, efforts have focused on reducing emissions, such as CO2, from the perspective of global environmental protection. The automotive industry has taken steps to reduce emissions by decreasing vehicle body weight and thus improving fuel efficiency. One technique for reducing body weight is thinning the steel sheets used in automobiles by increasing their strength. However, it is known that increasing the strength of steel sheets degrades their ductility, highlighting the need for a steel sheet that offers a good balance between high strength and ductility.Furthermore, steel sheets whose mechanical properties vary in the longitudinal direction (rolling direction) will degrade the reproducibility of the shape fix, thus reducing the reproducibility of the springback and making it difficult to maintain the shape of the parts. Therefore, a steel sheet is needed that does not vary its mechanical properties in the longitudinal direction and that is notable for its material uniformity. In response to this need, for example, Patent Literature 1 proposes a high-strength steel sheet containing, by mass percent, C: 0.05 to 0.3%, Si: 0.01 to 3%, and Mn: 0.5 to 3%, with a ferrite volume fraction of 10 to 50%, a martensite volume fraction of 50 to 90%, a ferrite and martensite volume fraction of 97% or greater, and the steel sheet having a small strength variation in the longitudinal direction of the steel sheet, as a result of controlling a winding temperature difference between a front and middle part of the steel sheet at 0°C or more and 50°C or less, and controlling a winding temperature difference between a back and middle part of the steel sheet at 50°C or more and 200°C or less. Patent Literature 2 proposes a hot-rolled steel sheet having a chemical composition containing, in mass %, C: 0.03 to 0.2%, Mn: 0.6 to 2.0%, and Al: 0.02 to 0.15%, with a ferrite volume fraction of 90% or greater, and the steel sheet having a small strength variation in the longitudinal direction of the steel sheet, as a result of cooling control after winding. List of appointments Patent Literature Patent Literature 1: JP 2018-16873 A Patent Literature 2: JP 2004-197119 A Brief description of the invention Technical problem In accordance with the technique disclosed in Patent Literature 1, excellent material uniformity is achieved through a ferrite-martensite microstructure and by controlling the winding temperature to reduce microstructural differences along the longitudinal direction of the steel sheet. However, the precipitate variation along the longitudinal direction of the steel sheet was not controlled, leaving the issue of yield strength variation unresolved. In accordance with the technique disclosed in Patent Literature 2, the resistance variation in the longitudinal direction of the steel sheet is reduced by using ferrite as the dominant phase and by controlling the composition and cooling before winding. However, precipitation elements such as Nb or Ti are not added, so the aforementioned reduction in resistance variation is conceptually different from the present invention, which is based on controlling the precipitate variation in the longitudinal direction of the steel sheet to which the precipitation elements are added. Therefore, it is an object of the present invention to provide a high-strength steel sheet and a high-strength member, as well as methods for manufacturing the same, all of which are intended to achieve a high yield ratio and excellent material uniformity by suitably adjusting the chemical composition in the presence of added precipitating elements, such as Nb and Ti, which can affect precipitation hardening to achieve a high yield ratio, creating a ferrite-martensite microstructure, controlling the total Nb and Ti content contained in a precipitate having a particle size in the longitudinal direction of the steel sheet of less than 20 nm (also referred to as microprecipitate hereafter), and controlling the variation of the amount of microprecipitate in the longitudinal direction of the steel sheet. Solution to the problem The present inventors carried out extensive studies in order to solve the aforementioned problem. Consequently, the present inventors discovered that, for greater strength and a higher coefficient of performance, it is necessary to control the total Nb and Ti content in the precipitate having a particle size less than 20 nm at 25 ppm by mass or more and 220 ppm by mass or less of the steel sheet. Furthermore, for less variation in the mechanical properties along the longitudinal direction of the steel sheet, it is necessary to control the difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate having a particle size less than 20 nm, along the longitudinal direction of the steel sheet, at less than 20 ppm by mass. As described above, the present inventors discovered, after our meticulous investigations aimed at solving the aforementioned problems, that a steel sheet having a specific chemical composition and a steel microstructure composed mainly of ferrite and martensite is obtainable as a high-strength steel sheet with a high coefficient of performance and excellent material uniformity by controlling the total Nb and Ti content in the microprecipitate and by controlling the variation of the total Nb and Ti content in the microprecipitate along the longitudinal direction of the steel sheet (hereinafter referred to simply as the variation of the amount of microprecipitate). The abstract of the present invention is as follows. [1] High-strength steel sheet having a chemical composition in % by mass containing: C: 0.06% or more and 0.14% or less, If: 0.1% or more and 1.5% or less, Mn: 1.4% or more and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, To: 0.01% or more and 0.20% or less, N: 0.10% or less, Nb: 0.015% or more and 0.060% or less, and Ti: 0.001% or more and 0.030% or less, S, N, and Ti contents satisfying formula (1) below, an equilibrium being Fe and an unavoidable impurity, including, in terms of area fraction relative to a complete steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total retained pearlite, bainite, and austenite, a total Nb and Ti content contained in a precipitate having a particle size less than 20 nm that is equal to or greater than 25 ppm by mass and equal to or less than 220 ppm by mass, and the difference between a maximum and a minimum value of the total Nb and Ti content contained in the precipitate having a particle size less than 20 nm, in a longitudinal direction of the steel sheet, being less than 20 ppm by mass, Formula (1): [ %Ti] - (48 / 14)[ %N] - (48 / 32)( %S] < 0, in Formula (1), [ %Ti] represents the content (in % by mass) of the component element Ti, [ %N] represents the content (in % by mass) of the component element N, and [ %S] represents the content (in % by mass) of the component element S. [2] The high-strength steel sheet conforming to [1], wherein the chemical composition further contains, in % by mass, one of, or two or more of Cr: 0.01% or more and 0.15% or less, rnz Lnn / zznz / E / YiAi Mo: equal to or greater than 0.01% and less than 0.10%, and V: 0.001% or more and 0.065% or less. [3] The high-strength steel sheet conforming to [1] or [2], wherein the chemical composition further contains, in % by mass, B: 0.0001% or more and less than 0.002%. [4] High-strength steel sheet conforming to any one of [1] to [3], wherein the chemical composition further contains, by mass %, one or two of Cu: 0.001% or more and 0.2% or less, and Ni: 0.001% or more and 0.1% or less. [5] The high-strength steel sheet conforming to any one of [1] to [4], having a laminated layer on one surface of the steel sheet. [6] A high-strength member comprising high-strength steel sheet conforming to any of points [1] to [5], subjected to at least one forming or welding process. [7] A method for manufacturing a high-strength steel sheet, comprising: a hot rolling process in which a steel plate having the chemical composition conforming to any one of [1] to [4] is heated to a heating temperature T (°C) satisfying the following formula (2) for 10 hours or more, then cooled from the heating temperature to a rolling start temperature at an average cooling rate of 2 °C / s or faster, then finished rolling to a finished delivery temperature of 850 °C or more, then cooled from the finished delivery temperature to a temperature range of 500 °C or more and 650 °C or less at an average cooling rate of 10 °C / s.or faster, and then rolled in the temperature range; and an annealing process in which the hot-rolled steel sheet obtained in the hot-rolling process is heated to an annealing temperature that is the transformation temperature AC1 or higher and (transformation temperature AC3 + 20 °C) or lower, is held at the annealing temperature for a holding time t (second) satisfying the following formula (3), and then cooled:. Formula (2): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.75 x (2.4 - 6700 / T) In formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% mass) of component element Nb, [%C] represents the content (% mass) of component element C, and [%N] represents the content (% mass) of component element N. Formula (3):1500 < (AT + 273) x logt < 3000 In formula (3), AT represents the annealing temperature (°C), and t represents the holding time (seconds) at the annealing temperature. [8] A method for manufacturing a high-strength steel sheet, comprising: a hot rolling process in which a steel plate having the chemical composition conforming to any one of [1] to [4] is heated to a heating temperature T (°C) satisfying the following formula (2) for 10 hours or more, then cooled from the heating temperature to a rolling start temperature at an average cooling rate of 2 °C / s or faster, then finished rolling to a finished delivery temperature of 850 °C or more, then cooled from the finished delivery temperature to a temperature range of 500 °C or more and 650 °C or less at an average cooling rate of 10 °C / s or faster, and then coiled within the temperature range; a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold rolled; and an annealing process in which the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature that is the transformation temperature AC1 or higher and (transformation temperature AC3 + 20 °C) or lower, held at the annealing temperature for a holding time t (second) that satisfies the following formula (3), and then cooled: Formula (2): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.75 χ (2.4 - 6700 / T) In formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% mass) of component element Nb, [%C] represents the content (% mass) of component element C, and [%N] represents the content (% mass) of component element N. Formula (3):1500 < (AT + 273) x logt < 3000 In formula (3), AT represents the annealing temperature (°C), and t represents the holding time (seconds) at the annealing temperature. [9] The method for manufacturing a high-strength steel sheet in accordance with [7] or [8], which further includes a rolling process to provide a coating, following the annealing process.

[10] A method for manufacturing a high-strength member, which includes subjecting the high-strength steel sheet manufactured by the method for manufacturing a high-strength steel sheet in accordance with any one of [7] to [9], at least to forming or welding. Advantageous effects of the invention The present invention controls the microstructure of the steel and the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet by adjusting the chemical composition and manufacturing method. The high-strength steel sheet of the present invention therefore has a high coefficient of performance and excellent material uniformity. The high-strength steel sheet of the present invention, when applied, for example, to a structural member of an automobile, can give the automotive steel sheet both high strength and material uniformity. That is, the present invention can maintain the parts in good shape and can improve the performance of the automobile body. Description of the modalities The embodiments of the present invention are described below. The present invention is not limited to the embodiments described below. First, the chemical composition of the high-strength steel sheet (which may occasionally be referred to as the steel sheet of the present invention) will be explained. In the description of the chemical composition of the steel sheet that follows, “%” is used as the unit of content for each component “% by mass”. Note that high strength in the context of the present invention means a tensile strength of 590 MPa or higher. It should also be noted that the steel sheet of the present invention is essentially a steel sheet obtained, at least, by heating a steel plate in a heating furnace, hot rolling each plate, and then coiling it. The steel sheet of the present invention has a high uniformity of material in the longitudinal direction (rolling direction) of the steel sheet. That is, the steel sheet stands out in terms of material uniformity with respect to each individual steel sheet (coil). C: 0.06% or more and 0.14% or less Carbon (C) is an element that improves hardenability and is necessary to obtain a predetermined area fraction of martensite and microprecipitate. C is also necessary for improving the strength of the martensite to achieve a tensile strength (TS) greater than 590 MPa. A C content below 0.06% will not allow the predetermined strength to be achieved. Therefore, the C content is set at 0.06% or higher. The C content is preferably 0.07% or higher. On the other hand, a C content above 0.14% will increase the area fraction of martensite, leading to excessive strength. Furthermore, the amount of carbide production increases, which impairs the control of the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet and degrades the uniformity of the material. Therefore, the C content is set at 0.14% or lower. The C content is preferably 0.13% or lower. Yes: 0.1% or more and 1.5% or less Silicon (Si) is a strengthening element that strengthens the solid solution. To achieve this effect, the Si content is set at 0.1% or more. The Si content is preferably 0.2% or more, and more preferably 0.3% or more. Meanwhile, Si has a suppressive effect on cementite formation, so an excessive Si content will suppress cementite production, and unprecipitated carbon will form carbide with niobium (Nb) or titanium (Ti) and become rough, thus degrading the uniformity of the material. Therefore, the Si content is set at 1.5% or less. The Si content is preferably 1.4% or less. rnz ίηη / ζζηζ / Ε / γίΛΐ Μη: 1.4% or more and 2.2% or less Manganese (Mn) is included to improve the hardenability of the steel and to achieve a predetermined area fraction of martensite. A manganese content below 1.4% makes it difficult to obtain a predetermined amount of microprecipitate, as pearlite or bainite forms during quenching. Therefore, the manganese content is set at 1.4% or higher. The preferred manganese content is 1.5% or higher. On the other hand, an excessive manganese content will increase the martensite area fraction, resulting in excessive strength. Furthermore, the formation of MnS reduces the total amount of nitrogen (N) and sulfur (S) to less than the amount of titanium (Ti). This fails to suppress the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet and degrades the uniformity of the material. Therefore, the manganese content is set at 2.2% or lower. The preferred manganese content is 2.1% or lower. P: 0.05% or less Phosphorus (P) is an element that can strengthen steel, but excessive amounts will cause segregation at the grain boundaries, degrading workability. Therefore, P content is controlled to 0.05% or less to achieve the minimum required level of workability for automotive applications. The P content is preferably 0.03% or less, and more preferably 0.01% or less. While there is no specific lower limit for P content, a currently industrially viable lower limit is approximately 0.003%. S: 0.0050% or less Sulfur (S) degrades workability by forming MnS, TiS, Ti(C,S), etc. Therefore, the S content must be controlled to 0.0050% or less to achieve the minimum required level of workability when applied to automobiles. The S content is preferably 0.0020% or less, more preferably 0.0010% or less, and even more preferably 0.0005% or less. Although the lower limit for S content is not specifically defined, a currently industrially viable lower limit is approximately 0.0002%. To: 0.01% or more and 0.20% or less Aluminum is added to promote complete deoxidation and reduce coarse inclusions in the steel. This effect occurs with an aluminum content of 0.01% or higher. The preferred aluminum content is 0.02% or higher. However, with an aluminum content exceeding 0.20%, the carbide produced during winding after hot rolling will have less opportunity to dissolve during the annealing process, resulting in coarse inclusions or carbide, and a degraded yield coefficient. Therefore, the aluminum content is set at 0.20% or lower. The preferred aluminum content is 0.17% or lower, and more preferably 0.15% or lower. N: 0.10% or less Nitrogen (N) is an element that forms coarse inclusions in steel, based on nitrides or carbonitrides, such as TiN, (Nb, Ti)(C, N), or AlN. With a N content exceeding 0.10%, the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet cannot be suppressed, thus degrading the material's uniformity. Therefore, the N content should be controlled to 0.10% or less. The N content is preferably 0.07% or less, and more preferably 0.05% or less. Although the lower limit for N content is not specifically defined, a currently industrially viable lower limit is approximately 0.0006%. Nb: 0.015% or more and 0.060% or less Nitrogen (Nb) contributes to precipitation hardening through the production of microprecipitates and increases the yield strength. To achieve this effect, the Nb content must be 0.015% or higher. Ideally, it should be 0.020% or higher, and even more preferably 0.025% or higher. However, a high Nb content increases the variation in the amount of microprecipitate along the longitudinal axis of the steel sheet, thus degrading the material's uniformity. Therefore, the Nb content is set at 0.060% or lower. The Nb content is preferably 0.055% or less, and more preferably 0.050% or less. Ti: 0.001% or more and 0.030% or less Titanium (Ti) contributes to precipitation hardening through the production of microprecipitates and to an increased yield strength. To achieve this effect, the Ti content must be 0.001% or more. Preferably, the Ti content is 0.002% or more, and more preferably 0.003% or more. However, a high Ti content increases the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet, thus degrading the material's uniformity. Therefore, the Ti content is set at 0.030% or less. Preferably, the Ti content is 0.020% or less, more preferably 0.017% or less, and even more preferably 0.015% or less. The contents of S, N and Ti satisfy the following formula (1): Formula (1): [ %Ti] - (48 / 14)( %N] - (48 / 32)( %S] < 0, in Formula (1), [ %Ti] represents the content (in % by mass) of the component element Ti, [ %N] represents the content (in % by mass) of the component element N, and [ %S] represents the content (in % by mass) of the component element S. rnz Lnn / zznz / E / YiAi By controlling the amount of Ti so that it is no greater than the total amount of N and S in atomic proportion, the production of Ti-containing carbide that might occur during winding can be suppressed, thereby suppressing the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet. To achieve this effect, [%Ti] - (48 / 14)(%N) - (48 / 32)(%S] is 0 (0.0000) or less, preferably less than 0 (0.0000), and more preferably -0.001 or less. The lower limit of [%Ti] - (48 / 14)(%N) - (48 / 32)(%S), although not specifically limited, is preferably -0.01 or greater, in order to suppress inclusion production that might be attributed to excessive N and S content. The steel sheet of the present invention contains the aforementioned components, and the remainder, in addition to the aforementioned components, has a chemical composition containing Fe (iron) and an unavoidable impurity. However, the steel sheet of the present invention preferably contains the aforementioned components, and the remainder preferably has a chemical composition consisting of Fe and an unavoidable impurity. The steel sheet of the present invention may also contain the components listed below as freely selectable components. Note that any of the freely selectable components listed below, if its content is less than the lower limit value, is understood to be contained as an unavoidable impurity. Any one of, or two or more of, Cr: 0.01% or more and 0.15% or less; Mo: 0.01% or more and less than 0.10%; and V: 0.001% or more and 0.065% or less They may contain Cr, Mo, and V to improve the hardenability of the steel. To achieve this effect, the Cr and Mo content is preferably 0.01% or more, and more preferably 0.02% or more. The V content is preferably 0.001% or more, and more preferably 0.002% or more. However, it should be noted that any of these elements, when present in excess, can degrade the uniformity of the material by producing carbides. Therefore, the Cr content is preferably 0.15% or less, and more preferably 0.12% or less. The Mo content is preferably less than 0.10%, and more preferably 0.08% or less. The V content is preferably 0.065% or less, and more preferably 0.05% or less. B: 0.0001% or more and less than 0.002% Boron (B) is an element that improves the hardenability of steel, and when present, it exhibits a martensite-producing effect with a predetermined area fraction, even if the manganese (Mn) content is low. To achieve this effect, the B content is preferably 0.0001% or higher. More preferably, it is 0.00015% or higher. Furthermore, B content above 0.002% will form nitride with nitrogen, and excessive titanium will readily form carbide during winding, thus degrading the material's uniformity. Therefore, the B content is preferably less than 0.002%. More preferably, it is less than 0.001%, and more preferably, 0.0008% or lower. rnz ίηη / ζζηζ / Ε / γίΛΐ One of, or two of, Cu: 0.001% or more and 0.2% or less, and Ni: 0.001% or more and 0.1% or less Copper (Cu) and nickel (Ni) demonstrate corrosion resistance enhancement effects in the automotive environment and suppress hydrogen penetration into the steel sheet by coating the steel sheet surface with corrosion products. To achieve the minimum required level of corrosion resistance for automotive use, the Cu and Ni contents are preferably 0.001% or more, and more preferably 0.002% or more. However, to prevent surface defects due to excessive Cu and Ni content, the Cu content is preferably 0.2% or less, and more preferably 0.15% or less. The Ni content is preferably 0.1% or less, and more preferably 0.07% or less. Note that the steel sheet of the present invention may contain Ta, W, Sn, Sb, Ca, Mg, Zr or REM as the other element, without impairing the effect of the present invention, with a content of each of these elements being acceptable at 0.1% or less. The microstructure of the steel sheet of the present invention will now be explained. The steel sheet of the present invention contains, in terms of area fraction relative to a complete steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total pearlite, bainite, and retained austenite. Furthermore, the total Nb and Ti content in a precipitate having a particle size of less than 20 nm is 25 ppm by mass or more and 220 ppm by mass or less, and the difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate having a particle size of less than 20 nm, in the longitudinal direction of the steel sheet, is less than 20 ppm by mass. The ferrite area fraction is 30% or more and 100% or less Carbon barely forms a solid solution with ferrite and migrates to be expelled from it, but upon cooling, the carbon forms carbide before being expelled. The ferrite area fraction is important in determining the location of precipitate formation, and when controlled at 30% or higher, it allows sufficient microprecipitate to form, resulting in a high yield ratio and improved strength through a synergistic effect of structural hardening due to martensite and precipitation hardening due to microprecipitate. Therefore, the ferrite area fraction is specified at 30% or higher. The ferrite area fraction is preferably 35% or higher, more preferably 40% or higher, and even more preferably 50% or higher.The upper limit of the ferrite area fraction is not specifically limited and can even be 100%, provided that a sufficient level of strength can be achieved by precipitation hardening with the aid of microprecipitates. However, since a large ferrite area fraction tends to increase the variation in the amount of microprecipitate in the longitudinal direction of the steel sheet, the ferrite area fraction is preferably 95% or less, and more preferably 90% or less. rnz Lnn / zznz / E / YiAi The area fraction of martensite is equal to or greater than 0% and less than 70%. With a martensite area fraction exceeding 70% relative to the entire microstructure, the strength will be excessive. Therefore, it is specified that the martensite area fraction, relative to the entire microstructure of the steel, be 70% or less. The martensite area fraction is preferably 65% ​​or less, and more preferably 60% or less. The lower limit of the martensite area fraction is not specifically limited and may even be 0%, provided that a sufficient level of strength can be achieved by precipitation hardening with the aid of microprecipitates. The martensite area fraction is preferably 5% or greater, and more preferably 10% or greater, from the standpoint of suppressing the variation in the amount of microprecipitate in the longitudinal direction of the steel sheet, as previously suggested. The equilibrium distinct from ferrite and martensite includes retained austenite, bainite, and pearlite, and is acceptable if their area fraction represents less than 20%. The area fraction of the equilibrium is preferably 10% or less, and more preferably 7% or less. The area fraction of the equilibrium may even be 0%. In the present invention, ferrite is a microstructure that results from the transformation of austenite at relatively high temperatures and is composed of crystal grains having a BCC lattice. Martensite refers to a hard microstructure that results from austenite at low temperatures (at or below the martensite transformation temperature).Bainite is a hard microstructure that forms from austenite at relatively low temperatures (at or above the martensite transformation temperature), in which fine carbide is dispersed within the ferrite in needle-like or plate-like forms. Pearlite refers to a microstructure that also forms from austenite and is composed of lamellar ferrite and cementite. Retained austenite results from the decrease in the martensite-austenite transformation temperature to or below room temperature due to the concentration of carbon or another element within the austenite. The area fraction values ​​of the individual structures in the steel microstructure employed here are those obtained by measurement in accordance with the methods described later in the Examples. The total Nb and Ti content in the precipitate with a particle size less than 20 nm is equal to or greater than 25 ppm by mass and equal to or less than 220 ppm by mass. The total Nb and Ti content in the precipitate, which has a particle size of less than 20 nm, can be easily measured using a method described later in the Examples. The total content (mass ppm) in the context of the present invention means the mass proportion of Nb and Ti contained in the precipitate, which has a particle size of less than 20 nm, relative to the steel sheet. Reinforcement with the aid of the microprecipitate is necessary to increase the strength and the coefficient of performance. To achieve this effect, the total Nb and Ti content in the precipitate, which has a particle size of less than 20 nm, is necessarily controlled to 25 ppm by mass or higher. The total content is preferably 27 ppm by mass or higher, and more preferably 30 ppm by mass or higher.Meanwhile, with a total content exceeding 220 ppm by mass, not only does the strength become excessive, but the amount of carbide production also increases. This leads to a failure to control the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet, degrading the material's uniformity. The total Nb and Ti content in the precipitate with a particle size of less than 20 nm is specified at 220 ppm by mass or less. The total content is preferably 215 ppm by mass or less, and more preferably 210 ppm by mass or less. The difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate with a particle size less than 20 nm, in the longitudinal direction of the steel sheet, is less than 20 ppm by mass. Since the amount of microprecipitate directly affects strength, excellent material uniformity can be achieved by suppressing the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet. To achieve this effect, the difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate with a particle size smaller than 20 nm, along the longitudinal direction of the steel sheet, is specified as less than 20 ppm by mass. The total content is preferably 18 ppm by mass or less, and more preferably 15 ppm by mass or less. The lower limit of the total content, although not specifically limited, may even be 0 ppm by mass.The “difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate having a particle size of less than 20 nm, in the longitudinal direction of the steel sheet, is specified to be less than 20 ppm by mass” in the context of the present invention means that the difference between the maximum and minimum values ​​of the total content is less than 20 ppm by mass, along the entire length of the longitudinal direction (rolling direction) of the steel sheet, with respect to each individual steel sheet (coil). The difference can be measured by a method described later in the Examples. The steel sheet of the present invention may have a laminated layer on its surface. The coating layer is typically an electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvanized layer, without any particular limitation. The properties of the high-strength steel sheet of the present invention will be explained below. The steel sheet of the present invention has a tensile strength of 590 MPa or greater, when measured by a method described later in the Examples. The tensile strength, although not specifically limited, is preferably less than 980 MPa, from the standpoint of easy balance with other properties. The steel sheet of the present invention has a high yield ratio. More specifically, the yield coefficient calculated from the tensile strength and yield strength measured by a method described later in the Examples is 0.70 or greater. The yield coefficient rnz Lnn / zznz / E / YiAi is preferably 0.72 or greater, and more preferably 0.75 or greater. The upper limit of the yield ratio, although not specifically limited, is preferably 0.9 or less, from the standpoint of easy balance with other properties. The steel sheet of the present invention is distinguished by the uniformity of the material. More specifically, the difference between the maximum and minimum values ​​of the yield ratio (AYR) in the longitudinal direction of the steel sheet, calculated from the tensile strength and yield strength measured by a method described later in the Examples, is 0.05 or less. The difference is preferably 0.03 or less, and more preferably 0.02 or less. A method for manufacturing the high-strength steel sheet of the present invention will now be explained. The manufacturing method for the high-strength steel sheet of the present invention includes a hot rolling process, an optional cold rolling process, and an annealing process. The temperature at which the steel plate (the raw steel material), sheet, or similar material is heated or cooled, as described below, is understood to be the surface temperature of the plate (the raw steel material), sheet, or similar material, unless otherwise specified. Hot rolling process A hot rolling process is a process in which a steel plate with the chemical composition described above is heated to a heating temperature T (°C) that satisfies the following formula (2) for 1 hour or more, then cooled from the heating temperature to the rolling start temperature at an average cooling rate of 2 °C / s or faster, then finished rolling to a finishing machine delivery temperature of 850 °C or more, then cooled from the finishing machine delivery temperature to a temperature range of 500 °C or more and 650 °C or less at an average cooling rate of 10 °C / s or faster, and then coiled within that temperature range. Formula (2): log{[ %Nb] χ ([ %C] + 12 / 14[ %N])} < 0.75 χ (2.4 - 6700 / T) In formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% mass) of component element Nb, [%C] represents the content (% mass) of component element C, and [%N] represents the content (% mass) of component element N. The above formula (2) is satisfied during slab heating. If the above formula is not satisfied, an excess of Nb-containing carbonitride is produced during slab heating, causing the amount of Ti to exceed the total amount of N and S, and degrading the uniformity of the material. Therefore, the heating temperature of the roughings is determined to satisfy the aforementioned formula (2). The heating temperature T (°C) of the steel plate preferably satisfies the following formula (2A), and more preferably the following formula (2B). Formula (2A): log{[ %Nb] χ ([ %C] + 12 / 14[ %N])} < 0.77 χ (2.4 - 6700 / T) Formula (2B): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.80 x (2.4 - 6700 / T) The upper limit of the slab heating temperature is not specifically restricted, but is preferably 1500 °C or less. The soaking time is specified as 1.0 hour or more. A soaking time of less than 1.0 hour is insufficient for the Nb- and Ti-containing carbonitrides to fully solubilize, so the Nb-containing carbonitride will remain excessively during slab heating. Therefore, the amount of Ti will be greater than the total amount of N and S, thus degrading the uniformity of the material. Therefore, the soaking time is specified as 1 hour or more, and preferably 1.5 hours or more. The upper limit of the soaking time, although not specifically restricted, is usually 3 hours or less. The heating rate when heating a cast steel slab to the slab heating temperature, although not specifically restricted, is preferably controlled at 5 to 15 °C / min. The average cooling rate from the slab heating temperature to the rolling start temperature is 2 °C / s or faster. If the average cooling rate from the plate heating temperature to the rolling start temperature is less than 2 °C / s, an excess of Nb-containing carbonitride will be produced, resulting in a higher amount of Ti than the total amount of N and S during winding, which will degrade material uniformity. Therefore, the average cooling rate from the plate heating temperature to the rolling start temperature is specified as 2 °C / s or higher. The average cooling rate is preferably 2.5 °C / s or faster, and more preferably 3 °C / s or faster. The upper limit of the average cooling rate, while not specifically restricted from the standpoint of improving material uniformity, is preferably specified as 1000 °C / s slower, from the standpoint of saving energy in the cooling system. The delivery temperature of the finisher is 850 °C or higher If the finisher's delivery temperature is below 850 °C, cooling requires a longer time, during which carbonitride containing Nb or Ti may form. Consequently, the amount of N is reduced, the production of Ti-containing precipitates that may occur during winding is not suppressed, the variation in the amount of microprecipitates along the longitudinal direction of the steel sheet increases, and the uniformity of the material is degraded. Therefore, the finisher's delivery temperature is specified as 850 °C or higher. The finisher's delivery temperature is preferably 860 °C or higher. Meanwhile, the upper limit of the finisher's delivery temperature, although not specifically limited, is preferably 950 °C or lower, and more preferably 920 °C or lower, to avoid difficulties in cooling to the winding temperature. The winding temperature is 500 °C or more and 650 °C or less If the winding temperature exceeds 650 °C, a large amount of precipitate is produced during winding. This makes it impossible to suppress the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet, thus degrading the material's uniformity. Therefore, the lower limit for the winding temperature is specified as 650 °C or lower. The winding temperature is preferably 640 °C or lower. Conversely, if the winding temperature is below 500 °C, the amount of precipitate produced is reduced, preventing precipitation hardening and decreasing the coefficient of performance. Therefore, the winding temperature is specified as 500 °C or higher. The winding temperature is preferably 520 °C or higher. The average cooling rate from the finisher delivery temperature to the winding temperature is 10 °C / s or faster. If the average cooling rate from the finishing machine delivery temperature to the winding temperature is slow, excess carbonitride containing Nb or Ti is produced before winding. This reduces the amount of N, fails to suppress the production of Ti-containing precipitate resulting from winding, increases the variation in the amount of microprecipitate along the longitudinal direction of the steel sheet, and degrades the uniformity of the material. Therefore, the average cooling rate from the finishing machine delivery temperature to the winding temperature is specified at 10 °C / s or more. The average cooling rate is preferably 20 °C / s or faster, and more preferably 30 °C / s or faster.The upper limit of the average cooling rate, although not specifically limited from the point of view of improving material uniformity, is preferably specified to be 1000 °C / s slower, from the point of view of saving energy in the cooling installation. Hot-rolled steel sheet can be pickled. Pickling conditions are not specifically limited. Cold rolling process The cold rolling process is a process for cold rolling hot-rolled steel sheet obtained from the hot rolling process. The reduction ratio of the cold rolling, although not specifically limited, is preferably specified as 20% or more, from the standpoint of improving surface flatness and increasing the uniformity of the microstructure. The upper limit of the reduction ratio, although not specifically limited, is preferably 95% or less, considering the cold rolling load. Note that the cold rolling process is not essential and is omittable if the microstructure and mechanical properties of the steel satisfy the present invention. Annealing process An annealing process is a process in which cold-rolled steel sheet or hot-rolled steel sheet is heated to an annealing temperature which is the transformation temperature AC1 or higher and (the transformation temperature AC3 + 20 °C) or lower, held at the annealing temperature for a holding time t (second) that satisfies the following formula (3), and then cooled. Formula (3):1500 < (AT + 273) χ logt < 3000 In formula (3), AT represents the annealing temperature (°C), and t represents the holding time (seconds) at the annealing temperature. The annealing temperature is C1 the AT transformation temperature or higher and (C3 AT transformation temperature + 20 °C) or lower If the annealing temperature is lower than the C1 transformation temperature A, the microprecipitate that can be produced during annealing is less likely to form due to cementite production, making it difficult to obtain the necessary amount of microprecipitate to achieve adequate strength. Therefore, the annealing temperature is specified to be the AC1 transformation temperature or higher. The annealing temperature is preferably (AC1 transformation temperature + 10 °C) or higher, and more preferably (AC1 transformation temperature + 20 °C) or higher. On the other hand, if the annealing temperature is higher than (AC3 transformation temperature + 20 °C), the precipitate becomes coarser to reduce the amount of microprecipitate, so precipitation hardening becomes ineffective, and the yield ratio decreases.Therefore, it is specified that the annealing temperature be (AC3 transformation temperature + 20 °C) or lower. The annealing temperature is preferably (AC3 transformation temperature + 10 °C) or lower, and more preferably AC3 transformation temperature or lower. Note that the transformation temperature AC1 and the transformation temperature AC3 are calculated using the following formulas. Note also that (element symbol in %) represents the content (% by mass) of each element in the following formulas. AC1 (°G)=723+22[ %S¡]-18[ %Mn]+17[ %Cr]+4.5[ %Mo]+16[ %V] AC3(°C)=910-203 / [ %C]+45[ %S¡]-30[ %Mn]-20[ %Cu]15[ %N¡]+11[ %Cr]+32[ %Mo]+104[ %V]+400[ %Ti]+460[ %AI] rnz Lnn / zznz / E / YiAi The holding time t (second) at the annealing temperature AT (°C) satisfies formula (3). A short holding time at the annealing temperature makes the reverse transformation to austenite less likely, so the microprecipitate that can occur during annealing becomes less probable due to cementite production, making it difficult to obtain the necessary amount of microprecipitate to achieve adequate strength. On the other hand, a long holding time at the annealing temperature makes the precipitate coarser to reduce the amount of microprecipitate, so precipitation hardening becomes ineffective, and the coefficient of performance decreases. Therefore, the holding time t (seconds) at the annealing temperature AT (°C) satisfies formula (3). The holding time t (seconds) at the annealing temperature AT (°C) preferably satisfies the following formula (3A), and more preferably the following formula (3B). Formula (3A): 1600 < (AT + 273) x logt < 2900 Formula (3B): 1700 < (AT + 273) χ logt < 2800 The cooling rate during cooling after holding at the annealing temperature is not specifically limited. It should be noted that hot-rolled steel sheet, after the hot rolling process, can be subjected to heat treatment to soften the microstructure, and the annealing process can be followed by quench rolling to control the shape. The annealing process may be followed by a rolling process, provided the properties of the steel sheet remain unchanged. Rolling consists, for example, of subjecting the surface of the steel sheet to electrogalvanizing, hot-dip galvanizing, or hot-dip galvanizing. When the surface of the steel sheet is subjected to hot-dip galvanizing, a hot-dip galvanized layer is preferably formed on the surface of the steel sheet, typically by immersing the steel sheet obtained as described above in a galvanizing bath at 440 °C or higher and 500 °C or lower. The coating weight is then checked, usually by gas purging. The steel sheet after hot-dip galvanizing may be alloyed.The hot-dip galvanized layer, when alloyed, is preferably alloyed within the temperature range of 450°C or higher to 580°C or lower, holding it for 1 second or more to 60 seconds or less. When subjecting the surface of the steel sheet to electrogalvanizing, the process conditions can be adjusted to those of any of the conventional methods, without particular limitation. According to the aforementioned manufacturing method of this modality, it is now possible, by controlling the hot rolling conditions and the annealing temperature and time, to control the proportion of the microstructure, the amount of microprecipitate, and the variation of the amount of microprecipitate in the longitudinal direction of the steel sheet, and to obtain high-strength steel sheet that has a high ratio of rnz Lnn / zznz / E / YiAi performance and excellent material uniformity. The high-strength member and its manufacturing method of the present invention will now be explained. The high-strength member of the present invention is the high-strength steel sheet of the present invention subjected to at least one of the two forming or welding processes. Furthermore, the method for manufacturing the high-strength member includes subjecting the high-strength steel sheet manufactured by the method for manufacturing a high-strength steel sheet of the present invention to at least one of the two forming or welding processes. Since the high-strength steel sheet of the present invention is well-balanced between high strength and material uniformity, the high-strength member obtained using the high-strength steel sheet of the present invention can maintain good shape. Therefore, the high-strength member of the present invention is conveniently applicable, for example, to automotive structural members. The forming process can be based on any of the usual forming methods, such as press working, without limitation. The welding process can be based on any of the common welding methods, such as spot welding or arc welding, without limitation. Examples Example 1 The present invention will now be described specifically with reference to the Examples. Note that the scope of the present invention is not limited to the following Examples. 1. Manufacturing of steel sheets for evaluation Each steel with the chemical composition indicated in Table 1, and the balance including Fe and unavoidable impurities, was melted in a vacuum melting furnace and bloomed to obtain a bloomed material 27 mm thick. The bloomed material thus obtained was hot-rolled to a thickness of 4.0 mm. The hot-rolling process conditions are summarized in Table 2. A sample of each hot-rolled steel sheet, intended for subsequent cold rolling, was then ground to reduce the thickness to 3.2 mm and cold-rolled according to a reduction ratio indicated in Table 2 to produce each cold-rolled steel sheet. Each of the hot-rolled and cold-rolled steel sheets was then annealed under the conditions indicated in Table 2 to produce each steel sheet. Sample n.Sample No. 55 in Table 2 is a steel sheet whose surface was hot-dip galvanized after annealing. Sample No. 56 in Table 2 is a steel sheet whose surface was hot-dip galvanized after annealing. Sample No. 57 in Table 2 is a steel sheet whose surface was electro-galvanized after annealing and subsequent cooling to room temperature. Note that the blank cells in Table 1 represent items that were not intentionally added but were not always 0% by mass, occasionally allowing unavoidable content. Meanwhile, the cold rolling cells in Table 2 represent that the steel sheet was not cold rolled. Again in Table 2, 1: Lower limit of slab heating temperature calculated from Formula (2) represents the values ​​calculated using the aforementioned Formula (2): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.75 x (2.4 - 6700 / T). In formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% mass) of component element Nb, [%C] represents the content (% mass) of component element C, and [%N] represents the content (% mass) of component element N. rnz Lnn / zznz / E / YiAi O CN Table 1 co CN co LO o co co CO 00 o CN 00 co co o co X- 00 co X- 00 co co x— CN co co 00 00 00 co 00 00 00 co 00 o 00 co oo CN LO o LO xr LO CN o 00 xf xf o O o O CN o σ> o X- CN oooo CN < CO co f'-x. bx.CO σ> LO o X- CN CO CN CN xf co co co OO co co |S, o CN oo O ooo OO CN CO ooo O ooooooo OO CN ooo O χ— oooooooo OO ooo O co oozo σ> o □ ooo LO ooom ó 00 ooo co oozo LO O o ó co LO CN 00 CN LO CN 00 LO CO 00 co O oo X- ooox— o CN ox— oox— oooooooo O ooooo )— ó ó ó ó ó ó ó Ó ó ó ó ó ó LO LO 00 LO LO LO LO LO LO LO LO LO LO co LO co xt oo O oo O o O o O oooo O zoo ó ooo O oooooox— CN co CO O) LO co oo CN co co co CN co CN CN CN co CN LO o CO CN CN co o O oo OO oo CO OO ooo O oo OO oo O oooooz ó O ó ó Ó ó o ó ó oo ó ó ó LO CN LO LO xt co LO co co 00 LO LO o O ooooooooo < ó Ó ó ó ó ó ó ó ó ó ó ó ó ó ó 'c? co σ> 00 00 co 00 oo co 00 00 LO σ> ω ooooooo co oooooo (ü ooooooooooooo E ooooooooooooooc ω oo ó ooooooooooooo χθ o'· I*»» O) o co σ> co f»x.co cooooo xf oooooooo 0) ooooo O oooooooo CL oo ó oo ó oo ó oooooo E =5 oo CN 00 00 -- o fx». co co co CN c co 00 o >- 00 co 00 00 Γ*'- co O ¡ciór Σ X— x— X- CN x— CN x— x— X- x— x— x— x— x— ω o Q o 00 LO [s^_ o LO CN oo co o co co co E LO xr co xf Xt X- CN xr oo LO Xt LO O ω oo ó x— ooo X— X- O ooox— O o CN CN co co o O) CN o co oo co co O) CO co co O) 00 O) CN co O) LO LO co o O ooooooooo O ó ooooo O ooooo φ TJ O o Q. i- Φ o Cü < co o Q LJJ | | of T “5 k: _l z. rnz Lnn / zznz / E / YiAi 812 857 933 837 835 834 834 OJ co O σ) X- O oo OO co f''·». co co V— oo co ooooooooo LO ooooooooo ó ó co 00 co co 00 LO co oooooo co T“ ooooooo ó ó ó ó ó ó ó LO LO LO o LO LO o O o O ooo ó o ó oo ó ó LO o O) LO CXI CXI CX CLOX O J OJO ooo O o O o ó ó ó ó o Ó o co LO LO o O O CXI oooo ó Ó Ó ó ó ó ó ó co 00 co σ> 00 co oo O oooo CXI o O oooooo O ooooo ó O ó ooo ó CO ooooooooooooo ó oo T ó co 00 o co I Γ·- I'·— 00 00 co cxi T— T- τ— τ— τ— τ— co τ— LO 00 co LO LO ó O ó O o Ó o T- CO (M LO co oo O) O) O σ> 00 O) O) OO oooo ó oo o σ- O σ I = σ I rnz ίηη / ζζηζ / Ε / γίΛΐ CN CN Table 2 Observations Example of invention Example of invention Example of invention Example of invention Comparative example Example of invention Example of invention Example of invention Comparative example Example of invention Example of invention Example of invention Comparative example Comparative example Example of invention Example of invention Annealing condition Lp 1657 1657 1657 2146 2042 1908 1688 1657 1672 2231 1908 1396 1657 1626 1657 * Second LO CO LO CO LO CO O o T- O co O co LO CO LO CO LO CO O CN O co O CN LO CO LO CO LO CO Annealing temperature OOO CO oo CO oo CO O o 00 oo co oo co o CN CO oo CO o 00 OO oo oo 00 OO CO oo CO o co oo 00 Cold rolling Reduction ratio xO cr CO LO CO LO CO LO co LO co LO co LO CO LO co LO co LO co LO co LO CO LO co LO co LO co LO Hot rolling co to O 0 O CO o CO o CO o co o co o co O co o co o co o co o co O CO o co o co o co Winding temperature o 009 009 009 009 009 009 009 009 009 009 009 009 009 009 009 TemperatureDelivery of the finisher or 0 O CO CO O CO CO O CO CO O co co O co co O co co O co co O co CO O 00 00 O 00 00 O 00 00 088 O CO CO 098 006 CN to p LO LO LO LO LO LO LO LO LO LO LO T- CN LO Slab heating time Time (h) CN X- CN CN t- CN T- CN CN o CN or CN T- CN CN CN CN x— 1202 1202 1202 1202 1157 1157 1157 1157 1235 1235 1235 1235 1198 1198 1198 Slab heating temperature O □ 1250 1300 1250 1250 1130 1200 1250 O o co 1280 1280 O 00 CN 5— O CO CN 1250 1250 1250 Type of steel < m OQ 0 ,— CN co LO CO CO σ> OT“ CN CO LO rnz Lnn / zznz / E / YiAi ccccc C c C cccccc 'O :o 'O 'O 'O 'O 'O 'O o 'O c cu ccc cu oco C CU cu cocccccc cu occ cu cu c Φ Φ Φ Φ cu Φ Φ s cu Φ Φ Φ Φ Φ Φ CU Φ Φ s cu Φ CQF ccc CX F c C CX FOF ccccccc a F cc ex F d F c Φ o Φ Φ Φ o Φ oo Φ Φ Φ Φ Φ Φ Φ oo Φ ο φ oo φ φ φ φ φ o φ φ oo φ ooooooooooooooooooooooo CX Q CX QQ CX QQ CX Q ex Q ex QQQOQQ ex ex ddbbbbbbbbbbbbbbbbbbb bbbb Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ lu LU llT LJJ l±l ÜT LU LU llT LU llT LU llT LU LU LU LU llT llT LU LU 00 O) x— CM CO LO CO CM CO LO 00 x— LO LO LO P^.LO LO LO ρ«^ LO CM LO CT> CO CM CM CO CT> LO CO x— CO x— x— CO x— co x— CO x— co x— x— co x— CO x— CO x— CO x— co x— 00 x— CO x— LO x— x— O CM σ» x— σ> x— 22 CO x— x— O LO LO o LO LO LO LO LO LO LO LO LO LO CM o CM p^, o O CO CO CO co CO CO co CO co CO co LO CO CO CO X- 00 LO p— CM VOO oooo OO ooooo O ooo O o OOO o O CM oooo σ» O oooo O o co LO O LO LO O CM 00 00 00 CO CO 00 p»^. 00 00 CO 00 00 00 00 CO CO P*w 00 σ> 00 00 CO co co co co co co co co co co ooo CO o O co CO CO LO LO LO LO LO LO LO LO LO LO LO co | co p^. LO LO LO LO LO oooooooo LO o LO LO ooooo O o O oo O co co co co co co co CM co CO X- co 00 co co co CM co co CO co co co ooooooo O oooooooo O oo O ooooo LO o co 00 LO op^.oo LO oooo O oo O ooo co co LO co co LO co co co co LO co co co co CO co co CO co co co oooooooooooooooooooooooo 00 co LO CO CM co 00 00 co CO co 00 00 00 co CO co 00 00 co co co co 00 00 00 CO o co 00 00 co CO co 00 00 00 co CO co 00 00 00 co co co ox— LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO x— x— x— x— 00 00 00 co oo CD ooooo CD oop^, p<^. p^. p^.ooo O co co co CO CM x— CM x— x— x— x— x— x— x— x— CM x— CM x— CM x— CM x— CM x— x— oo O ooo O ooo LOoo LOoo LOoo LOoo LO LO LO LO LO LO LO 00 00 co co LO LO CM x— CM x— CM x— CM x— CM x— CM x— CM x— CM x— CM X— CM x— CM x— CM x— CM x— CM X— CM X— x— CM x— CM x— CM CM x— LU 0 T ”3 with 00 σ> or CM CO LO with p^ 00 σ> ox— CM 00 LO with p^, CO X- X- X- CM CM CM CM CM CM CM CM with CO CO co co CO co CO co. rn? ίηη / ζζηζ / Ε / γίΛΐ CXI cooc C ooo O oooo OO occ 'O 'O 'O c 75 ra cc 75 75 75 75 75 75 75 75 75 75 75 ccc Φ ra m Φ Φ 01 ro mm ai 05 03 03 01 01 01 QX Φ C Φ QX g CQΦ d CX Q. ex o. Q α g FFFFFFFFFFFFF Φ OO Φ Φ OOOOOOOOOOO Φ Φ Φ φ oo Φ Φ oooooooooooooo φ φ φ ooo OO oooooooooooooo CX Q CX QQ CXQ QQ QQ ex bbbbbbbbbbbbbbbbbbb Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ LU llT LU l±l llT LU LU llT LU llT LUT llT LUCM LUCM ll 00 CO in LO in LO LO LO 00 LO LO LO LO T— LO CM LO LO CO τ— LO LO CD τ— o CO CD τ— CO τ— co x— τ— CD r— CD τ— CD T— CD τ— T— CD τ— 00 T— CD τ— CD T— CD T— τ— CD τ— r— o LO o LO LO LO o LO LO LO LO CD LO o LO LO LO CD LO LO CO co co CO co CO CO co CO xf CO LO CO CO xr CO O oooooo O CD o CD CD CD O CD CD O CD OO ooo co o CD CD CD O CD CD CD CD CM CD CD 000 CO 00 00 00 CO CO CO 00 00 CD <0 co co co co co CO CO CO CO CO CD CO CO CO CO CO CDCO lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo lo O oooooo O CD OOO <D O O O O <D CD co co co CO co co co CO CO co CO co CO CO CO co CO LO o o o o o o o O O o o o O O O o o O O o o o o o o o O O o o o O O O o o O LO ¢0 CD CO <0 CD CD C0 CO CO CD CD CO CD C0 CO co CD CD LO O o o o o o CD CD CD CD CD CD CD CD CD O O 00 00 co CO CO co 00 00 CO CO CO 00 00 00 CO CO CO 00 00 00 00 00 CO CO co 00 00 CO CO CO 00 00 00 CO co CO 00 00 LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO LO CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM O T- T- T- T- T“ T- T- T“ x- T- T- t- T- O O CM CXI CM CXJ CD CD CO LO O CXJ CXJ CM LO LO O o O O CD CD CD CD CXJ CD CD CD CD CD τ— r— CM τ— CM τ— CXI τ— CXJ τ— CXJ τ— τ— CM τ— CXI T— CM x— CXJ τ— CXJ τ— CXI Λ— CXI τ— CXI τ— CXJ T— CXJ τ— CXI r— O O O O O O o O O o O CD O CD O CD o O O LO LO o LO o LO CM LO LO LO LO CD O LO LO LO LO LO CM τ— CM τ— CM 5— CM 5— co T— CM T— CO T— CM 5— CM 5— CM 5— CM T— CM T— CM τ— CO 5— CM 5— CM τ— CM T— CM T—CXI T— x¿ _1 z O CL σ Oí <Z) 1- Z) < en O CM co LO CD C0 en O τ— CXI co LO co oo LO LO LO LO LO LO LO LO Ί: Lower limit of the slab heating temperature calculated from formula (2) '2: Average cooling rate from the forging heating temperature to the rolling start temperature '3: Average cooling rate from the finisher delivery temperature to the winding temperature tu ίχ CNN IT CXJ '4: Holding time (t) at annealing temperature (AT) '5: (AT + 273) x logt rnz Lnn / zznz / E / YiAi 2. Evaluation methods The microstructures of steel sheets obtained under different manufacturing conditions were analyzed to investigate the proportion of the microstructure, and they were subjected to tensile tests to evaluate tensile properties, including tensile strength. The methods for the individual evaluations are as follows. (Fractions of ferrite and martensite area) Samples were taken from the steel sheets in the rolling direction, and L-shaped cross-sections taken in the thickness direction and parallel to the rolling direction were mirror-polished. The cross-sections taken in the thickness direction were etched with a nital solution to expose the microstructure and then observed using a scanning electron microscope (SEM). The ferrite and martensite area fractions were examined using the dot-counting method, whereby a 16 x 15 mesh with a 4.8 pm interval was superimposed on an 82 pm × 57 pm actual length area on a 1500x SEM image, and the number of mesh points falling on the individual phases was counted. Each area fraction was determined by averaging three area fraction values ​​obtained from independent 1500x SEM images.Ferrite has a black microstructure, and martensite has a white microstructure. The area fraction of the balance, other than ferrite and martensite, was calculated by subtracting the total area fraction of ferrite and martensite from 100%. In the present invention, the balance was considered to represent the total area fraction of pearlite, bainite, and retained austenite. The area fraction of the balance is given in the column titled “Other” in Table 3. Area fractions were measured using a test sample sampled at the center in both the longitudinal direction (rolling direction) and the width direction of the steel sheet. (Total Nb and Ti content in the precipitate with a particle size less than 20 nm). Five grams of each steel sheet were placed in a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution for electrolytic extraction, and the solution was filtered through a 20 nm pore size filter. The filtrate was dried, and nitric, perchloric, and sulfuric acids were added. The mixture was heated to dissolve the compounds until a white mist of sulfuric acid was produced. The solution was allowed to cool, and hydrochloric acid was added, followed by dilution with pure water. The dilute solution was subjected to elemental analysis using an ICP emission spectrophotometer. From the elemental analysis results, the mass ratio (ppm) of the total Nb and Ti content in the precipitate with a particle size smaller than 20 nm was calculated relative to the steel sheet. Samples were taken individually from a front, a middle, and a back portion of the steel sheet along the longitudinal (rolling direction) axis and analyzed using the aforementioned extraction residue method to determine the total (ppm by mass) Nb and Ti content in the precipitate with a particle size smaller than 20 nm for each individual portion. The difference between the maximum and minimum values ​​measured in the three portions was determined. Note that the measurements for the front, middle, and back portions along the longitudinal (rolling direction) axis of the steel sheet were taken at the center of each portion, respectively. Note that the measurement on the front, in the longitudinal direction of the steel sheet, was taken at a position 1 m from the front end towards the center. Similarly, the measurement on the rear end, in the longitudinal direction of the steel sheet, was taken at a position 1 m from the rear end towards the center. In the present invention, the difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate having a particle size of less than 20 nm, calculated after measurement in the leading, middle, and trailing portions of the steel sheet in the longitudinal (rolling direction), was assumed to be the difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate having a particle size of less than 20 nm, in the longitudinal direction of the steel sheet. The differences between the maximum and minimum values ​​are summarized in Table 3. The winding temperature tends to be higher and the cooling rate after winding tends to be slower in the central portion along the longitudinal direction of the steel sheet; meanwhile, the winding temperature tends to be lower and the cooling rate after winding tends to be faster at the leading and trailing ends along the longitudinal direction of the steel sheet. Therefore, the microprecipitate containing Nb and Ti tends to be less abundant in the central portion along the longitudinal direction of the steel sheet, while it tends to be more abundant at the leading and trailing ends. Therefore, the higher of the measured value obtained at the leading or trailing end along the longitudinal direction of the steel sheet was assumed to be the maximum value.Meanwhile, the measured value obtained in the central portion along the longitudinal direction of the steel sheet was assumed to be the minimum value. Therefore, in the present invention, the difference between the maximum and minimum values ​​of the total Nb and Ti content along the longitudinal (rolling direction) direction of the steel sheet is calculated as the difference between the maximum and minimum values ​​of the measured values ​​obtained at three points: the front, the central portion, and the back, along the longitudinal (rolling direction) direction of the steel sheet. Meanwhile, in the present invention, the total Nb and Ti content in the precipitate having a particle size of less than 20 nm, measured in the central portion in both the longitudinal and width directions of the steel sheet, was specified as the total Nb and Ti content in the precipitate having a particle size of less than 20 nm. The total contents are summarized in Table 3. rnz Lnn / zznz / E / YiAi (Tensile test) JIS No. 5 gauge samples with a length of 50 mm and a cross-sectional width between gauge marks of 25 mm were taken from individual steel sheets in the vertical direction relative to the rolling direction and subjected to a tensile test at a pulling speed of 10 mm / min, in accordance with the requirements of JIS Z 2241 (2011). The tensile strength (denoted as TS in Table 3) and yield strength (denoted as YS in Table 3) were measured by the tensile test. The yield strength (denoted as YR in Table 3) was calculated by dividing YS by TS. Note that the tensile strength (TS), yield strength (YS), and coefficient of elasticity (YR) summarized in Table 3 are values ​​obtained by measuring each sampled specimen of the steel sheet in the central part both in the longitudinal direction (rolling direction) and in the width direction. (Uniformity of the material) The tensile test mentioned above was performed individually on the front, middle, and rear sections in the longitudinal direction of the steel sheet, and the uniformity of the material was evaluated based on the difference (denoted as AYR in Table 3) between the maximum and minimum values ​​of the measured yield ratio (YR) in these three sections. Note that the measurements on the front, middle, and rear sections in the longitudinal direction of the steel sheet were performed individually on the middle section in the width direction. The measurement in the present invention on the front end section in the longitudinal direction of the steel sheet was performed at a position 1 m from the front end toward the middle section.Furthermore, the measurement in the present invention on the rear end in the longitudinal direction of the steel sheet was carried out at a position of 1 m from the rear end towards the central part. 3. Evaluation Results The results of the evaluation are summarized in Table 3. rn? Lnn / zznz / E / YiAi or CN Table 3 Observations Example of the invention Example of the invention Example of the invention Example of the invention Comparative example Example of the invention Example of the invention Example of the invention Comparative example Example of the invention Example of the invention Comparative example Comparative example Example of the invention Example of the invention Example of the invention Mechanical properties AYR 0.02 τO O 0.02 0.02 ZOO LO OO 0.02 00Ό ZOO 0.02 0.03 0.02 80Ό 90Ό 0.02 0.02 YR 8ZO 8ZO O 00 or 0.76 or 0.75 9ZO 0.78 0.75 0.75 0.74 99Ό O 00 or 9ZO 0.79 0.79 SI MPa 638 645 636 645 602 602 604 t09 724 722 721 725 705 90Z 669 Z69 YS MPa 498 503 509 452 746 5 478 564 539 552 551 Microstructure CN * ppm mass 10 00 X- O X- 25 X- X- 25 O CN X~ OX~ CN CO X- O X- O ppm mass O co o 00 o 180 80 or co Other ox LO X~ LO X- LO CN O CN CN CN o CN O CN co xP 59 58 co LO 09 65 65 62 09 Type of steel < co OQ 0 z X— CN 00 LO CO 00 OO x— x— CN x— 00 x— x— LO x— CO X- rnz 'O 'O -O Ό 'O -o *o kO 'O “O -o 'O 'OOQ oooooooo Ü oo 0 Q o QQO > C ccccccccccccc CE O 0) Φ cu Φ Φ cc Φ Φ co Φ Φ Φ Φ cc Φ ccccccc CE cc CE CL £ CE CE with Q, £ CE CE CL £ CL £ CE CE CE CE CL £ CE CE Q. Ό 0 0 0 0 0 0 o O Ό O ooo O o O ooo OOOOO o O o OO oo CL CL CL QQ CL Q.CL Q_ CL CL Q. CL CL CL CL CL Q. Q_ FFFEFFFFFFFFFFFFFFF Φ O Φco ó ó ó o ó ó ó ó ó ó ó o o ó ó ó ó co 04 co τ— co co o σ> o co 00 o σ> 00 LO Λ— LO OGG) σ> OI OJ OJ OI co OI co 04 V LO G) o LO LO LO m oo 00 00 co co co co co co CO co LO co co T- co OJ co co G OI o co o LO LO LO LO LO LO LO co G) co oo OJ co CO o OJ co LO co co co LO LO LO LO o co OI OI co co LO σ> co OJ oo co OJ co OI OI T- y— T- X~ OJ co T- T- T“ χ— T- T- X- T- T- o OO o LO LO O LO LO LO LO LO ooo co < ​​co o O ooo OI ΤΓ O CO CO T— co 00 co OI OJ τ— T- 00 co LO τ— OI τ— T- OJ OI O OI VX~ o co T- OI OI o O LO o T- o T- CO co LO co co co co o T~ o OO T- T- X- T- Ti- (NI OJ OJ O! co CO OJ CO co CO CO co OI O o LO LO LO OI o CO o LO G) co 00 00 co co LO LO LO LO O·. o* co CO G) CO OI LU 0 00 σ> o OJ co LO CO oo σ> o OJ co LO co T- T- τ— CNI OJ OJ OI CNI OI OJ OJ OI OI co co co co co CO co. rnz Lnn / zznz / E / YiAi m OJ O ción ción oo uop U0I0 ooooooo OOOO ción ción upp > cc > > cc > > > > > > > > > > ccc ω > ro S > φ Φ φ co φ φ Φ φ Φ Φ Φ re cc re cc re re re re re re re re re re re ccc Q. F (0 CÜ QF Q. F Q. F Q. F Q. F Q. F Q. F Q. F Q. F Q. F Q. F Q. F co Φ Φ o Φ 0) oo φ Φ ooooooooooo Φ Φ Φ o Φ φ oo Φ φ ooooooooooo φ φ φ oooooooooooooooooooo O o Q_ Q. QQQ Q_ QQ Q_ Q_ Q. Q_ Q. Q_ Q_ CL Q. CL CL Q. FFFFFFFFFFFFFFFFFFFFF Φ O Φ Φ Φ Φ φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ Φ llT LlT llT LU LU LU llT LU LU llT llT LU llT llT llT llT llT llT llT llT LU CO CO CO CO CM O CM CM CO CM CM CO co CM CM CO ooooo O o OOO o OOOO oo OOO o ó ó ooo CD OO o O oo OO o c5 co o OO c5 O) co co co LO LO co o O o CO co o 00 o en CO O) co I'·— co r*·— co co CO [···.Γ'— CO r- co co I'-— f''» ó ó ó ó Ó ó ó Ó ó ó ó O ó ó ó ó ó ó en o co o CO co CO co o O en X— 00 00 co LO LO co co co LO CM CM LO co CO co LO co co co co co co co co CO co CO co LO co co Μ co co CO co co co co CO X- en co co o co CO co co o co co LO O CO CO co co CM LO CM co o CM CM co oo LO LO LO co LO co CO LO LO LO LO LO co co CM co oo CM ooo CM 00 O) o co X- X- X~ x- CM X~ co X- CM T- CM CM X- 7“ CM CM CM CM X~ T- X~ OOO o LO o O o LO LO LO oo O o O LO O CM CM oooo X- co CM I*·» 7— X- CO CO co CM X— CO CO 00 00 co 7— CO co o x- CM 3- co o LO CM O CM o T- CM CO LO LO LO en en CM X- X- en co CO en O en 7“ en 7“ CO CO co CM CM CO CO CO CM CM x- X- LO 7— co CM CO CO co CM CM CM LO 00 co CO en en en O LO en oo en CO CO co co co co co co CO co co co co “O _1 zo CL σ oc en 1- D < 00 en o CM co LO co 00 σ> O CM 00 LO CO CO co co M M- M M- M- LO LO LO LO LO LO LO LO. en? Lnn / zznz / E / YiAi m CM ID CO a: Ferrite area fraction, M: Martensite area fraction. Others: Total area fraction of retained pearlite, bainite, and austenite. *1: Total Nb and Ti content in the precipitate with a particle size less than 20 nm 2: Difference between the maximum and minimum values ​​of the total Nb and Ti content in the precipitate with a particle size less than 20 nm, in rnz Lnn / zznz / E / YiAi In this modality, steel sheets with a TS of 590 MPa or greater, a YR of 0.70 or greater and an AYR of 0.05 or less were considered acceptable and were listed as inventive examples in Table 3. Conversely, steel sheets that do not meet at least one of these requirements were considered rejected and were listed as a comparative example in Table 3. Example 2 Steel sheet No. 1 of Example 1, listed in Table 3, was formed by pressing to manufacture a member of this example invention. Furthermore, steel sheet No. 1 of Example 1, listed in Table 3, and steel sheet No. 2 of Example 1, listed in Table 3, were spot-welded to manufacture a member of this example invention. It was confirmed that, since the high-strength steel sheet of this example invention is well-balanced between high strength and material uniformity, the high-strength member obtained using the high-strength steel sheet of this example invention can maintain good shape, and that the steel sheet is suitable for automotive structural members.

Claims

1. A high-strength steel sheet having a chemical composition by mass % containing: C: 0.06% or more and 0.14% or less, Si: 0.1% or more and 1.5% or less, Mn: 1.4% or more and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or more and 0.20% or less, N: 0.10% or less, Nb: 0.015% or more and 0.060% or less, and Ti: 0.001% or more and 0.030% or less, the S, N, and Ti contents conforming to formula (1) below, an equilibrium being Fe and an unavoidable impurity, comprising, in terms of area fraction relative to a full steel microstructure, 30% or more and 100% or less ferrite, a 0% or more and 70% or less of martensite, and less than 20% in total of pearlite, bainite and retained austenite; a total Nb and Ti content contained in a precipitate with a particle size less than 20 nm that is equal to or greater than 25 ppm by mass and equal to or less than 220 ppm by mass,and a difference between a maximum and a minimum value of the total Nb and Ti content contained in the precipitate with a particle size less than 20 nm, in a longitudinal direction of the steel sheet, less than 20 ppm by mass, Formula (1): [%Ti] - (48 / 14)[%N] - (48 / 32)[%S] < 0, in Formula (1), [%Ti] represents the content (in % mass) of the component element Ti, [%N] represents the content (in % mass) of the component element N, and [%S] represents the content (in % mass) of the component element S.

2. The high-strength steel sheet according to claim 1, wherein the chemical composition further contains, by mass %, one, or two or more of Cr: 0.01% or more and 0.15% or less, Mo: equal to or greater than 0.01% and less than 0.10%, and V: 0.001% or more and 0.065% or less.

3. The high-strength steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in % by mass, B: 0.0001% or more and less than 0.002%.

4. The high-strength steel sheet according to any of claims 1 to 3, wherein the chemical composition further contains, by mass %, one or two of Cu: 0.001% or more and 0.2% or less, and Ni: 0.001% or more and 0.1% or less.

5. The high-strength steel sheet according to any of claims 1 to 4, having a coating layer on one surface of the steel sheet.

6. A high-strength member comprising the high-strength steel sheet according to any of claims 1 to 5 subjected at least to forming or welding.

7. A method for manufacturing a high-strength steel sheet, comprising: a hot rolling process in which a steel plate having the chemical composition according to any of claims 1 to 4 is heated to a heating temperature T (°C) that meets the following Formula (2) for 1.0 hours or more, then cooled from the heating temperature to a rolling start temperature at an average cooling rate of 2°C / s or faster, then finished rolling to a final delivery temperature of 850°C or more, then cooled from the final delivery temperature to a temperature range of 500°C or more and 650°C or less at an average cooling rate of 10°C / s or faster, and then wound in the temperature range; and an annealing process in which the hot-rolled steel sheet obtained in the hot rolling process is heated to an annealing temperature which is the transformation temperature Aci or higher and (transformation temperature Aca + 20 °C) or lower, is held at the annealing temperature for a holding time t (second) that satisfies the following formula (3), and is then cooled: Formula (2): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.75 x (2.4 - 6700 / T) in Formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (in % mass) of the component element Nb, [%C] represents the content (in % mass) of the component element C, and [%N] represents the content (in % mass) of the component element N; and Formula (3): 1500 < (AT + 273) χ logt < 3000, in Formula (3), AT represents the annealing temperature (°C), and t represents the holding time (second) at the annealing temperature.

8. A method for manufacturing a high-strength steel sheet, comprising: a hot rolling process wherein a steel plate having the chemical composition according to any of claims 1 to 4 is heated to a heating temperature T (°C) meeting the following Formula (2) for 1.0 hour or more, then cooled from the heating temperature to a rolling start temperature at an average cooling rate of 2°C / s faster, then finished rolling to a final delivery temperature of 850°C or more, then cooled from the final delivery temperature to a temperature range of 500°C or more and 650°C or less at an average cooling rate of 10°C / s faster, and then coiled within the temperature range; rnz Lnn / zznz / E / YiAi a cold rolling process in which the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled;and an annealing process in which the cold-rolled steel sheet obtained from the cold rolling process is heated to an annealing temperature that is the transformation temperature Aci or higher and (the transformation temperature Acs + 20 °C) or lower, at an average heating rate of 600 °C to 700 °C 8 °C / s slower, held at the annealing temperature for a holding time t (second) that satisfies the following Formula (3), and then cooled: Formula (2): log{[ %Nb] x ([ %C] + 12 / 14[ %N])} < 0.75 x (2.4 - 6700 / T) in Formula (2), T represents the heating temperature (°C) of the steel sheet, [ %Nb] represents the content (in % mass) of the component element Nb, [ %C] represents the content (in % mass) of the component element C, and [ %N] represents the content (in % mass) of component element N;and Formula (3): 1500 < (AT + 273) x logt < 3000, in Formula (3), AT represents the annealing temperature (°C), and t represents the holding time (seconds) at the annealing temperature.; 9. The method for manufacturing a high-strength steel sheet according to claim 7 or 8, further comprising a coating process for providing a coating after the annealing process.

10. A method for manufacturing a high-strength member, comprising subjecting the high-strength steel sheet manufactured by the method for manufacturing a high-strength steel sheet according to any of claims 7 to 9, at least to forming or welding.