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

Abstract

The subject matter of the present invention is to provide a high-strength steel sheet and a high-strength member having 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 total 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. The ferrite contains, in terms of area fraction relative to a total microstructure, 0% or more and 10% or less of unrecrystallized ferrite, with a difference in the area fraction of unrecrystallized ferrite in the longitudinal direction of the steel sheet of 5% or less.

Description

HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH MEMBER AND METHODS FOR MANUFACTURING THE SAME ινΐΛ / a / zuzz / uu 11 ou 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 them. More specifically, the present invention relates to a high-strength steel sheet and a high-strength member having excellent material uniformity, and methods for manufacturing them. BACKGROUND OF THE TECHNIQUE 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 the weight of vehicle bodies and thus improving fuel efficiency. One technique for reducing body weight, for example, is to thin the steel sheets used in automobiles by increasing their strength. However, it is known that steel sheets lose ductility as their strength increases, highlighting the need for a steel sheet that balances high strength and ductility. Furthermore, steel sheets with varying mechanical properties along the longitudinal direction will degrade the reproducibility of the form setting, thereby reducing the reproducibility of the spring rate and making it difficult to maintain the shape of the parts.Therefore, a steel sheet is needed that does not vary its mechanical property in the longitudinal direction of the steel sheet, and that stands out for the uniformity of the material. 5 In response to this need, for example, Patent Literature 1 proposes a high-strength steel sheet containing, by mass %, 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 total 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 portion of the steel sheet to 0°C or greater and 50°C or less, and controlling a winding temperature difference between a rear and middle portion of the steel sheet to 50°C or greater and 200°C or less. Patent Literature 2 proposes a hot-rolled steel sheet that 35 has a chemical composition containing, by 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 variation in strength in the longitudinal direction of the steel sheet, as a result of the cooling control after winding. LIST OF REFERENCES 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 the microstructural difference in the longitudinal direction of the steel sheet. However, the problem of large variation in the yield strength remains. 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 controlling the composition and cooling before winding. However, precipitation elements such as Nb or Ti are not added, so the reduction in resistance variation mentioned is different in its conception from the present invention, which is based on controlling the variation of the fraction of the unrecrystallized ferrite area 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 and methods for manufacturing the same, all of which are intended to improve the uniformity of the material by suitably adjusting the chemical composition in the presence of an added precipitation element, such as Nb or Ti, which can affect precipitation hardening to achieve a high yield ratio, and to create a ferrite-martensite microstructure to control the variation of the fraction of the unrecrystallized ferrite area 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 the addition of Nb or Ti is necessary to achieve high strength, as well as a high strength-to-weight ratio. s N c NNC performance, and also that the difference between the maximum and minimum value of the fraction of the S —I unrecrystallized ferrite area in the longitudinal direction of the steel sheet is controlled or necessarily to 5% or less, in order to reduce the variation of the mechanical property in the longitudinal direction of the steel sheet. As described above, the present inventors discovered, after exhaustive research aimed at solving the aforementioned problems, that a steel sheet having a specific chemical composition and a steel microstructure composed primarily of ferrite and martensite can be obtained as a high-strength steel sheet that excels in material uniformity by controlling the variation of the fraction of unrecrystallized ferrite area in the longitudinal direction of the steel sheet. A brief description of the present invention is as follows. [1] A high-strength steel sheet with 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 conforming to Formula (1) below, an equilibrium being Fe an unavoidable impurity, including, in terms of area fraction relative to a full 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, and ferrite containing, in terms of area fraction relative to a full microstructure, 0% or more and 10% or less of unrecrystallized ferrite, with a difference between a maximum and a minimum value of the fraction of the area of ​​unrecrystallized ferrite in a longitudinal direction of the steel sheet of 5% or less: 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 conforming to [1], wherein the chemical composition further contains, by mass %, one of, or two or more of Cr: 0.01% or more and 0.15% or less, Mo: 0.01% or more and 0.10% or less, 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 0.002% or less. [4] The high-strength steel sheet conforming to any of [1] to [3], where the chemical composition also contains, in % 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 of 15 [1] to [4], having a plating layer on one surface of the steel sheet. [6] A high-strength member including high-strength steel sheet conforming to any of [1] to [5] subjected to at least one forming or welding process. [7] A method for manufacturing a high-strength steel sheet, which includes: a hot rolling process in which a steel plate having the chemical composition conforming to any of [1] to [4] is heated to a heating temperature T (°C) that complies with Formula (2) below for 1.0 hours or more, then it is cooled from the heating temperature to the rolling start temperature at an average cooling rate of 2 °C / s faster, then it is finished rolling at a finish supply temperature of 850 °C or more, then it is cooled from the finish supply temperature to 650 °C or less at an average cooling rate of 10 °C / s faster, and then it is wound at 650 °C or less; and an annealing process in which the hot-rolled steel sheet obtained from the hot rolling process is heated to an annealing temperature which is the upper transformation temperature Ac and (the transformation temperature Ac + 20 °C) or lower, at an average heating rate of 8 °C / s slower, held at the annealing temperature for a holding time t (second) that complies with the following Formula (3), and then cooled:. Formula (2): 0.80 * (2.4 - 6700 / T) < log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 35 6700 / T) In Formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% by mass) of the component element Nb, [%C] represents the content (% by mass) of the component element C, and [%N] represents the content (% by mass) of the component element N. Formula (3):1500 < (AT + 273) χ logt < 5000 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 in which a steel plate having the chemical composition conforming to any of [1] to [4] is heated to a heating temperature T (°C) conforming to the following Formula (2) for 1.0 hour or more, then cooled from the heating temperature to the rolling start temperature at an average cooling rate of 2 °C / s faster, then finished rolling to a finish delivery temperature of 850 °C or more, then cooled from the finish delivery temperature to 650 °C or less at an average cooling rate of 10 °C / s faster, and then coiled at 650 °C or less; 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 Aci or higher and (the transformation temperature Acs + 20 °C) or lower, at an average heating rate of 8 °C / s slower, held at the annealing temperature for a holding time t (second) that complies with Formula (3) below, and then cooled: 5 Formula (2): 0.80 χ (2.4 - 6700 / T) < log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 6700 / T) In Formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% by mass) of the component element Nb, [%C] represents the content (% by mass) of the component element C, and [%N] represents the content 0 (% by mass) of component element N. Formula (3):1500 < (AT + 273) χ logt < 5000 In Formula (3), AT represents the annealing temperature (°C), and t represents the holding time (second) at the annealing temperature. [9] The method for manufacturing a high-strength steel sheet in accordance with [7] or [8], which further includes a plating process to provide a plating, 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 of [7] to [9], at least to forming or welding. ADVANTAGES OF THE INVENTION The present invention controls the microstructure of the steel and the variation of the fraction of unrecrystallized ferrite area in the longitudinal direction of the steel sheet by adjusting the chemical composition and manufacturing method. Consequently, the high-strength steel sheet of the present invention is distinguished by its 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a steel sheet of the present invention taken in the thickness direction, observed under a scanning electron microscope. Description of the modalities The embodiments of the present invention are described below. The present invention is not limited to the embodiments described below. 5 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 resistance to 3. Traction of 590 MPa or higher. It should also be noted that the steel sheet of the present invention basically refers to 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 high material uniformity in the longitudinal direction (rolling direction) of the steel sheet. That is, the steel sheet exceeds s 7 N c NNC in material uniformity, with respect to each individual steel sheet (coil). S —IC: 0.06% or more and 0.14% or less. Carbon (C) is necessary to achieve a tensile strength (TS) greater than 590 MPa, either by improving the strength of the martensite or through precipitation hardening using fine precipitate. 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 the martensite, leading to excessive strength. Such a content will also increase the amount of carbide produced, making recrystallization less likely and thus degrading 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 higher. The Si content is preferably 0.2% or higher, and more preferably 0.3% or higher. Si also has a suppressive effect on cementite formation, so an excessive Si content will suppress cementite production. Furthermore, unprecipitated carbon forms carbide with niobium (Nb) or titanium (Ti) and becomes coarse, thus degrading the material's uniformity. Therefore, the Si content is set at 1.5% or lower. The Si content is preferably 1.4% or lower. Mn: 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% will decrease the amount of fine precipitate, as pearlite or bainite forms during quenching, hindering the achievement of the required strength. Therefore, the manganese content is set at 1.4% or higher. Preferably, the 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 increases the variation of the precipitate along the longitudinal direction of the steel sheet, increasing the variation of the unrecrystallized ferrite area fraction and thus degrading the uniformity of the material. Therefore, the manganese content is set at 2.2% or lower. The Mn content is preferably 2.1% or less. P: 0.05% or less Phosphorus (P) is an element that can strengthen steel, but an excessive content will lead to segregation at the grain boundaries, which will degrade workability. Therefore > s Therefore, the P content is controlled to 0.05% or less in order to achieve the minimum necessary workability level when applied to the automobile. The P content is preferably of the 0.03% or less, and more preferably 0.01% or less. Although the lower limit for P content is not specifically limited, an industrially viable lower limit at present is approximately 0.003%. S: 0.0050% or less Sulfur (S) degrades workability by forming MnS, TiS, Ti(C,S), etc. S also suppresses recrystallization, further degrading material uniformity. Therefore, the S content should be controlled to 0.0050% or less. Preferably, the S content is 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 is observed at an aluminum content of 0.01% or higher. The preferred aluminum content is 0.02% or higher. Furthermore, at an aluminum content above 0.20%, the carbide produced during winding after hot rolling will be less susceptible to becoming solute during annealing, suppressing recrystallization and thus reducing material uniformity. 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 of the precipitate along the longitudinal direction of the steel sheet cannot be suppressed. This increases the variation in the fraction of unrecrystallized ferrite along the longitudinal direction and degrades the uniformity of the material. 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 limited, an industrially viable lower limit is currently approximately 0.0006%. Nb: 0.015% or more and 0.060% or less Nitrogen (Nb) contributes to precipitation hardening by producing fine precipitates. To achieve this effect, the Nb content must be 0.015% or higher. Preferably, the Nb content is 0.020% or higher, and more preferably 0.025% or higher. On the other hand, a high Nb content increases the variation in the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet, thus degrading the material's uniformity. Therefore, the Nb content is set at 0.060% or lower. Preferably, the Nb content is 0.055% or lower, and more preferably 0.050% or lower. Ti: 0.001% or more and 0.030% or less Titanium (Ti) contributes to precipitation hardening by producing fine precipitates. To achieve this effect, the Ti content must be 0.001% or higher. Preferably, the Ti content is 0.002% or higher, and more preferably 0.003% or higher. On the other hand, a high Ti content increases the variation in the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet, thus degrading the material's uniformity. Therefore, the Ti content is set at 0.030% or lower. Preferably, the Ti content is 0.020% or lower, more preferably 0.017% or lower, and even more preferably 0.015% or lower. The contents of S, N and Ti comply with the following Formula (1): Formula (1): [%T¡] - (48 / 14)[%N] - (48 / 32)[%S] < 0, in Formula (1), [%T¡] 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. 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. This allows for the suppression of variations in the amount of fine precipitate along the longitudinal direction of the steel sheet. Since the fine precipitate affects the recrystallization behavior during the annealing process, suppressing the variation in the amount of fine precipitate along the longitudinal direction of the steel sheet can reduce the variation in the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet, thus achieving excellent material uniformity. 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] 3 3 (48 / 32)[%S]”, although not specifically limited, is preferably -0.01 or higher, in order to suppress inclusion production that may possibly be attributed to excessive N and deS content. The steel sheet of the present invention contains the aforementioned components, and the remainder, which is not the aforementioned components, has a chemical composition 35 containing Fe (iron) and an unavoidable impurity. Now, the steel sheet of the present > s The 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 their content is less than the lower limit value, is understood to be contained as an unavoidable impurity. Any of, or two or more of: Cr: 0.01% or more and 0.15% or less; Mo: 0.01% or more and 0.10% or less; 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 production effect with a predetermined area fraction, even if the manganese (Mn) content is low. To obtain this effect, the B content is preferably 0.0001% or more. More preferably, it is 0.00015% or more. On the other hand, 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 less. 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 and nickel demonstrate effects of improving corrosion resistance in the automotive environment, and of suppressing hydrogen penetration into the steel sheet, by covering the surface of the steel sheet with corrosion products. In order to achieve the minimum required level of corrosion resistance for automotive use, the copper (Cu) and nickel (Ni) contents are preferably 0.001% or more, and more preferably 0.002% or more. 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, an acceptable content of each of these elements being 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 of ferrite, 0% or more and 70% or less of martensite, and less than 20% in total of pearlite, bainite, and retained austenite. The ferrite contains, in terms of area fraction relative to a complete microstructure, 0% or more and 10% or less of unrecrystallized ferrite, with a difference between the maximum and minimum area fractions of unrecrystallized ferrite in the longitudinal direction of the steel sheet of 5% or less. 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 the precipitate to form completely, thereby improving strength through a synergistic effect of structural hardening due to the martensite and precipitation hardening due to the precipitate. 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% as long as a sufficient level of strength can be achieved by precipitation hardening with the aid of the fine precipitate. However, since a large ferrite area fraction tends to increase the variation in the amount of fine precipitate in the longitudinal direction of the steel sheet, the ferrite area fraction is preferably 95% or less, and more preferably 90% or less. 3 The area fraction of martensite is 0% or greater and 70% or less With the martensite area fraction exceeding 70% relative to the total microstructure, the strength will be excessive. This also increases the amount of precipitate produced in the ferrite to suppress recrystallization, thus increasing the variation in the unrecrystallized ferrite area fraction along the longitudinal direction of the steel sheet and degrading the material's uniformity. Therefore, the martensite area fraction, relative to the total microstructure of the steel, is specified to be 70% or less. The martensite area fraction is preferably 65% ​​or less, and more preferably 60% or less. The lower limit for 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 fine precipitates.The martensite area fraction is preferably 5% or greater and more preferably 10% or greater, from the point of view of suppressing the variation of the unrecrystallized ferrite area fraction, by suppressing the variation of the amount of fine precipitate in the longitudinal direction of the steel sheet, as previously suggested. The remainder other than ferrite and martensite includes retained austenite, bainite, and pearlite, and is acceptable if its area fraction represents 20% or less. The area fraction of the remainder 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 results from austenite at relatively low temperatures (at or below the martensite transformation temperature). 3 above it), in which fine carbide is dispersed in the ferrite in the form of needles or plates. Pearlite refers to a microstructure that develops from austenite and is composed of lamellar ferrite and cementite. Retained austenite occurs as a result of the decrease in the transformation temperature of martensite to austenite to or below room temperature due to the concentration of C or another element in the austenite. 5 Ferrite contains 0% or more and 10% or less of unrecrystallized ferrite, in terms of area fraction relative to the whole microstructure Non-recrystallized ferrite in the context of the present invention refers to a ferrite particle containing a sub-boundary within the crystal grain. The sub-boundary can be observed by a method described later in the Examples. Figure 1 is a cross-sectional view of a steel sheet of the present invention taken along the thickness direction, observed practically under a scanning electron microscope. In Figure 1, an illustrative site where non-recrystallized ferrite resides is surrounded by a dashed line, where the non-recrystallized ferrite contains a sub-boundary within the crystal grain. Unrecrystallized ferrite, which recrystallizes during annealing to become ferrite, can cause variation in the recrystallization rate along the longitudinal direction of the steel sheet, and degradation of the material's uniformity, if its area fraction relative to the entire microstructure exceeds 10%. By controlling the unrecrystallized ferrite area fraction relative to the complete microstructure to 10% or less, the variation in recrystallization can be suppressed, and therefore the variation in the yield ratio can be reduced. Therefore, of the ferrite area fraction, the unrecrystallized ferrite area fraction relative to the entire microstructure should be 10% or less, preferably 9% or less, and more preferably 8% or less. The lower the amount of unrecrystallized ferrite, the better; it can even be 0%. The values ​​of the area fraction of the individual structures in the 10 microstructure of the steel employed here are those obtained by measurement in accordance with the methods described later in the Examples. The difference between the maximum and minimum values ​​of the fraction of the unrecrystallized ferrite area in the longitudinal direction of the steel sheet is 5% or less. Since the fraction of uncrystallized ferrite area directly affects strength, suppressing the variation in the amount of fine precipitate along the longitudinal direction of the steel sheet can reduce the variation in the fraction of uncrystallized ferrite area, resulting in excellent material uniformity. To achieve this effect, the difference between the maximum and minimum values ​​of the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet is specified as 5% or less. The difference is preferably 4% or less, and more preferably 3% or less. The lower limit of the difference is not specifically limited and may even be 0%. The “difference between the maximum and minimum values ​​of the fraction of unrecrystallized ferrite area in the longitudinal direction of the specified steel sheet to 5% or less” in the context of the present invention means that the difference between the maximum and minimum values ​​of the fraction of unrecrystallized ferrite area is 5% or less, along the entire length of the longitudinal (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 plating layer on its surface. The plating layer is normally an electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvanized and annealed 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 > s Nc NNC 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 or 980 MPa, from the point of view of easy equilibrium 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 (YR) 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 method for manufacturing 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 external temperature of the plate (the raw steel material), sheet, or similar material, unless otherwise specified. <Proceso de laminación en caliente> A hot rolling process is a process in which a steel plate having the chemical composition described above is heated to a heating temperature T (°C) that complies with 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 finish delivery temperature of 850 °C or more, then cooled from the finish delivery temperature to 650 °C or less at an average cooling rate of 10 °C / s faster, and then coiled at 650 °C or less. Formula (2): 0.80 χ (2.4 - 6700 / T) <Ξ log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 6700 / T) In Formula (2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content (% by mass) of the component element Nb, [%C] represents the content (% by mass) of the component element C, and [%N] represents the content (% by mass) of the component element N. If the heating temperature of the steel sheet is low, an excess of Nb-containing carbonitride is produced during heating, resulting in a higher amount of Ti than the combined amounts of N and S, thus degrading the material's uniformity. Conversely, if the heating temperature is high, a large amount of precipitate forms during winding, making it impossible to control the variation in the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet, further degrading the material's uniformity. Therefore, the heating temperature of the steel sheet is determined to comply with Formula (2). The heating temperature T (°C) of the steel sheet preferably complies with Formula (2A) below, and more preferably with Formula (2B) below. Formula (2A): 0.79 χ (2.4 - 6700 / T) < Log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.67 χ 10 (2.4-6700 / T) Formula (2B): 0.78 χ (2.4 - 6700 / T) < Log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.70 χ (2.4 - 6700 / T) The soaking time is specified as 1.0 hour or more. A soaking time of less than 1 hour is insufficient for the Nb- and Ti-containing carbonitrides to fully dissolve, so the Nb-containing carbonitride will remain in excess during the heating of the plates. Therefore, the amount of Ti will be greater than the total amount of N and S, which will degrade 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 for soaking time, although not specifically limited, is usually 3 hours or less. 3 The heating rate when heating a molten steel plate to the plate heating temperature, although not specifically limited, is preferably controlled at 5 to 15 °C / min. The average cooling rate from the plate heating temperature to the lamination start temperature is 2°C / s faster 5. 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, so the amount of Ti will be greater than the total amount of N and S during winding, which will degrade the uniformity of the material. Therefore, the average cooling rate from the plate heating temperature to the rolling start temperature The rolling start temperature is specified as 2 °C / s faster. The average cooling rate is preferably 2.5 °C / s faster, and more preferably 3 °C / s 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 as 1000 °C / s slower, from the point of view of energy savings in the cooling system. iviA / a / ¿u¿¿ / uui i ou > s N c NNC The finish delivery temperature is 850 °C or higher 5 —I If the delivery temperature for the finished product is below 850 °C, a longer cooling time is required, 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 fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet increases, and the uniformity of the material is degraded. Therefore, the delivery temperature for the finished product is specified at 850 °C or higher. The delivery temperature for the finished product is preferably 860 °C or higher. Meanwhile, the upper limit of the delivery temperature for the finished product, 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 650 °C or lower If the winding temperature exceeds 650 °C, a large amount of precipitate is produced as a result of the winding process. This makes it impossible to suppress the variation in the fraction of unrecrystallized ferrite area along the longitudinal direction of the steel sheet, thus degrading the material's uniformity. Therefore, the winding temperature is 650 °C or lower, and preferably 640 °C or lower. While not specifically limited, the lower limit for the winding temperature is preferably 400 °C or higher, and more preferably 420 °C or higher, to obtain the precipitate that contributes to precipitation hardening. The average cooling rate from the finished supply temperature to the winding temperature is 10 °C / s faster If the average cooling rate from the finished 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 fraction of unrecrystallized ferrite area in the longitudinal direction of the steel sheet, and degrades the uniformity of the material. Therefore, the average cooling rate from the finished delivery temperature to the winding temperature is specified to be 10 °C / s faster. The average cooling rate is preferably 20 °C / s faster, and more preferably 30 °C / s 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. > s N c NNC Hot-rolled coiled steel sheet can be pickled. The pickling conditions are not specifically limited.<Proceso de laminación en frío> 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 higher, from the standpoint of improving surface flatness and also making the microstructure more uniform. The upper limit of the reduction ratio, although not specifically limited, is preferably 95% or lower, 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 comply with 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 Aci or higher and (the transformation temperature Acs + 20 °C) or lower, at an average heating rate of 600 °C to 700 °C of 8 °C / s slower, held at the annealing temperature for a holding time t (second) that complies with the following Formula (3), and then cooled. Formula (3):1500 < (AT + 273) x logt < 5000 In Formula (3), AT represents the annealing temperature (°C), and t represents the holding time (second) at the annealing temperature. The average heating rate from 600°C to 700°C is 8°C / s slower The recrystallization temperature falls within the range of 600 °C to 700 °C, so the average heating rate within this temperature range must be slow to promote recrystallization. If the average heating rate from 600 °C to 700 °C exceeds 8 °C / s, the amount of unrecrystallized ferrite increases, thus altering the recrystallization rate along the longitudinal direction of the steel sheet and degrading the material's uniformity. Therefore, the average heating rate from 600 °C to 700°C is specified as 8°C / s slower. The average heating rate is preferably 7°C / s slower, and more preferably 6°C / s slower. The lower limit of the average heating rate, although not specifically limited, is usually 0.5 °C / so greater. The annealing temperature is transformation temperature Aci or higher and (transformation temperature Aca + 20 °C) or lower. If the annealing temperature is below the Ac-i transformation temperature, the fine precipitate that can be produced during annealing is less likely to form due to cementite production, making it difficult to obtain the necessary amount of fine precipitate to achieve adequate strength. Furthermore, recrystallization is suppressed, so the variation of the unrecrystallized ferrite area fraction in the longitudinal direction of the steel sheet cannot be controlled, thus degrading the material's uniformity. Therefore, the annealing temperature is specified to be the Am transformation temperature or higher. The annealing temperature is preferably (Aci transformation temperature + 10 °C) or higher, and more preferably (Aci transformation temperature + 20 °C) or higher.On the other hand, if the annealing temperature is higher than (transformation temperature Aca + 20 °C), the martensite area fraction becomes greater than 70%, leading to excessive strength. This also increases the amount of precipitate produced in the ferrite to suppress recrystallization, thereby increasing the variation in the unrecrystallized ferrite area fraction in the longitudinal direction of the steel sheet and degrading the material's uniformity. Therefore, the annealing temperature is specified to be (transformation temperature Aca + 20 °C) or lower. The annealing temperature is preferably (transformation temperature Acs + 10 °C) or lower, and more preferably a transformation temperature Acs or lower. Note that the transformation temperature Aci and the transformation temperature Ac3 are calculated using the following formulas. Note also that the element symbol %) represents the mass percentage of each element in the following formulas. Aci (°C) = 723 + 22[%Si] - 18[%Mn] + 17[%Cr] + 4.5[%Mo] + 16[%V] Ac3 (°C) = 910 - 203^[%C] + 45[%S¡] - 30[%Mn] - 20[%Cu] - 15[%NI] + 11 [%Cr] + 2 5 32[%Mo] + 104[%V] + 400[%Ti] + 460[%AI] The holding time t (second) at the annealing temperature AT (°C) complies with Formula (3). A short holding time at the annealing temperature makes the reverse transformation to austenite less likely, so the fine precipitate that can occur during annealing is less likely to form due to cementite production, making it difficult to obtain the necessary amount of fine precipitate to achieve adequate strength. On the other hand, a long holding time at the annealing temperature increases the amount of precipitate produced in the ferrite, thus suppressing recrystallization, increasing the variation of the unrecrystallized ferrite area fraction in the longitudinal direction of the steel sheet, and degrading the uniformity of the material. Therefore, the holding time t (seconds) at the annealing temperature AT (°C) satisfies Formula (3).The holding time t (second) at the annealing temperature AT (°C) preferably meets the following Formula (3A), and more preferably the following Formula (3B). Formula (3A): 1600 < (AT + 273) χ logt < 4900 Formula (3B): 1700 < (AT + 273) χ logt < 4800 The cooling rate during cooling after holding at the annealing temperature is not specifically limited. Note that hot-rolled steel sheet, after the hot rolling process, can undergo heat treatment to soften its microstructure. It should also be noted that the annealing process may be followed by hot rolling to control the shape. The annealing process may be followed by a plating process, provided that the properties of the steel sheet remain unchanged. Plating consists, for example, of subjecting the surface of the steel sheet to electrogalvanizing, hot-dip galvanizing, or hot-dip galvanizing and annealing. 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. 3. Next, the weight of the plating is checked, usually by gas purging. The steel sheet after hot-dip galvanizing can be alloyed. When alloying the hot-dip galvanized layer, it is preferably done in the temperature range of 450 °C or higher and 580 °C or lower, holding it for 1 second or more and 60 seconds or less. When subjecting the surface of the steel sheet to electrogalvanizing, the conditions of the 5 processes can be adjusted to those of any of the conventional methods, without any particular limitation. In accordance with the aforementioned manufacturing method of this modality, it is now possible, by controlling the hot rolling conditions and the annealing temperature and time, to suppress variations in the microstructure ratio and in the fraction of the unrecrystallized ferrite area in the longitudinal direction of the steel sheet, and to obtain the high-strength steel sheet that excels in 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 may be based on any of the common forming methods, such as press working, without limitation. The welding process may 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 3 Each steel with the chemical composition indicated in Table 1, and the remainder including Fe and unavoidable impurities, was melted in a vacuum melting furnace and coated to obtain a coated material 27 mm thick. The coated material thus obtained was hot-rolled to a thickness of 4.0 mm. The hot-rolling process conditions are summarized in Table 2. Below, a sample of each sheet of Five hot-rolled steel sheets, intended for subsequent cold rolling, were milled to reduce their 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 No. 55 Sample No. 3 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, after annealing, was hot-dip galvanized and then hot-annealed. Sample No. 57 in Table 2 is a steel sheet whose surface, after annealing and subsequent cooling to room temperature, was electro-galvanized. Note that the blank cells in Table 1 represent items that were not intentionally added, but were not always 0% by mass, occasionally allowing for 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 the iron heating temperature calculated from Formula (2)” represents the values ​​calculated using Formula (2-1), a part of Formula (2). Again in Table 2, “2: upper limit of the iron heating temperature calculated from Formula (2)” represents the values ​​calculated using Formula (2-2), a part of Formula (2). Formula (2): 0.80 χ (2.4 - 6700 / T) < log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 6700 / T) Formula (2-1): log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 - 6700 / T) Formula (2-2): 0.80 χ (2.4 - 6700 / T) < log{[%Nb] χ ([%C] + 12 / 14[%N])} In Formula (2), Formula (2-1) and Formula (2-2), T represents the heating temperature (°C) of the steel plate, [%Nb] represents the content of the component element Nb (% by mass), [%C] represents the content of the component element C (% by mass) and [%N] represents the component element N (% by mass). 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Evaluation methods The microstructures of the 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 the tensile properties, including tensile strength. The methods for individual assessments are as follows. (Fractions of the area of ​​ferrite, martensite and unrecrystallized ferrite) Test samples were taken individually from a portion of the front end, a central portion, and a portion of the rear end in the longitudinal (rolling direction) of the steel sheet, in the rolling direction. L-shaped cross-sections were taken in the thickness direction and parallel to the rolling direction with a mirror finish. The front, central, and rear portions in the longitudinal (rolling direction) of the steel sheet, where the samples were taken, were individually centered in the width direction. The cross-sections taken in the thickness direction were pickled with a nital solution to expose the microstructure and then observed using a scanning electron microscope (SEM).The area fractions of ferrite, martensite, and unrecrystallized ferrite were examined using the dot-counting method, in which a 16 x 15 mesh with a 4.8 pm interval was superimposed on an area of ​​82 pm x 57 pm of actual length in a 1500 SEM image, and the number of mesh points falling within the individual phases was counted. Each area fraction was determined by a mean of three area fraction values ​​obtained from independent 1500 SEM images. The ferrite and martensite area fractions in the present invention were given by values ​​determined in the central portion in the longitudinal direction of the steel sheet. The unrecrystallized ferrite area fraction was given by the difference between the maximum and minimum values ​​of the measured values ​​obtained at the three points, which are the front, the central portion, and the back portion.The ferrite and unrecrystallized ferrite microstructures are black, and the martensite microstructure is white. The unrecrystallized ferrite has white subboundaries within its crystal grain. The fraction of the remaining area, other than ferrite and martensite, was calculated by subtracting the fraction of the total area of ​​ferrite and martensite from 100%. In the present invention, the equilibrium area was considered to represent the fraction of the total area of ​​pearlite, bainite, and retained austenite. The equilibrium area fraction is given in the column titled "Other" in Table 3. Note that the measurement at the front end, 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 at the rear end, in the longitudinal direction of the steel sheet 35, 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 unrecrystallized ferrite area fraction values, measured individually on the front, middle, and back portions in the longitudinal (rolling direction) direction of the steel sheet, was designated as “the maximum and minimum value difference 5 of the unrecrystallized ferrite area fraction in the longitudinal direction of the steel sheet.” 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 tends to be faster at the leading and trailing ends along the longitudinal direction of the steel sheet. Therefore, fine precipitate tends to be less abundant in the central portion along the longitudinal direction of the steel sheet, and unrecrystallized ferrite tends to be less abundant. Conversely, fine precipitate tends to be more abundant at the leading and trailing ends along the longitudinal direction of the steel sheet, and unrecrystallized ferrite tends to be more abundant.Therefore, the measured value obtained at the front end or the rear end in the longitudinal direction of the steel sheet, whichever is greater, was assumed to be the maximum value. Meanwhile, the measured value obtained in the middle portion in the longitudinal direction of the steel sheet was assumed to be the minimum value. Thus, in the present invention, the difference between the maximum and minimum values ​​of the fraction of the unrecrystallized ferrite area in the longitudinal direction of the steel sheet can be given by the difference between the maximum and minimum values ​​of the measured values ​​obtained at three points, which are the front end, the middle portion, and the rear end in the longitudinal (rolling direction) direction of the steel sheet. (Traction test) 5. 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 to the rolling direction and subjected to tensile testing at a pulling rate of 10 mm / min, in accordance with the requirements of JIS Z 2241 (2011). The tensile strength (indicated as TS in Table 3) and yield strength (indicated as The tensile strength (TS) and yield strength (YS) values ​​summarized in Table 3 were obtained by measuring each sampled specimen of the steel sheet at its center in both the longitudinal (rolling direction) and width directions. (Uniformity of the material) The aforementioned tensile test was performed individually on the front, middle, and rear sections in the longitudinal direction of the steel sheet, and the material uniformity was evaluated based on the difference (denoted as AYR in Table 3) between the maximum and minimum values ​​of the yield ratio (YR) measured in these three sections. The yield ratio (YR) was calculated by dividing YS by TS. Note that the five 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 carried out at a position 1 m from the front end toward the middle section.Furthermore, the measurement in the present invention on the rear end part 10 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. Table 3 ινΐΛ / a / zuzz / uu ii ου ω <D c O c •o o c O) c c •o c 4) c invención | c •o u c 4) c o S <6 n invención | invención | invención | o 2 fü Ω invención | invención I invención | o >S Ω invention | invention | invention | or > 2 Ω invention | invention | invention | invention | invention | invention IO > 2 Ω or 2 re Ω invention | invention | invention | invention I invention | invention | invention | O > 2 rü Ω invention I 0 Π3 £<D (Λ Λ O J2 4) Ό O •a E 4> UJ 1 Example of the Example of the । J2 4) Ό O a E 4> UJ E ooo QE 4) UJ I Example of the । Example of the Example of the i E ooo Ω E 4) UJ | Example of the I Example of the । Example of the E ooo QE 4) UJ Example of the । Example of the । Example of the । E ooo Ω E 4) iu Example of the i Example of the । I Example of the Example of the । I Example of the । I Example of the i E ooo Ω E 4) UJ E ooo Ω E 4) UJ I Example of the i Example of the । | Example of the I Example of the । Example of the I Example of the i Example of the । E ooo QE 4) UJ 1 Example of the । micas 1«AV CM OO 001 | oo ni oo δ o 0 05 | 0 02 | ooo δ o O o 0 03 I 0 02 | CO OO 0 05 | oo 0 02 | 0 06 I 0 05 | 0 02 | 0 05 I 0 02 | 0 02 | 0 03 I | 90 0 | lio 0 05 | 0 03 | O o 0 02 I 001 | ni oo 0 03 | | 90 0 0 03 1 o O) E ω 4) Ό 1 TS 1 | MPa | CO en co I sw | I 636 | UT) 3 1 602 1 I 602 | I 604 | 1 601 | And *24 | I 224 | And t24 | I 521 | I 705 | I 706 | | 669 | I 697 | I 596 II 592 | I 593 | And tes | I 823 | I 428 | I 826 II 824 II 630 | I 629 | I 630 | I 626 | I 648 II 650 | And 6*9 | 1,748 | 1 595 | 1 601 I Propia vs 1 MPa | ¢0 en 503 I 509 | CM 5 466 1 1 *s* 1 29* IMI SW 545 | 535 II 84* I *95 539 | 552 | I ISS I tst I *5* I es* 451 | I 4SS 642 | 653 I 634 | I 86* 484 | 499 | 501 | I 215 520 | I 88* 580 | 1 40V I *2* CJ 5? en ni CO O r- ni — ni CO 'T CJ ni CO CO CM CM CO UO ni — CM rj CO O x ni ni x 'T ni ni CO x Otros 1 «O θ' un X- tn x- UD CN o r- CN CN CN o ΓN o CO o CN 't r- n- X- o CO X- (N CN 'TO X- en un o O Π3 jnr licroestruc 2 s? 3 I 26 I 23 CM 1 20 1 20 3 CO X“ I39 OO os ΙΛ CO CO CO *£ I x— x- x- I 43 5* II 46 I 46 *2 II 23 I 23 3 o en x— en o co en σ> r en CO CO CO X“ CO X CM X- CO ΓΝ CM CN X“ σ> CM CN CO CO en un r- (N CN r- CO xx X en x— en en (NX“ r- x- I 84 1 ZL 1 54 1 84 1 64 I 54 59 | 58 | 58 IO CO 65 | 65 | I 29 60 | 83 I CO I 28 80 | 50 | I *s I *5 Xun I 54 I 54 I 54 I 24 o 68 | 80 | 1 58 1 46 σ> Tipo de o ü υ < CO OQ UJ u_ 0 I Núm. — ni CO un CO r- CO O) O x— CO UD CO r- CO σ> I 20 I rj rj I 23 I 3 I 25 | I 26 | δ I 82 | 1 29 1 I 30 | CO 1 32 | 1 88 | I *ε I Example of the invention Comparative example | I Comparative example I Example of the invention | Example of the invention | Comparative example | Comparative example | Example of the invention | I Example of the invention I Example of the invention | Comparative example | Comparative example | Comparative example | Comparative example | I Comparative example I Comparative example | Comparative example | Comparative example | Comparative example | Comparative example | 1 Example of the invention I Example of the invention | Example of the invention | Comparative example | co oo I 0 07 II 0 07 | co oo co oo | 60 0 | I 0 07 | | 0 02 | O <z>| 0 02 | θ o | 0 02 | | 0 06 | I 0 02 I | 0 06 | I 0 07 I | 0 06 | | 0 08 | <z>1 0 02 1 | 0 02 | | 0 02 | | 0 06 | 647 un I 659 I | 099 | | 663 | I 661 | | 650 | | 646 | I II 621 | | 826 | I 576 | I 677 I | 853 | I 469 I | 639 | I 639 | I 651 | | 668 | | 668 | 1 635 1 1 651 | | 640 | I 612 | un I 487 II 9St | 511 | un | 449 | I 504 | $ co I 411 I | 663 | | 460 | I 521 | | 9S9 | I 326 II 436 | I 498 | I 520 | I lis 1 534 1 1 tos 1 I 513 | I 498 | un s oí r- co CO un CO CO OI 04 OI — CO CO O ·* O- CO CO o> en en en r- θ r- co CO o T- 04 CO en un 04 O 04 OI m co en un i martensita I 65 | I 29 II 29 | I rt | en T” en 29 | I 28 I 48 | I 40 | 59 | W en I 29 | en 36 | 36 | o <n I 28 | 04 del área de la OI en r- 04 04 en 04 04 en 04 04 OI en un Fracción 69 28 | S9 68 | 68 | 68 | 67 67 69 I 89 | 47 79 60 | ín 65 I 69 | 70 67 CO 60 | 64 1 64 | co : la ferrita a de E "5 o Q. σ OH en 1— oo ;ción del; 35 36 37 38 39 40 oj co 44 un 46 47 48 o> 50 íñ 52 53 54 55 56 57 ss Frac Ó Other: Fraction of the total area of ​​pearlite, bainite and retained austenite *1: Fraction of the area of ​​unrecrystallized ferrite in relation to the total microstructure *2: Difference between the maximum and minimum value of the fraction of the area of ​​unrecrystallized ferrite in the longitudinal direction of the steel sheet In this modality, steel sheets with a TS of 590 MPa 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 of the 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 of the invention. It was confirmed that, since the high-strength steel sheet of this example of the invention is well-balanced between high strength and material uniformity, the high-strength member obtained using the high-strength steel sheet of this example of the invention can maintain good shape, and that the steel sheet is suitable for automotive structural members.< / z> < / z>

Claims

1. A high-strength steel sheet having a chemical composition in % by mass containing: 5 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, 10 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 meeting formula (1) below, 15 an equilibrium being Fe an unavoidable impurity, comprising, in terms of area fraction relative to a full steel microstructure, 30% or more and 100% or less than ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite, and the ferrite containing in terms of area fraction relative to a complete microstructure, 0% or more and 1023% or less unrecrystallized ferrite,with a difference between a maximum and a minimum value of the area fraction of unrecrystallized ferrite in a longitudinal direction of the steel sheet of 5% or less: Formula (1): [%Ti] - (48 / 14)[%N] - (48 / 32)[%S] < 0, in Formula (1), [%Ti] represents the content (in % mass) of component element Ti, [%N] represents the content (in % mass) of component element N, and [%S] represents the content (in % mass) of component element S.

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

3. The high-strength steel sheet according to claim 35 1 or 2, wherein the chemical composition further contains, in % by mass, Β: 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: 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 plating 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. 15 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 starting temperature of 2 3 lamination at an average cooling rate of 2 °C / s faster, then finished laminating to a final delivery temperature of 850°C or more, then cooled from the final delivery temperature to 650 °C or less at an average cooling rate of 10 °C / s faster, and then wound at 650 °C or less; and an annealing process in which the hot-rolled steel sheet obtained 2 5 in the hot rolling process is heated to an annealing temperature which is the transformation temperature Am or higher and (the transformation temperature Acs + 20°C) or lower, at an average heating rate of 600 °C to 700 °C of 8 °C / so slower, held at the annealing temperature for a holding time t (second) that satisfies the following formula (3), and then cooled: 3 3 Formula (2): 0.80 * (2.4 - 6700 / T) < log{[%Nb] x ([%C] + 12 / 14[%N])} < 0.65 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 < 5000, in formula (3), AT represents the annealing temperature (°C), and t represents the holding time (seconds) at the annealing temperature.

5.

8. 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) 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 higher, then cooled from the final delivery temperature to 650 °C or lower at an average cooling rate of 10 °C / s faster, and then coiled at 650 °C or lower; a cold rolling process in which the hot-rolled steel sheet 15 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 Aci or higher and (the transformation temperature Aca + 20 °C) or lower, at an average heating rate of 600 °C to 700 °C of 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): 0.80 χ (2.4 - 6700 / T) < log{[%Nb] χ ([%C] + 12 / 14[%N])} < 0.65 χ (2.4 6700 / T), in Formula (2), T represents the heating temperature (°C) of the steel sheet, [%Nb] represents the content (in % of 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 < 5000, in formula (3), AT represents the annealing temperature (°C), yt 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 plating process to provide plating, following 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.