Steel sheet, steel strip, and methods for producing same

Abstract

The present invention provides a steel sheet, a steel strip, and methods for producing the steel sheet and the steel strip, the steel sheet having a TS of 590-1180 MPa and having little variation in mechanical characteristics (YS, TS, El, λ), i.e., having exceptional stability of mechanical characteristics.　This steel sheet has a component composition containing specific amounts of C, Si, Mn, P, S, Al, and N, and includes, by area ratio, 30.0-90.0% of soft phases and 10.0-70.0% of hard phases. From among the hard phases, a hard phase for which [C content of hard phase] / [C content of steel sheet] is 1.50 or greater represents 65% or more, and a hard phase for which [Mn content of hard phase] / [Mn content of steel sheet] is 1.30 or less represents 10% or more.

Description

Steel plate, steel strip and their manufacturing method

[0001] The present invention relates to a steel sheet and a steel strip that are used in various applications such as automobiles and home appliances, and that have excellent mechanical properties and stability of mechanical properties, which contribute to improving yields in the production of structural members in particular, and to a method for producing the same.

[0002] CO2 emissions from improved fuel efficiency due to thinner and lighter steel sheets used in automobile bodies ２ In order to achieve both reduction in CO emissions and improvement in collision safety, efforts are being made to increase the strength of steel sheets for automobiles. ２ New strict regulations on emissions and crashworthiness are being introduced one after another, and as a result, there has been an increase in the use of high-strength steel sheets, particularly high-strength steel sheets with a tensile strength (hereinafter simply referred to as TS) of 590 MPa or more, for the main structural members and reinforcing members (hereinafter also referred to as automotive frame structural members) that are assembled to the framework of an automobile cabin in order to increase the strength of the automobile body.

[0003] High-strength steel sheets with TS ranging from 590 MPa to 780 MPa are increasingly being used for automotive structural components, such as front and rear side members, to absorb impact energy. High-strength steel sheets with TS of 980 MPa or higher are increasingly being used for cabin-related structural components for occupant protection, such as center pillars. Impact energy-absorbing components require improvements in YS, which correlates with impact energy absorption performance, and in deformation performance (El), which allows the steel to deform bellows-like and absorb energy without cracking (fracturing) during a collision. Furthermore, cabin-related structural components are becoming increasingly strong, with 980 MPa-class high-strength steel sheets being increasingly used. Therefore, in addition to the aforementioned YS and TS, improvements in El, which represents the in-plane press formability of the steel sheet, are also required. Furthermore, improvements in λ, which represents the press formability of the steel sheet edges, are also required.

[0004] From the viewpoint of design (designability) and for the purpose of improving load-bearing capacity and body rigidity to ensure collision safety, the skeletal structural members of automobile bodies are becoming complex in shape. In order to manufacture automobile bodies without defects in press forming of such complex shapes, it is important to reduce the variation in the mechanical properties (YS, TS, El, λ) of the steel sheet (improving the stability of the mechanical properties). Therefore, there is a demand for the development of steel sheets with excellent stability of mechanical properties.

[0005] As an example of a steel sheet that serves as a material for such automotive components, Patent Document 1 discloses a method for producing a high-strength cold-rolled steel sheet having a composite structure mainly composed of ferrite and tempered martensite in a continuous annealing line, in which the continuous annealing line sequentially undergoes a heating process, a slow cooling process to a quenching start temperature Tq, a quenching process by rapid cooling, and a tempering process by reheating. The ferrite fraction Vf of the steel sheet is measured by magnetic properties immediately after the quenching process, and compared with a predetermined target value Vf0 of the ferrite fraction necessary to achieve target values ​​of the mechanical properties of the product steel sheet. The quenching start temperature Tq is adjusted so that the deviation ΔVf from the target value Vf0 of the ferrite fraction ΔVf = Vf - Vf0 approaches 0. This method is characterized by adjusting the quenching start temperature Tq. This method is capable of reducing structural variations associated with fluctuations in the heat treatment temperature in the continuous annealing line and variations in mechanical properties caused by the structural variations.

[0006] Patent No. 5374479

[0007] However, Patent Document 1 discloses a method for producing a high-strength cold-rolled steel sheet having a composite structure mainly composed of ferrite and tempered martensite in a continuous annealing line, in which the continuous annealing line sequentially undergoes a heating step, a slow cooling step to a quenching start temperature Tq, a quenching step by rapid cooling, and a tempering step by reheating. The ferrite fraction Vf of the steel sheet is measured by magnetic properties immediately after the quenching step, and compared with a predetermined target value Vf0 of the ferrite fraction necessary to achieve target values ​​of the mechanical properties of the product steel sheet. The quenching start temperature Tq is adjusted so that the deviation ΔVf from the target value Vf0 of the ferrite fraction ΔVf = Vf - Vf0 approaches 0. However, this method is limited to cold-rolled steel sheets, and there are cases in which the variation in mechanical properties cannot be reduced by controlling only the ferrite fraction.

[0008] For these reasons, the steel sheet and the manufacturing method thereof disclosed in Patent Document 1 cannot necessarily be said to reduce the variations in the mechanical properties (YS, TS, El, λ) of the steel sheet. In other words, there has been a demand for the establishment of a new technology that reduces the variations in the mechanical properties of the steel sheet.

[0009] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a steel plate, a steel strip, and a method for manufacturing the same, which have a TS of 590 MPa or more and less than 1180 MPa, excellent mechanical properties (YS, TS, El, λ), and small variations in the mechanical properties (YS, TS, El, λ), i.e., excellent stability of the mechanical properties.

[0010] In the present invention, a steel strip refers to a product wound into a coil with a mass of 5 t or more and a width of 500 mm or more. The longitudinal length of the steel strip is more than 10 m. A steel plate refers to a portion extracted from the steel strip, and refers to a product with a width of 500 mm or more and a longitudinal length of 10 m or less, preferably less than 1 m.

[0011] Further, in the present invention, being excellent in mechanical properties (YS, TS, El, λ) means that the yield stress (YS) and total elongation (El) measured in a tensile test in accordance with JIS Z 2241 (2011), and the hole expansion ratio (λ) measured in a hole expansion test in accordance with the Japan Iron and Steel Federation Standard JFST 1001 satisfy the following formulas 1 to 3 according to the tensile strength (TS) measured in the tensile test. Equation 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Equation 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Equation 3: 45% ≦ λ when 590 MPa ≦ TS < 780 MPa 30% ≦ λ when 780 MPa ≦ TS < 980 MPa 20% ≦ λ when 980 MPa ≦ TS < 1180 MPa

[0012] Further, in the present invention, a steel sheet having excellent stability of mechanical properties refers to a steel sheet having ΔTS, ΔYS, ΔEl, and Δλ obtained from the differences between the maximum and minimum values ​​of TS, YS, El, and λ at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel sheet, i.e., ΔTS (maximum value of TS−minimum value of TS), ΔYS (maximum value of YS−minimum value of YS), ΔEl (maximum value of El−minimum value of El), and Δλ (maximum value of λ−minimum value of λ), satisfying the following formulas 4, 6, 8, and 10, and also the standard deviations σTS, σYS, σEl, and σλ obtained from TS, YS, El, and λ at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel sheet satisfy the following formulas 5, 7, 9, and 11. Further, in the present invention, a steel strip having excellent stability of mechanical properties is a steel strip in which, at each of the 1 / 4 position, 1 / 2 position and 3 / 4 position in the width direction of the steel strip, the differences between the maximum and minimum values ​​of TS, YS, El and λ at each of the positions of the leading edge 10 m position, the tail edge 10 m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position in the longitudinal direction of the steel strip, i.e., ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔEl (maximum value of El - minimum value of El), Δλ (maximum value of λ - minimum value of λ) satisfy the following formulas 4, 6, 8 and 10, This means that at each of the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel strip, the standard deviations σTS, σYS, σEl and σλ obtained from TS, YS, El and λ at each of the longitudinal positions of the steel strip, i.e., the leading 10 m position, the trailing 10 m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position, satisfy the following equations 5, 7, 9 and 11.Equation 4: When 590 MPa ≦ TS < 780 MPa, ΔTS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔTS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔTS ≦ 60 MPa Equation 5: When 590 MPa ≦ TS < 780 MPa, σTS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σTS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σTS ≦ 30 MPa Equation 6: When 590 MPa ≦ TS < 780 MPa, ΔYS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔYS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔYS ≦ 60 MPa Equation 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σYS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σYS ≦ 30 MPa Formula 8: When 590 MPa ≦ TS < 780 MPa, ΔEl ≦ 4.0% When 780 MPa ≦ TS < 980 MPa, ΔEl ≦ 4.5% When 980 MPa ≦ TS < 1180 MPa, ΔEl ≦ 5.0% Formula 9: When 590 MPa ≦ TS < 780 MPa, σEl ≦ 2.0% When 780 MPa ≦ TS < 980 MPa, σEl ≦ 2.5% When 980 MPa ≦ TS < 1180 MPa, σEl ≦ 3.0% Formula 10: If 590 MPa ≦ TS < 780 MPa, then Δλ ≦ 25% If 780 MPa ≦ TS < 980 MPa, then Δλ ≦ 25% If 980 MPa ≦ TS < 1180 MPa, then Δλ ≦ 25% Equation 11: If 590 MPa ≦ TS < 780 MPa, then σλ ≦ 7% If 780 MPa ≦ TS < 980 MPa, then σλ ≦ 7% If 980 MPa ≦ TS < 1180 MPa, then σλ ≦ 7%.

[0013] The present inventors conducted extensive research to achieve the above-mentioned objectives and have made the following discoveries: (1) By using a predetermined composition, controlling the area fraction of the soft phase (one or more selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) to 90.0% or less and the area fraction of the hard phase (one or more selected from fresh martensite and tempered martensite) to 10.0% or more, a TS of 590 MPa or more can be ensured. (2) By using a predetermined composition, controlling the area fraction of the soft phase (one or more selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) to 30.0% or more and the area fraction of the hard phase (one or more selected from fresh martensite and tempered martensite) to 70.0% or less, a TS of less than 1180 MPa can be achieved. (3) By controlling the structure fraction of the soft phase and the hard phase as described above with a predetermined component composition, the following formulas 1 to 3 can be realized. Equation 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Equation 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Equation 3: 45% ≦ λ when 590 MPa ≦ TS < 780 MPa 30% ≦ λ when 780 MPa ≦ TS < 980 MPa 20% ≦ λ when 980 MPa ≦ TS < 1180 MPa (4) Furthermore, by controlling the area ratio of the hard phases in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel] is 1.50 or more to 65% or more, and the area ratio of the hard phases in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] is 1.30 or less to 10% or more, it becomes possible to reduce the variations in the mechanical properties (YS, TS, El, λ) (improving the stability of the mechanical properties).(5) In the annealing / soaking process, a structure containing ferrite (unrecrystallized ferrite and recrystallized ferrite) and austenite is formed, and then the steel is rapidly heated from the annealing / soaking temperature to a temperature above 10°C (annealing / soaking temperature + 10°C) at an average heating rate of 10°C / sec or more. In this rapid heating process, the amount of diffusion of the diffusion-substitutional element Mn from the ferrite to austenite is small, so a large amount of austenite is generated and grown, mainly due to the diffusion of the interstitial element C. The region where this austenite is generated and grown, mainly due to the diffusion of C, has a high-C, low-Mn composition. Therefore, the final structure formed after the subsequent cooling process becomes a hard phase with a high-C, low-Mn composition. This hard phase with a high-C, low-Mn composition is a new structure realized through the annealing / soaking process and the rapid heating process. By generating an appropriate amount of this hard phase with a high-C, low-Mn composition and controlling the structure fraction of the hard phase and the soft phase, it is possible to manufacture a steel sheet with excellent stability of mechanical properties. That is, ΔTS, ΔYS, ΔEl, and Δλ obtained from the differences between the maximum and minimum values ​​of TS, YS, El, and λ at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate, i.e., ΔTS (maximum value of TS−minimum value of TS), ΔYS (maximum value of YS−minimum value of YS), ΔEl (maximum value of El−minimum value of El), and Δλ (maximum value of λ−minimum value of λ), satisfy the following formulas 4, 6, 8, and 10, and σTS, σYS, σEl, and σλ, which are standard deviations obtained from TS, YS, El, and λ at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate, satisfy the following formulas 5, 7, 9, and 11.Furthermore, the steel strip is such that, at each of the 1 / 4 position, 1 / 2 position and 3 / 4 position in the width direction, the differences between the maximum and minimum values ​​of TS, YS, El and λ at each of the longitudinal positions of the steel strip, i.e., the leading 10 m position, the trailing 10 m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position, i.e., ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔEl (maximum value of El - minimum value of El), Δλ (maximum value of λ - minimum value of λ), satisfy the following formulas 4, 6, 8 and 10, At each of the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel strip, the standard deviations σTS, σYS, σEl and σλ obtained from TS, YS, El and λ at each of the longitudinal positions of the steel strip, 10 m position at the leading end, 10 m position at the tail end, 1 / 4 position, 1 / 2 position and 3 / 4 position, satisfy the following equations 5, 7, 9 and 11.Equation 4: When 590 MPa ≦ TS < 780 MPa, ΔTS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔTS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔTS ≦ 60 MPa Equation 5: When 590 MPa ≦ TS < 780 MPa, σTS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σTS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σTS ≦ 30 MPa Equation 6: When 590 MPa ≦ TS < 780 MPa, ΔYS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔYS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔYS ≦ 60 MPa Equation 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σYS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σYS ≦ 30 MPa Formula 8: When 590 MPa ≦ TS < 780 MPa, ΔEl ≦ 4.0% When 780 MPa ≦ TS < 980 MPa, ΔEl ≦ 4.5% When 980 MPa ≦ TS < 1180 MPa, ΔEl ≦ 5.0% Formula 9: When 590 MPa ≦ TS < 780 MPa, σEl ≦ 2.0% When 780 MPa ≦ TS < 980 MPa, σEl ≦ 2.5% When 980 MPa ≦ TS < 1180 MPa, σEl ≦ 3.0% Formula 10: If 590 MPa ≦ TS < 780 MPa, then Δλ ≦ 25% If 780 MPa ≦ TS < 980 MPa, then Δλ ≦ 25% If 980 MPa ≦ TS < 1180 MPa, then Δλ ≦ 25% Equation 11: If 590 MPa ≦ TS < 780 MPa, then σλ ≦ 7% If 780 MPa ≦ TS < 980 MPa, then σλ ≦ 7% If 980 MPa ≦ TS < 1180 MPa, then σλ ≦ 7%.

[0014] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows: [1] A steel sheet has a component composition containing, by mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, wherein the structure at a 1 / 4 position of the sheet thickness has an area ratio of a soft phase: 30.0% or more and 90.0% or less, and an area ratio of a hard phase: 10.0% or more and 70.0% or less, wherein the hard phase is the area ratio of the hard phase in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet] is 1.50 or more is 65% or more, and the area ratio of the hard phase in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel sheet] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, the following formulas 1 to 3 are satisfied at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel sheet, Furthermore, ΔTS, ΔYS, ΔEl, Δλ, which are the differences between the maximum and minimum values ​​of TS, YS, El, and λ at each of the measurement positions, i.e., ¼ position, ½ position, and ¾ position in the width direction of the steel sheet, respectively, (maximum value of TS−minimum value of TS), (maximum value of YS−minimum value of YS), (maximum value of El−minimum value of El), and (maximum value of λ−minimum value of λ), satisfy the following formulas 4, 6, 8, and 10, respectively; and further, σTS, σYS, σEl, σλ, which are the standard deviations obtained from TS, YS, El, and λ at each of the measurement positions, i.e., ¼ position, ½ position, and ¾ position in the width direction of the steel sheet, respectively, satisfy the following formulas 5, 7, 9, and 11, respectively. The steel sheet has a tensile strength of 590 MPa or more and less than 1180 MPa. Formula 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa; 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa; 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Formula 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa; 17.0% ≦ El when 780 MPa ≦ TS < 980 MPaWhen 980MPa≦TS<1180MPa, 11.0%≦El Equation 3: When 590MPa≦TS<780MPa, 45%≦λ When 780MPa≦TS<980MPa, 30%≦λ When 980MPa≦TS<1180MPa, 20%≦λ Equation 4: When 590MPa≦TS<780MPa, ΔTS≦40MPa When 780MPa≦TS<980MPa, ΔTS≦50MPa When 980MPa≦TS<1180MPa, ΔTS≦60MPa Equation 5: When 590MPa≦TS<780MPa, σTS≦20MPa When 780MPa≦TS<980MPa, σTS≦25MPa When 980MPa≦TS<1180MPa, σTS≦30MPa Equation 6: When 590MPa≦TS<780MPa, ΔYS≦40MPa When 780MPa≦TS<980MPa, ΔYS≦50MPa When 980MPa≦TS<1180MPa, ΔYS≦60MPa Equation 7: When 590MPa≦TS<780MPa, σYS≦20MPa When 780MPa≦TS<980MPa, σYS≦25MPa When 980MPa≦TS<1180MPa, σYS≦30MPa Equation 8: When 590MPa≦TS<780MPa, ΔEl≦4.0% When 780MPa≦TS<980MPa, ΔEl≦4.5% When 980MPa≦TS<1180MPa, ΔEl≦5.0% Equation 9: When 590MPa≦TS<780MPa, σEl≦2.0% When 780MPa≦TS<980MPa, σEl≦2.5% When 980MPa≦TS<1180MPa, σEl≦3.0% Equation 10: When 590MPa≦TS<780MPa, Δλ≦25% When 780MPa≦TS<980MPa, Δλ≦25% When 980MPa≦TS<1180MPa, Δλ≦25% Equation 11: When 590MPa≦TS<780MPa, σλ≦7% When 780MPa≦TS<980MPa, σλ≦7% [2] The composition further contains, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less,Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, [3] The steel sheet according to [1] or [2], which has a plating layer on a surface of the steel sheet, and the plating layer is any one of a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer, and a hot-dip aluminum plated layer. [4] A steel strip having a chemical composition containing, by mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more but less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, wherein the structure at a 1 / 4 position of the sheet thickness of the steel strip has an area ratio of a soft phase: 30.0% or more and 90.0% or less, and an area ratio of a hard phase: 10.0% or more and 70.0% or less, wherein the hard phase is the area ratio of the hard phase in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel strip] is 1.50 or more is 65% or more, and the area ratio of the hard phase in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel strip] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, the following formulas 1 to 3 are satisfied at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel strip, at each of the leading edge 10 m position, the tail edge 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip, and further, at the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the width direction of the steel strip,The differences between the maximum and minimum values ​​of TS, YS, El, and λ at the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position of the longitudinal direction of the steel strip, i.e., (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), respectively, satisfy the following formulas 4, 6, 8, and 10, respectively; and further, at the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel strip, A steel strip in which σTS, σYS, σEl, and σλ, which are standard deviations obtained from TS, YS, El, and λ, respectively, at the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip, satisfy the following formulas 5, 7, 9, and 11, respectively, and the tensile strength is 590 MPa or more and less than 1180 MPa. Equation 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Equation 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Equation 3: 45% ≦ λ when 590 MPa ≦ TS < 780 MPa 30% ≦ λ when 780 MPa ≦ TS < 980 MPa 20% ≦ λ when 980 MPa ≦ TS < 1180 MPa Equation 4: When 590MPa≦TS<780MPa, ΔTS≦40MPa When 780MPa≦TS<980MPa, ΔTS≦50MPa When 980MPa≦TS<1180MPa, ΔTS≦60MPa Formula 5: When 590MPa≦TS<780MPa, σTS≦20MPa When 780MPa≦TS<980MPa, σTS≦25MPa When 980MPa≦TS<1180MPa, σTS≦30MPa Formula 6: When 590MPa≦TS<780MPa, ΔYS≦40MPa When 780MPa≦TS<980MPa, ΔYS≦50MPa When 980MPa≦TS<1180MPa, ΔYS≦60MPa Formula 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPaWhen 780MPa≦TS<980MPa, σYS≦25MPa When 980MPa≦TS<1180MPa, σYS≦30MPa Equation 8: When 590MPa≦TS<780MPa, ΔEl≦4.0% When 780MPa≦TS<980MPa, ΔEl≦4.5% When 980MPa≦TS<1180MPa, ΔEl≦5.0% Equation 9: When 590MPa≦TS<780MPa, σEl≦2.0% When 780MPa≦TS<980MPa, σEl≦2.5% When 980MPa≦TS<1180MPa, σEl≦3.0% Equation 10: When 590MPa≦TS<780MPa, Δλ≦25% when 780MPa≦TS<980MPa, Δλ≦25% when 980MPa≦TS<1180MPa, Δλ≦25% Equation 11: when 590MPa≦TS<780MPa, σλ≦7% when 780MPa≦TS<980MPa, σλ≦7% when 980MPa≦TS<1180MPa, σλ≦7% [5] The chemical composition further contains, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less [6] The steel strip according to [4] above, which contains at least one selected from the group consisting of: a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer, and a hot-dip aluminum plated layer.[7] A hot rolling process is provided in which a steel slab having the chemical composition according to [1] or [2] is hot rolled under the condition of a coiling temperature after finish rolling of 350°C or more and 650°C or less to obtain a hot-rolled steel sheet; an annealing and soaking process is provided in which the hot-rolled steel sheet is heated and annealed and soaked under the conditions of an annealing and soaking temperature of 720°C or more and 860°C or less for a holding time of 20 seconds or more; a rapid heating process is provided in which the steel sheet is rapidly heated from the annealing and soaking temperature to (the annealing and soaking temperature + 10°C) or more under the condition of an average heating rate of 10°C / second or more, and the temperature ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at each of the 1 / 4 position, 1 / 2 position and 3 / 4 position in the width direction of the steel sheet after the rapid heating is 25°C or less; and a cooling process is provided in which the steel sheet is cooled from the reached temperature to 550°C under the condition of an average cooling rate of 4°C / second or more after the rapid heating process. [8] The method for producing a steel sheet according to [7], further comprising: a cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet after the hot rolling step and before the annealing and soaking step; or a plating step of performing any one of hot-dip galvanizing, galvannealing, electrogalvanizing, and hot-dip aluminum plating on the hot rolled steel sheet or the cold rolled steel sheet after the cooling step; or a reheating holding step of cooling the hot rolled steel sheet or the cold rolled steel sheet to a cooling stop temperature of 0°C or higher and 250°C or lower, heating the hot rolled steel sheet to a temperature range of (the cooling stop temperature + 50°C) or higher and 460°C or lower, and holding the temperature range for a tempering time of 10 seconds or higher and 2000 seconds or lower. [9] The method for manufacturing a steel sheet according to [7] above, wherein the ultimate temperature is determined based on information on the austenite fraction of the steel sheet measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

[10] The method for manufacturing a steel sheet according to [7] above, wherein the ultimate temperature is determined based on information on the chemical composition, information on the coiling temperature in the hot rolling step, and information on the austenite fraction of the steel sheet measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

[11] A steel slab having the chemical composition according to [4] or [5] above,The method includes a hot rolling process in which hot rolling is performed under conditions where the coiling temperature after finish rolling is 350°C or higher and 650°C or lower to obtain a hot-rolled steel strip; an annealing and soaking process in which the hot-rolled steel strip is heated and annealed and soaked under conditions of an annealing and soaking temperature of 720°C or higher and 860°C or lower and a holding time of 20 seconds or longer; a rapid heating process in which the hot-rolled steel strip is rapidly heated from the annealing and soaking temperature to (the annealing and soaking temperature + 10°C) or higher at an average heating rate of 10°C / second or higher, and further in which the reached temperature ΔT (maximum reached temperature - minimum reached temperature) at each of the 1 / 4, 1 / 2 and 3 / 4 positions of the width of the steel strip after the rapid heating is 25°C or lower; and a cooling process in which, after the rapid heating process, the hot-rolled steel strip is cooled from the reached temperature to 550°C at an average cooling rate of 4°C / second or higher.

[12] The method for producing a steel strip according to

[11] above, further comprising: a cold rolling step of cold rolling the hot rolled steel strip to obtain a cold rolled steel strip after the hot rolling step and before the annealing and soaking step; or a plating step of applying any one of hot-dip galvanizing, galvannealing, electrogalvanizing, and hot-dip aluminum plating to the hot rolled steel strip or the cold rolled steel strip after the cooling step; or a reheating holding step of cooling the hot rolled steel strip or the cold rolled steel strip to a cooling stop temperature of 0°C or higher and 250°C or lower, heating it to a temperature range of (the cooling stop temperature + 50°C) or higher and 460°C or lower, and holding it in the temperature range for a tempering time of 10 seconds or higher and 2000 seconds or lower.

[13] A method for manufacturing a steel strip according to

[11] , wherein the ultimate temperature is determined based on information on the austenite fraction of the steel strip measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

[14] A method for manufacturing a steel strip according to

[11] , wherein the ultimate temperature is determined based on information on the chemical composition, information on the coiling temperature in the hot rolling step, and information on the austenite fraction of the steel strip measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

[0015] According to the present invention, it is possible to provide a steel plate, a steel strip, and a method for manufacturing the same, which have a TS of 590 MPa or more and less than 1180 MPa, excellent mechanical properties (YS, TS, El, λ), and small variations in the mechanical properties (YS, TS, El, λ), i.e., excellent stability of the mechanical properties.

[0016] Figure 1a is an SEM image showing the soft phases of recrystallized ferrite, unrecrystallized ferrite, and bainitic ferrite, and the hard phase of fresh martensite. Figure 1b is an SEM image showing the soft phase of recrystallized ferrite and the hard phase of fresh martensite. Figure 1c is an SEM image showing the soft phases of recrystallized ferrite, transformed ferrite, and epitaxial ferrite, and the hard phase of fresh martensite. Figure 1d is an SEM image showing the soft phase of bainitic ferrite and the hard phases of fresh martensite and tempered martensite. Figure 2(a) shows the structure (1) of Invention Example No. 2 (Steel B) at the measurement position 8 in Figure 4, as well as the C map (2) and Mn map (3) obtained by EPMA measurement. Figure 2(b) shows Comparative Example No. 2 (Steel B). 4 shows the structure (1) of Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 4 , and the C map (2) and Mn map (3) obtained by EPMA measurement. Figure 3a shows an SEM image ((1), backscattered electron image) of Example No. 2 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 5 , and the results of C line analysis (quantitative) (2) and Mn line analysis (quantitative) (3) obtained by EPMA measurement. Figure 3b shows an SEM image ((1), backscattered electron image) of Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 5 , and the results of C line analysis (quantitative) (2) and Mn line analysis (quantitative) (3) obtained by EPMA measurement. This is a schematic diagram of a steel sheet viewed from the top to illustrate the measurement positions of YS, TS, and El of the steel sheet. This is a schematic diagram of a steel strip viewed from the top to illustrate the measurement positions of YS, TS, and El within the steel strip. Fig. 6a is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), and a reheating / holding step (E). Fig. 6b is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing treatment step (D1), and a reheating / holding step (E). Fig. 6c is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing treatment step (D1), an alloying treatment step (D11), and a reheating / holding step (E).Fig. 6d is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), an electrogalvanizing step (D2), and a reheating / holding step (E). Fig. 6e is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip aluminum plating step (D3), and a reheating / holding step (E).

[0017] The present invention will be described based on the following embodiments.

[0018] [1] Steel plate and steel strip A steel plate according to one embodiment of the present invention contains, in mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance being Fe and unavoidable impurities. The steel plate has a component composition in which the structure at a 1 / 4 position of the plate thickness has an area ratio of a soft phase of: The hard phase has an area ratio of 30.0% to 90.0% and an area ratio of the hard phase is 10.0% to 70.0%. The area ratio of the hard phase is 65% or more, and the ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet] is 1.50 or more. The area ratio of the hard phase is 10% or more, and the ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel sheet] is 1.30 or less. When the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, the following formulas 1 to 3 are satisfied at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel sheet, and further, the differences between the maximum and minimum values ​​of TS, YS, El, and λ at each of the measurement positions of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel sheet (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (λ and ΔTS, ΔYS, ΔEl, and Δλ, which are the sum of the maximum value of TS, YS, El, and λ (minimum value of λ) satisfy the following formulas 4, 6, 8, and 10, respectively; and further, σTS, σYS, σEl, and σλ, which are the standard deviations obtained from TS, YS, El, and λ, respectively, at measurement positions 1 / 4, 1 / 2, and 3 / 4 in the width direction of the steel sheet, satisfy the following formulas 5, 7, 9, and 11, respectively; and the steel sheet has a tensile strength of 590 MPa or more and less than 1,180 MPa.

[0019] Further, a steel strip according to one embodiment of the present invention has a composition containing, in mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and the structure at a 1 / 4 position of the sheet thickness of the steel strip has an area ratio of a soft phase: 30.0% or more and 90.0% or less, and an area ratio of a hard phase: 10.0% or less. % or more and 70.0% or less, and among the hard phases, the area ratio of hard phases in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel strip] is 1.50 or more is 65% or more, and the area ratio of hard phases in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel strip] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, The following formulas 1 to 3 are satisfied at each of the positions of the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position of the steel strip, and further, at the 1 / 4 position, the 1 / 2 position and the 3 / 4 position of the steel strip in the width direction, the differences between the maximum and minimum values ​​of TS, YS, El and λ at each of the positions of the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position of the steel strip in the longitudinal direction are (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El) and (maximum value of λ - minimum value of λ) The steel strip has ΔTS, ΔYS, ΔEl, and Δλ, which satisfy the following formulas 4, 6, 8, and 10, respectively, and further has standard deviations σTS, σYS, σEl, and σλ, which are obtained from TS, YS, El, and λ at the leading edge 10 m position, the tail edge 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip, at the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position, respectively, satisfy the following formulas 5, 7, 9, and 11, respectively, and has a tensile strength of 590 MPa or more and less than 1180 MPa.

[0020] Equation 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Equation 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Equation 3: 45% ≦ λ when 590 MPa ≦ TS < 780 MPa 30% ≦ λ when 780 MPa ≦ TS < 980 MPa 20% ≦ λ when 980 MPa ≦ TS < 1180 MPa Equation 4: When 590MPa≦TS<780MPa, ΔTS≦40MPa When 780MPa≦TS<980MPa, ΔTS≦50MPa When 980MPa≦TS<1180MPa, ΔTS≦60MPa Formula 5: When 590MPa≦TS<780MPa, σTS≦20MPa When 780MPa≦TS<980MPa, σTS≦25MPa When 980MPa≦TS<1180MPa, σTS≦30MPa Formula 6: When 590MPa≦TS<780MPa, ΔYS≦40MPa When 780MPa≦TS<980MPa, ΔYS≦50MPa When 980MPa≦TS<1180MPa, ΔYS≦60MPa Formula 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σYS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σYS ≦ 30 MPa Formula 8: When 590 MPa ≦ TS < 780 MPa, ΔEl ≦ 4.0% When 780 MPa ≦ TS < 980 MPa, ΔEl ≦ 4.5% When 980 MPa ≦ TS < 1180 MPa, ΔEl ≦ 5.0% Formula 9: When 590 MPa ≦ TS < 780 MPa, σEl ≦ 2.0% When 780 MPa ≦ TS < 980 MPa, σEl ≦ 2.5% When 980 MPa ≦ TS < 1180 MPa, σEl ≦ 3.0% Formula 10: When 590 MPa ≦ TS < 780 MPa, Δλ ≦ 25% When 780 MPa ≦ TS < 980 MPa, Δλ ≦ 25% When 980 MPa ≦ TS < 1180 MPa, Δλ ≦ 25% Equation 11: When 590 MPa ≦ TS < 780 MPa, σλ ≦ 7%When 780 MPa ≦ TS < 980 MPa, σλ ≦ 7% When 980 MPa ≦ TS < 1180 MPa, σλ ≦ 7%

[0021] First, the chemical composition of a steel sheet according to one embodiment of the present invention will be described. The chemical composition of a steel strip is also the same as that of the steel sheet. The following description of the chemical composition of the steel sheet can be applied to the chemical composition of the steel strip. Note that the unit of chemical composition is always "mass %", but hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0022] C: 0.030% or more and 0.250% or less. C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, and retained austenite to ensure a TS of 590 MPa or more. Here, if the C content is less than 0.030%, the area ratio of the soft phases of unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite increases excessively, making it difficult to achieve a TS of 590 MPa or more. On the other hand, if the C content exceeds 0.250%, the area ratio of the hard phases of fresh martensite and tempered martensite increases excessively, resulting in a decrease in El. Therefore, the C content is set to 0.030% or more and 0.250% or less. The C content is preferably 0.050% or more. The C content is also preferably 0.130% or less.

[0023] Si: 0.01% or more and 2.50% or less Si promotes the formation of unrecrystallized ferrite and recrystallized ferrite in the soft phase during the temperature rise to the annealing soaking temperature and in the annealing soaking step. That is, Si is an element that affects the area fraction of the soft phase. Here, if the Si content is less than 0.01%, the area fraction of the soft phase decreases, and El decreases. On the other hand, if the Si content exceeds 2.50%, it becomes difficult to form and grow a large amount of austenite in the rapid heating step, in which rapid heating is performed from the annealing soaking temperature to (annealing soaking temperature + 10°C) or more at a heating rate of 10°C / sec or more. Therefore, it is not possible to achieve an area ratio of 65% or more of the hard phases in which the ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel] is 1.50 or more, and an area ratio of 10% or more of the hard phases in which the ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] is 1.30 or less. This makes it difficult to reduce the variation in the mechanical properties (YS, TS, El, λ) of the steel sheet (improving the stability of the mechanical properties). Therefore, the Si content is set to 0.01% or more and 2.50% or less. The Si content is preferably 0.10% or more. The Si content is also preferably 1.80% or less.

[0024] Mn: 1.30% or more and less than 3.50% Mn is an element that can adjust the area ratio of fresh martensite and tempered martensite in the hard phase. Here, if the Mn content is less than 1.30%, the area ratios of unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, and epitaxial ferrite in the soft phase increase, making it difficult to achieve a TS of 590 MPa or more. On the other hand, if the Mn content is 3.50% or more, the martensitic transformation start temperature (hereinafter simply referred to as the Ms point or Ms) decreases, making it difficult to obtain a hard phase with a high C / low Mn composition. This makes it difficult to reduce the variation in the mechanical properties (YS, TS, El, λ) of the steel sheet (improving the stability of the mechanical properties). Therefore, the Mn content is set to 1.30% or more and less than 3.50%. The Mn content is preferably 1.60% or more. The Mn content is also preferably 3.00% or less.

[0025] P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of steel sheets. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. As a result, during tensile testing, voids are generated and cracks propagate along the prior austenite grain boundaries, preventing the desired El from being obtained. Therefore, the P content is set to 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

[0026] S: 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, voids will form and cracks will grow from the sulfides during tensile testing, preventing good local elongation and the desired El. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. There is no particular lower limit for the S content, but due to production technology constraints, the S content is preferably 0.0001% or more.

[0027] Al: 0.010% or more and 2.000% or less Al promotes the formation of soft phase unrecrystallized ferrite and recrystallized ferrite during the temperature rise to the annealing soaking temperature and in the annealing soaking step. That is, Al is an element that affects the area fraction of the soft phase. Here, if the Al content is less than 0.010%, the area fraction of the soft phase decreases, and El decreases. On the other hand, if the Al content exceeds 2.000%, it becomes difficult to form and grow a large amount of austenite in the rapid heating step, in which rapid heating is performed from the annealing soaking temperature to (annealing soaking temperature + 10°C) or more at a heating rate of 10°C / sec or more. Therefore, it is not possible to achieve an area ratio of 65% or more of the hard phases in which the ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel] is 1.50 or more, and an area ratio of 10% or more of the hard phases in which the ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] is 1.30 or less. This makes it difficult to reduce the variation in the mechanical properties (YS, TS, El, λ) of the steel sheet (improving the stability of the mechanical properties). Therefore, the Al content is set to 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. The Al content is also preferably 1.000% or less.

[0028] N: 0.0100% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.0100%, voids will form and cracks will propagate from the nitrides during tensile testing, preventing good local elongation and the desired El. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less. There is no particular lower limit for the N content, but due to production technology constraints, the N content is preferably 0.0005% or more.

[0029] The basic chemical composition of a steel sheet according to one embodiment of the present invention has been described above. However, the steel sheet according to one embodiment of the present invention has a chemical composition containing the basic chemical components, with the balance other than the basic chemical components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of a high-strength steel strip according to one embodiment of the present invention contains the basic chemical components, with the balance consisting of Fe and unavoidable impurities. The steel sheet according to one embodiment of the present invention may contain, in addition to the basic chemical components, at least one selected from the optional chemical components listed below. Note that the effects of the present invention can be obtained as long as the optional chemical components listed below are contained in amounts up to the upper limit amounts listed below, so no lower limit is particularly set. Note that when the optional chemical elements listed below are contained in amounts less than the preferred lower limit values ​​described below, the elements are considered to be included as unavoidable impurities. Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0. 200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200 %, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and at least one selected from REM: 0.0200% or less

[0030] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0031] Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0032] V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling and annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0033] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B controls the formation of soft-phase transformed ferrite and epitaxial ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, it becomes difficult to ensure a soft-phase area ratio of 30.0% or more, which may result in poor local elongation and the failure to obtain the desired El. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0034] Cr: 1.000% or less Cr is an element that improves hardenability. The addition of Cr generates appropriate amounts of the hard phases of fresh martensite and tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.000%, the area ratio of the hard phase increases, which can lead to void formation and crack propagation originating from the hard phase of fresh martensite during tensile testing. This can result in poor local elongation and the failure to achieve the desired El. Therefore, when Cr is included, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less.

[0035] Ni: 1.000% or less. Ni is an element that improves hardenability, and the addition of Ni generates appropriate amounts of hard phase fresh martensite and tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of the hard phase increases, and during tensile testing, voids may form and cracks may grow from the hard phase fresh martensite, preventing good local elongation and the desired El. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

[0036] Mo: 1.000% or less Mo is an element that improves hardenability. Adding Mo generates appropriate amounts of hard phase fresh martensite and tempered martensite, thereby increasing TS and YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, the area ratio of the hard phase increases, and during tensile testing, voids may form and cracks may grow from the hard phase fresh martensite, resulting in poor local elongation and the possibility of not achieving the desired El. Therefore, when Mo is added, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less.

[0037] Sb: 0.200% or less Sb is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 590 MPa or more. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, the surface layer hardness varies within the steel sheet, and there is a risk of large variations in TS and YS. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

[0038] Sn: 0.200% or less Like Sb, Sn is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 590 MPa or more. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, the surface layer hardness varies within the steel sheet, and there is a risk of large variations in TS and YS. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.030% or less.

[0039] Cu: 1.000% or less Cu is an element that improves hardenability. The addition of Cu generates appropriate amounts of hard phase fresh martensite and tempered martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of the hard phase may increase excessively. In addition, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively formed hard phase fresh martensite and coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase fresh martensite during tensile testing, which may prevent good local elongation and the desired El. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.300% or less.

[0040] Ta: 0.200% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.200%, a large amount of coarse precipitates and inclusions may form. In such cases, voids may form and cracks may propagate from the hard fresh martensite phase during tensile testing, which may result in poor local elongation and the desired El. Therefore, when Ta is added, the Ta content is preferably 0.200% or less. The Ta content is more preferably 0.100% or less.

[0041] W: 0.500% or less. W is an element that improves hardenability. Adding W increases the formation of large amounts of the hard phases of fresh martensite and tempered martensite, thereby increasing TS and YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of the hard phase increases. During tensile testing, voids may form and cracks may propagate from the hard phase of fresh martensite. This may result in poor local elongation and the failure to achieve the desired El. Therefore, when W is added, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less.

[0042] Mg: 0.0200% or less Mg is an element effective in spheroidizing inclusions such as sulfides and oxides and increasing the local elongation of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase of fresh martensite during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less.

[0043] Zn: 0.0200% or less Zn is an element effective in spheroidizing the shape of inclusions and increasing the local elongation of the steel sheet. To achieve this effect, the Zn content is preferably 0.0010% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase fresh martensite during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.

[0044] Co: 0.0200% or less Like Zn, Co is an effective element for spheroidizing inclusions and increasing the local elongation of steel sheets. To achieve this effect, the Co content is preferably 0.0010% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase fresh martensite during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.

[0045] Zr: 0.1000% or less Like Zn and Co, Zr is an element that effectively spheroidizes the shape of inclusions and increases the local elongation of steel sheets. To achieve this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase of fresh martensite during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less.

[0046] Ca: 0.0200% or less Ca exists as inclusions in steel. Here, if the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the hard phase fresh martensite during tensile testing, which may result in poor local elongation and the desired El not being obtained. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0040% or less. While the lower limit of the Ca content is not particularly limited, a Ca content of 0.0005% or more is preferred due to production technology constraints. Furthermore, a Ca content of 0.0010% or more is more preferred due to production technology constraints.

[0047] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for increasing the local elongation of steel sheet. To achieve this effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% or if the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the fresh martensite hard phase during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are each 0.0200% or less, and the content of As is 0.0500% or less.

[0048] In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably La and / or Ce.

[0049] Inevitable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing facilities, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include O.

[0050] Next, the steel structure of a steel plate according to one embodiment of the present invention will be described. The steel structure of a steel strip is also the same as that of the steel plate. The following description of the steel structure of the steel plate can be applied to the steel structure of the steel strip.

[0051] <Structure at 1 / 4 of the steel plate thickness> Area ratio of soft phase: 30.0% or more and 90.0% or less The soft phase referred to here refers to one or more types selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite. The soft phase (one or more types selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) is a phase (structure) that improves El. To ensure a desired El, the area ratio of the soft phase is set to 30.0% or more. The area ratio of the soft phase is preferably 35.0% or more. On the other hand, if the area ratio of the soft phase increases excessively, it becomes difficult to achieve a TS of 590 MPa or more. Therefore, the area ratio of the soft phase is set to 90.0% or less. The area ratio of the soft phase is preferably 80.0% or less.

[0052] Here, both unrecrystallized ferrite and recrystallized ferrite are ferrites that are generated during heating up to the annealing soaking temperature and during annealing soaking. Unrecrystallized ferrite is ferrite that has a crystal orientation similar to that of the processed ferrite grains in the structure before heating and contains subgrain boundaries within the crystal grains. Recrystallized ferrite is ferrite that has a crystal orientation different from that of the processed ferrite grains in the structure before heating and does not contain subgrain boundaries within the crystal grains. Transformed ferrite and epitaxial ferrite are both ferrites that are generated during cooling to 550°C after the rapid heating process. Transformed ferrite is ferrite that is generated when ferrite nucleates with a new crystal orientation from austenite generated during the annealing soaking process (heating up and annealing soaking) and the rapid heating process, and the ferrite grains further grow. Epitaxial ferrite is ferrite that has been transformed while having the same crystal orientation as the unrecrystallized ferrite and recrystallized ferrite adjacent to the austenite produced in the annealing and soaking process (heating and annealing and soaking) and the rapid heating process. Transformed ferrite and epitaxial ferrite are harder than recrystallized ferrite. Bainitic ferrite is ferrite that is produced during cooling and holding at temperatures below 550°C after the rapid heating process. Bainitic ferrite is ferrite that has been transformed from the austenite produced in the annealing and soaking process and the rapid heating process, and has a relatively high dislocation density within the crystal grains and contains fresh martensite, retained austenite, cementite, or carbides such as cementite within the crystal grains.

[0053] Area fraction of hard phase: 10.0% or more and 70.0% or less. The hard phase referred to here refers to one or more selected from fresh martensite and tempered martensite. The hard phase (fresh martensite and tempered martensite) is a phase (structure) that improves TS and YS. In order to ensure a TS of 590 MPa or more and a desired YS, the area fraction of the hard phase is set to 10.0% or more. The area fraction of the hard phase is preferably 20.0% or more. On the other hand, if the area fraction of the hard phase increases excessively, it becomes difficult to ensure the desired El. Therefore, the area fraction of the hard phase is set to 70.0% or less. The area fraction of the soft phase is preferably 65.0% or less.

[0054] The area ratio of the remaining structure other than the soft layer and hard phase is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 5.0% or less. Alternatively, the area ratio of the remaining structure may be 0.0%.

[0055] The remaining structure is not particularly limited, and examples thereof include retained austenite, pearlite, carbides such as cementite, other precipitates, and oxides. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

[0056] Here, the area ratios of the soft phase (one or more selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) and the hard phase (one or more selected from fresh martensite and tempered martensite) are measured at a 1 / 4 position in the plate thickness direction of the steel plate as follows.

[0057] That is, a sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface. Next, the observation surface of the sample is polished with diamond paste, and then finish-polished using alumina. Next, the observation surface of the sample is etched with 3 vol.% nital to reveal the structure. Next, the observation position is set to 1 / 4 of the plate thickness of the steel plate, and five fields of view, 25 μm × 35 μm, are observed at a magnification of 3000 times using an SEM. From the obtained structure image, using Adobe Photoshop from Adobe Systems, the area ratios of the soft phase unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite, which are the constituent structures, and the hard phase fresh martensite and tempered martensite are divided by the measured area to calculate the area ratio for five fields of view, and these values ​​are averaged to obtain the area ratio of each structure.

[0058] The soft phases of unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite, as well as the hard phases of fresh martensite and tempered martensite, are distinguished as follows: Recrystallized ferrite: An example of recrystallized ferrite is shown in Figures 1a, 1b, and 1c (see symbol F2). Recrystallized ferrite is a black region and has a blocky morphology. Recrystallized ferrite contains few subgrain boundaries and carbides. Unrecrystallized ferrite: An example of unrecrystallized ferrite is shown in Figure 1a (see symbol F1). Unrecrystallized ferrite is a black region and has a blocky morphology. Unrecrystallized ferrite contains many subgrain boundaries and may also contain carbides. Transformed ferrite: An example of transformed ferrite is shown in Figure 1c (see symbol F3). Transformed ferrite is a black region adjacent to fresh martensite and retained austenite. Epitaxial ferrite: An example of epitaxial ferrite is shown in Figure 1c (see symbol F4). Epitaxial ferrite is a black region adjacent to either unrecrystallized ferrite or recrystallized ferrite. Bainitic ferrite: An example of bainitic ferrite is shown in Figures 1a and 1d (see symbol F5). Bainitic ferrite is a black to dark gray region and is lumpy or amorphous. Bainitic ferrite also contains relatively small amounts of carbides. Tempered martensite: An example of tempered martensite is shown in Figure 1d (see symbol TM). Tempered martensite is a gray region and is amorphous. Tempered martensite also contains a relatively large number of carbides. Fresh martensite + retained austenite: Examples of fresh martensite and retained austenite are shown in Figures 1a, 1b, 1c, and 1d (see symbol FM). Fresh martensite and retained austenite are white to light gray regions with amorphous morphology. Fresh martensite and retained austenite do not contain carbides. Cementite (carbide): Cementite is a white region with dotted or linear morphology.Cementite is included in tempered martensite (see symbol TM in FIG. 1d). Remaining structure: In addition to the above-mentioned retained austenite and cementite (carbide), the remaining structure includes pearlite, other precipitates, oxides, and the like, and the forms thereof are as known.

[0059] The area ratio of retained austenite is measured as follows.

[0060] Specifically, the substrate steel sheet is mechanically ground in the thickness direction (depth direction) to a position corresponding to one-quarter of the sheet thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0061] The area fraction of fresh martensite is calculated by subtracting the area fraction of retained austenite from the area fraction of fresh martensite + retained austenite calculated as described above: [Area fraction of fresh martensite (%)] = [Area fraction of fresh martensite + retained austenite (%)] - [Area fraction of retained austenite (%)]

[0062] The area ratio of the remaining structure is determined by subtracting the area ratios of the soft phases (unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) and hard phases (fresh martensite and tempered martensite) determined as described above from 100.0%: [Area ratio (%) of remaining structure] = 100.0 - [Area ratio (%) of soft phase] - [Area ratio (%) of hard phase]

[0063] The area ratio of the hard phases in which the ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet (steel strip)] is 1.50 or more is 65% or more, and the area ratio of the hard phases in which the ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel sheet (steel strip)] is 1.30 or less is 10% or more. Generally, the hardness of the hard phases (fresh martensite and tempered martensite) is determined by the C content (hereinafter also simply referred to as C content) in the hard phase, and the higher the C content in the hard phase, the higher the hardness of the hard phase. When the ratio of the hard phases in which the ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet (steel strip)] is 1.50 or more is less than 65%, that is, when there are few hard phases with sufficient hardness, TS decreases. Therefore, the area ratio of hard phases having a ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet (steel strip)] of 1.50 or more is set to 65% or more of the total hard phases. Furthermore, when the Mn content (hereinafter also referred to simply as Mn content) in the hard phase is low, that is, when austenite with a low Mn content is generated in the rapid heating process, a hard phase with a high C content and a low Mn content is generated in the subsequent cooling process. When this high-C, low-Mn hard phase is present in an appropriate amount, a good hole expansion ratio is easily obtained, and the effect of reducing the variation in mechanical properties (TS, YS, El, λ) is obtained. Although the details are not clear, it is thought that the following is true. In other words, in the case of a structure consisting of a typical high-C, high-Mn hard phase and a soft phase, the large difference in hardness between the structures makes it easy for voids to occur between the structures, leading to a decrease in the hole expansion ratio and variation in mechanical properties. In this regard, since a high-C, low-Mn hard phase is relatively softer than a general high-C, high-Mn hard phase, the difference in hardness between structures is reduced, the occurrence of voids is suppressed, and this contributes to an improvement in the hole expansion ratio and a reduction in the variation in mechanical properties. When the hard phase having a ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel plate (steel strip)] of 1.30 or less is 10% or more, the above-mentioned effects can be obtained.

[0064] Here, [C content (mass%) in hard phase] / [C content (mass%) in steel sheet (steel strip)] and [Mn content (mass%) in hard phase] / [Mn content (mass%) in steel sheet (steel strip)] are measured as follows.

[0065] A sample was cut out so that the observation surface was the thickness cross section (L cross section) parallel to the rolling direction of the steel plate, and the observation surface of the sample was polished with diamond paste, followed by finish polishing using alumina. The characteristic X-ray intensity and concentration of C and Mn were then measured at a quarter of the steel plate thickness. The characteristic X-ray intensity and concentration of C and Mn were obtained by mapping analysis (qualitative values) and line analysis (quantitative values) using a field-emission electron probe microanalyzer (FE-EPMA). In the mapping analysis, the measurement range was 16.6 μm × 16.6 μm, the number of measurement points was 256 × 256 points, the acceleration voltage was 9 kV, and the acquisition time was 200 ms, and the characteristic X-ray intensity (counts: qualitative values) of C and Mn was obtained. In the line analysis, the measurement is performed under the conditions of a measurement length of 16.6 μm, the number of measurement points is 256, the acceleration voltage is 9 kV, and the acquisition time is 2 s, and the concentrations of C and Mn (mass %: quantitative values) are obtained.

[0066] In Fig. 2, (a) shows the structure (1) of Inventive Example No. 2 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5, as well as the C map (2) and Mn map (3) obtained by EPMA measurement. Also, in Fig. 2, (b) shows the structure (1) of Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5, as well as the C map (2) and Mn map (3) obtained by EPMA measurement. Also, Fig. 3a shows an SEM image ((1), backscattered electron image) of Inventive Example No. 2 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5, as well as the results of line analysis (quantitative) of C (2) and line analysis (quantitative) of Mn (3) obtained by EPMA measurement. Also, Fig. 3b shows Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5. 39 (Steel B) at the measurement position 8 in FIG. 5 ((1), backscattered electron image), and the results of line analysis (quantitative) of C (2) and line analysis (quantitative) of Mn (3) obtained by EPMA measurement are shown. In FIG. 2, the soft phase is the region shown in white in (1) of FIG. 2(a) and (1) of FIG. 2(b). The soft layer is the region with a low C content (no peak) in (2) of FIG. 3a and (2) of FIG. 3b. In FIG. 2, the hard phase with a high C, low Mn composition is the region shown in gray in (1) of FIG. 2(a). The hard phase with a high C, low Mn composition is the region with a high C content and a low Mn content (a peak of C content exists and no peak of Mn content exists) in (2) and (3) of FIG. 3a. In Figure 2, the hard phase with a high C / Mn composition is the region shown in black in (1) of Figure 2(a) and (1) of Figure 2(b). It also refers to the region where both the C content and the Mn content are high (where peaks exist) in (2) and (3) of Figure 3(a) and (2) and (3) of Figure 3(b).

[0067] The above mainly describes the chemical composition and structure of the steel sheet of the present invention, but the steel strip can also be described by replacing the steel sheet with the steel strip in all of the above content described in terms of the steel sheet.

[0068] Next, the mechanical properties of the steel plate and steel strip according to one embodiment of the present invention will be described.

[0069] Tensile Strength (TS) The tensile strength (TS) of the steel plate and steel strip according to one embodiment of the present invention is 590 MPa or more and less than 1180 MPa.

[0070] Yield stress (YS), total elongation (El), hole expansion ratio (λ) Furthermore, the yield stress (YS) and total elongation (El) of the steel plate and steel strip according to one embodiment of the present invention satisfy the following formulas 1 and 2. This makes it possible to obtain desired collision energy absorption performance and press formability. Furthermore, the hole expansion ratio (λ) of the steel plate and steel strip according to one embodiment of the present invention satisfies the following formula 3. Equation 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Equation 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Equation 3: 45% ≦ λ when 590 MPa ≦ TS < 780 MPa 30% ≦ λ when 780 MPa ≦ TS < 980 MPa 20% ≦ λ when 980 MPa ≦ TS < 1180 MPa

[0071] The tensile strength (TS), yield stress (YS), and total elongation (El) are measured by a tensile test conforming to JIS Z 2241 (2011), which will be described later in the Examples. The hole expansion ratio (λ) is measured by a hole expansion test conforming to the Japan Iron and Steel Federation standard JFST 1001. The measurement positions of TS, YS, El, and λ of the steel sheet are shown in FIG. 4, and the measurement positions of TS, YS, El, and λ in the steel strip (coil) are shown in FIG. 5. For a steel sheet according to one embodiment of the present invention, TS of 590 MPa or more and less than 1180 MPa, the above formula 1 for YS, the above formula 2 for El, and the above formula 3 for λ are satisfied at each measurement position shown in FIG. 4. For a steel strip according to one embodiment of the present invention, TS of 590 MPa or more and less than 1180 MPa, the above formula 1 for YS, the above formula 2 for El, and the above formula 3 for λ are satisfied at each measurement position shown in FIG. 5.

[0072] Variations in mechanical properties (TS, YS, El, λ) A steel sheet and steel strip according to one embodiment of the present invention satisfy the following formulas 4 to 11. In this case, it is possible to perform press forming without defects on a frame structural member of an automobile body having a complex shape. TS in each of the following formulas (formulas 4 to 11) is the average value of TS at each measurement position shown in Figure 4 (see position a indicated by reference numeral 31, position b indicated by reference numeral 32, and position c indicated by reference numeral 33) for a steel sheet. Furthermore, TS in each of the following formulas (formulas 4 to 11) is the average value of TS at each measurement position shown in Figure 5 (see reference numerals 1 to 15) for a steel strip.Equation 4: When 590 MPa ≦ TS < 780 MPa, ΔTS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔTS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔTS ≦ 60 MPa Equation 5: When 590 MPa ≦ TS < 780 MPa, σTS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σTS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σTS ≦ 30 MPa Equation 6: When 590 MPa ≦ TS < 780 MPa, ΔYS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔYS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔYS ≦ 60 MPa Equation 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σYS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σYS ≦ 30 MPa Formula 8: When 590 MPa ≦ TS < 780 MPa, ΔEl ≦ 4.0% When 780 MPa ≦ TS < 980 MPa, ΔEl ≦ 4.5% When 980 MPa ≦ TS < 1180 MPa, ΔEl ≦ 5.0% Formula 9: When 590 MPa ≦ TS < 780 MPa, σEl ≦ 2.0% When 780 MPa ≦ TS < 980 MPa, σEl ≦ 2.5% When 980 MPa ≦ TS < 1180 MPa, σEl ≦ 3.0% Formula 10: If 590 MPa ≦ TS < 780 MPa, then Δλ ≦ 25% If 780 MPa ≦ TS < 980 MPa, then Δλ ≦ 25% If 980 MPa ≦ TS < 1180 MPa, then Δλ ≦ 25% Equation 11: If 590 MPa ≦ TS < 780 MPa, then σλ ≦ 7% If 780 MPa ≦ TS < 980 MPa, then σλ ≦ 7% If 980 MPa ≦ TS < 1180 MPa, then σλ ≦ 7%.

[0073] For a steel plate, ΔTS is the difference between the maximum and minimum values ​​of TS (maximum value of TS - minimum value of TS) at each of the 1 / 4, 1 / 2, and 3 / 4 positions (see position a, 31, position b, and 33, respectively, in FIG. 4) in the width direction of the steel plate (see reference numeral 21 in FIG. 4, a direction perpendicular to the longitudinal direction 22 of the steel plate). Also, for a steel plate, ΔYS is the difference between the maximum and minimum values ​​of YS (maximum value of YS - minimum value of YS) at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Also, for a steel plate, ΔEl is the difference between the maximum and minimum values ​​of El (maximum value of El - minimum value of El) at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, for a steel plate, Δλ is the difference between the maximum and minimum values ​​of λ (maximum value of λ - minimum value of λ) at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, for a steel plate, σTS is the standard deviation of TS at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, for a steel plate, σYS is the standard deviation of YS at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, for a steel plate, σEl is the standard deviation of El at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, for a steel plate, σλ is the standard deviation of λ at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate.

[0074] Furthermore, for the steel strip, ΔTS is measured at 1 / 4, 1 / 2 and 3 / 4 positions (refer to reference numerals 31, 32 and 33, respectively, in FIG. 5) in the width direction of the steel strip (see reference numeral 21 in FIG. 4, a direction perpendicular to the longitudinal direction 22 of the steel plate), from the leading end position 23 in the longitudinal direction of the steel strip to the tail end position 24 in the longitudinal direction of the steel plate, at positions 10 m from the leading end position in the longitudinal direction of the steel strip (see reference numerals 1, 6 and 11 at position 41 in FIG. 5). ), the tail end 10 m position (see symbols 5, 10, and 15 at position 45 in Figure 4), the 1 / 4 position (see symbols 2, 7, and 12 at position 42 in Figure 5), the 1 / 2 position (see symbols 3, 8, and 13 at position 43 in Figure 4), and the 3 / 4 position (see symbols 4, 9, and 14 at position 44 in Figure 5). Also, for a steel strip, ΔYS is the difference between the maximum and minimum values ​​of YS (maximum value of YS - minimum value of YS) at the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel strip, and at the leading edge 10 m position, the tail end 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip. For a steel strip, ΔEl is the difference between the maximum and minimum values ​​of El (maximum El - minimum El) at the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1 / 4, 1 / 2 and 3 / 4 positions in the longitudinal direction of the steel strip. For a steel strip, Δλ is the difference between the maximum and minimum values ​​of λ (maximum λ - minimum λ) at the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1 / 4, 1 / 2 and 3 / 4 positions in the longitudinal direction of the steel strip. Furthermore, for a steel strip, σTS is the standard deviation of TS at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1 / 4, 1 / 2, and 3 / 4 positions in the longitudinal direction of the steel strip. Furthermore, for a steel strip, σYS is the standard deviation of YS at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1 / 4, 1 / 2, and 3 / 4 positions in the longitudinal direction of the steel strip.Furthermore, for a steel strip, σEl is the standard deviation of El at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the tail edge 10 m, the 1 / 4, 1 / 2, and 3 / 4 positions in the longitudinal direction of the steel strip. Furthermore, for a steel strip, σλ is the standard deviation of λ at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the tail edge 10 m, the 1 / 4, 1 / 2, and 3 / 4 positions in the longitudinal direction of the steel strip.

[0075] Plating Layer The steel sheet and steel strip may have a plating layer on their surface. Possible types of plating layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, an electrogalvanized layer, and a hot-dip aluminum plated layer. The plating layer may be provided on only one surface of the steel sheet or steel strip, or on both surfaces.

[0076] The galvanized layer referred to here refers to a plating layer containing zinc (Zn) as the main component (Zn content of 50.0% or more), such as a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. The aluminum plated layer referred to here refers to a plating layer containing aluminum (Al) as the main component (Al content of 50.0% or more), such as a hot-dip aluminum plated layer.

[0077] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% to 1.0 mass% Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

[0078] The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass to 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass to 3.5% by mass. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities. The electrogalvanized layer is preferably composed of, for example, Zn as the main component (Zn content of 70.0% or more). The electrogalvanized layer may optionally contain one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Cr, Mg, Si, Zr, V, Cu, Pb, Sb, Sn, Ca, Li, Ti, Be, Bi and REM in a total amount of 0.0 mass % or more but less than 30.0 mass %, where the remainder other than the above elements is unavoidable impurities.

[0079] In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g / m ２ 80g / m or more ２ It is preferable that the content of the aluminum plating layer is less than 25.0 mass%. The type of the aluminum plating layer is not particularly limited, but a typical example is one that is preferably composed of Al and less than 50.0 mass% Fe. The hot-dip aluminum plating layer may optionally contain one or more elements selected from the group consisting of Zn, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total content of 0.0 mass% to 25.0 mass%. The Fe content of the hot-dip aluminum plating layer is more preferably less than 25.0 mass%. The remainder other than the above elements is unavoidable impurities. The coating weight of the aluminum plating layer per side is not particularly limited, but is preferably 20 g / m. ２ 120g / m or more２ It is preferable to do the following:

[0080] The coating weights of the zinc-plated layer and the hot-dip aluminum-plated layer are measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the coating layer. The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the portion that was covered with the coating) to determine the coating weight (g / m ２ ) is calculated.

[0081] The thickness of the steel plate and steel strip according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.

[0082] [2] Manufacturing Method of Steel Plate and Steel Strip Next, a manufacturing method of a steel plate according to one embodiment of the present invention will be described. The manufacturing method of a steel plate according to one embodiment of the present invention includes a hot rolling process in which a steel slab having the above-mentioned chemical composition is hot rolled under the condition of a coiling temperature after finish rolling of 350°C or more and 650°C or less to obtain a hot-rolled steel plate, an annealing and soaking process in which the hot-rolled steel plate is heated and annealed and soaked under the conditions of an annealing and soaking temperature of 720°C or more and 860°C or less for a holding time of 20 seconds or more, a rapid heating process in which the hot-rolled steel plate is rapidly heated from the annealing and soaking temperature to (annealing and soaking temperature + 10°C) or more under the condition of an average heating rate of 10°C / second or more, and the temperature ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel plate after the rapid heating is 25°C or less, and an average cooling rate of 4°C / second or more from the reached temperature to 550°C after the rapid heating process. and a cooling step of cooling the hot-rolled steel sheet under the above conditions, or further comprising a cold-rolling step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet after the hot-rolling step and before the annealing and soaking step, or further comprising a plating step of subjecting the hot-rolled steel sheet or cold-rolled steel sheet to any one of hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating after the cooling step, or further comprising a reheating and holding step of cooling the hot-rolled steel sheet or cold-rolled steel sheet to a cooling stop temperature of 0°C or more and 250°C or less, heating it to a temperature range of (cooling stop temperature + 50°C) or more and 460°C or less, and holding it in that temperature range for a tempering time of 10 seconds or more and 2000 seconds or less after the cooling step or the plating step.

[0083] Furthermore, a method for producing a steel strip according to one embodiment of the present invention includes: a hot rolling step in which a steel slab having the aforementioned chemical composition is hot rolled under conditions in which the coiling temperature after finish rolling is 350°C or higher and 650°C or lower to obtain a hot rolled steel strip; an annealing and soaking step in which the hot rolled steel strip is heated and annealed and soaked under conditions of an annealing and soaking temperature of 720°C or higher and 860°C or lower for a holding time of 20 seconds or longer; a rapid heating step in which the hot rolled steel strip is rapidly heated from the annealing and soaking temperature to (annealing and soaking temperature + 10°C) or higher under conditions of an average heating rate of 10°C / second or higher, and further in which the reached temperature ΔT (maximum reached temperature - minimum reached temperature) at each of the ¼, ½ and ¾ positions of the width of the steel strip after the rapid heating is 25°C or lower; and a cooling step in which, after the rapid heating step, the steel strip is cooled from the reached temperature to 550°C under conditions of an average cooling rate of 4°C / second or higher. Alternatively, the method for producing a steel strip further includes a cold rolling step of cold rolling the hot rolled steel strip to obtain a cold rolled steel strip after the hot rolling step and before the annealing and soaking step, or a plating step of applying any one of hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating to the hot rolled steel strip or the cold rolled steel strip after the cooling step, or a reheating and holding step of cooling the hot rolled steel strip or the cold rolled steel strip to a cooling stop temperature of 0°C or more and 250°C or less, heating it to a temperature range of (the cooling stop temperature + 50°C) or more and 460°C or less, and holding it in the temperature range for a tempering time of 10 seconds or more and 2000 seconds or less after the cooling step or the plating step.

[0084] Figures 6a to 6e are graphs showing examples of heat treatment patterns in a method for producing a steel sheet and a steel strip according to an embodiment of the present invention, with time on the horizontal axis and temperature on the vertical axis. Specifically, Figure 6a is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), and a reheating / holding step (E). Figure 6b is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing step (D1), and a reheating / holding step (E). Figure 6c is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing step (D1), an alloying step (D11), and a reheating / holding step (E). 6d is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), an electrogalvanizing step (D2), and a reheating / holding step (E). Also, FIG. 6e is a schematic diagram showing an example of a heat treatment pattern including an annealing / soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip aluminum plating step (D3), and a reheating / holding step (E). In FIGS. 6a to 6e, two patterns are shown for each of the reheating / holding steps (E).

[0085] The manufacturing method of the steel strip is the same as that of the steel plate. Note that the temperature in the manufacturing method refers to the surface temperature of the steel plate or steel strip. Furthermore, unless otherwise specified, the temperature in the manufacturing method refers to the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions of the width of the steel plate or steel strip. Furthermore, for the steel strip, the temperatures in the hot rolling process, annealing and soaking process, rapid heating process, and cooling process refer to the temperatures at the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip. Furthermore, for the steel strip, the temperatures in processes other than the hot rolling process, annealing and soaking process, rapid heating process, and cooling process (plating process, reheating and holding process, etc.) refer to the temperatures at the 1 / 2 position in the longitudinal direction of the steel strip.

[0086] In the present invention, the method for producing the steel material (steel slab) is not particularly limited, and any known production method, such as a converter or electric furnace, is suitable. Furthermore, to prevent macrosegregation, the steel slab is preferably produced by a continuous casting method. However, it can also be produced by an ingot casting method or a thin slab casting method. Furthermore, in addition to the conventional method of cooling the produced steel slab to room temperature and then reheating it, energy-saving processes such as direct rolling, in which the hot slab is charged into a heating furnace without being cooled to room temperature, or in which the slab is rolled immediately after a short heat retention period, can also be applied without any problems. When heating the slab, the slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, the slab heating temperature is preferably 1300°C or lower to prevent an increase in scale loss. The slab heating temperature is the temperature of the slab surface. Furthermore, the slab is made into a sheet bar by rough rolling under normal conditions, but if the heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0087] [Hot Rolling Step] The finish rolling temperature of the hot rolling is preferably 820° C. or higher. If the finish rolling temperature of the hot rolling is lower than 820° C., the rolling load increases, and the reduction ratio in the non-recrystallized state of austenite increases, which may result in the development of an abnormal structure elongated in the rolling direction, resulting in a decrease in El of the final material.

[0088] Coiling temperature after finish rolling: 350°C or higher and 650°C or lower If the coiling temperature after finish rolling is less than 350°C, a large amount of fresh martensite and tempered martensite are formed during the cooling process to room temperature after coiling, and the strength at those positions increases significantly, resulting in large variations in the mechanical properties (YS, TS, El, λ) of the final steel sheet. Therefore, the coiling temperature after finish rolling is set to 350°C or higher. The coiling temperature after finish rolling is preferably 400°C or higher. If the coiling temperature after finish rolling exceeds 650°C, a large amount of ferrite is formed at the time of coiling, and the strength at those positions decreases significantly, resulting in large variations in the mechanical properties (YS, TS, El, λ) of the final steel sheet. Therefore, the coiling temperature after finish rolling is set to 650°C or lower. The coiling temperature after finish rolling is preferably 600°C or lower.

[0089] During hot rolling, the rough-rolled sheets may be joined together and continuously subjected to finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material quality. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0090] The hot-rolled steel sheet thus produced is then subjected to pickling. Pickling is important for ensuring good chemical conversion treatability and plating quality in the final steel sheet product, as it can remove oxides from the steel sheet surface. Pickling may be performed once or multiple times.

[0091] When the hot-rolled pickled steel sheet obtained as described above is subjected to cold rolling as needed, the cold rolling may be performed as is after the hot-rolled pickled steel sheet, or after heat treatment. Optionally, the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling. Cold rolling is performed by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0092] When cold rolling is performed as needed, the reduction ratio of the cold rolling is not particularly limited, but is preferably 80% or less. If the reduction ratio of the cold rolling exceeds 80%, the steel sheet is likely to have a defective shape and the amount of zinc coating may become non-uniform. Note that the reduction ratio referred to here is a cumulative reduction ratio.

[0093] [Annealing and Soaking Step] In the annealing and soaking step (A) shown in Figs. 6a to 6e, the hot-rolled steel sheet is heated and annealed and soaked at an annealing and soaking temperature of 720°C to 860°C for a holding time of 20 seconds or more.

[0094] Annealing soaking temperature: 720°C or higher and 860°C or lower. If the annealing soaking temperature is lower than 720°C, the proportion of austenite generated during soaking in the two-phase region of ferrite and austenite will be insufficient. Therefore, during the annealing soaking process, austenite with little Mn enrichment (close to the Mn content of the steel) will occupy the majority. After the subsequent rapid heating process, the austenite will transform into transformed ferrite and epitaxial ferrite during the cooling process. This prevents the desired hard phases (fresh martensite and tempered martensite) from being generated, resulting in a failure to obtain a TS of 590 MPa or higher. Furthermore, the area fraction of ferrite will excessively increase after annealing soaking, preventing the desired TS from being obtained. On the other hand, if the annealing soaking temperature exceeds 860°C, a large amount of austenite will be generated during annealing soaking, preventing the ferrite area fraction from being 30.0% or higher, and thus preventing the desired El from being obtained. Therefore, the annealing soaking temperature is set to 720°C or higher and 860°C or lower. The annealing soaking temperature is preferably 740° C. or higher. The annealing soaking temperature is preferably 830° C. or lower. The annealing soaking temperature is the maximum temperature reached in the annealing soaking step.

[0095] Holding time: 20 seconds or more If the holding time (also referred to as annealing holding time or annealing time) is less than 20 seconds, the proportion of austenite generated during soaking in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the holding time is set to 20 seconds or more. The holding time is preferably 30 seconds or more. There are no particular limitations on the upper limit of the holding time, but it is preferably set to 900 seconds or less, and more preferably set to 400 seconds or less. The holding time is the holding time in a temperature range of (annealing soaking temperature - 30°C) or more and the annealing soaking temperature or less. That is, the holding time includes not only the holding time at the annealing soaking temperature but also the residence time in a temperature range of (annealing soaking temperature - 30°C) or more and the annealing soaking temperature or less during heating before reaching the annealing soaking temperature.

[0096] Here, a radiant tube furnace is preferably used as the heat treatment furnace for the annealing and soaking step. A radiant tube furnace heats the steel sheet with radiant heat from a radiant tube and a furnace wall, so the volume of the heat source is very large and the thermal inertia is large. Therefore, it is difficult to quickly follow changes in the set temperature, and the responsiveness to temperature control commands is low.

[0097] The annealing soaking temperature is obtained by a thermometer that measures the surface temperature of the steel sheet. There are no particular limitations on the temperature measurement method, but for example, a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet is suitable. In the case of a radiation thermometer, since it may be affected by reflected light of infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel sheet. Furthermore, since it may be affected by the emissivity of the steel sheet surface, a multiple reflection measurement method that utilizes the wedge-shaped space between the transport roll in the furnace and the steel sheet may be adopted.

[0098] [Rapid Heating Step] After the annealing and soaking step (A), in the rapid heating step (B) shown in Figs. 6a to 6e, rapid heating is performed from the annealing and soaking temperature to a temperature equal to or higher than (annealing and soaking temperature + 10°C) under conditions of an average heating rate of 10°C / sec or more, and among the temperatures reached at each of the ¼ position, ½ position and ¾ position in the width direction of the steel sheet after the rapid heating, (maximum reached temperature - minimum reached temperature) is 25°C or less.

[0099] Rapid heating to (annealing soaking temperature + 10°C) or higher at an average heating rate of 10°C / sec or higher In the rapid heating process, the steel is rapidly heated from the annealing soaking temperature to an ultimate temperature of (annealing soaking temperature + 10°C) or higher. The ultimate temperature is preferably (annealing soaking temperature + 20°C) or higher. The ultimate temperature is preferably (annealing soaking temperature + 150°C) or lower. If the average heating rate is less than 10°C / sec, coarsening of austenite grains and the diffusion of Mn into austenite by ferrite are promoted, making it difficult to control the mechanical properties. As a result, it becomes impossible to set the tensile strength (TS), yield stress (YS), total elongation (El), the difference between the maximum and minimum values ​​of TS (ΔTS), the difference between the maximum and minimum values ​​of YS (ΔYS), the difference between the maximum and minimum values ​​of El (ΔEl), the difference between the maximum and minimum values ​​of λ (Δλ), the standard deviation of TS (σTS), the standard deviation of YS (σYS), the standard deviation of El (σEl), and the standard deviation of λ (σλ) of the steel sheet within the desired range. Therefore, the average heating rate is set to 10°C / sec or more. The average heating rate is preferably 20°C / sec or more, more preferably 30°C / sec or more. If the average heating rate is 30°C / sec or more, the diffusion of Mn from ferrite to austenite is suppressed, and only the diffusion of C from ferrite to austenite is likely to occur. In addition, the line length can be further shortened.

[0100] If the average heating rate exceeds 200°C / s, local high-temperature areas may occur in the sheet width direction, and uniformity may not be maintained. Therefore, the average heating rate is preferably 200°C / s or less. The average heating rate is more preferably 170°C / s or less, and even more preferably 150°C / s or less. The average heating rate is preferably 120°C / s or less. If the average heating rate is 170°C / s or less, the risk of buckling deformation of the steel sheet due to thermal stress can be further reduced. Furthermore, from the viewpoint of reducing the risk of buckling deformation of the steel sheet due to thermal stress, the average heating rate is preferably 300°C / s or less, and more preferably 150°C / s or less. Here, the average heating rate (°C / s) is obtained by dividing the difference (°C) between the annealing soaking temperature (heating start temperature) and the final temperature in rapid heating by the heating time (seconds) from the annealing soaking temperature to the final temperature. The annealing soaking temperature (heating start temperature) and ultimate temperature for determining the average heating rate refer to the average temperatures at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel sheet. Also, the annealing soaking temperature (heating start temperature) and ultimate temperature for determining the average heating rate refer to the average temperatures at the 1 / 2 position in the longitudinal direction of the steel strip and the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip.

[0101] ΔT of the temperature reached (maximum value of the temperature reached - minimum value of the temperature reached) at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel sheet after rapid heating: 25°C or less If ΔT exceeds 25°C, the ratio of ferrite and austenite generated during heating in the two-phase region of ferrite and austenite becomes non-uniform in the width direction of the steel sheet, and in the subsequent cooling process, variations occur in the ratios of soft phase and hard phase generated, making it impossible to keep all of ΔTS, ΔYS, ΔEl, Δλ, σTS, σYS, σEl, and σλ within the desired ranges.

[0102] The temperature reached during rapid heating has a significant impact on the YS, TS, El, and λ of the final product. Therefore, for rapid heating, it is preferable to use an induction heating (IH) device, which has high responsiveness to temperature control commands. The induction heating (IH) device adjusts its output to rapidly heat the steel sheet so that the temperature of the steel sheet is between 725°C and 940°C. Furthermore, when heating in this temperature range, it is preferable that the induction heating (IH) device be of the transverse type.

[0103] Method for determining the temperature to be reached in the rapid heating step In the rapid heating step, the appropriate temperature to be reached after rapid heating varies depending on the chemical composition of the steel sheet. Therefore, it is preferable that the suitable temperature range of the temperature to be reached is predicted in advance by measurement, calculation, or simulation, and set in consideration of the temperature of the steel sheet in the annealing and soaking step.

[0104] An example of a method for determining the ultimate temperature in the rapid heating step is a method for determining the ultimate temperature based on the aforementioned information on the chemical composition of the steel sheet and information on the coiling temperature in the hot rolling step. Specifically, as shown in Figures 6a to 6e, an example of a method for determining the ultimate temperature in a feedforward control step F is a method for determining the ultimate temperature based on information on the chemical composition of the steel sheet and information F0 on the coiling temperature after finish rolling in the hot rolling step. Here, the information F0 may include information on the thickness and width of the steel sheet and the reduction rate in cold rolling (cumulative reduction rate) in addition to information on the chemical composition of the steel sheet and information on the coiling temperature after finish rolling in the hot rolling step.

[0105] Another method for determining the ultimate temperature in the rapid heating step is to determine the ultimate temperature based on information about the austenite fraction of the steel sheet measured with a transformation rate meter or the like in a temperature range of 150° C. or higher and 600° C. or lower after the rapid heating step. Specifically, as shown in Figures 6a to 6e, a method for determining the ultimate temperature in the feedback control step (G) is to determine the ultimate temperature based on information G0 about the austenite fraction of the steel sheet measured with a transformation rate meter or the like in a temperature range of 150° C. or higher and 600° C. or lower.

[0106] Further, examples of a method for determining the ultimate temperature in the rapid heating step include a method for determining the ultimate temperature based on information on the chemical composition of the steel sheet described above, information on the coiling temperature in the hot rolling step, and information on the austenite fraction of the steel sheet measured with a transformation rate meter or the like in a temperature range of 150° C. or higher and 600° C. or lower after the rapid heating step. That is, the ultimate temperature may be determined based on both the process in the feedforward control step (F) and the process in the feedback control step (G).

[0107] The temperature reached in the rapid heating process is obtained by a thermometer that measures the surface temperature of the steel sheet. The temperature measurement method is not particularly limited, but a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet is suitable. In the case of a radiation thermometer, since it may be affected by reflected infrared light emitted by the surrounding furnace body, a cover may be provided between the measurement unit of the radiation thermometer and the detection unit of the steel sheet. Furthermore, since it may be affected by the emissivity of the steel sheet surface, a multi-reflection measurement method that utilizes the wedge-shaped space between the furnace transport roll and the steel sheet may also be employed. Furthermore, it is preferable to use a scanning radiation thermometer to measure the temperature at each position in the width direction of the steel sheet.

[0108] For the information on the austenite fraction, a phase fraction prediction model is constructed, and performance data is collected offline. As performance data, thermal histories and phase fractions are obtained for various sets of operating conditions, and multiple sets of training data based on the performance data are stored in a storage device or the database.

[0109] The machine learning model for utilizing the above-mentioned performance data is not limited to a specific generation method, as long as it can provide predictions with the required accuracy for practical use. For example, commonly used methods such as neural networks (including deep learning), decision tree learning, random forests, and support vector regression may be used. An ensemble model combining multiple methods may also be used. Furthermore, as the phase fraction prediction model, a machine learning model with binarized output may be used, which determines whether the phase fraction of a steel sheet is within a predetermined allowable range (pass or fail) rather than a calculated value of the phase fraction. In this case, a classification model such as k-nearest neighbor or logistic regression may be used. The operating conditions are adjusted based on the phase fraction during annealing obtained by the material property prediction model and the phase fraction prediction model to obtain a steel sheet with excellent mechanical property stability. By performing such control, the annealing temperature of the steel sheet is adjusted, enabling the production of a product with stable mechanical properties. The material property prediction model used may be not only a physical model obtained by offline laboratory experiments or numerical analysis, but also a machine learning model obtained from accumulated manufacturing experience.

[0110] [Cooling Step] After the rapid heating step (B), in the cooling step (C) shown in Figs. 6a to 6e, cooling is performed from the above-mentioned ultimate temperature to 550°C at an average cooling rate of 4°C / sec or more.

[0111] Average cooling rate from the temperature reached in the rapid heating step to 550°C: 4°C / sec or more The average cooling rate from the temperature reached in the rapid heating step to 550°C is 4°C / sec or more. Preferably, it is 5°C / sec or more. The average cooling rate is preferably 100°C / sec or less, more preferably 60°C / sec or less. Furthermore, when slowly cooled in the medium temperature range of 650°C or less, fine austenite at the ferrite grain boundaries coalesces with adjacent ferrite with similar orientations to form a single ferrite grain, and fine isolated austenite islands are left within the ferrite grain, ultimately increasing the proportion of isolated fine island-shaped hard second phases (martensite + retained austenite) within the ferrite grains. Therefore, the average cooling rate from 650°C to 500°C is preferably 14°C / sec or less, more preferably 12°C / sec or less. Here, the average cooling rate (°C / sec) is obtained by dividing the difference (°C) between the cooling start temperature and the cooling stop temperature in the target temperature range by the cooling time (sec) from the cooling start temperature to the cooling stop temperature. The cooling start temperature and cooling stop temperature for calculating the average cooling rate here refer to the average temperatures at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel sheet. The cooling start temperature and cooling stop temperature for calculating the average cooling rate here refer to the average temperatures at the leading edge 10 m, the trailing edge 10 m, the 1 / 4, 1 / 2, and 3 / 4 positions in the longitudinal direction of the steel strip at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip. The cooling start temperature and cooling stop temperature for calculating the average cooling rate here refer to the average temperatures at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip at the 1 / 2 position in the longitudinal direction of the steel strip.

[0112] It is preferable that cooling of the steel sheet is initiated within 10 seconds after the temperature reached in the rapid heating step (heating temperature reached). The time from reaching the temperature reached in the rapid heating step until cooling is initiated is preferably within 5 seconds, more preferably within 3 seconds.

[0113] Cooling means that can be used include gas jet cooling, roll cooling, water cooling (water quenching), etc. The cooling zone may be divided into multiple sections and different cooling means may be combined, or the cooling conditions of the same type of cooling means may be changed, thereby controlling the thermal history of the steel sheet during cooling.

[0114] [Plating Treatment Step] After the cooling step (C), the hot-rolled steel sheet may be subjected to any of hot-dip galvanizing, galvannealing, electrogalvanizing, and hot-dip aluminum plating as the plating treatment step (D) shown in Figures 6b to 6e if the cold-rolling step is not included, or to the cold-rolled steel sheet if the cold-rolling step is included. Figure 6b shows an example of a heat treatment pattern when the plating treatment step (D) is a hot-dip galvanizing treatment step (D1) in which hot-dip galvanizing treatment is performed. Figure 6c shows an example of a heat treatment pattern when the plating treatment step (D) is a galvannealing treatment step in which hot-dip galvanizing treatment is performed. The galvannealing treatment step includes a hot-dip galvanizing treatment step (D1) in which hot-dip galvanizing treatment is performed and an alloying treatment step (D11) in which alloying treatment is performed after the hot-dip galvanizing treatment step (D1). Figure 6d shows an example of a heat treatment pattern when the plating treatment step (D) is an electrogalvanizing treatment step (D2) in which electrogalvanizing treatment is performed. FIG. 6e shows an example of a heat treatment pattern when the plating treatment step (D) is a hot-dip aluminum plating treatment (D3) that performs hot-dip aluminum plating.

[0115] As described above, among the plating processes, examples of the galvanizing process include hot-dip galvanizing, galvannealed hot-dip galvanizing, and electrogalvanizing.

[0116] In the case of hot-dip galvanizing treatment, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower, and then adjust the coating weight by gas wiping, etc. The hot-dip galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use, for example, a plating bath having an Al content of 0.10 mass % or higher and 0.23 mass % or lower, with the balance consisting of Zn and unavoidable impurities.

[0117] In the case of alloying hot-dip galvanizing treatment, it is preferable to perform the hot-dip galvanizing treatment as described above, and then heat the hot-dip galvanized steel sheet to an alloying temperature of 500°C or higher and 600°C or lower to perform the alloying treatment. If the alloying temperature is lower than 500°C, the Zn-Fe alloying rate will be slow, and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite will transform to pearlite, making it difficult to achieve a TS of 590 MPa or higher. The alloying temperature is preferably 510°C or higher. The alloying temperature is preferably 570°C or lower.

[0118] The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is 20 to 80 g / m per side. ２ The plating weight can be adjusted by gas wiping or the like.

[0119] In the case of electrogalvanization, for example, an electroplating solution containing 80 g / L of divalent zinc ions as sulfate is used, and the current density is set to 10 to 80 A / dm ２ By adjusting the electrolysis time, the amount of zinc plating layer deposited can be adjusted to 20 g / m ２ 80g / m or more ２ It can be controlled as follows:

[0120] When hot-dip aluminum plating is performed, the cold-rolled steel sheet is immersed in an aluminum plating bath at 660 to 730°C, and the hot-dip aluminum plating is performed, and then the coating weight is adjusted by gas wiping or the like. The coating weight is 20 to 120 g / m per side. ２ The hot-dip aluminum plating bath is not particularly limited as long as it has the composition of the aluminum plating layer described above, but it is preferable to use, for example, a plating bath having a composition in which the Zn content is 1.0 mass % or more and 20.0 mass % or less, with the balance being Al and unavoidable impurities.

[0121] [Reheating and Holding Step] In the reheating and holding step (E) shown in Figs. 6a to 6e, after the cooling step if a plating treatment step is not included, or after the plating treatment step if a plating treatment step is included, the hot-rolled steel sheet or the cold-rolled steel sheet can be cooled to a cooling stop temperature of 0°C or more and 250°C or less, heated to a temperature range of (cooling stop temperature + 50°C) or more and 460°C or less, and held in the temperature range for a tempering time of 10 seconds or more and 2000 seconds or less.

[0122] Reheating temperature: (Cooling stop temperature + 50°C) or higher and 460°C or lower. Reheating time: 10 seconds or higher and 2000 seconds or lower. Reheating to a temperature above (the cooling stop temperature + 50°C) and holding for 10 seconds or longer releases diffusible hydrogen from the steel. This also reduces the area ratio of fresh martensite in the final structure, ensuring an appropriate amount of tempered martensite. This also reduces the amount of austenite immediately after the cooling process, reducing the area ratio of retained austenite in the final structure. As a result, the desired λ, R / t, ST, and SFmax are obtained. On the other hand, if the reheating temperature exceeds 460°C, when a plating process is performed, the zinc plating or other coating may partially dissolve and adhere to the roll, preventing the steel sheet from being uniformly plated. Furthermore, if the reheating time is less than 10 seconds, the desired amount of diffusible hydrogen from the steel may not be released.

[0123] The steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity. Furthermore, temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned processes (offline). Furthermore, the number of times temper rolling may be one, two, or more. Furthermore, rolling using a leveler or the like may be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0124] Other conditions of the manufacturing method are not particularly limited, but from the viewpoint of productivity, it is preferable that the series of processes such as the annealing, hot-dip galvanizing, and alloying treatment of the galvanizing be carried out in a continuous galvanizing line (CGL). After the hot-dip galvanizing process, wiping can be performed to adjust the coating weight of the galvanizing. Note that the plating conditions other than those described above can be based on conventional methods.

[0125] The above mainly describes the method for manufacturing a steel sheet of the present invention, but the method for manufacturing a steel strip can also be described by replacing the steel sheet with the steel strip in all of the methods for manufacturing a steel sheet described above.

[0126] Steel materials having the chemical compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and then cast into steel slabs by continuous casting. In Table 1, "-" indicates the content of unavoidable impurities.

[0127] The obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to rough rolling and hot rolling, and then coiled at the coiling temperature after hot rolling shown in Table 2 (Table 2-1, Table 2-2) to form a coil, thereby obtaining a hot-rolled steel strip. Next, No. 1 to No. 82 of the obtained hot-rolled steel strips were subjected to pickling and cold rolling (cumulative reduction: 63%) to obtain cold-rolled steel strips with the thicknesses shown in Tables 2 and 3 (Tables 3-1 to 3-10). Furthermore, No. 83 to No. 91 of the obtained hot-rolled steel strips were subjected to pickling to obtain hot-rolled steel strips (white skin) with the thicknesses shown in Tables 2 and 3. Next, the obtained cold-rolled steel strips or hot-rolled steel strips (white skin) were subjected to an annealing soaking step, a rapid heating step (IH rapid heating step), and a cooling step under the conditions shown in Table 2 to obtain steel strips. In addition, the steel strips were subjected to treatments in an annealing soaking step, a rapid heating step, and a cooling step, and were further subjected to treatments in a plating step as required, and further to treatments in a reheating holding step as required. Each temperature in Table 2 represents an average value at 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip.

[0128] Here, the plating process included hot-dip galvanizing, galvannealed hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating to obtain hot-dip galvanized steel strip (GI), galvannealed hot-dip galvanized steel strip (GA), electrogalvanized steel strip (EG), and aluminum-plated steel strip (Al). In Table 2, the types of plating processes are also indicated as "GI," "GA," "EG," and "Al." When no plating process was performed, it is indicated as "CR." In Table 2, for CR, GI, EG, and Al, no alloying process was performed, so the alloying temperature is indicated as "-."

[0129] The zinc plating bath temperature for GI and GA in the above plating treatment step was 470°C. When producing GI, the zinc plating coverage was 45 to 72 g / m per side. ２ (Double-sided plating) and when manufacturing GA, 45 g / m per side ２ The zinc coating layer of the finally obtained steel strip (hot-dip galvanized steel strip) contained 0.1 to 2.0 mass% Fe for GI, with the balance being Zn and unavoidable impurities. The zinc coating layer of the finally obtained steel strip (hot-dip galvanized steel strip) contained 8.0 to 12.0 mass% Fe for GA, with the balance being Zn and unavoidable impurities. When producing EG, the zinc coating weight was 45 g / m per side. ２ In the case of EG in which the zinc plating layer is a zinc (Zn)-nickel (Ni) plating layer, the Ni content in the zinc plating layer is set to 9 mass % or more and 25 mass % or less (double-sided plating), and the zinc plating coating weight is set to 45 g / m per side. ２ In the case of producing Al, the aluminum plating bath temperature was 700°C, and the aluminum plating coating weight was 45 g / m per side. ２ (double-sided plating).

[0130] After each measurement of the steel strip described below, the steel strip was cut into a 1 m length in the longitudinal direction so as to include the longitudinal half position (see symbols 3, 8, and 13 in Figure 5) as the center in the longitudinal direction (rolling direction) to obtain a steel plate.

[0131] Using the obtained steel strips (CR, GI, GA, EG, and Al), the steel microstructures were identified at each position of the steel strip (positions 1 to 15 in Figure 5) using the method described above. The measurement results are shown in Table 3. In Table 3, the percentages of the steel microstructures are the average values ​​for all measurement positions (positions 1 to 15). The soft phases are unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite, the hard phases are fresh martensite and tempered martensite, RA is retained austenite, P is pearlite, and θ is carbide. The steel microstructures of the steel plates were also identified at half positions in the longitudinal direction of the steel strips (see symbols 3, 8, and 13 in Figure 5) using the same method as for the steel strips.

[0132] The tensile tests were conducted in accordance with JIS Z 2241 (2011). That is, JIS No. 5 test specimens were taken from each of the positions marked 1 to 15 in Fig. 5 within each of the obtained steel strips (CR, GI, GA, EG, and Al) so that the longitudinal direction was perpendicular to the rolling direction of the steel strip. Using the taken test specimens, tensile tests were conducted at a crosshead speed of 10 mm / min, and TS, YS, and El were measured. From the TS, YS, and El at each position of symbols 1 to 15 in each obtained steel strip (CR, GI, GA, EG, Al), the difference between the maximum and minimum values ​​of TS (ΔTS), the difference between the maximum and minimum values ​​of YS (ΔYS), the difference between the maximum and minimum values ​​of El (ΔEl), and the standard deviation of TS (σTS), the standard deviation of YS (σYS), and the standard deviation of El (σEl) were calculated. In addition, λ at each position of symbols 1 to 15 of the steel strip was measured using a hole expansion test in accordance with the Japan Iron and Steel Federation standard JFST 1001. Then, from the λ at each position, the difference between the maximum and minimum values ​​of λ (Δλ) and the standard deviation of λ (σλ) were calculated. The results are shown in Table 3. As an example of the evaluation of the variation of the steel sheet, the difference between the maximum and minimum values ​​of TS (ΔTS), the difference between the maximum and minimum values ​​of YS (ΔYS), the difference between the maximum and minimum values ​​of El (ΔEl), as well as the standard deviation of TS (σTS), the standard deviation of YS (σYS), and the standard deviation of El (σEl) at 1 / 2 positions in the longitudinal direction of the steel strip (symbols 3, 8, and 13) were also shown. In addition, the difference between the maximum and minimum values ​​of λ (Δλ) and the standard deviation of λ (σλ) were also shown as the evaluation of the variation of the steel sheet.

[0133] ・TS 〇 (Pass): 590 MPa or more and less than 1180 MPa × (Fail): Less than 590 MPa, 1180 MPa or more ・YS 〇 (Pass): 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa × (Fail): 360 MPa > YS when 590 MPa ≦ TS < 780 MPa 460 MPa > YS when 780 MPa ≦ TS < 980 MPa 580 MPa > YS when 980 MPa ≦ TS < 1180 MPa ・El 〇 (Pass): 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa If 780MPa≦TS<980MPa, 17.0%≦El If 980MPa≦TS<1180MPa, 11.0%≦El × (Fail): If 590MPa≦TS<780MPa, 23.0%>El If 780MPa≦TS<980MPa, 17.0%>El If 980MPa≦TS<1180MPa, 11.0%>El ·λ 〇 (Pass): If 590MPa≦TS<780MPa, 45%≦λ If 780MPa≦TS<980MPa, 30%≦λ If 980MPa≦TS<1180MPa, 20%≦λ × (Fail): If 590MPa≦TS<780MPa, 45%>λ When 780 MPa ≦ TS < 980 MPa, 30% > λ When 980 MPa ≦ TS < 1180 MPa, 20% > λ

[0134] ・ΔTS ○ (Pass): When 590MPa≦TS<780MPa, ΔTS≦40MPa When 780MPa≦TS<980MPa, ΔTS≦50MPa When 980MPa≦TS<1180MPa, ΔTS≦60MPa × (Fail): When 590MPa≦TS<780MPa, ΔTS>40MPa When 780MPa≦TS<980MPa, ΔTS>50MPa When 980MPa≦TS<1180MPa, ΔTS>60MPa ・ΔYS ○ (Pass): When 590MPa≦TS<780MPa, ΔYS≦40MPa When 780MPa≦TS<980MPa, ΔYS≦50MPa When 980MPa≦TS<1180MPa, ΔYS≦60MPa × (Fail): When 590MPa≦TS<780MPa, ΔYS>40MPa When 780MPa≦TS<980MPa, ΔYS>50MPa When 980MPa≦TS<1180MPa, ΔYS>60MPa ・ΔEl ◯ (Pass): When 590MPa≦TS<780MPa, ΔEl≦4.0% When 780MPa≦TS<980MPa, ΔEl≦4.5% When 980MPa≦TS<1180MPa, ΔEl≦5.0% × (Fail): When 590MPa≦TS<780MPa, ΔEl>4.0% When 780MPa≦TS<980MPa, ΔEl>4.5% When 980MPa≦TS<1180MPa, ΔEl>5.0% ・Δλ ○ (Pass): When 590MPa≦TS<780MPa, Δλ≦25% When 780MPa≦TS<980MPa, Δλ≦25% When 980MPa≦TS<1180MPa, Δλ≦25% × (Fail): When 590MPa≦TS<780MPa, Δλ>25% When 780MPa≦TS<980MPa, Δλ>25% When 980MPa≦TS<1180MPa, Δλ>25%

[0135] σTS ○ (Pass): When 590MPa≦TS<780MPa, σTS≦20MPa When 780MPa≦TS<980MPa, σTS≦25MPa When 980MPa≦TS<1180MPa, σTS≦30MPa × (Fail): When 590MPa≦TS<780MPa, σTS>20MPa When 780MPa≦TS<980MPa, σTS>25MPa When 980MPa≦TS<1180MPa, σTS>30MPa σYS ○ (Pass): When 590MPa≦TS<780MPa, σYS≦20MPa When 780MPa≦TS<980MPa, σYS≦25MPa When 980MPa≦TS<1180MPa, σYS≦30MPa × (Fail): When 590MPa≦TS<780MPa, σYS>20MPa When 780MPa≦TS<980MPa, σYS>25MPa When 980MPa≦TS<1180MPa, σYS>30MPa σEl ○ (Pass): When 590MPa≦TS<780MPa, σEl≦2.0% When 780MPa≦TS<980MPa, σEl≦2.5% When 980MPa≦TS<1180MPa, σEl≦3.0% × (Fail): When 590MPa≦TS<780MPa, σEl>2.0% If 780MPa≦TS<980MPa, σEl>2.5% If 980MPa≦TS<1180MPa, σEl>3.0% ・σλ ◯ (Pass): If 590MPa≦TS<780MPa, σλ≦7% If 780MPa≦TS<980MPa, σλ≦7% If 980MPa≦TS<1180MPa, σλ≦7% × (Fail): If 590MPa≦TS<780MPa, σλ>7% If 780MPa≦TS<980MPa, σλ>7% If 980MPa≦TS<1180MPa, σλ>7%

[0136] The underlined parts in Tables 1, 2 and 3 indicate values ​​outside the appropriate range of the present invention.

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] As shown in Table 3, in all of the inventive examples, the tensile strength (TS), yield stress (YS), total elongation (El), hole expansion ratio (λ), difference between the maximum and minimum values ​​of TS (ΔTS), difference between the maximum and minimum values ​​of YS (ΔYS), difference between the maximum and minimum values ​​of El (ΔEl), difference between the maximum and minimum values ​​of λ (Δλ), standard deviation of TS (σTS), standard deviation of YS (σYS), standard deviation of El (σEl), and standard deviation of λ (σλ) all passed.

[0151] On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (El), hole expansion ratio (λ), difference between the maximum and minimum values ​​of TS (ΔTS), difference between the maximum and minimum values ​​of YS (ΔYS), difference between the maximum and minimum values ​​of El (ΔEl), difference between the maximum and minimum values ​​of λ (Δλ), standard deviation of TS (σTS), standard deviation of YS (σYS), standard deviation of El (σEl), and standard deviation of λ (σλ) in the steel strips and steel plates (CR, GI, GA, EG, Al) was insufficient.

[0152] According to the present invention, it is possible to produce steel sheets and steel strips having a TS of 590 MPa or more and less than 1180 MPa and small variations in mechanical properties (YS, TS, El, λ), i.e., having excellent stability of mechanical properties. By applying the steel sheets and steel strips having excellent stability of mechanical properties obtained according to the method of the present invention to structural members of automobile bodies, for example, the yield during press-forming of the steel sheets in the automobile body manufacturing process and during welding and assembly of press-formed members is greatly improved, contributing to a reduction in the environmental load in the automobile manufacturing process and therefore having extremely great industrial utility value.

[0153] F1 Unrecrystallized ferrite F2 Recrystallized ferrite F3 Transformed ferrite F4 Epitaxial ferrite F5 Bainitic ferrite FM Fresh martensite TM Tempered martensite A Annealing and soaking process B Rapid heating process C Cooling process D Plating process D1 Hot-dip galvanizing process D11 Alloying process D2 Electrogalvanizing process D3 Hot-dip aluminum plating process E Reheating and holding process F Feedforward control process F0 Information on the steel sheet composition, sheet thickness, sheet width, reduction rate in cold rolling, and coiling temperature after finish rolling in hot rolling G Feedback control process G0 Information on austenite fraction a 1 / 4 position in the width direction of the steel sheet b 1 / 2 position in the width direction of the steel sheet c 3 / 4 position in the width direction of the steel sheet 1 At a position 10 m from the tip of the steel strip and at a position 1 / 4 in the width direction 2 1 / 4 position in the longitudinal direction and 1 / 4 position in the width direction of the steel strip 3 1 / 2 position in the longitudinal direction and 1 / 4 position in the width direction of the steel strip 4 3 / 4 position in the longitudinal direction and 1 / 4 position in the width direction of the steel strip 5 10 m position from the tail end of the steel strip and 1 / 4 position in the width direction 6 10 m position from the head end of the steel strip and 1 / 2 position in the width direction 7 1 / 4 position in the longitudinal direction and 1 / 2 position in the width direction of the steel strip 8 1 / 2 position in the longitudinal direction and 1 / 2 position in the width direction of the steel strip 9 3 / 4 position in the longitudinal direction and 1 / 2 position in the width direction of the steel strip 10 10 m position from the tail end of the steel strip and 1 / 2 position in the width direction 11 10 m position from the head end of the steel strip and 3 / 4 position in the width direction 12 1 / 4 position in the longitudinal direction and 3 / 4 position in the width direction of the steel strip 13 1 / 2 position in the longitudinal direction and 3 / 4 position in the width direction of the steel strip 14 3 / 4 position in the longitudinal direction and 3 / 4 position in the width direction of the steel strip 15 10 m position from the tail end of the steel strip and 3 / 4 position in the width direction 21 Width direction of the steel plate and steel strip 22 Longitudinal direction of the steel plate and steel strip 23 Leading end position in the longitudinal direction of the steel strip 24 Tail end position in the longitudinal direction of the steel strip 31 1 / 4 position in the width direction of the steel plate and steel strip 32 1 / 2 position in the width direction of the steel plate and steel strip 33 3 / 4 position in the width direction of the steel plate and steel strip 41 10 m position from the leading end in the longitudinal direction of the steel strip42 1 / 4 position in the longitudinal direction of the steel strip 43 1 / 2 position in the longitudinal direction of the steel strip 44 3 / 4 position in the longitudinal direction of the steel strip 45 10 m position from the tail end in the longitudinal direction of the steel strip P Line analysis location P1 Hard phase P2 Soft phase C0 C content (0.088%) of Examples No. 2 (Steel B) and No. 39 (Steel B) C1 1.50 times the C content (0.088%) of Examples No. 2 (Steel B) and No. 39 (Steel B) C2 Region where [C content at measurement position] / [C content of steel] is 1.50 or more M0 Mn content (2.71%) of Examples No. 2 (Steel B) and No. 39 (Steel B) M1 1.50 times the C content (0.088%) of Examples No. 2 (Steel B) and No. 39 (Steel B) 1.30 times the Mn content (2.71%) of No. 39 (Steel B) M2 Region where [Mn content at measurement position] / [Mn content of steel] is 1.30 or less

Claims

1. A steel sheet having a chemical composition containing, by mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more but less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, wherein the structure at a 1 / 4 position in the sheet thickness direction of the steel sheet has an area ratio of a soft phase: 30.0% or more and 90.0% or less, and an area ratio of a hard phase: 10.0% or more and 70.0% or less, wherein the hard phase is the area ratio of the hard phase in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel sheet] is 1.50 or more is 65% or more, and the area ratio of the hard phase in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel sheet] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, the following formulas 1 to 3 are satisfied at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel sheet, Furthermore, ΔTS, ΔYS, ΔEl, Δλ, which are the differences between the maximum and minimum values ​​of TS, YS, El, and λ at each of the measurement positions, i.e., ¼ position, ½ position, and ¾ position in the width direction of the steel sheet, respectively, (maximum value of TS−minimum value of TS), (maximum value of YS−minimum value of YS), (maximum value of El−minimum value of El), and (maximum value of λ−minimum value of λ), satisfy the following formulas 4, 6, 8, and 10, respectively; and further, σTS, σYS, σEl, σλ, which are the standard deviations obtained from TS, YS, El, and λ at each of the measurement positions, i.e., ¼ position, ½ position, and ¾ position in the width direction of the steel sheet, respectively, satisfy the following formulas 5, 7, 9, and 11, respectively. The steel sheet has a tensile strength of 590 MPa or more and less than 1180 MPa. Formula 1: 360 MPa ≦ YS when 590 MPa ≦ TS < 780 MPa 460 MPa ≦ YS when 780 MPa ≦ TS < 980 MPa 580 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa Formula 2: 23.0% ≦ El when 590 MPa ≦ TS < 780 MPa 17.0% ≦ El when 780 MPa ≦ TS < 980 MPa 11.0% ≦ El when 980 MPa ≦ TS < 1180 MPa Formula 3:When 590 MPa ≦ TS < 780 MPa, 45% ≦ λ When 780 MPa ≦ TS < 980 MPa, 30% ≦ λ When 980 MPa ≦ TS < 1180 MPa, 20% ≦ λ Formula 4: When 590 MPa ≦ TS < 780 MPa, ΔTS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔTS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔTS ≦ 60 MPa Formula 5: When 590 MPa ≦ TS < 780 MPa, σTS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σTS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σTS ≦ 30 MPa Formula 6: When 590 MPa ≦ TS < 780 MPa, ΔYS ≦ 40 MPa When 780 MPa ≦ TS < 980 MPa, ΔYS ≦ 50 MPa When 980 MPa ≦ TS < 1180 MPa, ΔYS ≦ 60 MPa Formula 7: When 590 MPa ≦ TS < 780 MPa, σYS ≦ 20 MPa When 780 MPa ≦ TS < 980 MPa, σYS ≦ 25 MPa When 980 MPa ≦ TS < 1180 MPa, σYS ≦ 30 MPa Formula 8: When 590 MPa ≦ TS < 780 MPa, ΔEl ≦ 4.0% When 780 MPa ≦ TS < 980 MPa, ΔEl ≦ 4.5% When 980 MPa ≦ TS < 1180 MPa, ΔEl ≦ 5.0% Formula 9: When 590 MPa ≦ TS < 780 MPa, σEl ≦ 2.0% When 780 MPa ≦ TS < 980 MPa, σEl ≦ 2.5% When 980 MPa ≦ TS < 1180 MPa, σEl ≦ 3.0% Equation 10: When 590 MPa ≦ TS < 780 MPa, Δλ ≦ 25% When 780 MPa ≦ TS < 980 MPa, Δλ ≦ 25% When 980 MPa ≦ TS < 1180 MPa, Δλ ≦ 25% Equation 11: When 590 MPa ≦ TS < 780 MPa, σλ ≦ 7% When 780 MPa ≦ TS < 980 MPa, σλ ≦ 7% When 980 MPa ≦ TS < 1180 MPa, σλ ≦ 7% 2. The chemical composition further contains, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, 2. The steel sheet according to claim 1, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

3. The steel sheet according to claim 1 or 2, which has a plating layer on its surface, the plating layer being any one of a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer and a hot-dip aluminum plated layer.

4. A steel strip having a composition containing, by mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, wherein the structure at the 1 / 4 position of the sheet thickness of the steel strip has an area ratio of soft phase: 30.0% or more and 90.0% or less, and an area ratio of hard phase: 10.0% or more and 70.0% or less, wherein the hard phase is the area ratio of the hard phase in which [C content (mass%) in the hard phase] / [C content (mass%) in the steel strip] is 1.50 or more is 65% or more, and the area ratio of the hard phase in which [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel strip] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is λ, the following formulas 1 to 3 are satisfied at each of the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel strip, at each of the leading edge 10 m position, the tail edge 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip, and further, at the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the width direction of the steel strip, The differences between the maximum and minimum values ​​of TS, YS, El, and λ at the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position of the longitudinal direction of the steel strip, i.e., (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), respectively, satisfy the following formulas 4, 6, 8, and 10, respectively; and further, at the 1 / 4 position, 1 / 2 position, and 3 / 4 position in the width direction of the steel strip, A steel strip having a tensile strength of 590 MPa or more and less than 1180 MPa, wherein σTS, σYS, σEl, and σλ, which are standard deviations obtained from TS, YS, El, and λ at the leading 10 m position, the tail 10 m position, the 1 / 4 position, the 1 / 2 position, and the 3 / 4 position in the longitudinal direction of the steel strip, satisfy the following formulas 5, 7, 9, and 11, respectively: Formula 1: When 590 MPa≦TS<780 MPa, 360 MPa≦YS When 780 MPa≦TS<980 MPa, 460 MPa≦YSWhen 980MPa≦TS<1180MPa, 580MPa≦YS Equation 2: When 590MPa≦TS<780MPa, 23.0%≦El When 780MPa≦TS<980MPa, 17.0%≦El When 980MPa≦TS<1180MPa, 11.0%≦El Equation 3: When 590MPa≦TS<780MPa, 45%≦λ When 780MPa≦TS<980MPa, 30%≦λ When 980MPa≦TS<1180MPa, 20%≦λ Equation 4: When 590MPa≦TS<780MPa, ΔTS≦40MPa When 780MPa≦TS<980MPa, ΔTS≦50MPa When 980MPa≦TS<1180MPa, ΔTS≦60MPa Equation 5: When 590MPa≦TS<780MPa, σTS≦20MPa When 780MPa≦TS<980MPa, σTS≦25MPa When 980MPa≦TS<1180MPa, σTS≦30MPa Equation 6: When 590MPa≦TS<780MPa, ΔYS≦40MPa When 780MPa≦TS<980MPa, ΔYS≦50MPa When 980MPa≦TS<1180MPa, ΔYS≦60MPa Equation 7: When 590MPa≦TS<780MPa, σYS≦20MPa When 780MPa≦TS<980MPa, σYS≦25MPa When 980MPa≦TS<1180MPa, σYS≦30MPa Equation 8: When 590MPa≦TS<780MPa, ΔEl≦4.0% When 780MPa≦TS<980MPa, ΔEl≦4.5% When 980MPa≦TS<1180MPa, ΔEl≦5.0% Equation 9: When 590MPa≦TS<780MPa, σEl≦2.0% When 780MPa≦TS<980MPa, σEl≦2.5% When 980MPa≦TS<1180MPa, σEl≦3.0% Equation 10: When 590MPa≦TS<780MPa, Δλ≦25% When 780MPa≦TS<980MPa, Δλ≦25% If 980 MPa ≦ TS < 1180 MPa, then Δλ ≦ 25%. Equation 11: If 590 MPa ≦ TS < 780 MPa, then σλ ≦ 7% If 780 MPa ≦ TS < 980 MPa, then σλ ≦ 7% If 980 MPa ≦ TS < 1180 MPa, then σλ ≦ 7% 5. The chemical composition further contains, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, 5. The steel strip according to claim 4, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

6. A steel strip according to claim 4 or 5, having a plating layer on the surface of the steel strip, the plating layer being any one of a hot-dip galvanized layer, a galvannealed layer, an electrogalvanized layer and a hot-dip aluminum plated layer.

7. A method for manufacturing a steel slab having the chemical composition defined in claim 1 or 2, comprising: a hot rolling process in which a steel slab is hot rolled under the condition of a coiling temperature after finish rolling of 350°C or more and 650°C or less to obtain a hot-rolled steel sheet; an annealing and soaking process in which the hot-rolled steel sheet is heated and annealed and soaked under the conditions of an annealing and soaking temperature of 720°C or more and 860°C or less for a holding time of 20 seconds or more; a rapid heating process in which the steel sheet is rapidly heated from the annealing and soaking temperature to (the annealing and soaking temperature + 10°C) or more at an average heating rate of 10°C / second or more, and the temperature ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at each of the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel sheet after the rapid heating is 25°C or less; and a cooling process in which the steel sheet is cooled from the reached temperature to 550°C at an average cooling rate of 4°C / second or more after the rapid heating process. Alternatively, the method for manufacturing a steel sheet further includes a cold rolling step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet after the hot-rolling step and before the annealing and soaking step, or a plating step of performing any one of hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating on the hot-rolled steel sheet or the cold-rolled steel sheet after the cooling step, or a reheating and holding step of cooling the hot-rolled steel sheet or the cold-rolled steel sheet to a cooling stop temperature of 0°C or higher and 250°C or lower, heating to a temperature range of (the cooling stop temperature + 50°C) or higher and 460°C or lower, and holding in the temperature range for a tempering time of 10 seconds or higher and 2000 seconds or lower.

8. The method for producing a steel sheet according to claim 7, wherein the ultimate temperature is determined based on information on the chemical composition and information on the coiling temperature in the hot rolling process.

9. A method for manufacturing a steel plate according to claim 7, wherein the ultimate temperature is determined based on information on the austenite fraction of the steel plate measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

10. A method for manufacturing a steel sheet according to claim 7, wherein the ultimate temperature is determined based on information on the chemical composition, information on the coiling temperature in the hot rolling process, and information on the austenite fraction of the steel sheet measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating process.

11. A method for manufacturing a steel slab having the chemical composition defined in claim 4 or 5, comprising: a hot rolling process in which a hot-rolled steel strip is obtained by hot-rolling the steel slab under conditions in which the coiling temperature after finish rolling is 350°C or higher and 650°C or lower; an annealing and soaking process in which the hot-rolled steel strip is heated and annealed and soaked under conditions of an annealing and soaking temperature of 720°C or higher and 860°C or lower and a holding time of 20 seconds or longer; a rapid heating process in which the steel strip is rapidly heated from the annealing and soaking temperature to (the annealing and soaking temperature + 10°C) or higher at an average heating rate of 10°C / second or higher, and further in which the temperature ΔT (maximum value of the temperature to be reached - minimum value of the temperature to be reached) at each of the 1 / 4, 1 / 2 and 3 / 4 positions of the width of the steel strip after the rapid heating is 25°C or lower; and a cooling process in which the steel strip is cooled from the temperature to 550°C under conditions of an average cooling rate of 4°C / second or higher, Alternatively, the method for manufacturing a steel strip further includes a cold rolling step of cold rolling the hot rolled steel strip to obtain a cold rolled steel strip after the hot rolling step and before the annealing and soaking step, or a plating step of applying any one of hot-dip galvanizing, alloyed hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating to the hot rolled steel strip or the cold rolled steel strip after the cooling step, or a reheating and holding step of cooling the hot rolled steel strip or the cold rolled steel strip to a cooling stop temperature of 0°C or higher and 250°C or lower, heating to a temperature range of (the cooling stop temperature + 50°C) or higher and 460°C or lower, and holding in the temperature range for a tempering time of 10 seconds or higher and 2000 seconds or lower.

12. The method for producing a steel strip according to claim 11, wherein the ultimate temperature is determined based on information on the chemical composition and information on the coiling temperature in the hot rolling process.

13. A method for manufacturing a steel strip according to claim 11, wherein the temperature to be reached is determined based on information on the austenite fraction of the steel strip measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.

14. A method for producing a steel strip according to claim 11, wherein the ultimate temperature is determined based on information on the chemical composition, information on the coiling temperature in the hot rolling process, and information on the austenite fraction of the steel strip measured in a temperature range of 150°C or higher and 600°C or lower after the rapid heating process.

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