HIGH-STRENGTH STEEL SHEET AND METHOD FOR PRODUCING IT

Abstract

The present invention provides a high-strength steel sheet, which can be used in various applications, including automotive parts, and exhibits excellent collision safety and excellent formability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to one aspect of the present invention meets a predetermined chemical composition and has a metallographic microstructure having a ferrite fraction of 0% to 10%, an MA fraction of 0% to 30%, a hard phase other than ferrite and MA of 70% to 100% in terms of area ratio, and a retained austenite fraction of 5% to 30% in terms of volume ratio. In the high-strength steel sheet, the IQ asymmetry as analyzed by the EBSD method is from -1.2 to -0.3 when the asymmetry is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are considered as an aggregation of regions having an area of ​​0.05 µm2.

Description

The present invention relates to a high-strength steel sheet that is applied to various applications including automotive parts. BACKGROUND OF THE INVENTION Steel sheets used in automotive and similar parts are required to have enhanced strength to ensure collision safety, protect passengers, and reduce vehicle body weight, thereby improving fuel efficiency and reducing environmental impact. Mechanical properties that significantly affect collision safety include, for example, tensile strength and yield strength. Furthermore, excellent moldability is also required when using steel sheets as components for parts with complex shapes. Mechanical properties that greatly contribute to moldability include, for example, ductility, hole expansion properties, and bending capacity. However, it is difficult to improve both collision safety and moldability simultaneously. To date, techniques for improving collision safety and techniques for improving moldability have each been proposed as separate approaches (Patent Literatures 1 and 2). However, it is difficult to improve both collision safety and moldability simultaneously, even with the techniques proposed so far. This is considered to be because increasing the strength of the steel sheet (e.g., to improve tensile strength) is a requirement for improving collision safety, and this requirement conflicts with the requirement for improving moldability (e.g., ductility and hole expansion properties). From the perspective of passenger protection in the event of a car collision, it is important to prevent passengers from coming into contact with the vehicle body deformed by the collision. The present inventors have considered that it is possible to reduce the amount of deformation of the parts and eliminate contact between the vehicle body and the passenger in the event of a collision by increasing the load at the initial deformation stage (load required for initial deformation) in the event of a collision. Based on this idea, the present inventors have studied high-strength steel sheets that have a high load requirement for initial deformation from various perspectives, in addition to their moldability, for various applications, including automotive parts. Specific examples of the properties required for high-strength steel sheets that have characteristics as described above include the following requirements (1) to (5). zoicnn / zznz / E / Y (1) Tensile strength TS: 780 MPa or more (2) Yield ratio YR expressed as the ratio (YS / TS) of yield strength YS to tensile strength TS: 0.70 or more (3) Tensile strength TS x total elongation EL: 13000 MPa-% or more (4) Tensile strength TS x hole expansion ratio λ: 40000 MPa-% or more (5) Load / sheet thickness where the load deforms the test piece to a bending angle of 10° in the bending test by the German Association of Automotive Industry (VDA): 3.0 kN / mm or more The present invention has been made in view of circumstances as described above, and one object thereof is to provide a high-strength steel sheet having properties required at high levels and both excellent collision safety and excellent moldability, as well as a useful method for manufacturing this high-strength steel sheet. List of References Patent Literature Patent Literature 1: JP 5,610.102 B2 Patent Literature 2: JP 5,589,893 B2 BRIEF DESCRIPTION OF THE INVENTION A high-strength steel sheet according to one aspect of the present invention contains each of 0:0.10% to 0.35%; Si + Al: 0.5% to 3.0%; Mn: 1.0% to 3.0%; P: more than 0% and 0.05% or less; S: more than 0% and 0.01% or less in terms of mass percent; and unavoidable iron and impurities as remainder, wherein the high-strength steel sheet has a metallographic microstructure having a ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area ratio and retained austenite fraction: 5% to 30% in terms of volume ratio, and an IQ asymmetry as analyzed by an EBSD method that is -1.2 to -0.3 when the asymmetry is expressed by the following Formula (1) in a case where the crystal grains having a bcc structure and a bct structure are considered as an aggregation of regions having an area of ​​0.05 μm2. [Mat. 2] zoicnn / zznz / E / Y I Cnn / 77n7 / E / YIAI Each variable in Formula (1) indicates the following. n: Total number of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2s: Standard deviation of IQ in a region that has an area of ​​0.05 μm2x¡: IQ of a region i that has an area of ​​0.05 μm2xave: Average IQ of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2 As another aspect of the present invention, a method for manufacturing a high-strength steel sheet is also included, and this manufacturing method is a method for manufacturing a high-strength steel sheet as described above, and the method includes, in this order: After heating a steel material having the above chemical composition, then subject the steel material to hot rolling, cool and coil the sheet material after hot rolling is completed, and then subject the steel material to pickling and cold rolling, a step to heat the steel material to a temperature T1 of a transformation point A of the sheet or more and 950°C or less and hold the steel material for a time t1 of 5 seconds or more and 1800 seconds or less in this temperature range for austenitization; a step to carry out cooling from a rapid cooling start temperature T2 of 700°C or more to a cooling detection temperature T3a in a temperature range of 300°C or more and 500°C or less at an average cooling rate CR2 of 10°C / sec or more; a step to carry out retention for a time t3 of 10 seconds or more and less than 300 seconds at an average cooling rate of 10°C / sec or less in a temperature range of 300°C or more and 500°C or less; a step for carrying out the cooling from a final holding temperature T3b of 300°C or more to a cooling detection temperature T4 in a temperature range of 100°C or more and 300°C or less at an average cooling rate CR3 of 10°C / sec or more; and a step for carrying out the heating from the cooling holding temperature T4 to a reheating temperature T5 that meets the following requirement in a temperature range of 300°C or more and 500°C or less and carrying out the holding for a time t5 of 350 seconds or more and 1800 seconds or less in the temperature range of the reheating temperature T5, a difference between the reheating temperature T5 and an average temperature of the cooling detection temperature T3a and the final holding temperature T3b that is 50°C or less. The objects, features, and advantages of the present invention will be evident from the following detailed description and the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a schematic diagram illustrating an example of a heat treatment pattern carried out on a raw sheet. DETAILED DESCRIPTION OF THE INVENTION The present inventors have diligently studied the microstructure to obtain a high-strength steel sheet exhibiting both excellent collision safety and excellent moldability. As a result, it has been found that a high-strength steel sheet exhibiting both excellent collision safety and excellent moldability can be obtained by appropriately prescribing the IQ distribution state as analyzed by the EBSD method for relatively soft bainitic ferrite and hard recent bainite and martensite, which are the main microstructures having a bcc and a bct structure, in a case where the crystal grains are considered as an aggregation of regions having an area of ​​0.05 μm², and the present invention has been completed in this manner. According to the present invention, it is possible to provide a high-strength steel sheet that exhibits both excellent collision safety and excellent moldability, and a useful method for manufacturing this high-strength steel sheet. High-strength steel sheet The reason why the chemical composition is prescribed for the high-strength steel sheet of the present type is as follows. In the following chemical composition, “%” means “% by mass”. C: 0.10 to 0.35% Carbon (C) is an effective element for achieving a desired microstructure, such as retained austenite, and for ensuring properties such as tensile strength (TS) x total elongation (EL). To achieve these effects, the amount of C must be set at 0.10% or more. Preferably, the amount of C is 0.13% or more, more preferably 0.15% or more, and even more preferably 0.18% or more. However, when the amount of C becomes excessive and exceeds 0.35%, the martensite-austenite (MA) and retained austenite become coarser, and both the deep drawing property (zoicnn / zznz / E / Y) and the hole expansion property, as assessed by the hole expansion ratio (λ), decrease. Excessive C also adversely affects weldability. Therefore, the amount of C is required to be set at 0.35% or less, and is preferably 0.33% or less, more preferably 0.30% or less.Si + Al: 0.5 to 3.0%. Both Si and Al have the function of suppressing cementite precipitation and promoting the formation of retained austenite. In order to effectively exert this action, the total amount of (Si + Al) from Si and Al must be set at 0.5% or more. Preferably, Si + Al is 0.7% or more, and more preferably 1.0% or more. However, when Si + Al exceeds 3.0% and becomes excessive, tempered martensite and bainite cannot be ensured, and the martensite and retained austenite become coarse. Therefore, Si + Al must be set at 3.0% or less. Preferably, Si + Al is 2.5% or less, and more preferably 2.0% or less. Between Si and Al, Al may be added in a sufficient amount to function as a deoxidizing element, particularly at least 0.1%. Alternatively, for the purpose of suppressing cementite formation and increasing the amount of retained austenite, for example, Al may be contained in a large amount of 0.6% or more. Mn: 1.0 to 3.0% Manganese (Mn) is a necessary element to suppress ferrite formation and ensure the production of tempered martensite, bainite, and similar non-ferrite formations. To effectively achieve these effects, the amount of Mn is set at 1.0% or more. Preferably, the amount of Mn is 1.5% or more, and more preferably 1.7% or more. However, when the amount of Mn becomes excessive and exceeds 3.0%, the transformation of bainite during the manufacturing process is suppressed, relatively soft bainite is not obtained in sufficient quantities, and consequently, the high overall elongation of the alloy cannot be ensured. Therefore, the amount of Mn is required to be set at 3.0% or less, and preferably 2.7% or less, and more preferably 2.5% or less. P: More than 0% and 0.05% or less P is an impurity that is inevitably mixed in. When the amount of P becomes excessive and exceeds 0.05%, the total elongation (EL) and the hole expansion ratio (λ) decrease. Therefore, the amount of P must be kept to 0.05% or less. Preferably, the amount of P is 0.03% or less. It is impossible to reduce the amount of P to 0% in industrial production, and P is typically present at approximately 0.001% or more. S: more than 0% and 0.01% or less Sulfur (S) is an impurity that is inevitably mixed in. When the amount of S becomes excessive and exceeds 0.01%, sulfide-based inclusions such as MnS form, and these inclusions become the starting point for cracking and decrease the hole expansion property. Therefore, it is desirable to keep the amount of S at 0.01% or less. Preferably, the amount of S is 0.005% or less. It is impossible to reduce the amount of S to 0% in industrial production, and S is typically contained at approximately 0.0005% or more. The prescribed chemical composition of the high-strength steel sheet of this form is as described above, with the remainder being iron and unavoidable impurities other than phosphorus (P) and sulfur (S). These unavoidable impurities, such as trace elements like arsenic (As), antimony (Sb), and tin (Sn), are introduced depending on the raw materials, manufacturing equipment, and other factors and are allowed to mix in. For example, there are elements, which are impurities, whose content is usually preferably smaller than the P and S described above, and whose composition range is prescribed separately as follows. Therefore, the term "unavoidable impurities" that constitute the remainder means the concept excluding the elements whose composition range is prescribed separately. Nitrogen (N) is also present as an impurity, but its quantity is preferably as follows. N: 0.01% or less Nitrogen (N) is an impurity mixed into steel, and when the amount of N becomes excessive, it forms a coarse nitride that impairs bending strength and hole expansion properties, or causes blowholes during welding. For this reason, it is preferable to have a lower amount of N, such as P and S, and ideally 0.01% or less. Reducing the amount of N is costly, and the cost increases significantly when attempting to reduce it to less than 0.0005%. Therefore, the lower limit for the amount of N is preferably set at 0.0005% or more. As a preferred embodiment of the present invention, in addition to the elements mentioned above, it is also effective to contain (a) at least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.5% or less, (b) at least one selected from the group consisting of Ni: more than 0% and 2% or less, Cr: more than 0% and 2% or less, and Mo: more than 0% and 0.5% or less, (c) B: more than 0% and 0.005% or less, (d) at least one selected from the group consisting of Mg: more than 0% and 0.04% or less, REM: more than 0% and 0.04% or less, and Ca: more than 0% and 0.04% or less, and similar in the high-strength steel sheet of the present embodiment, if It is necessary. The properties of high-strength steel sheet are further improved depending on the types of elements it contains. At least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.5% or less Titanium (Ti) is an element that exerts the effects of precipitation strengthening and microstructure micronization, and contributes to improving the strength of steel sheets. Therefore, Ti may be present. These effects increase as the Ti content increases, and the amount of Ti is preferably 0.01% or more, more preferably 0.02% or more, in order to effectively exert these effects. However, when the amount of Ti exceeds 0.2% and becomes excessive, excessive Ti carbonitride precipitates, and the formability of the steel sheet decreases. Therefore, the amount of Ti is preferably 0.2% or less, more preferably 0.10% or less, and even more preferably 0.06% or less. Like titanium, niobium (Nb) is an element that exerts precipitation strengthening and microstructure micronization effects, contributing to improved strength in steel sheets. Therefore, niobium may be present. These effects increase with increasing niobium content, preferably 0.005% or more, and more preferably 0.010% or more, to effectively exert these effects. However, when the niobium content exceeds 0.2% and becomes excessive, niobium carbonitride precipitates excessively, and the formability of the steel sheet decreases. Therefore, the niobium content is preferably 0.2% or less, more preferably 0.10% or less, and even more preferably 0.06% or less. Like Ti and Nb, V is an element that exerts precipitation strengthening and microstructure micronization effects, contributing to improved strength in steel sheets. Therefore, V may be present. These effects increase with increasing V content, preferably 0.01% or more, and more preferably 0.02% or more, to effectively exert these effects. However, when the amount of V exceeds 0.5% and becomes excessive, excessive V carbonitride precipitates, decreasing the formability of the steel sheet. Therefore, the amount of V is preferably 0.125% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. Each of Ti, Nb and V can be contained individually, or two or three of Ti, Nb or V can be contained concurrently. At least one selected from the group consisting of Ni: more than 0% and 2% or less, Cr: more than 0% and 2% or less, and Mo: more than 0% and 0.5% or less Ni is an effective element that contributes to increasing the strength of steel sheets, stabilizes retained austenite, and ensures a desired amount of retained austenite. Therefore, Ni may be contained. These effects increase as the Ni content increases, and the amount of Ni is preferably 0.001% or more, more preferably 0.01% or more, in order to effectively exert these effects. However, when the amount of Ni exceeds 2% and becomes excessive, the formability during hot rolling decreases. Therefore, the amount of Ni is preferably zoicnn / zznz / E / Y 2% or less, more preferably 1.0% or less. Like nickel, chromium (Cr) is an effective element that contributes to increasing the strength of steel sheets, stabilizes retained austenite, and ensures a desired amount of retained austenite. Therefore, Cr can be contained. These effects increase as the Cr content increases, and the amount of Cr is preferably 0.001% or more, more preferably 0.01% or more, in order to effectively exert these effects. However, when the amount of Cr exceeds 2% and becomes excessive, the formability during hot rolling decreases. Therefore, the amount of Cr is preferably 2% or less, more preferably 1.0% or less. Like Ni and Cr, Mo is an effective element that contributes to increasing the strength of steel sheets, stabilizes retained austenite, and ensures a desired amount of retained austenite. Therefore, Mo can be contained. These effects increase as the Mo content increases, and the amount of Mo is preferably 0.001% or more, more preferably 0.01% or more, in order to effectively exert these effects. However, when the amount of Mo exceeds 0.5% and becomes excessive, the formability during hot rolling decreases. Therefore, the amount of Mo is preferably 0.5% or less, more preferably 0.20% or less. Each of Ni, Cr and Mo can be contained individually, or two or three of Ni, Cr or Mo can be contained concurrently. B: more than 0% and 0.005% or less B is an effective element for suppressing ferrite transformation by improving workability. Therefore, B may be contained. These effects increase as the B content increases, and the amount of B is preferably 0.0001% or more, more preferably 0.0010% or more, in order to effectively exert these effects. However, when the amount of B exceeds 0.005% and becomes excessive, workability during hot rolling decreases. Therefore, the amount of B is preferably 0.005% or less, more preferably 0.0030% or less. At least one selected from the group consisting of Mg: more than 0% and 0.04% or less, REM: more than 0% and 0.04% or less, and Ca: more than 0% and 0.04% or less Magnesium (Mg), rare earth elements (REM), and calcium (Ca) form fine oxides or sulfides, suppressing the decrease in hole expansion property caused by coarse oxides or sulfides. Therefore, Mg, REM, or Ca can be present in any arbitrary combination. These effects increase as the contents of Mg, REM, and Ca increase, with the amounts of Mg, REM, and Ca each preferably being 0.0005% or more, and more preferably 0.0010% or more, to sufficiently achieve these effects. However, when the amounts of Mg, REM, or Ca exceed 0.04% and become excessive, the oxide or sulfide becomes coarse, and the hole expansion property decreases. Therefore, the amount of Mg, the amount of REM, and the amount of Ca are each preferably 0.04% or less, more preferably 0.010% or less.REM includes a total of 17 classes of rare earth elements from the lanthanide series from La (atomic number 57) to Lu (atomic number 71) plus Se and Y, and the amount of REM means the total amount of these 17 classes of elements. Next, the metallographic microstructure of the high-strength steel sheet of the present type will be described. The area ratio and volume ratio below are the proportions occupied in the overall metallographic microstructure, respectively. Ferrite fraction: 0% to 10% by area in terms of area ratio Ferrite is generally excellent in malleability but has a low strength problem. As a result, the yield-to-rule ratio (YR) decreases when the amount of ferrite is large. For this reason, the ferrite fraction is set at 0% to 10% by area. The upper limit for the ferrite fraction is preferably 5% by area or less, more preferably 3% by area or less. The ferrite fraction can be determined by observing a nital-etched steel sheet under a scanning electron microscope (SEM) and measuring the proportions of black areas that do not contain carbide by counting spots. MA fraction: 0% to 30% per area in terms of area proportion MA is a complex microstructure of fresh martensite and retained austenite. Since MA contains retained austenite, increasing the amount of MA is effective in improving malleability, as indicated by the total elongation (EL). However, MA also contains extremely hard fresh martensite, and thus the hole expansion ratio (λ) decreases as the MA fraction increases. For this reason, the MA fraction is set at 0% to 30% by area. The lower limit of the MA fraction is preferably 3% by area or more, more preferably 5% by area or more. The upper limit of the MA fraction is preferably 25% by area or less, more preferably 20% by area or less. The MA fraction can be determined by observing a nital-etched steel sheet under SEM and measuring the gray, non-carbide-containing portions by point counting. Hard phase different from ferrite and MA: 70% to 100% by area in terms of area ratio The hard phase is an important microstructure for ensuring the desired tensile strength (TS) and yield ratio (YR). The high-strength steel sheet of the present embodiment may contain, for example, bainitic ferrite, bainite, tempered martensite, fresh martensite, and similar components as the hard phase. In the high-strength steel sheet of the present embodiment, the area ratio of the hard phase other than ferrite and MA is set at 70% to 100% by area to ensure the desired tensile strength (TS) and yield ratio (YR). The area ratio of the hard phase other than ferrite and MA is preferably 75% by area or more, and more preferably 80% by area or more. The area ratio of the hard phase other than ferrite and MA can be determined as the area ratio of the portions excluding ferrite and MA mentioned above. Austenite fraction retained: 5% to 30% by volume in terms of volume ratio The retained austenite undergoes a TRIP (transformation-induced plasticity) phenomenon, transforming into martensite during similar pressing operations, and can provide a large total elongation (EL). The martensite formed has high hardness and can thus increase the tensile strength (TS x total elongation EL) (hereafter abbreviated as TS x EL). Since the yield ratio of the retained austenite is low, it is necessary to prevent its excessive introduction to ensure the desired high yield ratio. For this reason, the volume proportion of retained austenite is set at 5% to 30% by volume. The preferred fraction is 7% by volume or more and 20% by volume or less. The fraction of retained austenite can be measured, for example, by X-ray diffraction. In this method, for instance, a portion of the steel sheet surface at 1 / 4 of the sheet thickness can be removed by mechanical and chemical polishing, and Co-Kα rays can be used as the characteristic X-rays. Then, the volume fraction of retained austenite can be determined from the integrated intensity ratio of the diffraction peaks attributed to the body-centered cubic (bcc) lattice phase and the body-centered tetragonal (bct) and face-centered cubic (fcc) lattice phases. The asymmetry of IQ as analyzed by the EBSD method is -1.2 to -0.3 when the asymmetry is expressed by the following Formula (1) in the case where the crystal grains that have the bcc structure and the bct structure are established as an aggregation of regions that have the area of ​​0.05 um2_____*_____V p - . Formula ~~ 2) \ 5 / Each variable in Formula (1) indicates the following. n: Total number of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2s: Standard deviation of IQ in a region that has an area of ​​0.05 μm2x¡: IQ of a region i that has an area of ​​0.05 μm2xave: Average IQ of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2la Formula (1) is an analyzed and defined parametric equation based on experimental results. In the high-strength steel sheet of this type, the metallographic microstructures exhibiting both bcc and bct structures, such as bainitic ferrite, bainite, and quenched martensite, are largely similar, making it difficult to unambiguously define their distinct forms. Therefore, the EBSD (Electron Back Scattering Diffraction) method, a crystal analysis technique using SEM, is employed to analyze the image quality (IQ) of the crystal grains with both bcc and bct structures. IQ indicates the sharpness of the EBSD pattern and is generally known to be affected by the amount of dislocation in the crystal. Specifically, the more dislocations present in the crystal, the lower the IQ. In the high-strength steel sheet of the present modality, the IQ at each measurement point is not adopted as IQ. Instead, the region enclosed by the boundary where the difference in crystal orientation between the measurement points exceeds 300 is defined as a crystal grain, and the average IQ is adopted for each crystal grain that has a bcc and a bct structure. In the IQ analysis, a measurement point with an OI < 0.1 is considered unreliable and is excluded from the analysis. The Cl (Confidence Index) is an index that shows whether the EBSD pattern detected at each measurement point matches the database of the specified crystal system (bcc or fcc in the case of iron), and indicates the reliability of the data. The relatively soft bainitic ferrite and bainite, and the hard recent martensite and quenched martensite are quenched microstructures that have a bcc and a bct structure. Formula (1) expresses the IQ asymmetry as analyzed by the EBSD method in a case where crystal grains with a bcc and a bct structure are considered as an aggregation of regions that have the same IQ as the crystal grains and have an area of ​​0.05 μm². According to Formula (1), the area-IQ relationship can be clarified for bainitic ferrite and bainite, and for recent martensite and quenched martensite. The area of ​​crystal grains is not always an integral multiple of 0.05 μm². For this reason, the value obtained by dividing the area of ​​the crystal grains by 0.05 μm² and rounding to the first decimal place is considered to be the number of regions with an area of ​​0.05 μm² contained within the crystal grains. As a result of the present inventors' study, it has been revealed that both excellent collision safety and excellent moldability can be achieved by controlling the IQ symmetry calculated by Formula (1) to -1.2 to -0.3 for the aggregation of regions with an area of ​​0.05 μm² in crystal grains with a bcc and a bct structure. All the reasons why the high-strength steel sheet of the present zoicnn / zznz / E / Y modality exhibits excellent collision safety and excellent moldability have not been made clear, but can be considered as follows. In particular, it is presumed that it is possible to suppress the concentration of strain in the soft microstructure in the initial state of strain while utilizing the effect of improving moldability by the soft structure in a composite microstructure steel sheet with microstructures that have different hardness by appropriately controlling the area of ​​the microstructure that has a bcc structure and a bct structure and the state of the IQ distribution. Where the IQ asymmetry is less than -1.2, this means that the proportion of soft microstructures with high IQ is high, and the average IQ is relatively high compared to the desired distribution. At this point, as the soft microstructures increase, the load at the 10° bending angle in the VDA / sheet thickness bending test decreases, and the desired collision safety cannot be achieved. Since the difference in hardness between the microstructures is large, and the locations that become the point of crack initiation in the hole expansion test increase, a tensile strength of TS x hole expansion ratio λ (hereafter abbreviated as TS χ λ) of 40,000 MPa% or higher cannot be guaranteed. When the IQ asymmetry exceeds -0.3, this indicates that the proportion of hard microstructures with relatively low IQ is high compared to the desired distribution state. At this point, as the production of soft microstructures decreases, moldability, particularly total elongation (EL), also decreases and the desired TS × EL cannot be achieved. In the high-strength steel sheet of the present embodiment, properties such as (1) tensile strength TS, (2) yield ratio YR, (3) TS χ EL, (4) TS χ λ, and (5) load / sheet thickness where the load deforms the test piece to a bending angle of 10° in the bending test by the German Association of Automotive Industries (VDA) are all at high levels. These properties of the high-strength steel sheet of the present embodiment will be described below. (1) Tensile strength TS: 780 MPa or more The high-strength steel sheet of this embodiment preferably has a tensile strength (TS) of 780 MPa or more. The tensile strength (TS) is more preferably 880 MPa or more, and even more preferably 980 MPa or more. The higher tensile strength (TS) is more preferred, but the upper limit of the tensile strength (TS) is approximately 1600 MPa or less, considering the chemical composition and manufacturing conditions of the high-strength steel sheet of this embodiment. (2) YR performance ratio: 0.70 or more zoicnn / zznz / E / Y In the high-strength steel sheet of the present embodiment, the performance ratio YR, expressed as the ratio (YS / TS) of the yield strength YS to the tensile strength TS, is preferably 0.70 or higher. This enables a high yield strength to be achieved in combination with the high tensile strength TS mentioned above. As a result, deformation can be suppressed under load, and collision safety is improved. The performance ratio YR is more preferably 0.75 or higher, and even more preferably 0.80 or higher. From a collision safety perspective, the higher performance ratio YR is preferable, but the upper limit of the performance ratio YR is generally 0.95 or lower, considering the chemical composition and manufacturing conditions of the high-strength steel sheet of the present embodiment. (3) TS x EL: 13000 MPa-% or more The high-strength steel sheet of the present embodiment preferably has a TS x EL of 13,000 MPa-% or more. Having a TS x EL of 13,000 MPa-% or more makes it possible to achieve both excellent strength and excellent press formability. TS x EL is more preferably 14,000 MPa-% or more, and even more preferably 15,000 MPa-% or more. It is more preferable for the TS x EL to be higher, but the upper limit for TS x EL is approximately 25,000 MPa-% or less, considering the chemical composition and manufacturing conditions of the high-strength steel sheet of the present embodiment. (4) TS x λ: 40,000 MPa-% or more The high-strength steel sheet of the present embodiment preferably has a tensile strength (TS χ λ) of 40,000 MPa-% or higher. Having a TS χ λ of 40,000 MPa-% or higher allows for both excellent strength and excellent press formability. A TS χ λ of 50,000 MPa-% or higher is more preferably 50,000 MPa-% or higher, and even more preferably 60,000 MPa-% or higher. A higher TS χ λ is preferable, but the upper limit for TS χ λ is approximately 150,000 MPa-% or lower, considering the chemical composition and manufacturing conditions of the high-strength steel sheet of the present embodiment. (5) Load / sheet thickness where the load deforms the test piece to a bending angle of 10° in the bending test by German Association of Automotive Industry (VDA): 3.0 kN / mm or more. In the high-strength steel sheet of the present embodiment, the load / sheet thickness ratio at which the load deforms the test piece to a bending angle of 10° in the VDA bending test is preferably 3.0 kN / mm or more. Since the load / sheet thickness ratio at which the load deforms the test piece to a bending angle of 10° in the VDA bending test is preferably 3.0 kN / mm or more, it is possible to reduce the amount of deformation of the steel sheet in a collision or similar event, or to improve collision safety. The load / sheet thickness ratio is more preferably 3.1 kN / mm or more, and even more preferably 3.2 kN / mm or more.It is more preferable since the load / sheet thickness value is higher, but the upper limit of the load / sheet thickness value where the load deforms the test piece to a bending angle of 10° in the VDA bending test is approximately 5.0 kN / mm or less considering the chemical composition and manufacturing conditions of the high-strength steel sheet of the present modality. Manufacturing method The high-strength steel sheet of the present embodiment can be manufactured by following the procedure described below. The present inventors have found that a high-strength steel sheet having the desired steel microstructure described above, and consequently having the desired mechanical properties mentioned above, can be obtained by carrying out the heat treatment described in detail below on the original sheet having a predetermined chemical composition. Examples of the original sheet that undergoes heat treatment include a hot-rolled steel sheet obtained by hot-rolling a steel material, and a cold-rolled steel sheet obtained by further pickling and cold-rolling the hot-rolled steel sheet, and the conditions for hot rolling, pickling, and cold rolling are not particularly limited. An example of a heat treatment pattern carried out on an original sheet is illustrated schematically in Figure 1. Step A: Step to heat the steel sheet to a temperature T1 of the transformation point AC3 or higher and 950°C or lower and hold the steel sheet for a time t1 of 5 seconds or more and 1800 seconds or less in this temperature range for austenitization. In Step A in Figure 1, the steel sheet is heated to a temperature T1 (heating temperature T1) of the transformation point AC3 or higher and 950°C or lower and held for a time t1 (holding time t1) of 5 seconds or more and 1800 seconds or less in the temperature range. This makes it possible to sufficiently reverse the transformation of the microstructure of the steel sheet to austenite. The average heating rate HR1 ([1] in Figure 1) when the steel sheet is heated to the heating temperature T1 of the Acs transformation point or higher and 950°C or lower is not particularly limited, and the steel sheet can be heated at an arbitrary average heating rate HR1. For example, the steel sheet can be heated from room temperature to the heating temperature T1 by setting the average heating rate HR1 to 1°C / sec or higher and 100°C / sec or lower. When the heating temperature T1 ([2] in Figure 1) is lower than the transformation point AH, the reverse transformation to austenite becomes insufficient, ferrite remains, and thus the yield ratio YR decreases. Consequently, the heating temperature T1 is preferably set at or above the transformation point AH, and is preferably AH + 5°C or more, or more preferably AH + 10°C or more. When the heating temperature T1 exceeds 950°C, the crystal grains of the steel sheet microstructure can become coarser, and the hole expansion ratio λ can decrease; therefore, the heating temperature T1 is preferably set at or below 950°C. When the holding time t1 at the heating temperature T1 ([2] in Figure 1) is less than 5 seconds, the reverse transformation to austenite becomes insufficient, ferrite remains, and thus the yield ratio YR decreases. Consequently, the holding time t1 is preferably set at 5 seconds or more, and preferably 10 seconds or more, and most preferably 20 seconds or more. When the retention time t1 exceeds 1800 seconds, in addition to decreased productivity, the crystal grains of the steel sheet microstructure can become coarse, leading to a reduction in properties such as the hole expansion ratio λ. Therefore, the retention time t1 is preferably set at 1800 seconds or less, and more preferably 1500 seconds or less, and more preferably 1000 seconds or less. Step B: Step to carry out the cooling from the rapid cooling start temperature T2 of 700°C or more to the cooling detection temperature T3a in a temperature range of 300°C or more to 500°C or less at the average cooling rate of 10°C / sec or more In step B in Figure 1, the steel sheet, which has been heated and held in step A, is cooled from the rapid cooling start temperature T2 of 700°C or higher to the cooling detection temperature T3a of 300°C or higher and 500°C or lower at an average cooling rate of 10°C / sec or higher. This makes it possible to suppress ferrite precipitation during the cooling process. In the manufacturing method of the present embodiment, the average cooling rate CR1 ([3] in Figure 1) from the heating temperature T1 to the rapid cooling start temperature T2 is not particularly limited. Examples of the average cooling rate include cooling at 0.1 °C / sec or more and 5 °C / sec or less. When the rapid cooling start temperature T2 is below 700°C, ferrite precipitates and the yield ratio YR decreases. Therefore, the rapid cooling start temperature T2 is preferably set at 700°C or higher, and preferably 750°C or higher, and more preferably 800°C or higher. The upper limit of the rapid cooling start temperature T2 is not specifically limited and may be the heating temperature T1 in step A or lower. When the average cooling rate CR2 ([4] in Figure 1) from the rapid-start cooling temperature T2 to the cooling detection temperature T3a is less than 10 nC / sec, ferrite precipitates during cooling and the yield ratio YR decreases. Therefore, the average cooling rate CR2 is preferably set at 10°C / sec or higher, and is preferably 15°C / sec or higher, and more preferably 20°C / sec or higher. The upper limit of the average cooling rate CR2 is not specifically limited, but may be, for example, 100°C / sec or lower. The cooling detection temperature T3a is the temperature at which rapid cooling ends and also the retention start temperature in step C, as described later. When this cooling detection temperature T3a is below 300°C, the bainite that precipitates in the subsequent step becomes excessively hard, and the formation of the concentrated carbon portion does not proceed subsequently. The amount of retained austenite also decreases, and the desired TS x EL cannot be obtained. Therefore, the cooling detection temperature T3a is preferably set at 300°C or higher, and more preferably 320°C or higher, and even more preferably 340°C or higher. When the cooling detection temperature T3a exceeds 500°C, the bainite precipitated in the subsequent step becomes excessively soft, and the desired yield ratio YR cannot be achieved. As the concentrated carbon portion becomes coarse, retained austenite and MA form, and TS χ λ also decreases. Therefore, the cooling detection temperature T3a is preferably set at 500°C or lower, and more preferably at 480°C or lower, and more preferably at 460°C or lower. Step C: Step to carry out retention for time t3 of 10 seconds or more and less than 300 seconds at an average cooling rate of 10°C / seq or less in the temperature range of 300°C or more v 500°C or less In step C in Figure 1, the steel sheet cooled in step B is held for a time t3 (holding time t3: [5] in Figure 1) of 10 seconds or more and less than 300 seconds at an average cooling rate of 10°C / sec or less within a temperature range of 300°C or more and 500°C or less. This partially forms bainite. This bainite has a lower carbon solid solution boundary than austenite and thus expels the carbon zoicnn / zznz / E / Y that exceeds the solid solution boundary. As a result, a region of carbon-concentrated austenite forms around the bainite. This region becomes retained austenite after cooling in step D and reheating in step E, which will be described later. This retained austenite can improve TS x EL. When the temperature (holding temperature) at the time of retention in this step C is less than 300°C, the precipitated bainite becomes excessively hard, and the formation of the concentrated carbon portion does not proceed sufficiently. The amount of retained austenite also decreases, and the desired TS x EL cannot be obtained as described above. Therefore, the holding temperature is preferably set at 300°C or higher, and is preferably 320°C or higher, and more preferably 340°C or higher. When the holding temperature exceeds 500°C, the precipitated bainite becomes excessively soft, and the desired yield ratio (YR) cannot be achieved. As the carbon-concentrated portion thickens, retained austenite forms, and MA and TS x λ also decrease. Therefore, the holding temperature is preferably set at 500°C or lower, and more preferably at 480°C or lower, and more preferably at 460°C or lower. When the average cooling rate in step C exceeds 10°C / s, insufficient bainite transformation occurs. As a result, not enough carbon-rich concentrated regions form, the amount of retained austenite decreases, and the desired TS x EL cannot be achieved. Therefore, the average cooling rate in step C is preferably set at 10°C / s or less, and is preferably 7°C / s or less, and more preferably 3°C / s or less. Alternatively, the average cooling rate in step C can be 0°C / s, meaning the steel sheet can be held at a constant temperature. Within the above temperature range, the cooling rate can be changed, or cooling and holding at a constant temperature can be combined.In the holding step C, the temperature of the steel sheet can rise within the temperature of 300°C or more and 500°C or less due to heat generation due to the transformation of bainite. When the retention time t3 is less than 10 seconds, insufficient bainite transformation occurs. As a result, not enough carbon-concentrated regions form, the amount of retained austenite decreases, and the desired TS x EL cannot be achieved. Therefore, the retention time t3 at 300°C or higher and 500°C or lower is preferably set at 10 seconds or more, and preferably 20 seconds or more, and more preferably 30 seconds or more. zoicnn / zznz / E / Y However, when the holding time t3 is 300 seconds or more, the bainite transformation in step C progresses excessively. In this case, the carbon-concentrated portion becomes very large, and the retained austenite and MA in the microstructure of the annealed steel sheet become coarse. As a result, the hole expansion ratio λ decreases, and the desired TS χ λ cannot be achieved. Therefore, the holding time t3 at 300°C or more and 500°C or less is preferably set to less than 300 seconds, and is preferably 200 seconds or less, and more preferably 100 seconds or less. Step D: Step to carry out the cooling from the final holding temperature T3b to the cooling detection temperature T4 in the temperature range of 100°C or more v 300°C or less at the average cooling rate of 10°C / sec or more In step D in Figure 1, the steel sheet retained in step C is cooled from the final holding temperature T3b (corresponding to the cooling start temperature in step D) of 300°C or higher to the cooling detection temperature T4 over a temperature range of 100°C or higher to 300°C or lower at an average cooling rate CR3 ([6] in Figure 1) of 10°C / sec or higher. This allows a portion of the untransformed austenite to undergo martensitic transformation and the untransformed austenite that does not transform into martensite to be micronized. As a result, fine retained austenite and MA are obtained. The formed martensite is then hardened through the step described below, thereby improving the YR performance ratio and collision safety. When the average cooling rate CR3 is less than 10°C / s, the carbon-concentrated region expands more than necessary during cooling, MA becomes thicker, and thus TS χ λ decreases. Consequently, the average cooling rate CR3 is preferably set at 10°C / s or more, and is preferably 15°C / s or more, and most preferably 20°C / s or more. The upper limit of the average cooling rate CR3 is not specifically limited, but it can be, for example, 100°C / s or less. When the quench detection temperature T4 ([7] in Figure 1) exceeds 300°C, insufficient martensitic transformation occurs, MA in the microstructure of the annealed steel sheet becomes coarse, and the desired TS χ λ cannot be achieved. Simultaneously, a sufficient amount of quenched martensite cannot be introduced into the microstructure of the annealed steel sheet, and the yield ratio YR also decreases. Therefore, the quench detection temperature T4 is preferably set at 300°C or less, and more preferably at 280°C or less, and more preferably at 260°C or less. zoicnn / zznz / E / Y When the quench detection temperature T4 is below 100°C, the martensitic transformation proceeds excessively, and the desired amount of retained austenite cannot be ensured in the microstructure of the annealed steel sheet. As a result, the total elongation EL decreases, and the desired TS χ EL cannot be achieved. Therefore, the quench detection temperature T4 is preferably set at 100°C or higher, and is preferably 120°C or higher, and more preferably 140°C or higher. In this step D, after cooling is stopped, as illustrated in Figure 1 [7], the temperature can be held at the cooling detection temperature T4, but it is preferable to carry out the reheating step described below without holding at the cooling detection temperature T4. When holding is carried out at the cooling detection temperature T4, it is preferable that the holding time t4 be set to 1 second or more and 600 seconds or less. The properties are hardly affected when the holding time t4 at the cooling detection temperature T4 is prolonged, but productivity decreases when the holding time t4 exceeds 600 seconds. [Step E: step to carry out heating from the cooling detection temperature T4 to the heating temperature T5 in the temperature range of 300°C or more and 500°C or less and carry out holding with time t5 of 350 seconds or more and 1800 seconds or less in the temperature range of the reheating temperature T5] In step E in Figure 1, the steel sheet cooled in step D is heated to a reheat temperature T5 that meets the following requirement: 300°C or more and 500°C or less, and is held for a time (holding time t5) of 350 seconds or more and 1800 seconds or less within the temperature range of the reheat temperature T5 ([9] in Figure 1). This reheat temperature T5 needs to meet the requirement that the difference between the reheat temperature T5 and the average temperature [(T3a - T3b) / 2] of the cooling detection temperature T3a in step B (which corresponds to the holding start temperature in step C) and the final holding temperature T3b is 50°C or less. In step E, a portion of the transformed austenite undergoes bainitic transformation, and simultaneously, the martensite precipitated in step D is quenched. This allows for the control of the IQ asymmetry of the microstructure, which has both a bcc and a bct structure, to a desired value. At the same time, carbon in the bainite and martensite is released, promoting carbon concentration in the surrounding austenite, which can stabilize it. As a result, the final amount of retained austenite can be... 7QICnn / 77n7 / E / Y increase. The heating rate HR2 up to the reheating temperature T5 ([8] in Figure 1) is not particularly limited, and examples thereof include 1°C / sec or more and 50°C / sec or less. When the reheating temperature T5 is below 300°C, the martensite cannot be sufficiently quenched, the IQ symmetry exceeds -0.3, and the desired TS x EL cannot be achieved. Therefore, the reheating temperature T5 is preferably set at 300°C or higher, and is preferably 320°C or higher, and more preferably 340°C or higher. When the reheating temperature T5 exceeds 500°C, carbon precipitates as cementite, and a sufficient amount of retained austenite cannot be ensured. As a result, the total elongation EL decreases, and the desired TS x EL cannot be achieved. Therefore, the reheating temperature T5 is preferably set at 500°C or less, and more preferably at 480°C or less, and more preferably at 460°C or less. When the reheating temperature T5 is more than 50°C higher than the average temperature of the cooling detection temperature T3a and the final holding temperature T3b, the bainite precipitated in this step E becomes excessively banded and the IQ symmetry is less than -1.2. As a result, the desired collision safety cannot be obtained. When the reheat temperature T5 is more than 50°C lower than the average of the cooling detection temperature T3a and the final holding temperature T3b, the difference in hardness between the bainite precipitated in step C and the quenched bainite and martensite precipitated in step E becomes larger, and the hole expansion ratio λ decreases. For this reason, the difference between the reheat temperature T5 and the average of the cooling detection temperature T3a and the final holding temperature T3b is preferably 50°C or less, preferably 40°C or less, and most preferably 30°C or less. When the holding time t5 ([9] in Figure 1) after reheating in this step E is less than 350 seconds, the desired quenched state of hard microstructures such as martensite cannot be achieved, and the IQ asymmetry of crystal grains with a bcc and a bct structure is less than -1.2. As a result, the desired collision safety cannot be obtained. Therefore, the holding time t5 after reheating is preferably set at 350 seconds or more, and preferably 380 seconds or more, and more preferably 400 seconds or more. When the holding time t5 after reheating is greater than 1800 seconds, the mechanical properties are hardly affected, but a decrease in the zoicnn / zznz / E / Y productivity is caused.Therefore, the retention time t5 is preferably set to 1800 seconds or less, and is preferably 1200 seconds or less, most preferably 600 seconds or less. After maintaining the reheating temperature T5, cooling takes place, but the average cooling rate CR4 (

[10] in Figure 1) at this time is not particularly limited, and for example, the average cooling rate CR4 is 1°C / sec or more and 50°C / sec or less. In the high-strength steel sheet of the present embodiment, the steel sheet may undergo plating, such as electroplating and deposition plating, and may also undergo alloying after plating. The steel sheet may undergo surface treatments such as organic film formation, film rolling, organic salt treatment, inorganic salt treatment, and chromium de-icing. When hot-dip galvanizing is carried out on steel sheets as a coating, for example, the temperature of the steel sheet is raised or lowered to no less than the temperature of the galvanizing bath (-40°C) and no more than the temperature of the galvanizing bath (+50°C), and the steel sheet is allowed to pass through the galvanizing bath. Through this hot-dip galvanizing process, a steel sheet with a hot-dip galvanized layer on its surface can be obtained; that is, a hot-dip galvanized steel sheet. When alloying is carried out after hot-dip galvanizing, for example, hot-dip galvanized steel sheet is heated to a temperature of 460°C or higher and 600°C or lower. When the heating temperature is below 460°C, the alloying may be insufficient. When the heating temperature is above 600°C, the alloying may be excessive, and corrosion resistance may be compromised. Through alloying, a steel sheet with an alloyed hot-dip galvanized coating on the surface can be obtained; that is, an alloyed hot-dip galvanized steel sheet. It should be noted that the above embodiments are merely specific examples of carrying out the present invention, and the technical scope of the present invention should not be interpreted as limited by them. In other words, the present invention can be implemented in various ways without departing from the technical idea or its main features. This specification describes several aspects of a technique like the one described above, but the main techniques are summarized below. A high-strength steel sheet according to one aspect of the present invention contains zoicnn / zznz / E / Y each of C: from 0.10% to 0.35%; Si + Al: 0.5% to 3.0%; Mn: 1.0% to 3.0%; P: more than 0% and 0.05% or less; S: more than 0% and 0.01% or less in terms of mass percent; and unavoidable iron and impurities as remainder, wherein the high-strength steel sheet has a metallographic microstructure with a ferrite fraction 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area ratio and retained austenite fraction: 5% to 30% in terms of volume ratio, and an IQ asymmetry analyzed by an EBSD method is -1.2 to -0.3 when the asymmetry is expressed by the following Formula (1) in a case where the crystal grains having a bcc structure and a bct structure are considered an aggregation of regions having an area of ​​0.05 μm2. ” V (*l ~ . Formula (n~l)(n~2)ZA s / Each variable in Formula (1) indicates the following. n: Total number of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2s: Standard deviation of the Cl in a region with an area of ​​0.05 μm2x¡: IQ of a region i with an area of ​​0.05μ m2xave: Mean IQ of the regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2 This configuration makes it possible to provide a high-strength steel sheet that offers both excellent collision safety and excellent moldability. As a preferred embodiment of the present invention, it is also effective to further contain (a) at least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.5% or less, (b) at least one selected from the group consisting of Ni: more than 0% and 2% or less, Cr: more than 0% and 2% or less, and Mo: more than 0% and 0.5% or less, (c) B: more than 0% and 0.005% or less, (d) at least one selected from the group consisting of Mg: more than 0% and 0.04% or less, REM: more than 0% and 0.04% or less, and Ca: more than 0% and 0.04% or less, and the like in the high-strength steel sheet, if necessary. The properties of high-strength steel sheet are further enhanced depending on the types of elements it contains. Another preferred embodiment of the present invention also includes a high-strength steel sheet having a plating layer on the surface of the steel sheet. As another aspect of the present invention, a method for manufacturing a high-strength steel sheet is also included, and this manufacturing method includes, in this order, after heating a steel material having the aforementioned chemical composition, subjecting the steel material to hot rolling, cooling and rolling the steel material after hot rolling, and subjecting the steel material to pickling and cold rolling, a step of heating the steel material to a temperature T1 of a transformation point Ac of the steel or higher and 950°C or lower, and holding the steel material for a time t1 of 5 seconds or higher and 1800 seconds or lower in this temperature range for austenitization; a cooling step from a rapid cooling start temperature T2 of 700°C or more to a cooling stop temperature T3a in a temperature range of 300°C or more and 500°C or less at an average cooling rate CR2 of 10°C / sec. or more; a step in which retention is carried out for a time t3 of 10 seconds or more and less than 300 seconds at an average cooling rate of 10°C / sec. or less in a temperature range of 300°C or more and 500°C or less; a cooling step from a final holding temperature T3b of 300°C or more to a cooling stop temperature T4 within a temperature range of 100°C or more and 300°C or less at an average cooling rate CR3 of 10°C / sec or more; and a step in which heating is carried out from the cooling stop temperature T4 to a reheating temperature T5 that satisfies the following requirement within a temperature range of 300°C or more and 500°C or less and holding is carried out for a time t5 of 350 seconds or more and 1800 seconds or less within the temperature range of the reheating temperature T5, a difference between the reheating temperature T5 and an average temperature of the cooling stop temperature T3a and the final holding temperature T3b that is equal to or less than 50°C. Using this configuration, it is possible to manufacture a high-strength steel sheet that exhibits both excellent collision safety and excellent moldability, as described above. When manufacturing a high-strength steel sheet that has a plating layer on the surface of the steel sheet as described above, the manufacturing method is only required to include an additional step of applying a zoicnn / zznz / E / Y plating layer to the surface of the steel sheet. Hereafter, the present invention will be described more specifically with reference to the Examples, but the present invention is not limited to the following Examples, and of course, it is also possible to carry out the present invention by suitably adding changes within a range that is compatible with the aforementioned essence and the essence mentioned below, and all of them are included within the technical scope of the present invention. EXAMPLES The cast steels (steel types A to E) having the chemical compositions (remainder: iron and unavoidable impurities other than P and S) presented in Table 1 below were produced by vacuum casting, and the cast materials were hot-rolled to a veneer thickness of 30 mm, and then hot-rolled again. The transformation point Acs of each steel type presented in Table 1 below are the values ​​calculated by the following Formula (2) created with reference to Formula VII-20 described on page 273 of “The Physical Metallurgy of Steels: 1985, William C. Leslie”. In Table 1, the column indicates that no addition is made or that the value is below the measurement limit. Transformation point AC3 (°C) = 910 203 [-xC]1 / 215-.2 [xNi] + 44.7 [xSi] + 104 [xV] + 31.5 [xMo] 30 [-xMn] 11 [-xCr] + 700 [xP] + 400 [xAI] + 400 [xT¡] - - - Formula (2) In formula (2), [C], [Ni], [Si], [V], [Mo], [Mn], [Cr], [P], [Al] and [Ti] signify the mass percentages of C, Ni, Si, V, Mo, Mn, Cr, P, Al and Ti, respectively, and elements not included are calculated as “0% by mass”. zoicnn / zznz / E / Y Table 1 Steel Type Chemical Composition (% by mass) Transformation Point AC3 (°C) C Si Mn PS Al Ti BA 0.194 1.97 2.67 0.006 0.0008 0.040 0.017 0.0001 856 B 0.218 1.11 2.25 0.010 0.0012 0.045 0.031 0.0023 835 C 0.293 1.54 2.04 0.007 0.0010 0.039 - - 828 D 0.250 2.06 2.06 0.008 0.0010 0.037 - 0.0001 859 E 0.204 1.80 2.14 0.006 0.0008 0.048 - - 858 In the hot rolling process, the sample material was heated to 1250°C and then rolled to a veneer thickness of 2.6 mm in several stages. The final hot rolling temperature was set at 920°C. The sample material was then cooled to 600°C at an average cooling rate of 30°C / sec, placed in a 600°C furnace, held for 30 minutes, and then furnace cooled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was pickled to remove surface inclusions and then cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.4 mm as the original sheet. The original sheet obtained was subjected to the heat treatment presented in Table 2 (steps A to C illustrated in Figure 1) and in Table 3 (steps D to E illustrated in Figure 1) below to obtain a sample. In test n21 of Tables 2 and 3, after reheating and holding in step E, a heat treatment simulating hot-dip galvanizing was performed, consisting of holding at 460°C, corresponding to immersion in a coating bath, and holding at 500°C, corresponding to an alloying furnace. In Tables 2 and 3, the average cooling rates CR1, CR2, and CR3 are expressed as negatives to distinguish them from the heating rates. zoicnn / zznz / E / Y Table 2 Test No. Steel Type Step A Step B Step C Average heating rate HR1 (°C / sec) Heating temperature T1 (°C) Holding time t1 (seconds) Average cooling rate CR1 (°C / sec) Rapid cooling start temperature T2 (°C) Cooling stop temperature T3a (°C) Average cooling rate CR2 (°C / sec) Holding start temperature (corresponding to T3a) (°C) Holding end temperature T3b (°C) Holding time t3 (seconds) Average cooling rate (°C / sec) 1 A 5 930 126 -2 910 400 -12 400 400 71 0 2 B 4 860 161 -2 840 400 -50 400 400 80 0 3 C 13 930 295 -2 910 400 -50 400 400 45 0 4 D 50 930 180 -2 910 400 -50 400 400 50 0 5 E 15 930 246 -2 910 425 -50 425 425 37 0 6 E 3 800 144 -2 780 190 -50 None None None None 7 E 4 890 144 -2 870 190 -50 None None None Table 3 Test No. Steel Type Step D Step E Note: Cooling start temperature T3b (°C) Cooling stop temperature T4 (°C) Average cooling rate CR3 (°C / sec) Holding time t4 (seconds) Reheating rate HR2 (°C / sec) Reheating temperature T5 (°C) (T3aT3b-) / 2 - T5(°C) Holding time t5 (seconds) Average cooling rate CR4 (°C / sec) 1 A 400 200 -13 7 30 400 0 445 10 With alloy hot-dip galvanizing simulation 2 B 400 250 -31 5 30 400 0 384 10 - 3 C 400 250 -12 2 30 370 30 441 10 - 4 D 400 200 -50 50 50 400 0 450 10 - 5 E 425 200 -21 2 30 400 25 335 10 - 6 E 780 190 -50 None 30 420 - 402 10 - 7 E 870 190 -50 None 30 420 - 402 10 - zoicnn / zznz / E / Y The metallographic microstructure and IQ asymmetry of the sample obtained at the plating thickness position / 4 were determined using the methods mentioned. The results are presented in Table 4 below. Using the obtained sample, a tensile test, a hole expansion test, and a VDA bending test were performed using each of the following methods, and the mechanical properties of the steel sheet were measured. The results are presented in Table 5 below. Tensile Test JIS n.25 tensile test pieces were collected from the sample such that the direction perpendicular to the rolling direction of the steel sheet was the longitudinal direction, and were subjected to measurement of the yield strength YP, tensile strength TS, and total elongation EL in accordance with the method prescribed in JIS Z 2241: 2011. Hole expansion test The sample was subjected to the hole expansion test prescribed by the JFS T1001 standard to measure the hole expansion ratio X. VDA flex test Based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry, the bending test was performed under the following conditions, and the displacement with the maximum load measured in the bending test was converted into an angle based on the VDA standard to determine the bending angle. The load was then evaluated at a bending angle of 10°. Measurement conditions Test method: Roller support, punch push Roll diameter: φ30 mm Punch shape: Tip R = 0.4 mm Distance between rollers: 2.9 mm Punch thrust speed: 20 mm / min Dimensions of the test piece: 60 mm x 60 mm Bending direction: Perpendicular to the rolling direction Testing machine: SHIMADZU AUTOGRAPH (maximum load: 20 kN) Table 4 Test No. Steel Type Metallographic Microstructure Cl Asymmetry Ferrite Fraction MA Fraction Remaining Hard Phase Austenite Fraction (% by area) (% by area) (% by area) retained (% by volume) 1 A 1 9 90 7.5 -0.875 2 B 0 11 89 9.1 -0.883 3 C 0 16 84 12.8 -0.669 4 D 2 13 85 10.5 -0.620 5 E 0 13 87 12.9 -1.665 6 E 49 14 37 14.6 -1.409 7 E 3 7 90 6.2 -0.164 zoicnn / zznz / E / Y Table 5 Test No. Steel Type Mechanical Properties of Steel Sheet Note: Yield Strength YS (MPa) Tensile Strength TS (MPa) Yield Ratio YR (-) Total Elongation EL (%) TS ELx (MPa-%) Hole Expansion Ratio λ (%) TS (xMPa-%) Load with a bending angle of 10° in the bending test VDA / sheet thickness (kN / mm) 1 A 1007 1189 0.85 15.0 17835 88.0 104632 3.29 Example 2 B 1030 1169 0.88 14.4 16834 52.7 61606 3.37 Example 3 c 1028 1218 0.84 14.7 17905 51.2 62362 3.44 Example 4 D 1207 1374 0.88 12.6 17312 49.8 68425 3.56 Example 5 E 830 1045 0.79 16.8 17556 36.7 38352 2.56 Comparative Example 6 E 633 1043 0.61 23.2 24198 23.3 24302 2.08 Comparative Example 7 E 1136 1282 0.89 9.4 12051 53.4 68459 3.04 Comparative Example Based on these results, the following can be considered. First, test numbers 1 to 4 all satisfy the chemical composition present in the high-strength steel sheet of the present modality (steel types A to D in Table 1), are manufactured under the manufacturing conditions prescribed in the manufacturing method of the present modality to obtain the desired steel microstructure, and therefore exhibit excellent collision safety (load at a bending angle of 10° in the VDA / sheet thickness bending test) in addition to high tensile strength TS, high yield ratio YR and high moldability (TS ELx, TS )χλ. On the other hand, tests ns5 to 7 do not meet the manufacturing conditions prescribed in the manufacturing method of the present modality, so the desired steel microstructure has not been obtained, and as a result, some of the mechanical properties are lower.

[0124] Specifically, Test No. 5 is an example where the retention time t5 in step E is shorter than the desired condition, and the IQ asymmetry is less than the desired value. As a result, the desired mechanical properties with respect to TS and collision safety have not been achieved. Test nQ6 is an example where the heating temperature in step A is low (below the transformation point A) and step C is omitted. Because step C is omitted, soft bainite has not precipitated, but a large amount of ferrite (a microstructure with a high IQ) has precipitated in step A. The average IQ value has increased, and the IQ asymmetry has decreased. As a result, the amount of ferrite and the IQ asymmetry have deviated from the desired ranges, thus decreasing the YR creep ratio, TS, and collision safety, and the desired mechanical properties have not been achieved. Test ne7 does not include step C, so it is expected that the soft bainite has not precipitated but the amount of harder martensite (a microstructure that has a low IQ) has increased in step D. As a result, unlike in test ns6, the IQ asymmetry is greater than the desired value and the TS EL xha has decreased. This application is based on Japanese patent application ne2019-168392 filed on September 17, 2019, the contents of which are included in this application. In order to express the present invention, it has been adequately and sufficiently described above through the embodiments with reference to specific and similar examples. However, those skilled in the art should recognize that changes and / or improvements to the embodiments described above can easily be made. Accordingly, changes or improvements made by those skilled in the art shall be construed as being included within the scope of the claims, unless the changes or improvements are at a level that falls outside the scope of the appended claims. Industrial Applicability The present invention has broad industrial applicability in the technical field related to high-strength steel sheets and methods of manufacturing them.

Claims

1. A high-strength steel sheet comprising each of: C: 0.10% to 0.35%; Si + Al: 0.5% to 3.0%; Mn: 1.0% to 3.0%; P: more than 0% and 0.05% or less; S: more than 0% and 0.01% or less in terms of mass percent; and unavoidable iron and impurities as residue, wherein the high-strength steel sheet has a metallographic microstructure having a ferrite fraction: 0% to 10%, an MA fraction: 0% to 30%, a hard phase other than ferrite and MA: 70% to 100% in terms of area ratio and a retained austenite fraction: 5% to 30% in terms of volume ratio, and an IQ asymmetry as analyzed by an EBSD method is -1.2 to -0.3 when the asymmetry is expressed by the following Formula (1) in a case where the crystal grains having a bcc structure and a bct structure are considered as an aggregation of regions having an area of ​​0.05 μm2: TI J · A*************·*»******** j ♦ * ♦ (n — l)(n - 2)Zj \ s / Formula each variable in Formula (1) indicates the following: n: total number of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2, s: standard deviation of IQ in a region that has an area of ​​0.05 μm2, x¡: IQ of a region i that has an area of ​​0.05 μm2, y xave: average IQ of regions that have a bcc structure and a bct structure and an area of ​​0.05 μm2.

2. The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.5% or less.

3. The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Ni: more than 0% and 2% or less, Cr: more than 0% and 2% or less, and Mo: more than 0% and 0.5% or less.

4. The high-strength steel sheet according to claim 1, further comprising B: more than 0% and 0.005% or less.

5. The high-strength steel sheet according to claim 1, further comprising at least one selected from the group consisting of Mg: more than 0% and 0.04% or less, REM: more than 0% and 0.04% or less, and Ca: more than 0% and 0.04% or less.

6. The high-strength steel sheet according to claim 1, having a coating layer on a surface of the steel sheet.

7. A method of manufacturing high-strength steel sheet, which is a method of manufacturing high-strength steel sheet according to any one of claims 1 to 5, the method comprising, in this order: after heating a steel material having the chemical composition according to any one of claims 1 to 5, then subjecting the steel material to hot rolling, cooling and rolling the steel material after completing the hot rolling, and then subjecting the steel material to pickling and cold rolling, a step of heating the steel material to a temperature T1 of a transformation point A of steel or higher and 950°C or lower and holding the steel material for a time t1 of 5 seconds or higher and 1800 seconds or lower in this temperature range for austenitization;a step to carry out cooling from a rapid cooling start temperature T2 of 700°C or more to a cooling detection temperature T3a in a temperature range of 300°C or more and 500°C or less at an average cooling rate CR2 of 10°C / sec or more; a step to carry out holding for a time t3 of 10 seconds or more and less than 300 seconds at an average cooling rate of 10°C / sec or less in a temperature range of 300°C or more and 500°C or less; a step to carry out cooling from a final holding temperature T3b of 300°C or more to a cooling detection temperature T4 in a temperature range of 100°C or more and 300°C or less at an average cooling rate CR3 of 10°C / sec or more;and a step to carry out heating from the cooling hold temperature T4 to a reheat temperature T5 that meets the following requirement within a temperature range of 300°C or more and 500°C or less, and carrying out the hold for a time t5 of 350 seconds or more and 1800 seconds or less within the temperature range of the reheat temperature T5, a difference between the reheat temperature T5 and an average temperature of the cooling hold temperature T3a and the final hold temperature T3b that is 50°C or less. zoicnn / zznz / E / Y; 8. The manufacturing method according to claim 7, which is a method for manufacturing the high-strength steel sheet according to claim 6, the method further comprising a step for forming a coating layer on a surface of the steel sheet.