HIGH STRENGTH STEEL SHEET

Abstract

A high-strength steel sheet according to the present invention contains a predetermined chemical composition, in a metallographic structure, the total area ratio of quenched martensite and bainite is 80% or more, at a position 1 / 4 of the sheet thickness of a cross section parallel to a rolling direction and perpendicular to a rolled surface, the standard deviation of the numerical densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5 × 1010 numbers / mm3, and the tensile strength is 780 MPa or more.

Description

HIGH STRENGTH STEEL SHEET Technical field of the invention [1] The present invention relates to a high-strength steel sheet that has excellent tensile strength, total elongation and bending capacity, and that is excellent in terms of material quality stability. Priority is claimed over Japanese Patent Application No. 2019-128590, filed in Japan on July 10, 2019, the contents of which are incorporated herein by reference. Related technique [2] Hot-rolled steel sheet, manufactured by hot rolling, is widely used as a relatively inexpensive structural material and as a material for structural elements of vehicles or industrial equipment. Specifically, the reinforcement of hot-rolled steel sheet, used in suspension parts, bumper parts, or impact-absorbing parts of vehicles, is progressing, while also possessing excellent workability, so that hot-rolled steel sheet can withstand forming into complex shapes, is necessary for hot-rolled steel sheet. [3] So far, low-strength steel sheets have had a relatively simple structural configuration in which a ferrite structure is the main component and strength is ensured with a small amount of solid solution strengthening element as required, whereas in high-strength steel, a low-temperature transformation structure such as bainite or martensite or a precipitate such as Tic is used to ensure strength and thus the structure configuration of high-strength steel becomes complex.These transformation, precipitation, or similar phenomena are significantly affected by the temperature history. In a single manufacturing step for hot-rolled steel sheet, the temperature history can vary in the width and length directions due to irregularities in the cooling water application method in the width direction, irregularities in the cooling rate depending on the coil's position after winding, or similar factors. In high-strength hot-rolled steel sheets, it is important to suppress formability instability (irregularity in mechanical properties in either the width or length direction of a coil) caused by the aforementioned temperature irregularities. [4] Patent document 1 reports on a technique in which both high strength and excellent formability are obtained by surface rolling of a hot-rolled steel sheet and heating the hot-rolled steel sheet in a temperature range of 600 to 750°C to precipitate fine carbides. [5] Incidentally, with respect to material quality stability, Patent Document 2 reports a technique in which, in a hot-rolled steel sheet having a tensile strength of 780 MPa or more, the amount of added Ti and V is controlled to be within a certain range, so that fine carbides precipitate uniformly during hot rolling and coiling and, consequently, the material quality of the hot-rolled steel sheet is stabilized. Previous technique document Patent document [6] Patent Document 1: International Publication No. W02010 / 137317 Patent Document 2: Unexamined Japanese Patent Application, First Publication No. 2013-100574 Description of the invention Problems that must be solved by the invention [7] However, the inventors of the present invention found that the prior art cannot achieve sufficient stability in material quality. An object of the present invention is to provide a high-strength, hot-rolled steel sheet that has excellent tensile strength, total elongation, and bending capacity, and that is excellent in terms of material quality stability. Material quality stability means that the variation in tensile strength and total elongation is small in each part of a steel sheet. Means to solve the problem [8] (1) A high-strength steel sheet according to an aspect of the present invention contains, as a chemical composition, in % by mass, C: 0.030% to 0.280%, Si: 0.05% to 2.50%, Mn: 1.00% to 4.00%, Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, O: 0.0100% or less, Ti: 0% to 0.20%, Nb: 0% to 0.20%, total Ti and Nb: 0.04% to 0.40%; B: 0% to 0.010%; V: 0% to 1,000%, Cr: 0% to 1,000%, Mo: 0% to 1,000%, Cu: 0% to 1,000%, Co: 0% to 1,000%, W: 0% to 1,000%, Ni: 0% to 1,000%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.0100%, and a remainder that includes Fe and impurities, wherein, in a metallographic structure, a total area ratio of quenched martensite and bainite is 80% or more, at a position 1 / 4 of the sheet thickness of a cross section parallel to a rolling direction and perpendicular to a rolled surface, a standard deviation of number densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5 × 1010 numbers / mm3, wherein the number densities are measured at 10 points every 50 mm in a width direction, and the tensile strength is 780 MPa or more. (2) In high-strength steel sheet conforming to (1), the standard deviation of the surface roughness Ra may be 1.0 pm or less, wherein the surface roughness Ra is measured at 10 positions at 50 mm intervals along the width direction. (3) High-strength steel sheet conforming to (1) or (2) may contain, as its chemical composition, in % by mass, at least one of the group consisting of B: 0.001% to 0.010%, V: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Co: 0.005% to 1.000%, W: 0.005% to 1.000%, Ni: 0.005% to 1.000%, Ca: 0.0003% to 0.0100%, Mg: 0.0003% to 0.0100%, REM: 0.0003% to 0.0100% and Zr: 0.0003% to 0.0100%. (4) In high-strength steel sheet conforming to any of (1) to (3), the total elongation may be 10% or more, and R / t, which is a value calculated by dividing the limit bending radius by the thickness, may be 2.0 or less. Effects of the invention [9] In accordance with the aspect described above, it is possible to obtain a high-strength steel sheet that has excellent tensile strength, total elongation and bending capacity and that is excellent in terms of material quality stability. Brief description of the drawings

[10] Figure 1 is a conceptual diagram showing a section observed to evaluate a metallographic structure. Figure 2 is a conceptual diagram showing an observed section for evaluating the standard deviation of the numerical densities of precipitates. Modalities of the invention

[11] The inventors herein sought a method for stabilizing the material quality of a high-strength steel sheet. A hot-rolled steel sheet is coiled after hot rolling into a coil shape, and the cooling rate of the hot-rolled steel sheet after coiling can vary depending on its position on the coil. Due to this variation in cooling rate, the volume ratio of a transformation structure, the numerical density of precipitates, or similar properties can vary considerably depending on the position on the coil. The inventors herein found that such a phenomenon can cause instability in the material quality. On the other hand, when hot-rolled steel sheet is cooled to a relatively low temperature (500°C or less) in the quenching zone after the final hot-rolling stage, and then coiled, the overall structure of the hot-rolled steel sheet becomes a low-temperature transformation structure (such as bainite, martensite, or similar), and the strength-contributing precipitates of substitute elements (Ti, Nb) do not precipitate extensively. The inventors of the present invention found that, in this case, the uneven volume ratio of the transformation structure and the uneven number density of the precipitates are minimal, and therefore the material quality can be stabilized. However, the structure obtained by the method described above is primarily composed of the low-temperature transformation structure, which has low work hardenability.Therefore, the total elongation of the steel sheet obtained by the method described above can be relatively low, such as less than 10%, or 9% or less. To broaden the range of parts for which the steel is applicable, further improvement of its formability is desirable.

[12] The inventors hereof attempted to temper the hot-rolled steel sheet, which was coiled at the low temperature described above, to a temperature of 500°C or higher. Consequently, the dislocation introduced during transformation was recovered, and the hot-rolled steel sheet had an excellent property in which the total elongation was 10% or more. However, tempering the transformation structure at low temperature decreases the strength. Therefore, the inventors hereof induced precipitation hardening in the steel sheet by incorporating alloying elements such as Ti and Nb into the steel sheet, which precipitate at 550°C or higher, and improved both the total elongation and the strength.

[13] However, it was found that when the surface of hot-rolled steel sheet prior to quenching has roughness irregularities due to scale removal irregularities during finishing rolling, these roughness irregularities cause irregularities in emissivity during the temperature increase for quenching, and the heating temperature can vary depending on the position. Such temperature irregularity leads to an uneven precipitation density and, consequently, to instability in the material quality.

[14] Therefore, the inventors of the present repeated intensive studies and invented a method that can reduce the surface roughness of hot-rolled steel sheet before tempering by properly controlling the temperature during hot rolling, the steel sheet component and the descaling method, and reduce temperature irregularities caused by surface roughness during tempering to obtain a high-strength steel sheet that is excellent in terms of material quality stability.

[15] A high-strength steel sheet according to one embodiment of the present invention is described in detail below. The present invention is not limited to the embodiment described herein and may be modified in various ways within the scope of the substance of the present invention. Furthermore, the numerical limiting ranges described below include the lower and upper limits of the ranges. Numerical values ​​expressed as 'more than' or 'less than' are not included in the numerical ranges. With respect to the quantity of each element, it means % by mass.

[16] In a high-strength steel sheet 1 conforming to the present modality, a rolling direction RD, a thickness direction TD, and a width direction WD shown in Figure 1 and Figure 2 are defined as described below. The rolling direction RD means a direction in which the steel sheet is moved by a roller during rolling. The thickness direction TD is a direction perpendicular to a rolled surface 11 of the steel sheet. The width direction WD is a direction perpendicular to the rolling direction RD and the thickness direction TD. The rolling direction RD can be easily specified based on the grain size distribution of the steel sheet. Therefore, the rolling direction RD can be specified even for a steel sheet cut from a sheet of rolled material.

[17] In high-strength steel sheet conforming to this modality, the ratio of total area of ​​quenched martensite to bainite is regulated. The area ratio of the metallographic structure is measured in a cross-section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 (see Figure 1). Hereafter, there will be cases where the cross-section parallel to the rolling direction RD and perpendicular to the rolled surface 11 will be referred to simply as the cross-section parallel to the rolling direction RD. A detailed method for evaluating the metallographic structure will be described below.

[18] In high-strength steel sheet conforming to this modality, the standard deviation of the numerical densities of precipitates (precipitates including Ti / Nb) having a diameter of 10 nm or less and including one or both Ti and Nb is regulated. The numerical density of precipitates including Ti / Nb is measured on a sheet thickness of 1 / 4 at position 121 of cross-section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 (see Figure 2). Ten cross-sections 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 are produced every 50 mm along the width direction WD, and the standard deviation of the 10 numerical densities measured on these surfaces is considered the standard deviation of the numerical densities of precipitates including Ti / Nb in accordance with this modality.

[19] The 1 / 4-thickness position is a position at a depth of 1 / 4 of the thickness of steel sheet 1 from the rolled surface 11 of steel sheet 1. In Figure 1 and Figure 2, only the position at a depth of 1 / 4 of the thickness of steel sheet 1 from the upper rolled surface 11 of steel sheet 1 is shown as the 1 / 4-thickness position. However, it goes without saying that the position at a depth of 1 / 4 of the thickness of steel sheet 1 from the lower rolled surface 11 of steel sheet 1 can also be treated as the 1 / 4-thickness position. Furthermore, Figure 2 shows only some of the 10 measuring surfaces for the numerical density.Furthermore, Figure 2 simply shows the numerical density measurement points conceptually, and there is no need to form the numerical density measurement surfaces as shown in Figure 2 as long as a predetermined requirement is met. A detailed method for evaluating the standard deviation of the numerical densities of precipitates, including Nb / Ti, will be described below.

[20] High-strength steel sheet The high-strength steel sheet conforming to this specification contains, as its chemical composition, in % by mass, C: 0.030% to 0.280%, Yes: 0.05% to 2.50%, Mn: 1.00% to 4.00%, In the sun: 0.001% to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, 0: 0.0100% or less, Ti: 0% to 0.20%, Nb: 0% to 0.20%, total of Ti and Nb: 0. B: 0% to 0.010%, V: 0% to 1.000%, Cr: 0% to 1,000%, Mo: 0% to 1,000%, Cu: 0% to 1,000%, Co: 0% to 1,000%, W: 0% to 1,000%, Ni: 0% to 1,000%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.0100% and 0.4% to 0.40%; The remainder: Fe and impurities in a metallographic structure, a total area ratio of quenched martensite and bainite is 80% or more at a position 1 / 4 of the sheet thickness of a cross section parallel to a rolling direction and perpendicular to a rolled surface, a standard deviation of the number densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5 × 1010 numbers / mm3, wherein the number densities are measured at 10 points every 50 mm in a width direction, and a tensile strength of 780 MPa or more.

[21] 1. Chemical Composition The composition of the high-strength steel sheet according to this specification will be described in detail below. The high-strength steel sheet according to this specification contains, as its chemical composition, basic elements and one optional element as required, and the remainder includes Fe and impurities.

[22] C: 0.030% or more and 0.280% or less Carbon is an important element for ensuring the strength of steel sheets. When the carbon content is less than 0.030%, a tensile strength of 780 MPa or higher cannot be guaranteed. Therefore, the carbon content is adjusted to 0.030% or higher, preferably 0.050% or higher, 0.100% or higher, or 0.120% or higher.

[23] On the other hand, when the C content is above 0.280%, since the weldability becomes poor, the upper limit is adjusted to 0.280%. The C content is preferably 0.250% or less, or 0.200% or less, and very preferably 0.150% or less, 0.140% or less, 0.130% or less, or 0.120% or less.

[24] If: 0.05% or more and 2.50% or less Silicon (Si) is an important element that can improve the material's strength by strengthening the solid solution. When the Si content is less than 0.05%, the yield strength deteriorates, and therefore the Si content is adjusted to 0.05% or higher. The Si content is preferably 0.10% or higher, and very preferably 0.30% or higher, 1.00% or higher, or 1.20% or higher.

[25] On the other hand, when the Si content is greater than 2.50%, since deterioration of surface properties occurs, the Si content is adjusted to 2.50% or less. The Si content is preferably 2.00% or less, very preferably 1.80% or less, 1.50% or less, or 1.30% or less.

[26] Mn: 1.00% or more and 4.00% or less Manganese (Mn) is an effective element for increasing the mechanical strength of steel sheets. When the Mn content is less than 1.00%, a tensile strength of 780 MPa or higher cannot be guaranteed. Therefore, the Mn content is adjusted to 1.00% or higher. The Mn content is preferably 1.50% or higher, and very preferably 1.80% or higher, 2.00% or higher, or 2.20% or higher.

[27] On the other hand, when an excess of Mn is added, the structure becomes uneven due to Mn segregation, and the flexural workability deteriorates. Therefore, the Mn content is adjusted to 4.00% or less, preferably 3.00% or less, very preferably 2.80% or less, 2.60% or less, or 2.50% or less.

[28] In the sun: 0.001% or more and 2.000% or less Aluminum (Al) is an element that deoxidizes steel, making the steel sheet solid. When the Al content is less than 0.001%, since sufficient deoxidation is not possible, the Al content is adjusted to 0.001% or more. However, in cases where sufficient deoxidation is required, it is desirable to add 0.010% or more of Al. More desirable Al content levels are 0.020% or more, 0.030% or more, or 0.050% or more.

[29] On the other hand, when the Al sol. content exceeds 2.000%, the degradation of weldability becomes significant, and the number of oxide-based inclusions increases, significantly degrading the surface properties. Therefore, the Al sol. content is adjusted to 2.000% or less and is preferably 1.500% or less, very preferably 1.000% or less, and even more preferably 0.090% or less, 0.080% or less, or 0.070% or less. Al sol. means acid-soluble Al that does not convert to an oxide such as Al₂O₃ and is soluble in acids.

[30] Total Ti and Nb: 0.04% to 0.40% In the present invention, Ti and Nb are important elements, as they contribute to strength as precipitates formed during the quenching of hot-rolled steel sheet. To achieve this effect, a total of 0.04% or more of Ti and Nb is required. When the total content of Ti and Nb is less than 0.04%, sufficient strength cannot be obtained. The total content of Ti and Nb is preferably 0.08% or more, and very preferably 0.10% or more, 0.12% or more, or 0.15% or more. On the other hand, when Ti and Nb are added in excess, recrystallization during hot rolling is suppressed, and a texture with a specific crystal orientation develops, thus impairing the hole expansion capacity, which is one of the formability indices of steel sheet for vehicles. Therefore, the total content of Ti and Nb must be 0.40% or less. Ti and Nb are preferably 0.35% or less, and very preferably 0.32% or less, 0.30% or less, or 0.25% or less in total.

[31] Ti: 0.20% or less As described previously, when excess Ti is added, recrystallization during hot rolling is suppressed, and a texture with a specific crystal orientation develops, thus impairing hole expansion capacity, which is one of the formability indices of automotive steel sheets. Therefore, the Ti content must be 0.20% or less. The Ti content can be 0.18% or less, 0.15% or less, or 0.10% or less. The lower limit for Ti content is not individually defined and is determined by the total Ti and Nb content described above. Therefore, the Ti content can be 0%. Alternatively, the Ti content can be defined as 0.01% or more, 0.02% or more, or 0.05% or more.

[32] Nb: 0.20% or less As described previously, when excess Nb is added, recrystallization during hot rolling is suppressed, and a texture with a specific crystal orientation develops, thus impairing hole expansion capacity, which is one of the formability indices of automotive steel sheets. Therefore, the Nb content must be 0.20% or less. The Nb content can be 0.18% or less, 0.15% or less, or 0.10% or less. The lower limit for Nb content is not individually limited and is defined in relation to the total Ti and Nb content described above. Therefore, the Nb content can be 0%. Alternatively, the Nb content can be defined as 0.01% or more, 0.02% or more, or 0.05% or more.

[33] High-strength steel sheet conforming to this modality contains impurities as part of its chemical composition. Impurities refer, for example, to elements accidentally present in the raw materials or scrap metal, or from the manufacturing environment or similar factors during the industrial production of the steel. Impurities include, for example, elements such as P, S, and N. These impurities are preferably limited as described below to ensure that the effect of this modality is sufficiently realized. Furthermore, since the quantity of impurities is preferably small, it is not necessary to limit the lower limit, and the lower impurity limit may be 0%.

[34] P: 0.100% or less Phosphorus (P) is normally an impurity found in steel, but it increases tensile strength and can therefore be present in positive amounts. However, when the P content exceeds 0.100%, weldability deterioration becomes significant. Therefore, the P content is limited to 0.100% or less. Preferably, the P content is limited to 0.080% or less, 0.070% or less, or 0.050% or less.

[35] Although the lower limit of the P content is not particularly restricted, to more reliably obtain the effect of the action described above, the P content may be adjusted to 0.001% or more, 0.002% or more, or 0.005% or more.

[36] S: 0.0200% or less Sulfur (S) is an impurity in steel, and its content is preferably as low as possible from a weldability standpoint. When the S content exceeds 0.0200%, weldability deteriorates significantly, the amount of precipitated MnS increases, and low-temperature toughness deteriorates. Therefore, the S content is limited to 0.0200% or less. The S content is preferably 0.0100% or less and very preferably limited to 0.0080% or less, 0.0070% or less, or 0.0050% or less.

[37] Although the lower limit for the S content is not particularly restricted, from a desulfurization cost standpoint, the S content may be adjusted to 0.0010% or more, 0.0015% or more, or 0.0020% or more.

[38] N: 0.01000% or less Nitrogen (N) is an impurity in steel, and its content is preferably as low as possible from a weldability standpoint. When the N content exceeds 0.01000%, weldability degradation becomes significant. Therefore, the N content is limited to 0.01000% or less and may preferably be 0.00900% or less, 0.00700% or less, or 0.00500% or less. The lower limit for N content is not particularly restricted, but it may be adjusted to, for example, 0.00005% or more, 0.00010% or more, or 0.00020% or more.

[39] O: 0.0100% or less Oxygen (O₂) is an impurity found in steel, and its content is preferably as low as possible from a weldability standpoint. When the O₂ content exceeds 0.0100%, weldability degradation becomes significant. Therefore, the O₂ content is limited to 0.0100% or less and is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit for O₂ content is not particularly restricted, but it can be adjusted to, for example, 0.0005% or more, 0.0008% or more, or 0.0010% or more.

[40] High-strength steel sheet conforming to this modality may contain an optional element in addition to the basic elements and impurities described above. For example, instead of some Fe, which is the remainder described above, B, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr may be contained as optional elements. These optional elements may be contained according to the purpose. Therefore, it is not necessary to limit the lower limits of these optional elements, and the lower limits may be 0%. Furthermore, even when these optional elements are contained as impurities, the effects described above are not impaired.

[41] B: 0% or more and 0.010% or less Boron (B) can suppress the roughness of the punched cross-section by segregating at the grain boundary and improving grain boundary strength. Therefore, B can be included. When the B content exceeds 0.010%, the effect described above becomes saturated, which is economically disadvantageous; therefore, the B content is 0.010% or less. The B content is preferably 0.005% or less, and very preferably 0.003% or less. To preferably achieve the effect described above, the B content can be 0.001% or more.

[42] V: 0% or more and 1,000% or less Cr: 0% or more and 1,000% or less Mo: 0% or more and 1,000% or less Cu: 0% or more and 1,000% or less Co: 0% or more and 1,000% or less W: 0% or more and 1,000% or less Neither: 0% or more and 1,000% or less V, Cr, Mo, Cu, Co, W, and Ni are effective elements for ensuring stable strength. Therefore, these elements may be included. However, even when the content exceeds 1,000% of each element, the effect of the action described above is likely to become saturated, and there are cases where including such elements becomes economically disadvantageous. Therefore, it is preferable that the content of V, Cr, Mo, Cu, Co, W, and Ni be set at 1.0% or less or 1,000% or less. The upper limit for each of the contents of V, Cr, Mo, Cu, Co, W, and Ni may be set at 0.500% or less, 0.300% or less, or 0.100% or less.

[43] To more reliably obtain the effect of the action described above, at least one of V: 0.005% or more, 0.008% or more, or 0.010% or more, Cr: 0.005% or more, 0.008% or more, or 0.010% or more, Mo: 0.005% or more, 0.008% or more, or 0.010% or more, Cu: 0.005% or more, 0.008% or more, or 0.010% or more, Co: 0.005% or more, 0.008% or more, or 0.010% or more, W: 0.005% or more, 0.008% or more, or 0.010% or more, and Ni: 0.005% or more, 0.008% or more, or 0.010% or more is preferably contained.

[44] Ca: 0% or more and 0.0100% or less Mg: 0% or more and 0.0100% or less REM: 0% or more and 0.0100% or less Zr: 0% or more and 0.0100% or less Calcium (Ca), magnesium (Mg), mineral redox (MRE), and zinc (Zr) are all elements that contribute to inclusion control, particularly the fine dispersion of inclusions, and have a toughness-enhancing effect. Therefore, one or more of these elements may be present. However, when the amount exceeds 0.0100% for any of these elements, there are instances where deterioration of surface properties occurs. Therefore, the amount of each element is preferably set to 0.01% or less, or 0.0100% or less. The upper limit for the amounts of Ca, Mg, MRE, and Zr may be set to 0.0080%, 0.0050%, or 0.0030%. To more reliably achieve the effect of the action described above, the amount of at least one of these elements is preferably set to 0.0003% or more, 0.0005% or more, or 0.0010% or more.

[45] Here, REM refers to a total of 17 elements including Se, Y, and lanthanides, and is at least one of them. The REM content means the total amount of at least one of these elements. Industrially, the lanthanides are added in metal misch form.

[46] High-strength steel sheet conforming to the present modality preferably contains, as a chemical composition, % by mass, at least one of Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, REM: 0.0003% or more and 0.0100% or less, and Zr: 0.0003% or more and 0.0100% or less.

[47] The steel composition described above can be measured using an ordinary steel analysis method. For example, the steel composition can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured using an infrared absorption method after combustion, N can be measured using an inert gas fusion thermal conductivity method, and O can be measured using an inert gas fusion non-dispersive infrared absorption method.

[48] ​​2. Metallographic structure In the high-strength steel sheet conforming to the present modality, in the metallographic structure, the ratio of total area of ​​quenched martensite to bainite is 80% or more.

[49] The ratio of total area of ​​tempered martensite to bainite is 80% or more In the present invention, to minimize the structural and property irregularities caused by variations in cooling rates during coil winding of hot-rolled steel sheet, it is important to adjust 80% or more of the structure to consist of bainite and martensite, which are low-temperature transformation structures. This is achieved, for example, by cooling the hot-rolled steel sheet to 500°C or less in a quenching zone after hot rolling. The martensite then becomes quenched martensite during subsequent quenching. Consequently, the ratio of total area of ​​quenched martensite and bainite to the overall structure is 80% or more. When the total area ratio is less than 80%, the material quality becomes uneven, which is undesirable.The total area ratio of quenched martensite to bainite can be 85% or more, 90% or more, or 95% or more. It is not necessary to define an upper limit for the total area ratio of quenched martensite to bainite; for example, the total area ratio of quenched martensite to bainite can be 100%. On the other hand, ferrite or similar metals may be included in the steel sheet as part of the metallographic structure. Therefore, the total area ratio of quenched martensite to bainite can be 98% or less, 95% or less, or 92% or less.

[50] In the present invention, the remainder of the metallographic structure may include ferrite, pearlite, residual austenite, fresh martensite and / or cementite.

[51] Method for measuring metallographic structure. The identification of the structures described above, the confirmation of their positions, and the measurement of their area fractions are carried out using the following methods.

[52] First, a cross-section parallel to the rolling direction (i.e., a cross-section parallel to the rolling direction and perpendicular to the rolled surface) is corroded using a reagent from Nital and a reagent described in Unexamined Japanese Patent Application, First Publication No. S59-219473. Regarding the corrosion of the cross-section, specifically, a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol is used as solution A, a solution prepared by dissolving 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water is used as solution B, solution A and solution B are mixed in a 1:1 ratio to prepare a liquid mixture, and nitric acid is added and mixed in a ratio of 1.5% to 4% with respect to the total amount of this liquid mixture, thus preparing a pretreatment liquid.Furthermore, the pretreatment liquid described above is added to and mixed with a 2% Nital liquid at a ratio of 10% to the total amount of 2% Nital liquid, thus preparing a posttreatment liquid. The cross-section parallel to the rolling direction (i.e., the cross-section parallel to the rolling direction and perpendicular to the rolled surface) is immersed in the pretreatment solution for 3 to 15 seconds, washed with alcohol, dried, and then immersed in the posttreatment solution for 3 to 20 seconds, then washed with water and dried, thereby corroding the cross-section. Next, as shown in Figure 1, at a depth of 1 / 4 of the sheet thickness from the surface (rolled surface 11) of steel sheet 1 and at the center in the width direction WD, at least three regions of 40 gm × 30 gm are observed at a magnification of 1,000 to 100,000 times using a scanning electron microscope, thus identifying the metallographic structure, confirming the locations of presence, and measuring the area fractions. In any case where the object of measurement is a steel sheet that is not subjected to any special machining after manufacture (in other words, a steel sheet that is not cut from a coil) or a steel sheet that is cut from a coil, the central position in the width direction is a position that is substantially equidistant from both ends of steel sheet 1 in the width direction WD.

[53] It is difficult to distinguish between lower bainite and tempered martensite using the measurement method described above. Therefore, in the present modality, it is not necessary to distinguish between them. That is, the total area fraction of bainite and tempered martensite is obtained by measuring the area fractions of upper bainite and lower bainite or tempered martensite. Upper bainite is an aggregate of laths and a structure containing a carbide between the laths. Lower bainite is a structure containing iron-based carbides having principal axes of 5 nm or more and extending in the same direction. Tempered martensite is an aggregate of lath-like crystal grains and a structure containing iron-based carbides having principal axes of 5 nm or more and extending in different directions.

[54] At the position of 1 / 4 of the sheet thickness of the cross section parallel to the rolling direction and perpendicular to the rolled surface, the standard deviation of the numerical densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5 × 1010 numbers / mm3, wherein the numerical densities are measured at 10 points every 50 mm in the width direction. In the present invention, precipitates containing one or both Ti and Nb (hereinafter referred to as Nb / Ti precipitates) are important for ensuring elongation and bending capacity, as well as for ensuring strength. Generally, the strength of a steel sheet tends to be inversely proportional to its elongation and bending capacity. However, by using Nb / Ti precipitates, the strength of the steel sheet can be improved without compromising its elongation and bending capacity. On the other hand, strength and elongation vary depending on the amount of Nb / Ti precipitates. Therefore, it is important that the amount of Nb / Ti precipitates distributed within the material be uniform along the width (i.e., perpendicular to the rolling direction). When the standard deviation of the numerical densities of the Ti / Nb precipitates is 5 × 10¹⁰ numbers / mm³ or higher, variations in mechanical properties occur, and material quality stability cannot be achieved. Therefore, the standard deviation of the numerical density of Nb / Ti precipitates is adjusted to less than 5 × 10¹⁰ numbers / mm³, and preferably less than 4 × 10¹⁰ numbers / mm³, or even less than 3 × 10¹⁰ numbers / mm³. Provided that the chemical composition and standard deviation of the number density of precipitates, including Nb / Ti, are within the range described above, it is assumed that a sufficient quantity of precipitates containing Nb / Ti can be obtained to ensure elongation and bending capacity. Therefore, it is not necessary to limit the upper and lower limits of the number density of precipitates containing Nb / Ti. On the other hand, the number density of precipitates containing Nb / Ti can be defined as 3.5 × 10¹⁰ numbers / mm³ or higher, 3.8 × 10¹⁰ numbers / mm³ or higher, or 4.0 × 10¹⁰ numbers / mm³ or higher.

[55] The standard deviation of the numerical densities of precipitation, including Ti / Nb, is measured using the following method. A replica sample fabricated according to a method described in the unexamined Japanese patent application, First Publication No. 2004-317203, is extracted from the position 1 / 4 of the thickness of sheet 121 of cross-section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 shown in Figure 2, and observed using a transmission electron microscope. The magnification of the observed section is 50,000x, and in 3 observed sections, the number of precipitates including Ti / Nb is counted, where the value obtained as the square root (approximate value of the equivalent diameter of the circle) of<eje mayor χ eje menor > is 10 nm or less. After that, the counted number of precipitates that include Ti / Nb is divided by the volume of the electrolyzed sample to calculate the total density of the precipitates.The contribution of precipitates with an equivalent circular diameter greater than 10 nm to precipitation hardening is small and does not significantly affect the properties obtained by the present invention. Therefore, the number density of precipitates with an equivalent circular diameter greater than 10 nm is not limited. Replicate samples are taken at 10 points every 50 mm along the WD width direction (see Figure 2), and the numerical densities of the precipitation, including Ti / Nb, are obtained for each sample. The average of these numerical densities is then assumed to be the numerical density of the precipitation, including Ti / Nb, in each of the 10 replicate samples. Furthermore, the standard deviation of the numerical densities of the precipitation, including Ti / Nb, in each of the 10 replicate samples is assumed to be the standard deviation of the numerical densities of the precipitation, including Ti / Nb, in the steel sheet. When the size of the steel sheet, which is the object of future measurement, is sufficiently large along the width direction, the measurement points for the standard deviation of the numerical precipitation densities, including Ti / Nb, can be arranged in a straight line along the width direction. On the other hand, when the size of the steel sheet, which is the object of future measurement, is less than 450 mm along the width direction, the measurement points for the standard deviation of the numerical precipitation densities, including Ti / Nb, can be arranged in two or more straight lines along the width direction.When measuring the standard deviation in the width direction of features other than precipitation numerical densities that include Ti / Nb (e.g., surface roughness and the like), the measurement points can be arranged as described above.

[56] 3. Standard deviation of surface roughness Ra The standard deviation of the surface roughness Ra measured at 10 points every 50 mm along the width direction is preferably 1.0 pm or less The steel sheet conforming to this specification is not particularly restricted provided that the chemical composition, metallographic structure, and tensile strength described below are within predetermined ranges. When the surface roughness Ra of the rolled surface 11 is measured at 10 points every 50 mm along the width direction (i.e., a direction at right angles to the rolling direction), the standard deviation of the surface roughness Ra can be set to 1.0 pm or less. By suppressing a variation in the surface roughness Ra, it is possible to suppress a variation in bending workability and further improve the stability of the material quality. Therefore, the standard deviation is preferably set to 1.0 pm or less. Here, the surface roughness of the steel sheet can be changed at will through further processing.For example, after manufacturing a high-strength steel sheet with excellent material quality stability using a preferred manufacturing method described below, surface roughness modification, such as fine machining, can be performed on this high-strength steel sheet. From this perspective, setting the standard deviation of the surface roughness (Ra) within the range described above is not essential.

[57] For surface roughness Ra, a roughness curve 5 mm long in the width direction is acquired at each measurement position using a contact-type roughness gauge (SURFTEST SJ-500 manufactured by Mitutoyo Corporation), and the arithmetic mean of the Ra roughness is obtained by the method described in JIS B0601: 2001. The standard deviation of the surface roughness Ra is obtained using the value of the arithmetic means of the Ra roughness at each measurement position obtained as described above.

[58] Furthermore, if a surface treatment membrane, such as a plating or coating, is applied to the surface of the steel sheet, the surface roughness Ra of the steel sheet refers to the surface roughness measured after removing the surface treatment membrane. In other words, the surface roughness Ra of the steel sheet is the surface roughness of the base metal. The method for removing the surface treatment membrane can be appropriately selected based on the type of surface treatment membrane, ensuring that the surface roughness of the base metal remains unaffected. For example, if the surface treatment membrane is a zinc plating, the galvanized layer must be removed using dilute hydrochloric acid with an added inhibitor.This allows only the galvanized layer of the steel sheet to be exfoliated. The inhibitor is an additive used to suppress a change in surface roughness attributed to the prevention of excessive dissolution of the base metal. For example, a substance obtained by adding IBIT No. 700BK hydrochloric acid pickling corrosion inhibitor, manufactured by Asahi Chemical Co., Ltd., to hydrochloric acid diluted 10 to 100 times, so that the concentration reaches 0.6 g / L, can be used as an exfoliation medium for the galvanized layer.

[59] 4. Mechanical properties Tensile strength TS: 780 MPa or more High-strength steel sheet conforming to this modality has, as sufficient strength to contribute to vehicle weight reduction, a tensile strength (TS) of 780 MPa or more. The tensile strength of the steel sheet may be 800 MPa or more, 900 MPa or more, or 1000 MPa or more. Meanwhile, it is assumed that achieving a tensile strength greater than 1470 MPa with the configuration of this modality is difficult. Therefore, it is not necessary to specifically specify the upper limit of the tensile strength, but the substantial upper limit of the tensile strength in this modality may be set at 1470 MPa. Furthermore, the tensile strength of the steel sheet may be set at 1400 MPa or less, 1300 MPa or less, or 1200 MPa or less.

[60] A tensile test can be performed in the following order according to JIS Z 2241 (2011). JIS No. 5 test pieces are picked from 10 positions on the high-strength steel sheet at 50 mm intervals in the width direction. Here, the width direction of the steel sheet and the longitudinal direction of the test pieces are aligned. Furthermore, the individual test pieces are picked from alternate positions in the rolling direction of the steel sheet so that the picking positions of the individual test pieces do not interfere with each other. Tensile tests are performed on these test pieces according to JIS Z 2241 (2011), the tensile strengths TS (MPa) are obtained, and the average value is calculated. This average value is considered the tensile strength of the high-strength steel sheet.

[61] Furthermore, the high-strength steel sheet conforming to the present modality may have the following characteristics in terms of elongation and hole expansion capacity as a formability index. These mechanical properties are obtained due to a variety of properties of the high-strength steel sheet conforming to the present modality described above.

[62] Total elongation EL: 10% or more High-strength steel sheet conforming to this modality may have a total elongation of 9% or more, or 10% or more in the tensile test as an index of formability. Meanwhile, it is difficult to obtain a total elongation of more than 35% with the configuration of this modality. Therefore, the substantial upper limit of total elongation may be set at 35%.

[63] Limit bending radius R / t (bending capacity): 2.0 or less If the R / t value is obtained by dividing the limiting radius of curvature R (mm) by the sheet thickness t (mm) as the flexural capacity index, the high-strength steel sheet conforming to this modality may have an R / t of 2.0 or less. However, it is difficult to establish the flexural capacity R / t index at 0.1 or less with the configuration of this modality. Therefore, the substantial lower limit of the flexural capacity R / t index may be set at 0.1.

[64] The limiting bending radius R is obtained by repeatedly performing bending tests at various bending radii. In the bending test, bending is performed according to JIS Z 2248 (2006) (90° V-block bending test). The bending radius (specifically, the inside bending radius) is changed in 0.5 mm increments. As the bending radius decreases in the bending test, cracking and other defects in the steel sheet become more likely. The minimum bending radius at which cracking and other defects do not occur in the steel sheet, as determined in this test, is considered the limiting bending radius R. Furthermore, a value obtained by dividing this limiting bending radius R by the thickness t of the steel sheet is used as the R / t index to evaluate bending capacity.

[65] In high-strength steel sheet conforming to the present specification, as an index of material quality stability, among the tensile test results measured at 10 points every 50 mm in the width direction (i.e., a direction at right angles to the rolling direction), the standard deviation of TS may be 50 MPa or less, and the standard deviation of EL may be 1% or less. The method for obtaining the standard deviation TS and the standard deviation EL is the same as the tensile test method described above for obtaining the average value of the tensile strengths. The standard deviation TS and the standard deviation EL can be obtained by obtaining the standard deviation of the results of 10 tensile tests using the method described above.

[66] Furthermore, in the high-strength steel sheet conforming to the present modality, the standard deviation of R / 1 (limit bending radius R (mm), sheet thickness t (mm)) measured at 10 points every 50 mm along the width direction can be adjusted to 0.2 or less.

[67] 5. Manufacturing method An example of a preferred method for manufacturing high-strength steel sheet in accordance with this modality will now be described. However, it should be noted that the method for manufacturing high-strength steel sheet in accordance with this modality is not particularly restricted. Any steel sheet that meets the requirements described above is considered to be steel sheet in accordance with this modality, regardless of the manufacturing methods used.

[68] The manufacturing step preceding hot rolling is not particularly restricted. That is, after melting in a blast furnace, electric furnace, or similar facility, a variety of secondary melting is carried out, and then casting can be carried out by a method such as ordinary continuous casting, ingot casting, or thin plate casting.In the case of continuous casting, a cast plate can be hot-rolled after being cooled to a low temperature and then reheated, or the cast plate can be hot-rolled as is after casting without cooling to a low temperature. Scrap can be used as raw material.

[69] A heating step is performed on the cast plate. In this heating step, the plate is heated to a temperature of 1100°C or higher and 1350°C or lower, and then held for 30 minutes or more. In a case where Ti and / or Nb are included, it is heated to a temperature of 1200°C or higher and 1350°C or lower, and then held for 30 minutes or more. If the heating temperature is lower than 1200°C, the Ti and / or Nb, which are precipitation elements, do not dissolve sufficiently so that, in the subsequent hot rolling, a sufficient amount of precipitation hardening can be achieved, and the Ti and / or Nb is retained as coarse carbides, impairing formability, which is undesirable. Therefore, in a case where Ti and / or Nb are included in the iron, the heating temperature of the iron is 1200°C or more.On the other hand, if the heating temperature exceeds 1350°C, the amount of scale formation increases, thus decreasing efficiency. Therefore, the heating temperature should be 1350°C or lower. A holding time of 30 minutes or more is preferable to ensure sufficient dissolution of Ti and / or Nb. Furthermore, to prevent excessive scale loss, a holding time of 10 hours or less is preferable, and 5 hours or less is highly preferable.

[70] Next, a rough rolling step is carried out to rough roll the heated plate to produce a rough rolled sheet. In rough rolling, the conditions are not particularly limited as long as the plate has the desired dimensions and shape. The thickness of the rough-rolled sheet affects the amount of temperature reduction from the tip to the tail of the hot-rolled steel sheet during the initial rolling process until the completion of the final rolling step, and is therefore preferably determined taking this into account.

[71] The finishing lamination is carried out on the unrolled sheet. In this finishing lamination step, a multi-stage finishing lamination is performed. In the present embodiment, the finishing lamination is carried out within a temperature range of 850°C to 1200°C under conditions that satisfy the following formula (1). K' / Si* >2.50 ... (1) Here, when Si > 0.35, Si* is set to 140Vsi and, when Si <0.35, Si* is set to 80. Si represents the Si content (% by mass) of the steel sheet.

[72] Furthermore, K' in formula (1) is represented by the following formula (2). K' = D χ (DT - 930) χ 1.5 + Σ ( (FTn- 930) χ Sn) ... (2) Here, D is the amount sprayed per hour (m3 / min) of hydraulic descaling before the start of the finish rolling, DT is the temperature of the steel sheet (°C) at the time of hydraulic descaling before the start of the finish rolling, FT is the temperature of the steel sheet (°C) at the nth stage of the finish rolling, and S is the amount sprayed per hour (m3 / min) at the time of spraying water to the steel sheet by spraying between the Nlth stage and the nth stage of the finish rolling.

[73] Si* is a parameter related to a steel sheet component that indicates the ease with which irregularities attributed to inclusions are generated. When the amount of Si in a steel sheet component is large, the inclusions generated in the surface layer during hot rolling change from wüstite (FeO) to fayalite (Fe2SiO4). Wustite flakees off relatively easily and is unlikely to produce irregularities in the steel sheet, while fayalite grows and becomes embedded in the steel sheet and is likely to produce irregularities. Therefore, as the amount of Si increases—that is, as Si* increases—the likelihood of irregularities forming in the surface layer becomes more pronounced. Here, the addition of Si to facilitate the formation of irregularities in the surface layer becomes significantly effective, particularly when 0.35% by mass or more of Si is added. Therefore, when 0.35% by mass or more of Si is added...When 35% by mass or more of Si, Si* acts as a function of Si; however, when 0.35% by mass or less of Si is added, Si* acts as a constant.

[74] K' is a manufacturing condition parameter indicating the difficulty in forming irregularities. The first item in formula (2) indicates that when hydraulic descaling is performed before the start of finish rolling to suppress the formation of irregularities, as the hourly sprayed amount of hydraulic descaling agent increases and as the temperature of the steel sheet increases, the hydraulic descaling becomes more effective. When descaling is performed multiple times before the start of finish rolling, the descaling value closest to the finish rolling time is used.

[75] The second item in formula (2) is an item that indicates the descaling effect, during finish rolling, of scale that has not been completely exfoliated by descaling before finishing or scale that forms again during finish rolling, and indicates that spraying a large amount of water on the steel sheet at high temperatures facilitates descaling.

[76] When the ratio between the manufacturing condition parameter K', which indicates the difficulty in forming irregularities, and the parameter Si*, which relates to the steel sheet component and indicates the ease in forming a scale defect portion, is 2.50 or more, it is possible to suppress irregularities and a temperature variation during tempering. Therefore, K' / Si* is set to 2.50 or more, preferably 3.00 or more, and most preferably 3.50 or more.

[77] To establish the standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the width direction (i.e., a direction at a right angle to the rolling direction) at 0.5 pm or less, which is a preferred form in the present invention, it is preferable that K' / Si* > 3.00.

[78] After finishing rolling, cooling is carried out at an average cooling rate of 50°C / s or faster, and winding is performed at a winding temperature of 450°C or lower. This is because the property inequalities caused by the temperature history after winding are suppressed by including bainite and martensite, which are low-temperature transformation structures, as the main structures as described above. Here, the average cooling rate is a value obtained by dividing the temperature difference between the start of cooling and before winding by the time between them. When the average cooling rate is slower than 50°C / s, it becomes difficult to establish the ratio of total area of ​​quenched bainite and martensite to 80% or more of all structures.

[79] Similarly, when the rolling temperature is above 450°C, it becomes difficult to establish the ratio of total bainite to quenched martensite area in 80% or more of all structures. From this perspective, the rolling temperature is set to 450°C or lower, preferably 400°C or lower, and most preferably 200°C or lower. Furthermore, setting the rolling temperature to 450°C or lower also has the effect of suppressing the formation of internal oxide on the surface of the steel sheet after rolling and an increase in surface roughness.

[80] Pickling is carried out on the high-strength steel sheet manufactured in this way in order to remove oxide from the surface of the steel sheet. Pickling can be performed, for example, with hydrochloric acid having a concentration of 3% to 10% at a temperature of 85°C to 98°C for 20 to 100 seconds.

[81] Furthermore, soft reduction can be carried out with a rolling reduction of 20% or less on the manufactured hot-rolled steel sheet. The purpose of soft reduction is to introduce dislocations, which act as precipitation sites during tempering. In cases where soft reduction is performed, the strength can be easily obtained, and the shape correction effect is achieved; therefore, it is preferable. Soft reduction can be performed before or after pickling. Soft reduction carried out after pickling has the effect of further reducing the surface roughness. To set the standard deviation of the surface roughness Ra to 0.5 pm or less when the surface roughness Ra is measured at 10 positions at 50 mm intervals in the width direction (i.e., a direction at a right angle to the rolling direction), which is a preferred form in the present invention, it is necessary to perform the smooth reduction after pickling.

[82] The resulting steel sheet is quenched (heated) from 550°C to 750°C for 10 to 1000 seconds. The quenching process aims to recover dislocations in the low-temperature transformation structure to enhance elongation, as well as to precipitate elements including Ti and / or Ni to obtain strength.

[83] When the quenching temperature is below 550°C, sufficient elongation cannot be ensured, nor can sufficient strength be guaranteed, which is undesirable. When heating is carried out at a quenching temperature above 750°C, hardening of the precipitates occurs, such that sufficient strength cannot be guaranteed, which is also undesirable. Therefore, in the method of manufacturing high-strength steel sheet according to the present modality, the quenching temperature is from 550°C to 750°C.

[84] When the heating time is less than 10 seconds, sufficient elongation and strength cannot be ensured, which is undesirable. When heating is carried out for a time greater than 1000 seconds, the effect of increased elongation due to dislocation recovery and the effect of increased strength due to precipitation become saturated, and therefore, for the sake of productivity, the heating time is 1000 seconds or less. Therefore, in the manufacturing method for high-strength steel sheet according to the present method, the quenching time is from 10 seconds to 1000 seconds.

[85] Hot-dip galvanizing or hot-dip galvanizing-annealing can be performed after heating. By reducing the surface roughness using the technique according to this patent, the wettability of the hot-dip galvanized steel sheet is improved, and the effect of applying a uniform coating can be achieved.

[86] High-strength steel sheet conforming to this modality can be manufactured using the manufacturing method described above. Examples

[87] The high-strength steel sheet according to the present invention will now be described more specifically with reference to examples. The following examples are examples of the high-strength steel sheet of the present invention, and the high-strength steel sheet of the present invention is not limited to the following aspects. The conditions in the examples described below are illustrative conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these illustrative conditions. The present invention is capable of adopting a variety of conditions within the scope of the essence of the present invention, provided that the object of the present invention is achieved.

[88] The steels having the chemical compositions shown in Table 1 were cast, and after casting, the plates were heated to a temperature range of 1200°C to 1350°C, either as such or after having been cooled once to room temperature and then held, and then the plates were as-rolled at temperatures of 1100°C or higher, producing as-rolled steel sheets. In Table 1, the values ​​outside the scope of the invention are underlined.

[89] Table 1 Steel Material Chemical Composition (Unit: % by mass, the remainder is Fe and impurities) c Si Mn sol.Al Ti Nb P s N 0 Other Ti+Nb Category A 0.061 1.21 2.59 0.100 0.11 0.02 0.010 0.0020 0.00200 0.00100 B:0001 0.13 Example B 0.060 0.10 2.49 0.030 0.00 0.05 0.010 0.0010 0.00200 0.00100 0.05 Example C 0.069 0.80 2.20 0.050 0.12 0.02 0.010 0.0010 0.00300 0.00200 Ca:0.002 0.14 Example D 0.050 0.40 2.09 0.030 0.10 0.00 0.008 0.0010 0.00300 0.00100 B:0.002 0.10 Example E 0.061 1.49 2.20 0.030 0.11 0.02 0.010 0.0020 0.00300 0.00100 0.13 Example F 0.080 2.00 2.00 0.025 0.09 0.01 0.010 0.0010 0.00300 0.00200 Cr:0.4 0.10 Example G 0.059 0.70 1.81 0.030 0.10 0.01 0.011 0.0010 0.00300 0.00100 V:0.01 0.11 Example H 0.121 1.30 1.80 0.020 0.09 0.01 0.012 0.0010 0.00300 0.00100 Mo:0.01 0.10 Example 1 0.040 1.10 1.61 0.020 0.11 0.01 0.010 0.0010 0.00200 0.00200 Cu:0.01 0.12 Example J 0.061 1.02 1.79 0.030 0.10 0.02 0.010 0.0010 0.00300 0.00100 Co:0.1 0.12 Example K 0.080 0.90 1.88 0.029 0.16 0.02 0.010 0.0010 0.00300 0.00100 B:0.001, W:0.01 0.18 Example L 0.071 1.79 1.09 0.020 0.11 0.01 0.012 0.0030 0.00300 0.00100 N¡:0.8 0.12 Example M 0.110 1.20 1.79 0.021 0.10 0.03 0.013 0.0010 0.00200 0.00100 Mq:0.002 0.13 Example N 0.079 0.87 1.30 0.030 0.08 0.02 0.011 0.0020 0.00300 0.00100 ROW.001 0.10 Example. 0 0.089 1.43 1.79 0.130 0.12 0.03 0.014 0.0010 0.00200 0.00100 Zr:0.002 0.15 Example P 0.050 0.90 1.60 0.030 0.03 0.04 0.010 0.0030 0.00300 0.00100 B:0.002 0.07 Example Q 0.048 1.10 1.59 0.030 0.01 0.01 0.010 0.0030 0.00300 0.00100 B:0.002 0.02 Comparative Example R 0.150 1.10 3.01 0.030 0.15 0.10 0.010 0.0030 0.00300 0.00100 W:0.05, Ca:0.005 0.25 Example

[90] On the raw rolled sheets, a multi-stage finishing lamination was carried out which included a total of seven stages under the conditions shown in Table 2 and Table 3. After that, cooling and winding after the final lamination were carried out under the individual conditions shown in Table 4 and Table 5. After that, pickling was carried out for all conditions; however, for some conditions, gentle reduction was performed in a step before or after pickling. Following this, the steel sheets were heated to the quenching temperature at heating rates of 30°C / s to 150°C / s, and then quenched at the quenching temperatures and times described in Table 4 and Table 5. Afterward, for some conditions, hot-dip annealing or hot-dip galvanizing was performed. In the plating step, the steel sheets were at a temperature range of 400°C to 520°C.

[91] Table 2 Observations á Ti pode acero (xiu) josed^ Quantity of Si 1%) Temperature of the steel lar na of steel fQ in the n-ésirra stage of finishing lar nade Quantity removed per hour during hydraulic descaling (rrf / rrin) tf tf ÍE tf te tE tf Before finishing lar nade cry Lh <Λ cd cd cd id §errplo ccnparativo 1 A 28 1.21 1005 995 986 977 968 ​​959 950 1 04 0 1 0 0 oo 0 E^errplo ccnparativo 2 A 28 1.21 1009 1000 991 982 972 963 954 1 04 0 1.2 0 0 02 24 Corporate E^errplo 3 A 28 1.21 1004 995 986 977 968 ​​958 949 1 04 1 1.2 1 0 1.5 24 Corporate E^erplo 4 A 28 1.21 1006 997 988 979 970 960 951 1 04 1 1.2 1 0 1.5 24 Corporate error 5 A 28 1.21 1010 1001 992 982 973 964 955 1 04 1 1.2 1 0 1.5 24 Corporate error 6 A 28 1.21 1007 998 989 980 970 961 952 1 04 1 1.2 1 0 1.5 24 ^errpt 7 A 20 1.21 1010 1001 992 983 974 965 955 1 04 1 24 1 0 1.5 24 ^errpt 8 A 24 1.21 1001 991 982 973 964 955 946 3 04 1 24 1 0 1.5 24 E^erplo 9 A 1.2 1.21 1002 992 983 974 965 956 947 1 04 1 24 1 0 1.5 24 ^errpto 10 A 1.2 1.21 1004 995 986 977 968 959 949 1 04 1 24 1 0 1.5 24 ^errpto 11 A 1.2 1.21 1005 996 987 978 969 959 950 1 04 1 24 1 0 1.5 24 ^errpto 12 A 20 1.21 1002 992 983 974 965 956 947 1 04 1 24 1 0 1.5 24 ^errpto 13 A 20 1.21 1005 996 987 978 968 959 950 1 04 1 24 1 0 1.5 24 ^errpto 14 A 20 1.21 1000 990 981 972 963 954 945 1 04 1 24 1 0 1.5 24 Ejerrplo 15 A 20 1.21 1005 996 987 977 968 959 950 1 04 1 24 1 0 1.5 24 ^errplo 16 A 3.8 1.21 1004 995 986 977 968 958 949 1 04 1 24 1 0 1.5 24 Egerrplo 17 A 20 1.21 1004 995 985 976 967 958 949 1 04 1 24 1 0 1.5 24 ^errpto 18 A 20 1.21 1010 1000 991 982 973 964 955 1 04 1 24 1 0 1.5 24 Ejerrplo 19 A 20 1.21 1008 999 990 981 972 962 953 1 04 1 24 1 0 1.5 24 Ejerrplo 20 A 22 1.21 1067 1048 1029 1010 991 972 952 1 04 0 1.2 0 0 0 24 ^errpto 2I B 20 Q10 1027 1008 986 971 944 927 905 1 0 0 1.8 0 0 04 04 Ejerrplo 22 B 20 Q10 1029 1010 988 973 946 929 907 1 0 0 1.8 0 0 04 04 genrpl 23 C 29 080 1069 1050 1031 1012 993 974 954 1 04 0 1.2 0 0 0 24.

[92] Table 3 Observed enes 5 11 steel can Amount of S ¢4) Temperature of the steel larri na fQ in the n-th stage of finishing larri nado Quantity rocked per hour before the de-encrusted! hicbáulica (rrf / rrin) Ir £ Ir tr Ir ÍE t£ Altes del larri nado de ababado στ ιΛ di d? ιΛ ιΛ E^errplo corporativo 24 D 29 0.40 1008 999 990 980 971 962 953 1 04 0 1.2 0 0 0 0 ^errplo 25 D 29 Q40 1002 98359 4965 956 947 1 04 0 1.2 0 0 0 24 E^errplo 26 D 29 040 1083 1067 1052 1037 1022 1007 992 1 04 0 1.2 0 0 0 24 E^errplo 29 20 20 plo 1082 1067 1052 1037 1021 1006 991 1 04 0 1.2 0 0 0 24 E^errplo 28 D 29 040 1079 1063 1048 1033 1018 1093 80 1020 1.28 0 0 24 ^errplo 29 E 29 1.49 1063 1044 1025 1006 987 967 948 1 04 1 1.2 0 0 0 24 E^errplo 30 F 29 200 1076 1057010101019 980 961 1 04 1 1.2 0 0 0 24 §errplo 31 G 29 070 1073 1054 1035 1015 996 977 958 1 04 0 1.2 0 0 0 0 24 Ejerplo 23 39 101 H. 1041 1022 1003 984 965 946 2 04 0 1.2 0 0 0 24 Ejerrplo 33 1 29 1.10 1055 1036 1017 998 978 959 940 1 04 0 1.2 1 1 1 24 Ejerrplo 34 J 1 24 Ejerrplo 35 K 29 090 1064 1044 1025 1006 987 968 ​​949 1 04 0 1.2 1 1 1 24 Ejerrplo 36 L 1 04 1 24 1 1 1.5 24 ^errplo 37 M 29 1.20 1002 993 985 973 966 957 947 1 04 1 24 1 0 1.5 24 Ejerrplo 7171 308 N 1012 993 974 955 1 04 0 1.2 0 0 0 0 0 24 Example 39 0 29 1.43 1059 1040 1021 1001 982 963 944 2 09 40 0 ​​P 0 1.2 090 1070 1051 1032 1013 993 974 955 1 04 0 1.2 0 0 0 24 Exerrplo corparativo 41 Q 1 1 0 24 Example 42 R 20 1.10 1009 1002 993 983 975 965 956 1 04 1 1.8 1 0 1.5 24 Example 43 A 29 950 1 04 1 1.2 1 0 1.5 24.

[93] Table 4 Observations OR Z Average cooling rate after finishing rolling (°C / second) Rolling temperature (°C) Soft reduction ratio (%) Soft reduction time Tempering temperature (°C) Tempering time (seconds) Plating type Comparative example 1 100 <100°C 5 After pickling 649 100 Hot-dip galvanized-annealed Comparative example 2 100 <100°C 5 After pickling 652 100 Hot-dip galvanized-annealed Comparative example 3 30 <100°C 5 After pickling 650 100 Hot-dip galvanized-annealed Comparative example 4 60 500°C 5 After pickling 654 100 Hot-dip galvanized-annealed Comparative example 5 60 <100°C 5 After pickling 772 100 Hot-dip galvanized-annealed Comparative example 6 60 <100°C 5 After pickling 652 5 Hot-dip galvanized-annealed Example 7 100 <100°C 5 After pickling 651 100 Hot-dip galvanized-annealedHot-dip galvanized Example 8 100 <100°C 5 After pickling 652 100 Hot-dip galvanized-annealed Example 9 100 430°C 5 After pickling 647 100 Hot-dip galvanized-annealed Example 10 100 <100°C 0 - 653 100 Hot-dip galvanized-annealed Example 11 100 <100°C 5 Before pickling 648 100 Hot-dip galvanized-annealed Example 12 100 <100°C 10 After pickling 652 100 Hot-dip galvanized-annealed Example 13 100 <100°C 10 Before pickling 652 100 Hot-dip galvanized-annealed Example 14 100 <100°C 5 After pickling 568 100 Hot-dip galvanized-annealed Example 15 100 <100°C 5 After pickling 717 100 Hot-dip galvanized-annealed Example 16 100 250°C 5 After pickling 650 30 Hot-dip galvanized-annealed Example 17 100 <100°C 5 After pickling 650 500 Hot-dip galvanized-annealedExample 18 60 <100°C 5 After pickling 649 100 Without plating Example 19 100 <100°C 5 After pickling 646 100 Hot-dip galvanized Example 20 100 <100°C 5 After pickling 646 100 Hot-dip galvanized-annealed Example 21 100 <100°C 5 After pickling 647 100 Hot-dip galvanized-annealed Example 22 100 <100°C 5 After pickling 500 100 Hot-dip galvanized-annealed Example 23 100 150°C 5 After pickling 649 100 Hot-dip galvanized-annealed

[94] Table 5 Observations OR Z Average cooling rate after finishing rolling (°C / second) Rolling temperature (°C) Soft reduction ratio (%) Soft reduction time Tempering temperature (°C) Tempering time (seconds) Plating type Comparative example 24 100 <100°C 5 After pickling 648 100 Hot-dip galvanized-annealed Example 25 100 <100°C 5 After pickling 650 100 Hot-dip galvanized-annealed Example 26 100 <100°C 0 - 648 100 Hot-dip galvanized Example 27 100 <100°C 5 Before pickling 650 100 No plating Example 28 100 <100°C 10 After pickling 648 100 Hot-dip galvanized-annealed Example 29 100 <100°C 5 After pickling 648 100 Hot-dip galvanized-annealed Example 30 100 <100°C 5 After pickling 650 100 Hot-dip galvanized-annealed Example 31 100 <100°C 5 After pickling 647 100 Hot-dip galvanized-annealedHot-dip galvanized Example 32 100 <100°C 5 After pickling 652 100 Hot-dip galvanized-annealed Example 33 100 <100°C 5 After pickling 648 100 Hot-dip galvanized-annealed Example 34 100 <100°C 5 After pickling 652 100 Hot-dip galvanized-annealed Example 35 100 <100°C 5 After pickling 646 100 Hot-dip galvanized-annealed Example 36 100 <100°C 5 After pickling 646 100 Hot-dip galvanized-annealed Example 37 100 <100°C 5 After pickling 651 100 Hot-dip galvanized-annealed Example 38 100 <100°C 5 After pickling 648 100 Hot-dip galvanized-annealed Example 39 100 <100°C 5 After pickling 647 100 Hot-dip galvanized-annealed Example 40 100 <100°C 5 After pickling 646 100 Hot-dip galvanized-annealed Comparative example 41 100 <100°C 5 After pickling 649 100Hot-dip galvanized-annealed Example 42 100 <100°C 5 After pickling 650 100 Hot-dip galvanized-annealed Example 43 60 <100°C 5 After pickling 652 15 Hot-dip galvanized-annealed

[95] The metallographic structures of the high-strength steel sheets obtained were observed using the following method. First, a cross-section parallel to the rolling direction and perpendicular to the rolled surface was corroded using a Nital reagent and a reagent described in the unexamined Japanese patent application, first publication No. S59-219473. Regarding the corrosion of the cross-section, specifically, a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as solution A, a solution prepared by dissolving 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water was used as solution B, solution A and solution B were mixed in a 1:1 ratio to prepare a liquid mixture, and more nitric acid was added and mixed in a ratio of 1.5% to 4% with respect to the total amount of this liquid mixture, thus preparing a pretreatment liquid.Furthermore, the pretreatment liquid described above was added to and mixed with a 2% Nital liquid at a ratio of 10% to the total amount of the 2% Nital liquid, thus preparing a posttreatment liquid. The cross-section parallel to the rolling direction and perpendicular to the rolled surface was immersed in the pretreatment solution for 3 to 15 seconds, washed with alcohol, dried, and then immersed in the posttreatment solution for 3 to 20 seconds. It was then washed with water and dried, thereby corroding the cross-section.

[96] Next, at a position at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet and in the center in the width direction, at least three 40 pm × 30 pm regions were observed with a magnification of 1000 to 100,000 times using a scanning electron microscope, thus identifying the metallographic structure, confirming the positions of presence and measuring the area fractions. The total area fraction of bainite and tempered martensite was obtained by measuring the area fractions of upper bainite and lower bainite or tempered martensite.

[97] The numerical densities and standard deviations of precipitation including Ti / Nb are measured using the following method. A replica sample, fabricated according to a method described in the unexamined Japanese Patent Application, First Publication No. 2004-317203, was extracted from the 1 / 4-thickness position of sheet 121 from cross-section 12, parallel to the rolling direction RD and perpendicular to the rolled surface 11 shown in Figure 2, and observed using a transmission electron microscope. The magnification of the observed section was 50,000x, and in 3 observed sections, the number of precipitates, including Ti / Nb, where the value obtained as the square root (approximate value of the equivalent diameter of the circle) of<eje mayor χ eje menor > If it is 10 nm or less, it was counted. After that, the counted number is divided by the volume of the electrolyzed sample to calculate the total density of the precipitate.

[98] Replicate samples were extracted at 10 points every 50 mm in the width direction, and the numerical densities of the precipitation, including Ti / Nb, were obtained for each sample. The average of the numerical densities of the precipitation, including Ti / Nb, in each of the 10 replicate samples was then assumed to be the numerical density of the precipitation, including Ti / Nb, of the steel sheet. Furthermore, the standard deviation of the numerical densities of the precipitation, including Ti / Nb, in each of the 10 replicate samples was assumed to be the standard deviation of the numerical densities of the precipitation, including Ti / Nb, of the steel sheet.

[99] The standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the direction perpendicular to the rolling direction was obtained in the following order. A 5 mm long roughness curve in the direction perpendicular to the rolling direction was acquired at each measurement position using a contact-type roughness gauge (SURFTEST SJ-500 manufactured by Mitutoyo Corporation), and the arithmetic mean of the Ra roughness was obtained by the method described in JIS B0601:2001. The standard deviation of the surface roughness Ra was obtained using the arithmetic mean values ​​of the Ra roughness at each measurement position obtained as described above.

[100] Regarding tensile strength, a tensile test was carried out in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece taken from the high-strength steel sheet such that the direction (C direction) perpendicular to the rolling direction was along the longitudinal direction, and the tensile strength TS (MPa) and the elongation at the end (total elongation) EL (%) were obtained. The samples were taken from 10 positions on the steel sheet at 50 mm intervals in the width direction. The average value of the tensile strengths of the 10 test pieces was considered as the tensile strength TS of the steel sheet, and if TS > 780 MPa was achieved, the steel sheet was determined to be a high-strength hot-rolled steel sheet and was evaluated as passing.

[101] In addition, the standard deviations of TS and EL were obtained at 10 positions at 50 mm intervals in the width direction on the steel sheets. A steel sheet that had a TS standard deviation of 50 MPa or less and an EL standard deviation of 1% or less was determined to be a steel sheet that had excellent material quality stability.

[102] A bending test was carried out in accordance with 5 JIS Z 2248 (90° V-block bending test), and the bending R (mm) was tested in 0.5 mm steps. In addition, R / t was measured at 10 positions at 50 mm intervals in the width direction (direction perpendicular to the rolling direction), and the standard deviation thereof was obtained.

[103] Table 6 Observations Ó Z * ώ k K' / Si* Total area ratio of quenched martensite and bainite (%) Average numerical density of precipitates (10,D numbers / mm3) Standard deviation of precipitates (10,D numbers / mm3) Standard deviation of roughness (pm) Average tensile strength TS (MPa) Standard deviation of TS (MPa) Average total elongation EL (%) Standard deviation of EL (%) Average limiting radius of curvature R / t Standard deviation R / t Comparative example 1 154 198 1.28 95 12.1 7.0 1.2 1050 75 14.3 1.3 1.5 0.3 Comparative example 2 154 286 1.86 96 11.1 6.0 1.1 1050 70 14.6 1.2 1.5 0.3 Comparative Example 3 154 409 2.66 69 11.5 6.5 0.8 950 63 17.2 1.2 1.4 0.3 Comparative Example 4 154 426 2.77 49 11.6 6.8 1.3 930 61 18.5 1.3 1.4 0.4 Comparative Example 5 154 459 2.98 95 3.3 2.3 0.7 750 35 8.5 0.3 1.2 0.3 Comparative Example 6 154 434 2.82 95 2.2 2.3 0.7 750 34 8.8 0.3 1.2 0.3 Example 7 154 559 3.63 96 10.5 2.0 0.4 1050 5 14.7 0.3 1.2 0.1 Example 8 154 668 4.34 96 11.0 2.1 0.4 1050 12 14.5 0.4 1.2 0.1 Example 9 154 466 3.03 82 11.3 2.6 0.4 1010 7 16.2 0.4 1.1 0.1 Example 10 154 496 3.22 95 10.9 4.1 0.7 970 28 15.0 0.7 1.1 0.3 Example 11 154 504 3.27 94 11.0 4.1 0.6 1050 37 15.0 0.7 1.1 0.3 Example 12 154 465 3.02 96 11.5 2.7 0.3 1070 15 14.7 0.3 1.1 0.1 Example 13 154 503 3.27 95 12.0 4.0 0.6 1070 36 15.0 0.6 1.1 0.3 Example 14 154 445 2.89 95 11.6 2.7 0.4 1010 13 15.0 0.3 1.1 0.1 Example 15 154 501 3.25 94 11.7 2.8 0.3 1000 11 15.6 0.4 1.1 0.1 Example 16 154 494 3.21 85 11.1 2.5 0.4 1000 9 14.9 0.3 1.1 0.1 Example Example 17 154 488 3.17 96 11.1 2.7 0.4 1010 5 15.0 0.4 1.1 0.1 Example 18 154 550 3.57 91 11.5 2.0 0.3 1050 9 14.7 0.4 1.2 0.1 Example 19 154 536 3.48 94 11.3 2.2 0.3 1050 11 14.8 0.3 1.2 0.1 Example 20 154 434 2.82 94 12.3 4.2 0.7 1050 30 15.5 0.7 1.2 0.3 Example 21 80 232 2.89 94 6.2 4.1 0.6 820 27 14.0 0.6 1.2 0.3 Comparative example 22 80 238 2.98 94 3.3 3.6 0.6 750 23 8.8 0.2 1.2 0.3 Example 23 125 445 3.55 95 10.5 2.0 0.3 1100 13 13.1 0.2 1.2 0.1.

[104] Table 7 Observations Ó Z ώ k K' / Si' Total area ratio of tempered martensite and bainite (%) Average numerical density of precipitates (10ID numbers / mm3) Standard deviation of precipitates (1010 numbers / mm3) Standard deviation of roughness (pm) Average tensile strength TS (MPa) Standard deviation of TS (MPa) Average total elongation EL (%) Standard deviation of EL (%) Average limiting radius of curvature R / t Standard deviation R / t Comparative example 24 89 220 2.48 96 9.8 5.5 1.1 980 60 15.3 1.2 1.0 0.3 Example 25 89 243 2.74 94 9.7 4.3 0.7 980 35 15.2 0.7 1.0 0.3 Example 26 89 585 6.60 94 4.0 4.2 0.6 930 33 15.2 0.7 1.0 0.3 Example 27 89 581 6.56 94 9.9 4.1 0.7 980 34 15.2 0.7 1.0 0.3 Example 28 89 562 6.35 96 10.0 4.3 0.3 980 10 15.2 0.3 1.0 0.1 Example 29 171 525 3.07 95 10.2 2.0 0.2 1050 10 13.1 0.2 1.0 0.1 Example 30 198 610 3.08 96 11.0 2.1 0.3 1050 11 14.5 0.3 1.0 0.1 Example 31 117 464 3.96 95 10.5 2.0 0.2 1030 11 13.5 0.2 1.0 0.1 Example 32 160 592 3.71 94 10.Example 33: 147,511 3.48 95 10.3 2.2 0.3 850 8 18.0 0.3 1.0 0.1 Example 34: 141,524 3.70 94 10.5 2.3 0.3 1000 10 17.0 0.3 1.0 0.1 Example 35: 133,583 4.39 95 20.2 2.8 0.2 1190 11 17.7 0.3 1.2 0.1 Example 36: 187,618 3.30 95 10.6 2.1 0.3 1120 Example 37: 153,475,3.10, 96,10.1,2.3,0.3,1210,11,14.5,0.2,1.2,0.1 Example 38: 131,447,3.43,95,10.1,2.0,0.3,1150,11,14.9,0.2,1.0,0.1 Example 39: 167,581,3.47,96,10.6,2.5,0.3,1200,12,14.5,0.2,1.2,0.1 Example 40: 133,448,3.38,94,7.3,2.6,0.2,803,7,20.1,0.3,1.0 0.1 Comparative Example 41 147 530 3.61 95 1.6 1.4 0.2 760 7 22.2 0.4 1.0 0.1 Example 42 147 523 3.56 95 20.1 2.0 0.3 1090 5 14.6 0.3 1.2 0.1 Example 43 154 427 2.77 95 4.5 2.1 0.6 845 35 10.8 0.4 1.8 0.3.

[105] In Table 6 and Table 7, the values ​​outside the scope of the invention are underlined. As shown in the tables, in the examples where all 5 conditions of the present invention were met, the tensile strength, total elongation, bending capacity, variation in tensile strength, and variation in total elongation were all excellent. On the other hand, in the comparative examples where at least one of the conditions of the present invention was not met, at least one property of the tensile strength (average tensile strength TS described in the Table), total elongation (average total elongation EL described in the Table), bending capacity (average limiting radius of curvature R / t described in the Table), variation in tensile strength (standard deviation of TS described in the Table), and variation in total elongation (standard deviation of EL described in the Table) was insufficient.

[106] Specifically, in Comparative Example 1 and Comparative Example 2, the standard deviation (Precipitate Standard Deviation described in the Table) of the numerical densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb, measured at a position 1 / 4 of the sheet thickness from a cross-section parallel to a rolling direction and perpendicular to a rolled surface, was large. Therefore, in Comparative Example 1 and Comparative Example 2, the TS standard deviation and the EL standard deviation were not good. This is assumed to be because Comparative Example 1 and Comparative Example 2 were manufactured under conditions where K' / Si* was insufficient and the surface roughness of the steel sheets after hot rolling was not small.

[107] In Comparative Example 3, the total area ratio of quenched martensite to bainite was insufficient, and the standard deviation of the precipitates was large. Therefore, in Comparative Example 3, the TS standard deviation and the EL standard deviation were not good. This is assumed to be because Comparative Example 3 was manufactured under conditions where the average cooling rate after the finishing roll was insufficient, and the property irregularities caused by the temperature history after rolling were not suppressed.

[108] In Comparative Example 4, the total area ratio of quenched martensite to bainite was insufficient, and the standard deviation of the precipitates was large. Therefore, in Comparative Example 4, the TS standard deviation and the EL standard deviation were not good. This is assumed to be because Comparative Example 4 was manufactured under conditions where the winding temperature was too high, and the formation of internal oxide on the surface of the steel sheet and the increase in surface roughness were not suppressed.

[109] In Comparative Example 5, the average tensile strength TS was insufficient and the average total elongation EL was insufficient. This is assumed to be because Comparative Example 5 was manufactured under conditions where the tempering temperature was too high.

[110] In Comparative Example 6, the average tensile strength TS was insufficient and the average total elongation EL was insufficient. This is assumed to be because Comparative Example 6 was manufactured under conditions where the tempering time was insufficient.

[111] In Comparative Example 22, the average tensile strength TS was insufficient and the average total elongation EL was insufficient. This is assumed to be because Comparative Example 22 was manufactured under conditions where the tempering temperature was insufficient.

[112] In Comparative Example 41, the total amount of Ti and Nb was insufficient, and the average tensile strength TS was insufficient. This is assumed to be because, in Comparative Example 41, the amount of Ti and Nb that are precipitate material, including Ti / Nb, was insufficient, and precipitation hardening was not induced. Brief description of the reference symbols

[113] 1 High-strength steel sheet (steel sheet) Laminated surface Cross-section parallel to the rolling direction and perpendicular to the rolled surface 121 Position of 1 / 4 of the sheet thickness of the cross-section parallel to the rolling direction and 5 perpendicular to the rolled surface RD Laminating Department TD Thickness direction WD width direction

Claims

1. A high-strength steel sheet comprising, as chemical composition, in % by mass: C: 0.030% to 0.280%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%; Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.01000% or less; O: 0.0100% or less; Ti: 0% to 0.20%; Nb: 0% to a total of 0.20%; Ti and Nb: 0.04% to 0.40%; B: 0% to 0.010%; V: 0% to 1.000%; Cr: 0% to 1.000%; Mo: 0% to 1,000%; Cu: 0% to 1,000%; Co: 0% to 1,000%; W: 0% to 1,000%; Ni: 0% to 1,000%; Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; REM: 0% to 0.0100%; Zr: 0% to 0.0100%; and a remainder that includes Fe and impurities, wherein, in a metallographic structure, the total area ratio of quenched martensite and bainite is 80% or more, at a position 1 / 4 of the sheet thickness of a cross section parallel to a rolling direction and perpendicular to a rolled surface, a standard deviation of the number densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5 × 1010 numbers / mm3, wherein the number densities are measured at 10 points every 50 mm in a width direction, and a tensile strength is 780 MPa or more.

2. The high-strength steel sheet according to claim 1, wherein a standard deviation of the surface roughness Ra is 1.0 pm or less, wherein the surface roughness Ra is measured at 10 positions at 50 mm intervals in the width direction.

3. The high-strength steel sheet according to claim 1 or 2, comprising, as its chemical composition, in % by mass, at least one of the group consisting of: B: 0.001% to 0.010%; V: 0.005% to 1.000%; Cr: 0.005% to 1.000%; Mo: 0.005% to 1.000%; Cu: 0.005% to 1.000%; Co: 0.005% to 1.000%; W: 0.005% to 1.000%; Ni: 0.005% to 1.000%; Ca: 0.0003% to 0.0100%; Mg: 0.0003% to 0.0100%; REM: 0.0003% to 0.0100%; and Zr: 0.0003% to 0.0100%.

4. The high-strength steel sheet according to any of claims 1 to 3, 15 wherein a total elongation is 10% or more, and R / t, which is a value calculated by dividing a limit bending radius by a thickness, is 2.0 or less.