HIGH-STRENGTH THIN STEEL SHEET AND METHOD FOR ITS MANUFACTURE

Abstract

A thin, high-strength steel sheet with a tensile strength of 1180 MPa or more is provided, exhibiting excellent malleability, excellent delayed fracture resistance from a base steel sheet, and excellent delayed fracture resistance from a projection weld, along with a method for its manufacture. The thin, high-strength steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, and N, with the remainder consisting of Fe and unavoidable impurities, and a complex structure containing ferrite, quenched martensite, and bainite, wherein a volume fraction of the total quenched martensite and bainite contains five or more carbides with a particle size of 0.1 µm or more.0 µm or less in a grain with respect to the total quenched martensite and bainite is 85% or more, whereas the mass % of C and the mass % of Mn in a region of 20 µm or less in the thickness direction from a surface of the steel sheet is 20% or less with respect to the mass % of C and the mass % of Mn in a region of 100 µm or more and 200 µm or less from the surface of the steel sheet.

Description

HIGH-STRENGTH THIN STEEL SHEET AND METHOD FOR ITS MANUFACTURE Lnn / zznz / E / Yi TECHNICAL FIELD This disclosure relates to a thin sheet of high-strength steel and a method for its manufacture and, in particular, relates to a thin sheet of high-strength steel suitable as an element for structural components of automobiles or the like and a method for its manufacture. BACKGROUND In recent years, CO2 emissions regulations have become stricter due to increasingly frequent environmental problems, and in the automotive sector, reducing the weight of vehicle bodies has become a key issue in order to reduce fuel consumption. Therefore, the thickness of structural components is being reduced by using high-strength steel sheets in automotive parts, and in particular, by using thin sheets of high-strength steel with a tensile strength (TS) of 1180 MPa or more. High-strength steel sheets used for structural and automotive reinforcement components are required to have excellent malleability. Particularly when forming complex shapes, high-strength steel sheets are needed that excel in all their properties, such as elongation and hole-expansion forming capacity, rather than solely in individual properties. Furthermore, there is concern that high-strength steel sheets with a tensile strength (TS) of 1180 MPa or higher may suffer delayed fracture (hydrogen embrittlement) caused by hydrogen entering from the operating environment. Therefore, thin sheets of high-strength steel used in the automotive field are required to have high formability as well as excellent resistance to delayed fracture. Furthermore, a car body is primarily assembled using resistance spot welding, with some areas inaccessible to a resistance spot welding gun being joined by stud welding. Stud welding is also frequently used when assembling dissimilar materials. In stud welding, a nut with a protruding portion is first projected onto a steel sheet, and then a stud is passed through the nut to join the materials. In cars manufactured using stud welding, tension is also applied to the projection weld to maintain the overall rigidity of the car body. Therefore, the properties of the projection weld are also important. Examples of conventional methods for improving the malleability of a steel sheet and the delayed fracture resistance of a base steel sheet include a method for controlling the forms of martensite and bainite, as described in JP patent 6032173 B (Patent Literature 1). Additionally, examples of methods for improving spalling resistance in a projection weld include a technique for controlling welding conditions to improve spalling resistance, as described in JP patent 2012-157900 A (Patent Literature 2). APPOINTMENT LIST Patent Literature Patent Literature 1: JP 6032173 B Patent Literature 2: JP 2012-157900 A BRIEF DESCRIPTION OF THE INVENTION Technical problem We recognize the challenge of improving not only the delayed fracture resistance of a base steel sheet, but also the delayed fracture resistance of a projection weld. A thin sheet of high-strength steel that comprehensively satisfies all the malleability, delayed fracture resistance properties of a base steel sheet, and delayed fracture resistance of a projection weld has not yet been developed. Therefore, it might be useful to provide a thin, high-strength steel sheet with a tensile strength of 1180 MPa or more that has excellent malleability, excellent delayed fracture resistance from a base steel sheet, and excellent delayed fracture resistance from a projection weld, as well as a method for its manufacture. In this disclosure, the term thin steel sheet means a steel sheet that is 0.6 mm or more and 2.8 mm or less thick. Furthermore, the term excellent malleability means that the material has both excellent elongation and excellent hole expansion formability. “Excellent elongation” means that the elongation (EL) is 14% or more. Excellent hole expansion formability means that the hole expansion ratio (λ) is 50% or more. An “excellent delayed fracture resistance of a base steel sheet” means that no cracking occurs even when the entire steel sheet is subjected to a constant load test and electrolytically charged for 100 hours. Furthermore, excellent delayed fracture resistance of a projection weld means that no cracking occurs even when the projection weld is subjected to a constant load test and electrolytically loaded for 100 hours. In the following description, the delayed fracture resistance of a base steel sheet and the delayed fracture resistance of a projection weld may be collectively and simply referred to as delayed fracture resistance. Lnn / zznz / B / Yi Solution to the Problem As a result of intensive studies, it was discovered that it is possible to obtain a thin, high-strength steel sheet that largely satisfies all the malleability and delayed fracture resistance properties of a base steel sheet and the delayed fracture resistance of a projection weld by controlling the volume fractions of ferrite, quenched martensite, and bainite in the steel sheet to specific proportions, refining the average grain size of each microstructure of the steel sheet, softening the hard martensite that can impair malleability and delayed fracture properties, and reducing the concentrations of carbon and manganese in a surface layer of the steel sheet. Our findings are listed below. (1) When the hardness difference between soft ferrite and hard martensite is high during punching in a hole expansion test, voids form at the interface, and a greater number of voids impairs the formability by hole expansion. On the other hand, it was found that the hardness difference between ferrite and quenched martensite can be reduced by quenching and softening the martensite, which reduces void formation and improves the malleability of a steel sheet. (2) Hydrogen penetration into steel causes the formation and propagation of fractures, resulting in what is known as delayed fracture. Intensive studies have revealed that hard martensite is a region where fractures occur in steel with a complex structure. It has been found that fracture formation can be reduced by quenching the martensite. (3) Furthermore, it was discovered that when the alloy content in the steel is increased to ensure strength, the strength during projection welding also increases, and microvoids form at the weld interface. It was also discovered that fractures propagate from these microvoids when stress is applied or when hydrogen penetrates the steel with microvoids. As a result of intensive studies, it was found that by appropriately specifying the dew point within a temperature range of 600 °C or higher during annealing, along with the C and Mn contents in the steel, and by reducing the C and Mn concentrations in a surface layer of the steel sheet, it is possible to increase the initial current efficiency during projection welding and eliminate the aforementioned microvoids. It was found that the delayed fracture resistance of a projection weld can be improved in this way. (4) It was also found that by using carbides in steel as hydrogen capture sites, hydrogen diffusion from the steel surface can be suppressed, and the delayed fracture resistance of a base steel sheet and a projection weld can be significantly improved. Some carbides formed during heating and hot rolling are still present as large carbides after final annealing. It was found that since large carbides contribute little to delayed fracture resistance, a predetermined amount of fine carbides is needed to act as hydrogen capture sites to further improve delayed fracture resistance. Additionally, it was found that to obtain a predetermined amount of fine carbide, the annealing process must be carefully controlled to quench the martensite and form a predetermined amount of bainite.According to our findings, the Lnn / zznz / B / Yi carbides that serve as hydrogen capture sites are primarily found in quenched martensite and bainite grains, where the carbon content is higher than that of ferrite. The amount of precipitated carbide is low in ferrite grains, where the carbon content is low. Therefore, it was found to be important to control the volume fraction of total quenched martensite and bainite grains containing a predetermined amount of carbide relative to the total number of quenched martensite and bainite grains in the steel sheet. This ensures the presence of carbides that serve as hydrogen capture sites and improves delayed fracture resistance. This disclosure is based on previous findings. Based on the above, the following information is provided. [1] A thin sheet of high-strength steel comprising: a chemical composition containing (consisting of), in % by mass, C: 0.10% or more and 0.22% or less. Yes: 0.5% or more and 1.5% or less. Mn: 1.2% or more and 2.5% or less. P: 0.05% or less, S: 0.005% or less, At: 0.01% or more and 0.10% or less, and N: 0.010% or less, while the remainder corresponds to Fe and unavoidable impurities, and a complex structure containing 5% or more and 35% or less ferrite by volume fraction, 50% or more and 85% or less of tempered martensite by volume fraction, and 0% or more and 20% or less of bainite by volume fraction, wherein the ferrite has an average grain size of 5 pm or less, the quenched martensite has an average grain size of 5 pm or less, a volume fraction of a total of quenched martensite and bainite containing five or more carbides with a particle size of 0.1 pm or more and 1.0 pm or less in a grain, with respect to a total of quenched martensite and bainite of 85% or more, and the mass % of C and the mass % of Mn in a region of 20 pm or less in the thickness direction from a surface of the steel sheet is 20% or less with respect to the mass % of C and the mass % of Mn in a region of 100 pm or more and 200 pm or less from the surface of the steel sheet. [2] The thin, high-strength steel sheet in accordance with paragraph [1], wherein the chemical composition further contains, in terms of % by mass, at least one of the elements selected from the group consisting of: Ti: 0.05% or less, V: 0.05% or less, and Nb: 0.05% or less. Lnn / zznz / E / Yi [3] The thin, high-strength steel sheet in accordance with paragraph [1] or [2], wherein the chemical composition further contains, in terms of % by mass, at least one of the elements selected from the group consisting of: Mo: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, B: 0.0030% or less, Ca: 0.0050% or less, Rare earth elements (REM): 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, Co: 0.020% or less, and Zn: 0.020% or less. [4] A method for manufacturing a thin sheet of high-strength steel, comprising: subjecting a steel plate having the chemical composition according to any of subsections [1] to [3] to hot rolling under the condition of a finished supply temperature of 850°C or more and 950°C or less to obtain a hot-rolled sheet, then cooling the hot-rolled sheet at a first average cooling rate of 30°C / µg or more to a coiling temperature of 550°C or less and then coiling the hot-rolled sheet at the coiling temperature, then subjecting the hot-rolled sheet to a pickling process, then subjecting the hot-rolled sheet after pickling to cold rolling with a rolling reduction of 30% or more to obtain a cold-rolled sheet, subsequently,Heat the cold-rolled sheet at an average heating rate of 3°C / s plus and 30°C / s minus to a first immersion temperature of 800°C or more and 900°C or less with a dew point of -40°C or more and 10°C or less in a temperature range of 600°C or more, and hold the cold-rolled sheet at the first immersion temperature for 30 seconds or more and 800 seconds or less, then cool the cold-rolled sheet from the first immersion temperature to a second immersion temperature of 350°C or more and 475°C or less at a second average cooling rate of 10°C / s plus, and hold the cold-rolled sheet at the second immersion temperature for 300 seconds or less, Lnn / zznz / B / Yi then cool the cold-rolled sheet to room temperature at a third average cooling rate of 100 °C / s or more, then reheat the cold-rolled sheet to a third immersion temperature of 200 °C or more and 400 °C or less, and hold the cold-rolled sheet at the third immersion temperature for 180 seconds or more and 1800 seconds or less, and then subject the cold-rolled sheet to a pickling process. Favorable Effect Pursuant to this disclosure, it is possible to provide a high-strength thin steel sheet having a tensile strength of 1180 MPa or more and possessing excellent malleability, excellent delayed fracture resistance in a base steel sheet and excellent delayed fracture resistance in a projection weld, as well as a method for its manufacture. DETAILED DESCRIPTION The following describes one form of this disclosure. It should be noted that this disclosure is not limited to the following form. First, the appropriate range of chemical composition for a basic steel sheet and the reasons for its limitation will be explained. The following “%” representations indicating the chemical composition of the steel sheet are presented as “% by mass,” unless otherwise specified. C: 0.10% or more and 0.22% or less Carbon (C) is an element that effectively increases the strength of a steel sheet and contributes to the formation of martensite and bainite, the second phase. In the following description, the term "second phase" refers to martensite and bainite, unless otherwise specified. When the C content is less than 0.10%, it is difficult to guarantee tensile strength, as the ferrite volume fraction increases. When the C content is less than 0.10%, hole expansion formability deteriorates. The C content is preferably 0.12% or higher. On the other hand, when the C content exceeds 0.22%, the hardness of a weld interface in a projection weld increases excessively, thus deteriorating the delayed fracture resistance of the projection weld. Furthermore, the delayed fracture resistance of the base steel sheet is also impaired. Additionally, when the C content exceeds 0.At 22%, the ferrite volume fraction decreases. Furthermore, elongation and hole expansion formability deteriorate. The carbon content is preferably 0.21% or less, and optimally 0.20% or less. Lnn / zznz / E / Yi Yes: 0.5% or more and 1.5% or less Silicon (Si) is an element that strengthens ferrite through solid solution, contributing to increased strength in steel sheets. When the Si content is below 0.5%, the required strength cannot be guaranteed, and the hardness difference between ferrite and martensite increases, impairing the hole expansion ratio. Furthermore, when the Si content is below 0.5%, the ferrite volume fraction increases, and the delayed fracture resistance of the base steel sheet and the projection weld deteriorates. Therefore, the Si content is set at 0.5% or higher, preferably 0.6% or higher. Excessive Si addition reduces the weld interface strength of a projection weld and impairs its delayed fracture resistance.Excessive addition of silicon increases the ferrite volume fraction, increases the average ferrite grain size, and decreases the quenched martensite volume fraction. Furthermore, excessive silicon addition decreases the percentage of fine carbides, tensile strength, hole expansion formability, and delayed fracture resistance of a base steel sheet. Therefore, the silicon content is set at 1.5% or less. The silicon content is preferably 1.4% or less. Mn: 1.2% or more and 2.5% or less Manganese is an element that contributes to increasing the strength of a steel sheet by promoting solid solution hardening and the formation of the second phase. Mn also has the effect of stabilizing austenite during annealing. To obtain these effects, the Mn content should be 1.2% or more. The preferred Mn content is 1.4% or more. On the other hand, when the Mn content is excessive, banded microsegregation (Mn banding) occurs, resulting in impaired elongation, hole expansion formability, and delayed fracture resistance. Therefore, the Mn content is set at 2.5% or less. The preferred Mn content is 2.4% or less. P: 0.05% or less Phosphorus (P) contributes to increasing the strength of a steel sheet by solid solution hardening. However, when P is added in excess, segregation at grain boundaries becomes significant, leading to grain boundary brittleness and impaired delayed fracture resistance. Therefore, the P content is set at 0.05% or less. The P content is preferably 0.04% or less. No lower limit for the P content is specified. However, the P content is preferably 0.0005% or more, as manufacturing costs increase when the P content is extremely low. S: 0.005% or less When the S content is high, a large amount of sulfide, such as MnS, is formed, and delayed fracture occurs in the area near the sulfide, leading to a deterioration of tensile strength. Lnn / zznz / B / Yi delayed fracture. Therefore, the S content is set at 0.005% or less. The S content is preferably 0.0045% or less. No lower limit for the S content is specified. However, the S content is preferably 0.0002% or more, as the manufacturing cost increases when the S content is extremely low. Al: 0.01% or more v 0.10% or less Aluminum (Al) is a necessary element for deoxidation. To achieve this effect, the Al content should be 0.01% or higher. When the Al content exceeds 0.10%, the effect is saturated. Therefore, the Al content is set at 0.10% or lower. Ideally, the Al content should be 0.06% or lower. N: 0.010% or less Nitrogen forms large nitrides and impairs hole expansion formability and delayed fracture resistance. Therefore, the nitrogen content is set at 0.010% or less. The nitrogen content is preferably 0.008% or less. The lower limit for the nitrogen content is not specifically defined, although it is preferably 0.0005% or more due to limitations in production technologies. Optional Components In addition to the above components, the high-strength steel thin sheet of the present disclosure may additionally contain, in terms of % by mass, at least one component selected from the group consisting of Ti: 0.05% or less, V: 0.05% or less and Nb: 0.05% or less. Ti: 0.05% or less Titanium (Ti) is an element that further increases the strength of a steel sheet by forming fine carbides, nitrides, or carbonitrides. Ti can be added as needed, since the growth of fine carbonitrides during annealing can be adequately controlled by its addition. To achieve these effects, the Ti content is preferably 0.001% or more, and optimally 0.01% or more. Conversely, when adding Ti, its content is preferably 0.05% or less to obtain better elongation. The Ti content is preferably 0.04% or less. V: 0.05% or less The vanadium (V) further increases the strength of a steel sheet by forming fine carbonitrides. To achieve this effect, the V content is preferably 0.001% or more, and optimally 0.01% or more. Alternatively, when adding V, its content is preferably 0.05% or less, so that the weld interface strength of a projection weld is increased, further improving the delayed fracture resistance of the projection weld. The V content is preferably 0.03% or less. Lnn / zznz / B / Yi Nb: 0.05% or less Nitrogen (Nb), like vanadium (V), further increases the strength of a steel sheet by forming fine carbonitrides. To achieve this effect, the Nb content is preferably 0.001% or more, and optimally 0.01% or more. On the other hand, when Nb is added, its content is preferably 0.50% or less, so that the weld interface strength of a projection weld is increased, further improving the delayed fracture resistance of the projection weld. The Nb content is preferably 0.05% or less. In addition to the above chemical composition, the high-strength steel thin sheet of the present disclosure may additionally contain, in terms of % by mass, at least one component selected from the group consisting of Mo: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, B: 0.0030% or less, Ca: 0.0050% or less, Rare Earth Elements (REM): 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, Co: 0.020% or less and Zn: 0.020% or less. Mo: 0.50% or less Molybdenum (Mo) promotes the formation of a second phase, further increasing the strength of the steel sheet. It also stabilizes austenite during annealing and is necessary to control the volume fraction of the second phase. To achieve these effects, the Mo content is preferably 0.010% or more, and optimally 0.05% or more. Conversely, when Mo is added, its content is preferably 0.50% or less to prevent excessive second phase formation and further improve elongation and hole expansion formability. The Mo content is preferably 0.3% or less. Cr: 0.50% or less Chromium (Cr) promotes the formation of a second phase, further increasing the strength of the steel sheet. To achieve this effect, the Cr content is preferably 0.010% or more, and optimally 0.1% or more. Conversely, when Cr is added, its content is preferably 0.50% or less to avoid excessive second phase formation, further improving elongation and flexural strength, and to prevent excessive surface oxide formation, thus further enhancing chemical conversion. The Cr content is preferably 0.3% or less. Cu: 0.50% or less Copper (Cu) is an element that further increases the strength of the steel sheet by hardening the solid solution and forming a second phase, and can be added as needed. To achieve this effect, the Cu content is preferably 0.05% or more, and optimally 0.1% or more. However, when the Cu content exceeds 0.50%, the effect is saturated. Therefore, When Cu is added, its content is preferably 0.50% or less. The Cu content is preferably 0.3% or less. Ni: 0.50% or less Nickel (Ni), like copper (Cu), further increases the strength of the steel sheet by hardening the solid solution through the formation of a second phase. It can be added as needed. To achieve this effect, the Ni content is preferably 0.05% or more, and optimally 0.1% or more. Furthermore, it is preferable to add Ni along with Cu, as this suppresses surface defects caused by Cu. When adding Ni, its content is preferably 0.50% or less, increasing the weld interface strength of a projection weld and further improving its delayed fracture resistance. The Ni content is preferably 0.3% or less. B: 0.0030% or less Boron (B) promotes the formation of a second phase, further increasing the strength of the steel sheet. It also ensures hardenability without lowering the martensitic transformation threshold. Furthermore, it segregates at grain boundaries to enhance their strength, effectively increasing delayed fracture resistance. To achieve these effects, the B content is preferably 0.0002% or more, and optimally 0.0005% or more. Conversely, when adding B, its content is preferably 0.0030% or less to increase hardness and further improve delayed fracture resistance. The B content is preferably 0.0025% or less. Ca: 0.0050% or less Calcium (Ca) is an element that reduces the adverse effect on hole-expansion formability through sulfide spheroidization and can be added as needed. To achieve this effect, the Ca content is preferably 0.0005% or higher. However, when the Ca content exceeds 0.0050%, the effect is saturated. Therefore, when Ca is added, its content is preferably 0.0050% or lower. The ideal Ca content is 0.003% or lower. Rare earth elements (REM): 0.0050% or less Rare earth elements (REE), such as calcium, reduce the adverse effect on hole expansion formability caused by sulfide spheroidization and can be added as needed. To achieve this effect, the REE content is preferably 0.0005% or higher. However, when the REE content exceeds 0.0050%, the effect is diminished. Lnn / zznz / E / Yi saturated. Therefore, when rare earth elements (REM) are added, their content is preferably 0.0050% or less. The rare earth element (REM) content is preferably 0.0015% or less. Ta: 0.100% or less Ta further increases the strength of the steel sheet by forming fine carbonitrides. To achieve this effect, the Ta content is preferably 0.001% or more, and optimally 0.010% or more. Conversely, when Ta is added, its content is preferably 0.100% or less, so that the weld interface strength of a projection weld is increased, further improving the delayed fracture resistance of the projection weld. The Ta content is preferably 0.050% or less. W: 0.500% or less The W further increases the strength of the steel sheet by forming fine carbonitrides. To achieve this effect, the W content is preferably 0.001% or more, and optimally 0.010% or more. On the other hand, when W is added, its content is preferably 0.500% or less, so that the weld interface strength of a projection weld is increased, further improving the delayed fracture resistance of the projection weld. The W content is preferably 0.300% or less. Sn: 0.200% or less Tin (Sn) is an element that suppresses oxidation on the surface of steel sheets during annealing, more effectively controls the thickness of the softened surface layer, and reduces the adverse effects of hole expansion on formability. It can also be added as needed. To achieve these effects, the Sn content is preferably 0.001% or more, and optimally 0.005% or more. Alternatively, when adding Sn, its content is preferably 0.200% or less to increase the weld interface strength of a projection weld, further improving its delayed fracture resistance. The Sn content is preferably 0.050% or less. Sb: 0.200% or less Antimony (Sb) is an element that suppresses oxidation on the surface of steel sheets during annealing, more effectively controls the thickness of the softened surface layer, and reduces the adverse effect of hole expansion on formability. It can also be added as needed. To achieve these effects, the Sb content is preferably 0.001% or more, and optimally 0.005% or more. Conversely, when Sb is added, its content is preferably 0.200% or less, thereby increasing the weld interface strength of a projection weld. Lnn / zznz / B / Yi further improves the delayed fracture resistance of the projection weld. The Sb content is preferably 0.050% or less. Mg: 0.0050% or less Magnesium (Mg) is an element that reduces the adverse effect on hole-expansion formability through spheroidization of sulfides, and it can be added as needed. To achieve this effect, the Mg content is preferably 0.0005% or higher. However, when the Mg content exceeds 0.0050%, the effect is saturated. Therefore, when adding Mg, its content is preferably 0.0050% or lower. The ideal Mg content is 0.0030% or lower. Zr: 0.1000% or less Zinc (Zr) is an element that reduces the adverse effect on hole expansion formability by spheroidizing inclusions, and it can be added as needed. To achieve this effect, the Zr content is preferably 0.001% or higher. However, when the Zr content exceeds 0.1000%, the effect is saturated. Therefore, when Zr is added, its content is preferably 0.1000% or lower. The Zr content is preferably 0.0030% or lower. Co: 0.020% or less Cobalt (Co) is an element that reduces the adverse effect on hole expansion formability by spheroidizing inclusions, and it can be added as needed. To achieve this effect, the Co content is preferably 0.001% or higher. However, when the Co content exceeds 0.020%, the effect is saturated. Therefore, when adding Co, its content is preferably 0.020% or lower. The ideal Co content is 0.010% or lower. Zn: 0.020% or less Zinc (Zn) is an element that reduces the adverse effect on hole expansion formability by spheroidizing inclusions, and it can be added as needed. To achieve this effect, the Zn content is preferably 0.001% or higher. However, when the Zn content exceeds 0.020%, the effect is saturated. Therefore, when adding Zn, its content is preferably 0.020% or lower. The Zn content is preferably 0.010% or lower. The rest, in addition to the components mentioned above, corresponds to Fe and unavoidable impurities. The following is a description of the microstructure of the high-strength thin steel sheet of this disclosure. The microstructure of the high-strength thin steel sheet of this disclosure is a complex structure containing 5% or more and 35% or less by Lnn / zznz / E / Yi ferrite volume fraction, 50% or more and 85% or less by quenched martensite volume fraction, and 20% or less by bainite volume fraction. The average ferrite grain size is 5 pm or less, and the average quenched martensite grain size is 5 pm or less. The volume fraction, as set forth herein, refers to a volume fraction relative to the overall structure of the steel sheet, and this definition may be applied in the following description. Furthermore, the average grain size, as set forth herein, refers to an equivalent circular crystalline grain size. Ferrite volume fraction: 5% or more and 35% or less It is difficult to obtain a tensile strength of 1180 MPa or more in a microstructure where the ferrite volume fraction exceeds 35%. The ferrite volume fraction is preferably 30% or less. On the other hand, when the ferrite volume fraction is less than 5%, elongation deteriorates due to excessive formation of the second phase. Therefore, the ferrite volume fraction is set at 5% or more. The ferrite volume fraction is preferably 10% or more and, optimally, 15% or more. The ferrite volume fraction is preferably 30% or less and, optimally, 28% or less. Average ferrite grain size: 5 pm or less When the average ferrite grain size exceeds 5 µm, the strength of a weld interface deteriorates due to increased oiling of the crystalline grains during projection welding, leading to a decrease in delayed fracture resistance. Therefore, the ferrite crystalline grain size is typically set at 5 µm or less. The average ferrite grain size is preferably 4 µm or less. Volume fraction of quenched martensite: 50% or more v 85% or less To ensure a tensile strength of 1180 MPa or higher, the quenched martensite volume fraction is set at 50% or higher. However, when the quenched martensite volume fraction exceeds 85%, the number of fracture sites increases during delayed fracture, resulting in a deterioration of the delayed fracture strength of the base steel sheet and projection weld. Therefore, the upper limit for the quenched martensite volume fraction is set at 85% or lower. The quenched martensite volume fraction is preferably 75% or lower. The quenched martensite volume fraction is preferably 60% or lower. Average size of tempered martensite grains: 5 pm or less When the average grain size of quenched martensite exceeds 5 pm, the crystalline grains become further oiled during projection welding, resulting in reduced strength and delayed fracture resistance. Additionally, voids that form at the interface between martensite and ferrite tend to connect, leading to impaired hole expansion formability. Therefore, the upper limit is set at 5 pm. The average grain size of quenched martensite is preferably 4.5 pm or less, and optimally 4 pm or less. Bainite: 0% or more and 20% or less by volume fraction The bainite content can be 20% or less by volume fraction to further increase the strength of the steel sheet. However, because bainite has a high dislocation density, excessive void formation occurs after punching in a hole expansion test if the volume fraction exceeds 20%, resulting in impaired hole expansion formability. Therefore, the bainite volume fraction is typically set at 20% or less. The bainite volume fraction can also be 0%. The bainite volume fraction is preferably 15% or less. The volume fractions of ferrite, quenched martensite, and bainite are measured as follows. First, the steel sheet is cut so that a cross-section along the thickness direction, parallel to the rolling direction (L-section), becomes an observation surface. This section is polished and then etched with 3% by volume of nital to obtain an observation surface. Using a scanning electron microscope (SEM) and a field emission scanning electron microscope (FE-SEM), the observation surface is analyzed at 3000x magnification to obtain a micrograph. The area ratio of each phase is measured using the dot-counting method (in accordance with ASTM E562-83 (1988)), and the area ratio is considered the volume fraction. The average size of the ferrite and tempered martensite grains is obtained by importing data in which ferrite and tempered martensite grains have been identified from the SEM and FE-SEM micrograph mentioned above in Media Cybernetics' Image-Pro, calculating the equivalent circular diameter of all ferrite and tempered martensite grains in the micrograph, and averaging the values. In the microstructure of the thin sheet of high-strength steel described herein, the volume fraction of total quenched martensite and bainite containing five or more carbides with a particle size of 0.1 µm or larger and 1.0 µm or smaller in the grain, relative to the total quenched martensite and bainite, is 85% or more. With this microstructure, the fine carbides with a particle size of 0.1 µm or larger and 1.0 µm or smaller can function as hydrogen-capturing sites that penetrate the steel, thereby increasing the delayed fracture resistance of a base steel sheet and a projection weld. As described above, the volume fraction of bainite may be 0%, in which case the volume fraction of the total quenched martensite containing five or more carbides with a particle size of 0.1 pm or more and 1.0 pm or less is 85% or more with respect to the total quenched martensite.Ferrite is not taken into account in the measurement of carbides, since carbides hardly precipitate on ferrite. Lnn / zznz / B / Yi When the volume fraction of total quenched martensite and bainite containing five or more carbides with a particle size of 0.1 µm or larger and 1.0 µm or smaller is less than 85% of the total quenched martensite and bainite, the number of carbides serving as capture sites is insufficient, impairing the delayed fracture resistance of a base steel sheet and a projection weld. Furthermore, when the carbide particle size is less than 0.1 µm, the total surface area of ​​the carbides serving as capture sites is small. As a result, the amount of trapped hydrogen is insufficient, and delayed fracture resistance is impaired. On the other hand, when the carbide particle size exceeds 1.0 µm, the locations of stable capture sites are limited.Hydrogen eventually diffuses even if it is temporarily trapped, resulting in a deterioration of delayed fracture resistance. Furthermore, when the number of carbides in the quenched martensite and bainite grains is less than five, the amount of carbide available as trapping sites is insufficient, further impairing delayed fracture resistance. The volume fraction of the total quenched martensite and bainite containing five or more carbides with a particle size of 0.1 µm or larger and 1.0 µm or smaller, relative to the total quenched martensite and bainite, is preferably 88% or more and optimally 90% or more. The volume fraction of quenched martensite and bainite grains containing carbides with particle sizes of 0.1 µm or larger and 1.0 µm or smaller, relative to the total quenched martensite and bainite, is measured as follows. First, the microstructure of the steel sheet is observed using a transmission electron microscope (TEM) at 20,000x magnification at a position 1 / 4 of the thickness from the steel sheet surface, and the particle size and number of carbides present in all quenched martensite and bainite grains within the field of view are calculated. The carbide particle size is obtained by importing data in which carbides have been identified into Media Cybernetics Image-Pro and by calculating the equivalent circular diameter. The total volume of quenched martensite and bainite grains containing five or more carbides with a particle size of 0.1 µm or smaller is then calculated.1 pm or more and 1.0 pm or less in the grain. The total volume of the quenched martensite and bainite is also calculated. The total volume of the quenched martensite grains and the bainite grains containing five or more carbides with a particle size of 0.1 pm or more and 1.0 pm or less in the grain is divided by the total volume of the quenched martensite and bainite to calculate the volume fraction of the quenched martensite grains and the bainite grains containing carbides with a particle size of 0.1 pm or more and 1.0 pm or less with respect to the total of the quenched martensite and bainite. Furthermore, in the high-strength thin steel sheet of this disclosure, the mass percent of C and the mass percent of Mn in a region 20 µm or less in the thickness direction from the surface of the steel sheet each correspond to 20% or less with respect to the mass percent of C and the mass percent of Mn in a region 100 µm or more and 200 µm or less from the surface of the steel sheet. Reducing the mass percent of C and the mass percent of Mn in a region 20 µm or less in the thickness direction from the surface of the steel sheet, i.e., in a surface layer of the sheet, For steel, the efficiency of the initial current during projection welding can be increased to suppress the formation of microvoids. When the mass percent of carbon (C) and mass percent of manganese (Mn) in a region 20 µm or less in the thickness direction from the steel sheet surface exceeds 20% of the mass percent of C and mass percent of Mn in a region 100 µm or more and 200 µm or less from the steel sheet surface, microvoids exist at a weld interface during projection welding, which impairs the delayed fracture resistance of a projection weld. The mass percent of C in a region 20 µm or less in the thickness direction from the steel sheet surface is preferably 15% or less, and optimally 10% or less, of the mass percent of C in a region 100 µm or more and 200 µm or less from the steel sheet surface.Furthermore, the mass percent of Mn in a region 20 µm or less in the thickness direction from the surface of the steel sheet is preferably 5% or less and, optimally, 10% or less of the mass percent of Mn in a region 100 µm or more and 200 µm or less from the surface of the steel sheet. The lower limit of the ratio of the mass percent of C in a region 20 µm or less in the thickness direction from the surface of the steel sheet to the mass percent of C in a region 100 µm or more and 200 µm or less from the surface of the steel sheet is not specified, although preferably it is 1% or more. The lower limit of the ratio of the mass % of Mn in a region of 20 pm or less in the thickness direction from the surface of the steel sheet with respect to the mass % of Mn in a region of 100 pm or more and 200 pm or less from the surface of the steel sheet is not specified, although preferably it is 1% or more. The ratio of the mass percent of carbon (C) to the mass percent of manganese (Mn) in a region 20 µm or less along the thickness direction from the steel sheet surface, relative to the mass percent of C and mass percent of Mn in a region 100 µm or more and 200 µm or less from the steel sheet surface, is measured as follows. First, a sample is cut so that a cross-section along the thickness direction parallel to the rolling direction of the steel sheet (section L) becomes an observation plane, and then the observation plane is polished with diamond paste. Subsequently, the observation plane is given a final polish with alumina.Using an electron probe microanalyzer (ERMA), a line analysis is carried out at three locations in a region of 200 pm or less in the thickness direction from the surface of the steel sheet in the observation plane. The ratio of the mass % of C and the mass % of Mn in a region of 20 pm or less in the thickness direction from the surface of the steel sheet is calculated with respect to the mass % of C and the mass % of Mn in a region of 100 pm or more and 200 pm or less from the surface of the steel sheet at each location, and the average of the three locations is also determined. In addition to ferrite, quenched martensite, and bainite, the microstructure of the high-strength thin steel sheet of this disclosure may contain retained austenite, pearlite, and unrecrystallized ferrite. However, the volume fraction of retained austenite is preferably 10% or less and, optimally, 5% or less. The volume fraction of pearlite is preferably 10% or less, and The volume fraction of unrecrystallized ferrite is preferably 10% or less and, optimally, 5% or less. The retained austenite volume fraction is measured as follows. First, the steel sheet is polished in the thickness direction (depth direction) to 1 / 4 of the sheet thickness to obtain an observation plane. The observation plane is observed using the X-ray diffraction method. The integrated intensity of the diffracted X-rays from the ferrite

[200] ,

[211] , and

[220] planes and the iron austenite

[200] ,

[220] , and

[311] planes is measured using an X-ray diffractometer (RINT2200 manufactured by Rigaku) ​​at an accelerating voltage of 50 keV with a MoKa source as the radiation source. Using these measured values, the retained austenite volume fraction is determined with the formula described in “Handbook of X-ray Diffraction” (2000) Rigaku Corporation, p. 26, 62-64. The methods for measuring the volume fractions of pearlite and unrecrystallized ferrite are as follows. First, the steel sheet is cut so that a cross-section along the thickness direction, parallel to the rolling direction (L-section), becomes an observation surface. This section is polished and then etched with 3% by volume of nital to obtain an observation surface. Using a scanning electron microscope (SEM) and a field emission scanning electron microscope (FE-SEM), the observation surface is analyzed at 3000x magnification to obtain a micrograph. The area ratio of each phase is measured using the dot-counting method (in accordance with ASTM E562-83 (1988)), and this area ratio is considered the volume fraction. The thin sheet of high-strength steel described in this disclosure may also include a coating or plating layer. The composition of the coating or plating layer is not specified and may be a common composition. The coating or plating layer may be formed by any method and may be a hot-dip coating or an electroplating layer, for example. The coating or plating layer may be alloyed. The type of metal for the coating or plating is not specified and may be a zinc coating or plating, an aluminum coating or plating, or a similar alloy. The following describes a method for manufacturing the high-strength steel thin sheet described in this disclosure. For the method of manufacturing high-strength steel thin sheets, each temperature range refers to the surface temperature of a steel plate or sheet, unless otherwise specified. In the method for manufacturing the high-strength steel thin sheet of the present disclosure, a steel plate having the chemical composition described above is subjected to hot rolling under the condition of a finished supply temperature of 850°C or more and 950°C or less to obtain a hot-rolled sheet, the hot-rolled sheet is then cooled at a first average cooling rate of 30°C / s more to a rolling temperature of 550°C or less and subsequently rolled at the rolling temperature, Lnn / zznz / E / Yi Next, the hot-rolled steel sheet undergoes a pickling process. Next, the hot-rolled sheet after pickling is subjected to cold rolling with a rolling reduction of 30% or more to obtain a cold-rolled sheet. Subsequently, the cold-rolled sheet is heated at an average heating rate of 3°C / s plus and 30°C / s minus to a first immersion temperature of 800°C or more and 900°C or less with a dew point of -40°C or more and 10°C or less within a temperature range of 600°C or more. The cold-rolled sheet is held at the first immersion temperature for 30 seconds or more and 800 seconds or less. Afterward, the cold-rolled sheet is cooled from the first immersion temperature to a second immersion temperature of 350°C or more and 475°C or less at a second average cooling rate of 10°C / s plus. The cold-rolled sheet is held at the second immersion temperature for 300 seconds or less, then,The cold-rolled sheet is cooled to room temperature at a third average cooling rate of 100°C / s or more, subsequently the cold-rolled sheet is reheated to a third immersion temperature of 200°C or more and 400°C or less, and the cold-rolled sheet is held at the third immersion temperature for 180 seconds or more and 1800 seconds or less, and then the cold-rolled steel sheet is subjected to a pickling process. First, a steel plate with the chemical composition described above is produced. The steel materials are melted to obtain molten steel with the aforementioned chemical composition. The melting method is not specified, and any known melting method may be used, such as converter or electric arc furnace casting. The resulting molten steel is then solidified to produce a steel plate. The method for producing the steel plate from the molten steel is not specified, and continuous casting, ingot casting, thin-plate casting, or other similar techniques may be used. Continuous casting is preferred to avoid macrosegregation. The resulting steel plate is then hot-rolled at a finished supply temperature of 850°C or higher and 950°C or lower to produce a hot-rolled sheet. Alternatively, the steel plate produced in this way can be cooled to room temperature and then subjected to a heating and rolling process. The heating temperature of the plate is preferably 1100°C or higher to facilitate carbide dissolution and reduce the rolling load. The heating temperature is preferably 1300°C or lower to minimize scale loss. Alternatively, hot rolling can be carried out using so-called energy-saving processes. Examples of energy-saving processes include direct rolling, in which the resulting steel plate, without being fully cooled to room temperature, is placed in a heating furnace and hot-rolled as a hot plate, as well as Lnn / zznz / B / Yi direct rolling, whereby the resulting steel plate is subjected to heat retention for a short period and then immediately subjected to rolling. Hot rolling finish delivery temperature: 850 °C or higher and 950 °C or lower The finishing roll of hot-rolled steel must be performed in a single-phase austenite region to improve the delayed fracture resistance of the base steel sheet and projection weld after annealing. This increases the uniform refinement of the microstructure in the steel sheet and reduces material anisotropy. Therefore, the delivery temperature for finishing hot-rolled steel is set at 850 °C or higher. On the other hand, when the delivery temperature for finishing exceeds 950 °C, the microstructure of the hot-rolled sheet becomes oily, and the crystalline grains after annealing also become oily, resulting in impaired hole expansion formability and delayed fracture resistance of the base steel sheet and projection weld. Therefore, the delivery temperature for finishing hot-rolled steel is set between 850 °C and 950 °C.The delivery temperature for hot-rolled finishing is preferably 880 °C or higher. The delivery temperature for hot-rolled finishing is preferably 920 °C or lower. First average cooling rate: 30 °C / s more The hot-rolled sheet is then cooled to a rolling temperature of 550 °C or lower at an initial average cooling rate of 30 °C / s or more. After hot rolling, the austenite transforms into ferrite during cooling. However, the ferrite becomes oily if the cooling rate is too slow, so rapid cooling is carried out after hot rolling to homogenize the microstructure. Therefore, after hot rolling, the hot-rolled sheet is cooled to 550 °C or lower at an initial average cooling rate of 30 °C / s or more. After hot rolling, the hot-rolled sheet is preferably cooled to 550 °C or lower at an initial average cooling rate of 35 °C / s or more. When the initial average cooling rate is less than 30 °C / s, the ferrite becomes oily.As a result, the microstructure of the hot-rolled sheet becomes heterogeneous, and both the hole expansion formability and delayed fracture resistance of the base steel sheet and projection weld deteriorate. Although the upper limit of the first average cooling rate is not specified, this rate is preferably 250 °C / s and, optimally, 100 °C / s or less due to production technology constraints. Winding temperature: 550 °C or less The hot-rolled sheet, which has been cooled to a rolling temperature of 550 °C or higher, is then rolled at a rolling temperature of 550 °C or lower. When the rolling temperature exceeds 550 °C, ferrite and pearlite form excessively in the microstructure of the hot-rolled sheet. A uniform fine microstructure cannot be obtained, and the average grain size of the ferrite and martensite in the microstructure of a thin, high-strength steel sheet becomes oily, resulting in a non-homogeneous microstructure and impaired hole expansion formability, delayed fracture resistance of the base steel sheet, and delayed fracture resistance of a projection weld. The rolling temperature is preferably 500 °C or lower. No lower limit for the rolling temperature is specified.However, when the rolling temperature is too low, excessive hard martensite forms, increasing the cold rolling load. Therefore, the rolling temperature is preferably 300 °C or higher. Next, the hot-rolled sheet undergoes a pickling process after coiling and before cold rolling to remove any inclusions from its surface. Pickling conditions can be adjusted as needed. The hot-rolled sheet, after pickling, is then subjected to cold rolling with a roll reduction of 30% or more to obtain a cold-rolled sheet. In this disclosure, cold rolling is performed with a roll reduction of 30% or more. This is because when the roll reduction is less than 30%, ferrite recrystallization is not favored, and both ferrite and martensite become oily, resulting in impaired hole expansion formability, delayed fracture resistance, and elongation. Although no upper limit for the roll reduction is specified, such reduction is preferably 95% or less due to limitations in production technologies. Next, the cold-rolled sheet undergoes an annealing process to promote recrystallization and form fine ferrite, martensite, and bainite in the steel sheet's microstructure to increase strength. Specifically, the cold-rolled sheet is heated at an average heating rate of 3°C / s plus or minus 30°C / s to an initial immersion temperature of 800°C or more and 900°C or less with a dew point of -40°C or more and 10°C or less within a temperature range of 600°C or more, and is held at the initial immersion temperature for 30 seconds or more and 800 seconds or less.subsequently, it is cooled at a second average cooling rate of 10 °C / s more from the first immersion temperature to a second immersion temperature of 350 °C or more and 475 °C or less, held at the second immersion temperature for 300 seconds or less, then cooled to room temperature at a third average cooling rate of 100 °C / s more; then reheated to a third immersion temperature of 200 °C or more and 400 °C or less, and held at the third immersion temperature for 180 seconds or more and 1800 seconds or less. First, the cold-rolled sheet is heated at an average heating rate of 3°C / second plus or minus 30°C / second to an initial immersion temperature of 800°C or more and 900°C or less with a dew point of -40°C or more and 10°C or less within a temperature range of 600°C or more, and is held at the initial immersion temperature for 30 seconds or more and 800 seconds or less. In the following description, the process of holding the sheet at the initial immersion temperature of Lnn / zznz / B / Yi 800 °C or more and 900 °C or less for 30 seconds or more and 800 seconds or less is also known as the first dip. Average heating rate: 3 °C / s plus and 30 °C / s minus By heating the cold-rolled sheet to an initial immersion temperature of 800 °C or higher and 900 °C or lower at an average heating rate of 3 °C / s or higher and 30 °C / s or lower, it is possible to refine the crystalline grains obtained after annealing. Rapid heating of the cold-rolled sheet hinders recrystallization and results in anisotropic crystalline grains. Furthermore, the ferrite volume fraction increases, while the quenched martensite volume fraction decreases. As a result, it is difficult to achieve a tensile strength of 1180 MPa or higher, and the elongation, hole expansion formability, and delayed fracture resistance of a base steel sheet and a projection weld deteriorate. Therefore, the average heating rate is set at 30 °C / s or lower.When the heating rate is too low, the ferrite and martensite grains become oily, the predetermined average grain size cannot be achieved, and the hole expansion formability and delayed fracture resistance of a base steel sheet and projection weld deteriorate. Therefore, the average heating rate is set at 3°C / s or higher. The average heating rate of the cold-rolled sheet at the first dip temperature of 800°C or higher and 900°C or lower is preferably 5°C / s or higher. Dew point in a temperature range of 600 °C or more: -40 °C or more and 10 °C or less To reduce the mass percent of carbon (C) and manganese (Mn) in the surface layer of a steel sheet after annealing, the dew point at a temperature range of 600°C or higher is set at -40°C or higher and 10°C or lower during heating, up to the first dip temperature and the first dip. In an annealing furnace, when the dew point at a temperature range where the surface temperature of the steel sheet is 600°C or higher is -40°C or higher and 10°C or lower, it is considered to be below -40°C. When the dew point is below -40°C, the mass percent of carbon (C) and manganese (Mn) in the surface layer increase, and the delayed fracture resistance of a projection weld deteriorates. The dew point in a temperature range of 600 °C or more is preferably -30 °C or more.By setting the dew point at -30 °C or higher, the mass percent of carbon in a region 20 gm or less in the thickness direction from the surface of the steel sheet is less than 10% of the mass percent of carbon in a region 100 gm or more and 200 gm or less from the surface of the steel sheet, further increasing resistance to delayed fracture. On the other hand, when the dew point exceeds 10 °C, the mass percent of manganese in the surface layer of the steel sheet after annealing increases, and the resistance to delayed fracture of a projection weld deteriorates. The dew point in a temperature range of 600 °C or higher is preferably 5 °C or lower. Lnn / zznz / B / Yi First immersion temperature: 800 °C or more and 900 °C or less The first dip temperature is a predetermined temperature set within a temperature range corresponding to a dual-phase region of ferrite and austenite. When the first dip temperature is below 800 °C, the ferrite fraction increases and the volume fraction of quenched martensite decreases, making it difficult to guarantee strength. Therefore, the first dip temperature is set at 800 °C or higher. Conversely, when the dip temperature is too high, the dip occurs in a single-phase austenite region, and the austenite crystal grains grow excessively, resulting in grain oiling.As a result, the average grain size of the resulting quenched martensite increases, the volume fraction of the quenched martensite increases, and the elongation, hole expansion formability, and delayed fracture resistance of the base steel sheet and projection weld deteriorate. Therefore, the initial dip temperature is set at 900 °C or lower. The initial dip temperature is preferably 880 °C or lower. Holding time at the first immersion temperature: 30 seconds or more and 800 seconds or less. The steel sheet is held at the first immersion temperature for 30 seconds or more to allow recrystallization to occur and for a portion of the microstructure to undergo austenite transformation. When the holding time at the first immersion temperature is less than 30 seconds, the ferrite volume fraction increases and the quenched martensite volume fraction decreases, resulting in a deterioration of tensile strength. On the other hand, when the holding time at the first immersion temperature exceeds 800 seconds, Mn microsegregation is favored, which impairs hole expansion formability and delayed fracture resistance of a base steel sheet and a projection weld. Therefore, the holding time at the first immersion temperature is set at 800 seconds or less. The holding time is preferably 600 seconds or less.By setting the holding time to 600 seconds or less, the mass % of Mn in a region of 20 pm or less in the thickness direction from the surface of the steel sheet is less than 10% of the mass % of Mn in a region of 100 pm or more and 200 pm or less from the surface of the steel sheet, further increasing the delayed fracture resistance. The cold-rolled sheet is then cooled from the first immersion temperature to a second immersion temperature of 350°C or higher and 475°C or lower at a second average cooling rate of 10°C / second or more; it is then held at the second immersion temperature for 300 seconds or less and subsequently cooled to room temperature at a third average cooling rate of 100°C / second or more. In the following description, the process of holding the sheet at the second immersion temperature for 300 seconds or less is also referred to as the “second immersion.” Lnn / zznz / E / Yi Second average cooling rate: 10 °C / s more After the first immersion, the steel sheet is cooled from the initial immersion temperature to room temperature at a second average cooling rate of 10 °C / s or less. When the average cooling rate is less than 10 °C / s, ferrite transformation progresses during cooling, increasing the ferrite volume fraction and impairing tensile strength and hole expansion formability. Although no upper limit is specified for the second average cooling rate, it is preferably less than 200 °C / s, more preferably less than 100 °C / s, and optimally less than 50 °C / s due to production technology constraints. Second immersion temperature: 350 °C or higher and 475 °C or lower When the final cooling temperature after immersion is below 350 °C, some austenite grains transform into martensite, and subsequent quenching further thickens the carbides. As a result, the carbides that serve as hydrogen capture sites are insufficient, and delayed fracture resistance deteriorates. When the final cooling temperature after immersion exceeds 475 °C, excessive pearlite forms. As a result, the volume fraction of quenched martensite decreases, the volume fraction of ferrite increases, and tensile strength and hole expansion formability deteriorate. The second immersion temperature is preferably 450 °C or lower. Holding time at the second immersion temperature: 300 seconds or less After the quenching described above, the steel sheet is held at the predetermined second immersion temperature of 350°C or higher and 475°C or lower for 300 seconds or less to form bainite. When the holding time exceeds 300 seconds, the bainite volume fraction increases, and hole expansion formability deteriorates. Furthermore, the number of carbides with a particle size of 0.1 µm or higher and 1.0 µm or lower contained in the quenched martensite and bainite grains decreases, and the delayed fracture resistance of a base steel sheet and a projection weld deteriorates. Therefore, the holding time at the second immersion temperature is set at 300 seconds or less. The holding time at the second immersion temperature is preferably 200 seconds or less.The lower limit of the holding time at the second immersion temperature is not specified, and may be 0 seconds. Third average cooling rate: 100 °C / s more This is an extremely important feature of the present disclosure. After the second dip, the cold-rolled sheet is cooled at a third average cooling rate of 100 °C / s to transform the remaining austenite into martensite. When the third average cooling rate is below 100 °C / s, the carbides are oiled by the subsequent quenching treatment. As a result, the amount of fine carbide serving as a hydrogen capture site is insufficient, and Lnn / zznz / B / Yi impairs the delayed fracture resistance of a base steel sheet and a projection weld. The third average cooling rate is preferably 150 °C / s or more and, optimally, 200 °C / s or more. The cooling method can be any method by which a third average cooling rate of 100 °C / s or more can be obtained, and some examples include gas cooling, mist cooling, and water cooling. Water cooling is preferred from the standpoint of low cost. Although the upper limit of the third average cooling rate is not specified, this rate is preferably 2000 °C / s or less and, optimally, 1200 °C / s or less due to limitations in production technologies. Next, the cold-rolled sheet, which has cooled to room temperature, is reheated to a third immersion temperature of 200 °C or higher and 400 °C or lower, and held at this third immersion temperature for 180 seconds or more and 1800 seconds or less. The quenching treatment improves delayed fracture resistance by tempering the martensite. Third immersion temperature: 200 °C or more and 400 °C or less When the third immersion temperature is below 200 °C or above 400 °C, fine carbides with a sufficient particle size of 0.1 µm or larger and 1.0 µm or smaller cannot be obtained. As a result, the carbides that serve as hydrogen capture sites are insufficient, and the delayed fracture resistance of a base steel sheet and a projection weld deteriorates. Holding time at the third immersion temperature: 180 seconds or more and 1800 seconds or less. When the third immersion temperature is less than 180 seconds or greater than 1800 seconds, fine carbides with a sufficient particle size of 0.1 µm or larger and 1.0 µm or smaller cannot be obtained. As a result, the carbides that serve as hydrogen capture sites are insufficient, and the delayed fracture resistance of a base steel sheet and a projection weld is impaired. The holding time at the third immersion temperature is preferably 1500 seconds or less. Pickling treatment Next, the cold-rolled sheet, after the tempering treatment, undergoes a pickling process. Pickling is performed to remove oxides of Si, Mn, and other similar elements concentrated in the surface layer of the steel sheet. Without pickling, these oxides cannot be sufficiently removed, and alloying elements such as Si and Mn become excessively concentrated on the surface of the steel sheet, leading to a deterioration of the delayed fracture resistance of a projection weld. No pickling conditions are specified, and any of the common pickling methods using hydrochloric acid, sulfuric acid, or similar acids may be applied. However, it is preferable to perform pickling under pH conditions of 1.0 or higher and 4.0 or lower. Lnn / zznz / E / Yi temperature of 10 °C or more and 100 °C or less, plus an immersion time of 5 seconds or more and 200 seconds or less. After pickling, the thin sheet of high-strength steel can undergo coating or plating treatment. The type of metal for the coating or plating is not specified, but zinc is an example. Examples of galvanizing treatments include hot-dip galvanizing and galvanic annealing, where the alloying treatment is carried out after hot-dip galvanizing. When hot-dip galvanizing is applied, the temperature of the thin sheet of high-strength steel immersed in the molten bath is preferably between -40°C and +50°C.If the temperature of the thin, high-strength steel sheet immersed in the molten bath is -40°C or higher (hot-dip galvanizing bath temperature -40°C), the solidification of the molten zinc can be more effectively prevented, thus improving the coating's appearance. If the temperature of the thin, high-strength steel sheet immersed in the molten bath is +50°C or lower (hot-dip galvanizing bath temperature +50°C), mass production can be further increased. After hot-dip galvanizing, an alloying treatment can be applied to the zinc coating at temperatures between 450°C and 600°C. This treatment results in an iron (Fe) concentration in the zinc coating of 7% to 15%, which improves the adhesion of the hot-dip galvanizing and the corrosion resistance after coating. In hot-dip galvanizing, a galvanizing bath containing 0.10% or more and 0.20% or less of Al is preferably used. After the galvanizing process, the steel sheet may be subjected to washing to adjust the coating weight. After pickling, the thin high-strength steel sheet can be subjected to temper rolling. After pickling, when the thin high-strength steel sheet is subjected to temper rolling, the elongation index of the tempered roll is preferably 0.05% or more and 2.0% or less. EXAMPLES The following are some examples of this disclosure. This disclosure is not limited in any way by the examples described below and may be implemented with appropriate modifications without departing from the spirit of this disclosure. All such modifications are included within the technical scope of this disclosure. Steel materials with the chemical compositions listed in Table 1 were prepared using steelmaking processes and subjected to continuous melting to produce steel plates. The steel plates were then subjected to hot rolling with a The hot rolling temperature (Lnn / zznz / E / Yi) was set to 1250 °C, and the finished delivery temperature (FDT) was specified in Table 2, to produce hot-rolled sheets. These hot-rolled sheets were then cooled to the coiling temperature (CT) at the first average cooling rate (cooling rate 1) specified in Table 2 and coiled at the coiling temperature. After pickling, the hot-rolled sheets were then cold-rolled at the reduction ratio specified in Table 2 to produce cold-rolled sheets (thickness: 1.4 mm). These cold-rolled sheets were then conveyed to a continuous annealing line (CAL) and subjected to further annealing.First, the cold-rolled sheets were heated at the average heating rate indicated in Table 2 and annealed at the first immersion temperature for the immersion time (first holding time) indicated in Table 2. Next, the cold-rolled sheets were cooled to the second immersion temperature and the second average cooling rate (cooling rate 2) indicated in Table 2. Subsequently, the cold-rolled sheets were held at the second immersion temperature for the time indicated in Table 2 (second holding time) and then cooled to room temperature at the third average cooling rate (cooling rate 3).Next, as a tempering treatment, the cold-rolled sheets were reheated to the third immersion temperature, held at the third immersion temperature for the time indicated in Table 2 (the third holding time), and then subjected to pickling to obtain steel sheets. A JIS No. 5 tensile test piece was collected from each of the steel sheets obtained, so that the direction orthogonal to the rolling direction corresponded to the longitudinal direction (tension direction), and both the tensile strength (TS) and the elongation (EL) were measured by a tensile test in accordance with JIS Z2241 (1998). The hole expansion ratio was measured in accordance with JIS Z2256 (2010). 10 mm diameter holes were drilled at 12.5% ​​spacing, and a testing machine was configured so that the turning occurred on the die side. The holes were then opened with a 60-degree conical punch, and the amount of hole diameter expansion when a fracture at the hole edge penetrated in the thickness direction at least once was expressed as the ratio of the hole diameter at the time of fracture to the initial hole diameter. This ratio was defined as the hole expansion ratio (λ). A steel sheet with a λ (%) value of 50% or higher was considered to have good hole expansion formability. The delayed fracture resistance of a base steel sheet was measured as follows. Initially, a 30 mm x 100 mm steel piece was cut from each of the produced steel sheets, with the rolling direction corresponding to the longitudinal direction. The end face of the steel piece was ground. Two bolt holes were then provided in opposite positions when the steel piece was U-bent in the longitudinal direction to obtain a test piece. The test piece was subjected to a 180-degree U-bend with a 10 mm radius of curvature at the punch end using a press forming machine. After the bending process, the steel was... The test piece was deformed due to elastic recovery, so that the opposing surfaces separated (the U-bend opened outwards). A bolt was inserted into the bolt holes of the spring-rebound test piece, secured so that the distance between the opposing surfaces was 20 mm or 25 mm, and tension was applied to the test piece. The bolted test piece was immersed in a 3.0% NaCl and 0.3% NH4SCN solution at 25 °C, and an electrolytic charging process was carried out, with the test piece acting as the cathode to allow hydrogen to penetrate the steel. The current density was set to 1.0 mA / cm², and the counter electrode was platinum.It was determined that a test piece with a distance of 25 mm between opposite surfaces and that did not fracture even after 100 hours of immersion exhibited good resistance to delayed fracture of a base steel sheet (good resistance), as well as it was determined that a test piece with a distance of 20 mm between opposite surfaces and that did not fracture even after 100 hours of immersion exhibited particularly good resistance to delayed fracture of a base steel sheet (excellent resistance). The delayed fracture resistance of a projection weld was measured as follows. First, a 50 mm x 150 mm test piece was cut from each of the steel sheets, and a 10 mm diameter hole was drilled in the center. The test piece and an M6 welding nut with four protruding portions were placed in an AC welding machine so that the center of the hole in the test piece and the center of the hole in the nut were aligned. The test piece and the welding nut were then subjected to projection welding using a servo-motor-type AC (50 Hz) welding gun attached to the AC welding machine to produce a test piece with a projection weld. The electrode tips used in the welding gun were 30 mm diameter flat electrodes.The welding conditions were as follows: an electrode force of 3000 N, a welding time of 7 cycles (50 Hz), a welding current of 12 kA, and a holding time of 10 cycles (50 Hz). A bolt was secured in the nut hole of the test piece using spray welding, and the test piece was placed on a spacer. A compression pull-off test was then performed in accordance with JIS B 1196 (2001), whereby the bolt was screwed into the welded nut, a compressive load was gradually applied to the bolt tip so that the center of the load coincided with the center of the bolt as closely as possible, and the load was measured when the nut pulled off the steel sheet. The pull-off strength was defined as PS. Test pieces with a fixed bolt were prepared in the same way as before and loaded with 0.5 x PS and 0.7 x PS.Next, the test pieces were immersed in a hydrochloric acid solution (pH = 2.2) at room temperature, and the time until the nut detached from the steel sheet was measured. Under a load of 0.5 x PS, a test piece that showed no fracture after 100 hours was determined to exhibit good delayed fracture resistance of a projection weld (good resistance), and under a load of 0.7 x PS, a test piece that showed no fracture after that time was determined to exhibit good delayed fracture resistance. Lnn / zznz / E / Yi of 100 hours exhibited a particularly good resistance to delayed fracture of a projection weld (excellent resistance). The volume fractions of ferrite, quenched martensite, and bainite, as well as the average grain size of ferrite and quenched martensite in the resulting steel sheets, were calculated according to the methods described above. The volume fractions of retained austenite, pearlite, and unrecrystallized ferrite were also calculated according to the methods described above. The volume fractions of quenched martensite grains and bainite grains containing carbides with particle sizes of 0.1 µm or larger and 1.0 µm or smaller, relative to the total quenched martensite and bainite, were calculated according to the method described above. In addition, the ratio of the mass percent of carbon (C) and the mass percent of manganese (Mn) in a region 20 µm or smaller in the thickness direction from the steel sheet surface, relative to the mass percent of C and the mass percent of Mn in a region 100 µm or larger and 200 µm or smaller from the steel sheet surface, was measured according to the method described above. The results of measurements of the steel sheet microstructure, tensile strength, elongation, hole expansion formability, and delayed fracture resistance of a base steel sheet and a projection weld are listed in Table 3. nofr Lnn / zznz / E / Yi Tabla t ID Steel Sample Chemical Composition (% by mass.) Observations c Si Mn ps .Al N Other Components A 0.14 1.11 2.21 0.01 0.001 0.03 0.002 - Formed Steel B 16 1.45 1.29 0.01 0.001 0.02 0.003 Ti:0.03.Nb:0.02,B:0.0015 Formed Steel C 0.12 0.54 2.43 0.01 0.002 0.03 0.002 V:Ü.O2. Mo:0.12, Ca:0.0011 Formed Steel D 0.21 0.84 1.85 0.01 0.001 0.03 0.003 Cu;0. 15. XrO.19 Formed Steel E 0.15 0.95 1.54 0.OÍ 0.001 0.02 0.002 Cr:0.22. REMUGOOS Formed Steel 0.24 L44 2.14 0.01 0.002 0.03 0.003 - Comparative Example G 0.09 1.15 1.98 0.01 0.002 0.03 0.002 Ti:0.03 Comparative Example H 0.15 1.66 1.58 0.01 0.002 0.03 0.003 Mo:0.2t Comparative Example T 0.14 0.44 2.33 0.01 0.002 0.03 0.003 Cu.0.25 Comparative Example J 0.15 1 7? -63 0.01 0.002 0.03 0.003 V0.03 Comparative Example K 0.14 1.05 1.05 0.01 0.002 0.03 0.003 - Comparative Example L 0.13 1.05 ».12 0.01 0.001 0.02 0.002 Ta:0.020. W:0.020 Formed Steel M 0.15 <77 1.95 0.01 0.001 0.03 0.003 Ti:0.02.Sn:0.025.Sb;0.025 Acero Conformado N 0.14 i 16 0.01 0.002 0.02 0.002 Mg;0.0015 Zr:0.0015 Acero Conformado O ΰΛΊ 1.38 2.39 0.01 0.002 0.03 0.003 Nb:0.02 .Co:0.005,Zn: 0.005 Acero Conformado. The highlighted indicates that it is outside, of the appropriate rank of the current disclosure. Tabla 2 ΰ g Λ é o 1 pT tí § iiT í ií? « Tí § o § w1 *S. a ó μΤ tí t ¡T o 1 'ó ot wT Spmpfo Comparativo 1 ro & 8 Q o Tí w tí pt C •í. 'noi 3 S4 p £ ¡Tí 1 uf Zjeosplo Comparativo | O •S Í N' tí $ f £ M' 1 § '0 UT | rvl I :T tí g cí t Q & N' tí i μϊ' MO '£ w ¨0 tz5 <i £ s -p th 'ti o δ λ 4j 1 -e íc p v· n ('ί o m re ra c» c 4 ί r-i ’x1 ñ ó tfl «¿? cl>”i 'p / , TÍ '§ ? > W -·' swora w So re 1» C> o wo CZJ ou re 00 «0 co i· s | ¡3 ta I pra V. O «"1 § tM Vi Vi 8 re re V v. V! Ci re 2 o re & -8 ta " p / ilo 4- re Ί· re t-· Φ '3· Ó Ó re O ν'! $ c> •ig -t «D r3l ? Ci C) *r ó re ¡i; re 00 Λ 2 2 O re Τ' Λ 2 reti w ΐ a κ b ™ ; ,8 w re ti 5? <D Ó re c¡ re s (D re V, oí ó w| 8 o 'D o re o Cl ré 5 ó g o Ci Cl re V.w — ~ *P v> !< re v, re o V» $ $ re re mi Sitíre vi Sir s re s'i $ * -ts es ,« HJ ¿i 1 E VI E- 2 O Φ Φ a. 0ί O 8 Φ r> Φ w § a Φ ¿.i Q § ra l·?! ra o, o Ch s -5 O Οί n $ i R Sj $ Λ -3⁄4 Ή Lj a K u 0 < < 0) W < U CQ for υ -« a ,-Η ra tn ί- re r «j θ' Q re re X! *0 re s 2 ra. :8 £ noe Lnn / zznz / Ε / γΐΛΐ 8 Ñ S ¿i M δ 1’ i V py i t Q t it? o 6 & & δ _ci t ícT 1 I tí I w1 hjeínpío Ccmp2i3hvc| $ J5 M t Q * 1 ιχΐ -i σι & δ O *& iq1 S ϋ I < § w i él I o B* n VT i i i in (3 K δ O o Ϊ iiT 1-5 & I 8 w p tó t9 I o Λ. δ ρΤ I Φ 1 ίΐΐ 'm ra t Q 1 iíT •i SI & g H Έ, 1 puf Cí Fá f tí ’S. w1 í uf jy I w' í •R 1.1 3 -8 el Ñ 1? n • ^1 to Uí ¡Λ w £ ίΛ ¿1 * »Λ to to to to to to to to tí ϋ rZ „ t á .s bu .¡v 5· o> H ' Λΐ ró tr, fe o o fe fe Fr, g fe O § fe vi © sT| fe fe fe! ? l | -' fe fe O o ó o g Λ fe Tp a Ό fe ¢4 ? s fe tu fe fe f-í G T, Ί? m Ό L·? vJ 9 > (el O «> g Q tFl o r4 fe w« fe o UJ fe «I fe IZJ fe fe to fe to to o <1 to fe Φ O 1 8. d 'o .u $ $ H « bu o °? Q Γ4 | fe fe- ώ> o fe fe fe to fe íc 2 fe 2 Sí e i fe £ 55 τ p fe 'N । fe fe fe fe fe fe g fe o fe fe <5 o fe fe V-| Ή C1 λ"> 9 > ¿1 -- V C 4 C4 G» C -1 tN «1 ¢4 •5f C4 2 ¢4 Vi 2 2 Vi V d '£ M o ω ·μ fe & S ñ » JS bu t* n> H fo- ¿3 φ fe δ ró Q rn ίο a í 'Z> to fe φ to en fe fe § O M 5 "< :S fe i ? E W O fe w w fe § fe 1« w fe fe >7? W fe O to o to to w «± fe i» 3 Λ c] u L Φ fJ t? c cu '- S φ o o ’" 2 fe Q fe 2 2 2 o 2 fe 2 Γ4 2 u’ ej £ fe· Lt g JE o $ « O x O Vi 2 o 2 2 £Λ 2 o Uh »«Ί G' o 2 2 fe o 2 2 2 y 1 Έ ?-· fe ft vi o VI «1 o fe fe Vi fe fe fe Vi fe fe s & fe 2 VI © Laminado sa caLsníe lu w fe u K, fe VI V «i fe 35 fe I^i fe Λ fe O Vi fe fe fe V! fe fe fe fe 0.' r-l ..-, vi 9 > ÚJ V! % v fe fe o s (D Ti Vi ó> νΊ fe •41 fe Ci fe ó T Vi Vi f. <l fe © vi •ύ t3 í 1 f v) •u -tí í £ to o o. § s o os 81 —. 8 θ'. si 8¡ rí ai m c a ® q co u w ω < nú oi v. ή -s *1 sj 1**1 n ό r l <n ir, r- ró ti fef •8 1 'δ. i noe Lnn / zznz / Ε / γΐΛΐ Lnn / zznz / E / γΐΛΐ Lnn / zznz / E / γΐΛΐ The examples proved superior in all aspects: tensile strength, elongation, formability by hole expansion, delayed fracture resistance of a base steel sheet, and delayed fracture resistance of a projection weld. On the other hand, the comparative examples were inferior in at least one of the following aspects: tensile strength, elongation, formability by hole expansion, delayed fracture resistance of a base steel sheet, and delayed fracture resistance of a projection weld.< / l>

Claims

1. A high-strength steel sheet comprising: a chemical composition containing, in terms of % by mass, C: 0.10% or more and 0.22% or less. Si: 0.5% or more and 1.5% or less. Mn: 1.2% or more and 2.5% or less. P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.10% or less, and N: 0.010% or less, while the remainder corresponds to Fe and unavoidable impurities, and a complex structure containing 5% or more and 35% or less of ferrite by volume fraction, 50% or more and 85% or less of quenched martensite by volume fraction, and 0% or more and 20% or less of bainite by volume fraction, wherein the ferrite has an average grain size of 5 pm or less, the quenched martensite has an average grain size of 5 pm or less, a volume fraction of a total of quenched martensite and bainite containing five or more carbides with a particle size of 0.1 pm or more and 1.0 pm or less in a grain, with respect to a total of quenched martensite and bainite of 85% or more, and the mass % of C and the mass % of Mn in a region of 20 pm or less in the thickness direction from a surface of the steel sheet is 20% or less with respect to the mass % of C and the mass % of Mn in a region of 100 pm or more and 200 pm or less from the surface of the steel sheet.

2. The high-strength thin steel sheet according to claim 1, wherein the chemical composition further contains, in terms of % by mass, at least one of the elements selected from the group consisting of: Ti: 0.05% or less, V: 0.05% or less, and Nb: 0.05% or less.

3. The high-strength thin steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in terms of % by mass, at least one of the elements selected from the group consisting of: Lnn / zznz / E / Yi Mo: 0.50% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, B: 0.0030% or less, Ca: 0.0050% or less, Rare Earth Elements (REM): 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0050% or less, Zr: 0.1000% or less, Co: 0.020% or less, and Zn: 0.020% or less.

4. A method for manufacturing a thin sheet of high-strength steel, comprising: subjecting a steel plate having the chemical composition according to any one of claims 1 to 3 to hot rolling under the condition of a finished supply temperature of 850°C or more and 950°C or less to obtain a hot-rolled sheet, then cooling the hot-rolled sheet at a first average cooling rate of 30°C / µg or more to a coiling temperature of 550°C or less and then coiling the hot-rolled sheet at the coiling temperature, subsequently subjecting the hot-rolled sheet to pickling, then subjecting the hot-rolled sheet after pickling to cold rolling with a rolling reduction of 30% or more to obtain a cold-rolled sheet, subsequently,Heat the cold-rolled sheet at an average heating rate of 3°C / s plus and 30°C / s minus to a first immersion temperature of 800°C or more and 900°C or less with a dew point of -40°C or more and 10°C or less in a temperature range of 600°C or more, and hold the cold-rolled sheet at the first immersion temperature for 30 seconds or more and 800 seconds or less, then cool the cold-rolled sheet from the first immersion temperature to a second immersion temperature of 350°C or more and 475°C or less at a second average cooling rate of 10°C / s plus, and hold the cold-rolled sheet at the second immersion temperature for 300 seconds or less, not? Lnn / zznz / E / Yi then cool the cold-rolled sheet to room temperature at a third average cooling rate of 100 °C / s more, subsequently,Reheat the cold-rolled sheet to a third immersion temperature of 200°C or more and 400°C or less, and hold the cold-rolled sheet at the third immersion temperature for 180 seconds or more and 1800 seconds or less, and then subject the cold-rolled sheet to a pickling process.