STEEL SHEET AND METHOD FOR PRODUCING IT

Abstract

A high-ductility, high-softening-resistance steel sheet from a single weld and a method for producing the steel sheet are provided. The steel sheet has a specific chemical composition, wherein the steel microstructure contains ferrite: 6% to 90% by area, a microstructure composed of one or more layers of upper bainite, fresh martensite, quenched martensite, lower bainite, and retained carbon: 10% to 94% by area in total, and retained: 3% to 20% by volume, a ratio (SUB / S2nd) x 100 (%) of a SUB area ratio of a top bainite with a width in the range of 0.8 to 7 µm, a length in the range of 2 to 15 µm, and an aspect ratio of 2.2 or more in contact with the retained ?UB with a grain width in the range of 0.17 to 0.80 µm and an aspect ratio in the range of 4 to 25 to an S2nd area ratio of the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained ? varies from 2.0% to 15%, grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 µm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained ? have a distribution density N? of 7 / µm2 or less (including 0 / µm2), and grains with an equivalent circular diameter in the range of 1.3 to 20 µm and an aspect ratio of 3 or less have a total area ratio S?Block of 5% or less (including 0%).

Description

STEEL SHEET AND METHOD FOR PRODUCING IT TECHNICAL FIELD The present invention relates to a steel sheet that can be suitably applied to press forming and is used through a press forming process for automobiles, household appliances and the like, and a method for producing the steel sheet. BACKGROUND OF THE TECHNIQUE In recent years, the increasing need to reduce the weight of automotive bodies has driven the use of high-strength steel sheets, ranging from 980 to 1470 MPa, in automotive frame components, bumpers, seating components, and similar parts. Furthermore, laser welding is being used to reduce component weight and increase their rigidity. This includes, for example, using custom-made blanks by joining steel sheets of varying thicknesses or strengths before press forming, creating a closed-section structure by laser welding the edges or flanges of pressed components, shortening flanges, and increasing weld strength compared to conventional spot welding. However, when high-strength steel sheets (grades 590 to 1470 MPa) are applied to automotive parts, a reduction in their ductility tends to cause cracking during pressing. In many of these steel sheets, hard martensite forms within the microstructure to enhance strength and ductility. Therefore, laser welding of these steel sheets results in significant softening in a heat-affected zone (hereafter referred to as HAZ) due to the softening of the martensite. This leads to the problem that a softened portion of the HAZ will rupture during press forming or break in a disadvantageous way during component deformation, thus decreasing the component's strength. Therefore, it is desirable for these high-strength steel sheets to have greater formability and less HAZ softening than before. In such circumstances, for example, TRIP steel, which contains and disperses TRIP in the microstructure of steel sheets, has been developed as a technique to improve the ductility of steel sheets. For example, Patent Literature 1 reveals that high-ductility steel sheets with a tensile strength (TS) of 80 kgf / mm² or more and a TS x El >2500 kgf / mm²-% can be produced by annealing steel containing C: 0.10% to 0.45%, S: 0.5% to 1.8%, and Mn: 0.5% to 3.0% and holding the steel in the range of 350°C to 500°C for 1 to 30 minutes to form and retain the steel. Patent Literature 2 reveals that steel sheets of excellent ductility El and draw-flanging capability λ can be produced by annealing steel containing C: 0.10% to 0.25%, Si: 1.0% to 2.0% and Mn: 1.5% to 3.0%, cooling the steel to a temperature in the range of 450°C to 300°C in 10°C / s more, and holding the steel for 180 to 600 seconds so that the retained austenite is 5% by area or more, the bainitic ferrite is 60% by area or more and the polygonal ferrite is 20% by area or less. / CCLLn / LZnZ / E / Yli Patent Literature 3 reveals that high ductility steel sheets can be produced by annealing steel containing C: 0.10% to 0.28%, Si: 1.0% to 2.0% and Mn: 1.0% to 3.0% in the temperature range of the A3 transformation temperature or higher, then slowly cooling the steel to a temperature in the range of A3 - 250°C to As - 20°C at a cooling rate in the range of 1°C to 10°C to form ferrite and then cooling the steel to the bainite transformation temperature range (320°C to 450°C) at a cooling rate of 11°C / s or higher avoiding ferrite transformation. Steel sheets contain bainitic ferrite: 30% to 65% by area, polygonal ferrite: 30% to 50% by area, and retained austenite: 5% to 20% by area, and have TS x El > 23000 % MPa. As a technique for improving the softening resistance of HAZ, for example, Patent Literature 4 describes a technique for preventing quench softening of martensite in a steel sheet, which has a microstructure containing martensite: 5% to 40% and has a tensile strength of 780 MPa or more, forming fine carbide in a heat-affected zone allowing the steel sheet to contain C: 0.05% to 0.20%, Si: 0.005% to 1.3%, Mn: 1.0% to 3.2%, Mo: 0.05% to 0.5%, and one or two or more of Nb: 0.005% to 0.05% and Ti: 0.001% to 0.05%. Patent Literature 5 reveals that steel sheets excellent in ductility, extensibility, and weldability can be produced by annealing a steel sheet containing C: 0.01% to 0.3%, Si: 0.005% to 2.5%, Mn: 0.01% to 3%, Mo: 0.01% to 0.3%, and Nb: 0.001% to 0.1% in a high-temperature region to have a near-one and single-phase microstructure, and then cooling and holding the steel sheet in the temperature range of 200°C to 450°C to form 50% to 97% bainite or bainitic ferrite as the main phase and 3% to 50% austenite as a second phase. List of Appointments Patent Literature PTL 1: Publication of Examined Japanese Patent Application No. 6-35619 PTL 2: Japanese Patent No. 4411221 PTL 3: Japanese Patent No. 4716359 PTL 4: Japanese Patent No. 3881559 PTL 5: Japanese Patent No. 3854506 BRIEF DESCRIPTION OF THE INVENTION Technical Problem The conventional TRIP steel described in Patent Literature 1 has a high yield strength, but it has the problem that the HAZ softening resistance of a weld is significantly reduced. In other words, plastic deformation in a softened HAZ portion of a laser-welded steel sheet breaks the steel sheet into HAZ, which includes a portion softer than the base material. This makes laser welding difficult and necessitates measures such as establishing a weld line in a portion that is not deformed. / CCLLn / LZnZ / E / Yli The technique described in Patent Literature 2 primarily uses bainitic ferrite as its microstructure, and the amount of ferrite is kept small. Therefore, the technique has the disadvantage that ductility is not necessarily high despite the high draw-forming capacity. The technique described in Patent Literature 3 uses ferrite as a microstructure to improve ductility, but it has the drawback that the use of ferrite makes it difficult to increase strength, particularly to 980 MPa or higher. Furthermore, the softening resistance of HAZ is significantly reduced. The technique described in Patent Literature 4 aims to improve the quench softening resistance of martensite, but it is insufficient to improve the softening resistance of HAZ. The technique cannot adequately prevent martensite softening in HAZ and has an insufficient improvement effect, particularly in steel sheets with a tensile strength of 980 MPa or higher. The technique described in Patent Literature 5 aims to improve high-strength materials and is effective at high laser welding speeds and with low heat input. However, it suffers from significant beam softening in the 4–6 kW laser output range at normal or low welding speeds, such as 3–5 mpm, and from beam breakage in a tensile test of a joint with a laser weld line perpendicular to the tensile axis. Furthermore, ductility is not always high, and further ductility improvement is desired. Additionally, the technique requires the addition of a large amount of expensive Mo or Nb, so cost reduction is also a concern. As described above, in the related art, in high-strength steel sheets, particularly with a tensile strength of 980 MPa or more, no steel sheet has high ductility and high resistance to HAZ softening. The present invention has been made to solve such problems and aims to provide a steel sheet with high ductility and still high resistance to softening of a weld and a method for producing the steel sheet. SOLUTION TO THE PROBLEM The present inventors have intensively studied the means to simultaneously achieve high ductility and high resistance to softening of HAZ and have reached the following conclusions. (i) Form a predetermined amount of coarse upper bainite adjacent to the retained y-saturation (Y-saturation) with a grain width in the range of 0.17 to 0.80 pm and an aspect ratio in the range of 4 to 25 (retained y-saturation) in the steel microstructure. The coarse upper bainite is soft, and carbon in the upper bainite can be transferred to the retained y-saturation by maintaining the coarse upper bainite adjacent to the retained y-saturation. Therefore, it is possible to form a region composed of retained y-saturation, which is less susceptible to thermal effects, and soft bainite with a low carbon content, which is also less susceptible to thermal effects. Such a region also has the effect of improving ductility. / CCLLn / LZnZ / E / Yli (i) Adjust the grain distribution density Ne of grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm to be 7 / pm2 or less in a microstructure composed of one or two or more of upper bainite, fresh martensite, quenched martensite, lower bainite, and retained martensite. A region where the grains have a high distribution density is composed of quenched martensite or lower bainite. In this region, welding further accelerates quenching and tends to produce softening. Therefore, the softening resistance of HAZ can be improved by decreasing a region with a high grain density so that the microstructure is composed mainly of a region with a low grain density, which is less likely to soften on quenching. (iii) Adjust the total SYBiock area ratio of grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less to be 5% or less. These relatively coarse grains, which are often fresh martensite, significantly increase the strength of the base material, but tend to cause softening due to heat input and significantly reduce the softening resistance of HAZ. (iv) This microstructure can be formed by controlling the heating rate in an annealing step and, in a cooling step after annealing, holding it at approximately 450°C for a predetermined time and then cooling to approximately 200°C, and then reheating and holding it at approximately 400°C. This can improve both the ductility and softening resistance of HAZ. The present invention is based on the above findings and more specifically provides the following. [1] A steel sheet containing, as a chemical composition, in mass percentage, C: 0.06% to 0.25%, Yes: 0.1% to 2.5%, Mn: 2.0% to 3.2%, P: 0.02% or less, S: 0.01% or less, Sun. Al: less than 1.0% (including 0%), and N: less than 0.015%, where the total content of Si and sol. Al: Si + sol. Al ranges from 0.7% to 2.5%, the remainder being composed of Fe and incidental impurities, a steel microstructure contains ferrite: 6% to 90% by area; a microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite; and retained: 10% to 94% by area in total; and retained γ: 3% to 20% by volume, the ratio (SuB / Sznd) x 100 (%) of the area ratio Sub of an upper bainite with a width in the range of 0.8 to 7 pm, a length in the range of 2 to 15 pm, and an aspect ratio of 2.2 or more in contact with retained γυε with a grain width in the range of 0.17 to 0.80 pm and an aspect ratio of / CCLLn / LZnZ / E / Yll in the range of 4 to 25 to the area ratio S2nd of the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ ranges from 2.0% and 15%, grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ having a distribution density Νθ of 7 / pm2 or less (including 0 / pm2), and grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less have a total SyBiock area ratio of 5% or less (including 0%). [2] The steel sheet according to [1], wherein the grains with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, quenched martensite, lower bainite and retained γ have a distribution density Npine in the range of 0.03 to 0.4 / pm2. [3] The steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass percentage, one or two selected from Ti: 0.002% to 0.1% and B: 0.0002% to 0.01%. [4] The steel sheet according to any of [1] to [3], wherein the chemical composition further contains, in mass percentage, one or two or more selected from Cu: 0.005% to 1%, Ni: 0.01% to 1%, Cr: 0.01% to 1.0%, Mo: 0.01% to 0.5%, V: 0.003% to 0.5%, Nb: 0.002% to 0.1%, Zr: 0.005% to 0.2% and W: 0.005% to 0.2%. [5] The steel sheet according to any of [1] to [4], wherein the chemical composition further contains, in mass percentage, one or two or more selected from Ca: 0.0002% to 0.0040%, Ce: 0.0002% to 0.0040%, The: 0.0002% to 0.0040%, Mg: 0.0002% to 0.0030%, Sb: 0.002% to 0.1% and Sn: 0.002% to 0.1%. [6] The steel sheet according to any of [1] to [5], wherein the steel sheet has a tensile strength in the range of 590 to 1600 MPa. [7] The steel sheet according to any of [1] to [6], having a galvanized layer over / CCLLn / LZnZ / E / Yli a surface of the steel sheet. [8] A method for producing a steel sheet comprising: hot rolling and cold rolling a steel slab having a chemical composition in accordance with any of [1] to [5], then on a continuous annealing line, heating the cold-rolled steel sheet at 1°C / s to 6°C / s in the temperature range of 660°C to 740°C, heating the cold-rolled steel sheet at 1°C / s to 6°C / s in the temperature range of 740°C to 770°C, annealing the cold-rolled steel sheet in an annealing temperature range of 770°C to 850°C, then cooling the cold-rolled steel sheet at an average cooling rate in the range of 1°C / s to 2000°C / s in the temperature range of 770°C to 700°C, further cooling the cold-rolled steel sheet at an average cooling rate in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C,then hold the cold-rolled steel sheet in the temperature range of 500°C to 405°C for 13 to 200 seconds, then cool the cold-rolled steel sheet from 405°C to a cooling stop temperature Tsq in the range of 170°C to 270°C at an average cooling rate in the range of 1°C / s to 50°C / s, then heat the cold-rolled steel sheet in the temperature range of the cooling stop temperature Tsq to 350°C at an average heating rate of 2°C / s or more, hold the cold-rolled steel sheet at 350°C to 500°C for 20 to 3000 seconds, and then cool the cold-rolled steel sheet to room temperature, wherein a holding time in the temperature range of 170°C to 250°C between cooling after annealing and heating at a rate The average heating rate is 2°C / s or more in 50 seconds or less. [9] The method for producing a steel sheet in accordance with [8], wherein in cooling from 405°C to the cooling stop temperature Tsq in the range of 170°C to 270°C, a cooling rate in the range of 320°C to 270°C is 0.3°C / s plus or minus 20°C / s.

[10] The method for producing a steel sheet in accordance with [8] or [9], wherein a dew point temperature in annealing in an annealing temperature range of 770°C to 850°C is 45°C or more.

[11] The method for producing a steel sheet in accordance with any of [8] to

[10] , wherein the galvanizing treatment or the galvano-annealing treatment is carried out between cooling at an average cooling rate in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C and holding in the temperature range of 500°C to 405°C for 13 to 200 seconds.

[12] The method for producing a steel sheet in accordance with any of [8] to

[10] , wherein holding at 350°C to 500°C for 20 to 3000 seconds is followed by a galvanizing treatment or galvano-annealing treatment. ADVANTAGEOUS EFFECTS OF THE INVENTION The present invention can provide a steel sheet with high ductility and high resistance to HAZ softening of a weld. The present invention can also achieve an increase in strength. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an example of a SEM image. Figure 2 is an explanatory view of aspect ratio, grain width, and grain length. DESCRIPTION OF MODALITIES The present invention is specifically described below. The present invention is not limited to the following embodiments. A steel sheet according to the present invention has a particular chemical composition and a particular steel microstructure. Therefore, a steel sheet according to the present invention is described below in order of its chemical composition and steel microstructure. A steel sheet according to the present invention contains the following components. The unit “%” of the component content in the following description means “% by mass”. C: 0.06% to 0.25% Carbon can increase the strength of a fusion zone and a mitigated zone in a welded joint, thereby reducing deformation in the HAZ and improving its softening resistance. Carbon is included to ensure the area ratio of quenched martensite to guarantee a predetermined strength, to ensure the volume ratio of retained martensite to improve ductility, and to concentrate it in the retained martensite to stabilize the retained martensite and improve ductility. These effects cannot be sufficiently achieved with a carbon content below 0.06%. Therefore, the lower limit is 0.06%, preferably 0.09% or more, and more preferably 0.11% or more. A carbon content of more than 0.06%A carbon content of 25% results in a delay in the transformation of the upper bainite into the intermediate retention during cooling and hinders the formation of a sufficient amount of upper bainite adjacent to the retained yub. This reduces the ductility and softening resistance of HAZ. Therefore, the upper limit for carbon content is 0.25%. From the perspective of improving the softening resistance and ductility of HAZ, a carbon content of 0.22% or less is desirable. From the perspective of improving the softening resistance and ductility of HAZ, a carbon content of 0.20% or less is more desirable. Yes: 0.1% to 2.5% If it can suppress carbide formation in martensite or bainite and increase the amount of solid solution strengthening, which is less susceptible to thermal effects, thus improving the resistance to softening of HAZ in a weld and improving the stability of the retained material to improve ductility. From these perspectives, the Si content is preferably 0.6% or more, more preferably 0.8% or more, and even more preferably 1.1% or more. A Si content of more than 2.5% results in an extremely high rolling load and makes it difficult to produce a thin sheet. This also impairs the chemical conversion treatment capability and the toughness of a weld. Therefore, the Si content is 2.5% or less. The Si content is preferably less than 2.0% from the perspective of ensuring the chemical conversion treatment capability and toughness of a base material and a weld. The Si content is preferably 1.8% or less, more preferably 1.5% or less, from the perspective of ensuring the toughness of a weld. Mn: 2.0% to 3.2% Mn is an important element from the perspective of ensuring a predetermined area ratio of quenched martensite and / or bainite to ensure strength, from the perspective of stabilizing retained γ by lowering the Ms temperature of retained γ to improve ductility, from the perspective of reducing carbide formation in the bainite to improve ductility in the same way as with Si, and from the perspective of increasing the volume ratio of retained γ to improve ductility. To produce these effects, the Mn content is 2.0% or more. From the perspective of stabilizing retained γ to improve ductility, the Mn content is preferably 2.3% or more, more preferably 2.5% or more, and even more preferably 2.7% or more. A Mn content of more than 3%A 2% Mn content results in a delay in the transformation of the upper bainite during cooling and hinders the formation of a sufficient amount of upper bainite adjacent to the retained yub. This reduces the ductility and softening resistance of HAZ. Therefore, the Mn content is 3.2% or less. The Mn content is preferably 3.0% or less from the perspective of promoting bainite transformation to achieve high ductility, more preferably 2.9% or less. P: 0.02% or less Although phosphorus (P) is a steel-strengthening element, a high P content results in poor spot weldability. Therefore, the P content is 0.02% or less. From the perspective of improving spot weldability, the P content is preferably 0.01% or less. While P is not strictly required, from a production cost perspective, the P content is preferably 0.001% or more. S: 0.01% or less Although sulfur (S) improves scale removal during hot rolling and suppresses nitriding during annealing, it has significant adverse effects on spot weldability, bending capacity, and draw-flanging capacity. To reduce these adverse effects, the S content is 0.01% or less. In the present invention, spot weldability tends to deteriorate due to the high C, Si, and Mn contents. From the perspective of improving spot weldability, the S content is preferably 0.0020% or less, more preferably less than 0.0010%. Although S is not necessarily included, from a production cost perspective, the S content is preferably 0.0001% or more. Sun. Al: less than 1.0% (including 0%) Aluminum can suppress carbide formation and promote the transformation of the upper bainite, thereby improving the softening resistance of HAZ. It is desirable that the lower limit of the aluminum sol. content be, but not limited to, 0.01% or more for stable deoxidation. On the other hand, an aluminum sol. content of 1.0% or more results in very low strength of the base material and adversely affects the ability to be treated by chemical conversion. Thus, the aluminum sol. content is less than 1.0%. The aluminum sol. content can be 0%. To increase strength, the aluminum sol. content is preferably less than 0.50%, and more preferably 0.10% or less. N: less than 0.015% Nitrogen (N) is an element that forms nitrides, such as BN, AlN, or Tin, in steel. It decreases hot ductility and impairs the surface quality of the steel. In steel containing boron (B), N has the detrimental effect of negating the effects of B through the formation of BN. A content of 0.015% or higher results in very poor surface quality. Therefore, the N content is typically less than 0.015%. Although N is not strictly required, from a production cost perspective, the N content is preferably 0.0001% or higher. Si + sol. Al: 0.7% to 2.5% Silicon (Si) and aluminum solute (Al) can improve the softening resistance of HAZ. To sufficiently produce these effects, the Si and Al solute content should be 0.7% or more, preferably 0.8% or more, more preferably 0.9% or more, and particularly preferably 1.1% or more, in total. An excessively high Si and Al solute content results in an excessive increase in ferrite or upper bainite and causes a significant decrease in strength. Thus, the Si and Al solute content should be 2.5% or less, preferably 2.0% or less, in total. The chemical composition of a steel sheet according to the present invention may contain the following optional elements in addition to the components described above. Ti: 0.002% to 0.1% Ti fixes N as TiN in steel and has the effect of improving hot ductility and activating the hardenability enhancement by B. Ti also improves the softening resistance of HAZ through the precipitation of TiC and a resulting finer microstructure. To produce these effects, a Ti content of 0.002% or more is desirable. From the perspective of sufficiently fixing N, a Ti content of 0.008% or more is preferable, and even more preferably 0.010% or more. On the other hand, a Ti content above 0.1% results in a higher rolling load and lower ductility due to a greater amount of precipitation strengthening. Therefore, a Ti content of 0.1% or less is desirable, and preferably 0.05% or less. To achieve high ductility, a Ti content of 0.03% or less is even more preferable. B: 0.0002% to 0.01% Boron (B) is an element that improves the hardenability of steel and has the advantage of facilitating the formation of a predetermined area proportion of quenched martensite and / or bainite. B also improves hardenability in the vicinity of a weld by forming a hard microstructure near the weld and thus improving the softening resistance of HAZ. B also improves delayed fracture toughness. To produce such effects of B, the B content is preferably 0.0002% or more. More preferably, the B content is 0.0005% or more, and even more preferably 0.0010% or more. However, a B content above 0.01% results not only in the saturation of the effects, but also in very low hot ductility, leading to surface defects. Therefore, the content of B is preferably 0.01% or less, more preferably 0.0050% or less, even more preferably 0.0030% or less. Cu: 0.005% to 1% Copper (Cu) improves corrosion resistance in the automotive operating environment. A copper corrosion product is effective at coating the surface of a steel sheet and preventing hydrogen penetration. Copper is incorporated when scrap metal is used as a raw material. Allowing the incorporation of copper enables the use of recycled materials as raw materials and can reduce production costs. From this perspective, the copper content is preferably 0.005% or higher. Furthermore, from the perspective of improving delayed fracture resistance, a copper content of 0.05% or higher is more desirable, and preferably 0.10% or higher. However, an excessively high copper content results in surface defects. Therefore, a copper content of 1% or less is desirable, preferably 0.4% or less, and preferably 0.2% or less. Ni: 0.01% to 1% Like copper, nickel (Ni) is an element that can improve corrosion resistance. Ni can reduce the occurrence of surface defects, which tend to occur in the presence of copper. Therefore, a Ni content of 0.01% or more is desirable, preferably 0.04% or more, and more preferably 0.06% or more. However, an excessively high Ni content results in uneven scale formation in a heating furnace and causes surface defects. An excessively high Ni content also results in increased costs. Therefore, a Ni content of 1% or less is desirable, preferably 0.4% or less, and more preferably 0.2% or less. Cr: 0.01% to 1.0% Cr can be added to improve the hardenability of steel and to suppress carbide formation in upper / lower martensite or bainite. Cr also improves hardenability in the vicinity of a weld, forms a hard phase near the weld, and improves resistance to HAZ softening. To produce this effect, a Cr content of 0.01% or more is desirable, preferably 0.03% or more, and more preferably 0.06% or more. However, an excessively high Cr content results in low pitting corrosion resistance. Therefore, a Cr content of 1.0% or less is desirable, preferably 0.8% or less, and more preferably 0.4% or less. Mo: 0.01% to 0.5% Molybdenum (Mo) may be included to improve the hardenability of steel and to suppress carbide formation in upper / lower martensite or bainite. Molybdenum also improves hardenability in the vicinity of a weld by forming a hard phase near the weld and thus improving the softening resistance of HAZ. To produce this effect, the Mo content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.06% or more. However, Mo significantly impairs the chemical conversion treatability of a cold-rolled steel sheet. Therefore, the Mo content is preferably 0.5% or less. From the perspective of improving chemical conversion treatability, the Mo content is more preferably 0.15% or less, and even more preferably less than 0.10%. V: 0.003% to 0.5% Zinc (V) can be contained to improve the hardenability of steel, to suppress carbide formation in upper / lower martensite or bainite, to create a finer microstructure, and to precipitate carbide and improve resistance to delayed fracture. Boron (B) also improves hardenability in the vicinity of a weld by forming a hard phase near the weld and thus improving the softening resistance of HAZ. To produce these effects, a zinc content of 0.003% or more is desirable, preferably 0.005% or more, and more preferably 0.010% or more. However, a large amount of zinc results in significantly low castability. Therefore, a zinc content of 0.5% or less is desirable, preferably 0.3% or less, and more preferably 0.1% or less. Nb: 0.002% to 0.1% Nitrogen (Nb) can be added to create a finer steel microstructure and increase steel strength, promote bainite transformation through grain refinement, improve bending ability, and enhance delayed fracture resistance. Nb also improves hardenability in the vicinity of a weld by forming a hard phase near the weld, thus improving HAZ softening resistance. To achieve these effects, a Nb content of 0.002% or more is desirable, preferably 0.004% or more, and more preferably 0.010% or more. However, a high Nb content results in excessive precipitation strengthening and reduced ductility. A high Nb content also results in increased rolling load and impaired castability. Therefore, a Nb content of 0.1% or less is desirable, preferably 0.05% or less, and more preferably 0.010%.0.3% or less. Zr: 0.005% to 0.2% Zinc (Zr) can be retained to improve the hardenability of steel, suppress carbide formation in bainite, create a finer microstructure, and precipitate carbide to improve delayed fracture resistance. To achieve these effects, a Zr content of 0.005% or more is desirable, preferably 0.008% or more, and more preferably 0.010% or more. However, a high Zr content results in a greater amount of coarse precipitate, such as ZrN or ZrS, which remains unresolved during slab heating prior to hot rolling, leading to reduced delayed fracture resistance. Therefore, a Zr content of 0.2% or less is desirable, preferably 0.15% or less, and more preferably 0.08% or less. W: 0.005% to 0.2% Water (W) can be retained to improve the hardenability of steel, suppress carbide formation in bainite, create a finer microstructure, and precipitate carbide to improve delayed fracture resistance. To achieve these effects, a W content of 0.005% or more is desirable, preferably 0.008% or more, and more preferably 0.010% or more. However, a high W content results in a greater amount of coarse precipitate, such as WN or WS, which remains unresolved during slab heating prior to hot rolling, leading to reduced delayed fracture resistance. Therefore, a W content of 0.2% or less is desirable, preferably 0.15% or less, and more preferably 0.08% or less. Ca: 0.0002% to 0.0040% Calcium (Ca) binds sulfur (S) as CaS and contributes to improved flexural strength or delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. However, a high Ca content results in poor surface quality or flexural strength. Therefore, it is desirable for the Ca content to be 0.0040% or less, preferably 0.0035% or less, and more preferably 0.0020% or less. Ce: 0.0002% to 0.0040% Like calcium, phosphate (Ce) also binds sulfur (S) and contributes to improved flexural strength or delayed fracture resistance. Therefore, the Ce content is preferably 0.0002% or more, more preferably 0.0004% or more, and even more preferably 0.0006% or more. However, a high Ce content results in poor surface quality or flexural strength. Therefore, it is desirable for the Ce content to be 0.0040% or less, preferably 0.0035% or less, and more preferably 0.0020% or less. The: 0.0002% to 0.0040% Like calcium, lanolin also binds sulfur and contributes to improved flexural strength or delayed fracture resistance. Therefore, the lanolin content is preferably 0.0002% or more, more preferably 0.0004% or more, and even more preferably 0.0006% or more. However, a high lanolin content results in poor surface quality or flexural strength. Therefore, it is desirable for the lanolin content to be 0.0040% or less, preferably 0.0035% or less, and more preferably 0.0020% or less. Mg: 0.0002% to 0.0030% Magnesium (Mg) fixes oxygen (O) as MgO and contributes to improved delayed fracture resistance. Therefore, the Mg content is preferably 0.0002% or more, more preferably 0.0004% or more, and even more preferably 0.0006% or more. However, a high Mg content results in poor surface quality or flexural strength. Therefore, it is desirable for the Mg content to be 0.0030% or less, preferably 0.0025% or less, and more preferably 0.0010% or less. Sb: 0.002% to 0.1% Sb suppresses the oxidation or nitriding of a surface layer of a steel sheet and reduces the decrease in the carbon or boron content of the surface layer. A smaller decrease in carbon or boron content results in the suppressed formation of ferrite in a surface layer of a steel sheet, which increases strength and improves delayed fracture resistance. From this perspective, an Sb content of 0.002% or more is desirable, preferably 0.004% or more, and more preferably 0.006% or more. However, an Sb content of more than 0.1% results in impaired castability and impaired delayed fracture resistance of one face of the cut end due to segregation at a γ grain boundary. Therefore, it is desirable that the Sb content be 0.1% or less, preferably 0.04% or less, more preferably 0.03% or less. Sn: 0.002% to 0.1% Sn suppresses the oxidation or nitriding of a surface layer of a steel sheet and reduces the decrease in the carbon or boron content of the surface layer. A smaller decrease in carbon or boron content results in the suppressed formation of ferrite in a surface layer of a steel sheet, leading to increased strength and improved delayed fracture resistance. From this perspective, a Sn content of 0.002% or more is desirable, preferably 0.004% or more, and more preferably 0.006% or more. However, a Sn content of more than 0.1% results in impaired castability. This also results in impaired delayed fracture resistance of a cut end face due to Sn segregation at a γ grain boundary. Therefore, a Sn content of 0.1% or less is desirable, preferably 0.04% or less, and more preferably 0.03% or less. The remainder consists of iron and incidental impurities. When the aforementioned optional components are contained below their respective lower limits, the optional elements below their lower limits do not diminish the advantages of the present invention. Therefore, if present, the optional elements below their lower limits are considered incidental impurities. The steel microstructure of a steel sheet according to the present invention is described below. Ferrite: 6% to 90% To achieve high ductility, the ferrite area ratio is 6% or more. For the sake of high ductility, the ferrite area ratio is preferably 8% or more, and more preferably more than 10%. However, an excessive increase in ferrite results in lower strength and suppressed upper bainite formation. Therefore, the ferrite area ratio is 90% or less. For the sake of strength, the ferrite area ratio is preferably 85% or less, and more preferably 70% or less. In this document, ferrite refers to polygonal ferrite. Microstructure composed of one or two or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 10% to 94% To ensure predetermined strength and ductility, the total area ratio of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ in the remainder, excluding polygonal ferrite, ranges from 10% to 94%. The lower limit is preferably 15% or more, and more preferably 30% or more. The area ratio of each microstructure—upper bainite, fresh martensite, tempered martensite, and lower bainite—typically falls within the following range: Upper bainite ranges from 1% to 30%; fresh martensite ranges from 0% to 10%; tempered martensite ranges from 2% to 60%; and lower bainite ranges from 2% to 70%. / CCLLn / LZnZ / E / Yli And retained: 3% to 20% To achieve high ductility, the retained γ volume fraction is 3% or more of the entire steel microstructure, preferably 6% or more, and more preferably 8% or more. The retained γ content includes that of the retained γ formed adjacent to the upper bainite and the retained γ formed adjacent to the martensite or lower bainite. An excessively high retained γ content results in lower strength, lower drawability, and impaired delayed fracture resistance. Therefore, the retained γ volume fraction is 20% or less, preferably 15% or less. The “volume fraction” can be considered as the “area ratio.” The ratio (SuB / S2nd) x 100 (%) of the Sub area ratio of an upper bainite with a width in the range of 0.8 to 7pm, a length in the range of 2 to 15pm, and an aspect ratio of 2.2 or more in contact with the retained yub with a grain width in the range of 0.17 to 0.80pm and an aspect ratio in the range of 4 to 25 to the S2nd area ratio of the microstructure composed of one or two or more upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ ranges from 2.0% to 15%. In a production method described later, a predetermined amount of retained yub adjacent to the low-carbide-containing upper bainite (bainitic ferrite) can be formed by holding it at an intermediate temperature range of 500°C to 405°C in a cooling step. The retained yub grains have a grain width in the range of 0.17 to 0.80 pm and an aspect ratio in the range of 4 to 25. The retained yub adjacent to the coarse upper bainite allows carbon in the upper bainite to transfer to the retained yub and become locally concentrated. Consequently, the upper bainite contains little solute carbon and is therefore less susceptible to thermal effects. Furthermore, carbon transfer from the upper bainite to the adjacent retained yub can form retained γ, which is less susceptible to thermal effects.The upper bainite, which is effective in improving the softening resistance of HAZ, is in contact with the retained yub surface and has a width in the range of 0.8 to 4 pm, a length in the range of 2 to 15 pm, and an aspect ratio of 2.2 or greater. It is necessary to control the ratio between the Sub area ratio of the upper bainite and the total S2nd area ratio of the microstructure composed of one or more layers of fresh martensite, quenched martensite, lower bainite, and retained γ. In the present invention, the softening resistance of HAZ is improved when the (SuB / S2nd) ratio x 100 (%) is 2.0% or greater. Ductility is also improved. From the perspective of improving the softening resistance of HAZ, (SuB / S2nd) x 100 (%) is preferably 3.0% or greater. An excessively high proportion of upper bainite sub-area results in a significant reduction in the strength of the base material.This also results in lower ductility, stretch flanging capacity, and delayed fracture resistance. Therefore, the (Sub / S2nd) x 100 (%) is 15% or less, preferably 12% or less, more preferably 10% or less. The Sub and S2nd area ratios refer to the area ratios relative to the entire steel microstructure. Grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ have a distribution density Νθ of / CCLLn / LZnZ / E / Yli 7pm2 or less (including 0pm2). Grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm, in a microstructure composed of one or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ, are composed primarily of carbide. The region where the carbide is densely distributed is tempered martensite and / or lower bainite and tends to soften under thermal effects. Therefore, from the perspective of improving the softening resistance of HAZ, grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25pm ​​in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ have a distribution density Ne of 7 / pm2o less, preferably 6 / pm2o less, more preferably 4 / pm2o less.It is desirable that the lower limit of the grain distribution density be, but not limited to, 0 / pm2. Carbide forms significantly when the cooling rate in the temperature range of 320°C or less is too high or when the holding time at a low temperature of 250°C or less is too long. Grains with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ have a distribution density NF¡ne in the range of 0.03 to 0.4 / pm2 (preferred conditions). Grains with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30 pm in a microstructure composed of one or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ are primarily retained γ formed in the lower bainite (and its periphery). These grains have both the effect of improving ductility and the effect of improving the softening resistance of HAZ. To produce the effects, grains with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ preferably have a distribution density Npine of 0.03 / pm2o or more, more preferably 0.04 / pm2o or more, and even more preferably 0.05 / pm2o or more. An excessively large number of such grains results in a lower λ.Therefore, the grains preferably have a distribution density Nrine of 0.4 / pm2o less, more preferably 0.3 / pm2o less. Grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less have a total SYBiock area ratio of 5% or less (including 0%). This microstructure is composed primarily of fresh martensite. Conventional TRIP steel produced by austempering or bainitic quenching contains a large amount of these grains and has low HAZ softening resistance. In the quenching step after annealing, rapid cooling to 405°C or lower, followed by further cooling to 270°C or lower at a moderately slow cooling rate in a low-temperature region of 320°C or lower, can decrease the amount of massive microstructure. To achieve high HAZ softening resistance, / CCLLn / LZnZ / E / Yli grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less should have a total SYBiock area ratio of 5% or less, preferably 4% or less, and more preferably 3% or less. The following describes a method for measuring a steel microstructure. The morphology and area ratio of ferrite, retained yub, and upper bainite were measured by cutting a cross-section in the thickness direction parallel to the roll direction, mirror-polishing the cross-section, etching it with 3% nital by volume, and observing eight fields with a 1 / 4 position thickness using a scanning electron microscope (SEM) at 5000x magnification. The ferrite observed was equiaxed polygonal ferrite with little carbide in the interior and an aspect ratio of less than 2.2. This appears as the darkest region when viewed with the SEM. Retained yub appears as white grains when viewed with the SEM. The upper bainite contains little carbide in the interior, similar to ferrite, and is a region that appears black when viewed with the SEM. A region with an aspect ratio > 2.2 is classified as upper bainitic (bainitic ferrite) for calculating the Sub area ratio.As illustrated in Figure 2, the aspect ratio a / b was calculated from the major axis length a, which is the longest grain length, and the minor axis length b, which is the longest grain length in the direction perpendicular to the major axis. In the case of a plurality of grains in contact with each other, the grains are divided approximately uniformly along the dashed line shown in Figure 2 in a region where individual grains are in contact, and the size of each grain is measured. The S2nd area ratio of the microstructure composed of one or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained material was measured in the same way as for ferrite. This area ratio is the area ratio of the region distinct from ferrite. The area ratio included the carbide area ratio because the carbide area ratio was very small. The volume fraction of retained austenite was determined by X-ray diffractometry after chemical polishing at a position 1 / 4 thickness from the surface layer. The incident X-rays were from a Co-Kα radiation source. The area ratio of retained austenite was calculated from the intensity ratios of the ferrite (200), (211), and (220) planes and the austenite (200), (220), and (311) planes. Because retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffractometry is equal to the area ratio of austenite retained in the steel microstructure. Additionally, the grain distribution density (N) of grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure composed of one or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained grains was also determined from a SEM photograph. The area to be measured for the grain distribution density is within the aforementioned microstructure and does not include ferrite. Thus, the number of grains of the relevant size in the microstructure (and its periphery) is determined and divided by the area of ​​the microstructure to obtain the grain distribution density (N). The equivalent circular size (equivalent circular diameter) was determined by observing individual / CCLLn / LZnZ / E / Yli grains with a SEM, determining the area ratio and calculating the equivalent circular diameter. Furthermore, the distribution density (NF) of grains with an aspect ratio in the range of 3.6 to 40 and a grain width in the range of 0.14 to 0.30 pm in the microstructure composed of one or more layers of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained grains was also determined from a SEM photograph. The area to be measured for the distribution density is the aforementioned microstructure and does not include ferrite. Thus, the number of grains of the relevant size in the microstructure (and its periphery) is determined and divided by the area of ​​the microstructure to obtain the distribution density (NF). Additionally, the proportion of total SYBiock area of ​​grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less was also determined from an SEM photograph. Figure 1 is an example of a SEM photograph. In the case of a steel sheet used for observation in Figure 1, 0.18% C-1.5% Si-2.8% Mn steel was heated to 630°C at 20°C / s and then heated from 630°C to 800°C at a constant heating rate of 3°C / s. The steel was annealed at 800°C, then cooled to 450°C at a constant cooling rate of 20°C / s, then held at 450°C for 30 seconds, then cooled to 320°C at a constant cooling rate of 15°C / s, then cooled from 320°C to 270°C at 6°C / s, and then cooled from 270°C to 200°C at 5°C / s. After reaching 200°C, the steel was immediately heated to 400°C at 15°C / s, held at 400°C for 10 minutes, and then cooled to 100°C or below at 10°C / s. A vertical cross-section was ground to a thickness of 1 / 4 in the rolling direction, etched in 3% nital, and examined by SEM. Upper bainite, fresh martensite, tempered martensite, and retained lower bainite are individually evaluated in an SEM photograph. Upper bainite (a) is a microstructure containing little carbide, with almost no striated strain (latted interface) visible within it. It is ferrite-black and has a width in the range of 0.8 to 7 pm, a length in the range of 2 to 15 pm, and an aspect ratio of 2.2 or greater. Grains with a grain width in the range of 0.17 to 0.80 pm and an aspect ratio in the range of 4 to 25 are retained (b). Tempered martensite (c) is a region containing 2.0 to 20 per pm² of fine carbides with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure. Lower bainite (d) is a region containing 0 to 4 per pm2 of film-like grains with a grain width in the range of 0.14 to 0.30 pm and an aspect ratio in the range of 3.6 to 15 and contains 0 to 1.9 per pm² of fine carbide grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure. The tempered martensite and lower bainite, the striated strain (latted interface) seen within, are slightly grayer than the ferrite or upper bainite. Fresh martensite (e) includes massive white grains with an aspect ratio of approximately 3 or less and an equivalent circular diameter of 0.26 pm or more. (e) also includes some massive and flaky material. A black region with little carbide and an aspect ratio of 2.1 or less is identified as polygonal ferrite (f). / CCLLn / LZnZ / E / Yli A steel sheet according to the present invention preferably has a tensile strength of 590 MPa or more, more preferably 980 MPa or more, and even more preferably 1180 MPa or more. The upper limit of the tensile strength is preferably 1600 MPa or less, and more preferably 1450 MPa or less, from the perspective of ensuring other characteristics. A steel sheet according to the present invention may have a galvanized layer on its surface. The galvanized layer may be a galvano-annealed layer formed by an alloying treatment. The following describes a method for producing a steel sheet according to the present invention. Hot rolled A slab can be hot-rolled by heating and then rolling the slab, by directly rolling an unheated continuous-cast slab, or by briefly heating a continuous-cast slab and then rolling it. Hot rolling can be carried out in the usual way. For example, the slab heating temperature ranges from 1100°C to 1300°C, the immersion time varies from 20 to 300 minutes, the final rolling temperature ranges from the transformation temperature Ara to Ara + 200°C, and the winding temperature ranges from 400°C to 720°C. The winding temperature preferably ranges from 430°C to 530°C to reduce thickness variations and ensure consistently high strength. Cold rolled In cold rolling, the reduction in thickness can range from 30% to 85%. To ensure consistently high strength and reduce anisotropy, the reduction in thickness is preferably between 45% and 85%. For high rolling loads, the softening annealing treatment can be performed at 450°C to 730°C in a continuous annealing line (CAL) or a box annealing furnace (BAF). Continuous annealing line A steel slab with a predetermined chemical composition is hot-rolled and cold-rolled, and then annealed on a continuous annealing line under the conditions specified below. Although the annealing equipment is not particularly limited, a continuous annealing line (CAL) or a continuous galvanizing line (CGL) is preferred from the perspective of ensuring productivity, a desired heating rate, and a desired cooling rate. Heating rate in the temperature range of 660°C to 740°C: 1°C / s to 6°C / s. Slow heating at 1°C / s to 6°C / s in this temperature range improves the softening resistance of HAZ. An excessively high heating rate in this temperature range results in the formation of austenite in the pre-recrystallization state by reverse transformation, producing excessively fine austenite. In finely dispersed austenite, even when held at approximately 450°C, a subsequent cooling step results in insufficient coarse upper bainite (CCLLn / LZnZ / E / Yli) and does not produce the effect of improving the softening resistance of HAZ. To produce such an effect, the heating rate in the temperature range of 660°C to 740°C should be 6°C / s or less, preferably 5°C / s or less. An excessively low heating rate reduces productivity.Therefore, the heating rate is 1°C / s more, preferably 2°C / s more. Heating rate in a temperature range of 740°C to 770°C: 1°C / s to 6°C / s. Slow heating at 1°C / s to 6°C / s in this temperature range improves the softening resistance of HAZ. An excessively high heating rate in this temperature range results in an excessively large number of austenite nucleation sites, causing excessively fine austenite. In finely dispersed austenite, even when held at approximately 450°C, a subsequent cooling step forms insufficient coarse upper bainite and does not produce the effect of improving HAZ softening resistance. The heating rate is preferably 5°C / s or less. Annealing temperature: 770°C to 850°C To ensure a predetermined area ratio of quenched martensite and / or bainite and a predetermined volume fraction of retained martensite, the annealing temperature ranges from 770°C to 850°C. An annealing temperature below 770°C results in a decrease in the amount of upper bainite and reduced resistance to softening of the hardened steel. An annealing temperature above 850°C results in decreased ferrite formation and reduced ductility. The dew point temperature during annealing in an annealing temperature range of 770°C to 850°C is -45°C or higher (preferred conditions) The dew point temperature during annealing in the 770°C to 850°C annealing temperature range is adjusted to -45°C or higher to promote the formation of a decarburized layer on the surface and decrease the distribution density (Ne) of grains (primarily carbide) with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure composed of one or two or more layers of upper bainite, fresh martensite, quenched martensite, lower bainite, and retained martensite. This suppresses excessive quenching softening in the surface layer and improves the softening resistance of the HAZ. To achieve these effects, the dew point temperature during annealing in the 770°C to 850°C annealing temperature range is preferably -45°C or higher, more preferably -40°C or higher, and even more preferably -35°C or higher.Due to the possible degradation of the roll caused by pickup at a dew point temperature of more than 10°C, the dew point temperature is preferably 10°C or less. Average cooling rate in the temperature range of 770°C to 700°C: 1°C / s to 2000°C / s After annealing, cooling is carried out at an average cooling rate of 1°C / s to 2000°C / s within the temperature range of 770°C to 700°C. An average cooling rate of less than 1°C / s results in the formation of a large amount of ferrite and a decrease in the amount of upper bainite, causing lower strength, lower resistance to softening of the HAZ, and lower λ. Rates of 3°C / s or higher are preferred. On the other hand, an excessively high average cooling rate results in a poor sheet shape. Therefore, the average cooling rate is 2000°C / s or lower, preferably 100°C / s or lower, and more preferably less than 30°C / s. Average cooling rate in the temperature range of 700°C to 500°C: 8°C / s to 2000°C / s Cooling is performed at a rate of 8°C / s or less within the temperature range of 700°C to 500°C. An average cooling rate of less than 8°C / s results in the formation of a large amount of ferrite and a decrease in the upper bainite subunit, causing lower strength, lower resistance to HAZ softening, and a lower λ. A rate of 10.0°C / s or more is preferred. On the other hand, an excessively high average cooling rate results in a poor sheet shape. Therefore, the average cooling rate is 2000°C / s or less, preferably 100°C / s or less, and more preferably less than 30°C / s. Galvanizing treatment or galvano-annealing treatment (preferred conditions) The galvanizing or electro-annealing treatment can be performed by cooling at an average cooling rate in the range of 8°C / s to 2000°C / s within a temperature range of 700°C to 500°C and holding at that temperature in the range of 500°C to 405°C for 13 to 200 seconds, as described below. The galvanizing treatment is preferably carried out by immersing a steel sheet in a hot-dip galvanizing bath at 440°C to 500°C and then adjusting the coating weight by gas cleaning or similar means. When the zinc coating is further alloyed, alloying is preferably carried out in the temperature range of 460°C to 580°C for a holding time of 1 to 120 seconds. Cooling at an average cooling rate in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C may be followed by heating to 500°C or more, if necessary.The zinc coating is preferably formed in a galvanizing bath with an aluminum content of 0.08% to 0.25% by mass. The galvanized steel sheet may undergo a coating treatment, such as a resin, grease, or oil coating. The retention time in the temperature range of 500°C to 405°C is 13 to 200 seconds. Holding this temperature range for a predetermined time can form upper bainite with little carbide precipitation. Additionally, retained yub with a high concentration of carbon can form adjacent to it. Holding the temperature range can achieve (SuB / S2nd) x 100 (%) = 2.0% or more and improve the softening resistance of HAZ. From this perspective, the holding time in the temperature range of 500°C to 405°C is 13 seconds or more, preferably 15 seconds or more. However, a retention time of more than 200 seconds produces almost no further bainite formation, promotes the concentration of untransformed bulk carbon, and leads to an increase in the amount of residual bulk microstructure. This reduces the softening resistance of HAZ.Therefore, the holding time in the temperature range of 500°C to 405°C varies from 13 to 200 seconds. From the perspective of improving drawability, the holding time in the temperature range of 500°C to 405°C is preferably 100 seconds or less. Such holding time in this temperature range corresponds to decreasing the average cooling rate to 7.3°C / second or less within this temperature range. From the perspective of improving ductility, the holding temperature range is preferably 410°C or higher, more preferably 430°C or higher, and preferably 490°C or lower, more preferably 480°C or lower. Average cooling rate from 405°C to the cooling stop temperature Tsq in the range of 170°C to 270°C: 1°C / s to 50°C / s Moderately slow cooling is performed in the temperature range of 405°C to a cooling stop temperature (Tsq) in the range of 170°C to 270°C. This can concentrate carbon in and adjacent to the underlying material while simultaneously promoting the formation of martensite and lower bainite. It also reduces the formation of carbide with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm, and reduces the formation of massive fresh martensite with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less. This softens the martensite and lower bainite, thereby improving the softening resistance of HAZ. It also improves ductility. Based on these considerations, the average cooling rate in the temperature range varies from 1°C / s to 50°C / s.From the perspective of suppressing carbide formation, it is desirable that the average cooling rate in the temperature range be less than 15°C / s, preferably less than 10°C / s. Average cooling rate in the range of 320°C to 270°C when cooling from 405°C to the cooling stop temperature Tsq in the range of 170°C to 270°C: 0.3°C / s plus or minus 20°C / s (preferred conditions) Slow cooling in the range of 320°C to 270°C is preferable for distributing martensitic and / or bainite carbon below γ and thus suppressing carbide formation with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm, and having a grain distribution density Npine with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30 pm in the range of 0.03 to 0.4 pm². From such a perspective, it is desirable that the cooling rate in the temperature range be 0.3°C / s plus or minus 20°C / s, or more ideally 0.3°C / s plus or minus 10°C / s. Cooling stop temperature Tsq: 170°C to 270°C The cooling stop temperature Tsq should range from 170°C to 270°C for grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less to have a total SyBiock area ratio of 5% or less and to ensure the amount of γ retained. Average heating rate in the temperature range from cooling stop temperature Tsq to 350°C: 2°C / s more Additional heating for a short period within the temperature range of the quenching stop temperature Tsq up to 350°C can suppress carbide precipitation and achieve high ductility. Therefore, the average heating rate within the temperature range of the quenching stop temperature Tsq to 350°C is 2°C / s or more. Ideally, the average heating rate should be 5°C / s or more, preferably 10°C / s or more. The upper limit of the average heating rate is preferably, but not limited to, 50°C / s or less, and more preferably 30°C / s or less. Holding time in the temperature range of 170°C to 250°C in the step from cooling after annealing to heating at an average heating rate of 2°C / s or more: 50 seconds or less Holding the material in the temperature range of 250°C or lower retards carbon diffusion from the lower martensite and / or bainite and promotes carbide precipitation. This hardens these microstructures and reduces the softening resistance of HAZ. Therefore, the holding time in the temperature range of 170°C to 250°C during the step from cooling after annealing to heating at an average heating rate of 2°C / s, as described later, should be 50 seconds or less. From this perspective, the holding time is preferably 30 seconds or less. Holding time at 350°C to 500°C: 20 to 3000 seconds Retention in the temperature range of 350°C to 500°C for 20 to 3000 seconds is performed from the perspective of distributing retained carbon (C) to retained carbon (yub) formed by intermediate retention (retention in the temperature range of 500°C to 405°C for 13 to 200 seconds) and retained carbon (y) formed adjacent to lower martensite or bainite to stabilize them and improve the ductility and softening resistance of HAZ, and from the perspective of subdividing massively distributed regions as untransformed by bainite transformation and improving λ. The retention time in the temperature range is preferably 240 seconds or more from the perspective of further improving ductility, and more preferably 300 seconds or more from the perspective of improving the softening resistance of HAZ. The steel sheet is then cooled to room temperature and can undergo a production rolling process, or "skin pass," to stabilize its formability under pressing, such as by adjusting the surface roughness or flattening the sheet shape, and to increase the yield strength (YS). The elongation percentage during skin pass is preferably between 0.1% and 0.5%. The sheet shape can be flattened using a leveler. When galvanizing or electro-annealing is not performed between cooling at an average cooling rate in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C and holding in the temperature range of 500°C to 405°C for 13 to 200 seconds, galvanizing or electro-annealing may be performed after holding in the temperature range of 350°C to 500°C for 20 to 3000 seconds (preferred conditions). When performed, galvanizing is preferably carried out by immersing the steel sheet in a hot-dip galvanizing bath at 440°C to 500°C and then adjusting the coating weight by gas cleaning or similar means. When the zinc coating is further alloyed, the alloy is preferred in the temperature range of 460°C to 580° for a holding time of 1 to 120 seconds.From the perspective of preventing the decomposition of retained γ, 550°C or lower is preferred. The zinc coating is preferably formed in a galvanizing bath with an Al content of 0.08% to 0.25% by mass. The galvanized steel sheet may undergo a coating treatment, such as a resin, grease, or oil coating. From the perspective of improving drawability by flanging, after the heat treatment described above or skin pass rolling, a low-temperature heat treatment can be performed in the range of 100°C to 300°C for 30 seconds to 10 days. This treatment removes the hydrogen that has penetrated the steel sheet during quenching or annealing of the martensite formed by final cooling or skin pass rolling. The low-temperature heat treatment can reduce the hydrogen content to less than 0.1 ppm. Electroplating can also be performed. Electroplating is preferably followed by low-temperature heat treatment to further reduce the hydrogen content of the steel. Work examples may have TS x El > 17000 MPa-%, which is important as an index of the formability of a component with a complex shape that includes stretch forming and flange stretch forming, and can prevent a rupture originating from a weld beam. EXAMPLE 1 A cold-rolled steel sheet 1.4 mm thick with a chemical composition listed in Table 1 was treated under the annealing conditions listed in Table 2 to produce steel sheets in accordance with the present invention and comparative examples. Some of the steel sheets (cold-rolled steel sheets: CR) underwent hot-dip galvanizing after being held at a temperature range of 350°C to 500°C to form hot-dip galvanized steel sheets (Gl). More specifically, one steel sheet was immersed in a galvanizing bath at a temperature range of 440°C to 500°C for hot-dip galvanizing. Subsequently, the coating thickness was adjusted by gas cleaning or a similar method. The hot-dip galvanizing was carried out in a galvanizing bath with an aluminum content ranging from 0.10% to 0.22%. After hot-dip galvanizing, some of the hot-dip galvanized steel sheets underwent an alloying treatment to form electro-annealed steel sheets (GA). The alloying treatment was carried out at a temperature range of 460°C to 580°C.Part of the steel sheets (cold-rolled steel sheets: CR) were subjected to electroplating to form electrogalvanized steel sheets (EG). The microstructure of the steel was determined using the method described above. Table 3 shows the measurement results. / CCLLn / LZnZ / E / Yli JIS No. 5 tensile test samples were taken from the steel sheets and subjected to a tensile test (in accordance with JIS Z 2241). Table 3 shows TS and El. Two steel sheets, 150 mm thick perpendicular to the rolling direction and 125 mm thick in the rolling direction (end surfaces ground), were placed opposite and adjacent to each other in the rolling direction and laser-welded at the abutment. The gap between the abutting surfaces was 0 mm in condition A or 0.15 mm in condition B. In condition B, the fusion zone has a smaller cross-sectional area, and therefore the evaluation is more stringent. An Nd:YAG laser was used for the laser welding. The spot diameter at the focal point was 0.6 mm, the focal point was 4 mm above the steel sheet, the shielding gas was argon (Ar), the laser output was 4.2 kW, and the welding speed was 3.7 m / min. A JIS tensile test sample was taken.5. The welded member was positioned so that the weld line was perpendicular to the tensile axis and placed at the longitudinal center of the test specimen (in accordance with JIS Z 2241). A fracture with a break point separation from the weld line of 2.0 mm or more (separation of a uniform portion of the weld by more than 2.0 mm) was considered a base material fracture. A fracture with a break point separation from the weld line of less than 2.0 mm and a fracture with a crack along the weld line (a crack in a HAZ or fusion zone) were considered a weld fracture. Base metal fracture under the laser welding condition A(O) was considered to have high resistance to HAZ softening. In the working examples, TS x El > 17000 MPa-% is met, and the laser-welded portion may also have a fracture of the base material. Work samples numbers 1, 11, 12, 18, 26, 29, 32, 33 and 37 have (SuB / S2nd) x 100 (%) of 3.0 or more, Ne of 5 / pm2o less, Npine of 0.03 / pm2o more, and SYBiock of 5% or less, have fracture of the base material even under condition B, including a gap in the butt weld, have a ductility that meets TS x El > 19000 MPa-%, and are therefore particularly good. / CCLLn / LZnZ / E / Yli TABLE 1 No. 1 Steel. Chemical aosition {% by mass) Note c If Mn PS sdAJ N Other SksoLÁ A 0.198 1.50 2.65 0.094 0.0002 3.972 0.3029 Nb 0 096 110.015, BQ QC11 1.6 Example B Oil 0.240 0.54 2.90 0.007 0.0012 0.040 0.0036 HbO.012, Ti :0.035. 8:0.0012 6.6 Comparative Example Steel C 0 118 1.76 2.83 0.010 0.0007 0.272 0 003' - 2 0 Example Steel D 0.264 1.6? 2.45 0.005 0.0005 3 197 0.0056 - 1.9 Comparative Example Steel E 0.053 1.02 2.92 0.002 0.0019 0.010 0.0045 - 1.0 Comparative Exampleb Steel F 0.168 1.71 276 0.011 0.0018 0.137 0.0068 Τ®.01δ, ΒΌ.0Ο25 1.8 Sample Steel G 0.216 2.55 2.81 0.004 0.0016 0.411 0.0035 1ΊΌΌ24. B:0.00® 3.0 Comparative Example Steel H 0.132: 0.08 2.99 0.010 0.0018 0.193 0.0051 Ti:0.950, ΒΌ.0Ο13 0.3 Comparative Example Steel I 0.242 0.31 2.07 0.010 0.0011 0.401 0.3047 Cr0.02, Ms:0.05 0.7 Example Steel J 0.210 1.62 2.78 0 008 0.0018 0.367 0.0040 V:0.010. a:0.307. W:0.011 2.0 Example Steel K 0.134 1.85 3.25 0.008 0.0012 0.242 0.0066 OrO.OWt NLO.02, Cr0.02, Nb:0.094 2.1 Comparative Example Steel L 0.201 1.64 191 0.005 0.0014 0.371 0.0038 νΟΟΙΟ 2.0 Comparative Example Steel 0.088 3 48 3.14 0 008 0.0019 0.020 0.0042 CaO.Í»06. Ce.O.CWS, Lamí 2.5 Example Steel N 0.189 2.® 274 0.009 0.0005 □ 261 0.3060 Mg:0.0007, Sb:O..O1, SO.91 2.3 Example Steel 0 0.126 1.65 2.65 0.007 0.0035 1 100 0.3044 Sb;0.08 2.7 Example Oil Comparative. * The underlined values ​​are outside the scope of the present invention. / CCLLn / LZnZ / E / Yli TABLE 2 » ΰΤ ¿ i Φ y 1 the s r-> έ i § cí | IC I ¿3 TS. faith íaT faith Al $ your §. faith uT § £ 1 ic I rñ * faith UJ 'JM 1 § í | § £ 1 ¿J § * faith UJ yours £ c.· ·§. faith uT house? w << U? Yes iXT! I to to 1 UJ §. faith uT $ g M \ A' faith Ψ Γ ijJ' £ ¡j7 faith φ ÚT If Ψ Cu' '0 ij r* ií* y. tí ύϊ Cu' ¡..l. UJ y. you 111' s? ijJ' ifr & Φ ΛΓ fe Cu y. ¡i iY(' w ij J IjT fe CÍi' cu ¡JJ* ¡y 51 Cu” ω IJj' ¿i Cu IJ J u ws £ c<' c-> c¡jss « £ c<' Hear here 55 £ S uí ES X o re 55 s RRR w «' <-> cio 55 £ RRS<c c¿ C> § $ V % $ M « Cíe $ cp % M g <-· § g V$ RM % <- S¥ $ r-> c. R § g % % § g § ~í § RM '4 O-- ^1 l.-.l « A' - S ·£ ;·' £ C? & UJ U.' is.· UJ w \a I» UJ UJ UJ is. UJ UJ & c ,3⁄4 ·» .& >7 ' Ί £ CU iM Al 3 SO so £ S7 so MA! CJ is OI of <¿l iM A) R is Al RR vi CM is Al RR with R with Sd RR h ϕ Λ! a: ^.. $ í δ & CU f. í.ÍÑ 'Ñ rÑ s OJ oí tM £ ÍÑ OI gsg eo rM g ÍÑ R g íu 10 uó tu i-Λ CU LS •M s £ CJ S? R UJ 'Λ CU — cu LS •SI R te. Cú rM Γ Al W« oí o¡ A*» 0Φ os Ai A. Oí «< M 8 A^ £ A*» W oí A. « O)<r¡ A*» ΊΟ u> Λ5 w Oí X; A*» R cr, «> fe 17 CU CU - ÍJ 17 17 Sí 17 £ ÍJ 4 17 17 ΓΛ id cu 17 17 CU Sí Sí R id Si 17 i? Cu - R id id 17 id id K Mú l·' g Vj >ri LA o> $ fó ΙΛ Oí M N- Sñ 5^1 ΙΛ o; $ I5 ΙΛ O LA o> $ fó $ VJ R LA c> Ü5 $ «1 RU> IA rj R co $ R & IA ró & RORRS CJ ol SR c? RR CJ o ri RRRRRR c? RRRRRRR CJ RRRRRRR CJ R CJ R QJ uj ir. w 4? cj , to w 0? uj, r. ir: w '-rs ir. LO UJ UJ ir· UJ 05 ir. W. UJ UJ LO R UJ u: IO io UJ UJ io ce UJ !<: o 8 .. - «í R s £ IX «> •n $ & XRRX ss § § XRRXXR § SXR ?> XR § XX § / · 3 o H $ s V 3 S 'Λ .. 3 s VV $ í? o V $ so 3 RVOHR co OVR í? o RR 5' $ R ► · RR ir. >2 ccj s $ r $ R $ $? lf. R ss & & R ir. >2 r>j UJ R! R .>'> £í £ s 3 & íí' R $ K-; RR >3 UJ R so fe RR '•-S 'ir x -. ΐί| CU ... o, iN ·.. tJ OJ o:.O: ... OI O ... ;. is -1 ... C-. CU C'l ... cu eu ... CU w cu ... :. that A c. X IX g| - Go - - o. - - - *+ 'C. cr 1. re that ΛΙ L. c.. ce .. r.. that that L. that that 1. ω that that c.. that A t. ω Y s < < “ΐ s< < < < < < “X vt < < < < < -l vt < < i»l ω OI UJl CJI ttl idl -HX OI ss X? - f J CJ -.1 ir. cc ae: < r. s? £ S: £ ÍO oj R 55 05 WRRSRR ÍS R 5? R $ o-· $ s? V \f. * The underlined values ​​are beyond the scope of the present invention. * 1: Heating rate in the temperature range of 660°C to 740°C * 2: Heating rate in the temperature range of 740°C to 770°C * 3: Average cooling rate in the temperature range of 770°C to 700°C * 4: Average cooling rate in the temperature range of 700°C to 500°C * 5: Holding time in the temperature range of 500°C to 405°C * 6: Average cooling rate in the temperature range of 405°C to the cooling stop temperature Tsq * 7: Average cooling rate in the temperature range of 320°C to 270°C * 8: Holding time in the temperature range of 170°C to 250°C between cooling after annealing and heating * 9: Average heating rate in the temperature range of the cooling stop temperature Tsq at 350°C * 10: holding time in the temperature range of 350°C to 500°C * 11: CR: cold-rolled steel sheet (withoutcoating treatment), GA: galvanized steel sheet, Gl: hot-dip galvanized steel sheet (without zinc alloy coating treatment), EG: electrogalvanized steel sheet / CCLLn / LZnZ / E / Yli TABLE 3 and. A£8?3 fe CanK&HS&as 8fea Είφ. Afea (3⁄4) ugly Ares asá® ÚÚ Frixión vetee sfey «e&náfe (3⁄4} s^SoS Ν'λϊ yra) Á TSs Wa%} Frastsas under esrefeóft be ^áserA Ffaotsrs zXtóBÓft w&s1 S iafe OOS 1233 108 2^'34 OGA 8 & og 03 0.05 g 1288 12.6 15§'?7 NG NG C<:rí^ra^vc A 8 92 & OL 03 0.05 g ^270 12.5 ®ΰ? 4 A 10 80 v< 0.4 8.1 9.08 _ 1258 ·$} '7 15478 &G NG Cüi^faí ms A 7_ ϕ: 2“__ ¿ p: 01 2 fe 12&3 1573⁄4΄3⁄4 kc ϕ2&΀Yes” § A 15 85 § OL 0.2 O / 6 1170 153 17550 NG EjeoTjsío Q?p ^gs=Ss® 7 A quí 38 s 0.7 $ 008 S 1175 110 φ10 NG Ώ8 AΟ η η § Q7 82 014 1251 134 18763 NG NG Axes?^ § A 10 80 § OL 013 •y 1243 133 1g812_ OG 383 Axis^xs Íbrtwatíí» 2J 10 Ί 10 1212 14 S 17338 Q NG Ejsrnpb 11 A 15 Sí· 11 02 8.10 'J 1239 15.4 1» 0 O Ejsmjs® A 18 So 12 ¿ § 02 8:85 so1.fe 1235 Ej 'iii 0.2 009 3 1224 15.7 y O 883 14 A § 91 10 0; 0.2 003 1^13 17.7 21559 NG Ejsí^ks OGí^ra^yc: 15 A 10 §í: 12 0.0 8.24 _ 1214 1S2 1® KG NG A 38 8 V .<' O0S 1249 17738 G NO A $ 91 ¿.sí 1L 8.12 1253 kS M 15343 NG NG 15 A 10 90 13 2.§ 0.0 :Λ •v 12® 19.1 22520 O o Example »' .*.*> ·.·. J Λ & )V ií? bt**-k ι<ϋο i¿Xí ϊιί &&J VDn^f-^G as A 10 ^•: b -3⁄4 p: 180 Ufe Yes 1255 13.5 1g§43 NG NG §8^S CSW®^® 21 A 10 80 ¿_ ¿J 08 022 V 13.1 19518 NG Exercise ÜGP^gs=sΟΌ 22 A 9 91 ¿ 2.B í 3 -í 008 1^4 18421 NG NG Exercise Compare 22 A 10 w ¿L 2 03 8;^ 1238 í á: <¡ 17703 O 11G Example 24 A SO i T> $ $ os 123S 155 19158 O 883 g SV *“__ > ___ 2.4 808 < 1170 NG NO Eísoijsb &snpKa§¥c· 28 G 43 57 £ 02 8.03 10® 135 1SS® O u Eempss ¿.f D 11 2.3 1 ? θ -< 1821 35 13778 MG 883 5ea^o Qxnpaiíaí fes ?§ 31 § >5 13.8 2.0 8.01 $ 542 ¿2.7 12303 NG NG E$5?s^jg Ocn^afe 25 55 45 & !v 0.2 8:85 -í sl¿ 253 293® QG ^npls ÍÜ G •n 21 0.1 015 1175 W 21158 NG 883 Eiso^fe Gsv^fsí fes 31 H ?? 79 £_ 2.4 138 8.03 3 1281 124 1« KG iNG §8!^S .^¿ g 84 í.?4 011 J 1225 104 ¿¿54^ 0 Q Ejsrnpfc 34 KA 92 § '·r___ 20 8:^ Yes W 15.1 18444 0 NG Ejgissps Cssffiparaáw 35 = 32 *· 17.4 8.85 ::¿ 11:ts¿ -y -| 183S4 o NO Οαιηρβκ^® 351 57 33' §5 0.7 &41 23.9 17948 0 NO Eíe^fe 37 N 13 18 $ ·? 007 1&5 173 22381 of} E.srr5jsb ​​38 i=í 0. 1% 11 A, A; 3S 8.15 24 1301 í 15742 0 NG $3 §8 2.8 8.8 002 § 15W 95 14345 NO ^1^0 Stomparasphes 40 87 •JJ· 7.4 008 34 1509 6.S 18553 o NO Example Comparable® 41 84 40 »7 7 8 8;03 3 540 235 15S® G NG Ej&:;^s Csn'psaO® 42 S5 ·> 8.1 5.4 1.12 562 2? $1®24 O NG Osnwas*®. 1CC L Ln / Lznz / Ε / ΥΙΛΙ * The underlined values ​​are beyond the scope of the present invention. OR: fracture of the base material NG: fracture of the weld * 12: Microstructure composed of one or two or more upper bainite, fresh martensite, tempered martensite, lower yy retained bainite. EXAMPLE A cold-rolled steel sheet 1.4 mm thick with a chemical composition listed in Table 1 was treated under the annealing conditions listed in Table 4 to produce steel sheets in accordance with the present invention and comparative examples. The galvanizing or galvano-annealing treatment was performed by cooling at an average cooling rate of 8°C / s to 2000°C / s within a temperature range of 700°C to 500°C, followed by holding at a temperature range of 500°C to 405°C for 13 to 200 seconds. More specifically, a steel sheet was immersed in a galvanizing bath at a temperature range of 440°C to 500°C for hot-dip galvanizing. Subsequently, the coating thickness was adjusted by gas cleaning or a similar method. Hot-dip galvanizing was carried out in a galvanizing bath with an aluminum content ranging from 0.10% to 0.22%. After hot-dip galvanizing, some of the hot-dip galvanized steel sheets underwent an alloying treatment to form galvano-annealed (GA) steel sheets.The zinc coating alloy treatment was performed in the temperature range of 460°C to 580°C. The determination of the steel microstructure, the tensile test, and the evaluation of the HAZ softening resistance of the steel were performed in the same way as in Example 1. Table 5 shows the results. In the working examples, TS x El > 17000 MPa-% is met, and the laser-welded portion may also have a fracture of the base material. Example work No. 3 has (SuB / Sznd) x 100 (%) of 3.0 or more, Ne of 5 / pm2o less, Nππβ of 0.03 / pm2o more, and SYBiock of 5% or less, has fracture of the base material even in condition B, including a gap in the butt weld, has a ductility that meets TS x El > 19000 MPa-%, so it is especially good. / CCLLn / LZnZ / E / Yli TABLE 4 tea of ​​faith. Yes rq *4 w w&. W Mot <5 Wj 77 ^333 re; and; o 1 # £ i 7 46 5 7 ¿45 465 & Λ x 5 § ;.X jx 4íi· & Λ x •¿5 5 <· í 5 26 4ÍÍ §66 GA 1 A ¿ 2 395 42 § § 25 A$ 4 * Underlined values ​​are beyond the scope of the present invention. * 1: Heating rate in the temperature range of 660°C to 740°C * 2: Heating rate in the temperature range of 740°C to 770°C * 3: Average cooling rate in the temperature range of 770°C to 700°C * 4: Average cooling rate in the temperature range of 700°C to 500°C * 5: Holding time in the temperature range of 500°C to 405°C * 6: Average cooling rate in the temperature range of 405°C to the cooling stop temperature Tsq * 7: Average cooling rate in the temperature range of 320°C to 270°C * 8: Holding time in the temperature range of 170°C to 250°C between cooling after annealing and heating * 9: Average heating rate in the temperature range of the cooling stop temperature Tsq at 350°C * 10: holding time in the temperature range of 350°C to 500°C * 11: CR: cold rolled steel sheet(without coating treatment), GA: galvanized steel sheet, Gl: hot-dip galvanized steel sheet (without zinc alloy coating treatment), EG: electrogalvanized steel sheet TABLE 5 1CC L Ln / Lznz / E / YILI Na Asara Ares efe volite refea <ía í^j ^f^. ‘ ¡¿η?!: syssxj.» í%} (kf="J" es í» ffaátútci bajo <okáiaéfl de sótíedurs ;sser a {fe setífed-ára láser s 3 $ 31 ís 22 υ 5 8.s5 1222 í6.? 254374 n mió 2 6c 8 í. v ¿ í7.8 1s8íg,8 o ei&ttfpfe 4t 5¾ 12 34 υ2 0.08 i 1^0 <s8$i eistipto 4* 88 7 87 0 4 1232 fio s883í *•'0 ejetopfo ocsispsfsftv®* The underlined values ​​are outside the scope of the present invention. O: fracture of the base material, NG: weld fracture * 12: Microstructure composed of one or two or more of upper bainite, fresh martensite, quenched martensite, lower and retained bainite, and INDUSTRIAL APPLICABILITY The present invention provides high ductility and high resistance to softening of HAZ and is conveniently applicable to pressure forming used through a pressure forming process in automobiles, household appliances and the like.

Claims

1. A steel sheet comprising, as chemical composition, in mass percent, C: 0.06% to 0.25%, Si: 0.1% to 2.5%, Mn: 2.0% to 3.2%, P: 0.02% or less, S: 0.01% or less, Sol. Al: less than 1.0% (including 0%), and N: less than 0.015%, wherein a total content of Si and Sol. Al: Si + Sol. Al ranges from 0.7% to 2.5%, a remainder consisting of Fe and incidental impurities, a steel microstructure containing ferrite: 6% to 90% by area; a microstructure composed of one or two or more of upper bainite, fresh martensite, quenched martensite, lower bainite; and retained: 10% to 94% by area in total; and retained γ: 3% to 20% by volume, a ratio (Sue / S2nd) x 100 (%) of a Sub area ratio of a superior bainite with a width in the range of 0.8 to 7 pm, a length in the range of 2 to 15 pm, and an aspect ratio of 2.2 or more in contact with retained yub with a grain width in the range of 0.17 to 0.80 pm and an aspect ratio in the range of 4 to 25 to an S2nd area ratio of the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ ranging from 2.0% to 15%, grains with an aspect ratio of 3.5 or less and an equivalent circular diameter in the range of 0.02 to 0.25 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite and retained γ having a distribution density Ne of 7 / pm2 or less (including 0 / pm2), and grains with an equivalent circular diameter in the range of 1.3 to 20 pm and an aspect ratio of 3 or less have a total SyBiock area ratio of 5% or less (including 0%).

2. The steel sheet according to claim 1, wherein the grains with an aspect ratio in the range of 3.6 to 15 and a grain width in the range of 0.14 to 0.30 pm in the microstructure composed of one or two or more of upper bainite, fresh martensite, quenched martensite, lower bainite and retained γ have a distribution density Npine in the range of 0.03 to 0.4 / pm2.

3. The steel sheet according to claim 1 or claim 2, wherein the chemical composition further comprises, in mass percentage, one or two selected from Ti: 0.002% to 0.1% and B: 0.0002% to 0.01%.

4. The steel sheet according to any of claims 1 to 3, / CCLLn / LZnZ / E / Yli wherein the chemical composition further comprises, in mass percentage, one or two or more selected from Cu: 0.005% to 1%, Ni: 0.01% to 1%, Cr: 0.01% to 1.0%, Mo: 0.01% to 0.5%, V: 0.003% to 0.5%, Nb: 0.002% to 0.1%, Zr: 0.005% to 0.2% and W: 0.005% to 0.2%.

5. The steel sheet according to any one of claims 1 to 4, wherein the chemical composition further comprises, in mass percentage, one or two or more selected from Ca: 0.0002% to 0.0040%, Ce: 0.0002% to 0.0040%, La: 0.0002% to 0.0040%, Mg: 0.0002% to 0.0030%, Sb: 0.002% to 0.1% and Sn: 0.002% to 0.1%.

6. The steel sheet according to any of claims 1 to 5, wherein the steel sheet has a tensile strength in the range of 590 to 1600 MPa.

7. The steel sheet according to any of claims 1 to 6, comprising a galvanized layer on a surface of the steel sheet.

8. A method for producing a steel sheet comprising: hot rolling and cold rolling a steel slab having a chemical composition according to any one of claims 1 to 5, then in a continuous annealing line, heating the cold-rolled steel sheet at 1°C / s to 6°C / s in the temperature range of 660°C to 740°C, heating the cold-rolled steel sheet at 1°C / s to 6°C / s in the temperature range of 740°C to 770°C, annealing the cold-rolled steel sheet in an annealing temperature range of 770°C to 850°C, then cooling the cold-rolled steel sheet at an average cooling rate in the range of 1°C / s to 2000°C / s in the temperature range of 770°C to 700°C, further cooling the cold-rolled steel sheet at a rate of average cooling in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C,then hold the cold-rolled steel sheet in the temperature range of 500°C to 405°C for 13 to 200 seconds, then cool the cold-rolled steel sheet from 405°C to a cooling stop temperature Tsq in the range of 170°C to 270°C at an average cooling rate in the range of 1°C / s to 50°C / s, then heat the cold-rolled steel sheet in the temperature range of the cooling stop temperature Tsq to 350°C at an average heating rate of 2°C / s or more, hold the cold-rolled steel sheet at 350°C to 500°C for 20 to 3000 seconds, and then cool the cold-rolled steel sheet to room temperature, wherein a holding time in the temperature range of 170°C to 250°C between cooling after annealing and heating at an average heating rate of 2°C / s or more is 50 seconds or less.

9. The method for producing a steel sheet according to claim 8, wherein in cooling from 405°C to the cooling stop temperature Tsq in the range of 170°C to 270°C, a cooling rate in the range of 320°C to 270°C is 0.3°C / s plus or minus 20°C / s.

10. The method for producing a steel sheet according to claim 8 or claim 9, wherein the dew point temperature in annealing in an annealing temperature range of 770°C to 850°C is -45°C or more.

11. The method for producing a steel sheet according to any of claims 8 to 10, wherein the galvanizing treatment or the galvano-annealing treatment is carried out between cooling at an average cooling rate in the range of 8°C / s to 2000°C / s in the temperature range of 700°C to 500°C and holding in the temperature range of 500°C to 405°C for 13 to 200 seconds.

12. The method for producing a steel sheet according to any of claims 8 to 10, wherein holding at 350°C to 500°C for 20 to 3000 seconds is followed by a galvanizing treatment or galvano-annealing treatment.