STEEL SHEET AND METHOD FOR ITS PRODUCTION

Abstract

A steel sheet and a method for producing the steel sheet are provided. The present invention provides a steel sheet with a specific composition, wherein a steel microstructure contains ferrite: 5% or less by area percent, a microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained: 95% to 100% by area percent, and retained: 4% to 15% by volume percentage, ?UB retained with a grain width in the range of 0.25 to 0.60 µm, a grain length in the range of 1.0 to 15 µm, and an aspect ratio in the range of 3.1 to 25 has an area percentage S?UB in the range of 0.2% to 7.0%, ?LB retained with a grain width in the range of 0.08 to 0.24 µm, a grain length in the range of 0.6 to 15 µm, and an aspect ratio in the range of 4 to 40 has a distribution number N?LB in the range of 10 to 120 per 100 µm2, fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 µm and an aspect ratio of 3 or less and / or retained grains with an equivalent circular grain diameter of 0.4 to 1.0 µm and an aspect ratio of 3 or less have a total area percentage S?Fine in the range of 0.4% to 5.0%, and fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 µm and an aspect ratio of 3 or less and / or grains of Retained with an equivalent circular grain diameter in the range of 1.2 to 20 µm and an aspect ratio of 3 or less have a total area percentage S?Block in the range of 4% or less (including 0%).

Description

STEEL SHEET AND METHOD FOR PRODUCTION THEREOF FIELD OF THE INVENTION The present invention relates to a steel sheet and a method for producing the steel sheet, which can preferably be applied to press molding used through a press molding process in automobiles, household appliances, and the like. BACKGROUND OF THE INVENTION In recent years, with the increasing need for weight reduction in automotive bodies, the application of high-strength steel sheets (980 to 1180 MPa grade) to automotive frame and seat components has been progressing. However, when these high-strength steel sheets are applied to automotive components, cracking tends to occur during the pressing process due to a decrease in ductility or expansibility. Therefore, it is desirable for these high-strength steel sheets to have greater formability than conventional steel sheets. In this context, a TRIP steel containing and retained dispersed in the microstructure of steel sheets has been developed as a technique to improve the ductility of steel sheets. For example, Patent Document 1 discloses that ductile steel sheets with a tensile strength (TS) of 80 kgf / mm2 or more and 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 hold. Patent Document 2 discloses that steel sheets with high ductility El and expansibility 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 to 10°C / s greater, and holding the steel for 180 to 600 seconds such that retained austenite is controlled to 5% or more by percentage area, bainitic ferrite to 60% or more by percentage area, and polygonal ferrite to 20% or less by percentage area. Patent Document 3 discloses that a steel sheet can be provided with high ductility and expansibility by annealing a steel sheet of a specific composition, cooling the steel sheet to a temperature in the range of 150°C to 350°C, and then reheating and holding the steel sheet at approximately 400°C to form a microstructure containing ferrite, tempered martensite, and retained austenite. This utilizes the Quenching and Partitioning (Q&P) principle, which involves a cooling process of cooling once to a temperature range between a martensitic transformation start temperature (Ms point) and a martensitic transformation completion temperature (Mf point), and then reheating and holding to stabilize the retained γ.In recent years, this principle has been used to develop high-strength steels with high ductility and expansibility. Patent Document 4 discloses an improved method of the Q&P treatment. More specifically, it aims to achieve high ductility and expansibility by annealing steel of a specific composition at a temperature of Ae3-10°C or higher to reduce the polygonal ferrite to 5% or less, and then stopping the cooling at a relatively high temperature in the range of Ms-10°C to Ms-100°C to form superior bainite when reheated to approximately 400°C. Patent Document 5 discloses a method for using low-temperature formed bainite and high-temperature formed bainite to produce a steel sheet with high ductility and low-temperature toughness. More specifically, a steel sheet with high ductility and low-temperature toughness is produced by annealing steel containing 0.10% to 0.5% carbon, cooling the steel to a temperature in the range of 150°C to 400°C at a cooling rate of 10°C / s or greater, holding the steel in this temperature range for 10 to 200 seconds to form low-temperature bainite, reheating the steel to the upper temperature range of 400°C to 540°C or lower, and holding the steel for 50 seconds or more to form high-temperature bainite. List of Appointments Patent Documents PTL 1: Publication of Examined Japanese Patent Application No. 6-35619 PTL 2: Japanese Patent No. 4411221 PTL 3: Japanese Patent No. 5463685 PTL 4: Japanese Patent No. 3881559 PTL 5: Japanese Patent No. 3854506 BRIEF DESCRIPTION OF THE INVENTION Technical Problem However, the TRIP steel known as described in Patent Document 1 has very low expansibility, although it has high El. The technique described in Patent Document 2 primarily uses bainitic ferrite as its microstructure, and the amount of ferrite is small. This results in good expandability, but not necessarily high ductility. Therefore, its application to difficult-to-form components requires further improvement in ductility. The technique described in Patent Document 3 achieves relatively higher ductility and expansibility than known TRIP steel and steel containing bainitic ferrite. However, fracture was observed when forming difficult-to-form components, such as center pillars, and further improvement in ductility was required. Among the ductilities, local ductility (L.I.), which is a measure of ductility under local deformation at a flange end face or similar, tends to deteriorate in known TRIP steel and needs improvement. The technique described in Patent Document 4 reduces the amount of polygonal ferrite formed to decrease the amount of massive martensite, and therefore sufficient ductility cannot be ensured. The technique also establishes a relatively high cooling stop temperature to improve the εi and leaves a large amount of untransformed γ when cooling is stopped. In this way, massive martensite tends to remain. Although the technique described in Patent Document 5 uses both low-temperature transformed bainite and high-temperature transformed bainite to improve ductility, the low-temperature transformed bainite contributes little to the improvement, and the use of high-temperature transformed bainite tends to leave a bulky microstructure. Therefore, it is difficult to achieve both high ductility and high expansibility simultaneously. Thus, steel sheets with sufficiently high ductility and expansibility have not been produced in the related art. The present invention has been made to solve such problems and aims to provide a steel sheet with very high ductility and high expansibility even with a breaking stress at the level of 780 to 1450 MPa and to provide a method for the production of the steel sheet. Steel sheets, as used herein, include galvanized steel sheets that undergo surface galvanizing. Solution to the Problem The inventors of the present have thoroughly studied a means of providing very high ductility and high expansibility and have obtained the following conclusions. First, the inventors hereof examined the causes of (1) insufficient expansibility of an austenitized TRIP steel and (2) insufficient ductility of a steel using Q&P. The possible cause of (1) is described below. In the austenitized TRIP steel, carbon diffuses from bainite to untransformed austenite during austenitizing at approximately 400°C, and the bainite transformation is delayed when the amount of carbon in austenite approaches the composition To at which the free energy of a bcc phase and an fcc phase become equal. The delayed transformation leaves a massive microstructure composed of hard martensite in which carbon is concentrated only near the composition To and retained. The possible cause of (2) is described below.In the steel used by Q&P, although the cooling stop temperature can be lowered sufficiently to reduce the amount of massive microstructure, carbide precipitation or carbon stabilization in martensite prevents carbon from being supplied to an austenite phase and prevents the retained carbon from being sufficiently stabilized. Phenomenon (1) is also unavoidable when a large amount of upper bainite is generated in the final tempering stage of the Q&P process. More specifically, it is difficult in a previously proposed heat treatment method to achieve both the utilization of the stable retained material formed adjacent to the upper bainite transformation and the reduction in the amount of bulk microstructure. Thus, it is difficult in the related technique to deviate from certain ductility and expansibility ranges. Furthermore, the inventors of the present technique have discovered a novel heat treatment method that can impart characteristics beyond the characteristic range of previous techniques by achieving both the utilization of stable retained material formed adjacent to the upper bainite and a reduction in the amount of bulk microstructure. It is based on the following. (i) In a cooling process after annealing, high-temperature bainite forms by retaining the transformation tip of the upper bainite at approximately 450°C (405°C to 490°C) for 10 to 200 seconds with little carbide precipitation. Such intermediate retention in the high-temperature region forms plate-like retained bainite (rod-like in the cross-sectional microstructure), which contributes to improved ductility in the final microstructure, and bainite with less stress adjacent to the plate-like retained bainite. The bainite is essential for supplying carbon to the plate-like retained bainite. (ii) In a remaining untransformed γ region, secondary cooling is initiated before the carbon concentration reaches composition To, which can cause the formation of a massive microstructure, and rapid cooling is carried out to 310°C at a cooling rate of 10°C / s greater. (iii) Cooling is carried out continuously from 310°C to a cooling stop temperature of 220°C or higher and lower than 255°C to divide the remaining untransformed γ region by martensite transformation or lower bainite transformation to decrease the amount of bulk microstructure. (iv) In this cooling process, a second retention takes place after slow cooling of less than 20°C / s in the low temperature range of 310°C to 255°C, thereby forming lower bainite to extremely finely partition the remaining y and simultaneously cause carbon partitioning to form a film-like y (acicular in cross-section microstructure) that contributes to enhanced ductility, particularly enhanced local ductility. (v) The cooling stop temperature is set at 220°C or higher and lower than 255°C (220°C to 254°C) to disperse fine martensite or fine retained γ with an equivalent circular diameter in the range of 0.4 to 1.0 pm, which reduces a decrease in λ or local ductility and contributes to improved ductility. Furthermore, it reduces massive martensite or massive γ, which greatly affects local ductility. (vi) Subsequently, the two types of retained γ, i.e., the retained plate-type γεB adjacent to the upper bainite formed by retention while cooling and the retained film-type γεB remaining adjacent to the lower martensite or bainite formed during secondary cooling, are stabilized by reheating and retention at approximately 400°C to convert the martensite into tempered martensite and simultaneously by partitioning the carbon into the retained plate-type γεB and the retained film-type γεB. (vii) In the case of a composite forming that includes draw forming and drawn flange forming in one component, increasing the weld bead tension in pressing suppresses the entry of a steel sheet and tends to induce cracking in a draw-formed portion, and decreasing the weld bead tension increases the entry of a steel sheet and tends to induce cracking in a flange portion. To avoid any cracking, the form stability of a component that even requires strict form on a flange end face is significantly improved by satisfying TS x El >17000 MPa%, preferably TS x El > 18000 MPa%, more preferably TS x El > 19000 MPa%, where El is a measure of ductility, and ensuring λ> 50% or more, preferably λ > 55% or more, for TS: 780 to 1319 MPa, or λ > 40% or more, preferably λ > 45% or more, for TS: 1320 to 1450 MPa, where λ is a measure of expansibility. In this way, the utilization of stable retained γ and the reduction in the amount of bulk microstructure, which were previously difficult to achieve, can be accomplished simultaneously through a two-stage cooling process. This process utilizes the upper bainite transformation prior to the martensite transformation and controls the remaining amount of bulk microstructure through the Q&P process, and through slow cooling for carbon partitioning within an intermediate range of 310°C to 255°C. Consequently, a steel sheet with very high ductility and expansibility can be obtained. The present invention can also achieve reinforcement. The phrase “with very high ductility and expansibility,” as used herein, refers to TS x El > 17000 MPa%, λ > 50% or more for TS: 780 to 1319 MPa, and λ > 40% or more for TS: 1320 to 1450 MPa.The term “upper strength”, as used herein, refers to TS > 780 MPa. The present invention is based on such conclusions and provides more specifically the following. [1] A steel sheet having a composition containing, on a mass percent basis, C: 0.06% to 0.25%, Si: 0.6% to 2.5%, Mn: 2.3% to 3.5%, P: 0.02% or less, S: 0.01% or less, Al: less than 0.50%, N: less than 0.015%, and a remainder consisting of iron and incidental impurities, wherein a steel microstructure contains ferrite: 5% or less by area percent, a microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained: 95% to 100% by area percent, and retained: 4% to 15% by volume percent, with a grain width in the range of 0.25 to 0.60 pm. a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25 has a SyUB area percentage in the range of 0.2% to 7.0%, and LB retained with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40 has a distribution number NvLBθη in the range of 10 to 120 per 100 pm2, fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and / or retained grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a total area percentage SvF¡na θη in the range of 0.4% to 5.0%, and fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and / or retained grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less have a total area percentage SYBioque θη the interval of 4% or less (including 0%). [2] A steel sheet having a composition containing, on a mass percent basis, C: 0.06% to 0.25%, Si: 0.6% to 2.5%, Mn: 2.3% to 3.5%, P: 0.02% or less, S: 0.01% or less, Al: less than 0.50%, N: less than 0.015%, and a remainder consisting of iron and incidental impurities, wherein a steel microstructure contains ferrite: 5% or less by area percent, a microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, retained: 95% to 100% by area percent, retained: 4% to 15% by volume percent, retained with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.5 to 15 pm, and an aspect ratio in the range of 4 to 25 has a percentage of area SvUb θη in the range of 0.2% to 7.0%, and retained with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40 has a distribution number NvLB in the range of 10 to 120 per 100 pm2, fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a total area percentage SvFina in the range of 0.4% to 5.0%, and fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less have a percentage of total SvBioque area in the range of 4% or less (including 0%). [3] The steel sheet in accordance with [1] or [2], wherein a ratio of a percentage of area SUB of ferrite or upper bainite adjacent to the retained yUB with respect to the percentage of area SvUB satisfies SUB / SvUB> 3.5. [4] The steel sheet in accordance with any of [1] to [3], wherein a region having a C concentration in the range of 0.6% to 1.3% with an adjacent region having a C concentration of 0.07% or less in the microstructure has a total area percentage Sc concentration θη in the range of 0.1% to 5%. [5] The steel sheet in accordance with [4], wherein the region with a C concentration in the range of 0.6% to 1.3% with the adjacent region having a C concentration of 0.07% or less is and retained. [6] The steel sheet in accordance with [5], wherein the region with a C concentration in the range of 0.6% to 1.3% with the adjacent region having a C concentration of 0.07% or less are retained yub grains. [7] The steel sheet conforming to any of [3] to [6], wherein the adjacent region contains upper bainite. [8] The steel sheet conforming to any of [1] to [7], wherein the composition additionally contains, on a mass percent basis: one or two selected from Ti: 0.002% to 0.1% and B: 0.0002% to 0.01%. [9] The steel sheet conforming to any of [1] to [8], wherein the composition additionally contains, on a mass percent basis: 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%.

[10] The steel sheet conforming to any of [1] to [9], wherein the composition additionally contains, on a mass percent basis: 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%.

[11] The steel sheet in accordance with any of [1] to

[10] , wherein the steel sheet has a breaking stress in the range of 780 to 1450 MPa.

[12] The steel sheet conforming to any of [1] to

[11] , which includes a galvanized layer over a surface of the steel sheet.

[13] A method for producing a steel sheet, the method includes: hot rolling and cold rolling a block of steel having the composition described in any of [1], [2], and [8] to

[10] and annealing the cold-rolled steel sheet to an annealing temperature in the range of 810°C to 900°C; then cool the steel sheet at an average cooling rate of 1°C / s to 2000°C / s in the temperature range of 810°C to 700°C and cool the steel sheet at an average cooling rate of 10°C / s to 2000°C / s in the temperature range of 700°C to cyefrnn / Lznz / e / YiAi 490°C; hold the steel sheet in the temperature range of 490°C to 405°C for 10 to 200 seconds; cool the steel sheet at an average cooling rate of 10°C / s to 100°C / s in the temperature range of 405°C to 310°C; cool the steel sheet at an average cooling rate of 0.4°C / s greater and less than 20°C / s in the temperature range of 310°C to 255°C, cool the steel sheet at an average cooling rate of 2°C / s greater and less than 30°C / s in the temperature range of 255°C to a cooling stop temperature Tsq in the range of 254°C to 220°C; Heat the steel sheet at an average heating rate of 2°C / s greater in the temperature range of Tsq to 350°C, and hold the steel sheet at 350°C to 550°C for 20 to 3000 seconds; and cool the steel sheet to a temperature in the range of 350°C to 50°C or lower at an average cooling rate of 0.1°C / s greater. Advantageous Effects of the Invention The present invention can provide a steel sheet with high ductility and very high expansibility. The present invention can also achieve superior strength in the steel sheet. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is an example of a SEM image. Fig. 2 is an explanatory view of aspect ratio, grain width, and grain length. Fig. 3 is an example of a graph showing the relationship between C concentration and analysis length. DETAILED DESCRIPTION OF THE INVENTION 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 composition and a particular steel microstructure. Accordingly, a steel sheet according to the present invention is described below in order of composition and steel microstructure. A steel sheet according to the present invention contains the following components. The unit “%” of component content in the following description refers to “% by mass”. C: 0.06% to 0.25% Carbon (C) is included from the perspective of ensuring the percentage of tempered martensite area to guarantee a predetermined strength, from the perspective of ensuring the percentage of retained γ volume to improve ductility, and from the perspective of being concentrated in the retained γ to stabilize the retained γ and improve ductility. A C content of less than 0.06% results in insufficient strength and ductility of the steel sheet. The lower limit for C content is therefore 0.06%, preferably 0.09% or more, and more preferably 0.11% or more. A C content of more than 0.25% results in a delay in the transformation of upper bainite during intermediate retention during quenching and makes it difficult to form plate-like retained γεB adjacent to a predetermined amount of upper bainite transformation. This results in decreased ductility.This also results in an increase in massive retained martensite and a decrease in expansibility. This also results in a significant degradation of several characteristics of the steel sheet, such as spot weldability, bending capacity, and hole expansion capacity. Therefore, the upper limit for carbon content is 0.25%. A carbon content of 0.22% or less is desirable to improve ductility and spot weldability. Furthermore, a carbon content of 0.20% or less is desirable to further improve ductility and spot weldability. Yes: 0.6% to 2.5% Silicon (Si) is included to harden the ferrite, increasing its strength, and to reduce carbide formation in martensite or bainite, thereby improving the stability of retained gamma and enhancing ductility. The Si content is 0.6% or higher to reduce carbide formation and improve ductility. Preferably, the Si content is 0.8% or higher, and more preferably 1.1% or higher, to improve ductility. A Si content greater than 2.5% results in an extremely high rolling load and makes it difficult to produce thin sheets. This also affects the chemical conversion treatment capability and the toughness of a weld. Therefore, the Si content is 2.5% or less. Preferably, the Si content is less than 2.0% to ensure the chemical conversion treatment capability and toughness of both the material and the 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: 2.3% to 3.5% Manganese (Mn) is an important element from the perspective of ensuring a predetermined percentage of tempered martensite and / or bainite area to guarantee strength, from the perspective of lowering the Ms point of retained γ to stabilize the retained γ and improve ductility, from the perspective of reducing carbide formation in bainite to improve ductility in the same way as silicon (Si), and from the perspective of increasing the volume percentage of retained γ to improve ductility. To produce these effects, the Mn content is 2.3% or higher. In a method that uses bainite transformation as a final step among known heat treatment methods, an Mn content of 2.3% or higher results in a large amount of residual bulk microstructure containing hard martensite and retained γ, and results in decreased expansibility.In the present invention, however, due to a microstructure formed using a heat treatment method described below, a massive microstructure can be reduced even in the presence of a large amount of Mn, and the stabilizing effects of retained γ and the effects of increasing the volume percentage of Mn can be achieved. From the perspective of stabilizing retained γ to improve ductility, the Mn content is preferably 2.5% or more, preferably 2.6% or more, and more preferably 2.7% or more. An Mn content of more than 3.5% results in a considerable delay in the bainite transformation and makes it difficult to ensure high ductility. An Mn content of more than 3.5% also makes it difficult to avoid the formation of massive coarse martensite and results in a decrease in expansibility. Thus, the Mn content is 3.5% or less. The Mn content is preferably 3.2% or less from the perspective of promoting bainite transformation to ensure high ductility. The Mn content is more preferably 3.1% or less. P: 0.02% or less Although phosphorus (P) is a hardening element in steel, a high P content results in poor spot weldability. Therefore, the P content should be 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 preferable to 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 hole expansion 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 very high carbon (C), silicon (Si), and manganese (Mn) contents. From the perspective of improving spot weldability, the S content is preferably 0.0020% or less, more preferably less than 0.0010%. While S is not necessarily included, from the perspective of production costs, the S content is preferably 0.0001% or more, more preferably 0.0005% or more. In the sun: less than 0.50% Aluminum (Al) is included for deoxidation or to stabilize the material and is retained in place of silicon (Si). It is desirable, but not limited, that the lower limit of Al content be 0.01% or more for stable deoxidation. On the other hand, 0.50% or more of Al content results in very low material strength and negatively affects the ability to undergo chemical conversion treatment. Therefore, the Al content is kept below 0.50%. To achieve high strength, the Al content is preferably below 0.20%, 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 and affects both hot ductility and surface quality. In steel containing boron (B), N has the detrimental effect of negating the effects of B through the formation of BN. A N content of 0.015% or more results in very poor surface quality. Therefore, the N content is less than 0.015%, preferably 0.010% or less. Although N is not strictly required, from a production cost perspective, the N content is preferably 0.0001% or more. The 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% Titanium (Ti) fixes nitrogen (N) as TiN in steel and has the effect of improving hot ductility and improving boron hardenability. The precipitation of TiC is effective in refining the microstructure. To produce these effects, a Ti content of 0.002% or more is desirable. From the perspective of sufficient nitrogen fixation, the Ti content is preferably 0.008% or more, and more preferably 0.010% or more. On the other hand, a Ti content of more than 0.1% results in an increase in rolling load and a decrease in ductility due to an increase in the amount of precipitation hardening. Thus, a Ti content of 0.1% or less is desirable. The Ti content is more preferably 0.05% or less. To ensure high ductility, the Ti content is even more preferably 0.03% or less. 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 percentage of tempered martensite and / or bainite. Residual boron in solid solution improves delayed fracture resistance. To produce such effects, the B content is preferably 0.0002% or more. More preferably, 0.0005% or more, and even more preferably, 0.0010% or more. On the other hand, a B content of more than 0.01% results not only in saturation of the effects but also in very low hot ductility and surface defects. Therefore, the B content is preferably 0.01% or less, more preferably 0.0050% or less, and 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 effectively coats the surface of a steel sheet and suppresses hydrogen invasion. Copper is incorporated when scrap metal is used as a raw material. Incorporating copper allows recycled materials to be used as raw materials and can reduce production costs. From this perspective, the copper content is preferably 0.005% or higher. To improve delayed fracture resistance, a copper content of 0.05% or higher, preferably 0.10% or higher, is desirable. However, an excessively high copper content results in surface defects. Therefore, a copper content of 1% or less, preferably 0.4% or less, and even more preferably 0.2% or less, is desirable. 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 develop 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. An excessively high Ni content, however, results in uneven scale formation in a furnace and leads to surface defects. An excessively high Ni content also results in higher costs. Therefore, the Ni content is 1% or less, preferably 0.4% or less, and more preferably 0.2% or less. Cr: 0.01% to 1.0% c / ptrnn / Lznz / e / YiAi Chromium (Cr) may be included to improve the hardenability of steel and to reduce carbide formation in upper / lower martensite or bainite. To achieve these effects, 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 resistance to pitting corrosion. 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 reduce carbide formation in upper / lower martensite or bainite. To achieve these effects, 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 affects the chemical conversion treatment capability of a cold-rolled steel sheet. Therefore, the Mo content is preferably 0.5% or less. From the perspective of improving chemical conversion treatment capability, the Mo content is most preferably 0.15% or less. V: 0.003% to 0.5% Zinc (V) may be included to improve the hardenability of steel, to reduce carbide formation in upper / lower martensite or bainite, to refine the microstructure, and to precipitate carbide and improve delayed fracture resistance. To produce such effects, a zinc content of 0.003% or more is desirable, preferably 0.005% or more, and more preferably 0.010% or more. A high zinc content, however, results in a large decrease in 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) may be included to refine the steel microstructure and increase its strength, to promote bainite transformation through grain refinement, to improve bending capacity, and to enhance delayed fracture 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 hardening and decreased ductility. A high Nb content also results in increased rolling load and decreased castability. Therefore, a Nb content of 0.1% or less is desirable, preferably 0.05% or less, and more preferably 0.03% or less. Zr: 0.005% to 0.2% Zinc (Zr) may be included to improve the hardenability of steel, to reduce carbide formation in bainite, to refine the microstructure, and to precipitate carbide and improve delayed fracture resistance. To produce such effects, a Zr content of 0.005% or more is desirable, preferably 0.008% or more, and more preferably 0.010% or more. A high Zr content, however, results in a greater amount of coarse precipitates, such as ZrN or ZrS, which remain undissolved during block heating prior to hot rolling, resulting in decreased delayed fracture resistance. Thus, 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) may be included to improve the hardenability of steel, reduce carbide formation in bainite, refine the 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 precipitates, such as WN or WS, remaining undissolved during block heating prior to hot rolling, thus decreasing 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 enhanced resistance to delayed fracture. Therefore, the Ca content is preferably 0.0002% or more, more preferably 0.0005% or more, and more preferably 0.0010% or more. However, a high Ca content results in poor surface quality or reduced 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, carbon also binds sulfur and contributes to improved flexural strength or enhanced resistance to delayed fracture. Therefore, the carbon content is preferably 0.0002% or more, more preferably 0.0004% or more, and even more preferably 0.0006% or more. A high carbon content, however, results in poor surface quality or low flexural strength. Therefore, it is desirable for the carbon 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 enhanced resistance to delayed fracture. 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 low 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 resistance to delayed fracture. 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 low 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. cyefrnn / Lznz / e / YiAi 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 carbon or boron content in that surface layer. A smaller decrease in carbon or boron content results in suppressed ferrite formation in a surface layer of a steel sheet, increased strength, and improved resistance to delayed fracture. 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. An Sb content greater than 0.1%, however, results in decreased castability, segregation at a pre-γ grain boundary, and decreased resistance to delayed fracture of a cut end face. Therefore, an Sb content of 0.1% or less is desirable, preferably 0.04% or less, and more preferably 0.03% or less. Sn: 0.002% to 0.1% Tin (Sn) suppresses the oxidation or nitriding of a surface layer of a steel sheet and reduces the decrease in carbon (C) or boron (B) content in that layer. A smaller decrease in C or B content results in suppressed ferrite formation in a steel sheet surface layer, resulting in higher steel sheet strength and improved resistance to delayed fracture. From this perspective, a Sn content of 0.002% or more is desirable, preferably 0.004% or more, and even more preferably 0.006% or more. However, a Sn content greater than 0.1% results in decreased castability. This also leads to Sn segregation at a grain boundary and a decrease in the delayed fracture resistance of a cut end face. Thus, it is desirable that the Sn content be 0.1% or less, preferably 0.04% or less, more preferably 0.03% or less. When these optional components are contained below their respective lower limits, the optional elements below their lower limits do not reduce the advantages of the present invention. A steel sheet according to the present embodiment contains these components, and the remainder other than these components includes Fe (iron) and incidental impurities. The remainder preferably consists of Fe and incidental impurities. The steel microstructure of a steel sheet according to the present invention is described below. Ferrite: 5% or less To ensure a high λ, the ferrite is 5% or less by area percentage, preferably 4% or less, and more preferably 2% or less. Ferrite, as used herein, refers to polygonal ferrite. Microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100% To ensure predetermined strength, ductility, and expansibility, the total area percentage of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ in the remainder other than polygonal ferrite ranges from 95% to 100%. The lower limit is preferably 96% or higher, and more preferably 98% or higher. The area percentage of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ was observed in a SEM photograph. The content of each microstructure is often found in the following c / ptrnn / Lznz / e / YiAi interval. The area percentage of upper bainite ranges from 1% to 30%. The area percentage of fresh martensite ranges from 0% to 20%. The area percentage of tempered martensite ranges from 3% to 40%. The percentage of lower bainite area varies from 5% to 70%. And Retained: 4% to 15% To ensure high ductility, retained γ constitutes 4% or more by volume percent, preferably 5% or more, and more preferably 7% or more, of the entire steel microstructure. The retained γ content includes γ formed adjacent to upper bainite and γ formed adjacent to martensite or lower bainite. An excessive increase in retained γ content results in decreased strength, decreased expansibility, and decreased delayed fracture resistance. Therefore, the volume percent of retained γ is 15% or less, preferably 13% or less. The "volume percent" can be considered as "area percent." Yub Retained with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25 has a SyUB area percentage in the range of 0.2% to 7.0%. In a production method described later, retained plate-like γυB formed adjacent to low-carbide upper bainite (bainitic ferrite) can be produced by retention in the intermediate temperature range of 490°C to 405°C during a quenching process. Ductility is further enhanced by the formation of a minimal amount of retained Yub with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25. This effect is achieved by ensuring a retained γυB area percentage (SvUB) of 0.2% or more. Thus, the SyUB is 0.2% or more. It is desirable for the SyUB to be 0.3% or more because it greatly improves ductility. The SyUb is preferably 0.4% or more. To ensure greater ductility, the retained Yub preferably has a grain width in the range of 0.25 to 0.60 pm, and a grain length in the range of 1.5 to 15 pm, and an aspect ratio in the range of 4 to 25. It should be noted here that, even in a steel microstructure with the same grain width, grain length, and aspect ratio, a low level of C concentration results in fresh martensite and not only makes a very small contribution to improved ductility but also greatly affects expansibility. This microstructure is one of the microstructures designated as MA, and the microstructures conforming to the present specification are γ-stable with highly concentrated C, which is different from and should be distinguished from MA. Thus, as described below, the microstructures conforming to the present specification are only those identified as an fcc structure by EBSD. An excessively large amount of retained plate-like yub results in excessive carbon consumption and a large reduction in strength.This also results in decreased expansibility and decreased delayed fracture resistance. Therefore, the SvUb is 7.0% or less, preferably 5.0% or less, and more preferably 4.0% or less. The percentage area refers to the percentage of area relative to the entire steel microstructure. The percentage area of ​​retained Yub can be distinguished from other metallic phases (bcc) by acquiring phase map data using EBSD and measuring an fcc microstructure. The ratio of the SLb area percentage to the SyUb area percentage of ferrite or adjacent upper bainite retained satisfies SUB / SyUB> 3.5. The ductility-enhancing effect of the retained yUB can be improved by controlling the area ratio relative to the ferrite or upper bainite formed adjacent to the retained yUB. To ensure high ductility, a SUB / SyUB ratio of 3.5 or greater is desirable. From the perspective of improving ductility, a SUB / SyUB ratio of 4.0 or greater is preferable. Although there is no upper limit, for current thermal history, a ratio of 15 or less is preferred. yLBRetained with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40 has a NyLB distribution number in the range of 10 to 120 per 100 pm2. In a production method described later, film-type ylbretenide (sometimes also referred to as ylbretenide grains) formed adjacent to martensite and lower bainite can be produced by providing a second intermediate hold-down that slows the cooling rate in the temperature range of 310°C to 255°C in a quenching process. Film-type ylbretenide grains are grains with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40. The grains are composed mainly of γ-retained γ and partially contain carbide and / or martensite. Film-type ylbretenide grains were identified by their shape in a SEM photograph. From the perspective of improving ductility, the ylbretenide grain distribution number (NyLBs) is 10 or more per 100 pm².From the perspective of improving ductility, the NyLB is preferably 20 or more, and more preferably 30 or more, per 100 pm². An NyLB of more than 120 per 100 pm² results in excessive hardness and a decrease in ductility. Therefore, the NyLB is 120 or less per 100 pm². From the perspective of improving ductility, the NyLB is preferably 100 or less, and more preferably 80 or less, per 100 pm². As described above, those with a grain width of 0.25 pm or more are referred to as plate type. On the other hand, those with a grain width of 0.24 pm or less are referred to as film type. Fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a total SyFina area percentage in the range of 0.4% to 5.0%. Fresh fine martensite or retained γ grains (sometimes also referred to as retained γ) with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a small decreasing effect on λ or L and a large increasing effect on L. Thus, the total area percentage SvFina of fresh martensite and retained γ grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less is 0.4% or more. From the perspective of improving ductility, the SyFina is preferably 0.7% or more. An excessively increased SyFina is responsible for a decrease in λ. Thus, the area percentage is 5.0% or less. From the perspective of improving λ, the total area percentage is preferably 4.0% or less. Fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less have a total area percentage in the range of 4% or less. Excessive bainite transformation in a final tempering process has resulted in a large amount of massive martensite or retained γ. To avoid this, the Mn content has been reduced to 2% or less to promote bainite transformation. However, a low Mn content has reduced the stabilizing effect or the effect of increasing the volume percentage of retained γ and has negatively impacted ductility. On the other hand, both utilizing bainite transformation and reducing the amount of massive microstructure are possible in the present invention, where an appropriate quenching treatment is applied to a steel sheet containing a large amount of Mn. A massive microstructure that negatively affects expansibility is fresh martensite with a circular grain diameter equivalent in the range of 1.2 to 20 pm and an aspect ratio of 3 or less, and retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less. In this way, the total area percentage SvB|Oque can be reduced to 4% or less to ensure high expansibility and high local ductility. The SvB|Oque is preferably 3% or less to ensure high expansibility and high local ductility. The SvB|Oque can be 0%. In the case where either fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less, or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less, is contained alone, the area percentage of which it is contained is taken as the total area percentage. A region with a C concentration in the range of 0.6% to 1.3% with an adjacent region that has a C concentration of 0.07% or less has a total area percentage Sc concentration in the range of 0.1% to 5%. The percentage of area of ​​a region with a higher C concentration than the surrounding area can be controlled to improve ductility. More specifically, the percentage of total area (Sc) of the region with a C concentration in the range of 0.6% to 1.3%, with the adjacent region having a C concentration of 0.07% or less, can be adjusted to the range of 0.1% to 5% to improve ductility. The adjacent region refers to a region with a C concentration in the range of 0.6% to 1.3% and adjacent to a region with a C concentration of 0.07% or less. From the perspective of improving ductility, the region with a C concentration in the range of 0.6% to 1.3%, with the adjacent region having a C concentration of 0.07% or less, is preferably γ retained, more preferably retained Yub grains (sometimes also referred to as γυB retained). Part or all of the adjacent region preferably contains upper bainite. In the following description, the region with a C concentration in the range of 0.6% to 1.3%, with the adjacent region having a C concentration of 0.07% or less, is γυB retained, and the adjacent region is upper bainite. When the region is γυB retained, and the adjacent region is upper bainite, the Sc concentration is referred to as SyUB. In the retained γυB formed adjacent to the upper bainite, at least one side of the grains tends to have a very low carbon content. The carbon in bainite (bainitic ferrite) formed at high temperatures in the range of 405°C to 490°C is readily released from the austenite and efficiently concentrated in the retained plate-like yUb. Consequently, the carbon content of the retained plate-like yUb ranges from 0.6% to 1.3%, thus contributing to improved ductility. The carbon content of the surrounding upper bainite region is reduced to 0.07% or less. To further improve ductility, the percentage of SyUb* area of ​​a retained yUb region with such a carbon distribution state preferably ranges from 0.1% to 5%. A SyUB* of 0.2% or more results in a significant increase in ductility. Therefore, a SyUB* of 0.2% or more is even more desirable. The upper limit is preferably 4% or less, and more preferably 3% or less. A method for measuring a steel microstructure is described below. The ferrite area percentage was measured by cutting a cross-section in the thickness direction parallel to the rolling direction, mirror-polishing the cross-section, etching the cross-section with 3% nital, and observing 10 fields at a 1 / 4-thickness position with a SEM at 5000x magnification. The target ferrite was a relatively equiaxed polygonal ferrite with little carbide in its interior. This is the darkest region on the SEM. When it was difficult to distinguish whether the microstructure on either side of the retained plate-like yUB was upper bainite or ferrite, the area percentage was calculated by considering a polygonal ferrite region with an aspect ratio < 2.0 as ferrite and a region with an aspect ratio > 2.0 as upper bainite (bainitic ferrite). As illustrated in Fig.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 a 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 Fig. 2 in a region where the individual grains are in contact with each other, and the size of each grain is measured. The percentage area of ​​a microstructure composed of one, two, or more of the following was measured in the same way as for ferrite. This percentage area is the percentage area of ​​the region other than ferrite. The percentage area included the percentage area of ​​carbide, since the percentage area of ​​carbide was very small. The volume percentage of retained austenite (y) was determined by X-ray diffraction after chemical polishing at a depth of 1 / 4 of the thickness from the surface layer. The incident X-ray was from a cobalt-Ka radiation source. The area percentage of retained austenite was calculated from the intensity ratios of the ferrite (200), (211), and (220) planes to the austenite (200), (220), and (311) planes. Because the retained austenite is randomly distributed, the volume percentage of retained austenite determined by X-ray diffraction is equal to the area percentage of retained austenite in the steel microstructure. The shape and percentage of retained plate-like yub formed adjacent to the upper bainite were determined by electropolishing a cross-section in the thickness direction parallel to the rolling direction of the steel sheet at a 1 / 4-thickness position, acquiring phase map data using EBSD, and measuring an fcc microstructure. The measured region was 30 µm x 30 µm, and 10 fields separated by 50 µm or more were measured. Previous methods for measuring grain size and aspect ratio were used to determine grain length (major axis length), grain width (minor axis length), and aspect ratio. The percentage of grain area with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25 or with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.5 to 15 pm, and an aspect ratio in the range of 4 to 25 was determined as SyUb- The same fields were subjected to chemical etching with 3% nital, and the percentage of total SUB area of ​​ferrite or bainite adjacent to one or both sides of plate-type retained yUB was determined. The following was also determined from SEM photographs in the same way: the distribution number of retained yLB with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40; the percentages of fresh martensite area with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and retained y grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less; and the shapes (length, aspect ratio) and percentages of fresh martensite area with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and retained y grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less. 3 or less. The volume percentage of retained y refers to the volume percentage relative to the entire steel sheet. SYUB, ​​SYFina, and SYB|OqUe refer to the area percentages relative to all regions in the microstructure. NyLb refers to the number distribution density in a region composed of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained y (other than ferrite). The equivalent circular grain size (equivalent circular grain diameter) was determined by observing individual grains with a SEM, determining the percentage of area, and calculating the equivalent circular diameter. The carbon concentration (% by mass) of the region with a carbon concentration in the range of 0.6% to 1.3%, with the adjacent region having a carbon concentration of 0.07% or less, and the carbon concentration (% by mass) of the adjacent region were measured by line analysis in a cross-section in the thickness direction parallel to the rolling direction at a position 1 / 4 of the thickness using a JXA-8500F field emission electron probe microanalyzer (FE-EPMA) manufactured by JEOL Ltd. at an accelerating voltage of 6 kV, an irradiation current of 7 x 10⁸ A, and a minimum beam diameter. The analysis length was 6 pm. Carbon profile data were randomly collected at 20 positions separated by 10 pm or more to acquire average information on the microstructure.To eliminate the influence of contamination, the background was subtracted to match the average C value obtained in each line analysis with the carbon content of the base material. More specifically, when the average measured carbon content was greater than the carbon content of the base material, the increase was considered to be contamination, and the value obtained by subtracting the increase from the analytical value at each position was taken as the actual C content at that position. For the percentage of total area Sc concentration of the region with C in the range of 0.6% to 1.3% adjacent to the region with a C concentration of 0.07% or less, assuming that a region with the C content at the base of the C peak being 0.07% or less had a random distribution state, the ratio of the region with C in the range of 0.6% to 1.3% in the line analysis result was taken as the percentage of area. Fig.Figure 3 is an example of a graph of the relationship between the measured C concentration and the length of analysis. In Fig. 3, the region with a C concentration in the range of 0.6% to 1.3% with the adjacent region having a C concentration of 0.07% or less is a Sc concentration-i. A graph like the one shown in Fig. 3 is derived at 30 positions to obtain the percentage of total area Sc concentration-i. The shape of the microstructure (and retained plate-type, retained film-type) marked with in Fig. 3 is determined in an SEM photograph. The concentration level of the plate-retained Yub can be determined using the analytical method described above. Thus, when the concentration level of the Yub varies from 0.6% to 1.3% in the characteristic evaluation, a metallic phase with that concentration level can be identified as the plate-retained γυB. Fig. 1 is an example of a SEM photograph. A steel sheet used in the observation of Fig. 1 was produced by annealing 0.18%C-1.5%Si-2.8%Mn steel at 840°C, in which a single γ phase was formed, cooling the steel at 20°C / s in the range of 810°C to 700°C, cooling the steel at 20°C / s in the range of 700°C to 490°C, cooling the steel at 20°C / s in the range of 490°C to 450°C, isothermal holding of the steel at 450°C for 30 seconds, cooling the steel at 15°C / s in the range of 450°C to 310°C, cooling the steel from 310°C to 255°C at 5°C / s, cooling the steel from 255°C to 245°C to 6°C / s, heating the steel from 245°C to 350°C at 8°C / s, holding the steel at 400°C for 1080 seconds, and cooling the steel at 15°C / s in the range of 350°C to 50°C or lower. A vertical cross-section at a position of 1 / 4 of the thickness in the rolling direction was polished, etched with 3% nital, and observed with a SEM. Upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ are evaluated individually in an SEM photograph. Upper bainite (a) is a microstructure containing little carbide, with almost no internal streaking (latted interface), a ferrite-like black appearance, and a minor axis width of 0.4 pm or more. Plate-like retained γ (b) has a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25 adjacent to upper bainite or ferrite. Tempered martensite (c) is a region containing 2.0 to 20 per 1 pm² of fine carbide grains with an aspect ratio of 4 or less and an equivalent circular diameter in the range of 0.03 to 0.3 pm in the microstructure. Lower bainite (d) is a region containing 0.1 to 4 per 1 pm² of film-retained γ grains (e) with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 pm or more and 15 pm, and an aspect ratio in the range of 4 to 40, or 0.2 to 1.9 per 1 pm2 of fine carbide grains with an aspect ratio of 4 or less and an equivalent circular diameter in the range of 0.03 to 0.3 pm in the microstructure. Tempered martensite and lower bainite contain streaks (latted interface) and are slightly grayer than ferrite or upper bainite. Fresh martensite or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less remain in a region where the bainite transformation or the martensite transformation was insufficiently developed. Also remaining are fresh martensite or retained γ grains (1) with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less.A black region with little carbide and an aspect ratio of 2.0 or less is polygonal ferrite (g). A steel sheet according to the present invention preferably has a tensile strength of 780 MPa or more, more preferably 980 MPa or more. The upper limit of the tensile strength is preferably 1450 MPa or less, more preferably 1400 MPa or less, from the perspective of compatibility with other characteristics. The formability stability of a steel sheet in accordance with the present invention is significantly improved by ensuring a hole expansion ratio λ > 50% or more, preferably 55% or more, in the TS: grade of 780 to 1319 MPa, or λ > 40% or more, preferably 45% or more, in the TS: 1320 to 1450 MPa. A method for producing a steel sheet in accordance with the present invention is described below. Hot Rolling A steel block is hot-rolled by a method of heating followed by rolling the block, a method of directly rolling a continuously cast block without heating, or a method of heat-treating a continuously cast block for a short time followed by rolling the block. Hot rolling can be carried out in the usual way. For example, the block heating temperature varies from 1100°C to 1300°C, the holding temperature varies from 20 to 300 minutes, the finishing temperature varies from the Ar3 transformation point to the Ar3+ transformation point 200°C, and the coiling temperature varies from 400°C to 720°C. The coiling temperature preferably varies from 430°C to 530°C to reduce thickness variations and ensure consistently high strength. Cold Rolling In cold rolling, the reduction by rolling can vary from 30% to 85%. From the perspective of ensuring consistently high strength and reducing anisotropy, the reduction by rolling preferably ranges from 45% to 85%. When the rolling load is high, a softening annealing treatment can be carried out at 450°C to 730°C in a continuous annealing line (CAL) or a box annealing furnace (BAF). Annealing After hot rolling and cold rolling, a steel block with a predetermined composition is annealed under the conditions specified below. Although the annealing equipment is not particularly limited, a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) is preferred to ensure productivity, a desired heating rate, and a desired cooling rate. Annealing Temperature: 810°C to 900°C To ensure a predetermined area percentage of tempered martensite and / or bainite and a predetermined volume percentage of retained ferrite, the annealing temperature ranges from 810°C to 900°C. To reduce polygonal ferrite to 5% or less, the annealing temperature is adjusted to achieve single-phase annealing of ferrite. 815°C or higher is preferred, and 880°C or lower is preferred. Average Cooling Rate in Temperature Range from 810°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 in the temperature range of 810°C to 700°C. An average cooling rate lower than 1°C / s results in the formation of a large amount of ferrite and a decrease in strength and λ. A rate higher than 3°C / s is preferred. On the other hand, an excessively high average cooling rate results in a poor sheet shape. Therefore, the average cooling rate is lower than 2000°C / s, preferably lower than 100°C / s, and more preferably lower than 30°C / s. Average Cooling Rate in Temperature Range from 700°C to 490°C: 10°C / s to 2000°C / s Cooling is carried out at a rate of 10°C / s greater in the temperature range of 700°C to 490°C. An average cooling rate of less than 10°C / s results in the formation of a large amount of ferrite and a decrease in strength and λ. A rate of 15°C / s greater is preferred. On the other hand, an excessively high average cooling rate results in poor sheet shape. Therefore, the average cooling rate is 2000°C / s less, preferably 10065°C / s less, and more preferably less than 30°C / s. A rate of 29°C / s less is preferred because this results in a sheet shape with a good level (a sheet deformation of 15 mm or less, described later in an example). Furthermore, a lower average cooling rate of 14°C / s is preferred because this results in a sheet shape with a better level (a sheet deformation of 10 mm or less described later in an example). Holding Time in Temperature Range of 490°C to 405°C: 10 to 200 Seconds This temperature range can be held for a predetermined time to form upper bainite with minimal carbide precipitation and retained plate-like yUB with a high carbon concentration adjacent to the upper bainite. This temperature range can also be held to control the percentage area ratio of SUB / SvUB of these microstructures within a predetermined range. From these perspectives, the temperature range of 490°C to 405°C is held for 10 seconds or more. From the perspective of forming retained plate-like yUB and improving ductility, this temperature range is preferably held for 14 seconds or more. A retention time of more than 200 seconds, however, results in the slow formation of retained plate-like yUB, the progression of carbon concentration to untransformed massive γ, and an increase in the amount of residual massive microstructure.Thus, the holding time in the temperature range of 490°C to 405°C varies from 10 to 200 seconds. From the perspective of improving expansibility, the holding time in the temperature range of 490°C to 405°C is preferably 40 seconds or less. Holding in this temperature range corresponds to a decrease in the average cooling rate of 9°C / s lower in the temperature range. From the perspective of improving ductility, the holding temperature range is preferably 410°C or higher, more preferably 420°C or higher, and preferably 470°C or lower, more preferably 460°C or lower. Average Cooling Rate in Temperature Range of 405°C to 310°C: 10°C / s to 100°C / s After holding in the 405°C to 490°C range, rapid cooling to 310°C is necessary to avoid excessive carbon concentration at y. Holding above 310°C results in the concentration of massive untransformed γ carbon, suppresses bainite transformation in a subsequent quenching or tempering process, and increases the amount of retained massive oy martensite. This results in a decrease in λ. To improve λ, the average cooling rate in the 405°C to 310°C temperature range is 10°C / s greater, preferably 12°C / s greater, and more preferably 15°C / s greater. An excessively high cooling rate in this temperature range results in poor sheet shape.Thus, the cooling rate in this temperature range is 100°C / s lower, preferably less than 30°C / s, more preferably less than 20°C / s. Cooling Rate in Temperature Range from 310°C to 255°C: 0.4°C / s greater and less than 20°C / s A second retention step is carried out by slow cooling in the range of 310°C to 255°C. This allows the formation of lower martensite or bainite simultaneously with the concentration of adjacent carbon, thus forming film-type retained γLB adjacent to the lower martensite or bainite. This improves ductility. The retention at lower temperatures decreases the amount of bulk microstructure and forms fine retained γLB martensite with an equivalent circular grain size in the range of 0.4 to 1.0 pm. From the perspective of improving ductility, the average cooling rate in this temperature range is 0.4°C / s greater than and less than 20°C / s. From the perspective of increasing the amount of film-type retained γLB formed and improving ductility, it is desirable that the average cooling rate in this temperature range be less than 15°C / s, preferably less than 10°C / s, and particularly preferably less than 7°C / s. The cooling rate is reduced most effectively in the range of 310°C to 301°C, and it is particularly important to specify this temperature range as the cooling rate range, the preferred cooling rate range, or the most preferred cooling rate range. Average Cooling Rate in Temperature Range from 255°C to Cooling Stop Temperature Tsq in Range from 254°C to 220°C: 2°C / s greater and less than 30°C / s The bulk microstructure is further reduced by rapid cooling in the temperature range of 255°C to a cooling stop temperature Tsq in the range of 220°C to 254°C. In this temperature range, carbide precipitates and prevents the concentration of retained γ carbon, thus requiring rapid cooling. From the perspective of avoiding a decrease in ductility and reducing the amount of bulk microstructure to improve λ, the average cooling rate in this temperature range should be 2°C / s greater than and less than 30°C / s. From the perspective of reducing carbide formation, it is desirable that the average cooling rate in this temperature range be 3°C / s greater, preferably 5°C / s greater.An excessively high cooling rate in this temperature range results in insufficient carbon partitioning during bainite formation, a decrease in the amount of film-like yl, a decrease in ductility, and a decrease in λ due to a hardened second phase. Therefore, it is desirable for the cooling rate in this temperature range to be 15°C / s lower, preferably less than 10°C / s. Cooling Stop Temperature Tsq: 220°C to 254°C To disperse fine fresh martensite or fine retained γ with a circular grain diameter equivalent in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less, ensuring high ductility and an appropriate amount of retained γ, the quenching stop temperature (Tsq) should range from 220°C to 254°C. A quenching stop temperature below 220°C results in a decrease in the amount of fine martensite or fine retained γ and suppressed partitioning of carbon to retained γ due to carbide precipitation within the lower martensite or bainite, even during a short retention time. Therefore, the quenching stop temperature should be 220°C or higher, preferably 230°C or higher. A quenching stop temperature above 254°C results in the presence of residual bulk microstructure and the failure to stably achieve a high λ.In this way, the cooling stop temperature is 254°C or lower, preferably 250°C or lower. Average Heating Rate in Cooling Stop Temperature Range Tsq at 350°C: 2°C / s greater Heating for a short time within the temperature range of the quench stop temperature to 350°C can suppress carbide precipitation and ensure high ductility. When the martensite or lower bainite formed by quenching is reheated to 350°C or higher as a core, higher bainite forms. These effects cannot be achieved when the average heating rate to 350°C is low. This results in a decrease in the amount of retained γ and a decrease in ductility. Therefore, the average heating rate within the temperature range of the quench stop temperature to 350°C is 2°C / s greater. From the perspective of reducing carbide precipitation and forming higher bainite during reheating, it is desirable for the average heating rate to be 5°C / s greater, preferably 10°C / s greater.The upper limit of the average heating rate is preferably, but not limited to, 50°C / s lower, more preferably 30°C / s lower. Holding Time in Range of 350°C to 550°C: 20 to 3000 Seconds From the perspective of partitioning C to retained film-type Yub formed adjacent to retained plate-type Yub, martensite, or lower bainite formed by intermediate retention to stabilize them, and from the perspective of making a bulk-distributed region as untransformed γ finer by bainite transformation and improving Λ, the temperature range of 350°C to 550°C is retained for 20 to 3000 seconds. For a retention time in the range of 60 to 3000 seconds in the temperature range of 350°C to 550°C, the region with a concentration of C in the range of 0.6% to 1.3% with the adjacent region having a concentration of C of 0.07% or less has a total area percentage SCconcentration θη in the range of 0.1% to 5%, and the ductility is further improved. From the perspective of utilizing the effect of refining untransformed γ through bainite transformation and improving λ, it is desirable that the temperature range of 350°C to 550°C be retained for 180 seconds or more. Due to this retention, the region with a C concentration in the range of 0.6% to 1.3%, with the adjacent region having a C concentration of 0.07% or less, has a total area percentage Sc-concentration in the range of 0.2% to 5%, and the ductility is further improved. 1500 seconds or less is preferred, and 1200 seconds or less is even more desirable. The temperature range of 350°C to 550°C can also be used for hot-dip galvanizing. In hot-dip galvanizing, a steel sheet is preferably immersed in a galvanizing bath at a temperature range of 440°C to 500°C, and then the coating thickness is adjusted by gas purging or similar means. A galvanizing bath with an aluminum content of 0.10% to 0.22% is preferably used for hot-dip galvanizing. Hot-dip galvanizing can be followed by zinc alloy coating. Zinc alloy coating is preferably carried out at a temperature range of 470°C to 550°C. After that, cooling can be carried out to a temperature in the range of 350°C to 50°C or lower at an average cooling rate of 0.1°C / s greater, and the steel sheet can be tempered to stabilize formability by pressing, such as adjusting surface roughness or flattening the sheet shape, or to increase YS. The tempering elongation percentage preferably ranges from 0.1% to 0.5%. The sheet shape can also be flattened with a leveler. A rate of 5°C / s greater is preferred, and a rate of 100°C / s less is preferred. From the perspective of improving expansibility, after heat treatment or tempering, a low-temperature heat treatment can be carried out in the temperature range of 100°C to 300°C for 30 seconds to 10 days. This treatment causes the tempering of the martensite formed by the final cooling or tempering, or it causes the hydrogen introduced into the steel sheet during annealing to be removed. Low-temperature heat treatment can reduce the hydrogen content to less than 0.1 ppm. Electrodeposition can also be carried out. Electrodeposition is preferably followed by low-temperature heat treatment from the perspective of further reducing the hydrogen content of the steel. Examples of the present invention can provide steel sheets with very high formability that satisfy TS x El > 17000 MPa%, preferably TS x El > 18000 MPa%, more preferably TS x El > 19000 MPa% and λ > 50% or more, preferably λ > 55% or more, for TS: 780 to 1319 MPa, or λ > 40% or more, preferably λ > 45% or more, for TS: 1320 to 1450 MPa. The steel according to the present invention has a high local ductility (L. El) of 6.0% or more in a grade of 780 to 1179 MPa or 5.0% or more in a grade of 1180 to 1450 MPa. EXAMPLE 1 A cold-rolled steel sheet 1.2 mm thick with a composition indicated in Table 1 was processed under the annealing conditions indicated in Table 2-1 to produce steel sheets in accordance with the present invention and comparative examples. Some of the steel sheets (cold-rolled steel sheets) were also subjected to hot-dip galvanizing to produce hot-dip galvanized steel sheets (Gl). 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 purging or a similar method. A galvanizing bath with an aluminum content ranging from 0.10% to 0.22% was used for hot-dip galvanizing. After hot-dip galvanizing, some of the hot-dip galvanized steel sheets underwent zinc alloy coating treatment to produce galvanically annealed steel sheets (GA).The zinc coating alloy treatment was carried out in the temperature range of 470°C to 550°C. Part of the steel sheets (cold-rolled steel sheets) were subjected to electrodeposition to produce electrogalvanized (EG) steel sheets. The steel microstructure was measured using the method described above. Table 22 shows the measurement results. The percentage of retained plate-like γυB area formed adjacent to the upper bainite was determined as the percentage of SyUB area of ​​γ grains with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.5 to 15 pm, and an aspect ratio in the range of 4 to 25. JIS No. 5 tensile test specimens were taken from the steel sheets and subjected to a tensile test (in accordance with JIS Z 2241). Table 2-2 shows TS, El, and L. El. The expansibility was evaluated in a hole expansion test in accordance with the specifications of the Japan Iron and Steel Federation standard JFST 1001. More specifically, a 100 mm × 100 mm square sample was drilled with a punching tool having a punch diameter of 10 mm and a die diameter of 10.3 mm (13% clearance). The hole was then enlarged with a tapered punch with a 60-degree apex angle such that a burr formed around the drilled hole remained outside until a crack occurred that penetrated the sample. Hole expansion ratio λ (%) = {(d - d0) / d0} * 100, where d0 indicates the initial hole diameter (mm) and d indicates the hole diameter (mm) when a crack occurred. Example Nos. 1, 7, 8, 9, 10, 14, 15, 16, 20, 21, 24, 27, 29, 30, 32, and 33 satisfy TS χ El > 17000 MPa% and have a hole expansion capacity (λ) of 50% or more at the 1180 MPa grade. In contrast, the comparative examples were inferior in at least one of the characteristics. Examples with an average cooling rate in the range of 15°C / s to 29°C / s within the temperature range of 700°C to 490°C exhibited good sheet strain in the range of 11 to 15 mm, as measured by the following method. Examples with an average cooling rate in the range of 5°C / s to 14°C / s exhibited better sheet strain in the range of 10 mm or less, as measured by the following method. Sheet strain for sheet shape evaluation was assessed by taking a 1500 mm long cut sample from an annealed steel sheet, placing the sample on a flat horizontal table, and measuring the maximum value (unit: mm) of the strain heights on all four sides. When the sample was cut in the longitudinal direction, the clearance of a mechanical shear blade was 4% (the upper limit of the control range was 10%). cyefrnn / Lznz / e / YiAi A ω W Μ O 01 O Ul (n Tabla 1 > B h C h c c 4 C * t Steel No. Composition (% by mass) Note C Si Mn PS Sunlight. N other A 0.188 1.50 2.65 0.004 0.0002 0.060 0.0029 Ti:0.015, B:0.0012, Nb:0.005 Example sidewalk B 0.230 1.52 2.04 0.007 0.0012 0.040 0.0036 Ti:0.018, B:0.0012, Nb:0.020 Comparative example steel C 0.115 1.17 3.04 0.006 0.0013 0.018 0.0040 • Example sidewalk D 0.262 1.04 2.39 0.004 0.0002 0.019 0.0063 Comparative example steel E 0.126 1.33 2.82 0.007 0.0003 0.018 0.0095 T:0.025, B:0.0019 Example sidewalk F 0.132 0.51 2.00 0.003 0.0011 0.005 0.0069 Ti:0.103,B: 0.0014 Comparative example steel G 0.219 1.30 2.83 0.006 0.0020 0.018 0.0076 Ti:0.015,B:0.0018,Cu:0.22,Ni:0.05,Cr:0.05,Mo:0.04 Example steel H 0.181 1.20 2.53 0.007 0.0004 0.004 0.0036 V:0.009, Zr:0.009, W:0.008 Example steel I 0.049 1.01 2.32 0.006 0.0013 0.005 0.0014 B:0.0028, Nb:0.013, V:0.021 Comparative example steel J 0.187 0.85 2.70 0.006 0.0011 0.006 0.0070 T¡:0.009, B:0.0045, Ca:0.0007, Ce:0.0005, La:0.001 Example steel K 0.194 1.92 2.56 0.008 0.0011 0.009 0.0034 B:0.0028,Mg:0.001,Sb:0.01,Sn:0.01 Example sidewalk L 0.206 2.70 3.57 0.003 0.0012 0.003 0.0034 Ti:0.008,B:0.0028,V:0.013,Mg:0.001 Comparative example steel. Β h C hcc Table 2-1 No. Steel No. Annealing Conditions Note Annealing Temperature (ΐ) Holding Time (s) CR1 '1 (°C / s) CR2 *2 (°C / s) Holding Time '3 (s) CR3 *4 m CR4 '5 (°C / s) CR5 '6 (°S / s) Cooling Stop Temperature (°C) Holding Time at Cooling Stop Temperature (s) Heating Rate Holding Temperature (ΐ) Holding Time '8 (s) CR6 '9 (s) Coating *10 1 A 840 180 3 30 20 30 8 10 250 2 15 400 800 20 Example 2 A 805 180 8 30 20 30 8 10 250 2 15 400 800 20 Comparative Example 3 A 850 180 0.5 30 20 30 8 10 250 2 15 400 800 20 Comparative Example 4 A 850 180 3 2 20 30 8 10 250 2 15 400 800 20 Comparative Example 5 A 850 180 8 30 2 30 8 10 250 2 15 400 800 20 Comparative Example θ A 850 180 3 30 7 30 8 10 250 2 15 400 800 20 Comparative Example 1 A 850 180 3 30 10 30 8 10 250 2 15 400 800 20 Example 8 A 850 180 3 30 14 30 8 10 250 2 15 400 800 20 Example 5 A 850 180 3 30 30 30 8 10 250 2 15 400 800 10 Example 10 A 850180 8 30 60 30 8 7 250 2 15 365 1200 1 Example 11 A 850 180 3 30 500 30 8 10 250 2 15 400 800 20 Comparative Example 12 A 850 180 7 30 20 2 8 10 250 2 15 400 800 20 Comparative Example 13 A 850 180 8 30 20 30 20 1 250 30 15 400 800 20 Comparative Example 14 A 850 180 3 30 20 30 '4 10 250 2 15 400 800 20 Example 15 A 850 180 3 30 35 30 5 4 250 10 15 400 800 20 Example 16 A 850 180 3 20 35 30 0.5 2 250 20 3 400 800 20 Example 17 A 850 180 7 30 30 30 8 7 250 10 1 400 800 20 Comparative Example 18 A 850 180 3 30 20 30 8 7 250 2 15 330 180 20 Comparative Example 19 A 850 180 8 30 20 30 8 7 250 2 15 400 15 20 Comparative Example 20 A 850 180 8 30 20 30 8 6 250 2 15 400 60 20 Example 21 A 850 180 3 30 20 30 8 6 250 2 15 400 180 20 Example 22 B 870 180 30 30 20 30 8 8 250 2 15 400 800 20 Comparative Example 23 B 870 180 50 50 20 30 5 8 250 2 15 400 800 20 Comparative Example 24 0 850 180 30 30 35 16 8 10 230 2 5 400 800 50 Example 25 C 910 180 3 30 35 30 8 10 230 2 15 400 800 20 ExampleComparative Example 26 D 850 180 8 30 34 30 8 9 222 2 15 400 800 20 Comparative Example 27 E 850 180 130 30 34 30 8 10 230 2 10 400 800 100 Example 28 F 850 ​​180 3 30 35 29 7 10 250 2 15 400 800 20 Comparative Example 29 G 850 180 8 100 35 30 8 10 225 2 15 530 800 20 GA Example 30 H 850 180 3 30 180 30 8 10 235 2 15 400 800 20 Gl Example 31 I 850 180 3 32 35 30 8 8 250 2 15 400 800 20 Comparative Example 32 J 850 180 3 30 34 30 8 14 235 2 30 400 2800 20 EG Example 33 K 850 180 8 30 35 30 8 27 235 2 50 400 800 20 Example 34 L 850 180 7 30 35 29 7 10 230 2 15 400 800 20 Comparative Example *1: Average cooling rate in the temperature range of 810-700°C *2: Average cooling rate in the temperature range of 700-490°C *3: Holding time in the temperature range of 490-405°C *4: Average cooling rate in the temperature range of 405-310°C *5: Average cooling rate in the temperature range of 310-255°C *6: Average cooling rate at the temperature level of 255°C to the cooling stop temperature Tsq°C, Tsq: 220°C-254°C 7: Average heating rate at the cooling stop temperature level of 350°C *8: Holding time at the temperature level of 350-550°C *9: Average cooling rate to a temperature of 350-50°C or lower Ί 0: GA: non-annealed ga Iva steel sheet, Gl: hot-dip galvanized steel sheet (without Zn coating alloy treatment), EG: electrogalvanized steel sheet at ω ω μ ro Ο ϋι Ο υι Ο ϋι (Λ Table 2-2 No. Steel No. Microstructure Characteristics Note Percentage of ferrite area (%) Percentage of remaining area (%) Percentage of retained volume (%) SyjB (%) Concentration (%) NYlb Pprr?) SyFina (%) SyBioqua (%) BibγεB IS (MPa) El (%) L.EI (%) (%) ISxEI (MPa%) 1 A 2 98 9 0.6 0.5 55 2.0 2 5.0 1212 15.6 6.4 55 18907 Example 2 A 11 89 4 0.6 0.5 25 4.0 8 15.0 1263 13.0 5.0 38 16419 Comparative example 3 A 8 92 5 0.4 0.3 30 4.0 7 13.0 1242 13.4 5.4 42 16643 Comparative Example 4 to 8 92 5 0.4 0.3 35 4.0 5 12.0 1244 13.4 5.2 46 16670 Comparative Example 5 to 1 99 6 0.1 0.0 80 0.4 2 3.2 1228 13.4 6.3 60 16455 Comparative Example 6 to 1 99 6 0.1 0.0 78 0.3 2 3.4 1223 13.4 6.1 57 16388 Comparative Example 7 to 1 99 8 0.2 0.1 67 2.0 0 4.0 1215 14.1 6.0 57 17132 Example 8 A 1 99 10 0.5 0.4 62 2.0 1 4.0 1219 15.0 5.9 57 18285 Example 9 A 1 99 11 1.1 0.9 50 3.0 2 6.0 1224 16.7 5.8 56 20441 Example 10 A 1 99 11 1.0 0.8 37 4.0 3 8.0 1219 17.5 5.5 51 21333 Example 11 A 1 99 8 0.6 0.5 15 5.0 θ 12.0 1210 15.8 5.3 44 19118 Comparative Example 12 A 1 99 8 0.6 0.4 40 4.0 θ 6.0 1208 14.5 5.2 46 17516 Comparative Example 13 A 1 99 5 0.5 0.4 8 1.0 3 6.0 1239 13.6 5.2 48 16850 Comparative Example 14 A 1 99 5 0.5 0.4 18 2.0 2 6.0 1229 14.4 5.4 51 17698 Example 15 A 1 99 12 0.8 0.8 64 2.0 0 6.0 1209 16.6 6.5 60 20069 Example 16 A 1 99 13 1.1 0.9 88 2.0 0 6.0 1217 16.8 7.5 67 20446 Example 17 A 1 99 5 0.3 0.2 72 0.4 0 5.0 1231 13.4 5.4 61 16495 Comparative Example 18 A 1 99 3 0.1 0.0 133 0.2 0 3.2 1249 13.0 5.4 55 16237 Comparative Example 19 A 1 99 3 0.1 0.0 98 2.0 3 3.3 1248 13.2 5.4 50 16474 Comparative Example 20 A 1 99 5 0.3 0.1 82 2.0 2 3.5 1239 14.4 5.7 56 17842 Example 21 A 1 99 7 0.4 0.2 64 2.0 1 4.0 1224 15.6 6.0 56 19094 Example 22 B 14 86 7 0.1 0.1 24 2.0 8 12.0 1170 16.0 5.3 38 18720 Comparative Example 23 B 3 97 7 2.8 2.5 8 0.4 6 6.0 1198 15.5 5.2 48 18569 Comparative Example 24 C 5 95 8 1.0 0.7 53 3.0 3 13.0 1066 20.2 8.7 62 21531 Example 25 C 3 97 3 0.5 0.4 32 1.1 1 7.0 1110 14.1 5.2 53 15614 Comparative Example 26 D 2 98 5 0.7 0.5 41 1.7 2 8.0 1511 8.7 3.2 18 13135 Comparative Example 27 E 2 98 14 4.0 3.0 76 4.2 4 5.0 1046 19.1 7.0 59 19999 Example 28 F 2 98 7 0.9 0.6 123 0.6 5 6.0 1119 13.8 5.1 54 15389 Comparative Example 29 G 2 98 10 1.2 0.8 60 4.1 3 8.0 1361 14.1 5.2 42 19144 Example 30 H 3 97 7 0.9 0.6 48 2.8 2 5.0 1254 16.4 6.0 51 20610 Example 31 I 1 99 2 0.1 0.0 8 0.2 1 22.0 565 15.8 5.8 33 8933 Comparative Example 32 J 3 97 7 0.9 0.7 103 3.1 2 4.0 1262 15.5 5.7 57 19548 Example 33 K 2 98 10 1.2 0.8 60 4.8 2 6.0 1275 16.0 5.9 53 20358 Example 34 L 2 98 18 2.0 1.4 92 2.1 13 5.0 1325 13.4 4.9 26 17804 Comparative example. N 00 *11: Micro structure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and tempered martensite EXAMPLE 2 A 1.2 mm thick cold-rolled steel sheet with a composition indicated in Table 1 was processed under the annealing conditions indicated in Table 3-1 to produce steel sheets in accordance with the present invention and comparative examples. The measurement of the steel microstructure and the evaluation of the mechanical characteristics of the steel sheets were carried out as described above. Table 3-2 shows the results. The percentage of retained plate-like yUB area formed adjacent to the upper bainite was determined as the percentage SyUB area of ​​γ grains with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25. Example Nos. 1, 2, 3, 4, 5, and 9 satisfy TS x El > 17000 MPa% and have a hole expansion capacity (A) of 50% or more at the 1180 MPa grade. In contrast, the comparative examples were inferior in at least one of the characteristics. c / efrnn / Lznz / e / YiAi £ ω ω μ ο οι ο υι I O 01 Table 3-1 No. Steel No. Annealing Conditions Note Annealing Temperature (°C) Holding Time (°C / s) CR1 *1 (°C / s) CR2 '2 (°C / s) Holding Time (°C / s) CR3 '4 (°C / s) CR4 *5 (°C / s) CR5 '6 (°C / s) Cooling Stop Temperature (°C) Holding Time at Cooling Stop Temperature Heating Rate '7 (T / s) Holding Temperature (T) Holding Time *8 (s) CR6 (°C / s) 1 A 815 180 5 29 20 30 10 9 250 2 5 400 800 20 Example 2 A 875 180 2 29 20 31 10 9 250 2 10 400 800 20 Example 3 A 845 180 8 12 20 30 10 10 240 2 15 400 800 20 Example 4 A 845 180 7 30 20 12 8 10 240 2 25 400 800 20 Example 5 A 845 180 8 35 20 28 8 9 222 2 15 400 800 20 Example 6 A 850 180 8 35 20 29 8 10 200 2 15 400 800 20 Comparative Example 7 A 850 180 8 28 20 28 8 8 230 2 15 600 800 20 Comparative Example 8 A 850 180 8 30 20 28 8 5 230 2 15 400 3600 20 Comparative Example 9 0 860 180 8 28 20 15 8 9 240 2 10 400 800 20 Example 10 D 825 180 8 30 20 15 8 9 240 2 10 400 80020 Comparative Example Ί: Average cooling rate in the temperature range of 810-700°C '2: Average cooling rate in the temperature range of 700-490°C '3: Holding time in the temperature range of 490-405°C '4: Average cooling rate in the temperature range of 405-310°C '5: Average cooling rate in the temperature range of 310-255°C '6: Average cooling rate in the temperature range of 255°C to cooling stop temperature Tsq°C, Tsq: 220°C-254°C '7: Average heating rate in the temperature range of cooling stop temperature to 350°C '8: Holding time in the temperature range of 350-550°C '9: Average cooling rate to a temperature of 350-50°C or below ω ω ο σι o hJ M I heard O O01> Β h C hcc C * t Table 3-2 No. Steel No. Microstructure Characteristics Note Percentage of ferrite area (%) Percentage of remaining area (%) Percentage of retained volume (%) Syue (%) To be treated (%) Nylb (71000m2) Sypra (%) Sy^loque Sub / Syub TS (MPa) El (%) LEI (%) (%| TSxEI (MPa%) 1 A 5 95 12 0.4 0.2 40 1.0 4 4.0 1182 17.0 6.0 52 20094 Example 2 A 4 96 11 1.3 0.5 62 2.0 2 5.0 1189 16.5 5.5 53 19619 Example 3 A 4 96 11 1.4 0.6 57 30 3 4.0 1185 16.8 5.4 57 19908 Example 4 A 3 97 10 0.9 0.4 55 2.0 3 6.0 1198 14.2 5.1 51 17012 Example 5 A 1 99 θ 0.5 0.2 30 0.7 1 5.0 1278 14.4 5.3 60 18403 Example 6 A 1 99 3 0.1 0.0 21 0.2 3 4.0 1286 13.1 4.5 65 16847 Comparative Example 1 A 1 99 7 0.4 0.2 88 3.0 7 5.0 1162 14.5 4.0 39 16849 Comparative Example 8 A 1 99 6 0.2 0.1 70 2.0 8 6.0 1165 14.1 3.9 35 16427 Comparative Example 9 C 3 97 10 1.2 0.8 65.0 3.0 3 5.0 1055 18.7 7.5 58 19729 Example 10 D 1 93 16 0.9 0.8 51.0 47 8 8.0 1411 9.1 2.9 22 12840 Comparative Example '10: Microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained martensite Industrial Application The present invention provides very high ductility and high expansibility and is preferably applicable to press molding used through a press molding process in automobiles, household appliances, and the like. c / ptrnn / Lznz / e / YiAi

Claims

I. A steel sheet with a composition characterized in that it comprises, on a mass percent basis: C: 0.06% to 0.25%, Si: 0.6% to 2.5%, Mn: 2.3% to 3.5%, P: 0.02% or less, S: 0.01% or less, Al: less than 0.50%, N: less than 0.015%, and a residue consisting of iron and incidental impurities, wherein a steel microstructure contains ferrite: 5% or less by area percent, a microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100% by area percent, and retained γ: 4% to 15% by volume percent, retained γυB with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.0 to 15 pm, and an aspect ratio in the range of 3.1 to 25 has a SyUB area percentage in the range of 0.2% to 7.0%, YLB retained with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40 has a NyLB distribution number in the range of 10 to 120 per 100 pm2, fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a total area percentage SyF¡na θη in the range of 0.4% to 5.0%, and fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less have a total area percentage SyB|O that in the interval of 4% or less (including 0%).

2. A steel sheet with a composition characterized in that it comprises, on a mass percent basis: C: 0.06% to 0.25%, Si: 0.6% to 2.5%, Mn: 2.3% to 3.5%, P: 0.02% or less, S: 0.01% or less, Al: less than 0.50%, N: less than 0.015%, and a residue composed of iron and incidental impurities, wherein a steel microstructure contains ferrite: 5% or less by area percent, a microstructure composed of one or two or more of the following: upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100% by area percent, and retained γ: 4% to 15% by volume percent, γυB retained with a grain width in the range of 0.25 to 0.60 pm, a grain length in the range of 1.5 to 15 pm, and an aspect ratio in the range of 4 to 25 has an area percentage SvUB in the range of 0.2% to 7.0%, Ylb retained with a grain width in the range of 0.08 to 0.24 pm, a grain length in the range of 0.6 to 15 pm, and an aspect ratio in the range of 4 to 40 has a NyLB distribution number in the range of 10 to 120 per 100 pm2, fresh martensite with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 0.4 to 1.0 pm and an aspect ratio of 3 or less have a total area percentage SvFina in the range of 0.4% to 5.0%, and fresh martensite with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less and / or retained γ grains with an equivalent circular grain diameter in the range of 1.2 to 20 pm and an aspect ratio of 3 or less have a total area percentage SvB|OqUe θη the interval of 4% or less (including 0%).

3. The steel sheet according to claim 1 or 2, further characterized in that a ratio of a percentage of upper ferrite or bainite area SUB adjacent to the retained γυB with respect to the percentage of area SyUB satisfies SUB / SyUB > 3.

5.

4. The steel sheet according to any of claims 1 to 3, further characterized in that a region having a C concentration in the range of 0.6% to 1.3% with an adjacent region having a C concentration of 0.07% or less in the microstructure has a total area percentage Sc concentration in the range of 0.1% to 5%.

5. The steel sheet according to claim 4, further characterized in that the region with a C concentration in the range of 0.6% to 1.3% with the adjacent region having a C concentration of 0.07% or less is γ retained.

6. The steel sheet according to claim 5, further characterized in that the region with a C concentration in the range of 0.6% to 1.3% with the adjacent row having a C concentration of 0.07% or less are retained γυB grains.

7. The steel sheet according to any of claims 3 to 6, further characterized in that the adjacent region contains upper bainite.

8. The steel sheet according to any of claims 1 to 7, further characterized in that the composition further comprises, on a mass percentage basis: one or two selected from Ti: 0.002% to 0.1% and B: 0.0002% to 0.01%.

9. The steel sheet according to any of claims 1 to 8, further characterized in that the composition additionally comprises, on a mass percent basis: 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%.

10. The steel sheet according to any one of claims 1 to 9, further characterized in that the composition additionally comprises, on a mass percentage basis: 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%.

11. The steel sheet according to any of claims 1 to 10, further characterized in that the steel sheet has a breaking strength in the range of 780 to 1450 MPa.

12. The steel sheet according to any of claims 1 to 11, further characterized in that it comprises a galvanized layer on a surface of the steel sheet.

13. A method for producing a steel sheet, the method characterized in that it comprises: hot rolling and cold rolling a block of steel having the composition described in any one of claims 1, 2, and 8 to 10 and annealing the cold-rolled steel sheet to an annealing temperature in the range of 810°C to 900°C; then cooling the steel sheet at an average cooling rate of 1°C / s to 2000°C / s in the temperature range of 810°C to 700°C and cooling the steel sheet at an average cooling rate of 10°C / s to 2000°C / s in the temperature range of 700°C to 490°C; holding the steel sheet in the temperature range of 490°C to 405°C for 10 to 200 seconds; cool the steel sheet at an average cooling rate of 10°C / s to 100°C / s in the temperature range of 405°C to 310°C; cool the steel sheet at an average cooling rate of 0.4°C / s greater and less than 20°C / s in the temperature range of 310°C to 255°C, cool the steel sheet at an average cooling rate of 2°C / s greater and less than 30°C / s in the temperature range of 255°C to a cooling stop temperature Tsq in the range of 254°C to 220°C; heat the steel sheet at an average heating rate of 2°C / s greater in the temperature range of Tsq to 350°C, and hold the steel sheet at 350°C to 550°C for 20 to 3000 seconds; and cool the steel sheet to a temperature in the range of 350°C to 50°C or lower at an average cooling rate of 0.1°C / s greater. cyefrnn / Lznz / e / YiAi.