HIGH-STRENGTH STEEL SHEET AND METHOD FOR ITS PRODUCTION

Abstract

A high-strength steel sheet with a low yield strength and a method for its production are provided. In the present invention, a high-strength steel sheet has a chemical composition containing, in % by mass, C: 0.06% or more and 0.120% or less, Si: 0.3% or more and 0.7% or less, Mn: 1.6% or more and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or more and 0.20% or less, and N: 0.010% or less, the remainder being Fe and incidental impurities, and the high-strength steel sheet has a microstructure in which ferrite is present as a major phase, and martensite is present in an area fraction of 10% or more and less than 50% in relation to an area of ​​a total of the microstructure, wherein the martensite has an average grain diameter of 3.0 µm or less; in a whole of the martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or more; and the martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or more and 0.90% or less by mass %.

Description

HIGH-STRENGTH STEEL SHEET AND METHOD FOR ITS PRODUCTION FIELD OF INVENTION The present invention relates to a high-strength steel sheet and a method for producing it. The high-strength steel sheet is suitable for use in structural parts of automobiles and similar applications. More specifically, the present invention relates to a high-strength steel sheet having a low yield strength and excellent surface properties, and to a method for producing it. BACKGROUND OF THE INVENTION In recent years, from a global environmental protection perspective, efforts have been made to reduce exhaust emissions, such as CO2. The automotive industry has been developing approaches to reduce the weight of vehicle bodies to improve fuel efficiency, thereby reducing the amount of exhaust emissions. One technique for reducing vehicle body weight is to increase the strength of the steel sheets used in motor vehicles, thus reducing their thickness. Furthermore, it is known that as the strength of a steel sheet increases, its ductility decreases; therefore, there is a need for steel sheets that possess both high strength and ductility. Additionally, among automotive components, floor panels, for example, must have excellent surface properties.In addition, in many cases, floor parts are shaped into a complex form, and therefore there is a need for a steel sheet that has a low yield strength, does not exhibit cracking during shaping, and does not easily lose its shape. In response to the need, Patent Document 1, for example, discloses a high-strength galvanized steel sheet having a low yield ratio, which has a composition containing, in mass %, C: 0.05 to 0.20%, Si: 0.3 to 1.8%, and Mn: 1.0 to 3.0% and has a microstructure in which ferrite is present in a volume fraction of 60% or more, martensite is present in a volume fraction of 5% or more, retained austenite is present in a volume fraction of 2% or more, and the ferrite has an average grain diameter of 5 pm or greater, the high-strength galvanized steel sheet, therefore, has a tensile strength of 590 MPa or greater, a strength-elongation balance of 21000 MPa % or greater, and a yield ratio of 65% or less. Furthermore, Patent Document 2 discloses a high-strength steel sheet, which has a chemical composition containing, in % by mass, C: 0.07 to 0.2%, Si: 0.005 to 1.5%, Mn: 1.0 to 3.1%, P: 0.001 to 0.06%, S: 0.001 to 0.01%, Al: 0.005 to 1.2%, and N: 0.0005 to 0.01% and has a metallurgical structure formed by ferrite and martensite, the high-strength steel sheet, therefore, has a breaking strength of 590 MPa or higher and has improved workability. Furthermore, Patent Document 3 discloses a high-strength steel sheet, which has a chemical composition containing, in % by mass, C: 0.05 to 0.13%, Si: 0.6 to 1.2%, Mn: 1.6 to 2.4%, P: 0.1% or less, S: 0.005% or less, Al: 0.01 to 0.1%, and N: less than 0.005% and has a microstructure in which 80% or more ferrite is present, 3 to 15% martensite is present, and 0.5 to 10% pearlite is present, each in a volume fraction, the high-strength steel sheet RRnRnn / iznz / e / Yi resistance, load-bearing, has a breaking strength of 590 MPa or higher and a yield ratio of 70% or less. Furthermore, Patent Document 4 discloses a high-strength steel sheet, which has a chemical composition containing, in mass %, C: 0.06 to 0.12%, Si: 0.4 to 0.8%, Mn: 1.6 to 2.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.1%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.5%, and N: 0.005% or less and has a metallurgical structure in which equiaxed ferrite is present in a volume fraction of 50% or more, martensite is present in a volume fraction of 5 to 15%, a retained austenite phase is present in a volume fraction of 1 to 5%, the retained austenite phase having an average grain diameter If the austenite phase has an aspect ratio of 10 pm or less, and the retained austenite phase has an aspect ratio of 5 or less, the high-strength steel sheet therefore has a breaking strength of 590 MPa or higher, a total elongation of 30% or more, and a hole expansion index of 60% or more. List of Appointments Patent Documents PTL 1: Publication of Unexamined Japanese Patent Application No. 2001-192767 PTL 2: Publication of Unexamined Japanese Patent Application No. 2011-144409 PTL 3: Publication of Unexamined Japanese Patent Application No. 2012-177175 PTL 4: Publication of Unexamined Japanese Patent Application No. 2014-19928 BRIEF DESCRIPTION OF THE INVENTION Technical Problem In the technology disclosed in Patent Document 1, listed above, a ferrite-martensite structure is used. The ferrite grain diameter is limited, resulting in a low yield strength and improved ductility. However, the annealing stages are performed twice to obtain a coated steel sheet. Unfortunately, as a result of performing the annealing stages twice, the surface of the steel sheet is susceptible to oxide formation, and therefore, excellent surface properties are not achieved. Furthermore, in the technology disclosed in Patent Document 2, listed above, ferrite is used as a main phase, and, consequently, workability is improved; however, since there is no disclosure of a martensite grain diameter, it can be assumed that the martensite grain diameter is not controlled, and, as a result, a low creep ratio is not achieved. Furthermore, the technology disclosed in Patent Document 3, listed above, utilizes a ferrite-martensite structure, and consequently, a low creep ratio is achieved, as disclosed. However, the creep ratio disclosed in Patent Document 3 is greater than the limitation of the present invention, which is 63% or less. Presumably, one reason for this is a failure to control the martensite grain diameter. The annealing temperature and the cooling stop temperature for controlling the martensite grain diameter disclosed in Patent Document 3 differ from the limitations of the present invention.Furthermore, steel sheets having a yield strength of 63% or less disclosed in Patent Document 3 have higher Si and Mn contents than those of the present invention, and, therefore, it can be assumed that the steel sheets do not have excellent surface properties. Furthermore, in the technology disclosed in Patent Document 4, listed above, a ferrite-martensite structure is used, limiting the volume fraction and average grain diameter of retained austenite, and consequently achieving a low creep ratio and improved workability; however, Cr and V are added to ensure hardenability. Unfortunately, Cr and V are known to degrade surface properties. Achieving the excellent surface properties sought by the present invention requires a chemical composition in which the content of these elements is reduced. The present invention has been made in view of the problems described above, and the objectives of the present invention are to provide a high-strength steel sheet having a low yield strength and excellent surface properties and to provide a method for producing the same. Solution to the Problem The inventors of the present invention diligently conducted studies to solve the problems described above. As a result, it was discovered that a high-strength steel sheet with a low yield strength can be obtained by adjusting the chemical composition to achieve a specific composition, utilizing a microstructure composed of ferrite and martensite components, and controlling the martensite grain diameter, aspect ratio, and carbon concentration. Accordingly, the present invention was completed. That is, the inventors of the present invention discovered that achieving the strength sought by the present invention requires that martensite be present in an area fraction of 10% or more, and achieving the low creep ratio sought by the present invention requires that martensite be present in an area fraction of less than 50%, that martensite having an aspect ratio of 3 or less be present in an amount of 60% or more of the total martensite, that the martensite having an aspect ratio of 3 or less have a carbon concentration of 0.3% or more and 0.9% or less by mass percent, and that the martensite have an average grain diameter of 3.0 pm or less. Note that the aspect ratio is a value calculated by dividing a larger dimension by a smaller dimension. The present invention was made based on the discoveries described above, and a summary of the present invention is as follows. [1] A high-strength steel sheet, the high-strength steel sheet has a chemical composition containing, in mass %, C: 0.06% or more and 0.120% or less, Si: 0.3% or more and 0.7% or less, Mn: 1.6% or more and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or more and 0.20% or less, and N: 0.010% or less, the remainder being Fe and incidental impurities, the high-strength steel sheet has a microstructure in which ferrite is present as a major phase, and martensite is present in an area fraction of 10% or more and less than 50% relative to an area of ​​a total of the microstructure, wherein the martensite has an average grain diameter of 3.0 pm or less; In the totality of martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or more; and martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or more and 0.90% or less by mass %. [2] The high-strength steel sheet in accordance with [1], wherein the chemical composition additionally contains, in % by mass, one or more selected from Cr: 0.01% or more and 0.20% or less, Mo: 0.01% or more and less than 0.15%, and V: 0.001% or more and 0.05% or less. [3] High-strength steel sheet conforming to [1] or [2], wherein the chemical composition additionally contains, in % by mass, one or more selected from Groups A to C described below: Group A: one or two selected from Nb: 0.001% or more and 0.02% or less and Ti: 0.001% or more and 0.02% or less, Group B: one or two selected from among Cu: 0.001% or more and 0.20% or less and Ni: 0.001% or more and 0.10% or less, and Group C: B: 0.0001% or more and 0.002% or less. [4] The high-strength steel sheet conforming to any of [1] to [3], wherein a surface of the steel sheet has a coating layer. [5] A method for producing a high-strength steel sheet, the method comprising heating a block of steel having the chemical composition in accordance with any of [1] to [3] and subsequently subjecting the block of steel to a hot rolling stage; and subjecting a hot-rolled steel sheet obtained from the hot rolling stage to an annealing stage in which the hot-rolled steel sheet is held at an annealing temperature of Aci or higher and a lower Acao temperature for 30 seconds or more, a resulting steel sheet being cooled under conditions in which an average cooling rate in a range from the annealing temperature to 350°C is 5°C / second or greater, and a cooling stop temperature is 350°C or lower, and subsequently,The resulting steel sheet is held under conditions in which a holding time for a temperature range from 350 °C to 300 °C is 50 seconds or less, and a holding time for a temperature range from less than 300 °C to a temperature T1 (°C) is 1000 seconds or less, where the temperature T1 (°C) is a selectable temperature within a temperature range of 200 to 250 °C. [6] A method for producing a high-strength steel sheet, the method includes heating a block of steel having the chemical composition in accordance with any of [1] to [3] and subsequently subjecting the block of steel to a hot rolling stage; after that, subjecting a hot-rolled steel sheet obtained from the hot rolling stage to a cold rolling stage; and subjecting a cold-rolled steel sheet obtained from the cold rolling stage to an annealing stage in which the cold-rolled steel sheet is held at an annealing temperature of Aci or higher and Acs or lower for 30 seconds or more, a resulting steel sheet being cooled under conditions in which an average cooling rate in a range from the annealing temperature to 350 °C is 5 °C / second or higher, and a cooling stop temperature is 350 °C or lower,and subsequently, the resulting steel sheet is held under conditions in which a holding time for a temperature range from 350 °C to 300 °C is 50 seconds or less, and a holding time for a temperature range from less than 300 °C to a temperature T1 (°C) is 1000 seconds or less, where the temperature T1 (°C) is a selectable temperature within a temperature range of 200 to 250 °C. [7] The method for producing a high-strength steel sheet in accordance with [5] or [6], the method additionally includes carrying out a coating process after the annealing stage. Advantageous Effects of the Invention In the present invention, by adjusting a chemical composition and a production method, a microstructure is controlled, and furthermore, a martensite grain diameter, a martensite aspect ratio, and a martensite carbon concentration are controlled. As a result, the high-strength steel sheets of the present invention have excellent surface properties and a low creep ratio. Furthermore, in cases where a high-strength steel sheet of the present invention is used in an automotive structural member, the high strength and low yield strength of an automotive steel sheet can be achieved in combination. That is, with the present invention, the performance of motor vehicle bodies can be enhanced. DETAILED DESCRIPTION OF THE INVENTION Description of Modalities The embodiments of the present invention will be described below. Note that the present invention is not limited to these embodiments. First, a chemical composition of a high-strength steel sheet of the present invention (hereafter sometimes referred to as a “steel sheet of the present invention”) will be described. In the following description of the chemical composition, the unit “%” used to indicate a component content means “% by mass”. C: 0.06% or more and 0.120% or less Carbon (C) is an element that improves hardenability and is necessary to ensure a predetermined area fraction of martensite. Furthermore, C increases the strength of martensite and is therefore necessary to ensure a tensile strength (TS) of 590 MPa or higher, which is the objective of the present invention. If the C content is less than 0.06%, the aforementioned predetermined strength cannot be achieved. Therefore, it is specified that the C content be greater than or equal to 0.06%. The C content is preferably greater than or equal to 0.065% and more preferably greater than or equal to 0.070%. Moreover, if the C content is greater than 0.120%, the area fraction of martensite increases, and therefore the creep ratio increases. Consequently, it is specified that the C content be less than or equal to 0.120%. The C content is preferably less than or equal to 0.115% and more preferably less than or equal to 0.11%. Yes: 0.3% or more and 0.7% or less Silicon (Si) is an element that enables hardening through solid solution hardening. To produce the effects described above of the present invention, it is specified that the Si content be greater than or equal to 0.3%. The Si content is preferably greater than or equal to 0.35% and more preferably greater than or equal to 0.40%. Furthermore, if the Si content is too high, the ferrite strength increases, and therefore the creep ratio increases. In addition, if an excessive amount of Si is present, an oxide forms on a surface of the steel sheet, and consequently, a surface property is significantly degraded. Therefore, it is specified that the Si content be less than or equal to 0.7%. The Si content is preferably less than or equal to 0.64% and more preferably less than or equal to 0.60%. Mn: 1.6% or more and 2.2% or less Manganese (Mn) must be present to improve the hardenability of the steel and ensure the predetermined area fraction of martensite. If the Mn content is less than 1.6%, ferrite forms on a portion of the surface layer of the steel sheet, consequently degrading its strength. Additionally, pearlite or bainite forms during cooling, consequently increasing the creep ratio. Therefore, the Mn content is specified to be greater than or equal to 1.6%. The Mn content is preferably greater than or equal to 1.65%, and more preferably greater than or equal to 1.70%. On the other hand, if an excessive amount of Mn is present, an oxide forms on a surface of the steel sheet, consequently significantly degrading a surface property. Therefore, the Mn content is specified to be less than or equal to 2.2%. The Mn content is preferably less than or equal to 2.14% and more, preferably less than or equal to 2.10%. P: 0.05% or less Phosphorus (P) is an element that hardens steel. However, if the P content is high, the P segregates at the grain boundaries and thus degrades workability. Therefore, it is specified that the P content be less than or equal to 0.05% to achieve at least the minimum workability necessary for using the steel sheet of the present invention as an automotive steel sheet. The P content is preferably less than or equal to 0.03% and more preferably less than or equal to 0.01%. Note that the lower limit of the P content is not particularly restricted; currently, an industrially feasible lower limit is approximately 0.003%. Therefore, it is preferably specified that the P content be greater than or equal to 0.003%. More preferably, the P content is greater than or equal to 0.005%. S: 0.0050% less Sulfur (S) degrades workability by forming MnS and / or similar compounds. Furthermore, where titanium (Ti) is present with sulfur, sulfur can degrade workability by forming TiS, Ti(C,S), and / or similar compounds. Therefore, it is specified that the sulfur content be less than or equal to 0.0050% to achieve at least the minimum workability necessary for using the steel sheet of the present invention as an automotive steel sheet. The sulfur content is preferably less than or equal to 0.0020%, more preferably less than or equal to 0.0010%, and even more preferably less than or equal to 0.0005%. Note that the lower limit for the sulfur content is not particularly restricted; currently, an industrially feasible lower limit is approximately 0.0002%. Therefore, it is preferably specified that the sulfur content be greater than or equal to 0.0002%. More preferably, the S content is greater than or equal to 0.0005%. To: 0.01% or more and 0.20% or less Aluminum (Al) is added to achieve sufficient deoxidation and reduce coarse inclusions present in the steel. This effect is exhibited when the Al content is greater than or equal to 0.01%. Preferably, the Al content is greater than or equal to 0.02%. More preferably, the Al content is greater than or equal to 0.03%. On the other hand, if the Al content is greater than 0.20%, Fe-based carbides, such as cementite, which form during coiling after hot rolling, do not readily dissolve during annealing, and therefore coarse inclusions and carbides form; as a result, workability is degraded. Accordingly, it is specified that the Al content be less than or equal to 0.20% to achieve at least the minimum workability necessary to use the steel sheet of the present invention as an automotive steel sheet. The Al content is preferably less than or equal to 0.03%.17% and more, preferably less than or equal to 0.15%. N: 0.010% or less Nitrogen (N) is an element that forms coarse nitride inclusions, such as AlN, in steel and degrades workability by forming such inclusions. Furthermore, in cases where titanium (Ti) is present with N, N is an element that forms coarse inclusions. Examples of these inclusions include nitride and carbonitride inclusions, such as Tin and (Nb,Ti)(C,N); consequently, N can degrade workability by forming these inclusions. Therefore, it is specified that the N content be less than or equal to 0.010% to achieve at least the minimum workability necessary for using the steel sheet of the present invention as an automotive steel sheet. The N content is preferably less than or equal to 0.007% and more preferably less than or equal to 0.005%.Note that the lower limit for N content is not particularly restricted; currently, an industrially feasible lower limit is approximately 0.0006%. Therefore, it is preferably specified that the N content be greater than or equal to 0.0006%. More preferably, the N content is greater than or equal to 0.0010%. The components described above are the basic components of the steel sheet used in the present invention. The steel sheet used in the present invention has a chemical composition containing the basic components described above, with the remainder, other than the components described above, including Fe (iron) and incidental impurities. It is preferable that the steel sheet of the present invention have a chemical composition containing the components described above, with the remainder consisting of Fe and incidental impurities. The steel sheet of the present invention may contain the following components as optional components, in addition to the components described above. Note that in the present invention, where any of the following optional components is present in an amount less than the lower limit thereof, the component shall be considered to be present as an incidental impurity, which will be described later. One or more selected from among Cr: 0.01% or more and 0.20% or less, Mo: 0.01% or more and less than 0.15%, and V: 0.001% or more and 0.05% or less Cr, Mo, and / or V may be included to produce a hardenability-enhancing effect in steel. Where Cr and / or Mo are included, it is preferable that the Cr content be greater than or equal to 0.01%, and / or the Mo content be greater than or equal to 0.01%, to produce this effect. More preferably, the contents are greater than or equal to 0.02%, and even more preferably, greater than or equal to 0.03%, each. Where V is included, it is preferable that the V content be greater than or equal to 0.001% to produce the effect described above. More preferably, the content is greater than or equal to 0.002%, and even more preferably, greater than or equal to 0.003%. However, if the content of any of these elements is excessive, an oxide formation reaction involving the generation of hydrogen ions can be induced. As a result, an increase in the pH of the base metal surface is prevented, which in turn prevents the precipitation of a zinc phosphate crystal, and consequently, conversion coating failure can occur. Therefore, where Cr is to be included, it is preferable that the Cr content be less than or equal to 0.20%. More preferably, the Cr content is less than or equal to 0.15%, and even more preferably, less than or equal to 0.10%. Where Mo is to be included, it is preferable that the Mo content be less than 0.15%. More preferably, the Mo content is less than or equal to 0.1%, and even more preferably, less than or equal to 0.05%.Where V is to be included, it is preferable that the V content be less than or equal to 0.05%. More preferably, the V content is less than or equal to 0.03%, and even more preferably, less than or equal to 0.01%. One or two selected from among Nb: 0.001% or more and 0.02% or less and Ti: 0.001% or more and 0.02% or less Nitrogen (Nb) and titanium (Ti) contribute to increased strength by refining the previous grain size and forming fine precipitates. Where one or two of these elements are selected from Nb and Ti, it is preferable that the Nb content be greater than or equal to 0.001%, and / or the Ti content be greater than or equal to 0.001%, to achieve this effect. More preferably, the contents are greater than or equal to 0.0015%, and even more preferably, greater than or equal to 0.0020%, respectively. Furthermore, if Nb and / or Ti are included in large quantities, a surface property may be degraded. Therefore, where one or two of these elements are selected from Nb and Ti, it is preferable that the Nb content be less than or equal to 0.02%, and / or the Ti content be less than or equal to 0.02%. More preferably, the contents are less than or equal to 0.017%, separately, and even more preferably, less than or equal to 0.015%, separately. One or two selected from among Cu: 0.001% or more and 0.20% or less and Ni: 0.001% or more and 0.10% or less Copper (Cu) and nickel (Ni) have the effect of improving corrosion resistance in motor vehicle environments and forming a corrosion product that coats the surface of a steel sheet, thereby preventing hydrogen from penetrating the steel sheet. When one or both of these elements are selected, it is preferable that the Cu content be greater than or equal to 0.001%, and / or the Ni content be greater than or equal to 0.001%, to achieve these effects. More preferably, the contents are greater than or equal to 0.002%, and even more preferably, greater than or equal to 0.003%, each. However, if the Cu and / or Ni content is too high, a surface defect may occur, and consequently, a surface property may be degraded. Therefore, in cases where Cu will be included, it is preferable that the Cu content be less than or equal to 0.20%.More preferably, the copper content is less than or equal to 0.15%, and even more preferably, less than or equal to 0.1%. Where nickel is included, it is preferable that the nickel content be less than or equal to 0.10%. More preferably, the nickel content is less than or equal to 0.07%, and even more preferably, less than or equal to 0.05%. B: 0.0001% or more and 0.002% or less Boron (B) is an element that improves the hardenability of steel. When B is present, it produces the effect of forming a predetermined area fraction of martensite even when the Mn content is low. In cases where B is to be included, it is preferable that the B content be greater than or equal to 0.0001% to produce this effect. More preferably, the B content is greater than or equal to 0.0003%, and even more preferably, greater than or equal to 0.0005%. On the other hand, if the B content is greater than 0.002%, the thickening of Mn oxides is promoted, and consequently, a surface property may be degraded. Therefore, in cases where B is to be included, it is preferable that the B content be less than or equal to 0.002%. More preferably, the content of B is less than or equal to 0.0015%, and even more preferably, less than or equal to 0.0010%. Now, a microstructure of the high-strength steel sheet of the present invention will be described. The steel sheet of the present invention has a microstructure in which ferrite is present as a major phase, and martensite is present in an area fraction of 10% or more and less than 50% relative to the total area of ​​the microstructure. The martensite has an average grain diameter of 3.0 pm or less. In the total martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or more. The martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or more and 0.90% or less by mass percent. Note that in the descriptions that follow, “area fraction” refers to an area fraction relative to the total area of ​​the microstructure. In the present invention, ferrite is present as a major phase. In the present invention, the “major phase” refers to a component present in an area fraction ranging from 50% to 100% relative to the total area of ​​the microstructure. Accordingly, “ferrite is present as a major phase” means that ferrite is present in an area fraction of 50% to 90% relative to the total area of ​​the microstructure. In the present invention, it is necessary for ferrite to be present as a major phase in order to reduce the yield strength and achieve a good creep ratio. The lower limit of the ferrite area fraction is preferably 55% or more, and more preferably 60% or more. The upper limit is preferably 85% or less, and more preferably 80% or less.“Ferrite”, as referred to herein, is recrystallized ferrite and does not include non-recrystallized ferrite, which does not recrystallize. Fraction of Martensite Area: 10% or more and less than 50% To achieve a high tensile strength of 590 MPa or higher in terms of TS in the steel sheet of the present invention as described above, it is specified that the area fraction of martensite relative to the total area of ​​the microstructure be greater than or equal to 10%. The area fraction is preferably greater than or equal to 15% and more preferably greater than or equal to 20%. Furthermore, if the area fraction of martensite relative to the total area of ​​the microstructure is greater than or equal to 50%, the martensite is present as a major phase; therefore, the carbon content of the martensite is reduced, and as a result, the yield strength is increased. Accordingly, it is specified that the area fraction of martensite be less than 50%. The area fraction is preferably less than or equal to 45% and more preferably less than or equal to 40%. Note that in the present invention, the remaining components, other than ferrite and martensite, are one or more selected from retained austenite, bainite, unrecrystallized ferrite, and pearlite, and a total allowable amount thereof is less than or equal to 10.0% in terms of area fraction. With respect to the remaining components other than ferrite and martensite, the total amount of the one or more selected from retained austenite, bainite, unrecrystallized ferrite, and pearlite is preferably less than or equal to 7.0% and more preferably less than or equal to 5.0% in terms of area fraction. Note that the area fraction of the remaining components may be 0%. In the present invention, ferrite is a component that forms at a relatively high temperature as a result of the transformation of austenite and is composed of BCC network grains. Non-recrystallized ferrite is a component containing elongated white streaks remaining within the ferrite grains. Martensite is a hard component that forms from austenite at a low temperature (a temperature lower than or equal to the martensitic transformation temperature). Bainite is a hard component that forms from austenite at a relatively low temperature (a temperature higher than or equal to the martensitic transformation temperature) and includes acicular or plate-like ferrite and fine carbides dispersed therein. Pearlite is a component that forms from austenite at a relatively high temperature and is composed of lamellar ferrite and cementite.Retained austenite is a component that forms when the enrichment of an element such as C in austenite causes the martensitic transformation temperature to change to a temperature lower than or equal to room temperature. Note that in the present invention, the value of the area fraction of each of the components in the microstructure is a value obtained by performing a measurement according to a method that will be described in the Examples section below. Average Martensite Grain Diameter: 3.0 pm or less Achieving the low creep ratio sought by the present invention requires reducing the ferrite strength and increasing the martensite strength. An effective way to achieve this is by reducing the average grain diameter of the martensite. Producing the effects described above requires that the average grain diameter of the martensite be less than or equal to 3.0 pm. The average grain diameter is preferably less than 3.0 pm, more preferably less than or equal to 2.7 pm, and even more preferably less than or equal to 2.0 pm. The lower limit of the average grain diameter of the martensite is not particularly restricted and is preferably 0.5 pm or more, and more preferably 0.8 pm or more. Note that in the present invention, the average grain diameter of the martensite in the microstructure is a value obtained by performing a measurement according to a method that will be described in the Examples section below. Proportion of Martensite That Dyes Aspect Ratio of 3 or Less in Total Martensite: 60% or more Unlike acicular martensite, martensite with an aspect ratio of 3 or less has high strength. Therefore, martensite with an aspect ratio of 3 or less is an important component in achieving the low yield strength sought by the present invention. In cases where the area fraction of martensite with an aspect ratio of 3 or less is less than 60% relative to the area fraction of the whole martensite, this fraction is insufficient to achieve the low yield strength sought by the present invention. Therefore, it is specified that the proportion of martensite with an aspect ratio of 3 or less in the whole martensite be 60% or more in terms of area fraction. The proportion is preferably greater than or equal to 65%, and more preferably greater than or equal to 70%.The upper limit of the proportion of martensite having an aspect ratio of 3 or less in the total martensite is not particularly restricted and can be 100%. More preferably, the upper limit is 90% or less. Note that in the present invention, the aspect ratio of the martensite in the microstructure is a value obtained by performing a measurement according to a method that will be described in the Examples section below. Carbon Concentration of Martensite Having an Aspect Ratio of 3 or Less: 0.30% or more and 0.90% or less by mass % Increasing the strength of martensite and achieving the low creep ratio sought by the present invention requires increasing the carbon concentration of martensite with an aspect ratio of 3 or less. To produce the effects described above, the carbon concentration of martensite with an aspect ratio of 3 or less must be greater than or equal to 0.30% by mass. The carbon concentration is preferably greater than or equal to 0.35%, and more preferably greater than or equal to 0.40%. On the other hand, if the carbon concentration of martensite with an aspect ratio of 3 or less is greater than 0.90% by mass, austenite remains, without undergoing a martensitic transformation; as a result, the area fraction of martensite is less than 10%, and therefore, the strength is reduced.Therefore, it must be specified that the carbon concentration of martensite having an aspect ratio of 3 or less is less than or equal to 0.90% by mass. The carbon concentration is preferably less than or equal to 0.85% and more preferably less than or equal to 0.8%. Note that in the present invention, the carbon concentration of martensite having an aspect ratio of 3 or less in the microstructure is a value obtained by performing a measurement according to a method that will be described in the Examples section below. With respect to measurement positions, in the present invention, the microstructure described above is uniform across a region of sheet thickness, excluding a region of an outermost layer measuring 10 pm in the sheet thickness direction. Therefore, with respect to sheet thickness measurement positions, measurements can be taken at any position within the region where the microstructure is uniform. In the steel sheet of the present invention, a surface of the steel sheet may have a coating layer. Preferably, the coating layer may be a galvanized layer (hereinafter sometimes referred to as “Gl”), a galvano-annealed layer (hereinafter sometimes referred to as “GA”), or an electrogalvanized layer (hereinafter sometimes referred to as “EG”). Please note that the coating metal can be a metal other than zinc. For example, an aluminum or similar coating can be used. It is preferable that the iron content of the coating layer be within a range of 7 to 16% by mass. If the iron content is less than 7% by mass, uneven alloying may occur and / or degrade the spalling property. Conversely, if the iron content is greater than 16% by mass, the peel strength may be degraded. Now, the properties (mechanical properties) of the high-strength steel sheet of the present invention will be described. As described above, the steel sheet of the present invention has high strength. Specifically, the steel sheet has a tensile strength (TS) of 590 MPa or higher as measured according to a method that will be described in the Examples section below. Note that the upper limit of the tensile strength is not particularly restricted; preferably, the tensile strength is less than or equal to 780 MPa because in such a case, a balance with other properties is readily achieved. Furthermore, the steel sheet of the present invention has a low yield ratio (YR). Specifically, the steel sheet has a yield ratio (YR = YS / TS) of 0.63 or less. The yield ratio is calculated from the tensile strength (TS) and yield strength (YS) values, which are measured according to a method that will be described in the Examples section below. The yield ratio is preferably less than or equal to 0.61 and more preferably less than or equal to 0.59. Note that the lower limit of the yield ratio is not particularly restricted; preferably, the yield ratio is greater than or equal to 0.4 because in such a case, a balance with other properties is easily achieved. More preferably, the yield ratio is greater than or equal to 0.45. Note that in the steel sheet of the present Invention, the properties of a yield ratio of 0.63 or less and a tensile strength of 590 MPa or higher can be achieved in cases where an annealing temperature of Aci or higher and a temperature Aca or lower and a quenching stop temperature of 350 °C or lower are employed. Furthermore, the steel sheet of the present invention has excellent surface properties. As mentioned herein, the “surface property” refers to chemical convertibility when the steel sheet is hot-rolled or cold-rolled, and the “surface property” refers to coating adhesion when the steel sheet is coated. Specifically, when the steel sheet was either hot-rolled or cold-rolled, an assessment was made of whether or not excellent chemical convertibility was achieved. This assessment was performed by calculating a coverage rate of the measured conversion crystals using a method for evaluating chemical convertibility, which was carried out according to a method that will be described in the Examples section below. In the present invention, where the coverage rate, which is a rate in terms of an area fraction, was 95% or more, the symbol “O” was assigned; where the coverage rate was 90% or more and less than 95%, the symbol “Δ” was assigned; and where the coverage rate was less than 90%, the symbol “x” was assigned.It was determined that the symbols “O” and “Δ” represented cases in which good chemical convertibility was exhibited (i.e., excellent chemical convertibility was exhibited). When the steel sheet was coated, an assessment was made of whether or not excellent coating adhesion was achieved by visually examining its appearance. In the present invention, steel sheets free from peeling spot defects were assigned the symbol “O”, steel sheets exhibiting a peeling spot defect were assigned the symbol “x”, and steel sheets that were free from peeling spot defects but had a non-uniform or similar coating appearance were assigned the symbol “Δ”. Note that a “peeling spot defect” refers to an exposed, uncoated region of a steel sheet on the order of approximately several micrometers to several millimeters.It was determined that the symbols “O” and “Δ” represented cases where the coating was sufficiently adhered, and therefore good coating adhesion was achieved (i.e., excellent coating adhesion was achieved). Now, the methods of the present invention for the production of a high-strength steel sheet will be described. The methods of the present invention for producing a high-strength steel sheet include a hot-rolling stage, described below, a cold-rolling stage, which is optional, and an annealing stage. Note that in the following description, "temperature" refers to the surface temperature of the steel sheet unless otherwise specified. The surface temperature of the steel sheet can be measured using a radiation pyrometer or similar device. Hot Rolling Stage A steel starting material (steel block) with the chemical composition described above is subjected to a hot rolling stage. Note that it is preferable for the steel block to be produced by a continuous casting method to avoid macro-segregation of a component. Alternatively, the steel block can be produced by an ingot casting method or a thin-block casting method. The preferred conditions for the hot rolling stage of the present invention are, for example, the following. First, a steel block having the chemical composition described above is heated. If the heating temperature for the steel block is below 1200 °C, a sulfide may precipitate, which can degrade workability. Therefore, in order to achieve at least the minimum workability necessary to use a high-strength steel sheet produced by the present invention as an automotive steel sheet, it is preferable that the heating temperature for the steel block be greater than or equal to 1200 °C. More preferably, the heating temperature is greater than or equal to 1230 °C, and even more preferably, greater than or equal to 1250 °C.Please note that the upper limit of the heating temperature for the steel block is not particularly restricted and is preferably 1400 °C or lower. More preferably, the upper limit is 1350 °C or lower. Furthermore, it is preferable that the average heating rate for the steel block be 5 to 15 °C / minute, and that the temperature holding time for the steel block be 30 to 100 minutes. As stated herein, the “average heating rate for the steel block” is the average of the heating rates over a period beginning from the start of heating until the surface temperature of the steel block reaches the aforementioned heating temperature. The “temperature holding time for the steel block” is the period from the time the aforementioned heating temperature is reached until hot rolling begins. It is preferable that, after the steel block has been heated, hot rolling is carried out under the conditions described below. It is preferable that the finisher exit temperature be greater than or equal to 840 °C. If the finisher exit temperature is less than 840 °C, it takes a long time to reduce the temperature to a rolling temperature, which can lead to oxidation of the base metal surface and, consequently, degradation of the surface properties. Therefore, it is preferable that the finisher exit temperature be greater than or equal to 840 °C. More preferably, the finisher exit temperature is greater than or equal to 860 °C. On the other hand, the upper limit of the finisher exit temperature is not particularly restricted. It is preferable that the finisher exit temperature be less than or equal to 950 °C because, otherwise, cooling the steel sheet to a rolling temperature, which will be described later, is difficult. More preferably, the finisher exit temperature is less than or equal to 920 °C. It is preferable that the reduction percentage for the rolling finish be greater than or equal to 70%, from the standpoint of achieving a martensite aspect ratio of 3 or less. It is preferable that the reduction percentage be less than or equal to 95%, from the standpoint of ensuring the ferrite area fraction. If the rolling temperature exceeds 700 °C, the surface of the base metal may undergo decarburization, resulting in a difference in microstructure between the interior of the steel sheet and its surface. This can lead to uneven alloy concentration. Furthermore, decarburization causes ferrite to form in a surface layer of the steel sheet, reducing its tensile strength. Therefore, it is preferable for the rolling temperature to be less than or equal to 700 °C. More preferably, the rolling temperature should be less than or equal to 670 °C. The lower limit for the rolling temperature is not particularly restricted. In cases where cold rolling follows hot rolling, it is preferable for the rolling temperature to be greater than or equal to 550 °C to avoid a decline in cold rolling capability.In cases where cold rolling is not carried out, it is preferable that the rolling temperature be greater than or equal to 300 °C because if the rolling temperature is less than 300 °C, rolling the hot-rolled steel sheet is difficult. Hot-rolled steel sheet can be pickled after coiling. In this case, the pickling conditions are not particularly restricted. Note that pickling of hot-rolled steel sheet may not be possible after hot rolling. Cold Rolling Stage The cold rolling stage is a stage in which the hot-rolled steel sheet obtained in the hot rolling stage is subjected to cold rolling as required. In cases where the cold rolling stage is carried out, it is preferable that the cold rolling be carried out under the conditions described below in the present invention. The reduction percentage for cold rolling is not particularly limited; however, if the reduction percentage is less than 20%, the flatness of the steel sheet surface is degraded, and the resulting structure may be non-uniform. Therefore, it is preferable that the reduction percentage be greater than or equal to 20%. More preferably, the reduction percentage is greater than or equal to 30%. Even more preferably, the reduction percentage is greater than or equal to 40%. On the other hand, if the reduction percentage is greater than 90%, unrecrystallized ferrite may remain. Therefore, it is preferable that the reduction percentage be less than or equal to 90%. More preferably, the reduction percentage is less than or equal to 80%. Even more preferably, the reduction percentage is less than or equal to 70%. Note that in the present invention, the cold rolling step is not an essential step; the cold rolling step can be omitted provided that the microstructure and mechanical properties of the present invention described above can be achieved. Annealing Stage The annealing stage is a stage in which annealing is carried out on the hot-rolled steel sheet obtained from the hot rolling stage described above or on the cold-rolled steel sheet obtained from the cold rolling stage described above. In the present invention, the annealing stage is carried out under the conditions described below. The annealing stage is a stage in which the hot-rolled steel sheet obtained or the cold-rolled steel sheet obtained is held at an annealing temperature of Aci or higher and Acs or lower for 30 seconds or more; subsequently, the resulting steel sheet is cooled under conditions in which an average cooling rate in an interval from the annealing temperature to 350 °C is 5 °C / second or higher, and a cooling stop temperature is 350 °C or lower;and subsequently, the resulting steel sheet is held under conditions in which a holding time for a temperature range from 350 °C to 300 °C is 50 seconds or less, and a holding time for a temperature range from less than 300 °C to a temperature T1 (°C) is 1000 seconds or less, where the temperature T1 (°C) is a selectable temperature within a temperature range of 200 to 250 °C. Hot-rolled or cold-rolled steel sheet is heated to an annealing temperature of Aci or higher and Acs or lower and then held within the temperature range. If the annealing temperature is lower than the Aci temperature, an excessive amount of cementite forms, and consequently, the resulting area fraction of martensite is less than 10%. Therefore, it is specified that the annealing temperature be greater than or equal to the Aci temperature. Preferably, the annealing temperature is greater than or equal to (the Aci temperature + 10 °C). On the other hand, if the annealing temperature is higher than the Acs temperature, the resulting area fraction of martensite is greater than 50%, and the resulting average grain diameter of the martensite is greater than or equal to 3.0 pm, which increases the creep ratio.Furthermore, when the martensite area fraction is large, the carbon concentration of the martensite with an aspect ratio of 3 or less is reduced, which lowers the martensite's strength; consequently, the creep ratio increases. Therefore, the annealing temperature is specified to be less than or equal to the Acs temperature. Preferably, the annealing temperature is less than or equal to (the Acs temperature - 10°C). Please note that the Aci temperature and the Acá temperature referred to herein are calculated using the following equations. Aci (°C) = 723 + 22(%Si) - 18(%Mn) + 17(%Cr) + 4.5(%Mo) + 16(%V) ΛβηΛηη / ιζηζ / Β / γι Ac3(°C) = 910 - 203(%C)1 / 2+ 45(%Si) -30(%Mn) - 20(%Cu) - 15(%N¡) + 11 (%Cr) + 32(%Mo) + 104(%V) + 400(%T¡) + 460(%AI) In each of the equations, “(% chemical symbol)” represents a content (% by mass) of the chemical symbol in the steel, and when it is not present, the content is 0. It is specified that the retention time associated with the annealing temperature (annealing retention time) be greater than or equal to 30 seconds. If the annealing retention time is less than 30 seconds, ferrite recrystallization does not develop sufficiently; consequently, the ferrite is unrecrystallized ferrite, which increases the creep ratio. Furthermore, carbon diffusion is not promoted; consequently, the carbon concentration of martensite with an aspect ratio of 3 or less is low, which increases the creep ratio. Therefore, it is specified that the annealing retention time be greater than or equal to 30 seconds. Preferably, the annealing retention time is greater than or equal to 35 seconds. More preferably, the annealing retention time is greater than or equal to 50 seconds. The upper limit of the annealing retention time is not particularly restricted.From the standpoint of inhibiting the oiling of austenite grain diameters, thereby preventing an increase in the creep ratio that can be caused by a coarse martensite grain diameter, it is preferable that the annealing holding time be less than or equal to 900 seconds. More preferably, the annealing holding time is less than or equal to 500 seconds, and even more preferably, less than or equal to 300 seconds. After being held at the annealing temperature, the hot-rolled or cold-rolled steel sheet is cooled under conditions where the average cooling rate over the range from the annealing temperature to 350 °C is 5 °C / second or greater, and the stop-cooling temperature is 350 °C or lower. If the stop-cooling temperature is above 350 °C, balnite and / or pearlite form at a later stage, increasing the creep ratio. Therefore, the stop-cooling temperature is specified to be less than or equal to 350 °C. Preferably, the stop-cooling temperature is less than or equal to 320 °C. More preferably, the stop-cooling temperature is less than or equal to 300 °C. If the average cooling rate in the range from the annealing temperature to 350 °C is less than 5 °C / second, large amounts of balnite and / or pearlite form, increasing the creep ratio. Therefore, it is specified that the average cooling rate be greater than or equal to 5 °C / second. Preferably, the average cooling rate is greater than or equal to 7 °C / second, and more preferably, greater than or equal to 10 °C / second. The upper limit of the average cooling rate is not particularly restricted. Preferably, the upper limit is 40 °C / second or less. More preferably, the average cooling rate is less than or equal to 30 °C / second. Note that in cases where the cooling stop temperature is below 350 °C, the average cooling rate in the range from below 350 °C to the cooling stop temperature is not particularly limited. In such cases, from the standpoint of inhibiting pearlite and / or balnite formation, thereby achieving a good creep ratio, it is preferable for the average cooling rate to be greater than or equal to 5 °C / second and less than or equal to 40 °C / second. «Rn«nn / iznz / e / Yi Subsequently, the hot-rolled or cold-rolled steel sheet is held under the following conditions. First, the hot-rolled or cold-rolled steel sheet is held under conditions where the holding time for the temperature range from 350°C to 300°C is less than or equal to 50 seconds. In the temperature range from 350°C to 300°C, pearlite and / or bainite form, which reduces the martensite having an aspect ratio of 3 or less; consequently, the strength is reduced, and therefore the yield ratio is increased. Therefore, achieving the yield ratio sought by the present invention requires that the holding time for the temperature range be short. If the holding time for the temperature range from 350°C to 300°C is greater than 50 seconds, pearlite and / or bainite form.For these reasons, it is specified that the retention time for the temperature range from 350 °C to 300 °C be less than or equal to 50 seconds. Preferably, the retention time for the temperature range is less than or equal to 45 seconds, and more preferably, less than or equal to 40 seconds. The lower limit of the retention time for the temperature range is not particularly restricted and may be 0 seconds. Preferably, the retention time for the temperature range is greater than or equal to 5 seconds, and more preferably, greater than or equal to 8 seconds. Subsequently, the resulting steel sheet is held under conditions where the holding time for the temperature range from below 300 °C to temperature T1 (°C) is less than or equal to 1000 seconds. At temperatures below 300 °C, pearlite or bainite does not readily form; however, if the holding time is prolonged, bainite forms, reducing the martensite with an aspect ratio of 3 or less and consequently increasing the creep ratio. Furthermore, one reason why temperature T1 (°C) is selected within the 200–250 °C temperature range is that the temperature range at which bainite forms varies depending on the conditions for the annealing stage, including the annealing temperature, cooling rate, cooling stop temperature, and holding time for the temperature range from 350 °C to 300 °C.Therefore, it is specified that the retention time for the temperature range from less than 300 °C to temperature T1 (°C) is less than or equal to 1000 seconds. Preferably, the retention time is less than or equal to 900 seconds, and more preferably, less than or equal to 800 seconds. The lower limit is not particularly restricted and may be 0 seconds. The retention time for the temperature range is preferably greater than or equal to 10 seconds, and more preferably greater than or equal to 50 seconds. Note that in the present invention, the hot-rolled steel sheet that has undergone the hot rolling stage can be further subjected to heat treatment to soften the structure, before being cold-rolled, and / or the hot-rolled steel sheet that has undergone the hot rolling stage or the cold-rolled steel sheet that has undergone the cold rolling stage can be tempered to adjust a shape, after the annealing stage. Furthermore, a coating process can be carried out after the annealing stage as long as the properties of the steel sheet are not altered. In cases where a steel sheet with a coating is produced, the following process can be used: After the steel sheet is held at a temperature between less than 300 °C and T1 (°C) for 1000 seconds or less in the annealing stage described above, the steel sheet, before cooling, is heated to a temperature between 400 °C or higher and 500 °C or lower, and then a coating process is carried out. An alloying process can also be performed on the sheet after the coating process. In cases where an alloying process is carried out, the steel sheet is heated to a temperature between 500 °C and 600 °C or lower, for example, and then the alloying process is performed. An electrogalvanizing process can be carried out after cooling. For example, in cases where a hot-dip galvanizing process is carried out on annealed steel sheet (hot-rolled steel sheet or cold-rolled steel sheet), it is preferable that the hot-dip galvanizing process be carried out by immersing the steel sheet in a galvanizing bath having a temperature of 420 °C or higher and 500 °C or lower, and subsequently, the coating weight is adjusted by gas purging or similar. Furthermore, in cases where an alloying process is carried out on the galvanized coating after the hot-dip galvanizing process, it is preferable that the alloying process be carried out within a temperature range of 500 °C or higher and 600 °C or lower. In cases where electrogalvanizing is performed on annealed steel sheet (hot-rolled or cold-rolled), the process involves immersing the steel sheet in a galvanizing bath or a zinc-nickel bath, adjusted to a pH of 1 to 3 at room temperature, and then applying a current. In this case, it is preferable to adjust the coating weight by regulating the current, the duration of electrolysis, and / or similar factors. In the production method described above of the present invention, the annealing temperature, the cooling stop temperature, the holding temperature, and the holding time of the annealing stage are controlled. Consequently, the martensite grain diameter, aspect ratio, and carbon concentration of the resulting high-strength steel sheet are controlled, thus enabling the production of a high-strength steel sheet with a low yield strength. Furthermore, the high-strength steel sheet with a low yield strength of the present invention has excellent surface properties and is therefore suitable for use in automotive structural members. EXAMPLES [Example 1] The present invention will be described in detail with reference to examples. Note that the present invention is not limited to the examples described below. 1. Production of Steel Sheets for Evaluation Steels with the chemical composition shown in Table 1, the remainder being Fe and incidental impurities, were produced in a vacuum melting furnace. These steels were then rough-ground into blocks 27 mm thick. The resulting blocks were hot-rolled to a sheet thickness of 4.0 mm under the conditions shown in Table 1. RRnRnn / iznz / e / Yi 2-1 to Table 2-3, and in this way, hot-rolled steel sheets were produced. Note that the percentage reduction for the rolling finish was within a range of 80 to 90% for all conditions. Next, some of the hot-rolled steel sheets obtained were cold-rolled. The samples that would be subjected to cold rolling were those obtained by grinding the hot-rolled steel sheets to a sheet thickness of 3.2 mm. Subsequently, the samples were cold-rolled to a sheet thickness of 2.24 to 0.8 mm under the conditions shown in Table 2-1 to Table 2-3, and in this way, cold-rolled steel sheets were produced.Next, each of the hot-rolled and cold-rolled steel sheets obtained as described above was annealed under the conditions shown in Tables 2-1 through 2-3, and in this way, steel sheets were produced. Note that the blanks in Table 1 (spaces with in Table 1) indicate that there was no intentional addition of the element; that is, the element may have been incorporated incidentally, and thus the content may not have been 0% by mass. Table 1 RRnRnn / iznz / e / Y Type of maple Chemical composition (% mass) Temperature AC1 (°C) Temperature AC3 ('C) C Si Mn PS Al N Cr Mo V Nb Ti Cu Ni BA 0.090 0.50 1.80 0.007 0.0008 0.05 0.0021 - - - - - - - - 702 841 B 0.062 0.48 1.84 0.007 0.0009 0.05 0.0022 - - - - - - - - 700 848 C 0.067 0.49 1.82 0.008 0.0008 0.05 0.0023 - - - - - - - - 701 848 D 0.118 0.51 1.74 0.007 0.0008 0.05 0.0022 - - - - - - - - 703 834 E 0.112 0.50 1.84 0.007 0.0007 0.05 0.0021 - - - - - - - - 701 833 F 0.088 0.67 1.76 0.007 0.0008 0.05 0.0021 - - - - - - - - 706 851 G 0.093 0.32 1.80 0.006 0.0008 0.04 0.0031 - - - - - - - - 698 827 H 0.093 0.43 1.62 0.009 0.0045 0.04 0.0026 - - - - - - - - 703 837 I 0.102 0.51 1.66 0.007 0.0008 0.05 0.0024 - - - - - - - - 704 842 J 0.087 0.50 2.13 0.006 0.0009 0.19 0.0033 - - - - - - - - 696 896 K 0.083 0.45 2.18 0.007 0.0008 0.04 0.0029 - - - - - - - - 694 825 L 0.089 0.42 1.88 0.007 0.0008 0.03 0.0025 - - - - - - - - 698 826 M 0.092 0.56 1.81 0.006 0.0008 0.05 0.0023 0.04 - - - - - - - 703 843 N 0.095 0.51 1.76 0.007 0.0009 0.04 0.0030 0.12 - - - - - - - 705 837 0 0.086 0.53 1.88 0.005 0.0007 0.05 0.0022 0.19 - - - - - - - 704 843 P 0.091 0.55 1.84 0.007 0.0009 0.06 0.0033 - - 0.005 - - - - - 702 846 Q 0.096 0.50 1.74 0.006 0.0008 0.05 0.0021 - - 0.020 - - - - - 703 843 R 0.090 0.48 1.88 0.007 0.0006 0.04 0.0025 - - 0.040 - - - - - 700 837 S 0.092 0.52 1.79 0.006 0.0007 0.05 0.0019 0.03 0.02 0.004 - - - - - 703 843 T 0.086 0.49 1.76 0.007 0.0009 0.18 0.0034 - - - 0.006 0.004 - - - 702 904 u 0.088 0.46 1.87 0.009 0.0046 0.06 0.0024 - - - - - 0.02 0.009 - 699 842 V 0.090 0.50 1.83 0.007 0.0008 0.04 0.0027 - - - - - - - 0.0006 701 835 w 0.140 0.56 1.76 0.006 0.0008 0.05 0.0024 - - - - - - - - 704 830 X 0.053 0.48 1.88 0.007 0.0005 0.04 0.0023 - - - - - - - - 700 847 Y 0.088 0.86 1.92 0.006 0.0009 0.05 0.0033 - - - - - - - - 707 854 z 0.091 0.46 2.40 0.007 0.0008 0.03 0.0025 - - - - - - - - 690 811 AA 0.093 0.51 1.49 0.006 0.0008 0.05 0.0021 - - - - - - - - 707 850 AB 0.108 0.51 1.72 0.070 0.0008 0.05 0.0023 - - - - - - - - 703 838 AC 0.095 0.51 1.81 0.007 0.0080 0.05 0.0019 - - - - - - - - 702 840 AD 0.102 0.51 1.71 0.007 0.0008 0.25 0.0021 - - - - - - - - 703 932 AE 0.095 0.51 1.76 0.007 0.0009 0.04 0.0150 - - - - - - - - 703 836 AF 0.083 0.45 1.88 0.007 0.0008 0.04 0.0029 0.25 - - - - - - - 703 837 AG 0.090 0.48 1.88 0.007 0.0006 0.04 0.0025 - - 0.060 - - - - - 701 839. Table 2-1 No. Steel Type Hot Rolled Cold Rolled Annealing Conditions Notes Block Heating Temperature Finisher Exit Temperature Rolling Temperature Percentage Reduction Annealing Temperature Annealing Hold Time Average Cooling Rate *1 Cooling Stop Temperature Hold Time for Range from 350 to 300 °C Hold Time for Range from Less than 300 °C to Temperature T1 °C °C °C % °Cs °C / s °Css 1 A 1250 880 650 75 800 35 15 320 40 300 Example of the Invention 2 1250 880 650 75 800 35 15 320 30 300 Example of the Invention 3 1250 880 650 75 800 35 15 320 10 300 Example of the Invention 4 1250 880 650 75 800 35 15 320 60 300 Comparative example 5 1250 880 650 75 800 35 15 320 40 300 Example of the invention 6 1250 880 650 75 800 35 15 320 40 500 Example of the invention 7 1250 880 650 75 800 35 15 320 40 700 Example of the invention 8 1250 880 650 75 800 35 15 320 40 0 ​​Example of theInvention 9 1250 880 650 75 720 300 25 260 20 400 Example of the invention 10 1250 880 650 75 710 100 25 260 10 100 Example of the invention 11 B 1280 880 650 56 650 35 15 320 40 300 Comparative example 12 1280 880 650 56 720 35 15 320 40 300 Example of the invention 13 1280 880 650 56 800 35 15 320 40 300 Example of the invention 14 1280 880 650 56 850 35 15 320 40 300 Comparative Example 15 C 1250 860 650 60 800 150 15 320 40 300 Example of the Invention 16 1250 860 650 60 800 300 15 320 40 300 Example of the Invention 17 1250 860 650 60 800 60 15 320 40 300 Example of the Invention 18 1250 860 650 60 800 20 15 320 40 300 Comparative Example 19 D 1300 880 650 56 800 35 15 320 40 300 Example of the Invention 20 1300 880 650 56 800 35 10 320 40 300 Example of the invention 21 1300 880 650 56 800 35 7 320 40 300 Example of the invention 22 1300 880 650 56 800 35 3 320 40 300 Comparative example 23 E 1250 880 670 40 800 35 15 200 0 300 Example of the invention 24 1250 880 670 30 800 35 15 250 0 300 ExampleExample of the invention 25 1250 880 670 50 800 35 15 320 40 300 Example of the invention 26 1250 880 670 60 800 35 15 380 40 300 Comparative example 27 F 1250 880 650 75 800 35 15 320 70 300 Comparative example 28 1250 880 650 75 800 35 15 320 40 300 Example of the invention 29 1250 880 650 75 800 35 15 320 30 300 Example of the invention 30 1250 880 650 75 800 35 15 320 10 300 Example of the invention 31 G 1250 900 650 75 800 35 15 320 70 200 Comparative example 32 1250 900 650 75 800 35 15 320 40 300 Example of the invention 33 1250 900 650 75 800 35 15 320 30 100 Example of the invention 34 1250 900 650 75 800 35 15 320 60 100 Comparative example 35 H 1250 880 600 75 650 35 15 320 40 300 Comparative example 36 1250 880 600 75 750 35 15 320 40 300 Example of the invention 37 1250 880 600 75 800 35 15 320 40 300 Example of the invention 38 1250 880 600 75 850 35 15 320 40 300 Comparative example 39 I 1350 880 650 75 800 28 15 320 40 300 Comparative example 40 1350 880 650 75 800 80 15 320 40 300 Example of theInvention 41 1350 880 650 75 800 200 15 320 40 300 Example of the invention 42 1350 880 650 75 800 280 15 320 40 300 Example of the invention *1: The average cooling rate (°C / s) is an average cooling rate over a range from the annealing temperature to 350 °C. *2: The temperature T1 (°C) is a selectable temperature (°C) within a temperature range of 200 to 250 °C. RRnRnn / rznz / e / Y Table 2-2 No. Steel Type Hot Rolled Cold Rolled Annealing Conditions Notes Block Heating Temperature Finisher's Exit Temperature Rolling Temperature Percentage Reduction Annealing Temperature Annealing Hold Time Average Cooling Rate *1 Cooling Stop Temperature Hold Time for Interval from 350 to 300 °C Hold Time for Interval from Less than 300 °C to Temperature T1 *2 °C °C °C % CC s ° C / s °C ss 43 J 1250 880 620 70 800 35 30 320 40 300 Example of the Invention 44 1250 880 620 70 800 35 20 320 40 300 Example of the Invention 45 1250 880 620 70 800 35 5 320 40 300 Example of Invention 46 1250 880 620 70 800 35 2 320 40 300 Comparative example 47 K 1250 880 650 70 800 35 15 320 40 300 Example of the invention 48 1250 880 650 70 800 35 15 300 40 300 Example of the invention 49 1250 880 650 70 800 35 15 340 40 300 Example of the invention 50 1250 880 650 70 800 35 15 370 40 300Comparative Example 51 L 1250 880 650 70 680 35 15 320 40 300 Comparative Example 52 1250 880 650 70 740 35 15 320 40 300 Example of the Invention 53 1250 880 650 70 800 35 15 320 40 300 Example of the Invention 54 1250 880 650 70 860 35 15 320 40 300 Comparative Example 55 1250 880 650 70 800 35 15 270 0 300 Example of the Invention 56 1250 880 650 70 800 35 15 320 40 300 Example of the invention 57 1250 880 650 70 800 35 15 370 40 300 Comparative example 58 1250 880 650 70 800 35 15 400 40 300 Comparative example 59 M 1250 920 650 70 800 35 15 320 40 300 Example of the invention 60 1250 920 650 70 800 35 15 320 20 300 Example of the invention 61 1250 920 650 70 800 35 15 320 60 300 Comparative example 62 1250 920 650 70 800 35 15 320 80 300 Comparative Example 63 N 1250 880 650 70 800 35 15 320 40 300 Example of the Invention 64 1250 880 650 70 800 35 15 320 40 500 Example of the Invention 65 1250 880 650 70 800 35 15 320 40 200 Example of the Invention 66 1250 880 650 70 800 35 15 320 40 100Example of invention 67 O 1250 880 570 70 710 35 15 320 40 300 Example of invention 68 1250 880 570 70 740 35 15 320 40 300 Example of invention 69 1250 880 570 70 800 35 15 320 40 300 Example of invention 70 1250 880 570 70 840 35 15 320 40 300 Example of invention 71 P 1250 880 650 65 800 35 15 370 40 300 Comparative example 72 1250 880 650 65 Example of the invention 73 1250 880 650 65 800 35 15 320 40 300 Example of the invention 74 1250 880 650 65 800 35 15 280 0 300 Example of the invention 75 O 1250 880 650 65 800 35 15 320 50 300 Example of the invention 76 1250 880 650 65 800 35 15 320 40 300 Example of the invention 77 1250 880 650 65 800 35 15 320 30 300 Example of the invention 78 1250 880 650 65 800 35 15 320 60 300 Comparative example 79 R 1250 880 650 65 800 35 15 320 40 300 Example of the invention 80 1250 880 650 65 800 35 15 320 40 0 ​​Example of the invention 81 1250 880 650 65 800 35 15 320 40 230 Example of the invention 82 1250 880 650 65800 35 15 320 40 250 Example of the invention *1: The average cooling rate (°C / s) is an average cooling rate over an interval from the annealing temperature to 350 °C. *2: The temperature T1 (°C) is a selectable temperature (°C) within a temperature range of 200 to 250 °C. ΛβΠΛηη / ιζηζ / Β / γ Table 2-3 No. Steel Type Hot Rolled Cold Rolled Annealing Conditions Notes Block Heating Temperature Finisher Exit Temperature Rolling Temperature Percentage Reduction Annealing Temperature Annealing Hold Time Average Cooling Rate *1 Cooling Stop Temperature Hold Time for Interval from 350 to 300 °C Hold Time for Interval from Less than 300 °C to Temperature T1 *2 °C °C °C % °C s °C / s °css 83 S 1200 880 650 56 800 35 15 320 40 300 Example of the Invention 84 1220 880 650 56 800 35 15 320 40 300 Example of the Invention 85 1240 880 650 56 800 35 15 320 40 300 Example Example of invention 86 1250 880 650 56 800 35 15 320 40 300 Example of invention 87 T 1250 840 650 56 800 35 15 320 40 300 Example of invention 88 1250 850 650 56 800 35 15 320 40 300 Example of invention 89 1250 880 650 56 800 35 15 320 40 300 Example of invention 90 1250 920 650 56 800 35 15 320 40300 Example of the invention 91 u 1250 880 700 56 800 35 15 320 40 300 Example of the invention 92 1250 880 650 56 800 35 15 320 40 300 Example of the invention 93 1250 880 600 56 800 35 15 320 40 300 Example of the invention 94 1250 880 550 56 800 35 15 320 40 300 Example of the invention 95 V 1250 880 650 - 800 35 15 320 10 300 Example of the invention 96 1250 880 650 - 750 200 15 320 40 100 Example of the invention 97 1250 880 650 - 800 35 15 260 40 300 Example of the invention 98 1250 880 650 - 800 35 25 320 40 400 Example of the invention 99 w 1250 880 650 56 800 35 15 320 40 300 Comparative example 100 X 1250 880 650 56 800 35 15 320 40 300 Comparative example 101 Y 1250 880 650 56 800 35 15 320 40 300 Comparative example 102 z 1250 880 650 56 800 35 15 320 40 300 Comparative Example 103 AA 1250 880 650 56 800 35 15 320 40 300 Comparative Example 104 AB 1250 880 650 56 800 35 15 320 40 300 Comparative Example 105 AC 1250 880 650 56 800 35 15 320 40 300 Comparative Example 106 AD 1250 880650 56 800 35 15 320 40 300 Comparative Example 107 AE 1250 880 650 56 800 35 15 320 40 300 Comparative Example 108 AF 1250 880 650 56 800 35 15 320 40 300 Comparative Example 109 AG 1250 880 650 56 800 35 15 320 40 300 Comparative Example RRnRnn / iznz / e / Y *1: The average cooling rate (°C / s) is an average cooling rate over a range from the annealing temperature to 350 °C. *2: The temperature T1 (°C) is a selectable temperature (°C) within a temperature range of 200 to 250 °C. 2. Evaluation Methods Steel sheets produced under different manufacturing conditions were subjected to microstructural analysis, which investigated the component fractions, and to a tensile test, which evaluated mechanical properties such as tensile strength. The investigation of component fractions and the evaluations were carried out in the following ways. <Fracciones de Área de Ferrita y Martensita> Ferrite and martensite were examined as follows: a specimen was cut from each of the steel sheets, along one rolling direction and one direction perpendicular to the rolling direction. A cross-section of sheet thickness L from this section, parallel to the rolling direction, was mirror-polished and chemically etched with a denital solution to reveal the microstructure, which was then examined using a scanning electron microscope (SEM). In an SEM image at 1500x magnification, a 16 × 15 grid with a spacing of 4.8 pm was placed over a region of 82 pm × 57 pm (actual lengths), and the ferrite and martensite area fractions were investigated (measured) using a dot-counting method, in which the number of dots in each phase is counted.The area fractions were each an average of three area fractions determined from separate SEM images at 1500x magnification. Martensite was one component that appeared white, and ferrite was one component that appeared black. Note that the microstructure of the steel sheets of the present invention is uniform in one sheet thickness direction across the sheet thickness positions, excluding a region extending 10 pm from a surface layer in the sheet thickness direction. Therefore, with respect to the sheet thickness measurement positions, measurements can be taken at any position within the region where the microstructure is uniform. In the present invention, the microstructure was examined at a position 1 / 4 of the sheet thickness in the sheet thickness direction. <Diámetro de Grano Promedio de Martensita y Relación de Aspecto de Martensita> The average grain diameter of the martensite and its aspect ratio were examined as follows: a specimen was cut from each of the steel sheets, along the rolling direction and perpendicular to the rolling direction. A cross-section of sheet thickness L from this section, parallel to the rolling direction, was mirror-polished and chemically etched with a nital solution to reveal the microstructure, which was then examined using a scanning electron microscope. All major and minor dimensions of the martensite within a SEM image were measured at 1500x magnification, and the average of these measurements was calculated and designated as the average grain diameter of the martensite. The aspect ratio of the martensite was calculated by dividing the measured major dimension by the measured minor dimension. Note that the microstructure of the steel sheets of the present invention is uniform in the sheet thickness direction across the sheet thickness positions, excluding a region extending 10 pm from a surface layer in the sheet thickness direction. Therefore, with respect to the sheet thickness measurement positions, measurements can be taken at any position within the region where the microstructure is uniform. In the present invention, the microstructure was examined at a position 1 / 4 of the sheet thickness in the sheet thickness direction. <Concentración de Carbono de Martensita Que Tiene Relación de Aspecto de 3 o Menos> The carbon concentration of the martensite was measured by X-ray diffraction analysis as follows: after each of the steel sheets was ground to a position of 1 / 4 sheet thickness, a specimen was cut, and a cross-section of sheet thickness L thereof, parallel to the rolling direction, was mirror polished and used. The X-rays used were Co-Ka radiation. In the present invention, a 22.5 pm x 22.5 pm region was measured for three fields of view using an electron probe microanalyzer (ERMA) under conditions including an accelerating voltage of 7 kV and a distance between measurement points of 80 nm, and the measured data were converted to a C concentration using a standard curve method.The SEM images obtained simultaneously, which were obtained with an immersion lens detector, were used for a comparison to distinguish the types of martensite, and an average of the martensite carbon concentrations that. RRnRnn / iznz / e / Yi had an aspect ratio of 3 or less within the measurement field of view. It was calculated for three fields of view, and the values ​​were averaged to perform the calculation. Note that the microstructure of the steel sheets of the present invention is uniform in the sheet thickness direction across the sheet thickness positions, excluding a region extending 10 pm from a surface layer in the sheet thickness direction. Therefore, with respect to the sheet thickness measurement positions, measurements can be taken at any position within the region where the microstructure is uniform. In the present invention, the microstructure was examined at a position 1 / 4 of the sheet thickness in the sheet thickness direction. <Fracciones de Área de Componentes Restantes> The remaining components described above were examined as follows: a specimen was cut from each of the steel sheets, along the rolling direction and perpendicular to the rolling direction, and a cross-section of sheet thickness L from the same, parallel to the rolling direction, was mirror-polished and chemically etched with a nital solution to reveal the microstructure, which was then examined using a scanning electron microscope. In a 1500x SEM image, a 16x15 grid with a spacing of 4.8 pm was placed in a region of 82 pm × 57 pm (actual lengths), and the area fractions of the remaining components were investigated (measured) using the dot-counting method, in which the number of dots found in each of the phases is counted.The area fractions were each an average of three area fractions determined from separate SEM images at 1500x magnification. Pearlite is a component containing ferrite and cementite precipitated within it in a lamellar form; bainite is a component containing ferrite and cementite precipitated within it in a globular form; and retained austenite is a component that appears black. Note that the microstructure of the steel sheets of the present invention is uniform in the sheet thickness direction across the sheet thickness positions, excluding a region extending 10 pm from a surface layer in the sheet thickness direction. Therefore, with respect to the sheet thickness measurement positions, measurements can be taken at any position within the region where the microstructure is uniform. In the present invention, the microstructure was examined at a position 1 / 4 of the sheet thickness in the sheet thickness direction. <Propiedades Mecánicas> A JIS No. 5 specimen with a gauge length of 50 mm, a gauge width of 25 mm, and a sheet thickness of 1.4 mm was cut from each of the steel sheets along the rolling direction, and a tensile test was performed at a crosshead speed of 10 mm / minute. Using each specimen, the tensile strength (referred to as “TS” in Tables 3-1 through 3-3) and the yield strength (referred to as “YS” in Tables 3-1 through 3-3) were measured. The yield ratio (referred to as “YR” in Tables 3-1 through 3-3) was calculated by dividing YS by TS. <Convertibil¡dad Química> Each of the steel sheets was degreased with a commercially available alkaline degreasing agent, the steel sheet was then immersed in a modifying agent The surface was coated with RRnRnn / iznz / e / Yi, and subsequently, a chemical conversion was carried out in which the steel sheet was immersed in a phosphating agent (PALBOND PB-L3080, manufactured by Nihon Parkerizing Co., Ltd.) under conditions that included a bath temperature of 40 °C and a process time of 120 seconds. The coverage rate of the conversion crystals was calculated by visually examining the surface of the steel sheet that had undergone the chemical conversion. Cases where the coverage rate of the conversion crystals, which was a rate in terms of a fraction of area, was 95% or more were indicated with the symbol “O”, cases where the coverage rate was 90% or more and less than 95% were indicated with the symbol “Δ”, and cases where the coverage rate was less than 90% were indicated with the symbol “χ”.It was determined that cases with the symbol “O” or “Δ” represented cases in which uniform conversion crystals were formed, and therefore, good chemical convertibility was exhibited. 3. Evaluation Results The results of the investigations and evaluations described above are shown in Table 3-1 to Table 3-3. Table 3-1 No. Steel Type Microstructure Mechanical Properties Notes Ferrite Martensite Average Martensite Grain Diameter Martensite Having An Aspect Ratio of 3 or Less / Full Martensite Concentration of Martensite Having An Aspect Ratio of 3 or Less Remaining Components YS TS YR Chemical Convertibility % Area % Area pm % % by Mass % Area MPa MPa 1 A 66 30 2.4 70 0.43 4 378 638 0.59 O Example of the Invention 2 60 35 2.3 75 0.35 5 360 642 0.56 O Example of the Invention 3 63 35 2.9 75 0.42 2 351 640 0.55 O Example of the Invention 4 67 25 3.0 55 0.56 8 410 641 0.64 or Comparative Example 5 66 30 2.9 70 0.44 4 346 638 0.54 or Example of the Invention 6 55 40 2.5 70 0.37 5 362 642 0.56 or Example of the Invention 7 53 40 2.4 80 0.42 7 331 639 0.52 or Example of the Invention 8 53 45 2.3 60 0.51 2 390 640 0.61 or Example of the Invention 9 88 12 1.8 85 0.88 0 303 621 0.49 or Example of the Invention 10 70 30 2.1 100 0.83 0 326 638 0.51 or Example of the invention 11 B 88 8 2.4 95 1.00 4 360 563 0.64 or Comparative Example 12 80 15 2.7 70 0.66 5 377 617 0.61 or Example of the invention 13 61 35 3.0 70 0.49 4 335 614 0.55 or Example of the invention 14 40 55 3.3 65 0.21 5 395 616 0.64 or Comparative Example 15 c 68 30 2.2 70 0.45 2 349 618 0.56 or Example of the invention 16 64 35 2.8 70 0.33 1 380 622 0.61 or Example of the invention 17 66 30 2.4 70 0.40 4 354 620 0.57 or Example of the invention 18 63 30 2.0 50 0.23 7 405 621 0.65 or Comparative example 19 D 60 35 2.4 70 0.46 5 376 678 0.55 or Example of the invention 20 56 40 2.6 70 0.45 4 360 682 0.53 or Example of the invention 21 58 35 2.7 60 0.38 7 370 680 0.54 or Example of the invention 22 57 35 2.0 55 0.43 8 445 681 0.65 or Comparative example 23 E 63 35 2.4 70 0.43 2 349 693 0.50 or Example of the invention 24 64 35 3.0 70 0.34 1 361 697 0.52 or Example of the invention 25 55 40 2.6 70 0.41 5 338 694 0.49 or Example of the invention 26 56 40 2.7 50 0.42 4 465 695 0.67 or Comparative example 27 F 63 30 2.9 50 0.38 7 407 618 0.66 or Comparative Example 28 61 35 2.6 70 0.43 4 358 622 0.58 or Example of the invention 29 60 35 2.6 80 0.40 5 334 619 0.54 or Example of the invention 30 63 35 2.7 85 0.34 2 360 620 0.58 or Example of the invention 31 G 58 35 2.4 55 0.45 7 411 643 0.64 or Comparative Example 32 66 30 2.0 70 0.45 4 366 647 0.57 or Example of the invention 33 63 35 2.0 65 0.38 2 357 644 0.55 or Example of the invention 34 57 35 2.4 40 0.42 8 426 645 0.66 or Comparative example 35 H 88 7 1.8 70 0.96 5 360 518 0.69 or Comparative example 36 83 13 2.2 70 0.84 4 334 602 0.55 or Example of the invention 37 60 35 2.4 70 0.46 5 373 599 0.62 or Example of the invention 38 41 55 3.4 70 0.22 4 385 600 0.64 or Comparative example 39 I 63 30 2.0 55 0.25 7 410 618 0.66 or Comparative Example 40 65 30 2.9 65 0.34 5 368 622 0.59 or Example of the invention 41 63 35 2.4 70 0.40 2 363 619 0.59 or Example of the invention 42 74 25 2.7 75 0.45 1 333 620 0.54 or Example of the invention. *I: The remaining components are one or more of retained austenite, bainite, and pearlite. RRnRnn / iznz / e / Y Table 3-2 No. Steel Type Microstructure Mechanical Properties Notes Ferrite Martensite Average Martensite Grain Diameter Martensite Having An Aspect Ratio of 3 or Less / Full Martensite C Concentration of Martensite Having An Aspect Ratio of 3 or Less Remaining Components '1 YS TS YR Chemical Convertibility % Area % Area % % by Mass % Area MPa MPa 43 J 68 30 2.4 70 0.47 2 333 668 0.50 Δ Example of the Invention 44 64 35 2.4 70 0.34 1 332 672 0.49 Δ Example of the Invention 45 53 40 2.2 65 0.48 7 359 669 0.54 Δ Example of the Invention 46 52 40 2.4 50 0.44 8 435 671 0.65 Δ Comparative example 47 K 61 35 2.6 70 0.35 4 360 678 0.53 Δ Example of the invention 48 55 40 2.1 75 0.45 5 346 682 0.51 Δ Example of the invention 49 56 40 2.0 65 0.48 4 364 679 0.54 Δ Example of the invention 50 60 35 3.0 55 0.40 5 435 680 0.64 Δ Comparative example 51 L 86 8 2.0 80 0.98 6 366 568 0.64 O Comparative example 52 81 15 2.2 70 0.76 4 345 662 0.52 O Example of the invention 53 55 40 2.4 70 0.48 5 363 659 0.55 o Example of the invention 54 37 60 3.5 60 0.23 3 453 660 0.69 o Comparative example 55 68 30 2.8 70 0.46 2 375 658 0.57 o Example of the invention 56 66 30 2.2 70 0.44 4 346 662 0.52 o Example of the invention 57 65 30 2.9 55 0.45 5 423 659 0.64 o Comparative example 58 61 35 2.7 50 0.36 4 426 661 0.64 or Comparative Example 59 M 60 35 2.4 70 0.40 5 353 648 0.54 Δ Example of the invention 60 58 40 2.1 70 0.48 2 367 652 0.56 Δ Example of the invention 61 58 35 2.7 50 0.35 7 418 649 0.64 Δ Comparative Example 62 57 35 2.9 50 0.46 8 425 651 0.65 Δ Comparative Example 63 N 56 40 2.4 70 0.46 4 353 638 0.55 Δ Example of the invention 64 60 35 2.4 80 0.48 5 352 642 0.55 Δ Example of the invention 65 56 40 2.2 65 0.34 4 374 639 0.59 Δ Example of the invention 66 63 35 2.5 65 0.45 2 400 641 0.62 Δ Example of the invention 67 O 80 15 2.3 70 0.72 5 377 613 0.62 Δ Example of the invention 68 71 25 2.6 70 0.51 4 372 632 0.59 Δ Example of the invention 69 65 30 2.8 70 0.44 5 339 649 0.52 Δ Example of the invention 70 59 35 3.0 70 0.42 6 364 656 0.55 Δ Example of the invention 71 P 62 35 2.8 55 0.40 3 420 653 0.64 Δ Comparative example 72 51 45 2.5 65 0.35 4 330 657 0.50 Δ Example of the invention 73 60 35 2.1 70 0.48 5 380 654 0.58 Δ Example of the invention 74 58 40 2.0 75 0.47 2 346 656 0.53 Δ Example of the invention 75 0 56 40 3.0 65 0.47 4 380 633 0.60 Δ Example of the invention 76 55 40 2.0 70 0.35 5 353 638 0.55 Δ Example of the invention 77 51 45 2.3 70 0.46 4 361 634 0.57 Δ Example of the invention 78 53 40 2.5 50 0.43 7 413 636 0.65 Δ Comparative example 79 R 60 35 2.2 70 0.45 5 332 663 0.50 Δ Example of the invention 80 66 30 2.8 60 0.43 4 332 667 0.50 Δ Example of the invention 81 55 40 2.6 65 0.34 5 365 664 0.55 Δ Example of the invention 82 66 30 2.8 70 0.48 4 350 666 0.53 Δ Example of the invention. Ί: The remaining components are one or more of retained austenite, bainite, and pearlite. RRnRnn / iznz / e / Y Tabla 3-3 No. Steel Type Microstructure Mechanical Properties Notes Ferrite Martensite Average grain diameter of martensite Martensite having an aspect ratio of 3 or less / full martensite C concentration of martensite having an aspect ratio of 3 or less Remaining components Ί YS TS YR Chemical convertibility % area % area pm % % by mass % area MPa MPa 83 s 60 35 2.9 70 0.42 5 347 638 0.54 Δ Example of the invention 84 61 35 2.6 70 0.40 4 377 642 0.59 Δ Example of the invention 85 65 30 2.0 70 0.37 5 349 639 0.55 Δ Example of the invention 86 59 35 2.2 70 0.43 6 371 641 0.58 Δ Example of the invention 87 T 67 30 2.7 70 0.42 3 356 618 0.58 Δ Example of the invention 88 66 30 3.0 70 0.41 4 360 622 0.58 Δ Example of the invention 89 65 30 2.6 70 0.43 5 365 619 0.59 Δ Example of the invention 90 66 30 2.7 70 0.35 4 331 621 0.53 Δ Example of the invention 91 u 59 35 2.9 70 0.49 6 378 658 0.57 Δ Example of the invention 92 65 30 2.8 70 0.45 5 362 662 0.55 Δ Example of the invention 93 66 30 2.0 70 0.48 4 371 659 0.56 Δ Example of the invention 94 55 40 2.2 70 0.33 5 333 661 0.50 Δ Example of the invention 95 V 57 40 2.6 70 0.47 3 337 648 0.52 Λ Example of the invention 96 74 25 2.2 70 0.44 1 379 652 0.58 Δ Example of the invention 97 56 40 2.2 65 0.47 4 372 649 0.57 Δ Example of the invention 98 63 35 2.6 65 0.42 2 356 651 0.55 Δ Example of the invention 99 w 40 55 2.2 70 0.66 5 480 728 0.66 O Comparative example 100 X 61 35 2.1 70 0.43 4 361 582 0.62 O Comparative example 101 Y 55 40 2.1 70 0.48 5 445 668 0.67 X Comparative example 102 z 51 45 2.8 70 0.40 4 360 761 0.47 X Comparative example 103 AA 70 25 2.9 70 0.37 5 365 549 0.66 o Comparative example 104 AB 66 30 2.1 70 0.43 4 438 672 0.65 or Comparative Example 105 AC 59 35 2.1 70 0.45 6 420 648 0.65 or Comparative Example 106 AD 62 35 2.5 70 0.26 3 414 641 0.65 or Comparative Example 107 AE 65 30 2.4 70 0.37 5 427 639 0.67 or Comparative Example 108 AF 56 40 2.6 70 0.43 4 371 652 0.57 X Comparative example 109 AG 50 45 2.3 70 0.46 5 378 663 0.57 X Comparative example. *1: The remaining components are one or more of retained austenite, bainite, and pearlite. RRnRnn / iznz / e / Y In Example 1, steel sheets having a TS of 590 MPa or higher, a YR of 0.63 or less, and good chemical convertibility were considered “passed” and are listed as “Example of the Invention” in the “Notes” column in Tables 3-1 through 3-3. On the other hand, steel sheets having at least one TS of less than 590 MPa, a YR of more than 0.63, and low chemical convertibility were considered “failed” and are listed as “Comparative Example” in the “Notes” column in Tables 3-1 through 3-3. [Example 2] 1. Production of Steel Sheets for Evaluation Hot-rolled steel sheets produced by hot rolling and cold-rolled steel sheets produced by hot rolling followed by cold rolling were annealed under the conditions shown in Table 4. The steels rolled were those of Steel Type A, F, or Y shown in Table 1. The annealed steel sheets were then subjected to a galvanizing process, thus producing coated steel sheets. Note that the percentage reduction for the roll finish in hot rolling was within the range of 80 to 90% for all conditions. In Table 4, “Gl” indicates a galvanized steel sheet, “GA” indicates a galvanoannealed steel sheet, and “EG” indicates an electrogalvanized steel sheet. With regard to galvanized steel sheet, in carrying out the hot-dip galvanizing process on annealed steel sheet (hot-rolled steel sheet or cold-rolled steel sheet), the hot-dip galvanizing process was carried out by immersing the steel sheet in a galvanizing bath that had a temperature of 420 °C or higher and 500 °C or lower, and subsequently, the coating weight was adjusted by gas purging or similar. Furthermore, with regard to the galvano-annealed steel sheet, in carrying out the alloying process in the galvanized coating after the hot-dip galvanizing process, the alloying process was carried out within a temperature range of 500 °C or higher and 600 °C or lower. Furthermore, with regard to electrogalvanized steel sheet, in carrying out the electrogalvanizing process on annealed steel sheet (hot-rolled steel sheet or cold-rolled steel sheet), the electrogalvanizing process was carried out by immersing the steel sheet in a galvanizing bath or a zinc-nickel bath, which had been adjusted to a pH of 1 to 3 at room temperature, and then supplying a current. RRnRnn / iznz / e / Yi Table 4 No. Steel Type Hot Rolling River Rolling Annealing Conditions Coating Process Notes Block Heating Temperature Finisher Exit Temperature Rolling Temperature Percentage Reduction Annealing Temperature Annealing Holding Time Average Cooling Rate Cooling Stop Temperature Holding Time for Range from 350 to 300 °C Holding Time for Range from Below 300 °C to Temperature TΓ2 Process Temperature Coating Type °C °C °C % °C s °C ss °C 1 A 1250 880 650 75 800 35 15 310 40 300 550 GA Example of the Invention 2 1250 880 650 75 790 35 15 320 30 300 510 GA Example of the Invention 3 1250 880 650 75 800 35 15 320 10 300 570 GA Example of the invention 4 1250 880 650 75 770 35 15 320 60 300 530 GA Example with parameter 5 1250 880 650 75 800 35 15 320 40 200 460 Gl Example of the invention 6 1250 880 650 75 800 35 15 320 40 500 470 Gl Example of the invention 1 1250 880650 75 800 35 15 320 40 800 450 Gl Example of the invention 8 1250 880 650 75 800 35 15 320 40 0 ​​25 EG Example of the invention 9 F 1250 880 650 75 720 300 25 260 20 400 25 EG Example of the invention 10 Y 1250 880 650 75 710 100 25 260 10 200 25 EG Example with parameter 11 A 1250 880 650 740 70 8 320 20 600 540 GA Example of the invention 12 1250 880 650 780 70 15 440 20 200 480 Gl Example with parameter Ί: The speed of finamiento pwwdio ('C^sm speed '2: The temperature T! |C) is a selectable temperature (°Cj within a temperature range of 200 to 250 °C. 2. Evaluation Methods Steel sheets (coated steel sheets) produced under different manufacturing conditions were subjected to microstructural analysis, which investigated the component fractions, and to tensile testing, which evaluated mechanical properties such as tensile strength. The investigation of component fractions and the evaluations were performed in a manner similar to that described in Example 1. Coating Adhesion The appearance of the coated steel sheets was visually examined. Steel sheets free of peeling spot defects were assigned the symbol “O,” steel sheets exhibiting a peeling spot defect were assigned the symbol “x,” and steel sheets free of peeling spot defects but with a non-uniform or similar coating appearance were assigned the symbol “Δ.” Note that a “peeling spot defect” refers to an exposed, uncoated region of a steel sheet on the order of several micrometers to several millimeters. Cases with the symbol “O” or “Δ” were determined to represent instances where the coating was sufficiently adhered, thus achieving good coating adhesion. 3. Evaluation Results The results of the investigations and evaluations described above are shown in Table 5. RRnRnn / iznz / e / Yi Tabla 5 o si o ro m o si o si o01 No. Steel Type Microstructure Mechanical Properties Notes Ferrite Martensite Average grain diameter of martensite Martensite having an aspect ratio of 3 or less / complete martensite C concentration of martensite having an aspect ratio of 3 or less Remaining components*! YS TS YR Coating Adhesion % of Area % of Area μm % % by Mass % of Area MPa MPa 1 A 66 30 2.4 70 0.43 4 376 638 0.59 0 Example of mention 2 57 39 2.3 75 0.35 4 358 632 0.57 A Example of mention 3 63 35 2.9 75 0.42 2 352 640 0.55 0 Example of mention 4 63 29 3.0 55 0.56 8 408 629 0.65 0 Comparative Example 5 66 30 2.9 70 0.44 4 348 638 0.55 0 Example of mention 6 55 40 2.5 70 0.37 5 360 642 0.56 0 Example of mention 7 53 38 2.4 80 0.42 9 333 634 0.53 0 Example of mention 8 53 45 2.3 60 0.51 2 387 640 0.60 0 Example of mention 9 F 88 12 1.8 85 0.88 0 305 621 0.49 A Example of mention 10 Y 70 30 2.1 100 0.83 0 328 638 0.51 X Comparative example 11 A 64 34 2.8 70 0.66 2 381 643 0.59 0 Example of mention 12 57 41 2.9 50 0.49 2 396 657 0.65 0 Comparative example. Ί: The remaining components are one or more of retained austenite, bainite, and pearlite. In Example 2, steel sheets having a TS of 590 MPa or higher, a YR of 0.63 or less, and good coating adhesion were considered “approved” and are indicated as “Example of the Invention” in the “Notes” column in Table 5. On the other hand, steel sheets having at least one of a TS of less than 590 MPa, a YR of more than 0.63, and low coating adhesion were considered “failed” and are indicated as “Comparative Example” in the “Notes” column in Table 5.

Claims

1. A high-strength steel sheet, the high-strength steel sheet having a chemical composition characterized in that it comprises, in mass %, C: 0.06% or more and 0.120% or less, Si: 0.3% or more and 0.7% or less, Mn: 1.6% or more and 2.2% or less, P: 0.05% or less, S: 0.0050% or less, Al: 0.01% or more and 0.20% or less, and N: 0.010% or less, with a remainder of Fe and incidental impurities, the high-strength steel sheet having a microstructure in which ferrite is present as a major phase, and martensite is present in an area fraction of 10% or more and less than 50% relative to the area of ​​the whole microstructure, wherein the martensite has an average grain diameter of 3.0 pm or less. martensite, a proportion of martensite that has an aspect ratio of 3 or less is 60% or more, and martensite that has an aspect ratio of 3 or less has a carbon concentration of 0.30% or more and 0.90% or less by mass.

2. The high-strength steel sheet according to claim 1, further characterized in that the chemical composition additionally comprises, in % by mass, one or more selected from Cr: 0.01% or more and 0.20% or less, Mo: 0.01% or more and less than 0.15%, and V: 0.001% or more and 0.05% or less.

3. The high-strength steel sheet according to claim 1 or 2, further characterized in that the chemical composition additionally comprises, in % by mass, one or more selected from Groups A to C described below: Group A: one or two selected from Nb: 0.001% or more and 0.02% or less and Ti: 0.001% or more and 0.02% or less, Group B: one or two selected from Cu: 0.001% or more and 0.20% or less and Ni: 0.001% or more and 0.10% or less, and Group C: B: 0.0001% or more and 0.002% or less.

4. The high-strength steel sheet according to any of claims 1 to 3, further characterized in that a surface of the steel sheet has a coating layer.

5. A method for producing a high-strength steel sheet, the method characterized in that it comprises: heating a block of steel having the chemical composition according to any one of claims 1 to 3 and subsequently subjecting the block of steel to a hot rolling stage; and subjecting a hot-rolled steel sheet obtained from the hot rolling stage to an annealing stage in which the hot-rolled steel sheet is held at an annealing temperature of Aci or higher and a temperature Aca or lower for 30 seconds or more, a resulting steel sheet is cooled under conditions in which an average cooling rate in a range from the annealing temperature to 350 °C is 5 °C / second or greater, and a cooling stop temperature is 350 °C or lower, and subsequently,The resulting steel sheet is held under conditions in which a holding time for a temperature range from 350 °C to 300 °C is 50 seconds or less, and a holding time for a temperature range from less than 300 °C to a temperature T1 (°C) is 1000 seconds or less, where the temperature T1 (°C) is a selectable temperature within a temperature range of 200 to 250 °C.

6. A method for producing a high-strength steel sheet, the method characterized in that it comprises: heating a block of steel having the chemical composition according to any one of claims 1 to 3 and subsequently subjecting the block of steel to a hot rolling stage; thereafter subjecting a hot-rolled steel sheet obtained from the hot rolling stage to a cold rolling stage; and subjecting a cold-rolled steel sheet obtained from the cold rolling stage to an annealing stage in which the cold-rolled steel sheet is held at an annealing temperature of Aci or higher and a temperature Aca or lower for 30 seconds or more, a resulting steel sheet being cooled under conditions in which an average cooling rate in a range from the annealing temperature down to 350 °C is 5 °C / second or greater.and a cooling stop temperature of 350 °C or lower, and subsequently, the resulting steel sheet is held under conditions in which a holding time for a temperature range from 350 °C to 300 °C is 50 seconds or less, and a holding time for a temperature range from less than 300 °C to a temperature T1 (°C) is 1000 seconds or less, where the temperature T1 (°C) is a selectable temperature within a temperature range of 200 to 250 °C.

7. The method for producing a high-strength steel sheet according to claim 5 or 6, further characterized in that the method additionally comprises carrying out a coating process after the annealing stage.