Abstract: Provided herein is a plated steel having high strength with excellent elongation, excellent hole expansibility, and excellent material uniformity, and a method for producing such a plated steel. The steel sheet provided herein has a specific composition, and a steel structure that contains ferrite as a primary phase, and 2 to 12% of perlite, and 3% or less of martensite by volume, and in which the remainder is a low-temperature occurring phase. The ferrite has an average crystal grain diameter of 25 ?m or less. The perlite has an average crystal grain diameter of 5 ?m or less. The martensite has an average crystal grain diameter of 1.5 ?m or less. The perlite has a mean free path of 5.5 ?m or more. The steel sheet has a tensile strength of 440 MPa or more.

Abstract: Provided herein is a steel sheet of a specific composition that has a micro structure containing, by volume, 75 to 95% of ferrite, 3 to 15% of martensite, 0.5 to 10% of perlite, 10% or less of unrecrystallized ferrite, and 21.5% or less of a low-temperature occurring phase representing the remainder, and in which the ferrite has an average crystal grain diameter of 6 ?m or less, and the martensite has an average crystal grain diameter of 3 ?m or less, and an average aspect ratio of 4.0 or less, and in which a Nb base precipitate having an average grain diameter of 0.10 ?m or less is contained. The steel sheet has a tensile strength of 590 MPa or more.

Abstract: A high-strength cold-rolled steel sheet has a chemical composition containing, by mass %, C: 0.15% or more and 0.30% or less, Si: 0.8% or more and 2.4% or less, Mn: 2.4% or more and 3.5% or less, P: 0.08% or less, S: 0.005% or less, Al: 0.01% or more and 0.08% or less, N: 0.010% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0002% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, a microstructure including ferrite having an average grain diameter of 3 ?m or less and a volume fraction of 5% or less (including 0%), retained austenite having a volume fraction of 10% or more and 20% or less, martensite having an average grain diameter of 4 atm or less and a volume fraction of 20% or less (including 0%), and the balance including bainite and/or tempered martensite.

Abstract: Provided is a resistance spot welding method that inhibits, in accordance with the degree of axis misalignment between electrodes, the occurrence of cracking in a weld regardless of the steel grade. In resistance spot welding methods according to the present invention, H (ms) is an electrode force retaining time after completion of current passage, D (mm) is an amount of axis misalignment between the electrodes, t (mm) is a total sum of sheet thicknesses of a plurality of overlapping steel sheets, T (MPa) is a tensile strength of a steel sheet having a highest tensile strength among the plurality of steel sheets, F (N) is an electrode force, and d (mm) is a tip diameter of one electrode of the pair of electrodes that has a smaller tip diameter. The electrode force retaining time H is specified to be a predetermined value or greater.

Abstract: A steel sheet includes a microstructure containing a volume fraction of 20% to 55% of ferrite having an average grain size of 7 ?m or less, a volume fraction of 5% to 15% of retained austenite, a volume fraction of 0.5% to 7% of martensite having an average grain size of 4 ?m or less, and a structure composed of bainite and/or tempered martensite and having an average grain size of 6 ?m or less, and a difference in nano-hardness between ferrite and the structure composed of bainite and/or tempered martensite being 3.5 GPa or less and a difference in nano-hardness between the structure composed of bainite and/or tempered martensite and martensite being 2.5 GPa or less.

Abstract: A high strength cold rolled steel sheet with a low yield ratio has a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which the average grain size of ferrite is 15 ?m or less, the volume fraction of ferrite is 70% or more, the volume fraction of bainite is 3% or more, the volume fraction of retained austenite is 4% to 7%, the average grain size of martensite is 5 ?m or less, and the volume fraction of martensite is 1% to 6%, wherein the average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less, and the tensile strength is 590 MPa or more.

Abstract: A high-strength cold-rolled steel sheet has a composition and a microstructure. The microstructure comprises: ferrite having an average grain size of 5 ?m or less and a volume fraction of 3% to 20%, retained austenite having a volume fraction of 5% to 20%, and martensite having a volume fraction of 5% to 20%, the remainder being bainite and/or tempered martensite. The total number of retained austenite with a grain size of 2 ?m or less, martensite with a grain size of 2 ?m or less, or a mixed phase thereof is 150 or more per 2,000 ?m2 of a thickness cross section parallel to the rolling direction of the steel sheet.

Abstract: A resistance spot welding method inhibits, in accordance with the electrode angle, the occurrence of cracking in the weld regardless of the steel grade. The resistance spot welding method satisfies relationships: 2·A·(t·T/F)1/2?H when 0?A<1 (3·A?1)·(t·T/F)1/2?H when 1?A<10 (A+19)·(t·T/F)1/2?H when 10?A<20 where H (ms) is an electrode force retaining time after completion of current passage, A (degrees) is an electrode angle of the electrodes, t (mm) is a sheet thickness of a steel sheet having a largest sheet thickness among the steel sheets, T (MPa) is a tensile strength of a steel sheet having a highest tensile strength among the two or more steel sheets, and F (N) is the electrode force.

Abstract: A high strength cold rolled steel sheet has a chemical composition including, by mass %, C: 0.06 to 0.13%, Si: 1.2 to 2.3%, Mn: 0.6 to 1.6%, P: not more than 0.10%, S: not more than 0.010%, Al: 0.01 to 0.10% and N: not more than 0.010%, the balance comprising Fe and inevitable impurities. The steel sheet includes a microstructure containing not less than 90% in terms of volume fraction of ferrite with an average grain diameter of less than 20 ?m and 1.0 to 10% in terms of volume fraction of pearlite with an average grain diameter of less than 5 ?m. The ferrite has an average Vickers hardness of not less than 130. The steel sheet has a yield ratio of not less than 65% and a tensile strength of not less than 590 MPa.

Abstract: A high-strength cold-rolled steel sheet excellent in terms of elongation (EL) and hole expansion ratio (A) having a low yield ratio (YR), as well as a method for manufacturing the steel sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al: 0.10% or less, N: 0.010% or less, and the balance being Fe and inevitable impurities, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 5 ?m or less in an amount of 30% to 55% in terms of volume fraction, retained austenite having an average crystal grain diameter of 2 ?m or less in an amount of 5% to 15% in terms of volume fraction, and tempered martensite having an average crystal grain diameter of 2 ?m or less in an amount of 30% to 60% in terms of volume fraction, in which the number of grains of the retained austenite existing in an area of 1000 ?m2 is 10 or more.

Abstract: Provided are a high-strength cold-rolled steel sheet having excellent formability, excellent ductility, excellent hole expansibility, and high yield ratio and a method for producing the same. The high-strength cold-rolled steel sheet contains 0.05% to 0.15% C, 0.10% to 0.90% Si, 1.0% to 2.0% Mn, 0.005% to 0.05% P, 0.0050% or less S, 0.01% to 0.10% Al, 0.0050% or less N, and 0.010% to 0.100% Nb, which are chemical components, on a mass basis, the balance being Fe and unavoidable impurities; has a microstructure which is a multi-phase structure containing 90% or more of a ferrite phase and 0.5% to less than 5.0% of a martensite phase on a volume fraction basis, the remainder being low-temperature transformation phases; and has a yield ratio of 70% or more.

Abstract: A high-strength cold-rolled steel sheet having a tensile strength (TS) of 1150 MPa or more and excellent elongation and delayed fracturing resistance, as well as a method for manufacturing the steel sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.16% to 0.30%, Si: 1.20% to 2.20%, Mn: 1.50% to 2.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007% or less, at least one of Ti and Nb, and the balance being Fe and inevitable impurities, wherein the combined mass % of Ti and Nb is 0.04% to 0.15%, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 3 ?m or less in an amount of 45% to 65% in terms of volume fraction, tempered martensite having an average crystal grain diameter of 2 ?m or less in an amount of 35% to 55% in terms of volume fraction, and optionally non-recrystallized ferrite having an average crystal grain diameter of 5 ?m or less in an amount of 5% or less.

Abstract: A steel sheet contains C: 0.15% to 0.22%, Si: 1.0% to 2.0%, Mn: 1.7% to 2.5%, P: 0.05% or less, S: 0.02% or less, Al: 0.01% to 0.05%, N: 0.005% or less, O: 0.01% or less, and the balance being iron and unavoidable impurities while satisfying [Si]/[Mn]?0.5 ([Si] and [Mn] represent Si and Mn contents, wherein the steel sheet has a structure including, in terms of area fraction, 60% to less than 100% of tempered martensite, 5% or less including 0% of untransformed austenite, and the balance being ferrite, and the ferrite has an average crystal grain size of less than 3.5 ?m, wherein less than 10 particles/100 ?m2 of Si—Mn compound oxide particles having a circle equivalent diameter of 5 ?m or less are present on a surface of the steel sheet, and the surface is covered with Si-based oxide at a coverage of 1% or less.

Abstract: A high-strength cold-rolled steel sheet having a specified chemical composition and a microstructure including ferrite having an average crystal grain diameter of 2 ?m or less in an amount of 10% to 25% in terms of volume fraction, retained austenite in an amount of 5% to 20% in terms of volume fraction, martensite having an average crystal grain diameter of 2 ?m or less in an amount of 5% to 15% in terms of volume fraction, and the balance being a multi-phase structure including bainite and tempered martensite having an average crystal grain diameter of 5 ?m or less, in which a relational expression, 0.35?V2/V1?0.75 (1), is satisfied, where V1 is a volume fraction of phases which are different from ferrite and V2 is a volume fraction of tempered martensite.

Abstract: A high-strength cold-rolled steel sheet has a composite structure containing 0.15 to 0.25% by mass of C, 1.8 to 3.0% by mass of Mn, and 0.0003 to 0.0050% by mass of B, and having a ferrite volume fraction of 20% to 50%, a retained austenite volume fraction of 7% to 20%, a martensite volume fraction of 1% to 8%, and the balance containing bainite and tempered martensite, and in the composite structure, ferrite has an average crystal grain diameter of 5 ?m or less, retained austenite has an average crystal grain diameter of 0.3 to 2.0 ?m and an aspect ratio of 4 or more, martensite has an average crystal grain diameter of 2 ?m or less, a metal phase containing both bainite and tempered martensite has an average crystal grain diameter of 7 ?m or less, the ratio of the volume fraction of tempered martensite to the volume fraction of a metal structure other than ferrite is 0.60 to 0.85, and the average C concentration in retained austenite is 0.65% by mass or more.

Abstract: A high-yield-ratio, high-strength cold-rolled steel sheet has a composition containing, in terms of % by mass, C: 0.13% to 0.25%, Si: 1.2% to 2.2%, Mn: 2.0% to 3.2%, P: 0.08% or less, S: 0.005% or less, Al: 0.01% to 0.08%, N: 0.008% or less, Ti: 0.055% to 0.130%, and the balance being Fe and unavoidable impurities. The steel sheet has a microstructure that contains 2% to 15% of ferrite having an average crystal grain diameter of 2 um or less in terms of volume fraction, 5 to 20% of retained austenite having an average crystal grain diameter of 0.3 to 2.0 ?m in terms of volume fraction, 10% or less (including 0%) of martensite having an average grain diameter of 2 ?m or less in terms of volume fraction, and the balance being bainite and tempered martensite, and the bainite and the tempered martensite having an average crystal grain diameter of 5 ?m or less.

Abstract: A hot rolled steel sheet has a chemical composition including, by mass %, C: 0.060% to 0.120%; Si: 0.10% to 0.70%; Mn: 1.00% to 1.80%; P: 0.10% or less; S: 0.010% or less; Al: 0.01% to 0.10%; N: 0.010% or less; Nb: 0.010% to 0.100%, wherein Nb is contained so that content of solute Nb is 5% or more relative to the total Nb content; the balance being Fe and incidental impurities. The hot rolled steel sheet has a microstructure containing ferrite of not more than 15 ?m in average crystal grain diameter by a volume fraction of not less than 75%, the balance being low-temperature-induced phases. The hot rolled steel sheet can be suitably utilized for manufacturing a cold rolled steel sheet or hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more, excellent in material homogeneity and capable of giving excellent cold rolling property.

Abstract: A high-strength cold-rolled steel sheet having good ductility, hole expandability, and delayed fracture resistance and being excellent in material homogeneity. The steel sheet has a chemical composition containing, in mass %, C: 0.15 to 0.25%, Si: 1.2 to 2.2%, Mn: 1.7 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.10%, N: 0.006% or less, Ti: 0.003 to 0.030%, B: 0.0002 to 0.0050%, and Fe and inevitable impurities. The steel sheet has a microstructure that includes ferrite having an average crystal grain diameter of 4 ?m or less at a volume fraction of 5 to 20%, retained austenite at a volume fraction of 5% or less, including 0%, and tempered martensite at a volume fraction of 80 to 95%. Additionally, the mean free path of the ferrite is 3.0 to 7.5 ?m.

Abstract: A high-strength cold-rolled steel sheet has a chemical composition containing, by mass %, C: 0.15% or more and 0.30% or less, Si: 0.8% or more and 2.4% or less, Mn: 2.4% or more and 3.5% or less, P: 0.08% or less, S: 0.005% or less, Al: 0.01% or more and 0.08% or less, N: 0.010% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0002% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, a microstructure including ferrite having an average grain diameter of 3 ?m or less and a volume fraction of 5% or less (including 0%), retained austenite having a volume fraction of 10% or more and 20% or less, martensite having an average grain diameter of 4 ?m or less and a volume fraction of 20% or less (including 0%), and the balance including bainite and/or tempered martensite.

Abstract: A zinc or zinc alloy coated steel excellent in various properties including corrosion resistance and adhesion properties and, in particular, good conductivity is provided. A method for manufacturing a zinc or zinc alloy coated steel sheet includes: preparing a surface-treatment liquid for a the steel sheet, containing following components, by blending the including: (A) a specific resin emulsion; (B) a tetraalkoxysilane; (C) at least one type of silane coupling agent; (D) a chelating agent; (E) a vanadate compound; (F) a titanium compound; and water; and applying by coating the surface-treatment liquid to a surface of the steel sheet, heating and drying the surface of the steel sheet such that a coating amount per one surface is 200 to 1,000 mg/m2 to form a surface treatment coating film on the surface.