Abstract: Provided are a high-strength steel sheet, which is suitable as a material of automotive parts, and a method for producing the high-strength steel sheet. In the high-strength steel sheet, the C content is 0.15% or less, the area ratio of ferrite is 8% to 45%, the area ratio of martensite is 55% to 85%, the proportion of martensite grains adjacent to only ferrite grains in the entire microstructure is 15% or less, the average crystal grain sizes of ferrite and martensite are each 10 ?m or less, and the area ratio of ferrite grains having a size of 10 ?m or more to all the ferrite grains included in a portion of the steel sheet which extends from 20 to 100 ?m below the surface thereof is less than 5%.

Abstract: A hot dip galvanized steel sheet includes a steel sheet, a hot dip galvanizing layer, and an intermetallic compound, in which the steel sheet has a chemical composition containing, by mass %, C: 0.03% or more and 0.70% or less, Si: 0.10% or less, Mn: 0.5% or more and 0.9% or less, P: 0.020% or more and 0.050% or less, S: 0.010% or less, Nb: 0.010% or more and 0.035% or less, N: 0.005% or less, Al: 0.10% or less, and the balance being Fe and inevitable impurities, in which the hot dip galvanizing layer contains 0.3% or more and 0.6% or less of Al, and in which the intermetallic compound contains 0.12 gm?2 or more and 0.22 gm?2 or less of Al and Fe2Al5 having an average grain diameter of 1 ?m or less.

Abstract: A method for producing high-strength galvannealed steel sheets having excellent workability and fatigue resistance. The method comprises subjecting a steel sheet to an oxidation treatment, the oxidation treatment including heating the steel sheet in an upstream stage at a temperature in the range of 400° C. to 750° C. in an atmosphere having an O2 concentration of not less than 1000 ppm by volume and a H2O concentration of not less than 1000 ppm by volume and a downstream stage at a temperature in the range of 600° C. to 850° C. in an atmosphere having an O2 concentration of less than 1000 ppm by volume and a H2O concentration of not less than 1000 ppm by volume. The method further comprises subjecting the steel sheet to reducing annealing, hot-dip galvanizing treatment and alloying treatment.

Abstract: A hot-dip galvanized steel sheet having a good appearance and good adhesion to a coating, the hot-dip galvanized steel sheet having a composition containing, on a mass basis: C: 0.08% or more and less than 0.20%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01% to 3.00%, and S: 0.200% or less, a remainder being Fe and incidental impurities, wherein the hot-dip galvanized steel sheet includes an internal oxidation layer and a decarburized layer, the internal oxidation layer having a thickness of 5 ?m or less, the decarburized layer having a thickness of 20 ?m or less, and 50% or more by area of the internal oxidation layer is composed of a Si oxide containing Fe and/or Mn represented by Fe2XMn2-2XSiOY, wherein X ranges from 0 to 1, and Y is 3 or 4.

Abstract: A hot-dip galvanized steel sheet having a good appearance and good adhesion to a coating, the hot-dip galvanized steel sheet having a composition including, on a mass basis: C: 0.20% to 0.50%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01% to 3.00%, and S: 0.200% or less, a remainder being Fe and incidental impurities, wherein the hot-dip galvanized steel sheet includes an internal oxidation layer and a decarburized layer, the internal oxidation layer having a thickness of 4 ?m or less on a ferrite side from an interface between ferrite and a galvanized layer, the decarburized layer having a thickness of 16 ?m or less on the ferrite side from the interface between the ferrite and the galvanized layer, and 50% or more by area of the internal oxidation layer is composed of a Si oxide containing Fe and/or Mn represented by Fe2XMn2-2XSiOY, wherein X ranges from 0 to 1, and Y is 3 or 4.

Abstract: A high-strength hot-dip galvanized steel sheet is produced by a method that includes a first heating step of holding a steel sheet at a temperature for a time in an atmosphere having a H2 concentration and a dew point, a cooling step of cooling the steel sheet after the first heating step, an electrolytic treatment step of subjecting the steel sheet after the cooling step to electrolytic treatment under specific conditions, a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature for a time in an atmosphere having a H2 concentration and a dew point, and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.

Abstract: A method of manufacturing a galvanized steel sheet having a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.

Abstract: A method of manufacturing a high strength galvanized steel sheet has a first heating step including heating to 400° C. to 750° C. in an atmosphere containing O2: 0.1 to 20 percent and H2O: 1 to 50 percent and heating to 600° C. to 850° C. in an atmosphere containing O2: 0.01 to less than 0.1 percent and H2O: 1 to 20 percent is applied to a steel sheet, a second heating step includes holding the steel sheet in an atmosphere containing H2: 1 to 50 percent and having a dew point of 0° C. or lower at 750° C. to 900° C. for 15 to 600 s, cooling to a temperature of 450° C. to 550° C., and holding is performed at that temperature for 10 to 200 s, and a galvanization treatment is applied.

Abstract: A method for producing a high-strength hot-dip galvanized steel sheet includes a steel sheet containing, in percent by mass, 0.01% to 0.18% of C, 0.02% to 2.0% of Si, 1.0% to 3.0% of Mn, 0.001% to 1.0% of Al, 0.005% to 0.060% of P, 0.01% or less of S, and the balance being Fe and incidental impurities, and a galvanized coating layer on each surface of the steel sheet with a coating weight of 20 to 120 g/m2 per surface, in which, when the steel sheet is subjected to annealing and a hot-dip galvanizing treatment in a continuous hot-dip galvanizing line, the dew point of the atmosphere is controlled to ?40° C. or lower in the annealing furnace temperature range of 750° C. or higher.

Abstract: There is provided a method for manufacturing a high-strength galvannealed steel sheet having excellent coating adhesiveness and corrosion resistance whose base material is a high-strength steel sheet containing Si and Mn. The method includes performing an oxidation treatment on a steel sheet including Si and Mn in a first zone having an atmosphere of an oxygen concentration, thereafter performing an oxidation treatment in a second zone having an atmosphere of an oxygen concentration, thereafter performing reduction annealing and galvanizing, and further performing an alloying treatment by heating the galvanized steel sheet.

Abstract: Provided is a method for manufacturing a high-strength galvanized steel sheet, made from a steel sheet containing Si and/or Mn, having excellent exfoliation resistance during heavy machining. When a steel sheet containing 0.01% to 0.18% C, 0.02% to 2.0% Si, 1.0% to 3.0% Mn, 0.001% to 1.0% Al, 0.005% to 0.060% P, and 0.01% or less S on a mass basis, the remainder being Fe and unavoidable impurities, is annealed and galvanized in a continuous galvanizing line, a temperature region with a furnace temperature of A° C. to B° C. (600?A?780 and 800?B?900) is performed at an atmosphere dew-point temperature of ?5° C. or higher in a heating process.

Abstract: A galvanized steel sheet has a composition containing, by mass %, C: 0.05% or more and 0.5% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100% or less, S: 0.02% or less, Al: 0.010% or more and 0.5% or less, B: 0.0002% or more and 0.005% or less, Ti: 0.05% or less, a relationship of Ti>4N being satisfied, and the balance comprising Fe and inevitable impurities, and a microstructure containing 60% or more and 95% or less of tempered martensite in terms of area ratio and 5% or more and 20% or less of retained austenite in terms of area ratio.

Abstract: A high-strength galvanized steel sheet whose base steel is a high-strength steel sheet contains Si, Mn, and B and has good coating adhesiveness. A base steel sheet containing Si, Mn, and B is oxidized at a heating temperature of steel sheet T that satisfies the formula, and then subjected to reduction-annealing and galvanizing T?58.65×[Si]+29440×[B]?13.59×[O2]+548.1 [Si]: amount of Si in the steel on a mass percent basis [B]: amount of B in the steel on a mass percent basis [O2]: O2 concentration on a volume percent basis in an atmosphere at oxidizing treatment.

Abstract: A hot dip galvanized steel sheet includes a steel sheet, a hot dip galvanizing layer, and an intermetallic compound, in which the steel sheet has a chemical composition containing, by mass %, C: 0.03% or more and 0.70% or less, Si: 0.10% or less, Mn: 0.5% or more and 0.9% or less, P: 0.020% or more and 0.050% or less, S: 0.010% or less, Nb: 0.010% or more and 0.035% or less, N: 0.005% or less, Al: 0.10% or less, and the balance being Fe and inevitable impurities, in which the hot dip galvanizing layer contains 0.3% or more and 0.6% or less of Al, and in which the intermetallic compound contains 0.12 gm?2 or more and 0.22 gm?2 or less of Al and Fe2Al5 having an average grain diameter of 1 ?m or less.

Abstract: A method for manufacturing a galvanized steel sheet includes heating a base steel sheet in a heating zone such that the surface of the base steel sheet is heated at a temperature of 600° C. or higher and 790° C. or lower while a furnace temperature T° C. in the heating zone of an annealing furnace is controlled based on the water vapor partial pressure PH2Oin Air of air fed into the heating zone, the base steel sheet having a chemical composition consisting of, by mass %, C: 0.05% or more and 0.25% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.001% or more and 0.10% or less, Al: 0.01% or more and 3.00% or less, S: 0.200% or less, and the balance being Fe and inevitable impurities, heating the base steel sheet in the heating zone such that the surface of the base steel sheet is heated at a temperature of 630° C. or higher and 850° C.

Abstract: A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol % H2+balance N2 gas atmosphere with a dew point of ?25° C. or less.

Abstract: Provided is a galvannealed steel sheet with excellent anti-powdering property when press forming is performed, without controlling the contents of chemical elements in steel which are effective for strengthening a steel sheet, such as Si and P, to be low in order to achieve required material properties and without increasing cost due to, for example, processes being complicated. A galvannealed steel sheet with excellent anti-powdering property has a coated layer taking in grains of a base steel sheet such that the grains constitute 2.0% or more and 15.0% or less of the coated layer in terms of cross section area ratio.

Abstract: A galvannealed steel sheet includes a zinc coating layer containing 7 to 15% Fe on the surface of steel sheet including, C: 0.02% to 0.30%, Si: 0.01% to 2.5%, Mn: 0.1% to 3.0%, P: 0.003% to 0.08%, S: not more than 0.01%, Al: 0.001% to 0.20%, one or more selected from Ti: 0.03% to 0.40%, Nb: 0.001% to 0.2%, V: 0.001% to 0.2%, Mo: 0.01% to 0.5% and W: 0.001% to 0.2%. In the zinc coating layer, carbides with average particle size of 1 nm to 20 nm including one or more selected from titanium, niobium, vanadium, molybdenum and tungsten are present with a density of five or more particles per segment that is defined by the thickness of the coating layer and also by dividing a cross section of the coating layer at intervals of 1 ?m in a direction perpendicular to the thickness of the coating layer.

Abstract: A method of manufacturing a high-strength galvanized steel sheet includes hot-rolling a slab to form a steel sheet; during continuous annealing, heating the steel sheet to a temperature of 750° C. to 900° C. at an average heating rate of at least 10° C./s at a temperature of 500° C. to an A1 transformation point; holding that temperature for at least 10 seconds; cooling the steel sheet from 750° C. to a temperature of (Ms point—100° C.) to (Ms point—200° C.) at an average cooling rate of at least 10° C./s; reheating the steel sheet to a temperature of 350° C. to 600° C.; holding that temperature for 10 to 600 seconds; and galvanizing the steel sheet.

Abstract: A high-strength galvanized steel sheet has excellent mechanical properties such as a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more and a method for manufacturing the same. A high-strength galvanized steel sheet excellent in formability contains 0.05% to 0.5% C, 0.01% to 2.5% Si, 0.5% to 3.5% Mn, 0.003% to 0.100% P, 0.02% or less S, and 0.010% to 0.5% Al on a mass basis, the remainder being Fe and unavoidable impurities, and has a microstructure which contains 0% to 10% ferrite, 0% to 10% martensite, and 60% to 95% tempered martensite on an area basis as determined by structure observation and which further contains 5% to 20% retained austenite as determined by X-ray diffractometry.