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 method for producing high-strength galvanized steel sheets having excellent coating adhesion, workability and appearance. The method comprises hot rolling a slab comprising, by mass %, C: 0.05 to 0.30%, Si: 0.1 to 2.0% and Mn: 1.0 to 4.0%, then coiling the steel sheet into a coil at a specific temperature TC, and pickling the steel sheet, cold rolling the hot-rolled steel sheet resulting from the hot rolling, annealing the cold-rolled steel sheet resulting from the cold rolling under specific conditions, and galvanizing the annealed sheet resulting from the annealing in a galvanizing bath containing 0.12 to 0.22 mass % Al.

Abstract: A high strength galvanized steel sheet excellent in coating adhesiveness is made from a base material that is a high strength steel sheet containing Si, Mn, and Cr. A method includes performing an oxidation treatment on steel containing Si, Mn, and Cr in an oxidation furnace under the condition that a selected exit temperature T, reduction annealing and a galvanizing treatment, or optionally, further an alloying treatment under conditions that heating is performed at a temperature of 460° C. or higher and 600° C. or lower for an alloying treatment time of 10 seconds or more and 60 seconds or less.

Abstract: A method of producing a galvannealed steel sheet includes: annealing a steel strip by conveying the steel strip through a heating zone including a direct fired furnace, a soaking zone, and a cooling zone in this order in an annealing furnace; hot-dip galvanizing the steel strip discharged from the cooling zone; and heat-alloying a galvanized coating formed on the steel strip. Mixed gas of humidified gas and dry gas is supplied into the soaking zone from at least one gas supply port located in a region of lower ½ of the soaking zone in a height direction so that a dew point measured in a region of upper ? of the soaking zone in the height direction and a dew point measured in a region of lower ? of the soaking zone in the height direction are both ?20° C. or more and 0° C. or less.

Abstract: A high-strength hot-dip galvanized steel sheet having a surface coated with a hot-dip galvanized coating, the steel sheet having a chemical composition comprising C: more than 0.10% and less than 0.18%, Si: 0.01 to 1.00%, Mn: 1.5 to 4.0%, P: 0.100% or less, S: 0.020% or less, Al: 0.010 to 0.500%, Cr: 0.010 to 2.000%, Nb: 0.005 to 0.100%, Ti: 0.005 to 0.100%, B: more than 0.0005% and 0.0030% or less, and Fe and incidental impurities, and a method for producing the same. The steel sheet has a microstructure including ferrite having an area fraction in the range of 0 to 10%, martensite having an area fraction in the range of 15 to 60%, tempered martensite having an area fraction in the range of 20 to 50%, and bainitic ferrite having an area fraction in the range of 20 to 50%.

Abstract: Provided are a method for controlling a dew point in a reducing furnace and a reducing furnace in which, even in the case of galvanizing Si-added steel, coating adhesion can be secured, alloying treatment can be performed without increasing the alloying temperature excessively, and it is possible to obtain a hot-dip galvanized steel sheet having an excellent coating appearance. When a steel sheet is subjected to annealing and hot-dip galvanizing treatment using continuous hot-dip galvanizing equipment including at least a radiant tube-type reducing furnace, a mixed gas of a dry gas and a humidified gas by a humidifying device having a water vapor permeable membrane is used as a gas to be supplied into the reducing furnace. The mixed gas is supplied into the reducing furnace, thereby controlling the dew point in the reducing furnace.

Abstract: Provided are a high-strength galvanized steel sheet which can preferably be used as a material for automobile parts and a method for manufacturing the steel sheet. The steel sheet has a C content of 0.15% or less, in which an area ratio of ferrite is 10% or less, an area ratio of bainitic ferrite is 2% or more and 30% or less, an area ratio of martensite is 60% or more and 98% or less, an area ratio of retained austenite is less than 2%, an average grain diameter of martensite adjacent to bainite is 15 ?m or less, a proportion of massive martensite adjacent only to bainite to the whole metallographic structure is 10% or less, and a value (?Hv) calculated by subtracting the Vickers hardness at a position located at 20 ?m from the surface of the steel sheet from the Vickers hardness at a position located at 100 ?m from the surface of the steel sheet is 30 or more.

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 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: 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: 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 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 having high corrosion resistance after painting includes, on a mass percent basis, C: 0.05% to 0.15%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.7%, Al: ?1.00%, P: ?0.025%, S: ?0.025%, and Cr: 0% to 0.8% as chemical components, the remainder being Fe and incidental impurities. The galvannealed steel sheet includes a galvannealed layer in an amount of 20 to 120 g/m2 per side, a percentage of exposed zinc metal of the galvannealed layer surface being 20% or more and less than 80%, and an amount of internal oxidation in a region disposed not more than 5 ?m from the surface of the steel sheet is 0.02 g/m2 or more and 0.1 g/m2 or less: Si?0.68%, Cr??1.25Si+0.85, and Si>0.68%, Cr=0, wherein Si and Cr denote respective contents (mass %) of Si and Cr.

Abstract: A galvannealed steel sheet having high corrosion resistance after painting includes, on a mass percent basis, C: 0.05% to 0.15%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.7%, Al: 1.00% or less, P: 0.025% or less, S: 0.025% or less, and Cr: 0% to 0.8% or less as chemical components, the remainder being Fe and incidental impurities, wherein the galvannealed steel sheet includes a galvannealed layer on at least one surface of a steel sheet in an amount of 20 to 120 g/m2 per side, a percentage of exposed zinc metal of the galvannealed layer surface being 20% or more and less than 80%, and an amount of internal oxidation in a region disposed not more than 5 ?m from the surface of the steel sheet is 0.02 g/m2 or more and 0.1 g/m2 or less: Si?0.68%, Cr??1.25Si+0.85, and Si>0.68%, Cr=0, wherein Si and Cr denote respective contents (mass %) of Si and Cr.

Abstract: A high strength galvanized steel sheet excellent in coating adhesiveness is made from a base material that is a high strength steel sheet containing Si, Mn, and Cr. A method includes performing an oxidation treatment on steel containing Si, Mn, and Cr in an oxidation furnace under the condition that a selected exit temperature T, reduction annealing and a galvanizing treatment, or optionally, further an alloying treatment under conditions that heating is performed at a temperature of 460° C. or higher and 600° C. or lower for an alloying treatment time of 10 seconds or more and 60 seconds or less.

Abstract: A zinc-based metal plated steel sheet is excellent in tribological properties during press forming. An oxide layer containing crystalline 3Zn(OH)2.ZnSO4.xH2O is formed on a plated surface. The oxide layer has a thickness of 10 nm or more. The crystalline oxide layer is composed of 3Zn(OH)2.ZnSO4.3-5H2O.

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: A zinc-based metal plated steel sheet is excellent in tribological properties during press forming. An oxide layer containing crystalline 3Zn(OH)2·ZnSO4·xH2O is formed on a plated surface. The oxide layer has a thickness of 10 nm or more. The crystalline oxide layer is composed of 3Zn(OH)2·ZnSO4·3-5H2O.

Abstract: A zinc-based metal plated steel sheet is excellent in tribological properties during press forming. An oxide layer containing crystalline 3Zn(OH)2·ZnSO4·xH2O is formed on a plated surface. The oxide layer has a thickness of 10 nm or more. The crystalline oxide layer is composed of 3Zn(OH)2·ZnSO4·3-5H2O.