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, the decarburized layer having a thickness of 16 ?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 high-strength galvanized steel sheet that includes a chemical composition containing, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and Fe and inevitable impurities. The steel sheet having a microstructure containing, by an area percentage basis, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases containing a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%).

Abstract: A high strength galvanized steel sheet has a composition including, C: 0.02% or more and 0.30% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.003% or more and 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% or more and 0.40% or less and the balance being Fe and inevitable impurities, and a zinc-coated layer having a coating weight per surface of 20 g/m2 or more and 120 g/m2 or less. The concentration ratio of C to Ti (C/Ti) in a portion within 10 ?m from the surface of the base steel sheet is 0.8 or more and 1.5 or less, and the total amount of oxides of one or more selected from Fe, Si, Mn, P, Al and Ti formed in a surface portion within 100 ?m from the surface of the base steel sheet is 0.05 g/m2 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: Provided is a hot-rolled steel sheet having a composition containing 0.04 mass percent to 0.20 mass percent C, 0.7 mass percent to 2.3 mass percent Si, 0.8 mass percent to 2.8 mass percent Mn, 0.1 mass percent or less P, 0.01 mass percent or less S, 0.1 mass percent or less Al, and 0.008 mass percent or less N, the remainder being Fe and inevitable impurities. Internal oxides containing one or more selected from the group consisting of Si, Mn, and Fe are present at grain boundaries and in grains in a base metal. The internal oxides present at the grain boundaries in the base metal are located within 5 ?m from the surface of the base metal. The difference between the depths at which the internal oxides are formed in the cross direction of the steel sheet is 2 ?m or less.

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 comprising a cold-rolled steel sheet, an intermetallic compound formed on the cold-rolled steel sheet, and a galvanizing layer formed on the intermetallic compound, the cold-rolled steel sheet having a specific composition and a microstructure having a martensite area ratio of 7% or more and less than 25% and a ferrite area ratio of 50% or more and which includes a base metal surface portion in which the amount of internal oxides per single side is 0.05 g/m2 or less.

Abstract: A high-strength galvanized steel sheet comprising a cold-rolled steel sheet, an intermetallic compound formed on the cold-rolled steel sheet, and a galvanizing layer formed on the intermetallic compound, the cold-rolled steel sheet having a specific composition and a microstructure having a martensite area ratio of 7% or more and less than 25% and a ferrite area ratio of 50% or more and which includes a base metal surface portion in which the amount of internal oxides per single side is 0.05 g/m2 or less.

Abstract: A galvannealed steel sheet having a galvanized layer on a surface thereof is provided, having a composition which contains C: 0.10% to 0.35%, Si: 0.3% 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 on a mass basis, the remainder being Fe and incidental impurities. The steel sheet has a SiC/SiO2 ratio of more than 0.20, the SiC/SiO2 ratio being a ratio of SiC amount to SiO2 amount at a depth of 1 ?m or less in the steel sheet from an interface between the steel sheet and the galvanized layer, and Fe in the galvanized layer constitutes 8% to 13% by mass.

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 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 method for manufacturing a high-strength galvanized steel sheet. The method includes a first heating step of holding the steel sheet in a temperature range of 750° C. to 880° C. for 20 s to 600 s in an atmosphere having an H2 concentration of 0.05% to 25.0% by volume and a dew point of ?45° C. to ?10° C., a cooling step, a rolling step of rolling the steel sheet with a rolling reduction of 0.3% to 2.0%, a pickling step of pickling the steel sheet with a pickling weight loss of 0.02 gram/m2 to 5 gram/m2 in terms of Fe, a second heating step of holding the steel sheet in a temperature range of 720° C. to 860° C. for 20 sec. to 300 sec. in an atmosphere having an H2 concentration of 0.05% to 25.0% by volume and a dew point of ?10° C. or lower, and a galvanizing step.

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 method for manufacturing such steel sheet includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ?0.01%, the balance being represented by Fe and inevitable impurities, in such a manner that the dew point of the atmosphere is controlled to become not more than ?45° C. during the course of soaking when the annealing furnace inside temperature is in the range of not less than 820° C. and not more than 1000° C. as well as that the dew point of the atmosphere is controlled to become not more than ?45° C. during the course of cooling when the annealing furnace inside temperature is in the range of not less than 750° C.

Abstract: The present invention provides a method for manufacturing a high-strength galvannealed steel sheet. The method includes hot-rolling a slab to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet, continuously annealing the cold-rolled steel sheet by heating to 750° C. to 900° C. at an average heating rate of 8° C./s or more from 500° C. to an A1 transformation point, holding the steel sheet for 10 seconds or more, and then cooling the steel sheet to a temperature region of 300° C. to 530° C. at an average cooling rate of 3° C./s or more from 750° C. to 530° C., galvanizing the steel sheet, and further coating-alloying the steel sheet in a temperature region of 540° C. to 600° C. for 5 to 60 seconds.

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: The invention provides a high strength steel sheet which exhibits excellent chemical convertibility and corrosion resistance after electrodeposition coating even in the case where the steel sheet has a high Si content, and a method for manufacturing such steel sheets. The method includes continuous annealing of a steel sheet which includes, in terms of mass %, C at 0.01 to 0.18%, Si at 0.4 to 2.0%, Mn at 1.0 to 3.0%, Al at 0.001 to 1.0%, P at 0.005 to 0.060% and S at ?0.01%, the balance being represented by Fe and inevitable impurities, while controlling the dew-point temperature of the atmosphere to become not more than ?40° C. when the annealing furnace inside temperature is in the range of not less than 750° C.

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.