Abstract: Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area% or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Abstract: Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area % or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Abstract: An ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Si, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 1 area % or more of a region in which martensite constitutes 90 area % or more, residual austentite constitutes 0.5 area % or more, and the local Mn concentration is at least 1.2 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

Abstract: Disclosed is a high strength steel sheet having excellent hydrogen embrittlement resistance. The steel sheet has a tensile strength of 1180 MPa or more, and satisfies the following conditions: with respect to an entire metallographic structure thereof, bainite, bainitic ferrite and tempered martensite account for 85 area % or more in total; retained austenite accounts for 1 area % or more; and fresh martensite accounts for 5 area % or less (including 0 area %).

Abstract: Disclosed is a high-strength cold-rolled steel sheet having improved stretch-flange formability and excellent hydrogen embrittlement resistance. In addition to Fe, C, Si, Mn, P, S, N, and Al, the steel sheet contains V or at least one element of Nb, Ti and Zr. The contents of the at least one element of Nb, Ti and Zr, if present, satisfy the expression of [% C]?[% Nb]/92.9×12?[% Ti]/47.9×12?[% Zr]/91.2×12>0.03. The steel sheet has an area ratio of tempered martensite of 50% or more with ferrite as the remainder. The number of precipitates having a circle-equivalent diameter of 1 to 10 nm is 20 particles or more per 1 ?m2 of the tempered martensite. The number of precipitates containing V or the at least one element of Nb, Ti and Zr and having a circle-equivalent diameter of 20 nm or more is 10 particles or less per 1 ?m2 of the tempered martensite.

Abstract: A galvannealed steel sheet obtained by subjecting a base steel sheet to hot-dip galvanization and then alloying the galvanization layer, the base steel sheet being obtained by hot rolling a specified steel. A method for producing the galvannealed steel sheet.

Abstract: The present invention is the thin steel sheet containing C, Si, Mn, P, S, Al, Mo, Ti, B, and N wherein a value Z calculated by the equation described below is 2.0-6.0, an area ratio against all the structure is 1% or above for retained austenite and 80% or above for total of bainitic ferrite and martensite, a mean axis ratio of the retained austenite crystal grain is 5 or above, and tensile strength is 980 MPa or above where Value Z=9×[C]+[Mn]+3×[Mo]+490×[B]+7×[Mo]/{100×([B]+0.001),and the thin steel sheet has 980 MPa or above tensile strength and enhanced hydrogen embrittlement resistance properties.

Abstract: Disclosed is a high strength steel sheet having excellent hydrogen embrittlement resistance. The steel sheet has a tensile strength of 1180 MPa or more, and satisfies the following conditions: with respect to an entire metallographic structure thereof, bainite, bainitic ferrite and tempered martensite account for 85 area % or more in total; retained austenite accounts for 1 area % or more; and fresh martensite accounts for 5 area % or less (including 0 area %).

Abstract: Disclosed is a high-strength cold-rolled steel sheet which has improved stretch-flange formability while keeping excellent hydrogen embrittlement resistance. The cold-rolled steel sheet comprises 0.03 to 0.30% by mass of C, 3.0% by mass or less (including 0% by mass) of Si, more than 0.1% by mass and not more than 2.8% by mass of Mn, 0.1% by mass or less of P, 0.005% by mass or less of S, 0.01% by mass or less of N, and 0.01 to 0.50% by mass of Al. The cold-rolled steel sheet additionally comprises V in an amount of 0.001 to 1.00% by mass or one or more elements selected from Nb, Ti and Zr in the total amount of 0.01% by mass or more, with the remainder being made up by iron and unavoidable impurities, wherein the contents of one or more elements selected from Nb, Ti and Zr fulfils the requirement represented by the following formula: [% C]?[% Nb]/92.9×12?[% Ti]/47.9×12?[% Zr]/91.2×12>0.03.

Abstract: Disclosed is a galvannealed steel sheet having an excellent surface appearance, wherein plating failure and non-uniform alloying are suppressed. Also disclosed is a method for producing such a galvannealed steel sheet. The galvannealed steel sheet is obtained by hot-dip galvanizing a base steel, and then alloying the plating layer. The base steel is obtained by hot rolling a steel which contains 0.02-0.25 mass % of C, 0.5-3 mass % of Si, 1-4 mass % of Mn, 0.03-1 mass % of Cr, not more than 1.5 mass % of Al (excluding 0 mass %), not more than 0.03 mass % of P (excluding 0 mass %), not more than 0.03 mass % of S (excluding 0 mass %) and 0.003-1 mass % Ti, and additionally contains 0.25-5.0 mass % of Cu and 0.05-1.0 mass % of Ni, while satisfying formula (1), with the balance being made up of iron and unavoidable impurities. [Cu]/[Ni]?5 In formula (1), [ ] represents the content (mass %) of each element.

Abstract: The invention relates to an ultrahigh-strength thin steel sheet excellent in the hydrogen embrittlement resistance, the steel sheet including, by weight %, 0.10 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less of Al, 0.003 to 2.0% of Cr, and a balance including iron and inevitable impurities; in which grains of residual austenite have an average axis ratio (major axis/minor axis) of 5 or more, the grains of the residual austenite have an average minor axis length of 1 ?m or less, and the grains of the residual austenite have a nearest-neighbor distance between the grains of 1 ?m or less.

Abstract: The present invention is the thin steel sheet containing C, Si, Mn, P, S, Al, Mo, Ti, B, and N wherein a value Z calculated by the equation described below is 2.0-6.0, an area ratio against all the structure is 1% or above for retained austenite and 80% or above for total of bainitic ferrite and martensite, a mean axis ratio of the retained austenite crystal grain is 5 or above, and tensile strength is 980 MPa or above. Value Z=9×[C]+[Mn]+3×[Mo]+490×[B]+7×[Mo]/{100×([B]+0.001)} Thus a high strength thin steel sheet with 980 MPa or above tensile strength and enhanced hydrogen embrittlement resistance properties can be provided. Also, in accordance with the present invention, the hot-rolled steel sheet for cold-rolling capable of manufacturing the high strength thin steel sheet with good productivity and having improved cold-rollability can be provided.

Abstract: Disclosed is an alloyed hot-dip galvanized steel sheet containing 2.0 to 3.5 percent by mass of Mn. The steel sheet includes a base steel sheet and a galvanized zinc-coat layer thereon, in which MnO particles are present in an average number of 10 or less per micrometer on a straight line lying in an interface between the galvanized zinc-coat layer and the steel sheet, an Fe—Al—O alloy layer is present at the interface between the MnO particles and the steel sheet, and the length of the Fe—Al—O alloy layer is less than 10% of the overall length of the interface. The alloyed hot-dip galvanized steel sheet, even though having a high Mn content, is resistant to uneven alloying and excels in surface appearance, because the amounts of the MnO particles and the Fe—Al—O alloy layer that cause uneven alloying are controlled.

Abstract: The invention relates to an ultrahigh-strength thin steel sheet excellent in the hydrogen embrittlement resistance, the steel sheet including, by weight %, 0.10 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less of Al, 0.003 to 2.0% of Cr, and a balance including iron and inevitable impurities; in which grains of residual austenite have an average axis ratio (major axis/minor axis) of 5 or more, the grains of the residual austenite have an average minor axis length of 1 ?m or less, and the grains of the residual austenite have a nearest-neighbor distance between the grains of 1 ?m or less.

Abstract: The present invention provides a high strength steel used for spring steel that has excellent hydrogen embrittlement resistance. The high strength steel which spring steel having excellent hydrogen embrittlement resistance comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and balance of iron and inevitable impurities and the structure includes: 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and also the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the steel tensile strength is 1860 MPa or higher.

Abstract: The present invention provides a high strength bolt that has excellent hydrogen embrittlement resistance. The high strength bolt having excellent hydrogen embrittlement resistance which bolt comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and balance of iron and inevitable impurities, and the structure includes: 1% or more residual austenite; 80% or more in total content of bainitic ferrite and martensite; and 10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and also the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the bolt has tensile strength of 1180 MPa or higher.

Abstract: The present invention provides a high strength steel used for spring steel that has excellent hydrogen embrittlement resistance. The high strength steel which spring steel having excellent hydrogen embrittlement resistance comprises 0.20 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, higher than 0% and not higher than 1.5% of Al, 0.15% or less P, 0.02% or less S, and balance of iron and inevitable impurities and the structure includes: 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 10% or less (may be 0%) in total content of ferrite and pearlite in the proportion of area to the entire structure, and also the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher and the steel tensile strength is 1860 MPa or higher.

Abstract: The purpose of the present invention is to provide a high strength thin steel sheet that has high hydrogen embrittlement resisting property. In order to achieve the above purpose, a high strength thin steel sheet having high hydrogen embrittlement resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; and the metal structure comprises: residual austenite; 1% by area or more in proportion to the entire structure; bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.

Abstract: The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 ?m or less; minimum distance between the residual austenite grains is 1 ?m or less; and the steel has tensile strength of 1180 MPa or higher.

Abstract: A steel suitable for a fabric with or without painting and has good corrosion resistance with reproducibility, even if the steel is composed mainly of ordinary carbon steel or low alloy steel, wherein the surface of the steel is coated with rust comprising one or more selected from Ti, Nb, Ta, Zr, V and Hf in the total amount of 0.01 wt % or more. In the steel, the fraction of &agr;-FeOOH and an amorphous rust is 35 wt % or more, and the fraction of &bgr;-FeOOH is 20 wt % or less.