Abstract: The present invention relates to a steel plate with excellent sour resistance, HAZ toughness and HAZ hardness, including, in terms of mass %, C: 0.02-0.20%, Si: 0.02-0.50%, Mn: 0.6-2.0%, P: over 0% and 0.030% or less, S: over 0% and 0.004% or less, Al: 0.010-0.08%, N: 0.001-0.01%, Nb: 0.002% or more and less than 0.05%, O: over 0% and 0.0040% or less, REM: 0.0002-0.05%, and Zr: 0.0003-0.020% with the remainder consisting of iron and inevitable impurities, in which an expression 10,000×[Nb]+31×Di?82?0 (where Di=([C]/10)0.5×(1+0.7×[Si])×(1+3.33×[Mn])×(1+0.35×[Cu])×(1+0.36×[Ni])×(1+2.16×[Cr])×(1+3×[Mo])×(1+1.75×[V])×1.115) is satisfied, and, in the composition of inclusions with 1 ?m or more width contained in steel, Zr amount is 1-40%, REM amount is 5-50%, Al amount is 3-30%, and S amount is over 0% and less than 20%.

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 cold-rolled steel sheet of the present invention which has a composition containing, in terms of % by mass, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%), and Al: 0.001-0.10%, and remainder being mainly iron, and which has a structure comprising, in terms of area ratio, 10-80% ferrite, less than 5% (including 0%) of the sum of retained austenite and martensite, and a hard phase as the remainder. The steel sheet gives a KAM value frequency distribution curve in which the relationship between the proportion of frequency having a KAM value ?0.4, XKAM?0.4°, and the area ratio of ferrite, V? satisfies XKAM?0.4°/V??0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, XKAM=0.6-0.8° is 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 ?m or larger exist so that three or less such cementite grains are dispersed per ?m2 of the hard phase.

Abstract: The invention provides a high-strength cold-rolled steel sheet which is improved in elongation and stretch-flangeability and exhibits more excellent formability. The high-strength cold-rolled steel sheet has a composition which contains by mass C: 0.03 to 0.30%, Si: 0.1 to 3.0%, Mn: 0.1 to 5.0%, P: 0.1% or below, S: 0.005% or below, N: 0.01% or below, and Al: 0.01 to 1.00% with the balance consisting of iron and unavoidable impurities. The high-strength cold-rolled steel sheet has a structure which comprises at least 40% (up to 100% inclusive) in terms of area fraction of tempered martensite having a hardness of 300 to 380 Hv and the balance ferrite. The cementite particles in the tempered martensite take such dispersion that 10 or more cementite particles having equivalent-circle diameters of 0.02 to less than 0.1 ?m are present per one ?m2 of the tempered martensite and three or fewer cementite particles having equivalent-circle diameters of 0.

Abstract: Provided are the following cold-rolled steel sheets: 1) a cold-rolled steel sheet having higher stretch flangeability than conventional steels; 2) a cold-rolled steel sheet having a higher balance between elongation and stretch flangeability than conventional steels; and 3) a cold-rolled steel sheet heightened in all of yield stress, elongation, and stretch flangeability. The cold-rolled steel sheets are characterized by containing 0.03-0.30 mass % carbon, up to 3.0 mass % (including 0 mass %) silicon, 0.1-5.0 mass % manganese, up to 0.1 mass % phosphorus, less than 0.01 mass % sulfur, up to 0.01 mass % nitrogen, and 0.01-1.00 mass % aluminum and having a structure which comprises tempered martensite in an amount of 50% or more (including 100%) in terms of areal proportion and in which the remainder is ferrite.

Abstract: A cold-rolled steel sheet of the present invention which has a composition containing, in terms of % by mass, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%), and Al: 0.001-0.10%, and remainder being mainly iron, and which has a structure comprising, in terms of area ratio, 10-80% ferrite, less than 5% (including 0%) of the sum of retained austenite and martensite, and a hard phase as the remainder. The steel sheet gives a KAM value frequency distribution curve in which the relationship between the proportion of frequency having a KAM value ?0.4, XKAM?0.4°, and the area ratio of ferrite, V? satisfies XKAM?0.4°/V??0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, XKAM=0.6-0.8° is 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 ?m or larger exist so that three or less such cementite grains are dispersed per ?m2 of the hard phase.

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: Provided are the following cold-rolled steel sheets: 1) a cold-rolled steel sheet having higher stretch flangeability than conventional steels; 2) a cold-rolled steel sheet having a higher balance between elongation and stretch flangeability than conventional steels; and 3) a cold-rolled steel sheet heightened in all of yield stress, elongation, and stretch flangeability. The cold-rolled steel sheets are characterized by containing 0.03-0.30 mass % carbon, up to 3.0 mass % (including 0 mass %) silicon, 0.1-5.0 mass % manganese, up to 0.1 mass % phosphorus, less than 0.01 mass % sulfur, up to 0.01 mass % nitrogen, and 0.01-1.00 mass % aluminum and having a structure which comprises tempered martensite in an amount of 50% or more (including 100%) in terms of areal proportion and in which the remainder is ferrite.

Abstract: The invention provides a high-strength cold-rolled steel sheet which is improved in elongation and stretch-flangeability and exhibits more excellent formability. The high-strength cold-rolled steel sheet has a composition which contains by mass C: 0.03 to 0.30%, Si: 0.1 to 3.0%, Mn: 0.1 to 5.0%, P: 0.1% or below, S: 0.005% or below, N: 0.01% or below, and Al: 0.01 to 1.00% with the balance consisting of iron and unavoidable impurities. The high-strength cold-rolled steel sheet has a structure which comprises at least 40% (up to 100% inclusive) in terms of area fraction of tempered martensite having a hardness of 300 to 380 Hv and the balance ferrite. The cementite particles in the tempered martensite take such dispersion that 10 or more cementite particles having equivalent-circle diameters of 0.02 to less than 0.1 ?m are present per one ?m2 of the tempered martensite and three or fewer cementite particles having equivalent-circle diameters of 0.