Kobe Steel, Ltd.

Akira Ibano

Toshio Murakami

Hideo Hata

Kenji Saito

Fumio Yuse

Junichiro Kinugasa

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]&#x2212;[% Nb]/92.9&#xd7;12&#x2212;[% Ti]/47.9&#xd7;12&#x2212;[% Zr]/91.2&#xd7;12&#x3e;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 &#x3bc;mof 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 &#x3bc;mof the tempered martensite.

Cold-rolled steel sheet

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 &#x2266;0.4, X, and the area ratio of ferrite, Vsatisfies X/V&#x2267;0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, Xis 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 &#x3bc;m or larger exist so that three or less such cementite grains are dispersed per &#x3bc;mof the hard phase. The steel sheet has improved balance between elongation and stretch flangeability and has better formability.

Cold-rolled steel sheet and method for producing the same

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 &#x3bc;m are present per one &#x3bc;mof the tempered martensite and three or fewer cementite particles having equivalent-circle diameters of 0.1 &#x3bc;m or above are present per one &#x3bc;mof the tempered martensite.

High-strength cold-rolled steel sheet

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. The steel sheets are further characterized in that at least one of the following structural factors has been regulated: the proportions of cementite particles and of the ferrite grains in the tempered martensite and the dislocation density in all structures.

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Kobe, Japan

Akira Ibano