HIGH-STRENGTH COLD-ROLLED STEEL SHEET WITH THE ULTIMATE TENSILE STRENGTH OF 780 MPa OR MORE HAVING THE EXCELLENT LOCAL DEFORMABILITY AND TIME-DELAYED RISE OF THE HARDNESS OF THE WELDING POINT Russian patent published in 2007 - IPC C22C38/04 

FIELD: metallurgy industry; production of the high-strength cold-rolled steel sheet with the ultimate tensile strength of 780 MPa or more.

SUBSTANCE: the invention is pertaining to the field of metallurgy, in particular, to the high-strength cold-rolled steel sheet with the ultimate tensile strength of 780 MPa or more. The steel sheet has the excellent local deformability and time-delayed rise of the hardness of the welding point and contains the following components (in mass %): C - 0.05-0.09, Si - 0.4-1.3, Мт - 2.5-3.2, Р - 0.001-0.05, N - 0.0005-0.006, А1 - 0.005-0.1, Ti - 0.001-0.045, S - within the range defined by the expression (A), Fe - the rest and the imminent impurities. At that the steel sheet microstructure consists of 7% or more - bainite and rest - ferrite, martensite, hardened martensite and the residual austenite or their combination; and the contents of the indicated components in the steel sheet meets the following expressions: S ≤0.08·(Ti(%)-3.43·N (%)) +0,004 (A), at that at the negative value of the term [Ti (%) - 3.43·N(%)] the indicated value is taken equal to null, 950 ≤ Mneg./(C(%)-Si(%)/75)))·the bainite area(%)(C), where Mneg. = Mn (%)-0.29·Si(%)+6.24·C(%) (B), (C(%)+(Si(%)/20)+(Mn (%)/18)≤ 0.30 (D). The steel sheet has the high strength, the time-delayed rise of the hardness of the welding point and the good weldability.

EFFECT: the invention ensures the steel sheet high strength, the time-delayed rise of the hardness of the welding point and the good weldability.

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