HIGH TENSILE STRENGTH STEEL HAVING EXCELLENT PLASTIC DEFORMABILITY AFTER COLD WORKING, AND METHOD F

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PROBLEM TO BE SOLVED : To provide a high strength steel having a tensile strength of ≥780 MPa and a yield ratio of ≤90%, and to provide a method for producing the same. SOLUTION : The high tensile strength steel comprising, by mass, 0.02 to 0.2% C, 0.01 to 0.5% Si, 0.4 to 2.5% Mn, 0.1 to 1% Cr, 0.1 to 1% Mo, 0.01 to 0.035% Ti, 0.0003 to 0.005% B and 0.001 to 0.1% Al, and the balance Fe with impurities in which the content of P is ≤0.05%, the content of S is ≤0.008% and the content of N is ≤0.01%, also, the value expressed by the following formula (a) is ≤85, and the ratio of martensite is ≥80% : the formula (a) is 44.6+1086×Nb(%)+773×Ti(%)+2.44×(RFe/C(%)); wherein the atomic symbol in the formula is the content (mass%) of the element; and RFe is the content (mass%) of Fe present as precipitates measured by an extraction residue method. The steel may contain one or more kinds selected from Nb, Cu, Ni, V, Ca, Mg, rare earth metals and Zr as well. In the production method, specified slab heating temperature, quenching conditions and heat recuperating conditions are used. COPYRIGHT : (C)2007, JPO&INPIT