| 摘要 |
PROBLEM TO BE SOLVED: To produce low yield ratio high tensile strength steel widely usable as general welded steel, having low yield ratio and low cracking sensitivity and excellent in notch toughness and soft resistance in the heat-affected zone, and to provide its production method excellent in cost effectiveness. SOLUTION: Steel having a composition containing, by weight, <=0.15% C, <=0.6% Si, <=2.0% Mn, <=0.02% P, <=0.002% S, <=0.06% N, 0.01 to 0.04% Al, <=0.06% Nb and <=0.02% Ti and the balance iron with inevitable impurities, in which weld cracking parameter Pcm is <=0.20, is subjected to thermomechanical treatment by rolling without performing quenching and tempering heat treatment, thereby a three phase composite structure composed of ferritic colonies of <1μm, ferrite of 1 to 10μm and martensite or bainite is formed while the quantity ratio of each phase is controlled to form the low yield ratio high tensile strength steel having tensile strength of >=680 MPa and yield ratio of 0.50 to 0.85.
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