LOW YIELD RATIO AND HIGH TENSION STEEL MATERIAL EXCELLENT IN TOUGHNESS OF WELD HEAT-AFFECTED ZONE, AND PROCESS FOR PRODUCING THE SAME

摘要

A steel material which is excellent in toughness of weld heat-affected zone and exhibits a low yield ratio of 80% or less in a high strength range of 590 MPa or more in the case wherein a welding operation with a heat input of 30 kJ/mm or more is performed, and a method for producing the steel material are provided. A steel material comprises, by mass percent, 0.03 to 0.2% of C, 0.02 to 0.5% of Si, 1.0 to 2.0% of Mn and 0.002 to 0.01% of N, satisfies 0.02% or less of P, 0.015% or less of S and 0.01% or less of Al, and contains at least one of 0.001 to 0.1% of a REM(rare earth metal) and 0.0003 to 0.005% of Ca and 0.001 to 0.05% of Zr respectively, wherein the steel material contains at least one of an oxide of the REM and CaO, and Zr2O as oxides in steel, and comprises 4 to 24% of a fraction of ferrite occupied in the entire structure of the steel material, and the balance of at least one of a bainite structure and a martensite structure. A steel material comprises, by mass percent, 0.03 to 0.12% of C, 0.02 to 0.5% of Si, 1.4 to 1.8% of Mn and 0.002 to 0.01% of N, satisfies 0.02% or less of P, 0.015% or less of S and 0.01% or less of Al, and contains at least one of 0.001 to 0.1% of a REM(Rare Earth Metal) and 0.0003 to 0.02% of Ca and 0.001 to 0.05% of Zr respectively, wherein the steel material contains at least one of an oxide of the REM and CaO, and Zr2O as oxides in steel, and comprises 1.1% or less of a fraction of island-shaped martensite occupied in the entire structure of the steel material, and the balance of a bainite structure.