| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet which has excellent fatigue crack propagation resistance, and has excellent toughness in a weld zone even when subjected to welding operation. SOLUTION: The steel sheet has a composition containing, by mass, 0.03 to 0.2% C, 0.05 to 0.6% Si, 0.3 to 2% Mn and 0.001 to 0.1% sol.Al, and, if required, containing one or more metals selected from Cu, Ni, Cr, Mo, V, Nb and Ti, and the balance Fe with impurities, and also satisfying the inequality of 1.3×C(%)+0.04<=Si(%)<=6.4×C(%)+0.1. In the steel sheet, the ratio occupied by a pearlitic structure is <=5% by area, and a mixed structure of ferrite and bainite is contained in >=85% by area, and in the structure, the area ratio of the structure having an aspect ratio of <=1.7 to be occupied is 15 to 45%, and the difference in hardness between the structure having an aspect ratio of <=1.7 and the structure having an aspect ratio of >1.7 is <=150 by Vickers hardness. |
