STEEL MATERIAL

摘要

A steel material that has a chemical composition which contains, in terms of mass%, 0.05-0.2%, excluding 0.05%, C, 1-3% Mn, 0.5-1.8%, excluding 0.5%, Si, 0.01-0.5% Al, 0.001-0.015% N, more than 0.1% to 0.25% Ti or the sum of V and Ti, at least 0.001% Ti, 0-0.25% Cr, and 0-0.35% Mo, with the remainder comprising Fe and impurities, and that has a steel structure which is a dual-phase structure comprising at least 50% by area main phase comprising ferrite and a second phase comprising one or more constituents selected from the group consisting of bainite, martensite, and austenite, the second phase having an average nanohardness less than 6.0 GPa. In cases when a boundary where the difference in orientation between the crystals is 2º or more is defined as a grain boundary and the region surrounded with this grain boundary is defined as a crystal grain, all crystal grains in the main phase and second phase have an average grain diameter of 3 µm or less and the proportion of the length of small-angle grain boundaries where the difference in orientation is 2-15º, excluding 15º, in the length of all grain boundaries is 15% or higher.