| 摘要 |
A steel material having a chemical composition of, by mass%, C: greater than 0.05% to 0.2%, Mn: 1% to 3%, Si: greater than 0.5% to 1.8%, Al: 0.01% to 0.5%, N: 0.001% to 0.015%, Ti or a sum of V and Ti: greater than 0.1% to 0.25%, Ti: 0.001% or more, Cr: 0% to 0.25%, Mo: 0% to 0.35%, and a balance: Fe and impurities, includes a steel structure being a multi-phase structure having a main phase made of ferrite of 50 area% or more, and a second phase containing one or two or more selected from a group consisting of bainite, martensite and austenite, in which an average nanohardness of the above-described second phase is less than 6.0 GPa, and when a boundary where a misorientation of crystals becomes 2° or more is defined as a grain boundary, and a region surrounded with the grain boundary is defined as a crystal grain, an average grain diameter of all crystal grains in the above-described main phase and the above-described second phase is 3 µm or less, and a proportion of a length of small-angle grain boundaries where the misorientation is 2° to less than 15° in a length of all grain boundaries is 15% or more. |
