Patent ID: 8128762

Claim:
A steel sheet comprising: 0.03 to 0.13 percent by mass of carbon (C), 0.02 to 0.8 percent by mass of silicon (Si), 1.0 to 2.5 percent by mass of manganese (Mn), 0.03 percent by mass or less of phosphorus (P), 0.01 percent by mass or less of sulfur (S), 0.01 to 0.1 percent by mass of aluminum (Al), 0.01 percent by mass or less of nitrogen (N), and at least one member selected from the group consisting of 0.004 to 0.1 percent by mass of titanium (Ti) and 0.004 to 0.07 percent by mass of niobium (Nb), with the remainder including iron and inevitable impurities, wherein the steel sheet structurally has a ferrite matrix structure and bainitic and martensitic second phase structures, the steel sheet has a ferrite fraction of from 50 to 86 percent by area, a bainite fraction of from 10 to 30 percent by area, and a martensite fraction of from 4 to 20 percent by area based on the entire structure, wherein the bainite area fraction is larger than the martensite area fraction, and wherein the ferrite has an average grain size of 2.0 to 5.0 μm, and the ratio of the average hardness (Hv) of the ferrite to the tensile strength (MPa) of the steel sheet is equal to or more than 0.25.