HOT-ROLLED STEEL SHEET HAVING EXCELLENT COLD WORKING PROPERTIES AND HARDENING PROPERTIES, AND METHOD FOR PRODUCING SAME

摘要

Disclosed is a hot-rolled steel sheet which has excellent cold working properties and excellent hardening properties. Specifically, a steel material, which has a composition containing, in mass%, 0.18-0.29% of C, 0.0050% or less of N, 0.002-0.05% of Ti and 0.0005-0.0050% of B with adequate amounts of Si, Mn, P, S and Al, is hot rolled while setting the finish rolling ending temperature to 800-900°C, and after completing the hot rolling, the steel material is cooled at an average cooling rate of 20°C/s or less and wound up at a winding temperature (CT) of 500°C or more. Consequently, the hot-rolled steel sheet has a structure that is composed of a ferrite phase and a pearlite phase, and the ferrite phase has an average crystal grain size of 7.0-15.0 µm and a structural fraction of 50% by volume or more. The hot-rolled steel sheet has a tensile strength of 500 MPa or less, and tensile strength variations in the width direction including those in the edges of the steel sheet are 60 MPa or less. As a result, the hot-rolled steel sheet has achieved both excellent cold working properties and excellent hardening properties. In addition to the above described composition, the hot-rolled steel sheet may contain one or both of Nb and V, one or two elements selected from among Ni, Cr and Mo, and one or both of Sb and Sn.