| 摘要 |
The present invention provides a hot-rolled steel sheet in which it is possible to prevent any reduction in the strength at the plate-thickness center of the steel sheet during heat treatment, even when there is minimal processing of the steel sheet and the rate of work hardening is low. This hot-rolled steel sheet contains, in percent by mass, 0.040-0.150% of C, 0-0.500% of Si, 0.10-1.50% of Mn, 0-0.050% of P, 0-0.020% of S, 0.010-0.050% of Al, 0.0010-0.0060% of N, 0.008-0.035% of Nb, 0-0.10% of Cu, 0-0.10% of Ni, 0-0.02% of Cr, 0-0.020% of Mo, 0-0.20% of V, 0-0.0100% of Ca, and 0-0.0050% of B as chemical components, and also contains 0.005-0.030% of Nb in solid solution, the balance being iron and unavoidable impurities. The ferrite structure has a surface-area ratio of 85% or higher, the balance having a cementite and/or perlite structure, and the average crystal grain diameter of the ferrite being 5-20 μm. |
