| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel sheet excellent in punchability. <P>SOLUTION: The high-strength hot-rolled steel sheet has a composition containing 0.050-0.15% C, 0.1-1.5% Si, 1.0-2.0% Mn, 0.03% or less P, 0.0030% or less S, 0.01-0.08% Al, 0.05-0.15% Ti and 0.005% or less N, and comprises a structure having a bainitic phase at an area ratio of more than 95%. Furthermore, in the structure, an average grain size of bainite in a region from the surface to a position at 1/4 of the sheet thickness is 5 μm or less in a cross section in the L direction, and 4 μm or less in a cross section in the C direction, and additionally, in the central part of the sheet thickness, the number of crystal grains having an aspect ratio of 5 or more and extending in the rolling direction is 7 or less. Accordingly, the hot-rolled steel sheet has: high strength of tensile strength TS of 780 MPa or more; a punched edge face having fine and uniform ductile fracture; excellent punched edge face properties; and excellent punchability. <P>COPYRIGHT: (C)2012,JPO&INPIT |
