| 摘要 |
<p>Provided is a high-strength hot rolled steel sheet having excellent toughness even if the tensile strength is strengthened to at least 780 MPa. The sheet contains, by mass%, 0.04-0.12% C, 0.5-1.2% Si, 1.0-1.8% Mn, no greater than 0.03% P, no greater than 0.0030% S, 0.005-0.20% Al, no greater than 0.005% N, and 0.03-0.13% Ti, the remainder being a composition of Fe and unavoidable impurities. The structure is such that the area ratio of a bainite phase is over 95%, the average grain size of the bainite phase being no greater than 3 µm. Furthermore, the difference (?Hv1) between the Vickers hardness at a position 50 µm from the surface and the Vickers hardness at a position at 1/4 of the sheet thickness is no greater than 50, the difference (?Hv2) between the Vickers hardness at the position at 1/4 of the sheet thickness and the Vickers hardness at a position at 1/2 of the sheet thickness is no greater than 40, the sheet thickness is 4.0 to 12 mm inclusive, and the tensile strength is at least 780 MPa.</p> |
