| 摘要 |
PROBLEM TO BE SOLVED: To provide a thick hot-rolled steel sheet which has a tensile strength of 440-640 MPa, preferably 490-590 MPa, and an elongation of≥20%, is excellent in homogeneity in the sheet thickness direction, has a high strength, is excellent in workability, is excellent in strength and toughness after a heat-treatment and is suitably used as a structural member of an automobile or a construction machine, and a method for producing the same. SOLUTION: The method for producing the thick hot-rolled steel sheet comprises the steps of: subjecting a steel raw material comprising 0.10-0.20% C, appropriately adjusted amounts of Si, Mn, Al, P, S and N, 0.01-0.15% Ti and 0.0010-0.0050% B to hot rolling in which a rolling finishing temperature is 820-880°C in finish rolling; subsequently cooling the same at a cooling rate of 15-50°C/s to a cooling stop temperature of 500-600°C; and winding the same into a coil. The method yields the thick hot-rolled steel sheet which comprises a structure which is homogenous in the sheet thickness direction and comprises a bainitic ferrite phase, is excellent in homogeneity in the sheet thickness direction represented by a difference in hardness in the sheet thickness direction of within 10% from an average and satisfies a tensile strength of 440-640 MPa and an elongation of≥20%. By further subjecting the steel sheet to a hot-treatment, a large, thick, high-strength, high-toughness structural member having a tensile strength of≥980 MPa and a vTrs of≤-60°C can be manufactured. COPYRIGHT: (C)2009,JPO&INPIT
|
