| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-rolled thin steel plate which has a high tensile strength of 440 MPa or higher, superior workability, further superior strength and toughness after heat treatment, and a plate thickness of less than 6 mm, and to provide a manufacturing method therefor. SOLUTION: A base steel material for the hot-rolled thin steel plate comprises 0.10 to 0.20% C, Si, Mn, Al, P, S and N controlled to an appropriate range, and further 0.01 to 0.15% Ti and 0.0005 to 0.0050% B. The method for manufacturing the steel plate comprises the steps of: hot-rolling the steel material so that the ending temperature in the finish-rolling process can be 820 to 880°C; cooling the hot-rolled plate at a surface cooling speed of 15 to 50°C/s until the surface temperature reaches a temperature range of 550 to 650°C; and winding the cooled plate into a shape of a coil in the temperature range. Thereby manufactured hot-rolled steel plate shows a structure which is uniform in the plate thickness direction and comprises a bainitic ferrite phase, and acquires such high strength as is shown by a tensile strength of 440 to 640 MPa and such high ductility as is shown by an elongation of 20% or higher; and when further being subjected to a working process and heat treatment, can be converted into a product which has such high strength as is shown by a tensile strength of 980 MPa or higher and such high toughness as is shown by a vTrs of -100°C or lower. COPYRIGHT: (C)2008,JPO&INPIT
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