Hot-rolled steel sheets excellent both in workability and in strength and toughness after heat treatment and process for production thereof

Abstract

The invention provides a hot-rolled steel sheet of less than 6mm in thickness which has high strength corresponding to a tensile strength of 440MPa or above and is excellent both in workability and in strength and toughness after heat treatment; and a process for the production of the sheet. The process comprises subjecting a raw steel material which contains C: 0.10 to 0.20%, Si, Mn, Al, P, S and N: properly adjusted contents, Ti: 0.01 to 0.15%, and B: 0.0005 to 0.0050% to hot rolling wherein the finishing temperature of finish rolling is 820 to 880 DEG C, cooling the hot-rolled steel material at a surface cooling rate of 15 to 50 DEG C / s until such a temperature region that the surface temperature is 550 to 650 DEG C, and winding up it in a coil in the temperature region. The process can give a high -strength and -ductility hot-rolled steel sheet which has a bainitic ferrite structure homogeneous in the thicknesswise direction and exhibits a tensile strength of 440 to 640MPa and an elongation of 20% or above. Further, high-strength and -toughness products having tensile strength of 980MPa or above and vTrs of -100 DEG C or below can be produced by subjecting the hot-rolled steelsheet additionally to working and heat treatment.

Description

technical field [0001] The present invention relates to a hot-rolled thin steel sheet suitable for use as an automobile member, in particular, suitable for use as an airbag component, having a tensile strength of 440 to 640 MPa, excellent workability, and excellent strength and toughness after heat treatment after processing A hot-rolled steel sheet and a manufacturing method thereof. In addition, the "thin steel sheet" referred to here refers to a steel sheet having a thickness of less than 6 mm, preferably a steel sheet having a thickness of 1 mm or more. Background technique [0002] In recent years, from the viewpoint of protecting the global environment, the exhaust gas regulation of automobiles has been strengthened, and the weight reduction of automobile bodies has been promoted in order to improve gallon mileage (gasoline mileage). The automotive components are no exception, and the weight reduction of automotive components is also strongly demanded. Among automoti...

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Problems solved by technology

[0010] However, although the thin steel sheets manufactured by the techniques described in Patent Document 1 and Patent Document 2 have excellent strength characteristics after heat treatment, there is a problem that the toughness after heat treatment is insufficient, not only failing to meet the latest toughness requirements, but also due to the The strength is too low, and sometimes the strength is insufficient in the part where no heat treatment is performed

Especially when the strength of the portion not subjected to heat treatment requires a strength of 490 MPa or more, this problem is remarkable.

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