Thick hot-rolled steel sheet excellent in workability and excellent in strength and toughness after heat-treatment, and method for producing the same

Abstract

Disclosed is a thick hot-rolled steel sheet which has a tensile strength of 440 to 640 MPa, preferably 490 to 590 MPa, an elongation of 20% or more, excellent uniformity in the thickness-wise direction, high strength, excellent processability, and excellent strength / toughness after a thermal treatment, and is therefore suitable for use as a structural member for an automobile, a construction machine or the like. Also disclosed is a method for producing the steel sheet. A steel material is provided, which contains 0.10 to 0.20% of C, in which the contents of Si, Mn, Al, P, S and N are adjustedwithin proper ranges, and which further contains 0.01 to 0.15% of Ti and 0.0010 to 0.0050% of B. The steel material is subjected to the hot-rolling under the conditions of a rolling finishing temperature for the finish rolling of 820 to 880 DEG C. The resulting product is cooled at a cooling rate of 15 to 50 DEG C / s and a cooling finishing temperature of 500 to 600 DEG C, and is then wound-up in a coli-like shape. In this manner, the steel sheet can have a structure which is uniform in the thickness-wise direction and is composed of a bainitic ferrite phase. The steel sheet has excellent uniformity in the thickness-wise direction so that the fluctuation in the hardness in the thickness-wise direction falls within the range up to 10% above or below the average value, and has a tensile strength of 440 to 640 MPa and an elongation of 20% or more. The steel sheet may be subjected to a further thermal treatment, thereby producing a highly-strong, highly-tough, thick and large-sized structural member having a tensile strength of 980 MPa or more and a vTrs of -60 DEG C or lower.

Description

technical field [0001] The invention relates to a thick-walled hot-rolled steel plate and a manufacturing method thereof. The "thick-walled hot-rolled steel sheet" referred to here refers to a relatively thick hot-rolled steel sheet having a thickness of not less than 6 mm and not more than 12 mm. This thick-walled hot-rolled steel sheet is suitable as a material for manufacturing structural parts of automobiles and construction machinery (hereinafter sometimes referred to as construction machinery). 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 increase mileage per gallon (gasoline mileage). Automotive parts are no exception, and weight reduction of automotive parts is strongly demanded. In addition, structural components such as construction machinery are also strongly requir...

AI Technical Summary

Problems solved by technology

If raw materials for parts are cut from such hot-rolled steel sheets (coils), non-uniform deformation occurs (when cutting in the width direction, it is called lifting), and the dimensional accuracy of the cut raw materials decreases. As a result, there is a problem that the dimensional accuracy of parts decreases

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