High strength hot rolled steel sheet having excellent stretch flangeability and its production method

Abstract

Disclosed is a hot rolled steel sheet in which it includes a steel having a chemical composition of 0.03 to 0.10% (the percent hereinafter representing % by mass) of C, 0.2 to 2.0% of Si, 0.5 to 2.5% of Mn, 0.02 to 0.10% of Al, 0.2 to 1.5% of Cr, 0.1 to 0.5% of Mo, and the residue of iron and inevitable contaminants, and in this steel sheet, at least 80% by area in longitudinal cross section has a martensitic structure. As a consequence, a high strength hot rolled steel sheet having a tensile strength of the level as high as 980 MPa or higher simultaneously with an excellent forming workability, and in particular, excellent stretch flangeability is provided at a relatively low cost.

Description

technical field [0001] The present invention relates to a high-strength hot-rolled steel sheet having, for example, a tensile strength of 980 MPa or higher and excellent stretch flangeability, and a method for producing the same. The hot-rolled steel sheet is suitable as, for example, automobile members and bumpers , Beam (pillar) and other reinforcement raw materials. Background technique [0002] Recently, development of a hot-rolled steel sheet having a high tensile strength on the order of 980 MPa or higher and having improved stretch-flangeability from the viewpoint of formability has been advanced. As a method for obtaining high elongation flangeability, there are methods of making the metal structure a single-phase structure such as ferrite and bainitic ferrite, and simultaneously making the structure a composite structure of ferrite and bainite. As a method of suppressing the difference in strength between the two phases as much as possible, in order to secure such ...

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Problems solved by technology

That is, in the method of utilizing the precipitation strengthening by the addition of microalloy disclosed in the above-mentioned document, since a large amount of Ti or the like is added, the heating temperature of the slab before hot rolling has to be increased, and there is a thermal difficulty.

Furthermore, when the strength is increased by precipitation strengthening by adding Ti or the like, since the yield ratio is significantly increased, the shape freezing property after press working is deteriorated

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