High-strength hot-rolled steel sheet and method for producing same

Abstract

The present invention provides a high-strength hot-rolled steel sheet having both excellent strength and excellent workability (particularly, bending workability), and a method of producing the same.The steel sheet of the present invention has a certain composition as well as microstructures such that an area ratio of ferrite phase is 95% or more, an average grain size of the ferrite phase is 8 μm or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm. The steel sheet of the present invention also has a tensile strength of 980 MPa or more.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This is the U.S. National Phase application of PCT / JP2013 / 000257, filed Jan. 21, 2013, which claims priority to Japanese Patent Application No. 2012-013592, filed Jan. 26, 2012, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to a high-strength hot-rolled steel sheet that has both high strength, or a tensile strength (TS) of 980 MPa or more, and excellent workability (particularly, bending workability), and is usefully applied in automobile members, and a method for producing the same.BACKGROUND OF THE INVENTION[0003]In recent years, to reduce CO2 emission from the viewpoint of global environment protection, there is an increasing demand in the entire automobile industry for improved fuel efficiency of automobiles. To improve fuel efficiency of automobiles, it is most effective to reduce the weight of automob...

AI Technical Summary

Benefits of technology

[0013]The present invention has been made in view of these situations, and an object of the present invention is to provide a high-strength hot-rolled steel sheet that has a tensile strength of 980 MPa or more, and still exhibits excellent bending workability.

[0017]As a result, the inventors have conceived an idea of adding vanadium (V) as reinforcing means. It is hard for V to precipitate when added alone due to its high solubility in steel, whereas it becomes easier for V to precipitate when coupled with Ti carbides. As a result, when Ti and V are added in combination to the steel material of the hot-rolled steel sheet in an appropriate amount, the strength of the steel sheet is dramatically increased as compared to the case where Ti or V is added alone, whereby a hot-rolled steel sheet having a tensile strength of 980 MPa or more is obtained.

[0019]In addition, the inventors of the present invention have searched for ways to impart excellent bending workability to a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more, to which Ti and V have been added in combination as described above, while maintaining the strength of the steel sheet. As a result, to give bending workability, it was found advantageous to improve the surface appearance quality of the steel sheet, and furthermore, reduce solute elements that would deteriorate the workability of the steel sheet and reduce inclusions as much as possible that would serve as the origins of voids. As a result of further investigations, it was revealed that a hot-rolled steel sheet that has a tensile strength of 980 MPa or more and exhibits excellent bending workability may be obtained by arranging the steel sheet to have a component composition with optimized contents of C, Mn, Ti and V, while reducing Si content as much as possible.

[0059]According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent bending workability that is suitably applicable to automobile structural members or the like, that is highly advantageous in, for example, being capable of reducing the weight of automobile members, forming automobile members or the like, and that enables the even wider application of high-strength hot-rolled steel sheets, thereby causing a significantly advantageous effect in industrial terms.

Problems solved by technology

However, the technique proposed by PTL 1 involves substantially enhancing the strength of a steel sheet by dispersing NbC, and it is difficult to obtain a steel sheet having a tensile strength of 980 MPa or more by using this technique utilizing NbC.

This is because while the degree of precipitation strengthening achieved by dispersing precipitates increases with increasing carbide volume fraction, it is not possible to increase the carbide volume fraction due to a small solubility product in steel and a large atomic density of NbC.

In addition, in the technique proposed by PTL 2, Ti and V are added to steel as precipitation-strengthening elements, but Ti and V for forming carbides are contained in the steel in a small amount, or added to the steel in an inappropriate manner, in which case, again, the tensile strength of the steel sheet does not reach 980 MPa.

As described above, in the conventional techniques, it was difficult to obtain a high-strength steel sheet having a tensile strength of 980 MPa or more.

Moreover, it was not possible to impart excellent bending workability to the steel sheet, while retaining such high strength of the steel sheet.