High strength steel sheet excellent in deep drawing characteristics and method for production thereof

Abstract

The present invention provides a high-strength steel sheet useful for applications to automobile steel sheets and the like and having excellent deep drawability, a tensile strength (TS) of as high as 440 MPa or more, and a high r value (average r value 1.2), and a process for producing the steel sheet. The steel sheet has a composition containing, by % by mass, 0.010 to 0.050% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, 0.005 to 0.1% of P, 0.01% or less of S, 0.005 to 0.5% of A1, 0.01% or less of N, and 0.01 to 0.3% of Nb, the Nb and C contents in steel satisfying the relation, (Nb / 93) / (C / 12) = 0.2 to 0.7, and the balance substantially including Fe and inevitable impurities. The steel microstructure contains a ferrite phase and a martensite phase at area ratios of 50% or more and 1% or more, respectively, and the average r value is 1.2 or more.

Description

technical field [0001] The present invention proposes a high-strength steel sheet with high tensile strength (TS) of 440 MPa or more, high r-value (average r-value ≥ 1.2), excellent deep drawability, and a manufacturing method thereof, which are useful for use in automobile steel sheets. Background technique [0002] In recent years, from the viewpoint of protecting the global environment, due to regulations on CO 2 Emissions are required to improve the fuel consumption of automobiles. Furthermore, in order to ensure the safety of passengers during a collision, it is also required to improve the safety centering on the collision characteristics of the automobile body. This actively contributes to the lightweight and high strength of the automobile body. [0003] In order to satisfy both the lightweight and high strength of the automobile body, it is generally believed that it is effective to increase the strength of the component raw materials and reduce the thickness of t...

AI Technical Summary

Problems solved by technology

[0008] However, in the technology of adding solid-solution strengthening elements using such ultra-low carbon steel as a raw material, when producing so-called high-strength steel sheets with a tensile strength of 440 MPa or more, or further 500 or 590 MPa or more, the amount of alloying elements added increases. , causing problems in surface appearance, deterioration of plating performance, and obvious brittleness of secondary processing.

Furthermore, since the r-value deteriorates when a large amount of solid-solution strengthening components is added, there is a problem that the level of the r-value decreases as the strength is increased.

In addition, in order to reduce the amount of C to the ultra-low carbon region in which the amount of C specifically disclosed in Citation 1 is less than 0.010%, vacuum degassing is required in the steelmaking process, that is, a large amount of CO is generated during the manufacturing process. 2 , from the perspective of protecting the earth's environment, it is hardly a good technology

According to this method, after cold rolling, it is necessary to perform annealing for obtaining a good r value and heat treatment for forming a structure separately, and in the annealing process, it is necessary to carry out a long holding time of 1 hour or more based on box annealing. Maintain, there is a problem of poor productivity in the process (in terms of time)

Moreover, due to the high percentage of the second phase of the resulting structure, it is difficult to stably ensure an excellent balance of strength and ductility

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