Dual phase hot rolled steel sheet having excellent formability and stretch flangeability

Abstract

The present invention provides a process of producing a family of hot rolled dual phase steel sheets having excellent formability and stretch flangeability, with yield strengths of from about 500 MPa to about 900 MPa from a single steel chemistry consisting of, by weight percent, 0.02-0.15% of C, 0.3-2.5% of Mn, 0.1-2.0% of Cr, 0.01-0.2% Al, 0.001-0.01% Ca, not more than 0.1% P, not more than 0.03% S, not more than 0.2% Ti, not more than 0.2% V, not more than 0.2% Nb, not more than 0.5% Mo, not more than 0.5% Cu, not more than 0.5% Ni, the balance being Fe and unavoidable impurities. A slab or ingot of this composition is reheated to a temperature of between 1050° C. and 1350° C. and held at this temperature for at least 10 minutes, then hot rolled, completing the hot rolling at between 800° C. and 1000° C. The sheet is cooled, immediately after completion of hot rolling, at a rate of not lower than 10 C / sec., without requiring specific cooling patterns, and coiled at a temperature of not less than 450 C. The cooling temperature is controlled to produce the desired yield strength within the range of from about 500 MPa to about 900 MPa.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to hot-rolled dual phase-structure (ferrite / martensite) steel sheet products a method for their production. In particular, the hot rolled steel sheet has excellent formability and stretch flangeability, as well as improved surface quality, weldability and fatigue property.[0003] 2. Brief Description of the Prior Art[0004] Currently, applications of high strength steel sheets to automotive parts are increasing in order to reduce vehicle weight and thus improve the fuel efficiency. Among these high strength steels, dual phase steel, which possesses a microstructure consisting of martensite islands embedded in a ferrite matrix, is attracting more and more attention due to its superior combination of high strength, excellent ductility, continuous yielding and low yield ratio. In particular, dual phase steels help to improve vehicle crashworthiness and durability and therefore improve passenger safety and vehicle life. ...

AI Technical Summary

Benefits of technology

[0013] As a principal object, the present invention has thereof been made in order to advantageously avoid the above described problems of the prior methods and has the provision of an alloy design and manufacturing method, which has less demanding or restrictive facility and processing requirements, for producing hot rolled dual phase steel sheet at higher coiling temperatures without any specific annealing and / or cooling processes following the conventional hot rolling and coiling steps.

[0056] Finally, the microstructure of the presently invented hot rolled steel sheets was examined. One of the typical micrographs obtained using a "LePera" etching technique is given in FIG. 4. As is illustrated by and can be readily observed in this micrograph, fine martensite islands (white) are uniformly distributed in the fine-grained ferrite matrix. It is such a dual phase structure that provides not only the excellent formability but also the excellent stretch flangeability.

Problems solved by technology

Although the hot rolled dual phase steel sheets obtained by the previous patents or methods exhibit low yield ratio and good ducility, no improved stretch flangeability has been demonstrated.

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