High-strength cold rolled steel sheet having excellent deep drawability and bake hardenability and method for manufacturing the same

Abstract

A high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability, with a tensile strength ≧440 MPa, an average r-value ≧1.20 and a bake hardening value ≧40 MPa, is obtained by subjecting a steel raw material having a chemical composition including C: 0.010-0.06 mass %, Si: more than 0C:\Users\ejensen\Desktop\!TEMP\.5 mass % but not more than 1.5 mass %, Mn: 1.0-3.0 mass %, Nb: 0.010-0.090 mass %, Ti: 0.015-0.15 mass % and satisfying(Nb / 93) / (C / 12)<0.20 to hot rolling, cold rolling and then to annealing including steps of heating to a temperature of 800-900° C. while a temperature region of 700-800° C. is an average heating rate <3° C. / s, and soaking and thereafter cooling at a rate ≧5° C. / s from the soaking temperature to a cooling stop temperature ≦500° C. to thereby form a microstructure including ferrite phase with an area ratio ≧70% and martensite phase with an area ratio ≧3%.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength cold rolled steel sheet suitable for use in an outer panel and the like of an automobile body and having a tensile strength TS of not less than 440 MPa, an average r-value of not less than 1.20 and a Bake hardening (BH) value of not less than 40 MPa and excellent deep drawability and bake hardenability as well as a method for manufacturing the same.BACKGROUND ART[0002]Recently, it is strongly demanded to reduce the weight. of the automobile body for improving a fuel consumption of an automobile and reducing CO2 discharge amount from a viewpoint of the protection of global environment. At the same time, it is strongly demanded to increase the strength of the automobile body in view of ensuring safeness of crew member(s) at the time of impact. In order to meet these demands, it is required to simultaneously satisfy the weight reduction and the increase in strength of the automobile body. To this end, it is effective...

AI Technical Summary

Benefits of technology

The present invention relates to a high-strength cold rolled steel sheet with excellent deep drawability and bake hardenability. The steel sheet has a tensile strength of not less than 440 MPa, an average r-value of not less than 1.20, and a BH value of not less than 40 MPa. The C content is controlled to a level of 0.010-0.06 mass% and the relation between Nb addition amount and C content is restricted to less than 0.20 to reduce the negative effects on the deep drawability caused by solute C. By controlling the C content and adding a sufficient amount of Nb and Ti, the steel sheet has an adequate amount of solute C for the development of {111} recrystallization texture and the formation of martensite, which increases the r-value and strength. The high-strength cold rolled steel sheet can be used for automobile parts to increase strength and improve collision safety while reducing weight.

Problems solved by technology

As previously mentioned, the conventional techniques utilizing solid solution enhancement for increasing the strength of mild steel sheets having an excellent deep drawability are necessary to add a greater amount or excessive amount of alloying elements and have problems in not only r-value and BH property but also raw material cost.

Also, the technique for increasing the strength by utilizing microstructure enhancement has problems in production that the prolonged annealing is necessary, and another heat treatment is necessary after the annealing for forming the desired microstructure, and the high-speed cooling equipment is necessary and so on.

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