Method for producing a galvanized steel sheet

Abstract

A method for producing a galvanized steel sheet including (a) melting a steel having a steel composition comprising 0.005 to 0.04 mass % C, 1.5 mass % or lower Si, 1.0 to 2.0 mass % Mn, 0.10 mass % or lower P, 0.03 mass % or lower S, 0.01 to 0.1 mass % Al, less than 0.008 mass % N and 0.2 to 1.0 mass % Cr, wherein Mn (mass %)+1.29 Cr (mass %) is 2.1 to 2.8, and the balance being iron and unavoidable impurities, (b) hot rolling and cold rolling the steel from step (a) to provide a steel sheet, and (c) annealing the steel sheet from step (c) at an annealing temperature of at least the Ac1 point and not more than the Ac3 point, wherein the galvanized steel sheet has a structure which includes a ferrite phase and a martensite phase with a volume fraction being at least 3.0% and less than 10%, the average particle diameter of the ferrite is larger than 6 μm and not more than 15 μm, and 90% or more of the martensite phase exists in a ferrite grain boundary.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a Divisional application of U.S. application Ser. No. 12 / 084,173 filed Apr. 25, 2008, which is the United States national phase application of International application PCT / JP2006 / 326320 filed Dec. 25, 2006. The entire contents of each of U.S. application Ser. No. 12 / 084,173 and International application PCT / JP2006 / 326320 are hereby incorporated by reference herein.[0002]The present invention relates to galvanized steel sheets that are applicable in fields including automobiles and home appliances, have favorable press-formability and are-excellent in terms of strength-ductility balance and bake-hardenability, as well as methods for producing such galvanized steel sheets.BACKGROUND ART[0003]Recently, improvement in fuel efficiency of automobiles has been demanded from the perspective of global environment sustainability, and safety improvement of automobile bodies has also been desired from the perspective of protectin...

AI Technical Summary

Benefits of technology

The solution provides galvanized steel sheets with enhanced strength-ductility balance and bake-hardenability, suitable for applications in automobiles and home appliances, ensuring high formability and improved surface quality while maintaining productivity.

Problems solved by technology

However, the background arts described above have the following problems.

In the case of a strength being equal to or higher than 440 MPa, large quantities of Si and P should be added and thus issues deterioration of on the surface characteristics, such as difficulties in alloying, red scales or plating failures, are significant.

It is therefore difficult to apply these techniques to outer panels of automobiles requiring stringent control of surface quality.

So this technique cannot be easily applied to outer panels of automobiles mainly formed by stretch forming, such as doors and hoods.

This may affect the productivity.

As a result, pearlitic or bainitic transformation progressed during the cooling step and it was difficult to achieve 90% or higher content ratio of martensite in the second phase consistently.

This result indicates that steel sheets with excellent strength-ductility balance cannot be easily obtained by using the components and production methods described in Patent Document 3 or 4 because the ductility may be decreased as the result of pearlite or bainite generation in the second phase.

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