High-strength hot-dip galvanized steel sheet with excellent processability and impact resistance and process for producing same

Abstract

Provided is a hot-dip galvanized steel sheet which has a tensile strength TS of 590 MPa or higher and excellent processability and which, even when no strain is introduced thereinto by pressing, highly absorbs energy in a low-strain range up to about 5% and has excellent collision resistance. Also provided is a process for producing the steel sheet. The galvanized steel sheet has a composition which contains, in terms of mass%, 0.04-0.13% C, 0.7-2.3% Si, 0.8-2.0% Mn, up to 0.1% P, up to 0.01% S, and 0.01-0.1% Al, with the remainder comprising iron and incidental impurities, and has a structure which comprises, in terms of areal proportion, at least 75% ferrite phase, at least 1% bainitic ferrite phase, and 1-10% pearlite phase, has a martensite phase content of up to 10% in terms of areal proportion, and satisfies (areal proportion of martensite)/((areal proportion of bainitic ferrite)+(areal proportion of pearlite))<=0.6, and in which the ratio of the Mn concentration in the ferrite phase to the Mn concentration in the second phase is 0.70 or above.

Description

technical field [0001] The present invention relates to a high-strength galvanized steel sheet excellent in impact resistance for use as a steel sheet for automobiles. Background technique [0002] In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of protecting the global environment. Therefore, there is an active trend toward thinning the wall by increasing the strength of vehicle body materials and reducing the weight of the vehicle body itself. However, increasing the strength of a steel sheet leads to a decrease in ductility, that is, a decrease in formability, and therefore, development of a material having both high strength and high formability has been desired. In addition, when the car crashes, the strain rate of each part reaches about 10 3 / s, therefore, the impact resistance characteristics in this high speed range become particularly important. Furthermore, recently, there has been an increasing dem...

AI Technical Summary

Problems solved by technology

However, the high impact absorption capacity of DP steel sheet with low yield strength is due to the fact that the work hardening caused by stamping is large, and when the processing strain is introduced, the yield strength is greatly reduced due to strain aging in the subsequent coating and sintering process. As a result, there is a problem that parts with a small amount of processing such as bending may not exhibit sufficient shock absorption capacity.

In addition, for DP steel, it has the characteristics of high impact absorption energy and excellent impact resistance in the high strain region of about 10% to about 30%, and is suitable for frontal collision parts that are deformed to a certain extent during collisions. A part that absorbs collision energy, but from the viewpoint of ensuring space for the crew, its characteristics are not sufficiently satisfactory for parts that require high energy absorption in a low-strain region such as a side collision part

[0004] In addition, Patent Document 2 discloses a technique for improving the impact resistance of TRIP steel utilizing transformation-induced plasticity of residual γ, but has the same problems as the above-mentioned DP steel.

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