Dual-phase steel sheet excellent in stretch flange formability and production method thereof

Abstract

Disclosed is a dual-phase steel sheet having low yield ratio, excellent in the balance for strength-elongation and for strength-stretch flange formability, and also excellent in bake hardening property containing (on the mass% basis). C: 0.01-0.20%, Si: 0.5% or less, Mn: 0.5-3%, sol. Al: 0.06% or less (inclusive 0%), P: 0.15% or less (exclusive 0%), and S: 0.02% or less (inclusive 0''), and in which the matrix phase contains tempered martensite; tempered martensite and ferrite; tempered bainite; or tempered bainite and ferrite, and the second phase comprises 1 to 30% of martensite at an area ratio based on the entire structure.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to a dual-phase steel sheet excellent in bake hardening property capable of ensuring high strength by applying coating and baking (hardening property after bake coating, hereinafter sometimes referred to as BH (Bake Hardening) property) and a stretch flange formability. Specifically, this invention relates to a high strength dual-phase steel sheet excellent in the bake hardening property described above and having a low yield ratio, as well as excellent in the balance for strength-elongation and the balance for strength-stretch flange formability.[0003] 2. Description of Related Art[0004] Steel sheets used by pressing in the industrial fields such as for automobiles, electric apparatus and machineries are required to have both excellent strength and ductility together and such demands for the characteristics have been increased more and more in recent years.[0005] As steel sheets intended for making the strength an...

AI Technical Summary

Benefits of technology

0091] Further, depending on the chemical compositions of the steel species used, fine retained austenite may sometimes remain.

0092] Then, basic chemical compositions constituting the steel sheet according to this invention are to be described. All the units for the chemical compositions are based on mass %.

0094] C is an element essential to the formation of the martensite contributing to the improvement of the strength, and the strength of the steel sheet in this invention is mainly determined by the area ratio and the hardness of martensite. In this invention, after heating to 2-phase region (.alpha.+.gamma.) in the final heat treatment step ["continuous annealing step or galvanization step" in (1) or (3) described above, or "second continuous annealing step or galvanization step" in (2) or (4) described above], it is cooled to transform the .gamma. phase into the martensite. The area ratio of the .gamma. phase during heating (that is, the martensite area ratio after cooling) is greatly effectuated, for example, by the amount of C in the steel and it is difficult to ensure the necessary strength when the amount of C is small. The 2-phase region (.alpha.+.gamma.) is narrowed, particularly, at 0.01% or less to worsen the productivity. Accordingly, the lower limit is defined as 0.01% (preferably, 0.02%). However, when C exceeds 0.20%, the spot weldability is deteriorated remarkably, as well...

Problems solved by technology

However, since the amount of solid solution C that can be present at super saturation in the ferrite is limited, it was difficult to obtain BH property above a certain level.

Further, while the DP steel sheet has high tensile strength (TS) at low yield ratio and also has excellent elongation (EI) property, since coarse martensite i...

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