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

[0090] The steel sheet according to this invention may comprise only the matrix phase and the second phase but it may contain bainite as other different kind of structure within a range as not deteriorate the effect of the invention. The bainite structure can be remained naturally, for example, in the production process according to this invention to be described later [for example, in a step of cooling at an average cooling rate of 3.degree. C. / s or more down to Ms point or lower in "continuous annealing step or galvanization step" in (1) or (3) described above, or "second continuous annealing step or galvanization step" in (2) or (4)"; or in a step of alloying after the method (1)-4) described above]. It is preferred that less bainite structure is contained.

[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 as the increase of the martensite area ratio in the steel sheet not only deteriorates the workability but also increases the yield ratio. Accordingly, the upper limit is defined as 0.20% (preferably, 0.15%).

[0096] Si is an element contributing to the improvement of ductility such as elongation by decreasing the amount of the solid solution C in the .alpha. phase. In order to effectively provide such an effect, it is preferably added by 0.05% or more (more preferably, 0.1% or more). However, since galvanization failure occurs, for example, in a case of zinc galvanization when Si is added in excess of 0.5%, the upper limit is defined as 0.5% (preferably, 0.3%).

[0098] Mn is useful as a solid solution strengthening element and also is an element necessary for stabilizing the .gamma. phase by suppressing transformation in the cooling process. Further, it is useful for forming a desired martensite phase. In order to effectively provide such an effect, it is added by 0.5% or more (preferably, 0.7% or more and, more preferably, 1% or more). However, since Mn deteriorates the galvanization property upon zinc galvanization when added in excess of 3%, the upper limit is defined as 3% (preferably, 2.5% or less and, more preferably, 2% or less).

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 induces trigger for fracture, it was poor in the stretch flange formability (local ductility: .lambda.).

However, it has been found that the steel sheet involves problems that it is difficult to obtain a high ductility (high elongation) at a level identical with that of the existent DP steel sheet and the yield ratio is increased somewhat.

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