Microstructure, Work Hardening and Ductility of Continuously Annealed Dual Phase Steel

Abstract

Dual phase steel sheets offer potential for weight reduction in automotive components, because of combination of excellent formability and high strength.
In this paper, a study is made on the effect of heating temperature on the properties of hot rolled dual phase steel, which is obtained by continuous annealing of a steel containing 0.06%C-2.6%Mn.
The result can be summarized as follows.
1) The steel heated up-to about Ac₃ has very fine ferrite/martensite structure and also excellent strength-ductility combination. This superior ductility is caused by the improvement of local elongation related to its unique microstructure.
2) Retained austenite is observed in dual phase steels and its volume fraction is 3-5%. However the retained austenite can be easily transformed to martensite by tensile strain and completely disappears after small strain of 0.05.
3) Work hardening of dual phase steel consists of three stages up-to necking. The first stage of work hardening is related to volume fraction and mean diameter of second phase. The next one is related to volume fraction of retained austenite and the last one is only related to volume fraction of second phase.