Abstract
Composite ferrite–lath martensite dual phase steel is an alternative to high carbon pearlitic steel for producing high strength wire and may be drawn with no intermediate patenting treatments because of its high work hardening rate and formability. This paper presents the study of the relationships between the void density in drawn wires, the drawing limit, and the initial structure. An Fe–2Si–0·1C (wt-%) dual phase steel was treated to produce three different martensite morphologies at constant volume fraction, then wire drawn. The void density varied linearly with total drawing strain in all three conditions. High magnification scanning electron microscopy revealed that voids were initiated by decohesion at ferrite/martensite interfaces and by shear cracking of martensite particles. The voids grow to form cracks parallel to the drawing direction and affect the drawing limit. The character of the martensite, either dislocated lath or twinned plate, was determined by transmission electron microscopy. The void density was least in the fibrous lath martensite structure, where the lath martensite has a high degree of initial structural coherency with the surrounding ferrite, which hindered void formation and hence delayed the drawing limit.
MST/1024
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