Strength and Ductility of Ferrite-Bainite-Martensite Steels

The influence of the bainite structure and the ratio of the volume fraction of bainite fB to that of total second phase f on the tensile properties was investigated in the ferrite-bainite-martensite steels with f=0.11-0.33.(1) Continuous yielding, low yield ratio of around 0.4 and the best combination of tensile strength and total elongation were achieved in the ferrite-upper bainite-martensite steel with fB/f=0.2-0.7. This steel will have much superior stretch formability to that of ferrite-martensite steels.(2) Ferrite-bainitic ferrite steel excluding martensite islands (fB/f=1.0) had high yield ratio of about 0.7 and relatively good combination of tensile strength and total elongation compared with other ferrite-bainite steels. (3) Deformation model for the ferrite-bainite-martensite steels, which was composed of the stages of preferential yielding of the ferrite, successive yielding of the bainites and yielding of the martensite, was proposed to explain the interesting tensile properties.