Effect of Ferrite/Martensite on Microstructure Evolution and Mechanical Properties of Ultrafine Vanadium Dual-Phase Steel

Ultrafine-grained dual-phase ferrite/martensite steel produced through intercritical annealing at 765, 775 and 795 degrees C. The microstructures at all temperatures consisted of ultrafine ferrite, martensite and carbides. Carbides were found in two different morphologies, alloy carbides and V(C, N). The grain size of ferrite was decreased to 0.83 +/- 0.3 mu m when the intercritical temperature was increased to 795 degrees C. Higher kinetics of phase transition from ferrite to austenite and ferrite grains growth restriction by alloy carbides and V(C, N) carbides reduced the ferrite size. The maximum yield strength of 1710 +/- 15 MPa with total elongation of 11.5 +/- 0.3% was achieved at 795 degrees C. The larger volume fraction of martensite, smaller ferrite grain size and smaller (FeMnCr)3C particles improved the yield strength. Despite the higher ferrite grain size and higher carbon content in martensite, the maximum strain hardening rate was attained at 765 degrees C. Higher amount of carbides increased the strain hardening rate at 765 degrees C. The strengthening mechanism of dual-phase steels at each intercritical temperature was studied and strength contribution from each strengthening factor was calculated. The calculated results at each temperature were agreed well with the experimental results.