Cold-rolling and inter-critical annealing of low-carbon steel: Effect of initial microstructure and heating-rate

The effects of starting microstructure and heating rate on the inter-critical annealing treatment have been investigated by 80% cold-rolling of different initial microstructures namely ferrite-pearlite and ferrite-martensite with blocky and fibrous martensitic morphologies, followed by inter-critical annealing treatment using two different heating rates (∼0.5 °C/s and ∼300 °C/s). Sub-critical annealing has also been carried out to understand the effect of starting structures on cold-rolled and completely recrystallized microstructures. Due to the fine-scale lamellar structure comprised of alternate layers of ferrite and martensite, ferrite-fibrous martensite starting structure showed the finest ferrite grain sizes (3-6 μm) and more uniform distribution of martenisitc islands compared to ferrite-pearlite and ferrite-blocky martensite after the inter-critical annealing. Slow heating resulted in a coarser ferrite grain size with more uniform distribution of martensite, compared to rapid heating. Recrystallization-transformation interaction and the avoidance of ferrite grain growth contributed to the finer grain sizes after rapid annealing. Nature and distribution of θ-particles and the austenite islands played an important role in controlling the ferrite recrystallization and grain growth. Finer microstructural constituents offered superior combination of strength, ductility and strain-hardening ability to ferrite-fibrous martensite starting structures after rapid annealing, compared to other structures. Rapidly heated samples showed higher strength than the slowly heated samples.