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

Abstract

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 (3–6μm) 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.