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 1C/s and 300 1C/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 mm) and more uniform distribution of martenisitc islands compared to ferrite–pearlite and ferriteblocky
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 y-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 ferritefibrous
martensite starting structures after rapid annealing, compared to other structures. Rapidly
heated samples showed higher strength than the slowly heated samples.