Temperature changes in hot flat rolling of steels at low strain rates and low reduction

Abstract

A finite difference (two-dimensional) model developed to describe the heat flow at low strain rates (0.08–1.5 s−1) and low reduction (∼10 per cent) for a hot flat rolled slab both during air cooling and during roll contact has been used in conjunction with experimentally measured temperatures through specimen thickness during roll contact after reheating to temperatures in the range 1000–1200°C for mild steel, low carbon SS316 and high carbon SS316 (with Nb, V and Ti). At high strain rates (∼1.5 s−1) a steep temperature gradient was produced in the specimen near the surface whereas for low strain rates (∼0.08 s−1) this temperature gradient penetrated deep into the thickness, leading to a large drop in the mean rolling temperature. Roll chilling, leading to higher values of the Zener-Holloman parameter, Z, resulted from a decrease in the mean rolling temperature and the large temperature gradient during roll contact. Temperature changes due to material composition, reheating temperature, contact time and rolling conditions led to precipitation strengthening and roll chilling effects which have accounted for the excess load and torque observed experimentally and industrially.