Influence of Hot Deformation and Precipitates on the Recrystallization of Nb-V-Ti Free-Cutting Steel

Abstract

Nb, V, and Ti were added to free-cutting steel to improve its mechanical properties by means of precipitation strengthening and fine grain strengthening. The process parameters during the hot deformation of Nb-V-Ti free-cutting steel were studied at strain rates of 0.01–10 s−1 and temperatures of 850–1250 °C. The isothermal compression test results showed that the temperature rise at low deformation temperature and high strain rate has a great influence on the softening of the steel. The processing maps at strains of 0.3–0.6 were established based on a dynamic material model (DMM). When the strain was 0.6, the optimum hot-working window was at a temperature in the range of 1150–1250 °C and at a strain rate in the range of 0.01–0.1 s−1. The instable regions were mainly located at low temperature and high strain rate. The instable characteristics included the mixed grains around the MnS phase, flow localization, and intense deformation. In general, the existence of MnS leads to a decrease in the toughness of the steel. The MnS phase was easy to be broken when the compression tested at a lower temperature, e.g., 850 °C and at a higher strain rate, e.g., 10 s−1; its morphology changed from a long-rod shape to a discontinuous shape, and then, to a dot-like shape with the decrease in temperature from 1250 to 850 °C and the increase in strain rate from 0.01 to 10 s−1. The nucleation mechanism of this steel was grain boundary bulging. The size of fine (Nb,Ti) (C,N) precipitates is less than 10 nm, inhibiting austenite recrystallization and leading to austenite strengthening during hot deformation at 850 °C. Moreover, the dislocation motion and grain boundary migration were greatly inhibited by the Ti-rich(C,N) and MnS throughout the entire hot deformation process.