The phase evolution behavior and the key mechanisms responsible for strengthening in a 750 MPa yield strength Nb–Ti microalloyed High‐strength steel were explored through the use of a Gleeble‐3500 thermal‐mechanical simulation system. The experimental data reveal that within the cooling rate interval of 0.5–5°C/s, ferrite constituted the primary phase. At cooling rates between 10 and 50°C/s, bainite was observed to form the prevailing phase in the microstructure. Furthermore, finer bainitic structures and higher hardness were obtained with increasing cooling rates. Under relatively low cooling rates ranging from 0.5 to 5°C/s, the principal strengthening mechanisms were identified as grain refinement and precipitation strengthening. The dominant precipitates responsible for this effect consisted of nanoscale (Ti, Nb)C particles.
Yang et al. (Mon,) studied this question.