Study on Prior Austenite Grain Size and Softening Fraction in Governing Microstructure and Mechanical Behavior of Medium Mn Steel after Intercritical Annealing

The effects of the prior austenite grain (PAG) size and softening fraction on the microstructure and mechanical properties of 0.15C-7Mn steel after intercritical annealing (IA) are investigated. The hot-forged alloy is subjected to double-hit compression under different conditions to obtain different PAG sizes and softening fractions. Comparing the samples with the maximum (67 mu m) and minimum (33 mu m) PAG sizes, different grain morphology and size of reversed austenite, and carbide re-dissolution degree are noted after IA. The ultimate tensile strength (UTS), total elongation (TE), and product of UTS and TE increase linearly with decreasing PAG size. The decrease in PAG size increases the nucleation rate and volume fraction of the retained austenite. The softening fraction affects the morphology of the microstructure and formation of carbide particles after IA, and the austenite formation kinetics. In particular, a smaller PAG size and higher softening fraction can yield austenite grains with more suitable mechanical stability and higher volume fraction, which is conducive to the transformation-induced plasticity effect and the improvement of mechanical properties.