Mechanism analysis of grain refinement caused by deformation and the improvement of strength and ductility of CLAM steel

The mechanical behaviors and deformation mechanisms of Chinese low activation martensitic (CLAM) steel under extreme loading conditions were systematically studied. The mechanical experiments were performed at a wide range of strain rate (from 0.001 to 3500 s^-1) and temperature (from 300 to 1073 K). The main results show that the strength of the CLAM steel shows an apparent positive strain rate and temperature softening effect. In particular, at quasi-static loading conditions, the elongation of CLAM steel first decreases (300–673 K) and then increases (673–1073 K) with the temperature rising. Under dynamic conditions, the elongation of the CLAM steel is positively correlated with temperature rising and is larger than that under quasi-static loading conditions. The microstructure characterization results indicate that grain refinement during deformation and the positive strain rate effect on elongation are primarily governed by changes in grain size, especially at high temperatures. The relationship between the plasticity capability, precipitates and grain refinement are also analyzed. The obvious competitive mechanisms under different loading conditions in the recrystallization process of the CLAM steel. In summary, precipitates contribute to grain refinement in martensitic structures by providing nucleation sites for new grains and by obstructing dislocation movement, thereby raising the local strain and promoting dynamic recrystallization (DRX). Both of these mechanisms result in a finer and more uniform grain structure, which enhances the mechanical properties of the material, such as strength and toughness.