Influence of second phase on the formation of mixed grain structures in 22Cr-13Ni-5Mn after deformation and solid solution

• Z phase dominates the formation of mixed grain structures in 22Cr-13Ni-5Mn steel during solution treatment after deformation. • The precipitation, dissolution and coarsening of the Z phase are dominated by both deformation temperature and strain. • An accumulative dislocation density was defined to describe the role of deformation in Z phase nucleation sites. • The mixed grain structures after solution can be suppressed by controlling the grain-to-particle size ratio. As an important structural material, 22Cr-13Ni-5Mn steel often faces mixed grain issues during solution treatment after forging. This study applied isothermal double-cone compression tests over a wide range of strains and temperatures, followed by solution treatments, to investigate the formation mechanism of mixed grain structures. Microstructural characterization revealed that the formation of mixed grains is strongly correlated with Z-phase particle size, which in turn is influenced by temperature and strain. An accumulative dislocation density parameter, accounting for dislocation evolution through recrystallization and work-hardening, was proposed to evaluate the contribution of deformation process to the available nucleation sites of the Z phase. The results of precipitation kinetics indicated that deformation promotes nucleation of the Z phase. At lower deformation temperatures, increased strain leads to more Z-phase nucleation sites but smaller particle size. However, as strain increases, some very fine particles may dissolve, thereby inducing coarsening of the remaining particles. Consequently, coarse particles inhibit grain boundary migration, while very fine particles tend to induce particle-assisted abnormal grain growth, leading to mixed grain structures. A grain-to-particle size ratio was defined to predict the uniformity of post-solution grains, and a temperature-dependent threshold of this ratio was determined to suppress the formation of mixed grains.