Mechanism of improving solidification structure of super austenitic stainless steel by feeding strip into molten steel

Feeding steel strip into molten steel is an effective technique for refining the solidification structure of steel, especially for high-alloy steel. This study experimentally and numerically investigated the effects of feeding strip on the solidification structure of super austenitic stainless steel S32654. The results revealed that feeding strip noticeably increased the proportion of equiaxed grains, refined the dendritic structure, and improved Mo segregation and the precipitation of σ phase. The relevant mechanism was clarified as follows. Firstly, feeding strip promoted the formation and survival of a large number of floating dendrites by cooling the molten steel. As the temperature decreased, these floating dendrites sharply increased their quantity by repeated splitting and then acted as nucleation particles. Numerous equiaxed grains grew around these floating dendrites, significantly refining the central dendritic structure. Meanwhile, the doubling of the equiaxed grains has two effects on Mo segregation. First, it prevented the growth of columnar grains and the movement of Mo segregation interface towards the center, thus reducing the Mo macrosegregation. On the other hand, it dispersed and refined the residual liquid phase (Mo segregation regions), thereby reducing the Mo microsegregation. Additionally, the reduced Mo segregation lowered the σ phase precipitation temperature, allowing it to form in solid austenite with a shortened growth duration. The lighter Mo segregation and slower Mo diffusion in solid austenite led to insufficient Mo supply for the σ phase growth. Moreover, dendritic refinement limited the growth space of the σ phase. Finally, less and finer σ phase particles precipitated in the S32654 steel fed with strip. The improvement of feeding strip on the solidification structure will reduce the temperature and time required for high-temperature homogenization, which is beneficial for the subsequent hot working.