Precipitation and growth of MnS inclusion in an austenitic hot-work die steel during ESR solidification process

Abstract

This study follows a previous presentation of metallurgical results and operating parameters about the comparison between conventional ESR and continuous unidirectional solidification of electroslag remelting (ESR-CDS) and more precisely the effect of cooling rate on the precipitation and growth of inclusions in an austenitic hot-work die steel [Y.F. Qi, J. Li, C.B. Shi, R.M. Geng, J. Zhang, Effect of directional solidification in electroslag remelting on the microstructure and cleanliness of an austenitic hot-work die steel, ISIJ Int. 58, 1275–1284 (2018)]. A thermodynamic study, a classical solidification model and a growth model for MnS inclusions were used orderly to interpret the metallurgical results. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize microstructure and inclusions (i.e. number, size, morphology and distribution) in the as-cast ingots. The results showed that the as-cast microstructure was refined and the dendritic arm spacings were reduced through ESR-CDS process. The characteristics of inclusions showed that the number density and the size of inclusions in ingot were much more reduced through ESR-CDS process in comparison with ESR process. The starting time for the precipitation of MnS inclusion is retarded, which is due to the segregation of elements Mn and S lowered by higher cooling rate in ESR-CDS in comparison with that in ESR. The starting time for the precipitation of MnS inclusion in ESR was fs = 0.92, while that in ESR-CDS was fs = 0.95.