In view of the requirements for mechanical properties and service life above 650 °C, a high-Mn austenitic hot work die steel, instead of traditional martensitic hot work die steel such as H13, was developed in the present study. The effect of heat treatment on the microstructure and mechanical properties of the newly developed work die steel was studied. The results show that the microstructure of the high-Mn as-cast electroslag remelting (ESR) ingot is composed of γ-Fe, V(C,N), and Mo₂C. V(C,N) is an irregular multilateral strip or slice shape with severe angles. Most eutectic Mo₂C carbides are lamellar fish-skeleton-like, except for a few that are rod-shaped. With increasing solid solution time and temperature, the increased hardness caused by solid solution strengthening exceeds the effect of decreased hardness caused by grain size growth, but this trend is reversed later. As a result, the hardness of specimens after various solid solution heat treatments increases first and then decreases. The optimal combination of hardness and austenitic grain size can be obtained by soaking for 2 h at 1170 °C. The maximum Rockwell hardness (HRC) is 47.24 HRC, and the corresponding austenite average grain size is 58.4 μm. When the solid solution time is 3 h at 1230 °C, bimodality presented in the histogram of the austenite grain size as a result of further progress in secondary recrystallization. Compared with the single-stage aging, the maximum impact energy of the specimen after two-stage aging heat treatment was reached at 16.2 J and increased by 29.6%, while the hardness decreased by 1–2 HRC. After two-stage aging heat treatment, the hardness of steel reached the requirements of superior grade H13, and the maximum impact energy was 19.6% higher than that of superior grade H13, as specified in NADCA#207-2003.

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 f s = 0.92, while that in ESR-CDS was f s = 0.95.

The phase transformation behavior of crack-arrest steel during continuous cooling either with or without deformation at high temperatures was investigated. By carefully examining the microstructures of continuous cooled samples, we found that pearlite, quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), upper bainite, lath-like bainite and martensite/austenite will exist depending on cooling rates and transformation temperatures in both conditions of deformation and without deformation after austenization. The transformation curves of AF and GB moved toward the left in deformation condition in comparison with that of without deformation condition, which indicated that deformation promoted the formation of QPF and AF; meanwhile, deformation inhibited the formation of bainite. Finer bainite and the accompanying M/A could be obtained by reducing the bainite nucleation activation energy as follows: (i) increasing the undercooling by increasing the cooling rate; (ii) increasing the stored energy by deformation of the austenite.