The degeneration of the heterogeneous nucleation for the large precipitates (Ti, Nb, V)(C, N) in ESR H13+Nb tool steel with low-MgO slag

The characteristics of the large heterogeneous precipitates (Ti, Nb, V)(C, N) with oxide nucleus in H13+ Nb tool steel produced through ESR process were studied, and compared with the ones produced without the remelting process. ESR process with slag 50wt% CaF2-35wt% Al2O3-10wt% CaO-5% wt% MgO has a negative effect on controlling the primary carbonitride for the larger amount of ones with size > 5 mu m, which is partly due to the different heterogeneous nucleation characteristics. 100% of the nucleant oxide in the NON-ESR samples is Al2O3-MgO spinel with stoichiometric composition and the size is mostly located in 0.5 similar to 1.5 mu m, which shows a strong selectivity on the nucleant inclusions. However, the trend is not obvious for the ESR samples in which the average value of Mg/Al atomic ratio in nucleant spinel is just 0.135. The behavior of heterogeneous nucleation on spinel with low Mg/Al ratio in ESR sample is closer to that on alumina, which leads to the degeneration of the heterogeneous nucleation. It is a part of reason for the increment of the precipitates size. The thermodynamic analyses show that it should keep the total content of Mg in liquid steel larger than 4 ppm if the oxide is located in the stoichiometric spinel phase region.