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

¹ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, P.R. China
e-mail: xieyou_metallurgy@126.com
² XINING SPECIAL STEEL CO., LTD, 52 Chaidamu West Road, Xining Qinghai 810005, P.R. China

Received: 6 July 2015
Accepted: 28 January 2016

Abstract

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 μm, 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 ~ 1.5μm, 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.