Grain Refinement and Delta Ferrite Reduction of High Cr Steel Ingots by Thermal Control

High Cr steel (X12CrMoWVNbN10-1-1) ingots were prepared by thermal control by varying three parameters: the melt pouring temperature, the mold temperature and the melt superheating temperature. The effects of these parameters on the macro-/micro-structure of the ingots were investigated through a group of orthogonal experiments. The results showed that superheating temperature was the dominating parameter that affects the grain size and fraction of delta ferrite. The optimized parameters for the sound ingot were 1893 K for superheating temperature, 1073 K for mold temperature and 1873 K for pouring temperature. Under that condition, the ingot was composed of fine equiaxed grain with a radius of 1.1 mm, and the fraction of the delta ferrite was less than 2%. Mechanisms that explain the increment in the proportion of the equiaxed fraction and the grain refinement were proposed, and the mechanisms for the delta ferrite reduction were explained.