Generation mechanism of heterogeneous (Ti, Nb, V) (C, N) in H 13 steel modified by niobium

The precipitation of carbonitride in H13 hot working die steel would be detrimental to the strength, hardness, toughness and fatigue resistance properties of steel. The characteristics of heterogeneous precipitates (Ti,Nb,V)(C,N) with the size larger than 1μm in a Nb containing H13 forging bar pre-produced by electro-slag remelting (ESR) process were studied. The generation mechanism was analyzed through calculation by Thermo-Calc software. Most of the precipitates observed consisted of three-layer structures. The center was an oxide core which was mainly MgO- Al2O3 spinel or Al2O3, the middle layer was a Ti-V-rich (Ti,Nb,V)(C,N) phase and the outer layer was a Nb-rich (Ti, Nb,V)(C, N) layer. The normalized compositional distributions of Ti, Nb and V in the Ti-V-rich and Nb-rich layers were concentrated. The average composition of Ti-V-rich layer was (Ti0.477 Vo. 336 Nb0.187 ) C, Ny and that of Nb-rich layer was (Ti0. 128 V0. 255 N0. 617 ) C, Nu. The size of the heterogeneous precipitates could be up to 10μm. The generation mechanism of the precipitates could be well speculated through Tbermo-Calc calculation. The spinel and alumina have already existed in the liquid steel before solidification. During solidification the Ti-rich phase will firstly start to precipitate on the oxide when the solid fraction reaches 0.823. Then the V-rich phase and Nb-rich phase will precipitate successively. During the subsequent heat treatment process, the V-rich phase will be dissolved in Ti-rich phase, resulting in the appearance of large precipitate with three-layer structure.