Precipitation Behavior And Control Measures For The Large Carbonitrides During Solidification In Nb/Ti Containing H13 Tool Steel

AISI H13 is a widely used hot working tool steel, and now more and more attention has been paied attention on the niobium-microalloyed H13 for the superior performance. However, large primary precipitates can be found in H13 and microalloyed H13 steels, affecting the properties of steel products. The size of these precipitates could be even up to tens of microns. The precipitation behavior of primary precipitates in Nb/Ti containing H13 steel were studied in this paper, combining with the analysis results of actual Nb-Ti containing H13 bar produced with or without ESR process, small ingot experiment and Thermo-Calc software calculation. Based on the study above, the content of niobium in Nb-microalloyed H13 was further optimized. The results are listed as follows.A great number of precipitates larger than 3?m are found in H13 bar with 0.054 wt% and 0.060 wt% Nb, no matter the bar is produced with or without ESR process. The precipitates could be classified into three types:Ti-V-N-rich carbonitrides, Nb-C-rich carbonitrides and V-C-rich carbonitrides. They exist in single form or combine with each other to form two-layer, three-layer or even four-layer structures. The compositional distributions of the primary precipitates are concentrated and have little relationship with producing process and cutting positions of one sample. These phases are stable at high temperature and ESR process has little refining effect on the size. The three types of large carbonitrides are generated in the liquid steel during solidification.The characteristics of primary precipitates are closely related with the addition of Nb and Ti in H13. During solidification of H13 ingot without Nb and Ti, V-rich (V,Mo,Cr,Fe)C precipitates first and Mo-Cr-rich (V,Mo,Cr,Fe)C precipitates at the end of solidification. Homogeneous Ti-rich and inhomogeneous Ti-V-rich (Ti,V)(C,N) precipitate first when 0.034% Ti is added, while Nb-rich carbonitride precipitates first in H13 with 0.051 wt% Nb. When H13 tool steel is modified with 0.064 wt% Nb and 0.036 wt% Ti, Ti-N-rich (Ti,Nb,V)(C,N) precipitates first; with the consumption of Ti and N and enrichment of Nb, V and C, Nb-C-rich and V-C-rich (Ti,Nb,V)(C,N) will precipitate successively.Mo-Cr-rich carbides are unstable at 1150? and will dissolve after holding a short time. V-rich carbonitrides are relatively more stable, but the dissolution can also be observed after holding a longer time at appropriate temperature. Ti-rich, Nb-rich and Ti-Nb-rich carbonitrides are stable at 1150? and 1250?. For inhomogeneous Ti-V-rich carbonitrides, the ones with low atomic ratio of Ti/V start to decompose into a distribution of small precipitates after holding a short time, while the transformation of the ones with higher ratio of Ti/V will need more time.Nb-rich precipitates in Nb-containing H13 steel could not be controlled effectively by heat treatment for the high stability at high temperature. Then selecting an appropriate content of Nb is important. The content of Nb in H13 was evaluated through calculation by Thermo-Calc and further small ingot experiment and shop test. The result shows that controlling the content of Nb no more than 0.03 wt% is effective to avoid the precipitation of Nb-rich carbonitrides.