8%Crダイス鋼の1次炭化物およびポロシティ形成に及ぼす凝固条件の影響

抄録

Cold tool steels contain relatively larger amount of C and Cr to utilize carbide for increase of hardness and wear-resistance. However, in the large-scale ingots, coarsening of carbides caused by microsegregation would degrade steel properties. Also, micro-porosity generated during ingot casting affects the risk of cracking and the occurrence of unclosed defects in the subsequent forging process. Therefore, it is important to predict and control its occurrence. In order to investigate the effect of solidification conditions on the formation of carbides and micro-porosity of 8Cr type tool steel (Fe-1C-8Cr-2mass%Mo), unidirectional solidification experiment with different solidification conditions were conducted. As a result, primary carbides in the interdendritic zone were formed and estimated to be M₇C₃ and M₆C. The distribution coefficients were analyzed using random sampling method. The solid fraction, at which dendrite growth terminated and carbides began to crystallize, changed to higher solid phase fraction as the pull-down speed decreased. G/√R was calculated using the temperature gradient G and cooling rate R, and the relationship between the micro-porosity area fraction was investigated. The results showed that porosity occurrence was minimal in the range of 1.6 to 7.0 (K-s)^1/2/mm. In the larger range of G/√R, the porosity formation is considered to be caused by insufficient supply of liquid phase due to higher solid phase ratio and lower permeability to the final solidification zone when the eutectic phases start to crystallize.