Mo系高速度工具鋼の炭化物粒度および分布に及ぼす凝固組織の影響

抄録

Solidification structures and their hot-worked structures of the AISI M2 type high speed tool steel have been studied under the various solidification conditions by means of the metallographic observation and thermal analysis. The melts in a laboratory scale were unidirectionally solidified for the sake of simplifying the freezing process. Also the cooling curves were obtained on 300kg ingots to know the differences in the solidification structure and the forged one between the unidirectionally frozen ingots and the commercial service ingots.
The results obtained are summarized as follows:
1. The secondary arm spacing of dendrite in columnar crystals, S_II(μ), and the grain diameter of equiaxed crystals in 300kg ingots, l(μ), varied with the average cooling rate, R (°C/min), during the solidification, showing the relationship of S_II=100R^-0.28 and l=220R^-0.29. Therefore, the columnar structure had more fine carbide net work than the equiaxed one at the same cooling rate.
2. The morphology of the dendrite changed from the equiaxed to the cellular type with decreasing liquidus isotherm drift rate, V(cm/min), and with increasing temperature gradient, G(°C/cm), across the solidified layer.
3. The carbide size and distribution in the forged structures became fine precipitously with increasing cooling rate, R, until 100°C/min. Also they became fine with the solidification structure change from the globular to the cellular type at the same cooling rate.