TiNを核とした鋼の等軸晶凝固

抄録

Equiaxed solidification of steel was investigated with emphasis on the nucleation on titanium nitride (TiN). Ferritic stainless steels bearing different amounts of titanium and nitrogen were remelted using a gas-tungsten-arc welding, and equiaxed grain formation was examined in the as-solidified welds. It was found that the precipitation of TiN in the melts prior to solidification is a prerequisite for columnar-to-equiaxed transition (CET) to occur. The most distinct CET and the largest number density of equiaxed grains are obtained when the TiN precipitation initiates approximately 30 K above the liquidus temperature of the melt. When the TiN precipitation is at even higher temperatures, on the other hand, the CET is adversely affected even though the amount of TiN formed prior to solidification increases. This is probably due to a decrease in the number of effective TiN particles since higher precipitation temperature increases time and space for TiN particles to coarsen, ripen and float out before they reach the solidification front. Liquid-tin quenching was applied during the solidification to directly observe the solidification front. Equiaxed grain formation on TiN is clearly identified in the melt within 200 to 300 μm ahead of the solidification front. The solutal undercooling ahead of the front is also a prerequisite for the equiaxed grain formation, but the number of TiN particles supplied to this region as a nucleating agent should be more dominant for CET and for higher density of equiaxed grains.