As-cast Austenite Grain Structure in Al Added 0.2wt% Carbon Steel

Effects of AI addition on as-cast gamma-austenite grain structure in 0.2wt%C-0.035wt%P steel with AI concentration ranging from 0.04 to 1.04wt% were studied by means of furnace cooling and casting experiments. In the furnace cooling experiment with a cooling rate of 0.03 degrees C/s, the as-cast gamma grain structure consisted of equiaxed grains and the gamma grain size was not affected by the increase in Al concentration up to 0.54wt%. In the casting experiment of the sample with 0.04wt% Al, on the other hand, the as-cast gamma grain structure consisted of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Equiaxed Grain (EG) regions, sequentially, from the mold side to the center of the ingot. The increase in AI concentration leads to increase in the fraction of FCG region at the expense of both CCG and EG regions. Even in the samples with high AI concentrations, AlN particles were rarely found and also AI segregation did not occur substantially. Instead, P segregated in interdendritic regions. The concentration of the segregated P increased from the CCG region to the FCG region. It was suggested based on a thermodynamic calculation that the segregation of P is enhanced by AI addition and the high P concentration stabilizes the high temperature phase such as liquid or delta-ferrite, depending on AI concentration at lower temperatures. This stabilized high temperature phase is considered to retard the gamma grain boundary migration. Therefore, the increase of FCG region and the decreases of CCG and EG regions due to AI addition should be attributable to pinning effect of the stabilized high temperature phase.