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

Abstract

Figure 3 shows a phase diagram of Fe-0.2wt%C-0.035wt%P-0.8wt%Mn-0.2wt%Si-0.006wt%N-xAl system calculated from CALPHAD method. 14) We employed the thermodynamic database, PanIron for all the calculations. 15) The horizontal axis is Al concentration. The liquidus temperature does not substantially change with the Al concentration. It should be noticed that the type of steel changes from hyperperitectic to hypoperitectic one at 0.53 wt% Al. When Al concentration is less than 0.53 wt%, the solidus temperature, in other words, T g decreases very slightly with Al concentration. On the other hand, in the range of Al concentration for hypoperitectic steel, d phase is stabilized at lower temperatures by increasing Al concentration, leading to drastic decrease in T g . As mentioned in the introduction, it has been reported that g grains grow rapidly below T g and, therefore, lowering T g leads to finer g grain structure. However, the decrement of T g up to 0.54 wt% Al is quite small. Therefore, the Al addition does not lead to significant change in the g grain structure during slow cooling process, as is consistent with the result of Figs. 1 and 2. In Fig. 3, also, the temperature for formation of AlN phase increases with the increase in Al concentration. However, the AlN phase forms at much lower temperatures than T g in all samples of the present experiments. Hence, it is considered that the AlN phase precipitates from g phase after g grain growth substantially occurs at the high temperature region. In the present SEM/EPMA analysis, AlN particles were rarely detected. Casting ExperimentFigure 4(a) shows the as-cast g grain structure in the sample with 0.04 wt% Al. As described in the Sec. 2, the g grain boundary is decorated with film-like proeutectoid a phase. The bottom of the micrograph corresponds to the mold side, while the top part represents the center of the ingot. It is seen that the columnar grains develop from the mold side to the center of the ingot along the thickness direction, which coincides with the direction of heat flow in the present casting condition, and the equiaxed grains form in the central region. It is noted that very fine columnar grains form between the columnar grain region and the equiaxed grain region. Therefore, this as-cast g grain structure can be characterized by three regions, Coarse Columnar Grain (CCG) region near the mold side, Equiaxed Grain (EG) region in the central region of the ingot, and Fine Columnar Grain (FCG) region in between the CCG and the EG regions.Figure 4(b) shows the d dendrite structure at the same location. The d dendrite structure consists of columnar dendrites developing from the mold side to the inward of the sample and the equiaxed dendrites in the central region. By comparing the as-cast g grain and d dendrite structures (Figs. 4(a) and 4(b)), the CCG and FCG are found to originate from the columnar dendrites. The cooling rates at various positions from the mold side toward the center can be evaluated by the empirical equation, l 2 ϭ710(60Ṫ) Ϫ0.39 , wher...

Effects of Al and P Additions on As-cast Austenite Grain Structure in 0.2 mass% Carbon Steel

“…As was consistent with our previous study on the effects of Al addition, 14) the present EPMA analysis showed that the AlN particles did not exist in the samples with 0.04 and 0.1 mass% Al, and they were rarely dispersed in the samples with 0.54 and 1.04 mass% Al. Consequently, the AlN should not be responsible for the refinement described in Fig.…”

“…In our previous study, we investigated the effects of Al addition on the as-cast g grain structure in 0.2 mass% C steel. 14) It was observed that the Al addition leads to the refinement of the g grain structure obtained by permanent mold casting. The g grain structure consisted of Coarse Columnar Grain (CCG), Fine Columnar Grain (FCG) and Coarse Equiaxed Grain (CEG) regions.…”

“…Several mechanisms were proposed to explain this phenomenon, e.g., increase in partial solidification time, 23) increase in dendrite tip undercooling, 25) decrease in partition coefficient 20,22,25) and change in morphology of d dendrite structure. 14) Furthermore, the convection flow in the liquid due to pouring, thermal convection or density difference should promote mass transfer from the solid/liquid solidification front toward the center of ingot, which lead to increase in the average concentration in the liquid and accordingly increment of the degree of microsegregation. Although it remains to clarify which mechanism is dominant, the degree of microsegregation of P increases from the mold side to the center of ingot in the present casting experiment.…”

“…[8][9][10][11][12][13][14] The addition of d-ferrite stabilizing elements such as Nb, V and Mo extends the dϩg region in the phase diagram toward lower temperatures, which leads to refinement of as-cast g grain structure due to pinning effect of the d phase on the g grains. 15,16) Moreover, it was reported that P addition led to the refinement of as-cast g grain structure in 0.1 mass% C steel.…”

Effects of Second Phase Particle Dispersion on Kinetics of Isothermal Peritectic Transformation in Fe–C Alloy

“…Therefore, the decrease in Tγ corresponds to a decrease in the temperature for onset of rapid grain growth, thereby retarding γ grain growth at high temperatures. In fact, it has been reported in several studies [2][3][4][5] that reduction of Tγ by alloying element addition leads to grain refinement of as-cast γ structure. It should be noted that these early works focused on the reduction of Tγ due to stabilization of liquid or δ phase in a low temperature region.…”