Interdendritic TiN Precioitation And Dendrite Growth Under Forced Convection During Steel Solidification

TiN inclusions will inevitably precipitate during the solidification of molten steel.The shape,size,and distribution of TiN in steel affect the strength,toughness and fatigue properties of steel.For example,TiN can cause cracks in steel,and a small TiN can improve the yield stress of steel.In order to visualize the whole process of TiN inclusions from nucleation to growth in the solidification process of molten steel,a cross-scale model for simulating dendrite growth and TiN precipitation in the solidification process of molten steel is established based on the cellular automata(CA)model.In this model,the macro temperature field grid,mesoscopic matrix growth grid,and TiN precipitated micro grid are coupled by linear interpolation.A heterogeneous nucleation model is used to describe the TiN nucleation process to accurately describe the TiN nucleation process.The flow field has an important effect on the growth of dendrites.Coupled with the CA dendrite growth model and lattice Boltzmann flow field model,and under forced convection is developed and applied to the solidification process of Fe-C-Ti-N alloy system under forced convection.The conclusion is as follows:(1)The equiaxed growth of Fe-0.33%C-0.044%Ti-0.0145%N alloy was simulated by the dendrite growth model,and the results were compared with those of the classical LGK analytical model.The results show that the growth rate of dendrite tip in the undercooled melt decreases rapidly and reaches equilibrium;with the increase of undercooling degree,the growth rate of dendrite tip increases,while the tip radius decreases;the solute concentration around the equiaxed crystal decreases from the S/L interface to the distance,and the solute concentration at the root of dendrite is the highest;the calculated value of dendrite growth model is in good agreement with the predicted result of LGK analytical model.(2)The precipitation of TiN in the solidification process of Fe-0.33%C-0.044%Ti-0.0145%N and Fe-1.01%C-0.0078%Ti-0.0049%N quaternary alloys was calculated by using the multi-scale CA model,and the results were compared with the experimental results.The results show that the calculated results of the model are in good agreement with the volume and morphology of TiN observed in the experiment;the simulation shows that TiN begins to precipitate mainly near the cooling wall,and TiN dissolution and re-precipitation are found in the two alloys,respectively.(3)Using the established multi-scale CA model,the precipitation of TiN in the columnar region of the two alloy systems Fe-0.33%C-0.09%Ti-0.02%N(High-rat)and Fe-0.33%C0.06%Ti-0.03%N(Low-rat)with the same initial concentration product was calculated,and the effect of different Ti/N ratio on TiN precipitation was analyzed.The results show that Low-rat and High-rat alloy system TiN precipitate almost at the same time,but the solid fraction of TiN in Low-rat alloy is 45%higher than that in High-rat alloy,and re-precipitation will occur.N element is the controlling factor of re-precipitation.(4)The eccentric square algorithm greatly reduces the influence of grid anisotropy on the growth of dendrites and can simulate the growth of equiaxed crystals in all directions.The precipitation process of tin in the equiaxed region of the Fe-0.33%C-0.06%Ti-0.03%N alloy system was calculated by the multi-scale CA model,and the TiN precipitation in the equiaxed region and the columnar region was compared.The results show that the columnar and equiaxed grains coexist,the solid phase ratio of TiN precipitation increases from 0.60 to 0.65,and TiN nucleates preferentially in the columnar region.The appearance of the equiaxed region reduces the total volume fraction of TiN by 70%,but the average volume of TiN nucleated in the equiaxed region is larger.(5)The growth and evolution of equiaxed grains in Fe-0.33%C-0.044%Ti-0.0145%N alloy system were simulated by the established flow field model.The results show that the dendrites on the upstream side are more developed than those on the vertical side and the downstream side;with the increase of flow velocity,there are two symmetrical vortices on the downstream side;in the range of flow velocity 0.01-0.05 m·s-1,the y-axis dendrites become coarser and longer.