Numerical Simulation Of Equiaxed Crystal Nucleation And Growth Of Stainless Steel Under Vibration And Chill Conditions

As resource-saving materials,ferritic stainless steel can replace the relatively expensive austenitic stainless steel and is used in household appliances,kitchen utensils and other daily necessities.The wrinkles and edge cracking defects are easily formed in the roll forming process of ferritic stainless steel,resulting in the decrease of yield of rolled sheets,which limits its widely application.In recent years,based on the basic theory of solidification process,a vibration-induced nucleation technique has been proposed.Although previous studies have found that this technology can significantly improve the equiaxed crystal rate of solidification structure of ingot,the nucleation and growth conditions and mechanism of equiaxed crystals are still unclear.However,the solidification of ferritic stainless steel is a high-temperature thermoforming process,and the nucleation and growth process cannot be directly observed in real time.Therefore,the study of numerical simulation is of great significance.In this paper,the numerical simulation of solidification process of Cr17 ferritic stainless steel under vibration quenching was carried out.In macroscopic simulation,three-dimensional geometric model was built by Gambit software,and numerical simulation was carried out by Fluent software.Temperature field distribution,flow field distribution,grain and fluid trajectory under different degree of undercooling and vibration frequencies were analyzed.In microscopic simulation,two-dimensional mathematical models of nucleation(continuous nucleation model)and growth(KGT model)were established by cellular automaton(CA)method,and programmed by Visual C++ software.Microsoft Visual Studio 2010 software was used conduct numerical simulation calculation.The grain morphology and structure at differentdegrees of undercooling were analyzed.The law and condition of grain nucleation,stripping and growth during solidification of ferritic stainless steel were preliminarily discussed.The results show that the temperature field distributes symmetrically along the direction of vibration,and diffuses outward with positive temperature gradient,the temperature perpendicular to the direction of vibration is lower than that of vibration direction,high degree of undercooling is beneficial to the diffusion of temperature field,low degree of undercooling and high vibration frequency can effectively supress the rapid formation of solidified shell on the surface of nucleation generator.Under the condition of the 540 K degree undercooling,not only a large number of grains can be formed rapidly on the surface of nucleation generator,but also the formation of solidified shell can be effectively prevented,which is favorable for the stripping of the grains.The flow field distributes symmetrically along the vibration direction and diffuses outward with a heart-shaped.The melt fluidity perpendicular to the vibration direction is better than that in the direction of vibration.The influence of vibration frequency on the flow field is greater than that of degree of undercooling.The uniform distribution of grains can be achieved at three different frequencies of vibration,but the time needed to achieve uniform distribution decreases first and then increases with the increase of vibration frequency.Under the condition of 1000 Hz frequency,uniform distribution can be achieved as soon as possible.Considering the temperature gradient in the melt,the forming conditions of solidified shell on the surface of nucleation generator and the grains and fluid trajectories,the optimum parameters for nucleation and growth of equiaxed crystals perpendicular to the vibration direction are degree of undercooling of 540 K and vibration frequency of 1000 Hz,respectively.