Effect Of Ti On Solidification Structure And Inclusions Of Ferrite Steel

Grain refinement is of great significance for improving the mechanical properties of steel.Solidification refinement refers to increasing the proportion of equiaxed crystal regions and refining equiaxed grains during solidification.For cast steel of non-deformable materials or high-alloy ferritic steels or austenitic steels without phase change,solidification refinement is particularly important for improving mechanical properties.Previous studies have shown that the addition of rare metal Ti can refine the as-cast microstructure of steel,and it is believed that the inclusion of high-melting Ti can be used as the core of the nucleation of the primary phase of the molten steel.For ferritic steel,solid phase transformation occurs during cooling to room temperature after solidification,and it is difficult to retain the primary phase to room temperature,which makes the microscopic analysis of heterogeneous nucleation difficult.Therefore,the relevant mechanism lacks experimental basis.In this paper,low-carbon high-silicon single-phase ferritic steel is selected to maintain single-phaseδ-ferrite during cooling to room temperature after solidification,and the refinement effect of Ti on the solidification structure of ferritic steel is studied.By understanding the influence of Ti on the behavior of inclusions during solidification,the heterogeneous nucleation of high-melting Ti inclusion phase is discussed.By comparing the macrostructure and microstructure of the experimental steel before and after the addition of Ti,the refinement effect of Ti addition on the solidification structure of ferritic steel was investigated.Scanning electron microscopy（SEM）combined with energy spectrum（EDS）was used to add Ti to the experimental steel.The inclusions were statistically observed and analyzed.The inclusions were identified by electron backscatter diffraction（EBSD）combined with EDS analysis to study the effect of Ti addition on the behavior of inclusions.The crystallographic theory calculation of heterogeneous nucleation effect was discussed.The possibility and utility of heterogeneous nucleation of high melting point Ti inclusions.The results show that:0.065%（mass fraction）Ti is added,and the solidification structure of the experimental steel changes from the surface fine grain zone+columnar crystal to 100%equiaxed crystal,indicating that the addition of Ti promotes the heterogeneous nucleation process;adding 0.065%（mass fraction）Ti,the inclusions may have Ti₃O₅ and TiN;theoretical analysis and combined phase identification show that the inclusions of TiN or TiN inclusions with Al-Ti-O as the core are mainly composed of inclusions.Square,and the size of inclusions is mainly concentrated between 3⁴μm;the theoretical calculations using two-dimensional mismatch and edge-edge matching model show that Ti3O5 can not be used asδ-ferrite heterogeneous nucleation core;TiN can As an effective core of theδ-ferrite heterogeneous nucleation,the solidified structure of the experimental steel is changed from the equiaxed crystal of the surface layer to the columnar crystal to become 100%equiaxed crystal.