Fundamental Research On Precipitation Of Ultrafine TiO_x Inclusions During Solidification Of Steel

For steels,many efforts have been made to remove the nonmetallic inclusions because of their harmful effects to material quality.However,fine dispersed nonmetallic inclusions(＜3μm) can improve the properties of steel.The fine Ti oxides combined with MnS and TiN may refine the crystal by acting as heterogeneous nucleation sites of intragranular ferrite(IGF),consequently, enhance the strength and toughness of steel.To effectively utilize the ultraflne inclusions,the key issue is to obtain the fine dispersed Ti oxide inclusions.Inclusions precipitated during solidification are relatively small.In the paper,the precipitation of Ti oxides during solidification was studied systematically.The theoretical and experimental results showed that fine dispersed Ti oxide inclusions could be obtained during solidification of steel. The effect of oxygen content,alloying elements contents on the type of inclusion was studied through thermodynamic calculation.Ti oxide is favorable for IGF nucleation,the A1/Ti ratio should be controlled to ensure that Ti oxides precipitate during solidification of steel.Ti₃O₅ is the stablest among all Ti oxides in the range of solidification temperature.[A1]content should be less than 34ppm when[Ti]content is 0.02%at 1873K.[Mn]in liquid steel can affect the type of Ti oxide for given oxygen content.Inclusions are mainly Ti oxide when oxygen content is 10ppm;they are Ti oxide and Mn-Ti oxide when it is 20ppm;Mn-Ti oxide dominates when it is 30ppm.The content and diameter of Ti₃O₅ precipitated during solidification were calculated at different oxygen content and cooling rate by a coupled mathematical model.The microsegregation of[Ti] and[O]in front of the liquid-solid interface leads to the precipitation of Ti₃O₅ during solidification of steel.Ti₃O₅ inclusions were engulfed by the solid phase immediately resulting in fine dispersed inclusions in steel.The finite difference method was introduced to the calculation.The calculation domain is half of the secondary dendrite arm spacing.The mass transmission is only controlled by diffusion.The consuming of[Ti]and[O]for the precipitation Ti₃O₅ was considered in the model.Results showed that the diameter of Ti₃O₅ is 2.5μm at cooling rate of 10K/min which is in the condition of In-mode casting and it is 0.7μm at cooling rate of 1000K/min which is in the condition of thin slab continuous casting.It proved theoretically that ultrafine inclusions can be formed during solidification.Based on theoretical analysis,the effect of oxygen content and cooling rate on number,type and size of inclusions were researched by directional solidification experiment.The solute dements diffused along one direction and the microsegregation of them resulted in the precipitation of inclusions in unidirectional solidification.Mn-Fe,Si-Fe,Ti-Fe or the mixture of Mn-Fe/Si-Fe Mn-Fe/Ti-Fe were added in Fe to investigate the precipitation of oxide inclusions during solidification.The results showed that the diameters of inclusions are 1.3μm,1.21μm,1.1μm when Mn-Fe,Si-Fe and Mn-Fe/Si-Fe were added to steel.The amount of the inclusions is the most with Mn-Fe addition and the least with Si-Fe addition.When Mn-Fe/Ti-Fe was added to steel,the type of Ti oxide was affected by oxygen content.Inclusions are mainly Ti oxide when oxygen content is 10ppm;Ti oxide and Mn-Ti oxide when it is 20ppm;Mn-Ti oxide dominates when it is 30ppm.The size of Ti oxides is mainly affected by cooling rate and it decreases with the increase of cooling rate.The average diameters are about 2.0μm,1.2μm and 1.0μm at the cooling rates of 9K/min,89K/min and 143K/min,respectively.The result is consistent with the calculation result of the coupled model.It proved experimentally that ultrafine inclusions formed during solidification.To extract effectively and detect exactly the ultrafine oxide inclusions in steel sample made by directional solidification,Nuclepore Track-Etch Membrane was introduced to improve the traditional acid dissolution and electrical extraction methods in filtering process.Threedimensional morphology and the structure of TiOx inclusions were detected by scanning electromicroscopy(SEM) and transmission electromicroscopy(TEM).Other information such as the size and composition etc.were also obtained.The structural detection results showed that the type of Ti oxide precipitated during solidification is Ti₃O₅ when oxygen content is controlled at a reasonable level.When[Mn]exists in steel,reaction of Ti₃O₅ with[Mn]results in the formation of Ti₂O₃ and MnO ultrafine composite inclusions in the following production process,which is advantageous to the composite of MnS and the formation of Mn depleted zone,which is advantageous for nucleation of IGF.The mechanism for IGF nucleation was developed.