Effect Of Lanthanum And Titanium Composite Treatment On Non-metallic Inclusions In TRIP Steel

Transformation-induced plasticity（TRIP）steel is promising for automotive applications due to its high strength,high plasticity and lightweight characteristics.However,the high aluminium content in TRIP steel leads to inclusions during the smelting process,such as Al₂O₃ and AlN,which affect the product performance.In this paper,Fe-1.8%Mn-1.0%Al-0.8%Si-0.08%C TRIP steel was studied using a combination of high temperature experiments and thermodynamic calculations.The influences of alloy addition sequence,lanthanum treatment,titanium metal treatment,lanthanum and titanium composite treatment on the inclusions were systematically researched,and the control methods of typical inclusions were explored.The results of this study can provide a theoretical reference for the production of high cleanliness TRIP steels.The main studies and results are as follows:（1）The effect of two deoxidation methods on inclusions in TRIP steels was investigated by adjusting the addition sequence of Si and Al.The results showed that the number density and mean equivalent circle diameter of inclusions were larger when Si was added first followed by Al.While single-phase AlN inclusions were produced when Al was added before Si.And the final deoxidation product species were basically the identical between two deoxidation methods.（2）The effects of lanthanum addition（0.005 wt%,0.01 wt%,0.02 wt%,0.05 wt%and 0.1 wt%）and reaction time（1,5,10,30,and 60 minutes）on inclusions were investigated in TRIP steels.The results showed that lanthanum can reform Al₂O₃ into La-Al-O inclusions at low lanthanum content（mass fraction of 0-0.0041 wt%）,and the reforming was completed within 30 minutes.While lanthanum can modify MnS and Al₂O₃ to La-O-S or La-S inclusions at high lanthanum content（0.0089 wt%-0.0403wt%）,and the modification was completed within 1 minute.With the increase of lanthanum content,the path of lanthanum modifing Al₂O₃ was Al₂O₃→La Al₁₁O₁₈→La AlO₃→La₂O₂S→La₂S₃.To effectively regulate inclusions in TRIP steel,the recommended amount of lanthanum addition was 0.004 wt%-0.005 wt%.（3）The effects of titanium addition（0.02 wt%,0.05 wt%,and 0.12 wt%）and the addition time（the sequence of titanium and aluminum addition）on inclusions in TRIP steel were investigated.The results showed that when titanium was added before aluminium,the inclusions in the steel were TiO_x,Ti_xAl_yO,TiN,TiO_x-TiN after adding titanium,and then Al₂O₃,TiN,Al₂O₃-TiN after adding aluminium.When aluminium was added before titanium,the inclusions were Al₂O₃ and Al₂O₃-AlN after aluminium addition,and then were Al₂O₃ and TiN after titanium addition.The addition of titanium over 0.05 wt%can effectively inhibit the formation of AlN inclusions.（4）The effects of lanthanum and titanium composite treatment on inclusions in TRIP steel were investigated by considering the sequence of lanthanum and titanium additions,treatment time,lanthanum addition,and other factors.The results showed that:the lanthanum and titanium composite treatment can transform Al₂O₃ to La-containing inclusions and AlN into TiN.And the number density of inclusions was smaller when titanium was added before lanthanum,and lanthanum can inhibit the formation of TiN.Whereas the size of inclusions was relatively smaller when lanthanum was added first and then titanium,and titanium can inhibit the precipitation of lanthanum inclusions.What’s more,the addition sequence of lanthanum and titanium had no effect on the kinds of final inclusions.