Study On The Inclusions Evolution Behavior In Marine Steel Treated With Magnesium And Titanium

On the basis of oxide metallurgy technology,the utilization of beneficial inclusions in steel is an important method to improve the weldability of Marine steel.The inclusion of Mg O and Mg O-Ti₃O₅-Al₂O₃ composite oxides in the control steel can be used to nulate austenite grain boundaries and induce intragranular ferrite（IAF）nucleation,which has important application potential for improving the weldability of offshore steel.Therefore,the oxide metallurgy technology with Mg-Ti composite deoxidation as the core has attracted wide attention from researchers.In this paper,the effects of Mg-Ti composite treatment on the evolution of inclusions in EH420 steel have been studied.The main work and results are as follows:（1）On the basis of Fact Sage thermodynamic calculation,the effects of coupling changes of Mg-Al-Ti deoxidation system and oxygen content on the types,conditions and amounts of inclusions precipitated in steel at 1873 K were systematically studied.The calculation results show that when the content of Mg,Al and Ti in the control steel is 0.0020wt%,0.0080-0.010 wt%and 0.010-0.020 wt%respectively,when the content of T.O is0.0030 wt%,Mg O-Ti₃O₅-Al₂O₃ composite inclusions and Mg O inclusions were precipitated.When T.O.content is 0.0060 wt%,Mg O-Ti₃O₅-Al₂O₃ composite inclusions are precipitated.（2）The effect of oxygen,Ti and Mg contents on inclusions in steel was studied by using high temperature melting experiment and sequential treatment of Mg-Ti in molten steel.The results show that:1)the liquid oxygen level of steel before Mg-Ti treatment has a significant effect on the type of inclusions,and the inclusions evolve with the decrease of oxygen:Mn O-Mg O-Ti₃O₅ inclusions→high Mg O and Mg O-Ti₃O₅-Al₂O₃ inclusions→high Mg O inclusions.The number of inclusions density decreased from 233/mm² to 98/mm²,and the proportion of inclusions larger than 3μm decreased significantly.2)When Mg content is at the level of 0.0030 wt%,the inclusions in steel are mainly affected by Mg element,mainly Mg O-Ti₃O₅-Al₂O₃ inclusions.The increase of Ti content makes the inclusions distribution slightly shift towards Ti₃O₅ direction,and the number density of inclusions increases from 98/mm² to 129/mm².3)With the increase of Mg content in steel,the proportion of Mg O in steel inclusions increases gradually,the number density of inclusions increases from 47/mm² to 116/mm²,and the proportion of inclusions less than2μm increases significantly.（3）The effect of Ti and Mg content on inclusions in steel was studied by using high temperature melting experiment and sequential treatment of Ti-Mg in molten steel.The results show that:1)When the Mg content was at the level of 0.0015 wt%,the inclusions in steel changed from Al₂O₃-Mg O to high Mg O+Mg O-Ti₃O₅-Al₂O₃ with the increase of Ti content,the number of inclusions in steel increased from 149/mm² to 223/mm²,and the number of large-size inclusions increased slightly.2)When the Ti content is at 0.0120 wt%,the inclusions of high Ti₃O₅ inclusion changed to high Mg O inclusion with the increase of Mg content in steel,and the number density of inclusions increases from 85/mm² to 365/mm² with 2μm inclusions are predominant.（4）Compared with Mg-Ti sequential treatment,under Ti-Mg sequential treatment condition,When the content of T.O.and Al are in the range of 0.0030-0.0060 wt%and0.005-0.010 wt%,the content of Ti is at the level of 0.0120 wt%and the content of Mg is at the level of 0.0015 wt%,A large number of Ti₃O₅-Mg O-Al₂O₃ system inclusions and high Mg O inclusions were obtained which were conducive to inducing IAF nucleation and nadding-rolled grain boundaries.