Research On The Evolution Mechanism Of Inclusions In Calcium Treated Steels And The Optimization Of Ca-treatment Process

Calcium?Ca?treatment is widely applied to solve the nozzle clogging problems induced by aluminum deoxidation products.In order to increase the yield of Ca and improve the modification effect of inclusion,three steel grades,i.e.Q235,SPA-H,DC01 produced by BOF-LF-CC process in a CSP plant was studied in this article.The evolution mechanism of inclusions in Ca treated steels has been investigated by characterization of inclusions in refining and continuous casting process,and the thermodynamic conditions on the formation of nonmetallic inclusions were calculated.On this basis,the Ca treatment process was optimized.The typical inclusions in the tested steels were observed by SEM+EDS,and the results show that when arriving at LF,the inclusions are dominated by spherical,angular shaped and clustered Al₂O₃.While,most of the Al₂O₃ inclusions were modified to Mg O·Al₂O₃ spinel with high melting point in the LF refining process before calcium treatment,and single Al₂O₃ inclusions only accounted for a small part.Three minutes later after Ca treatment,the main inclusions are turned into Al₂O₃-Ca O-MgO+?CaS?,Ca O-Al₂O₃+?Ca S?.In tundish and casting slab samples,a majority of inclusions are transformed to low melting point Al₂O₃-Ca O-MgO composite oxides.The transformation path of major inclusions in Ca treated steels can be summarized as follows: Al₂O₃?Mg O·Al₂O₃?·xCa O??Al₂O₃·x Ca O?·y Mg O?+?CaS??Al₂O₃·x Ca O?·y Mg O?.Thermodynamic calculation indicates that the Al₂O₃ inclusions can be modified to MgO·Al₂O₃ spinel by trace Mg content?0.3ppm?with the Al content?0.010⁰.05%?of the tested steel.When MgO content is lower than 15%,and Al₂O₃ content is between 42%⁶8%,the Al₂O₃-CaO-MgO composite oxides may exist in the form of liquid under the steelmaking temperature.At 1873 K,the [Ca] content in Q235,SPA-H,DC01 steel needs to be 0.0018%,0.0014%,0.0016% respectively,inorder to turn Al₂O₃ inclusions into liquid 12C·7A.When the percentage of [S] in steel is 0.0034%,Ca S inclusion will generate if the [Ca] content is 0.0050%,however,when the mass fraction of [S] is dropped to 0.0010%,the [Ca] content should be up to 0.0169% to produce CaS.The Al-S equilibrium curve shows that when the equilibrium transferred from C/L to C·A /C·2A,the higher [S] content is required to form Ca S,and the Ca S will be consumed when contacting with Al₂O₃ or Ca Al2O4.During the cooling and solidification process,Ca S inclusions started to precipitate at the solidification front in SPA-H and DC01 steel when the temperature was decreased to 1775.7K,1800.8K respectively.Nevertheless,Ca S inclusions has already formed before the temperature downs to the liquidus line in Q235 steel,which indicates that the S content was too high in the Q235 steel.The Ca treatment process was optimized by sample analysis and theoretical research.The gas flow rate of purging plug below the wire feeding point should be controlled around 120NL/min,while the argon flow rate and outlet pressure of another purging plug ought to be about 520NL/min,0.33 Mpa.According to the calculation,the wire feeding velocity of CaAl and Ca Si wire should between2.4³.6m/s,2.1³.1m/s,respectively.Comparing to the calculated value,the practical velocity of CaAl wire was proper while the Ca Si wire was a little large.The pure Al₂O₃ inclusions and calcium aluminates with high melting point caused the nozzle clogging,which means that adequate Ca-cored wire is still necessary on the basis of partly transformation of inclusions,and the amount of Ca Al or Ca Si wire should be increased in SPA-H and Q235 steel to improve the modification effect of inclusions.It is found that the sulfur capacity of slag is closely related to slag basicity,thus the refining slag is supposed to have high basicity.When the percentages of Mg O was increased from 5% to 10%,the liquid region of quarternary system CaO-Al₂O₃-SiO2-MgO was expanded under the same temperature,while Al₂O₃ and SiO2 could decrease the melting point of refining slag to some extent.Ifw?Ca O?/w?Si O2?and w?Ca O?/w?Al₂O₃?are located between 4¹0,1.5².5,respectively,the activity of Al₂O₃ is small while the activity of CaO is large,which is conducive to the absorption of Al₂O₃ by refining slag.