Recently,large sized CaO-Al2O3 inclusions with low melting temperature have been the main reason for lowering mechanical properties of high strength low alloy(HSLA)steel plates.New philosophy,i.e.refining by top slag with relatively low basicity and Al2O3 content,was proposed to control such kind of inclusions.Firstly,the characteristics of refining slag,such as component activities and sulphide capacity(CS)of CaO-Al2O3-SiO2-MgO slag,were studied through thermodynamic calculation.Then,slag-metal equilibrium experiments were carried out in laboratory to investigate the exact chemical composition of refining slag together with thermodynamic analysis.Finally,industrial trials were done to verify the desulphurization ability and inclusions control in steel refined by the new slag.Thermodynamic calculations indicated that the slags with basicity of 3.5and Al2O3 content of 20%and basicity of 5.0and Al2O3 content of 20% or 25% have high values of CaO activity,sulphide capacity and ratio of MgO activity to Al2O3 activity.Laboratory equilibrium experiments showed that the slag with basicity of about 4.5and Al2O3 content of about 20%is helpful for increasing the melting temperature of inclusions in steel.After introducing such kind of refining slag in industrial trials,the sulphur content in eight heats of steel is below 20×10-6,which meets the requirement of HSLA steel,and most inclusions distribute in relatively high melting zone(≥1 773K)of CaOAl2O3-MgO(-SiO2)quasi-ternary diagram.New philosophy of top refining slag is feasible to control low melting point inclusions of CaO-Al2O3 system in HSLA steel.
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