Modification Effects Of Mg, Ca Treatment On Non-metallic Inclusions In Molten Steel

In Al deoxidized process of steelmaking, abundant A12O3 inclusions would be produced which was harmful to rolling performance and casting capability. The common method of modifying A12O3 inclusions was Ca treatment. But actually, Ca treatment still has some deficiencies. As a new modification way, Mg treatment draws many people’s attention. Magnesium can purify molten steel, minimize inclusions, and reduce the harm of non-metallic inclusions on ferrous material at the same time. But research on industrial application of Mg treatment is still rarely.In this paper, combined with relevant thermodynamic calculations, by taking SPHC steel as test material, magnesium and calcium treatments industrial test was carried out. And molten steel, billet and rolled samples was taken at different time nodes.Through the method of oxygen and nitrogen determination, SEM-EDS and quantitative metallography detection, the effect of magnesium and calcium treatment on characteristics of non metallic inclusions and molten steel cleanliness were investigated and compared. Through these experiment and analysis, main conclusions were proposed as follows:(1) A12O3 inclusions in molten steel can be modified to magnesia-alumina spinel, after feeding slow-release magnesium alloy cored wires into 120t ladle.3 min after feeding, atomic percent of magnesium in inclusions can reach 10%-14%. And it remained in this range from leaving LF node to casting blank. Inclusions was transformed from irregular shape to near-spherical shape, and particle size is smaller than 2 μm. After the sample solidified, a layer of sulphide covered on the surface of the inclusion.(2) Magnesium treatment can significantly improve the cleanliness of molten steel, T.O removal rate can reach 70% after feeding wires. But in the case of high initial T.O, the particle size of MgO·A12O3 inclusion is large, which can easily cause nozzle clogging.(3) The rate of calcium treatment modifying inclusions is relatively slower.3 min after calcium treatment, the metamorphic reaction still didn’t achieve a balance. After soft blowing 7 min, the composition of inclusions reached liquid phase region. But the rate of magnesium modifying inclusions is relatively faster.3 min after feeding magnesium alloy cored, all of the inclusions in molten steel had been completely modified into MgO·A12O3.(4) Within 3min after feeding alloy cored wires, whether in magnesium treatment test steel or in calcium treatment test steel, the particle size of inclusions is reduced. And after soft blowing 7min, particle size rises again because of collision and aggregation between the inclusions. But relatively speaking, inclusions after magnesium treatment rise slightly, still less than the level before the treatment,Inclusions in calcium treatment increase greatly, and the number and particle size of inclusions even exceed the level before the treatment.