Effect of Ultra-high Magnesium on SKS51 Liquid Steel Cleanliness and Microstructure

Abstract

This study emphasizes that ultra-high Mg ([%Mg]>0.03) content is very difficult to be achieved under conventional smelting conditions. The content of Mg in steel has significant influence on the cleanness of molten steel and microstructure. As the content of Mg increases, the content of O and S in steel decreases significantly, with O content as low as 0.0002% and S content as low as 0.0008%. Almost all inclusions in ultra-high Mg steel are magnesium-bearing oxide, sulfide, even carbide. As the content of Mg increases, the number of inclusions in the steel increases and the size decreases. But if too much Mg is added into the steel, the size and number of inclusions will rapid increase. The as-cast secondary dendrite spacing of steel decreased obviously with the increase of Mg content in steel. As the content of Mg increases, the as-cast microstructure changes from lamellar pearlite to granular pearlite. The phase diagram of SKS51 steel was calculated by Thermal-calc software. The calculated results showed that MgC₂ was precipitated in the austenite before the austenite was converted into perlite. MgC₂ may become the nuclear core, leading to perlite transformation. Suspected MgC₂ was found in spheroidized annealing Fe₃C core.