Comparative study of magnesium and calcium treatments on Q420NE high-strength low-alloy steel

Abstract

In Q420NE high-strength low-alloy steel, the type, size, and distribution of non-metallic inclusions critically influence the strength-toughness, formability, and weldability. Industrial trials on Mg-treatment were conducted to improve the steel cleanliness and inclusion control of Q420NE high-strength low-alloy steel, with comparative studies combining Ca-treatment and untreated experiments. Samples were collected at four critical stages: after RH degassing, wire feeding, soft blowing, and tundish stages. Results show that Mg-treated steel exhibits smaller average equivalent diameters and lower area fractions of inclusions compared to both Ca-treated and untreated steels. After Mg-treatment, the total oxygen (T.O) content decreased to 15 ppm, representing reductions of 25% and 51.6% compared to untreated and Ca-treated steels, respectively. Untreated steel contained Al₂O₃, CaO-Al₂O₃, MnS, and CaS. Ca-treatment modified Al₂O₃ into spherical CaO-Al₂O₃ and transformed MnS into (Ca,Mn)S, forming composite inclusions. In Mg-treated steel, Al₂O₃ inclusions are transformed into dispersive distributed MgO·Al₂O₃ spinel coated with MgS.