Characteristics analysis of inclusion of 60Si2Mn–Cr spring steel via experiments and thermodynamic calculations

Abstract

A plant trial of the production of 60Si2Mn–Cr spring steel using silicon–manganese combined with aluminium to deoxidise was performed, and the characteristics of inclusions during ladle furnace refining, calcium treatment and in billets were investigated by scanning electron microscope–energy dispersive spectroscopy and thermodynamic calculations. The formation mechanisms of oxide and CaS inclusions are discussed. The experimental observation and thermodynamic analysis showed that calcium treatment cannot entirely modify large-size MgO·Al2O3 spinel inclusions into homogeneous CaO–MgO–Al2O3 inclusions, but formed a liquid xCaO·yAl2O3 layer on its surface. When the Al content was 0.05 mass%, [Mg], [Ca] and [O] in molten steel could be controlled at 2.7∼5 ppm, 2.5∼8 ppm and 4.1∼5.2 ppm, respectively, to achieve inclusions in the low melting point region. A large amount of CaS was generated in the present process due to a higher sulphur concentration in the molten steel and an excessive amount of Ca–Si wire. To avoid/reduce its formation, the sulphur concentration should be controlled to below 70 ppm.