Hot Deformation Characteristics of Oxide/Sulphide Inclusions in Low Carbon Steels

Abstract

Studies have been made of the effect of sulphur contents between 0.02 and 0.2% on the composition of the oxides and oxy-sulphides produced by deoxidation of a low carbon (<0.04%) steel with aluminium-manganese-silicon alloys. The inclusions were analysed by Quantimet for size distribution and Microscan V for composition. Hot rolling of the suction cast bars was achieved on a 2 high-rolling mill. Samples were taken at 50%, 70% and 90% total reductions in thickness and temperature of rolling was between 800 and 1300°C. Deformation of the inclusions was measured by optical microscopy. It was shown that at 0.56% Mn in the steel increasing sulphur decreased the manganese oxide content of the inclusions but with higher manganese contents (>1.5% Mn) little or no effect was observed. The average size of both the oxide and sulphide inclusions increased with increasing sulphur content in the Steel. The relative plasticity indices of inclusions have been related to inclusion size, matrix strain, composition and the rolling temperature. It was shown that inclusions less than about 2 um in diameter did not exhibit any deformation characteristics under all working conditions. It was also shown that the relative plasticity index value was lowered as the matrix strain increased. Spheroidisation of the inclusions partly contributed towards the decrease in relative plasticity index with decreasing inclusion size. Oxide inclusions rolled at the lower temperatures behaved in a rigid or brittle manner. There was a rapid transition from non-deformable to deformable behaviour over a narrow range of temperature. The transition temperatures for the inclusions were observed to be well below the liquidus temperatures of any of the oxide compositions. The non-deformable/deformable transition temperature was decreased with increasing sulphur content of the steel, reflecting the change in composition of oxides shown above. A ternary oxide (Al 0.-MnO-SiO ) diagram has been produced to predict the transition temperatures for the oxide inclusions within the composition range studied. Deformation of sulphide inclusions decreased with increasing rolling temperatures. It was shown that the equations (for estimation of inclusion strain in a third plane if the inclusion strain in the other planes are known; width spread, ez: plane strain plasticity index, (νpξ*)) are valid and can be useful for volume plasticit?® index determination. It was found that ductility of the steel decreased with increasing percentage area of both oxide and sulphide inclusions. Directionality was found to be pronounced in steels containing type II MnS inclusions.