EFFECT OF SILICON ON MICROSTRUCTURES AND SOME MECHANICAL-PROPERTIES OF LOW-CARBON STEELS

Three vacuum melted, controlled rolled low carbon steels, varying principally in silicon content (0·31–1·03 wt-%) have been heat treated under various conditions to achieve different grain sizes, which in turn influences properties. The microstructures have been studied by optical and transmission electron microscopy. The interlamellar spacing of the cementite in the pearlite colony was observed to decrease up to 0·78 wt-%Si. At about 1·03 wt-%Si the cementite tends to fragment and spheroidise; this also occurs at the higher austentising temperatures. The volume fraction of pearlite in these steels remains essentially the same up to 0·78 wt-%Si; however, at 1·03 wt-%Si this volume fraction is reduced by about 2%. Furthermore, silicon additions to the steel were found to increase the extent of decarburisation. The incidence of ledged boundaries was rarely observed, but a higher ledge density was noted with increased silicon levels. Only partially coherent (with ferrite) α-Si3N4 precipitates were observed in all the conditions, observation and identification being easier for the higher silicon steels (in the annealed condition) and the aged condition. In the aged condition, the morphology of the α-Si3N4 precipitates is flaky in the grain interior and rather angular at the grain boundaries. It is inferred that the effect of silicon on the tensile properties of these steels is via these microstructural features, but its effect on the impact behaviour requires a more satisfactory theory than those so far proposed in the literature. © 1990 The Institute of Metals.