THE EFFECT OF GRAIN-SIZE ON THE ALLOY-SOFTENING OF LOW-CARBON STRUCTURAL-STEELS BY SILICON

The effect of grain size on the "softening" of five vacuum-melted controlled-rolled steels (principally varying in silicon content) was studied under various test temperature and strain rate conditions. The resulting structures were characterized using optical and transmission electron microscopes. The effective (thermal) stress, sigma*, the dislocation velocity exponent, m* and the dislocation frequency factor, upsilon, were used to interpret the results. "Softening" due to silicon alone was separated from the "softening" due to the combined effect of silicon plus grain size. It was observed that refining the grains promoted "softening" through the reduction of sigma*. This effect was not so obvious at grain sizes > 20-mu-m and small strain rates, epsilon(.). Generally, m* and epsilon increased with increasing silicon additions and with reduction in grain size, provided there was a true "softening". True "softening" was observed to depend on the silicon content, test temperature, strain rate and grain size. Activation parameter studies were correlated with previous studies on the effect of silicon on the impact behaviour of this grade of steels. These suggested that higher silicon contents than those currently used in these steels could enhance the strength, and the impact properties, provided the grain sizes are tailored.