The strengthening and "softening" of low-carbon structural steels by silicon
The variation of tensile properties and grain sizes, on adding silicon, up to 1.03 wt% in five grades of low carbon structural steel was studied. The reduced sensitivity of the lower yield stress, [σy], at sub-zero test temperatures, otherwise called "softening", of these steels with reduced grain sizes, brought about by the silicon additions was also investigated. Optical and transmission electron microscopes were used to characterize the ferrite-pearlite structures, decarburized layers, random interlamellar spacings in the pearlite constituent, slip line, twin and dislocation structures. The precipitates observed were studied qualitatively with the transmission electron microscope, and microanalysis was carried out by using the dispersed energy of the X-rays (EDAX). Dilatometric studies were done to establish the critical temperatures of the steel grades. Uniaxial tensile tests were carried out at between 77 and 350 K, with strain rates of 1.7x10⁻⁴, 3.3x10⁻⁴s and 0.33 s⁻¹. The data from these tests were analyzed in terms of the contribution silicon and nitrogen make to the tensile properties, and were used to determine the thermal activation parameters, hence "softening". It was found that silicon additions beyond 0.31 wt% in these steels inhibit grain growth, due to the effect of silicon on the grain nucleation kinetics. In conformity to previous reports, the Hall-Patch slope, Ky was reduced to a limit, on the initial additions of silicon, but further to this observation, it was found that low silicon steels, below 14 μm possess higher strength than high silicon steels, within 0.31 to 0.78 wt% Si. In contrast with previous reports, in which SiN precipitates were identified in similar. steel grades, under aged conditions, low temperature (∞) Si₃N₄ precipitates were observed in both the annealed and the aged samples of the steels in the present study. It was also found that reduced grain sizes increase the "softening" tendency of these steels. These results we correlated with those from limited reports found in the literature, regarding the effect of silicon (above 0.7 wt%) on the impact behaviour of these grades of steel. From the correlation, it is suggested that a new theory should be sought to explain how the initial additions of silicon, with reduced grain sizes, improve the impact behaviour. Hypotheses are advanced linking the improvement of impact behaviour with the "softening" phenomenon. It is also suggested that a higher silicon to manganese ratio, with silicon not exceeding 1 wt%, may improve not only the strength and the impact behaviour, but also-the cost indices of these grades of steel.