Improved grain size control in heat treatable steels : final report

This research, involving CENIM, CORUS, Sidenor and SIMR examined the effect of compositional and processing variables on the grain coarsening characteristics of steels. Compositional variables examined included nitrogen (typical of BOS and EAF steels), aluminium, niobium, titanium and vanadium. In Al-treated steels it was shown that the nitrogen content was equally important in controlling grain coarsening temperature (GCT). In steels with ~0.004 %N the GCT was about 950°C maximum and in excess of 1 050°C with ~0.010 %N. Processing history was shown to have a marked influence on GCT. The variables examined included start and finish hot working temperatures, cold working, normalising and heating rate for austenitising. The highest GCT, in hot worked Al-treated steels, was achieved with high reheat temperatures (for complete AIN dissolution), and high finish working temperatures to help retain AIN in solution during cooling. Precipitation of the maximum amount of fine AIN pinning particles was achieved by slow heating (e.g. through 680°C). Abnormal grain growth, characterised by a mixture of very coarse and fine grains, occurred at the GCT and above. At much higher temperatures, Al-treated steels were generally uniformly coarse grained, but these were finer than the very coarse grains in the abnormal grain growth region. Work also examined the effect of grain size, Mn and residuals on the hardening characteristics of plain carbon steels. Grain refinement markedly reduced the depth of hardening achievable, with a lack of surface hardness in extreme instances. The results and data presented should be of major value to the heat treatment industry.