Austenite grain coarsening and formation of intragranular ferrite in HSLA steels deoxidized with Ti and Al

The grain coarsening behaviour of hot rolled plates of Ti and Al killed low alloy high strength steels was studied. It was found that the grain growth characteristics of the two steels were markedly different. The Ti treated steel showed normal grain growth in the range 1173-1473 K whereas the conventional Al killed steel exhibited virtually no grain growth from 1173-1373 K, followed by abnormal grain growth at a temperature of 1373 K. Abnormal grain growth occurred at a much higher temperature (> 1473 K) in the Ti steel. Microstructural investigation revealed that the Ti steel contained more large particles, which were identified to be an oxide phase rich in Ti and Al and together with smaller TIN particle. The Al killed steel contained relatively few large particles, but had a larger volume fraction of fine TiN particles compared with the Ti steel under identical treatment conditions. It is concluded that the fine precipitates in the Al steel effectively prevent normal grain growth, but that selective dissolution and coarsening promote abnormal grain growth as the temperature increases above 1273 K. In contrast, the low density of precipitates did not prevent normal grain growth in the Ti steel at temperatures below 1473 K. However, the precipitates evidently restrained grain growth up to 1473 K, wherein dissolution and coarsening triggered unpinning and resulted in abnormal grain growth. For a controlled cooling rate over the temperature range of 1073 to 773 K, ferrite formation was found to be stimulated by large oxide particles within coarse austenite grains in the Ti killed steel, confirming previous work on the ferrite nucleating effect of TiO particles.