Austenite grain abnormal growth in the microalloyed steel 38MnSiVS5 and determination of driving and pinning forces

Several castings of 38MnSiVS5 steel have been manufactured with different titanium and, aluminium contents and the austenitic grain size has been determined at temperatures between 900 and 1200 degreesC. The volume fraction of normal and abnormal grain growth have been quantified. The results show that abnormal growth of the austenitic grain occurs irrespective of the titanium and aluminium contents, due fundamentally to the partial dissolution of TiN precipitates. The steels with high aluminium contents have presented worse behaviour due to the formation of a second type of precipitates, namely AlN, which quickly dissolve between 1000 and 1100 degreesC, causing a drastic decline in the local pinning forces which gives rise to more pronounced abnormal growth of the affected grains. In parallel, a study of the precipitates has been carried out by transmission and scanning microscopy, allowing the calculation of pinning forces according to Zener and Gladman expressions and the establishment of a comparison with driving forces. This comparison makes it possible to explain the differences encountered between the different castings.