Influence of Al and Nb on optimum Ti/N ratio in controlling austenite grain growth at reheating temperatures

Abstract

On the basis of steel 38MnSiVS5 a high number of castings were manufactured varying only the Ti and Al contents and obtaining ingots in which the Ti/Al ratio was different in each case. The specimens will be heated in air furnace to different austenitization temperatures, between 900°C and 1200°C, and subsequently quenched in water. The most important achievement would be that Al plays a harmful role as an austenite grain controller at high temperatures (>1 050°C), as a consequence of the quick dissolution of AIN particles and therefore the reduction in inhibition forces at the grain boundaries which are intercepted by them. The best austenite grain control was shown by steel with contents of Ti=0.044, Al=0.009, N=0.0131 (mass%) and a Ti/N ratio of 3.36. But the interpolation of the results allows it to be concluded that the best Ti/N ratio would be close to 2.5. It was also found that the control of austenite grain size improved with an addition of Nb. In paralell, a study of precipitate sizes was carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).