Effect of titanium additions on austenite grain growth kinetics of medium carbon V–Nb steels containing 0·008–0·018%N

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Abstract
An investigation has been carried out into the effects of multiple additions of the microalloying elements V, Ti, and Nb on the austenite grain growth characteristics of 0·4C–1·6Mn (mass-%) steels. It has been shown that simultaneous additions of V, Ti, and Nb inhibit grain growth and increase the grain coarsening temperature markedly. In both V and Ti steels, increased Nb contents refine further the austenite grain size. Using a thermodynamic model, good agreement has been obtained between the measured and calculated volume fractions of carbonitride particles at the solidus temperature, although such particles are too large for effective grain boundary pinning. Afeature of interest is that increased Nb contents in Ti–V steels decrease the size of the larger Ti rich carbonitride particles and inhibit their formation during solidification of the steel. The stability of the carbonitrides increases with increasing Ti content especially. The N dissolved in the austenite decreases with simultaneous additions of Ti, Nb, and V, which may have effects on other properties. It is concluded that increased N in the steels results in a further refinement of the austenite grain size by increasing the volume fraction of carbonitride, and such increased N may decrease the microalloy content dissolved in the austenite to such an extent that carbo nitride particle coarsening rates are decreased and more efficient grain refinement is produced. On the basis of the results obtained, suggestions are made for the optimisation of grain boundary pinning and grain refinement. MST/1332

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