The effect of micro alloying elements (vanadium, titanium) additions on the austenite grain growth behavior in medium carbon steel containing nitrogen

The austenite grain growth behavior of microalloying elements free steel (nitrogen steel) and micro alloyed steel (V-N steel and V-Ti-N steel) was investigated. The equilibrium dissolving behavior of precipitates was calculated by thermodynamic software and the morphology was observed by tunneling electron microscope. Moreover, grain growth kinetics was analyzed through theoretical calculation. Results show that the austenite grain size of V-N steel and V-Ti-N steel are significantly refined by the undissolved precipitates compared to nitrogen steel. Due to higher dissolving temperature of (vanadium, titanium)(carbon, nitrogen), the austenite grain of V-Ti-N steel keeps fine and increases slowly from 900 degrees C to 1250 degrees C. The difference of activation energy between V-N steel and V-Ti-N steel was supposed to come from the effect of different kinds of precipitates on the austenite grain growth. Compared to the vanadium rich (vanadium, titanium)(carbon, nitrogen), the titanium rich (vanadium, titanium)(carbon, nitrogen) is larger and more stable in view of its existence at higher temperature. The decrease of pinning forces can be attributed to the decrease of volume fraction and increase of radius of (vanadium, titanium)(carbon, nitrogen). Compared with critical grain sizes, the larger experimental grain sizes lead to grain growth from 900 degrees C to 1250 degrees C.