Prediction for the Austenite Grain Size of Ti/Nb Microalloyed X70 Linepipe Steel

Abstract

Austenite grain size is an important factor in determining the strength and toughness of a material. It is the initial condition for the subsequent phase transformation during cooling and thus affects the final microstructure and resulting mechanical properties. Alloying elements such as Nb, Ti, V, Al can be used for retarding austenite grain growth. These elements form secondary phases with interstitial elements such as C and N in steel and they work as an inhibitor of moving boundaries. So it is important to investigate the change of effects of precipitate on grain growth varying time and temperature. The purpose of the present study is to propose a model for predicting austenite grin size which can deal more than a kind of secondary phase such as TiN and NbC which precipitates different temperature. Also, the fractions of precipitates are calculated based on thermodynamic database, and considering abnormal grain growth after dissolving of NbC at high temperatures. Finally, proposed model will be tested in continuous heating condition.