Effects of two different types of MX carbonitrides on austenite growth behavior of Nb-V-Ti microalloyed ultra-high strength steel

The size evolution of two different types of MX carbonitrides and its influence on austenite grain growth behavior of Nb-V-Ti microalloyed ultra-high strength steel has been investigated. Based on microstructure examination, the spherical Nb-rich MX is much finer than the cuboidal or rectangle shape Ti-rich MX. The average size of austenite grain and two types of MX carbonitrides sluggishly increased with temperature in the low temperature region. However, a rapid growth has been observed at relative high austenitizing temperatures due to the dissolution and coarsening of precipitates. A grain growth model was developed in view of the pinning effect of different types of carbonitrides. The pinning contribution of two types of MX carbonitrides was calculated using pinning parameter equation, and the pinning effect of Nb-rich MX is much larger than that of Ti-rich MX. This suggests that fine Nb-rich carbonitrides play a more predominant role in retarding austenite grain growth.