Mechanism of boundary induced transformation and its application in the grain refinement of large-size structural steels

The conventional thermal-mechanical control processing technology governed by a heavy reduction at a relatively lower temperature has been applied successfully in the refinement of the final microstructure of hot-rolled steels. However, it might not be the best method for the manufacture of large-size structural steels due to their limited reduction ratio and relatively high temperature during hot rolling. Boundary induced transformation (BIT), is proposed in the present paper as an alternative approach for the grain refinement of large-size structural steels. The austenite grain size during reheating and rough rolling is controlled by Ti microalloying with the aim of reducing the critical strain for the onset of dynamic recrystallization (DRX) in the subsequent rolling. The precipitation of NbC particles and their inhibition of static recrystallization during the interpass interval is responsible for the accumulation of strain to ensure the occurrence of DRX in finish rolling. Consequently, the large amount of austenite grain boundaries resulted from the formation of fine DRXed grains will provide potent nucleation sites for the phase transformation from austenite to ferrite and then lead to an effective grain refinement. The experimental results demonstrate that the ferrite grain size can be significantly refined via this approach, illustrating that BIT mechanism is very applicable to refine grain size effectively for the large-size structural steels with limited reduction ratio.