Inconsistent flow stress in low carbon boron steels during finishing

The influence of deformation parameters and composition on flow stress behaviour of boron-containing steels is described. High boron steels displayed a consistent flow stress during finishing due to the boron concentration at austenite grain boundaries having achieved steady state followed by the precipitation of BN. Higher flow stresses than those found in boron-free steel were due to solute boron at grain boundaries, which delayed the onset and the rate of dynamic recrystallisation (DRX) and consequently enhanced the solute drag effect. In low B steels, inconsistent flow stress behaviour at finishing temperatures above the BN solubility temperature was attributed to deformation at or near the maximum non-equilibrium grain boundary segregation (NGS) of boron, which occurred rapidly. The flow stress in low B steels was consistent when DRX occurred during roughing and early finishing and was promoted by weak NGS and a finer grain size when the reheat temperature was decreased. Flow stress consistency was also promoted by avoiding a NGS peak by deforming at temperatures below the BN solubility temperature. A high driving force for AlN precipitation in austenite increased flow stress inconsistency through the protection of solute boron for NGS and increased solute drag.