Effect of Residual Elements on the Strength and Recrystallisation Kinetics in Nb-Based Microalloyed Steels During Hot Deformation

Abstract Residual elements such as Cu and Sn have been introduced into steel due to increasing usage of steel scrap, which may influence the microstructural evolution during processing. This study investigates the solute drag effect of Cu (0.5 wt pct), Sn (0.3 wt pct) and Nb (0.035 wt pct) on recrystallisation kinetics in low carbon microalloyed steels during hot deformation. Double hit compression tests have been carried out at 1000 °C to 1050 °C and strain rates of 2 s −1 , to measure the solute retardation parameter (SRP). Results reveal that Nb exhibits the strongest individual retardation (SRP ≈ 383), consistent with literature, whilst the SRP has been measured to be 6 for Cu and 86 for Sn. In terms of the synergetic effect, Cu and Nb solute drag effect act additively, but Sn and Nb exhibit a negative synergy, reducing the combined SRP by 36 pct compared to a linear-addition prediction. This has been attributed to Sn having a high diffusivity and binding energy, which may saturate grain boundary vacancies and diminish Nb’s effectiveness. Stress–strain analysis reveals that the Sn addition increases the work hardening rate with increasing strain, unlike Cu and Nb. The observed two distinct solute–dislocation interaction behaviours during deformation further supports the hypothesis that Sn tends to have a stronger affinity to grain boundaries and dislocations at deformation temperatures of 1000 °C and 1050 °C, compared to Nb and Cu addition.