Effects of residual elements on the microstructure and mechanical properties of a Q&P steel

Producing steel requires large amounts of energy to convert iron ores into steel, which often comes from fossil fuels, leading to carbon emissions and other pollutants. Increasing scrap usage emerges as one of the most effective strategies for addressing these issues. However, typical residual elements (Cu, As, Sn, Sb, Bi, etc.) inherited from scrap could significantly influence the mechanical properties of steel. In this work, we investigate the effects of residual elements on the microstructure evolution and mechanical properties of a quenching and partitioning (Q&P) steel by comparing a commercial QP1180 steel (referred to as QP) to the one containing typical residual elements ($\text{Cu}+\text{As}+\text{Sn}+\text{Sb}+\text{Bi}<0.3\text{wt}\%$) (referred to as QP-R). The results demonstrate that in comparison with the QP steel, the residual elements significantly refine the prior austenite grain ($9.7\mu \mathrm{m}$ vs. $14.6\mu \mathrm{m}$) due to their strong solute drag effect, leading to a higher volume fraction (13.0% vs. 11.8%), a smaller size (473 nm vs. 790 nm) and a higher average carbon content (1.26 wt% vs. 0.99 wt%) of retained austenite in the QP-R steel. As a result, the QP-R steel exhibits a sustained transformation-induced plasticity (TRIP) effect, leading to an enhanced strain hardening effect and a simultaneous improvement of strength and ductility. Grain boundary segregation of residual elements was not observed at prior austenite grain boundaries in the QP-R steel, primarily due to continuous interface migration during austenitization. This study demonstrates that the residual elements with concentrations comparable to that in scrap result in significant microstructural refinement, causing retained austenite with relatively higher stability and thus offering promising mechanical properties and potential applications.