Soft Phase Control of Yield Strength and Impact Toughness in a Q&P-Processed Medium-Carbon Steel

The yielding behavior and impact toughness of a 0.44 wt.% C steel subjected to quenching and partitioning (Q&P) at 400 °C for 20–1000 s were investigated. The steel has a multiphase microstructure containing blocky retained austenite (RA) and carbide-free bainitic ferrite (BF). Using the continuum composite approach, we show that plastic deformation at the 0.2% offset strain can be described as initiating via dislocation multiplication in RA and BF, while martensite remains elastic. The yield strength (YS) calculated from solid solution and dislocation strengthening agrees with experimental values (1110–1210 MPa) within 3% for all partitioning times. The classical composite model overestimates YS unless only RA and BF are considered as plastically deforming; the combined RA + BF fraction remains nearly constant, explaining the stable YS. The product of strength and elongation reaches 35.3 GPa×% at 300 s. The Charpy impact toughness peaks at 34 J/cm2 at 500 s, due to full martensite decomposition, and decreases to ~30 J/cm2 at 1000 s, attributed to excessive carbon enrichment of RA (1.63 wt.%), which reduces its plastic accommodation capacity and promotes earlier cleavage initiation.