Structure and Mechanical Properties of V, Nb-Added TRIP-Assisted Steel After Q&P Treatment with Near Ac3 Austenitization

The article is devoted to studying the effect of Q&P heat treatment parameters after intercritical annealing near the Ac3 critical temperature on the phase-structural state and mechanical properties of 0.2 wt. % C-Mn-Si-Cr-Mo TRIP-assisted steel micro-alloyed with Nb and V. The investigations were conducted using optical microscopy (OP) and electron microscopy (SEM, TEM), X-ray diffraction (XRD) analysis, and mechanical properties testing. It was found that intercritical annealing at 900 ℃ followed by the quenching to 235 ℃ and the “partitioning” at 350–450 ℃ promoted the formation of a multiphase structure consisting of martensite, bainite, and minor amounts of proeutectoid ferrite (about 5 vol. %) and retained austenite (5.4–7.4 vol. %). The strength of steel decreased with the holding at the “Partitioning” stage. Retained austenite has a film-like morphology and performs different tendencies to the TRIP effect depending on the “Partitioning” temperature. The optimal was the “Partitioning” at 400 ℃ for 10–20 min, ensuring the advanced combination of properties: UTS of 1020–1122 MPa; YTS of 908–1020 MPa, TEL of 21.0–23.5%, impact toughness (KCV) of 115 J/cm2, and the PSE of 25 GPa⋅%.