Tailoring strength/ductility combination in 2.5 wt% Si-alloyed middle carbon steel produced by the two-step Q-P treatment with a prolonged partitioning stage

Abstract

The present work aimed at studying the effect of the two-step “Quenching-and-Partitioning” (Q-P) processing with a prolonged partitioning stage on the mechanical properties of middle carbon steel with the composition of 0.56 wt% С, 2.50 wt% Si, 1.70 wt% Mn, 0.50 wt% Cr, 0.21 wt % Mo, 0.12 wt% V, 0.05 wt% Nb. A total of 28 regimes with the quenching temperature of 160 °C and 200 °C, the partitioning temperatures of 200–300 °C and the partitioning time of 10–300 min were applied. Under Q-P treatment the UTS value ranged between 1535 and 2374 MPa, the total elongation ranged between 8 and 23% and the U-notched impact toughness ranged between 48 and 117 J/cm2, and TEL and KCU values were twice those of the “Quenching-Tempering” (Q-T) treatment. The Q-P-treated specimens possessed a multi-phase structure consisting of 38–61 vol% of tempered martensite (with transitional carbide precipitates), lower carbide-free bainite and 11–28 vol% of retained austenite enriched with 0.80–1.23 wt% carbon. The tensile strength gradually increased with the decrease in the partitioning temperature and partitioning time. In contrast, the ductility and the impact toughness tended to increase with the increasing partitioning temperature and partitioning time, and this phenomenon could be attributed to martensite tempering, bainite transformation and the increase in volume fraction of retained austenite. Q-P regimes were tailored for different strength/ductility combinations. The Q-P treatment with the regime of quenching at 160 °C, and partitioning at 270 °C for 50–120 min ensured the most advanced mechanical properties with the combination of 1743–1830 MPa UTS, 20–21% TEL, 36.6 GPa⋅% PSE, 75–79 J/cm2 KCU. The combination of the ultra-high tensile strength of 2374 MPa with 9% TEL and 61 J/cm2 KCU was obtained by quenching at 160 °C and partitioning at 200 °C for 90 min. The structural characteristics of the Q-P-treated steel were analyzed by SEM, TEM, XRD, nanoindentation, and dilatometric measurements.