Quenching and partitioning response of carbon-manganese-silicon sheet steels containing nickel, molybdenum, aluminum and copper additions

Abstract

In order to produce passenger vehicles with improved fuel economy and increased passenger safety, car manufacturers are in need of steels with enhanced strength levels and good formability. Recently, promising combinations of strength and ductility have been reported for several, so-called third generation advanced high-strength steels (AHSS) and quenching and partitioning (Q&P) steels are increasingly being recognized as a promising third generation AHSS candidate. Early Q&P research used conventional TRIP steel chemistries and richer alloying strategies have been explored in more recent studies. However, systematic investigations of the effects of alloying elements on tensile properties and retained austenite fractions of Q&P steels are sparse. The objective of the present research was to investigate the alloying effects of carbon, manganese, molybdenum, aluminum, copper and nickel on tensile properties and microstructural evolution of Q&P heat treated sheet steels. Seven alloys were investigated with 0.3C-1.5Mn-1.5Si (wt pct) and 0.4C-1.5Mn-1.5Si alloys used to study carbon effects, a 0.3C-5Mn-1.6Si alloy to study manganese effects, 0.3C-3Mn-1.5Si-0.25Mo and 0.3C-3Mn-1.5Si-0.25Mo-0.85Al alloys to study molybdenum and aluminum effects and 0.2C-1.5Mn-1.3Si-1.5Cu and 0.2C-1.5Mn-1.3Si-1.5Cu-1.5Ni alloys to study copper and nickel effects. Increasing alloy carbon content was observed to mainly increase the ultimate tensile strength (UTS) up to 1865 MPa without significantly affecting total elongation (TE) levels. Increasing alloy carbon content also increased the resulting retained austenite (RA) fractions up to 22 vol pct. Measured maximum RA fractions were significantly lower than the predicted maximum RA levels in the 0.3C-1.5Mn-1.5Si and 0.4C-1.5Mn-1.5Si alloys, likely resulting from transition carbide formation. Increasing alloy manganese content increased UTS, TE and RA levels, and decreased yield strength (YS) and austenite carbon content (Cγ) levels. Retained austenite fractions up to 28 pct and UTS levels exceeding 1500, 1600, and 1700 MPa with TE levels of 20, 17 and 12 pct respectively were measured for the 0.3C-5Mn-1.6Si alloy. Molybdenum and aluminum additions to a 0.3C-3Mn-1.5Si alloy were not observed to significantly affect the resulting tensile properties at a partitioning temperature of 400 °C. Partitioning at 450 °C increased the TE levels of the 0.3C-3Mn-1.5Si-0.25Mo-0.85Al alloy and decreased the TE levels of the 0.3C-3Mn-1.5Si-0.25Mo alloy while similar UTS levels were measured in both alloys. The aluminum addition potentially increased strain hardening. RA fractions were observed to be close to predicted levels in both alloys. Copper and nickel additions were not observed to increase the resulting maximum UTS·TE levels. Copper and nickel additions increased RA levels and the measured RA levels were observed to be close to the predicted values.