Quenching and Partitioning of Ni-Added High Strength Steels

Abstract

High strength steels containing significant fractions of retained austenite have been developed in recent years, and are the subject of growing commercial interest when associated with the TRIP phenomenon during deformation. A new process concept “quenching and partitioning” (Q&P) has been proposed by CSM/USA, and the results show the potential to create a new kind of steel microstructure with controlled amounts of retained austenite, enriched by carbon partitioning. Four steels containing C, Si, Mn, Ni, Cr and Mo, were designed with variation in the Ni and C content, aiming to decrease Bs temperature and to suppress carbide formation during the partitioning treatment. Several heat-treatment procedures were performed in specimens previously machined for tensile testing, while x-ray diffraction was used to determine the fraction of retained austenite. The tensile test results showed that except for the high C high Ni alloy, most of the processing conditions resulted in strengths superior to those of advanced high strength steels (AHSS), although it is importantly recognized that higher alloy additions were used in this study, in comparison with conventional AHSS grades.. A variety of strength and ductility combinations were observed, confirming the potential of the Q&P process and illustrating the strong influence of the final microstructure on the mechanical properties. Experimental results for samples partitioned at 400 °C indicate that higher ultimate tensile strength is associated with higher fraction of retained austenite for multiple heat treatments of each alloy investigated. The amount of retained austenite obtained was generally lower than that predicted by the model. Further studies are in progress to understand the influence of alloying and processing parameters (time/temperature) on the partitioning of carbon and precipitation of transition carbides.