Optimization of processing, microstructure, and performance of Q&P steels

Abstract

Quenched and partitioned (Q&P) steels are third generation advanced high strength steels (AHSS) designed to maintain (or increase) strength and improve ductility (relative to the second generation of AHSS) while utilizing leaner alloyed steels. Q&P steels are of particular interest in the automotive industry as a way to improve safety and produce lighter weight vehicles that are more fuel efficient. Though there is an abundance of studies that attempt to maximize retained austenite contents in Q&P steels, as retained austenite has been shown to promote promising combinations of strength and ductility, few sources are available that take into consideration the influence of chemical and morphological characteristics (such as morphology, stability, and size) of the retained austenite on performance. In this work, consideration of retained austenite characteristics and the influence of prior processing (hot band thickness, cold reduction, and coiling temperature), and thus prior microstructure, on the heat-treating response, resulting microstructures, and property performance of Q&P steels are investigated in a 0.17C-2.8Mn-1.5Si steel through modeling, dilation simulations, and mechanical testing.

The Koistinen-Marburger (KM) relationship was modified to incorporate variations in composition and austenite grain size to model optimal quench temperatures. Through the explicit incorporation of grain size, it was implied that austenite could be fully stabilized at higher quench temperatures, dependent on the applied heat treatment and parameters used. Dilation experiments exploring the effect of quench temperature and partitioning time, involving careful monitoring of secondary martensite formation during final quenching, were performed. It was found that a general Q&P heat treatment could be applied to a material of the same composition but with variations in the prior processing and result in similar microstructures and amounts of retained austenite. Heat treatment parameters as determined by the dilation experiments were applied to sub-size ASTM E8 tensile specimens and mechanically tested. Substantial variation in tensile properties were found in the different processing conditions. These variations are not fully understood and may have arisen due to inconsistent temperature control during heat treatment.