Quenched and Partitioned Microstructures Produced via Gleeble Simulations of Hot-Strip Mill Cooling Practices
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TRODUCTION
QUENCHED and partitioned (Q&P) steels exhibit excellent combinations of strength and ductility, because they contain a significant fraction of martensite along with austenite that can transform to martensite during deformation leading to enhanced work hardening and improved elongation.[1–6] Previous researchers created Q&P microstructures via thermal profiles that are perhaps most suited to an annealing step following cold rolling, as shown schematically in Figure 1(a). In these profiles, the steel is austenitized (either fully or partially) and then quenched to a temperature between the martensite start (Ms) and martensite finish (Mf) temperatures to create a controlled mixture of martensite and austenite (and any intercritical ferrite present during annealing). The steel is then held isothermally at the quench temperature (QT) (one-step partitioning) or at an elevated temperature (two-step partitioning) to allow carbon to partition into austenite from martensite. Carbon-enriched austenite, stable at room temperature, is capable of contributing to mechanical properties in the same manner as it does in transformation-induced plasticity (TRIP) steel. Other microstructural changes due to conventional tempering reactions (e.g., carbide formation or carbon atom trapping at defects in the martensite), decomposition of austenite into other transformation products such as bainite, or interface
G.A. THOMAS, PhD Student, and J.G. SPEER and D.K. MATLOCK, Professors, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] Manuscript submitted November 15, 2010. Article published online March 9, 2011 3652—VOLUME 42A, DECEMBER 2011
migration are recognized as potential competing mechanisms during the partitioning step. The present authors previously proposed a processing route that would allow for the production of Q&P microstructures directly off of the hot-strip mill,[7] as shown schematically in Figure 1(b). Here, the quenching step is accomplished during run-out table cooling, similar to that in dual-phase steels (step cooling may be used prior to quenching if primary ferrite is desired). Further, the partitioning step may be accomplished nonisothermally during slow cooling of the wound coil, similar to that in austempering of hot-rolled TRIP steel. Under these conditions, the coiling temperature (CT) dictates critical attributes of the process. First, the coiling temperature serves as the QT, which sets the initial fraction of martensite and austenite prior to partitioning. Second, the coiling temperature serves as the initial partitioning temperature (PTi) and ultimately establishes the extent of partitioning that can occur during coil cooling. Because these two important parameters are both controlled by the coiling temperature, the hot-rolled Q&P process is fundamentally more constrained than the profile shown in Figure 1(a) in which the QT and partitioning temperature (and time) can be independently controlled. Figure 2 sho
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