The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mech
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TRODUCTION
ADVANCED high-strength steels (AHSS) containing substantial amounts of retained austenite have been the subject of many recent investigations due to their good combination of strength and ductility.[1–4] Recently, Hsu[5] proposed a novel quenching-partitioning-tempering (Q-P-T) process, with the microstructure of Q-P-T steels consisting of lath martensite, fine complex alloying carbides, and flakelike carbon-enriched retained austenite. Similar to quenching and partitioning (Q&P) technology previously proposed by Speer et al.,[3,6] the Q-P-T process includes a quenching treatment from the austenite state to a quenching temperature (TQ) between the start temperature of martensitic transformation (Ms) and the final temperature (Mf). Then, an isothermal holding (partitioning/tempering) treatment is operated, during which carbon diffuses from the supersaturated martensite into the neighboring untransformed austenite, thereby stabilizing retained austenite during sequent cooling to room temperature; meanwhile, stable alloying carbides (such as NbC) precipitate from the martensite matrix and provide the potential of precipitation strengthening, which was excluded in the Q&P process for satisfying the ‘‘Constrained Carbon ParaS. ZHOU, K. ZHANG, and Y. WANG, Doctoral Students, and J.F. GU and Y.H. RONG, Professors, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 8, 2011. Article published online October 19, 2011 1026—VOLUME 43A, MARCH 2012
equilibrium’’ theory proposed by Speer et al.[3,6] Our previous research[7,8] on cold-rolled sheets showed that steels subjected to Q-P-T processes exhibited an excellent combination of strength and ductility. Great efforts were made for understanding the relationship between the microstructure and the mechanical property of Q-P-T steels. However, the mechanism of high strength-ductility of Q-P-T steels and the stability of retained austenite at elevated temperatures were not investigated. Therefore, the aim of the present article is to study the mechanical properties of the Q-P-T steel designed at elevated temperatures and the stability of the retained austenite so as to reveal the mechanism of high strength-ductility and to evaluate the temperature range to which the Q-P-T steel studied can be subjected.
II.
EXPERIMENTAL PROCEDURE
The chemical compositions of the Q-P-T steel investigated are listed in Table I, together with Ac3, Ms, and Mf temperatures, which were determined by a Gleeble-3500 thermal simulator (Gleeble, Dynamic Systems Inc. (DSI), Poestenkill, NY). Addition of Si element is to suppress the formation of cementite (Fe3C). Elements Mn and Ni are used to stabilize austenite and lower Ms temperature. Element Nb can effectively not only refine original austenite grains but also stabilize carbide formation, which leads to grain-refinement strengthening and precipitation strengthening. Experimental samples with a dimension of 105 9 22 9 2.5 m
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