Effect of Partitioning Treatment on the Mechanical Property of Fe-0.19C-1.47Mn-1.50Si Steel with Refined Martensitic Mic
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MICROSTRUCTURAL optimization is very important to achieve excellent mechanical property of steels. Many efforts have been paid to obtain good combination of strength, plasticity, and high impact toughness by means of phase transformation or grain refinement.[1–4] In the aspect of phase transformation, one of the promising advances is the development of steels with complex microstructures, in which the relatively hard phase such as martensite or bainite affords strength while the softer phase such as ferrite or retained austenite is responsible for plasticity. The outstanding achievements include dual-phase (DP), transformation-induced plasticity (TRIP), quenching and partitioning (Q&P)[5] and its modified version, i.e., quenching-partitioning-tempering (Q-P-T) steels.[6] Among them, the novel Q&P steel exhibits excellent comprehensive mechanical property, FENG HUANG and JILAN YANG, Ph.D. Students, ZHENGHONG GUO, Associate Professor, and SHIPU CHEN, YONGHUA RONG, and NAILU CHEN, Professors, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. Contact e-mail: zhenghongguo@ sjtu.edu.cn Manuscript submitted May 18, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
mainly because it contains lath martensite as matrix along with considerable amount of retained austenite (‡10 vol pct) at room temperature.[7] To realize this goal, a special cooling procedure was designed based on constrained carbon paraequilibrium (CCE) theory[5] and Koistinen–Marburger (K–M) equation.[8] That is, the steel was austenitized at first, then quenched to a temperature between the martensite start (Ms) and finish (Mf) temperatures, and held at the quenching stop or a slightly higher temperature for a suitable duration to allow the partitioning of carbon from supersaturated martensite to retained austenite, which can thereby be stabilized down to room temperature. To meet the requirement of microstructural control, especially to avoid the potential decomposition of austenite at partitioning temperatures, the composition of Q&P steels usually contains cheap Mn and Si/Al, taking the benefits that Mn decreases Ms and stabilizes austenite, while Si/ Al suppresses the formation of brittle iron carbides. Q&P steel is a martensitic-based steel in nature. Its strengthening mechanism includes the contribution of solute elements especially carbon (solid solution strengthening), dislocation density (similar to work hardening), effective martensitic size (grain boundary strengthening), and possible alloy carbide/nitride (precipitation hardening).[3] To define effective martensitic
Table I. Chemical Composition (Weight Percent) and Characteristic Temperatures of the Steel Investigated C 0.190
Mn
Si
P
S
Fe
Ac3 (K)
Ms (K)
Mf (K)
1.460
1.500
0.0070
0.0038
balance
1158 ± 5
679 ± 5
503 ± 5
size, previous studies on directly quenched low-carbon steels have found the laths formed within a prior austenite grain group themselves into several packets that are further subdivided into blocks.[9] The i
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