Revealing the Conditions of Bainitic Transformation in Quenching and Partitioning Steels
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THE development of the third generation of advanced high-strength steels (3G-AHSSs) for automotive applications has drawn much attention, especially in the last two decades. The key philosophy of 3G-AHSSs is to obtain superior mechanical properties by tailoring the stability of metastable austenite and its corresponding TRIP (Transformation-Induced Plasticity) effect upon deformation. The quenching and partitioning (Q&P) process, which was first proposed by Speer et al. in 2003,[1] is one of the most successful concepts for producing 3G-AHSSs. The retained austenite is controlled following the CCE model,[2,3] wherein the following essential assumptions are made: (1) carbon is fully partitioned from martensite into austenite during the partitioning process; (2) competing reactions,
SHAN CHEN, CHENCHONG WANG, LINGYU SHAN, YONG LI, XIANMING ZHAO, and WEI XU are with the State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, Heping District, Shenyang 110819, P.R. China. Contact e-mails: wangchenchong@ ral.neu.edu.cn, [email protected] Manuscript submitted February 28, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
including carbide precipitation and austenite decomposition, are suppressed during the partitioning process. The abovementioned assumptions have been validated in 0.2C-4.0Mn-1.6Si-1.0Cr steel[4] by partitioning at 450 C for 300 seconds after quenching to a temperature from 70 C to 250 C and in 0.3C-1.6Si-3.5Mn steel[5,6] by partitioning at 400 C for 200 seconds after quenching to 180 C or 200 C. The retained austenite in the final microstructure is therefore calculated according to the austenite fraction upon first quenching with full enrichment of carbon partitioned from athermal martensite. However, in recent years, a substantial amount of evidence has shown that austenite decomposition may occur during partitioning and this unneglectable phenomenon could be one important reason for the disagreement of retained austenite fractions between model predictions and experimental observations.[7–9] The isothermal transformation below the martensite start (Ms) temperature has been reported in various systems, despite arguments regarding the nature of transformation,[10,11] i.e., isothermal martensitic or bainitic transformation. Nevertheless, decomposition below the Ms temperature was mostly considered as isothermal bainite in C-Mn-Si steels when the C content was less than 1 wt pct.[12–17] Regarding general bainitic
transformation above the Ms temperature, the transformation kinetics and the final transformed amount strongly depend on the composition of the steel. The effects of alloying elements on bainitic transformation below the Ms temperature were basically the same as those above the Ms temperature; for example, C and Mn retard bainitic transformation,[18,19] whereas Si delays cementite precipitation,[20,21] which effectively enriches C from martensite to retained austenite rather than precipitating. From the processing point of vi
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