The Role of Retained Austenite Stability on Low-Temperature Mechanical Behaviors of a Quenching and Partitioning Steel
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NTRODUCTION
MANY advanced high-strength steels (AHSS) have been newly developed for lightweight automotive applications.[1–6] Despite the excellent balance of strength and ductility, the unsatisfactory in-use properties keep these new automotive steels from practical applications.[1–3,7] Among various in-use properties, low-temperature mechanical behaviors are essential to ensure the safety of automotive steels. As temperature decreases, it becomes more difficult for dislocations to move inside the microstructure, which can induce undesired brittle fracture.[8,9] The phenomenon is called cold brittleness. In many cities around the world, winter temperatures can be much lower than 0 °C, the temperature where cold brittleness is commonly observed in steels. Therefore, to promote their industrial applications, AHSS should be designed carefully to make sure that the mechanical properties do not degrade too much at low temperatures. Quenching and partitioning (Q&P) steels belong to the third generation of AHSS, containing a multiphase microstructure of martensite matrix (sometimes partially ferrite and bainite) and 10 to 20 pct retained austenite.[4–7,10] Due to the susceptibility of martensite matrix to cold brittleness, the low-temperature mechanical behaviors of Q&P steels should be studied carefully for practical applications. Besides, the behaviors of retained austenite may change significantly along with
Z. WANG and M.X. HUANG are with the Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China and also with the Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen 518000, China. Contact e-mail: [email protected] Manuscript submitted June 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the in-use temperature.[11–14] Research shows that low temperature may reduce the stability of retained austenite and promote deformation-induced martensite transformation (DIMT),[14] which influences the yield strength, work hardening, fracture behaviors, and other mechanical properties.[11–13] When the retained austenite stability decreases, the risk of brittle fracture is also likely to increase.[15] In a word, the low-temperature deformation and fracture of Q&P steels can be even more complicated due to the presence of the metastable phase. Despite the potential risks and complicated mechanisms, studies on low-temperature behaviors of Q&P steels are still very limited. In the present work, the role of retained austenite stability on low-temperature mechanical properties is studied for a Q&P steel. Retained austenite stability was measured by X-ray diffraction (XRD), based on which the evolutions of yield strength, work hardening, and fracture behaviors at low temperatures are analyzed. The purpose of the present work is to draw attention to the low-temperature embrittlement risks of Q&P steels, based on which further work could be conducted to promote their industrial applications.
II.
EXPERIMENTAL PROCEDURE
The chemical composition of the studied Q&P st
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