Resetting the Austenite Stability in a Medium Mn Steel via Dislocation Engineering
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INTRODUCTION
HIGH-PERFORMANCE steels with high strength and excellent ductility are desirable to construct lightweight structural components for applications in varied industrial sectors.[1,2] To achieve high strength and excellent ductility, the transformation-induced plasticity (TRIP) effect is frequently employed in different advanced high-strength steels (AHSSs),[3] such as carbide-free bainite steel,[4–6] quenching and partitioning (Q&P) steel,[7–9] and medium Mn TRIP steel.[10–13] The TRIP effect takes place when the metastable retained austenite grains transform to martensite during plastic deformation. The TRIP effect can not only provide the additional dislocation density but also offer the dynamic strain partitioning to improve the work hardening behavior.[14,15] However, it is believed that either the early occurrence of TRIP effect at small strain regime due to low austenite stability or the suppressed TRIP effect at large strain regime due to high austenite stability is detrimental to the mechanical properties.[16] In other words, the stability of retained austenite grains should be optimized to provide a well-controlled TRIP effect to enhance the strain hardening behavior of steels. In general, the stability of retained austenite grains is governed by different factors, including chemical compositions,[17,18] grain size,[19,20] morphology,[8] and matrix.[21] These factors can be manipulated to optimize
B.B. HE, M. WANG, and M.X. HUANG are with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China, and also with Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen 518000, China. Contact e-mail: [email protected] Manuscript submitted May 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the stability of austenite grains during thermal-mechanical processing. For example, the chemical compositions and grain size of retained austenite in a medium Mn steel can be tuned by using different intercritical annealing parameters such as duration and temperatures.[22,23] However, for a given steel product, it is difficult to reset the mechanical stability of retained austenite grains as the above factors are inherent to the thermal-mechanical processing. Nevertheless, there may be a necessity to reset the mechanical stability of retained austenite grains for some steel products. For instance, it has been demonstrated that a substantial volume fraction (~ 0.2) of retained austenite grains transform to martensite at small strain regime (~ 5 pct) due to their low mechanical stability in a dual-phase medium Mn steel.[24] In this contribution, we propose to reset the mechanical stability of retained austenite grains in a medium Mn steel by dislocation engineering concept.[25] The dislocations are introduced by using a warm rolling process at a temperature where the retained austenite grains are mostly deformed by the dislocation plasticity.[26] These dislocations tend to stabilize the retained austenite grains during plastic deformation at room temperature.
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