Microstructure and Properties of a Low Carbon Bainitic Steel Produced by Conventional and Inverted Two-Step Austempering Processes

Junyu Tian; Wei Wang; Guang Xu; Xiang Wang; Mingxing Zhou; Hatem Zurob

doi:10.1007/s12540-022-01316-3

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자료유형: 학술저널

저자정보: Junyu Tian (Wuhan University of Science and Technology) Wei Wang (Central Research Institute of BaoShan Iron & Steel Co. Ltd.) Guang Xu (Wuhan University of Science and Technology) Xiang Wang (McMaster University) Mingxing Zhou (Wuhan University of Science and Technology) Hatem Zurob (McMaster University)

저널정보: 대한금속·재료학회 Metals and Materials International Metals and Materials International Vol.29 No.5

발행연도: 2023.5

수록면: 1,298 - 1,310 (13page)

DOI: 10.1007/s12540-022-01316-3

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The microstructure evolution and strain hardening behaviour of a low-carbon carbide-free bainitic steel prepared by eithersingle-step austempering (420 °C or 365 °C), conventional two-step austempering (420 °C then 365 °C) or inverted two-stepaustempering (365 °C then 420 °C) treatments were investigated. The results show that when the total isothermal holdingtime was the same, the inverted two-step austempering treatment (first completing bainitic transformation at low-temperatureand then annealing at high-temperature austempering) led to the highest toughness (30.7 GPa%) due to the finer bainiticmicrostructure and higher fraction of film-like retained austenite. Grain refinement and transformation-induced plasticityallowed the material to achieve high ductility without sacrificing strength. Comparing single-step austempering at 365 °Cwith the inverted two-step austempering process indicates that annealing at a higher temperature after completion of thebainitic transformation resulted in better tensile properties because of a lower dislocation density and more stabler retainedaustenite. In addition, the samples heat-treated by the conventional two-step austempering process exhibited slower bainitetransformation kinetics and the worse tensile properties than the sample which was heat-treated using a single-step austemperingtreatment at 365 °C or the one which was heat-treated using an inverted two-step heat treatment. Through the analysisof the orientation relationships, it is observed that the original austenite and the bainitic plates mainly followed the K-Sorientation relationships regardless of whether the bainite plates were formed in the first or the second heat-treatment step.

#Bainitic steel · Microstructure · Orientation relationships · Strain hardening · Two-step austempering

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