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Intercritical annealing to achieve a positive strain rate sensitivity of mechanical properties and suppression of plastic instabilities in multi-phase medium-Mn steels

Published

Author(s)

Jake Benzing, William E. Luecke, Steven Mates, Dirk Ponge, Dierk Raabe, James Wittig

Abstract

The present work investigates the room temperature tensile properties of a cold-rolled Fe-12Mn- 3Al-0.05C (% by mass) steel designed to have a multi-phase microstructure and positive strain rate sensitivities. An increase in intercritical annealing time from 0.5 h to 8 h (585 °C) increases the amount of ultrafine-grained (UFG) reverted austenite from 2 to 35 volume %, but the remainder of the microstructure is composed of UFG ferrite and recovered 𝛼′-martensite that resembles the cold-rolled state. Servo-hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static to low-dynamic strain rates ( ̇ = 10-4 s-1 to  ̇ = 103 s-1). Mechanical testing results yield a positive strain-rate sensitivity of the yield strength (YS) and ultimate tensile strength (UTS) for both annealing times, indicating the flow stress predominantly depends on respective short-range barriers in the multi-phase microstructure. Tensile properties, thermal imaging and digital image correlation indicate there is suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (an area of improvement for many multi-phase medium-Mn steels). Also, fracture surfaces of specimens annealed for 0.5 h show some instances of cleavage fracture (approximately 30 µm wide areas and lath-like ridges), while specimens annealed for 8 h maintain a substantially greater product of strength and elongation. The relevant important processing-structure-property relationships are discussed in the context of design strategies for multi-phase steels such that maintain homogeneous deformation behavior and positive strain rate sensitivities are maintained.
Citation
Journal of Alloys and Compounds

Keywords

Medium-Mn steel, strain-rate sensitivity, servo hydraulic tension, Kolsky bar tension

Citation

Benzing, J. , Luecke, W. , Mates, S. , Ponge, D. , Raabe, D. and Wittig, J. (2020), Intercritical annealing to achieve a positive strain rate sensitivity of mechanical properties and suppression of plastic instabilities in multi-phase medium-Mn steels, Journal of Alloys and Compounds, [online], https://doi.org/10.1016/j.msea.2020.140469 (Accessed July 31, 2021)
Created October 28, 2020, Updated January 19, 2021