Temperature-Dependent Macroscopic Mechanical Behaviors and Their Microscopic Explanations in a Medium Mn Steel
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t years, medium Mn steels, as promising candidates for the third-generation advanced high strength steels with excellent combinations of high strength and large elongation (compared to common TRIP steels[1]), have been investigated extensively.[1–5] Their excellent mechanical properties mainly originate from the large volume fraction of metastable austenite, typically ranging from 20 to 40 pct, which will transform into martensite during deformation and provide extra work hardening ability to improve the elongation by delaying the necking stage. However, some plastic
JIAWEI MA, HAITING LIU, and YAO SHEN are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] QI LU is with the China Science Lab, General Motors Global Research and Development, Shanghai 201206, China. Contact e-mail: [email protected] YONG ZHONG and LI WANG are with the State Key Lab of Development and Application Technology of Automotive Steels, Baosteel Research Institute, Shanghai 201900, China. Manuscript submitted 18 November 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
instability phenomena, such as Lu¨ders bands and/or Portevin-Le Chaˆtelier (PLC) bands, have been reported in cold-rolled medium Mn steels when deformed at different temperatures.[6–10] These phenomena, unfortunately, may give rise to stretcher-strain marks on steel surfaces and influence sheet forming performance.[11] In order to suppress or even eliminate these undesirable phenomena, it is imperative to investigate the different macroscopic mechanical behaviors involved at various deformation temperatures and the underlying microscopic explanations. When cold-rolled medium Mn steels are deformed at various temperatures during quasi-static uniaxial tensile tests, the competition/coordination between the Lu¨ders band and the PLC band has been frequently observed.[6–8] At certain temperatures, both types of bands may co-exist, while at some other temperatures, one type of bands will take the dominant role. For example, when Wang et al.[7] examined the tensile behaviors of a Fe-7Mn-0.14C-0.23Si (wt. pct) steel at room temperature (RT), 100 °C and 300 °C, respectively, they found a Lu¨ders band at each of these temperatures, yet only PLC bands (corresponding to the remarkable stress serrations on the tensile curve) at RT after the propagation of the Lu¨ders band. Also, when
investigating the tensile behaviors of a Fe-7Mn-0.3C-2Al (wt. pct) steel and a Fe-10Mn-0.1C-2Al (wt. pct) steel at -50 °C, RT and 100 °C, respectively, Yang et al.[6] and Zhang et al.[8] reported similar findings. To better understand these plastic instability phenomena, some microscopic explanations have been proposed. So far it has been generally accepted that Lu¨ders bands are formed due to the lack of mobile dislocations, and the amount of mobile dislocations might be reduced after the intercritical annealing process in cold-rolled medium Mn steels.[4,12,13] To maintain the propagation of Lu¨ders bands, sufficient strain h
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