Effect of Tempering on the Microstructure and Tensile Properties of a Martensitic Medium-Mn Lightweight Steel
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I.
INTRODUCTION
IMPROVEMENTS of both fuel efficiency and passenger safety of vehicles can be achieved by employing high-strength lightweight steels with ultimate tensile strength (UTS) above 1.0 GPa. To obtain ultrahigh strength alongside moderate ductility, various strengthening mechanisms, such as grain refinement, dislocation hardening, TRansformation-Induced Plasticity (TRIP) and TWinning-Induced Plasticity (TWIP), and their coupling, have been studied.[1–6] To reduce the weight of
SUKJIN LEE is with the Department of Materials Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea and also with the ASPPRC, Colorado School of Mine, Golden, CO 80401. SEOK-HYEON KANG, JAE-HOON NAM, and YOUNG-KOOK LEE are with the Department of Materials Science and Engineering, Yonsei University. Contact e-mail: [email protected] SANG-MIN LEE is with the Department of Materials Science and Engineering, Yonsei University and also with the Sheet Productions Design Team, R&D Center, Hyundai Steel, Dangjin, 31719, Republic of Korea. JAEBOK SEOL is with National Institute for Nanomaterials Technology, POSTECH, Pohang, 37673, Republic of Korea. Manuscript submitted November 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steel, a large amount of Al (> 5 wt pct) is added as a lightweight alloying element.[7–38] The high-strength lightweight steels reported until now can be categorized into four groups based on their matrix microstructure: austenite, ferrite, ferrite + austenite (dual-phase), and ferrite + austenite + martensite (triple-phase). As shown in Table I, high-Mn austenitic lightweight steels are generally stronger (UTS > 1.0 GPa) and lighter than medium-Mn ferritic, dual-phase, and triple-phase lightweight steels. For example, Kim et al.[20] reported that high UTS (~ 1.55 GPa) and high specific UTS (SUTS = 227 MPa (gcm-3)-1) could be obtained in an Fe-16.1Mn-9.6Al-0.86C-4.9Ni (wt pct) austenitic lightweight steel by controlling the distribution and morphology of Fe-Al type brittle intermetallic compounds (B2) in the austenite matrix. However, because austenitic lightweight steels have a high amount of alloying elements (Mn and Ni) and high material cost, and also undergo difficult fabrication processes,[7–20] medium-Mn lightweight steels with Mn concentration less than 10 wt pct are more attractive.[21–38] When Mn concentration is less than 10 wt pct, matrix microstructure changes from austenite to ferrite, dual-phase or triple-phase. Ferritic medium-Mn lightweight steels exhibit yield strength
Table I.
Chemical Compositions and Tensile Properties of Various Lightweight Steels Categorized by Their Matrix Microstructure Chemical Composition (Weight Percent)
Matrix Microstructure Austenite (+ j-Carbide) Ferrite (+ j-Carbide) Ferrite + Austenite (+ j-Carbide) Ferrite + Austenite + Martensite (+ j-Carbide)
Mn
Al
C
Ni
YS (MPa)
UTS (MPa)
El (Pct)
References
12.0–30.0 2.0–6.0 3.0–12.0 9.0–10.0
5.0–12.0 4.0–8.0 5.0–10.0 5.0–6.5
0.6–2.00 0.1–0.7 0.2–1.2 0.15–0.25
5.0 — — —
355–1355 565–762 561–6
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