Microstructure Evolution and Age-Hardening Behavior of Microalloyed Austenitic Fe-30Mn-9Al-0.9C Light-Weight Steels
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TRODUCTION
IN recent years, the automotive industry has focused its efforts on reducing energy consumption and CO2 emissions through weight reduction of vehicles.[1] In the literature, Fe-Mn-Al-C steels have been investigated as candidate materials for light-weight automotive steel over the past several decades.[1–15] Steels of these grades still have many problems that must be addressed before they can be applied in the automotive industry. For example, the high carbon content has made it difficult to weld automotive structures and its high production cost is also an obstacle to its broader application. Nevertheless, many studies have attempted to develop high-performance light-weight Fe-Mn-Al-C steel due to its
JOONOH MOON, SEONG-JUN PARK, TAE-HO LEE, and CHANG-HOON LEE are with the Advanced Metallic Materials Division, Ferrous Alloy Department, Korea Institute of Materials Science, 797 Changwondae-ro, Seongsan-gu, Changwon, Gyeongnam, 642-831, Republic of Korea. Contact e-mail: [email protected] CHANGHEE LEE is with the Division of Materials Science and Engineering, Hanyang University, Wangsimni-ro, Seongdong-gu, Seoul, 133-791, Republic of Korea. HEUNG NAM HAN is with the Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Republic of Korea. Manuscript submitted September 5, 2015.
METALLURGICAL AND MATERIALS TRANSACTIONS A
excellent mechanical properties and low density.[1–16] A previous study reported that the addition of 1 wt pct Al has the effect of reducing the density by approximately 0.1 g/cm3.[1] In addition, high-performance light-weight FeMn-Al-C steels may be considered as structural materials for military vehicles because mobility is very important for such vehicles. The use of light-weight steel can improve the mobility of vehicles through weight reductions. Fe-Mn-Al-C steels can be categorized as austenitic, duplex, or ferritic base steels, according to their matrix phase constituents.[1] Austenitic Fe-Mn-Al-C steels cover Fe-(15-30)Mn-(8-12)Al-(0.5-1.2)C (in wt pct)[1] and show a combination of high strength and good total elongation.[2–8] Lin et al.[2] investigated the effect of C on the microstructures and tensile properties of Fe30Mn-8.5Al-C steels and reported that the yield strength was clearly improved by an increase in the C content, with nearly equivalent ductility. From a TEM analysis, they confirmed that this was due to the precipitation of the fine j-carbide of (Fe,Mn)3AlC.[2,8] Yoo and Park[3] studied the correlation between the room-temperature tensile behavior and the deformed microstructure of Fe-27.8Mn-9.1Al-0.79C steel and showed that high ductility with continuous strain hardening occurred due to microband-induced plasticity (MBIP). GutierrezUrrutia and Raabe[4] introduced a type of high-performance
austenitic Fe-30.5Mn-8.0Al-1.2C steel, known as Simplex steel. This steel showed an outstanding strain-hardening capacity resulting from dislocation substructure refinement and the subsequent activation of deformation twinning
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