Development of New Third-Generation Medium Manganese Advanced High-Strength Steels Elaborating Hot-Rolling and Intercrit
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IN recent years, the established environmental policies to reduce the greenhouse gas emissions have urged the car manufacturers to decrease the vehicle body weight by developing new lightweight and high-strength materials.[1] Having a combination of high strength and ductility, the advanced high-strength steels (AHSSs) were developed to use down-gauged sections with reduced product weight, while taking advantage of the excellent productibility of steels.[2] Generally, their properties are inherited from a complex microstructure of austenite, ferrite, martensite and different precipitates and their presence in the microstructure can be controlled by chemical composition and/or heat treatment MOHAMMAD EMAMI, MOHSEN ASKARI-PAYKANI, and HAMID REZA SHAHVERDI are with the Department of Materials Engineering, Tarbiat Modares University, Jalal e ale Ahmad Hw., P.O. Box 14115-143, Tehran, Iran. Contact e-mail: shahverdi@ modares.ac.ir EHSAN FARABI and HOSSEIN BELADI are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] Manuscript submitted November 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
cycles. These factors have a significant effect on the stacking fault energy (SFE), stability and fraction of austenite, thus altering the dominant deformation mechanism such as transformation-induced plasticity (TRIP), twining-induced plasticity (TWIP) and dislocation glide.[3] In particular, the third generation of AHSS has been under the spotlight for several years because of their remarkably better combination of strength/ductility [i.e., a ultimate tensile strength (UTS) 9 El. pct value > 30 GPa pct] compared with the first-generation counterparts (i.e., a UTS 9 El. pct value of 15 ± 10 GPa pct).[4] Moreover, a cheaper final product is expected as a smaller number of costly elements such as Ni, Cr and Mn are present in their production. However, the UTS 9 El. pct value for the second-generation category is about 50 ± 10 GPa pct, which is higher than that of the third-generation group.[4] Medium manganese steels containing 3 to 12 wt pct Mn are promising candidates as third-generation AHSSs due to the superior combination of strength and ductility.[5–8] In these alloys, the austenite plays a crucial role in controlling the mechanical properties and the enhancement of ductility and work hardening.[8–12] To obtain austenite, an intercritical annealing, namely
the austenite reverted transformation (ART), can be performed.[8,9] During annealing, the segregation of Mn and C to grain boundaries promotes nano-laminate austenite to nucleate at martensite lath boundaries.[9] In fact, the austenite is stabilized at room temperature through the partitioning of carbon and manganese to the austenite grains, while the martensite is being reverted to austenite.[9] Lee et al.[11] showed that the Mn partitioning effectively took place in the ultrafine-grained austenite through 180 seconds intercritical annealing of 6 pct Mn steel at a temperature range of 64
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