Effect of Mn Addition on Microstructural Modification and Cracking Behavior of Ferritic Light-Weight Steels
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TRODUCTION
IN order to increase fuel efficiency and to decrease CO2 emission, the reduction in vehicle’s weight has been focused in research areas of automotive steels.[1–3] A large amount of Mn and Al has been added to automotive steels to achieve the light-weight effect as well as excellent strength and ductility.[4–8] This addition leads to about 10 pct of weight saving when compared with transformation-induced plasticity steels or twinning-induced plasticity (TWIP) steels. Mn promotes the formation of austenite because it acts as an austenite stabilizer.[9–13] Al, a ferrite stabilizer, helps form a duplex structure of ferrite and austenite at high temperatures, and promotes the precipitation of j-carbides (composition: (Fe,Mn)3-Al-C, perovskite structure) during the cooling.[9–12] The amount of j-carbides varies with contents of Mn and Al as well as C. Ferritic Fe-Mn-Al-C steels containing 3 to 12 wt pct of Mn and 5 to 9 wt pct of Al generally have the duplex SEOK SU SOHN, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. BYEONG-JOO LEE and SUNGHAK LEE, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also jointly appointed with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] JAI-HYUN KWAK, Senior Principal Researcher, is with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Kwangyang 545-090, Korea. Manuscript submitted March 13, 2013. Article published online August 14, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure of ferrite and austenite. The austenite can be transformed to j-carbide or martensite because of the low thermal or mechanical stability of austenite.[14–16] Since j-carbides are transformed from the austenite by the eutectoid reaction, they exist in a lamellar shape in the ferrite matrix.[15,16] j-carbides precipitated from the austenite are coherent with the matrix, and ferrite and j-carbides nucleated from the austenite have a Nishiyama–Wasserman orientation relationship.[15,16] According to Suh et al.,[12] the austenite was retained at room temperature when an Fe-5.8Mn-3.1Al-0.12C-0.47Si steel was annealed at temperatures of 993 K (720 C) or higher, and was transformed to a¢-martensite during the deformation. It was also reported from the recent studies on ferritic light-weight steels that the retained austenite was not sufficiently stable due to the relatively low stacking-fault energy because of the lower Mn content than that of TWIP steels.[13,17–19] However, detailed microstructures or deformation mechanisms of the ferritic Fe-Mn-Al-C steels are relatively unknown. In particular, studies on how alloying elements such as Mn affect the size, volume fraction, and distribution of secondary phases such as j-carbide, austenite, and martensite are rarely performed. Furthermore, j-carbides, martensite, or austeni
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