The Influence of Cr and N Additions on the Mechanical Properties of FeMnC Steels
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INTRODUCTION
AS a part of a global interest in cost-effective steel with superior strength-ductility properties, high Mn austenitic steel grades are being developed, which, in contrast to traditional austenitic steels based on the Fe-Cr-Ni alloy system, contain little or no Ni. In these alternative ferrous alloy systems, the main austenite stabilizing elements are Mn, C, and N. Most of the research in this field has been done on stainless steel grades.[1–5] For some applications, e.g., structural parts in the automotive industry, the corrosion resistance is less important than the deformation behavior and Cr-free austenitic steels are actively being studied.[6,7,8] The automotive applications are mainly related to passive passenger safety systems requiring high energy absorption in the event of a crash. This requires steels with a high strength, a high ductility, and a sustained degree of strain hardening during deformation. A gradual increase of the flow stress is therefore necessary, especially at low strains in the range of what is applied during the forming operations of a structural part. In the Fe-Mn alloy system, the microstructure may contain ferrite (bcc), austenite (fcc), e-martensite (hcp), and a¢-martensite (bcc or bct)[9,10,11] when the Mn content is lower than 30 mass pct. In binary Fe-Mn alloys and in most high Mn (~20 mass pct Mn) ferrous alloys, the formation of e-martensite plays an essential role during plastic deformation. Because the chemical driving force DGc-e for the L. BRACKE, Researcher, formerly with the Department of Metallurgy and Materials Science, Ghent University, is with the Corus Research Department, NL-1970 CA, Ijmuiden, The Netherlands. J. PENNING, Professor, is with the Laboratory for Iron and Steelmaking, Department of Metallurgy and Materials Science, Ghent University, B-9052, Zwijnaarde, Belgium. Contact e-mail: [email protected]. N. AKDUT is with the OCAS, Arcelor Industry Research Center, B-9060 , Zelzate, Belgium. Manuscript submitted June 7, 2006. Article published online April 13, 2007.
520—VOLUME 38A, MARCH 2007
transformation of austenite into e-martensite is proportional to the intrinsic stacking fault energy (ISFE) of the alloy,[12] it is expected that the ISFE will be the most important parameter controlling the transformation behavior of unstable austenitic steels. Several studies relate the occurrence of strain-induced austenite to emartensite to a low ISFE in austenitic steels. According to Allain et al.,[13] increasing the ISFE leads to a transition from e-martensite formation to microtwinning during plastic deformation. Their results indicate that strain-induced e-martensite can be formed when the ISFE is lower than 18 mJm)2 and that mechanical microtwinning can occur for ISFE values between 12 and 35 mJm)2, which confirmed earlier results by Schumann.[14] Sato et al.[15] report that alloys with an ISFE lower than approximately 20 mJm)2 will transform into e-martensite during plastic deformation, while alloys with higher ISFE will form deformation microtwins.
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