Investigation of the Delayed Fracture Phenomenon in Deep-Drawn Austenitic Manganese-Based Twinning-Induced Plasticity St
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INTRODUCTION
THE use of Advanced High Strength Steels in the automotive industry has increased significantly in the last decade in order to improve safety and fuel economy. One of the latest developments is Mn-based TwinningInduced Plasticity (TWIP) steels that combine a very high strength with a high uniform elongation.[1] These superior mechanical properties are a result of deformation mechanisms combining slip of dislocations with (micro)twinning (TWIP effect) or strain-induced martensite transformation (TRIP effect).[2] The formation of microtwins and/or strain-induced martensite leads to a strongly increased and sustained strain hardening, resulting in high uniform elongation and high strength.[3] On the other hand, in addition to usual application issues like formability and weldability, a problem encountered with austenitic Mn-based TWIP steels is delayed fracture. This is such a phenomenon that even after successful forming, fracture may still occur after some time. The observed delay time ranges from seconds to weeks in a stainless steel.[4] The assessment of delayed fracture in thin sheet material is R.T. VAN TOL, Principal Researcher and Ph.D. Student, formerly with Tata Steel Research Development & Technology, Wenckebachstraat 1, 1970 CA IJmuiden, The Netherlands, and with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands, and also with the Materials Innovation Institute, is now with Shell Global Solutions International B.V., Grasweg 31, 1031 HW Amsterdam, The Netherlands. Contact e-mail: [email protected] L. ZHAO, Research Fellow, is with the Department of Materials Science and Engineering, Delft University of Technology, and also with the Materials Innovation Institute. L. BRACKE, Research Engineer, formerly with Tata Steel Research Development & Technology, is now with OCAS N.V., Technologiepark 935, 9052 Zwijnaarde, Belgium. P. KO¨MMELT, Principal Researcher, is with Tata Research Development & Technology. J. SIETSMA, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted October 16, 2012. Article published online June 5, 2013 4654—VOLUME 44A, OCTOBER 2013
currently not defined in a universal testing standard. Testing usually comprises deep drawing with a specific deep-drawing ratio and monitoring the appearance of cracks in a specified time frame. Most research in the field of delayed fracture has been performed on austenitic stainless steels, like AISI 301 and 304 series.[4,5] The phenomenon observed in austenitic steels is mainly related to three potential causes: (1) the limited stability of austenite, (2) the residual stress/ strain state, and (3) the environmental condition (related to the presence of hydrogen). The first cause, the austenite stability, is indicated in the literature by the stacking fault energy (SFE). The two other potential causes relate delayed fracture to stress corrosion cracking and hydrogen embrittlement.[6] The combination o
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