On the Relation of Microstructure and Texture Evolution in an Austenitic Fe-28Mn-0.28C TWIP Steel During Cold Rolling
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CURRENT trends in the automobile industry focus on improved passenger safety, reduced vehicle weight for lower CO2 emission, and improved formability to permit cost-effective processing and production of complex parts. While light materials have become increasingly important in modern car designs, steel is still the material of choice when it comes to structural applications due to its high strength. Therefore, 60 to 70 pct of the body-in-white weight consists of steel. In order to meet the customer’s expectations, car manufacturers emphasize increased specific strength along with high formability. Apart from the well-established advanced high strength steels (AHSS), such as dual phase (DP) and transformation-induced plasticity (TRIP) steels, Ultra-AHSS, such as twinning-induced plasticity (TWIP) and microband-induced plasticity (MBIP) steels, have moved recently into the focus of worldwide steel research. The main reasons are their superior mechanical properties compared to AHSS with a typical UTS—ef product of >50,000 MPa pct,[1–5] CHRISTIAN HAASE, Doctoral Candidate, LUIS A. BARRALESMORA, Research Fellow and Group Leader of ‘‘Parallel Material Models’’, DMITRI A. MOLODOV, Professor and Group Leader of ‘‘Interface Dynamics’’, and GU¨NTER GOTTSTEIN, Professor and Director, are with the Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University, Aachen 52074, Germany. Contact e-mail: [email protected] SANDIP GHOSH CHOWDHURY, Senior Principal Scientist, is with the CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India Manuscript submitted August 16, 2012. Article published online November 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
where UTS denotes the tensile strength and ef the fracture strain. Research on TWIP steels was initiated by the work of Gra¨ssel and Frommeyer[6–8] based on the process of deformation by twin formation[9] and previous investigations on high manganese steels.[10] Excellent strength, ductility, and work-hardening rate in these materials are attributed to a dynamic Hall–Petch effect due to twin formation, which drastically reduces the effective glide distance of dislocations. These properties depend on the stacking fault energy (SFE), C, of the particular austenitic steel since this parameter determines the active deformation mechanism. In turn, the SFE depends on chemical composition (e.g., Al, Si, Mn, C content) and temperature.[11–15] According to Allain et al.[16] and Sato et al.,[17] SFE values of 20 and 18 mJ/m2 were defined as the upper limit for e-martensite formation, while twin formation was reported to occur for SFE values in the range between 12 and 35 mJ/m2[16] or 14 and 50 mJ/m2.[18] For materials with SFE >50 mJ/m2, formation of twinning is largely suppressed and microbands are formed by planar glide in order to accommodate plastic deformation.[4,19] In contrast to the large number of investigations based on the correlation between microstructure and mechanical behavior of TWIP steels, only little effort has been put into the analysis of
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