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0903-Z05-40.1

SUBMICRON/NANO GRAINED STAINLESS STEELS WITH SUPERIOR MECHANICAL PROPERTIES S. Rajasekhara1, M. C. Somani2, M. Koljonen2, L. P. Karjalainen2, A. Kyröläinen3, P. J. Ferreira1 1

Materials Science and Engineering, The University of Texas at Austin, Austin, Texas – 78712, USA 2

Department of Mechanical Engineering, University of Oulu, 90014 Oulu, Finland 3

Outokumpu Stainless Oy, 95400 Tornio, Finland

Abstract Metastable austenitic stainless steels may transform to martensite when subjected to cold rolling. Upon subsequent annealing the martensite reverts back to ultra-fine grained austenite. Based on this concept, nano/submicron austenitic grains have been produced in a 63% cold rolled commercial AISI 301LN subjected to annealing treatments at 600°C, 800°C and 1000°C for 1, 10 and 100 seconds. Transmission Electron Microscopy (TEM) observations show the formation of equiaxed austenitic grains as small as ~ 200nm in samples annealed at 800°C, and a dramatic increase in grain size as the annealing temperature and duration is increased. Additional tensile tests indicate that samples annealed at 800°C for 1 second exhibit a yield strength of ~ 740 MPa and an total elongation of ~ 45%. This combination of strength and ductility is excellent exceeding those of conventionally annealed steels (σy=350 MPa; Ductility ~ 40%) or cold-rolled steels (σy=650 MPa; Ductility ~ 30%). Finally, a correlation between the observed grain sizes and mechanical properties, in particular the yield strength, is obtained. Preliminary analysis indicates that the Hall-Petch equation can satisfactorily relate the observed yield strength with corresponding grain sizes. 1. Introduction Austenitic stainless steels (SS) are frequently selected in applications where good corrosion properties and aesthetic considerations are important. Examples of these applications include pipelines for the chemical industry, components for space applications, automobile parts, kitchenware and architectural facades. For these applications, austenitic SS sheets need to be subjected to cold forming processes, such as deep-drawing and stamping. However, when deformed under those conditions, austenitic SS exhibit a large variation in yield strength (above 50%), and undesirable yield strength in regions devoid of deformation [1]. In these areas the material can be easily scratched, dented and/or deformed in service and thus its corrosion properties and appearance strongly deteriorate. A feasible solution to resolve this long time problem has not been achieved, mainly because deformation during forming processes result in formation of hard martensite in areas where high strength is not required; and regions where high strength is necessary are not subjected to deformation, leaving soft austenitic regions

0903-Z05-40.2

In this context, there has been a growing interest in developing a process involving heavy cold reduction of metastable austenitic stainless steels and subsequent annealing to produce ultrafine grained austenite [2-4]. However, this work has