Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels
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stainless steels (SS) are frequently selected in applications where good corrosion properties and aesthetic considerations are important. However, in cases where an austenitic SS sheet needs to be subjected to cold forming, such as deep-drawing or stamping, it exhibits a large variation in yield strength (above 50 pct) and undesirable yield strength in regions absent of deformation. The main cause of this variation is related to the cold-working process, during which the deformed regions transform from the soft parent face-centered-cubic (fcc) austenite (g) phase into the hard martensite (a9) phase. In low-carbon austenitic SS, the tetragonality of the martensite is basically absent, and thus the martensite phase can be considered as exhibiting a body-centered-cubic (bcc) structure. A feasible solution to resolve the variation in yield strength associated with the cold-working process has not been achieved, mainly because in the forming processes, deformation and hard martensite formation occur in areas where high strength is not required, while regions where high strength is necessary are not subjected to deformation, leaving soft austenitic regions. In these soft areas the material can be easily scratched, dented, and deformed in service, and thus its corrosion properties and appearance strongly deteriorate. A tempting solution to this problem is to use a fully martensitic SS for the forming operation instead of an ausD.L. JOHANNSEN (now D.L. Rittermann), formerly a Graduate Research Assistant in the Materials Science and Engineering Program at the University of Texas at Austin, Austin, TX 78712, is now with Intel, Oregon. A. KYROLAINEN, Senior Researcher, is with Research Laboratories, Outokumpu Stainless Steel Oy, Tornio, Finland. P.J. FERREIRA, Assistant Professor, is with the Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712. Contact e-mail: [email protected] Manuscript submitted October 29, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
tenitic SS. In this fashion, the undeformed regions maintain the high strength and hardness of the martensite phase, whereas the deformed regions acquire the high strength and hardness associated with the martensite phase. However, due to the poor formability of the martensitic structure, products manufactured in this fashion may fracture. To overcome the aforementioned limitations, a research plan has been designed to develop, in cold-rolled SS sheets, regions (10 to 20 mm) with nano/submicron austenite grain size, exhibiting high strength and high formability properties, while the sheet remains in the martensite phase, outside the narrow regions. Using this concept, a single SS sheet can be produced such that undeformed regions maintain the strength and hardness of the martensite, while deformed regions have enhanced strength and hardness due to the reļ¬ned austenite grain structure produced within the narrow regions (Figure 1). In particular, the idea consists of heavily cold-rolling an AISI 301 austenitic SS sheet (;90 pct
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