Mechanism of Oxidation of Austenitic Stainless Steels under Conditions of Hot Rolling in Steckel Mills
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INTRODUCTION
THE oxidation of austenitic stainless steels at high temperatures shows the development of different surface structures corresponding to alternative modes of oxidation. The protective behavior by selective oxidation of chromium leads to the formation of a single layer of Cr2O3 and a chromium depletion zone in the substrate adjacent to the oxide-metal interface. Cr2O3 constitutes the thermodynamically stable scale for austenitic stainless steels; however, its establishment and stability require favorable kinetic conditions. When these conditions are not satisfied, a nonprotective behavior is favored with the development of duplex scales formed by spinel phases FeCr2O4 and Fe3O4 growing externally and internally simultaneously by outward diffusion of Fe and inward diffusion of oxygen.[1] A transition from protective to nonprotective behavior can take place on a single specimen occurring gradually or suddenly by breakaway oxidation. Breakaway scales develop by rapid internal oxidation into localized oxide nodules. Occurrence of breakaway oxidation following a protective period has been frequently reported.[2,3] The mode of oxidation is determined by several factors, most significantly the steel chemical composition and the oxidizing conditions. Particular interest has been raised by steel producers in the oxidation behavior of these S.J. COBO, Research Engineer, is with ArcelorMittal R&D, F-57283 Maizie`res-les-Metz, France. Contact e-mail: sebastian. [email protected] E.J. PALMIERE and W.M. RAINFORTH, Professors, are with the Institute of Microstructural and Mechanical Process Engineering - The University of Sheffield (IMMPETUS), SI 3JD Sheffield, United Kingdom. Manuscript submitted August 21, 2007. Article published online July 16, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
steels under conditions of multipass hot rolling in which high-temperature oxidation is coupled with mechanical working. Although the high-temperature oxidation behavior of austenitic stainless steels is well documented,[4,5] few experimental studies examined the effect of thermomechanical processing, and there are no references to the oxidation under multipass hot rolling conditions. The successive application of heavy deformation on the scale and metal substrate is expected to significantly modify oxide development. The limited ductility of chromium and Fe oxide phases[6] would, in principle, favor fracture over plastic deformation, leading to spallation.[7] The surface state of the alloy is expected to change, altering its chemistry, microstructure, and surface finish, all factors that highly influence oxidation phenomena. In this work, experimental results describe the surface development of an austenitic stainless steel during multipass hot rolling under conditions imposed by the Steckel Mill operation. This finishing mill, shown schematically in Figure 1, is a single stand reversing mill where five to seven passes are employed to achieve final thickness of the steel strip. With processing times up to 700 seconds, the temperature dec
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