Factors Affecting the Development of Oxide Scales on Austenitic Stainless Steels during Hot Rolling in Steckel Mills
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HOT rolling of austenitic stainless steels in Steckel Mills, involving successive stages of heavy mechanical deformation and long residence time inside furnaces in between rolling passes, introduces particular characteristics to the development of tertiary scales. Tertiary oxide scales are those formed on the surface of steel strips during the finishing operation of the hot rolling process. The Steckel Mill, a single stand reversing mill, uses five to seven passes to reach the final thickness of the strip with processing times up to 700 seconds. To compensate the temperature decline, the strip spends the interpass time coiled inside furnaces at 950 C to 1100 C exposed to a combustion gas atmosphere.[1] Thick tertiary scales are formed, having a negative impact on surface quality of the product downstream.[2] An investigation of the influence of the main processing parameters in hot Steckel rolling on the development of those tertiary oxide scales is presented in this work for a particular stainless steel grade. The effect of temperature, applied deformation, and furnace atmosphere composition on the mechanism of high-temperature oxidation is analyzed, discussing alternatives for controlling oxide scale growth during the process. Different surface structures are formed in austenitic stainless steels corresponding to different modes of S.J. COBO, Research Engineer, is with ArcelorMittal R&D, F-57283 Maizie`res-les-Metz, France. Contact e-mail: sebastian. [email protected] W.M. RAINFORTH, Professor, is with the Institute of Microstructural and Mechanical Process Engineering - The University of Sheffield (IMMPETUS), S1 3JD Sheffield, United Kingdom. Manuscript submitted August 21, 2007. Article published online July 16, 2008 2486—VOLUME 39A, OCTOBER 2008
oxidation. Selective oxidation of Cr leads to the formation of a protective single layer of Cr2O3, which constitutes the thermodynamically stable scale in contact with these alloys. The establishment and stability of the Cr2O3 film requires favorable kinetic conditions, i.e., enough time after nucleation to grow and sufficient rate of supply of Cr to the surface by outward diffusion in the alloy. When these conditions are not satisfied, nonprotective duplex scales develop, formed mainly by spinel phases FeCr2O4 and Fe3O4 growing externally and internally by outward diffusion of Fe and inward diffusion of oxygen.[3] A transition from protective to nonprotective behavior may take place occurring gradually or suddenly by breakaway oxidation. This transition has often been reported in austenitic stainless steels with the formation of localized nodules at random positions.[4,5] The dominant mode of oxidation depends on several factors, namely, chemistry, surface state, and oxidizing conditions. Multipass hot rolling imposes a complex set of conditions having an effect that has not been yet examined in detail for these materials.
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EXPERIMENTAL
Laboratory simulation of the main process conditions in hot Steckel rolling was carried out using a single stand reversible mill 2
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