Effect of Al Additions on Scale Structure and Oxidation Kinetics of 430-Ferritic Stainless Steel Reheated in a Combustion Atmosphere

Al is sometimes added to ferritic 430 type stainless steel to improve the surface appearance of cold rolled bright annealed products. High-surface quality is required for such applications. However, during continuous casting, slab reheating, and hot rolling, complex oxides can penetrate deep into the matrix and form highly adherent subsurface scale layers that resist removal using high pressure water descaling. Therefore, an understanding of the oxidation phenomena during reheating in the combustion atmosphere of the slab reheating process is critically important for control and elimination of potential scale related surface defects on the product. In this article, the kinetic aspects of oxidation and scale formation on a standard and an Al alloyed 430 ferritic stainless steel were investigated using several experimental and modeling methods. Experiments were performed in a TGA apparatus that replicates the combustion gas atmosphere and temperature in an industrial slab reheat furnace. Oxidized samples were characterized using optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy to document the specific changes in the scale morphology and microstructure in the base and Al alloyed 430 ferritic stainless steels. Focused ion beam (FIB) sectioning of the scale layer, followed by high resolution transmission electron microscopy (TEM), revealed details of the phase, composition and structure of the oxides formed in the subsurface region of the ferritic matrix. Thermodynamic simulations were used to predict phases in the multilayer scale structure. Differences in oxidation kinetics, phase, composition, topology, and microstructure of the multi-layered scale formed during re-heating of the investigated alloys are discussed.