Study On Composition Of Oxide Scale On 430 Stainless Steel Surface And Interface Bonding Strength

After hot rolling, only through full recrystallizing by annealing process in the batch, can the stainless steel be put into use. However, during hot rolling, cooling and annealing process, a complex oxide structure layer will form on the surface of stainless steel due to oxidation. Oxide layer not only decreases the appearance quality of stainless steel parts but also influences following processing, even properties of products, so pickling is needed after annealing process to remove the surface oxide layer. Composition of the oxide scale formed on the stainless steel surface and the bonding strength between the layer and substrate directly determines the difficulty of pickling.In this paper, 430 ferritic stainless steel with annealing imprint on surface through annealing treatment in the batch is studied. Scanning electron microscope, X-ray photoelectron spectroscopy, glow discharge spectrum analyzer were used to analyze the morphology, microstructure of the oxide layer as well as its formation mechanism. Because microstructure of the stainless steel/oxidizing layer interface directly relates to the effect of pickling, based on the experimental analysis, the stainless steel/oxidizing layer interface was constructed using the first principles density functional theory pseudopotential plane wave method. The bonding strength of pure iron or 430 stainless steel matrix and oxide interface as well as the influence of alloy elements were studied from the atomic level in order to provide a guiding to experiment study in theory.Analysis results of annealing imprint on the surface of 430 stainless steel show that oxidation layers exist in the samples of annealing imprint, and black strip of different thickness is found in the stainless steel matrix and oxide interface. It's turned out to be compounds rich in silicon. There are four layers from the outer surface to the matrix in the oxide layers of annealing imprint, Fe-O layer (mainly are high iron oxides, such as Fe3O4,Fe2O3), Fe-Cr-O layer (mainly are low iron oxides and Cr oxides), Cr-O layer (mainly compact layer of Cr2O3) and Si-O layer (mainly are SiO2 and FeO). Calculation of 430 stainless steel matrix/oxide interface indicates that bonding strength between Fe-Cr/FeO interface is close to that between Fe/FeO interface, that is to say, Cr has little effect on the combined ability of stainless steel matrix/oxide interface, Fe-Ni/FeO interface has a relatively lower bonding strength, it can be removed relatively easily by pickling. The segregation of Si on the stainless steel matrix/oxide interface improves bonding strength obviously, thus increases the difficulty of pickling.