Oxide scale on stainless steels and its effect on sticking during hot-rolling

Abstract

The failure of oxide scales on ferritic stainless steels (STSs) has been known to cause an unexpected and severe sticking problem during the hot-rolling process. The oxide scale fragments formed during hot-rolling are stuck on the surfaces of the working roll, deteriorating the surfaces of both the roller and the rolled materials. In this study, we investigate the effects of the compositions and microstructures of thermally grown oxide scales on the degree of sticking during hot-rolling in three different kinds of commercial ferritic STSs and one austenitic STS. The STS samples are oxidized at around 1280 °C for 2 h, and the oxide scales formed on the STSs are identified by X-ray diffraction (XRD). Additionally, the cross-sectional microstructure and chemical compositions of the oxide scales on the STSs are examined by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM), respectively. The oxide scales adjacent to the interfaces with the bulk STS are analyzed by high-resolution transmission electron microscopy (HR-TEM). The Gleeble test, which is a high-temperature (1250 °C) tensile test, is conducted to analyze the effects of the stress induced in the oxide scales during hot-rolling on the fracture of the scales. According to the microstructural analysis and mechanical test results, a brittle Cr2O3 layer, where cracks are easily formed and propagate through the scale thickness under the stress induced by hot-rolling, is formed along the interfaces of STSs with high Cr contents. This effect increases the probability of sticking failure in high-Cr STSs. In contrast, scales with a composite-like structure of Fe- and Cr-oxides are formed on the surface of STSs containing low and medium levels of Cr. According to the Gleeble test results, the composite-like oxide scales show higher mechanical resistance to tensile stress than the Cr2O3 layers in the high-Cr STSs. In addition, our microstructural analysis results reveal that FexNiy intermetallic compounds are observed as a form of a composite-like structure inside the oxide scales in the austenitic STS and that this structure also delays crack propagation through the oxide scales and reduces the degree of sticking.