Oxide Scales Behaviour During Descaling and Hot Rolling

The impact of the descaling process on the surface finish of steel products in hot rolling has been established over the years. However, most of the studies completed on the effect of the secondary scales and the influence of the primary scales has been unnoticed. The aim of this research was to study the behaviour of oxide scales during the descaling and hot rolling of two steel grades, particularly the influence of primary scales. In this research, the oxide scale behaviour of two key steel compositions was studied. High carbon rail steel and high silicon electrical steel (DSP) provided by Tata Steel Europe as-cast condition. The project consisted of two main sections: 1. Assessment of the oxidation kinetics and characterisation of the oxide scales microstructure developed under different conditions. 2. Evaluation of the mechanical properties of the steel grades at elevated temperatures.
Isothermal oxidation experiments were conducted to evaluate different oxidation parameters, including the atmosphere, temperature and alloy compositions. All isothermal oxidation experiments were conducted under controlled environment using dry or humid atmospheres. The characteristic zones and microstructure of the oxide layer were studied by different techniques, including X-ray diffraction (XRD) and backscattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDX) to assist the identification of the oxide phases. It was found that the compositions of the oxide scales were influenced by the oxidation parameters, including temperature, time and oxidant atmosphere composition.
Furthermore, it was confirmed that the parent material chemistry not only significantly influenced the oxidation kinetics but also it affected the microstructure and composition of the oxide scales. In long isothermal oxidation of both grade Wüstite and was the predominant phase and the amount of Hematite was small. In addition, a considerable amount of Magnetite was developed under the same oxidation condition. The spinel phase of Fayalite was detected at the oxide-metal interface during oxidation of both alloys; however, the amount of this phase was significantly higher in DSP steel. In the oxidation of two alloys under dry condition, it was observed that the oxide outer scales completely detached from the parent materials. The amount of Wüstite phase in DSP samples was negligible when oxidised under a dry atmosphere.
A high temperature compression test was designed to evaluate the failure mode of the oxide scales under compressive stresses at elevated temperatures. All thermomechanical experiments were carried under a dry air atmosphere using the Arbitrary Strain Path (ASP) rig at The University of Sheffield. The results confirmed that FeO had higher plasticity than Fe3O4 and Fe2O3. However, it was observed that Wüstite behaved in a brittle manner at 850˚C.
A new tensile test designed based on previous works of Krzyzanowski and Beynon. The test allowed direct measurement of the ultimate tensile strength of the oxide scale. The oxide scale failed in two different manners, as reported previously by the above researchers. The first mode was in transverse direction due to cracks initiation and propagation perpendicular to the oxide-metal interface, and the second failure mode was due to the slide of the oxide scale along the oxide-metal interface. However, it was found the test deformation temperature is not governing the failure mode exclusively. The alloying elements of the parent metal and the temperature of the oxidation had great importance in the failure mode of the oxide under tensile load.
These observations confirmed that the scale failure in an industrial steel rolling mill is a complex failure which potentially includes both brittle and ductile failure modes.