Behaviour of scales during through processing of steels

Steel remains the largest tonnage product world-wide. The majority of steel is used in the wrought condition, i.e. it has been hot worked after casting. During hot working it is inevitable that the steel oxidises, with primary oxide scales being formed during the initial reheat, and secondary scales being formed during the hot rolling following the removal of the primary scale. The oxide scale has to be removed throughout the process because of the detrimental effects that it has, not only the surface quality, but also the significant changes in chemical composition in the steel in the surface region, a direct result of the oxidation process. Despite the central importance of the oxide scale in steel processing, very little is known about the through- process behaviour of the scale. There is a good batch of knowledge on the formation of secondary scales for basic “mild” steel, but surprisingly little information on primary scales and how they change during the hot rolling, or on the effect of alloy composition. The latter is particularly important as shown by the major changes in scale structure and adherence to the substrate that occurs as a result of small increases in silicon in the steel. This project will work with one the world’s leading plant manufacturers, Siemens, to understand the through-process behaviour of oxide scales on a range of steels, such as line-pipe steels used high pressure transfer of oil and gas.
The PhD will firstly extend the experimental skills in characterization. A laboratory facility will be established that allows controlled oxidation experiments to be undertaken, with a gas atmosphere that has a similar composition to that found in industrial processing. Oxidation kinetics will be established through thermal gravimetric analysis (TGA). The mechanical behaviour of the primary scales will be investigated through two approaches. Firstly, laboratory rolling experiments will be used to simulate the industrial process. Secondly, a new mechanical test method has recently been established that allows the mechanical properties of the scale to be measured as a function of temperature. Interestingly, oxide scales behave in the classic brittle manner at lower hot working temperatures, but undergo a brittle to ductile transition at a temperature defined by the oxide composition. It is crucial to understand whether the scale is behaving as a brittle or ductile solid, and how this behaviour changes during the hot rolling process, which will typically start at high (~1250oC) temperature and finish at much lower temperature (e.g. 850oC).
A key variable that needs to be better understood is the effect of alloy composition. Even small changes in key elements can have a strong effect on the adherence of the scale. Silicon is the most notorious of the alloy additions as its presence leads to the formation fayelite, which forms as a low melting point eutectic structure, which dramatically alters the adherence of the scale. It is not only silicon, but also many other elements that have a strong effect. Of particular note is the micro-alloy additions of Nb, V and Ti that also have a strong effect on scale structure and adherence, but little is known as to how additions of <0.1 weight % can have such a dramatic effect. Answering this question will require detailed advanced electron microscopy on a range of steels processed under a range of conditions. The outcomes of the experimental study of oxide structure, mechanical properties and adherence will be fed into sophisticated 3D models that have been developed for mild steel, but urgently need upgrading for commercially important advanced high strength steels.