Investigation of the formation, constitution and properties of scale formed during the finishing, rolling, cooling and coiling of thin hot strips

Understanding the oxidation behaviour of steels is often a key for controlling the surface quality especially of steel flat products. Within that scope the project aims to determine the relationship between processing conditions, scale characteristics and scale properties in terms of scale formation and scale adherence of hot strips. The investigations have been considered scale which is formed during finishing rolling, cooling and coiling. Furthermore the effect of scale constitution and properties on pickling and direct hot dip galvanizing of unpickled hot strips were evaluated. The first part of the project was essentially about establishing suitable test methods to evaluate scale adherence / coherence, and about making an inventory of scale properties in relation to the hot rolling. That means a series of shop trials changing finishing and coiling temperature as well as chemical composition of steels was done to understand the influences of scale formation and adherence. Simultaneously a series of laboratory trials including laboratory hot rolling, high temperature X-ray diffraction (HTXRD) in situ, annealing and thermobalance experiments were performed to get more basic knowledge in scale formation, wustite decomposition, oxide reduction and hot rolling behaviour as a precondition for industrial test variants. The second part includes consequences of scale properties in downstream processes in which is exploited the knowledge gained in the first part. In that scope hot dip galvanizing of unpickled hot strips, pickling, forming, welding and cutting experiments were performed. Evaluating scale adherence well known tests, like tensile test, cupping test and bending test were modified with a sticked foil (scotch tape) to get the amount of detached scale after steel deformation. Simple quantification was done by defining levels of scale adherence based on visible differences in the amount of detached scale. The application and quantification of scale adherence test methods were improved by image analysis of area fraction of detached scale sticked at the foil using optical microscopy. Accurate quantification of scale adherence can be also reached by measuring grey values of the sticked foils in back reflection or evaluating the light weakening of scale particles sticked at the foil in transmission mode. Tensile - and cupping - scotch tape - test were performed as standard forming test variants. Bending test was used for thicker strips (> 8 mm). The amount of scale detachment depends strongly on local strain which was calculated by FEM-simulations. The tensile overlap and peeling test is routinely used in the field of coating technology. The shear strength (force required for failure at the overlap area) or peeling force are measured. So the result of a successful tensile overlap test is a shear strength value combined with an assessment of the failure mode which is predominantly subdivided in adhesively or cohesively. The peeling test was found to be unsuitable, since the tested oxide layers were better capable of withstanding the peeling forces than the adhesive layers. The tensile overlap test is reasonably suitable for providing information on scale adhesion and cohesion, provided that the samples are sufficiently flat and a strong type of adhesive is used. So it was concluded that the tensile overlap test is only suitable for non-elongated hot rolled samples. Various scale adherence test methods do not give exactly the same ranking, since different aspects of scale adherence/coherence strength were tested; scale particles detaching under heavy deformation (e.g. in the cupping test) is clearly something different than scale particles detaching under shear (e.g. in the tensile overlap test). Which test method is most suitable to evaluate scale integrity will depend on the final application of the hot rolled strip by the end user. For steel strips that should undergo substantial deformation (forming), the Erichsen cupping t