Descaling of steel in hot rolling processes is an application of high-pressure spray nozzles. The impact force and pressure of theses nozzles can be determined using Newton's 2nd and 3r d axiom. This allows calculating the impact pressure approximately by a simple formula. Direct measurement of the impact is possible with a force transducer, scanning the area of direct impingement of the spray. Droplet size of the spray jet seems to be a secondary factor, because pure mechanical considerations lead to a sufficient model of the impact. However, measurement of the area, covered by the spray, shows that the water film of the jet has been disintegrated, when impinging on the surface, but has not been atomized completely. This can be proved by short-time pho tos. Different investigations substantiate the hypothesis, that thermal shock due to high gradients of surface temperature changes under the spray is a dominant physical mechanism of descaling, especially for secondary scale. That is why two additional types of experiments are necessary to characterize the influence of the water jet on the hot surface, which is the measurement of the heat transfer coefficient of the impinging water and the descaling test under laboratory conditions. The study of metallurgical and surface quality parameters of the steel before and after the test allows quantifying the descaling efficiency.
Descaling of steel in hot rolling processes is an application of high-pressure nozzles of the flat jet type. Scale is a layer of oxide build up on the surface of the steel due to high temperatures, up to 1250° C, and the presence of oxygen and other gases. It can be created as primary scale in the atmosphere of the oil or gas fired furnace, or as secondary scale under ambient conditions during the rolling process. Primary scale has a porous structure and a thickness in the range of 1 to 5 mm, whereas secondary scale has a more compact microstructure and a thickness between 60 and 100 µm [2]. The latter is d ifficult to remove and usually a layer of 5 to 10 µm remains.
Descaling of steel in hot rolling processes is an application of high-pressure nozzles of the flat jet type. Scale is a layer of oxide build up on the surface of the steel due to high temperatures, up to 1250° C, and the presence of oxygen and other gases. It can be created as primary scale in the atmosphere of the oil or gas fired furnace, or as secondary scale under ambient conditions during the rolling process. Primary scale has a porous structure and a thickness in the range of 1 to 5 mm, whereas secondary scale has a more compact microstructure and a thickness between 60 and 100 µm [2]. The latter is d ifficult to remove and usually a layer of 5 to 10 µm remains.
The usage of high-pressure nozzles for descaling in production processes is an indispensable measure for the quality of the steel. In the literature the mechanism of descaling is explained in different ways. Mechanical forces due to high impact pressures of a Ğrazor bladeğ like jet are hold responsible for removal of the scale from the surface. On the other hand the water jet causes an intensive cooling of the surface and a high temperature gradient in the area close to the surface is created. This can cause break-up of the oxide layer due to different thermal expansion rates of the scale and the steel, i.e. high shearing forces between scale and steel and in the scale as well.
Flat jet nozzles with rectangular impact distribution and spray angles between 22° and 40° are preferred for the generation of high impact pressures at a sufficient working width of the spray. The spray p ressure ranges from 80 to 500 bars with flow rates between 10 and 200 l/min. The spray distance typically covers 50 to 200 mm.
The physical mechanism of descaling still is subject of controversial discussions [1,2]. Scale is a layer of iron oxides, a material with the characteristic of a ceramic, i.e. a low thermal expansion coefficient and good heat insulation properties. The steel on the other hand has a high thermal expansion coefficient. FIGURE 10 shows results from a REM analysis of a layer of primary scale - typically Fe2O3 on the steel. When a high-pressure water jet hits the surface of a scale/steel compound, not only a sharp peak of mechanical pressure is induced but also a high gradient of the surface temperature change, due to intensive cooling by the water jet. This creates enormous shear fo rces between steel and scale and in the scale itself. It can be observed under laboratory conditions that secondary scale breaks up explosion-like. Estimations of the mechanical stress, induced by the impact of the jet, and the shear stress, created by the thermal shock, show that the latter can be about 500 times higher and, thus, is dominating the process [2]. This substantiates the need for thermal measurements in addition to the pure mechanical determination of the impact pressure.
A valid model of the descaling mechanism can only be obtained completing the mechanical (impact) and thermal (heat transfer) experiments by the study o f the descaling itself under laboratory conditions. This requires pro ducing scale under controlled conditions and then to perform the descaling p rocess. The latter is similar to the heat transfer test: A ho t steel plate with scale is moved under the jet of the high-pressure nozzle and the scale is removed more or less co mpletely. FIGURE 2 (b) shows two steel plates after descaling under different conditions and with different descaling success. The scale has been produced under atmospheric conditions, which equals the so-called secondary scale in the plant. Study of metallurgical and surface quality parameters of the steel allows quantifying the result of the descaling process. In order to get a defined scale quality, heating conditions, surface quality and grade of the steel has to be controlled carefully. Evaluation of the test result, i.e. the quantity of the removed scale and the surface quality of the steel then is done by several methods, as there are: optical or electron microscopy, image analysis, gravimetric methods or magneto-inductive measurements.
Descaling of steel still is a process, which needs intensive research to understand the underlying physical mechanisms. Energy and momentum of the jet of descaling nozzles are dominating spray parameters, which create the impact on the steel surface. Though is can be sho wn, that the liquid is disintegrating, complete atomization does not take place under descaling conditions and the droplet size is not a parameter which characterizes the descaling process. The counteraction of the liquid and the hot surface is of the mechanical as well as the thermo-dynamical type. The thermodynamics of descaling can be investigated with heat transfer measurements. Heat transfer from the hot surface causes high temperature gradients in the scale/steel compound and is a dominant reason for break-off of the scale. The thermal effect of the spray jet covers a wider area on the surface than the pure mechanical impact.
Correlation between mechanical and thermal parameters can only be found by combinatio n of results from mechanical and thermal experiments. The effectiveness of descaling can be studied under labo ratory co nditions with controlled descaling experiments. This work will be continued in order to get a comp lete model of descaling.