Heat Transfer and Fluid Flow Laboratory studies hydraulic descaling of steel experimentally. Experimental work in the lab is done to optimize descaling process in steel plants and the results are used for development of theory of descaling as well. Hydraulic descaling should be incorporated into the production processes as continuous casting or hot rolling and must be designed to achieve product quality requirements with minimal cost.
Oxides are created on steel surfaces during continuous casting or hot rolling. The oxide scales are generally unwanted because of dissimilar properties to the underlying material, which is often designed to have special characteristics. Scales also act as a thermal barrier for cooling during rolling and have a negative effect on surface quality. Hydraulic descaling is used to remove oxide scales while the surface is impinged by high pressure water that disrupts scales by impact pressure and by thermal shock as well.
The laboratory work is focused on descaling in relation to the surface quality and also in relation to heat transfer. A test bench for measuring water impact forces is available. The scales on tested surfaces are created in the laboratory with specific parameters provided in advance or during experimentation according to process specifications.
Heat transfer tests are carried out for test plates without oxide scales. The final heat transfer results (heat transfer coefficient distribution) are compiled using correlations based on additional heat tests of surfaces covered by scales.
The key heat transfer measurements are done on a linear test bench. The test plate is connected to a driving mechanism and to temperature sensors. The sensors record temperatures while running through the descaling section. The experiments are carried out for variable parameters that generally include water pressure, inclination angle, type of nozzles, distance of the nozzle from surface, and sizes of the overlapping areas. The heat transfer coefficient distribution is obtained by using the IHCP (inverse heat conduction problem) as a function of the spray parameters
- An electric furnace heats the test plate to the initial experimental temperature.
- The plunger water pump is switched on and the water pressure is adjusted.
- A driving mechanism moves the test plate through the spray. After recovering the temperature field in the plate, the movement of the plate under the spray is repeated.
- The sensors measure the temperature at a depth of less than 1 mm from the cooled surface.
- The positions of the test plate and the sensors (in the direction of movement) are recorded together with the temperature values.
The goal of quality tests is to evaluate the quality of descaling for given descaling nozzles, spray parameters, speed and chemical content of the plate.
- An electric furnace heats the test plate to the initial experimental temperature. When required, the examined surface of the test plate is covered by an additional plate to avoid scale formation during furnace heating.
- A plunger water pump is switched on and the water pressure is adjusted.
- The test plate is exposed to air in the furnace for required time by removing the additional plate.
- The test plate is placed on the linear test bench and the driving mechanism moves the test plate through the spray.
- The test plate cools in a protective atmosphere to avoid additional scale formation.
The final surface quality after hydraulic descaling is studied. The thickness of scale is measured by an ultrasonic thickness gauge or by using an electron microscope. The surface roughness is determined by surface analysis. Image analysis is used for evaluating the percentage of remaining scale.
When it is necessary to obtain more types of descaled surfaces at one test plate, the entire experimental procedure is done twice, where the test plate is rotated 90˚ around its center before the second descaling. This way, four different types of surfaces are achieved.
Impact pressure is one of the basic nozzle spray parameters, and is especially important for the characterization of descaling nozzles. To obtain the impact pressure distribution on the sprayed surface, a test bench was developed in the Heat Transfer and Fluid Flow Laboratory.