The cooling of work rolls is an important process in the hot rolling technology and cannot be taken as a marginal issue. The Heat Transfer and Fluid Flow Laboratory designs cooling of rolls to reach their long working life while rolled product is still well heat treated. There are a few aspects of concern that are reflected when roll cooling is designed by the lab. Especially, wearing of rolls where high temperature decreases the durability of the surface layer, and thermal deformation of rolls.
Proper cooling design is critical for the shape and tolerance of flat products. Finding the optimum coolant pressure and flow rate is a difficult task. In regards to water quantity, the measurement shows that the phrase “more is better” is not always valid here. In other word – an increase in the amount of water can cause decrease in cooling intensity.
The Heat Transfer and Fluid Flow Laboratory developed a specialized rotating test bench with experimental roll diameter of Ø 650 mm equipped by pump station with total power of 150 kW to test cooling system configurations identical with the configurations used in the industry. The second available test bench with experimental roll diameter of Ø 350 mm is a part of closed coolant loop and is used mainly for testing cooling characteristics of various coolants. The cooling system is tested in many possible configurations (given by nozzle types, geometry, coolant pressure and temperature) that are subsequently evaluated and compared to design new cooling technology with higher efficiency. The improvement of the efficiency of new desing of cooling technology is experimentally proved after.
For roll cooling investigation the surface temperature or heat transfer coefficient at the roll needs to be determined. When roll works in the industry, to get its surface temperatures or temperatures under the surface is usually impossible. The rotational aspect of the cooled object makes the problem even more complicated. The only reliable method how to describe heat transfer under the above complex conditions is to perform experiment either during rolling campaign or in the lab. However, the experiments carried out in the lab by using eligible experimental methods are much cheaper and less time consuming. Experimental roll used in our lab is equipped by a test segment with several thermal sensors connected to the data logger. The temperatures measured by the thermal sensors are used in the inverse heat conduction problem (IHCP) to calculate the heat flux on the roll surface and subsequently the heat transfer coefficient and surface temperature.
- An external electric heater heats the test segment of roll equipped by thermal sensors
- When uniform starting temperature is reached, the heater is removed from the roll, and the roll is covered by deflector.
- The speed of the roll and coolant pressure is set up while the roll is still covered by the deflector.
- When all experimental conditions are set up, the deflector is opened and cooling starts.
- The temperatures are recorded by thermal sensors during cooling and saved to a data logger
The data obtained by the thermocouples during the roll cooling experiment gives information about temperature history at positions under the roll surface. Each data point is also connected to information about the angle of cylinder rotation and thus the data collected in “time” order can be converted into position order. The measured temperatures are used for calculation of heat transfer coefficient and surface temperature history.
The Heat Transfer and Fluid Flow Laboratory developed software to simulate a rolling campaign. The software, called “SimRoll”, enables the computation of spray patterns, temperatures on the surface and inside the working roll and thermal crown during plate rolling. “SimCool” enables the computation of temperatures on the surface and inside the slab, spray, and HTC patterns. The tools are not available commercially and are for internal purposes only. The simulations are used to design new cooling systems with higher efficiency.
Thermal characteristics in the roll gap are affected by extensive set of industrial conditions that cannot be easily moved to the lab. In this case to carry out the experiments in the real rolling mill is the best way how to get proper results. The experimental method in the roll mill is similar to the method employed in the lab. The subsurface temperatures are measured by several temperature sensors built in the roll and used for computation of heat transfer and thermal loads in the roll gap.
Numerical modelling of the thermal behaviour of rolls during rolling can be done using experimental results. The heat transfer coefficient distribution on the roll surface can be used as a thermal boundary condition for the numerical model that can simulate several hours of roll work. The model provides information about temperature field evolution and about changes in roll shape due to thermal expansion.
Cyclic thermal boundary conditions produce cyclic stresses mainly in the surface layer of the roll. This cyclic load can initialize thermal cracking and, together with oxidation and abrasion, wearing of the roll surface. From the point of view of crack growth, the dominant direction is circumferential. The surface layer of the roll goes into a compression stress in the rolling gap, then the cycle continues into tension stress in the area of the cooling zone. The cyclic character of load is shown on circumferential stress-strain charts. The internal area of these curves is proportional to the deformation energy.
To obtain the impact pressure distribution on the sprayed surface, a test bench was developed in the Heat Transfer and Fluid Flow Laboratory.