Analysis of roll gap heat transfers in hot steel strip rolling through roll temperature sensors and heat transfer models. Key Engineering Materials, 2012Materials, , 504-506, pp.1043Materials, -1048 Abstract. This paper presents an analysis of roll bite heat transfers during pilot hot steel strip rolling. Two types of temperature sensors (drilled and slot sensors) implemented near roll surface are used with heat transfer models to identify interfacial heat flux, roll surface temperature and Heat Transfer Coefficient HTC roll-bite in the roll bite. It is shown that: -the slot type sensor is more efficient than the drilled type sensor to capture correctly fast roll temperature changes and heat fluxes in the bite during hot rolling but its life's duration is shorter. -average HTC roll-bite is within the range 15-26 kW/m 2 /K: the higher the strip reduction (e.g. contact pressure) is, the higher the HTC roll-bite is.-scale thickness at strip surface tends to decrease heat transfers in the bite from strip to roll. -HTC roll-bite is not uniform along the roll-strip contact but seems proportional to contact pressure. -this non uniform HTC roll-bite along the contact could contribute to decrease thermal shock (so roll thermal fatigue) when the work roll enters the roll bite, in comparison to a uniform HTC roll-bite . -Heat transfer in the roll bite is mainly controlled by heat conduction due to the huge roll-strip temperature difference, while heat dissipated by friction at roll-strip interface seems negligible on these heat transfers.

The general term “spray cooling” is for the research presented here limited to the spray cooling of hot surfaces with film boiling, starting at temperatures of about 1200 °C and finishing at the Leidenfrost point where cooling intensity changes rapidly. This is typical area of secondary cooling in continuous casting. Herein, a correlation for Heat Transfer Coefficient (HTC). The most frequently used parameter of water impingement density is in the presented correlation used together with impact pressure to get good results. This study uses both water and mist nozzles. It is shown and experimentally verified why equations based only on the water impingement density cannot provide sufficiently precise predictions of HTC.

Abstract.A temperature sensor with a thermocouple placed at ~0.5 mm from roll surface is used in hot rolling conditions to evaluate by inverse calculation heat transfers in the roll bite. Simulation analysis under industrial hot rolling conditions with short contact lengths (e.g. short contact times) and high rolling speeds (7 m./sec.) show that the temperature sensor + inverse analysis with a high acquisition frequency (> 1000 Hz) is capable to predict accurately (5 to 10% error) the roll bite peak of temperature as well as the roll surface temperature evolution all around the roll rotation. However as heat flux is more sensitive to noise measurement, the peak of heat flux in the bite is under-estimated (20% error) by the inverse calculation and thus the average roll bite heat flux is also interesting information from the sensor (these simulation results will be verified with an industrial trial that is being prepared). Rolling tests on a pilot mill with low rolling speeds (from 0.3 to 1.5 m./sec.) and strip reductions varying from 10 to 40% have been performed with the temperature sensor. Analysis of the tests by inverse calculation show that at low speed (<0.5 m./sec.) and large contact lengths (reduction: 30 to 40%), the roll bite peak of heat flux reconstructed by inverse calculation is correct. At higher speeds (1.5 m./sec.) and smaller contact lengths (reduction : 10-20%), the reconstruction is incorrect: heat flux peak in the bite is under-estimated by the inverse calculation though its average value is correct. The analysis reveals also that the Heat Transfer Coefficient HTC roll-bite (characterizing heat transfers between roll and strip in the bite) is not uniform along the roll bite but is proportional to the local rolling pressure. Finally, based on the above results, simulations with a roll thermal fatigue degradation model under industrial hot rolling conditions show that the non-uniform roll bite Heat Transfer Coefficient HTC roll-bite may have under certain rolling conditions a stronger influence on roll thermal fatigue degradation than the equivalent (e.g. same average) HTC roll-bite taken uniform along the bite. Consequently, to be realistic the roll thermal fatigue degradation model has to incorporate this non-uniform HTC roll-bite .2

Knowledge of temperature distribution in the roll is fundamental aspect in cold rolling. An inverse analytical method has been previously developed to determine interfacial heat flux and surface temperature by measuring the temperature with a thermocouple (fully embedded) at only one point inside the roll. On this basis some pilot mill tests have been performed. The temperature sensor, the calibration procedure and rolling tests at different strip rolling conditions (5%, 10%, 15% and 20%) are described. Results show a good agreement with well-known theoretical models. Moreover the CPU times of the method (around 0.05 s by cycle) enable an online control of the rolling process.

Knowledge of temperature distribution in the roll is fundamental aspect in cold rolling. An inverse analytical method has been previously developed to determine interfacial heat flux and surface temperature by measuring the temperature with a thermocouple (fully embedded) at only one point inside the roll. On this basis some pilot mill tests have been performed. The temperature sensor, the calibration procedure and rolling tests at different strip rolling conditions (5%, 10%, 15% and 20%) are described. Results show a good agreement with well-known theoretical models. Moreover the CPU times of the method (around 0.05 s by cycle) enable an online control of the rolling process.