Metal rolling is by friction which develops at metal-to-roll interfaces during the rolling process. But, friction at the metal-to-roll interface during the metal rolling process can cause roll surface damage if not controlled. Over time, friction results in downtime and repair of the mill. Therefore, lubrication is essential to control the metal-to-roll interface coefficient of friction. It is important to understand the conditions at the metal-to-roll interface to minimize energy loss and improve the strip surface finish.
In this work, a new method for measurement of metal-to-roll interface conditions, based on the reflection of ultrasound, is evaluated during the cold metal rolling operation. The method is a pitch-catch sensor layout arrangement. Here, a piezoelectric element generates an ultrasonic pulse which is transmitted to the metal-to-roll contact interface. This method is non-invasive to both roll and strip during the process.
The wave reflection from the metal-to-roll interface is received by a second transducer. The amplitude of the reflected waves is processed in the frequency domain. The reflection coefficient values are used to study the metal-to-roll interface conditions at different rolling parameters like, roll speeds and rolling loads. The results show that the reflection coefficient increases with increasing roll speed. This is because of reduction in the roll-bite contact area or increase in the frictional resistance of interface during the roll speed increment. However, the reflection coefficient decreases with increasing rolling load due to either increase in the roll-bite contact area or pressure.
The reflection coefficient determines the oil film thickness formation at the metal-to-roll interface. In-addition, the Time-of-Flight of the reflected wave obtained from this technique is used to estimate strip thickness and roll-bite length during the rolling process. The oil film thickness in the range of 1.25µm to 3.05µm was measured during the rolling process. The film thickness increases with increasing roll speed and reduces with increasing rolling load. The roll-bite value of 5.5mm was measured during the process.
The results from this study show that this ultrasonic technique can measure the metal-to-roll interface conditions (roll-bite, oil film and strip thickness) during the rolling process. This ultrasonic technique has the advantage of minor roll modification. Additionally, the experimental roll condition values obtained from the ultrasonic reflection method agree with theoretical values. The technique shows promising results as a research tool, and with further development, could be used for lubricant monitoring. Also, it can be utilized in the control system of a working mill for reduction of friction losses in the metal rolling process
