Ultrasonic Measurements of the Strip Thickness, Lubricant Film Thickness, Roll Deflection and Roll Stress in the Roll Bite in the Cold Rolling of Steel

In cold rolling the interface between the roll and strip, known as the roll bite, is key to the finish and geometry of rolled products. This interface is complex and operates in the mixed regime with part asperity contact and part fluid film. The factors which affect this are of interest to the cold rolling community. The inaccessibility of the interface has made direct measurement of its condition difficult. In this thesis ultrasonic approaches have been developed to measure the state of the roll bite, in-situ and in real time.

The thesis starts by introducing the background theory underpinning modern cold rolling. The theory behind the proposed ultrasonic measurements is developed. It is proposed that the proportion of longitudinal and shear waves reflected by the asperity and lubricant film components of the mixed mode interface are dependent on their respective stiffnesses. From these the lubricant layer thickness can be calculated. It is also shown how time-of-flight measurements can be used to measure strip thickness, roll stress, roll deflection and roll material properties.

Ultrasonic sensor systems were incorporated into pilot and semi-industrial cold-rolling mills. Measurements were taken as steel was rolled under a range of lubrication conditions, with rolling velocities from 25 m/min to 1200 m/min and elongations from 5% to 50%. Stiffness and lubricant film thickness measurements were demonstrated for films from 0.3 µm to ~6 µm. Roll bite contact lengths of 9.13mm to 15.34mm were recorded for elongations from 9.7% to 40%. For these same elongations average radial roll stresses of 180 MPa to 340 MPa and roll deflections of 30 µm to 55 µm at the roll bite centre were measured. Time-of-flight measurements yielded thickness profiles of strip reduced from 2.8mm to 2.154mm.

Validation was provided by multiple numerical models which showed good agreement with the ultrasonic results.