Adjusting Rolling Speed and Thickness to Help Ensure Metal Strip Quality

Sheet metal is thin steel, aluminum, or other alloyed metal that is used in both manufacturing and construction. It has strength and flexibility, and is the ideal material for industrial and consumer applications, like buildings, cars, transport vehicles, heating and air conditioning systems, power generation, medical devices, siding, signs, etc.

Because it is used in so many different products and industries, sheet metal endures cutting, bending, drilling of holes for screws and bolts, welding, riveting, and soldering. Not only does the ‘recipe’ of the material need to be correct, but the thickness of the metal strip must be exact as well.

Many believe that metal strip quality begins in the meltshop, and that is not far from the truth. Controlling the chemistry and thermal history of the steel not only assures the grade produced will meet the mechanical properties desired of the final product, but that the strip will handle the tons of pressure and tensions of the high speed rolling process.

According to BusinessDictionary, cold-rolled steel is a metal forming process in which “the shape and structure of steel is altered through drawing, extruding, hammering, pressing, rolling, spinning, and/or stretching at temperatures below the steel’s recrystalization point (usually room temperature). These operations cause changes in the metal’s composition which permanently increase its hardness and tensile strength, and enhance surface finish, but decrease ductility and impact resistance.”

In roughing mills, equipment is used to reduce the thickness of the steel while at the same time extending the overall length, to form the metal to roughly the final thickness. The Hot Finish mill forms the strip to the final dimension. Not only does temperature plays a crucial part in these processes as it dictates the quality of the surface finish and sheet thickness, but so does speed. As I mentioned, the metal is subjected to tons of pressure and tensions of the high speed rolling process.

The following table summarizes the typical thicknesses and rolling speeds for each mill type. Not surprisingly, as the material gets thinner, the speeds increase dramatically.

	Hot Roughing Mill	Hot Finish Mill	Cold Rolling Mill
Maximum Thickness	> 400 mm	20-50 mm	3-20 mm
Minimum Thickness	20-50 mm	3-20 mm	0.100 mm
Rolling speeds (meters per min)	~ 100	~1000	~2000

Automatic Gauge Control (AGC) is an essential component of modern rolling mills. Comprehensive AGC algorithms incorporate readings from dozens of sensors around the mill to control thickness to high tolerances..

Some of the key AGC input parameters are speed and tension. Conservation of mass dictates that the mass per unit time entering the mill must equal the mass per unit time exiting the mill. So as the material is rolled thinner, the speeds must increase. If the drive motors are off, even by a few centimeters per minute, the strip may break or cobble with catastrophic results. There is a delicate balance between the reduction caused by the mill force, and the reduction caused by drawing (extruding) the material through the gap.

Metals coating weight and thickness measurement gauges provide precise, real-time measurements that help meet specifications of various applications while maximizing raw material use. Thickness gauges are primarily used to look for dramatic deviations in thickness – which could affect the mechanical properties of the finished product. Small changes can be corrected by slight adjustments in speed and tension, as the motor power can be controlled very quickly. Larger changes can be compensated by the hydraulic cylinders in the mill stand, which can react in a matter of milliseconds.

X-ray gauges are the optimal thickness gauge technology to provide the speed and accuracy required to operate a cold rolling mill. X-ray based sensor systems provide extremely accurate high speed/low noise measurements, permitting producers to realize material savings and quality improvements.

NOTE: To read additional details about High Speed X-ray Thickness Gauges, including illustrations, read the white paper, Optimization of a Cold Rolling Mill with a High Speed X-ray Thickness Gauge.