Cold Rolling Mill

A cold rolling mill reduces the thickness of metal strip below the recrystallisation temperature, usually at room temperature with emulsion cooling. The strip arrives as a coil already hot rolled and pickled, typically 2-4 mm thick, and leaves at 0.2-0.5 mm with a controlled gauge and a bright, smooth surface. Because the metal is not softened by heat, it work hardens as it deforms, which raises the rolling force and the required drive power well above what a hot mill of the same width needs. The mill earns its place for three reasons: it hits thickness tolerances that hot rolling cannot reach, it produces the surface finish that coated and exposed automotive sheet require, and it raises the strength of the strip through the cold work itself.

Mill stand and housing

The Mill Stand / Housing carries the full separating force back into the foundation. Two Housing Post members and two Housing Cap members form a closed frame, clamped together by pre-tensioned Housing Tie Rod rods so the housing does not breathe under load. The frame sits on Sole Plate plates bolted to the mill foundation. Inside each window, replaceable Window Liner Plate wear plates and Window Guide guides locate the roll chocks and absorb the rubbing as they move. The housing stiffness matters directly: any frame stretch under force adds to the roll gap and shows up as a gauge error the gauge control then has to remove.

Roll set and chocks

The Roll Set uses the 4-high arrangement. Two small Work Roll rolls touch the strip; they are forged from a high-chromium steel, hardened to 60-65 HRC, and ground to a fine finish that transfers to the product. A small work-roll diameter lowers the rolling force needed for a given reduction, but a thin roll deflects, so two large Back-up Roll rolls back it up and keep the gap flat across the width. Each roll neck runs in a chock: the Work Roll Chock carries Ball Bearing bearings, while the heavier Back-up Roll Chock carries an Oil-Film Bearing that floats the neck on a hydrodynamic film through a bronze Bearing Bushing running on a Bearing Sleeve. A Roll Neck Seal and an Oil Seal keep emulsion out of the bearings.

Hydraulic gap control and automatic gauge control

Gauge is set by the Hydraulic Gap Control. A short-stroke HGC Capsule Cylinder on each side, driven through a fast Servo Valve, positions the roll gap, while an electric screwdown through Screwdown Screw screws, Screwdown Nut nuts, and a Screwdown Gearbox handles the coarse pass-line setting. A Position Sensor reads cylinder stroke and a Roll Force Load Cell reads rolling force. The AGC Electronics closes the loop: it compares measured thickness and force against the target and trims the cylinder position within a few milliseconds. Force feedback corrects for housing stretch and roll eccentricity faster than the gauge alone can, since the gauge sits some distance downstream of the bite.

Main drive

The Main Drive supplies the torque. A Main Drive Motor with its Stator Assembly, Rotor Assembly, and Copper Winding in a Motor Housing feeds a Pinion Stand that splits torque through a Helical Gear Pair to two Drive Spindle spindles. Universal-joint spindles and gear Spindle Coupling couplings transmit torque to the work-roll necks while allowing the rolls to move as the gap changes. On a tandem mill each stand has its own drive, and stand speeds are matched so strip mass is conserved between stands.

Coil handling and strip tension

Cold rolling depends on tension as much as on the gap. The Coil Handling & Tension group runs a Payoff Reel (Uncoiler) paying off the incoming coil under back tension and a Tension Reel (Recoiler) winding the product under forward tension, each on an expanding Reel Mandrel driven by a Reel Drive Motor. Sets of Bridle Roll and Deflector Roll rolls build and steer the tension, a Belt Wrapper starts the coil on the reel, and a Tensiometer Roll measures the actual strip tension. Tension reduces the roll force needed for a given reduction and helps keep the strip flat and tracking centred.

Emulsion and coolant

The Emulsion System does three jobs at once: it cools the rolls, lubricates the bite to lower friction and force, and washes away iron fines. A Emulsion Pump circulates emulsion from the Emulsion Tank to Spray Header manifolds carrying many Spray Nozzle nozzles. A Emulsion Filter removes the fines and a Plate Heat Exchanger holds the temperature near 50-60 C. Switching individual Zone Control Valve zones along the barrel lets the operator control thermal camber, since a locally hotter roll grows in diameter. A separate Oil-Mist Lube Unit lubricates the bearings and gearboxes.

Controls, gauging, and variants

The Control System ties the loops together through a Control Panel, the Drive Control converter, and a X-Ray Thickness Gauge X-ray or isotope gauge on entry and exit, with a Gauge Radiation Source and a Gauge Detector. Flatness is held by the Shape Control group, where Work-Roll Bending Cylinder bending cylinders, a Roll-Shift Actuator for profiled rolls, and a Shapemeter Roll roll with many Shapemeter Zone Sensor sensors map and correct strip shape across the width. Mills come in two layouts. A reversing mill passes the strip back and forth through a single stand for many passes, suited to lower volumes and a wide product mix. A tandem mill places three to six stands in line so the strip reaches final gauge in one pass through the line, suited to high tonnage of a narrower product range.