Skin Pass Mill

Role of the skin pass

A skin pass mill applies the final light pass to cold-rolled or coated steel strip. The reduction is small, typically 0.3% to 2.0% of permanent extension, and the goal is not gauge reduction but surface and shape. The pass does three jobs at once. It flattens the strip so it lies down after coiling, it transfers a controlled surface roughness from the Work Roll to the strip, and it works the surface layer enough to suppress yield-point elongation. Without that last effect, deep-drawn parts show Luders bands (stretcher strain marks) when the customer presses them. A single pass through the Skin Pass Mill removes that risk for several months of aging.

The unit is built around one rolling stand. That makes it look like a temper mill, and the rolling mechanics are the same, but the application differs. A skin pass mill usually sits inline inside a continuous annealing line or a hot-dip galvanizing line, after the furnace and coating sections, where it handles galvanized, galvannealed and electro-coated strip in addition to bare cold-rolled coil. Because it is inline, its speed is tied to the rest of the line, and the strip arrives already under tension from the line bridles and accumulators rather than from a payoff and coiler local to the stand.

Mill stand and housing

The Mill Stand & Housing carries the roll-separating force across the full strip width. Two cast Housing Post uprights are closed by a Housing Cap at the top and clamped together with pre-tensioned Tie Rod members so the stand stays stiff when force changes. Each post sits on a grouted Sole Plate. The window in which the roll chocks slide is faced with a replaceable Window Liner Plate and kept square by a Window Guide. Stand stiffness matters here more than in a heavy reduction mill: the elongation is so small that gauge and shape are sensitive to any spring in the housing.

Roll set and chocks

The Roll Set is the part that does the surface work. Two hardened Work Roll rolls contact the strip. Their finish, either electro-discharge texture (EDT) or a fine bright grind, is copied onto the strip surface in the bite, which is why work-roll roughness and the wear of that texture are tracked closely. Two large Back-Up Roll rolls sit behind the work rolls to resist bending and hold the gap flat across the width. Each roll neck runs in a Roll Chock. The back-up necks carry the highest load and run in a hydrodynamic Oil-Film Bearing, while the work-roll positions use Ball Bearing units for location and thrust. A Roll-Neck Seal and an Oil Seal keep water, solution and scale out of the bearings.

Roll-force and elongation control

Elongation control is the defining function of the mill. The Roll-Force & Elongation Control sets the small permanent extension by trimming roll force against measured strip tension and speed. A hydraulic Hydraulic Loading Cylinder on each side applies the force through a Servo Valve, a Roll-Gap Position Sensor reads the roll gap, and a Load Cell reads actual force per side. A Roll-Bending Cylinder set bends the work-roll necks to correct flatness across the width. The Elongation Controller computes percent extension from the entry and exit strip speed, then trims force and tension to hold the target. Because the strip is loaded both by roll force and by line tension, the controller treats the two together: at light elongation, more of the work can come from tension; at heavier settings, force dominates.

Strip tension, bridles and threading

The Strip Tension & Threading holds the strip under tension on both sides of the stand. A driven Entry Bridle sets back tension and a driven Exit Bridle sets forward tension, isolating the stand from coiler or downstream pull. A Deflector Roll steers the path, a Strip Centering Guide keeps the strip centered, and a Tensiometer Roll measures actual tension for the elongation loop. A Strip Speed Encoder on each side gives entry and exit speed; the ratio of the two is the measured elongation. A Strip Wiper clears residual liquid and debris before the strip leaves.

Drive, hydraulics and controls

The Drive Train turns the work rolls through a Main Drive Motor, a Gear Distributor that splits torque to top and bottom, two Drive Spindle members and a Drive Coupling at each end. Separate Bridle Drive Motor units drive the bridles. The Lubrication & Hydraulics group runs a circulating Lube Oil Unit for the bearings and a Hydraulic Power Unit for the loading and bending cylinders. The Control System coordinates roll force, elongation, roughness and line speed; it carries a Control Panel, a Drive Control Cabinet cabinet for line-speed coordination, a Roughness Monitor, plus Pressure Sensor and Load Cell inputs, a Wire Bundle, Relay outputs and redundant Power Supply units.

Wet skin-pass and variants

In wet mode the Wet Skin-Pass & Surface System meters a dilute solution into the bite from a Solution Tank through a Solution Pump, a Applicator Header and a row of Spray Nozzle jets. A Wringer / Squeegee Roll and a Blow-Off / Drying Header remove the liquid afterward, a Solution Filter keeps the loop clean, and a Roll-Cleaning Brush keeps the roll texture free of pickup. The wet pass gives a more uniform finish and reduces work-roll wear on long campaigns. Dry skin passing is used where a specific dry texture is wanted. The mill runs bare cold-rolled coil as well as galvanized and galvannealed strip, where the pass also flattens and smooths the zinc coating. Against a standalone temper mill, which is a dedicated batch stand with its own payoff and recoiler, this stand earns its keep by being inline: it picks up strip already conditioned by the furnace and coating sections and matches its speed to them.