Temper Rolling Mill

The Temper Rolling Mill is a single 4-high rolling stand that takes annealed strip and applies a very light reduction, typically 0.5-3% elongation, to finish its mechanical and surface properties. It does not change gauge in any meaningful way. The job is to remove the yield-point elongation that annealing leaves in low-carbon steel, set a controlled surface roughness, and improve flatness before the strip is shipped or sent to coating. Because the reduction is small, every part of the mill is built around precise control of a light load rather than the brute force of a cold-reduction mill.

Purpose of temper rolling

Fully annealed low-carbon steel has a sharp yield point. When it is later formed, that yield point releases unevenly and leaves stretcher-strain marks, also called Luders bands, on the part surface. A pass through the roll set at a small fixed elongation works the surface grains just enough to eliminate the discontinuous yield, so the steel deforms smoothly when the customer presses it. The same pass transfers a defined roughness from the work rolls to the strip, which controls how the strip takes paint and how it behaves in the press. The third function is flatness: light rolling under tension flattens residual shape defects such as edge wave and center buckle. The elongation controller ties these together, since the amount of extension sets how completely the yield point is killed.

Mill stand and housing

The mill stand is a closed frame. Two cast-steel housing posts form the load window, closed at the top by two housing caps and pulled together by four pre-tensioned tie rods so the assembly reacts the roll-separating force as one stiff loop. Frame stiffness matters more here than peak strength, because a flexible stand would let the gap drift and spoil the elongation. Sole plates align the housing to the foundation. Bronze-faced liner plates and window guides line the chock windows so the chocks move only on the vertical axis, with worn liners replaced rather than the casting.

Roll set and chocks

Two work rolls do the rolling, backed by two large back-up rolls that limit work-roll deflection across the width. The work-roll barrel carries an electro-discharge texture, a pattern of fine pits ground in by EDT, which transfers a target roughness to the strip. A bright finish roll is fitted instead when a low-Ra surface is wanted. The work rolls run in work-roll chocks on ball bearings, while the heavily loaded back-up necks run in back-up-roll chocks carrying hydrodynamic oil-film bearings. Roll neck seals and oil seals keep bearing oil in and temper solution out of the necks.

Roll-force, gap, and elongation control

This is the defining control problem of the mill. The strip leaves slightly longer than it enters, and that difference must be held to a fraction of a percent across the whole coil. Two hydraulic gap control cylinders, fed through high-response servo valves, set the gap and the roll-separating force. Gap position sensors and roll-force load cells close the loop on each side. The elongation controller reads the stretch encoders at entry and exit, computes the actual extension from the speed ratio, and trims roll force and strip tension to hold the setpoint to about +/-0.05%. Four roll bending cylinders bend the work rolls to correct shape across the width while the elongation loop runs.

Tension and coil handling

Tension is part of the rolling mechanism, not just transport. The payoff reel pays the annealed coil off under back-tension and the tension reel recoils the strip under front-tension, both on segmented reel mandrels driven by reel drive motors. Between them, an entry and an exit bridle of rubber-lagged bridle rolls set the strip tension by controlled speed difference, which decides how much of the elongation comes from tension versus roll force. Deflector rolls and strip guides hold the pass line, a tensiometer measures actual tension, and the stretch encoders feed the elongation loop.

Solution, surface, and the rest of the line

In wet temper rolling the temper solution system lays a thin film on the strip so friction and texture transfer stay even. A solution pump draws from the solution tank through filters to applicator headers and spray nozzles; wringer rolls and a blow-off header dry the strip, and cleaning brushes keep the work-roll texture clear. A dry skin-pass mill omits this system. The main motor drives through a gear distributor and two spindles. The lube unit and hydraulic power unit feed the bearings and cylinders, and the control system runs drive, gauge, and elongation control with pressure sensors, load cells, and a roughness monitor. The same stand is run as a temper mill for soft tempers, as a skin-pass for very light passes, or as a double-cold-reduction mill when a heavier extension is wanted after annealing.