Cold Rolling Mills

What is cold rolling?

Cold rolling starts where hot rolling ends. The input is a hot-rolled coil (HRC) — typically 1.8mm to 6mm thick — that has been pickled (acid-washed to remove oxide scale) and oiled. This pickled coil is then passed through rolls at room temperature, reducing its thickness by 40% to 90% to produce cold-rolled strip as thin as 0.15mm — or for foil, as thin as 0.006mm.

Because the steel is below its recrystallisation temperature, the crystal grains cannot reform during deformation. Each pass through the rolls distorts the grain structure further, making the steel progressively harder, stronger, and less ductile — a phenomenon called work hardening. After significant cold reduction, the steel must be softened by annealing — heating to 600–700°C in a controlled atmosphere to recrystallize the grains.

The trade-off is worth it. Cold-rolled steel has surface roughness below 1 micron (hot-rolled is 12–25 microns), thickness tolerance of ±0.01mm (hot-rolled is ±0.1mm), and yield strength 20–40% higher than hot-rolled steel of the same grade. These properties make it essential for automotive body panels, appliance casings, beverage cans, electrical transformers, and any application where appearance, precision, or formability matters.

Cold rolling is a finishing process, not a primary shaping process. Every cold-rolled coil began life as a hot-rolled coil on a hot rolling mill. The hot mill determines the starting thickness; the cold mill determines the final thickness, surface, and mechanical properties.

The cold rolling process

The cold rolling line has four major stages: pickling (acid wash to remove scale), cold reduction (the main thickness reduction), annealing (heat treatment to restore ductility), and skin pass/temper rolling (light finishing pass). Some products add a fifth stage — coating (galvanizing, tin plating, or painting).

Pickling removes the oxide scale layer using hydrochloric acid (HCl) baths. Modern continuous pickling lines process strip at speeds up to 300 m/min. Without pickling, scale would embed into the strip surface during cold rolling.

Cold reduction is the main thickness reduction step, covered in detail in the mill types section below. Total reduction ranges from 40% to 90% depending on the final product.

Annealing restores ductility by recrystallizing the work-hardened grain structure. Batch annealing stacks 3-5 coils under a bell furnace for 20-30 hours. Continuous annealing (CAL) passes strip through a furnace at 200-400 m/min — faster and more uniform, but higher capital cost.

Skin pass / temper rolling applies a very light reduction (0.5-2%) after annealing. This eliminates yield point elongation (the stretcher strain defect that appears during stamping), sets the final surface texture, and improves flatness. Skin pass mills are typically single-stand 4-high or 2-high.

Cold rolling mill types

Tandem cold mill (TCM)

The tandem cold mill is the highest-productivity cold rolling configuration. It consists of 4 to 6 stands in series, with the strip passing through all stands in one continuous pass. Each stand reduces thickness by 20-40%, and cumulative reduction reaches 70-90%. Strip speed at the last stand can exceed 2,500 m/min (150 km/h).

Tandem mills are used by large steelmakers producing over 500,000 MTPA. Each stand is typically a 4-high configuration. Roll coolant (water-oil emulsion) is sprayed continuously to manage heat from deformation and friction. Automatic gauge control (AGC) holds thickness to ±0.005mm across the entire coil.

Single-stand reversing cold mill

For producers with lower volumes (under 300,000 MTPA) or frequent grade/width changeovers, a single-stand reversing cold mill is more economical. The strip passes back and forth through one 4-high or 6-high stand, recoiled after each pass, typically 3 to 5 passes. Capital cost is 30-40% of a tandem mill but productivity is lower. Maximum speed: 600-1,200 m/min. Common for stainless steel, silicon steel, and specialty grades.

Skin pass mill (temper mill)

A skin pass mill applies 0.5% to 2% reduction after annealing — not for thickness, but for three specific purposes: eliminating yield point elongation (Luders lines during stamping), imparting surface roughness (using EDT, SBT, or laser-textured rolls), and improving flatness. Often integrated inline with the continuous annealing line.

Stand configurations for cold rolling

4-High stand

The standard workhorse. Two small work rolls (400-600mm diameter) backed by two large backup rolls (1200-1500mm diameter). Modern stands include hydraulic roll bending, CVC shifting, and roll coolant zoning for ±0.005mm thickness control.

6-High stand

Adds intermediate rolls between work and backup rolls. The intermediate rolls shift axially, providing superior edge drop control and flatness. Used for automotive exposed sheet, electrical steel, and tinplate — premium products where quality justifies the extra cost.

Cluster mill (Sendzimir 20-high)

Two extremely small work rolls (as small as 25mm diameter) backed by 18 rolls in a cluster. The tiny work rolls are essential for very hard materials (stainless steel, titanium) and ultra-thin gauges down to 0.001mm (1 micron). Smaller work rolls = smaller contact arc = lower rolling force — the only way to reduce hard materials effectively.

Trade-offs: slow (100-600 m/min) and narrow (max 1600mm width). Used exclusively for high-value products where the premium price justifies low productivity.

Cold-rolled products

Hot rolling vs cold rolling -- side by side