Process Overview
Cold rolling is performed on hot-rolled, pickled strip at room temperature — below the recrystallisation temperature of steel. The process reduces gauge by 50–85% in total, tightening dimensional tolerances from ±0.1–0.2 mm (hot rolled) to ±0.01–0.03 mm (cold rolled), developing a bright, oxide-free surface finish, and increasing yield strength through work hardening. Because the steel is heavily cold-worked, it must be annealed after rolling to restore ductility before forming. Cold rolling feeds downstream processing lines — continuous annealing, batch annealing, hot-dip galvanising, electrogalvanising, and tin plating — that convert full-hard cold-rolled coil into the final products used in automotive body panels, white goods, food packaging, and construction cladding.
Pickling: Scale Removal Before Rolling
Before cold rolling, the hot-rolled coil must be descaled. The surface layer of iron oxides (mill scale) formed during hot rolling and coiling is brittle, abrasive to mill rolls, and prevents the surface bonding needed for metallic coatings. In a continuous pickling line (CPL), the strip passes through tanks of hot hydrochloric acid (HCl, 12–18% concentration, 70–85 °C) at strip speeds of 80–200 m/min, with a residence time in acid of 15–40 seconds. After acid tanks, the strip is rinsed and dried before coiling or direct entry to the cold mill. Modern pickling + tandem cold mill lines (PL-TCM) combine both operations in a single continuous pass, eliminating the coil storage step and improving yield through reduced surface damage.
Tandem Cold Mill vs Reversing Mill
Two mill configurations dominate cold rolling. In the tandem cold mill (TCM), 4–6 four-high or six-high rolling stands are arranged in series and the strip passes through all stands in a single continuous pass at speeds up to 30 m/s at the last stand. Total reduction is achieved in one pass; productivity is typically 400,000–1,000,000 t/yr per line. In a reversing cold mill (Z-high or four-high), a single stand is used and the strip is rolled back and forth in 5–9 passes. Reversing mills are lower productivity (100,000–300,000 t/yr) but more flexible — they suit a wider range of gauges, widths, and specialty grades with lower volume. Modern six-high TCM stands include an intermediate roll that can be shifted axially to control strip crown and edge drop, enabling tighter flatness tolerances than conventional four-high mills.
Work Hardening and Annealing
Cold rolling work-hardens the steel: dislocation density increases by 3–4 orders of magnitude, yield strength rises from ~250 MPa (hot rolled) to 500–900 MPa (full hard), and elongation falls below 5%. Full-hard material is too brittle for press forming and must be annealed to restore ductility by allowing the deformed microstructure to recrystallise. Continuous annealing lines (CAL) are used for automotive and appliance grades: the strip is heated to 750–880 °C in a radiant-tube furnace, held for 20–120 seconds, then cooled at 10–100 °C/s. The controlled rapid cooling enables production of advanced high-strength steels (AHSS) — dual-phase (DP), complex phase (CP), and transformation-induced plasticity (TRIP) grades — by controlling the proportion of martensite formed. Batch annealing furnaces (BAF) are used for deep-drawing grades: coils are stacked in a bell furnace under a hydrogen atmosphere and annealed for 24–72 hours at 650–720 °C. The long, slow cycle produces fully recrystallised, coarse-grained steel with exceptional formability.
Temper Rolling (Skin Passing)
After annealing, cold-rolled strip passes through a temper rolling mill at a light reduction of 0.3–2.0%. This serves three purposes: (1) it eliminates the distinct yield point (Lüders bands) introduced during annealing — without temper rolling, press-forming the steel would cause visible surface markings on automotive panels; (2) it sets the surface roughness, which controls paint adhesion, press-forming lubrication, and panel appearance; and (3) it corrects minor shape and flatness defects. The work-roll surface texture — shot-blasted for general grades, electron discharge textured (EDT) or laser-textured for automotive exposed quality — is transferred directly to the strip surface during skin passing.
Typical Tandem Cold Mill Parameters
| Parameter | Typical value |
|---|---|
| Input gauge (hot rolled, pickled) | 1.5–6.0 mm |
| Output gauge (cold rolled, full hard) | 0.5–2.0 mm |
| Total cold reduction | 50–85% |
| Last-stand rolling speed | up to 30 m/s (108 km/h) |
| Rolling force per stand (wide TCM) | 15,000–30,000 kN |
| Work roll diameter (four-high TCM) | 450–650 mm |
| Gauge tolerance (cold rolled) | ±0.01–0.03 mm |
| CAL annealing temperature | 750–880 °C |
| CAL cooling rate (AHSS grades) | 10–100 °C/s |
| Temper rolling reduction | 0.3–2.0% |
Operating parameters for a tandem cold mill processing automotive-grade low-carbon steel.
Most Cold-Rolled Steel Is Further Coated Before Use
Bare cold-rolled coil is a minority of the final product mix. Hot-dip galvanising (HDG / CGL) applies a zinc coating of 60–275 g/m² by passing the annealed strip through a molten zinc bath at ~460 °C — this is the dominant corrosion protection route for automotive (GI/GA) and construction (Z275) grades. Electrogalvanising (EG) applies a thinner, more uniform zinc layer (20–80 g/m²) by electrodeposition for the most demanding automotive exposed surfaces. Electrolytic tinplate (ETP) deposits 1.1–8.4 g/m² of tin onto cold-rolled steel for food and beverage cans. Organic coating lines apply paint or polymer films for roofing and white goods. A fully integrated cold rolling complex therefore combines the cold mill, annealing lines, and multiple coating lines as a single production unit.