Application Guide for Operation Procedures of Six-High Rolling Mill
A modern 6 hi cold rolling mill (six-high cold rolling mill) is a precision piece of equipment used to produce high-quality strip and foil with tight dimensional tolerances and excellent surface finish.
The following application guide focuses on practical operation procedures, process parameters, and quality control methods for six-high mills, providing production reference for engineers, operators, and maintenance staff.
This guide is applicable to a wide range of six-high configurations, including 6-hi HC mills (high crown), reversible six-high cold rolling mills, and continuous 6-high tandem cold rolling mills used for carbon steel, stainless steel, copper, aluminum, and specialty alloys.
1. Overview of Six-High Cold Rolling Mills
A six-high cold rolling mill is an advanced configuration where each work roll is supported by an intermediate roll and a backup roll.
Compared with four-high mills, the 6 hi cold rolling mill offers improved control of strip crown, shape, and gauge, especially for thin and ultra-thin strip.
1.1 Typical Six-High Mill Configuration
The standard roll stack configuration from top to bottom is:
- Top backup roll (BUR)
- Top intermediate roll (IMR)
- Top work roll (WR)
- Strip (rolled material)
- Bottom work roll
- Bottom intermediate roll
- Bottom backup roll
With additional devices such as work roll bending, intermediate roll shifting, and automatic gauge and shape control (AGC/AFC), the six-high mill can achieve very high flatness quality (I-units < 10) and thickness tolerances down to ±0.5–1.0% for many products.
1.2 Typical Technical Parameters of a 6 Hi Cold Rolling Mill
| Item | Typical Value (Carbon Steel 6-Hi Reversible Mill) |
|---|---|
| Strip width range | 600 – 1,250 mm |
| Entry thickness | 1.8 – 4.0 mm (pickled hot band) |
| Exit thickness | 0.18 – 1.2 mm (depending on grade) |
| Maximum rolling force | 12 – 18 MN (1,200 – 1,800 tons) |
| Maximum strip speed | 800 – 1,600 m/min (reversible often 600 – 1,200 m/min) |
| Work roll diameter | 350 – 520 mm (steel strip); 200 – 280 mm for thinner foil lines |
| Intermediate roll diameter | 500 – 650 mm |
| Backup roll diameter | 900 – 1,200 mm |
| Main motor power | 2 × 1,200 – 2 × 2,000 kW (mill stand) |
| Thickness tolerance (typical) | ±(0.5 – 1.0)% of strip thickness for most grades |
Values vary by manufacturer, product range, and whether it is a single-stand reversible 6-hi cold mill or a multi-stand 6-hi tandem cold rolling mill.
2. Operator Skill Requirements and Safety Preparation
2.1 Skills and Knowledge for 6 Hi Cold Rolling Mill Operators
Before independently operating a six-high cold rolling mill, the operator should master the following:
- Understand the full process flow: entry coil → uncoiling → welding (if applicable) → threading → rolling → recoiling → coil handling.
- Familiarity with product specifications: grade, width, entry thickness, exit thickness, thickness schedule, and flatness requirements.
- Knowledge of mechanical components: roll stack, chocks, screw-down (or hydraulic AGC), coolant/emulsion system, fume extraction, drive and motor control, hydraulic and pneumatic units.
- Ability to interpret mill setup sheets and process cards, including reduction schedule, rolling force limits, speed pattern, and tension levels.
- Understanding of safety devices: emergency stop, interlocks, doors, guards, strip break detection, coil car and mandrel safety.
- Basic knowledge of metallurgy and work hardening to adjust reductions and annealing cycles.
2.2 Pre-Shift Checks and PPE
Before each shift, operators should:
- Wear appropriate personal protective equipment (PPE): safety helmet, safety shoes, cut-resistant gloves, protective glasses or face shield, hearing protection where needed.
- Receive handover from the previous shift, review operating log, discuss remaining coils, issues, and maintenance interventions.
- Check the condition of tools: feeler gauges, micrometers, calipers, infrared thermometer, surface roughness comparator, etc.
- Verify function of safety systems: emergency stops, door interlocks, light curtains (if equipped).
- Ensure housekeeping: no loose scrap, tools, or oil on walkways or near rotating equipment.
3. Process Flow of Six-High Cold Rolling Mill
The typical process flow for a reversible 6 hi cold rolling mill is:
Pickled hot band (entry coil) → Coil preparation → Uncoiling → Threading / Feeding → Reversible cold rolling with multiple passes → Exit recoiling → Coil inspection and packaging
For an industrial 6 hi cold rolling mill line, a pickling line, degreasing/cleaning, skin-pass mill, and tension leveler may be integrated, especially in continuous tandem lines, but the core rolling stand operation is similar.
3.1 Raw Material and Coil Data
Before loading each coil, verify:
- Grade (e.g., low-carbon IF steel, high-strength steel, stainless, copper, aluminum)
- Entry thickness (tin) and width
- Planned exit thickness (tout) and tolerance
- Coil weight and inner diameter
- Surface requirements (bright, matte, special roughness)
Example: For a low carbon steel coil with 2.0 mm entry thickness and 1,000 mm width, the target final thickness may be 0.35 mm. The total reduction is therefore:
Reduction = (2.0 − 0.35) / 2.0 = 1.65 / 2.0 = 82.5%
Such a reduction is usually achieved in 4–6 passes on a reversible 6-hi mill, depending on grade and allowable work hardening.
3.2 Coil Loading and Uncoiling
Standard procedures for loading and uncoiling:
- Use coil car to position coil onto uncoiler mandrel.
- Check mandrel expansion, coil centering, and side guide settings.
- Remove coil straps only after coil is securely clamped and guards are in place.
- Remove edge defects or welds at the head if required.
- Ensure correct connection to threading device (pinch roll, threading table, or bar).
3.3 Threading and Reversible Rolling
Threading typically follows a low-speed mode:
- Set mill in threading mode with low roll gap, low speed (around 10–30 m/min), and low tension.
- Engage entry pinch rolls and guide strip into the roll gap, then to the coiler on the opposite side.
- Once tail is safely on the exit mandrel, increase tensions and speed to rolling mode.
In a reversible 6 hi cold rolling mill, the strip travels back and forth through the stand. The reduction schedule and direction changes are controlled automatically.
Operators must monitor tension, thickness deviations, shape readings, and surface appearance continuously.
4. Pass Schedule and Reduction Strategy
One of the most important tasks is designing a suitable pass schedule for each grade and final thickness. Excessive single-pass reduction may exceed rolling force limits, cause strip breakage, or produce poor shape.
Too many passes reduce productivity and increase costs.
4.1 Example Pass Schedule for Low Carbon Steel
Example: Reversible 6-hi cold rolling mill rolling low carbon steel from 2.0 mm to 0.35 mm.
| Pass No. | Entry Thickness (mm) | Exit Thickness (mm) | Reduction (%) | Typical Speed (m/min) |
|---|---|---|---|---|
| 1 | 2.00 | 1.50 | 25% | 200 – 400 |
| 2 | 1.50 | 1.05 | 30% | 250 – 500 |
| 3 | 1.05 | 0.73 | 30.5% | 300 – 700 |
| 4 | 0.73 | 0.50 | 31.5% | 400 – 800 |
| 5 | 0.50 | 0.35 | 30% | 500 – 900 |
The actual schedule must consider mill capability (force, torque, speed) and material properties (yield strength, work hardening exponent n, elongation).
For high-strength steels, each pass may be limited to 20–25% reduction, while soft IF steel can tolerate 30–40% in early passes.
4.2 Tension Settings
Proper entry and exit tension improve shape and reduce rolling force. Typical tension levels for low carbon steels:
| Location | Typical Range | Notes |
|---|---|---|
| Entry tension | 5 – 15 kN (for 1,000 mm width) | Lower during threading and tail-out; higher in steady rolling. |
| Exit tension | 15 – 40 kN (for 1,000 mm width) | Must not exceed material yield; too high may cause necking or breakage. |
| Tension ratio (exit/entry) | 1.5 – 3.0 | Optimized by grade and thickness to stabilize strip shape. |
Tension is usually controlled by AC/AC or DC/AC drives on uncoilers and coilers, synchronized with the mill stand main motors.
5. Key Operation Parameters in Six-High Rolling
The main parameters to be set and monitored for a 6 hi cold rolling mill include:
- Roll gap / reduction
- Rolling force (hydraulic screw-down pressure)
- Mill speed
- Entry and exit tension
- Coolant/emulsion flow, concentration, and temperature
- Roll bending forces and intermediate roll shifting
- AGC (Automatic Gauge Control) and AFC (Automatic Flatness Control) settings
5.1 Emulsion and Cooling Parameters
Proper lubrication and cooling are crucial for strip surface quality and roll life.
| Parameter | Typical Range | Remarks |
|---|---|---|
| Emulsion concentration | 2 – 5% for carbon steel; 5 – 8% for stainless | Concentration too low increases roll wear; too high reduces heat removal. |
| Emulsion temperature | 25 – 35 °C | Stable temperature improves shape and flatness control. |
| Flow rate | 150 – 400 L/min per nozzle zone (varies by mill size) | Should be balanced across width to avoid thermal crown issues. |
| Filtration fineness | 50 – 150 μm | Prevents roll scoring from solid particles. |
For thin gauge rolling (<0.3 mm), emulsion cleanliness and stable temperature become even more critical to avoid chatter and surface defects.
5.2 Roll Bending and Intermediate Roll Shifting
Six-high mills are known for excellent flatness control using roll bending and shifting:
- Work roll bending: positive bending (push ends towards strip) to reduce center crown; negative to correct edge wave.
- Intermediate roll shifting: lateral movement of IMR to change the effective roll crown, improving shape over varying widths.
Typical bending forces can reach 200–400 kN per side for medium-width mills. Operators must follow the shape meter readings and adjust bending within allowed limits to avoid excessive stress on the roll systems.
6. Gauge Control and Thickness Measurement
Modern 6 hi cold rolling mills use AGC (Automatic Gauge Control) and X-ray or isotope thickness gauges to achieve high precision. Operators must understand the control modes:
- Feed-forward AGC: Based on entry thickness measurement and known mill modulus.
- Feedback AGC: Based on exit thickness measurement, adjusting screw-down in real time.
- Mass flow AGC: Uses mass conservation (tin × vin = tout × vout) to control thickness.
Exit thickness is often measured continuously with ±0.1–0.2% accuracy. The control system then adjusts hydraulic cylinders every few milliseconds, maintaining a consistent thickness even with entry thickness variations or temperature changes.
7. Surface Quality and Defect Prevention
Maintaining excellent surface quality is one of the main advantages of a well-running 6 hi cold rolling mill. Some common defects and prevention measures are summarized below.
| Defect Type | Possible Cause | Prevention / Countermeasure |
|---|---|---|
| Roll marks / chatter marks | Roll surface damage, vibration, incorrect grinding, excessive roll wear | Regular roll inspection and grinding; check bearing clearance; optimize speed to avoid natural frequencies; adjust tension and reduction. |
| Scratches / scoring | Hard particles in emulsion, damaged guides, burrs on edges | Improve filtration; clean guides; remove sharp edges; control debris from previous coils. |
| Edge cracks | Excessive reduction, poor edge condition, material with low ductility | Reduce reduction per pass; edge trim if necessary; adjust annealing cycle; verify steel grade and chemistry. |
| Wavy edges / center buckle | Incorrect roll bending or shape control, non-uniform cooling, excessive tension | Adjust roll bending and IMR shifting; correct coolant balance; adjust entry/exit tension; use shape meter feedback. |
| Oil stains / spots | Dirty emulsion, leakage of hydraulic oil, poor drainage at exit | Improve emulsion management; fix leakages; use air knives or squeegee rolls at exit. |
If strip surface quality degrades, especially due to roll defects, the work rolls should be replaced or reground immediately to prevent large quantities of downgraded coils.
8. Routine Checks and Maintenance During Operation
Stable operation of a six-high cold rolling mill relies on planned checks during each shift and regular preventive maintenance. Operators should:
- Monitor oil and lubrication levels; check for leaks, unusual temperatures, or noise at bearings and gearboxes.
- Record key process parameters for each coil: rolling force, speed, tensions, cooling temperature, AGC performance.
- Inspect entry and exit coil heads and tails for deformation, slippage, or telescoping.
- Check roll temperatures using infrared thermometer periodically to avoid thermal cracks.
- Verify the calibration of thickness gauges and measuring instruments as per defined intervals.
8.1 Work Roll Management
Work roll surface and profile are critical. Typical guidelines for a steel 6 hi cold rolling mill:
- Work roll roughness (Ra): 0.6 – 1.2 μm for general steel sheet; 0.2 – 0.6 μm for bright surface or tinplate base.
- Maximum strip tonnage per roll campaign: 500 – 2,000 tons depending on grade and thickness.
- Maximum permissible wear: typically 0.2 – 0.4 mm on diameter before regrinding.
- Regularly inspect roll ends and necks for cracks or spalls.
When strip shape deteriorates or surface marks appear, the work rolls may need to be changed even if the tonnage limit has not been reached.
9. Example Operating Data for 6 Hi Cold Rolling Mill
The table below provides a set of realistic reference parameters for different materials rolled on six-high mills. These are indicative values and must be adapted to specific equipment.
| Material / Product | Entry Thickness (mm) | Exit Thickness (mm) | Total Reduction (%) | Number of Passes | Max Speed (m/min) |
|---|---|---|---|---|---|
| Low carbon steel sheet | 2.0 | 0.40 | 80% | 4–5 | 700–900 |
| Deep-drawing IF steel | 1.8 | 0.30 | 83.3% | 4–6 | 800–1,000 |
| Stainless steel strip (304) | 3.0 | 0.80 | 73.3% | 5–7 | 400–700 |
| Aluminum strip | 1.2 | 0.20 | 83.3% | 3–4 | 1,000–1,600 |
| Copper strip | 1.0 | 0.10 | 90% | 4–6 (with intermediate annealing if needed) | 600–900 |
These figures highlight the broad capability of the six-high configuration for various metals. In many plants, a single 6 hi cold rolling mill line is dedicated either to carbon steel, stainless, or non-ferrous to optimize roll materials and emulsion chemistry.
10. Avoiding Quality and Safety Incidents
Ensuring safe and stable production on a 6 hi cold rolling mill requires strict operational discipline. Some key rules include:
- Verify coil identification and process card before start; do not roll wrong grade or thickness.
- Do not exceed maximum permissible rolling force, tension, or speed specified for each product.
- Keep stable strip speed during steady rolling; avoid frequent acceleration/deceleration which may cause strip wandering or poor flatness.
- Use appropriate roll profile and bending strategy when changing from wide to narrow coils or vice versa.
- Stop immediately if abnormal vibration, noise, or surface defects appear and cannot be corrected promptly.
- Record and analyze every significant incident (strip break, clogging of coolant nozzles, roll spalling) to prevent recurrence.
11. Long-Tail Application Scenarios for 6 Hi Cold Rolling Mills
In addition to standard automotive and appliance sheet production, six-high mills are increasingly used in specialized applications:
- Ultra-thin stainless steel foil production (0.05 – 0.15 mm) with tight flatness requirements for heat exchangers and flexible pipes.
- Battery foil and electrode materials for lithium-ion batteries, where thickness uniformity directly affects battery performance.
- Precision copper and brass strip for connectors, electronics, and lead frames.
- High-strength steel strip for structural components, where controlled work hardening during cold rolling is critical.
For these segments, the advanced shape control and lower rolling force requirement per unit reduction of a six-high mill are particularly advantageous, supporting tighter tolerances and higher yield.
12. Summary
A 6 hi cold rolling mill combines high precision, high productivity, and excellent shape control, making it an ideal choice for modern strip and foil production lines.
By following systematic operation procedures—pre-shift inspection, accurate setup according to product cards, scientifically designed pass schedules, careful tension and emulsion control, and rigorous surface and shape monitoring—plants can achieve:
- Stable production of thin and ultra-thin strip with tight gauge tolerances.
- High flatness quality suitable for demanding downstream processes.
- Improved roll life and reduced maintenance downtime.
- Lower scrap rates and better coil output per campaign.
For equipment selection, process optimization, or troubleshooting of an existing 6 hi cold rolling mill line, collecting accurate operating data and combining it with sound metallurgical and mechanical knowledge is essential.
The guidelines and typical parameters in this application guide provide a practical reference framework that can be further refined based on each plant’s specific conditions and product portfolio.
