Introduction to 4 Hi Cold Rolling Mill

A 4 hi cold rolling mill is one of the most widely used configurations in strip and sheet rolling for steel, stainless steel, aluminum, copper and other non‑ferrous metals. It is designed to achieve precise thickness reduction, excellent surface quality and high strip flatness at relatively high speeds. Because of its versatility and compact structure, the 4 hi mill is commonly selected for medium and precision cold rolling, as well as for pilot and production lines in metal service centers, rolling plants and downstream processing facilities.

This article provides a detailed, engineering‑oriented introduction to the 4 hi cold rolling mill, including working principle, mill configuration, layout, core components, typical technical parameters, application ranges and key design/selection considerations. The intention is to offer real production reference for engineers, technicians and decision‑makers planning or operating a 4 high cold rolling mill line.

1. What Is a 4 Hi Cold Rolling Mill?

A 4 hi cold rolling mill (4‑high cold rolling mill) is a rolling stand with four rolls arranged vertically:

• Two smaller work rolls in the middle which directly contact and deform the strip.
• Two larger backup rolls located above and below the work rolls to provide bending stiffness and reduce roll deflection.

Compared with a 2 hi cold rolling mill, the 4 hi configuration allows the work rolls to be significantly smaller in diameter. Small work rolls reduce the rolling force required to achieve a given reduction, improve thickness control, and enable rolling to much thinner gauges without excessive roll bending. Backup rolls support the work rolls, preventing them from flattening under the high rolling load.

Depending on the overall mill layout and coil handling scheme, a 4 hi cold rolling mill can be:

• Reversible (single-stand) 4 hi cold rolling mill: strip passes back and forth through the same stand; direction is reversed after each pass.
• Tandem 4 hi cold rolling mill: several 4 hi stands arranged in series to perform multiple reductions in a single pass (often integrated with pickling for continuous pickling and rolling).

In practical engineering documents, you may also see the 4 hi mill described as:

• 4‑high reversing cold rolling mill
• 4 hi reversible cold mill
• 4 hi strip cold rolling line

2. Typical Applications of 4 Hi Cold Rolling Mills

A 4 hi cold rolling mill is commonly used for medium and precision cold rolling of strip and coil in the thickness range from about 0.05 mm to 3.0 mm (depending on material, stand size and installed power). Typical applications include:

• Cold rolling of low carbon steel and carbon structural steel for general sheet, automotive, appliance and building materials.
• Rolling of alloy steel and high‑strength low‑alloy (HSLA) steel for structural and automotive applications.
• Cold rolling of stainless steel strip (e.g. 304, 316L, 430) for kitchenware, chemical, medical and architectural products.
• Non‑ferrous metals such as aluminum strip and sheet, copper strip, brass and nickel alloys.
• Rolling of precision strip used in electronics, battery casing, heat exchangers and other high value applications.

In many plants, the 4 hi cold rolling mill is used for:

• Intermediate rolling after hot rolling and pickling, to bring the strip to a medium gauge.
• Finish rolling to final gauge for certain products where an ultra‑high precision 6‑hi or 20‑hi mill is not required.

3. Main Components and Layout of a 4 Hi Cold Rolling Mill

A complete 4 hi cold rolling mill line typically consists of the following subsystems:

3.1 Pay‑off and Recoiling Section

The coil handling equipment at the entry and exit side includes:

• Uncoiler (pay‑off reel) – feeds the hot‑rolled or intermediate cold‑rolled coil into the mill.
• Recoiler (coiler) – rewinds the strip into a tight coil after rolling.
• Hydraulic expanding mandrel – adapts to different inner coil diameters and ensures reliable clamping.
• Strip threading and guiding devices – entry pinch roll, steering guides, side guides and threading table.

For higher automation, coil cars, coil upenders, banding machines and coil weighing devices are normally added.

3.2 4 Hi Mill Stand

The mill stand is the core of the 4 hi cold rolling mill and consists of:

• Two work rolls (upper and lower), diameter typically 80–160 mm for small and medium mills.
• Two backup rolls, diameter typically 320–460 mm or larger, depending on mill size.
• Chocks and bearings designed for high radial load and high speed.
• Housing and window which carry the vertical rolling force.
• Roll gap adjustment system, usually hydraulic screwdown with automatic gauge control (AGC).
• Hydraulic roll bending and shifting (optional) to improve strip flatness and crown control.

Work rolls are often made from high‑quality alloy steel such as 86CrMo7 or other Cr‑Mo bearing grades, which offer high hardness, good wear resistance and resistance to thermal fatigue. Backup rolls are usually forged alloy steel with tough core and hardened surface.

3.3 Drive System

The drive system of a 4 hi cold rolling mill includes:

• Main motor – DC motor or AC vector motor with power typically from 100 kW up to several MW depending on mill size.
• Reduction gearbox – transfers torque to the rolls and sets speed ranges.
• Spindle and universal joint couplings – cross‑type universal shafts are widely used to connect gearbox and roll necks.
• Pinion stand or herringbone gear housing – splits torque symmetrically to upper and lower work rolls.

For precise thickness control and efficiency, the drive system must provide stable torque, wide speed range, fast acceleration and deceleration, as well as accurate speed regulation combined with tension control.

3.4 Hydraulic and Lubrication System

The hydraulic system serves multiple functions:

• Roll gap adjustment (AGC hydraulic screwdown).
• Backup roll and work roll bending (if equipped).
• Coil clamping and mandrel expansion at uncoiler/recoiler.
• Hydraulic actuators for pinch rolls, side guides, entry/exit threading devices.

Separate lubrication systems are provided for bearings, gears and hydraulic components, usually with online filtration and temperature control to ensure reliable operation.

3.5 Electrical and Automation System

Modern 4 hi cold rolling mills are equipped with:

• Digital main drive control – DC or AC drive with fieldbus or Ethernet communication.
• Automatic Gauge Control (AGC) – adjusts roll gap and/or speed to maintain strip thickness tolerance.
• Tension control – controls entry and exit strip tensions via uncoiler and recoiler drives.
• Level 1 PLC control – logic, interlocks, safety, and basic sequencing.
• Level 2 process control (for advanced lines) – setup calculation, pass schedule optimization and data logging.
• Thickness gauges – usually X‑ray or isotope gauges for continuous measurement.
• Flatness measuring rolls – used in precision strip rolling to monitor shape and flatness.

4. Working Principle of a 4 Hi Cold Rolling Mill

The core of cold rolling is plastic deformation of the strip under compressive stress between rotating rolls. In a 4 hi cold rolling mill:

1. The strip is threaded from the uncoiler through entry guides and into the roll bite between the work rolls.
2. The work rolls, driven by the main motor, pull the strip through the roll gap at a controlled speed, simultaneously applying compressive force normal to the strip.
3. Under this force, the strip thickness decreases, while its length and width increase slightly (within control limits).
4. Backup rolls provide support to the work rolls, greatly restrict their deflection and help maintain uniform thickness across the strip width.
5. The reduced strip exits the stand and is wound onto the recoiler under controlled tension.

Process lubrication (rolling oil or water‑based coolant) is sprayed onto the roll–strip contact zone to reduce friction, remove heat and improve surface finish. Friction conditions, together with roll diameter, strip strength and reduction, determine the rolling force and torque.

5. Technical Parameters of a Typical 4 Hi Cold Rolling Mill

Technical parameters of a 4 hi cold rolling mill vary depending on material, width, target thickness and production capacity. The following table summarizes a set of representative parameters (for reference only, not tied to any specific manufacturer).

Parameters like roll diameters, rolling forces and motor power are interrelated. For example, higher rolling force capacity allows higher reductions per pass, but also requires stronger mill housings, larger hydraulic cylinders and more robust foundations.

6. Process Flow in a 4 Hi Reversible Cold Rolling Mill

A typical process flow in a single‑stand reversible 4 hi cold rolling mill is as follows:

1. Entry coil preparation: hot‑rolled pickled coil is inspected, end cropped and welded if necessary.
2. Loading on uncoiler: coil is loaded via coil car, mandrel expands to grip the coil ID.
3. Threading: strip head is manually or automatically threaded through entry guides and mill stand to the exit side and attached to the recoiler mandrel.
4. Pass 1 (forward direction): rolling begins with defined reduction, entry and exit tensions, and speed ramp; AGC and tension control are activated.
5. Direction reversal: once the coil is fully transferred onto the recoiler, the mill stops, roll gap and speed are adjusted, and the rolling direction is reversed.
6. Pass 2 (reverse direction): strip is rolled back through the stand with further reduction; this continues for several passes until the target thickness is achieved according to the rolling schedule.
7. Final recoiling: after the last pass, the finished strip remains on the designated recoiler and is banded and removed.
8. Offline processes: annealing, skin‑passing, tension leveling, slitting or cutting to length, depending on the final product requirements.

For higher productivity, multi‑stand tandem 4 hi cold rolling mills can reduce the strip from hot‑band gauge to final gauge in a single pass without reversing, but they require more complex control, higher investment and longer line length.

7. Advantages of 4 Hi Cold Rolling Mills

Compared with 2 hi mills and some other configurations, the 4 hi cold rolling mill offers several significant advantages:

• Higher gauge reduction capability – smaller work rolls reduce the required rolling force, allowing greater total reduction.
• Improved thickness accuracy – reduced work roll deflection and AGC systems enable tight gauge tolerances.
• Better flatness and strip shape – backup rolls and optional roll bending help control crown and edge waves.
• Versatility – suitable for a variety of materials (carbon steel, stainless steel, aluminum, copper, etc.) and a wide thickness range.
• Reasonable investment level – compared with 6‑hi or 20‑hi mills, the 4 hi mill has simpler structure and lower installation cost, while still meeting many precision requirements.
• Compact footprint – especially in reversible configuration, a single 4 hi stand can serve multiple product grades with limited building space.

8. 4 Hi Cold Rolling Mill vs. 6 Hi and 20 Hi Mills

When selecting a cold rolling mill type, it is common to compare 4 hi, 6 hi and 20 hi (cluster) mills. The choice depends mainly on required thickness, strip width, material strength and flatness tolerance.

For many service centers and integrated mills, a 4 hi cold rolling mill is an optimal compromise between flexibility, investment and achievable quality, especially when combined with appropriate annealing and finishing lines.

9. Key Design Considerations for a 4 Hi Cold Rolling Mill

When designing or selecting a 4 hi cold rolling mill, engineers must consider a range of mechanical, electrical and process factors. Some of the most important are summarized below.

9.1 Mill Strength and Stiffness

The mill housing, tie rods, screws and chocks must be sized for the maximum rolling force, with adequate stiffness to limit elastic deflection. Excessive mill stretch leads directly to thickness variation and makes AGC less effective. Finite element analysis (FEA) is often used to optimize housing design.

9.2 Roll Diameter and Material

Work roll diameter determines, to a large extent, the rolling force and bite angle:

• Smaller diameter reduces rolling force and improves reduction capability but increases roll deflection.
• Larger diameter improves stiffness and surface life but requires more force for the same reduction.

Materials like 86CrMo7 or other high‑alloy forged steel grades are chosen for their hardness (often 60–65 HRC at the surface), resistance to wear and good toughness. Heat treatment must be controlled to achieve a strong core while maintaining surface hardness.

9.3 Automation Level and Control Precision

For modern 4 hi cold rolling mill lines, automation level is a critical factor:

• AGC type – hydraulic AGC (HAGC), electric screwdown or a combination, with feed‑forward and feedback loops.
• Tension control accuracy – especially important for thin strip and high‑strength materials.
• Shape control – roll bending, coolant pattern control and possibly automatic flatness control based on measurement feedback.
• Data acquisition and diagnostic functions – for process optimization and predictive maintenance.

9.4 Rolling Oil and Coolant System

Cold rolling generates considerable heat due to plastic deformation and friction. A well‑designed rolling oil/coolant system is essential to:

• Maintain roll and strip surface temperature within target range.
• Ensure stable friction conditions for consistent reduction and surface finish.
• Provide adequate filtration to keep solid contaminants below acceptable limits (often <10 µm).

The composition of rolling oil (base oil plus additives) is chosen according to material and desired surface quality.

9.5 Safety and Maintenance

Safety features for a 4 hi cold rolling mill line normally include:

• Emergency stop circuits covering all critical zones.
• Interlocks on coil cars, mandrel expansion, threading devices and doors.
• Overload protection for drive, hydraulic and mechanical components.
• Fire detection and rolling oil fire suppression measures.

Ease of maintenance is also essential, including roll change systems, accessible lubrication points, removable covers and sufficient space around the stand for lifting operations.

10. Typical Product Examples and Process Capability

The following are some representative products that can be produced on a well‑equipped 4 hi cold rolling mill line.

• Cold rolled low carbon steel strip
  Input: hot rolled pickled strip 2.0 mm × 400 mm
  Output: 0.25 mm × 400 mm cold rolled strip
  Tolerance: ±0.003–0.005 mm (with AGC)
  Applications: household appliances, automotive parts, stamping.
• Stainless steel strip
  Input: hot rolled and annealed 1.8 mm × 300 mm
  Output: 0.15 mm × 300 mm after multi‑pass rolling and intermediate annealing
  Applications: kitchenware, flexible hoses, precision components.
• Aluminum strip
  Input: 1.5 mm × 500 mm cold rolled
  Output: 0.30 mm × 500 mm
  Applications: heat exchangers, building panels, packaging.
• Copper strip
  Input: 1.0 mm × 250 mm
  Output: 0.08 mm × 250 mm
  Applications: electrical components, connectors, electronic shielding.

Actual pass schedules, reduction per pass, rolling speeds and inter‑pass annealing cycles are determined based on material grade, mechanical properties, mill stiffness and customer specifications.

11. Long‑Tail Considerations: Choosing a 4 Hi Cold Rolling Mill for Your Plant

From a practical standpoint, when searching for or specifying a 4 hi cold rolling mill for stainless steel strip, 4 hi cold rolling mill for copper strip or a 4 hi reversible cold rolling mill for narrow coil, the following questions are important:

• What are the input and output thickness ranges and strip widths?
• What materials and grades will be rolled (e.g. low carbon, martensitic stainless, ferritic stainless, aluminum alloys)?
• What are the required thickness tolerances and flatness classes?
• Will the line be standalone, or integrated with pickling, annealing or skin‑pass mills?
• What is the target annual production capacity and coil size range (weight, OD, ID)?
• What level of automation (AGC, automatic pass schedule, remote diagnostics) is expected?

Answering these questions provides a basis for defining mill force capacity, roll size, motor power and auxiliary equipment, ensuring the selected 4 hi cold rolling mill can meet current requirements and allow for future product development.

12. Conclusion

The 4 hi cold rolling mill remains a key piece of equipment in modern strip and sheet production, combining robust mechanical design with increasing levels of automation and precision control. With properly configured work and backup rolls, sufficient rolling force and reliable AGC and tension control, a 4 hi mill can deliver high‑quality cold rolled products across a wide range of materials and dimensions.

For engineers and plant planners, understanding the structure, working principle and technical parameters of a 4 hi cold rolling mill is essential when designing new lines, upgrading existing facilities or optimizing pass schedules and maintenance strategies. By aligning mill capabilities with target product mix and quality requirements, it is possible to achieve both stable production and competitive operating costs in cold strip rolling.