Basic Knowledge of 4 Hi Cold Rolling Mill

A 4 hi cold rolling mill is one of the most widely used configurations in strip and sheet rolling, especially for carbon steel, stainless steel, copper and aluminum alloys. Understanding its structure, working principle, process parameters and typical application ranges is essential for engineers who are involved in manufacturing, equipment selection, process optimization or maintenance of cold rolling lines.

This article introduces the basic knowledge of 4 hi cold rolling mills from a practical and engineering perspective. It covers the mill layout, main components, rolling theory, pass schedule design, typical parameters and real-world configuration examples. Long-tail topics such as “4 hi cold rolling mill for stainless steel strip”, “4 hi reversing cold mill pass schedule” and “AGC and roll bending in 4 high mills” are also discussed for deeper process reference.

1. What Is a 4 Hi Cold Rolling Mill?

A 4 hi (4-high) cold rolling mill is a type of rolling mill consisting of two smaller-diameter work rolls and two larger-diameter backup rolls. The strip is reduced in thickness as it passes between the two work rolls, while the backup rolls support the work rolls and limit their elastic deflection. This configuration allows higher rolling forces, improved shape control and better dimensional accuracy compared with a 2-hi mill.

In most cases, a 4 hi cold rolling mill is used after hot rolling and pickling. The feedstock is usually hot-rolled coil that has been pickled to remove scale, sometimes annealed depending on the grade and final product requirements.

1.1 Basic Configuration

A typical single-stand 4 hi cold rolling mill includes:

• Two work rolls (small diameter, in direct contact with the strip)
• Two backup rolls (large diameter, supporting the work rolls)
• Mill housing/frame and chocks
• Roll gap adjustment system (screw-down or hydraulic capsules)
• Drive system (main motors, gearboxes, spindles or universal joints)
• Strip threading and guiding equipment (entry and delivery guides, pinch rolls)
• Thickness control system (AGC – Automatic Gauge Control)
• Flatness and shape control systems (roll bending, work roll shifting, coolant distribution)
• Coolant and lubrication system (emulsion or rolling oil)
• Fume extraction and safety interlocks

4 hi mills can be configured as reversing mills (the strip passes multiple times back and forth through the same stand) or as tandem mills (multiple 4 hi stands arranged in series to reduce thickness continuously).

1.2 Typical Application Range

Depending on the design, a 4 hi cold rolling mill can handle a wide variety of metals, strip widths and thicknesses. The following table shows typical ranges for common industrial 4 hi mills used in steel service centers and integrated plants.

These values are representative and vary with design capacity, installed power, strip grade and mill stiffness. When specifying a 4 hi reversing cold mill or a 4 hi tandem cold mill, engineers usually define these ranges clearly in the technical specification.

2. Main Components and Design Features

The performance of a 4 hi cold rolling mill strongly depends on detailed mechanical design of the roll system, housing stiffness, drive power and hydraulic systems. This section outlines key components and typical engineering values used in practice.

2.1 Work Rolls and Backup Rolls

Work rolls are the main deformation tools and directly contact the strip. They have relatively small diameter to improve biting conditions and reduce rolling force. Backup rolls have much larger diameter; their function is to support the work rolls and control roll bending.

Rolls are ground periodically to maintain profile and surface quality. In high-precision 4 hi cold rolling mills, CVC (Continuously Variable Crown) or other contour designs may be applied on work rolls to assist shape control.

2.2 Mill Housing and Screw-down System

The mill housing needs sufficient rigidity to withstand the extremely high rolling forces encountered in cold reduction. For a medium-gauge 4 high mill, maximum rolling force can reach 10–18 MN per stand (1–1.8 × 10⁶ kgf).

There are two main types of roll gap adjustment (screw-down) in modern 4 hi cold rolling mills:

• Mechanical screw-down: driven by large screw jacks and electric motors; traditional but robust.
• Hydraulic screw-down (HGC capsules): provides fast response, high sensitivity and is integrated with AGC for closed-loop thickness control.

In newer mills, hydraulic gap control (HGC) cylinders are placed above or below the mill stand. Position sensors (LVDT or similar) monitor the gap and allow automatic corrections in milliseconds based on entry thickness measurement and load cells.

2.3 Roll Bending and Shape Control

4 hi cold rolling mills often incorporate hydraulic positive and negative work roll bending systems. These systems apply controlled forces at roll ends to adjust roll gap profile and thus influence strip flatness.

Typical bending force ranges for a medium-width 4 hi mill:

• Work roll bending force: 400 – 1,000 kN per side
• Backup roll bending (if available): 800 – 2,000 kN per side

Combined with tapered rolls, CVC rolls or roll shifting, these bending systems allow the operation to compensate for thermal crown growth, wear and load distribution during long rolling campaigns.

2.4 Drive System and Installed Power

The main drive of a 4 hi cold rolling mill is sized according to maximum rolling force, strip speed, strip width and reduction schedule. Modern mills typically use AC motors with vector control and high-power drives (inverters), whereas older mills may still operate with DC motors.

The mill stand can be driven by either the work rolls only or by both work and backup rolls depending on design. Universal spindles and robust gearboxes are required to transmit torque reliably under fluctuating loads during cold reduction.

3. Working Principle of a 4 Hi Cold Rolling Mill

Cold rolling is a plastic deformation process conducted below the recrystallization temperature of the metal. In a 4 hi cold rolling mill, the strip is compressed between two rotating work rolls, reducing its thickness and strengthening the material by work hardening.

3.1 Deformation and Load Distribution

As the strip enters the roll gap, friction between the work rolls and strip surface pulls the material into the bite. The compressive stress generated in the roll gap exceeds the yield strength of the metal, causing plastic deformation. The elastic deformation of the rolls and stand leads to an overall mill “spring” that must be accounted for when setting the roll gap.

Key variables that determine rolling force (F) include:

• Entry thickness (h₀) and exit thickness (h₁)
• Strip width (b)
• Flow stress of the material (function of composition, prior processing, temperature and strain rate)
• Friction coefficient between rolls and strip
• Roll diameter and contact length

Engineering models (such as the Sims model or Orowan theory) are used to estimate rolling force and torque during pass schedule design. These models help ensure that the required reductions remain within installed power and stiffness limits of the 4 hi mill.

3.2 Thickness Reduction and Pass Schedule

The total reduction from hot band to final cold strip is typically between 50% and 80%, depending on product and process route. In a 4 hi reversing cold mill, this total reduction is achieved through multiple passes. Each pass has a certain percentage reduction based on mill power, material grade and desired final properties.

Typical single-pass reductions for low-carbon steel in a 4 hi reversing mill:

• Roughing passes: 25 – 40%
• Intermediate passes: 15 – 30%
• Finishing passes: 10 – 25% (to maintain surface and shape quality)

High-strength steels and stainless steels usually require more passes with lower reduction per pass due to higher flow stress and more pronounced work hardening.

3.3 Example Pass Schedule in a 4 Hi Reversing Cold Mill

The following example illustrates a pass schedule for low-carbon steel strip in a 4 hi reversing cold rolling mill. The actual values will be optimized in practice based on mill capability and product requirements, but the example provides a realistic engineering reference.

This example shows how thickness decreases and strip speed increases pass by pass. Tension of entry and exit coilers is controlled to maintain strip stability and thickness uniformity.

4. Hot Rolling vs. Cold Rolling in a 4 Hi Mill

Strictly speaking, a 4 hi cold rolling mill is designed for cold rolling at or near room temperature. However, with specific modifications some mills can perform warm rolling or even hot rolling within a limited temperature range.

4.1 Why Standard 4 Hi Mills Are for Cold Rolling

Conventional 4 hi cold rolling mills are built with:

• Work rolls and backup rolls optimized for low-temperature lubrication with water-based emulsions or rolling oils
• Housings, seals and hydraulic systems designed for moderate thermal loads
• Surface finish requirements suitable for cold rolled products

At hot rolling temperatures (800–1,000 °C for steel), the lubrication, roll materials and thermal expansion characteristics are completely different. Therefore, a standard cold mill cannot be directly used as a hot rolling mill without major modifications.

4.2 Modified 4 Hi Mills for Warm Rolling

For special applications, some 4 hi mills are designed for warm rolling (typically 200–600 °C). In such cases:

• Roll materials are selected for elevated-temperature operation
• Heat-resistant lubrication and cooling systems are used
• Strip heating furnaces or induction heaters are installed upstream
• Thermal crown compensation becomes more critical

However, this type of installation is less common than standard cold rolling lines, and detailed engineering evaluation is required before using a 4 hi configuration for warm or hot rolling.

5. Control Systems: AGC, Tension and Shape Control

One of the key advantages of modern 4 hi cold rolling mills is the high level of automation and process control. Automatic Gauge Control (AGC), tension control, roll bending and flatness control systems work together to ensure precise product dimensions and good strip shape.

5.1 Automatic Gauge Control (AGC)

AGC systems continuously adjust the roll gap during rolling to maintain the target strip thickness. Common AGC modes in 4 hi cold rolling mills include:

• Feed-forward AGC: uses entry thickness and mass flow conservation to predict required exit thickness and pre-adjust the gap.
• Feedback AGC: uses an X-ray or isotope thickness gauge at the exit to measure actual thickness and correct deviations.
• Mass flow AGC: combines roll speed and load measurements to infer thickness variations.

These systems are usually implemented using digital controllers and advanced control algorithms. In some installations, fuzzy control or model predictive control is used for improved dynamic response, especially in mills with hydraulic gap control and roll bending systems.

5.2 Tension Control

Entry and exit strip tensions are controlled by motor torque of coilers, payoff reels and bridle rolls. Proper tension helps stabilize the strip, improve thickness profile and prevent strip breaks.

Typical tension ranges for low-carbon steel strip in a 4 hi cold mill:

• Entry tension: 30 – 80 MPa
• Exit tension: 60 – 120 MPa

Tension is often set as a percentage of the material’s yield strength to avoid necking or strip breakage. Proper tension also reduces rolling force by adding longitudinal stress to the strip.

5.3 Flatness and Shape Measurement

A 4 hi cold rolling mill typically uses flatness measuring rolls or optical flatness sensors at the exit. These devices measure lengthwise strain distribution across the strip width and feed data to the shape control system. Based on measured shape errors (edge wave, center buckle, quarter buckle, etc.), the control system adjusts:

• Work roll bending forces
• Roll shifting (in mills equipped with CVC or shifting mechanisms)
• Coolant distribution across width

This closed-loop flatness control is particularly important for high value-added products such as automotive exposed panels, tinplate, electrical steel and stainless steel strip.

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

The following table summarizes key technical parameters for a typical 4 hi reversing cold rolling mill designed for carbon steel strip with maximum width of 1,250 mm. These values are representative and often used as reference when designing or selecting equipment.

When designing a cold rolling plant, parameters like these must be aligned with upstream hot strip mill capabilities and downstream annealing, pickling or galvanizing lines to achieve consistent production flow.

7. Types of 4 Hi Cold Rolling Mills

Although all 4 hi mills share the same basic four-roll configuration, different process arrangements are used depending on production volume, product mix and investment level. The main types are:

7.1 4 Hi Reversing Cold Rolling Mill

In a 4 hi reversing mill, the strip passes back and forth through the same stand multiple times. Entry and exit sides are equipped with coilers, and the direction of the strip is reversed after each pass.

Main characteristics:

• Flexible for small to medium production volumes
• Suitable for many grades, including stainless and specialty steels
• Simple layout and relatively moderate investment
• Lower productivity than multi-stand tandem mills

A reversing 4 hi cold rolling mill is often chosen by regional service centers or plants that produce diversified products in smaller lots.

7.2 4 Hi Tandem Cold Rolling Mill

A tandem cold mill typically consists of 4–6 stands of 4 hi mills arranged in series. The strip passes through all stands in a single direction, with reduction distributed across stands.

Main characteristics:

• High productivity (line speeds often above 1,500 m/min)
• Suitable for large production quantities of automotive, appliance and packaging steels
• Higher investment and more complex automation
• Typically integrated with pickling lines (PL-TCM or PL-TCM/TCL)

In tandem mills, interacting control loops coordinate speed, tension and reduction among stands to achieve target gauges and shape with high productivity.

7.3 4 Hi Cold Rolling Mill for Non-ferrous Metals

For copper, brass and aluminum strip, 4 hi cold rolling mills are also widely used. Although the principle is the same, there are some differences:

• Smaller work roll diameters for thin non-ferrous strip
• Different roll materials to handle soft metals and minimize sticking
• Rolling oils with specific additives for non-ferrous alloys
• Often integrated with annealing furnaces and slitting lines

As demand grows for high-precision copper and aluminum foil, some lines use a combination of 4 hi and 6 hi or 20 hi cluster mills to achieve ultra-thin gauges.

8. Process Route and Auxiliary Equipment

A 4 hi cold rolling mill is normally part of a complete processing route. To produce high-quality cold rolled products, auxiliary equipment and processes are required before and after the mill.

8.1 Typical Process Flow

A typical process route for cold rolled carbon steel strip using a 4 hi mill is:

1. Hot rolling (HR strip)
2. Pickling line (to remove scale and clean surface)
3. Edge trimming and coil inspection
4. 4 hi cold rolling mill (reversing or tandem)
5. Batch annealing or continuous annealing (for deep drawing grades)
6. Skin-pass mill / temper mill (light reduction to improve surface and shape)
7. Oiling, cutting and slitting lines
8. Packaging and shipping

For stainless steels, an additional bright annealing and pickling stage may be required. For galvanizing products, the cold strip is fed into a hot-dip galvanizing line or an electrolytic coating line after cold reduction and annealing.

8.2 Coolant and Filtration System

Coolant plays a dual role in a 4 hi cold rolling mill: it provides lubrication between rolls and strip, and removes heat generated by deformation and friction. Water-based emulsions are widely used for carbon steel, while rolling oils are often used for high surface quality products and non-ferrous metals.

A typical coolant system includes:

• Emulsion preparation and concentration control
• Filtration units (vacuum filters, magnetic separators, bag filters)
• Heat exchangers or cooling towers
• Circulation pumps and flow control valves

Proper management of coolant quality is crucial for roll life, strip surface cleanliness and environmental compliance.

9. Comparison of 4 Hi, 6 Hi and 20 Hi Cold Rolling Mills

When planning a cold rolling line, engineers often need to choose between different mill configurations. 4 hi mills occupy an important position between simpler 2 hi mills and cluster mills such as 6 hi and 20 hi designs.

In many plants, 4 hi cold rolling mills coexist with 6 hi or 20 hi mills, each serving different product ranges and final thicknesses.

10. Practical Considerations for Operating a 4 Hi Cold Rolling Mill

From an operational standpoint, stable and efficient performance of a 4 hi cold rolling mill depends on a combination of mechanical condition, roll management, process setup and operator skills.

10.1 Roll Management and Grinding

Key points in roll management include:

• Monitoring work roll wear and surface defects
• Planning roll campaigns based on tonnage per roll pair
• Grinding roll profile and crown with high precision
• Maintaining backup roll surface condition to prevent marking

In a typical 4 hi mill, work roll change may be required after several hundred to a few thousand tons depending on strip grade and reduction schedule. Backup rolls last significantly longer between grinding cycles.

10.2 Setup and Scheduling

Efficient scheduling of coils through a 4 hi cold rolling mill includes:

• Grouping coils by grade, thickness and width to reduce setup changes
• Planning pass schedules according to incoming hot band conditions
• Adjusting coolant and rolling oil parameters according to product
• Using mill models and databases to estimate rolling force and torque

For plants aiming to optimize yield and energy consumption, advanced scheduling and simulation tools are often integrated with the mill’s level 2 automation system.

10.3 Safety and Maintenance

Because 4 hi cold rolling mills involve high forces, high speed strip and heavy coils, safety is always a priority. Key aspects include:

• Regular inspection of mill stands, chocks and spindles for fatigue
• Monitoring hydraulic systems for leaks and pressure stability
• Ensuring interlocks and emergency stop systems are functional
• Training operators on threading procedures and strip break handling

Preventive maintenance plans, including vibration monitoring and periodic non-destructive testing of critical components, help avoid unexpected shutdowns and improve overall availability of the 4 hi mill.

11. How to Specify a 4 Hi Cold Rolling Mill for a New Project

For engineers involved in new plant design or upgrading existing facilities, a clear technical specification for a 4 hi cold rolling mill is essential. Typical items to define include:

• Target product range: materials (carbon steel, stainless, non-ferrous), width, thickness and grades
• Annual production capacity and coil size (weight, ID/OD)
• Required thickness tolerances and flatness index
• Desired automation level (AGC, AFC, roll bending, shape meter, coil tracking)
• Compatibility with existing upstream and downstream equipment
• Energy efficiency, water and oil consumption targets
• Safety standards and environmental regulations to be met

Based on these requirements, equipment suppliers can propose a 4 hi reversing cold mill or a 4 hi tandem mill with appropriate roll sizes, drive power, hydraulic systems and automation packages.

12. Summary

4 hi cold rolling mills are a core technology in the production of high-quality metal strip and sheet. By combining small-diameter work rolls with larger backup rolls, they achieve high rolling forces and excellent thickness and shape control. Through proper design of roll systems, housing stiffness, hydraulic gap control, AGC and flatness control, modern 4 hi mills can produce a wide variety of products—from low-carbon automotive sheet to stainless steel strip and non-ferrous alloys.

For process engineers and plant designers, understanding the basic configuration, technical parameters, rolling theory and control systems of 4 hi cold rolling mills provides a solid foundation for equipment selection, process optimization and long-term reliable operation. When integrated with upstream pickling lines and downstream annealing, temper rolling and slitting, a well-designed 4 hi cold rolling mill forms the heart of a flexible and efficient cold strip production line.