Technical Overview: CuSn7Zn4Pb7-C-GZ Flange Bronze Bushing for HSM Work Roll Chocks

In a Hot Skin Pass Mill (also known as a temper mill), the Work Roll Chocks are subjected to massive, pulsating radial forces, heavy impact loads during strip entry, and continuous exposure to cooling water, scale, and high ambient temperatures.

Selecting CuSn7Zn4Pb7-C-GZ (Continuous Cast Leaded Bronze / RG7 equivalent) with solid lubricant integration offers an optimized balance of low friction, wear resistance, and emergency running properties.

The Severe Tribological Environment of HSM Work Roll Chocks

Before examining the material characteristics of CuSn7Zn4Pb7, it is critical to quantify the stress environment inside a hot skin pass mill roll chock:

• Dynamic Impact Loads: As the leading edge of a thick hot-rolled strip enters the roll bite, a severe shock load propagates directly from the work rolls into the chocks.
• Abrasive Fluid Ingress: Continuous spraying of high-pressure cooling water combined with airborne iron oxide scale creates a highly abrasive slurry that easily bypasses secondary seals.
• Boundary Lubrication Conditions: The oscillation speeds and high specific pressures mean that bearings frequently transition from hydrodynamic lubrication to boundary lubrication, risking metal-to-metal contact.

Material Architecture: CuSn7Zn4Pb7-C-GZ (CC493K)

CuSn7Zn4Pb7 is a multi-component leaded tin bronze standardized under EN 1982. It strikes an optimal metallurgical compromise between mechanical strength, wear compliance, and anti-seizure properties.
Internationally, it is widely cross-referenced with ASTM B505 / UNS C93200 (SAE 660) and JIS H5121 (CAC406C).

Chemical Composition Analysis

The performance profile of this alloy is directly driven by its elemental distribution:

• Tin (Sn, 6.0% – 8.0%): Forms a solid solution with copper, providing the fundamental structural hardness and tensile strength necessary to resist deformation under rolling forces.
• Lead (Pb, 5.0% – 8.0%): Lead is virtually insoluble in the copper matrix. During solidification, it segregates into microscopic globules uniformly distributed across the microstructure. If the primary oil or grease film
  fails, these lead globules smear across the mating surface, providing an immediate, emergency dry lubrication barrier that prevents catastrophic shaft welding (seizure).
• Zinc (Zn, 2.0% – 5.0%): Acts as a deoxidizer during casting, improves fluid castability, and refines the grain boundary structure to handle high-frequency mechanical vibration.

Engineering Insight: Although aluminum bronze alloys such as C95400 provide higher tensile strength (≥ 550 MPa), they offer limited embeddability. In harsh HSM environments contaminated with iron scale, aluminum bronze tends to trap abrasive particles, which can damage and score the roll neck surfaces. In contrast, CuSn7Zn4Pb7 delivers superior embeddability. Its softer, more compliant matrix allows fine debris particles to become embedded within the bushing material, helping protect the costly roll shaft from wear and scoring.

Material Selection — The Engineering Decision

Primary Alloy Options

Which one for yourapplication?

Practical rule of thumb: For Skin Pass Mill work-roll chock auxiliary pivots — C93200 (SAE 660)​ is the default “safe” choice because its lead content gives natural anti-galling even when lubrication temporarily fails. If the mill has chronic wash-out problems at that point, upgrade to graphite-plugged aluminum bronze (C954 base)​ for oilless reliability.

Flange Design and Geometry Optimization for Roll Chocks

In work roll chock assemblies, flanged bushings are the preferred configuration due to their ability to manage both axial loads and positional stability. The integrated flange performs two critical functions. First, it acts as a thrust surface that absorbs axial movement generated by strip steering forces and uneven rolling tensions. Second, it mechanically secures the bushing inside the chock housing, preventing axial movement or “walking” during high-vibration skin pass mill operations.

Precision Clearance Control to Reduce Roll Chatter

Roll chatter is one of the most serious issues affecting strip surface quality and dimensional accuracy. To maintain stable rolling performance, engineers apply strict tolerance control between the housing, bushing, and roll neck.

Typically, the housing bore is machined to an H7 tolerance, while the bushing outer diameter is produced with a p6 or s6 interference fit to ensure secure retention. The running clearance between the bushing inner diameter and the roll neck is commonly controlled within e7 or f7 tolerances. This optimized fit improves system rigidity while minimizing the risk of thermal seizure during operation.

Extending Service Life: Graphite Plugs vs. Engineered Oil Grooves

CuSn7Zn4Pb7 flanged bronze bushings can be customized with different lubrication systems depending on the rolling mill’s maintenance strategy and operating conditions.

1. Graphite Plugged Self-Lubricating Bushings

For applications where conventional grease lubrication is unreliable, the bushing can be manufactured with strategically positioned solid lubricant plugs made from graphite or graphite-MoS₂ compounds. During operation, these plugs release a thin lubricating film onto the shaft surface, enabling low-friction performance without continuous grease supply.

This self-lubricating design is especially effective in hot skin pass mills, where elevated temperatures can cause traditional grease to dry out, carbonize, or block lubrication channels.

2. Double-Loop (“8”-Shape) Oil Groove Design

For systems equipped with centralized lubrication units, engineered oil groove patterns are recommended. Double-loop or “8”-shaped grooves distribute lubricant evenly across the entire 360° bearing surface, ensuring consistent lubrication under heavy loads.

In addition to lubricant distribution, these groove channels also help remove contaminants such as cooling water, iron scale, and debris that may pass through the sealing system, improving operational reliability and reducing wear.

Conclusion: A Material Choice Focused on Operational Reliability

For steel mill operators and metallurgical engineers, selecting the correct bearing material is essential for maintaining production stability and protecting expensive roll assemblies. CuSn7Zn4Pb7-C-GZ offers an ideal balance of embeddability, thermal conductivity, anti-seizure performance, and load-carrying capability required for demanding Hot Skin Pass Mill work roll chock applications.

By using precision-machined, centrifugally cast flanged bronze bushings, mills can reduce unplanned downtime, improve strip surface consistency, and lower overall maintenance and operating costs throughout the rolling system lifecycle.