Roll Cracking Causes Guide

1. Introduction

In modern steel manufacturing, the stability and durability of rolling mill rolls directly determine production efficiency, product quality, and operational costs. However, many cold rolling mills frequently experience roll cracking, roll bursting, and surface spalling, which result in unexpected shutdowns, increased maintenance costs, and delivery delays.

This article provides a comprehensive analysis of roll cracking causes in cold rolling mills, based on real production data from a six-high reversible rolling mill. It also proposes practical and proven prevention strategies to enhance roll life and improve production stability.

2. Roll Bursting and Cracking Phenomena

During cold rolling operations, roll failures can occur in multiple stages:

2.1 Roll Failure During Production

Unexpected roll bursting may occur during normal rolling, often accompanied by:

• Sudden strip breakage

• Loud abnormal noise

• Instant mill shutdown

Key characteristics include:

• Severe cracking of the roll body

• Large-area surface spalling

• Significant damage to intermediate rolls

• Secondary damage to work rolls

This leads to:

• Material waste

• Coil rejection

• Production interruption

2.2 Roll Failure During Replacement or Storage

Roll bursting is not limited to operation. It may also occur:

• During roll changing

• Immediately after replacement

• During storage or handling

In severe cases:

• Roll fragments may eject several meters

• Safety risks increase significantly

• Rolls become completely unusable

3. Root Cause Analysis of Roll Cracking

3.1 Stress Analysis of Rolling Mill Rolls

(1) Bending Stress

Rolls are subjected to significant bending forces:

• Rolling force up to 10 MN

• Positive bending force: 300 kN

• Negative bending force: 200 kN

Although loads are generally evenly distributed, problems arise when:

• Roll shifting distance is excessive

• Local load concentration occurs

• Edge contamination accumulates

This leads to:

• Local stress concentration

• Crack initiation at contact zones

• Progressive surface spalling

(2) Fatigue Stress

During each rotation:

• Rolls experience alternating tensile and compressive stress

• Stress concentrates in the roll core and surface defects

Over time:

• Microcracks form at inclusions

• Cracks propagate along stress directions

• Surface peeling eventually occurs

This is a major contributor to roll fatigue failure.

(3) Thermal Shock Fatigue

Rolls continuously cycle between:

• High-temperature deformation zones

• Low-temperature cooling zones

This results in:

• Repeated thermal expansion and contraction

• Surface microcracks

• Accelerated crack growth

Thermal fatigue is a critical factor in roll spalling in steel rolling.

3.2 Process-Related Causes

(1) Insufficient Cooling

Field inspections revealed:

• Uneven emulsion spray distribution

• Blocked nozzles

• Inconsistent pressure

Consequences:

• Roll temperatures exceeding 300°C

• Thermal stress imbalance

• Axial crack formation

Additional issues include:

• Low emulsion concentration

• Poor cleanliness

• Improper temperature control

(2) Rolling Process Slippage

Improper rolling parameters may cause:

• Slippage between strip and roll surface

• Increased friction and vibration

Effects:

• Rapid temperature rise

• Surface damage

• Crack initiation

3.3 Production Accidents

Approximately 90% of roll bursting incidents are associated with strip breakage.

When strip breakage occurs:

• Sudden thermal shock damages rolls

• Steel adhesion causes surface indentation

• High-speed impact generates extreme stress

If not corrected:

• Defects transfer between rolls

• Cracks expand rapidly

• Catastrophic roll failure occurs

3.4 Roll Quality and Maintenance Issues

(1) Improper Hardness Matching

Typical hardness configuration:

• Work roll: 90–95 HSD

• Intermediate roll: 75–80 HSD

• Backup roll: 60–65 HSD

Problems occur when:

• Hardness mismatch exists

• Hardness changes during service

This leads to:

• Uneven wear

• Stress concentration

• Induced cracking in adjacent rolls

(2) Inadequate Grinding and Maintenance

If grinding is insufficient:

• Fatigue layers remain

• Microcracks are not removed

Under cyclic stress:

• Cracks propagate rapidly

• Surface spalling occurs

4. Effective Prevention Measures

4.1 Optimize Roll Grinding and Inspection

• Establish strict grinding standards

• Increase grinding allowance for damaged rolls

• Perform non-destructive testing (NDT) after grinding

• Ensure complete removal of microcracks

4.2 Ensure Proper Roll Matching

• Match rolls by diameter, hardness, and service life

• Ensure compatibility between work, intermediate, and backup rolls

• Maintain bearing stability for backup rolls

4.3 Improve Emulsion System Management

• Maintain optimal concentration and cleanliness

• Prevent nozzle blockage

• Monitor system performance in real time

• Avoid contamination from hydraulic oils

4.4 Implement Scientific Roll Changing Cycles

Typical reference:

• Work rolls: 30 passes / 100 tons

• Intermediate rolls: 120 passes / 400 tons

• Backup rolls: 670 passes / 2000 tons

Replace rolls immediately when:

• Vibration marks appear

• Surface defects are detected

4.5 Apply Roll Preheating System

• Preheating time: 30–40 minutes

• Pressure stabilization: 4–5 minutes

Recommended emulsion temperatures:

• Summer: 45–50°C

• Winter: 50–55°C

Benefits:

• Reduces thermal shock

• Improves roll stability

4.6 Optimize Rolling Process Parameters

Adjust rolling strategies based on:

• Product thickness

• Material properties

• Customer requirements

Key methods:

• High-speed rolling

• High-reduction rolling

• High-tension rolling

4.7 Establish Integrated Coordination Mechanism

• Real-time communication between departments

• Data sharing between rolling, maintenance, and repair teams

• Rapid response to upstream and downstream changes

5. Conclusion

Roll cracking and bursting in cold rolling mills are complex issues caused by a combination of mechanical stress, thermal fatigue, process instability, and maintenance deficiencies.

By implementing:

• Proper roll maintenance

• Optimized cooling systems

• Scientific rolling parameters

• Effective operational coordination

Steel plants can significantly:

• Extend roll service life

• Reduce downtime

• Improve product quality

• Lower overall production costs