Key words：HSS High Speed Steel roller/work rolls/forged rolls /Infinite Chilled Cast Iron Roller/Graphite Steel Roller
Ⅰ. Causes of Roll Fracture and Failure
1. Manufacturing and Material Defects
(1) Metallurgical Inclusions: Large inclusions inside the roll disrupt the continuity of the metal matrix, becoming the source of fatigue cracks.
(2) Excessive Residual Austenite: In hot-rolled high-speed steel rolls, if the residual austenite content is too high, it will transform into martensite after being heated on the mill, generating huge structural stress and causing surface cracking.
(3) Casting Hot Spots: In thick areas such as the roll neck, the casting cooling rate is slow, forming a coarse "hot spot zone," reducing fatigue strength and making it prone to brittle fracture under impact loads.
2. Thermal Stress and Overload Fracture
(1) Abnormal Temperature Difference: This is a common cause of hot-rolled roll fracture. For example, during mill commissioning, an emergency stop and cooling water shutdown cause a rapid temperature rise on the roll surface (even resulting in needle-like martensite overheating), while the roll core temperature remains low. The enormous thermal stress (calculated value up to 335 MPa) exceeds the strength limit, leading to fracture.
(2) Mechanical Overload: During cold rolling, if the raw material wedge is too large or its hardness is uneven, it will cause uneven stress on both sides of the mill. Local stress will exceed the roll's strength limit, crushing the roll edge.
II. Preventive Measures
1. Source Control: Incoming Roll and Manufacturing Optimization
(1) Strict Flaw Detection: New rolls and repaired rolls undergo 100% ultrasonic and surface magnetic particle testing. Rolls with internal inclusions, shrinkage cavities, or "hot spots" are strictly rejected.
(2) Optimized Design: For easily broken roll neck areas, the transition fillet radius should be increased to avoid stress concentration caused by abrupt right-angle changes.
(3) Material Selection and Hardness Matching: Based on the mill type (cold rolling/hot rolling) and the type of steel being rolled, carefully select the roll material (e.g., 9Cr2Mo alloy steel for cold rolling work rolls, and high-nickel cast iron or high-speed steel for hot rolling). It is essential to ensure that the hardness of the roll working layer matches the rolling load—excessive hardness can lead to brittle fracture, while insufficient hardness results in inadequate wear resistance.
2. Process Monitoring: Rolling Technology and Operating Procedures
(1) Hot Rolling Cooling Management: It is strictly forbidden to immediately shut off the cooling water when the strip is jammed. Cooling should continue after shutdown, or the rolls should be raised to prevent localized overheating leading to thermal stress fracture.
(2) Cold Rolling Raw Material Monitoring: Strengthen the inspection of edge cracks and wedges in hot-rolled incoming materials to prevent sudden changes in rolling force due to raw material issues that could damage the rolls.
(3) Tension and Strip Shape Control: Optimize the coiler tension setting and bending roll force to prevent folding and crushing damage to the roll surface caused by tension loss or waviness (e.g., prevention of "diagonal lines" defects in the hot-rolled coiling section).
III. Conclusion
Roll management is a systematic project. Fractures often originate from thermal stress or manufacturing defects, while spalling often stems from unremoved fatigue cracks. The optimal maintenance strategy is preventative maintenance: ensuring quality control of rolls before they are put on the mill through rigorous non-destructive testing, releasing roll stress through scientific grinding and resting, and reducing the impact of sudden accidents on the roll surface through optimized rolling processes.
