Au cours des dernières années, multiple technical improvements including rolling mill process optimization and roll device performance upgrading have effectively boosted roll hardness, thermal crack resistance and impact resistance. These measures cut roll consumption and significantly prolong roll service life. This paper mainly analyzes typical roll failures in hot narrow strip production: irregular spalling at the center of roll grooves, roll ring cracking and pitting defects on roll grooves, and proposes targeted optimized roll cooling solutions matching actual production line conditions. The optimized cooling equipment and water supply system greatly improve roll cooling efficiency. Entre-temps, appropriately widening roll rings, selecting proper roll materials and adopting matched auxiliary devices can effectively eliminate cracking and spalling defects.
This integrated optimization system can eliminate irregular block peeling in the middle of roll grooves, solve roll ring cracks and surface pitting, and lift the overall stable operation efficiency of rolls.
Rolls are core critical equipment in metallurgical production. Directly contacting steel blanks, rolls deform blanks under rolling pressure to manufacture steel products meeting specified quality standards. Donc, rolls require outstanding fatigue resistance and wear resistance. With growing market demand for rolled steel products, excessive roll consumption pushes up production costs year by year. Against this background, how to boost rolling productivity, reduce roll replacement frequency and extend roll service life has become a key concern across the metallurgical industry.
Rolls work under harsh service conditions: high temperature, heavy pressure, alternating hot-cold cycles and iron oxide scale abrasion. Roll barrels on each mill stand bear combined mechanical stress, friction, thermal stress and impact loads, which easily trigger wear, cracking and spalling and drastically shorten roll service life. As a core component of rolling mills, rolls involve high procurement costs, and roll consumption is a vital economic index directly linked to total production cost.
1 Significance of Extending Roll Service Life
Rolls are high-consumption working components that continuously perform plastic deformation of metal in rolling mills, accounting for a large proportion of total production expenses. A complete roll management system enables operators to track the full service status and lifecycle of every single roll. Standardized roll matching and application, rational grinding allowance control reduce roll fatigue damage, ensure stable mill operation, extend roll service life and cut unplanned roll replacement downtime. These improvements lift rolling line productivity and economic benefits. A sound roll management system and standardized management procedures are core factors to lower rolling production costs and prolong roll service life.
Dedicated staff shall be assigned to roll management with a full roll lifecycle tracking system. Complete records must be retained for roll warehouse incoming inspection, pairing application, crack detection and abnormal roll removal. Systematic whole-process monitoring of roll service status and usage trajectory is critical to reducing roll consumption and extending roll service life.
2 Common Roll Defects in Production
2.1 Irregular Spalling at the Center of Roll Grooves
Excessive copper content in the chemical composition of roll grooves is the primary cause of irregular central spalling. When the central zone of roll grooves is heated, copper continuously migrates on the inner surface of the groove wall. Copper features a low melting point, which drastically reduces the hot plasticity of the roll surface and initiates surface microcracks. These cracks expand and propagate with continuous copper migration, eventually causing irregular block spalling.
Taking hot narrow strip rolling as an example, inadequate on-site supervision allows finish operators to arbitrarily adjust rolling tonnage for each groove, leading to inconsistent wear across different roll grooves. During roll reconditioning, residual microcracks fail to be fully removed by turning. When the reworked roll is put back into production, microcracks rapidly expand and interconnect into macroscopic cracks, resulting in groove metal spalling.
2.2 Roll Ring Cracking
Roll rings bear superimposed assembly stress, thermal stress and rolling stress during production. The tangential tensile stress at the inner diameter of roll rings reaches a high level during rolling, and single-material roll rings cannot deliver long service life. Local stress concentration combined with other alternating stresses easily triggers irreversible roll ring cracking damage.
2.3 Pitting Defects on Roll Grooves
Piqûres (also known as spot blemishes) is a widespread surface defect featured by rough, uneven groove surfaces. Pitting defects appear in continuous strips, partial patches or scattered spots. Minor pitting is permissible as long as its depth does not exceed the product thickness tolerance. Main root causes of pitting are listed below:
- Worn finish pass or intermediate pass grooves, or fragmented iron oxide scale embedded on roll surfaces;
- Fragmented iron oxide pressed and peeled onto the surface of rolled stock;
- Chemical erosion on roll surface;
- Severe surface oxidation of steel blanks during reheating.
2.4 Roll Fracture
Impacts, tail whipping, steel jamming and other abnormal production incidents generate internal and surface cracks plus soft spots on rolls. These defects severely disrupt stable rolling and shorten roll service life. Severe penetrating cracks cause premature large-area roll spalling and force early roll retirement.
3 Root Cause Analysis of Roll Failures
Roll defects arise from multiple factors: improper manual operation, insufficient cooling performance, unreasonable rolling process parameters and inherent roll material quality flaws. Inadequate or mismatched cooling creates drastic temperature differences on roll surfaces, inducing thermal stress that accelerates surface spalling. Excessively high roll temperature degrades roll strength and wear resistance, triggering groove bursting, metal spalling, thermal cracking and even complete roll fracture.
Additional failure drivers include improper roll material selection mismatched with rolled steel grades and production processes, as well as operator errors such as steel winding and stock piling, which lead to groove bursting, metal peeling and roll breakage.
Metallic brittle inclusions inside steel blanks (SiO₂, Al₂O₃, silicate impurities) also severely shorten roll service life. The hazard level varies with inclusion quantity, size and morphology: higher volume fraction and larger particle size bring more severe damage, especially angular sharp-edged inclusions.
4 Optimized Technical Countermeasures
4.1 Upgrade Roll Cooling Devices
Optimizing cooling equipment improves cooling water utilization efficiency and heat dissipation performance to extend roll lifespan. Mass heat accumulates during rolling, so continuous cooling is required to stabilize roll temperature, extend roll change cycles and guarantee controlled processing temperature. The roll cooling system consists of primary and secondary cooling water tanks. Primary cooling water is axially sprayed through slits to the rolling zone for lubrication and cooling of copper tubes. Secondary cooling rapidly lowers tube temperature, prevents air from entering the rolling sleeve and avoids copper tube oxidation. Water spray rings are adopted for direct roll surface cooling.
Main drive speed exceeds 1300 rpm and auxiliary roll drive runs above 700 rpm under high-speed rolling. Precise control of cooling water flow and pressure is mandatory, with continuous sufficient cooling supply. Rolls with over-limit temperature must be replaced immediately to avoid thermal fatigue cracking. Stable roll temperature requires cooling water flow above 3500 L/h and pump pressure controlled below 0.8 MPa.
The original square perforated water tank structure is replaced with solid cylindrical nozzles. Two rows of elliptical water slits are added at splitting wedge positions, covering a wedge width of 5~8 mm. The water supply volume at splitting wedges is tripled compared with other zones to strengthen local cooling and extend the service life of splitting wedge roll grooves.
4.2 Cooling Water Parameter Optimization & Water Supply System Retrofit
Cooling water temperature must be controlled within 40~60 ℃. Excessively high water temperature accelerates roll fracture, while ultra-low temperature disrupts steel recrystallization and downgrades finished product quality; extreme high or low temperatures both aggravate thermal fatigue damage.
The cooling water pipeline system is reconstructed: low-turbidity water supply is upgraded to medium-turbidity water supply, and variable frequency drive motors are equipped for water pumps to realize automatic adjustable hydraulic pressure stabilized at 0.8 MPa.
4.3 Pass Schedule Optimization
Limited cooling efficiency affects φ550 hot narrow strip mill performance. Based on the original pass design, forced spreading principle is applied to the No.1 mill stand. Given the high temperature of K1 finished stock, forced spreading passes are arranged on K3 and K4 stands, with straight-line bottom profiles for spreading grooves. The pass inclination angle of forced spreading grooves is set to 17.7° to avoid surface wrinkles and unqualified finished surface quality after spreading on K3 and K4. Corresponding dimensional adjustments are implemented on K6 roll grooves to eliminate tail pulling defects.
4.4 Matériau de l'anneau de roulement & Dimension Optimization
Taking Q215 hot narrow strip steel as an example, the initial design sets a 19.0 mm central groove spacing and 7.8 mm intermediate roll ring width, meeting theoretical design standards. Cependant, the narrow roll ring width easily generates fatigue cracks and full ring fracture under cyclic loads. To solve this problem, the roll gap is redesigned to 22.0 mm, and intermediate roll ring width is increased to 10.8 mm.
4.5 Automatic Thickness Control System Modification
Rolling force fluctuates with entry thickness, tension, friction coefficient and deformation resistance, while deformation curves directly affect exit strip thickness. Contrôle automatique de la jauge (CAG) adopts model algorithms to automatically regulate strip thickness, rolling mill status and external disturbances, realizing intelligent setting of rolling pressure, line speed and roll gap for precise thickness tolerance control. Hydraulic screwdown systems equipped in AGC feature high control accuracy and fast response. Precise slab thickness control relies on reliable mill quality control and sufficient hydraulic AGC units. For the No.6 finishing mill of strip line, target controlled thickness ranges from 2.7 mm à 27 mm with allowable deviation rate of 10%.
4.6 Standardize Operating Procedures to Avoid Human-Induced Damage
Improper operation is a common cause of roll damage; standardized operation specifications are formulated as follows:
- Prevent direct contact between guides and roll surfaces;
- Strictly follow cooling water switching procedures to avoid dry running after roll replacement;
- Strengthen regular equipment inspection; immediately stop production once steel winding occurs, and wait for wound stock to cool to ambient temperature before disposal;
- Control rolled stock shape strictly per technical standards and forbid excessive single-pass reduction;
- Fully shear black oxidized stock at strip heads and prohibit feeding severely oxidized black steel into mills.
4.7 Ensure Sufficient Roll Turning Reconditioning Allowance
Stable tonnage output per roll groove relies on complete removal of residual cracks during regrinding. Unremoved residual cracks drastically reduce rolling capacity and contaminate finished product surface quality. For Q215 hot narrow strip finished rolls, the minimum turning allowance is specified as 600 mm. Rolls with residual microcracks that cannot be fully eliminated after turning shall be scrapped and prohibited from reuse.
5 Conclusion
Implementing the above integrated optimization measures can greatly extend roll service life, reduce roll replacement frequency and cut labor intensity for workers. The optimized system eliminates irregular spalling at roll groove centers and resolves roll ring cracking and pitting defects. Cooling-induced groove wear is nearly eliminated, finished product quality is improved, and average mill effective operation efficiency is lifted significantly.
