Description:
Cold-rolled steel strips and sheets typically range from 0.1-3mm in thickness and 100-2000mm in width. They offer advantages such as a smooth surface, high flatness, precise dimensions, and good mechanical properties. These products are widely used in various industries including automobile manufacturing, electrical appliances, building materials, packaging, and household appliances. With technological progress, quality requirements for cold-rolled strips and sheets continue to increase across all sectors. The automobile industry in particular has reached unprecedented quality standards for steel sheets. To meet these challenges, various advanced rolling mills with different rolling methods and roll configurations have been developed. The evolution from 4-high to 6-high, 8-high, 12-high, and even 20-high mills demonstrates a clear trend toward multi-roll configurations with smaller roll diameters. This paper analyzes how work roll diameter variations affect rolling performance from a theoretical perspective, aiming to establish useful reference patterns for researchers.
Keyword: Rolling roll, work roll, cold rolling mill
Related product: Work roll
Stress State Analysis in Cold Rolling Process
Rolling Process Overview
The steel strip rolling process involves plastic and elastic deformation as the strip passes through the gap between upper and lower work rolls under rolling pressure, resulting in thickness reduction. The significant extrusion force during rolling flattens and elongates steel grains, distorts the crystal lattice, and breaks grains, leading to work hardening characterized by decreased plasticity and increased hardness and strength. Simultaneously, the rolling process generates substantial deformation heat, raising temperatures of both strip and rolls. Excessive roll surface temperatures can reduce work roll hardened layer hardness, affecting strip surface quality and roll life. Uneven temperature distribution across rolls disrupts normal roll contour, directly impacting strip shape and dimensional accuracy. Therefore, effective cooling measures using emulsions are essential for normal cold rolling production.
During rolling, thickness fluctuations occur due to several factors: uneven strip chemical composition and structure; original thickness variations; roll thermal expansion; roll wear; roll eccentric operation; gaps between rolling mill contact surfaces; and elastic deformation of the mill itself. Constant roll gap control maintains consistent strip thickness longitudinally, where thickness sensors feedback to hydraulic systems that dynamically adjust rolling pressure. This method requires high mill rigidity.
While constant roll gap control ensures consistent middle portion thickness, it compromises edge accuracy due to dynamic rolling pressure changes causing work roll bending variations across the strip width. This limitation led to the development of constant pressure control, which maintains consistent rolling pressure without adjusting for incoming strip thickness variations, resulting in optimal strip shape. Constant pressure control is typically used in leveling operations to improve sheet shape.
Additional dynamic control methods like HC, CVC, and UPC roll contour control can also be employed for shape control during rolling.
Work Roll Force Analysis
N0 – Rolling pressure
N0′ – Strip reaction force on work roll
f Friction – Static friction reaction force from strip on work roll
Fx – X-direction bearing reaction force
Fy – Y-direction bearing reaction force
M drive – Drive torque
With constant roll gap control, rolling force changes rapidly, making N0 and N0′ dynamic alternating forces. This causes sharp variations in Fx and Fy, which also become alternating forces. When strip rolling speed (V0) remains constant:
M drive = f friction · r (torque balance, where r is work roll radius)
From a contact perspective, work rolls contact both the steel strip and backup rolls (or intermediate rolls) in line contact, experiencing sharply alternating stress with significant stress concentration at contact points.
Considering elastic deformation, work rolls undergo dynamic local elastic deformation on their upper and lower contact surfaces under the alternating pressures of N0 and N0′.
Detailed Analysis of Rolling Deformation Zone
Assuming infinite work roll rigidity with no elastic deformation under rolling force, the rolling deformation zone corresponds to the theoretical deformation zone shown in the figure.
In reality, work rolls develop a hardened surface layer through quenching, providing high surface hardness. However, rolling force still causes deformation, with amount A representing flattening deformation.
After exiting the roll gap, the strip undergoes elastic recovery deformation, with amount B.
Effect of Roll Diameter Changes on Mill Stiffness
Force analysis reveals that work rolls experience sharply alternating stress with significant local stress concentration at contact lines, causing local elastic deformation.
In actual production, surface quenching typically creates a hardened layer on larger diameter work rolls, or surface treatments like carburizing and nitriding improve surface hardness. With larger diameters, quenching depth cannot reach the core, resulting in softer core material. Under sharply alternating stress, the hard shell and soft core create internal stress gradients along the contact line, causing inconsistent internal elastic deformations that facilitate accumulation and expansion of internal defects, leading to fatigue cracks or surface spalling. Combined with rolling heat effects, roll surface failure accelerates, directly affecting strip quality and production schedules.
Deformation zone analysis shows that the actual deformation zone length exceeds the theoretical length. When using larger diameter work rolls with hard shell/soft core characteristics, elastic deformation at the contact line exceeds that of fully hardened rolls, further increasing the actual deformation zone length. Under constant pressure, unit contact pressure decreases, but the strip’s yield limit remains fixed. Therefore, greater rolling force is required to increase unit contact pressure and achieve ideal rolling effects.
Reducing work roll diameter provides several benefits:
Smaller work roll diameters facilitate through-hardening, creating uniform hardness throughout. This reduces elastic deformation under rolling force, decreasing mill vibration, improving mill stiffness, and enhancing thickness control. The rolling deformation zone length also shortens, increasing unit contact pressure and enabling ideal rolling effects with relatively smaller rolling forces. In multi-stand rolling, this helps overcome work hardening from previous stands and ensures consistent sheet thickness.
Reduced work roll diameter increases fatigue strength, extends service life, and lowers consumption. (Reference 2)
Smaller work roll diameters reduce mill vibration and improve stiffness. With identical housing wall thickness, the same rolling force produces smaller elastic deformation in mills with smaller window heights compared to those with larger window heights, further reducing vibration and improving stiffness.
Reduced work roll diameter with improved stiffness enables rolling of thinner steel strips, expanding processable thickness ranges.
Conclusion
Reducing work roll diameter supports energy savings and consumption reduction, improves strip thickness control, and enables rolling of thinner steel strips.
