Roll crown decides whether cold-rolled strip comes out flat. The result starts with the roll you select. This article covers roll selection and surface condition, crown selection and grinding, crown distribution across the rolls, hydraulic bending for shape control, and the tension settings that hold the profile in place.
1. Roll Selection
1.1 Roll Surface Condition
A work roll needs a surface free of defects and a finish that meets specification. It needs hardness at the surface, toughness at the core, and a long service life. Manufacturers generally forge work rolls from alloy steel. Body hardness reaches Shore 95–100 or above. Neck hardness reaches Shore 45–50 or above.
A backup roll needs to sit softer than the work roll, so it does not mark the work roll surface. Its body hardness generally reaches Shore 60–65, and its neck Shore 42. A mill normally changes and regrinds each backup roll once every six months.
Rolling generates very high contact stress and frequent alternating stress. Because of this load, a backup roll fatigues after a period of service, and its surface begins to spall. If the spalled area stays small, the roll can sometimes return to service after cleaning. However, some mills change the roll as soon as spalling appears, whatever its area or depth, and grind away the spalled section before the roll returns to the mill. Therefore, to prevent spalling, a mill should give the backup roll a tempering recovery treatment after a set period of service, generally six months.
1.2 Roll Crown Selection and Roll Grinding
Cold rolling changes the crown of a roll in two ways. First, rolling force bends the roll, and the bending produces the pressure crown. Second, uneven heating expands the roll unevenly, more at the centre than at the edges, and the difference produces the thermal crown.
To compensate for both effects, a mill grinds the roll with a camber in advance. This camber is the roll crown. On a cold mill the pressure crown dominates, so the selected crown usually runs convex.
In addition, the amount of convexity depends on several factors. These include the rolling reduction, the yield strength and width of the rolled material, the heating conditions of the roll, the materials of both roll and strip, the tension during rolling, and the properties of the lubricant.
Therefore, a mill grinds its rolls on a dedicated roll grinder. Grinding removes dents, bruises, scratches, aluminium pickup and similar surface defects. It also produces the required crown and roughness. After grinding, the maximum crown sits at the mid-point of the body length. From there it tapers smoothly and symmetrically towards both ends.
1.3 Roll Crown Configuration Methods
Crown configuration decides how a mill distributes the total crown it needs among the individual rolls. The mill can split the crown evenly between the top and bottom work rolls. It can also grind the crown into the top roll alone, or into the bottom roll alone.
Configuration should weigh two practical points. How easily do the rolls grind, and how easily does the crown adjust? Each mill settles the question for its own case, on the premise that product quality stays guaranteed. The usual arrangements are:
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First, on a two-high cold mill, most installations crown both the top and the bottom roll. Only a very small number crown the top roll alone and leave the bottom roll cylindrical.
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Second, on a four-high cold mill, most installations crown the work rolls and leave the backup rolls cylindrical. If the total crown stays small, only the top work roll takes the crown and the bottom work roll stays cylindrical. If the crown runs larger, the total crown splits evenly between the top and bottom work rolls.
2. Roll Crown Control
In strip rolling, fluctuations in rolling pressure, deformation heat and other factors change the crown considerably. The strip then develops uneven thickness or waves. Waves sometimes appear in the middle of the strip, and sometimes at the edges.
When a wave shows up, the crown has already changed. The pressure distribution across the strip width has turned uneven. Waves hurt rolling. Once they grow too large, the strip may end up as scrap, or it may break during rolling. A mill therefore has to control strip shape in good time.
2.1 General Methods of Roll Crown Control
When rolling speed stays low, adjust the spray, the roll bending, the tension, the reduction and the speed. Then set each one according to the size and position of the wave.
2.2 Hydraulic Roll Bending for Shape Control
When waves appear during rolling, they hinder continued rolling, whether they sit at the edge or in the middle. Although the general measures above do adjust shape, they act slowly and their range stays small. Tension is one example. If the mill widens the adjustment range too far, the strip tears, and a strip break follows.
In recent years mills have fitted hydraulic devices to the backup roll chocks and the work roll bearing boxes. Hydraulic cylinders force the backup roll or the work roll to bend. The mill thereby controls the roll crown it needs. This method is hydraulic roll bending.
Once a wave appears, hydraulic bending removes it quickly and effectively, particularly during high-speed rolling. Flatness improves noticeably.
The control logic runs as follows. When a wave appears at the strip edge, increase the P force or reduce the Q force, and the roll crown increases. This adjustment is positive bending. When a wave appears in the middle of the strip, increase the Q force or reduce the P force, and the roll crown decreases. This adjustment is negative bending.
2.3 Automatic Roll Crown Control
A modern cold rolling mill carries a shape meter. Through a microcomputer, the meter links hydraulic roll bending, roll tilting, rolling force, coolant and tension into a closed loop. The loop forms an automatic shape control system. The shape meter sends out a signal, and the computer controls strip shape on that basis.
3. Tension Control
3.1 What Entry and Exit Tension Do
Modern strip mills carry a powerful entry and exit tension system. Front tension can exceed the yield strength of the parts that elongate less. Because of this, tension alone stretches even a section with little elongation. As a result, uneven local elongation cannot produce waves, and the rolled strip comes out flatter.
Tension also lowers the rolling force. However, an increase in back tension lowers it more than the same increase in front tension.
In addition, tension improves the centring of the strip during rolling. Moreover, it prevents or reduces the camber that appears when a mill rolls without tension. Thus it creates favourable conditions for high-speed rolling.
3.2 Limits on Back Tension and Fluctuation
If back tension runs too high, it causes excessive backward slip. Then forward slip falls, and may even reach zero. The strip then slips on the rolls, and a stable rolling process becomes difficult to establish. A mill must therefore keep back tension below a certain limit.
Tension rolling still needs adjustment to the specific conditions. If the mill adjusts it badly, the strip breaks. Otherwise the coil layers shift against each other and scratch the strip surface.
During rolling, tension on the strip should stay balanced and constant. When fluctuation stays mild, it shifts the coil layers and scratches the strip surface. When fluctuation turns severe, it breaks the strip. The permissible fluctuation range generally runs to ±1–3%.
3.3 Unit Tension and Adjustment Range
In theory the maximum tension in cold rolling should not exceed the yield strength of the rolled material. When rolling aluminium alloy strip, the unit tension generally runs at 50–60% of the yield strength of that pass. The plasticity of the material and the condition of the edges both affect the choice.
The tension adjustment range is the ratio of maximum to minimum tension. It generally runs from 10:1 to 20:1. Within this range a mill can set a given tension value accurately.
As the number of passes increases, work hardening of the strip increases. The yield strength of the rolled material rises with it, so the unit tension selected for each pass should rise accordingly.
Finally, uncoiler tension should stay below the tension used for coiling in the previous pass. Otherwise the coil layers shift and scratch the strip surface when the coil uncoils for rolling.
