Cold Rolling Process on Metals | Industries | Metallurgy

In this article we will discuss about the cold rolling process which is performed on metals.

Sliding and Sticking in Rolling:

At surface contact in rolling:

(i) Very high local pressures and abnormal material flow occur at the nose and tail of the workpiece,

(ii) The contact zone between the workpiece and the roll changes continuously,

(iii) The direction of interface sliding reverses, at the neutral point.

The continuity of an incompressible material can be maintained only if the product of thickness and velocity is constant at any point along the zone of contact in rolling (assuming that width remains constant).

The constancy of volume demands that the exit speed of the strip should be increased in proportion to the pass reduction. Further the rolls will have to move at some speed intermediate between entry and exit speed. It may be noted that speed of strip is lower at entry and high at exit.

There is only one point in the arc of contact where strip velocity is same as roll velocity (neutral point) and there is no relative slip between the two surfaces at this point. Fig. 6.3 shows the distribution of velocity and pressure between the rolls for complete sliding condition.

Other than at neutral point, at all other points along the arc of contact the strip is moving either slower than the rolls (V₁) (backward slip between entry and neutral point) or faster than the roll (V₂) (forward slip between neutral point and exit). Refer shaded area in Fig. 6.3 for the speed difference between roll and strip surface. Condition shown in Fig. 6.3 occurs for full sliding condition.

In a typical rolling process, forward slip may be around 55% and rest of speed differential occurs in backward slip zone. Therefore, the strip enters the roll gap at a high interface sliding velocity which diminishes very rapidly to match speed of rolls in the neutral zone. It then picks up again as the strip leaves the roll gap. This relative sliding and its reversal, is helpful in effecting reduction.

The angle between the neutral and exit planes is dependent upon the pass geometry and the coefficient of friction.

As soon as the product of interface pressure (which is maximum at neutral zone) and coefficient of friction reaches the yield strength of the workpiece material in shear, sticking of the interface sets in and deformation occurs by subsurface shear in the bulk of the workpiece material. In such a case the neutral plane broadens into a neutral zone and reduces forward slip [Refer Fig. 6.4 (a)].

The position of the neutral plane is very sensitive to the presence of tensions. When tension imbalance exists and back tension predominates, backward slip increases and the neutral plane moves forward. Interface friction also affects the position of the neutral plane. It also determines the total roll force and power requirements. Interface pressure rises at the neutral plane because of friction.

In sticking phenomena, because of strain hardening and friction being high enough to arrest relative sliding between roll and workpiece around the neutral plane, the peak of the friction hill becomes flattened. In hot rolling and with high friction, the sticking zone could widen over entire arc of contact, giving greatly flattened friction hill.

When sliding or sticking friction prevails, new surfaces are generated on the strip during its passage through the rolls. The generation of new surfaces occurs along the arc of contact when sliding friction predominates. The surfaces breakup just before entering the roll gap when sticking friction prevails.

Forces in Cold Rolling:

When a small slice of strip metal passes through the roll gap, the normal vertical pressure at points along the arc of contact is given as below:

It is important to remember that the thickness of metal rolled by cold rolling is often considerable in excess of the initial roll setting, because the various mill parts get deformed elastically under the load encountered on the mill during cold rolling. When cold rolling strip, the thickness must be controlled to within close limits.

Rolling Load:

F is given by summing the normal roll pressure across the arc of contact and multiplying by the width of the strip.

Condition for the Strip to be Accepted by the Rolls without Slipping:

Without applying external force, the horizontal component of the frictional force at the roll surface must exceed the horizontal component of the normal roll force at the entry position for acceptance of strip by the roll.

Thus Δt = t_i – t_e ≤ µ²R.

Position of Neutral Plane:

On inlet side the thickness of strip decreases, metal’s length increases and thus rolls travel faster than stock. On exit side, stock moves faster than the rolls. At neutral plane, rolls and stock travel at same speed. At this plane the direction of the frictional force changes and thus friction is zero.

Problem 1:

Determine the rolling load to cold roll an aluminium strip of 1500 mm width from 4 mm to 3.3 mm thickness on a 4 high mill. The diameter of rolls is 500 mm. Plane strain of aluminium at inlet as used is 0.6 and corresponding plane stress is 140 N/ mm². Take value of as 0.06. Take strain at the end of rolling as 0.8 when plane stress is 148 N/mm².

Solution:

w = width of strip = 1500 mm

t_i = thickness at inlet = 4 mm, t_e = 3.3 mm

Δt = 4 – 3.3 = 0.7 mm, R = 250 mm.

Problem 2:

Determine the position of neutral plane by two methods.

Solution:

Rolling Loads Corrected for Roll Flattening:

When strip passes through the rolls, they get separated due to separating force and are deformed elastically. Rolls get deformed to larger radius R’ along the contact arc.

Effect of Inlet and Exit Tension:

To obtain good gauge, inlet and exit tensions are applied to cold strip. These have the effect of reducing the rolling load which may be beneficial to the rolling of very thin gauge material. The normal vertical pressures in this case get multiplied by factors

Torque Required for Rolling:

Torque for rolling can be calculated by determining turning moment applied to the roll barrel necessary to produce the frictional force experienced at the roll surface by the stock.

An alternative empirical approach is to assume that the total load acts downwards at a single plane within the arc of contact at a certain distance √Δt X R from the line of roll centres, then applied torque T required to overcome the true rolling separating force F is given by

R’ = roll radius due to deformation under load

and R = roll radius without deformation. Value of λ’ can be taken as 0.43 for early passes in cold rolling where rolls are rough and high frictional condition exist, and λ’ = 0.48 during finishing stands where rolls are highly polished and friction is low.

Power for Rolling Mill:

As the rolls rotate, both the upper and lower rolls work against the roll separating force.

Problem 3:

(a) What is angle of bite and when the rolls bite the workpiece in rolling operation?

(b) What is the value of bite angle for various materials in rolling operation?

(c) If D is diameter roll and a the angle of bite, then what is the reduction of thickness At in rolling? How much is maximum reduction?

(d) What is backward slip and forward slip in roiling?

(e) What is angle of nip?

Ans. (a) Angle of bite is the angle between the entrance plane (at point where workpiece makes first contact with the roller) and centre lines of the rolls.

Rolls will bite a workpiece when the angle of friction is greater than the angle of bite.

(b) Bite angle is 3 to 4° for cold rolling of steel with well ground rolls, 6 – 8° with lubricated rolls, 18 – 22° for hot rolling steel sheets.

(c) Δt = D(1 – cos α), Maximum reduction = µ²D/2.

(d) Due to decrease in thickness of metal in rolling, outlet velocity of metal is higher after rolling. There is a neutral point in zone of rolling where speed of rolling wheel matches with speed of metal (V_n). Before this workpiece experiences backward slip due to its slower speed (V_i) and after neutral plane, it experiences forward slip due to higher speed of metal (V₀). Backward slip is (V_n – V_i / V_n) and forward slip = (V₀ – V_n / V_n)

(e) Angle of nip is twice the angle of bite. Initially if strip is made to enter rolls by tapering the front end of strip, then angle of bite can be increased two times (angle of nip).

Control of Rolled Strip Thickness during Cold Rolling:

Roll force obviously depends upon the reduction in thickness required. A typical curve between variation of thickness and roll forces is shown in Fig. 6.3 and such a curve is called the plastic line. If the material to be rolled can be easily deformed, i.e. it has low yield strength, then roll force required for same thickness reduction will reduce. In other words, slope of plastic line will reduce.

It will also reduce with low friction between work and rolls, and with increased applied tension. The thickness of rolled strip t₂ can be determined by intersection of the elastic line (dotted line Fig. 6.5) and plastic line as shown in Fig. 6.5. Strip thickness may change due to variation in stock thickness, material hardness or roll speed or roll tension.

In order to maintain the correct gauge thickness under all condition, rolling conditions must be changed rapidly and automatically by either adjusting the roll gap or changing the tension applied to the strip, the latter being more sensitive and commonly used method.