Cold-rolling methods

The input to cold-rolling mills is normally ‘hot band’ steel some 2–4 mm thick and approximately 1 m wide. Coils are typically of 20 tonne weight. Figure 4.7 depicts such a coil. A rolling mill could be thought of, crudely, as an oversize mangle in which strip to be reduced passes through a pair of rolls which are compressed onto it

Figure 4.7 Hot rolled coil – hot band

as shown in Figures 4.8(a) and (b). To secure considerable reduction of thin strip the physics of the roll bite requires rolls to be of comparatively small diameter. Small-diameter rolls are more susceptible to bending under the loads applied to them, so that the centre region of strip being rolled can be reduced in thickness less than the edges. To counter this effect back-up rolls are used to restrain deformation of the

‘work’ rolls (Figure 4.8c).

More complex arrays of multiback-up roll systems are used in special Zendsimir mills used for very heavy reductions. Other expedients involve rolls with non-parallel profiles and the facility of internal inflation of rolls by hydraulics.

Rolling, both hot and cold, is a very extensive technology and its ramifications need not concern steel users.

Mills may be tandem or reversing. In the tandem mill steel is progressively reduced in thickness roll-stand to roll-stand whereas in a reversing mill the metal passes from left to right through one mill stand, then back again, through several ‘passes’ to attain the final desired thickness.

The surface condition of hot-rolled steel is never as smooth as that attainable by cold rolling. The achievement of the cold (reducing) mill is assessed in terms of:

• Speed of working

• Exactitude of output thickness (gauge)

Manufacturing methods 39

Strip Work roll

Pressure

Back-up roll Pressure

Pressure Strip

Rolls

Strip Roll

Pressure (a)

(c)

(b)

Figure 4.8 a Outline of a rolling mill b Pressure applied to roll necks

c Use of back-up rolls to stiffen a rolling mill

Figure 4.9 Example of shape aberration: loose middle with tight edge

• Perfection of ‘shape’ (see Chapter 10)

• Cleanliness

• Strip profile

• Surface roughness.

The output thickness is important to the motor builder who expects to have a clear appreciation of how many laminations will produce a stator stack of specified height.

Chapter 13 examines the steps taken to procure exact thickness control.

‘Shape’ describes the state of deformation or internal stress of the strip. The rolling process can operate so that the filaments of metal nearer to the edge of the strip become less longitudinally extended than the centre regions (Figure 4.9). This state is known

Wavy edge Tight middle

Figure 4.10 Example of shape aberration: tight middle, wavy edge

as loose middle or tight edge. Similarly the edge regions can be over-extended leading to a wavy edge (Figure 4.10). These ‘shape’ variations are often invisible when strip is under tension, and are present in the form of differential internal stress.

Punching strip of unfavourable shape into laminations can produce defects such as round holes (as punched) becoming oval when relaxed. Such an effect is clearly bad for motor stator laminations where a close-tolerance round hole is vital. Fortunately effective methods of continuous shape control can now be applied on mills. Output shape can be sensed at various positions across the width of the strip and signals applied to a range of corrective mechanisms.

Cleanliness: Clean strip requires clean rolling, clean lubricant and clean ingoing strip. Chapter 10 looks at cleanliness assessment.

Surface roughness: It might be thought that the more perfectly smooth a steel’s surface could be the better it would be. However, too smooth a surface can provoke sticking together (by micro-welds) of laminations during stack annealing processes, and will inhibit the penetration of gas between laminations during a final anneal when access for gas to produce decarburisation may be desired. To control surface roughness final cold rolling may be carried out using ‘textured’ rolls which have been precision roughened to give a desirable surface roughness to steel strip.

Strip profile considers the variation of thickness of strip across its width. There are two main components of such variation – crown and edge drop. Crown refers to the overall growth of thickness towards the centre of the strip, and edge drop considers the more rapid fall away of thickness in the last 100 mm near the strip edge. For motor laminations the variation in thickness side-to-side of a lamination is a factor of first importance. If edge drop is severe, sloping stack profiles can arise (Figure 4.11).

The progressive rotation of laminations as they are stamped to even out stack shape is a useful procedure but is less widely practised than would be most useful.

Of course by discarding some edge material the effect of edge drop can be minimised, but this is a very expensive option. Strip profile is much more easily controlled during hot rolling than during cold rolling, so that specificational liaison between hot and cold mills is essential. The provision of near-perfect profile is expensive since rolls wear in use and despite many compensational ploys will require regrinding earlier if

Manufacturing methods 41

Pile of laminations tmin

tmax tav

100 mm

Crown = tmax–tav. ‘Edge drop’ is the thickness change over the edge region A–B. The thickness variations are shown on a greatly exaggerated scale.

A B

A lamination pile cut from steel with taper will not pile level unless individual laminations are rotated.

Figure 4.11 Edge drop and crown

best profile is continuously demanded. It should be noted that metal of perfect ‘shape’

can have severe crown or edge drop and vice versa. The two problems have differing origins and different solutions.