D ESCRIPCIÓN DE LOS CU

Figure 4.4 shows the outline of a steel strip process route. Liquid steel is continuously cast into slabs which are hot rolled down to some 2–4 mm thick coil. This coil is called

‘hot band’. Hot rolling to thicknesses less than 2 mm is problematical as accurate

Manufacturing methods 33

Slag Vacuum Inject

Applied vacuum

Metal (a)

(b)

Figure 4.3 a Vacuum degassing installation b Vacuum degassing units

thickness control and good surface condition are harder to maintain so treatment is continued cold. A few hot mills can approach 0.7–1.0 mm for hot-rolled final gauge (thickness), but this is not common.

Modern cold mills take in hot-band coils and roll them progressively thinner to a final thickness of 0.2–0.7 mm (Figure 4.5). Before cold rolling, coils will be exposed to a shot-blast, side-trim and pickling process to remove scale and provide a surface suitable for cold rolling. A strand anneal may precede first cold rolling to homogenise the internal structure of the metal. In a strand anneal a coil of steel is fed into a long furnace as it is unrolled, then coiled up again at the far end. Furnaces may be hundreds of metres long.

Liquid steel

Solid slab Hot saw

Cut-off slab Water

cooled mould

Reheat furnace

Slab

Multi-stand hot strip mill

Slab

Coiler coiling ‘hot band’

Figure 4.4 Steel casting and slabbing

4.2.1 Decarburisation

When steel is rolled to thin final thicknesses such as 0.5 mm or less it is possible to decarburise it by exposure to a decarburising atmosphere as the strip passes through a strand annealing furnace. To obtain effective decarburisation, exposure to wet hydro-gen at 800+^◦C for a minute or two is effective. Carbon diffuses to the surface and reacts with the furnace atmosphere gas to produce hydrocarbons, carbon monoxide and carbon dioxide.

If decarburisation is not performed during steel-making and not carried out dur-ing a later strand anneal treatment, it must be tolerated or removed from the finally punched lamination before use. A considerable proportion of steel for motors is annealed after punching to remove internal stress and the effects of a final cold reduc-tion. Remember that internal stress inhibits easy domain moreduc-tion. This final anneal will be carried out at a motor builders plant or a lamination supply house which offers finished laminations to the market. Decarburisation at this final stage of heat treatment requires a long cycle at 800+^◦C set up to ensure that decarburising gases can penetrate to every part of a stack of laminations in the furnace load and that the products of reaction have time to diffuse out. Additionally, heat must reach all parts of the furnace charge so that metal is hot enough for long enough. Figure 4.6 outlines the strand decarburising process, in this case one in which a final oxide coating is being applied as part of a grain orientation process.

Manufacturing methods 35

Roller leveller Shears Work

rolls Back up

rolls

Tension reel

Tension reel Mill

stand

Figure 4.5 Cold rolling mill, schematic and photograph

Coiler Shears

Welder

Entry loops S Wrap

rolls Decarb furnace

Jet coolers

Water seal

Squeegee rolls

loopsExit

MgO coating

unit

Drying oven

Coolers

Figure 4.6 Photo and schematic of a decarburising line

Manufacturing methods 37 In the process shown below (iron to final product) decarburising can be applied at stages 3, 5 or 7, or the effort spread over the three processes:

1. Iron-making

Depending on the duty for which steel is required and the excellence of magnetic performance desired, decarburisation can be applied with varying degrees of effort (and cost).

Long ago decarburisation was achieved by reaction of the metal when hot with air (oxygen). This formed a surface oxide scale in which carbon was incorporated and which could be pickled away. Of course iron was lost in the scale also, so it was never a popular process.

Early attempts to reduce carbon levels before the cold-rolling stage involved wind-ing coils of hot band (as-hot-rolled some 3 mm thick) steel with an interlap wire mesh and heating them in a decarburising atmosphere using a batch process. The interlap mesh permitted gas penetration between the laps of a coil, and the annealing treatment could be continued long enough to allow good carbon removal. Speed of removal of carbon varies approximately as the inverse square of steel thickness so that a long box treatment of coils was made. This ‘open coil annealing’ treatment had some merit, but the provision and recovery of an interlap mesh was a troublesome process. It may be noted that steel very low in carbon and other hardening elements is very soft and of low mechanical ‘Q’ so that a piece is readily bent by hand and if dropped emits a thud rather than a clang.

4.2.2 Desulphurisation

Sulphur levels have traditionally been kept low by management of slags during steel-making so that sulphur is encouraged to migrate into supernatent liquid slag and be removed. Much lower levels of sulphur (e.g. less than 0.002%) can be attained by treatment at the secondary steel-making stage with calcium silicide or other com-pounds. The cost of this treatment is considerable, but very low-sulphur metal, when of low carbon content also, gives excellent magnetic performance.