Fabrication of Sheet and Strip

Cakes from the cast shop are in most cases the starting material for the fabrication of sheets and strips. The key elements of the fabrication process for flat products are the hot and subsequent cold rolling operations. In detail the fabrication process comprises the following steps, which are also illustrated in some detail in Figure 3-17.

- Pre-heating,

- hot rolling and milling, - intermediate annealing, - cold rolling,

- pickling, washing and drying, - re-rolling and strand annealing, - Finishing, and

- for sheets: Cutting to length, - for strips: Cutting to width.

Preheating Hot Rolling Milling

Intermed.

Annealing Preheating Pickling Washing

Drying

Rerolling Strand

Annealing Finishing

Cutting to Length

Cutting to Width

Cooling Water Systems

Protection Gas Supply

Lubricant Systems

Pickling Systems Slabs/Cakes

Sheets Copper & Alloy

Strips Copper & Alloy

Effluent Treatment Spent

Lubricant

Sludge Sulfates (Cu, Ni, Zn, etc)

Neutalisation

Figure 3-17: Fabrication of sheets and strips of copper and copper alloys

Normally the slabs/cakes are pre-heated prior to hot rolling in gas- or oil-fired furnaces. In some plants the off-gas is used for heat recovery.

The hot rolling is usually done with a dual rolling mill equipped with benches up to 200 m and a final coiling device. The width of the rolled product is up to 1,400 mm, the thickness of the slabs is reduced from approximately 250 mm down to approximately 10 mm. The cooling water for the rolls has small amounts of lubricant added for improving the attachment to the steel rolls. The vapour generated is vented and the vent gas is demisted prior to release to atmosphere.

The hot rolling of the cast slabs takes only a short time per slab and is determined by the slab weight. There is no loss of metal during hot rolling so slab and coils weight are identical. The hot rolling mill does not require protection gas. The noise development during rolling is controlled by protective measures. The surface and side milling units installed after hot rolling are equipped with ventilation and dust filtration systems. The loss of material is around 5 %,

the chips are recycled for metal recovery. The surface milling requires protective measures against noise emissions.

Further cold rolling operations result in a hardened metal. In most cases the coil is annealed prior to cold rolling. Annealing is done under reducing conditions to avoid oxidation. As protection gas exogas or nitrogen/hydrogen mixtures are in use. Exogas is produced at site from natural gas in a special reactor which is indirectly fired. N2 and H2 are purchased and stored at site in special tanks. The N2/H2 protection gas mixtures are produced from the storage tanks by mixing the components in the ration required. For annealing before cold rolling usually bell type furnaces are used with electric heating or indirectly fired by natural gas or fuel oil. Tower type furnaces are applied for intermediate annealing of pre-rolled coils.

They are heated by natural gas and are operated with protection gas. Strand annealing furnaces are in use for final and also for intermediate annealing. This method is a so-called touchless annealing process, surface oxidation is avoided by applying protection gas. In some cases, heat content of the off-gases is utilised to pre-heat combustion air or to generate steam. Prior to annealing the material will be degreased after the coiling by washing with water which contains detergents. The used solutions are cleaned by ultra-filtration.

After annealing the material is normally pickled in line in order to remove residual surface contaminants and oxides. For pickling purposes systems of sulphuric acid as well as mixtures from sulphuric and nitric acid are in use. The burden generated during usage of nitric acid is treated in a gas cleaning system (scrubbing with caustic soda). The pickling is done in continuous lines or in automatic processing, the acid is changed from time to time, the criteria is the pickling result. The spent acid is given to recovery. The pickling is vented for workers protection. Connected with the pickling bath a rinsing system is attached working with rotating brushes, water and at last with deionised water to get a clean and acid free surface.

The rinsing water is given to the waste water treatment plant. Sludges are separated from the pickling media and disposed of or recycled. After washing the handling steps following are drying and coiling.

The sheet thickness is further reduced by stepwise cold rolling operations on different reversing mills. For cold rolling different types of mills called Duo, Quarto, Sexto, Sendzimir (12 rolls) are in use. Rolling mill designs as single-stand which are combined to an in-line multi-stand rolling mill are also applied. It depends on the thickness of the sheet on the coils and on the finishing status the coil has already been reached which combination of the different types is applied.

During cold rolling an emulsion or oil is used for roll protection. Therefore, the roll stands are vented and the ventilation gases are cleaned by mechanical filters, wet electrostatic precipitators or scrubbing. The emulsion and the oil is cleaned from metal and cracked oil particles by paper or textile band filters. Alternatively, the rolling mill coolant liquid is mixed

with inert adsorbing agents and filtered. The duration of use for the original emulsion or oil may reach several years before it has to be replaced.

Also after cold rolling the surface of the rolled products is degreased by water containing washing agents. The washing solution is cleaned by ultra-filtration and reused. The oil separated from the washing water is given to authorised external companies for further treatment.

For strip fabrication special cutting units are available to cut the rolled and finished sheet coils into strips of the defined width. Table 3-53 compiles some typical data for the fabrication of brass strips by hot and cold rolling.

Table 3-53: Typical data of a facility to roll brass strips

Hot rolling

Type of mill Mainly Duo

Dimensions start 250 - 130 mm thickness, 450 - 1.000 mm width Final dimensions 15 - 12 mm thickness, 450 - 1.000 mm width Temperature 750 - 800 °C

Rolling force Abt. 10 - 12 kN/mm sheet width Surface milling 0,3 - 0,7 mm cutting of both surfaces 1^st Cold rolling

Type of mill Mainly Quarto

Rolling force Abt. 15 - 20 kN/mm sheet width Dimension reduction and

rolling speed

Reduction 15 mm to 4 mm thickness in multiple passes, speed 100-200 m/min

Conversion ratio 70 - 80 % Annealing^*) (recrystallisation)

Temperature 550 - 600 °C 2^nd and final rolling

Type of mill Mainly Quarto, alternatively, depending on sheet thickness, Sexto or stands with 20 rolls are used Rolling force Abt. 2 - 1o kN/mm sheet width, depending of type of

mill used

Rolling speed Reduction 4 to 1 mm: 300 - 500 m/min, multiple passes

Reduction 1 to 0,1 mm: 500 - 1000 m/min, multiple passes

Cutting of sheets to strips

Type of equipment Longitudinal strip cutting machine

*) Annealing required after every conversion ratio of more than 70-80 %.

Annealing temperatures selected depending of material properties to be achieved.

The described concept for sheet and strip fabrication, based on hot and cold rolling corresponds to the conventional concept and is also suitable for high production rates.

However, there are concepts industrially applied or under development for direct casting of sheets or strips followed by cold rolling. Such concepts bypass the hot rolling operation. One method is the horizontal casting used for copper alloy strip production. These concepts are also applied for rod and profile casting. Furthermore, different other systems have been proposed for continuous strip and sheet casting and some are under development.