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Compact strip coating plantsThe demand for high-quality, corrosion-resistant thin steel sheet for the mass manufacture of consumer goods and construction is growing in newly industrialising countries like Bulgaria. Recently Tenova supplied a 550kt/yr push-pull pickling line with acid regeneration plant, together with a new 250kt/yr hot dip galvanising line as part of compact cold rolling and finishing plant for the Intertrust group near Sofia.

The transformation of hot rolled steel coils into cold rolled products, such as galvanised, painted,

tinplated, or simply annealed and tempered, low carbon steel coil, is growing rapidly in newly industrialised countries. Compact cold rolling and coating complexes are often built close to end-user markets and are gaining in popularity because they offer four main advantages:

` Based on consolidating a range of technologies, they are easier to operate than large steel production complexes.

` The hot rolled coils they use as feedstock are widely available on the global market, and are easily transported and stored.

` Compact plants have a lower impact on local communities, and they are designed to be environmentally friendly, with full provision for waste treatment.

` They use highly automated equipment, such as on-line flatness measurement for the cold mill or automatic inspection systems in the main process lines, and sensors to maintain a consistently high level of quality.

The demand for high-quality, corrosion-resistant thin steel sheet for the mass manufacture of consumer goods and construction is growing in Bulgaria, where the Intertrust group has steel manufacturing as one of its major businesses. In 2007, the company decided to invest in a modern compact cold rolling complex to be installed near the Kremikovtzi steelworks to produce pickled and galvanised low carbon steel coils.

As part of this project, Tenova Strip Processing, Italy and Tenova Key Technologies, Austria are supplying a 400-550kt/yr push-pull pickling line with acid regeneration plant, together with a new hot dip galvanising line, which will produce 250kt/yr of various types of galvanised sheet.

Authors: Pierluigi Curletto, Gerhard Frithum and Stefano MarelliTenova

r Table 1 Details of push-pull pickling line at Intertrust

r Table 2 Details of galvanising line at Intertrust

Finishing processes

Production capacity, kt/y 400-550 Material processed Hot rolled carbon steel stripStrip thickness, mm 1.5-5.0 Strip width, mm 900-1,550 Maximum coil weight, t 30 Process section speed, m/min 140 Pickling agent Hydrochloric acid

Production capacity, kt/y 250Material processed Cold rolled and hot rolled pickled coilsStrip thickness, mm 0.2-2.2 Strip width, mm 750-1,500 Maximum coil weight, t 25 Process section speed, m/min 150 Steel grades Full hard, commercial quality, drawing quality, deep drawing qualityZinc coating weight, g/m2 80-350 Chemical treatment Chromate passivation

PUSH-PULL PICKLINGPush-pull pickling lines were developed in the 1960s mainly to process heavy gauge strip (up to 12mm), however, the Intertrust line can handle 1.5-5mm thick strip (see Table 1) and has three main sections:

` Hot rolled coils are transferred from storage to the entry section by an overhead crane, which lays them down on an entry storage saddle where the operator removes the storage straps.

` The entry coil car then automatically loads the coil onto the pay-off reel, before a coil opener threads the head end into a flattener machine which levels the strip before it enters a shear to remove defective parts at the leading edge. a

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` The strip is then threaded into a stitcher if its thickness is up to 2.5mm, or sent directly to the process tanks if more than 2.5mm.

The shallow process tanks are lined internally with acid-resistant granite, within which a high hydrochloric acid flow rate performs the chemical reactions that transform the iron oxide to iron chloride and water.

The last part of the pickling process is a five-stage acid rinsing section to clean the strip and remove acid, while minimising water consumption. Strip emerges free from scale and ready for the last two operations in the exit section: side trimming and surface oiling. The side trimmer cutting heads have very accurate automatic position control. Similar equipment is shown in Figures 1 and 2.

ACID REGENERATION PLANTThe acid regeneration process (ARP) generates virtually no waste and has no external fluids. The process achieves 99.9% recovery of metal chloride solutions, based on the principle of pyro-hydrolysis. High quality iron oxide in powder form is a by-product, which is in strong demand for the ferrite and pigment industries.

The plant has a capacity for 2,500 litres per hour of waste acid and uses:

` a tank farm optimised to serve the regeneration plant and the pickling line.

` a refractory lined spray-roasting reactor and spray-booms with nozzles for waste acid injection.

` a high-efficiency venturi circulation system for cooling reactor off-gas and concentrating waste acid.

` a high-efficiency absorption system to obtain 18% regenerated acid at a constant level.

` off-gas cleaning, including another venturi system for dust elimination to minimise emissions, and

` a fully automated control room to reduce labour costs.

HOT-DIP GALVANISINGThe design of the galvanising line has evolved from Aetna Standard technology, which was acquired by Italimpianti, and now Tenova. The entry section has two pay-off reels, one narrow lap-welder, and a horizontal looper. Space has been left for future installation of a cleaning section.

The 98m long furnace has a maximum capacity of 48.4t/hr and has six main sections:

` A non-oxidising furnace with open flame burners with a preheating chamber where the strip is heated from ambient temperature to about 670°C.

` Radiant P-type tube heating, where the strip is heated to the final temperature.

` A soaking section where the strip is maintained at the

r Fig 1 Side trimmer in PPPL Duferco

r Fig 2 Duferco PPPL general view

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Finishing processes

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annealing temperature for a time dictated by the final metallurgical properties.

` A cooling section, where a six-jet cooler reduces the strip temperature from the annealing temperature to about 450°C.

` A hot steering bridle to increase the strip tension in the coating pot from about 6 to 18MPa.

` The snout, where the strip is dipped in the induction heated zinc melting pot.

After coating, the strip is cooled by a vertical air cooler from 460°C to 150°C, before final cooling by water spray to 40°C. The line is similar to that shown in Figure 3. (See also Table 2.) A surface conditioning section performs the last three main operations on the coated strip:

` Skin passing to improve the yield behaviour, improve the flatness, and provide the correct surface roughness to the strip. This is achieved with a four-high skin pass mill with a maximum force of 500t capacity and a 30t work-roll in-out bending force (the roll bit is wetted by a special lubrication system).

` A tension leveller with four flex unit cassettes, one anti-crossbow unit, one anti-coil set unit, deflector rolls and tensiometer roll are used to prepare the strip for entry to the chemical coater.

` The strip passes vertically through the coater which applies coatings with two rolls in a direct or reverse roll application to both sides of the strip simultaneously. An electrical infrared oven is used to dry the strip, similar to that shown in Figure 4.

Finally, the exit section has a vertical looper, horizontal and vertical inspection, an electrostatic oiler (which can be used as an alternative to chemical coating), a shear with capacity for sample collection, a recoiler, a coil car with a weighing scale and a banding machine. MS

Pierluigi Curletto is Proposal & Sales Manager for Tenova Strip Processing in Genoa, Italy, and Stefano Marelli is Marketing & Sales Manager in the division’s Milan office. Gerhard Frithum is Senior Sales Manager for Tenova Key Technologies, Klosterneuburg, Austria.

CONTACT: stripprocessing@it.tenovagroup.com

r Fig 3 HDG line at Lilama, Vietnam

r Fig 4 Coater at Ternium Siderar

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