Heavy mill stand pieces to be machined in UK

Voest-Alpine Industrieanlagenbau and VAI UK has placed a GBP 2m contract with AK Heavy Engineering for machining and test assembling a 5m wide finishing mill stand, destined for China.

AK Heavy Engineering, part of the Kvaerner Group, has gained a GBP 2 million contract for machining and test assembling a 5m wide steel finishing mill stand, destined for China, which will weigh over 1,000 tonnes when fully erected.

Comprising two massive 4-piece bolted housings, the finishing stand will form the centrepiece of a 'greenfield' heavy plate rolling mill project for Chinese steelmaker Zhangjiagang Hongchang Plate Co, known as Shagang, and the order was placed by global engineering and plant building group Voest-Alpine Industrieanlagenbau (VAI) and its Sheffield subsidiary VAI UK.

Each consisting of two 15m high legs and 6m wide top and bottom tie beams, the bolted housings are among the largest ever produced and will require the extensive milling, boring, craneage and assembly resources of AK Heavy Engineering's giant Sheffield works, one of the largest production capacities in Western Europe.

With a total finished height of around 17m and each weighing 525 tonnes (the equivalent of 75 London buses), the two housings will be test assembled in a large 4m deep pit, employing specially made concrete blocks and cast iron beds to support the vertical structures, before being dismantled for shipping to PR China, where they will be erected on site using massive 300mm diameter pre-tensioned bolts.

Representatives of AK Heavy Engineering and VAI presented to Shagang management, during which the technical and logistical merits of bolted housings over single-piece castings or multi-piece welded structures were explained.

In addition to cost and other economies related to procurement, handling, material upgrading and transportation, bolting together smaller castings ensures greater integrity and the elimination of potential electroslag welding defects on such large pieces.

The eight separate rough castings were sourced from Sheffield Forgemasters and Skoda in the Czech Republic.

The Shagang Group is one of China's leading steel producers and its new mill, built near Shanghai in the Jiangsu Province, will produce rolled steel plates in widths up to 5m and will be only the second plant in China with this capacity.

It will produce steel plate for construction, civil engineering, shipbuilding, pipeline, machinery and other applications, the wider widths requiring fewer welded joints and introducing economies of scale throughout the manufacturing cycle.

After long and intense negotiations, VAI secured the EUR 70 million contract for the engineering, key equipment supply, installation and commissioning of the new 5m plate mill facility.

The project, undertaken in consortium with ABB for the main mill motors and drives, will include the supply of full automation systems and the plant will eventually have a capacity of 1.5m tonnes/year of hot rolled steel plate.

In the process, steel slabs up to 500mm thick are charged into a re-heat furnace, where they are raised to temperatures up to 1000 deg C, then passed through a primary descaler, before rolling and thermo-mechanical processing in the combined roughing and finishing mill stand.

A 4-high reversing plate mill, with two large back-up rolls and two smaller-diameter works rolls, this has a maximum rolling load of 10,000 tonf (10MN) and produces plates between 5mm and 150mm gauge and widths between 1500mm and 4900mm, at exit speeds up to 7.3m/sec.

After accelerated cooling, the rolled plate then moves down the line to a vertical edger and a hot leveller, then finally to a shear for cutting the plate to required lengths, with further levellers and shears available to process the plate to greater degrees of accuracy and finish, depending on the final application.

AK Heavy Engineering has a long-standing relationship with VAI; the latter acquired Kvaerner Metals Equipment in 1999, which strengthened its rolling mill technology skills and formed the basis of VAI UK.

AK also has one of the few facilities in Europe able to undertake projects on this scale, with sophisticated CNC machining centres, milling machines and vertical/horizontal borers, as well as the capacity to handle single lifts up to 300 tonnes and assemblies over 1,000 tonnes.

It also had relevant recent experience of machining and erecting 300 tonne single-piece mill housings, also for China, and of producing 4-piece bolted 525 tonne mill housings, for a 3m wide stand at Oregon Steel in America.

This and numerous earlier steel industry contracts puts AK Heavy Engineering at the forefront of rolling mill housing technology, with experience of single- and multi-piece castings.

The company contends that smaller, less complex castings can reduce the amount of upgrading or weld repair necessary, which may account for as much as 50% of final value.

Also, these are easier to inspect with non-destructive ultrasonic testing, greatly increasing confidence in the integrity of the casting material.

The capability of being able to machine the individual components on several machines concurrently additionally streamlines the internal workflow.

It also enables erection of the mill housings on site in China with a much smaller crane.

Transport logistics are also simplified, as single-piece 525 tonne housings are extremely difficult and expensive to ship across the world.

Despite having world-class welding skills and quality control and inspection regimes, AK Heavy Engineering identified further potential risks associated with welding multi-piece housings, due to the difficulties of properly testing and analysing the welds that might contain hidden defects.

This was demonstrated to Shagang technicians using photographs and micrographic images of the electroslag welding process; as a result, a 4-piece bolted solution was preferred.

Nevertheless, machining the individual legs of the housings, each with a cross-section of 1,000mm and weighing around 200 tonnes, requires AK's largest milling and boring facilities, with huge vertical borers similarly needed for the 90 tonne tie beams.

Extremely tight geometric and parallel tolerances further demand that machining is performed very accurately, with frequent cross checking; while employing the latest CNC technology, this still requires highly specialised and experienced operators and is a tribute to the skilled Sheffield workforce.

Assembling the components in a vertical position in the heavy fitting bay similarly required special facilities and procedures.

This has entailed the design and manufacture of special equipment, including a 4.5 tonne lifting bracket and involving two overhead cranes, as well as the production of 40 tonne concrete supports, 38 tonne cast iron beds and a fabricated temporary tie beam, all designed to achieve the precise assembled dimensions.

Detailed method statements and risk assessment studies have also been drawn up to ensure the trial erection and assembly phase is completed successfully.

This will involve craning the 15m legs into the pit, raising them to the vertical, then manoeuvring through 90 degrees, before securing them to the top and bottom tie beams, using 72mm diameter temporary bolts and washer plates, also fabricated by AK Heavy Engineering.

On completion and final inspection by VAI technicians, the finished housings will be stripped down, rust protected and the components individually clad with timber, before being transported by road to an east coast port, for shipping to China.

On site, local labour will undertake final assembly, using pre-tensioned 300mm diameter supplied bolts, which will be heated prior to torquing up, then cooled down for optimum tightness.

Delivery is scheduled later this year and already AK Heavy Engineering has received an enquiry from VAI for similar 4-piece bolted housings, this time destined for India.

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