New horizons – steel processing

Materials World magazine
,
14 Jul 2008

Mike Brammer, Director, Plate Mills at Siemens VAI Metals Technologies Ltd, UK, describes recent developments in process technology.

At the end of the 20th century steel seemed to have reached a ceiling in terms of global capital investment. Few new plants were built, and the industry’s technologists were increasingly engaged in incremental developments. Radical equipment and process concepts were few and far between.

Now all that has changed. In every year of the millennium so far, more steelplant has been commissioned than was put to work in the whole of the previous decade. The market for steel products has changed forever. New capacity in emerging economies is not the only consequence of this capital development boom – steelplant engineering has also been energised.

Technical innovation did not stop during the industry investment slowdown of the 1980s and 90s. But because there was little demand for exploitation, new concepts were stored away, waiting for an investment climate that would generate a worthwhile return. These ideas are now being realised, exploiting design and manufacturing techniques hitherto neglected in the steel industry. They aim for operating volumes last considered in the high days of Japanese steel in the early 1980s, and they take on equipment duties in a modern-age of high-strength, high-purity steel that have never been approached before.

Ironing things out

Reduction of ore to produce iron will likely involve a modern blast furnace. In metallurgical terms, the process is complex, involving high temperatures and pressures, multiple phase and counter-current chemical reactions.

The modern furnace is cooled with copper and cast iron staves, and incorporates extensive instrumentation for monitoring and model-based control. This includes ensuring the correct distribution of the burden. Fields of active development include rotating charging devices and hydraulic taphole opening equipment.

A modern blast furnace will also include a whole range of environmental control devices such as stockhouse dust and casthouse fume collection, a condensing slag granulation plant and cleaning of the off-gas gas using a high efficiency cyclone to allow recycling of dry dust.

The primary feedstock for the rolling mill is produced by continuous casting whereby liquid steel is converted into a wide variety of semi-finished products. During this conversion, a continuous casting machine will remove energy at a rate of around 500kJ/kg. For a slab caster this equates to around 37.5MW. To ensure correct slab dimensions and optimum product quality this must be done in a carefully controlled way.

Although a relatively mature technology, casters are continuously being developed to meet increasing demands. Modern conventional slab casters are state-of-the-art in terms of productivity, product quality and operator safety.

A typical slab caster will include over 2,000t of precision engineered equipment. This includes 40-60t segments which are aligned, around a curve, to accuracies within 0.2mm. Slab thicknesses of up to 400mm and slab widths of up to 3,250mm are being cast. Productivity rates of over 1.5Mt/yr/strand are becoming routine. This is achieved by increasing casting speeds, reliability and improving caster flexibility through on-line slab width adjustment and rapid thickness change between casts.

Product quality is also increasing due to improved control and automation. With good operation, breakout prediction systems can almost eliminate sticker type breakouts, while dynamic roller gap adjustment ensures optimum soft reduction even during transient casting periods. Moveable spray nozzles ensure optimum cooling over the full width range and on-line quality assessment gives detailed quality predictions as soon as the slab is cut.

To maximise operator safety and eliminate human error, casters are starting to be equipped with robotic systems. These are especially useful in the liquid steel area where consistency of operation improves product quality. These developments will continue and the boundaries will expand further.

Ready to roll

The most powerful metalworking machines anywhere in industry are rolling mills used to manufacture steel plate. Most new rolling mills are five metres wide, and the number of such mills worldwide has more than doubled in the last five years. Each typically delivers 40MW of drive power at full load and contains roll separating forces of around 10,000t. The elastic deflection of the housings at these loads is about 100 times the plate thickness tolerances to which these mills operate.

One of the key modern developments in the mill stand itself is the appearance of modular housings. The size of the mills was previously limited by foundry capacity, to a largest weight for single cast components of up to 500t. Modular housings have escaped this limit with bolted construction supplanting earlier welded variants.

Some of the most significant developments in the plate mill are in the ancillary equipment. The secret of high strength plate-making with good weldability and superior toughness lies in close control of the microstructure, which in turn demands high rates of cooling beyond the mill.

However, the engineering of cooling systems is complex because the finished plate has to remain flat and exhibit minimal residual stress. Some systems, such as the MULPIC illustrated, incorporate sophisticated mechanisms for modulating cooling at plate edges and ends, since full-surface uniformity is as important as the precision of control.

As the development of the cooling systems progresses, product properties that were until recently only achievable by off-line quench and temper treatments are increasingly being realised in the in-line process. The motivation for this is energy and inventory savings. Cases where investment return comes from the cost reductions that attend process simplification provide the main incentives for R&D in the commodity materials processing sector.

Shear hard work

Another plate processing field in which technology is improving is in shearing. Mechanical shearing is the main method of cutting plates to delivered size, but the trend to a wider range of plate thicknesses and strengths places exceptional demands on the machinery.

The ideal geometry of the cut varies with both these parameters, so a conventional shear with a fixed blade path is becoming increasingly compromised. An ingenious solution to this limitation has emerged in the form of hydraulic shears with programmable blade paths. The maintenance and operability of these new-generation machines are also improved.

Moreover, rolled steel products are traditionally proved and certified by taking samples from the mill and performing tensile and impact tests. Standards dictate test practice and frequency, and the mechanical properties are assumed to describe a complete production batch.

Until recently, no other approach was possible, but metallurgical modelling has now evolved to the point where product properties resulting from a given steel composition and process route can be predicted with great accuracy.

Some hot strip mills have already discontinued physical testing on general low carbon grades, since model-based is more accurate and gives better batch coverage, eliminates costs and reduces delivery lead-times.

In the plate mill, with its more complex process metallurgy and critical application standards, model-based control systems are still limited to identifying process windows for target properties which can then be navigated in general control.

Even in plate, near-future progress in model-based control is set to challenge concepts of materials certification. The virtual test-house will ultimately become the norm throughout the steel industry and all of its products.

Peaks and troughs

The current high level of capital development in the global steel industry will inevitably subside in time, but it is not slowing yet. Its momentum and geographical reach has already exceeded the economists’ early predictions. The investment cycle is now self-sustaining. New assets are so superior to the old, that the global industry is having to re-equip to compete.

Further information:

Siemens VAI Metals Technologies Ltd