Pictures of the Future      Spring 2008

A Polish Steel Plant that’s on a Roll

Thanks to Europe’s most modern hot strip mill, which was built by Siemens, the world’s leading steel company, ArcelorMittal, has been able to boost production in Kraków, Poland, and manufacture even harder and more resistant grades of steel.

The architects who planned and built Kraków’s eastern suburb, Nowa Huta, in the late 1940s borrowed heavily from the historic architecture of the city’s older districts. That’s why the rows of apartment buildings lining modern Nowa Huta’s main thoroughfares include many structures adorned by Renaissance-style arcades.

At the heart of the city’s 18th district is the former Huta T. Sendzimira steel combine, which covers an area of 1,000 ha. The combine was fronted by an administrative building with an Italian-style facade that locals nicknamed "The Doge’s Palace." That’s where steel manufacturer ArcelorMittal has resided since 2005.

ArcelorMittal, the world’s leading steel company, began operating Europe’s most modern hot strip mill at the site in the summer of 2007. The facility, which includes a rolling line nearly 400 m in length for producing steel strip, is housed in a 580-m-long hall. Visitors to the building are treated to an impressive view and the warmth of steel slabs coming out of an oven heated to a temperature of 1,200 °C. The furnace opens once every three minutes to disgorge a slab. Depending on requirements, these steaming and hissing tongues of raw steel are between six and 12 m long, 70 to 210 cm wide, and 22 to 25 cm thick. After the slabs emerge, powerful rollers squeeze and shape them, and the process concludes with steel strips measuring over 1 km in length being wound into coils.

ArcelorMittal produces around eight million tons of steel annually in Poland, making it the country’s largest steel producer. Altogether, it has invested more than $380 million in the Kraków hot strip mill to date, which is the biggest investment the European steel sector has seen in the last decade.

With its 4,750 employees, the mill has become one of the most important steel manufacturing locations in Europe. What is more, it  meets all European Union environmental norms and stands out thanks to its relatively moderate noise emissions and its energy-saving technology. Furthermore, despite its huge size, the mill still has room for growth, with its infrastructure being capable of of supporting an increase in capacity from 2.4 to 4.8 mill. t per year.

A little more than three years ago, ArcelorMittal decided to abandon its original plan to modernize the old hot rolling mill. Instead, the company opted to build a completely new facility in order to significantly boost productivity and product quality. There was good reason for this decision, as ArcelorMittal expects demand for steel to rise over the next few years, especially in the 12 Eastern EU member states, where the need for high-tensile steel in industries such as the automotive sector is continually increasing.

Sensitive Steel. As a leading engineering and plant construction company for the global iron and steel industry, Siemens VAI (now a part of Siemens’ Metal Technologies division) constructed the complete rolling mill in Kraków in a turnkey project that included all hot strip mill equipment and all electrical and automation systems. The company also built the production bay, the roll shop, and other facilities, including a water treatment plant. Siemens is the only company in the world able to offer complete iron and steel mills and strip mill facilities from a single source in packages that include training, as well as monitoring services for production, assembly, and commissioning. That’s why Siemens was also responsible for commissioning the Kraków Unit hot strip mill, and why it continues today to train operating and maintenance personnel as well. Siemens handed over the 73,000-m² turnkey facility in the summer of 2007, just 23 months after the contract was signed and four weeks ahead of schedule.

Steel is a very sensitive material, which is why many coordinated individual steps are required to shape it properly in a rolling mill. Cracks can form, for example, if temperatures in the individual steps are not exactly correct. But undesired temperature fluctuations can be avoided, for instance, by moving transfer bars into what is called an "isolation tunnel" before the rolling process. Temperature losses are kept low in this tunnel, achieving a homogenous temperature distribution.

For instance, the so-called Encopanel tunnel developed by Siemens VAI prevents steel from cooling down too rapidly. "Placing steel in such a tunnel before the rolling process begins allows us to roll it more efficiently and use less energy," says Adam Dziedzic, deputy director of the Kraków hot strip mill. "This generates indirect energy savings, since the slabs don’t need to be heated up as much at the start of the process."

During the next step, the resulting strip is sent to the "heart" of the facility—the finishing mill, where the temperature of the steel (previously over 1,000 °C) drops to 875 °C. The finishing mill is equipped with stands weighing 200 t each that hold work rolls operated by eight-megawatt motors. The six work rolls squeeze the transfer slabs down to a final thinness of anywhere between 1.2 and 25.4 mm, depending on the steel’s intended application. Very thin bars are used, for example, in white goods and automobiles—both of which are important markets for ArcelorMittal in Poland. 25-mm bars, on the other hand, are utilized in industries such as construction and shipbuilding .

It takes only a few minutes for the once bulky metal to be transformed into a delicate-looking strip of steel up to 1.7 km in length and as much as 2.10 m wide. Rollers then transport the steel strips to a 40-m-long cooling section, where directed water cooling is used to abruptly reduce the final rolling temperature down to 700 °C in order to achieve required material properties.

Whether the steel ends up flexible for motor vehicle production or rigid for use in construction depends on how fast it cools and how long it remains at a given temperature.

Controlled Cooling. Directed cooling is achieved via a complex control system that applies water to the metal strip from above and below as needed. Water is stored in a tank with a capacity of over 900 m³. The tank is located above the cooling system, which is divided into different zones. Thanks to the height difference, the water hits the metal at a constant pressure. Siemens developed this cooling system, which is known as QuickSwitch.

Jan Staniewski, deputy director of Hot Strip Mills at ArcelorMittal, is very proud of the cooling system: "Whatever mechanical property you’re looking for—whether tensile strength, hardness, tenacity, or structure—it can be achieved with extreme precision using a clever cooling strategy." This means that in addition to making steel for automotive applications, the Kraków Unit is ideal for producing special quality classes for specific customer requirements. These include steel for containers exposed to very high pressure, or extremely resistant steel grades for oil and gas pipes in regions subject to extreme temperature fluctuations, such as Siberia.

Digital Quality Assurance. At the end of the roll line, the steel strip is rolled up into coils and sent to an integrated measuring and sampling station, where it is checked for quality. The extensive benefits of the hot strip mill, whose products are used with transformer components in the automotive industry and as stamped components out of black plate (a low carbon steel that can accept a finish of chromium or tin), are clear. "We produce extremely hard and resistant materials that also guarantee a high degree of safety, which is important in natural gas pipes, for example, as well as in ships and automobiles," says Staniewski.

The plant’s advanced technology allows it to produce strips with a maximum width of 2.10 m—among the widest anywhere in Europe. This is important because it opens the door to the key market segment of wide, hard steel strip for the shipbuilding and pipeline industries in particular. After all, the wider the steel, the fewer the welding seams and rivets needed.

Staniewski also reports that even better quality assurance than before is now achieved at the new hot strip mill, thanks to the extensive use of computer control systems. All production parameters remain transparent at all times and are continually optimized by process computers. "Still, it’s also a more demanding task to manage a state-of-the-art facility like this one," says Staniewski.

It’s easy to understand why ArcelorMittal has been successful in its search for suitably qualified employees in Nowa Huta. The district, which has a population of 250,000, has always attracted skilled workers from all over the country and will remain the center of Polish steel production in the future. It also helps
that the city of Kraków is home to one of Europe’s oldest universities, the world-famous Akademia Górniczo-Hutnica (AGH), which has been Poland’s most respected and renowned center of education and training for the metallurgical sector since 1919.
                                                                                                                    Thomas Veser