Ovako Steel’s 2018 investment in a vacuum tank degassing (VTD) facility is a critical development for the Smedjebacken mill. Removal of hydrogen by a state-of-the-art vacuum process now enables Ovako to address the needs of forgers that require high-performance, reliable and cost-effective steel for demanding applications such as those in the automotive industry.
Why Forges Demand Vacuum Degassed Steel
The vacuum degassing process is an intermediate step that takes place between the molten steel leaving the ladle furnace and being cast into billet form. The VTD unit is effectively a giant vacuum cleaner that allows the production of steel with enhanced properties by sucking dissolved gases such as hydrogen, oxygen and nitrogen from the melt, leaving only minimal quantities behind.
Hydrogen, in particular, is the enemy of steel. It must be removed to prevent the two potential issues of hydrogen embrittlement and delayed cracking. Delayed cracking can occur sometime after casting when individual hydrogen atoms diffuse through the steel to tiny voids, where they combine to form molecules of hydrogen gas (H₂). This gas creates localized pressure that can increase to the point where the metal has reduced ductility and tensile strength and can eventually crack open. Therefore, emphasis is placed on removing hydrogen from billets intended for the forging of components used in demanding applications.
VTD is a Streamlined Process
Historically, the Smedjebacken mill has become very experienced in eliminating the effects of hydrogen from its steel products. VTD, however, has streamlined what was a long, expensive process. Previously, hydrogen was removed in an annealing stage. This involves putting the cast steel into a furnace at high temperatures (typically 600˚C) for many hours. The prolonged heating enables the hydrogen to diffuse to the surface.
VTD now enables the same high quality to be achieved much faster and as an integral part of the main steel production process. The facility, which includes a total of 12 vacuum pumps, was designed, manufactured and installed by SMS group.
The VTD facility can receive the full 125-ton load of molten steel from the ladle. It takes up to seven minutes for the pumps to achieve the required vacuum. Samples are taken to check that the required level of 2 ppm (parts per million), or lower, of hydrogen has been achieved and the temperature is correct. Then, following any final trimming that might be required, the melt goes on to casting.
The whole VTD process takes only around 45 minutes. In theory, vacuum degassing could be applied to nearly all the steel grades produced at Smedjebacken. In practice, it will be applied to hydrogen-sensitive steel grades and when customer requirements demand vacuum degassing, which will be around half of the mill’s output.
Sustainability Benefits and Clean-Steel Production Capability
In addition to its essential role in producing vacuum degassed steel, the VTD facility is integral to Ovako’s overall approach to sustainability. All production is based on steel scrap, making this company the biggest scrap recycler in the Nordics. Since steel can be recycled an infinite number of times without its properties deteriorating, it is possible to manufacture steel with a lower climate impact without compromising its quality. This is one of the reasons why the company’s products have an 80% lower carbon footprint “cradle-to-gate” compared to the global average.
Ovako is steadily improving its environmental impact through ongoing development of its production processes. Some of the measures adopted include the use of oxyfuel burners that have cut NOx emissions by 75%, carbon dioxide (CO₂) by 20% and reduced energy usage by 50%. Furnaces have also been converted from oil to liquid petroleum gas (LPG) or electricity, saving around 5,000 tons per year of CO₂.
To demonstrate its commitment to sustainability, Ovako published Environmental Product Declarations (EPD) that cover the full environmental impact of hot-rolled steel bars from its Hofors, Imatra, Smedjebacken and Boxholm (Smebox) mills.
Göran Nyström, Ovako’s EVP of Marketing and Technology, explained how the VTD offers some significant sustainability and quality benefits.
“The VTD is powered by electricity supplied from the Nordic region’s green energy mix, and it eliminates the need to heat the annealing furnaces with fossil fuels. We estimate it will save approximately 3,500 tons per year of CO₂. That helps to further reduce the overall carbon footprint and energy consumption of the Smedjebacken site. It is also an important contribution to our overall strategy to minimize our cradle-to-gate carbon footprint. In fact, we already saved 2 tons of CO₂ for every ton of steel produced, giving Ovako the lowest carbon footprint of any steel producer in the world.
“While the removal of hydrogen is the critical element in the VTD process, it also removes oxygen and nitrogen. Oxygen removal is particularly important because it forms nonmetallic inclusions that impact the fatigue strength of the steel. Controlling the size and distribution of these inclusions is central to the creation of clean steels that deliver consistent quality and improved fatigue strength.”
Vacuum Degassing is a Critical Element for Forges
For customers that produce performance and safety-critical components for the automotive industry, vacuum degassing is regarded as essential. It is mandatory for them to purchase steel that has been processed in a VTD facility. Following Ovako’s investment in a new casting machine and the additional control station, the VTD facility is the final piece of the quality jigsaw. Together with the company’s emphasis on sustainability, exciting new areas of business are opening up for the Smedjebacken mill, especially in forging. Ovako has already seen early success. A major automotive customer has approved steel processed by the VTD facility at Smedjebacken for use in the production of forged crankshafts.