Refining Quality with Secondary Metallurgy

As more steel producers transition to electric arc furnace-based steel production, a crucial step in conventional steelmaking—­secondary metallurgy—is becoming increasingly important. From advanced Ladle Furnaces, VD/VOD, and RH-Degassing units, ­Primetals ­Technologies is bringing groundbreaking solutions for secondary metallurgy to steel producers worldwide.

Since the early 20th century, steel producers have refined steel in a secondary process following the primary melting units, i.e., LD converters (BOF). Today, both LD converter- and electric arc furnace-based meltshops are integrated with secondary metallurgy aggregates, improving the quality of steel to the desired grades for end users. Moreover, as more producers adopt electric arc furnaces to meet environmental regulations and reduce the environmental impact of steel plants, additional treatment steps during secondary metallurgy enable them to deliver high-quality, advanced steel grades without sacrificing yield or quality.

In the mid-twentieth century, following World War II, an increased demand for high-quality, “clean” steel led to the broad adoption of ladle treatment processes. Developments from 1950 to 1960 used argon to stir the liquid in the ladle, reducing temperature variations and pushing slag to the surface. Commercial vacuum pump availability increased during this period, allowing for the inclusion of ladle stirring with vacuum chambers to remove hydrogen.

During the mid-1960s, ladle furnaces began to enter the industry, featuring two lids: one for vacuum treatment and the other for three-electrode heating. During this time, the Finkl-Mohr process, which utilized a single vacuum-tight lid through which electrodes were inserted, was also introduced. The Finkl-Mohr process was relatively demanding, and in most cases, heating using the ladle furnace under atmospheric conditions is more common.

As production methods changed, vacuum tank degassing and Ruhrstahl Heraeus (RH) degassing units became standard. RH degassing plants were first developed and installed in Germany in the 1950s. For vacuum degassing and RH degassing plants, the introduction of oxygen blowing advanced these units to accommodate various steel grades. Today, VD/VOD plants and RH degassing units are gaining importance, particularly as producers adopt electric arc furnaces, enabling new routes to meet quality requirements.

With the latest secondary metallurgical treatment solutions, Primetals Technologies supports steel producers worldwide in producing high-quality, high-grade steel. In conventional steel production, secondary metallurgy often focuses on specific refinement processes, such as vacuum treatment, to achieve certain characteristics. In some cases, ladle furnaces and twin ladle furnaces are used only to provide additional heating to reach the required casting temperature.

In electric steelmaking, however, secondary metallurgy becomes increasingly essential to compensate for variations in the quality of raw materials, including scrap and ore-based material (OBM) such as direct reduced iron (DRI), hot-briquetted iron (HBI), or pig iron. The additional steps in secondary metallurgy play a vital role in balancing the chemical composition of steel, producing cleaner grades, compensating for tramp elements from scrap, and controlling sulfur, nitrogen, and carbon levels.

When designing metallurgical processes, the desired steel grade and target composition must be defined. The reason is that standards often allow a broad range of chemical compositions, whereas the requirements of customers, casters, and downstream facilities are often much narrower. When producing steel using an electric arc furnace, a ladle furnace, a vacuum tank degasser (VD), or an RH degassing unit, it is essential to achieve the appropriate chemical composition for specific grades, particularly those that without it have unique requirements, such as API grades, ultra-low carbon interstitial-free (ULC-IF), and non-grain-oriented silicon steel (NGO-SI).

For steel grades such as commodity and HSLA, treatment following tapping from an electric arc furnace may require only a ladle furnace and a single vacuum treatment step—i.e., either a VD or RH unit. Commodity-grade steel, for example, utilizes a ladle furnace to reach the optimal casting temperature and does not require vacuum treatment. Alloys can be added during tapping and ladle furnace treatment to achieve target chemical compositions, and desulfurization occurs during those steps.

HSLA steel grades follow a similar process. However, ladle treatment steps are extended to keep the molten steel temperature high enough to account for heat loss during subsequent vacuum treatment and to reduce sulfur content. Reduced sulfur content is required to reduce nitrogen in the subsequent vacuum treatment. Micro alloys such as niobium (Nb) or titanium (Ti) are added either during vacuum treatment using the material-handling system or after vacuum treatment using wires. Additional aluminum is added after vacuum treatment to compensate for losses during vacuum treatment.

API steel grades follow a similar route to HSLA, with longer ladle-furnace treatment, which helps reduce overall sulfur content. Due to the need for very low sulfur levels in API grades, the VD unit can achieve sulfur levels that are much lower than those of an RH unit. Therefore, an RH degassing unit would require additional treatment in the ladle furnace to reach the appropriate sulfur level. Nevertheless, vacuum treatment in either a VD or RH degassing unit is necessary.

Meanwhile, ULC-IF steels require very low carbon concentrations. During secondary treatment, oxygen must be present in the steel to achieve these low concentrations. However, oxygen cannot be present during the ladle furnace treatment, as oxygen hinders desulfurization. Therefore, the steel is killed during tapping to achieve the sulfur target in the ladle furnace. The steel is then heated to compensate for the long vacuum treatment time.

With the addition of oxygen blowing (OB) for decarburization, the VD-OB process is longer than the RH-OB process, meaning longer heating times in the ladle furnace for the VD-OB route. Additionally, deslagging is required before vacuum treatment for the VD-OB route. Due to more efficient decarburization in the RH than in the VD, it is possible to reduce vacuum treatment times. This leads to less superheating of the ladle furnace than previously required.

The vacuum treatment starts for RH-OB and VD-OB with a degassing step to reduce nitrogen and hydrogen. Afterwards, oxygen needs to be added to the steel to reduce carbon via oxygen blowing. During oxygen blowing, carbon is reduced, but to achieve the lowest carbon levels, an additional deep decarburization step at absolute pressures below five mbar is required. After decarburization is finished, aluminum is added to deoxidize the steel. Similar to other steel grades, micro alloys can be added either during vacuum treatment with the material handling system or after vacuum treatment using wires.

For other specialized steels, such as non-grain-oriented silicon steels (NGO-Si), treatment times are longer due to the addition of silicon and aluminum to achieve the appropriate chemical composition.

New and additional treatment steps raise further logical investigations for steel producers, especially for maintaining yield or extending product mixes. When transitioning steel production to an electric arc furnace, a close look at the treatment steps reveals that product mix variety can be maintained while maintaining yield if appropriate investments are made in additional production aggregates. Experts at Primetals Technologies examined various product mixes and determined varying arrangements of secondary treatment steps to meet the demands of even the most diverse product mixes.

Experts concluded that to maintain even a diverse product mix, additional secondary treatment units, including VD and RH units, are required to meet the desired casting schedule. For a producer casting a variety of steel grades, from commodity to NGO-Si, two twin VD stands and RH units would be required to meet the casting schedule and maintain annual productivity. With experience in both greenfield and brownfield supply and construction, specialists at Primetals Technologies have delivered secondary metallurgy solutions to producers worldwide that fit the specifications and footprint of existing plants. Moreover, patented technologies, such as the Combined Vessel and Ladle Lifting System, expedite degassing procedures in RH units.

Vacuum treatment steps play a vital role in balancing the chemical composition of steel, producing cleaner grades. Looking back to the beginnings of secondary metallurgy, additional treatment steps were necessary to meet changes in quality demands. Changes in steel production routes toward electric steelmaking and the growing demand for high-quality steel have once again brought secondary metallurgical treatment to the minds of steel producers worldwide. Specialists at Primetals Technologies are ready to advise and deliver optimal secondary metallurgy aggregates to meet the challenges of quality and yield for steel producers.