Steel-making processes

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Fundamentals of Modern Steel Manufacturing

The production of steel has evolved significantly since its industrial inception, employing various methods to convert raw materials into high-quality steel products. The primary processes - Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency, and crucible processes - each offer distinct advantages and applications in modern steel manufacturing. These methods differ primarily in their heat generation, impurity removal techniques, and final product characteristics.

Crucible and High-Frequency Induction Methods

The traditional Huntsman crucible process has largely been superseded by modern high-frequency induction furnaces, representing a significant advancement in steel manufacturing technology. In these induction furnaces, heat generation occurs within the metal itself through eddy currents, induced by magnetic fields created by alternating current flowing through water-cooled coils surrounding the crucible. The process operates optimally with input current alternating between 500 to 2000 hertz, with the eddy currents' effectiveness increasing proportionally to the square of the frequency. Higher frequencies result in shallower current penetration, necessitating careful frequency selection based on furnace size to ensure adequate current penetration and optimal heating.

A key advantage of this method is the automatic vertical circulation of the melt, driven by eddy currents, which ensures uniformity in the steel's chemical composition. The process can efficiently handle charges from 1 to 5 tonnes, making it particularly suitable for producing high-quality specialty steels, including ball bearing, stainless, magnet, die, and tool steels. The controlled environment prevents contamination from furnace gases, resulting in superior product quality.

Acid and Basic Steelmaking: Principles and Applications

The fundamental distinction in steel manufacturing lies in the use of either acid (siliceous) or basic (limey) slag systems, each requiring specific furnace linings and producing different results in impurity removal. Acid processes, utilizing silica linings, effectively remove silicon, manganese, and carbon through oxidation. However, they require raw materials with minimal phosphorus and sulfur content, as these impurities cannot be effectively removed through acid slag processes.

In contrast, basic processes, employing magnesite or dolomite linings with lime in the charge, can remove a broader range of impurities, including silicon, manganese, carbon, phosphorus, and sulfur. Basic processes typically work with raw materials containing low silicon but high phosphorus contents. The final steel quality in basic processes heavily depends on the degree of oxidation before deoxidizers (such as ferro-manganese, ferro-silicon, and aluminum) are introduced to remove soluble iron oxide and form insoluble oxides:

2Al + 3FeO (soluble) ⇔ 3Fe + Al₂O₃ (solid)

Figure 1. Furnaces used for making pig iron and steels. RH side of open hearth furnace shows use of oil instead of gas

The Evolution of Bessemer Steel Processing

The Bessemer process, available in both Acid and Basic (Thomas) variants, represents a significant milestone in steel manufacturing history. This process is characterized by its unique refining method, where molten pig iron is processed by forcing air through it in a specialized converter vessel. These egg-shaped converters, capable of handling 15-25 tonnes of material, revolutionized steel production efficiency.

Key Process Characteristics:

• The oxidation of impurities generates sufficient heat to maintain the process temperature
• Raw material composition is crucial: acid process requires approximately 2% silicon, while the basic process needs 1.5-2% phosphorus
• The "blowing" phase, marked by an intense flame at the converter mouth, typically takes about 25 minutes
• Due to the rapid processing time, precise control can be challenging

Historical Development and Modern Applications:

The Acid Bessemer process gradually declined as the Acid Open Hearth steel process gained prominence, primarily due to economic factors. This transition was further accelerated by the emergence of basic electric arc furnaces coupled with vacuum degassing technology. However, the Basic Bessemer process maintains its significance, particularly in Continental Europe, where it's used to produce economical steel grades suitable for:

• Ship plates
• Structural sections
• Steel castings (using the modified Tropenas converter)

A notable modification of the basic process is the Tropenas converter, which features side tuyeres rather than bottom-mounted air inputs. This variation is particularly valuable in steel casting applications, where the raw material is typically melted in a cupola and carefully measured before charging into the converter (Fig. 1).

The Siemens Open-Hearth Process: Advancing Steel Production

The Siemens process, available in both acid and basic variants, represents a significant advancement in controlled steel production. This method distinguishes itself through its sophisticated heat management system and longer processing time, allowing for better control over the final product quality.

The process's technical innovation lies in its heat generation through oil or gas combustion, supported by an innovative regenerator system utilizing two chambers on each furnace side. These regenerators are essentially chambers filled with checker brickwork, designed to maximize heat exchange efficiency. The system preheats gas and air by alternately capturing and utilizing waste gas heat, significantly improving overall energy efficiency.

Production capacity varies significantly between furnace types, with fixed furnaces capable of handling up to 600 tonnes and tilting furnaces managing up to 200 tonnes. A key advantage of the Siemens process is its extended processing time of 6-14 hours per charge, allowing for precise control over the steel's composition and properties.

The process demonstrates remarkable flexibility in raw material management, effectively handling both cold and molten pig iron, steel scrap, and lime (in the basic process). Iron ore is charged into the melt to promote impurity oxidation, though modern facilities increasingly employ oxygen lancing for enhanced oxidation efficiency.

While the Basic Open Hearth process historically dominated bulk steel production, current industry trends show a significant shift toward large arc furnaces. This transition is particularly evident in scrap melting operations and single slag processing, often integrated with vacuum degassing systems for superior quality control (Fig. 1).

The Electric Arc Process: Modern Steel Refinement

The electric arc process represents a significant advancement in precision steel manufacturing, distinguished by its unique heat generation method using electric arcs between carbon electrodes and the metal bath (Fig. 1). This process has become fundamental in producing high-quality, specialized steel grades.

The process begins with carefully graded steel scrap melted under an oxidizing basic slag, specifically designed to eliminate phosphorus. The furnace's tilting capability allows for the removal of impure slag, followed by the introduction of a second limey slag phase. This dual-slag approach serves two crucial purposes: removing sulfur and deoxidizing the metal directly in the furnace. The result is an exceptionally pure steel product, provided that excessive temperatures, which could lead to unwanted gas absorption, are carefully controlled.

Modern applications have expanded through the integration of oxygen lancing technology, particularly valuable for carbon removal in chromium-rich environments. This advancement has made it possible to efficiently recycle stainless steel scrap, contributing to both economic and environmental benefits.

One notable characteristic of the electric arc process is its nitrogen content profile. Steels produced through this method typically contain nitrogen levels between 0.01-0.25%, notably higher than open hearth steels, which maintain levels between 0.002-0.008%. This distinction becomes particularly relevant when selecting steel for specific applications.

The process has found its niche in the production of highly alloyed steels, including:

• Stainless steel
• Heat-resisting steel
• High-speed steel varieties

Oxygen Processes: Revolutionary Advancements in Steel Production

The evolution of oxygen-based steelmaking processes marks a significant advancement in addressing the limitations of traditional Bessemer steel, particularly its high nitrogen content which poses challenges in cold forming applications. Continental steel works have developed several innovative oxygen-based processes, each offering unique advantages in modern steel production.

The LID (Linz-Donawitz) process, developed in Austria, represents a breakthrough in converting low phosphorus pig iron into high-quality steel. This method employs top blowing with an oxygen lance in a basic lined vessel (Fig. 2b). To maintain optimal temperature control, manufacturers incorporate scrap or ore additions. The result is exceptional quality steel with remarkably low hydrogen and nitrogen content, typically achieving nitrogen levels as low as 0.002%. An enhanced variant, the OLP process, introduces lime powder to the oxygen jet, enabling the processing of higher phosphorus pig iron.

Sweden's contribution to oxygen steelmaking comes in the form of the Kaldo process, which combines top blowing oxygen with a rotating furnace design operating at 30 revolutions per minute (Fig. 2a). This innovative approach ensures efficient mixing while allowing simultaneous removal of carbon and phosphorus from high-phosphorus pig iron (typically containing 1.85% phosphorus). The process is complemented by strategic additions of lime and ore.

The German Rotor process offers yet another variation, utilizing a rotary furnace equipped with dual oxygen nozzles – one submerged in the metal and another positioned above it (Fig. 2c). Meanwhile, traditional basic Bessemer processes have been modernized through the combination of oxygen with steam, effectively producing low nitrogen steel while managing temperature control.

Figure 2. Comparative diagrams of (a) Kaldo, (b) LID, and (c) Rotor processes

Emerging technologies in this field include the Fuel-oxygen-scrap (FOS) process and spray steelmaking. The latter employs an innovative approach where iron is poured through a specially designed ring fitted with oxygen and flux jets, creating an atomization effect. This technique achieves rapid chemical refining through an enhanced surface-to-mass ratio, resulting in efficient steel conversion.

These modern oxygen-based processes consistently produce steel with remarkably low levels of nitrogen, sulfur, and phosphorus, achieving quality standards that compete favorably with traditional open-hearth steel while offering improved efficiency and control.

Vacuum Degassing and Advanced Refining Techniques

The implementation of vacuum degassing technology represents a significant leap forward in steel refinement, particularly for specialized alloy production. This sophisticated process has evolved into approximately 14 distinct methods, broadly categorized into stream, ladle, mold, and circulation techniques, with DH and RH processes being notable examples of the circulation method.

The vacuum degassing process achieves multiple critical objectives in steel refinement. Under vacuum conditions, the process effectively removes hydrogen and atmospheric impurities, while also eliminating volatile elements such as tin, copper, lead, and antimony. The vacuum environment promotes the reduction of metal oxides through the carbon-oxygen reaction and facilitates the removal of oxides formed during standard deoxidation processes. This comprehensive purification allows for precise control of alloy composition within stringent parameters.

The benefits of vacuum degassing are particularly evident in the enhanced properties of the final product:

• Improved transverse properties in rolled products
• Significantly enhanced fatigue life in bearing steels
• Achievement of lower carbon contents in stainless steels
• Exceptional consistency in metal quality

Figure 3. Methods of degassing molten steel

Vacuum Melting and Electroslag Refining (ESR) have emerged as crucial processes in meeting the demanding requirements of aerospace applications. These advanced techniques address specific challenges in producing complex alloy steels, particularly regarding macro-segregation control and the management of non-metallic inclusions. They also enable precise control of reactive elements such as titanium, aluminum, and boron.

Three primary advanced melting processes have been developed:

1.Vacuum Induction - Melting operates within a sealed tank, primarily used for super alloy production. This method is particularly effective with nickel and cobalt-based alloys, often serving as a preliminary step for investment casting. The process excels in removing volatile impurities while maintaining material purity.

2. The Consumable Electrode - Vacuum Arc Re-melting process, initially developed for titanium processing, has proven invaluable in eliminating hydrogen and various types of segregates. Its unique solidification pattern produces exceptionally clean steel, making it ideal for aircraft engine components where strength, uniformity, and freedom from impurities are crucial.

Figure 4. Typical vacuum arc remelting furnace

3. Electroslag Refining (ESR), an expansion of welding technology, remelts pre-formed alloy electrodes in a water-cooled crucible. The process utilizes electrical resistance heating within a molten slag pool, ensuring vertical unidirectional freezing from the base through a protective slag layer.

Figure 5. Electroslag remelting furnace