Steel Production: An Overview of Furnace Processes and Alloying Elements
Steel Production Processes
Open Hearth Furnace
One of the most popular steel production methods, the open hearth furnace, can hold 10 to 540 tons of metal. Its shallow depth allows flames to directly heat the load. Fuel sources include gas, tar, or oil. These regenerative furnaces often have side fireplaces to expel heated gases used for air and fuel heating. Open hearth furnaces typically have basic linings, but acid linings (silica and clay brick walls) also exist. Basic linings offer better control over phosphorus, sulfur, silicon, magnesium, and carbon removal, while acid linings primarily control carbon. However, basic linings are more expensive.
Electric Arc Furnace
Electric arc furnaces primarily melt high-quality scrap steel for producing tool steels, high-quality steels, temperature-resistant steels, or stainless steels. These furnaces always have basic brick linings for high-quality steel production. Some electric arc furnaces can hold up to 270 tons of molten material. Melting 115 tons takes roughly three hours and 50,000 kWh. Pure oxygen is injected through a lance. These furnaces operate with three graphite electrodes, which can be up to 760mm in diameter and 12m long. The furnace body, a steel pot or plate lined with refractory material, is supported by a water-cooled steel belt. During loading, electrodes move to open the vault, allowing a crane to deposit the load into the crucible. Electric arc furnaces are prevalent in small to medium-sized industries producing steel for specific purposes like corrugated profiles or special alloys.
Refining Furnaces
Refining furnaces come in various forms and use air or oxygen to control carbon content in iron. Two prominent examples are:
Air or Crucible Furnace
The oldest casting process, the air furnace, uses fragile clay and graphite crucibles placed within a container holding solid fuel like coal or combustion products. Crucibles are less common today, except for melting nonferrous metals, with capacities between 50 and 100 kg.
Cupola Furnace
Cupola furnaces are cost-effective and low-maintenance, used for making cast iron. These furnaces are over 4 meters long with diameters ranging from 0.8 to 1.4 meters. They are loaded from the top with layers of scrap iron, coke, and limestone. High-pressure fans inject air for coke combustion through nozzles at the bottom. These furnaces can also process iron ore, pig iron pieces, or solid iron. For every kilogram of coke, 8 to 10 kg of iron are processed. One ton of iron requires 40kg of limestone and 5.78 cubic meters of air at 100 kPa and 15.5°C. Cupola furnaces can produce up to 20 tons of castings every three hours. Similar to blast furnaces but smaller, their main drawback is the high cost of emission control equipment, often exceeding the furnace’s cost, leading to uncontrolled dust emissions and operational restrictions.
Steel Identification and Casting
AISI/SAE Steel Identification
The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) have standardized steel identification using four or five digits. The AISI system also includes a letter prefix indicating the production process.
- First digit: Predominant alloying element (1=carbon, 2=nickel, 3=nickel-chromium, 4=molybdenum, 5=chromium, 6=chromium-vanadium, 8=triple alloy, 9=silicon-magnesium).
- Second digit: Approximate percentage of the primary alloying element.
- Third and fourth digits: Average carbon content in hundredths.
- Prefix (AISI): Production process (A=basic open hearth, B=acid Bessemer, C=basic oxygen converter, D=acid open hearth, E=electric furnace).
For example, 2540 indicates a nickel alloy steel with 5% nickel and 0.4% carbon.
Ingot and Continuous Casting
To create usable metal objects like wires, bars, sheets, plates, tubes, and structural shapes, iron undergoes rolling. This involves passing the metal through shaped rollers to achieve the desired form. The metal is first cast into ingots or blooms, which can be rectangular, square, or round, and range from 25 kg to several tons.
Continuous Casting
For large quantities of material with a constant cross-section, continuous casting is used. Molten metal flows from a crucible through a mold with the desired shape. A water cooling system solidifies the metal as it passes through the mold. Rollers then shape the material as it’s drawn out of the system. Once the material reaches the required length, it is cut and stored. This method produces profiles, rods, bars, sheets, and plates of various sizes.
Chemical Elements in Ironworks
Several chemical elements influence the engineering properties of ferrous alloys:
- Carbon: Up to 4% in low-quality iron, carbon contributes to hardness and defines various alloy properties and machinability.
- Silicon: Up to 3.25%, silicon softens iron and influences carbon content. Above 3.25%, it acts as a hardener. Low-silicon castings respond better to heat treatments.
- Manganese: Above 0.5%, manganese removes sulfur from iron. The manganese-sulfur compound floats as slag due to its low density. It also improves fluidity, strength, and hardness.
- Sulfur: An undesirable impurity that should be removed.
- Phosphorus: Increases fluidity and reduces melting temperature.