Over 1.5 billion tons of steel are produced every year to make products as diverse as sewing needles and structural beams for skyscrapers.
Carbon steel is made from iron and carbon and is the most common category of steel, making up approximately 85% of all steel production in the US. As its name implies, the carbon content of the product (in the range 0–2%) gives the metal carbon like properties due to the influence of the carbon on the microstructure. Carbon steel can also contain small amounts of manganese, silicon, and copper.
Mild steel is a commercial term for low carbon steel (i.e. steel with a carbon content in the range 0.04– 0.3%). It is a general classification that is not covered by a standard specification.
Carbon and mild steel are manufactured in a 3-stage process:
- Primary steelmaking
- Secondary steelmaking
These are followed by various finishing techniques that have a direct effect on the final product characteristics.
Steel can either be made from 100% recycled material or from a combination of recycled material and virgin steel.
Virgin steel is produced in a blast furnace from iron ore, coke (produced from coal), and lime. The raw materials are added to the top of the furnace, which operates at 3000 degrees Fahrenheit. As the iron ore melts and mixes with the burning coke, carbon is released into the molten product. Impurities are absorbed by lime into a slag on the surface, which can be skimmed from the liquid steel. The product at this stage contains about 4% carbon and still has some impurities present. Molten virgin steel is transferred to the Basic Oxygen Furnace (BOF), which already contains recycled scrap metal. Pure oxygen is blown through the liquid steel to oxidize the excess carbon, forming a finished product with between 0% and 1.5% carbon content.
Recycled scrap steel can be reprocessed without the addition of virgin steel in an Electric Arc Furnace. High power electric arcs melt the metal at temperatures up to 3000 degrees Fahrenheit. As the scrap steel melts, further batches of scrap can be added to the furnace up to its capacity. Once a flat bath of molten steel is achieved, oxygen is blown through in the same manner as the BOF.
In both cases, molten steel is tapped from the furnace into ladles or steel baths for further processing, while the surface slag containing impurities is removed.
Market demands for higher quality steel products and consistent properties have fueled the development of secondary steelmaking processes. Steel composition is altered by adding or removing individual components or by manipulating the temperature. The following are the most common types of secondary processing:
- Stirring: Electromagnetic fields are used to induce turbulent currents in the ladle. This method easily separates non-metallic inclusions, which float to the surface, while ensuring a homogeneous mixture and composition of the steel.
- Ladle furnace: The ladle acts as a secondary electrode furnace enabling precise temperature control and the measured injection of alloy components.
- Ladle injection: Inert gas is injected into the bottom of the steel bath. As the gas heats up and rises through the molten steel, a stirring effect is achieved.
- Degassing: Removal of hydrogen, oxygen and nitrogen, while also reducing the sulphur content of the product. Various techniques using vacuums, inert gas injection and temperature are used to degas molten steel.
- Composition Adjustment by Sealed Argon Bubbling with Oxygen Blowing (CAS-OB): Stirring is achieved by injecting argon gas into the sealed steel bath. A snorkel arrangement prevents the slag from being disturbed while hydrogen content is reduced and oxide inclusions are floated to the surface. Oxygen is fed to the bath through a lance and aluminum is added through the snorkel giving an increased level of temperature control and accurate final composition.
A critical aspect of secondary steelmaking is the removal of oxygen. The presence of oxygen in molten steel as it begins to solidify results in a reaction with carbon to release carbon monoxide gas. Controlling deoxidation can be used to alter the characteristics of the finished product and therefore the suitability of the steel to be used for different applications.
Rimming steels are non-deoxidized or partially deoxidized steels. High levels of carbon monoxide are produced during solidification resulting in a good surface quality but the presence of a large number of blow holes.
Capped steels follow the same pattern as rimming initially, but after about a minute, the mold is capped to suppress the formation of carbon monoxide.
Semi-killed steels have been partially deoxidized prior to pouring into the mold and usually have a carbon content in the range of 0.15–0.3%.
Killed steels have been fully deoxidized such that there is no formation of carbon monoxide at all during solidification. The finished product has a homogeneous structure and no blowholes. Aluminum is added into the ladle or mold as a primary deoxidizer to “kill” the formation of carbon monoxide; however, there are applications where the addition of aluminum to the finished product is undesirable. Alternatives to aluminum are ferroalloys of manganese and silicon or calcium silicide.
Traditional casting methods involve the lifting of the ladle by crane so that molten steel can be teemed into individual molds mounted on rail cars. Ingot molds are tapered slightly to facilitate removal of the ingots after solidification. Ingots are transferred to soaking pits where they are reheated for hot rolling.
Casting machines enable continuous casting of molten steel into shapes more suitable for downstream processing. Ladles are lifted to an elevated platform where they discharge the molten steel into a tundish, which feeds the casting machine. Molten steel is fed from the tundish into a water-cooled mold with a movable bottom plate. As the steel skin solidifies, the plate is slowly lowered allowing more molten steel to enter the mold. Steel is formed into slabs, blooms, or billets in a continuous casting machine. The solidified product is pulled by rollers before being straightened and cut at the end of the machine. This process can continue for days or weeks without interruption.
Finishing processes for carbon steel
Solid cast ingots must be rolled into more useful shapes and sizes like those produced by continuous casting. Steel is compressed and pulled by rotating rolls. Because the rolls are rotating at a faster pace than the steel as it enters the machine, the rolls thrust the steel forward and compress it between them. The process can be carried out on hot, warm, or cold steel with two main categories:
- In hot forming, steel is heated above the recrystallization temperature to break up the as-cast microstructure. This yields a more uniform grain size and even distribution of carbon within the steel.
- Cold forming is carried out below the recrystallization temperature. This process increases the strength through strain hardening by up to 20% while improving the finish and allowing tighter tolerances.
Steel emerges from the rolling process as semi-finished products in the form of blooms, billets, or slabs depending on the final dimensions. A bloom is a very thick rectangular slab, a billet has a similar thickness but a narrower width, and a slab is a thinner and wider product.
Semi-finished products are further processed to intermediate products in a rolling mill to make them ready for manufacturing and final processing by downstream companies.
- Blooms are rolled into beams for structural applications.
- Rolled bars are narrow, thin strips used for machine building and construction.
- Rails can be rolled directly from blooms.
- Plates are rolled from slabs and are defined by their thickness being above 1/4 inch. Steel plates are used in heavy manufacturing like boilers, bridges industrial vessels, tanks, and ships.
- Sheets are also rolled from slabs with a thickness less than 1/4 inch and are used for car bodies, household appliances, office equipment, beverage cans, and more.
- Round or square rods are used in construction and frameworks, braces, shafts, and axles.
Once the steel leaves the rolling mill, downstream companies use many different secondary processing techniques to prevent corrosion and improve the properties of the metal with the predominant technique being heat treatment.
The purpose of heat treatment is to manipulate the mechanical properties of steel by changing the distribution of carbon in the product and the internal microstructure. When manipulating the mechanical properties of steel, an increase in ductility always results in a reduction of hardness and strength and vice versa.
Normalizing involves heating steel to a temperature about 130 degrees Fahrenheit above the upper critical temperature. The temperature is held until the entire product is uniformly heated, after which it is air-cooled. This is the most common form of heat treatment and gives the steel a high strength and hardness.
Annealing raises the temperature of steel into the solid solution state for 1 hour before cooling at a rate of 70 degrees Fahrenheit per hour by allowing it to cool in the furnace once turned off. A soft and ductile steel results with no internal stresses.
Quenching is a similar process to normalizing, but cooling is accelerated by quenching the steel in water, brine, or oil. The resulting product is very hard (up to 4 times harder than normalized steel), but very brittle, making it susceptible to breaking and cracking. For this reason, quenching to a predetermined temperature is normally followed by a controlled cooling rate down to room temperature in a process called tempering or stress relieving.
By designing the temperature and cooling rate parameters during heat treatment, the properties of steel can be precisely controlled.
Surface treating carbon steel
Approximately one third of steel produced is treated with a surface coating to inhibit corrosion, and improve weldability and paintability.
Hot dip galvanizing is a process of applying a surface coat of zinc to steel. The steel is heated before entering a zinc bath where liquid zinc layers the surface of the product. The thickness of the coating is controlled with gas-knives. To prevent the zinc coating from cracking, a small amount of aluminum is added to the zinc solution.
Electrolytic galvanizing is another process for applying a zinc coat to steel products. Zinc is deposited onto the surface of the steel through controlling the current in an electrolyte solution. This technique enables better control of coating thickness and can be used to apply differential coatings (coatings with different thicknesses on either side of a product) or zinc alloy coatings to optimize the desired characteristics.
Downstream secondary processing techniques
Downstream companies further process their steel raw materials into finished products using techniques like machining (for example, drilling) and joining (for example, welding).
Categorizing of carbon steels
Carbon steel can be categorized depending on the chemical composition and the characteristics of the product. Plain carbon steel is free from alloys and can be described in these categories:
- Low carbon steel has a 0.04–0.3% carbon content and is the most common grade of carbon steel. It is ductile, highly formable, and can be used for automobile body parts, plate, and wire products. At the higher end of the low carbon content range, and with the addition of manganese up to 1.5%, mechanical properties are changed to become suitable for stampings, forgings, seamless tubes, and boiler plates.
- Medium carbon steel has a carbon range from 0.31–0.6% and a manganese range from 0.6–1.65%. This steel can be heat treated and quenched to further adjust the microstructure and mechanical properties. Popular applications include shafts, axles, gears, rails, and railway wheels.
- High carbon steel contains carbon in the range 0.6–1% with a 0.3–0.9% manganese content. Properties of high carbon steels make it suitable for use as springs and high-strength wires. These products cannot be welded unless a detailed program of heat treatment is included in the welding procedure.
- Ultra-high carbon steels contain between 1.25–2% carbon and are known as experimental alloys. Tempering can produce a steel with a great hardness level, which is useful for applications like knives, axles, or punches.
Recycling of carbon steel
The recycling of steel is one of the success stories of sustainable living and minimizing the impact of human activities on the environment. In fact, steel is the most recycled material on the planet. The sources for recycled steel are as follows:
- Scrap from the steel mill itself
- Scrap from secondary manufacturers through offcuts and other waste material
- Scrap from steel products which reach the end of their life
Once a product leaves its production facility, it is often a long time before it reaches the end of its life. As such, there is not enough recycled steel to meet the manufacturing demand. Therefore, there is almost always a combination of virgin and recycled steel in the production of finished products.
Recycling of steel is also economically attractive as it brings down the cost of finished products. For this reason, the steel industry has been actively involved in promoting and establishing recycling networks to make it easy for end of life products to be recycled.