A Comprehensive Guide to Steel

Steel is an exceptional material that has played a crucial role in modern society. As the alloy of iron and carbon, steel demonstrates superior strength and fracture resistance compared to other forms of iron. This versatile material is used in various industries, including construction, transportation, manufacturing, and weaponry. This article will discuss steel’s definition, properties, types, and applications and explore its history and composition.

A Comprehensive Guide to Steel

The History of Steel

The history of steel begins with the history of iron. The discovery and rise to prominence of iron, which we now call the Iron Age, began around the 12th century BCE, although the timing varied depending on the geographical location. In Europe, it is considered to have started around the 5th century BCE. Early steels, which consist of iron with added carbon to increase strength, were made in China around that time but generally had too low an iron content to be considered true steel. In India, around 400 BCE, true steel was created by melting iron and charcoal in small crucibles.

Various steel and cast iron types, which contain a higher carbon content, were developed worldwide during medieval times and into modern history. A vital advancement was made by Benjamin Huntsman in England in 1751 when he used coal from the earth’s crust instead of charcoal from timber to heat the crucibles. An even more significant breakthrough was achieved by Henry Bessemer, who obtained a British patent in 1855 for a steel manufacturing process. His process involved blowing air directly through the molten iron and its additives, which became essential to modern commercial steel production.

It wasn’t until around 1912-1914 that stainless steel was first developed with the addition of chromium and nickel, and these materials were gradually refined throughout the end of World War II.

The History of Steel

Definition of Steel

Steel is an alloy primarily made from iron and carbon, with carbon content commonly ranging from 0.2% to 2.1% by weight. Carbon alters iron’s properties, making it stronger, harder, and more durable. Other elements, such as manganese, nickel, chromium, and molybdenum, are often added to enhance specific steel properties. For example, adding 11% chromium can make stainless steel resistant to corrosion and oxidation.

Steel is an incredibly useful and common ferrous alloy in modern use due to its high strength, substantial toughness, and ability to be significantly altered by heat treatment. Besides, due to its high tensile strength and low cost, steel is utilized in construction, tools, marines, trains, cars, bicycles, machines, electrical appliances, and furniture.

Definition of Steel

Chemical Composition of Steel

Carbon steels, low-carbon steels, and high-carbon steels are the three common types of steel based on their chemical composition. Common alloying elements include manganese, chromium, nickel, molybdenum, vanadium, boron, and silicon. Trace pollutants from the steelmaking process appear in all steels. These involve sulfur and phosphorus, which are extracted from the fuel and rock in the blast furnace, and manganese, aluminum, and silicon, which are eliminated during the ladle’s deoxidation process. Copper and other metals, considered as residuals, are brought into the steelmaking kiln via scrap. These components commonly account for less than 1% of steel, making them neither alloys nor alloy precursors.

The Producing Process of Steel

When steel is produced, it undergoes complex processes which transform raw materials into the finished product. Two main steelmaking methods exist: the traditional Blast Furnace and modern Electric Arc Furnace processes. Here’s a detailed explanation of each process:

Blast Furnace Process

  1. Iron Ore Extraction: The first step in the Blast Furnace process is to extract iron ore. Iron ore usually contains iron oxide and impurities like silica, alumina, and other elements. Miners extract iron ore from mines and then transport it to steel mills.
  2. Preparation of Iron Ore: The iron ore is extracted, crushed into small pieces, and refined to remove impurities to create a concentrated iron ore containing a high iron content.
  3. Charging the Blast Furnace: The blast furnace is a large structure, usually a tall cylindrical tower lined with refractory materials to endure high temperatures. Iron ore, coke (a carbon-rich material derived from coal), and limestone are loaded into the top of the blast furnace.
  4. Reduction of Iron Ore: As the raw materials descend into the Blast Furnace, hot air is blown in from the bottom. The coke reacts with the iron ore, reducing the iron oxide to molten iron, and the limestone reacts with impurities, forming slag.
  5. Collection of Molten Iron: The molten iron, now separated from impurities and called pig iron, accumulates at the bottom of the blast furnace due to its higher density. The lighter slag floats on top of the molten iron.
  6. Tapping: Periodically, the molten iron and slag are removed from the Blast Furnace. The iron is then poured into molds to form pig iron, while the slag is processed for various applications.
  7. Further Processing: Pig iron contains a high percentage of carbon and other impurities, and it is further processed in an Electric Arc Furnace or a Basic Oxygen Furnace to produce steel with the desired properties.

Electric Arc Furnace (EAF) Proces

  1. Scrap Collection: The Electric Arc Furnace process primarily utilizes recycled steel, known as scrap, as the primary raw material. Scrap encompasses various steel products, including appliances, old cars, and industrial waste.
  2. Charging the Furnace: Scrap is loaded into the electric arc furnace, a large container lined with refractory materials. The furnace is equipped with graphite electrodes, which conduct an electric current.
  3. Melting: An electric arc is formed between the electrodes and the scrap, creating high-intensity heat that melts the scrap into liquid steel. This melting method is quicker and more energy-efficient than the Blast Furnace technique.
  4. Alloying and Refining: Alloying elements can be added to molten steel to achieve specific properties; fluxes are also added to remove impurities, and oxygen is blown into the furnace for further refinement.
  5. Tapping: Once the molten steel reaches the desired composition and temperature, it is extracted from the Electric Arc Furnace and then shaped or processed further.
  6. Continuous Casting or Forming: The molten steel can be transformed into semi-finished products like billets or slabs through constant casting or finished products through forging, rolling, or other shaping processes.
  7. Heat Treatment: The final steel products may undergo heat treatment processes like tempering and quenching to improve their mechanical properties.

The Blast Furnace and Electric Arc Furnace processes are vital in steel production. Each method has its advantages and applications. The method option depends on factors such as cost considerations, the desired steel properties, and the availability of raw materials.

Steel FlowlinesSteel Finishing

Source: AISI

The Characteristics of Steel

The properties of steel can vary significantly based on its composition and processing. Some critical characteristics of steel include:

Strength: Steel is known for its high strength compared to other building materials like aluminum or plastic. This allows it to endure heavy loads and resist deformation. Steel’s tensile strength ranges from 200 MPa for mild low-carbon steels to over 1500 MPa for certain martensitic steels that have been tempered and hardened. Steel’s strength is attributed to its carbon content, with higher-carbon steels being stronger than lower-carbon steels.

Hardness: Steel’s carbon content enhances its hardness compared to pure iron. This hardness improves wear resistance and the capacity to maintain a sharp cutting edge. Similar to strength, hardness grows as the carbon content increases. Hardness can generally vary from 60 HB for mild steels to over 650 HB for specialized steels.

Toughness: Additional carbon improves strength and hardness but reduces toughness. Toughness measures a material’s ability to absorb energy before fracturing, crucial for withstanding sudden impacts and preventing brittle fractures. Mild steels with lower carbon content generally have higher toughness than ultra-high-strength steels.

Durability: Steel is very durable and can withstand tough environmental conditions, making it suitable for long-lasting structures. Compared to pure iron, steel’s carbon content increases its hardness. This hardness improves wear resistance and the ability to maintain a sharp cutting edge. Like strength, hardness increases as the carbon content increases. Hardness ranges from 60 HB for mild steels to over 650 HB for specialized steels.

Thermal Conductivity: Steel is an efficient conductor of heat, making it an ideal option for applications requiring effective heat transfer.

Weldability and Ductility: Low-carbon steels are extremely weldable, while higher-carbon steels can crack during welding. Special precautions have to be taken when welding high-carbon or alloy steels.

Formability: Lower-carbon steels can be more easily bent, forged, or stamped into shape. Higher-carbon steels are less ductile and harder to form.

Corrosion Resistance: Pure iron and mild steels have low corrosion resistance and readily rust. However, alloying elements like chromium, nickel, and copper can significantly improve corrosion resistance. Specific types of steel, such as stainless steel, exhibit outstanding corrosion resistance, making them perfect for applications in corrosive environments.

Electrical conductivity: While not as conductive as aluminum or copper, steel still has reasonable electrical conductivity, making it useful in electrical applications.

Recycling: Steel is fully recyclable, and due to its value, a large amount of steel is recycled globally.

The Properties of Steel

Physical Properties of Steel

PropertiesValueUnitExamples
Density( kg/m3)7,700 – 8,050Kilograms per cubic meterCarbon Steel, Alloy Steel
Melting Point(°C)1,370 – 1,540Degrees CelsiusHigh Carbon Steel, Stainless Steel
Tensile Strength(MPa)370 – 2,070 Megapascals304 Stainless Steel, 4340 Alloy Steel
Thermal Expansion( µm/m·K)10.8 – 13.5Micrometers per meter·KelvinMedium Carbon Steel, Alloy Steel
Thermal Conductivity(W/m·K)15 – 50 Watts per meter·KelvinCarbon Steel, 316 Stainless Steel
Young’s Modulus(GPa)190 – 210 GigapascalsLow Carbon Steel, Tool Steel
Crystal StructureFace-centered cubic (FCC) or body-centered cubic (BCC)N/ACarbon Steel, Stainless Steel
Hardness( HRC)40 – 70Rockwell hardness scaleD2 Tool Steel, 4140 Alloy Steel

Chemical Properties of Steel

Steel TypeOxidationCorrosion ResistanceReactivityStabilityMagnetism
Carbon SteelProneModerate to lowReacts with certain chemicalsStableNon-magnetic
Alloy SteelVaries based on alloying elementsVaries based on alloying elementsVaries based on alloying elementsVaries based on alloying elementsVaries based on alloying elements
Stainless SteelResistantHighResists chemical reactionsStableNon-magnetic (some types can be slightly magnetic)

Different Types of Steel

There are various types of steel, each with its specific properties and applications. Here are several typical types:

1. Carbon Steel

It is the most basic form of steel, primarily composed of iron and carbon. Depending on its carbon content, carbon steel could be further grouped into low-carbon steel, medium-carbon steel, and high-carbon steel.

2. Stainless Steel

Stainless steel is recognized for its outstanding corrosion resistance because of the presence of chromium and other alloying elements. It is extensively utilized in applications requiring resistance to rust and staining, like kitchen appliances, architectural structures, and medical instruments.

3. Alloy Steel

It contains additional alloying elements beyond carbon, like manganese, nickel, chromium, or molybdenum. These elements enhance specific properties like hardness, strength, and heat resistance. Alloy steels are commonly utilized in machinery, automotive components, and construction equipment.

4. Tool Steel

Tool steel is designed for making tools, molds, and dies. It is hard, tough, and wear-resistant for high-temperature and high-stress applications. It is commonly used in drills, cutting tools, and industrial machinery.

5. High-Strength Low-Alloy (HSLA) Steel

Compared to carbon steel, HSLA steel demonstrates enhanced mechanical properties, including improved formability and higher strength. This is achieved by incorporating small quantities of alloying elements like vanadium, titanium, or niobium. HSLA steel is frequently utilized in structural applications, pipelines, and automotive components.

6. Weathering Steel

Also known as corten steel, it is a type of steel that develops a protective rust-like surface when exposed to the elements. It is extremely resistant to atmospheric corrosion and is commonly used in outdoor structures, bridges, and artistic sculptures.

 7. Electrical Steel

Also known as silicon steel, it is designed specifically for use in electric power generation, transmission, and distribution. Its low electrical conductivity and minimal magnetic losses make it perfect for electrical transformers, generators, and motors.

Different Types of Steel

Applications of Steel

Construction: Steel is an essential material in construction, utilized for structural frames, columns, beams, and reinforcement bars.

Transportation: Due to its strength and durability, it is used in the manufacturing of automobiles, trains, ships, and aircraft.

Infrastructure: Bridges, tunnels, and pipelines often rely on steel for their structural integrity and load-bearing capacity.

Machinery and Equipment: Steel is crucial for manufacturing tools, machinery, and equipment across various industries, from agriculture to manufacturing.

Energy Sector: Used in producing wind turbines, oil and gas pipelines, and power transmission infrastructure.

Consumer Goods: Steel is commonly used in everyday items, from household appliances to cutlery, due to its durability and resistance.

Advantages & Disadvantages

Advantages

Strength and Durability: Steel is famous for its high strength and durability, making it appropriate for heavy-duty uses.

Versatility: Steel is easily shaped for versatile design and manufacturing.

Sustainability: Steel is a fantastically recyclable material, which reduces its environmental impact and promotes sustainability.

Dimensional stability: Steel is highly rigid and resistant to deformation.

Fire Resistance: Steel is an excellent choice for structures requiring fire protection due to its outstanding fire-resistant properties.

Cost-Effective: Despite its many advantages, steel remains cost-effective because of its availability and recyclability.

Disadvantages

Corrosion: Some types of steel are susceptible to corrosion, so they need to be protected with coatings or corrosion-resistant alloys.

Weight: Steel is denser than many other materials, which can be a drawback in weight-sensitive uses.

Energy Requirement: Although modern techniques have improved its energy footprint, steel production consumes significant energy and natural resources.

Thermal conductivity: Steel conducts heat well, complicating insulation, cooling, and heating of large steel buildings.

Cost: Steel, especially specialized grades and stainless steel, is more expensive than many other materials.

Summary

Steel is an essential construction material due to its abundance, affordability, strength, and diverse properties, which shape the modern industrial world. Metallurgists can tailor its hardness, toughness, and corrosion resistance through carbon and alloying additions to suit any application. Steel serves as a middle ground between pure iron’s softness and cast iron’s brittleness, enabling the construction of infrastructure, transportation, energy, consumer goods, machinery, and defense applications that are integral to our lives.