Steel is a fundamental material in modern infrastructure, but it is not a single substance. It is an iron alloy whose properties are dramatically altered by small adjustments in its chemical composition or processing methods. This variability necessitates a robust classification system to ensure the correct material is selected for a given application. Steel classification generally relies on three main approaches: chemical makeup, manufacturing processes, and standardized naming systems.
Categorizing Steel by Chemical Makeup
The most direct way to classify steel is by examining its chemical recipe, with the percentage of carbon being the most influential component. Steel is commonly grouped into three categories based on carbon content, which directly dictates the metal’s mechanical properties. Low-carbon steel, often called mild steel, typically ranges from 0.05% to 0.30% carbon by weight. This composition results in a highly ductile, easily welded, and malleable material, making it the most common form used in structural applications and general sheet metal.
Medium-carbon steel contains between 0.30% and 0.60% carbon, providing a balance between the strength of high-carbon steel and the ductility of low-carbon steel. This increased carbon content allows the steel to be heat-treated to improve strength and hardness. It is well-suited for automotive components, machinery parts, and railway tracks. As the carbon level rises, the material becomes less ductile and more challenging to weld.
High-carbon steel contains more than 0.60% carbon, sometimes reaching up to 1.5% in specialized grades. This high percentage significantly increases the steel’s hardness, tensile strength, and wear resistance, allowing it to hold a sharp edge. It is used for tools, springs, and high-strength wires. This added hardness comes at the expense of ductility, making the material more brittle and difficult to form or weld.
Beyond carbon, the inclusion of other elements creates three major steel families. Carbon steel is defined by the absence of significant amounts of alloying elements other than carbon, silicon, and manganese. Alloy steel is purposefully combined with elements like nickel, molybdenum, and vanadium to enhance specific characteristics such as strength, toughness, and resistance to high temperatures. For example, molybdenum increases strength at elevated temperatures, while nickel improves toughness.
Stainless steel is a specific type of alloy steel defined by the presence of a minimum of 10.5% chromium. The chromium forms a passive, self-healing oxide layer on the surface that protects the underlying metal from corrosion and rust. Other elements, such as nickel, are often added to further improve corrosion resistance and stabilize the microstructure.
Categorizing Steel by Manufacturing Process
Steel can also be classified by the methods used to physically shape and treat the metal, which significantly alters its internal structure and surface qualities. The temperature at which steel is formed distinguishes between hot-rolled and cold-rolled products. Hot-rolled steel is processed at temperatures above the steel’s recrystallization point, typically exceeding 1,700°F.
Rolling the steel while highly malleable allows large shapes to be formed quickly and cost-effectively, resulting in a product free from internal stresses. However, as the steel cools and shrinks non-uniformly, hot-rolled material often has a rough, scaled surface and less precise dimensional tolerances. Conversely, cold-rolled steel is processed at or near room temperature, typically as a secondary process performed on cooled hot-rolled material.
This cold-working process requires much more force but results in a smoother, brighter surface finish and tighter dimensional accuracy. The mechanical deformation at lower temperatures also increases the steel’s hardness and tensile strength through strain hardening. However, this process introduces internal stresses that may require subsequent heat treatment before the steel is used.
Another classification based on processing is the deoxidation practice, which refers to how much oxygen is removed from the steel before casting. The presence of oxygen can react with carbon to form gas bubbles during solidification. Killed steel is fully deoxidized by adding strong agents like aluminum and silicon, which prevents gas evolution and causes the steel to solidify quietly, or “kill” itself.
Killed steel is characterized by a high degree of chemical uniformity and is used for high-quality applications like forging and alloy production. Semi-killed steel is partially deoxidized, allowing for controlled gas evolution that helps compensate for solidification shrinkage, thus minimizing the internal pipe cavity. Rimmed steel is poorly deoxidized, which leads to significant gas evolution and results in a chemically varied ingot with an outer rim of high purity and a core of higher impurity.
Industry Systems for Naming Steel Grades
To communicate the precise composition and type of steel, industry relies on standardized, codified naming systems. The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) developed a widely used four-digit numerical system for classifying carbon and alloy steels. This system conveys the chemical makeup without a long list of element percentages.
In this four-digit code, the first digit indicates the main alloying element present in the steel. For instance, the 1xxx series denotes a plain carbon steel, while 4xxx indicates a molybdenum steel, and 5xxx is a chromium steel. The second digit provides information about the modifying alloying elements or their concentration within the group.
The final two digits of the four-digit code are the most precise, representing the nominal carbon concentration in hundredths of a percent by weight. For example, the designation 1045 refers to a plain carbon steel containing approximately 0.45% carbon, a medium-carbon grade used for shafts and axles. The use of these standardized codes is essential for trade, engineering specifications, and regulatory compliance across global industries.
A secondary but more comprehensive system is the Unified Numbering System (UNS), which serves as a cross-referencing tool for virtually all metals and alloys. Each UNS designation consists of a single-letter prefix followed by five digits, creating a unique identifier that correlates various national and regional numbering systems. The letter prefix indicates the general metal family, such as ‘S’ for stainless steels or ‘T’ for tool steels.
The UNS system does not establish material specifications or requirements for quality, heat treatment, or form. Rather, it provides a consistent, standardized name for a specific chemical composition. This unified approach is particularly valuable in global supply chains and data management, ensuring that engineers and manufacturers worldwide are referring to the same material.