Steel is an alloy primarily composed of iron and a small percentage of carbon, which provides its strength and structure. Because steel is used in applications ranging from surgical tools to skyscraper beams, its properties and performance vary widely. Systematic classification methods are necessary to allow manufacturers and engineers to accurately identify and communicate the exact composition and characteristics of any given steel grade.
Classification by Chemical Composition
The most fundamental way to classify steel is by its chemical composition, as the elements present directly determine its mechanical properties. Adding various elements to the iron-carbon mixture allows metallurgists to tailor the steel for specific applications like corrosion resistance or extreme hardness. The four main types categorized this way are carbon, alloy, stainless, and tool steels.
Carbon Steels
Carbon steels are mainly composed of iron and carbon, with only trace amounts of other elements. These steels are further classified by their carbon content, which dictates their hardness and ductility. Low carbon steel, often called mild steel, contains a carbon range of approximately 0.04% to 0.30% and is highly ductile and weldable, making it suitable for structural components and sheet metal.
Medium carbon steel (0.31% to 0.60% carbon) provides a balance of strength and toughness, making it suitable for automotive parts and machinery components. High carbon steel (over 0.60% carbon) is the strongest and hardest of the carbon steels. Although less ductile and more difficult to weld, it is used in cutting tools and high-strength wires.
Alloy Steels
Alloy steels are intentionally modified with elements beyond iron and carbon to enhance specific properties. Elements like nickel, molybdenum, manganese, and vanadium are added in varying proportions to manipulate characteristics such as hardenability, strength, and corrosion resistance. For instance, molybdenum increases strength at elevated temperatures, while nickel improves toughness.
The combination and quantity of these alloying elements differentiate grades, optimizing them for pipelines, specialized machinery, or aircraft components. These steels often require specific heat treatments to achieve their superior mechanical properties.
Stainless Steels
Stainless steels are defined by the addition of a minimum of 10.5% chromium by weight, which provides corrosion resistance. Chromium reacts with oxygen in the atmosphere to form a thin, stable, and self-repairing passive oxide layer on the steel surface. This protective film prevents rust and staining, making it highly valued for food processing, medical equipment, and architectural finishes.
Stainless steels are divided into three main microstructural families. Austenitic steels, which often contain nickel, are non-magnetic and cannot be hardened by heat treatment, making them common in kitchenware. Ferritic steels contain chromium but little or no nickel, are magnetic, and are used for automotive trim and exhaust systems. Martensitic steels contain less nickel and more carbon, making them magnetic and heat-treatable for high-strength applications like knives and surgical instruments.
Tool Steels
Tool steels are used in manufacturing tools for cutting, forming, and shaping other materials. They are engineered for extreme hardness, resistance to abrasion, and the ability to maintain strength and shape at high temperatures. Their composition typically includes high levels of elements like tungsten, molybdenum, vanadium, and cobalt.
These steels are classified based on their primary applications and quenching methods, such as water-hardening, air-hardening, or high-speed tool steels. High-speed steel (HSS) contains elements like tungsten and molybdenum to retain a sharp cutting edge even when the friction of cutting causes the tool to become red-hot.
Classification by Deoxidation Practice
Classification by deoxidation practice focuses on how oxygen is controlled and removed from the molten metal before casting. This practice significantly influences the internal structure, uniformity, and surface quality of the final product. Deoxidation refers to adding elements like silicon and aluminum to react with the dissolved oxygen.
Killed Steel
Killed steel is fully deoxidized before casting using strong deoxidizers like silicon and aluminum. The term “killed” refers to the molten metal lying quietly in the mold because no carbon monoxide gas evolves during solidification. This complete deoxidation results in a highly uniform composition throughout the ingot, with minimal internal voids or porosity.
Due to its uniform internal structure and properties, killed steel is used for high-performance applications such as forgings, alloy steels, and any part requiring superior soundness. The disadvantage is that a shrinkage cavity, known as a pipe, forms at the top of the ingot as the steel solidifies, which must be cut off, reducing the overall yield of usable material.
Semi-Killed Steel
Semi-killed steel represents a practice intermediate between fully killed and non-deoxidized steels. A controlled, limited amount of deoxidizer is added, allowing some gas to evolve during solidification, which partially offsets the natural shrinkage of the steel. This practice yields a material with a moderate degree of uniformity and soundness. This type of steel is often used for general structural purposes and typically contains a carbon content between 0.15% and 0.25%. The partial gas evolution helps to minimize the size of the shrinkage cavity compared to killed steel, improving the material yield while still providing acceptable mechanical properties for many common applications.
Rimmed Steel
Rimmed steel, sometimes called “boiling steel,” undergoes minimal deoxidation before casting. The molten steel contains oxygen that reacts with carbon to produce carbon monoxide gas during solidification, causing a characteristic “boiling” action in the mold. This vigorous gas evolution prevents the formation of a large central shrinkage cavity, increasing the yield.
The gas evolution pushes impurities toward the center, creating a relatively pure, low-carbon outer layer, or “rim,” which results in an excellent surface finish. However, the core of the material is chemically non-uniform due to segregation of impurities. Rimmed steel is typically used for applications like sheet metal, where superior surface finish and high ductility are prioritized over internal homogeneity.
Standardized Naming Systems
Standardized naming systems are used globally to communicate steel classifications unambiguously. These systems translate the chemical composition and manufacturing process into a concise alphanumeric code for consistent identification across industries. Organizations like the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) established one of the most recognized systems.
The common AISI/SAE four-digit system is based on the steel’s chemical makeup. The first digit identifies the main class of steel (e.g., ‘1’ for carbon steel or ‘4’ for molybdenum steel), and the second digit indicates the concentration of the primary alloying element.
The last two digits represent the average carbon content, expressed in hundredths of a percent by weight. For example, ‘1040’ designates a plain carbon steel (’10xx’ series) containing 0.40% carbon. This systematic coding allows engineers to immediately understand the material’s basic composition and expected properties.
For alloy steels, the first digit identifies the main alloying element (e.g., ‘2xxx’ for Nickel steels or ‘3xxx’ for Nickel-Chromium steels). This structure provides a quick reference to the steel’s intended function. Additional letters can be added as prefixes or suffixes, such as an ‘H’ suffix to denote a special grade where hardenability is a major requirement.