Classification of steel
Steel is an iron-carbon alloy (with a carbon content of less than 2.11%) and other alloying elements. Due to its excellent mechanical properties, machinability, and cost-effectiveness, it has become the most widely used metal material in modern industry. Steel can be classified in numerous ways, including by chemical composition, quality grade, application, smelting method, and microstructure. These classifications reflect the diverse properties and applications of steel. An accurate understanding of steel classification facilitates optimal material selection and proper use. Understanding the steel classification system is crucial for material selection and process development in fields such as machinery manufacturing, construction engineering, and the automotive industry.
Steel is most fundamentally classified by chemical composition, falling into two main categories: carbon steel and alloy steel. Carbon steel (abbreviated as carbon steel) is steel with a carbon content of less than 2.11% and no intentionally added alloying elements. Its performance is primarily determined by its carbon content. Higher carbon content increases strength and hardness, but also reduces plasticity and toughness. Carbon steel can be categorized by carbon content into low-carbon steel (carbon content ≤ 0.25%), medium-carbon steel (carbon content 0.25%-0.60%), and high-carbon steel (carbon content > 0.60%). Low-carbon steels, such as Q235, exhibit excellent plasticity and weldability and are commonly used in the manufacture of bolts, plates, and other components. Medium-carbon steels, such as 45 steel, offer moderate strength and toughness, and can be improved through quenching and tempering. They are widely used in shaft components. High-carbon steels, such as T10 steel, offer high hardness and wear resistance, making them suitable for manufacturing tools, springs, and other components. Alloy steel is carbon steel to which one or more alloying elements (such as chromium, nickel, manganese, silicon, molybdenum, etc.) are deliberately added. The addition of alloying elements can improve the steel’s hardenability, strength, toughness, wear resistance and other properties. According to the total content of alloying elements, it can be divided into low-alloy steel (total content ≤5%), medium-alloy steel (total content 5%-10%) and high-alloy steel (total content >10%). For example, 20CrMnTi is a commonly used low-alloy carburizing steel, and 1Cr18Ni9Ti is a typical high-alloy stainless steel.
Steel can be categorized by quality grade into standard, high-quality, and premium steel. This classification is primarily based on the content of harmful impurities (sulfur and phosphorus) and the quality of the steel’s smelting. Standard steel has a sulfur content of ≤0.050% and a phosphorus content of ≤0.045%. Impurity levels are not specifically controlled during the smelting process. For example, carbon structural steel Q235 is primarily used for structural parts subject to low loads. High-quality steel has sulfur and phosphorus contents of ≤0.035%. Degassing is required during smelting to ensure uniform chemical composition and stable mechanical properties. Examples include 45 high-quality carbon structural steel and 40Cr alloy structural steel, which are widely used in the manufacture of critical mechanical parts. Premium steel has a sulfur content of ≤0.025% and a phosphorus content of ≤0.030%. This further reduces impurity levels compared to high-quality steel. Some steel grades also undergo vacuum smelting. The steel is typically marked with an “A” on the surface, such as 38CrMoAlA, a high-quality alloy structural steel. It is suitable for manufacturing high-precision, complex parts subject to high loads, such as turbine main shafts and precision gears.
Steel can be categorized by application into three main groups: structural steel, tool steel, and special performance steel. This is the most commonly used classification in industrial production. Structural steel is used to manufacture various engineering structures and mechanical parts. It can be further divided into engineering structural steel (such as carbon structural steel and low-alloy high-strength structural steel) and machinery manufacturing steel (such as high-quality carbon structural steel and alloy structural steel). Engineering structural steel emphasizes weldability and toughness, such as Q355 used in bridges and building steel structures; machinery manufacturing steel emphasizes strength and wear resistance, such as 20Cr used in carburized gears. Tool steel is used to manufacture various cutting tools, molds, and gauges. It can be divided into cutting tool steel (such as T12A), mold steel (such as Cr12), and gauge steel (such as CrWMn). Tool steel requires high hardness, high wear resistance, and sufficient toughness, and typically undergoes quenching and tempering. Special performance steel is steel with special physical or chemical properties, such as stainless steel (such as 1Cr13) with corrosion resistance, heat-resistant steel (such as 1Cr18Ni9Ti) with high temperature strength and oxidation resistance, wear-resistant steel (such as ZGMn13) with high impact toughness and wear resistance. Special performance steel is widely used in special environments such as chemical industry, aviation, mining, etc.
Steel can be subdivided by smelting method, smelting equipment, degree of deoxidation, and other factors, reflecting the characteristics of the steel production process. Based on smelting equipment, it can be divided into converter steel, electric furnace steel, and open-hearth steel. Converter steel offers high production efficiency and low cost, making it the most produced steel type. Electric furnace steel (especially electric arc furnace steel) allows for precise control of chemical composition, making it suitable for producing high-alloy and high-quality steel. Open-hearth steel has been gradually replaced by converter steel and electric furnace steel due to its long production cycle and high energy consumption. Based on the degree of deoxidation, it can be divided into rimmed steel, killed steel, and semi-killed steel. Rimmed steel is incompletely deoxidized, generating large amounts of gas during solidification, resulting in a high number of bubbles and good plasticity, but with uneven composition and low cost. Killed steel is completely deoxidized, resulting in a calm solidification of the molten steel, resulting in a dense structure and uniform properties, but at a higher cost. Semi-killed steel has a deoxidation level somewhere in between, with intermediate performance and cost. The smelting method directly affects the purity and stability of the steel. The selection should be considered in conjunction with application requirements and budget.
