Low-alloy steel is a type of steel that contains a small percentage of alloying elements, typically less than 5%. These alloying elements are added to enhance specific properties of the steel, such as strength, toughness, and corrosion resistance, without significantly increasing the overall cost. Low-alloy steels are widely used in various industries due to their improved mechanical and physical characteristics compared to plain carbon steels. There are several classification systems for low-alloy steels based on the types of alloying elements present and their percentage in the steel composition. Some common low-alloy steel classifications include:

1. Based on Alloying Elements: a. Low-Alloy Steels with Chromium (Cr): These steels contain chromium as the primary alloying element. Chromium improves hardenability, strength, and corrosion resistance. b. Low-Alloy Steels with Nickel (Ni): Nickel is added to increase toughness, impact resistance, and corrosion resistance in low-alloy steels. c. Low-Alloy Steels with Molybdenum (Mo): Molybdenum enhances the strength and high-temperature properties of low-alloy steels. d. Low-Alloy Steels with Vanadium (V): Vanadium improves the hardenability and wear resistance of low-alloy steels. e. Low-Alloy Steels with Titanium (Ti): Titanium is added to improve grain refinement and strengthen the steel.

2. Based on Usage: a. Low-Alloy Structural Steels: These steels are used in construction and engineering applications, providing higher strength and improved structural integrity. b. Low-Alloy High-Strength Steels (HSLA): These steels have higher strength-to-weight ratios, making them ideal for weight-sensitive applications, such as automotive and aerospace components. c. Low-Alloy Pressure Vessel Steels: These steels are designed for use in pressure vessels, such as boilers and storage tanks, where resistance to high pressures and temperatures is crucial. d. Low-Alloy Wear-Resistant Steels: These steels are formulated to withstand abrasion and wear, making them suitable for mining, construction, and earth-moving equipment.

3. Based on ASTM/ASME Standards: The American Society for Testing and Materials (ASTM) and the American Society of Mechanical Engineers (ASME) have developed specific standards for classifying low-alloy steels based on their properties and applications. Some common designations include ASTM A588, ASTM A709, and ASTM A572 for structural steels, and ASTM A387 for pressure vessel steels.

Low-alloy steels offer a wide range of mechanical properties and are utilized in critical applications where high strength, toughness, and corrosion resistance are required. The choice of low-alloy steel depends on the specific application, design requirements, and desired performance characteristics.