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structural steel[′strək·chə·rəl ¦stēl]
a general term for a group of steels designed for the manufacture of building structures and of components for machines and mechanisms. Structural steel used for building structures must have high weldability; the carbon content must not exceed 0.25 percent. A distinction is made between carbon steel and high-strength low-alloyed steel (less than 5 percent alloying elements); structural steel is also distinguished according to purpose (steel for bridge construction and for the frames of high-rise buildings).
Structural steel used in mechanical engineering is classified in terms of chemical content as carbon steel or alloyed steel (chromium, chromium-nickel, and so on); in terms of method of manufacture, as wrought or foundry steel; and in terms of working conditions, as construction, heat-resistant, stainless (corrosion-resistant), or wear-resistant steel. Depending on the carbon content, a distinction is made between mild case-hardened steel (0.1-0.25 percent carbon) and so-called refined steel (0.25-0.45 percent carbon). Steel with a higher carbon content (0.5-0.65 percent carbon) is used for certain components (for example, springs). Depending on the degree of alloying, steel for mechanical engineering is divided into low-alloyed (less than 5 percent alloying elements), medium-alloyed steel (5-10 percent), and high-alloyed steel (more than 10 percent). Machine components manufactured from steel undergo heat treatment. Depending on the magnitude and nature of loads being absorbed by the component, the steel must satisfy requirements for ultimate strength (σt may reach 2.5-3.0 giganewtons per sq m, or 250–300 kilograms-force per sq mm), plasticity, impact resistance, endurance limit, weldability, and hardenability.
Steel used in engineering structures, usually manufactured by either the open-hearth or the electric-furnace process. The exception is carbon-steel plates and shapes whose thickness is 7/16 in. (11 mm) or less and which are used in structures subject to static loads only. These products may be made from acid-Bessemer steel. The physical properties and chemical composition are governed by standard specifications of the American Society for Testing and Materials (ASTM). Structural steel can be fabricated into numerous shapes for various construction purposes.