Low alloy structural steel: steel with a total alloy composition of less than 5%

Low alloy structural steel refers to alloy structural steel with total alloy composition less than 5%. The steel is similar in carbon content to mild steel and is strengthened by a small amount of alloying elements to improve toughness and weldability. Its strength is much higher than that of carbon steel with the same carbon respect. Widely used in pressure vessels, chemical equipment, boilers, Bridges, vehicles, ships and large steel structures. Alloy elements manganese, silicon, molybdenum inch to cause solution strengthening. Vanadium and niobium can refine grain and improve toughness. Molybdenum can play a role in improving hardenability, promoting the acquisition of bainite structure, but also improve the thermal strength.

Brand name and its representation

According to the provisions of the national standard (GB1591-1994), there are five brands of low-alloy structural steel in China. The elements added mainly include manganese, silicon, vanadium, titanium, aluminum, chromium, nickel and rare earth elements. Its brand is represented by the yield point letter Q, yield point value, quality grade (A, B, C, D, E five grades). It is divided into five grades, which are expressed as follows: Yield grade - Quality grade. Yield point rating: Q295, Q345, Q390, Q420, Q460.

Performance requirement

1. Good comprehensive mechanical properties. Common low alloy structural steel should have high yield limit first, but also have good comprehensive mechanical properties because of the complexity of its working conditions. For example, it can withstand various stresses in use (such as stress caused by temperature difference, alternating fatigue load, etc.), and can withstand shearing, cold bending, welding and other processing processes in the manufacturing process, as well as the aging brittleness that may be generated.

2, good process performance. It is required that common low-alloy steel should have good machining and forming properties, and use common processing methods such as cutting, stamping, hot bending and welding to produce good quality finished products. For the boiler, pressure vessel, steel structure and other general welding method, so the steel should have good flame cutting performance and welding performance, the performance of the heat affected zone near the weld is small, the weld and the nearby area will not crack, the comprehensive mechanical properties of the welding joint should not be less than (or rarely less than) the base material. In addition, the steel is required to have good cold stamping performance.

3, good corrosion resistance. Due to the ordinary low alloy steel and strength is much higher than carbon steel, with its manufacturing pressure vessel, steel structure wall thickness is much smaller than carbon steel, so must correspondingly improve atmospheric corrosion (especially Marine atmospheric corrosion) caused by the loss rate, so that it has a good corrosion resistance under various atmospheric conditions. Therefore, the corrosion resistance test of steel should be carried out in addition to the laboratory, but also in the field of use. Of course, it is essential to adopt appropriate external anticorrosion technology for carbon steel, low alloy steel and other materials.

application

Due to the strengthening effect of alloying elements, low alloy structural steel not only has higher strength, but also has better plasticity, toughness and weldability. The comprehensive performance of Q345 steel is better, is the common grade of steel structure, Q390 is also recommended to use the grade. Compared with carbon structural steel Q235, low alloy high strength structural steel can save 20%~30% of steel, and has good dynamic load and fatigue resistance. Low alloy structural steel is mainly used for rolling various types of steel, steel plate, steel pipe and reinforcement, widely used in steel structure and reinforced concrete structure, especially suitable for all kinds of heavy structure, long-span structure, high-rise structure and bridge engineering, bearing dynamic load and impact load structure.

Low alloy structural steel is a kind of steel with low carbon structure, the content of alloying elements is less, generally less than 3%, mainly plays the role of refining grain and improving strength. This type of steel is significantly stronger than carbon steel with the same carbon content, so it is often called low-alloy high-strength steel. It also has good toughness, plasticity and good weldability and corrosion resistance. Originally used in bridge, vehicle and ship industries, its application scope has expanded to boiler, high pressure vessel, tubing, large steel structure and automobile, tractor, earth-moving machinery and other products.