Low alloy structural steel refers to the alloy structural steel with total alloy composition less than 5%. The carbon content of this kind of steel is similar to that of low carbon steel, and it is mainly strengthened by a small amount of alloy elements to improve toughness and weldability. Its strength is much higher than that of the same carbon steel. Widely used in pressure vessels, chemical equipment, boilers, bridges, vehicles, ships and large steel structures. Alloy elements such as manganese, silicon and molybdenum cause solution strengthening. Vanadium and niobium can refine grains and improve toughness. Molybdenum can improve the hardenability, the bainite structure and the thermal strength.
Brand and its representation
Low alloy structural steel grades and their expression: there are five grades of low alloy structural steel in China, and the main elements are manganese, silicon, vanadium, titanium, sharp, chromium, nickel and rare earth elements. Its trademark is composed of yield point letter Q, yield point value and quality grade (Grade A, B, C, D, e). It is divided into five grades, which are expressed as follows: yield point grade – quality grade. Yield point grade: q295, Q345, Q390, Q420, Q460.
1. Good comprehensive mechanical properties. Ordinary low alloy structural steel should have high yield limit at first, but due to the complexity of its working conditions, it should also have good comprehensive mechanical properties. For example, it can bear the effects of various stresses in use (such as temperature difference stress, stress produced by alternating fatigue load, etc.), and can withstand the processing procedures such as shearing, cold bending, welding, etc. in the manufacturing process, as well as the aging brittleness that may be produced therefrom.
2. Good process performance. It is required that ordinary low-alloy steel should have good processing and forming performance, and use common methods such as shearing, stamping, hot bending and welding to manufacture finished products with good quality. For boiler, pressure vessel, steel structure and so on, welding method is generally adopted, so the steel shall have good flame cutting performance and welding performance, the performance change of heat affected zone near the weld joint is small, the weld joint and its adjacent area shall not produce cracks, and the comprehensive mechanical performance of the welded joint shall not be less than (or rarely less than) the base metal. In addition, the steel is required to have good cold stamping performance.
3. Good corrosion resistance. Because ordinary low alloy steel and its strength are much higher than carbon steel, and the wall thickness of pressure vessel and steel structure made of it is much smaller than that of carbon steel, the loss rate caused by atmospheric corrosion (especially marine atmospheric corrosion) must be increased accordingly, so that it has a good ability to resist corrosion under various atmospheric conditions.. Therefore, the corrosion resistance test of steel should be carried out not only in the laboratory, but also in the field. Of course, it is necessary to adopt appropriate external anti-corrosion technology for carbon steel, low alloy steel and other materials.
The role of alloying elements
The common low alloy steel widely used in pressure vessel is mostly ferrite pearlite structure. The final properties are obtained by hot rolling or normalizing, and its structure is accepted by the equilibrium structure of steel. The main alloy element in steel is carbon. Increasing the carbon content can increase the quantity of pearlite and increase the yield limit and strength limit. However, there is a certain limit to increase the carbon content, because the increase of carbon content will affect the welding performance and other properties of steel (such as stamping performance, etc.), so that the brittleness transition temperature increases and the cold brittleness goes bad. Therefore, the carbon content of low alloy structural steel for pressure vessels is generally limited to less than 0.20%. When the carbon content is limited, the increase of the strength of this kind of steel mainly depends on the addition of a small amount of various alloy elements (the total addition is less than 5%, generally less than 3%, mostly 1% – 2%). For the low alloy structural steel with ferrite pearlite structure, the effects of adding alloy elements on its strength are as follows:
① the same solution strengthening of ferrite;
② increasing the relative amount of pearlite;
③ controlling the grain size;
④ affecting the dispersion of pearlite;
⑤ precipitation hardening.
Manganese and silicon are both solid soluble in ferrite, which have significant solution strengthening effect. Other elements include chromium, nickel, copper, cobalt, etc. Considering the cost saving and resource saving conditions, manganese and silicon are commonly used alloy elements in low alloy steel in China. Under the condition of low carbon, when the content of manganese is less than 1.8%, not only the strength of steel can be improved, but also the plasticity and toughness can be maintained. In addition, manganese can enlarge the austenite zone and make the eutectoid point of the steel move to the left and down, so that it has more pearlite structure with finer structure and the strength of the steel is increased accordingly.
The silicon content in low alloy structural steel is generally in the range of 0.2% – 1.7%, which will reduce the toughness. Chromium and nickel are also solid solution strengthening elements of ferrite, and nickel has a good effect on improving low temperature toughness; phosphorus strengthening ferrite has a significant effect, but due to the increase of cold brittleness, the maximum content should be limited to 0.15%, and the total content of phosphorus and carbon should be limited to less than 0.25%.
According to the national standard (low alloy high strength structural steel) (GB 1591), the chemical composition and mechanical properties of each grade of low alloy high strength structural steel are specified. Due to the strengthening effect of alloy elements, low alloy structural steel not only has higher strength, but also has better plasticity, toughness and weldability. Q345 steel has good comprehensive performance and is a common brand of steel structure. Grade Q390 is also a recommended brand. Compared with carbon structural steel Q235, low-alloy high-strength structural steel can save steel 20% ~ 30%, and has good dynamic load and fatigue resistance. Low alloy structural steel is mainly used for rolling various sections, steel plates, steel pipes and steel bars. It is widely used in steel structures and reinforced concrete structures, especially in various heavy-duty structures, long-span structures, high-rise structures and bridge projects, structures bearing dynamic and impact loads, etc.
Low alloy structural steel is a kind of low-carbon structural steel. The content of alloy elements is less than 3%, which is mainly used to refine grains and improve strength. The strength of this kind of steel is significantly higher than that of carbon steel with the same carbon content, so it is often called low alloy high strength steel. It also has good toughness, plasticity, weldability and corrosion resistance. Originally used in bridges, vehicles, ships and other industries, its application scope has been expanded to boilers, high-pressure vessels, oil pipes, large steel structures, automobiles, tractors, earth moving machinery and other products.