Low alloy structural steel
1. Purpose
Mainly used to manufacture bridges, ships, vehicles, boilers, high-pressure vessels, oil and gas pipelines, large steel structures, etc.
2. Performance requirements
(1) High strength: Generally, its yield strength is above 300MPa.
(2) High toughness: The required elongation is 15% ~ 20%, and the room temperature impact toughness is greater than 600kJ/m ~ 800kJ/m. For large welded components, higher fracture toughness is also required.
(3) Good welding performance and cold forming performance.
(4) Low cold-brittle transition temperature.
(5) Good corrosion resistance.
3. Ingredient characteristics
(1) Low carbon: Due to the high requirements on toughness, weldability and cold forming performance, its carbon content does not exceed 0.20%.
(2) Add alloying elements mainly manganese.
(3) Add auxiliary elements such as niobium, titanium or vanadium: A small amount of niobium, titanium or vanadium forms fine carbides or carbonitrides in the steel, which is beneficial to obtaining fine ferrite grains and improving the strength and toughness of the steel. .
In addition, adding a small amount of copper (≤0.4%) and phosphorus (about 0.1%) can improve the corrosion resistance. Adding a small amount of rare earth elements can desulfurize and degas, purify steel, and improve toughness and process performance.
4. Commonly used low alloy structural steels
16Mn is the most widely used and most produced steel type among low-alloy high-strength steels in my country. The structure in the used state is fine-grained ferrite-pearlite. Its strength is about 20% to 30% higher than that of ordinary carbon structural steel Q235, and its atmospheric corrosion resistance is 20% to 38% higher.
15MnVN The most commonly used steel grade among medium strength steels. It has high strength, good toughness, weldability and low-temperature toughness, and is widely used in manufacturing large structures such as bridges, boilers, and ships.
After the strength level exceeds 500MPa, the ferrite and pearlite structures cannot meet the requirements, so low carbon bainite steel was developed. Adding Cr, Mo, Mn, B and other elements is beneficial to obtain bainite structure under air cooling conditions, resulting in higher strength, better plasticity and welding performance. It is mostly used in high-pressure boilers, high-pressure vessels, etc.
5. Heat treatment characteristics
This type of steel is generally used in a hot-rolled air-cooled state and does not require special heat treatment. The microstructure in use is generally ferrite + sorbite.
