low alloy high strength steel

Low alloy high strength steel

Low alloy high strength steel is a kind of engineering structural steel developed by adding a small amount of Mn, Si and a small amount of Nb, V, Ti, Al and other alloy elements on the basis of carbon structural steel. The so-called low alloy means that the total amount of alloying elements in steel does not exceed 3%. High strength is relative to carbon engineering structural steel. The development principle of low alloy high strength steel is to use as few alloy elements as possible to obtain as high comprehensive mechanical properties as possible, so as to meet the purpose of use and low cost. Low alloy high strength steel can meet the requirements of various structures (such as large bridges, pressure vessels and ships) in engineering, with large bearing capacity, reducing structural dead weight, improving reliability and saving materials and resources.

This kind of steel is mainly used to manufacture various engineering structures with high strength requirements, such as bridges, ships, vehicles, high-pressure vessels, oil and gas pipelines, large steel structures, etc. This kind of steel can obtain high strength without complex treatment process or even heat treatment, which greatly reduces the quality of engineering structure. Therefore, this kind of steel is used to replace the general carbon structural steel.

Composition characteristics

Low carbon content is to meet the requirements of plasticity, toughness, weldability and cold deformation of engineering structural steel; Adding a small amount of alloy elements dominated by Mn can improve the mechanical properties. Mn not only has a significant strengthening effect on ferrite, but also can reduce the cold brittle temperature of steel, increase the number of pearlite in steel and further improve the strength; In order to further improve and improve the properties of steel. The addition of trace V, Ti, Nb, AI and other fine grain elements not only further improves the strength, but also improves the toughness of the steel. Rare earth element re is sometimes added to this kind of steel to eliminate harmful impurities, improve the morphology and distribution of inclusions and weaken its cold brittleness.

Performance characteristics

1. High yield limit and good plasticity and toughness

The most remarkable feature of low alloy high strength steel is high strength. In hot rolling or normalizing state, the strength of low alloy high strength steel is generally 30% ~ 50% higher than that of corresponding carbon engineering structural steel. Therefore, it can bear large load. Engineering structures are generally large or giant, and the weight of the component itself often becomes an important part of the load. When the strength of structural materials is improved, the self weight of the component can be significantly reduced and its ability to bear other loads can be further improved. Moreover, this good effect also greatly improves the compactness of engineering components, further improves its reliability, reduces raw material consumption, reduces cost and saves resources.

The elongation of low alloy high strength steel is 15% ~ 23%, and the impact absorption energy at room temperature

2. Good welding performance and atmospheric corrosion resistance

Welding is the most common method of building engineering structures, so the steel used in engineering structures is required to have good welding properties. Low alloy high strength steel has low carbon content, low alloy element content and good plasticity. It is not easy to produce quenching structure and cracks in the weld area, and the added Ti, Nb and V can also inhibit the grain growth in the weld area. Therefore, most of these steels have excellent welding performance and generally do not carry out heat treatment after welding.

Most engineering structures are used in atmospheric or marine environment. A small amount of Cu, Ni, Cr, P and other elements are added to low-alloy high-strength steel, which effectively improves the corrosion resistance of engineering structures to atmosphere, seawater and soil. If 0.2% ~ 0.5% copper, 0.05% ~ 0.1% phosphorus and aluminum are added, the corrosion resistance of steel can be significantly improved, and the effect of adding copper and phosphorus at the same time is the best.   [1]  

Development trend

Based on controlled rolling technology and microalloying metallurgy, many low alloy high strength steels have been developed. The main development directions of low alloy high strength steel are as follows.

(1) Low carbon, ultra-low carbon and high purity purification.

(2) Microalloyed steel technology.

(3) Controlled rolling and controlled cooling processes are adopted.

(4) Ultrafine grain refinement, computer control and performance prediction.