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Influence of Chemical Composition on Seamless Steel Tubes

Posted: 05/10/2023 19:29:22 Hits: 4

A large number of seamless steel pipes of various specifications and materials are used in pipelines, valve chambers, station process pipelines, pressure vessels and pressure pipeline equipment in natural gas gathering and transportation pipeline projects, including gas gathering pipeline projects and gas transmission pipeline projects. Although some seamless steel pipes are of the same steel grade, because they are manufactured according to different seamless steel pipe standards, their performance and quality vary greatly. Therefore, according to the design parameters, including design pressure, design temperature and medium conditions of natural gas gathering and transportation pipeline engineering, external environmental conditions, other quality requirements and investment status, seamless steel pipes should be properly selected to ensure project safety and save investment.

1.1 Contents of phosphorus, sulfur and aluminum
The phosphorus and sulfur elements in the chemical composition of seamless steel pipes are harmful elements remaining in the steel, but they are unavoidable elements.

1. 1. 1 Phosphorus
Phosphorus is a harmful element that causes serious segregation of steel, and can increase the brittleness of steel, especially low-temperature brittleness. The reason why phosphorus causes segregation in steel is that because phosphorus expands the distance between the solidus line and liquidus line of iron, the iron alloy needs more time to solidify.

In this process, the molten iron with a great phosphorus content gathers in the dendrite cracks that have become a solid phase, and becomes a segregation of phosphorus. Since the diffusion rate of phosphorus in iron is slow, the steel with phosphorus segregation promotes the formation of banded structure in the rolled product. It is impossible to eliminate segregation by forging or heat treatment, and it is difficult to obtain a uniform structure. Studies have shown that to obtain the best steel surface and internal properties, the optimum chemical composition of low-carbon steel for seamless steel pipes contains no more than 0.005% phosphorus. Phosphorus increases the brittleness of low carbon steel mainly reflects a decrease in the impact toughness of the steel, and when the phosphorus content is high, the ductile-brittle transition temperature of the steel is increased, which is very unfavorable to the performance of the steel.

1.1.2 Sulfur
Sulfur is an element that causes the macrostructure of steel to be extremely uneven, and is also the main element that forms non-metallic inclusions, thereby reducing the ductility and toughness of steel, and having a serious adverse effect on the hot deformation processability of steel. In the process of forging and rolling, reticular cracks will be generated, even leading to large cracks and waste products, which is not conducive to the weldability of steel, and may easily lead to hot cracking of welding seams. The solubility of sulfur in δ iron γ iron is very small. In the solidification process of the steel ingot, the sulfur accumulates in the unsolidified liquid part, and there are relatively many sulfur and sulfides inclusions in the core, upper part and dendritic structure of the steel ingot that finally solidifies, making the macrostructure of steel extremely uneven. In addition to forming the compound FeS with iron, sulfur in steel is more likely to form compounds with some other metal elements in steel. The affinity between sulfur and manganese is much greater than that between sulfur and iron. Therefore, in the steel ingot, sulfur is distributed in the interior of the grain or on the grain boundary as a high melting point compound such as manganese sulfide, forming a discontinuous net structure. The net structure of this sulfide reduces the ductility and toughness of the steel, and reduces the impact value of the test. Studies have shown that the internal defects of seamless steel pipes are reduced with the decrease in sulfur and nitrogen content in steel and Mn/S ratio. When the average manganese content is 0.40%, the sulfur content in steel should not exceed 0.020%.

1.1.3 Aluminum
Aluminum has a good affinity with oxygen and nitrogen elements, and is an excellent deoxidizer in steel. Its function is to deoxidize and fix nitrogen, refine grains, inhibit the aging of low-carbon steel, and improve the toughness of steel at low temperatures. The reason why aluminum refines essential grains in steel is that aluminum and other elements, mainly N form a finely dispersed refractory compound AlN in steel to act as a restraint. Studies have shown that the aluminum content in AIN should be controlled above 0.008%. The purpose is to refine the grains of the steel. When the aluminum content is lower than this value, the grains will be coarse, but if the aluminum content is increased, it is impossible to refine grains.

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