The factors affecting the properties of steel include chemical composition, melting and casting, rolling and heat treatment processes, etc., with chemical composition as the main factor.
Among the factors affecting the properties of steel, the content of sulfur and phosphorus directly affects the performance of the steel plate in the thickness direction. Sulfur is the most segregated element in continuous casting slabs. Sulfur can cause hot embrittlement of steel, causing cracks in steel during high temperature forging. A lot of porosity and pores are produced during welding.
Phosphorus is second only to sulfur in the high segregation element in the continuous casting slab of steel, and the diffusion rate of phosphorus in iron solid solution is very small, so the segregation of phosphorus is difficult to eliminate, which seriously affects the performance of steel.
Phosphorus dissolves in ferrite in the form of solid solution. This solid solution is very brittle, and the phosphorus-rich zone formed makes the steel brittle, reducing the plasticity, toughness and weldability of the steel. It is easy to cause cracking of steel during hot working, and cracks are easy to occur during welding. Phosphorus is an element that reduces the surface tension of steel. With the increase of phosphorus content, the surface tension of molten steel decreases significantly, thereby reducing the crack resistance of steel.
Therefore, steel plates for offshore platforms have strict requirements on the content of sulfur and phosphorus, among which the content of sulfur is strictly controlled.
