At present, the four international standards for marine platform steels, namely En10225 and BS7191 (European standard), API (US standard), Norsok (North Sea Standard), have clear regulations on the mechanical properties and design and manufacture of steel for offshore platforms.
The main grades of high-strength steel for world marine platforms are yield strengths of 355, 420, 460, 500, 550, 620, 690 MPa, and low temperature performance requirements of at least -40°C, or even -60°C, resistance to lamellar tearing performance Z To 35%, the corrosion resistance is good.
Requirements for chemical composition of steel for marine engineering
The factors affecting the performance of steel are: chemical composition, smelting and casting, rolling and heat treatment processes, etc., which are mainly composed of chemical components; sulfur and phosphorus content directly affect the performance of the thickness direction of the steel plate. Sulfur is the most severe element of segregation in continuous casting billets. Sulfur can cause the steel to be hot and brittle, causing cracks in the steel during high temperature forging. Many loose and vent holes are produced during welding.
Phosphorus is an element which is second only to the high degree of segregation of sulfur in the continuous casting billet of steel, and the diffusion rate of phosphorus in the iron solid solution is small, so the segregation of phosphorus is difficult to eliminate, thereby seriously affecting the performance of the steel. Phosphorus is dissolved in ferrite in the form of a solid solution. This solid solution is very brittle, and the phosphorus-rich region formed promotes brittleness of steel and reduces the plasticity, toughness and weldability of steel. It is easy to cause cracking of steel during hot working, and cracks are likely to occur during welding. Phosphorus is an element that lowers the surface tension of steel. As the phosphorus content increases, the surface tension of the molten steel decreases significantly, thereby reducing the crack resistance of the steel.
Therefore, the steel plate for offshore platforms has strict requirements on the content of sulfur and phosphorus, and the content of sulfur is strictly controlled.
Performance requirements for marine engineering steel
Because the marine platform is immersed in seawater all the year round, it has to withstand all kinds of bad sea conditions. Therefore, the technical indexes of steel for offshore platforms are extremely high, not only have high resistance to atmospheric corrosion and seawater corrosion, but also require good mechanics and processing properties, etc. The performance requirements for steel for marine platforms include:
(1) It has high strength and resists wind current impact above the water surface.
(2) It has good low temperature impact properties.
(3) It has good welding performance.
(4) Steel purity requirements.
(5) Requirements for corrosion resistance.