In the context of the continuous advancement of offshore oil exploration work, more steel is needed for domestic offshore platforms, and the use of high strength ship plates is increasing year by year.

The Marine platform is in the ocean for a long time, the operating environment is changeable, and the service period is more than 2 times that of ordinary ships. Under the conditions of long-term low temperature, light and seawater erosion, the materials used in the platform structure planning should have the characteristics of high strength, strong toughness, easy impact, fatigue resistance and good welding effect.

Chemical Composition Analysis Of High Strength Ship Plates

On a global scale, there are many industry standards for shipbuilding and steel production. China, the United States, France and other countries have developed their own shipbuilding standards. Under the background of the continuous updating of offshore oil exploration and shipbuilding technology, the offshore platform with higher quality and the cargo ship with larger scale have gradually come into being. Such offshore facilities require the use of better quality hull panels.

• Carbon

Less carbon is used in Offshore Platform Steel Plate. Generally, the carbon content of steel is high, and more pearlite phase is gathered, which will produce a large number of galvanic cells, indirectly enhancing the corrosion rate of steel and weakening the toughness of steel.

• Manganese

When the manganese content increases, it will indirectly increase the strength and hardness of the steel, and appropriately weaken the toughness of the steel. Mainly manganese and sulfur will produce MnS, causing manganese segregation, which reduces the toughness of steel.

• Sulfur and Phosphorus

When the content of sulfur and phosphorus is too much, it will form hot brittleness, resulting in cracks in the steel under high temperature conditions, loose holes and pores in welding production. Therefore, the content of sulfur and phosphorus needs to be strictly controlled.

Mechanical Performance Analysis Of High Strength Ship Plates

1. Mechanical requirements

“DH/EH” indicates high strength steel. “32”, “36” and “40” indicate the number of kilograms of sheet metal. In the standards of various countries, the mechanical performance of high strength is as shown in the above table. Only the United States specifications are slightly different, mainly reflected in the elongation. In the United States code, the elongation of the three high-strength plates is 20%, without difference requirements. In terms of impact energy, France and the United States have limited the thickness of the plate to within 100mm, and the thickness of the plate given by other types of specifications is ≤150mm.

2. Mechanical properties requirements of high-performance ship plate steel

(1). High strength

Under the safety requirements of ship service, the production and processing of high-performance ship plate steel should ensure the technical effect. Refining grain and precipitated phase is the key to ensure the strength of steel. In most cases, various technical methods such as “microalloy” and “controlled rolling and cooling” are adopted to ensure the degree of grain refinement.

(2). Strength and toughness

In recent years, the use of ships in extremely cold environment is larger. Under normal circumstances, under lower temperature conditions, the toughness of steel will be weakened. Therefore, it is a key technical task to enhance the toughness of steel in the extremely cold area. Ship plate steel should have strong toughness, according to the characteristics of its use environment, both crack toughness, to ensure the balance of strength and toughness.

(3). Strong corrosion resistance

Enhance the corrosion resistance of steel, so that it has a longer service capacity, help control the expenditure of ship production, operation and maintenance.

(4). Excellent welding performance

During ship production, ship welding tasks may occur. Shipbuilding efficiency mainly depends on welding technology. The original welding technology is difficult to meet the needs of shipbuilding. High wire energy welding has the characteristics of fast production speed and strong technical safety. This new welding technology has gradually become the main production technology of ship plate steel. In the thermal influence range formed by welding process, softening, embrittlement and other phenomena may occur, which indirectly weakens the toughness of weld.

Example – Thermodynamic Analysis Of DH36 Ship Plates

In the mechanical calculation period, the strain and position change of high strength plate welding production stage are judged by finite element model. Before the mechanical analysis, all kinds of thermal and mechanical attribute information are added to the finite element model. The key parameters of mechanical calculation include elastic modulus, thermal expansion coefficient and so on. When the temperature increases, the thermodynamic properties of DH36 sheet are as follows:

In the range of 0~1600%C, the elastic modulus and yield strength of DH36 high-strength ship plate will gradually decrease to zero, Poisson’s ratio will not change, and the coefficient of thermal expansion will gradually increase. The figure shows the technical diagram of the welded joint of DH36 steel plate used in this stress analysis.

In the Figure, the long side of the specimen is 40mm, the wide side is 150mm, and the thickness is 20mm. The welding method of 6 layers is adopted, and a total of 12 welds are set, that is, numbers 1~12 in the figure.

Conclusions

(1). High-performance ship plate contains a variety of chemical components, according to its function, content specifications, effective control of the dosage, is the key measure to ensure the mechanical properties of the ship plate.

(2). During production, the factory should refer to the requirements of the structural steel standards for shipbuilding and offshore engineering to ensure the overall performance of shipbuilding.

(3). When the temperature of DH36 ship plates rise from 0 to 1600℃, the elastic modulus and yield strength will gradually decrease until it does not. Poisson’s comparison is stable, and the coefficient of thermal expansion will increase slightly. In the welding process, the stress behavior along the weld line of DH36 ship plate steel fluctuates, so as to determine the weld joint, which is helpful to improve the overall mechanical property of the plate.