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Steel for ships
Marine profiles, plates and pipes are the most widely used materials in shipbuilding, and there are many specifications and models, so the selection requirements are high. People engaged in the shipbuilding industry must be familiar with and master their models, specifications and performance, so as to use them reasonably in actual production, so as to meet actual needs and save costs.
When selecting steel for hull structure, the first thing to be encountered is the yield strength index of steel. According to the yield strength of steel, the safety factor required by the structure is considered, and the cross-sectional dimensions of the hull components are determined to ensure that the structure has sufficient strength, rigidity and stability.
For military ships, sometimes in order to improve their combat performance, such as to ensure the explosion resistance of the ship, under the condition of meeting the static strength design, steel should be selected according to the explosion resistance index.
(1) The raw materials and production processes of the selected steel grades should take into account convenient supply and sufficient supply. Domestic raw materials are preferred over imported raw materials.
(2) According to the purpose, tonnage and conditions of use of the ship, select hull structural steel with various technical properties that meet the requirements to ensure the safety and reliability of the ship.
(3) The selected steel type and technical and economic performance should be reasonable. On the premise of meeting the technical requirements, low-priced steel types should be selected to reduce the construction cost of the ship.
Substitution principle The specific principles of steel substitution in specific construction are as follows:
(1) Replace lower strength grades with higher strength grades, such as replacing carbon steel with low alloy steel. The individual substitution scale remains unchanged. If a large number of substitutions are made, the calculation should be recalculated and the consent of the relevant parties should be obtained.
(2) Replace higher strength grades with lower strength grades. If non-load-bearing components are allowed to be substituted, if a large number of substitutions are made, the calculation should be recalculated and the consent of the relevant parties should be obtained.
(3) The substitution of steel types should consider the possibility of welding and pay attention to the electrochemical corrosion effects of different steel types. After substitution, the completed parts should be marked..
Classification Marine steel can generally be divided into three types: hull structure steel, boiler and pressure vessel steel, and mechanical structure steel.
1. Hull structure steel All hull structure steels shall be produced by steel mills approved by the classification society, and qualified products shall be stamped with the classification society's seal. The steel smelting method can be open-hearth, electric furnace or basic oxygen-blown converter steelmaking method. If other methods are used, they shall be specially approved by the classification society. The chemical composition of the steel shall meet the corresponding requirements of the specification, and the mechanical properties test of the steel shall be carried out according to the regulations. The test items mainly include: tensile test, impact test, bending test and Z-axis tensile test.
Hull structure steel is divided into two types according to chemical composition and performance: general strength hull structure steel and high strength hull structure steel.
1) General strength hull structure steel General strength hull structure steel is the former marine carbon steel (low carbon steel), which is divided into four grades: A, B, D and E. It is suitable for steel plates and wide flat steels with a thickness not exceeding 100mm and steel sections and bars with a thickness not exceeding 50mm. Grade A is rimming steel, Grade B is killed steel, Grade D and Grade E are fully killed fine grain (aluminum treated) steel, and the manganese content in Grade E steel is higher than that in Grade D steel, while the carbon content is lower than that in Grade D steel.
Carbon, phosphorus and sulfur content in four grades of steel for general strength hull structure
Carbon, phosphorus and sulfur content in four grades of steel for general strength hull structure
The carbon, phosphorus and sulfur content in four grades of steel for general strength hull structure is shown in the table. The carbon content in steel directly affects its strength. The higher the carbon content, the higher the strength, but the toughness and ductility become worse. Phosphorus and sulfur are harmful components in steel. Phosphorus will increase the cold brittleness of steel, reduce ductility, and reduce impact toughness. When the phosphorus content reaches 3%, the impact toughness is almost reduced to zero; sulfur and iron will form iron sulfide in the crystallization of steel, which is easy to cause cracks and tearing in steel. This phenomenon is called hot brittleness, which makes the welding performance of steel worse.
At present, general strength hull structure steel is widely used in the construction of small and medium-sized ships.
((1) Strength level
Chemical composition of high-strength hull structure steel
Chemical composition of high-strength hull structure steel
High-strength hull structure steel is also called marine low-alloy steel. This kind of steel is divided into strength levels according to its minimum yield point stress, and each strength level is divided into four levels: A, D, E and F according to its impact toughness.
There are six grades of A32, D32, E32, A36, D36 and E36 for steel plates and wide flat steel with a thickness not exceeding 100mm, and six grades of A40, D40, E40, F32, F36 and F40 for steel plates, wide flat steel, steel sections and bars with a thickness not exceeding 50mm. The "32", "36" and "40" in the grades respectively indicate that the minimum yield stress value should be greater than 314N/mm2, 353N/mm2 and 392N/mm2. According to the specification, when the yield stress of steel is greater than or equal to 265N/mm2, it is considered high-strength steel. High-strength hull structure steel should be killed steel after grain refinement treatment, and the chemical composition of its barrel sample is shown in the table. (2) Characteristics Since high-strength hull structural steel is smelted by adding a small amount of alloy elements such as manganese, niobium, vanadium, aluminum and silicon to general-strength hull structural steel, its strength, mechanical properties, weldability, corrosion resistance and wear resistance are all better than general-strength hull structural steel. Although the steel itself is expensive, due to its above-mentioned properties, the amount of steel used can be reduced during shipbuilding, thereby reducing the weight of the hull structure and reducing the shipbuilding cost. The final economic indicators are similar to those of general-strength hull structural steel.
3) Requirements for hull structural steel at normal temperature Material grade and use of steel grade Material grade and use of steel grade In order to prevent hull fracture, hull components in different parts of the ship are divided into three categories according to the stress they bear, namely, secondary category, main category and special category, and the corresponding three material grades are Ⅰ, Ⅱ and Ⅲ. The material grade of each strong component of the hull should not be lower than the provisions listed in the table. When the ship length is greater than or equal to 90m, the steel used for the hull structure shall meet the requirements of the above table. When the ship length is less than 90m, the steel used for the hull structure can generally use A/AH steel grade. For hull components of different material grades, the steel grade can be selected according to Tables 2-4 according to the plate thickness. When the plate thickness is greater than the thickness required by the specification, the steel grade should be selected according to the table according to the actual plate thickness. Steel grades required for each material grade Steel grades required for each material grade The plates used to manufacture the stern column, rudder, rudder arm and tail shaft bracket should generally not be lower than the steel grade corresponding to material grade II. For rudder structures that bear concentrated forces (such as the lower rudder bearing of a semi-balanced rudder or the upper part of a balanced rudder), material grade III should be used. The material grade of the deck plate, sheer top plate and upper plate of the longitudinal bulkhead in the 0.4L area in the middle of the ship should remain unchanged at the front end of the poop and both ends of the bridge. The material grade used for the strong deck, side strake and torsion box structure in the middle 0.4L area of the container ship should remain unchanged throughout the cargo hold area. 2. Steel for boilers, pressure vessels and mechanical structures The steels for boilers and pressure vessels include 360 A, 360B, 410A, 410B, 460A, 460B, 490A, 490B, 1Cr0.5Mo and 2.25Cr1Mo, etc. They are named after their lower limit of tensile strength, and the unit is N/mm2. The difference between grade A steel and grade B steel is mainly the deoxidation method. Grade A steel is killed steel, and grade B steel is killed and refined grain steel. 1Cr0.5Mo and 2.25Cr1Mo grade steels are killed alloy steels. The "Rules for Classification and Construction of Seagoing Steel Ships" has specific requirements for the deoxidation, chemical composition and heat treatment of the steel, and requires tensile and impact mechanical properties tests on the specimens, and high-temperature tensile mechanical properties tests should be carried out for steel used at working temperatures not less than 50°C. Mechanical structural steel can be made of general hull structural steel, high-strength hull structural steel and boiler and pressure vessel steel. For important mechanical components with working temperatures above 50°C, the mechanical properties data at the designed high temperature should be submitted to the classification society for reference.
Application types of marine steel 1. Application types of marine steel In order to meet the different needs of various parts of the hull structure, marine steel is mainly of the following types in actual application: 1) Steel plate Steel plate (plate) is the main component of the hull structure, accounting for about 60% to 65%, such as shell plate, deck plate and compartment partition, etc. Generally, steel plates with a thickness of 4mm or less are called thin plates, and those with a thickness of more than 4mm are called thick plates. The size range of marine steel plates is generally: 6-10mm thick, 1200-3000mm wide, and 6000-14000mm long. 2) Section steel Section steel accounts for about 35% to 40% of the hull structure, second only to steel plates, and is mainly used as the hull skeleton. According to its cross-sectional shape, it can be divided into: flat bar, bulb bar, angle bar – A/B, I-bar, T-bar, and channel bar. 3) Cast steel and forged steel The bow and stern columns, anchors, fairleads, bollards, and stern tubes of ships are often made of cast steel, while simple-shaped components such as rudders and shafts of ships are mostly made of forged steel. The mechanical strength and toughness of forged steel are better than those of cast steel, but due to the limitations of processing technology, its component structure should not be too complicated.