In the world practice of shipbuilding and ship repair, there is one generally accepted principle: carbon or low-alloyed steel can be used as hull steel (shipbuilding plate, profile, etc.). Hull steel is smelted in oxygen converters with purging of pure oxygen from above, open-hearth or electric furnaces. Methods of deoxidation and heat treatment affect such qualities of hull steel as weldability, ductility, and brittle fracture resistance.
TYPES OF SHIP STEEL AND THEIR USE
According to the degree of deoxidation, steel is subdivided into rimmed, semi-killed steel and killed steel.
Killed steel is produced by fully deoxidizing metal with the necessary amount of aluminum, ferromanganese, and ferrosilicon. Killed steel is widely used in shipbuilding. There are practically no non-metallic inclusions in it, there is a great homogeneity of the chemical composition, fine grains (due to deoxidation with aluminum) significantly increase the resistance of steel to brittle fracture (the toughness of fine-grained steel is 2-3 times higher than that of coarse-grained).
Semi-killed steel is less deoxidized by silicon, in contrast to killed steel, it has lower toughness at low temperatures; for other qualities, it is not inferior to killed steel. It is widely used as shipbuilding steel of several categories (up to a thickness of 25 mm).
Rimmed steel contains a large amount of oxygen in the form of compounds with iron, manganese and other elements. Rimmed steel has high ductility and deep-drawing ability due to the almost complete absence of silicon and low manganese content. Its disadvantages include a large heterogeneity of the chemical composition, coarse-grained, the possibility of separation due to the presence of, a greater tendency to aging, cold brittleness (abrupt transition from a viscous to a brittle state as temperature decreases). Therefore, the use of rimmed steel in shipbuilding is limited to just one category (up to a thickness of 12.5 mm).
The chemical composition of the shipbuilding steel is determined by the manufacturer for the ladle analysis (ie, from each ladle of each heat) and is given in the factory quality document. In the marking of rolled steel, the number of melting must be included.
The mass fraction of carbon (upper limit) determines the indicators of weldability and the degree of probability of hot cracks in the heat-affected zone of welds. The carbon equivalent, determined from the ladle analysis, gives an idea of the level of weldability of the metal.
In steel of category A, the highest mass fraction of carbon is allowed – 0.23% (with sheet thickness up to 12.5 mm). It is not allowed to use the steel of category A for structures with a high level of stress concentration subjected to dynamic loads. For icebreakers, the steel of category E is used (for shell in the area of the ice belt along the entire length of the vessel.
For shipbuilding steels, effective corrosion protection is required. For the effective corrosion protection the use of the following methods is required:
- for the underwater part of the hull – comprehensive protection using paint coatings and electrochemical protection;
- for a belt of variable waterlines – any paint and varnish coatings approved for use with a safety period of up to 80-85% after 2 years of service;
- for ballast and cargo ballast tanks – comprehensive methods of protection against corrosion or paint coatings with a service life of at least 8 years.