Investigation On Preparation And Properties Of A New Low Temperature-ductile Steel

Mooring-chain steel is mainly used for the fabrication of senior mooring chain in offshore oil platforms, marine engineering, aircraft carriers and other military engineering. These chains are often immersed into seawater and may undergo the impact from hurricane or huge waves, as well as withstanding a -20℃low temperature environment; it should also possess the good properties of cold roll forming, weldable and heat treatment. On the basis of the developed R3 level mooring-chain steel (HZR3) and the requirements under GB/T20848-2007, we redesign the composition of steel base on strengthening and toughening theory. In addition, new technique with high efficiency and low energy consumption, double refined and high clean smelting are applied in developing R4-class mooring-chain steel (HZR4). We also have investigated the microstructure, grain size and non-metallic inclusions ratio, continuous cooling transformation curve (CCT), mechanical properties and sensitivity of heat treatment on HZR4 steel.The results show that, the production process of "Pretreatment→Converter→Refining (LF refining furnace + RH vacuum degassing)→Continuous casting→Continuous rolling→Finishing" used in Huaian steel mill is leading in China. The properties of HZR4 steel are exceeded national standards, and meeted requirements of customer. HZR4 steel has a lower carbon (0.21%), high Ni content (0.98%), and micro-alloying (Nb) compared with that of HZR3 steel. All these methods mentioned above can refine the grains further, and their average grain size is grade of 7. The yield strength and tensile strength increase obviously, but the elongation and area reduction rate reduce.There is no pearlite transformation area in CCT curve of HZR4 steel. When the cooling rate is greater than 1℃/s, the martensitic transformation can occur. The critical martensite transformation rate is significantly decreased and thus hardenability is improved. For HZR3 steel, however, the martensitic transformation may occur when the cooling rate is only higher than 20℃/s. Under temperatures of 0℃, -20℃and -40℃, the impact energy of HZR4 steel is higher than that of HZR3 steel, showing excellent impact toughness, particularly the low temperature impact resistance. The brittle transition temperature of HZR4 steel is under -40℃.The change of strength resulted from the quenching temperature and tempering time shows a lower sensitivity, while the relationship between plasticity and annealing time is more sensitive. The effect of temperature of tempering on strength and plasticity is large. With the tempering temperature increasing, from the general trend, the strength is decreased, while the plastic is increased; but the variety of the strength alters a little, showing a good temper resistance. Furthermore, the effect of tempering temperature on impact toughness of HZR4 steel is not obvious.