New Steel Plate for Liquefied-Natural-Gas Storage Tank
A new steel plate for LNG storage tanks has been developed by optimizing chemical composition and applying recent thermomechanical-control-process (TMCP) technology.
With demand for LNG rising, construction of above-ground LNG storage tanks is expected to increase. 9% Ni steel plate has excellent strength and cryogenic toughness. For LNG storage tanks, a double-integrity structure has been proposed to prevent peremptory destruction.
Although a high safety standard is demanded for such steel plates, in terms of saving construction costs of LNG tanks, a reduction in the amount of nickel used was desired. The new steel plate, equivalent to conventional 9% Ni steel, has been developed by adopting a TMCP to obtain the refined microstructure and a large amount of retained austenite.
Development of New Steel Concept
In the development of the new steel, key technologies are application of the TMCP and optimization of chemical compositions. The properties of the base plate and welded joint of the new steel are equivalent to those of 9% Ni steel, with excellent brittle-crack-initiation resistance and brittle-crack-propagation-arresting capability.
A TMCP is a production process wherein the rolling temperature and cooling rate after rolling are controlled. TMCP technology, which improves strength, toughness, and weldability, was developed for use in shipbuilding steel or line-pipe steel. TMCP technology has been applied to plates for offshore structures, high-rise buildings, bridges, and several other structural applications. Microstructures obtained with TCMP technology are finer than those obtained with conventional processes.
The production process of the new steel is a combination of controlled rolling, accelerated cooling, and appropriate heat treatment (intermediate heat treatment, known as lamellarizing). A very fine martensitic microstructure is formed by controlling the previous austenite grain size in the heating process and rolling conditions in the uncrystallized zone and quenching in the accelerated cooling process after rolling. Retained austenite, which improves toughness, is also formed by lamellarizing and tempering after direct quenching. The amount of retained austenite of the new steel is greater than that of 9% Ni steel.
Compositionally, the new steel reduces silicon (Si) and adds manganese (Mn), chromium (Cr), and molybdenum (Mo). By decreasing Si, precipitation of cementite and autotempering during cooling at welding are promoted. The toughness of the heat-affected zone (HAZ) is improved. Furthermore, contents of Mn, Cr, and Mo are controlled to ensure appropriate hardenability of the HAZ. According to research about high-tensile-strength steel, the HAZ microstructure should be a mixture of martensite and lower bainite for improved toughness. The same trend is noticed in the new steel; that is, when hardenability is high, martensite is formed in the HAZ and autotempering is suppressed. In the case of low hardenability, upper bainite, which deteriorates the toughness of the HAZ, is formed.
Owing to the optimization of the production process and chemical composition described here, the new steel for LNG storage tanks has properties of the base plate and welded joint equivalent to those of 9% Ni steel.
Mechanical Properties of Base Plate and Welded Joint.
To evaluate the fitness of the new steel for the inner material of the LNG storage tank, test plates were manufactured in actual production equipment, reflecting the findings mentioned in the preceding subsection. The test plate thicknesses are 6, 10, 25, 40, and 50 mm. Results of the tensile test and Charpy impact test of base plates met established standard requirements for 9% Ni steel. The crack-tip-opening-displacement values of the new steel are of a high level and are equivalent to those of conventional 9% Ni steel.
Large-Scale Fracture Test
To evaluate the safety of LNG storage tanks, large-scale fracture tests were conducted.
The resistance to brittle-crack initiation was evaluated with a cross-weld notchwide plate tensile test simulating the T-cross welded part of an actual LNG storage tank. Brittle fracture was not observed in any specimen. All specimens yielded thoroughly and fractured over maximum load. The fracture-stress values of all specimens at –165°C exceeded 750 MPa and were at the same level as those of 9% Ni steel. It was confirmed that the resistance to brittle-crack initiation of the new steel is of a high level and is equivalent to that of conventional 9% Ni steel.
Brittle-crack-propagation-arresting properties were evaluated with a duplex test. It was confirmed that a brittle crack was immediately arrested after penetrating the test plate from an embrittled plate under an applied stress of 393 MPa, which is equivalent to design stress. It was confirmed that the new steel had excellent brittle-crack-propagation-arresting properties, similar to those of 9% Ni steel.
Approach to Practical Application of New Steel. Assuming that the new steel and 9% Ni steel are welded together, properties of the welded joint of 7% Ni steel and 9% Ni steel were also evaluated. It was confirmed that the welded-joint properties of different materials were equivalent to those of identical materials. Considering the actual construction work of a tank, the influence of repair welding on welded-joint toughness was evaluated and no faulty result was found. Fatigue properties were also evaluated and were found to be equivalent to those of 9% Ni steel. Furthermore, physical constants were required. Young’s modulus, Poisson’s ratio, and thermal-expansion rate of new steel have been evaluated and were found to be equivalent to those of 9% Ni steel.
Construction and Operation of an LNG Storage Tank Made of New Steel
First Application of New Steel for an LNG Storage Tank
For the first time, the new steel with Ni composition of 7.0–7.5% was adopted by Osaka Gas for application to the above-ground LNG storage tank in Senboku Terminal 1, having a capacity of 230 000 m3 and being the largest of its kind in the world (Fig. 2). The LNG storage tank has a diameter of 90 m and a height of 60 m.