Abstract: Liquefied natural gas is an important new clean energy, and the construction of 200 000 cubic meters of LNG storage tanks is currently leading the development of LNG storage tanks. Meanwhile, 9Ni steel has gradually become the mainstream product of LNG storage tank construction by virtue of its excellent comprehensive mechanical properties. The effect of microstructure on the mechanical properties of the tested steel was investigated by means of X-ray diffraction, electron probe and transmission electron microscopy to unravel the underlying mechanisms behind the ‘high strength and low yield ratio’ reinforcement exhibited by the tested steel. The tested results show that the grain size of tempered sorbite and the volume fraction of reversed austenite are crucial microstructural parameters influencing the yield ratio of the tested steel. Following treatment with QLT process, compared with the QT process, the grain size of tempered martensite in the tested steel is coarsened from 8.1 μm to 13.1 μm, while the volume fraction of reverse transformation austenite is increased from 5% to 10.2%. Consequently, its yield strength is improved to 622 MPa, while maintaining a stable tensile strength and reducing the yield ratio to 0.87. These changes primarily stem from localized enrichment of nickel elements within the tested steel, leading to variations in parameters associated with multi-scale levels of tempered sorbite and reversed austenite. As a result, these alterations indirectly effect its mechanical properties.

Key words: 9Ni steel, tempered sorbite, reversed austenite, yield ratio, localized segregation of nickel element