Development Of A Nickel Saving 7Ni Steel For LNG Projects And Investigation On Mechanisms Of Strengthening And Toughening

The global demand for liquefied nature gas(LNG),which is more efficient and cleaner than coal and oil as an energy source,is growing quickly in recent years.Many countries,including China,are constructing or planning a large number of LNG projects.9Ni steel is the most common cryogenic steel for constructing LNG storage tanks.With increasing of nickel price,the cost of Ni alloying for 9Ni steel accounts for a larger and larger proportion,thus,saving and low cost have become an important development direction of Ni series cryogenic steel.On condition of lowering down Ni content from 9Ni steel,how to keep the good cryogenic mechanical properties of steel by chemical composition and process optimization,which could be a key problem for developing a nickel saving cryogenic steel.The aim of this paper is to develop a 7Ni steel,which can be equivalent to the conventional 9Ni cryogenic steel as a nickel saving cryogenic steel,and explore its strengthening and toughening technique and mechanism,which could be a reference for industrial production of nickel saving cryogenic steel.In this paper,composition design and heat treatment process thinking were roughly determined by literature researches and preliminary experiments.Then,the effect of different parameters of QLT process and important alloy elements on microstructure and properties of 7Ni steel,as well as the toughning mechanism were studied.And TMCP process of 7Ni was initially researched.Finally,the 7Ni steel,having excellent cryogenic mechanical properties,was developed,and the chief results in the paper are as follows:1.The 7Ni steels with different contents of Mn,Si,Nb and Cu were initially designed and cryogenic toughness of 7Ni steels under different compositions and processes were compared.The experimental steels with low Mn,high Si content and trace Nb showed a good cryogenic toughness.And on this basis,we determined the idea for composition design of 7Ni steel.The temperatures of Ac1 and Ac3 of 7Ni steel were determined.The procedure of continuous cooling transformation was researched,and the static and dynamic CCT curves of the steel were done.2.Analysis methods of OM(Optical Microscope),SEM(Scanning Electron Microscope),TEM(Transmission Electron Microscope)and XRD(X-ray diffraction)were conducted to study micro structure evolution of 7Ni steel under different parameters of QLT process,as well as the influence of formation,amount,morphology and stability of reversed austenite on mechanical properties of the steel was researched.Both cryogenic strength and toughness of experiment steel obviously decreased with increasing quenching temperature.At quenching temperature of 830 ?,the grains and martensite packets were the finest,and there are the highest amount of reversed austenite in the steel.With increasing intercritical quenching temperature,the cryogenic toughness of steel increased at first and then decreased,and the best combination of cryogenic mechanical properties was achieved when intercritical quenching temperature was 690 ?.The size of microstructure did not change a lot when intercritical quenching temperature was increased.The variation of toughness was mainly caused by combined influence of the amount and stability of reversed austenite.Moderate amount of reversed austenite with high stability would be more beneficial for toughness improvement.When tempering temperature was increased to 580 ?,cryogenic mechanical properties as well as micro structure were improved,and higher amount of reversed austenite were obtained.3.The effects of Mn,Nb and Si contents on microstructure and properties of 7Ni steel were studied.When Mn content ranged in a low level,the size of prior austenite grain didn't change a lot.With increasing Mn content,the amount of reversed austenite in steel becomes higher and higher,however,when Mn content was beyond a certain value,the stability of reversed austenite began to decrease.The distribution of reversed austenite would change with increasing Mn content.Compared with cryogenic strength,the change of Mn content has a more significant effect on cryogenic toughness.Prior austenite grains and microstructure of 7Ni steel could be effectively refined by addition of trace Nb content.When Nb content increased from 0.025 wt.%to 0.05 wt.%,fine niobium precipitates with nanoscale size were observed and they tend to be spherical and locate at grain boundaries or in the vicinity of grain boundaries.Higher amount of reversed austenite could be observed in the specimen with higher Nb content,they also mainly distribute at grain boundaries.But Nb does not affect the formation of the reversed austenite directly.With increasing Nb content,both tensile strength and yield strength increased at first and then decreased,while,cryogenic toughness presented a continuous rising trend.With appropriate increase of Si content,the microstructure of steel could be refined,moreover,cryogenic strength and toughness could be simultaneously improved.4.In addition,the rolling process of 7Ni steel was preliminary studied by thermal simulation experiment.The dynamic recrystallization behaviors of 7Ni steel in process of high temperature deformation were studied through high temperature compression test.The activation energy of dynamic recrystallization was calculated as 322 KJ/mol,and RTT curves of dynamic recrystallization were drown.Meanwhile,mathematic model for high temperature deformation resistance of 7Ni steel was established.The influence of parameters of TMCP processing on microstructure and properties of 7Ni steel was investigated by use of thermal simulation machine.The results show that increasing of rolling reduction and cooling rate would refine the micro structure of samples,however,the effects of rolling reduction and cooling rate on tensile properties were different.5.By analyzing the effect of micro structure evolution on cryogenic mechanical properties,the toughning mechanism of 7Ni steel was found.Refinement of grain and martensite packet could hinder crack propagation and blunt it.The toughness was affected by combination of amount and stability of reversed austenite.Without serious damage to stability,the higher amount of reversed austenite could improve toughness of the steel.Film-like reversed austenite has a stronger effect on inhibition of crack propagation and would be more beneficial for toughness.Moreover,the reversed austenite at grain boundaries could purify boundaries,and improved their toughness.6.The micro structure and mechanical properties of optimized 7Ni steel were compared to 9Ni steel.As a result,there are no significant changes of microstructure between 7Ni steel and 9Ni steel,and mechanical properties of 7Ni steel could reach the equal level of 9Ni steel.The Main mechanical properties index of 7Ni steel could meet the standard demands of 9Ni steel.In conclusion,the 7Ni steel in this paper not only reduced the alloying cost,but also had a high safety in low serving temperature environment.So,there would be a great application potential for 7Ni steel in LNG projects.