Study On Microstructure And Mechanism Of Strengthening And Toughening Of Nickel Saving Steel For Low Temperature Storage

With the continuous development of lower Ni or high Mn content cryogenic steels by Japan,South Korea,Europe and the United States,and the demand for steels for cryogenic storage tanks in China,it is necessary to study low-cost cryogenic steels and analyze the mechanism of strengthening and toughening.In order to solve the high cost and complicated process of Ni-containing cryogenic steels,two kinds of medium-manganese cryogenic steels:5Mn and 5Mn-1.5Ni steels were designed by the idea of“substitution of Mn for Ni".The QLT process was used to obtain the thin steel plates with lath martensite,ferrite and reverse austenite microstructure at room temperature with excellent strength and cryogenic toughness.The microstructure evolution and mechanical properties during heat treatment were systematic investigated,besides,the best process parameters were acquired,the strengthening and toughening mechanism were analyzed.The main conclusions are as follows:(1)According to the principle "the substitution of Mn for Ni”,Mn was added to the steel to replace Ni.Meanwhile,Cr and Mo were added to the steel to achieve solid solution strengthening,refine grains,improve austenite stability.The content of P and S were limited to ensure the cryogenic toughness of steel.The volume fraction of each phase at different temperature and phase transformation temperature of the experimental steel were calculated by using the thermodynamic software Thermo-Calc.Finally two kinds of medium-manganese cryogenic steels were designed.The composition were:0.05C-5Mn-0.15Ni-0.40Cr-0.20Mo and 0.05C-5Mn-1.50Ni-0.40Cr-0.20Mo.The continuous cooling phase transformation behavior of experimental steel was studied and the phase transformation temperature of two experimental steels were by using the fully automatic phase transformation instument,and the static CCT curve and dynamic CCT curve were plotted.(2)The hot rolling process of two kinds of experimental steels have been studied.The ingot was hot rolled in eight passes to 12mm thickness plate,the yield strength of both experimental steels exceeded 900MPa,the tensile strength exceeded 1100MPa,the elongation exceeded 19%,the room temperature impact energy was about 90J,and the impact energy at-100℃ was greater than 40J.(3)The high temperature quenching and tempering treatment were processed on the experimental steels.The influence of temperature and time on the microstructure and mechanical properties of the high temperature quenching were analyzed.The optimal high temperature quenching parameters of 5Mn steel was 780℃-180min.The yield and tensile strength were 623MPa and 790MPa,respectively,the elongation was 34.7%,the impact energy at-100℃ was 141.8J,the PSE was 27.41GPa·%.The optimal high temperature quenching parameters of 5Mn-1.5Ni steel was 750℃-180min.The yield and tensile strength were 625MPa and 858MPa,the elongation was 34.5%,the impact energy at-100℃ was 147.5J,the PSE was 29.60GPa·%.(4)The influence of temperature and time on the microstructure and mechanical properties of the tempering were analyzed.The optimal tempering parameters for 5Mn steel was 780℃-180min-680℃-60min-600℃-180min.The yield and tensile strength were 591MPa and 782MPa,respectively,the elongation was 36.8%,the impact energy at-100℃ was 162.8J,the PSE was 28.78GPa·%,and the reversed austenite content at room temperature reached 19.2%;The optimal tempering parameters for 5Mn-1.5Ni steel was 750℃-180min-650℃-60min-590℃-180min.The yield and tensile strength were 626MPa and 792MPa,respectively,the elongation was 36.1%,the impact energy at-100℃ was 170.8J.The PSE was 28.59GPa·%,the reversed austenite content at room temperature was 19.8%.(5)After the two experimental steels were treated by QLT process,combining with the results of EPMA surface scanning,some bright lath and depressions were reversed austenite.The reversed austenite formed between the martensite laths was film-like,and the reverse austenite distributed in the prior austenite grain boundaries and the martensite packet boundaries was mostly block-shaped.The formation of the reverse austenite purified the matrix and improved the plastic deformation ability of the material.In the cryogenic impact process,it relieved the stress concentration at the tip and hindered of expansion crack.Meanwhile,the microstructure was refined,the ratio of the large-angle grain boundaries were higher,the ability against crack propagation of material was stronger.Compared with the two experimental steels,the mechanical properties of 5Mn-1.5Ni steel was more excellent,and the FATT was lower than-120℃.