Investigation On Strengthening And Toughening Mechanisms Of Cryogenic Application9Ni Steels And Development Of Their Industrial Manufacturing Technology

With the progressing of society and increasing emphasis on environmental protection, LNG as a kind of clean energy, increases year by year in the field of energy, as a result, the demand pressure for9Ni steel which is LNG storage tank structural material is increasing. Therefore, the development of independent intellectual property right9Ni steel is very great significance for the adjustment of Chinese energy structure. The microstructure of9Ni steel at room temperature was designed as tempered martensite matrix with a small amount of stable reversed austenite so as to have high strength and toughness, especially for the cryogenic toughness, because the LNG liquefying temperature is as low as-162℃. Up to now, there are many issues originated from industrial production need to be thoroughly resolved, which arttributed to the immature research on evlotion of dual phase microstructure and the strengthening and toughening mechanism of9Ni steel in domestic.In this paper, based on the systematic investigation on hot deformation behavior, phase transformation law, influence of heat treatment parameters on microstructure and properties and evolution of microstructure of different heat treatment processes, the toughening mechanism of9Ni steel of different heat treatment process is analyzed, and, high quality9Ni steel plates are manufactured in industrial-scale on the3500mm steckel mill line. Based on these work, a creative shot-flow processing (DLT) is developed.The chief original work and results are as follows:1. The dynamic and static recrystallization behavior of9Ni steel high temperature austenite were investigated by single and double hit compression tests, and, the activation energy of dynamic recrystallization and static recrystallization were determined as346kJ/mol and194kJ/mol, respectively. Morever, kinetic mathematic model for both recrystallization behaviors and deformation resistance model of9Ni steel were established. In addition, the high temperature plastic region of9Ni steel was determined through high temperature tensile experiment.2. The equilibrium phase diagram of9Ni steel was determined by Therm-calc software, which shows that liquidus temperature and A3line temperature are1502℃and690℃, respectively. The phase transformation laws of9Ni steel during continuous cooling process were explored, and, the results show that the room temperatue microstructures consisted of some ferrite and Widmanstatten whose size and ratio increase with cooling rate decreasing, and, alloy element C, Ni and Mn enriched in martensie during cooling process, when cooling rate is lower than1℃/s. The structure of bainite and martensite can be abtained with a cooling rate from1℃/s to5℃/s. When cooling rate is higher than5℃/s, the structure of whole martensite was abtained at abient temperature, in which the alloy elements are homogeneous except for carbon which segregates at high angle grain boundaries regardless of cooling rate. If the high temperature austenite was deformed before continuous cooling, the γâ†'α transformation temperature should increase, and the y phase successively transform to bainite and martensite with a cooling rate from1℃/s to20℃/s, but the proportion of martenaite increase with cooling rate increasing.3. The influence of QT process parameters on microstructure and properties were investigated. It is determined that the key points for toughening are improving materrial’s plasticity deformation capacity and resistance to crack growth through the analyzing on the fracture process. The relationship between toughness and martensite matrix was investigated based on EBSD detecting on tempered plates, the results show that the impact energy increase with the average misorientation angle increasing. If the austenitizing for rolled plates are sufficient, the prior austenite grains are equiaxed and homogeneous in size, the impact absorbed energy linearly increase with the average misorientation angle increasing if no significant difference of tensile properties was detected. Therefore, the cryogenic toughness of9Ni steel plates can be improved by dispersing martensite lath orientation.4. The precipitation, stability of reversed austenite of QT process and its influence on mechanical properties were investigated by SEM, XRD and EMPA. The results show that there are few small size retained austenite collected C and Si in austenitized and quenched plates. During tempering process, the reversed austenite firstly precipitates at prior austenite grain boundaries and collects austenite stability elements, synchronously. The reversed austenite precipitates at high angle grain boundaries within the prior austenite grain collect C and Si firstly, and can collect Ni and Mn element if increase tempering temperature(≥600℃for1h) or tempering time(≥3h at580℃). However, the stability of reversed austenite decreasing with the amount and size increasing, which should deteriorate toughness of plates. The main contribution of reversed austenite on toughness lies in:1)the reversed austenite can scavenge harmful elements for grain boundaries to toughening the high angle grain boundaries,2)the reversed austenite collect alloy elements from martensite matrix to improving the plastisc deformation capacity of matrix.5. The evolution of microstructure and strengthening and toughening mechanism of QLT process were investigated. The results show that:the precipitation of y phase between laths in high Ni region during holding process at670℃, are benefited to increasing the average grain boundaries misorientation, refinement of microstructure and unclear sites for reversed austenite. Meanwhile, the alloy elements C, Si, Mn and Ni enrich in y phase, which result in element enrichment martensite and reversed austenite after quenching and tempering so as to optimizing the structure. With the dispersion of reversed austenite nucleation sites, the amount of austenite increases, which improve the purification degree of martensite matrix, especially for interstitial element scavenging, which can improve plastic deformation capacity of the material. So, the toughening mechanism for double phase region heat treatment is summarized as:1)the hinder crack propagation and increasing propagation path function of the matrix is strengthened by more dispersively distributed martensite lath and more refined grain derived from the maximum average misorientation;2) the best plastic of the material attributed to the highest degree purification of matrix and the most amount of reversed austenite, could result in increasing plastic deformation region ahead of the crack tip to toughening the material;3) the structure was optimized by the formation of second martensite, which can hinder formation and propagation of cracks by the remission of stress concentration.6. Influence of rolling parameters on as rolled prior austenite grain was investigated to generating uniform and fine grains by rolling process, and then, a new creative short-flow processing (DLT) was developed. The influence of holding time and tempering time on microstructure and properties were studied, the results shown that:the highest impact toughness was obtained attributed to the fine effective grain size, when holding time was40min at670℃. With the increasing of tempering time, the cryogenic toughness increase accompanied an obvious drop in strength. The mechanical properties of plates manufactured by DLT process meet the GB24510-2009requirements. Comparing to QT process, the austenitizing process is omited in DLT process. Comparing to QLT process, not only the omitting of ausetenitizing process, but the lamellarizing time for optimal toughness are shorter. So, the DLT process is an energy saving and consumption reduction process.