Development Of Heavy 9Ni Steel Plate Product Process For Ultra-low-temperature Application

With the increasing tradition of LNG, the demand for 9 Ni steel as LNG structural materials is growing. It's great significance to develop 9Ni steel with independent intellectual property rights. The chemical composition, product process and process parameters were identified by a large number of experiments. The austenitic deformation behavior and CCT were studied by thermal simulation experiments. The low-temperature toughness and other mechanical properties were studied by Charpy impact test and tensile test and the micro-structure was analysed by OM and SEM, The main achievements of this present work were as follows:1. The austenitic dynamic softening behavior was studied and deformation resistance model before the peak strain was established by single-pass compression experiment. The dynamic recrystallization activation energy is 273.96 KJ/mol. The static and dynamic CCT curves were got by thermal expansion experiments. Whether undeformed or deformed 40%, the main micro-structure is bainite in the cooling rate range of 1 to 30℃/s. The Bs point of dynamic CCT is lower than the static CCT and the Bf point of dynamic CCT is higher.2. The casting and continuous casting and rolling process routes were repeated by process experiments, and developed a new route which named as heat treatment online process route which be realized in two ways:(1) quenching and tempering after duplex district finishing; (2) quenching and tempering after insulation in the duplex district for 15 minutes which finishing temperature is in austenitic district.3. Through adjust the reduction distribution ratio in the different temperature ranges of rolling experiments, the conclutions were got that plates should be heat treated by DQ+T and got the best result when large reduction ratio be distributed at fully austenitic recrystallization district and plates should be heat treated by DQ+QT or DQ+QLT and got the good result when large reduction ratio be distributed at austenitic incomplete recrystallization district. The plates should be heat treated by QLT when the finishing temperature is in the lower temperature of austenitic district and should be heat treated by DQ+T and got some ferrite when the finishing temperature is in the duplex district.4. The microstructure of 9Ni steel are martensite matrix and some austenitic after DQ+T, DQ+QT or QT. The microstructure of 9Ni steel are martensite matrix and some austenitic and ferrite after DQ+QLT or QLT. The crogenic toughness increased by the order DQ+T, DQ+QT and DQ+QLT and the yield strength and tensile strength conversely.