Manufacturing Process, Microstructures And Mechanical Properties Of Pressure Vessel Steel07MnNiMo VDR

Research and development of cryogenic pressure vessel steel to increase strategic petroleum storage is an important strategic task in China in order to meet the requirement for large scale equipment and security. Pressure vessel steel07MnNiMoVDR (07MnNiMoDR) is a kind of quenched and tempered steel with low weld cracking susceptibility, which is widely used in manufacturing low-temperature spherical tank. With the implementation of the new national standard GB19189-2011, the requirement for the cryogenic properties of this steel is more stringent, which brings a challenge to the enterprises that produce the limit thickness of quenched and tempered steel plate and meet the requirements of low temperature performance and low weld cracking susceptibility. The research and development on cryogenic pressure vessel steel can not only have considerable economic benefits for the enterprises, but also have a positive effect on upgrading the technological level of high quality steel.Based on the results of trial production and the existing equipments in factory, the composition and process of quenched and tempered steel plate is optimized and the suitable rolling and tempering process for production is obtained in this thesis by theoretical calculations and experimental studies. Process for producing07MnNIMoVDR steel with high strength, high ductility and good low temperature impact toughness is developed and the mechanical properties of the steel meet the requirements of new standard GB1989-2011. The main contents in the thesis are summarized as follows.(1) The equilibrium phase and the law of precipitation phase formation are analyzed and the performance of the material before and after changing the composition is calculated using thermodynamic software Thermo-Calc and JMatPro software, respectively. Chemical composition of trial production steel is optimized without altering the phases in phase equilibrium diagram. The steel was smelted experimentally.(2) The static and dynamic CCT curves are studied on the phase transition analyzer and thermo-mechanical simulator. The phase transformation of undercooled austenite under different cooling conditions is studied using hardness measurement and microstructural analysis. (3) The flow stress curve of the experimental steel deformed at elevated temperatures is obtained by a single-pass compression experiment in thermo-mechanical simulator. The influences of deformation conditions on the dynamic recrystallization and flow stress are also studied, and the flow stress model is obtained with regression analysis as follows.(4) Productive rolling process is implemented by using φ450mm hot rolling mill in laboratory. The influence of different process parameters on microstructure and mechanical properties of the experimental steel are studied and the variations of the microstructure and mechanical properties after quenching and tempering process are abtained.(5) Quenching and tempering process for the steel with different thickness is studied by microstructural observation and performance test after quenching and tempering process. And the mechanical properties of the steel can meet the new standard GB19189-2011. The quenching and tempering process for steel plate with a thickness of27mm (corresponding to steel plate of60mm in thickness with the same compression ratio) is as follows. Quenching temperature is930℃, quenching holding time is40min and the tempering process is610℃×60min、630℃×30min、630℃×40min or630℃×50min. The quenching and tempering process for steel plate with a thickness of12mm should be quenched at temperature of930℃for20min and the tempering process is610℃×40min or630℃×30min.