Research On Low-temperature Pressure Vessel Steel 07MnNiMoVDR Rolled And Quenched And Tempered Technology

With the development of industrial technology, such as the production, stockpiling and transfer of all kinds of liquefied petroleum gas, liquid ammonia, liquid oxygen, liquid hydrogen, liquid nitrogen, the properties requirements and demand of low-temperature pressure vessel steel ever-increasing.In the background of ordinary steel excess demands in China, the development of this steel has good market prospects and considerable economic benefits.Based on the test platform of some plant and State Key Laboratory of Rolling Technology and Automation, Northeastern University, We developed the steel.with the combination of laboratory tests and industrial test rolling.The developed 07MnNiMoVDR steel has a good low-temperature impact toughness, strength, elongation and other indicators in-40℃.the indicators have reached or surpassed GB19189-2003.The main point in theses are as follows:1.Using of thermal expansion of austenitic thermal simulation testing machine in the MMS-300.we got static austenitic CCT curve and change rules of organization in different cooling conditions.The results showed that:In a wide range of cooling rate between 1℃/s and 30℃/s, the steel has bainite organizations;cooling rate more than 50℃/s, martensitic transformation occurs.If cooling rate more than 150℃/s, the great majority of organizations is martensite;When the cooling rate between 1℃/s and 30℃/s, with the cooling rate between the transition temperature of bainite started to increase first and then decreased;With cooling rate of increasing, the termination transition temperature of bainitic change little in pre-Period, with the cooling rate increased, the termination temperature rise to hight direction.2.Through the thermal expansion of the pilot, combination with methods of TEM and metallographic detection, we stuy on the laws of dynamics of phase transformation and carbide precipitation of 07MnNiMoVDR in quenching and tempering process.The results showed that:organizational structure change occurred in process of pre-heating tempering phase.residual austenite chang into martensite or bainite between 200℃and 300℃. Carbon atoms of Martensite through the continuous decomposition, long-distance spread of formationθ-carbide;when the temperature reaches above 400℃, martensite through the long-range proliferation formed grain boundary and grainθ-carbide;When the temperature rose to 650℃, further segregation of carbon atoms,θ-carbide change into granular cementite, lathαPhase recrystallization have taken place with the formation of tempering sorbite.3.In the same conditions of hot rolling, through changes the parameters of quenched and tempered:quenching temperature, quenching time, tempering temperature and tempering time, to study the law of changes of mechanical properties in tempering process. We got the P quality parameter and P curveof tempering.The results showed that:when the quenching and tempering in 920℃:hardnessH=735.97173-27.81462*p;when the quenching in 970℃:hardness H=711.12514-27.48573*p4.Useing the same system of rolling and cooling system, we studied the laws of how the different quenching temperature and tempering temperature influence carbide precipitation of alloying elements and mechanical properties of test steel.The results showed that:with the increase of quenching temperature, solid solution alloy in the martensite content increased, heating in the tempering process, the volume of carbide precipitation increased making the steel yield strength increased and beam width of martensite lath bigger,toughness droped;impurity elements such as P, S diffussed to the austenite grain boundary.Grain boundaries enriched relative more impurity elements and carbide of the alloy.they retains in the ultimate organization making the impact energy decline. As result of the ball carbide Spheroidized and glowed up, the impact energy eclined intensity when tempering temperature unexceed 650℃.;tempering temperature exceeds 650℃, the temper sorbite recrystallization occurrence,resulting in strength decreased.Tempering temperature exceeds 680℃ferrite austenite occurs,the impact energy and yield strength. sharp decline.5.Thought changing different programs of rollng and cooling, we studed the change law of the mechanical properties of different programs and comparing the microstructure and properties of the different programs under different parameters of quenching and tempering.The results showed that:Two-stage rolling wth cooling after rolled,rough rolling finishing temperature more than 1000℃, the total reduction ratio of rough rolling more than 67%, fist stage finish rolling temperature in about 880℃, the final rolling temperature control in about 840℃, finishing down reduction ratio more than 50%, on average of deformation between 20% and 30% in every mill, and cooling rate after rolled about 10℃/s,Quenching temperature between 910℃and 970℃, time solution between 0.5 and 1h, tempering temperature between 620 and 680℃, tempering holding time between 0.5 and 1h, the mechanical properties of steel is well, surplusng the national standard of GB19189-2003. 6.Based on the laboratory research and production in 4300mm plate mill of steel plant,the results show that:The developed technology of 07MnNiMoVDR is reasonable.The chemical composition of the steel, product performance,dimensions and surface qualities more than meet the requirements of GB19189-2003.