Processing,Microstructures And Mechanical Properities Of 07MnNiMoDR Pressure Vessel Steel Plate

07MnNiMoDR is a low-temperature pressure vessel steel with low weld crack sensitivity and is widely used in the manufacturing of petroleum,natural gas,ethylene and propylene spherical tanks.At present,there exist some problems,e.g.unstable tensile strength and low temperature impact toughness,in industrial production of this steel.This seriously restricts the promotion of the steel product.In this thesis,theoretical analysis and experimental studies have been carried out on the chemical composition design,optimization,production processing,mechanical properties and microstructure control of 07MnNiMoDR steel.Finally,the 07MnNiMoDR steel plate with excellent low temperature impact toughness and mechanical properties matching was obtained.The main research contents and results are summarized as follows.(1)The equilibrium precipitation phases of 07MnNiMoDR steel at various temperatures were simulated by JMatPro thermodynamics software.The influence of the main alloying element contents on the precipitation phase and hardenability of the steel plate was analyzed.The results show that the main precipitates in 07MnNiMoDR steel are M(C,N),M23C6,M7C3,M2(C,N)and G_PHASE phases.When the C content is lowered,the strength of the steel sheet is ensured by the precipitation strengthening of the microalloying elements such as Nb,V,and Ti,and the fine grain strengthening during rolling.The elements Mn and Mo can significantly improve the hardenability of the steel sheet and promote the transformation of granular bainite.Ni is mainly dissolved in the matrix to refine grains.(2)The static and dynamic CCT curves of 07MnNiMoDR steel were measured and drawn up on the basis of thermal expansion method.The effects of various cooling rates and supercooled austenite pre-deformation on the phase transformation and microstructure of the experimental steel were investigated.The results show that the granular bainite microstructure can be obtained at a cooling rate of 2?10?/s.When 50%deformation is applied to supercooled austenite,the granular bainite and lath bainite structure can be obtained at a cooling rate of 2?50?/s.Pre-deformation of supercooled austenite between 800? and 900? can increase the critical cooling rate of austenite,increase the phase transition temperature of ferrite and bainite,enlarge the temperature range of microstructure transition,and induce the formation of ferrite.It is easy to obtain bainite microstructure in the phase transformation of the experimental steel.(3)The flow stress curves of 07MnNiMoDR steel were measured and drawn up by thermal simulation single pass compression experiment.The effects of various deformation conditions on dynamic recrystallization and austenite grain size of experimental steel were analyzed.The thermal deformation activation energy and constitutive equation of the experimental steel are obtained.The results show that the higher the deformation temperature,the smaller the strain rate and the larger the deformation,the more likely the experimental steel to undergo dynamic recrystallization.The austenite grain size decreases first and then increases with the increase of deformation temperature.When deformed at 1050?1100?,and the austenite grain size is the smallest.When the deformation temperature is lowered,the austenite undergoes incomplete dynamic recrystallization.As the strain rate increases,the austenite grains become coarse.When the deformation temperature is high,the austenite undergoes complete dynamic recrystallization.As the strain rate and deformation increase,the austenite grain size decreases.The thermal deformation activation energy of the steel for the low carbon low alloy pressure vessel was calculated to be 559.4 kJ/mol.(4)The on-site rolling process parameters for the experimental steel were determined.The reheating temperature of the continuous cast slab is set as 1200?.Two-stage rolling processes can be adopted where the first stage rolling temperature is 1100? followed by air cooling,and the second stage rolling temperature is 850? and finishing temperature is 810?,then water cooled after rolling.The red temperature control is about 500? and finally air cooled to room temperature.(5)The effects of tempering temperature and holding time on the mechanical properties and microstructure of the experimental steel were studied.The results show that with the increase of tempering temperature,the strength of the experimental steel gradually decreases,and the impact toughness first increases and then decreases.With the prolonged tempering holding time,the strength of the experimental steel gradually decreased.When the 07MnNiMoDR steel is tempered at high temperature,the M-A island is mainly decomposed,the bainitic ferrite lath bundle is coarsened,and the carbide grows at the grain boundary.The tensile strength of 07MnNiMoDR steel plate can reach 670 MPa,the yield strength is 610 MPa,and low-temperature impact energy is 325 J at-50 ?.This can meet the customer's requirements for this steel.