| The medium-high temperature vessel pressure steel is widely used in petroleum,chemical engineering and other important national economic fields.With the rapid growth of the national economy,the demand of developing pressure vessels is rising constantly.12Cr2Mo1 steel is a classical medium-high temperature vessel pressure steel.The container part produced by 12Cr2Mol steel has not only high strength,good ductility and good weldability,but also good working and high temperature endurance performance,which ensures the safety during long term service.The conventional process of 12Cr2Mo1 steel plate is hot rolling,normalizing and high temperature tempering.In view of the practical problems of low efficiency and high cost in the production of the steel,NanSteel and Northeastern University put forward the feasibility study of using thermo-mechanical control process+high temperature tempering instead of conventional rolling+normalization+high temperature process.Based this background,in this paper,the microstructure and mechanical properties of 12Cr2Mo1 steel during hot rolling and heat treatment were systematically studied.The main works of this dissertation were listed as follows.(1)The high temperature deformation behavior of the tested steel was studied by high temperature single pass compression test.The thermal deformation activation energy was determined.The constitutive relationship and deformation resistance model of the steel were established(2)The continuous cooling transformation of the static and dynamic curves were measured by the thermal expansion method.The results show that the deformation makes the ferrite and pearlite region move to the left,which promotes the eutectoid ferrite transformation and improves the bainite transformation temperature.The phase transition zone of the experimental steel is divided into three parts:high temperature phase transition zone,medium temperature phase transition zone and low temperature phase transition zone.Cr,Mo and other elements in the experimental steel increase the stability of austenite,so bainite can be obtained in a wide range of cooling rates.(3)The microstructure and mechanical properties of the steel during conventional hot rolling+normalizing+high temperature tempering were studied.The dissolution behavior of carbides and the behaviors of microstructure evolution at different normalizing temperatures were analyzed.The results show that there was a large volume of undissolved carbides at low normalizing temperature,the undissolved carbides will aggregate and grow during the subsequent tempering process.At higher normalizing temperature,the austenite grains begin to coarsen,which promotes the formation of bainite and martensite,and reduces the nucleation sites of precipitates.The lower tempering temperature leads to the precipitation of irregular carbides.When the tempering temperature exceeds 700 ℃,the carbides begin to gather,grow up and some carbides appear to dissolve.When the normalizing temperature is 930℃ and the tempering temperature is 700℃,the room temperature matrix of the tested steel is composed of ferrite and fine bainite,the precipitates are finely dispersed.The yield strength and tensile strength are 535 MPa and 634 MPa,respectively,the ductile-brittle transition temperature is-52.6℃,which shows good mechanical properties.(4)The microstructure evolution during the simulated thermo-mechanical control process of the experimental steel was studied.The results show when the finishing rolling temperature decreased from 860℃ to 800℃,which promotes the formation of bainite,the microhardness increases from 296 HV to 400 HV;Increasing the cooling rates during the hot rolling stage reduces the grain size of the M/A island and can refine the bainite structure;When the final cooling temperature is 650℃,the matrix is granular bainite;When the final cooling temperature is 600℃,the microstructure was composed of fine and diffuse M/A island;When the final cooling temperature is 550℃,the matrix was composed of lath bainite.(5)The microstructure and mechanical properties were studied after thermo-mechanical control process with high temperature tempering of the tested steel.The results show the bainite and the M/A island is smaller after thermo-mechanical control process than that after conventional hot rolling,the yield strength is 580 MPa,the tensile strength is 790 MPa,and the elongation is 18.1%,the ductile-brittle transition temperature is-19.7℃.When tempering at 700℃ and the tempering time is 30 min,the irregularly shaped M/A islands cannot be decomposed.When the tempering time is 50 min,M/A islands began to decompose,and nano scale precipitates were distributed in the matrix.When the tempering time is 70 min,a part of bainite decompose into polygonal ferrite,and some bainite laths merge,the maximum size reaches to 450 nm,and the precipitates grow up.The best mechanical properties were obtained at tempering temperature is 700℃ and tempering time is 50 min.The yield strength is 495 MPa,the tensile strength is 562 MPa,and the elongation is 21.5%,the ductile-brittle transition temperature is-51.7℃.(6)The effects of different welding heat inputs on microstructure and impact toughness were investigated.The results show when the welding heat input is 20 kJ/cm,the coarse grain zone in the heat affected zone of the experimental steel is lath martensite+bainite,and the impact energy at room temperature is 64 J.When the welding heat input is 50 kJ/cm,the coarse grain zone in the heat affected zone of the experimental steel is bainite+acicular ferrite,the impact energy at room temperature is 102 J.when the welding heat input is 80 kJ/cm,the coarse grain zone in the heat affected zone of the experimental steel is coarse bainitic,and the impact energy at room temperature is 31 J.When the welding heat input is 50 kJ/cm,the hardness of the welding heat affected zone is equivalent to the base material matrix,and the precipitates are finely dispersed and has good impact toughness.(7)The effects of the minimum simulated post-weld heat treatment,the maximum simulated post-weld heat treatment and the step cooling treatment on the microstructure and mechanical properties of the experiment steel were studied.The results show the matrix structure is ferrite after the minimum simulated post-weld heat treatment.The yield strength and the tensile strength is 490 MPa and 552 MPa respectively,the ductile-brittle transition temperature is-50.1℃.After the maximum simulated post-weld heat treatment,the matrix is decomposed from bainite to ferrite completely.The yield strength and the tensile strength is 475 MPa and 540 MPa respectively,the ductile-brittle transition temperature is-34.5℃;And after the step cooling treatment,the matrix is ferrite.The yield strength and the tensile strength is 460 MPa and 520 MPa respectively,the ductile-brittle transition temperature is-34.5℃.The prolongation of heat treatment time promoted the spheroidization of the precipitates and bainite decomposition,which resulted in the decrease of strength and toughness.The heat treatment after simulated welding reduced the internal stress of the matrix,the precipitation of carbides such as M23C6 and M6C3 increased the strength and maintained the stability of the matrix.(8)The long-term rupture time of the experimental steel was tested at the temperature of 500℃,550 ℃,600 ℃ as well as the stress of 160～350 MPa.Combined with the extrapolation method,the high temperature endurance strengths of the experimental steel were calculated as 234.0 MPa,196.6 MPa,163.3 MPa,127.0 MPa,90.7 MPa and 56.5 MPa when they served for 105 hours at 450 ℃,475 ℃,500 ℃,525 ℃ and 550 ℃ respectively.The high temperature endurance strengths reach the standard value,indicating the ideal high temperature lasting performance of the experimental steel. |
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