Research And Development Of 1000 MPa Grade Hydroelectric Steel Plate

At present,the 1000 MPa grade hydroelectric steel used in China is still dependent on import.In order to change this status,it is necessary to develop 1000 MPa grade hydropower steel.Based on the performance requirements of large thickness and high strength hydroelectric steel,this thesis was focused on the chemical composition design,production and processing technology,microstructural and mechanical properties evaluation in laboratory for this steel,and the 1000 MPa grade hydroelectric steel plate was obtained with requirement for the mechanical performance.The main research contents and results obtained in this thesis are summarized as follows:(1)The chemical composition of 1000 MPa grade hydropower steel was optimized by JMatPro software.The equilibrium precipitation phases of experimental steel at different temperatures were calculated.The influence of main alloying elements on the precipitation phase and mechanical behaviors was analyzed.In order to meet the mechanical performance requirements,a reasonable chemical composition was designed.In this case,the carbon equivalent Ceq and the cold crack sensitivity index Pcm of this steel are 0.56%and 0.28%,respectively,which meet the welding requirement.(2)The continuous cooling transformation behavior of the experimental steel was studied by thermal expansion and metallographic methods.The static and dynamic CCT curves and SH-CCT curves of HAZ were measured and plotted.The results show that after hot deformation,full martensitic microstructure could be obtained when the cooling rate is higher than 40℃/s for this steel and the highest hardness is 411 HV.For the undeformed steel,a martensitic microstructure can be obtained when the cooling rate is only higher than 20℃/s and the maximum hardness is 406 HV.When the cooling rate is lower than 20℃/s(t8/3>25 s)in the heat affected zone,a part of martensitic microstructure was obtained with a maximum hardness of380HV.(3)The single-pass compression test was carried out on the experimental steel by thermomechanical simulator,and the true stress-strain curve was obtained.The influence of deformation conditions on dynamic recrystallization was analyzed.Finally,flow stress equation of the experimental steel was obtained.The results show that when the strain rate is 0.1 s-1,the deformation temperature is 1050-1150℃ and the true strain is greater than 0.7,complete dynamic recrystallization can occur in this experimental steel.(4)Based on the research results of continuous cooling transformation and dynamic recrystallization behavior,a reasonable rolling and heat treatment processes were formulated.First,a two-stage rolling schedule can be used in the laboratory,i.e.hot rolling could be carried out in recrystallization zone and non-recrystallization zone.After rolling,fine lath martensite microstructure can be obtained by direct quenching to room temperature.Finally,the rolled steel sheet is subjected to tempering heat treatment at different holding temperatures and times.After tempering,the steel has the mixed microstructure of tempered sorbite and tempered martensite.In this case,a good comprehensive mechanical behavior was obtained for the experimental steel.And the room temperature tensile strength is 982 MPa,the yield strength is 912 MPa,the impact energy(-60℃)is 85 J and the elongation is 18.3%.The comprehensive mechanical properties meet the performance requirements of 1000 MPa grade hydroelectric steel plates.