Quenching And Tempering Process,Microstructure And Properties Of 800MPa Grade Steel Plate For Hydropower Station

With the development of large capacity pumped storage power station,in order to reduce the wall thickness of the pipe,spiral and fork tube,and to reduce the difficulty of construction and welding,most of new hydropower stations have bagun using 800MPa grade hydropower steel.At present,the 800MPa grade hydropower steel produced by Shougang and Wugang or any other domestic iron and steel enterprises has been in the market.Compared with foreign congeneric products,there is still a certain gap on the low temperature impact performance,hardenability and so on.This paper will be based on previous studies,combined with industrial trial and the production process of the 800MPa grade hydropower steel will be further optimized.On the basis of the guarantee of the microstructure and properties,the process flow will be reduced and the production efficiency will be improved as far as possible.The main search contents are shown as follows:(1)The equilibrium phase diagrams and hardenability curves of WEL-TEN780 steel,WSD690E steel and experimental steel were established by using Thermo-Calc software and JMatPro software.Analyze the influence of the elements on the precipitated phase during high temperature tempering and the hardenability of the experimental steel.Taking hardenability,obdurability and welding properties into consideration,the chemical compositions of the experimental steel were optimized.(2)Using thermal expansion method,the phase transformation temperature,static CCT curves and dynamic CCT curves were measured.The relationship between different cooling rates and microstucture were also obtained.Compare static CCT curves with dynamic CCT curves,we can find that deformation increased the cooling rate of obtaining martensite.The critical quenching rate of direct quenching is bigger than that of reheat quenching.(3)Using the thermal simulation experiment for single press test,the flow stress curves of the experimental steel were measured.The influence of the deformation temperature,strain rate and deformation degree on the flow stress of the steel was researched.According to the deformation resistance of Jihua Zhou,the mathematical model of deformation resistance was established.According to the flow stress curves,calculated the DRX fration,then the recrystallization zone map was estableished.(4)The experimental steels were obtained by two stage rolling in ?450 mm two-high reversing hot rolling mill.The influence rule of beginning temperature,end temperature,starting cooling temperature and such technological parameters on microstucture and properties was investigated.It was concluded that reduce the holding time and cool to room temperature after rolling quickly could prevent microstructure from coarsing.(5)Use different quenching process after hot rolling.It was shown that the yield strength and the tensile strength of DQ+T was higher than that of RQ+T,and the plasticity and the low temperature impact performance were almost the same.Direct quenching eliminates the reheating process,use the heat after rolling and save energy.Taking all above into consideration,we choose direct quenching process.(6)The experimental steel must use proper tempering treatment after quenching.Set different tempering temperature and tempering time for tempering test.Analyze the influence of different tempering parameters on microsturcture and properties.Finally,the optimal tempering temperature is 610? and the tempering time is 90 min.