Research On Microstructure And Properties Of 800MPa Grade Steel Heavy Plate For Hydropower Station

In recent years, Chinese hydropower industry is developing rapidly. In order to reduce wall thickness of the pipe, spiral casing, branch pipe and reduce the difficulty of construction and welding, the majority of pumped storage power station have already begun using hydropower 800MPa grade steel because of the higher HD value. About 800MPa grade hydropower steel, there is no national standard. But the quality of domestic products compared with abroad, while on the strength level is relatively close to, but in the plasticity index, low temperature impact performance, there are certain gaps.This paper will combine the laboratory test, theoretical analysis and industrial trial production. According to some domestic steel equipment and process conditions, formulate reasonable laboratory test scheme, ensure that the thickness of 800MPa hydropower 60mm steel after quenching and tempering treatment up to the technical index requirement of the Nanjing iron&steel. Paper’s main work and achievements are shown as follows:(1) The design of test steel composition provides guaranteed performance needed in the subsequent processing. Using the Thermo-Calc software to calculate the effect of main elements on the equilibrium precipitates. The JMatPro software is used to calculate the effect of these elements on hardenability. Combined with the cost, beneficial effect of the chemical composition is fully developed. It ensures that the 60mm thick plate hardenability is good. The test steel not only has good mechanical properties but also good weldability.(2) Expansion experiment is used to measure test steel static CCT curves and dynamic CCT curves. The effect of cooling rate and strain on CCT curve was studied. According to the CCT curves of the test steel, it showed good hardenability; the off-line quenching is more likely to get martensite organization than online quenching; deformation microstructure at 20℃/s can be transformed into martensite which undeformation microstructure only was 5℃/s.(3) Through the single press test on the thermal simulation experiment, the stress-strain curve of the test steel is measured, the influence of the deforming temperature, deforming rate and deforming degree on the deformation resistance of the steel is researched, and the mathematical model of deformation resistance is established. Through the double press test on the thermal simulation experiment, the rule of static recrystallization is discussed; the influence of different deformation temperature and the time interval of the pass on static recrystallization of the steel were researched.(4) The austenite grain coarsening temperature of test steel were studied. Results showed that microalloy carbonitride prevented austenite grain growth at the temperature below 1200℃. Along with the precipitates dissolve, austenite grain size grows obviously also occurred, and the microalloying elements raised the activation energy of recrystallization of test steel. Hot rolling heating temperature of test steel is set to 1200℃.(5) Different cooling mode, i.e. water cooling, air cooling and water cooling to about 500℃ followed with air cooling to room temperature continuously, were employed after rolling respectively. The microstructures and properties of each rooling status and heating treatment status were detected. The results showed that, the water cooling got martensite which air cooling and control cooling got granular bainite, the strength and toughness of low carbon martensite is best. After rolling only the intensity exceed the requirements of standard, but plasticity and toughness are not meet requirements. The microstructure was tempered sorbite organization which is fine equiaxed a phase matrix on the distribution of the diffuse granular carbides after quenching and tempering, so the plasticity and toughness of test steel increased obviously. Comparison of the on-line quenching, the microstructure of the off-line quenching is more uniform so that its performance is better.(6) Having studied the quenching and tempering process of experimental steel systematically, It showed that 50min holding time when the heat treatment of steel quenching at 920℃ has good mechanical properties. Reducing quenching temperature cause elements cannot solid solution sufficiently, thus the amount of precipitation decreased after tempering and result in mechanical properties got much worse. Increasing the quenching temperature can enlarge the grain size and lead to the decreases of toughness; Shorting quenching holding time made the grain size heterogeneous, thus the mechanical properties decreased. Though extending the holding time make grain size uniformly, precipitates size enlarged and easily precipitate along the grain boundary interfaces and cause the decrease of mechanical properties. Choosing the best quenching process and then tempering at 650℃ for 1h, raising tempering temperature and extending tempering time can make the translation of tempered sorbite more adequately, the reduction of dislocation density, the increase of the carbides size therefore leads to the decrease of the strength and the increase of the toughness; Similarly lowing tempering temperature and shorting tempering time can make the increase of the strength and the decrease of the toughness. After optimization of the experimental laboratory program,800MPa hydro steel can meet the requirements of mechanical properties eventually and provide guidance for the practical production.