Processing Technology, Microstructures And Mechanical Properties Of A High Strength Hydroelectric Steel Plate

In recent years, with the quick construction of hydroelectric plants in China, especially the construction of large capacity pumped storage power plant, and with the emergence of high water-head, large diameter and pressure steel pipe of high HD value, the requirement is put forward for hydroelectric steel with higher toughness and weldability. In order to reduce the wall thickness of the pipe, spiral and fork tube, and to reduce the difficulty of construction and welding, high strength steel of 800 MPa grade has been used to these high-parameter, large-scale penstock, especially bifurcated pipe. However, the high strength steel for hydroelectric plants is generally used with higher prices and added values. Therefore, there is a need to develop steel with higher toughness and weldability, which is one of the urgent problems to be solved. This is also significant to the engineering applications. In this thesis, the chemical compositions of 800 MPa grade high strength steel for hydroelectric plant weredesigned and optimized in laboratory experiment. The effects of the controlled rolling and cooling and heat treatment processing parameters on the mechanical properties and microstructures were discussed and the main contents are as follows.(1) According to simulation result of the mechanical properties by JMatPro software, in order to ensure the mechanical properties of the experimental steels and minimize the amount of multi-alloying elements, an optimal chemical composition of the experimental steel have been calculated. Also in order to ensure the weldability, carbon equivalent was controlled at the lowest level, elements of Cr, Mo and Mn weer added to improve quenchability. The final chemical composition meets the requirement for design.(2) The static CCT diagram of the experimental steel was measured by using thermal expansion in thermal dilatometer and metallographical method. The influence of cooling rate on the microstructure and hardness of the experiment steel was discussed and the results reveal that the full lath martensitic microstructure could be obtained when the experimental steel was cooled at rate in the range of 10～40℃ after rolling and the highest hardness is 468 HV.(3) The SH-CCT diagram of the experimental steel was also measured by using thermal expansion in thermal dilatometer and metallographical method. The influence of cooling rate on microstructure and hardness of heat-affected zone (HAZ) was determined and the results reveal that not all martensitic microstructure could be obtained when the cooling rate is less than 10℃/s, i.e. t8/5>30s. In such a case, the maximum hardness is 414HV.(4) Two-stage hot rolling schedule was employed to obtain the experimental steel plate in a laboratoryΦ450 mm hot rolling mill. The microstructure of experimental steel after hot rolling was analyzed and the result show that the microstructure of the experimental steel after hot rolling consisted of fine and granular bainite.(5) The experiments relating quenching and tempering were performed to obtain the optimal heat treatment schedule. The microstructure and mechanical properties were also studied. Optimal quenching and tempering processing parameters were proposed to meet the mechanical performance requirements. Quenching process should be conducted when the heating temperature is 910～930℃ and hold for 20 min, while the tempering should be carried out at 650℃×40 min for the steel plate of 12 mm in thickness (equivalent to the 38 mm thick steel plate for the same compression ratio).