Effect Of Composition And Heat Treatment Process On Microstructure And Mechanical Properties Of Enameling Steels

After firing at 800℃～900℃, the yield strength of enameling steels used in water heater tank will decreases, due to phase transformation during firing process. In this thesis, experiments and thermodynamic calculations are carried out to study the effects of heat treatment temperature, cooling rate, chemical composition, cold rolling and tempering process on the microstructures and mechanical properties of the cold rolled and hot rolled enameling steels. The strengthening mechanisms of cold rolled steels are analyzd. The main work and results are as follows:According to the thermodynamic calculations and thermo-mechanical analysis (TMA) results, the start and end temperatures of ferrite-austenite transformation in enameling steels studied in the present work are about 725℃ and 885℃. In the temperature range of 700℃～900℃, ferrite, austenite, Fe3C, TiN or AlN are the main equilibrium phases. The microstructures of the as-rolled hot rolled steel are ferrite and a small quantity of lamellar pearlite distributed at the grain boundaries. After heat treatment at 871℃, the yield strength decreases as the ferrite-austenite transformation temperature, the content of Si element and the cooling rate increase. The microstructures of the as-rolled cold rolled steel are ferrite and dispersion cementite particles. The yield strength of the cold rolled steel is lower than that of the hot rolled steel. The reason is that after cold rolling and annealing, the lamellar pearlite in hot rolled steel transforms into cementite particles, recrystallization occurs; and grain grows, leading to the yield strength decreasing. After different cold rolling and annealing processes, the relationship between grain size and yield strength is obtained, based on which the coefficient of fine-grain strengthening is determined as 12.2 N/mm3/2. Using this coefficient, and the results of thermodynamic calculations and microstructure analysis, the numerical estimates of the different strengthening contributions of the cold rolled steels with various compositions and processes are determined based on an empirical formula.After heat treatment at different temperatures with air cooling, the yield strength of the cold rolled and hot rolled steels with relative high contents of C and Mn will decrease first and then increase as the heat treatment temperature increases, namely, there is a yield strength dip at 732℃～815℃. In this range, the carbon-rich bainite-ferrite phase forms along the grain boundaries. The solid-solution strengthen becomes lower, leading to the yield strength decreases. When the cooling rate is lower than 1.5℃/s, the formation of carbon-rich bainite-ferrite phase can be avoided. Thus, the yield strength increases. After tempering at 400℃～700℃, the bainite-ferrite phase will transform into pearlite, and the yield strength increases to be higher than 380MPa. In addition, the samples are cooled from 815℃ to 600℃ by air cooling and then heated at 600℃ for lmin～5min can also avoid the formation of carbon-rich bainite-ferrite phase and increase the yield strength.