Investigation Of Microstructures And Mechanical Properties Of Enameling Steels By Experiment And Simulation

Water heater tanks built with enameling steel encounter deformation from temperature and stresses imposed during service,which requires good yield strength.However,when the steel is exposed to the thermal cycle to fire the glass enamel,the yield strength may fall.In this thesis,the impact of different heat treatments on the microstructure and yield strength of enameling steel is studied by experiments and cellular automaton?CA?modeling.In order to figure out the mechanism of the lump phase formation and low yield strength exhibited in the air cooled sample,the microstructure,chemical composition,and nanohardness of the lump phase exhibited in the air cooled sample are investigated in detail.The influence of lump phase on yield strength is also studied.Additionally,experiments are carried out to investigate the possibility of refining enameling steel grains and improving yield strength through cyclic heat treatments.The main results are as follows.The results show that after the 12#cold-rolled enameling steel is held at an intercritical temperature of 780°C for 5 min and then air cooled to room temperature,there is a small amount of lump phase distributing at the ferrite/ferrite grain boundaries and the air cooled sample exhibits a lower yield strength compared to the as-rolled sample.Scanning electron microscopic?SEM?,transmission electron microscopic?TEM?and 3-D atom-probe tomographic?APT?analyses show that the lump phase is basically a complex martensite,which is transformed from the austenite enriched in elements C and Mn.Due to the composition differences in the parent austenite grains with different sizes,such martensite contains both the lath-like feature of low carbon martensite and twins normally formed in high carbon martensite.After tempering the air cooled sample at 100°C,the martensitic morphology is nearly unchanged.When the tempering temperature is increased to400°C,the decomposition of martensite is nearly complete and the yield strength increases evidently.The nanohardness of martensite is higher than that of ferrite for all the heat treated samples.After tempering,the nanohardness of martensite decreases.According to the CA simulation,after holding at 780°C for 5 min and then cooling to room temperature at 8.8°C/s,the C distribution in the ferrite matrix is non-uniform and the average C concentration of the ferrite matrix is about 0.028 mol.%.After tempering at 400°C for 5 min,the C distribution in the ferrite matrix becomes more homogeneous and the average C concentration of the ferrite matrix increases to 0.04 mol.%.The above results imply that martensite enriched in C reduces the amount of C atoms in solution in the ferrite matrix,and thus fewer dislocations can be pinned to form Cottrell atmospheres,which is considered to be the main reason for the low yield strength of the air cooled sample.The water-quenching experiments indicate that the complete austenitizing temperature(A_c3)of14#hot-rolled enameling steel is slightly below 950°C with the heating rate of 95¹15°C/s,and the austenite?ferrite transformation start temperature(A_r3)is around 830⁸40°C with the cooling rate of 4.0⁶.0°C/s.While for the austenite?perlite transformation start temperature(A_r1),it is estimated around 700⁷20°C when cooling at 1.5².0°C/s.After cyclic heat treated in the temperature range of A_c3-A_r3,the grains become larger and the sizes are more non-uniform;additionally,there are some pearlite islands exhibiting in the ferrite matrix.While after cyclic heat treated in the temperature range of A_c3-A_r1,despite the similar microstructure consisting of ferrite and a small amount of pearlite,the grain size is smaller than the case heat treated between A_c3-A_r3.Therefore,lowering critical temperature,shortening the holding time and increasing the cooling rate of cyclic heat treatment appropriately are beneficial for obtaining finer grains.After heat treated between 950⁸40°C for 2 cycles,the relative good mechanical properties of samples are obtained.However,it is found that the grain refinement can hardly be achieved by cyclic heat treatment for the enameling steel studied in this work,due to the fact that the original grain size is relatively small and the maximum cooling rate is limited by the practical enamel-firing process.The yield strength obtained by cyclic heat treatment is similar to that by conventional heat treatment.