Study On The Effects Of Hot Rolling Process On Microstructures And Mechanical Properties Of Low Alloy Enameling Steels

The object of this study is enameling steel manufactured for the water heater tank. Because of the certain amount of temperature and pressure during usage period of the tank, the yield strength of enameling steels should meet a higher requirement. Enamel-firing process will change the mechanical properties of enameling steels. Hot rolling process affects the mechanical property of enameling steels as rolled and after enamel-firing. In this thesis, experiments and finite element modelling are carried out to study enameling steels with the same composition and different hot rolling processes and investigate the relationship among hot rolling parameters, microstructures and mechanical properties of enameling steels. The main results are as follows:Steels rolled by the dynamic strain induced transformation (DSIT) process, controlling start temperature of the last three passes near Ar3 temperature, can still obtain relatively small grain size and high yield strength after heat treatment. The reason is that the critical nucleation size of DSIT process is smaller than that of the normal austenite-ferrite transformation. Steels have been through the processes of DSIT and ferrite-austenite transformation for several times to obtain finer grain, release strain energy, avoid grain growth during heat treatment and lead to the high yield strength. It can be also found that increasing cooling range and cooling rate after rolling and decreasing final cooling temperature can avoid grain growth during cooling process. Steel 4 is rolled by DSIT process that sets the start temperature of the last three passes as 840±10℃; the cooling range as 340℃～360℃; the final cooling temperature as 440℃～460℃; the cooling rate as 21℃/s～23℃/s. The grain sizes of Steel 4 before and after heat treatment are all about 6μm. After heat treatment, the yield strength of Steel 4 increases from 370MPa to 380MPa-390MPa and the property of Steel 4 is close to that of the steel produced by the continuous casting and rolling process.It can be found from the experiment and simulation results that when the heat transfer coefficient between the slab and rollers is taken as 5～10kw/m2·k and convective heat transfer coefficient between the slab and air is taken as 0.02～0.05kw/m2·k, the simulated strip shape, maximum roll force and start temperature of each pass agree reasonably well with those measured experimentally. Roll force increases with the decrease of rolling temperature, and increases with the increase of reduction, but rolling speed has less influence on roll force. For the DSIT process, the roll force during the last three passes is about 2-3 times larger than that during the first three passes. Thus, we must consider rolling temperature and reduction to guarantee that roll force is permissible for rollers. Before pass 4 of the DSIT process, the temperature of slab center is higher than that of slab surface. During pass 4 to pass 6, the temperatures of slab center and surface converge, but rise about 60℃ and higher than the setting temperature. Thus, setting start temperatures of the last three passes lower than Ar3 can obtain finer and more uniform grains and higher strength. It can be also found that when reduction exceeds 60%, the rolling process is relatively unstable and it may cause the damage of slab and rollers. When rolling speed is lower than 750mm/s, the rolling process is also relatively unstable because of the large stress fluctuation and temperature decreasing.