Effects Of Hot-rolling Process And Alloy Compositions On Microstructures And Mechanical Properties Of Emameling Steels

Enameling steels used for water heater tanks are studied in this thesis.Because water tanks will sustain a certain amount of pressure during the usage period,the yield strength of enameling steels should meet a certain requirement.Since the mechanical properties of the enameling steels may change during the enamel-firing process as a result of grain coarsening,solid-solid phase transformations and solute diffusion,it is significant to study the effects of alloy elements and hot-rolling process on micro structures and mechanical properties of enameling steels before and after firing.In this thesis,experiments,finite-element simulation and thermodynamic calculation are adopted to investigate enameling steels with different compositions,and produced by different hot-rolling processes.The influences of two different hot rolling processes,namely,the continuous rolling and dynamic strain induced transformation(DSIT),and alloying elements of Si and Mn are studied.The results can be used to optimize both the composition and the hot-rolling process to improve the final mechanical properties of a enameling steel.The main results are summarized as follows.Compared to the samples produced by the continuous rolling,the grain size of samples rolled by DSIT process is smaller due to the cyclic dynamic strain induced transformation occurred during the DSIT rolling.However,when the start rolling temperature of the last three passes is relatively high,the as-rolled yield strength of the samples produced by DSIT is close to that hot rolled by the continuous rolling and the yield platform becomes shorter,owing to the inhomogeneity of the nucleation sites during phase transformation and the large number of lump phase appearing in the final microstructure,While controlling the start rolling temperature of the last three passes close to Ar3 temperature,the microstructure of DSIT samples becomes more homogeneous,and the yield strength increases.The yield strengths of samples rolled by the continuous rolling decrease about 20～30 MPa after heat treated at 815℃-871℃,while the yield strengths of DSIT samples increase to different levels.The variation of yield strengths is in accordance with the microstructure changes after different heat treatments.The results of finite-element simulation show that the temperature at slab center is higher than that at slab surface during the first a few passes.The difference between temperatures at slab center and slab surface decreases as the rolling proceeds.Besides,the rolling force in DSIT process is larger than that of the continuous rolling,and the temperature fluctuations are more obvious during the DSIT process.The sample with lower Si concentration has no obvious yield platform and the micro structure contains ferrite with larger grain size and more pearlite phase.After heat treated at 815℃～871℃,the yield platform appears and yield strength increases in the sample with lower Si concentration.However,the yield strength is affected by the firing temperature.The yield strength of the sample heat treated at a high temperature is lower than those heat treated at tower temperatures,which may result from the fect that the grain size of pearlite increases fter heat treating at a high temperature.According to the thermodynamics calculations,either the decrease of Si or the increase of Mn reduces the ferrite-austenite transformation temperature.Thus the amount of austenite will increase during firing at a certain temperature,which may be the reason why the firing temperature has a significant effect on the morphology of the pearlite phase.Besides,the amount of Mn23C6 precipitates increases in the sample with higher Mn concentration,which will consume more carbon atoms and reduce the strengthening of Cottrell atmospheres formed by the carbon solution in the ferrite matrix,resulting in a relative low after-fire yield strength of the sample with higher Mn concentration.