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

Enameling steels used for water heater tanks are studied in this thesis.The yield strength of enameling steels should meet a certain requirement since the tank will bear high pressure during the usage period.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.The compositions and hot rolling process are the key factors affecting the mechanical property of as-rolled and after-fire enameling steels,therefore,it is significant to study the effects of compositions and hot-rolling process on microstructures and mechanical properties of enameling steels before and after firing.In this thesis,experiments are adopted to investigate enameling steels with different compositions,and produced by different hot-rolling processes,to figure out the effects of compositions of Mn and Si,and dynamic strain induced transformation(DSIT)process on mechanical properties and microstructures of enameling steels.The results could be used to optimize compositions and hot rolling process to improve the final mechanical properties of enameling steels.The main results are summarized as follows.It is found that the mechanical properties and microstructure of enameling steels before and after firing has certain rules with the change of the Mn/Si ratio of components by the comparison of the mechanical properties and microstructure of the enameling steels with different Mn and Si components.With the Mn/Si ratio from 5 to 1.6,the as-rolled yield strength of enameling steel increases gradually for about 140 MPa,and there appears obvious yield platform from no yield platform on the tensile curve.When the Mn/Si ratio is relatively higher,the second phase in the microstructure of the as-rolled steels are coarser and the content of the element C is higher,while the Mn/Si ratio is relatively lower,the second phase is finer and the content of the element C is lower,which may be the reason of that the as-rolled yield strength increases gradually and there is obvious yield platform from no yield platform on the tensile curve with the decrease of the Mn/Si ratio.After heat treatment at 815~871oC,with the decrease of Mn/Si ratio,the yield strength of steel first increases and then decreases,and when the Mn/Si ratio is 2.4,the strength of steel reaches the maximum value at about 400 MPa.According to the results of Gleeble thermal simulation test,the deformation temperature and strain rate have great influence on the microstructure and stress-strain curves during the deformation process.When the deformation temperature is the same,the greater the strain rate is,the more the finer deformed ferrite grains appeared in microstructure and the more uneven the grain size is,and the stress of the steel increases with the increase of the strain rate during the compression process.When the compression temperature is 860 oC,there distributes a lot of finer ferrite grains around coarse ferrite grains.When the compression temperature is 910 oC,the grain size distribution is more uniform than that of the sample microstructures at 860 oC.In addition,the stress of steel during deformation increases with the increases of deformation temperature.It is found that the last pass of DSIT process is not obvious,and the fourth and fifth passes of DSIT process are more effective during the actual rolling process.