Microstructures Of Dynamic Strain Induced Transformation In Enameling Steels:Experiment And Modeling

The enameling steels used for water heater tank are studied in this thesis.The yield strength of enameling steels should meet a certain requirement since the tank will bear high pressure during use.The mechanical property of the enameling steels may worsen after the enamel-firing process due to the phase transformations.The compositions and hot rolling process are the key factors affecting the mechanical property of as-rolled and after-fire enameling steels.In this thesis,the enameling steels with different compositions produced by dynamic strain induced transformation(DSIT)hot rolling are investigated using both experiments and modeling,to figure out the effects of compositions,DSIT process and heat treatment on mechanical properties and microstructures of enameling steels.The results will 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.The steels with a lower Cu composition have a higher as-rolled yield strength,and the yield strength(410MPa)decreases after firing,while the steels with a higher Cu composition have a lower as-rolled yield strength and a higher after-fire yield strength(440MPa).The reason may be that the as-rolled steels with a higher Cu composition contain lump phase enriched with C,Mn and Si,which impair yield strength.After firing the lump phase transforms to pearlite,leading to an increase in the yield strength.For the as-rolled steels with a higher Cu composition,the yield strength increases evidently after aging at 500? or 550? for 3h.According to transmission electron microscopic(TEM)analysis,Cu-rich precipitates in nano size appear in the as-rolled steels with a higher Cu composition after aging.These Cu-rich precipitates distribute in ferrite matrix and neighborhood of dislocations,improving yield strength through combination with ferrite matrix,and interaction with dislocation stress field to obstruct the movement of dislocation.According to finite element simulation,temperature at the surface of slab is lower than that of center before Pass 4 in the DSIT process while the temperatures of these two locations become nearly the same after Pass 4.In the last three passes,temperatures at slab surface and center both fluctuate in the range of 810?-900?.Effective stress at slab surface is higher than that of slab center in Pass 1,and they tend to be the same in Pass 4.According to cellular automaton(CA)modeling,with the decreasing temperature before Pass 4,the growth rate of ferrite fraction is relatively high at first and then decreases gradually.During the short period of Pass 4,microstructures change little and the ferrite fraction is much higher than the equilibrium value at the end of Pass 4 due to the insufficient time for phase transformation.As the temperature continues to decrease after Pass 4,ferrite fraction decreases at first and then increases,which means that ferrite?austenite reverse transformation takes place in this period.In the actual rolling process,steels undergo austenite?ferrite transformation and reverse transformation for several times,which releases strain energy and avoids grain growth during firing,leading to a high yield strength.In addition,the grain size of surface microstructure is smaller and more uniform compared to that of center microstructure according to CA modeling,which agrees well with the experimental observations.