Analysis On The Fracture Of The Stainless Steel During Twin-roll Strip Casting Process And Numerical Simulation On Its Temperature And Stress Fields

The specimen was taken from the strip of the304stainless steel after twin-roll stripcasting. The microstructure and fractograph of the specimen was observed by OM andSEM. The phase structure of the specimen were deterimined by XRD. Meanwhile, the DSCcurve of304stainless steel during the cooling process was measured. The stress-straincurves of304stainless steel at different temperatures were measured by thermo-simulatingmachine and its dilatometric curve was determined too.The residual stress fields of the column specimens after quenching and air coolingwere measured, and the FEM of them was built and the stress fields were simulated. Thesimulated results were compared with the measured one.The FEM of the coupling fields between the temperature and fluid during the stripcasting of304stainless steel was built in this work. By using this model, the temperatureand fluid fields during the strip casting of304stainless steel were simulated. Meanwhile,the temperature of the304stainless steel strip at the exit during twin-roll strip castingprecess was measured. By using this model, the effect of the different technologyparameters on the solidification of the stainless steel was simulated too. Based on abovementioned model, combining simulated temperature field of the304stainless steel strip, thestress field of the304stainless steel strip was simulated too and the effect of differenttechnology parameters on the stress field was discussed in this work.The crack analysis of the304stainless steel strip during twin-roll strip casting processshows that, the cracks occur at the dent of the strip surface and develop along the dendriticgrain boundary. The large numbers of oxides can been found on the fractograph, whichindicates that the cracks have been appeared at higher temperature. By means of themicrostructure of304stainless steel at different temperatures, the analysis of XRD result,DSC curve and the Fe-C phase diagram, the phase transformation at high temperature fromδ to γ has been carried out. With the increasing of the temperature, the strength of thestainless steel is decreased, while, the plasticity is increased.The measured residual stresses of the column specimens after quenching and aircooling were compared with the simulated ones and they are closer to each other, whichproves the validity of this model. By using this model, the temperature and fluid fields atdifferent times and at different location of the specimen were simulated. The simulated results show that, for quenching specimen, at the beginning of the cooling process, thetangential tensile stress appears on the surface, while, tangential compress one at the center.With the increasing of the time, the stress is changed to zero, then, the tangential compressstress is appeared on the surface, while, tangential tensile one at the center, and the stress ischanged greatly. When the time is90s, the stress is the residual one. For the air coolingspecimen, at the begining of the cooling process, the tangential tensile stress is appeared onthe surface, while, tangential compress one at the center. With the increasing of the time,both of the stresses on the surface and center are decreased. When the time is1800s, thoseare very small, which are close to zero, and can be taken as residual one.The measured temperature of the304stainless steel strip at the exit during twin-rollcasting process was compared with the simulated one, and they are closer to each other,which proves the validity of this model. By using this model, the effect of differenttechnology parameters on the temperature field was simulated. The results show that, thefaster the casting speed is, the higher the temperature of the strip at the exit and the lowerthe position of the freezing point are. Meanwhile, the higher the casting temperature is, thehigher the temperatures of the whole melting pool and the exit of the strip, and the lowerthe position of the freezing point are. Moreover, the larger the radius of the roller is, thelower the temperatures of the whole melting pool and the higher the position of the freezingpoint are. In addition, the thicker the strip is, the higher the temperatures of the wholemelting pool and the lower the position of the freezing point are.Based on the temperature field simulated by the coupling ones during the twin-rollstrip casting of304stainless steel, the stress fields both upper and under the exit of the stripwere simulated. The simulated result of the stress field upper the exit of the strip shows that,the largest tensile stress is appeared on the contact interface between strip and the roller,which is near the exit, while, the largest compress stress is appeared at the location of thestrip center. The effect of different technology parameters on the stress field upper the exitof the strip was simulated. The faster the casting speed is, the lower the stress along thecontact interface and the lower both the tensile and compress stresses at the exit of the stripare. Meanwhile, the higher the casting temperature is, the lower the stress along the contactinterface is, however, little effect can be found at the exit of the strip. Moreover, the largerthe radius of the roller is, the higher the stress along the contact interface is, however, littleeffect can be found at the exit of the strip. In addition, the thicker the strip is, the lower thelocation of the roller radian is, at which the largest stress is appeared and the higher the stress along the direction of the strip thickness is.The simulated result of the temperature and stress fields under the exit of the stripshows that, for the temperature field, at the beginning of the cooling process, thetemperature at the strip center is higher than that on strip surface. With the increasing of thecooling time, the temperatures at both strip center and surface are decreassed. For the stressfield, at the beginning of the cooling process, the tensile stress is appeared on the stripsurface, while, the compress one at the center.. With the increasing of the cooling time, thetensile stress on the strip surface is decreased into zero, then increased into compress stress,however, the compress stress at the strip center is decreased into zero, then increased intotensile stress.