| Twinning Induced Plasticity(TWIP)steels are being developed for automobile during the last decade,attributed to its combination of very high strength and ductility at room temperature.Therefore,Fe-Mn-C(-Al)TWIP steels have been recognized as the next generation material in automobile.However,the additions of such high percent of Mn and Al in TWIP steels have posed challenges to casting and rolling processes.Moreover,there is limited literature which focuses on the metallurgical research of TWIP steels.Based on this condition,the present study have carried out the systematic investigation on the metallurgical fundamental,solidification characteristics,and hot ductility of Fe-Mn-C(-Al)TWIP steels.Research works in this paper are summarized as follows:(1)Blowing the mixed CO2+O2 gas during the smelting of TWIP steels to achieve decarburization and manganese conservation was first proposed.Moreover,equilibrium thermodynamic calculation,software simulation,and experiments have carried out to investigate the influence of CO2 on the decarburization and manganese conservation.Accroding to the Factsage software calculation,the manganese evaporation and oxidation loss could be effectively reduced by inserting a certain proporation of CO2 gas..In addition,the experiments were conducted in the tube-type resistance furnace to investigate the influence of different CO2 proportion,gas flow,and smelting temperature on the decarburization and manganese conservation.The results show that the manganese conservation would be improved as the CO2 proportion increased when the carbon content in molten steel decreased to the similar level.And,the smelting temperature and gas flow rate have little influence on the manganese conservation in TWIP steels.(2)The liquidus temperature calculation formula of Fe-(16-20)Mn-(0.4-0.8)C-(0?3)Al TWIP steels were determined by the Thermo-calc software using the improved Scheil model under the non-equilibrium condition.From the comparison,the calculation results are very close to the measured values,and error margin is only plus or minus 8?.On the other hand,the solidification mode of Fe-(12?20)Mn-(0.02?1)C-(0.05?5)Al TWIP steels were investigated by Themo-calc software.(3)On the one hand,the macro-segregation of elements was investigated by the chemical analysis of drilling cutting of TWIP steels.The results show that there is no apparent macro-segregation from the edge to the center in TWIP ingot.On the other hand,the micro-segregation was analyzed by using the JEOL JXA-8230 electron probe microanalyzer in combination with the micro-segreagtion model in differnet Mn and C containing TWIP steels.It coud be conclude from the results that the Mn micro-segregation ratio would be increased as the solidification processed.While the Al micro-segregation ratio would be decreased as the solid fraction increased.The C micro-segregation ratio shows little change,the value of which is around 1.In addition,the C micro-segregation ratios are 0.85?1.16,0.76?1.22,and 0.74?1.32 in 14.94 wt%,18.21 wt%,and 23.6 wt%Mn containing TWIP steels,respectively.This indicated that the carbon micro-segragation would be more serious with the addition of manganese in TWIP steels.(4)The characteristics of the inclusions in four TWIP steels with different A1 contents(0.002%-1.590%)as well as the influence of Al content on the precipitation of AIN inclusions were investigated by using scanning electron microscopy,energy-dispersive X-ray spectroscopy,and an automated program called "INCAFeature." In addition,systematic thermodynamics calculations of AIN formed in TWIP steel were carried out using the appropriate thermodynamic data for high-Mn-Al TWIP steel.The results show that AIN would begin to precipitate and locally precipitate around the MnS(Se)-Al2O3 inclusions when the Al content in the steel reaches 0.75%.The thermodynamics calculations show that AIN could already form in the liquid TWIP steel at an A1 content of 1.07%.Then,AIN would locally precipitate around the MnS(Se)inclusions,thus forming MnS(Se)-AlN aggregates.When the Al content increases to 1.59%,the precipitation temperature of AIN is 42 ? higher than the liquidus temperature of the TWIP steel.Furthermore,precipitated AIN inclusions in the liquid TWIP steel could act as heterogeneous nuclei for MnS(Se)inclusions,thus forming MnS(Se)-AlN inclusions.Moreover,according to the thermodynamics calculation,the lowest N content for AIN formation in the liquid Fe-18.21%Mn-0.64%C-1.59%Al steel is just 0.0043%.Therefore,the N content should be kept as low as possible to avoid the formation of excessive AIN inclusions during melting of Fe-Mn-C(-Al)TWIP steel.(5)Hot tensile tests were conducted using Gleeble-3500 thermomechanical simulator in the testing temperature range of 700?1200?.The hot ductility and influenced mechanism were revealed based on the discussion and analysis of matrix phase,matrix homogeneity,fracture behavior,precipitated particles,and dynamic recrystallization(DRX).The result shows that the high Mn and Al alloying delayed the onset of DRX,and intergranular cracking as a result of grain boundary sliding was recognized as the main fracture mechanism during hot tensile tests for TWIP steel.Futhermore,the excessive number of fine AIN particles in 18Mn1.5Al steel,and the average size of which were only 564nm.They could effectively pin the grain boundaries,inhibiting the occurrence of DRX and simultaneously promote grain boundary sliding,causing the hot ductility decrease. |
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