Effects Of Cr And N Alloying And Grain Microstructure On The Corrosion Resistance And Mechanical Properties Of Twinning-induced Plasticity Steels

Twinning induced plasticity(TWIP)steels have excellent mechanical properties and particularly the plasticity,showing prospective applications in aerospace,defense,transportation and energy,etc.However,the corrosion resistance of TWIP steels is relatively weak,leading to limited or unfavorable performance in corrosive or aqueous environments.In order to overcome this disadvantage,Cr and N alloying were utilized in the present dissertation to improve the corrosion resistance of TWIP steels through increasing the electrode potential of austenite matrix and forming a passivation film on the grains.The grain boundary engineering means were also used to change the proportions of special grain boundaries so as to reduce the corrosion rate of grain boundaries.Moreover,the controlling technologies of grains and relevant changes in the mechanical properties of new TWIP steels were investigated to provide guidelines for disclosing the evolution laws of microstructures and properties of TWIP steels alloyed by Cr and N.The main conclusions are as follows:A series of TWIP steels with the composition of Fe-25Mn-xCr-0.3C(x=0,3,6,9 and 12 in mass fraction)were prepared in an electric ARC furnace under argon atmosphere.It is found that,by the examination of microstructures and corrosion behavior,with increasing the Cr addition,the corrosion potential(Ecorr)of matrix increased while the density of corrosion current(Icorr)decreased,demonstrating that the Cr additions do improve the corrosion resistance of TWIP steels.In addition,the electrochemical impedance spectroscopy analysis shows that the charge transfer resistance(Rct)was increased and the electric capacitance of double layers Y0 value was reduced as the Cr content was increased.This suggests that,during the electrode reaction at the electrode/solution interface,the hindering effect of Cr alloying on the ion diffusion and migration were gradually enhanced,resulting in decreased corrosion rate of TWIP steels.The mechanisms would be the enrichment of Cr oxides and depletion of Fe and Mn oxides because the former and the passivation film are the two main reasons for increased corrosion resistance of TWIP steels.The study on the N alloying shows that the addition of N makes the density of corrosion current(Icorr)decreased and the charge transfer resistance(Rct)increased,thus is also favorable for improving the corrosion resistance of TWIP steels.It is found that joint alloying by Cr and N in TWIP steels exhibits better corrosion resistance.In terms of related thermodynamic model,the stacking fault energies of Fe-25MnxCr-0.3C-0.3N(x=12 and 18,all in mass percentage)TWIP steels are calculated to be about 30 mJ·m-2,meeting the requirement of twinning deformation.Compared with common Fe-25Mn-0.6C TWIP steel,the TWIP steel containing 12Cr and 0.3N has the highest yield strength,tensile strength and plasticity,which are 496 MPa,904 MPa and 67.2%,respectively.When the steel was deformed to a strain of 15%,less deformation twins appeared while high density of dislocations in the form of walls can be observed.When the strain was increased to 45%,the number of deformation twins increased but the density was still not very high and the thickness of twin lamella was very thin.The evolution of deformation microstructures demonstrated that the TWIP steels alloyed with Cr and N still deformed predominantly in the form of twinning but the strain hardening rate was relatively low.As the location where the defects are most concentrated,the grain boundaries are also the place to be most prone to corrode.In light of this feature,hot and cold rolling with varied reduction rates were carried out using Fe-25Mn-12Cr-0.3C TWIP steel to get different grain morphologies and sizes.It is found that,as the reduction rate of cold rolling increased,both the corrosion potential(Ecorr)and the charge transfer resistance(Rct)were increased,while the corrosion current density(Icorr)was reduced,suggesting that the corrosion resistance of TWIP steel was improved.The further observation of grain boundaries demonstrated that the proportion of ∑3 grain boundaries increased but the proportion of other special grain boundaries was very low and had only slight changes.The increase of Σ3 grain boundaries would be one reason leading to improved corrosion resistance.Experiments were conducted to tailor the grain structures and to understand the relevant mechanical properties of Cr and N alloyed TWIP steels.The Fe-25Mn-15Cr0.3C-0.3N TWIP steel was firstly cold rolled to the reduction rate of 66.7%,then heated to different temperatures ranged from 650℃ to 1050℃ for keeping 10 min and finally water quenched.The observations of microstructures found that the recovery,recrystallization start and end temperature are 650℃,700℃ and 850℃,respectively.If the annealing temperature reached or over 1050℃,the grains rapidly grown.The TWIP steel in the cold rolled state has the higher yield and tensile strength but lower plasticity.As the annealing temperature increased,the strength decreased and the plasticity increased.When annealed at a temperature above 850℃,the yield strength of TWIP steel meets the Hall-Petch relationship.The corrosion resistance of recrystallized TWIP steel tended to decrease as the annealing temperature increased,because the Σ3 grain boundaries decreased with increasing the annealing temperature.As is known,the Σ3 grain boundaries are beneficial to corrosion resistance of TWIP steels.