| The composition design of low Ni type duplex stainless steel adopts Mn to take place Ni to enlarge the austenitic phase region to stabilize the austenitic phase.Due to the difference of stacking fault energy between ferritic and austenitic phase,the flow softening mechanism of two phases is different in the hot deformation,which results in the difference of grain size,the number and distribution of grain boundaries,and then affects the pitting corrosion,intergranular corrosion and passivation behavior.Based on the hot compression test,the effects of Mn additions(6.3%,10.3%,14.1%)on pitting,intergranular corrosion behavior and passivation film composition of23%Cr low Ni type duplex stainless steel were studied by means of potentiodynamic polarization curve,AC impedance spectrum,double loop electrochemical potentiodynamic reactivation and XPS,aims to find excellent corrosion resistant performance matching of hot processing,to provide reference for rolling process formulation and to broaden the application field of low Ni type duplex stainless steel.When compressed at 800?/0.01-1 s-1,the deformed microstructure analysis found that ferrite recrystallized dynamically,and austenite was in the nucleation stage of recrystallization.When the specimen compressed at 900-1050?/0.1-10 s-1,the main softening mechanism of ferrite is DRV,while that of austenite is DRX.Compression at 900?/0.1-1 s-1 and 1050?/0.1-10 s-1 is beneficial to the recrystallization and refinement of the microstructure of the tested steel.The addition of 6.3%and 10.3%Mn can accelerate the recrystallization softening of austenite,while the addition of 14.1%Mn will weaken the effect of austenite recrystallization.The pitting potential of the specimen fluctuated greatly under different deformation conditions,and the pitting potential of the deformed specimen was lower than the solid solution specimen.After compressed at 0.1 and 1 s-1,the pitting potential of the 6.3%Mn specimen was higher.While the strain rate up to 10 s-1,the pitting potential of the 10.3%Mn tested steel was higher.Under the condition of 900-1050?/0.1-1 s-1,the recrystallization refinement degree of6.3%Mn and 10.3%Mn tested steel is higher than 14.1%Mn,which is conducive to the diffusion of Cr to form a continuous passivation film,increase the resistance and density of the passive film,and improve the pitting corrosion resistance of the tested steel.Too high Mn addition weakens the recrystallization process and weakens the diffusion of Cr.At the same time,Mn addition reduces the tested steel pitting equivalent and is not conducive to improving the material pitting performance.The pitting pits are mainly distributed at the?/???/?grain boundary,?/?phase boundary and near the grain boundary and phase boundary.At the same deformation temperature,the corrosion degree increases with the increase of strain rate.At the same strain rate,with the increase of deformation temperature from 900 to 1050?,the number of corrosion pits decreased and the degree of corrosion decreased.The intergranular corrosion sensitivity of solid solution samples gradually increased with the increase of Mn addition,and the intergranular corrosion resistance of 14.1%Mn specimen was poor.When deformed at 0.1 s-1/900-1150?,14.1%Mn tested steel has the lowest intergranular corrosion sensitivity.After compression at 1-10 s-1/900-1150?,the intergranular corrosion sensitivity of 10.3%and 14.1%Mn steel is the lowest,and the intergranular corrosion resistance of the specimen is better.Medium and high Mn deformation samples have excellent intergranular corrosion resistance.The increase of Mn content can make the intergranular corrosion resistance of hot compressed specimen superior to the corresponding Mn addition of solid solution specimen.The intergranular corrosion pits expanded along the interface of?/?,?/?and the grain boundary,and gradually formed“necklace-shaped”and“step-shaped”corrosion gullies and corrosion steps.The increase of Mn addition can improve the intergranular corrosion resistance of the tested steel to a certain extent.The composition of passivation film contains Fe2O3,Fe3O4,Fe OOH,Cr2O3,Cr(OH)3,Mn O,Mn OOH,Ni O and other metallic oxides/hydroxides,as well as Fe,Cr,Mn,Ni and other metal elements.The content of each oxide varies with the change of Mn addition.The passivation film of tested steel is rich in Fe OOH,Cr(OH)3 and Mn O,and the inner layer mainly contains Fe2O3,Cr2O3 and Mn OOH.When compressed at 900?/0.1 s-1,the DRX refinement of 10.3%Mn sample is up to the best,and the increase of grain boundary number is conducive to the increase of passivation film thickness. |
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