| The Ti-Nb dual stablized Fe-17Cr ferritic stainless steels studied in this paper are based on traditional ferritic stainless steels and are produced by the combination of refining and stabilizing elements Ti and Nb to reduce the contents of C and N in the steel.New ferritic stainless steels emerged.The addition of stable elements can improve the corrosion resistance of stainless steel,but at the same time it will cause precipitation of?Ti,Nb?N,?Ti,Nb?C and other high-melting and hard-granular inclusions in the solidification process of steel,affecting fatigue properties,toughness and surface quality of the stainless steel.Therefore,it is particularly important to study the control of the precipitation behavior of these inclusions.At first,the SEM and EDS composition analysis results indicate that the white streak defect on the Ti-Nb-stabilized Fe-17Cr ferritic stainless steel rolled plate surface is caused by a large amount of 1-2?m-sized TiN aggregates,and TiN tends to be added due to the addition of Nb.The?Ti,Nb?C,N,which forms a regular morphology composite structure,first forms a TiN core,and the outer layer forms a NbC composite coating TiN.Secondly,the thermodynamic calculation of the inclusions in the steel shows that the MgO·Al2O3spinel is preferentially precipitated in the smelting process.When the Mg content in the molten steel drops below 2.196×10-5wt%,it enters the Al2O3 precipitation dominant zone.The initial precipitation temperatures of TiN,NbN,and NbC were 1759K,1570K,and1455K,respectively.The lower precipitation temperature allows NbN and NbC to nucleate around TiN.Orientation relationships between the TiN precipitates and ferrite grains in the solidified microstructure of Fe-17Cr stainless steel were systematically investigated.Remelting experiments were carried out to obtain more TiN precipitates within an electron backscattered diffraction?EBSD?observed zone.Pole figures by EBSD show that seven orientation relationships between the TiN precipitates and ferrite grains,including Kurdjumov–Sachs and other six ones,exist in the solidified samples after remelting.Based on the misfits and orientation relationships between the TiN precipitates and ferrite grains obtained EBSD,the preferential growth direction of ferrite grains is[111]along[001]and[110]in?001?of the TiN precipitates.The refinement mechanism of ferrite grains by the TiN precipitates in the steel was then discussed according to the experimental results and the orientation relationships between the TiN precipitates and ferrite grains.The comparison of corrosion resistance of Ti stabilized and Ti-Nb dual stabilization ferritic stainless steel was investigated by the pitting test and electrochemical measurements.It was found that with the addition of Nb content,the quantity and density of precipitates in steel are increased and solid solution state of Nb was observed on the surface of steel.Electrochemical measurements shows the corrosion resistance of Ti-Nb dual stabilization ferritic stainless steel is better than Ti stabilized and increasing Ti content was effective on improving the corrosion resistance.It is suggested that the positive effect on corrosion resistance of Nb is stronger than Ti. |
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