The Effect Of Rare Earth Slag On The Inclusion And Mechanical Properties Of AF-3 Steel In The Electroslag Remelting

Retaining ring is one of the largest components under stress in coal-fired power units,safe and reliable operation requires sufficiently high strength,good ductility and toughness,favourable corrosion resistance and minimal residual stresses.Due to the excellent mechanical property and corrosion resistance,AF-3 steel is a suitable single-phase austenitic retaining ring steel which can meet the requirement of comprehensive properties of grommet in large generator set.However,as the hot forging cracks of AF-3 steel during hot forging are prone to occur,it is difficult for mass production.As the addition of rare earth elements can improve the steel's mechanical property remarkably,sothis paper attempts to add rare earth in AF-3 steel by electroslag remelting?ESR?reduction method to investigate the influence of rare earth slag on the inclusions and mechanical properties of AF-3 steel in ESR process.The rare earth content,inclusions and mechanical properties of normal temperature and high temperature in the end point steel sample of the rare earth smelting slag were researched by the following analysis methods whichincluded the nitrogen-oxygen analyzer,direct reading spectrometer,optical microscope,scanning electron microscopy and tensile test.The results showed that:?1?Rare earth Ce cannot be added into steel by reduction decomposition of rare earth oxides.When Al and Ca-Si were added into the rare earth slag,the content of Ce in the end point steel sample reached 98×10-6 and 110×10-6 respectively.The type of inclusions in AF-3 steel had changed with different slags,the inclusions in the original electrode were mainly SiO₂-MnO.The inclusions in the steel smelted with the pre-melted slag were mainly MgO-Al₂O₃ in irregular shape.The inclusions in AF-3 steel smelted with rare earth slag including reductants were mainly MgO-Al₂O₃ and MgO-SiO₂.The inclusions in AF-3 steel with rare earth slag in which A1 and Ca-Si were added were mainly Al₂O₃,MgO-Al₂O₃ and MgO-Al₂O₃,SiO₂-Al₂O₃,and a few Ce₂O₃-Al₂O₃ in irregular shape.?2?After ESR process,the ingot surface quality and feeding effect were better,but a small amount of porosity and small cracks existed at the bottom of the ingot after the pre-melted slag electroslag remelting.An equal amount of reducer Ca-Si with different batches was added into the slag in the ESR process which was beneficial for obtaining a uniform distribution of rare earth elements in the electroslag ingot.The inclusion area percentage in AF-3 steel increased with the increase of rare earth content after the reduction.There were TiN,MgO-Al₂O₃,Al₂O₃ and TiN compound inclusions in AF-3steel after the pre-meltied slag electroslag remelting.There were TiN,MgO-Al₂O₃ and Ce₂O₃-Al₂O₃ inclusions in AF-3 steel after the Ca-Si reduction of rare earth residue electroslag remelting.There were no other types of rare earth inclusions have been found in electroslag ingot in addition to the Ce₂O₃-Al₂O₃ inclusion.?3?The tensile strength and yield strength of AF-3 steel at normal temperature firstly decreased and then increased with the increase of the content of rare earth.When rare earth content was 0.0174%,the tensile strength and yield strength of AF-3 steel fell to the minimum value with 890 MPa and 520 MP,respectively,which were reduced by 2.73%and 10.34%respectively compared with that after the pre-melted slag electroslag remelting.The addition of rare earth could make the elongation percentage and reduction of area of AF-3 steel at normal temperature increase with the increase of rare earth content.Under the condition of 800?C and 900?,the tensile strength and yield strength of AF-3 steel firstly decreased and then increased with the increase of the content of rare earth.The high temperature elongation percentage and reduction of area also increased with the increase of content of rare earth which was the same with the tensile results of AF-3 steel at room temperature.