| With the rapid development of refining process and alloying technology,the comprehensive performances of gear steels have been improved ceaselessly.Cleanliness is no longer the key factor to restrict the service life of gears,but the addition of alloying elements makes gear steels compositions and micro structure inhomogeneous,worsening the mechanical properties of gear steels.The banded structure which is due to the micro-segregation of alloying elements would reduce the plasticity of gear steels and make it crack in the process of cold heading;besides,the asymmetric ingot-shaped segregation zone at the cross section of gear steels would make gears heat treatment deformation uneven,reduce their machinability and meshing accuracy,and even break due to the uneven stress during working.Due to the rapid development of automobile industry,the demand for gear steels has also increased massively and its processing cost has been paid more attention,consequently,the automobile gears need good cold heading performance and freecutting performance.In order to have good cold heading property,gear steels must have good plasticity.Therefore,it is very necessary to study the appropriate spheroidizing annealing process for gear steels to obtain good granular pearlite+ferrite structure.Gear steels need sulfur to improve its free-cutting performance.Sulfur is prone to segregate and sulfide often presents long-striped shape and aggregated distribution in gear steels,resulting in poor mechanical and free-cutting properties.The distribution of alloying elements and sulfide and their improvements during diffusion annealing process were studied in detail,so did the evolution of ingot-shaped macro-segregation zone and sulfide during rolling process;For improving the banded structure defects and obtaining good granular pearlite,the structure analysis and mechanism of spheroidizing annealing process were studied.According to the research on sulfide in gear steel FAS3420H,it is found that the dot-chain gathered type ? MnS in bloom is dominating,lying on grain boundary,which would distribute in the rolling direction in a form of long-striped shape after being rolled.The calculation result of Fe-MnS pseudo-binary phase diagram shows that the precipitation types of MnS are mainly monotectic reaction and eutectic reaction.Based on the result of thermodynamic calculation,with the progress of solidification,Mn and S will be gradually enriched in liquid,and type ? MnS will precipitate among austenite when the solubility product of MnS is reached,with large quantity and small size,as a result of a eutectic reaction;type ? MnS will precipitate,with relatively uniform distribution and less quantity,which is a divorced eutectic reaction;type ? MnS is mainly distributed at the edge of the bloom and precipitates by a monotectic reaction at a faster cooling rate.In addition,alloying elements(except for Al)in the studied steel will be gradually enriched in liquid,and its micro-segregation will be formed after solidification.Type ? MnS in continuous casting bloom grew up first and then broke during diffusion annealing process,which was much related to the homogenization of Mn and S in dendrite.On account of the diffusion coefficient difference between Mn and S,S firstly reached the homogenization status in the entire dendrite,and combined with the inter-dendritic enriched Mn,so MnS inclusions would precipitate on the surface of original MnS inclusions and make Type ? MnS grow up into long-striped inclusions.Soon afterwards,Mn also homogenized within the entire dendrite,it would combine with S and new MnS inclusions would precipitate at high energy sites such as grain boundary,sub-grain boundary or dislocation.Short-rod or long-striped MnS inclusions were not stable during long time diffusion annealing process to break and spheroidize.Eventually small size and large quantity MnS inclusions were gained,its distribution was also improved.With the extension of diffusion annealing time to 5h,the segregation of Mn and Cr in dendrites was improved.During hot deformation process,the elongation and fragmentation of MnS alternately reciprocated.Relatively small size and large quantity of MnS can be gained when the gear steel was rolled at high temperatures such as 1200?,and its relative plasticity was also low,since sufficient dynamic recrystallization happened in matrix at this condition.The deformation of matrix and the migration of grain boundaries would promote the fracture of long-striped MnS,at this time,the fragmentation of MnS was mainly dominated,and finally the size of MnS became smaller,the number became larger,and its distribution could be slightly improved.Simultaneously due to the large diffusion coefficients of Mn and S at high temperatures,the fragmentation and spheroidization of MnS were promoted.The three-dimensional coupled numerical simulation results of temperature,stress and microstructure of gear steel FAS3420H shows that the rolling process parameters obviously affected the final shape of the ingot-shaped segregation zone at the center of the round bar after rolling.The different metal flow degree in horizontal and vertical directions was the reason that the squared segregation zone in continuous casting bloom finally became rectangular.After the optimization of the fifth roller spacing,the effective strain of the cross section of the round steel was more uniform,finally an ingot-shaped segregation zone was gained which was squared.The model was verified by the statistics of macrostructure and grain size in the rolled round bar.The temperature and time of sub-temperature spheroidizing annealing plays a decisive role in obtaining granular pearlite in gear steel FAS3420H.After normalizing with austenitizaion at 850? for 30min+705? sub-temperature spheroidizing annealing process for 16h,firstly after being normalized,the banded structure disappeared and the structure was refined,so that the fine pearlite was evenly distributed in the ferrite matrix,reducing the diffusion path required by carbon elements.With the extension of holding time in sub-temperature section to 16h,the matrix of gear steel was granular pearlite+ferrite,and the spheroidization grade was grade 6.The spheroidization model of cementite shows that the dislocation and sub-grain boundary defects existing in cementite would result in grooves on the surface of cementite.Carbides would precipitate during the subtemperature annealing at the plane near the groove where the free energy of heterogeneous nucleation needed is zero,and the micro zone balance of carbon would be broken,which would promote its diffusion in the groove to the nearby plane,lead to the dissolution of cementite in the groove,and finally the cementite would be fractured. |
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