Precipitation And Evolution Mechanism Of TiN Inclusion In GCr15 Bearing Steel

Titanium nitride（TiN）is one of the common high hardness and brittle inclusions in high carbon chromium bearing steel.Because of its high hardness and brittleness,TiN is easy to become the source of fatigue cracks during rolling and bearing service,which seriously endangers the structure and properties of materials.Due to the obvious difference of titanium content in alloy raw materials used by domestic bearing steel production enterprises,the stability of production process fluctuates greatly,resulting in the uneven control level of titanium inclusions in bearing steel products.In this work,GCr15 bearing steel produced by BOF-LF-CC and BOF-LF-VD-CC processes in a domestic steel plant was taken as the object of study.The mechanism of precipitation and evolution（compounding,aggregation,dissolution-precipitation）of TiN inclusions in the steel was studied,which provided a theory for formulating relevant processes to control the size and distribution of TiN-containing second phase particles and improve the structure and properties of bearing steel.First,TiN and related complex particles in GCr15 bearing steel were observed and precipitation mechanisms of different particles were analyzed.Morphology,size and element distribution of TiN,TiN-MnS,TiN-MgO-MgAl₂O₄,TiN-MgAl₂O₄-MnS,TiN-MgO-MgAl₂O₄-MnS in metallographic specimen and the carbides（MC_x）and TiN-MC_x extracted by electrolysis were observed by field emission scanning electron microscopy（FESEM）and energy dispersive spectrometer（EDS）.The size of single-particle TiN inclusion is 2-12μm,that of multi-particle polymerized TiN inclusion is 10-20μm,and that of complex TiN inclusions are less than 10μm.Thermodynamic calculation shows that the order of precipitation of different inclusions is as follows:MgAl₂O₄,MgO,TiN and MnS.Combined with FactSage calculation and XRD analysis,it is confirmed that the main carbides in steel are M₃C,M₇C₃（M=Fe,Cr,Mn）and trace Cr₃C₂.Precipitation mechanism of TiN-MC_x complex particles with different TiN volume fraction was analyzed based on the diffusion and segregation of elements in solidification process and the movement characteristics of inclusions at solid-liquid interface.TiN-MC_x complex particles with higher TiN volume fraction can precipitate out of the early stage of solidification process,TiN growth size is large,and after TiN is swallowed by solid-liquid interface,carbides can growth on the surface of TiN with little amount.TiN-MC_x complex particles with lower TiN volume fraction can precipitate out of the late stage of solidification process,TiN growth size is small,and after TiN is engulfed by solid-liquid interface,carbides can growth on the surface of TiN with large amount.Second,the segregation,nucleation-growth,aggregation and dissolution-precipitation mechanisms of TiN were studied.The theoretical precipitation radius of TiN is calculated to be 1-6μm under the conditions of w[Ti]_%=0.0060-0.0079 and w[N]_%=0.0049-0.0070,precipitation temperatures of 1640 K,1630 K,1620 K and local cooling rates of 0.5 K/s to 10 K/s,which is in agreement with the experimental observation of the size of single particle TiN inclusion.The precipitation,aggregation,dissolution-precipitation behavior of TiN inclusions at GCr15 bearing steel level was observed by high temperature confocal laser scanning microscopy（CLSM）.The formation mechanism of multi-particle polymerized TiN inclusions is confirmed by CLSM.The formation process of multi-particle polymerized TiN inclusions formed by single-particle TiN polymerized by cavity-bridge force can be divided into three stages:initial stage,Ti and N diffuse in the solid matrix and sinter in the solid phase;in intermediary stage,when they contact at the active angle;In the last stage,Ti and N exist in the molten steel,which widens the neck area of the solid phase sintering zone and diffuses to the edge until it is stable.When the atomic percentage ratio of Ti and V in TiN is about 18:1,the inclusion may dissolve at about 1688 K.The dissolution of TiN inclusions is inversely related to the size of inclusions:the larger the size,the smaller the dissolution rate.Two adjacent TiN inclusions can be dissolved or grown by Ostwald ripening mechanism.Dissolved TiN forms[Ti]and[N],a part of[Ti]and[N]grow to form long strip TiN inclusions,while the other part precipitates to form TiN inclusions less than 3μm during solidification.Finally,Based on the TiN-MnS inclusions observed by FESEM,the structural evolution of TiN-MnS and TiN-Fe composite interface growth was simulated by first principle simulation software Material Studio 2017.The results show that the most active surface of TiN crystal structure is（111）surface,which is the charge contributed by the 3d orbital of Ti atom at the apex and the 2p orbital of N atom at the bulk center.In TiN-MnS and TiN-Fe interfacial systems,N atoms are converted into interfacial common atoms,which form ionic bonds with metal atoms at both ends of the interface,and the ionicity of bonding with Fe is stronger than that of Mn.In the interfacial system of MnS and Fe grown on TiN（111）surface,the"template effect"of TiN coherent epitaxy is about four atomic layers with atomic spacing of 6-7?.In the interfacial system of TiN-Fe,there is a transition layer from ionic to metallic,with thickness of 5.67?.