Mechanism Of Inducing Intragranular Ferrite From Controllable Inclusions

The oxide metallurgy uses inclusions to induce IGF nucleation refining the steel structure.In order to study the law of inclusions induced IGF nucleation and mechanism,adding TiO₂ particles into medium-low-carbon steel to investigate,by SEM and TEM.By Fact Sage,the calculation indicates the precipitation order is TiO₂>Ti₃O₅>Ti N>Ti₂O₃>Ti O with the increasing of temperature because of content of Ti and O elements in steel and the temperature.DH36 steel with TiO₂ as the research object,O-Al-Si-Ti-Mn-S composite inclusions were formed inducing IAF and IPF,the content of titanium is higher than base steel,and the total number and size of inclusions increase.In order to further explore,by TEM it was found the typical composite inclusion is Mn S+3Mn O·Al₂O₃·3Si O₂+Ti₃O₅+Ti N,3Mn O·Al₂O₃·3Si O₂ and Ti N are IAF nucleation sites.The formation process is:at 1573 K,TiO₂ transformed into Ti₃O₅ and combined with Mn S forming Ti₃O₅+Mn S;the precipitated 3Mn O·Al₂O₃·3Si O₂ also collides with Mn S and Ti₃O₅+Mn S to grow up,and finally precipitate Ti N.To calculate the composite inclusions impacting IAF mechanism,3Mn O·Al₂O₃·3Si O₂,Mn S,Ti N and Ti₃O₅ as the base phase respectively,δless than 6%can stimulate the IGF nucleation;Mn S has a solute-poor zone of Mn and the interface diffusion rate is faster than Fe,which conforms to the solute-poor zone mechanism;during the solidification process,the stress and strain energy of inclusions are 1/1000 times the nucleation energy of IGF,which can be ignored.The addition of TiO₂ particles can strengthen the induction of IGF,and the primary ferrite can stimulate the nucleation of secondary ferrite.The sympathetic nucleation is edge-face type.The formation of nucleation points is a shear transformation,which is short-range from carbon atoms.Migration reduces the energy of the system to complete instantly.Afterwards,the interface-controlled diffusion transition is realized by reducing the nucleation energy through carbon solute enrichment in the dislocation area and discontinuous diffusion.Figure 59;Table 21;Reference 60...