Research On Nucleation And Threedimensional Morphology Control Of Intragranular Ferrite

The effect of inclusions size and phase composition on the nucleation of intragranular ferrite was analyzed by SEM and laser roman spectrometer.The role of continuous cooling rate and austenite grain size on intragranular ferrite nucleation were investigated separately by high temperature quenching and confocal scanning laser microscope.At the same time,the three-dimensional model of intragranular ferrite was established by focused ion beam(FIB)and computer-aided reconstruction to reveal the formation of interlocking microstructure and the morphology for intragranular ferrite.Besides,the orientation relationship of ferrite was analyzed by EBSD.Finally,the nucleation mechanism of acicular ferrite was further understood.The probability of inclusion for nucleating ferrite increased gradually with increasing the inclusion size.The optimal size of inclusion was between 3?m and 4?m.The typical effective inclusion was composed by Ti2O3 and Al oxide inside,and MnS covered outside.This leaded to the formation of manganese-depleted zone.The range of cooling rate obtained acicular ferrite was 0.3?/s~3?/s.The best cooling rate was 1?/s.The average austenite grain size was proportional to holding time.Instead of a single unit,but the larger ferrite consisted of several small ferrites.The boundary of fusion-growth ferrite was straight and smooth.And the surface was flattened.There was no clear-cut boundary between every little unit.The boundary of impactgrowth ferrite was not straight and smooth.Interlocking microstructure of intragranular ferrite was formed because of sympathetic nucleation,impingement and intersect.Meanwhile,it is found that the interior misorientation was low,for the most part,misorientation was within 2°.However,the boundary misorientation was very high.These ferrites were separated from each other by high angle grain boundary.There were four mechanisms of intragranular ferrite formation in the test steel.The inclusion as an inert surface could provide nucleation sites and nucleation driving force for intragranular ferrite.The low mismatch between MnS and ferrite was effective in nucleating the intragranular ferrite.The precipitation of MnS on Ti2O3 might result in Mn depletion,which was effective in nucleating the acicular ferrite.Sympathetic nucleation was also the important factor to promote intragranular ferrite.