Effect Of Mg-ti Treatment On Peritectic Transformation And Inclusion Formation In Peritectic Steel

Corner transverse crack is a common crack defect in peritectic steel.Because of the structural difference betweenδphase andγphase,whenδphase changes intoγphase during solidification,it is accompanied by volume shrinkage,which leads to stress concentration and easy to produce corner transverse cracks.At the same time,the proeutectoid ferrite film is easy to precipitate at the grain boundary of coarse original austenite formed by peritectic transformation,resulting in corner transverse cracks.In this thesis,Q235A peritectic steel is taken as the research object.Firstly,the effect of adding magnesium and titanium on peritectic phase transformation is studied by in-situ observation,and then the effects of adding magnesium and titanium on the microstructure and inclusions of steel are compared and analyzed,and the formation mechanism of inclusions is discussed.The peritectic transformation process of remelted Q235A steel and Mg-Ti treated Q235A steel was observed in situ by high temperature laser confocal microscope,and the surface roughness of solid samples was measured.The results show that the solidification path of remelted Q235A steel is the same as that of Mg-Ti treated Q235A steel,which is L→L+δ→L+δ+γ→L+γ→γ.Mg-Ti treatment reduces the starting temperature of peritectic reaction by 12.3℃,slows down the peritectic transformation rate,and leads to the reduction of peritectic transformation shrinkage by 10%.Thermodynamic calculation shows that Mg-Ti can reduce the peritectic reaction shrinkage and solidification shrinkage characteristic index,which is consistent with experimental observation.The microstructure of steel at room temperature was observed by Zeiss optical microscope.It was found that the microstructure of remelted Q235A steel was"polygonal ferrite+side lath ferrite+pearlite",and that of Mg-Ti treated Q235A steel was"polygonal ferrite+pearlite".The harmful side lath ferrite disappeared after Mg-Ti treatment.Field emission scanning electron microscope（FESEM）shows that the remelted Q235A steel mainly contains Al₂O₃inclusions,Mn S inclusions and Al₂O₃-Mn S inclusions.Mg-Ti treated Q235A steel mainly contains MgO inclusions,MgS inclusions,（Mg,Mn）S inclusions,MgO-（Mg,Mn）S inclusions,MgO-TiN inclusions,（Mg,Mn）S-TiN inclusions and MgO-（Mg,Mn）S-TiN inclusions.Mg-Ti treatment significantly changes the types and sizes of inclusions in steel.The automatic statistical analysis system of inclusions found that after Mg-Ti treatment,the proportion of inclusions smaller than 2μm in steel increased from 20%to 40%,and the number of inclusions smaller than 5μm was more than 80%.Mg-Ti treatment significantly refines the size of inclusions.Thermodynamic calculation shows that the precipitation order of inclusions in steel is MgO>MgS>TiN>Mn S,and two-dimensional mismatch calculation shows that the mismatch between MgO and MgS is 0.82%,and the mismatch between MgS and TiN is 1.51%,which indicates that MgO and TiN are easy to be heterogeneous nucleation cores.Considering three nucleation mechanisms（uniform nucleation,grain boundary nucleation,dislocation nucleation）,the nucleation mechanism of TiN is calculated.The three critical nucleation sizes are basically the same,and the critical nucleation work of grain boundary nucleation is the smallest and the relative nucleation rate is the largest.Under the cooling rates of 0.5 K/s,1 K/s,5 K/s and 10K/s,the final growth radius of TiN inclusions is 1.39-6μm.