Three Dimensional Morphology And Nucleation Mechanism Of Intragranular Ferrite In Non-quenched And Tempered Steel

Non quenched and tempered medium carbon steel has higher strength and lower toughness because of its higher carbon content.It is found that the fine and dispersed intragranular ferrite in the steel structure can refine the steel structure and improve its strength and toughness.Therefore,this paper is devoted to the finding of the factors affecting the nucleation of the intragranular ferrite.The influence factors of intragranular ferrite nucleation in non quenched and tempered medium carbon steel were investigated by means of metallographic microscope,scanning electron microscope and energy spectrum analyzer.The experimental results showed that the size of inclusions in shaft shaped iron 2 ~ 4 ?m this interval induced ferrite most.Inclusion induces the growth of intragranular equiaxed ferrite in all directions,and finally the inclusion is embedded in the intragranular ferrite,which is most favorable to induce nucleation and growth of intragranular equiaxed ferrite.The composition analysis shows that the core of inclusion is Al2O3,and the outer part is Ti N and V(C,N).The segregation of Mn S is found by further scanning the inclusions.It is speculated that the nucleation mechanism of medium axial ferrite in the sample is the result of the mechanism of poor Mn zone and the minimum mismatch mechanism.The effect of cooling rate and austenite grain size on intragranular equiaxed ferrite was also investigated.The experimental results showed that the cooling rate affects the microstructure and crystal steel inside the ferrite transformation temperature,cooling rate and increase the phase transition temperature is lowered,there is a inverse relationship between cooling rate,etc.to get axial ferrite at 0.05?/s~0.1?/s this interval range.Different holding time affect the size and is proportional to the austenite grain size of austenite,the difference also affects the nucleation of ferrite induced ferrite nucleation health only reasonable austenite size can,holding time at 600 s,austenite size moderate,ferrite nucleation speed for ferrite intragranular nucleation.The three-dimensional shape model of equiaxed ferrite in non quenched and tempered medium carbon steel was obtained by focused ion beam,electron backscattered diffraction and kernel average misorientation technique,and the complex 3D model of equiaxed ferrite was obtained.Found such axial ferrite nucleation stimulated nucleation.It is found equiaxed shape ferrite is composed of several different orientation of small sub grains.It is found that the equiaxed ferrite is composed of several sub grains with small misorientation,while the orientation difference between ferrite is larger.In summary,the mechanism of the poor Mn zone and the minimum mismatch mechanism,and the other nucleation modes such as excitation nucleation and so on,the equiaxed ferrite in the non quenched and tempered medium carbon steel has been formed.