The Studies On Directional Recrystallization And Its Kinetics

It is well known that microstructure dramatically influences the materials properties. Directional microstructure with improved grain boundaries can improve and/or enhance material properties.The best example is the remarkable achievements of directional solidification.However,there are some shortages for directional solidification technique. Firstly,this technique can not be applied to produce those products without liquid-solid phase transformation,such as powder metallurgy products,etc..Moreover,metal melt must be contained in mould at high temperature for a long time during directional solidification,this leads to a high energy consumption and materials contamination. Therefore,it is important and significant to study directional recrystallization technique, because this technique matches the requirement of producing directional microstructure and material properties controlling simultaneously under solid state.In the present work, pure iron was selected as a model material to investigate directional recrystallization process,the influencing factors of directional recrystallization,the kinetics and mechanisms of directional recrystallization.And then the directional recrystallization of Fe-6.5wt%Si high silicon steel and its effect on magnetic properties were investigated.The directional recrystallization of cold-rolled iron with thickness reduction of 0%, 70%and 85%were investigated.It was found that the columnar-grained structure could be produced in the specimens with different deformation degree at proper processing conditions.However,the aspect ratios of columnar grains decrease with increasing degree of deformation.This is inconsistent with traditional recrystallization results.The further studies found that the larger the deformation degree is,the stronger is the deformation texture,which leads to a much higher probability of forming low angle or twin boundaries during directional grain boundary migration.The phenomenon that large degree of deformation hinders directional recrystallization was explained using orientation pinning.Based on these results,it was proposed that the mechanism of directional recrystallization essentially can be described as selective growth and/or competitive migration of the grain boundaries."Island grains" formation in columnar grains was also explained.The.experimental results of kinetics show that a columnar-grained structure can successfully grow from small aquiaxed grains without texture.The experimental results prove the existence of both a lower and an upper limit of the withdrawing velocity for columnar grain growth at each hot zone temperature(HZT).Importantly,there is an optimum withdrawing velocity at which the largest aspect ratio of columnar grains can be obtained.As the HZT increases,the largest aspect ratio of columnar grain as well as the optimum withdrawing velocity increases.Based on the systematic experiments,a kinetic model was proposed:there is a optimum withdrawing velocity at each HZT.The optimum withdrawing velocity corresponds to the velocity of grain boundary migration at the corresponding HZT.In the region of hyper-optimum withdrawing velocity,the directional grain boundary migration is a thermal activation process;In the region of sub-optimum withdrawing velocity,directional grain boundary migration is controlled by the effective withdrawing velocity.Based on the kinetic model,a kinetic approach was suggested to correlate the microstructure to processing parameters.The apparent activation energy of directional grain boundary migration is calculated as 144kJ/mol for iron,which is close to that of the grain boundary self-diffusion of pure iron.In addition, there is a critical HZT(675℃for iron),below which the columnar grain structure cannot be produced.The applicability of the kinetic model proposed in this work was checked in Fe-6.5wt%Si high silicon steel.The results prove the applicability of the kinetic model to Fe-based solid solution materials.The directional microstructure in high silicon steel were also successfully produced by the secondary recrystallization.The critical HZT for directional recrystallization is about 1050℃.The apparent activation energy of directional grain boundary migration is about 332kJ/mol,which is far higher than that of iron,144kJ/mol,which was obtained by the same method.This indicates that,compared with in iron,the higher energy barrier must be conquered in high silicon steel during directional recyrstallization.The solute distribution,especially Si and B on grain boundaries of high silicon steel during directional recrystallization was investigated using X-ray energy dispersive spectroscopy(EDS)and Auger electron spectroscopy(AES).It was found that the solute of Si is depleted on the grain boundary.This phenomenon can be explained by equilibrium segregation(depletion)theory.The B enrichment was observed on the moving grain boundaries ahead of columnar grains.This enhances the depletion of Si on grain boundaries.The B enrichment on the moving boundaries is attribute to the non-equilibrium segregation.The existence of solute elements on the grain boundary retards the grain boundary migration dramatically,leading to the high HZT and high apparent activation energy for directional recrystallization in high silicon steel.Directional microstructure has been produced successfully in recrystallized Fe-6.5wt%Si high silicon steel through directional recrystallization.Furthermore, orientation controlling of magnetic properties was carried out.With the columnar-grained structure developing,the selective growth of columnar grains determines the texture evolution during directional recrystallization through forming the low energy boundaries. This leads to the formation of {111}<110> and near{110}<111> components and the decrease of coercivity in the direction with 60°along the direction of the heat flow.