Microstructure Evolution And Deformation Mechanism Of Bainite-Retained Austenite Steel During Cold Drawing

Steel wire products are one of the most important steel products.At present,high strength steel wires are usually made of pearlite which been cold drawn several passes.Several times of lead bath quenching treatment is needed during processing,which has great environmental pollution.At the same time,the carbon content of material composition tends to increase with the enhancement of strength.The increase of carbon content is one of the important reasons for plastic deterioration and torsion stratification of steel wire.Aiming at the problems existing in the steel wire production process,this topic combining the research achievements of the new generation of high strength steel,studied new bainite-rerained austenite steel suited for cold drawing.With considering the process of production and application requirements,the aim of new bainite-rerained austenite steel wire is the reduction of environmental pollution and the benefit of deformation during processing.The components system design,the heat treatment process and microstructure evolution and deformation mechanism during cold drawing were researched in this paper.Cold drawing belongs to a large plastic deformation process,the composition of low temperature bainite steel used as the structural material needed to be redesigned for the cold drawing process.Through thermodynamic calculation and thermal expansion test,the designed experimental steel composition with compound addition of Si,Mn,Al,P,fully separated the pearlite phase zone and bainite phase zone,effectively restrain the decomposition of supercooled austenite,can get a banded bainite-retained austenite organization.At the same time,the dynamic calculation shows that double peak phenomena of the carbon concentration in austenite appeared near the phase interface at the early stage of isothermal bainitic treatment due to the difference of carbon concentration.And it is found that the movement of bainite/austenite interface during the isothermal bainite process shows a bi-directional result which was also occured at the martensite/austenite interface during the partitioning process in the Q&P steel.The distribution trend of elements measured by electron probe further proves the theoretical calculation results.The cold drawing experiment of the new bainite-rerained austenite steel shows that the obvious uneven deformation appeared,and large accumulative deformation occurred near the surface and the deformation degree decreases rapidly far away from the surface.Microstructure can be roughly divided into several plastically deforming regions from surface to center.The microhardness test and numerical simulation show that the peak value of plastic deformation appears at the sub-surface due to the reason that additional shear plastic deformation caused by reverse flow direction of microstructure around the drawing die.The uneven deformation of cold drawing has obvious influence on the fracture morphology of the sample.Most of dimple at the deformation region approached a circle with much shallower depth.The dimples at undeformed region,by contrast,have the oval form and well-developed tearing ridges.The reason is an increase in the number of micro voids due to the grain refinement by local deformation.With the increase of drawing deformation,the microstructure of experimental steel gradually turns to the drawing direction,being broken and stretched.The microstructural evolution during cold drawing deformation can be divided into two stages,one is the bend and tilt stage and the other is the broken and stretched stage.The main deformation mechanism during bend and tilt stage is the slide upon each other of band.And the reason for the subsequent broken and stretched stage is the segmentation of band by dislocation bands which caused by the movement of multiple slip lines resulted from large deformation.The surface layer structure of bainite-rerained austenite steel wire after big plastic deformation leads to the kink of crack and decrease the stress intensity factor of crack tip when the crack extends to the center,reduces the effective driving force of crack propagation,significantly delayed the crack extension.The effects of different stress states on the stress and strain states of the microstructure were investigated using the RVE model based on actual microstructure.Results showed that:1.The position of deformation induced martensite discontinuous extension for the reason that the alternate soft and hard phase and the deformation induced phase transform change the stress state and makes the stress distribution complex in micro-region.While under the effect of combined stress,the retained austenite transformation location has more obvious directivity and is not affected by the distribution of the original retained austenite strip,but related to the stress direction.2.The regulating effect of microcell stress distribution by retained austenite was found to be related to the morphology and distribution of retained austenite.Stress of bainite matrix reduced when the bainite matrix was divided by banded retained austenite,while a high stress microcell will appear in continuous matrix by contrast.3.The study shows that the high stress zone and low stress zone have been transferred with the change of the retained austenite.In the early stage of deformation,the high stress area is mainly distributed in the bainite ferrite matrix,and the low stress area is the retained austenite with no transformation.And in the late deformation,the martensite transformed from retained austenite becomes the primary high stress zone,and the low stress area is still the retained austenite with no transformation,while the stress of bainite ferrite between the two.4.Under the combined stress during cold drawing,the extension of transformated retained austenite and stress distribution of the surface microstructure has a certain angle with the deformation zone surface of wire drawing die.The stress distribution in the microstructure is layered and the deformation microstructure is a lamellar structure due to the stress regulating effect of retained austenite.The stress is concentrated at the continuous bainite matrix in the core at first,then gradually become more homogeneous.The final position of stress concentration is mainly near the tip of the transformated martensite.