Study On Equal-channel Angular Extrusion Technique For Steel Plate And Strip

In this dissertation,a new method is proposed to enlarge the size of metal material which can be processed by severe plastic deformation(SPD)technology,and achieve grain refinement of steel plate and strip,which is called semicontinuous equal-channel angular extrusion(SC-ECAE)method.The corresponding prototype and pilot plant are developed to determine the optimal equipment conditions and process windows.The influence of SC-ECAE process on microstructure,texture and properties of the low carbon steel plate is analyzed,which verify the effectiveness of this novel method.Main content is as follows:(1)In order to develop a new method for steel industrial application,continous frictional angular extrusion(CFAE)method is applied to the processing of steel material for the first time.The grain refinement of steel material by CFAE method is verified and the advantages and disadvantages of this method are studied,and the feasibility of its industrial application is also analyzed.It provides inspiration and sound foundation for the development of the new one.(2)Due to the driving difficulties and surface problems of the workpiece in the practice of CFAE process,SC-ECAE method is first proposed.In contrast to the external forces that are exerted on specimens in traditional ECAE,the driving forces are applied on the dies in the novel SC-EACE process.The main benefits of SC-ECAE are its capability to process bulk materials to various extents,high productivity,low scrap rate,and reduced extrusion force requirement.In particular,it is possible to realize SPD processing of a workpiece with a high length/thickness ratio,which provides a new solution for the industrial application.(3)A three-dimensional finite element model of SC-ECAE process is established for validation of processes and optimization of parameters.The different extursion angles,workpiece dimensions and friction conditions are simulated respectively.And the impact of the process parameters on deformation is analyzed and the optimum process windows are determined.(4)The SC-ECAE equipments are designed and developed.which can achieve the continuous severe plastic deformation of plate and strip.Based on the core functional design and mechanical analysis,the prototype is developed firstly,and then the pilot machine is developed successfully after the function optimization and the large-scale innovation.(5)The maximum diameter of the steel workpieces prepared by traditional ECAE methods is about 10mm.And the maximum width of conventional ECAE methods for steel plate and strip,such as CFAE,is about 20mm.However,80-180mm wide steel sheets can be carried out by the pilot machine of SC-ECAE which developed by this thesis.The SPD process of 180mm wide steel sheet was realized for the first time,which solved the limitation of workpiece successfully and expended workpieces size greatly.(6)The influence of SC-ECAE process on the microstructure of low carbon steel is studied by SEM,TEM,EBSD.Plastic deformation is found to occur,essentially by simple shear,in the region along the intersectional shear plane of the two extrusion channels.With the application of 10 passes(a total strain of 4.8 in the 10th pass),the spacing between most HABs decreases continuously and the fraction of final HABs remain above 90%,whereas the overall grain boundary spacing,including both HABs and LABs,is down to about 0.55?m.(7)The influence of SC-ECAE process on the texture of low carbon steel is also studied by SEM and EBSD.In the process of SC-ECAE,the main texture of low carbon steel includes{111}<110>,{110}<112>,{112}<111>,{110}<111>and {110}<001>.In the initial stage,the increasing dislocation induces new texture and increases the intensity.With further deformation,LABs are formed between dislocation cell bands which making some of dislocation cell bands to change their orientation.Therefore,the intensity of texture begins to decrease.After more passes,the intensity of texture keeps decreasing with HABs and sub grains in dislocation cell bands increasing continuously.(8)Mechanical properties of ultrafine-grained low carbon steel sheets,which are obtained by the method and equipment of SC-ECAE,are estimated by tensile tests after different passes at room temperature.After 10 passes deformation,yield strength and tensile strength of the unprocessed low carbon steel workpiece are increased from 81.OMPa and 264MPa to 638.7MPa and 710.3MPa,respectively,total elongation reduces from 51.7%to 12.6%.In addition,the results do not indicate noticeable differences in the mechanical properties among different directions.At the dimensions of the present investigation,the mechanical properties in different directions are similar and significant anisotropy is not observed after SC-ECAE.Through this work,the subject breaks through the workpiece size restrictions of the existing SPD methods.A novel SPD method is developed and realized,which also achieves a 180mm wide grain refined steel sheet by SPD processes for the first time.It is a big promotion for the SPD theory to the actual industrial application.