METALLURGICAL ASPECTS AND GRAIN REFINEMENT TECHNIQUES OF STEELS STATE OF ART OF THERMOMECHANICAL PROCESSING TMP AND SEVERE PLASTIC DEFORMATION SPD

In the last years, ultrafine grained materials have attracted considerable research interest because they tend to possess high strength without sacrificing toughness and ductility. The role of grain size refinement in improving both strength and toughness is well known. The development of ultrafine grained steels through thermomechanical processing mid severe plastic deformation is being considered by many researcher groups around the word. It is possible to refine the grain size trough control of the transformation reaction or through recrystallization processing. The various processes currently being researched are reviewed and summarised. It appears that it is relatively easy to form a uniformly fine structure through strain-induced trans' formation, although how to implement this industrially is still an issue. The success of TMP in achieving suitable final ferrite grain refinement depends upon two effects: (i) obtaining the proper austenite conditioning during hot rolling, which implies having the correct microstructure and composition in the austenite at the point of transformation; and (ii) using the optimum post-rolling cooling path. The earliest forms of TMP inolved controlled rolling where coarse-grained austenite was pancaked by rolling below the non static recrystallization temperature Tnr. The controlled recrystallization processes, both dynamic and static, can also be used to either refine the austenite prior transformation or to refine transformed ferrite. A controlled rolling and accelerated cooling have a strong role in grain refinement. In microalloyed steels it is now common to have ferrite grain sizes after TMP of approximately 5 pm compared with conventional values of 20 mu m for a low; C steel rolled without control over the processing conditions. The effects of the different parameters such as rolling temperature, reduction per pass, finishing temperature and cooling rate were also treated. Advanced Thermomechanical processes were also illustrated, these new processes lead to have very fine ferrite grains about 3 mu m and in some process even less than lion. It has been shown that the ductility decreases dramatically when grain size is below 1 mu m. The optimum compromising between yielding, toughness and ductility is reached for ferrite grain size around 2-3 mu m. It has been already well known also that severe plastic deformation (SPD) of metallic materials is capable of producing ultrafine grained (UFG) materials With submicrometer or nanometer grain size. Many techniques were illustrated. Since ECAP was introduced in the literature as an innovative technology of manufacturing bulk UFG metallic materials, many research groups worldwide have devoted effort to discover not only the processing characteristics but also the microstructural and mechanical characteristics of ultrafined materials. Early investigations using ECAP processing were very often focused on pure aluminium and copper or their alloys. Very recently, significant interest has shifted to the use of ECAP in processing of UFG low carbon steels. This interest has been motivated in part by the fact that UFG low carbon steels con be used in many applications as structural materials and in part by ECAP capability to improve the strength of steels without a need to change their chemical composition. It was observed that the ultimate tensile strength (UTS) increased with increased mumber of passes. On the other hand, the number of research works on SPD of commercial medium carbon steels is still limited and at laboratory scale, probably because systemaic SPD processing in industrial production is relatively difficult hi steels with higher flow stresses and low productivity.