TECHNICAL STEEL RESEARCH NEW BOOK FOR MECHANICAL AND METALLURGY

ULTRA FINE GRAINED STEEL BY INNOVATIVE

DEFORMATION CYCLES.

SUMMARY 

Recently, much attention has been devoted world wide in search of materials with superior strength and ductility combination. Although many methods are available to improve strength, grain size is perhaps the only method, as of known today, to improve the conflicting demands in properties as, for example, strength and toughness. 

This project was aimed to establish the best processing routes to promote the formation of ultrafine ferrite (preferably 2-3 µm) and mixed (e.g. ferrite-cementite, ferrite-martensite) microstructures either in the bulk or the external layers of strips/plates and rods for a wide range of chemical compositions (0.05-1.6 %C). The experimentation was mainly focused on processing routes that do not require extreme strains and significant plant changes, making use of microalloying additions (e.g. niobium) when necessary, for producing these novel microstructures. 

In addition, two severe plastic deformation techniques to produce ultrafine grains (Accumulative Roll Bonding and Equal Channel Angular Pressing) were tested. Of course different experiments were performed depending on material carbon content and for this reason different work packages were created. 

For low C steels (0.05-0.1%C), experiments were mainly carried out to assess the hot deformation conditions for producing fine grain sizes through Strain Induced Dynamic Transformation mechanism (SIDT) and to establish the influence of chemical composition, prior austenitic grain size, strain and deformation temperature on grain refinement. SIDT is a dynamic phase transformation occurring during deformation at temperature slightly higher 

than Ar3 due to the strain energy accumulated in austenite phase that induces an early γ−α 伊phase transformation. 

Deformation-dilatometry tests confirmed that increasing the strain the amount of strain-induced transformed ferrite and grain size refinement increase. The critical strain, required for SIDT to occur is related to the deformation conditions and chemical composition. The increase of carbon content in solution retards the SIDT and increases the critical strain. Small prior austenite grain size leads to an increase of strain induced ferrite volume fraction and ferrite grain refinement. After the determination of SIDT parameters, laboratory rolling tests were carried out to reproduce as closely as possible the deformation schedules used in dilatometer experiments in order to get ultra fine grains on sheets surface. 

For microalloyed low C steels the static and dynamic precipitation effect of Nb on grain refinement was studied. These experiments were aimed at defining the processing window for hot rolling of Nb steels by adjusting the steel composition and the process parameters (in particular the time between the last rolling pass and the on-set of accelerated water cooling in the run-out table) in order to obtain the finest ferrite grains. The effects of soluble B and P additions were also investigated. 

From the results it was concluded that to obtain fine grains in hot rolled Nb steel, all the Nb must be dissolved at 1250 °C, strain accumulation must occur below Tnr and above Ar3 to induce dynamic recrystallisation of deformed austenite grains. To reduce the NbC precipitation during cooling, and also the austenite static recrystallisation, the last pass must be followed by fast cooling to the γ−α transformation temperature

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