Investigations Of Warm Deformation Behavior And Microstructure Evolution Of Initially Quenched Carbon Steels

Refinement of microstructure is the key technology of new steel, current techniquefor microstructure refinement can just obtain micron-size ferrite grains. In order toproduce ultrafine grain steels，it need new theory and new technology.The aim for this dissertation is to obtain ultrafine grain steels by warm deformationof initially quenched steels at low temperature and low flow stress, and investgate thewarm deformation behavior of quenched carbon steels and the microstructure evolutionduring the warm deformation.The warm deformation behavior of initially quenched steels, such as10,20, T8,T10A and T12, at temperatures below Ac1was studied by Gleeble-3500thermal simulationmachine, and constitutive equations of the warm deformation were set up. Microstructurecharacteristics during the warm deformation with different processes were examined byoptical microcopy, scanning electron microscopy （SEM） and transmission electronmicroscopy （TEM）. Based on the theory of processing map, the power dissipationefficiency and processing maps were established to determine the region of plasticinstability.Experiment results show that the flow stress of the warm deformation of the initiallyquenched steels decreases with the deformation temperature elevating and the strain ratereducing. The flow stress increases with the increase of carbon content at low deformationtemperature and high strain rate(600℃，0.1-1s^-1); which decrease at high deformationtemperature and low strain rate(700℃，0.01-0.001s^-1) with high carbon content. Thewarm deformation activation energy decrease with increase of carbon content（<0.78wt%）and increase with increase of carbon content（>0.78wt%）. The maximum of powerdissipation efficiency of warm deformation at strain of0.5increases with increase ofcarbon content at same deformation conditions. Ultrafine microstructures with equiaxialultrafine/submicron-grained ferrite and nano cementite particles can be obtained by warmdeformation of initially quenched steels. Dynamic recrystallization of ferrites andcementites precipitation can be seen during warm deformation of initially quenched steels.The main softening mechanism during warm deformation of initially quenched T12steel is dynamic recrystallization, with higher work-softening effect than that of initiallyspheroidized T12steel, and the power dissipation efficiency of initially quenched steel ishigher tan that of initially spheroidized steel. The warm compression promotes thefragmentation and the spheroidization of lamellar cementites in the initially quenchedsteels. The fragmentation of lamellar cementites was due to the extension ofsub-grainboundary in the cementite, and the spheroidization of cementites depended onthe diffusion of carbon atoms at the tip of bended and breakup cementites. Themicrohardness of initially quenched T8steel after warm deformation shows a trend fromdescent to ascent with the increase of strain, and microhardness reaches minimum at strainof0.5, with fully globularize cementites.