Materials Science Forum Vols. 584-586

Abstract: The ferrite grain size refining is the unique mechanism for increasing both mechanical strength and formability of steels. Steel with an ultra-fine ferrite grained structure must show a good relation between mechanical strength, ductility and toughness, while the low carbon content enhances good welding characteristics. The objective of this work is to investigate the influence of warm rolling on the evolution of texture in a microalloyed low carbon-manganese (0.11%C, 1.41%Mn, 0.028%Nb and 0.012%Ti) steel with ultra-fine grains produced through out quenching, warm rolling, followed by sub and intercritical annealing. The evolution of restoration process - recovery and recrystallization - was followed by optical and scanning microscopy. After subcritical annealing, the microstructure was formed by spheroidal iron carbides and a ferritic recovered matrix. Otherwise, after intercritical annealing, the microstructure was composed mainly by ultrafine grain polygonal ferrite, MA (martensite-austenite) constituent and carbides. The mechanical behaviour of the steel was evaluated using tensile tests. The mechanical properties have been correlated with the evolution of texture in the ultra-fine grained ferrites.

Abstract: Samples of nanostructured and ultrafine grained steels with carbon content ranging from 0.05 to 0.55%wt. have been obtained by a warm consolidation process from mechanically milled powders and subsequent heat treatments. In general, homogeneous grain size distributions were obtained except for the low-carbon steel in which a bimodal grain size distribution was observed when it was heat treated at high temperatures. The stress-strain response has been studied by means of compression tests. Nanostructured materials showed high strength but poor results in terms of ductility. In the low-ultrafine range (mean grain size between 100-500 nm) the three materials showed an increase in the ductility with strain softening. Finally, when the average grain size was close to 1 µm samples showed larger ductility and strain hardening.

Abstract: The structure, mechanical and functional properties of ultrafine-grained low-carbon steels have been studied after severe plastic deformation (SPD) by high pressure torsion (HPT) and equalchannel angular pressing (ECAP). It is revealed that HPT of low carbon steels at a temperature below 0.3 Tm leads to the formation of nanocrystalline structure with a grain size of <100 nm or a mixture of oriented substructure and nanograins. ECAP under similar conditions leads to the formation of submicrocrystalline structure with a grain size of 200-300 nm. The initial martensitic state compared with the initial ferritic-pearlitic state of the low-carbon steels results in formation of finer structure after SPD and less intense grain growth upon heating, i.e., results in a higher thermal stability. Low-carbon low-alloy steels after ECAP are characterized by high strength (UTS > 1000 MPa) and plasticity (EL = 10-15%). The high-strength state after ECAP is retained upon tensile test testing up to a temperature of 500°C. The submicrocrystalline low-carbon steels after ECAP processing and subsequent heating is characterized by an increased impact toughness at test temperatures down to -40°C.

Abstract: Interstitial-free steel sheets have been processed using a novel severe plastic deformation technique - continuous frictional angular extrusion (CFAE), in order to produce ultrafine grained structures. The deformation was carried out at room temperature and individual sheet specimens were repeatedly processed to various passes. An overall grain size of 200nm was achieved after 8 passes (or an equivalent total strain of 5.3). The present paper reports the evolution of microstructures during deformation, which were examined and characterized using electron backscatter imaging and high resolution EBSD in a field emission gun SEM. The mechanisms of grain refinement are discussed.

Abstract: The enhancement of toughness at low temperatures in fine-grained low carbon steel was studied, basing on the theory of crack-tip shielding due to dislocations. Low carbon steel was subjected to an accumulative roll bonding (ARB) process for grain refining. The grain size perpendicular to the normal direction was decreased to approximately 200nm after the ARB process. The fracture toughness of low carbon steel with the ARB process was measured at 77K by four-point bending, comparing with the fracture toughness of those without the ARB. It was found that the value of fracture toughness at 77K was increased by grain refining due to the ARB process, indicating that the ARB process enhances toughness at low temperatures and that the brittle-to-ductile transition (BDT) temperature shifted to a lower temperature. Quasi-two-dimensional simulations of dislocation dynamics, taking into account crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicates that the BDT temperature is decreased by decreasing the dislocation source spacing.

Abstract: The article deals with the problems of strengthening and cold resistance increase of steels through combining equal channel angular pressing (ECAP) with thermal processing (TP). Dependence of cold resistance on the state of the material structure determined by regimes of equal channel angular pressing and thermal processing is shown. Dependence of specific work of plastic deformations, stored and dissipated energy of steel on its plasticity is stated.

Abstract: Ultrafine grained low carbon steel processed by high pressure torsion (HPT) has been investigated. Depending on initial state (ferritic-pearlitic state after normalization at 950°C, or martensitic ones after quenching from 950°C and 1180°C), the evolution of the microstructure and the mechanical properties was investigated after HPT and annealing at 400-600°C using transmission electron microscopy and X-ray analysis. It has been shown that HPT of martensitic low carbon steel provides a finer structure then that for ferritic-pearlitic initial state, and the initial martensitic morphology and phase composition is strongly dependent on the temperature of quenching. The initial structure was refined by HPT to 95nm in ferritic-pearlitic state and up to 65 and 50 nm in martensitic ones (after quenching from 950°C and 1180°C, respectively). Such ultrafine grained structures demonstrate substantial mechanical properties and possess a high thermal stability up to 500°C in all investigated states. Annealing for 1 h at 500°C results in grain growth up to 860nm for ferritic-pearlitic initial state and 150-450 nm for martensitic ones.

Abstract: Severe Plastic Deformation (SPD) is known to be an effective method of producing nanocrystalline materials, for instance by HPT and ECAP. These techniques are also capable of reproducing microstructures which arise naturally when high pressure and friction is involved, for example in wheel-rail contact problems. The resulting deformation layers build the origin point for fatigue cracks. For that reason the knowledge of the mechanical properties of these deformation layers are of vital importance. In the framework of this study a baintic rail steel quality was deformed by High Pressure Torsion up to distinctive equivalent strains at a nominal pressure of 6 GPa up to a final equivalent strain of 16. Afterwards the evolution of the resulting microstructure was investigated by Scanning Electron Microscopy, by microhardness measurements and X-ray diffraction. The bainitic structure showed a strong alignment and fragmentation into the shear direction with increasing strain, which was accompanied by an increase in hardness as well. X-ray diffraction measurements showed that the amount of retained austenite decreases dramatically after small amounts of strain, which indicates that retained austenite cannot be stabilized by high pressures. Torque measurements during deformation showed after strong hardening at the beginning, a saturation behaviour for higher strains, whereas for instance pearlitic rail steel qualities show further hardening.

Abstract: Linear flow splitting is a new cold forming process for the production of branched sheet metal structures. It induces severe plastic strain in the processing zone which results in the formation of an UFG microstructure and an increase in hardness and strength in the flanges. Inbuilt deformation gradients in the processing zone lead to steep gradients in the microstructure and mechanical properties. In the present paper the gradients in the UFG microstructure and the mechanical properties of a HSLA steel (ZStE 500) processed by linear flow splitting are presented, as well as a calculation of local strength from hardness measurements on the basis of the Ludwikequation. In order to investigate the thermal stability of the UFG microstructure heat treatments below the recrystallization temperature were chosen. The coarsening process and the development of the low angle to high angle grain boundary ratio in the gradient UFG microstructure were monitored by EBSD measurements. It is shown that heat treatment can lead to a grain refinement due to a strong fragmentation of elongated grains while only little coarsening in the transverse direction occurs. A smoothing of the gradients in the UFG microstructure as well as in the mechanical properties is observed.

Abstract: The influence of severe warm deformation on the microstructural evolution and flow behavior of a plain carbon steel has been investigated through torsion testing. The specimens were severe deformed within the ferritic region up to strain of 70 with constant strain rate of 0.1 s 1 − . Microstructural evolutions have been investigated using high resolution electron backscatter diffraction (EBSD). True stress-true strain curves exhibit a single and smooth maximum, followed by a slow but significant softening stage. A steady state is observed at very large strains. This finding suggests that the relative balance between comparable work hardening and dynamic work softening results in the occurrence of warm ductility during large deformation. The initial grain structure was equiaxed, but at low strains the grains become elongated in the torsion direction. However, at large strains the grain aspect ratio decreases and finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The flow stress, as well as all the average microstructural parameters, then remains independent of strain. The mechanisms operating during such warm flow behavior and structure changes are discussed in detail.