Materials Science Forum Vols. 715-716

Abstract: In this work result of the theoretical analyses, witch the main purpose was modelling of the microstructure change during round bars rolling was presented. To determination of the austenite diameter during numerical modelling of the rolling process it is necessary to assign mathematical models of the microstructure change, relationship making the value of yield stress dependent on deformation parameters, temperature and strain range. Theoretical analysis was made in computer program Forge2008®, based on the finite-element method. An analysis was made for two cases: traditional - without accelerated cooling during rolling process and normalizing with one section of the accelerated cooling during rolling process for the ø26 mm round bars. Modification of the ø26 mm round bars rolling technology with accelerated cooling, affect the reduction of average austenite diameter, which cause improving impact resistance of the final product. Results of the theoretical analysis were verified in industrial conditions, in one of polish steelwork.

Abstract: The development of dynamic recrystallization in a 304-type austenitic stainless steel during warm multi-pass screw rolling was studied. The fraction recrystallized depended significantly on the distance from the centre of rolled rods. The highly elongated original grains were mainly observed in the centre of processed samples, while the portions close to the sample edge were characterized by relatively fast development of dynamic recrystallization leading to formation of almost equiaxed fine grains with a size of about 1 μm. The volume fraction of fine grains in the edge areas increased with straining and exceeded 0.7 after rolling to 63% cross area reduction. The structural mechanism responsible for the grain refinement was considered as continuous dynamic recrystallization. The difference in the recrystallization kinetics across the sample is associated with the variation in strain intensity.

Abstract: Structural changes in a 9%Cr martensitic steel after 1%, 4% creep and creep rupture test at 650°C and stress of 118 MPa were examined. Heat treatment provided the formation of tempered martensite lath structure (TMLS) in the steel. The precipitations of second phase particles along block and lath boundaries provide effective stabilization of the TMSL under annealing/aging condition. This structure hardly changed under creep conditions in grip portion of crept sample. Significant coarsening of both the second phase particles and the martensite laths takes place in neck portion. In addition, the latter ones lose their original morphology and are replaced by large strain-induced subgrains. It should be noted that the increase of subgrain size is in almost direct proportion to the particle growth during the creep to 4% strain. The rapid growth of martesite laths followed by their evolution to deformation subgrains takes place within the tertiary creep regime.

Abstract: The characteristics of grain boundary motion and evolution are of fundamental importance in material science. Optical microscopy is used to analyse grain boundary fluctuations in two-dimensional colloidal crystals. Colloidal systems are particles (colloids) on the order of 1µm dispersed in a solvent where they display rich phase behaviour of colloidal 'crystal', liquid' and 'gas' phases. They are widely used as a model system to study many fundamental issues in condensed matter physics and statistical mechanics. The intrinsic slowness and increased length scales of colloidal systems make them an excellent model system to study grain boundaries as an analogy to atomic systems. Static and dynamic correlation functions are compared with capillary wave theory to calculate the grain boundary mobility and stiffness. These fundamental properties of grain boundaries determine the kinetics of curvature-driven grain growth.

Abstract: Dynamic recrystallization (DRX) of ferrite in a low carbon steel with the (α+θ) duplex microstructures was investigated using hot compression tests in combination with SEM, TEM and EBSD, and the effect of the size of cementite particles was analyzed. The results indicated that during hot deformation of the low carbon steel DRX of ferrite took place and the increase in the average size of cementite particles was of benefit to DRX. The formation of DRX grains was attributed to particle stimulated nucleation (PSN), by the well development of the subgrain near cementite particles. At the beginning of hot deformation, DRX grains were mainly formed near cementite particles with the size of about 1μm or above. With the increase in the strain, such grains were also formed around pairs or groups of particles with the size of 0.5μm to 1μm.

Abstract: Plate steel S460 is intercritically rolled during the final stages of industrial processing. A series of experiments to represent the preliminary stages of an intercritical simulation were completed and the isothermal austenite to ferrite transformation kinetics investigated. The growth of the ferrite grains was interpreted using the classic JMAK model and the effect of processing history also examined. A double austenite deformation at 1323 K, to a true strain of 0.2, led to the most acceptable starting microstructure for the extended simulation. The role of niobium in this preliminary simulation is also considered.

Abstract: Austenitic stainless steels of the AISI 304 and 316 grades, amongst over other hundred compositions of stainless steels available in the market, are the most frequently used ones worldwide. They are selected for numerous applications due to their favorable combination of characteristics such as low price, moderate to good corrosion resistance, excellent ductility and toughness along with good weldability. Their major limitation is in the yield strength, which is relatively low (about 200 MPa), in the annealed condition. Through cold working, this value can be easily multiplied by a factor of up to six, however ductility drops. The cold worked sub-structure of the austenitic stainless steels is formed by a planar array of dislocations and strain induced martensites, α (BCC) and ε (HCP). The microstructure evolution of austenitic stainless steels, AISI 304L and 316L, during cold rolling and subsequent annealing was studied (maximum thickness reduction - 90%). Samples were initially solution annealed at 1100°C for one hour with subsequent water quenched. Following, they have been cold rolled at room temperature, with cold reductions varying between 10 and 90%. After rolling, samples with approximately 90% thickness reduction have been submitted to annealing treatments in order to study martensite reversion, recovery and recrystallization. Annealing treatments have been performed between 200 and 900°C, with 100°C interval for one hour. The resulting microstructures were investigated by optical microscopy, scanning electron microscopy (with EBSD), magnetic measurements and hardness evaluation. As received (hot rolled) austenitic stainless steel sheet presented recrystallized equiaxial grains with austenite and islands of delta ferrite, in larger quantities mainly in the centre of the sheet. The solution annealing at 1100°C for one hour eliminated delta ferrite. During rolling, the austenite partially transforms into α martensite. The 50% αmartensite reversion temperature is close to 550°C for both steels. This temperature is practically independent of the amount of αmartensite present in the steel. The 50% recrystallization temperature of the 304L steel is lower than that of the 316L steel, about 700 and 800°C, respectively. The 316L steel shows a higher recrystallization resistance, due to its higher SFE and lower storage deformation energy than the 304L steel. Recrystallization temperature is about 150°C higher that the αmartensite reversion temperature. The percentage of αmartensite has a strong influence on the recrystallized grain size, the higher the percentage of this phase the smaller will be the grain size.

Abstract: The pancake-shaped growth of Goss grains in Fe-3%Si steel is approached by solid-state wetting mechanism. The fraction of grains with Σ9 relation with Goss grains in the 0.3 mm thick specimens after primary recrystallization is higher near the surface than at the center. This result indicates that the probability of solid-state wetting is higher near the surface than at the center, leading to the pancake-shaped growth.

Abstract: Texture formation of AA5182 for compressive deformation with a range of temperatures from 673K to 823K and strain rates from 5.0×10^-4 to 5.0×10^-2s^-1 is experimentally investigated by EBSD technique and X-ray diffraction. Fiber textures are observed in all deformation conditions. Stress regions are divided into a low stress region (10~55MPa) and a high stress region (above 55MPa) on the basis of the relationship between stress and grain size. In the low stress region, it is found that the main component of the fiber texture is {001}(compression plane). In this case, the pole density at {001} is increased with increasing temperature at the same stress level. It is concluded that development of {001} component is attributed to grain boundary migration. For the high stress region, the main component of the fiber texture is {011}. It is considered that the formation of {011} component is attributed to the slip deformation by {111}<110> system.

Abstract: Examination of the SIBM mechanism based on the dislocation substructure at the interior of the Goss oriented grain was carried out by changing the grain size prior to the temper rolling. The following results were obtained. 1) SIBM significantly increased Goss orientation during the eminent grain growth with the initial grain sizes from 18 to 55μm. 2) When the initial grain sizes were large, i.e. 37μm and 55μm, the rolling with the reduction beneath the critical value could not promote SIBM, even the normal grain growth could also be hindered. Consequently a proposal was made that the nucleation of the recovery appeared among substructure domains containing sluggish strain. There exists an adequate size of the domain which varies with the change both of the rolling reduction and the initial grain size.