Papers by Keyword: Intercritical Rolling

Abstract: Effect of austenite as a harder second phase on ferrite substructure evolution by intercritical rolling has been investigated using 0.12C-0.3Si-1.35Mn steel. Slab was reheated at 780°C (ferrite; α + austenite; γ phase region) and rolled with 90% reduction down to 12mm in thickness using laboratory mill. 700°C (ferrite; α + cementite; θ phase region) rolled plate was also prepared to compare with 780°C rolled plate. Microstructure distribution along the plate thickness has been observed in detail by SEM and EBSD. Microstructure showed mixed structure of fine-grained ferrite and elongated ferrite for both of the plates. The ratio of fine-grained ferrite region was around 50% at the plate surface, however, fine-grained ferrite formation hardly occurred at mid thickness for 700°C rolled plate. FEM analysis revealed that fine-grained ferrite distribution along the thickness can be well explained by equivalent plastic strain distribution along the plate thickness for 700°C rolled plate. Equivalent plastic strain showed maximum value near the plate surface due to shear strain component, and it could help substructure evolution and fine grained ferrite formation. On the other hand, the ratio of fine-grained ferrite region reached to 50% all through the thickness for 780°C rolled plate. Austenite as a harder second phase seems to promote ferrite substructure evolution even at mid thickness of the plate.

Abstract: Series of trials were conducted on a laboratory rolling mill to evaluate the influence of intercritical rolling on the microstructure and texture of steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V. Two parallel rolling schedules A and B were designed on the base of the experimentally deduced CCT diagram of the steel. In rolling schedule A the material was subjected to accelerated cooling and coiling simulation after final rolling in the intercritical region, whereas in rolling schedule B the last rolling pass in the intercritical region was replaced by a water quench at the same temperature of the intercritical rolling pass in schedule A. Microstructure and texture were characterized by means of light optical microscopy, scanning electron microscopy, EBSD and XRD. It was found that the average grain diameter and the texture depend significantly on the final rolling temperature in the intercritical region. The decrease of the intercritical rolling temperature leads to an increase of the {111}〈uvw〉 /{001}〈uvw〉 ratio, but at the same time the increase of the average ferrite grain size was also observed. A phenomenological model based on the K–S orientation relationships was used to predict the texture formation in the intercritical region.

Abstract: In order to simplify the production routine and to save cost of hot-rolled TRIP steels, the intercritical rolling process was investigated using thermo-mechanical simulation technology. Both 0.2C-1.5Mn-1.5Si and 0.2C-1.5Mn-1.5Si-0.5Cu TRIP steels were chosen so as to study the effect of 0.5 pct Cu addition on microstructure and retained austenite volume fraction of 0.2C-1.5Mn-1.5Si TRIP steel. It is found that grain size refinement happened during intercritical deformation, and multi-phase microstructure could be achieved after isothermal treatment, so intercritical annealing can be eliminated to avoid grain size coarse and to keep the refine grain size in intercritical-deformed TRIP steels.

Abstract: The softening kinetics of a 0.19 wt% C 1.5 wt% Mn steel deformed at two intercritical temperatures have been characterised using the stress relaxation technique. Recrystallisation of intercritical austenite has been modelled using a single grain model (Chen et al., 2002 [1]), whilst recovery of both intercritical austenite and ferrite has been modelled using a model in the literature [Verdier et al., 1999 [2]). The models are combined to predict the overall softening kinetics with a rule of mixtures formulation. Comparison of the model with experiment shows significant deviations. The reasons are discussed with reference to the mixture rule and to the local stress-strain distribution which exists in the deformed samples. A simple modification to the model is proposed which takes into account the effect of a local stress distribution in deformed austenite.

Abstract: The distribution of the characteristic texture components between the ferrite grains of different size classes has been studied in a steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V after different rolling schedules with a final rolling temperature above or below Ar3. Microstructures and textures were characterized by means of optical microscopy and orientation microscopy. A strong grain refining effect together with a bimodal grain size distribution was observed in the steel both after final rolling in the intercritical region or in the austenite region, close to the Ar3 d temperature. The differences in grain size were interpreted on the basis of three potentially acting mechanisms: (i) transformation- induced recrystallization, (ii) increased mobility of specific grain boundaries and (iii) fast nucleation of ferrite grains on specific sites of the parent austenite microstructure. The experimental data clearly favoured the third of these assumptions as the responsible mechanism for the observed bimodal grain size distributions.