Papers by Keyword: Ultra-Fine Ferrite

Abstract: Plane-strain compression testing on HSLA steel samples using single-pass and multi-pass hot-deformation schedules showed that heavy deformation of metastable austenite below A_e3, developed ultra-fine ferrite grains (<3 µm), increased the fraction of high-angle (>15º misorientation) boundaries (>70%), and encouraged the formation of beneficial g-fibre ((ND//<111>) components along with {332}<113> and {554}<225> texture components, minimising the intensity of harmful ‘cube’ texture.

Abstract: During large strain deformation of materials, the high angle grain boundary spacing approaches the order of mean thermal diffusion distances for given deformation conditions. Based on the results of microstructural and grain size analysis in low carbon steel subjected to large strain-high Z deformation, the evolved ferrite grain size was found to be controlled by the Zener-Hollomon parameter and grain boundary diffusion was found to be the controlling mechanism.

Abstract: The microstructural change was observed during large strain high Z deformation with high strain rate in high temperature range using ultra low carbon steel. The finer grains were obtained as decreasing the deformation temperature and increasing the strain rate. And the fraction of high angle grain boundaries became higher in low deformation temperature and strong texture of ferrite recrystallized dynamically was measured such as ND//<100>,<111> and RD//<110>. The change of grain size could be analyzed by Zener-Hollomon parameter, whereas the duration has large effect on the deviation of expected grain size in deformation with high strain rate.

Abstract: Several studies concerning ferrite grain refinement have been developed in recent the last years due to the recognised influence of such microstructures on steels properties. This work was focused on the evaluation of the microstructure and mechanical properties of an ultrafine grained CMn steel obtained by hot torsion deformation and intercritical annealing. After 5 min soaking at 900 and 1200°C, the samples of low carbon steel were quenched and then reheated. Hot torsion deformation was conducted at temperatures of 700 or 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total true strain of ≈1 and generating an ultrafine and inhomogeneous microstructure with grain sizes of the order of 1-m, formed by strain-induced dynamic transformation (SIDT). The samples were heated up to 800oC and held for 1, 2 and 3 h. A more homogeneous microstructure and ferrite grain size were obtained after annealing The microhardness tests showed the reduction in hardness with the increase in annealing time. They also highlighted the effects of the ferrite grain size and the volume fractions of the microstructure constituents.

Abstract: We examined the microstructure development in low carbon steel (0.15% C) during heavy warm deformation (HWD) using field emission scanning electron microscopy (FESEM) and electron back-scattering diffraction (EBSD). Plane strain compression tests have been conducted in the temperature range of 773-923 K at strain rates of 0.01 s-1 and 1 s-1 with the specimens deformed to 25% of their original thickness. We summarize the strain rate and temperature into the Zener-Hollomon parameter and investigate its variation with plastic strain on the basis of the evolved microstructures and grain boundary character with a view to understanding the critical conditions for forming ultrafine grains and classifying them. Once established, these compressive strain-Z parameter plots simplify the selection of processing parameters (such as strain, strain rate, and temperature), towards achieving tailor-made microstructures in industrial components.

Abstract: The low carbon steel of 0.22wt% carbon was tested to estimate the dynamic phase transformation behavior from austenite to ferrite. The samples were deformed at just above Ar3 temperature by hot torsion at condition of strain rate (0.5/sec) and strain (5.0). The flow curve obtained at just above Ar3 significantly differed from others due to dynamic transformation. Based on the analysis of flow stress curve and observation of micro-structure evolution, the initiation and finish points of strain induced dynamic transformation (SIDT) could be determined. An inflection point observed at early deformation range (0.2–0.3) from the work-hardening rate and stress plot meant that new ferrite grains were nucleated in austenite matrix and these nuclei could be also confirmed by optical microscope. Subsequently in strain range of 0.7-1.0, the flow stress had the maximum value and new fine ferrite grains were dynamically generated inside untransformed austenite grains as well as prior austenite grains. The dynamic phase transformation induced by deformation made eventually fine ferrite grains under 3 ㎛ and decreased stress level with a fixed gradient.

Abstract: During large strain deformation of materials, the width of the initial high angle grain boundaries approaches the order of mean diffusion distances encountered during elevated temperature deformation. Since the evolution of ultrafine grains is attributed to thermally activated processes, the role of interfaces in determining the grain size is significant. In order to investigate into this role, microstructure development in low carbon steel (0.15% C) subjected to large strain deformation was studied with specific reference to the controlling mechanism of ferrite grain size evolution. Plane strain compression tests have been conducted in the temperature range of 773-923K at strain rates of 0.01 s -1 and 1 s-1 and the specimens were deformed to 25% of the original thickness and the Microstructural evolution is studied. Based on the results obtained, diffusion along grain boundaries was found to be the mechanism controlling ferrite grain size in this material processed through large strain deformation.

Abstract: Ultrafine-grained structures formed dynamically through simple compression at warm deformation temperatures were investigated in a 0.15%C- 0.4%Si-1.5%Mn steel. The effects of strain, strain rate and deformation temperature on the microstructural evolution were examined using an isothermal plane strain compression technique with a pair of anvils. The maximum strain was 4, the deformation temperature was below the AC1 temperature, and the Zener-Hollomon parameter (Z) ranged between 1012 s-1 and 1016 s-1. Ultrafine ferrite grains surrounded by high angle boundaries are generated by simple compression when the strain exceeded a critical value. The number of newly generated ultrafine grains increased with the strain; however, the average sizes were found to be independent of strain. The grain size, `d`, was found to depend on Z parameter. An equation, d (μm) =102.07Z-0.16, was found to satisfy the experimentally obtained data. This study demonstrates the possibility of obtaining ultrafine ferrite through multi-pass caliber rolling as a high Z- large strain deformation technique for producing bulk engineering components. It was also noted that the empirical relation established based on single pass compression tests is valid for multi-pass caliber rolling.

Abstract: In the last years, several studies concerning ultra refinement of ferrite grains have been conducted using different experimental techniques (ECAP, ARB, HPT). The aim of all investigations was to provide an optmized relationship between mechanical properties and microstructure of steels. The present work, likewise, deals with strain induced dynamic transformation of ferrite. Samples of low C-Mn steel were intensely deformed in hot torsion aiming at the production of ultrafine grains of ferrite thereby enhancing the mechanical properties when compared to hot rolled products. After soaking during 5min at 900°C, the samples were quenched and then reheated and submitted to hot torsion deformation at temperatures of 700 and 740°C. The torsion schedule consisted of 7 isothermal passes leading to a total strain of ≈1, generating an ultrafine microstructure with grain sizes of the order of 1µm. The shape of stress-strain curves so obtained suggested that ferrite refinement occurred by dynamic recrystallization. The various constituents present in the microstructure as well as ferrite grain size and morphology were examined by optical and scanning electron microscopy. Microhardness tests were performed to evaluate mechanical properties.