Nanostructure characterisation of flow-formed Cr-Mo-V steel using transmission Kikuchi diffraction technique.

Nowadays flow-forming has become a desired near net shape manufacturing method as it provides excellent mechanical properties with improved surface finish and significant manufacturing cost reduction. However, the material is subjected to excessive plastic deformation during flow-forming process, generating a very fine and complex microstructure. In addition, the intense dislocation density and residual stress that is generated in the component during processing makes the microstructure characterisation using conventional micro-analytical tools challenging. Thus, the microstructure/property relationship study in such a material is rather difficult. In the present study a flow-formed Cr-Mo-V steel nanostructure and crystallographic texture were characterised by means of Transmission Kikuchi Diffraction (TKD). Here, TKD is shown to be a powerful technique in revealing very fine martensite laths within an austenite matrix. Moreover, fine precipitates in the order of 20-70 nm on the martensite lath boundaries were clearly imaged and characterised. This greatly assisted in understanding the preferable site formation of the carbides in such a complex microstructure. The results showed that the actual TKD spatial resolution was in the range of 5-10 nm using 25 kV for flow-formed Cr-Mo-V steel.

Texture evolution in grain-oriented electrical steel during hot band annealing and cold rolling.

The optimization of magnetic and physical properties of electrical steel is imperative for many engineering applications. The key factors to improve magnetic properties are the steel composition as well as control of the crystallographic orientation and microstructure of the steel during processing. However, this requires careful control of processing at all stages of production. Under certain conditions of deformation and annealing, electrical steel can be produced to have favourable texture components. For grain-oriented (GO) electrical steels that are used in most transformer cores, a pronounced {110} <001> Goss texture plays a vital role to achieve low power losses and high permeability. Essentially, Goss texture develops during secondary re-crystallization in GO electrical steels; however, the mechanism of the abnormal Goss grain growth is still disputed in the literature. In the current study, the influence of the annealing conditions on the development of annealing, cold rolling and re-crystallization textures of hot-rolled GO electrical steel were investigated in detail following each processing step. Furthermore, the orientation data from electron backscatter diffraction were used to evaluate the orientation-dependent stored energy of deformed grains after hot rolling. In the light of new findings in the present study, annealing and deformation texture development mechanisms were critically reviewed.

Oxide formation and alloying elements enrichment on TRIP steel surface during inter-critical annealing.

The gas atmosphere in continuous annealing and galvanizing lines alters both composition and microstructure of the surface and sub-surface of sheet steels. The alloying element enrichments and the oxide morphology on transformation-induced plasticity steel surfaces are strongly influenced by the dew point of the furnace atmosphere and annealing temperature. The formation of a thin oxide film and enrichment of the alloying elements during annealing may result in surface defects on galvanized sheet products. The present contribution reports on the use of microanalysis techniques such as electron backscatter diffraction, glow discharge optical emission spectroscopy and electron probe micro-analysis for the detailed surface analysis of inter-critically annealed transformation-induced plasticity steel such as oxide phase determination, microstructure and microtexture evolutions.

Phase determination and microstructure of oxide scales formed on steel at high temperature.

Even in simple low-alloy steels the oxide scales that form during hot working processes are often a complex mixture of three iron oxide phases: haematite, magnetite and wustite. The mechanical properties, and hence descalability, are intimately linked with phase distribution and microstructure, which in turn are sensitive to both steel composition and oxidation conditions. In this study electron backscatter diffraction in the SEM has been used to characterize the microstructures of oxide scales formed on two compositions of low-alloy steel. The technique can unambiguously differentiate between the candidate phases to provide the phase distribution within the scale. This is used to investigate grain orientation relationships both within and between phase layers. It has been found that the strength of the orientational relationship between the magnetite and wustite layers is dependent on steel composition, and in particular Si content. In a low-Si (0.01 wt%) alloy only a very weak relationship was found to exist for a range of oxidation temperatures (800-1000 degrees C), whereas for the higher Si (0.37 wt%) alloy a strong relationship was observed under the same oxidation conditions. These orientational relationships are particularly important because, in this temperature range, the majority of oxide scale growth occurs at the magnetite/wustite interphase boundary.

Microstructural and microtextural characterization of oxide scale on steel using electron backscatter diffraction.

High-temperature oxidation of steel has been extensively studied. The microstructure of iron oxides is, however, not well understood because of the difficulty in imaging it using conventional methods, such as optical or electron microscopy. A knowledge of the oxide microstructure and texture is critical in understanding how the oxide film behaves during high-temperature deformation of steels and more importantly how it can be removed following processing. Recently, electron back-scatter diffraction (EBSD) has proved to be a powerful technique for distinguishing the different phases in scales. This technique gives valuable information both on the microstructure and on the orientation relationships between the steel and the scale layers. In the current study EBSD has been used to investigate the microstructure and microtexture of iron oxide layers grown on interstitial free steel at different times and temperatures. Heat treatments have been carried out under normal oxidation conditions in order to relate the results to real steel manufacturing in industry. The composition, morphologies, microstructure and microtexture of selected conditions have been studied using EBSD.