Bevis Hutchinson

Orientation dependence of secondary recrystallisation in silicon-iron

Experiments and analyses have been carried out to reach a better understanding of the mechanism of Goss texture formation during the secondary recrystallisation of silicon steel processed by the single cold reduction route. A new experimental approach demonstrated the effect of misorientation on the growth rates of secondary grains and it is shown that these rates are controlled by the proportion of matrix grains having Σ9 CSL relationships to growing secondary grains. It is considered that the Σ9 boundaries have lower energy than general grain boundaries and so are less strongly inhibited by Zener drag. The relative infrequency of Σ9 boundaries around the periphery of secondary grains is seen as evidence for their sacrificial behaviour. Other experiments involving growth of randomly oriented nuclei provide independent support for the important role of Σ9 boundaries during secondary recrystallisation in this steel.

Deformation modes and anisotropy in magnesium alloy AZ31

Abstract A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300 °C. Structures have been examined by optical and transmission electron microscopy and also by atomic force microscopy to quantify surface displacements seen at grain boundaries. Plastic anisotropy varies strongly with test temperature as was observed previously by Agnew and Duygulu. The present findings do not support the view that crystallographic becomes a major contributor to deformation at higher temperatures. Rather, the material behaviour reflects an increasing contribution from grain boundary sliding despite the relatively high strain rate (10– 3 s– 1) used in the mechanical tests.

A re-evaluation of the mechanism of SIBM

The process for creating a new recrystallized grain by the bulging out of an existing grain boundary within the deformed structure was recognized by Beck and Sperry and described as strain induced boundary migration, often now as SIBM. For many years SIBM seemed to be accorded less significance than the subgrain growth models for nucleation of recrystallization. However, the importance of SIBM is increasingly recognized, for example in the case of cube texture development in aluminum alloys. There is increasing evidence that high angle boundaries in deformed metals are not only residues of the prior grain structure but may also be generated by the deformation itself. These also participate in the nucleation of recrystallization on annealing and must do so by some form of SIBM mechanism.

The Cube Texture Revisited

The cube texture in rolled and annealed fcc metals and alloys has long fascinated metallurgists because of its high symmetry and extreme sharpness. This paper demonstrates and analyses the texture perfection that is developed in a copper sample. Reasons that have been advanced to explain the development of the texture during recrystallisation and grain growth are discussed. Orientation selectivity is favoured during growth but more particularly in the nucleation stage. Especial attention is paid to the rapid recovery which cube oriented crystals undergo on heating after plane strain deformation and which is the basis for its uniquely preferential nucleation. Various metallurgical factors are known to affect the strength of the cube texture in practice and explanations for some of these are presented.

The production mechanism of extensively sharp Goss orientation in HI-B material

The production mechanism of highly sharpened Goss orientation in HI-B material was investigated in a comparison with the conventional. Both materials involve multistage orientation selection processes during the secondary Goss grain evolution, notably, the nucleation stage and the later growth stage generate distinctive selections. In HI-B material, each selection process produces considerably different orientation and consequently very small tolerance of the deviation angle is allowed.

Vanadium microalloying for ultra-high strength steel sheet treated by hot-dip metallising

Ultra-high strength steel sheets have been subjected to heat treatments that simulate the thermal cycles in hot-dip galvanising and galvannealing processes and evaluated with respect to their resulting mechanical properties and microstructures. The steels contained suitable contents of carbon (∼0.2%), manganese (1.2%) and chromium (0.4%) to ensure that they could be fully transformed to martensite after austenitisation followed by rapid cooling in a continuous annealing line, prior to galvanising. Different contents of vanadium (0–0.1%) and nitrogen (0.002–0.012%) were used to investigate the possible role of these microalloying elements on the strength of the tempered martensite. Vanadium, especially when in combination with a raised nitrogen content, helps to resist the effect of tempering so that a larger proportion of the initial strengthening is preserved after the galvanising cycle, giving tensile strength levels exceeding 1000 MPa. Different deoxidation practices using aluminium or silicon have also been included. These showed similar strength levels at corresponding carbon contents but the bendability of the Si-killed steel sheet was considerably superior. Microstructural examinations have been made on the annealed steels but the reason for the beneficial effect of vanadium is still not fully explained. It is concluded that microalloying with vanadium is a very promising approach in the development of corrosion-resistant ultra-high strength steel sheet products.

Development of Boundary Misorientations During Grain Growth in Silicon Steels

Changes in grain size, texture and misorientation distributions have been monitored during extensive normal grain growth in 3%Si steels. The boundary misorientation distributions deviate significantly from the Mackenzie relationship. Comparisons of correlated and uncorrelated distributions show large excesses of low angle boundaries. However, these are not a result of low energy boundaries being favoured during grain growth since the deviation diminishes as growth proceeds. The effect originates in the nucleation of grains in colonies of similar orientation during primary recrystallisation. A slight tendency for promotion of 60º boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel.

Elasticity and wave velocity in fcc iron (austenite) at elevated temperatures – Experimental verification of ab-initio calculations

&NA; High temperature crystal elasticity constants for face centred cubic austenite are important for interpreting the ultrasonic properties of iron and steels but cannot be determined by normal single crystal methods. Values of these constants have recently been calculated using an ab‐initio approach and the present work was carried out to test their applicability using laser‐ultrasonic measurements. Steel samples having a known texture were examined at temperatures between 800 °C and 1100 °C to measure the velocity of longitudinal P‐waves which were found to be in good agreement with modelled values. HighlightsEvaluating new data for fcc iron from ab‐initio modelling.Texture in high temperature austenite phase.Laser‐ultrasonics measurements at high temperatures.Close agreement between measured and modelled velocities of longitudinal P‐waves.

Observations of dynamic transformation products in a commercially hot-rolled steel

ABSTRACT Sufficient evidence has now accumulated to show that dynamic transformation (DT) is a real phenomenon in steels and can take place over a wide range of temperatures in the austenite regime. During plastic deformation, some of the austenite is transformed to ferrite despite austenite being the chemically more stable equilibrium phase. Occurrence of DT has been demonstrated in various laboratory tests but apparently not in commercially hot-rolled steels. In this note, we review some old results on high strength low alloy strip steel that contained microstructural features that appear to have been caused by DT. We deduce that DT occurred early in the rolling schedule, leading to thin-pancaked sheets of ferrite in the final product. The presence of these pre-existing ferrite nuclei frequently gave rise to adjacent regions of coarse polygonal ferrite grains during cooling.

Characterizing Grain-Oriented Silicon Steel Sheet Using Automated High-Resolution Laue X-ray Diffraction

Controlling texture in grain-oriented (GO) silicon steel sheet is critical for optimization of its magnetization performance. A new automated laboratory system, based on X-ray Laue diffraction, is introduced as a rapid method for large scale grain orientation mapping and texture measurement in these materials. Wide area grain orientation maps are demonstrated for both macroetched and coated GO steel sheets. The large secondary grains contain uniform lattice rotations, the origins of which are discussed.