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Hutchinson B.,SwereaKIMAB | Lindell D.,SwereaKIMAB | Nave M.,Deakin University | Rollett A.,Carnegie Mellon University
Materials Science Forum | Year: 2013

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 60° boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel. © (2013) Trans Tech Publications, Switzerland.

Hutchinson B.,SwereaKIMAB | Martin D.,SwereaKIMAB | Karlsson O.,SwereaKIMAB | Lindberg F.,SwereaKIMAB | And 3 more authors.
Materials Science and Technology (United Kingdom) | Year: 2016

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. © 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group

Nazarov A.,French Corrosion Institute | Diler E.,French Corrosion Institute | Persson D.,SwereaKIMAB | Thierry D.,French Corrosion Institute
Journal of Electroanalytical Chemistry | Year: 2014

ZnO films of different thicknesses were prepared by thermal oxidation of zinc. The oxide covered surfaces were characterized by Scanning Kelvin Probe (SKP) and Scanning Kelvin Probe-Surface Photovoltage (SKP-SPV) techniques, Infrared Reflection Absorption Spectroscopy (IRRAS), contact angle measurements, and dc voltammetry. The influence of the thickness of ZnO on the absorption of the light, water and oxygen was evaluated. SKP and dc electrochemistry were used to estimate the mechanism of electron exchange between the zinc surface and an aqueous solution containing a red-ox system [Fe(CN)6]2 -/[Fe(CN)6]3 -. It was shown that ZnO/Zn electrodes with a thick ZnO film nobled the Volta potential that enhanced the electron transfer from the bulk zinc to the molecule of the oxidizer- [Fe(CN)6]3 -. Atmospheric corrosion of oxidized zinc surface was investigated after deposition of a single droplet of NaCl aqueous electrolyte. Thicker ZnO films promote the oxygen reduction and the spreading area of the cathodic reaction from the local NaCl contamination. It enlarged the area of metal surface participating in the cathodic reaction and consequently accelerated the atmospheric corrosion. The ability to enhance the oxygen reduction was discussed from the point of view of the band structure and the semiconducting properties of the ZnO layer. © 2014 Elsevier B.V. All rights reserved.

Haghdadi N.,Deakin University | Martin D.,SwereaKIMAB | Hodgson P.,Deakin University
Materials and Design | Year: 2016

A detailed understanding of the hot deformation and work hardening behavior of LDX 2101 dual phase steel has been obtained through a wide range of hot compression tests with strain rates from 0.01 to 50 s-1 and temperatures from 900 to 1250 °C. In most of the cases, the material showed typical dynamic recrystallization (DRX) behavior i.e., a peak followed by a gradual decrease to a steady state stress. The work hardening rate showed a two stage behavior i.e., a transient sharp drop at low stress values followed by a gradual decrease at higher stresses. Using the work hardening rate behavior at the latter stage, the saturation stress was calculated for different hot working conditions. Regression methods were used to develop a hyperbolic-sine equation linking the saturated stress to the deformation conditions. A physically-based Estrin-Mecking (EM) constitutive equation was then employed to model the flow behavior in the work hardening (WH)-dynamic recovery (DRV) regime. Finally, the Avrami equation to describe the evolution of the softening fraction was coupled to the EM model to extend the model to the dynamic recrystallization region. The results show that the model which is based on the stress-strain and work hardening behavior accurately predicts the flow behavior of this microstructurally complex steel. © 2016 Elsevier Ltd.

Hutchinson B.,SwereaKIMAB | Siwecki T.,SwereaKIMAB | Komenda J.,SwereaKIMAB | Hagstrom J.,SwereaKIMAB | And 3 more authors.
Ironmaking and Steelmaking | Year: 2014

Hot rolled strips having a minimum yield stress of 700 MPa have been produced in full scale processing. Alloy additions (1·5%Mn, 1%Cr and 0·3%Mo) together with rapid cooling on the runout table produce fully bainitic microstructures which confer strength and toughness. Microalloying with 0·08%V has been shown to be essential for achieving stable strength levels that are independent of the coiling temperature since the necessary low coiling temperatures are difficult to reproduce accurately under steelworks conditions. The products have excellent bendability and HAZ toughness. Typical microstructures and textures of the hot rolled strips are also presented. © 2014 Institute of Materials, Minerals and Mining.

Hutchinson B.,SwereaKIMAB | Komenda J.,SwereaKIMAB | Rohrer G.S.,Carnegie Mellon University | Beladi H.,Deakin University
Acta Materialia | Year: 2015

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500°C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V-N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V-N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V-N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {1 1 0} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {1 1 0} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V-N steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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