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EM sensor modelling for prediction of phase transformation fractions

Researchers: Jialong Shen, Dr Lei (Frank) Zhou

Participants: Primetals Technologies Limited, University of Warwick, University of Manchester

Electromagnetic (EM) sensors are used during steel processing for quality control and property predictions. One of the

The present project aims to establish a predictive approach, based on modelling for the EMspecTM sensor and microstructures of interest, to obtain the ferrite fraction during transformation at elevated temperatures from EMSpecTM sensor measurements. The work will involve the following activities:most recent systems, EMspecTMsensor technology from the University of Manchester, has been licensed and developed for industrial application by Primetals Technology Ltd., , It is currently used in the cooling zone on the run out table of the hot strip mill for monitoring phase transformation. EM models have been developed at WMG for predicting room temperature magnetic properties (for example low field permeability) from steel microstructures, including for different phase fractions. In addition, the variation in low field permeability with temperature has been established for simple ferromagnetic microstructures. EM models to obtain magnetic properties (low field permeability) from EM sensor measurements have also been developed at WMG and the University of Manchester.

  • Modelling of high temperature permeability values for dual phase microstructures.
  • Modelling of the EMspecTM sensor and verification of the model.
  • Modelling for the relationship between sensor output and EM properties for microstructures of interest.
  • Compilation of the outputs into a database for obtaining the ferrite transformed fraction based on input data of the zero crossing frequency, temperature and grade.

Micro-alloyed high strength steel featuring non-metallic precipitate

Researcher: Dr Yongjun Lan

This project researches new micro-alloyed high strength steels. This research focuses on how to control the features of precipitates in order to maximise the strengthening effect of these small particles. Another emphasis of this work is placed on understanding the precipitation behaviour under industrial conditions, e.g., coiling after hot rolling and annealing after cold rolling.

Related project: Micro-structuring micro-alloyed steels via non-metallic precipitate formation

Non-Destructive Evaluation of Steel Microstructures using Electromagnetic Sensors

Researcher: Dr Jun Liu
Project partners: University of Manchester, NML (India), Tata Steel, Primetals Technologies Limited, NPL

Part of the EPSRC sponsored project ‘ASAP - Advanced electromagnetic sensors for assessing property scatter in high value steels’.

A variety of electromagnetic (EM) sensors / techniques have been developed or commercialised for evaluating/monitoring microstructure, mechanical properties or creep damage in steels during industrial processing, heat treatment or service exposure in a non-destructive and non-contact fashion. These sensors/techniques are sensitive to different microstructures based on the principle that microstructural changes in steels alter their electrical and magnetic properties. The major challenge is the interpretation of the EM sensor signals and the correlation to microstructure and mechanical properties, which is currently more or less qualitative, empirical and/or general.

The present project aims to establish a quantitative link between the selected microstructural parameters of interest and relevant EM properties to help identify the most appropriate EM properties to be measured for greatest sensitivity to the features of interest. This will determine the type of sensors or techniques to be used for a given application, and enable accurate evaluation of the microstructural parameters and microstructural feature distribution. Major / minor magnetic hysteresis loop measurements will be carried out to measure various magnetic properties of model and/or commercial steels with different microstructural feature distributions, e.g. precipitate size and inter-particle spacing distribution. Major / minor loop models are being developed to consider the microstructural parameter distribution, which can, in turn, be inversely evaluated by fitting with experimental measurements. EM sensor outputs are also modelled by Finite Element method using Comsol Multiphysics to look at the link between EM sensor signals and relevant EM properties. To improve understanding of the fundamentals we also observe the magnetic domain structure (i.e. magnetic microstructure) and domain processes (i.e. development of domains in an applied magnetic field) to look at their interactions with microstructural features.

Phase Transformation and Precipitation of Carbides in Duplex and Austenitic Steels

Researcher: Dr Alireza Rahnama

Duplex and austenite based LDS (Low Density Steels) can provide much higher strength and ductility. The poor cold formability in ferritic LDS can be overcome by introducing austenite, which does not have disorder to order transition. The amount of austenite and the stability of the austenite as well as the morphology and the distribution of the austenite play an important role in duplex and austenitic LDS alloys, which can be controlled by process to get various combinations of strength and ductility. The most benefit that can be obtained is the nano κ-carbide precipitation strengthening, which can be also modified by process control.

Therefore, the study of the phase transformation behaviour of the austenite and the precipitation behaviour of κ-carbides in duplex and austenitic LDS alloys will provide a basic understanding for developing LDS alloys with improved properties. The phase transitions need to be understood in the context of the T-t and mechanical conditions that are present during thermos-mechanical processing and further development of the product into coated and formed sheets and even further when joined to structures.

The objective of the project is to investigate the microstructural evolution during processing (deformation, annealing and joining) of duplex and austenitic LDS alloys. Specifically:

  • Study of microstructure formation during deformation, annealing and resistance spot welding.
  • Characterisation of the resulting microstructures and related to the process.
  • Modelling for microstructure formation to provide better understanding.

Product Uniformity Control (PUC) – Electromagnetic sensors

Researcher: Dr Lei (Frank) Zhou, Research Fellow, with Prof Claire Davis

Participants: Tata Steel, ArcelorMittal, Thyssenkrupp Steel, Salzgitter, Swerea KIMAB AB, TNO, Chalmers University of Technology, SSSA, CEIT, The University of Warwick, The University of Manchester, WMG Universite Joseph Fourier Grenoble, CEA, Cedrat.

This is an EU funded RFCS (Research Fund for Coal and Steel) project with an overall objective to optimise the product (steel strip) uniformity by the employment of online systems that continuously and non-destructively measure electromagnetic (EM) and ultrasonic parameters that relate to the steel microstructure.

Electromagnetic sensors have previously been shown to be able to quantitatively monitor phase transformation in steels (e.g. austenite to ferrite) and are sensitive to other microstructural changes e.g. decarburisation in rail and rod steels and thermal exposure for power generation steels. In order to use on-line systems to assess microstructural inhomogeneity then detailed understanding of the sensitivity of sensors signals to microstructural changes is required.The main role of University of Warwick in this project involves modelling of the EM inductance properties for microstructures in steels.

The research focuses on the link between the microstructure and the EM signal via the influence of the materials electromagnetic properties. Finite element models (in 2D and 3D) will be established to model the effect of microstructure features (including phase balance, grain size, texture, dislocation density, precipitates, applied / residual stress) on the EM properties (primarily the relative permeability). Initially models for individual microstructural parameters will be developed with complex multi-component microstructures then being considered.

Simulation of the Rapid Solidification Rates seen in Thin Slab and Strip Casting of Steel

Researcher: Dr Carl Slater

Part of a EPSRC sponsored project “ASSURE – Advanced Steel Shaping Using Reduced Energy”

Since the implementation of continuous casting for steels in the 1960’s, the use of this technique has increased dramatically. Today about 95% of the steel produced worldwide is fabricated by this method. In particular thin slab cast direct rolling and strip casting offers major advantages in term of efficiency through the reduced capital needed for reheating and reduced rolling requirements. As production of steels moves closer to net shape casting, a dramatic increase in the initial cooling rates seen during solidification is observed. For thick and thin slab casting typical cooling rates are 12 and 50 °C/s respectively. Whereas strip casting can see cooling rates up to 1700 °C/s at the strip surface. Whilst these rates have been produced in full-scale casters, reproducing these controllably on a lab-based scale is difficult. Within this project several techniques have been explored in order to replicate the cooling conditions of these accelerated casting processes.

The figure (right) shows an example of one of these techniques: using a Gleeble 3500, tungsten electrodes are used to pass a large electrical current through a steel sample encased in an alumina crucible, which heats the sample at a rate of around 50 °C/s to just above the liquidus temperature. The sample is then controllably cooled at rates up to 100 °C/s. The advantage of this technique over the conventional splat or immersion test is the ability to have direct temperature measurement of the molten steel (compared to the extrapolated values obtained from the other methods). In addition to this controllable thermal profiles can be applied post solidification, such as reheats/dwells, to study the formation of precipitates. This method also incorporates the electromagnetic stirring seen in practise as well as the use of much larger samples compared to the other techniques.

Alongside the experimental investigations modelling is being carried out using both COMSOL (a multiphysics simulation software) and ThermoCalc (for phase prediction) to understand the thermal profiles in the samples and to predict the segregation behaviour.


The Role of Annealing Conditions on Crack Depth of Zn coated Boron Steels

Researchers: Dr Vit Janik; Prof Sridhar Seetharaman
Partners: Tata Steel / High Value Manufacturing Catapult

Introduction: Zn coated 22MnB5 steels offer automotive manufacturers a corrosion resistant high-strength material with potential for light weighting. Direct hot-press forming enables an efficient route to process and shape this material into high-strength components. A primary hindrance to allow wide application of this process technology is caused by cracking inside the Zn coating that cause cracks to penetrate into the substrate material during hot press forming. Low melting point of Zn (420oC) causes the coating to partially melt during the hot-forming and allows Zn to infiltrate into the underlying steel substrate during deformation, a phenomenon known as liquid metal assisted cracking (LMAC); a special case of LMAC is called Solid Metal Induced Embrittlement(SMIE) and could operate even no liquid phases are present and embrittlement atoms are distributed from a solid face.

Methodology: High Resolution Transmission Electron Microscopy (HR-TEM) with Energy Dispersive Spectroscopy (EDS) and Focused Ion Beam (FIB) lift-out method to characterize Zn distribution and morphology of microstructural features and cracks inside the coating and the substrate material was applied.

Conclusions: Increased annealing time before the forming reduces susceptibility to deep micro-cracks formed during the mechanical loading. SIME is likely to be active during short times of annealing when enriched Zn is present on α-Fe(Zn) grain boundaries and in the pockets and mean Zn content is relatively high in the α-Fe(Zn) coating, SMIE is responsible for the deep V-shape cracks. On contrary, enriched Zn in α-Fe(Zn) is homogenized and mean Zn concentration is lowered in the α-Fe(Zn) coating during long time annealing, thus no SMIE is active due to lower availability of solid Zn atoms in the α-Fe(Zn) and the blunt U-shape cracks are due to external loads only.