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Steels Processing Group PhD Projects

Details of active research projects being undertaken by postgraduate researchers within the Steels Processing Research Group.

A Sengupta

Figure showing that we aim to develop a linkage between the fluid flow results from numerical modelling and the final solidification structure

A Multiscale Approach for Fluid Flow Effect on Microstructure and Segregation

PhD Student: Arunava Sengupta
Start date: October 2015
Academic supervisors: Dr Michael Auinger, Prof Sridhar Seetharaman
Industrial supervisor: Dr Begona Santillana (Tata Steel Netherlands)
Partner: Tata Steel Netherlands

The objective of the study is to investigate the effect of the fluid flow (convection) on the dendritic structure and hence gives an impression about the influence of the flow field on the overall microstructure in a solidified steel slab. Therefore, we aim to elaborate a better connection between the local composition of dendrites during solidification and its effect on the overall grain structure (and the possible danger of segregation) in the final product of the continuous casting process.

It is a well-known fact that the dendrite growth direction in a flowing melt tends to bend towards the upstream direction leading to a curvature that points towards the region in the mould, where the material solidifies at the latest. The mechanisms responsible for this bending are mainly due to difference in temperature and concentration gradients occurring at the solidification front. For free dendrites, that is for a pure metal and growing in an undercooled substance, the thermal field around the dendrite tip determines the growth direction. The temperature around the tip in the downstream direction is higher than in the upstream direction due to the latent heat released on the surface. For columnar dendrites, that is for an impure metal and growth with a positive temperature gradient, the solute volume around the dendrite tip plays an important role in determining the growth direction. It is particularly important from an industrial point of view to correlate the dendritic deflection behaviour with fluid flow so that one can tune the casting process parameters to achieve the desired solidification structure. The methodology will involve development of solidification model coupled with fluid flow backed up by experimental verification.

Plaster torch at TTC

Plasma torch at TTC

Coal Characterisation for Blast Furnace Pulverised Coal Injection

PhD student: Ian Moore
Start date: September 2014
Academic Supervisors: Prof Sridhar Seetharaman (WMG); Prof Kerry Kirwan (WMG), Zushu Li
Industrial Supervisor: Colin Atkinson (MPI)

Blast Furnace Pulverised Coal Injection (PCI) is the injection of pulverised coal into the ironmaking Blast Furnace to act as a reducing agent in the process of reducing iron ore to iron. The use of coal as a reducing agent decreases the dependency upon coke in the ironmaking process. This has numerous advantages including:


• Environmental e.g. reduced emissions from the cokemaking process
• Economic e.g. increased rate of production
• Technical e.g. increased operational flexibility


It is known that coal particles swell when subjected to temperatures above around 400 °C yet the effect of this swelling upon Blast Furnace operations has not been well studied. It is also known that the degree of swelling is dependent upon coal type therefore understanding the swelling behaviour of a particular coal and how that impacts upon the overall Blast Furnace process may be an important consideration in selecting the most cost effective coals for Blast Furnace injection.
The Confocal Scanning Laser Microscope (CSLM) allows direct observation of individual coal particles as they are heated, revealing the transient swelling behaviours of the particles and enabling differences between coal types to be measured. This project aims to improve the understanding of how the swelling of coal particles impacts Blast Furnace performance.

DP600

Fig. 1 Optical micrograph of a dual phase steel, DP600, microstructure consisting of a matrix of ferrite and islands of martensite.

Development of an electromagnetic sensor system for characterisation of advanced high strength strip steels

PhD Student: Mohsen Aghadavoudi Jolfaei
Supervisor: Prof Claire Davis
Start date: February 2015

It is important to be able to detect microstructural changes in the production line during steel processing as the mechanical properties of steels are linked to their microstructure. Parameters such as grain size, phase balance and precipitates play a vital role in controlling the properties of steel. Conventional characterisation methods for microstructures, such as optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are generally destructive, requiring a small sample to be taken, and time consuming, due to the sample preparation processes and image analysis especially when a quantification of the phase fractions is required.

Electromagnetic (EM) sensors are a nondestructive method that has been shown to be able to measure the ferrite and martensite phase fractions in dual-phase steels, figure 1. Correlations between the EM sensor signal and mechanical properties (tensile and yield stress) have also been found for a limited number of DP600, DP800 and DP1000 samples. However, a much larger range of samples needs to be studied to enable the development of a calibrated sensor that can provide quantitative measurements for the full range of DP steels. Such a sensor would allow a fast and cheap quantitative, yet non-destructive, evaluation of the phase fractions, from which both product development and quality control can benefit greatly. In addition, extending the technique to a broader range of steel grades, where other microstructural features (compared to ferrite and second phase fraction) provide a significant contribution to strength, requires greater understanding of the role of the different microstructure parameters on the magnetic properties, and hence EM sensor signal. This project will assess a wide range of laboratory heat treated and commercial DP steels, including considering variability due to small changes such as across strip width and between coils. Other steel grades, such as microalloyed and interstitial free steels, will also be considered.

grains

Grain sizes in a high stength low alloy (HSLA) steel that has been continuously cast as slab (left) and a high Al steel that has been laboratory ingot cast (right).

Direct Casting of High Al Steels

PhD Student: Neil Hollyhoke
Start date: October 2014
Supervisors: Dr Carl Slater; Prof Claire Davis

Most strip steels produced today are made using continuous casting techniques, where a continuous slab is produced, typically between 50 and 250mm thick and is sectioned then later reheated and hot rolled down to strip. This is an energy intensive process and energy savings of around 80% are possible though casting directly, or at least close to, the end products required thickness. However, ensuring that the required properties are achieved in the as-cast (minimally deformed) strip is a challenge. Additionally some steel grades, such as those with a large amount of aluminium, show promising properties for strip applications (e.g. low density for automotive use) but are extremely difficult or expensive to produce at an industrial scale with conventional continuous casting techniques; direct casting may offer a way to make these steel grades more commercially viable.

Most metal objects are made of many individual crystals called grains and the size of these grains plays an important role in determining the mechanical properties, a reduced grain size improves both strength and ductility. Conventional techniques cast metal with moderately sized grains then as it is rolled the grain size is refined by recrystallisation. Another way to reduce grain size is to solidify the material faster; direct casting to strip gives cooling rates several orders of magnitude higher than in continuous casting, leading to smaller grain sizes. While this processing route offers a lot of potential in terms of energy saving and producing new steel grades, direct strip casting is very technically challenging, hence its current underdeveloped state. One issue is that the combination of high surface area to volume ratio and increased atmospheric exposure of both the liquid and just solidified steel make atmospheric interactions far more serious so an inert atmosphere must be maintained. Another is the level of control and consistency necessary to produce an acceptable quality product, even very small inconsistencies in the strip can lead to major quality issues. The drastically reduced thickness also means that a much greater length of material must be produced in a given time in order to compete with the volume of output from a conventional continuous casting facility.

In this work the effect of high solidification rates on the microstructure development (grain size and segregation) in high strength low alloy (HSLA) and high Al steel grades are being considered.

EBSD

EBSD images showing the deformed austenite grain structure (left) and the recrystallised grain structure (right) in the Fe-30Ni-0.44Nb steel.


TEM

TEM image showing precipitates in a partially recrystallised sample.

Effect of precipitation during recrystallisation of HSLA steels

PhD Student: Mo Ji
Supervisor: Prof Claire Davis
Start date: October 2013

The excellent mechanical properties (both strength and toughness) of HSLA steels has meant they are used extensively in the structural and transport industries and for oil and gas pipelines. This is achieved by generating a fine uniform grain microstructure via the precise control of recrystallization during the rolling process. HSLA steels often contain small quantities of Nb, V and Ti, i.e. strong carbo-nitride formers, which affect the grain size development through grain boundary pinning on reheating and retardation of recrystallization by precipitation pinning and solute drag. Niobium is the most influential element on recrystallisation in HSLA steels.

The crystalline defects generated during the rolling process provide the driving force for recrystallization, and can also considerably accelerate precipitation, i.e. strain-induced precipitation (SIP), since the dislocations act as nucleation sites for precipitates. The precipitation nucleation rate, density, size and dispersion properties for SIP differ from that of precipitation in undeformed austenite. The interaction between SIP and the deformation substructures affects the evolution of recrystallisation. Therefore, to precisely control the final mechanical properties of HSLA steels through grain size refinement via recrystallisation, the effect of SIP evolution, preceding or during recrystallisation, on the recrystallisation process needs to be understood.

The aim of this project is to investigate the nucleation and growth mechanism of strain induced precipitation during and after deformation and to quantify the interaction between recrystallisation and precipitation. The work is using a Fe-30Ni alloy, containing Nb, as this material retains its austenitic structure to room temperature allowing the deformation structures seen during high temperature rolling to be retained. Examples of deformed and recrystallised grain structures are shown below, along with initial observation of strain induced precipitates.

Figure 1: Schematic of hot strip mill

Fig. 1: Schematic of hot strip mill

Figure 2: EM (eddy current) sensor integrated into the roller table of a hot strip mill

Fig. 2: EM (eddy current) sensor integrated into the roller table of a hot strip mill

Quantification of Microstructure in Plate Steels using EM sensor Technology

PhD Student: Will Jacobs
Start date: October 2015
Supervisors: Prof Claire Davis and Frank Zhou
Partner: Primetals Technology Ltd

The ability to non-destructively characterise metal microstructures in-situ during hot processing offers enormous advantages to metals producers for dynamic feedback control and optimisation. Hot rolling and controlled cooling during steel production is used to develop the desired microstructures and currently feedback is achieved via temperature measurement coupled to mill models to indirectly determine microstructure development. The ability to directly measure microstructural parameters on-line therefore represents a significant step forwards; in this project electromagnetic (EM) sensors for on-line inspection are considered.

EM sensor signals are sensitive to changes in the relative permeability of a steel sample, where the relative permeability is determined by the steel microstructure, temperature and presence of applied or residual stresses. EM based sensors are currently used in the steel industry to assess the condition of strip steels off-line or in mill locations where the strip is cold. Primetals Technology Limited is currently developing an EM system for hot steel inspection, with systems being trialled in the cooling zone after hot strip rolling for phase transformation fraction measurement. The key to developing this technique in the industrial environment for a broader range of microstructural characterisation is to understand and interpret the meaning of the EM sensor signal in the context of the steel microstructure. This is not straightforward, as it has been demonstrated that a number of microstructure constituents influence the measured signal. For example grain size, precipitates, texture, phase balance and distribution. In addition, when considering plate and sections the influence of non-uniform through thickness microstructure (and temperature for in-situ testing during hot deformation or cooling) needs to be considered.

SEM Image of a Low Density Steel sample, oxidised in Air for ~200mins, showing Al and Fe oxide layers.

Fig. 1 SEM Image of a Low Density Steel sample, oxidised in Air for ~200mins, showing Al and Fe oxide layers.

Surface Evolution and Coatability of Low Density Steels

PhD Student: Jack Isaacs
Start date: February 2016
Supervisor: Prof Sridhar Seetharaman; Prof Barbara Shollock; Dr Michael Auinger
Partner: Tata Steel

A high amount of Al (~7 wt%) and Mn (~6 wt%) in low density FeMnAlC steels make them very different from the conventional steels. The highly alloyed Al and Mn particularly influences the ability of galvanising because of the formation of more stable (Mn,Al) O type oxides. Therefore, the coating ability of FeMnAlC steels will be different from the conventional steels.

In addition, the Al in the steel may also react with N, which can be present as an impurity in the bulk steel or in the gas-atmosphere in which the steel is cooled, re-heated and rolled. Aluminium nitride precipitates could affect the coating ability and result in detrimental mechanical properties. This project aims to investigate the surface microstructure and its effect on coatability in ferritic and duplex LDS alloys with composition ranges being Al 2 to 10 wt% and Mn up to 6 wt%.

Specific objectives are:

  • To develop a fundamental understanding of the selective oxidation behaviour of low density steels with high Al and low to medium Mn in annealing atmospheres
  • To understand the formation mechanism of nitride inclusions in annealing atmospheres.
  • To investigate the effect of the selective oxides and nitride inclusions on the coating behaviour and surface quality

The outcome of the project will be the gained knowledge of the surface evolution in high Al steels under conditions relevant to industrial conditions. This should aid in the strategic goal for developing coated LDS.

ISS

A schematic diagram of Materials Science Laboratory Electromagnetic Levitator (MSL-EML) on the ISS



Thermophysical properties of low-density steels under microgravity conditions

PhD Student: Antonia Betzou
Supervisors: Prof Sridhar Seetharaman, Dr Prakash Srirangam; Dr Michael Auinger
Industrial Partner: Tata Steel Europe
Organisation Partner: European Space Agency (ESA)

Lightweight materials are of growing importance in automotive applications to meet the challenges of fuel efficiency and CO2 emissions. Therefore, steel is facing a great challenge from aluminium and magnesium alloys. Fe-Al & Fe-Al-Mn based low density steels that are defined as Advanced High Strength Steels (AHSS) are being considered.

Fe-Al & Fe-Al-Mn alloys typical consist of aluminium in the range of 3-12% with low amounts of carbon. The presence of aluminium in steel significantly reduces the density of the steel. The concept looks simple, but there are lot of challenging issues to be understood, for example chemical containing reactivity in the liquid state, solidification behaviour, cracking during casting and hot rolling, high temperature oxidation etc. Since the amount of aluminium is high, the steel will have a tendency to form oxides and nitrides etc. which would affect the casting and solidification conditions. Oxide inclusions in steel melt are harmful and lead to problems in process control and product quality in steel castings. Oxide inclusion is usually a result of unwanted mechanical or chemical interactions between the molten metal and its surroundings in the refractory containers. The evolution of the oxide precipitates in steel melts, in terms of structure, is an important issue, since oxide size is known to be an important factor in mechanical properties of the final steel. It was also observed that different morphologies of oxides and nitrides were formed at different oxygen and nitrogen levels. The formation of oxides and inclusions alter the melt physical properties is essential for improving the physical, mechanical and performance properties of these steels.

In this project, the basic objective is to determine the thermophysical properties of Fe-Al & Fe-Al-Mn alloys in terrestrial conditions and compare them with the thermophysical properties under micro-gravity conditions where there will be no convection, no chemical reactivity and no oxygen- effect. The first stage of the project is to measure the thermophysical properties of Fe-Al & Fe-Al-Mn alloys at different aluminium concentrations and temperatures above the liquidus temperature in terrestrial conditions. Further, these alloys will be characterised to understand the microstructure and mechanical properties. The same set of alloys will be used to determine the density and viscosity in micro-gravity conditions under controlled oxygen environments. These alloys after being tested at ISS, will be characterised to understand the microstructural changes to micro-gravity.

Yuyi Zhu

Effect of internal oxidation in bendability of UHSS

PhD Student: Yuyi Zhu
Start date: January 2015
Supervisors: Prof. Barbara Shollock, Daniela Proprentner
Industrial supervisor: Dr. Wanda Melfo, Dr. Vladimir Basabe
Partner: Tata Steel Netherlands

Ductile Ultra High Strength Steels (UHSS), the so-called third generation of Advanced High Strength Steels (AHSS), are increasingly used for automotive applications. However, the strengthening alloying additions in these steels may have consequences for the production of the final product. The internal oxidation zone (IOZ) formed initially during coiling of the hot rolled product will lead to surface defects of the galvanized steel sheet. This phenomenon is known to affect the formability of the steel sheet.
This project focuses on simulation of the surface and subsurface deformation during the steel sheet forming process by using the in-situ 3-point bending stage and the digital image correlation (DIC) concurrently. The microscopic strain distribution and failure mechanism are monitored to establish a link between the near surface conditions and the formability of the final product. The internal oxidation depth varies within the steel coil owning to the uneven cooling rate at different locations. Samples with different internal oxidation depth are tested to find out the critical conditions that lead to failure during forming process.
On the other hand, simulation of surface and subsurface reactions that lead to the formation of the internal oxides. The internal oxidation depth largely depends on the temperature, atmosphere, top surface composition and alloying additions. Rolling, hot coil cooling, pickling, cold rolling and annealing were simulated under lab conditions to create a better understanding of how internal oxides develop during the whole strip steel production process. Microstructural characterization, including EDS, EBSD and STEM, were used to determine the nature of the external and internal oxides that formed throughout the steel processing.

Lewis Yule

Figure. (a) Schematic of the SECCM set up. (b) SEM image of an SECCM scanning area. (c) Tafel plots of individual scanning points on highlighted line in (b).

Scanning Electrochemical Cell Microscopy: A new method to carry out highly localised corrosion measurements on steel surfaces.

PhD Student: Lewis Yule
Start date: October 2015
Supervisors: Prof. Barbara Shollock, Prof Pat Unwin

Corrosion is an age-old problem that continues to plague modern society. It has well-known detrimental effects on human health and environmental safety, in addition to being a great economic burden worldwide. Indeed, it has been reported that 3-4% of global GDP is spent on issues caused or related to corrosion. Corrosion is caused by the formation of local cathodic/anodic sites or “corrosion cells” on a metal surface, which arise due to the presence of local heterogeneities (e.g., grains of different orientations, grain boundaries, inclusions and surface defects) that exhibit different electrochemical behaviour when exposed to a corrosive (electrolyte) solution. Despite being an area of ongoing and active research, the corrosion mechanisms occurring at these heterogeneities are not completely understood, largely due to the highly localised nature of these features.
Corrosion is traditionally studied with macroscopic electrochemical techniques (e.g., cyclic polarization and electrochemical impedance spectroscopy), carried out on polycrystalline metal surfaces on the millimetre to centimetre length scale. The response measured from such an experiment is an ‘average’ of the entire exposed surface and thus do not provide any local electrochemical information, which, as highlighted above, is crucial for understanding the role of certain surface features in the overall corrosion process. For this reason, in recent years, there has been a drive to develop localized techniques that are able to extract spatially resolved electrochemical (i.e., corrosion) information on the nanometer to micrometer length scales.
Scanning electrochemical cell microscopy (SECCM), developed by the Unwin group, works on the principle of confining electrochemical measurements to a small area of the surface, allowing direct, localised (spatially resolved) investigation of (semi)conductive electrode substrates. SECCM has been successfully applied in the study of the relationship between structure and function in a wide range of materials at the nanoscale (e.g., electrocatalysts and sp2 carbon materials), but has not yet been employed to study corrosion. With its previous success in other fields, it follows that SECCM should be the ideal tool for carrying out direct micro- and nanoscale corrosion measurements, particularly when used in conjunction with other microscopy techniques in a correlative multi-microscopy approach.
This project investigates the suitability and potential of SECCM as a tool to investigate highly localised corrosion mechanisms with the long-term aim of applying the method to industrial interests, for example, testing the effectiveness of steel coatings.

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fanfu_wu_2.jpg

EBSD misorientation map showing the strain partitioning effect into the grain structure at heavily deteriorated cut edge. (Local Misorientation Maps, the cut edge is at the bottom of each map. Green colour is representative of higher local misorientation only)

The effect of microstructure, stress, and temperature on the magnetic properties of electrical steel

PhD Student: Fanfu Wu
Supervisor: Prof Claire Davis, Dr Frank Lei Zhou
Start date: February 2017

Electrical steel is an excellent magnetic soft material that has specific desired magnetic properties: such as low losses, and high permeability. There are two types of electrical steels, grain oriented (GO) and non-grain oriented (NGO), for two main applications: transformers and rotating machines. Grain oriented electrical steels have excellent flux-magnifying properties in certain direction, therefore GO electrical steel are used as the cores of transformers where high rates of flux change can be achieved to construct the efficient transformers. Non-grain oriented electrical steels have excellent isotropic magnetic properties, which give improved magnetization ability in any spatial direction. Thus, NGO electrical steels are used to construct the rotor and stator cores of the electric motors.
Microstructure controls the intrinsic materials properties of NGO electrical steel, with texture and grain size being the most influential parameters. Stress / strain and temperature affect the magnetic properties of the material and it is known that during manufacturing of NGO electrical steel stators and rotors, mechanical cutting or shear cutting can introduce plastic shear stress and residual stress into the material. The working environment of NGO electric steel in motors also involves additional (elastic) stress levels and temperature change. Current motor designs make assumptions and simplifications to account for the above factors, which causes inaccuracy in performance prediction with larger performance fitting factors. Therefore, the focus of this work is on how the variable magnetic properties due to differences in microstructure and environment can be modelled, both in a microstructure magnetic model and in a component level model, which could be included in motor designs.
This project’s research focus is on NGO electrical steel in an electric motor, in particular the influence of microstructure (grain size and texture), component processing (cutting / stamping) and operating conditions (stress and temperature) on the magnetic performance

james_mathew.png

Recyclability of aluminium and aluminium alloys

PhD Student: James Mathew
Start date: 03/10/2016
Supervisors: Dr Prakash Srirangam, Professor Mark Williams, Dr Greg Gibbons

Recycling of Al is gaining importance in automotive and aerospace industries. Presence of iron in Al alloys hinders the application of recycled alloys in automotive and aerospace. Iron in aluminium alloys will be present as hard and brittle intermetallics which hampers the mechanical and performance properties. So, it is essential to understand the behavior and effect of iron intermetallics in order to make them useful for industrial applications. The aim of the project is to characterise the effect of iron content on the microstructure and mechanical properties of Al alloys.

We seek to use DFT modelling to design an optimal smooth hardness transition zone (Red) for heat treated materials. Avoiding sharp decreases in hardness which can result in the 'egg shell effect' (Black), or scenarios where the surface hardness barely differs from the core hardness rendering the treatment redundant (Blue).aurash_karimi.png

Modelling of Nitrogen Transport in Iron Lattices with Point Defects

PhD Student: Aurash Karimi
Start date: October 2016
Supervisors: Prof Claire Davis; Dr Michael Auinger

The application of heat treatment methods, such as Nitriding and Nitrocarburizing, increase resistance against wear and corrosion for iron surfaces, whilst maintaining ductility in the core metal. Such properties are, in essence, achieved as a result of the iron being left with a hard surface and a soft interior after treatment. Problems can occur when the transition from the hard surface to the soft core does not happen over sufficient depth, so that the hardness profile of the iron contains sharp decreases. In particular, a prevalent issue in heat treatment is the so called ‘egg shell effect’, where the hard brittle surface resulting from heat treatment lacks sufficient support, and hence will fracture under relatively low impact. Models that accurately predict hardness profiles given heat treatment parameters and microstructural details of the material to be treated could reduce the occurrence of such undesirable effects, as they allow for finer control over the so-called hardness transition zone.
This project seeks to adopt Density Functional Theory (DFT) to initiate a new wave of robust modelling options for Nitriding and Nitrocarburizing. Although DFT requires considerable computational effort, it appears to have advantages over the phenomenological and empirical models in current use. For example, the effect of point defects (primarily vacancies) and nitrogen displacement by carbon are thought to have a significant effect on nitrogen transport during treatment, but these effects have yet to be incorporated into existing models. This could be because reliance of empirical data at the macroscale in order to model microscopic effects is inherently problematic, as existing data can deviate from theoretical predictions, and experiments to generate new data are costly. In contrast, poorly understood atomic level effects could be incorporated into models in a natural manner using DFT, as it can be used to simulate microscopic events such that their influence at the macroscale can be directly interpolated. This approach should lead to more advanced modelling options for heat treatment processes, which could then be advantageously be adopted at the industrial level.