Oxidation and Descalability of Electrical Steels
PhD
Supervisor: Prof Barbara Shollock
Project:
Processing strip steel products from as-cast reheated slabs with optimum surface quality poses
unique challenges especially for the case of electrical steels, where attainment of a defect-free
surface combined with excellent magnetic properties is of critical importance.
The challenges arise from numerous factors, e.g. the feedstock/strip moves several metres per
second through the rolling stages and also experiences a range of reheat temperatures and gas
environments with varying dew points together with cycles of cooling and deformation. These
diverse conditions impact the surface of the steel by influencing the oxides formed and their
subsequent behaviour.
The nature of the oxides formed during reheating is mainly influenced by (i) alloy composition,
especially Si and Al in the case of electrical steels and (ii) the heating profile - time (which can be
from seconds to hours), temperature, heating rate and atmosphere. The evolution of these oxides
during steel processing is also influenced by high-pressure descaling, growth, deformation and
transformation during rolling and subsequent cooling. In some steels, external and internal
oxidation occurs, and understanding the oxidation processes, the metal/oxide interface and the
effect of oxidation on the underlying steel microstructure will help gain insight into the mechanical
response of the steel in terms of descalability, oxide scale behaviour under deformation and
downstream finishing processes (pickling, annealing and cold rolling).
This project will study, through a combined approach of experimental and modelling work, the
factors influencing the kinetics and development of oxides on steels grades, especially NGO electrical
steels (Non Grain Oriented) during primary reheating (1100-1200oC) in both natural and mixed gas.
The research provides a basis for understanding the descalability and regrowth of secondary scale of
these steels and their decomposition/internal oxidation upon cooling. The PhD will especially focus
on interface and internal oxidation issues with the view to develop key descriptors and underlying
mechanisms that will feed into alternative and beneficial routes for controlling oxidation of electrical
steels.
To understand this, the diffusion of the oxidizing and the oxidizing species through the metal matrix
or the oxide scale will be studied to determine the rate controlling process. This will be
accomplished using tracer gases and secondary ion mass spectrometry in simulated process
conditions. The morphology of the internal and external oxidation will be examined using FIBFEGSEM microscopy and details of elemental segregation will be determined using chemical analysis
in the STEM. Thermo-dynamic predictions of oxide stability will also be carried out to inform
internal oxidation/diffusion models developed at Tata Steel UK Swinden Technology Centre. The
influence of internal oxidation and formation of intrusions at the scale-metal interface on
bendability will be examined using in-situ testing in the SEM with digital image correlation.
From knowledge developed during reheating , descaling and cooling, guidelines will be developed
leading to more robust prediction and control of oxidation which will complement ongoing work
carried out at Tata Steel Europe R&D.
Entry Requirements :
Due to funding regulations application is restricted to UK students only.
Applicants should possess a first degree of at least a second class honours standard or the
equivalent.