Cambridge 7th to 9th September
Presenting Author:
Bridget Stewart
<bridget.stewart@mpiuk.com>
article posted 27 Apr 2015
Bridget Stewart
Dr Bridget Stewart is a Principal Scientist with the Materials Processing Institute.
She studied Chemistry at the University of Newcastle followed by a PhD Scholarship
in Glass Chemistry at the University of Aberdeen. Her PhD thesis, supervised by
Prof. John Duffy, concerned the optical basicity of sites for metal ions in oxide-based
glasses. The work was linked to metallurgical slags and she joined British Steel as a
slag chemist at the Scunthorpe Works before moving to the British Steel Research
centre in Teesside. She has since gone on to lead Corus's and Tata Steel's research
work in her specialist fields of continuous casting mould powders, as-cast surface
quality and oxide scale formation. This has included many international multi-partner
projects, with other research institutes, universities and steel companies in Europe,
which she has led as the Project Coordinator.
Dr Stewart spent two years as the Senior Lecturer in Materials Engineering at Teesside
University and combines her current role at the Institute with that of Visiting Lecturer
at Teesside. She is an Expert Evaluator in the Steel Area for the European Commission
and in 2011/12, she was President of the Cleveland Institution of Engineers (Affiliated
with IOM3).
Scattering light using glasses and crystals in order to optimise
the surface quality of continuously cast steels
Bridget Stewart (née Harris)
Materials Processing Institute, Eston Road, Grangetown,
Middlesbrough, TS6 6US, UK .
<www.mpiuk.com>
A target of all continuous casting operators is the ability to cast all grades free of surface defects.
Mould powders are key to the control of initial metal solidification and optimisation of
continuous casting mould operations. These powders are a mixture of several mineralogical
components and carbon. Typically they comprise
CaO, SiO2, Na2O,
Al2O3, F and C
with minor amounts of metal oxides such as MnO, Li2O,
MgO and Fe2O3.
The powders are fed onto the surface of the molten steel, where upon they begin to melt,
forming first a sinter layer then a mushy layer and eventually a liquid flux pool.
Liquid slag from this pool infiltrates the gap between the mould and newly solidified
steel strand. Some of this lubricates the newly-formed steel shell. However, most of the
liquid freezes against the water-cooled copper mould, to form a slag film, which may
crystallise over time. The phase transformations and glass:crystalline ratio in this film
are critical to the rate of heat transfer from steel to mould. The aim of the mould powder
investigator is to tune the powder properties to suit the casting conditions and steel type.
Traditionally, fluorine has been added in order to generate cuspidine
(3CaO.2SiO2.CaF2)
as the major crystalline phase precipitating from the predominantly
CaO-SiO2
glassy slag. However, fluorine has been found to have a detrimental effect on the
environment and equipment life. This paper discusses recent and current approaches
to optimising the scattering of radiation in order to control the rate of heat transfer,
with a focus on reducing the need for fluorine.