Crack prevention in continuous casting

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EUROPEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
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technical steel research
Steelmaking
Crack prevention in
continuous casting
h
Report
9
EUR 18558 EN STEEL RESEARCH EUROPEAN COMMISSION —ri
Edith CRESSON, Member of the Commission
responsible for research, innovation, education, training and youth
HB'
DG XII/C.2 — RTD actions: Industrial and materials technologies —
Materials and steel
Contact: Mr H. J.-L. Martin
^ 4 Address: European Commission, rue de la Loi 200 (MO 75 1/10),
B-1049 Brussels — Tel. (32-2) 29-53453; fax (32-2) 29-65987 European Commission
technical steel research
Steelmaking
Crack prevention in continuous casting
B. Patrick, M. W. Short, R. Walmsley, B. Barber
British Steel Teesside Technology Centre
PO Box 11
Eston Road
Grangetown
Middlesbrough
United Kingdom
K. Harste
AG der Di I linger Hüttenwerke
Postfach 1580
D-66748 Dillingen
K.-H. Tacke, I. Steinert
Max-Planck-Institut für Eisenforschung
Max-Planck-Straße 1
D-40237 Düsseldorf
Contract No 7210-CA/833/168/167
1 July 1993 to 30 June 1996
Final report
Directorate-General
Science, Research and Development
1998 EUR 18558 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-4902-3
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER Final Project Summary
The project has the objective to investigate cracking on continuously cast steel
and to find ways to reduce surface and internal cracking. The methods of this
research include measurements and observations on casting machines as well as
mathematical modelling approaches.
At British Steel Technology levels of strain have been determined from casting
parameters and machine condition and related to levels of intercolumnar cracking
during slab casting, thus enabling critical levels of strain which cause intercolumnar
cracking to be determined.
The project involved measurements of machine condition (from strand condition
monitor data) and strand bulging, along with corresponding casting conditions.
These measurements, together with those of sump position and strand support
movement and strains during casting, have been used to obtain a quantitative
understanding of the factors affecting strains at the solid/liquid interface during
normal casting operations. Internal quality data have been collected over a six year
period to relate calculated levels of strain to intercolumnar cracking with respect
to steel grade.
State of the art computer models have been developed for the calculation of strand
temperatures, shell thicknesses, bulging and roll misalignment strains. Therefore,
from the measurements of machine condition and casting parameters it was possible
to calculate the resulting levels of strain at specific roll positions. When these
calculated strain levels were compared with the data on internal quality, critical
levels of strain which cause intercolumnar cracking could be determined. These
data will be of use in machine design and enhancement studies and in improving
current operations.
Dillinger Hüttenwerke has established thermal analysis of the casting process to
investigate the causes of surface and internal cracks. A nozzle test device has
been used to measure the water impact below spray nozzles. The profiles have
been fitted by Gaussian functions. The thermal computation was then performed
in high resolution including the features of individual nozzles. The model results
have been checked by temperature measurements at various locations in the caster;
they were found to be in good agreement.
Three nozzle arrangements of a billet caster have been numerically studied. The
thermal state of the strands strongly depends on specific casting conditions and
nozzle types. Overcooled regions can be distinguished at the corners in the first case
and near the face centres in the second case. Crack observations on cast billets are
related to these findings. Internal cracking observations have been analysed from sulphur prints.
Further work has focused on the investigation of surface cracks (micro cracks) on
slabs cast on two vertical machines. The appearance of these cracks has been
described. The influence of A1N precipitation on the embrittlement of the steel
and the crack susceptibility has been investigated by thermal modelling which was
coupled with the actual ductility behaviour of the steel. Also, high resolution
thermal analysis of both casters has been carried out. The comparison of both
casting machines has demonstrated how micro cracking is related to the water
cooling and how they can be avoided by suitable secondary cooling patterns.
Max-Planck-Institut has developed mechanical modelling to investigate the effect
of the secondary cooling on the crack formation in continuous casting. The finite
element model tracks a section of the strand through the machine. Using high
resolution temperature fields the stress and deformation state of the strand section
was analysed. Realistic material properties have been applied with special regard
to phase transitions; plastic deformations were modelled by creep laws. Contact
boundary conditions are introduced to model the strand support in the mould and
at roller positions. Two visualization methods were developed to identify criti­
cal regions for surface cracking; both use the simultaneous occurrence of elevated
tensile stresses and low material ductility as cracking criterion. Internal cracking
problems were evaluated by plotting strains at the phase front.
The model has been applied to study crack formation on billet and slab sections.
The mechanical models predict stresses and can be used to explicitly locate nozzles
and ranges of the strand which are critical for surface cracking. The predicted
cracking patterns were in agreement with the empirical observations. Also, regions
of the strand with increased internal cracking risk became visible. They were
correlated with empirical cracking observations.
When the model was applied to slab casting, surface regions of increased cracking
probability could be identified. The predicted cracking width profile agreed quali­
tatively with the findings on slabs. Contents
List of Tables 9
List of Figures 11
1 Introduction'
2 Work done at British Steel Teesside Technology Centre 20
2.1 Measurement of Machine Condition 2
2.1.1 Roll Gap Error and Roll Bend0
2.1.2 Backface and Topface Misalignment1
2.2 Collection of Plant Data4
2.2.1 Selecting Casts for Analysis
2.2.2g the Strain Value 2
2.3 Analysis of Results
2.4 Calculation of Strain6
2.4.1 Development of Methods of Deriving Simple Equations for
Rapid Calculation of Strains
2.4.2 Calculated Strains (Equation Method) and Slab Quality . . 28
2.4.3d Strains (Finite Element Method) and Slab Quality 28
2.4.4 Comparison of Strains Calculated by the Finite Element and
Equation Methods 29
2.5 On-Plant Measurements 31
2.5.1 Sump Position
2.5.2 Strain Gauge Measurements
2.5.3 Measurement Technique2
2.5.4 Measurements before Enhancement
2.5.5s aftert 33
2.6 Conclusions 38
3 Work done at Dillinger Hüttenwerke 40
3.1 Thermal Model 4
3.1.1 Nozzle Test Device1
3.1.2 Material Properties2 3.1.3 Boundary Conditions 42
3.1.4 Model Check3
3.1.5 Results and Discussion4
3.2 Surface Cracking5
3.2.1 Corner Cracks . 4
3.2.2 Face Centered Cracks6
3.2.3 Optimized Cooling
3.3 Micro Cracking 47
3.3.1 Micro Cracks - Look and Occurrence 4
3.3.2 Influence of A1N Precipitation8
3.3.3e of Secondary Cooling on Micro Cracking 50
3.3.4 3-D Thermal Modelling for Slab Casting 52
3.3.5 Conclusions 5
3.4 Internal Cracking3
4 Work done at Max-Planck-Institut4
4.1 Numerical Model
4.1.1 Basic Assumptions
4.1.2 Modelling of the Growing Shell 56
4.1.3 Element Type and Element Discretization7
4.1.4 Implicit Creep Algorithm, Convergence Acceleration 5g
4.1.5 Boundary Conditions9
4.2 Modelling of Material Behaviour 61
4.2.1 Transformation Temperatures
4.2.2 Young's Modulus, Poissons's Ratio 52
4.2.3 Density, Thermal Strains go
4.2.4 Creep Law¿
4.3 Results gg
4.3.1 Surface Cracking of Billet Sections
4.3.2 Internalg of Billets
4.3.3 Slab Analysis 71
4.4 Conclusions ~* Table of Symbols
References 79
Keywords 83
Figures5