Advanced Inspection of Surface Quality in Continuously Cast Products by Online Monitoring

Continuously Cast Products by Online
Monitoring
2018
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Acknowledgements
I would like to express my gratitude to the European School of Materials (EUSMAT)
for the financial support to carry out this Master Programme, and to the AMASE
Master Program Secretary for their constant support during these two years.
I would like especially thank my supervisors Esa Vuorinen and Pavel Ramirez Lopez
for their guidance and support during this semester. In addition, I would like to give
my sincere thanks to Rosa Pineda and Pooria Jalali that have always been there to
help, for their endless support.
In addition, I would like to thank Swerea MEFOS for the opportunity to carry out my
master thesis in this research institute and the support during the project.
Finally, to everyone that has contributed to the development of this project,
technicians at Swerea MEFOS and friends and colleagues both at MEFOS and LTU,
whom not only contributed with productive discussions but also have been there to
support me along the way.
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Abstract
The present Master’s Thesis is dedicated to the study of a laser scanning system and
its applicability for the detection of surface defects at the end of a casting machine in
a steel production plant. Both room and high temperature trials were carried out on
different carbon and stainless steel billets and slabs. For the high temperature tests,
the samples were heated until a maximum temperature of 1200 C. For all trials, the
surfaces were scanned with a blue laser sensor in order to generate a 3D
representation of the as-cast product surface.
The applicability of a blue laser sensor was proven for carbon and stainless steel
surfaces, both at room and high temperature. Defects such as depressions and
oscillation marks were detected, as well as some small corner cracks. Furthermore,
the full transversal section of billets and slabs was reconstructed from different scans
of the faces of the products.
The effect of different scanning parameters on the resolution of the scans and the
final results were analyzed and discussed with special focus on the scanning
strategies that would be optimal for the industrial application of the sensor.
A microstructural analysis was carried out in order to correlate the subsurface
microstructure with the presence of depressions in the edges of a duplex stainless
steel slab. The as-cast structure of columnar and equiaxed grains was clearly
observed, and some special features were analyzed and discussed. Nevertheless, no
clear correlation between the subsurface microstructure and the presence of the
defect was found.
List of Figures
Figure 1 Schematic representation of a continuous casting machine. After (4). ...................................... 15
Figure 2 Typical possible process routes in the steel production. ............................................................. 17
Figure 3 Typical process route for the stainless steel production. Adapted from (9). ............................. 19
Figure 4 High-temperature region of a pseudobinary phase diagram for duplex stainless steel
compositions. The shaded region represents the composition of commercial alloys. Extracted from
(19). ............................................................................................................................................................... 21
Figure 5 Typical solidification structure observed in the transversal section of an as-cast steel. Chill
zone, columnar zone, and equiaxed center region. Extracted from study material from the master
program course “Tecnología Metalúrgica” at Universitat Polytecnica de Catalunya 2017. ..................... 23
Figure 6 As cast microstructures of a stainless steel (a) 316 (austenitic) and (b) 430 (ferritic). The
presence of the equiaxed central region can be observed in the ferritic as-cast structure while it is not
observed in the austenitic. Extracted from (23). ........................................................................................ 23
Figure 7 Schematic representation of the laser scanning equipment. Adapted from (31). ..................... 27
Figure 8 ScanCONTROL 3D-View 3.0 software interface (Micro-Epsilon). ............................................ 28
Figure 9 Schematic representation of the ”z coordinate” and ”moment 0” color codings. It is possible to
note how the ”moment 0” representation provides a realistic image of the object. Examples provided
by Senso Test. ............................................................................................................................................... 29
Figure 10 ScanCONTROL 2960-100/BL measuring range. Dimensions in mm. Extracted from (31). . 30
Figure 11 Examples of measuring fields defined in the sensor ScanCONTROL 2960-100/BL manual. 30
Figure 12 Maximum collection frequency for some of the defined measuring fields. Note that the
smaller the measuring field, the higher the allowed collection frequency. .............................................. 32
Figure 13 Set up for the small scale tests. The equipment is placed in a cooling jacket (prepared for
high temperature tests) and installed in a milling table arm. Test sample is placed in the milling drill
table. ............................................................................................................................................................. 34
Figure 14 Schematic representation of the grinding machine used for the full scale trials. Provided by
Swerea MEFOS............................................................................................................................................. 36
Figure 15 Top and lateral schematic view of the elements during the scanning of the top surfaces of the
samples. ........................................................................................................................................................ 36
Figure 16 Schematic view of the elements during the lateral scanning of the samples. .......................... 37
Figure 17 Image of the grinding table were the blue laser is installed. ..................................................... 37
Figure 18 Images of some of the defects that were present on the surface of the steel products. Images
in the right side show a depression in the inner bow (wide face) of a stainless steel slab, with a sharp
edge at approximately 120 mm from the border (from the narrow face). Images in the left show a
depression in the outer bow (wide face) of the same stainless steel slab, where the sharp edge is not
present, but a more continuous curvature can be seen. In the interior of both depressions some
oscillation marks are visible. ....................................................................................................................... 38
Figure 19 Image of the slab from which samples were taken for metallographic analysis. ..................... 41
Figure 20 Schematic representation of the area from which the samples were taken for metallographic
analysis. ........................................................................................................................................................ 42
Figure 21 Results of a variation in the frequency of data collection. Both images show the same region
on the surface where a small piece of scale can be seen. Both images were collected at a scanning speed
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of 1 m/s and with 240 mm working distance; (a) 400 profiles per second, the image in the scanning
direction is constructed with approximately 280 points; (b) 50 profiles per second, the image in the
scanning direction is constructed with approximately 35 points.............................................................. 43
Figure 22 Example of how different scanning parameters can lead to the same resolution in the
scanning direction. (a) Collected with 1 m/s and 400 profiles per second; (b) 0.2 m/s and 80 profiles
per second. .................................................................................................................................................... 44
Figure 23 Resolution in the direction of the laser line remains constant even with a change in scanning
parameters. Both images were collected at a scanning speed of 1 m/s and with 240 mm working
distance; (a) 400 profiles per second; (b) 50 profiles per second. ............................................................ 44
Figure 24 (a) 2D representation of results for the smalls cale trials; (b) augmentation of a small area in
(a). The color coding represents the height of the sample. Dark areas correspond to valleys while of
brigth areas are associated with peaks. Oscillation marks are easily observed. ....................................... 45
Figure 25 Area of the sample with corner cracks; color coding indicates in dark regions of the sample
with low z values and in brigth regions of the surface with high z value. ................................................. 45
Figure 26 Area of the sample with corner cracks; color coding ”Moment 0” provides a realistic
representation of the surface. ...................................................................................................................... 46
Figure 27 Image of the grinding machine during the high temperature trials (left) and image of the
blue laser on the surface of a high temperature sample during the trials (right). ................................... 47
Figure 28 Results from high temperature full scale trials. Area around a depression in the inner bow of
2101 steel slab. .............................................................................................................................................. 47
Figure 29 Results from full scale high temperature trials. Complete scan of a 2101 steel slab with
depressions. The surface corresponds to the inner bow of the slab and the width of the scan is
equivalent to the width of the blue laser line.............................................................................................. 48
Figure 30 Results for room temperature full scale trials. Region of the surface of a 304 stainless steel
slab in which oscillation marks are visible. ................................................................................................ 48
Figure 31 Results for high temperature full scale trials. Region of the surface of a carbon steel billet.
Some scale at the bottom of the scan can be recognized and variations in the surface profile are also
detected......................................................................................................................................................... 49
Figure 32 Slab cross section constructed from multiple profiles of different faces for one of the grade
2101 slab sample. The height variations in the profiles are exaggerated to generate a better perspective.
Dimensions in mm. ...................................................................................................................................... 49
Figure 33 Billet cross section constructed for multiple profiles from different faces. The height
variations in the profiles are exaggerated to generate a better perspective. Dimensions in mm. ........... 50
Figure 34 Slab cross section constructed for multiple profiles from different faces for the grade 304
slab sample. The height variations in the profiles are exaggerated to generate a better perspective.
Dimensions in mm. ...................................................................................................................................... 50
Figure 35 Macro etching results for a 200x200 mm section of a stainless steel slab (left); augmentation
of an area in the narrow face in which a transition inside the columnar zone can be distinguished
(right). Marble’s reagent etching................................................................................................................. 51
Figure 36 Area of the cross section presented in Fig.30 in which the angle between columnar grains
and surface of the slab can be clearly seen. Marble’s reagent etching. ..................................................... 51
Figure 37 Microstructure of an area of the sample close to the surface (a); and microstructure in the
equiaxed region of the material (b). Etching with Beraha’s: Ferrite dark, austenite white. .................... 53
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Figure 38 Microstructure of the material in a large region (aprox. 30 mm from the surface). The
surface of the material is on the left while the equiaxed region can be noted on the right. Beraha’s
etching: Ferrite dark, austenite white. ........................................................................................................ 53
Figure 39 Microstructure of the material in a large region including the surface of the material in the
wide face. Beraha’s etching: Ferrite dark, austenite white. ....................................................................... 53
Figure 40 Results from a Thermocalc simulation of the amount of phases against temperature for a
steel within the compositional ranges presented in Table 5...................................................................... 54
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dy Distance between consecutive collected profiles in the scanning direction
vs Scanning speed
Cr Chromium weight percent
Mo Molybdenum weight percent
Nb Niobium weight percent
Ni Nickel weight percent
C Carbon weight percent
N Nitrogen weight percent
Nieq Nickel equivalent number
δ Delta ferrite phase
γ Austenite phase
V Velocity at which the solid phase grows from the liquid
K Thermal gradient
ASTM American Society for Testing and Materials
FEPA Federation of European Producers of Abrasives
CCD Charged Coupled Device
3. Background .............................................................................................................19
3.2. Stainless Steel Solidification during Continuous Casting ................................. 20
3.2.1. Solidification Structure ................................................................................ 22
3.2.2. Surface Quality ............................................................................................. 24
3.3.2. Laser Scanning ............................................................................................. 26
3.4. Metallographic Techniques ............................................................................... 33
4. Experimental Procedures ...................................................................................... 33
4.1.2. Procedure ..................................................................................................... 35
4.2.1. Apparatus ..................................................................................................... 35
4.2.2. Materials ...................................................................................................... 37
4.2.3. Procedure ..................................................................................................... 38
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1. Introduction
Continuous casting (CC) is an established technology for the production of steel and
is responsible for the solidification of most of the millions of tons of steel that are
produced in the world every year (1). In the continuous casting process, the liquid
steel is poured into a tundish and from the tundish it flows to a bottomless copper
mold, Figure 1. Once in the mold, the molten steel solidifies against the water-cooled
copper walls and forms a solid shell (1). The solidifying metal is withdrawn from the
mold at a given casting speed, which also matches the liquid metal flow into the mold
(1). The tundish holds the molten steel and provides a control flow of metal into the
mold (2), and while in slab casting usually one mold is served by one tundish, several
billet molds can be supplied by the same tundish. The mold level is a key parameter
in the casting process since it exerts a large influence on the liquid flow to the mold
and especially in the formation of vortex in the tundish, which could incorporate air
or slag in the melt (2). The liquid steel is transported to the mold trough pouring
nozzles located along the bottom of the tundish. The design of these nozzles controls
the volume and flow of the steel to the mold and also plays a key role in the fluid
control. Nevertheless, the mold is the most important component in the continuous
casting process (2). It has the primary function of extracting heat from the molten
steel as efficiently as possible. It is also continuously oscillating in the vertical
direction in order to avoid the adherence of the solidified metal to the copper surface,
with oscillation frequencies that can be in the order of 100-200 cycles per minute (2).
The frequency of mold oscillation together with the oscillation characteristics (mold
velocity, mold displacement, and time for upward and downward movement) has a
strong influence on the formation of surface defects, such as oscillation marks (3).
Figure 1 Schematic representation of a continuous casting machine. After (4).
The continuous casting makes possible not only high productivity but also lower
energy consumption, better labor efficiency and quality assurance when compared
Ladle
Tundish
Mold
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with earlier ingot casting techniques (1-3). Since its first appearance in the last
century, an extensive amount of work has been devoted to improve the productivity
as well as the quality and cost of steel products. From the first mold oscillation
introduced by Junghans (7) (with the purpose of overcoming the sticking of the
initially solidified shell) to the more recent introduction of continuous casting
modeling to improving the yield and quality of steel production (8), the list of
technological improvements is vast.
The development of refractories with better performance, the invention of refining
methods such as argon oxygen decarburization (AOD) and the vacuum oxygen
decarburization (VOD) are milestones in the development of continuous casting
technologies; as have also been the electromagnetic stirring, the better understanding
and design of the secondary cooling zone and, nowadays, the implementation of
advanced modelling to control and predict the quality of the steel produced (9).
Nevertheless, there are plenty of defects that still plague the industry and that affect
both the quality of the final product and productivity (9).
In a conventional steel plant, the continuous casting of slabs is usually followed by
the hot rolling, which not only reduces its width to approach the dimensions of the
final product but also breaks the casting structure and promotes chemical and
microstructural homogenization. After the liquid steel with the proper composition
has been obtained, the liquid is continuously cast to form billets, blooms or slabs. The
following step is the hot rolling of the steel product and different process routes
between the casting and the hot rolling are possible, Figure 2. The most energetically
efficient routes are the direct rolling, which consists the hot rolling of the material
directly after the casting, and the direct rolling, which involves an intermediate stage
in a furnace in order to control the temperature and add time and flexibility to the
processing. Other possible routes include the storage of the material either for a short
or a long term. Since the presence of severe defects in the cast products can lead to
problems during the hot rolling, both the direct rolling and the direct charging
require a cast product with a high surface quality (5). As a consequence, it is very
common that the billets and slabs after the casting are cooled down to room
temperature, inspected, ground if necessary, and then reheated to be hot rolled with
an evident increase in energy consumption (10).
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Figure 2 Typical possible process routes in the steel production.
An extensive effort has been done in the past years to better understand and predict
the formation of surface defects, which are usually generated in the mold or in the
secondary cooling zone (11, 12). The evolution of computer systems and the
invaluable work of technologists and researchers have made it possible to gain
understanding in the complexity of the high temperature solid-liquid interaction of
slag, molten steel and copper mold.
More than two decades ago, Brimacombe dreamed with the idea of the “intelligent
mold”, capable of “thinking” and taking process decisions based on the temperature
“feelings” and the “observation” of mold level and other process parameters (13).
Motivated by the necessity to empower the workforce with the knowledge of the
fundamental rules that govern the solidification of the metal and determine the
quality of the product, he had worked and developed the expert system CRAC/X. This
computer program, focused on both internal and external cracks, condensed a large
amount of information regarding billet defects, and linked the presence of one (or
more) of them with the casting parameters to help the machine operator in correcting
operational problems and improve the product quality (14). Naturally, the crack
length(s), and type(s) are user’s inputs in the program.
Further work has been done in this direction in the past years. The general concept is
that, with the help of computers, quality predictions can be me made considering the
real, on-time, casting parameters (15). The result is that not only decisions can be
made with respect to the process downstream (hot rolling, direct charging, grinding,
discard) but it is also possible to control the system to eradicate future defects.
Nevertheless, in order to correlate defects with operating conditions, the model has to
be trained to associate a specific defect and its appearance with different casting
parameters, and nowadays this can only be done with the invaluable help of
experienced operators (15).
2. Aim and Objectives
At the present time, there are no commercially available systems capable of assessing
the as cast surface quality in a reproducible and reliable way, and this task is usually
performed in the plant by experienced operators. Nevertheless, even with the best-
trained personnel, it is not possible to inspect the product online, since the visual
inspection can only be performed at room temperature after the as-cast products
have been cooled down, which necessary means time and additional cost. Even more,
different defect severity criteria may still be present as they depend on the operator
and in some cases the severity of the defect is not assessed. For example, a common
practice is to divide the slab surface into 3 areas, left edge, center and right edge. A
typical report contains the slab number (from the cast sequence) and a statement
about the presence of defects in the different areas of the slab, such as “longitudinal
cracks”, “inclusions”, and “deep oscillation marks”, “depressions” “transversal
cracks”, etc., always associated with the position on the slab in which they are
observed (15). If a severity criterion is also included, and depending on the plant or
industry standards; a classification into severe, medium and light can be established
for cracks based on the opening of the mouth, the crack density or the crack length, or
for example based on the depth for oscillation marks. It is important to note that the
fact that the severity is assessed by visual inspection may lead to differences between
the results when different persons are performing the inspection.
All these inconveniences can be overcome with the use of adequate inspection
techniques in the form of systems installed on-line in the plant. Although some
surface inspection systems have been developed for their use in rolling mills or
mapping samples offline (16-18) transferring them to online monitoring of
continuous casting is difficult because of the higher topography variations on the as-
cast surface compared with the more defined surface after rolling. In addition, the
high temperature of the solidified slab,…