Prof.MiroslawGlowacki
DepartmentofAppliedComputerScienceandModelling
FacultyofMetalEngineeringandIndustrialComputerScience
AGHUniversityofScienceandTechnology
Al.Mickiewicza30,30–059Kraków
tel.0/prefix/126173641
fax.0/prefix/126172092
e-mail:[email protected],[email protected]
Kraków,21.08.2012
CRITICALREVIEW
ofaPhDdissertationunderthetitle:
“ELUCIDATION OF FORMATION MECHANISM OF
GRAIN
STRUCTURES IN AS-CAST AND SEMI-SOLID CARBON STEELS
INCLUDING VISCOSITY BEHAVIOR OF THE SEMI-SOLID STEEL”
Author:SHINGO TSUCHIYA
Subject of the review
ThesubjectofthereviewcopyofPhDthesis:“Elucidation of formation
mechanism of
grain structures in as-cast and semi-solid carbon steels
including viscosity behaviour of the semi-solid steel”byShingoTsuchiyawhich
wassubmittedtomyevaluationistheanalysisoftheformationmechanismofascastmicrostructureincarbonsteelsduringcontinuouscasting.Thedissertation
presentstheresultsofexperimentalandtheoreticalworkdescribingtheevolution
of grainmicrostructureinlaboratoryconditions.Themainaimoftheworkwasto
developprinciplesforcastingtechnologythatallowsavoidingofas-castcoarse
columnar grainmicrostructureforsteelsinwiderangeofcarboncontent.The
authorwas inspired byrecentsevererequirementsforthereductionoftheenergy
consumptionandtheemissionofgreenhouseeffectgasinsteelsindustry.Newhigh
strengthsteelshavestartedtobeproducedinintegratedcastingandrolling
processes.A particular characteristic of thiskindofsteelmanufacturingisveryhigh
levelofmaterialtemperatureresultinginsemi-solidstateofsteel.
In such new and very profitable rolling technologies the control of the as-cast γ grain
structure is very important for the improvement of quality of steel plants products. The
presented investigation is a valuable contribution to the understanding of a variety of
mechanisms that have crucial influence on as-cast grain microstructure evolution.
The dissertation contains 114 pictures, 11 tables and it refers to 227 publications.
The 210-page text consists of an abstract, table of content and 9 chapters. Its layout
differs slightly from most other polish PhD theses. Although all the chapters form a
coherent whole each of them is a separate entity with its own bibliography and final
conclusions. The last, ninth chapter summarises all the remaining chapters and contains
recapitulation of the thesis conclusions.
Overall assessment of the dissertation
One of the main problems discussed in the study is as-cast microstructure. The size
and shape of austenite grains is of great importance in improving the productivity of
continuous casting, hot rolling and semi-solid steel processing. Hence, the control of
grain structure is regarded as an important issue. The formation mechanism of as-cast
grain structure evolution still requires further explanations. The study focuses on
investigation leading to clarification of this mechanism. The author proposes original
methods for controlling the microstructure. The thesis contains a discussion of both the
semi-solid structure and the viscous steel behaviour affecting the steel flowability,
which is important for semi-solid steel processing.
The first chapter comprises introduction. The author presents the bibliographical
review concerning steel making processes and their progress in past years indicating
the integrated processes as high productivity and stable production methods. A part of
the chapter is dedicated to control rolling. The author has shown the importance of
application of semi-solid method to manufacturing of steel slabs and significance of
initial as-cast grain size and chemical composition of steel for the properties of rolling
products. He also points to material cracks as one of the most important problems in
continuous casting processes. A variety of crack types occur on the surface and inside of
semi-products. The formation of cracks depends on the external force applied to the cast
and the strength of the cast. The work focusses on the second of the mentioned reasons
of cracking. At the end of the first chapter the author states the purpose of the entire
study. He has focused on the development of as-cast and semi-solid grain
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microstructure paying attention to formation mechanisms of the structure development
in carbon steels. Another goal of the work was the investigation of semi-solid structure
development during reheating and cooling processes. The author has also analysed the
effect of alloying element and heating rate for coarse grain microstructure formation
for 100Cr6 steel and finally the viscosity behaviour of semi-solid steel as an important
factor for the application of steel to semi-solid processing.
The second chapter is dedicated to formation of high temperature microstructure in
carbon steels. It presents literature on high temperature elements of microstructure. The
focus is placed on dendrite structure, as-cast grain evolution and semi-solid structure.
The author analyzes the solidification process in the casting process of carbon steels. He
presents the observation that the dendrite structure solidifies as first from molten liquid
and the development of as-cast grain structure starts during or after solidification. The
main purpose of the chapter is to review and classify previous contributions according to
the solidification conditions and morphology of grain structure. The author describes
the evolution of as-cast structure development in real non-equilibrium processes on the
basis of classical nucleation theory and principle of minimum energy. Methods of
fabrication of semi-solid raw metals and the development of semi-solid structure in
steels were explained, as well.
Steels having carbon content of 0,2% and 0,35% are subject of the third chapter.
The author has examined the formation mechanism of coarse columnar
grains (CCG)
using their own dedicated equipment. Taking into consideration results of laboratory
experiment and real continuous casting process he draws conclusion that the equipment used
in his work is able to reproduce cooling corresponding to practical process. Both the dendrite
and the as-cast
grain microstructure are similar in the cast slabs and in the laboratory
samples. The so called “rapid unidirectional solidification equipment” was next the basis of
further investigation. According to the author fine columnar γ grains (FCG) always are
formed in the front of the CCG region and the CCG region develops toward the FCG region.
There exists a boundary between the FCG and CCG structure and the minor axis diameter of
FCG is quite comparable to the primary dendrite arm spacing. The FCG region corresponds
to the two-phase field (liquid +
phase). The author concludes that the difference in the
minor axis diameters between the CCG and FCG is the driving force for the development of
the CCG structure. The grain growth along the minor axis of CCG takes place isothermally at
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temperature close to the γ transformation temperature (T ). Near the FCG/CCG boundary and
below T the CCG does not grow.
In chapter 4 the author presents the results of investigation of hyperperitectic carbon
steels using the experimental technique which was described in chapter 3. He states that in
hyperperitectic steels having carbon concentration less than 0.38 the as-cast structures
consist of equiaxial grains (EG) and CCG whose average minor axis diameter varies from 1
to 3 mm. When the concentration is higher than 0.38 columnar grains (CG) appear. The
average diameter along the minor axis of such grains is in the range from 200 to 500 µm and
they appear near the copper mould. After certain distance from the mould the microstructure
suddenly changes from CG to CCG. According to the author the length of CG region
gradually increases with increase in carbon content. For steels with carbon concentration
higher than 0.42% nearly the entire structure consists of the CG. The Author concludes that
the positions of
grain boundary is in coincidence with the positions of boundaries of
dendrite clusters in CG region and there is no such correlation between the
grain region and
the dendrite structures in EG and CCG regions. The last conclusion concerns the FCG region
with grain diameter smaller than 200 µm. Such a region is observed in front of CCG and is
missing in front of the CG region. In CG region one can observe a correlation between
crystallographic orientation of
grains and dendrites. The FCG region does not exhibit such a
relationship. This fact was confirmed by my numerical analyses, as well.
In chapter 5, the author summarizes the formation mechanism of as-cast grain
structure in steels with the carbon concentration of less than 0.15 mass%. In steels with
the carbon content between 0.1 and 0.15%, CCG structure is observed near the surface
of the sample. Samples having 0.1% of carbon that are cooled to room temperature show
a delay in CCG development and FCG at the front of CCG does not grow significantly. In
steels with the carbon concentration of less than 0.1 mass%, neither the CCG nor FCG
structures are observed. Columnar and equiaxial grain (CEG) microstructure is
observed instead. Grain size of such a structure is smaller than that in CCG and larger
than that in FCG in the whole observation area. Having observed of steels with 0.05%
carbon content cooled in furnace conditions the author draws conclusion that the CEG
structure is formed by the massive-like transformation mechanism without diffusion.
Chapter 6 was written in order to show the practical methods of as cast grain size
and region control. At the beginning the author summarizes the morphology and
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principles of grain formation in carbon steels. He focuses on alloying elements as
means of CCG structure control. Supplement of 0.05 mass % of Ti or Nb removes the
CCG structure completely. Instead, the sample consists only of FCG. Taking into
consideration the phase diagram the author expects the formation of TiN particles
formation above
. Supplement of Nb stabilizes phase in wide range of temperature.
According to authors investigation pinning phases such as TiN and phase prevent the
formation of CCG structure. In contrast to his expectations supplementary amount of Mn
decreased the length of CCG region. The formation of CCG structure depends largely on
carbon concentration. Hence, the strict control of carbon concentration is also
important. The dispersion of pinning phases stable at low temperatures can change CCG
structure into FCG structure with the size of a few hundred micrometers. The final
conclusion in the chapter concerns the size and morphology of the FCG. It is affected by
phase dendrite structure. The transition of the dendrite morphology from columnar to
equiaxial results in the formation of fine equiaxial grain microstructure.
Chapter 7 is dedicated to control of a semi-solid structure in a selected steel. The
effect of heating rate, holding time and the addition of Nb in 100Cr6 steel during its
reheating was investigated. The author concludes that the remelting reaction starts
above solidus temperature at grain boundaries and inter-dendritic positions
regardless of Nb concentration. The growth of grain structure during reheating
process below solidus gives significant effect on the grain size in semi-solid structure.
According to the author the size of globular grain in semi-solid structure was smaller
in samples with Nb content, especially in the case of a slow reheating conditions. He
concludes that it is an effect of suppression of grain growth by NbN particles. In semisolid conditions Nb did not have significant effect on the coarsening kinetics. The author
explains the growth behaviour of semi-solid structure on basis of Ostwald ripening
theory.
Viscosity of liquid 100Cr6 steel and liquid+ phase is the subject of investigations
presented in chapter 8. To perform the measurements the author used high temperature
rotational viscometer. He concludes that superheat of around 20oC showed nonNewtonian liquid steel behaviour in which viscosity decreases with increase in shear
rate. According to him the viscosity of semi-solid is affected by the breakdown of the
atomic structure with increase in shear rate. During experiments thixotropy was clearly
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observed. The last conclusion concerns the temperature influence. The viscosity of semisolid steel dramatically increases with even small decrease in temperature. The precise
control of temperature is required in practical application of semi-solid processing of
steels.
In the last chapter the author recapitulates the thesis content. He shortly presents
the results of the study and appropriate conclusions pointing to the most important
aspects of the work.
The analysis presented to my review study by Shingo Tsuchiya is presented in a
comprehensive manner. The author has examined the problem of grain
microstructure development, which takes place during the continuous casting as well as
the viscous properties of liquid and semi-solid steel. He has outlined the main factors
influencing both the phenomena. Practical methods of grain formation control have
been proposed. The influence of supplementary elements like Ti, Nb and Mn as well as
the cooling conditions were analysed. The experimental work was supported by
theoretical calculations. All the author’s conclusions seem to be relevant and supported
by appropriate experimental work and theoretical considerations. The proposed
solution should be helpful for technologists working on development of continuous
casting and integrated casting and rolling processes. Hence, the work should be
regarded as an interesting and valuable contribution to the theory and practice of
continuous casting.
Author’s main achievements and own solutions
The text of the dissertation is clear and facilitates proper assessment of the ideas
presented in the study. The objective of the dissertation is plainly defined in chapter no
1 in section 1.5 entitled “Purpose” and it was recapitulated in final summary in
chapter 9. The main purpose of the dissertation is to explain the formation mechanism of
γ grain structures in both the as-cast and semi-solid carbon steels. The author has
examined the effect of alloying elements and heating rate on the microstructure. The
study deals also with semi-solid processing of steels and with the viscous behaviour of
semi-solid steel under different solid fractions.
In contrast to the work purpose, the author does not dedicate a separate chapter to
the main thesis of the dissertation. However, it is evidently suggested in the study,
6
especially by abstract and by the first three chapters. It is also recapitulated in the last
chapter.
The author states that the driving force for the development of the CCG structure is
different for steels with different carbon content. The size and shape of grain agree
with dendrite assemblage in CG region. However, there is no such a correlation between
the grain and the dendrite structures in EG and CCG regions. According to the author,
the grain growth along the minor axis of CCG isothermally takes place at temperature
close to
only near the FCG/CCG region boundary. He has found that the CCG does not
coarsen below
, which is not consistent with previous works concerning continuous
casting process. The content of 0.05mass% of Ti or Nb in in many cases (e.g. in 0.1%C
steels) removed CCG region completely and the whole structure consisted of FCG
structure. Contrary to the expectation, the addition of Mn decreases the length of CCG
region.
The main point of the work is the explanation of the phenomena accompanying the
solidification of carbon steels during continuous casting. The study focuses on
morphology and the microstructure of steels with different carbon content and the
development of semi-solid structure during reheating process. The author has examined
the effect of alloying elements and cooling/heating rates of both the phenomena and the
viscous behavior of semi-solid steel under different solid fractions.
The work presented in the dissertation is innovatory. The next author’s achievement
is the development of useful laboratory equipment allowing the testing of steel
behaviour during solidification. It is also useful for conducting industrial experiments
designed to recognition the microstructure of
phase nascent during real continuous
casting process. Comparison of results of both the types of experiments allowed the
verification of laboratory equipment as a tool that allows the reconstruction of industrial
conditions in laboratory.
Another achievement of the study is a very good bibliographical review of three
main phenomena: as-cast grain formation, semi-solid steel behaviour and liquid steel
viscosity. The author of the dissertation has summarised all the collected information
and has supplemented it with original thoughts concerning the behaviour of carbon
steels. His theoretical conclusions were relevant and have provided a good basis for
planning relevant experimental work.
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Is should also be pointed out that the results of laboratory tests were analysed
thoughtfully and thoroughly. The author not only draws conclusions directly from the
experimental data but also plans and conducts theoretical support for the collected
results based on thermodynamic models of the investigated phenomena. Thanks to these
calculations the author is able to confirm or reject the questionable conclusions coming
from the analysis of results of the experimental work, which in many cases are
inconsistent with previously published knowledge.
Further achievement of the dissertation is its scope. The author has tested steels in
a wide range of carbon content and has found the intervals of carbon content in which
the as-cast grain stricter formation mechanisms differ from one another. He also has
investigated the influence of cooling conditions and supplementary elements on the CCG
microstructure formation and has drawn conclusions allowing the suppression of
formation of this kind of structure. He has also tested a selected 100Cr6 grade steel
composed of 1% of C and 1.37% of Cr in experiments designed to reveal the influence of
Nb content on the size of globular γ grains in semi-solid structure different reheating
conditions. The semi-solid steel viscous behaviour of 100Cr6 steel in both liquid and
binary liquid+γ phase states has been investigated as well.
In my opinion the main goals of the work – the explanation of formation mechanism
of γ grain structures in both as-cast and semi-solid carbon steel conditions as well as the
effect of alloying elements and heating rate on the microstructure formation mechanism
– have been achieved. As a result of the work, a system of microstructure control during
continuous casting of steels has been created and tested. It can be very useful for
designers of both continuous casting and integrated casting and rolling technologies by
providing information about the processes run in varied conditions. The author has
proved his suppositions presenting detailed results of experimental investigation and its
theoretical support.
Substantive comments
The work does not contain any significant errors of substance. The text is clear, and
the thoughts are formulated clearly. The author provides a number of statements, which
generally are not in doubt. A thorough analysis of the text, however, showed that it failed
to avoid certain errors – mainly mistakes of editorial character. In each chapter
equations, figures and tables are numbered independently. In the text of dissertation
8
there are several equations with the same numbers. It is not a good practice. I suppose
that in all cases the author refers to equations in the same chapter. This convention will
apply to my comments, as well. If I will be forced to comment an equation located in
different chapter its number will be preceded by the chapter number – e.g. (3.5). In my
opinion all the thesis equations figures and tables should be numbered in this manner.
1. Page 27, equation (1). The author uses symbol G (with different sub- and superscripts) for description of many physical quantities (cf. e.g. (11) on page 40). This
is a little bit confusing. I propose to change symbols for other quantities in
further studies. The Gibbs free energy should be denoted by G which is the
traditional symbol for free energy.
2. Page 32, equation (9), Fig 4, and the text around. This time G denotes the
temperature gradient (as in equation (4.1) on page 108). Because both the
equation and figure show schematically the temperature distribution at front of
solid and liquid steel, the symbol may easily be confused with Gibbs energy. In
one-dimensional problems I suggest the traditional derivative symbol
/
.
Moreover, in subsequent paragraph the author uses the symbol G instead of the
name of the quantity and an inattentive reader could be disoriented.
3. Page 32, equation (10). The derivative symbol
coordinate
/
|
. It is obvious that the
= 0 denotes the liquid-solid phases boundary, but the
axis
should be clearly defined in either the neighbouring text or figures.
4. Page 40, equation (11). In this equation and surrounding text symbol
expressly seems to denote Gibbs energy while the author uses it for grain
growth linear rate. Equation (18) defines the quantity in details. I suggest the
symbol . Operator “” usually defines the derivative with respect do time.
5. Pge39, line 15. A sentence should start with capital letter – “They showed …”
instead of “they showed …”
6. Pge39, line 19. The author has written: “ ,,, in this paper …”. But should be „ … in
this dissertation.. “ or “ … in this thesis … “
7. Page 57, section 3.3.1 and 3.3.2. The author states: “Direct finite difference
method was applied to calculate solidification process in the rapid unidirectional
solidification equipment. In this method.” My question concerns the method.
Why the finite difference method was selected? The method is not very good for
9
discretization such kind of problems. In many cases it does not converge,
especially for big temperature gradients that appear during contact of liquid steel
with cold copper surface. There are much better methods – finite element
method or a number of meshless methods. Another question is if the applied
method is a real FDM? Such a method requires the definition of type of method
(explicit, implicit or Crank–Nicolson method) type of differences (backward,
central or forward), control of numerical stability and many other aspects. The
applied method looks like a kind of a simple energy balance method that can also
provide uncertain results. This problem requires explanation and wider
discussion.
8. Page 58, line 2. Why the author uses following units: “g/m3” or “J/g K”? The base
unit for mass in International System of Units is “kg”. It should be “kg/m3” and
“J/kg K”.
9. Page 58, first and second paragraph. The author uses indices: , and + 1 for
elements presented in Fig. 5. The figure shows two-dimensional heat flow.
Double index should describe an element – one index per one dimension: e.g.
, + 1; i+1, j+1 etc. Single index does not clearly denote the elements.
10. Page 58, line 2 from the bottom. The author writes: “The heat flow from a
material to outer fluid, e.g. air, was defined as equation (4) based on Newton’s
law of cooling.” The Newton law of cooling also seems to be not very adequate.
The law does not apply if the heat transfer takes place by radiation, convection or
conduction accompanied by a change of state. Here a more general FourierKirchhoff’s equation should be applied. There are many commercial systems
allowing suitable computation of temperature evolution.
11. Page 60, table 2, row 6. The unit of heat transfer coefficient is W/m2K. For
contact: air-metal it usually ranges from 10 to 100 W/m2K. Why the value of
1100 in the author’s study?
12. Page 60, table 2, rows 7-9. The unit of heat conductivity is W/m2K.
13. Page 68, line 2 and 3 from the bottom. The author uses a symbol “
” for
describing equivalent carbon concentration. On page 58 in equation (1) and
below the same symbol denotes the specific heat. Different symbols should be
chosen.
10
14. Page 69, Fig. 15. A symbol is missing in second line of legend. Probably the
crossed square should be placed here.
15. Page 79, line 16. Why the value of
(kinetic constant for the grain boundary
mobility) is the half of the one presented in Ref. (3-4)?
16. Page 80, line 8. The author writes: “. We calculated this isothermal process …”.
We means who?
17. Page 83, line 2 from the bottom and equation (7). Another notation collision. This
time ! denotes the radius of curvature. In previous sections ! was the density.
18. Page 84 equations (8) and (9). Probably:
"
#$
"
"
"
+ # instead of # + # .
%
$
$
19. Page 85, Fig. 26. The author refers to the figure for the first time on page 83. It
should be placed earlier for convenience.
20. Page 91, lines 3 and 4. A steel with carbon content of 0.15% is hyper- or hypoperitectic?
21. Page 104, Fig. 9. The figure is placed one page earlier than it is referred to.
22. Page 108, line 0. The Peclet number is calculated from a relationship:
&'( = )*/(2 ( ). Why the factor 2 in denominator? The Peclet number is defined
as &- = .*/ , where . is the characteristic length, * the velocity and
the mass
diffusion coefficient.
23. Page 111, Fig. 12. The figure is placed one page too far.
24. Page 113, line 3. Symbol
is used one more time but this time it is less
misleading as before because Δ
0
denotes the activation energy. So it plays the
same role in Arrhenius equation as increment of Gibbs free energy in similar
Eyring–Polanyi equation.
25. Page 130, line 2. The author refers to Fig. 8 but this figure is missing in the
chapter.
26. Page 174, equation (3). The equation describes the Herschel–Bulkley fluid model
with yield pointy 1 . The effective viscosity 2 = 2 3457468
9:"
, where 2 is the
so called flow consistency index, is not the same quantity as constant 2 in
equation (2) and should be either represented by another symbol or
subscripted.
11
27. Page 175, lines 7 and 9 from the bottom. The author writes: “In a concentrated
dispersion system, such as concentrated choroid, the particles …” As I suppose he
has in mind “colloid” not “choroid”.
28. Page 190, line 5 and Fig. 14 on page 191. Next notation collision – this time G(x)
denotes the radius distribution function.
The remaining text does not contain substantial errors and is written rather
carefully, without significant editorial shortcomings.
Final conclusion
Summarizing my opinion on the dissertation by Shingo Tsuchiya submitted to my
revision I certify the following. The author has chosen to resolve an important scientific
problem. Main targets of the work have been achieved. On the basis of extensive
experimental work and theoretical support the author was able to explain the problems
of formation mechanism of γ grain microstructures in both the as-cast and semi-solid
carbon steels. The control system of microstructure formation in carbon steels
developed by the author allows for predicting the influence of important parameters of
continuous casting process, which has been confirmed by experimental studies and
theoretical verification of experimental results. The author has demonstrated
knowledge of issues related to continuous casting of steels and methods for the process
control. It confirms his ability to independently resolve scientific problems.
The work contains a small number of faults and inaccurate scientific formulations.
However, they do not affect the value of substantive work, and only require increased
attention from the reader.
Therefore, in my opinion the presented dissertation meets the conditions
specified by the Act on Academic Degrees and Scientific Title and I put request for
admission Mr Shingo Tsuchiya to further proceeding within the process of PhD
degree conferment.
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