Florian Vercruysse annealing Third generation advanced high strength steel by ultrafast

167 Pages·2016·14.34 MB·English

Third generation advanced high strength steel by ultrafast
annealing
Florian Vercruysse
Supervisors: Prof. dr. Roumen Petrov, Prof. dr. ir. Patricia Verleysen
Counsellors: Felipe Manuel Castro Cerda, Ahmed Elmahdy
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in Sustainable Materials Engineering
Department of Materials Science and Engineering
Chair: Prof. dr. ir. Joris Degrieck
Faculty of Engineering and Architecture
Academic year 2015-2016

Preface
First of all, I would like to express my appreciation and thanks to my promotors professor Dr. Ir.
Roumen Petrov and professor Dr. Ir. Patricia Verleysen for giving me the opportunity to carry out the
research on this master thesis project. Their guidance as well as the constructive discussions were an
essential part in writing this dissertation.
A second and most sincere 'thank you' should be addressed to Ir. Felipe Castro as he was the mentor
during this project. His overall help/assistance and the many/daily and challenging/stimulating
conversations related to the subject had obviously its influence on the depth to which this research
has been carried out.
Thirdly I would like to express my gratitude to Ir. Ahmed Elmahdy for the joint effort that has been
made in improving the quality of the mechanical testing.
The moral support and the personal guidance of my friends and my family during this very intense
and most exciting year of my student's life are highly appreciated as well.
Florian Vercruysse
The author gives permission to make this master dissertation available for consultation and to copy
parts of this master dissertation for personal use. In the case of any other use, the copyright terms
have to be respected, in particular with regard to the obligation to state expressly the source when
quoting results from this master dissertation.

Ghent University
Department of materials science and engineering
Third generation advanced high strength steel by ultrafast annealing
by
Florian Vercruysse
Supervisors: Prof. Dr Ir. R. Petrov, Prof. Dr. Ir. P. Verleysen
Counsellors: Ir. F. Castro, Ir. A. Elmahdy
Academic year: 2015-2016
Outline
“Third generation advanced high strength steel by ultra fast annealing” written by Florian Vercruysse
This dissertation concerns the influences ultra fast annealing has on the microstructure and
mechanical properties of low carbon and dual phase steel. Influences on recovery, recrystallization as
well as phase transformation will be elaborated on. Grain refinement, as the optimal aim to improve
mechanical properties, was to be studied as function of the annealing parameters. General reduction
on the grain size was observed in LC steel, as long as the annealing treatment is carried out below A
c1
as well as in the case of DP steel (in intercritical region) when heating rates are increased.
Consequently an increase in mechanical properties could be observed in LC steel. Due to the
influence of the heating rate on the phase transformation kinetics in DP steel, increases in strength
are less unambiguously observed. A retardation of austenite transformation by recrystallization at
400°C/s was observed whereas this behaviour was absent when recrystallization was hindered at
1000°C/s. Grain refinement of DP steel due to UFA-treatment was observed by analysing the
maximum size of the martensite grain formed as a correlation between prior austenite and
martensite is expected.
Keywords: Ultra fast annealing, Recrystallization, Phase transformation, Grain refinement,
Mechanical properties
In the first chapter a short introduction on recrystallization and phase transformation is given.
Additionally, the results that have been published in literature concerning prior research carried out
in the field of UFA have been compiled in a summarizing overview. As the range of types of steel that
have been tested via this annealing technique is quite broad it has been opted for to extract the
research that is closely related to the LC and DP material to be investigated in this dissertation or
research that shows general insights.
The second chapter of this dissertation describes the general experimental set-up and design of
experiments. The annealing parameters we have chosen for as well as the microstructural and
mechanical characterization that is needed in order to evaluate the influence of the heating rate on
LC and DP steels are illustrated in this chapter.
The third chapter is made up of the results that are extracted from the experiments being described
in chapter 2. First/small remarks on early observations are included as well.
The fourth chapter of this dissertation comprises discussions on the information that has got
extracted from the tests. Hypotheses that have been stated earlier are confirmed or adapted where
needed.
In a fifth chapter additional experiments are proposed that would help to further the research that
has been started in this project of master thesis. New treatments and/or other analysing techniques
are presented that could be useful in obtaining a better understanding of the effects an ultra fast
annealing treatment has on materials. Moreover, these new treatments and/or other analysing
techniques may further strengthen the statements made during this dissertation or may result into
new insights that change the perception obtained here.
A final chapter is devoted to the summarizing of the most important observations and conclusions
that can be extracted from the research performed in the framework of this master thesis.
Third generation advanced high strength steels via ultrafast annealing
Florian Vercruysse,
Ghent University, Belgium
Abstract The changes in microstructural and mechanical properties after ultra fast annealing on cold
rolled low carbon and mixed ferritic/pearlitic initial microstructure were investigated. The LC steel
has been treated in ferritic, intercritical and austenitic region to evaluate the influence of UFA on the
recrystallization behaviour. Based on EBSD data it could be concluded that recrystallization kinetics
are shifted to higher temperatures when heating rates are increased. A smoothing out of the
preferential orientation created during conventional annealing is observed as well as a reduction in
grain size of 15%, caused by the shift in recrystallization kinetics to higher temperature. The
ferrite/pearlite initial structure exhibits retardation of recrystallization into the intercritical region,
creating an interaction between recrystallization and phase transformation and resulting into a
higher amount of grain refinement. It has been observed that phase transformation can be hindered
by recrystallization at intermediate to high heating rates (400°C/s); recrystallization will not be
observable at higher heating rates (1000°C/s) as it is more severely hindered itself.
Keywords: Ultra fast annealing, recrystallization hindering, phase transformation, grain refinement,
mechanical properties
duration of the UFA cycle also has the potential to have
the production cost of the steel reduced; the needed
1 Introduction length of the annealing section can be significantly
The search for a continuous improvement of the reduced when compared to the rather long continuous
mechanical properties in steel is often driven and/or annealing line being standard today[52].
accelerated by the needs of the automotive industry.
Ever increasing strength, deformability, weldability
2 Experimental procedure
combined with a decreasing mass of the material are
tendencies asked by this industry. As the emission
regulation becomes more stringent the need for more 2.1 Materials
lightweight cars is constant; in addition higher passenger Two different types of steel were tested in this
safety is expected as well. Due to these reasons dissertation. A lean alloyed LC steel was prepared to
advanced high strength steels are often used in the car better understand the recrystallization behaviour of
industry and they are evolving at a rapid pace[3,4]. steels and to study the influence of carbon when high
Increasing mechanical properties by the correct alloying heating rates are applied. The second material was
strategy is a popular route to follow. However as alloying aimed to have a dual phase structure created. In order
elements are becoming more and more scarce the need to accomplish latter structure higher amounts of carbon
for other processing techniques is present and will only were added as well as other alloying elements (like
gain in importance in the future. manganese and niobium) to interfere with
recrystallization. The chemical composition of both types
Ultra fast annealing might be a solution for this problem. of steel is indicated in table 1 and 2. Both materials were
Grain refinement has been reported by the hindering of 50% cold rolled to have a sufficient amount of stored
recrystallization when higher heating rates are applied energy/dislocations introduced to allow for the
and the correct initial microstructure (carbide, investigation of recrystallization as a result of different
precipitates etc) is used without the need for large annealing strategies. The initial microstructure was a
amounts of expensive alloying elements[3,6-8]. largely deformed ferrite structure in case of the LC steel
Hindering of this recrystallization allows for phase and a mixed deformed structure of ferrite and pearlite
transformation starting from an unrecrystallized for the DP steel.
structure and resulting in the finer-grained material that
raises the strength level of the material. The short
Table 1: low carbon steel - chemical composition in DP steel. Separation was based on grain average IQ as
IQ is influenced by the differences in dislocation density
C Si Mn P S in ferrite and martensite. Parallel to this inspection
𝒙̿ 𝒘𝒕% 0.005 0.012 0.124 0.016 0.004 method, visual separation of martensite and ferrite was
N O Nb Cr Mo also carried out via quantitative metallography methods.
0.003 0.026 0.006 0.015 0.001 The mechanical properties of the materials were tested
by means of quasi static tensile tests and Hopkinson split
bar experiments as well to visualize the high strain rate
Table 2: dual phase steel - chemical composition validity of the DP and LC material which is often used in
the car industry. The same geometry was used in the
C Si Mn P S tensile test (testing rate of 0.0033s-1) and in the
𝒙̿ 𝒘𝒕% 0.110 0.031 1.876 0.011 0.002 Hopkinson test. A width of 4mm and gauge length of
N O Nb Cr Mo 5mm was selected. It was opted for to work with test
0.009 0.015 0.027 0.453 0.179 samples having these dimensions in order to eliminate
any influences of geometry when comparing different
2.2 Annealing strategy
strain rates. Secondly the small homogeneous
The strips of metal were cut in a direction parallel to the temperature zone created in the gleeble set-up did not
rolling direction before being heat-treated via a Gleeble, allow for larger specimen without running into the risk
current resistance heating device. The heating rates of testing tensile specimen with gradients in mechanical
imposed in this work were 10 °C/s as a reference for the properties. Additional DIC techniques were included to
conventional continuous heating lines and 400°C/s, overcome the non-availability of a small extensometer
800°C/s and 1000°C/s respectively. After reaching the (being able to cope with latter small samples) and to
peak temperature set, the samples were immediately provide local strain measurements as well.
quenched via helium or water. Isothermal holding is
limited (<0.2s) to preserve the changes that have been
induced by using the varying heating rate. The 3 Results and discussion
temperatures chosen in this research were 650, 750,
800, 870, 910 and 1050°C for the LC steel. By means of 3.1 LC steel
latter scheme several data points were obtained in the
ferritic, intercritical and austenitic region. For the DP 3.1.1 Influence recrystallization
steel a focus was set on the intercritical region in order The recrystallization fraction of the specimen was
to have a 50/50 martensite ferrite microstructure analysed by means of EBSD. Grain average
formed. As shifts in phase transformation temperatures misorientation made it possible to split up recrystallized
are observed with increasing heating rates, it was chosen and unrecrystallized ferrite after annealing. Recovery,
for to have the samples with a predicted 50/50 decrease in the dislocation density by annihilation etc,
distribution [based on thermo-dynamical calculations will procede recrystallization and influence the driving
(thermocalc)] tested at 770°C and at 790°C and 825°C as forces for recrystallization. With increasing heating rate
well to allow for aimed phase fractions even in material a decrease in recrystallized fraction was observed(Figure
heated at higher heating rates. 1). The limited time for dislocations and (sub)grain
boundaries to move as well as the possible reduction in
2.3 Microstructural and
driving force for recrystallization by the bifurcation of
mechanical analysis recovery can explain the shift in recrystallization kinetics
Microstructural analysis of the specimen was carried out to higher temperatures in these an-isothermal annealing
in TD plane. LOM and SEM microscopy was used after experiments. The start temperature is situated ≈650°C as
grinding and polishing of the sample. The etching agent nuclei were observed and seems to be rather unaffected
used to reveal the microstructure of the DP material is a by the heating rate. When heating rates do exceed some
4% nital solution; LC steels are treated with a 2% nital heating threshold value it was noted that shifts in
followed by 4% picric acid and a final emersion into finishing temperature do exist. This phenomenon has
Marshall’s reagent. Additional polishing with OPU was been observed in samples heated at 1000°C/s where still
carried out to allow EBSD analysis. This technique has unrecrystallized grains(Figure 2) were visible when
been used to evaluate the changes in texture and compared to the fully recrystallized structures being
recrystallization fractions in LC steel. A grain average imposed to lower heating rates of 800°C/s.
misorientation of 0.7° was set a threshold value; below
this value, the grains were assumed to be recrystallized.
EBSD was used to divide between martensite and ferrite
Figure 1: Recrystallized fraction of LC steel a.f.o.
Figure 3: Grain size of RX fraction LC steel
annealing parameters
The grain refinement by over 17% (Figure 3) that was
observed in the ferritic region was absent when the
annealing temperature was increased above 870°C. If
(part of) ferrite is transformed into austenite when being
heated above A a grain coarsening has been noticed
c1,
with increasing temperature as has been reported
before[6]. When crossing this temperature a grain
coarsening rather than grain refinement was observed
with increasing heating rate. It was hypothesized that
this flip-over in behaviour is caused by heterogeneous
nucleation/growth of austenite due to an insufficient
amount of time available for full homogenization of
carbon. These local gradients in carbon concentration
can cause an overall changing tendency for nucleation.
An overall reduction of the amount of nuclei formed
when increasing the heating rate will result into the
Figure 2: Indication of unrecrystallized grains at 870°C
observed coarsening rather than the refinement as it
with 1000°C/s HR
was the case during ferritic annealing. This hypothesis is
3.1.2 Heating rate influence on the grain based on the slow(er) dissolution of carbides present
size and on insufficient diffusion of carbon.
Recrystallization was completed below A , restricting
c1
3.1.3 Texture changes due to UFA
the possible range in which grain refinement can take
During recrystallization of ferrite the typical γ-fibre
place as phase transformation starts from a
dominated texture is created. Increasing the heating rate
recrystallized microstructure. This larger amount of
does not have an influence on the overall trend of
nucleation sites due to the increasing heating rate will
orientations that are favoured during recrystallization.
generate the smaller average grain size observed in the
The larger intensities of specific orientations like <112>
material when annealed in the ferritic region. Due to the
(111) is more pronounced when conventional heating
shorter annealing cycle, a lower amount of grain growth
rates are applied. The ‘smoothing–out’ of these peaks to
is expected and this phenomenon can enhance the
the general γ-fibre is explained by the hypothesized shift
refinement effect as well. The annealing cycle has to be
in recrystallization to higher temperatures. The reason
kept short however as it was mentioned in literature
for specific orientations to get preferentially
that grain growth at these higher heating rates will have
recrystallized can be found in a heterogeneous
the reduction in grain size destroyed (even in case of
distribution of stored energy within different
short holding times).
orientations, a link with the taylor map and the stored
energy/dislocation density can be assumed[71]. The
orientations corresponding to higher stored energy will
tend to nucleate and recrystallize earlier/easier as
driving forces are higher. As the recrystallization is
suppressed to higher temperatures the higher driving
force for specific orientations to nucleate first becomes During straining dislocations will be created (Frank read
of less(er) importance as the overall tendency to source, interacting dislocations, etc) and an increase in
recrystallize will have significantly increased by the dislocation density will be the result[12]. As
increasing temperature. The question whether or not dislocations influence each other an additional amount
oriented nucleation or oriented growth is on the origin of dislocations will result into a more difficult
of this phenomenon observed is still be answered; this environment to allow for further dislocation motion. The
topic has to be investigated more extensively but the additional force that is needed to act on the dislocation
limited amount of time available during UFA might to get it moved and to allow for further deformation will
however might point into oriented nucleation as the be observed as the hardening of the material in stress
preferential theory. strain curves. The formation of (new) dislocations
however is not the only phenomenon influencing the
overall dislocation density. Dislocations of opposite sign
however can attract each other and annihilate, resulting
into the disappearance of dislocations as well. The final
dislocation density of the system will be governed by the
increase by straining but by the annihilation mechanism
as well. During low strain rate, annihilation can play a
significant role in reducing the growth in dislocation

density. When strain rates are increased the annihilation
phenomena are reduced since shorter timescales being
Figure 4 Texture plot of recrystallized fraction heated at
present will lead to a smaller distance a dislocation can
(a)10°C/s and (b) 800°C/s
travel through to annihilate. This process results into an
3.1.4 Mechanical properties overall more significant increase in dislocation density
Changes in microstructure will have an influence on the that will explain differences[78] in the observed UTS and
manner in which the material will behave during loading. total elongation. Strain rate dependency on the critical
The influence of grain size reduction on the increase of resolved shear stress will influence the activation of slip
strength without having to face a reduction in ductility systems too and this phenomenon further explains why
(as often is the case with other strengthening yield strength and UTS are (highly) sensitive to the strain
techniques) is proposed by the Hall Petch rate.
relationship[43]. A grain refinement effect should result
into an increase in strength of the material. An increase
in UTS of 5% has been observed when higher heating
rates are applied. As the grain refinement was solely
based on recrystallization and consequently limited, only
small improvements in strength were to be expected.
Additionally, Hopkinson split bar tests were conducted
as well in order to evaluate the mechanical properties of
this type of LC steel when higher strain rates were
applied, as it would be the case in car crashes e.g. . It can
be seen that a shift to higher stresses at yield and UTS is
noticeable with increasing strain rate. This finding
coincides with a decrease in strain at fracture. An Figure 5: UTS and strain at fracture of LC steel obtained
increase in UTS of 30 % and a decrease of strain at at low (0.0033/s) and high (500/s) strain rate
fracture was seen if the strain rate was increased from
3.2 DP steel
0.0033 s-1 to 500 s-1(Figure 5). This behaviour can be
explained by the increase in dislocation density during
3.2.1 Recrystallization/phase
straining. When materials are stressed above their yield
transformation
point strength plastic deformation will occur. In most of
The impact of increasing heating rates on the shift in
the types of steel latter deformation is achieved by
temperature of transformation has been studied. The
dislocation motion. Specific alloying strategies
phase fractions of ferrite and martensite present as well
(influencing the stacking fault energy) and (very) high
as the extent of recrystallization was analysed via EBSD
strain rates (as often reached in explosions) can promote
and SEM images(Figure 6). An increase in the amount of
a shift in deformation mechanism from dislocation
martensite formed can be observed when increasing the
motion to twinning. However, in this case studied
annealing temperature. As larger fractions of austenite
dislocations will govern the properties of the material.
will be formed with increasing annealing temperature