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@PHDTHESIS{Kumnorkaew:951588,
author = {Kumnorkaew, Theerawat},
othercontributors = {Bleck, Wolfgang and Lian, Junhe},
title = {{U}nderstanding and improving thermodynamic stability of
austenite in low carbon carbide free bainitic steels via
ausforming process},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-02072},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2023},
abstract = {Carbide-free bainitic (CFB) steel has become a new
forefront of advanced high-strength steels owing to their
outstanding balance in mechanical properties. Due to a
thermodynamic instability of austenite in low carbon CFB
steels, formations of only primary phase bainitic ferrite
and secondary carbon enriched retained austenite phase are
impracticable. The untransformed austenite at high
temperatures could partially transform into fresh martensite
during cooling operation, depending on the local carbon
concentration in the austenite. A general consequence is
that an excessive formation of fresh martensite may
deteriorate ductility, despite the enhanced strength of the
steel. Thus, controlling the thermodynamic stability of
austenite has been a challenging issue in developing
low-carbon carbide-free bainitic (CFB) steels, besides
increasing mean carbon content and chemical compositions.
Ausforming as a thermomechanical heat treatment process is
applied to compromise the formation of fresh martensite and
to balance the phase constituent of the steels. This process
combines plastic deformation of the untransformed austenite
with the conventional process of isothermal heat treatment.
Parameters of ausforming, such as deformation temperature,
strain, and strain rate, are of significant importance in
defining appropriate conditions for desirable
microstructures and mechanical properties. The correlation
between the ausforming conditions throughout the kinetics
behavior of isothermal bainitic transformation, factors
inherent in the martensite transformation, hardness, and
tensile properties have been established. A unified
physics-based model has been developed based on nucleation
rate theory to provide a better understanding of how
ausforming influences the variations of activation energy,
corresponding driving energy, and the evolution of carbon
enrichment in austenite. In addition, the impact of the
chemical compositions has been conducted to reveal a
limitation of ausforming with respect to the deformation
strain on improving the thermodynamic stability of austenite
against the formation of fresh martensite. Throughout the
dissertation, a systematic investigation in heterogeneous
microstructure and mechanical properties subjected to
ausforming conditions allows for establishing advanced
high-strength steels with reasonable hardness and improved
strength and ductility.},
cin = {522110 / 520000},
ddc = {620},
cid = {$I:(DE-82)522110_20180901$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-02072},
url = {https://publications.rwth-aachen.de/record/951588},
}
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