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@PHDTHESIS{Blankart:975531,
author = {Blankart, Charline},
othercontributors = {Krupp, Ulrich and Brandt, Robert},
title = {{A}nwendbarkeit unterschiedlicher {W}ärmebehandlungsrouten
beim {P}resshärten von {M}ittelmanganstahl},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-12074},
pages = {1 Online-Ressource : Illustrationen},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2024; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2023},
abstract = {Press-hardening of manganese-boron steels is one of the
most efficient production processes for high strength
automotive components. However, the residual formability of
these sheet components is greatly limited by the formation
of a fully martensitic microstructure. To extend the
application of press-hardened components also to impact
energy-absorptive parts of the vehicle body, the use of
third-generation advanced high strength steels, especially
medium manganese steels, is gaining increasing attention.
The alloying concept of these steels allows the critical
cooling rate and the Ac3-temperature to be significantly
lowered compared to manganese-boron steels, while
introducing a certain amount of retained austenite improves
the ductility of the material. Therefore, the present
research analyzes the application potential of
press-hardening of a medium manganese steel
(Fe-0,3C-5Mn-1,5Si) in combination with an intercritical
annealing or quenching $\&$ partitioning treatment. These
heat treatments lead in general to ultrafine-grained
multiphase microstructures containing retained austenite and
different body-centered cubic phases, such as fresh
martensite, tempered martensite, and ferrite, with different
carbon content each. Since, depending on the phase
fractions, strongly different mechanical properties are
expected, the qualitative distinction but also the
quantitative determination of the body-centered cubic phases
are of extraordinary importance to be able to understand the
correlation between microstructure and mechanical
properties. In the present study, it is shown that using
phase maps combined with grain average band slope of
electron backscatter diffraction measurements is a suitable
method to distinguish quantitatively fresh and tempered
martensite as well as martensite and ferrite which was
validated by electron probe micro analysis. Thermodynamic
simulation and dilatometer experiments were performed and
analyzed to understand the phase transformation kinetics of
Fe-0,3C-5Mn-1,5Si. Based on dilatation results and electron
backscatter diffraction analysis, the Koistinen-Marburger
equation was adapted to fit the investigated medium
manganese steel. According to the findings, suitable heat
treatment process windows for intercritical annealing and
quenching $\&$ partitioning were determined and reproduced
in both the dilatometer and salt baths. Regarding the
quenching $\&$ partitioning conditions, quenching
temperature has turned out to be an important influencing
parameter determining the phase fractions. It was shown that
an exceptional property combination of high strength and
ductility could be achieved when the phase fraction of fresh
martensite was less than $20\%,$ which was reached with a
quenching temperature ≤150 °C. Higher amounts of fresh
martensite lead, due to the difference in the plasticity of
the different phases, to increased stresses at the
interfaces and brittle intergranular fracture.Unlike the
quenching $\&$ partitioning conditions, the intercritically
annealed samples show discontinuous yielding behavior with
Lüders-plateau and serrations. The Lüders-plateau could
only be attenuated in the condition where also fresh
martensite, due to insufficient thermal stability of the
austenite, existed. Consequently, more mobile dislocations
were present in the microstructure, which carry the plastic
deformation. Selected heat treatments which reached
promising mechanical properties were reproduced in a
laboratory-scale press-hardening system equipped with a
heated hat-shaped tool. Even though the optimum temperature
range of quenching temperature could not yet be precisely
set in the press hardening tests carried out, initial
experiments show that a multiphase microstructure with
retained austenite fractions of more than $10\%$ can be
achieved by combining press hardening with quenching $\&$
partitioning. To match the optimum time for component
removal and thus achieve better mechanical properties, the
integration of improved sensor technology, is indispensable.
The evaluation of sensor data in combination with approaches
of integrated computational materials engineering would also
allow to predict properties based on process control and to
identify production scrap or to avoid it by an adapted heat
treatment.},
cin = {522110 / 520000},
ddc = {620},
cid = {$I:(DE-82)522110_20180901$ / $I:(DE-82)520000_20140620$},
pnm = {WS-B1.II-neu - Press Hardening (X080067-WS-B1.II-neu) / EXC
2023: Internet of Production (IoP)},
pid = {G:(DE-82)X080067-WS-B1.II-neu / G:(GEPRIS)390621612},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-12074},
url = {https://publications.rwth-aachen.de/record/975531},
}
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