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@PHDTHESIS{Mosecker:660383,
author = {Mosecker, Linda},
othercontributors = {Bleck, Wolfgang and Mayer, Joachim},
title = {{M}aterials design of high nitrogen manganese austenitic
stainless {TWIP} steels for strip casting},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2016-05336},
pages = {1 Online Ressource (91 Seiten) : Illustrationen},
year = {2016},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2016},
abstract = {High nitrogen manganese austenitic stainless TWIP steels
achieve attractive mechanical properties and excellent
strain hardening behavior. However, high nitrogen steel
melting methods are generally associated with high pressures
to enhance the nitrogen solubility in the melt. Thin strip
casting offers an attractive option that not only shortens
the process route but also allows the alloying with nitrogen
at atmospheric pressure. In the present work, the materials
design of austenitic Fe-Cr-Mn-N steels for the production by
strip casting is presented. A thermodynamics based model
using CALPHAD method was developed to predict and control
the thermal and mechanical stability of the austenitic phase
by calculating the Gibbs free energy change (ΔGγ→ε) and
the stacking fault energy (SFE). The application of a
non-constant composition-dependent interfacial energy,
ϭγ/ε, is introduced and the effect of higher ordered
interaction parameter and strain energy term on SFE is
discussed. Fe-Cr-Mn-N alloys with nominal chemical
composition in the range of 13-14 $wt.\%$ Cr, 20-26 $wt.\%$
Mn and 0.4-0.6 $wt.\%$ N were melted and processed by strip
casting in laboratory and industrial scale. The solubility
of nitrogen in the melt and the phase stability during
solidification at atmospheric pressure are predicted by
thermodynamic model calculations as function of balanced
chromium to manganese concentration. The as-cast and
cold-rolled microstructure is characterized by light optical
microscopy and electron probe microanalysis to analyze the
secondary dendrite arm spacing, grain size distribution and
microstructure segregation. The phase distribution and
deformation substructures with respect to character and
location of grain and sub-grain boundaries, distribution of
grain orientation and local variations in residual strain
are determined by X-ray diffraction and electron
back-scatter diffraction. The deformation mechanisms and
mechanical properties of the investigated Fe-Cr-Mn-N alloys
are discussed to depend on temperature, SFE and ordered
microstructural phenomena like short range ordering (SRO).
Differences in the strain hardening behavior of the as-cast
and cold-rolled/recrystallized strip material are explained
by the change in grain size, differences in the density and
distribution of the dislocation substructure and the
critical stress for the onset of deformation twinning. The
flow behavior is homogenous and no serrations in the flow
stress occur during tensile deformation in the temperature
range from -150 to 250°C. The absence of dynamic strain
aging is attributed to the type of SRO and the activation
energy for reorientation of the point defects, rather than
the SFE. The RT SFE of the examined steels is determined
between 24-31 mJ/m² which defines mechanical twinning as
the dominant secondary deformation mechanism, resulting in
high work hardening rate and formability. The occurrence of
high temperature deformation twinning at 250°C is explained
by the effect of nitrogen on the dislocation arrangements
and the probability of Cr-N SRO. It is assumed that with
increasing the interstitial nitrogen content the effect of
SFE on the activity and character of mechanical twinning
becomes less dominant, and the temperature sensitivity of
the yield strength within the thermal and athermal
temperature range increases. In comparison to conventional
high-Mn TWIP steels, the investigated high nitrogen Fe-Cr-Mn
alloys exhibit extra-ordinary high flow stress and strain
hardening behavior, with YS up to 660 MPa and total
elongation A50 of 47 $\%,$ meeting the requirements for
application in automobile industry.},
cin = {522110 / 520000},
ddc = {620},
cid = {$I:(DE-82)522110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2016-053366},
url = {https://publications.rwth-aachen.de/record/660383},
}
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