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Austenite decomposition of a HSLA-Nb/Ti steel and an A1-TRIP steel during continuous cooling Lottey, Kulwinder Kaur
Abstract
The ability to control and predict the mechanical properties of hot rolled steel depends strongly on the thermomechanical processes which the steel undergoes. The phase transformation that occurs on the run-out table of a hot strip mill is a critical processing step which significantly influences the final microstructure, and thus, the properties of the hot rolled steel. This work examines the austenite-to-ferrite phase transformation of a high-strength low-alloy steel (HSLA-90) microalloyed with niobium and titanium and a transformation-induced plasticity steel alloyed with aluminium (Al- TRIP). The austenite decomposition kinetics have been investigated using a Gleeble 3500 thermomechanical simulator equipped with a dilatometer. The effect of cooling rate on the austenite decomposition kinetics were quantified for both steels with initial microstructures comprised of austenite. For the Al-TRIP an additional initial microstructure consisting of a mixture of austenite (67%) and ferrite (33%) phases was also investigated. It was shown that accelerated cooling lowers the transformation temperatures in addition to refining the resulting ferrite grains. Microstructural analyses revealed a decrease in polygonal ferrite fraction with accelerated cooling and the formation of acicular products for the HSLA-90 steel. A transition from high temperature products such as, polygonal ferrite and pearlite, to low temperature products such as, bainite and martensite, was seen for the Al-TRIP with accelerated cooling where there was an increase in bainite and martensite fractions. The effect of initial austenite grain size and a pancaked austenite microstructure was investigated for the HSLA-90 steel. Increasing the austenite grain size resulted in a shift to lower transformation start temperatures and an associated decrease in the polygonal ferrite fraction. However, accelerated cooling and smaller austenite grain sizes resulted in refining the ferrite grains. Additional ferrite grain refinement was obtained with the prior deformation of the initial austenite microstructure which increased the ferrite nucleation rate by introducing additional nucleation sites both on the austenite grain boundary and within the deformed grains at crystallographic defects. The transformation start temperatures and polygonal ferrite fraction were significantly increased by the retained strain. A previously developed sequential transformation model was applied to describe the austenite-to-polygonal ferrite transformation which consisted of sub-models to predict the transformation start temperature, ferrite growth and ferrite grain size under continuous cooling conditions. The first sub-model predicted transformation start temperature by combining corner nucleation of ferrite with early growth. The subsequent ferrite growth was described using a model that employed the Avrami equation (or JMAK model) which was adapted to non-isothermal transformations, i.e. continuous cooling, by using the Scheil equation of additivity with an Avrami exponent of n = 0.85 and a rate constant b which depends exponentially on temperature; the effect of austenite grain size on the transformation kinetics was captured with a suitable grain size exponent m. The ferrite grain size was predicted by employing a model that expressed ferrite grain size as a function of initial austenite grain size and transformation start temperature. The combined effect of austenite grain size and retained strain was incorporated into the transformation start, transformation kinetics and ferrite grain size models by employing an effective grain size, i.e. d[sub eff] = d[sub γ] exp(- ε).
Item Metadata
Title |
Austenite decomposition of a HSLA-Nb/Ti steel and an A1-TRIP steel during continuous cooling
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2004
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Description |
The ability to control and predict the mechanical properties of hot rolled steel
depends strongly on the thermomechanical processes which the steel undergoes. The
phase transformation that occurs on the run-out table of a hot strip mill is a critical
processing step which significantly influences the final microstructure, and thus, the
properties of the hot rolled steel. This work examines the austenite-to-ferrite phase
transformation of a high-strength low-alloy steel (HSLA-90) microalloyed with niobium
and titanium and a transformation-induced plasticity steel alloyed with aluminium (Al-
TRIP). The austenite decomposition kinetics have been investigated using a Gleeble 3500
thermomechanical simulator equipped with a dilatometer.
The effect of cooling rate on the austenite decomposition kinetics were quantified
for both steels with initial microstructures comprised of austenite. For the Al-TRIP an
additional initial microstructure consisting of a mixture of austenite (67%) and ferrite
(33%) phases was also investigated. It was shown that accelerated cooling lowers the
transformation temperatures in addition to refining the resulting ferrite grains.
Microstructural analyses revealed a decrease in polygonal ferrite fraction with accelerated
cooling and the formation of acicular products for the HSLA-90 steel. A transition from
high temperature products such as, polygonal ferrite and pearlite, to low temperature
products such as, bainite and martensite, was seen for the Al-TRIP with accelerated
cooling where there was an increase in bainite and martensite fractions.
The effect of initial austenite grain size and a pancaked austenite microstructure
was investigated for the HSLA-90 steel. Increasing the austenite grain size resulted in a
shift to lower transformation start temperatures and an associated decrease in the
polygonal ferrite fraction. However, accelerated cooling and smaller austenite grain sizes
resulted in refining the ferrite grains. Additional ferrite grain refinement was obtained
with the prior deformation of the initial austenite microstructure which increased the
ferrite nucleation rate by introducing additional nucleation sites both on the austenite
grain boundary and within the deformed grains at crystallographic defects. The transformation start temperatures and polygonal ferrite fraction were significantly
increased by the retained strain.
A previously developed sequential transformation model was applied to describe
the austenite-to-polygonal ferrite transformation which consisted of sub-models to predict
the transformation start temperature, ferrite growth and ferrite grain size under
continuous cooling conditions. The first sub-model predicted transformation start
temperature by combining corner nucleation of ferrite with early growth. The subsequent
ferrite growth was described using a model that employed the Avrami equation (or
JMAK model) which was adapted to non-isothermal transformations, i.e. continuous
cooling, by using the Scheil equation of additivity with an Avrami exponent of n = 0.85
and a rate constant b which depends exponentially on temperature; the effect of austenite
grain size on the transformation kinetics was captured with a suitable grain size exponent
m. The ferrite grain size was predicted by employing a model that expressed ferrite grain
size as a function of initial austenite grain size and transformation start temperature. The
combined effect of austenite grain size and retained strain was incorporated into the
transformation start, transformation kinetics and ferrite grain size models by employing
an effective grain size, i.e. d[sub eff] = d[sub γ] exp(- ε).
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Extent |
12008549 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-12-03
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0078775
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2005-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.