Transcript PowerPoint
GRAIN GROWTH BEHAVIOUR OF NIOBIUM-ALLOYED
DIRECT QUENCHED STEELS DURING SLAB REHEATING
CASR-seminar 19.12.2013
Materials engineering laboratory/ Jaakko Hannula
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CONTENT
• Introduction
• Purpose of the study/ experimental
• Results
• Summary
PROJECT
• This study is part of the 2-year project called ”Optimum Use of
Niobium in Modern Direct Quenched Steels”
• Started 1.10.2012, ending 30.09.2014
• Funded by Ruukki Oy (Original funder CBMM)
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PURPOSE OF THE STUDY
• Effect of Nb on grain growth behaviour of cast material
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4 different temperatures: 1050°C, 1100°C, 1150°C, 1200°C
3 different compositions
Slow heating rate
Austenite grain size measurements
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COMPOSITIONS
DQP = Direct Qunched and Partitioned
w-%
Comp.
DQP
C
Si
Mn
P
S
Cr
Ni
Mo
Ti
Nb
Cu
Al
B
O2
N2
0.236 0.73
1.7
0.011 0.0015 1.2
0.018 0.11 0.0022 0.0048 0.013 0.65 0.0003 0.0012 0.0018
DQP+0.02
Nb
0.236 0.73
1.7
0.011 0.0015 1.2 0.0087 0.097 0.002 0.032 0.014 0.66 0.0003 0.0012 0.0018
DQP+0.05
Nb
0.232 0.73
1.7
0.011 0.0015 1.1 0.0088 0.1 0.0016 0.058 0.014 0.64 0.0003 0.0012 0.0018
• high carbon, silicon and aluminium
contents stabilize austenite
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QUENCH & PARTITIONING PROCESS
• carbide precipitation and decomposition of austenite are suppressed by
appropriate alloying
• the carbon partitions from the supersaturated martensite phase to the
untransformed austenite phase increasing the stability of the residual
austenite
microstructures with martensite/austenite combinations
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SLAB REHEATING BEFORE HOT ROLLING
1200
1100
Temperature [°C]
1000
900
800
700
600
Heating 1
500
Heating 2
400
300
200
100
0
Time [s]
EXPERIMENTAL HEATING PROCEDURE
~1200 °C
~1000 °C
~700 °C
30
min
Hot
furnace
30
min
30
min
~10 °C/min
~1150 °C
30
min °C/min
~1.6
~5 °C/min
Water
quenching
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AUSTENITE GRAIN SIZE: DQP
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AUSTENITE GRAIN SIZE: DQP
Test temperature: 1050 °C
25 mm
DQP+0.05 Nb
DQP+0.02Nb
DQP
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ABNORMAL α TO γ
TRANSFORMATION BEHAVIOUR
•The same coarse austenite grains as those before austenitizing emerge when
martensitic or bainitic steel with coarse grains is reheated to an austenite
region at a slow reheating rate.
• It can be also called as γ grain memory
(1) Coarse γ grains and fine globular γ grains that nucleate along the
coarse γ grain boundaries are formed when bainite or martensite steel is
reheated above the AC3 temperature (2) Coarse γ grains are formed by
the growth, impingement, and coalescence of acicular γ grains that
corresponds to retained γ between laths. (3) Abnormal α to γ
transformation is suppressed by decreasing the amount of retained γ and by
increasing the amount of cementite before reheating
•α to γ transformation behavior is governed by competition between the
nucleation and growth of newly formed γ from the dissolution of cementite
and the growth and coalescence of retained γ.
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ABNORMAL α TO γ
TRANSFORMATION BEHAVIOUR
Normal grain growth
Abnormal grain growth
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ABNORMAL α TO γ
TRANSFORMATION BEHAVIOUR
• Nb prevents cementite to precipitate in bainite carbon can
enrich to austenite + Si (0,73 %) and Al (0,65 %) will stabilize
formed retained austenite
normal grains can’t nucleate by dissolution of cementite,
because there are not any cementite particles abnormal,
very large grains will grow from retained austenite between
martensite/bainite laths very coarse austenite grain
structure
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ABNORMAL α TO γ
TRANSFORMATION BEHAVIOUR
DQP+0.05 Nb
850 °C
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EFFECT OF PRIOR HEAT TREATMENT
•Prior heat treatment at 650 °C for 90 minutes were used to transform the
retained austenite to ferrite and carbides
•Formed carbides (cementite) act as a new nucleation sites for new grains
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EFFECT OF PRIOR HEAT TREATMENT
DQP+0.02Nb
1050 °C
Composition
DQP
DQP+0.02 Nb
DQP+0.05 Nb
650 °C (90 min) + 1050 °C
Vol. Fraction of retained austenite [%]
As cast
650°C, 90 min
7.14*
6.78*
6.77*
2.54*
* Results from XRD-measurements
EFFECT OF PRIOR HEAT TREATMENT
As cast
After 650 °C for 90 min
EFFECT OF STARTING AUSTENITE GRAIN SIZE
TO FINAL GRAIN SIZE
• Grain sizes after hot rolling
• 4 passes above Tnr - temperature
D0 [γm]
3000
ε
0.2
Dr [γm]
691
691
258
134
0.2
0.2
0.2
258
134
86
D0 [γm]
50
ε
0.2
Dr [γm]
44
44
41
39
0.2
0.2
0.2
41
39
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Sellars: Dr=1.1*D0^0.67*ε^-0.67
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SUMMARY
• Abnormally coarse austenite grains were detected in DQPcompositions, when Nb was used as an alloying element
• Abnormal α to γ transformation-phenomenon was detected
• Nb prevents cementite precipitation in bainite no nucleation of
new austenite grains coarse austenite grains can grow
• This can be suppressed by decreasing the amount of retained
austenite or by increasing the amount of cementite precipitates
this was achieved by heat treatment at 650 °C for ~90 minutes
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Thank you!
Questions?