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Report
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?

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