grain growth behaviour of niobium-alloyed direct quenched steels during slab reheating casr-seminar...
TRANSCRIPT
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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
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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• 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
Comp. C Si Mn P S Cr Ni Mo Ti Nb Cu Al B O2 N2
DQP 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
w-%
DQP = Direct Qunched and Partitioned
• high carbon, silicon and aluminium contents stabilize austenite
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QUENCH & PARTITIONING PROCESS
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• 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
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025
050
075
010
0012
5015
0017
5020
0025
0030
0035
0040
0043
5055
2057
6059
4062
4065
4067
8073
200
100
200
300
400
500
600
700
800
900
1000
1100
1200
Heating 1Heating 2
Time [s]
Tem
pera
ture
[°C
]
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EXPERIMENTAL HEATING PROCEDURE
Hot furnace
~10 °C/min ~700 °C
~1000 °C
~1150 °C
~5 °C/min
~1200 °C
~1.6 °C/min
Waterquenching
30 min
30 min
30 min
30 min
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AUSTENITE GRAIN SIZE: DQP
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AUSTENITE GRAIN SIZE: DQP
Test temperature: 1050 °C
DQP+0.05 Nb DQP+0.02Nb DQP
25 mm
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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 thecoarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature (2) Coarse γ grains are formed bythe 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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Normal grain growth
Abnormal grain growth
ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR
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• 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
ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR
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DQP+0.05 Nb
850 °C
ABNORMAL α TO γ TRANSFORMATION BEHAVIOUR
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EFFECT OF PRIOR HEAT TREATMENT
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• 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
1050 °C 650 °C (90 min) + 1050 °C
DQP+0.02Nb
Composition Vol. Fraction of retained austenite [%] As cast 650°C, 90 min
DQP 7.14* DQP+0.02 Nb 6.78* DQP+0.05 Nb 6.77* 2.54*
* Results from XRD-measurements
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EFFECT OF PRIOR HEAT TREATMENT
As cast After 650 °C for 90 min
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EFFECT OF STARTING AUSTENITE GRAIN SIZE TO FINAL GRAIN SIZE
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Sellars: Dr=1.1*D0^0.67*ε^-0.67
D0 [γm] ε Dr [γm]50 0.2 44
44 0.2 4141 0.2 3939 0.2 38
D0 [γm] ε Dr [γm]3000 0.2 691
691 0.2 258258 0.2 134134 0.2 86
• Grain sizes after hot rolling• 4 passes above Tnr - temperature
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SUMMARY
• Abnormally coarse austenite grains were detected in DQP-compositions, 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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