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Effect of Intercritical Annealing Timeon γ→α Transformation during Coolingin Cold-rolled Dual Phase Steels
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Effect of Effect of IntercriticalIntercritical Annealing TimeAnnealing Timeon on Transformation during CoolingTransformation during Cooling
in Coldin Cold--rolled Dual Phase Steelsrolled Dual Phase Steels
Yuki Toji, Katsumi Nakajima, Takako Yamashita, Kaneharu Okuda,Hiroshi Matsuda, Kohei Hasegawa and Yasushi Tanaka
2008/3/28
#155th ISIJ Meeting
Steel Research Laboratory, JFE Steel Corporation
Int.14
2IntroductionIntroductionColdCold--rolled Dual Phase steelsrolled Dual Phase steelsDP steelDP steelexcellent balance with high strength and good formabilityexcellent balance with high strength and good formability
from manufacturing prospective, from manufacturing prospective, the stability of mechanical properties is importantthe stability of mechanical properties is important to achieve high accuracy to achieve high accuracy of car partsof car parts
M
F
FFerriteMMartensite
3IntroductionIntroductionTo control the mechanical properties,Precise microstructural control during continuous annealing linesis necessary
ecrystallization of
transformation
transformationtransformationpartitioning of alloying elementpartitioning of alloying element
transformationtransformationredistribution ofredistribution ofalloying element alloying element
A3
A1
ferrite + pearlite ferrite + martensite
dissolution of
4Effect of Effect of intercriticalintercritical annealing time on TSannealing time on TS
TS decreases with a decrement of annealing timeDifference of the TS between specimens with different
annealing time is larger at a lower Tq
1073KAir Cooling (A.C.)
Tq=873973K
W.Q.
ta=1001000s
Toji et al. (IS3-2007)
950
1000
1050
1100
1150
823 873 923 973 1023Quench start temperature(Tq) / K
T
S
/
M
P
a
100s250s1000s
Annealing time(ta)
Tempering condition573K600s
0.13%C-1.4%Si-2%Mn (mass%)
5Effect ofEffect of intercriticalintercritical annealing time on microstructureannealing time on microstructure
Change in VM shows a good agreement with the influence of annealing time on TSChange in TS with the annealing time is mainly caused by a change in VM
V
o
l
u
m
e
f
r
a
c
t
i
o
n
o
f
m
a
r
t
e
n
s
i
t
e
(
V
M
)
/
%
6035 40 45 50 55VM / %
950
1000
1050
1100
1150
T
S
/
M
P
a
Toji et al. (IS3-2007)1073K A.C.Tq=873973K
W.Q.
ta=1001000s
Tq / K
1000s
38
42
46
50
54
58
823 873 923 973 1023
250s
100s
ta
A.C.
IntercriticalIntercritical annealing time at 1073K affects the mechanical properties ofannealing time at 1073K affects the mechanical properties ofcoldcold--rolled DP steels through the change of microstructurerolled DP steels through the change of microstructure
6Objective of this studyObjective of this study
(1(1) ) transformation during transformation during intercritical intercritical annealingannealing(2) Subsequent(2) Subsequent transformationtransformation during coolingduring cooling
To clarify the effect of theTo clarify the effect of the intercriticalintercritical annealing time on annealing time on microstructures of coldmicrostructures of cold--rolled DP steelsrolled DP steels
7Experimental proceduresExperimental procedures
C Si Mn P S Al N Fe0.125 1.40 1.98 0.016 0.001 0.032 0.0034 Bal.
Chemical compositionChemical composition
MicrostructureMicrostructure SEM the volume fraction of martensite
Elemental analysisElemental analysis EPMA the distribution of alloying elements
ColdCold--rolled steel sheetrolled steel sheet(1.4mmt, ferrite(1.4mmt, ferrite--pearlitepearlite))
1073K (+ region)Air cooling (A.C.)
873973K
Water quenching (W.Q.)
ta=01000s
HeatHeat--treatmenttreatment
(mass%)
8ContentsContents
(1(1) ) transformation duringtransformation during intercriticalintercritical annealingannealing
(2) Subsequent(2) Subsequent transformationtransformation during coolingduring cooling
9 transformation during annealingtransformation during annealingBefore annealing
cold-rolled steel
annealing time
1073K
W.Q.
0s
100s 250s 1000s
M
F10m
P M
F2m
RD
ND
FP
F
10
Relationship between VRelationship between V and annealing timeand annealing time
Volume fraction of Volume fraction of is almost saturated more than 250sis almost saturated more than 250sIt canIt cant be explained only by the volume fraction of t be explained only by the volume fraction of during annealingduring annealingChange in the distribution of Change in the distribution of MnMn,,SiSi during annealing was investigatedduring annealing was investigated
Equilibrium fraction (Thermo-Calc)
202530354045505560
0 200 400 600 800 1000 1200Annealing time / s
V
o
l
u
m
e
f
r
a
c
t
i
o
n
o
f
/
%
W.Q.
annealing time
1073K
1023Tq / K
V
M
/
%
1000s250s
100s
ta
A.C.38
42
46
50
54
58
823 873 923 973
100s250s 1000s
11
Distribution of Distribution of MnMn,,SiSi before annealingbefore annealing
Si mapping Mn mapping
Before annealing, Si,Mn are distributed almost uniform
5m
SEM
F
P
RD
ND
12
Partitioning of Partitioning of MnMn during annealingduring annealing
M
M
F
F
F
F
F
F
M
M
F
30s 100s 250s 1000sMnMn mappingmapping
Annealing time at 1073K
SEMSEM--BEIBEIsame area of each mappingssame area of each mappings
MnMn concentrates into concentrates into (M) following the nucleation and growth of (M) following the nucleation and growth of
M
M
M
RD
ND
5m
13
Partitioning of Partitioning of SiSi during annealingduring annealing
M
M
F
F
F
F
F
F
M
M
F
SiSi mappingmapping
SEMSEM--BEIBEIsame area of each mappingssame area of each mappings
SiSi concentrates into concentrates into following the nucleation and growth of following the nucleation and growth of
M
M
M
RD
ND
5m
30s 100s 250s 1000sAnnealing time at 1073K
14
Change in Change in fraction and fraction and MnMn distribution during annealingdistribution during annealing
Before 500s, Before 500s, is considered to be in a is considered to be in a metastable metastable statestate
Equilibrium frac. (Thermo-Calc)
20
25
30
35
40
45
50
55
60
0 200 400 600 800 1000 1200Annealing time / s
V
o
l
u
m
e
f
r
a
c
t
i
o
n
o
f
/
%
1.8
2.0
2.2
2.4
2.6
2.8 Mn
concentration in / w
t%Mn content in the steel
Equilibrium conc.(Thermo-Calc)
Ta=1073K
Difference ofDifference of MnMn concentration in concentration in affects affects thethe transformationtransformation
15
ContentsContents
(1(1) ) transformation duringtransformation during intercriticalintercritical annealingannealing
(2) Subsequent(2) Subsequent transformationtransformation during coolingduring cooling
16
Influence of Influence of MnMn distribution on distribution on transformation during coolingtransformation during cooling
V
o
l
u
m
e
f
r
a
c
t
i
o
n
o
f
/
%
35
40
45
50
55
60
0 2 4 6 8 10 12 14 16 18 20
ta=100s(MnMn in in ==2.3%2.3%)
ta=1000s (Mn in Mn in ==2.55%2.55%)
Cooling time
973K 923K 873K1073K
Air cooling (10K/s)
1073K for 100s or 1000sAir cooling
873973K
WQ
transformation rate is fasterin the specimen with low Mn concentraiton in
17
ConclusionConclusionTo clarify the effect of the intercritical annealing time on microstructures of cold-rolled DP steels, (1) transformation during intercritical annealing, and (2) Subsequent transformation during cooling were investigated
At the early stage of transformation during annealing at 1073K, substitutional alloying elements hardly diffuses, and substitutionalscontinue to partition even after the volume fraction of reaches toequilibrium value.
Although the volume fraction of is almost same before cooling,the transformation kinetics differs depending on the Mn
concentration in . It was found that the specimen with lower Mnconcentration in after annealing shows the faster transformation behavior.
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