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

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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.