Incomplete transformation of bainite in

microalloyed high strength low alloy steels

The 155th ISIJ meeting (2008.3.28)

International organized session

“Effects of alloying elements on

microstructure formation in steels

and other materials”

T. Furuhara, K. Takahashi, G. Miyamoto

Inst. Mater. Res., Tohoku Univ., Japan

Industrial importance of bainitic steel - example

Welded structural steels

but increase in the amount of MA

e.g., Bridge, Building, Vessel, Pipeline etc.

Transformation during fast cooling

from coarse g grains

Formation of Martensite/Austenite (MA) constituent

Degradation of toughness

Nb bearing steel - Incomplete transformation of bainite

Microalloying (B, Mo, Nb) for increasing in hardenability

• Decrease in carbon content down to less than 0.1mass%

for better weldability

• Use of bainitic structure to obtain high strength

Transformation stasis (Incomplete transformation)

Fe-0.19C-1.81Mo(mass%)

Reynolds, Li, Shui, Aaronson : Met. Trans. A. 1990

Mo segregation at g/a interfaceExplanation of the transformation

stasis below the bay by SD(L)E

100

0

Restriction of ferrite

growth by SD(L)E

Interphase boundary

carbide precipitation

Sympathetic nucleation

of degenerate ferrite

Cease of

sympathetic nucleation

Log t

Aust

enit

e tr

ansf

orm

ed (

%)

(Reynolds et al : Met. Trans. A. 1990)

（Humphery et al: MMTA, 2004.)

Bainite transformation in Si-added steels

Bhadeshia, Edmonds: Met. Trans., 10A(1979), 895.

Enrichment of carbon up to T0’

→ Loss of driving force for

displacive transformationSuppression of Fe3C precipitation in g

→ Enrichment of carbon in g

Increase of retained g TRIP

Incomplete

transformation

γstable

(C-rich)

γunstable

BF

Carbide

（Tsuzaki, Maki: Netsushori, 32(1992)，70.）

(Experimental)

Fe-(0.05, 0.15)C-1.5Mn -0.2Si-(0, 0.030)Nb (mass%)

Fe-(0.05, 0.15)C-1.5Mn -(0, 0.5)Mo - (0, 0.001)B

Materials & Heat treatment

Isothermal holding : 723 ~ 973 K ~86.4ks

Isothermal holding : 773 ~ 873 K ~ 1036.8 ks

Microstructure observation : OM, SEM, TEM

Measurement of phase fraction : Point counting method

To clarify effects of microalloying (Nb, Mo, B) on

transformation behavior of Fe-low C-1.5Mn steels

(Objective)

Effect of Nb addition

Isothermal transformation (OM) PF : Polygonal Ferrite

B : Bainite

M : MartensiteP :Pearlite

MA :Martensite-Austenite constituent

Fe-0.15C-1.5Mn (Nb free alloy)

0.03%Nb added alloy

973K 823K 773K

973K 823K 773K

Fe-0.15C-1.5Mn-(0, 0.03Nb) , transformed at 773K

Nb free 0.03%Nb added

Bainite transformation with cementite precipitation

No difference in the transformation kinetics with Nb addition

Fe-0.15C-1.5Mn-(0, 0.03Nb), transformed at 853K

stasis

Nb free 0.03%Nb added

Transformation stasis appears by Nb addition.

(Formation of carbide-free bainitic ferrite)

BF

MA

1μm

P

BFα

α

Fe-0.15C-1.5Mn-0.03Nb transformed at 853K for 10.8ks (TEM)

θ

BF

BF : Bainitic Ferrite

θ: Cementiteα : Ferrite

P :Pearlite

MA :Martensite-Austenite constituent

Dislocation-free ferrites of new

orientations containing q forms

after the stasis.

Nb free 0.03%Nb

Fe-0.05C-1.5Mn alloys, transformed at 853K

A higher BF fraction at the stasis

for a lower C content

Fe-1.5Mn-C (para)

phase diagram

○： Initial C content

●： C contents estimated

at the stasis by

a lever rule

NPLE

Mechanism for transformation stasis with Nb addition

T0’ : Upper limit of BF formation

Ae3

Carbon %

Te

mp

era

ture

Initial carbon content

T0’

773K : No differnce with Nb addition

853K : Stasis with Nb addition

853K

773K

Acm

θ

B

c

c

c

c

BF

BF

BF

γ

• BF formation

• C enrichment

in g

Diffusional

decomposition

Suppression by Nb

Stasis

BF

BF

BF

θ

No stasis

θ

BF

BF

BF

γ γ

BF

BF

BF

773K 853K

Suppression of ferrite transformation with Nb addition

On nucleation

Decrease in g grain bounday energy with Nb segregation

On growth

Solute drag effect by Nb

Pinning by Nb(C,N) precipitation

(M. Suehiro, Z. -K. Liu, J. Ågren：Acta Mater., 44 (1996), 4241)

Decrease in diffusion coefficient of carbon(S. Nanba, H. Morimoto, G. Anami, T. Towada:

Kobe steel Eng. Rep., 47 (1997), 8)

(M. Enomoto, N. Nojiri, Y. Sato：Mater. Trans., JIM, 35 (1994), 859)

Decrease in BF/g bounday energy with Nb segregation

Effect of (B, Mo) addition

Base 10s

10mm

P

BF

M(A)

Base 10.8ks

P

BF

α

MA

1 10 102

103

104

105

1060

20

40

60

80

100

Time (s)

Fra

ction o

f auste

nite

transfo

rmed (

vol.%

)

B-added

Base

(B,Mo)-

added

Mo-added

10mm

Isothermal transformation at 873K (OM)

Mo-added 0.6ks Mo-added 259.3ks

BF

α

BF

M(A)

M(A)

10mm 20mm

BF: Bainitic Ferrite, α: Ferrite MA: Martensite-Austenite constituent

P: Pearlite M(A): Mertensite(untransformed austenite)

Addition of Mo

Suppression of diffusional

transformation

Negligible amount of

carbide after the stasis

BF: Bainitic Ferrite θ: Cementite Martensite-Austenite constituent

M(A): Martensite (untransformed austenite) P: Pearlite DP: Degenerate Pearlite

10mm 10mm 10mm

Mo-added 10s

Before stasis

Mo-added 86.4ks

During stasis

Mo-added 1036.8ks

After stasisM(A)

BF

MA

carbide

P

BF

MA

P

BF

θ

Base 10s Base 0.6ks

10mm 10mm

BF

P

θ

BFDP

MA

DP

Time (s)

Fra

ction o

f auste

nite

transfo

rmed (

vol.%

)

B-addedBase

1 10 102

103

104

105

106

0

20

40

60

80

100

Mo-added

(B,Mo)-added

Isothermal transformation at 823K (OM)

M(A)

TEM microstructure transformed at 823K

Mo-added alloy

(B,Mo)-added alloy

BF: Bainitic Ferrite MA: Martensite-Austenite constituent α: Ferrite θ: Cementite M23C6 carbide

500nm

1mm 500nm

(B,Mo)-added alloy

αMA

BF

BF

α

BF

M23C6

BF

BF

BF

M23C6

10.8 ks (during staisis) 1036.8 ks (after stasis)

θuntransformed austenite

During the stasis

→ remainig as interlath MA

After the stasis

→ decomposing to ferrite+carbide

<diffusional transformation>

Fast transformation kinetics

without much difference

Isothermal transformation at 773K (SEM)

BF: Bainitic Ferrite MA: Martensite-Austenite constituent θ: Cementite DP: Degenerate Pearlite

Time(s)

Fra

ction o

f auste

nite

transfo

rmed (

vol.%

)

B-added

Base

Mo-added

(B,Mo)-added

1 10 102

103

104

105

0

20

40

60

80

100

B-added 60s (B,Mo)-added 60s

BF

10mm

interlathθ

DP

interlath θ

MA

5mm

intralathθ

intralathθ

BF

Bainite transformation

with θ precipitation

No transformation stasis

T0’(⊿G=100Kj/mol)

T0’(⊿G=200Kj/mol)T0’(⊿G=300Kj/mol)

T0’(⊿G=400Kj/mol)T0’(⊿G=500Kj/mol)

Fraction of transformation at 823K

■0.15%C-(B,Mo)-added alloy

60%

■0.05%C-(B,Mo)-added alloy

92%

Evaluation %C from the lever rule

α γ

0.0236 0.358

para-α T0‘ content

0.0226 0.348

0 0.2 0.4 0.6 0.8

773

873

973

1073

%C

Tem

pera

ture

(K

)

para-Ae3

T0

NPLE-γ

Fe-C-1.5mass%Mn (para)

Time (s)

Fra

ction o

f auste

nite

transfo

rmed (

vol.%

)

B-addedBase

Mo-added

(B,Mo)-added

1 10 102 103 104 105 1060

20

40

60

80

100

Fe-0.15C-1.5Mn(-100ppmB-0.5Mo)

Incomplete transformation by

Mo addition

1 10 102

103

104

105

106

0

20

40

60

80

100

Time (s)

Fra

ction o

f auste

nite

transfo

rmed (

vol.%

)B-added

Base Mo-added

(B,Mo)-added

823K

873K

Restart by a formation at 873K

by M23C6 precipitation at 823K

873K，823K：Carbide-free BF formation

→ Stasis

773K：Bainite transformation

with cementite (no stasis)

(Summary)

1. A transformation stasis in upper bainite transformation

appears in Mn-containing low-alloy low-carbon steels

microalloyed with Nb and Mo.

2. Dislocation-free ferrites of which orientations are often

different from those of adjacent BFs form with carbide

precipitation after the stasis.

3. Mo or Nb in solution suppress the nucleation of ferrite

at BF/g interphase boundary in a temperature range

where the incomplete transformation of bainite occurs.