ó Effects of combining Nb and Mo in

HSLA Steels: From austenite

conditioning to final microstructure

N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga

puranga@ceit.es

CEIT and TECNUN (University of Navarra)

Donostia-San Sebastian, Basque Country, Spain

Summary

• High strength

• Low temperature toughness

Nb-Mo microalloyed steels

Niobium

Strain Accumulation

Microstructural refinement

Molybdenum

Increase of hardenability

Nb-Mo MICROALLOYED STEELS

MULTIPLE MICROALLOYING

Combined effect of Nb-Mo on

• Austenite Conditioning

– Softening kinetics

– Non-recrystallization temperature

• Phase Transformation

– CCT Diagrams

– Unit size and microstructural homogeneity

• Mechanical Properties

– Tensile tests

– Strengthening contributions

CHEMICAL COMPOSITIONS

Steel C Mn Si Nb Mo Al N

CMn 0.05 1.58 0.05 - 0.01 0.03 0.005

3NbMo0 0.05 1.6 0.06 0.029 0.01 0.028 0.005

3NbMo16 0.05 1.58 0.04 0.03 0.16 0.027 0.005

3NbMo31 0.05 1.57 0.05 0.028 0.31 0.028 0.005

6NbMo0 0.05 1.56 0.05 0.06 0.01 0.028 0.004

6NbMo16 0.05 1.6 0.05 0.061 0.16 0.03 0.005

6NbMo31 0.05 1.57 0.05 0.059 0.31 0.031 0.005

Materials

1. AUSTENITE CONDITIONING

Nb-Mo Steels during hot-working

• The use of Nb is well known because of its effect retarding recrystallization.

• The addition of Mo to Nb microalloyed steels may introduce significant changes in the microstructural evolution during hot working.

• For example, it has been reported that Mo in solid solution produces a strong retardation effect on dynamic and static recrystallization.

• Therefore, the combination of both elements enhances strain accumulation prior to final cooling strategy.

156.0

08.1 DDSRX

for Nb microalloyed steels

for 0.03% Nb-Mo microalloyed steels

for 0.06% Nb-Mo microalloyed steels

eff

DNb

TRTDxt 185

275000exp

180000exp1092.9 53.06.5

0

11

5.0

15.00

NbNb eff

MoNbNb eff 09.019.1

MoNbNb eff 032.019.1

0

10

20

30

40

50

0 10 20 30 40 50t0.5 (exp.)

t 0.5 (

ca

l.)

3Nb3Nb-Mo6Nb6Nb-Mo166Nb-Mo31

Static Recrystallization Kinetics

950

975

1000

1025

1050

1075

1100

0 10 20 30 40

Interpass time (s)

T nr

(ºC

)

3Nb

3Nb-Mo31

6Nb-Mo31

6Nb

Dependence of Tnr as a function of the

interpass time ( = 0.4)

Low Nb (0.03%Nb)

0

20

40

60

80

100

7 7.5 8 8.5 9

10000/T (1/K)

Fra

cti

on

al

So

fte

nin

g (

%)

T nr = 1026 ºC

T nr = 985ºC

tip = 10 s, = 0.4

Precipitation

solute drag

3Nb

3Nb-Mo31

0

20

40

60

80

100

7 7.5 8 8.5 9

10000/T (1/K)

Fra

cti

on

al

So

ften

ing

(%

)

T nr =1030ºC

T nr = 1045ºC

tip = 30 s, = 0.4

6Nb

6Nb-Mo31

High Nb (0.06% Nb)

Precipitation

2. PHASE TRANSFORMATIONS

Thermomechanical schedule

CONTINUOUS COOLING TRANSFORMATION STUDY

Cooling rates: 0.1-200ºC/s

Precipitate dissolution

Austenite conditioning

Continuous cooling

Cycle A→ Undeformed austenite

Cycle B→ Deformed austenite (Strain = 0.4)

Cycle C → Deformed austenite (Strain = 0.8)

Transformation Products

BAINITIC FERRITE

MARTENSITE

BF

M

QUASIPOLIGONAL FERRITE

GRANULAR FERRITE

GF

QF

POLYGONAL FERRITE

DEGENERATED PEARLITE

DP

PF

0

200

400

600

800

1000

0.1 1 10 100 1000 10000

Te

mp

era

ture

(ºC

)

Time (s)

Cycle A

6NbMo0

6NbMo31

PF

GF+QF

DP

BF

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 221 220 240 196 178 170 167 158 154 139285 253 229 210 207 199 197 196 176 136

Continuous Cooling Transformation diagrams (CCT)

Effect of the addition of Mo

EFFECT OF CHEMICAL COMPOSITION

Mo↑ Transformation start

temperature ↓

0

200

400

600

800

1000

0.1 1 10 100 1000 10000

Te

mp

era

ture

(ºC

)

Time (s)

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 192 180 155 145 133 131 136 121 131 107 256 239 202 201 181 194 180 155 156 150277 292 256 223 220 204 200 176 162 130

Cycle A

CMn

3NbMo06NbMo0

PF+P

GF+QF

BF

DP

HIGH Nb Nb↑ Transformation start temperature ≈

Steel D0 (μm) Sv (mm-1)

Cycle A Cycle B

CMn 19 105.3 121.6

3NbMo0 17 117.6 135.9

6NbMo0 14 142.9 165

3NbMo31 12 166.7 192.5

6NbMo31 14 142.9 165

LOW Nb Nb↑ Transformation start temperature ↓

Continuous Cooling Transformation diagrams (CCT)

EFFECT OF CHEMICAL COMPOSITION

Effect of the addition of Nb

0

200

400

600

800

1000

0.1 1 10 100 1000 10000

Te

mp

era

ture

(ºC

)

Time (s)

ºC/s 200 100 50 20 10 5 2 1 0.5 0.16NbMo31

ε=0 (Cycle A) HV 285 253 229 210 207 199 197 196 176 136ε=0.4 (Cycle B) 246 246 232 220 215 204 192 183 181 155 ε=0.8 (Cycle C) 246 261 247 229 217 205 199 186 175 153

QF+GF

BFMs

PF

DP

Continuous Cooling Transformation diagrams (CCT)

EFFECT OF THE THERMOMECHANICAL SCHEDULE

Effect of the amount of deformation in austenite

ACCUMULATED DEFORMATION ↑

Transformation start temperature ↑

EBSD Quantification

6NbMo0 (1ºC/s) RECRISTALLYZED γ

DEFORMED γ

EBSD Quantification

Accumulation of deformation in γ

Cycle B Microstructural

refinement

Mean crystallographic unit sizes

0

4

8

12

16

0.01 0.1 1 10 100 1000

Me

an

Gra

in S

ize

(µ

m)

Cooling Rate (K/s)

6NbMo0_Cycle A

6NbMo0_Cycle B

6NbMo31_Cycle A

6NbMo31_Cycle B

4º

(a)0

4

8

12

16

0.01 0.1 1 10 100 1000M

ea

n G

rain

Siz

e (

µm

)

Cooling Rate (K/s)

6NbMo0_Cycle A

6NbMo0_Cycle B

6NbMo31_Cycle A

6NbMo31_Cycle B

15º

(b)

0

2

4

6

8

10

12

0.01 0.1 1 10 100 1000

Dc

20

% /

Dm

ean

(15

º)

Cooling Rate (K/s)

6NbMo0_Cycle B

6NbMo31_Cycle B

6NbMo0_Cycle C

6NbMo31_Cycle C

EBSD Quantification

Dc20% Cut off grain size at 80% area fraction

in a grain size distribution histogram

Microstructural heterogeneity → Dc20%/Dmean

FORMATION OF BAINITIC

FERRITE

FERRITIC MICROSTRUCTURES

3. MECHANICAL PROPERTIES

Thermomechanical schedules

Coiling temperatures:

650ºC, 550ºC, 450ºC

Precipitate dissolution

Austenite conditioning

Isothermal maintenance

COILING SIMULATIONS – PLANE STRAIN COMPRESSION

Final Microstructures

POLYGONAL FERRITE

PERLITE

QUASIPOLIGONAL FERRITE

GRANULAR FERRITE

6NbMo0 550ºC 650ºC

3. MECHANICAL PROPERTIES

Strength

300

400

500

600

700

400 450 500 550 600 650 700

Yie

ld S

tre

ng

th/T

en

sile

str

en

gth

(M

Pa

)

Coiling temperature (ºC)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

TS

YS

Mechanical properties

YIELD STRENGTH - TENSILE STRENGTH

Mechanical properties

CORRELATION BETWEEN MICROSTRUCTURE-MECHANICAL

PROPERTIES

),,,,σ(σ 0y pptgsssf

Composition

Dislocation

Density Grain/Unit Size

Precipitation

Mechanical properties

A. Iza-Mendia, I. Gutierrez, Materials Science and Engineering A, vol. 561, 2013, pp, 40-51

Grain Size

2

1

2º

θ

i

15ºθ2

iigs d·f10

πθfbαMμ*1.05σ

15ºii

Low angle

boundary fraction High angle

boundary fraction

350

400

450

500

3 4 5 6

Yie

ld S

tre

ng

th (

MP

a)

Mean grain size 2º (μm)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

Mechanical properties

• Mechanical strentgh ↑

•Toughness ↑

MICROSTRUCTURAL REFINEMENT

Nb-Mo

Nb

Grain Size

Mechanical properties

Dislocation Density

ραMμbσ ρ

ub

2ρ

Kernel Average

misorientation for θ<2º

Mechanical properties

6

7

8

9

400 450 500 550 600 650 700

Me

an

siz

e o

f p

rec

ipit

ate

s (

nm

)

Coiling Temperature (ºC)

3NbMo0

3NbMo31

6NbMo0

6NbMo31

6NbMo31

)106.125

xln(

x

f10.8σ

4

0.5

vppt

550ºC 650ºC

100 nm 100 nm

Precipitation

Hardening

16% 13% 14%

21%18%

22%

0% 12% 5%

63%57% 59%

0%

20%

40%

60%

80%

100%

650 550 450

σy

(MP

a)

Coiling temperature (ºC)

6NbMo31MOD 457 482 461EXP 426 434 473

16% 14% 14%

20%19% 17%

4% 8% 10%

60% 59% 59%

0%

20%

40%

60%

80%

100%

650 550 450

σy

(MP

a)

Coiling temperature (ºC)

6NbMo0MOD 428 454 445EXP 406 410 408

18% 15% 14%

19% 22% 26%

0%7% 4%

63%56% 57%

0%

20%

40%

60%

80%

100%

650 550 450

σy

(MP

a)

Coiling temperature (ºC)

3NbMo31MOD 431 435 456EXP 397 457 460

17% 17% 17%

20% 20% 20%

2% 2%5%

61% 62% 58%

0%

20%

40%

60%

80%

100%

650 550 450

σy

(MP

a)

Coiling temperature (ºC)

3NbMo0MOD 423 426 407EXP 387 401 414

UNIT SIZE

PRECIPITATION

DISLOCATIONS

COMPOSITION

Contributions to Strength

CONCLUSIONS

Conclusions

• Nb and Mo show synergetic mechanisms

ideal for:

– Strain accumulation during austenite

conditioning

– Transformation start temperature control

– Microstructural refinement after transformation

• The formation of low-angle boundary

substructure is the main contribution to

strength

ACKNOWLEDGEMENTS

Acknowledgements

• IMOA and CBMM

• Prof. Hardy Mohrbacher

• Spanish Government MINECO (MAT2009-

09250 and MAT2012-31056)

• Basque Government (PI2011-17)

• Thermomechanical Treatments Group at CEIT

ó Effects of combining Nb and Mo in

HSLA Steels: From austenite

conditioning to final microstructure

N. Isasti, B. Pereda, B. López, J.M. Rodriguez-Ibabe and Pello Uranga

puranga@ceit.es

CEIT and TECNUN (University of Navarra)

Donostia-San Sebastian, Basque Country, Spain