Effect of thermomechanical process on the austenite transformation in Nb-Mo microalloyed steels

Titulo de la presentaciónEffect of composition and thermomechanical

process on the austenite transformation in Nb-Mo microalloyed steels

October 21, 2010 – Houston, Texas

N. Isasti, B. López and P. Uranga

puranga@ceit.es

(CEIT and TECNUN, Univ. Navarra)

A. Leff, E. Toby, M. Hartshorne, C. Wrinkler and M. Taheri (Drexel U.)

M. Grimes (Lehigh U.)

Introduction

• Multiple microalloying for high performance grades:– High strength and low-T toughness

• Combined effect – synergies Nb-Mo• Effect of thermomechanical route:

– Recrystallized austenite– Deformed austenite

• Reducing grain size (unit size) in transformed phases:– strength and toughness improved

EXPERIMENTALSteel compositions and Techniques

Chemical compositions of steels (wt%)

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

Chemical compositions of steels (wt%)

Steel C Mn Si Nb Mo Al N

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

Experimental Procedure

• Bähr DIL805D deformation dilatometer

• Optical Microscopy

• Philips XL30cp Scanning Electron Microscope (SEM), using TSL (TexSEM laboratories) MSC 2002 equipment.

• Field Emission Scanning Electron Microscope (FEG-SEM) Jeol JSM-7000F, using HKL Channel5 EBSD

• Vickers hardness (HV 1 kg)

 1250ºC, 5min

1150ºC, ε=0.3, =1s-1

900ºC, ε=0.4, =1s-1

Cycle A Cycle BTem

pera

ture

(ºC

)

Time (s)

12s

20ºC/s5s

20ºC/sε&

ε& 11,4.0 −== sεε &

11,3.0 −== sεε &→

→

11,3.0 −== sεε &

11,4.0 −== sεε &

Schematics of thermomechanical schedules

RESULTSMicrostructural Characterization and Dilatometry Curves

0.1 ºC/s

0.5 ºC/s

PF+GF(+P)

PF+QF+GF

Optical Micrographs 6NbMo0Cycle BCycle A

10 ºC/s

100 ºC/s

QF+GF

QF+GF+BF+M

Optical Micrographs 6NbMo0Cycle BCycle A

Dilatometry curves

ΔL/L0 vs Temperature (ºC) Transformed Fraction vs Temperature (ºC)

6NbMo31

Cycle B

-0.012

-0.008

-0.004

0

0.004

0 200 400 600 800

ΔL/

Lo

Temperature (ºC)

0.1ºC/s 0.5ºC/s 10ºC/s 100ºC/s

0

20

40

60

80

100

0 200 400 600 800

Tran

sfor

med

fra

ctio

n (%

)Temperature (ºC)

0.1ºC/s 0.5ºC/s 10ºC/s 100ºC/s

Dilatometry curves – Transfomation Rates6NbMo31 Cycle B

0

20

40

60

0 200 400 600 800

Tran

sfor

mat

ion

Rat

e

Temperature (ºC)

10ºC/s 100ºC/s

0

0.2

0.4

0.6

0 200 400 600 800

Tran

sfor

mat

ion

Rat

e

Temperature (ºC)

0.1ºC/s 0.5ºC/s

CCT DIAGRAMSPhase Stability Regions

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000

Tem

pera

ture

(ºC

)

Time (s)

A PF

GF+QF

BF

GF

M

15% 88%

Ms

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 279 275 272 240 220 207 204 202 197 1916NbMo0 Cycle A

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000Te

mpe

ratu

re (º

C)

Time (s)

PF

GF+QF

92%

45% 65%

25%

BF

25%

Ms

P

M

A

6NbMo0Cycle B

GF

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 268 265 257 227 215 210 209 197 185 160

CCT Diagrams 6NbMo0

Cycle BCycle A

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000

Tem

pera

ture

(ºC

)

Time (s)

PF

GF+QF

BF

Ms

6NbMo16Cycle A

M

87%

15%

A

GF

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 292 280 271 240 229 219 217 211 203 199

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000Te

mpe

ratu

re (º

C)

Time (s)

PF

GF+QF

BF

Ms

85%

6NbMo16Cycle B

85%

55%

M

GF

A

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 270 268 264 248 234 219 213 212 204 157

CCT Diagrams 6NbMo16

Cycle BCycle A

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000

Tem

pera

ture

(ºC

)

Time (s)

70%PF

GF+QF

BF

GF

MMs

A

6NbMo31Cycle A

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

HV 311 298 292 260 236 220 214 213 212 210

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000Te

mpe

ratu

re (º

C)

Time (s)

A

GF+QF

PF93%

19% 41% 65%

75%

Ms

A

PF93%

19% 41% 65%

75%BF

Ms M

6NbMo31Cycle B

GF

ºC/s 200 100 50 20 10 5 2 1 0,5 0,1

HV 299 292 282 241 232 215 213 201 176 147

CCT Diagrams 6NbMo31

Cycle BCycle A

CCT ANALYSISAlloying and Retained Strain Effects

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000

Tem

pera

ture

(ºC

)

Time (s)

PF

GF+QF

BF

GF

MMs

A

6NbMo31

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

Effect of Retained Strain on CCT 6NbMo31

0

100

200

300

400

500

600

700

800

900

1000

0.1 1 10 100 1000 10000

Tem

pera

ture

(ºC

)

Time (s)

PF

GF+QF

BF

GF

MMs

A

Cycle B

ºC/s 200 100 50 20 10 5 2 1 0.5 0.1

Mo Effect on CCT Cycle B

LOW COOLING RATE PHASESEBSD / SEM-FEGSEM

Cooling Rate= 0.1 ºC/s

Low CR-EBSD

Effect of Mo addition

Cycle B

6NbMo0 6NbMo16 6NbMo31

Cooling Rate= 0.1 ºC/s

0

0.2

0.4

0.6

0.8

1

1.2

5 15 25 35 45 55 65 75 85 95 105

115

125

135

145

155

Accu

mul

ated

Are

a Fr

actio

n

Grain Size (Diameter µm)

6NbMo0

6NbMo16

6NbMo31

0.1ºC/s

EBSD

Effect of MoCycle B

IDENTIFICATION OF SECONDARY PHASES

EBSD / SEM-FEGSEM

Fe3C-1

MAD 0.538

Degenerated Pearlite

EBSD

Identification of second phases

1 ºC/s0.5 ºC/s

6NbMo16

Cycle B

M/A Islands

HIGH COOLING RATE PHASESBainite + Martensite + (GF+QF)

=50 µm; Map15; Step=0.2 µm; Grid700x700=50 µm; Map15; Step=0.2 µm; Grid700x700

High CR products – FEGSEM EBSD6NbMo16

100 ºC/s20 ºC/s

Image Quality (IQ)

1-15º>15º

=50 µm; Map15; Step=0.2 µm; Grid700x700=50 µm; Map15; Step=0.2 µm; Grid700x700

EBSD (Electron Backscatter Diffraction) 6NbMo16

100 ºC/s20 ºC/s

Misorientation Map

=50 µm; Map15; Step=0.2 µm; Grid700x700=50 µm; Map15; Step=0.2 µm; Grid700x700

EBSD (Electron Backscatter Diffraction) 6NbMo16

100 ºC/s20 ºC/s

Inverse Pole Figure IPF (z)

Final Remarks

• Effect of Mo and retained strain has been evaluated– Slight effect of Mo: increase in ferrite size– Strong effect of strain

• EBSD unit size measurements may be useful for toughness predictions

• The generated data will be the input for modeling

Acknowledgements

• Science and Innovation Ministry of Spain (MAT2009-09250)

• NSF and TMS. Conference Registration Fee Funding

• N . Isasti for all the experimental results and analysis (1st year PhD student)

• D. Jorge-Badiola: help on EBSD

Titulo de la presentaciónEffect of composition and thermomechanical

process on the austenite transformation in Nb-Mo microalloyed steels

October 21, 2010 – Houston, Texas

N. Isasti, B. López and P. Uranga

puranga@ceit.es

(CEIT and TECNUN, Univ. Navarra)

A. Leff, E. Toby, M. Hartshorne, C. Wrinkler and M. Taheri (Drexel U.)

M. Grimes (Lehigh U.)