Precipitation of Nb in ferrite after

austenite conditioning

I. Gutierrez, A. Iza-Mendia, A. Altuna, B. Pereda

CEIT and Tecnun (University of Navarra),

Manuel de Lardizábal 15, 20018

Donostia-San Sebastián Spain

the other co-authors

Amaia Iza-Mendia Alazne Altuna Beatriz Pereda

Donostia-San Sebastian

CEIT is a non profit Research Centre working

in partnership with the University of Navarra

www.ceit.es

www.tecnun.es

Reheating Hot rolling Run-out Coiling

table

INTRODUCTION

Strip hot rolling mill

The hot rolling mill transforms as-cast steel into finished or

semifinished products

it can be considered as a tool for getting tailored

combinations of mechanical properties

Ferrite grain

refinement

Reheating Hot rolling Run-out Coiling

table

INTRODUCTION

Strip hot rolling mill

• Austenite conditioning

Nb:

- Solute drag

- Precipitation

The main role of Nb microalloying is the control of the

austenite microstructure in the hot rolling mill

Reheating Hot rolling Run-out Coiling

table

INTRODUCTION

Strip hot rolling mill

Over the last 30 years most of the research on Nb

microalloyed steels has concentrated on the hot rolling mill

Higher requirements in terms of mechanical property at

minimum cost

INTRODUCTION

Need for process and additions

optimisation

S Vervynckt et al. International Materials Review, 2012

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Nb solubility in

austenite

INTRODUCTION Tem

pera

ture

Reheating

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Nb in solution

INTRODUCTION Tem

pera

ture

Reheating Hot rolling

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Nb in solution

INTRODUCTION Tem

pera

ture

Reheating Hot rolling

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Nb in solution

INTRODUCTION Tem

pera

ture

Reheating Hot rolling

Strain induced

precipitation

Run-out

table

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Nb in solution

INTRODUCTION

Tem

pera

ture

Reheating Hot rolling Run-out

table

Coiling

Reheating Hot rolling Run-out Coiling

table

INTRODUCTION

Strip hot rolling mill

Focus: extra strengthening associated with

Nb above that expected from grain

refinement

• Interphase precipitation

• Homogeneous precipitation

• Cluster formation

• Hardenability (transformation dislocations)

Nb in solution after austenite conditioning

Complex interactions, depending

on Nbfree and cooling strategies

INTRODUCTION

INTRODUCTION

“There is still a certain disagreement with regard to the

morphology of the precipitates”

“There is no definite answer to the question whether

precipitation of Nb carbides significantly contribute to

the strength or not”

Steel Reseach, 2004

OBJECTIVES

• Study the potential precipitation of Nb in ferrite during coiling.

• Estimate the precipitation strengthening

EXPERIMENTAL

C Si Mn Al Nb N

0.06 0.35 1.00 0.047 0.056 0.006

Plane strain compression + simulated coiling

Modelling assisted design of

the thermomechanical

sequences

EXPERIMENTAL

C Si Mn Al Nb N

0.06 0.35 1.00 0.047 0.056 0.006

Mechanical testing &

microstructural characterisation

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Tem

pera

ture

Coiling

1250ºC, 15 min

EXPERIMENTAL

1100ºC, 1s-1, =0.3; + 20s holding

Recrystallized

&

Nb in solution

Sequence S1:

750ºC

300ºC

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Tem

pera

ture

Coiling

1250ºC, 15 min

EXPERIMENTAL

1000ºC, 1s-1, =0.3

Strained

&

Some strain induced

precipitation of Nb

Sequence S2:

750ºC

300ºC

1100ºC, 1s-1, =0.3; + 20s holding

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Tem

pera

ture

Coiling

(650ºC)

1250ºC, 15 min

EXPERIMENTAL

1100ºC, 1s-1, =0.3

870ºC; 1h

Rex or Strained

&

full Nb precipitation

S1 and S2 Reference tests

1100ºC, 1s-1, =0.3; + 20s holding

870ºC

Nb precipiation

model predictions

B. López et al., 2006

PRECIPITATION IN AUSTENITE: REFERENCE TEST

TEM ANALYSIS

S1-R S2-R

Reference tests

Coiling T

750ºC

600ºC

500ºC

MICROSTRUCTURE

Bainitic

microstructures

Tcoiling<600ºC

0.06%C-0.056%Nb

S1

TENSILE PROPERTIES

TENSILE PROPERTIES + FERRITE GRAIN SIZE

CONTRIBUTIONS TO YIELD STRENGTH

pptdssy

dk 2/1

0

lattice

friction

solid

solution

grain size

dislocations

precipitates

Pickering, 1993

2/14.17)(5544833254 dCNSiMn freefreeyF

CONTRIBUTIONS TO YIELD STRENGTH

yFerimentalexpyyNb

2/1

freefreeyF d4.17)CN(5544Si83Mn3254

Cfree equilibrium

Nfree was determined by modelling

EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb

EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb

Full

precipitation

in

EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb

0.13%C-0.02%Nb Reducing the Nb content

Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE

y Nb (MPa) 90 150

S1

Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE

y Nb (MPa) 90 150

S1

Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE

y Nb (MPa) 90 150

S1

Coiling at 750ºC

Precipitation at the transformation front

GB

TEM ANALYSIS

Thin foil

CCT CURVES

CCT + INTERPHASE PTT CURVES

T. Sakuma and R.W.K. Honeycombe: 1984

Coiling at 750ºC

CCT + INTERPHASE PTT + COILING CURVES

Coiling at 750ºC

TEM ANALYSIS

• Nb-rich precipitates:

• in few regular parallel rows (spacing

1300 nm)

• in some segments of rows

• irregularly distributed

• AlN nucleated at dislocations

Discontinuous interphase precipitation

CCT + INTERPHASE PTT + COILING CURVES

Coiling at 600ºC

Coiling at 600ºC

Homogeneous precipitation in ferrite

TEM ANALYSIS

Thin foil

0 2 4 6 8 10 12 14 16 18 20

keVFull Scale 68 cts Cursor: 19.536 (0 cts)

foto 3 grande

Cu

Cu Nb

NbAl

CuFe

Carbon extraction replica TEM ANALYSIS

Coiling at 600ºC

From the

Cu-grid

410125.6ln8.10)(

d

d

fMPa v

ppt

PRECIPITATION STRENGTHENING

ASHBY-OROWAN'S EQUATION

fv: volume fraction

d: average precipitate diameter in m

PRECIPITATION STRENGTHENING

Coiling at 650ºC

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300

Tem

pera

ture

1250ºC, 15 min

1100ºC, 1s-1, =0.3; + 20s holding

Sequence S1:

600ºC; 24 or 48h

Long holding at Coiling

temperature

STABILITY

S1 + holding at 600ºC

TENSILE

The tensile properties are not affected by a prolonged holding at 600ºC

S1 + 48h holding at 600ºC

TEM ANALYSIS

Baker-Nutting orientation relationship

• Coiling at 600ºC produces in this steel an homogeneous, extremely fine and stable general precipitation of NbC in ferrite, leading to maximum strengthening:

• proportional to the Nb left in solution in austenite;

• in agreement with the predictions from Ashby-Orowan’s equation.

CONCLUSIONS

• Considering homogeneous precipitation of NbC in ferrite as a possible strengthening mechanism opens the way to a better design of the composition and thermomechanical sequences for an improved use of Nb additions.

• To the Institute of Materials, Minerals and Mining (IOM3)

• To Companhia Brasileira de Metalurgia e Mineração (CBMM)

• To Beta Technology

• To Naila Croft, Ben Mico and Georgia Gomes Bemfica.

ACKNOWLEDGEMENTS

Modelling to Optimise the Processing of Niobium Steels

50