Precipitation of Nb in ferrite after austenite conditioning I. - - PowerPoint PPT Presentation

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Precipitation of Nb in ferrite after austenite conditioning I. - - PowerPoint PPT Presentation

Dec 13, 2022 472 likes •985 views

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 Lardizbal 15, 20018 Donostia-San Sebastin Spain the other co-authors Amaia

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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

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SLIDE 2

the other co-authors

Amaia Iza-Mendia Alazne Altuna Beatriz Pereda

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SLIDE 3

Donostia-San Sebastian

CEIT is a non profit Research Centre working in partnership with the University of Navarra

www.ceit.es www.tecnun.es

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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

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SLIDE 5

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

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SLIDE 6

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

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Higher requirements in terms of mechanical property at minimum cost

INTRODUCTION

Need for process and additions

  • ptimisation

S Vervynckt et al. International Materials Review, 2012

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Nb solubility in austenite

INTRODUCTION

Temperature

Reheating

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SLIDE 9

200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Nb in solution

INTRODUCTION

Temperature

Reheating Hot rolling

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Nb in solution

INTRODUCTION

Temperature

Reheating Hot rolling

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Nb in solution

INTRODUCTION

Temperature

Reheating Hot rolling

Strain induced precipitation

Run-out table

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Nb in solution

INTRODUCTION

Temperature

Reheating Hot rolling Run-out table Coiling

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SLIDE 13

Reheating Hot rolling Run-out Coiling table

INTRODUCTION

Strip hot rolling mill Focus: extra strengthening associated with Nb above that expected from grain refinement

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  • Interphase precipitation
  • Homogeneous precipitation
  • Cluster formation
  • Hardenability (transformation

dislocations)

Nb in solution after austenite conditioning

Complex interactions, depending

  • n Nbfree and cooling strategies

INTRODUCTION

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SLIDE 15

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

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OBJECTIVES

  • Study the potential precipitation of Nb in

ferrite during coiling.

  • Estimate the precipitation strengthening
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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

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SLIDE 18

EXPERIMENTAL

C Si Mn Al Nb N

0.06 0.35 1.00 0.047 0.056 0.006

Mechanical testing & microstructural characterisation

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SLIDE 19

200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Temperature

Coiling 1250ºC, 15 min

EXPERIMENTAL

1100ºC, 1s-1, =0.3; + 20s holding

Recrystallized  & Nb in solution Sequence S1:

750ºC  300ºC

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SLIDE 20

200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Temperature

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

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Temperature

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

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870ºC Nb precipiation model predictions

  • B. López et al., 2006

PRECIPITATION IN AUSTENITE: REFERENCE TEST

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TEM ANALYSIS

S1-R S2-R Reference tests

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Coiling T 750ºC 600ºC 500ºC MICROSTRUCTURE

Bainitic microstructures Tcoiling<600ºC

0.06%C-0.056%Nb

S1

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SLIDE 25

TENSILE PROPERTIES

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TENSILE PROPERTIES + FERRITE GRAIN SIZE

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SLIDE 27

CONTRIBUTIONS TO YIELD STRENGTH

ppt d ss y

d k     

 

   

 2 / 1

lattice friction solid solution grain size dislocations precipitates Pickering, 1993

2 / 1

4 . 17 ) ( 5544 83 32 54

      d C N Si Mn

free free yF

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SLIDE 28

CONTRIBUTIONS TO YIELD STRENGTH

yF erimental exp y yNb

     

2 / 1 free free yF

d 4 . 17 ) C N ( 5544 Si 83 Mn 32 54

      

Cfree equilibrium Nfree was determined by modelling

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EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb

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EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb

Full precipitation in 

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EXTRA STRENGTHENING DUE TO Nb

0.06%C-0.056%Nb 0.13%C-0.02%Nb

Reducing the Nb content

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Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE y Nb (MPa)  90 150

S1

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SLIDE 33

Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE y Nb (MPa)  90 150

S1

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SLIDE 34

Coiling at : 750ºC 600ºC

IQ-EBSD

EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE y Nb (MPa)  90 150

S1

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Coiling at 750ºC

Precipitation at the transformation front GB

TEM ANALYSIS

Thin foil

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CCT CURVES

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CCT + INTERPHASE PTT CURVES

  • T. Sakuma and R.W.K.

Honeycombe: 1984

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Coiling at 750ºC

CCT + INTERPHASE PTT + COILING CURVES

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SLIDE 39

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

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SLIDE 40

CCT + INTERPHASE PTT + COILING CURVES

Coiling at 600ºC

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Coiling at 600ºC

Homogeneous precipitation in ferrite

TEM ANALYSIS

Thin foil

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2 4 6 8 10 12 14 16

Cu Cu Nb Nb Al Cu Fe

Carbon extraction replica

TEM ANALYSIS

Coiling at 600ºC

From the Cu-grid

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4

10 125 . 6 ln 8 . 10 ) (

  d d f MPa

v ppt

PRECIPITATION STRENGTHENING

ASHBY-OROWAN'S EQUATION

fv: volume fraction d: average precipitate diameter in m

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SLIDE 44

PRECIPITATION STRENGTHENING

Coiling at 650ºC

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200 400 600 800 1000 1200 1400 50 100 150 200 250 300

Temperature

1250ºC, 15 min 1100ºC, 1s-1, =0.3; + 20s holding

Sequence S1:

600ºC; 24 or 48h

Long holding at Coiling temperature

STABILITY

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SLIDE 46

S1 + holding at 600ºC

TENSILE

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

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SLIDE 47

S1 + 48h holding at 600ºC

TEM ANALYSIS Baker-Nutting orientation relationship

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  • 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

  • f Nb additions.
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  • 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

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Modelling to Optimise the Processing of Niobium Steels

50