1/37

Control of Microstructure during Solidification & Homogenization of Thin-Slab Cast Direct-Rolling

(TSCDR) Microalloyed Steels

Tihe (Tom) Zhou

Supervisors: Dr. Hatem. S. Zurob, Dr. Nikolas. Provatas

February 27, 2007702 (Part 1)

2/37

Outline• Introduction • Objectives• Literature Review• Solidification• δ To γ Phase Transformation• Austenite γ Coarsening Process4. Experiment Approach5. Preliminary Results • Heat Transfer Model• Effect of Cooling Rate• Effect of Dipping Time • Verification of the simulation process6. Summary and Future work

3/37

Introduction

Holding furnace Rolling Stands

caster

Layout of the most common SMS-Demag Compact Strip Production Mill

C. Klinkenberg et al, Mater. Sci. Forum, 2005, 500-501, 235 J. Muller et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 240

Thin Slab Casting Direct Rolling Process (TSCDR)

Started In 1989 NucorThird Revolution of Production of Steel Slab thickness from 50mm to 70mm50 Installations, 55 million tons, 14% of the world output

4/37

Low Capital Costs: Shot length, Less high and less space for casting and rolling

equipment, eliminating rough rolling mills

Energy Savings: No re-heating stage, short production time, economically

variable at low capacities, more flexible than CCS, products can change easier and quicker

Environmental Advantages: Electric arc furnace using scraps, reduce energy consumption

D. Shi et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 59G. Megahed et al, 33rd McMaster Symposium on Iron & Steelmaking, 2005, 292

Benefits of TSCDR Process:Introduction

5/37

Comply with American Petroleum Institute (API) standardsMicroalloy steel used for oil and gas pipelines must meet the requirements:High strength, High toughness, Low ductile-to-brittle transition temperature, Good weldability, Corrosion resistance.

TSCDR Process for Microalloyed Steel

Examples of plants Developing Nb-(Ti)-API tube grades

C. Klinkenberg et al, Mater. Sci. Forum, 2005, 500-501, 235

Introduction

6/37

Challenges of TSCDR Microalloyed Steels- Non-uniform as Cast Microstructure/Grains

Dendrite morphology and SDAS (Second Dendrite Arm Spacing ) of cast structure

Wang et al, Mater. Sci. Forum, 2005, 500-501, 29

400 um

Introduction

7/37

Challenges of TSCDR Microalloyed Steels- Non-uniform as Cast Austenite Grains

Wang et al, Mater. Sci. Forum, 2005, 500-501, 29

Austenite grains in the as cast slab

Introduction

8/37

Microstructure evolution during the solidification and subsequent cooling

N. S. Pottore et al, Metall. Trans A, 1991,vol. 22A, pp, 1871-1879

Challenges of TSCDR Microalloyed Steels- Rapid Coarsening of Austenite Grains

1520ºC

1380ºC1430ºC1460ºC

1480ºC1500ºC

γ

γγ γγγ

Introduction

9/37

Challenges with TSCDR Microalloyed Steel- Persistence of Large Austenite Grains

P. Uranga et al., Mater. Sci, Forum, 2005, 500-501, 245.

not eliminate the non-uniformity large grains still exist refinement limited by number of passes

Average grain size reduced by thermo-mechanical processes

Introduction

10/37

Research ObjectivesObtain finer & more uniform microstructure

Simulation of initial solidification process Study the mechanism of coarsening in the solid state

- delta dendrite coarsening - delta grain growth

- gamma phase transformation- austenite grain coarsening

Find methods to refine microstructure and prevent grain coarsening

11/37

1. Liquid 2. Delta dendrites

3. Delta grains 4. Austenite Grains 5. Alpha ferrite grains

(1) Liquid/Solid, (2) δ-ferrite/γ-austenite, (3) γ-austenite/α-ferrite

C%

Literature Review

Microstructure Evolution

12/37

Solidification

Theories and Models- Heat flow

- Mass flow

- Solute redistribution

- Liquid-solid interface

- Processing parameters and microstructure parameters

Literature Review

M. C. Flemings, Solidification Processing, McGraw-Hill Inc., 1974W. Kurz and D. J. Fisher, Trans Tech Publication Ltd., Switzerland, 1998Bruce Chalmers, John Wiley & Sons, Inc., 1964W. C. Winegard, Institute of Metals, London, 1964

13/37

)( TTck

tT ∇⋅∇=

∇⋅∇=∂∂ α

ρ

- Merton C. Flemings Model- First finite difference Model- Incorporated two-phase changes- 3-D Mode: steady and unsteady state- Thin slab casting process Model

α: thermal diffusivity cm²/sK: thermal conductivity cal/(cm-ºC-s)ρ: density g/cm³c: specific heat, cal/(g-ºC)

Solidification

M. C. Flemings, Solidification Processing, McGraw-Hill Inc., 1974.B. G. Thomas, Metall. Mater. Trans B, 2002, Vol. 33B, No. 12, 795.M. Gonzalez et al, Metall. Mater. Trans B, 2003, Vol.34B, No. 8, 455S. Louhenkilpi, Mater. Sci. Eng. A 413–414, 2005, 135J.E. Camporredondo S et al Metall. Mater. Trans B, 2004, Vol.35B, No. 6, 541.

Heat Transfer Model

Literature Review

14/37

Solidification- Affects influence As-cast Microstructure

• Control Processing Parameters- Increase cooling rate (thinner slab 20mm)

2. Addition of inoculation substances- Particles tend to accumulate at liquid steel

3. Electromagnetic stirring (EMS) fields- Need special set-up

4. In TSCDR process core reduction- Break the dendrite arms - Homogenize the as cast microstructure

Literature Review

Microstructure evolution in the solid state: Coarsening, transformation

From liquid to solid

15/37

Which stage dominate the coarsening process?Dendrite arm spacing, delta grains, delta to austenite phase transformation, austenite coarsening before and inside the soaking furnace

Grain Growth Model

( )dtCrt

∫ +=ℜ

0 20

4αλ

γ

γ: surface energy

r0 :initial grain size

C: solute concentration

t: time

λ and α: constant

ℜ :dimensionless grain size

Time (S)

Gra

in s

ize

(μm

)

H. S. Zurob et al, Acta Meter. 2002, 50 ,3075

Literature Review

16/37

Delta δ to γ Austenite Phase Transformation- Nucleation of γ-grains

H. Yin et al Acta mater. 1999, Vol. 47, No. 5,1523

At the Triples Points of δ GBs Along the δ GBs

Literature Review

17/37

Austenite γ-grains refinement methods

• Thermomechanical treatment during δ to γ austenite phase transformation

F. Zarandi and S. Yue, Mater. Sci. Forum, 2006, 500-501, 115.

100μm 400μm

Literature Review

18/37

2. Various oxide particles act as heterogeneous nucleation sites for austenite.

H. Suito et al, ISIJ Int., 2006, Vol. 46, 840

Austenite γ-grains refinement methodLiterature Review

19/37

Austenite γ-grains refinement method

3. Precipitates pin grain boundaries

Literature Review

C. J. Tweed et al, Acta Metall., 1407

20/37

Literature Summary

From the literature review, the best chance to produce fine and uniform initial microstructure is in the solid state.

Research approach:

• Step 1: Produce as cast microstructure which is resemble the Thin Slab Cast slab in the industry

• Step 2: Examine the coarsening process of delta grains, delta to austenite phase transformation and austenite grains.

Literature Review

21/37

γ-austenite

Liquid

δ-ferrite

α-ferrite

Tem

pera

ture

(o C)

Time (hr)

References Method Current Method

Real time simulation of the TSCDR Process in the industries is impossible

15300C

15000C11500C

Experimental approach Experiment Process

22/37

Equipment

Furnace Chamber

Pressure Gauge

Gas Inlet

Gas Outlet

Diffusion Pump

Mechanical Pump

ADL Model-MP Crystal Growing Furnace Sketch of ADL Model-MP Furnace

Experimental approach

Courtesy Mr. John Thomson Courtesy Dr. Dmitri Malakhov

23/37

Dipping bar

Graphite

Crucible

Thermocouples

Experiment Set-up

TransverseLongitudinal

Thermocouples

Doll Pin

Dipping Block

Experimental approach

24/37

qq q

h

h

h

Preliminary Results

K: conduction C: convection, R: radiation

Complicated solution- geometries- variation of the properties with temperature- unstable S-L interfaces

- Heat Transfer Model

25/37

Heat Transfer Model (cont.)

Temperature Measurement

Preliminary result

0

200

400

600

800

1000

70 80 90 100 110 120Time(s)

Tem

pera

ture

( C)

top bottom center

0

Dipping Bar

Thermocouple

26/37

)( TTck

tT ∇⋅∇=

∇⋅∇=∂

∂ αρ

2

2

zT

tT

∂∂=

∂∂ α

)2(

0

0

txerf

TTTT

i α=

−−

Boundary conductions: z=0, T = T0

z=S, T=TM

tS sαγ2=

Heat Transfer Model (cont.)

α: thermal diffusivity cm²/sK: thermal conductivity cal/(cm-ºC-s)ρ: density g/cm³c: specific heat, cal/(g-ºC)γ: depends on the properties of mold and solid

Chill Solid Liquid

Preliminary result

27/37

Pre-Heat Treatment: 970°C, 30 mins; 3%Picral, 1 min

Etching Solution & Pre-heat Treatment( Ø0.75˝×3˝ (Cu) ,t= 6s )

100μm

100μm100μm

100μm

Preliminary result

28/37

Processing and Microstructural Parameters

nmVGA −−= 11λ

G: Temperature gradient, V: Solidification rate

Effect of Cooling Rate

nVGB −= )(12λ

B1 and n are constants

R. W. Cahn et al, Physical Metallurgy, 1983, 478

Using the heat transfer model: Control the processing parameters, the microstructural parameters can be predicted

Preliminary result

29/37

Ø0.75˝×3˝ (Cu), 6s

Ø0.5˝×3˝ (Cu), Al2O3 coating 6sØ0.5˝×3˝ (Fe-pipe), 6s

Ø0.5˝×3˝ (Cu), 6s

Effect of Cooling Rate

100μm 100μm

100μm100μm

Preliminary result

30/37

Ø0.75˝×3˝ (Cu), t= 4s, 7s, 10s, 15s

Effect of Dipping Time

10S100μm 15S100μm

7S100μm4S100μm

Preliminary result

31/37

Preliminary Results Ø1˝- Ø0.5×3˝(Steel) , t= 7s

100μm

100μm

100μm

100μm

Preliminary result

32/37

Wang et al, Mat. Sci. Forum, 2005, 500-501, 29

1. Microstructures are the same as the references 2. Primary arm spacing has the same scale as the references:

100~150um: In all the samples

TSCDR: 120~180um: H. J. Diepers, Mater. Sci. Forum 2006, Vol. 508, 145

3. Solidification rate is approximately 1 mm/s

TSCDR is1mm/s. J. E. Camporredondo, Metall. and Mater. Trans B, 2004, vol.35B, 541.

A. H. Castillejos et al, 33rd McMaster Symposium on Iron &Steelmaking 2005, P,47

This set-up successfully simulates TSCDR Process.

Verification of the solidification simulation process

100μm 100μm

Preliminary result

33/37

Summary

2. Unique etching technique was established to reveal the

dendrite microstructure of the as-cast microalloyed steel

3. Secondary dendrites arms were generated during the

simulation process

4. 1-D heat transfer model can predict the relationship

between the processing and microstructural parameters

5. Experimental set-up successfully simulated the initial

solidification stage in the thin slab casting process

34/37

2. Build a set-up to study the kinetics of grain growth and its contribution to the final coarsening- δ-ferrite growth- δ-ferrite to γ-austenite phase transformation - Simulate the coarsening process of γ-austenite grains

2. Refine δ-ferrite and γ-austenite grains

Future Work

35/37

Thermocouples

Heating elements

Solidified shell

Graphite crucible

Alumina crucible

Dipping barThermocouples

Experimental Set-upFuture work

36/37

Acknowledgements

Dr. Hatem. S. Zurob Dr. Sumanth. Shankar Dr. Nikolas. Provatas Dr. Mani. SubramanianMs. Connie Barry Mr. Jim GarrettMr. Martin Vanooste Mr. John Thomson

Mr. Doug CulleyMr. John Roddaand Mr. Ed McCaffery

Financial support

Steel Research Center, McMaster University

Courtesy John Thomson

37/37

Thank you

Questions??