28 Casting Processes 3 Continuous Casting of Steel - Solidification.20101112.4cdda08c2c28b1.24761975

1ME450

Continuous Casting:Solidification phenomena

B.G. Thomas

University of Illinois at Urbana-Champaign Tenaris University Continuous Casting Course BG Thomas 1

Director, Continuous Casting ConsortiumWilkins Professor of Mechanical Engineering

University of Illinois at Urbana-Champaign

Overview Solidification structure formation

Nucleation, Dendrites, and grain growth, , g g Effect of EMS

Segregation Microsegregation (between dendrites) Macrosegregation (centerline vs surface)

Tenaris University Casting Course, 2007 Solidification BG Thomas 2

Hot-tear cracks and ductility loss

2Billet casting process

Ladle

Tundish

Molten Steel

T b ldMolten steel stream

Mold

Torch Cutoff Point

Liquid Pool

Solidifying

Meniscus

z

Tube mold Small cross sectionSmall aspect ratioOpen pour (with oil lubrication)Or submerged nozzle Foot rolls


Spray Cooling

Billet

Metallurgical Length

Strand

Solidifying Shell

Drive roll

(with mold flux)

The mold is the heart of the caster

Transforms liquid to a shaped cross-sectionR h t t lidif lt t l h ll Removes heat to solidify molten steel shell

Determines productivity Breakouts Casting speed

Determines quality


Determines quality Creates surface! Affects internal cleanliness and structure

3Billet mold

meniscus

Steel Jacket

Mold

Water Channelst

eel


Side view Top view

Sol

idify

ing

Solidification Front Structure

Liquidus (100% liquid)Cell

Solidus (100% solid)

Solid

mold Columnar dendrite

Nucleus

Mushy region

Primary dendrite arm spacing


shellEquiaxed grain

4Final Solidification Structure

Showering crystals initiate columnar-equiaxed transitionGravity causes earlier transition on outer radius

Variations in equiaxed grain pile-up traps liquid pockets, leading to porosity and centerline


Ed Szekeres, Brimacombe course notes

porosity and centerline segregation

Radial Streaks

Segregation DefectsCenterline Segregation (and Porosity)

casting


Longitudinal Sectioncracks filled with segregated liquid

Ed Szekeres

5Final Solidification Structure

Typical grain structure in a billet cross section


Development of Microstructure Nucleation

Undercooling needed to overcome energy barrier g gyto initiate nucleation

number of nucleation sites controls # of grains Growth

Competitive growth of columnar grains from walls (Certain growth directions are preferred)


Equiaxed grains nucleate in central liquid Final macrostructure

depends on competition between columnar vs. equiaxed grains

6Dendrite Growth

Dendrites start from nucleation sites Branched, 3-D, tree-like structures [100] growth direction [100] growth direction

Secondary dendrite arm spacing (SDAS, ) has important effect on material properties

Secondary arms


Columnar solidification video equiaxed solidif. video

Segregation Caused by:

Alloy partitioning during solidification creates enriched liquid Fluid flow (from liquid shrinkage bulging convection etc ) Fluid flow (from liquid shrinkage, bulging, convection, etc.)

Microsegregation Small scale (in between dendrites)

Macrosegregation Scale of entire cast section center to surface Filling in of internal cracks and porosity with enriched liquid Cannot be removed!


Worse at higher superheat need superheat < 10C to avoid segregation Need some squeezing to match liquid shrinkage

(0.3 2 mm/m machine taper, or soft reduction)

7Segregation Caused by:

Alloy partitioning during solidification creates enriched liquid Fluid flow (from liquid shrinkage bulging convection etc ) Fluid flow (from liquid shrinkage, bulging, convection, etc.)

Microsegregation Small scale (in between dendrites)

Macrosegregation Scale of entire cast section center to surface Filling in of internal cracks and porosity with enriched liquid Cannot be removed!


Worse at higher superheat need superheat < 10C to avoid segregation Need some squeezing to match liquid shrinkage

(0.3 2 mm/m machine taper, or soft reduction)

Equilibrium (Very Slow) Cooling Non-Equilibrium Solidification

Phase diagrams explain alloy partitioning and segregation

Tenaris University Casting Course, 2007 Solidification BG Thomas 14Insufficient diffusion causes compositional changes: segregation.

From: W.D. Callister, Materials Science and Engineering,An Introduction (6th Ed.) , Wiley and Sons, 2003, pp. 257,259.

8Phase Diagram Composition between Dendrites

Phase diagrams explain alloy partitioning and segregation

Primary dendrite arm0.2

Liq id

4

32

1

1.0

12.6%S

1

Liquid

L23

Tem

pera

ture

o C

4

12.6


Insufficient diffusion causes compositional changes: segregation.

00.2

12.6Weight % SFe

1.00.2

Continuous Casting Short Course Iron-Carbon Phase Diagram Prof. Brian G. Thomas


91.0

Scheil

0.13%C-0.35%Si-1.52%Mn-0.016%P-0.002%S CR = 0.25 oC/sec

Microsegregation: composition variations between dendrites

0.4

0.6

0.8

Clyne-Kurz & Ohnaka(2) Ohnaka(4) & simple model present FDM Lever rule Brody-Flemings

C in

Liq

uid

Phas

e, w

t%


0.5 0.6 0.7 0.8 0.9 1.00.0

0.2

= 3.773k = 0.19Co = 0.13

C

Solid Fraction

0.34%Si-1.52%Mn-0.012%P-0.015%S

Non-equilibrium Phase Diagram(from Microsegregation Model)

1400

1450

1500

1550

+L ++L

+L

Liquid

fS=0.0

fS=0.75

mpe

ratu

re, o

C


0.0 0.2 0.4 0.6 0.8

1300

1350 fS=0.9

fS=1.0

cooling rate (oC/sec) 1 10 100

Tem

Carbon Content, wt%

10

Macrosegregation in a rolled billetSegregation is: composition variations between regions of a casting


Macrosegregation cannot be removed!

Rotary Mold EMS

Rotary Mold EMS most common- Controls superheatp- Excessive EMS can entrapsurface scum

- lowers temp gradients in liquid- mixes liquid


Rotating magnetic field stirs liquid

- mixes liquid- favors equiaxed grains

11

2X Mold EMS

(M)

12

Fluid flow causes bands


Faint dark & light bands caused by fluid flow (eg. EMS)(indicate shell thinning due to the local reduction in cooling rate caused bya surface depression).

Casting Macrostructures: range (depends on conditions & composition)

Fullycolumnar

Fullyequiaxed


13

Control of solidification structure Larger equiaxed zone with:

Low superheatLow superheat Electromagnetic stirring High alloy content (larger freezing range) Add grain refiners (easier nucleation)


Columnar grain boundaries are weakSurface (chill)

Hot Tearing cracks- Occur due to tension stress on mushy zone-Intergranular:- Follow between dendrites &


grain boundaries

Austenite Grain Boundaries

14

Measuring ductilityDuctility is the ability of metal to draw down or neck and avoid brittle fracture by plastic flow

Ductility is measured by reduction in area, %RA., in 1D tensile testsy y

%100..% xA

AAAR

o

fo


Behavior in actual 3D stress - strain states must be inferred indirectly

Ductility Problems in Steel

crack

ctilit

y tio

n in

Are

a)

hot tearing

intermediate temperature ductility loss

S, P segregation

liquid

empe

ratu

re

pera

ture

riu

m)

100

sulfide or nitride precipitates

grain boundary

dendrite


300 600 900 1200 1500Temperature (C)

Duc

(% R

educ

t

+ Fe3C

Liqu

idus

Te

Sol

idus

Tem

(non

-equ

ilibr

0

Temperature (oC)

15

High temperature embrittlement


Hot Tear Crack Formation


Tensile test of solidifying Succino-nitrile

M. Rappaz,JOM-e, Jan 2002

16

Grain Boundary Embrittlement


Hot Tear Crack: Closeup

Scanning electron micrograph of exposed

f f id ksurface of midway crack showing smooth contour of dendrite arms that were covered liquid film when crack formed. Precipitates are MnS that likely solidified into lumps afterwards (the thin liquid film forms beads


liquid film forms beads due to surface tension)

Vandrunen, Brimacombe, and Weinberg, Ironmaking and Steelmaking, Vol. 2, 125, 1975

17

High temperature zone of embrittlement Zero ductility point (~1340C) - Solidus hot tearing

M h i Mechanism:Liquid film formation at grain boundaries due to segregation to interdendritic liquid of residual elements: S, P, Cu, Sn, Sb, Zn

most important zone for continuous casting cracks (particularly internal cracks)ff t f ll i l t


effect of alloying elements:S, Cu, Sn, Sb, Zn - badMn - helpsP - worsens embrittlement at high carbon content

Iron-Sulfur Phase Diagram

Effect of alloys

Adding residual element lowers melting point

Same applies to other residuals:

P


Fe S

SnBi SbCuZn

18


Effect of S

Adding Mn traps S as MnS precipitates

Raises solidus temperature


Fe

http://riodb.ibase.aist.go.jp/pdsul/e_index_top.html


Effect of Mn1%Mn

LiquidL+

Adding Mn traps S as MnS precipitates

Raises solidus temperature

+nS

+MnS

++MnS


Fe

http://riodb.ibase.aist.go.jp/pdsul/e_index_top.html

+MnS

19

Liquidus

Location of Crack Formation

Solidusx2

x1

x2

x1

Internal crack

start

end

Strand surface


High Temperature

end

Casting Direction

Crack with ferrite network

Crack following prior austeniteprior austenite grain boundaries

Tenaris University Casting Course, 2007 Solidification BG Thomas 38B.G. Thomas, PhD Thesis, 1985

20

Ferrite network crack: zoom-in


B.G. Thomas, I.V. Samarasekera and J.K. Brimacombe, "Investigation of Panel Crack Formation in Steel Ingots, Part II: Off-Corner Panel Cracks," Metall. Trans. B,Vol. 19B (2), 1988, 289-301.

Grain boundary fracture mechanism


Init. Sol talk

28 Casting Processes 3 Continuous Casting of Steel - Solidification.20101112.4cdda08c2c28b1.24761975

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Transcript of 28 Casting Processes 3 Continuous Casting of Steel - Solidification.20101112.4cdda08c2c28b1.24761975