Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties

DSC

process

variables

and

cast

strip quality 103

control loops, working simultaneously in a real time basis to ensure a well-

controlled casting process (Kim

et

al.

2001):

[J

Mould level control;

[J

Constant gap or constant thickness control;

[J

Roll separation force control coupled

with

roll speed,

and

[J

Edge

dam

load or position control.

The mould level is critical to the process to ensure even solidification across the

width

of the strip

and

may be controlled using a meniscus level feedback to

control the tundish outlet flow. To maintain a precise cast strip thickness, the

roll position

must

be controlled rapidly and

with

high

precision

with

possible

changes in position needed as a result of roll thermal expansion

and

roll

eccentricity. The roll separating force

may

need constant adjustment to

accommodate any changes in casting speed etc. The aims of the edge-dam

control are to prevent leakage of the melt from the mould

and

to minimise

wear (§4.3.2.3). In addition to these main control loops, various other control

systems are needed during casting as well as downstream processing where in-

line furnaces, rolling

and

coiling operations

may

be operative

in

one

continuous process (Kim

et

al

. 2001).

HOLDING FURNACE

METAL

FLOW CONTROL

...

41--------------------

....

........

··

..

..........

..

·

....

·

...

·

..

·

....

··

...........

.

LIQUID METAL

FLOW CONTROL

SOLID

METAL

SPEED CONTROL

Figure 4.

2.

Main process control loops in a typical Hazelett casting operation,

with kind permission of Hazelett

(2006)

.



104

Direct

strip

casting

of

metals

and

alloys

Twin

belt

casting

The control system of the Hazelett-type strip caster is similar in complexity to

the twin roll process, although control of the process is slightly less critical

since the strip produced is considerably thicker. This provides enough scope

for downstream processing to reduce the strip to the required thickness

and

improve both its internal and surface integrity. Figure 4.2 is a schematic

diagram of a typical Hazelett belt caster showing the various control systems

in

operation; these may be divided into liquid metal flow control

and

solid metal

feed control. Similarly to

lRC,

this process delivers molten metal

in

a highly

controlled manner into the counter-rotating belts (mould) to produce the

solidified strip; the casting process is often linked to further downstream

annealing, rolling, shearing and coiling facilities, as shown in Figure 3.22.

4.3 Important process variables in DSC

While

not

exhaustive, Table

4.1

provides a list of important factors that need to

be considered

in

order to produce high quality, low-cost strip products. These

factors may be divided into those that affect the melting, casting and

downstream processing stages.

MELT

MOULD

SECONDARY

MANAGEMENT

CHARACTERISTICS

PROCESSES

AND

CASTING

PROCESSING

t t t

Feeding

liquid

metal

into

Mould

design

and

its

thermal

Secondary

cooling

of

cast

strip

the

mould

and

mechanical

properties

Liquid

metal

containment

Mould

wear,

deformation

and

Withrawal,

guiding

and

coiling

in

the

mould

oxidation

of

strip

Resistance

of

components

Further

surface

oxidation

of

to

attack

by

liquid

metal

Oxidation

of

as~st

strip

as-cast

strip

Control

of

oxidation

and

impurities

Roll

separating

force

(TRC)

In-line

rolling

and

control

of

internal

in

the

melt

Casting

velocity

and

point

structure

and

surface

quality

Control

of

metal

flow

and

of

final

solidification

In-line

shearing

and

coiling

melt

atmosphere

Strip

thickness

and

flatness

procedures

Mould

level

control

In-line

or

batch

annealing

t

FINAL STRIP ATTRIBUTES

AND QUAILITY

Table 4.1. Some factors affecting

the

DSC process.



DSC

process

variables

and

cast

strip quality 105

4.3.1 Melt management

This section considers the introduction of molten metal into the mould from

various holding furnaces

and

its containment

in

the mould during casting.

4.3.1.1

Mould

level

control

and

melt

quality

An important factor affecting strip surface quality in vertical TRC is the control

of the height of the melt pool

(Cramb

1995;

Guthrie

and

Tavares

1998;

Blejde

et

al.

2000b;

Zapuskalov

2003).

Variations in pool height

(h±M)

affect the

metallostatic pressure of the melt since

M'

= pgM. The flow of liquid metal

into the mould

must

therefore be extremely consistent to maintain a constant

metallostatic pressure and, hence, contact pressure between the strip and the

mould wall. This generates a uniform interfacial heat flux throughout casting

and alleviates problems with variations

in

strip thickness

and

surface quality

(Cramb

1988,

1995).

Other factors affecting cast strip quality include surface

waves

and

variations

in

fluid flow patterns

in

the mould

due

to general pool

turbulence (see §4.3.2.3). Internal pool dams are

an

effective method for

suppressing wave formation, as well as stabilising the meniscus

and

minimising contact between any inclusions

in

the melt

and

the mould surface.

Inclusions are a potential problem

in

DSC

and

may arise

by

the incorrect choice

of melt chemistry, poor degassing and filtration systems

in

the various holding

furnaces or

due

to oxidation of the melt and disintegration of the ceramic

nozzle or edge dams

in

the mould. Inclusions may cause nozzle blockages and

poor metal feeding leading to mould level variations and,

if

present

in

the

mould, can interact with the meniscus region during casting to cause localised

decreases

in

heat flux leading to the formation of depressions

or

cracks

on

the

strip surface.

It

is therefore imperative, particularly with casting of thin-gauge

strip, that the number and size of inclusions

in

the melt be carefully controlled

to minimise clogging and other inclusion-related casting problems. For

example, inclusions are reduced

in

aluminium strip casting

by

passing the

molten metal through degassing

and

filtration units (Merchant

et

al.

1989;

Menet

et

al.

1997).

4.3.1.2

Melt

atmosphere

In the various DSC processes, molten metal is contained

in

a tundish and

transferred to the mould that usually consists of moving belts

or

rolls. Since

liquid metal is

in

direct contact with the atmosphere, oxidation of the melt

and

the formation of oxide films

on

the mould surface need to be minimised

by

shrouding the melt with some type of inert gas.

In vertical

TRC

of the design shown in Figure 3.12c(i),

it

is particularly

important to shield the meniscus since this region is the initial point of

solidification

on

the rolls (see Figure 3.25a). The formation

and

subsequent

entrapment of scum

in

this region results in various defects

in

the cast strip and



106

Direct

strip

casting

of

metals

and

alloys

must be avoided

at

all costs to ensure adequate strip quality.

It

is pertinent to

note that the type of gas shielding above the meniscus

not

only minimises

oxidation problems

but

has a notable influence

on

the heat flux between the

solidifying metal and the surface of the moving mould

(§4.5.3.3). In addition to

inert gas shrouding, the propensity for scum entrapment

in

the meniscus

region can be reduced

by

implementing weirs

in

the melt pool between the

nozzle and roll faces (Cramb

1995).

In the Hazelett belt casting process the atmosphere is also controlled in the

vicinity of the mould interface

by

the use of

an

inert gas layer which is injected

via small ports directly onto the incoming belt surface immediately prior to

contact with the molten metal. The gases infiltrate the mould interface evenly

to minimise oxidation, and more importantly, to control the rate of heat

transfer from the melt through the belts with the control of gas composition

being important as this has a marked influence

on

heat flux (Hazelett

2006).

4.3.1.2 Liquid metal containment

in

the mould

Edge containment in strip casting is extremely important as it minimises liquid

metal seepage

and

ensures good edge quality. Edge containment is more

critical

in

DSC of steel than

in

aluminium

due

to the greater metallostatic

pressures of the melt

and

material limitations of the side dams

due

to the

higher operating temperatures. This area of development has been the focus of

intense research and, for reasons of commercial sensitivity, information

in

the

open literature is limited. Nevertheless, any material suitable for edge

containment

must

have good mechanical properties at high temperature, must

allow sufficient thermal insulation

and

be resistant to wear

and

thermochemical corrosion (Legrand

et

al.

1997).

An effective mode of liquid metal containment

in

vertical TRC is shown

schematically

in

Figure 4.3 which shows the placement of ceramic dams to seal

the roll end faces. To minimise seepage of liquid metal between the dams

and

the rolls, the caster is designed with some type of well-controlled edge

dam

loading system (not shown).

In

the case of

TRC

of steel, the edge dams

must

be

highly resistant to heat, wear and corrosion, and are therefore usually made of

a ceramic material such as fused silica,

BN,

Si3N4

-BN

and

Sialon-BN (Fournier

and Platon

2000;

Moon

et

al.

2002).

Edge containment

in

horizontal

TRC

is

different to vertical

TRC

with the former involving the flow of liquid metal into

the roll bite using a precision-machined nozzle (see

e.g.

Figure 4.7).

Nevertheless, the nozzle must also be heat, corrosion

and

wear resistant.

In

vertical

TRC

of steel, edge dams must be replaced regularly

due

to wear,

corrosion and mechanical stress and correspond to a large fraction of the

running costs of the caster (Cramb

1995).

Recent work

at

pasco has shown

that edge

dam

life

in

TRC of stainless steel can be extended

by

using a lubricant

to reduce wear between the frictional faces of the

dam

wall

and

the roll edge.



DSC

process

variables

and

cast

strip quality 107

One of the most notable factors limiting edge

dam

life is skull formation in the

region where the

dams

are in direct contact

with

the rolls

or

belts. This also

causes problems

with

edge

and

surface integrity of the strip. Ideally, this

problem

would

be eliminated if the edge dams were adiabatic barriers to heat

flow,

but

may

be alleviated somewhat

by

dam

preheating

and

supplementary

heating of the pool in their vicinity. A potential method of avoiding skull

formation during strip casting and problems in general

with

liquid metal

containment in the

mould

is

by

electromagnetic

edge

containment. For these

reasons, electromagnetic edge dams are

under

intense development in

both

the

ferrous

and

non-ferrous industries (Blazek

et

al.

1998;

Whittington

et

al.

1998).

--7''--Roll

Figure

4.3.

Schematic diagram showing the roll setup and possible configuration

of

edge darns of a vertical twin roll caster (inset shows the roll/liquid interfacial region).

In the Hazelett belt casting process, liquid metal containment is complicated

by

the design of the caster

and

the large liquid metal

sump

that

must

be contained

within the moving belts

(§4.4.3). Liquid metal is generally contained

by

an

endless chain of edge

dam

blocks held against each side of the solidifying strip

by

adjustable spring-loaded guides. Since the

edge

dams

are constantly

revolving

with

the moulds, adjacent blocks in the

dam

must

key together to

minimise intrusion of molten metal between abutting blocks (Hazelett

2006).

4.3.2 M?uld characteristics and casting

4.3.2.1 Style

of

mould

and

materials

Solidification occurs

at

the interface between the liquid metal

and

the mould to

generate strip of particular dimensions

and

quality. The

mould

is therefore one



108

Direct

strip

casting

of

metals

and

alloys

of the most important items of equipment in strip casting

and

may be either

stationary

or

moveable (§3.2.3). In general, the choice of mould material is

dependent

on

the metal to be cast with resistance to oxidation, deformation and

wear and the rate of heat transfer being important variables (Cramb 1995). The

mould may also

be

surface treated to alter its heat transfer characteristics as

well as improving resistance to wear and thermochemical corrosion (Guthrie

et

al.

2000).

The production of aluminium strip by

TRC

requires a set of robust rolls capable

of withstanding the high roll torques and roll separation forces needed for

casting high quality thin-gauge strip (Thomas

2003).

In

this circumstance,

internally water-cooled steel-sleeved moulds are preferred. For example, FATA

Hunter Inc. utilises a composite construction for their rolls that involves a steel

core with cooling water grooves

on

which a high strength steel shell is shrunk

(Vangala

et

al.

1992).

The surfaces of the rolls in aluminium TRC are usually

lubricated during casting with recent developments at Pechiney involving the

continuous spraying of a water-base suspension of graphite

or

BN

that acts as

both a lubricant and parting agent (Menet

et

al.

2001).

In

contrast,

TRC

of steel does not involve large separation forces and the

preferred roll sleeve material is a high conductivity copper alloy. Usually there

is no lubrication during casting (although

BN

spray may

be

used (Birat and

Steffen

1990»

but

the roll surface may be textured to control the rate of heat

transfer during casting (§4.5.3.2). Roll surface texturing may be achieved

by

a

range of methods such as knurling, chemical etching, electric discharge

machining, laser ablation

or

shot blasting (Moon

et

ai.

2002). For reasons

discussed earlier, the rolls may also be surface treated.

Cast metal

Gas layer

Ceramic coating

Caster belt

High velocity

cooling

water

Temperature

rC)

RT

MP

O.5mm

........................

:;.:

..

~

..

~

..

---

Distance

Figure

4.4.

Schematic diagram of the interfacial region between solidifying metal and

the belt of the Hazelett caster, adapted from Hazelett

(2006).



DSC

process

variables

and

cast

strip quality 109

In

Hazelett casting of

wide

strip, the belts are usually

made

from 1-2

mm

gauge

low carbon steel

and

are water-cooled. The key components of the

mould/metal interface of the Hazelett caster are shown in Figure

4.4;

liquid

metal is separated from each belt

by

a ceramic-based coating which is

thermally sprayed onto the belt surface and lasts the life of the belt. An inert

gas layer is used to control both oxidation

and

the rate of heat transfer. The

articulated chill block caster shown in Figure

3.10 has a

mould

configuration

consisting of a set of counter-rotating, internally water-cooled cast iron or

copper alloy chilling blocks that may also be coated

with

a ceramic

or

cermet

material.

4.3.2.2

Molten

metal

delivery

The method of feeding molten metal into the mould of a strip caster depends

on

the geometry of the mould which may be single

or

twin rolls

in

various

configurations

or

single

or

twin belt casters, Figure 3.12. The molten metal feed

system is critical for the operation of the caster as

it

must

deliver metal

uniformly over time, achieve uniform flow across the

width

of the mould and

minimise defects in the cast strip.

(a)

(d)

Solidified strip

Water-cooled

rolls

(b) (c)

(e)

(f)

Figure 4.5. Some of the possible molten feeding systems employed

in

twin roll casting,

after

Li

(1995a) (with kind permission of Metals, Minerals

and

Materials Society, USA).

Metal

feed

systems

in

twin

roll

casters

A range of possible methods for feeding molten metal into the

mould

region of

the twin roll caster are given in Figure

4.5

which shows some of the wide



110

Direct

strip

casting

of

metals

and

alloys

variety of nozzle designs that have emerged. The submerged nozzle design

shown

in

Figure 4.5a is an adaptation of the feeding techniques for

conventional ingot casting and is often used for vertical

TRC.

The two main

types of submerged nozzle designs for vertical casting are the round

and

slot

types which are used depending

on

the casting situation. To avoid uneven

solidification

in

steel strip casting, it has been suggested that this type of nozzle

configuration should extend across the full width of the roll (Guthrie

and

Isac

1999).

The nozzle, particularly for steel casting, is usually made of a thermally

resistant ceramic material such as alumina-graphite refractory.

Roll surface

Above

0,1

c:

E

:B

til

'0

0,05

~

CD

'0

:!2

i:i)

(f)

"5

0,03

(f)

Below

Roll surface

Above

0,1

<::

E

0

III

til

13

'0

0,05

~

CD

'0

:!2

i:i)

(f)

"5

0,03

(f)

Below

Roll surface

Figure 4.6. The influence of nozzle

shape

on

liquid

metal

flow fields

and

solid fraction

profile

at

a horizontal

plane

100

mm

below

the

melt

surface for: (a) vertical slot nozzle

(half

width

of

roll)

and

(b) vertical

tubular

nozzle, after

Guthrie

and

Tavare (1998)

(with

kind

permission of Elsevier Limited).

Guthrie and Tavares

(1998)

studied the effect of molten metal delivery into the

mould of both a vertical twin roll caster

and

horizontal belt caster.

Computational fluid dynamics modelling was used to simulate 3-D turbulent

flows, heat transfer

and

solidification

in

these casters. Figure

4.6

shows, for

vertical

TRC,

the computed flow fields as well as the solid fraction contours

100



DSC

process

variables

and

cast

strip quality 111

nun

below the free surface of the melt for a vertical slot nozzle traversing half

the roll

width

and

a tubular nozzle, respectively. Their

work

demonstrates the

significant influence of nozzle design in TRC

on

velocity fields

and

temperature distribution in the melt.

Casting roll

o

Roll

Width

o

Spout

Molten metal

(a)

(b)

o

Figure

4.7.

Schematic diagram showing:

(a)

cross section of the nozzle of a typical

aluminium

twin

roll caster,

and

(b)

plan

view of a typical nozzle showing the tip

orifice spacers across the roll width, adapted from Merchant

et

al.

(1989).

In horizontal

TRC,

the nozzle can be made from

AI

2

0

3

/Si0

2

fibre composite or

similar material

and

is a precision-machined slot traversing the

width

of the

rolls (Figure 4.7). This is a typical arrangement for TRC of aluminium alloys.

Metal feeding in this case is critical

in

the sense that the shape of the

solidification front and, hence, the integrity of the solidified strip, is governed

by

the velocity field

and

temperature distribution

at

the outlet of the tip that

brings the metal into the roll bite.

Uniform metal flow is readily achieved by

the insertion of a baffle

and

through the use of carefully designed tip orifice

spacers (Figures 4.7a

and

b). These spacers are necessary

due

to the

width

of

the nozzle

and

need to be streamlined, symmetrical

and

reasonably spaced to

maintain uniform flow of molten metal.

It

is relevant to note that high-speed,

thin strip casting requires a different nozzle design to that

shown

in

Figure 4.7b

due

to turbulence

and

non-uniform temperature gradients that lead to various



112

Direct

strip

casting

of

metals

and

alloys

casting defects. The

SpeedcasterTM

developed

by

FATA Hunter Inc. overcame

these problems via a molten metal feed system that incorporates a distributor

box with a flow distribution baffle

in

conjunction with a baffle-less tip with

adjustable orifice (Beals

et

al.

1998).

This allows strip to be cast

at

high speed

by

generating a uniform, laminar flow of metal into the roll bite.

Metal

feed

systems

in

twin

belt

casters

The mode of feeding liquid metal into the moving belts of a Hazelett caster

depends

on

the type of metal to be cast (Hazelett

2006).

When casting low

melting point metals such as aluminium, zinc

and

lead, a fully closed, pool

injection metal feed system is employed, Figure 4.8a. Here, the metal is

introduced

under

slight pressure into the closed mould cavity with metal flow

controlled by maintaining the preset level in the tundish. In contrast, high

melting point metals such as copper and steel cannot be cast

by

this method

due

to the lack of suitable refractory materials. In this case, the

upper

belt

pulley is offset (Figure 4.8b),

and

metal flows through

an

open

tundish into a

standing pool of molten metal formed

at

the convergence of the belts. During

casting, metal flow can be regulated

by

means of an electromagnetic level

indication flow control system.

Figure

4.8.

(a) Closed and (b) open molten metal feed systems of the Hazelett caster

showing the position of the molten metal feed tip

and

angle of caster,

with kind permission of Hazelett

(2006).

4.3.2.3

Mould

integrity

-

wear,

deformation

and

oxidation

The integrity of the mould surface is critical for the production of high quality

strip free of major surface defects and cracks. Furthermore, the rolls and belts

are expensive components of a strip caster

and

maintaining their integrity over

prolonged periods of casting will reduce the cost of producing the as-cast strip.



Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties

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