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

DSC

process

variables

and

cast

strip quality 113

Mould

wear

Mould wear can be a major problem in strip casting, particularly

in

TRC of iron

alloys where the high melting point metal interacts

with

the relatively soft

copper rolls (Zapuskalov

and

Vereschagin

2000).

Mould wear

may

be

defined

as damage to the mould surface via progressive loss of material

due

to the

relative motion between surfaces of the roll and strip.

It

is

an

inherently

complex process which accompanies both removal of material from the mould

surface

and

a redistribution of topography

on

the surface; this can result

in

progressive roughening of the surface over a period of casting.

Melt

_Strip

_Roll

!

(a)

Roll

Melting

(b)

Roll

Skidding

(c)

Strip Digging

(d)

Strip Slipping

Figure

4.9.

Schematic

of

the interaction

of

steel with the roll surface

of

a twin roll

caster at different stages

of

strip formation:

(a)

Roll

surface melting by liquid

steel;

(b)

Roll

skidding on the strip surface;

(c)

Strip digging into the roll surface, and

(d)

Strip slipping on the roll surface, adapted from Zapuskalov and Vereschagin

(2000).

Zapuskalov and Vereschagin

(2000)

have shown

that

roll

wear

in

TRC of iron

alloys is associated with the interaction of the roll

with

both

molten metal and

rolling deformation of the solidified strip. Figure

4.9

shows schematically the

possible modes of roll wear:

(a)

the interaction of molten metal

with

the rolls in

the very early stages of casting causing localised melting

in

micro-areas where

the rolls are

in

direct contact with molten metal; (b) roll skidding relative to the

solidifying shells of the strip as they approach the roll bite;

(c)

strip digging

(imprinting) into the roll surfaces, and (d) strip slipping relative to the moving

rolls. The first process was found to

be

the major contributor to roll wear

but

increasing the roll separating force

(§4.4.2)

promotes the friction-related

mechanisms. The mechanisms shown

in

Figure

4.9

tend to increase roll

roughness and will therefore affect interfacial heat transfer and, hence, cast

strip properties (§4.5.3.2).

Roll wear is less problematic

in

TRC of aluminium alloys since localised

melting of the steel rolls is not a major problem

and

the frictional effects

described above can

be

alleviated by mould lubrication. Mould wear

in

belt

casters is also

not

a major problem since the mould is usually separated from



114

Direct

strip

casting

of

metals

and

alloys

the solidifying strip

by

both a ceramic-based coating

on

the

mould

surface

and

inert gas interfacial layer (§4.3.2.1).

Mould

deformation

An

important parameter

in

DSC of thin-gauge strip is the strip profile and the

existence of factors such as strip crown, edge

drop

and

thickness variations

in

both the longitudinal

and

transverse directions of the strip (Blejde

et

aI.2000b).

Adequate strip profile is critical for further downstream processing such as

hot

and

cold rolling and may be achieved

by

adequate control of roll gap

dimensions at the nip

and

taking into consideration the strength of the rolls

and their thermal distribution during casting. The overall geometry of the roll

gap

can be adequately maintained

by

machining a roll crown that compensates

for the thermal crown developed during casting. In the 4-Hi TRC casting

configuration shown

in

Figure

3.18,

a designated force can

be

generated in the

backup rolls to adjust the bend (positive

or

negative) of the caster rolls to

ensure that the roll camber is controlled during casting of high quality

thin-

gauge strip. Figure

4.10

shows the marked effect of roll

bend

in

the 4-Hi caster

on

as-cast strip profile.

Profile (%)

+0.64

+1.30

+1.54

Edge

Centre

line

+52

+38

+29

+18

Roll bend (%)

Figure

4.10.

As-cast

strip profile in

4-Hi

twin roll casting

of

aluminium, after

Thomas

(2003)

(with kind permission

of

Institute

of

Physics,

UK).

In

the Hazelett process, the belts form the mould surfaces

and

are held

under

tension

on

the casting machine

but

there are problems

with

the interaction of

the cold belts with molten metal as they enter the mould region; this may cause

buckling and thermal distortion of the belts (Hazelett

2006). These problems

can

be

overcome

by

utilising

an

induction belt preheating system that heats the

belts immediately prior to entering the casting mould. This preheating process,



DSC

process

variables

and

cast

strip quality

115

in

conjunction

with

the belt coatings and inert gas injection, tends to control the

rate of heat transfer during casting and, hence, the as-cast microstructure (see

chapter

5).

Mould

surface

oxidation

Strip casting may continuously contaminate a moving

mould

with

oxides

or

elements from the melt and these have to be minimised

or

removed to maintain

the cleanliness of the cast strip.

In

TRC,

it

is possible to use in-line roll surface

conditioning consisting of brush rolls to remove these

unwanted

films.

4.3.3

Secondary processes

and

processing

During casting, the strip leaves the mould

and

may pass through straightening

(pinch) rolls, various types of furnace

and

one

or

more in-

line rolling mills. The rolling mill

must

be configured to take into account the

entry speed of the strip, the desired rolling temperature

and

the degree of

deformation etc. The overall process is therefore similar

in

complexity to a

conventional rolling mill and successful operation relies

on

automated control

systems. After rolling, the strip is usually coiled

in

an

uninterrupted process at

an

appropriate temperature to generate the desired microstructure

and

properties.

4.4 Strip formation

during

DSC

4.4.1

Single rolVbelt casters

In casters of the types shown schematically in Figures 3.16a-c, molten metal is

introduced onto a rotating belt

or

roll to produce the as-cast strip. As indicated

in §3.2.4, these processes have been used extensively

in

the production of

rapidly-quenched ribbons in a continuous manner since the invention

by

Pond

and Maddin

(1969)

of the rotating substrate method for producing amorphous

alloy strip. The liquid/solid front

in

chill

block

melt

spinning

and

planar

flow

casting

are shown schematically

in

Figure

4.11;

both processes are characterised

by

the flow of liquid metal onto a moving substrate

with

the planar flow

casting process generating flow

by

a pouring nozzle

with

a special lip profile.

Here, the bottom of the tip is located a short distance,

g

(<<

1 mm) from the roll

surface.

In

the solidification process,

it

is necessary to ensure adequate heat

extraction while maintaining the melt

puddle

until solidification of the strip is

complete.



116

Direct

strip

casting

of

metals

and

alloys

The nucleation

and

growth of grains during chill block melt spinning can be

summarised as follows (van Rooijen

et

al

.

1985):

Q For metals that crystallize

during

casting, heterogeneous nucleation is

expected to occur predominantly

at

the meniscus region

on

the substrate

side of the flowing melt to generate a very fine grained structure.

Q The nuclei that form will produce a solid shell that moves along

with

the

roll

and

further dendritic growth of favourably-oriented nuclei produces a

columnar solidification structure. Due to the relative flow of the melt

with

the growing dendrites, the columnar grains are inclined towards the

casting direction

by

a rotation about the transverse direction, as described

in

§2.3.3.

2.

Q Dendritic growth may

not

complely consume the full thickness of the strip

and

a zone with an equiaxed grain structure may form

at

the top side (free

surface) of the strip.

It

has been found that the major contributor to the

formation of equiaxed grains is constitutional supercooling ahead of the

solid-liquid interface which tends to favour nucleation

and

growth of

equiaxed crystals.

(b)

~Nozzle

s

Mould~

Figure 4.

11.

Schematic diagram of the liquid pool profile in: (a) chill block melt

spinning,

and

(b)

planar flow casting (showing nozzle dimensions).

4.4.2 Twin roll casters

There are two principal modes of strip formation in

TRC:

(i)

casting with little

or

no deformation (i.e. ferrous alloys), and

(ii)

casting

with

substantial

concomitant deformation

(Le

. aluminium alloys).

Limited

deformation

during

casting

In

this situation, there is little

or

no deformation of the strip

during

casting

and

solidification is carefully controlled to ensure that the required strip thickness

is produced

on

exiting the rolls. Consequently, the strip thickness depends



DSC

process

variables

and

cast

strip quality

117

principally

on

the thickness of the solidifying shells that form

on

the rolls. Due

to the minimal solid-state deformation, the roll separating forces are

not

excessive

and

the rolls can

be

produced from high conductivity copper sleeves

which ensures a

high

rate of heat transfer from the metal

through

the rolls

(§4.5.3.2). Nevertheless, the casting rolls

must

be designed to withstand

thermal fatigue

and

oxidation

due

to high melt temperatures

and

any roll

deflections that

may

occur during casting (§4.3.2.1).

Roll

ni

p

(

PO

int 2)

Meniscus region

(Point 1)

Figure 4.

12

. Schematic diagram of strip formation during vertical twin roll casting of

steel showing the roll separating forces associated with casting,

adapted from Kopp

et

al.

(1998).

A conceptual model of strip formation in vertical TRC (particularly for steel) is

given in Figure 4.

12.

At

steady state, the rolls are counter-rotating

at

a given

velocity

and

nuclei continually form

at

the meniscus region of each roll (point

I)

with

further dendritic growth generating two solid shells that thicken as they

move

with

the rolls. As the shells approach the roll

nip

(point

2),

their



118

Direct

strip

casting

of

metals

and

alloys

dendritic networks interact

and

combine to form the final solidified strip. The

optimum

location of the solidification endpoint for steel strip casting is

different to that for aluminium (see Figure

4.14)

and

is situated just prior to

reaching the bite of the rolls which allows only a very small reduction in

thickness (Figure 4.13a).

If

the solidification endpoint is located too far

up-

stream from the roll bite (Figure 4.13b), there is

an

excessive roll separating

force and semi-molten material will be squeezed

out

resulting in problems

with

segregation (Fujita

et

al.

1989;

Shibuya

and

Ozawa 1991).

If

the solidification

endpoint is located too far down-stream, the two solidifying shells will

not

be

adequately

bonded

and

the strip will leave the caster in the semi-molten state

(Figure 4.13c), resulting in metal flowing through the roll gap.

(b) SQUEEZING

Roll separating

force

2

(a)

STABLE

CASTING

3

Roll gap (mm)

(c)

BREAKOUT

4 5

Figure 4.

13.

Influence of roll gap

on

the stable casting region

in

vertical

twin

roll

casting of steel, adapted from Kawakami

et

al.

(1987).

Deformation

during

casting

In

this case, the strip is cast between the rolls where some

hot

deformation is

possile

during

the process. Figure 4.14 is a schematic diagram showing the

position of the nozzle

and

the liquid

and

solid zones in the roll gap of an

aluminium twin roll caster. Solidification usually commences

by

nucleation

at

the roll surface as the molten metal exits the distribution nozzle

and

proceeds

by

dendritic growth which is usually completed within 10 to

20

mm

from the

nose of the nozzle

(L

and

L+S

zones). The distance required to solidify the strip

as

it

exits the nozzle is the

sump

depth,

f3

with the important parameter, a,

termed the caster feed-metal tip setback. The

sum

of the casting

and

rolling

zones contribute to the total contact length over which heat is extracted from

the strip to the rolls.



DSC

process

variables

and

cast

strip

quality

119

......

f--------a--------I

..

~I

Roll

nip

S+L

Figure 4.14. Schematic diagram of the position of the nozzle

in

horizontal

TRC

showing the liquid metal

sump

(L), solid + liquid region (5+L)

and

solid

region (5), adapted from Bercovici (1985).

In conjunction with the roll diameter,

D,

the tip setback determines the extent

of deformation in twin roll casting

in

which the strip of initial thickness, D

T

,

at

the solidification

end

point is reduced to the thickness, d

T

,

as it emerges from

the caster. The degree of hot rolling during casting,

e(%) , is estimated to be

(Westengen

and

Nes

1984):

100

1

:-----:::::::::::;;;:===......,

~75

c:

o

n

-5

50

~

Cl

c:

'E

25

0:::

50

100 150

2000

Tip setback (mm)

(4.1)

50

100

150

200

Tip setback (mm)

Figure

4.15.

Effect of tip setback (given as a -

f3

where p =

20

mm)

on

the degree of

rolling reduction

in

twin roll casting:

(a)

varying roll gap, d

T

(D =

500

mm);

and

(b) varying roll diameter, D

(d

T

= 10 mm).



120

Direct

strip

casting

of

metals

and

alloys

Figure 4.15 shows the effect of tip setback

on

the degree of

hot

rolling

during

horizontal TRC

and

the significant influence of roll diameter

and

roll

gap

on

hot

deformation through the roll bite. For the case of TRC of aluminium, the

strip is always solidified

at

a predetermined setback which causes

it

to

be

reduced

in

thickness

by

at

least 20-30% (Merchant

et

ai

.

1989).

Flow stress

.

...

.

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

·

·········

······

·

··

···T··

.....

Distance from sump

Figure

4.16.

Schematic diagram showing the pressure distribution through the roll

bite of a horizontal twin roll caster. The dashed line is the flow stress of the

solidified metal, adapted from Iricibar

and

Jin

(1984).

Roll

separating

forces

In horizontal TRC of aluminium alloys, solid

or

semisolid metal enters the

rolling region

at

a temperature close to the melting point

and

exits the roll

gap

at

temperatures as low as

250

°C (Merchant

et

ai.

1989). The substantial

hot

rolling

in

this process generates a steep pressure distribution along the arc of

contact of the metal

and

the rolls with the maximum pressure being several

times greater than the flow stress of the material. Figure 4.16 shows the

estimated pressure distribution in horizontal twin roll casting incorporating a

certain tip setback

in

the process. The figure shows

that

the contact pressure is

considerally greater than the flow stress of the solidified strip which results

in

plastic deformation of the strip through the roll bite.

It

is useful to note that

TRC for aluminium is actually designed to withstand roll loads of

up

to

20

tonnes

in

order

to maintain good thickness

and

crown control in the as-cast

strip (Thomas

2003)

.



DSC

process

variables

and

cast

strip quality 121

In

TRC

of steel, the minimal deformation of the solidifying strip results

in

a less

severe pressure hill compared to that generated during aluminium strip

casting. Figure

4.12 shows schematically the increase

in

roll pressure as the

solidifying steel strip passes through the rolls of the caster. There is a moderate

increase

in

pressure

in

the semi-solid zone which increases more sharply on

complete solidification at the roll nip. In this case, the pressure should be

sufficient for the two shells to weld together without too much rolling of the

solidified strip (Kopp

et

al.

1998). The dashed line

in

Figure 4.12 illustrates the

effect of

hot

rolling on the contact pressure where, as expected, a more intense

pressure distribution is generated.

4.4.3

Twin belt and chill block casters

The solidification behaviour of metal strip produced

by

twin belt/block casting

is considerably different to that in roll casting. Strip

up

to 10-20

mm

in

thickness is generated

and

the process is probably better described as a thin

slab casting process (§3.2.2). In belt

(Le.

Hazelett) or chill block (i.e. Alusuisse

II)

casting, molten metal solidifies onto each of the counter-rotating belts,

forming two shells separated

by

a deep liquid sump as the material proceeds

through the caster. Hence, the shape of the sump is

an

important distinction

between a twin-roll caster and block or belt casters. Compared with the

shallow sump of a twin roll caster (Figure

4.14), block

and

belt casting

processes have a

sump

often exceeding 1000

mm

in

length (Figure 4.17).

Solidification therefore takes considerable time and is

not

complete until

-70-

80% of the mould length.

At the mould/metal interface of the Hazelett and articulated chill block casters

the solidifying metal is separated from the mould

by

a ceramic-based coating

and

a film of inert gas (see

e.g.

Figure

4.4).

These interface configurations result

in substantially slower cooling rates compared with TRC where the latter

usually involves direct metal/mould contact. Consequently, roll

and

belt

casting produce different solidification microstructures (Chapter

5).

+----------------Sump--------------~~I

depth

Figure

4.17.

Schematic diagram of the solidification conditions

in

beIt/block caster

showing the shape of the solidifying zone.



122

Direct

strip

casting

of

metals

and

alloys

4.4.4 Influence

of

casting

speed

on

strip

characteristics

4.4.4.1

Liquid

sump

profile

The

mould

speed

during

DSC has a substantial influence

on

the shape of the

liquid metal

sump

(Merchant

et

ai

. 1989). The effect of casting parameters

on

the shape of the

sump

in

horizontal TRC of aluminium alloys has been

modelled

by

Hunt

and

co-workers (Bagshaw

et

ai.

1986; Bradbury

and

Hunt

1995). Their numerical model utilises a generalized energy equation

in

the

liquid, semi-solid

and

solid zones of a

wedge

shaped

moving

mould

and

has

been

used

to investigate the effect of varying casting parameters

on

the thermal

efficiency

of

the process, heat line formation, sticking, segregation

and

productivity. Figure 4.18 shows a particular outcome of the model showing the

change

in

sump

shape with casting speed for the production of 4

mm

gauge Al

strip.

It

can be seen that

an

increase

in

casting speed generates a deeper liquid

and

semi-solid zone

with

a concomitant increase

in

exit temperature of the

strip. The shape of the

sump

in

Figure 4.18 is

in

general accordance

with

the

predictions of Draskovic

and

Mitrovic (1982), Roadman

et.

ai

(1984), Sahai

and

Saxena

(2002)

and

Sarioglu

and

Thevoz

(2003)

.

Fraction Solidified

Speed

= 2m/min

Exit Temperature = 338°C

Speed

= 4m/min

Exit Temperature

= 520°C

Figure 4.

18.

Predicted fraction solidified

and

strip exit temperature as a function of

casting velocity for twin roll casting of 4

mm

gauge aluminium strip, after Thomas

(2003) (with kind permission of Institute of Physics, UK).

The effect of casting speed, tip setback

and

roll diameter

on

sump

depth

were

experimentally determined

by

Bercovici

(1985);

the

sump

depth

was

directly

measured

by

the insertion of temperature probes. Figure 4.

19

shows, for

various tip setbacks, that a deeper

sump

depth

is generated as the casting

speed is increased. These findings

would

be

expected

but

nevertheless

significant since

an

increase

in

sump

depth

beyond

a critical value

may

result

in

localised solidification defects

on

the strip surface

in

the form of

run

out



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

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