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

Secondary

processing

and

fabrication

233

Strip-cast

aluminium

alloys

Most

fcc metals

produced

by

cold rolling

and

annealing

have

a

low

r -value

«0.8)

and

a

high

M -value (-0.6)

which

results

in

limited

deep

drawability

and

problems

with

earing

(Humphreys

and

Hatherly

2004). To enhance the

drawability, either a reduction

in

{100}<001>

or

an

increase

in

{11I} <

uvw

> is

desirable but, unfortunately,

the

latter

does

not

form easily

in

these materials.

There

have

been

several

attempts

at

improving

the

formability

parameters

in

strip-cast

aluminium

alloys

and

the

following discussion outlines a few of the

more

recent.

Uppperdie

Lower die

Figure

6.31.

Schematic illustration of the continuous strip shearing process based on

DCAP, after Han

et

al.

(2002)

(with kind permission of Elsevier Limited).

Table

6.4.

Texture evolution and calculated formability parameters as a function of

processing route for strip-cast

AA1050,

adapted from Han

et

al.

(2002).

Process Effect

Major texture

r-value

M-value

Strip

casting

<110>//ND

i

<110>//ND

Moderate

(0.9)

Very low

(0.05)

DCAP

<ll1>//ND

i

<111>//ND

{lOO}<Ol1>

i

{lOO}<Ol1>

High (1.1)

High

(0.6)

<110>//ND!

DCAP + annealing

{lOO}<Ol1>

!

<111>//ND

High

(LOS)

Very low

(0.05)

{lOO}<OOl>

i

{lOO}<Ol1>

{lOO}<OO1>

Han

et

al.

(2002)

have

developed a novel technique for

improving

the

strength

of

the

{11I} <

uvw

> texture

in

strip-cast (1.55

mm

gauge) AA1050

Al

alloys

by

incorporating dissimilar channel

angular

pressing

(DeAP)

as a secondary

processing step, Figure 6.31.

In

this process,

the

as-cast

strip

is subjected to



234

Direct

strip

casting

of

metals

and

alloys

shear deformation through the roll bite of a rolling mill which tends to generate

a range of important texture components after annealing (Table

6.4).

DCAP

generates

<ll1>//ND

and {l00}<011> texture components through the shearing

action of the die with the sheet which is similar to the surface shear texture that

is produced

in

twin roll cast Al alloy strip (see

e.g.

Figures

5.35

to

5.37).

On

annealing, the major texture components include a combination of

<l1l>//ND,

{100}<011>

and

{l00}<001> and generates sheet material

with

a high r -value

and very low

M-value; this equates to an overall improvement

in

formability.

It

is pertinent to note, however, that plastic straining

by

DCAP will have a

deleterious effect

on

the surface condition of the strip as considerable contact

occurs between the die wall and the strip surface.

GallerneauIt and Lloyd

(2002) investigated the effect of secondary processing of

twin roll cast

AA5754

on mechanical behaviour (see Table

6.3)

and compared their

data with material produced by conventional

DCC!I'MP. Figure

6.32

shows the

effect of processing variables on formability of both types of material; formability

is assessed as a limiting dome height before splitting using a lubricated bulge

tester with a

100

mm

diameter die. Despite the elaborate secondary processing

schedules, the strip cast material exhibits slightly inferior formability compared

with DCC material; this result was tentatively attributed to differences in the

distribution of intermeta1lic phases in the alloy and not considered to be a result of

differences

in

texture.

50,----------------------------------,

~~----~--------~--------~------~

1.0 2.0 3.0 4.0

Sheet thickness (mm)

Figure

6.32.

Comparison of limiting dome height for conventionally-produced

and

10

mm

gauge

twin

roll cast AA5754 alloy processed further

by

the following routes, after

Gallemeault and Lloyd

2002

(with kind permission of Trans Tech Publishers):

Process 1 - Warm roll to 6 mm, cold roll to 3.5, 2.0

and

1.0

mm

and

recrystallize.

Process 2 - Warm roll to 3.5, 2.0

and

1.0

mm

and

recrystallize.

Process 3 - Warm roll to 6 mm, interanneal, cold roll to 3.5, 2.0

and

1.0

mm

and

recrystallize.

Process 4 - Cold roll to 3.5, 2.0

and

1.0

mm

and

recrystallize.



Secondary

processing

and

fabrication

235

6.7.2

Surface treatments and coatings

Sheet metal products are often surface treated

or

coated for decorative purposes

and/or protection against wear, oxidation

and

corrosion. A range of surface

treatments which may be classified as either metallic or non-metallic coatings

are shown in Figure

6.33.

Some examples relevant to

DSC

include

electrochemical coatings such as anodising of aluminium

and

magnesium,

conventional coatings such as tin plating

and

galvanising of low carbon steel

and polymeric or ceramic coatings

on

both aluminium

and

low carbon steel.

,---------

-----------1

i Anodising, Phosphate, Chromate, Oxide J

,------------------------------------

Figure

6.33.

Possible coatings for sheet metal products.

The process of anodising is important

in

the aluminium sheet metal industry

and

involves the generation of a coherent and pore-free oxide layer

on

the metal

substrate that makes the underlying material impervious to chemical attack.

The mechanism of anodising is a chemical conversion of the surface of the

aluminium to produce a

hard

oxide layer during immersion of the material in

an acidic solution. These anodic layers have varying degrees of porosity and, to

improve corrosion resistance, a sealing operation is carried

out

by

exposure to

steam. During this stage, dyes can be used to produce a corrosion and wear

resistant coating in a range of colours for decorative applications.

There are difficulties

in

generating a high quality surface in strip-cast Al alloys

which often lead to a poor finish after anodising

and

preclude the production of

components where surface quality is paramount. For example, surface

segregation

on

strip-cast aluminium alloys results

in

a grey surface appearance

after anodising (Emley

1976).

For thicker gauge strip produced

by

belt casting

(Figure

3.9),

there is scope for substantially altering the cast structure by hot

and cold rolling which favours the ability to etch and anodise the final product.

The ability to produce a satisfactory finish

in

DSC Al alloys is expected to

extend the market range of this material.



236

Direct

strip

casting

of

metals

and

alloys

The processes of

hot

dip

galvanising and tin plating are cost effective means of

protecting mild steel against corrosion

and

involve the application of a layer of

zinc

or

its alloys and tin, respectively, to the steel surface. For sheet products,

these processes are carried

out

as a continuous process (Llewellyn 1994). The

application of paint to a strip surface (that may

be

galvanised) is similar to the

previous processes

and

is generally carried

out

as a batch

or

continuous process

(Llewellyn 1994).

In

galvanised steel, the zinc alloy layer acts as

both

a physical

barrier

and

sacrificial anode and thus, cathodically protects the underlying steel

if there is any surface damage.

In

contrast, tin plating

and

painting generates

an

impervious surface layer more resistant to chemical attack than the

underlying steel.

Figure 6.34 is a schematic diagram showing a possible

plant

setup for the

continuous processing of galvanised steel sheet products from a strip caster;

similar configurations are likely for tin plating

and

painting. Strip-cast low

carbon steel is highly amenable to surface modification

by

these processes and

will make them important additions to any DSC process since possible

applications of strip-cast steel sheet include structural decking, light gauge steel

framing, tubular goods, office furniture, appliances, food packaging and

exposed automotive panels.

Twin roll caster

Inert atmosphere

Single/multi-strand

Rolling

/

Heat treatment

furnace

\

Coiler

Zinc bath

Figure

6.34.

Schematic diagram

of

a possible configuration

of

a twin roll casting,

cold rolling and galvanising plant.

In summary, the final quality of any surface treated product is strongly

dependent

on

the structure

and

properties of the sheet material to

be

treated.

The various types of surface defects described in §4.6.2 need to

be

minimised or,

preferably, eliminated

in

order to generate high quality coated products

produced

by

strip casting.



Appendix A

Simulation

and

Modelling

of

Direct

Strip

Casting

A.I Introduction

Direct strip casting is a complex process involving solidification of liquid metal

directly into thin-gauge strip.

To

control both the process and the microstructure

and properties of the strip, an understanding of various phenomena such as heat

transfer, fluid flow and thermomechanical behaviour is important. A more

.

complete understanding of these (and many other) aspects of

DSC

can be

achieved using casting simulators and/or mathematical modelling. To be

effective, however, the outcome of these methods must compare well with actual

plant data. As it is impossible to depict the overall strip casting process by a

single model, the process is generally broken

up

into component parts. Examples

include the prediction of molten metal flow, interfacial heat transfer or the

temperature profile of the strip during casting. This appendix describes some

useful laboratory-based techniques for simulating strip casting together with

relevant mathematical techniques for modelling heat transfer, fluid flow and

microstructural evolution etc. The appendix precludes a detailed discussion of

the very wide range of mathematical

and

computer-based techniques available for

modelling

DSC and does not cover models and simulations associated with

machine dynamics and process automation.

A.2 Experimental techniques

A.2.1 Splat quenching

Quenching a molten droplet onto a substrate is a simple

but

useful method for

understanding rapid solidification behaviour. The droplet may be levitated above

237



238

Direct

strip

casting

of

metals

and

alloys

the substrate or injected from a small nozzle containing the melt. Other rapid

quenching techniques are possible such as sandwiching a molten droplet between

two platens. A schematic diagram of the interaction of a droplet with a substrate

is given in Figure

At.

Using this technique,

it

is possible to study systematically

the effect of solidification variables (melt superheat, melt atmosphere and alloy

composition) on cast structure development

and

heat transfer. Furthermore,

changes

in

the type, surface condition, topography and temperature of the

substrate are easily made. Splat quenching is also useful for investigating the

cooling rate

and

undercooling associated with the early stages of solidification

and is a particularly valuable technique for investigating interfacial heat flux.

The latter is made possible

by

carefully inserting thermocouples

at

various

positions within the substrate (Figure

AI)

or using optical pyrometry to

measure the droplet surface temperature.

I

Droplet maintained

by levitation or ejected

from

a nozzle

Droplet

Embedded

Thermocouples

Figure

A.t.

Schematic diagram of the interaction of a molten droplet with a substrate.

(a typical method of temperature measurement is also shown).

A notable downside of splat quenching is the difficulty

in

comparing directly

the microstructure and heat flux data with that generated

by

DSC.

For

example, the microstructures are not the same as those produced

by

strip

casting since the latter involves nucleation and growth of solidifying grains that

are affected

by

molten metal flow and the movement of the substrate relative to

the melt (see §5.2.I).

A.2.2 Substrate immersion

Substrate immersion to produce as-cast metal coupons is another useful

technique for simulating the solidification processes associated with strip

casting. Figure

A2

is a typical example of the technique whereby a water-



Simulation

and

modelling

of

direct

strip

casting

239

cooled paddle containing one

or

more types of substrate is

dipped

at

a certain

velocity into a pool of molten metal which is then retracted from the melt to

produce a solidified coupon

on

the substrate.

Slider Assembly

"

compumotors"

D

Immersion Paddle

Inert

Gas

Atmosphere

"

cy

1

..

........

.

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

.

..

....

...

..

...

.

....

......

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

..

........

Copper Substrates

~Induction

Furnace

75kW - 3kHz

Figure A.2. Schematic diagram of a typical immersion apparatus showing the melt,

furnace

and

substrates. The inset shows the types of substrate surface

topography discussed

in

the book: smooth, ridged

and

pyramid,

adapted from Strezov

and

Herbertson

(1998).

Compared with splat quenching, substrate immersion is a better DSC analogue

since the melt/substrate contact during dipping closely approximates the

melt/roll contacting geometry of the meniscus region of a single-

and

twin-roll

caster

and

produces as-cast microstructures comparable to that produced

by

these casting processes (Strezov

and

Herbertson 1998). The technique is also

capable of investigating systematically the same casting variables as splat

quenching

but

is more versatile since casting velocity is

an

additional variable.

Furthermore, several different substrates can be used concurrently during

casting (see inset of Figure

A2) which is particularly useful as

it

allows for the



240

Direct

strip

casting

of

metals

and

alloys

unambiguous interpretation of the effect of substrate type on heat flow and

microstructural development. Substrate immersion can also be used in

conjunction with optical pyrometry, Figure

A.3.

This allows temperature

fluctuations to be monitored during solidification and subsequent cooling and can

be used to investigate phase transformations in the as-cast strip. This

is useful in

the study of solidification behaviour of iron and titanium alloys due to the various

types of phase transformation that occur (see chapter

2).

High

volume

gas

flow

valve

~

Inert

gas

Data

acquisition

~UPPIY

and

quench

------'IIVI

control

Pyrometer

--a,

o

/

Strip sample

Melt

Immersion I withdrawal

Substrate

o

Figure A.3. Schematic representation of

an

experimental apparatus

used

to study

microstructural evolution

in

strip casting, after Evans and Strezov (2000a) (with

kind permission of The Metals, Minerals and Materials Society,

USA).

A.2.3 Laboratory- and pilot-scale strip casters

While a scaled-down strip caster

is

more capital intensive and expensive than the

foregoing techniques, it is a powerful method for investigating a wide range of

important variables pertaining to strip casting. For example, a laboratory-scale

twin roll caster is useful for correlating casting performance with a wide range of

variables including alloy type, melt superheat, molten metal delivery, mould level

control, furnace atmosphere, casting speed and roll properties (material type,

surface structure, roll shape, roll cooling, roll separating force and edge

containment) etc. Considering the substantial benefits of a scaled-down strip

caster, a large number of organisations have been active in developing research

programs over the past few decades.

Li

(1995b)

has compiled a comprehensive

list of the types of laboratory- and pilot-scale

twin roll strip casters used for

process development as well as some casting parameters and alloys investigated.

For obvious reasons of confidentiality, the list is incomplete

but

nevertheless,

highlights the extent of the research throughout the world.



Simulation

and

modelling

of

direct

strip

casting

241

A.3

Mathematical Techniques

Mathematical modelling of DSC

has

been a major contributor

to

our

understanding of various phenomena involved

in

the process

and

is a useful

tool for

the

prediction of productivity

and

for process optimisation. While a

large

number

of

mathematical techniques are available for modelling strip

casting, this section is restricted to models discussed

in

the

book

for

understanding

phenomena

such as

heat

transfer, fluid flow

and

microstructural

development.

A.3.1 Modelling of heat transfer

The successful operation of a strip caster is highly dependent

on

the optimal

thermal management of the casting process. Depending

on

the information

required, various heat transfer models have been developed including one

dimensional (I-D) transient models

or

two dimensional (2-D) steady state

or

transient models. For I-D models, it is assumed that heat flow is normal to the

~

mould wall

and

negligible

in

other directions; such models are easiest

to

develop

and

provide a reasonably good description of DSC. 2-D models are more

powerful

and

can generate information such as the spatial temperature

distribution throughout the strip

and

mould during casting (Figure

4.18)

and

temperature hysteresis behaviour (Figure

4.21).

An important aspect of all

transient

heat transfer models is the ability to calculate

the

time variation of

interfacial

heat

flux

during

casting (see

e.g.

Chapter 4).

Transient

heat

flux

calculations

for

a

stationary

mould

The transient interfacial

heat

flux can

be

determined from a time modified

version

of

Eq.

4.7:

(A.l)

where

Tm(t)

and

TM(t) is the metal

and

mould

surface temperature,

respectively,

and

q(t)

the

interfacial

heat

flux

at

a given time

t.

Heat

flux associated

with

the initial contact of molten metal

with

a

mould

can

be

calculated directly only if the interfacial temperature can

be

measured.

There are obvious difficulties associated

with

such a

measurement

in

DSC

and

heat

flux is usually calculated

in

conjunction

with

the

more

simplified casting

procedures

of

splat quenching

and

substrate immersion. Here, temperature

changes can

be

measured

by:

(i)

thermocouples placed carefully

at

a various

depths

below the surface of the substrate (Strezov

and

Herbertson 1998)

and

(ii)

optical

pyrometry

at

the surface of the melt (Wang

and

Matthys 2002)

or

at

the

metal/substrate interface via a specially machined hole

in

the

substrate

(Mizukami

et

al.

1992).



242

Direct

strip

casting

of

metals

and

alloys

The transient heat flux in the initial stages of melt/substrate contact is strongly

time-dependent, so

it

is essential to solve Fourier's time-dependent heat flow

equation

(Eq.

2.47):

The calculation of interfacial heat flux is

not

trivial

but

solutions of the heat flow

equation are possible using inverse numerical techniques. These techniques

estimate the interfacial heat flux based

on

careful temperature measurements in

the interior of the mould. The most commonly used techniques are based on

the finite difference method proposed initially

by

Beck

and

co-workers (Beck

et

ai.

1985).

Here, the temperature

at

one or several locations below the surface of

the mould is measured accurately and iterations of

an

initial estimate of the

interfacial heat flux are made until the difference between the computed and

calculated temperatures

at

prescribed positions in the mould fall below a

prescribed value. This iterative process is carried

out

over a range of

temperatures to calculate the interfacial heat flux as a function of time.

It

is

pertinent to note that small errors in the measured

data

can lead to large

uncertainties in the calculated value of heat flux. For example,

if

a

thermocouple is placed too close to the mould surface, the heat flux calculated

from the thermocouple

data

corresponds to the local heat flux across the

interface rather than an overall value.

On

the other hand,

if

the thermocouple is

positioned too far below the interface, it is less sensitive to the rapid changes in

heat flux associated

with

the initial period of contact between the melt

and

mould surface. Errors can be minimised both

by

the correct choice of

thermocouple

and

optimising its position within the mould.

30.-----------------------,

OL-

__

-L

____

L-

__

~

____

L_

__

~

o

10

20

30

40

50

Time

(ms)

Figure A.4. Variation

in

interfacial

heat

flux

during

substrate immersion, after Strezov

and

Herbertson (1998) (with kind permission of

the

Iron

and

Steel Institute of Japan).



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

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