FIELD OF THE INVENTION The present invention relates to the production of iron-based specialty alloys, and to specialty alloys so-produced. The present invention also relates to the processing of cast-specialty alloys produced in accordance with the present invention.

BACKGROUND OF THE INVENTION

Herein the expression term "iron-based specialty alloys" is used to denote iron-based metal alloys that have special physical properties such as mechanical, electrical, thermal or magnetic properties. In general terms, such iron-based specialty alloys are known in the art. The extent to which these properties are present is largely determined by the exact proportions of alloying elements and by the alloy microstructure. Generally, the properties of iron-based specialty alloys are attributable to the inclusion of relatively high proportions of alloying element levels, e.g. Cr, Ni, Cu, Si and Al.

For the avoidance of doubt, this class of alloys does not include "mainstream" carbon steels, stainless steels or silicon steels that are continuously cast and manufactured in large production volumes in the steel industry.

Iron based specialty alloys are of significant value from a market/end user perspective due to their desirable properties. Iron-based specialty alloys tend to be produced in relatively low volumes, and then as thin strips (0.1 -3mm thick), foils (~50μηι thick) or as wire.

Conventionally, iron-based specialty alloys are usually cast as ingots, requiring substantial and complex hot deformation and heat treatments before the material is capable of being cold rolled to final products, such as thin sheet, with final properties. Thermo-mechanical , processing of cast ingots to the point where the material is suitable for cold rolling is difficult due to the inherent problems of macro-segregation in high alloy systems, coarse cast microstructures in ingots, surface oxidation during slow solidification/cooling/reheating as well as stresses (and associated cracking) caused by solid state, high temperature phase transformations (ferrite to austenite or vice versa). The production process therefore typically involves high yield losses (as much as 50%), energy inefficiency and high conversion costs.

Against this background it would be desirable to produce iron-based specialty alloys by a different approach that avoids these disadvantages and that is more suited to the manufacturing challenges associated with iron-based specialty alloys.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a process for producing an iron-based specialty alloy intermediate product, which method comprises:

-

(a) forming a melt of desired alloy composition; and

(b) casting the melt under solidification conditions to produce an iron-based specialty alloy intermediate product that has a microstructure and other properties that renders it suitable for a finishing operation.

This embodiment of the invention produces an iron-based specialty alloy in the form of cast intermediate product that is well suited to processing in a subsequent (direct) finishing operation to produce a finished product, such as a thin sheet or wire, having desired final properties and metallurgical characteristics (e.g. a fully recrystallised microstructure).

Herein the expressions "iron-based specialty alloy intermediate product", "cast alloy", "intermediate product" and "cast intermediate product" denote the as-cast product that has yet to undergo any finishing operation. The expression "finished product" is used herein to denote the product following application of one or more finishing operations to the cast intermediate product. For example, when the "finished product" is a thin sheet, this may be made by deformation and heat treatment of the intermediate product, such as cold rolling and annealing.

In accordance with the invention the intermediate product has been produced under casting conditions that are designed to render the intermediate product inherently suitable for application of a subsequent finishing operation. The finished product itself will invariably require further processing (e.g. shaping, welding etc) to produce a final article or final component from the finished product. The process for producing the cast intermediate product per se may be regarded as providing a feedstock for a subsequent finishing operation. This finishing operation may be carried out immediately following formation of the cast intermediate product, for example as an extension of the production process. Alternatively, the finishing operation may be carried out subsequently, for example by someone other than by the producer of the cast intermediate product. In this respect the cast intermediate product might be made by one entity and provided to another entity for subsequent cold working. It is not essential the finishing operation applied to the cast intermediate product be directly linked to the manufacture of that product. Accordingly, in another embodiment, the present invention provides a process for producing an iron-based specialty alloy finished product which comprises subjecting an iron-based specialty alloy intermediate product that has been produced in accordance with the process of the present invention to a finishing operation. The finishing operation per se is conventional and will typically include deformation and heat treatment, such as cold rolling and annealing.

The present invention also provides a cast intermediate iron-based specialty alloy product, and an iron-based specialty alloy finished product, produced in accordance with the present invention. The finished product may be of conventional form, such as a thin sheet or wire. The present invention also provides a final article or final component that has been made from the finished product. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

BRIEF DISCUSSION OF DRAWINGS The present invention is illustrated with reference to the accompanying non-limiting drawings in which:

Figure 1 is a schematic diagram of a typical dipping/immersion apparatus showing the furnace/melt and the paddle arrangement containing substrates;

Figures 2-5 are phase diagrams for various iron-based specialty alloys;

Figures 6-9 show microstructures of cast alloys produced in accordance with the invention and Figure 13 shows an example of a finished product microstructure;

Figures 10 shows the surface appearance of a cast alloy produced in accordance with the invention and Figure 11 shows a finished product sample; and

Figure 12 shows heat flux curves for an alloy composition (Fe-Ni). DETAILED DISCUSSION OF THE INVENTION

In accordance with the present invention it has been found that the solidification conditions under which the specialty melt is cast are critical to avoiding undesirable metallurgical features in the cast intermediate product that would otherwise present problems when it comes to a subsequent finishing operation that is applied to the cast alloy to influence its microstructure and thus its final properties.

Generally, a finishing operation will be applied to control microstructural features such as grain size (preferably the cast alloy has a fine grain microstructure, with grain width of less than ΙΟΟμηι), grain orientation and precipitates. In turn, the microstructure following the finishing operation will be responsible for the final properties of the specialty alloy, as described above. The finishing operation typically involves mechanical deformation of the cast alloy, possibly with one or more heat-treatment steps. Such finishing operations are per se well known in the art. Typically, the finishing operation will involve cold rolling and annealing, although depending upon alloy composition warm (or hot) rolling may be called for instead of cold rolling.

Central to the present invention is the philosophy that is applied to solidification of the melt during casting in order to achieve a desirable outcome in terms of metallurgical features in the cast intermediate product. Thus, the solidification conditions are selected and controlled to provide in the cast intermediate product a relatively fine microstructure (especially compared to ingot casting) and other properties (the intermediate product should be essentially free of macro-segregation, surface oxidations and cracks and have thin sections) that renders the intermediate product especially well suited to subsequent finishing, taking into account the nature of that finishing and the intended outcome of it in terms of microstructural features and final properties.

In practice a number of variables may be controlled in order to produce a cast intermediate product having desirable properties in accordance with the present invention. These variables are discussed below. < Casting comprises contacting the melt with a suitable substrate, with the melt-substrate contacting and initial solidification conditions promoting sufficient cooling and nucleation during casting to . produce a fine microstructure and to minimise or avoid macro-segregation of alloy components and surface oxidation.

The cooling rate that is applied to the melt during casting should be sufficiently high to promote nucleation in order to produce a suitably fine microstructure and to minimise or avoid macro-segregation of alloy components and surface oxidation.

Related to the cooling rate is the requirement for suitably high surface heat transfer. The casting methodology used should provide a sufficiently high surface heat transfer to support the cooling rate without being too high to create cracks and other solidification defects.

The melt composition should be chosen based on the properties it is intended to achieve in the finished and final products to be produced from the intermediate product, taking into account proposed finishing operations etc. The melt composition should also be selected to avoid high temperature solid-state phase transformations that will lead to volume changes and residual stresses as solidification and cooling proceeds. Such residual stresses can cause cracking in the cast intermediate alloy and this is to be avoided. Such solid-state phase transformations are related to the composition of the alloy and can be understood for a given alloy composition from a phase diagram for that alloy system (illustrated in Figures 2 to 5). The problematic solid state phase transformations tend to occur at relatively high temperature, for example above about 900°C.

It is also preferable to cast the alloy as a thin section (typically about 2 mm or less), i.e. with relatively large surface are to volume ratio. Casting the alloy in this form is useful as it supports the high cooling rate regime that is called for, and provides dimensions that are suitable for subsequent finishing operations, such as direct cold rolling. It may also be preferred to cast the alloy with dimensions that are close to the final dimensions required of the finished product as this can reduce the extent of mechanical deformation that is subsequently required in a finishing operation, typically up to 85%.

The cast intermediate product produced in accordance with the invention is suitable for subsequent finishing by conventional methodologies (such as cold rolling, warm rolling, hot rolling and/or annealing), for example to form a thin sheet or coupon of the alloy. Preferably, the cast intermediate product can be subjected to significant thickness reduction >50%, in a finishing operation without the appearance of cracks. Typically, the alloys are amenable to heavy cold reduction achieving >70%, e.g., >70 to 85% reduction without the appearance of cracks. This is demonstrative of good ductility in the intermediate product.

In summary the following variables are relevant to consider in casting a specialty alloy in accordance with the present invention:

• Cooling rate. This must be high enough to promote nucleation for relatively fine cast microstructures and to avoid macro-segregation and limit surface oxidation. The microstructure generally has a cast grain width <250 μπι, for example <150 μιη, such as from 100 to <150 um.

A typical cooling rate regime to support the required properties of the cast intermediate products is as follows:

1. Nucleation cooling rate > 10 ⁴ °C/s

2. Solidification cooling rate (near surface) >10 ³ °C/s

3. Solidification cooling rate (final) > 10 ² °C/s Surface heat transfer. This must be high enough to support the required cooling rate regime (as above), but not too high to create cracks and other solidification defects.

Typical heat flux requirements to support the required properties of the cast intermediate products:

1. Peak heat flux >10 MW/m ² - sufficient wetting, acceptable and consistent product quality

2. Peak heat flux <20 (maximum of 30) MW/m ² - localised very high heat fluxes could contribute to solidification defects

• The alloy is cast as thin sections (typically 2 mm or less - high surface area to volume ratio). This supports the high cooling rate regime and provides dimensions suitable for direct finishing operations (typically involving cold rolling).

• Alloy composition. This is carefully selected from regions of the alloy phase diagram which avoid stresses and cracks from high temperature solid state phase transformations.

The properties of the cast specialty alloy that render it capable of finishing (typically involving direct cold rolling) to produce a finished product includes:

• Essentially free of macro-segregation.

• Relatively fine cast microstructure.

• Little or no surface oxidation.

• Essentially free of cracks and solidification defects. • Near final product dimensions.

In an embodiment of the invention involves casting an Fe-based specialty alloy having a relatively high alloying element level. Typical alloying elements include Cr, Ni, Cu, Si and Al.

The present invention has been found to give good results in relation to a number of specialty alloys as follows:

• Fe-Cr-Al or Fecral alloys (such as Fe-15Cr-4Al and Fe-20Cr4-5Al). Fecral alloys withstand high temperatures and exhibit high electrical resistance (e.g. from 1100-1300°C). They are used in heating elements and catalytic converters.

• Fe-Ni alloys (such as Fe-36Ni or invar and Fe-41Ni). Invar and other similar Fe-Ni alloys are controlled expansion alloys (extremely small thermal expansion over wide range of temperatures) and are used in glass-to-metal seals in electron tubes, transistors, headlights, thermostats, and other similar applications.

• Fe-Cu and Fe-Cu-Cr alloys (such as Fe-20Cu, Fe-30Cu, Fe-40Cu and Fe-40Cu- 3Cr). Fe-Cu alloys have excellent electrical conductivity and abrasion resistance and often used for a sliding contact element or the like. Fe-Cu-Cr alloys have improved corrosion resistance due to the inclusion of chromium and exhibit excellent heat conductivity and electrical conductivity. They are particularly suitable for high-strength lead frame of a semiconductor Integrated Circuit or for a pin grid array.

• Fe-Al-Cr or iron-aluminide based alloys (such as Fe-15.9Al-2.2Cr and Fe-15.9A1- 5.5Cr). Iron aluminide alloys have low density, excellent corrosion resistance (oxidising, carburising and sulfidising atmospheres, and also against molten salts), high temperature strength/wear resistance and high electrical resistivity. They are used as high temperature structural materials (automotive components) and also in heating elements, gas filters and fasteners.

• Fe-Si alloys. These alloys have beneficial electrical and magnetic properties. In accordance with the present invention it may be possible to produce alloys with a Si content higher than is achievable by conventional continuous casting methodologies. For example, in accordance with the invention it may be possible to produce alloys having a Si content up to 4%, and possibly higher. Such alloys are useful as electrical steels.

Embodiments of the present invention are illustrated in the following non-limiting examples.

Example 1 - Fecral Alloys

Products in the form of strips (0.4 to 100 mm wide and > 0.1 mm thick) and thin foils (typically 50 μπι thick) are commonly available.

Conventionally produced by ingot metallurgical processing, i.e., ingot casting, hot forging/hot rolling, and cold rolling and annealing. Modern powder metallurgical techniques have also been trialled.

Two alloys were tested in this program - Fe-15Cr-4Al (Fecral 125) and Fe-20Cr-5Al (Fecral 135). Chemical compositions of these alloys are given in the following table.

Melting was carried out under inert conditions to prevent oxide/slag accumulation (induction furnace, tightly controlled under argon atmosphere); standard casting practices were employed using a dip tester (smooth substrate, wire brush cleaned, nitrogen/argon atmosphere, 1 m/s casting speed).

Castability:

Both alloys were castable during dip/immersion testing and behaved reasonably similarly - satisfactory surface quality, except for some slag patches observed on a few samples; no stress related surface distortions or cracks were observed. The trials revealed a range of acceptable initial contact/wetting conditions (alloy type and dipping atmosphere had some impact), nucleation patterns and heat fluxes; and satisfactory productivity (as indicated by K-factor or solidification rate).

Relatively fine "cast" columnar grain structure (60-150 μηι wide and 500-1000 μπι long - a wide range impacted by alloy composition and dipping atmosphere); moderate alignment of {100} planes with solidification direction; sparsely populated, sub-micron size particles which were predominantly aluminium nitrides and manganese sulphides; macro- segregation of elements and surface oxidation were not observed in noticeable levels.

Product Properties:

Cast products were sufficiently ductile; could be subjected to conventional processing conditions (80% cold reduction and 30 min annealing at 900 °C) to produce fully recrystallised microstructure (mixture of coarse and fine grains - average grain size of 19 μ ^η ί). Note that 20Cr-5Al alloy was warm rolled at around 200 °C as this allowed the strip to be above its ductile to brittle transition temperature.

Mechanical properties of the as-cast and annealed products (400-500 MPa YS, 700-800 MPa UTS and 160-180 VHN microhardness) are not significantly different from the data reported for the conventionally produced alloys. Note: Yield Strength (YS); Ultimate Tensile Strength (UTS) and Vickers Hardness Number (VHN).

Key findings:

1. Casting/stream-lined production of two Fecral alloys - casting a wide of range of compositions is possible.

Several specific factors were related to castability - melt chemistry (tight atmospheric control)/surface quality, acceptable productivity, sensitivity to process variables (alloy chemistry and dipping atmosphere), dipping atmosphere/surface cleanliness/heat flux control, etc.

Good initial contact/wetting conditions and lack of solid-state phase transformations (following solidification) contribute to castability (compared to low-C or stainless steels for example).

Macro-segregation of elements and surface oxidation were not observed in noticeable levels and hot forging/hot rolling is not needed - important factor for minimising yield loss.

Ductile cast products are amenable for heavy cold reduction - 70-80% reduction without intermediate anneal (warm rolling needed for 20Cr-5Al alloy).

Cold rolling and annealing conditions are similar to those conventionally practiced.

7. Product mechanical properties (both as-cast and annealed) are comparable to the data reported for conventionally produced alloys. Example 2 - Iron-Nickel Alloys

Typically specialty alloys of this type contain 35-50 wt% Ni. Products in the form of strips/sheets coils (0.1 -3.0 mm thick), plates and rods are commonly available.

The alloy is conventionally produced by ingot casting, hot forging/hot rolling, and cold rolling and annealing. However, serious problems with elemental segregation, oxidation and cracking plague alloys produced by conventional manufacturing routes. For example, there is a need for grinding before final cold rolling and as a result yield could be as low as 50%. Roll compaction of elemental powders have also been used to produce these alloys (up to 2 mm thick and 350 mm wide).

Two alloys were tested in this program - Fe-36Ni (Invar or Invar 36) and Fe-41Ni (Alloy 42). Chemical compositions of these alloys are given in the following table.

Melt practice could be managed with relative ease (induction furnace under argon); standard casting practices using a dip tester were used (smooth substrate, wire brush cleaned, nitrogen atmosphere, 1-2 m/s casting speed).

Castabilitv:

Both alloys were highly castable during dip/immersion testing and behaved similarly - excellent surface quality of cast products (the best results obtained among all the alloy systems studied under this program); insensitive to process variables such as surface cleanliness; good initial contact/wetting conditions, nucleation patterns and ideally suited heat fluxes; and satisfactory productivity (casting rates). Relatively fine "cast" columnar grain structure (100 μιη wide and 200 to 800 μπι long); strong alignment of {100} planes with solidification direction; sparsely populated, sub-micron size particles, which predominantly had Mn, S, Si, O and N; macro- segregation of elements and surface oxidation were not observed in noticeable levels.

Product Properties:

Cast products were highly ductile; could be subjected to conventional processing conditions (80% cold reduction and 30 min annealing at 900 °C) to produce fully recrystallised microstructure (mixture of coarse and fine grains - average grain size of 14 μπι).

Mechanical properties of the annealed products (300-340 MPa YS, 520-540 MPa UTS and 100-130 VHN microhardness) are not significantly different from the data reported for the conventionally produced alloys.

Key findings:

1. Casting/stream-lined production of two Fe-Ni alloys - casting a wide range of compositions is possible.

2. Several specific factors are related to castability - melt chemistry control (atmospheric control), acceptable productivity, excellent surface quality, insensitivity to process variables (surface cleaning), ideally suitable heat fluxes, etc.

3. Good initial contact wetting conditions and lack of solid-state phase transformations (following solidification) contribute to castability (compared to low-C steels or stainless steels for example). Macro-segregation of elements and surface oxidation were not observed in noticeable levels and hot forging/hot rolling is not needed - important factor for minimising yield loss.

Ductile cast products are amenable for heavy cold reduction - 80-85% reduction without intermediate anneal.

6. Cold rolling and annealing conditions are similar to those conventionally practiced. 7. Product mechanical properties (both as-cast and annealed) are comparable to the data reported for conventionally produced alloys.

Example 3 - Iron-Copper Alloys Four alloys with the following chemical compositions were tested in this program:

Melting was carried out under inert conditions to prevent oxide/slag accumulation (induction furnace, under argon); standard casting practices using a dip tester were used (smooth substrate, wire brush cleaned, argon atmosphere, predominantly at 0.75 m/s casting speed).

All four alloys were castable during dip/immersion testing and behaved reasonably similarly - satisfactory surface quality, except for some slag patches observed on a few samples; no stress related surface distortions or cracks. Acceptable productivity (casting rate) was achieved for all alloys; productivity was relatively higher for the alloys that had around 38% copper.

Relatively fine "cast" columnar grain structure (80-120 μπι wide and 500-1000 μπι long); macro-segregation of elements and surface oxidation were not observed in noticeable levels.

Mechanical properties of the as-cast products were typically 350-420 MPa YS and 440-470 MPa UTS.

Cast products were sufficiently ductile, and could be easily cold rolled to 70% reduction in thickness. Combined annealing and aging treatments will be needed to achieve desired final product properties. Example 4 - Iron-Aluminium Alloys

Two variations of iron aluminide alloys were tested in this program. Chemical compositions of these alloys are shown below.

Melting was carried out under inert conditions to prevent oxide/slag accumulation (induction furnace, under argon); standard casting practices using a dip tester were used (smooth substrate, wire brush cleaned, argon atmosphere, predominantly at 0.75 m/s casting speed); possible cracking of the solidified samples during cooling were overcome by quickly transferring the cast samples to a furnace set at 550 °C and by cooling the samples thereafter over several hours to reach room temperature. Both alloys were castable during dip/immersion testing and behaved reasonably similarly - excellent surface quality was achieved; slag patches were sometimes observed on a few samples; no stress related surface distortions or cracks formed (once the furnace cooling procedure was established). Acceptable productivity (casting rate) was achieved for both alloys.

Moderately fine "cast" columnar grain structure (170-260 μηι wide and around 1000 μπι long); macro-segregation of elements and surface oxidation were not observed in noticeable levels.

These alloys are brittle at room temperature, and therefore subjected to hot rolling at around 800 °C to produce the final product. Around 50% reduction in thickness was achieved in 2 to 3 passes. Mechanical properties of the products (both cast and further processed) were typically 610-760 MPa YS and these were comparable to the data reported for conventionally produced alloys.

General Experimental Details

Experimental aspects include (1) the evaluation of the selected alloys for their suitability to be cast under rapid solidification conditions and (2) a preliminary evaluation of the cast products with respect to their amenability for down-stream processing and the resulting product properties. A systematic, standard methodology was adopted for these evaluations and the details are provided below.

Alloys tested:

Four separate casting campaigns, involving more than 100 individual tests, were conducted (see Table 1 for alloys tested). Table 1 : Fe-based specialty alloys tested during the experimental program

Evaluation of castability: (a) Dip testing facility and melt practice

Experiments were carried out to assess the suitability of the selected alloys to be produced under rapid solidification conditions. Figure 1 shows the schematic arrangement of the dip testing technique whereby a paddle, containing one or more types of substrate, is dipped at a certain velocity into a pool of molten metal, and then retracted to produce a solidified coupon on the substrate.

A 100 kW induction furnace, with a nominal capacity to hold -100 kg Fe-based alloys, was used for melting. Different amounts of alloying additions (in high purity form) needed to achieve the target chemical composition were prepared in advance for easy addition during melting. The total required amount of iron was melted first (usually added in the form of A06 low-carbon steel plates), and this was followed by de-oxidation of the melt by additions of aluminium, silicon and manganese. Main alloying elements were added next, and necessary adjustments were made. Sufficient time was allowed between additions to ensure complete melting of all the preceding additions. Following complete melting and homogenisation, the chemical composition of the melt was analysed using spectroscopy, and minor adjustments were made to attain the target alloy chemistry.

The chemical composition of the melt was periodically analysed to ensure that the melt composition did not drift away from the target. The top surface of the melt, during some campaigns, appeared cloudy with the presence of slag (e.g., Fecral tests), and in such cases "Slax" additions were made and slag was scooped out of the melt surface as much as practical. (b) Experimental setup and solidification studies

The paddle immersed into the melt had two different copper substrates. One copper substrate was instrumented with a 300 μπι K-type thermocouple placed approximately 500 μηι beneath the surface on the middle of the substrate. The other copper substrate was chrome coated with a 10- 15 μπι thick flash coating. Both substrates were 38 mm x 38 mm in size, and had a smooth surface finish (~0.4 Ra).

Prior to commencing each series of experiments with new alloy chemistry, the substrate surfaces were chemically cleaned using 10% phosphoric acid. In between tests, substrate surfaces were cleaned using a wire brush consisting of fine brass bristles. As the solidified samples emerged from the melt, these were rapidly cooled with an argon blast to minimise the oxidation of the surfaces.

The paddle immersion profiles were carefully controlled through a servo-motor arrangement. Typically, substrates were dipped into the melt at 1 m/s, with a total immersion time of around 150 to 250 ms. Substrate temperature data acquired during solidification (typically increased from -100 °C to -160 °C) were used to calculate heat transfer rates during the period of initial contact and solidification (the first 20 ms are critical). For each variation in chemical composition, a total of ten dips were usually carried out. Solidification tests were performed either under nitrogen or argon atmosphere (Note: in between tests and at all other times during the campaign, the melt was kept under argon atmosphere to minimise oxidation of the melt and slag formation).

Characterisation of cast samples:

The as-cast coupons were subjected to detailed investigations and the test procedure is summarised below.

1. Selected samples were scanned (both substrate side and melt side) to keep a record of the surface condition. Surface conditions were also visually assessed for the presence of slag patches, distortions and cracks. When surface distortions were observed, simple size estimations were made and a distortion size was allocated to each sample.

2. Typically ten thickness measurements were made across the width of the sample along a line 10 mm below the top edge of the sample (Note: position corresponding to around 150 ms immersion time under most testing conditions). K-factor (or solidification rate) was calculated based on the average thickness and the immersion time, by assuming t° ⁵ correlation.

3. Heat transfer rates during solidification were calculated from the temperature data using an inverse procedure. "Peak Heat Flux" and "Average Heat Flux" values were deduced from the heat flux curves.

4. Sample surfaces (substrate side) were observed under optical microscope to assess ' the initial contact or nucleation behaviour of the melt under the imposed solidification conditions.

5. Transverse cross-sections along the width of the samples, i.e. normal to the dipping direction, were taken around the same position where the strip thickness was measured. The sections were prepared using standard metallographic procedures (mounting, grinding and polishing), were either chemically or electrolytically etched, and then examined using light optical microscope to study solidification and as-cast product microstructures.

Selected metallographic samples (the same sections used for optical microscopy) were additionally subjected to: a. SEM-EDX analysis to observe and analyse inclusions and precipitates; and b. SEM-EBSD analysis to study crystallographic orientations of the as-cast grains.

Mechanical properties of the as-cast samples were assessed using: a. Micro-hardness testing which was conducted on the polished sections prepared for metallographic analysis. b. Shear punch testing. Samples taken from the as-cast samples were slightly ground on both sides using emery paper to flatten them, as well as to remove the thin oxide surface layer (if any) on the melt side of the samples. A shear punch test fixture consisting of a 3 mm diameter die was used to make the punch/deformation on the samples (in a direction perpendicular to the sample), with the whole setup attached to an Instron Universal Tensile Testing Machine.

Selected dip samples were cold rolled to assess the suitability of the as-cast strips to be converted into thin sheets. Cold-rolled sheet samples were then annealed, and the final sheets products were characterised for microstructure and mechanical properties. Downstream processing of strips - cold rolling and annealing:

Strips cut form the as-cast coupons were cold rolled in a hand mill. Samples were rolled in a direction parallel to the dipping direction ("longitudinal direction"). Typically, 70 to 85% reduction in thickness was achieved in 5 to 8 passes. In the case of 20Cr-5Al Fecralloy, the cast strip was warm rolled at an estimated temperature of 200 °C. In the case of iron aluminides, the samples were hot rolled at around 800 °C.

Coupons cut from the cold rolled sheets were annealed in a tube furnace under inert conditions (argon flow). For example, Fecral and Fe-Ni alloys were annealed at 900 °C for 30 min and rapidly cooled in air at the end of the treatment.

Assessment of final sheet product: , The cold-rolled and annealed sheet samples were subjected to detailed investigations. The standard procedure adopted is summarised below.

1. Optical microscopic assessment of metallographic sections taken along the rolling direction of the cold-rolled and annealed samples ("longitudinal sections").

2. SEM-EDX and SEM-EBSD analysis of the selected annealed samples.

3. Shear punch testing (on sheet samples) and micro-hardness testing (on longitudinal metallographic sections) of the cold rolled and annealed samples. Summary:

The full list of observations and measurements made and the key parameters derived with regards to castability and product assessment are summarised in the following table. Table 2: List of measurements made and key parameters derived for characterising castability and product properties

CHARACTEROBSERVATIONS /

KEY PARAMETERS ISTICS MEASUREMENTS

Heat flux

Quality of melt/substrate contact and productivity. measurements

Castability Visual/macro

Surface quality including presence of slag patches inspection of dip

and cracks.

samples

Measurements on dip Thickness profiles; K-factors and productivity; and samples surface distortions

Surface (nucleation density) and through-thickness

Microstructure and solidification structures; as-cast microstructures

As-cast product texture analysis of dip (grain sizes) and crystal lographic textures; and characterisation samples analysis of inclusions/precipitates and elemental segregation.

Micro-hardness measurements and shear punch

Mechanical properties

testing (shear properties were then converted to of dip samples

tensile properties).

Process-ability of Thermal-mechanical

Suitability for cold rolling, and annealing response cast products processing of dip

(under inert conditions).

samples

Microstructure and Through-thickness microstructures (grain sizes) and

Sheet product texture analysis of crystal lographic textures; and analysis of characterisation sheet products inclusions/precipitates and elemental segregation.

Micro-hardness measurements and shear punch

Mechanical properties

testing (shear properties were then converted to of sheet products

tensile properties).