Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/20—Measures not previously mentioned for influencing the grain structure or texture; Selection of compositions therefor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D25/00—Special casting characterised by the nature of the product
  • B22D25/06—Special casting characterised by the nature of the product by its physical properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D7/00—Casting ingots, e.g. from ferrous metals
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium

Definitions

• 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.
• 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.
• 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.
• 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 ( ⁇ 50 ⁇ thick) or as wire.
• 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.
• the present invention provides a process for producing an iron-based specialty alloy intermediate product, which method comprises:
• 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).
• iron-based specialty alloy intermediate product denotes the as-cast product that has yet to undergo any finishing operation.
• finishing product is used herein to denote the product following application of one or more finishing operations to the cast intermediate product.
• 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.
• 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.
• the finishing operation may be carried out subsequently, for example by someone other than by the producer of the cast intermediate product.
• the cast intermediate product might be made by one entity and provided to another entity for subsequent cold working.
• 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.
• 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;
• Figure 12 shows heat flux curves for an alloy composition (Fe-Ni). DETAILED DISCUSSION OF THE INVENTION
• 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.
• 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 ⁇ ), grain orientation and precipitates.
• 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.
• 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.
• 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.
• ⁇ 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.
• 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.
• the alloy 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.
• the cast intermediate product can be subjected to significant thickness reduction >50%, in a finishing operation without the appearance of cracks.
• 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.
• the microstructure generally has a cast grain width ⁇ 250 ⁇ , for example ⁇ 150 ⁇ , 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:
• 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.
• 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).
• the properties of the cast specialty alloy that render it capable of finishing (typically involving direct cold rolling) to produce a finished product includes:
• an Fe-based specialty alloy having a relatively high alloying element level.
• Typical alloying elements include Cr, Ni, Cu, Si and Al.
• 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 have beneficial electrical and magnetic properties.
• Such alloys are useful as electrical steels.
• Products in the form of strips (0.4 to 100 mm wide and > 0.1 mm thick) and thin foils (typically 50 ⁇ thick) are commonly available.
• 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).
• Relatively fine "cast” columnar grain structure (60-150 ⁇ wide and 500-1000 ⁇ 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.
• 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 ⁇ ⁇ ⁇ ). 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.
• 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).
• 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.
• serious problems with elemental segregation, oxidation and cracking plague alloys produced by conventional manufacturing routes.
• 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).
• 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).
• 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 ⁇ ).
• Ductile cast products are amenable for heavy cold reduction - 80-85% reduction without intermediate anneal.
• 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).
• 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.
• 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.
• 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 paddle immersed into the melt had two different copper substrates.
• One copper substrate was instrumented with a 300 ⁇ K-type thermocouple placed approximately 500 ⁇ beneath the surface on the middle of the substrate.
• the other copper substrate was chrome coated with a 10- 15 ⁇ thick flash coating. Both substrates were 38 mm x 38 mm in size, and had a smooth surface finish ( ⁇ 0.4 Ra).
• the substrate surfaces 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.
• 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).
• 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).
• Selected metallographic samples 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.
• 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).
• 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.
• Table 2 List of measurements made and key parameters derived for characterising castability and product properties
• Sheet product texture analysis of crystal lographic textures and analysis of characterisation sheet products inclusions/precipitates and elemental segregation.

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