Patent Application
20150267272
1. Field of the Invention
The invention concerns a cored wire intended to be fed into a bath of molten metal to perform metallurgical treatment, the cored wire comprising:
The molten metal is iron or steel for example. The objective of metallurgical treatment may be to add at least one substance for example to the molten metal, the substance being intended to regulate the composition of the molten metal and/or the composition of the precipitates or non-metallic inclusions it contains.
2. Description of Related Art
In metallurgy it is known to provide a said substance by means of “cored wires” wound in coils. The cored wire is generally composed of a fill comprising the active substance in powder form packed in a metal sheath formed of metal having a composition compatible with that of the molten metal to be treated. When treating molten steel, this sheath is itself advantageously in steel.
The cored wire is fed into the bath of molten metal using an injection device, generally automatic, which injects a precise length of cored wire at suitable speed.
For example, in the foundry sector to produce spheroidal graphite cast iron it is known to use a cored wire to conduct nodularisation. This concerns the adding of magnesium to change the shape of the graphite particles of the iron, from lamellar to spheroidal. The added substance is generally a magnesium ferrosilicon alloy in powder form.
According to another example, in the steel-making industry, it is known to treat steels with calcium. For example this treatment is intended to modify the chemical composition of endogenous inclusions of alumina type to obtain inclusions of calcium aluminate type. These inclusions, dispersed in the molten steel, are liquid at casting temperature. They therefore cannot adhere to the nozzle walls of a ladle or tundish of a continuous casting installation. Castability is thereby improved as is the final quality of the steel produced.
Numerous types of cored wire exist having a fill formed of pure calcium or calcium alloy powder, or a mixture of calcium and iron, even aluminium powders. For example, the alloy commonly called CaSi (calcium silicide) or the mixture of calcium and iron powders (generally called CaFe) are widely used to fill cores.
Although the adding of a cored wire to a bath of molten metal is a clever way of adding the active substance to the molten metal, the efficacy thereof is sometimes limited. For cored wires containing CaFe powders used in steel-making for example the yield of calcium addition, defined as the amount of calcium found in the steel after injection of the cored wire divided by the amount of calcium injected by the consumed cored wire, is generally in the order of 10% to 15%, sometimes much less. The low efficacy of calcium is essentially due to its low vaporisation temperature. In the region of 1480° C., this vaporisation is generally lower than the working temperature of the liquid steel which means that the calcium vaporises when being added to the liquid steel.
It is one objective of the invention to provide a cored wire achieving more efficient metallurgical treatment whilst remaining competitively priced.
For this purpose the subject of the invention is a cored wire of the type described above wherein the fill comprises:
According to particular embodiments, the device may comprise one or more of the following characteristics taken alone or in any possible technical combination:
By “equivalent diameter” of an element is meant the diameter of a disc having a surface equal to the surface presented by the element in cross section. If the given element has a circular cross section the equivalent diameter is equal to the ordinary diameter.
When the notion of equivalent diameter is used for an element it is implied that the element is locally substantially cylindrical but does not necessarily have a circular base.
By “metallurgical treatment” is meant for example:
The invention further concerns a method for metallurgical treatment of a molten metal bath, the method comprising the step of inserting a cored wire such as described above in the molten metal bath.
According to particular embodiments, the method may comprise one or more of the following characteristics taken alone or in any possible technical combination:
The invention will be better understood on reading the following description given solely as an example with reference to the appended drawings in which:
With reference to
Similarly, a transverse plane T is defined perpendicular to the longitudinal axis L. It is understood that the transverse plane T is transverse for the illustrated portion of cored wire 1, i.e. locally transverse.
The cored wire 1 is intended for example to be fed into a bath of molten iron (not illustrated).
The cored wire 1 comprises a fill 2 and an outer sheath 4 both extending longitudinally.
The outer sheath 4 forms a peripheral portion of the cored wire 1, intended to be in contact with the molten metal bath when the cored wire 1 is inserted in the bath of molten metal.
The outer sheath 4 is advantageously formed of metal strip 6 folded over around the longitudinal axis L.
The strip 6 is in steel, copper, aluminium, nickel or zinc for example, or else a mixture of two or more of these elements.
The strip 6 advantageously comprises two longitudinal folds 6a, 6b (
The fill 2 comprises an extruded bar 8 extending longitudinally and an intermediate layer 10 extending longitudinally between the extruded bar 8 and the outer sheath 4.
The extruded bar 8 is advantageously substantially cylindrical with circular base. The extruded bar 8 has a diameter D1 in the transverse plane T, with D1 advantageously between 2 and 10 mm, e.g. 8 mm.
The extruded bar 8 comprises an active substance to treat the molten iron. The active substance is magnesium for example. Advantageously the extruded bar 8 mostly contains magnesium.
By “mostly” is meant that the extruded bar 8 comprises at least 50% by weight of magnesium, preferably at least 90% by weight of magnesium.
In the example, the extruded bar 8 is formed of magnesium having industrial purity e.g. 99.8 weight %.
The extruded bar subsequent to treatment 8 is not a mere cluster of powder material compacted at the time the cored wire 1 is closed, nor an agglomerate of powder grains (powder material) bound together by a binder irrespective of kind. The extruded bar 8 is obtained for example by extrusion of a solid cylinder of material (billet) through a die by means of a press. The bar 8 can also be obtained directly using a continuous casting method, the liquid material being solidified in the form of a continuous bar. The porosity of the extruded bar 8 is considered to be practically zero, the apparent density of the bar being close to the true density of the material.
The intermediate layer 10 extends for example in the space lying between the extruded bar 8 and the outer sheath 4.
The intermediate layer 10 has an equivalent outer diameter D2. For example D2 is such that the ratio D2/D1 is between 1.3 and 6.2.
The intermediate layer 10 is advantageously formed of a powder. The intermediate layer 10 may also comprise a thermally insulating layer covering the bar 8.
The intermediate layer 10 also mostly contains, as defined above, an active substance for metallurgical treatment, e.g. a ferrosilicon alloy. Advantageously the intermediate layer 10 may also comprise up to 12% by weight of calcium, barium and rare earths (lanthanum, cerium).
The composition of the powder of the intermediate layer 10 is evidently dependent on the metallurgical treatment to be conducted. It may be neutral i.e. having no metallurgical effect on the bath of liquid metal to be treated, in this case the powder solely acts as thermal insulator of the bar 8. It may also take direct part in metallurgical treatment and thereby assume a dual role of thermal insulator and active treatment element.
A description is now given of the functioning and use of the cored wire 1 to carry out nodularisation and inoculation of the iron melt to obtain spheroidal graphite cast iron.
The iron to be treated is in the form of a molten metal bath contained for example in a vessel such as a ladle.
The cored wire 1 is fed into the iron melt bath under conditions known per se.
The adding of magnesium to the iron melt leads to a series of physical and chemical conversions, some of which are simultaneous and others successive:
The desulfurizing reaction is the following:
Mg+FeS->Fe+MgS.
Simultaneously there occurs intense mixing of the iron melt by gaseous Mg and deoxidation of the iron melt. Magnesium is an energetic deoxidizer. Once desulfurizing and deoxidation are completed, the remaining magnesium is incorporated in the iron melt. Evidently throughout the entire duration of the process, some of the magnesium vapours formed escape from the surface of the molten iron and are fully lost by oxidation in the slag or atmosphere, giving rise to the formation of magnesium oxide for example.
The yield of magnesium addition is defined as the ratio between first the difference in Mg content effectively found in the melt after injecting the cored wire 1 and the Mg content in the melt before adding the cored wire 1, and second the theoretical amount of Mg added to the melt by means of the cored wire 1 if 100% of the added Mg should effectively be found in the melt.
The magnesium contained in the extruded bar 8 plays a nodularising role i.e. it allows particles of spheroidal graphite to be obtained in the cast iron.
By means of the above-described characteristics the cored wire 1 achieves more efficient metallurgical treatment, in this example nodularisation of the cast iron, whilst remaining competitively priced.
The intermediate layer 10 acts as heat protection for the extruded bar 8, slowing down the rise in temperature of the magnesium contained in the extruded bar 8. The positioning of the intermediate layer 10 around the extruded bar 8 provides protection thereto. At the time of injection of the cored wire 1 into the molten iron bath, the rise in temperature of the magnesium is delayed through slowed heat transfer. The cored wire 1 can therefore be inserted more deeply into the column of liquid iron. The contact time of the magnesium gas with the liquid iron is thereby lengthened and therefore improves the yield of magnesium addition.
In addition, in the extruded bar 8, the magnesium has a reduced specific surface area for heat transfer compared with the surface of a fill composed of mere powder grains. The specific surface area is no longer the surface of the grains but the side surface of the extruded bar 8. This slows down the vaporisation of the magnesium, thereby improving the addition yield and moderating the reaction of Mg with the iron melt.
In the example, the intermediate layer 10 also plays a metallurgical role. The intermediate layer 10 comprises a second active substance for metallurgical treatment, namely a ferrosilicon alloy for example. This second active substance acts as inoculant. As is known inoculation regenerates graphite seeding potential after treatment with magnesium. This allows prevention of the formation of cementite and contributes towards obtaining the desired content of spheroidal graphite. For ferrite iron grades it also promotes the formation of ferrite by increasing the density of spherules.
In addition, since the substance that is active for nodularisation i.e. magnesium is contained in the extruded bar 8, it is possible to add the active substance in more regular fashion than with prior art cored wires containing powder magnesium for which the density and compaction are difficult to control. The small variation in magnesium weight per metre in the cored wire 1 advantageously in the order of +/−2%, allows a reduction in the dispersion of residual magnesium in the treated iron melt and hence lowering of the consumption of cored wire for one same quantity of Mg effectively added to the melt, and an improvement in the quality of the parts cast from the treated iron melt in particular via a reduction in porosities and/or oxide films. The weight per metre of magnesium in the cored wire of the invention is much more precise than the weight per metre of magnesium contained in standard cored wires i.e. produced from magnesium powder.
Finally, with a view to imparting maximum metallurgical treatment efficacy to the cored wire 1, the ratio of the diameters D2/D1 is between 1.3 and 6.2. This range was determined using the following criteria.
For the insulating intermediate layer to be sufficiently efficient, it must be sufficiently thick. The space between the extruded bar and the outer sheath must therefore be sufficiently wide to contain the powder. A D2/D1 ratio of 1.3 or higher guarantees the minimum space required for sufficient thermal protection of the powder in the intermediate layer.
A D2/D1 ratio of 6.2 or lower is based on both metallurgical and economic considerations. It allows a guaranteed minimum proportion of active substance (extruded bar 8) in relation to the insulating substance. If the imbalance is too great this will generate major heat losses in the molten metal bath to be treated (too much powder added having regard to the added active substance), but also an increase in the cost price of the cored wire.
Samples P1 to P8 were prepared and tested. The results of the tests are given in Tables 1, 2a and 2b below. The objective of these tests was to prove the advantage of the cored wire 1 of the invention in terms of yield of magnesium addition when treating cast iron melt.
Samples P1 to P4 were commercially available, conventional cored wires filled with powder material having the following characteristics:
Samples P5 to P8 were taken from a cored wire conforming to the invention and having the following characteristics:
All the samples were tested under the same conditions namely in ladles having a height to diameter ratio of 0.8. The injection rates were adapted for each of the wires in order to maintain one same quantity of magnesium added per unit of time. For each sample, the yield of magnesium addition was calculated (Tables 2a and 2b).
The yield of magnesium addition obtained with samples P5 to P8 of the invention was a mean of 26.7%, compared with 19.3% for samples P1 to P4, i.e. an improvement of +38%. In Tables 2a and 2b the “range” is the difference between the highest addition yield and the lowest addition yield. For samples P5 to P8, the range was reduced by 42% compared with samples P1 to P4. This improves the predictability of magnesium injection obviating with greater certainty the need for a second injection due to insufficient Mg addition, and hence allows lesser consumption of active product and avoids extending production time. This shows that over and above the positive impact on magnesium yield at the time of injection, the use of an extruded wire provides for better regularity of magnesium weight per metre, the treatment is more regular and results for residual magnesium after treatment are less dispersed. This makes it possible to reduce the nominal magnesium content and therefore, in addition to the economic advantage of reduced wire consumption, the quality of the molten metal will be improved since it is known that a high level of magnesium which is nonetheless necessary to obtain a GS structure, has negative side effects e.g. increased shrinkage propensity.
The intermediate layer 10 may also play a major metallurgical role, for example allowing limited the fading of magnesium over time i.e. a decrease in the magnesium content of the iron melt after injecting the cored wire. Since the solubility of magnesium in iron melt is limited and since this solubility is a function of temperature, its concentration decreases continuously as a function of time.
Magnesium is also a powerful deoxidizer and desulfurizer. Mg has a tendency to combine with oxygen to form inclusions of MgO, thereby becoming increasingly less efficient for nodularisation of graphite spherules. The effect of spheroidization decreases, at times until it becomes insufficient. The graphite then changes over from a perfectly spheroid shape to an irregular, ragged shape and finally vermicular shape if the magnesium content is too low. The term iron degeneration is then used.
To oppose this fading of magnesium in the iron melt and to improve graphite spheroidization, several technical solutions were applied. For example, the intermediate layer 10 may contain deoxidizing elements other than magnesium, e.g. cerium, but also chemical elements from Groups IIA and IIIA in the Periodic Table and/or inoculating elements such as silicon. The multiplication of graphite nodule seeding sites allows many more spherules to be obtained and thereby limits the degeneration essentially affecting the large graphite nodules.
Table 3 lists the tested samples and reference is made to graph 3 giving the results in which the curves C1 to C4 correspond to the change in time of the proportion of Mg remaining in the iron melt compared with the Mg added via the cored wire:
Curve C5 corresponds to a lower limit below which it is not desired to exceed in order to guarantee optimal quality of the cast parts.
The four types of cored wires were tested under the same operating conditions 15 treatments per wire), namely:
It was found that the cored wires PFT25, PFT32 and PFT40 according to one preferred variant of the invention promote slower magnesium fading than the reference. The addition of 6% barium to the intermediate layer therefore allows lengthening of the lifetime of the treated melt (limited residual value of Mg guaranteeing the quality of the cast parts) by 15 minutes (compared with the reference for which it is only 20 minutes).
The intermediate layer 10 surrounding the extruded bar 8 allows a significant reduction in magnesium fading over time. It was shown that an intermediate layer 10 formed of a powder comprising a combination between the elements cerium, calcium and barium allows a longer magnesium residence time to be obtained in the iron melt.
This also allows guaranteed homogeneous quality of all parts cast from the iron melt treated in this manner, from the first to the last. The casting process may last several tens of minutes and it is therefore important that the magnesium content should be above the limit value when the last part is cast from the same ladle of treated iron melt.
According to one variant not illustrated, the cored wire 1 may comprise an insulating layer extending longitudinally between the extruded bar 8 and the intermediate layer 10.
The insulating layer may comprise paper for example, or moistened paper, metallized paper or metal. The insulating layer allows adjustment of the global heat transfer coefficient between the molten metal bath and the extruded bar 8. Advantageously the insulating delays the complete melting of the cored wire 1.
With reference to
The cored wire 100 mostly differs through it chemical composition and use.
The cored wire 100 is intended for example to be injected into a bath of molten steel (not illustrated).
For the cored wire 100, the outer sheath 4 is in steel. As a variant, it may be in aluminium, nickel, zinc or copper.
The extruded bar 8 mostly contains calcium. Preferably the extruded bar 8 is formed of calcium having industrial purity of 98.5%. According to one variant (not illustrated) the extruded bar 8 can be encased in a thermally insulating layer extending longitudinally.
The intermediate layer 10 comprises iron powder. As a variant it may comprise powders of aluminium, magnesium and/or oxides such as slag.
For example:
the outer sheath 4 has a thickness of about 0.4 mm;
the weight per metre of the iron powder is about 300 g/m;
the weight per metre of the extruded bar 8 is about 85 g/m and has a diameter of about 8.5 mm.
The cored wire 100 is used in similar manner to the cored wire 1 e.g. to treat a molten steel bath with calcium.
One advantage of the cored wire 100 is that it develops the same weight per metre of calcium as a standard 30% CaFe cored wire (mixture of calcium and iron powders in proportions of: 30% Ca-70% Fe). It can therefore be used as a direct replacement of standard CaFe cored wires, with an increased performance level in terms of yield of calcium treatment in the ladle of liquid steel and reduced standard deviation of yield i.e. improved predictability.
Standard calcium treatment of a ladle of molten steel was conducted by injecting a prior art CaFe cored wire A ladle of 245 tonnes was used. The targeted calcium content of the steel before being sent for continuous casting was 27 ppm.
The outer sheath of the cored wire had a thickness of 0.4 mm. The fill of the cored wire was a mixture of calcium and iron powders in a weight proportion of 30:70. The weight per metre of the powder mixture was 275 g/m.
The mean length of the injected cored wire was 620 m, at an injection rate of 290 m/min.
The mean addition yield was 12.9%. The standard deviation obtained in the tests was 7.6% (absolute percentage).
152 ladles of the same molten steel were then treated with the cored wire 100 of the invention.
The sheath 4 of the cored wire 100 had a thickness of 0.4 mm. The fill of the cored wire 100 was a bar 8 of calcium having a diameter D1 of 8.5 mm and weight per metre of 85 g/m and an iron powder surrounding this bar 8 having a weight per metre of 300 g/m.
The diameter D of the cored wire 100 was similar to the diameter of the standard cored wire, namely 13.6 mm.
The mean length of the injected cored wire 100 was about 374 m, at the same injection rate of 290 m/min.
A mean addition yield of 20.8% was obtained with a standard deviation of 4.3% (absolute percentage).
The treatment time of the steel was reduced by means of the cored wire 100. On average the treatment lasted less than 80 seconds with the cored wire 100, compared with 130 seconds for the prior art CaFe wire.
Much higher mean addition yields were obtained: from 12.9% to 20.8%, i.e. an improvement of about +60%, with a lower standard deviation of 4.3%.
The reduction in the amount of consumed cored wire 100 represents major savings in the cost of metallurgical treatment.
Finally a reduction in churning of the liquid metal was observed in the ladle at the time of injecting the cored wire 100. This reduced churning allows for easier treatment of the ladles, the ladles having a low guard height (distance between the upper edge of the ladle and the surface of liquid metal), without the risk of metal splashes. The cored wire 100 also allows a reduction in the maintenance frequency of the ladle cover since less metal remains adhering to the walls subsequent to spraying of liquid metal. Additionally it reduces the reuptake of hydrogen in the liquid steel and re-oxidation thereof during calcium treatment, again due to the lesser churning of the liquid metal which reduces its exposure to the surrounding atmosphere.
As is conventional, a cored wire contains a powder or mixture of powders for which the weight per metre and composition must be controlled throughout production. A conventional method for obtaining a cored wire particularly comprises the following steps:
These three steps determine the quality of the obtained cored wire.
The proportioning step of each of the powders allows the final proportion to be heeded for each of the chemical elements forming the fill. However depending on the type of powder, this proportioning can be easily perturbed. For example, when using a moving belt for transfer it is possible that one of the powders may be added in excess via drop-over effect. When the belt stops, the powder that has reached the end of the belt may continue to flow due to its inertia. This is all the more possible the greater the flowability of the powder.
The mixing step is the most complex. Most mixers on cored wire production lines are of “ploughshare” type. Blades secured to a central rotating shaft mix the different powders that were proportioned upstream. However mixers of this type easily induce segregation phenomena of the powders they are meant to mix. Depending on the densities of the powders considered in relation to one another, some powders have a tendency to accumulate in dead spots of the mixer which locally modifies the composition of the mixture. In addition, demixing phenomena between powders may also occur.
Finally, the depositing of the mixture in the cored wire at times causes heterogeneities in the mixture. At the time of deposit of the mixture, segregation sometimes occurs due in particular to the different pathways taken by the powder particles or to the phenomenon of elutriation.
The use of an extruded bar of the invention reduces the risk of poor proportioning of powders and poor mixing. The weight per metre of the extruded bar is much better controlled. Therefore the weight per metre of the active substance is much better controlled. For example this weight per metre is independent of variations in density of the powders used.
In the present application, by “thermally insulating layer” is meant an additional layer around the extruded bar. The additional layer allows delaying of heat transfer from outside the cored wire towards the core when the cored wire is fed into a bath of molten metal. The additional layer is adapted to form an additional heat barrier between the outside medium of the cored wire (liquid metal) and the extruded bar. The propagation of heat is slowed due to the presence of the additional layer. The rise in temperature of the extruded bar is therefore delayed.
The efficacy of the thermally insulating layer varies in particular in relation to type. Examples of thermally insulating layers are given in application FR-A-2871477 by the Applicant.
The fact that the thermally insulating layer is advantageously located on the extruded bar, and for example surrounds it completely, further improves the thermal protection of the extruded bar.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1260678 | Nov 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/073350 | 11/8/2013 | WO | 00 |
