In metallurgy, it is known to treat steels by using cored wires. Cored wires comprise a garnissage (in this case, metal calcium) wrapped in an envelope made of a cold rolled low carbon strip.
This method of adding calcium is much more efficient than spraying, especially in terms of the kinetic energy of the particles. The resulting calcium metal cored wire is hard, which reduces the loss of powder during feeding.
In metallurgy, it is known to provide such an active substance by means of cored wires 1 which are introduced into a bath of liquid metal.
The method involves injecting a cored wire 1 into the bath of liquid steel in such a way that it is not entirely immersed. The cored wire 1 comprises a lining comprising the active substance in pulverulent form, which is contained within a metallic envelope made of a metal whose composition is compatible with that of the molten metal to be treated. This envelope is advantageously made of steel.
The resulting magnesium addition yield (calcium content in the liquid iron after injection of the cored wire 1) is defined as the ratio, on the one hand, between the calcium actually found in the liquid steel after the introduction of the cored wire 1 and, on the other hand, the theoretical calcium content which could be expected to be present in the liquid metal after the introduction of the cored wire 1. The intermediate layer 10 can also play a major metallurgical role, for example by limiting the fading of the magnesium in the molten iron after the cored wire 1 has been introduced.
The cored wire according to the invention is mainly made of calcium aluminum iron powder. This is an important factor in achieving good magnesium addition yield during the treatment of liquid cast iron, as well as an improvement in the quality of the castings obtained from this type of cast iron (reduction of porosities and oxide sails).
The outer casing 4 forms a portion peripherique of the cored wire 1 and is intended to be introduced into a bath of metal en fusion in a manner known per se. This outer casing 4 is advantageously constituted by a steel strip 6 folded on itself around the longitudinal axis L.
The filling of the cored wire consists of a mixture of calcium and iron powders in the proportion by mass 30/70, and its metric weight is 275 g/m2. The diameter D of the cored wire is comparable to that of standard cored wire, namely about 13.6 mm.
The cored wire is introduced into the molten metal bath by means of an injection device, generally automatic, introducing a precise length of the cored wire at an appropriate speed. This considerably reduces the amount of calcium consumed, which represents a significant saving in the cost of the metallurgical treatment. It also allows for an improvement in the quality of cast iron, by reducing porosities and oxide sails in the steel.
The cored wire 1 also enables a reduction in the hydrogen reprise from the liquid steel and in its reoxydation during the calcium treatment, thanks to the slight agitation of the liquid metal which diminishes its exposure to the ambient atmosphere. It has been possible to obtain average addition efficiencies that are significantly better than those obtained with standard flux-cored wires: from 12.9% to 20.8%, an improvement of around + 60%. The results are illustrated in Tables 1, 2a and 2b ci-dessus.
Several types of cored wires are currently in use, whose packing consists of a mixture of pure calcium powders or calcium alloys, or even a mix of iron and calcium (commonly known as CaFe). However, these kinds of packed wires sometimes present problems such as segregation, due to the different trajectories of the powder particles during deposition.
In the present invention, a metal cored wire with a better segregation resistance is provided. The process is based on the preparation of a melange of powders in a mixing device, and then depositing it on the cored wire. Afterwards, the melange is removed, thereby forming a lining within the cored wire.
The outer envelope 4 forms a portion peripherique of the cored wire, intended to come into contact with the bath of molten metal when the cored wire is introduced into it. The outer casing is advantageously constituted by a steel strip 6 folded on itself around the longitudinal axis L of the cored wire.
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