EP0329611B1 - Process for continuously coating a filamentary steel article by immersing the article in a bath of the molten coating metal - Google Patents

Description

The present invention relates to a process for the continuous coating of a threadlike steel substrate by immersion of this substrate in a bath of molten coating metal, according to which a coating metal is chosen whose melting point is greater than the austenization temperature of the steel, the steel substrate is preheated to a temperature lower than that of said bath, it is passed through this bath to coat it and bring it at the same time to its austenization temperature.

The continuous coating of a filiform substrate by immersion involves the rapid passage of this substrate, whose temperature is lower than that of the molten coating metal, through a spout of a crucible filled with this molten metal which solidifies quickly in contact with this relatively cooler substrate.

Many solutions have already been proposed based on this principle, for example in GB-982.051 or in FR 1,584,626. These methods generally have in common that they cross the beak of the crucible containing the molten metal by moving from bottom to top, the speed, the cross-section of the passage and the wettability of the beak preventing the flow of the molten metal.

This technique has already been used to form a coating on a wire whose cross section is greater than that desired, this wire then being re-woven once coated to bring it to the final section. In the case of steel wires, it is necessary that the crystal structure of the steel is sufficiently softened, which implies that this wire has previously undergone heating to its austenization temperature followed by controlled cooling as a function of the composition of the steel in order to give it the desired crystalline structure. So far, this technique has been applied with coating metals whose melting point was below the temperature austenizing the steel, so that the steel wire was subjected to the heat treatment intended to form the structure necessary to make it drawable, prior to coating, since this coating was carried out at a temperature below that of austenization. Under these conditions, the cooling of the wire after coating can be carried out very quickly by passage through a liquid, without modifying the crystalline structure of the steel obtained prior to coating. Since the coating process occurs by passing the wire vertically from bottom to top, rapid cooling of the wire makes it possible to reduce the height of the installation, especially with high speeds of advance of the wire.

There are however important applications from the economic point of view where it would be necessary to produce steel wires of small section, coated with metals whose melting point is significantly higher than the austenization temperature of steel. On the one hand, the section is too small for the steel wire to be able to mechanically resist hot the tensile forces necessary to make it pass through the bath of molten metal; on the other hand, with a section sufficient to withstand the operating conditions, the uncontrolled cooling of the coated wire would generate in the steel wire a crystalline structure which would render it incapable of undergoing further drawing, so that this wire could no longer be brought to the desired section.

It has already been proposed in EP-A-60,225 to continuously coat a steel wire with 0.7% carbon by immersion in a bath of molten metal whose melting point is higher than the austenization temperature of steel, in particular in a brass bath and in a copper bath, the steel wire being previously heated to 500 ° C.

It has also been proposed in JP-5782467 to carry out the austenization of a steel strip by passing this strip through a bath of a molten metal such as copper whose temperature is higher than the temperature d austenization.

None of these documents envisages carrying out a controlled cooling of the steel substrate thus coated, with a view to it confer a softened crystal structure making it possible to reduce the section of the coated substrate by re-drawing or re-rolling. This is precisely the aim of the present invention.

To this end, the subject of this invention is a process for the continuous coating of a filiform steel substrate, by immersion of this substrate in a bath of molten coating metal according to claim 1.

The accompanying drawing illustrates, schematically and by way of example, an embodiment of an installation for implementing the method.

Figure 1 is an elevational view of an installation for the implementation of this method.

Figures 2 and 3 are T.T.T. (transformation-temperature-time) of two types of steels.

The installation illustrated in FIG. 1 comprises a supply coil 1 of steel wire 2. This steel wire 2 passes over a first guide roller 3 to go through different treatment stations 4, 5 and 6 intended respectively for cleaning, rinsing and drying the wire 2. A drawing capstan 3a brings the steel wire 2 to the below a graphite tank 7 of a crucible 8 containing a bath 9 of molten metal heated by a heating body 10 housed in the wall of the crucible 8.

Before crossing the spout 7 of the crucible provided with two openings 11 and 12 aligned vertically for this purpose, the steel wire 2 passes through a tubular conduit 13 whose entry is controlled by a seal 14. This tubular conduit is connected to a source of protective gas 15 for example H₂ + N₂ and is surrounded by an electric coil 16 for preheating supplied by a high frequency HF source. The maximum wire temperature is a function of the preheating temperature and the thickness of the deposited layer.

Depending on the type of steel used to form the filiform substrate 2, cooling is carried out relatively quickly for mild steels with less than 0.1% carbon. For steels with a higher carbon content, too rapid cooling is not acceptable since these steels must be maintained at a temperature of the order of 550 ° C., corresponding to the maximum temperature of the TTT curve, for ten seconds to obtain the desired fine-grained ferrite-perlite crystal structure. Generally, this temperature is obtained by passing the steel wire coated with copper or brass in a bath of molten lead. However, given the fact that the coating process according to the invention takes place along a vertical path, this solution is difficult to implement, this is the reason why it is proposed to use a fluidized bed 17, which can be supplied by an air circuit 18 associated with a heating device 19. Part of the heat required comes directly from the wire 2 itself. A thermal probe 20 makes it possible to regulate the temperature of the air as a function of the quantity of heat necessary to maintain the temperature of the fluidized bed at 540 ° C.

A second cooling system 21 with circulation of water is disposed above the fluidized bed 17 to complete the cooling of the wire 2 before it passes over a guide roller 3b which is suspended by means of a elastic tension regulation system 22 of wire 2 which is used to regulate the drawing capstan 3a, so as to obtain a low tension during coating. From this roller, the wire 2 is led to a storage drum 23. Since a mild steel wire heated to 700 ° C-800 ° C becomes very fragile in contact with molten copper in particular, the tension exerted by the voltage regulator 22 must not exceed 15 MPa.

Different metals and alloys have been deposited on steel wires of different types, the common point between the examples which follow is to give the steel a fine ferrite-perlite crystal structure, thanks to controlled cooling. As will be seen in these examples, in the case of mild steels with less than 0.1% carbon, a simple air cooling can be slow enough to obtain the desired crystalline structure, so that in this case the fluidized bed 17 can be omitted, a sufficient distance being provided between the outlet of the spout 7 and the cooling system 21 to allow the desired crystalline structure to be obtained. On the other hand, with steels with a higher carbon content, having greater quenchability, it is necessary to maintain the wire at a temperature of 540 ° C. for a few seconds to avoid quenching in ambient air and to obtain a crystalline structure. fine ferrite-perlite. The diagrams of FIGS. 2 and 3 respectively show and schematically the T.T.T. (transformation-time-temperature) of mild steel and steel with higher carbon content. The curve for controlled cooling of the steel wire coated with a metal whose melting point is higher than the austenization temperature of the steel is plotted on each of these diagrams.

In the following examples, three metals and alloys are used, namely, copper, brass and silver. The mild steel wire coated with copper finds applications in the electrical field, as telephone wire, electrically conductive spring, ground wire of an electrical transmission conduit for example. The 0.7% carbon steel wire covered with brass finds particular application as a reinforcement wire for tires with radial carcass. Finally the mild steel wire coated with silver finds electronic applications. In each of these cases, the coated wire has a section substantially greater than that of the finished wire so that the thickness of the coating metal decreases at the same time as the diameter of the wire during the re-drawing of this wire. This operation does not cause deterioration of the deposited metal layer if it adheres well to this wire.

EXAMPLE 1

This example relates to the deposition of a layer of copper on a mild steel wire.

A steel wire with less than 0.1% carbon, 1 mm in diameter, is used for this purpose. The first operation consists of an alkaline electrochemical degreasing at 60 ° C followed by an attack in an HCl bath and drying. Following this phase of preparation of the substrate, begins the actual coating phase which consists of preheating the wire 2 using the coil 16 supplied with high frequency current, the wire 2 passing at this time the tubular conduit 13 in which prevails an atmosphere of 20% H₂ + N₂ at a pressure of 5 mm of water column. The temperature of the steel wire 2 is thus brought to 740 ° C. at the moment when it enters the beak 7 of the crucible 8 through the opening 11. The beak of the crucible contains 70 g of liquid Cu at the temperature of 1120 ° C. corresponding to a 5 mm thick liquid bath.

Then the wire is cooled in air for 10 seconds before entering the water cooling enclosure 21. The running speed of the wire 2 is 30 m / min. The copper layer obtained is a 200 μm concentric layer adhering around the steel wire 2. The wire can then be drawn with an 80% reduction in its section.

EXAMPLE 2

The steel wire used in this example is a 0.7% carbon steel wire with a diameter of 1 mm. The preparation of this wire is identical to that of the wire of Example 1 as well as its preheating.

The spout 7 of the crucible 8 contains a layer of 40 mm of brass comprising 60% Cu and 40% Zn at a temperature of 1000 ° C.

At the outlet of the spout 7, the brass-coated wire enters the fluidized bed 17, the temperature of which is maintained at 540 ° C. The wire feed speed is 30 m / min and the fluidized bed offers a path length of 5 m so that the wire is kept at this temperature of the order of 550 ° C for 10 s, the time to bring this steel in the fine-grained ferrite-cementite zone. The layer obtained at a thickness of 15 μm formed concentrically around the steel wire is adhered to its surface.

EXAMPLE 3

A mild steel wire with less than 0.1% carbon 1 mm in diameter is coated with a layer of Ag.

This wire is cleaned and preheated under the same operating conditions as those of the previous examples.

The spout 7 of the crucible contains 70 g of liquid Ag at 990 ° C in an atmosphere of 10% H₂ + N₂.

The cooling takes place in air as in Example 1 and an adherent and concentric layer of Ag 50 μm thick is obtained.

Each of the wires obtained according to one of the preceding examples has a diameter several times greater than the desired diameter. Thus, for example, the wire of Example 2 is then re-drawn to be brought to a final diameter of 0.25 mm.

It should also be noted that from an economic point of view, the annealing of the steel at the same time as its coating makes it possible to eliminate an operation and therefore to reduce the production costs in a non-negligible proportion.

Claims (3)

1. A method of continuously coating a filiform steel substrate by immersing this substrate in a bath of molten coating metal, in which a coating metal is chosen which has a melting point higher than the austenitisation temperature of the steel, the steel substrate is preheated to a temperature lower than that of the said bath, it is introduced into this bath in order to coat it and at the same time to bring it to its austenitisation temperature, characterised in that the said filiform steel substrate is coated in the non-softened state and, in order to give the steel of this substrate a softened crystalline structure, this substrate thus coated is cooled at a controlled rate and drawn in order to give it the required cross-section.

2. A method according to claim 1, characterised in that a filiform substrate of soft steel with leas than 0.1 % carbon is coated and in that this substrate is then cooled at a rate chosen in order to obtain a ferrite-perlite structure.

3. A method according to claim 1, characterised in that a filiform steel substrate containing more than 0.2% carbon is coated and in that the temperature of this coated substrate is decreased rapidly to a temperature in the order of 550°C, in that the substrate is then maintained at this temperature until the transformation into a fine-grained ferrite-perlite structure and the cooling of the substrate is than brought to an end.

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