Method and apparatus for manufacturing netting of steel wire

Abstract: The invention relates to a method for manufacturing a netting of metal wire, whereby starting from drawn wire of steel with < 0,25 % C, this wire is annealed and a netting is formed of the thus annealed wire, whereby the wire is continuously drawn from a spool, conducted through an annealing apparatus and thereupon formed into a netting.

Description: The invention relates in the first place to a method for manufacturing a netting of metal wire, whereby starting from drawn wire of steel with a carbon content of less than 0,25 wt. pct., this wire is annealed and a netting is formed of the thus annealed wire. A method for manufacturing netting is for instance known from American patent specification 2.000.788. In this case, netting is to be understood as referring to a mesh-comprising material formed of wire which can be made by weaving or braiding or welding. Further, an extra mechanical or welding-technical joining operation may have been carried out in the places where the wires of the netting touch each other. The netting formed can have many shapes, such as, for instance, square, rhombic, hexagonal etc. and is generally made of annealed mild steel wire. Mild steel wire is to be understood as referring to steel wire with a carbon content that is lower than 0,25 wt. pct. and preferably lower than 0,10 wt. pct. Generally, such a netting is made of drawn mild steel wire with a diameter of between 0,3 and 2,5 mm; preferably 0,5 to 2 mm. The drawn mild steel wire is obtained by reducing the diameter of a thicker wire rod by means of a drawing process until a desired diameter is obtained. Starting from a wire rod with a diameter of 5,5 mm, for instance, drawing is carried out in a number of steps until a diameter of 1,15 mm is obtained, which corresponds to a reduction of: Such a drawn wire is called a hard-drawn mild wire. The tensile strength of the wire has become higher because of the drawing process, higher than 800 Newton/mm², for instance. Such a hard-drawn mild wire has the drawback that it cannot be worked into a netting as the wire has lost its flexibility because of the drawing process, as a result of which processing into a netting has become virtually impossible. Generally, to restore the wire's flexibility, the wire is subjected to an annealing treatment the particulars of which will be discussed hereinafter. The annealing treatment applied is a treatment of the wire at a temperature of ± 700°C, as a result of which the wire recovers its ductility. In a specific case, the annealing treatment is carried out in a so-called bell furnace or pot furnace; in that case, processing time is very long, of the order of a few hours. Such a discontinuous annealing treatment has drawbacks in the sense that it involves special treatments, an expensive installation, long waiting times, annealed wire windings sticking together because of drawing soap rests present between the windings on the spool, attendant problems during unwinding with the possibility of wire rupture, intermediate stocks, etc. It is the object of the present invention to eliminate the drawbacks mentioned hereinbefore and to provide a method that on the one hand effects a major saving of time and on the other reduces the aforesaid drawbacks of sticking and consequent wire rupture. To that end, the invention is characterized in that the wire is continuously drawn from a spool, conducted through an annealing apparatus and thereupon formed into a netting. The fact is that it has been found very well possible to draw wire in a continuous method from a spool, conduct it through an annealing apparatus, whereby the residence time needed for annealing is adjusted, and, thereupon, continuously work the wire thus made flexible into a netting. By thus incorporating a continuous annealing step into the method for manufacturing netting, a very considerable simplification of the method is obtained, combined with a very great saving of time and a greatly reduced risk of wire rupture. In connection with the invention, reference is furthermore made to American patent specification 1.984.335, which also describes a method by which a wire material is continuously annealed to give the wire ductility properties. Contiguously to this heat treatment, the wire is then cleaned and provided with a zinc coating in one or more steps. This publication does not describe that, contiguously to the annealing treatment, the wire can be subjected to a treatment for forming a netting product. As regards the annealing apparatus to be applied in the method in accordance with the invention, a bath of molten metal such as a conventional lead bath can be chosen, for instance. With advantage, however, the annealing apparatus is an apparatus for heating wire by means of induction. Induction heating of steel wire has the advantage that such a treatment is a no-contact treatment and whereby a very short cycle of heating, maintaining at temperature and cooling can be obtained because of the small mass of a separate wire, which is very advantageous for a fast progress of the whole process. If induction heating is chosen, the frequency applied will generally be in the range of from 1 to 450 kHz, more specifically in the range of from 10 to 100 kHz. Suitably, a protective gas such as hydrogen or nitrogen, for instance, will be present when applying induction heating, to prevent oxidation of the wire in the annealing apparatus. As mentioned hereinbefore, connection of the wires composing the netting at the intersections thereof, by mechanical or welding operations for instance, can be taken care of in the method for manufacturing netting. The method in accordance with the present invention is particularly advantageous in the case that the wires of the netting are joined by a welding operation. Especially in those cases where induction heating has taken place involving application of a protective gas, a wire is obtained that is very little or preferably not oxidised at the surface, as a result of which a very good welding character is maintained. Very advantageously, the method for manufacturing netting in accordance with the invention as described hereinbefore is followed by the application to the netting of a covering layer of a metal or a metal alloy, such as for instance a covering layer of zinc. Alternatively, the netting formed can in addition be provided with a covering layer of synthetic material such as polyvinyl chloride. It is of course also possible to apply first a zinc layer to the steel of the netting, followed by a synthetic covering layer. In general, such covering layers are applied to protect the netting from corrosion during use, whilst they furthermore improve the decorative appearance of the netting. The invention is also embodied in an installation for forming netting out of steel wire, at least comprising one or more wire spools, wire guiding means and a netting-forming device. Such an installation is for instance known from the aforesaid American patent specification 2.000.788. Now, according to the invention, such a known apparatus further comprises an annealing apparatus through which the wire that has been drawn from a spool, is conducted for the continuous execution of a heat treatment on the wire. As mentioned hereinbefore, a very considerable speeding up of the total netting-forming process is obtained by incorporating such an annealing apparatus, the risk of wire rupture during the whole process furthermore being reduced considerably. In particular, the annealing apparatus in the installation as described hereinbefore is followed by a means for cooling the wire, for instance by using a cooling roller that is cooled with a cooling agent such as water. If so desired, the wire's tensile strength can still be increased considerably by an accelerated cooling of the wire. Quench ageing with cold water can for instance increase tensile strength to 600-700 N/mm² with an elongation of some 15 %. This increase in tensile strength and decrease in elongation only sets in through ageing or some time after the formation of the netting. The annealing apparatus incorporated into the installation can for instance be a conventional bath of molten metal such as a lead bath or a fluidised bed; with great advantage, however, the annealing apparatus is an induction annealing apparatus, heating of the wire conducted through such an annealing apparatus taking place at a frequency of between 1 and 450 kHz, in particular of between 10 and 100 kHz. Fluidised-bed annealing apparatus are known and are described i.a. in European patent specification 0181653 of the present applicant. The invention will hereinafter be described with reference to the accompanying drawing in which figure 1 shows a schematic side view of an apparatus for manufacturing a netting of steel wire; figure 2 is a schematic representation of an induction annealing apparatus as it can be applied for treating several wires in the method in accordance with the invention; and figure 3 shows a section along line III-III′ in figure 2. In figure 1 is shown that wires 1 are drawn from wire holders 2 such as reels or spools and conducted via guiding means 3 through an annealing apparatus 5 and thereupon supplied to a netting-forming device 4 via a cooling roller 6. The netting-forming device 4 can be a weaving apparatus but can also be an apparatus for forming netting by welding. The annealing apparatus 5 can be a conventional annealing apparatus such as a lead bath, a fluidised-bed annealing apparatus, a radiant furnace or preferably an annealing apparatus energised by an induction generator, which will be described in more detail. In figure 2 is shown an induction furnace for treating a large number of wires 1 and which consists of ceramic tubes 7 the number of which corresponds to the number of wires and around which the windings of the induction coil 8 are wound. The windings of the coil 8 are energised by a network 9 at a high frequency the workings of which will not be discussed within the scope of the present application but will be clear to those skilled in the art; anyway, the network 9 does not differ from the usual types of network for energising such induction coils. Figure 3 is a section along line III-III′ clearly showing the ceramic tubes 7 and the wires 1 running through them, the coil windings being indicated with 8. Naturally, the number of tubes and the size of the induction coil can be varied according to the desired capacity of such an annealing furnace. The induction coil 8 is for instance a wound copper tube with a sectional size of 6 x 8 mm with an inside height of 24 mm (dimension A) and a width of 540 mm (dimension B) in figure 3 into which have been fitted 33 ceramic tubes 7 of a size of 11 x 15 mm. Here follows a description of a comparative test whereby the tensile strength of so-called pot-annealed wire of a diameter of 4,5 mm was compared with the tensile strength of intermediate-frequency annealed wire of the same diameter. To carry out the discontinuous annealing cycle, use was made of an electric bell furnace into which a charge with a total weight of 4.500 kg was placed. The temperature of the wire charge had to attain 700°C; to that end, the temperature of the furnace was set at 900°C at the top and at 925°C at the bottom. In the appended graph A is sketched the time-temperature course inside the bell furnace, the reference numbers indicating the position of measurement inside the furnace. As can be seen clearly, the total heat treatment extends over a period of more than 12 hours, the temperature of 700°C required for adequate tempering being maintained for 6 hours. In a following test, the same type of wire was continuously annealed involving application of induction heating, heating in a short time to about 720°C to recrystallise the ferrite in the wire. The duration of the treatment depends on the speed of the processing machine following the annealing apparatus; if, for instance, a hexagonal-netting machine is applied with a braiding or weaving speed of 6 m/min., an induction furnace will be applied, for instance, with a length of ± 1 m, the heat treatment duration being about 10 sec. . Generally, the annealing treatment will not be more than a few minutes and preferably less than 1 min., more preferably still less than 20 sec. Instead of using induction heating, it is also possible to use a lead bath, a fluidised-bed annealing apparatus or a radiant furnace. The following tables give the difference in tensile strength depending on the way of annealing, for a conventional wire type. 1. 0,81 mm dia. before intermediate-frequency annealing tensile strength (N/mm²) elongation (100 mm) 1105 2,8 1105 2,7 1105 2,8 2. 0,81 mm dia. after intermediate-frequency annealing (22 kHz) tensile strength (N/mm²) elongation (100 mm) 468 24 470 24,6 465 23,8 3. 0,81 mm dia. pot-annealed 700°C 6 h tensile strength (N/mm²) elongation (100 mm) 386 27,7 380 32 404 28,5 As is apparent from the above tables, a considerable improvement in tensile strength can be observed as a result of the way of annealing the wire. It appears from the foregoing that the increase in tensile strength is coupled with a small decrease in elongation before rupture. As mentioned hereinbefore, tensile strength can be increased additionally by carrying out a quenching treatment with cold water on the annealed wire; elongation decreases in the process. Contiguously to the annealing treatment, the steel wire is worked into a wire netting in the known way after which the wire netting is thereupon subjected to a coating treatment such as coating with a metal layer such as a zinc layer or a zinc-alloy layer and/or coating with a synthetic layer such as polyvinyl chloride. Coating with a metal layer or metal-alloy layer following the netting-forming process has the advantage that the intersection of the wires, which has been reinforced by welding or a mechanical operation for instance, is enveloped by a zinc covering layer, which results in a further reinforcement of the junction. In the case of prior-art type nettings made starting from continuously annealed wire, for instance, which has been galvanised contiguously to the annealing treatment, as described in American patent specification 1.984.335, and then worked into netting, the junction or intersection or torsion of the wires is not enveloped by a zinc covering layer. The nettings in accordance with the present invention are clearly distinguished from this type of prior-art nettings by the fact that the junction, intersection or torsion of the wires is enveloped by a zinc covering layer.

Claims: Method for manufacturing a netting of metal wire, whereby starting from drawn wire of steel with < 0,25 % C, this wire is annealed and a netting is formed of the thus annealed wire, characterized in that the wire (1) is continuously drawn from a spool (2), conducted through an annealing apparatus (5) and thereupon formed into a netting. Method in accordance with claim 1, characterized in that the annealing apparatus (5) is a bath of molten metal, such as a lead bath, maintained at annealing temperature. Method in accordance with claim 1, characterized in that the annealing apparatus (5) is a fluidised-bed annealing apparatus. Method in accordance with claim 1, characterized in that the annealing apparatus (5) is an apparatus for heating the wire (1) by means of induction. Method in accordance with claim 4, characterized in that the annealing apparatus (5) works with a frequency chosen in the range of from 1 to 450 kHz. Method in accordance with claim 4-5, characterized in that the frequency is chosen in the range of from 10 to 100 kHz. Method in accordance with claim 4-6, characterized in that to prevent oxidation of the wire in the annealing apparatus (5), a protective gas is present for induction heating of the wire (1). Method in accordance with claim 7, characterized in that the netting is composed by welding operations starting from the annealed wire (1). Method in accordance with claim 1, characterized in that the netting formed is in addition provided with a covering layer of a metal or metal alloy. Method in accordance with claim 9, characterized in that the covering layer is a zinc layer. Method in accordance with claim 1, characterized in that the netting formed is in addition provided with a covering layer of synthetic material, such as of polyvinyl chloride. Installation for forming netting out of steel wire (1) at least comprising one or more wire spools (2), wire guiding means (3) and a netting-forming device (4), characterized in that into the installation has been incorporated an annealing apparatus (5) through which the wire (1) that has been drawn from a spool (2), is conducted for the continuous execution of a heat treatment on the wire (1). Installation in accordance with claim 12, characterized in that following the annealing apparatus (5), there is a means for cooling the wire (1) such as a cooling roller (6). Installation in accordance with claim 12 and 13, characterized in that the annealing apparatus (5) is an induction annealing apparatus.

Inventors: NYS MARC, NYS, MARC
Applicants: BEKAERT SA NV, N.V. BEKAERT S.A.
Application year: 1990
Main classification: C21D8
IPC codes: C21D8, B21F27, C21D9