DESCRIPTION

FIELD OF THE INVENTION

The present invention concerns a process and a plant for continuously manufacturing a steel wire, as well as such a steel wire and a cord formed with a plurality of such steel wires.

PRIOR ART

The steel wires or cords can be used as structural reinforcement elements in the manufacture of elastomeric materials, like for example semifinished products intended for manufacturing tyres, tubes, conveyor belts, driving belts and cables. The steel wires can also be used for cutting materials.

A cord is typically made by cording a plurality of suitable steel wires.

Typically, the steel wires intended for manufacturing tyres comprise a steel core coated by at least one metal layer which provides for both protecting the underlying steel from corrosion and for providing a suitable adhesion of the metal wire or of the cord comprising said wire to the elastomeric material with which the wire or the cord are rubber coated, in addition to facilitating and improving the drawing process which the wire undergoes.

In the technological field of the manufacture of steel wires, documents US6228188, US7354493, and US2009030803 refer to processes and systems for manufacturing highly resistant wires through heat treatment.

SUMMARY OF THE INVENTION

The manufacture of steel wire typically provides a series of continuous machining and treatments, comprising at least: a first drawing, during which the steel wire is brought to a diameter that is suitable for the machining and for the subsequent treatments; austenitizing, during which the steel wire is heated to a temperature of 950-1100 °C and is kept at such a temperature for a period of time that is sufficient for the steel structure to entirely become austenite; a patenting, during which the steel wire is quickly cooled to about 550-600 °C and is kept at such a temperature for a period of time that is sufficient for the steel structure to become substantially pearlitic; a coating treatment, for example brassing, in which the steel wire is coated with metal or metal alloys for example with copper and zinc; a drawing, in which the diameter of the steel wire is reduced to the final desired value. In the rest of the description, we shall also use the term austenite to refer to steel with a substantially entirely austenitic structure, pearlite to refer to steel with a substantially entirely pearlitic structure.

During the patenting, the desired phase transformation of the structure of the steel wire from austenite (solid solution of carbon in gamma iron) to pearlite (formed by lamellae of ferrite and of cementite) is indeed carried out as long as the initial cooling is quick enough, otherwise the austenite does not transform into pearlite but into bainite (acicular aggregate formed by ferrite and particles of cementite or complex carbides), which is a steel structure that is not desired, since it is not suitable for the subsequent drawing: by this it is meant that the mechanical characteristics of the steel wire after drawing are not optimal if the structure thereof comprises a considerable amount of bainite.

The Applicant has vice versa found that trying to obtain a very quick cooling can, in reality, be counterproductive due to the inherent difficulties in carrying out a correct removal of heat capable of obtaining, in the wire, the desired microstructure in the austenite-pearlite transformation.

For example a very quick cooling from the austenitizing temperatures makes it very difficult to control the structural uniformity between the surface part of the wire and the inner part in a radial direction.

The Applicant has found that it is possible to keep the austenitic structure of the steel wire even when subjecting it to slow cooling, after being austenitized at high temperature.

The Applicant has thus found that it is possible to slowly lower the temperature of the steel wire, keeping its austenitic structure, and then quickly cool it in a simpler manner -thanks to the lower starting temperature- causing the desired transformation from austenite to pearlite, forming a wire which -once drawn- has excellent mechanical characteristics (for example high resistance, high elongation and high torsional and fatigue resistance).

The Applicant has found that pearlite forming the wire mainly has a lamellar microstructure rather than a fragmented microstructure and again that such a lamellar structure is fine; by the term "fine" it is meant to indicate a lamellar microstructure in which the spacing is lower than about 100 nm.

The Applicant has attributed to this fine lamellar pearlitic microstructure a greater capability of the wire of withstanding the subsequent drawing.

In accordance with a first aspect thereof, the invention concerns a process for manufacturing a steel wire, comprising:

- providing a steel wire;

austenitizing said steel wire;

patenting said steel wire.

Preferably said process comprises

drawing said steel wire.

In the process for manufacturing a steel wire the patenting of said steel wire comprises:

slow cooling said steel wire to a first predetermined temperature; quick cooling the steel wire to a second predetermined temperature.

Patenting said steel wire comprises keeping said steel wire at said second predetermined temperature.

The Applicant has been able to find that during the slow cooling, the steel wire substantially keeps its austenite structure unaltered; with the subsequent quick cooling, the steel wire is brought to the conditions wherein it can begin changing phase in which the austenitic structure is transformed into pearlitic structure.

Since the quick cooling is started when the steel wire has a relatively low temperature, i.e. the first predetermined temperature (720-800°C), with respect to the second predetermined temperature (900-1000 °C) which it had at the end of the austenitization, the beginning of the change of state can also occur at a relatively low temperature, at the second predetermined temperature of (550- 600 °C), i.e. at the ideal conditions for there to be the substantially total desired transformation into pearlite.

In a second aspect thereof, the invention concerns a plant for the production of steel wire,

said plant comprises:

an inlet section of the steel wire;

an austenitizing section of the steel wire;

a patenting section of the steel wire.

Said plant further comprises

a drawing section of the steel wire.

The patenting section comprises:

a first subsection, suitable for slow cooling said steel wire to a first predetermined temperature;

- a second subsection, separate and different from the first subsection, suitable for quick cooling the steel wire and subsequently keep it at said second predetermined temperature.

Further aspects of the invention concern a steel wire obtained according to the process of the first aspect of the invention, a process for manufacturing a steel cord which uses such a steel wire and a steel cord obtained with such a process.

Preferably the steel wire for austenitizing has a diameter of 0.5-3.5 mm.

Preferably the austenitizing of the steel wire, is carried out at a temperature of 900-1000 °C.

Preferably the slow cooling of the steel wire is carried out to a first predetermined temperature ranging from about 720 to about 800 °C.

Preferably, the slow cooling is carried out in a time period of 4- 0 s.

Preferably, drawing the steel wire is carried out until the wire has a diameter of 0.1-2 mm, even more preferably of 0.1-0.6 mm.

Preferably, the quick cooling of the steel wire is carried out to a second predetermined temperature ranging from about 550 °C to about 600 °C.

Preferably, the quick cooling is carried out in a time period of 0.5-2 s.

Preferably, keeping the steel wire substantially at said second predetermined temperature ranging from about 550 °C to about 600 °C is carried out for a time period of at least 3 s.

Preferably, prior to drawing the patented steel wire, the process comprises coating the steel wire with at least a metal or metal alloy layer.

Preferably, such a coating of the steel wire is carried out in order to brass said steel wire.

Preferably, the slow cooling is carried out in air. By air, in the present description, it is meant a gaseous atmosphere which mainly comprises air, but it is not excluded for there to be other secondary gaseous substances, for example due to the machining previously carried out or that occur immediately after, or that come from the steel itself in those temperature conditions. The slow cooling therefore, does not require a complex cooling system; it is sufficient to ensure an exchange of air that is suitable for taking away the heat necessary for the desired reduction of temperature.

In a preferred embodiment of the invention, the quick cooling is carried out in molten lead bath. This system makes it possible to obtain, in a relatively simple manner, the desired quick lowering of the temperature thanks to the thermal conductivity of lead and thanks to the possibility of suitably adjusting the temperature of molten lead.

In a preferred embodiment of the invention, the quick cooling is carried out by spraying cooling liquid. This system allows a better control of the temperature in the subsequent sections of the plant during the quick cooling.

In a preferred embodiment of the invention, the keeping of the wire at a predetermined temperature is carried out in molten lead bath. This system makes it possible to take away, in a relatively simple manner, the great amount of heat necessary for the transformation of austenite into pearlite.

In a preferred embodiment of the invention, the keeping of the wire at a predetermined temperature is carried out by spraying cooling liquid. This system allows the best control of the temperature during the keeping of the temperature during the phase change from austenite to pearlite, making it possible to keep track of the generated heat while the transformation is taking place.

In a preferred embodiment of the invention, during austenitization the steel wire reaches a maximum temperature of 950°C. Indeed, it has been found that the presence of the subsequent slow cooling makes it possible to carry out the austenitization at a temperature that is slightly lower than what is commonly used; it is thought -without by this wishing to give an interpretation of the physical phenomena which can occur during the process according to the invention- that the austenitization can be completed during the slow cooling. More preferably, during austenitization the steel wire reaches a maximum temperature of 930°C, even more preferably of 920°C. It has been found that with these temperatures the steel wire is substantially completely austenitic when the transformation of phase into pearlite starts. With this condition ensured, the advantage in terms of manufacturing costs which are obtained by reducing the temperature during austenitization, are evident.

In a preferred embodiment of the invention, during the slow cooling the steel wire reaches a minimum temperature of about 740-760°C.

Preferably, during the slow cooling the steel wire reaches a minimum temperature of about 750°C.

In a preferred embodiment of the invention, in the quick cooling the steel wire reaches a minimum temperature of 550-650°C.

Preferably, in the quick cooling the steel wire reaches a minimum temperature of 580-600°C.

In a preferred embodiment of the invention, the coating of the steel wire comprises:

applying a copper coating,

applying a zinc coating,

thermally diffusing copper and zinc applied.

In a preferred embodiment of the invention, the drawing is a drawing in wet conditions.

In a preferred embodiment of the invention, the first subsection of the patenting section comprises a chamber in controlled atmosphere, passed through by the steel wire.

In a preferred embodiment of the invention, the second subsection of the patenting section comprises a molten lead bath, passed through by the steel wire.

In a preferred embodiment of the invention, the second subsection of the patenting section comprises a chamber with at least one sprayer, for spraying the steel wire with a cooling liquid spray.

In a preferred embodiment of the invention, the second subsection of the patenting section comprises a molten lead bath, passed through by the steel wire, followed by a chamber with at least one sprayer, for spraying the steel wire with a cooling liquid spray.

Further characteristics and advantages of the invention shall become clearer from the following description of preferred embodiments thereof, given with reference to the attached drawings. In such drawings:

figure 1 is a diagram of the plant according to the invention; figure 2 is a diagram of part of the plant of fig. 1 , according to an embodiment of the invention;

figure 3 is a diagram of part of the plant of fig. 1 , according to another embodiment of the invention;

figure 4 is a diagram of part of the plant of fig. 1 , according to yet another embodiment of the invention; figure 5 is a diagram showing the course of the temperature of the steel wire during the process according to the invention;

figure 6 is a table showing the results of tests carried out on steel wires.

A plant 100 for manufacturing high resistance steel wire F comprises an inlet section 110 of the steel wire, an austenitizing section 120, a patenting section 130, a brassing section 140 and a drawing section 150. The patenting section 130, in turn, comprises a first subsection 131 and a second subsection 135, separate and different from the first subsection 131.

In the first subsection 131 , the steel wire F is slow cooled to a first predetermined temperature ranged between 720 and 800 °C in a time period of 4-10 s. In the second subsection 135, the steel wire is quick cooled to a second predetermined temperature ranged between 550 and 600 °C in a time period of 0.5-2 s and then is kept substantially at said second predetermined temperature ranged between 550 and 600 °C for a time period of at least 3 s.

By the expression "kept substantially at said second predetermined temperature" it is meant kept in a temperature range within about 30 °C or preferably 20°C, of the predetermined temperature. Such a temperature variation is connected to the exothermicity of the reaction of austenite-pearlite phase transformation which leads to the recalescence phenomenon, the increase tendency of the temperature of the wire during the phase transformation.

Preferably, the first subsection 131 of the patenting section 130 comprises a controlled atmosphere chamber 132, passed through by the steel wire F.

In a preferred embodiment, shown in figure 2, the second subsection 135 of the patenting section 130 comprises a molten lead bath 136, passed through by the steel wire F.

In another preferred embodiment, shown in figure 3, the second subsection 235 of the patenting section 230 comprises a chamber 237 with at least a sprayer 238, for spraying the steel wire F with a cooling liquid spray.

In yet another preferred embodiment, shown in figure 4, the second subsection 335 of the patenting section 330 comprises a molten lead bath 336, passed through by the steel wire F, followed by a chamber 337 with at least one sprayer 338, for spraying the steel wire F with a cooling liquid spray.

With the plant 100 it is possible to manufacture a high resistance steel wire F for reinforcing elastomeric materials, for example tyres, according to processes in accordance with the invention. The steel used is a steel containing 0.2%- 1.0% of C (Carbon), preferably 0.6%-0.95% of C.

More in particular, a process according to the invention provides: providing a steel wire F, having a diameter of 0.5-3.5 mm; its austenitization, at a temperature of 900-1000 °C; its patenting; its brassing; its drawing, to a diameter of 0.1-2 mm, preferably 0.1-0.6 mm. Moreover, patenting comprises slow cooling of the steel wire F at a temperature of 700-800 °C in a time period of 4-10 s; its quick cooling at a temperature of 550-600 °C in a time period of 0.5-2 s; keeping it at a temperature of 550-600 °C for a time period of at least 3 s.

The patenting achieved in such a way ensures that the steel structure of the wire after patenting is mostly pearlitic with a fine lamellar microstructure.

Preferably, the arrangement of the steel wire F occurs in the inlet section 110, its austenitization in the austenitizing section 120, its patenting in the patenting section 130, the slow cooling in the subsection 131 , the quick cooling and keeping in the subsection 135.

Preferably, the slow cooling is carried out in air, in the chamber 132.

In a preferred embodiment, the quick cooling is carried out in the molten lead bath 136.

In a preferred embodiment, the quick cooling is carried out by spraying cooling liquid, in the chamber 237 by the sprayers 238.

In a preferred embodiment, the keeping of the temperature is carried out in the molten lead bath 136, 336.

In a preferred embodiment, keeping of the temperature is carried out by spraying cooling liquid, into the chamber 237, 337 by the sprayers 238, 338. Preferably the cooling liquid is water.

Preferably, in the austenitization the steel wire F reaches a maximum temperature of 950°C, more preferably of 930 °C and even more preferably of 920°C.

Preferably, during the slow cooling the steel wire F reaches a first predetermined minimum temperature of 740-760X, more preferably of about 750°C.

Preferably, in the quick cooling the steel wire F reaches a second predetermined minimum temperature of 550-650°C, more preferably of 580- 600°C.

Preferably, brassing comprises the application of a copper coating, the application of a zinc coating, and thermally diffusing the copper and the zinc applied.

Preferably, drawing is a drawing in wet conditions.

EXAMPLES

Tests have been carried out with different manufacturing conditions, so as to verify the effects of the invention. In particular, the results obtained by subjecting steel wire AISI SAE 1080 (C=0.80%) to the following patenting and drawing steps were compared:

1*) a quick patenting according to the prior art (quick cooling in about 2 s until beginning of the pearlitic transformation and keeping it in molten lead at 600° until completion of the pearlitic transformation, carried out directly upon leaving the austenitization treatment);

2 ^* ) a quick patenting according to the prior art (quick cooling for 1 s in molten lead at 580 °C and keeping it in molten lead at 620°C until completion of the pearlitic transformation, carried out directly upon leaving the austenitization treatment);

3 ^* ) a slow patenting according to the prior art (initial cooling for about 1 s in molten lead at 590 °C and keeping it in air, until completion of the pearlitic transformation; 4 ^Λ ) a patenting according to the invention, with initial slow cooling in 5 s to 750°C followed by a quick cooling in 1 s to 580 °C by spraying cooling liquid and keeping it as such until the completion of the pearlitic transformation;

5 ^Λ ) a patenting according to the invention, with initial slow cooling in 5 s to 750°C followed by a quick cooling in 1 s in molten lead to 590 °C and keeping it as such until the completion of the pearlitic transformation.

In the table of figure 6, the following data is shown.

Before patenting: initial diameter of the wire in mm (D ₀ ).

After patenting and before brassing: tensile strength in N (Fm); tenacity in N/mm ² (Rm); % elongation to rupture (At); % lamellar pearlite (PI); % fragmented pearlite (Pf); % bainite (B); spacing in nm (sp).

After drawing: final diameter in mm (D traf); tensile strength in N (Fm traf); tenacity in N/mm ² (Rm traf); % elongation to rupture % (At traf).

Determining of the test parameters was carried out according to ISO standard 6892-1 :2009.

Tests 1 ^* , 2 ^* , 3 ^* carried out on wires made with the processes according to the prior art compared with tests 4 ^Λ , 5 ^Λ carried out on wires made with the process according to the invention make it possible to highlight an increase in the mechanical qualities both before patenting and at the end in wires patented following the process according to the present invention. It should also be noted how the microstructures in the wires made with the process according to the present invention are decidedly better, in terms of fine pearlite and in terms of lamellar spacing.

The Applicant has also been able to obtain a substantial improvement in the cording process thanks to the wires manufactured with the process according to the invention.

For example: for a cord 2+1x0.22HT i.e. a cord formed by three basic wires with a diameter of 0.22 mm of coated high resistance steel twisted together, it has been found that a rupture occurs every 279 km of cord produced, with respect to a rupture every 150 km for manufacturing with conventional patenting. Consequently, the number of welds to be carried out for 1000 km of cord manufactured has dropped to 3.6 with respect to the standard of 6.7 welds.

Advantageously, the process for manufacturing a steel wire according to the invention makes it possible

- to work steel with a lower C content (for example 0.7% instead of 0.8%) while still obtaining the final mechanical characteristics;

- to carry out the process on a material substantially having the same C content but starting with a wire rod having a diameter that is smaller so as to draw with a lower number of steps or obtain a smaller reduction of the diameter of the wire, and hence less hardening thereof.