EA045914B1 - STEEL CORD WITH BRASS COATING AND INCREASED IRON CONTENT ON THE SURFACE - Google Patents

Description

Technical field

The invention relates to a brass-coated steel cord used for reinforcing rubber products such as tires, hoses, conveyor belts and similar products, hereinafter grouped under the name rubber products.

State of the art

The vulcanized rubber composite, which uses brass-coated steel cords to provide the strength and rigidity of resilient tuff vulcanized rubber, is exceptional. This composite plays a key role in the performance of rubber products that are used in harsh and extreme conditions, such as tires.

For a composite to be strong and durable, it is critical that the adhesion between the low modulus vulcanized rubber and the high modulus steel cord is strong from the start and remains adequate throughout the life of the rubber product.

When brass-coated steel cords are vulcanized into rubbers with a sulfur-based vulcanization system, _a remarkable reaction occurs between the copper in the brass and the sulfur in the rubber, producing Cu( _2.x )S in the rubber, where x is about 0.2, growing as dendrites, like wood, in rubber. The resulting adhesion layer has a thickness of less than 250 nm or even less than 100 nm. This ensures an initially strong connection. Initially, the main oxide on the surface of the brass layer is zinc oxide. This zinc oxide slows down the reaction of copper with sulfur donors during vulcanization. Indeed, growth of stoichiometric Cu ₂ S should be prevented as this results in brittle crystals, resulting in lower initial adhesion. After vulcanization, some zinc sulfide is usually formed, together with Cu( _2-x )S, while a layer of zinc oxide has grown as an intermediate layer between the sulfide and the remaining brass metal.

On the other hand, with prolonged use of a rubber product, particularly in hot and humid conditions, this adhesive layer breaks down and the adhesion becomes less strong. One of the main mechanisms of this deterioration is the dezincification of brass due to the diffusion of zinc ions (Zn ²⁺ ) from the brass layer to the adhesion layer to form zinc oxides and hydroxides, which leads to a weakening of the adhesive bond.

To overcome this problem, organic cobalt-based salts such as, for example, cobalt naphthenate, cobalt stearates or cobalt boron decanoate complexes are added to the rubber compound in addition to other additives such as carbon black, sulfur, accelerators, oils, antioxidants, activators, etc. Rubber compound intended for rubber compound used to encapsulate steel cords. These organic cobalt salts (1) inhibit the formation of weaker zinc sulfide (ZnS) bonding bridges between brass and rubber, thereby promoting the formation of non-stoichiometric dendritic copper sulfide during bond formation, and (2) inhibit the diffusion mechanism of zinc ion release from the brass layer into hot and humid conditions, thereby improving adhesion retention.

However, the addition of cobalt-based organic salts also has the disadvantages that they act as a catalyst for the oxidation of diene bonds in the rubber, thereby accelerating the aging of the rubber, which can ultimately lead to rubber failure in the vicinity of the steel cord. However, the main disadvantage of these organic cobalt-based salts is that they are suspected of being carcinogenic and therefore their use is increasingly being restricted.

To avoid the use of organic cobalt-based salts, it has been proposed to include cobalt in metallic form in the brass coating rather than in organic form in the rubber. See, for example, US 4,255,496 and US 4,265,678. Such ternary alloy layers do provide very favorable adhesion retention results in hot and humid conditions. However, they did not completely remove organic cobalt salts from rubber. The applicant's recent work, published in WO 2011/076746, WO 2013/117248, WO 2013/117249, further proposes solutions allowing the use of ternary alloy coatings also without cobalt-containing compounds. Although the use of cobalt in tires is significantly reduced with this technology, cobalt is still present in the steel cord and thus cobalt remains in the production environment.

Other coating metals instead of cobalt were also considered, such as nickel and iron (see, for example, Rubber-brass bonding, Chapter 6 by W.J. van Ooij of the Handbook of Rubber Bonding, Rapra Technology Limited, page 176). However, these metals are considered too slow-acting in the case of iron, or even too passive in the case of nickel.

However, US 4,446,198 suggests using a copper-iron-zinc triple plating instead of a copper-cobalt-zinc plating. It is known that a certain amount of iron gets from the steel substrate into the rubber on the surface of the steel cord and helps to maintain and enhance adhesion (see page 429 Mechanism and theories of rubber adhesion - steel tire cords - an overview, W.J. van Ooij, RUBBER CHEMISTRY AND TECHNOLOGY , Volume 57, pages 421-456, 1984). One way to increase the amount of iron on the surface of the steel cord is to reduce the amount of brass, thereby exposing parts of the steel backing to rubber. However, this has its limitations, since sufficient brass must remain on the surface of the steel cord to ensure

- 1 045914 adhesion cookies.

Thus, the inventors set out to improve the distribution of iron to provide improved adhesion retention.

The essence of the invention

Thus, the main objective of the invention is to overcome the main problem associated with the use of cobalt in rubber products: firstly, by eliminating cobalt from the steel cord and, secondly, by allowing the use of rubber compounds that are substantially free of cobalt . Another goal of the invention is to create a steel thread with the correct distribution of iron on its surface. Another object of the invention is to provide a steel thread in which the iron depth profile is selected depending on improved adhesion retention. Another object of the invention is to provide a steel cord in which some or all of the strands have a preferred iron depth profile. It is yet another object of the invention to provide a manufacturing process that produces the steel cord according to the invention.

In accordance with the first aspect of the invention, a steel thread is provided with the features of the preamble of claim 1 of the claims. A steel thread is included in a steel cord for reinforcing rubber products, as explained in the second aspect of the invention.

The steel thread has a diameter d, expressed in millimeters. For the purposes of this invention, the diameter d is the diameter of an imaginary circle having the same surface area as the cross section perpendicular to the steel thread. For example, a steel thread may have a round, circular cross-section, with the diameter d being equal to the diameter of that circular thread.

Alternatively, the thread may have a cross-section that deviates from a circle, such as an ellipse, a regular or irregular polygon, or a mixed polygon in which there are straight edges and curved boundaries, such as an originally round wire that has been flattened from one or two and more sides. These are all possible perpendicular cross-sections of the steel thread, the only requirement is that they be convex.

The steel thread includes a steel backing and a brass coating. In the present invention, the term brass refers to an alloy formed by copper and zinc, i.e. consists of copper and zinc. Any other element, whether metal or non-metal, intentionally (such as phosphorus or iron) or unintentionally (such as oxygen) added to the coating is not considered part of the brass.

The coating has an average thickness of 450xd nanometers or more, which is determined by the total mass amount of copper and zinc in said brass. For example, a wire with a diameter of 0.30 mm will have an average coating thickness of 135 nanometers or more. By average we mean that the thickness is determined over its entire circumference and over a significant length (for example, along a length 100 times the diameter d). In other words, the average thickness is a general and not a local measure of thickness.

Brass has a mass copper content of 61-75 mass percent relative to the total mass of copper and zinc in the brass. For the purposes of this invention, the weight percentage will be abbreviated as wt%. More preferably, the mass percentage of copper is greater than 62% by weight, or greater than 63% by weight, or even greater than 64% by weight. If the mass percentage of copper becomes too low, there is a risk of excessive β-brass formation. β-brass is a harder phase of brass that is more difficult to deform than the more ductile α-brass, which forms at a higher mass percentage of copper. When the mass percentage of copper in brass exceeds 75 wt.%, there is a risk of formation of brittle copper sulfide. Therefore, a lower weight percentage of copper is preferable, for example less than 73 wt%, or less than 71 wt%, or even less than 69 wt% copper in the brass.

The amount of copper and zinc in the coating is determined by wet chemical analytical methods. These methods are known to those skilled in the art. In these methods, a mass of steel filament is collected and weighed, copper and zinc are removed from the steel substrate using a stripping solution, the stripping solution is diluted to a standard volume, and the mass content of copper and zinc is determined by one of the following methods:

a) X-ray fluorescence spectroscopy;

b) inductively coupled plasma spectroscopy;

c) atomic absorption spectroscopy.

Details of such a technique can be found in BISFA, Internationally agreed methods for testing of steel tire cord, articles E11/1, E11/2 and E11/4. BISFA is the International Bureau for the Standardization of man-made fibers.

The mass percentage of copper (0^(^00.%)) is then calculated as:

mass Si

Si(% Miiuc.i 10()-- , mass C and * mass /n

If the sum of the masses of copper and zinc per unit mass of steel cord is B grams per kilogram, then the average coating thickness t, expressed in nanometers, for a thread with an equivalent diameter

- 2 045914 meter d can be calculated using the formula:

t=231.8xBxd.

The characteristic of steel thread is that the coating has an unusual amount of iron on the top surface. The amount of iron on a surface is measured using X-ray photoelectron spectroscopy, a surface analysis technique commonly known by the acronym XPS. This technique is also called ESCA, which stands for Electron Spectroscopy for Chemical Analysis. More specifically, the coating has an average iron content of 4 or more atomic percent in the first layer, with this first layer extending from the surface of the filament to a depth of 3 nm below the surface. Iron content is expressed as atomic percentage (at%) relative to the total amount of iron, copper and zinc. No other elements should be counted in the denominator.

In XPS, electrons from the surface of a sample are emitted by surface atoms after being excited by photons from a focused X-ray beam. From the kinetic energy distribution of these photoelectrons, conclusions can be drawn regarding the composition of the surface atoms under study (and their chemical bonding). The diameter of a substantial circular X-ray beam is about 100 µm. The wavelength of X-ray photons corresponds to the wavelength of the K-alpha line of aluminum. The depth of analysis is only a few nanometers below the surface. Thus, the analyzed volume corresponds to a very thin circular disk. To obtain the depth profile, surface atoms are sputtered using an argon ion beam for standardized periods of time between X-ray photoelectron measurements. The intensity of the argon ion sputtering beam is calibrated such that 1 nanometer of alpha iron is sputtered in 10 seconds. For the purposes of this invention, a spray time ratio of 10 seconds per nanometer of surface layer will be maintained. Based on the kinetic energy of the photoelectrons, the types of atoms on the surface can be identified.

Although XPS analysis can easily identify various elements such as carbon, oxygen, sulfur, for the purposes of this invention only metals such as copper, zinc and iron are measured. Since the number and energy of photoelectrons are respectively a measure of the content and atomic number of atoms on the surface, it becomes possible to trace the atomic percentage of iron, indicated by the sign (Fe) at various depths xi with index i (index '0' is measured before deposition, the last measurement point is indicated by 'N ' at depth 'Δ'):

(Hc)HH) - , , where #Fe, #Cu and #Zn represent the number of photoelectrons determined for the corresponding energies associated with these elements.

The thus obtained depth profile (Fe)i as a function of depth xi must then be averaged between surfaces to a sputtering depth of Δ 3 nm. It is preferable to carry out at least 4 measurements at different depths from the surface to a depth of 3 nm. For example, at spray times of 0, 6, 12, 18, 24 and 30 seconds, the content of iron atoms (Fe)i can be determined, where the i numbering is 0, 1, 2, 3, 4 to 5. Preferably, the measurement depths are equidistant or, at least evenly distributed. The average iron content is then obtained by integrating the iron profile over depth using the trapezoidal rule and dividing by the depth Δ:

This gives an indication of how many iron atoms are present in the disk-shaped volume analyzed by the X-ray beam at a specified depth.

Although the focused X-ray photon beam already averages out many variations in surface iron content due to its relatively large beam size, it is best to take measurements at four different points on the surface of the steel filament and use the average of the four numbers obtained as the final iron content in the top 3 nm coating layer.

The steel substrate is preferably made from unalloyed carbon steel with a composition within the following ranges (all percentages are percentages by weight, abbreviated as wt%):

carbon content in the range of 0.60-1.20 wt.%, for example 0.80-1.1 wt.%;

manganese content in the range of 0.10-1.0 wt.%, for example 0.20-0.80 wt.%;

silicon content in the range of 0.10-1.50 wt.%, for example 0.15-0.70 wt.%;

sulfur content below 0.03 wt.%, for example below 0.01 wt.%;

the phosphorus content is below 0.03% by weight, for example below 0.01% by weight. By subjecting steel to strain hardening, similar to wire drawing, it is possible to obtain threads with a tensile strength in excess of 2500 MPa, or above 3000 MPa, or even above 3500 MPa.

- 3 045914

Microalloying steel can help produce threads with even higher tensile strength. The mass percentages of alloying elements are within the following limits: chromium: 0.10-1.0 wt.%; nickel: 0.05-2.0 wt.%, cobalt: 0.05-3.0 wt.%; vanadium: 0.05-1.0 wt.%; molybdenum: 0.05-0.60 wt.%; copper: 0.10-0.40 wt.%; boron: 0.001-0.010 wt.%; niobium: 0.001-0.50 wt.%; titanium: 0.001-0.50 wt.%; antimony: 0.0005-0.08 wt.%; calcium: 0.001-0.05 wt.%; tungsten: for example, in an amount of about 0.20% by weight; zirconium: for example, in an amount of 0.01-0.10 wt.%; aluminum: preferably in amounts less than 0.035 wt.%, for example less than 0.015 wt.%, for example less than 0.005 wt.%; nitrogen: in an amount less than 0.005 wt.%; rare earth metals (wt.%. REM): in an amount of 0.010-0.050 wt.%.

Microalloying makes it possible to achieve tensile strength above 3500 MPa, above 3700 and even up to 4000 MPa and above.

An alternative approach is to use low carbon steels that have been deep drawn to achieve sufficient tensile strength. In this case, typical steel compositions have a carbon content of less than 0.20 wt.%. An example would be carbon content in the range of 0.04-0.08 wt.%, silicon content 0.166 wt.%, chromium content 0.042 wt.%, copper content 0.173 wt.%, manganese content 0.382 wt.%, molybdenum content. 0.013 wt.%, nitrogen content 0.006 wt.%, nickel content 0.077 wt.%, phosphorus content 0.007 wt.%, sulfur content 0.013 wt.%, all percentages are percentage by weight. The tensile strength of these threads is significantly lower: above 1200 MPa or even above 1400 MPa, but they have a lower carbon footprint due to the elimination of intermediate heat treatments.

In a further improved first embodiment, the amount of iron found in this disc-shaped volume is at least 4 atomic percent iron or even 5 atomic percent iron. In any case, it never exceeds 10 atomic percent and is possibly below 7.5 atomic percent.

The advantage of increased iron content on the surface is that adhesion retention in hot and humid conditions is improved with cobalt-free compounds as well as with currently used cobalt-containing compounds.

Preferably, the iron content increases steadily with increasing measurement depth from the already high level in the first layer. According to a second embodiment, the average iron content in the second layer extending from the surface to a depth of 9 nanometers below the surface is greater than 5 atomic percent or even greater than 6 atomic percent.

When sprayed to a depth of 20 nanometers below the surface of the filament - the third layer - the brass coating has an average iron content of 6, 8 or 10 atomic percent or more, third implementation.

According to the fourth embodiment, the average iron content in this third layer is less than 20 atomic percent iron, or less than 15 at.%, or even less than 13 at.%, or less than 11 at.%, relative to the sum of the iron, zinc and copper atoms found in the third layer. Indeed, too high an iron content can lead to a lack of brass, which is ultimately needed to create adhesion. It turned out that an iron concentration of 8-11 at.% in the third layer (0 to 20 nm) provides the best adhesion balance. Although values greater than 11 at.% result in improved adhesion retention under hot and humid conditions, initial adhesion is less satisfactory. A value below 8% results in adequate initial adhesion, but less than optimal adhesion gain in hot and humid conditions.

The balance between iron and brass is sensitive: too much brass on the steel cord can lead to other adhesion maintenance problems, so it is best to keep the average brass thickness below 1350xd or less, with d being the diameter of the strand in millimeters. For example, the average thickness may remain below 1200xd or less or even below 1000xd or less.

In another embodiment, the iron is further finely dispersed within the brass coating. The way to verify this is to analyze the surface of the filament with a scanning electron microscope operating in backscattering electron (BSE) mode. These electrons are elastically scattered (reflected) from the nuclei of the surface under study and indicate the average atomic mass of the surface atoms under study. Heavier elements such as copper and zinc are more efficient at returning electrons than lighter atoms such as iron. Therefore, areas of the surface of a brass-plated thread that are thicker will appear lighter in gray tone than thin brass coatings on a bare steel surface, which will therefore appear darker.

Now, when analyzing the surface of the thread in BSE mode, alternating thin brass strips and thick brass strips, oriented along the length of the steel thread, will be visible on the coating. Thick brass strips, which contain more copper and zinc than iron, appear relatively lighter than thin brass strips, which have more iron atoms. Thus, in the context of this invention, the terms lighter and darker should be understood to be related to each other in the same way,

- 4 045914 as the terms thin and thick. Even an inexperienced electron microscope operator can easily adjust the electron beam to obtain maximum BSE contrast on the filament surface.

A feature of this embodiment is that in said thick brass strips, iron is present at least on the surface. The average iron content is at least 4 atomic percent or more relative to the total amount of iron, copper and zinc. The average value is taken for the top 3 nanometers from the surface of the thick brass strip.

In this case, the amount of iron is determined using scanning Auger electron spectroscopy (AES), which makes it possible to probe the surface in a very narrow space (about the size of the incident electron beam, i.e. 100 nm diameter) at a very shallow depth: for the elements in question this about 0.5 nm. As with XPS measurements, the depth profile is created by sequentially separating the top layer by sputtering atoms using argon ions. By then integrating the iron profile using the trapezoidal rule and dividing by the total depth of the profile, the average iron content can be determined.

Alternatively, the average iron content found in the top 3 nm layer of the thick brass strips is 6 atomic percent or more relative to the total amount of iron, copper and zinc.

Additionally and alternatively, the average iron content detected at a depth within the upper 9 nanometers from the surface of said thick brass strip is equal to or greater than 8 atomic percent or more relative to the total amount of iron, copper and zinc.

According to a second aspect of the invention, a steel cord is provided. Steel cord consists of one, two or more strands joined together. The steel threads correspond to one or more of the previously described embodiments.

In another preferred embodiment of the invention, the steel cord consists of a single thread. This single thread can be used in the tire, for example in the bead area for bead reinforcement or in the belt area as a reinforcement for the belt (monofilament). Such monofilaments can be made with a larger diameter, for example, 0.25-0.70 mm, for example, 0.30-0.35 mm, for reinforcing the tire breaker, or with a diameter of 0.70 to 2.10 mm for reinforcing the beads. Alternatively, the single steel thread according to the invention can also be used as the reinforcing braid of the hose.

The term including means that, in addition to steel threads, other non-steel threads such as aramid fibers or high-density polyethylene fibers may be mixed with the steel threads. Alternatively, the steel cord may also consist solely of steel threads.

The steel cords are assembled in accordance with prior art techniques known as:

a) single-ply cord, such as nxd, in which n strands each of diameter d are twisted together with the lay direction and lay pitch forming the cord, n can be 2-5, maximum 6, and d varies in the range of 0.10-0.50 mm . It is possible that the threads are deformed to create an open cord in which the threads form individual spirals that are in weak contact with each other. Typical is 3x0.30 open cord;

b) in multi-layer cords, the layers are wound with a certain length and pitch around the core, which can be a single thread or a separate bundle. They are usually denoted as core+mxd, where the core is, for example, equal to nxd threads or one thread with diameter d. Examples: 1+6, i.e. 0.32+6x0.30 or 3+9 for example 3x0.22+9x0.20 or a three-ply cord such as (3+9+15)x0.175 in which all the threads have the same diameter. When the lay direction and lay pitch of all layers are equal to each other, the yarns form a compact configuration known as a compact cord, i.e. 27x0.175 mm.

c) when different strands are combined and twisted around each other, a multi-strand cord of MxN type is obtained, for example, type 4x(1+6), in which 4 strands out of 7 strands are twisted together with the pitch and lay direction of the cord. When the direction of the bundle lay is opposite to the direction of the cord lay, i.e. the bundle in the s direction and the cord in the z direction is called ordinary lay. When the direction of lay of the cord and external bundles is the same, it is called Lang lay. The latter cords can reach much higher elongations;

d) a special type of multi-core cord is a cord consisting of two bundles L and T, having at least different lay pitches. A twisted strand T (twisted) has a shorter lay pitch than a strand L (long lay), the latter, for example, having an almost infinite lay pitch. The two strands are twisted together with the pitch and direction of the cord lay. In a particularly preferred embodiment, the strand T has a pitch and lay direction that is equal to the pitch and lay direction of the cord. Particularly preferred examples are 2+2, 2+3, 2+4, 3+2, 3+3, where the first digit refers to the number of threads in the L: strand and the second digit refers to the number of threads in the T strand.

In accordance with the third aspect of the invention there is provided a rubber product reinforced with steel cords as defined in the description above. The rubber product may be a tire, such as a car, truck, van or off-road vehicle tire, hose, belt, etc.

- 5 045914 as a conveyor belt or elevator belt, or any other rubber-based product that can be reinforced with steel cord. All of these products are manufactured and assembled by methods known or that will become known in the relevant art. The only difference is that the steel cord used for reinforcement includes filaments with an average iron content of 4 atomic percent or more in the first layer extending from the surface of the filament to a depth of 3 nanometers below the surface, the iron content being expressed as atomic percentage relative to to the total amount of iron, zinc and copper, as determined by X-ray photoelectron spectroscopy at four points on the surface of the steel thread.

A specific embodiment of the invention in accordance with the third aspect is a rubber product in which the vulcanized or unvulcanized rubber contains substantially no cobalt or cobalt-containing compounds. Indeed, steel cord was invented to be compatible with adhesive rubber compounds, known as rubber compounds, which contain virtually no cobalt or organic cobalt compounds added to the rubber.

The term substantially free means that in the vulcanized rubber the amount of cobalt detectable by x-ray fluorescence is less than 100 micrograms per gram of rubber (0.01 wt% Co in rubber, wt%) or less than 50 μg micrograms. per gram of rubber (0.005 wt.% Co) or even below 20 (0.002 wt.% Co) or 10 (0.001 wt.% Co) micrograms per gram of rubber.

Since the rubber coating mixture typically contains organic cobalt compounds, the analysis is best performed in the rubber near the steel cord, for example, on the remaining rubber that adheres to the steel cord when the steel cord is pulled from the rubber product. This is where the highest concentration of cobalt is expected.

A particular advantage of the invention is that it is still compatible with currently used cobalt-containing compounds with positive effects regarding adhesion and adhesion retention compared to currently used brass steel cords.

According to a fourth aspect of the invention, there is provided a method for producing steel thread as described above, comprising the steps of:

a) selection of wire rod with the steel composition suggested in the product description above. The steel wire rod has a diameter of about 5.5 mm or more. The wire rod is cleaned of scale and coated with a soap carrier in accordance with the prior art;

b) the wire rod is dry drawn to the intermediate wire. The intermediate wire has a diameter of 0.50-3.20 mm, for example 1.20-2.50 mm;

c) the intermediate wire is then patented. Patenting is an operation known to those skilled in the art. When patenting, the wire is brought to a temperature above the austenitization temperature of the steel, which is usually 950-1050°C to dissolve the carbon in the iron lattice. Then the intermediate wire is kept at a holding temperature of 550-660°C, thereby restoring the original fine-pearlite steel structure of the wire rod. This structure is particularly preferred because it can be easily drawn to smaller diameters. The resulting wire is a patented wire;

d) this wire is then etched to remove excess oxides or drawing residues by passing the wire through an acid bath. Typically, hydrogen sulfate or hydrogen chloride or similar strong acids are used for this purpose;

e) the patented wire is additionally electrolytically coated with copper in a known manner, in an amount adjusted to achieve the proposed compositions and weight. Typical copper plating baths are copper sulfate or copper pyrophosphate baths having compositions, concentrations, pH values and operating at current densities known to the person skilled in the art;

f) the copper clad wire is then electrolytically coated with zinc metal in an amount sufficient to achieve the proposed brass composition. A typical galvanizing bath is a zinc sulfate bath of composition, concentration and pH values, operating at current densities known in the art;

e) at the next stage, brass is formed by diffusion of zinc and copper when heated in a known manner. Heat can be generated by resistive heating of the wire, either by electric current passing through the steel wire or by electromagnetic induced eddy currents;

g) by wet drawing of brass-coated wire on a drawing mill, a steel thread is formed according to the invention. In this operation, a brass-coated wire is immersed in a lubricant and pulled through a series of dies with gradually smaller holes.

Characteristic of the process is that the intermediate wire has a circular middle

- 6 045914 arithmetic deviation Ra or circular roughness in the future, which is more than 0.40 micrometers, for example more than 0.50 or even more than 0.55 micrometers. If the roughness is below 0.40 micrometer, the positive effect on adhesion will be less pronounced. The circular arithmetic mean deviation does not exceed 1.00 micrometers, for example, below 0.95 micrometers. Too high a circular roughness leads to loss of fatigue properties.

The definitions set out in the international standard ISO 4287:1997 Geometric Product Characteristics (GPS) - Surface Texture: Profile Method - Terms, definitions and surface texture parameters apply. The arithmetic mean deviation Ra is defined as:

J - r(c)!dd where L _e represents the length of the estimate in the circular direction ξ of the wire,

represents the absolute deviation from the average value in the direction radial to the wire axis, where

represents the average value of the radial deviation along the length of the estimate L _e . Since the physical measurement will be represented in a numerical format, the integral can be replaced by a discrete summation performed over the entire length of the estimate covered by N discrete deviation measurement ri, i ranging from 1 to N.

Roughness should preferably be determined using stylus type equipment in which the stylus measures changes in the radial direction as the wire rotates. The recommendations of ISO 4288:1996 should be followed in that for an expected roughness of 0.40-1.00 micrometers, a minimum rating length L _e of 4 mm should be observed, with a cut-off length λ ₀ of 0.80 mm. The cut-off length λ ₀ used in the analysis distinguishes between profile waviness and actual roughness. If the wire is less than 1.27mm in diameter, overlap must be introduced to achieve the required rating length. Optical methods based on a perpendicular cross-section of the wire can also be used, provided the same analysis technique is followed.

In accordance with an improvement in the method of making steel filament, one or more dies including diamond are used in the final passes of wet wire drawing. Non-exhaustive examples of what is meant by a diamond-incorporating die include dies made from one natural diamond, one artificial diamond, a compact of diamond particles sintered together (sintered diamond), carbonado (black diamond), or polycrystalline diamond (polycrystalline diamond dies). PCD). It has been the inventors' experience that, coupled with the increased circumferential roughness on the intermediate wire, the amount of iron on the surface is increased when using diamond dies compared to the commonly used and conventional tungsten carbide dies.

Another improvement of the method is that the intermediate wire etching step is carried out in hydrochloric acid, where the concentration of iron(III) is 6-15 grams per liter, or even 7-15 grams per liter, or 8-13 grams per liter. These are unusually high concentrations of iron(III) ions in hydrochloric acid. The inventors have found that such high concentrations lead to increased attack on the wire surface and, consequently, increased roughness. Increased roughness results in more iron on the surface.

In another method, after the step of electroplating the patented and copper-coated wire with a zinc coating (step e), the wire is passed through an acid bath containing iron(II) cations. Only after this do copper and zinc diffuse into the brass (stage e). Zinc atoms on the surface are replaced by iron atoms and the presence of iron on the surface increases.

In another method, after the step of electroplating the patented and copper-coated wire with a zinc coating (step e), the wire is passed through an acid bath containing iron(I) cations. The zinc atoms on the surface are replaced by iron atoms. After this, an additional layer of zinc is electrolytically deposited on top of the layer that was exchanged with iron. Only after this is diffusion carried out. This option has the advantage that the added iron is hidden in a layer of zinc.

Possible compositions of acid baths containing iron(II) cations are:

ferric chloride solutions;

ferrous sulfate solutions;

solutions of ferrous ammonium sulfate;

ferric fluoroborate solutions;

- 7 045914 solutions of iron sulfamate;

mixed sulfate-chloride baths.

Brief description of drawings

Fig. 1 - profile of the concentration of atoms (Fe), iron relative to the sum of Fe, Cu and Zn atoms for the reference sample and various samples according to the invention;

fig. 2 is a profile of the concentration of iron atoms (Fe) relative to the sum of Fe, Cu and Zn atoms for a reference sample and various samples according to another embodiment of the invention;

fig. 3 - BSE SEM image showing that the thick brass strips appear relatively light and the thin brass strips appear relatively dark;

fig. 4 shows an enlarged and 180° rotated portion of FIG. 3 through secondary electron imaging in a scanning Auger microscope, where sampling points for elemental analysis are indicated;

fig. 5 - shows the difference in the distribution of iron in thin brass strips (i.e. darker) for the reference sample and two samples according to the invention, measured by Auger depth analysis;

fig. 6 shows the difference in iron distribution in the thick brass strips (i.e. they appear lighter) for the reference sample and two samples of the invention, measured by Auger depth analysis.

Carrying out the invention

To produce samples according to the invention, the inventors started from high-carbon steel wire rods with a carbon grade of 0.80 wt.%. C and nominal diameter 5.5 mm.

The wire was dry drawn to an intermediate diameter of 1.85 mm. Care was taken to obtain a sufficiently high circular roughness R _a of about 0.90 µm. The circular roughness of the intermediate wire R _a can be increased by increasing the inclusion of soap powder in the dry drawing stage, decreasing the reduction on the last die in the dry drawing stage, decreasing the die angle in the dry drawing stage, or a combination of any of the above. Thus, a circular roughness R _{a of} 0.80-1.00 μm was obtained on the intermediate wire.

The wire was then cleaned by etching in a manner known to one skilled in the art. The commonly used etching acid is hydrochloric acid. However, by maintaining the concentration of iron(III) cations above 6 grams per liter and below 15 grams per liter, which is unusual for the specialist, the circular roughness can be further increased.

The wire is then coated with copper by electrolytic deposition in a bath of copper pyrosulfate. After proper washing and drying, the wire is electrolytically coated with zinc in a zinc sulfate bath. These methods are known to those skilled in the art.

After zinc precipitation, iron can be precipitated from an acidic electrolytic solution containing iron(II) cations through an exchange reaction with zinc. Since zinc is a less electropositive metal than iron, zinc cations will go into solution and iron(II) cations will precipitate to maintain charge neutrality. Ferrous sulfate solution seems most suitable for precipitating iron, since the acid is compatible with the acid of the zinc electrolyte. The amount of deposited iron will depend on the time of immersion of the wire.

After this, copper and zinc diffuse when the wire is heated by resistive heating or mid-frequency induction heating, while the iron remains on the surface after the exchange.

Patenting, copper plating, galvanizing, iron deposition and diffusion are carried out sequentially through an installation in which a reel of intermediate wire is unwound, guided through the installation, and the resulting brass-plated wire is wound onto a take-up reel.

At the next stage, a steel thread with a diameter of 0.28 mm is obtained from the wire by wet drawing. Thanks to the use of diamond dies in one or more last passes, i.e. on dies located closer to the exit from the drawing mill, the wire can be easily drawn. In particular, drawability is an issue when brass becomes enriched in iron through a zinc-iron exchange reaction.

By varying the various parameters mentioned above, a series of samples were prepared with increasing concentrations of iron on the surface. The standard used was ordinary brass wire treated in the same way, but with a surface roughness of the intermediate wire less than 0.40 microns, the etching conditions were standard (4-7 grams per liter of iron(I) cations concentration), there was no additional addition of iron, and drawing was carried out in ordinary vidia dies. This sample is designated Ref for comparison.

Using XRFS, the mass of copper and zinc per unit mass of steel cord was determined in accordance with a method known to one skilled in the art. From the total mass of copper and zinc, the average coating thickness (expressed in nm) can be calculated. The mass fraction of copper is calculated as the ratio of the mass of copper to the total mass of copper and zinc.

The distribution of iron in the top layer of brass coating was measured using X-ray photoelectron spectroscopy. The equipment used was a K-Alpha X-ray photoelectron spectrometer (XPS) system supplied by Thermo Fisher Scientific. Usually the depth of

- 8 045914 sample lysis is 2-5 nm with a beam area of about 8000 µm ² . From the distribution of the kinetic energy of the emitted electrons, information about the atoms being examined can be obtained, that is, the number of elements (peak energy) and the number of atoms present (peak height). Although many elements can be determined, only the contents of copper, zinc and iron are retained. By stepwise removal of the upper layer of atoms using an argon gun, a depth profile of the distribution of atoms in the upper layer is obtained. Examples of curves can be found in Fig. 1 and 2, which show the amount of (Fe)i in atomic percentage relative to the total amount of Cu, Zn and Fe atoms, determined at different total sputtering times. For a calibrated argon beam, sputtering for 10 seconds equals about 1 nm of material removed. From the curves it can be seen that the amount of iron present on the surface increases significantly in the wires according to the invention compared to the reference wire (designated Ref”).

Using the trapezoidal rule, the average iron content in the layer extending from the surface 0 to a depth of x nanometers is calculated, where x takes values 3, 9 and 20 nm, corresponding to the first, second and third layers. This procedure is repeated on four sections at different points on the surface of a single steel strand to prevent measurement errors due to local variations in the coating.

Thus, the following table is compiled. I (numbers in italics correspond to the claims):

Table I __Average iron concentration by depth____________________

Sample 0 - 3 nm 0-9 nm 0 - 20 nm t (nm) % wt. Si Voloka Ref 1.3 1.7 2.2 254 63.2 W S2 3.3 3.6 4.3 249 64.3 D S3 4.4 4.7 5 5 244 64.9 D S4 6.5 7.2 8.2 238 66.5 D S5 7.7 8.6 9.9 234 68.6 D S6 10.5 AND 2 12.6 220 70.2 D S7 6.8 7.3 8.2 225 69.6 D S9 2.5 h,o 3.8 253 64.3 W S10 z, z 3.2 3.8 249 64.9 W S11 5.4 6.0 6.6 241 66.7 W S12 7.6 8.0 8.8 236 68.5 W S13 6.5 6.0 6.4 230 69.7 W

In table The only difference between samples (S2, S9), (S3, S10), (S4, S11), (S5, S12) and (S6, S13) is that the first element of the pair is drawn using a diamond die (D ), and the drawing of the second element is performed using a conventional tungsten carbide die (W). It should be noted that the use of diamond-containing dies always leads to an increased presence of iron on the surface, which corresponds to the invention, namely to an increase in the presence of iron on the surface.

Fig. 3 shows an image of the surface of a brass filament formed by backscatter electrons in the BSE mode of a scanning electron microscope (FEI Inspect model). This image shows alternating lighter and darker stripes in the coating in the direction of the wire. The lighter stripes correspond to thicker brass strips, the point of which is indicated by + 2, while the darker stripes correspond to a relatively thin coating, for example, at the point indicated by + 1. The scratch is made deliberately so that analysis can be carried out at the same point with a scanning Auger microscope.

Fig. 4 represents the same area, but in secondary electron imaging (SEI) mode, observed by a PHI scanning Auger microscope. Note that the orientation is rotated 180° and the magnification is quadrupled compared to FIG. 3. At the indicated points 1 and 2, the Auger electron profiles were measured using a PHI-700 Scanning Auger Nanoprobe supplied by ULVAC-PHI. Using this procedure, depth profiles obtained in both areas can be compared with each other for different samples. Auger analysis differs from XPS in that the area of analysis is very small, typically less than 100 nm ² , while for XPS it is a few square micrometers.

Fig. 5 shows the presence of iron relative to the total amount of iron, copper and zinc in an area that appears dark in the BSE SEM mode. As expected, since the brass coating is thin, the amount of iron present increases dramatically when it reaches the steel substrate, which typically occurs within 10 nm. Both the samples of the invention and the reference can show a noticeable presence of iron in the first few nanometers.

In contrast, when analyzing the presence of iron in an area that appears bright in the BSE SEM mode on a reference sample, there is only a very limited presence of iron relative to the total amount of iron, copper and zinc in the first 3, 10 or even 15 nm from the surface: on average it remains below 0.03 or 3 atomic percent. However, in invention samples S3 and S4 the presence of iron is already noticeable, found even in the area of thick brass even at very shallow depths. At

- 9 045914 calculating the average atomic percentage of iron in various samples, the following results were obtained (Table II):

Table II

Sample 0 - 3 nm (% at, Fe) 0 - 9 nm (% at, Fe) 0 - 15 nm (% at, Fe) Ref. 1.3 2.0 2.2 S3 4.1 3.1 3.2 S4 12.2 10.2 9.9

The presence of increased amounts of iron on the surface results in improved adhesion properties with cobalt salt containing compounds as well as non-cobalt containing compounds, as will be demonstrated later.

Three 0.28 mm strands of each type of reference thread and sample thread are twisted together to form a 3 x 0.28 mm steel cord. These steel cords are used to carry out adhesion tests on a large number of different adhesive joints, mainly falling into two groups:

group I includes 6 different compounds, the common feature of which is the deliberate addition of organic cobalt salts;

group II contains 6 different compounds that do not contain intentionally added cobalt.

For each of the twelve compounds, the conventional cure (RC) conditions were TC90 plus 5 min, where TC90 represents the time at which a particular rubber reaches 90% of its maximum torque on the rheometer curve obtained at the vulcanization temperature.

To determine adhesion retention on RC-cured specimens, the following aging conditions are applied:

after exposure to humidity conditions (CH): RC samples are maintained at a temperature of 93 ° C in an environment with a relative humidity of 95% for 14 days;

after steam exposure (SA): in which RC samples are steamed at 120°C for 2 days.

In the following, each of the vulcanization conditions RC, CH or SA will be referred to as a condition.

Adhesion test results are peel strength determined in accordance with ASTM D2229-04, as described in more detail in the BISFA (Bureau International Standards Manmade Fibers) brochure Internationally Harmonized Test Methods for Tire Steel Cord. Edition 1995, D12 Determination of static adhesion to rubber compounds. In this test, steel cords are embedded in rubber blocks and pulled axially from the rubber after vulcanization. The maximum force achieved (in N) is noted. The average of several (at least four) individual maximum force measurements (in N) is reported as the pull-out force (POF) for one sample, one group, one combination of conditions.

The results of adhesion tests are presented in table. III and table. IV below as a Z-score relative to the reference mean (RA). The reference average value RA is equal to the weighted average value of the reference sample, i.e. conventional brass plating obtained by drawing in tungsten carbide dies, with all cobalt-containing compounds of group I, and this for a specific condition according to the column heading. The reference standard deviation (RSTD) is equal to the statistical standard deviation of all results obtained on a reference sample with Group I compounds under normal conditions. Briefly, positive or negative deviations are calculated relative to a known brass steel cord drawn using tungsten carbide dies, tested in cobalt containing rubber, for each of the various conditions.

For each of groups I and II and for the sample sample (Samples) table. II pull-out force is determined for each condition. The pullout forces are averaged to a weighted sample mean (SA), and a statistical standard deviation called the sample standard deviation (SSTD) for that family and condition is calculated.

The Z score of a sample in a group of compounds for a particular condition is equal to the difference between the mean of the sample in that group and the condition minus the mean of the standard for that condition, divided by the combined standard deviation of the standard deviation of the standard and the standard deviation of the sample. Briefly:

SA - IA

Z :((N _S - ] JSS [ ]У - (X _R - DRSTiy) , N _s -X _r -2 where N _S represents the number of results summed to obtain SA and SSTD, and N _R is the number of results summed for receiving RA and RSTD.

The Z-score indicates how much deviations from the mean are statistically significant compared to the standard mean, i.e. current state of technology in specific

- 10 045914 group and condition in which the sample was tested:

Z-scores below -2 indicate statistically significant deterioration relative to the benchmark mean;

Z-scores of -2 to -1 indicate possible deterioration but are not statistically significant;

Z-scores between -1 and +1 indicate no statistically significant deterioration or improvement in the mean for the reference;

Z-scores between +1 and +2 indicate possible improvement that is not statistically significant;

Z-scores above +2 represent a statistically significant improvement over the prior art.

In table III summarizes the results of Z-scores on selected samples obtained with compounds of group I.

Conclusions.

With conventional cure (RC), overall results are neutral or marginally lower than the reference for average iron concentrations below 7.5 at.% Fe in the 0-3 nm range.

Samples with an average iron concentration above 7.5 at.% Fe (S6, S12) show a significantly lower result compared to the standard (double underline).

Thus, too much iron on the surface will negatively affect the results of conventional curing.

For humidity exposure (HH), the results are generally better than the standard reference (all values are positive).

The results for samples (S4, S5, in bold) show a significant improvement in CH results with cobalt-containing compounds.

However, samples drawn using conventional tungsten carbide dies show poorer results.

Thus, the average iron concentration above 4 at.% Fe on the surface of the steel thread has a positive effect on maintaining adhesion when exposed to humidity conditions.

When exposed to steam (SA), an overall positive improvement was found, which was significant at an average iron concentration above 4 at.% Fe on the surface of the steel thread.

Samples obtained by drawing with conventional tungsten carbide dies give lower values.

Samples with 4-7.5 at.% Fe in the upper part of a steel thread with a thickness of 0-3 nm perform better under three conditions of adhesion testing of rubber, including cobalt compounds.

In table IV summarizes the results of Z-scores obtained on selected samples of compounds of group II.

Conclusions.

With conventional cure (RC), overall results are neutral to slightly negative compared to the reference for average iron concentrations below 7.5 at.% Fe within 0-3 nm of surface area for specimens drawn with diamond-containing dies.

Samples obtained by drawing using conventional tungsten carbide dies have significantly lower performance.

When exposed to humid conditions (CH), all samples of the invention perform better than the reference, and significantly better than the reference, for average iron concentrations above 4 at.% Fe in the range of 0-3 nm.

When exposed to steam (SA), all samples according to the invention showed significantly higher results than the standard.

In conclusion, samples of the invention with average iron content above 4 at.% Fe and below 7.5 at.% Fe within 0-3 nm from the surface show similar results when cured normally and improved results when exposed to humidity and environmental conditions. steam in adhesive compositions including cobalt salts. When testing adhesives that contain virtually no cobalt, the results after exposure to humidity and steam conditions are significantly better, while the results of conventional curing are only slightly lower. The use of one or more diamond-containing dies in one or more final passes of wet wire drawing further improves these results.

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Claims (6)

Table III Reverse R.C. CH S.A. Rjjf 0 0 0 S2 J),3 J ,11 IC S3 J),l ],s 2x2 S4 HJ.O 1.3 3.9 S? -].fl 1.3 4 JI S.C. _ Ί 3 ].s 4.9 st -0.1 ],s 2.7si -] .0 1.2 0.7 Si? ____пь-____ ],4 2.4 Table IV Of.i|Xl k'U R.C. CH S.A. Ru'f J, 4 -1.4 3.1 S2 -J.1) 1.1 3.7 S3 -1.5 2.6 4.H_S4 4.] ft.l S3 zkZ 5.5 S6 J.7 3.S 6.6 S7 -(1.6 2.9 4.0 Si i 1.4 5.5 S1 2 -Al '.S 5.0 CLAIM 1. A steel thread with a diameter d, expressed in millimeters, for reinforcing rubber products, comprising a steel base, coated with brass, said brass consisting of copper and zinc; said coating has an average thickness of 450xd nanometers or more, which is determined by the total mass of copper and zinc of said brass; the specified brass has a mass copper content of 61-75 mass percent relative to the total mass of copper and zinc in the specified brass, the specified thickness of the specified coating and the specified copper content in the specified brass are determined by wet chemical analysis methods, characterized by the fact that the specified coating has an average iron content of 4 or more atomic percent to 10 or less atomic percent in a first layer, said first layer extending from the surface of said filament to a depth of 3 nanometers below said surface, said iron content expressed as atomic percent relative to the total iron, zinc and copper content of said first layer, said iron, zinc and copper contents are determined by X-ray photoelectron spectroscopy, said average value is determined at the depth of said first layer and at 4 different points on the surface of said steel thread. 2. The steel thread of claim 1, wherein said brass coating has an average iron content of 5 atomic percent or more in a second layer, said second layer extending from the surface of said thread to a depth of 9 nanometers below said surface. 3. The steel thread of claim 2, wherein said brass coating has an average iron content in the third layer of 6 atomic percent or more, said third layer extending from the surface of said thread to a depth of 20 nanometers below said surface. 4. The steel thread according to any one of claims 1 to 3, wherein said brass coating has an average iron content of 20 atomic percent or less in a third layer, said third layer extending from the surface of said thread to a depth of 20 nm below said surface. 5. The steel thread according to any one of claims 1 to 4, wherein said brass coating has an average thickness of 1350xd nanometers or less. 6. Steel thread according to any one of claims 1 to 5, in which on the specified coating alternating thin brass strips and thick brass strips are visible, oriented along the length of the steel thread, and these stripes can be distinguished by a scanning electron microscope operating in electron backscattering mode, wherein the thick brass strips appear relatively light and said thin brass strips appear relatively dark, further characterized in that said thick brass strips have an average iron content of 4 or more atomic percent relative to the total iron, copper and zinc content, said average the value is taken by depth within the upper 3 nanometers from the surface of the specified thick brass strip, the content of the specified iron, zinc and copper is determined from scanning data -