EA045969B1 - STEEL CORD WITH BRASS COATING ENRICHED IN IRON PARTICLES - Google Patents

Description

Field of technology to which the invention relates

The invention relates to a steel cord for reinforcing rubber products such as tires, hoses, conveyor belts and other devices.

Prior Art

In 2019, it is predicted that approximately 2 billion steel-cord vehicle tires will be produced worldwide. The steel cord itself consists of steel threads covered with a brass coating. Steel and brass are relatively harmless to the environment and human health.

However, to stabilize the adhesion between the rubber cord compound and the steel cord, tire manufacturers add organic cobalt-based salts, such as cobalt naphthenate, cobalt stearates, or cobalt-boron decanoate complexes to the rubber in addition to other additives such as gas soot, sulfur, accelerators, oils, antioxidants, activators, etc. Some of these organic cobalt-based salts are considered carcinogenic and their use is increasingly being restricted.

How organic cobalt salt acts in the adhesion system has been the subject of extensive scientific research, in which the late W. J. van Ooij played a prominent role. For the purposes of this application, his review of Rubber and Brass Bonding, Chapter 6 of the Handbook of Rubber Bonding, Rapra Technology Limited, 2001, will be retained as the Basic Reference (BR). The growth of dendritic non-stoichiometric copper sulfide (Cu _x S, with x about 1.8) into the rubber network before and during vulcanization is believed to be the primary adhesion mechanism. This layer is an adhesion layer and has a thickness of less than 250 nm or even only 100 nm.

Organic cobalt salts serve two purposes:

firstly, to suppress the growth of less binding zinc sulfide (ZnS), thereby promoting the formation of non-stoichiometric dendritic copper sulfide during bond formation;

It is generally accepted that loss of adhesion under hot and humid conditions is due to dezincification of brass due to the diffusion of zinc ions (Zn ²⁺ ) into the adhesion layer, forming zinc oxides and hydroxides and thereby weakening the adhesion layer. The second purpose of adding organic cobalt salts is to improve the maintenance of the steel cord-rubber bond under hot and humid conditions by inhibiting this diffusion mechanism;

The disadvantage is that organic cobalt salts act as a catalyst for the oxidation of diene rubber bonds, thereby accelerating the aging of rubber, which can ultimately lead to rubber failure.

To avoid the use of organic cobalt salts in rubber, in the late seventies of the last century it was proposed to include cobalt in the brass layer of the steel cord rather than in the rubber. See, for example, US 4255496 and US 4265678. Such three-component alloy layers indeed give very good adhesion retention results in hot and humid conditions. However, they did not allow the complete removal of organic cobalt salts from rubber. Recent work by the current applicant, published in WO 2011/076746, WO 2013/117248, WO 2013/117249, has further presented solutions allowing the use of ternary alloy coatings also in cobalt-free mixtures.

With this technology, the total amount of cobalt introduced into one tire, when cobalt is included in the brass coating, is reduced by approximately one-fifth to one-tenth of the amount of cobalt (in terms of metal) when added as an organic cobalt salt to the rubber. This means a significant reduction in cobalt use and reduced environmental burden.

However, the problem of handling cobalt-containing mixtures in production conditions has passed from the tire manufacturer to the steel cord manufacturer.

Another challenge facing the tire industry is that eliminating the use of organic cobalt salts for rubber adhesion represents a major shift in production strategy, making tire production more difficult. Additional cobalt-free rubber compounds must be introduced and segregation with conventional rubbers must be absolute.

In addition, cobalt has become a strategic material in the production of rechargeable batteries, for example used in electric vehicles. Consequently, cobalt prices have risen sharply and the balance between market supply and demand is not expected to reach equilibrium in the coming years. Thus, completely eliminating the use of cobalt will not only be good for the environment and solve health problems for workers, but will also have an impact on the overall cost of the tire.

In conclusion: completely eliminating cobalt from a tire is beneficial to the price of the tire, to worker health, and to the environment. Therefore, other metals that are less harmful than cobalt should be considered.

Based on the interaction of the vulcanization accelerator in rubber with the metal surface, van Ooij suggests in BR, page 176: All metals that can react with the accelerator should in principle bind to the rubber. These metals include the transition metals cobalt, copper, iron, nickel and zinc. Of these, copper and cobalt are very active and form strong bonds. Other metals are virtually unbound because sulfide growth is slow (in the case of iron or zinc), or

- 1 045969 the metal is passive (in the case of nickel), or the sulfide does not form dendrites, as is the case with copper and cobalt.

Therefore, the growth of the sulfide layer on the steel substrate coating is expected to be slower when using iron as the third metal than when using cobalt. However, even in ordinary brass-coated steel cord, some iron - the steel backing - is present on the surface, and this iron has been found to help maintain and enhance adhesion. See page 429 Mechanism and theories of rubber adhesion to steel tire cords - an overview, W.J. van Ooij, RUBBER CHEMISTRY AMD TECHNOLOGY, Volume 57, pages 421-456, 1984. Thus, thin brass coatings have improved adhesion properties and adhesion retention.However, the thinness of the brass coating has its limits, since sufficient copper and zinc must be present on the surface, and the thread must be drawn.

Although the use of a brass-iron ternary alloy for adhesive purposes was proposed in US 4,446,198, implementation of this coating was never considered because other problems prevented its use, as will be explained below.

Disclosure of the invention

The inventors set out to overcome the problems associated with the prior art. The main goal of the invention is to completely eliminate the use of cobalt in the tire. In particular, the inventors overcame the problems of introducing iron into the brass coating. In addition, the inventors demonstrate that the use of an iron-rich brass coating provides adequate initial adhesion and adhesion retention in conventional aging tests using a completely cobalt-free rubber compound. The inventors have also found that their proposed steel cord coating performs equally well in cobalt-containing rubber compounds, thereby reducing the risk that rubber substitution may inadvertently occur.

According to the first aspect of the invention, summarized in paragraph 1 of the claims, a steel cord is provided. The steel cord includes one or more strands including a steel strand backing and a coating that partially or completely covers the steel strand backing. The coating includes brass, which - to achieve the objectives of the invention - consists of copper and zinc. The coating is enriched with iron. A characteristic feature of the coating is that iron is present in brass in the form of particles whose size is 10-10,000 nanometers.

Steel cord gives a rubber product, such as a tire, hose or belt, tensile and compressive strength combined with flexibility. To form a rubber-steel cord composite, the threads are provided with a rubber-adhesive coating. The threads can be tied into a bundle or twisted into a strand. Strands or strands, in turn, can be twisted into cords. Strands, bundles and cords are collectively called steel cords. Since single strands of steel, commonly referred to as monofilaments, are also currently offered for tire reinforcement, the single strands are also considered steel cord for purposes of this application. In addition, the use of the term steel cord does not exclude the possibility that other non-steel threads or filamentary materials are mixed with the steel threads. The addition of, for example, high-performance organic man-made fibers such as Twaron® or aramid-based Kevlar®, or ultra-high molecular weight polyethylene fibers such as Dyneema®, can provide additional functionality to the steel cord.

By steel thread-like substrate is meant an elongated steel element with a length exceeding its width and thickness dimensions, the length, width and thickness being oriented mutually orthogonally to each other. For example, the length is several kilometers, and the width and thickness are less than one millimeter. The orthogonal cross-section of the steel filamentary substrate may be square, rectangular or polygonal, but preferably circular with a diameter d. The diameter of the thread is 0.10-0.50 mm. Larger diameters, such as 0.275-0.40 mm, are mainly used to strengthen the tire belt because these fibers are relatively stiff. The threads are collected in a structure with a small number of threads (less than 9) or even as a monofilament. Finer fibers, for example 0.10-0.275 mm, assembled into assemblies containing nine or more fibers, are preferably used to strengthen the tire carcass. Strength, flexibility and fatigue resistance are more important here, which are easier to achieve with thinner threads.

The steel from which the steel filament substrate is preferably made of unalloyed carbon steel, the composition of which is within the following limits (all percentages are percentages by weight, abbreviated as wt%.

carbon content 0.60-1.20 wt.%, for example, 0.80-1.1 wt.%;

manganese content 0.10-1.0 wt.%, for example, 0.20-0.80 wt.%;

silicon content 0.10-1.50 wt.%, for example, 0.15-0.70 wt.%;

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

the phosphorus content is below 0.03 wt.%, for example, less than 0.01 wt.%.

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By subjecting steel to a strain hardening operation such as wire drawing, a tensile strength of more than 2500 MPa or more than 3000 MPa or even more than 3500 MPa can be obtained.

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, even up to 4000 MPa and above.

An alternative approach may be to use low carbon steels that have been highly drawn to achieve sufficient tensile strength. Typical steel compositions have a carbon content of less than 0.20 wt.%. An example is carbon content 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 mass percent. The tensile strength of these threads is much lower: above 1200 MPa or even above 1400 MPa, but they have a reduced carbon footprint due to the elimination of intermediate heat treatments.

The substrate of steel threads is partially or completely covered with a coating. The term partially coated means that certain areas of the steel filament backing are beyond the coating. Usually these sections are longitudinal and arise from the drawing of the wire into the thread. Sometimes they are visible as lines. It is possible, but in rare cases the filament substrate will be completely coated.

The coating is brass enriched with iron. The term iron-rich means that the iron is not derived from the steel filament backing. Iron is added to brass. It is characteristic that iron is present in the coating in the form of particles measuring 10-10,000 nanometers. For the purposes of this application: particle size means the maximum distance between any two points on the surface of a particle.

The inventors have found that the presence of particles plays an important role in the adhesion of steel cord to rubber compounds. The presence of iron particles in the brass plating results in a finer-grained brass plating. It is assumed that finer-grained brass coating is better for promoting adhesion, since copper can effectively diffuse through grain boundaries and aggregates of lattice defects during the formation of the initial adhesion layer. As a result, the initial adhesion characteristics are higher than those of conventional brass coatings. In this regard, it is preferable that the iron particles are small and numerous. In addition, the resulting brass lattice structure, which contains iron particles, results in markedly improved adhesion retention in hot and humid conditions.

On the other hand, it has been found that the iron particles cannot be too large as this leads to processing problems during wet wire drawing. Therefore, in a further improved embodiment of the invention, the iron particles have a size of 20-5000 nanometers or even 20-3000 nanometers, more particularly 20-2000 nanometers, for example 20-1000 nm.

In another preferred embodiment, some iron particles are pressed into a steel thread-like substrate. Iron particles may coat the coating. Some iron particles, especially larger ones, may have a flat surface. The shape of the particles is preferably oblate, i.e. the particles are disk-shaped rather than needle-shaped.

Particles can be observed using the following procedure (Procedure 1):

P1 (a) steel cord weighing about 0.2 grams is cut into 1-2 cm samples and weighed. The weight W is noted. The samples are stored in a glass;

P1 (b) Add 10 ml of brass removal solution to a beaker and keep in an ultrasonic bath for five minutes.

The brass removal solution (removal solution) includes 16 g of ammonium persulfate (NH ₄ ) ₂ S ₂ O ₈ with an aqueous solution of ammonia NH3-H2O in 1 liter of aqueous solution.

P1 (c) Rinse the treated thread with clean water, holding the thread with plastic tweezers. Collect the wash water in the removal solution contained in the beaker. Dry the treated thread.

P1 (d) To observe particles, particles can either be filtered on a filter paper or extracted with a magnet and then dried in an inert atmosphere.

Particles can be observed preferably using a scanning electron microscope

- 3 045969 or optical microscope.

An alternative way to observe the size of larger (greater than 1000 nm) particles is to detect iron particles that have been pressed into a steel filament substrate on a dry treated filament.

The thread coating includes brass. In this case, brass is an alloy consisting of zinc and copper. It is a substitution alloy in the sense that copper or zinc atoms can replace each other in the crystal lattice. Within the coating, the composition of the brass can vary from almost pure zinc on the radial outer side of the coating to almost pure copper near the filamentary substrate. Preferably, the total copper content of the brass is 63 percent by weight or higher, based on the total weight of copper and zinc in the plating (excluding any other element in the plating). More preferably, the ratio of the mass of copper to the total mass of copper and zinc is greater than 65 percent by weight, or even greater than 67 percent by weight. When the amount of copper in brass exceeds 63 percent by mass, the formation of beta(b)-brass is avoided in favor of alpha(a)-brass. Beta brass is a harder phase of brass and is more difficult to deform.

The amount of iron added to the plating is greater than or equal to 1% and less than 10% of the total mass of brass and iron. When the weight of iron for enrichment is less than 1% of the weight of brass and iron, no improvement in adhesive performance is observed. When the weight of added iron exceeds 10%, wire drawing becomes difficult.

In a preferred embodiment, the amount of iron in the coating is greater than or equal to 2 wt.%, and less than 10 wt.%, compared to the total weight of brass and iron. In an even more preferred embodiment, the amount of iron is 3-9 wt.%, compared to the total weight of brass and iron.

The amount of iron, copper and zinc in iron-fortified brass plating can be measured using a second test procedure (Procedure 2), which is P2(a) - P2(c).

Perform steps P1 (a) to P1 (c) of procedure 1;

P2 (d) the solution in the glass is converted from alkaline to acidic by adding 5 ml of 37% hydrochloric acid, HCl, and the solution in the glass is stirred;

P2 (e) transfer the solution from the beaker to a volumetric flask, cool to room temperature and dilute with demineralized water to 100 ml;

P2(e) measure the concentration of iron, copper and zinc in solution using inductively coupled plasma optical emission spectroscopy (ICP-OES) using standard solutions (Cu; Fe; Zn) (0; 0; 0), (2; 0 .02; 1), (5; 0.1; 2), (10; 0.5; 5) mg/l all in a matrix of 10 ml of removal solution, 5 ml of 37% HCl per 100 ml of demineralized water.

Preferred ranges for the respective elements in the coating are 6269 weight percent copper and 1-10 weight percent iron, with the balance being zinc based on the total weight of copper, iron and zinc. Most preferably, of the total mass of copper, iron and zinc, 6266 weight percent is copper and 2-10 or even 3-9 weight percent is iron, with the balance being zinc. One preferred composition consists of 64 wt.% copper and 8 wt.% iron, the balance zinc. Copper, iron and zinc concentrations are measured using ICP-OES.

In another preferred embodiment, the coating contains substantially no iron-zinc alloy. Iron-zinc alloys are formed in several phases: eta-(p), zeta-©, delta-©, gamma1-(G1) and gamma-(G) phases with increasing iron content. This (p)phase, containing only 0.03 wt.% iron, should still be considered pure zinc, it is soft like zinc, and for the purposes of this application is not considered an iron-zinc alloy. The presence of iron-zinc alloy layers or particles is unacceptable because they have a higher hardness, which is not suitable for wet wire drawing.

The total amount of coverage, i.e. the sum of copper, zinc and iron on the steel cord filaments relative to the total weight of the steel cord - coating weight (MCW) - is preferably 1-6.5 grams of coating per kg of filament (1-6.5 g/kg). More preferably, 3-5 grams of coating per kg of fiber (3-5 g/kg), for example, 3.5-4 grams of coating per kg of fiber (3.5-4 g/kg).

In another preferred embodiment, the amount of phosphorus and iron present on the surface of the steel thread is controlled. The amount of phosphorus present on the surface is denoted by P _s and is expressed in milligrams per square meter (mg/m ² ), the amount of iron on the surface is denoted by Fe _s and is also expressed in mg/m ² . The amount of phosphorus and iron present on the surface is determined by gently etching the surface of the fibers with a weak acid that dissolves phosphorus and iron, according to the following measurement procedure (Procedure 3):

P3 (a) about 5 grams of steel cord are taken, cut into samples of about 5 cm in size and placed in a test tube;

P3 (b) add 10 ml of 0.01 molar hydrochloric acid HCl;

P3 (c) shake the sample with the acid solution for 15 seconds;

P3 (d) measure the amount of Fe and P present in the solution using ICP-OES;

P3 (e) express the result as the mass of Fe and P per unit surface area of the steel filaments in milligrams per square meter (mg/m ² ). The result is denoted as Fe _s and P _s .

- 4 045969

The inventors have found that the best adhesion results are achieved when the amount of phosphorus present on the surface is less than 4 mg/m ² but greater than zero: 0 < Ps < 4 mg/m ² . Higher Ps content slows down the growth of the adhesion layer. At Ps below 4 mg/m ² the adhesion layer is formed immediately after the start of the vulcanization cycle, but before the cross-linking of the rubber begins. Ps can be below 3 mg/ ^m2 or even below 1.5 mg/ ^m2 .

In another preferred embodiment, the amount of iron present on the surface of Fes is greater than or equal to 30 mg/m ² (Fes >30 mg/m ² ). Even more preferably, more than 35 mg/ ^m2 or even more than 40 mg/ ^m2 iron is present on the surface. The inventors assume that a sufficient amount of iron must be present on the surface of the filaments in order to have sufficient adhesion retention. It is believed that the presence of iron particles on the surface will inhibit the diffusion of Zn ²⁺ ions to the adhesion layer and thus lead to improved adhesion retention under hot and humid conditions, as will be demonstrated below.

Preferably, the ratio between the amount of iron present on the surface of the Fes fiber and the amount of phosphorus present on the surface of the same Ps fiber is greater than 27 or even greater than 30. When this ratio is satisfied, a sufficient amount of iron is present on the surface while the amount of phosphorus is sufficient few.

Since the amount of iron is related to the mass of the coating, an improved measure of the relative amount of iron is to first divide the amount of iron Fe _s present on the surface by the mass of the SCW surface coating before finding the relationship with the amount of phosphorus Ps present on the surface. Surface coating mass is expressed in grams per square meter (g/m ² ) to correspond to the units P _s and Fe _s . The relationship between the surface coating mass SCW (in g/ ^m2 ) and the coating mass (in g/kg) MCW for a thread of diameter d, expressed in millimeters mm, is:

SCW= 1.97 x d xMCW for steel wire and coating thickness according to the present invention.

Preferably, the ratio (Fes/SCW)/Ps is equal to Fes/(SCW x Ps) - that is, the ratio of Fes to the product of P _s and SCW - is greater than 13. Above this value, the best results are obtained for maintaining adhesion. It's even better if the ratio is higher than 14. You can get values up to 25.

In another preferred embodiment of the invention, the steel cord consists of a single thread. Such thread can be used in the tire, for example, in the bead area to strengthen the bead or in the belt area to enhance the rigidity of the belt (monofilament). Alternatively, the single steel strand according to the invention can also be used as a reinforcing wire for a hose.

According to a second aspect of the invention, there is provided a rubber product comprising vulcanized rubber reinforced with steel cord according to any of the above claimed embodiments. The rubber product may be a tire, for example, for a car, truck, van, or off-road vehicle. Alternatively, the rubber product may be a hose, such as a hydraulic hose, or a belt, such as a conveyor belt, timing belt, or elevator belt. All of these products are manufactured and assembled by methods that are known or become known in the relevant art. The only difference is that the steel cord used for reinforcement has a brass coating enriched with iron, where the iron is present as particles in the brass coating and where the particles are 10-10,000 nanometers in size. The advantage of the invention is that it is compatible with currently used cobalt-containing mixtures without adversely affecting adhesion and adhesion retention.

However, steel cord was invented in particular to be compatible with adhesive rubber compounds known as rubber cord compounds, which contain substantially no cobalt or organic cobalt compounds added to the rubber. The term substantially free means that the amount of cobalt in the vulcanized rubber, as determined by x-ray fluorescence, is less than 100 micrograms per gram of rubber (0.01 percent Co by weight of rubber, wt.%) or less than 50 micrograms per gram. rubber (0.005 wt.% Co) or even less than 20 (0.002 wt.% Co) or 10 (0.001 wt.% Co) micrograms per gram of rubber. Since typically only the rubber cord compound contains organic cobalt compounds, the analysis is best performed on the rubber in the vicinity of the steel cord, such as remaining rubber that adheres to the steel cord when the steel cord is pulled from the rubber product. This is where the highest concentrations of cobalt are expected.

Also claimed is the use of steel cord in accordance with any of the above embodiments for reinforcing a rubber product—preferably a rubber product that is substantially free of cobalt.

According to a third aspect of the invention, there is provided a method for producing steel cord thread as described above. The method includes the following stages:

(a) producing an intermediate steel wire having an intermediate diameter D: first intermediate steel wire. The intermediate diameter is selected based on the final diameter

- 5 045969 thread, steel composition, in particular carbon content, final tensile strength to be achieved. Typical sizes are 0.5-3.2 mm;

(b) Electroplating, also called galvanizing, of intermediate steel wires with copper, iron and zinc. Preferably, the metals, copper, iron and zinc, are applied in layers;

(c) heat treating the intermediate steel wire coated with copper, iron and zinc to diffuse the zinc into the copper at a temperature of at least 420°C, the melting point of zinc. The formation of the iron-zinc zeta© phase can be avoided if the temperature is kept below 530°C: the iron does not melt and an iron-zinc alloy does not form. The temperature must be maintained for at least 2 seconds to allow the zinc to diffuse into the copper, 10 seconds of diffusion is sufficient. The resulting wire is an intermediate steel wire with a brass coating enriched with iron particles;

(d) optionally, zinc oxide and iron oxide are removed from the surface of the intermediate steel wire with a brass coating enriched in iron particles. It is preferable to do this in an acid bath.

Alternatively, the formation of zinc oxide and iron oxide can be avoided by performing the diffusion step (c) in an inert atmosphere such as a nitrogen atmosphere;

(e) the intermediate steel wire coated with brass enriched with iron particles is subjected to a wet wire drawing operation, thereby obtaining a thread according to the invention. The final diameter of the thread will be indicated by the letter d.

A characteristic feature of the invention is that during the wet wire drawing operation, the iron particles present on the intermediate steel wire with a brass coating enriched with iron particles are reduced to a size of less than 10,000 nm or 10 μm and more than 10 nm.

In another preferred embodiment of the method, step (b) of electrolytically coating the intermediate steel wire with copper, iron and zinc is carried out in the following order:

(b1) electrolytic coating of the intermediate wire with copper;

(b2) electroplating the copper-clad intermediate wire with iron;

(b3) electrolytic coating of intermediate wire coated with copper and iron, zinc.

Following this order has the advantage that the previously applied coating does not dissolve in the bath of the subsequently applied layer.

Step (b2) can be performed in any of the group consisting of the following electroplating solutions:

ferric chloride solutions;

ferrous sulfate solutions;

solutions of ferrous ammonium sulfate;

ferric fluoroborate solutions;

ferrous sulfamate solutions;

mixed sulfate-chloride baths.

In another preferred embodiment, in step (e) - where the intermediate steel wire with a brass coating enriched with iron particles is subjected to a wet wire drawing operation, thereby obtaining a thread according to the invention - the wire drawing is carried out to a true elongation of at least 3.5 . The true elongation s obtained by wet wire drawing is equal to:

ε = 2 1η (D / d)

When the true elongation ε is greater than 3.5, greater than 3.7, or even greater than 3.9, and even greater than 4, which is currently within the limits achievable for steel, the iron particles elongate, crumble, grind to a size of less than 10,000 nm or even less than 5000 nm, for example less than 3000 nm or 2000 nm.

The inventors have found that when wet wire drawing is performed using dies incorporating diamond, additional beneficial surface properties arise that enhance the adhesion and adhesion retention of the steel cord made from the filaments. Non-exhaustive examples of what is meant by dies including diamond are dies made from one natural diamond, one artificial diamond, pressed diamond particles sintered together (sintered diamonds), carbonado (black diamond), or polycrystalline diamonds (PCD dies).

At a minimum, the die that determines the final diameter of the thread, called the head, is a die that includes diamond. Alternatively, one, two, three or more dies located upstream of the wire drawing direction may also be diamond dies, and the remaining dies are conventional carbide dies, such as tungsten carbide dies. Optionally, all dies are dies containing diamond, although this is generally considered too expensive and not necessary to implement this preferred method of the invention.

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Brief description of drawings

Fig. 1a shows iron particles produced from a coating that is pressed against a steel substrate.

Fig. 1b shows an iron particle present in the brass coating as determined by HAADF-STEM.

Fig. 2a shows the peel force adhesion results obtained for Group I mixtures under scorch conditions.

Fig. Figure 2b shows the peel force adhesion results obtained for Group I mixtures under conventional vulcanization conditions.

Fig. 2c shows the peel force adhesion results obtained for Group I mixtures under curing conditions.

Fig. 2d shows the results of adhesion with peel force obtained for Group I mixtures after vulcanization with humidity aging.

Fig. 2e shows the peel force adhesion results obtained for Group I mixtures after steam aging vulcanization.

Fig. 3a shows the results of adhesion with peel force obtained for mixtures of group II under scorching conditions.

In fig. 3b shows the peel force adhesion results obtained for Group II mixtures under conventional vulcanization conditions.

Fig. 3c shows the peel force adhesion results obtained for Group II mixtures under over-vulcanization conditions.

Fig. 3d shows the results of adhesion with peel force obtained for mixtures of group II after vulcanization with humidity aging.

Fig. 3e shows the peel force adhesion results obtained for Group II mixtures after steam aging vulcanization.

Group I mixtures are five different mixtures that contain organic cobalt salt that is currently used in industry. Group II mixtures represent five different mixtures that do not contain added cobalt.

Each point in Fig. 2a-2e, fig. 3a-3e represents the average value for five different mixtures in the corresponding group according to different vulcanization conditions (a-c) or aging conditions (d-e).

In fig. 2a-2e and 3a-3e, the reference value 0 represents the average value for conventional brass-coated steel cord produced using set W dies obtained with Group I mixtures.

Carrying out the invention

The invention was implemented on a structure with increased tensile strength 3x0.28. The term super strength means that the tensile strength of a single thread is at least 3265 N/mm ² with a target value of 3440 N/mm ² .

The thread is prepared as follows:

A 0.80C grade steel rod was selected, which means the steel has a minimum carbon content of 0.80 wt% and a maximum carbon content of 0.85 wt%. Other elements were present as described in paragraphs [0019] - [0020] of this application (composition of unalloyed carbon steel). The steel wire was dry drawn to a diameter of 1.98 mm;

This steel wire is patented by preheating the wire to temperatures above 950°C to achieve complete austenitization. Subsequently, the wire is cooled in a water-air-water patenting unit known in the art. This is an intermediate steel wire having an intermediate diameter according to the claimed method;

This intermediate steel wire is electroplated with a layer of copper by passing the wire through a copper pyrophosphate bath containing complexes of Cu ²⁺ cations and P2O7 ^4- anions in an aqueous alkaline bath containing Cu ²⁺ in the concentration range of 22-38 g/l pyrophosphate ( P2O7 ^4- ) in the concentration range of 150-250 g/l, NO3 nitrate ^- in the concentration range of 5-10 g/l and ammonia NH3 with a concentration of 1-3 g/l. The bath operates at a pH of 8.0-9.0 and a current density of 1-9 A/dm ² . The amount of copper applied is adjusted depending on the desired composition of the final coating;

The copper-coated intermediate wire is then passed through a solution of iron sulfamate (Fe(OSO ₂ NH ₂ ) ₂ ) with the following composition: 75 g/l iron (II), ammonium sulfamate in the concentration range 30-38 g/l, sodium chloride 37- 45 g/l at pH 2.7-3.0, at a temperature of 50-60°C and a current density of 5-6 A/dm ² . The amount of iron applied is adjusted depending on the desired composition of the final coating.

Using a solution of ferrous sulfamate in an electrolyte results in a stable and well-controlled bath.

The copper-iron coated intermediate wire is then passed through a bath of an aqueous solution of zinc sulfate (ZnSO ₄ -7H ₂ O) containing 40-90 g/l zinc at pH 3-3.7. A layer of zinc is applied

- 7 045969 at a current density of 20-30 Adm ² ;

The intermediate wire coated with copper, iron and zinc is then heated through an intermediate heating step followed by a temperature insulation zone. Care was taken to ensure that the temperature did not exceed 530°C to prevent the formation of iron-zinc carbide. The resulting wire is an intermediate steel wire with a brass coating enriched with iron particles;

In the next step, the zinc oxide and iron oxide formed during the heat treatment are removed by immersion in phosphoric acid. Depending on the immersion and cleaning time, the amount of phosphorus present on the surface can be adjusted;

In the first experimental design, the coating composition with 62, 64, 66, 68 wt.% Cu in combination with an iron content of 1, 2, 3, 4, 5 wt.% Fe were combined, and the rest was zinc. The weight percentages refer to the total amount of coating. The results showed that better adhesion results were obtained at higher Fe concentrations. Therefore, a second scheme of experiments was started with even higher iron content.

In the second experimental design, the following coating compositions and masses were obtained on an intermediate steel wire with a brass coating enriched with iron particles (Table I):

Table I

Sample Copper (wt.%) Iron (wt.%) Zinc (wt.%) Total (g/kg) Cu/(Cu+Zn) (wt.%) Comp. 62.5 - 37.5 4.28 62.5 S64-1 64.1 1.2 34.6 4.12 64.9 S64-2 63.9 2.4 33.8 4.17 65.4 S64-4 64.5 4.0 31.4 4.135 67.2 S64-6 64.2 6.6 29.3 4.17 68.7 S64-8 63.1 9.4 27.5 4.14 69.7 S64-10 62.5 I,7 25.7 4.21 70.8

Comp. this is a standard that is brass coated wire with no planned addition of iron. The claimed wires are marked with the letter S at the beginning.

Dissolution of the coating on the intermediate steel wire according to Procedure 1 indicates the presence of iron particles. The X-ray diffraction pattern shows that there was no beta(b)-brass peak at two theta (2θ) angle of 43.3°, where a peak would be expected in the presence of beta-brass, and this is for all samples of the invention.

After this, the intermediate wire with a brass coating enriched with iron particles is drawn to a final diameter of 0.28 mm by wet drawing through smaller dies in a lubricant. The lubricant contains high-pressure additives, which usually contain phosphorus in organic compounds. During wet wire drawing, two types of dies were compared.

Set W: All drawing dies are made of tungsten carbide; including the last three portages, the last of which is a composite portage.

Set D: At least the composite die is a sintered diamond die, and the remaining dies are tungsten carbide dies.

Thus, the total true elongation applied to an intermediate steel wire with a brass coating enriched with iron particles is 3.91.

If we consider a particle of iron in the brass coating of an intermediate wire, this particle undergoes an elongation (D/d) ² in the direction of the wire, that is, in the longitudinal direction. At the same time, the particle is compressed in the radial and circumferential directions with a coefficient (d/D). This is assuming that the iron is incompressible. This means that the iron particles present in the 1.98 mm intermediate wire are elongated by approximately 50 times when drawn to a diameter of 0.28 mm. Since iron cannot withstand such high elongation, larger particles of the intermediate wire are ground, crushed, broken into particles of 10-10000 nm, which can be confirmed by procedure 1 above.

Fig. 1 shows the surface of S64-8-D filament after brass removal under a scanning electron microscope (SEM). Various iron particles 102 (7.5 µm), 102' (6.8 µm), 102 (8.5 µm), 104 (1.0 µm), 104' (1.0 µm) were detected, which were pressed onto thread-like backing. The particle size - the points most distant from each other - is no more than 8.5 microns.

Smaller iron particles (104, 104') can be even smaller than the specified size of 1.0 µm when other techniques such as High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) are used. Particles with a size of 120 nm can be found inside the brass coating: see Fig. 1b, which shows the concentration of iron in the coating. An iron particle is visible, indicated by an arrow. A dotted line has been added to make the outer edge of the cover easier to see.

There appears to be a correlation between the size of the largest particles and the amount of iron included in the brass coating of the intermediate steel wire: the more iron included,

- 8 045969 the larger the particles.

Three steel strands were twisted into 3x0.28 ST cord, and the surface phosphorus and iron residues in the resulting cord were determined in accordance with Procedure 3. The results are presented in Table II:

Table II

Sample Ps (mg/ ^m2 ) Fe _s (mg/m ² ) Fes/Ps sew (g/m ² ) Fes/(SCWxPs) Ref-W 1.5 36.2 24.0 2.15 11.2 S64-1-W 1.5 32.2 21.1 2.13 9.9 S64-2-W 1.7 38.4 22.6 2.09 10.8 S64-4-W 1.9 43.2 22.4 2.01 10.8 S64-6-W 2.6 52.3 20.3 2.11 9.6 S64-1-D 0.8 34.3 41.9 2.00 21.0 S64-2-D 0.8 34.5 42.1 1.98 21.2 S64-6-D 1.0 35.7 35.8 1.96 18.3 S64-8-D 1.5 46.9 31.3 1.98 15.8 S64-10-D 1.9 55.5 29.5 1.91 15.4

Ref-W is a standard, which is a brass-coated wire without iron particles, drawn in W set dies.

It is clear from Table II that set W dies result in more phosphorus as well as more iron on the surface. Set D dies result in lower phosphorus and iron concentrations for the same amount of iron added to the coating. The Fe/Ps ratio is always higher than 27 for yarns drawn with Set-D dies. When using the ratio (Fe _s /SCW)/P _s the difference is even more obvious and the relative range of values is reduced. All values for wire drawn using Set D have this ratio above 14 and even 15, while for wire drawn using Set W this ratio is below 13 and even below 11.

In the series of figs. 2a-2e, 3a-3e present the results of determining the adhesion of various samples of the table. II in five mixtures of Group I (which contain organic cobalt salts) and Group II (which do not contain intentionally added cobalt). Adhesion measurement results are peel forces determined in accordance with ASTM D2229-04, which is described in more detail in the BISFA (Bureau International Standards for Manmade Fibers) brochure Internationally Harmonized Test Methods for Steel Cord. Edition 1995, D12 Determination of static adhesion to rubber compounds in accordance with specified conditions (pre-vulcanization, conventional vulcanization, re-vulcanization). In this test, steel cords are embedded in rubber in the form of a block and pulled axially from the rubber after vulcanization. The maximum force achieved (in H) is noted. The average of the 24 individual maximum forces (in H) is referred to as the pull-out force (POf).

For each of the ten compounds, conventional cure (RC) conditions are specified as the time TC90 plus 5 minutes, TC90 being the time at which a particular rubber reaches 90% of its maximum torque on the rheometer curve taken at the cure temperature. An over-vulcanized (OC) condition occurs when the rubber is vulcanized significantly longer than the normal vulcanization time, in this application twice the normal vulcanization time. Under-vulcanization (UC) is carried out by vulcanizing rubber only half the time of normal vulcanization.

To determine adhesion retention, the following aging conditions are applied to RC vulcanized specimens:

Post-vulcanization humidity (CH): RC specimens are kept at 93°C and 95% relative humidity for 14 days.

After steam aging (SA): RC samples are steam aged at 120°C for 2 days.

Hereinafter, any of the vulcanization conditions UC, RC or OC or any of the aging conditions CH or SA will be referred to as a Condition.

The adhesion test results are presented as the Z-score deviation from the reference average (RA) in FIG. 2a-2e and 3a-3e. The reference average RA value, designated 0 in all figures, is equal to the sample weighted average Ref-W value for all Group I cobalt-containing mixtures and for the specific Condition according to this figure. The statistical standard deviation of all results obtained on the Ref-W sample in Group I mixtures for a particular Condition is calculated and called the reference standard deviation (RSTD) for that condition. Briefly: the standard is a known brass (Specimen Ref-W) - cobalt-containing rubber system (Group I) under each of the conditions mentioned in the figure legend.

For each of groups I and II and for each of the samples (Samples) table. II pull-out force is determined for each Condition. The pullout forces are the weighted sample averages (SA) and

- 9 045969 calculated statistical standard deviation, called sample standard deviation (SSTD) for this Group and Condition.

The Z-score of a sample in a Mixture Group for a particular Condition is thus equal to the difference between the sample mean for that Group and the Condition minus the reference sample mean for that Condition, divided by the pooled standard deviation of the reference standard deviation and the sample standard deviation. Short:

—YA

Z = --((¾ - IJSSTD ² + (Ay JN _s + N _R -2

Where N _S is the number of results aggregated to obtain SA and SSTD, and NR is the number of results aggregated to obtain RA and RSTD.

Z-scores indicate how much deviations from the means are statistically significant compared to the reference mean, that is, the current condition for the particular Group Condition in which the sample was tested:

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

Z-scores between -2 and -1 indicate possible deterioration but are not statistically significant;

Z-scores between -1 and +1 indicate that no statistically significant deterioration or improvement in the reference mean can be concluded;

Z-scores of +1 - +2 indicate possible improvement but are not statistically significant;

Z-scores above +2 represent a statistically significant improvement in the current condition.

Regarding group I, i.e. cobalt-containing mixtures, the following conclusions can be drawn:

Fig. 2a: In the scorch state, the presence of iron particles in the brass coating....

..does not result in statistically significant improvement or deterioration compared to the reference average when using Set-W dies;

... can lead to improvement over the reference average when using Set-D dies;

The best results under UC conditions are achieved when less iron is introduced into the coating.

Fig. 2b: Under normal vulcanization conditions, the presence of iron particles in the brass coating...

...does result in marginal improvement when using Set-W dies.

...really improves when using Set-D dies. However, the improvement is not statistically significant.

The number of iron particles included does not have a significant effect.

Fig. 2c: Under over-vulcanization conditions, the presence of iron particles in the brass coating does not lead to statistically improved results compared to the prior art. However, there is no indication that the invention may lead to a decrease in quality: all Z-scores are positive throughout.

Fig. 2d: After curing under wet conditions, use of the invention results in a statistically significant improvement when more iron (8-10 wt%) is introduced into the brass coating using Set-D dies. There is no significant improvement on other samples. In general, the use of the invention does not lead to deterioration of the results.

Fig. 2e: the invention leads to a high, statistically significant improvement in adhesion retention results after steam aging when using Set-D dies. The results improve even further with increasing iron content in the coating. Using Set-W dies does lead to an improvement, but it is not statistically significant.

The inventors concluded that their invention could be used to replace currently used steel cord with currently used cobalt-containing mixtures without the risk of possible deterioration in adhesion results or problems maintaining improved adhesion. On the contrary: when using Set-D dies, the results of adhesion retention after steam aging are significantly and statistically significantly improved.

With regard to group II, that is, mixtures that do not contain intentionally added cobalt, the following conclusions can be drawn:

Fig. 3 a: When subvulcanizing, the invention does not lead to significant improvement or deterioration compared to the prior art (ie: brass-coated Set-W steel cord in cobalt-containing mixtures). Overall, there is a slight tendency that higher iron content may lead to poorer prevulcanization results. This tendency is less pronounced when using Set-D dies.

Fig. 3b: Under normal vulcanization conditions, all Z-scores are positive, indicating

-

Claims (15)

that no undesirable effect of the invention should be expected. The improvements are not statistically significant. Regarding the results of re-vulcanization, the same conclusions can be drawn as in the case of Fig. 3c: The steel cord according to the invention is better, but the improvement is not statistically significant. Fig. 3d: The invention shows a statistically significant improvement in vulcanization results under humid conditions at higher iron contents (6 wt%, 8 wt%, and 10 wt%) and using Set-D drawing dies. The remaining results remain statistically insignificant. Fig. 3e: The invention shows a noticeable and statistically significant improvement over the prior art for Set-W and Set-D dies after steam aging. There is a clear indication that increased iron content in the coating leads to improved results, but only up to 8 wt.% iron. In fig. 3a-3e: Brass refers to results obtained with Set-W reference wires when tested in Group II mixtures. In conclusion, it has been demonstrated that the inclusion of iron particles in the brass coating results in improved adhesion retention in mixtures that do not intentionally contain added cobalt, as well as in mixtures that do contain cobalt. The invention was specially developed for the reinforcement of rubber products such as tires, hoses or belts, to completely eliminate the presence of cobalt in rubber, as well as in the coating of steel cord. CLAIM 1. A steel cord containing one or more threads, said threads containing a steel thread-like substrate and a coating partially or completely covering said steel thread-like substrate, said coating containing brass consisting of copper and zinc, said coating enriched with iron, characterized in that said iron is present in said brass in the form of particles, said particles having a size of 10-10,000 nm. 2. Steel cord according to claim 1, in which said particles have a size of 20-5000 nm. 3. Steel cord according to claim 1 or 2, wherein said brass contains at least 63% copper by weight and the balance is zinc. 4. Steel cord according to any one of claims 1 to 3, wherein the amount of iron in said coating is greater than or equal to 1% by weight and less than 10% by weight compared to the total weight of brass and iron. 5. The steel cord of claim 4, wherein the amount of iron in said coating is greater than or equal to 3% by weight and less than 9% by weight compared to the total weight of the brass and iron. 6. Steel cord according to any one of claims 1 to 5, wherein said coating is substantially free of a zinc-iron alloy. 7. The steel cord according to any one of claims 1 to 6, wherein phosphorus is present on the surface of said thread, wherein the amount of phosphorus present on the surface of said thread is P _s and the amount of iron present on the surface of said thread is Fe _s wherein said P _s and Fe _s are determined using the 3 P _s and Fe _s procedure as defined herein and expressed in milligrams per square meter, wherein said P _s is less than or equal to 4 mg/m ² and greater than zero. 8. The steel cord according to claim 7, wherein the amount of iron present on the surface of Fe _s is greater than or equal to 30 mg/m ² . 9. Steel cord according to claim 7 or 8, in which the ratio of Fe _s to P _s exceeds 27. 10. The steel cord according to any one of claims 7 to 9, wherein the mass of the surface coating SCW is the sum of the mass of brass and iron present in said coating per unit surface area, said mass of the coating is expressed in grams per square meter, wherein the ratio Fe _s to product P _s and SCW coating weight greater than 13. 11. Steel cord according to any one of paragraphs. 1-10, in which the specified steel cord consists of a single thread. 12. A rubber product comprising vulcanized rubber reinforced with steel cord according to any one of claims 1 to 11, wherein said rubber product is one of the group consisting of a tire, a passenger car tire, a truck tire, a van tire, a tire for SUV, hose, hydraulic hose, belt, timing belt, conveyor belt, elevator belt. 13. The rubber product of claim 12, wherein said vulcanized rubber is substantially free of cobalt. 14. The use of steel cord according to any one of claims 1-11 for reinforcing a rubber product. 15. A method for producing steel cord thread according to any one of claims 1-11, including the following stages: -