Brass coated steel cord with increased surface iron content

Abstract

The present invention provides a steel wire for twisting into a steel cord to reinforce rubber products such as tires, hoses and conveyor belts. The steel wire includes a steel substrate coated with a coating comprising brass. The coating differs in that the iron content on its surface is significantly higher than that of the steel wire of the prior art. In a layer extending from the surface to a depth of 3nm below the surface, the average iron content of the coating is 4 atomic percent or more compared to the total amount of iron, zinc and copper atoms. The present invention describes a steel cord made from the steel wire and a rubber product comprising the steel cord. The present invention provides a method for manufacturing a steel wire with an increased surface iron content and an alternative method. The steel wire of the present invention shows improved adhesion retention under hot and wet conditions, which is true in rubbers containing organic cobalt compounds and rubbers that are substantially free of cobalt. The use of the present invention can extend the life of rubber products.

Description

Technical Field

[0001] The present invention relates to a brass coated steel cord for use in reinforcing rubber products such as tires, hoses, conveyor belts and the like, hereinafter referred to as "rubber products". Background Art

[0002] High-performance vulcanized rubber compounds are special compounds in which brass-coated steel cords provide strength and stiffness to elastomeric tuff vulcanized rubber. This compound is critical to the performance of rubber products used in harsh, extreme environments, such as tires.

[0003] For a composite to last, it is of utmost importance that the adhesion between the low modulus vulcanized rubber and the high modulus steel cord is high initially and remains good throughout the service life of the rubber article.

[0004] When vulcanizing brass coated steel cord in sulfur-based vulcanization system rubber, a significant reaction occurs between the copper in the brass and the sulfur in the rubber. (2-x) S (where "x" is about 0.2) grows into the rubber like a tree. The thickness of the "bonding" layer formed is less than 250nm or even less than 100nm. This provides an initial strong bond. First, the main oxide on the surface of the brass layer is zinc oxide. This zinc oxide slows down the reaction between copper and sulfur-donating substances during the vulcanization process. In fact, the stoichiometric growth of Cu2S should be prevented because it will produce brittle crystals and thus lead to low initial adhesion. After vulcanization, it is usually with Cu (2-x) S produces some zinc sulfide together with the zinc oxide layer growing as an intermediate layer between the sulfide and the remaining metallic brass.

[0005] On the other hand, during the long-term use of rubber products, especially in hot and humid environments, the adhesive layer deteriorates and the bond is no longer strong. One of the main mechanisms of this deterioration is the "dezincification" of brass, which is due to the zinc ion (Zn 2+ ) diffuses from the brass layer to the bonding layer, forming zinc oxide and hydroxide in the bonding layer, resulting in the weakening of the bonding layer.

[0006] To overcome this problem, cobalt-based organic salts such as cobalt naphthenate, cobalt stearate or cobalt borodecanoate compounds are added to skim compounds, in addition to other additives such as carbon black, sulfur, accelerators, oils, antioxidants, activators, etc. Skim compounds are special rubber mixtures used to encapsulate steel cords. These cobalt-based organic salts can: (1) inhibit the formation of zinc sulfide (ZnS) bridges that have poor bonding between brass and rubber, thereby promoting the formation of non-stoichiometric dendritic copper sulfide during bond formation, and (2) inhibit the mechanism of zinc ion diffusion out of the brass layer under hot and humid conditions, thereby improving bond retention.

[0007] However, the addition of cobalt-based organic salts also has its disadvantages, because cobalt-based organic salts act as oxidation catalysts for diene rubber bonds, thereby accelerating rubber aging and ultimately leading to rubber failure near the steel cord. However, the main disadvantage of these cobalt-based organic salts is that they may be carcinogenic, so their use is increasingly restricted.

[0008] In order to avoid the use of cobalt-based organic salts, it is recommended to incorporate cobalt in metallic form into the brass coating rather than incorporating cobalt in organic form into the rubber. See, for example, US4255496 and US4265678. This ternary alloy layer does provide a very good adhesion retention effect under hot and humid conditions. However, they cannot completely eliminate the organic cobalt salts in the rubber. The recent work of the applicant (disclosed in WO2011 / 076746, WO2013 / 117248, WO2013 / 117249) further provides a solution that can use a ternary alloy coating in a cobalt-free compound. Although the use of cobalt in the tire is significantly reduced by this technology, the steel cord still contains cobalt, so cobalt is still present in the production environment.

[0009] Other coating metals have also been considered as alternatives to cobalt, such as nickel and iron (see, for example, Chapter 6 "Rubber-brass bonding" by WJ van Ooij, "Handbook of Rubber Bonding", Lapner Technologies Ltd., 2001, p. 176). However, these metals are considered too slow in the case of iron or too inactive in the case of nickel.

[0010] However, US4446198 suggests using a ternary copper-iron-zinc coating instead of a copper-cobalt-zinc coating. It is well known that the rubber on the surface of the steel cord can be exposed to a certain amount of iron (derived from the steel substrate), which helps to maintain and increase adhesion (see page 429 of "Mechanism and theories of rubber adhesion to steel tire cords–an overview", WJ van Ooij, RUBBER CHEMISTRY AND TECHNOLOGY, Vol. 57, pp. 421-456, 1984). One way to increase the iron content on the surface of the steel cord is to reduce the brass content, thereby exposing part of the steel substrate to the rubber. However, this has its limitations because the surface of the steel cord must retain enough brass to increase adhesion.

[0011] The inventors therefore set themselves the task of improving the iron distribution in order to improve the bond retention. Summary of the invention

[0012] Therefore, the main object of the present invention is to overcome this main problem of using cobalt in rubber products: firstly, to eliminate cobalt in the steel cord; secondly, to use a rubber compound that is substantially free of cobalt. Another object of the present invention is to provide a steel wire having a suitable iron distribution on its surface. Another object of the present invention is to provide a steel wire whose iron depth distribution is selected based on improved adhesion retention. Still another object of the present invention is to provide a steel cord wherein part or all of the steel wire has a preferred iron depth distribution. Still another object of the present invention is to provide a method of manufacturing the steel cord of the present invention.

[0013] According to a first aspect of the invention there is provided a steel wire having the features of the preamble of claim 1. As stated in a second aspect of the invention the steel wire is comprised in a steel cord for reinforcing a rubber article.

[0014] The diameter of the wire is "d" (expressed in millimeters). For the purposes of this application, the diameter "d" is the diameter of an imaginary circle having the same surface area as a vertical cross-section of the wire. For example, the wire may have a circular cross-section, and the diameter "d" is equal to the diameter of the circular wire.

[0015] Alternatively, the wire may present a cross section other than circular, such as an ellipse, or a regular or irregular polygon, or a mixed polygon with straight sides and curved borders (e.g. an initially circular wire flattened on one, two or more sides). These are all possible vertical cross sections of the wire, the only requirement being that they are convex.

[0016] The steel wire comprises a steel substrate and a coating comprising brass. For the purposes of this application, the term "brass" relates to an alloy formed from, i.e. comprising, copper and zinc. Any other element, whether metallic or non-metallic, added to the coating intentionally (e.g. phosphorus or iron) or unintentionally (e.g. oxygen) is not considered to be part of the brass.

[0017] The average thickness of the coating, determined by the total mass of copper and zinc in the brass, is 450 x d nanometers or more. For example, the average coating thickness for a wire with a diameter of 0.30 mm is 135 nm or more. "Average" means that the thickness is determined over the entire circumference of the wire and over a considerable length (e.g., a length of 100 times the diameter "d"). In other words, the "average thickness" is an overall measurement of the thickness, not a local measurement.

[0018] The mass of copper in the brass is between 61 and 75 mass percent, compared to the total mass of copper and zinc in the brass. For the purposes of this application, the mass percentage will be abbreviated as "wt %". More preferably, the mass percentage of copper is above 62 wt %, or above 63 wt %, or even above 64 wt %. If the mass percentage of copper is too low, there is a risk of forming too much β-brass. β-brass is a harder brass phase that is more difficult to deform than the more ductile α-brass formed at a higher mass percentage of copper. When the mass percentage of copper in the brass is above 75 wt %, there is a risk of forming brittle copper sulfides. Therefore, a lower mass percentage of copper is preferred, for example a brass containing less than 73 wt %, or less than 71 wt %, or even less than 69 wt % copper.

[0019] The copper and zinc content of the coating is determined by wet chemical analysis methods. These methods are well known to those skilled in the art. In these methods, a large number of steel wires are sampled and weighed, the copper and zinc are stripped from the steel substrate using a stripping solution, the stripping solution is diluted to a standard volume, and the mass of copper and zinc is determined by one of the following techniques:

[0020] ax-ray fluorescence spectroscopy;

[0021] b. Inductively coupled plasma spectroscopy;

[0022] c. Atomic absorption spectroscopy.

[0023] Details of this procedure may be found in entries E11 / 1, E11 / 2 and E11 / 4 of BISFA's "Internationally agreed methods for testing of steel tyre cord". BISFA is "The International Bureau for the Standardisation of man-made fibres".

[0024] Then, the mass percentage of copper (Cu (wt%)) is calculated as follows:

[0025]

[0026] If the sum of the masses of copper and zinc per unit mass of steel cord is "B" g / kg, the average coating thickness "t (expressed in nanometers)" of a steel wire with an equivalent diameter "d" can be calculated by the following formula:

[0027] t=231.8×B×d

[0028] The steel wire is characterized in that the top surface of the coating has an abnormal iron content. The surface iron content is determined by X-ray photoelectron spectroscopy, a surface analysis technique commonly abbreviated as "XPS". This technique is also known as "ESCA", which stands for "Electron Spectroscopy for Chemical Analysis". More specifically, the coating has an average iron content of 4% or more atomic percentage in a first layer extending from the surface of the steel wire to a depth of 3 nm below the surface. The iron content is expressed as atomic percentage (at%) relative to the total amount of iron, copper and zinc. No other factors are taken into account in the denominator.

[0029] In XPS, electrons from the sample surface are emitted from surface atoms after being excited by photons in a focused X-ray beam. From the kinetic energy distribution of these "photoelectrons", conclusions can be drawn about the composition of the probed surface atoms (and their chemical bonding state). The diameter of the roughly circular X-ray beam is about 100 μm. The wavelength of the X-ray photons corresponds to the wavelength of the K-α line of aluminum. The depth of detection is only a few nanometers below the surface. Therefore, the volume detected is equivalent to a very thin disk. In order to obtain the depth distribution, the surface atoms are sputtered away by an argon ion sputtering beam during the standardized time period between X-ray photoelectron measurements. The intensity of the argon ion sputtering beam is calibrated so that 1nm of α-iron is sputtered away in 10 seconds. For the purpose of this application, a ratio of 10 seconds of sputtering time per nanometer surface layer will be maintained. Based on the kinetic energy of the photoelectrons, the atomic species of the surface can be identified.

[0030] Although XPS analysis can easily identify various elements such as carbon, oxygen, and sulfur, for the purposes of this application, only metallic copper, zinc, and iron were determined. Since the number and energy of the photoelectrons are measures of the abundance and atomic number of the surface atoms, respectively, the atomic percentage of iron can be tracked using the different depths (x i ), the measurement before sputtering is represented by a count of "0", and the last measurement point is represented by "N" at a depth of "Δ":

[0031]

[0032] Among them, #Fe, #Cu, #Zn represent the counts of photoelectrons filtered corresponding to the corresponding energies of these elements.

[0033] Afterwards, a distance "x" must be obtained between the surface and the sputtering depth "Δ" of at most 3 nm. i "Function (Fe) i The depth distribution of the Fe atoms can be averaged. Preferably, at least 4 measurements at different depths below the surface up to a depth of 3 nm must be performed. For example, the Fe atoms (Fe) can be determined at sputtering times of 0, 6, 12, 18, 24 and 30 seconds. i The abundance of iron is calculated, with "i" being counted from 0, 1, 2, 3, 4 to 5. Preferably, the measured depths are equidistant or at least uniformly distributed. The iron depth distribution curve is then integrated by the trapezoidal rule and divided by the depth "Δ" to obtain the average value of the iron abundance:

[0034]

[0035] In this way, an indication can be obtained of how many iron atoms are present within the disk-shaped volume probed to the above-mentioned depth by the X-ray beam.

[0036] Although a focused X-ray photon beam will average out large differences in iron on the surface due to its relatively large beam size, it is better to take measurements at four different points on the wire surface and use the average of the four numbers obtained as the final measure of the iron content in the top 3nm layer of the coating.

[0037] Preferably, the steel in the steel substrate is made of plain carbon steel, and its composition is within the following range (all percentages are percentages by mass, abbreviated as "wt %"):

[0038] - a carbon content of 0.60 wt% to 1.20 wt%, for example 0.80 wt% to 1.1 wt%;

[0039] - a manganese content of 0.10 wt% to 1.0 wt%, for example 0.20 wt% to 0.80 wt%;

[0040] - a silicon content of 0.10 wt% to 1.50 wt%, for example 0.15 wt% to 0.70 wt%;

[0041] - a sulfur content of less than 0.03 wt%, for example less than 0.01 wt%;

[0042] - A phosphorus content below 0.03 wt%, such as below 0.01 wt%.

[0043] By subjecting the steel to a strain hardening operation, such as wire drawing, it is possible to obtain steel wire having a tensile strength exceeding 2500 MPa or above 3000 MPa or even above 3500 MPa.

[0044] Microalloying of steel helps to obtain steel wire with higher tensile strength. The mass percentages of alloying elements are in the following ranges: chromium: 0.10wt% to 1.0wt%; nickel: 0.05wt% to 2.0wt%; cobalt: 0.05wt% to 3.0wt%; vanadium: 0.05wt% to 1.0wt%; molybdenum: 0.05wt% to 0.60wt%; copper: 0.10wt% to 0.40wt%; boron: 0.001wt% to 0.010wt%; niobium: 0.001wt% to 0.50wt%; titanium: 0.001wt% to 0.50wt% t%; Antimony: 0.0005wt% to 0.08wt%; Calcium: 0.001wt% to 0.05wt%; Tungsten: for example, a content of about 0.20wt%; Zirconium: for example, a content of 0.01wt% to 0.10wt%; Aluminum: preferably a content below 0.035wt%, such as below 0.015wt%, such as below 0.005wt%; Nitrogen: a content of less than 0.005wt%; Rare earth metals (wt% REM): a content ranging from 0.010wt% to 0.050wt%. Microalloying allows a tensile strength of more than 3500MPa, or higher than 3700MPa, or even greater than or equal to 4000MPa to be achieved.

[0045] In another approach, a low carbon steel can be used which has been deeply drawn in order to achieve sufficient tensile strength. A typical steel composition would then have a carbon content of less than 0.20 wt%. For example, the carbon content is between 0.04 wt% and 0.08 wt%, the silicon content is 0.166 wt%, the chromium content is 0.042 wt%, the copper content is 0.173 wt%, the manganese content is 0.382 wt%, the molybdenum content is 0.013 wt%, the nitrogen content is 0.006 wt%, the nickel content is 0.077 wt%, the phosphorus content is 0.007 wt%, and the sulfur content is 0.013 wt%, all percentages being by mass. The ultimate tensile strength of these wires is considerably lower: it is above 1200 MPa or even above 1400 MPa, but the carbon footprint of these wires is reduced due to the omission of intermediate heat treatments.

[0046] In a further improved first embodiment, the iron content in the disc-shaped volume is at least 4 atomic percent, or even 5 atomic percent. In any case, the iron content does not exceed 10 atomic percent, and may be less than 7.5 atomic percent. The advantage of the increased surface iron content is that the adhesion retention under wet heat conditions is improved both in compounds without any added cobalt and in the cobalt-containing compounds currently used.

[0047] Preferably, the iron content, starting from an already high level in the first layer, increases steadily with increasing measured depth. According to a second embodiment, the average iron content in the second layer extending from the surface to a depth of 9 nm below the surface is therefore higher than 5 atomic percent or even higher than 6 atomic percent.

[0048] According to a third embodiment, the brass coating has an average iron content of 6, 8 or 10 atomic percent or more when sputtered to a depth of 20 nm below the surface of the steel wire (third layer).

[0049] According to a fourth embodiment, the average iron content in the third layer is less than 20 atomic percent, or less than 15 at%, or even less than 13 at%, or less than 11 at%, which percentage refers to the percentage of the total amount of iron, zinc and copper atoms detected in the third layer. In fact, too high an iron content may lead to a lack of brass, which is required for the final adhesion. In the third layer (0-20nm), it seems that an iron concentration of 8-11 at% brings the best balance for adhesion. Although values ​​above 11 at% increase the adhesion retention in a wet and hot environment, its initial adhesion is not very satisfactory. Values ​​below 8% produce good initial adhesion, but the increase in adhesion in a wet and hot environment is not optimal.

[0050] The balance between iron and brass is delicate: too much brass on the steel cord may cause other adhesion retention problems, so the average thickness of brass is best kept below 1350×d or less, where "d" is the diameter of the steel cord in millimeters. For example, the average thickness can be kept below 1200×d or less, or even below 1000×d or less.

[0051] In another embodiment, additionally, the iron is finely dispersed within the brass coating. A method of verifying this is to observe the surface of the steel wire under a scanning electron microscope operated in backscattered electron mode (BSE). These electrons are elastically backscattered ("reflected") from the nuclei of the atoms on the probed surface and represent the average atomic weight of the atoms on the probed surface. Heavier elements such as copper and zinc are able to return electrons more efficiently than lighter atoms such as iron. Therefore, on a gray tone, a thicker brass coating portion on the surface of the steel wire will appear lighter than a thin brass coating on a bare steel surface, and therefore a thin brass coating on a bare steel surface will appear darker.

[0052] When the surface of the steel wire is now observed in BSE mode, the coating will show alternating thin brass bands and thick brass bands along the length of the steel wire. The thick brass bands contain more copper and zinc than iron and appear relatively lighter than the thin brass bands, which contain more iron atoms. Therefore, in the context of this application, the terms "lighter" and "darker" should be understood as being related to each other, similar to the terms "thin" and "thick". Even an inexperienced electron microscope user can easily adjust the electron beam to obtain maximum BSE contrast on the steel wire surface.

[0053] The special feature of this embodiment is that in the thick brass strip, iron is present at least on the surface. The average iron content is at least 4 atomic percent or more of the total amount of iron, copper and zinc. The average is taken from the top 3nm of the surface of the thick brass strip.

[0054] In this case, the iron content is determined by scanning Auger electron spectroscopy (AES), which allows a spatially very narrow probing (approximately the size of the impinging electron beam, i.e. a diameter of 100 nm) from the surface to a very small depth (for the element in question, this depth is about 0.5 nm). As in the XPS measurement, the depth profile is formed by subsequent ablation of the top layer by means of argon ion sputtering of atoms. By subsequently integrating the iron profile using the trapezoidal rule and dividing by the total depth of the profile, the average iron content can be determined.

[0055] Alternatively, the average iron content detected in the top 3 nm layer of a thick brass strip is equal to 6 atomic percent or more of the total amount of iron, copper and zinc.

[0056] Additionally or alternatively, the average iron content detected within a depth of 9 nm from the top of the surface of the thick brass strip is equal to or higher than 8 atomic percent or more of the total amount of iron, copper and zinc.

[0057] According to a second aspect of the present invention, there is provided a steel cord. The steel cord comprises one, two or more filaments assembled together. The steel cord is the steel cord according to one or more of the above embodiments.

[0058] In another preferred embodiment of the invention, the steel cord comprises a single filament. Such a single filament can be used in a tire, for example as a bead reinforcement in the bead area, or as a belt reinforcement in the belt area ("monofilament"). Such a monofilament can be made into a larger diameter, for example 0.25 to 0.70 mm, for example 0.30 to 0.35 mm, for reinforcement of the tire belt, or a diameter of 0.70 to 2.10 mm for bead reinforcement. Optionally, a single steel wire according to the invention can also be used as a hose reinforcement wire.

[0059] "Comprising" means that in addition to steel wires, other non-steel wires, such as aramid or high-density polyethylene fibers, may also be mixed with the steel wires. Alternatively, the steel cord may also consist of steel wires only.

[0060] Assembling steel cords according to known processes and techniques:

[0061] a. A single-layer cord, such as "n×d", in which "n" filaments of diameter "d" are twisted together to form a strand with a certain twist direction and twist length. "n" can be 2-5, up to 6, and "d" varies between 0.10-0.50 mm. The filaments may be deformed to obtain an "open cord", in which the filaments form independent spirals that are loosely in contact with each other. An exemplary cord is 3×0.30OC;

[0062] b. In a multilayer cord, each layer is wound around a core with a certain twist length and twist direction, which can be a single filament or a single strand. Multilayer cords are usually expressed as "core + m × d", where "core" is equal to "n × d" or a single filament with a diameter of "d", for example. For example, 1 + 6: such as 0.32 + 6 × 0.30; or 3 + 9: such as 3 × 0.22 + 9 × 0.20, or a three-layer cord, such as (3 + 9 + 15) × 0.175, where all filaments have the same diameter. When the twist direction and twist length of all layers are equal to each other, the filaments form a compact structure called "compact cord", for example 27 × 0.175 mm.

[0063] c. When different strands are combined and twisted around each other, a multi-strand cord of the M×N type is obtained, for example a multi-strand cord of the 4×(1+6) type, in which 4 strands of 7 filaments are twisted together with a cord twist pitch and a cord twist direction. When the strand twist direction is opposite to the cord twist direction, for example the strands are in the "s" direction and the cords are in the "Z" direction, this is called a conventional twist direction. When the cord has the same twist direction as the outer strands, it is called a "Lang twist direction". The latter cord can achieve a much higher elongation;

[0064] d. A special multi-strand cord is a cord comprising at least two strands "L" and "T" having different lay lengths. The lay length of the twisted strand "T" ("twisted") is shorter than the lay length of the strand "L" ("long lay length"), the latter having, for example, an almost infinite lay length. The two strands are intertwined with each other at a cord lay length and a cord twist direction. A particularly preferred embodiment is that the lay length and the twist direction of the strand "T" are equal to the lay length and the twist direction of the cord. Particularly preferred examples are 2+2, 2+3, 2+4, 3+2, 3+3, where the first number refers to the number of filaments in the "L" strand and the second number refers to the number of filaments in the "T" strand.

[0065] According to a third aspect of the invention, a rubber product is claimed which is reinforced with a steel cord as defined above. The rubber product may be, for example, a hose, a belt (e.g. a conveyor belt or an elevator belt), a tire for a passenger car, a truck, a van or an off-road machine, or any other rubber-based article which may be reinforced with a steel cord. All of these products are manufactured and assembled in a manner known or which will become known in their respective fields. The only difference is that the steel cord used for reinforcement comprises filaments having an average iron content of 4 atomic percent or more in a first layer extending from the surface of the filaments to a depth of 3 nm below the surface, the iron content being expressed as an atomic percent relative 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 cord.

[0066] According to a third aspect, a particular embodiment of the invention is a rubber product, the vulcanized or unvulcanized rubber of which is substantially free of cobalt or cobalt-containing compounds. In fact, the steel cord is invented to be combined with adhesive rubber compounds, called "skim compounds", which substantially do not add cobalt or organic cobalt compounds to the rubber.

[0067] "Essentially not added" means that in the vulcanized rubber, the cobalt content detectable by X-ray fluorescence is less than 100 μg / g rubber (0.01 wt% cobalt relative to the mass of rubber), or less than 50 μg / g rubber (0.005 wt% cobalt), or even less than 20 μg / g rubber (0.002 wt% cobalt relative to the mass of rubber) or 10 (0.001 wt% cobalt) μg / g rubber.

[0068] Since skim compounds often contain organic cobalt compounds, it is best to analyze the rubber near the steel cord, such as the residual rubber that adheres to the steel cord when it is pulled from the rubber product. This is where the cobalt content is highest.

[0069] A particular advantage of the present invention is that it is still compatible with currently used cobalt-containing compounds, which has a positive effect on adhesion and adhesion retention compared to currently used brass steel cords.

[0070] According to a fourth aspect of the present invention, a method for producing the above-mentioned steel wire comprises the following steps:

[0071] a. Select a wire rod having a steel composition as set forth in the product description above. The steel wire rod has a diameter of about 5.5 mm or greater. Descale the wire rod and coat it with a soap carrier according to techniques known in the art;

[0072] b. The wire rod is dry drawn into an intermediate wire. The diameter of the intermediate wire is between 0.50 mm and 3.20 mm, for example, 1.20 mm to 2.50 mm;

[0073] c. The intermediate wire is then lead-quenched (patent). Lead-quenching is an operation known to those skilled in the art. In the lead-quenching operation, the wire is heated to above the austenitizing temperature of the steel, which is typically between 950°C and 1050°C, to dissolve carbon into the iron lattice. The intermediate wire is then held at a soaking temperature between 550°C and 660°C, thereby restoring the original fine pearlitic steel structure of the wire. This structure is very popular because it can be easily drawn to a finer diameter. The resulting wire is lead-quenched wire;

[0074] d. The wire is then pickled to remove any excess oxides or drawing residues by passing it through a tank containing an acid. Typically, bisulfate or hydrogen chloride or similar strong acids are used for this purpose;

[0075] e. Further electrolytically coating the lead bath quenched wire with copper by known means, adjusting the copper content to achieve the recommended composition and quality. Typical copper plating baths are copper sulfate baths or copper pyrophosphate baths, which have compositions, concentrations, pH values ​​known to those skilled in the art, and are operated at current densities known to those skilled in the art;

[0076] f. Subsequently, the copper-plated wire is electrolytically coated with zinc metal in an amount sufficient to achieve the proposed brass composition. A typical zinc plating bath is a zinc sulfate bath, the composition, concentration, pH value of which are known in the art, and which is operated at a current density known in the art;

[0077] g. In a further step, the brass is formed by diffusing the zinc and copper by heating in a known manner. The heat may be generated by resistive heating of the wire: by an electric current driven through the wire, or by electromagnetically induced eddy currents;

[0078] h. The steel wire according to the present invention is formed by drawing the brass-plated wire in a wet wire drawing machine. In this operation, the brass-plated wire is immersed in a lubricant and drawn through a series of dies with progressively smaller holes.

[0079] The process is characterized in that the intermediate wire has a circumferential arithmetic mean deviation R greater than 0.40 microns. a (or "circumferential roughness" hereinafter), for example greater than 0.50 micrometers or even greater than 0.55 micrometers. If the "roughness" is lower than 0.40 micrometers, its beneficial effect on adhesion will be small. The circumferential arithmetic mean deviation is not higher than 1.00 micrometers, for example lower than 0.95 micrometers. Too high circumferential roughness will lead to a loss of fatigue performance.

[0080] The definitions specified in the international standard ISO 4287:1997 "Geometric product specification (GPS) - Surface structure: Profile method - Terms, definitions and surface structure parameters" apply. "Arithmetic mean deviation R a ” is defined as:

[0081]

[0082] Among them, L e is the calculated length of the wire in the circumferential direction ξ, is the absolute deviation from the radial mean value of the wire axis, where is the calculated length L e Physically, the measured value will be expressed in digital format by replacing the integral with a discrete summation over the entire calculation length, where N discrete deviation measurements r i (i from 1 to N) spans the entire calculation length.

[0083] Preferably, the roughness is determined by a stylus device, where the stylus measures the change in the radial direction of the wire as it rotates. The recommendations of ISO 4288:1996 should be followed, and for expected roughness between 0.40 microns and 1.00 microns, a minimum calculation length L must be followed. e The requirement is 4 mm, and the cut-off length λ c The cutoff length λ used in the analysis is 0.80 mm. c Used to distinguish between waviness and actual roughness of the profile. If the wire diameter is less than 1.27 mm, an overlap must be introduced to achieve the required calculation length. Optical methods based on vertical cross-sections of the wire can also be used if the same analysis method is followed.

[0084] According to an improvement of the method for producing steel wire, one or more dies containing diamond are used in one or more of the final passes of wet drawing. Non-exhaustive examples of the meaning of "die containing diamond" include: dies made of a single natural diamond, a single synthetic diamond, a compact of diamond particles sintered together ("sintered diamond"), carbonaceous ("black diamond") or polycrystalline diamond ("PCD die"). The experience of the inventors is that when using dies containing diamond, combined with an increased circumferential roughness on the intermediate metal wire, the iron content of the surface increases compared to the commonly used and accepted tungsten carbide dies.

[0085] Another improvement of the method is that the pickling step of the intermediate wire is carried out in hydrochloric acid, wherein the concentration of iron (III) is between 6 and 15 g / l, or even between 7 and 15 g / l, or between 8 and 13 g / l. The concentration of iron (III) ions in hydrochloric acid is abnormally high. The inventors have experienced that such a high concentration leads to increased corrosion of the surface of the metal wire and thus to increased roughness. The increase in roughness leads to an increase in surface iron.

[0086] In another alternative of the method, after the step of electrolytically coating the lead-bath-quenched copper-plated wire with a zinc coating (step "f"), the wire is led through an acidic bath containing iron(II) cations. Only after this, copper and zinc diffuse into the brass (step "g"). The surface zinc atoms are replaced by iron atoms and the surface iron content is increased.

[0087] In another alternative of the method, after the step of electrolytically coating the lead-bath-quenched copper-plated wire with a zinc coating (step "f"), the wire is led through an acidic bath containing iron(II) cations. The surface zinc atoms are replaced with iron atoms. Thereafter, an additional zinc layer is electrolytically deposited on top of the iron replacement layer. Only after this is a diffusion step performed. The advantage of this alternative is that the added iron is buried in the zinc layer.

[0088] Possible acidic bath compositions containing iron (II) cations are:

[0089] - ferrous chloride solution;

[0090] - ferrous sulfate solution;

[0091] - ferrous ammonium sulfate solution;

[0092] - ferrous fluoroborate solution;

[0093] - Ferrous sulfamate solution;

[0094] - mixed sulfate-chloride bath; BRIEF DESCRIPTION OF THE DRAWINGS

[0095] Figure 1is the atomic concentration distribution curve of Fe relative to the sum of Fe, Cu and Zn atoms for the reference sample and different samples according to the present invention (Fe i ;

[0096] Figure 2 is an atomic concentration distribution curve of Fe relative to the sum of Fe, Cu and Zn atoms of a reference sample and different samples according to another embodiment of the present invention (Fe i ;

[0097] Figure 3 is a BSE SEM photograph showing a relatively light-looking thick brass strip and a relatively dark-looking thin brass strip;

[0098] Figure 4 Secondary electron imaging in a scanning Auger microscope revealed Figure 3 The enlarged and 180° rotated part of the figure shows the analysis points for elemental analysis;

[0099] Figure 5 The difference in the distribution of iron in a thin brass strip (i.e., appearing darker) of a reference sample and two samples of the invention as measured by Auger depth analysis is shown;

[0100] Figure 6 The difference in the distribution of iron in a thick brass strip (i.e., appearing lighter) of a reference sample and two samples of the invention as measured by Auger depth analysis is shown; DETAILED DESCRIPTION

[0101] To manufacture the samples of the present invention, the inventors used high carbon steel wire rods having a carbon grade of 0.80 wt% C and a nominal diameter of 5.5 mm.

[0102] The wire was dry drawn to a median diameter of 1.85 mm. Care was taken to obtain a sufficiently high circumferential roughness R of about 0.90 μm. a The circumferential roughness R of the intermediate wire can be increased by increasing the amount of soap powder suction in the dry wire drawing step, reducing the amount of depression of the last die in the dry wire drawing step, reducing the drawing speed during the dry wire drawing process, reducing the die angle during the dry wire drawing process, or a combination of any of the above methods. a Thus, a circumferential roughness R between 0.80 μm and 1.00 μm was obtained on the intermediate wire. a .

[0103] Subsequently, the wire is cleaned by a pickling method known to those skilled in the art. The acid commonly used for pickling is hydrochloric acid. However, the circumferential roughness can be further increased by keeping the concentration of iron (III) cations above 6 grams per liter and below 15 grams per liter (which is not a conventional condition for those skilled in the art).

[0104] Thereafter, the wire is coated with copper by electrolytic deposition in a copper pyrosulfate deposition bath. After appropriate rinsing and drying, the wire is electrolytically coated with zinc deposited from a zinc sulfate bath. These are techniques known to those skilled in the art.

[0105] After zinc deposition, iron can be deposited from an acidic electrolytic solution containing iron (II) cations by a displacement reaction with zinc. Since zinc is less inert than iron, zinc cations will enter the solution and iron (II) cations will precipitate to maintain charge neutrality. Ferrous sulfate solutions appear to be most suitable for precipitating iron, as this acid is compatible with the acid in the zinc electrolyte. The amount of iron deposited will depend on the soaking time of the wire.

[0106] Thereafter, the wire is heated by resistance heating or medium frequency induction heating, causing the copper and zinc to diffuse, and the displaced iron to remain on the surface.

[0107] Lead bath quenching, copper plating, zinc plating, iron deposition and diffusion are carried out in-line on a through-type device, wherein an unwound intermediate wire reel is guided through the device and the resulting brass-plated wire is wound onto a take-up reel.

[0108] In the subsequent step, the wire is wet drawn into a wire with a diameter of 0.28 mm. The wire is well drawn by using a diamond die in one or more of the last passes (i.e. the die is located at the exit of the wet drawing machine). In particular, drawability becomes a problem when brass is enriched with iron by zinc-iron substitution.

[0109] By varying the different parameters described above, a series of samples were prepared with progressively higher surface iron concentrations. As a reference, a conventional brass wire made along the same route was used, but with a surface roughness of less than 0.40 μm for the intermediate wire, conventional pickling conditions (iron (III) cation concentration between 4 and 7 g / l), no additional iron addition, and drawing in a conventional widia die. This sample is indicated as "Ref" for reference.

[0110] The mass of copper and zinc per mass unit of steel cord is determined by XRFS according to methods known to those skilled in the art. From the total mass of copper and zinc, the average thickness of the coating (expressed in nm) can be calculated. The mass percentage of copper is calculated by the ratio of the mass of copper to the total mass of copper and zinc.

[0111] The iron distribution of the top layer of the brass coating was determined by X-ray photoelectron spectroscopy. The equipment used was a K-Alpha X-ray Photoelectron Spectrometer (XPS) system available from ThermoFisher Scientific. Typically, the sample was analyzed to a depth of 2 nm to 5 nm with a beam area of ​​approximately 8000 μm 2 From the kinetic energy distribution curve of the emitted electrons, information about the detected atoms can be obtained based on the number of elements (energy position of the peak) and the number of atoms present (height of the peak). Although many elements can be detected, only the number of copper, zinc and iron is retained. The top layer of atoms is gradually removed by an argon gun, and the depth curve of the atomic distribution at the top of this layer is obtained. Figure 1 and Figure 2 See the example traces in Figure 1 and Figure 2 The total amount of Cu, Zn and Fe atoms detected at different total sputtering times is shown in Figure 2. i The content is expressed as atomic percent. For a calibrated argon beam, 10 seconds of sputtering is equivalent to removing about 1 nm of material. It can be clearly seen from the trace that the iron content present at the surface is increased more in the wire of the present invention than in the reference wire (denoted by "Ref").

[0112] Using the trapezoidal rule, the average iron content over a layer extending from the surface "0" to a depth of "x" nanometers is calculated, with "x" being 3, 9, and 20 nm for the first, second, and third layers, respectively. This step is repeated on four traces at different points on the surface of a wire to prevent local variations in the coating from causing measurement bias.

[0113] In this way the following Table I is constructed (the numbers in italics fall within the terms of the claims):

[0114] Table I

[0115]

[0116] In Table 1, the only difference between samples (S2, S9), (S3, S10), (S4, S11), (S5, S12) and (S6, S13) is that the first of each pair of samples was pulled by a diamond die ("D") and the second was pulled by a conventional tungsten carbide die ("W"). It should be noted that it can be seen that the use of a diamond-containing die always results in an increase in surface iron content, which is consistent with the present invention, i.e., the ability to increase surface iron content.

[0117] Figure 3A photograph of the surface of a brass wire generated by backscattered electrons in the BSE mode of a scanning electron microscope (FEI Inspect model) is shown. In this figure, the coating shows lighter and darker bands alternating along the wire direction. The lighter bands correspond to thicker brass bands (points indicated by reference mark "+2") and the darker bands correspond to relatively thinner coatings, such as the points indicated by "+1". Scratches were deliberately formed to allow analysis of the same points in a scanning spiral microscope.

[0118] Figure 4 The same region is shown as observed in a scanning Auger microscope at PHI, but in "Secondary Electron Imaging" (SEI) mode. Figure 3 compared to, Figure 4 The direction in the image is rotated 180 degrees and the magnification is 4 times. At the indicated points "1" and "2", the Auger electron distribution is determined by a "PHI-700 Scanning Auger Nanoprobe" available from ULVAC-PHI. Through this process, the depth distributions obtained in these two areas of different samples can be compared with each other. Auger analysis differs from XPS in that the analysis area of ​​Auger analysis is very small, usually less than 100nm 2 , while the analysis area of ​​XPS is several square microns.

[0119] Figure 5 The iron content in the dark areas relative to the total amount of iron, copper and zinc is shown in the BSE mode of the SEM. As expected, due to the thinness of the brass coating, the iron content rises sharply when reaching the steel substrate, which usually occurs within 10 nm. Both the samples according to the invention and the reference samples show the presence of iron clearly in the first few nanometers.

[0120] In contrast, when analyzing the iron content in the areas that appear shallow in the BSE mode of the SEM on the reference samples, the iron content is very limited in the first 3 nm, 10 nm or even 15 nm from the surface relative to the total amount of iron, copper and zinc: on average, the iron content remains below 0.03 or 3 atomic percent. However, in the samples S3 and S4 of the invention, the presence of iron is already clearly detected in the thick brass areas even at very shallow depths. When calculating the average atomic percentage of iron in the different samples, the following results are obtained (Table II):

[0121] Table II

[0122] sample 0-3nm(Fe at%) 0-9nm(Fe at%) 0-15nm(Fe at%) Reference 1.3 2.0 2.2 S3 4.1 3.1 3.2 S4 12.2 10.2 9.9

[0123] The increase in surface iron content leads to improved adhesion of cobalt salt compounds as well as compounds without cobalt, as demonstrated below.

[0124] Three filaments of each type of 0.28 mm, of the reference and sample filaments, were twisted together to form 3 x 0.28 mm steel cords. These steel cords were used for bonding tests in a number of different bonding compounds, which were broadly divided into two groups:

[0125] - Group I contains 6 different compounds, the common feature of which is that they all contain intentionally added organic cobalt salts;

[0126] - Group II contains 6 different compounds, none of which have intentionally added cobalt.

[0127] For each of the twelve compounds, conventional cure (RC) conditions were set to TC90 time plus 5 minutes, where TC90 is the time required for a particular rubber to reach 90% of its maximum torque on the rheological curve at the cure temperature.

[0128] To determine the bond retention, the following aging conditions were applied to the RC cured samples: - Post-cure humidity aging (CH): The RC samples were placed in an environment at 93°C and 95% relative humidity for 14 days

[0129] - Post steam ageing (SA): RC samples were steamed at 120°C for 2 days. In the following, each of the vulcanization conditions RC, CH or SA will be referred to as "condition". The adhesion result is the pull-out force measured according to the ASTM D2229-04 standard, as further detailed in the BISFA ("The International Bureau for Standardisation of Man-made fibres") handbook "Internationally agreed methods for testing of steel tyre cord", 1995 edition, and "D12 Determination of static adhesion to rubber compounds". In this test, the steel cord is embedded in a block of rubber and pulled out of the rubber in the axial direction after vulcanization. The maximum force obtained (in N) is recorded. For one sample, one group, one combination of conditions, the average of several (at least four) measurements of the single maximum force (in N) is recorded as the "Pull-out force" (POF).

[0130] In Tables III and IV below, adhesion test results are expressed as Z-scores relative to a reference average ("RA"). The reference average RA is equal to the weighted average of the "reference" samples, i.e., conventional brass coatings drawn with tungsten carbide dies, present in all cobalt-containing compounds of Group I, the specific conditions of which are indicated in the column headings. The reference standard deviation ("RSTD") is equal to the statistical standard deviation of all results obtained under the specific conditions based on the reference samples present in Group I compounds. In short: positive or negative deviations are calculated relative to known brass steel cords drawn in tungsten carbide dies and tested in cobalt-containing rubber (under various conditions).

[0131] For each of Groups I and II, and for the samples selected in Table II ("Samples"), the pull-out forces for various conditions have been determined. The pull-out forces were weighted averaged as the sample mean ('SA') and the statistical standard deviation was calculated, called the sample standard deviation ('SSTD') for that series and condition.

[0132] The Z-score for a sample in a group of compounds under a particular condition is equal to the mean of the samples in that group and condition minus the reference mean for that condition, divided by the combined standard deviation of the reference standard deviation and the sample standard deviation. In short:

[0133]

[0134] Among them, N S is the number of results combined to obtain SA and SSTD, N R is the number of results combined to obtain RA and RSTD.

[0135] The Z score indicates how statistically significant the deviation of the mean from the reference mean (i.e. the state of the art) is for a particular group and under the particular conditions of the test sample:

[0136] -Z scores below "-2" indicate a statistically significant deterioration compared to the reference mean;

[0137] -Z scores between "-2" and "-1" indicate possible deterioration, but it is not statistically significant;

[0138] - Z scores between "-1" and "+1" indicate that no statistically significant deterioration or improvement relative to the reference mean can be inferred;

[0139] - Z scores between "+1" and "+2" indicate possible improvement but are not statistically significant;

[0140] - Z-scores above "+2" indicate a statistically significant improvement over the current state of the art.

[0141] Table III summarizes the Z-score results obtained on selected samples from Group I compounds.

[0142] - In conventional sulfiding (RC), for average iron concentrations below 7.5 at% Fe in the range 0 nm to 3 nm, the overall results are neutral to insignificant reductions compared to the reference values.

[0143] Samples with average iron concentrations above 7.5 at% Fe (S6, S12) show results significantly lower than the reference values ​​(double underline).

[0144] Therefore, too high a surface iron content has an adverse effect on conventional vulcanization results;

[0145] - For sulfur humidity aging (CH), the results are generally better than the standard reference values ​​(all positive signs).

[0146] The results for samples (S4, S5, in bold) show a significant improvement in the CH results for the cobalt-containing compounds. However, the samples drawn with conventional tungsten carbide dies show lower results.

[0147] Therefore, the average iron concentration on the steel wire surface above 4 at% Fe has a positive effect on the adhesion retention during sulfidation humidity aging;

[0148] - In steam ageing (SA) there is an overall positive improvement, which is significant for average iron concentrations above 4 at% Fe on the wire surface.

[0149] Samples drawn with tungsten carbide dies yielded lower values.

[0150] In the adhesive rubber containing cobalt compounds, samples with iron contents of 4-7.5 at% Fe in the top 0-3 nm of the steel wire performed best among the three conditions.

[0151] Table IV summarizes the Z-score results obtained on selected samples from Group II compounds.

[0152] - In conventional sulphidation (RC), for samples pulled with diamond containing dies and with average iron concentrations below 7.5 at% Fe in 0-3 nm below the surface, the overall results are neutral to non-significantly negative compared to the reference values.

[0153] The fractions for samples pulled with tungsten carbide were significantly lower;

[0154] - In the curing humidity aging (CH), all the samples of the present invention outperformed the reference values ​​in score, and significantly outperformed the reference values ​​when the average iron concentration in 0-3nm was higher than 4at%Fe;

[0155] - In the steam aged samples (SA), all the inventive samples scored significantly better than the reference values.

[0156] Conclusion: Samples of the invention having an average iron content above 4 at% Fe and below 7.5 at% Fe in the range 0-3 nm below the surface show equivalent results under conventional curing and improved results under sulphurized humidity aging and steam aging in a cobalt salt-containing bonding compound. When tested in a bonding compound substantially free of cobalt, the results after sulphurized humidity aging and steam aging are significantly better, while the conventional curing results are only slightly lower. The use of one or more diamond-containing dies in one or more of the final passes of wet drawing can further improve these results.

[0157] Table III

[0158]

[0159] Table IV

[0160]

Claims

1. A steel wire for reinforcing a rubber product, the diameter of the steel wire being expressed as "d" in millimeters, and comprising: A steel substrate coated with a coating comprising brass, The brass comprises copper and zinc; The coating has an average thickness of 450×d nanometers or greater, the average thickness being determined by the total mass of copper and zinc of the brass; The content of copper in the brass is 61 to 75 mass percent relative to the total mass of copper and zinc in the brass. Determining the thickness of the coating and the copper content of the brass by wet chemical analysis, "d" being the diameter of an imaginary circle having the same surface area as a vertical cross-section of the wire; It is characterized in that The first layer of the coating has an average iron content of 4-10 atomic percent, the first layer extending from the surface of the steel wire to a depth of 3 nm below the surface, and the iron content refers to the atomic percentage relative to the total content of iron, zinc and copper in the first layer, The iron, zinc and copper contents were determined by X-ray photoelectron spectroscopy, the average iron content being taken at the depth of the first layer and at 4 different points on the surface of the steel wire.

2. The steel wire according to claim 1, wherein The average iron content in the second layer of the brass coating is 5 atomic percent or more, and the second layer extends from the surface of the steel wire to a depth of 9 nm below the surface.

3. The steel wire according to claim 2, wherein: The third layer of the brass coating has an average iron content of 6 atomic percent or more, and the third layer extends from the surface of the steel wire to a depth of 20 nm below the surface.

4. The steel wire according to any one of claims 1 to 3, wherein The third layer of the brass coating has an average iron content of 20 atomic percent or less, and the third layer extends from the surface of the steel wire to a depth of 20 nm below the surface.

5. The steel wire according to any one of claims 1 to 3, wherein The brass coating has an average thickness of 1350×d nanometers or less.

6. The steel wire according to any one of claims 1 to 3, wherein The coating exhibits alternating thin brass bands and thick brass bands along the length of the wire, the bands being discernible in a scanning electron microscope operating in a backscattered electron mode, wherein the thick brass bands appear relatively light and the thin brass bands appear relatively dark, Its further characteristic is that In the thick brass strip, the average iron content is 4 atomic percent or more of the total content of iron, copper and zinc, the average iron content being taken within a depth range of the top 3 nm from the surface of the thick brass strip, and the iron, zinc and copper contents are determined by scanning Auger electron spectroscopy.

7. The steel wire according to claim 6, wherein In the thick brass strip, the average iron content is 6 atomic percent or more of the total content of iron, copper and zinc, the average iron content being taken within a depth range of the top 3 nm from the surface of the thick brass strip, and the iron, zinc and copper contents are determined by scanning Auger electron spectroscopy.

8. The steel wire according to claim 7, wherein In the thick brass strip, the average iron content is 8 atomic percent or more of the total content of iron, copper and zinc, the average iron content being taken within a depth range of the top 9 nm from the surface of the thick brass strip, the iron, zinc and copper contents being determined by scanning Auger electron spectroscopy.

9. A steel cord comprising one, two or more steel wires according to any one of the preceding claims.

10. A rubber product reinforced with the steel cord according to claim 9, wherein the rubber product is a tire, a hose, a belt or any other rubber product reinforced with a steel cord.

11. The rubber product according to claim 10, wherein: The rubber does not contain cobalt or cobalt-containing compounds.

12. A method for manufacturing a steel wire according to any one of claims 1 to 8, the method comprising the following steps: a. Select wire rod; b. forming an intermediate wire by dry drawing the wire rod; c. quenching the intermediate wire in a lead bath to obtain a lead bath quenched wire; d. pickling the lead bath quenched wire; e. electrolytically coating the lead bath quenched wire to provide a copper coating; f. electrolytically coating the lead bath quenched wire that has been coated with copper to provide it with a zinc coating; g. diffusing the copper coating and the zinc coating on the lead bath quenched wire to form a brass coating, thereby forming a brass-plated wire; h. forming a steel wire by wet drawing the brass-plated wire; It is characterized in that The circumferential roughness R of the intermediate wire a Greater than 0.40 microns; During the drawing process of the brass-plated wire, one or more dies containing diamond are used in one or more final passes.

13. The method according to claim 12, wherein: In step "d", pickling is carried out in hydrochloric acid having an iron (III) ion concentration of 6-15 g / L.

14. The method according to any one of claims 12 to 13, wherein: After step "f" and before step "g", the wire is led through an acidic bath containing iron (II) cations for replacing zinc atoms with iron atoms in the top layer of the zinc coating.

15. The method according to claim 14, wherein: The acid bath is one of the group comprising: - ferrous chloride solution; - ferrous sulfate solution; - ferrous ammonium sulfate solution; - ferrous fluoroborate solution; - Ferrous sulfamate solution; - Mixed sulphate-chloride bath.

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