Description

Technical Field

[1 ] The invention relates to a steel cord for rubber reinforcement. And the invention also relates to a rubber product reinforced by steel cords.

Background Art

[2] Steel cords are widely used for reinforcing the rubber product, such as rubber belt, rubber tire or hose, since the steel cords can provide sufficient strength to the rubber product and have good adhesion with the rubber.

[3] In the recent years, with the development of the light tire, the steel cord with smaller diameter and sufficient breaking load becomes more and more popular. Consequently the steel wires for steel cord are required to have higher tensile strength, from High Tensile (HT) to Super Tensile (ST), Ultra Tensile (UT) and even Mega Tensile (MT). With the higher tensile strength, the steel wires are finer while maintaining the breaking load of a steel cord, thereby the steel cord is finer, and the rubber ply reinforced by such steel cords is thinner, and thereby the tire is lighter. A lighter tire saves the energy consumption and the cost.

[4] However, with the increase of the tensile strength of the steel wires, the fracture rate of the steel wire increases during the production of the steel cord. As a result, the production cost of a steel cord increases quite a lot.

[5] JP3542489 discloses a steel wire with high tensile strength being at least 3500MPa, wherein the steel wire has a hydrogen amount which is released from room temperature to 300°C of not more than 0.5ppm. The fatigue strength of the steel wire is significantly improved with the hydrogen amount of more than 0.5ppm.

[6] However, in practice, the fracture problem of the steel wire with high tensile strength is still unacceptable. As a result, the production cost of a steel cord is too high. Disclosure of Invention

[7] The primary object of the invention is to provide a steel cord comprising high tensile strength steel wire with a lower wire fracture rate during manufacturing and a lower production cost.

[8] Another object of the invention is to provide a rubber product reinforced by the steel cords comprising high tensile strength steel wire with a lower production cost.

[9] According to the first aspect of the invention, a steel cord for rubber reinforcement is provided, wherein the steel cord comprises two or more steel wires that are twisted together, the steel wires have a carbon content of more than 0.70% in weight, at least one of the steel wires has a diffusible hydrogen with an amount ranging from 0.001 ppm to 0.35 ppm and a non-diffusible hydrogen with an amount ranging from 0.001 ppm to 0.60 ppm. According to the invention “ppm” are parts per million in mass.

[10] The “diffusible hydrogen” is the hydrogen released from the steel wire within a temperature being not more than 300°C.

[11 ] The “non-diffusible hydrogen” is the hydrogen released from the steel wire within a temperature being higher than 300°C and no more than 600°C.

[12] The measuring method of the “diffusible hydrogen” and the “non- diffusible hydrogen” is:

- untwisting the steel wires out from the steel cord, cutting the steel wire with a certain weight - about 2 gram - to make a sample, removing the oxide and the coating of the sample surface to expose the bare surface of the steel substrate by sand paper, cleaning the steel substrate in ethanol by ultrasonic cleaning, and then drying the steel substrate by heating to have a clean bare surface of the steel substrate to avoid any grease;

- heating the steel substrate in vacuum at 10’ ⁹ Pa level to release the hydrogen from the steel substrate, the heating speed is 100°C per hour, measuring the released hydrogen by the quadrupole mass spectrometry, the mass of the hydrogen released every 5 seconds is captured and recorded by being divided by the sample weight as an amount expressed in ppm, the total amount of the captured and recorded hydrogen within a temperature being no more than 300°C is deemed as the “diffusible hydrogen”, and the total amount of the captured and recorded hydrogen within a temperature being higher than 300°C and no more than 600°C is deemed as the “non-diffusible hydrogen”.

The hydrogen measurement can be done with a device called “Hydrogen Thermal Desorption Analyzer”, with a model of HTDS-002 supplied by R-DEC Co., Ltd.

[13] By controlling both the diffusible hydrogen amount and the non- diffusible hydrogen amount of the steel wire, the fracture of the steel wire during the manufacturing of a steel cord is significantly reduced. The control of the diffusible hydrogen and the non-diffusible hydrogen can be done in two phases, firstly in the process for making a steel wire from a wire rod and secondly in the process for making a steel cord from steel wires. The control of the diffusible hydrogen and the non-diffusible hydrogen in the process for making a steel wire from a wire rod improves the drawability of the steel wire and also contributes to the reduction of steel wire fracture rate during the twisting process following. Furthermore the control of the diffusible hydrogen and the non-diffusible hydrogen in the process for making a steel cord from steel wires reduces the fracture of the steel wires. When developing the invention, the inventors discovered that the hydrogen enters into the steel substrate during the twisting process that is for making a steel cord, i.e. the cabling or bunching process, and this increases the fracture of the steel wire during the twisting process. The invention controls the entering of the hydrogen during the twisting process, and therefore the fracture of the steel wire during the twisting process is decreased.

[14] The defects of the steel wire absorb the diffusible hydrogen with the effect of the hydrogen dislocation, and the concentration of the diffusible hydrogen makes the defects easier to break. The non- diffusible hydrogen mostly exists in the defects, and the reduction of the non-diffusible hydrogen can be one index showing the reduction of the defects of the steel wire. Therefore, the control of both the diffusible hydrogen amount and the non-diffusible hydrogen amount of the steel wire can reduce the fracture of the steel wire during the twisting process for making a steel cord.

[15] Preferably, the diffusible hydrogen of the steel wire is present in an amount that is less than or equal to 0.25 ppm. More preferably, the diffusible hydrogen of the steel wire is present in an amount that is less than or equal to 0.20 ppm. Most preferably, the diffusible hydrogen of the steel wire is present in an amount that is less than 0.06 ppm.

[16] Preferably, the non-diffusible hydrogen of the steel wire is present in an amount that is less than or equal to 0.55 ppm. More preferably, the non-diffusible hydrogen of the steel wire is present in an amount that is less than or equal to 0.45 ppm.

[17] With the preferable solutions, the fracture of the steel wires is reduced a lot during the twisting process for making a steel cord.

[18] According to the present invention, the steel wire has a high tensile strength. The tensile strength of a wire is the breaking load of the wire, that is the force (expressed in newton) at which the wire breaks, divided by the area of the perpendicular cross section of the wire. The steel cord formed by such steel wires results in the desired weight reduction of a tire. Preferably, the steel wire has a tensile strength TS satisfying: TS>3900-2000xD MPa, D is the diameter of the steel wire and expressed in mm. More preferably, TS satisfies: TS>4200-2000xD MPa. Even more preferably, TS>4600-2000xD MPa. The tensile strength of the steel wire is measured when the steel wire is untwisted out from the steel cord. The steel wires of one steel cord may have same or different tensile strength.

[19] Depending on the various applications of the steel cord, the steel cord and the corresponding steel wires have a proper diameter. Preferably, the steel wire has a diameter D being less than 0.6mm. The steel wires of one steel cord may have same or different diameter. [20] The steel wire has a carbon content of more than 0.70% in weight. Preferably, the steel wire is high carbon steel wire, wherein the carbon content is not less than 0.80% in weight. The steel wire preferably has a nickel content being less than 0.005% in weight.

[21 ] Preferably, each of the steel wires of the steel cord has a diffusible hydrogen with an amount ranging from 0.001 ppm to 0.35 ppm and a non-diffusible hydrogen with an amount ranging from 0.001 ppm to 0.60 ppm. Alternatively, the steel cord may have one or more steel wires having a diffusible hydrogen and a non-diffusible hydrogen out of the range as mentioned above.

[22] The steel cord comprises two or more steel wires, wherein the two or more steel wires are twisted together in any known manner. The steel cord has a construction as any one of existing constructions, such as nx1 , nxm, n+m or n+m+l.

[23] According a second aspect of the invention, a tire is provided. The tire comprises at least one belt layer, at least one carcass layer, at least one tread layer and a pair of bead portions, at least one steel cord is embedded in the belt layer and/or said carcass layer of the tire, wherein at least one steel cord comprising two or more steel wires twisted together, the steel wires having a carbon content that is more than 0.70% by weight, at least one of the steel wires has a diffusible hydrogen with an amount ranging from 0.001 ppm to 0.35 ppm and a non-diffusible hydrogen with an amount ranging from 0.001 ppm to 0.60 ppm.

Brief Description of Figures in the Drawings

[24] Figures 1 a-1 b describe the diagrams showing the diffusible hydrogen and the non-diffusible hydrogen of the invention.

Mode(s) for Carrying Out the Invention

[25] The steel wires for steel cord are made from a wire rod.

[26] The wire rod is firstly cleaned by mechanical descaling and I or by chemical pickling in a H2SO4 or HCI solution in order to remove the oxides present on the surface. The wire rod is then rinsed in water and is dried. The dried wire rod is then subjected to a first series of dry drawing operations in order to reduce the diameter until a first intermediate diameter.

[27] At this first intermediate diameter D1 , e.g. at about 3.0 to 3.5 mm, the dry drawn steel wire is subjected to a first intermediate heat treatment, called patenting. Patenting means first austenitizing until a temperature of about 1000 °C followed by a transformation phase from austenite to pearlite at a temperature of about 600 - 650 °C. The steel wire is then ready for further mechanical deformation.

[28] Thereafter the steel wire is further dry drawn from the first intermediate diameter D1 until a second intermediate diameter D2 in a second number of diameter reduction steps. The second diameter D2 typically ranges from 1.0 mm to 2.5 mm.

[29] At this second intermediate diameter D2, the steel wire is subjected to a second patenting treatment, i.e. austenitizing again at a temperature of about 1000 °C and thereafter quenching at a temperature of 600 to 650 °C to allow for transformation to pearlite.

[30] If the total reduction in the first and second dry drawing step is not too big a direct drawing operation can be done from wire rod till diameter D2.

[31] After this second patenting treatment, the steel wire is usually provided with a brass coating: copper is plated on the steel wire and zinc is plated on the copper. A thermo-diffusion treatment is applied to form the brass coating. Alternatively, the steel wire can be provided with a ternary alloy coating, including copper, zinc and a third alloy of cobalt, titanium, nickel, iron or other known metal.

[32] The brass-coated or the ternary alloy coated steel wire is then subjected to a final series of cross-section reductions by means of wet drawing machines. The final product is a steel wire with a carbon content higher than 0.70 percent by weight, or no less than 0.80 percent by weight, or even higher than 0.90 percent by weight, with a tensile strength (TS) typically above 3000 MPa and adapted for the reinforcement of rubber products. The steel wire has a nickel content being less than 0.005% in weight, even no nickel, i.e. the nickel content of the steel wire is very limited and thereafter undetectable.

[33] Steel wires adapted for the reinforcement of tires typically have a final diameter ranging from 0.05 mm to 0.60 mm, e.g. from 0.10 mm to 0.40 mm. Examples of wire diameters are 0.10 mm, 0.12 mm, 0.15 mm, 0.175 mm, 0.18 mm, 0.20 mm, 0.22 mm, 0.245 mm, 0.28 mm, 0.30 mm, 0.32 mm, 0.35 mm, 0.38 mm, 0.40 mm. Better that the diameter the steel wire is in the range of 0.10mm-0.50mm.

[34] Two or more steel wires are twisted by the existing steel cord making process, i.e. cabling or bunching process, to make a steel cord.

[35] The control of the diffusible hydrogen and the non-diffusible hydrogen is done in two phases, firstly in the process for making a steel wire from a wire rod and secondly in the process for making a steel cord from steel wires.

[36] The following methods can be used for controlling the diffusible hydrogen and the non-diffusible hydrogen of a steel wire in the process for making a steel wire, including controlling the diffusible hydrogen and the non-diffusible hydrogen of the wire rod, controlling the time and the temperature of pickling, adding an additive into the pickling solution for inhibiting the entering of the hydrogen, cancelling the use of pickling and/or improving the PH value of the plating solution. As a result, the steel wire has a significant improvement of the drawability, i.e. almost 10% reduction of wire fracture rate, and furthermore this is also in favour of the following twisting process.

[37] Decreasing the temperature of the twisting process, i.e. cabling or bunching process, can be used for controlling the diffusible hydrogen and the non-diffusible hydrogen of the steel wire in the process for making a steel cord from steel wires. By doing these, the steel cord has a reduced wire fracture during the twisting process.

[38] Finally, a steel cord is obtained, wherein the steel wires of the steel cord have a diffusible hydrogen with an amount ranging from 0.001 ppm to 0.35 ppm and a non-diffusible hydrogen with an amount ranging from 0.001 ppm to 0.60 ppm. [39] Figures 1 a-1 b are the diagrams showing the diffusible hydrogen and the non-diffusible hydrogen of the invention. For Figure 1a, the abscissa is the temperature with a heating rating of 100°C per hour, and the ordinate is the hydrogen releasing speed in ppm per 5 seconds. The heating speed is 100°C per hour, and the hydrogen released every 5 seconds is captured and recorded by being divided by the sample weight as one amount. The diagram is formed by these amounts. For Figure 1 b, the abscissa is the temperature, and the ordinates is the accumulative hydrogen amount. The total amount of the hydrogen released within a temperature being no more than

300°C is the “diffusible hydrogen”, and the total amount of the hydrogen released within a temperature being higher than 300°C and no more than 600°C is the “non-diffusible hydrogen”. For the example in Figure 1a and Figure 1 b, the diffusible hydrogen is 0.08ppm while the non-diffusible hydrogen is 0.38ppm.

[40] Following Table 1 summarizes the comparison between the invention and the references.

[41] Table 1 [42] The fracture of the wire is observed by comparing the number of the fracture of the wire, including the break and serious crack, during the twisting process for the production of one ton steel cord. In Table 1 the amount of fractures per ton as occurring during the production of Reference 1 was taken as the reference (100%).

[43] It is clear that the controlling of both the diffusible hydrogen and the non-diffusible hydrogen of the steel wire can reduce the fracture of the steel wire during the twisting process for making a steel cord. With the reduction of the fracture of the steel wire, the production cost of a steel cord is reduced.

[44] The tensile strength of the steel wire is measured in the following method:

- Unravel/untwist the steel wires out from the steel cord,

- Calculate the tensile strength of individual steel wire by dividing the steel wire breaking load by the steel wire cross-sectional area. The steel wire breaking load is measured according to the principle mentioned in the standard ISO6892-1 :2009 with some particular setting like the clamp length being 250mm and the test speed being 100mm/min, test 5 times for each steel wire, and the average value of the 5 times tests is the steel wire breaking load. The steel wire diameter for calculating the cross-sectional area is measured by microcalliper: firstly positioning the steel wire in the middle of the anvils, and then measuring the diameter to obtain the first value; secondly measuring the diameter at +/- 60° rotation of the wire to obtain the second value; thirdly measuring the diameter at another 60° rotation of the wire to obtain the third value; finally calculate the average diameter of the three values to obtain the steel wire diameter.