Steel cord for rubber reinforcement and method for manufacturing the same

Abstract

The present invention relates to a steel cord for rubber reinforcement. In the steel cord of the present invention, cobalt is contained by 0.001 ppm to 0.1 ppm within a 4 nm top-surface of the brass-plated steel wire. A method of manufacturing the steel cord includes: providing a brass-plated steel wire; mixing a cobalt compound in a wet lubricant filled in a wet drawing bath provided with a plurality of drawing dies between one pair of multi-stage drawing cones such that the concentration of the cobalt compound becomes 0.1 ppm to 100 ppm; and causing the cobalt to be contained by 0.001 ppm to 0.1 ppm within a 4 nm top-surface of the brass-plated steel wire after the brass-plated steel wire passes through a final die by causing the cobalt to be attached to a surface of the brass-plated steel wire and alloyed with a brass layer while the brass-plated steel wire is passing through the drawing cones and the drawing dies to be subjected to multi-stage drawing.

Claims

1. A steel cord for rubber reinforcement comprising one or more brass-plated steel wires, wherein cobalt is alloyed with the brass plating layer by a process of wet drawing of the brass-plated steel wire and is present in an amount of about 0.001 ppm to 0.1 ppm within a 4 nm top-surface depth from the surface of the plating layer of the brass-plated steel wire.

2. The steel cord of claim 1, wherein two or more brass-plated steel wires are stranded with each other.

3. A method of manufacturing a steel cord for rubber reinforcement, the method comprising: providing a brass-plated steel wire; mixing a cobalt compound in a wet lubricant filled in a wet drawing bath provided with a plurality of drawing dies between one pair of multi-stage drawing cones such that the concentration of the cobalt compound becomes 0.1 ppm to 100 ppm; and passing the brass-plated steel wire through the drawing cones and the drawing dies and subjecting to multi-stage drawing, wherein cobalt attached to the surface of the brass-plated steel wire is alloyed with the brass plating layer while passing through the drawing cones and the drawing dies and subjected to multi-stage drawing in the wet drawing bath such that cobalt is present in an amount of about 0.001 ppm to 0.1 ppm within a 4 nm top-surface depth from the surface of the plating layer of the brass-plated steel wire.

4. The method of claim 3, wherein the cobalt compound is cobalt boroacylate, cobalt naphthenate, cobalt stearate, cobalt neodecanoate, cobalt borocarboxylate, cobalt acetyl acetate, or cobalt abietate.

5. The method of claim 3, wherein the cobalt is present in the brass plating layer only as a ternary alloy of CuZnCo.

6. The steel cord of claim 1, wherein the cobalt is present in the brass plating layer only as a ternary alloy of CuZnCo.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic sectional view illustrating a wet drawing bath for use in a method of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

(3) The manufacturing method of the present invention including the above-described objects and technical features of the present invention may be fully understood through the following embodiments. The present embodiment is included in desirable examples provided for understanding of the present invention, and the protection scope of the present invention is not limited or restricted by the embodiment.

(4) First a brass-plated steel wire with a wire diameter of 1.70 mm was prepared. The brass plating layer on the brass-plated steel wire was composed of 64 wt % copper and 36 wt % zinc. In order to allow a structure, hardened after the drawing to the above-mentioned wire diameter, to be drawn again, a heat treatment step and a patenting step were performed, and thus, the structure was transformed into a 100% pearlite structure. During the step of forming the plating layer of the steel wire, a current density of a copper bath and a zinc bath was adjusted so that the attachment amount of the plating layer became 4.0 g/kg to 5.0 g/kg.

(5) Next, the temperature of the lubricant solution within a wet drawing bath where the drawing is to be performed on the brass-plated steel wire was maintained in a range of 40 C. to 50 C., the concentration of the wet lubricant component in the lubricant solution was maintained in a range of 6% to 9%, and a pH of the lubricant solution was maintained in a range of 6 to 9. In addition, the concentration of the cobalt compound added to the lubricant solution was set to 0.1 ppm to 100 ppm.

(6) In the wet drawing bath as described above, the brass-plated steel wire was wired to sequentially pass one pair of drawing cones and dies placed between respective stages of the drawing cones, and thus the brass-plated steel wire was subjected to multi-stage drawing. During the multi-stage drawing, the cobalt dissolved in the lubricant liquid within the drawing bath was coated and pressed on the surface of the brass-plated steel wire, and thus, a ternary alloy of brass-cobalt was formed.

(7) The wire diameter of the final plated steel wire drawn while passing through the wet drawing bath was 0.30 mm.

(8) The plated steel wires subjected to the drawing step were stranded with each other in a wire strander to manufacture steel cord specimens of a 12 structure.

(9) First, a measurement was performed to as to check the coated amount of the cobalt included in the plating layers of the specimens. The concentration of cobalt on a steel cord obtained through a conventional method, in which the cobalt is bonded to the surface of a steel cord through an existing plating process or through drawing performed by placing separate independent baths inside and outside a wet drawing bath, is considerably higher than the concentration attached to the steel cord of the present invention. Thus, when the steel cord obtained through the conventional method is analyzed using conventional wet analysis equipment, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy), or dry analysis equipment, EDX (Energy Dispersive X-ray), AES (Auger Electron Spectroscopy), or XPS (X-ray Photoelectron Spectroscopy), cobalt is detected in a concentration of several ppm or more or 0.01 to several atomic %. Whereas, since the content of cobalt in the steel cord of the present invention was too small, it was difficult to detect cobalt through the conventional methods using the equipment described above.

(10) Meanwhile, when analysis is performed using the ratio of the amount of a specimen and the amount of solution (testing material 5 g: acidic solution 20 ml), i.e. the C value (g/ml) (the amount of testing material (g)/the amount of acidic solution (ml)) in the step of dissolving the specimen using the ICP-AES equipment, the cobalt concentration detected at the C value of 0.25 g/ml cobalt is several ppm or more in the concentration range of cobalt contained in the cobalt-containing plating layer formed on the steel cord by the conventional method. However, the cobalt concentration detected from the specimen according to the present invention at the C value of 0.25 g/ml was 0.1 ppm or less that exceeded a detection limit so that it was impossible to obtain a correct concentration (attached amount).

(11) Thus, in order to obtain the cobalt concentration value of the steel cord specimen according to the present invention, the inventor of the present invention set the C value to be 0.5<C<2.5 by concentrating and dissolving the specimen at a ratio 2 to 10 times higher than the conventional case and then performed the analysis. In other words, the cobalt concentration in the plating layer of the steel cord according to the present invention is in the range of infinitesimal amount which can be detected only through a special analysis beyond a conventional analysis, as described above.

(12) Table 1 below shows results of ICP-AES analysis for respective top-surface cobalt concentrations of plating layers of steel cord specimens.

(13) TABLE-US-00001 TABLE 1 Comp. Comp. Comp. Items Example 1 Example 2 Example 1 Example 2 Example 3 Example. 3 Remarks Cobalt concentration 0 0.0001 0.001 0.01 0.1 1.0 within 4 nm top- surface of plating layer (ppm) ICP (C = 0.25 g/ml) 0.00 0.00 0.01 0.01 0.12 0.50 Unit: ppm ICP (C = 0.5 g/ml) 0.00 0.00 0.01 0.18 0.30 1.10 Unit: ppm ICP (C = 1.25 g/ml) 0.00 0.02 0.13 0.41 0.72 2.10 Unit: ppm

(14) In table 1 above, when the ratio of a conventional test material and the amount of an acidic solution is 0.25 gl/ml (C=0.25 g/mm), in the case of Comparative Example 2, and Examples 1 and 2,in which the concentration within the 4 nm top-surface of the plating layer steel cord was 0.01 ppm or less, it was impossible to obtain correct detected values since all the values obtained by analyzing the specimens were equal to or lower than the detection limit. However, in the pre-treatment process of dissolving the specimens in acid for the purpose of wet analysis, analyzing solutions were prepared by dissolving the specimens to be highly concentrated such that the C values became 0.5 and 1.25, respectively, and then the solutions were analyzed using ICP-AES, which enabled the analysis on the specimens of Comparative Example 2 and Examples 1 and 2.

(15) Meanwhile, the cobalt concentration of the specimens of the examples according to the present invention was not detected using conventional dry analysis equipment such as EDX, AES, ESCA. It was possible to analyze the cobalt concentration of the specimens of the examples according to the present invention in an XAS (X-ray Absorption Spectroscopy) that uses a synchrotron radiation accelerator as an analysis energy source and analyzes only a component of a specific atom within a 5 nm top-surface. Table 2 below shows results of top-surface cobalt concentration analysis of plating layers of the steel cord specimens performed using the dry analysis equipment and XAS analysis equipment.

(16) TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Items Example 1 Example 2 Example 1 Example 2 Example 3 Example 3 Remarks Cobalt concentration 0 0.0001 0.001 0.01 0.1 1.0 within 4 nm top- surface of plating layer (ppm) EDS 0.00 0.00 0.01 0.01 0.01 0.01 Unit: atomic % AES 0.00 0.00 0.01 0.02 0.02 0.02 Unit: atomic % XAS 0.00 0.01 0.002 0.012 0.090 0.200 Unit: %

(17) As in Table 2, in the dry analysis equipment, EDS and AES, no cobalt component was detected in all the specimens. In the case of XAS, analysis on Examples 1 to 3 and Comparative Example 3 was enabled. However, in the case of Comparative Example 2, it was impossible to obtain a detected value since the concentration was equal to or less than the detection limit value.

(18) Through the analysis results of Tables 1 and 2, the cobalt concentration range of the plating layer of the steel cord claimed in the present invention is an infinitesimal concentration range which cannot be analyzed using conventional analysis equipment or analysis methods. Due to this, the cobalt concentration range is a region which has not drawn attention in the existing technical field from the start or has been excluded as being considered ineffective in the aging adhesive strength improvement by the addition of cobalt.

(19) Meanwhile, Table 3 below shows results of initial and hygrothermal aging adhesive strength tests. The initial adhesive strength test was performed for 15 minutes at 140 C. according to ASTM D-2229, and the aging adhesive strength test was performed as the hygrothermal aging adhesive strength test, in which the specimens were stored for 7 days at 105 C.100% RH.

(20) TABLE-US-00003 TABLE 3 Comp. Comp. Comp. Items Example 1 Example 2 Example 1 Example 2 Example 3 Example 3 Cobalt concentration 0 0.0001 0.001 0.01 0.1 1.0 within top-surface 4 nm of plating layer (ppm) Number of times of 2.1 2.1 2 2.2 4.5 23.9 snapping of drawn wire (times/ton) Initial relative adhesive 100 100 101 100 98 93 strength (%) hygrothermal aging 100 100 115 120 114 95 relative adhesive strength

(21) In Table 3 above, the initial and hygrothermal aging adhesive strengths refer to relative adhesive strengths when the measurement values of Comparative Example 1 are considered 100.

(22) From Table 3, it can be seen that since the specimens of Examples 1 to 3 of the present invention exhibit initial adhesive strengths which are substantially the same as that of Comparative Example 1, which was not coated with the cobalt compound, and Comparative Example 2 in which the concentration of the cobalt compound was 0.0001 ppm, the cobalt compound does not contribute greatly to the improvement of the initial adhesive strength.

(23) However, in the hygrothermal aging adhesive strength, it can be seen that the specimens of the examples of the present invention exhibit superior adhesive strengths as compared to the specimens of the comparative examples. Meanwhile, it can be seen that Comparative Example 3, in which the cobalt concentration is high as compared to the specimens of the examples of the present invention, exhibits a considerably high snapping rate of drawn wire as compared to the specimens of the examples of the present invention.

(24) From the measurement results of Table 3 above, it can be seen that the cobalt existing in the infinitesimal amount range in a plating layer of a steel cord contributes to the aging adhesion improvement of the steel cord. It can also be seen that the aging adhesion becomes poor when the content of cobalt is less than or exceeds the concentration range of cobalt defined in the present invention.