JP2756003B2 - High strength steel cord excellent in corrosion fatigue resistance and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a steel wire used for reinforcing rubber or the like, and more particularly, a rubber product reinforced with a steel wire is repeatedly used in a humid environment or an environment where it is exposed to water. The present invention relates to a high-strength steel cord having improved corrosion fatigue resistance when receiving a fluctuating input, and a method for manufacturing the same.

(Conventional technology) Steel wires used for rubber reinforcement are particularly required to have high strength in order to reduce the weight of products and reduce manufacturing costs. There is a phenomenon that the corrosion fatigue property is reduced. That is, when the strength of the steel wire is increased, when the rubber product reinforced with the steel wire receives a fluctuating stress in a corrosive environment, there is a problem that the life of the product is shortened, and the improvement has been conventionally made. For example, JP-A-57-149578 discloses that a metal cable having excellent mechanical fatigue characteristics can be obtained by compressing the residual stress on the surface of a metal wire. Japanese Patent Application Laid-Open No. 60-183202 discloses that the corrosion fatigue resistance is improved by the pearlite structure having an average cementite lamellar spacing of 300 to 500 ° in the strand of the steel cord and the refinement of the pearlite crystal grains. ing. Japanese Patent Application Laid-Open No. 62-203615 discloses a method of improving corrosion fatigue properties by applying a brass plating after applying an iron or iron-nickel plating to a steel wire instead of applying a brass plating to a steel wire. .

(Problems to be Solved by the Invention) Although it is described that the mechanical fatigue characteristics are improved in the metal cable disclosed in Japanese Patent Application Laid-Open No. 57-149578, the corrosion fatigue characteristics are not discussed. without the present invention it has been different in purposes and tensile strength of the wire as in example is less than 330kgf / mm ^2, 330kgf / m
There is a disadvantage that desired corrosion fatigue characteristics cannot be obtained only by compressing the residual stress on the surface of a wire having a tensile strength of m ² or more. Next, in JP-A-60-183202,
As in the example, the tensile strength of the strand is 300 kgf / m
are those less than m ^2, pearlite in odds and disadvantages of 330kgf / mm ² or more tensile strength wires for the purpose of weight reduction of the tire and a large amount requires a steel cord in order to ensure the safety factor of the tire However, a desired corrosion fatigue property cannot be obtained only by improving the pearlite crystal grains. Furthermore, Japanese Patent Application Laid-Open No. 57-149578 discloses that corrosion fatigue resistance is improved by providing a steel wire with iron or iron-nickel plating and then brass plating and providing a protective layer against corrosion. However, in order to obtain a protective layer having good covering properties, iron or iron-nickel plating with a thickness of 10 μm or more is required, resulting in a decrease in product tensile strength and a decrease in productivity. Since the steel cord is often used as a combined steel cord, the brass plating and the protective plating are worn away due to fretting between the wires of the steel cord during use of the tire, and there is a disadvantage that the protective action from a corrosive environment is lost.

As described above, there is a strong demand for steel wires having high strength and excellent corrosion fatigue resistance, but it has been difficult to manufacture them easily at low cost.

(Means for Solving the Problems) The present inventors used SW as specified in JIS G 3506 as steel.
As a result of intensive studies using RH82A or a material equivalent thereto, a steel cord having high strength and excellent corrosion fatigue resistance was obtained without using a special steel material. That is, the first invention of the present invention provides a steel wire having a tensile strength of 330 to 390 kgf /
A mm ^2, or less difference is 0.05% by weight of the carbon content and the carbon content of the internal steel wire surface layer from the surface of the steel wire having a thickness of 10 minutes of the steel wire diameter and the final Cementite lamellar spacing of the metal structure of the steel material after heat treatment is 1000-1400
In the range of Å, the cementite lamellar spacing of the metallographic structure of the steel wire after the final drawing of this steel material is 120-190 、,
A steel wire with improved corrosion fatigue resistance, characterized in that the residual stress in the region from the steel wire surface to a quarter of the steel wire diameter in the depth direction is a compressive stress of -70 mm or more and -20 mm or less. It is a high-strength steel cord with two or more strands.

A second invention of the present invention relates to a method for producing a steel cord having the above-described improved corrosion fatigue resistance, and the production method is as follows. The difference between the carbon content of the surface layer having a thickness of one-tenth of the steel wire diameter from the surface of the steel wire and the carbon content inside the steel wire in the heat treatment for improving the wire drawing workability in the next step In the heat treatment, the temperature in the heating furnace and the residence time of the steel in the furnace are set so that the maximum temperature of the steel is preferably 880 to 970 ° C, and the furnace atmosphere is 2% or less. The atmosphere of CO gas reduction, and before the final heat treatment, clean the steel surface in advance,
Suppress the decrease in carbon content of the surface layer due to decarburization, then quench at a rate of 140 ° C or more per second to a temperature of 630 ° C or more and less than 660 ° C, and maintain the above-mentioned temperature for at least 12 seconds and By performing the final heat treatment for transformation, the cementite lamellar interval of the metal structure of the steel material is set in the range of 1000 to 1400 °, then pickled by a usual method, and subjected to plating treatment, and then 330 to 390 kgf / mm ^{2 in} the next step. In order to obtain tensile strength, the steel material is subjected to final drawing at a surface reduction rate of 97% or more to form a steel wire, and the steel wire is provided with 70 kgf / mm ² or more, preferably 70 kgf / mm ² or more.
While applying a tensile force equivalent to ~ 170 kgf / mm ² , the steel wire is repeatedly passed through rolls arranged in a staggered manner so that the bending strain on the surface of the steel wire becomes 1% or more.

When performing one or two or more wire drawing operations before the final heat treatment, reduce the number of intermediate heat treatments as much as possible, specifically, at least one of the wire drawing operations. Processing is preferably performed without going through a heat treatment step.

(Operation) The steel material of the steel cord is JIS G
3506 Hard steel wire type code: SWRH82A, carbon content 0.79 to 0.86% by weight, silicon content 0.15 to 0.35% by weight, manganese content 0.30 to 0.60% by weight, phosphorus content 0.030
% And a sulfur content of 0.030% by weight or less. Further, in order to improve the disconnection in the wire drawing and the stranded wire processing, the smaller the amount of the nonmetallic inclusions, especially the nonductile inclusions, the better. To increase the strength of the steel cord, it is desirable to have a high carbon content, but a pearlite structure with excellent workability cannot be obtained. When the composition is changed, the above-mentioned steel material is used because the price of the steel material increases.

The reason for limiting the patenting temperature to the range of 630 ° C or more and less than 660 ° C in the heat treatment (patenting treatment) in the final stage is that if the patenting temperature is within the above range, the metallographic structure of the steel after the patenting treatment The cementite lamellar spacing of the steel wire is in the range of 1000 ° to 1400 °, and accordingly, the steel wire drawn from this steel material has the cementite lamellar spacing of the conventional lamellar spacing patented at 600 ° C. (100 °). As a result, the crack propagation speed in the corrosion fatigue properties in the region from the surface to the depth of one-quarter of the steel wire diameter in the depth direction is reduced, thereby increasing the fracture life. Can be improved.

The reason why the propagation speed in the crack propagation process decreases is presumed to be that the stress at the crack tip can be relaxed by increasing the lamella spacing of the steel wire. In addition, by performing the above heat treatment, the cementite shape can be smoothed and the lamellar spacing is widened, so that the machinability of the steel material is improved as compared with the conventional heat treatment, and the drawability in the next drawing process is improved. It is possible to increase the diameter of the steel material in the final heat treatment, so that the wire drawing reduction rate can be reduced in the wire drawing process prior to the final heat treatment process, and the productivity is improved, which is industrially advantageous. It is.

If the lamellar spacing in the final heat treatment is less than 1000 °, the desired corrosion resistance of the product after wire drawing cannot be obtained, and the machinability deteriorates.
If it exceeds 00 °, the tensile strength will not reach the desired value, and in addition, a long patenting time will be required, and the productivity will decrease.

After heating the steel material to the maximum heating temperature: 880 to 970 ° C and solution-treating it, it was cooled at a rate of 140 ° C / sec or more because the desired temperature before the start of pearlite transformation was used to achieve a uniform lamellar structure. This is to suppress the precipitation of ferrite on the surface layer of the steel material, which has poor mechanical properties when the cooling rate is lowered, in order to obtain a high cooling rate.

The reason why the patenting holding time is set to 12 seconds or more is to terminate the pearlite transformation at a desired temperature in order to obtain a uniform lamellar structure.

In the heat treatment for improving the drawability, the difference between the carbon content of the surface layer having a thickness of one tenth of the steel wire diameter from the surface of the steel wire and the carbon content inside the steel wire is 0.05% by weight or less. The reason is that if the decarburized layer of the surface layer becomes thicker, the mechanical strength of the surface layer of the product strand will decrease, and the mechanical fatigue and corrosion fatigue resistance will deteriorate. It is desirable to reduce the chance of decarburization, that is, reduce the number of intermediate heat treatments as much as possible.

In the heat treatment of steel, it is generally known to control decarburization by adjusting the temperature, time, and atmosphere to appropriate values (eg, steel heat treatment revised 5th edition, The Iron and Steel Institute of Japan, Showa)
Published in 1944, pages 28-39, "1.4 Furnace Atmosphere"), in the final heat treatment of the present invention, the maximum temperature of steel
The reason for setting the heating furnace temperature and steel stay time in the furnace so that the temperature is 880-970 ° C is that if the temperature is lower than 880 ° C, sufficient solution treatment is not performed, and if the temperature exceeds 970 ° C, the decarburization reaction is promoted. Yes, and the reason why the furnace atmosphere was set to the reducing atmosphere is to suppress the oxidation reaction, and to suppress the decarburization reaction and the decarburization reaction by adjusting the temperature, atmosphere, and time to the appropriate ranges described above. This is because the scale quality and the amount of the scale can be generated, and the solution treatment can be performed under the condition that decarburization does not occur.

In addition, by performing a hot water washing process before performing the final heat treatment, a compound such as a lubricant used in a wire drawing process prior to the final heat treatment process that promotes decarburization attached to the steel material surface is removed. Thus, a desired surface layer in which decarburization is suppressed can be obtained.

Reduced area reduction rate by wire drawing of steel material after final heat treatment
The reason for setting it to 97% or more is to obtain a tensile strength higher than a desired value.

Drawing the steel material 70 kgf / mm ² or more in a predetermined diameter, preferably while adding tensile force corresponding to 70~170kgf / mm ^2, passes through the rolls 1 arranged in a staggered shape as shown in FIG. 2 The reason why the bending process is applied so that the bending strain of the steel wire surface becomes 1% or more is that the residual stress of the surface layer can be made more compressive by adding a tensile stress as shown in FIG. 3, and This is because the residual stress in the region from the surface to the depth of not less than の of the steel wire diameter in the depth direction can be compressed. By applying a constant compressive residual stress from the surface to the inside of the steel material, as shown in Fig. 1, crack propagation in the region from the steel wire surface to one quarter of the steel wire diameter in the depth direction is achieved. Can be reduced and the rupture life can be improved by about 25%.

If the tensile stress exceeds 170 kgf / mm ² , disconnection when bending strain is applied increases, and scratches on the wire surface increase, and mechanical fatigue characteristics tend to decrease, which is not desirable. Therefore, the tensile stress is preferably 70 to 170 kgf / m
It was m ^2.

The reason for setting the residual stress between −70 mm and −20 mm is as follows:
If it is less than mm, the bending and tensile force must be increased, which is not desirable because damage to the surface of the steel wire increases and mechanical and corrosion fatigue properties deteriorate.

(Examples) Hereinafter, the present invention will be described specifically with reference to examples.

Examples 1 and 2, Comparative Examples 1 to 4 and Conventional Examples 1 and 2 In Examples 1 and 2, SWRH82A having a diameter of 5.5 mm and having a stermore treatment having the chemical composition shown in Table 1 was subjected to normal dry drawing. After the borax treatment used, the steel material diameter at the time of the final heat treatment shown in Table 2 was obtained by primary drawing in a conventional manner, and in the final heat treatment step, it was previously immersed in warm water of 90 ° C. for 2 seconds. After the cleaning treatment, the steel material is heated and held in a heating furnace set at the average furnace temperature shown in Table 2 so that the maximum temperature of the steel material is about 910 ° C. in a 0.5% CO gas reducing atmosphere. (Holding time: 30 seconds), then cool and hold at a rate of 140 ° C per second to a patenting temperature of 650 ° C (holding time:
12 seconds) A final heat treatment was performed to complete the pearlite transformation.

In addition, in a normal process, it is common to perform one or more wire drawing processes before the final heat treatment, but in Examples 1 and 2, both the primary wire drawing processes performed before the final heat treatment are performed. This was performed without going through a normal patenting process (hereinafter referred to as "intermediate heat treatment").

Further, in Conventional Examples 1 and 2, 5.5 mm diameter SWRH82A and SWRH72A each having the chemical composition shown in Table 1 were first drawn to a 3.0 mm diameter, subjected to an intermediate heat treatment, and then SWRH82A was subjected to 1.3 mm.
After 2nd wire drawing of SWRH72A to 2mm diameter and 1.00mm diameter,
In the final heat treatment step, the steel material is heated and held in a heating furnace set to the average furnace temperature shown in Table 2 in a 0.5% CO gas reducing atmosphere (holding time: 34 seconds), and subsequently, at 140 ° C./sec. A final heat treatment of cooling and holding at a patenting temperature of 600 ° C. at a speed (holding time: 12 seconds) was performed to terminate the pearlite transformation.

Further, Comparative Examples 1 to 3 are examples in which the same intermediate heat treatment as in Conventional Examples 1 and 2 was performed and then the final heat treatment was performed under the conditions shown in Table 2, and Comparative Example 4 was a case in which bending was performed. Example 1 except that the tensile stress was less than 70 kgf / mm ²
This is an example of the case where the final heat treatment is performed under substantially the same conditions as in FIG.

Each of the above-mentioned steel materials subjected to the final heat treatment was pickled by a usual method, plated, and then finally drawn to a diameter of 0.19 mm by a conventional method. To apply a predetermined bending strain while applying a tensile force of 1.5 to 3.7 kgf to the drawn steel wire, a second
Roller 1 having a diameter of 11 mm as shown in the figure is d1 = 13 mm, d2 =
Rollers arranged in a zigzag pattern at 5 mm and subsequently rollers arranged in the same manner as described above so that the bending directions of the steel wires differ from each other by 90 ° were continuously passed. The tensile strength of steel wire is 14
The measurement was performed after a heat aging treatment at 0 ° C. for 40 minutes. Observation of the lamellar spacing and the cementite shape of the steel material subjected to the final heat treatment was performed by a scanning electron microscope after etching the steel material using a 1% nital solution. After measuring the residual stress of the drawn steel wire, a 100 mm long steel wire was coated with a corrosion-resistant resin so that half of the outer circumference of the steel wire was continuously coated in the length direction of the steel wire. The bending displacement of the steel wire was measured by immersing the steel wire in a half-circumferential direction at a predetermined concentration in the depth direction from the surface by immersing the steel wire in a nitric acid solution having a concentration of 0.1%. Here Evaluation of corrosion fatigue resistance of the steel wire a small amount of chlorine ions steel wire, nitrate ions, was immersed in neutral solution containing sulfate ions, repetition of 30 kgf / mm ² at a rate of 1000 revolutions per minute bending stress The steel wire was immersed in a sulfuric acid solution at a concentration of 1% in order to evaluate the corrosion fatigue index obtained by measuring the number of rotations up to fracture and the hydrogen embrittlement characteristics of the steel wire in corrosion fatigue. Hydrogen embrittlement resistance obtained by applying 40 kgf / mm ² bending stress repeatedly at a rate of 1000 revolutions per minute while cathodic electrolysis at a current density of 0.01 mA / dm ² and measuring the number of revolutions before breaking. It was evaluated using an index.

Table 2 shows test results such as a heat treatment method, a metal structure, a bending strain applying method, a residual stress value, and a corrosion fatigue property for each of the conventional example, the comparative example, and the example.

Comparison between Conventional Example 1 and Conventional Example 2 in Table 2 shows that Conventional Example 2, which is about 25% stronger than Conventional Example 1 by the usual method, has a 20 to 30% reduction in corrosion fatigue resistance. I understand.

In Conventional Example 2, the lamellar interval of the steel material after the final heat treatment was smaller than the appropriate range of the present invention, but Comparative Example 1 in which the lamellar interval of the steel material was increased to the appropriate range of the present invention, compared with Conventional Example 2. Although the corrosion fatigue resistance is improved by 10 to 20%, the corrosion fatigue resistance is still inferior to that of the conventional example 1.

In Comparative Example 2, since the lamellar spacing of the steel material after the final heat treatment was larger than the proper range of the present invention, the tensile strength of the steel wire after wire drawing was smaller than that of Comparative Example 1, and the internal residual stress of the present invention was also small. Since it is outside the appropriate range, sufficient corrosion fatigue resistance has not been obtained.

Comparative Example 3 is an example showing that the corrosion resistance is poor because the difference between the carbon content of the surface layer portion and the carbon content inside the steel wire is large and the internal residual stress is outside the proper range of the present invention.

In each of Conventional Examples 1 and 2 and Comparative Examples 1 to 3, since the intermediate heat treatment was performed, the difference in carbon content between the surface layer portion and the inside of the steel wire was relatively high (0.05 to 0.17 weight). %), Comparative Example 4 was not subjected to the intermediate heat treatment, so that the difference in the carbon content was low (0.
Since the internal residual stress is a tensile stress as the tensile stress during bending is smaller than the proper range of the present invention, the corrosion fatigue resistance is reduced. .

On the other hand, in Example 1, the difference in carbon content between the surface layer portion and the inside of the steel wire was as small as 0.02%, the lamellar spacing of the steel material after the patenting treatment was 1190 °, and the residual stress was −60.
mm, the corrosion fatigue resistance of the steel wire is improved, and the tensile strength of the steel wire is 25
It can be seen that it has increased by about%.

Example 2 is an example in which the corrosion fatigue resistance is particularly emphasized. Compared with the conventional example 1, the tensile strength of the steel wire is about 11%, and the increase rate is smaller than that of the example 1. It can be seen that the fatigue properties are further improved than in Example 1.

(Effect of the Invention) As described above, the present invention reduces the thickness of the decarburized layer on the surface of the steel wire as much as possible, increases the lamella spacing of the pearlite structure, and increases the depth from the surface in the steel wire of the steel cord. The strength of the steel wire is reduced compared to the conventional product by reducing the corrosion fatigue resistance of the steel cord inexpensively and easily by making the predetermined compressive residual stress in the area up to one-fourth of the steel wire diameter in the direction. Thus, the rubber product reinforced with steel wire can be reduced in weight and production cost without impairing its durability, and is industrially useful.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows Example 1 (a), Comparative Example 1 (B) in Table 1;
And a graph showing the relationship between the number of repeated bending times of the corrosion fatigue treatment and the crack depth generated by the corrosion fatigue treatment for the steel wire of the conventional example 2 (c) for each steel wire. FIG. 2 shows the steel used in the examples. FIG. 3 is a layout view of rollers of a device for applying a predetermined bending strain to a wire, and FIG. 3 is a diagram showing a steel wire having a large tensile stress (a) and a small tensile stress (b) after being subjected to bending strain processing. It is the diagram which calculated | required the state of the stress inside a steel wire by calculation. 1 ... Laura

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-109104 (JP, A) JP-A-60-183202 (JP, A) JP-A-62-203615 (JP, A) JP-A-58-1983 16033 (JP, A) JP-A-62-77442 (JP, A) JP-A-57-149578 (JP, A)

Claims (4)

(57) [Claims] 1. The carbon content of a surface layer having a tensile strength of 330 to 390 kgf / mm ² and a thickness of one tenth of the diameter of a steel wire from the surface of the steel wire, and the carbon content of the steel wire inside the steel wire. The difference with the amount is 0.
The cementite lamellar interval of the metal structure of the steel wire after the final drawing of the steel wire is set to the range of 1000 to 1400 mm. Excellent corrosion fatigue resistance, characterized by 190 mm and residual stress in the region from the steel wire surface up to a quarter of the steel wire diameter in the depth direction is a compressive stress of -70 mm or more and -20 mm or less. High strength steel cord. 2. The steel surface is cleaned before the final heat treatment.
After that, by setting the temperature in the heating furnace in a CO gas reducing atmosphere of 2% or less and the stay time of the steel material in the furnace, and heating the steel material to a maximum heating temperature: 880 to 970 ° C., 140 ℃
By performing a final heat treatment of quenching to a temperature of 630 ° C or more and less than 660 ° C at a cooling rate of at least 630 ° C and holding for 12 seconds or more, the cementite lamellar interval of the metal structure of the steel is in the range of 1000 to 1400 °. And then 330
In order to obtain a tensile strength of ~ 390 kgf / mm ² , the steel material was subjected to final drawing at a reduction rate of 97% or more to form a steel wire, and then a tensile force equivalent to 70 kgf / mm ² or more was applied to the steel wire. A method for producing a high-strength steel cord excellent in corrosion fatigue resistance, wherein a bending process is performed such that a bending strain on the surface of the steel wire is 1% or more in a state where the steel cord is added. 3. A wire drawing is performed one or more times before a final heat treatment, and at least one of the wire drawing is performed.
The method for producing a high-strength steel cord excellent in corrosion fatigue resistance according to claim 2, which is performed without a heat treatment step. 4. The method for producing a high-strength steel cord excellent in corrosion fatigue resistance according to claim 3, wherein one wire drawing is performed before the final heat treatment, and the wire drawing is performed without going through a heat treatment step. .