KR20080082362A - Ultra high tensile steel cord for tire - Google Patents

Abstract

An ultra-high strength steel cord and a method for manufacturing the same are provided to prevent or minimize the delamination of wires when stranding or drawing the wires, thereby significantly improving the productivity of the steel cord. An ultra-high strength steel cord for reinforcing a tire is formed by stranding a plurality of wires. Each of the wires contains carbon of 0.80-2.0 wt% and chrome of 0.15-0.50 wt%. The wire has a tensile strength of 3500 MPa or more. A peak is shown at a temperature section of 80-120 at a DSC(Differential Scanning Calorimeter), wherein an area of the peak is 5.00 uNs/mg or less. A final wet wire drawing process is divided into three steps according to the transformation quantity, and then a delta parameter, which is a relationship between a dice cut angle and a dice cut ratio, is varied when a wire drawing process is performed every the steps.

Description

Ultra high tensile steel cord for tire reinforcement and its manufacturing method {Ultra high tensile steel cord for tire}

1 is a differential scanning sequence analysis graph of element wires.

Figure 2 shows the first embodiment of the present invention,

  (A) is the organization photograph after brass plating,

  (B) is a photograph of the tissue after the final wet drawing.

Figure 3 shows a comparative example 1 wire for Example 1 of the present invention,

  (A) is the organization photograph after brass plating,

  (B) is a photograph of the tissue after the final wet drawing.

4 is a view showing a wire of Comparative Example 2 according to Example 1 of the present invention;

  (A) is the organization photograph after brass plating,

  (B) is a photograph of the tissue after the final wet drawing.

The present invention, the ultra-high strength for the tire reinforcement made of the element wire containing 0.80 to 2.0wt% and 0.15 to 0.50wt% of carbon and chromium, the tensile strength is 3500MPa or more, and no delamination phenomenon in the stranded work A steel cord and a method of manufacturing the same.

In general, among various kinds of reinforcement materials used for reinforcement of various rubber products including tires of automobiles and industrial belts, steel cords have high strength, modulus, heat resistance, heat transfer rate, fatigue resistance, and adhesiveness. As the most suitable material for the required characteristics, in particular, the amount of use is rapidly increasing with the increase of automobiles.

Steel cord widely used throughout the industry for reinforcement of various rubber products including automobile tires and industrial belts is a collection of several strands of steel wire, that is, a collection of wires in which a plurality of wires form a twisted structure. It has a cross-sectional shape, and is a twisted structure in which a plurality of wires are stranded at least one layer on its outer circumferential surface with at least one elementary wire as a core wire.

That is, the steel cord is subjected to pickling treatment to remove the surface scale of the wire rod, which is a material, and then subjected to sequential processes such as primary dry drawing, primary heat treatment, secondary dry drawing, secondary heat treatment, brass plating, and wet drawing. The wire is manufactured by twisting the wire, and a plurality of wires are twisted at a constant pitch, and some processes may be added or subtracted depending on the type of steel cord or working conditions.

At this time, the heat treatment is a very dense perlite (sorbite) or sorbite by immersing in a molten lead which is maintained at a temperature of about 550 ℃ in the state of heating the steel material to the austenite zone to induce constant temperature transformation and then air-cooled A patting heat treatment to obtain) tissue is used.

The tensile strength of the wire which envisions the steel cord as described above should be at least 2700 MPa. In the case of ultra high strength steel cord, the wire should have a tensile strength of 3500 MPa or more. As the tensile strength of the wire increases, the steel cord and the steel The tire in which the cord is embedded can be made lighter, and finally, the fuel efficiency of the car can be improved.

No less important than the high strength of steel cords, however, the disconnection of wires should not occur during wire drawing and high speed stranding. In particular, in the case of ultra-high strength steel cords, the wires have a tensile strength of 3500 MPa. For high carbon steels containing 0.80wt% or more of carbon, steel materials containing about 0.2wt% of chromium, which is a work hardening strengthening element, are used as materials.

The ultra-high strength steel cord is not only excellent in strength but also excellent in various properties such as heat resistance, thermal shear rate, fatigue resistance, and rubber adhesiveness, but its usage is increasing day by day, but the ultra-high strength steel cord has an alloying element such as chromium. When the added high carbon steel is used as a material, when an excessive deformation amount is applied in the drawing process, there is a problem in that the delamination of breaking the wire occurs due to incorrect stress in the stranding process.

As described above, the delamination phenomenon that prevents the manufacture of ultra-high strength steel cords means that surface cracks are generated along the longitudinal direction when an element wire subjected to work deformation by drawing is subjected to lateral stress (twist) of the wire, and this surface crack is instantaneous. Spiral fracture failure caused by wires that are propagated in wire rods or stranded wires, which occurs mainly when high-carbon steels or high-alloy steels are processed under severe conditions or when the temperature rises excessively due to heat generated during processing. Known.

In addition, in the related art, since the ductility of the element wire is reduced as cracks are generated in the vicinity of a hard phase such as cementite during machining as the strength of the wire increases, the delamination has been described. In order to increase the strength without increasing the total reduction rate for the wire rods, alloying elements were added.

In addition, by using a die design and a lubricant appropriately to suppress the temperature rise of the wire surface due to the surface friction, while reducing the surface roughness of the wire by minimizing the surface roughness of the die to reduce the delamination phenomenon.

However, the above conventional methods for preventing or reducing the delamination phenomenon have the following problems.

First, in the case of increasing the strength by adding an alloying element, the reduction of the limit of reduction rate in which the delamination occurs according to the addition of the alloying element is lowered, and the effect of increasing the strength due to the addition of the alloying element is canceled out.

Second, when the die reduction rate and lubricant are properly used, the slip type drawing machine has a limit on the die reduction rate that can be adjusted to prevent the surface damage of the wire rod by the surface friction, and the method of lowering the die angle to uniform internal deformation is a lubricant. It is difficult to enter, thereby increasing the friction between the wire rod and the die, there is a limit.

Third, in the case of optimizing the lubricant and lowering the surface roughness of the wire rod, there is a slight effect, but cannot be a fundamental solution to the delamination phenomenon.

The present invention was devised to solve the problems of the conventional ultra-high strength steel cord and its manufacturing method, in which a delamination phenomenon frequently occurs in the manufacturing process, and may not only add an alloying element but also increase the final drawing amount. It is an object of the present invention to provide an ultra-high strength steel cord in which a plurality of element wires having a tensile strength of 3500 MPa or more without twisting occurring are stranded.

The above object of the present invention is achieved by differentially applying a die reduction angle and a die reduction ratio in accordance with the amount of deformation during wire drawing.

The element wire constituting the ultra-high strength steel cord for tire reinforcement of the present invention has a composition containing 0.15 to 0.50 wt% of chromium in high carbon steel containing 0.80 to 2.0 wt% of carbon, and differential scanning thermal analyzer (DSC) It is a technical feature to control the final fresh condition so that the area of the peak occurring in the 80 ~ 120 ℃ section during the analysis to 5.00㎶s / mg or less.

The element wire constituting the steel cord of the present invention as described above, the size of cementite in the wire rod structure and the interlayer spacing of the pearlite before the final drawing is controlled by the control of the pre-finding parting heat treatment conditions, the final after the heat treatment By controlling the drawing processing conditions, delamination does not occur in the final drawing process or in the stranding step after the final drawing.

In this case, when the size of cementite and the interlayer spacing of pearlite are not properly adjusted by heat treatment, the area of the peak occurring at 80 to 120 ° C. in the differential scanning thermal analysis of the final fresh wire is 5.00 μs / mg. In such a case, delamination is likely to occur in the wire drawn in the final drawing or stranding process.

The reason why the content ratio of carbon and chromium contained in the element wire constituting the steel cord of the present invention is limited to the above range is basically to make the tensile strength of the element wire be 3500 MPa or more, and the carbon content is 0.80. If the wt% is not reached, the tensile strength is likely to be insufficient. If the wt% is exceeded, the wire rod is too hard to significantly increase the frequency of disconnection during drawing or stranding, and increases the occurrence rate of disconnection due to excessive carbide formation. More preferred content range is 0.9-1.3 wt%.

In addition, chromium is a representative work hardening strengthening element. When the content is less than 0.15 wt%, the tensile strength is insufficient as in the case where the carbon content is insufficient, and when the content is more than 0.40 wt%, the excess high diameter precipitated in the base Doro chromium carbide is not dissolved in the matrix even by the diffusion during heat treatment, causing the wire to break when drawn or stranded, more preferred content range is 0.20 to 0.30wt%.

The element wire constituting the steel cord of the present invention having the above composition is largely carbon steel, and components other than carbon and chromium follow the component range of general carbon steel.

In the differential scanning thermal analysis of the final drawn wire, the area of the peak occurring in the 80 to 120 ° C section is limited to 5.00 μs / mg or less. This is to make it easy to grasp the degree of delamination.

In other words, the wire rod subjected to the parting heat treatment has a fine pearlite structure, and according to the thickness of cementite constituting the pearlite and the interlayer spacing of the pearlite, the delamination may not occur or occur during the final drawing or stranding. In order to prevent the delamination phenomenon, the cementation thickness of cementite forming the pearlite structure of the wire rod is adjusted to 40 to 60 nm, and the interlayer spacing of pearlite is within the range of 70 to 100 nm by adjusting the pre-finding parting heat treatment condition. Preferably, in this case, by combining the final drawing conditions, the area of the peak in the differential scanning sequence analysis of the final fresh wires appears to be 5.00 μs / mg or less.

 At this time, if the cementite thickness of the heat-treated wire rod and the interlayer spacing of pearlite are less than 40 nm and 70 nm, respectively, spherical cementite is formed as carbon is decomposed from cementite among the wires to be described below, thereby reducing freshness and ductility. When the cementite thickness and the interlayer spacing of pearlite exceed 60 nm and 100 nm, respectively, the brittleness of the enlarged cementite also decreases freshness and ductility.

In particular, the presence of spherical cementite in elementary wire structure increases the interfacial energy between cementite and ferrite, and causes defects such as voids and voids at the boundary, which causes stress concentration. The failure to suppress the propagation of the crack before the maximum stress results in delamination.

In other words, when the wire rod is drawn in harsh conditions, spherical cementite produced in the wire rod tissue in the wire rod acts as a major factor for the occurrence of delamination.

In addition, an appropriate perlite interlayer spacing can be determined by analyzing a carbon decomposition behavior at a temperature at which dynamic aging of fresh wires occurs, using a differential scanning thermal analyzer. It is related to the dynamic aging caused by the re-decomposition of, that is, the transition of carbon present in the cementite into the ferrite region. If the peak area is large, it can be determined that the re-decomposition of cementite takes place.

Therefore, in a pearlite structure having a peak area of 5.00 μs / mg or less, re-decomposition of cementite is difficult, so that no delamination occurs. Thus, ultra-high strength wire can be produced without breaking wires.

In addition, in order to obtain such a pearlite structure, not only the parting before the final drawing, but also the control of the final wet drawing condition that deforms the structure obtained by the parting is important. For this, the "delta parameter" is used.

The delta parameter is a parameter defined by the relation between the die reduction angle and the die reduction ratio, and is expressed as in Equation 1 below.

Where "Δ" is the delta parameter, "α" is the dice half angle in radians,

         "γ" is a die reduction rate.

The die half angle and the die reduction rate are adjusted to change the delta parameter represented by Equation 1 according to the deformation amount range. When the deformation amount is less than 0.75, the delta parameter is decreased from 3.0 to 1.5, and the deformation amount is 0.75 to 2.25. The delta parameter is maintained at 1.5 and the delta parameter is gradually increased from 1.6 to 3.0 when the strain exceeds 2.25.

In addition, by performing final drawing to adjust the delta parameter as described above, the metal flow accompanying the plastic working during drawing is made uniform, and at the same time, the surface temperature of the wire being drawn is controlled to 180 ° C or lower. If the surface temperature of the wire rod in the final drawing exceeds 180 ℃, the decomposition of cementite in the drawing is promoted.

That is, in general, the final wet temple is composed of 10 or more passes. For example, assuming 10 passes of fresh processing, 1 to 3 passes for the first stage, 4 to 7 passes for the second stage, and 8 to 10 passes. In the third step, the fresh pass schedule is determined such that the deformation amount of each pass of the first step is less than 0.75, the deformation amount of each pass of the second step is between 0.75 and 2.25, and the deformation amount of each pass of the third step is more than 2.25.

In the first stage, as the pass progresses, the delta parameter of the first pass is set to 3.0, and then the delta parameter is reduced for each pass so that it is reduced to 1.5 from the last pass of the first pass. Keep the delta parameters for each pass at 1.5, and in the third step, increase the delta parameters for each pass from 1.6 to 3.0 as the pass progresses.

In this case, the deformation amount is equal to the following equation (2).

Here, ε is deformation amount, D ₁ is die lead wire diameter, and D ₂ is die lead wire diameter.

When the final wet drawing process is performed in the same manner as described above, in the first step having a deformation amount of 0.75 or less, the pearlite is rearranged as the colonies existing irregularly along the fresh direction rotate in the [001] direction. By gradually lowering the delta parameters per pass, it is possible to promote rearrangement of pearlite.

In the intermediate strain of 0.76 to 2.25, the cementite of the colony parallel to the fresh axis is thinned by plastic deformation, and the interlayer spacing of pearlite is also narrowed. As the cementite and the fine pearlite are changed, the delta parameter of each pass is maintained at a level of 1.5, so that the plastic deformation can be uniformly performed to the inside of the wire rod.

In addition, in the section where the amount of deformation exceeds 2.25, all of the vertical colonies are rearranged in the direction of [001] so that the deformation is completed. By increasing the delta parameter for each pass, the friction between the fresh cone and the wire is minimized by minimizing the difference with the mechanical reduction rate. This prevents the brittleness of the wire rod from increasing due to heat while minimizing the surface damage of the wire rod that may be caused by the wire rod.

The characteristics of the present invention steel cord as described above will be described through the following examples.

Example

A wire rod having a diameter of 5.5 mm containing 1.02 wt% of carbon and 0.20 wt% of chromium was pickled, and then freshly prepared at a diameter of 3.05 mm and then freshly drawn at a diameter of 1.47 mm with a patented heat treatment.

Cementite thickness and pearlite interlayer spacing in each tissue were measured after the secondary heat treatment and brass plating on the lead wires with different temperatures, respectively, and the cementite thickness and pearlite interlayer spacing after final wet drawing were measured. The results are as follows, and the respective tissue photographs are shown in FIGS. 2 to 4. (The patterning heat treatment structure is hardly changed by brass plating.)

division           After heat treatment           After wet freshness   Cementite Thickness (nm)    Pearlite Interlayer Spacing (nm)    Cementite Thickness (nm)     Pearlite Interlayer Spacing (nm) Example 1      47.1      73.4       3.73       22.9 Comparative Example 1      33.9      56.9       3.25       20.9 Comparative Example 2      37.5      65.6       4.23       41.5

Only Example 1 wire rod of Table 1 meets the criteria of cementite thickness and pearlite interlayer spacing after heat treatment.

In addition, after the copper plating of the wire rod of Example 1 and the wire rods of Comparative Examples 1 and 2, respectively, was finally wet drawn to a diameter of 0.18 mm, the final wet wire reduced the thickness of cementite and the spacing between pearlite layers. It can be confirmed from Table 1 above.

At this time, the wire rods of Example 1 and Comparative Example 1 were drawn with different delta parameters for each section as in the final drawing method described above.

One of the wire rods of Comparative Example 2 (hereinafter referred to as "Comparative Example 2" element wire of Table 2), as in the wire rods of Example 1 and Comparative Example 1, was drawn with different delta parameters for each section, the other one (Table below) 2 "comparative example 3" wire | wire) was wired in the same manner as the conventional wire drawing method, by reducing the amount of reduction | deduction for each section.

The mechanical properties of the final fresh wires were examined as described above, and differential scanning sequence analysis was performed on the wires of Example 1 and the wires of Comparative Examples 1 and 2, and the results are shown in Table 2 and FIG. same.

  division  unit  Example 1  Comparative Example 1  Comparative Example 2  Comparative Example 3 The tensile strength   MPa   4,505   4,549   4,491   4,425 Hunter fatigue  cycle  87,456  15,492  18,785   4,572 Peak area S / mg   3.666   5.255   5.190     - Delamination    -   none   has exist   has exist   has exist

* Hunter fatigue value is the result of experiment with fatigue load as 1550MPa.

* The peak area appears in the range of 80 ~ 120 ℃ when analyzed by differential scanning sequence analyzer

   Is the area of the peak.

Referring to Tables 1 and 2, delamination did not occur only when the wire rod of Example 1, which met the criteria of the thickness of cementite and the perlite interlayer spacing, was fresh, and the wire rod of Example 1 was finally fresh. Only the peak area group in the differential scanning sequence analysis was found, and the fatigue characteristics were found to be the best.

The steel cord was manufactured by stranding each of the three wires of Table 2, and the mechanical properties of each steel cord are shown in Table 3 below.

   division   unit   Example 1   Comparative Example 1   Comparative Example 2  Comparative Example 3  Steel cord    mm     0.37      0.37     0.37      -    Code B / S     N     325      326     324      -   3-roll fatigue   cycle    28,903     6,785    12,153      -   Stiffness    TSU      7       8      9      -  Unit weight    g / m    0.604     0.603    0.604      -  Twisted pair   Times / ton      6       85      70   Can not be followed

* The 3 roll fatigue value is the result of experiment with the fatigue load 3.3kgf.

Looking at Table 3, it can be seen that in the case of the steel cord stranded by the wire of Example 1 of the present invention, the occurrence rate of disconnection at the time of stranded wire is extremely low compared to 1 or Comparative Example 2, and in the case of Comparative Example 3 wire The twisted pair was impossible.

In this case, in the two wires of Comparative Examples 2 and 3 in which only the final twisted wire conditions are different while the same parting is performed, the reason why only the wires of Comparative Example 3 cannot be stranded depends on the fresh wire condition. Means that it must be fresh in a different way.

As described above, in the steel cord of the present invention, since the delta parameter is differentially applied for each deformation amount section when the wires constituting the wire are finally drawn, the delamination at the time of drawing or stranding despite the addition of alloying elements or harsh processing amount. By preventing or minimizing the phenomenon, not only the mechanical properties are excellent, but also the productivity is high, which reduces the manufacturing cost, and the commercial production of ultra-high strength steel cords wired with small wires of 3500 MPa or more enables future tire weight reduction and improved fuel economy of the automobile. It is expected to have a significant effect on.

Claims (2)

In the steel cord for a tire reinforcement which wired many element wires, The element wire is a brass plated high carbon steel having a tensile strength of 3500 MPa or more containing 0.80 to 2.0 wt% of carbon and 0.15 to 0.50 wt% of chromium, and has an area of 5.00 μs at a peak of 80 to 120 ° C. in a differential scanning thermal analysis. Ultra-high strength steel cord for tire reinforcement, characterized in that / mg or less. In the manufacturing method of the steel cord for tire reinforcement material consisting of a sequential process of pickling, primary dry drawing, primary heat treatment, secondary dry drawing, secondary heat treatment, brass plating, final wet drawing and stranded wire as a material , The final wet drawing is divided into a first drawing step having a deformation amount of less than 0.75 represented by Equation 2 below, a second drawing step having a deformation amount of 0.75 to 2.25, and a third drawing step having a deformation amount exceeding 2.25, In the first drawing step, the delta parameter for each pass represented by Equation 1 below is gradually reduced from 3.0 to 1.5, in the second drawing step, the delta parameter for each pass is maintained at 1.5, and in the third drawing step, the delta parameter for each pass is delta. Ultra-high strength steel cord for tire stiffeners, characterized by an incremental parameter from 1.6 to 3.0. Equation 1

Where "Δ" is the delta parameter, "α" is the dice half angle in radians,          "γ" is a die reduction rate. Equation 2

Here, ε is deformation amount, D ₁ is die lead wire diameter, and D ₂ is die lead wire diameter.

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