JPH06346190A - Extra fine steel wire excellent in fatigue characteristic - Google Patents

Abstract

PURPOSE:To improve the fatigue cgaracteristics of an extra fine steel wire by subjecting a medium to high carbon steel wire rod to final patenting, applying wiredrawing strain within a prescribed range corresponding to carbon content, and specifying the structure after wiredrawing. CONSTITUTION:A medium to high carbon steel wire rod of 0.6-1.0wt.% C is subjected to final patenting to form a fine pearlitic structure, and a true strain of wiredrawing is applied in the range between (3-1.82C) and (4.1-1.82C). A structure, in which >=90% of pearlite colonies are in parallel to a wiredrawing direction and pearlite lamellar spacing is in the range between 0.02 and 0.06mum and, further, the rate of breakage of cementite constituting the pearlite colonies is regulated to <=2O%, is formed. By this method, the fatigue characteristics of the extra fine steel wire of (180 to 250)kgf/mm<2> tensile strength can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention can be applied to, for example, a bead wire for reinforcing a tire, an extra fine steel wire such as a steel cord to be drawn, and more particularly to an extra fine steel wire having excellent fatigue characteristics.

[0002]

2. Description of the Related Art Conventionally, a typical steel cord manufacturing method is 0.8% C eutectoid carbon steel with a final patenting with a wire diameter of 0.8 to 1.6 mm to obtain a wire drawing strain (hereinafter, referred to as true strain). It is 300-340 by performing strong processing of 3 or more)
It has a tensile strength of about kgf / mm ² . Steel cords are required to have high strength and, at the same time, to be required to improve fatigue properties. Up to now, the fatigue characteristics have been mainly studied from the viewpoint of cord construction, such as improvement of rubber permeability (JP-A-60-42028) and optimization of filament type ratio (JP-A-59-124404). There is. The present invention also improves the fatigue characteristics of cords by improving the fatigue characteristics of the strands of ultrafine steel wires such as steel cords.

As an example of improving the cord characteristics by adding alloy elements, Japanese Examined Patent Publication No. 2-10220 discloses a specific amount of C in hypereutectoid steel.
Addition of o improves the limit of delamination occurrence, and Japanese Examined Patent Publication No. 3-23674 uses 0.75 to 0.90% C
There is one that aims to improve fatigue characteristics in a corrosive environment by adding specific amounts of Ni, Cu, and V to steel. However, the former is limited to the improvement of the twisting workability and there is no idea of improving the fatigue characteristics, and the latter is the improvement of the fatigue characteristics in a corrosive environment due to the improvement of the corrosion resistance, and the atmospheric fatigue characteristics have not been evaluated.
Since both of them contain expensive alloy elements, there is a problem of high cost.

As an example of improvement in the patenting method, in JP-A-4-289148, the pearlite structure of 0.7 to 0.9% C steel is adjusted to an angle within 45 ° in the longitudinal direction, and the strength of the ultrafine wire is adjusted. Although it is said that the ductility is improved, the ductility improvement is limited to the drawing.

As an example of improvement in the structure and wire drawing conditions, Japanese Patent Application Laid-Open No. 4-131323 discloses that 0.2 containing an alloying element such as V.
It is said that 0 to 0.50% C steel has a fine ferrite-pearlite structure and is subjected to drawing with a surface reduction rate of 10 to 40% to improve fatigue resistance and wear resistance, but the C content is low and heat treatment is continuous. Since it is cooled, the tensile strength of the wire drawing material is 90kgf /
There is a low problem of about mm ² .

[0006]

By processing the true strain of 3 or more, the tensile strength of the ultrafine steel wire increases, while the fatigue strength tends to decrease. Although the controlling factors of fatigue strength have not been clarified, it is believed that the fatigue strength decreases due to an increase in surface tensile residual stress, an increase in defects in microstructure of pearlite, an increase in surface flaw susceptibility, and the like.

[0007]

The inventors of the present invention have found that if a wire drawing method is improved in a specific true strain range, there is a region in which fine defects do not occur in the pearlite structure. Depends on the C content, the structure after wire drawing is characterized by the directionality of pearlite colonies, the pearlite lamellar spacing, and the cementite fragmentation rate. Within that condition range, the fatigue properties are extremely excellent and the surface tension The present invention has been achieved by finding that the residual stress is also reduced.
That is, a medium-high carbon steel wire rod having a carbon content of 0.6 to 1.0% is finally patented to form a fine pearlite structure, and a wire drawing strain is -1.82 [% C] + 3 ≤ true strain ≤. −
In the range of 1.82 [% C] +4.1, 90% or more of the pearlite colonies are parallel to the drawing direction, and the pearlite lamellar spacing after drawing is 0.02 to 0.06 μm.
The ultrafine steel wire having a tensile strength of 180 to 250 kgf / mm ² and an excellent fatigue property, which has a structural characteristic that the cementite constituting the pearlite colony has a fragmentation rate of 20% or less.

The reasons for limiting the present invention are as follows. If the carbon content is less than 0.6%, a high strength of 180 kgf / mm ² or more cannot be obtained even if a predetermined true strain is applied, while if the carbon content exceeds 1.0%, the pearlite structure is Since the pro-eutectoid cementite is deposited on and the rotatability of pearlite colonies is hindered during wire drawing,
The carbon content was limited to the range of 0.6 to 1.0%.

The composition other than carbon is not particularly limited, and normally, the composition similar to that of the wire of this type may be used.
For example, Si: 0.3-0.6%, Mn: 0.4-0.
7%, P: 0.005 to 0.015%, S: 0.005
˜0.015%, the balance being Fe and unavoidable impurities. If necessary, alloy elements such as Cr, Mo, Ni and V may be contained. The patenting heat treatment can be performed using a conventional lead bath furnace, fluidized bed, or the like, and is not particularly limited as long as a fine pearlite structure can be obtained.

The wire drawing strain (true strain) is -1.82 [% C] +3 or more and -1.82 [% C] + depending on the carbon content.
It was defined in the range of 4.1 or less. The change behavior of the grain structure of pearlite steel during wire drawing is schematically shown in Figs.
In the initial stage of wire drawing, the process of aligning the pearlite tissues in the longitudinal direction of the wire drawing by rotation of the pearlite colony, the process of stretching the aligned pearlite colony itself in the middle stage of drawing, and the process of cutting with cementite fragmentation in the final stage of drawing. Exists. FIG. 3 shows the effect of true strain on the hunter fatigue strength of steel types having different C contents. It is considered that the higher the amount of C, the faster the alignment of pearlite colonies in the drawing direction and the early separation of cementite, but it was found that the peak at which the fatigue strength becomes maximum differs depending on the amount of C, and the above range was defined. .

That is, when the true strain is less than -1.82 [% C] +3, the pearlite colonies are insufficiently aligned in the longitudinal direction of wire drawing, while the true strain of -1.82 [% C] +4.1 or more is true. With strain, the fragmentation rate of cementite in pearlite is 20.
%, The fatigue strength is considered to be reduced. Here, the fragmentation rate of cementite was defined as the number of cementites having a fragmentation site in the pearlite colony / the total number of cementites in the colony.

It was defined that 90% or more of the pearlite colonies were parallel to the drawing direction as the structure after drawing. The pearlite colonies are oriented in random directions immediately after patenting. It is considered that the higher the proportion of ferrite and cementite arranged in the wire drawing direction in the layered structure, the greater the effect of preventing crack propagation in the direction perpendicular to the wire drawing direction, and the better the fatigue properties. However,
About 10% of pearlite colonies are parallel to the wire drawing direction (parallel to the wire drawing direction refers to within ± 10 ° of the wire drawing direction)
Even if it is not, the effect can be secured, so 90% or more is specified.

The lamellar spacing after wire drawing is 0.02
0.06 μm was specified. If the lamellar spacing is less than 0.02 μm, the thickness of the ferrite is thin, and a slip band is generated inside the ferrite to facilitate crack propagation and fragmentation of cementite is also likely to occur.
The above is specified. On the other hand, if the lamellar spacing is more than 0.06 μm, the desired high strength of 180 kgf / mm ² or more cannot be obtained, so 0.06 μm or less is specified.

Next, the fragmentation rate of the cementite constituting the pearlite colony was specified to be 20% or less. Here, the fragmentation rate refers to the ratio of the number of cementites that have even one site of fragmentation to the total number of cementites that compose a pearlite colony. Voids are not always formed at the places where cementite is divided, but the surrounding ferrite is filled, but it becomes a crack propagation path in the direction perpendicular to the wire drawing direction, especially when the cementite separation rate exceeds 20%. Therefore, 20% or less is specified. In addition, to adjust to such a pearlite structure,
Normal wet drawing method can be used, but the surface layer of steel wire
This can be achieved by arranging a wire drawing die so that the degree of deformation between the middle and the center is as constant as possible, optimizing the lubricant, and improving the prevention of heat generation of the steel wire during wire drawing.

By combining the above conditions, for example, 0.82% C-0.20% Si-0.50Mn-
It is a high carbon steel composition system containing 0.008% P-0.009% S, and the final patenting is 1.0 mm and the finished wire diameter is 0.30 mm (true strain 2.4). 92% parallelism, lamellar spacing 0.041 μm,
Cementite dividing ratio of 8%, fine steel wire having a high fatigue properties Hunter fatigue strength 120 kgf / mm ² in fine steel wire of tensile strength 245kgf / mm ² was obtained.

[0016]

EXAMPLE Based on the present invention, an ultrafine steel wire of 0.30 to 0.60 mm was made by trial using the steels of the five kinds of components shown in Table 1. The final patenting treatment was carried out in a lead bath furnace, followed by ultrafine wire drawing. Various methods are conceivable for the ultra-fine wire drawing method, for example, a method of gradually reducing the area reduction rate to 14% at the beginning of wire drawing to about 9% toward the finishing wire, and within 2 steps from the final step Was set at a die approach angle of 10 ° to relax the tensile stress at the center of the steel wire and completely immersed in a water-soluble lubricating liquid to prevent heat generation of the steel wire during wire drawing.

Table 2 also shows the measurement results of the mechanical properties of the final LP material, the mechanical properties of the finished line, the structural characteristics, the fatigue strength, and the surface residual stress. Here, the parallelism of pearlite colonies showing the structural characteristics, the lamellar spacing, and the fragmentation rate of cementite were measured by observing the final drawn wire with a transmission electron microscope. The fatigue strength was measured by a Hunter-type rotary bending fatigue tester in an atmospheric environment with a humidity of 50% at a repetition number of 10 ⁷ or more, and the surface residual stress of the finished line was determined by X-ray diffraction.

[0018]

[Table 1]

[0019]

[Table 2]

Symbols A to D are examples of the present invention, and symbols E to L are shown.
Is a comparative example. In the examples of the present invention, the hunter fatigue strength of the wire was excellent at 100 kgf / mm ² or more, and the tensile residual stress on the surface was as low as 20 kgf / mm ² or less. Since the true strain of Comparative Example E was as small as 1.5, the tensile strength of the finished wire was 15
This is an example in which the fatigue strength was low because the parallelism of the pearlite colony was 90% or less as low as 2.7 kgf / mm ² . On the contrary, since the true strain of Comparative Example F was as large as 3.0,
This is an example in which the fatigue strength was lowered because the tensile strength of the finished wire exceeded 250 kgf / mm ² and the cementite fragmentation rate was as high as 30%. In this case, the surface tensile residual stress is also 60 kgf /
mm ² was extremely large.

Although Comparative Example G was subjected to a predetermined true strain, the parallelism of pearlite colonies was 90 because of poor drawing conditions.
This is an example in which the fatigue strength was lowered because the content was less than%. Although the comparative example H was subjected to a predetermined true strain, the lamellar spacing was 0.015 μm, which was too thin. On the contrary, the comparative example I was a lamellar spacing, which was 0.080 μm, which was too thick. Is. Comparative Example J is an example in which the fatigue strength was lowered because the fracture rate of cementite exceeded 20% and the tensile strength also exceeded 250 kgf / mm ² at the same time, although the predetermined true strain was applied, the wire drawing conditions were poor. Comparative Example K had a low C content of 0.48% and thus had proeutectoid ferrite after patenting, and Comparative Example L had a high C content of 1.05% and thus had proeutectoid cementite after patenting. In this example, the rotation of pearlite colonies was inhibited and the pearlite colony parallelism and the cementite fragmentation ratio in the specified range were not obtained, and the fatigue strength decreased.

[0022]

The present invention has a tensile strength of 180 to 250 kgf /
Since the fatigue strength of the ultrafine steel wire of mm ² can be dramatically improved,
It can be applied to applications such as bead wires for reinforcing tires and steel cords, where fatigue strength is particularly required. In addition, since the true strain is relatively small, the tensile residual stress in the surface layer is low, and the defects in the internal microstructure are also low, so disconnection during wire drawing and twisting can be suppressed to a very low level, and manufacturing of die basic units etc. The cost can also be improved.

[Brief description of drawings]

1A, 1B and 1C are schematic views showing the relationship between changes in mechanical properties of an ultrafine steel wire and pearlite colonies associated with drawing for obtaining an ultrafine steel wire according to the present invention.

FIG. 2 is a table showing the relationship between true strain, tensile strength, and drawing.

FIG. 3 is a chart showing a relationship between true strain and hunter fatigue strength of steel types having different C amounts.

Claims (1)

[Claims] 1. A medium to high carbon steel wire rod having a carbon content of 0.6 to 1.0% by weight is finally patented to form a fine pearlite structure, and a wire drawing strain is -1.82 [% C. ] +3
≤ true strain ≤ -1.82 [% C] + 4.1, 90% or more of the pearlite colonies are parallel to the drawing direction, and the pearlite lamellar spacing after drawing is 0.02
An ultrafine steel wire having a tensile strength of 180 to 250 kgf / mm ² and excellent fatigue properties, which has a structural characteristic that the cementite constituting a pearlite colony has a fragmentation rate of 20% or less in the range of 0.06 μm.