JPH08156514A - High strength steel wire excellent in twisting crack resistance - Google Patents

Abstract

PURPOSE: To provide an extra-high tension extra fine steel wire excellent in twisting crack resistance which is used for automobile tire steel cord. CONSTITUTION: This high strength steel wire excellent in twisting crack resistance is a carbon steel having a carbon content of 0.40-1.10% by weight formed of a tissue of two phases of ferrite/pearlite, pearlite single phase, or two phases of pearlite/cementite, wherein the hardness distribution of a steel wire having a wire diameter of 0.60-0.05mm and a strength 3300-4500MPa satisfies the condition of 0.960<=HV<=1.03 in R=0, R=0.8, R=0.95. (When the radius of the steel wire is r0 , and the distance between an optional position and the center of the steel wire is r, R shows R=r/r0 . When the hardness in the position of R=0.5 is HV0.5 , and the hardness in the position R is HVR, HV shoes HV=HVR/HV0.5 ).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength ultrafine steel wire used for steel cords of automobile tires. More specifically, a wire diameter of 0.60 to 0.05 mm and a strength of 3300 to 60, which has been reinforced by cold drawing using a die.
The present invention relates to an ultra-fine steel wire of 00 MPa class.

[0002]

2. Description of the Related Art In automobile tire steel cord wires, the demand for higher tensile strength of steel wires has recently been increasing more and more due to the demand for weight reduction of tires. As a result of energetic development of many studies to meet such demands,
It has been found that the biggest challenge in increasing the tensile strength of a steel wire is to establish a technique for suppressing the ductility of the steel wire, particularly the occurrence of cracks that occur in the longitudinal direction of the steel wire during a torsion test. It was

On the other hand, WIRE JOURNAL
VOLUME 16, 1 of INTERNATIONAL
983, NO. Page 50 of No. 4 describes that cracking in the longitudinal direction of the galvanized steel wire can be suppressed by controlling the cementite lamella spacing of the pearlite structure constituting the steel wire to an appropriate size. In addition, Japanese Examined Patent Publication No. 60-26805 and Japanese Examined Patent Publication No. 60-26806.
Japanese Patent Laid-Open Publication No. 20-29200 discloses a bluing treatment (20) for removing strain after processing such as twisted wire processing or spring winding processing.
However, cracking in the longitudinal direction in the twist test poses a problem because the toughness of the steel wire decreases, but after the wire drawing or during wire drawing, straightening under certain conditions is performed. It is described that this processing can suppress the cracks in the longitudinal direction.

[0004]

However, according to the research conducted by the present inventors, the wire diameter is 0.60 to 0.05 mm and the strength is 3300 to 6000 MP even with the above-mentioned technique.
It has been found that when an a-class extra fine steel wire is to be produced, the occurrence of longitudinal cracks in the twist test cannot be suppressed.

The present invention has been made for the purpose of establishing a technique for suppressing the occurrence of cracks in the longitudinal direction during a twisting test of an ultra-high tensile ultrafine steel wire used for automobile tire steel cords. is there.

[0006]

In order to solve the above problems, the present invention controls the hardness distribution in the steel wire and the residual stress in the surface layer so that the ductility of the high-strength ultrafine steel wire, especially during the twist test. In the above, the occurrence of cracks in the longitudinal direction is suppressed. That is, the gist of the present invention is as follows.

(1) Carbon content in% by weight, C: 0.4
0 ~ 1.10% carbon steel with ferrite / pearlite 2
Phase or pearlite single phase or pearlite / cementite dual phase structure and wire diameter of 0.60 to 0.05 m
m, the hardness distribution in the steel wire having a strength of 3300 to 4500 MPa is R = 0, R = 0.8, R = 0.95,
A high-strength steel wire having excellent resistance to twist cracking, which satisfies the condition of 0.960 ≦ HV ≦ 1.030.

However, R is R = r / r _O where r _O is the radius of the steel wire and r is the distance between the arbitrary position and the center of the steel wire.
Indicates. HV is HV _0.5 when the hardness at the position of R = 0.5 is HV _0.5 and HV _R is the hardness at the position R, then HV = HV _R /
HV _0.5 is shown. (2) Carbon content is% by weight, C: 0.40 to 1.10
% Carbon steel, consisting of a ferrite / pearlite dual phase or a pearlite single phase or a pearlite / cementite dual phase, and having a wire diameter of 0.60 to 0.05 mm and a strength of 3
The hardness distribution in the steel wire of 300 to 6000 MPa is R =
0, R = 0.8, R = 0.95, 0.960 ≦
A high-strength steel wire excellent in torsional cracking resistance, which satisfies the condition of HV ≦ 1.030 and has a residual stress of the surface layer controlled within a range of +200 MPa on the tensile side to −1000 MPa on the compression side.

[0009] However, if R is the radius of the steel wire r _O, the distance between the arbitrary position and the center of the steel wire and r, R = r / r _O
Indicates. HV is HV _0.5 when the hardness at the position of R = 0.5 is HV _0.5 and HV _R is the hardness at the position R, then HV = HV _R /
HV _0.5 is shown.

[0010]

The present invention is a steel which is subjected to a patenting heat treatment or the like to form a ferrite / pearlite dual phase or a pearlite single phase or a pearlite / cementite dual phase structure, and which is given a good wire drawability and then is drawn. It is intended for lines. FIG. 1 shows the hardness distribution of a steel wire obtained by wire-drawing a carbon steel wire rod containing 0.8% of C to a wire diameter of 0.30 mm and a strength of strength 3800 MPa class and the occurrence rate of vertical cracks during a twist test (N = The relationship of the vertical crack occurrence ratio of 30 samples) is shown. It can be seen that cracking can be suppressed only within the range of the present invention.

In the twist test, the shear stress becomes maximum at the surface layer of the steel wire and decreases as it goes inward. When a vertical crack occurs in the twist test, as shown in FIG.
Since it occurs in the initial stage of twisting and immediately after yielding, it is particularly important to secure ductility in the vicinity of the surface layer of the steel wire.
The strength of the steel wire is considered to correspond to the average hardness in the steel wire, but in order to secure the ductility of the surface layer, it is necessary to suppress the hardness in the vicinity of the surface layer from becoming abnormally high.
The inventors of the present invention have found that it is important to control the hardness distribution within the scope of the present invention in order to achieve both high strength of the steel wire and ensuring ductility in the vicinity of the surface layer.

For example, when the pulling resistance is high and a large shear stress is generated in the surface layer of the steel wire during drawing of the die due to insufficient lubricity of the lubricant in the wire drawing process, work hardening near the surface layer is preferentially promoted. The hardness in the vicinity of the surface layer becomes abnormally high, the ductility deteriorates, and vertical cracking occurs during the twisting test. Further, during the patenting treatment, particularly if the solution treatment conditions are not appropriate, when decarburization and decarburization layers occur in the vicinity of the surface layer, even if the internal hardness is uniform, the surface layer hardness is abnormally reduced and falls outside the range of the present invention. Due to the extreme lack of strength of the surface layer, vertical cracking occurs during the twisting test.

Controlling the hardness distribution within the range of the present invention means that the wire drawing strain is uniformly applied,
It has the effect of reducing the tensile residual stress on the surface of the steel wire, which eliminates the potential cause of the occurrence of vertical cracks and leads to the improvement of the twisting property. Further, it is effective to suppress the vertical cracking by positively applying the compressive residual stress. FIG.
In a steel wire with a C content of 0.8% and a wire diameter of 0.05 mm, the relationship between the surface layer residual stress when the hardness distribution is controlled within the range of the present invention and the critical strength at which vertical cracking does not occur during the twisting test is shown. It is a thing. When the residual stress is not actively controlled, the strength is limited to the level of the 4500 MPa class of claim 1, but by controlling the residual stress within the range of claim 2, iron which has hitherto been unattainable. It is possible to achieve a half level of the ideal intensity of.

The twisting characteristics are also improved at lower strength levels. As shown in FIG. 4, 0.8
% C, wire diameter 0.30 mm, strength 3800 MPa class It is possible to increase the twisting speed of the steel wire. However, even if the residual stress exceeds a value of -1000 MPa of compression, the effect is saturated and the treatment is economically expensive. Therefore, the range of the present invention is defined as above.

The hardness distribution of the present invention is achieved by a combination of several means described below. It suppresses decarburization, decarburization, and bainite layer formation near the surface layer by optimizing the patenting treatment conditions. The area reduction rate near the final stage of the wire drawing process is set to 11% or less.
In particular, the reduction rate of the final stage and at most up to the fourth stage before the final stage is 11% or less. Further, the surface reduction rate of the final stage is divided into two dies, the final stage is 1.0% or more and 5.0% or less, and the front side is 5% or more and 11% or less.

Bearing length 0.20 to 0.4d (d
Is a die hole diameter), and a die with a die approach angle of 12 to 5 degrees is used. In particular, when the above-mentioned reduced surface area drawing is performed near the final stage, a die of 10 degrees or less is used. In addition to wire drawing with reduced surface ratio, roll straightening is performed after or during wire drawing, or after wire drawing and during wire drawing.

Further, as a method for controlling the residual stress, there are the following means. Shot peening is performed after wire drawing. After wire drawing, straightening is performed with back tension. In the final stage of ultrafine wire drawing, wire drawing with a reduced area ratio of less than 4% is performed. The content of C in the present invention is defined in the range of 0.40 to 1.10% because if the content is less than 0.40%, the proportion of ferrite in the microstructure after patenting treatment becomes too large.
This is because if it exceeds 10%, the amount of pro-eutectoid cementite at the grain boundaries is excessively precipitated, and good twisting characteristics cannot be secured.

The effects of the present invention will be described in more detail below with reference to examples.

[0019]

Examples Table 1, Table 2 (continued from Table 1), Table 3 and Table 4 (continued from Table 3) show twisted longitudinal cracks of steel wires with controlled C content and hardness distribution within the scope of the present invention. The occurrence situation of was shown.
Tables 5 and 6 (continuation of Table 5) show the characteristics when the values are out of the range of the present invention. As shown in Tables 1 to 4, it is obvious that controlling the hardness distribution within the range of the present invention by wire drawing of a reduced surface, straightening, etc. is effective in suppressing the occurrence of cracks. In addition, Table 7 and Table 8 (Table 7
(Continued) shows an example of controlling the residual stress, but it is clear that the effect of suppressing vertical cracking at high strength is great.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

[0025]

[Table 6]

[0026]

[Table 7]

[0027]

[Table 8]

[0028]

As described in detail above, the inventors of the present invention have conducted research on a technique for preventing the occurrence of longitudinal cracks that occur during the twisting test of a steel wire, and as a result, the hardness of the steel wire It was found that controlling distribution and surface residual stress is extremely effective in suppressing the occurrence of cracking. According to the present invention, it is possible to manufacture a steel wire having a higher tensile strength, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of hardness distribution on the occurrence of vertical cracks during the twisting test of a steel wire.

FIG. 2 is a diagram showing a relationship between a twist angle and torque when a vertical crack occurs during a twist test of a steel wire.

FIG. 3 is a diagram showing the effect of surface residual stress on the critical strength at which vertical cracking does not occur during the twisting test of a steel wire.

FIG. 4 is a diagram showing the effect of surface residual stress on the twisting speed of a steel wire.

Claims (2)

[Claims] 1. The carbon content in% by weight, C: 0.40-
1.10% carbon steel consisting of ferrite / pearlite dual phase or pearlite single phase or pearlite / cementite dual phase structure, and wire diameter of 0.60 to 0.05 mm,
The hardness distribution in the steel wire having a strength of 3300 to 4500 MPa is 0 .. at R = 0, R = 0.8, and R = 0.95.
A high-strength steel wire having excellent resistance to twist cracking, which satisfies the condition of 960 ≦ HV ≦ 1.030. Here, R in the case where the radius of the steel wire was r _O, the distance between the arbitrary position and the center of the steel wire is r, shows a R = r / r _O. HV is R = 0.5
If the hardness of the position of the HV _0.5, the hardness of the position R and the HV _R, indicating the HV = HV _R / HV _0.5. 2. The carbon content in% by weight, C: 0.40-
1.10% carbon steel consisting of ferrite / pearlite dual phase or pearlite single phase or pearlite / cementite dual phase structure, and wire diameter of 0.60 to 0.05 mm,
The hardness distribution in a steel wire having a strength of 3300 to 6000 MPa is 0 .. at R = 0, R = 0.8, and R = 0.95.
The condition of 960 ≦ HV ≦ 1.030 is satisfied, and the residual stress of the surface layer is +200 MPa on the tensile side to compression −1000 MP.
A high-strength steel wire having excellent resistance to twist cracking, which is controlled within the range of a. However, R is R = r, where r _O is the radius of the steel wire and r is the distance between the arbitrary position and the center of the steel wire.
/ R _O is shown. HV is the hardness at the position of R = 0.5
And _0.5, if the hardness of the position R and the HV _R, HV = H
V _R / HV _0.5 is shown.