JPH11286748A - Steel wire excellent in fatigue property, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimally reduce the fatigue failure starting point as well as improve the material strength to obtain a steel wire having high fatigue characteristics by regulating the lattice strains of steel wire having a pearlite structure the specified in ratio of the contents of C and Si, to be in a specified range. SOLUTION: This steel wire is the one having a pearlite structure and having chemical components contg., by weight, 0.7 to 1.0% C and 0.5 to 1.5% Si, in which, in the case where the lattice constant is defined as (a), the lattice strains ΔaLS is allowed to satisfy 0.01×a<=ΔaLS<=0.002×a. At this time, the chemical components of the steel wire are preferably incorporated with Mn and Cr respectively by about<=1%, and the range of the lattice constant (a) is suitably regulated to be in the range of 2.8670 to 2.8705 Å. It is optimal that this steel wire is formed into a spring for automotive parts requiring fatigue strength or is utilized for reinforcing such as PC steel stranded wire by executing spring working and stranding working. Moreover, in the case of spring working, it is preferably that the surface residual stress is regulated to about <=100 MPa or is the compressive stress.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire having excellent fatigue characteristics, which is optimal for a spring or a PC steel wire, and a method for producing the same.

[0002]

2. Description of the Related Art Important properties of a steel wire include high tensile strength, high toughness, and high fatigue strength. However, in a steel wire to be drawn, high tensile strength and fatigue strength are not always compatible.

In general, the higher the degree of wire drawing, the higher the tensile strength. Further, the fatigue strength does not increase unless the tensile strength is high to some extent. However, increasing the degree of working increases the number of micro defects in the material due to plastic working, and when micro defects are gathered, the starting point causes fatigue fracture at an early stage.

[0004]

Therefore, in order to improve the fatigue characteristics, it is important to improve the strength by introducing a suitable defect and to avoid a suitable concentration of the defect. Conventionally, a steel wire to be drawn has been subjected to a heat treatment after drawing in order to enhance the toughness and to emphasize the tensile strength. Although the strength at that time was finally determined as the product strength as the final product, no particular attempt was made to improve fatigue.

An object of the present invention is to provide a steel wire capable of obtaining high fatigue characteristics by improving the material strength and optimally reducing the starting point of fatigue fracture, and a method of manufacturing the same.

[0006]

The steel wire of the present invention achieves the above-mentioned object, and has a characteristic feature of having a pearlite structure, a chemical composition of C: 0.7-1.0% by weight%, Si:
It is a steel wire containing 0.5 to 1.5% and the lattice strain Δa _LS satisfies the following condition when the lattice constant is a. 0.001 × a ≦ Δa _LS ≦ 0.002 × a

Here, it is desirable that the chemical composition of the steel wire contains 1% or less of each of Mn and Cr. Such a steel wire is subjected to spring processing or twisting to be used as a spring for automobile parts requiring fatigue strength, or as a steel wire used for reinforcement, such as a PC steel stranded wire, a control cable, a steel cord, or a parallel wire. It is optimal to use. When spring processing is performed, the surface residual stress is desirably 100 MPa or less as a tensile stress or a compressive stress. The lattice constant a is appropriately in the range of 2.8670 to 2.8705 °.

Further, the steel wire of the present invention has a pearlite structure, and has a chemical component of C: 0.7 to 1.0% by weight, Si:
A steel wire containing 0.5 to 1.5%, wherein the lattice constant Δa _LS satisfies the following condition when the lattice constant is a. 0.0025 × a ≦ Δa _LS ≦ 0.0045 × a In this case, the lattice constant a is 2.8670 to 2.8710 °.
It is preferred that

Further, the method of the present invention, which is most suitable for producing the above-mentioned steel wire, comprises the following:
%, Si: steel of pearlite structure containing 0.5% to 1.5% cold working, the lattice constant after cold working when the a _1, a same lattice strain .DELTA.a _LS1 within the following a step of, subjected to heat treatment _{0.0025 × a 1 ≦ Δa LS1 ≦} 0.0045 × a 1 obtained steel wire, the lattice constant when the a _2, a lattice strain .DELTA.a _LS2 within the following And a step of performing 0.001 × a ₂ ≦ Δa _LS2 ≦ 0.002 × a ₂

Here, the chemical components of the steel wire include Mn and Cr.
Is desirably contained at 1% or less. Examples of the cold working include wire drawing, roller dies, swaging, rolling, and forging. As the range of a ₁ from 2.8670 to 2.8
The range of 710 ° and a ₂ is 2.8670 to 2.87.
About 05 ° is appropriate. Improving the strength by introducing appropriate strain by cold working and removing the strain appropriately by subsequent heat treatment, avoiding the concentration of micro defects and eliminating the starting point of fatigue fracture Improve. Incidentally, the conventional steel wire, the lattice constants a ₃ after cold working 2.8665～2.8710A, lattice strain .DELTA.a _LS3 is 0.
001 was _{× a 3 × 0.0045 × a 3} . The lattice constants a ₄ after heat treatment from 2.8665 to 2.8695
Å, lattice strain Δa _LS4 is 0.0015 × a ₄ or more;
Fatigue strength was low.

The drawing (cold working) conditions after patenting are such that the smaller the die approach angle is,
The smaller the degree of work and the smaller the angle at which the steel wire is drawn, the smaller the variation in lattice strain. As the heat treatment conditions after the wire drawing, the higher the heat treatment temperature, the smaller the variation in lattice strain. Furthermore, the larger the amount of Si, the larger the lattice constant, the smaller the degree of cold working, the greater the variation in the lattice constant, and the higher the heat treatment temperature, the greater the variation in the lattice constant.

As will be apparent from experiments described later, it has been found that the fatigue properties are dramatically improved by defining the lattice constant and the lattice strain as described above. In other words, for the first time, the correlation between lattice strain and fatigue can be clarified, and it can be seen that controlling the lattice strain to an appropriate range can remove a defect that can be a starting point of fatigue and improve fatigue characteristics.

The lattice constant itself is a value obtained by a conventional material (however, it is not controlled). However, a range of lattice strain suitable for this lattice constant has not been specified so far. That is, conventionally, based on the idea that if the tensile strength is simply high, the fatigue strength will be improved, the pearlite strength UP (patenting temperature reduction), the wire drawing degree UP, the material strength UP = high C, etc. However, the fatigue strength did not improve.

On the other hand, it has been found that the average amount of strain for improving the fatigue strength and its distribution can be controlled. The average strain amount is a lattice constant a of 2.8670-
2.8705 ° is desirable, and the strain distribution is lattice strain Δ
It has been found that it is preferable if a _LS is 0.001 × a ≦ Δa _LS ≦ 0.002 × a. These indicate that the fatigue does not increase only by the patenting conditions, the working degree, the components, etc. as in the conventional case, and that the fatigue strength is not determined only by the tensile strength of the final product.

The lattice constant can be determined by the X-ray diffraction method. The lattice strain is also obtained by the X-ray diffraction method, but the analysis based on the half width of a general diffraction peak is qualitative,
Even if the half width is quantified, its meaning is ambiguous in material terms. Therefore, as a result of earnestly studying a method capable of evaluating these with high accuracy, it was possible to clarify a material range in which the fatigue characteristics can be improved. In this method, the lattice strain is separated from the crystallite size by a calculation called the Wilson method for the conventional general X-ray diffraction.

First, the lattice distortion will be described. This is caused by non-uniform deformation, rotation, displacement, processing, etc. of the unit cell inside the crystal, and is microscopically caused by point defects or dislocations. The size of the unit cell is larger or smaller than an ideal size without distortion, and a tensile force or a compressive force remains in terms of stress. When the size of the lattice is measured by X-ray diffraction for such a material, the diffraction peak is not sharp but wide. By evaluating the half width of this width (measuring the width at a height position that is half the peak height), the magnitude of the distortion can be roughly determined.

However, this width is increased not only by the size of the unit cell but also by the expansion and crystallite size (X-ray crystal grain size) inherent to the apparatus. Therefore, in order to accurately evaluate the variation in the size of the unit cells, they need to be separated. This is precisely measured by lattice distortion.

A method for measuring lattice distortion will be described. This method is commonly used for evaluating ceramics and the like. The half width of several diffraction peaks was determined, and Wilson
The lattice strain and the crystallite size are calculated separately by a calculation called a method. Several diffraction peaks are measured to determine a half width (integral width). This time, 110, 200, 211,
Five of 20, 311 are measured. The instrument constant is calibrated using the half width of the same diffraction peak of the standard sample (in this case, pure iron powder), and the half width is determined only by the influence of lattice strain and crystallite size. The horizontal axis is plotted as [(Δ2θ) / (tanθ ₀ sinθ ₀ )], and the vertical axis is plotted as [(Δ2θ) ² / tan ² θ ₀ ] to obtain an intercept (expansion due to crystallite size is determined by Cauchy function, lattice distortion Spread is approximated as Gaussian function). The value obtained by dividing the obtained square root of the intercept by 4 is the value of the lattice distortion obtained here.

The number of diffraction peaks need not be five. Further, although it is not necessary to use the same diffraction peak as this time, the accuracy increases as the number of diffraction peaks increases. In the evaluation, a value indicating the distribution state of the strain is used, and the evaluation is shown in anonymous (or%). Note that Δ2θ is a half-value width (integral width) in the unit of “radian”,
θ ₀ is the diffraction angle and the unit is “degree”. By controlling the lattice strain for a predetermined amount of C and a predetermined amount of Si by such an evaluation, it is possible to achieve a high fatigue characteristic which cannot be obtained by the conventional evaluation based on the X-ray half-value width.

In the steel wire and the method of manufacturing the same according to the present invention, the chemical composition and structure of the steel wire are limited for the following reasons. C (0.7% or more, 1.0% or less) is the most effective element for increasing the strength of a steel wire. If it is less than 0.7%, sufficient strength cannot be obtained, and if it exceeds 1.0%, segregation problem occurs and it is not practical. Si (more than 0.5%, 1.5% or less) basically has the effect of a deoxidizing agent and is necessary for reducing nonmetallic inclusions. 0.5% over 0.5%
The effect of solid solution strengthening is greater than the following, and the fatigue properties are further improved. Mn has a deoxidizing effect similarly to Si. 1
%, The hardenability increases, the time for pearlite transformation increases, and the productivity decreases. Cr has an effect on the strength UP, but since the hardenability increases like Mn, 1% or less is appropriate. The reason for using pearlite steel is that when wire drawing is performed, the balance between strength and toughness is good.

[0021]

Embodiments of the present invention will be described below. In each example, the method of obtaining the lattice distortion was performed by the method described above. (Example 1) A test material having the following components (all units are% by weight) was melted and cast, hot cast and hot rolled, and then subjected to a drawing process and a patenting treatment. Further, a steel wire was manufactured by performing a cold small diameter working heat treatment. The obtained steel wire was subjected to a fatigue test, and the lattice strain was measured by X-ray diffraction.

Component C Si Mn Cr Conventional material 0.82 0.21 0.51 0.05 Developed material 1 0.80 0.89 0.28 0.11 Developed material 2 0.80 1.21 0.22 0 .13 Comparative material 1 0.81 1.87 0.25 0.12 Comparative material 2 0.80 0.05 0.32 0.12

The size in each step is 5.5 after hot rolling.
mm and 3.6 mmφ after drawing. The patenting was 570+ (Si% × 30) ° C. Further, the cold working was performed by drawing with a hole die. Development materials,
The wire drawing condition of the comparative material is 8
° The surface reduction rate per processing was set to 18 to 15%. The wire drawing speed was 10 m / min or less, and the drawing direction from the die exit to the contact with the pot was controlled within 0.5 ° from the center axis of the die hole. After the wire was processed from 3.6 mmφ to 1.6 mmφ by the wire drawing, the wire was straightened and heat-treated. This heat treatment is 350-450
The test was performed at a temperature of 20 ° C for 20 minutes. The production conditions of the developed material and the comparative material are the same except for the chemical components.

On the other hand, in the wire drawing of the conventional material, an approach angle of 11 °, a reduction in area per work of 20 to 17%, and a wire drawing speed of 30 to 500 m / min are selected. The angle was set to about 1 ° (in order to add a line pattern). The heat treatment condition after drawing is 300 to 350 ° C. × 20 minutes.

A Hunter-type rotary bending fatigue test was performed on the test material obtained by the above method to determine the fatigue strength, and the lattice constant and lattice strain by X-ray diffraction. Table 1 shows the lattice constant and lattice strain after drawing and after heat treatment, respectively.

[0026]

[Table 1]

The results are shown in the graph of FIG. 1 together with the results of the fatigue characteristics. As is clear from this graph, when the developed materials 1 and 2 which are the steel wires of the present invention have a lattice constant of a,
It can be seen that the fatigue limit is high and the fatigue characteristics are excellent in the range of 0.001 × a ≦ Δa _LS ≦ 0.002 × a.
On the other hand, the conventional material and the comparative materials 1 and 2 have poor fatigue characteristics. From these facts, before heat treatment, that is, after cold working, the lattice strain is 0.0025a to 0.0045a.
It is clear that the lattice strain should be within the range of 0.001a to 0.002a after the heat treatment.

(Example 2) The developed material 1 was processed in the same manner as in Example 1 until wire drawing to 1.6 mmφ was performed, then processed into a coil spring, and subjected to a fatigue test. When the heat treatment conditions after coiling were changed from 300 ° C to 450 ° C,
The residual stress varied from 280 MPa in tension to 30 MPa in compression. FIG. 2 is a graph showing the results of a fatigue test performed on the springs obtained under the respective heat treatment conditions using a star-type fatigue tester. As a result, when the lattice constant is a, 0.001 ×
In the range of a ≦ Δa _LS ≦ 0.002 × a, 1
A particularly high fatigue limit was exhibited when the residual stress was less than or equal to 00 MPa.

(Example 3) The steel type of the component of the developed material 1 was 1
It was rolled to 1.5 mmφ and immediately thereafter cooled in boiling water to perform pearlite transformation. This wire rod was 4.22 mmφ and 4.
Wire drawing to 35mmφ, centering 4.35mmφ steel wire,
Stranding was performed using 4.22 mmφ steel wires as side wires (six bundles). After the stranding, heat treatment was performed at a temperature in the range of 350 to 450 ° C. to increase the yield point, thereby forming a PC steel strand. It can be easily presumed that the same effect may be obtained by cooling with lead, salt, mist, strong wind or the like, instead of cooling with boiling water.

The drawing conditions were basically the same as those in Example 1 except for the size. A tensile fatigue test of the thus obtained PC strand was performed. 86.4kg fatigue test
The magnitude (σA) of the total amplitude load up to breakage was determined with a maximum load of / mm ² . The breaking life was 2 million times. In addition, the lattice constant and the lattice strain were also obtained in the same manner as in Example 1.

As a result, assuming that the lattice constant is a, the total amplitude load (σA) is large in the range of 0.001 × a ≦ Δa _LS ≦ 0.002 × a and the fatigue characteristics are excellent. Recognize.

(Example 4) In the components of the developed material 1,
After patenting at 65 mmφ, the degree of wire drawing and the heat treatment conditions after wire drawing were changed to examine the respective fatigue strengths. Except for the wire diameter (working degree), the fatigue test conditions, wire drawing conditions, and heat treatment conditions are the same as those in Example 1. FIG. 4 shows the relationship between fatigue properties and the lattice constants after wire drawing and heat treatment, where a ₁ and a ₂ are the lattice constants and Δa _LS1 and Δa _LS2 are the lattice strains, respectively.
Shown in

[0033] As is apparent from this graph, when the lattice constant after wire drawing was set to a _1, the lattice strain .DELTA.a _LS1 is 0.
In the range of 0025 × a ₁ ≦ Δa _LS1 ≦ 0.0045 × a ₁ , when the lattice constant of the steel wire after the heat treatment is a ₂ , the lattice strain Δa _LS2 is 0.001 × a ₂ ≦ Δa _LS2 ≦ 0. 002
When it is within the range of × a ₂ , it can be seen that high fatigue characteristics are provided.

[0034]

As described above, according to the steel wire of the present invention, the fatigue characteristics of the steel wire can be remarkably improved by specifying the lattice constant and the lattice strain. Therefore, the steel wire of the present invention can be effectively used as a spring or a PC steel stranded wire. Further, the production method of the present invention is an optimal method for producing the steel wire of the present invention, and can obtain a steel wire in which the lattice constant and the lattice strain are specified in a predetermined range.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between lattice strain / lattice constant and fatigue limit in steel wires having different chemical components, drawing conditions and heat treatment conditions.

FIG. 2 is a graph showing the relationship between lattice strain / lattice constant and fatigue limit in a steel wire processed into a coil spring.

FIG. 3 shows the lattice strain in a steel wire processed into a PC stranded wire /
5 is a graph showing a relationship between a lattice constant and a total amplitude stress up to a fatigue limit.

FIG. 4 is a graph showing the relationship between lattice strain / lattice constant and fatigue limit in a drawn steel wire and the relationship between lattice strain / lattice constant and fatigue limit in a steel wire after heat treatment.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Koji Yamaguchi 1-1-1, Kunyokita, Itami-shi, Hyogo Pref. Within Itami Works, Sumitomo Electric Industries, Ltd.

Claims (7)

[Claims] 1. A steel wire having a pearlite structure and containing, by weight%, C: 0.7 to 1.0% and Si: 0.5 to 1.5%, wherein a lattice constant is a And the lattice strain Δ
a A steel wire having excellent fatigue properties, wherein the _LS satisfies the following conditions. 0.001 × a ≦ Δa _LS ≦ 0.002 × a 2. The spring according to claim 1, wherein the steel wire according to claim 1 is subjected to spring processing, and the surface residual stress thereof is not more than 100 MPa in tensile stress or compressive stress. 3. A twisted steel wire obtained by further processing the steel wire according to claim 1. 4. The lattice constant a is 2.8670 to 2.870.
The steel wire having excellent fatigue resistance according to claim 1, wherein the steel wire has an angle of 5 °. 5. A steel wire having a pearlite structure and containing 0.7 to 1.0% of C and 0.5 to 1.5% of Si by weight of a chemical component, wherein the lattice constant is a And the lattice strain Δ
a A steel wire having excellent fatigue properties, wherein the _LS satisfies the following conditions. 0.0025 × a ≦ Δa _LS ≦ 0.0045 ×
a 6. The lattice constant a is 2.8670 to 2.871.
The steel wire excellent in fatigue resistance according to claim 5, wherein the angle is 0 °. 7. C: 0.7 to 1.0 by weight of a chemical component.
%, Si: steel of pearlite structure containing 0.5% to 1.5% cold working, the lattice constant after cold working when the a _1, a same lattice strain .DELTA.a _LS1 within the following a step of, subjected to heat treatment _{0.0025 × a 1 ≦ Δa LS1 ≦} 0.0045 × a 1 obtained steel wire, the lattice constant when the a _2, a lattice strain .DELTA.a _LS2 within the following And producing a steel wire having excellent fatigue properties. 0.001 × a ₂ ≦ Δa _LS2 ≦ 0.002 × a ₂