JP6736950B2 - Steel wire and method for manufacturing the steel wire - Google Patents

Description

The present invention relates to a steel wire and a method for manufacturing the steel wire.

An ultra-fine steel wire having a high strength and a wire diameter of 0.38 mm or less is used for reinforcing a rubber or an organic material. For example, in general, ultra-fine steel wire (also referred to as “filament” below) used as a reinforcing material for tires is obtained by subjecting pearlite-transformed high-carbon wire to intermediate patenting and dry drawing by hot rolling. After the wire diameter is adjusted to be adjusted to a target tensile strength, final patenting, copper plating or brass plating treatment is performed, and then wet drawing is performed.

Since the filaments thus produced are used as a reinforcing material, a twisted wire process for twisting a plurality of filaments is generally performed for the purpose of imparting structural elongation. For this reason, the shape of each individual filament is a loose spiral (coil).

When a tire in which a stranded wire of this form is embedded in rubber is subjected to actual running, the stranded wire is mainly subjected to repeated tensile loads. At this time, when viewed as a filament alone, a state in which forward and reverse twists are repeatedly applied is obtained. Therefore, the filament having a spiral shape is required to have durability against repeated twists in the forward and reverse directions.

With respect to the durability against repeated forward and reverse twists of the filament, many developments have been made to improve the characteristics of the steel wire using delamination that occurs when twisting only in one direction as an evaluation index.

For example, in Patent Document 1, the heating temperature upper limit during patenting is specified, and the surface layer portion temperature of the wire rod is made lower than the internal temperature thereof after the start of the cooling step and before the pearlite transformation, whereby the average pearlite nodule size of the surface layer portion. Has proposed a technique for improving the twisting characteristic of the steel wire by making the structure smaller than the inside by 0.3 μm or more.

Further, in Patent Document 2, in the forced cooling stage from the austenitizing temperature at the time of patenting, after cooling to 500 to 560° C. once and then reheating and then performing pearlite transformation, a cross-section of the drawn wire is obtained. A technique is proposed in which the product (Da×Db) of the major axis length (Da) and the minor axis length (Db) of ferrite grains is controlled to a certain value or less to suppress vertical cracking in a high strength material.

Further, in Patent Document 3, the maximum diameter of the voids contained in the steel wire is defined as an upper limit in relation to the shear yield stress of the steel wire, and the tensile residual stress value of the surface layer and the upper limit value of the variation in the circumferential direction thereof are respectively set. By prescribing it, we propose a technology to manufacture steel wire with excellent delamination resistance.

Further, in Patent Document 4, the tensile residual stress of the steel wire surface is defined as the wire diameter by defining the strength of the final patenting material, the area ratio of pro-eutectoid ferrite/cementite, and the wire drawing method after patenting. We propose a technology that suppresses delamination by keeping the value below the corresponding value.

On the other hand, there have been many developments to improve the characteristics of the steel wire by focusing on the difference in structure between the surface layer and the center and the degree of integration, regarding the durability of the filament against repeated twists.

For example, in Patent Document 5, by applying a patenting treatment and an appropriate plastic deformation in a warm working region of 550 to 650° C., a difference in the surface layer and the center structure of the high carbon steel wire rod before wire drawing is created. We have proposed a method for manufacturing high carbon steel wire rods that have excellent wire drawing workability.

Further, in Patent Document 6, the heating temperature, rolling temperature, and cooling rate in the wire rod/bar steel manufacturing process are regulated to create a structural difference between the resurface layer and the D/4 part in the same manner as in JP 2011-219829 A, and twist at low temperature. We propose a method for manufacturing wire rods and steel bars with excellent characteristics.

Further, in Patent Document 7, the component range and the finish rolling temperature are defined in the rolling process, the difference between the surface layer and the center of the pearlite block size is defined, and the {110} plane integration degree is defined to improve the mechanical descaling property. It specifies the structure of excellent wire rods and steel bars.

JP-A-11-241280 JP-A-11-199978 JP 2001-279380 A JP 2001-279381 A JP, 2011-219829, A JP, 2009-120906, A WO2011/05574

By the way, a steel wire having a wire diameter of 0.38 mm or less has a very low rigidity against bending, and it is very difficult to accurately grip the center axis of rotation when carrying out a torsion test. It has not been possible to accurately evaluate the durability (that is, vertical cracking resistance) against repeated twists in the forward and reverse directions, which is required in the actual running of a tire in which a wire is used as a reinforcing material.

Therefore, none of the above technical documents has proposed a steel wire excellent in longitudinal crack resistance against repeated forward and reverse twists and a manufacturing method thereof. That is, with any of the above techniques, it is necessary to impart a tensile strength of 3300 MPa or more to a steel wire having a wire diameter of 0.38 mm or less, and sufficiently secure longitudinal cracking resistance against repeated forward and reverse twists. In reality, the tensile strength of the original steel wire cannot be fully exerted as a steel wire in a stranded wire.

Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide a steel wire excellent in vertical crack resistance against repeated forward and reverse twists, and a method for manufacturing the same. ..

The present inventors have developed a mechanism for accurately gripping a steel wire in accordance with the rotation axis, repeatedly applying forward and reverse twists, and continuously recording the response characteristics (torque fluctuation) of the steel wire during that time. For the first time, it has become possible to quantitatively evaluate the longitudinal crack resistance of steel wire against repeated forward and reverse twists.

Then, the inventors conducted an evaluation experiment on the steel wire prototyped by various processing methods with the evaluation method. Thereby, the present inventors have found that in order to impart excellent longitudinal crack resistance to repeated forward and reverse twisting to the steel wire, a metal structure located between the center of the steel wire and the surface of the steel wire. In, the width of the elongated crystal grain, the inclination difference of the crystal orientation between adjacent crystal grains, and the degree of integration of the iron cubic {110} plane on the great circle of the positive point diagram of the iron cubic {110} plane. It has been found that it is important to keep the angle between the maximum value and the length direction of the steel wire within a certain range.
Furthermore, in order to impart this specific metallographic structure to the steel wire, the present inventors carry out twisting, which is a plastic deformation mode other than wire drawing, to change the texture of the elongated crystal grains. We have also found that is important.

The present invention was made based on the above findings, and the gist thereof is as follows.

<1>
In a steel wire having a circular cross section with a wire diameter of 50 to 380 μm and a tensile strength of 3300 to 3900 MPa,
In mass %, C: 0.85 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.0%, Cr: 0.01 to 1.30%, Nb: 0 to 0.5%, V: 0 to 0.5%, and Mo: 0 to 0.2%, and the balance consisting of Fe and impurities,
Metal observed in a cross section of the steel wire which is parallel to the length direction of the steel wire and includes the center of the steel wire, and which is located between the center of the steel wire and the surface of the steel wire. The structure has a crystal grain width of 50 to 200 nm measured on a radial line segment of a steel wire, a crystal orientation inclination difference of 5 to 30° between adjacent crystal grains, and an iron cubic crystal { A steel wire having an angle of 1° to 45° between the direction in which the degree of integration of the iron cubic {110} face is maximum and the length direction of the steel wire, on the great circle of the positive electrode point diagram of the 110} face.

<2>
In the repeated twist test of constant strain amplitude performed at 90% of the strain corresponding to the 0.2% proof stress point of the steel wire measured in the one-way twist test, the number of repetitions until the steel wire breaks is 100 times or more. The steel wire according to 1>.

<3>
In mass %, C: 0.85 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.0%, Cr: 0.01 to 1.30%, Nb: A steel wire rod containing 0 to 0.5%, V: 0 to 0.5%, and Mo: 0 to 0.2%, and the balance being Fe and impurities, was subjected to one or more first elongations. A first wire drawing process in which wire drawing is performed to obtain a wire drawn material;
A twisting step of subjecting the drawn wire to a twisting process of 10 to 200 times per length of the drawn wire obtained by multiplying the diameter D of the drawn wire by 100; and
A second wire drawing process for obtaining a steel wire by subjecting the intermediate wire drawing material to a second wire drawing process once or a plurality of times under conditions of 1.2 or more as a true wire drawing line strain, and
A method for manufacturing a steel wire having:

<4>
The method for manufacturing a steel wire according to <3>, wherein in the twisting step, twisting is performed 25 to 75 times per length of the drawn wire obtained by multiplying the diameter D of the drawn wire by 100.

According to the present invention, it is possible to provide a steel wire excellent in longitudinal crack resistance against repeated twists in the forward and reverse directions, and a manufacturing method thereof.

It is a schematic diagram for demonstrating the cross section (observation surface) of the steel wire which observes a metal structure. It is a schematic diagram for demonstrating the method of identifying a crystal grain. It is a schematic diagram for demonstrating that a crystal grain is regarded as a substantially rectangular shape in the cross section (observation surface) of the steel wire for observing the metal structure. A schematic diagram for explaining the angle formed by the direction in which the degree of integration of the iron cubic {110} plane is the maximum and the length direction of the steel wire on the great circle of the positive point diagram of the iron cubic {110} plane. Is. It is process drawing which shows the manufacturing method of the steel wire which concerns on this embodiment. It is a schematic diagram for demonstrating twist processing. It is a schematic block diagram which shows an example of the evaluation apparatus used for the longitudinal cracking resistance evaluation method (torsion test) which concerns on this embodiment. It is a schematic diagram which shows the torque curve obtained when a one-way twist is added with respect to a steel wire. It is a schematic diagram which shows the "relationship between a twist (rotation angle) and a torque" at the time of performing the operation|movement of giving a twist to a normal direction. It is a schematic diagram which shows "the relationship between the operation|movement of the twist|twisting in the forward/reverse direction and torque" when the operation|movement of the twist|twisting in the normal direction is repeated. It is a schematic diagram which shows the relationship of FIGS.

Hereinafter, an embodiment which is an example of the present invention will be described.

<Steel wire>
The steel wire according to the present embodiment is a steel wire having a circular cross section with a wire diameter of 50 to 380 μm and a tensile strength of 3300 to 3900 MPa, and in mass%, C: 0.85 to 1.20. %, Si: 0.05 to 2.00%, Mn: 0.2 to 2.0%, Cr: 0.01 to 1.30%, Nb: 0 to 0.5%, V: 0 to 0. A steel wire (drawn perlite steel) containing 5% and Mo: 0 to 0.2%, and the balance being Fe and impurities. And, the center of the steel wire is parallel to the length direction of the steel wire. The metallographic structure observed in the cross section of the steel wire including the part, which is located between the center of the steel wire and the surface of the steel wire, is measured on the radial line segment of the steel wire. The width of the formed crystal grains is 50 to 200 nm, the tilt angle difference of the crystal orientations between the adjacent crystal grains is 5 to 30°, and the iron The angle formed by the direction in which the degree of integration of the cubic {110} plane is maximum and the length direction of the steel wire is 1° to 45°.

With the above configuration, the steel wire according to the present embodiment is a steel wire having excellent resistance to longitudinal cracking against repeated forward and reverse twists. The reason for this is that in the metallographic structure, the width of the elongated crystal grains, the tilt angle difference of the crystal orientation between adjacent crystal grains, the iron cubic {110} plane on the great circle of the positive electrode dot diagram of the iron cubic {110} By setting the angle between the direction in which the degree of accumulation of the plane is maximum and the length direction of the steel wire within the above range, the direction in which cracks tend to occur shifts from the longitudinal direction of the steel wire, and vertical cracking resistance of the steel wire It is presumed to improve the sex.

When the steel wire according to the present embodiment is used as a reinforcing material for a tire, etc., it withstands stress fluctuations due to repeated forward and reverse twists well, and vertical cracks are less likely to occur. Since the fatigue strength corresponding to the strength can be exerted, the durability as a reinforcing material is improved.

(Chemical composition)
The action of each element of the chemical composition of the steel wire will be described below. In addition, "%" indicates mass% unless otherwise specified.

"C: 0.85 to 1.20%"
If the C content is less than 0.85%, sufficient strength during patenting and a wire-drawing work hardening rate cannot be obtained, so that the strength of 3300 MPa or more, which is the object of the present invention, can be stably obtained. Can not. Therefore, the C content is 0.85% or more. On the other hand, when the C content exceeds 1.20%, the original strength is high, and the work hardening amount increases, which makes it difficult to secure the ductility of the steel wire. Therefore, the C content is 1.20. % Or less.

"Si: 0.05-2.00%"
Si is an element effective for strengthening ferrite in pearlite and for deoxidizing steel. However, if the Si content is less than 0.05%, the above effect cannot be expected, and if the Si content exceeds 2.00%, the effect is saturated. Therefore, the Si content is set to 0.05 to 2.00%.

"Mn: 0.2-2.0%"
Mn is an element effective not only for deoxidation and desulfurization but also for improving the hardenability of steel and increasing the tensile strength of the steel wire after heat treatment. However, if the Mn content is less than 0.2%, the above effect cannot be obtained. On the other hand, if the Mn content exceeds 2.0%, Mn segregation occurs and the workability of the steel wire rod deteriorates, and the wire drawing work is performed. It not only causes breakage, but also deteriorates ductility. Therefore, the Mn content is 0.2 to 2.0%.

"Cr: 0.01-1.30%"
Cr is an effective element that refines the lamellar spacing of pearlite, increases the tensile strength after heat treatment, and particularly improves the cold work hardening rate. However, if the Cr content is less than 0.01%, the effect is small. On the other hand, if the Cr content exceeds 1.30%, the pearlite transformation end time during the heat treatment is long and the productivity is reduced. Therefore, the Cr content is 0.01 to 1.30%. The preferable Cr content is more preferably 0.01 to 1.00%.

"The rest"
The balance of the steel wire is Fe and impurities. Here, the impurity means a component contained in the raw material or a component mixed in the manufacturing process and not intentionally contained in the steel wire.

"Other elements"
The steel wire according to the present embodiment is based on a chemical composition in which the above elements and the balance are Fe and impurities, but for the purpose of improving mechanical properties such as strength, toughness, and ductility, Nb:0 in mass%. .About.0.5%, V:0 to 0.5%, and Mo:0 to 0.2% may be contained alone or in combination.

(Metal structure)
The metallographic structure of the steel wire according to this embodiment will be described.

-Cross section of steel wire for observing metal structure-
As shown in FIG. 1, the cross section of the steel wire for observing the metallographic structure is a cross section of the steel wire that is parallel to the length direction of the steel wire and includes the central portion of the steel wire. It is a cross section of a steel wire located in the middle of the surface of the wire (hereinafter also referred to as "observation surface"). However, the observation surface is an area having a size of 4 μm in the longitudinal direction (Z direction) of the steel wire×40 μm in the radial direction (r direction) of the steel wire. The observation surface is an area in which the center portion of the length of the steel wire in the radial direction (r direction) of the area of this size is located between the center portion of the steel wire and the surface of the steel wire.

The observation surface is formed by cutting the target steel wire by wet cutting so that the temperature does not rise above 30° C., and then cutting off the shaving margin (milling thickness) by about 50 to 100 μm with an Ar ion beam. Then, the time for shaving with an Ar ion beam is adjusted, and Ar ion milling is performed so that data can be collected at least from the observation surface. The crystal orientation data can be collected by using an EBSD (electron backscattering diffraction image) measuring device attached to the FE-SEM. In the present embodiment, a JSM-7100F, which is an FE-SEM manufactured by JEOL Ltd., is used to photograph an electron backscattering pattern with a CCD camera installed in the SEM lens barrel, and TSL data acquisition software OIM-DC (Data Collection). The crystal orientation data was collected by. The processing of creating a crystal orientation map of crystal grains from the collected data was performed on OIM-Analysis of ESL analysis software of TSL.

Here, a steel wire obtained by drawing a steel wire rod having a body-centered cubic crystal such as ferrite has a strong {110}//DD texture, that is, many crystal grains are longitudinally body-centered. It has a structure in which cubic {110} planes are oriented. However, as a result of the investigation, the crystal orientation of the crystal grains in the longitudinal section of the steel wire, which is the observation surface, differs in units corresponding to the crystal grains of the steel wire rod (patenting material or rolled wire rod, etc.) that is the raw material. ,know.

Even after the wire drawing of the steel wire, it is possible to clearly identify the crystal grain boundaries on the crystal orientation map as one stretched rectangular section having substantially the same crystal orientation even in the longitudinal section of the steel wire which is the observation surface. Therefore, this method is used to identify the crystal grains.
Specifically, the crystal grains are identified as follows. In the EBSD method, generally, the observation surface is divided into regular hexagonal elements (pixels), the crystal orientation information of the ferrite at the center position of the regular hexagonal elements is acquired, and the orientation information of the pixels is represented. On the analysis software, a color corresponding to the crystal orientation is added to these orientation information.
When it is possible to visually determine that the two pixels have different colors, the pixels are determined to belong to different crystal grains, and the boundaries of the crystal grains are defined.

Since the boundary line of the crystal grains obtained by such an operation is along the hexagonal element (EBSD pixel) of the EBSD method, it looks like a polygonal line in detail, but the polygonal line should be connected smoothly by visual observation. Thereby, a crystal grain interface is obtained. The region identified at this crystal grain interface is defined as a crystal grain (extended pearlite colony) (see FIG. 2).
The crystal grains (extended pearlite colonies) obtained by such an operation are the spindles that are elongated in the longitudinal direction of the steel wire in the longitudinal section (circumferential surface of the steel wire (θ surface)) of the steel wire that is the observation surface. It has a shape like. However, the length of one continuous crystal grain is sufficiently long with respect to the length of the observation surface, that is, the length of the steel wire in the longitudinal direction (Z direction) of about 4 μm, and within the area of the observation surface, The crystal grains can be regarded as a substantially rectangular shape (see FIG. 3).

-Crystal grain width-
The width of the crystal grain observed on the observation surface by the above operation (the width of the crystal grain measured on the line segment in the radial direction of the steel wire) is 50 to 200 nm. The width of the crystal grains is the average value of the widths of all the crystal grains observed on the observation surface.
If the width of the crystal grains is less than 50 nm, the deformability near the surface layer of the steel wire will be reduced, and the initial crack will be introduced early in response to the application of lateral eccentric stress. In many cases, the strength of the steel wire is deteriorated, and the strength originally possessed by the steel wire cannot be exhibited, resulting in fracture with low stress. On the other hand, when the width of the crystal grain exceeds 200 nm, the effect of work hardening by wire drawing is not sufficiently obtained, and the tensile strength is less than 3300 MPa. Therefore, the width of the crystal grain is 50 to 200 nm.
In addition, the width of the crystal grains is preferably 70 μm or more in order to improve the lateral eccentric stress relaxation property and more exert the strength of the steel wire. On the other hand, the width of the crystal grains is preferably 120 μm or less in order to sufficiently obtain the effect of work hardening by wire drawing.

− Difference in crystal orientation between crystal grains −
The tilt difference in crystal orientation between adjacent crystal grains observed on the observation surface by the above operation is 5 to 30°. Note that the tilt difference of the crystal orientation is the average value of the tilt differences between all the adjacent crystal grains observed on the observation surface. However, when the observation surface includes an end portion of a crystal grain that extends in the length direction, the end portion is not a measurement target.
When the difference in crystal orientation between crystal grains is less than 5°, the drawing strength is insufficient and work hardening is not sufficiently obtained, so that the tensile strength may be less than 3300 MPa. On the contrary, when the tilt angle difference between the crystal grains exceeds 30°, peeling is likely to occur between the elongated crystal grains, and longitudinal cracking is likely to occur against repeated forward and reverse twists. Therefore, the tilt angle difference of the crystal orientation between the crystal grains is set to 5 to 30°.
The difference in crystal orientation between crystal grains is preferably 9° or more in order to obtain tensile strength more stably. On the other hand, the difference in crystal orientation between crystal grains is set to 20° or less in order to make it more difficult for the elongated crystal grains to separate and to prevent vertical cracking due to repeated forward and reverse twists. Is preferred.

-Pole figure of iron cubic {110} plane-
In a steel wire that has undergone cold plastic deformation such as wire drawing, the part where the degree of integration of the iron cubic {110} plane is maximum on the positive pole diagram of the iron cubic {110} plane is the steel wire. Clearly appears in the length direction (Z direction) side. Then, on the great circle of the positive point diagram of the iron cubic {110} plane, the angle formed by the direction in which the degree of accumulation of the iron cubic {110} plane is maximum and the length direction of the steel wire (Z direction) ( Hereinafter, it is also referred to as “angle of maximum integration degree of iron cubic {110} plane”) is 1° to 45°.

When the angle of the maximum degree of integration of the iron cubic {110} plane is less than 1, vertical cracking is likely to occur due to repeated forward and reverse twists. On the contrary, when the angle of the maximum integration degree of the iron cubic {110} plane exceeds 45°, it becomes difficult to obtain the tensile strength of 3300 MPa or more, which is the characteristic of the steel wire. Therefore, the angle of the maximum integration degree of the iron cubic {110} plane is 1° to 45°.
In addition, the angle of maximum integration degree of the iron cubic {110} plane is preferably 5° or more in order to more clearly obtain the effect that vertical cracking is less likely to occur with respect to forward and reverse repeated twists. It is more preferable that the angle is not less than °. On the other hand, the angle of the maximum degree of integration of the iron cubic {110} plane is preferably less than 30° in order to obtain the tensile strength of the steel wire more efficiently.

Here, the positive electrode diagram (integration degree distribution) is obtained as follows.
First, in the crystal orientation map of the texture to be evaluated, the pole point of the specific crystal plane of each pixel is projected on the stereo projection diagram expressing the material coordinates (one for each pixel). Note that the stereo projection diagram is a representation of the material coordinate space (three-dimensional) on a plane (circular, two-dimensional).
Next, the number of poles existing in a certain range centered on the position on the stereo projection is counted ((A) value). In the present embodiment, the total is calculated within the range of the solid angle of 5° or less.
On the other hand, the poles of a specific crystal plane of the texture, which are oriented completely in random directions, are projected on a stereo projection diagram, and the number of poles is counted in the same manner as above ((B) value)).
Then, the (A) value/(B) value obtained above is calculated.
Such an operation is performed for the entire stereo projection diagram (all directions of material coordinates), and the distribution of (A)/(B) is obtained, which is the positive electrode point diagram (distribution of integration degree).

That is, the degree of integration of the iron cubic {110} plane in the positive electrode diagram of the iron cubic {110} plane is the iron cubic {110} when the crystal grains are oriented in completely random directions. It is an index of how many times the surface exists.
The direction in which the degree of integration of the iron cubic {110} plane is maximum is defined by a color in which the degree of integration of the iron cubic {110} plane is maximum in the positive pole figure of the iron cubic {110} plane. It is a direction along a straight line X that passes through the center of the arc located on the great circle in the positive pole figure and the center of the great circle in the positive pole figure. That is, the angle of the maximum degree of integration of the iron cubic {110} plane indicates the angle A formed by the straight line X and the longitudinal direction (Z direction) of the steel wire (see FIG. 4 ). In FIG. 4, θ indicates the circumferential direction of the steel wire and r indicates the radial direction of the steel wire.

The maximum integration degree of the iron cubic {110} plane is defined as follows.
Based on the distribution of the degree of integration obtained from the positive point diagram (distribution of degree of integration), contour lines (contour line mode) of the degree of integration in 1° steps are drawn. The maximum point of the maximum degree of integration is displayed with an accuracy of 1° as the maximum point in the positive point map (integration degree distribution), as the top of the mountain is shown in the topographic map. Several local maximum points appear in the positive point diagram (accumulation degree distribution). Then, the integration degree of the largest point is set as the maximum integration point (that is, the maximum integration degree).

-Characteristics of steel wire-
The steel wire according to the present embodiment has a circular cross section with a wire diameter of 50 to 380 μm. When a steel wire having a wire diameter of 50 to 380 μm is provided with the above-mentioned specific metal structure, the effect of vertical crack resistance against repeated forward and reverse twists is easily exhibited. A steel wire having a wire diameter of 50 to 380 μm is a steel wire required for a steel cord used for reinforcement of rubber and organic materials in applications such as tires, belt cords and high pressure hoses.

The tensile strength of the steel wire according to this embodiment is 3300 to 3900 MPa. When a steel wire having a tensile strength of 3300 to 3900 MPa is imparted with the above-mentioned specific metallographic structure, the effect of longitudinal cracking resistance against repeated forward and reverse torsion is likely to be exhibited. A steel wire having a tensile strength of 3300 to 3900 MPa is a steel wire required for a steel cord used for reinforcing tires, belt cords, high pressure hoses and the like, and for reinforcing rubber and organic materials.

In the steel wire according to the present embodiment, a repeated twisting test with a constant strain amplitude performed at 90% of the strain corresponding to the 0.2% proof stress point of the steel wire measured in the one-way twisting test is performed until the steel wire breaks. The number of repetitions is preferably 100 times or more, more preferably 150 times or more. When the number of repetitions until the steel wire is broken is 100 or more, the steel wire has further excellent vertical cracking resistance against forward and reverse repeated twists.
The method for measuring the number of repetitions until the steel wire is broken will be described in the method for evaluating vertical crack resistance according to this embodiment described later.

The steel wire according to the present embodiment may be subjected to a well-known plating treatment such as brass plating on the surface.

<Steel wire manufacturing method>
The manufacturing method of the steel wire according to the present embodiment is, in mass %, C: 0.85 to 1.20%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, and A first wire drawing step in which a steel wire containing Cr: 0.01 to 1.30% and the balance consisting of Fe and impurities is subjected to wire drawing one or more times to obtain a wire drawn material; The wire drawing material is subjected to a twisting process 10 to 200 times per length of the wire drawing material obtained by multiplying the diameter D of the wire drawing material by 100, and a twisting process for obtaining an intermediate wire drawing material and a true wire drawing strain Then, the second wire drawing is performed once or a plurality of times under the condition of 1.2 or more to obtain a steel wire (see FIG. 5 ).

In the method for manufacturing a steel wire according to the present embodiment, the first wire drawing material obtained by drawing the steel wire material having the above chemical composition is subjected to predetermined twisting and second wire drawing to obtain the steel according to the present embodiment. A wire (that is, a steel wire having the above-mentioned specific metal structure) is obtained. That is, in the method for manufacturing a steel wire according to the present embodiment, it is possible to obtain a steel wire having excellent resistance to longitudinal cracking against repeated forward and reverse twists.

The details of each step will be described below.

-First wire drawing process-
In the first wire drawing process, the steel wire rod is subjected to the first wire drawing process once or plural times. The chemical composition of the steel wire rod is the same as the chemical composition described for the steel wire according to the present embodiment.

In the first wire drawing step, the steel wire rod used for the first wire drawing is not particularly limited, but as an example, it is manufactured by the following method.

First, by hot rolling, a steel slab (billet) as a raw material is heated to 1100 to 1200° C. to obtain a hot rolled wire rod having a metal structure of an austenite structure. In hot rolling, the cross-sectional area is gradually reduced by alternately changing the rolling direction by 90 degrees for each stand of rolling, and the steel strip is cut to a desired final wire diameter (for example, final wire diameter of 5.5 mm). To roll. At this time, the steel slab is heated to a very high temperature due to the processing heat due to a very high processing strain rate, but the maximum temperature reached by the steel slab is controlled within the range of 1050-1150° C. on the surface.

Next, the hot-rolled wire rod having the obtained austenite structure is water-cooled within a range of 800 to 750° C. within 5 seconds before reaching the winding machine. After that, the air flow of the airflow cooling and the conveying speed are adjusted so that the pearlite transformation temperature is in the range of 620° C. to 660° C. on the cooling conveyor.

Next, in order to remove the oxide scale on the surface of the hot rolled wire rod after cooling, the hot rolled wire rod is pickled with hydrochloric acid. After that, wire drawing using a dry lubricant and patenting are appropriately combined to obtain a wire diameter suitable for finishing the target steel wire. After the final patenting, pickling, and Cu electroplating and Zn plating as wet electroplating are performed in this order on the drawn material having a predetermined wire diameter, the patenting material is subjected to thermal diffusion treatment by high frequency heating. Brass plating is formed on the surface of.

Then, after forming the brass plating, the oxide film is removed with oxalic acid to obtain a plated wire rod to be subjected to the first wire drawing process.
When producing a hot rolled wire rod, the heating temperature during hot rolling is set to 1050 to 1150° C. and the pearlite transformation temperature is set to 620° C. to 660° C., so that the size of the crystal grains of the steel wire rod (pearlite colony). It is preferable to control the size) to be 15 to 25 μm in terms of equivalent circle diameter. This makes it easier to obtain the desired metallographic structure and to improve the vertical cracking resistance against repeated forward and reverse torsion.

Next, the plated wire thus obtained is subjected to first wire drawing. The first wire drawing process is performed, for example, by a wet method. In the wet wire drawing, for example, wire drawing is performed by a method in which a die is completely submerged in a lubricant in which a fine solid lubricant is dispersed as an emulsion in water.

In the first wire drawing process, the first wire drawing process is carried out once or a plurality of times, but it is selected that the work area reduction rate at each time (each stage) is 15 to 22%, and the bearing length is the line length. This may be done using a nominal 0.3D die for diameter D. It should be noted that the management of the die angle is preferably performed by inspecting it with a die shape measuring device manufactured by Conoptica before use and confirming that it is within the above range.

In the first wire drawing process, from the viewpoint of reducing heat generation during wire drawing, for example, the approach angle of the die is 10 to 12° in all angles, and wire drawing in the latter half of wet drawing is 0.9 mm or more. It is better to use diamond dies. In addition, it is preferable that the temperature of the steel wire is 100° C. or less by setting the work area reduction ratio of each wire drawing process (each step) to 15 to 22%.

-Twisting process-
In the twisting process, the wire drawing material obtained by the first wire drawing is twisted. Specifically, for example, after the first wire drawing process, one end of the wound wire drawing material is gripped by a jig, the wire drawing material is pulled out while being rotated, and the wire drawing material is twisted. This twisting process imparts a twisting plastic deformation to the drawn wire.

In the twisting process, the number of twists of the wire drawing material is 10 to 200 times per length of the wire drawing material obtained by multiplying the diameter D of the wire drawing material by 100 (that is, per length of the wire drawing material of 100×D) (Fig. 6).
If the number of twists is less than 10, vertical cracks will occur due to repeated twists. On the other hand, if the number of twists exceeds 200, ductile fracture occurs during the twisting process and the wire drawing material breaks. Therefore, the number of twists is 10 to 200.
Further, the number of twists is preferably 25 times or more, more preferably 40 times or more in order to more effectively suppress vertical cracking due to repeated forward and reverse twists. On the other hand, the number of twists is preferably 100 times or less, and more preferably 75 times or less in order to more effectively suppress breakage of the drawn wire during twisting.

Here, the number of twists per length of a wire drawing material of 100×D is counted as “1 time” when one end of the wire drawing material to be gripped for twisting is rotated once, and the number of rotations is 100×. It is calculated by dividing by the length of the drawn wire material of D (that is, the formula: number of twists=rotation speed of one end of the drawn wire material/100×D length of the drawn wire material).

In the twisting step, the number of rotations (rotational speed) for rotating the wire drawn material when twisting the wire drawn material is not particularly limited, and may be, for example, in the range of 10 to 300 rpm.

-Second wire drawing process-
In the second wire drawing process, the second wire drawing process is performed once or a plurality of times under the condition that the true wire drawing strain is 1.2 or more.
When the true strain of the second wire drawing is less than 1.2, vertical cracking occurs due to repeated twists in the forward and reverse directions. Therefore, the true strain of the second wire drawing is 1.2 or more.
In addition, the true strain of the second wire drawing is preferably 1.5 or more in order to more effectively suppress vertical cracking against repeated twists in the forward and reverse directions. On the other hand, if the true strain of the second wire drawing becomes too large, the effect of twisting will be diminished during the second wire drawing, and the effect of vertical cracking resistance against repeated forward and reverse twisting will appear. Since it may be difficult to be carried out, it is preferable to suppress it to less than 2.3.

Here, the true strain of the second wire drawing is calculated by the following formula.
Formula: True strain ε=2×ln (DT/DF)
(In the formula, DT represents the wire diameter of the drawn wire (twisted wire obtained by the first wire drawing) before twisting. DF is the wire diameter of the drawn wire after the second wire drawing ( The target final wire diameter) is shown.)

However, when the second wire drawing is carried out a plurality of times, the true strain of the second wire drawing is the total of the true strains in each wire drawing.
Specifically, for example, when performing the second wire drawing process twice, the true strain of the first wire drawing process is DT is "the wire diameter of the wire drawing material before the twisting process", and DF is It is calculated by the above formula as "the wire diameter of the wire drawing material after the first wire drawing". On the other hand, the true strain of the second wire drawing is DT as "the wire diameter of the wire drawing material after the first wire drawing", and DF is the wire diameter of the wire drawing material after the second wire drawing ( The final wire diameter of the steel wire)” is calculated by the above formula. Then, the true strain of the second wire drawing is calculated as the sum of the true strains of the first and second wire drawing.

The conditions of the second wire drawing process are not particularly limited except for true strain, and for example, a well-known dry die drawing process may be performed.

Through the above steps, a steel wire having a desired wire diameter, tensile strength and metallic structure can be obtained.

<Evaluation method of vertical crack resistance>
The longitudinal cracking resistance evaluation method according to the present embodiment is a method for evaluating the vertical cracking resistance of a steel wire against repeated forward and reverse twists. In other words, the resistance to longitudinal cracking of the steel wire against forward and reverse repeated twists is confirmed by the repeated twist test with a constant strain amplitude performed at 90% of the strain corresponding to the 0.2% proof stress point of the steel wire measured in the one-way twist test. Is a method of evaluating.

Specifically, the longitudinal cracking resistance evaluation method according to the present embodiment is performed on a steel wire with 90% of the strain corresponding to the 0.2% proof stress point of the steel wire measured by the one-way torsion test. By repeatedly applying twist in the forward and reverse directions and obtaining the "number of repetitions of the twisting motion in the forward and reverse directions" until the steel wire breaks, the resistance to longitudinal cracking of the steel wire against forward and reverse repeated twists is calculated. Is a method of using

The evaluation device 12 used in the method for evaluating vertical cracking resistance (torsion test) according to the present embodiment is, for example, a rotary chuck that grips one end of a steel wire 10 so as to match the rotation axis, as shown in FIG. 7. 14, a chuck 16 with a torque sensor that rotatably grips the other end of the steel wire 10 on an extension line of its rotation axis, a rotating machine 18 that rotates the rotary chuck 14, and a chuck 16 with a torque sensor. A movable base 20 (a base with wheels) that is movable in the longitudinal direction of the steel wire 10 is connected to the movable base 20 via a wire rod 24, and the movable base 20 is moved to apply tension to the gripped steel wire 10. And a weight 22 for The evaluation device 12 also includes a control unit 26 that is connected to each unit of the device and controls each unit.

Then, using this evaluation device 12, a repeated torsion test of a constant strain amplitude performed at 90% of the strain corresponding to the 0.2% proof stress point measured by the one-way torsion test was carried out by the following method. Evaluate the vertical cracking resistance of steel wire against forward and reverse repeated twisting.

First, as a preliminary test, a one-way twist test is performed in which the steel wire 10 to be evaluated is twisted until it breaks. Specifically, for example, one end and the other end of the steel wire 10 are respectively held by the rotary chuck 14 and the chuck 16 with the torque sensor so that kinks (twisting, twisting, entanglement, etc.) do not occur, and the weight 22 moves the load. The table 20 is moved to apply a tension of 1% of the tensile breaking load of the steel wire 10 measured in advance.
In this state, the rotating chuck 14 is rotated by the rotating machine 18, and the steel wire 10 is twisted in one direction. When twisting in one direction is applied to the steel wire 10, for example, a predetermined torque curve (see FIG. 8: horizontal axis: rotation angle (torsion) [rad], vertical axis: torque [N·m]) is obtained. There is a point beyond the elastic deformation range of the line 10 where the shape cannot be restored.
Then, a point at which 0.2% or more of plastic deformation is introduced by applying a twist in one direction to the steel wire 10 to make it a shear strain is regarded as a starting point of plastic deformation, and this point is set to 0. Record as 2% proof point (see Figure 8).

Next, as a main test, a twisting test is performed in which the steel wire 10 to be evaluated is twisted in the forward and reverse directions. Specifically, one end and the other end of the steel wire 10 are gripped by the rotary chuck 14 and the chuck 16 with the torque sensor, respectively, so that the steel wire 10 is in a state similar to the preliminary test.
Next, with respect to the steel wire 10, a twist corresponding to 90% torque (constant strain) of the 0.2% proof stress point of the steel wire 10 measured in the preliminary test (hereinafter referred to as “0.2% proof stress of the steel wire 10”). "Twist of 90% of strain corresponding to a point" is repeatedly applied in the forward and reverse directions.
Specifically, a 90% twist of the strain corresponding to the 0.2% proof stress point of the steel wire 10 measured in the preliminary test is applied to the steel wire 10 in the positive direction. Next, the steel wire 10 is twisted in the opposite direction and past the neutral point (twisting start point), and the strain corresponding to the 0.2% proof stress point of the steel wire 10 measured in the preliminary test is applied to the steel wire 10. A 90% twist is applied in the opposite direction. Then, the steel wire 10 is twisted in the forward direction again and returned to the neutral point (twisting start point). This twisting operation in the forward and reverse directions (constant strain amplitude of twisting) is set as one set and repeated, and the torque sensor detects the maximum point of the torque generated during this period and continues recording.
Note that FIG. 9 shows the “relationship between twist (rotation angle) and torque” when the action of imparting twist in the forward and reverse directions (constant strain amplitude of twist) is performed.

Then, when the operation of imparting the twist in the forward and reverse directions is repeatedly applied, the steel wire 10 is damaged depending on the vertical crack resistance thereof, and the steel wire 10 is inferior in vertical crack resistance (inferior material). , The maximum point of torque clearly starts to decrease with a small number of repetitions. On the other hand, the more the steel wire 10 (predominant material) which is superior in vertical cracking resistance, the greater the number of repetitions in which the maximum point of torque starts to clearly decrease. At this time, the “number of repetitions of the twisting motion in the forward and reverse directions” per length of the steel wire 10 (that is, the length of the 100×D steel wire 10) obtained by multiplying the diameter D of the steel wire 10 by 100 , Repeated torsion test of constant strain amplitude performed at 90% of strain corresponding to 0.2% proof stress point of steel wire, and recorded as the number of repetitions until the torque decreases (“torque lowering point”) (see FIG. 10). ).

Further, if the operation of applying the twist in the forward and reverse directions is repeatedly applied, the steel wire 10 will be broken. At this time, the “number of repetitions of the twisting motion in the forward and reverse directions” per length of the steel wire 10 (that is, the length of the 100×D steel wire 10) obtained by multiplying the diameter D of the steel wire 10 by 100 , Repeated torsion test of constant strain amplitude performed at 90% of strain corresponding to 0.2% proof stress point of steel wire, and recorded as the number of repetitions until the steel wire breaks ("break point") (see FIG. 10). ..

As described above, in the longitudinal cracking resistance evaluation method according to the present embodiment, the operation of applying the twist in the forward and reverse directions is repeatedly applied, the torque applied to the steel wire is continuously measured, and the torque lowering point and the fracture The points are determined and the vertical crack resistance of the steel wire against forward and reverse repeated twists is evaluated (see FIG. 11). Note that FIG. 11 is a schematic diagram showing the relationship between FIGS. 8 to 10.

Hereinafter, the present invention will be described more specifically with reference to Examples. However, each of these examples does not limit the present invention.

(Production of steel wire)
First, a steel slab (billet) having a chemical composition shown in Table 1 and a square cross section of 122 mm square and a length of 18 m was heated to about 1100° C. to change the metal structure to an austenite structure, and a wire diameter of 5 was obtained by hot rolling. A 0.5 mm hot rolled wire rod is obtained. In this hot rolling, the finishing rolling speed was 80 m/sec. In addition, water cooling was performed between rolling to prevent the structure after rolling from becoming coarse, and the maximum temperature reached during finish rolling was adjusted to about 1050°C.

Next, the hot-rolled wire rod having the obtained austenite structure was water-cooled within a range of 850° C. within 5 seconds before reaching the winder. After that, the air volume and the conveying speed of the blast cooling were adjusted so that the pearlite transformation temperature was in the range of 620°C to 660°C on the cooling conveyor.
Next, in order to remove the oxide scale on the surface of the hot-rolled wire rod after cooling, the hot-rolled wire rod was pickled using an aqueous hydrochloric acid solution having a hydrochloric acid concentration of 18 mass% and a temperature of 60°C. Then, a wire drawing material having a wire diameter of 1.5 mm was obtained by performing drawing a plurality of times using a dry lubricant and patenting by introducing a hot bath at 580° C. between the drawing wires. Final drawing, pickling, and Cu plating and Zn plating as wet electroplating are performed in this order on a drawn wire having a wire diameter of 1.5 mm by introducing a hot bath at 580°C, and then 500°C by high frequency heating. Then, the surface of the hot-rolled wire was brass-plated by heat diffusion treatment.
After forming the brass plating, the oxide film was removed with oxalic acid to obtain a plated wire rod for the first wire drawing process.

Next, the obtained plated wire is wet-drawn (first wire drawing) with a die that is completely submerged in a lubricant in which a fine solid lubricant (wax) is dispersed as an emulsion in water. Then, a wire drawing material having a wire diameter of 200 μm was obtained.
The wet wire drawing process (first wire drawing process) was performed once. The wire drawing conditions were such that the area reduction rate was 20%, the approach angle was 10°, and the bearing length was nominally 0.3D with respect to the wire diameter D. In addition, the management of the die angle was carried out by inspecting the die shape measuring device manufactured by Conoptica before use, and after confirming that it was generally within the above range, it was used.

Next, with respect to the wire drawing material obtained by the wet wire drawing (first wire drawing), the number of twists per length of the 100×D wire drawn material shown in Table 1 (“twisting number” in the table Notation)", and the twisting process was performed at a rotation speed of 20 to 300 rpm.

Next, the intermediate wire drawing material obtained by the twisting was subjected to dry wire drawing (second wire drawing) under the conditions that the wire drawing true strain shown in Table 1 was obtained.
The dry wire drawing process (second wire drawing process) was performed once. The wire drawing conditions were the same as those of the first wire drawing step except for the true strain in the wire drawing process.

In this way, steel wires having the wire diameters shown in Table 1 were obtained.

(Measurement/Evaluation)
-Various measurements-
With respect to the obtained steel wire, the width of the elongated crystal grains (indicated as "crystal grain width" in the table) and the tilt angle difference of the crystal orientations between adjacent crystal grains ("Crystal orientation between adjacent crystal grains in the table" Inclination difference"), the angle between the direction in which the degree of integration of the iron cubic {110} plane shows the maximum and the length direction of the steel wire on the great circle of the positive point diagram of the iron cubic {110} plane. (Indicated as "angle of maximum integration degree of iron cubic {110} plane" in the table) was measured according to the method described above.
The tensile strength of the obtained steel wire was also measured according to JIS-Z2241 (2015).

-Evaluation of repeated torsion test-
The obtained steel wire was repeatedly subjected to a torsion test according to the above-described evaluation method for longitudinal crack resistance according to the present embodiment. Then, in a repeated twist test of constant strain amplitude performed at 90% of the strain corresponding to the 0.2% proof stress point of the steel wire measured in the one-way twist test, the number of repetitions until the steel wire was broken was obtained and evaluated. .. The evaluation criteria are as evaluated.
-Evaluation criteria-
A (◎): The number of repetitions until the steel wire breaks is 140 times or more B (○): The number of repetitions until the steel wire breaks is 120 times or more and less than 140 times C (△): The number of repetitions until the steel wire breaks Is 100 times or more and less than 120 times D(x): The number of repetitions until the steel wire is broken is less than 100 times

From the notation results, with the steel wires of Examples 1 to 16, the tensile strength was satisfied at 3300 to 3900 MPa, but the number of repetitions until the steel wire ruptured in the repeated torsion test was large, and against the forward and reverse torsions. It can be seen that the steel wire has high vertical crack resistance.
On the other hand, in the steel wires of Comparative Examples 1 and 2, the number of twists in the twisting process was small, and therefore the number of repetitions until the steel wire was broken was less than 100 in the repeated twist test, and the steel wire was not twisted in the forward and reverse twists. Has low vertical cracking resistance.
Further, in the steel wire of Comparative Example 3, the true strain of the second wire drawing after the twisting was insufficient, so the number of repetitions until the steel wire was broken was less than 100 times in the repeated twisting test. Low resistance to vertical cracking of steel wire against repeated twisting and low tensile strength. Even in the steel wires of Comparative Examples 4 to 9, the number of repetitions until the steel wire was broken was less than 100 in the repeated twist test, and it was found that the vertical resistance to longitudinal cracking of the steel wire against forward and reverse repeated twists was high. The steel wire of Comparative Example 10 has a high C content of steel and an excessively high tensile strength, and therefore has a low resistance to longitudinal cracking. The steel wire of Comparative Example 11 has a high resistance to longitudinal cracking because of the low C content of steel, but has a low tensile strength.

The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to such examples. It is obvious to those skilled in the art that various changes or modifications can be conceived within the scope of the idea described in the claims, and naturally, they also belong to the technical scope of the present invention. Understood.

10 Steel Wire 14 Rotating Chuck 16 Chuck with Torque Sensor 18 Rotating Machine 20 Moving Stand 22 Weight

Claims (4)

In a steel wire having a circular cross section with a wire diameter of 50 to 380 μm and a tensile strength of 3300 to 3900 MPa,
In mass %, C: 0.85 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.0%, Cr: 0.01 to 1.30%, Nb: 0 to 0.5%, V: 0 to 0.5%, and Mo: 0 to 0.2%, and the balance consisting of Fe and impurities,
Metal observed in a cross section of the steel wire which is parallel to the length direction of the steel wire and includes the center of the steel wire, and which is located between the center of the steel wire and the surface of the steel wire. The structure has a crystal grain width of 50 to 200 nm measured on a radial line segment of a steel wire, a crystal orientation inclination difference of 5 to 30° between adjacent crystal grains, and an iron cubic crystal { A steel wire having an angle of 1° to 45° between the direction in which the degree of integration of the iron cubic {110} face is maximum and the length direction of the steel wire, on the great circle of the positive electrode point diagram of the 110} face. A repeated twist test of a constant strain amplitude performed at 90% of the strain corresponding to a 0.2% proof stress point of the steel wire measured by the one-way twist test, and the number of repetitions until the steel wire breaks is 100 times or more. Item 1. The steel wire according to Item 1. In mass %, C: 0.85 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.0%, Cr: 0.01 to 1.30%, Nb: A steel wire rod containing 0 to 0.5%, V: 0 to 0.5%, and Mo: 0 to 0.2%, and the balance being Fe and impurities, was subjected to one or more first elongations. A first wire drawing process in which wire drawing is performed to obtain a wire drawn material;
A twisting step of subjecting the drawn wire to a twisting process of 10 to 200 times per length of the drawn wire obtained by multiplying the diameter D of the drawn wire by 100; and
A second wire drawing process for obtaining a steel wire by subjecting the intermediate wire drawing material to a second wire drawing process once or a plurality of times under conditions of 1.2 or more as a true wire drawing line strain, and
The method for manufacturing a steel wire according to claim 1 or 2, further comprising : The method for manufacturing a steel wire according to claim 3, wherein in the twisting step, twisting is performed 25 to 75 times per length of the drawn wire obtained by multiplying the diameter D of the drawn wire by 100.