KR20160147041A - Steel wire material - Google Patents

Abstract

An object of the present invention is to provide a steel wire rod having high strength and low electrical resistivity. This steel wire rod contains C, Si, Mn, Cr, and Al as chemical components, and limits N, P, and S, and optionally contains one or more elements selected from the group consisting of Mo, V, Ti, Nb and B And the remainder contains Fe and impurities. The structure of the steel wire rods is a pearlite having an area ratio of 85% or more and an average lamella spacing of 50 nm to 100 nm. When the Si content is [% Si], the content of Cr contained in the cementite in the pearlite is [% Cr?], And the content of Cr contained in the ferrite in the pearlite is [% Cr?], ([% Cr?] / [% Cr?])? (2.0 + [% Si] x 10).

Description

Steel wire {STEEL WIRE MATERIAL}

The present invention relates to a steel wire rod.

The present application claims priority based on Japanese Patent Application No. 2014-114429 filed on June 2, 2014, which is incorporated herein by reference.

Twisting wires of steel wire are used in the inner core material for strength reinforcement in aluminum twisted wire used for power transmission lines and the like. Such a twisted wire is generally called an aluminum conductor STEEL-reinforced cable. Hereinafter, the twisted aluminum wire is referred to as ACSR.

The steel wire used in the core material of the ACSR is often produced by cold drawing using piano wire materials such as SWRS72B and SWRS82B of the JIS standard. In the production of ACSR, during the cold drawing or the cold drawing, a heat treatment called "putting" or an aluminum plating treatment is carried out if necessary.

In the ACSR, electricity flows not only on the aluminum wire but also on the steel wire. Therefore, if the electrical resistivity of the steel wire is large, the electrical resistivity in the entire ACSR becomes large, and the heat generation during transmission becomes large. As a result, power transmission efficiency is lowered.

When the strength of the steel wire is low, it is necessary to increase the steel wire having a higher electrical resistivity than that of aluminum to reinforce the strength. However, since electricity also flows through the steel wire, the electrical resistivity of the entire ACSR is increased as a result.

In view of this, there is a demand for a steel wire having a low electrical resistivity, a high strength steel wire and a high strength steel wire material, which is low in electrical resistivity.

The electrical resistivity of the steel generally rises with increasing elemental content in the steel. Therefore, in the steel material disclosed in Patent Document 1, the electrical resistivity is reduced by reducing the content of major elements such as C, Mn, and Cr. However, in this steel material, the content of C and Si in the steel is small because it is intended to reduce the electric resistivity and to improve the cold-hardening. As a result, the tensile strength is insufficient.

Further, in the steel sheet for spring disclosed in Patent Document 2, when the content of C, Si, and Mn in the steel is less than a value expressed by a predetermined formula, the electrical resistivity of the steel is lowered. However, in this spring steel sheet, since the structure is not optimized and Cr is not contained, the tensile strength can not be increased. As a result, the balance between the securing of the strength and the reduction of the electrical resistivity is insufficient.

Further, in the high-strength and high-strength hyper-graphite wire disclosed in Patent Document 3, the content and the structure of C, Si, Mn and the like in the steel are specified, thereby securing tensile strength and drafting degree. However, in the high-strength and high-tough-hardening steel wire, the Si content is not less than 0.5%, and the structure is not optimized for lowering the electrical resistivity, so that the electrical resistivity is high.

Patent Document 4 discloses a high carbon steel wire rod in which a spheroidizing heat treatment time is shortened by increasing the Cr content in the carbide. However, since the high carbon steel wire rod has a Cr content of not less than 6.0% by mass in the carbide, it is not possible to achieve both reduction of electrical resistivity and improvement of tensile strength.

Japanese Patent Application Laid-Open No. 2003-226938 Japanese Patent Application Laid-Open No. 2004-156120 Japanese Patent Application Laid-Open No. 6-271937 International Publication No. 2012/144630

In view of the foregoing, it is an object of the present invention to provide a steel wire rod having high strength and low electrical resistivity.

In the steel wire of the present invention, in particular, the Si content which raises the electrical resistivity is limited, and the Cr content in the cermitate in the pearlite is increased in the range that both the electrical resistivity and the tensile strength are compatible, By decreasing the amount of Cr contained in the film. In addition, it is a steel wire material in which the average lamellar spacing of pearlite is made small to improve the tensile strength, thereby reducing the electrical resistivity and improving the tensile strength.

The steel wire material of the present invention is a material before cold drawing, and includes a hot-rolled wire material and a steel material obtained by heat-treating a hot-rolled wire material.

Means for Solving the Problems The inventors of the present invention have researched and studied in detail the distribution state of chemical components, textures, and alloying elements of steel wire rods in order to obtain a steel wire rod having high strength and low electrical resistivity by solving the above problems. As a result, they found the following knowledge (a) to (c).

(a) Since the electrical resistivity of the steel wire rod at room temperature does not change much by cold drawing, the presence of the steel wire rod and the structure or alloy element before cold drawing is determined by the electrical resistivity and tensile strength .

(b) When the Si content and the Cr content are increased, the strength of the steel wire rod is increased by solid solution strengthening, but the increase of the Si content significantly increases the electrical resistivity. On the other hand, although the increase in the Cr content increases the electrical resistivity, the effect is smaller than that of Si. When Cr is dissolved in ferrite, the electrical resistivity is increased. Therefore, if the Cr content in the cementite is increased and the Cr content in the ferrite is reduced, it is possible to suppress the increase in electrical resistivity. In other words, it is possible to suppress the rise of the electric resistivity by increasing the Cr content in the cementite in the pearlite and reducing the Cr content in the ferrite in the pearlite.

Further, by controlling the Cr content in the cementite in accordance with the Si content contributing to solid solution strengthening, it is possible to control both the improvement of the tensile strength and the reduction of the electric resistivity with high efficiency.

Hereinafter, the " Cr content in the cementite in the pearlite " and the " Cr content in the ferrite in the pearlite " may simply be expressed as " Cr content in the cementite "

(c) In order to achieve both the improvement of the tensile strength of the steel wire rod and the reduction of the electrical resistivity, it is effective to make the structure of the steel wire rod into a structure containing pearlite. Further, when ferrite and cementite are pearlites having a lamellar structure, the tensile strength can be improved by reducing the average lamellar spacing. On the other hand, since the effect of the average lamellar spacing on the pearlite is not so large, the average lamellar spacing can be reduced in order to achieve both the improvement of the tensile strength and the reduction of the electrical resistivity.

The present invention is based on the above knowledge, and its main points are as follows.

(1) A steel wire rod according to one aspect of the present invention comprises, as a chemical component, 0.8 to 1.1% of C, 0.02 to 0.30% of Si, 0.1 to 0.6% of Mn, 0.3% , Mo: not more than 0.20%, V: not more than 0.15%, and Al: not more than 0.005%, P: not more than 0.03% , Ti: not more than 0.050%, Nb: not more than 0.050%, and B: not more than 0.0030%, the balance being Fe and impurities; The structure includes pearlite, the area ratio of the pearlite is 85% or more; The average lamellar spacing of the pearlite is 50 nm to 100 nm; Wherein a content of Si is [% Si], a content of Cr contained in the pearlite is [% Cr?], A content of Cr contained in the ferrite in the pearlite is represented by [% Cr? ], [% Si], [% Cr?] And [% Cr?] Satisfy the following formula (a).

(2) The steel wire according to the above item (1), wherein the chemical component comprises 0.02 to 0.20% of Mo, 0.02 to 0.15% of V, 0.002 to 0.050% of Ti, 0.002% To 0.050% and B: 0.0003% to 0.0030%.

(3) The steel wire rod according to (1) or (2), wherein the tensile strength TS of the steel wire rod is 1350 MPa or more and the absolute value of the tensile strength TS of the steel wire rod is May be 64 times or more of the absolute value of the electric resistivity?

According to each of the above-mentioned forms (1) to (3), it is possible to provide a steel wire rod having high strength and low electrical resistivity. Particularly, the steel wire rod of the above form is suitable as a material of a steel wire which is used for reinforcing the strength of a transmission line and has a small power loss.

Further, the steel wire obtained by performing cold drawing on the steel wire of the above-described shape, and after cold drawing, if necessary, is subjected to an aluminum plating treatment, has high strength and low electrical resistivity. Therefore, when the ACSR is manufactured using this steel wire, a predetermined strength can be secured, and an ASCR having a small electrical resistivity can be obtained. Therefore, industrial contribution is very remarkable.

The steel wire rod according to the present embodiment will be described.

First, the reason for limiting the chemical composition of the steel wire rod in this embodiment will be described. In the following description,% means mass%.

C: 0.8% to 1.1%

C is an element effective for increasing the tensile strength by making the metal structure of the steel wire material pearlite.

When the C content is less than 0.8%, it becomes difficult to stably impart a high strength, for example, a tensile strength of about 1350 MPa, to the steel wire material. Therefore, the lower limit of the C content is set to 0.8%. In order to obtain more uniform pearlite and increase the tensile strength, the C content is preferably 0.9% or more, and more preferably 1.0% or more.

On the other hand, if the C content is too large, the steel wire rod becomes hard and deteriorates the drawing workability. Particularly, when the C content exceeds 1.1%, it is difficult to industrially stably suppress the production of cementite precipitated along the old austenite grain boundary, that is, the generation of cemented cementite, so that the drawability is significantly lowered. Therefore, the upper limit of the C content is set at 1.1%.

Si: 0.02% to 0.30%

Si is an element effective for increasing the strength of a steel wire rod by solid solution strengthening and is also an element necessary for deoxidizing agent.

When the Si content is less than 0.02%, these effects are not sufficient. Therefore, the lower limit of the Si content is set at 0.02%. The Si content is preferably 0.05% or more in order to secure the strength by solid solution strengthening and also to smell the deoxidation effect more stably.

On the other hand, if the Si content is increased, the electrical resistivity increases. Particularly, when the Si content exceeds 0.30%, improvement in tensile strength and reduction in electrical resistivity can not be achieved at the same time. Therefore, the upper limit of the Si content is set to 0.30%. In order to obtain a lower electrical resistivity, the Si content is preferably 0.20% or less, and more preferably 0.10% or less.

Mn: 0.1% to 0.6%

Mn is an element having an effect of increasing the strength of the steel wire rod and fixing S in the steel wire rod as MnS and preventing hot brittleness.

When the Mn content is less than 0.1%, these effects are not sufficient. Therefore, the lower limit of the Mn content is set to 0.1%. Further, in order to secure strength and further prevent hot brittleness, the Mn content is preferably 0.2% or more, and more preferably 0.3% or more.

On the other hand, if the Mn content increases, the electric resistivity increases. Particularly, when the Mn content exceeds 0.6%, improvement in tensile strength and reduction in electrical resistivity can not be achieved at the same time. Therefore, the upper limit of the Mn content is set to 0.6%. In order to obtain a lower electrical resistivity, the Mn content is preferably 0.5% or less, and more preferably 0.4% or less.

Cr: 0.3% to 1.5%

Cr has an effect of reducing the average lamellar spacing of pearlite and increasing the tensile strength of the steel wire rods. When Cr is dissolved in ferrite in pearlite, the electrical resistivity is increased. Therefore, the Cr content in the cementite is increased, and the Cr content in the ferrite is relatively lowered, thereby suppressing an increase in electrical resistivity.

If the Cr content is less than 0.3%, sufficient tensile strength of the steel wire rod can not be ensured and the Cr content in the cementite can not be increased. Therefore, in order to improve the tensile strength and reduce the electrical resistivity, the Cr content must be 0.3% or more. In order to obtain higher tensile strength, the Cr content is preferably 0.4% or more, and more preferably 0.5% or more.

On the other hand, when the Cr content is more than 1.5%, the drawing workability of the steel wire rod is deteriorated. Therefore, the upper limit of the Cr content is set to 1.5%. In order to further suppress deterioration of the drawability, the Cr content is preferably 1.0% or less, and more preferably 0.8% or less.

Al: 0.01% to 0.05%

Al is an element having a deoxidizing effect and is an element necessary for reducing the amount of oxygen in the steel wire rods.

When the Al content is less than 0.01%, this effect is insufficient. Therefore, the lower limit of the Al content is set to 0.01%. In order to obtain more deoxidation effect, the Al content is preferably 0.02% or more.

On the other hand, Al is an element which forms hard oxide inclusions and deteriorates the ductility of the steel wire rods. Particularly, when the Al content exceeds 0.05%, coarse oxide inclusions are easily formed, so that the drawing workability of the steel wire rod is significantly lowered. Therefore, the upper limit of the Al content is set to 0.05%. The Al content is preferably 0.04% or less, more preferably 0.03% or less, so as not to further lower the drawing workability of the steel wire rod.

In the steel wire rod according to the present embodiment, N, P and S need to be further limited as follows.

N: not more than 0.008%

N is an element that sticks to the potential in the steel during cold drawing to lower the drawability. Particularly, when the N content exceeds 0.008%, the draw processability deteriorates remarkably. Therefore, the N content is limited to 0.008% or less. Preferably 0.005% or less, and more preferably 0.004% or less.

The lower limit of the N content includes 0%. However, considering the current refining technology and manufacturing cost, the lower limit of the N content is preferably 0.0001%.

P: not more than 0.03%

P is an element that is segregated at grain boundaries and deteriorates the drawability. Particularly, when the P content exceeds 0.03%, deterioration of the drawing workability becomes remarkable. Therefore, the P content is limited to 0.03% or less. , Preferably not more than 0.02%, and more preferably not more than 0.01%.

The lower limit of the P content includes 0%. However, considering the current refining technology and manufacturing cost, the lower limit of the P content is preferably 0.001%.

S: not more than 0.02%

S, like P, is an element that lowers the drawability. Particularly, when the S content exceeds 0.02%, the deterioration of the drawing processability becomes remarkable. Therefore, the S content is limited to 0.02% or less. It is preferably not more than 0.01%.

The lower limit of the S content includes 0%. However, considering the current refining technology and manufacturing cost, the lower limit of the S content is preferably 0.001%.

The above is the basic composition of the steel wire according to the present embodiment, and the balance is iron and impurities. The term " impurities " in " the remainder is Fe and impurities " means inevitably incorporated from ores as raw materials, scrap, or a manufacturing environment when industrially producing steel.

However, in the steel wire rod according to the present embodiment, at least one selected from the group consisting of Mo, V, Ti, Nb and B may be optionally contained in place of the rest of the Fe component in addition to the basic component.

Mo: 0.20% or less

The addition of Mo is optional, and the lower limit of its content is 0%.

However, by adding Mo, the effect of increasing the balance between the tensile strength and the electrical resistivity of the steel wire rod can be stably perfected. In order to obtain this effect, it is preferable to add Mo in an amount of 0.02% or more. More preferably, the Mo content is 0.05% or more.

On the other hand, when the Mo content exceeds 0.20%, the martensite structure is likely to be formed in the steel, and the drawing processability may be lowered. Therefore, the upper limit of the Mo content is preferably 0.20%. More preferably, the upper limit of the Mo content is 0.10%.

V: 0.15% or less

The addition of V is optional, and the lower limit of its content is 0%.

However, V has the effect of reducing the pearlite block size by forming carbide or carbonitride in the steel wire rods. As a result, the drawing processability can be improved by adding V. In order to obtain this effect, V is preferably added in an amount of 0.02% or more. More preferably, the V content is 0.05% or more.

On the other hand, when the V content exceeds 0.15%, coarse carbides or carbonitrides are likely to be formed in the steel wire rod, resulting in deterioration of wire drawing workability. Therefore, the upper limit of the V content is preferably 0.15%. More preferably, the upper limit of the V content is 0.08%.

Ti: not more than 0.050%

The addition of Ti is optional, and the lower limit of its content is 0%.

However, Ti has the effect of reducing the pearlite block size by forming carbide or carbonitride in the steel wire rods. Therefore, the addition of Ti can improve the drawing processability. In order to obtain this effect, Ti is preferably added in an amount of 0.002% or more. More preferably, the Ti content is 0.005% or more.

On the other hand, if the Ti content exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed in the steel wire rod, resulting in deterioration of wire drawing workability. Therefore, the upper limit of the Ti content is preferably 0.050%. More preferably, the upper limit of the Ti content is 0.030%.

Nb: not more than 0.050%

The addition of Nb is optional, and the lower limit of its content is 0%.

However, Nb has the effect of reducing the pearlite block size by forming carbide or carbonitride in the steel wire rod. As a result, the drawing workability can be improved by adding Nb. In order to obtain this effect, Nb is preferably added in an amount of 0.002% or more. More preferably, the Nb content is 0.005% or more.

On the other hand, when the Nb content exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed in the steel wire rod, resulting in poor drawing workability. Therefore, the upper limit of the Nb content is preferably 0.050%. More preferably, the upper limit of the Nb content is 0.020%.

B: not more than 0.0030%

The addition of B is optional, and the lower limit of its content is 0%.

However, B has an effect of forming BN by binding with N dissolved in the steel wire material, thereby reducing the solute N. Therefore, the fresh workability can be improved by the addition of B. To obtain this effect, B is preferably added in an amount of 0.0003% or more. More preferably, the B content is 0.0007% or more.

On the other hand, when the B content exceeds 0.0030%, coarse carbides are easily formed in the steel wire rod, and the drawing workability is sometimes lowered. Therefore, the upper limit of the B content is preferably 0.0030%. More preferably, the upper limit of the B content is 0.0020%.

Next, the structure of the steel wire rod according to the present embodiment will be described.

The structure of the steel wire rod according to the present embodiment includes pearlite in which ferrite and cementite have a lamellar structure in a layered form. The main structure of the steel wire rod according to the present embodiment is pearlite. Here, " main structure " means a structure occupying 85% or more at an area ratio C on an end face C perpendicular to the longitudinal direction of the steel wire rod or parallel to the longitudinal direction of the steel wire rod. The area ratio of the pearlite can be obtained by subtracting the area ratio of the non-pearlite structure from 100%. The area ratio of pearlite is 85% or more, preferably 90% or more, and more preferably 95% or more. The area ratio of pearlite may be 100%.

The remainder of the structure of the steel wire rod according to the present embodiment, that is, the structure other than pearlite, includes a non-pearlite structure such as pro-eutectoid ferrite, bainite, pseudo-pearlite and corner stone cementite. If the area ratio of the non-pearlite structure exceeds 15%, the drawability is deteriorated. Therefore, the area ratio of the non-pearlite structure is 15% or less. The area ratio of the non-pearlite structure is preferably 10% or less, and more preferably 5% or less. The area ratio of the non-pearlite structure may be 0%.

The area ratio of pearlite can be obtained as follows.

For example, in a specimen of a steel wire rod as described in a later-described embodiment, the C section perpendicular to the longitudinal direction of the steel wire rod is mirror-polished, and the C section is corroded.

Subsequently, a specimen corroded by the break-off is photographed at an arbitrary point at a magnification of 5,000 times using an SEM at 10:00. The area per field of view is set to 3.6 x 10 ^-4 mm ² .

Using the obtained SEM photographs of the respective fields of view, the area ratio of pearlite in each field of view can be obtained by a normal image analysis method.

Then, the area ratio of the pearlite of the steel wire rod is obtained by averaging the area ratio of the obtained pearlite of 10 field of view.

Average lamellar spacing of pearlite: 50 nm to 100 nm

The tensile strength of the steel wire rod can be improved by reducing the average lamellar spacing of the pearlite described above. Also, the influence of the average lamellar spacing on the electrical resistivity is not so large. Therefore, in order to achieve both the improvement of the tensile strength of the steel wire rod and the reduction of the electrical resistivity, it is necessary to reduce the average interlamellar spacing. If the average lamellar spacing of pearlite exceeds 100 nm, the effect of improving the tensile strength becomes insufficient. Therefore, in the steel wire rod according to the present embodiment, in order to obtain this effect, the average lamellar spacing of the pearlite is set to 100 nm or less. The average lamellar spacing of the pearlite is preferably 75 nm or less.

On the other hand, in order to make the average lamellar spacing of pearlite less than 50 nm, it is necessary to transform at low temperature. However, when the steel is transformed at a low temperature, the area ratio of the non-pearlite structure such as bainite exceeds 15%, and the wire drawing workability of the steel wire rod is lowered. Therefore, the average lamellar spacing of the pearlite is 50 nm or more. The average lamellar spacing of the pearlite is preferably 55 nm or more.

The average lamellar spacing of pearlite can be measured as follows. For example, as described in the later-described embodiment, the section C of the steel wire rod is polished, and then the section C is etched to expose the pearlite. Subsequently, the C-section crossed with the pearlite is photographed in a plurality of fields by a scanning electron microscope (SEM) to obtain a tissue photograph of the sample. By means of this obtained tissue photograph, the average lamellar spacing of pearlite can be measured.

Specifically, it can be measured by the following method. First, a 10-field tissue photograph is taken using an SEM. Among the 10 photographed tissue photographs taken, a range where the direction of the lamella is aligned within the field of view is selected in a range where five positions of the lamella can be measured. A straight line is drawn perpendicularly to the lamella at a plurality of selected positions, and the length of five intervals of the lamella is obtained. Subsequently, two points are selected from the smallest one of the selected plurality of points in the length of five intervals. Then, by dividing the length of the five intervals of the measured lamella by 5 in each of the two selected positions, the interval of the lamella at each position can be obtained. That is, two intervals of the lamella can be obtained with one field of view. The average value of the lamellar spacings of 20 positions in total, as obtained in this manner, can be regarded as " average lamellar spacing of pearlite " of the sample.

As described above, in order to improve the tensile strength of the steel wire material, it is effective to reduce the average lamella spacing of the pearlite. As described above, in order to reduce the average lamellar spacing of pearlite, it is preferable to set the cooling rate in the cooling step after hot rolling to 50 DEG C / sec or more, and then pearlite transformation at a low temperature of about 600 DEG C or so.

Electricity flows mainly in the ferrite portion of the pearlite. Cr has an effect of increasing the electrical resistivity when it is dissolved in ferrite in pearlite. Therefore, if the Cr content contained in the ferrite in the pearlite can be lowered, the electrical resistivity can be reduced. That is, it is possible to suppress the increase in the electrical resistivity of the steel wire rod by reducing the Cr content in the ferrite in the pearlite by concentrating Cr in the cementite in the pearlite. Further, Cr is an element which is easily concentrated in cementite in pearlite. Therefore, by controlling the heat treatment conditions, it is possible to increase the Cr content in the cementite in the pearlite and lower the Cr content in the ferrite in the pearlite.

In addition, Si is an element contributing to solid solution strengthening. As a result, if the Si content in the steel wire rods increases, the strength of the steel wire rods can be improved. However, if the Si content in the steel wire rods increases, the electrical resistivity increases. Therefore, by increasing the Cr content in the cementite in the pearlite as the Si content increases, high tensile strength and low electrical resistivity can be achieved.

In order to obtain these effects, in the steel wire rod according to the present embodiment, the Si content, the Cr content in the cementite in the pearlite, and the Cr content in the ferrite in the pearlite satisfy the following formula (1) It is important. Here, the Cr content in the cementite in the pearlite is [% Cr?] And the Cr content in the ferrite in the pearlite is [% Cr? ]to be.

In the steel wire rod according to the present embodiment, by controlling the Cr content in the cementite in the pearlite according to the content of Si contributing to solid solution strengthening so as to satisfy the above formula (1), the tensile strength is improved and the electrical resistivity So that the reduction can be controlled to be compatible with each other.

The Cr content in the cementite in the pearlite, that is, [% Cr?] In the above formula (1) can be obtained, for example, by chemical analysis of the residue extracted by electrolysis. Specifically, it can be obtained by the following method. First, the steel wire rod according to the present embodiment was cut into a size suitable for electrolysis, and electrolysis was performed by setting the current density to 250 to 350 A / m < 2 > using a 10% AA- The solution is extracted. Subsequently, the extracted solution is filtered with a filter having a mesh size of 0.2 mu m to obtain a residue. The filtrate, that is, the residue, can be obtained by performing general chemical analysis. As a general chemical analysis, for example, a method of dissolving a residue by an acid solution and analyzing the solution by ICP emission spectroscopy can be mentioned. In the steel wire rod according to the present embodiment, the metallic elements contained in the cementite in the pearlite are substantially Fe, Mn and Cr, Fe and Cr are less likely to be extracted from the cementite other than Mn, The Cr content in the cementite in the pearlite, that is, [% Cr?] Can be calculated by using the following formula (2). Here, the Cr content, the Fe content, and the Mn content in the residue are expressed as [% Cr Cr], [% Fe Fe], and [% Mn Mn], respectively, in the following formula (2) The S content contained in the steel wire rods is defined as [% S].

Further, in order to improve the tensile strength and reduce the electric resistivity, the Cr content in the cementite in the pearlite is preferably 0.80% to 5.80% by mass%.

The Cr content in the ferrite in the pearlite is calculated on the premise that C is hardly dissolved in the ferrite. For example, the Cr content in the entire steel wire, i.e., [Cr], the Cr content in the cementite in the pearlite, That is, [% Cr [theta]] and the volume fraction of cementite determined by the C content, i.e., [phi [theta]]. Specifically, it is known that the volume fraction of cementite in pearlite is generally determined by the following formula (3) since C is hardly soluble in ferrite. In the following formula (3), the C content is expressed as [% C] by mass%, and the volume fraction of cementite in pearlite is defined as [[phi] [theta]].

The coefficient of 0.149 in the following formula (3) can be obtained from the composition of cementite of 6.69 mass% C and the density of cementite of 7.68 g / cm3.

Then, the volume fraction of ferrite in pearlite, that is, [phi], is obtained by the following formula (4).

From the above, the Cr content, i.e., [% Cr?], Contained in the ferrite in the pearlite can be calculated using the following formula (5).

As described above, in order to suppress an increase in the electrical resistivity of the steel wire rods, it is effective to concentrate Cr in the cementite. Thus, in order to concentrate Cr in cementite, it is preferable to complete the pearlite transformation from the austenite, hold it in the temperature region, and concentrate Cr in cementite. However, if the holding time in the temperature region is long after the completion of the pearlite transformation, the cementite in the pearlite is spheroidized, and the strength of the steel wire rod may be lowered.

The tensile strength TS of the steel wire rod according to the present embodiment is preferably 1350 MPa or more. Further, it is preferable that the absolute value of the tensile strength TS of the steel wire rod is 64 times or more of the absolute value of the electrical resistivity p expressed in units of 占 占. M.

When the steel wire rod according to the present embodiment is applied to the core material of the ACSR, if the strength of the steel wire rod is low, it may be necessary to increase the steel wire rod to reinforce the strength. In this case, it is assumed that the electric resistivity in the entire ACSR becomes large. Therefore, the tensile strength TS of the steel wire rod according to the present embodiment is preferably 1350 MPa or more, more preferably 1400 MPa or more, and further preferably 1500 MPa or more. Further, when the tensile strength TS of the steel wire rod is 1350 MPa or more, for example, when the wire rod diameter is 11 mm to 5 mm, the tensile strength of the steel wire after cold drawing is 1,900 MPa or more .

In the steel wire rod according to the present embodiment, from the viewpoint of achieving both high strength and low electrical resistivity of the steel wire rod, in the relationship between the absolute value of the tensile strength TS and the maximum value of the electrical resistivity rho expressed in units of 占 占, m, Is preferably within the numerical range.

Generally, the maximum value of the tensile strength TS of the wire rod produced under the general hot rolling conditions using the steel having the chemical composition of SWRS72B and SWRS82B specified in JIS G 3502, which is used for the core material of the ACSR, It is about 55 times the absolute value. The tensile strength unit of the wire is ㎫, and the unit of electrical resistivity is μΩ · cm.

Therefore, assuming that the absolute value of the tensile strength TS of the wire rod is 55 times the absolute value of the electric resistivity? Expressed in units of 占 占 m, that is, 55 times the reference value, In steel wire rods, it is preferable that the absolute value of the tensile strength TS be 64 times or more of the absolute value of the electrical resistivity p expressed in units of 占 占 m so as to be 15% or more of the reference value. Further, it is more preferable that the absolute value of the tensile strength TS is 67 times or more of the absolute value of the electrical resistivity p expressed in units of 占 占 · m so as to be 20% or more of the reference value.

Thus, while the tensile strength TS of the steel wire rod is 1350 MPa or more and the absolute value of the tensile strength TS of the steel wire rod is 64 times or more of the absolute value of the electrical resistivity rho expressed in units of 占 占 m, The strength can be increased and the electrical resistivity can be reduced. As a result, the number of reinforcement when the steel wire material is applied to the core material of the ACSR can be reduced. Further, it is possible to suppress an increase in the electrical resistivity in the entire ACSR, thereby suppressing heat generation during transmission, and securing stable transmission efficiency.

In the steel wire rod according to the present embodiment, the absolute value of the electric resistivity p expressed in units of 占 · m is not particularly limited. That is, in the steel wire rod according to the present embodiment, it is preferable that the absolute value of the tensile strength TS and the absolute value of the electric resistivity p expressed in units of 占 占 ㎝ m satisfy the following formula (6).

Excess value of tensile strength TS ≥ Exact value of electrical resistivity ρ × 64 ... (6)

By using the steel wire material satisfying the above formula (6), a significantly higher tensile strength than the conventional one can be secured. As a result, it is possible to reduce the number of reinforcement when the steel wire rod is applied to the core material of the ACSR and to reinforce the electric resistivity of the entire ACSR.

In addition, the lower the electric resistivity? Of the steel wire rod in the present embodiment is, the better, and the larger the tensile strength TS is, the better.

By satisfying the above-described chemical composition and structure, it is possible to obtain a steel wire having both improved strength and reduced electrical resistivity. In order to obtain the steel wire rod described above, the steel wire rod may be manufactured by a manufacturing method described later. Next, a preferable manufacturing method of the steel wire rod according to the present embodiment will be described.

The steel wire rod according to the present embodiment can be manufactured as follows. The method of manufacturing a steel wire rod described below is an example for obtaining the steel wire rod according to the present embodiment and is not limited to the following procedure and method. Any method that can realize the structure of the present invention, It is possible to adopt it.

First, the steel is melted so as to have the chemical composition described above, and then a steel strip is produced by continuous casting and hot rolling is performed. After continuous casting, crushing may be performed. When the obtained slab is subjected to hot rolling, the slab is heated by a general method so that the center portion of the slab becomes 1000 占 폚 to 1100 占 폚, and the finish temperature is 900 占 폚 to 1000 占 폚. After finishing rolling, the hot-rolled wire rod is first cooled down to 700 占 폚 or less by combining water cooling and air cooling by air. The average cooling rate in the primary cooling is preferably 50 DEG C / second or more. After the primary cooling, the wire rod is immersed in a molten salt of a nitrate system at a temperature of 500 ° C to 530 ° C and is secondarily cooled to 590 ° C to 620 ° C for pearlite transformation. Then, the wire after the secondary cooling is maintained in the molten salt bath at a temperature of 550 ° C to 570 ° C for 30 seconds to 50 seconds, whereby Cr can be concentrated in the cementite. Thereafter, the molten salt is removed by spraying water, and after the third cooling to room temperature, winding is performed. The winding may be performed immediately after the primary cooling or the secondary cooling.

In the above-mentioned secondary cooling, the average cooling rate is preferably 30 占 폚 / second or more. Further, in the holding after the secondary cooling, it is preferable that the temperature of the wire rod is maintained in the range of 600 DEG C to 550 DEG C for 30 seconds to 50 seconds. For example, a hot bath or a fluidized bed furnace may be used. In the case of using a hot water bath, it is not necessary to lower the temperature to 700 DEG C during the primary cooling, and the secondary cooling and the heating may be performed in the same ladle. In this case, it is preferable to keep the flask at a temperature of 550 to 600 DEG C for 35 to 60 seconds.

As the cooling and holding method after the finish rolling, cooling and holding can be performed by only laydown. For example, when the temperature of the wire rod after the finish rolling is in the range of 900 to 700 占 폚, the average cooling rate of the wire rod when immersed in a ladle of 640 占 폚 to 500 占 폚 in the temperature of the ladle is 100 占 폚 / / Sec.

When the temperature of the wire after finish rolling is in the range of 700 占 폚 to 620 占 폚 and the temperature of the hot water is 590 占 폚 to 600 占 폚, the average cooling rate of the wire rod is 40 占 폚 / sec to 50 占 폚 / sec. When the temperature is 550 to 560 占 폚, the average cooling rate of the wire rod is 60 to 70 占 폚 / sec. When the temperature of the hot rolling is 490 to 500 占 폚, the average cooling rate of the wire rod is 90 占 폚 / Lt; 0 > C / sec.

The finishing temperature in the above-described hot rolling refers to the surface temperature of the steel wire immediately after finish rolling. The average cooling rate in cooling after finish rolling indicates the surface cooling rate of the steel wire rod.

Example

Hereinafter, the effects of the steel wire rod according to the present embodiment will be described in more detail with an example of the steel wire rod according to the present invention. However, the conditions in the examples are examples of conditions employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to the following examples. It is also possible to carry out the present invention without departing from the gist of the present invention as long as the object of the present invention is achieved. Therefore, the present invention can adopt various conditions, all of which are included in the technical features of the present invention.

Strengths A to Y having the chemical compositions shown in Table 1 were dissolved in a 50 kg vacuum melting furnace and then cast as an ingot. The chemical composition of the steel V satisfied SWRS82B of JIS standard.

Each of the above ingots was heated at 1250 占 폚 for 1 hour and hot forged to a diameter of 15 mm so that the finish temperature was 950 占 폚 or higher and then cooled to room temperature to obtain a hot forging material. This hot forging material was cut and cut to obtain a cutting material having a diameter of 10 mm and a length of 1000 mm.

Subsequently, each of the obtained cutting materials was heated for 15 minutes in a nitrogen atmosphere at 1050 캜 to set the center temperature of the cutting material to 1000 캜 or higher. Thereafter, the wire was hot-rolled to a cutting diameter of 7 mm such that the finishing temperature was in the range of 950 캜 to 1000 캜, thereby obtaining a wire rod. Further, in the state where the temperature of the wire rod was 900 DEG C or higher, it was immersed and kept in a ladle under the conditions shown in Table 2. Thereafter, the wire rod was taken out from the hot water bath and cooled to room temperature to obtain a steel wire rod.

When the temperature of the wire after the finish rolling was in the range of 900 to 700 占 폚, the average cooling rate of the wire was 100 to 200 占 폚 / sec when the temperature of the hot water was 640 to 500 占 폚.

When the temperature of the wire after the finish rolling is in the range of 700 占 폚 to 620 占 폚, the average cooling rate of the wire is 40 占 폚 / sec to 50 占 폚 / sec when the temperature of the hot water is 590 占 폚 to 600 占 폚, The average cooling rate of the wire rod was 90 deg. C / sec to 100 deg. C / sec when the temperature of the ladle was 490 deg. C to 500 deg.

For comparison, a part of the obtained cutting material was subjected to hot rolling at a diameter of 7 mm to obtain a wire rod, and then the wire rod was cooled down to room temperature by air cooling in the air or air cooling by a fan, To obtain a steel wire rod. The average cooling rate of the wire rod when it is cooled in the atmosphere is 7 ° C / sec to 8 ° C / sec in the range of 900 ° C to 700 ° C in the temperature of the wire after the finish rolling, Lt; 0 > C to 5 [deg.] C / sec. The average cooling rate of the wire rod when air-cooled by an electric fan is 12 to 14 占 폚 / sec in the range of 900 占 폚 to 700 占 폚, and the temperature of the wire rod after finish rolling is 700 占 폚 to 700 占 폚 And 6 ° C / sec to 7 ° C / sec in the range of 620 ° C.

The steel wires of Test Nos. 1 to 48 prepared under the above-mentioned respective conditions were subjected to the following tests and evaluated.

For each steel wire rod, the C section perpendicular to the longitudinal direction of the steel wire rod was mirror-polished and then corroded.

In order to obtain the area ratio of pearlite, a specimen corroded with the borehole was photographed at a magnification of 5000 times at an arbitrary point using a SEM at 10:00. The area per field of view was 3.6 x 10 < ^-4 >

Then, in the photographs of each field of view, the area ratio of the pearlite portion was determined by a method of image analysis in the ordinary way. The average value of the area ratio of the pearlite in the 10-view field was taken as the area ratio of the pearlite of the steel wire rod.

In addition, in order to obtain the average lamellar spacing of pearlite, a specimen corroded with the exudate was photographed at SEM at a magnification of 10000 times at an arbitrary point at 10:00. The area per field of view was 9.0 × 10 ^-5 mm 2.

Then, in the photographs of the respective fields of view, a range in which the directions of the lamellas of the pearlite were aligned was selected. Subsequently, a straight line was drawn perpendicularly to the lamella, and the length of five intervals of the lamella was obtained for a portion where the five intervals of the lamella were measurable, and the portion with the smallest lamella interval and the portion with the smallest lamella interval were found. Then, by dividing the obtained length of five intervals of the lamella by 5, the pearlite lamella interval of each place was obtained. The average value of the lamellar spacings of 20 places in total, as determined in the above manner, was taken as the average lamellar spacing of the pearlite of the steel wire rod.

After the steel wire rod was cut to a diameter of 6 mm, the current density was set to 250 to 350 A / m < 2 > using an electrolytic solution of 10% AA, which is a general condition of electrolytic polishing, and electrolysis was conducted to extract the solution. The above-mentioned 10% AA-based electrolytic solution was a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution. Subsequently, the extracted solution was filtered with a filter having a mesh size of 0.2 탆 to obtain a residue, and the residue was dissolved by an acid solution. The solution was analyzed by ICP emission spectroscopy to determine the Cr content [% Cr Cr] (% Fe residue) and Mn content (% Residue Mn) were obtained. The Cr content in the cementite in pearlite, that is, [% Cr?] Was calculated using the following formula (A). Further, "the metallic element in the cementite contains substantially Fe, Mn and Cr", "Fe and Cr are not extracted from other than cementite" and "when S is contained in the steel, Mn is cementite To form MnS earlier than before ". Here, the Cr content, the Fe content and the Mn content contained in the residue are expressed as [% Cr Cr], [% Fe Fe] and [% Mn Mn], respectively, in mass% The S content contained in the steel wire rods was defined as [% S].

The Cr content in the ferrite was calculated as follows. First, the volume fraction [phi] [theta] of cementite in pearlite was determined by the following formula (B), and then the volume fraction [phi alpha] of the ferrite in the pearlite was determined by the following formula (C). Thereafter, the Cr content [% Cr?] In the ferrite was calculated by the following formula (D).

In the following formula (B), the C content of the entire steel wire rod is [% C] and the Cr content of the entire steel wire rod is [% Cr ].

Two tensile test specimens each having a diameter of 3.2 mm (a circle having a radius of 1.6 mm around the center of the cross section of C) and a length of 18 mm were taken from the central portion of the C section of each steel wire rod by cutting, and JIS Z 2241, tensile strength at room temperature was measured, and tensile strength TS was measured. Then, the measured average value was defined as the tensile strength TS of the steel wire rod. The tensile strength TS is in units of MPa.

As a test piece for measuring the electrical resistivity p, a test piece of a rectangular parallelepiped having a size of 3.0 mm x 4.0 mm x 60 mm was taken from the center of each steel wire rod and the electrical resistivity was measured at a temperature of 20 캜 by a normal four-terminal method. The unit of the electrical resistivity ρ obtained is μΩ · cm.

The obtained evaluation results are shown in Tables 3 and 4. In addition, [% Cr?] And [% Cr?] In Tables 3 and 4 indicate mass%, Cr content in cementite in pearlite, and Cr content in ferrite in pearlite, respectively.

In Table 3 and Table 4, the case where the above-mentioned formula (1) is satisfied is shown as " & cir & "

Tables 3 and 4 show the tensile strength TS of the steel wire rod whose unit is expressed in MPa, the maximum value of the electrical resistivity rho expressed in units of 占 · m and the value of 64 times the maximum value of the electrical resistivity rho.

In Table 3 and Table 4, the case where the absolute value of the tensile strength TS is equal to or larger than 64 times the absolute value of the electrical resistivity? Expressed in units of 占 占 is determined as " good & Was judged to be " poor ", and " x " was indicated.

From Table 3 and Table 4 it can be seen that in the case of Test Nos. 1, 3, 6, 7, 9 to 11, 14, 17, 18, 20 to 22, 24, 26, 27, 30, 33, 34, 36 and 44 to 47 At least one of the technical characteristics of the chemical composition, the structure, the average lamellar spacing of the pearlite, the Si content, the Cr content in the cementite in the pearlite, and the content of Cr in the ferrite in the pearlite specified in the present invention is not satisfied . In Test Nos. 45 and 47, the drawability was poor.

In contrast, Test Nos. 2, 4, 5, 8, 12, 13, 15, 16, 19, 23, 25, 28, 29, 31, 32, 35, 37 to 43 and 48, Chemical composition, texture, average lamellar spacing of pearlite, Si content, Cr content in cementite in pearlite, and Cr content in ferrite in pearlite.

≪ Industrial applicability >

INDUSTRIAL APPLICABILITY According to the present invention, a steel wire rod having a high strength and a low electrical resistivity can be obtained, and the contribution to the industry is remarkable.

Claims (3)

As a chemical component, in mass%
C: 0.8% to 1.1%
0.02 to 0.30% Si,
Mn: 0.1% to 0.6%
Cr: 0.3% to 1.5%
Al: 0.01% to 0.05%
N: 0.008% or less,
P: 0.03% or less,
S: 0.02% or less, and optionally,
Mo: 0.20% or less,
V: 0.15% or less,
Ti: 0.050% or less,
Nb: 0.050% or less and
And B: 0.0030% or less,
The remainder including Fe and impurities,
The structure includes pearlite, the area ratio of the pearlite is 85% or more,
The average lamellar spacing of the pearlite is 50 nm to 100 nm,
Wherein a content of Si is [% Si], a content of Cr contained in the pearlite is [% Cr?], A content of Cr contained in the ferrite in the pearlite is represented by [% Cr? , [% Si], [% Cr?] And [% Cr?] Satisfy the following formula (1).

3. The composition according to claim 1, wherein, as the chemical component,
Mo: 0.02% to 0.20%
V: 0.02% to 0.15%,
Ti: 0.002% to 0.050%
Nb: 0.002% to 0.050% and
And B: 0.0003% to 0.0030%. The steel wire rod according to any one of claims 1 to 3, wherein the tensile strength TS of the steel wire rod is 1350 MPa or more, and the absolute value of the tensile strength TS of the steel wire rod satisfies an electrical resistivity ρ Of the total length of the steel wire.