JP2017115176A - Hot rolling wire material for wire drawing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolling wire material for wire drawing suitable for applications capable of manufacturing an ultra fine steel wire excellent in strength and durability without conducting a patenting treatment by using a hot rolled wire material as a raw material.SOLUTION: There is provided a hot rolling wire material for wire drawing containing, by mass%, C:0.3 to 0.5%, Si:0.1 to 1.0%, Mn:0.3 to 1.0%, Cr:0.25 to 0.7%, Mn+2Cr:0.90 to 1.8% and the balance Fe with inevitable impurities with Al, N, P and S in the impurities limited to predetermined ranges, 70% of which consists a pearlite structure and having average cementite thickness t in the pearlite structure calculated by the formula (1) of 15 to 40 nm, maximum particle diameter of TiN in measurement area of 125 mmof less than 15 μm and diameter of 4.0 to 6.0 mm. t=average pearlite lamellar spacing×{100/(100-volume fraction of proeutectoid ferrite)}×C content×0.153 Formula (1).SELECTED DRAWING: None

Description

  The present invention relates to a hot-rolled wire for wire drawing suitable for applications that can be produced without performing a patenting process when producing an ultrafine steel wire excellent in strength and ductility using a hot-rolled wire as a raw material. Is.

  In general, a thin high-strength steel wire used as a steel cord or sewing wire used as a reinforcing material for a radial tire, various belts, or a hose of an automobile has a wire diameter (diameter) of 5 to 5 adjusted and cooled after hot rolling. 6 mm steel wire (hereinafter referred to as “steel wire” is simply referred to as “wire”) is subjected to primary wire drawing to a wire diameter of 3 to 4 mm, followed by primary patenting and then secondary wire drawing. It is manufactured through a process in which the wire diameter is set to 1 to 2 mm by processing, further subjected to secondary patenting treatment and brass plating, and then the wire diameter is set to 0.1 to 0.4 mm by final wet drawing. The thin high-strength steel wire (extra fine steel wire) manufactured in this way is twisted into a “twisted steel wire” by, for example, twisting to form a steel cord or the like.

Incidentally, where as the patenting process is well known, after the entire organization is heated to the austenite temperature region and austenite structure, lead bath maintained at a temperature below the A ₁ transformation point, the fluidized bed In such a process, the substrate is rapidly cooled to a temperature range mainly composed of a pearlite structure, and maintained for a predetermined time in the temperature range.

From the viewpoint of reducing manufacturing costs and CO ₂ reduction, there is a strong demand for reducing the number of patenting processes, and a technique for reducing the patenting process that has been performed twice in the past to one time has already been widely implemented.

  In recent years, there has been a demand from the industry to make this patenting process zero. However, if a steel wire having a diameter of 5 to 6 mm is drawn to a diameter of 0.1 to 0.4 mm without performing heat treatment in the middle, even if breakage occurs frequently or even if the wire can be drawn, the ductility is insufficient. For this reason, breakage often occurs during the subsequent stranded wire.

  Therefore, there has been a strong demand for a hot-rolled wire for wire drawing that can produce a steel cord without subjecting the hot-rolled wire to heat treatment including patenting.

  In order to meet the above demand, for example, techniques described in Patent Documents 1 and 2 shown below have been proposed.

  Patent Document 1 includes C: 0.2 to 0.6%, B: 0.0003 to 0.01%, and the like, and is characterized by a pearlite area ratio, proeutectoid ferrite content, and lamellar cementite morphology. A wire rod for a strength steel wire is disclosed. However, since the thickness of the lamellar cementite is not taken into consideration, it was not satisfactory as a means for stably suppressing disconnection during wire drawing or stranded wire.

  Patent Document 2 includes a method for producing a steel wire for rubber reinforcement including C: 0.35 to 0.9%, etc., characterized by the area ratio of pro-eutectoid ferrite and the tensile strength after wire drawing. It is disclosed. However, in the example of Patent Document 2, a heat treatment called bluing is performed in the middle of wire drawing, and the stretched lamella structure formed by drawing is divided, so that a predetermined strength is obtained after wire drawing. There was a problem that it was difficult and the ductility deteriorated.

JP 2014-55316 A JP-A-9-49018

  The present invention has been made against the background of the above circumstances, and is suitable as a production material for steel cords, sawing wires, etc., and suitable for applications that can be produced without performing patenting treatment when producing them. An object of the present invention is to provide a hot-rolled wire rod for wire drawing that has a high strength of, for example, 3000 MPa or more, is excellent in ductility, and can be stably manufactured.

  In order to solve the above problems, the present inventors first investigated the influence of the chemical composition, microstructure, and inclusions of the steel wire on wire breakage during wire drawing, and tensile strength and ductility after wire drawing. After repeated research and careful analysis of the results, the following findings were obtained.

a) In order to improve the ductility after wire drawing, it is effective to reduce the amount of C and add Cr.
b) When the cementite in the pearlite structure in the hot-rolled wire rod stage is thick, wire breakage tends to occur during wire drawing, and ductility after wire drawing decreases. If the cementite in the pearlite structure is too thin, the pearlite lamella spacing will be too small as a result, and the tensile strength will increase excessively, so that the wire will be easily broken during wire drawing.
c) In order to obtain a pearlite structure having an appropriate cementite thickness stably with a hot-rolled wire, it is necessary that the phase transformation after finish rolling occurs stably in the target temperature range, and for that purpose, the amount of Mn It is necessary to contain the Cr content appropriately, and the degree of influence of Cr should be about twice that of Mn.
d) When the amount of Al as an inevitable impurity is appropriately controlled, the inclusion that becomes the starting point at the time of disconnection is TiN. Therefore, if the maximum particle size of TiN is reduced, disconnection can be prevented.

  As a result of further detailed experiments and research based on the findings of a) to d), the amount of alloying elements and impurity elements of steel is appropriately adjusted or regulated, and at the same time, the conditions of the metal structure mainly composed of pearlite. In particular, by adjusting the average thickness of cementite in pearlite and the maximum particle size of TiN within appropriate ranges, the above-mentioned problems can be solved without applying a patenting treatment. Has been found to have a high tensile strength of, for example, 3000 MPa or more and excellent ductility, and can be stably manufactured even in a mass production process while ensuring such excellent performance. It was.

(1) In mass%,
C: 0.3 to 0.5%,
Si: 0.1 to 1.0%,
Mn: 0.3 to 1.0%,
Cr: 0.25 to 0.70%,
Mn + 2Cr: 0.90 to 1.8%
And the balance consists of Fe and inevitable impurities, and Al, Ti, N, P, S and O in the impurities are each Al: 0.002% or less,
Ti: 0.002% or less,
N: 0.005% or less,
P: 0.02% or less,
S: 0.01% or less,
O: 0.003% or less,
In addition, as a metal structure, 70% or more by volume ratio is composed of a pearlite structure, the average cementite thickness t in the pearlite structure obtained from the formula (1) is 15 to 40 nm, and TiN in a measurement area of 125 mm ² A hot rolled wire rod for wire drawing having a diameter of 4.0 to 6.0 mm, wherein the maximum particle size is less than 15 μm.
t = average pearlite lamella spacing (nm) × {100 / (100-volume fraction of pro-eutectoid ferrite (%))} × C content (%) × 0.153 (1)
(2) Furthermore, in mass%,
The hot-rolled wire rod for wire drawing according to (1), characterized by containing any one or both of Mo: 0.02 to 0.20% and B: 0.0003 to 0.0030% .

  According to the present invention, it is possible to stably use steel cords and sawing wires having high tensile strength of, for example, 3000 MPa or more and excellent ductility without high patentability. Thus, it is possible to produce the same, which brings about extremely useful effects in the industry.

  The chemical composition of the hot-rolled wire rod for wire drawing according to the present invention and the conditions of the metal structure will be described in more detail.

<Ingredient composition>
C: C is an effective component for increasing the tensile strength of steel. However, when the content is less than 0.3%, it is difficult to stably impart a high strength such as a tensile strength of 3000 MPa to the final product. Furthermore, it is effective to increase the C content in order to stably obtain a high-strength final product, and in order to obtain 3200 MPa or more, the C content is preferably set to 0.35% or more. On the other hand, if the C content is too large, the steel material is hardened, leading to wire breakage and ductility reduction during wire drawing. In particular, if the C content exceeds 0.5%, the influence becomes remarkable, and stable mass production becomes industrially difficult. Therefore, the C content is determined to be within a range of 0.3 to 0.5%. Desirably, it is 0.35% or more and 0.45% or less.

  Si: Si is also an effective component for increasing the strength of the steel material, and is also a necessary component as a deoxidizer. However, if the Si content is less than 0.1%, the effect of adding Si is not sufficient. On the other hand, if the Si content exceeds 1.0%, ductility after wire drawing decreases. Therefore, the Si content is determined to be in the range of 0.1 to 1.0%. Since Si is an element that also affects the hardenability of the steel material and the formation of proeutectoid cementite, the Si content is 0.2 to 0.5 from the viewpoint of ensuring a desired microstructure stably in the steel wire material. It is more desirable to adjust within the range of%.

  Mn: Mn affects the phase transformation time from austenite and is an effective component for obtaining a stable pearlite structure with a hot-rolled wire. However, if the Mn content is less than 0.3%, the effect by the above action is not sufficient. On the other hand, Mn is an element that is easily segregated. If it is contained in an amount exceeding 1.0%, Mn is segregated particularly in the central portion of the wire, martensite is generated in the segregated portion, and breaks during wire drawing. It becomes easy. Therefore, the Mn content is determined to be in the range of 0.3 to 1.0%.

  Cr: Cr greatly affects the phase transformation time from austenite and is effective for obtaining a stable pearlite structure with hot-rolled wire, and at the same time reduces the lamella spacing of pearlite and increases the strength and ductility of the final product. There is an effect. In order to stably obtain excellent ductility when the final product has a tensile strength of 3000 MPa or more, a Cr content of 0.25% or more is required. However, if the Cr content exceeds 0.70%, Cr is segregated particularly in the central portion of the wire, martensite is generated in the segregated portion, and breakage is likely to occur during wire drawing. Therefore, the Cr content is determined to be in the range of 0.25 to 0.70%. Desirably, the Cr content is 0.50% or less.

  Mn + 2Cr: As described above, Mn and Cr greatly affect the phase transformation time from austenite, and are effective in obtaining a stable pearlite structure with a hot-rolled wire, so control the cementite thickness in the pearlite structure. Also effective. The effect of Cr is about twice that of Mn. When Mn + 2Cr is less than 0.90%, the cementite in the pearlite structure in the hot-rolled wire becomes thick and cannot be stably mass-produced within the range specified in the present invention. In order to mass-produce more stably, 1.00% or more is preferable. On the other hand, as described above, both Mn and Cr are likely to segregate particularly in the central portion of the wire, and martensite is generated in the segregated portion, and breakage is likely to occur during wire drawing. Therefore, Mn + 2Cr is determined to be in the range of 0.90 to 1.8%. Desirably, it is 1.00% or more, and more desirably 1.10% or more.

  The balance with respect to each of the above elements may basically be inevitable impurities and Fe. However, in the present invention, the contents of impurities Al, Ti, N, P, S, and O are further set as follows. Regulate on the street.

Al: Al is an element that forms oxide inclusions containing Al ₂ O ₃ as a main component and reduces wire drawing workability. In particular, if the Al content exceeds 0.002%, the oxide inclusions are coarsened, and breakage occurs frequently during wire drawing, resulting in a significant reduction in wire drawing workability. Therefore, the Al content is restricted to 0.002% or less.

Ti: When Ti contains N, TiN is easily formed, and TiN is very hard and is not deformed by hot rolling or wire drawing, so it is likely to become a disconnection starting point during wire drawing. Even if the manufacturing method is taken into consideration, if the Ti content exceeds 0.002%, it is difficult to make the maximum TiN particle size in the measurement area 125 mm ² less than 15 μm, and breakage is likely to occur during wire drawing. . Therefore, the Ti content is limited to 0.002% or less. Preferably, it is 0.001% or less.

N: If N contains Ti, TiN is likely to be formed, and TiN is very hard and is not deformed by hot rolling or wire drawing, so it is likely to be a disconnection starting point during wire drawing. Even if the manufacturing method is taken into consideration, if the N content exceeds 0.005%, it is difficult to make the maximum TiN particle size in the measurement area 125 mm ² less than 15 μm, and breakage is likely to occur during wire drawing. . Therefore, the N content is regulated to 0.005% or less. Preferably, it is 0.004% or less.

  P: P is an element that segregates at the grain boundary and lowers the wire drawing workability. In particular, if the P content exceeds 0.02%, the wire drawing workability is significantly lowered. Therefore, the P content is restricted to 0.02% or less.

  S: S is also an element that reduces wire drawing workability. And when S content exceeds 0.01% especially, since the fall of wire drawing workability will become remarkably, we decided to regulate S content to 0.01% or less.

O: O is an element that easily forms an oxide, and when Al is present, it is an element that forms a hard oxide-based inclusion mainly composed of Al ₂ O ₃ and reduces wire drawing workability. . In particular, if the O content exceeds 0.003%, even if the Al content is within the range of the present invention, the oxide inclusions are coarsened and wire breakage occurs frequently during wire drawing, and wire drawing workability is increased. The reduction of the becomes remarkable. Therefore, the O content is limited to 0.003% or less.

  Furthermore, in this invention, in addition to the component demonstrated above, you may contain any one or both of Mo: 0.02-0.20% or B: 0.0003-0.0030%.

  Mo: The addition of Mo is optional, but if Mo is added, the effect of increasing the balance between tensile strength and ductility after wire drawing can be more stably exhibited. In order to obtain this effect, the Mo content is preferably 0.02% or more. However, if the Mo content exceeds 0.20%, a martensite structure is likely to be generated, and the wire drawing workability may be reduced. Therefore, the Mo content when Mo is positively added is preferably in the range of 0.02 to 0.20%. More preferably, the Mo content is 0.10% or less. On the other hand, from the viewpoint of obtaining a balance between the tensile strength and ductility of the wire drawing material, the Mo content is more preferably 0.04% or more.

  B: Addition of B is optional, but if B is added, the effect of increasing the balance between tensile strength and ductility after wire drawing can be more stably exhibited. In order to obtain this effect, the B content is preferably 0.0003% or more. However, if the B content exceeds 0.0030%, coarse BN is likely to be generated, and the wire drawing workability may be lowered. Therefore, the B content when B is positively added is preferably in the range of 0.0003 to 0.0030%. More preferably, the B content is 0.0020% or less. On the other hand, from the viewpoint of obtaining a balance between the tensile strength and ductility of the wire drawing material, the B content is more preferably 0.0005% or more.

<Volume ratio of pearlite structure>
In order to stably secure a high tensile strength of 3000 MPa or more and excellent ductility without performing a patenting treatment, the volume ratio of the pearlite structure of the hot-rolled wire rod is set to 70% or more. There is a need to. The remaining structure is one or more of pro-eutectoid ferrite and bainite. The volume ratio of the pearlite structure is more preferably 80% or more.

<Average thickness of cementite in pearlite structure>
In order to stably secure a high strength of 3000 MPa or higher in the final product without performing a patenting process, to obtain excellent ductility, and to prevent disconnection during wire drawing, The average thickness of cementite in the pearlite structure of the rolled wire must be 40 nm or less. On the other hand, if the average thickness of cementite in the pearlite structure is less than 15 nm, the strength after wire drawing becomes too high, so that the ductility is lowered and breakage during wire drawing tends to occur. Therefore, the average thickness of cementite in the pearlite structure was set in the range of 15 to 40 nm. The average thickness of cementite in the pearlite structure is preferably in the range of 15 to 35 nm, more preferably in the range of 20 to 35 nm.

<Maximum particle size of TiN>
When Al was 0.002% or less and O was 0.003% or less, inclusions other than TiN, that is, sulfides and oxides were not observed at the starting point of the disconnection. When the maximum TiN particle size in the measurement area of 125 mm ² was 15 μm or more, disconnection occurred during wire drawing even when other requirements were satisfied. Therefore, the maximum TiN particle size in the measurement area of 125 mm ² was set to less than 15 μm. The maximum TiN particle size in the measurement area of 125 mm ² is preferably 12 μm or less, more preferably 10 μm or less.

<Diameter of hot rolled wire rod for wire drawing>
If the diameter of the hot-rolled wire rod for wire drawing exceeds 6.0 mm, even if the other requirements of the present invention are satisfied, the wire cannot be drawn to the target diameter of 0.32 mm or can be drawn. Even in this case, the target ductility of the present invention cannot be obtained. On the other hand, if the diameter of the hot-rolled wire is less than 4.0 mm, the production efficiency in hot rolling is greatly reduced and the cost is increased, and the merit of eliminating the patenting process is lost. Therefore, the diameter of the hot-rolled wire rod for wire drawing is set within a range of 4.0 to 6.0 mm. The thickness is preferably in the range of 4.5 to 6.0 mm, more preferably in the range of 4.5 to 5.5 mm.

<Metallic structure measurement method>
Next, a measurement method for each condition of the metal structure defined in the present invention will be described.

The volume ratio of the pearlite structure is measured by the following method. First, the cross section of the hot-rolled wire rod (that is, the cut surface perpendicular to the length direction) is mirror-polished, then corroded with picral, and at any position using a field emission scanning electron microscope (FE-SEM). Take 10 photos at a magnification of 3000x. The area per field of view is 5.0 × 10 ⁻⁴ mm ² (vertical 20 μm, horizontal 25 μm). Next, the area ratio of the tissue other than the pearlite tissue is obtained by normal image analysis using the photograph. Since this area ratio is the same as the volume ratio, the value obtained by removing the area ratio other than the pearlite structure from 100 is defined as the volume ratio of the pearlite structure. At this time, the volume ratio of the pro-eutectoid ferrite structure was also determined by the same method.

The average thickness of cementite in the pearlite structure is calculated by the following method. First, the cross section of the hot-rolled wire rod was mirror-polished and then corroded with picral. Using a field emission scanning electron microscope (FE-SEM), 10 fields of interest were photographed at a magnification of 5000 times. The area per field of view is 1.8 × 10 ⁻⁴ mm ² (vertical 12 μm, horizontal 15 μm). Next, in the range where the direction of the pearlite lamella in the field of view is aligned, the lamella extends at the place where the lamella interval is measurable and the lamella interval is the smallest and the second smallest lamella interval. A length perpendicular to the direction was drawn by drawing a straight line with respect to the direction, and a length corresponding to 5 lamellas was obtained and divided by 5 to obtain a pearlite lamella spacing at each location. The average value of the lamella spacing for the 10 visual fields thus obtained (total of 20 locations) was taken as the average pearlite lamella spacing of the hot-rolled wire rod.
The average thickness t of cementite in the pearlite structure was calculated by the following formula using the average pearlite lamella spacing determined by the above method and the volume fraction of pro-eutectoid ferrite.

  t (nm) = average pearlite lamella spacing (nm) × {100 / (100-volume fraction of pro-eutectoid ferrite (%))} × C content (%) × 0.153 Formula (1)

Here, the constant 0.153 was used because the volume fraction when all the C became cementite was 0.153 in a steel material having a C content of 1.00%. A cross section perpendicular to the rolling direction was cut out from the hot-rolled wire rod, the cross section perpendicular to the rolling direction was mirror-polished, and inclusions were measured using an optical microscope. In addition, observation with said optical microscope was performed for every range of 2.5 mm x 2.5 mm, and the long diameter and the short diameter were measured about the largest TiN in this range. This measurement was performed on each sample for 20 fields of view, and the maximum TiN particle size in a measurement area of 125 mm ² was determined. The particle size was calculated from the following formula.

Particle size = (major axis ² + minor axis ² ) ^1/2

  Moreover, since TiN exhibits a gold color when observed with an optical microscope, it can be easily distinguished from other inclusions.

<Manufacturing method>
Next, an example of a method for producing the hot-rolled wire rod for wire drawing according to the present invention will be described. However, it goes without saying that the method for producing the hot-rolled wire rod for wire drawing according to the present invention is not limited to the method described below.

  When manufacturing the hot-rolled wire rod for wire drawing according to the present invention, the volume ratio of pearlite, the average thickness of cementite in the pearlite structure, and the maximum particle size of TiN can surely satisfy the above-mentioned conditions. In addition, the process and each process condition may be set according to the steel component composition, target performance, wire diameter, etc., but here 0.3 to 0.5% C, 0.1 to 1.0% Si, 0.3 to 1.0% Mn, 0.25% to 0.70% Cr, and 0.90 to 1.8% Mn + 2Cr, Al as an inevitable impurity: 0.002% or less, Ti : An example of a production method when steel containing 0.002% or less and N: 0.005% or less is shown.

In the case of melting for small-scale experiments, if the steel is 50 kg or less, the raw material is evacuated for 20 minutes or more after melting, the material is cast iron, and the casting is cast in a mold having an inner average cross-sectional area of 120 cm ² or less. A portion of 15% by volume from both ends of the later ingot is not used.

  Further, in the case of continuous casting, it is preferable to sufficiently carry out electromagnetic stirring of the molten steel, set the average cooling rate from the start of solidification to the end of solidification to 5 ° C./min or more, and further reduce during the solidification.

  In the case of a small ingot cast by the above method, a steel piece is obtained by hot forging after heating to 1200 to 1250 ° C., and in the case of a slab manufactured by continuous casting, by piece rolling after heating to 1200 to 1250 ° C. .

  The steel slab manufactured by the above method is heated so that it may become 1050-1150 degreeC, a finishing temperature shall be 900-1000 degreeC, and it hot-rolls to (phi) 4.0-6.0mm. After finish rolling, water cooling and air cooling by air are combined to cool an average cooling rate of 50 ° C./second or more until it enters a temperature range of 680 to 730 ° C., and then the air cooling by air is used to reduce the average cooling rate to 10 to 10 It cools until it enters into the temperature range of 630-590 degreeC at 20 degreeC / sec, and it cools until it becomes below 500 degreeC.

  In addition, the heating temperature of the steel slab in the present specification refers to the surface temperature of the steel slab, the rolling finish temperature refers to the surface temperature of the steel wire immediately after the finish rolling, and the cooling rate after the finish rolling, Refers to the surface cooling rate of steel wire.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. It is not something. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  First, steels A to U having chemical compositions shown in Table 1 were cast into 50 kg or 150 kg ingots. The casting conditions at this time are also shown in Table 1. In the case of an ingot manufactured under the conditions shown in Table 1, after heating to 1230 ° C., a steel piece having a diameter of 80 mm was formed by hot forging and then allowed to cool to room temperature.

Further, steels V to X having chemical compositions shown in Table 2 were melted by a converter and then continuously cast. During casting, the molten steel was sufficiently agitated, the average cooling rate from the start of solidification to the end of solidification was 6 ° C./min, and further reduced during the solidification. The slabs having the components shown in Table 2 were cooled to 700 ° C. or lower, heated to 1250 ° C., and then subjected to block rolling to obtain 122 mm square steel slabs.
The steel slab manufactured by the above method was hot-rolled on the conditions of Table 3.

About the hot-rolled wire thus obtained, the volume ratio of the pearlite structure, the volume ratio of pro-eutectoid ferrite, the average thickness of cementite in the pearlite structure, and the maximum in the measurement area of 125 mm ² by the following method The TiN particle size was measured.

The volume ratio of the pearlite structure is determined by mirror-polishing the cross section of the hot-rolled wire rod (that is, the cut surface perpendicular to the length direction) and then corroding with picral, using a field emission scanning electron microscope (FE-SEM). Then, take 10 pictures at an arbitrary position at a magnification of 3000 times. The area per field of view is 5.0 × 10 ⁻⁴ mm ² (vertical 20 μm, horizontal 25 μm). Next, the area ratio of the tissue other than the pearlite tissue is obtained by normal image analysis using the photograph. Since this area ratio is the same as the volume ratio, the value obtained by removing the area ratio other than the pearlite structure from 100 is defined as the volume ratio of the pearlite structure. At this time, the volume ratio of the pro-eutectoid ferrite structure was also determined by the same method.

Next, the average thickness of cementite in the pearlite structure is calculated by the following method. First, the cross section of the hot-rolled wire rod was mirror-polished and then corroded with picral. Using a field emission scanning electron microscope (FE-SEM), 10 fields of interest were photographed at a magnification of 5000 times. The area per field of view is 1.8 × 10 ⁻⁴ mm ² (vertical 12 μm, horizontal 15 μm). Next, in the range where the direction of the pearlite lamella in the field of view is aligned, the lamella extends at the place where the lamella interval is measurable and the lamella interval is the smallest and the second smallest lamella interval. A length perpendicular to the direction was drawn by drawing a straight line with respect to the direction, and a length corresponding to 5 lamellas was obtained and divided by 5 to obtain a pearlite lamella spacing at each location. The average value of the lamella spacing for the 10 visual fields thus obtained (total of 20 locations) was taken as the average pearlite lamella spacing of the hot-rolled wire rod.

  The average thickness t of cementite in the pearlite structure was calculated by the following equation (1) using the average pearlite lamella spacing determined by the above method and the volume fraction of pro-eutectoid ferrite.

  t (nm) = average pearlite lamella spacing (nm) × {100 / (100-volume fraction of pro-eutectoid ferrite (%))} × C content (%) × 0.153 Formula (1)

The maximum TiN particle size in a measuring area of 125 mm ² is obtained by cutting a section perpendicular to the rolling direction from a hot-rolled wire, mirror-polishing the section perpendicular to the rolling direction, and measuring inclusions using an optical microscope. Went. In addition, observation with said optical microscope was performed for every range of 2.5 mm x 2.5 mm, and the long diameter and the short diameter were measured about the largest TiN in this range. This measurement was performed on each sample for 20 fields of view, and the maximum TiN particle size in a measurement area of 125 mm ² was determined. The particle size was calculated from the following formula.

Particle size = (major axis ² + minor axis ² ) ^1/2

  In addition, when observed with an optical microscope, TiN exhibited a gold color and thus could be distinguished from other inclusions.

  The hot rolled wire rod was subjected to surface scale removal, brass plating, and wire drawing to obtain a steel wire having a diameter of 0.32 mm. In addition, the wire drawing to a diameter of 2.0 mm was performed by a pulse schedule in which the area reduction rate of each die was 18% on average on a wire rod provided with a lubricant by a normal method. Subsequently, wet wire drawing (final wire drawing) to a diameter of 0.32 mm is applied to the wire rod that has been drawn to a diameter of 2.0 mm, with a pulse schedule in which the area reduction rate of each die is 15% on average. went. In this wet wire drawing (final wire drawing), wire drawing workability was evaluated, and the results are shown in Table 4. That is, the final wire drawing was performed for 20 kg for each steel wire, and the number of wire breaks at that time was recorded. Further, when the number of wire breaks was 2, the wire drawing up to a diameter of 0.32 mm and evaluation thereafter were stopped. In addition, when the number of wire breaks when 20 kg wet drawing to a diameter of 0.32 mm is 0 or less, the wire drawing workability is evaluated as good. When the number of wire breaks is 1 or more, the wire drawing workability is Rated bad.

  Further, the strength and ductility were examined as follows. That is, the steel wire that could be drawn to a diameter of 0.32 mm was subjected to a tensile test for each three wires, the tensile strength and the drawing were measured, and the average value of the three wires is shown in Table 3. The frequency of breakage at the time of stranded wire correlates with the drawing in the tensile test, and if the drawing is 30% or more, disconnection at the time of stranded wire hardly occurs. Therefore, if the drawing is 30% or more, ductility Was good.

  The target performance of the hot-rolled wire rod for wire drawing according to the present invention is 0 when the hot-rolled wire rod having a diameter of 4.0 to 6.0 mm is wet-drawn to a diameter of 0.32 mm by 20 kg. That is, the tensile strength with a diameter of 0.32 mm is 3000 MPa or more, preferably 3200 MPa or more, more preferably 3400 MPa or more, and the drawing in the tensile test is 30% or more.

From Table 4, it is apparent that at least one characteristic described above does not reach the target value at a test number outside the conditions defined in the present invention.
On the other hand, it is clear that the test numbers satisfying all the conditions defined in the present invention reach the target values for all the above-described characteristics.

  The preferred embodiments and examples of the present invention have been described above. However, these embodiments and examples are merely examples within the scope of the present invention and do not depart from the spirit of the present invention. Thus, addition, omission, replacement, and other changes of the configuration are possible. That is, the present invention is not limited by the above description, is limited only by the description of the scope of claims, and can be changed as appropriate within the scope.

Claims (2)

% By mass
C: 0.3 to 0.5%,
Si: 0.1 to 1.0%,
Mn: 0.3 to 1.0%,
Cr: 0.25 to 0.70%,
Mn + 2Cr: 0.90 to 1.8%
And the balance consists of Fe and inevitable impurities, and Al, Ti, N, P, S and O in the impurities are each Al: 0.002% or less,
Ti: 0.002% or less,
N: 0.005% or less,
P: 0.02% or less,
S: 0.01% or less,
O: 0.003% or less,
In addition, as a metal structure, 70% or more by volume ratio is composed of a pearlite structure, the average cementite thickness t in the pearlite structure obtained from the formula (1) is 15 to 40 nm, and TiN in a measurement area of 125 mm ² A hot rolled wire rod for wire drawing having a diameter of 4.0 to 6.0 mm, wherein the maximum particle size is less than 15 μm.
t = average pearlite lamella spacing (nm) × {100 / (100-volume fraction of pro-eutectoid ferrite (%))} × C content (%) × 0.153 (1) Furthermore, in mass%,
The hot-rolled wire rod for wire drawing according to claim 1, characterized by containing any one or both of Mo: 0.02 to 0.20% and B: 0.0003 to 0.0030%. .