JP6648516B2 - Hot rolled wire for wire drawing - Google Patents

Description

  The present invention relates to a hot-rolled wire rod for wire drawing that is suitable for applications that can be manufactured without applying a patenting process, when a hot-rolled wire material is used as a raw material to produce an ultrafine steel wire having excellent strength and ductility. Things.

  In general, small-diameter high-strength steel wires used as reinforcing materials for radial tires, various belts and hoses of automobiles, and used as sawing wires, have a wire diameter (diameter) adjusted and cooled after hot rolling of 5 to 5 mm. A 6 mm steel wire rod (hereinafter, the “steel wire rod” is simply referred to as a “wire rod”) is subjected to primary drawing to have a wire diameter of 3 to 4 mm, and then subjected to primary patenting and then to secondary drawing. It is manufactured through a process of reducing the wire diameter to 1 to 2 mm, further applying a secondary patenting process and brass plating, and then reducing the wire diameter to 0.1 to 0.4 mm by final wet drawing. Then, the thin-diameter high-strength steel wire (ultra-fine steel wire) manufactured in this manner is twisted by, for example, twisting to form a “twisted steel wire”, which is 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 This is a process of rapidly cooling to a temperature range in which the pearlite structure is predominant by immersing in immersion or the like, and maintaining the temperature in the temperature range for a predetermined time.

There is a strong demand for reducing the number of patenting processes from the viewpoint of reducing manufacturing costs and CO ₂ , and techniques for reducing the number of patenting processes that have been conventionally performed twice to once have been widely implemented.

  In recent years, the industry has requested that the patenting process be performed zero times. However, when a steel wire having a diameter of 5 to 6 mm is drawn to a diameter of 0.1 to 0.4 mm without being subjected to heat treatment in the middle, disconnection frequently occurs on the way, or even if the wire can be drawn, insufficient ductility. Therefore, disconnection often occurs during 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 a patenting process.

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

  Patent Literature 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, an amount of proeutectoid ferrite, and a lamella cementite morphology. A wire for a high strength steel wire is disclosed. However, because the thickness of the lamella cementite is not considered, it has not been satisfactory as a means for stably suppressing disconnection during drawing or twisting.

  Patent Document 2 discloses a method for producing a rubber reinforcing steel wire containing C: 0.35 to 0.9% and characterized by the area ratio of proeutectoid ferrite and the tensile strength after drawing. It has been disclosed. However, according to the example of Patent Document 2, a heat treatment called bluing is performed during drawing, and the stretched lamella structure formed by drawing is divided, so that a predetermined strength is obtained after drawing. However, there is a problem that the ductility is deteriorated and the ductility is deteriorated.

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

  The present invention has been made in view of the above circumstances, and is suitable as a material for manufacturing steel cords, sewing wires, and the like, and is suitable for applications that can be manufactured without applying a patenting process when manufacturing these. It is an object of the present invention to provide a hot-rolled wire for wire drawing having high strength of, for example, 3000 MPa or more, excellent ductility, and stable production.

  In order to solve the above-mentioned problems, the present inventors first investigated the effects of the chemical composition, microstructure, and inclusions of the steel wire on the breaking during wire drawing, and the tensile strength and ductility after wire drawing. As a result of repeated studies and detailed analysis of the results, the following findings were obtained.

a) In order to improve ductility after wire drawing, it is effective to reduce the amount of C and add Cr.
b) When the cementite in the pearlite structure at the stage of the hot-rolled wire rod is thick, the wire is easily broken during the wire drawing and the ductility after the wire drawing is reduced. If the cementite in the pearlite structure is too thin, the resulting pearlite lamella spacing will be too small, resulting in an excessive increase in tensile strength, which will easily cause breakage during wire drawing.
c) In order to obtain a pearlite structure with an appropriate cementite thickness in a hot-rolled wire in a stable manner, it is necessary that the phase transformation after finish rolling occurs stably in a target temperature range. And the amount of Cr must be appropriately contained, and the degree of influence of Cr is about twice that of Mn.
d) When the amount of Al as an unavoidable impurity is appropriately controlled, the inclusion serving as a starting point at the time of disconnection is TiN. Therefore, disconnection can be prevented by reducing the maximum particle size of TiN.

  As a result of further detailed experiments and studies based on these findings a) to d), the amounts of alloying elements and impurity elements of steel are appropriately adjusted or regulated, and at the same time, the conditions of the metal structure mainly composed of pearlite In particular, the average thickness of cementite in pearlite and the maximum particle size of TiN are adjusted to be within appropriate ranges, thereby solving the above-mentioned problems, and performing wire drawing without applying a patenting treatment. Have a high tensile strength of, for example, 3000 MPa or more, and have excellent ductility, and have been found to be able to be stably manufactured even in a mass production process while securing such excellent performance. Was.

(1) In mass%,
C: 0.3-0.5%,
Si: 0.1 to 1.0%,
Mn: 0.3-1.0%,
Cr: 0.25 to 0.70% ,
And the balance is made up of Fe and unavoidable 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
Tona is,
Mn% + 2 × Cr%: 0.90 to 1.8%,
Further, 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 the TiN content in the measurement area of 125 mm ² is A hot-rolled wire for wire drawing having a diameter of 4.0 to 6.0 mm, having a maximum particle size of less than 15 µm.
t = average pearlite lamellar interval (nm) × {100 / (100-volume ratio of pro-eutectoid ferrite (%))} × C content (%) × 0.153 formula (1)
(2) Further, in mass%,
Mo: 0.02 to 0.20%, B: 0.0003 to 0.0030%, or both of them, characterized by being characterized in that it contains: .

  ADVANTAGE OF THE INVENTION According to this invention, without applying patenting processing, it has high tensile strength, for example, 3000 MPa or more. Industrially extremely useful effects are obtained, for example, it is also possible to manufacture them.

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

<Component composition>
C: C is a component effective for increasing the tensile strength of the steel material. However, when the content is less than 0.3%, it is difficult to stably impart a high strength such as 3000 MPa in tensile strength to the final product. Further, it is effective to increase the C content in order to stably obtain a high-strength final product, and it is desirable to make the C content 0.35% or more in order to obtain 3200 MPa or more. On the other hand, if the C content is too large, the steel material is hardened, resulting in disconnection during wire drawing and reduction in ductility. In particular, if the C content exceeds 0.5%, the effect becomes remarkable, and stable mass production becomes industrially difficult. Therefore, the C content is set in the range of 0.3 to 0.5%. Desirably, it is not less than 0.35% and not more than 0.45%.

  Si: Si is also an effective component for increasing the strength of the steel material, and is also a necessary component as a deoxidizing agent. However, if the content of Si is less than 0.1%, the effect of adding Si is not sufficient, while if it exceeds 1.0%, the ductility after drawing is reduced. Therefore, the content of Si is determined to be in the range of 0.1 to 1.0%. Since Si is an element that also affects the hardenability of steel and the formation of proeutectoid cementite, the Si content is 0.2 to 0.5 from the viewpoint of stably securing a desired microstructure in the steel wire. % Is more desirably adjusted.

  Mn: Mn affects the phase transformation time from austenite and is an effective component for obtaining a stable pearlite structure in a hot-rolled wire. However, if the Mn content is less than 0.3%, the effect of the above-mentioned 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 the wire breaks during drawing. It will be easier. 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 in a hot-rolled wire rod, and at the same time, reduces the lamella spacing of pearlite to increase the strength and ductility of the final product. There is action. 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 segregates particularly in the central portion of the wire, and martensite is generated in the segregated portion, so that the wire is easily broken at the time of 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 for obtaining a stable pearlite structure in a hot-rolled wire, so that the thickness of cementite in the pearlite structure is controlled. It is also effective. The effect of Cr is about twice that of Mn. If Mn + 2Cr is less than 0.90%, the cementite in the pearlite structure in the hot-rolled wire becomes too thick to be stably mass-produced within the range specified in the present invention. For more stable mass production, the content is preferably 1.00% or more. On the other hand, as described above, both Mn and Cr tend to segregate particularly at the center of the wire, and martensite is generated at the segregated portion, and the wire tends to break during drawing. Therefore, Mn + 2Cr is determined to be in the range of 0.90 to 1.8%. Preferably, it is at least 1.00%, more preferably at least 1.10%.

  The balance for each of the above elements may be basically unavoidable impurities and Fe. In the present invention, the contents of the impurities Al, Ti, N, P, S, and O are further defined as follows. Regulate on the street.

Al: Al is an element that forms an oxide-based inclusion containing Al ₂ O ₃ as a main component and reduces wire drawing workability. In particular, when the Al content exceeds 0.002%, the oxide-based inclusions are coarsened, and the wire is frequently broken during wire drawing, and the wire drawing workability is significantly reduced. Therefore, the Al content is restricted to 0.002% or less.

Ti: When Ti contains N, Ti easily forms TiN, and TiN is very hard and is not deformed by hot rolling or wire drawing, so that it tends to be a starting point of wire breakage during wire drawing. Even if the production method is considered, if the Ti content exceeds 0.002%, it is difficult to reduce the maximum TiN particle size in the measurement area of 125 mm ^{2 to} less than 15 μm, and the wire is easily broken during wire drawing. . Therefore, the Ti content is restricted to 0.002% or less. Preferably, it is 0.001% or less.

N: N easily forms TiN when it contains Ti, and TiN is very hard and is not deformed by hot rolling or wire drawing, and thus tends to be a starting point of wire breakage during wire drawing. Even if the production method is taken into consideration, if the N content exceeds 0.005%, it is difficult to reduce the maximum TiN particle size in the measurement area of 125 mm ^{2 to} less than 15 μm, and the wire tends to break during wire drawing. . Therefore, the N content is restricted 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 drawability. In particular, if the P content exceeds 0.02%, the wire drawing workability is significantly reduced. Therefore, the P content is restricted to 0.02% or less.

  S: S is also an element that lowers the drawability. If the S content exceeds 0.01% in particular, the wire drawing workability significantly decreases, so the S content is restricted to 0.01% or less.

O: O is an element that easily forms an oxide, and when Al is present, is an element that forms an oxide-based inclusion mainly composed of hard Al ₂ O ₃ and lowers the drawability. . In particular, if the O content exceeds 0.003%, even if the Al content is within the range of the present invention, the oxide-based inclusions are coarsened, and the wire breaks frequently during wire drawing, and the wire drawing processability is increased. Is significantly reduced. Therefore, the O content is restricted to 0.003% or less.

  Further, in the present invention, in addition to the components described above, one or both of Mo: 0.02 to 0.20% and B: 0.0003 to 0.0030% may be contained.

  Mo: The addition of Mo is optional, but the addition of Mo can more stably exhibit the effect of enhancing the balance between tensile strength and ductility after drawing. To obtain this effect, the content of Mo is preferably set to 0.02% or more. However, when the content of Mo exceeds 0.20%, a martensite structure is easily generated, and the wire drawing workability may be reduced. Therefore, when Mo is positively added, the Mo content 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 drawn wire, the Mo content is more preferably 0.04% or more.

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

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

<Average thickness of cementite in pearlite structure>
In order to ensure high strength of 3000MPa or more in tensile strength of the final product without performing patenting treatment, to obtain excellent ductility and to prevent breakage during wire drawing, it is necessary to use hot working. It is necessary that the average thickness of cementite in the pearlite structure of the rolled wire is 40 nm or less. On the other hand, if the average thickness of the cementite in the pearlite structure is less than 15 nm, the strength after the wire drawing becomes too high, so that the ductility is reduced and the wire is easily broken during the wire drawing. Therefore, the average thickness of the cementite in the pearlite structure is 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, and 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, no inclusion other than TiN, that is, sulfide or oxide was 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 drawing even if 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 for wire drawing>
If the diameter of the hot-rolled wire for wire drawing exceeds 6.0 mm, even if other requirements of the present invention are satisfied, the wire cannot be drawn to the target diameter of 0.32 mm of the present invention, or the wire cannot be drawn. However, 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, so that there is no merit of eliminating the patenting process. Therefore, the diameter of the hot-rolled wire for wire drawing is set in the range of 4.0 to 6.0 mm. It is preferably in the range of 4.5 to 6.0 mm, and more preferably in the range of 4.5 to 5.5 mm.

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

The volume ratio of the pearlite structure was measured by the following method. First, the cross section of a hot-rolled wire (that is, a cut surface perpendicular to the longitudinal direction) is mirror-polished, then corroded by piclar, and is arbitrarily determined using a field emission scanning electron microscope (FE-SEM). Photograph 10 locations at 3000x magnification. The area per visual field is 5.0 × 10 ⁻⁴ mm ² (length 20 μm, width 25 μm). Next, using the photograph, the area ratio of a structure other than the pearlite structure is determined by ordinary image analysis. 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 fraction of the proeutectoid ferrite structure was also obtained by the same method.

The average thickness of cementite in the pearlite structure was calculated by the following method. First, after the cross section of the hot-rolled wire was mirror-polished, it was corroded by piclar, and 10 visual fields were photographed at arbitrary positions at a magnification of 5000 using a field emission scanning electron microscope (FE-SEM). The area per visual field is 1.8 × 10 ⁻⁴ mm ² (length 12 μm, width 15 μm). Next, in the range in which the orientation of the pearlite lamella in the photograph of each visual field is aligned, the lamella can be measured at intervals of 5 lamellas, and the lamella extends at the location where the lamella interval is the smallest and the location where the lamella interval is the second smallest. A straight line perpendicular to the direction was drawn to determine the length of the lamella at five intervals, and the length was divided by 5 to determine the pearlite lamella interval at each location. The average value of the lamella intervals for 10 visual fields (total of 20 places) thus obtained was defined as the average pearlite lamella interval of the hot-rolled wire.
The average thickness t of cementite in the pearlite structure was calculated by the following equation using the average pearlite lamellar spacing obtained by the above method and the volume fraction of proeutectoid ferrite.

  t (nm) = average pearlite lamellar interval (nm) x {100 / (100-volume fraction of pro-eutectoid ferrite (%)) x C content (%) x 0.153 ... formula (1)

Here, the constant 0.153 was used because, in a steel material having a C content of 1.00%, the volume fraction when all the C became cementite was 0.153. A cross section perpendicular to the rolling direction was cut out from the hot-rolled wire, the cross section perpendicular to the rolling direction was mirror-polished, and inclusions were measured using an optical microscope. In addition, the observation by the above-mentioned optical microscope was performed every 2.5 mm x 2.5 mm, and the major axis and the minor axis were measured for the largest TiN within this range. This measurement was performed for each sample in 20 visual fields, and the maximum TiN particle size in a measurement area of 125 mm ² was obtained. The particle size was calculated from the following equation.

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

  In addition, when observed with an optical microscope, TiN has a golden color, and thus can be easily distinguished from other inclusions.

<Production method>
Next, an example of a method for producing a hot-rolled wire 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 producing 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-described conditions. The process and each process condition may be set according to the composition of the steel, the target performance, the wire diameter, and the like. Here, C of 0.3 to 0.5%, 0.1 to 1.0% Si, 0.3 to 1.0% Mn, 0.25% to 0.70% Cr, 0.90 to 1.8% Mn + 2Cr, Al as an inevitable impurity: 0.002% or less, Ti : An example of the production method when steel containing 0.002% or less and N: 0.005% or less is used.

In the case of melting for a small amount of experiment, if the steel is 50 kg or less, evacuate the raw material for at least 20 minutes after melting, cast in a mold made of cast iron and have an internal average cross-sectional area of 120 cm ² or less. The 15% by volume from both ends of the later ingot is not used.

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

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

  The steel slab produced by the above method is heated to 1050 to 1150 ° C, and hot-rolled to φ4.0 to 6.0 mm at a finishing temperature of 900 to 1000 ° C. After the finish rolling, water cooling and air cooling by air are combined to cool the average cooling rate at a rate of 50 ° C./sec or more to a temperature range of 680 to 730 ° C., and thereafter, the average cooling rate is 10 to 10 by air cooling by air. After cooling at a rate of 20 ° C./sec to a temperature in the range of 630 to 590 ° C., it is allowed to cool to 500 ° C. or less.

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

  Next, an example of the present invention will be described. However, the conditions of the example are one example of conditions adopted to confirm the operability and effects of the present invention, and the present invention is not limited to this one example of conditions. 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 the 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 the ingot manufactured under the conditions shown in Table 1, after heating to 1230 ° C., the steel was turned into a steel slab having a diameter of 80 mm by hot forging and then allowed to cool to room temperature.

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

The hot-rolled wire thus obtained was subjected to the following method to determine the volume ratio of the pearlite structure, the volume ratio of the proeutectoid ferrite, the average thickness of the cementite in the pearlite structure, and the maximum value in the measurement area of 125 mm ^2. Was measured for TiN particle size.

The volume ratio of the pearlite structure is determined by using a field emission scanning electron microscope (FE-SEM) after the cross section of the hot-rolled wire (that is, a cut surface perpendicular to the length direction) is mirror-polished and then corroded by piclar. 10 photos at 3000 times magnification at arbitrary positions. The area per visual field is 5.0 × 10 ⁻⁴ mm ² (length 20 μm, width 25 μm). Next, using the photograph, the area ratio of a structure other than the pearlite structure is determined by ordinary image analysis. 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 fraction of the proeutectoid ferrite structure was also obtained by the same method.

Next, the average thickness of the cementite in the pearlite structure was calculated by the following method. First, after the cross section of the hot-rolled wire was mirror-polished, it was corroded by piclar, and 10 visual fields were photographed at arbitrary positions at a magnification of 5000 using a field emission scanning electron microscope (FE-SEM). The area per visual field is 1.8 × 10 ⁻⁴ mm ² (length 12 μm, width 15 μm). Next, in the range in which the orientation of the pearlite lamella in the photograph of each visual field is aligned, the lamella can be measured at intervals of 5 lamellas, and the lamella extends at the location where the lamella interval is the smallest and the location where the lamella interval is the second smallest. A straight line perpendicular to the direction was drawn to determine the length of the lamella at 5 intervals, and the length was divided by 5 to determine the pearlite lamella interval at each location. The average value of the lamella intervals for 10 visual fields (total of 20 places) thus obtained was defined as the average pearlite lamella interval of the hot-rolled wire.

  The average thickness t of the cementite in the pearlite structure was calculated by the following equation (1) using the average pearlite lamellar spacing obtained by the above method and the volume fraction of proeutectoid ferrite.

  t (nm) = average pearlite lamellar interval (nm) x {100 / (100-volume fraction of pro-eutectoid ferrite (%)) x C content (%) x 0.153 ... formula (1)

The largest particle size of TiN in the measurement 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. Was done. In addition, the observation by the above-mentioned optical microscope was performed every 2.5 mm x 2.5 mm, and the major axis and the minor axis were measured for the largest TiN within this range. This measurement was performed for each sample in 20 visual fields, and the maximum TiN particle size in a measurement area of 125 mm ² was obtained. The particle size was calculated from the following equation.

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

  In addition, by observation with an optical microscope, TiN was golden, and thus could be distinguished from other inclusions.

  The hot-rolled wire was subjected to surface scale removal, brass plating, and wire drawing to obtain a steel wire having a diameter of 0.32 mm. The wire drawing up to a diameter of 2.0 mm was performed on a wire rod to which a lubricant was attached by an ordinary method, using a pulse schedule in which the area reduction rate of each die became 18% on average. Subsequently, the wire drawn to 2.0 mm in diameter is subjected to wet drawing (final drawing) to 0.32 mm in diameter on a pulse schedule in which the area reduction rate in each die is 15% on average. went. In this wet drawing (final drawing), the drawability was evaluated, and the results are shown in Table 4. That is, the final wire drawing was performed for each steel wire at 20 kg, and the number of disconnections at that time was recorded. When the number of disconnections became two, the wire drawing to a diameter of 0.32 mm and the subsequent evaluation were stopped. In addition, when the number of breaks when performing 20 kg wet drawing to 0.32 mm in diameter is 0 or less, the wire drawing workability is evaluated as good. Rated bad.

  Further, strength and ductility were examined as follows. That is, three tensile tests were performed on each of the steel wires that could be drawn to a diameter of 0.32 mm, and the tensile strength and the drawing were measured. Table 3 shows the average value of each of the three wires. Note that the frequency of disconnection at the time of stranded wire is correlated with the drawing at the tensile test. If the drawing is 30% or more, almost no disconnection at the time of twisting occurs. Was good.

  The target performance of the hot-rolled wire for wire drawing according to the present invention is that the number of times of disconnection when the hot-rolled wire having a diameter of 4.0 to 6.0 mm is wet-drawn to a diameter of 0.32 mm by 20 kg is 0 times. That is, the tensile strength at 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 a tensile test is 30% or more.

From Table 4, it is apparent that at least one of the above-mentioned characteristics does not reach the target value in the test number out of the conditions specified in the present invention.
On the other hand, it is clear that the test numbers satisfying all the conditions specified in the present invention have reached the target values in all the characteristics described above.

  Although the preferred embodiments and examples of the present invention have been described above, these embodiments and examples are merely examples within the scope of the present invention, and do not depart from the scope of the present invention. Thus, addition, omission, substitution, and other changes of the configuration are possible. That is, the present invention is not limited by the above description, but is limited only by the description of the claims, and it is needless to say that the present invention can be appropriately changed within the scope.

Claims (2)

In mass%,
C: 0.3-0.5%,
Si: 0.1 to 1.0%,
Mn: 0.3-1.0%,
Cr: 0.25 to 0.70% ,
And the balance is made up of Fe and unavoidable 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
Tona is,
Mn% + 2 × Cr%: 0.90 to 1.8%,
Further, 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 the TiN content in a measurement area of 125 mm ² is A hot-rolled wire having a maximum particle size of less than 15 μm and having a diameter of 4.0 to 6.0 mm for wire drawing.
t = average pearlite lamellar interval (nm) × {100 / (100-volume ratio of pro-eutectoid ferrite (%))} × C content (%) × 0.153 formula (1) Furthermore, in mass%,
The hot-rolled wire rod for wire drawing according to claim 1, characterized by containing one or both of Mo: 0.02 to 0.20% and B: 0.0003 to 0.0030%. .