JP2014189855A - Wire rod for high-strength steel wire excellent in cold-drawability and high- strength steel wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a wire rod for a high-strength steel wire good in cold-drawability and also exhibiting prescribed high strength; a high-strength steel wire obtained from the wire rod for the high-strength steel wire; and a high-strength galvanized steel wire.SOLUTION: The wire rod for a high-strength steel wire contains each C:0.80 to 1.3%, Si:0.1 to 1.5%, Mn:0.1 to 1.5%, P:0.03% or less (excluding 0%), S:0.03% or less (excluding 0%), Ti:0.02 to 0.2%, Al:0.01 to 0.10% and N:0.001 to 0.006% and the balance iron with inevitable impurities and satisfies a relationship of the following (1) expression. 0.05%≥[Ti*]≥(0.0023×[C])...(1), where [Ti*]=(All Ti amount-compound-type Ti amount having a size of 0.1 μm or more and [C] represents the content of C (mass%).

Description

  The present invention relates to a high-strength steel wire useful as a material for a galvanized steel wire used for a rope for a bridge, and the like, and particularly to a high-strength steel wire wire for obtaining such a high-strength steel wire. The present invention relates to a wire material for high-strength steel wire that has good workability when drawn without heat treatment later.

  Steel wires (or steel stranded wires) subjected to hot dip galvanizing to increase corrosion resistance are used for ropes used for bridges and the like. As a material of such a steel wire, for example, JIS G 3548 shows a steel wire having a wire diameter of 5 mm and a tensile strength TS of about 1500 to 1700 MPa. The material steel is mainly described in JIS G 3506. Carbon steel is used.

  By the way, in the steel wire that is the material of the hot dip galvanized steel wire, in addition to reducing the manufacturing cost, it gives merit such as reduction of the amount of steel used due to high strength and improvement of freedom of bridge design, that is, high strength. In addition, development of low-cost steel wire is required.

  In producing a galvanized steel wire, the following method is generally adopted. First, a wire rod (steel wire rod) manufactured by hot rolling is placed in a ring shape on a cooling conveyor, subjected to pearlite transformation, and then wound into a coil shape to obtain a wire rod coil. Next, a patenting process is performed to improve the strength and homogenize the structure. This patenting process is a kind of heat treatment. Generally, a wire rod is heated to about 950 ° C. using a continuous furnace to austenite, and then immersed in a refrigerant such as a lead bath maintained at about 500 ° C. To obtain a fine and uniform pearlite structure.

  Thereafter, cold drawing is performed, and a steel wire having a predetermined strength is obtained by utilizing the work hardening effect of pearlite steel. After that, plating treatment is performed by immersion in a hot dip zinc bath maintained at around 450 ° C. to obtain a galvanized steel wire. After the galvanizing treatment, finish drawing may be further performed. As a cable for a bridge, a parallel wire (PWS) in which they are bundled or a twisted galvanized steel strand is used.

In such a series of manufacturing processes, the patenting process is a factor in increasing the manufacturing cost. The patenting treatment is effective for increasing the strength of the wire and making the quality uniform, but raises the manufacturing cost, and also has environmental problems such as the emission of CO ₂ and the use of environmentally hazardous substances. The merit is great if the rolled wire can be drawn and commercialized (steel wire) without heat treatment. Drawing a wire after rolling without heat treatment is called “raw drawing”.

  In order to achieve high strength by raw material, it is necessary to use hypereutectoid steel with an increased C content in order to compensate for the decrease in strength when the patenting treatment is omitted. However, as the C content is increased, pro-eutectoid cementite is precipitated at the grain boundaries, and there is a problem that wire drawing workability is lowered. For this reason, even when the C content is increased for higher strength, it is excellent in properties that can be submerged while suppressing the effects of pro-eutectoid cementite (this property is referred to as “rawness”). Realization of a wire rod is desired.

Various techniques for improving wire drawing workability have been proposed so far. For example, Patent Document 1 proposes a technique for improving wire drawing workability by performing cooling after hot rolling in a molten salt bath. This technique is called a direct patenting process. However, the direct patenting process in the molten salt bath has a problem that the manufacturing cost is higher than that of the blast cooling, and the maintainability of the equipment is also low. Moreover, the wire drawing workability of the obtained steel material is as low as about 80% in terms of the reduction in area, and the strength level of the wire (steel wire) remains at about 180 to 190 kgf / mm ² (1764 to 1862 MPa).

  On the other hand, Patent Document 2 discloses a technique for improving the wire strength by controlling the cooling conditions after hot rolling and omitting the patenting process. However, the wire drawing workability of the steel material obtained by this technique is as low as about 50% in terms of area reduction, and the strength level of the wire is also about 1350 to 1500 MPa.

JP 04-289128 A Japanese Patent Laid-Open No. 05-287451

  The present invention has been made under such circumstances, and an object of the present invention is to provide a high-strength steel wire that has good stretchability and can also achieve a predetermined high strength, and such a high-strength steel wire. The object is to provide a high-strength steel wire obtained from a wire, and a high-strength galvanized steel wire.

The wire rod for high-strength steel wire of the present invention capable of achieving the above object is C: 0.80 to 1.3% (meaning of mass%, the same applies to the component composition hereinafter), Si: 0.1 to 0.1% 1.5%, Mn: 0.1 to 1.5%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Ti: 0.0. 02 to 0.2%, Al: 0.01 to 0.10%, and N: 0.001 to 0.006%, respectively, the balance is made of iron and inevitable impurities, and the relationship of the following formula (1) It is characterized by satisfying.
0.05% ≧ [Ti *] ≧ (0.0023 × [C]) (1)
However, [Ti *] = (total Ti amount−compound type Ti amount having a size of 0.1 μm or more), and [C] represents the C content (mass%).

  In the wire material for high-strength steel wire of the present invention, it is preferable that the metal structure is a pearlite phase having an area ratio of 90% or more and the maximum length of proeutectoid cementite is 15 μm or less. Further, the amount of solute N in the wire is preferably 0.0005% or less (not including 0%).

  In the chemical composition of the wire for high-strength steel wire, if necessary, (a) B: 0.010% or less (not including 0%), (b) Cr: 0.5% or less (not including 0%) ), (C) V: 0.2% or less (not including 0%), (d) Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (including 0%) No), Mo: 0.5% or less (not including 0%), Co: 1.0% or less (not including 0%), and Nb: 0.5% or less (not including 0%) It is also effective to contain one or more selected from the above, and the properties of the wire are further improved depending on the type of component to be contained.

The present invention also includes a high-strength steel wire obtained by drawing (for example, drawing) the above-described high-strength steel wire. Moreover, in the high-strength galvanized steel wire produced by hot-dip galvanizing this high-strength steel wire, it is preferable that the tensile strength TS is equal to or higher than the tensile strength TS * defined by the following equation (2).
TS * = − 87.3D + 2234 (MPa) (2)
However, D shows the wire diameter (mm) of a high-strength galvanized steel wire.

  According to the present invention, a wire material for a high-strength steel wire that has excellent stretchability and achieves high strength can be obtained by strictly defining the chemical composition while taking into account the fine TiC precipitation state. Steel wires obtained from such high-strength steel wire rods are extremely useful as materials for hot-dip galvanized steel wires and steel stranded wires that are used as rope materials used in bridges and the like.

  In order to solve the above-mentioned problems, the present inventors have studied the relationship between the wire material structure and wire drawing workability. In particular, the precipitation mechanism of pro-eutectoid cementite in hypereutectoid steel was also investigated. As a result, it was found that precipitation of proeutectoid cementite can be suppressed by precipitating fine TiC in the vicinity of the grain boundary. The most effective is fine TiC having a size of 0.1 μm or less, and it is necessary to ensure a sufficient amount of precipitation of fine TiC. As the C content of the steel material is higher, cementite is more likely to precipitate, so more fine TiC is required. Coarse TiC is unlikely to have such an effect, so it is necessary to deposit as much fine TiC as possible. It is extremely important to appropriately control the amount of TiC deposited and the size distribution.

  By precipitating fine TiC having a size of 0.1 μm or less in the vicinity of the austenite grain boundary as described above, the grain boundary energy can be reduced and precipitation of proeutectoid cementite can be suppressed. Although a great amount of labor and cost are required to directly evaluate fine TiC, it can be easily evaluated by using electrolytic extraction residue measurement. That is, the total amount of Ti in the steel is a compound such as TiC or TiN at room temperature, and the size of TiN is about 5 to 10 μm. Therefore, the amount of compound type Ti having a size of 0.1 μm or more (specifically, compound type Ti in the residue filtered through a mesh having an opening of 0.1 μm) (the amount of Ti present as a compound) is measured, and steel When the value subtracted from the total Ti in the inside is [Ti *], this [Ti *] represents the amount of fine TiC that has passed through the mesh.

  As the C content in the steel increases, pro-eutectoid cementite is more likely to be precipitated, so a large amount of fine TiC is required. From this relationship, the above [Ti *] is 0.0023 × [C] or more, preferably 0.0023 × [C] + 0.001% or more, more preferably, when the C content is [C]. An amount of 0.0023 × [C] + 0.005% or more is required. On the other hand, when a large amount of fine TiC is precipitated, the grain boundaries become brittle and the toughness of the wire is lowered, causing vertical cracks during wire drawing. From such a viewpoint, the upper limit of [Ti *] is 0.05% or less, preferably 0.03% or less, more preferably 0.01% or less.

  The wire for a steel wire of the present invention satisfies the basic components as a wire, and it is necessary to appropriately adjust the chemical component composition in order to appropriately control the precipitation state of TiC. From such a viewpoint, the reason for setting the range of the chemical composition of the wire is as follows.

(C: 0.80 to 1.3%)
C is an element effective for increasing the strength, and the strength of the steel wire after cold working improves as the C content increases. In order to achieve the desired strength level of the present invention, the C content needs to be 0.80% or more. However, when the C content is excessive, pro-eutectoid cementite precipitates at the grain boundaries and inhibits wire drawing workability. From such a viewpoint, the C content needs to be 1.3% or less. The preferable lower limit of the C content is 0.84% or more (more preferably 0.90% or more), and the preferable upper limit is 1.2% or less (more preferably 1.1% or less).

(Si: 0.1-1.5%)
Si is an effective deoxidizer and exhibits the effect of reducing oxide inclusions in the steel. Moreover, while raising the intensity | strength of a wire, there exists an effect which suppresses the cementite granulation accompanying the heat history at the time of hot dip galvanization, and suppresses a strength fall. In order to exhibit such an effect effectively, it is necessary to contain Si 0.1% or more. However, if the Si content is excessive, the toughness of the wire is reduced, so it is necessary to set it to 1.5% or less. The preferable lower limit of the Si content is 0.15% or more (more preferably 0.20% or more), and the preferable upper limit is 1.4% or less (more preferably 1.3% or less).

(Mn: 0.1 to 1.5%)
Since Mn greatly increases the hardenability of the steel material, it has the effect of lowering the transformation temperature during blast cooling and increasing the strength of the pearlite structure. In order to exhibit these effects effectively, the Mn content needs to be 0.1% or more. However, Mn is an element that is easily segregated. If it is excessively contained, the hardenability of the Mn segregated portion is excessively increased and there is a risk of generating a supercooled structure such as martensite. Considering these effects, the upper limit of the Mn content is set to 1.5% or less. The minimum with preferable Mn content is 0.2% or more (more preferably 0.3% or more), and a preferable upper limit is 1.4% or less (more preferably 1.3% or less).

(P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%))
P and S segregate at the prior austenite grain boundaries, embrittle the grain boundaries, and reduce fatigue characteristics. Therefore, the lower the better, but the upper limit is set to 0.03% or less for industrial production. All of these contents are preferably 0.02% or less (more preferably 0.01% or less). P and S are impurities inevitably contained in the steel material, and it is difficult to make the amount 0% in industrial production.

(Ti: 0.02-0.2%)
Ti is an extremely important element for the wire of the present invention, and exhibits the effect of suppressing the precipitation of proeutectoid cementite by being finely precipitated in the form of TiC in the vicinity of the grain boundary. This is due to the action of fixing C in the vicinity of the grain boundary in the form of TiC and locally lowering the C content, and the action of relaxing grain boundary energy by fine TiC of 0.1 μm or less and preventing cementite nucleation. Is. Ti, like Al, also has the effect of crystal grain refinement and toughness improvement due to the formation of nitrides. In order to exhibit such an effect, it is necessary to contain Ti 0.02% or more. However, when the Ti content is excessive, TiC is excessively precipitated, embrittles the grain boundaries, and the toughness decreases. From such a viewpoint, the Ti content needs to be 0.2% or less. The preferable lower limit of the Ti content is 0.03% or more (more preferably 0.04% or more), and the preferable upper limit is 0.18% or less (more preferably 0.16% or less).

(Al: 0.01-0.10%)
Al has a strong deoxidizing effect and has an effect of reducing oxide inclusions in the steel. Further, a crystal grain fine effect due to the pinning action of nitride and a reduction effect of solid solution N can be expected. In order to exert such an effect, Al needs to be contained by 0.01% or more. However, when the Al content becomes excessive, Al-based inclusions such as Al ₂ O ₃ increase, which causes problems such as increasing the disconnection rate during wire drawing. In order to prevent this, the Al content needs to be 0.10% or less. The preferable lower limit of the Al content is 0.02% or more (more preferably 0.03% or more), and the preferable upper limit is 0.08% or less (more preferably 0.06% or less).

(N: 0.001 to 0.006%)
When N dissolves in steel as an interstitial element, it causes embrittlement due to strain aging and lowers the toughness of the wire. Therefore, the upper limit of the N content (total N) in the steel is set to 0.006% or less. However, it is the solute N dissolved in the steel that causes such an adverse effect, and the compound type N precipitated as a nitride does not adversely affect the toughness. Therefore, it is desirable to control the amount of solute N dissolved in steel separately from N in steel (total N), and the amount of solute N is preferably 0.0005% or less (more preferably). 0.0003% or less). On the other hand, since it is difficult to reduce N in steel to less than 0.001% in industrial production, the lower limit of the N content in steel is set to 0.001% or more. In addition, the upper limit with preferable N content in steel is 0.004% or less (more preferably 0.003% or less).

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and as the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. If necessary, (a) B: 0.010% or less (not including 0%), (b) Cr: 0.5% or less (not including 0%), (c) V: 0.2 % Or less (not including 0%), (d) Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Mo: 0.5% or less (Not including 0%), Co: 1.0% or less (not including 0%), and Nb: 0.5% or less (not including 0%), each of one or more selected from the group It is also effective to contain them in appropriate combinations, and the properties of the wire are further improved according to the types of components to be contained. The reason for setting the range when these elements are contained is as follows.

(B: 0.010% or less (excluding 0%))
B has an effect of preventing the formation of pro-eutectoid ferrite and pro-eutectoid cementite and making it easy to control the structure to a uniform pearlite structure. Moreover, by fixing N in steel in the form of BN, strain aging is suppressed and the toughness of the wire is improved. In order to effectively exhibit these actions, B is preferably contained in an amount of 0.0003% or more. More preferably, it is 0.0005% or more (more preferably 0.0008% or more). However, if the B content is excessive, a compound with iron (B-constituent) precipitates and causes cracking during hot rolling, so the upper limit is preferably made 0.010% or less. A more preferable upper limit of the B content is 0.008% or less (more preferably 0.006% or less).

(Cr: 0.5% or less (excluding 0%))
Cr has the effect of reducing the lamella spacing of pearlite and increasing the strength and toughness of the wire. Moreover, similarly to Si, it has the effect of suppressing the strength reduction of the wire during galvanization. However, even if the Cr content is excessive, the effect is saturated and it is economically wasteful. Therefore, it is preferable that the appropriate content is 0.5% or less. In order to effectively exhibit the effect of Cr, Cr is preferably contained in an amount of 0.001% or more (more preferably 0.05% or more). Moreover, the upper limit with more preferable Cr content is 0.4% or less (more preferably 0.3% or less).

(V: 0.2% or less (excluding 0%))
V produces fine charcoal / nitrides (carbides, nitrides and carbonitrides), and therefore has an effect of increasing strength and refinement of crystal grains, and also suppresses aging embrittlement by fixing solute N. I can expect. In order to effectively exhibit the effect of V, V is preferably contained in an amount of 0.001% or more (more preferably 0.05% or more). However, even if the V content is excessive, the effect is saturated and economically wasteful. Therefore, the appropriate content is preferably 0.2% or less. More preferably, it is 0.18% or less (more preferably 0.15% or less).

(Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Mo: 0.5% or less (not including 0%), Co: 1. 1% or more selected from the group consisting of 0% or less (not including 0%) and Nb: 0.5% or less (not including 0%)
Ni is an element effective for increasing the toughness of the steel wire after wire drawing. In order to effectively exhibit the effect of Ni, Ni is preferably contained in an amount of 0.05% or more (more preferably 0.1% or more). However, even if the Ni content is excessive, the effect is saturated and is economically wasteful, so the appropriate content is preferably 0.5% or less. More preferably, it is 0.4% or less (more preferably 0.3% or less).

  Cu and Mo are effective elements for enhancing the corrosion resistance of the steel wire. In order to exhibit such an effect effectively, it is preferable to contain all 0.01% or more (more preferably 0.05% or more). However, if the Cu content is excessive, Cu reacts with S to segregate CuS at the grain boundary part and generate soot in the wire manufacturing process, so the upper limit may be 0.5% or less. preferable. More preferably, it is 0.4% or less (more preferably 0.3% or less).

  On the other hand, Mo, like Cu, is an element effective for improving the corrosion resistance of steel wire. However, if the Mo content is excessive, a supercooled structure is likely to occur during hot rolling, and ductility is also degraded. To do. For these reasons, the upper limit of the Mo content is preferably 0.5% or less. More preferably, it is 0.4% or less (more preferably 0.3% or less).

  Co has the effect of reducing proeutectoid cementite and making it easy to control the structure to a uniform pearlite structure. However, even if Co is contained excessively, the effect is saturated and it is economically wasteful, so the upper limit is preferably made 1.0% or less. More preferably, it is 0.8% or less (more preferably 0.5% or less). In order to effectively exhibit the effect of Co, it is preferable to contain 0.05% or more, more preferably 0.1% or more (more preferably 0.2% or more).

  Nb, like Ti, contributes to crystal grain refinement by forming a nitride, and can also be expected to suppress aging embrittlement by fixing solute N. However, even if Nb is excessively contained, the effect is saturated and economically wasteful, so the upper limit value is preferably set to 0.5% or less. More preferably, it is 0.4% or less (more preferably 0.3% or less). In order to effectively exhibit the effect of Nb, it is preferable to contain 0.05% or more, more preferably 0.1% or more (more preferably 0.2% or more).

  In the wire for high-strength steel wire of the present invention, the metal structure is preferably mainly composed of pearlite phase (for example, 90% or more in area ratio), but other phases (for example, pro-eutectoid ferrite and bainite) are partly. (10 area% or less) mixing is permissible.

  In the present invention, it is preferable that the length of pro-eutectoid cementite is also controlled. This is because the pro-eutectoid cementite deposited on the center side from D / 4 of the wire (D: diameter of the wire) causes cracks during wire drawing and causes a broken cut. Cementite (lamellar cementite) that forms a pearlite lamellar structure rotates in accordance with the wire drawing and has the property of being oriented in the longitudinal direction of the wire. However, pro-eutectoid cementite cannot rotate in synchronism with the surrounding structure and generates cracks from the interface. The factor governing this rotation is the length of proeutectoid cementite. When the length (maximum length) of proeutectoid cementite is larger than 15 μm, it becomes difficult to rotate and it becomes a source of cracks, but a short one is easy to rotate, so it does not hinder the wire drawing workability so much. From this point of view, the length (maximum length) of proeutectoid cementite is preferably 15 μm or less. More preferably, it is 13 micrometers or less, More preferably, it is 10 micrometers or less. In addition, the minimum of the length of pro-eutectoid cementite is not specifically limited, For example, about 0.1 micrometer may be sufficient.

  When manufacturing the wire material for high-strength steel wire of the present invention, the steel piece having the chemical composition adjusted as described above may be used according to the normal manufacturing conditions. However, preferable production conditions for appropriately adjusting the structure and the like of the wire are as follows.

In the manufacturing process of high carbon steel wire, generally, a steel piece adjusted to a predetermined chemical composition is heated to austenite, and after obtaining a wire with a predetermined wire diameter by hot rolling, it is cooled on a cooling conveyor. A perlite structure is formed in the process. At this time, a fine austenite structure accompanying dynamic recrystallization is obtained during hot rolling, but TiC can be finely dispersed in the vicinity of the grain boundary by precipitating TiC simultaneously with the recrystallization. Here, when the reduction strain in the final rolling four passes (four passes from the final pass to the fourth pass) having the greatest influence on the crystal grain size is ε, the reduction strain ε is 0.4 or more. By doing so, crystal grains can be made sufficiently fine and TiC can be finely dispersed. Here, the area reduction strain ε is represented by ε = ln (S ₁ / S ₂ ) (S ₁ : shows the cross-sectional area of the wire on the rolling roll entering side, and S ₂ : shows the cross-sectional area of the wire on the exit side). Is done. A preferable range of the area reduction strain ε is 0.42 to 0.8, and a more preferable range is 0.45 to 0.6.

  Further, in the cooling process after rolling, coarsening of finely precipitated TiC proceeds. At this time, an important requirement is the placement temperature of the wire. It is preferable to control the mounting temperature to 850 to 950 ° C. because a desired TiC precipitation state can be obtained. When this mounting temperature exceeds 950 ° C., TiC becomes coarse, and when it is less than 850 ° C., TiC remains excessively fine. The upper limit of the mounting temperature is more preferably 940 ° C. or less, and further preferably 930 ° C. or less. The lower limit of the mounting temperature is more preferably 870 ° C. or higher, and further preferably 880 ° C. or higher.

  In the cooling process after rolling, cooling is performed by blast cooling. However, if the cooling rate (average cooling rate) at this time becomes too high, bainite and the like are likely to be mixed, and the structure mainly composed of the pearlite phase. become unable. From such a viewpoint, it is preferable that the average cooling rate within the range of the mounting temperature is 20 ° C./second or less. More preferably, it is 18 degrees C / sec or less (more preferably 14 degrees C / sec or less). The lower limit of the cooling rate at this time is preferably 3 ° C./second or more from the viewpoint of reducing the precipitation of pro-eutectoid cementite. More preferably, it is 4 ° C./second or more (more preferably 5 ° C./second or more).

The high carbon steel wire (wire material for high strength steel wire) of the present invention has a good stretchability, and a high strength steel wire that exhibits desired properties (strength, twist value) by drawing. Will be obtained. Such high-strength steel wires are generally used as high-strength galvanized steel wires by galvanizing the surface. In the steel wire after the drawing process, the smaller the wire diameter, the higher the strength. The tensile strength TS of this high-strength galvanized steel wire is preferably not less than the tensile strength TS * defined by the following formula (2), more preferably not less than TS * + 50 (MPa), and still more preferably TS * + 100. (MPa) or more. In addition, the relationship of the following (2) Formula is calculated | required by experiment.
TS * = − 87.3D + 2234 (MPa) (2)
However, D shows the wire diameter (mm) of a high-strength galvanized steel wire.

  Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.

  Using steel slabs (cross-sectional shape: 155 mm × 155 mm) with the chemical composition (steel types A to S) shown in Table 1 below, they are hot-rolled and processed to a predetermined wire diameter, and mounted on a cooling conveyor in a ring shape Then, after performing pearlite transformation by controlled cooling by blast cooling, various rolled material coils were obtained by winding in a coil shape. In Table 1, “-” means no addition.

  About the obtained rolled material, after truncating the unsteady portion of the terminal, a good terminal was collected to evaluate the rolled material (rolled material wire diameter, [Ti *], solute N amount, maximum length of proeutectoid cementite , Structure and tensile strength TS) were evaluated by the following methods. The “heating temperature” in Table 2 is the furnace temperature before hot rolling, and the surface reduction strain ε is the total reduction in the final rolling 4 passes (total 4 passes from the final pass to the 4th pass). Surface distortion. The “average cooling rate” is an average of cooling rates from placement to 800 ° C. However, test no. For No. 5, the average cooling rate from the placement to 750 ° C. was taken.

(TiC distribution, evaluation of solute N content)
[Ti *] and the amount of solute N were evaluated by electrolytic extraction residue measurement. In this measurement, extraction was performed using a 10% acetylacetone solution, and a mesh having a size of 0.1 μm was used. The amount of compound-type Ti, the amount of compound-type N and the amount of compound-type B in the residue were measured using ICP emission analysis, and the amount of AlN was measured using the bromester method. The sample amount used for the bromoester method was 3 g, and the sample amount used for the absorption spectroscopy was 0.5 g. In addition, since the precipitation state of TiC does not change unless it is subjected to a heat treatment of at least 1000 ° C., it may be measured with a steel wire after drawing or hot dip galvanizing. From these values, [Ti *] = total Ti amount−based on the amount of compound type Ti having a size of 0.1 μm or more, [Ti *] amount is measured, and solid solution N = total N amount−compound type N The amount of solute N was measured from the amount.

(Tensile strength TS of rolled material, evaluation of structure)
A tensile test was performed on the end sample of the rolled material, and the tensile strength TS of the rolled material was measured. At this time, an average value of three times (n = 3) was obtained. Moreover, the state of pro-eutectoid cementite was similarly evaluated by embedding a terminal sample in resin and observing with a scanning electron microscope (SEM). A cross section (transverse cross section) perpendicular to the longitudinal direction of the wire was observed, and the maximum length of plate-like pro-eutectoid cementite observed in the center side from 4 / D (D: diameter of wire) in the cross section was measured. In addition, when the tip of pro-eutectoid cementite was branched into a plurality, the total value of the lengths of the branches was adopted.

  The production conditions and evaluation results at this time are shown in Table 2 below. Table 2 also shows the value of 0.0023 × [C] of the rolled material (C is the C content of the rolled material).

  Each rolled material obtained above was processed to a predetermined wire diameter by cold drawing, and immersed in a hot-dip zinc bath at 440 to 460 ° C. for about 30 seconds to obtain a hot-dip galvanized steel wire. The tensile strength TS of the wire (hot galvanized steel wire) was evaluated by a tensile test. At this time, the average value of 3 times (n = 3) was measured. Moreover, the twist value was measured by the twist test, and also the presence or absence of the vertical crack was determined from observation of the fracture surface shape. For the twist value, the number of twists required until breakage was standardized with a distance between chucks of 100 mm, and an average value of three times (n = 3) was calculated. In the case where even one vertical crack was observed in three twist tests, it was determined that there was a vertical crack.

  The evaluation results of the hot dip galvanized steel wire (wire diameter, area reduction during cold drawing, tensile strength TS, tensile strength TS * determined by the above equation (2), presence or absence of longitudinal cracks) are shown in Table 3 below. Shown in

  From these results, it can be considered as follows. That is, test no. 1 to 3 and 8 to 19 satisfied all the requirements defined in the present invention, and 90% by area or more of the structure was a pearlite phase. Also, no abnormality such as wire breakage is observed during wire drawing, and the wire strength and twisting properties after hot dip galvanizing are good. Among these, test No. 16 and 19, the amount of solute N was slightly increased, and the twist value was slightly decreased.

  In contrast, test no. 4 to 7, 20 to 23 are examples that do not satisfy any of the requirements (or preferable requirements) defined in the present invention, and abnormalities such as wire breakage are observed during wire drawing, or after hot dip galvanizing treatment It can be seen that either the wire strength or the twisting property is inferior.

  Among these, test No. In No. 4, the placement temperature was as high as 1000 ° C., and the amount of [Ti *] was reduced (TiC was coarsened: the maximum length of pro-eutectoid cementite was more than 15 μm). Was not able to be suppressed, and the wire was broken in the middle of wire drawing. Test No. In No. 5, since the mounting temperature was as low as 800 ° C. and the amount of [Ti *] was excessive (TiC was excessively refined), the grain boundaries became brittle and vertical cracks occurred.

  Test No. No. 6, the reduction strain ε in the final four passes was reduced, the crystal grains were not sufficiently refined, and the amount of [Ti *] was reduced (TiC was not refined: maximum length of proeutectoid cementite Therefore, the pro-eutectoid cementite could not be sufficiently suppressed, and the wire was broken in the middle of wire drawing. Test No. In No. 7, since the cooling rate was increased and the rolled material structure was a mixed structure of pearlite and bainite (area ratio of bainite: 40%), the drawability was lowered and the wire was broken during drawing.

  Test No. No. 20 is an example using a steel material (steel type P) with a low C content, and the strength of the steel wire was lowered. Test No. No. 21 is an example using excessive C content (steel type Q), and it was not possible to suppress proeutectoid cementite, which was disconnected.

  Test No. No. 22 is an example using a steel material (steel type R) with a small Ti content, and the proeutectoid cementite could not be suppressed, and was broken. No. No. 23 is an example using an excessive Ti content (steel type S), the amount of [Ti *] is excessive, and vertical cracks occurred.

Claims (9)

C: 0.80 to 1.3% (meaning mass%, component composition is the same hereinafter),
Si: 0.1 to 1.5%,
Mn: 0.1 to 1.5%
P: 0.03% or less (excluding 0%),
S: 0.03% or less (excluding 0%),
Ti: 0.02 to 0.2%,
Al: 0.01-0.10%, and N: 0.001-0.006%,
Each of which consists of iron and inevitable impurities,
A wire material for high-strength steel wire excellent in stretchability characterized by satisfying the relationship of the following formula (1).
0.05% ≧ [Ti *] ≧ (0.0023 × [C]) (1)
However, [Ti *] = (total Ti amount−compound type Ti amount having a size of 0.1 μm or more), and [C] represents the C content (mass%).   The wire rod for high-strength steel wire according to claim 1, wherein the metal structure is a pearlite phase having an area ratio of 90% or more and the maximum length of proeutectoid cementite is 15 µm or less.   The wire rod for high-strength steel wire according to claim 1 or 2, wherein the amount of solute N is 0.0005% or less (not including 0%).   Furthermore, B: 0.010% or less (0% is not included) The wire for high strength steel wires in any one of Claims 1-3.   Furthermore, Cr: 0.5% or less (excluding 0%) is contained, The wire for high-strength steel wires in any one of Claims 1-4.   Furthermore, V: 0.2% or less (0% is not included) The wire for high-strength steel wires according to any one of claims 1 to 5.   Further, Ni: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Mo: 0.5% or less (not including 0%), Co: 1 One or more selected from the group consisting of 0.0% or less (not including 0%) and Nb: 0.5% or less (not including 0%). The wire material for high strength steel wires as described.   A high-strength steel wire obtained by drawing a wire for a high-strength steel wire according to any one of claims 1 to 7. A high-strength galvanized steel wire produced by subjecting the high-strength steel wire according to claim 8 to hot dip galvanization, wherein the tensile strength TS is equal to or higher than the tensile strength TS * defined by the following formula (2): A high-strength galvanized steel wire.
TS * = − 87.3D + 2234 (MPa) (2)
However, D shows the wire diameter (mm) of a high-strength galvanized steel wire.