Galvanized steel wire

Abstract

To provide a galvanized steel wire that satisfies both tensile strength and twisting characteristics.SOLUTION: A galvanized steel wire i contains, as chemical components other than iron (Fe), by mass%, 0.85 to 1.00% carbon (C), 1.00 to 1.40% silicon (Si), 0.10 to 0.40% manganese (Mn), 0.030% or less phosphorus (P), 0.030% or less sulfur (S), 0.40 to 1.00% chromium (Cr), 0.020 to 0.080% aluminum (Al), and 0.0010 to 0.0100% nitrogen (N). In the galvanized steel wire, in a cross section parallel to a longitudinal direction including an axis, an average circle equivalent radius of spheroidal cementite present at a depth of 1 / 4 of a radius from a circumferential face is 8.6 to 12.6 nm.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a steel wire coated by zinc plating.

Background Art

[0002] Zinc-plated steel wires are used, for example, as cables for bridges of suspension bridges. In recent years, suspension bridges have been growing longer and there are also many demands for shortening the construction period. Therefore, not only weight reduction but also high strength is required for the cables for bridges (zinc-plated steel wires) used.

[0003] Zinc-plated steel wires are strengthened by patenting and wire drawing on rolled materials, and further zinc-plated for the purpose of imparting rust prevention, and finally bundled into the number that can withstand the required load for use.

[0004] As a method for increasing the strength of zinc-plated steel wires, there is work hardening by wire drawing, but as another method, an increase in the strength of steel wires by adding alloy components is also conceivable.

[0005] However, regarding "work hardening by wire drawing", when the wire drawing diameter is increased, the occurrence of longitudinal cracks (hereinafter, delamination) during twisting due to embrittlement cannot be denied. Conversely, when the wire drawing diameter is decreased, there are concerns about an increase in processing costs and an extension of the construction period due to an increase in the number of bundled wires. Regarding "addition of alloy components", simply adding alloy components increases hardenability, and supercooled structures such as martensite and bainite are formed after patenting, and there is a risk that the wire drawing property and ductility of the steel wire cannot be ensured. Thus, since the steel wire itself becomes brittle due to high strength and the risk of delamination increases, measures to maintain and improve ductility are desired for high strength.

[0006] As patent documents that are considered to contribute to the solution of the above problems, the following Patent Documents 1 to Patent Documents 3 can be cited.

[0007] Patent Document 1 discloses a technique for preventing delamination (achieving good twist characteristics) in a steel wire having a specific component composition, where the area ratios of the pearlite structure in the interior and surface layer are 90% or more and 80% or more, the area ratio of the lamellar pearlite structure with an average cementite length of 1.0 μm or more in the overall structure is 30% to 65%, and the area ratio of the fragmented pearlite structure with an average cementite length of 0.30 μm or less is 20% to 50%. In other words, Patent Document 1 discloses a technique for improving twist characteristics by reducing non-pearlite in the steel wire and defining the structural fractions of lamellar pearlite with different cementite lengths.

[0008] Patent Document 2 discloses a technology for high-strength hot-dip galvanized steel wire containing, by weight percent, C: 0.75-1.1%, Si: 0.5-2.0%, Mn: 0.2-2.0%, and also containing two or more of the following elements: Ni: 0.1-1.0%, P: 0.03-0.5%, and Al: 0.05-1.0%, with a total amount of 0.2% or more, and the remainder consisting of Fe and unavoidable impurities. In other words, as a method for increasing the strength of high-strength hot-dip galvanized steel wire, a technology is disclosed in which the fragmentation and spheroidization of cementite during hot-dip galvanizing are suppressed by actively adding two or more of Ni, Al, and P.

[0009] Patent Document 3 discloses a high-strength galvanized steel wire having a pearlite structure after wire drawing, with a tensile strength of 2000 MPa or more, containing, by mass%, C: 0.8~1.1%, Si: 0.8~2.0%, Mn: 0.2~1.0%, Cr: 0.1~1.0%, with the remainder being Fe and unavoidable impurities, wherein the ratio of the C concentration in ferrite in the surface layer of the steel wire to the C concentration in ferrite in the center of the steel wire is 5 or less, and furthermore, the ratio of the alloying element concentration in cementite to the alloying element concentration in ferrite is, for the main components Si, Mn, and Cr, Si: 0.1~0.5, Mn: 1.5~8.0, and Cr: 1.5~8.0. In other words, a technology has been disclosed that prevents the fragmentation and spheroidization of layered cementite, thereby achieving both strength and ductility, by controlling the carbon concentration in the ferrite and alloy concentration in the cementite on the surface of the steel wire through a combination of processing conditions. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Patent No. 6485612 [Patent Document 2] Japanese Patent Application Publication No. 11-293394 [Patent Document 3] Patent No. 3302213 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, even after referring to the aforementioned Patent Documents 1 to 3, no technology is found that aims to achieve both increased strength and improved twist characteristics in galvanized steel wire.

[0012] The technology disclosed in Patent Document 1 does not allow for optimal alloy composition, and the spheroidization of cementite is insufficient, resulting in a problem where both tensile strength and torsional properties cannot be achieved.

[0013] The technology disclosed in Patent Document 2 clearly fails to ensure torsional properties because the spheroidization of cementite is insufficient (because the cementite cannot be controlled at the nano-order). Furthermore, the addition of components disclosed in Patent Document 2 may lead to increased hardenability and an increase in non-metallic inclusions, which could worsen the productivity of steel wire.

[0014] Even with the technology disclosed in Patent Document 3, the spheroidization of cementite is not controlled at the nano-order level, and it is highly questionable whether it can increase the strength of steel wire.

[0015] The present invention has been made in view of the above-mentioned problems, and aims to provide a galvanized steel wire that satisfies both tensile strength and torsional properties. [Means for solving the problem]

[0016] The zinc-plated steel wire according to the present invention is ,quality In percentage terms, carbon (C) is 0.85-1.00%, silicon (Si) is 1.00-1.40%, manganese (Mn) is 0.10-0.40%, phosphorus (P) is 0.030% or less, sulfur (S) is 0.030% or less, chromium (Cr) is 0.40-1.00%, aluminum (Al) is 0.020-0.080%, and nitrogen (N) is 0.0010-0.0100%. The composition is such that %, the remainder is Fe and unavoidable impurities. The material is characterized in that, in a cross-section parallel to the longitudinal direction including the axis, the average equivalent diameter of the spherical cementite located at a depth of 1 / 4 of its diameter from the circumferential surface is 8.6 to 12.6 nm.

[0017] Furthermore, galvanized steel wire The above component composition further It may contain one or more elements from the group consisting of 0.01-0.20% copper (Cu), 0.01-0.20% nickel (Ni), 0.001-0.100% vanadium (V), and 0.0001-0.0050% boron (B). [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a galvanized steel wire that satisfies both tensile strength and torsional properties.

Brief Description of the Drawings

[0019] [Figure 1] FIG. 1 is a diagram showing the process of manufacturing a zinc-plated steel wire. [Figure 2] FIG. 2 is an enlarged cross-sectional view (photograph by TEM) of the zinc-plated steel wire.

Embodiments for Carrying Out the Invention

[0020] Embodiments of the zinc-plated steel wire according to the present invention will be described with reference to the drawings. The embodiments described below are an example of embodying the present invention, and do not limit the configuration of the present invention with that specific example.

[0021] The zinc-plated steel wire is manufactured according to the process shown in FIG. 1.

[0022] First, a steel piece (rolled material) with a corner of 155 mm that has been block-rolled is heated to 850 to 1200 °C and held at this temperature for 60 to 240 minutes. Heating such a steel piece is performed to dissolve the grain boundary cementite existing in the steel piece and to fully austenitize it. The heating conditions are set to raise the temperature to 850 to 1200 °C and hold this temperature for 60 to 240 minutes, but the range of the heating conditions is not limited to this. That said, if the heating temperature is less than 850 °C, the grain boundary cementite remains undissolved and the austenitization is insufficient, so the risk of wire breakage during rolling increases.

[0023] After heating the steel piece, the steel piece is rough-rolled and finish-rolled (caliber rolling) to obtain a wire rod with a diameter of φ10 to 15 mm. The temperature of the finish rolling is 850 to 1000 °C, and after rolling, it is cooled to the transformation completion temperature by stelmor (air cooling).

[0024] In finish rolling, the rolling temperature is set to 850-1000°C, but it is not limited to this range. However, if the finish temperature is below 850°C, the austenite grains become finer and the grain boundary area increases, so grain boundary cementite is formed and becomes the starting point for delamination in the steel wire. On the other hand, if the finish temperature exceeds 1000°C, the austenite grains become coarser and the hardenability increases, so the pearlite transformation is not completed in the Stermor and a supercooled structure is formed, increasing the risk of wire breakage during transport to the next process.

[0025] Subsequently, the wire is heated at 900-1000°C for 180-360 seconds, and then immersed in a molten lead bath at 500-650°C for 120-250 seconds to perform lead patenting. This patenting process can transform the wire's structure into a uniform and fine pearlite structure. Patenting may also be performed in a salt bath or fluidized bed instead of lead.

[0026] Regarding patenting, if the steel billet is heated to less than 900°C for less than 180 seconds, austenitization is insufficient in the wire structure, resulting in a decrease in tensile strength after patenting and insufficient tensile strength of the wire.

[0027] If heating during patenting exceeds 1000°C and exceeds 360 seconds, the austenite grains coarseen, increasing hardenability. During the subsequent cooling and holding period, the pearlite transformation may not be completed, and some parts may become martensite, potentially leading to wire breakage during drawing. Therefore, the preferred patenting conditions are 925-995°C and 190-350 seconds, and even more preferred conditions are 950-990°C and 200-320 seconds. Furthermore, if the cooling conditions during patenting are below 500°C and less than 120 seconds, bainite or martensite structures may form, potentially leading to wire breakage during drawing. If the cooling conditions exceed 650°C, the lamellar spacing of the pearlite structure becomes large, potentially resulting in insufficient tensile strength. Cooling conditions exceeding 250 seconds are not desirable because the patenting process time becomes too long, reducing productivity. Therefore, the cooling conditions for patenting are preferably 520-630°C for 130-240 seconds, and more preferably 530-610°C for 140-220 seconds. Although a lead bath is preferable for patenting, a fluidized bed or salt bath patenting may also be used.

[0028] Next, the wire is pickled with an aqueous solution of a strong acid such as sulfuric acid or hydrochloric acid to remove surface oxides. After rinsing with water, a bonder treatment is applied to form a phosphate coating on the surface of the wire, and the treated wire is drawn to a diameter of φ5.0 to 7.0 mm. The total reduction ratio in this wire drawing process is 65 to 95%.

[0029] Regarding the conditions for wire drawing, in order to satisfy a tensile strength of 1960 MPa or more for galvanized steel wire and suppress the occurrence of delamination, it is essential to perform wire drawing with a total reduction ratio of 65 to 95%. If the total reduction ratio is less than 65%, work hardening will be insufficient, and the desired strength may not be obtained. If the total reduction ratio exceeds 95%, the steel wire will become brittle and delamination will occur. Taking these factors into consideration, the total reduction ratio in wire drawing is preferably 68 to 93%, and more preferably 70 to 90%.

[0030] After the wire drawing process, the steel wire was straightened, then aged by immersion in molten lead at 450-550°C for 25-180 seconds, followed by immersion in molten zinc at 420-450°C for 25-180 seconds. Note that the aging treatment described above is not limited to a lead bath; a salt bath or fluidized bed can also be used.

[0031] By immersing drawn pearlite in molten lead at 450-550°C for 25-180 seconds, the layered cementite can be grown into appropriately fine spherical cementite. Regarding the aging treatment conditions, below 450°C, the spherical cementite in the structure becomes finer, leading to brittleness and delamination of the steel wire. Above 550°C, the spherical cementite coarses, reducing tensile strength. Considering these factors, the preferred aging treatment conditions are 455-540°C for 30-160 seconds, and more preferred conditions are 460-530°C for 35-140 seconds.

[0032] Finally, as shown in Figure 1, hot-dip galvanizing is performed after the aging treatment. For the hot-dip galvanizing treatment, the steel wire surface is immersed in molten zinc at 420-450°C for 25-180 seconds to coat it with a zinc layer. The aging treatment can also be performed by galvanizing under the same conditions as the aging treatment.

[0033] Regarding the steel wire described above, or in other words, galvanized steel wire, the components other than iron (Fe) are within the ranges shown below. The numerical values ​​for the content of each component are expressed in mass percent.

[0034] In other words, the composition of steel wire consists of, in mass percent, carbon (C) as a chemical component other than iron (Fe). The composition is as follows: 0.85-1.00% of sodium (P), 1.00-1.40% of silicon (Si), 0.10-0.40% of manganese (Mn), 0.030% or less of phosphorus (P), 0.030% or less of sulfur (S), 0.40-1.00% of chromium (Cr), 0.020-0.080% of aluminum (Al), and 0.0010-0.0100% of nitrogen (N).

[0035] The reason for specifying the composition of the steel wire as described above is as follows:

[0036] First, the carbon (C) content is 0.85 to 1.00%. In order to obtain higher tensile strength by increasing the pearlite fraction in galvanized steel wire and the cementite fraction in pearlite, the carbon content should be at least 0.85%, preferably 0.90%, and more preferably 0.92%. However, excessive addition of carbon (C) does not contribute to tensile strength and forms protereminate cementite, which reduces the ductility of the steel wire. Therefore, it is best to keep the carbon content at most 1.00%, preferably 0.98%, and more preferably 0.96%.

[0037] Silicon (Si) contributes to solid solution strengthening of ferrite, suppression of protereminate cementite, and refinement of spherical cementite during hot-dip galvanizing. The Si content is at least 1.00%, preferably 1.10%, and more preferably 1.15%. However, excessive silicon leads to ferrite embrittlement and reduces the ductility of the steel wire, so its content should be 1.40% or less, preferably 1.30%, and even more preferably 1.25% or less.

[0038] Manganese (Mn) combines with sulfur (S) to form ductile manganese sulfide (MnS). The manganese content is at least 0.10%, preferably 0.13%, and more preferably 0.15%. However, if the manganese content is excessive, the effect of improving hardenability is large and supercooled structures tend to form during patenting, so the manganese content should be 0.40% or less, preferably 0.35%, and more preferably 0.30% or less.

[0039] Because phosphorus (P) segregates in steel wire and adversely affects ductility, its content is preferably 0.030% or less, more preferably 0.020% or less, and more preferably 0.015% or less. However, reducing the phosphorus content requires a long molten steel treatment, which leads to decreased productivity and increased costs. Therefore, it is preferably 0.002% or more, and more preferably 0.003% or more.

[0040] Sulfur (S) combines with manganese to form ductile manganese sulfide. High sulfur content increases the risk of wire breakage during drawing, starting from the manganese sulfide, and also reduces the ductility of the steel wire. Therefore, a sulfur content of 0.020% or less is preferable, and 0.015% or less is more preferable. On the other hand, reducing sulfur requires a longer molten steel treatment, leading to decreased productivity and increased costs. Therefore, a sulfur content of 0.002% or more is preferable, and 0.003% or more is preferable.

[0041] Chromium (Cr) contributes to the refinement of the lamellar spacing of pearlite and the refinement of spheroidal cementite during aging treatment. In high-carbon steel, an increase in hardenability is suppressed when the chromium content is between 0.40% and 1.00%. To achieve refinement of spheroidal cementite, the chromium content must be 0.40% or higher, preferably 0.45% or higher, and more preferably 0.48% or higher.

[0042] On the other hand, excessive addition of chromium leads to a significant increase in hardenability and makes it easier for a supercooled structure to form during patenting; therefore, its content is 1.00% or less, preferably 0.90% or less, and more preferably 0.85% or less.

[0043] Aluminum (Al) combines with nitrogen (N) in the steel wire to form fine aluminum nitride (AlN), which increases hardenability. The aluminum content is 0.020% or more, preferably 0.030% or more, and more preferably 0.035% or more, in order to suppress the coarsening of austenite grains during patenting. On the other hand, if it is included in excess, coarse aluminum nitride (AlN) and nonmetallic inclusions such as aluminum oxide (Al2O3) are formed, leading to a decrease in the ductility of the steel wire. Therefore, the aluminum content is 0.080% or less, preferably 0.075% or less, and more preferably 0.070% or less.

[0044] Nitrogen (N) combines with aluminum in the steel wire to form fine aluminum nitride (AlN), increasing hardenability. The nitrogen content should be 0.0010% or more, preferably 0.0020% or more, and more preferably 0.0030% or more, in order to suppress grain coarsening during patenting, which leads to increased hardenability. On the other hand, if nitrogen is present in excess in the steel wire... Because the amount of dissolved nitrogen increases and age embrittlement occurs during the wire drawing process, reducing ductility, the nitrogen content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0070% or less.

[0045] Copper (Cu) may be added as needed to enhance the corrosion resistance of steel wire and contribute to its resistance to hydrogen embrittlement. However, excessive addition of copper reduces hot ductility and worsens manufacturability, so its content should be 0.20% or less, preferably 0.10% or less, and more preferably 0.05% or less.

[0046] Nickel (Ni) may be added as needed to suppress hydrogen penetration from the steel wire surface and improve hydrogen embrittlement resistance. However, since a high nickel content increases hardenability and raises the risk of overcooling during patenting, a nickel content of 0.20% or less is appropriate, preferably 0.10% or less, and more preferably 0.05% or less.

[0047] Vanadium (V) may be added as needed because it forms fine carbides within the steel wire and contributes to precipitation strengthening. The vanadium content is preferably 0.010% or more, more preferably 0.015% or more. However, since a high vanadium content reduces the ductility of the steel wire and increases the risk of overcooling during patenting due to increased hardenability, the vanadium content is appropriate at 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.

[0048] Boron (B) may be added as needed during patenting to suppress protereminate ferrite formation by segregating at austenite grain boundaries and improve the ductility of the steel wire. However, a high boron content can lead to cracking during casting and worsen manufacturability, so a boron content of 0.0050% or less is appropriate, preferably 0.040% or less, and more preferably 0.0035% or less.

[0049] A galvanized steel wire is manufactured by performing the process shown in Figure 1 on a steel billet with such a composition. The galvanized steel wire of the present invention has the following characteristic configuration.

[0050] In other words, when the cross-section of the zinc-plated steel wire of the present invention is observed with a scanning electron microscope (SEM), a drawn pearlite structure is observed in which plate-like cementite and ferrite are alternately layered. However, the applicant conducted diligent research and performed microstructural observation at the nano-order level on the cross-section of the steel wire after immersion using a transmission electron microscope (TEM). As a result, it was found that what was recognized as plate-like cementite by SEM was actually an aggregate of fine spherical cementite through TEM analysis. The reason for this is thought to be that when heat is applied to the steel wire due to aging treatment after drawing, the cementite that was plastically deformed by drawing is transformed into metastable spherical nanoparticles.

[0051] The applicant furthered their research and discovered that high levels of tensile strength and torsional properties can be achieved by controlling the particle size of nano-order spherical cementite within a certain range. They also discovered that the particle size of spherical cementite can be controlled within a certain range by optimizing the steel wire composition and the heat treatment conditions after wire drawing.

[0052] Specifically, we found that by setting the average equivalent diameter of the spherical cementite located at a depth of 1 / 4 of its diameter from the circumferential surface in a cross-section parallel to the longitudinal direction including the axis of the steel wire to 8.6 to 12.6 nm, it is possible to obtain a galvanized steel wire that satisfies both tensile strength and torsional properties. [Examples]

[0053] The applicant has conducted several experiments regarding the ability to obtain galvanized steel wire that satisfies both tensile strength and torsional properties by setting the average equivalent circle diameter of spheroidal cementite in the steel wire structure to 8.6 to 12.6 nm. The results are described as examples.

[0054] The steel wires (test materials) used in the experiment consisted of two types: No. 1 (inventive example) and No. 2 (comparative example). The steel composition is as shown in Table 1.

[0055] [Table 1]

[0056] The rolling conditions for the steel wire are as shown in Table 2.

[0057] [Table 2]

[0058] Rolling condition 1 is steel Piece 93 in a furnace at 946°C minutes The material is heated and roughly rolled, then finished at 901°C. The wire diameter after finish rolling is 15 mm.

[0059] Rolling condition 2 is steel Piece 88 in a furnace at 919°C minutes The material is heated and roughly rolled, then finished at 898°C. The wire diameter after finish rolling is 14 mm.

[0060] Table 3 lists the patenting conditions for steel wire, and Table 4 shows the drawing conditions for steel wire.

[0061] [Table 3]

[0062] [Table 4]

[0063] Table 5 lists the aging treatment conditions for steel wire, and Table 6 shows the zinc plating treatment conditions for steel wire.

[0064] [Table 5]

[0065] [Table 6]

[0066] Steel wires 1 through 7 were manufactured under varying conditions, and various tests were conducted. The results are shown in Table 7.

[0067] [Table 7]

[0068] The tensile strength measurements in Table 7 were performed in accordance with JIS Z 2241 (2011). The tensile strength was measured with a gauge length (GL) of 200 mm and a tensile speed of 0.008 S. -1 The test was conducted as follows. The tensile strengths listed in Table 7 were calculated by averaging the measurement results of the three test specimens.

[0069] Torsion tests (torsion tests) to evaluate the presence or absence of delamination in Table 7 were performed in accordance with JIS H 3521 (1991) at GL = 100 × D (D: diameter mm), and the fracture surface was observed visually.

[0070] Torsion tests were performed on 3 to 6 test specimens each. The presence or absence of delamination was determined by whether or not characteristic longitudinal cracks of delamination were present on the fracture surface after fracture. Specimens in which no longitudinal cracks were observed were evaluated as having no delamination (if even one crack occurred, delamination was detected).

[0071] When the fractured cross-section of a drawn galvanized steel wire is observed with a scanning electron microscope (hereinafter abbreviated as "SEM"), a drawn pearlite structure is observed, in which plate-like cementite and ferrite are alternately layered.

[0072] In galvanized steel wire, the structure of the cementite microstructure and the occurrence of delamination during torsional deformation are closely related. To generalize the presence or absence of delamination in relation to the degree of structural change of the cementite lamellae, a more detailed observation of the pearlite microstructure after deformation in galvanized steel wire is effective.

[0073] Therefore, we used a transmission electron microscope (TEM), which has a higher resolution than a scanning electron microscope (SEM), to investigate the relationship between the microstructure of galvanized steel wire after manufacturing and delamination.

[0074] Figure 2 shows photographs of the test specimens taken by TEM. Figure 2(a) is a photograph of No. 1 (steel wire 1) in Table 7, and (b) is a photograph of No. 4 (steel wire 4), with spheroidal cementite indicated by arrows in both cases.

[0075] Test specimens taken from galvanized steel wire were imaged using a TEM (TalosF200X, manufactured by FEI Japan, with an acceleration voltage of 200kV), and the equivalent diameter of the spherical cementite circle was determined from the obtained images.

[0076] Specifically, a test specimen was taken using a thin-film method on a plane parallel to the central axis of a galvanized steel wire, including the central axis, at a position D / 4 of the diameter D inward from the circumferential surface. This specimen was then photographed in three fields at an observation magnification of 320,000x. The granular contrast observed within the fields was identified as spherical cementite. In Table 7, the equivalent diameter of spherical cementite circles was calculated by determining the equivalent diameter of each circle from the area of ​​spherical cementite present in the field of view, and then calculating the average value.

[0077] The applicant observed the structure at the nanoscale using a transmission electron microscope and found that the plate-like cementite is an aggregate of fine spherical cementite particles.

[0078] This is thought to be because the cementite, which was plastically deformed during wire drawing, was transformed into metastable spherical nanoparticles when heat was applied to the steel wire during aging treatment after wire drawing.

[0079] Further intensive research revealed that if the particle size of nano-order spherical cementite is within a certain range, it is possible to achieve both high tensile strength and good torsional properties simultaneously. This can be achieved by optimizing the steel wire composition and the heat treatment conditions after wire drawing.

[0080] As shown in Table 7, the examples of the invention (steel wires 1 to 3) have a tensile strength of 1960 MPa or higher and do not exhibit delamination. In other words, the composition and the equivalent circular diameter of the spheroidal cementite in steel wires 1 to 3 are within the scope of the claims, resulting in good tensile strength and torsional properties.

[0081] On the other hand, in steel wire 4, the equivalent circular diameter of the spherical cementite exceeds the upper limit, resulting in reduced tensile strength. In steel wires 5 to 7, the equivalent circular diameter of the cementite falls below the lower limit, and although the tensile strength meets the desired value, delamination is observed.

[0082] In summary, the following findings can be obtained regarding the average equivalent diameter of spherical cementite.

[0083] First, regarding spheroidal cementite, the aging process has the effect of reducing dislocations excessively introduced into the steel wire during wire drawing and straightening, thereby improving the ductility of the steel wire. However, at the same time, there are concerns that the tensile strength may decrease as the lamellar cementite becomes spheroidal and coarser, or that delamination may occur because the ductility does not recover due to insufficient spheroidization.

[0084] As a result of diligent research, the applicant has found that if the average equivalent diameter of spherical cementite after aging treatment is 8.6 to 12.6 nm, it is possible to achieve a high level of both strength and ductility. If the average equivalent diameter is less than 8.6 nm, the recovery of ductility is insufficient and there is a risk of delamination. On the other hand, if it exceeds 12.6 nm, softening progresses excessively, and the tensile strength falls below 1960 MPa. Therefore, the average equivalent diameter of spherical cementite is preferably 8.6 to 12.6 nm, more preferably 9.0 to 12.4 nm, and more preferably 9.2 to 12.2 nm.

[0085] It should be noted that the embodiments disclosed herein are illustrative and not restrictive in all respects. In particular, matters not explicitly stated in the embodiments disclosed herein, such as operating conditions, handling conditions, dimensions and weight of components, etc., can be easily selected by a person skilled in the art by referring to the problems to be solved, solutions, operations and effects of the present invention disclosed herein. [Industrial applicability]

[0086] This invention can be used, for example, in galvanized steel wire used in bridge cables.

Claims

1. Having a composition in mass%, C: 0.85-1.00%, Si: 1.00-1.40%, Mn: 0.10-0.40%, P: 0.030% or less, S: 0.030% or less, Cr: 0.40-1.00%, Al: 0.020-0.080%, N: 0.0010-0.0100%, with the remainder being Fe and unavoidable impurities, In a cross-section parallel to the longitudinal direction including the axis, the average equivalent diameter of the spherical cementite located at a depth of 1 / 4 of its diameter from the circumferential surface is 8.6 to 12.6 nm. A zinc-plated steel wire characterized by the following features.

2. The component composition further includes one or more from the group consisting of Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, V: 0.001 to 0.100%, and B: 0.0001 to 0.0050%. The zinc-plated steel wire according to feature 1.

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