KR20170012467A - Wire material for steel wire, and steel wire - Google Patents

Abstract

A wire for a steel wire excellent in low-cycle fatigue characteristics and useful as a material for a high-strength steel wire such as a wire rope or a PC steel wire, and a steel wire capable of exhibiting such characteristics. The steel wire rod according to the present invention is characterized in that it contains 0.70 to 1.3% of C, 0.1 to 1.5% of Si, 0.1 to 1.5% of Mn, 0.001 to 0.006% of N, 0.001 to 0.10% 0.205 to 0.010%, P: 0 to 0.030%, S: 0 to 0.030%, the balance being iron and inevitable impurities, the pearlite being the main phase, The hydrogen diffusion coefficient D in the steel satisfies the following expression (1).
D? 2.5 × 10 ^-7 (cm ² / sec) ... (One)

Description

{WIRE MATERIAL FOR STEEL WIRE, AND STEEL WIRE}

TECHNICAL FIELD The present invention relates to a wire for a steel wire used as a material of a high strength steel wire used for a wire rope or a PC steel wire, and to such a steel wire.

In a strand to which repeated bending stress is applied, such as an elevator rope or a hoisting rope of a crane, the bending fatigue characteristic of a strand is an important factor determining the design strength and life of the rope. 2. Description of the Related Art In recent years, there has been a demand for a reduction in the weight of ropes accompanied by an increase in the speed of elevators and the miniaturization of cranes, and there is a demand for a high strength steel wire rod excellent in bending fatigue characteristics. Also, a wire rod for a high strength steel wire excellent in bending fatigue characteristics is also useful as a material for PC (Prestressed Concrete) steel wire. Concretely, it is required that such a wire for a steel wire does not cause occurrence of low-cycle fatigue occurring at a repetition frequency of 10 ⁴ to 10 ⁵ times.

As a technique for improving the characteristics of the wire rod, various proposals have been made so far. For example, Patent Document 1 discloses a technique of improving the fatigue strength by finely precipitating BN inclusions in steel.

Patent Document 2 discloses a technique for improving the fatigue characteristics evaluated by the rolling bending fatigue test of 10 ⁷ cycles by reducing the amount of hydrogen in the ultrafine steel wire having the drawn pearlite structure.

Japanese Patent Application Laid-Open No. 2011-225990 Japanese Patent Laid-Open No. 11-256274

The characteristic problem in the technique of Patent Document 1 is the high cycle fatigue occurring near the fatigue limit of the number of repetitions of 10 ⁷ , and the mechanism is different from the low cycle fatigue. In a product exposed to a long-term external atmosphere such as a wire rope, fatigue cracks are likely to occur due to oxidation of the surface layer or friction between wire elements, and crack propagation is promoted by hydrogen penetrating into the steel. The material is destroyed. Therefore, measures against hydrogen are required.

Also, the fatigue characteristic considered in the Patent Document 2 is high cycle fatigue. In the case of a product having hydrogen intrusion from the outside such as a wire rope or a PC steel wire, the low cycle fatigue characteristic is deteriorated by hydrogen invading from the outside Therefore, it is not sufficient to simply reduce the amount of hydrogen in the steel as in Patent Document 2.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel wire rope excellent in low cycle fatigue characteristics and useful as a material of a high strength steel wire such as a wire rope or a PC steel wire, To provide a steel wire.

The steel wire rod according to the present invention, which can solve the above-described problems, comprises, by mass%, 0.70 to 1.3% of C, 0.1 to 1.5% of Si, 0.1 to 1.5% of Mn, 0.001 to 0.006% of N, 0.001 to 0.10% Wherein the balance contains iron and inevitable impurities, and the pearlite is in the form of a pellet. , And the hydrogen diffusion coefficient D in the steel at 300 DEG C satisfies the following expression (1).

D? 2.5 × 10 ^-7 (cm ² / sec) ... (One)

On the other hand, " pearlite as a main phase " means that at least 95% by area of the metal structure is a pearlite structure.

The high strength steel wire rod according to the present invention comprises, by mass%

(a) at least one of Cr: more than 0% to 1.0% and V: more than 0% to 0.5%

(b) Ni: not less than 0% and not more than 0.5%, and Nb: not less than 0% and not more than 0.5%

(c) Co: more than 0% and not more than 1.0%

(d) Mo: not less than 0% but not more than 0.5%, and Cu: not less than 0% and not more than 0.5%

And the like.

The present invention also includes a steel wire having a chemical composition of the aforementioned steel and having a hydrogen diffusion coefficient D in a steel at 300 캜 satisfying the following formula (1).

D? 2.5 × 10 ^-7 (cm ² / sec) ... (One)

According to the present invention, hydrogen diffusion in a steel is inhibited by the hydrogen trapping effect of Ti-based inclusions such as TiC finely dispersed, and the diffusion coefficient is reduced, whereby a steel wire rod excellent in fatigue characteristics can be obtained. Particularly, excellent characteristics are exhibited for low cycle fatigue caused by a cyclic stress load of about 10 ⁴ to 10 ⁵ times.

Fig. 1 is a schematic explanatory view showing the execution state of the four-point bending fatigue test. Fig.

The inventors of the present invention investigated the factors that determine the low-cycle fatigue characteristics of steel wire rods, which are metal structures having pearlite as a main phase. The degradation of the fatigue property caused by hydrogen is caused by the hydrogen in the steel being diffused toward a minute crack generated in the steel due to the application of repetitive stress and causing the structure around the crack to brittle. In addition to the hydrogen occluded in the steel in the manufacturing process, hydrogen invading from the outside world likewise lowers the fatigue characteristics. Therefore, the hydrogen diffusion in the steel is inhibited, and the amount of hydrogen accumulated around the crack is reduced, thereby improving the fatigue characteristics. Specifically, the fatigue property is enhanced by a hydrogen diffusion coefficient D in the steel at 300 ℃ to below 2.5 × 10 ^-7 (cm ² / sec).

The hydrogen diffusion coefficient D is a temperature dependent property value, but in the present invention, the hydrogen diffusion coefficient D in the steel at 300 캜 is used as an index. This reason comes from the method of measuring the hydrogen diffusion coefficient. In the measurement of the hydrogen diffusion coefficient D, a method of analyzing the release curve of the hydrogen gas obtained by raising the sample in the measuring instrument is employed. However, the low temperature portion of the release curve is susceptible to disturbance, . This is because the hydrogen released at a low temperature is called a diffusible hydrogen, and diffusion at room temperature can not be ignored, so it is affected by the storage state of the sample provided for the measurement. On the other hand, the hydrogen diffusion coefficient D is preferably 2.3 × 10 ^-7 (cm ² / sec) or less, more preferably 2.0 × 10 ^-7 (cm ² / sec) or less.

As a means for inhibiting the diffusion of hydrogen, it is effective to finely disperse Ti-based inclusions such as TiC having the effect of adsorbing hydrogen in the steel.

The wire rope according to the present invention, when applied to a wire or the like, is required to appropriately adjust its chemical composition even when it exhibits its basic characteristics. Its chemical composition is as follows. On the other hand, "%" in chemical composition is "mass%".

(C: 0.70 to 1.3%)

C is an element effective for increasing the strength. With the increase in the amount of C, the strength of the wire (steel wire) before cold working and the steel wire after cold working is improved. Therefore, the amount of C was set to 0.70% or more. The amount of C is preferably 0.74% or more, and more preferably 0.78% or more. However, if the amount of C becomes excessively excessive, cornerstone cementite (hereinafter abbreviated as " corner stone ") may precipitate and cause breakage during drawing processing. Therefore, the amount of C was set to 1.3% or less. The amount of C is preferably 1.2% or less, and more preferably 1.1% or less.

(Si: 0.1 to 1.5%)

Si has an action as a deoxidizing agent and also has an effect of improving the strength of the wire rod. In order to effectively exhibit these effects, the amount of Si is set to 0.1% or more. The amount of Si is preferably 0.15% or more, and more preferably 0.18% or more. On the other hand, if the amount of Si becomes excessively excessive, the cold drawing property is deteriorated and an increase in the monodispersity is caused. Therefore, the amount of Si was set to 1.5% or less. The amount of Si is preferably 1.4% or less, and more preferably 1.3% or less.

(Mn: 0.1 to 1.5%)

Mn has a deoxidizing action similarly to Si, but has an action of increasing the toughness and ductility of steel by fixing S in the steel as MnS. In order to effectively exhibit these effects, the amount of Mn is set to 0.1% or more. The amount of Mn is preferably 0.15% or more, and more preferably 0.20% or more. However, Mn is an element that is easily segregated, and if it is added in excess, the hardenability of the Mn segregation portion excessively increases, and there is a fear that a supercooled structure such as martensite may be formed. Therefore, the amount of Mn was set to 1.5% or less. The Mn content is preferably 1.4% or less, and more preferably 1.3% or less.

(N: 0.001 to 0.006%)

N combines with B in the steel to form BN, which causes the effect of B to be lost. In addition, N in the solid state causes deterioration of the sintering property due to deformation aging at the time of drawing, and, in the case of remarkable case, vertical cracking. In order to prevent these harmful effects, the N content should be 0.006% or less. The N content is preferably 0.005% or less, and more preferably 0.004% or less. On the other hand, if it is a small amount, there is an effect of increasing the ductility of the wire rod by making the crystal grains finer by the nitride such as TiN or AlN. In order to achieve such an effect, the N content should be 0.001% or more. The amount of N is preferably 0.0015% or more, and more preferably 0.0020% or more.

(Al: 0.001 to 0.10%)

Al is an effective deoxidizing element. In addition, it has an effect of forming a nitride such as AlN and making crystal grains finer. In order to effectively exhibit such an effect, the amount of Al is 0.001% or more. The amount of Al is preferably 0.002% or more, and more preferably 0.003% or more. On the other hand, when Al is added in excess, an oxide such as Al ₂ O ₃ is formed, thereby increasing disconnection at the time of drawing. From this point of view, the amount of Al is 0.10% or less. The amount of Al is preferably 0.09% or less, and more preferably 0.08% or less.

(Ti: 0.02 to 0.20%)

Ti has the function of forming a carbide such as TiC and lowering the diffusion coefficient of hydrogen to improve the fatigue characteristics of the steel wire. It also has a function of forming a nitride such as TiN in combination with N in the steel to prevent deterioration of the characteristics of the sintering by N. In order to effectively exhibit these effects, the amount of Ti should be 0.02% or more. The amount of Ti is preferably 0.03% or more, and more preferably 0.04% or more. On the other hand, when the amount of Ti becomes excessive, Ti inclusions such as TiC and TiN are precipitated in a large amount to increase disconnection at the time of drawing. Therefore, the amount of Ti should be 0.20% or less. The amount of Ti is preferably 0.15% or less, and more preferably 0.10% or less.

(B: 0.0005 to 0.010%)

B has a function of suppressing pro-eutectoid ferrite (hereinafter abbreviated as " precious stone alpha ") precipitated in grain boundaries, and is effective for improving fatigue characteristics. Further, by forming BN, an effect of fixing the solid solution N in the steel and improving the salt characteristics can be expected. In order to effectively exhibit the effect of B, the amount of B is required to be 0.0005% or more. The lower limit of the preferable amount of B is 0.0007% or more, and more preferably 0.001% or more. On the other hand, when the amount of B becomes excessive, the amount of B needs to be 0.010% or less since an Fe-B based compound such as FeB _{2, which} is a compound with Fe, precipitates and causes cracking during hot rolling. The amount of B is preferably 0.008% or less, and more preferably 0.006% or less.

(P: 0% or more and 0.030% or less)

P is segregated in the old austenitic system to embrittle the grain boundaries to lower the fatigue strength. Therefore, the smaller the content of P is, the better. Therefore, the amount of P is 0.030% or less. The P content is preferably 0.025% or less, and more preferably 0.020% or less. The P content may be 0%, but is usually contained in an amount of 0.001% or more.

(S: not less than 0% and not more than 0.030%)

S, like P, segregates in the old austenitic system to embrittle the grain boundaries to lower the fatigue strength. Therefore, the smaller the content, the better. Therefore, the amount of S is 0.030% or less. The amount of S is preferably 0.025% or less, and more preferably 0.020% or less. The amount of S may be 0%, but is usually contained in an amount of 0.001% or more.

The basic components of the wire of the present invention are as described above, and the remainder is substantially iron. However, it is a matter of course that inevitable impurities included in raw materials, materials, manufacturing facilities, and the like are included in the steel.

Further, in order to further improve the characteristics such as strength, toughness and ductility, the wire material of the present invention may further contain,

(a) at least one of Cr: more than 0% to 1.0% and V: more than 0% to 0.5%

(b) Ni: not less than 0% and not more than 0.5%, and Nb: not less than 0% and not more than 0.5%

(c) Co: more than 0% and not more than 1.0%

(d) Mo: not less than 0% but not more than 0.5%, and Cu: not less than 0% and not more than 0.5%

And the like.

(At least one of Cr: more than 0% to 1.0% and V: more than 0% to 0.5%

Cr and V are useful elements for increasing the strength (tensile strength) of the wire rod, and they may be contained in combination of one kind or two kinds.

In particular, Cr has an effect of increasing the strength and toughness of the wire rod by making the lamellar spacing of pearlite small. In order to effectively exhibit such an effect, the amount of Cr is preferably 0.05% or more. The amount of Cr is more preferably 0.10% or more, and still more preferably 0.15% or more. On the other hand, if the Cr amount is excessively excessive, the hardenability is improved, and the risk of generating a supercooled structure during hot rolling increases, so that the Cr content is preferably 1.0% or less. The amount of Cr is more preferably 0.8% or less, and still more preferably 0.6% or less.

V has the effect of improving the strength of the wire by forming carbonitride. In addition, like Nb, there is an effect of suppressing aging embrittlement by fixing solute N in addition to contributing to grain refinement by forming excess N and nitride after AlN has been precipitated. In order to effectively exhibit such an effect, the amount of V is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more. However, since V is an expensive element and the effect is saturated even when it is added in excess, the amount of V is preferably 0.5% or less, more preferably 0.4% or less, still more preferably 0.2% or less to be.

(Ni: at least one of more than 0% and 0.5% or less and Nb: more than 0% and 0.5% or less)

Ni and Nb are useful elements for increasing the toughness of the steel wire, and they may be used either singly or in combination.

In particular, Ni is an element that increases the toughness of the steel wire after the drawing. In order to effectively exhibit such an effect, the amount of Ni is preferably 0.05% or more, more preferably 0.1% or more, and still more preferably 0.2% or more. However, even if Ni is added excessively, the effect is saturated, which is economically disadvantageous. Therefore, the amount of Ni is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Nb forms nitrides similarly to Ti or Al to contribute to the improvement of toughness of the steel wire by making crystal grains finer and also has an effect of suppressing aging embrittlement by fixing solid solution N. [ In order to effectively exhibit such an effect, the amount of Nb is preferably 0.01% or more, more preferably 0.03% or more, and still more preferably 0.05% or more. However, since Nb is an expensive element and its effect is saturated even when it is added in excess, it is not economically useless. Therefore, the amount of Nb is preferably 0.5% or less, more preferably 0.4% or less, still more preferably 0.3% to be.

(Co: more than 0% to 1.0% or less)

Co has an effect of reducing the generation of crushed stone cementite, especially when the amount of C is large, and making the structure uniform pearlite structure. In order to effectively exhibit this action, the amount of Co is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.2% or more. However, even if Co is added excessively, the effect is saturated, which is economically disadvantageous. Therefore, the amount of Co is preferably 1.0% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.

(Mo: at least one of more than 0% and 0.5% or less and Cu: more than 0% and 0.5% or less)

Mo is an element improving the corrosion resistance of the steel wire. In order to effectively exhibit such an effect, the amount of Mo is preferably 0.05% or more, more preferably 0.1% or more, and still more preferably 0.2% or more. However, if the amount of Mo becomes excessive, overcooled structure tends to be generated at the time of hot rolling, and ductility also deteriorates. Therefore, the amount of Mo is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Cu is an element that improves the corrosion resistance of the steel wire as well as Mo. In order to effectively exhibit such an effect, the amount of Cu is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. However, when the amount of Cu becomes excessive, CuS is segregated in the grain boundary part by reacting with S, and scratches are generated in the course of manufacturing wire rod. In order to avoid such influence, the amount of Cu is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Mo and Cu may be contained in combination of one kind or two kinds.

Next, a method of manufacturing the wire rope according to the present invention will be described.

The wire rod before cold drawing is usually manufactured by subjecting a steel material having a properly controlled chemical composition to solvent, crushing rolling and hot rolling, and further performing a patterning treatment as required. In producing the wire rod of the present invention while satisfying the requirements (diffusion coefficient of metal structure and hydrogen) defined in the present invention, the Ti content is properly controlled within the above-mentioned range, and then the precipitation of Ti-based inclusions such as TiC It is important to control the behavior properly.

First, in the crushing rolling, it is preferable to heat the cast steel to 1200 DEG C or higher to decompose the coarse TiC precipitated during casting. If the heating temperature is lower than 1200 占 폚, the TiC remaining on the wire remains, and the hydrogen diffusion coefficient can not be sufficiently reduced, so that the fatigue strength is lowered. The heating temperature is more preferably 1250 DEG C or higher, and more preferably 1300 DEG C or higher. However, if the heating temperature is excessively high, melting of the wire rod occurs, and therefore, it is usually set to about 1400 ° C.

In the subsequent hot rolling, the crucible is heated at a temperature of 1000 ° C or higher, and then the deformation rate at the final four passes of rolling is set to 0.5 sec ^-1 or more. The crystal grains are finely densified by dynamic recrystallization, It is preferable to precipitate it. If the deformation rate becomes smaller than 0.5 sec ^-1 , TiC can not be made sufficiently fine, and the hydrogen diffusion coefficient D can not be made sufficiently small. The deformation rate at this time is more preferably 0.8 sec ^-1 or more, and still more preferably 1.0 sec sec ^-1 or more. However, because of the problem of facility load, it is preferable that the strain rate is usually 5 sec ^-1 or less. On the other hand, the deformation speed Vε is a sum of the cross-sectional area S ₀ (m ² ) before entry to the first roll which is a roll immediately before the last pass, the cross-sectional area S ₄ (m ² ) after passing the final pass, Can be expressed by the following formula (2) using the passage time (rolling time) t (seconds).

V? = {In (S ₀ / S ₄ )} / t ... (2)

After the hot rolling, it is preferable to sufficiently cool by water cooling to control the placement temperature of the rolled material (wire rod) in the laying head to 800 to 1000 占 폚. If the setting temperature exceeds 1000 캜, the TiC may become coarse during cooling on the conveyor after placement. The deposition temperature is more preferably 980 占 폚 or lower, and still more preferably 950 占 폚 or lower. If the setting temperature is less than 800 占 폚, the deformation resistance of the wire rod is increased, and for example, there is a possibility that the coil springing can not be performed, and the loading defect in the laying head may occur. Therefore, it is preferable that the setting temperature is 800 DEG C or higher. The deposition temperature is more preferably 820 占 폚 or higher, and still more preferably 850 占 폚 or higher.

After the placement, the wire rod is cooled on the cooling conveyor to cause pearlite transformation during the cooling. However, it is preferable to quench the wire rod at an average cooling rate of 5 deg. C / sec or more before the pearlite transformation starts. If the average cooling rate at this time is slowed, TiC tends to coarsen and the hydrogen diffusion coefficient may increase. If the average cooling rate is less than 5 캜 / second, a structure in which lamellar spacing is locally called coarse pearlite is extremely precipitated, leading to deterioration of freshness. On the other hand, for the commencement of pearlite transformation, the temperature of the wire rod is measured, and a point at which the cooling curve changes (inflexion point) by transformation heat generation can be obtained. The average cooling rate is more preferably 10 ° C / second or more, and more preferably 15 ° C / second or more. The preferred upper limit of the average cooling rate is 100 DEG C / second or less, more preferably 50 DEG C / second or less.

The wire material obtained as described above can be used as a steel wire by drawing (cold working) as it is, but may be subjected to a patterning treatment before drawing. By performing such a patterning treatment before the drawing process, it is possible to increase the strength of the wire rod and to reduce the intensity gap.

Further, when it is expected that the degree of drawing processing will increase as in the case of manufacturing a thin steel wire, the patterning treatment is performed after a certain period of freshness from the rolled material to return the wire material structure to the raw pearlite structure, It is also useful to perform machining. At this time, if the TiC finely precipitated is retained by the reheating treatment, the state of low hydrogen diffusion coefficient D can be maintained even by the general patterning treatment.

The heating temperature (hereinafter sometimes referred to as " reheating temperature ") at the time of performing the patterning treatment is preferably about 900 to 1000 占 폚, more preferably 920 占 폚 to 980 占 폚. The reheating temperature is preferably 900 DEG C or higher from the viewpoint of preventing the remaining unreacted carbide and completely austenitizing the structure, but when the temperature becomes too high, TiC coarsens and the hydrogen diffusion coefficient D increases. The holding temperature in the patterning treatment is preferably about 530 to 600 占 폚, more preferably 550 占 폚 to 580 占 폚.

Since the wire material of the present invention has a sufficiently reduced hydrogen diffusion coefficient D in the steel, products such as wire ropes and PC steel wires that have been subjected to cold working or all or part of the steel wire are subjected to a fatigue characteristic great.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-136223 filed on July 1, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-136223 are hereby incorporated by reference.

Example

Hereinafter, the present invention will be described more specifically by way of examples. The present invention is not limited to the following embodiments, and it is of course possible to carry out the present invention by appropriately modifying it within a range that is suitable for the purpose of the latter, and they are all included in the technical scope of the present invention.

The steel ingots of the chemical composition shown in the following Table 1 were subjected to crushing rolling and hot rolling under the conditions shown in Table 2 below to form wire coil, and some of them were further subjected to the patterning treatment under the conditions shown in Table 3 below. The rolling line diameter shown in Table 2 below and the patterning line diameter shown in Table 3 shown below indicate heat treatment with intermediate drawing interposed therebetween.

The tensile test, the evaluation of the metal structure, and the measurement of the hydrogen diffusion coefficient D were carried out by the following methods using a sample taken from the wire rod before the finish drawing.

(Tensile test)

The tensile strength TS (tensile strength) of the collected samples was measured in accordance with JIS Z 2241 (2011). The steel wire was made in 8 pieces and the average value was obtained. The results are shown in Table 4 below.

(Evaluation of metal structure)

The metal structure was observed by an optical microscope in the cross section of the sample taken. On the other hand, in the item of the metal structure in the following Table 4, "P" indicates that the pearlite structure is at least 95% by area, that is, the pearlite is the main phase. Further, "P + α" or "P + θ" indicates a structure in which the pearlite structure is less than 95% by area and the ferrite (α) or cementite (θ) is mixed with more than 5% by area in addition to the pearlite structure have.

(Measurement of hydrogen diffusion coefficient D)

The collected sample was subjected to hydrogen filling to a saturated state, and a hydrogen release curve was obtained by the temperature increase analysis. The rate of temperature rise was 12 占 폚 / sec, and the amount of hydrogen emission was measured by an atmospheric pressure ionization mass spectrometer. The hydrogen filling conditions were as follows: H ₂ SO ₄ aqueous solution (pH 3) + KSCN: 0.01 mol / L was used as an electrolytic solution and charging was performed at a current density of 5 mA / cm ² for 48 hours. On the other hand, the sample after charging was stored in liquid nitrogen until the measurement, in order to prevent discretization of hydrogen as much as possible.

For the obtained hydrogen release curve, it is assumed that the hydrogen is uniformly distributed in the sample, and assuming that the sample shape is an infinite circle, fitting is performed by the hydrogen release curve obtained by numerical calculation using the diffusion coefficient as a parameter, D was obtained. The results are shown in Table 4 below. On the other hand, in order to make the infinite circumference approximation effective, the sample length was made five times or more of the diameter. Also, at this time, as the emission peak of hydrogen used for the fitting, a peak having a peak temperature of 200 DEG C or higher was used. The peak at a low temperature appearing at 200 占 폚 or less is called diffusible hydrogen and is also released by diffusion at room temperature. The influence of disturbance is considered. From the correlation curve between the temperature and the diffusion coefficient thus obtained, the diffusion coefficient at 300 캜 was obtained as the hydrogen diffusion coefficient D.

Next, wire rods (wires) were prepared by drawing the obtained wire rod coils and subjected to tensile test, evaluation of thin-film characteristics, evaluation of fatigue characteristics, and measurement of hydrogen diffusion coefficient D. Table 5 below shows the reduction ratio at the time of drawing and the wire diameter of the steel wire obtained by drawing.

(Tensile test)

The tensile strength TS of the steel wire and the yield point YP (Yield Point) were measured in accordance with JIS Z 2241 (2011). The steel wire was made in 8 pieces and the average value was obtained. The results are shown in Table 5 below. The values obtained by multiplying the tensile strength TS by 0.45 are shown in Table 5 below.

(Evaluation of Sorption Characteristics)

The salting characteristics were evaluated based on salting-out values (breaking-off number of seaweeds) necessary for the salting-out test to be broken. The salting-out values in Table 5 are average values of N = 5. At this time, the thinning rate was 52 times / minute and the tension was 500 gf (4.9 N). On the other hand, the saline value was standardized by converting the distance between the chucks (test line length) to 100 times (100 d) of the diameter d. In addition, the normal wavefront and longitudinal cracks are discriminated by the wavefront observation, and even if one of the five cracks is longitudinally cracked, it is described as " with longitudinal cracks "

(Evaluation of fatigue characteristics)

Fatigue characteristics were evaluated by performing a repeated four-point bending fatigue test with a jig having four points of support. In Fig. 1, reference numeral 1 denotes a test piece (wire rod), 2 denotes a direction in which a repetitive stress is applied, and? Denotes a supporting point. The test was carried out by a single bending, and the difference between the maximum stress and the minimum stress was defined as the stress amplitude. 100,000 cycles of repeated bending were carried out at various stress amplitudes, and it was judged that all of the test pieces which failed to be broken (disconnection) in the N = 3 tests were rejected even if one test piece was broken. The maximum stress amplitude in a sample determined as passing is defined as a fatigue strength of 100,000 cycles. The fatigue strength at 100,000 cycles is shown in Table 5 below. On the other hand, the stress waveform was a sine wave and the frequency was 10 Hz.

(Measurement of hydrogen diffusion coefficient D)

Also for the steel wire, the hydrogen diffusion coefficient D was determined as a reference value under the same conditions as above. The results are shown in Table 5 below.

From these results, it can be considered as follows.

First, 1 to 3 and 9 to 20 are all within the range specified in the present invention because of the chemical composition, the metal structure (pearlite area ratio) and the hydrogen diffusion coefficient D, A steel wire (wire) achieving a fatigue strength exceeding 0.45 times of the tensile strength TS after obtaining a tensile strength (in the standard, for example, a wire diameter of 7.0 mm, 1620 to 1770 MPa) exceeding "B" .

On the contrary, 4 to 8 and 21 to 26 are examples in which none of the requirements of the present invention is satisfied. Among them, 4, since the heating temperature at the time of crushing rolling was low, coarse TiC precipitated, and the hydrogen diffusion coefficient D became large, and the fatigue strength decreased.

Test No. 5, since the deformation rate of the final four passes during hot rolling was small, coarse TiC precipitated and the hydrogen diffusion coefficient D became large, and the fatigue strength decreased.

Test No. 6, since the reheating temperature after hot rolling was low, a poor failure occurred and no sample was obtained.

Test No. 7 shows that the placement temperature after hot rolling was high, 8, since the cooling rate after rolling was slow, TiC coarsened and the hydrogen diffusion coefficient D became large, and the fatigue strength was lowered.

Test No. 21 is an example using a steel P having a small amount of C, which is a mixed phase structure of ferrite and pearlite, has low tensile strength and low sintering characteristics, and fatigue strength also deteriorates.

Test No. 22 is an example using a steel grade Q having a large amount of C, and a large amount of crude stone cementite was precipitated, and therefore, it was broken during drawing.

Test No. 23 is an example using a steel type R having a small amount of Ti, but the amount of TiC is small and the hydrogen diffusion coefficient D is large and the fatigue strength is lowered.

Test No. 24 is an example using a steel grade S having a large amount of Ti, and a large amount of Ti-based inclusions was precipitated and broken in the drawing.

Test No. 25 is an example using a steel grade T having a large amount of B, but it was broken during hot rolling and a sample could not be obtained.

Test No. 26 is an example using a steel type U having a small amount of B, and the characteristics of the thinning and the fatigue strength were lowered. And the hydrogen diffusion coefficient D is also increasing.

Claims (3)

In terms of% by mass,
C: 0.70 to 1.3%
Si: 0.1 to 1.5%
Mn: 0.1 to 1.5%
N: 0.001 to 0.006%
Al: 0.001 to 0.10%
Ti: 0.02 to 0.20%
B: 0.0005 to 0.010%
P: not less than 0% and not more than 0.030%
S: not less than 0% and not more than 0.030%
Respectively, the remainder being iron and unavoidable impurities,
With pearlite as the main phase,
And the hydrogen diffusion coefficient D in the steel at 300 DEG C satisfies the following formula (1).
D? 2.5 × 10 ^-7 (cm ² / sec) ... (One) The method according to claim 1,
And further comprising at least one member selected from the following (a) to (d) in mass%.
(a) Cr: not less than 0% but not more than 1.0% and V: not less than 0% and not more than 0.5%
(b) at least one of Ni: not less than 0% and not more than 0.5% and Nb: not less than 0% and not more than 0.5%
(c) Co: more than 0% and not more than 1.0%
(d) Mo: not less than 0% but not more than 0.5%, and Cu: not less than 0% and not more than 0.5% A steel wire comprising the chemical composition of the steel according to claim 1 or 2, wherein the hydrogen diffusion coefficient D in the steel at 300 占 폚 satisfies the following expression (1).
D? 2.5 × 10 ^-7 (cm ² / sec) ... (One)

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