WO2016002414A1

WO2016002414A1 - Wire material for steel wire, and steel wire

Info

Publication number: WO2016002414A1
Application number: PCT/JP2015/065864
Authority: WO
Inventors: 友信石田; 智一増田
Original assignee: 株式会社神戸製鋼所
Priority date: 2014-07-01
Filing date: 2015-06-02
Publication date: 2016-01-07
Also published as: CA2951781A1; CN106471146B; EP3165623A1; JP2016014169A; KR20170012467A; CN106471146A; US20170130303A1; MX2016017015A; EP3165623A4

Abstract

Provided is a wire material for steel wire, the wire material having excellent low cyclic fatigue characteristics and being useful as a raw material for high-strength steel wire for wire rope, PC steel wire, or the like. Also provided is a steel wire that can exhibit said characteristics. A wire material for steel wire according to the present invention contains, by mass%, 0.70%-1.3% of C, 0.1%-1.5% of Si, 0.1%-1.5% of Mn, 0.001%-0.006% of N, 0.001%-0.10% of Al, 0.02%-0.20% of Ti, 0.0005%-0.010% of B, 0%-0.030% of P, and 0%-0.030% of S, the remainder being iron and unavoidable impurities, the main phase being pearlite, and the hydrogen diffusion coefficient (D) of the steel at 300 ℃ satisfying formula (1). (1) … D ≤ 2.5 x 10^-7 (cm²/sec).

Description

Steel wire rod and steel wire

The present invention relates to a wire for steel wire that is a material of a high-strength steel wire used for a wire rope, PC steel wire, and the like, and such a steel wire.

In steel strands that are repeatedly subjected to bending stress, such as elevator ropes and crane hoisting ropes, the bending fatigue characteristics of the strands are important factors that determine the design strength and life of the ropes. In recent years, there has been an increasing need for lighter ropes accompanying higher speed elevators and smaller cranes, and high-strength steel wire rods with excellent bending fatigue properties that can achieve these demands have been demanded. Moreover, the wire material for high-strength steel wires excellent in bending fatigue characteristics is also useful as a material for PC (Pressed Concrete) steel wires. Specifically, such a wire for a steel wire is required not to generate low cycle fatigue that occurs when the number of repetitions is 10 ⁴ to 10 ⁵ .

Various techniques have been proposed so far for improving the properties of the wire. For example, Patent Document 1 discloses a technique for improving fatigue strength by finely depositing BN inclusions in steel.

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

JP 2011-225990 A Japanese Patent Laid-Open No. 11-256274

The characteristic that is a problem in the technique of Patent Document 1 is high cycle fatigue that occurs near the fatigue limit of 10 ⁷ repetitions, and the mechanism is different from that of the low cycle fatigue. In products exposed to the outside air for a long time, such as wire rope, fatigue cracks are likely to occur due to oxidation of the surface layer and friction between the strands, and the crack growth is promoted by hydrogen that has penetrated into the steel. The material breaks down with a much lower lifetime. Therefore, measures against hydrogen are required.

In addition, the fatigue characteristics considered in Patent Document 2 are also high cycle fatigue, and in the case of products that have hydrogen penetration from the outside, such as wire rope and PC steel wire, Since the low cycle fatigue characteristics are lowered, it is not sufficient to simply reduce the amount of hydrogen in the steel as in Patent Document 2 above.

The present invention has been made in view of the circumstances as described above, and its purpose is excellent in low cycle fatigue characteristics and is useful as a material for high-strength steel wires such as wire ropes and PC steel wires. And providing a steel wire capable of exhibiting such characteristics.

The wire rod for steel wire of the present invention that can solve the above-mentioned problems is, in 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: 0 %, 0.030% or less, S: 0% or more, 0.030% or less, the balance being iron and inevitable impurities, pearlite as the main phase, and hydrogen diffusion coefficient in steel at 300 ° C D satisfies the following formula (1).
D ≦ 2.5 × 10 ⁻⁷ (cm ² / sec) (1)

Incidentally, “with pearlite as the main phase” means that 95% by area or more of the metal structure is a pearlite structure.

The wire material for high-strength steel wire of the present invention is further in mass%,
(A) Cr: more than 0%, 1.0% or less and V: more than 0%, 0.5% or less,
(B) Ni: more than 0%, 0.5% or less and Nb: at least one kind of more than 0%, 0.5% or less,
(C) Co: more than 0%, 1.0% or less,
(D) Mo: more than 0%, 0.5% or less and Cu: more than 0%, 0.5% or less,
Etc. are also preferable.

The present invention includes a steel wire having the above-described chemical composition of steel and having a hydrogen diffusion coefficient D in the steel at 300 ° C. satisfying the following expression (1).
D ≦ 2.5 × 10 ⁻⁷ (cm ² / sec) (1)

According to the present invention, hydrogen diffusion in steel is hindered by the hydrogen trap effect by Ti-based inclusions such as finely dispersed TiC, and a steel wire having excellent fatigue characteristics is obtained by reducing its diffusion coefficient. Can do. In particular, it exhibits excellent characteristics against low cycle fatigue caused by repeated stress loading of about 10 ⁴ to 10 ⁵ times.

FIG. 1 is a schematic explanatory view showing an implementation status of a four-point bending fatigue test.

The present inventors diligently investigated the factors that influence the low cycle fatigue characteristics in a steel wire material having a metal structure having pearlite as a main phase. The deterioration of the fatigue characteristics due to hydrogen is caused by the fact that hydrogen in the steel diffuses toward the minute cracks generated in the steel material due to repeated stress and embrittles the structure around the cracks. In addition to hydrogen occluded in the steel in the manufacturing process, hydrogen that has entered from the outside world also reduces fatigue properties. Therefore, fatigue characteristics can be improved by inhibiting hydrogen diffusion in the steel and reducing the amount of hydrogen accumulated around the cracks. Specifically, fatigue characteristics are improved by setting the hydrogen diffusion coefficient D in steel at 300 ° C. to 2.5 × 10 ⁻⁷ (cm ² / sec) or less.

Although the hydrogen diffusion coefficient D is a physical property value depending on temperature, in the present invention, the hydrogen diffusion coefficient D in steel at 300 ° C. is used as an index. The reason is derived from the method for measuring the hydrogen diffusion coefficient. In measuring the hydrogen diffusion coefficient D, a method of analyzing the hydrogen gas release curve obtained by raising the temperature of the sample in the measuring instrument is adopted, but the low temperature part of the release curve is easily affected by disturbance. Not suitable for accurate evaluation. This is because hydrogen released at a low temperature is called diffusible hydrogen, and diffusion at room temperature cannot be ignored, so it is affected by the storage state of the sample used for measurement. The hydrogen diffusion coefficient D is preferably 2.3 × 10 ⁻⁷ (cm ² / sec) or less, and 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 an effect of adsorbing hydrogen in the steel.

The steel wire rod according to the present invention needs to have its chemical composition adjusted appropriately in order to exert its basic characteristics when applied to a wire or the like. The chemical component composition is as follows. Note that “%” in the chemical composition is all “mass%”.

(C: 0.70 to 1.3%)
C is an element effective for increasing the strength, and the strength of the wire before cold working (steel wire) and the strength of the steel wire after cold working improves as the amount of C increases. Therefore, the C amount is set to 0.70% or more. The amount of C is preferably 0.74% or more, and more preferably 0.78% or more. However, when the amount of C becomes excessive, pro-eutectoid cementite (hereinafter sometimes abbreviated as “pre-deposition θ”) precipitates and causes wire breakage during wire drawing. Therefore, the C amount is set to 1.3% or less. The amount of C is preferably 1.2% or less, more preferably 1.1% or less.

(Si: 0.1-1.5%)
Si has an action as a deoxidizer and also has an action of improving the strength of the wire. In order to effectively exhibit these actions, the Si amount was determined to be 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, when the amount of Si becomes excessive, cold drawability is deteriorated, and the disconnection rate is increased. Therefore, the Si amount is 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 effect similar to Si, but has an effect of increasing the toughness and ductility of steel by fixing S in the steel as MnS. In order to effectively exhibit these actions, the amount of Mn is set to 0.1% or more. The amount of Mn is preferably 0.15% or more, more preferably 0.20% or more. However, Mn is an element that is easily segregated, and if added excessively, the hardenability of the Mn segregated portion is excessively increased, and a supercooled structure such as martensite may be generated. Therefore, the amount of Mn is set to 1.5% or less. The amount of Mn is preferably 1.4% or less, more preferably 1.3% or less.

(N: 0.001 to 0.006%)
N combines with B in the steel to form BN, and the effect of B is lost. Further, N in a solid solution state causes a decrease in torsional characteristics due to strain aging during wire drawing, and if it is remarkable, causes vertical cracks. In order to prevent these harmful effects, the N content is 0.006% or less. The N amount is preferably 0.005% or less, and more preferably 0.004% or less. On the other hand, if the amount is small, there is an effect that the crystal grains are refined by a nitride such as TiN or AlN and the ductility of the wire is increased. In order to exhibit such an effect, the N amount is set to 0.001% or more. The N amount is preferably 0.0015% or more, more preferably 0.0020% or more.

(Al: 0.001 to 0.10%)
Al is an effective deoxidizing element. It also has the effect of forming a nitride such as AlN to refine the crystal grains. In order to effectively exhibit such an effect, the Al content is set to 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 excessively, an oxide such as Al ₂ O ₃ is formed, and the disconnection at the time of wire drawing is increased. From such a viewpoint, the Al amount is set to 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 forms carbides such as TiC and has a function of reducing the diffusion coefficient of hydrogen and improving the fatigue characteristics of the steel wire. Moreover, it combines with N in the steel to form a nitride such as TiN, and has the function of preventing the twisting characteristics from being lowered by N. In order to effectively exhibit these effects, the Ti content is 0.02% or more. The amount of Ti is preferably 0.03% or more, more preferably 0.04% or more. On the other hand, when the amount of Ti becomes excessive, a large amount of Ti-based inclusions such as TiC and TiN are precipitated, increasing the disconnection at the time of wire drawing. Therefore, the Ti content is 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 sometimes abbreviated as “pre-deposition α”) precipitated at the grain boundaries, and is effective in improving fatigue characteristics. Moreover, by forming BN, the effect of fixing the solid solution N in steel and improving the twisting property can be expected. In order to effectively exhibit the effect of B, the amount of B needs to be 0.0005% or more. The lower limit of the preferable amount of B is 0.0007% or more, more preferably 0.001% or more. On the other hand, when the amount of B is excessive, Fe—B compounds such as FeB _{2, which} are compounds with Fe, precipitate and cause cracking during hot rolling, so the amount of B needs to be 0.010% or less. is there. The amount of B is preferably 0.008% or less, and more preferably 0.006% or less.

(P: 0% or more, 0.030% or less)
P segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and lowers fatigue strength. Therefore, the smaller the content, the better. Therefore, the P content is 0.030% or less. The amount of P is preferably 0.025% or less, and more preferably 0.020% or less. The amount of P may be 0%, but is usually contained at 0.001% or more.

(S: 0% or more, 0.030% or less)
S, like P, segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and lowers fatigue strength. Therefore, the content is preferably as small as possible. Therefore, the S amount 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 at 0.001% or more.

The basic components of the wire rod of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel.

In addition, the wire of the present invention further improves properties such as strength, toughness, ductility, etc.
(A) Cr: more than 0%, 1.0% or less and V: more than 0%, 0.5% or less,
(B) Ni: more than 0%, 0.5% or less and Nb: at least one kind of more than 0%, 0.5% or less,
(C) Co: more than 0%, 1.0% or less,
(D) Mo: more than 0%, 0.5% or less and Cu: more than 0%, 0.5% or less,
Etc. are also preferable.

(Cr: more than 0%, 1.0% or less and V: more than 0%, 0.5% or less)
Cr and V are elements useful in increasing the strength (tensile strength) of the wire, and these may be used alone or in combination of two or more.

Especially, Cr has the effect of increasing the strength and toughness of the wire rod by reducing the pearlite lamella spacing. In order to effectively exhibit such action, the Cr content 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 amount of Cr becomes excessive, the hardenability is improved and the risk of generating a supercooled structure during hot rolling increases, so the Cr amount 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. Further, in the same manner as Nb, nitride forms excessive solid solution N and nitride after precipitation of AlN and contributes to refinement of crystal grains, and also has an effect of suppressing aging embrittlement by fixing solid solution N. In order to effectively exhibit such an action, the V amount is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.03% or more. However, V is an expensive element, and even if added excessively, the effect is saturated and economically wasteful, so the V amount is preferably 0.5% or less, more preferably 0.4% or less, More preferably, it is 0.2% or less.

(Ni: more than 0%, 0.5% or less and Nb: more than 0%, 0.5% or less)
Ni and Nb are elements useful for increasing the toughness of the steel wire, and these may be used alone or in combination of two or more.

Especially Ni is an element that enhances the toughness of the steel wire after drawing. In order to effectively exhibit such an action, the Ni content is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.2% or more. However, even if Ni is added excessively, the effect is saturated and it is economically wasteful. Therefore, the Ni content is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Nb, like Ti and Al, forms a nitride, contributes to improving the toughness of the steel wire by refining crystal grains, and also has the effect of suppressing aging embrittlement by fixing solid solution N. In order to effectively exhibit such an action, the Nb content is preferably 0.01% or more, more preferably 0.03% or more, and still more preferably 0.05% or more. However, Nb is an expensive element, and even if added excessively, the effect is saturated and economically wasteful, so the amount of Nb is preferably 0.5% or less, more preferably 0.4% or less, More preferably, it is 0.3% or less.

(Co: over 0%, 1.0% or less)
Co has an effect of reducing generation of pro-eutectoid cementite and making the structure a uniform pearlite structure particularly when the amount of C is high. In order to effectively exhibit this action, the Co content 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 and it is economically wasteful. Therefore, the Co content is preferably 1.0% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.

(Mo: more than 0%, 0.5% or less and Cu: more than 0%, 0.5% or less)
Mo is an element that improves the corrosion resistance of the steel wire. In order to effectively exhibit such action, the Mo amount is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.2% or more. However, when the amount of Mo becomes excessive, a supercooled structure is likely to occur during hot rolling, and ductility also deteriorates. Therefore, the Mo amount is preferably 0.5% or less, more preferably 0.4% or less, and still more preferably 0.3% or less.

Cu, like Mo, is an element that improves the corrosion resistance of steel wires. In order to effectively exhibit such an action, the amount of Cu is preferably 0.05% or more, more preferably 0.08% or more, and further preferably 0.10% or more. However, when the amount of Cu becomes excessive, it reacts with S to segregate CuS at the grain boundary part, and generates soot in the wire manufacturing process. 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 or two kinds.

Next, a method for producing the steel wire rod according to the present invention will be described.

The wire rod before cold drawing is usually produced by melting, split-rolling and hot-rolling steel with appropriately controlled chemical components, and further performing patenting treatment as necessary. In producing the wire of the present invention while satisfying the requirements (metallographic structure, hydrogen diffusion coefficient) defined in the present invention, the Ti content is appropriately controlled within the above range, and then Ti system such as TiC is used. It is important to appropriately control the precipitation behavior of inclusions.

First, in the partial rolling, it is preferable that the slab is heated to 1200 ° C. or higher to decompose the coarse TiC precipitated during casting. When the heating temperature is lower than 1200 ° C., coarse TiC remains in the wire and the hydrogen diffusion coefficient cannot be sufficiently reduced, so that the fatigue strength is lowered. This heating temperature is more preferably 1250 ° C. or more, and further preferably 1300 ° C. or more. However, if the heating temperature becomes too high, melting of the wire material occurs, so the temperature is usually set to about 1400 ° C.

Subsequently, in performing hot rolling, after heating to 1000 ° C. or higher, the strain rate in the final four passes of rolling is 0.5 second ⁻¹ or more, and the crystal grains are refined by dynamic recrystallization, It is preferable to deposit fine TiC. If the strain rate is less than 0.5 sec ⁻¹ , TiC cannot be sufficiently miniaturized and the hydrogen diffusion coefficient D cannot be sufficiently reduced. The strain rate at this time is more preferably 0.8 sec ⁻¹ or more, and further preferably 1.0 sec ⁻¹ or more. However, from the viewpoint of equipment load, the strain rate is usually preferably 5 seconds ^-1 or less. Note that the strain rate Vε is equal to the cross-sectional area S ₀ (m ² ) before entering the first roll, four rolls before the final pass, and the cross-sectional area S ₄ (m ² ) after passing the final pass. And the total passage time (rolling time) t (seconds) of 4 passes can be expressed by the following equation (2).
Vε = {ln (S ₀ / S ₄ )} / t (2)

After the hot rolling, it is preferable to sufficiently cool with water cooling and control the mounting temperature of the rolled material (wire material) on the lay head to 800 to 1000 ° C. When the mounting temperature exceeds 1000 ° C., TiC may be coarsened during cooling on the conveyor after mounting. The mounting temperature is more preferably 980 ° C. or lower, and further preferably 950 ° C. or lower. Further, when the mounting temperature is less than 800 ° C., the deformation resistance of the wire is increased, and there is a possibility that a mounting failure at the laying head may occur, for example, coiling is impossible. Accordingly, the mounting temperature is preferably 800 ° C. or higher. The mounting temperature is more preferably 820 ° C. or higher, and further preferably 850 ° C. or higher.

After placing, the wire is cooled on a cooling conveyor, and pearlite transformation is caused during this cooling, but it is preferable to rapidly cool the pearlite transformation at an average cooling rate of 5 ° C./second or more. If the average cooling rate at this time is slow, TiC is likely to be coarsened and the hydrogen diffusion coefficient may be increased. Further, when the average cooling rate is less than 5 ° C./second, a structure with extremely rough lamellar spacing called “corse pearlite” is deposited locally, which may reduce the drawability. In addition, about the start of pearlite transformation, the temperature of a wire may be measured and the point (inflection point) where a cooling curve may change by transformation heat_generation | fever should just be calculated | required. The average cooling rate is more preferably 10 ° C./second or more, and further preferably 15 ° C./second or more. A preferable upper limit of the average cooling rate is 100 ° C./second or less, and more preferably 50 ° C./second or less.

The wire obtained as described above can be used as a steel wire after being drawn (cold working) as it is, but may be subjected to a patenting treatment before the drawing. By performing such a patenting process before wire drawing, the strength of the wire can be increased and the variation in strength can be reduced.

In addition, when it is expected that the degree of wire drawing will increase, as in the case of manufacturing a small diameter steel wire, after a certain degree of drawing from the rolled material, a patenting process is performed, and the wire material structure is converted into an unprocessed pearlite structure. It is also useful to perform wire drawing after returning to the above. At this time, if finely precipitated TiC is maintained even by the reheating process, the hydrogen diffusion coefficient D can be kept low even by a general patenting process.

The heating temperature when the patenting treatment is performed (hereinafter, this temperature may be referred to as “reheating temperature”) is preferably about 900 to 1000 ° C., more preferably 920 ° C. or more and 980 ° C. or less. The reheating temperature is preferably 900 ° C. or higher from the standpoint of preventing undissolved carbide from remaining and making the structure completely austenitic. However, when the temperature is too high, TiC becomes coarse and the hydrogen diffusion coefficient D increases. To do. The holding temperature in the patenting treatment is preferably about 530 to 600 ° C., more preferably 550 ° C. or higher and 580 ° C. or lower.

Since the wire diffusion material D of the present invention has a sufficiently reduced hydrogen diffusion coefficient D in steel, a steel wire obtained by cold working the wire, a wire rope using the steel wire in whole or in part, a PC steel wire, etc. These products have better fatigue properties than normal products.

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 incorporated herein by reference.

Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

Steel ingots having the chemical composition shown in Table 1 below are subjected to split rolling and hot rolling under the conditions shown in Table 2 below to form wire coils, and some of them are further subjected to the conditions shown in Table 3 below. A patenting process was performed. The difference between the rolling wire diameters shown in Table 2 below and the patenting wire diameters shown in Table 3 below indicates that the heat treatment was performed with the intermediate wire in between.

Using a sample collected from the wire before the final drawing, a tensile test, evaluation of the metal structure, and measurement of the hydrogen diffusion coefficient D were performed by the following methods.

(Tensile test)
Tensile strength TS (Tensile Strength) of the collected sample was measured according to JIS Z 2241 (2011). Eight steel wires were produced and the average value was calculated. The results are shown in Table 4 below.

(Evaluation of metal structure)
The metal structure was observed with an optical microscope for the cross section of the collected sample. In addition, in the item of the metal structure in Table 4 below, “P” indicates that the pearlite structure is 95 area% or more, that is, pearlite is the main phase. In addition, those indicated as “P + α” and “P + θ” have a pearlite structure of less than 95 area%, and a structure in which ferrite (α) and cementite (θ) larger than 5 area% are mixed in addition to the pearlite structure Show.

(Measurement of hydrogen diffusion coefficient D)
The collected sample was charged with hydrogen until saturated, and a hydrogen release curve was obtained by temperature analysis. The rate of temperature increase was 12 ° C./second, and the amount of hydrogen released was measured with an atmospheric pressure ionization mass spectrometer. The hydrogen charging conditions were as follows: H ₂ SO ₄ aqueous solution (pH 3) + KSCN 0.01 mol / L was used as the electrolyte, and charging was performed at a current density of 5 mA / cm ² for 48 hours. In addition, the sample after charge was stored in liquid nitrogen until measurement in order to prevent hydrogen from being dispersed as much as possible.

The obtained hydrogen release curve approximates that the hydrogen is uniformly distributed in the sample, assumes a sample shape of an infinite cylinder, and shows a hydrogen release curve obtained by numerical calculation using the diffusion coefficient as a parameter. The hydrogen diffusion coefficient D was determined by fitting. The results are shown in Table 4 below. In order to make the infinite cylinder approximation effective, the sample length was set to 5 times the diameter or more. At this time, the peak of hydrogen release used for fitting was a peak having a peak temperature of 200 ° C. or higher. The low temperature peak appearing at 200 ° C. or lower is called diffusible hydrogen, and is not used for evaluating the diffusion coefficient because it can be influenced by disturbances such as being released even at room temperature. From the correlation curve between the temperature thus obtained and the diffusion coefficient, the diffusion coefficient at 300 ° C. was determined as the hydrogen diffusion coefficient D.

Next, the obtained wire coil was drawn to produce a steel wire (wire), and a tensile test, a twist property evaluation, a fatigue property evaluation, and a hydrogen diffusion coefficient D were measured. Table 5 below shows the reduction in area during wire drawing and the wire diameter of the steel wire obtained by wire drawing.

(Tensile test)
The tensile strength TS and the yield point YP (Yield Point) of the steel wire were measured according to JIS Z 2241 (2011). Eight steel wires were produced and the average value was calculated. 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 twisting characteristics)
The twisting property was evaluated based on the twist value (number of times of rupture twist) required for rupture after conducting a twist test. The twist value in the following Table 5 is an average value of N = 5. At this time, the twisting speed was 52 times / minute, and the tension was 500 gf (4.9 N). The twist value was normalized by converting the distance between the chucks (test wire length) to 100 times the wire diameter d (100d). Moreover, the normal fracture surface and the vertical crack were discriminated by the fracture surface observation, and even one of the five vertical cracks was described as “with vertical crack” in Table 5 below.

(Evaluation of fatigue characteristics)
The fatigue characteristics were evaluated by repeatedly performing a four-point bending fatigue test using a jig that supported four points. In FIG. 1, 1 is a test piece (wire), 2 is a direction in which repeated stress is applied, and ◯ is a support point. The test was performed by half bending, and the difference between the maximum stress and the minimum stress was defined as the stress amplitude. Bending was performed 100,000 times with various stress amplitudes, and N = 3 tests that were not broken (disconnected) were all accepted, and even one that was broken was judged as unacceptable. The maximum stress amplitude in the sample determined to be acceptable was defined as 100,000 times fatigue strength. The 100,000 times fatigue strength is shown in Table 5 below. The stress waveform was a sine wave and the frequency was 10 Hz.

(Measurement of hydrogen diffusion coefficient D)
For the steel wire, the hydrogen diffusion coefficient D was determined as a reference value under the same conditions as described above. The results are shown in Table 5 below.

From these results, it can be considered as follows.

First, test No. 1 to 3 and 9 to 20 are described in JIS G 3522 (1991) IV because the chemical composition, metal structure (perlite area ratio), and hydrogen diffusion coefficient D are all within the ranges specified in the present invention. Steel wire that achieves a fatigue strength exceeding 0.45 times the tensile strength TS after obtaining a tensile strength exceeding the “Class B Piano wire” (standard, for example, 1620 to 1770 MPa when the wire diameter is 7.0 mm) (Wire) is obtained.

In contrast, test no. Examples 4 to 8 and 21 to 26 are examples in which any of the requirements of the present invention is not satisfied. Of these, test no. In No. 4, since the heating temperature at the time of the ingot rolling was low, coarse TiC precipitated, the hydrogen diffusion coefficient D increased, and the fatigue strength decreased.

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

Test No. In No. 6, since the mounting temperature after hot rolling was low, a mounting failure occurred and a sample was not obtained.

Test No. No. 7 has a high mounting temperature after hot rolling. In No. 8, since the cooling rate after rolling was slow, TiC coarsened, the hydrogen diffusion coefficient D increased, and the fatigue strength decreased.

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

Test No. No. 22 is an example using the steel type Q having a large amount of C. Since a large amount of pro-eutectoid cementite precipitated, the wire was broken during wire drawing.

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

Test No. No. 24 is an example using the steel type S with a large amount of Ti. A large amount of Ti-based inclusions were precipitated and disconnected during wire drawing.

Test No. 25 is an example using the steel type T having a large amount of B, and the sample was not obtained due to disconnection during hot rolling.

Test No. No. 26 is an example using a steel type U with a small amount of B, and the twisting characteristics and fatigue strength were lowered. The hydrogen diffusion coefficient D is also increased.

Claims

% 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: 0% or more, 0.030% or less,
S: 0% or more, 0.030% or less,
Each of which is iron and inevitable impurities,
With pearlite as the main phase,
A steel wire rod having a hydrogen diffusion coefficient D in steel at 300 ° C. that satisfies the following formula (1).
D ≦ 2.5 × 10 −7 (cm 2 / sec) (1)
The steel wire rod according to claim 1, further comprising one or more of the following (a) to (d) in mass%.
(A) Cr: more than 0%, 1.0% or less and V: more than 0%, 0.5% or less (b) Ni: more than 0%, 0.5% or less and Nb: more than 0% (C) Co: more than 0%, 1.0% or less (d) Mo: more than 0%, 0.5% or less and Cu: more than 0%, 0.5% or less At least one of
A steel wire comprising the chemical composition of the steel according to claim 1 or 2, wherein a hydrogen diffusion coefficient D in the steel at 300 ° C satisfies the following formula (1).
D ≦ 2.5 × 10 −7 (cm 2 / sec) (1)