WO2016002413A1

WO2016002413A1 - Wire material for steel wire, and steel wire

Info

Publication number: WO2016002413A1
Application number: PCT/JP2015/065863
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: EP3378964A1; EP3165625A4; EP3165625A1; JP2016014168A; KR20170013340A; MX2016017005A; CN106661687A; US20170198375A1; CA2951799A1; CN109576448A

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, the area ratio of proeutectoid ferrite being 1.0% or less, and the average thickness of the proeutectoid ferrite being 5 μm or less.

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, the need for weight reduction of ropes accompanying the increase in the speed of elevators and the miniaturization of cranes has increased, and a high-strength steel wire rod excellent in bending fatigue characteristics that realizes it has been demanded. In addition, a high-strength wire rod for steel wire excellent in bending fatigue characteristics is also useful as a material for a PC (Pressed Concrete) steel wire. 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 obtaining a high-strength wire by controlling the wire structure to a pearlite structure in which the area ratio of pro-eutectoid ferrite is 3% or less by directly performing a melt salt patenting treatment after hot rolling. It is disclosed.

Further, in Patent Document 3, the metal structure of the wire is a pearlite structure of 95% or more, and the maximum value and the average value of the pearlite block particle size of the pearlite at the center of the cross section perpendicular to the axial direction of the wire are within a predetermined range. A technique for obtaining a highly ductile wire rod by controlling is disclosed. This technique also discloses that it is useful to adjust the volume fraction of pro-eutectoid ferrite to 2% or less in order to improve the wire drawing workability.

JP 2011-225990 A JP 2007-39800 A International Publication No. 2007/139234

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 that are exposed to the outside air for a long time, such as wire rope, cracks are likely to occur in the surface layer due to oxidation of the surface layer, penetration of hydrogen, friction between strands, etc., far exceeding the original fatigue limit Therefore, it is necessary to take measures to suppress the crack growth.

Moreover, in the technique of the above-mentioned Patent Document 2, in order to obtain a high-strength wire, special equipment that can perform a patenting process directly after hot rolling is required, which increases capital investment. In addition, such equipment also has a disadvantage that it is inferior in productivity and maintainability as compared with a so-called steermore cooling equipment that cools a wire rod while being conveyed on a conveyor. In addition, only by reducing the area ratio of pro-eutectoid ferrite in the wire, sufficient effect of improving low cycle fatigue characteristics cannot be exhibited.

In addition, if only the requirements described in Patent Document 3 are specified, sufficient effects cannot be obtained with respect to the low cycle fatigue characteristics.

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 it is providing the steel wire which can exhibit such a characteristic.

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 and 0.030% or less, the balance being iron and inevitable impurities, pearlite as the main phase, and the area ratio of proeutectoid ferrite is 1. It has a gist in that it is 0% or less and the average thickness of pro-eutectoid ferrite is 5 μm or less.

Incidentally, “with pearlite as the main phase” means that 95% by area or more of the metal structure is a pearlite structure. The average thickness of pro-eutectoid ferrite means the average value of thickness in the width direction of pro-eutectoid ferrite when the pro-eutectoid ferrite is observed with an optical microscope.

The wire for steel wire of the present invention is further 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.

In the wire for steel wire of the present invention, the content of solute B is preferably 0.0003% or more.

The present invention also includes a steel wire having the chemical composition of the steel described above and having a 100,000 times fatigue strength σ satisfying the relationship of the following formula (1) with a tensile strength TS (Tensile Strength).
σ> 0.45TS (1)

According to the present invention, the bending fatigue strength of a steel wire after cold working (drawing) can be reduced by reducing the area ratio of proeutectoid ferrite in the steel wire before drawing and reducing the thickness thereof. It is possible to improve the fatigue characteristics. 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. FIG. 2 is a drawing-substituting micrograph showing an example of the observed pro-eutectoid ferrite grains.

The present inventors diligently investigated the factors that influence the low cycle fatigue characteristics in a steel wire material having a metal structure mainly composed of pearlite. As a result, it was found that pro-eutectoid ferrite slightly precipitated in the pearlite structure (hereinafter sometimes abbreviated as “pre-deposition α”) promotes the progress of fatigue cracks. In a high carbon steel with a carbon content of 0.70% or more, as shown in FIG. 2 to be described later, the pro-eutectoid α precipitates in a plate shape at the prior austenite grain boundaries. The present inventors completed the present invention by finding out that excellent low cycle fatigue characteristics can be exhibited by reducing the thickness after setting the content to 1.0% or less.

In steel wire rods with a pearlite structure as the main phase, voids are generated at the interface between pro-eutectoid α and pearlite, which promotes the development of fatigue cracks. Therefore, it is important to reduce the area ratio of proeutectoid α as much as possible and reduce the amount of the interface. Moreover, the effect which suppresses the vertical crack at the time of a twist test is also acquired by reducing the area ratio of pro-eutectoid (alpha). If a vertical crack occurs, the steel wire that is longitudinally cracked is judged to be defective because it cannot withstand the stranded wire processing. Considering these effects, it is necessary to make the area ratio of pro-eutectoid α 1.0% or less as a percentage of the entire metal structure. The area ratio of pro-eutectoid α is preferably 0.8% or less, and more preferably 0.6% or less.

B is effective to reduce the area ratio of proeutectoid α. The area ratio reduction effect of the pro-eutectoid α is exhibited when B exists as a solid solution B, and the amount deposited as a compound such as BN loses that effect. Therefore, in the steel wire rod of the present invention, it is necessary to control the N content and the B content within appropriate ranges, and it is preferable to manufacture them under manufacturing conditions in which BN hardly precipitates.

On the other hand, when the thickness of the pro-eutectoid α increases, the void expands due to the stress concentration on the void generated at the interface, which promotes the progress of fatigue cracks and decreases the fatigue strength. The pro-eutectoid α having a small thickness is deformed and rendered harmless by the wire drawing process, but the pro-eutect α having a large thickness remains after the wire drawing process and may be called bending fatigue strength (hereinafter simply referred to as “fatigue strength”). Lower). Specifically, the average thickness of the pro-eutectoid α needs to be 5 μm or less. The average thickness of pro-eutectoid α is preferably 4 μm or less, and more preferably 3 μm or less.

In order to reduce the average thickness of pro-eutectoid α, Ti inclusions such as TiC are finely dispersed in the steel, particularly in the vicinity of grain boundaries, and a large number of pro-eutectoid α precipitation nuclei are formed and the nuclei grow. It is effective to suppress this. For this purpose, it is necessary to control the amount of Ti in the steel wire to an appropriate range, and it is preferable that the Ti-based inclusions such as TiC are produced under production conditions that are likely to precipitate finely.

The steel wire rod according to the present invention needs to have its chemical composition appropriately adjusted from the viewpoint of exerting its basic characteristics when applied to a wire or the like. The chemical component composition including the amounts of B, N, and Ti described above 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. The amount of C also affects the amount of precipitation of pro-eutectoid α. If the amount of C is small, precipitation of pro-eutectoid α cannot be sufficiently suppressed. 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 0.20% 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 serves to reduce the particle size (thickness) of pro-eutectoid α. 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, more preferably 0.10% or less.

(B: 0.0005 to 0.010%, preferably 0.0003% or more as solute B)
B functions to prevent the generation of proeutectoid α and reduce the area ratio. However, such a function is not exhibited when a compound such as BN is formed. 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. Moreover, it is preferable to contain 0.0003% or more as solid solution B in steel, More preferably, it is 0.0005% or more.

(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 the effect of reducing the formation 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 rod of the present invention while satisfying the requirements (metal structure, pro-eutectoid α area ratio, pro-eutectoid α average thickness) defined in the present invention, the contents of Ti, B and N are within the above ranges. It is important to appropriately control the precipitation behavior of TiC and BN while appropriately controlling.

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 thickness of the pro-eutectoid α 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, when performing hot rolling, by heating to a temperature range of 1000 ° C. or higher, the coarse BN in the billet is sufficiently decomposed, and then sufficiently cooled by water cooling after rolling, and rolled material ( It is preferable to control the mounting temperature of the wire) on the laying head to 800 to 1000 ° C. When the mounting temperature exceeds 1000 ° C., a large amount of BN is precipitated in the wire during cooling on the conveyor after mounting, and there is a possibility that the solid solution B cannot be sufficiently secured. 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 increases, and there is a possibility that a mounting failure in the laying head may occur, for example, coiling cannot be performed. 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.

Further, when hot rolling is performed, it is preferable to set the strain rate in the final four passes of rolling to 0.5 second ⁻¹ or more, to refine crystal grains by dynamic recrystallization, and to precipitate fine TiC. . If the strain rate is less than 0.5 sec ⁻¹ , TiC cannot be sufficiently refined, and the average thickness of proeutectoid α 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 mounting, 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. When the average cooling rate at this time is slow, the pro-eutectoid α is likely to precipitate and become coarse at high temperatures, and the thickness of the pro-eutectoid α may not be sufficiently reduced. 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 by drawing (cold working) as it is, but it 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 thin steel wire, the wire material structure is subjected to a patenting treatment after drawing to some extent from the rolled material, and the wire material structure is converted into an unprocessed pearlite. It is also useful to perform further wire drawing after returning to the tissue. At this time, by performing a patenting treatment, the pro-eutectoid α obtained during hot rolling is lost, but if finely precipitated TiC and a sufficient amount of solute B are secured, general patenting can be performed. An appropriate area ratio and average thickness of pro-eutectoid α can be obtained depending on processing conditions.

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 more from the viewpoint of preventing the remaining of insoluble carbides and making the structure completely austenitic. However, when the temperature is too high, TiC becomes coarse or solute B is N. In some cases, the area ratio and average thickness of a predetermined pro-eutectoid α may not be obtained. 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.

The wire rod according to the present invention has a sufficiently reduced amount of pro-eutectoid α that promotes the generation and propagation of fatigue cracks, and the thickness thereof is controlled to be small. Products such as wire ropes and PC steel wires that use all or part of the wires have better fatigue properties than normal products. Generally, the tensile strength and fatigue strength are in a proportional relationship, but the steel wire manufactured from the wire of the present invention has a 100,000 times fatigue strength σ satisfying the relationship of the following formula (1) with the tensile strength TS. The present invention also includes such a steel wire. The present invention also includes products such as wire ropes manufactured using all or part of such steel wires.
σ> 0.45TS (1)

This application claims the benefit of priority based on Japanese Patent Application No. 2014-136222 filed on July 1, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-136222 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 samples taken from the wire before the finish drawing, tensile test, evaluation of metal structure (area ratio of pro-eutectoid α, pearlite area ratio, average thickness of pro-eutectoid α), measurement of solid solution B amount are as follows. Performed by method.

(Tensile test)
Tensile strength TS (Tensile Strength) of the collected sample was measured according to JIS Z 2241 (2011). The results are shown in Table 4 below.

(Evaluation of area ratio of proeutectoid α)
The area ratio of proeutectoid α was measured by embedding the collected sample in resin or the like, mirror polishing, observing with an optical microscope using a mixed solution of trinitrol phenol and ethanol, and measuring the area ratio by image analysis. . The part whitened by the above-mentioned corrosive liquid is the pro-eutectoid α. When the diameter of the wire was D, the D / 4 portion of the cross section was considered as the representative structure, and images were taken at a magnification of 400 times to evaluate a total of 5 fields of view. The “area ratio of pro-eutectoid α” shown in Table 4 below shows the average value. In addition, a cross section points out a surface perpendicular | vertical with respect to a wire rod longitudinal direction.

Moreover, the area ratio of pearlite was also measured by this method. 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. Moreover, what is indicated as “P + α” or “P + θ” indicates a structure in which the pearlite structure is less than 95 area%, and in addition to the pearlite structure, ferrite (α) and cementite (θ) are mixed.

(Evaluation of average thickness of proeutectoid α)
A sample polished in the same manner as described above was observed by SEM (Scanning Electron Microscope), and the thickness of 10 of the observed pro-eutectoid α grains was measured, and the average value was obtained to calculate the thickness per piece. did. The measurement was performed at D / 4 part of the cross section in the same manner as above. The results are shown in Table 4 below.

(Measurement of solid solution B amount)
The amount of solute B was evaluated by measuring the electrolytic extraction residue. Electrolytic extraction residue measurement using a 10% acetylacetone solution was carried out, and the amount of compound type B in the residue was measured by a bromester method using a mesh with an opening of 0.1 μm. The solid solution B amount was determined by subtracting the compound type B amount from the total B amount in the steel. The results are shown in Table 4 below. The sample used for the bromester method was 3 g. Moreover, since the amount of solid solution B does not change unless it receives a heat history of 900 ° C. or higher, the amount of solid solution B may be investigated with a steel wire after cold working.

Next, the obtained wire coil was drawn to produce a steel wire (wire), and a tensile test, a twisting property evaluation, and a fatigue property evaluation were performed. 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). 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 passed sample was defined as 100,000 times fatigue strength σ. The 100,000 times fatigue strength σ is shown in Table 5 below. The stress waveform was a sine wave and the frequency was 10 Hz.

From these results, it can be considered as follows.

First, test No. For 1-3, 10-21, the chemical component composition and the metal structure (perlite area ratio, area ratio of pro-eutectoid α, average thickness of pro-eutectoid α) are all within the ranges specified by the present invention. After obtaining a tensile strength (standard, for example, 1620 to 1770 MPa when the wire diameter is 7.0 mm) exceeding “Piano wire type B” described in G 3522 (1991), the relationship of the above equation (1) is obtained. A steel wire (wire) that achieves satisfactory fatigue strength is obtained.

In contrast, test no. Examples 4 to 9 and 22 to 27 are examples in which any of the requirements of the present invention is not satisfied. Of these, test no. As shown in Table 2, since the heating temperature at the time of the block rolling was low as shown in Table 2, coarse TiC was precipitated, and as shown in Table 4, the average thickness of proeutectoid α was increased and the fatigue strength was lowered.

Test No. As shown in Table 2, since the heating temperature during hot rolling was low as shown in Table 2, the area ratio of proeutectoid α increased as shown in Table 4, the solid solution B also decreased, and the fatigue strength decreased.

Test No. In Table 6, since the strain rate during finish rolling was small as shown in Table 2, coarse TiC was precipitated, and as shown in Table 4, the average thickness of pro-eutectoid α was increased, and the fatigue strength was lowered.

Test No. As shown in Table 2, since the mounting temperature after hot rolling was low as shown in Table 2, a mounting failure occurred and a sample was not obtained.

Test No. As shown in Table 2, since the mounting temperature after hot rolling was high as shown in Table 2 and TiC was coarsened, the average thickness of proeutectoid α was increased as shown in Table 4 and the fatigue strength was lowered.

Test No. As shown in Table 2, the average cooling rate after mounting was slow as shown in Table 2, the average thickness of proeutectoid α was increased as shown in Table 4, and the fatigue strength decreased.

Test No. No. 22 is an example using the steel type P having a small amount of C. As shown in Table 4, both the area ratio and the average thickness of the pro-eutectoid α were increased, and the twisting characteristics and fatigue strength were lowered.

Test No. No. 23 is an example using the steel type Q having a large amount of C. Since a large amount of proeutectoid cementite was precipitated, it was broken during wire drawing.

Test No. No. 24 is an example using the steel type R with a small amount of Ti. The amount of TiC was small, the average thickness of proeutectoid α was increased, and the fatigue strength was lowered.

Test No. No. 25 is an example using the steel type S with a large amount of Ti, and a large amount of Ti-based inclusions were precipitated and disconnected during wire drawing.

Test No. No. 26 is an example using the steel type T having a large amount of B, and a sample was not obtained due to disconnection during hot rolling.

Test No. 27 is an example using the steel type U with a small amount of B. The area ratio of the pro-eutectoid α was increased, and the twisting characteristics and fatigue strength were reduced.

FIG. 2 shows test No. 1 as an example. 3 is a drawing-substituting micrograph showing an example of pro-eutectoid α observed in FIG. An ellipse 3 shown in FIG. 2 indicates the deposition position of proeutectoid α. From FIG. 2, it can be seen that the pro-eutectoid α is precipitated in a plate shape, and the “width direction” and “length direction” of the grains can be easily distinguished.

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,
While the area ratio of pro-eutectoid ferrite is 1.0% or less,
A steel wire rod having an average thickness of pro-eutectoid ferrite of 5 μm or less.
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
The wire rod for steel wire according to claim 1 or 2, wherein the content of solute B is 0.0003% or more.
A steel wire comprising the chemical composition of the steel according to claim 1 or 2 and having a 100,000 times fatigue strength σ satisfying the relationship of the following formula (1) with the tensile strength TS.
σ> 0.45TS (1)