KR20110099749A - Wire rod, steel wire, and method for manufacturing wire rod - Google Patents

Abstract

The present invention is 0.90 to 1.30% by mass of C, 0.1 to 1.2% by mass of Si, 0.1 to 1.0% by mass of Mn, more than 0.1% by mass of less than 0.6% by mass of Al, and 0 to 0.02% by mass of P And a wire rod containing at least 0 to 0.02% by mass of S, 10 to 60 ppm of N, 10 to 40 ppm of O, and a remainder containing Fe and unavoidable impurities, the wire being perpendicular to the longitudinal direction of the wire. An area of 97% or more of the cross section is occupied by the pearlite structure, and an area of 0.5% or less of the central region of the cross section and an area of 0.5% or less of the first surface layer region of the cross section are occupied by the cornerstone cementite structure. Provide wire rods.

Description

Wire rod, steel wire and wire rod manufacturing method {WIRE ROD, STEEL WIRE, AND METHOD FOR MANUFACTURING WIRE ROD}

The present invention relates to a wire rod, a steel wire and a method for producing a wire rod. More specifically, the present invention relates to a rolled wire rod suitable for the use of a piano wire, a PC steel wire, a PC stranded wire, a rope, a bridge PWS wire, a production method thereof, and a steel wire obtained by stretching the rolled wire rod.

This application claims priority in 2010/25/25 based on patent application 2010-013278 for which it applied to Japan, and uses the content for it here.

Steel wires used as PC steel wires, PC stranded wires, ropes, PWS wires for bridges, and the like are generally wire-drawn wires having a diameter of 5 to 16 mm adjusted and cooled after hot rolling to have a diameter of 2 to 2 It is made into 8 mm, and it shape | molds to strand shape by performing hot dip galvanizing after drawing or in the middle of drawing as needed, and binding together without twisting each other or twisting each other.

In general, if longitudinal cracking (delamination) occurs when the wire rod is processed into steel wire, or when the wire is twisted, productivity and yield are greatly reduced. Therefore, it is strongly required that the wire rod and steel wire which belong to the said technical field do not disconnect at the time of a wire drawing, a twisting, or a binding process.

Such products have been required to secure strength of at least 1600 MPa and at the same time, to secure sufficient performance for toughness and ductility evaluated by torsion tests and the like.

For this reason, high intensity | strength is calculated | required with respect to the above-mentioned various products, and the desired high intensity | strength is not acquired by carbon steel wire rod etc. whose C content is less than 0.9 mass%. For this reason, the request | requirement to the steel wire of C content of 0.9 mass% or more is increasing. However, when the C content is increased, wire workability and torsion characteristics (radiation resistance) are lowered by the formation of the cornerstone cementite (hereinafter sometimes referred to as the cornerstone θ), thereby increasing the frequency of disconnection. For this reason, the C content is high, high strength can be ensured for a steel wire, and the demand for the wire rod excellent also in drawing property is very large.

With respect to the above-mentioned demands from the industry, a technique for producing high carbon wire rods in excess of 1% has been proposed.

For example, Patent Literature 1 discloses a "wire material for high strength high toughness microwires, high strength high toughness microwires, and a twist product using the microwires", which is made of steel having a specific chemical composition and defines the average area ratio of cementite cementite. twisted product) and a method for producing the ultrafine steel wire. However, since the wire rod proposed in this publication contains at least one of Ni and Co which are expensive elements as an essential component, manufacturing cost increases.

In patent document 2, the technique which suppresses generation | occurrence | production of the cornerstone cementite of the high carbon steel exceeding 1% by adding 0.6% or more of Al is proposed. However, Al is a strong deoxidation element, and since excessive addition increases the amount of hard inclusions which cause disconnection in drawing, it is difficult to apply it to the wire rod for high strength steel wire. In addition, since excessive Al addition promotes bainite formation, it is difficult to obtain a uniform pearlite structure.

On the other hand, in patent document 3, after heating a high carbon wire rod to an austenite temperature range, it cools to the temperature range of 823-1023K, and after carrying out the plastic working of 15-80% of the workability in this temperature range, 823-923K. A technique for suppressing saltpeter cementite has been proposed by constant temperature transformation in the temperature range of. However, in order to perform predetermined | prescribed processing in such a temperature range, large-scale equipment investment is needed and there exists a possibility of causing an increase in manufacturing cost.

Japanese Patent No. 2609387 Japanese Patent Application Publication No. 2003-193129 Japanese Patent Application Laid-open No. Hei 8-283867

This invention is made | formed in view of the said phenomenon, The objective is to provide the high-strength wire excellent in the ductility suitable for uses, such as PC steel wire, PC stranded wire, and PWS wire for bridges, at low cost with high productivity.

MEANS TO SOLVE THE PROBLEM This invention employs the following structures and methods, in order to solve the above-mentioned subject.

(1) The first aspect of the present invention is 0.90 to 1.30% by mass of C, 0.1 to 1.2% by mass of Si, 0.1 to 1.0% by mass of Mn, more than 0.1% by mass and less than 0.6% by mass of Al, 0 to 0.02 mass% P, 0 to 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0-0.5 mass% Co, 0-0.5 mass% V, 0-0.2 mass% Cu, 0-0.1 mass% Nb, 0-0.2 mass% Mo, 0-0.2 mass% W, 0 to 0.1% by mass of Ti, 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, Fe and It is a wire rod including a residual portion containing inevitable impurities. This wire rod has an area of 97% or more of the cross section perpendicular to the longitudinal direction, occupied by a pearlite structure, and has an area of 0.5% or less of the center region of the cross section, and 0.5% or less of the first surface layer region of the cross section. The area is occupied by the cornerstone cementite structure.

(2) The cross section of the wire rod described in (1) is occupied by the pearlite structure, the cornerstone cementite, the bainite structure, the pseudo pearlite structure, the ferrite structure, the grain boundary ferrite structure, and the martensite structure. You may be.

(3) The second aspect of the present invention is 0.90 to 1.30% by mass of C, 0.1 to 1.2% by mass of Si, 0.1 to 1.0% by mass of Mn, more than 0.1% by mass and less than 0.6% by mass of Al, 0 to 0.02 mass% P, 0 to 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0-0.5 mass% Co, 0-0.5 mass% V, 0-0.2 mass% Cu, 0-0.1 mass% Nb, 0-0.2 mass% Mo, 0-0.2 mass% W, 0 to 0.1% by mass of Ti, 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, Fe and Hot rolling is performed on the steel strip including the residual portion containing unavoidable impurities to obtain a rolled wire rod, a step of winding the rolled wire rod, and cooling of the rolled wire rod of 850 ° C or higher and 920 ° C or lower. , And the naengsok Y (℃ / s) for the rolled wire material is to be cooled from 850 to 650 ℃ ℃,

[Equation 1]

It is a manufacturing method of the wire rod as described in said (1) or (2) provided with the process of carrying out quenching by controlling to satisfy | fill, and carrying out a parting process by terminating pearlite transformation at the temperature of 500 degreeC or more and less than 650 degreeC.

(4) The third aspect of the present invention includes 0.90 to 1.30 mass% of C, 0.1 to 1.2 mass% of Si, 0.1 to 1.0 mass% of Mn, more than 0.1 mass% and less than 0.6 mass% of Al, 0 to 0.02 mass% P, 0 to 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0-0.5 mass% Co, 0-0.5 mass% V, 0-0.2 mass% Cu, 0-0.1 mass% Nb, 0-0.2 mass% Mo, 0-0.2 mass% W, 0 to 0.1% by mass of Ti, 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, Fe and The said winding about the process of hot-rolling a steel piece containing a remainder containing an unavoidable impurity, and obtaining a rolled wire rod, the process of winding up the said rolled wire rod, and the said rolled wire rod of 850 degreeC or more and 920 degrees C or less. By immersion, or forced cooling air (air blast cooling) directly to 500 ℃ to a molten salt (molten salt) in 600 ℃ immediately after the information, is naengsok Y (℃ / s) during which the cooling from 850 to 650 ℃ ℃

[Equation 1]

It is a manufacturing method of the wire rod as described in said (1) or (2) provided with the process of performing a parting process so that it may satisfy | fill.

(5) The fourth aspect of the present invention is 0.90 to 1.30% by mass of C, 0.1 to 1.2% by mass of Si, 0.1 to 1.0% by mass of Mn, more than 0.1% by mass and less than 0.6% by mass of Al, 0 to 0.02 mass% P, 0 to 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0-0.5 mass% Co, 0-0.5 mass% V, 0-0.2 mass% Cu, 0-0.1 mass% Nb, 0-0.2 mass% Mo, 0-0.2 mass% W, 0 to 0.1% by mass of Ti, 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, Fe and An area of not less than 97% of the cross section perpendicular to the longitudinal direction, including a residual portion containing unavoidable impurities, is occupied by the pearlite structure, and the area of not more than 0.5% of the center area of the cross section, and The area of the less than or equal to 0.5% of the surface area is obtained by a fresh pre-existing, which is occupied by the foundation cementite tissue liner. This steel wire has a tensile strength of 1800 MPa or more, and an area of 0.5% or less of the second surface layer region of the cross section perpendicular to the longitudinal direction of the steel wire is occupied by the cornerstone cementite.

(6) The steel wire as described in said (5) may have zinc plating or aluminum-zinc alloy plating.

According to the present invention, it is possible to provide a high-strength wire having excellent ductility suitable for applications such as PC steel wire, PC stranded wire, bridge PWS wire, and the like, with high yield at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS The figure which shows the example of the cornerstone cementite which generate | occur | produced in the surface layer area | region of a wire rod.
Fig. 2 is a graph showing the relationship between the amount of Al in the wire rod and the area ratio of the cornerstone θ in the first surface layer region of the wire rod.
3 is a diagram showing a relationship between the amount of Al in the wire rod and the area ratio of the cornerstone θ in the center region of the wire rod.
4 is a graph showing the relationship between the amount of C in the wire rod and the area ratio of the cornerstone θ in the first surface layer region of the wire rod.
5 is a graph showing the relationship between the amount of C in the wire rod and the area ratio of the cornerstone θ in the center region of the wire rod.
Fig. 6 is a diagram showing the effect of the cooling speed and the amount of C from 850 ° C to 650 ° C on the amount of precipitation of saltpeter θ;

MEANS TO SOLVE THE PROBLEM The present inventors investigated and studied the influence which the chemical composition and mechanical property of a wire rod have on wire workability, and, as a result, acquired the following knowledge.

(a) In order to raise tensile strength, what is necessary is just to increase content of alloying elements, such as C, Si, Mn, Cr. In particular, by increasing C to 1% by mass or more and relatively decreasing the work strain for obtaining the desired strength, the strength can be increased while maintaining the ductility of the steel wire.

(b) When the C content is increased, in the cooling process from the austenite region in the parting process, from the start of cooling to the start of pearlite transformation, as shown by the arrow in Fig. 1 in the subcooled austenite. Cornerstone cementite becomes easy to precipitate. This tendency becomes remarkable in the wire rod center region where the cooling rate is slowed down. If a large amount of cementite cementite is produced in the center region of the wire rod, it causes disconnection at the time of drawing.

(c) The limit cooling rate which can suppress the formation of saltpeter cementite can be expressed as a function of the amount of C. Formation of the cornerstone cementite in the wire rod center region where the cold speed becomes slow can be suppressed by cooling the mother phase austenite at a rate higher than this, and then performing constant temperature treatment.

(d) In a normal wire rod rolling line, the wire rod is wound at a constant temperature after the finish rolling, and conveyed by a conveyor to a parting treatment area such as stealmore. In the reheating parting line, there is no winding step of the wire rod, but some time is required for conveyance from the heating stand outlet side to the cooling stand for the parting. In the high C material, the cementite precipitation temperature (austenitic → austenite + cementite temperature) is high, and therefore, under the conventional heating and conveying conditions, in the region of a depth of several tens of micrometers of the wire rod outermost layer which contacts the atmosphere during conveyance. There is a possibility that the cornerstone cementite is produced in the wire outermost surface layer before the temperature is lowered and the cooling for the parting process is started. Fig. 1 shows an example of saltpeter cementite produced in the wire surface layer region. Since the cementite of such a surface layer area | region is a brittle structure, it causes a surface crack at the time of drawing, and causes the delamination of the steel wire obtained by drawing, and reduces ductility of a steel wire significantly.

(e) As a method of suppressing the cornerstone cementite in such a wire center area | region and a wire surface layer area | region, a comparatively small amount of Al of 0.1-0.6 mass% is added, and also 650 degreeC vicinity from 850 degreeC in a parting process. It is effective to increase the cooling rate of the mother phase austenite. The limit cooling rate for suppressing saltpeter cementite can be expressed as a function of the amount of C.

(f) When the C content of the wire rod is 0.9 to 1.1 mass% and the diameter is less than 10 mm, the case where the C content is 1.0 mass% or more and the diameter is 18 mm or less by Stelmore (air forced cooling), By DLP, the cooling rate more than the said limit cooling rate can be obtained.

In addition, DLP refers to the Direct In-line Patenting process of directly immersing a rolled wire rod in a molten salt to process the tenting.

(g) When the wire is drawn, in order to develop a fiber structure and to suppress delamination, it is preferable to set the true strain to 1.3 to 1.8.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention based on the knowledge mentioned above is described in detail.

(1st embodiment)

(Configuration of Wire Rod)

According to the first embodiment of the present invention, an area of 0.5% or less of the center region in the cross section perpendicular to the longitudinal direction of the wire rod and an area of 0.5% or less of the surface layer region (first surface layer region) in the cross section. This is a wire rod occupied by the cornerstone cementite structure.

According to the investigation by the present inventors, if there is a correlation between the cementite cementite rate of the wire rod surface area | region and wire center area | region before wire drawing, and the ductility of the steel wire after wire drawing, if the area ratio of the cementite of wire rod surface area | region can be suppressed to 0.5% or less, The delamination resistance of the steel wire obtained by drawing is improved, and drawing break can be suppressed by suppressing the area ratio of the cementite of a wire rod center area to 0.5% or less.

Here, the surface layer area | region (first surface layer area) of a wire rod means the area | region corresponded to the depth to 50 micrometers from the surface (peripheral part in a cross section) of a wire rod in the cross section perpendicular | vertical to the longitudinal direction of a wire rod.

The wire rod center region means a region having a radius of 100 μm from the center point of the cross section perpendicular to the longitudinal direction of the wire rod.

The cornerstone cementite means cementite having a small deformability having a thickness before wire drawing of 100 nm or more, which is formed at the old austenite grain boundary.

In addition, in the wire rod according to the present embodiment, 97% or more of the cross section perpendicular to the longitudinal direction of the wire rod is occupied by the pearlite structure. The remaining portion may be a cornerstone cementite, bainite structure, pseudo pearlite structure, ferrite structure, grain boundary ferrite structure, martensite structure, or the like.

(Manufacturing conditions of wire rod)

In order to suppress the cornerstone cementite in the surface layer area | region of the 0.9-1.3 mass% high C-rolled wire rod by said area ratio, when hot-rolling a steel slab (billlet) with a diameter of 7-18 mm, a salt bath or a stell It is necessary to make the wire rod temperature at the time of starting cooling for parting by mower etc. 850 degreeC or more. In order to do so, it is preferable to wind up in the temperature range higher than 850 degreeC. On the other hand, if the wire temperature at the time of starting cooling is too high, that is, the winding temperature is too high, the austenite grain size of the wire is coarsened and the ductility (cross-sectional shrinkage) is lowered, so the wire temperature at the start of cooling is 920. It is preferable to set it as degrees C or less.

The amount of cementite cementite in the wire rod center region depends on the cooling rate Y during cooling from 850 ° C to 650 ° C. The inventors of the present invention show that the cold content Y [° C / s] and the carbon content C% [mass%] of the wire rod are

[Equation 1]

It was discovered that it was effective to quench the rolled wire rod by a method satisfying the following, and then terminate the pearlite transformation at a temperature of 500 to 600 ° C.

(Base element)

The wire rod according to the present embodiment contains C, Si, Mn, Al, N, and O. Hereinafter, content of each component is demonstrated.

C: 0.90 to 1.30 mass%

C is an effective element for increasing the strength of the wire rod, and when the content is less than 0.90% by mass, it is difficult to stably provide high strength to the final product. On the other hand, when the content of C is excessively large, net-shaped cementite cementite is formed at the austenite grain boundary, whereby disconnection is likely to occur during the drawing process, and the toughness and ductility of the ultrafine wire rod after the final drawing is significantly degraded. . Therefore, content of C is prescribed | regulated to 0.90-1.30 mass%. In order to obtain a high strength steel wire, 0.95 mass% or more is preferable and 1.0 mass% or more is more preferable.

Si: 0.1-1.2 mass%

Si is an effective element for increasing the wire strength. There is also an effect of suppressing a decrease in strength during hot dip galvanizing of the steel wire. If it is less than 0.1 mass%, the effect of high strength is small. On the other hand, when the amount of Si is too large, precipitation of the cornerstone ferrite is promoted also in the roughened steel, and the limit workability in drawing is reduced. Therefore, content of Si is prescribed | regulated to 0.1-1.2 mass%.

Mn: 0.1-1.0 mass%

Mn, like Si, is an element useful as a deoxidizer. It is also effective for improving the hardenability and increasing the strength of the wire rod. In addition, Mn has a function of fixing S in the steel as MnS to prevent hot brittleness. If the content is less than 0.1% by mass, the above effects are hardly obtained. On the other hand, Mn is an element which tends to segregate, and when it exceeds 1.0 mass%, it is segregated especially in the central region of the wire rod, and martensite and bainite are formed in the segregated portion, which leads to deterioration in wire workability. Therefore, content of Mn is prescribed | regulated to 0.1-1.0 mass%.

Al: more than 0.1 mass% to less than 0.6 mass%

Al is an effective element for suppressing saltpeter cementite. It also increases the wire strength. On the other hand, when the amount of Al is too large, precipitation of the cornerstone ferrite is promoted also in the roughened steel, and the hard inclusions causing disconnection are increased. Therefore, content of Al is prescribed | regulated to more than 0.1 mass% and less than 0.6 mass%. Preferably it is 0.2-0.5 mass%, More preferably, it is 0.26-0.35 mass%.

N: 10 to 60 ppm

N produces | generates Al, Ti, B, and nitride in steel, and has the effect | action which prevents coarsening of the austenite particle size at the time of heating, and the effect is exhibited effectively by containing 10 ppm or more. However, when there is too much content, since there exists a possibility that solid solution N will accelerate the aging in a fresh wire, an upper limit is prescribed | regulated to 60 ppm.

O: 10ppm to 40ppm

By forming a composite inclusion with Si guitar, O becomes possible to form the soft inclusion which does not adversely affect a fresh characteristic. Such soft inclusions can be finely dispersed after rolling, miniaturizing the γ particle diameter due to the pinning effect, and improving the ductility of the parting wire. Therefore, the lower limit is prescribed | regulated to 10 ppm. If the O content is 20 ppm or more, the pinning effect can be obtained more strongly. However, when the content is too large, hard inclusions are formed and the freshness is degraded. Therefore, the upper limit of O is defined as 40 ppm.

(Inevitable impurities)

Although content of P and S contained as an impurity in the wire rod which concerns on this embodiment is not specifically prescribed, It is preferable to restrict to 0.02 mass% or less from a viewpoint of ensuring ductility, respectively. However, even if P and S are contained lower than 0.0005 mass%, respectively, the effect is limited.

(Optional element)

The wire rod according to the present embodiment is, in addition to the above elements, for the purpose of improving mechanical properties such as strength, toughness and ductility, Cr, Ni, Co, V, Cu, Nb, Mo, W, Ti, B, You may selectively contain 1 or more types of elements among REM, Ca, Mg, and Zr. Hereinafter, content of each component is demonstrated.

Cr: 0-0.5 mass%

Cr is an element effective in making the pearlite lamellar gap fine and improving the strength, the wire workability, and the like of the wire rod. In order to exhibit such an effect effectively, addition of 0.1 mass% or more is preferable. On the other hand, when the amount of Cr is too large, the transformation end time is long, there is a possibility that subcooled structures such as martensite and bainite may occur in the hot rolled wire, and the mechanical descaling property is also deteriorated. Therefore, the upper limit is defined as 0.5 mass%.

Ni: 0-0.5 mass%

Ni does not contribute much to the increase in strength of the wire rod, but is an element that increases the toughness of the wire rod. In order to exhibit such an effect effectively, addition of 0.1 mass% or more is preferable. On the other hand, when Ni is excessively added, the transformation end time becomes long, so the upper limit is defined as 0.5 mass%.

Co: 0-0.5 mass%

Co is an element effective in suppressing precipitation of the saltpeter cementite in the rolled material. In order to exhibit such an effect effectively, addition of 0.1 mass% or more is preferable. On the other hand, even if Co is added in excess, the effect is saturated and economically unnecessary, so the upper limit is defined as 0.5 mass%.

V: 0-0.5 mass%

V forms fine carbonitrides in ferrite, thereby preventing coarsening of the austenite grains during heating and contributing to the increase in strength after rolling. In order to exhibit such an effect effectively, addition of 0.05 mass% or more is preferable. However, when excessively added excessively, the formation amount of carbonitride becomes too large and the particle diameter of carbonitride becomes large, so an upper limit is prescribed | regulated as 0.5 mass%.

Cu: 0-0.2 mass%

Cu has the effect of improving the corrosion resistance of an ultrafine steel wire. In order to exhibit such an effect effectively, addition of 0.1 mass% or more is preferable. However, when excessively added, CuS is segregated during grain boundaries due to reaction with S, so that absorption occurs in steel ingots, wire rods and the like during the wire rod manufacturing process. In order to prevent such a bad influence, the upper limit is prescribed | regulated as 0.2 mass%.

Nb: 0 to 0.1 mass%

Nb has the effect of improving the corrosion resistance of the ultrafine steel wire. In order to exhibit such an effect effectively, addition of 0.05 mass% or more is preferable. On the other hand, when Nb is added excessively, transformation end time becomes long, and an upper limit is prescribed | regulated as 0.1 mass%.

Mo: 0-0.2 mass%

Mo is concentrated on the pearlite growth interface and has an effect of suppressing the growth of pearlite by a so-called solute drag effect. By adding an appropriate amount, it is possible to suppress only the growth of pearlite in the high temperature region of 600 ° C or higher, and suppress the formation of coarse pearlite having a large lamellar spacing. In addition, Mo suppresses the formation of ferrite, has the effect of improving the hardenability, and is effective in reducing the non-pearlite structure. If Mo is excessive, the pearlite growth in the entire temperature range is suppressed, requires a long time for patting, causes a decrease in productivity, and coarse Mo ₂ C carbides are precipitated, resulting in deterioration of fresh workability. Therefore, content of Mo is prescribed | regulated to 0.2 mass% or less. Preferable content is 0.005-0.06 mass%.

W: 0-0.2 mass%

W, like Mo, is concentrated at the pearlite growth interface and has the effect of suppressing the growth of pearlite by the so-called solute traction effect. By adding an appropriate amount, it is possible to suppress only the growth of pearlite in the high temperature region of 600 ° C or higher, and to suppress the formation of coke pearlite having a large lamellar spacing. W also suppresses the formation of ferrite, has the effect of improving the hardenability, and is effective in reducing the non-pearlite structure. When W is excessive, pearlite growth in the entire temperature range is suppressed, requires a long time for patting, leads to a decrease in productivity, coarse W ₂ C carbides are precipitated, and fresh workability is lowered. Therefore, content of W is prescribed | regulated to 0.2 mass% or less. Preferable content is 0.005-0.06 mass%.

Ti: 0 to 0.1 mass%

Ti is a deoxidation element and has the effect of fixing N which promotes the aging effect after drawing. Excess addition promotes precipitation of hard Ti carbide and leads to ductile deterioration and freshness deterioration of the steel wire, and is therefore defined as 0.1% by mass or less including O mass%.

B: 0 to 30 ppm

When B exists in austenite in a solid solution state, it is concentrated at grain boundaries and suppresses the formation of non-pearlite precipitation such as ferrite, pseudo pearlite, and bainite. When B content is 4 ppm or more, this effect can be acquired strongly. On the other hand, when B is excessively added, precipitation of coarse Fe ₃ (CB) ₆ carbide in austenite is promoted, adversely affecting freshness. In order to satisfy this, the upper limit of content of B is prescribed | regulated as 30 ppm. Preferable content is 4-15 ppm, More preferably, it is 8-12 ppm.

REM: 0 to 50 ppm

REM is effective for the detoxification of S. However, excessive addition causes oxide to form and causes disconnection, so the upper limit of the content is defined as 50 ppm.

Ca: 0-50 ppm

Ca is effective for reducing hard alumina inclusions, but excessive addition causes oxide to form and causes disconnection, so the upper limit of the content is defined as 50 ppm.

Mg: 0 to 50 ppm

Mg becomes a fine oxide and refines the steel structure to improve ductility. When 50 ppm is exceeded, since disconnection will arise easily from an oxide as a starting point, the upper limit of content is prescribed | regulated as 50 ppm.

Zr: 0 to 100 ppm

Since Zr becomes ZrO as austenite crystallized nucleus, ZrO has the effect of increasing the austenite equiaxed ratio and reducing central segregation. However, if it exceeds 100 ppm, disconnection tends to occur starting from the oxide, so the upper limit of the content is increased. It is prescribed by 10Oppm.

(2nd embodiment)

(Composition of steel wire)

2nd Embodiment of this invention is a steel wire whose tensile strength is 1800 Mpa or more obtained by cold drawing the wire rod of 1st Embodiment at a true strain rate of 1.3 or more. As for this steel wire, the area of 0.5% or less of the surface layer area | region (2nd surface layer area | region) in the cross section perpendicular | vertical to the longitudinal direction of a steel wire is occupied by cornerstone cementite.

Here, a 2nd surface layer area | region means the area | region from the surface layer (a peripheral part in a cross section) of a steel wire to 20 micrometers inside.

The steel wire thus obtained may be subjected to blueing, hot dip galvanizing, hot dip aluminum-zinc alloy plating, or the like as a final treatment.

<Examples>

Next, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples, but of course, it is also possible to add and modify appropriately within a range that may be suitable for the purpose of the present invention. They are all included in the technical scope of the present invention.

In Table 1 and Table 2, the chemical components of the steels A to V used in Examples 1 to 15 and Comparative Examples 1 to 19 of the present invention are shown. In addition, in Tables 1-6, the numerical value, unfavorable result, etc. which are not contained in the prescribed range mentioned above are underlined.

After heating the billet (steel piece) of the steel containing the chemical component shown in Table 1 and Table 2, it processes by the wire rod of 7-18 mm diameter by hot rolling, winds it to predetermined temperature, and then performs a parting process. It was.

After the rolled wire was wound in a ring shape, a parting process was performed by direct molten salt immersion (DLP) or stealmore. Regarding the cold speed Y from 850 ° C to 650 ° C, in the case of DLP, four test pieces having a length of 600 mm are made using wire rods of the same composition and diameter, and the thermocouples are embedded in the center thereof, and at least 850 ° C in an electric furnace. After heating to and immersing in a salt bath, time and temperature were measured and the average cooling rate at the time of cooling from 850 degreeC to 650 degreeC was computed. In the case of stelmore (air forced cooling), the temperature of the wire rod overlap was measured by a radiation thermometer on a conveyor, and the average cooling rate when cooling from 850 ° C to 650 ° C was calculated.

In order to measure the area ratio of the pearlite structure of the rolled wire rod and the cornerstone cementite area ratio, one ring having a diameter of 1.0 to 1.5 m in a ring-shaped wire rod was divided into eight equal parts to identify the site with the highest and lowest TS. Samples of 10 mm length are cut out from these portions of the continuous ring in a substantial portion, and resin is embedded so that the cross section perpendicular to the longitudinal direction (C cross section) can be observed, followed by polishing with alumina and corrosion with saturated pyral, SEM Observation was performed.

Regarding the area ratio of the pearlite structure, 90 degrees in the circumferential direction of a 200 × 200 μm square region of a 1 / 4D depth portion (D = diameter) is formed from the surface layer of the two portions (the highest and the lowest TS). Four locations were selected. The selected point is measured at 3000 times, and the pseudo pearlite part in which cementite is dispersed in granular form, the bainite part in which plate-like cementite is dispersed at roughly three times more lamellar spacing than the surroundings, the grain boundary ferrite part deposited along austenite, and the cornerstone The area ratio excluding the cementite portion was measured by image analysis as the area ratio of the pearlite structure, and the average value of four locations was obtained as the area ratio of the pearlite structure.

The SEM imaging location of the area ratio of the cornerstone cementite is demonstrated.

As for the wire rod center region, an area having a radius of 100 µm was selected from the center point in the cross section of the region where the TS was the lowest.

As for the wire surface layer area | region, four 50 micrometers x 50 micrometer square areas of the peripheral part vicinity in the cross section of the site | part where TS is highest were selected every 90 degrees in the circumferential direction.

These selected points were measured 5000 times, and the area ratio of the cornerstone cementite whose thickness is about 100 nm or more was measured by image analysis.

In addition, about the surface layer area | region, the maximum value of four measurement results was used.

The wire characteristic of the wire rod is obtained by removing the scale of the rolled wire rod by pickling, and then preparing a wire rod of 10 m in length to which a zinc phosphate coating is applied by binding treatment. Was performed to obtain a high strength wire rod having a diameter of 3 to 10 mm. At this time, the presence of breakage in the fresh wire was observed to confirm the freshness characteristics. In addition, fresh steel wire was subjected to bluing, hot dip galvanizing, hot dip aluminum-zinc alloy plating, and the like as necessary. After that, the twisted pair test was performed to confirm the occurrence of delamination.

In addition, for the measurement of the cornerstone cementite area ratio of fresh steel wire, a 10 mm long sample was cut out from the steel wire having a diameter of 3 to 8 mm, and the resin was embedded so that the cross section perpendicular to the longitudinal direction (C cross section) could be observed. Polished, corroded with saturated picral, and SEM observation was performed.

The SEM photographing location selected the rectangular area of 20 micrometers x 50 micrometers of the vicinity of the periphery part in the cross section of a steel wire.

This selected location was measured by 10,000 times, and the area ratio of the cornerstone cementite whose thickness is 50 nm or more was measured by image analysis.

As a result, it was confirmed that when the cornerstone cementite fraction of the surface layer area | region and center area | region of a rolled wire rod is suppressed, the disconnection at the time of delamination and drawing can be suppressed in the steel wire after drawing. The said measurement result was shown to Tables 3-6.

In Tables 3-6, the manufacturing conditions and the measurement result of the wire rod and steel wire in Examples 1-15 and Comparative Examples 1-19 are shown.

As can be seen from Examples 1 to 15 shown in Tables 3 and 4, when the amount of the element contained in the wire rod is properly controlled, the basement cementite fraction in the surface layer and the central region of the rolled wire rod is suppressed. The occurrence of delamination and wire breaking in the later steel wires could be suppressed. In addition, high final fresh TS was obtained.

In Comparative Example 1, B steel was used, but since the molten salt temperature was low, a large amount of bainite structure was generated, and delamination occurred.

In Comparative Examples 2, 4, 5, 8, and 9, G steel, H steel, I steel, K steel, and L steel were used, respectively, but the cooling rate of the austenite region (850 to 650 degrees C) was higher than the predetermined value. Since it was slow, the cornerstone cementite could not be suppressed in the wire surface layer area | region and center area | region.

In the comparative example 3, although G steel was used, since the coiling temperature and the wire temperature at the time of cooling start were too high, the ductility of the wire rod was deteriorated and it disconnected in the middle of drawing.

In Comparative Example 6, although I steel was used, since the wire rod temperature at the time of cooling start was high, the ductility of the wire rod decreased and disconnected in the middle of drawing.

In the comparative example 7, although J steel was used, since the coiling temperature and the wire temperature at the time of cooling start were too low, the wire temperature at the time of cooling start for parting became low, and cornerstone cementite was produced | generated. For this reason, delamination occurred in the disconnection or the wire rod during the drawing.

Moreover, in Comparative Examples 10-13, since M steel, N steel, O steel, and P steel whose Al amount was too small were used, respectively, generation | occurrence | production of cornerstone cementite could not be suppressed.

In Comparative Example 14, since Q steel having an excessively high Al amount was used, a large amount of bainite structure was generated, and delamination occurred.

In Comparative Example 15, since the amount of C used R steel which was excessive, the cornerstone cementite was produced | generated and was disconnected in the middle of drawing.

In Comparative Example 16, the steel wire could not satisfy the predetermined TS because the amount of C used was too small.

In Comparative Example 17, since T steel having an excessive amount of Si was used, a large amount of bainite structure was generated, and delamination occurred.

In Comparative Example 18, since U steel with excessive Mn amount was used, micromartensite was formed in the central segregation portion, the ductility of the wire rod was deteriorated, and disconnection occurred during drawing.

In Comparative Example 19, since V steel having an excessive amount of O was used, coarse inclusions were generated, and disconnection occurred during the drawing.

2 and 3 show the relationship between the amount of Al added and the cornerstone cementite area ratio of the wire rod surface region and the center region, and the relationship between the amount of C addition and the cornerstone cementite area ratio of the wire rod surface layer region and the center region in FIG. 4 and FIG. 5. By adding Al more than 0.1% by mass, the cornerstone cementite can be suppressed. 6 shows the relationship between the cooling rate and the amount of C added in the austenite region at 850 to 650 ° C. and the cornerstone cementite area ratio. Steel having the composition of the present invention,

[Equation 1]

It can be seen that by cooling and parting under the conditions satisfying the condition, it is possible to suppress the cornerstone cementite, and even if the steel having the composition of the comparative example is cooled and parted under the condition satisfying the equation (1), the cornerstone cementite cannot be suppressed. have.

According to the present invention, it is possible to provide a high-strength wire having excellent drawability suitable for applications such as PC steel wire, PC stranded wire, and PWS wire for bridges at a high yield with low yield. For this reason, this invention has sufficient industrial applicability.

Claims (6)

0.90 to 1.30 mass% of C,
0.1 to 1.2 mass% of Si,
0.1 to 1.0 mass% of Mn,
Al exceeding 0.1 mass% and less than 0.6 mass%,
0 to 0.02 mass% of P,
0 to 0.02 mass% of S,
10 to 60 ppm N,
10 to 40 ppm of O,
0-0.5 mass% of Cr,
0-0.5 mass% of Ni,
0 to 0.5 mass% of Co,
0 to 0.5 mass% of V,
0 to 0.2% by mass of Cu,
0 to 0.1 mass% of Nb,
0 to 0.2% by mass of Mo,
0 to 0.2% by mass of W,
0 to 0.1 mass% of Ti,
0-30 ppm of B,
0-50 ppm REM,
0-50 ppm of Ca,
0-50 ppm Mg,
0 to 100 ppm Zr,
Residual part containing Fe and unavoidable impurities
Wire rod, including
An area of 97% or more of the cross section perpendicular to the longitudinal direction of the wire rod is occupied by the pearlite structure,
An area of 0.5% or less of the central region of the cross section and an area of 0.5% or less of the first surface layer region of the cross section are occupied by the cornerstone cementite structure.
Wire rod, characterized in that. The method of claim 1,
The cross section of the wire rod,
The pearlite structure,
The cornerstone cementite,
Bainite tissue,
With pseudo pearlite tissue,
With ferrite tissue,
With grain boundary ferrite structure,
Martensite organization
Wire rod, characterized in that occupied by. It is a manufacturing method of the wire rod of Claim 1 or 2.
0.90-1.30 mass% C, 0.1-1.2 mass% Si, 0.1-1.0 mass% Mn, more than 0.1 mass% Al less than 0.6 mass%, 0-0.02 mass% P, 0- 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0 to 0.5 mass% Co, 0 to 0.5 mass% V, 0 to 0.2 mass% Cu, 0 to 0.1 mass% Nb, 0 to 0.2 mass% Mo, 0 to 0.2 mass% W and 0 to 0.1 mass% Ti And a steel strip containing 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, and a balance containing Fe and inevitable impurities. Hot rolling to obtain a rolled wire rod,
Winding the rolled wire rod;
Cooling is started for the said rolled wire rod of 850 degreeC or more and 920 degrees C or less, and cooling rate Y (degreeC / s) while the said rolling wire is cooled from 850 degreeC to 650 degreeC,
[Equation 1]

To quench by controlling to satisfy the condition, and the rolled wire is subjected to the parting process by terminating the pearlite transformation at a temperature of 500 ° C. or more and less than 650 ° C.
The manufacturing method of the wire rod characterized by including the. It is a manufacturing method of the wire rod of Claim 1 or 2.
0.90-1.30 mass% C, 0.1-1.2 mass% Si, 0.1-1.0 mass% Mn, more than 0.1 mass% Al less than 0.6 mass%, 0-0.02 mass% P, 0- 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0 to 0.5 mass% Co, 0 to 0.5 mass% V, 0 to 0.2 mass% Cu, 0 to 0.1 mass% Nb, 0 to 0.2 mass% Mo, 0 to 0.2 mass% W and 0 to 0.1 mass% Ti And a steel strip containing 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, and a balance containing Fe and inevitable impurities. Hot rolling to obtain a rolled wire rod,
Winding the rolled wire rod;
The cold wire Y during cooling from 850 ° C to 650 ° C by directly immersing in a molten salt of 500 ° C to 600 ° C or by air forced cooling for the rolled wire rod of 850 ° C or more and 920 ° C or less. ℃ / s)
[Equation 1]

Parting process to satisfy
The manufacturing method of the wire rod characterized by including the. 0.90-1.30 mass% C, 0.1-1.2 mass% Si, 0.1-1.0 mass% Mn, more than 0.1 mass% Al less than 0.6 mass%, 0-0.02 mass% P, 0- 0.02 mass% S, 10 to 60 ppm N, 10 to 40 ppm O, 0 to 0.5 mass% Cr, 0 to 0.5 mass% Ni, 0 to 0.5 mass% Co, 0 to 0.5 mass% V, 0 to 0.2 mass% Cu, 0 to 0.1 mass% Nb, 0 to 0.2 mass% Mo, 0 to 0.2 mass% W and 0 to 0.1 mass% Ti And, 0 to 30 ppm of B, 0 to 50 ppm of REM, 0 to 50 ppm of Ca, 0 to 50 ppm of Mg, 0 to 100 ppm of Zr, and a balance containing Fe and inevitable impurities, An area of 97% or more of the cross section perpendicular to the longitudinal direction is occupied by the pearlite structure, and an area of 0.5% or less of the central region of the cross section and 0.5% or less of the first surface layer region of the cross section. The area of is obtained by drawing the wire rod occupied by the cornerstone cementite structure,
Has a tensile strength of 1800 MPa or more,
An area of 0.5% or less of the second surface layer area of the cross section perpendicular to the longitudinal direction is occupied by the cornerstone cementite
Liner, characterized in that. The method of claim 5,
A steel wire, characterized by having zinc plating or aluminum-zinc alloy plating.

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