WO2020004570A1 - Flat steel wire, and wire rod for flat steel wire - Google Patents

Abstract

Provided is a flat steel wire containing, in mass%, 0.35-0.60% of C, more than 1.50% and less than 2.00% of Si, more than 0.65% and less than 1.50% of Mn, 0.010% or less of S, 0.010% or less of P, 0.005-0.60% of Cr, 0.005-0.080% of Al, 0.0020-0.0080% of N, 0.0002-0.0050% of Ca, 0.05-0.80% of Cu, and 0.05-0.60% of Ni, with the remainder comprising Fe and impurities, wherein Y1 represented by expression <1> satisfies expression <2>, expressions <5> and <6> are satisfied, the tensile strength is 1000-1350 MPa, the longitudinal tensile residual stress is 300 MPa or less, and the width/thickness ratio is 2.5-10.

Description

Flat steel wire and wire rod for flat steel wire

The present invention relates to a flat steel wire and a wire rod for a flat steel wire.
Priority is claimed on Japanese Patent Application No. 2018-124644 filed on June 29, 2018, the content of which is incorporated herein by reference.

平 Flat steel wire is used as a reinforcing material for flexible pipes for transporting high-pressure fluids such as natural gas and crude oil. In the development of offshore oil fields, the mining depth tends to be deeper with the increase in oil demand, and there is an increasing demand for reinforcement of flexible pipes. In addition, since the flexible pipe is used in a sour environment containing hydrogen sulfide, the flat steel wire used for the reinforcing material has a property of preventing hydrogen-induced cracking (HIC) from being caused by hydrogen-induced cracking. And sulfide stress corrosion cracking (SSC) which is a property not to cause sulfide stress corrosion cracking (Sulfide Stress Corrosion Cracking; SSC) is required. However, in general, hydrogen-induced cracking and sulfide stress corrosion cracking are more likely to occur in higher-strength wires, making it difficult to apply high-strength wires to components such as flexible pipes used in sour environments. . Heretofore, a technique for providing a high-strength wire used in such a sour environment has been proposed.

In Patent Document 1, C: 0.25 to 0.60%, Si: more than 0.50 and less than 2.0%, Mn: 0.20 to 1.50%, S: 0. 015% or less, P: 0.015% or less, Cr: 0.005 to 1.50%, Al: 0.005 to 0.080%, and N: 0.0020 to 0.0080%. : 0 to 0.0050% and Mg: 0 to 0.0050%, one or two of which are contained so as to satisfy [Ca] + [Mg]> 0.20 × [S], and have a tensile strength. Is 1000 MPa or more, and the average value of the Hv hardness measured in a cross section perpendicular to the longitudinal direction is 320 or more and less than 450, the standard deviation σHv of the measured value is 15 or less, and the width / thickness ratio is 1.5 or more and 10 or more. A high-strength flat steel wire excellent in resistance to hydrogen-induced cracking characterized by the following has been proposed.

Patent Document 2 discloses that, by mass%, C: 0.85% to 1.00%, Si: 0.80% to 1.30%, Mn: 0.30% to 0.90%, P : 0.017% or less, S: 0.010% or less, Cu: 0.20% or less, Al: 0% to 0.10%, Ti: 0% to 0.05%, B: 0% or more It contains 0.0040% or less, N: 0% or more and 0.0060% or less, Cr: 0% or more and 0.5% or less, and V: 0% or more and 0.50% or less. A flat steel wire having a rounded rectangular shape in which the short side of the cross section is 2 mm or more and 7 mm or less, the long side of the cross section is more than 8 mm and 56 mm or less, and the ratio of the long side to the short side is more than 4-8. The yield strength or 0.2% proof stress obtained by the tensile strength is 1600 MPa or more and 2000 MPa or less, and the tensile strength is A flat steel wire has been proposed, wherein the flat steel wire has a torsion value of 12 or more obtained by a torsion test under a condition of 900 MPa or more, elongation at break of 2% or more, and a distance between chucks of 500 mm. .

WO2017 / 154930 Japanese Patent No. 6116680

技術 The technology disclosed in Patent Document 1 is a flat steel wire that does not cause hydrogen-induced cracking in a sour environment with a pH of less than 5.5 even if the tensile strength exceeds 1000 MPa.

技術 According to the technique disclosed in Patent Document 2, a flat steel wire having high tensile strength of 1900 MPa or more and excellent in secondary workability can be obtained.

The present invention relates to a high-strength flat steel wire having a tensile strength of 1000 to 1350 MPa, and even in a harsh sour environment having a pH of less than 5.5, the surface of the flat steel wire is not subjected to a hydrogen-induced It is an object of the present invention to provide a flat steel wire which does not generate cracks and sulfide stress corrosion cracking and can be used as a reinforcing wire for a flexible pipe or the like having a deep mining depth and a wire for a flat steel wire suitable for the production thereof.

The present inventors conducted various studies on the effects of added elements on hydrogen-induced cracking and sulfide stress corrosion cracking in order to solve the above-mentioned problems, and obtained the following findings (a) to (d). Was.

(A) Hydrogen-induced cracking and sulfide stress corrosion cracking resistance of a flat steel wire occur starting from coarse sulfide contained in the flat steel wire. In particular, when sulfides such as MnS are coarse, when the primary wire drawing or flat pressure working after the primary wire drawing is performed as a step of forming a hot-rolled wire into a flat steel wire, Voids are formed around the coarse sulfide, which promotes hydrogen-induced cracking and sulfide stress corrosion cracking in severe sour environments with a pH of less than 5.5. Therefore, it is necessary to make the sulfide inevitably contained in the wire rod as fine as possible. For the refinement of sulfide, it is effective to add Ca to form MnS or CaS in which Ca is partially dissolved.

(B) In a high-strength flat steel wire having a tensile strength exceeding 1000 MPa, in order to simultaneously improve not only the resistance to hydrogen-induced cracking but also the resistance to sulfide stress corrosion cracking, contain Si in excess of 1.50%. After a large amount of Si is dissolved in the matrix, Cu: 0.05 to 0.80% and Ni: 0.05 to 0.60% are added in a total amount of 0 to satisfy Cu / Ni> 1. .10 to 1.0%.

(C) No cracking occurs in the steel material at the time of primary drawing and subsequent flat pressing to a flat steel wire, and hydrogen-induced cracking and sulfide stress corrosion cracking do not occur in a sour environment having a pH of less than 5.5. For this purpose, Y1 represented by the following formula <1> must contain C, Si, Mn, Cr, Cu, and Ni in a range that satisfies the formula <2>.
Y1 = 10 × √ [C] {(1 + 0.8 × [Si]) × (1 + 3 × [Mn]) × (1 + 2 × [Cr]) × (1 + 0.8 × [Cu]) × (1 + 0.7 × [Ni])} ... <1>
12 × D <Y1 <30 × D ... <2>
Here, [C], [Si], [Mn], [Cr], [Cu] and [Ni] in the above formulas <1> and <2> represent the content of each element in mass%, D represents the thickness (mm) of the flat steel wire.

(D) From a wire to a flat steel wire, for example, a rolled wire is subjected to primary wire drawing, followed by deformed wire drawing or cold rolling by a cold rolling machine. In the flat steel wire manufactured in such a process, tensile residual stress is generated in the longitudinal direction of the surface of the flat steel wire due to processing strain caused by cold working. In particular, in a flat steel wire having a tensile strength exceeding 1000 MPa, the residual tensile stress on the surface induces sulfide stress corrosion cracking, so that it must be minimized.

The present invention has been completed based on the above findings, and the gist thereof is a flat steel wire shown in the following (1) to (6) and a wire rod for a flat steel wire shown in the following (7).

(1)
In mass%,
C: 0.35 to 0.60%,
Si: more than 1.50% and less than 2.00%,
Mn: more than 0.65% and less than 1.50%,
S: 0.010% or less,
P: 0.010% or less,
Cr: 0.005 to 0.60%,
Al: 0.005 to 0.080%,
N: 0.0020 to 0.0080%,
Ca: 0.0002 to 0.0050%,
Cu: 0.05-0.80%,
Ni: 0.05 to 0.60%,
Ti: 0 to 0.100%,
Nb: 0 to 0.050%,
V: 0 to 0.50%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
And the balance consists of Fe and impurities,
Y1 represented by the following formula <1> satisfies the following formula <2>,
Satisfying the following expressions <5> and <6>,
The tensile strength is 1000 MPa or more and 1350 MPa or less,
The tensile residual stress in the longitudinal direction is 300 MPa or less,
A flat steel wire having a width / thickness ratio of 2.5 or more and 10 or less.
Y1 = 10 × √ [C] {(1 + 0.8 × [Si]) × (1 + 3 × [Mn]) × (1 + 2 × [Cr]) × (1 + 0.8 × [Cu]) × (1 + 0.7 × [Ni])} Formula <1>
12 × D <Y1 <30 × D Expression <2>
[Cu] / [Ni]> 1 Expression <5>
0.10 ≦ [Cu] + [Ni] ≦ 1.00 Expression <6>
However, [C], [Si], [Mn], [Cr], [Cu], and [Ni] in the above formulas <1>, <2>, <5>, and <6> are represented by mass% of each element. Represents the content, and D represents the thickness (mm) of the flat steel wire.
(2)
In mass%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
V: 0.01 to 0.50%,
The flat steel wire according to (1), wherein the flat steel wire contains one or more types selected from the group consisting of:
(3)
In mass%,
Mo: 0.01 to 1.00%,
B: 0.0002 to 0.0100%,
The flat steel wire according to (1) or (2), wherein the flat steel wire contains one or two types selected from the group consisting of:
(4)
In mass%,
REM: 0.0002 to 0.1000%,
Zr: 0.0002 to 0.100%,
Mg: 0.0002 to 0.0050%,
The flat steel wire according to any one of (1) to (3), characterized by containing one or more kinds selected from the group consisting of:
(5)
The flat steel wire according to any one of (1) to (4), further comprising a tempered martensite structure.
(6)
The flat steel wire according to any one of (1) to (4), further comprising a pearlite structure.
(7)
In mass%,
C: 0.35 to 0.60%,
Si: more than 1.50% and less than 2.00%,
Mn: more than 0.65% and less than 1.50%,
S: 0.010% or less,
P: 0.010% or less,
Cr: 0.005 to 0.60%,
Al: 0.005 to 0.080%,
N: 0.0020 to 0.0080%,
Ca: 0.0002 to 0.0050%,
Cu: 0.05-0.80%,
Ni: 0.05 to 0.60%,
Ti: 0 to 0.100%,
Nb: 0 to 0.050%,
V: 0 to 0.50%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
And the balance consists of Fe and impurities,
A wire rod for a flat steel wire, characterized by satisfying the following expressions <5> and <6>.
[Cu] / [Ni]> 1 Expression <5>
0.10 ≦ [Cu] + [Ni] ≦ 1.00 Expression <6>
However, [Cu] and [Ni] in the above formulas <5> and <6> represent the contents of each element in mass%.

The “impurity” in the “Fe and impurities” as the balance is a concept that includes, in addition to the components unintentionally contained in the steel material, other components contained within a range that does not impair the effects of the present invention. When the steel material is manufactured industrially, ore as a raw material, scrap, or a material mixed from a manufacturing environment is included.

The flat steel wire of the present invention has a high tensile strength of 1000 MPa or more and is unlikely to cause hydrogen-induced cracking and sulfide stress corrosion cracking even in a severe sour environment having a pH of less than 5.5. Can be used as reinforcement. The wire for flat steel wire of the present invention is suitable for manufacturing such flat steel wire.

(A) About chemical components:
Hereinafter,% for chemical components is% by mass.

C: 0.35 to 0.60%
C is an element that strengthens steel and must be contained in an amount of 0.35% or more. In order to achieve both excellent hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, a sufficiently high tensile strength is obtained even after high-temperature tempering treatment after quenching or heat treatment at high temperatures after processing into flat steel wire. In order to secure the strength, the content of C must be 0.35% or more. In order to further increase the strength, the content of C is preferably set to 0.38% or more, and more preferably 0.40% or more. However, when the content of C exceeds 0.60%, the strength of the joint becomes insufficient when flat steel wires are joined by welding. In addition, due to segregation, the structure of the steel material varies at a stage before being formed into a flat steel wire, and when the flat steel wire is subjected to flat pressure processing, a crack occurs in the wire. Therefore, a suitable C content is 0.35 to 0.60%. In order to ensure weldability, minimize segregation in the cross section of the flat steel wire, and increase the workability of the flat steel wire, the content is preferably 0.55% or less. In order to improve the material stress corrosion cracking resistance, it is desirable that the content is 0.50% or less.

Si: more than 1.50% and less than 2.00% Si forms a solid solution in the matrix, improves the strength of the flat steel wire, and is effective in improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. Element. In order to improve the resistance to sulfide stress corrosion cracking simultaneously with the resistance to hydrogen-induced cracking in a high-strength flat steel wire having a tensile strength exceeding 1000 MPa, Si must be contained in excess of 1.50%. However, when 2.00% or more is contained, problems occur such as cracking of the wire rod when flat-pressure processing is performed into a flat steel wire shape. Therefore, the content of Si is more than 1.50% and less than 2.00%. When it is desired to further increase the strength or to improve the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the Si content should be 1.60% or more, and the Si content should be 1.70% or more. More preferred. When it is desired to suppress cracking of the wire rod when processing into a flat steel wire, the content is preferably set to 1.80% or less.

Mn: more than 0.65% and less than 1.50% Mn is an element necessary for enhancing the hardenability of steel and increasing the strength. In a flat steel wire containing high Si, when performing heat treatment such as quenching, in order to suppress bending of the flat steel wire and to prevent high tensile residual stress from being generated on the surface, the content is set to be more than 0.65%. Must-have. However, when the content of Mn is 1.50% or more, the strength of the wire becomes too high, and there arises a problem that a crack occurs in the wire when it is processed into a flat steel wire. Therefore, the content of Mn in the present invention is more than 0.65% and less than 1.50%. In addition, when it is desired to further enhance the hardenability of the flat steel wire to suppress the bending of the wire or to increase the strength, Mn may be contained at least 0.70%, and may be contained at least 0.75%. More preferred. When it is desired to suppress cracking of the wire rod when working into a flat steel wire, Mn is preferably set to 1.30% or less, and more preferably 1.10% or less.

P: 0.010% or less P is contained as an impurity. However, when the content of P exceeds 0.010%, hydrogen-induced cracking and sulfide stress corrosion cracking are likely to occur, and in a flat steel wire having a tensile strength exceeding 1000 MPa, in a severe sour environment having a pH of less than 5.5. Hydrogen-induced cracking and sulfide stress corrosion cracking cannot be suppressed. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the content of P is preferably 0.008% or less, and more preferably less than 0.005%. The lower limit of the P content is not particularly limited. However, since excessive reduction leads to an increase in manufacturing cost, the lower limit of the P content may be set to 0.0005%.

S: 0.010% or less S is contained as an impurity. However, when the content of S exceeds 0.010%, MnS becomes coarse and deteriorates the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance. In order to improve the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking in flat steel wires with a tensile strength exceeding 1000 MPa, considering the balance with elements that easily form sulfides by combining with S, contain Ca I have to do it. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the S content is preferably 0.008% or less, and more preferably less than 0.005%. The lower limit of the S content is not particularly limited. However, since excessive reduction leads to an increase in manufacturing cost, the lower limit of the S content may be set to 0.0005%.

Cr: 0.005 to 0.60%
Cr is an element necessary for enhancing the hardenability of steel and increasing the strength, like Mn, and must be contained in an amount of 0.005% or more. However, if the content of Cr exceeds 0.60%, the strength of the wire becomes too high, causing problems such as cracking of the wire at the time of flat pressure working on a flat steel wire. Therefore, the appropriate Cr content in the present invention is 0.005 to 0.60%. In the case where the hardenability of the flat steel wire is further enhanced or the strength is increased, Cr may be contained at 0.05% or more, and more preferably at 0.10% or more. When it is desired to suppress cracking of the wire rod when performing flat pressure working on a flat steel wire, the content is preferably 0.50% or less, and more preferably 0.40% or less.

Al: 0.005 to 0.080%
Al not only has a deoxidizing effect, but also combines with N to form AlN, and its pinning effect has the effect of refining austenite grains during hot rolling. It has the effect of improving sulfide stress corrosion cracking resistance. For this reason, Al must be contained at 0.005% or more. From the viewpoint of improving the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance, the Al content is desirably 0.015% or more, and more desirably 0.020% or more. On the other hand, if the content of Al exceeds 0.080%, the effect is not only saturated, but also coarse AlN is generated, and the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance of the flat steel wire are changed. Lower. Therefore, the content of Al is preferably 0.060% or less, and more preferably 0.050% or less.

N: 0.0020 to 0.0080%
N dissolves in the matrix and has the effect of improving the strength of the flat steel wire. In addition, it combines with Al and Ti to form nitrides and carbonitrides, and has the effect of refining austenite grains during hot rolling, and has the effect of preventing hydrogen-induced cracking and sulfide stress corrosion of flat steel wires. It has the effect of improving cracking properties. In order to obtain these effects, N must be contained at 0.0020% or more, and more preferably 0.0030% or more. However, even if it is excessively contained, the effect is not only saturated, but also the productivity is deteriorated such as generation of cracks at the time of casting the steel. Therefore, the N content needs to be 0.0080% or less. is there. In order to ensure stable manufacturability, the content is preferably 0.0060% or less, and further preferably 0.0050% or less.

Ca: 0.0002-0.0050%
Ca is dissolved in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, it is possible to improve the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking even in a flat steel wire having a tensile strength exceeding 1000 MPa. In order to obtain these effects by Ca, Ca should be contained at 0.0002% or more, and when a higher effect is desired, Ca should be contained at 0.0005% or more. However, even when the content of Ca exceeds 0.0050%, the effect is saturated, and the oxide generated by reacting with oxygen in steel together with Al and Si becomes coarse, and on the contrary, the hydrogen-induced cracking resistance and This leads to a decrease in sulfide stress corrosion cracking resistance. Therefore, when Ca is contained, the appropriate Ca content is 0.0050% or less. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the Ca content is preferably 0.0030% or less, and more preferably 0.0025% or less.

In the flat steel wire of the present invention having excellent resistance to hydrogen-induced cracking and resistance to sulfide corrosion cracking, Cu: 0.05 to 0.80%, Ni: 0.05 to 0.60%, and Cu / Ni> In a range that satisfies the condition 1, Cu and Ni must be contained in a total amount of 0.10 to 1.00% (Equations <5> and <6>).

Cu: 0.05 to 0.80%
Cu has an effect of improving the sulfide stress corrosion cracking resistance of a flat steel wire having a tensile strength exceeding 1000 MPa, and is an essential additive element in the present invention. It also has the effect of increasing the hardenability of steel. In order to obtain the effect of improving the sulfide stress corrosion cracking resistance, the content must be 0.05% or more. However, when the content of Cu exceeds 0.80%, problems such as cracks occurring in the wire rod when processing into a flat steel wire occur. Therefore, the content of Cu is 0.05 to 0.80%. From the viewpoint of improving sulfide stress corrosion cracking resistance, the content of Cu contained is preferably 0.10% or more, and more preferably 0.20% or more. In consideration of the workability of the flat steel wire, the Cu content is preferably 0.70% or less, and more preferably 0.50% or less. In the present invention, Cu must be contained together with Ni. If Cu is contained alone at less than 0.05% of Ni, surface flaws occur during the hot rolling process for producing a wire. This causes cracking during primary drawing and flat pressure processing on flat steel wires, which makes it difficult to form flat steel wires.

Ni: 0.05 to 0.60%
Ni has an effect of improving the sulfide stress corrosion cracking resistance of a flat steel wire having a tensile strength exceeding 1000 MPa, and is an essential additive element in the present invention. It also has the effect of increasing the hardenability of steel. In order to obtain the effect of improving the sulfide stress corrosion cracking resistance, the content must be 0.05% or more. However, if the Ni content exceeds 0.60%, the strength of the wire becomes too high, and the wire tends to crack when subjected to flat pressing to a flat steel wire. Further, even if it can be processed, problems such as sulfide stress corrosion cracking are likely to occur. Therefore, the content of Ni is 0.05 to 0.60%. From the viewpoint of improving sulfide stress corrosion cracking resistance, the Ni content is preferably 0.07% or more, and more preferably 0.10% or more. The Ni content is preferably 0.50% or less, and more preferably 0.40% or less, in consideration of workability to a flat steel wire and sulfide corrosion cracking resistance. It is to be noted that Ni must be contained together with Cu, and when Cu is less than 0.05% and Ni is solely contained, the surface of the flat steel wire is subjected to a sour environment having a pH of less than 5.5 containing hydrogen sulfide. When tensile stress is applied to the steel sheet, fine cracks are likely to occur on the surface of the flat steel wire, and sulfide stress corrosion cracking resistance is reduced.

(4) By containing Cu and Ni in a range satisfying Cu / Ni> 1, an effect of suppressing sulfide stress corrosion cracking can be obtained (Equation <5>).

When the Cu / Ni ratio is 1 or less, that is, the Ni content is equal to or more than the Cu content, when a tensile stress is applied to the flat steel wire surface in a sour environment containing hydrogen sulfide, the flat steel wire surface Fine cracks are likely to occur, and sulfide stress corrosion cracking resistance is reduced. Therefore, Cu and Ni must satisfy Cu / Ni> 1. In order to improve the sulfide stress corrosion cracking resistance of the flat steel wire, the Cu / Ni ratio is preferably 1.5 or more, and more preferably 2 or more. Cu and Ni only have to satisfy Cu / Ni> 1, and the upper limit of the Cu / Ni ratio is not limited. However, generation of surface flaws and processing into a flat steel wire in a hot rolling process for manufacturing a wire rod are performed. In consideration of the properties, the Cu / Ni ratio is preferably 5 or less.

Furthermore, by containing Cu and Ni in a total amount of 0.10 to 1.00%, an effect of suppressing sulfide stress corrosion cracking can be obtained (Equation <6>). This is presumably because in a sour environment containing hydrogen sulfide, even if a tensile stress is applied to the surface of the flat steel wire, a film is formed on the surface of the flat steel wire, which has the effect of suppressing hydrogen intrusion.

When the total content of Cu and Ni is less than 0.10%, the above effects cannot be obtained. On the other hand, when the total content of Cu and Ni exceeds 1.00%, the strength of the steel material becomes too high, and when the flat steel wire is flat-pressed, cracks occur in the wire, or the pH 5 containing hydrogen sulfide is increased. Under a sour environment of less than 0.5, if tensile stress is applied to the surface of the flat steel wire, fine cracks are likely to occur on the surface of the flat steel wire, and the sulfide stress corrosion cracking resistance is reduced. From the viewpoint of improving sulfide stress corrosion cracking resistance, the total content of Cu and Ni is preferably 0.20 or more, and more preferably 0.40 or more. On the other hand, when it is desired to suppress the occurrence of cracks in the wire rod or the occurrence of minute cracks due to stress corrosion cracking during the production of flat steel wires, the total content of Cu and Ni is preferably 0.80% or less. , 0.50% or less is more preferable.

In the flat steel wire excellent in resistance to hydrogen-induced cracking and sulfide corrosion cracking resistance of the present invention, the Y1 value represented by the formula <1> needs to satisfy the formula <2>.
Y1 = 10 × √ [C] {(1 + 0.8 × [Si]) × (1 + 3 × [Mn]) × (1 + 2 × [Cr]) × (1 + 0.8 × [Cu]) × (1 + 0.7 × [Ni])} Formula <1>
12 × D <Y1 <30 × D Expression <2>
In the above formulas <1> and <2>, [C], [Si], [Mn], [Cr], [Cu], and [Ni] represent the content of each element in mass%, and D is Indicates the thickness (mm) of the flat steel wire.

Y1 is in a range that can be used as a flat steel wire having a tensile strength exceeding 1000 MPa, and is not necessary to impart hardenability to obtain sufficient strength without causing cracks in the wire when flat-pressing flat steel wire. Parameters.

Specifically, when manufacturing a flat steel wire, Y1 heats a flat steel wire having a thickness of D (mm) to a temperature of _three or more points of Ac, and performs a quenching process. This is a parameter that affects the fraction of the martensite structure obtained at a certain D / 2 (mm) position. When the tensile strength of the flat steel wire is adjusted by quenching and tempering, the value of Y1 is expressed by using the thickness D (mm) of the flat steel wire to obtain a uniform tempered martensite structure. It is necessary to exceed D. The cooling rate of the quenching treatment by oil cooling varies depending on the thickness D of the flat steel wire, but is generally about 30 to 50 ° C./sec. When the tensile strength is adjusted by primary drawing or flat pressure processing without quenching / tempering, it is necessary to apply a patenting process at the wire rod stage to obtain a uniform fine pearlite structure up to the center. Yes, the value of Y1 must exceed 12 × D. On the other hand, if Y1 exceeds 30 × D, martensite will be contained at the stage of the wire rod before being processed into a flat steel wire, and cracks will be generated when the flat steel wire is processed by cold working.

(B) Characteristics and manufacturing method:
In a sour environment, the higher the strength of the steel, the more likely it is for hydrogen-induced cracking and sulfide stress corrosion cracking to occur, but the flat steel wire in the present invention has improved hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. It is excellent, and can suppress hydrogen-induced cracking and sulfide stress corrosion cracking in a severe sour environment with a pH of less than 5.5 even when the tensile strength is 1000 MPa or more. If the production conditions are optimized by strictly adjusting the inclusions and components, hydrogen-induced cracking and sulfide stress corrosion cracking are less likely to occur even at a higher tensile strength. As long as hydrogen-induced cracking and sulfide stress corrosion cracking do not occur in a certain sour environment, the tensile strength of the flat steel wire is preferably 1100 MPa or more. However, when the tensile strength exceeds 1350 MPa, sulfide stress corrosion cracking occurs even when hydrogen-induced cracking does not occur.

The present invention controls the component segregation in the cross section perpendicular to the longitudinal direction of the wire rod by controlling the components and controlling the inclusions at the stage of melting the steel, and controlling the rolling and heating conditions. In addition, the present invention removes the processing strain imparted when processing the flat steel wire by heat treatment, and reduces the tensile residual stress generated in the longitudinal direction of the surface of the flat steel wire.

理由 The reason for reducing the residual tensile stress is that sulfide stress corrosion cracking occurs when the residual tensile stress measured in the longitudinal direction of the flat steel wire is 300 MPa or more. Therefore, when it is desired to suppress sulfide stress corrosion cracking in a severe sour environment having a pH of less than 5.5, the tensile residual stress in the longitudinal direction is preferably 250 MPa or less, and more preferably 100 MPa or less.

That is, in the present invention, in order to suppress sulfide stress corrosion cracking, not only the chemical components at the stage of smelting steel, but also the inclusions are controlled by controlling rolling and heating conditions, etc. Suppress component segregation in a cross section perpendicular to the direction. Further, in the present invention, the production conditions of the flat steel wire are controlled, for example, heat treatment is performed after the flat steel wire is processed, and the tensile residual stress measured in the longitudinal direction of the flat steel wire is controlled.

If the requirements of the present invention are satisfied, the effects of the present invention can be obtained irrespective of the method of manufacturing a flat steel wire. For example, a wire (a flat steel wire) is manufactured by the following manufacturing method. A flat steel wire may be manufactured using it as a material. The following manufacturing process is an example, and when a flat steel wire having a chemical composition and other requirements within the scope of the present invention is obtained by a process other than the following, the flat steel wire is included in the present invention.

Concretely, the chemical components such as C, Si, Mn, etc. are adjusted, and the steel ingots and slabs that are melted and cast by a converter or an electric furnace, etc., are subjected to a slab rolling process to be a material for product rolling. Slab. Before the product rolling, that is, during or before the heating of the slab rolling, the cast steel slab is subjected to a heat treatment at a temperature of 1250 ° C. or more for 12 hours or more. Thereby, a part of MnS forms a solid solution to be refined, and component segregation of the wire after product rolling can be suppressed.

After that, the slab is reheated and hot rolled, and finally finished into a bar or wire rod with a predetermined diameter.

The rolled wire (wire for flat steel wire) is processed into a flat steel wire after primary drawing. At this time, it is desirable that the total reduction in area when the rolled wire is processed into the flat steel wire is 80% or less. The flat steel wire is adjusted to a predetermined size by cold rolling the primary drawn wire using a cold rolling mill. In the state where the flat steel wire is kept cold-rolled, the flat steel wire is subjected to heat treatment because the tensile residual stress generated in the longitudinal direction of the flat steel wire is large. At this time, after reheating to the austenite region, oil quenching and tempering at a temperature of 460 ° C. or more may be performed. In this case, a flat steel wire including a tempered martensite structure is manufactured. In addition, without performing the quenching process, a heating process may be performed in which the heating temperature is 460 ° C. or more and the temperature is A ₁ point or less. In this case, a flat steel wire having a pearlite structure is produced.

平 When the flat steel wire is finished by cold rolling from a drawn round bar, both end faces in the thickness direction are parallel, and both end faces in the width direction have a semi-elliptical shape or an arc shape in a longitudinal vertical cross section, respectively. The same shape may be finished by wire drawing using a deformed die. If the ratio of the maximum width to the thickness in the width direction of the flat steel wire, the width / thickness ratio is less than 2.5, the thickness is large relative to the width of the flat steel wire, so the bending that occurs on the surface when the flat steel wire is bent Stress increases, and sulfide stress corrosion cracking is likely to occur. Furthermore, when the flat steel wire is to be incorporated into a flexible pipe, it becomes difficult to bend the flat steel wire, which causes problems such as cracks. On the other hand, when the flat steel wire has a width / thickness ratio of more than 10, after the flat steel wire is cold-rolled or after the heat treatment of the flat steel wire, the flat steel wire is warped and may be incorporated into a flexible pipe. It becomes impossible or the tensile residual stress increases, and sulfide stress corrosion cracking occurs. In addition, there is a problem in that when the wire is flat-pressed into a flat steel wire, a crack is easily generated in the steel.

(C) About optional components:
The high-strength flat steel wire of the present invention may contain, as necessary, Ti: 0 to 0.100%, Nb: 0 to 0.050%, V: 0 to 0.50%, Mo: 0 to 1.00%. , B: 0 to 0.0100%, REM: 0 to 0.1000%, Zr: 0 to 0.100%, and Mg: 0 to 0.0050% You may make it contain. Hereinafter, the effects of the optional elements Ti, Nb, V, Mo, B, REM, Zr, and Mg, and the reasons for limiting the contents will be described. Percentages for optional components are% by weight.

Ti: 0 to 0.100%
Ti combines with N and C to form carbides, nitrides or carbonitrides, and has an effect of miniaturizing austenite grains during hot rolling due to their pinning effect. It may be included because it has the effect of improving the properties and resistance to sulfide stress corrosion cracking. In order to obtain this effect, the content of Ti may be 0.001% or more. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the Ti content is desirably 0.005% or more, and more desirably 0.010% or more. On the other hand, if the content of Ti exceeds 0.100%, not only the effect is saturated, but also a large number of coarse TiN are generated, and the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance of the flat steel wire are rather reduced. Lower. Therefore, the content of Ti is preferably 0.050% or less, and more preferably 0.035% or less.

Nb: 0 to 0.050%
Nb combines with N and C to form carbides, nitrides or carbonitrides, and has an effect of miniaturizing austenite grains during hot rolling by their pinning effect. It may be included because it has the effect of improving the properties and resistance to sulfide stress corrosion cracking. In order to obtain this effect, Nb may be contained at 0.001% or more. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the content of Nb is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the content of Nb exceeds 0.050%, the effect is not only saturated, but also adversely affects the productivity of the steel, such as cracking of the steel slab in the step of slab-rolling the steel ingot or slab. Exert. Therefore, the Nb content is preferably 0.035% or less, and more preferably 0.030% or less.

V: 0 to 0.50%
V combines with C and N to form carbides, nitrides or carbonitrides, and can increase the strength of flat steel wires. For this purpose, 0.01% or more of V may be contained. However, if the content of V exceeds 0.50%, the strength of the flat steel wire increases due to the precipitated carbides and carbonitrides. Hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance are reduced. From the viewpoint of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking of the flat steel wire, the amount of V when contained is preferably 0.20% or less, and more preferably 0.10% or less. In order to stably obtain the above-described effect of V, the amount of V is preferably contained at 0.02% or more.

Mo: 0 to 1.00%
Mo is an element that enhances the hardenability of steel, and may be contained. However, in order to obtain the effect of improving the hardenability, the content may be 0.01% or more. However, when the content of Mo exceeds 1.00%, the strength of the wire becomes too high, and there arises a problem that a crack occurs in the wire when it is processed into a flat steel wire. Therefore, when Mo is contained, the content of Mo is 0.01 to 1.00%. When Mo is contained from the viewpoint of improving the hardenability, the content of Mo is preferably 0.02% or more, and more preferably 0.05% or more. In consideration of the workability of the flat steel wire, the content of Mo when it is contained is preferably 0.50% or less, and more preferably 0.30% or less.

B: 0 to 0.0100%
B is effective for increasing the hardenability of steel by adding a trace amount thereof, and may be contained in an amount of 0.0002% or more when an effect is desired. If the content exceeds 0.0100%, not only the effect is saturated, but also coarse nitride is generated, so that hydrogen-induced cracking and sulfide stress corrosion cracking are likely to occur. Therefore, when B is contained, the content of B is 0.0002 to 0.0100%. In order to further enhance the hardenability, the content of B may be set to 0.0005% or more, and more preferably 0.0010% or more. In consideration of hydrogen-induced cracking and sulfide stress corrosion cracking, the content of B in the case where B is contained is preferably 0.0050% or less, and more preferably 0.0030% or less.

REM: 0 to 0.1000%
REM is a general term for rare earth elements, and the content of REM is the total content of rare earth elements. REM, like Ca and Mg, has the effect of forming a solid solution in MnS and finely dispersing MnS. By dispersing MnS finely, resistance to hydrogen-induced cracking and resistance to sulfide stress corrosion cracking can be improved, and therefore MnS may be added. In order to obtain the effect of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking, REM should be contained at 0.0002% or more. To obtain higher effects, REM should be contained at 0.0005% or more. Just do it. However, even if the content of REM exceeds 0.1000%, the effect is saturated, the oxide generated by reacting with oxygen in the steel becomes coarse, and the resistance to hydrogen-induced cracking and sulfide stress corrosion are reduced. This leads to a decrease in cracking performance. Therefore, the content of REM when contained is 0.1000% or less. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the content of REM is preferably 0.0500% or less, and more preferably 0.030% or less.

Zr: 0 to 0.100%
Zr reacts with O to generate an oxide, and when added in a small amount, has an effect of finely dispersing the oxide and suppressing hydrogen-induced cracking and sulfide stress corrosion cracking. It may be added. In order to obtain the effect of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking, Zr should be contained at 0.0002% or more, and if a higher effect is desired, Zr should be contained at 0.001% or more. Just do it. However, when the content of Zr exceeds 0.100%, the effect is saturated and reacts with N and S in the steel to generate coarse nitrides and sulfides. Hydrogen-induced cracking and sulfide stress corrosion cracking resistance are reduced. Therefore, the content of Zr when it is contained is 0.100% or less. From the viewpoint of reducing inclusions that adversely affect the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the Zr content is preferably 0.080% or less, and more preferably 0.050% or less. preferable.

Mg: 0-0.0050%
Mg has an effect of forming a solid solution in MnS and finely dispersing MnS. By finely dispersing MnS, the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance can be improved even with a high-strength flat steel wire. Although Mg may not be contained (Mg: 0%), in order to obtain the effect of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking by Mg, Mg may be contained at 0.0002% or more. If a higher effect is desired, the content may be 0.0005% or more. However, even if the content of Mg exceeds 0.0050%, the effect is saturated, and the oxide generated by reacting with oxygen in steel together with Al and Ca becomes coarse, and on the contrary, the hydrogen-induced cracking resistance and This leads to a decrease in sulfide stress corrosion cracking resistance. Therefore, the proper content of Mg when it is contained is 0.0050% or less. From the viewpoint of improving the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking, the Mg content is preferably 0.0030% or less, and more preferably 0.0025% or less.

The balance is “Fe and impurities”. "Impurity" is a concept that includes, in addition to components unintentionally contained in steel materials, other components contained within a range that does not impair the effects of the present invention, and industrially manufactures steel materials. At this time, ore, scrap as a raw material, or those mixed from a production environment or the like are included.

Hereinafter, the present invention will be described specifically with reference to Examples.
Specifically, a steel rod having a chemical composition shown in Tables 1 and 2 was melted, and a steel slab was hot-rolled to produce a wire rod (wire rod for a flat steel wire). . The notation “-” in Tables 1 and 2 indicates that the content of the element is at the impurity level, and it can be determined that the element is not substantially contained.

Steels A and B having the chemical components shown in Table 1 were melted in an electric furnace, and the obtained steel ingot was heated at 1250 ° C. for 12 hours, and then slab-rolled into 122 mm square steel slabs to obtain rolling stock. . Next, the rolling material was heated at 1050 ° C. and rolled into a wire having a diameter of 12 mm. In the wire rolling, a patenting treatment was performed in which after coiling in a temperature range of an austenite single phase after finish rolling, the wire was directly immersed in a salt bath furnace maintained at 550 ° C. The rolled wire rod was subjected to a primary drawing process after lubricating the surface for primary drawing and then to a diameter of 11 mm. Thereafter, the drawn wire rod was rolled by a cold rolling mill to form a flat steel wire.
In order to produce flat steel wires having the same components but different tensile strengths and different tensile residual stresses in the longitudinal direction, test Nos. For A1 to A5 and A7, a flat steel wire cold-rolled to a width of 15 mm and a thickness of 5 mm using steel A was heated at 950 ° C. for 10 minutes, then immersed in cold oil and quenched. Tempering treatment was performed at a temperature of up to 600 ° C. for a predetermined time to produce flat steel wires having different tensile strengths. Test No. A1 to A5 and A7 are flat steel wires containing a tempered martensite structure. Test No. Regarding A6 and A8, a quenching treatment was not performed, and a heat treatment was performed at 450 ° C. and 580 ° C. for a predetermined time after cold rolling. Test No. For A9, no heat treatment was performed after cold rolling. Test No. A6, A8, and A9 are flat steel wires containing a pearlite structure.
Test No. For B1 to B5, flat steel wires having different shapes were produced by using a wire rod obtained by rolling steel B and changing the width and thickness when cold rolling to flat steel wires after primary drawing. Test No. B1 to B4 were processed into flat steel wires having different widths and thicknesses, heated at 950 ° C. for 10 minutes, immersed in cold oil, quenched, and tempered at 485 ° C. A flat steel wire having substantially the same tensile strength but different width and thickness was produced. Test No. For B5, a flat steel wire having a width of 18 mm and a thickness of 1.7 mm was prepared, heated at 950 ° C. for 10 minutes, immersed in cold oil and quenched, but a large warp occurred in the longitudinal direction of the flat steel wire. Subsequent tests were discontinued. Table 3 shows the heat treatment conditions after the cold rolling.

試 験 Test No. of the chemical component shown in Table 2. Samples Nos. 1 to 48 were melted in an electric furnace, and the obtained steel ingot was heated at 1250 ° C. for 12 hours, and then slab-rolled into 122 mm square steel slabs to obtain rolling slabs. Next, the material for rolling was heated at 1050 ° C. and rolled into a wire having a diameter of 12 mm (wire for flat steel wire). After the rolling, the surface of the wire was lubricated, and then subjected to a primary drawing to a wire having a diameter of 11 mm. Thereafter, the drawn wire rod was rolled by a cold rolling mill to form a flat steel wire having a width of 15 mm, a thickness of 4 mm or a width of 15 mm, a thickness of 5 mm, a width of 15 mm, and a thickness of 3 mm. Test No. For 1 to 26, 27, 29, 31, 33, 34, 36, 37, 39, 41, 42, 45 to 48, the formed flat steel wire was heated at 950 ° C. for 10 minutes after cold rolling. Thereafter, the steel sheet was immersed in cold oil to perform a quenching treatment, and a tempering treatment was performed at a temperature of 430 to 540 ° C. for 60 minutes. Test No. 28, 30, 32, 35, 38, 40, and 44 have any of the chemical components of steel outside the scope of the present invention, and when the flat steel wire was cold-rolled, cracks occurred in the flat steel wire. The subsequent tests were stopped without heat treatment. In Table 2, the underline indicates that the component composition is out of the range of the present invention.

About Y1 value and width and thickness, width / thickness ratio, tensile strength, tensile residual stress on the surface in the longitudinal direction of the flat steel wire, hydrogen-induced cracking resistance, and sulfide stress corrosion cracking resistance of the flat steel wire produced by the above method Tables 3 and 4 show the results of the investigation. In Tables 3 and 4, an underline indicates that the property is out of the range of the present invention, and "-" indicates a test for investigating various properties due to cracking when processing into a flat steel wire or the like. Not performed.

(4) The tensile strength, the tensile residual stress of the surface in the longitudinal direction, the resistance to hydrogen-induced cracking, and the resistance to sulfide stress corrosion cracking of the flat steel wire were investigated by the methods described below.

<1> Investigation of tensile strength of flat steel wire:
The tensile strength of the flat steel wire was measured by a breaking test described in JIS G 3546 (2012). A break test was performed at room temperature with a gauge length of 30 mm to determine the tensile strength. In addition, the cross-sectional area (S (mm ² )) of the flat steel wire was calculated using the following equation <3>, and was obtained by dividing the maximum test force until the test piece was broken by the cross-sectional area.
S = w × t−0.215t ² ... <3>
Here, w: width of flat steel wire (mm), t: thickness of flat steel wire (mm).

<2> Investigation of residual stress on longitudinal surface:
Here, the “longitudinal direction” refers to the thickness direction of the rolled flat steel wire, the length direction extending perpendicular to the width direction, and the “surface” refers to a depth of 50 μm from the surface of the flat steel wire toward the center of thickness. Point to the range. The residual stress is measured by a known X-ray method. Specifically, an X-ray stress measurement method using X-ray diffraction is used in accordance with JIS B 2711 (2013). The measurement was performed using the type of characteristic X-ray: MnKα ray, Cr filter, reference diffraction angle 2θ0: 152.0 °, η angle: 14.0 °, X-ray stress constant K: -336 MPa / ° X-rays were irradiated parallel to the longitudinal direction with the center position in the width direction as the center, and a diffraction pattern was obtained. Further, the residual stress was measured at six locations on the surface of the flat steel wire in which the distance between the measurement positions was at least 450 mm or more in the longitudinal direction, and the average value was obtained.

<3> Investigation of resistance to hydrogen-induced cracking:
Hydrogen-induced cracking resistance was evaluated using a flat steel wire cut to a length of 150 mm. The pH was adjusted to 5.0 by using HCl in a 5% NaCl + CH ₃ COOH solution. After degassing with nitrogen gas, a mixed gas of hydrogen sulfide (H ₂ S) and carbon dioxide (CO ₂ ) was introduced, and a flat steel wire was immersed in the solution to investigate the occurrence of cracks. At this time, the partial pressure of hydrogen sulfide is 0.01 MPa, the test temperature is 25 ° C., and the test time is 96 hours. After the test, the presence or absence of crack generation was confirmed by an ultrasonic test (UST: Ultra-sonic Test) in the thickness direction of the flat steel wire. The total area of the cracked portions determined to have cracked by ultrasonic flaw detection was determined by image analysis, and the hydrogen-induced cracking occurrence rate (χ (%)) was determined using the following equation <4>.

Here, A _f : the total area (mm ² ) of the crack occurrence part measured by UST, w: the width (mm) of the flat steel wire, and L: the length (mm) of the flat steel wire.

<4> Investigation of sulfide stress corrosion cracking:
The resistance to sulfide stress corrosion cracking was adjusted by applying a bending stress to a flat steel wire cut to a length of 150 mm using a four-point bending jig under the same conditions as those used in the investigation of hydrogen-induced cracking resistance. The flat steel wire was immersed together with the four-point bending jig to which the flat steel wire was fixed, and the presence or absence of cracks was examined. Specifically, strain gauges are attached to three places on the flat steel wire surface, bending strain is applied using a jig, and the maximum value of the bending strain read by the strain gauges attached to the three places is the flat steel wire. When a strain corresponding to a tensile stress of 90% of the yield strength was reached, the sample was fixed to a jig. The pH of the solution was adjusted to 5.0 by using 5% NaCl + CH ₃ COOH solution with HCl. After degassing with nitrogen gas, a mixed gas of hydrogen sulfide (H ₂ S) and carbon dioxide (CO ₂ ) is introduced, and immersed in the solution together with the 4-point bending jig fixing the flat steel wire to check for cracks. investigated. At this time, the partial pressure of hydrogen sulfide is 0.01 MPa, the test temperature is 25 ° C., and the test time is 96 hours. After the test, the presence or absence of cracks in the flat steel wire was visually determined. In addition, for the test material in which cracks were not visually confirmed, the longitudinal section at the position where the bending strain became maximum was filled with resin to check for the occurrence of minute cracks on the surface due to sulfide stress corrosion cracking. Then, the surface of the flat steel wire was inspected for the occurrence of minute cracks by an optical microscope. When a small crack having a depth of 20 μm or more was found on the surface of the flat steel wire, it was determined that sulfide stress corrosion cracking had occurred.

か ら From Table 3, Test No. which is an example of the present invention. A1 to A6 and B1 to B3 all satisfy the chemical components and the requirements of the present invention, and the production conditions of the steel material are appropriate. Therefore, the tensile strength is 1000 MPa or more, and hydrogen-induced cracking and sulfide No stress corrosion cracking occurred and no problem.

On the other hand, Test No. A7 has a tensile strength outside the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. Test No. A8 performed only heat treatment without performing quenching treatment. A9 was not heat-treated after processing into a flat steel wire. In each case, the tensile residual stress exceeded 300 MPa, and hydrogen-induced cracking and sulfide stress corrosion cracking occurred.
Test No. In B4, the width / thickness ratio of the flat steel wire is out of the range of the present invention, and sulfide stress corrosion cracking has occurred. Test No. B5 did not undergo a test such as a tensile test because a large warpage occurred in the longitudinal direction of the flat steel wire when the quenching treatment was performed.

か ら From Table 4, Test No. which is an example of the present invention. Nos. 1 to 26 all satisfy the chemical components and the requirements of the present invention and are suitable for steel production conditions. Therefore, the tensile strengths are all in the range of 1000 MPa to 1350 MPa, and hydrogen-induced cracking and sulfide corrosion cracking occur. Also has not occurred.

Test No. 27, 29, 31, 33, 34, 36, 37, 39, 41 to 42, and 45 to 48, any one of the chemical components, the formulas <5> and <6>, or the value of Y1 is the formula <2 >, Hydrogen-induced cracking and sulfide stress corrosion cracking have occurred.
Test No. 28, 30, 32, 35, 38, 40, and 44 are the chemical components of steel, any one of the formulas <5> and <6>, or the value of Y1 does not satisfy the formula <2>. , And when the flat steel wire was cold-rolled, the flat steel wire cracked. Therefore, the subsequent tests were stopped without performing the heat treatment.
Test No. 27 has a chemical component within the range of the present invention, but the value of Y1 does not satisfy the expression <2>, and sulfide stress corrosion cracking has occurred.
Test No. No. 28 has a chemical component within the range of the present invention, but the value of Y1 does not satisfy the formula <2>, cracks occur when processing into flat steel wire, and the test after heat treatment is stopped. I have.
Test No. In No. 29, Cu and Ni are out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 30, Ni was out of the range of the present invention, and cracks occurred during processing into flat steel wires, and the tests after the heat treatment were stopped.
Test No. In No. 31, Cu is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 32, Cu is out of the range of the present invention, and cracks occur when processing into flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 33, the Cu / Ni ratio was out of the range of the present invention, and sulfide stress corrosion cracking occurred.
Test No. In No. 34, Ni is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 35, the sum of Cu and Ni is out of the range of the present invention, a crack occurs when the flat steel wire is processed, and the test after the heat treatment is stopped.
Test No. In Nos. 36 and 37, the C content was outside the range of the present invention, and sulfide stress corrosion cracking occurred.
Test No. In No. 38, Si was out of the range of the present invention, and cracks occurred during processing into a flat steel wire, and the test after the heat treatment was stopped.
Test No. In No. 39, the content of Si is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 40, Mn was out of the range of the present invention, the value of Y1 did not satisfy the expression <2>, cracks occurred when the flat steel wire was processed, and the test after the heat treatment was stopped.
Test No. In No. 41, the content of Mn is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 42, the P content is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 44, Cr was out of the range of the present invention, the value of Y1 did not satisfy the expression <2>, cracks occurred when the flat steel wire was processed, and the test after the heat treatment was stopped.
Test No. In No. 45, the content of Al was out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking occurred.
Test No. In No. 46, the N content is out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking have occurred.
Test No. In No. 47, the Ca content is out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking have occurred.
Test No. In No. 48, the content of S is out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking have occurred.

Claims (7)

1. In mass%,
   C: 0.35 to 0.60%,
   Si: more than 1.50% and less than 2.00%,
   Mn: more than 0.65% and less than 1.50%,
   S: 0.010% or less,
   P: 0.010% or less,
   Cr: 0.005 to 0.60%,
   Al: 0.005 to 0.080%,
   N: 0.0020 to 0.0080%,
   Ca: 0.0002 to 0.0050%,
   Cu: 0.05-0.80%,
   Ni: 0.05 to 0.60%,
   Ti: 0 to 0.10%,
   Nb: 0 to 0.050%,
   V: 0 to 0.50%,
   Mo: 0 to 1.00%,
   B: 0 to 0.0100%,
   REM: 0 to 0.1000%,
   Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
   And the balance consists of Fe and impurities,
   Y1 represented by the following formula <1> satisfies the following formula <2>,
   Satisfying the following expressions <5> and <6>,
   The tensile strength is 1000 MPa or more and 1350 MPa or less,
   The tensile residual stress in the longitudinal direction is 300 MPa or less,
   A flat steel wire having a width / thickness ratio of 2.5 or more and 10 or less.
   Y1 = 10 × √ [C] {(1 + 0.8 × [Si]) × (1 + 3 × [Mn]) × (1 + 2 × [Cr]) × (1 + 0.8 × [Cu]) × (1 + 0.7 × [Ni])} Formula <1>
   12 × D <Y1 <30 × D Expression <2>
   [Cu] / [Ni]> 1 Expression <5>
   0.10 ≦ [Cu] + [Ni] ≦ 1.0
   0 ... Expression <6>
   However, [C], [Si], [Mn], [Cr], [Cu], and [Ni] in the above formulas <1>, <2>, <5>, and <6> are represented by mass% of each element. Represents the content, and D represents the thickness (mm) of the flat steel wire.
2. In mass%,
   Ti: 0.001 to 0.100%,
   Nb: 0.001 to 0.050%,
   V: 0.01 to 0.50%,
   The flat steel wire according to claim 1, comprising one or more selected from the group consisting of:
3. In mass%,
   Mo: 0.01 to 1.00%,
   B: 0.0002 to 0.0100%,
   The flat steel wire according to claim 1, comprising one or two selected from the group consisting of:
4. In mass%,
   REM: 0.0002 to 0.1000%,
   Zr: 0.0002 to 0.100%,
   Mg: 0.0002 to 0.0050%,
   The flat steel wire according to any one of claims 1 to 3, wherein the flat steel wire comprises one or more kinds selected from the group consisting of:
5. The flat steel wire according to any one of claims 1 to 4, wherein the flat steel wire includes a tempered martensite structure.
6. The flat steel wire according to any one of claims 1 to 4, wherein the flat steel wire has a pearlite structure.
7. In mass%,
   C: 0.35 to 0.60%,
   Si: more than 1.50% and less than 2.00%,
   Mn: more than 0.65% and less than 1.50%,
   S: 0.010% or less,
   P: 0.010% or less,
   Cr: 0.005 to 0.60%,
   Al: 0.005 to 0.080%,
   N: 0.0020 to 0.0080%,
   Ca: 0.0002 to 0.0050%,
   Cu: 0.05-0.80%,
   Ni: 0.05 to 0.60%,
   Ti: 0 to 0.100%,
   Nb: 0 to 0.050%,
   V: 0 to 0.50%,
   Mo: 0 to 1.00%,
   B: 0 to 0.0100%,
   REM: 0 to 0.1000%,
   Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
   And the balance consists of Fe and impurities,
   A wire rod for a flat steel wire, characterized by satisfying the following formulas <5> and <6>.
   [Cu] / [Ni]> 1 Expression <5>
   0.10 ≦ [Cu] + [Ni] ≦ 1.00 Expression <6>
   However, [Cu] and [Ni] in the above formulas <5> and <6> represent the contents of each element in mass%.