JP6897876B2 - Flat steel wire - Google Patents

Description

The present invention relates to flat steel wire .
The present application claims priority based on Japanese Patent Application No. 2018-124644 filed in Japan on June 29, 2018, the contents of which are incorporated herein by reference.

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 become deeper as the demand for oil increases, and there is an increasing demand for higher strength as reinforcing materials for flexible pipes. In addition, since flexible pipes are used in a sour environment containing hydrogen sulfide, hydrogen-induced cracking resistance, which is a characteristic that does not cause hydrogen-induced cracking (HIC) in flat steel wires used as reinforcing materials. , And sulfide stress corrosion cracking resistance, which is a property that does not cause sulfide stress corrosion cracking (SSC), is required. However, in general, the higher the strength line, the more likely it is that hydrogen-induced cracking and sulfide stress corrosion cracking will occur, making it difficult to apply the high-strength line to parts such as flexible pipes used in sour environments. .. So far, techniques for providing high-strength lines used in such a sour environment have been proposed.

Patent Document 1 describes in terms of mass%, C: 0.25 to 0.60%, Si: more than 0.50 and less than 2.0%, Mn: 0.25 to 1.50%, S: 0. It contains 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%, and further Ca. : 0 to 0.0050% and Mg: 0 to 0.0050%, one or two kinds are contained so as to satisfy [Ca] + [Mg]> 0.20 × [S], and tensile strength. Is 1000 MPa or more, the average value of 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 A high-strength flat steel wire having excellent hydrogen-induced cracking resistance, which is characterized by the following, has been proposed.

Patent Document 2 describes, in terms of mass%, C: 0.85% or more and 1.00% or less, Si: 0.80% or more and 1.30% or less, Mn: 0.30% or more and 0.90% or less, P. : 0.017% or less, S: 0.010% or less, Cu: 0.20% or less, Al: 0% or more and 0.10% or less, Ti: 0% or more and 0.05% or less, 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, V: 0% or more and 0.50% or less, and is viewed in a cross section perpendicular to the longitudinal direction. In this case, it is a flat steel wire having rounded corners, 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 and 8. Twisting under the conditions that the yield strength or 0.2% proof stress obtained by the tensile strength is 1600 MPa or more and 2000 MPa or less, the tensile strength is 1900 MPa or more, the breaking elongation is 2% or more, and the distance between chucks is 500 mm. A flat steel wire characterized in that the twist value obtained in the test is 12 times or more has been proposed.

WO2017 / 154930 Japanese Patent No. 6116680

The technique 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.

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

The present invention is a high-strength flat steel wire having a tensile strength of 1000 to 1350 MPa, and even in a harsh sour environment where the pH is less than 5.5, hydrogen induction is performed without coating the surface of the flat steel wire. It is an object of the present invention to provide a flat steel wire which does not cause cracking and sulfide stress corrosion cracking and can be used as a reinforcing wire material for a flexible pipe or the like having a deep mining depth.

In order to solve the above-mentioned problems, the present inventors have conducted various studies on the effects of additive elements on hydrogen-induced cracking and sulfide stress corrosion cracking, and obtained the following findings (a) to (d). It was.

(A) Hydrogen-induced cracking and sulfide stress corrosion cracking resistance of flat steel wire are generated starting from coarse sulfide contained in flat steel wire. In particular, when sulfides such as MnS are coarse, when the primary wire drawing process or the flat pressure process after the primary wire drawing process, which is necessary for forming a hot-rolled wire rod into a flat steel wire, is performed. Voids are formed around the coarse sulfide, which is a factor that promotes hydrogen-induced cracking and sulfide stress corrosion cracking in a severe sour environment with a pH of less than 5.5. Therefore, it is necessary to miniaturize the sulfide inevitably contained in the wire rod as much as possible. For the miniaturization of sulfide, it is effective to add Ca to obtain MnS or CaS in which Ca is partially dissolved.

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

(C) During primary wire drawing and subsequent cupronickel processing, the steel material does not crack, and hydrogen-induced cracking and sulfide stress corrosion cracking do not occur in a sour environment with a pH of less than 5.5. In order to do so, Y1 represented by the following formula <1> must contain C, Si, Mn, Cr, Cu, and Ni within a range satisfying 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 contents of each element in mass%. D represents the thickness (mm) of the flat steel wire.

(D) From the wire rod to the flat steel wire, for example, the rolled wire rod is first-lined and then processed by deformed wire drawing or cold rolling by a cold rolling mill. In the flat steel wire manufactured by such a process, tensile residual stress is generated in the longitudinal direction of the surface of the flat steel wire due to the processing strain caused by cold working. In particular, for flat steel wires having a tensile strength of more than 1000 MPa, the tensile residual stress on the surface induces sulfide stress corrosion cracking, so the tensile strength must be as small as possible.

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

(1)
By mass%
C: 0.35-0.60%,
Si: More than 1.50%, less than 2.00%,
Mn: Exceeds 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 to 0.80%,
Ni: 0.05 to 0.60%,
Ti: 0 to 0.100%,
Nb: 0 to 0.050%,
V: 0 to 0.50%,
Mo: 0-1.00%,
B: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
The balance consists of Fe and impurities.
Y1 represented by the following formula <1> satisfies the following formula <2>.
Satisfying the following formula <5> and the following formula <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])} ・ ・ ・ Equation <1>
12 × D <Y1 <30 × D
・ ・ ・ Formula <2>
[Cu] / [Ni]> 1
・ ・ ・ Formula <5>
0.10 ≤ [Cu] + [Ni] ≤ 1.00
... formula <6>
However, [C], [Si], [Mn], [Cr], [Cu], and [Ni] in the above formulas <1>, <2>, <5>, and <6> are based on the mass% of each element. The content is represented, and D represents the thickness (mm) of the flat steel wire.
(2)
By mass%
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
V: 0.01-0.50%,
The flat steel wire according to (1), which contains one kind or two or more kinds selected from the above.
(3)
By mass%
Mo: 0.01-1.00%,
B: 0.0002 to 0.0100%,
The flat steel wire according to (1) or (2), which contains one or two kinds selected from.
(4)
By 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), which contains one kind or two or more kinds selected from the above.
(5)
The flat steel wire according to any one of (1) to (4), which comprises a tempered martensite structure.
(6)
The flat steel wire according to any one of (1) to (4), which comprises a pearlite structure.

The term "impurities" in "Fe and impurities" as the balance is a concept that includes components unintentionally contained in the steel material and other components contained within a range that does not impair the effects of the present invention. Therefore, when steel materials are industrially manufactured, those mixed from ore, scrap, or the manufacturing environment as raw materials are included.

The flat steel wire of the present invention has a high tensile strength of 1000 MPa or more, and is less likely to cause hydrogen-induced cracking and sulfide stress corrosion cracking even in a severe sour environment with a pH of less than 5.5. Therefore, the tension of the flexible pipe It can be used as a reinforcing material .

(A) About chemical components:
Hereinafter,% of the chemical composition is mass%.

C: 0.35-0.60%
C is an element that reinforces steel and must be contained in an amount of 0.35% or more. For the purpose of achieving both excellent hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, the tensile strength is sufficiently high even if high-temperature tempering treatment after quenching or heat treatment at high temperature is performed after processing into flat steel wire. To ensure strength, the C content must be 0.35% or higher. If it is desired to further increase the strength, the C content is preferably 0.38% or more, and more preferably 0.40% or more. However, if the C content exceeds 0.60%, the strength of the joint portion becomes insufficient when the flat steel wires are joined by welding. In addition, segregation causes variations in the structure of the steel material before it is formed into a flat steel wire, causing cracks in the wire material when the flat steel wire is flat-pressed. Therefore, a suitable C content is 0.35 to 0.60%. If it is desired to ensure weldability, reduce segregation in the cross section of the flat steel wire as much as possible, and improve the workability of the flat steel wire, it is preferably 0.55% or less, and further, hydrogen-induced cracking resistance and sulfurization resistance. It is desirable to set it to 0.50% or less in order to improve the physical stress corrosion cracking property.

Si: Exceeds 1.50% and less than 2.00% Si dissolves in the matrix and is effective in improving the strength of flat steel wire and improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. It is an element. In order to improve hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance in high-strength flat steel wire having a tensile strength of more than 1000 MPa, Si must be contained in an amount of more than 1.50%. However, if it contains 2.00% or more, there arises a problem that the wire rod is cracked when flat-pressure processing is performed into the shape of a flat steel wire. Therefore, the Si content exceeds 1.50% and is less than 2.00%. When it is desired to further increase the strength or to improve the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance, Si may be contained in an amount of 1.60% or more, and 1.70% or more may be contained. More preferred. When it is desired to suppress cracking of the wire rod when processing it into a flat steel wire, it is preferably 1.80% or less.

Mn: Exceeding 0.65% and less than 1.50% Mn is an element necessary for enhancing the hardenability of steel and increasing its strength. In a flat steel wire containing high Si, in order to suppress bending of the flat steel wire during heat treatment such as quenching and not to generate high tensile residual stress on the surface, it should be contained in excess of 0.65%. necessary. However, if the Mn content is 1.50% or more, the strength of the wire rod becomes too high, and there arises a problem that the wire rod is cracked when it is processed into a flat steel wire. Therefore, the Mn content in the present invention exceeds 0.65% and is less than 1.50%. When it is desired to further improve the hardenability of the flat steel wire and suppress the bending of the wire rod or to increase the strength, Mn may be contained in an amount of 0.70% or more, and 0.75% or more. More preferred. When it is desired to suppress cracking of the wire rod when processing it into a flat steel wire, Mn is preferably 1.30% or less, and even more preferably 1.10% or less.

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

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

Cr: 0.005 to 0.60%
Like Mn, Cr is an element necessary for enhancing the hardenability of steel and increasing its strength, and must be contained in an amount of 0.005% or more. However, if the Cr content exceeds 0.60%, the strength of the wire rod becomes too high, causing problems such as cracks in the wire rod when flat-pressure processing the flat steel wire. Therefore, the appropriate Cr content in the present invention is 0.005 to 0.60%. When the hardenability of the flat steel wire is further increased or the strength is increased, Cr may be contained in an amount of 0.05% or more, more preferably 0.10% or more. When it is desired to suppress cracking of the wire rod during flat pressure processing into a flat steel wire, it is preferably 0.50% or less, and even 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. Therefore, Al must be contained in an amount of 0.005% or more. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, it is desirable that the Al content is 0.015% or more, and further preferably 0.020% or more. On the other hand, when the Al content exceeds 0.080%, not only the effect is 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. Decrease. Therefore, the Al content 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 has the effect of forming nitrides and carbonitrides by combining with Al and Ti to refine austenite grains during hot rolling, and has hydrogen-induced cracking resistance and sulfide stress corrosion resistance of flat steel wire. It has the effect of improving crackability. In order to obtain these effects, N must be contained in an amount of 0.0020% or more, and more preferably 0.0030% or more. However, even if it is excessively contained, not only the effect is saturated but also the manufacturability is deteriorated such as cracking when casting steel. Therefore, the N content should be 0.0080% or less. is there. In order to ensure stable manufacturability, it is preferably 0.0060% or less, and even more preferably 0.0050% or less.

Ca: 0.0002 to 0.0050%
Ca has the effect of being dissolved in MnS and finely dispersing MnS. By finely dispersing MnS, hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance can be improved even for flat steel wire having a tensile strength exceeding 1000 MPa. In order to obtain these effects by Ca, Ca may be contained in an amount of 0.0002% or more, and when a higher effect is desired, it may be contained in an amount of 0.0005% or more. However, even if the Ca content exceeds 0.0050%, the effect is saturated, and the oxide produced by reacting with oxygen in the steel together with Al and Si becomes coarse, and on the contrary, hydrogen-induced cracking resistance and cracking resistance Sulfide resistance Stress corrosion cracking is reduced. Therefore, the appropriate Ca content when contained is 0.0050% or less. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, the Ca content is preferably 0.0030% or less, and more preferably 0.0025% or less.

In the cupronickel wire excellent in hydrogen-induced cracking resistance and sulfide corrosion cracking resistance of the present invention, Cu: 0.05 to 0.80%, Ni: 0.05 to 0.60%, and Cu / Ni> A total of 0.10 to 1.00% of Cu and Ni must be contained within the range satisfying 1 (formula <5>, formula <6>).

Cu: 0.05 to 0.80%
Cu has the effect of improving the sulfide stress corrosion cracking resistance of 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 improving 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 Cu content exceeds 0.80%, there arises a problem that the wire rod is cracked when it is processed into a flat steel wire. Therefore, the Cu content is 0.05 to 0.80%. From the viewpoint of improving the sulfide stress corrosion cracking resistance, the content of Cu to be contained is preferably 0.10% or more, and more preferably 0.20% or more. In consideration of workability into flat steel wire, the Cu content is preferably 0.70% or less, and even more preferably 0.50% or less. In the present invention, Cu must be contained together with Ni, and if Cu is contained alone at a Ni content of less than 0.05%, surface defects occur in the hot rolling process for producing a wire rod. This makes it easier to form cracks during the subsequent primary wire drawing and flat pressure machining of flat steel wire, making it difficult to form into flat steel wire.

Ni: 0.05 to 0.60%
Ni has the effect of improving the sulfide stress corrosion cracking resistance of 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 improving 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 rod becomes too high, and cracks are likely to occur in the wire rod during flat pressure processing into a flat steel wire. Further, even if it can be processed, there arises a problem that sulfide stress corrosion cracking is likely to occur. Therefore, the Ni content is 0.05 to 0.60%. From the viewpoint of improving the 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 even more preferably 0.40% or less, in consideration of processability to flat steel wire and sulfide corrosion cracking resistance. Ni must be contained together with Cu, and when Cu is contained alone at less than 0.05%, the surface of the flat steel wire is subjected to a sour environment containing hydrogen sulfide and having a pH of less than 5.5. When tensile stress is applied to the steel wire, fine cracks are likely to occur on the surface of the flat steel wire, and the sulfide stress corrosion cracking resistance is lowered.

By containing Cu / Ni in a range satisfying Cu / Ni> 1, the 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 Cu content or more, when tensile stress is applied to the surface of the cupronickel wire in a sour environment containing hydrogen sulfide, the surface of the cupronickel wire becomes Fine cracking is likely to occur, and sulfide stress corrosion cracking resistance is reduced. Therefore, Cu and Ni must satisfy Cu / Ni> 1. In order to enhance 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 need only satisfy Cu / Ni> 1, and the upper limit of the Cu / Ni ratio is not limited, but surface defects occur in the hot rolling process for manufacturing wire rods and processing into cupronickel wire. Considering the properties, the Cu / Ni ratio is preferably 5 or less.

Further, when Cu and Ni are contained in the range of 0.10 to 1.00% in total, the effect of suppressing sulfide stress corrosion cracking can be obtained (Equation <6>). It is considered that this is because even if tensile stress is applied to the surface of the flat steel wire in a sour environment containing hydrogen sulfide, 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-mentioned effect 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, cracks occur in the flat steel wire during flat pressure processing, and pH 5 containing hydrogen sulfide is generated. In a sour environment of less than .5, when 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 lowered. From the viewpoint of improving the 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 generation of cracks in the wire rod and the generation of fine cracks due to stress corrosion cracking during the production of flat steel wire, the total content of Cu and Ni is preferably 0.80% or less. , 0.50% or less is even more preferable.

In the flat steel wire having excellent hydrogen-induced cracking resistance 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])} ・ ・ ・ Equation <1>
12 × D <Y1 <30 × D ・ ・ ・ Equation <2>
[C], [Si], [Mn], [Cr], [Cu], and [Ni] in the above formulas <1> to <2> represent the contents of each element in mass%, and D is Represents the thickness (mm) of a flat steel wire.

Y1 is in a range that can be used as a flat steel wire having a tensile strength of more than 1000 MPa, and is necessary to provide hardenability to obtain sufficient strength without cracking the wire rod when flat-pressure processing the flat steel wire. Parameter.

Specifically, when Y1 manufactures a flat steel wire, when the flat steel wire having a thickness of D (mm) is _{heated to a temperature of 3} points or more of Ac and hardened, it is formed at the center of the flat steel wire. It is a parameter that affects the fraction of martensite structure obtained at a certain D / 2 (mm) position. When adjusting the tensile strength of a flat steel wire by quenching / tempering, the value of Y1 is expressed using the thickness D (mm) of the flat steel wire in order to obtain a uniform tempered martensite structure, 12 × It is necessary to exceed D. The cooling rate of the quenching process by oil cooling varies depending on the thickness D of the flat steel wire, but is generally about 30 to 50 ° C./sec. In addition, when adjusting the tensile strength by primary wire drawing or flat pressure processing without quenching / tempering, it is necessary to perform patenting treatment 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. Further, when Y1 exceeds 30 × D, martensite is contained at the stage of the wire rod before being processed into a flat steel wire, and cracks occur 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 easily hydrogen-induced cracking and sulfide stress corrosion cracking occur. However, the flat steel wire in the present invention has hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. It is excellent, and even if the tensile strength is 1000 MPa or more, hydrogen-induced cracking and sulfide stress corrosion cracking can be suppressed in a severe sour environment with a pH of less than 5.5. If the manufacturing conditions are optimized by more 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. The tensile strength of the flat steel wire is preferably 1100 MPa or more as long as hydrogen-induced cracking and sulfide stress corrosion cracking do not occur under a certain sour environment. 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 component segregation in a cross section perpendicular to the longitudinal direction of a wire rod by adjusting the components at the stage of melting steel, controlling inclusions, and controlling rolling and heating conditions. Further, the present invention removes the processing strain applied when processing the flat steel wire by heat treatment to reduce the tensile residual stress generated in the longitudinal direction of the surface of the flat steel wire.

The reason for reducing the tensile residual stress is that sulfide stress corrosion cracking occurs when the tensile residual 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 even 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 melting the steel but also the inclusions are controlled by controlling the rolling and heating conditions, and the length of the wire rod. Suppresses component segregation within a cross section perpendicular to the direction. Further, in the present invention, the manufacturing conditions of the flat steel wire are controlled by applying heat treatment after processing the flat steel wire, 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 effect of the present invention can be obtained regardless of the method for manufacturing the flat steel wire. For example, a wire rod (wire rod for flat steel wire) can be manufactured by the manufacturing method shown below. Flat steel wire may be manufactured using it as a material. The following manufacturing process is an example, and when a flat steel wire whose chemical composition and other requirements are 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.

Specifically, steel ingots and slabs that have been melted and cast by adjusting chemical components such as C, Si, and Mn in a converter or electric furnace are subjected to a slab-rolling process to be used as a material for product rolling. It is a steel piece that becomes. Before rolling the product, that is, at the time of heating the bulk rolling or at a stage before that, the cast steel pieces are heat-treated at a temperature of 1250 ° C. or higher for 12 hr or higher. As a result, a part of MnS is solid-solved and made finer, and segregation of the components of the wire rod after rolling the product can be suppressed.

After that, the steel pieces are reheated and hotly rolled into a steel bar or wire rod having a predetermined diameter.

The rolled wire (wire for flat steel wire) is subjected to primary wire drawing and then processed into flat steel wire. At this time, it is desirable that the total reduced surface reduction rate when the rolled wire is processed into a 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 machine. In the cold-rolled state, the tensile residual stress generated in the longitudinal direction of the flat steel wire is large, so the flat steel wire is heat-treated. At this time, after reheating to the austenite region, oil quenching may be performed, and quenching / tempering treatment may be performed in which the tempering is performed at a temperature of 460 ° C. or higher. In this case, a flat steel wire containing a tempered martensite structure is produced. Further, the quenching treatment may not be performed, and the heating treatment may be performed so that the heating temperature is 460 ° C. or higher and A ₁ point or lower. In this case, a flat steel wire containing a pearlite structure is manufactured.

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 or arcuate vertical cross section in the longitudinal direction, respectively. The same shape may be finished by wire drawing using a deformed die. When the ratio of maximum width to thickness in the width direction of the flat steel wire and the width / thickness ratio are less than 2.5, the thickness with respect to the width of the flat steel wire is large, so the bending that occurs on the surface when the flat steel wire is bent. The stress increases and sulfide stress corrosion cracking is likely to occur. Further, when the flat steel wire is incorporated into the flexible pipe, it becomes difficult to bend the flat steel wire, which causes problems such as cracking. On the other hand, when the width / thickness ratio of the flat steel wire exceeds 10, the flat steel wire may be warped and incorporated into the flexible pipe after cold rolling on the flat steel wire or after heat treatment of the flat steel wire. It becomes impossible or the tensile residual stress becomes large and sulfide stress corrosion cracking occurs. In addition, there is a problem that the steel material is liable to crack when flat-pressure processing is performed from the wire rod to the flat steel wire.

(C) Optional ingredients:
The high-strength flat steel wire of the present invention has Ti: 0 to 0.100%, Nb: 0 to 0.050%, V: 0 to 0.50%, Mo: 0 to 1.00%, if necessary. , B: 0 to 0.0100%, REM: 0 to 0.1000%, Zr: 0 to 0.100%, and Mg: 0 to 0.0050%. It may be contained. Hereinafter, the action and effect of the optional elements Ti, Nb, V, Mo, B, REM, Zr, and Mg and the reason for limiting the content will be described. % For any component is mass%.

Ti: 0 to 0.100%
Ti combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect, and hydrogen-induced cracking of flat steel wire. Since it has the effect of improving properties and sulfide stress corrosion cracking resistance, it may be contained. In order to obtain this effect, Ti may be contained in an amount of 0.001% or more. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, it is desirable that the Ti content be 0.005% or more, and further 0.010% or more. On the other hand, when the Ti content exceeds 0.100%, not only the effect is saturated, but also a large number of coarse TiNs are generated, and on the contrary, the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance of the flat steel wire are obtained. To reduce. Therefore, the Ti content 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 the effect of refining austenite grains during hot rolling due to their pinning effect, and hydrogen-resistant cracking of flat steel wire. Since it has the effect of improving properties and sulfide stress corrosion cracking resistance, it may be contained. In order to obtain this effect, Nb may be contained in an amount of 0.001% or more. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, it is desirable that the Nb content is 0.005% or more, and further 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, not only the effect is saturated, but also the steel slab is cracked in the process of ingot rolling the ingot or slab, which adversely affects the steel manufacturability. To exert. Therefore, the Nb content is preferably 0.035% or less, and more preferably 0.030% or less.

V: 0 to 0.50%
V can combine with C and N to form carbides, nitrides or carbonitrides, increasing the strength of the flat steel wire. For this purpose, 0.01% or more of V may be contained, but if the V content exceeds 0.50%, the strength of the flat steel wire increases due to the precipitated carbides and carbonitrides, and rather, the strength of the flat steel wire increases. 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 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-mentioned effect of V, the amount of V is preferably 0.02% or more.

Mo: 0-1.00%
Mo is an element that enhances the hardenability of steel and may be contained. However, in order to obtain the effect of enhancing hardenability, it may be contained in an amount of 0.01% or more. However, if the Mo content exceeds 1.00%, the strength of the wire rod becomes too high, causing problems such as cracks in the wire rod when processing into flat steel wire. Therefore, the Mo content when contained is 0.01 to 1.00%. From the viewpoint of improving hardenability, the Mo content when contained is preferably 0.02% or more, and even more preferably 0.05% or more. In consideration of workability into flat steel wire, the Mo content when contained is preferably 0.50% or less, and even more preferably 0.30% or less.

B: 0 to 0.0100%
B is effective in enhancing the hardenability of steel by adding a small amount, and may be contained in an amount of 0.0002% or more if an effect is desired. Even if it is contained in excess of 0.0100%, not only the effect is saturated, but also coarse nitrides are generated, so that hydrogen-induced cracking and sulfide stress corrosion cracking are likely to occur. Therefore, the content of B when contained is 0.0002 to 0.0100%. If it is desired to further improve the hardenability, the content of B may be 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 when contained is preferably 0.0050% or less, and even 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. Like Ca and Mg, REM dissolves in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance can be improved, and thus MnS may be added. In order to obtain the effect of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking, REM may be contained in an amount of 0.0002% or more, and if a higher effect is desired, 0.0005% or more should be contained. Just do it. However, even if the REM content exceeds 0.1000%, the effect is saturated and the oxide produced by reacting with oxygen in the steel becomes coarse, resulting in hydrogen-induced cracking resistance and sulfide stress corrosion resistance. It causes a decrease in crackability. Therefore, the content of REM when contained is 0.1000% or less. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, the REM content is preferably 0.0500% or less, and more preferably 0.030% or less.

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

Mg: 0 to 0.0050%
Mg has the effect of being dissolved in MnS and finely dispersing MnS. By finely dispersing MnS, hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance can be improved even with high-strength flat steel wire. Although it is not necessary to contain Mg (Mg: 0%), in order to obtain the effect of suppressing hydrogen-induced cracking and sulfide stress corrosion cracking by Mg, it is sufficient to contain Mg in an amount of 0.0002% or more. If a higher effect is to be obtained, 0.0005% or more may be contained. However, even if the Mg content exceeds 0.0050%, the effect is saturated, and the oxide produced by reacting with oxygen in the steel together with Al and Ca becomes coarse, and on the contrary, hydrogen-induced cracking resistance and cracking resistance Sulfide resistance Stress corrosion cracking is reduced. Therefore, the appropriate Mg content when contained is 0.0050% or less. From the viewpoint of improving hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, the Mg content is preferably 0.0030% or less, and more preferably 0.0025% or less.

The rest is "Fe and impurities". The term "impurity" is a concept that includes a component unintentionally contained in a steel material and other components contained within a range that does not impair the effects of the present invention, and industrially manufactures a steel material. In some cases, ore as a raw material, scrap, or substances mixed from the manufacturing environment are included.

Hereinafter, the present invention will be specifically described with reference to Examples.
Specifically, the wire rod (wire rod for flat steel wire) produced by melting the steels of the chemical components shown in Tables 1 and 2 and hot rolling the steel pieces was made into a flat steel wire by the following method. .. The notation of "-" 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, the obtained ingots were heated at 1250 ° C. for 12 hours, and then ingot-rolled into 122 mm square steel pieces. .. Next, the rolling material was heated at 1050 ° C. and rolled into a wire rod having a diameter of 12 mm. The wire rod was rolled by coiling in a temperature range of austenite single phase after finish rolling, and then directly immersed in a salt bath furnace maintained at 550 ° C. for a patenting treatment. The surface of the rolled wire was lubricated for the primary wire drawing, and then the primary wire drawing process was performed so that the diameter was 11 mm. Then, the wire drawn wire was rolled by a cold rolling mill and formed into a flat steel wire.
In order to make flat steel wires with the same components but different tensile strength and longitudinal tensile residual stress, Test No. For A1 to A5 and A7, 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 for quenching, and then subjected to quenching treatment. A flat steel wire having different tensile strength was produced by performing a tempering treatment in which the material was held at a temperature of about 600 ° C. for a predetermined time. Test No. A1 to A5 and A7 are flat steel wires containing a tempered martensite structure. Test No. A6 and A8 were not subjected to quenching treatment, but were heat-treated by holding them at 450 ° C. and 580 ° C. for a predetermined time after cold rolling. In addition, the test No. No heat treatment was performed on A9 after cold rolling. Test No. A6, A8, and A9 are flat steel wires containing a pearlite structure.
Test No. For B1 to B5, a wire rod obtained by rolling steel B was used, and the width and thickness at the time of cold rolling into a flat steel wire after the primary wire drawing process were changed to produce flat steel wires having different shapes. Test No. For B1 to B4, after processing into flat steel wires having different widths and thicknesses, they are heated at 950 ° C. for 10 minutes, then immersed in cold oil for quenching treatment, and then tempered at a temperature of 485 ° C. , Flat steel wires having almost the same tensile strength but different widths and thicknesses were 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, and then immersed in cold oil for quenching. Subsequent tests were discontinued. Table 3 shows the heat treatment conditions after cold rolling.

Test Nos. Of the chemical components shown in Table 2. 1 to 48 were melted in an electric furnace, the obtained ingots were heated at 1250 ° C. for 12 hours, and then ingot-rolled into 122 mm square steel pieces, which were used as a rolling material. Next, the rolling material was heated at 1050 ° C. and rolled into a wire rod (wire rod for flat steel wire) having a diameter of 12 mm. After rolling, the surface of the wire rod was lubricated, and then primary wire drawing was performed so that the wire rod had a diameter of 11 mm. Then, the wire drawn wire 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 1-26, 27, 29, 31, 33, 34, 36, 37, 39, 41, 42, 45-48, the formed flat steel wire was heated at 950 ° C. for 10 minutes after cold rolling. After that, it was immersed in cold oil and subjected to quenching treatment, and then tempered treatment was carried out at a temperature of 430 to 540 ° C. for 60 minutes. Test No. 28, 30, 32, 35, 38, 40, 44 are because any of the chemical components of the steel is outside the scope of the present invention, and the flat steel wire is cracked when it is cold-rolled. , Subsequent tests were discontinued without heat treatment. In Table 2, the underline indicates that the component composition is out of the scope of the present invention.

Y1 value and width and thickness of flat steel wire produced by the above method, width / thickness ratio, tensile strength, tensile residual stress on the longitudinal surface of flat steel wire, hydrogen-induced cracking resistance, sulfide stress corrosion cracking resistance The results of the investigation are shown in Tables 3 and 4. In Tables 3 and 4, underlines indicate that the characteristics are out of the scope of the present invention, and "-" indicates that cracks occur during processing into flat steel wire, so a test for investigating various characteristics is performed. Indicates that the was not done.

The tensile strength of the flat steel wire, the tensile residual stress on the surface in the longitudinal direction, the hydrogen-induced cracking resistance and the sulfide stress corrosion cracking resistance 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 the fracture test described in JIS G 3546 (2012). A breaking test was carried out at room temperature with a reference point distance of 30 mm to determine the tensile strength. The cross-sectional area (S (mm ² )) of the flat steel wire was calculated using the following formula <3>, and the maximum test force until the test piece broke was divided by the cross-sectional area.
S = w × t-0.215t ² ... <3>
Here, w: the width of the flat steel wire (mm), t: the thickness of the flat steel wire (mm).

<2> Investigation of residual stress on the surface in the longitudinal direction:
Here, the "longitudinal direction" refers to the thickness of the rolled flat steel wire and the length direction extended in the direction perpendicular to the width direction, and the "surface" is a depth of 50 μm from the surface of the flat steel wire toward the center of thickness. Refers to the range up to that point. The residual stress is measured by a known X-ray method, and specifically, an X-ray stress measurement method using X-ray diffraction is used in accordance with JIS B 2711 (2013). For the measurement, the characteristic X-ray type: MnKα ray, Cr filter, reference diffraction angle 2θ0: 152.0 °, η angle: 14.0 °, X-ray stress constant K: -336 MPa / ° are used, and the surface of the flat steel wire is measured. A diffraction pattern was obtained by irradiating X-rays parallel to the longitudinal direction with the center position in the width direction as the center. Further, the residual stresses at 6 points were measured on the surface of the flat steel wire whose measurement positions were separated by at least 450 mm in the longitudinal direction, and the average value was obtained.

ここで、Ａ_ｆ：ＵＳＴで測定された割れ発生部の合計面積（ｍｍ^２）、ｗ：平鋼線の幅（ｍｍ）、Ｌ：平鋼線の長さ（ｍｍ）である。<3> Investigation of hydrogen-induced cracking resistance:
Hydrogen-induced cracking resistance was evaluated using a flat steel wire cut to a length of 150 mm. The pH of a 5% NaCl + CH ₃ COOH solution was adjusted with HCl to give a pH of 5.0. After deaeration with nitrogen gas, was introduced hydrogen sulfide (H 2 _S) + carbon dioxide (CO ₂₎ gas mixture was investigated the occurrence of cracks by immersing the flat steel wire in the solution. 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 was confirmed by an ultrasonic flaw detection test (UST: Ultra-sonic Test) in the thickness direction of the flat steel wire. The total area of the cracked portion determined to have been cracked by ultrasonic flaw detection was determined by image analysis, and the hydrogen-induced cracking occurrence rate (χ (%)) was determined using the following formula <4>.

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

<4> Investigation of sulfide-resistant stress corrosion cracking:
Sulfide stress corrosion cracking resistance was adjusted by applying bending stress to a flat steel wire cut to a length of 150 mm using a 4-point bending jig and under the same conditions as used in the investigation of hydrogen-induced cracking resistance. The presence or absence of cracking was investigated by immersing the flat steel wire together with the four-point bending jig fixed in the solution. Specifically, strain gauges are attached to three locations on the surface of the flat steel wire, bending strain is applied using a jig, and the maximum value of bending strain read by the strain gauges attached to the three locations is the flat steel wire. When the strain corresponding to the tensile stress of 90% of the yield strength was obtained, the strain was fixed to the jig. The pH of the solution was adjusted to 5.0 by using HCl in a 5% NaCl + CH _{3 COOH solution.} After deaeration with nitrogen gas, was introduced hydrogen sulfide (H 2 _S) + carbon dioxide (CO ₂₎ gas mixture, the solution was fixed soaked every 4-point bending jig the flat steel wire the occurrence or non-occurrence of 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. Furthermore, even for test materials for which cracks were not visually confirmed, the longitudinal cross section at the position where the bending strain was maximized was filled with resin in order to confirm the presence or absence of minute cracks on the surface due to sulfide stress corrosion cracking. Then, mirror polishing was performed, and the presence or absence of minute cracks on the surface of the flat steel wire was investigated with an optical microscope. When minute cracks with a depth of 20 μm or more were confirmed on the surface of the flat steel wire, it was judged that sulfide stress corrosion cracking had occurred.

From Table 3, Test No. which is an example of the present invention. Since A1 to A6 and B1 to B3 all satisfy the chemical composition and the requirements of the present invention and the manufacturing conditions of the steel material are appropriate, the tensile strength is 1000 MPa or more, and hydrogen-induced cracking and sulfides are formed. There is no stress corrosion cracking and there is no problem.

On the other hand, the test No. The tensile strength of A7 is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. A8 was not subjected to quenching treatment, only heat treatment was performed, and Test No. A9 was not heat-treated after being processed into a flat steel wire. In each case, the tensile residual stress exceeds 300 MPa, and hydrogen-induced cracking and sulfide stress corrosion cracking occur.
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 occurs. In addition, the test No. B5 was not subjected to a test such as a tensile test because a large warp occurred in the longitudinal direction of the flat steel wire when the quenching treatment was performed.

From Table 4, Test No. which is an example of the present invention. Since all of Nos. 1 to 26 satisfy the chemical composition and the requirements of the present invention and the manufacturing conditions of the steel material are appropriate, the tensile strength is in the range of 1000 MPa to 1350 MPa, and hydrogen-induced cracking and sulfide corrosion cracking are all caused. Has not occurred.

Test No. For 27, 29, 31, 33, 34, 36, 37, 39, 41-42, 45-48, the chemical composition, any of the formula <5>, the formula <6>, or the value of Y1 is the value of the formula <2. > Is not satisfied, so hydrogen-induced cracking and sulfide stress corrosion cracking occur.
Test No. 28, 30, 32, 35, 38, 40, 44 do not satisfy the chemical composition of steel, any of the formulas <5> and <6>, or the value of Y1 does not satisfy the formula <2>, and the present invention. Since the flat steel wire was cracked when it was cold-rolled, the subsequent tests were stopped without heat treatment.
Test No. In 27, the chemical composition is within the range of the present invention, but the value of Y1 does not satisfy the formula <2>, and sulfide stress corrosion cracking occurs.
Test No. In No. 28, the chemical composition is within the range of the present invention, but the value of Y1 does not satisfy the formula <2>, cracks occur during processing into a flat steel wire, and the test after the heat treatment is stopped. There is.
Test No. In No. 29, Cu and Ni are outside the scope of the present invention, and sulfide stress corrosion cracking occurs.
Test No. In No. 30, Ni is outside the scope of the present invention, cracks occur during processing into flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 31, Cu is outside the scope of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 32, Cu is outside the scope of the present invention, cracks occur during processing into flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 33, the Cu / Ni ratio is out of the range of the present invention, and sulfide stress corrosion cracking occurs.
Test No. In No. 34, Ni is outside the scope of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 35, the total amount of Cu and Ni is out of the range of the present invention, cracks occur during processing into flat steel wire, and the test after the heat treatment is stopped.
Test No. In 36 and 37, the C content is out of the range of the present invention, and sulfide stress corrosion cracking occurs.
Test No. In No. 38, Si is outside the scope of the present invention, cracks occur during processing into flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 39, the Si content is out of the range of the present invention, and sulfide stress corrosion cracking occurs.
Test No. In No. 40, Mn is out of the range of the present invention, the value of Y1 does not satisfy the formula <2>, cracks occur during processing into a flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 41, the Mn content is out of the range of the present invention, and sulfide stress corrosion cracking has occurred.
Test No. In No. 42, the content of P is out of the range of the present invention, and sulfide stress corrosion cracking occurs.
Test No. In No. 44, Cr is out of the range of the present invention, the value of Y1 does not satisfy the formula <2>, cracks occur during processing into a flat steel wire, and the test after the heat treatment is stopped.
Test No. In No. 45, the Al content is out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking occur.
Test No. In No. 46, the content of N is out of the range of the present invention, and hydrogen-induced cracking and sulfide stress corrosion cracking occur.
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 occur.
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 occur.

Claims (6)

By mass%
C: 0.35-0.60%,
Si: More than 1.50%, less than 2.00%,
Mn: Exceeds 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 to 0.80%,
Ni: 0.05 to 0.60%,
Ti: 0 to 0.10%,
Nb: 0 to 0.050%,
V: 0 to 0.50%,
Mo: 0-1.00%,
B: 0 to 0.0100%,
REM: 0 to 0.1000%,
Zr: 0 to 0.100%, and Mg: 0 to 0.0050%
The balance consists of Fe and impurities.
Y1 represented by the following formula <1> satisfies the following formula <2>.
Satisfying the following formula <5> and the following formula <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])} ・ ・ ・ Equation <1>
12 × D <Y1 <30 × D
・ ・ ・ Formula <2>
[Cu] / [Ni]> 1
・ ・ ・ Formula <5>
0.10 ≤ [Cu] + [Ni] ≤ 1.00
... formula <6>
However, [C], [Si], [Mn], [Cr], [Cu], and [Ni] in the above formulas <1>, <2>, <5>, and <6> are based on the mass% of each element. The content is represented, and D represents the thickness (mm) of the flat steel wire. By mass%
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.050%,
V: 0.01-0.50%,
The flat steel wire according to claim 1, wherein the flat steel wire contains one kind or two or more kinds selected from the above. By mass%
Mo: 0.01-1.00%,
B: 0.0002 to 0.0100%,
The flat steel wire according to claim 1 or 2, wherein the flat steel wire contains one or two selected from the above. By 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, which contains one kind or two or more kinds selected from the above. The flat steel wire according to any one of claims 1 to 4, which comprises a tempered martensite structure. The flat steel wire according to any one of claims 1 to 4, which comprises a pearlite structure.