EP3604590A1

EP3604590A1 - Wire rod and flat steel wire

Info

Publication number: EP3604590A1
Application number: EP18770555.3A
Authority: EP
Inventors: Toshihiko TESHIMA; Naoki Matsui; Hiroshi Ooba; Masahito Hanao; Shohei Kakimoto
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2017-03-24
Filing date: 2018-03-23
Publication date: 2020-02-05
Also published as: WO2018174270A1; JPWO2018174270A1; EP3604590A4; BR112019017993A2; KR20190119089A; JP6733808B2

Abstract

A wire rod according to an aspect of the present invention includes a predetermined chemical composition, in which an average value of a compositional ratio ε of a complex oxide satisfies 0.00 ≤ ε < 3.00, measured in a central portion, and an average value of an equivalent circle diameter of the complex oxide is 6.0 µm or less, measured in the central portion. A flat steel wire according to another aspect of the present invention includes a predetermined chemical composition, in which an average value of a compositional ratio ε of a complex oxide satisfies 0.00 ≤ ε < 3.00, measured in a center portion, and an average value of an equivalent circle diameter of the complex oxide is 3.0 µm or less, measured in the center portion.

Description

[Technical Field of the Invention]

The present invention relates to a wire rod and a flat steel wire.
Priority is claimed on Japanese Patent Application No. 2017-059111 , filed in Japan on March 24, 2017, the content of which is incorporated herein by reference.

[Related Art]

In flexible pipes that are used to transport natural gas, crude oil, and the like that are mined at the bottom of the sea at a high pressure, flat steel wires are used as a reinforcing material. Flat steel wires of this type are formed by being flattened to be 40% to 80% of a hot-rolled wire rod in thickness and used with the processed structure or after being quenched and tempered. In recent years, mining has taken place deep in the sea, and the transportation distance of mined substances has become longer, and thus there has been an intensifying demand for flexible pipes and flat steel wires that are a reinforcing material of flexible pipes to have a high strength. In addition, flexible pipes are used in a sour environment including hydrogen sulfide, and thus flat steel wires need to have a characteristic of resisting the occurrence of hydrogen induced cracking (HIC), that is, hydrogen induced cracking resistance.
However, in the case of providing a high strength to flat steel wires, the hydrogen susceptibility of the flat steel wires increases, and hydrogen induced cracking is accelerated. Furthermore, when a wire rod is flattened to be 40% to 80% in thickness, a sulfide present in the wire rod is stretched, and thus iron and the sulfide separate from each other, and hydrogen gathers in a void generated by the separation, which increases the probability of the occurrence of hydrogen induced cracking. Therefore, there is a demand for the development of a hot-rolled wire rod that can be used to manufacture flat steel wires which are applicable to flexible pipes that are used in a sour environment and has a high strength and excellent hydrogen induced cracking resistance. As a technique for providing the above-described high-strength material that is used in a sour environment, hitherto, Patent Document 1 has been proposed.
Patent Document 1 describes a technique for obtaining a high-strength flat steel wire having excellent hydrogen embrittlement resistance by carrying out a cold process on high-carbon steel having a pearlite structure and tempering the high-carbon steel for a short period of time.

[Prior Art Document]

[Patent Document]

[Patent Document 1] Published Japanese Translation No. 2013-534966 of the PCT International Publication

[Disclosure of the Invention]

[Problems to be Solved by the Invention]

Patent Document 1 discloses a flat steel wire having a tensile strength of 1,300 MPa or more and having excellent hydrogen induced cracking resistance in an environment with a pH of 5.6 or more; however, in a HIC test of this flat steel wire with a pH of 5.5 or less, cracks are formed even in a case where the tensile strength is set to 1,100 MPa. The present inventors considered that the lack of the hydrogen induced cracking resistance of the flat steel wire of Patent Document 1 results from the strong development of the degradation of hydrogen embrittlement resistance caused by the separation of inclusions.
The present invention has been made in consideration of the above-described circumstance, and an object of the present invention is to provide a wire rod from which a flat steel wire having a tensile strength of 1,100 MPa or more and having excellent hydrogen induced cracking resistance can be obtained.

[Means for Solving the Problem]

The summary of the present invention is as described below.

(1) A wire rod according to an aspect of the present invention includes, as a chemical composition, by mass%: C: 0.15% to 0.85%, Si: 0.10% to 2.00%, Mn: 0.30% to 1.50%, Al: 0.001 % to 0.080%, Ca: 0.0002% to 0.0050%, N: 0.0020% to 0.0080%, P: 0.020% or less, S: 0.020% or less, O: 0.0050% or less, Cr: 0% to 1.00%, V: 0% to 0.15%, Ti: 0% to 0.050%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Ni: 0% to 1.50%, Mo: 0% to 1.00%, B: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.1000%, and a remainder including Fe and impurities; in which an oxide including CaO and Al₂O₃ and satisfying Expression A and Expression B is defined as a complex oxide, an average value of a compositional ratio ε of the complex oxide that is defined as Expression C satisfies 0.00 ≤ ε < 3.00, measured in a central portion within a range of 1/10 of a diameter of the wire rod from a central axis of the wire rod in a cross section perpendicular to a rolling direction of the wire rod, and an average value of an equivalent circle diameter of the complex oxide is 6.0 µm or less, measured in the central portion of the cross section. $(Amount of oxide-forming element other than Ca and Al in oxide by unit$
$(O content in oxide by unit mol %) \geq (S content in oxide by unit mol %)$
$(Compositional ratio ε) = (concentration of CaO in the complex oxide by unit mass %) / (concentration of {Al}_{2} O_{3} in the complex oxide by unit mass %)$
(2) The wire rod according to (1) may include, as the chemical composition, by mass%: Cr: 0.05% to 1.00%.
(3) The wire rod according to (1) or (2) may include, as the chemical composition, by mass%: one or more selected from the group consisting of V: 0.02% to 0.15%, Ti: 0.002% to 0.050%, and Nb: 0.002% to 0.050%.
(4) The wire rod according to any one of (1) to (3) may include, as the chemical composition, by mass%: one or more selected from the group consisting of Cu: 0.01 % to 1.00%, Ni: 0.01 % to 1.50%, Mo: 0.01 % to 1.00%, and B: 0.0002% to 0.0100%.
(5) The wire rod according to any one of (1) to (4) may include, as the chemical composition, by mass%: one or both selected from the group consisting of REM: 0.0002% to 0.0100% and Zr: 0.0002% to 0.1000%.
(6) In the wire rod according to any one of (1) to (5), a tensile strength may be 600 MPa to 1,400 MPa.
(7) A flat steel wire according to another aspect of the present invention includes, as a chemical composition, by mass%: C: 0.15% to 0.85%, Si: 0.10% to 2.00%, Mn: 0.30% to 1.50%, Al: 0.001% to 0.080%, Ca: 0.0002% to 0.0050%, N: 0.0020% to 0.0080%, P: 0.020% or less, S: 0.020% or less, O: 0.0050% or less, Cr: 0% to 1.00%, V: 0% to 0.15%, Ti: 0% to 0.050%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Ni: 0% to 1.50%, Mo: 0% to 1.00%, B: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.1000%, and a remainder including Fe and impurities; in which an oxide including CaO and Al₂O₃ and satisfying Expression A and Expression B is defined as a complex oxide, an average value of a compositional ratio ε of the complex oxide that is defined as Expression C satisfies 0.00 ≤ ε < 3.00, measured in a center portion within a range of 1/7 of a minor axis length of the flat steel wire from a center axis of the flat steel wire in a cross section parallel to a rolling direction and a minor axis direction of the flat steel wire and including the center axis of the flat steel wire, and an average value of an equivalent circle diameter of the complex oxide is 3.0 µm or less, measured in the center portion of the cross section. $(Amount of oxide-forming element other than Ca and Al in oxide by unit mol %) < (1 / 3) \times (larger one of Ca content or Al content in oxide by unit mol %)$
$(O content in oxide by unit mol %) \geq (S content in oxide by unit mol %)$
$(Compositional ratio ε) = (concentration of CaO in the complex oxide by unit mass %) / (concentration of {Al}_{2} O_{3} in the complex oxide by unit mass %)$
(8) In the flat steel wire according to (7), a structure in the center portion may include 98 area% or more of tempered martensite.
(9) In the flat steel wire according to (7), the structure in the center portion may include 20 area% to 60 area% of ferrite and 40 area% to 60 area% of bainite.
(10) In the flat steel wire according to any one of (7) to (9), a tensile strength may be 1,100 MPa to 1,500 MPa.
(11) The flat steel wire according to any one of (7) to (10) may include, as the chemical composition, by mass%: Cr: 0.05% to 1.00%.
(12) The flat steel wire according to any one of (7) to (11) may include, as the chemical composition, by mass%: one or more selected from the group consisting of V: 0.02% to 0.15%, Ti: 0.002% to 0.050%, and Nb: 0.002% to 0.050%.
(13) The flat steel wire according to any one of (7) to (12) may include, as the chemical composition, by mass%: one or more selected from the group consisting of Cu: 0.01% to 1.00%, Ni: 0.01% to 1.50%, Mo: 0.01% to 1.00%, and B: 0.0002% to 0.0100%.
(14) The flat steel wire according to any one of (7) to (13) may include, as the chemical composition, by mass%: one or both selected from the group consisting of REM: 0.0002% to 0.0100% and Zr: 0.0002% to 0.1000%.

[Effects of the Invention]

The wire rod of the present invention can be used to manufacture a flat steel wire according to the present embodiment which has a tensile strength of 1,100 MPa or more and has excellent hydrogen induced cracking resistance in a severe sour environment with a pH of 5.5 or less. The flat steel wire according to the present embodiment has a tensile strength of 1,100 MPa or more and is excellent in terms of hydrogen induced cracking resistance and thus can be used as, for example, flat steel wires for the tension reinforcement of flexible pipes that are used in a severe sour environment.

[Brief Description of the Drawings]

FIG. 1 is a schematic view showing a C-section of a wire rod according to the present embodiment.
FIG. 2 is a schematic view showing a C-section of a flat steel wire according to the present embodiment.
FIG. 3 is a schematic view showing an L-section of the flat steel wire according to the present embodiment.

[Embodiments of the Invention]

The present inventors carried out a variety of studies in order to solve the above-described problem. In the related art, sulfides were detoxified by adding Ca or the like; however, even in such cases, there was a case where HIC occurred near the central axes of wire rods and flat steel wires. The present inventors found that, in a cracked portion near the central axis, the presence of a complex oxide including Al₂O₃ and CaO affects HIC. In addition, the present inventors found that HIC can be effectively prevented by controlling the composition and size of the complex oxide including Al₂O₃ and CaO near the central axis. That is, the following knowledge (a) to (d) was obtained.

(a) Hydrogen induced cracking in a flat steel wire occurs from a coarse sulfide in steel as a starting point. Particularly, in a case where a sulfide such as MnS is coarse, a void is generated around the sulfide at the time of flattening a hot-rolled wire rod and acts as a cause for accelerating hydrogen induced cracking in a severe sour environment with a pH of 5.5 or less.
(b) Therefore, it is necessary to refine a sulfide that is contained in a wire rod as much as possible. In order to refine the sulfide, it is effective to add a small Ca content or Mg and form MnS, CaS, or MgS partially including Ca or Mg.
(c) The addition of Ca generates an oxide including CaO and Al₂O₃ (hereinafter, referred to as the complex oxide in some cases) in some cases, and, during a flattening process, a void is generated around this complex oxide and acts as a cause for accelerating hydrogen induced cracking in a severe sour environment with a pH of 5.5 or less.
(d) Therefore, it is necessary to suppress the formation of a void around the complex oxide during the flattening process. In order for that, it is effective to control the size and compositional ratio of the complex oxide, thereby crushing the complex oxide during the flattening process. The crushed complex oxide is well adhered to a base metal, and thus the occurrence of hydrogen induced cracking in a severe sour environment with a high strength and a pH of 5.5 or less is significantly reduced.

A wire rod according to the present embodiment and a flat steel wire according to the present embodiment that is obtained by rolling the wire rod are completed on the basis of the above-described knowledge. Hereinafter, first, the wire rod according to the present embodiment will be described.

(A) Regarding chemical composition:

Hereinafter, the chemical composition of the wire rod according to the present embodiment will be described in detail. The amount of the chemical composition is "mass%".

C: 0.15% to 0.85%

C is an element that strengthens steel. In order to obtain this effect, 0.15% or more of C needs to be contained. When the C content exceeds 0.85%, the strength excessively increases, and thus cracks are formed in the wire rod during a flattening process, and the hydrogen induced cracking resistance deteriorates. Therefore, an appropriate C content is 0.15% to 0.85%. Furthermore, the C content is preferably set to 0.20% or more and, furthermore, preferably set to 0.30% or more, 0.35% or more, or 0.40% or more from the viewpoint of suppressing the formation of cracks. From the viewpoint of suppressing the cracking of a carbide, the C content is preferably set to 0.75% or less, and, furthermore, in order to improve the hydrogen induced cracking resistance, the C content is desirably set to 0.65% or less, 0.60% or less, or 0.50% or less.

Si: 0.10% to 2.00%

Si is an element that forms a solid solution in a matrix and improves the strength of the flat steel wire. In order to obtain this effect, 0.10% or more of Si needs to be contained. However, when the Si content exceeds 2.00%, cracks are generated in the wire rod during the flattening process. Therefore, the Si content is 0.10% to 2.00%. In a case where it is necessary to further increase the strength, 0.30% or more of Si needs to be contained, and it is more preferably that 0.50% or more, 0.55% or more, 0.60% or more, or 0.70% or more of Si needs to be contained. In a case where it is necessary to suppress the cracking of the wire rod during a process for producing the flat steel wire, the Si content is preferably set to less than 2.00% and more preferably set to 1.80% or less, 1.70% or less, or 1.50% or less.

Mn: 0.30% to 1.50%

Mn is an element that has an effect for enhancing the hardenability of steel and is necessary to increase the strength of the heat-treated flat steel wire. In order to obtain this effect, 0.30% or more of Mn needs to be contained. However, when the Mn content exceeds 1.50%, the strength of the wire rod excessively increases, and there is a problem in that cracks are generated in the wire rod at the time of processing the wire rod to the flat steel wire. Therefore, the Mn content in the wire rod according to the present embodiment is 0.30% to 1.50%. In order to increase the strength by enhancing the hardenability of the flat steel wire, the Mn content is preferably 0.40% or more and is more preferably 0.50% or more, 0.60% or more, 0.70% or more, 0.80% or more, or 0.90% or more. In a case where it is necessary to suppress the cracking of the wire rod during the process for producing the flat steel wire, the Mn content is preferably set to 1.30% or less and more preferably set to 1.10% or less, 1.05% or less, or 1.00% or less.

S: 0.020% or less

S is an impurity. When the S content exceeds 0.020%, MnS has a stretched form and degrades the hydrogen induced cracking resistance. In order to improve the hydrogen induced cracking resistance, it is necessary to control the upper limit of S being contained in consideration of the balance with an element that is likely to bond to S and generate a sulfide such as Ca. Therefore, the upper limit of the S content is set to 0.020% or less. From the viewpoint of improving the hydrogen induced cracking resistance, the S content is preferably less than 0.010% and more preferably less than 0.008% or less than 0.005%. From the viewpoint of the steel making costs, the S content may also be set to 0.001 % or more, 0.003% or more, or 0.005% or more.

P: 0.020% or less

P is an impurity. When the P content exceeds 0.020%, hydrogen induced cracking is likely to occur, and, in the flat steel wire, it is not possible to suppress hydrogen induced cracking in a severe sour environment with a pH of 5.5 or less. Therefore, the P content is set to 0.020% or less. From the viewpoint of improving the hydrogen induced cracking resistance, the P content is preferably 0.015% or less, more preferably less than 0.013% or less than 0.010%, and far more preferably less than 0.008%. From the viewpoint of the steel making costs, the P content may be set to 0.003% or more or 0.005% or more.

Al: 0.001% to 0.080%

Al is an element having a deoxidation action and is necessary to decrease the amount of oxygen in the wire rod. In order to obtain this effect, 0.001 % or more of Al needs to be contained. An Al content is preferably 0.002% or more and 0.005% or more, more preferably 0.015% or more, and still more preferably 0.020% or more or 0.025% or more. On the other hand, when the Al content exceeds 0.080%, a coarse oxide is generated, and the hydrogen induced cracking resistance deteriorates. Therefore, the Al content is set to 0.080% or less. In order to suppress the generation of a coarse oxide, the Al content is preferably 0.060% or less and more preferably 0.050% or less, 0.045% or less, or 0.040% or less.

N: 0.0020% to 0.0080%

N has an effect for forming a solid solution in ferrite and improving the strength of the flat steel wire. Furthermore, N has an effect for generating a nitride or a carbonitride by bonding to Al, Ti, and the like and refining an austenite grain during hot rolling and has an effect for improving the hydrogen induced cracking resistance of the flat steel wire. In order to obtain this effect, 0.0020% or more of N needs to be contained, and 0.0030% or more. 0.0035% or more, or 0.0040% or more of N is preferably contained. However, when N is excessively contained in the wire rod, the nitride or the carbonitride becomes coarse, the ductility degrades, and internal cracks are generated during a flat rolling process, and thus it is necessary to set the N content to 0.0080% or less. The N content is preferably 0.0060% or less and more preferably set to 0.0055% or less, 0.0050% or less, 0.0045% or less, or 0.0040% or less.

O: 0.0050% or less

O is an impurity. O forms a coarse oxide and degrades the hydrogen induced cracking resistance of steel. Therefore, the O content is preferably small. The O content is 0.0050% or less. The O content is preferably less than 0.0050%, more preferably less than 0.0040%, and still more preferably less than 0.0035%. From the viewpoint of the steel making costs, the O content may be set to 0.0007% or more or 0.0010% or more.

Ca: 0.0002% to 0.0050%

Ca has an effect for finely dispersing MnS when included in MnS. When MnS is finely dispersed, hydrogen induced cracking attributed to MnS can be suppressed. In order to obtain the effect of Ca for suppressing hydrogen induced cracking, 0.0002% or more of Ca needs to be contained, and, in a case where it is necessary to obtain a stronger effect, 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0015% or more of Ca needs to be contained. However, when the Ca content exceeds 0.0050%, the effect is saturated, an oxide that is generated by a reaction between oxygen in steel and Ca together with Al becomes coarse, and it becomes difficult to crush the oxide during the flattening process, which, inversely, leads to the degradation of the hydrogen induced cracking resistance. Therefore, an appropriate Ca content in the case of being contained is 0.0050% or less. From the viewpoint of improving the hydrogen induced cracking resistance, the Ca content is preferably 0.0040% or less and more preferably 0.0030% or less, 0.0025% or less, or 0.0020% or less.

(B) Regarding arbitrary components:

The wire rod according to the present embodiment may contain at least one or more elements selected from Cr, Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr as necessary instead of some of Fe that is a remainder described below. The wire rod according to the present embodiment is capable of solving the problem without containing these arbitrary components, and thus the lower limit value of the amount of the arbitrary element is 0%. Hereinafter, the actions and effects of Cr, Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr that are arbitrary elements and reasons for limiting the amounts thereof will be described. Regarding the arbitrary components, "%" indicates "mass%".

Cr: 0% to 1.00%

Similar to Mn, in order to increase the strength of the flat steel wire by enhancing the hardenability of steel, 0.05% or more of Cr may be contained in the wire rod. When the Cr content exceeds 1.00%, the strength of the wire rod excessively increases, and there is a problem in that, during the process for producing the flat steel wire, cracks are generated in the wire rod. Therefore, an appropriate Cr content in the wire rod according to the present embodiment is 1.00% or less. In the case of enhancing the hardenability of the flat steel wire, the Cr content is preferably 0.10% or more and more preferably 0.20% or more. In a case where it is necessary to suppress the cracking of the wire rod during a cold process for producing the flat steel wire, the Cr content is preferably set to 0.80% or less and more preferably 0.60% or less.

Ti: 0% to 0.050%

Ti has an effect for forming a carbide, a nitride, or a carbonitride by bonding to N or C and refining the austenite grain during hot rolling by a pinning effect thereof and has an effect for improving the hydrogen induced cracking resistance of the flat steel wire and thus may be contained. In order to obtain this effect, 0.002% or more of Ti is preferably contained. From the viewpoint of improving the hydrogen induced cracking resistance, the Ti content is preferably set to 0.005% or more and more preferably set to 0.010% or more. When the Ti content exceeds 0.050%, the effect is saturated, furthermore, a number of coarse TiN is generated and acts as a cause for the formation of cracks during the flattening process, and there is a possibility that the hydrogen induced cracking resistance may be deteriorated. Therefore, the Ti content is set to 0.050% or less and is more preferably 0.035% or less.

Nb: 0% to 0.050%

Nb has an effect for forming a carbide, a nitride, or a carbonitride by bonding to N or C and refining the austenite grain during hot rolling by the pinning effect thereof and has an effect for improving the hydrogen induced cracking resistance of the flat steel wire and thus may be contained. In order to obtain this effect, 0.002% or more of Nb is preferably contained. From the viewpoint of improving the hydrogen induced cracking resistance, the Nb content is preferably set to 0.005% or more and more preferably set to 0.010% or more. When the Nb content exceeds 0.050%, the effect is saturated, and, furthermore, there is an adverse influence on the manufacturability of steel such as the generation of cracks in a steel piece in a step of blooming a steel ingot or a slab into the steel piece. Therefore, the Nb content is set to 0.050% or less, and is preferably 0.035% or less and more preferably 0.030% or less.

V: 0% to 0.15%

V is capable of increasing the strength of the flat steel wire by bonding to C and N to form a carbide, a nitride, or a carbonitride. In order to obtain this effect, 0.02% or more of V is preferably contained. When the V content exceeds 0.15%, there is a case where the strength of the flat steel wire increases due to a carbide or a carbonitride to be precipitated and cracks are formed during the flattening process. Therefore, the V content is set to 0.15% or less. From the viewpoint of suppressing cracking during the flattening process, the V content is more preferably 0.10% or less and still more preferably 0.08% or less. In order to stably obtain the above-described effect of V, the lower limit of the V content is more preferably 0.03% or more.

Cu: 0% to 1.00%

Cu is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect for enhancing the hardenability, 0.01% or more of Cu is preferably contained. However, when the Cu content exceeds 1.00%, the strength of the wire rod excessively increases, and there is a problem in that cracks are generated in the wire rod at the time of being processed to the flat steel wire. Therefore, the Cu content in the case of being contained is 1.00% or less. From the viewpoint of improving the hardenability, the Cu content in the case of being contained is preferably 0.10% or more and more preferably 0.30% or more. The Cu content in the case of being contained is preferably set to 0.80% or less and is more preferably 0.50% or less in consideration of a property of being processed to the flat steel wire.

Ni: 0% to 1.50%

Ni is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect for enhancing the hardenability, 0.01% or more of Ni is preferably contained. However, when the Ni content exceeds 1.50%, the strength of the wire rod excessively increases, and there is a problem in that cracks are generated in the wire rod at the time of being processed to the flat steel wire. Therefore, the Ni content in the case of being contained is 1.50% or less. From the viewpoint of improving the hardenability, the Ni content in the case of being contained is preferably 0.10% or more and more preferably 0.30% or more. The Ni content in the case of being contained is preferably set to 1.00% or less and is more preferably 0.60% or less in consideration of a property of being processed to the flat steel wire.

Mo: 0% to 1.00%

Mo is an element that enhances the hardenability of steel and may be contained. In order to obtain the effect for enhancing the hardenability, 0.01% or more of Mo is preferably contained. However, when the Mo content exceeds 1.00%, the strength of the wire rod excessively increases, and there is a problem in that cracks are generated in the wire rod at the time of being processed to the flat steel wire. Therefore, the Mo content in the case of being contained is 1.00% or less. From the viewpoint of improving the hardenability, the Mo content in the case of being contained is preferably 0.02% or more and more preferably 0.05% or more. The Mo content in the case of being contained is preferably set to 0.50% or less and is more preferably 0.30% or less in consideration of a property of being processed to the flat steel wire.

B: 0% to 0.0100%

B is effective for enhancing the hardenability of steel when added in a small amount, and in a case where it is necessary to obtain this effect, 0.0002% or more of B may be contained. Even when more than 0.0100% of B is contained, the effect is saturated, and, furthermore, a coarse nitride is generated, and thus hydrogen induced cracking is likely to occur. Therefore, the B content in the case of being contained is 0.0100% or less. Furthermore, in a case where it is necessary to enhance the hardenability, the B content needs to be set to 0.0010% or more and is more preferably 0.0020% or more. In order to improve the hydrogen induced cracking resistance, the B content is preferably set to 0.0050% or less and is more preferably 0.0030% or less.

REM: 0% to 0.0100%

REM is a collective term of rare earth metals and, similar to Ca, has an effect for finely dispersing MnS when included in MnS. When MnS is finely dispersed, the hydrogen induced cracking resistance can be improved, and thus REM may be contained. In order to obtain the effect for suppressing hydrogen induced cracking, 0.0002% or more of REM needs to be contained, and, in a case where it is necessary to obtain a stronger effect, 0.0005% or more of REM needs to be contained. However, when the REM content exceeds 0.0100%, the effect is saturated, and an oxide that is generated by a reaction with oxygen in steel becomes coarse, which causes cracking during the flattening process. Therefore, an appropriate REM content in the case of being contained is 0.0100% or less. From the viewpoint of a property of being processed to the flat steel wire, the REM content is preferably 0.0050% or less and more preferably 0.0030% or less.
Rare earth metal (REM) is the collective term of two element of scandium (Sc) and yttrium (Y) and 15 (lanthanoid) elements of lanthanum (La) through lutetium (Lu). In the case of adding these elements, the elements may be added singly or may be added as a mixture. The REM content refers to the total value of the amounts of these 17 elements.

Zr: 0% to 0.1000%

Zr has an effect for generating an oxide by reacting with O and suppressing hydrogen induced cracking by finely dispersing the oxide when added in a small amount and may be contained in a case where it is necessary to obtain this effect. In order to obtain the effect for suppressing hydrogen induced cracking, 0.0002% or more of Zr needs to be contained, and, in a case where it is necessary to obtain a stronger effect, 0.0010% or more Zr needs to be contained. However, in a case where the Zr content exceeds 0.1000%, the effect is saturated, and Zr reacts with N or S in steel and generates a coarse nitride or sulfide, which, inversely, leads to degradation of the hydrogen induced cracking resistance. Therefore, the Zr content in the case of being contained is 0.1000% or less. From the viewpoint of decreasing an oxide having an adverse influence on the hydrogen induced cracking resistance, the Zr content is preferably 0.0800% or less and more preferably 0.0500% or less.
The remainder in the chemical composition of the wire rod includes Fe and an impurity. The "impurity" refers to an element that is mixed into an iron and steel material from an ore or a scrap as a raw material, a manufacturing environment, or the like at the time of industrially manufacturing the iron and steel material and substantially has no influence on the Properties of the wire rod according to the present embodiment.

(C) Regarding Properties of oxide:

The wire rod having the above-described components contains an oxide including CaO and Al₂O₃ in a predetermined amount or more. The present inventors found that a void that is generated around this oxide during the flattening process of the wire rod accelerates hydrogen induced cracking. In addition, the present inventors found that the oxide can be finely crushed during the flattening process by controlling the compositional ratio and the size of the oxide to appropriate ranges and the oxide migrates following the base metal during the crushing, which makes adhesion between the base metal and the oxide favorable and improves the hydrogen induced cracking resistance after the flattening process. In order to obtain this effect, it is necessary to strictly control the compositional ratio or the size of the oxide.
First, the oxide that is a control subject in the wire rod according to the present embodiment will be described. The oxide having an adverse influence on the hydrogen induced cracking resistance of the wire rod and the flat steel wire is an oxide that includes CaO and Al₂O₃ and satisfies Expression A and Expression B. Hereinafter, the "oxide including CaO and Al₂O₃ and satisfying Expression A and Expression B" will be abbreviated as the "complex oxide" in some cases. $(Amount of oxide-forming elements other than Ca and Al in oxide by unit mol %) < (1 / 3) \times (larger one of Ca content or Al content in oxide by unit mol %)$
$(O content in oxide by unit mol %) \geq (S content in oxide by unit mol %)$
"Oxide-forming elements other than Ca and Al" described in Expression A are, in the chemical composition of the wire rod according to the present embodiment, Si, Mg, and Mn.
The complex oxide causes hydrogen induced cracking and is regarded as an improvement subject in the wire rod according to the present embodiment. Therefore, in the wire rod according to the present embodiment, the composition and the size of the complex oxide are limited.
As far as the present inventors confirmed, a variety of inclusions other than the complex oxide substantially have no influences on hydrogen induced cracking. Therefore, in the wire rod according to the present embodiment, a variety of inclusions other than the complex oxide are not particularly limited.
For example, in the wire rod according to the present embodiment, the amount of oxides other than CaO and Al₂O₃ is small due to the chemical composition of the wire rod. Therefore, the oxides other than CaO and Al₂O₃ do not have any influences on hydrogen induced cracking. In addition, even among oxides including CaO and Al₂O₃, a complex oxide in which the amount (mol%) of the other oxide-forming elements such as Si, Mg, and Mn is 1/3 or more of the amount (mol%) or Ca or the amount (mol%) of Al is not present at a starting point of cracking in an evaluation test of the wire rod and the flat steel wire and is regarded to have no influences on hydrogen induced cracking. For the same reason, an inclusion in which the amount (mol%) of O is smaller than the amount (mol%) of S, that is, an inclusion not satisfying Expression B is also regarded to have no influences on hydrogen induced cracking.
In consideration of the above-described matter, in the wire rod according to the present embodiment, the complex oxide that is regarded as the control subject is limited to a compound that includes CaO and Al₂O₃ and satisfies Expressions A and B below.
In addition, the complex oxide that is regarded as the control subject in the wire rod according to the present embodiment may be limited to an oxide that is substantially made up of CaO and Al₂O₃.
The complex oxide is evaluated in the central portion of a C-section of the wire rod, that is, in the central portion of a cross section perpendicular to the rolling direction of the wire rod. As shown in FIG. 1, a central portion 11 of a C-section of a wire rod 1 refers to a range of 1/10 of a diameter d of the wire rod from the central of the wire rod in a case where the C-section of the wire rod 1 is substantially circular. That is, the central portion 11 of the C-section of the substantially circular wire rod 1 is a region in a concentric circle of the section of the wire rod 1 having a diameter of 1/5d (2/10d). In a case in which the C-section of the wire rod 1 is not substantially circular, a region that has a homothetic shape with a homothetic ratio of 1/5 of the C-section of the wire rod 1 and has a geometric central at the same location as that of the C-section of the wire rod 1 is regarded as the central portion 11 of the C-section of the wire rod 1. The complex oxide is likely to gather in the central portion of the slab, and thus, even in a wire rod that is obtained by rolling the slab, the complex oxide is likely to gather in the central portion. The composition of the complex oxide in the central portion is also substantially identical to that in the peripheral portion, and it is considered that the precipitation of a coarse complex oxide is also suppressed in the peripheral portion as long as that is suppressed in the central portion. For the above-described reason, the complex oxide is evaluated in the central portion of the C-section of the wire rod.
The complex oxide that is observed on the cross section perpendicular to the rolling direction of the wire rod embrittles in a case where a compositional ratio ε between CaO and Al₂O₃, which is computed using Expression C, satisfies 0.00 ≤ ε < 3.00, and thus it becomes possible to crush the complex oxide during the flattening process. $(Compositional ratio ε) = (concentration of CaO in oxide by unit mass %) / (concentration of {Al}_{2} O_{3} in oxide by unit mass %)$
Specifically, in a case where ε is close to 0.00, that is, a case where Al₂O₃ is a dominant component of the complex oxide or ε is smaller than 3.00, the complex oxide is crushed during the flattening process. Therefore, it is necessary to control the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod to be in the above-described range in order to improve the hydrogen induced cracking resistance of the flattened wire rod (that is, the flat steel wire). On the other hand, in a case where the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod is 3.00 or more, CaO is a dominant component of the complex oxide, and the hydrogen induced cracking resistance after the flattening process deteriorates regardless of the control of the size. For the above-described reason, the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod is regulated to be 0 or more and 3.00 or less. In order to obtain more stable hydrogen induced cracking resistance, the upper limit of the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod is preferably 1.00 or less and more preferably 0.60 or less. In addition, the lower limit of the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod may be set to 0.02, 0.05, 0.10, 0.15, or 0.20.
In addition, even when the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod satisfies the above-described conditions, in a case where there is a complex oxide having an equivalent circle diameter of more than 6.0 µm (coarse complex oxide) in the central portion of the wire rod, it is not possible to sufficiently improve the hydrogen induced cracking resistance of the flat steel wire. In a case where the coarse complex oxide is included in the wire rod, even after the wire rod is flattened, a complex oxide having an equivalent circle diameter of more than 3 µm remains in the flat steel wire, and the base metal and the complex oxide are separated from each other at the interface, and thus the hydrogen induced cracking resistance of the flat steel wire deteriorates. In a case where the wire rod is flattened at a working ratio of 40% or more, a complex oxide having a size of 6.0 µm or less is crushed to be approximately 3 µm or less. Therefore, the average value of the equivalent circle diameters of the complex oxide is set to 6.0 µm or less. The lower limit value of the average value of the equivalent circle diameters of the complex oxide is not particularly specified, but may be regulated to be 2.0 µm, 2.5 µm, 3.0 µm, 3.5 µm, or 4.0 µm.

(D) Regarding evaluation method

Next, methods for evaluating the complex oxide in the wire rod according to the present embodiment will be described. There is a case where the complex oxide in the wire rod is present in a cluster form, and, even in this case, individual complex oxides configuring the cluster are treated as independent oxides. There is no case where the entire cluster is regarded as one complex oxide.

(D-1) Chemical composition of complex oxide

The chemical composition of the complex oxide is considered to be substantially uniform in a wire rod regardless of the size. Therefore, it is possible to observe 10 views in the C-section of the central portion of the wire rod, analyze chemical compositions and compute compositional ratios ε only for complex oxides having the maximum equivalent circle diameter (complex oxides having a chemical composition that is most easily analyzed) in the respective views, and regard a value obtained by averaging the compute compositional ratios ε of the complex oxides in these 10 views as the average value of the compositional ratios ε of the complex oxide which are measured in the central portion of the wire rod. As long as this value satisfies the above-described requirement of the wire rod according to the present embodiment, the wire rod is regarded to satisfy the requirement of the wire rod according to the present embodiment. A specific method for analyzing the chemical composition of the complex oxide in the wire rod will be described below.
The C-section (that is, a cut surface perpendicular to the rolling direction of the wire rod) of the wire rod is mirror-polished, 10 places are observed using a field emission scanning electron microscope (FE-SEM) at a magnification of 1,000 times in a reflected electron image so that inclusions such as the complex oxide and the like can be observed, and photographs are captured. The area per view is set to 8,000 µm² (100 µm in height and 80 µm in width) or more. At this time, the chemical compositions of the respective inclusions are collectively measured using EDS, thereby determining whether or not the inclusions are the complex oxide that is regarded as the control subject in the wire rod according to the present embodiment. Next, the characteristic X-ray spectra of the complex oxides having the maximum size in the respective photographs are obtained using energy-dispersive X-ray spectroscopy (EDS), thereby analyzing elements. Therefore, the compositions of the complex oxides can be evaluated. From the peak energies of the obtained characteristic X-ray spectra, elements that are included in the complex oxides are specified, and the amounts (mol%) of the elements are determined from the heights of the peaks. In addition, with an assumption that Ca in the complex oxide is all present as CaO and Al is all present as Al₂O₃, the mass ratios CaO/Al₂O₃ of the complex oxides are computed, thereby obtaining the compositional ratios ε of the complex oxides having the maximum size in the respective views. In addition, these compositional ratios ε in 10 views are averaged, thereby computing the average value of the compositional ratios ε of the complex oxides which are measured in the central portion of the wire rod.
Here, an oxide in which the amount (mol%) of the other oxide-forming elements such as Si, Mg, and Mn is 1/3 or more of a larger one of the amount (mol%) or Ca and the amount (mol%) of Al (that is, an oxide not satisfying Expression A) is determined as, for example, an oxide that includes CaO, Al₂O₃, and SiO₂ and is not the control subject in the wire rod according to the present embodiment. In addition, an inclusion in which the amount (mol%) of O is smaller than the amount (mol%) of S (that is, an inclusion not satisfying Expression B) is determined as a sulfide-based inclusion and is not the control subject in the wire rod according to the present embodiment. Such an inclusion is ignored at the time of confirming the state of the complex oxide.

(D-2) Size of complex oxide

It is not realistic to measure the equivalent circle diameters of all of the complex oxide in the central portion of the wire rod. A value that is obtained using a method described below can be regarded as the average value of the equivalent circle diameters of the complex oxides which are measured in the central portion of the C-section of the wire rod.
The C-section (that is, a cut surface perpendicular to the rolling direction) of the wire rod is mirror-polished, 10 places are observed using a field emission scanning electron microscope (FE-SEM) at a magnification of 1,000 times in a reflected electron image so that inclusions can be observed, and photographs are captured. The area per view is set to 8,000 µm² (100 µm in height and 80 µm in width) or more. At this time, the chemical compositions of the respective inclusions are collectively measured using EDS, thereby determining whether or not the inclusions are the complex oxide that is regarded as the control subject in the wire rod according to the present embodiment. Next, regarding the respective obtained photographs, the areas of the complex oxides having the maximum size are measured by an ordinary image analysis from the respective photographs, and equivalent circle diameters that are obtained from the areas are computed. As the photographs for measuring the equivalent circle diameter, reflected electron images are preferably used. The average value of the equivalent circle diameters, which have been obtained using the above-described method, of the maximum complex oxides in the photographs of the 10 places is obtained, whereby the average value of the equivalent circle diameters of the complex oxides which are measured in the central portion of the C-section of the wire rod is obtained.
As long as the above-described requirements are satisfied, other configurations of the wire rod according to the present embodiment are not particularly limited. For example, the metallographic structure of the wire rod has no substantial influence on the hydrogen induced cracking resistance of the flat steel wire. This is because the states of a sulfide and the complex oxide are dominant regarding the hydrogen induced cracking resistance of the flat steel wire as described above. Therefore, the metallographic structure of the wire rod is not limited. However, in the case of taking the workability into account, the metallographic structure of the wire rod is preferably controlled to be a pearlite structure, a ferrite structure, or a bainite structure. Therefore, the metallographic structure of the wire rod may be regulated as a metallographic structure including a total of 99area% or more of a pearlite structure, a ferrite structure, and a bainite structure.
The diameter of the wire rod is also not particularly limited, and the diameters of wire rods for flat steel wires that are in circulation in the current market are generally set to 7 to 16 mm, and thus the diameter of the wire rod according to the present embodiment may also be regulated to be 7 to 16 mm.
The tensile strength of the wire rod is also not particularly limited. When the chemical composition of the wire rod is taken into account, it is considered that the tensile strength of the wire rod often becomes approximately 600 to 1,400 MPa. Therefore, the lower limit value of the tensile strength of the wire rod according to the present embodiment may be regulated to be 600 MPa or 700 MPa. In addition, the upper limit value of the tensile strength of the wire rod according to the present embodiment may be regulated to be 1,400 MPa or 1,350 MPa.
It is not necessary to limit the form and the like of the sulfide-based inclusion. This is because, in a case where the chemical composition of the wire rod and the chemical composition and grain diameter of the complex oxide are appropriately controlled, the sulfide-based inclusion is essentially finely dispersed and detoxified. In addition, according to an experiment by the present inventors, in a case where a sulfide was detoxified by Ca or the like, cracks starting from the sulfide was not generated in an evaluation test of the flat steel wire. In consideration of this fact as well, not limiting the form and the like of the sulfide-based inclusion is considered to be appropriate.

(E) Regarding manufacturing method

In a method for manufacturing the wire rod according to the present embodiment, in order to suppress hydrogen induced cracking, the compositional ratio ε of the oxide including CaO and Al₂O₃ is made to be appropriate by adding Ca in a molten steel stage, and, furthermore, the size of the complex oxide is controlled.
As long as the requirements of the wire rod according to the present embodiment are satisfied, the effect of the wire rod according to the present embodiment can be obtained regardless of the method for manufacturing the wire rod, but the wire rod needs to be manufactured according to, for example, a manufacturing method described below. The following manufacturing process is simply an example, and it is needless to say that a wire rod having a chemical composition and other requirements in the ranges of the wire rod according to the present embodiment which is obtained using a process other than the following manufacturing process is also considered as the wire rod according to the present embodiment.
Specifically, the component of molten steel is adjusted in a converter after the desulfurization of hot metal, a Ca alloy is added to the molten steel, and then a steel piece is obtained using continuous casting. After that, the steel piece is reheated to hot-roll a product and is finished to a steel material having a predetermined diameter. Hereinafter, an example of a method for manufacturing the molten steel will be described in more detail.
Desulfurization is carried out using a Kanbara reactor (KR) method in which a desulfurizer is added to hot metal tapped from a blast furnace and stirred, thereby removing sulfur, subsequently, dephosphorization and decarbonization are carried out in a converter. In addition, at the time of tapping molten steel from the converter to a molten steel pan, regarding elements except for Ca, REM, and Zr from a target chemical composition, the component of the molten steel is adjusted by adding an alloy of metallic Al or the like. Subsequently, the molten steel is degassed in Ruhrstahl-Heraeus (RH), and a Ca alloy is added to the molten steel. The composition of the Ca alloy is, for example, 40mass% of Ca and 60mass% of Si. In addition, regarding a method for adding the Ca alloy, a powder injection method in which the powder of the Ca alloy is blown into the steel together with an inert gas is used.
Here, a timing of adding the Ca alloy is set to 30 minutes or more and 60 minutes or less from the addition of the metallic Al. In a case where the Ca alloy is added within 30 minutes from the addition of the metallic Al, some of Ca added to the metal is consumed due to the reaction with coarse Al₂O₃ floating in the steel, and thus a sulfide-detoxifying effect of Ca cannot be obtained. In addition, in a case where the Ca alloy is added within 30 minutes from the addition of the metallic Al, coarse Al₂O₃ remains, and thus the equivalent circle diameter of the oxide in the wire rod after rolling does not reach 6.0 µm or less. On the other hand, in a case where the Ca alloy is added after 60 minutes from the addition of the metallic Al, the Al₂O₃ present in the steel becomes small, and it becomes difficult to control ε to be less than 3.00. In the case of adding either or both REM and Zr, an alloy including either or both REM and Zr is added at the same time or after the addition of the Ca alloy. This is because REM and Zr exhibit the same behavior as Ca in the relationship with Al.
In the molten steel manufactured by the above steps, the compositional ratio ε (mass% of CaO / mass% of Al₂O₃) of the complex oxide satisfies 0.00 ≤ ε < 3.00. This molten steel is turned into a steel piece using a continuous casting method. The casting rate at the time of turning the molten steel into the steel piece is preferably 0.6 m/min to 1.4 m/min. During casting, some of the inclusions float and do not remain in the steel piece, but the rest of the inclusions move down and remain in the steel piece. In the case of casting the molten steel at less than 0.6 m/min, the inclusions that have floated move down again, and thus there is a case where the amount of coarse inclusions increases in the slab. On the other hand, in the case of casting the molten steel at more than 1.4 m/min, the amount of the inclusions that move down increases, and thus there is a case where the amount of coarse inclusions increases in the steel piece.
The obtained steel piece is hot-rolled, thereby manufacturing a wire rod. The hot rolling is carried out by heating the steel piece at 1,020°C or more. The final finishing temperature of the hot rolling is set to 800°C to 960°C. In addition, the hot rolling is carried out so that the area ratio of the cross section of the steel piece before the hot rolling to the hot-rolled wire rod (the cross section area (mm²) of the steel piece / the cross section area (mm²) of the hot-rolled wire rod) reaches 100.0 or more. When the rolling temperature in final finishing rolling is less than 800°C or more than 960°C, or the area ratio of the cross section is less than 100.0, the crushing of the complex oxide during the hot rolling becomes insufficient, and the size of the complex oxide in the wire rod does not reach 6.0 µm or less. The size and compositional ratio of the complex oxide can be controlled using the above-described steps.
Next, the flat steel wire according to the present embodiment will be described below. The flat steel wire according to the present embodiment is a flat steel wire obtained by rolling the wire rod according to the present embodiment. The shape of a flat steel wire 2 is not particularly limited, but the shape of a C-section thereof is preferably a shape formed by pressing a circle.
In the flat steel wire 2 according to the present embodiment, the minor axis length of the C-section will be referred to as a thickness t of the flat steel wire 2, and the major axis of the C-section will be referred to as a width w of the flat steel wire 2.
In addition, an L-section of the flat steel wire 2 described below refers to a cross section that is parallel to the rolling direction and the minor axis direction of the flat steel wire and substantially includes the center axis of the flat steel wire. The center axis of the flat steel wire refers to an axis that passes through the center of the C-section and is parallel to the rolling direction. The minor axis direction of the flat steel wire refers to the minor axis direction of a cross section perpendicular to the rolling direction of the flat steel wire.
A center portion 21 in the L-section of the flat steel wire 2 refers to a region that is in a range of 1/7 of the minor axis length (the thickness t of the flat steel wire 2) of the flat steel wire 2 from the center axis of the flat steel wire 2 as shown in FIG. 3. In other words, the center portion 21 in the L-section of the flat steel wire 2 is a region at a depth of 5/14t or more from the surface of the flat steel wire in the L-section.
Hereinafter, the center portion 21 in the L-section of the flat steel wire 2 will be simply referred to as the "center portion" in some cases. In a case where the C-section of the flat steel wire 2 is substantially circular and the short axis and long axis of the C-section thereof cannot be specified, an arbitrary axis that passes through the center of the C-section of the flat steel wire and an axis perpendicular to the above-described axis may be regarded as the long axis and the short axis.
The chemical composition of the flat steel wire include, by mass%, C: 0.15% to 0.85%, Si: 0.10% to 2.00%, Mn: 0.30% to 1.50%, Al: 0.001% to 0.080%, Ca: 0.0002% to 0.0050%, N: 0.0020% to 0.0080%, P: 0.020% or less, S: 0.020% or less, O: 0.0050% or less, Cr: 0% to 1.00%, V: 0% to 0.15%, Ti: 0% to 0.050%, Nb: 0% to 0.050%, Cu: 0% to 1.00%, Ni: 0% to 1.50%, Mo: 0% to 1.00%, B: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.1000%, and a remainder including Fe and impurities. The flat steel wire is a flat steel wire obtained by rolling the wire rod, and thus the chemical composition of the flat steel wire according to the present embodiment is identical to the chemical composition of the wire rod according to the present embodiment. It is needless to say that the preferred upper limit value and the preferred lower limit value that have described regarding the respective elements in the chemical composition of the wire rod are also applicable to the chemical composition of the flat steel wire.
In the flat steel wire according to the present embodiment as well, the form of an oxide including CaO and Al₂O₃ (complex oxide) is regulated. The definitions of the complex oxide in the flat steel wire and a compositional ratio ε thereof are identical to the definitions of the complex oxide in the wire rod and the compositional ratio ε thereof. The average value of the compositional ratios ε of the complex oxide that is observed in the center portion of the flat steel wire is made to satisfy 0.00 ≤ ε < 3.00. The flat steel wire is a flat steel wire obtained by rolling the wire rod, and thus the chemical composition of the complex oxide in the flat steel wire according to the present embodiment is identical to the chemical composition of the complex oxide in the wire rod according to the present embodiment.
The average value of the equivalent circle diameters of the complex oxide that is observed in the center portion of the flat steel wire is set to 3.0 µm or less. In a case where the average value of the equivalent circle diameters of the complex oxide that is observed in the center portion of the flat steel wire exceeds 3.0 µm, the hydrogen induced cracking Properties of the flat steel wire is impaired by a void that is generated around the complex oxide.
As long as the above-described requirements are satisfied, other configurations of the flat steel wire according to the present embodiment are not particularly limited.
For example, the metallographic structure of the flat steel wire, similar to the metallographic structure of the wire rod, has no significant influence on the hydrogen induced cracking resistance of the flat steel wire. Therefore, the metallographic structure of the flat steel wire is not particularly limited. For example, in a case where the structure in the center portion of the flat steel wire includes 98area% or more of tempered martensite, it is possible to further improve the tensile strength of the flat steel wire, which is preferable. For example, in a case where the structure in the center portion of the flat steel wire includes 20% to 60area% of ferrite and 40% to 60area% of bainite, it is possible to improve the toughness and the like of the flat steel wire, which is preferable.
The width w and the thickness t of the flat steel wire are also not particularly limited. Generally, the widths of flat steel wires that are in circulation in the current market are set to 13 to 16 mm, and the thicknesses t are set to 2 to 7 mm, and thus the width and thickness of the flat steel wire according to the present embodiment may also be regulated as described above.
The tensile strength of the flat steel wire is also not particularly limited. When the use of the flat steel wire is taken into account, the tensile strength of the flat steel wire is desirably set to approximately 1,100 to 1,500 MPa, which can be achieved by appropriately adjusting a heat treatment condition of the flat steel wire.
A sulfide-based inclusion in the flat steel wire also does not need to be limited in terms of the form and the like for the same reason as the sulfide-based inclusion in the wire rod.
Methods for evaluating the complex oxide in the flat steel wire are based on the methods for evaluating the complex oxide in the wire rod. However, the methods for evaluating the complex oxide are different between the flat steel wire and the wire rod in terms of the fact that the complex oxide in the wire rod is evaluated in the central portion of the C-section of the wire rod, but the complex oxide in the flat steel wire is evaluated in the center portion of the L-section of the flat steel wire. The L-section of the flat steel wire in the present embodiment is a cross section including the center axis of the flat steel wire; however, at the time of evaluating the complex oxide, the complex oxide may also be evaluated using a cross section slightly away from the center axis of the flat steel wire as a measurement surface. In this case, the center portion in the measurement surface may be specified by regarding an axis parallel to the rolling direction of the measurement surface as the center axis of the flat steel wire. Even when there is a small gap between the measurement surface and the center axis of the flat steel wire, the evaluation results of the complex oxide are not substantially affected.
A method for manufacturing the flat steel wire according to the present embodiment includes a step of flattening the wire rod according to the present embodiment. The reduction of area in the flattening process is set to 40% or more. In a case where the reduction of area is less than 40%, the complex oxide in the wire rod is not sufficiently crushed, and thus it is difficult to set the maximum value of the equivalent circle diameter of the complex oxide in the flat steel wire to 3.0 µm or less. In order to adjust the tensile strength of the flat steel wire, the wire rod before the flattening process or the flat steel wire after the flattening process may be appropriately heat-treated. This is because the forms of the complex oxide and sulfides do not significantly change at a heat treatment temperature for ordinary steel.

[Examples]

Hereinafter, the present invention will be specifically described using examples.
Specifically, steels having a chemical composition in Table 1, Table 2-1, and Table 2-2 were melted, and wire rods and flat steel wires were produced using the following methods. A symbol "-" in these tables indicates that the amount of a corresponding element is on an impurity level and the element can be determined to be not contained.
Steels A and B having a chemical composition shown in Table 1 were manufactured using the following method. Desulfurization was carried out using a KR method on hot metal, and dephosphorization and decarbonization were carried out in a converter. After that, in order to adjust elements except for Ca, REM, and Zr in the above-described chemical composition, metallic A1 or the like was added to molten steel. A sample for an analysis was taken from the molten steel, a component analysis was carried out, and the chemical composition other than Ca, REM, and Zr was adjusted. After that, the molten steel was degassed in Ruhrstahl-Heraeus (RH), and a CaSi alloy was added to the molten steel. The composition of the CaSi alloy was 40 mass% of Ca and 60 mass% of Si. The CaSi alloy was added using a powder injection method in which the powder of the CaSi alloy was blown into the steel together with an inert gas. In Test Nos. A1, A4, A5, and B1, the CaSi alloy was added after 40 minutes from the addition of the metallic Al. In Test Nos. A2 and B2, the CaSi alloy was added after 25 minutes from the addition of the metallic Al. In Test Nos. A3 and B3, the addition timing of the CaSi alloy was after 70 minutes from the addition of the metallic Al.
The molten steel obtained as described above was cast, thereby producing a steel ingot. The casting rate was set to 0.9 m/min. After that, this steel ingot was reheated at 1,250°C for 12 hours and bloomed to a 122 mm × 122 mm steel piece, thereby producing a material for rolling. Next, regarding A1 to A3, A5, and B1 to B3, the material for rolling was heated to 1,050°C and rolled to a wire rod having a diameter of 12 mm. Regarding A4, the material for rolling was heated to 1,250°C, hot-rolled to a diameter of 16 mm, cut to a length of 1,500 mm, and ground to a diameter of 12 mm. After the rolling or the grinding, the surface of the wire rod was lubricated, then a primary wire drawing process was carried out so as to obtain a wire rod having a diameter of 11 mm. After that, regarding all steel materials, the wire-drawn wire rod was flat rolled by a cold rolling mill (flattening process) and formed to a flat steel wire having a width of 15 mm and a thickness of 3 mm. For Test Nos. A1 to A4 and Test Nos. B1 to B3, after produced, the flat steel wire was heated at 900°C for 15 minutes, then, quenched by being immersed in a cold oil, and tempered at a temperature of 450°C for 60 minutes. For Test No. A5, the wire rod was flat rolled (flattening process) and then annealed at 450°C for 60 minutes.
In addition, steels a to au having a chemical composition shown in Table 2-1 and Table 2-2 (Test Nos. 1 to 47 in Table 4-1 and Table 4-2) were melted using the same method as for the steel A1, the obtained steel ingots were heated at 1,250°C for 12 hours, and then steel pieces obtained by blooming 122 mm × 122 mm steel pieces were used as materials for rolling. Next, the materials for rolling were heated at 1,050°C and flat rolled (flattening process) to wire rods having a diameter of 12 mm. After that, the surfaces of the wire rods were lubricated, and a primary wire drawing process was carried out so as to obtain wire rods having a diameter of 11 mm. After that, the wire-drawn wire rods were rolled in a cold rolling mill and formed to flat steel wires having a width of 15 mm and a thickness of 3 mm. After cold-rolled, the formed flat steel wires were heated at 900°C for 15 minutes, then, quenched by being immersed in a cold oil, and heated at a temperature of 450°C for 60 minutes. For samples in which the flat steel wire broke at the time of cold-rolling the flat steel wire, the steps following the heat treatment were not carried out, and the test and the evaluation were stopped. A symbol "-" is given to cells in the "Evaluation result" column for the above-described samples. Underlined numerical values in Table 2-1 and Table 2-2 indicate that the component composition is not in the scope of the present invention.
The investigation results of the average values of the compositional ratios ε of the complex oxides, the average values of the equivalent circle diameters of the complex oxides, and the tensile strengths of the wire rods produced using the above-described method, and the average values of the equivalent circle diameters of the complex oxides, the structures, the tensile strengths, and the hydrogen induced cracking resistances of the flat steel wires are shown in Table 3-1 to Table 4-2. In Table 3-1, values deviating from the preferred manufacturing conditions are underlined. In Table 3-1 to Table 4-2, values outside the scope of the present invention are also underlined. In addition, in these tables, "complex oxide compositional ratio ε" indicates the average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod, and "average equivalent circle diameter" indicates the average value of the equivalent circle diameters of the complex oxide in the central portion of the wire rod or the center portion of the flat steel wire. The average value of the compositional ratios ε of the complex oxide in the center portion of the flat steel wire is substantially identical to that in the central portion of the wire rod and is thus not measured.
The average value of the compositional ratios ε of the complex oxide in the central portion of the wire rod and the average value of the equivalent circle diameters of the complex oxide in the central portion of the wire rod or the center portion of the flat steel wire were investigated using the above-described method. The tensile strength of the wire rod and the structure, tensile strength, and hydrogen induced cracking resistance of the flat steel wire were respectively investigated using methods described below.

(1) Tensile strength of wire rod

The wire rod was cut to a length of 340 mm, the top and bottom 70 mm portions were fixed using hydraulic chucks, and a tensile test was carried out. The tensile strength was computed by dividing the obtained maximum load by the cross section area of the wire rod. The tensile strength is preferably 600 MPa or more, and thus a wire rod having a tensile strength of 600 MPa or more was evaluated as an acceptable product.

(2) Structure of flat steel wire

An L-section of the flat steel wire was mirror-polished and eroded with picral, five arbitrary places in the center portion of the L-section were respectively observed using FE-SEM at a magnification of 2,000 times, and five photographs were captured. An OHP sheet was overlaid on each of the obtained photographs, and regions overlapping with a ferrite structure and a bainite structure in each of the transparent sheets were painted with color. Next, the area ratio of the "region painted with color" in each of the transparent sheets was obtained using image analysis software, thereby obtaining the area ratios of the ferrite structure and the bainite structure in each of the five places. In addition, the area ratios of the ferrite structure and the bainite structure in the five places were averaged, thereby computing the area ratios of the ferrite structure and the bainite structure in the flat steel wire. In addition, structures other than ferrite, bainite, and martensite were not substantially confirmed in any of the flat steel wires, and thus a value obtained by subtracting the area ratios of the ferrite structure and the bainite structure from 100% was regarded as the average value of a martensite structure.

(3) Tensile strength of flat steel wire:

The flat steel wire was cut to a length of 400 mm, the top and bottom 100 mm portions were fixed using hydraulic chucks, and a tensile test was carried out. The tensile stress was computed by dividing the obtained maximum load by the cross section area of the flat steel wire. The tensile strength is preferably 1,100 MPa or more, and thus a flat steel wire having a tensile strength of 1,100 MPa or more was evaluated as an acceptable product.

(4) Investigation of hydrogen induced cracking resistance of flat steel wire:

The hydrogen induced cracking resistance was evaluated using the flat steel wire cut to a length of 150 mm. As a solution for supplying hydrogen to a test specimen, a 5% NaCl+CH₃COONa aqueous solution was used, and the solution was used after the pH was adjusted to 5.0. After the solution was degassed with nitrogen gas, a hydrogen sulfide (H2S) and carbon dioxide (CO₂)-mixed gas was introduced thereto, and the flat steel wire was immersed in the solution, thereby investigating the generation of cracks. At this time, the pressure of the hydrogen sulfide was 0.1 MPa, the testing temperature was 25°C, and the testing time was 96 hours. After the test, the presence and absence of the generation of cracks was confirmed by an ultrasonic test (UST) with a frequency of 10 kHz in the thickness direction of the flat steel wire. The total of the areas of crack-generated portions in which cracks were determined to be generated by the ultrasonic test was obtained by an image analysis, and a hydrogen induced cracking occurrence percentage (χ (%)) was obtained using Expression D. A flat steel wire in which hydrogen induced cracking occurred was determined to fail in terms of the hydrogen induced cracking resistance. $χ = (Af / (w \times L)) \times 100$

Here, Af represents the total area (mm²) of the crack-generated portions measured by UST, w represents the width (mm) of the flat steel wire, and L represents the length (mm) of the flat steel wire.

[Table 3-1]

Test No.	Type of steel	Method for manufacturing wire rod		Properties of complex oxide in wire rod		Strength of wire rod [MPa]	Note
Test No.	Type of steel	Timing of addition of CaSi alloy [min.]	Hot rolling area ratio of cross section	Complex oxide compositional ratio ε	Average equivalent circle diameter [µm]	Strength of wire rod [MPa]	Note
A1	A	40	131.6	0.23	2.1	958	Invention Example
A2	A	25	131.6	0.17	7.6	965	Comparative Example
A3	A	70	131.6	4.23	2.4	951	Comparative Example
A4	A	40	74.1	0.41	9.3	944	Comparative Example
A5	A	40	131.6	0.53	1.8	956	Invention Example
B1	B	40	131.6	0.33	2.5	939	Invention Example
B2	B	25 25	131.6	0.21	8.2	960	Comparative Example
B3	B	70	131.6	5.52	3.0	957	Comparative Example

[Table 3-2]

Test No.	Type of Steel	Method for manufacturing flat steel wire		Average equivalent circle diameter of complex oxide in flat steel wire [µm]	Structures of flat steel wire (average value)			Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
Test No.	Type of Steel	Flattening by rolling	Quenching and tempering		Martensite fraction [area%]	Ferrite fraction [area%]	Bainite fraction [area%]	Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
A1	A	Yes	Yes	1.1	100	0	0	1284	0	Invention Example
A2	A	Yes	Yes	4.7	100	0	0	1288	70	Comparative Example
A3	A	Yes	Yes	2.5	100	0	0	1291	25	Comparative Example
A4	A	Yes	Yes	3.1	100	0	0	1281	85	Comparative Example
A5	A	Yes	No	1.4	0	34	66	1284	0	Invention Example
B1	B	Yes	Yes	1.6	100	0	0	1261	0	Invention Example
B2	B	Yes	Yes	4.1	100	0	0	1277	45	Comparative Example
B3	B	Yes	Yes	2.8	100	0	0	1283	40	Comparative Example

As shown in Table 3-1 and Table 3-2, in invention examples having an appropriate chemical composition and an appropriate manufacturing condition, hydrogen induced cracking did not occur even after the flattening process, and there was no problem.
In contrast, in Test Nos. A2 and B2, the time taken from the addition of the metallic A1 to the addition of the CaSi alloy was 25 minutes, and thus the complex oxide in the wire rod became coarse, and hydrogen induced cracking occurred in the flat steel wire.
In Test Nos. A3 and B3, the time taken from the addition of the metallic A1 to the addition of the CaSi alloy was 70 minutes, and thus the average value of the compositional ratios ε of the complex oxide in the wire rod reached 3.00 or more, and hydrogen induced cracking occurred in the flat steel wire.

In Test No. A4, the reduction amount of the cross section area during the hot rolling decreased, and the oxide was not sufficiently crushed during the hot rolling, and thus the maximum size of the complex oxide in the wire rod became outside of the scope of the present invention, and hydrogen induced cracking occurred in the flat steel wire.

[Table 4-1]

No. Test No.	Type of steel	Properties of complex oxide in wire rod		Strength of flat steel wire [MPa]	Average equivalent circle diameter of complex oxide in flat steel wire [µm]	Structure of flat steel wire (average value)	Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
No. Test No.	Type of steel	Complex oxide compositional ratio ε	Average equivalent circle diameter [µm]	Strength of flat steel wire [MPa]		Martensite fraction [area%]	Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
1	a	0.17	4.3	841	2.0	100	1256	0
2	b	1.69	39	819	1.2	100	1233	0
3	c	0.19	4.9	833	1.6	100	1301	0
4	d	0.25	5.1	941	1.0	100	1234	0
5	e	2.61	5.5	1172	1.2	100	1411	0
6	f	0.21	5.0	927	1.1	100	1316	0
7	g	0.23	5.1	934	1.6	100	1391	0
8	h	0.21	5.6	968	1.5	100	1369	0	Invention Examples
9	i	0.02	5.1	853	1.9	100	1299	0
10	j	0.91	5.0	879	1.6	100	1304	0
11	k	0.17	5.7	927	1.5	100	1267	0
12	l	0.16	3.7	1182	1.3	100	1406	0
13	m	0.17	4.4	1341	0.8	100	1459	0
14	n	0.16	5.8	966	1.3	100	1297	0
15	o	0.17	4.9	754	1.2	100	1194	0
16	p	0.18	5.6	836		-		-	Comparative Examples
17	q	0.17	5.2	1523	-		-
18	r	0.18	4.3	822	1.8	100	1249	85
19	s	0.00	11.4	940	5.1	100	1311	100
20	t	0.17	5.6	897	-	-	-	-
21	u	021	3.8	519	1.6	100	961	0
22	v	11.12	4.7	916	4.5	100	1321	70

[Table 4-2]

Test No.	Type of steel	Properties of complex oxide in wire rod		Strength of flat steel wire [MPa]	Average equivalent circle diameter of complex oxide in flat steel wire [µm]	Structure of flat steel wire (average value)	Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
Test No.	Type of steel	Complex oxide compositional ratio ε	Average equivalent circle diameter [µm]	Strength of flat steel wire [MPa]		Martensite fraction [area%]	Strength of flat steel wire [MPa]	Hydrogen induced cracking occurrence percentage [%]	Note
23	w	0.22	5.1	677	1.1	100	1106	0
24	x	0.36	4.4	973	1.2	100	1261	0
25	y	1.13	3.9	1015	1.6	100	1269	0
26	z	1.61	4.6	1170	1.9	100	1301	0
27	aa	1.65	3.7	973	1.5	100	1247	0
28	ab	0.57	5.6	965	1.4	100	1288	0
29	ac	0.88	5.5	984	1.6	100	1258	0
30	ad	0.36	4.9	926	1.3	100	1275	0
31	ac	0.41	5.1	802	1.0	100	1214	0
32	af	0.69	5.3	975	1.1	100	1301	0	Invention Examples
33	ag	0.44	4.1	914	1.6	100	1269	0
34	ah	0.36	3.1	1033	1.5	100	1366	0
35	ai	0.69	3.9	953	1.9	100	1288	0
36	aj	0.74	4.9	1092	1.4	100	1356	0
37	ak	1.16	5.4	1033	2.6	100	1323	0
38	al	1.69	5.4	1001	1.1	100	1312	0
39	am	2.94	5.6	1159	16	100	1348	0
40	an	1.36	4.8	1011	0.9	100	1318	0
41	ao	0.48	4.6	903	2.4	100	1264	0
42	ap	0.66	4.1	711	1.4	100	1064	0	Comparative Examples
43	aq	1.08	4.7	941	1.9	88	1088	0
44	ar	2.61	6.9	982	3.4	100	1301	75
45	as	0.21	7.6	865	3.8	100	1248	100
46	at	1.28	5.1	1017	-	-	-	-
47	an	0.08	8.4	926	4.3	100	1275	100

As shown in Table 4-1 and Table 4-2, in invention examples having an appropriate chemical composition and an appropriate manufacturing condition, hydrogen induced cracking did not occur even after the flattening process, and there was no problem.
In Test Nos. 16, 17, 20, and 46, the chemical composition was outside the scope of the present invention, cracks were generated in the flat steel wire at the time of cold-rolling to the flat steel wire (during the flattening process), and thus the steps following the heat treatment were not carried out, and the test was stopped.
In Test No. 16, the Mn content and the Ca content were outside the scope of the present invention, and cracks were generated during the flattening process.
In Test No. 17, the C content was outside the scope of the present invention, and cracks were generated during the flattening process.
In Test No. 20, the Si content was outside the scope of the present invention, and cracks were generated during the flattening process.
In Test Nos. 18 and 19, any of the chemical composition of the steel was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 18, the S content was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 19, Ca was not added, MnS was not refined, and the maximum size of the complex oxide was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 21, the C content and the N content were outside the scope of the present invention, and the tensile strength did not reach 1,100 MPa.
In Test No. 22, the Ca content was outside the scope of the present invention, additionally, the compositional ratio of the complex oxide was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 42, the Si content was outside the scope of the present invention, and the tensile strength did not reach 1,100 MPa.
In Test No. 43, the Mn content was outside the scope of the present invention, and the tensile strength did not reach 1,100 MPa.
In Test No. 44, the Al content and the O content were outside the scope of the present invention, the maximum size of the complex oxide was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 45, the Al content was outside the scope of the present invention, the maximum size of the complex oxide was outside the scope of the present invention, and hydrogen induced cracking occurred.
In Test No. 46, the N content was outside the scope of the present invention, and cracks were generated during the flattening process.
In Test No. 47, the Ca content was outside the scope of the present invention, MnS was not refined, and the maximum size of the complex oxide was outside the scope of the present invention, and hydrogen induced cracking occurred.

[Brief Description of the Reference Symbols]

1: WIRE ROD
11: CENTRAL PORTION
2: FLAT STEEL WIRE
21: CENTER PORTION

Claims

A wire rod comprising, as a chemical composition, by mass%:
C: 0.15% to 0.85%,

Si: 0.10% to 2.00%,

Mn: 0.30% to 1.50%,

Al: 0.001% to 0.080%,

Ca: 0.0002% to 0.0050%,

N: 0.0020% to 0.0080%,

P: 0.020% or less,

S: 0.020% or less,

O: 0.0050% or less,

Cr: 0% to 1.00%,

V: 0% to 0.15%,

Ti: 0% to 0.050%,

Nb: 0% to 0.050%,

Cu: 0% to 1.00%,

Ni: 0% to 1.50%,

Mo: 0% to 1.00%,

B: 0% to 0.0100%,

REM: 0% to 0.0100%,

Zr: 0% to 0.1000%, and

a remainder including Fe and impurities,

wherein an oxide including CaO and Al₂O₃ and satisfying Expression A and Expression B is defined as a complex oxide,

an average value of a compositional ratio ε of the complex oxide that is defined as Expression C satisfies 0.00 ≤ ε < 3.00, measured in a central portion within a range of 1/10 of a diameter of the wire rod from a central axis of the wire rod in a cross section perpendicular to a rolling direction of the wire rod, and

an average value of an equivalent circle diameter of the complex oxide is 6.0 µm or less, measured in the central portion of the cross section, $(amount of oxide-forming element other than Ca and Al in oxide by unit mol %) < (1 / 3) \times (larger one of Ca content or Al content in oxide by unit mol %)$
$(O content in oxide by unit mol %) \geq (S content in oxide by unit mol %)$
$(Compositional ratio ε) = (concentration of CaO in the complex oxide by unit mass %) / (concentration of {Al}_{2} O_{3} in the complex oxide by unit mass %)$
The wire rod according to claim 1 comprising, as the chemical composition, by mass%:
Cr: 0.05% to 1.00%.
The wire rod according to claim 1 or 2 comprising, as the chemical composition, by mass%:
one or more selected from the group consisting of

V: 0.02% to 0.15%,

Ti: 0.002% to 0.050%, and

Nb: 0.002% to 0.050%.
The wire rod according to any one of claims 1 to 3 comprising, as the chemical composition, by mass%:
one or more selected from the group consisting of

Cu: 0.01% to 1.00%,

Ni: 0.01% to 1.50%,

Mo: 0.01% to 1.00%, and

B: 0.0002% to 0.0100%.
The wire rod according to any one of claims 1 to 4 comprising, as the chemical composition, by mass%:
one or both selected from the group consisting of

REM: 0.0002% to 0.0100% and

Zr: 0.0002% to 0.1000%.
The wire rod according to any one of claims 1 to 5,
wherein a tensile strength is 600 MPa to 1,400 MPa.
A flat steel wire comprising, as a chemical composition, by mass%:
C: 0.15% to 0.85%,

Si: 0.10% to 2.00%,

Mn: 0.30% to 1.50%,

Al: 0.001% to 0.080%,

Ca: 0.0002% to 0.0050%,

N: 0.0020% to 0.0080%,

P: 0.020% or less,

S: 0.020% or less,

O: 0.0050% or less,

Cr: 0% to 1.00%,

V: 0% to 0.15%,

Ti: 0% to 0.050%,

Nb: 0% to 0.050%,

Cu: 0% to 1.00%,

Ni: 0% to 1.50%,

Mo: 0% to 1.00%,

B: 0% to 0.0100%,

REM: 0% to 0.0100%,

Zr: 0% to 0.1000%, and

a remainder including Fe and impurities,

wherein an oxide including CaO and Al₂O₃ and satisfying Expression A and Expression B is defined as a complex oxide,

an average value of a compositional ratio ε of the complex oxide that is defined as Expression C satisfies 0.00 ≤ ε < 3.00, measured in a center portion within a range of 1/7 of a minor axis length of the flat steel wire from a center axis of the flat steel wire in a cross section parallel to a rolling direction and a minor axis direction of the flat steel wire and including the center axis of the flat steel wire, and

an average value of an equivalent circle diameter of the complex oxide is 3.0 µm or less, measured in the center portion of the cross section, $(amount of oxide-forming element other than Ca and Al in oxide by unit mol %) < (1 / 3) \times (larger one of Ca content or Al content in oxide by unit mol %)$
$(O content in oxide by unit mol %) \geq (S content in oxide by unit mol %)$
$(Compositional ratio ε) = (concentration of CaO in the complex oxide by unit mass %) / (concentration of {Al}_{2} O_{3} in the complex oxide by unit mass %)$
The flat steel wire according to claim 7,
wherein a structure in the center portion includes 98 area% or more of tempered martensite.
The flat steel wire according to claim 7,
wherein a structure in the center portion includes 20 area% to 60 area% of ferrite and 40 area% to 60 area% of bainite.
The flat steel wire according to any one of claims 7 to 9,
wherein a tensile strength is 1,100 MPa to 1,500 MPa.
The flat steel wire according to any one of claims 7 to 10 comprising, as the chemical composition, by mass%:
Cr: 0.05% to 1.00%.
The flat steel wire according to any one of claims 7 to 11 comprising, as the chemical composition, by mass%:
one or more selected from the group consisting of

V: 0.02% to 0.15%,

Ti: 0.002% to 0.050%, and

Nb: 0.002% to 0.050%.
The flat steel wire according to any one of claims 7 to 12 comprising, as the chemical composition, by mass%:
one or more selected from the group consisting of

Cu: 0.01% to 1.00%,

Ni: 0.01% to 1.50%,

Mo: 0.01% to 1.00%, and

B: 0.0002% to 0.0100%.
The flat steel wire according to any one of claims 7 to 13 comprising, as the chemical composition, by mass%:
one or both selected from the group consisting of

REM: 0.0002% to 0.0100% and

Zr: 0.0002% to 0.1000%.