BR112019017993A2 - flat wire and steel wire - Google Patents

Abstract

a wire rod according to one embodiment of the present invention has a predetermined chemical composition and is configured so that: the average of the composition ratios ¿of the compound oxides as measured in its central part satisfies 0.00 ¿¿<3.00; and the mean of the equivalent circle diameters of the compound oxides as measured in their central part is 6.0 ¿m or less. a flat steel wire according to another embodiment of the present invention has a predetermined chemical composition and is configured so that: the average of the composition ratios of the compound oxides as measured in its central part satisfies 0.00 ¿<3.00; and the mean of the equivalent circle diameters of the compound oxides as measured in their central part is 3.0 ¿m or less.

Description

Descriptive Report of the Invention Patent for MACHINE WIRE AND FLAT STEEL WIRE.

Technical Field of the Invention

[001] The present invention relates to a wire rod and a flat steel wire.

[002] Priority is claimed over Japanese Patent Application No. 2017-059111, filed in Japan on March 24, 2017, the content of which is incorporated herein by reference.

Related Technique

[003] In flexible tubes that are used to transport natural gas, crude oil, etc., which are explored on the seabed under high pressure, flat steel wires are used as reinforcement material. Flat steel wires of this type are formed by being planed to be 40% to 80% in thickness of a hot rolled wire rod and used with the processed structure or after being cooled and tempered. In recent years, exploration has taken place deep in the ocean, and the transport distance of the explored substances has become longer, and so there has been an increasing demand that flexible pipes and flat steel wires are the reinforcement material for flexible pipes. have a high resistance. In addition, flexible tubes are used in an acidic environment including hydrogen sulfide, and therefore flat steel wires must have a characteristic of resisting the occurrence of hydrogen-induced fracture (HIC), that is, resistance to hydrogen-induced fracture.

[004] However, in the case of providing high resistance to flat steel wires, the susceptibility to hydrogen of flat steel wires increases, and the hydrogen-induced fracture is accelerated. In addition, when a wire rod is flattened to be 40% to 80% thick, a sulfide present in the wire rod is stretched, so that the iron and sulfide separate, and hydrogen accumulates in a gap

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2/54 generated by the separation, which increases the probability of the occurrence of hydrogen-induced fracture. Therefore, there is a demand for the development of a hot-rolled wire rod that can be used to produce flat steel wires that are applicable to flexible tubes that are used in an acidic environment and have high strength and excellent resistance to hydrogen-induced fracture. So far, as a technique for providing the high-strength material described above that is used in an acidic environment, Patent Document 1 has been proposed.

[005] Patent Document 1 describes a technique for obtaining a high strength flat steel wire that has excellent resistance to hydrogen embrittlement by performing a cold process on high carbon steel that has a pearlite structure and tempering the steel high carbon for a short period of time.

Prior art document

Patent Document

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

Description of the invention

Problems to be solved by the invention

[007] Patent Document 1 describes a flat steel wire having a tensile strength of 1,300 MPa or more and having excellent resistance to fracture induced by hydrogen 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, fractures are formed even in a case where the tensile strength is adjusted to 1,100 Mpa. The present inventors considered that the lack of resistance to fracture induced by hydrogen in the flat steel wire of Patent Document 1 results from the strong development of the degradation of resistance to hydrogen embrittlement caused by se

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3/54 stop inclusions.

[008] The present invention was made in consideration of the circumstance described above, and an objective of the present invention is to provide a wire rod from which a flat steel wire can be obtained which has a tensile strength of 1,100 MPa or more and which has excellent resistance to fracture induced by hydrogen.

Means to solve the problem

[009] The summary of the present invention is as described below.

[0010] (1) A wire according to an aspect of the present invention includes, as a chemical composition, in 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 the balance including Fe and impurities; in which an oxide including CaO and AI2O3 and which satisfies Expression A and Expression B is defined as a complex oxide, the average value of a composition ratio ε of the complex oxide that is defined by Expression C satisfies 0.00 <ε < 3.00, measured in a central portion within a range of 1/10 the diameter of the wire rod from the central axis of the wire rod in a cross section perpendicular to the rolling direction of the wire rod, and 0 average value of a diameter equivalent circle of complex oxide is 6.0 pm or less, measured in the central portion of the cross section.

(Number of oxide-forming elements other than Ca and Al in oxide per unit of mol%) <(1/3) χ (highest content between Ca content or Al content in oxide in units of mol%): Expression A ( content of O in oxide in units of mol%)> (content of S in

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4/54 oxide in units of mol%): Expression B (Composition ratio ε) = concentration of CaO in complex oxide in units of mass%) / (concentration of AI2O3 in complex oxide in units of mass%): Expression C [0011] (2) The wire rod according to 0 item (1) may include, as a chemical composition, in mass%: Cr: 0.05% to 1.00%. [0012] (3) The wire rod according to 0 item (1) or (2) may include, as a chemical composition, in mass%, one or more elements 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%.

[0013] (4) The wire rod according to any of items (1) to (3) may include, as a chemical composition, in mass%, one or more elements selected from the group consisting of Cu: 0.01% at 1.00%, Ni: 0.01% to 1.50%, Mo: 0.01% to 1.00%, and B: 0.0002% to 0.0100%.

[0014] (5) The wire rod according to any of items (1) to (4) may include, as a chemical composition, in mass%, one or both elements selected from the group consisting of REM: 0.0002 % at 0.0100% and Zr: 0.0002% at 0.1000%.

[0015] (6) In the machine wire according to any of items (1) to (5), the resistance to the feed can be from 600 MPa to 1,400 MPa.

[0016] (7) A flat steel wire according to another aspect of the present invention includes, as a chemical composition, in 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 the remainder including Fe and impurities, in which an oxide including CaO and AI2O3 and satisfied

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5/54 making Expression A and Expression B is defined as a complex oxide, the average value of the composition ratio ε of the complex oxide that is defined as Expression C satisfies 0.00 <ε <3.00, measured in one portion center within a range of 1/7 the length of a minor axis of the flat steel wire from the central axis of the flat steel wire in a cross section parallel to the rolling direction and the direction of the minor axis of the flat steel wire and including the central axis of the flat steel wire, and an average value of an equivalent circle diameter of the complex oxide is 3.0 pm or less, measured in the central portion of the cross section.

(Quantity of oxide-forming elements other than Ca and Al in the oxide in units of mol%) <(1/3) x (the highest content of the Ca content and the content of Al in the oxide in units of mol%): Expression A (O content in oxide in units of mol%)> (S content in oxide in units of mol%): Expression B (Composition ratio ε) = (CaO concentration in complex oxide in units of% by mass ) / (concentration of AI2O3 in the complex oxide in units of mass%): Expression C [0017] (8) In the flat steel wire according to 0 item (7), a structure in the central portion can include 98% in area or more of tempered martensite.

[0018] (9) In the flat steel wire according to item (7), the structure in the central portion can include 20% in area at 60% in area of ferrite and 40% in area at 60% in area of bainite .

[0019] (10) In the flat steel wire according to any of the items (7) to (9), the tensile strength can be from 1,100 MPa to 1,500 MPa.

[0020] (11) The flat steel wire according to any of the items (7) to (10) can include, as chemical composition, in% in

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6/54 mass, Cr: 0.05% to 1.00%.

[0021] (12) Cloth steel wire according to any of items (7) to (11) may include, as a chemical composition, one or more elements 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%.

[0022] (13) Flat steel wire according to any of items (7) to (12) may include, as a chemical composition, in mass%, one or more elements 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%.

[0023] (14) Flat steel wire according to any of items (7) to (13) may include, as a chemical composition, in mass%, one or both elements selected from the group consisting of REM: 0 , 0002% at 0.0100% and Zr: 0.0002% at 0.1000%.

Effects of the Invention

[0024] The wire rod of the present invention can be used to produce a flat steel wire according to the present embodiment which has a tensile strength of 1,100 MPa or more and has excellent resistance to fracture induced by hydrogen in a severe acid environment with a pH of 5.5 or less. The flat steel wire according to the present modality has a tensile strength of 1,100 MPa or more and is excellent in terms of resistance to fracture induced by hydrogen and thus can be used, for example, as flat steel wire for reinforcement of tension of flexible tubes that are used in a severe acid environment.

Brief Description of Drawings

[0025] Figure 1 is a schematic view showing a section C of a wire rod according to the present modality.

[0026] Figure 2 is a schematic view showing a section C of a flat steel wire according to the present modality.

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[0027] Figure 3 is a schematic view showing a section L of the flat steel wire according to the present modality. Modalities of the Invention

[0028] The present inventors have performed a variety of studies to solve the problem described above. In the relative technique, sulfides were purified by the addition of Ca or similar; however, even in such cases, there was a case in which ICH occurred close to the central axes of machine wires and flat steel wires. The present inventors have found that, in a fractured portion close to the central axis, the presence of a complex oxide including AI2O3 and CaO affects ICH. In addition, the inventors found that HIC can be effectively avoided by controlling the composition and size of the complex oxide containing AI2O3 and CaO close to the central axis. That is, knowledge (a) to (d) was obtained.

[0029] (a) The hydrogen-induced fracture in a flat steel wire occurs from a crude sulfide in the steel as a starting point. Particularly, in a case where a sulfide such as MnS is crude, a gap is generated around the sulfide at the time of flattening a hot-rolled wire and acts as a cause to accelerate hydrogen-induced fracture in an acidic environment severe with a pH of 5.5 or less.

[0030] (b) Therefore, it is necessary to refine a sulfide that is contained in a maximum possible 0 wire. To refine the sulfide, it is effective to add a small amount of Ca or Mg and form MnS, CaS or MgS including partially Ca or Mg.

[0031] (c) The addition of Ca generates an oxide that includes CaO and AI2O3 (hereinafter referred to as the complex oxide in some cases) in some cases and, during the planing process, a gap is generated around that oxide complex and acts as the cause for acceleration of hydrogen-induced fracture in an acidic environment if

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8/54 summer with a pH of 5.5 or less.

[0032] (d) Therefore, it is necessary to suppress the formation of a gap around the complex oxide during the planing process. For this, it is effective to control the size and composition ratio of the complex oxide, thereby compressing the complex oxide during the planing process. The compressed complex oxide adheres well to a base metal, so the occurrence of hydrogen-induced fracture in a severe acid environment with a high strength and a pH of 5.5 or less is significantly reduced.

[0033] A wire rod according to the present modality and a flat steel wire according to the present modality that is obtained by laminating the wire rod are completed based on the knowledge described above. Henceforth, initially, the machine wire according to the present modality will be described.

[0034] (A) Regarding the chemical composition:

[0035] Hereinafter the chemical composition of the wire rod in accordance with the present modality will be described in detail. The amount of the chemical composition is in% by mass.

[0036] C: 0.15% to 0.85%

[0037] C is an element that reinforces steel. To achieve this effect, 0.15% or more of C must be contained. When the C content exceeds 0.85%, the resistance increases excessively, and thus fractures are formed in the machine wire during the planing process, and the resistance to fracture induced by hydrogen deteriorates. Therefore, a suitable C content is 0.15% to 0.85%. In addition, the C content is preferably adjusted to 0.20% or more, and furthermore, preferably adjusted to 0.30% or more, 0.35% or more, or 0.40% or more of the view of suppressing fracture formation. From the point of view of suppressing a carbide fracture, the C content is preferably adjusted to 0.75% or less, and furthermore, to improve the

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9/54 resistance to hydrogen-induced fracture, the C content is desirably adjusted to 0.65% or less, 0.60% or less, or 0.50% or less.

[0038] Si: 0.10% to 2.00%

[0039] Si is an element that forms a solid solution in a matrix and improves the resistance of the flat steel wire. To achieve this effect, 0.10% or more of Si must be contained. However, when the Si content exceeds 2.00%, fractures are generated in the wire rod during the planing process. Therefore, the Si content is 0.10% to 2.00%. In a case where it is necessary to increase the resistance as well, 0.30% or more of Si needs to be contained, and it is more preferable than 0.50% or more, 0.55% or more, 0.60% or more, or 0.70% or more of Si is contained. In a case where it is necessary to suppress the fracture of the wire rod during the process to produce the flat steel wire, the Si content is preferably adjusted to less than 2.00% and more preferably adjusted to 1.80% or less, 1.70% or less, or 1.50% or less.

[0040] Mn: 0.30% to 1.50%

[0041] Mn is an element that has the effect of increasing the hardening capacity of steel and is necessary to increase the resistance of the heat-treated flat steel wire. To achieve this effect, 0.30% or more of Mn must be contained. However, when the Mn content exceeds 1.50%, the resistance of the wire rod increases excessively, and there is a problem due to the fact that fractures are generated in the wire rod when processing the wire rod to the flat steel wire. Therefore, the Mn content in the wire rod according to the present modality is 0.30% to 1.50%. To increase the resistance by increasing the hardening capacity 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,

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0.80% or more or 0.90% or more. In a case where it is necessary to suppress the fracture of the wire rod during the process for producing the flat steel wire, the Mn content is preferably adjusted to 1.30% or less and more preferably adjusted to 1.10% or less, 1.05% or less, or 1.00% or less.

[0042] S: 0.020% or less

[0043] S is an impurity. When the S content exceeds 0.020%, MnS has a stretched shape and degrades the fracture resistance induced by hydrogen. In order to improve the resistance to fracture induced by hydrogen, it is necessary to control the upper limit of the S content that is contained in the balance with an element that is capable of binding to the S and generating a sulfide such as Ca. Therefore, the limit higher than the S content is adjusted to 0.020% or less. From the point of view of improving hydrogen-induced fracture resistance, the S content is preferably less than 0.010% and more preferably less than 0.008% or less than 0.005%. From the point of view of steel production costs, the S content can also be adjusted to 0.001% or more, 0.003% or more, or 0.005% or more.

[0044] P: 0.020% or less

[0045] P is an impurity. When the P content exceeds 0.020%, the hydrogen-induced fracture is likely to occur, and in the flat steel wire, it is not possible to suppress the hydrogen-induced fracture in a severe acid environment with a pH of 5.5 or less . Therefore, the P content is adjusted to 0.020% or less. From the point of view of improving hydrogen-induced fracture strength, the P content is preferably 0.015% or less, more preferably less than 0.013% or less than 0.010%, and much more preferably less than 0.008%. From the point of view of steel production costs, the P content can be adjusted to 0.003% or more, or 0.005% or more.

[0046] Al: 0.001% to 0.080%

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[0047] Al is an element that has a deoxidizing action and is necessary to decrease the amount of oxygen in the machine wire. To achieve this effect, 0.001% or more of Al must be contained. The Al content is preferably 0.002% or more and 0.005% or more, more preferably 0.015% or more, and even more preferably 0.020% or more or 0.025% or more. On the other hand, when the Al content exceeds 0.080%, a crude oxide is generated, and the fracture resistance induced by hydrogen deteriorates. Therefore, the Al content is adjusted to 0.080% or less. To suppress the generation of a crude 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.

[0048] N: 0.0020% to 0.0080%

[0049] N has the effect of forming a solid solution in the ferrite and improving the resistance of the flat steel wire. In addition, N has the effect of generating a nitride or a carbonitride by bonding with Al, Ti and the like and refining the austenite grain during hot rolling and has the effect of improving the hydrogen-induced fracture resistance of the steel wire. plan. To achieve 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 carbonitride becomes crude, the ductility degrades, and internal fractures are generated during the flat rolling process, so it is necessary to adjust the N content to 0.0080 % or less. The N content is preferably 0.0060% or less and more preferably 0.0055% or less, 0.0050% or less, 0.0045% or less, or 0.0040% or less.

[0050] O: 0.0050% or less

[0051] O is an impurity. O forms a crude oxide and degrades the hydrogen-induced fracture resistance of steel. Therefore, the content

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12/54 O is preferably small. The O content is preferably 0.0050% or less. The O content is preferably less than 0.0050%, more preferably less than 0.0040%, and even more preferably less than 0.0035%. From the point of view of steel production costs, the O content can be adjusted to 0.0007% or more or 0.0010% or more.

[0052] Ca: 0.0002% to 0.0050%

[0053] Ca has the effect of dispersing MnS finely when included in the MnS. When MnS is finely dispersed, the hydrogen-induced fracture attributed to MnS can be suppressed. To obtain the Ca effect to suppress hydrogen-induced fracture, 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 must be contained. However, when the Ca content exceeds 0.0050%, the effect is saturated, an oxide that is generated by the reaction of the oxygen in the steel with Ca together with Al becomes crude, and it becomes difficult to compress the oxide during the planing process, which, conversely, leads to the degradation of resistance to fracture induced by oxygen. Therefore, an adequate Ca content if it is contained is 0.0050% or less. From the point of view of improving the resistance to fracture induced by hydrogen, 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 .

[0054] (B) Regarding arbitrary components:

[0055] The wire rod according to the present modality can contain at least one or more elements selected from Cr, Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr as needed instead of part of Fe which is the rest described below. The wire rod according to the present modality is capable of solving the problem without

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13/54 have these arbitrary components, and so the lower limit value of the arbitrary element is 0%. From now on, the actions and effects of Cr, Ti, Nb, V, Cu, Ni, Mo, B, REM, and Zr will be described, which are the arbitrary elements and the reasons for limiting their quantities. Regarding arbitrary components,% indicates% by mass. [0056] Cr: 0% to 1.00%

[0057] Similar to Mn, to increase the resistance of flat steel wire by increasing the steel's hardening capacity, 0.05% or more of Cr can be contained in the machine wire. When the Cr content exceeds 1.00%, the resistance of the wire rod increases excessively, and there is a problem that, during the process to produce the flat steel wire, fractures are generated in the wire rod. Therefore, an adequate Cr content according to the present modality is 1.00% or less. In the case of increasing the hardening capacity 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 fracture of the wire rod during the cold process to produce the flat steel wire, the Cr content is preferably adjusted to 0.80% or less and more preferably 0.60% or less.

[0058] Ti: 0% to 0.050%

[0059] Ti has the effect of forming a carbide, a nitride or a carbonitride to bond to the N or C and refine the austenite grain during hot rolling by its fixing effect and has the effect of improving fracture resistance hydrogen-induced flat steel wire and thus may be contained. To achieve this effect, 0.002% or more of Ti is preferably contained. In order to improve the resistance to fracture induced by hydrogen, the Ti content is preferably adjusted to 0.005% or more and more preferably adjusted to 0.010% or more. When the Ti content exceeds 0.050%, the

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14/54 effect is saturated, in addition a number of crude TiN is generated and acts as a cause of fracture formation during the planing process, and there is a possibility that the resistance to fracture induced by hydrogen may be deteriorated. Therefore, the Ti content is adjusted to 0.050% or less and is more preferably 0.035% or less.

[0060] Nb: 0% to 0.050%

[0061] Nb has the effect of forming a carbide, nitride or carbonitride by bonding with N or C and refining the austenite grain during hot rolling by its fixing effect and has the effect of improving the resistance to induced fracture hydrogen from the flat steel wire and thus may be contained. To achieve this effect, 0.002% or more of Nb is preferably contained. In order to improve the resistance to fracture induced by hydrogen, the Nb content is preferably adjusted to 0.005% or more and more preferably adjusted to 0.010% or more. When the Nb content exceeds 0.050%, the effect is saturated, and in addition, there is an adverse influence on the production capacity of the steel such as the generation of fractures in a piece of steel in a step of transforming an ingot or a plate steel in the steel part. Therefore, the Nb content is adjusted to 0.050% or less, and is preferably 0.035% or less, and more preferably 0.030% or less.

[0062] V: 0% to 0.15%

[0063] V is able to increase the resistance of the flat steel wire by bonding with C and N to form a carbide, nitride or carbonitride. To achieve this effect, 0.02% or more of V is preferably contained. When the V content exceeds 0.15% there is a case where the resistance of the flat steel wire increases due to a carbide or a carbonitride to be precipitated and fractures are formed during the planing process. Therefore, the V content is adjusted to 0.15% or less. From the point of view of suppressing the fracture

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During the planing process, the V content is more preferably 0.10% or less and even more preferably 0.08% or less. In order to stably obtain the effect described above from V, the lower limit of the V content is more preferably 0.03% or more.

[0064] Cu: 0% to 1.00%

[0065] Cu is an element that increases the hardening capacity of steel and can be contained. To obtain the effect to increase the curing capacity, 0.01% or more of Cu is preferably contained. However, when the Cu content exceeds 1.00%, the resistance of the wire rod increases excessively, and there is a problem that fractures are generated in the wire rod when it is processed in the flat steel wire. Therefore, the Cu content if it is contained is 1.00% or less. From the point of view of improving the curing capacity, the Cu content if it is 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 adjusted to 0.80% or less and is more preferably 0.50% or less in consideration of a property of being processed into flat steel wire.

[0066] Ni: 0% to 1.50%

[0067] Ni is an element that increases the hardening capacity of steel and can be contained. To obtain the effect to increase the curing capacity, 0.01% or more of Ni is preferably contained. However, when the Ni content exceeds 1.50%, the resistance of the wire rod increases excessively, and there is a problem due to the fact that fractures are generated in the wire rod when it is processed in the flat steel wire. Therefore, the Ni content if it is contained is 1.50% or less. From the point of view of improving the curing capacity, the Ni content if it is contained is preferably 0.10% or more and more preferably 0.30% or more.

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The Ni content, if contained, is preferably adjusted to 1.00% or less and is more preferably 0.60% or less in consideration of a property of being processed into flat steel wire.

[0068] Mo: 0% to 1.00%

[0069] Mo is an element that increases the hardening capacity of steel and can be contained. To obtain the effect of increasing the curing capacity, 0.01% or more Mo is preferably contained. However, when the Mo content exceeds 1.00%, the resistance of the wire rod increases excessively, and there is a problem that fractures are generated in the wire rod when it is processed in the flat steel wire. Therefore, the Mo content if it is contained is 1.00% or less. From the point of view of improving the curing capacity, the Mo content if it is contained is preferably 0.02% or more and more preferably 0.05% or more. The Mo content, if contained, is preferably adjusted to 0.50% or less and is more preferably 0.30% or less in consideration of a property of being processed into flat steel wire.

[0070] B: 0% to 0.0100%

[0071] B is effective in increasing the hardening capacity 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, in addition, a crude nitride is generated and thus the fracture induced by hydrogen is likely to occur. Therefore, the B content if it is contained is 0.0100% or less. In addition, in a case where it is necessary to increase the curing capacity, the B content needs to be adjusted to 0.0010% or more and is more preferably 0.0020% or more. To improve resistance

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17/54 due to hydrogen-induced fracture, the B content is preferably adjusted to 0.0050% or less and is more preferably 0.0030% or less.

[0072] REM: 0% to 0.0100%

[0073] REM is a collective term for rare earth metals and, similar to Ca, has the effect of finely dispersing MnS when included in MnS. When MnS is finely dispersed, the resistance to fracture induced by hydrogen can be improved, and thus REM can be contained. To achieve the effect of suppressing the hydrogen-induced fracture, 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 the steel becomes crude, which causes a fracture during the planing process. Therefore, an adequate content of REM if contained is 0.0100% or less. From the point of view of a property of being processed into flat steel wire, the REM content is preferably 0.0050% or less and more preferably 0.0030% or less.

[0074] Rare earth metals (REM) is the collective term of two elements of scandium (Sc) and yttrium (Y) and 15 (lantanoids) elements of lanthanum (La) through lutetium (Lu). In the case of adding these elements, the elements can be added alone or they can be added as a mixture. The REM content refers to the total value of the quantities of these 17 elements.

[0075] Zr: 0% to 0.1000%

[0076] Zr has the effect of generating an oxide by reacting with O and suppressing the hydrogen-induced fracture 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. To get

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18/54 the effect of suppressing the hydrogen-induced fracture, 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 of 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 the steel and generates a crude nitride or sulfide, which, conversely, leads to degradation of resistance to hydrogen-induced fracture. Therefore, the content of Zr if it is contained is 0.1000% or less. From the point of view of decreasing an oxide that has an adverse influence on hydrogen-induced fracture resistance, the Zr content is preferably 0.0800% or less and more preferably 0.0500% or less.

[0077] The remainder in the chemical composition of wire rod includes Fe and an impurity. Impurity refers to an element that is mixed in the material of iron and steel from an ore or scrap as a raw material, in the production environment, etc., at the time of industrially producing the material of iron and steel and which has no substantially influence on the properties of the wire rod according to the present modality.

[0078] (C) Regarding the properties of the oxide:

[0079] The machine wire having the components described above contains an oxide including CaO and AI2O3 in a predetermined amount or more. The present inventors have discovered that a gap that is generated around this oxide during the flattening process of the wire rod accelerates the hydrogen-induced fracture. In addition, the present inventors have found that the oxide can be finely compressed during the planing process by controlling the ratio of composition and the size of the oxide to suitable bands and the oxide migrates following the base metal during compression, which makes it stick. between the base metal and the favorable oxide and improves the resistance to fracture induced by hydrogen after the planing process

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19/54 to. To achieve this effect, it is necessary to strictly control the composition ratio or the size of the oxide.

[0080] Initially, the oxide that is the object of control in the wire rod according to the present modality will be described. The oxide that has an adverse influence on the hydrogen-induced fracture resistance of the machine wire and the flat steel wire is an oxide that includes CaO and AI2O3 and meets Expression A and Expression B. Hereinafter, the oxide including CaO and AI2O3 and which satisfies Expression A and Expression B will be abbreviated as complex oxide in some cases.

(Number of oxide-forming elements other than Ca and Al in the oxide in units of mol%) <(1/3) x (highest content between the Ca content and the content of Al in the oxide in units of mol%): Expression A (O content in oxide in units of mol%)> (S content in oxide in units of mol%): Expression B

[0081] 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 modality, Si, Mg and Mn.

[0082] The complex oxide causes the fracture induced by hydrogen and is considered as an object of improvement for the wire rod according to the present modality. Therefore, in the machine wire according to the present embodiment, the composition and size of the complex oxide are limited.

[0083] As far as the present inventors have confirmed, a variety of inclusions other than complex oxide have substantially no influence on hydrogen-induced fracture. Therefore, in the machine wire according to the present embodiment, a variety of inclusions other than complex oxide is not particularly limited.

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[0084] For example, in the wire rod according to the present modality, the amount of oxides other than CaO and AI2O3 is small due to the chemical composition of the wire rod. Therefore, oxides other than CaO and AI2O3 have no influence on hydrogen-induced fracture. In addition, even among oxides that include CaO and AI2O3, 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%) of Ca or the amount (mol%) of Al is not present at the starting point of the fracture in an assessment test for wire rod and flat steel wire and is considered to have no influence on the hydrogen-induced fracture. For the same reason, an inclusion in which the amount (mol%) of O is less than the amount (mol%) of S, that is, an inclusion that does not satisfy Expression B, is also considered to have no influence on the induced fracture by hydrogen.

[0085] In consideration of the subject described above, in the wire rod according to the present modality, the complex oxide that is considered as the object of the control is limited to a compound that includes CaO and AI2O3 and satisfies Expressions A and B below.

[0086] In addition, the complex oxide that is considered as the control object in the wire rod according to the present modality can be limited to an oxide that is substantially made of CaO and AI2O3.

[0087] The complex oxide is evaluated in the central portion of a wire section C, that is, the central portion of a cross section perpendicular to the rolling direction of the wire. As shown in Figure 1, the central portion 11 of a section C of a rod 1 refers to a 1/10 range of a diameter d of the wire rod from the center of the wire rod in a case where section C of the wire rod machine wire 1 is substantially circular. That is, the central portion 11 of the

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21/54 section C of the substantially circular wire 1 is a region in a concentric circle of the wire 1 section that has a diameter of 1 / 5d (2/1 Od). In a case where section C of machine wire 1 is not substantially circular, a region that has a homothetic shape with a homothetic ratio of 1/5 of section C of machine wire 1 and has a geometric center at the same location as section C of machine wire 1 is considered to be the central portion 11 of section C of machine wire 1. The complex oxide can be bonded to the central portion of the plate, and thus, even in a machine wire that is obtained by laminating the plate, the complex oxide is capable of binding 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 precipitation of a crude complex oxide is also considered to be suppressed in the peripheral portion as long as it is suppressed in the central portion. For the reason described above, complex oxide is evaluated in the central portion of section C of the machine wire.

[0088] The complex oxide that is observed in the cross section perpendicular to the rolling direction of the wire rod becomes fragile in a case where the composition ratio ε between CaO and AI2O3, which is computed using Expression C, satisfies 0, 00 <ε <3.00, and so it becomes possible to compress the complex oxide during the planing process.

(Composition ratio ε) = (CaO concentration in the oxide in units of mass%) / (AI2O3 concentration in the oxide in units of mass%): Expression C

[0089] Specifically, in a case where ε is close to 0.00, that is, a case where AI2O3 is a dominant component of the complex oxide or ε is less than 3.00.0 complex oxide is compressed during the process planing. Therefore, it is necessary to control the average value of the composition ε ratios of the complex oxide in the

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22/54 central portion of the wire rod to be in the range described above to improve the hydrogen-induced fracture resistance of the flattened wire rod (ie flat steel wire). On the other hand, in a case where the average value of the composition ratio ε of the complex oxide in the central portion of the wire rod is 3.00 or more, CaO is the dominant component of the complex oxide, and the resistance to fracture induced by hydrogen after the planing process deteriorates regardless of size control. For the reason described above, the average value of the composition ε ratios of the complex oxide in the central portion of the wire is set to be 0 or more and 3.00 or less. In order to obtain a more stable hydrogen-induced fracture resistance, the upper limit of the average value of the composition 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 mean value of the composition ε ratios of the complex oxide in the central portion of the wire rod can be adjusted to 0.02, 0.05, 0.10, 0.15, or 0.20.

[0090] In addition, even when the average value of the composition ratios ε of the complex oxide in the central portion of the wire rod satisfies the conditions described above, in a case where there is a complex oxide that has an equivalent circle diameter of more than 6.0 pm (crude complex oxide) in the central portion of the wire rod, it is not possible to sufficiently improve the hydrogen-induced fracture resistance of the flat steel wire. In a case where crude complex oxide is included in the machine wire, even after the machine wire is flattened, the complex oxide having an equivalent circle diameter of more than 3 pm remains in the flat steel wire, and the base metal and Complex oxides are separated from each other at the interface, and thus the hydrogen-induced fracture resistance of the flat steel wire deteriorates. In a case where the wire rod is flattened to a

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23/54 work of 40% or more, a complex oxide having a size of 6.0 pm or less is compressed to be approximately 3 pm or less. Therefore, the average value of the equivalent circle diameters of the complex oxide is adjusted to 6.0 pm or less. The lower limit value of the average value of the equivalent circle diameters of the complex oxide is not particularly specified, but can be set to be 2.0 pm, 2.5 pm, 3.0 pm, 3.5 pm, or 4.0 pm.

[0091] (D) Regarding the evaluation method

[0092] The following will describe methods for evaluating the complex oxide in the wire rod according to the present modality. There is a case in which the complex oxide in the machine wire is present in the form of a group, and even in this case, individual complex oxides that form the group are treated as independent oxides. There are no cases in which the entire group is considered as a complex oxide.

[0093] (D-1) Chemical composition of complex oxide

[0094] The chemical composition of the complex oxide is considered to be substantially uniform on a wire rod regardless of size. Therefore, it is possible to observe 10 views in section C of the central portion of the wire rod, analyze the chemical compositions and compute the composition ratios ε only for complex oxides having the maximum equivalent circle diameter (complex oxides having a chemical composition that is more easily analyzed) in the respective views, and consider a value obtained by taking the average of the computed ε composition ratios of the complex oxides in those 10 views as the average value of the complex oxide ε composition ratios that are measured in the central portion of the wire. As long as this value meets the requirements for the wire rod described above in accordance with the present embodiment, the wire rod is considered to satisfy the requirements of the wire rod.

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24/54 according to the present modality. A specific method for analyzing the chemical composition of the complex oxide in the wire rod will be described below.

[0095] Section C (that is, the cut surface perpendicular to the rolling direction of the machine wire) of the machine wire is mirror-polished, 10 locations are observed using a field-scanning electron microscope (FE-SEM) to a magnification of 1,000 times in a reflected electronic image so that inclusions such as complex oxide and the like can be observed, and photographs are taken. The area per view is adjusted to 8,000 pm ² (100 pm high and 80 pm wide) or more. At that time, the chemical compositions of the respective inclusions are measured collectively using EDS, thus determining whether or not the inclusions are the complex oxide that is considered as the control object in the wire rod according to the present modality. Next, the X-ray spectrum characteristics of complex oxides that have a maximum size in the respective photographs are obtained using scattered energy X-ray spectroscopy (EDS), thus analyzing the elements. Therefore, the compositions of complex oxides can be evaluated. From the peak energies of the characteristic X-ray spectra obtained, elements that are included in the complex oxides are specified, and the quantities (mol%) of the elements are determined from the peak heights. In addition, with the assumption that Ca in the complex oxide is all present as CaO and Al is all present as AI2O3, the CaO / AkOs mass ratios of the complex oxides are computed, thus obtaining the composition ratios ε of the complex oxides having the maximum height in the respective views. In addition, these composition ratios ε are averaged in the 10 views, thus computing the average value of the composition ε ratios of the complex oxides that are

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25/54 measured in the central portion of the machine wire.

[0096] 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 the highest amount (mol%) of the amount (mol%) of Ca and the amount (mol%) of Al (that is, an oxide that does not satisfy Expression A) is determined as, for example, an oxide that includes CaO, AI2O3, and S1O2 and is not the control object in the machine wire. according to the present modality. In addition, an inclusion in which the amount (mol%) of O is less than the amount (mol%) of S (ie, which does not satisfy Expression B) is determined as a sulfide-based inclusion and is not the object control in the machine wire according to the present modality. Such inclusion is ignored when confirming the state of the complex oxide.

[0097] (D-2) Size of complex oxide

[0098] It is not realistic to measure the equivalent circle diameters of all 0 complex oxides in the central portion of the wire rod. A value that is obtained using a method described below can be considered as the average value of the equivalent circle diameters of the complex oxides that are measured in the central portion of the wire section C.

[0099] Section C (that is, a cut surface perpendicular to the rolling direction) of the machine wire is mirror polished, 10 locations are observed using a field-scanning electron microscope (FE-SEM) at magnification 1,000 times in a reflected electronic image so that inclusions can be observed, and photographs are taken. The area per view is adjusted to 8,000 pm ² (100 pm high and 80 pm wide) or more. At that time, the chemical compositions of the respective inclusions are measured collectively using EDS, thus determining whether or not the inclusions are the complex oxide that is considered

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26/54 as the control object in the machine wire according to the present modality. Next, in relation to the respective photographs obtained, the areas of the complex oxides having the maximum size are measured by a common image analysis from the respective photographs, and the equivalent circle diameters that are obtained from the areas are computed. As photographs for measuring the equivalent circle diameter, reflected electronic images are preferably used. The average value of the equivalent circle diameters, which were obtained using the method described above, is obtained from the maximum complex oxides in the photographs of the 10 locations, with which the average value of the equivalent circle diameters of the complex oxides that are obtained is obtained. measured in the central portion of section C of the machine wire. [00100] As long as the requirements mentioned above are satisfied, other wire rod configurations in accordance with the present modality are not particularly limited. For example, the metallographic structure of the wire rod has no substantial influence on the hydrogen-induced fracture resistance of the flat steel wire. This is because the states of a sulfide and the complex oxide are dominant over the hydrogen-induced fracture 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 into account the working capacity, the wire rod metallographic structure is preferably controlled to be a pearlite structure, a ferrite structure or a bainite structure. Therefore, the metallographic structure of the wire rod can be regulated as a metallographic structure that includes a total of 99% in area or more than a pearlite structure, a ferrite structure and a bainite structure.

[00101] 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 on the market today are generally adjusted.

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27/54 to 7 to 16 mm, and thus the diameter of the wire rod according to the present modality can also be adjusted to be from 7 to 16 mm.

[00102] 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 value of the lower limit of the tensile strength of the wire rod according to the present modality can be adjusted to be 600 MPa or 700 MPa. In addition, the value of the upper limit of the tensile strength of the wire rod according to the present modality can be adjusted to be 1,400 MPa or 1,350 MPa.

[00103] It is not necessary to limit the shape, etc. inclusion of sulfide. 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 adequately controlled, the sulfide-based inclusion is essentially finely dispersed and purified. In addition, according to an experience of the present inventors, in a case where a sulfide was purified by Ca or similar, fractures starting from the sulfide were not generated in a flat steel wire evaluation test. Also in consideration of this fact, do not limit the shape etc. sulfide-based inclusions is considered to be adequate.

[00104] (E) Regarding the production method

[00105] In a wire rod production method according to the present modality, to suppress hydrogen-induced fracture, the oxide composition ratio ε including CaO and AI2O3 is made suitable by the addition of Ca in a steel step melted and, in addition, the size of the complex oxide is controlled.

[00106] Since the requirements of the wire rod according to

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28/54 present mode are satisfied, the effect of the machine wire according to the present mode can be obtained regardless of the method for producing the wire rod, but the wire rod must be produced according to, for example, a production method described below. The following production process is simply an example, and it goes without saying that a wire rod that has the chemical composition and other requirements in the wire rod ranges according to the present modality is obtained using a process different from the The following production is also considered as the wire rod according to the present modality.

[00107] Specifically the component of the molten steel is adjusted in a converter after desulphurization of the hot metal, a Ca alloy is added to the molten steel, and then a piece of steel is obtained using continuous casting. After that, the steel part is reheated to hot-roll a product and is finished in a steel material having a predetermined diameter. Hereafter an example of a method for producing molten steel will be described in greater detail.

[00108] Desulfurization is performed using a Kanbara reactor (KR) method in which a desulfurizer is added to the hot metal taken from a blast furnace and agitated, thus removing the sulfur, subsequently, dephosphorization and decarbonation are performed in a converter. In addition, when removing molten steel from the converter to a molten steel pan, in relation to elements except Ca, REM and Zr from and a desired composition, the molten steel component is adjusted by the addition of an Al alloy metallic or similar. Subsequently, the molten steel is degassed at Ruhrstahl-Heraeus (RH), and a Ca alloy is added to the molten steel. The composition of the Ca alloy is, for example, 40% by weight of Ca and 60% by weight of Si. In addition, in relation to the method for

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29/54 adding the Ca alloy, a powder injection method is used in which the Ca alloy powder is blown into the steel together with an inert gas.

[00109] Here, the Ca alloy addition time is adjusted 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 of adding metallic Al, part of the Ca added to the metal is consumed due to the reaction with AI2O3 floating in the steel, and thus a sulfide purification effect by Ca cannot be obtained. In addition, in a case where the Ca alloy is added within 30 minutes of adding metallic Al, crude AI2O3 remains, and so the equivalent circle diameter of the oxide in the wire after lamination does not reach 6.0 pm 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, ο AI2O3 present in the steel becomes small, and it becomes difficult to control ε to be less than 3.00. In the case of adding one or both between REM and Zr, a alloy including one or both between 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 0 Ca in relation to 0 Al.

[00110] In the molten steel produced by the above steps, the composition ratio ε (% by mass of CaO /% by mass of AI2O3) of the complex oxide satisfies 0.00 <ε <3.00. This molten steel is transformed into a piece of steel using a continuous casting method. The casting rate at the time of transformation of the molten steel into the steel part is preferably 0.6 m / min to 1.4 m / min. During the casting, some of the inclusions float and do not remain in the steel part, but the rest of the inclusions move downward and remain in the steel part. In the case of ingot the molten steel at less than 0.6 m / min, the inclusions that floated move down again, and so there is a case in which the amount of gross inclusions au

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30/54 mint on the plate. On the other hand, in the case of casting the molten steel at more than 1.4 m / min, the number of inclusions that move downwards increases, and so there is the case where the quantity of gross inclusions increases in the steel part.

[00111] The steel part obtained is hot rolled, thus producing a wire rod. Hot rolling is performed by heating the steel part to 1,020Ό or more. The fine finishing temperature I of the hot rolling mill is set to 800Ό to 960Ό. In addition, hot rolling is carried out in such a way that the area ratio of the cross section of the steel part before hot rolling to the hot rolled wire (the cross section area (mm ² ) of the steel part / the cross-sectional area (mm ² ) of the hot rolled wire rod reaches 100.0 or more. When the lamination temperature in the final finishing lamination is less than 800Ό or more than 960Ό, or the cross-sectional area ratio is less than 100.0, the compression of the complex oxide during hot rolling becomes insufficient, and the size of the complex oxide in the wire does not reach 6.0 pm or less. The size and ratio of the composition of the complex oxide can be controlled using the steps described above.

[00112] The flat steel wire according to the present modality will be described below. The flat steel wire according to the present modality is a flat steel wire obtained by rolling the wire rod according to the present modality. The shape of a flat steel wire 2 is not particularly limited, but the shape of its section C is preferably a shape formed by pressing a circle.

[00113] In flat steel wire 2 according to the present embodiment, the length of the minor axis of section C will be referred to as a thickness t of the flat steel wire 2, and the major axis of section C

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31/54 will be referred to as the width w of the flat steel wire 2.

[00114] In addition, a section L of the flat steel wire 2 described below refers to a cross section that is parallel to the rolling direction and the direction of the minor axis of the flat steel wire and substantially includes the central axis of the wire flat steel. The central axis of the flat steel wire refers to an axis that passes through the center of section C and is parallel to the rolling direction. The direction of the minor axis of the flat steel wire refers to the direction of the minor axis of a cross section perpendicular to the rolling direction of the flat steel wire.

[00115] The central portion 21 in section L of flat steel wire 2 refers to the region that is within a range of 1/7 of the length of the minor axis (the thickness t of flat steel wire 2) of the steel wire plane 2 from the central axis of flat steel wire 2 as shown in Figure 3. In other words, the central portion 21 in section L of flat steel wire 2 is a region at a depth of 5 / 14t or more from of the flat steel wire surface in section L.

[00116] Hereinafter, the central portion 21 in section L of the flat steel wire 2 will be referred to simply as the central portion in some cases. In a case where section C of flat steel wire 2 is substantially circular and the minor axis and major axis of its section C cannot be specified, an arbitrary axis that passes through the center of section C of the flat steel wire and an axis perpendicular to the axis described above can be considered as the major axis and the minor axis.

[00117] The chemical composition of flat steel wire includes, in weight%, 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

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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 the rest including Fe and impurities. Flat steel wire is flat steel wire obtained by laminating 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 goes without saying that the preferred upper limit value and the preferred lower limit value that have been described in relation to the respective elements in the chemical composition of the wire rod are also applicable to the chemical composition of the flat steel wire.

[00118] also in the flat steel wire according to the present modality, the shape of an oxide including CaO and AI2O3 (complex oxide) is regulated. The definitions of the complex oxide in the flat steel wire and its composition ratio ε are identical to the definitions of the complex oxide in the wire rod and its composition ratio ε. The average value of the complex oxide composition ratios that is observed in the central portion of the flat steel wire is made to satisfy 0.00 <ε <3.00. Flat steel wire is flat steel wire obtained by laminating the machine wire, 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 machine wire. according to the present modality.

[00119] The average value of the equivalent circle diameters of the complex oxide that is observed in the central portion of the flat steel wire is adjusted to 3.0 pm or less. In a case where the average value of the equivalent circle diameters of the complex oxide that is observed in the central portion of the flat steel wire exceeds 3.0 pm, the hydrogen-induced fracture properties of the flat steel wire are impaired by a span that is generated around the complex oxide.

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[00120] As long as the requirements described above are satisfied, other configurations of the flat steel wire according to the present modality are not particularly limited.

[00121] For example, the metallographic structure of the flat steel wire, similar to the wire structure, has no significant influence on the resistance to fracture induced by the hydrogen 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 central portion of the flat steel wire includes 98% area or more of tempered martensite, it is possible to also improve the tensile strength of the flat steel wire, which is preferable. For example, in a case where the structure in the central portion of the flat steel wire includes 20% to 60% in the area of ferrite and 40% to 60% in the area of bainite, it is possible to improve the toughness and similars of the steel wire plan, which is preferable.

[00122] The width w and thickness t of the flat steel wire are also not particularly limited. Generally, the widths of the flat steel wires that are in circulation on the market today are adjusted to 13 to 16 mm, and the thicknesses t are adjusted to 2 to 7 mm, and thus the width and thickness of the flat steel wire accordingly. with the present modality can also be regulated as described above.

[00123] The tensile strength of flat steel wire is also not particularly limited. When the use of flat steel wire is taken into account, the tensile strength of flat steel wire is desirably adjusted to approximately 1,100 to 1,500 MPa, which can be achieved by adequately adjusting the heat treatment condition of the flat wire. flat steel.

[00124] A sulphide-based inclusion in the flat steel wire also need not be limited in terms of shape, etc. for the same

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34/54 reason for the inclusion of sulphide in the wire rod.

[00125] Methods for evaluating complex oxide in flat steel wire are based on methods for evaluating complex oxide in wire rod. However, the methods for assessing complex oxide are different between flat steel wire and wire rod in terms of the fact that the complex oxide in the wire rod is evaluated in the central portion of section C of the wire rod, but the complex oxide in the wire flat steel wire is evaluated in the central portion of section L of the flat steel wire. The section L of the flat steel wire in the present embodiment is a cross section that includes the central axis of the flat steel wire; however, when assessing the complex oxide, the complex oxide can also be evaluated using a cross section slightly away from the central axis of the flat steel wire as the measurement surface. In this case, the central portion on the measuring surface can be specified in relation to an axis parallel to the rolling direction of the measuring surface as the central axis of the flat steel wire. Even when there is a small gap between the measuring surface and the central axis of the flat steel wire, the results of the evaluation of the complex oxide are not substantially affected.

[00126] A method for producing flat steel wire in accordance with the present embodiment includes a step of flattening the wire rod in accordance with the present embodiment. The area reduction in the planing process is adjusted to 40% or more. In a case where the reduction in area is less than 40%, the complex oxide in the wire rod is not compressed sufficiently, and thus it is difficult to adjust the maximum value of the equivalent circle diameter of the complex oxide in the flat steel wire to 3, 0 pm or less. To adjust the tensile strength of the flat steel wire, the wire rod before the planing process or the flat steel wire after the planing process

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35/54 can be heat treated appropriately. This is because the forms of the complex oxide and sulfides do not change significantly at the heat treatment temperature for common steel.

Examples

[00127] Hereinafter the present invention will be described specifically using examples.

[00128] Specifically, steels having the chemical compositions of Table 1, Table 2-1 and Table 2-2 were melted, and machine wires and flat steel wires were produced using the following methods. The symbol in these tables indicates that the amount of the corresponding element is at an impurity level and the element can be determined to be not contained.

[00129] A and B steels having the chemical compositions shown in Table 1 were produced using the following method. Desulfurization was performed using the KR method on the hot metal, and dephosphorization and decarburization were performed in a converter. After that, to adjust the elements except Ca, REM and Zr in the chemical composition described above, metallic Al or similar was added to the molten steel. A sample of the molten steel was taken for analysis, the component analysis was performed, and the chemical composition except Ca, REM and Zr was adjusted. After that, the molten steel was degassed at Ruhrstahl-Heraeus (RH), and a CaSi alloy was added to the molten steel. The composition of the CaSi alloy was 40% by weight of Ca and 60% by weight of Si. The CaSi alloy was added using a powder injection method in which the CaSi alloy powder was blown into the steel together with an inert gas. In tests ^Nos A1, A4, A5 and B1, CaSi alloy was added 40 minutes after the addition of metallic Al. In tests ^Nos A2 and B2, CaSi alloy was added 25 minutes after the addition of metallic Al. In tests ^Nos A3 and B3, the addition time of the CaSi alloy was 70 minutes after the addition of metallic Al.

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[00130] The molten steel obtained as described above was cast, thus producing a steel ingot. The casting rate was adjusted to 0.9 m / min. After that, this steel ingot was reheated to 1250Ό for 12 hours and transformed into a 122 mm x 122 mm steel piece, thus producing a rolling material. Then, in relation to A1 to A3, A5, and B1 to B3, the laminating material was heated to 1,050Ό and laminated on a wire rod having a diameter of 12 mm. In relation to A4, the laminating material was heated to 1250Ό, hot rolled to a diameter of 16 mm, cut to a length of 1,500 mm, and worn to a diameter of 12 mm. After lamination or wear, the surface of the wire rod was lubricated, and then a first wire transformation process was performed in order to obtain a wire rod with a diameter of 11 mm. After that, for all steel materials, the rebar turned into wire was laminated by a cold rolling mill (planing process) and formed into a flat steel wire having a width of 15 mm and a thickness of 3 mm. For tests n A1 to A4 and Tests ^Nos B1 to B3 after produced, the flat steel wire was heated at 900Ό for 15 minutes, then rapidly cooled by being immersed in a cold oil, and tempered at a temperature of 450Ό for 60 minutes. For test ² A5, the wire was laminated (flattening process) and then annealed at 450Ό for 60 minutes.

[00131] In addition, steels aa au having the chemical compositions shown in Table 2-1 and Table 2-2 (Tests ^Nos 1 to 47 in Table 4-1 and Table 4-2) was melted using the same method used for A1 steel, the steel ingots obtained were heated to 1250Ό for 12 hours, and then parts obtained by transformation, steel parts of 122 mm x 122 mm, were used as rolling materials. Then, the materials for lamination were heated to 1050Ό and there

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37/54 mined (flattening process) in machine wires having a diameter of 12 mm. After that, the surfaces of the wire rods were lubricated, and a primary wire transformation process was carried out in order to obtain wire rods having a diameter of 11 mm. After that, the machine wires transformed into wire were laminated in a cold rolling laminate and formed into flat steel wires having a width of 15 mm and a thickness of 3 mm. After being cold rolled, the flat steel wires were heated to 900Ό for 15 minutes, and then cooled quickly when immersed in a cold oil, and heated to a temperature of 450Ό for 60 minutes. For samples in which the flat steel wire broke at the time of cold rolling of the flat steel wire, the steps following the heat treatment were not carried out, and the testing and evaluation were interrupted. The symbol is given to the cells in the evaluation result column for the samples described above. Numerical values underlined in Table 2-1 and Table 2-2 indicate that the composition of the component is not within the scope of the present invention.

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[Table 11

Steel Chemical composition (% by mass): Fe balance and impurities Ç Si Mn P s Al N Here THE Cr V You Nb Ass Ni Mo B REM Zr THE 0.44 1.70 0.73 0.009 0.002 0.035 0.0032 0.0018 0.0033 - - - - - - - - - - B 0.42 1.68 0.77 0.009 0.002 0.030 0.0030 0.0022 0.0032 - - - - - - - - - -

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[Table 2-11

Steel Chemical composition (% by mass): Balance of Fe and im purities Note Ç Si Mn P s Al N Here THE Cr V You Nb Ass Ni Mo B REM Zr The 0.33 1.70 0.73 0.009 0.002 0.035 0.0032 0.0018 0.0031 - - - - - - - - - - Example of the Invention B 0.32 1.68 0.32 0.009 0.002 0.030 0.0035 0.0022 0.0032 - - - - - - - - - - ç 0.33 1.66 1.38 0.008 0.003 0.041 0.0036 0.0018 0.0033 - - - - - - - - - - d 0.52 0.73 0.71 0.008 0.003 0.041 0.0036 0.0018 0.0035 - - - - - - - - - - and 0.77 0.52 0.55 0.010 0.002 0.033 0.0037 0.0021 0.0033 - - - - - - - - - - f 0.41 1.41 0.77 0.010 0.003 0.037 0.0051 0.0020 0.0033 - - - - - - - - - - 9 0.42 1.51 0.71 0.008 0.003 0.039 0.0030 0.0020 0.0034 0.22 - - - - - - - - - H 0.63 0.66 0.35 0.008 0.003 0.039 0.0033 0.0021 0.0032 - 0.08 - - - - - - - - i 0.42 0.73 0.39 0.008 0.003 0.041 0.0041 0.0019 0.0033 - - 0.032 - - - - - - - i 0.49 0.77 0.74 0.010 0.002 0.030 0.0044 0.0013 0.0038 - - - 0.025 - - - - - - k 0.56 0.73 0.64 0.010 0.003 0.031 0.0038 0.0031 0.0036 - - - - 0.20 0.21 - - - - 1 0.71 0.63 0.65 0.008 0.002 0.035 0.0031 0.0021 0.0036 - - - - - - 0.05 - - - m 0.81 0.52 0.56 0.009 0.003 0.043 0.0042 0.0021 0.0033 - - - - - - - 0.0020 - - n 0.42 1.89 0.31 0.010 0.003 0.035 0.0031 0.0017 0.0035 - - - - - - - - 0.0021 - 0 0.29 1.36 0.88 0.009 0.003 0.033 0.0032 0.0014 0.0033 - - - - - - - - - 0.025 P 0.32 1.71 1.80 0.009 0.002 0.031 0.0034 0.0210 0.0033 - - - - - - - - - - Comparative Example q 1.03 1.76 0.71 0.010 0.003 0.033 0.0030 0.0018 0.0032 - - - - - - - - - - r 0.33 1.71 0.69 0.009 0.044 0.031 0.0036 0.0019 0.0034 - - - - - - - - - - s 0.41 1.72 0.72 0.009 0.002 0.032 0.0033 0.0000 0.0033 - - - - - - - - - - t 0.37 2.51 0.71 0.009 0.003 0.033 0.0034 0.0021 0.0033 - - - - - - - - - - u 0.11 0.67 0.71 0.010 0.002 0.030 0.0303 0.0019 0.0031 - - - - - - - - - - V 0.41 1.62 0.72 0.008 0.002 0.030 0.0034 0.0092. 0.0041 - - - - - - - - - -

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[Table 2-2]

Steel Chemical composition (% by mass): Fe balance and impurities Note Ç Si Mn P s Al N Here 0 Cr V You Nb Ass Ni Mo B REM Zr w 0.16 0.91 0.51 0.007 0.002 0.030 0.0033 0.0006 0.0035 0.11 - 0.013 - - - 0.08 0.0011 0.0015 - X 0.62 0.12 1.21 0.008 0.002 0.036 0.0031 0.0018 0.0034 - 0.13 - - 0.46 0.11 - - - 0.053 y 0.48 1.51 0.49 0.018 0.003 0.034 0.0030 0.0015 0.0029 - - - - - - - - - - z 0.71 1.16 0.74 0.010 0.019 0.002 0.0044 0.0017 0.0036 0.07 - - 0.017 - - 0.06 0.0029 - - aa 0.42 1.74 0.91 0.004 0.005 0.077 0.0034 0.0017 0.0039 - - - - - - - - - - ab 0.41 1.75 0.91 0.011 0.010 0.015 0.0033 0.0012 0.0036 - - - - - - - - - - B.C 0.44 1.67 0.88 0.007 0.003 0.021 0.0031 0.0013 0.0036 - - - - - - - - - - ad 0.53 0.51 1.48 0.013 0.003 0.036 0.0079 0.0011 0.0040 0.18 - 0.017 0.012 - - - - - - ae 0.21 1.91 1.10 0.006 0.003 0.034 0.0051 0.0003 0.0037 - - - - - - 0.08 0.0015 - - Example of the Invention af 0.44 1.57 0.75 0.007 0.005 0.032 0.0031 0.0047 0.0027 0.21 - - - - - - - 0.0017 - aq 0.36 1.64 0.81 0.009 0.002 0.036 0.0031 0.0019 0.0046 - - - - - - - - - - ah 0.74 0.29 1.04 0.008 0.003 0.031 0.0030 0.0017 0.0034 - - - - - - - - - - there 0.44 1.26 0.55 0.007 0.015 0.036 0.0034 0.0013 0.0033 0.15 0.11 - - - - - - - 0.026 a | 0.58 1.51 0.77 0.009 0.004 0.048 0.0039 0.0011 0.0035 - - - - - - - - - - ak 0.51 1.56 0.89 0.009 0.003 0.059 0.0036 0.0014 0.0036 - - - - - - - - - - al 0.45 1.70 0.71 0.006 0.002 0.034 0.0020 0.0016 0.0036 - - - - 0.44 - - - - - am 0.62 1.53 0.77 0.007 0.008 0.031 0.0033 0.0005 0.0033 - - - - - - 0.11 - - - an 0.47 1.64 0.74 0.008 0.006 0.028 0.0041 0.0044 0.0039 - - - - - - - 0.0017 - - to 0.33 1.77 0.75 0.009 0.002 0.005 0.0036 0.0018 0.0031 - - - - - - - - - - ap 0.52 0.08 0.71 0.008 0.003 0.044 0.0031 0.0013 0.0030 - - - - - 0.16 - - - - aq 0.36 1.74 0.29 0.007 0.003 0.031 0.0036 0.0010 0.0034 - - - - - - - - - - air 0.44 1.51 0.45 0.009 0.005 0.0003 0.0031 0.0014 0.0051 - - - - - - - - - - Example at 0.29 1.88 1.21 0.007 0.006 0.082 0.0039 0.0011 0.0032 0.11 - - 0.014 - - - - - - Comparative at 0.54 1.10 0.74 0.008 0.003 0.036 0.0085 0.0011 0.0036 - - - - - - - - - - au 0.44 0.96 0.44 0.009 0.006 0.037 0.0030 0.0001 0.0030 - - - - - - - - - -

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[00132] The results of the investigation of the average values of the composition ε ratios of the complex oxides, the average values of the equivalent circle diameters of the complex oxides, and the tensile strengths of the machine wires produced using the method described above, and the mean values of equivalent circle diameters of complex oxides, structures, tensile strengths, and hydrogen-induced fracture strengths of flat steel wires are shown in Tables 3-1 to Table 4-2. In Table 3-1, values that deviate from preferred production 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, the complex oxide composition ratio ε indicates the average value of the complex oxide composition ratios in the central portion of the wire rod and the mean 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 central portion of the flat steel wire. The average value of the composition ratios ε of the complex oxide in the central portion of the flat steel wire is substantially identical to that of the central portion of the wire rod and is thus not measured.

[00133] The average value of the composition 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 in the central portion of the flat steel wire were investigated using the method described above is used. The tensile strength of the wire rod and the structure, tensile strength and fracture resistance induced by hydrogen of the flat steel wire were respectively investigated using the methods described below.

[00134] (1) Tensile strength of wire rod

[00135] The machine wire was cut to a length of 340 mm,

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42/54 the upper and lower 70 mm portions were fixed using hydraulic chucks and a tensile test was performed. The tensile strength was computed by dividing the maximum load obtained by the cross-sectional area of the wire rod. The tensile strength is preferably 600 MPa or more, and so a wire having a tensile strength of 600 MPa or more has been assessed as an acceptable product.

[00136] (2) Flat steel wire structure

[00137] A section L of the flat steel wire was mirror-polished and corroded with picral, five arbitrary locations in the central portion of section L were observed respectively using a FE-SEM at a magnification of 2,000 times, and five photographs were taken . An OHP plate was superimposed on each of the photographs obtained, and overlapping regions with a ferrite structure and a bainite structure on each of the transparent sheets were painted with colors. Next, the area ratio of the region painted in color on each of the transparent sheets was obtained using image analysis software, thus obtaining the area ratios of the ferrite structure and the bainite structure in each of the five locations. In addition, the area ratios of the ferrite structure and the bainite structure were averaged at the five locations, thus computing the area ratios of the ferrite structure and the bainite structure in the flat steel wire. In addition, different structures of ferrite, bainite, and martensite were not substantially confirmed in any of the flat steel wires, and so the value obtained by subtracting the area ratios of the ferrite structure and the bainite structure from 100% was considered as the average value of the martensite structure.

[00138] (3) Tensile strength of flat steel wire:

[00139] The flat steel wire was cut to a length of 400 mm, the upper and lower portions 100 mm were fixed using

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43/54 if hydraulic chucks and a tensile test was performed. The tensile stress was computed by dividing the maximum load obtained by the cross-sectional area of the flat steel wire. The tensile strength is preferably 1,100 MPa or more, and so a flat steel wire having a tensile strength of 1,100 MPa or more has been assessed as an acceptable product.

[00140] (4) Investigation of hydrogen-induced fracture resistance of flat steel wire:

[00141] The resistance to fracture induced by hydrogen was evaluated using flat steel wire cut to a length of 150 mm. As a solution to supply hydrogen to a test specimen, a 5% aqueous solution of NaCI + CHsCOONa was used, and the solution was used after the pH was adjusted to 5.0. After the solution was degassed with nitrogen gas, a mixed gas of hydrogen sulfide (H2S) and carbon dioxide (CO2) was introduced there, and the flat steel wire was immersed in the solution, thus investigating the generation of fractures. At that time, the hydrogen sulfide pressure was 0.1 MPa, the test temperature was 25Ό, and the test time was 96 hours. After the test, the presence and absence of fracture generation was confirmed by an ultrasonic test (UST) with a frequency of 10 kHz in the direction of the thickness of the flat steel wire. The total portion areas with fractures generated in which the fractures were determined to be generated by the ultrasonic test were obtained by image analysis, and the percentage of occurrence of hydrogen-induced fractures (X (%)) was obtained using the Expression D. A flat steel wire in which hydrogen-induced fracture occurred was determined to fail in terms of hydrogen-induced fracture resistance.

X = (Af / (w x L)) x 100 Expression D

[00142] Here, Af represents the total area (mm ² ) of the portions with

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44/54 generated fractures measured by UST, w represents the width (mm) of the flat steel wire, and L represents the length (mm) of the flat steel wire.

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[Table 3-11

Test n ^Q Steel type Machine wire production method Properties of complex oxide in wire rod Wire resistance [MPa] Note CaSi alloy addition time [min.] Cross-section hot rolling area ratio Composition ratio of complex oxide ε Average equivalent circle diameter [pm] TO 1 THE 40 131.6 0.23 2.1 958 Example of the invention A2 THE 25 131.6 0.17 L6 965 Comparative example A3 THE ZQ 131.6 4.23 2.4 951 Comparative example A4 THE 40 Z4J. 0.41 9J3 944 Comparative example A5 THE 40 131.6 0.53 1.8 956 Example of the invention B1 B 40 131.6 0.33 2.5 939 Example of the invention B2 B 25 131.6 0.21 82 960 Comparative example B3 B ZQ 131.6 5.52 3.0 957 Comparative example

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[Table 3-2]

Test n ^Q Steel type Method for producing flat steel wire Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structures (average value) Flat steel wire resistance [MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Lamination planing Quick cooling and tempering Fraction of martensite [% in area] Ferrite fraction [% in area] Bainite fraction [% in area] TO 1 THE Yes Yes 1.1 100 0 0 1284 0 Example of the invention A2 THE Yes Yes 4Z 100 0 0 1288 70 Comparative example A3 THE Yes Yes 2.5 100 0 0 1291 25 Comparative example A4 THE Yes Yes 3J. 100 0 0 1281 85 Comparative example A5 THE Yes No 1.4 0 34 66 1284 0 Example of the invention B1 B Yes Yes 1.6 100 0 0 1261 0 Example of the invention B2 B Yes Yes 4J_ 100 0 0 1277 45 Comparative example B3 B Yes Yes 2.8 100 0 0 1283 40 Comparative example

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[00143] As shown in Table 3-1 and Table 3-2, in the examples of the invention that have an adequate chemical composition and an adequate production condition, the hydrogen-induced fracture did not occur even after the planing process, and there was no problems.

[00144] In contrast, in test ^Nos A2 and B2, the time elapsed since the addition of metallic Al by adding CaSi alloy was 25 minutes, and thus the complex oxide in the wire rod became rough, and the fracture hydrogen-induced occurred in the flat steel wire.

[00145] In Tests ^Nos A3 and B3, the time elapsed since the addition of metallic Al by adding CaSi alloy was 70 minutes and thus the average value of the ratios of ε composition of the complex oxide in the machine wire has reached 3 , 00 or more, and the hydrogen-induced fracture occurred in the flat steel wire.

[00146] In test n ^Q A4, the amount of reduction of the cross-sectional area during hot rolling decreased, and the oxide was not sufficiently compressed during hot rolling, and thus the maximum size of the complex oxide in the machine wire was became outside the scope of the present invention, and the hydrogen-induced fracture occurred in the flat steel wire.

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[Table 4-11

Test n ^Q Steel type Properties of complex oxide in wire rod Flat steel wire resistance [MPa] Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structure (average value) Flat steel wire resistance [MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Composition ratio of complex oxide ε Average equivalent circle diameter [pm] Fraction of martensite [% in area] 1 The 0.17 4.3 841 2 100 1256 0 2 B 1.69 3.9 819 1.2 100 1233 0 3 ç 0.19 4.9 833 1.6 100 1301 0 4 d 0.25 5.1 941 1.0 100 1234 0 5 and 2.61 5.5 1172 1.2 100 1411 0 Examples 6 f 0.21 5.0 927 1.1 100 1316 0 of the 7 g 0.23 5.1 934 1.6 100 1391 0 vention 8 H 0.21 5.6 968 1.5 100 1369 0 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

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Test n ^Q Steel type Properties of complex oxide in wire rod Flat steel wire resistance [MPa] Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structure (average value) Flat steel wire resistance [MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Composition ratio of complex oxide ε Average equivalent circle diameter [pm] Fraction of martensite [% in area] 12 | 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 0 0.17 4.9 754 1.2 100 1194 0 16 P 0.18 5.6 836 - - - - 17 q 0.17 5.2 1523 - - - - 18 r 0.18 4.3 822 1.8 100 1249 85 Examples 19 s 0.00 111 940 5J 100 1311 100 compare 20 t 0.17 5.6 897 - - - - assets 21 u 0.21 3.8 519 1.6 100 961 0 22 V 11.12 4.7 916 15 100 1321 70

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[Table 4-2]

Test n ^Q. Steel type Properties of complex oxide in wire rod Piano steel wire resistance [MPa] Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structure (average value) Flat steel wire strength [ MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Composition ratio of complex oxide ε Average equivalent circle diameter [pm] Fraction of martensite [% in area] 23 w 0.22 5.1 677 1.1 100 1106 0 Examples of the invention 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 B.C 0.88 5.5 984 1.6 100 1258 0 30 ad 0.36 4.9 926 1.3 100 1275 0 31 ae 0.41 5.1 802 1.0 100 1214 0

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Test n ^Q. Steel type Properties of complex oxide in wire rod Piano steel wire resistance [MPa] Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structure (average value) Flat steel wire strength [ MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Composition ratio of complex oxide ε Average equivalent circle diameter [pm] Fraction of martensite [% in area] 32 af 0.69 5.3 975 1.1 100 1301 0 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 there 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 1.6 100 1348 0 40 an 1.36 4.8 1011 0.9 100 1318 0 41 to 0.48 4.6 903 2.4 100 1264 0

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Test n ^Q. Steel type Properties of complex oxide in wire rod Piano steel wire resistance [MPa] Average equivalent circle diameter of the complex oxide in the flat steel wire [pm] Flat steel wire structure (average value) Flat steel wire strength [ MPa] Percentage of occurrence of hydrogen-induced fracture [%] Note Composition ratio of complex oxide ε Average equivalent circle diameter [pm] Fraction of martensite [% in area] 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 air 2.61 19 982 14 100 1301 75 45 at 0.21 L6 865 18 100 1248 100 46 at 1.28 5.1 1017 - - - - 47 au 0.08 14 926 £ 3 100 1275 100

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[00147] As shown in Table 4-1 and Table 4-2, in the examples of the invention that have an adequate chemical composition and an adequate production condition, the hydrogen-induced fracture did not occur even after the planing process, and there was no problems.

[00148] In tests ^Nos 16, 17, 20, and 46, the chemical composition was outside the scope of the present invention, fractures were generated in the flat steel wire for the cold rolling to flat steel wire (during planing process), and so the steps after the heat treatment were not carried out, and the test was interrupted.

[00149] In test n ^Q 16, the Mn content and the Ca content were outside the scope of the present invention, and fractures were generated during the planing process.

[00150] In test n ^Q 17, the C content was outside the scope of the present invention, and fractures were generated during the planing process.

[00151] In test n ^Q 20, the Si content was outside the scope of the present invention, and fractures were generated during the planing process.

[00152] In tests ^Nos 18 and 19, any of the steel chemical composition was outside the scope of the present invention, and fracture was induced by hydrogen.

[00153] In test n ^Q 18, the S content was outside the scope of the present invention, and hydrogen-induced fracture occurred.

[00154] In test n ^Q 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 fracture occurred.

[00155] In test n ^Q 21, the C content and the N content were outside the scope of the present invention, and the tensile strength did not reach

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1,100 MPa.

[00156] In test n ^Q 22, the Ca content was outside the scope of the present invention, in addition the composition ratio of the complex oxide was outside the scope of the present invention, and hydrogen-induced fracture occurred.

[00157] In test n ^Q 42, the Si content was outside the scope of the present invention, and the tensile strength did not reach 1,100 MPa.

[00158] In test n ^Q 43, the Mn content was outside the scope of the present invention, and the tensile strength did not reach 1,100 MPa.

[00159] In test n ^Q 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 fracture occurred.

[00160] In test n ^Q 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 fracture occurred.

[00161] In test n ^Q 46, the N content was outside the scope of the present invention, and fractures were generated during the planing process.

[00162] In test n ^Q 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 fracture occurred.

Brief Description of the Reference Symbols

WIRE MACHINE

CENTRAL PORTION

FLAT STEEL WIRE

CENTRAL PORTION

Claims (14)

1. Machine wire characterized by the fact that it comprises, as a chemical composition, in 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 the balance including Fe and impurities, where an oxide including CaO and AI2O3 and which satisfies Expression A and Expression B is defined as a complex oxide, The average value of the composition ratio ε of the complex oxide that is defined by Expression C satisfies 0.00 <ε <3.00, measured in a central portion within a range of 1/10 the diameter of the wire rod from the axis center of the wire rod in a cross section perpendicular to the direction of wire rod rolling, and Petition 870190084488, of 08/29/2019, p. 64/99 2/6 the average value of an equivalent circle diameter of the complex oxide is 6.0 pm or less, measured in the central portion of the cross section, (number of oxide-forming elements other than Ca and Al in the oxide in units of mol% ) <(1/3) χ (the highest content between the Ca content and the Al content in the oxide per units of mol%): Expression A, (O content in units of mol%)> (content of S in the oxide in units of mol%): Expression B, (ratio of composition ε) = (concentration of CaO in complex oxide in units of mass%) / (concentration of AI2O3 in complex oxide in units of mass%): Expression C . 2. Wire rod according to claim 1, characterized by the fact that it comprises, as a chemical composition, in mass%: Cr: 0.05% to 1.00%. 3. Wire rod according to claim 1 or 2, characterized by the fact that it comprises, as chemical composition, in mass%: one or more elements 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%. Machine wire according to any one of claims 1 to 3, characterized in that it comprises, as a chemical composition, in mass%: one or more elements selected from the group consisting of Cu: 0.01% to 1.00%, Petition 870190084488, of 08/29/2019, p. 65/99 3/6 Ni: 0.01% to 1.50%, Mo: 0.01% to 1.00%, and B: 0.0002% to 0.0100%. Machine wire according to any one of claims 1 to 4, characterized in that it comprises, as a chemical composition, in mass%: one or both of the elements selected from the group consisting of REM: 0.0002% to 0.0100% and Zr: 0.0002% to 0.1000%. Machine wire according to any one of claims 1 to 5, characterized in that the tensile strength is 600 MPa to 1,400 MPa. 7. Flat steel wire characterized by the fact that it comprises, as a chemical composition, in 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%, Petition 870190084488, of 08/29/2019, p. 66/99 4/6 Mo: 0% to 1.00%, B: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.1000%, and the balance including Fe and impurities, where an oxide including CaO and AI2O3 and which satisfies Expression A and Expression B is defined with a complex oxide, The average value of the composition 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 1/7 of the length of the minor axis of the steel wire plane from the central axis of the flat steel wire in a cross section parallel to the rolling direction and the direction of a minor axis of the flat steel wire and including the central axis of the flat steel wire, and an average diameter value of equivalent circle of complex oxide is 3.0 pm or less, measured in the central portion of the cross section, (number of oxide-forming elements other than Ca and Al in the oxide in units of mol%) <(1/3) χ (o highest content of 0 Ca content or 0 Al content in the oxide in units of mol%): Expression A (O content in the oxide in units of mol%)> (S content in the oxide in units of mol%): Expression B (composition ratio ε) = (concentration of CaO in the complex oxide in units of% by mass) / (concentration of AI2O3 in the complex oxide in un mass% ages): Expression C. 8. Flat steel wire according to claim 7, characterized by the fact that the structure in the central portion includes 98% in area or more of tempered martensite. 9. Flat steel wire according to claim 7, Petition 870190084488, of 08/29/2019, p. 67/99 5/6 characterized by the fact that the structure in the central portion includes 20% in area at 60% in area of ferrite and 40% in area at 60% in area of bainite. Flat steel wire according to any one of claims 7 to 9, characterized by the fact that the tensile strength is 1,100 MPa to 1,500 MPa. Flat steel wire according to any one of claims 7 to 10, characterized in that it comprises, as chemical composition, in mass%: Cr: 0.05% to 1.00%. Flat steel wire according to any one of claims 7 to 11, characterized in that it comprises, as a chemical composition, in mass%: one or more elements 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%. Flat steel wire according to any one of claims 7 to 12, characterized in that it comprises, as a chemical composition, in mass%: one or more elements 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%. Flat steel wire according to any one of claims 7 to 13, characterized in that it comprises, Petition 870190084488, of 08/29/2019, p. 68/99 6/6 as chemical composition, in% by mass: one or both elements selected from the group consisting of REM: 0.0002% to 0.0100% and Zr: 0.0002% to 0.1000%.