WO2014178303A1

WO2014178303A1 - Flat steel wire

Info

Publication number: WO2014178303A1
Application number: PCT/JP2014/061228
Authority: WO
Inventors: 大羽　浩; 比呂志谷田部; 新磯; 敏之真鍋; 雅嗣村尾; 耕一村尾; 憲小山田; 俊也池端; 充則尾崎
Original assignee: 新日鐵住金株式会社; ナミテイ株式会社
Priority date: 2013-04-30
Filing date: 2014-04-22
Publication date: 2014-11-06
Also published as: JPWO2014178303A1; EP2993246A4; EP2993246B1; DK2993246T3; EP2993246A1; JP6116680B2

Abstract

A flat steel wire has a rounded rectangular cross section when the cross section is taken in a direction perpendicular to the length direction, wherein the length of the shorter side of the cross section is 2 to 7 mm inclusive, the length of the longer side of the cross section is more than 8 mm and 56 mm or less, the ratio of the length of the longer side to the length of the shorter side is more than 4 and 8 or less, the tensile strength of the flat steel wire is 1900 MPa or more, and the twisting value of the flat steel wire is 12 times or more.

Description

Flat steel wire

The present invention relates to a high-strength flat steel wire excellent in secondary workability and used for improving pressure resistance and tension of power / communication wires, pipes / hoses and the like.
This application claims priority based on Japanese Patent Application No. 2013-095774 filed in Japan on April 30, 2013, the contents of which are incorporated herein by reference.

With regard to the high-strength flat steel wire, for example, Patent Document 1 discloses a high-strength flat steel wire containing 0.90% by mass or more of carbon. This flat steel wire is a high-strength flat steel wire having a tensile strength in the longitudinal direction of about 200 kgf / mm ² . However, generally, the secondary workability of a flat steel wire decreases as the strength of the flat steel wire increases. Even if the strength of the conventional flat steel wire is excellent, it has already reached the processing limit and is often difficult to be subjected to secondary processing such as bending and twisting.

Generally, flat steel wires are often used for reinforcing flexible pipes. For example, high pressure resistance and tension are required for a flexible pipe used when pulling crude oil from a deep-sea oil field. Therefore, the flat steel wire is required to have high strength. On the other hand, the flat steel wire is used by being deformed into a spiral shape with respect to the flexible pipe. Therefore, flat steel wires are required to have excellent secondary workability with respect to twisting and the like. In a flat steel wire with poor secondary workability, when it is deformed into a spiral shape, cracks may enter from the outer periphery and may be broken into vertical cracks. That is, when the flat steel wire is excellent in strength and secondary workability, a flexible pipe having high pressure resistance and tension can be realized.

Japanese Patent No. 3298688

An object of the present invention is to provide a high-strength flat steel wire excellent in secondary workability. The strength means tensile yield strength, 0.2% proof stress in a tensile test, maximum tensile strength, and the like, and the secondary workability means twisting property, tensile elongation at break, and the like.

The gist of the present invention is as follows.

(1) A flat steel wire according to an aspect of the present invention is a flat steel wire that is a rounded rectangle when viewed in a cross section perpendicular to the longitudinal direction, and a short side of the cross section is 2 mm or more and 7 mm or less, The long side of the cross section is 8 mm to 56 mm or less, the ratio of the long side to the short side is 4 to 8 or less, the yield strength or 0.2% yield strength obtained by a tensile test is 1600 MPa to 2000 MPa, and the tensile strength is 1900 MPa. As described above, the elongation at break is 2% or more, and the twist value obtained by the twist test under the condition that the distance between chucks is 500 mm is 12 times or more.

(2) In the flat steel wire according to the above (1), when viewed in the cross section, a region having a depth of 1 μm or less from the outline of the cross section toward the center of gravity of the cross section is defined in the flat steel wire. When the surface layer portion is used, an average value of the compressive residual stress in the longitudinal direction may be −200 MPa or less in the surface layer portion.

(3) In the flat steel wire according to the above (1) or (2), the chemical component is mass%, C: 0.85% to 1.00%, Si: 0.80% to 1.30. %: Mn: 0.30% or more and 0.90% or less, P: 0.017% or less, S: 0.010% or less, Cu: 0.20% or less, Al: 0% or more and 0.10% or less Ti: 0% or more and 0.05% or less, B: 0% or more and 0.0040% or less, N: 0% or more and 0.0060% or less, Cr: 0% or more and 0.5% or less, V: 0% or more It may contain 0.50% or less, and the balance may consist of Fe and impurities.

(4) In the flat steel wire according to any one of the above (1) to (3), the chemical component is, by mass%, Al: 0.005% or more and 0.10% or less, Ti: 0.00. It may contain at least one of 003% or more and 0.05% or less, B: 0.0005% or more and 0.0040% or less, and N: 0.0015% or more and 0.0060% or less.

(5) In the flat steel wire according to any one of the above (1) to (4), the chemical component is, by mass, Cr: 0.1% to 0.5%, V: 0.00. You may contain at least 1 of 005% or more and 0.50% or less.

(6) In the flat steel wire according to any one of the above (1) to (5), the short side of the cross section is 2 mm to 6 mm, and the long side of the cross section is more than 8 mm and 48 mm or less. Also good.

(7) In the flat steel wire described in (6) above, the short side of the cross section may be 2 mm or more and 5 mm or less, and the long side of the cross section may be more than 8 mm and 40 mm or less.

According to the above aspect of the present invention, since the shape of the flat steel wire and the like is preferably controlled, for example, during the twisting process, the occurrence of cracks from the outer peripheral surface of the flat steel wire and the vertical crack of the flat steel wire due to its progress Etc. are suppressed. As a result, it is possible to obtain a high-strength flat steel wire having high secondary workability as compared with conventional high-strength flat steel wires.

It is a schematic diagram at the time of seeing the flat steel wire concerning one embodiment of the present invention in the section perpendicular to the longitudinal direction. It is an external appearance photograph after the twist process of the flat steel wire which concerns on the same embodiment. It is a graph which shows the relationship between the average value of the residual stress of the surface layer part of a flat steel wire, and the twist value in a twist test.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In addition, the lower limit value and the upper limit value are included in the numerical limit range described below. However, the lower limit value does not include the lower limit value, and the upper limit value does not include the upper limit value.

The inventors of the present invention have a rounded rectangular shape when viewed in a cross section perpendicular to the longitudinal direction, and the ratio of the long side of the cross section to the short side of the cross section (long side / short side) is more than 4 and less than 8 It was found that the twisting characteristics of the flat steel wire can be improved by restricting to. It is considered that the twisting characteristics are improved by suppressing cracks from the outer peripheral surface of the flat steel wire, particularly corners, and suppressing non-uniform deformation.

Further, it has been found that the twisting characteristics of a flat steel wire are preferably improved by controlling the residual stress of the surface portion of the flat steel wire to a compressive stress state. Since the region where the residual stress of the surface layer portion of the flat steel wire is in the tensile stress state is likely to be a starting point of fracture, it is considered that the twisting characteristics are preferably improved by reducing the region that becomes the tensile residual stress in the surface layer portion.

Further, by limiting the C content to 1.00% or less by mass% as a chemical component of the flat steel wire, or by containing an alloying element that fixes N, the twisting characteristic of the flat steel wire is further increased. It has been found that it is preferably improved. By controlling the C content, a uniform and fine pearlite structure is formed even under heat treatment conditions that can be applied industrially, and thus it is considered that the twisting characteristics are more preferably improved. Also, fixing N in the steel suppresses the age hardening of the flat steel wire, thereby suppressing local hardening in the flat steel wire and the accompanying non-uniform deformation, further increasing the twisting characteristics. It is thought that it improves preferably.

The flat steel wire according to the present embodiment has a high strength of a tensile yield strength or 0.2% proof stress of 1600 MPa or more and a maximum tensile strength (tensile strength) of 1900 MPa or more, and at the same time a tensile breaking elongation of 2% or more. Also, it has excellent secondary workability such that the twist value in the twist test is 12 times or more.

Generally, structural steel is regarded as high-strength steel when the tensile strength is 1000 MPa or more. In general, in the case of a round steel wire, when the twist value is 12 times or more, it is considered that the twist characteristics are excellent. That is, it can be said that the flat steel wire according to the present embodiment simultaneously satisfies two contradictory properties of strength and secondary workability. For example, by using the flat steel wire according to the present embodiment, a flexible pipe having high pressure resistance and tension can be realized.

Hereinafter, the shape of the flat steel wire according to this embodiment will be described in detail.

The flat steel wire according to the present embodiment has a rounded rectangular shape when viewed in a cross section perpendicular to the longitudinal direction, the short side of the cross section is 2 mm or more and 7 mm or less, and the long side of the cross section is more than 8 mm and 56 mm. The ratio of the long side to the short side is more than 4 and 8 or less.

In general, in a flat steel wire, when viewed in a cross section perpendicular to the longitudinal direction of the flat steel wire, the shorter dimension (short side) is referred to as “thickness” and the longer dimension (long side) is referred to as “width”. . In the flat steel wire according to the present embodiment, the shorter side of the cross section that is a rounded rectangle is defined as the short side (thickness). The length of the short side is defined as the length from the corner to the corner when the corner of the cross section is assumed to be a substantially right angle without roundness. Similarly, in the flat steel wire according to the present embodiment, the longer side of the cross section that is a rounded rectangle is defined as the long side (width). The length of the long side is defined as the length from the corner to the corner when the corner of the cross section is assumed to be a substantially right angle without roundness. In FIG. 1, the short side (thickness) and the long side (width) in the said cross section of the flat steel wire 1 which concern on this embodiment are shown typically.

The flat steel wire 1 according to the present embodiment has a rectangular shape when viewed in a cross section perpendicular to the longitudinal direction, and each corner portion has an arc shape. That is, the cross-sectional shape is a rounded rectangle with rounded corners. Since the cross-sectional shape is a rounded rectangle, the breakage starting from the corner is suppressed during the secondary processing. Moreover, when the cross-sectional shape is a rounded rectangular shape, when the flat steel wire 1 is deformed in a spiral shape with respect to the pipe, interference between corner portions of the adjacent flat steel wires 1 is suppressed.

In the flat steel wire 1 according to the present embodiment, considering the surface stability of the rolling roll during production and the reaction force during rolling, and considering the control of the strength of the flat steel wire 1, the thickness of the flat steel wire 1 is The upper limit is 7 mm. Moreover, in order to perform a twisting process stably, the thickness of the flat steel wire 1 needs to be 2 mm or more. That is, the thickness of the flat steel wire 1 according to this embodiment is 2 mm or more and 7 mm or less.

In addition, as a result of studies to ensure a twist value of 12 times or more, the present inventors need to make the width / thickness ratio (ratio of long side to short side) more than 4 and 8 or less. I found. When the ratio of the long side to the short side is 8 or less, the non-uniform deformation is suppressed, so that the twisting characteristic is considered to be improved. Further, if the ratio of the long side to the short side is 4 or less, it is necessary to contain carbon and alloy elements more than necessary for securing the strength. As a result, the ductility and secondary workability of the flat steel wire 1 are improved. Reduce.

Based on the thickness (short side) of the flat steel wire 1 and the width / thickness ratio (ratio of the long side to the short side), the width (long side) of the flat steel wire 1 according to this embodiment is 8 mm. What is necessary is just to be super 56 mm or less.

Moreover, when the flat steel wire 1 which concerns on this embodiment is seen in the cross section perpendicular | vertical to a longitudinal direction, the short side of a cross section is 2 mm or more and 6 mm or less, and the long side of a cross section is 8 mm over 48 mm or less. More preferably, the short side of the cross section is 2 mm or more and 5 mm or less, and the long side of the cross section is more than 8 mm and 40 mm or less. Moreover, as for the flat steel wire 1 which concerns on this embodiment, it is preferable that the minimum of width / thickness ratio is 5 or more, and it is preferable that the upper limit of width / thickness ratio is 7 or less.

Hereinafter, the strength and twisting characteristics of the flat steel wire 1 according to the present embodiment will be described in detail.

The flat steel wire 1 according to the present embodiment has a tensile strength of 1900 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be 2400 MPa.

In the flat steel wire 1 according to this embodiment, the elongation at break is set to 2% or more in order to ensure ductility. The upper limit of the elongation at break is not particularly limited, but may be 5%. Furthermore, in the flat steel wire 1 which concerns on this embodiment, in order to ensure the extension after a yield, yield strength or 0.2% yield strength shall be 1600 Mpa or more. The upper limit of the yield strength or 0.2% proof stress is not particularly limited, but may be 2000 MPa. Moreover, it is preferable that yield strength or 0.2% yield strength is a value lower than tensile strength. Specifically, the ratio of yield strength or 0.2% yield strength to tensile strength (yield strength or 0.2% yield strength / tensile strength) is preferably 0.8 or more and 0.95 or less.

The above-described mechanical characteristics may be controlled according to manufacturing conditions such as rolling conditions. For example, the above-described mechanical characteristics may be controlled by controlling the wire diameter of the steel wire before rolling and the rolling temperature and rolling reduction during rolling. Moreover, you may control an above-described mechanical characteristic by controlling the chemical component of the flat steel wire 1 as needed.

Further, the flat steel wire 1 according to this embodiment has excellent secondary workability because the shape of the flat steel wire 1 and the like are preferably controlled. Specifically, in a twist test under the condition that the distance between chucks is set to 500 mm, a twist process of 12 times or more can be performed. Thus, in the flat steel wire 1 which concerns on this embodiment, intensity | strength and secondary workability are excellent simultaneously. FIG. 2 shows a flat steel wire 1 according to this embodiment after twisting. As shown in FIG. 2, it is confirmed that the flat steel wire 1 according to the present embodiment has excellent twist characteristics despite high strength. The upper limit of the twist value is not particularly limited, but may be 25 times.

Hereinafter, the residual stress state of the flat steel wire 1 according to the present embodiment will be described in detail. The residual stress means a tensile residual stress when the value is positive, and a compressive residual stress when the value is negative.

In the flat steel wire 1 according to the present embodiment, the twist characteristic of the flat steel wire 1 is preferably improved by controlling the average value of the residual stress of the surface layer portion 2 of the flat steel wire 1 to a compressive stress state. Specifically, when a region whose depth is within 1 μm from the contour line of the cross section toward the center of gravity of the cross section when viewed in a cross section perpendicular to the longitudinal direction is the surface layer portion 2 of the flat steel wire 1, In the surface layer portion 2, the average value of the residual stress in the longitudinal direction may be −200 MPa or less. When the average value of the residual stress of the surface layer portion 2 is −200 MPa or less, the twisting characteristics of the flat steel wire 1 are preferably improved. Note that the average value of residual stress being −200 MPa or less means that the average value of residual stress is −220 MPa or −250 MPa, for example. Further, the lower limit of the average value of the residual stress is not particularly limited, but may be −1200 MPa. That is, the average value of residual stress is preferably −1200 MPa or more and −200 MPa or less. In addition, even if the residual stress is in the tensile stress state in the local region of the surface layer portion 2 of the flat steel wire 1, the residual stress of the surface layer portion 2 of the flat steel wire 1 is, as an average, a compressive stress state, and its value When the value is −200 MPa or less, the above effect can be preferably obtained.

FIG. 1 schematically shows the surface layer portion 2 when viewed in a cross section perpendicular to the longitudinal direction. FIG. 3 shows the relationship between the average value of the residual stress of the surface layer portion 2 and the twist value in the twist test. FIG. 3 shows that the twisting characteristics of the flat steel wire 1 are critically improved when the average value of the compressive residual stress of the surface layer portion 2 is −200 MPa or less.

Since the region where the residual stress of the surface layer portion 2 of the flat steel wire 1 is in a tensile stress state is likely to be a starting point of fracture, it is preferable to control the surface layer portion 2 to a compressive residual stress having an average value of −200 MPa or less. In order to control the residual stress of the surface layer portion 2 of the flat steel wire 1 to a compressive stress state, it is preferable to perform shot blasting or the like. The average value of the compressive residual stress in the surface layer part 2 is more preferably −300 MPa or less or −400 MPa or less. Further, the surface layer portion 2 of the flat steel wire 1 is more preferably a region whose depth is within 2 μm or within 3 μm toward the center of gravity.

Hereinafter, preferred chemical components of the flat steel wire 1 according to this embodiment will be described in detail. In the following description, “%” of the content of each element means “mass%”.

The chemical composition of the flat steel wire 1 according to the present embodiment is mass%, C: 0.85% to 1.00%, Si: 0.80% to 1.30%, Mn: 0.30% 0.90% or less, P: 0.017% or less, S: 0.010% or less, Cu: 0.20% or less, Al: 0% or more and 0.10% or less, Ti: 0% or more and 0.05 %: B: 0% to 0.0040%, N: 0% to 0.0060%, Cr: 0% to 0.5%, V: 0% to 0.50%, the balance Is preferably composed of Fe and impurities.

Among the chemical components of the flat steel wire 1 according to this embodiment, C, Si, and Mn are basic elements.

C: 0.85% or more and 1.00% or less C (carbon) is an element that improves the strength of pearlite steel by increasing the cementite fraction of pearlite steel. In general, it is possible to improve the strength of pearlite steel by optimizing manufacturing conditions such as patenting and controlling the lamella spacing of pearlite, or by work hardening. However, it is preferable that the C content is 0.85% or more because the tensile strength can be 1900 MPa or more while ensuring ductility. Therefore, the lower limit of the C content may be 0.85%. Moreover, when C content is 1.00% or less, since the crack at the time of the process resulting from local segregation can be suppressed, it is preferable. Therefore, the upper limit of the C content may be 1.00%.

Si: 0.80% or more and 1.30% or less Si (silicon) is a deoxidizing element at the time of refining steel, and is an element that solidifies and strengthens ferrite. In order to obtain these effects, the lower limit of the Si content may be 0.80%. Moreover, when Si content is 1.30% or less, the nose temperature of the isothermal transformation at the time of heat processing can be controlled preferably. Therefore, the upper limit of the Si content may be 1.30%.

Mn: 0.30% or more and 0.90% or less Mn (manganese) is a solid solution strengthening element, and is an element that improves toughness and hardenability. In order to obtain these effects, the lower limit of the Mn content may be 0.30%. Moreover, when Mn content is 0.90% or less, since the transformation delay of the center part of the flat steel wire 1 can be suppressed, it is preferable. Therefore, the upper limit of the Mn content may be 0.90%.

The flat steel wire 1 according to the present embodiment contains impurities as chemical components. The “impurity” refers to a material mixed from ore as a raw material, scrap, or a production environment when steel is industrially produced. Among these impurities, P, S, and Cu are preferably limited as follows in order to sufficiently exhibit the above effects. Moreover, since it is preferable that there is little content of an impurity, it is not necessary to restrict | limit a lower limit and the lower limit of an impurity may be 0%.

P: 0.017% or less P (phosphorus) is an impurity. When the P content is 0.017% or less, it is preferable because the embrittlement of the steel can be suppressed and cracking during processing of the flat steel wire 1 can be suppressed. Therefore, the P content may be limited to 0.017% or less.

S: 0.010% or less S (sulfur) is an impurity. S combines with Mn in steel to form MnS. S segregates at the center of the steel slab during the refining-solidification process of the steel, and a large amount of MnS is formed at this center to embrittle the steel. When S content is 0.010% or less, since the fracture | rupture which started from the center part of the flat steel wire 1 can be suppressed, it is preferable. Therefore, the S content may be limited to 0.010% or less.

Cu: 0.20% or less Cu (copper) is an impurity mainly mixed from scrap or the like. Cu also has an effect of hardening steel by solid solution strengthening. However, when the Cu content is 0.20% or less, the workability of the flat steel wire 1 can be remarkably reduced, which is preferable. Therefore, the Cu content may be limited to 0.20% or less.

As described above, the chemical components of the flat steel wire 1 according to the present embodiment contain the basic elements and the balance Fe and impurities. However, the flat steel wire 1 according to the present embodiment may contain Al, Ti, B, N, Cr, and V as selective elements instead of a part of Fe that is the balance. These selective elements may be contained depending on the purpose. Therefore, it is not necessary to limit the lower limit values of these selected elements, and the lower limit value may be 0%. Moreover, even if these selective elements are contained as impurities, the above effects are not impaired.

Al: 0% or more and 0.10% or less Al (aluminum) is a deoxidizing element when refining steel, and is an element that forms a compound by combining with N in steel. The age hardening of the flat steel wire 1 is suppressed by Al fixing N in the steel. Moreover, when containing Al and B simultaneously, Al increases the amount of solute B in steel by fixing N in steel. Therefore, if necessary, the Al content may be 0% or more and 0.10% or less. The lower limit of the preferable Al content is 0.005%. Further, when the Al content is less 0.10%, since the cracking during processing due to _{the Al} 2 _{O 3} to form a cluster can be suppressed preferred.

In addition, since Ti also has the same effect as Al, Al content can be reduced according to Ti content. In this case, the same effect as described above can be obtained.

Ti: 0% or more and 0.05% or less Ti (titanium), like Al, is a deoxidizing element during refining of steel, and is an element that forms a compound by combining with N in steel. When Ti fixes N in the steel, age hardening of the flat steel wire 1 is suppressed. When Ti and B are contained simultaneously, Ti fixes N in the steel, thereby increasing the amount of solute B in the steel. Therefore, the Ti content may be 0% or more and 0.05% or less as necessary. The lower limit of the preferable Ti content is 0.003%. Moreover, when Ti content is 0.05% or less, since the crack at the time of the process resulting from TiC increasing can be suppressed, it is preferable.

B: 0% or more and 0.0040% or less B (boron) is an element that dissolves in austenite and improves hardenability. Therefore, the B content may be 0% or more and 0.0040% or less as necessary. The lower limit of the preferable B content is 0.0005%. Also, when the B content is less _{0.0040%, Fe 23 (C,} B) since the cracking during processing due to precipitation of such ₆ can be suppressed preferred.

N: 0% or more and 0.0060% or less N (nitrogen) is an element that combines with Al, Ti, and B in steel to form a nitride, and suppresses the coarsening of the austenite grain size during heating. Therefore, the N content may be 0% or more and 0.0060% or less as necessary. The lower limit of the preferable N content is 0.0015%. Further, when the N content is 0.0060% or less, age hardening due to excessive free N that does not bond with Al, Ti, B is suppressed, and as a result, the ductility reduction of the flat steel wire 1 is suppressed. preferable.

Cr: 0% or more and 0.5% or less Cr (chromium) is an element that reduces the lamella spacing of pearlite and improves the strength of pearlite steel. Cr is an element that contributes to an increase in strength during wire drawing. Therefore, the Cr content may be 0% or more and 0.5% or less as necessary. The lower limit of the preferable Cr content is 0.1%. Moreover, when Cr content is 0.5% or less, since the fall of productivity resulting from pearlite transformation completion time becoming long can be suppressed, it is preferable.

V: 0% or more and 0.50% or less V (Vanadium) is an element that combines with C in steel to precipitate carbide in ferrite. This precipitate hardens the ferrite. Therefore, the V content may be 0% or more and 0.5% or less as necessary. The lower limit of the preferred V content is 0.005%. Moreover, when V content is 0.50% or less, since the crack at the time of the process resulting from the production | generation of a coarse carbide | carbonized_material can be suppressed, it is preferable.

The flat steel wire 1 according to this embodiment may have a plating layer or a coating layer on the surface. Hereinafter, preferred plating layers and coating layers will be described in detail.

The flat steel wire 1 according to this embodiment may have a Zn or Ni-containing plating layer having a thickness of 10 μm or less on the surface. This plating layer is preferable because corrosion of the flat steel wire 1 can be suppressed, and delayed fracture due to hydrogen intrusion due to corrosion or hydrogen intrusion from the environment can be suppressed. Moreover, when the thickness of a plating layer is 10 micrometers or less, since peeling of the plating layer at the time of secondary processing can be suppressed, it is preferable.

Further, the flat steel wire 1 according to the present embodiment may have a coating layer of a resin or the like having an anticorrosive effect on the surface. The effect similar to a plating layer is acquired by this coating layer.

Hereinafter, a method for measuring tensile characteristics, twisting characteristics, and residual stress, which are important for the flat steel wire 1 according to the present embodiment, will be described in detail.

The tensile properties of the flat steel wire 1 according to the present embodiment may be obtained by a tensile test based on JIS Z2241: 2011 or ISO 6892-1: 2009. From the tensile test results, yield strength or 0.2% proof stress, maximum tensile strength (tensile strength), and elongation at break are determined.

The twist characteristics of the flat steel wire 1 according to the present embodiment may be obtained by a twist test in which one end is rotated after chucking both ends with a flat blade chuck. At that time, the flat steel wire 1 is twisted under the conditions that the distance between chucks is 500 mm, the twisting speed is 10 rpm, and the test temperature is room temperature. The number of twists until disconnection is defined is defined as the twist value.

The residual stress of the surface layer portion 2 of the flat steel wire 1 according to this embodiment may be obtained by X-ray diffraction. It is possible to obtain the residual stress in the longitudinal direction of the surface layer portion 2 of the flat steel wire 1 by performing X-ray diffraction using a plane parallel to the longitudinal direction of the flat steel wire 1 and analyzing the X-ray diffraction result. it can. In addition, what is necessary is just to measure the residual stress of the surface layer part 2 in multiple places, and to calculate the average value of a residual stress from these measurement results. Specifically, the long side (width) of the flat steel wire 1 is divided into four equal parts, and three points excluding both sides of the equally divided points are used as measurement points, and the short side (thickness) is divided into three equal parts. Of the equally divided points, two points excluding both sides are preferably used as measurement points. And it is preferable to determine a measuring point by the same method also about each measuring point about each opposite side, and make it a total of 10 measuring points. It is preferable to measure the residual stress in the longitudinal direction perpendicular to the cut surface at these 10 measurement points and set the average value as the residual stress in the longitudinal direction of the flat steel wire 1. The surface layer portion 2 of the flat steel wire 1 is defined as a region having a depth of 1 μm or less from the contour line of the cross section toward the center of gravity of the cross section. The target uses Cr to determine the diffraction angle. The relationship between the angle φ formed by the sample surface normal and the crystal surface normal and the X-ray diffraction angle θ is plotted, and the residual stress is obtained from the slope of the approximate straight line.

Hereinafter, the manufacturing method of the flat steel wire 1 which concerns on this embodiment is demonstrated.

The manufacturing method of the flat steel wire 1 according to the present embodiment is not particularly limited. For example, the flat steel wire 1 according to the present embodiment may be manufactured by a steel making process, a casting process, a wire rod rolling process, a constant temperature transformation process, a wire drawing process, and the like. Moreover, you may have a shot blast process and a surface treatment process after a wire-drawing process as needed.

In the steelmaking process, it is preferable to make steel in order to obtain molten steel composed of the basic elements, selective elements, and impurities described above. Although the steelmaking method is not particularly limited, the molten steel may be produced by a blast furnace method using ore or the like as a raw material and an electric furnace method using scrap or the like as a raw material. For example, steelmaking conditions may be that ΔT (degree of superheat) is 15 to 20 ° C. By the said conditions, the center segregation of the flat steel wire 1 can be controlled preferably.

As the casting process, it is preferable to cast the molten steel after the steel making process in order to obtain a slab. The casting method is not particularly limited, but a vacuum casting method, a continuous casting method, or the like may be used. For example, the central segregation of the flat steel wire 1 can be preferably controlled by suppressing concentrated segregated molten steel suction during solidification shrinkage by appropriate reduction of the crater end. Moreover, you may give soaking | spreading diffusion process, hot rough rolling (bundling rolling) etc. to the slab after a casting process and before a wire-rolling process as needed.

As the wire rod rolling step, it is preferable to roll the slab after the casting step in order to obtain a wire rod. The wire rolling conditions are not particularly limited. For example, the wire rolling start temperature is set in a temperature range of 1050 ° C. or higher and 1150 ° C. or lower, the wire rolling end temperature is set in a temperature range of 900 ° C. or higher and 1050 ° C. or lower, and accumulation in wire rolling is performed. The rolling reduction may be in the range of 98.7% to 99.8%. By the said conditions, the center segregation of the flat steel wire 1 can be controlled preferably. Moreover, you may wind up the wire before a constant temperature transformation process after a wire rolling process as needed.

As the constant temperature transformation (patenting) step, it is preferable to keep the wire after the wire rolling step at a constant temperature transformation temperature and to cool the wire to room temperature after the constant temperature transformation treatment. At this time, the wire rod after the wire rod rolling process may be directly subjected to the isothermal transformation treatment without cooling to room temperature, or the wire rod after the wire rod rolling step is cooled to room temperature and then reheated to perform the isothermal transformation treatment. Also good. The isothermal transformation condition is not particularly limited, but for example, the wire may be immersed in a molten salt maintained in a temperature range of 530 ° C. or higher and 550 ° C. or lower for a time of 60 seconds or longer and 90 seconds or shorter. Under the above conditions, the pearlite structure of the flat steel wire 1 can be preferably controlled. The flat steel wire 1 according to the present embodiment mainly includes a pearlite structure as a metal structure. In addition, it is not necessary to control the temperature of a wire to completely constant temperature during a constant temperature transformation process, and the temperature of a wire may fluctuate within the said temperature range. Moreover, when performing a constant temperature transformation process directly after a wire rod rolling process, as needed, the temperature from wire rod completion | finish temperature to a constant temperature transformation temperature is set to the cooling rate of 12 to 30 degree C / sec. In addition, the wire may be cooled.

In the wire drawing process, in order to obtain the flat steel wire 1, the wire after the isothermal transformation is drawn. At this time, when the flat steel wire 1 is viewed in a cross section perpendicular to the longitudinal direction, the short side of the cross section is 2 mm or more and 7 mm or less, the long side of the cross section is more than 8 mm and 56 mm or less, and the ratio of the long side to the short side is The wire after the isothermal transformation is drawn into a flat steel wire 1 so that becomes more than 4 and 8 or less. Although the wire drawing method is not particularly limited, for example, wire drawing by a wire drawing die and flat rolling may be combined. Specifically, after drawing the wire after constant temperature transformation with a normal wire drawing die, wire drawing with a round cross-section die or a rectangular cross-section die and flat rolling may be combined as necessary.

The strength of the flat steel wire 1 according to this embodiment increases as the processing amount in the wire drawing process increases. However, in order to satisfy both the strength and the secondary workability at the same time, it is preferable that the cross-sectional reduction rate in the wire drawing step is 50% or more and 90% or less. Here, the cross-section reduction rate in the wire drawing step is based on the cross section perpendicular to the longitudinal direction: cross-section reduction rate = 100− (cross-sectional area after wire drawing) / (cross-sectional area after isothermal transformation treatment) × 100 It is defined as

As needed, you may perform shot blasting to the flat steel wire 1 after a wire drawing process as a shot blasting process. The shot blasting conditions are not particularly limited. For example, an inorganic abrasive is used as shot blast particles, the shot blast particle size is set to 340 μm or more and 400 μm or less, the shot blast pressure is set to 1 kg / cm ² or more and 3 kg / cm ² or less, The blast time may be 10 seconds or more and 20 seconds or less. Specifically, the shot blast particle size may be 370 μm, the shot blast pressure may be 2 kg / cm ² , and the shot blast time may be 15 seconds. The above conditions are preferable because the average value of the compressive residual stress in the longitudinal direction of the surface layer portion 2 of the flat steel wire 1 can be controlled to be −200 MPa or less.

If necessary, as a surface treatment step, the flat steel wire 1 after the wire drawing step or the shot blasting step may be subjected to surface treatment. It is preferable to apply a plating layer or a coating layer to the flat steel wire 1 by surface treatment. The surface treatment conditions are not particularly limited, and general plating treatment or surface coating treatment may be performed.

The effects of one embodiment of the present invention will be described more specifically with reference to examples. However, the conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention It is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Invention Example Nos. Shown in Tables 1 to 4 1 to 18 and Comparative Example No. 19 to 30 flat steel wires were produced. The metal structure of the flat steel wire was adjusted to a fine pearlite structure in the constant temperature transformation process after the wire rod rolling process. Further, in order to increase the strength, in the wire drawing step, the wire was drawn halfway with a normal wire drawing die, and then formed by performing at least one of wire drawing using a round cross-section die or a rectangular cross-section die and flat rolling. Moreover, if necessary, shot blasting was performed on a flat steel wire as a shot blasting process. An inorganic abrasive is used as the shot blast grain, the shot blast particle size is 340 μm or more and 400 μm or less, the shot blast pressure is 1 kg / cm ² or more and 3 kg / cm ² or less, and the shot blasting time is 10 seconds or more and 20 seconds or less. Then, shot blasting was applied to the flat steel wire.

And about the invention example and the comparative example, the tensile characteristic, the twist characteristic, and the residual stress were measured.

Tensile properties were measured by performing a tensile test based on JIS Z2241: 2011 or ISO 6892-1: 2009. From the tensile test results, yield strength or 0.2% yield strength, tensile maximum strength (tensile strength), and elongation at break were determined.

The twisting property was measured by performing a twisting test in which one side was rotated after chucking both ends with a flat blade chuck. At that time, the flat steel wire was twisted under the conditions that the distance between chucks was 500 mm, the twisting speed was 10 rpm, and the test temperature was room temperature. Then, the number of twists until disconnection and separation was taken as the twist value.

Residual stress was determined by analyzing the X-ray diffraction results for the surface layer portion of the flat steel wire. Specifically, the long side (width) of the flat steel wire is divided into four equal parts, and the three points excluding both sides of the equally divided points are used as measurement points, and the short side (thickness) is divided into three equal parts, Two points of the equally divided points excluding both sides were taken as measurement points. The measurement points were determined by the same method for each opposite side, and X-ray diffraction was performed at a total of 10 measurement points. By analyzing these X-ray diffraction results, the average value of the residual stress in the longitudinal direction perpendicular to the cut surface was determined. Using Cr as a target, the relationship between the angle φ formed by the sample surface normal and the crystal surface normal and the X-ray diffraction angle θ was plotted, and the residual stress was obtained from the slope of the approximate straight line.

These measurement results are also shown in Tables 1 to 4. In addition, the numerical value shown with an underline in a table | surface shows that it is outside the range of this invention. Further, in the table, “-” for a chemical component indicates that it is not intentionally added or is a value below the detection limit. In Examples 1 to 18, the long side (width), short side (thickness), and width / thickness ratio (ratio of long side to short side) when viewed in a cross section perpendicular to the longitudinal direction are within the scope of the present invention. Satisfactory, tensile properties and twisting properties also satisfied the scope of the present invention.

On the other hand, in Comparative Examples 19 to 30, any of the long side (width), the short side (thickness), the width / thickness ratio (ratio of the long side to the short side), and the twisting property was insufficient.

According to the above aspect of the present invention, since the shape of the flat steel wire and the like is preferably controlled, for example, during the twisting process, the occurrence of cracks from the outer peripheral surface of the flat steel wire and the vertical crack of the flat steel wire due to its progress Etc. are suppressed. As a result, it is possible to obtain a high-strength flat steel wire having high secondary workability as compared with conventional high-strength flat steel wires. Therefore, industrial applicability is high.

1 Flat steel wire 2 Surface layer

Claims

A flat steel wire that is a rounded rectangle when viewed in a cross section perpendicular to the longitudinal direction,
The short side of the cross section is 2 mm or more and 7 mm or less,
The long side of the cross section is more than 8 mm and 56 mm or less,
The ratio of the long side to the short side is more than 4 and 8 or less,
Yield strength or 0.2% yield strength obtained by a tensile test is 1600 MPa or more and 2000 MPa or less, tensile strength is 1900 MPa or more, and elongation at break is 2% or more,
A flat steel wire characterized in that a twist value obtained by a twist test under a condition that the distance between chucks is 500 mm is 12 times or more.
When viewed in the cross-section, when the area of the flat steel wire is a surface layer portion having a depth of 1 μm or less from the outline of the cross-section toward the center of gravity of the cross-section,
The flat steel wire according to claim 1, wherein, in the surface layer portion, an average value of compressive residual stress in the longitudinal direction is -200 MPa or less.
The chemical composition of the flat steel wire is mass%,
C: 0.85% or more and 1.00% or less,
Si: 0.80% or more and 1.30% or less,
Mn: 0.30% or more and 0.90% or less,
P: 0.017% or less,
S: 0.010% or less,
Cu: 0.20% or less,
Al: 0% or more and 0.10% or less,
Ti: 0% or more and 0.05% or less,
B: 0% or more and 0.0040% or less,
N: 0% or more and 0.0060% or less,
Cr: 0% to 0.5%,
V: 0% or more and 0.50% or less
The flat steel wire according to claim 1 or 2, wherein the balance consists of Fe and impurities.
The chemical component of the flat steel wire is mass%,
Al: 0.005% or more and 0.10% or less,
Ti: 0.003% to 0.05%,
B: 0.0005% or more and 0.0040% or less,
The flat steel wire according to any one of claims 1 to 3, comprising at least one of N: 0.0015% to 0.0060%.
The chemical component of the flat steel wire is mass%,
Cr: 0.1% to 0.5%,
The flat steel wire according to any one of claims 1 to 4, comprising at least one of V: 0.005% to 0.50%.
The short side of the cross section is 2 mm or more and 6 mm or less,
The long side of the cross section is more than 8 mm and 48 mm or less,
The flat steel wire according to any one of claims 1 to 5, wherein:
The short side of the cross section is 2 mm or more and 5 mm or less,
The long side of the cross section is more than 8 mm and 40 mm or less,
The flat steel wire according to claim 6, wherein: