WO2018212327A1 - Wire, steel wire, and method for manufacturing steel wire - Google Patents

Abstract

A wire according to an embodiment of the present invention is configured such that: the wire has a chemical composition within a prescribed range; in both a surface layer part and a center part, the main structure is a pearlite structure; the surface area ratio of a ferrite structure is 45% or less, and the surface area ratio of a non-pearlite and non-ferrite structure is 5% or less; the sub-boundary density ρ1 when the angular difference in crystal orientation for lamellar ferrite within the pearlite structure is greater than or equal to 2° and less than 15° is 70/mm ≤ ρ1 ≤ 600/mm, and the density ρ2 of high angle boundaries where the angular difference in ferrite crystal orientation within the entire structure is 15° or greater is 200/mm or greater.

Description

Wire, steel wire, and method of manufacturing steel wire

The present invention relates to a wire, a steel wire, and a method for manufacturing a steel wire.
This application claims priority based on Japanese Patent Application No. 2017-099227 filed in Japan on May 18, 2017, the contents of which are incorporated herein by reference.

The present invention is widely used as a material for high-strength steel wires used as wires for reinforcing tires for automobiles, reinforcing wires such as aluminum transmission lines, PC steel wires, rope wires used for bridges, etc. It relates to the wire used. Moreover, this invention relates to the manufacturing method of the steel wire using this wire, and the steel wire obtained from this wire.

The wire is manufactured by hot rolling and processed into a wire by drawing to a predetermined wire diameter. Since the patenting process is performed once or twice in the middle of the wire drawing process to draw a thin steel wire, the wire material is required to have high wire drawing workability.

For example, in a reinforcing material having a wire diameter of 0.5 mm or more used for a tire for a large automobile, an improvement in productivity is required. There is a demand for a wire that can stably produce a steel wire having a wire diameter of 0.5 mm to 1.5 mm at a low cost from a wire having a wire diameter of 3.5 mm or more that can be produced by stable hot rolling. Therefore, the development of a wire rod that has a wire drawing workability that can omit an intermediate patenting step performed in the middle of the wire drawing process and that can exhibit a stable torsion characteristic after the wire drawing process has been underway.

However, in the case of a wire manufactured by a wire drawing process to a high wire drawing degree, breakage during wire drawing is more likely to occur. Furthermore, the steel wire drawn to a high degree of drawing tends to deteriorate the torsional characteristics. Furthermore, the fact that the wire diameter of the steel wire material is large is disadvantageous for the torsional characteristics of the steel wire.

Various methods for improving the structure of the wire have been proposed in order to prevent wire breakage during wire drawing. As such a technique, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2014-055316) discloses a high-strength steel wire in which 50% or more of lamellar cementite having an aspect ratio of 10 or more is present on the number basis with respect to the total number of lamellar cementite. A wire rod for high-strength steel wire has been proposed, which is a wire rod for use in such a lamellar cementite and prevents a reduction in wire drawing workability.

Patent Document 2 (Japanese Patent Application Laid-Open No. 2000-119756) includes pearlite in which the fraction of pro-eutectoid ferrite is 10% or less and the remainder is formed by discontinuous cementite. By configuring, a wire rod for high-strength steel wire that has prevented wire drawing workability from being deteriorated has been proposed.

These technologies are all wire drawing processes until a steel wire having a wire diameter of 0.1 to 0.4 mm is obtained by controlling the form of cementite of the wire to improve the wire drawing workability of the wire. In this case, no disconnection or the like is generated. However, the strength variation in the cross section of the steel wire cannot be suppressed only by controlling the form of cementite. Therefore, when a steel wire having a wire diameter of 0.5 mm or more is manufactured by the techniques disclosed in these patent documents, it is not possible to effectively achieve both the suppression of the occurrence of disconnection and the suppression of deterioration of the torsional characteristics, and the above-described problem May occur.

Therefore, wire breakage is difficult to occur in a process of manufacturing a steel wire having a large diameter (for example, wire diameter of 0.5 mm or more) by drawing to a high degree of wiredrawing, and the torsional characteristics after wire drawing are low. Realization of a good wire rod is desired.

Japanese Unexamined Patent Publication No. 2014-055316 Japanese Unexamined Patent Publication No. 2000-119756

The present invention has been made in view of the above circumstances, and is suitable as a material for large diameter wires and the like, and suppresses disconnection during wire drawing of a steel wire having high strength and excellent torsional characteristics. It is an object of the present invention to provide a wire that can be manufactured stably. Moreover, this invention makes it a subject to provide the steel wire which has high intensity | strength and the outstanding torsion characteristic, and its manufacturing method.

The present inventors conducted various studies in order to solve the above-described problems. As a result, the following knowledge was obtained.

(I) When a wire rod having a wire diameter of 5.5 mm or more is drawn to a level of 0.5 mm, the wire drawing strain is 4.5 or more. When drawing eutectoid steel or hypereutectoid steel with poor drawing workability at such a high drawing degree, patenting during drawing is essential. On the other hand, by using hypoeutectoid steel with a small cementite fraction as the material of the wire, it becomes possible to improve the wire drawing workability of the wire, and the wire is used for wire drawing with a wire drawing strain of 4.5 or more. It turns out that it will be possible.

(II) On the other hand, when the area ratio of ferrite exceeds 45% at the center in the cross section of the wire of hypoeutectoid steel (that is, the cut surface perpendicular to the length direction of the wire) It was found that the wire-drawing workability is insufficient even for hypoeutectoid steel wires because they are massive and coarse. In addition, when the crystal grain size becomes coarse (that is, when the large-angle grain boundary density is low), since the drawing value of the wire is low and the ductility is poor, a coarse crack is easily formed inside the wire during wire drawing. The wire drawing workability decreased. Further, it was found that in the cross section of the hypoeutectoid steel wire, when the area ratio of ferrite in the surface layer exceeds 45%, the torsional characteristics after the wire drawing process are deteriorated. This is considered because deformation concentrates on the ferrite structure.

(III) After the pearlite transformation, a large amount of subgrain boundaries are introduced into the layered ferrite (hereinafter referred to as lamellar ferrite) in the pearlite structure. The present inventors investigated the relationship between the subgrain boundary density of the wire and the torsion characteristics after the wire drawing of the wire (hereinafter sometimes simply referred to as torsion characteristics). As a result, it was found that basically, the better the torsional properties can be obtained as the subgrain boundary density in the pearlite structure increases. This is presumably because the greater the number of sub-grain boundaries, the more uniformly the processing strain is introduced during wire drawing and the strength variation in the cross section of the steel wire is reduced.

(IV) The present inventors examined means for improving the subgrain boundary density. Subgrain boundaries are considered to be introduced in order to eliminate misfits of both phases when lamellar ferrite and lamellar cementite (hereinafter referred to as lamellar cementite) in a pearlite structure grow cooperatively during pearlite transformation. The present inventors have found that the subgrain boundary density can be adjusted by using the pearlite transformation temperature and the content of an alloy element (for example, Si) that dissolves in lamella ferrite.

Specifically explaining the relationship between the pearlite transformation temperature and the subgrain boundary density, it was found that when the pearlite transformation temperature is 550 ° C. or less, the lower the pearlite transformation temperature, the lower the subgrain boundary density in the lamellar ferrite. This is probably because the number of points where the growth of lamellar cementite is interrupted increases. On the other hand, in the region where the pearlite transformation temperature is higher than 550 ° C., the subgrain boundary density tends to gradually decrease as the temperature increases. This is because when the pearlite transformation temperature is higher than 550 ° C., the lamellar spacing becomes coarser as the temperature increases, and the number of lamellar cementites decreases and the total amount of misfit decreases, so the subgrain density in lamellar ferrite decreases. It is thought that it will do. From these results, it was found that when the cooling conditions were controlled so that the pearlite transformation temperature was around 550 ° C., the largest amount of subgrain boundaries was introduced.

The relationship between the amount of alloying elements and subgrain boundary density will be specifically explained. Increasing the content of alloying elements such as Si increases the misfit at the interface between lamellar ferrite and lamellar cementite. However, it is considered that the subgrain boundary density increases.

(V) However, when experiments for increasing the subgrain boundary density in the pearlite structure were repeated based on the above-mentioned knowledge, a wire rod having low torsional characteristics after wire drawing was recognized despite the high subgrain boundary density. It was. The cause is not clear, but when the pearlite transformation was performed at a temperature lower than 600 ° C. to increase the subgrain boundary density, the number of twists after wire drawing tended to decrease. Therefore, by performing pearlite transformation at 600 ° C. to 620 ° C., not near 550 ° C., where the subgrain boundary density can be maximized, the subgrain boundary density in the lamellar ferrite is not increased excessively. It is considered that a wire material that achieves both torsional characteristics after processing can be obtained.

Based on the findings of (I) to (V) above, steel wires with high strength and excellent torsional properties that are suitable as materials for large-diameter wires, etc., can be manufactured stably while suppressing breakage during wire drawing. In order to realize a possible wire rod, it is necessary to use hypoeutectoid steel as the material of the wire rod. Furthermore, by adjusting the content of alloy elements and the adjustment cooling conditions after hot rolling, and adjusting the pearlite transformation temperature within an appropriate range, the ferrite fraction, large angle grain boundary density, and subgrain boundary density of the wire Needs to be controlled within an appropriate range. In this way, the wire obtained by adjusting the alloying element content and the adjustment cooling conditions after hot rolling and increasing the large-angle grain boundary density and sub-grain boundary density is another wire material having the same strength level. The present inventors have found that the wire drawing workability and the twisting property after wire drawing are superior.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) The wire according to one embodiment of the present invention has a chemical composition of mass%, C: 0.30 to 0.75%, Si: 0.80 to 2.00%, Mn: 0.30 to 1 0.00%, N: 0.0080% or less, P: 0.030% or less, S: 0.020% or less, O: 0.0070% or less, Al: 0 to 0.050%, Cr: 0 to 1 0.00%, V: 0 to 0.15%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, B: 0 to 0.0040%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0040%, the balance being Fe and impurities, the surface layer portion having a depth in the range of 150 to 400 μm from the surface of the wire, and the diameter of the wire from the central axis of the wire The main structure is a pearlite structure both in the center part that is within the range of / 10, and the transverse direction perpendicular to the length direction of the wire The area ratio of the ferrite structure in the plane is 45% or less, the area ratio of the non-pearlite and non-ferrite structure in the cross section is 5% or less, and the angle difference of the lamellar ferrite crystal orientation in the pearlite structure is 2 °. The density ρ1 of the sub-boundary that is less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and the density ρ2 of the large-angle grain boundary that has an angle difference of 15 ° or more of the ferrite crystal orientation in the entire structure is 200. / Mm or more.
(2) In the wire described in (1) above, the chemical composition may contain Al: 0.010 to 0.050% by mass.
(3) In the wire described in the above (1) or (2), the chemical composition may contain Cr: 0.05 to 1.00% by mass.
(4) In the wire according to any one of the above (1) to (3), the chemical composition is, in mass%, V: 0.005 to 0.15%, Ti: 0.002 to 0.00. One or more selected from the group consisting of 050% and Nb: 0.002 to 0.050% may be contained.
(5) In the wire described in any one of (1) to (4) above, the chemical composition may contain B: 0.0001 to 0.0040% by mass%.
(6) In the wire according to any one of (1) to (5), the chemical composition is Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0 by mass%. One or two selected from the group consisting of .0040% may be contained.
(7) In the wire according to any one of (1) to (6), the density ρ1 of the sub-boundary in the surface layer portion and the central portion of the wire satisfies the following formula 1. Also good.
220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1
(C) in the formula 1 is the C content in mass% in the chemical composition of the wire.
(8) In the wire described in any one of (1) to (7) above, the diameter of the wire may be 3.5 to 7.0 mm.
(9) The wire according to any one of (1) to (8) may be used as a material for a steel wire.
(10) A steel wire according to another aspect of the present invention is produced by drawing a wire according to any one of (1) to (9) above, and has a diameter of 0.5 to 1.. 5 mm.
(11) A method of manufacturing a steel wire according to another aspect of the present invention includes a step of drawing a wire according to any one of (1) to (9) to obtain a steel wire, The diameter of the steel wire is 0.5 to 1.5 mm.

According to the wire according to one aspect of the present invention, a steel wire having high strength suitable for a material such as a wire and excellent torsional characteristics can be stably manufactured while suppressing disconnection during wire drawing. Very useful. Since the steel wire which concerns on 1 aspect of this invention has high intensity | strength and the outstanding twist property, it is suitable as raw materials, such as a wire, and is very useful industrially. The method of manufacturing a steel wire according to an aspect of the present invention can stably manufacture a steel wire having high strength suitable for a wire material and excellent torsional characteristics while suppressing disconnection during wire drawing, It is extremely useful in industry.

It is the schematic which shows the surface layer part and center part of the wire which concern on this embodiment. It is explanatory drawing which shows an example of a pearlite structure | tissue.

Hereinafter, an embodiment which is an example of a wire according to the present invention will be described in detail.
As shown in FIG. 1, in the wire 1 according to the present embodiment, for convenience, a range of 150 to 400 μm in depth from the surface of the wire is defined as the surface layer portion 11, and the diameter d of the wire from the central axis of the wire. Is defined as the central portion 12. In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

The wire of the present embodiment is a steel wire material suitable as a material for a wire that is a reinforcing material for tires of automobiles, a reinforcing wire such as an aluminum power transmission line, a PC steel wire, a rope wire used for a bridge, etc. It can be used as a wire rod.
In addition, the wire drawing workability of a wire is an index indicating the difficulty of occurrence of disconnection when a wire is drawn to obtain a steel wire. The torsional characteristics after wire drawing of wire include the difficulty of delamination and the difficulty of torsional breakage when a torsion test is performed on a steel wire obtained by wire drawing. It is an index showing. The wire according to the present embodiment preferably has a wire drawing workability such that 50 kg of a wire having a diameter of 6.0 mm is prepared and the number of breaks when the wire is drawn to a diameter of 0.5 mm is zero. . Furthermore, it is preferable that the steel wire after wire drawing has a tensile strength of 2800 MPa or more. Moreover, it is preferable that the steel wire used for the wire has a torsion characteristic in which delamination does not occur even once even if ten torsion tests are performed and the average value of the number of twists is 23 times or more. It can be determined that a steel wire having a twist number of 23 or more has sufficient ductility not to break by handling such as straightening after wire drawing.

Next, the chemical composition and microstructure (metal structure) of the wire rod of this embodiment will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) About chemical composition First, the chemical composition of the wire of this embodiment is demonstrated. Hereinafter, the unit of the content of the chemical composition is mass%.

C: 0.30 to 0.75%
C is an element that strengthens steel. In order to obtain this effect, C must be contained in an amount of 0.30% or more. On the other hand, when the C content exceeds 0.75%, the cementite fraction increases and the wire drawing workability decreases. Therefore, the appropriate C content is 0.30% or more and 0.75% or less. Further, from the viewpoint of suppressing crack formation, the C content is preferably 0.35% or more, and more preferably 0.40% or more. On the other hand, from the viewpoint of improving wire drawing workability, the C content is preferably less than 0.75% or 0.70% or less, and more preferably 0.65% or less. The C content may be 0.42% or more, or 0.45% or more. The C content may be 0.60% or less, or 0.55% or less.

Si: 0.80 to 2.00%
Si is a component that not only increases the strength of the wire but also contributes to an increase in subgrain boundary density. However, if the Si content of the wire is less than 0.80%, the effect of increasing the sub-boundary density due to containing Si cannot be sufficiently obtained. On the other hand, if the Si content of the wire exceeds 2.00%, the ferrite fraction increases and the wire drawing workability decreases. Therefore, the Si content of the wire is determined to be in the range of 0.80 to 2.00%. In order to stably obtain a wire having a desired microstructure, the Si content of the wire may be 1.00% or more, 1.15% or more, 1.30% or more, or 1.50% or more. . The Si content of the wire may be 1.90% or less, 1.80% or less, 1.75% or less, or 1.70% or less.

Mn: 0.30 to 1.00%
Mn is an element having an effect of preventing hot brittleness of the steel wire by fixing S in the steel as MnS in addition to the effect of increasing the strength of the steel wire. However, if the Mn content is less than 0.30%, the above effect is not sufficient. For this reason, the lower limit of the Mn content is set to 0.30% or more. Furthermore, in order to realize the strength securing of the steel wire and the prevention of hot brittleness at a higher level, the Mn content is preferably 0.35% or more, more preferably 0.40% or more. . The Mn content may be 0.50% or more, or 0.55% or more.
On the other hand, Mn is an element that easily segregates. When Mn is contained exceeding 1.00%, Mn is concentrated particularly in the central portion, martensite and bainite are generated in the central portion, and wire drawing workability is lowered. In addition, the formation of coarse MnS also contributes to a decrease in wire drawing workability. Mn is preferably 0.90% or less, and more preferably 0.80% or less. The Mn content may be 0.75% or less, or 0.70% or less.

N: 0.0080% or less N is an element that lowers torsional characteristics while increasing the strength of the wire by fixing to dislocations during cold wire drawing. When the N content of the wire exceeds 0.0080%, the torsional characteristics are significantly deteriorated. Therefore, the N content of the wire is regulated to 0.0080% or less. The upper limit with preferable N content is 0.0060% or less or 0.0050% or less. The lower the N content, the better, and N may not be contained in the wire. The N content may be 0.0045% or less, or 0.0040% or less. The N content may be 0.0010% or more, or 0.0025% or more.

P: 0.030% or less P is an element that segregates at the grain boundary of the wire and deteriorates torsional characteristics. When the P content of the wire exceeds 0.030%, the torsional characteristics are significantly deteriorated. Therefore, the P content of the wire is regulated to 0.030% or less. The upper limit of the P content is preferably 0.025% or less. The lower the P content, the better. P may not be contained in the wire. The P content may be 0.020% or less, 0.015% or less, or 0.010% or less. The P content may be 0.002% or more, 0.005% or more, or 0.008% or more.

S: 0.020% or less S is an element that forms MnS and degrades the wire drawing workability. And when S content of a wire exceeds 0.020%, the fall of wire drawing workability will become remarkable. For this reason, the S content of the wire is regulated to 0.020% or less. The upper limit with preferable S content is 0.010% or less. The lower the S content, the better. S may not be contained in the wire. The S content may be 0.015% or less, 0.008% or less, or 0.005% or less. The S content may be 0.001% or more, 0.002% or more, or 0.005% or more.

O: 0.0070% or less O is an element that reduces the ductility of the wire by forming an oxide. When the O content of the wire exceeds 0.0070%, the torsional characteristics are significantly deteriorated. Therefore, the O content of the wire is regulated to 0.0070% or less. The upper limit of the O content is preferably 0.0050% or less. The lower the O content, the better, and O may not be contained in the wire. The O content may be 0.0005% or more, or 0.0010% or more. The O content may be 0.0045% or less, or 0.0040% or less.

(B) About structure of wire Next, the metal structure of the wire concerning this embodiment is explained. In addition, the requirements regarding the metal structure of the wire described below need to be satisfied in both the surface layer portion 11 and the center portion 12 of the wire 1.

The surface layer portion and the central portion of the wire have a pearlite structure as a main structure, and the cross-sectional area of the wire has an area ratio of 45% or less as a ferrite structure, and a non-pearlite and non-ferrite structure has an area ratio as 5% or less. The subgrain boundary density ρ1 at which the angle difference of the crystal orientation of the lamellar ferrite is 2 ° or more and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and the angle difference of the ferrite crystal orientation in the entire structure is 15 ° or more. It is necessary to have a metal structure with a large-angle grain boundary density ρ2 of 200 / mm or more. The “main structure” means a structure occupying the largest area ratio in the metal structure. The “area ratio” means an area ratio measured in a cross section perpendicular to the length direction of the wire, and the measurement method will be described later. In other words, the surface layer part and the center part of the wire according to the present embodiment include a pearlite structure in an area ratio of 50% or more.

The wire having such a metal structure in the surface layer portion and the center portion has a high drawing value during a tensile test and is excellent in wire drawing workability. Moreover, according to the wire having such a metal structure in the surface layer portion and the center portion, when this is drawn into a steel wire having a diameter of 1 mm or less and its tensile strength is 2800 MPa or more, excellent torsional characteristics are obtained. The steel wire which has is obtained. In the metal structure of the wire rod, the remaining main structure (non-pearlite and non-ferrite structure) excluding ferrite structure and pearlite structure is bainite, martensite, and the like.

Here, the grain boundary of the pearlite structure will be supplementarily explained.
In ordinary technical common sense, it is explained that pearlite exhibits a lamellar structure in which lamellar ferrite and lamellar cementite are arranged in layers by a eutectoid reaction generated from austenite, and a hierarchical substructure is formed therein. A region surrounded by a large-angle grain boundary is referred to as a block, and a region having the same lamella orientation in the block is referred to as a colony. In other words, it is recognized that a structure in which a cementite plate is dispersed in each grain of a ferrite structure while having some orientation is pearlite.

However, the actual perlite structure is not so simple. FIG. 2 shows a schematic diagram of an example in which the pearlite structure is simplified. In the metal structure shown in FIG. 2, a block surrounded by a curved large-angle grain boundary 22 is generated starting from an old γ grain boundary 21 (former austenite grain boundary), and a sub-grain boundary 23 is formed in the block. Is formed. The crystal orientation in the block is changed to many random orientations, and in the structure of FIG. 2, the total length of chain lines indicating the subgrain boundaries 23 can be recognized as the total length of the subgrain boundaries 23. Further, in the schematic diagram of FIG. 2, the total length of the large-angle grain boundaries 22 constituting the outer periphery of the block (the length of the thick solid line surrounding the block) can be recognized as the length of the large-angle grain boundaries 22. In FIG. 2, the layered structure of lamellar cementite 31 and lamellar ferrite 32 constituting the lamellar structure is enlarged and displayed.

The “pearlite structure” of the wire according to the present embodiment includes a so-called pseudo pearlite structure (a pearlite structure generated without the lamellar cementite 31 growing in a plate shape). The pseudo pearlite structure is different from a normal pearlite structure in that the lamella cementite 31 is observed to be divided in the block when observed with an SEM. However, in this embodiment, the pearlite structure and the pseudo pearlite structure are handled as the same.

In the actual steel material, in addition to the pearlite structure, other structures are mixed, and the structure is much more complex than the structure of FIG. 2. Therefore, in the wire according to the present embodiment, the subgrain boundaries and the large angle grain boundaries are It is defined as follows. In the pearlite structure, the boundary surface where the difference in crystal orientation between adjacent lamellar ferrites is 2 ° or more and less than 15 ° is called a sub-boundary, and the length of the sub-boundary per unit area of pearlite in the inspection visual field The total is called subgrain boundary density <ρ1>. In addition, the boundary surface where the angle difference between adjacent ferrite crystal orientations is 15 ° or more in the entire structure is called a large-angle grain boundary, and the total length of the large-angle grain boundaries per unit area of the inspection field is the large-angle grain boundary density. It is referred to as <ρ2>. The ferrite used for specifying the large-angle grain boundary includes both a normal ferrite structure and a lamellar ferrite constituting a pearlite structure. Each measuring method will be described later.

<Area ratio of ferrite structure and area ratio of non-pearlite and non-ferrite structure>
The area ratio of the ferrite structure in the cross section of the wire must be 45% or less for both the wire center and the surface layer. When it exceeds 45% at the center of the wire, ferrite is precipitated in a massive and coarse manner, so that the wire drawing workability is lowered. Further, when the area ratio of the ferrite structure is more than 45% in the surface portion of the wire, the number of twists after the wire drawing process is reduced. This is presumably because deformation concentrates on the ferrite portion of the surface layer portion. Note that it is not necessary to particularly define the lower limit value of the area ratio of the ferrite structure. The area ratio of the ferrite structure may be 0% in the center portion or the surface layer portion of the wire. The area ratio of ferrite may be 43% or less, 40% or less, 35% or less, or 30% or less in the center portion or the surface layer portion of the wire. In the center part or surface layer part of the wire, the area ratio of ferrite may be 10% or more, 15% or more, 20% or more, or 27% or more.

Also, the area ratio of non-ferrite and non-pearlite structure needs to be 5% or less. In other words, the total area ratio of the ferrite structure and the pearlite structure needs to be more than 95%. When the non-ferrite and non-pearlite structure exceeds 5%, a crack starting from the non-ferrite and non-pearlite structure is easily formed during the wire drawing process, and the wire drawing workability is lowered. The lower limit of the area ratio of non-ferrite and non-pearlite structure need not be specified. The area ratio of the non-ferrite and non-pearlite structure may be 0% in the center portion or the surface layer portion of the wire. That is, the total area ratio of the ferrite structure and the pearlite structure may be 100%. The area ratio of non-ferrite and non-pearlite structure is 4% or less, 3% or less, 2% or less, or 1% or less (that is, the total area ratio of ferrite structure and pearlite structure is more than 96%, more than 97%, 98% Or over 99%). The area ratio of the non-ferrite and non-pearlite structure may be 1% or more, or 2% or more (that is, the total area ratio of the ferrite structure and the pearlite structure is less than 99% or less than 98%).

<Density ρ1 of the sub-grain boundary where the angle difference of the lamellar ferrite crystal orientation in the pearlite structure is 2 ° or more and less than 15 °>
In the center part and the surface layer part of the wire rod, the subgrain boundary density ρ1 (subgrain boundary density at which the angle difference between the crystal orientations of the lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °) is 70 / mm to 600 / mm. Need to be. By using a wire having such a metal structure, a steel wire having a tensile strength of 2800 MPa or more after wire drawing and excellent twist characteristics can be obtained stably. By setting the subgrain boundary density to 70 / mm or more in the center part and the surface layer part of the wire, it is possible to suppress variations in the strength of the steel wire after wire drawing and to reduce localization of deformation during the torsion test. Therefore, good torsional characteristics can be obtained even with a high-strength steel wire. On the contrary, when the subgrain boundary density is less than 70 / mm at the center part and the surface layer part of the wire, the torsional characteristics are not improved when the tensile strength of the steel wire obtained after the wire drawing is 2800 MPa or more. Further, when the pearlite transformation temperature is less than 600 ° C., the torsional characteristics tend to decrease as described above, and the sub-boundary density at the center portion and the surface layer portion of the wire at this time was over 600 / mm. It is preferable to set the upper limit of 600 / mm. For this reason, the density of the sub-boundary where the difference in crystal orientation of the lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 ° in the center portion and the surface layer portion of the wire is in the range of 70 / mm to 600 / mm. To do. In the surface layer portion or the center portion of the wire, the subgrain boundary density is preferably 100 / mm or more, and more preferably 120 / mm or more. The subgrain boundary density may be 150 / mm or more, or 180 / mm or more in the surface layer portion or the center portion of the wire. The subgrain boundary density may be 550 / mm or less, 500 / mm or less, 400 / mm or less, or 350 / mm or less in the surface layer portion or the center portion of the wire.

In the surface layer portion and the center portion of the wire, the subgrain boundary density ρ1 preferably satisfies the following formula 1. (C) in Formula 1 is the C content in unit mass% in the chemical composition of the wire.
220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1
As the C content in the chemical composition of the wire increases, the area ratio of the ferrite structure in the surface layer portion and the center of the wire decreases, and the area ratio of the pearlite structure increases. It is considered that as the area ratio of the pearlite structure increases, the growth distance of cementite increases and subgrain boundaries are more easily introduced into the pearlite structure. Therefore, the present inventors considered that the preferable range of the subgrain boundary density depends on the C content in the chemical composition of the wire. According to the knowledge of the present inventors, when the subgrain boundary density satisfies the above formula 1 in the surface layer portion and the center portion of the wire, the twist characteristics are further improved by reducing the variation in the twist value of the wire. Is done.

<Density ρ2 of large-angle grain boundaries with an angle difference of 15 ° or more of ferrite crystal orientation in steel structure>
In the surface layer portion and the center portion of the wire rod, the large-angle grain boundary density ρ2 (the density of the large-angle grain boundary where the angle difference of the ferrite crystal orientation is 15 ° or more) needs to be 200 / mm or more. When the large-angle grain boundary density is sufficiently high, the ductility of the wire is high, and the formation of coarse cracks during wire drawing can be suppressed, so that wire drawing workability is improved. On the contrary, when the large-angle grain boundary density is less than 200 / mm in the surface layer portion and the center portion of the wire rod, the wire drawing workability is lowered. For this reason, in the surface layer part and the center part of the wire, the density of the large-angle grain boundaries having an angle difference of 15 ° or more in the ferrite crystal orientation is set in the range of 200 / mm or more. In the surface layer portion or the center portion of the wire, the large angle grain boundary density is preferably 230 / mm or more. The upper limit of the large-angle grain boundary density in the surface layer part or the center part of the wire is not particularly defined, but since it is difficult to produce a large-angle grain boundary density of 500 / mm or more, the large-angle grain in the surface layer part or the center part of the wire It is preferable that the upper limit of the field density is 500 / mm. The large-angle grain boundary density in the surface layer portion or the center portion of the wire may be 220 / mm or more, 250 / mm or more, or 280 / mm or more. The large-angle grain boundary density in the surface layer portion or the center portion of the wire may be 400 / mm or less, 380 / mm or less, or 350 / mm or less.

(C) Evaluation Method Next, a measurement method will be described for each condition of the metal structure of the wire according to this embodiment.

<Tissue area ratio>
The cross-section of the wire (that is, the cross section perpendicular to the length of the wire) is mirror-polished, then corroded with picral, and the surface layer portion is magnified 2000 times using a field emission scanning electron microscope (FE-SEM). 10 places are observed at arbitrary positions in the center, and photographs are taken. The area per field of view is 2.7 × 10 ⁻³ mm ² (vertical 0.045 mm, horizontal 0.060 mm).

Next, a transparent sheet (for example, an OHP (Over Head Projector) sheet) is overlaid on each obtained photograph. In this state, the “ferrite structure” in each transparent sheet is colored. Next, the area ratio of the “colored region” in each transparent sheet is obtained by image analysis software, and the average value is calculated as the average value of the area ratio of the ferrite structure. In this way, the ferrite area ratio can be obtained. Next, another transparent sheet is colored in “a region overlapping with a structure other than the pearlite structure and other than the ferrite structure”, and the area ratio is obtained. The area ratio of the non-pearlite and non-ferrite structure can be obtained by the above method. Since the ferrite structure and the pearlite structure are isotropic structures, the area ratio of the structure in the cross section of the wire is the same as the volume ratio of the structure of the wire. The area ratio of the pearlite structure can be calculated by subtracting the sum of the ferrite area ratio and the non-pearlite and non-ferrite area ratio from 100 area%.

<Subgrain boundary density in pearlite structure and large angle grain boundary density in all structure>
The cross section of the wire (that is, the cut surface perpendicular to the length direction) is mirror-polished, then polished with colloidal silica, and the surface of the wire (at the magnification of 400 times using a field emission scanning electron microscope (FE-SEM)) 4 fields of view are observed at the center and at a depth of 150 to 400 μm from the surface, and EBSD measurement (measurement by electron beam backscatter diffraction method) is performed. The area per field of view is 0.0324 mm ² (vertical 0.18 mm, horizontal 0.18 mm), and the measurement step is 0.3 μm.

Next, the total length of the lines having subgrain boundaries of 2 ° or more and less than 15 ° and the total length of lines having large-angle grain boundaries of 15 ° or more are measured for the obtained results in each measurement field. For example, by using OIM analysis (OIM: Orientation Imaging Microscopy), it is possible to obtain the total length of the lines of the sub-grain boundaries and the total length of the lines of the large-angle grain boundaries. Since the subgrain boundaries exist only in the pearlite structure, the value obtained by dividing the total length of the subgrain boundaries obtained in each measurement visual field by the pearlite area included in each measurement visual field is the subgrain boundary in each measurement visual field. It is defined as the grain boundary density ρ1.

Since the large-angle grain boundary is also present at the boundary between the ferrite structure and the pearlite structure, the value obtained by dividing the total line length of the large-angle grain boundary obtained in each measurement field by the area of each measurement field is the large angle in each measurement field. It is defined as the grain boundary density ρ2.
The average value of the analysis results of the surface layer portion and the central portion is the subgrain boundary density ρ1 of the ferrite crystal orientation angle difference of 2 ° or more and less than 15 ° in the pearlite structure of the surface layer portion and the central portion, and the surface layer portion and the central portion. The large-angle grain boundary density ρ2 at which the angle difference of the ferrite crystal orientation in the entire structure is 15 ° or more. Since the EBSD result is greatly influenced by noise, the result CI (confidence index) uses a result of 0.60 or more, and those with a CI of 0.10 or less are removed as noise. In addition, removal of CI below 0.10 is possible within OIM analysis.

As described above, the values of the sub-boundary density ρ1 and the large-angle grain boundary density ρ2 are not only in the above-described range in the surface portion of the wire (in the range of depth of 150 to 400 μm from the surface), but also in the center of the wire. It must be in range. Even when the subgrain boundary density ρ1 in the central portion of the wire is in the range of 70 / mm to 600 / mm and the large angle grain boundary density ρ2 is in the range of 200 / mm or more, the surface layer portion is not in the above range, or Even if it is within the range, if the central portion is not within the above range, the desired characteristics as a wire cannot be obtained. If it can be confirmed that ρ1 and ρ2 of the wire surface layer portion and ρ1 and ρ2 of the wire center portion are within the above range, it can be recognized that ρ1 and ρ2 are within the above range for the entire wire.

(D) Manufacturing Method In the manufacturing method of the wire according to the present embodiment, various conditions during pearlite transformation are optimized and the structure is controlled in order to improve the torsional characteristics of the wire.
The wire satisfying the above requirements of the wire according to the present embodiment can obtain the effects of the wire according to the present embodiment regardless of the manufacturing method. For example, according to the manufacturing method shown below, the wire according to the present embodiment can be obtained. What is necessary is just to manufacture a wire. In addition, the following manufacturing process is an example, and even when the wire composition whose chemical composition and other requirements are within the range of the wire material according to the present embodiment is obtained by a process other than the following, the wire material is included in the present invention. Needless to say, it is included.

First, after melting steel so that it may become the said component, a steel piece is manufactured by continuous casting and hot rolling is performed. In addition, you may perform lump rolling after casting. When the obtained steel slab is hot-rolled, the steel slab is heated to 1000 to 1250 ° C., and the finishing temperature is 900 to 1000 ° C. and hot-rolled to φ5.5 to 7.0 mm.
The heating temperature of the steel slab before hot rolling is 1000 ° C. or more and 1250 ° C. or less. This is because when the heating temperature of the steel slab is less than 1000 ° C., the reaction force during hot rolling increases, and when the heating temperature of the steel slab exceeds 1250 ° C., decarburization proceeds.
The finish rolling temperature of hot rolling is 900 ° C. or higher. This is because if the finish rolling temperature is less than 900 ° C., the reaction force of the finish rolling increases and the shape accuracy deteriorates. On the other hand, finish rolling temperature shall be 1000 degrees C or less. This is because, when hot rolling is performed at a temperature higher than 1000 ° C., the austenite grain size increases and the large-angle grain boundary density after pearlite transformation decreases.

After the hot rolling, the following four stages of cooling are performed to adjust the ferrite area ratio, subgrain boundary density, and large angle grain boundary density. In the primary cooling, the purpose is to suppress austenite grain growth and generate a fine austenite structure by cooling at a high cooling rate. The purpose of secondary cooling is to reduce the temperature in order to reduce the temperature difference between the wire surface layer part and the central part during the primary cooling, and to make the temperature uniform from the wire surface layer part to the central part. The purpose of the tertiary cooling is to cool the target pearlite transformation temperature to a target pearlite transformation temperature at a cooling rate capable of cooling as uniformly as possible from the surface portion of the wire rod to the center portion and suppressing the ferrite transformation. In quaternary cooling, the pearlite transformation is carried out so that the sub-grain boundary density and the large-angle grain boundary density are within the target ranges by performing cooling to make the pearlite transformation as uniform as possible from the surface of the wire to the center. With the goal. Details are shown below. The average cooling rate of primary to quaternary cooling described below is the amount of decrease in wire temperature from the start to the end of primary to quaternary cooling, expressed as the time from the start to the end of primary to quaternary cooling. Divided value. The ultimate temperature of primary to quaternary cooling is the temperature of the wire at the end of primary to quaternary cooling.

After the hot rolling, primary cooling is performed by water cooling to 830 to 870 ° C. within an average cooling rate of 50 to 200 ° C./second. In addition, the start and completion | finish of primary cooling are the start and completion | finish of spraying of a refrigerant | coolant (water).
When the average cooling rate in the temperature range of 870 ° C. or higher where the grain growth rate is high is less than 50 ° C./second and the time existing in this temperature range is long, the grain growth of austenite is promoted, so after pearlite transformation The large-angle grain boundary density will decrease. Although there is no upper limit of the average cooling rate in the primary cooling, an average cooling rate exceeding 200 ° C./second is difficult due to the limitations of the production equipment, and therefore the upper limit of the average cooling rate in the primary cooling is set to 200 ° C./second or less.
If the ultimate temperature in primary cooling is less than 830 ° C., ferrite transformation may proceed in a large amount only in the surface layer portion, and the ferrite area ratio in the surface layer portion increases, making it difficult to control to 45% or less. Therefore, the ultimate temperature in primary cooling is set to 830 ° C. or higher. When cooling is stopped at a temperature exceeding 870 ° C., austenite grains grow large and the large-angle grain boundary density after pearlite transformation decreases. Therefore, the ultimate temperature in the primary cooling is set to 870 ° C. or less.

Thereafter, secondary cooling is performed by air cooling in the atmosphere at an average cooling rate of less than 5 ° C./second to a range of 790 ° C. or more and 820 ° C. or less. Note that the start time of the secondary cooling is equal to the end time of the refrigerant blowing in the primary cooling, and the end time of the secondary cooling is equal to the start time of the refrigerant blowing in the tertiary cooling. The secondary cooling is cooling for reducing the temperature difference between the surface layer portion and the center portion of the wire that occurs during the primary cooling, and making the pearlite transformation temperature from the wire surface layer portion to the center portion uniform.
When the average cooling rate is 5 ° C./second or more in the secondary cooling, the temperature difference between the surface layer portion and the center portion remains, and after the pearlite transformation, the large-angle grain boundary density and subgrain boundary density of the surface layer of the wire. Can control the large-angle grain boundary density at the center of the wire. Therefore, the average cooling rate in the secondary cooling is less than 5 ° C./second.
If the ultimate temperature of the secondary cooling is less than 790 ° C., ferrite transformation may occur and the ferrite area ratio may be improved. Therefore, the ultimate temperature of the secondary cooling is 790 ° C. or higher. On the other hand, when the secondary cooling is stopped above 820 ° C., the temperature difference up to the pearlite transformation temperature between the surface layer portion and the center portion of the wire becomes large, and the temperature difference between the surface layer portion and the center portion again during the third cooling. Occurs. Therefore, the ultimate temperature for secondary cooling is set to 820 ° C. or lower. In steel types with a high Si content, the Ac1 temperature shifts to the high temperature side, so the ultimate temperature in secondary cooling is particularly important.
The secondary cooling time (elapsed time between the start and end of the secondary cooling) is preferably 5 seconds or more and 12 seconds or less. This is because when a secondary cooling time of more than 12 seconds is applied, grain growth of austenite grains is promoted. On the other hand, in the secondary cooling time within 5 seconds, a temperature difference in the wire may remain.

Thereafter, tertiary cooling is performed by blast cooling to an average cooling rate of more than 20 ° C./second and not more than 30 ° C./second to a range of 600 ° C. to 620 ° C. The start and end of the tertiary cooling are the start and end of air blowing. In tertiary cooling, cooling is performed to a pearlite transformation temperature at which optimum subgrain boundary density and large angle grain boundary density can be obtained at a cooling rate capable of suppressing ferrite transformation.
If the average cooling rate of the tertiary cooling is 20 ° C./second or less, ferrite transformation occurs and the ferrite area ratio becomes excessive. Therefore, the average cooling rate is over 20 ° C./second. On the other hand, when the tertiary cooling is performed at an average cooling rate exceeding 30 ° C./second, only the surface layer of the wire is cooled to the target temperature, and the fourth cooling is started in the state where the temperature of the center of the wire is excessive. . Therefore, an average cooling rate shall be 30 degrees C / sec or less.
When the ultimate temperature in the tertiary cooling is less than 600 ° C., the pearlite structure becomes excessively strong and the twisting characteristics are deteriorated. Therefore, the ultimate temperature of the tertiary cooling is 600 ° C. or higher. On the other hand, when the ultimate temperature of the tertiary cooling exceeds 620 ° C., the pearlite transformation temperature becomes high, the large-angle grain boundary density and the sub-grain boundary density are lowered, and the tensile strength after the pearlite transformation is also lowered. Therefore, the ultimate temperature of the tertiary cooling is set to 620 ° C. or less.

After that, quaternary cooling is performed to 550 ° C. or less at an average cooling rate of 10 ° C./second or less by air cooling with air. Note that the time point of starting the fourth cooling is equal to the time point of ending the air blowing in the third cooling. The end of the fourth cooling is the time when the air cooling is stopped, that is, the time when reheating or blowing of the refrigerant is started on the wire. However, when air cooling is performed until the temperature of the wire becomes 550 ° C. or less, the time when the temperature of the wire reaches 550 ° C. is regarded as the end of the fourth cooling. The purpose of quaternary cooling is to obtain a wire having uniform large-angle grain boundary density and sub-grain boundary density from the surface layer to the center by reducing the temperature difference in the cross section of the wire during pearlite transformation.
When the average cooling rate in the quaternary cooling is more than 10 ° C./second, the temperature change of the surface layer is large, and the subgrain boundary density decreases. Therefore, the average cooling rate in the fourth cooling is set to 10 ° C./second or less. The lower limit of the average cooling rate in the quaternary cooling is not limited, but the cooling rate when the wire is allowed to cool is usually 2 ° C./second or more. Therefore, 2 ° C./second may be set as the lower limit of the average cooling rate in the fourth cooling.
When the ultimate temperature of the fourth cooling exceeds 550 ° C., the pearlite transformation may not be completed. Therefore, the ultimate temperature of the fourth cooling is 550 ° C. or less. Since the influence of the cooling rate in the temperature range of 550 ° C. or less on the tissue is slight, accelerated cooling such as water cooling may be performed after the fourth cooling is performed to a temperature of 550 ° C. or less. In the examples to be described later, the present invention example is cooled to room temperature by cooling to 550 ° C. or lower by quaternary cooling, but the same applies even when cooled by other cooling means after completion of quaternary cooling. An organization is formed.

(E) About optional components:
The wire rod according to the present embodiment is replaced with at least one or two selected from the group consisting of Al, Cr, V, Ti, Nb, B, Ca, and Mg as necessary instead of a part of the remaining Fe. You may contain the above element. However, since the wire according to the present embodiment can solve the problem without including these optional elements, the lower limit value of these optional elements is 0%. Hereinafter, the effect of the optional elements Al, Cr, V, Ti, Nb, B, Ca, Mg and the reason for limiting the content will be described. % For optional ingredients is% by weight.

Al: 0 to 0.050%
Al may not be contained in the wire of this embodiment. Al is an element that precipitates as AlN and can increase the large-angle grain boundary density with an angle difference of 15 ° or more of the ferrite crystal orientation. In order to obtain the effect with certainty, it is preferable to contain 0.010% or more of Al. On the other hand, since Al is an element that easily forms hard oxide inclusions, when the Al content of the wire exceeds 0.050%, coarse oxide inclusions are remarkably easily formed. The deterioration of torsional characteristics becomes remarkable. Therefore, the upper limit of the Al content of the wire is 0.050%. The upper limit with preferable Al content is 0.040% or less, A more preferable upper limit is 0.035% or less, Furthermore, a preferable upper limit is 0.030% or less.

Cr: 0 to 1.00%
Cr may not be contained in the wire rod according to the present embodiment. Cr, like Mn, is an element that increases the hardenability of the steel and increases the strength of the steel. In order to reliably obtain this effect, it is preferable to contain 0.05% or more of Cr. On the other hand, when the Cr content exceeds 1.00%, the torsional characteristics deteriorate. Therefore, the Cr content is 1.00% or less. In addition, when raising the hardenability of steel, it is preferable to contain Cr 0.10% or more, and it is still more preferable to contain 0.30% or more. The upper limit of Cr is preferably 0.90% or less, and more preferably 0.80% or less.

V: 0 to 0.15%
V may not be contained in the wire rod of this embodiment. V combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect, improving the torsional properties of steel There is. In order to reliably obtain this effect, it is preferable to contain 0.005% or more of V. From the viewpoint of improving torsional characteristics, the V content is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the content of V exceeds 0.15%, not only the effect is saturated, but also the steel manufacturability such as cracking in the steel slab in the step of rolling the steel ingot or slab into the steel slab. V content is set to 0.15% or less. The V content is preferably 0.10% or less, and more preferably 0.07% or less.

Ti: 0 to 0.050%
Ti may not be contained in the wire rod of the present embodiment. Ti combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect, improving the torsional properties of steel There is. In order to obtain this effect with certainty, Ti is preferably contained in an amount of 0.002% or more. From the viewpoint of improving torsional characteristics, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, when the Ti content exceeds 0.050%, not only the effect is saturated, but also the steel manufacturability such as cracking in the steel slab in the step of rolling the steel ingot or slab into the steel slab. Adversely affect. Therefore, the Ti content is 0.050% or less. The Ti content is more preferably 0.025% or less.

Nb: 0 to 0.050%
Nb may not be contained in the wire rod of this embodiment. Nb combines with N and C to form carbides, nitrides or carbonitrides, and has the effect of refining austenite grains during hot rolling due to their pinning effect, improving the torsional properties of steel There is. In order to reliably obtain this effect, Nb is preferably contained in an amount of 0.002% or more. From the viewpoint of improving torsional characteristics, the Nb content is more preferably 0.003% or more, and even more preferably 0.004% or more. On the other hand, when the Nb content exceeds 0.050%, not only the effect is saturated, but also the steel productivity such as cracking in the steel slab in the step of rolling the steel ingot or slab into the steel slab. Nb content is 0.050% or less. The Nb content is more preferably 0.030% or less.

B: 0 to 0.0040%
B may not be contained in the wire rod of this embodiment. When B is contained in a small amount, there is an effect of reducing the ferrite structure of the steel. When it is desired to obtain the effect with certainty, it is preferable to contain 0.0001% or more of B. Even if more than 0.0040% of B is contained, not only the effect is saturated, but also a coarse nitride is generated, and the torsional characteristics are deteriorated. Therefore, if B is included, the B content is 0.0040% or less. In order to increase the area ratio of the pearlite structure, the B content is preferably 0.0004% or more, and more preferably 0.0007% or more. The content of B for improving torsional characteristics is preferably 0.0035% or less, and more preferably 0.0030% or less.

Ca: 0 to 0.0050%
Ca may not be contained in the wire rod of the present embodiment. Ca has the effect of dissolving in MnS and finely dispersing MnS. By finely dispersing MnS, disconnection during wire drawing due to MnS can be suppressed. In order to ensure the effect of Ca, Ca is preferably contained in an amount of 0.0002% or more. In order to obtain a higher effect, 0.0005% or more of Ca may be contained. However, when the Ca content exceeds 0.0050%, the effect is saturated. Furthermore, if the Ca content exceeds 0.0050%, the oxide produced by reaction with oxygen in the steel becomes coarse, which leads to a reduction in wire drawing workability. Therefore, the appropriate Ca content when contained is 0.0050% or less. The Ca content is preferably 0.0030% or less, and more preferably 0.0025% or less.

Mg: 0 to 0.0040%
Mg does not need to be contained in the wire rod of this embodiment. Mg is a deoxidizing element and generates an oxide. However, it also has an effect of finely dispersing MnS because it is an element having a correlation with MnS by generating sulfide. This effect can suppress disconnection during wire drawing due to MnS. In order to ensure the effect of Mg, Mg is preferably contained in an amount of 0.0002% or more. In order to obtain a higher effect, 0.0005% or more of Mg may be contained. However, when the Mg content exceeds 0.0040%, the effect is saturated and a large amount of MgS is generated, which leads to a decrease in wire drawing workability. Therefore, when Mg is contained, the appropriate Mg content is 0.0040% or less. The Mg content is preferably 0.0035% or less, and more preferably 0.0030% or less.

The balance of the chemical composition includes “Fe and impurities”. “Impurity” refers to what is mixed into steel materials from ores, scraps, or production environments as raw materials when industrially producing steel materials.

The diameter of the wire according to the present embodiment is not particularly limited, but the diameter of the wire currently distributed in the market is usually 3.5 to 7.0 mm, so this is the diameter of the wire according to the present embodiment. The upper and lower limit values may be used. When the diameter of the wire is 3.5 mm or more, it is preferable because the burden of hot rolling at the time of manufacturing the wire can be reduced. When the diameter of the wire is 7.0 mm or less, it is preferable because the amount of wire drawing strain during wire drawing of the wire can be suppressed.

The steel wire according to another aspect of the present invention is obtained by drawing a wire according to the present embodiment. The diameter of the steel wire is usually 0.5 to 1.5 mm in consideration of the application. Since the steel wire according to the present embodiment is the raw material, the chemical composition of the wire according to the present embodiment, the structure of the metal structure, the sub-boundary density ρ1, and the large-angle grain boundary density ρ2 are within the above-described ranges. Has excellent tensile strength and torsional properties.

In addition, since the steel wire regarding this embodiment is manufactured through a wire drawing process with a very large amount of strain, its metal structure has undergone significant deformation. For example, when the enlarged photograph of the cross section of the steel wire which concerns on this embodiment is seen, the phase surrounded by the grain boundary is crushed remarkably, and the kind cannot be distinguished. It is also extremely difficult to identify the existence of sub-grain boundaries and large-angle grain boundaries. That is, it is extremely difficult to specify the metal structure and other configurations of the steel wire according to the present embodiment by a normal structure specifying method (for example, taking a metal structure photograph and crystal structure analysis by EBSD). It is impossible or nearly impractical to directly identify the metal structure of the steel wire according to this embodiment by its structure or characteristics.

A method for manufacturing a steel wire according to another aspect of the present invention includes a step of drawing a wire according to the present embodiment. The wire drawing is performed with a surface reduction rate such that the diameter of the steel wire finally obtained is 0.5 to 1.5 mm. Since the chemical composition of the wire according to the present embodiment, the structure of the metal structure, the subgrain boundary density ρ1, and the large angle grain boundary density ρ2 are within the above-described ranges, the production of the steel wire according to the present embodiment using this is used. The method can suppress the number of breaks to an extremely low level, and can obtain a steel wire having excellent tensile strength and torsional characteristics.

Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited to the following examples.

Steels having chemical compositions shown in Tables 1 and 2 were melted, and wires were produced by the following method. The notation “-” in Tables 1 and 2 indicates that the content of the element is at the impurity level and it can be determined that the element is not substantially contained. The balance of the chemical composition of the steels shown in Tables 1 and 2 is iron and impurities.

First, after steel A having the chemical composition shown in Table 1 was melted by a converter, a billet having a square size of 122 mm was obtained by block rolling by a normal method. Next, the steel slab was heated to 1050 to 1150 ° C. and then hot-rolled to φ6 mm at a finishing temperature of 900 to 1000 ° C.

The adjustment cooling after finish rolling was performed under the conditions shown in (A1) to (A21) shown in Tables 3-1 to 3-3.
Specifically, with regard to (A1) to (A7), after cooling to 830 to 870 ° C. (primary cooling) within a range of an average cooling rate of 50 to 200 ° C./second by water cooling, air cooling by air is then performed. Was air-cooled (secondary cooling) to a range of 790 ° C. or higher and 820 ° C. or lower at an average cooling rate of less than 5 ° C./second. Thereafter, it is cooled to 20 ° C./second to 30 ° C./second to 600 to 620 ° C. (third cooling), is cooled to 550 ° C. or lower at 10 ° C./second (quaternary cooling), and then allowed to cool to room temperature. Cooled down to
With regard to (A8) to (A17), four types of adjustment cooling were performed under conditions different from the above cooling conditions to obtain a wire. The values with underline in Table 3-1 are inappropriate values in the production conditions of the wire according to the present invention.

Regarding (A18) to (A21), the four types of adjustment cooling were not performed, but the cooling was performed under the conditions shown in Table 3-2 to Table 3-4. Note that terms such as “primary cooling” in these tables are merely for distinguishing cooling stages, and are different from primary cooling to quaternary cooling included in the production method of the present invention.
Specifically, regarding (A18), immersion in a salt bath at 550 ° C. was performed instead of the tertiary cooling and the fourth cooling in the production method of the present invention.
Regarding (A19), reheating to 950 ° C. and holding for 60 seconds is performed on the wire after completion of the above hot rolling, and immersion in a 550 ° C. salt bath immediately after the end of this temperature holding. Carried out.
Regarding (A20), after implementing the primary cooling, cooling was performed by air blowing, cooling to 680 ° C. at an average of 1.0 ° C./sec, switching to standing cooling, and cooling to 550 ° C. or less.
Regarding (A21), after performing primary cooling, blast cooling was performed, the wire was cooled to 700 ° C. at 10 ° C./second, and then cooled to 550 ° C. or less at 5 ° C./second by air cooling.

Further, hot rolled wire rods were prepared from steels a to z having chemical compositions shown in Table 2 by the same method as (A1) in Table 3-1. Thereafter, dry wire drawing, plating, and wet wire drawing were performed to obtain a steel wire having a wire diameter of 0.5 mm. Values underlined in Table 2 are outside the desirable range of the present invention.

For the wires having test numbers A1 to A21 and test numbers 1 to 26 obtained as described above, tensile strength, drawing value, ferrite area ratio, non-pearlite and non-ferrite area ratio, subgrain boundary density ρ1 in the pearlite structure (Density of subgrain boundaries where the angle difference of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °), Large grain boundary density ρ2 (Large angle where the angle difference of the ferrite crystal orientation is 15 ° or more in the entire observation structure) Grain boundary density) was determined. The pearlite area ratio of each wire is a value obtained by dividing the ferrite area ratio and the non-pearlite and non-ferrite area ratio from 100%.

The results are shown in Tables 4-1 to 4-3 and Tables 5-1 to 5-3 below. Values underlined in Table 4-1, Table 4-2, Table 5-1, and Table 5-2 are values that are outside the scope of the present invention. Values underlined in Table 4-3 and Table 5-3 are values that do not satisfy the acceptance criteria of the present invention.

When the area ratio of the ferrite structure in the surface layer and the center of the wire, the area ratio of the non-ferrite and non-pearlite structure, the subgrain boundary density ρ1, the large angle grain boundary density ρ2, and the diameter of 6 mm are drawn to a diameter of 0.5 mm Wire breakage, wire strength before wire drawing and steel wire after wire drawing (steel wire strength), and torsional characteristics of wire after wire drawing (twisting, twisting variation, and delamination) The presence or absence) was investigated by the method described below.

<1> Area ratio of ferrite structure of wire, area ratio of non-ferrite and non-pearlite structure:
After the cross section of the wire was mirror-polished, it was corroded with picral, and the FE-SEM was used to observe any 10 locations on the surface and the center of the wire at a magnification of 2000 times, and photographed. The area per field of view is 2.7 × 10 ⁻³ mm ² (vertical 0.045 mm, horizontal 0.060 mm). Each photograph obtained was overlaid with an OHP sheet, and the “ferrite structure” and “regions that were non-perlite and overlapped with the non-ferrite structure” in each transparent sheet were painted. Subsequently, the area ratio of the “colored region” in each transparent sheet was obtained by image analysis software, and the average values were calculated as the average values of the area ratios of the ferrite structure, non-perlite and non-ferrite structure, respectively.

<2> Subgrain boundary density ρ1 and large angle grain boundary density ρ2 of the wire:
The cross section of the wire rod is mirror-polished, then polished with colloidal silica, and observed at four magnifications at the wire surface layer portion and the center portion using a FE-SEM at a magnification of 400 times, and EBSD measurement manufactured by TSL (Tex SEM Laboratories) Analysis was performed using the apparatus. The measurement area was 180 × 180 μm ² and the step was 0.3 μm. Next, for each result obtained, the total length of the sub-boundary line having an angle difference of 2 ° or more and less than 15 ° using the OIM analysis and the total length of the line of the large-angle grain boundary having an angle difference of 15 ° or more are used. Each was measured. Dividing the total length of sub-boundary lines having an angle difference of 2 ° or more and less than 15 ° by the average value of pearlite area ratio, the sub-boundary density is obtained, and the line of large-angle boundaries having an angle difference of 15 ° or more Was divided by the area of one field of view to determine the large-angle grain boundary density.

<3> Wiredrawing workability of wire The wiredrawing was performed on each 50 kg wire, and the number of wire breaks during wire drawing was recorded. In addition, when the frequency | count of disconnection was 3 times or more, the wire drawing process after the 3rd disconnection was stopped. And, when the number of breaks when drawing a 50 kg wire from a diameter of 6.0 mm to a diameter of 0.5 mm is 0, the wire drawing workability is evaluated as good, and when the number of breaks is 1 or more, It was evaluated that the wire drawing workability was poor. In addition, about the wire which stopped wire drawing, it judged that it was clearly unsuitable as a material of a steel wire, and the subsequent evaluation test was not implemented. Items that were not evaluated were marked with a symbol “-”.

<4> Tensile strength of wire rod and steel wire after wire drawing:
A wire and a steel wire were cut into a length of 200 mm, and a tensile test was performed by fixing the upper and lower sides 50 mm with a wedge chuck or an air chuck. The tensile strength was calculated by dividing the obtained maximum load by the cross-sectional area. After that, the wire diameter of the thinnest part after the tensile test of the wire is measured, and the amount of change in the cross-sectional area before and after the tensile test is divided by the cross-sectional area before the tensile test, and is drawn by multiplying by 100%. The value was calculated.

Since the steel wire used for the wire, which is a reinforcing material for automobile tires, preferably has a tensile strength of 2800 MPa or more, the tensile strength of 2800 MPa or more was evaluated as an acceptable product. In addition, regarding the tensile strength of the wire, no pass / fail criterion was provided.

<5> Torsional characteristics of steel wire after wire drawing:
In the torsion test, a steel wire 100 times as long as the wire diameter (diameter) was twisted until it was broken at 15 rpm, whether or not delamination occurred was judged by a torque (resistance force against torsion) curve, and the number of twists was measured. . The determination on the torque curve was performed by a method of determining that delamination occurred when the torque suddenly decreased before the disconnection. The torsion test was performed for 10 steel wires, and no delamination occurred. When the average number of twists of 10 steel wires was 23 times or more, it was evaluated that the torsional characteristics were good. .

Further, when the variation in the number of twists in the ten-time torsion test is small, it can be determined that the torsional characteristics are better. Therefore, variation in the number of twists of 10 steel wires (the difference between the maximum value of the number of twists of 10 steel wires and the above average value, and the minimum value of the number of twists of 10 steel wires and the above average value). The larger of the differences was calculated. A steel wire having a variation of 3 times or less was judged to have a good variation in the number of twists.
A steel wire in which the average value of the number of twists, the delamination, and the variation in the number of twists are all judged to be good has very good twist characteristics. However, even if the variation in the number of twists of the steel wire is more than three times, the steel wire judged to be good with respect to other torsional property evaluations has good torsional properties even in view of its assumed use. I can say that.

The results of each evaluation are summarized in the table below.

As shown in the table, the samples A1 to A7, which are examples of the present invention, all satisfy the requirements of the present invention, and the steel production conditions are appropriate. Therefore, the strength after wire drawing is 2800 MPa or more. In addition, the number of twists was 23 or more and no delamination occurred, and the wire was free from problems.

On the other hand, in the sample of A8, the average cooling rate in the primary cooling was low, and the austenite grain size was coarsened, so that ρ2 was lowered, disconnection occurred during wire drawing, and wire drawing workability was poor.
In the sample of A9, since the temperature reached in the primary cooling was low, the ferrite area ratio increased in the surface layer and the number of twists decreased.
In the sample of A10, the ultimate temperature in the primary cooling was high and the austenite grain size was coarsened, so that ρ2 was lowered and disconnection occurred.
In the sample of A11, the time for secondary cooling was long, and since the austenite grain size was coarsened, ρ2 was lowered and disconnection occurred.
In the sample of A12, since the ultimate temperature at the secondary cooling was low, the ferrite area ratio was high, the wire drawing workability was poor, and the steel wire strength and torsional characteristics were both low.
In the sample of A13, the average cooling rate in the tertiary cooling is small, the ferrite transformation proceeds, the ferrite area ratio is high, the wire drawing workability is poor, and the steel wire strength and torsional characteristics are both low.
In the sample of A14, the ultimate temperature of the tertiary cooling was high, the pearlite transformation was performed at a high temperature and both ρ1 and ρ2 were low, breakage occurred at the time of wire drawing, and the torsional characteristics were also bad.
In the sample of A15, the ultimate temperature in the third cooling was low and ρ1 was too high, so the torsional characteristics were poor.
In the sample of A16, the average cooling rate in the fourth cooling was high, ρ1 in the surface layer portion of the wire decreased, delamination occurred during the torsion test, and the torsional characteristics were poor.
The sample of A17 was obtained under the manufacturing conditions in which air cooling is stopped and blast cooling is started when the wire temperature reaches the temperature shown in the table in the fourth cooling. In the sample of A17, the ultimate temperature in the fourth cooling was high, the pearlite transformation was not completed, and the non-pearlite and non-ferrite area ratio was high, so that the wire drawing workability was lowered.
In the sample of A18, since the wire was immersed in a 550 ° C. salt bath after the secondary cooling, the wire was rapidly cooled to 550 ° C. As a result, in A18, ρ1 was high, delamination occurred during the torsion test, and the torsional characteristics were poor.
In the sample of A19, after reheating the wire and maintaining the temperature, the wire was immersed in a salt bath at 550 ° C., so that the wire was rapidly cooled to 550 ° C. As a result, ρ1 was high in A19, delamination occurred during the torsion test, and the torsional characteristics were poor.
In the sample of A20, the cooling rate after hot rolling was slow, and pearlite transformation occurred at a high temperature. Since the pearlite transformation temperature was high, both ρ1 and ρ2 were low in A20, breakage occurred during wire drawing, and the torsional characteristics were also poor.
In the sample of A21, the wire rod was cooled to 700 ° C by blast cooling after the secondary cooling, so the surface layer portion of the wire rod was rapidly cooled, ρ1 of the surface layer portion was increased, and delamination occurred during the torsion test. The torsional characteristics were bad.

Further, from the results shown in the table, the samples of Test Nos. 1 to 19 and 26, which are examples of the present invention, satisfy the desirable range of the present invention, and the wire production conditions are also appropriate. It has good workability, good torsional characteristics after wire drawing, and has the necessary tensile strength.

However, in the sample of test number 20, the C content was low, the ferrite area ratio was too large, and the steel wire was insufficient in strength.
The sample of Test No. 21 had a high C content, and the steel was hardened excessively, so that the wire drawing workability was lowered and wire breakage occurred during the wire drawing.
The sample of test number 22 had low ρ1 due to the low Si content, and delamination occurred during the torsion test.
In the sample of test number 23, the Mn content was too high, and there were many non-ferrite and non-pearlite structures.
The sample of test number 24 had a low Si content, low ρ1, and delamination occurred during the torsion test.
The sample of test number 25 had a low Mn content, a low ρ1, and delamination occurred during the torsion test.

From the results shown in the table, C, Si, Mn, N, P, and S are wires defined in the desirable ranges described above, the main structure is pearlite, the ferrite structure is 45% or less, and the non-ferrite And the non-pearlite structure is 5% or less, and the subgrain boundary density ρ1 at which the difference in crystal orientation of the lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, If the wire satisfying that the large-angle grain boundary density ρ2 at which the angle difference of the ferrite crystal orientation at 15 ° or more is 200 / mm or more is obtained, a high tensile strength is obtained after the wire drawing, and the wire drawing is performed. It has been found that it is possible to provide a wire rod for wire drawing with good wire drawing workability that can produce a steel wire that can be stably twisted without causing delamination during a torsion test after processing. That is, the present invention provides a wire rod for wire drawing capable of stably producing a steel wire having high strength suitable as a material such as a wire and further having excellent torsional characteristics while suppressing disconnection during wire drawing. I was able to.

DESCRIPTION OF SYMBOLS 1 Wire 11 Surface layer part 12 Center part 21 Old gamma grain boundary 22 Large angle grain boundary 23 Subgrain boundary 31 Lamellar cementite 32 Lamellar ferrite

Claims (11)

1. A wire,
   Chemical composition is mass%,
   C: 0.30% to 0.75%,
   Si: 0.80 to 2.00%
   Mn: 0.30 to 1.00%
   N: 0.0080% or less,
   P: 0.030% or less,
   S: 0.020% or less,
   O: 0.0070% or less,
   Al: 0 to 0.050%,
   Cr: 0 to 1.00%,
   V: 0 to 0.15%,
   Ti: 0 to 0.050%,
   Nb: 0 to 0.050%,
   B: 0 to 0.0040%,
   Ca: 0 to 0.0050%, and Mg: 0 to 0.0040%
   And the balance consists of Fe and impurities,
   The main structure is a pearlite structure in both the surface layer portion having a depth of 150 to 400 μm from the surface of the wire and the center portion having a range of 1/10 of the diameter of the wire from the central axis of the wire. The area ratio of the ferrite structure in the cross section perpendicular to the length direction of the wire is 45% or less, the area ratio of the non-pearlite and non-ferrite structure in the cross section is 5% or less, The density ρ1 of the sub-boundary where the angle difference of the crystal orientation of lamellar ferrite is 2 ° or more and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and the angle difference of the ferrite crystal orientation in the whole structure is 15 °. A wire rod having a large-angle grain boundary density ρ2 of 200 / mm or more.
2. The chemical composition is mass%,
   Al: 0.010 to 0.050%
   The wire according to claim 1, comprising:
3. The chemical composition is mass%,
   Cr: 0.05-1.00%
   The wire according to claim 1 or 2, characterized by comprising:
4. The chemical composition is mass%,
   V: 0.005 to 0.15%,
   Ti: 0.002 to 0.050%, and Nb: 0.002 to 0.050%
   The wire according to any one of claims 1 to 3, comprising one or more selected from the group consisting of:
5. The chemical composition is mass%,
   B: 0.0001 to 0.0040%
   The wire according to any one of claims 1 to 4, characterized by comprising:
6. The chemical composition is mass%,
   Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0040%
   The wire according to any one of claims 1 to 5, which contains one or two selected from the group consisting of:
7. The wire according to any one of claims 1 to 6, wherein the density ρ1 of the sub-boundary satisfies the following formula 1 in the surface layer portion and the central portion of the wire.
   220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1
   (C) in the formula 1 is the C content in mass% in the chemical composition of the wire.
8. The wire according to any one of claims 1 to 7, wherein the diameter of the wire is 3.5 to 7.0 mm.
9. The wire according to any one of claims 1 to 8, wherein the wire is used as a material of a steel wire.
10. It is manufactured by drawing a wire according to any one of claims 1 to 9,
    A steel wire having a diameter of 0.5 to 1.5 mm.
11. A step of drawing a wire according to any one of claims 1 to 9 to obtain a steel wire,
    A method for producing a steel wire, wherein the steel wire has a diameter of 0.5 to 1.5 mm.

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