TW201900888A - Wire, steel wire, and steel wire manufacturing method - Google Patents

Abstract

One embodiment of the present invention is a wire rod, which has a predetermined chemical composition, and in which, in both of a surface layer and a central layer, main structure is a pearlite structure, an area ratio of a ferrite structure is 45% or less, an area ratio of a structure which is not pearlite or ferrite is 5% or less, a density [rho]1 of sub-boundary in the pearlite structure, at which an angular difference between crystal orientation of lamellar ferrites is 2 DEG or more and less than 15 DEG, satisfies 70/mm ≤ [rho]1 ≤ 600/mm, and a density [rho]2 of high angle grain boundary in all structures, at which an angular difference between crystal orientation of ferrites is 15 DEG or more, satisfies 70/mm ≤ [rho]1 ≤ 600/mm, is 200/mm or more.

Description

Wire rod, steel wire, and manufacturing method of steel wire

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

BACKGROUND OF THE INVENTION The present invention relates to a wire widely used as a base material for high-strength steel wires. The aforementioned high-strength steel wires are metal wires for reinforcing materials used in tires of automobiles and the like, and reinforcing metal wires for aluminum transmission lines. , PC steel wire and metal wire for bridges and other ropes. The present invention also relates to a steel wire obtained from the wire rod and a method for manufacturing a steel wire using the wire rod.

The wire is manufactured by hot rolling, and is drawn to a predetermined wire diameter, thereby being processed into a metal wire. During the wire drawing process, about one or two times of toughening treatment is performed, and the wire drawing process is performed until it becomes a thin steel wire. Therefore, high wire drawing workability is required for the wire.

For a reinforcing material having a wire diameter of 0.5 mm or more used in, for example, large-scale automobile tires, productivity improvement is required. In addition, a wire rod capable of stably manufacturing a steel wire with a wire diameter of 0.5 mm to 1.5 mm from a wire rod with a wire diameter of 3.5 mm or more can be manufactured at a low cost. The wire rod with a wire diameter of 3.5 mm or more can be manufactured by stable hot rolling. Therefore, a wire is continuously being developed which has a drawability that can omit the intermediate toughening step performed during the draw processing and can exhibit stable torsional characteristics after the draw processing.

However, a metal wire manufactured through the steps of wire drawing processing to a high wire drawing processing degree may be more prone to wire breakage during wire drawing processing. In addition, the steel wire subjected to wire drawing to a high wire drawing degree tends to deteriorate in torsional characteristics. Moreover, the material of the steel wire, that is, the diameter of the wire is thick, which is disadvantageous for the torsional properties of the steel wire.

In order to prevent disconnection in the wire drawing process, various methods have been proposed to improve the structure of the wire. As such a technology, for example, Patent Document 1 (Japanese Patent Laid-Open No. 2014-055316) proposes a high-strength steel wire rod, which is present on a number basis based on the total number of lamellar skeletal carbon iron. There are 50% or more layered scheming carbon iron with an aspect ratio of 10 or more, and the wire for high-strength steel wire can prevent the wire drawing processability from being lowered by forming the layered skeletal carbon iron.

In addition, Patent Document 2 (Japanese Patent Laid-Open No. 2000-119756) proposes a wire rod for high-strength steel wires, which includes a fraction of primary iron particles of less than 10% and the remainder is cis-carbon Iron (cementite) is discontinuously formed by pearlite, which can prevent the wire drawability from being lowered.

These technologies are designed to control the wire shape of the clear carbon iron and make the wire drawability good, thereby preventing the cup shape from occurring in the wire drawing step until the steel wire with a wire diameter of 0.1 to 0.4 mm is obtained. Break (cuppy break), etc. However, merely controlling the morphology of Xueming carbon iron cannot suppress the strength variation in the cross section of the steel wire. 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 balance the occurrence of wire breakage and the deterioration of torsional characteristics, and the above-mentioned problems may occur.

Based on the above, it is desired to realize a wire rod that is unlikely to be broken in the steps of manufacturing a steel wire with a large diameter (for example, a wire diameter of 0.5 mm or more) such as wire drawing to a high wire drawing degree, and after the wire drawing processing The torsional characteristics are good.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 2014-055316 Patent Literature 2: Japanese Patent Laid-Open No. 2000-119756

SUMMARY OF THE INVENTION Problems to be Solved by the Invention The present invention has been made in view of the above circumstances, and an object thereof is to provide a wire that can stably manufacture steel wires while suppressing wire breaks during wire drawing processing. The steel wires are also suitable as thick-diameter metals. The base material of wire and the like has high strength and excellent torsional properties. Another object of the present invention is to provide a steel wire having high strength and excellent torsional characteristics, and a method for manufacturing the same.

Means for Solving the Problems The present inventors have conducted various studies in order to solve the aforementioned problems. As a result, the following knowledge insights were obtained.

(I) When drawing a wire having a wire diameter of 5.5 mm or more to a wire diameter of 0.5 mm, the wire drawing strain will be 4.5 or more. When the eutectoid steel or the hypereutectoid steel with poor drawability is used for the draw processing at the above-mentioned high draw processing degree, toughening during the draw processing may be required. On the other hand, it can be seen that by using a hypo-eutectoid steel with a small carbon-iron fraction as the material of the wire, the wire drawability can be improved, and the wire can be supplied to a wire drawing strain of 4.5 or more. Wire drawing processing.

(II) On the other hand, it can be seen that when the area ratio of the ferrous iron is greater than 45% in the central portion of the cross section of the wire of the hypoeutectoid steel (that is, the cut surface of the wire at right angles to the length direction), The structure becomes lumpy and coarse, so the wire drawability is not sufficient even for a wire of hypoeutectoid steel. In addition, when the crystal grain size becomes coarse (that is, when the high-angle grain boundary density is small), the shrinkage of the wire is low and the ductility is poor. Therefore, in the wire drawing process, it is easy to form coarse cracks inside the wire, leading to wire drawing. Degradability. In addition, it can be seen that when the cross section of the wire of the hypoeutectoid steel has an area ratio of ferrous iron in the surface layer portion of more than 45%, the torsional characteristics after the wire drawing process will decrease. This is thought to be due to the fact that the deformation is concentrated in the ferritic iron structure.

(III) After the metamorphosis of Plei iron, the secondary grain boundaries are introduced into a large amount of layered ferrous iron (hereinafter referred to as layered ferrous iron) in the Plei iron structure. The present inventors investigated the relationship between the secondary grain boundary density of the wire rod and the torsional characteristics (hereinafter sometimes simply referred to as torsional characteristics) after the wire rod is drawn. As a result, it was found that basically, the larger the secondary grain boundary density in the Plei iron structure, the better the torsional characteristics can be obtained. This is considered to be due to the fact that the more sub-grain boundaries, the more uniformly the processing strain is introduced in the wire drawing process, and the strength variation in the cross section of the steel wire is reduced.

(IV) The inventors of the present invention have studied the means for increasing the density of the secondary grain boundaries. The secondary grain boundary is considered to be eliminated when the lamellar ferrous iron and the lamellar carbon iron (hereinafter referred to as lamellar carbon iron) in the lamellar structure grow in coordination when the boron iron is transformed. The two phases are misfitted and introduced. The present inventors have found that the density of the secondary grain boundary can be adjusted by using the transformation temperature of Plei iron and the content of alloying elements (such as Si) in the layered ferrous iron.

If the relationship between the Plei iron transformation temperature and the secondary grain boundary density is specifically explained, it can be seen that when the Plei iron transformation temperature is below 550 ° C, the lower the Plei iron transformation temperature is, the lower the grain boundary density in the layered fertilizer iron is. Will decrease. This is thought to be due to an increase in the number of places where the growth of lamellar skimmer carbon iron is cut off. On the other hand, in a region where the abnormal temperature of the boron iron is higher than 550 ° C., as the temperature becomes higher and higher, the sub-grain boundary density tends to gradually decrease. It is considered that when the abnormal temperature of Plei iron is higher than 550 ° C, the higher the temperature, the coarser the lamellar interval will be, and the number of lamellar schiff carbon iron will decrease, leading to misfit. The total amount is reduced, so the secondary grain boundary density in the layered fertilized iron will gradually decrease. From these results, it was found that when the cooling conditions were controlled so that the Plei iron metamorphosis temperature was near 550 ° C, the largest number of secondary grain boundaries were introduced.

In addition, if the relationship between the amount of alloying elements and the density of the sub-grain boundaries is specifically explained, it is considered that by increasing the content of alloying elements such as Si, the interface between the lamellar ferrous iron and the lamellar skimmer carbon iron Mismatches increase and the density of the sub-grain boundaries increases.

(V) However, based on the above knowledge and knowledge, experiments were repeated to increase the secondary grain boundary density in the Plei iron structure, and it was confirmed that although the secondary grain boundary density was large, the torsional properties after wire drawing were still low. Although the reason is not clear, when the sporadic iron is deformed below 600 ° C. and the secondary grain boundary density is increased, it is confirmed that the number of twists after wire drawing tends to decrease. Therefore, it is believed that the density of the secondary grain boundaries is not around 550 ° C, which can maximize the density of the secondary grain boundaries, but that the wave iron is metamorphic at 600 ° C to 620 ° C, so that the secondary grain boundary density in the layered fertilizer iron does not increase excessively In this way, it is possible to obtain a wire that has both wire drawability and twist characteristics after wire drawing.

Based on the above-mentioned knowledge (I) ~ (V), in order to realize the wire rod that can stably manufacture the steel wire while suppressing the disconnection in the wire drawing process, it is necessary to use hypoeutectoid steel as the material of the wire rod. The aforementioned steel wire is also suitable as The base material of thick-diameter metal wire has high strength and excellent torsional properties. In addition, it is also necessary to adjust the cooling conditions after adjusting the alloying element content and hot rolling delay, and adjust the wave iron transformation temperature to an appropriate range, so as to adjust the ferrous grain iron fraction, high-angle grain boundary density, The grain boundary density is controlled in an appropriate range. The present inventors have found that the wire produced by adjusting the alloy element content and adjusting the cooling conditions after hot rolling as described above to increase the high-angle grain boundary density and the sub-grain boundary density has a higher strength level than other wires Wire rods are more excellent in wire drawability and torsional properties after wire draw.

The present invention has been completed based on the above knowledge and knowledge, and the gist thereof is as follows. (1) One aspect of the wire of the present invention has a chemical composition in terms of mass%: C: 0.30 ~ 0.75%, Si: 0.80 ~ 2.00%, Mn: 0.30 ~ 1.00%, N: 0.0080% or less, P: 0.030 % Or less, S: 0.020% or less, O: 0.0070% or less, Al: 0 ~ 0.050%, Cr: 0 ~ 1.00%, V: 0 ~ 0.15%, Ti: 0 ~ 0.050%, Nb: 0 ~ 0.050%, B: 0 ~ 0.0040%, Ca: 0 ~ 0.0050%, and Mg: 0 ~ 0.0040%, and the remainder is composed of Fe and impurities; the main structure of the surface layer and the central portion of the wire is the Pleistic iron structure The aforementioned surface layer portion ranges from a depth of 150 to 400 μm from the surface of the aforementioned wire, and the aforementioned central portion ranges from the central axis of the aforementioned wire to 1/10 of the diameter of the aforementioned wire; the transverse direction of the aforementioned wire at right angles to the length direction The area ratio of the ferrous iron structure in the cross section is less than 45%, and the area ratio of the non-pole iron and non-fertile iron structure in the aforementioned cross section is less than 5%; the crystal orientation of the lamellar fertilizer in the aforementioned iron structure The density ρ1 of the secondary grain boundary with an angle difference of 2 ° or more and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and the angle difference of the crystal orientation of the ferrous grains in the whole organization is 15 ° or more Density ρ2 angle grain boundaries is 200 / mm or more. (2) In the wire described in the above (1), the aforementioned chemical composition may also contain Al: 0.010 to 0.050% in terms of mass%. (3) In the wire described in the above (1) or (2), the aforementioned chemical composition may also contain Cr in a mass% of 0.05 to 1.00%. (4) In the wire described in any one of (1) to (3), the chemical composition may include one or two or more members selected from the group consisting of: : 0.005 to 0.15%, Ti: 0.002 to 0.050%, and Nb: 0.002 to 0.050%. (5) In the wire described in any one of (1) to (4) above, the aforementioned chemical composition may also contain B: 0.0001 to 0.0040% in mass%. (6) In the wire described in any one of (1) to (5) above, the aforementioned chemical composition may contain one or two selected from the group consisting of: Ca: 0.0002 ~ 0.0050%, and Mg: 0.0002 ~ 0.0040%. (7) In the wire described in any one of the above (1) to (6), in the surface layer portion and the center portion of the wire, the density ρ1 of the subgrain boundary may also satisfy the following formula 1. 220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1 (C) in the aforementioned Formula 1 is the C content in mass% in the aforementioned chemical composition of the aforementioned wire. (8) In the wire described in any one of (1) to (7), the diameter of the wire may be 3.5 to 7.0 mm. (9) The wire described in any one of the above (1) to (8) can also be used as a material for a steel wire. (10) Another aspect of the present invention is a steel wire manufactured by subjecting the wire described in any one of (1) to (9) to a wire drawing process, and the diameter of the steel wire is 0.5 to 1.5 mm . (11) A method for manufacturing a steel wire according to another aspect of the present invention includes a step of preparing a steel wire by subjecting the wire described in any one of (1) to (9) to a wire drawing process, and The diameter is 0.5 ~ 1.5mm.

ADVANTAGE OF THE INVENTION According to one aspect of the present invention, a wire can be stably manufactured while suppressing disconnection in the wire drawing process, which is extremely useful in the industry. The aforementioned steel wire is suitable as a base material for metal wires and has high strength and excellent twist. characteristic. The steel wire of one aspect of the present invention has high strength and excellent torsional properties, and is therefore suitable as a green material for a metal wire, for example, and is extremely useful in industry. The manufacturing method of one aspect of the present invention can stably manufacture the steel wire by suppressing the disconnection in the wire drawing process, so it is extremely useful in the industry. The aforementioned steel wire is suitable for the metal material and has high strength and excellent twist. characteristic.

Embodiments of the Invention Hereinafter, embodiments of an example of the wire of the present invention will be described in detail. As shown in FIG. 1, for convenience, in the wire 1 of this embodiment, a range of a depth of 150 to 400 μm from the surface of the wire is defined as the surface layer portion 11, and the diameter from the central axis of the wire to the wire diameter d is defined. A range of 1/10 is defined as the center portion 12. In addition, in this specification, a numerical range expressed using "~" means a range including numerical values described before and after "~" as a lower limit value and an upper limit value.

The wire of this embodiment is a wire that can be used as a steel wire material, and the aforementioned steel wire is suitable as a metal wire for reinforcing a tire of an automobile, a metal wire for reinforcing an aluminum transmission wire, a PC steel wire, and a bridge, etc. Metal materials such as wire for ropes. In addition, the drawability of a wire rod is an index showing how easy it is to break when a wire rod is drawn to obtain a steel wire. The torsional characteristics after wire drawing of a wire rod are indicators showing the ease with which layer peeling occurs and the ease with which twisted wire breakage occurs when a steel wire is subjected to a wire rope torsion test. The wire of this embodiment preferably has the following wire drawability: when 50 kg of a wire having a diameter of 6.0 mm is prepared and drawn to a diameter of 0.5 mm, the number of wire breaks is 0. Moreover, the tensile strength of the steel wire after the wire drawing process is preferably 2800 MPa or more. In addition, the steel wire used for the metal wire should preferably have the following torsional properties: Ten twist tests were performed on 10 wires without peeling off once, and the average number of twists was 23 or more. Steel wires with more than 23 twists can be judged to have sufficient ductility, and they will not break under processing such as correction after wire drawing.

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

(A) Chemical composition First, the chemical composition of the wire of the present embodiment will be described. Hereinafter, the content unit of the chemical composition is mass%.

C: 0.30 to 0.75% C is an element that strengthens steel. To obtain this effect, C must be contained in an amount of 0.30% or more. On the other hand, if the C content is more than 0.75%, the Xueming carbon-iron fraction becomes large, and the wire drawability decreases. Therefore, the appropriate C content is 0.30% or more and 0.75% or less. Furthermore, from the viewpoint of suppressing the formation of cracks, the C content is preferably set to 0.35% or more, and more preferably 0.40% or more. On the other hand, from the viewpoint of improving the drawability, it is preferable to set the C content to less than 0.75% or 0.70%, and more preferably 0.65% or less. The C content may be set to be 0.42% or more and 0.45% or more. In addition, the C content may be set to 0.60% or less and 0.55% or less.

Si: 0.80 to 2.00% Si is a component that not only improves the strength of the wire but also contributes to the density of the secondary grain boundaries. However, if the Si content of the wire is less than 0.80%, the effect of increasing the density of the secondary grain boundaries due to the Si content will not be fully obtained. On the other hand, if the Si content of the wire is more than 2.00%, the iron content of the fertilizer particles will increase, and the drawability will decrease. Therefore, the Si content of the wire is set in the range of 0.80 to 2.00%. In addition, in order to stably obtain a wire having a desired microstructure, the Si content of the wire may be set to 1.00% or more, 1.15% or more, 1.30% or more, or 1.50% or more. Also, the Si content of the wire can be set to 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 that, in addition to improving the strength of the steel wire, also has the effect of fixing S in the steel to MnS to prevent hot brittleness of the wire. However, if the Mn content is less than 0.30%, the above effects are insufficient. Therefore, the lower limit of the Mn content is set to 0.30% or more. In addition, in order to achieve a higher level of ensuring the strength of the steel wire and preventing hot brittleness, it is desirable to set the Mn content to 0.35% or more, and preferably to 0.40% or more. The Mn content may be set to 0.50% or more and 0.55% or more. On the other hand, Mn is an element liable to segregation. When it contains more than 1.00% of Mn, Mn will especially become thicker in the central part, and in the central part, Asada loose iron or toughened iron will be generated, resulting in a decrease in wire drawability. In addition, the formation of coarse MnS is also one of the reasons for reducing the drawability. Mn should preferably be 0.90% or less, and more preferably 0.80% or less. The Mn content may be set to 0.75% or less or 0.70% or less.

N: 0.0080% or less N is an element that can be fixed to the differential row in cold drawn wire processing to increase the strength of the wire, but also causes a reduction in torsional characteristics. If the N content of the wire is greater than 0.0080%, the torsional characteristics will be significantly reduced. Therefore, the N content of the wire is limited to 0.0080% or less. A preferable upper limit of the N content is 0.0060% or less or 0.0050% or less. In addition, the lower the N content, the better, and N may not be contained in the wire. The N content may be set to 0.0045% or less or 0.0040% or less. In addition, the N content may be set to be 0.0010% or more or 0.0025% or more.

P: 0.030% or less P is an element that segregates at the grain boundaries of the wire and causes a reduction in torsional characteristics. If the P content of the wire is greater than 0.030%, the torsional characteristics will be significantly reduced. Therefore, the P content of the wire is limited to 0.030% or less. The upper limit of the P content is preferably 0.025% or less. In addition, the lower the P content, the better, and P may not be contained in the wire. The P content may be set to 0.020% or less, 0.015% or less, or 0.010% or less. In addition, the P content may be set to 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 reduces the drawability. Moreover, if the S content of the wire is greater than 0.020%, the wire drawability will be significantly reduced. Therefore, the S content of the wire is limited to 0.020% or less. A preferable upper limit of the 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 set to 0.015% or less, 0.008% or less, or 0.005% or less. In addition, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

O: 0.0070% or less O is an element that forms oxides and reduces the ductility of the wire. If the O content of the wire is greater than 0.0070%, the torsional characteristics will be significantly reduced. Therefore, the O content of the wire is limited to 0.0070% or less. The upper limit of the O content is preferably 0.0050% or less. In addition, the lower the O content, the better, and O may not be contained in the wire. The O content may be set to be 0.0005% or more or 0.0010% or more. In addition, the O content may be set to 0.0045% or less and 0.0040% or less.

(B) Structure of wire rod Next, the metal structure of the wire rod according to this embodiment will be described. In addition, both the surface layer portion 11 and the center portion 12 of the wire rod 1 must satisfy the requirements for the metal structure of the wire rod described below.

The surface layer and the central part of the wire must have the following metal structure: the main structure is the Plei iron structure, the cross section of the wire, the area ratio of 45% or less is the fertile grain iron structure, and the non-pole iron and non-ferrous grain iron structure The area ratio is 5% or less, and the angular difference between the crystal orientations of the lamellar ferrous grain iron in the boron iron structure is 2 ° or more and less than 15 °. The secondary grain boundary density ρ1 is 70 / mm ≦ ρ1 ≦ 600 / mm, And the high-angle grain boundary density ρ2 of the angle difference of the crystal orientation of the ferrous grains in the whole structure is 15 ° or more is 200 / mm or more. The "major structure" means a structure that occupies the largest area ratio among metal structures. The "area ratio" means an area ratio measured in a cross section of a wire at a right angle to the length direction, and a measurement method thereof will be described in detail later. The above-mentioned requirements related to the amount of boron iron, in other words, the surface layer portion and the center portion of the wire of this embodiment include 50% or more of boron iron structure in terms of area ratio.

The wire having the above-mentioned metal structure in the surface layer portion and the central portion has a high shrinkage ratio during a tensile test and is excellent in wire drawability. In addition, according to the wire having the above-mentioned metal structure in the surface layer portion and the central portion, when it is drawn into a steel wire with a diameter of 1 mm or less and the tensile strength is 2800 MPa or more, a wire with excellent torsional properties can be obtained. Steel wire. In addition, among the metal structures of the wire rods, the main structures (non-poleite and non-ferrous iron) other than the ferrous iron structure and the non-ferrous iron structure are toughened iron and Asada loose iron.

Here, the grain boundaries of the boron iron structure will be explained. In common technical common sense, bolete is described as a layered lamellar structure of lamellar fat iron and lamellar scheming carbon iron due to the eutectoid reaction produced by Vostian iron. , And a hierarchical lower organization is formed inside. The area surrounded by the high-angle grain boundary is called a crystal block, and the area in which the layers have the same orientation is called a colony. In other words, a structure in which the Schmidt carbon iron plate has several orientations and is dispersed in each grain of the ferrous iron structure is identified as boron iron.

However, we do not think that the actual Pleiadian organization is so simple. FIG. 2 is a schematic diagram showing an example in which the boletide tissue has been purified. In the metal structure shown in FIG. 2, a crystal block surrounded by a curved high-angle grain boundary 22 is generated starting from the old γ grain boundary 21 (old Vostian iron grain boundary), and secondary crystals are formed in the crystal block.界 23. The crystal orientation in the crystal block changes into multiple random orientations, and the total length of the chain line length representing the sub-grain boundary 23 can be identified as the total length of the sub-grain boundary 23 in terms of the structure of FIG. 2. In addition, in the schematic diagram of FIG. 2, the length of the high-angle grain boundary 22 (the length of the thick solid line surrounding the crystal block) constituting the periphery of the crystal block and the like can be identified as the length of the high-angle grain boundary 22 in total. In addition, in FIG. 2, the lamellar structure of the lamellar skimmer carbon iron 31 and the lamellar fat iron 32 constituting the lamellar structure is enlarged and displayed in advance.

In addition, the "poleite structure" of the wire of the present embodiment is a substance containing a so-called pseudo-poleite structure (a poleite structure generated when the lamellar scheming carbon iron 31 does not grow into a plate shape). . The pseudo-wrought iron structure is different from the normal wrought iron structure in that the lamellar schiff carbon iron 31 can be confirmed to be cut in the crystal block when observed by SEM. However, in the present embodiment, the Pellet structure and the pseudo-Pellet structure are treated as the same thing.

In actual steel, in addition to the boron iron structure, there are other structures, and the structure is far more complicated than the structure in Fig. 2. Therefore, for the wire of this embodiment, the subgrain boundary is defined as follows With high-angle grain boundaries. The interface where the angle difference between the crystal orientations of adjacent lamellar pieces of ferrous iron in the Plei iron structure is 2 ° or more and less than 15 ° is called a subgrain boundary, and the per unit area of the Plei iron in the inspection field of view is The total length of the secondary grain boundaries is called the secondary grain boundary density <ρ1>. In addition, the interface where the angle difference of the crystal orientation of adjacent ferrous grains in the whole organization is more than 15 ° is called a high-angle grain boundary, and the total length of the high-angle grain boundary per unit area of the inspection field is called a high-angle grain boundary. Boundary density <ρ2>. The ferritic iron used to specify the high-angle grain boundary is a product containing both a normal ferrous iron structure and a lamellar ferrous iron constituting the polyiron structure. The respective measurement methods will be described later.

<Area ratio of ferrous iron structure and area ratio of non-polite iron and non-ferrous iron structure> The area ratio of ferrous iron structure in the cross section of the wire must be 45% or less in the central part and surface layer of the wire . When the central portion of the wire is larger than 45%, the ferrous iron precipitates massively and coarsely, and thus the drawability is reduced. In addition, when the area ratio of the ferrous grain iron structure in the surface layer portion of the wire is greater than 45%, the number of twists after the wire drawing process is reduced. This is thought to be due to the fact that the deformation is concentrated in the ferrous iron portion of the surface layer portion. In addition, the lower limit value of the area ratio of the ferrous iron structure need not be specified. In the central portion or the surface layer portion of the wire, the area ratio of the ferrous grain iron structure may also be 0%. The area ratio of the ferrous iron in the central part or the surface layer part of the wire rod may also be set to 43% or less, 40% or less, 35% or less, or 30% or less. In addition, the area ratio of the ferrous iron in the central portion or the surface layer portion of the wire rod may be set to 10% or more, 15% or more, 20% or more, or 27% or more.

In addition, the area ratio of non-fertilized iron and non-polite iron must be 5% or less. In other words, the total area ratio of the ferrous iron organization and the boron iron organization must be greater than 95%. When the non-fertilized iron and the non-pulletized iron structure is greater than 5%, cracks starting from the non-fertilized iron and the non-pulletized iron structure are likely to be formed in the drawing process, resulting in a decrease in the drawability. In addition, the lower limit value of the area ratio of non-fertilized iron and non-polite iron need not be specified. In the central part or the surface part of the wire rod, the area ratio of the non-fertilized iron and non-pole iron structure may also be 0%. That is, the total area ratio of the ferrous grain iron structure and the boron iron structure may be 100%. It is also possible to set the non-ferrous grain iron and the area ratio of the non-pole iron structure to 4% or less, to 3%, to 2% or to 1% or less (that is, to set the total area ratio of the ferrous iron structure and the pole iron structure). (Greater than 96%, greater than 97%, greater than 98%, or greater than 99%). In addition, the area ratio of non-ferrous iron and non-pole iron structure can be set to be 1% or more than 2%. %).

<Density ρ1 of the secondary grain boundaries of the lamellar ferrous grains in the Plei iron structure with an angular difference of 2 ° or more and less than 15 °> The density of the secondary grain boundaries ρ1 (Pola The angular difference between the crystalline orientations of the lamellar fertilizer particles in the structure and the sub-grain boundaries between 2 ° and 15 ° must be 70 / mm ~ 600 / mm. Since it is a wire having the above-mentioned metal structure, it is possible to stably obtain a steel wire having a tensile strength of 2800 MPa or more and excellent torsional properties after wire drawing. By setting the secondary grain boundary density to 70 / mm or more in the central portion and the surface layer portion of the wire, the variation in the strength of the steel wire after wire drawing can be suppressed, and the localization of deformation in the torsion test can be reduced. Good torsional properties can be obtained even for high-strength steel wires. On the contrary, if the density of the secondary grain boundaries in the central part and the surface layer part of the wire is less than 70 / mm, the torsional properties will not be improved when the tensile strength of the steel wire obtained after the wire drawing is above 2800 MPa. In addition, when the transformation temperature of Plei iron is lower than 600 ° C, the torsional characteristics tend to decrease as described above, and at this time, the density of the secondary grain boundaries in the central part and the surface layer part of the wire is greater than 600 / mm, so the upper limit is set Set to 600 / mm. Therefore, in the central part and surface layer part of the wire, the angle difference between the crystal orientations of the lamellar ferrous grains in the Plei iron structure is greater than 2 ° and less than 15 °. The density of the secondary grain boundaries is set at 70 / mm ~ 600. / mm range. In the surface layer portion or the central portion of the wire, the secondary grain boundary density should preferably be 100 / mm or more, and more preferably 120 / mm or more. It is also possible to set the secondary grain boundary density of the surface layer part or the center part of the wire to be 150 / mm or more or 180 / mm or more. In addition, the density of the secondary grain boundaries in the surface layer portion or the central portion of the wire may be 550 / mm or less, 500 / mm or less, 400 / mm or 350 / mm or less.

In the surface layer portion and the central portion of the wire rod, the subgrain boundary density ρ1 should preferably satisfy the following formula 1. (C) in Formula 1 is the C content in unit mass% in the chemical composition of a wire. 220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1 The larger the C content in the chemical composition of the wire, the more the area ratio of the ferrous iron in the surface layer and the center of the wire becomes. Is small, and the area ratio of the Plei iron structure becomes larger. In addition, it is considered that the larger the area ratio of the Plei iron structure, the greater the growth distance of the cuming iron, and the easier it is for the subgrain boundaries to be introduced into the Plei iron structure. Therefore, the present inventors believe that the preferred range of the sub-grain boundary density is related to the C content in the chemical composition of the wire. According to the knowledge of the present inventors, when the secondary grain boundary density of the surface layer portion and the central portion of the wire rod satisfies the above formula 1, the variation of the twist value of the wire rod becomes smaller, thereby further improving the torsional characteristics.

<Density ρ2 of the high-angle grain boundary where the angular difference of the crystal orientation of the ferrous iron in the steel structure is 15 ° or more> In the surface layer portion and the center of the wire, the high-angle grain boundary density ρ2 The density of high-angle grain boundaries at 15 ° or more must be 200 / mm or more. When the high-angle grain boundary density is large enough, the ductility of the wire is high, and the formation of coarse cracks in the wire drawing process can be suppressed, so the wire drawability is improved. On the contrary, if the density of the high-angle grain boundaries in the surface layer portion and the center portion of the wire is less than 200 / mm, the wire drawability will decrease. Therefore, in the surface layer portion and the central portion of the wire rod, the density of the high-angle grain boundaries having an angular difference of the crystal orientation of the ferrous grains of 15 ° or more is set within the range of 200 / mm or more. And in the surface layer part or the center part of the wire, the high-angle grain boundary density should be set to 230 / mm or more. Although the upper limit of the high-angle grain boundary density in the surface layer portion or the center portion of the wire rod is not specifically set, it will be difficult to manufacture the high-angle grain boundary density of 500 / mm or more. Therefore, the surface layer portion or the center portion of the wire rod is The upper limit of the medium-high angle grain boundary density is preferably set to 500 / mm. It is also possible to set the density of the high-angle grain boundary in the surface layer part or the center part of the wire to be 220 / mm or more, 250 / mm or more, or 280 / mm or more. In addition, the density of the high-angle grain boundary in the surface layer portion or the central portion of the wire rod may be 400 / mm or less, 380 / mm or 350 / mm or less.

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

＜ Area ratio of organization ＞ After the cross section of the wire (that is, the cross section of the wire at right angles to the length direction) is mirror-polished, it is corroded with a picral etchant, and a field emission scanning electron microscope (FE -SEM) Observing 10 points at any position of the surface layer part and the center part at a magnification of 2000 times, and taking photos. The area per one field of view is 2.7 × 10 ^-3 mm ² (0.045 mm in length and 0.060 mm in width).

Next, a transparent sheet (for example, OHP (Over Head Projector) sheet) is superimposed on each of the obtained photos. In this state, the color of "fat grain iron structure" was painted on each transparent sheet. Then, the area ratio of the "painted area" on each transparent sheet was calculated by the image analysis software, and the average value was calculated as the average value of the area ratio of the ferrous iron structure. By performing in the above manner, the area ratio of ferrous iron can be obtained. Next, on other transparent sheets, "area overlapping with a structure other than the boron iron structure and a structure other than the ferrous iron structure" was painted with a color, and the area ratio was calculated. By the above method, the area ratio of non-polite iron and non-fertilized iron can be obtained. In addition, since the ferrous iron structure and the boron iron structure are isotropic structures, the area ratio of the structure in the cross section of the wire rod is the same as the volume ratio of the wire rod structure. The area ratio of the boron iron structure can be calculated by subtracting the area ratio of ferrous iron from 100 area% and the area ratio of non-pole iron and non-ferrous iron.

<Second grain boundary density in the boron iron structure and high-angle grain boundary density in the whole structure> After the cross section of the wire (that is, the cut surface at right angles to the length direction) is mirror-polished, it is polished with colloidal silicon oxide. A field emission scanning electron microscope (FE-SEM) was used to observe four fields of view in the surface layer of the wire (with a depth ranging from 150 to 400 μm from the surface) and the center of the wire at a magnification of 400 times. Measurement by backscatter diffraction method). The area per field of view was set to 0.0324 mm ² (0.18 mm in length and 0.18 mm in width), and the interval between measurements was set to 0.3 μm.

Then, with respect to the obtained results of the respective measurement fields, the total length of the lines at the sub-grain boundaries of 2 ° or more and less than 15 °, and the total length of the lines at the high-angle grain boundaries of 15 ° or more were measured. The total line length of the sub-grain boundary and the total line length of the high-angle grain boundary can be obtained by using, for example, OIM analysis (OIM: Orientation Imaging Microscopy). Since the sub-grain boundaries exist only in the portion of the Plei iron structure, the value obtained by dividing the total length of the lines of the sub-grain boundaries obtained in each measurement field by the area of the Plei iron contained in each measurement field is defined as each measurement. Secondary grain boundary density ρ1 in the field of view.

Since the high-angle grain boundary also exists in the boundary between the ferrous grain iron structure and the boletic iron structure, the value obtained by dividing the total line length of the high-angle grain boundary obtained in each measurement field by the area of each measurement field is defined as High-angle grain boundary density ρ2 in each measurement field of view. The average value of the analysis results of the surface layer portion and the central portion is set as the subgrain boundary density ρ1 of the angle difference of the crystal orientation of the ferrous grains in the boron iron structure of the surface layer portion and the central portion is 2 ° or more and less than 15 ° The high-angle grain boundary density ρ2 of the angle difference of the crystal orientation of the ferrous grains in the entire structure of the surface layer portion and the central portion is 15 ° or more. Furthermore, the EBSD result is greatly affected by noise. Therefore, it is set to use a result with an average CI (confidence index) of 0.60 or higher, or remove a CI of 0.10 or lower as noise. The removal of CI below 0.10 can be performed in OIM analysis.

As described above, the values of the secondary grain boundary density ρ1 and the high-angle grain boundary density ρ2 must not only be in the foregoing range in the surface layer portion of the wire (a range from the surface depth of 150 to 400 μm), but also must be in the same range in the center portion of the wire. Even if the secondary grain boundary density ρ1 in the center portion of the wire rod is in the range of 70 / mm to 600 / mm and the high-angle grain boundary density ρ2 is 200 / mm or more, when the surface layer portion is not in the above range, or even in the wire rod surface layer When the part is in the above range but the center part is not in the above range, the characteristics sought for the purpose cannot be obtained as far as the wire is concerned. As long as it can be confirmed that ρ1 and ρ2 of the surface layer portion of the wire and ρ1 and ρ2 of the center portion of the wire are within the above range, it can be recognized that ρ1 and ρ2 in the entire wire are within the above range.

(D) Related manufacturing method In the manufacturing method of the wire of this embodiment, in order to improve the torsional characteristics of the wire, various conditions when the wave iron is deformed are appropriately adjusted to control the structure. The wire that satisfies the above-mentioned requirements of the wire of this embodiment can obtain the effect of the wire of this embodiment regardless of the manufacturing method. As long as the wire of this embodiment is manufactured by the manufacturing method shown below, for example, . In addition, the following manufacturing procedure is an example, and even when a wire having a chemical composition and other requirements within the scope of the wire of this embodiment is obtained by a procedure other than the following, the wire is naturally included in the present invention.

First, steel is melted so as to have the above-mentioned composition, and then a steel sheet is manufactured by continuous casting and hot rolled. Alternatively, block rolling may be performed after casting. When the steel sheet obtained by hot rolling is heated, the steel sheet is heated to 1000 to 1250 ° C, the completion temperature is set to 900 to 1000 ° C, and the hot rolling is rolled to φ5.5 to 7.0mm. The heating temperature of the steel sheet before hot rolling is set to 1000 ° C or higher and 1250 ° C or lower. This is because if the heating temperature of the steel sheet is lower than 1000 ° C, the reaction force of the hot rolling delay will increase, and if the heating temperature of the steel sheet is higher than 1250 ° C, decarburization will progress. The finishing rolling temperature of the hot rolling is set to 900 ° C or higher. This is because if the finishing rolling temperature is lower than 900 ° C, the reaction force of the finishing rolling will increase and the shape accuracy will deteriorate. On the other hand, the finishing rolling temperature is set to 1000 ° C or lower. This is because if hot rolling is performed at a temperature higher than 1000 ° C., the particle size of Vosstian iron will increase, and the high-angle grain boundary density after wave iron transformation will decrease.

After hot rolling, the following four stages of cooling are performed to adjust the area ratio of ferrous iron, the density of secondary grain boundaries, and the density of high-angle grain boundaries. The purpose of the first cooling is to suppress the grain growth of Vosstian iron by cooling at a faster cooling rate, and to generate a fine Vosstian iron structure. The purpose of the second cooling is to reduce the temperature difference between the surface layer portion and the central portion of the wire during the first cooling, so as to reduce the temperature difference between the surface portion of the wire and the central portion to a uniform temperature. The purpose of the third cooling is to cool as uniformly as possible from the surface layer portion to the center portion of the wire rod and to suppress the iron metamorphosis of the fertilizer grains, until the target wave iron metamorphosis temperature is reached. The purpose of the fourth cooling is to cool the surface to the center of the wire as much as possible to generate a deformed wave, and to cool it so that the secondary grain boundary density and the high-angle grain boundary density become the target ranges. Bole iron metamorphosis proceeds. The details are shown below. In addition, the average cooling rate of the first to fourth cooling described below means dividing the amount of wire temperature decrease from the beginning to the end of the first to fourth cooling by the first to the fourth cooling from the start to the end. Value from time to date. The so-called reaching temperature of the first to fourth cooling means the temperature of the wire at the end of the first to fourth cooling.

After the hot rolling, the first cooling is performed to 830 to 870 ° C by water cooling at an average cooling rate in a range of 50 to 200 ° C / sec. The start and end of the first cooling means the start and end of the injection of the refrigerant (water). If the average cooling rate is less than 50 ° C / sec in a temperature region where the grain growth rate is greater than 870 ° C, and a long time exists in this temperature region, the grain growth of Vostian iron will be promoted, so The high-angle grain boundary density will decrease after the boletic metamorphosis. Although there is no upper limit for the average cooling rate of the first cooling, it is difficult to achieve an average cooling rate of more than 200 ° C / sec. Due to the limitation of the manufacturing equipment. Therefore, the average cooling rate of the first cooling is set below 200 ° C / sec. Cap. When the reaching temperature of the first cooling is lower than 830 ° C, there is a fear that the transformation of the ferrous iron will only occur in a large amount in the surface layer portion, and the area ratio of the ferrous iron in the surface layer portion increases, which is difficult to control below 45%. Therefore, the reaching temperature of the first cooling is set to 830 ° C or higher. If the cooling is stopped at a temperature higher than 870 ° C, the Vostian iron particles will grow significantly, and the high-angle grain boundary density after the wave iron transformation will decrease. Therefore, the reaching temperature of the first cooling is set to 870 ° C or lower.

Then, by air cooling in the atmosphere, the second cooling is performed to a range of 790 ° C or higher and 820 ° C or lower at an average cooling rate of less than 5 ° C / sec. In addition, the time point when the second cooling is started is equal to the time point when the refrigerant injection is ended in the first cooling, and the time point when the second cooling is ended is equal to the time point when the refrigerant cooling is started in the third cooling. The second cooling is to reduce the temperature difference between the surface layer portion and the central portion of the wire generated during the first cooling, so that the abnormal transformation temperature of the wave iron from the surface portion to the central portion of the wire is uniformly cooled. In the second cooling, when the average cooling rate is set to 5 ° C / sec or more, a temperature difference between the surface layer portion and the center portion remains. After the boron iron metamorphosis, even if the high-angle grain boundary density and Secondary grain boundary density, but high-angle grain boundary density in the center of the wire will still decrease. Therefore, the average cooling rate of the second cooling is set to less than 5 ° C / second. If the reaching temperature of the second cooling is lower than 790 ° C, there is a possibility that the ferrous iron is deformed and the area ratio of the ferrous iron is increased. Therefore, the reaching temperature of the second cooling is set to 790 ° C or higher. On the other hand, if the second cooling is stopped at a temperature higher than 820 ° C, the temperature difference between the surface layer portion and the center portion of the wire to the transformation temperature of the wave iron will become large, and the surface layer portion and There will be another temperature difference between the center portions. Therefore, the reaching temperature of the second cooling is set to 820 ° C or lower. In steels with a large Si content, the temperature of the second cooling is particularly important because the temperature of Ac1 is shifted to the high temperature side. The second cooling time (elapsed time between the start and end of the second cooling) is preferably set to 5 seconds or more and 12 seconds or less. This is because if it takes a second cooling time of more than 12 seconds, the grain growth of Vostian iron particles will be promoted. On the other hand, if the second cooling time is within 5 seconds, the temperature difference in the wire may remain.

Then, the third cooling is performed to a range of 600 ° C or higher and 620 ° C or lower by an air-cooling at an average cooling rate of more than 20 ° C / sec and 30 ° C / sec or less. The start and end of the third cooling means the start and end of blowing air. In the third cooling, the cooling is performed at a cooling rate capable of suppressing the transformation of iron in the fertilized grains until the most appropriate secondary grain boundary density and high-angle grain boundary density are obtained. If the average cooling rate of the third cooling is 20 ° C./sec or less, the ferrous iron will be deformed and the ferrous iron area ratio will become excessive. Therefore, the average cooling rate is set to more than 20 ° C / second. On the other hand, when the third cooling is performed at an average cooling rate of more than 30 ° C / sec, the fourth portion of the wire starts to be cooled in a state where only the surface layer portion of the wire is cooled to the target temperature and the temperature of the center portion of the wire is too high. cool down. Therefore, the average cooling rate is set to 30 ° C / second or less. When the reaching temperature is lower than 600 ° C. during the third cooling, the strength of the Plei iron structure is excessively increased, and the torsional characteristics are reduced. Therefore, the reaching temperature of the third cooling is set to 600 ° C or higher. On the other hand, if the reaching temperature of the third cooling is higher than 620 ° C, the Plei iron transformation temperature will increase, and the high-angle grain boundary density and sub-grain boundary density will decrease, and the tensile strength after the Plei iron transformation will also increase. reduce. Therefore, the reaching temperature of the third cooling is set to 620 ° C or lower.

Then, by air cooling in the atmosphere, the fourth cooling is performed at an average cooling rate of 10 ° C./sec or less until 550 ° C. or less. In addition, the time point when the fourth cooling is started is equal to the time point when the air blowing is ended during the third cooling. The time point at which the fourth cooling ends is equal to the time point at which the air cooling is stopped, that is, the time point at which the wire is reheated or the refrigerant is started to be sprayed. However, when air cooling is performed until the temperature of the wire rod becomes 550 ° C or lower, the time point when the temperature of the wire rod becomes 550 ° C is regarded as the time point when the fourth cooling is completed. The purpose of the fourth cooling is to reduce the temperature difference in the cross section of the wire during the wave iron transformation to obtain a wire with uniform high-angle grain boundary density and sub-grain boundary density from the surface layer portion to the center portion. If the average cooling rate of the fourth cooling is greater than 10 ° C / second, the temperature of the surface layer changes greatly and the density of the sub-grain boundaries decreases. Therefore, the average cooling rate of the fourth cooling is set to 10 ° C./second or less. Although the lower limit of the average cooling rate of the fourth cooling is not limited, the cooling rate when the wire is cooled is generally 2 ° C / sec or more. Therefore, the lower limit of the average cooling rate of the fourth cooling may be 2 ° C./second. If the reaching temperature of the fourth cooling is higher than 550 ° C, there is a possibility that the wave iron transformation does not end. Therefore, the reaching temperature of the fourth cooling is set to 550 ° C or lower. In addition, since the cooling rate in the temperature range of 550 ° C or lower has a slight effect on the tissue, accelerated cooling such as water cooling may be performed after the fourth cooling is performed to a temperature of 550 ° C or lower. In the embodiment described later, the present invention is cooled to below 550 ° C by the fourth cooling, and then cooled down to room temperature by the cooling, but when it is cooled by other cooling means after the fourth cooling is completed, Will form the same organization.

(E) Optional components: The wire of this embodiment can also replace part of Fe in the remaining part, and optionally contains one or two or more elements selected from the group consisting of: Al, Cr, V , Ti, Nb, B, Ca, Mg. However, the wire of this embodiment can solve the problem without including any of these elements, so the lower limit of these arbitrary elements is 0%. In the following, the reasons for the effects and limitations of Al, Cr, V, Ti, Nb, B, Ca, and Mg of arbitrary elements will be described. % Of an arbitrary component is mass%.

Al: 0 to 0.050% The wire of this embodiment may not contain Al. Al is an element that precipitates into AlN, and can increase the density of high-angle grain boundaries with a difference in azimuth angle of the ferrous grain iron of 15 ° or more. When the effect is to be surely obtained, it should preferably contain more than 0.010% of Al. On the other hand, Al is an element that is liable to form hard oxide-based inclusions. Therefore, if the Al content of the wire is greater than 0.050%, coarse oxide-based inclusions are easily formed, and the reduction in torsional characteristics becomes significant. Therefore, the upper limit of the Al content of the wire is set to 0.050%. The upper limit of the Al content is preferably 0.040% or less, and a preferable upper limit is 0.035% or less, and a more preferable upper limit is 0.030% or less.

Cr: 0 to 1.00% The wire of this embodiment may not contain Cr. Cr is an element that can improve the hardenability of steel and increase the strength of steel, like Mn. In order to ensure this effect, Cr is preferably contained in an amount of 0.05% or more. On the other hand, if the Cr content is more than 1.00%, the torsional characteristics are deteriorated. Therefore, the Cr content is 1.00% or less. When the hardenability of steel is to be improved, Cr is preferably contained in an amount of 0.10% or more, and more preferably 0.30% or more. The upper limit of Cr should preferably be 0.90% or less, and more preferably 0.80% or less.

V: 0 to 0.15% V may not be contained in the wire of this embodiment. V will combine with N and C to form carbides, nitrides or carbonitrides, and by their pinning effect, it has the effect of minimizing Vostian iron particles during hot rolling delay, and has the effect of improving the steel Effect of torsional characteristics. In order to obtain this effect, it is preferable to contain 0.005% or more of V. From the viewpoint of improving torsional characteristics, it is preferable to set the V content to be 0.02% or more, and it is more preferable to contain V3 or more. On the other hand, if the V content is more than 0.15%, not only its effect will be saturated, but also the steel sheet will crack during the step of rolling the steel block or cast piece into pieces, which will also cause poor manufacturability of the steel. Effect, the V content should be set to 0.15% or less. The V content should preferably be 0.10% or less, and more preferably 0.07% or less.

Ti: 0 ~ 0.050% The wire of this embodiment may not contain Ti. Ti combines with N and C to form carbides, nitrides, or carbonitrides, and through its pinning effect, it has the effect of minimizing Vostian iron particles during hot rolling delay, and has the effect of improving the steel Effect of torsional properties. In order to obtain this effect, Ti should preferably contain 0.002% or more. From the viewpoint of improving the torsional characteristics, it is preferable to set the Ti content to 0.005% or more, and to contain 0.010% or more of Ti more preferably. On the other hand, if the Ti content is more than 0.050%, not only its effect will be saturated, but also in the step of rolling a steel block or a cast piece into a steel piece, the steel piece will crack and the like will cause poor manufacturability of the steel. influences. Therefore, the Ti content is set to 0.050% or less. And the Ti content is more preferably 0.025% or less.

Nb: 0 to 0.050% The wire of this embodiment may not contain Nb. Nb combines with N and C to form carbides, nitrides, or carbonitrides, and through its pinning effect, it has the effect of minimizing Vostian iron particles during hot rolling delay, and has the effect of improving steel Effect of torsional properties. In order to obtain this effect, Nb should preferably contain 0.002% or more. From the viewpoint of improving the torsional characteristics, it is more preferable to set the Nb content to be 0.003% or more, and to contain Nb of 0.004% or more. On the other hand, if the content of Nb is more than 0.050%, not only its effect will be saturated, but also the steel sheet will crack during the step of rolling the steel block or cast piece into pieces, which will also cause poor manufacturability of the steel. Effect, the Nb content should be set to 0.050% or less. And the Nb content is more preferably 0.030% or less.

B: 0 ~ 0.0040% The wire in this embodiment may not contain B. Containing a trace amount of B has the effect of reducing the iron and grain structure of steel. To ensure the effect, it is desirable to contain B in an amount of 0.0001% or more. Even if it contains more than 0.0040% of B, not only the effect will be saturated, but also coarse nitrides will be generated, so the torsional characteristics will be reduced. Therefore, the B content when contained is set to 0.0040% or less. When it is desired to increase the area ratio of the boron iron structure, it is preferable to set the B content to be 0.0004% or more, and it is more desirable to set the B content to be 0.0007% or more. In addition, the B content required for improving the torsional characteristics should be 0.0035% or less, and more preferably 0.0030% or less.

Ca: 0 to 0.0050% The wire of this embodiment may not contain Ca. Ca dissolves in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, it is possible to suppress wire breakage in the wire drawing process due to MnS. To ensure the effect of Ca, Ca should preferably be 0.0002% or more. In order to obtain a higher effect, it is sufficient to contain Ca of 0.0005% or more. However, if the Ca content is greater than 0.0050%, the effect is saturated. In addition, if the Ca content is more than 0.0050%, the oxide formed by the reaction with oxygen in the steel will become coarse, which will lead to a decrease in the wire drawability. Therefore, the appropriate Ca content when contained is 0.0050% or less. And the Ca content should be below 0.0030%, more preferably below 0.0025%.

Mg: 0 ~ 0.0040% The wire of this embodiment may not contain Mg. Mg is a deoxidizing element and generates oxides, but also generates sulfides. Therefore, Mg is an element having an interrelation with MnS, and has the effect of finely dispersing MnS. With this effect, it is possible to suppress wire breakage during wire drawing processing due to MnS. In order to obtain the effect brought by Mg, Mg should preferably contain 0.0002% or more. In order to obtain a higher effect, it is only necessary to contain Mg of 0.0005% or more. However, if the content of Mg is more than 0.0040%, the effect will be saturated and a large amount of MgS will be generated, but the drawability of the wire will be reduced. Therefore, the appropriate Mg content when contained is 0.0040% or less. And the Mg content should be below 0.0035%, more preferably below 0.0030%.

The remainder of the chemical composition contains "Fe and impurities". The so-called "impurity" refers to a substance that is mixed with steel from raw materials such as ore, waste materials, or from the manufacturing environment, etc. when industrially manufacturing steel materials.

The diameter of the wire rod of this embodiment is not particularly limited, but the diameter of the wire rod currently on the market is generally set to 3.5 to 7.0 mm, so it can also be set as the upper and lower limit of the diameter of the wire rod of this embodiment. When the diameter of the wire rod is 3.5 mm or more, it is preferable to reduce the burden of hot rolling during the production of the wire rod. When the diameter of the wire is set to 7.0 mm or less, it is preferable because the strain amount of the wire during wire drawing processing can be suppressed.

The steel wire according to another aspect of the present invention is obtained by subjecting the wire rod of this embodiment to wire drawing processing. If the application is considered, the diameter of the steel wire is generally set to 0.5 ~ 1.5mm. The steel wire of this embodiment has excellent tensile strength because the chemical composition of the raw material, that is, the wire composition of this embodiment, the structure of the metal structure, the sub-grain boundary density ρ1, and the high-angle grain boundary density ρ2 are set within the above-mentioned ranges. Torsion characteristics.

In addition, the steel wire related to the present embodiment is manufactured by wire drawing with a very large amount of strain, and therefore its metal structure is significantly deformed. When an enlarged photograph of the cross section of the steel wire of this embodiment is observed, for example, the phase surrounded by the grain boundary is obviously crushed, and the type cannot be discriminated. In addition, it is obviously difficult to identify the existence of sub-grain boundaries and high-angle grain boundaries. That is, it is extremely difficult to specify the metal structure and other structures of the steel wire of this embodiment by a general structure-specific method (for example, taking a picture of a metal structure and analyzing the crystal structure using EBSD, etc.). However, it is impossible to directly specify the metal structure of the steel wire of this embodiment according to its structure or characteristics, or it is completely impractical.

The manufacturing method of the steel wire of another aspect of this invention is provided with the process of drawing the wire of this embodiment. The wire drawing process is performed by reducing the surface area of the finally obtained steel wire diameter to 0.5 to 1.5 mm. Since the chemical composition, metal structure, sub-grain boundary density ρ1, and high-angle grain boundary density ρ2 of the wire of this embodiment are set within the above-mentioned ranges, the method for manufacturing the steel wire of this embodiment can be used to break the wire. The number of threads is suppressed to an extremely low level, and a steel wire having excellent tensile strength and torsional properties can be obtained. Examples

Hereinafter, the present invention will be specifically described by examples, but the present invention is not limited by the following examples.

Steels with chemical compositions shown in Tables 1 and 2 were melted, and wires were produced by the following method. The "-" mark in Tables 1 and 2 indicates that the element content is an impurity grade, and it can be judged that it is substantially not contained. The remainder of the chemical composition of the steels shown in Tables 1 and 2 is iron and impurities.

First, the steel A having the chemical composition shown in Table 1 was melted in a converter, and then a small steel billet of 122 mm square was obtained by block rolling by a general method. Next, the steel sheet is heated to 1050 to 1150 ° C, and then hot rolled to φ6 mm in the range of completion temperature of 900 to 1000 ° C.

The adjusted cooling after finishing rolling was cooled under the conditions shown in (A1) to (A21) shown in Tables 3-1 to 3-3. Specifically, (A1) to (A7) are cooled by water cooling to an average cooling rate of 50 to 200 ° C / sec to 830 to 870 ° C (the first cooling), and then the air is blown by air. It was cooled, and air-cooled at an average cooling rate of less than 5 ° C / sec to a range of 790 ° C to 820 ° C (second cooling). Thereafter, cooling is performed to 600 to 620 ° C (third cooling) at a temperature of more than 20 ° C / second and 30 ° C / second or less, and to 550 ° C or less (fourth cooling) at 10 ° C / second or less, and then cooling is performed by using Allow to cool to room temperature. Regarding (A8) to (A17), four types of adjustment cooling were performed under conditions different from the above-mentioned cooling conditions to obtain a wire. In addition, the underlined value in Table 3-1 is an inappropriate value in the manufacturing conditions of the wire of the present invention.

Regarding (A18) to (A21), four types of adjustment cooling were not performed, but cooling was performed under the conditions shown in Tables 3-2 to 3-4. In addition, the terms "first cooling" and the like in these tables are only used to distinguish the cooling stage, and are different from the primary cooling to the fourth cooling included in the manufacturing method of the present invention. Specifically, (A18) replaces the third cooling and the fourth cooling in the production method of the present invention, and the impregnation is performed in a salt bath at 550 ° C. Regarding (A19), the wire rod after the completion of the hot rolling was reheated to a temperature of 950 ° C and held for 60 seconds, and immediately after the temperature maintenance was completed, impregnation was performed in a salt bath at 550 ° C. Regarding (A20), the cooling is performed by the supply air after the first cooling is performed, and the cooling is switched to the cooling after being cooled to 680 ° C at an average of 1.0 ° C / sec, and the cooling is performed to 550 ° C or lower. Regarding (A21), air-cooling is performed after the first cooling, and the wire is cooled to 700 ° C. at 10 ° C./sec, and then cooled to 550 ° C. or lower at 5 ° C./sec by air cooling.

In addition, according to the steels a to z having the chemical compositions shown in Table 2, hot-rolled wire rods were produced in the same manner as in (A1) of Table 3-1. Then, a dry wire drawing process, a plating process, and a wet wire drawing process were performed to obtain a steel wire having a wire diameter of 0.5 mm. The underlined values in Table 2 are outside the preferred range of the present invention.

[Table 1]

[Table 2]

For the wires of test numbers A1 to A21 and test numbers 1 to 26 obtained as described above, the tensile strength, shrinkage ratio, area ratio of ferrous iron, area ratio of non-polite iron and non-ferrous iron, and wave area were calculated. Secondary grain boundary density ρ1 in the iron structure (density of the secondary grain boundaries with an angular difference in the crystalline orientation of the lamellar ferrous grains in the Plei iron structure of 2 ° or more and less than 15 °), and high-angle grain boundary density ρ2 ( Observe the density of high-angle grain boundaries where the angular difference of the crystal orientation of the ferrous grains in the whole tissue is 15 ° or more). In addition, the area ratio of boron iron of each wire rod is a value obtained by removing the area ratio of ferrous iron and the area ratio of non-baite iron and non-ferrous iron from 100%.

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

The area ratios of the ferrous grain iron structure, the non-ferrous grain iron and non-poled iron structure area ratios, the secondary grain boundary density ρ1, and the high-angle grain boundary density ρ2 were investigated by the methods described below, respectively. , The number of wire breaks when drawing a wire with a diameter of 6mm to a diameter of 0.5mm, the tensile strength of the wire before drawing and the steel wire after drawing (strength of the steel wire), and the steel after drawing The twist characteristics of the wire (the number of twists, the number of twists, and the presence or absence of layer peeling).

<1> Area ratio of ferrous grain iron structure, non-ferrous grain iron and non-pole iron structure area of wire: After cross-section of the wire is mirror-polished, it is corroded with a bitter acid etchant, and FE-SEM is used to Observe 10 times of the surface layer part and the center part of the wire at a magnification of 2000 times, and take a photo. The area per one field of view is 2.7 × 10 ^-3 mm ² (0.045 mm in length and 0.060 mm in width). On each of the obtained photographs, an OHP sheet was superimposed, and the "fertile grain iron structure" and "area overlapping with non-polite iron and non-fertile grain iron structure" of each transparent sheet were colored. Then, the area ratio of the "painted area" on each transparent sheet was obtained by using image analysis software, and the average value was calculated as the average of the area ratios of the ferrous iron structure and the non-poled iron and non-ferrous iron structure. value.

<2> Secondary grain boundary density ρ1 and high-angle grain boundary density ρ2 of the wire: After the mirror cross-section of the wire is polished, it is ground with colloidal silicon oxide, and the surface layer and center of the wire are 400 times at FE-SEM. Each of them was observed at 4 locations, and analyzed by using an EBSD measurement device manufactured by TSL (TexSEM Laboratories). The area at the time of measurement was set to 180 × 180 μm ² , and the pitch was set to 0.3 μm. Then, based on the obtained results, the total length of the sub-grain boundary lines with an angular difference of 2 ° or more and less than 15 ° and the total length of the high-angle grain boundary lines with an angular difference of 15 ° or more were measured using OIM analysis. Divide the total length of the sub-grain boundary lines with an angular difference of 2 ° or more and less than 15 ° by the average value of the area ratio of boron iron to obtain the sub-grain boundary density, and set a high angle with an angular difference of 15 ° or more. The total length of the grain boundary lines is divided by the area of one field of view to obtain the high-angle grain boundary density.

<3> Wire drawing processability Wire drawing was performed on 50 kg of each wire, and the number of wire breaks during wire drawing was recorded. In addition, when the number of disconnections is 3 or more, the wire drawing processing after the third disconnection is suspended. In addition, when 50 kg of wire is drawn from 6.0 mm in diameter to 0.5 mm in diameter, the number of wire breaks is 0, and the wire drawability is evaluated to be good. When the number of wire breaks is more than 1, the wire is evaluated to be drawn. Wire processability is poor. In addition, as for the wire material in which the wire drawing processing was suspended, it was determined to be obviously unsuitable as a steel wire material, and the subsequent evaluation test was not performed. The symbol "-" is recorded in the items that have not been evaluated.

〈4〉 Tensile strength of wire and steel wire after processing: The wire and steel wire were cut to a length of 200mm, and a wedge-shaped chuck or a pneumatic chuck was used to fix the upper and lower 50mm to perform a tensile test. The obtained maximum load was divided by the cross-sectional area to calculate the tensile strength. Then, measure the wire diameter where the wire diameter becomes the thinnest after the tensile test of the wire, divide the change in cross-sectional area before and after the tensile test by the cross-sectional area before the tensile test, and multiply by 100% to calculate the shrinkage. Face rate.

The steel wire used for the metal wire of the reinforcing material for automobile tires should have a tensile strength of 2800 MPa or more. Therefore, a tensile strength of 2800 MPa or more is evaluated as a qualified product. In addition, the elongation strength of the wire rod is not specifically set as a pass criterion.

<5> Torsion characteristics of steel wire after wire drawing: The torsion test is to twist a steel wire that is 100 times the wire diameter (diameter) at 15 rpm until the wire is broken, and use the torque (resistance to torsion) curve. The number of twists was measured to determine whether layer peeling occurred. The determination using the torque curve is performed by a method of judging that a sudden decrease in torque before the disconnection is considered to have occurred layer peeling. The torsion test was performed on each of the ten steel wires. When even one layer did not peel off and the average number of twists of the ten steel wires was 23 or more, it was evaluated that the torsional characteristics were good.

In addition, when the number of twists in the above-mentioned ten twist tests is small, it can be judged that the twist characteristics are more favorable. Therefore, the variation in the number of twists of the ten steel wires (the difference between the maximum number of twists of the ten steel wires and the above average value, and the difference between the minimum number of twists of the ten steel wires and the above average value is larger. By). And the steel wire with the number of times less than 3 times is judged to have good torsion times. The average value of the number of twists, the layer peeling, and the number of twists were all judged to be good steel wires, and their torsional characteristics were very good. However, even if the number of twists of the steel wire varies more than three times but other torsional characteristics are evaluated as good steel wires, considering its intended use, it can be said that its torsional characteristics are good.

The results obtained by the above individual evaluations are arranged in the following table.

[Table 3-1]

[Table 3-2]

[Table 3-3]

[Table 3-4]

[Table 4-1]

[Table 4-2]

[Table 4-3]

[Table 5-1]

[Table 5-2]

[Table 5-3]

As shown in the table, the samples of A1 to A7 of the examples of the present invention all meet the requirements of the present invention and the manufacturing conditions of the steel are appropriate. Therefore, the strength after wire drawing is 2800 MPa or more, and the number of twists is 23 or more. Layer peeling occurred, but there was no problem with the wire.

On the other hand, because the average cooling rate of the first cooling sample of A8 is low, the particle size of Vosstian iron is coarsened, so ρ2 is reduced, and wire breakage occurs during wire drawing, and the wire drawability is poor. Due to the low arrival temperature during the first cooling of the sample of A9, the area ratio of ferrous iron in the surface layer increased and the number of twists decreased. The sample of A10 has a large particle diameter due to the high arrival temperature during the first cooling, so ρ2 decreases and disconnection occurs. The sample of A11 had a large particle size in Vosstian iron due to the long time during the second cooling, so ρ2 decreased and disconnection occurred. The sample of A12 has a low arrival temperature during the second cooling, so the iron grain area ratio is high, and the wire drawability is poor, and the steel wire strength and torsional characteristics are also low. The average cooling rate of the sample A13 in the third cooling is small, which causes the ferrous iron to progress abnormally, resulting in a higher ferrous iron area ratio and poor wire drawability, and the strength and torsional properties of the steel wire are also lower. The sample of A14 has a high arrival temperature during the third cooling, and wave iron transformation at high temperature causes ρ1 and ρ2 to be low, resulting in wire breakage during wire drawing and poor torsional characteristics. The sample of A15 had a low reaching temperature during the third cooling, which caused ρ1 to become too high, and thus the torsional characteristics were poor. The average cooling rate of the A16 sample in the fourth cooling was high, which resulted in a decrease in ρ1 in the surface layer portion of the wire and layer peeling occurred during the torsion test, and the torsional characteristics were poor. The sample of A17 was prepared according to the following manufacturing conditions: At the time point when the temperature of the wire reached the temperature shown in the table in the fourth cooling, the air cooling was stopped and the air cooling was started. Due to the high arrival temperature during the fourth cooling of the sample A17, the transformation of Plei iron is not over, but the non-Plei iron and the area ratio of non-ferrous iron are high, so the drawability is reduced. Since the sample of A18 was immersed in a salt bath at 550 ° C after the second cooling, the wire was rapidly cooled to 550 ° C. As a result, ρ1 of A18 was high, layer peeling occurred during the torsion test, and the torsional characteristics were poor. After reheating and maintaining the temperature of the wire of A19, the wire was immersed in a salt bath at 550 ° C, so the wire was rapidly cooled to 550 ° C. As a result, ρ1 of A19 was high, layer peeling occurred during the torsion test, and the torsional characteristics were poor. As for the sample of A20, the cooling rate after hot rolling is slow, and the wrought iron metamorphosis occurs at high temperature. Due to the high metamorphic temperature of boron iron, ρ1 and ρ2 of A20 are both low and wire breakage occurs during wire drawing, and the torsional characteristics are also poor. In the sample of A21, the wire was cooled to 700 ° C by air cooling after the second cooling. Therefore, the surface layer portion of the wire was rapidly cooled, and the ρ1 of the surface layer portion became high. Layer peeling occurred during the torsion test, and the torsional characteristics were poor.

In addition, according to the results shown in the table, for the test numbers 1 to 19 and 26 of the examples of the present invention, the chemical composition satisfies the preferred range of the present invention, and the manufacturing conditions of the wire are also appropriate, so the wire drawing process Good torsion, good torsional properties after wire drawing, and has the required tensile strength.

However, the sample No. 20 had a low C content, an excessively large area of ferrous iron, and insufficient steel wire strength. In the sample No. 21, because the steel was excessively hardened due to the high C content, the drawability was reduced, and wire breakage occurred during the draw processing. The sample No. 22 had a low Si content and therefore a low ρ1, and peeling occurred during the torsion test. The sample No. 23 has a high Mn content, and has a large amount of non-ferrous iron and non-polite iron. Therefore, wire breakage occurs during wire drawing. The sample of test number 24 had a low Si content and a low ρ1, and peeling occurred during the torsion test. The sample No. 25 had a low Mn content and a low ρ1, and peeling occurred during the torsion test.

From the results shown in the table, as long as C, Si, Mn, N, P, and S have been specified in the previously described preferred range of wire, and the wire meets the following conditions: the main structure is boron iron, fertilizer The angle difference of the crystal orientation of the lamellar fertilized iron in the lamellar iron structure is less than 45%, the non-fertilized iron and the non-pulled iron structure is 5% or less, and the secondary grain boundaries are at least 2 ° and less than 15 °. The density ρ1 is 70 / mm ≦ ρ1 ≦ 600 / mm, and the high-angle grain boundary density ρ2 of the angle difference of the crystal orientation of the ferrous grains in the whole is more than 15 °, which can provide a steel wire that can be manufactured and The wire for wire drawing processing with good wire drawing workability. The aforementioned steel wire can obtain high tensile strength after wire drawing, and can be twisted stably without peeling of the layer during torsion test after wire drawing. That is, it is possible to provide a wire for wire processing capable of stably manufacturing a steel wire while suppressing breakage in the wire processing, and the steel wire has high strength suitable as a base material for a metal wire and the like, and further has excellent torsional characteristics.

1‧‧‧ Wire

11‧‧‧ Surface Department

12‧‧‧ Center

21‧‧‧Old γ grain boundary

22‧‧‧High-angle grain boundary

23‧‧‧ secondary grain boundaries

31‧‧‧Lamellar Shimmering Carbon Iron

32‧‧‧Layered flaky ferrous iron

d‧‧‧ diameter of wire

FIG. 1 is a schematic view showing a surface layer portion and a center portion of a wire rod according to this embodiment. FIG. 2 is an explanatory diagram showing an example of a boletite structure.

Claims (11)

A wire rod characterized by: its chemical composition in mass% contains: 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% Below, S: 0.020% or less, O: 0.0070% or less, Al: 0 ~ 0.050%, Cr: 0 ~ 1.00%, V: 0 ~ 0.15%, Ti: 0 ~ 0.050%, Nb: 0 ~ 0.050%, B : 0 ~ 0.0040%, Ca: 0 ~ 0.0050%, and Mg: 0 ~ 0.0040%, and the remaining part is composed of Fe and impurities; the main structure of the surface layer part and the center part of the wire is the Pleisteel structure, The surface layer portion ranges from a depth of 150 to 400 μm from the surface of the wire, and the center portion ranges from a central axis of the wire to 1/10 of the diameter of the wire; a cross section of the wire at a right angle to the length direction The area ratio of medium-fat grain iron structure is less than 45%, and the area ratio of non-poleite iron and non-fertile grain iron structure in the aforementioned cross-section is less than 5%; The density ρ1 of the secondary grain boundaries with a difference of more than 2 ° and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and Poor density ρ2 15 ° or high angle grain boundaries is 200 / mm or more. For example, the wire of claim 1, wherein the aforementioned chemical composition contains Al in an amount of 0.010 to 0.050% by mass. For example, the wire of claim 1 or claim 2, wherein the aforementioned chemical composition contains Cr in mass%: 0.05 to 1.00%. For example, the wire rod according to any one of claim 1 to claim 3, wherein the aforementioned chemical composition contains, by mass%, one or two or more kinds selected from the group consisting of: V: 0.005 to 0.15%, Ti : 0.002 to 0.050%, and Nb: 0.002 to 0.050%. For example, the wire rod of any one of claim 1 to claim 4, wherein the aforementioned chemical composition contains B in mass%: 0.0001 to 0.0040%. For example, the wire of any one of claim 1 to claim 5, wherein the aforementioned chemical composition contains, by mass%, one or two selected from the group consisting of: Ca: 0.0002 to 0.0050%, and Mg : 0.0002 ~ 0.0040%. For example, the wire rod according to any one of claim 1 to claim 6, wherein in the aforementioned surface layer portion and the aforementioned central portion of the aforementioned wire rod, the aforementioned density ρ1 of the aforementioned subgrain boundary satisfies the following formula 1, 220 × (C) +100 < ρ1 <220 × (C) +300: Formula 1 (C) in the aforementioned Formula 1 is the C content in mass% in the aforementioned chemical composition of the aforementioned wire. For example, the wire of any one of claim 1 to claim 7, wherein the aforementioned diameter of the aforementioned wire is 3.5 to 7.0 mm. The wire of any one of claims 1 to 8 is used as a material of a steel wire. A steel wire is characterized in that: it is manufactured by drawing a wire as in any one of claim 1 to claim 9, and has a diameter of 0.5 to 1.5 mm. A method for manufacturing a steel wire, comprising: a step of preparing a steel wire by drawing a wire material according to any one of claim 1 to claim 9, and a diameter of the aforementioned steel wire is 0.5 to 1.5 mm .