JP6528920B2 - Wire rod and method of manufacturing steel wire - Google Patents

Description

The present invention, wires, a method of manufacturing a 及 beauty steel wire.
Priority is claimed on Japanese Patent Application No. 2017-099227, filed May 18, 2017, the content of which is incorporated herein by reference.

The present invention is widely used as a material of high-strength steel wire used as a wire reinforcing material for tires such as automobiles, reinforcing wire for aluminum transmission wire, PC steel wire, wire for rope used for bridge etc. It relates to the wire used. The present invention also relates to a method of manufacturing a steel wire using this wire.

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

  For example, a reinforcing material having a wire diameter of 0.5 mm or more used for a large automobile tire or the like is required to improve productivity. From a wire rod having a wire diameter of 3.5 mm or more which can be produced by stable hot rolling, a wire rod capable of stably producing a steel wire having a wire diameter of 0.5 mm to 1.5 mm at low cost is required. Therefore, development of a wire rod having wire drawability capable of omitting an intermediate patenting step performed in the course of wire drawing and capable of exhibiting stable twisting characteristics after wire drawing processing has been advanced.

  However, in the case of a wire manufactured by the step of drawing to a high drawing degree, it is in a situation where a break occurs more easily in the middle of the drawing. Furthermore, the steel wire that has been drawn to a high drawing degree tends to have a deterioration in torsional characteristics. Furthermore, the fact that the wire diameter of the wire which is the material of the steel wire is large is disadvantageous for the torsion characteristics of the steel wire.

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

  In addition, 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 discontinuously formed of cementite. The wire rod for high strength steel wires which prevented the wire-drawing property fall is proposed by comprising.

  In these techniques, the wire drawing process up to obtaining a steel wire with a wire diameter of 0.1 to 0.4 mm by making the wire drawability of the wire better by controlling the form of cementite of the wire. In the above, it is intended to prevent the occurrence of a cutie break or the like. However, just controlling the form of cementite can not suppress the variation in strength within 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, coexistence of suppression of occurrence of breakage and deterioration of torsion characteristics is not effectively achieved, and the above problem May occur.

  Because of this, in the process of producing a steel wire with a large diameter (for example, a wire diameter of 0.5 mm or more) by wire drawing up to a high drawing rate, breakage does not easily occur, and the torsion characteristics after wire drawing It is desired to realize a wire rod that becomes good.

Japanese Patent Application Laid-Open No. 2014-055316 Japanese Patent Laid-Open Publication No. 2000-119756

The present invention has been made in view of the above-described circumstances, and suppresses breakage during wire drawing of a steel wire having high strength and excellent torsional characteristics, which is also suitable as a material for a large diameter wire or the like. It is an object of the present invention to provide a wire which can be stably manufactured. Another object of the present invention is to provide a method of manufacturing a steel wire having high strength and excellent torsional characteristics.

  The present inventors conducted various studies in order to solve the above-mentioned problems. As a result, the following findings 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 drawing strain is 4.5 or more. In the case of performing drawing with such a high drawing degree to eutectoid steel or hyper-eutectoid steel having poor drawability, patenting in the middle of drawing becomes essential. On the other hand, by using a hypoeutectoid steel with a small cementite fraction as the material of the wire, it is possible to improve the drawability of the wire, and provide the wire to drawing with a drawing strain of 4.5 or more. It turned out that it would be possible.

  (II) On the other hand, when the area ratio of ferrite exceeds 45% at the center of the cross section of the hypoeutectoid steel wire (that is, the cut surface perpendicular to the longitudinal direction of the wire), the ferrite structure becomes It turned out that even if it is a wire rod of hypoeutectoid steel, wire drawability runs short because of becoming massive and coarse. In addition, when the grain size becomes coarse (that is, when the large-angle grain boundary density is small), a large crack is likely to be formed inside the wire during wire drawing because the wire has a low reduction value and poor ductility. , Wire drawability decreased. Moreover, in the cross section of the wire rod of a hypoeutectoid steel, when the area ratio of the ferrite in a surface layer part becomes more than 45%, it turned out that the torsion characteristics after wire drawing processing fall. This is considered to be because deformation concentrates on the ferrite structure.

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

  (IV) The present inventors examined the means of improvement of the grain boundary density. Subgrain boundaries are considered to be introduced in order to eliminate the misfit of both phases when lamellar ferrite and layered cementite in the pearlite structure (hereinafter, lamellar cementite) grow cooperatively during pearlite transformation. The inventors of the present invention have found that the subgrain density can be adjusted using the pearlite transformation temperature and the content of an alloy element (eg, Si) dissolved in lamellar ferrite.

  The relationship between the pearlite transformation temperature and the subgrain boundary density is specifically described. It was found that when the pearlite transformation temperature is 550 ° C. or less, the subgrain boundary density in lamellar ferrite decreases as the pearlite transformation temperature decreases. This is considered to be due to an increase in locations where the growth of lamellar cementite is disrupted. On the other hand, in the region where the pearlite transformation temperature is higher than 550 ° C., the grain boundary density tends to gradually decrease as the temperature becomes higher. 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 lamella cementite decreases and the total amount of misfits decreases, so the subgrain density in lamellar ferrite decreases. It is thought that it will continue. From these results, it was found that when the cooling conditions are controlled so that the pearlite transformation temperature is around 550 ° C., the largest amount of subgrain boundaries are introduced.

  In addition, specifically describing the relationship between the amount of alloying elements and the density of subgrain boundaries, the misfit at the interface between lamellar ferrite and lamellar cementite increases by increasing the content of alloying elements represented by Si. And the grain boundary density is thought to increase.

  (V) However, based on the above findings, repeated experiments to increase the density of subgrain boundaries in pearlite structure show that a wire rod with low torsional characteristics after wire drawing although the density of subgrain boundaries is large The Although the cause is not clear, when the grain boundary density is increased by pearlite transformation at less than 600 ° C., the number of twists after wire drawing tends to decrease. Therefore, wire drawability and wire drawing can be achieved by perlite transformation at 600 ° C. to 620 ° C., not at around 550 ° C. where the grain boundary density can be maximized, so as not to increase the grain boundary density in lamellar ferrite too much. It is believed that a wire can be obtained that is compatible with the torsional properties after processing.

  Based on the findings of (I) to (V) above, steel wires having high strength and excellent twisting properties suitable as materials for large diameter wires and the like are stably manufactured by suppressing disconnection during wire drawing. In order to realize a possible wire rod, it is necessary to use hypoeutectoid steel as a 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 of the wire, the large angle grain boundary density, and the subgrain boundary density Needs to be controlled to an appropriate range. Thus, the wire obtained by adjusting the content of the alloying element and the adjustment cooling conditions after hot rolling and increasing the large angle grain boundary density and the sub-grain boundary density has the same strength level as the other wire The present inventors have found that wire drawability and torsional properties after wire drawing are superior to those of the prior art.

The present invention has been completed based on the above findings, the gist of which is as follows.
(1) The wire according to one aspect of the present invention has a chemical composition in 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%, the balance being Fe and impurities, and a surface layer portion having a depth of 150 to 400 μm from the surface of the wire, and 1 of the diameter of the wire from the central axis of the wire In both the central part, which is in the range of 10/10, the main structure is pearlite structure, and a transverse direction perpendicular to the longitudinal direction of the wire The area ratio of the ferrite structure in the plane is 45% or less, the area ratio of non-pearlite and non-ferrite structure in the cross section is 5% or less, and the crystal orientation angle difference of 2 ° of the lamellar ferrite in the pearlite structure The density 11 of the subgrain boundary which is less than 15 ° is 70 / mm ≦≦ 1 ≦ 600 / mm, and the density 22 of the large angle grain boundary which has an angular difference of 15 ° or more in the entire structure is 200 / Mm or more.
(2) In the wire described in the above (1), the chemical composition may contain Al: 0.010 to 0.050% by mass.
(3) In the wire rod according to (1) or (2), the chemical composition may contain, by mass%, Cr: 0.05 to 1.00%.
(4) In the wire rod according to any one of the items (1) to (3), the chemical composition is, in mass%, V: 0.005 to 0.15%, Ti: 0.002 to 0. You may contain 1 type, or 2 or more types selected from the group which consists of 050% and Nb: 0.002-0.050%.
(5) In the wire rod according to any one of the above (1) to (4), the chemical composition may contain B: 0.0001 to 0.0040% by mass.
(6) In the wire rod according to any one of the above (1) to (5), the chemical composition is, by mass%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0 It may contain one or two selected from the group consisting of .0040%.
(7) In the wire rod according to any one of (1) to (6), the density 11 of the subgrain boundary in the surface layer portion and the central portion of the wire rod satisfies the following formula 1 It is also good.
220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1
(C) in the said Formula 1 is C content in the mass% in the said chemical composition of the said wire.
(8) In the wire according to any one of the above (1) to (7), the diameter of the wire may be 3.5 to 7.0 mm.
(9) The wire rod according to any one of the above (1) to (8) may be used as a material of a steel wire .
( 10 ) A method of manufacturing a steel wire according to another aspect of the present invention includes the step of drawing a wire rod according to any one of the above (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 rod according to one aspect of the present invention, a steel wire having high strength and excellent twisting characteristics suitable as a material such as a wire can be stably manufactured by suppressing disconnection during wire drawing, and industrially It is extremely useful . The method of manufacturing a steel wire according to an aspect of the present invention can stably manufacture a steel wire having high strength and excellent twisting characteristics suitable as a material of the wire, 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 rod according to the present invention will be described in detail.
In addition, 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 range of 1/10 of is defined as the central portion 12. Moreover, in the present specification, a numerical range represented by using “to” means a range including numerical values described before and after “to” as the lower limit value and the upper limit value.

The wire according to the present embodiment is a steel wire material suitable as a wire reinforcement material for tires such as automobiles, a wire for reinforcement such as an aluminum transmission wire, a PC steel wire, a wire for rope used for a bridge, etc. It is a wire that can be used as
In addition, wire-drawing workability of a wire is a parameter | index which shows a hardness to generation | occurrence | production of a disconnection at the time of drawing a wire, and obtaining a steel wire. The twisting characteristics after wire drawing of wire rod means that it is difficult for delamination to occur when a steel wire obtained by wire drawing rod wire is subjected to a twisting test, and for wire breakage etc. Is an indicator that The wire according to the present embodiment preferably has a wire drawability so that the number of wire breakages when preparing a wire with a diameter of 6.0 mm to 50 kg and drawing this wire to a diameter of 0.5 mm is zero. . Furthermore, the steel wire after wire drawing preferably has a tensile strength of 2800 MPa or more. In addition, it is preferable that the steel wire used for the wire has a twisting property such that delamination does not occur even once even if ten twist 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 23 or more twists has sufficient ductility so as not to break in handling such as correction after wire drawing.

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

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

C: 0.30 to 0.75%
C is an element that strengthens steel. In order to obtain this effect, 0.30% or more of C must be contained. On the other hand, if the C content exceeds 0.75%, the cementite fraction increases and the wire drawability decreases. Therefore, the appropriate content of C is 0.30% or more and 0.75% or less. Furthermore, from the viewpoint of suppressing crack formation, the content of C is preferably 0.35% or more, and more preferably 0.40% or more. On the other hand, the C content is preferably less than 0.75%, or 0.70% or less, and more preferably 0.65% or less, from the viewpoint of improving wire drawability. 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 enhances the strength of the wire but also contributes to the increase in the density of grain boundaries. However, if the Si content of the wire is less than 0.80%, the effect of increasing the grain boundary density by containing Si can not be sufficiently obtained. On the other hand, when the Si content of the wire exceeds 2.00%, the ferrite fraction increases and the wire drawability decreases. Then, content of Si of a wire was defined 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 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 a function of fixing S in the steel as MnS to prevent hot brittleness of the steel wire in addition to the function of enhancing the strength of the steel wire. However, if the Mn content is less than 0.30%, the above effect is not sufficient. Therefore, the lower limit value of the Mn content is 0.30% or more. Furthermore, in order to realize the securing of the strength of the steel wire and the prevention of hot embrittlement 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 which tends to segregate. When Mn is contained in excess of 1.00%, Mn is particularly concentrated in the central portion, martensite and bainite are generated in the central portion, and wire drawability deteriorates. In addition, the formation of coarse MnS also contributes to the decrease in wire drawability. The Mn content 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 increases the strength of the wire by adhering to dislocations during cold wire drawing, but reduces the twisting characteristics. When the N content of the wire exceeds 0.0080%, the deterioration of the twisting characteristics becomes remarkable. Therefore, the N content of the wire is regulated to 0.0080% or less. The preferable upper limit of the 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 which is segregated in the grain boundaries of the wire and reduces the torsion characteristics. When the P content of the wire exceeds 0.030%, the deterioration of the twisting characteristics becomes remarkable. 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, and 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 reduces wire drawability. And when S content of a wire exceeds 0.020%, the fall of wire-drawing property becomes remarkable. From this, it was decided that the S content of the wire is regulated to 0.020% or less. The preferred upper limit of the S content is 0.010% or less. The lower the S content, the better, and 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 deterioration of the twisting characteristics becomes remarkable. 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) Structure of wire rod Next, the metal structure of the wire rod according to the present embodiment will be described. In addition, the requirement regarding the metal structure of the wire demonstrated below needs to be satisfy | filled in both the surface layer part 11 and the center part 12 of the wire 1. As shown in FIG.

  The main structure is a pearlite structure in the surface layer portion and the central portion of the wire, and a ferrite structure is 45% or less in area ratio in the cross section of the wire, and a non-perlite and non-ferrite structure is 5% or less in area ratio; Subgrain boundary density 11 which makes the angle difference of crystal orientation of lamellar ferrite in medium 2 ° or more and less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, and the angle difference of ferrite crystal orientation in all structures is 15 ° or more It is necessary to have a metallographic structure in which the large-angle grain boundary density な る 2 is 200 / mm or more. The "main structure" means a structure that occupies the largest area ratio in the metal structure. The "area ratio" means an area ratio measured in a cross section perpendicular to the longitudinal direction of the wire, and the measurement method will be described later. In other words, the surface layer portion and the central portion of the wire according to the present embodiment include the pearlite structure in an area ratio of 50% or more.

  A wire having such a metal structure in the surface layer portion and the central portion has a high reduction value at the time of a tensile test and is excellent in wire drawability. In addition, according to a wire having such a metal structure in the surface layer portion and the central portion, when it is drawn into a steel wire having a diameter of 1 mm or less and the tensile strength thereof is 2800 MPa or more, excellent torsional characteristics are obtained. The steel wire which it has is obtained. In the metal structure of the wire, the main structure (non-pearlite and non-ferrite structure) of the remaining portion excluding ferrite structure and pearlite structure is bainite, martensite or the like.

Here, the grain boundaries of the pearlite structure are additionally described.
In the ordinary technical common sense, perlite is described as having a lamellar structure in which lamellar ferrite and lamellar cementite are arranged in layers by the eutectoid reaction arising from austenite, and a hierarchical substructure is formed inside the lamellar structure. An area surrounded by large-angle grain boundaries is referred to as a block, and in the block, an area having the same lamellar orientation is referred to as a colony. In other words, it is recognized that the cementite plate dispersed in each grain of the ferrite structure has some orientation and is dispersed as pearlite.

  However, the actual pearlite organization is considered not so simple. FIG. 2 shows a schematic view of an example in which the pearlite structure is simplified. In the metal structure shown in FIG. 2, a block surrounded by curved large-angle grain boundaries 22 is generated starting from the old γ grain boundaries 21 (old austenite grain boundaries), and the subgrain boundaries 23 are formed in the block. It is formed. The crystal orientation in the block changes in many random orientations, and in the structure of FIG. 2, the sum of the lengths of the dashed lines indicating the subgrain boundaries 23 can be recognized as the total length of the subgrain boundaries 23. Further, in the schematic view of FIG. 2, it is possible to recognize the sum of the lengths (lengths of thick solid lines surrounding the block) of the large angle grain boundaries 22 constituting the outer periphery of the block and the like as the lengths of the large angle grain boundaries 22. In FIG. 2, the layered structure of lamella cementite 31 and lamella ferrite 32 constituting a lamella structure is enlarged.

  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-perlite structure is different from a normal perlite structure in that the lamellar cementite 31 is observed to be divided in the block when observed by SEM. However, in the present embodiment, the pearlite tissue and the pseudo-pearlite tissue are treated as the same.

  In the actual steel material, in addition to the pearlite structure, other structures are mixed, and the structure is much more complicated than the structure of FIG. 2, so in the wire rod according to this embodiment, subgrain boundaries and large angle grain boundaries It is defined as follows. The boundary surface where the angle difference between the crystal orientations of adjacent lamellar ferrites in the pearlite structure is 2 ° or more and less than 15 ° is referred to as a subgrain boundary, and the length of the subgrain boundary per unit area of pearlite in the inspection visual field The total is referred to as subgrain density <ρ1>. Also, the boundary where the angle difference between adjacent ferrite crystal orientations in all the structures is 15 ° or more is referred to as a large angle grain boundary, and the total length of large angle grain boundaries per unit area of inspection visual field is a large angle grain boundary density It is called <ρ2>. Ferrites used to identify large-angle grain boundaries include both normal ferrite structures and lamellar ferrites constituting pearlite structures. The respective measurement methods 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 needs to be 45% or less for both the wire core and the surface layer. If it exceeds 45% at the wire core, the drawability of the wire is reduced because ferrite precipitates in a massive and coarse manner. In addition, when the area ratio of the ferrite structure in the surface layer portion of the wire is more than 45%, the number of times of twisting after wire drawing decreases. It is considered that this is because deformation concentrates on the ferrite part of the surface layer part. The lower limit of the area ratio of the ferrite structure does not have to be particularly defined. The area ratio of the ferrite structure may be 0% in the central portion or the surface portion of the wire. The area ratio of the ferrite may be 43% or less, 40% or less, 35% or less, or 30% or less in the central portion or the surface portion of the wire. The area ratio of ferrite may be 10% or more, 15% or more, 20% or more, or 27% or more in the central portion or the surface layer portion of the wire.

  In addition, the area ratio of the 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 originating from the non-ferrite and non-pearlite structure is likely to be formed during wire drawing, and the wire drawability is reduced. The lower limit of the area ratio of the non-ferrite and non-pearlite structure does not have to be particularly defined. The area ratio of the non-ferrite and non-pearlite structure may be 0% in the central portion or the surface portion of the wire. That is, the total area ratio of the ferrite structure and the pearlite structure may be 100%. 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 more than 99%). The area ratio of non-ferrite and non-pearlite structures may be 1% or more, or 2% or more (that is, the total area ratio of ferrite structures and perlite structures may be less than 99%, or less than 98%).

<Density of subgrain boundary 粒 1> where angle difference of lamellar ferrite crystal orientation in pearlite structure is 2 ° or more and less than 15 °
Sub-grain boundary density 11 (density of sub-grain boundary where angle difference of crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more but less than 15 °) is 70 / mm to 600 / mm in the central part and the surface part of wire rod It needs to be. By being a wire having such a metal structure, a steel wire having a tensile strength of 2800 MPa or more after wire drawing and excellent in torsional characteristics can be stably obtained. By setting the grain boundary density to 70 / mm or more in the central portion and the surface portion of the wire, variation in strength of the steel wire after wire drawing can be suppressed, and localization of deformation during torsion test can be reduced. Therefore, even if it is a high strength steel wire, a good twisting characteristic can be obtained. Conversely, if the subgrain boundary density is less than 70 / mm in the central portion and the surface portion of the wire, the torsional characteristics do not improve when the tensile strength of the steel wire obtained after wire drawing is 2800 MPa or more. In addition, when the pearlite transformation temperature is less than 600 ° C., the twisting characteristics tend to deteriorate as described above, and the density of subgrain boundaries in the central portion and the surface portion of the wire at this time is more than 600 / mm. It is preferable to set the upper limit of to 600 / mm. For this reason, in the central portion and the surface portion of the wire, the density of the subgrain boundary which makes the angular difference of 2 ° or more and less than 15 ° of the crystal orientation of lamellar ferrite in the pearlite structure is within the range of 70 / mm to 600 / mm. Do. In the surface layer portion or the central portion of the wire, the subgrain boundary density is preferably 100 / mm or more, more preferably 120 / mm or more. The subgrain boundary density may be set to 150 / mm or more or 180 / mm or more in the surface layer portion or the central 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 central portion of the wire.

In the surface layer portion and the central portion of the wire, it is preferable that the sub-grain boundary density 11 satisfy the following equation 1. (C) in Formula 1 is C content in unit mass% in the chemical composition of a 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 central portion of the wire decreases, and the area ratio of the pearlite structure increases. As the area ratio of the pearlite structure increases, the growth distance of cementite increases, and it is considered that subgrain boundaries are more easily introduced into the pearlite structure. Therefore, the present inventors considered that the preferable range of subgrain density depends on the C content in the chemical composition of the wire. According to the findings of the present inventors, when the sub-grain boundary density in the surface layer portion and the central portion of the wire satisfies the above equation 1, the variation in the twist value of the wire is reduced, thereby further improving the torsion characteristics Be done.

<Density of large angle grain boundary と な る 2 where angle difference of crystal orientation of ferrite in steel structure is 15 ° or more>
The large angle grain boundary density 部 2 (the density of the large angle grain boundary which makes the angle difference of the ferrite crystal orientation 15 ° or more) in the surface layer portion and the central portion of the wire needs to be 200 / mm or more. When the large-angle grain boundary density is sufficiently large, the ductility of the wire is high, and the formation of coarse cracks during wire drawing can be suppressed, so the wire drawing workability is improved. Conversely, if the large-angle grain boundary density is less than 200 / mm in the surface layer portion and the central portion of the wire, wire drawability is reduced. Therefore, in the surface layer portion and the central portion of the wire, the density of the large angle grain boundary, which has an angle difference of 15 ° or more at the ferrite crystal orientation, is in the range of 200 / mm or more. In the surface layer portion or the central portion of the wire, the large angle grain boundary density is preferably 230 / mm or more. Although the upper limit of the large angle grain boundary density in the surface layer portion or central portion of the wire is not particularly defined, it is difficult in manufacture to set the large angle grain boundary density to 500 / mm or more, the large angle grain in the surface layer portion or central portion of the wire It is preferable to set the upper limit of the field density to 500 / mm. The large angle grain boundary density in the surface layer portion or central 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 central 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 the present embodiment.

<Area rate of organization>
After mirror polishing the cross section of the wire (that is, the cut surface perpendicular to the longitudinal direction of the wire), it is corroded with picral, and surface layer with 2000 times magnification using a field emission scanning electron microscope (FE-SEM) Observe and photograph 10 places at any position in the center. The area per visual field is 2.7 × 10 ⁻³ mm ² (0.045 mm in length, 0.060 mm in width).

  Next, a transparent sheet (for example, an OHP (Over Head Projector) sheet) is superimposed on each obtained photograph. In this state, the "ferrite structure" in each transparent sheet is colored. Next, the area ratio of the “colored area” in each transparent sheet is determined by image analysis software, and the average value is calculated as the average value of the area ratio of the ferrite structure. Thus, the ferrite area ratio can be determined. Next, a color is applied to another transparent sheet in the “area overlapping with the structure other than the pearlite structure and other than the ferrite structure”, and the area ratio is determined. The area ratio of non-pearlite and non-ferrite structures can be determined 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 whole structure>
After mirror polishing the cross section of the wire (that is, the cut surface perpendicular to the length direction), it is polished with colloidal silica, and the wire surface layer portion (magnification of 400 times using a field emission scanning electron microscope (FE-SEM) Each of the four fields of view is observed at a depth of 150 to 400 μm from the surface and in the center, and EBSD measurement (measurement by electron beam backscattering diffraction method) is performed. The area per field of view is 0.0324 mm ² (0.18 mm in length, 0.18 mm in width), and the measurement step is 0.3 μm.

  Next, for the results in each of the obtained measurement fields of view, the total length of the line having a subgrain boundary of 2 ° or more and less than 15 ° and the total length of the line having a large angle grain boundary of 15 ° or more are measured. For example, by using OIM analysis (OIM: Orientation Imaging Microscopy), it is possible to obtain the total line length of sub-grain boundaries and the total line length of large-angle grain boundaries. Since subgrain boundaries exist only in the part of pearlite structure, the value obtained by dividing the total length of the lines of the subgrain boundaries obtained in each measurement field of view by the pearlite area included in each measurement field of view is the subfield in each measurement field of view It is defined as grain boundary density 11.

Since the large angle grain boundary also exists at the boundary between the ferrite structure and the pearlite structure, the value obtained by dividing the total length of the line of the large angle grain boundary obtained in each measurement field by the area of each measurement field is large angle in each measurement field It is defined as grain boundary density 22.
Sub-grain boundary density 11 at an angle difference of 2 ° or more and less than 15 ° between ferrite crystal orientations in the pearlite structure of the surface layer part and the center part, and the surface layer part and the center part The large angle grain boundary density ρ2 is set such that the angle difference of the ferrite crystal orientation in the entire structure of the above is 15 ° or more. In addition, since the EBSD result is greatly affected by noise, the average CI (confidence index) uses a result of 0.60 or more, and CI of 0.10 or less is removed as noise. In addition, the removal of 0.10 or less of CI is possible within OIM analysis.

  As described above, the values of the subgrain boundary density 11 and the large angle grain boundary density 22 are not only in the above-mentioned range in the surface layer of the wire (in the range of 150 to 400 μm from the surface), but also in the wire center It is necessary to be a range. Even if the grain boundary density 11 at the wire core is 70 / mm to 600 / mm and the large angle grain boundary density 22 is 200 / mm or more, the surface layer is not in the above range, or the wire surface layer is Even if it is within the above range, when the central portion is not within the above range, the desired characteristics as a wire can not be obtained. If it can be confirmed that ρ1 and 22 in the surface portion of the wire and ρ1 and 22 in the center of the wire are within the above-mentioned range, it can be recognized that ρ1 and ρ2 fall within the above-mentioned range in the whole wire.

(D) Manufacturing Method In the method of manufacturing a wire according to this embodiment, various conditions at the time of pearlite transformation are optimized to control the structure in order to improve the twisting characteristics of the wire.
Although the wire satisfying the above requirements of the wire according to the present embodiment can obtain the effect of the wire according to the present embodiment regardless of its manufacturing method, for example, according to the manufacturing method shown below, the present embodiment A wire may be manufactured. The following manufacturing process is an example, and even if a wire having a chemical composition and other requirements in the range of the wire according to the present embodiment is obtained by a process other than the following process, the wire is used in the present invention. Needless to say, it is included.

First, steel is melted so as to become the above components, and then steel slabs are manufactured by continuous casting, and hot rolling is performed. Slab rolling may be performed after casting. When hot-rolling the obtained billet, it is heated so that a billet may become 1000-1250 ° C, and it hot-rolls to φ5.5-7.0 mm as finishing temperature 900-1000 ° C.
The heating temperature of the billet before hot rolling shall be 1000 ° C or more and 1250 ° C or less. When the heating temperature of the billet is less than 1000 ° C., the reaction force in the hot rolling increases, and when the heating temperature of the billet is more than 1250 ° C., decarburization proceeds.
The finish rolling temperature of hot rolling is 900 ° C. or more. When the finish rolling temperature is less than 900 ° C., the reaction force of the finish rolling is increased and the shape accuracy is deteriorated. On the other hand, the finish rolling temperature is set to 1000 ° C. or less. When hot rolling is performed at temperatures higher than 1000 ° C., the austenite grain size increases, and the large-angle grain boundary density after pearlite transformation decreases.

  After hot rolling, the following four stages of cooling are applied to adjust the ferrite area ratio, the density of subgrain boundaries, and the density of large angle grain boundaries. In primary cooling, it aims at suppressing grain growth of austenite by performing cooling at a rapid cooling rate, and generating a fine austenite structure. In secondary cooling, in order to make the temperature difference which exists in the wire surface layer part and the central part at the time of primary cooling small, it cools and aims to make uniform temperature from the wire surface part to the central part. In tertiary cooling, it is an object of the present invention to cool to a target pearlite transformation temperature at a cooling rate at which cooling can be performed as uniformly as possible from the surface portion of the wire rod to the center and ferrite transformation can be suppressed. In the fourth-order cooling, in order to perform pearlite transformation from the surface part of the wire rod to the central part as uniformly as possible, cooling is performed to progress the pearlite transformation so that the subgrain boundary density and the large angle grain boundary density become within the target range. With the goal. Details are shown below. The average cooling rate of primary to quaternary cooling described below refers to the amount of decrease in wire temperature from the start to the end of primary to fourth cooling in the time from the start to the end of primary to fourth cooling It is the 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. at an average cooling rate of 50 to 200 ° C./sec. The start and the end of the primary cooling are the start and the end of the spraying of the refrigerant (water).
If the average cooling rate in the temperature range of 870 ° C or higher where grain growth rate is large is less than 50 ° C / sec, and the time for which this temperature range exists is long, grain growth of austenite is promoted. Large angle grain boundary density will be reduced. Although there is no upper limit of the average cooling rate in primary cooling, the average cooling rate of more than 200 ° C./s is difficult due to the restriction of manufacturing facilities, so 200 ° C./s or less is made the upper limit of the average cooling rate in primary cooling.
When the temperature reached in the primary cooling is less than 830 ° C., there is a possibility that ferrite transformation proceeds in large amounts only in the surface layer, and the area fraction of ferrite in the surface layer 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 primary cooling was set to 870 ° C. or less.

Then, secondary cooling is performed to the range of 790 ° C. or more and 820 ° C. or less at an average cooling rate of less than 5 ° C./sec by air cooling with the atmosphere. The point of time of start of secondary cooling is equal to the point of end of spraying of the refrigerant in primary cooling, and the point of end of secondary cooling is equal to the point of start of spraying of the refrigerant in tertiary cooling. The secondary cooling is a cooling for reducing the temperature difference between the surface layer portion and the central portion of the wire rod which occurs at the time of primary cooling, and making the pearlite transformation temperature from the surface portion of the wire rod to the central portion uniform.
If the average cooling rate is 5 ° C / sec or more in secondary cooling, the temperature difference between the surface layer and the center remains, and after pearlite transformation, the large angle grain boundary density and subgrain boundary density of the surface layer of the wire And the large-angle grain boundary density at the center of the wire decreases. Therefore, the average cooling rate in 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 to improve the area fraction of ferrite. Therefore, the ultimate temperature of secondary cooling is 790 ° C. or higher. On the other hand, if secondary cooling is stopped above 820 ° C., the temperature difference to the pearlite transformation temperature between the surface layer portion and the central portion of the wire becomes large, and the temperature difference again between the surface layer portion and the central portion during tertiary cooling Will occur. Therefore, the ultimate temperature of secondary cooling is set to 820 ° C. or less. In a steel type having a high content of Si, the Ac1 temperature shifts to the high temperature side, so the ultimate temperature in secondary cooling is particularly important.
The secondary cooling time (the elapsed time between the start and the end of the secondary cooling) is preferably 5 seconds or more and 12 seconds or less. This is because grain growth of austenite grains is promoted when secondary cooling time of more than 12 seconds is applied. On the other hand, in the secondary cooling time within 5 seconds, the temperature difference in the wire may remain.

After that, third cooling is performed to a range of 600 ° C. or more and 620 ° C. or less at an average cooling rate of more than 20 ° C./s and less than or equal to 30 ° C./s. The start and the end of the tertiary cooling are the start and the end of the blowing of the atmosphere. In tertiary cooling, cooling is performed to a pearlite transformation temperature at which an optimum subgrain boundary density and large-angle grain boundary density can be obtained at a cooling rate at which ferrite transformation can be suppressed.
When the average cooling rate of tertiary cooling is 20 ° C./sec or less, ferrite transformation occurs and the ferrite area ratio becomes excessive. Therefore, the average cooling rate is more than 20 ° C./second. On the other hand, when tertiary cooling is performed at an average cooling rate of more than 30 ° C./second, only the wire surface layer portion is cooled to the target temperature, and quaternary cooling is started with the temperature of the wire core portion being excessive. . Therefore, the average cooling rate is set to 30 ° C./second or less.
When the temperature reached in the third cooling is less than 600 ° C., the pearlite structure is excessively strengthened and the twisting characteristics are degraded. Therefore, the ultimate temperature of tertiary cooling is set to 600 ° C. or more. On the other hand, when the ultimate temperature of tertiary cooling is over 620 ° C., the pearlite transformation temperature increases, the large angle grain boundary density and the subgrain boundary density decrease, and the tensile strength after the pearlite transformation also decreases. Therefore, the ultimate temperature of the third cooling was set to 620 ° C. or less.

Thereafter, fourth cooling is performed by air cooling with the atmosphere at an average cooling rate of 10 ° C./s or less to 550 ° C. or less. Note that the time of the start of the fourth cooling is equal to the time of the end of the air blowing in the third cooling. The end point of the fourth cooling is the point when the air cooling is stopped, that is, when the reheating of the wire or the blowing of the refrigerant is started. However, when the air cooling is performed until the temperature of the wire becomes 550 ° C. or less, the time when the temperature of the wire becomes 550 ° C. is regarded as the time of the end of the fourth cooling. In the fourth-order cooling, the object is to obtain a wire having uniform large-angle grain boundary density and sub-grain boundary density from the surface layer portion to the central portion by reducing the temperature difference in the wire cross section during pearlite transformation.
When the average cooling rate in the fourth-order cooling is more than 10 ° C./sec, the temperature change of the surface layer is large, and the grain boundary density decreases. Therefore, the average cooling rate in quaternary cooling is set to 10 ° C./second or less. The lower limit of the average cooling rate in the fourth-order cooling is not limited, but the cooling rate when the wire is allowed to cool is usually 2 ° C./sec or more. Therefore, 2 ° C./sec may be set as the lower limit of the average cooling rate in fourth-order cooling.
If the ultimate temperature of the fourth cooling is over 550 ° C., pearlite transformation may not end. Therefore, the ultimate temperature of the fourth cooling is set to 550 ° C. or less. Since the influence of the cooling rate in a temperature range of 550 ° C. or less on the structure is minor, accelerated cooling such as water cooling may be performed after quaternary cooling is performed to a temperature of 550 ° C. or less. In the examples described later, although the invention examples are cooled to 550 ° C. or lower by quaternary cooling and then allowed to cool to room temperature by cooling, the same is true even when cooled by other cooling means after completion of quaternary cooling. An organization is formed.

(E) Optional components:
The wire rod of the present embodiment is at least one or two types selected from the group consisting of Al, Cr, V, Ti, Nb, B, Ca, and Mg, as necessary, in place of a part of the remaining Fe. The above elements may be contained. 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%. The action and effects of the optional elements Al, Cr, V, Ti, Nb, B, Ca and Mg and the reasons for limiting the contents will be described below. % For optional components is mass%.

Al: 0 to 0.050%
It is not necessary to contain Al 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 at the ferrite crystal orientation. In order to obtain the effect surely, it is preferable to contain 0.010% or more of Al. On the other hand, Al is an element that easily forms hard oxide inclusions, so when the Al content of the wire exceeds 0.050%, coarse oxide inclusions are easily formed, The deterioration of the twisting characteristics is remarkable. 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, more preferably 0.035% or less, and still more preferably 0.030% or less.

Cr: 0 to 1.00%
It is not necessary to contain Cr in the wire of this embodiment. Cr, like Mn, is an element that enhances the hardenability of the steel and strengthens the steel. In order to ensure this effect, it is preferable to contain 0.05% or more of Cr. On the other hand, if the content of Cr exceeds 1.00%, the torsional characteristics deteriorate. Therefore, the content of Cr 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 further 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 of this embodiment. V combines with N and C to form carbides, nitrides or carbonitrides, and has an effect of refining austenite grains during hot rolling due to their pinning effect, and an effect of improving torsion characteristics 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 the twisting characteristics, the content of V is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, when the content of V exceeds 0.15%, not only the effect is saturated but also the steel pieces are cracked in the process of rolling the steel ingots and slabs into steel pieces, etc. The V content is made 0.15% or less because it adversely affects the The content of V is preferably 0.10% or less, and more preferably 0.07% or less.

Ti: 0 to 0.050%
It is not necessary to contain Ti in the wire of this embodiment. Ti combines with N and C to form carbides, nitrides or carbonitrides, and has an effect of refining austenite grains during hot rolling due to their pinning effect, and an effect of improving steel torsion characteristics There is. In order to ensure this effect, it is preferable to contain Ti in an amount of 0.002% or more. From the viewpoint of improving the torsional characteristics, the content of Ti is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, when the content of Ti exceeds 0.050%, the effect is not only saturated but also the steel pieces are cracked in the process of rolling the steel ingots and slabs into steel pieces, etc. Adversely affect Therefore, the content of Ti is set to 0.050% or less. Further, the content of Ti is more preferably 0.025% or less.

Nb: 0 to 0.050%
It is not necessary to contain Nb in the wire of this embodiment. Nb combines with N and C to form carbides, nitrides or carbonitrides, and has an effect of refining austenite grains during hot rolling due to their pinning effect, and an effect of improving steel torsion characteristics 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 the torsional characteristics, the content of Nb is more preferably 0.003% or more, and it is further 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 pieces are cracked in the process of rolling the steel ingots and slabs into steel pieces, etc. The content of Nb is made 0.050% or less. Further, the content of Nb is more preferably 0.030% or less.

B: 0 to 0.0040%
B may not be contained in the wire of this embodiment. B has an effect of reducing the ferrite structure of the steel by being contained in a small amount, and when it is desired to obtain the effect surely, it is preferable to contain B of 0.0001% or more. The inclusion of B in excess of 0.0040% not only saturates the effect but also results in the formation of coarse nitrides, which reduces the torsional properties. Therefore, the content of B when it is contained is made 0.0040% or less. When it is desired to increase the area ratio of pearlite structure, the B content is preferably 0.0004% or more, and more preferably 0.0007% or more. The content of B for improving the twisting property 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 is dissolved in MnS and has the effect of finely dispersing MnS. By finely dispersing MnS, it is possible to suppress disconnection during wire drawing caused by MnS. In order to obtain the effect by Ca certainly, it is preferable to contain Ca 0.0002% or more. In order to obtain a higher effect, 0.0005% or more of Ca may be contained. However, when the content of Ca exceeds 0.0050%, the effect is saturated. Furthermore, if the content of Ca exceeds 0.0050%, the oxide produced by reaction with oxygen in the steel becomes coarse, which in turn causes a reduction in wire drawability. Therefore, the appropriate content of Ca in the case of containing it is 0.0050% or less. The content of Ca is preferably 0.0030% or less, and more preferably 0.0025% or less.

Mg: 0 to 0.0040%
It is not necessary to contain Mg in the wire of this embodiment. Mg is a deoxidizing element and forms an oxide, but it is also an element having an interaction with MnS by forming a sulfide, and has an effect of finely dispersing MnS. This effect can suppress disconnection during wire drawing caused by MnS. In order to obtain the effect by Mg certainly, it is preferable to contain Mg 0.0002% or more. In order to obtain a higher effect, 0.0005% or more of Mg may be contained. However, when the content of Mg exceeds 0.0040%, the effect is saturated, a large amount of MgS is generated, and the wire drawability is lowered. Therefore, the appropriate content of Mg when it is contained is 0.0040% or less. The content of Mg is preferably 0.0035% or less, and more preferably 0.0030% or less.

  The remainder of the chemical composition contains "Fe and impurities". "Impurity" refers to what mixes in steel materials from the ore as a raw material, scrap, or a manufacturing environment etc., when manufacturing steel materials industrially.

  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 limits may be used. When the diameter of the wire is 3.5 mm or more, the load of hot rolling at the time of wire production can be reduced, which is preferable. When the diameter of the wire is 7.0 mm or less, it is preferable because the amount of wire drawing strain at the time of wire drawing of the wire can be suppressed.

  The steel wire which concerns on another aspect of this invention is obtained by wire-drawing the wire which concerns on this embodiment. The diameter of the steel wire is usually 0.5 to 1.5 mm in consideration of the application. In the steel wire according to the present embodiment, the chemical composition of the wire according to the present embodiment, which is the raw material, the configuration of the metal structure, the subgrain boundary density 11, and the large angle grain boundary density 22 are within the above ranges. Has excellent tensile strength and torsional properties.

  In addition, since the steel wire concerning this embodiment is manufactured through a wire drawing which has a very large amount of strain, its metal structure is significantly deformed. For example, when the enlarged photograph of the cross section of the steel wire according to the present embodiment is seen, the phase surrounded by the grain boundary is significantly crushed and the type can not be determined. It is also extremely difficult to identify the presence of subgrain boundaries and large angle grain boundaries. That is, it is extremely difficult to specify the metallographic structure and other configuration of the steel wire according to the present embodiment by a usual method of specifying the structure (for example, taking a photograph of metallographic structure, crystal structure analysis by EBSD, etc.). It is impossible or almost not practical to directly identify the metallographic structure of the steel wire according to the present embodiment by its structure or characteristics.

  The method for manufacturing a steel wire according to another aspect of the present invention includes the step of drawing a wire according to the present embodiment. The wire drawing is performed at a reduction of area such that the diameter of the finally obtained steel wire 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 11 and the large angle grain boundary density 22 are within the above ranges, the production of the steel wire according to the present embodiment using them The method can suppress the number of breaks to a very low level, and can obtain a steel wire having excellent tensile strength and torsional characteristics.

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

  Steels having the chemical compositions shown in Tables 1 and 2 were melted, and wire rods were produced by the following method. In addition, the description of "-" in Table 1 and Table 2 shows that content of the said element is an impurity level and it can be judged that it does not contain substantially. The balance of the chemical composition of the steels shown in Tables 1 and 2 is iron and impurities.

  First, a steel A having a chemical composition shown in Table 1 was melted by a converter, and a billet of 122 mm square was obtained by mass rolling in a usual manner. Next, after heating so that a steel piece might be set to 1050-1150 degreeC, it hot-rolled to (phi) 6 mm in the range of 900-1000 degreeC of finishing temperature.

The adjustment cooling after finish rolling performed cooling on the conditions shown to (A1)-(A21) shown by Table 3-1-Table 3-3.
Specifically, with regard to (A1) to (A7), after cooling (primary cooling) to 830 to 870 ° C. within the range of an average cooling rate of 50 to 200 ° C./sec by water cooling, then air cooling by the atmosphere Air cooling (secondary cooling) within the range of 790 ° C. or more and 820 ° C. or less at an average cooling rate of less than 5 ° C./sec. After that, cooling (third cooling) is applied to 600 to 620 ° C. at more than 20 ° C./s and not more than 30 ° C./s, cooling to 10 ° C./s or less to 550 ° C. or less (quaternary cooling). It cooled down.
About (A8)-(A17), four types of adjustment cooling were performed on the conditions different from said cooling conditions, and the wire was obtained. The underlined values in Table 3-1 are inappropriate values under the manufacturing conditions of the wire according to the present invention.

With regard to (A18) to (A21), four types of adjustment cooling were not performed, and cooling was performed under the conditions shown in Tables 3-2 to 3-4. The terms “primary cooling” and the like in these tables are merely to distinguish the cooling stages, and are different from the primary cooling to the fourth cooling included in the manufacturing method of the present invention.
Specifically, as for (A18), immersion in a salt bath at 550 ° C. was performed instead of the third cooling and the fourth cooling in the production method of the present invention.
With regard to (A19), reheating to 950 ° C. and temperature holding for 60 seconds are carried out on the wire after completion of the above-mentioned hot rolling, and immersion in a salt bath of 550 ° C. immediately after the temperature holding is completed. Carried out.
With regard to (A20), after primary cooling was carried out, cooling was performed by blowing air, and after cooling to 680 ° C. at an average of 1.0 ° C./sec, it was switched to free cooling and was cooled to 550 ° C. or less.
With regard to (A21), after primary cooling was carried out, blast cooling was performed to cool the wire to 700 ° C. at 10 ° C./sec, and then cooling was performed to 550 ° C. at 5 ° C./sec by air cooling.

  Moreover, the hot rolling wire rod was created from steel az of a chemical composition shown in Table 2 by the method similar to (A1) of Table 3-1. Thereafter, dry wire drawing, plating, and wet wire drawing were performed to obtain a steel wire with a wire diameter of 0.5 mm. The underlined values in Table 2 are outside the desirable range of the present invention.

  The tensile strength, reduction of area, ferrite area ratio, non-pearlite and non-ferrite area ratio, subgrain density パ ー 1 in pearlite structure for the wires of test numbers A1 to A21 and test numbers 1 to 26 obtained as described above (Density of subgrain boundary where angle difference of crystal orientation of lamellar ferrite in pearlite structure is 2 ° or more and less than 15 °), large angle grain boundary density 2 2 (large angle of angle difference of ferrite crystal orientation of 15 or more in whole observation structure) The density of grain boundaries 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. Underlined values in Tables 4-1, 4-2, 5-1, and 5-2 are values outside the scope of the present invention. Underlined values in Tables 4-3 and 5-3 are values that do not meet the acceptance criteria of the present invention.

  When area ratio of ferrite structure in surface layer part and center part of wire, area ratio of non-ferrite and non pearlite structure, subgrain boundary density 11, large angle grain boundary density 22, wire of diameter 6mm is drawn to 0.5mm in diameter Number of breaking of wire, wire rod before wire drawing and tensile strength of steel wire after wire drawing (steel wire strength), and torsion characteristics of steel wire after wire drawing (twisting number, twisting number variation, and delamination) The presence or absence) was investigated by the method described below, respectively.

Area ratio of ferrite structure of the wire <1>, 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 10 arbitrary points in the surface portion of the wire and in the center were observed at a magnification of 2000 times using FE-SEM, and photographed. The area per visual field is 2.7 × 10 ⁻³ mm ² (0.045 mm in length, 0.060 mm in width). An OHP sheet was overlaid on each of the obtained photographs, and colors were applied to the "ferrite structure" and "the area overlapping the non-perlite and non-ferrite structure" in each transparent sheet. Subsequently, the area ratio of the "colored area" in each transparent sheet was determined by image analysis software, and the average value was calculated as an average value of the area ratio of ferrite structure and non-perlite and non-ferrite structure, respectively.

Subgrain boundary density 11 and large angle grain boundary density 22 of wire rod:
The cross section of the wire is mirror-polished and then polished with colloidal silica, and four points in the surface portion of the wire and the center are observed at a magnification of 400 times using FE-SEM, and EBSD measurement made by TSL (TexSEM Laboratories) The analysis was performed using the device. The area at the time of measurement was 180 × 180 μm ² , and the step was 0.3 μm. Then, for each result obtained, using OIM analysis, the total length of subgrain boundaries with an angle difference of 2 ° or more and less than 15 °, and the total length of large angle grain boundaries with an angle difference of 15 ° or more Each was measured. Sub-grain boundary density is determined by dividing the total length of sub-grain boundary line having an angle difference of 2 ° or more and less than 15 ° by the average value of pearlite area ratio, and the line of large angle grain boundary having an angle difference of 15 ° or more Large-angle grain boundary density was determined by dividing the total length of H by the area of one field of view.

<3> Wire drawing processability of wire material A wire drawing process was performed on 50 kg of each wire material, and the number of disconnections during the wire drawing process 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 frequency | count of disconnection at the time of drawing a 50 kg wire from 6.0 mm in diameter to diameter 0.5 mm is 0 times, wire drawability is evaluated as favorable, and the frequency | count of disconnection is 1 or more times, It evaluated that wire drawability was bad. In addition, regarding the wire which stopped drawing, it was judged that it was obviously unsuitable as a material of a steel wire, and the subsequent evaluation test was not implemented. In the items which were not evaluated, the code "-" was described.

<4> Wire rod and tensile strength of steel wire after wire drawing:
The wire rod and the steel wire were cut to a length of 200 mm, and the upper and lower 50 mm were fixed by a wedge chuck or an air chuck and subjected to a tensile test. The tensile strength was calculated by dividing the obtained maximum load by the cross-sectional area. After that, measure the wire diameter at the narrowest point of the wire diameter after the tensile test, divide the amount of change in cross sectional area before and after the tensile test by the cross sectional area before the tensile test, and multiply by 100%. The value was calculated.

  Since it is preferable that the tensile strength is 2800 Mpa or more, the steel wire used for the wire which is a reinforcing material of a tire for motor vehicles evaluated the tensile strength 2800 Mpa or more as a passing goods. As for the tensile strength of the wire, no pass / fail criteria were set.

<5> Torsional characteristics of wire after wire drawing:
The torsion test was to twist a steel wire having a length of 100 times the wire diameter (diameter) until it broke at 15 rpm, and it was judged whether or not delamination occurred by a torque (resistance to 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 decreased sharply before disconnection. The torsion test was performed for each steel wire 10 each, no delamination occurred, and when the average value of the number of times of torsion of 10 steel wires was 23 or more, the torsion characteristics were evaluated as good. .

Moreover, when the dispersion | variation in the frequency | count of a twist in the above-mentioned 10 twist tests is small, it can be judged that a twist characteristic is still more favorable. Therefore, the 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 difference, whichever is larger, was calculated. It was judged that the variation in twisting number was good for a steel wire where the variation was 3 times or less.
A steel wire in which the average value of the number of times of twisting, the delamination, and the variation in the number of times of twisting are all judged to be good has very good twisting characteristics. However, even if the number of times of twisting of the steel wire is more than three times, it is considered that the steel wire judged to be good for the evaluation of other twisting characteristics has good twisting characteristics even in view of the expected application. I can say that.

  The results evaluated above are summarized in the following table.

  As shown in the table, all of the samples A1 to A7 according to the present invention satisfy the requirements of the present invention and the manufacturing conditions of the steel material are appropriate, so that the strength after wire drawing is 2800 MPa or more The number of twisting was 23 or more, and no delamination occurred, resulting in no problem.

On the other hand, in the sample of A8, the average cooling rate in primary cooling was low, and the austenite grain size was coarsened, so that ρ2 was decreased, wire breakage occurred during wire drawing, and wire drawability was poor.
In the sample of A9, the ferrite surface ratio increased in the surface layer and the number of times of twisting decreased because the temperature reached in the primary cooling was low.
In the sample of A10, the temperature reached in the primary cooling was high, and the grain size of the austenite was coarsened, so that が 2 decreased and a break occurred.
In the sample of A11, the time for secondary cooling was long, and since the austenite grain size was coarsened, ρ2 decreased and a break occurred.
In the sample of A12, the temperature reached in the secondary cooling was low, so the ferrite area ratio was high, the drawability was poor, and both the steel wire strength and the torsion characteristics were low.
In the sample of A13, the average cooling rate in tertiary cooling is small, ferrite transformation progresses, the ferrite area ratio becomes high, the wire drawability is poor, and both the steel wire strength and the torsion characteristics are low.
In the sample of A14, the ultimate temperature of the third-order cooling was high, pearlite transformation was performed at high temperature, both ρ1 and ρ2 were low, disconnection occurred at the time of wire drawing, and the torsional characteristics were also poor.
In the sample of A15, the ultimate temperature in the third-order cooling was low, and が 1 was too high, so that the torsional characteristics were poor.
In the sample of A16, the average cooling rate in the fourth-order cooling was high, ρ1 at the surface portion of the wire decreased, delamination occurred during the torsion test, and the torsion characteristics were poor.
The sample of A17 was obtained according to the manufacturing conditions in which air cooling was stopped and blast cooling was started when the temperature of the wire reached the temperature shown in the table in the fourth cooling. In the sample of A17, the ultimate temperature in the fourth cooling was high, and pearlite transformation did not end, and wire drawability decreased because the non-pearlite and non-ferrite area ratio were high.
In the sample of A18, since the wire was immersed in a salt bath of 550 ° C. after 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 torsion characteristics were poor.
In the sample of A19, since the wire was immersed in a salt bath of 550 ° C. after reheating and holding the temperature of the wire, the wire was rapidly cooled to 550 ° C. As a result, in A19, ρ1 was high, delamination occurred during the torsion test, and the torsion characteristics were poor.
In the A20 sample, the cooling rate after hot rolling was slow, and pearlite transformation occurred at high temperature. Since the pearlite transformation temperature was high, in A20, both 低 く 1 and で は 2 were low, disconnection occurred at the time of wire drawing, and the torsional characteristics were also poor.
In the sample of A21, since the wire is cooled to 700 ° C. by blast cooling after secondary cooling, the surface portion of the wire is rapidly cooled, and ρ1 in the surface portion is increased, and delamination occurs during the torsion test. , Torsional characteristics were bad.

  Further, from the results shown in the table, in the samples of the test numbers 1 to 19 and 26 of the present invention example, the chemical composition satisfies the desirable range of the present invention, and the production conditions of the wire are also appropriate. It has good processability, good torsional properties after wire drawing, and also has the necessary tensile strength.

However, the sample of Test No. 20 had a low content of C, the ferrite area ratio became too large, and the strength of the steel wire was insufficient.
The sample of Test No. 21 had a high content of C, and the steel was hardened excessively, so the wire drawability decreased, and a break occurred during wire drawing.
The sample of Test No. 22 had low ρ 1 due to the low content of Si, and delamination occurred during the torsion test.
In the sample of Test No. 23, the content of Mn was too high, and a large amount of non-ferrite and non-pearlite structure caused breakage during wire drawing.
The sample of Test No. 24 had a low content of Si and a low ρ 1, and delamination occurred during the torsion test.
The sample of Test No. 25 had a low content of Mn and a low ρ1, and delamination occurred during the torsion test.

  It is a wire which specified C, Si, Mn, N, P, and S in the desirable range described above from the results shown in the table, and the main structure is pearlite, and the ferrite structure is 45% or less, non-ferrite And the grain boundary density 11 at which the non-pearlite structure is 5% or less and the angular difference of the crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more but less than 15 ° is 70 / mm ≦ ρ1 ≦ 600 / mm, High tensile strength can be obtained after wire drawing if the wire material satisfies the fact that the large angle grain boundary density と な る 2 with an angle difference of 15 ° or more at a ferrite crystal orientation at 200 ° C. is 200 / mm or more It has been found that it is possible to provide a wire rod for wiredrawing, which is capable of producing a steel wire which can be stably twisted without occurrence of delamination during a torsion test after processing, and which has excellent wiredrawing properties. That is, there is provided a wire rod for wiredrawing processing which can stably produce a steel wire having high strength suitable as a material such as wire and further having excellent torsional characteristics while suppressing disconnection during wiredrawing. I was able to.

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

Claims (10)

Wire rod,
The chemical composition is in 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 pearlite in both the surface layer portion having a depth of 150 to 400 μm from the surface of the wire and the central portion ranging from the central axis of the wire to 1/10 of the diameter of the wire The area ratio of ferrite structure in a cross section perpendicular to the longitudinal direction of the wire is 45% or less, and the area ratio of non-pearlite and non-ferrite structure in the cross section is 5% or less, in the pearlite structure And the density ρ1 of the sub-grain boundary where the angular difference in the crystal orientation of lamellar ferrite is 2 ° or more and less than 15 ° is 70 / mm ≦≦ 1 ≦ 600 / mm, and the angle difference of ferrite crystal orientation in all the structures is 15 ° A wire characterized in that the density 22 of the large angle grain boundary which is the above is 200 / mm or more. The chemical composition is, in mass%,
Al: 0.010 to 0.050%
The wire according to claim 1, characterized in that The chemical composition is, in mass%,
Cr: 0.05 to 1.00%
The wire according to claim 1 or 2, characterized in that The chemical composition is, in 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, which contains one or more selected from the group consisting of The chemical composition is, in mass%,
B: 0.0001 to 0.0040%
The wire according to any one of claims 1 to 4, which contains The chemical composition is, in mass%,
Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0040%
The wire rod according to any one of claims 1 to 5, containing one or two selected from the group consisting of The wire according to any one of claims 1 to 6, wherein the density 11 of the subgrain boundary in the surface layer portion and the central portion of the wire satisfies the following Formula 1.
220 × (C) +100 <ρ1 <220 × (C) +300: Formula 1
(C) in the said Formula 1 is C content in the mass% in the said chemical composition of the said wire.   The wire according to any one of claims 1 to 7, wherein the diameter of the wire is 3.5 to 7.0 mm. The wire according to any one of claims 1 to 8, which is used as a material of a steel wire . A process of drawing a wire according to any one of claims 1 to 10 to obtain a steel wire,
The diameter of the said steel wire is 0.5-1.5 mm, The manufacturing method of the steel wire characterized by the above-mentioned.

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