CN110621799B - Wire rod, steel wire, and method for manufacturing steel wire - Google Patents

Abstract

A wire rod according to one aspect of the present invention has a chemical composition within a predetermined range, wherein the main structure of both the surface layer portion and the center portion is a pearlite structure, the area ratio of the ferrite structure is 45% or less, the area ratio of the non-pearlite and non-ferrite structure is 5% or less, the density ρ 1 of a subgrain boundary where the angle difference of crystal orientation of lamellar ferrite in the pearlite structure is 2 DEG or more and less than 15 DEG is 70/mm or less ρ 1 or less 600/mm, and the density ρ 2 of a large-angle grain boundary where the angle difference of ferrite crystal orientation in the entire structure is 15 DEG or more is 200/mm or more.

Description

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

Technical Field

The present invention relates to a wire rod, a steel wire, and a method for manufacturing the steel wire.

This application claims priority based on patent application No. 2017-099227, filed in 2017, month 5, and day 18, the contents of which are incorporated herein by reference.

Background

The present invention relates to a wire rod widely used as a raw material of a high-strength steel wire used for a wire used as a reinforcing material for tires of automobiles and the like, a reinforcing wire such as an aluminum wire, a PC steel wire, a wire for ropes used for bridges, and the like. The present invention also relates to a steel wire obtained from the wire rod, and a method for producing the steel wire using the wire rod.

Wire rods are manufactured by hot rolling and are processed into wires by wire drawing to a predetermined wire diameter. In the middle of wire drawing, a lead bath quenching (drawing) treatment is performed about 1 to 2 times to draw a thin steel wire, and thus a wire rod is required to have high wire drawing workability.

For example, a reinforcing material having a wire diameter of 0.5mm or more used for a large automobile tire or the like is required to have improved productivity. There is a need for a wire rod that can stably produce a steel wire having a wire diameter of 0.5mm to 1.5mm at low cost from a wire rod having a wire diameter of 3.5mm or more that can be produced by stable hot rolling. Therefore, development of wire rods having wire drawability that can omit the intermediate patenting step performed during wire drawing and that can exhibit stable torsional characteristics after wire drawing has been advanced.

However, there is a situation in which wire breakage during wire drawing is more likely to occur in a wire produced through a step of wire drawing to a high degree of wire drawing. Further, a steel wire drawn to a high degree of drawing tends to have deteriorated torsional characteristics. Further, the large wire diameter of the wire rod as the material of the steel wire is disadvantageous to the twisting characteristics of the steel wire.

In order to prevent breakage in wire drawing, methods for improving the structure of a wire have been proposed. As such a technique, for example, patent document 1 (japanese patent application laid-open No. 2014-055316) proposes a high-strength steel wire rod in which 50% or more of layered cementite having an aspect ratio of 10 or more is present in terms of number based on the total number of layered cementite, and by providing such layered cementite, the reduction in wire drawability is prevented.

Further, patent document 2 (japanese patent application laid-open No. 2000-119756) proposes a wire rod for a high-strength steel wire, which is configured such that the fraction of pro-eutectoid ferrite is 10% or less and the remaining portion includes pearlite (pearlite) in which cementite (cementite) is discontinuously formed, thereby preventing a reduction in wire drawability.

These techniques are all techniques for improving wire drawability by controlling the morphology of cementite of a wire rod, thereby preventing occurrence of longitudinal fracture or the like in a wire drawing process until a steel wire having a wire diameter of 0.1 to 0.4mm is obtained. However, merely controlling the form of cementite cannot suppress the variation in strength in the cross section of the steel wire. Therefore, when a steel wire having a wire diameter of 0.5mm or more is manufactured by the techniques disclosed in these patent documents, the suppression of the occurrence of wire breakage and the suppression of the deterioration of the twisting characteristics cannot be effectively achieved at the same time, and the above-described problem may occur.

From such a situation, it is desired to realize a wire rod which is less likely to cause wire breakage in a step of producing a steel wire having a large diameter (for example, a wire diameter of 0.5mm or more) through wire drawing to a high degree of wire drawing and which has excellent torsional characteristics after wire drawing.

Prior art documents

Patent document

Patent document 1 Japanese laid-open patent application publication No. 2014-055316

Patent document 2 Japanese laid-open patent publication No. 2000-119756

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a wire rod capable of stably producing a steel wire having high strength and excellent twisting characteristics, which is suitable as a raw material for a large-diameter wire or the like, by suppressing breakage during a wire drawing process. 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.

The present inventors have conducted various studies to solve the above problems. As a result, the following findings were obtained.

(I) When a wire rod having a wire diameter of 5.5mm or more is wire-drawn to a level of 0.5mm, the strain in wire drawing is 4.5 or more. When the eutectoid steel or hypereutectoid steel having poor wire drawability is subjected to wire drawing at such a high degree of wire drawing, patenting is required during the wire drawing. On the other hand, it is found that by using a hypoeutectoid steel having a small cementite fraction as a material of a wire rod, the wire rod can be improved in wire drawability, and the wire rod can be subjected to wire drawing with a wire drawing strain of 4.5 or more.

On the other hand, it is found that when the area ratio of ferrite at the center portion of the cross section (i.e., the cut surface perpendicular to the longitudinal direction of the wire rod) of the wire rod of the hypoeutectoid steel exceeds 45%, the ferrite structure becomes massive and coarse, and therefore, even the wire rod of the hypoeutectoid steel is insufficient in wire drawability. In addition, when the crystal grain size is coarse (that is, when the grain boundary density at a large angle is small), the wire rod has a low reduction of area value and poor ductility, and therefore coarse cracks are likely to be formed in the wire rod during wire drawing, and the wire drawing workability is deteriorated. Further, it is found that, when the area ratio of ferrite in the surface layer portion exceeds 45% in the cross section of the wire rod of the hypoeutectoid steel, the torsional characteristics after the wire drawing process are degraded. This is considered to be due to the concentration of deformation in the ferrite structure.

(III) after pearlite transformation, a large amount of lamellar ferrite (hereinafter referred to as lamellar ferrite) in the pearlite structure is introduced into the subgrain boundaries. The present inventors examined the relationship between the density of the subgrain boundary of the wire rod and the torsional characteristics after the wire rod is drawn (hereinafter, sometimes simply referred to as torsional characteristics). As a result, it was found that basically, the higher the density of the subgrain boundary in the pearlite structure, the better the torsional characteristics were obtained. This is considered to be because the more the subgrain boundary, the more uniformly the working strain is introduced during the drawing process, and the more the variation in strength in the cross section of the steel wire is reduced.

(IV) the present inventors have studied means for increasing the density of subgrain boundaries. It is considered that the subgrain boundaries are introduced to eliminate the mismatch between the two phases when lamellar ferrite and lamellar cementite (hereinafter referred to as lamellar cementite) in the pearlite transformation grow in a coordinated manner. The present inventors have found that the subgrain boundary density can be adjusted using the pearlite transformation temperature and the content of alloying elements (e.g., Si) solid-dissolved in the lamellar ferrite.

Specifically describing the relationship between the pearlite transformation temperature and the subgrain boundary density, it is known that: when the pearlite transformation temperature is 550 ℃ or lower, the subgrain grain density in the lamellar ferrite decreases as the pearlite transformation temperature decreases. This is considered to be due to an increase in the number of parts where the growth of lamellar cementite is interrupted. On the other hand, when the pearlite transformation temperature is in a temperature range higher than 550 ℃, the subgrain density tends to gradually decrease as the temperature becomes higher. This is considered to be because, when the pearlite transformation temperature is higher than 550 ℃, the lamellar spacing becomes larger as the temperature is higher, the number of lamellar cementite pieces decreases, the total amount of mismatching decreases, and therefore the subgrain boundary density in lamellar ferrite decreases. From these results, it is understood that when the cooling conditions are controlled so that the pearlite transformation temperature is near 550 ℃, the maximum amount of subgrain boundaries is introduced.

Further, specifically describing the relationship between the amount of the alloying element and the subgrain boundary density, it is considered that increasing the content of the alloying element represented by Si increases the mismatch at the interface between the lamellar ferrite and the lamellar cementite, and the subgrain boundary density increases.

However, as a result of repeating experiments for increasing the subgrain density in the pearlite structure based on the above findings, it was found that the wire rod after wire drawing had low torsional properties despite its high subgrain density. Although the reason is not clear, the number of twists after wire drawing tends to decrease when the density of the subgrain boundaries is increased by advancing pearlite transformation at a temperature of less than 600 ℃. Therefore, it is considered that a wire rod having both drawability and torsional characteristics after drawing can be obtained by not increasing the subgrain density in the lamellar ferrite by performing pearlite transformation at 600 to 620 ℃ rather than near 550 ℃ at which the subgrain density can be maximized.

From the above-described findings (I) to (V), in order to realize a wire rod capable of suppressing wire breakage during wire drawing and stably producing a steel wire having high strength and excellent torsional characteristics suitable even as a raw material for a large-diameter wire or the like, it is necessary to use a hypoeutectoid steel as a material for the wire rod. Further, it is also necessary to control the ferrite fraction, high angle grain boundary density and sub-grain boundary density of the wire rod within appropriate ranges by adjusting the content of the gold element and adjusting the cooling conditions after hot rolling to adjust the pearlite transformation temperature within appropriate ranges. The present inventors have found that a wire rod obtained by increasing the high angle grain boundary density and the subboundary density by adjusting the contents of the alloying elements and the cooling conditions after hot rolling as described above is superior in wire drawability and torsional properties after wire drawing to other wire rods having the same strength level.

The present invention has been completed based on the above-described findings, and the gist thereof is as follows.

(1) The wire rod according to one aspect of the present invention has a chemical composition containing, in mass%, C: 0.30 to 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 to 0.0040% and the balance being Fe and impurities, wherein the main structure is a pearlite structure, the area ratio of the ferrite structure in a cross section perpendicular to the longitudinal direction of the wire rod is 45% or less, the area ratio of the non-pearlite and non-ferrite structures in the cross section is 5% or less, the density ρ 1 of a sub-grain boundary in which the angle difference of the crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 ° is 70/mm or less 1 or less 600/mm, and the density ρ 2 of a large-angle grain boundary in which the angle difference of the crystal orientation of ferrite in the entire structure is 15 ° or more is 200/mm or more, in both a surface layer portion which is a range of a depth of 150 to 400 μm from the surface of the wire rod and a central portion which is a range of 1/10 of the diameter of the wire rod from the central axis of the wire rod.

(2) In the wire rod according to the above (1), the chemical composition may contain, in mass%, Al: 0.010-0.050%.

(3) In the wire rod according to the above (1) or (2), the chemical composition may contain, in mass%, Cr: 0.05 to 1.00 percent.

(4) The wire rod according to any one of the above (1) to (3), wherein the chemical composition may contain, in mass%, a chemical component selected from the group consisting of: 0.005-0.15%, Ti: 0.002 to 0.050%, and Nb: 0.002-0.050% of 1 or more than 2.

(5) The wire rod according to any one of (1) to (4) above, wherein the chemical composition may contain, in mass%, B: 0.0001 to 0.0040%.

(6) The wire rod according to any one of the above (1) to (5), wherein the chemical composition may contain, in mass%, a chemical component selected from the group consisting of Ca: 0.0002-0.0050% and Mg: 0.0002 to 0.0040% of 1 or 2 kinds.

(7) In the wire rod according to any one of the above (1) to (6), the density ρ 1 of the subgrain boundary may satisfy the following formula 1 in the surface layer portion and the central portion of the wire rod.

220 × (C) +100 < ρ 1 < 220 × (C) +300 … formula 1

The component (C) in the above formula 1 is a content of C in mass% in the above chemical composition of the wire rod.

(8) The wire rod according to any one of the above (1) to (7), wherein the diameter of the wire rod may be 3.5 to 7.0 mm.

(9) The wire rod according to any one of (1) to (8) above, which may be used as a material for a steel wire.

(10) A steel wire according to another aspect of the present invention is produced by drawing the wire rod according to any one of (1) to (9) above, and has a diameter of 0.5 to 1.5 mm.

(11) A method for producing a steel wire according to another aspect of the present invention includes a step of obtaining a steel wire by drawing the wire rod according to any one of (1) to (9), wherein the steel wire has a diameter of 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 torsional characteristics, which is suitable as a raw material for a wire or the like, can be stably produced while suppressing breakage during a wire drawing process, and is extremely useful industrially. The steel wire according to one aspect of the present invention has high strength and excellent torsional characteristics, and is therefore suitable as a raw material for, for example, a metal wire, and is extremely useful industrially. The method for producing a steel wire according to one aspect of the present invention is industrially extremely useful because it can stably produce a steel wire having high strength and excellent torsional characteristics suitable as a raw material for a metal wire while suppressing breakage during drawing.

Drawings

Fig. 1 is a schematic view showing a surface layer portion and a central portion of a wire rod according to the present embodiment.

Fig. 2 is an explanatory view showing an example of the pearlite structure.

Detailed Description

The following describes in detail an embodiment as an example of the wire rod according to the present invention.

As shown in fig. 1, in the wire rod 1 according to the present embodiment, for convenience, the range of the depth of 150 to 400 μm from the surface of the wire rod is defined as the surface portion 11, and the range of 1/10 from the central axis of the wire rod to the diameter d of the wire rod is defined as the central portion 12. In the present specification, the numerical range expressed by the term "to" means a range including numerical values described before and after the term "to" as a lower limit value and an upper limit value.

The wire rod of the present embodiment is a wire rod that can be used as a raw material of a steel wire that is suitable as a raw material of a metal wire used as a reinforcing material of a tire of an automobile or the like, a reinforcing metal wire such as an aluminum feed wire, a PC steel wire, a rope wire used for a bridge or the like, and the like.

The drawing workability of a wire rod is an index indicating the degree of difficulty in generating wire breakage when a wire rod is drawn to obtain a steel wire. The torsional characteristics after wire drawing of a wire rod are indicators indicating the degree of difficulty in delamination (delamination), the degree of difficulty in torsional breakage, and the like when a steel wire obtained by wire drawing of a wire rod is subjected to a torsion test. The wire rod according to the present embodiment preferably has the following wire drawability: the number of wire breaks when 50kg of a 6.0mm diameter wire rod was prepared and drawn to a diameter of 0.5mm was 0. Further, the steel wire after drawing preferably has a tensile strength of 2800MPa or more. The steel wire used for the metal wire preferably has the following torsional characteristics: even when 10 twisting tests were carried out, delamination did not occur 1 time, and the average value of the number of twists reached 23 or more. A steel wire twisted 23 times or more can be determined to have sufficient ductility so as not to break during a treatment such as straightening after drawing.

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

(A) With respect to chemical composition

First, the chemical composition of the wire rod of the present embodiment is explained. Hereinafter, the unit of the content of the chemical composition is mass%.

C：0.30～0.75％

C is an element for reinforcing steel. To obtain this effect, 0.30% or more of C must be contained. On the other hand, if the content of C exceeds 0.75%, the cementite fraction increases, and the wire drawability decreases. Therefore, the content of C is preferably 0.30% to 0.75%. Further, from the viewpoint of suppressing the formation of cracks, the content of C is preferably 0.35% or more, and more preferably 0.40% or more. On the other hand, from the viewpoint of improving the wire drawability, the C content is preferably less than 0.75%, or 0.70% or less, and more preferably 0.65% or less. The C content may be 0.42% or more, or 0.45% or more. The C content may be 0.60% or less, or 0.55% or less.

Si：0.80～2.00％

Si is a component that not only improves the strength of the wire rod, but also contributes to an increase in the density of the subgrain boundary. However, if the Si content of the wire rod is less than 0.80%, the effect of increasing the density of the subgrain boundary due to the Si content cannot be sufficiently obtained. On the other hand, if the Si content of the wire rod exceeds 2.00%, the ferrite fraction increases, and the wire drawability decreases. Therefore, the Si content of the wire rod is defined to be in the range of 0.80 to 2.00%. In order to stably obtain a wire rod having a desired microstructure, the Si content of the wire rod 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 rod may be 1.90% or less, 1.80% or less, 1.75% or less, or 1.70% or less.

Mn：0.30～1.00％

Mn is an element that has an effect of improving the strength of the steel wire, and also has an effect of fixing S in the steel as MnS to prevent hot brittleness of the steel wire. However, if the Mn content is less than 0.30%, the above effect is insufficient. Therefore, the lower limit of the Mn content is set to 0.30% or more. Further, in order to secure the strength of the steel wire and prevent hot shortness at a higher level, the Mn content is preferably 0.35% or more, more preferably 0.40% or more. The Mn content may be 0.50% or more, or 0.55% or more.

On the other hand, Mn is an element that is easily segregated. When Mn is contained in an amount exceeding 1.00%, Mn is concentrated particularly in the central portion, martensite and bainite are formed in the central portion, and wire drawability is lowered. Further, formation of coarse MnS also causes a reduction in wire drawability. The Mn content is preferably 0.90% or less, 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 rod by being fixed to dislocations during the wire drawing process in a cold state, but decreases the torsional characteristics. If the N content of the wire rod exceeds 0.0080%, the deterioration of the twisting characteristics becomes remarkable. Therefore, the N content of the wire rod is limited 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 content of N, the better, N may not be contained in the wire rod. 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: less than 0.030%

P is an element that segregates in the grain boundaries of the wire rod to degrade the torsional properties. If the P content of the wire rod exceeds 0.030%, the deterioration of the torsional characteristics becomes remarkable. Therefore, the P content of the wire rod is limited to 0.030% or less. The upper limit of the P content is preferably 0.025% or less. The lower the P content, the better, P may not be contained in the wire rod. 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. When the S content of the wire rod exceeds 0.020%, the drawability is remarkably reduced. Therefore, the S content of the wire rod is limited to 0.020% or less. The preferable upper limit of the S content is 0.010% or less. The lower the S content, the better, the wire rod may not contain S. 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 rod by forming an oxide. When the O content of the wire rod exceeds 0.0070%, the deterioration of the torsional characteristics becomes remarkable. Therefore, the O content of the wire rod is limited 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, O may not be contained in the wire rod. The O content may be 0.0005% or more, or 0.0010% or more. The O content may be 0.0045% or less, or 0.0040% or less.

(B) About the structure of the wire

Next, the metal structure of the wire rod according to the present embodiment will be described. The requirements for the metallic structure of the wire rod described below need to be satisfied in both the surface portion 11 and the central portion 12 of the wire rod 1.

The surface layer portion and the central portion of the wire rod need to have a metal structure as follows: the main structure is a pearlite structure, and in a cross section of the wire rod, the area ratio of the ferrite structure is 45% or less, the area ratio of the non-pearlite and non-ferrite structures is 5% or less, the subgrain boundary density rho 1 in which the angle difference of the crystal orientation of lamellar ferrite in the pearlite structure is 2 DEG or more and less than 15 DEG is 70/mm or less rho 1 or less 600/mm, and the large-angle grain boundary density rho 2 in which the angle difference of the crystal orientation of ferrite in the entire structure is 15 DEG or more is 200/mm or more. The "main structure" means a structure occupying the largest area ratio among metal structures. The "area ratio" means an area ratio measured in a cross section perpendicular to the longitudinal direction of the wire rod, and its measuring method will be described later. In other words, the surface layer portion and the central portion of the wire rod according to the present embodiment include a pearlite structure having an area ratio of 50% or more.

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

Here, the grain boundaries of the pearlite structure are described.

In the general technical common knowledge, pearlites are stated as: a lamellar structure in which lamellar ferrite and lamellar cementite are arranged in layers is formed by a eutectoid reaction from austenite, and a lower structure of a layer is formed inside the lamellar ferrite and the lamellar cementite. The region surrounded by high angle grain boundaries is called a block (block), and the region in which the orientation of the plies is the same in the block is called a cluster (colony). In other words, the structure in which the cementite sheet has several orientations and is dispersed in each grain of the ferrite structure is recognized as pearlite.

However, it is considered that the actual pearlite structure is not so simple. Fig. 2 is a schematic view showing an example in which a pearlite structure is purified. In the microstructure shown in fig. 2, a block surrounded by bent high angle grain boundaries 22 is generated starting from the original γ grain boundaries 21 (original austenite grain boundaries), and subboundaries 23 are formed in the block. The crystal orientation in the block changes to a large number of random orientations, and in the structure of fig. 2, the total length of the chain lines indicating the subgrain boundaries 23 can be identified as the total length of the subgrain boundaries 23. In the schematic diagram of fig. 2, the total of the lengths of the high angle grain boundaries 22 (the length of the thick solid line surrounding the block) constituting the outer periphery of the block and the like can be recognized as the length of the high angle grain boundaries 22. Fig. 2 shows an enlarged view of the layered structure of the layered cementite 31 and the layered ferrite 32 constituting the layered structure.

The "pearlite structure" of the wire rod according to the present embodiment includes a so-called pseudo pearlite structure (pearlite structure generated without lamellar cementite 31 growing in a plate shape). The pseudo-pearlite structure is different from a normal pearlite structure in that the lamellar cementite 31 is cut off in the block when observed by SEM. In the present embodiment, however, the pearlite structure and the pseudo pearlite structure are treated as the same structure.

In an actual steel material, other structures are present in addition to the pearlite structure, and the structure is far more complicated than the structure of fig. 2, and therefore, in the wire rod according to the present embodiment, the subgrain boundary and the high-angle grain boundary are defined as follows. A boundary surface in which 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 subboundary, and the total of the lengths of the subboundary per unit area of pearlite in the inspection field is referred to as a subboundary density < ρ 1 >. In addition, a boundary surface in which the angle difference between adjacent ferrite crystal orientations is 15 ° or more in the entire structure is referred to as a high angle grain boundary, and the total length of the high angle grain boundary per unit area in the inspection field is referred to as a high angle grain boundary density < ρ 2 >. The ferrite used for determining the high angle grain boundaries includes both a normal ferrite structure and lamellar ferrite constituting a pearlite structure. The respective measuring 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 rod needs to be 45% or less in both the center portion and the surface layer portion of the wire rod. If the amount of ferrite in the center portion of the wire rod exceeds 45%, the wire drawability is lowered because ferrite is deposited in a massive and coarse manner. In addition, when the area ratio of the ferrite structure of the surface layer portion of the wire rod exceeds 45%, the number of twists after the wire drawing process decreases. This is considered to be due to the concentration of deformation in the ferrite portion of the surface layer portion. Further, it is not particularly necessary to define the lower limit of the area ratio of the ferrite structure. The area ratio of the ferrite structure may be 0% in the central portion or the surface layer portion of the wire rod. The area ratio of ferrite may be 43% or less, 40% or less, 35% or less, or 30% or less in the central portion or the surface layer portion of the wire rod. 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 rod.

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%, cracks starting from the non-ferrite and non-pearlite structure are likely to form during wire drawing, and wire drawing workability is reduced. Further, it is not necessary to particularly define the lower limit of the area ratio of the non-ferrite and non-pearlite structures. The area ratio of the non-ferrite and non-pearlite structure may be 0% in the central portion or the surface layer portion of the wire rod. That is, the total area ratio of the ferrite structure and the pearlite structure may be 100%. The area ratio of the non-ferrite and non-pearlite structure may be 4% or less, 3% or less, 2% or less, or 1% or less (that is, the area ratio of the total of the ferrite structure and the pearlite structure may be more than 96%, more than 97%, more than 98%, or more than 99%). The area ratio of the non-ferrite and non-pearlite structures may be 1% or more, or 2% or more (that is, the area ratio of the total of the ferrite structure and the pearlite structure may be less than 99%, or less than 98%).

< density ρ 1 > -of subgrain boundaries in which the angle difference of the crystal orientations of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °

The subgrain density ρ 1 (the density of subgrain boundaries in which the angle difference of the crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °) needs to be 70/mm to 600/mm in the central portion and the surface layer portion of the wire rod. By using a wire rod having such a metal structure, a steel wire having a tensile strength of 2800MPa or more and excellent torsional characteristics can be stably obtained after wire drawing. By setting the subgrain density to 70/mm or more in the central portion and the surface layer portion of the wire rod, it is possible to suppress variations in the strength of the steel wire after drawing processing, to reduce localization of deformation in a torsion test, and to obtain good torsion characteristics even for a high-strength steel wire. On the contrary, if the subgrain density is less than 70/mm at the center and surface layer portions of the wire rod, the torsional properties are not improved when the tensile strength of the steel wire obtained after drawing is 2800MPa or more. Further, when the pearlite transformation temperature is less than 600 ℃, as described above, the torsional characteristics tend to be lowered, and the density of the subgrain boundary between the central portion and the surface layer portion of the wire rod in this case exceeds 600/mm, so it is preferable to set the upper limit thereof to 600/mm. Therefore, the density of the subgrain boundary in which the angle difference of the crystal orientation of the lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 ° is set to be in the range of 70/mm to 600/mm in the central portion and the surface layer portion of the wire rod. The density of the subgrain boundary is preferably 100/mm or more, more preferably 120/mm or more, in the surface layer part or the central part of the wire rod. The density of the subgrain boundaries may be 150/mm or more, or 180/mm or more in the surface layer portion or the central portion of the wire rod. The density of the subgrain boundaries in the surface layer or the central part of the wire rod may be 550/mm or less, 500/mm or less, 400/mm or less, or 350/mm or less.

Preferably: the subgrain density ρ 1 satisfies the following formula 1 in the surface layer portion and the central portion of the wire rod. The (C) in formula 1 is the C content in mass% in the chemical composition of the wire rod.

220 × (C) +100 < ρ 1 < 220 × (C) +300 … formula 1

The larger the C content in the chemical composition of the wire rod, the smaller the area ratio of the ferrite structure in the surface layer portion and the central portion of the wire rod, and the larger the area ratio of the pearlite structure. It is considered that the larger the area ratio of the pearlite structure, the larger the growth distance of the cementite, and the intergranular boundary is easily introduced into the pearlite structure. Therefore, the present inventors considered that the preferable range of the subboundary density depends on the C content in the chemical composition of the wire rod. According to the findings of the present inventors, when the density of the subgrain boundary at the surface layer portion and the central portion of the wire rod satisfies the above formula 1, the variation in the number of twists of the wire rod is reduced, and thus the twist characteristics are further improved.

< density ρ 2 > of large-angle grain boundary having an angular difference of ferrite crystal orientation of 15 ° or more in steel structure

The high angle grain boundary density ρ 2 (the density of a high angle grain boundary in which the angle difference of ferrite crystal orientation is 15 ° or more) needs to be 200/mm or more in the surface layer portion and the central portion of the wire rod. When the high-angle grain boundary density is sufficiently large, the wire rod has high ductility and can suppress the formation of coarse cracks during drawing, and thus the 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 rod, the wire drawability is lowered. Therefore, the density of the large-angle grain boundaries in which the angle difference between the ferrite crystal orientations is 15 ° or more is set to be 200/mm or more in the surface layer portion and the central portion of the wire rod. The high angle grain boundary density is preferably 230/mm or more in the surface layer portion or the central portion of the wire rod. The upper limit of the high angle grain boundary density in the surface layer portion or the central portion of the wire rod is not particularly limited, but it is difficult to set the high angle grain boundary density to 500/mm or more in view of production, and therefore it is preferable to set the upper limit of the high angle grain boundary density in the surface layer portion or the central portion of the wire rod to 500/mm. The high angle grain boundary density of the surface layer portion or the central portion of the wire rod may be set to 220/mm or more, 250/mm or more, or 280/mm or more. The high angle grain boundary density of the surface layer portion or the central portion of the wire rod may be 400/mm or less, 380/mm or less, 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 according to the present embodiment.

< area ratio of tissue >

The cross section of the wire rod (i.e., the cut surface perpendicular to the longitudinal direction of the wire rod) is cut intoAfter line mirror polishing, the film was etched with a bitter alcohol solution, and 10 portions of any of the surface layer portion and the central portion were observed at a magnification of 2000 times using a field emission scanning electron microscope (FE-SEM) to obtain photographs. The area of each 1 visual field is 2.7 multiplied by 10^－3mm ²(0.045 mm long and 0.060mm wide).

Next, a transparency (e.g., ohp (over Head projector) tile) is superimposed on each of the obtained photographs. In this state, "ferrite structure" in each transparent sheet was colored. Next, the area ratios of the "colored regions" in each transparent sheet were obtained by image analysis software, and the average value thereof was calculated as the average value of the area ratios of the ferrite structure. The ferrite area ratio can be obtained in this manner. Then, the "region overlapping with the structure other than the pearlite structure and the structure other than the ferrite structure" was colored on the other transparent sheet, and the area ratio thereof was obtained. The area ratio of the non-pearlite and non-ferrite structure can be determined by the above method. Further, in the structure in which the ferrite structure and the pearlite structure are isotropic, the area ratio of the structure in the cross section of the wire rod is the same as the volume ratio of the structure of the wire rod. 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 ratios from 100 area%.

< sub-grain boundary density in pearlite structure and high angle grain boundary density in whole structure >

After mirror polishing of the cross section (i.e., a cut surface perpendicular to the longitudinal direction) of the wire rod, polishing was performed using colloidal silica, and EBSD measurement (measurement by an electron beam backscatter diffraction method) was performed by observing 4 fields of view at 400 × magnification at the surface portion (in the range of 150 to 400 μm in depth from the surface) and the central portion of the wire rod using a field emission scanning electron microscope (FE-SEM). The area of each 1 visual field is set to be 0.0324mm²(0.18 mm in length and 0.18mm in width), and the step size in the measurement was set to 0.3. mu.m.

Next, the results of the obtained measurement fields were measured for the entire length of the line that the subgrain boundary of 2 ° or more and less than 15 ° had and the entire length of the line that the large-angle grain boundary of 15 ° or more had. For example, by using OIM analysis (OIM: organization Imaging Microcopy), the total length of lines that the subgrain boundaries have and the total length of lines that the high angle grain boundaries have can be obtained. Since the subgrain boundaries exist only in the pearlite structure, the value obtained by dividing the total length of the lines included in the subgrain boundaries obtained in each measurement field by the area of the pearlite included in each measurement field is defined as the subgrain boundary density ρ 1 in each measurement field.

Since the large angle grain boundaries are also present at the boundaries between the ferrite structure and the pearlite structure, the large angle grain boundary density ρ 2 in each measurement field is defined as a value obtained by dividing the total length of the lines included in the large angle grain boundaries obtained in each measurement field by the area of each measurement field.

The average value of the analysis results of the surface layer portion and the central portion is defined as a subgrain density ρ 1 in which the angle difference of ferrite crystal orientations in the pearlite structure of the surface layer portion and the central portion is 2 ° or more and less than 15 °, and a large-angle grain boundary density ρ 2 in which the angle difference of ferrite crystal orientations in the entire structure of the surface layer portion and the central portion is 15 ° or more. Since the EBSD result is greatly influenced by noise, the average CI (confidence index) is a result of 0.60 or more, and the noise is removed from the EBSD result with a CI of 0.10 or less. Further, removal of CI below 0.10 can be achieved within OIM analysis.

As described above, the values of the subgrain boundary density ρ 1 and the high angle subgrain boundary density ρ 2 need to be in the same ranges not only in the surface portion (the range of the depth of 150 to 400 μm from the surface) of the wire but also in the central portion of the wire. Even if the subgrain boundary density ρ 1 at the center of the wire rod is 70/mm to 600/mm and the high angle intergranular density ρ 2 is 200/mm or more, the surface layer part is not in the above range, and even if the surface layer part of the wire rod is in the above range, the center part is not in the above range, the wire rod cannot obtain the desired characteristics. If it can be confirmed that ρ 1 and ρ 2 in the surface layer portion and ρ 1 and ρ 2 in the central portion of the wire are both within the above-described ranges, it can be recognized that ρ 1 and ρ 2 are within the above-described ranges in the entire wire.

(D) Relating to the manufacturing method

In the method for manufacturing a wire rod according to the present embodiment, various conditions during pearlite transformation are optimized to control the structure in order to improve the torsional characteristics of the wire rod.

The wire rod satisfying the above requirements of the wire rod according to the present embodiment can obtain the effects of the wire rod according to the present embodiment without limiting the manufacturing method thereof, but the wire rod according to the present embodiment may be manufactured by, for example, the manufacturing method described below. It is to be noted that the following manufacturing process is an example, and it is needless to say that even when a wire rod having a chemical composition and other requirements within the range of the wire rod according to the present embodiment is obtained by a process other than the following process, the wire rod is included in the present invention.

First, after steel is melted so as to have the above-described composition, a billet is produced by continuous casting, and hot rolling is performed. Further, the casting may be followed by a preliminary rolling. When the obtained slab is hot-rolled, the slab is heated to 1000 to 1250 ℃ and hot-rolled to phi 5.5 to 7.0mm with the finish rolling temperature set to 900 to 1000 ℃.

The heating temperature of the billet before hot rolling is set to 1000 ℃ to 1250 ℃. The reason is that when the heating temperature of the slab is less than 1000 c, the reaction force at the time of hot rolling increases, and when the heating temperature of the slab exceeds 1250 c, decarburization is increased.

The finish rolling temperature of hot rolling is set to 900 ℃ or higher. The reason is that when the finish rolling temperature is less than 900 ℃, the reaction force of finish rolling increases and the shape accuracy deteriorates. On the other hand, the finish rolling temperature is set to 1000 ℃ or less. The reason is that when hot rolling is performed at a temperature exceeding 1000 ℃, the austenite grain size becomes large, and the large angle grain boundary density after pearlite transformation decreases.

After hot rolling, the following four-stage cooling was performed to adjust the ferrite area fraction, the subboundary density, and the high angle grain boundary density. The primary cooling is performed at a high cooling rate to suppress grain growth of austenite and generate a fine austenite structure. The secondary cooling is performed to reduce a temperature difference between the surface layer portion and the central portion of the wire rod during the primary cooling, and to achieve a uniform temperature from the surface layer portion to the central portion of the wire rod. The purpose of the tertiary cooling is to cool the wire rod to a target pearlite transformation temperature at a cooling rate that enables cooling from the surface layer portion to the central portion of the wire rod as uniformly as possible and that enables ferrite transformation to be suppressed. The purpose of the fourth cooling is to gradually cool the wire rod so as to cause pearlite transformation as uniformly as possible from the surface layer portion to the central portion of the wire rod, and to cause pearlite transformation so that the subgrain grain boundary density and high angle grain boundary density fall within a target range. The details are shown below. The average cooling rate of the primary to the fourth cooling described below is a value obtained by dividing the amount of decrease in the wire rod temperature from the start to the end of the primary to the fourth cooling by the time from the start to the end of the primary to the fourth cooling. The reaching temperature of the primary to fourth cooling is the temperature of the wire rod at the end of the primary to fourth cooling.

After hot rolling, the steel sheet is once cooled to 830 to 870 ℃ at an average cooling rate of 50 to 200 ℃/sec by water cooling. The start and end of primary cooling are the start and end of spraying of the cooling medium (water).

The average cooling rate in the temperature range of 870 ℃ or higher, in which the grain growth rate is high, is less than 50 ℃/sec, and when the time in which this temperature range exists is long, grain growth of austenite is promoted, so that the large angle grain boundary density decreases after pearlite transformation. The upper limit of the average cooling rate in the primary cooling is not limited, but it is difficult to realize an average cooling rate exceeding 200 ℃/sec due to the restrictions of the manufacturing equipment, and therefore 200 ℃/sec or less is used as the upper limit of the average cooling rate in the primary cooling.

When the temperature reached in the primary cooling is less than 830 ℃, ferrite transformation may progress only in a large amount in the surface layer portion, and the ferrite area ratio in the surface layer portion may increase, making it difficult to control to 45% or less. Therefore, the temperature reached in the primary cooling is set to 830 ℃ or higher. When the cooling is stopped at a temperature exceeding 870 ℃, austenite grains grow greatly, and the large angle grain boundary density after pearlite transformation decreases. Therefore, the temperature reached in the primary cooling is 870 ℃ or lower.

Then, the resultant is air-cooled in the atmosphere, and the resultant is secondarily cooled at an average cooling rate of less than 5 ℃/sec to a temperature in the range of 790 ℃ to 820 ℃. Further, the time point of the start of the secondary cooling is equal to the time point of the end of the spraying of the cooling medium in the primary cooling, and the time point of the end of the secondary cooling is equal to the time point of the start of the spraying of the cooling medium in the tertiary cooling. The secondary cooling is cooling for reducing a temperature difference between the surface layer portion and the central portion of the wire rod generated in the primary cooling and making the pearlite transformation temperature from the surface layer portion to the central portion of the wire rod uniform.

When the average cooling rate is set to 5 ℃/sec or more in the secondary cooling, the temperature difference between the surface layer portion and the central portion remains, and even if the high angle grain boundary density and the subgrain boundary density of the surface layer of the wire rod can be controlled after the pearlite transformation, the high angle grain boundary density in the central portion of the wire rod is reduced. Therefore, the average cooling rate in the secondary cooling is set to be less than 5 ℃/sec.

When the arrival temperature of the secondary cooling is less than 790 ℃, there is a possibility that ferrite transformation is generated and thus the ferrite area ratio is increased. Therefore, the temperature reached by the secondary cooling is 790 ℃ or higher. On the other hand, when the secondary cooling is stopped at a temperature exceeding 820 ℃, the temperature difference between the surface layer portion and the central portion of the wire rod up to the pearlite transformation temperature becomes large, and the temperature difference is generated again between the surface layer portion and the central portion in the tertiary cooling. Therefore, the temperature reached by the secondary cooling is set to 820 ℃ or lower. In steel grades containing a large amount of Si, the Ac1 temperature shifts to the high temperature side, and therefore the reaching temperature in the secondary cooling becomes particularly important.

It is preferable that the secondary cooling time (elapsed time between the start and end of secondary cooling) be 5 seconds or more and 12 seconds or less. The reason is that if the secondary cooling time of more than 12 seconds is taken, grain growth of austenite grains is promoted. On the other hand, if the secondary cooling time is within 5 seconds, a temperature difference may remain in the wire rod.

Thereafter, the steel sheet is cooled by air-blowing three times at an average cooling rate of more than 20 ℃/sec and 30 ℃/sec or less until the temperature is within a range of 600 ℃ to 620 ℃. The start and end of the third cooling are the start and end of the blowing of the air. In the third cooling, the cooling is performed at a cooling rate capable of suppressing ferrite transformation until a pearlite transformation temperature at which an optimum subgrain grain boundary density and high angle grain boundary density are obtained.

When the average cooling rate of the third cooling is 20 ℃/sec or less, ferrite transformation occurs and the ferrite area ratio becomes excessive. Therefore, the average cooling rate is set to exceed 20 ℃/sec. On the other hand, when the cooling is performed three times at an average cooling rate exceeding 30 ℃/sec, only the surface layer portion of the wire rod is cooled to a target temperature, and the four-time cooling is started in a state where the temperature of the central portion of the wire rod is excessive. Therefore, the average cooling rate is set to 30 ℃/sec or less.

When the arrival temperature in the tertiary cooling is less than 600 ℃, the pearlite structure is excessively strengthened, and the torsional characteristics are degraded. Therefore, the temperature reached by the third cooling is 600 ℃ or higher. On the other hand, when the reaching temperature of the tertiary cooling exceeds 620 ℃, the pearlite transformation temperature becomes high, the high angle grain boundary density and the subgrain boundary density decrease, and the tensile strength after pearlite transformation also decreases. Therefore, the temperature reached by the third cooling is 620 ℃ or lower.

Thereafter, the resultant was cooled four times to 550 ℃ or less at an average cooling rate of 10 ℃/sec or less by air cooling in the atmosphere. Further, the time point of the start of the four-time cooling is equal to the time point of the end of the blowing of the atmosphere in the three-time cooling. The time point at which the four cooling operations are finished is the time point at which the air cooling is suspended, that is, the time point at which reheating or spraying of the cooling medium to the wire rod is started. However, when air cooling was performed until the temperature of the wire rod became 550 ℃ or lower, the time point at which the temperature of the wire rod became 550 ℃ was regarded as the time point at which the four times of cooling was completed. The object of the quartic cooling is to obtain a wire rod having a uniform high 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 cross section of the wire rod during pearlite transformation.

When the average cooling rate exceeds 10 ℃/sec in the fourth cooling, the temperature change of the surface layer is large, and the subgrain boundary density is reduced. Therefore, the average cooling rate in the four-time cooling is set to 10 ℃/sec or less. The lower limit of the average cooling rate in the fourth cooling is not limited, and the cooling rate in the case of naturally cooling the wire rod is usually 2 ℃/sec or more. Therefore, 2 ℃/sec may be set as the lower limit of the average cooling rate in the four-time cooling.

When the arrival temperature of the quartic cooling exceeds 550 ℃, the pearlite transformation may not be completed. Therefore, the arrival temperature of the four-time cooling is 550 ℃. Since the cooling rate in the temperature range of 550 ℃ or lower has a slight influence on the tissue, accelerated cooling such as water cooling may be performed after four times of cooling to a temperature of 550 ℃ or lower. In the examples of the present invention described later, the temperature is cooled to 550 ℃ or lower by four times of cooling, and then the temperature is cooled to room temperature by natural cooling.

(E) With respect to the optional ingredients

The wire rod of the present embodiment may contain at least 1 or 2 or more elements selected from Al, Cr, V, Ti, Nb, B, Ca, and Mg, if necessary, in place of a part of the remaining Fe. However, since the wire rod according to the present embodiment can solve the problem without including any of these elements, the lower limit of any of these elements is 0%. The reasons for limiting the effects and contents of Al, Cr, V, Ti, Nb, B, Ca and Mg, which are optional elements, will be described below. The% of any component means mass%.

Al：0～0.050％

The wire rod of the present embodiment may not contain Al. Al is an element that precipitates as AlN and can increase the density of large-angle grain boundaries in which the difference in angle of ferrite crystal orientation is 15 ° or more. In order to reliably obtain the effect, it is preferable to contain 0.010% or more of Al. On the other hand, since Al is an element which easily forms hard oxide inclusions, when the Al content of the wire rod exceeds 0.050%, coarse oxide inclusions are easily formed remarkably, and the torsional characteristics are remarkably reduced. Therefore, the upper limit of the Al content of the wire rod 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：0～1.00％

The wire rod of the present embodiment may not contain Cr. Cr is an element that increases the hardenability of steel and increases the strength of steel, similarly to Mn. In order to obtain this effect reliably, 0.05% or more of Cr is preferably contained. On the other hand, if the Cr content exceeds 1.00%, the torsional characteristics deteriorate. Therefore, the content of Cr is 1.00% or less. When the hardenability of the steel is to be improved, the content of Cr is preferably 0.10% or more, and more preferably 0.30% or more. The upper limit of Cr is preferably 0.90% or less, more preferably 0.80% or less.

V：0～0.15％

The wire rod of the present embodiment may not contain V. V and N, C combine to form carbide, nitride or carbonitride, and by the pinning effect thereof, has an effect of refining austenite grains during hot rolling, and has an effect of improving the torsional properties of steel. In order to obtain this effect reliably, V is preferably contained in an amount of 0.005% or more. From the viewpoint of improving the torsional characteristics, the content of V is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the V content exceeds 0.15%, the effect is not only saturated, but also the productivity of the steel is adversely affected by, for example, cracking of the billet in the step of cogging and rolling the steel ingot or cast slab, and therefore the V content is 0.15% or less. The content of V is preferably 0.10% or less, and more preferably 0.07% or less.

Ti：0～0.050％

The wire rod of the present embodiment may not contain Ti. Ti combines with N, C to form carbide, nitride, or carbonitride, and has an effect of refining austenite grains during hot rolling due to the pinning effect thereof, and an effect of improving the torsional properties of steel. In order to obtain this effect reliably, it is preferable to contain 0.002% or more of Ti. 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, if the Ti content exceeds 0.050%, the effect is not only saturated, but also the steel billet is cracked in the step of cogging and rolling the steel ingot or cast slab into a billet, which adversely affects the manufacturability of the steel, so the Ti content is 0.050% or less. The Ti content is more preferably 0.0025% or less.

Nb：0～0.050％

The wire rod of the present embodiment may not contain Nb. Nb bonds with N, C to form carbide, nitride, or carbonitride, and has an effect of refining austenite grains during hot rolling due to the pinning effect thereof, and an effect of improving the torsional properties of steel. In order to obtain this effect reliably, Nb is preferably contained at 0.002% or more. From the viewpoint of improving the torsional characteristics, the Nb content is preferably 0.003% or more, and more preferably 0.004% or more. On the other hand, if the Nb content exceeds 0.050%, the effect is not only saturated, but also the steel billet is cracked in the step of cogging and rolling a steel ingot or a cast slab into a billet, which adversely affects the manufacturability of the steel, and therefore the Nb content is set to 0.050% or less. The Nb content is more preferably 0.030% or less.

B：0～0.0040％

The wire rod of the present embodiment may not contain B. B has an effect of reducing the ferrite structure of the steel by being contained in a small amount, and when the effect is to be obtained reliably, B is preferably contained in an amount of 0.0001% or more. When B is contained in an amount exceeding 0.0040%, the effect is saturated and coarse nitrides are generated, so that the torsional characteristics are degraded. Therefore, the content of B in the case of inclusion is set to 0.0040% or less. When the area ratio of the pearlite structure is to be increased, the content of B is preferably 0.0004% or more, more preferably 0.0007% or more. The content of B for improving the torsional characteristics is preferably 0.0035% or less, and more preferably 0.0030% or less.

Ca：0～0.0050％

The wire rod of the present embodiment may not contain Ca. Ca is solid-dissolved in MnS, and has an effect of finely dispersing MnS. By finely dispersing MnS, wire breakage in wire drawing due to MnS can be suppressed. In order to obtain the effect of Ca, it is preferable to contain 0.0002% or more of Ca. When a higher effect is desired, 0.0005% or more of Ca may be contained. However, if the content of Ca exceeds 0.0050%, the effect is saturated. When the content of Ca exceeds 0.0050%, oxides generated by reaction with oxygen in steel become coarse, and the wire drawing workability is adversely decreased. Therefore, the appropriate Ca content when contained is 0.0050% or less. The content of Ca is preferably 0.0030% or less, more preferably 0.0025% or less.

Mg：0～0.0040％

The wire rod of the present embodiment may not contain Mg. Mg is a deoxidizing element and produces oxides, but also produces sulfides, and therefore is an element having a correlation with MnS, and has an effect of finely dispersing MnS. This effect can suppress wire breakage in wire drawing due to MnS. In order to obtain the effect of Mg reliably, Mg is preferably contained in an amount of 0.0002% or more. When a higher effect is desired, 0.0005% or more of Mg may be contained. However, if the Mg content exceeds 0.0040%, the effect is saturated, and a large amount of MgS is produced, which leads to a decrease in wire drawability. Therefore, the content of Mg in the case of containing is preferably 0.0040% or less. The Mg content is preferably 0.0035% or less, more preferably 0.0030% or less.

The remaining amount of the chemical composition comprises "Fe and impurities". The "impurities" refer to substances mixed into a steel material from ores, scraps, or production environments, etc. as raw materials in the industrial production of a steel material.

The diameter of the wire rod according to the present embodiment is not particularly limited, and the diameter of the wire rod currently distributed in the market is usually 3.5 to 7.0mm, and therefore, the diameter may be set as the upper and lower limit values of the diameter of the wire rod according to the present embodiment. When the diameter of the wire rod is 3.5mm or more, the load of hot rolling during the wire rod production can be reduced, which is preferable. When the diameter of the wire rod is 7.0mm or less, the amount of strain in wire drawing during wire drawing of the wire rod can be suppressed, which is preferable.

A steel wire according to another aspect of the present invention is obtained by drawing the wire rod according to the present embodiment. The steel wire has a diameter of usually 0.5 to 1.5mm in consideration of the application. The steel wire according to the present embodiment has excellent tensile strength and torsional characteristics because the chemical composition, the composition of the metal structure, the subgrain density ρ 1, and the high angle intergranular density ρ 2 of the wire rod according to the present embodiment, which are the raw materials, are within the above-described ranges.

Further, since the steel wire according to the present embodiment is manufactured by drawing with a very large strain amount, the metal structure thereof is significantly deformed. For example, when an enlarged photograph of a cross section of the steel wire according to the present embodiment is viewed, the phase surrounded by the grain boundaries is significantly collapsed, and the type thereof cannot be discriminated. In addition, it is also significantly difficult to determine the presence of subgrain boundaries and high angle boundaries. That is, it is extremely difficult to specify the metal structure and other configurations of the steel wire according to the present embodiment by a general structure specifying method (for example, photographing of a photograph of the metal structure, analysis of an EBSD crystal structure, and the like). It is impossible or substantially impractical to directly determine the metal structure of the steel wire according to the present embodiment based on the structure or characteristics thereof.

A method for manufacturing a steel wire according to another aspect of the present invention includes a step of drawing the wire rod according to the present embodiment. The wire drawing is performed so that the steel wire finally obtained has a reduction of area of 0.5 to 1.5mm in diameter. Since the chemical composition of the wire rod, the structure of the metal structure, the subgrain boundary density ρ 1, and the high angle intergranular boundary density ρ 2 according to the present embodiment are set within the above ranges, the method for producing a steel wire according to the present embodiment using the wire rod can suppress the number of times of breakage to an extremely low level, and can obtain a steel wire having excellent tensile strength and torsional characteristics.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

Steels having chemical compositions shown in tables 1 and 2 were melted, and wire rods were produced by the following method. In tables 1 and 2, "-" indicates that the content of the element is at an impurity level and is judged to be substantially not contained. The balance of the chemical composition of the steels shown in tables 1 and 2 is iron and impurities.

First, steel a having a chemical composition shown in table 1 was melted in a converter, and thereafter, a 122mm square billet was obtained by cogging rolling by a usual method. Then, the billet is heated to 1050 to 1150 ℃, and then hot-rolled to phi 6mm at a finish rolling temperature in the range of 900 to 1000 ℃.

The conditioned cooling after the finish rolling was performed under the conditions shown in (A1) to (A21) shown in tables 3-1 to 3-3.

Specifically, (A1) to (A7) are cooled (1 time cooling) to 830 to 870 ℃ at an average cooling rate of 50 to 200 ℃/sec by water cooling, and then air-cooled (secondary cooling) to 790 ℃ or higher and 820 ℃ or lower at an average cooling rate of less than 5 ℃/sec by air cooling in the air. Then, the steel sheet is cooled to 600 to 620 ℃ at a temperature exceeding 20 ℃/sec and not more than 30 ℃/sec (third cooling), cooled to 550 ℃ or less at a temperature below 10 ℃/sec (fourth cooling), and then cooled to room temperature by natural cooling.

With respect to (A8) to (a17), four types of conditioned cooling were performed under different cooling conditions from those described above, and wire rods were obtained. The underlined values in table 3-1 are inappropriate values in the production conditions of the wire rod according to the present invention.

The cooling conditions shown in tables 3-2 to 3-4 were used for (A18) to (A21) without performing the four types of controlled cooling. The terms "primary cooling" and the like in these tables are used solely for distinguishing the cooling stages, and are different from the primary cooling to the quartic cooling included in the production method of the present invention.

Specifically, (a18) was immersed in a salt bath at 550 ℃.

With regard to (a19), the wire rod after the completion of the hot rolling was reheated to 950 ℃ and held at a temperature of 60 seconds, and immediately after the completion of the temperature holding, the wire rod was immersed in a salt bath at 550 ℃.

Regarding (a20), cooling was performed by air blowing after primary cooling, and after cooling to 680 ℃ at an average of 1.0 ℃/sec, cooling was performed by switching to natural cooling to 550 ℃ or lower.

In the case of (A21), after primary cooling, blast cooling was performed to cool the wire rod to 700 ℃ at 10 ℃/sec, and then cooling was performed to 550 ℃ or lower at 5 ℃/sec by air cooling.

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

The wire rods of test nos. a1 to a21 and test nos. 1 to 26 obtained as described above were subjected to determination of tensile strength, reduction in area, ferrite area ratio, non-pearlite and non-ferrite area ratios, subgrain density ρ 1 in the pearlite structure (density of subgrain boundaries in which the angle difference of the crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °), and high-angle subgrain density ρ 2 (density of high-angle subgrain which the angle difference of the crystal orientation of ferrite in the entire structure is 15 ° or more) as well as the results of the above-described measurements. The pearlite area ratio of each wire rod is a value obtained by subtracting the ferrite area ratio and the non-pearlite and non-ferrite area ratios from 100%.

The results are shown in tables 4-1 to 4-3 and tables 5-1 to 5-3 below. The underlined values in Table 4-1, Table 4-2, Table 5-1, and Table 5-2 are values outside the scope of the present invention. The underlined values in tables 4-3 and 5-3 are values that do not satisfy the criteria for passing or failing the present invention.

The area ratios of the ferrite structure, the area ratio of the non-ferrite and non-pearlite structure, the subgrain density ρ 1, the high angle grain boundary density ρ 2 in the surface layer and the central portion of the wire rod, the number of times of breakage when the wire rod having a diameter of 6mm was drawn to a diameter of 0.5mm, the tensile strengths (steel wire strengths) of the wire rod before drawing and the steel wire after drawing, and the torsional characteristics (the number of times of twisting, variations in the number of times of twisting, and the presence or absence of delamination) of the steel wire after drawing were examined by the methods described below.

Area ratio of ferrite structure, area ratio of non-ferrite and non-pearlite structure of the wire rod of < 1 >

After mirror polishing of the cross section of the wire rod, the wire rod was corroded with a bitter alcohol solution, 10 arbitrary portions of the surface portion and the central portion of the wire rod were observed at a magnification of 2000 times using an FE-SEM, and photographs were taken. The area of each 1 visual field is set to 2.7X 10^－3mm ²(0.045 mm long and 0.060mm wide). The OHP sheet was superimposed on each of the obtained photographs, and "ferrite structure" and "region overlapping with non-pearlite and non-ferrite structure" in each transparent sheet were colored. Next, the area ratios of the "colored regions" in each transparent sheet were obtained by image analysis software, and the average value thereof was calculated as the average value of the area ratios of the ferrite structure and the non-pearlite and non-ferrite structure.

Intergranular density rho 1 and high-angle intergranular density rho 2 of the (2) wire rod

The cross section of the wire rod was mirror-polished, polished with colloidal silica, and 4 portions of the surface layer and the central portion of the wire rod were observed at 400-fold magnification using FE-SEM, and analyzed using an EBSD measuring apparatus manufactured by tsl (texsem laboratories). The area during measurement was set to 180X 180. mu.m²The step size was set to 0.3 μm. Next, with respect to each of the obtained results, the line of the subboundary having the angle difference of 2 ° or more and less than 15 ° was measured using OIM analysisAnd the total length of the lines of the high-angle grain boundaries having an angle difference of 15 ° or more. The subgrain density was determined by dividing the total length of the lines of the subgrain boundaries having an angular difference of 2 ° or more and less than 15 ° by the average pearlite area ratio, and the high angle grain boundary density was determined by dividing the total length of the lines of the high angle grain boundaries having an angular difference of 15 ° or more by the area of 1 field.

Wire drawing processability of (3) wire rod

50kg of each wire rod was subjected to wire drawing, and the number of wire breaks during wire drawing was recorded. When the number of times of wire breakage is 3 or more, the wire drawing process after the 3 rd wire breakage is stopped. Further, when the number of wire breaks when 50kg of the wire rod was drawn from 6.0mm in diameter to 0.5mm in diameter was 0, the wire drawing workability was evaluated as good, and when the number of wire breaks was 1 or more, the wire drawing workability was evaluated as poor. The wire rod that had been subjected to the wire drawing process was judged to be a wire rod that was clearly unsuitable as a material for a steel wire, and the subsequent evaluation test was not performed. The symbol "-" is described in the item which is not evaluated.

Tensile strength of (4) wire rod and steel wire after wire drawing

The wire rod and the steel wire were cut into 200mm lengths, and 50mm in the upper and lower directions were fixed by a wedge chuck or an air chuck to perform a tensile test. The tensile strength was calculated by dividing the obtained maximum load by the sectional area. Then, the wire diameter of the portion of the wire rod having the smallest wire diameter after the tensile test was measured, and the amount of change in the cross-sectional area before and after the tensile test was divided by the cross-sectional area before the tensile test and multiplied by 100%, thereby calculating the cross-sectional shrinkage value.

Since the steel wire used as a wire for a reinforcing material of an automobile tire preferably has a tensile strength of 2800MPa or more, the steel wire is evaluated as a good product having a tensile strength of 2800MPa or more. In addition, the tensile strength of the wire rod is not particularly set as a criterion for passing or failing.

Torsion characteristics of steel wire after (5) drawing process

In the twisting test, a steel wire having a length 100 times the wire diameter (diameter) was twisted at 15rpm until breakage, and whether or not delamination occurred was judged by using a torque (resistance to twisting) curve, and the number of twists was measured. The determination using the torque curve is performed by a method of determining that delamination has occurred when the torque sharply decreases before the disconnection. The twisting test was performed on 10 steel wires each, and when 1 steel wire was not delaminated and the average value of the number of twists of 10 steel wires was 23 or more, the twisting characteristics were evaluated to be good.

In addition, when the variation in the number of twists in the 10-time twist test is small, it can be judged that the twist characteristic is more excellent. Therefore, the deviation of the number of twists of 10 wires (the larger one of the difference between the maximum value of the number of twists of 10 wires and the average value and the difference between the minimum value of the number of twists of 10 wires and the average value) was calculated. The steel wire having a deviation of 3 or less was judged to have a good deviation of the number of twists.

The steel wire was judged to have all good mean value of the number of twists, delamination, and variation in the number of twists, and the twisting characteristics were very good. However, even if the wire has a deviation of the number of twists exceeding 3, the wire judged to be good for other evaluations of the twisting characteristics can be said to be good in the twisting characteristics according to the intended use.

The results of the above evaluations are summarized in the following table.

TABLE 3-2

Tables 3 to 3

Tables 3 to 4

TABLE 4-1

TABLE 4-2

Tables 4 to 3

TABLE 5-1

TABLE 5-2

Tables 5 to 3

As shown in the table, the samples of invention examples A1 to A7 all satisfied the requirements of the present invention and the steel material was produced under appropriate conditions, and therefore the strength after drawing was 2800MPa or more, the number of twists was 23 or more, and delamination did not occur, and there was no problem in the wire rod.

On the other hand, the sample A8 had a low average cooling rate in the primary cooling and coarsened the austenite grain size, and therefore ρ 2 decreased, and wire breakage occurred during wire drawing, resulting in poor wire drawing workability.

Since the arrival temperature of the sample a9 was low in the primary cooling, the ferrite area ratio in the surface layer increased and the number of twists decreased.

The sample a10 had a high arrival temperature during primary cooling, and the austenite grain size coarsened, and therefore ρ 2 decreased, and wire breakage occurred.

The sample a11 had a long time in the secondary cooling, and the austenite grain size was coarsened, and therefore ρ 2 decreased, and wire breakage occurred.

The sample a12 had a high ferrite area ratio and poor wire drawability because of a low arrival temperature during secondary cooling, and had low wire strength and torsional characteristics.

The sample a13 had a small average cooling rate in the third cooling, increased ferrite transformation, increased ferrite area ratio, poor wire drawability, and low wire strength and torsional characteristics.

The sample a14 had a high arrival temperature at the third cooling, and underwent pearlite transformation at a high temperature, and both ρ 1 and ρ 2 were low, and broke during wire drawing processing, and the torsional characteristics were also poor.

The sample a15 had poor torsional properties because the arrival temperature in the third cooling was low and ρ 1 was too high.

The sample a16 had a high average cooling rate in the four-time cooling, and ρ 1 in the surface layer portion of the wire rod was decreased, causing delamination in the torsion test, and the torsion characteristics were poor.

The sample a17 was obtained under the production conditions in which air cooling was stopped and air-blast cooling was started at the time point when the wire temperature became the temperature shown in the table in the four times of cooling. The sample a17 had a high arrival temperature during the fourth cooling, did not complete pearlite transformation, and had a non-pearlite and non-ferrite area ratio, which lowered wire drawability.

The sample a18 was quenched to 550 ℃ by immersing the wire rod in a 550 ℃ salt bath after the secondary cooling. As a result, ρ 1 of a18 was high, delamination occurred during the torsion test, and the torsion characteristics were poor.

The sample a19 was heated and maintained at a temperature, and then the wire rod was immersed in a salt bath at 550 ℃. As a result, ρ 1 of a19 was high, delamination occurred during the torsion test, and the torsion characteristics were poor.

The sample A20 had a slow cooling rate after hot rolling and exhibited pearlite transformation at high temperatures. Since the pearlite transformation temperature is high, ρ 1 and ρ 2 of a20 are both low, and wire breakage occurs during wire drawing processing, and the torsional characteristics are also poor.

In the sample a21, the wire rod was cooled to 700 ℃ by air-blast cooling after the secondary cooling, and therefore the surface layer portion of the wire rod was rapidly cooled, and ρ 1 in the surface layer portion was increased, and delamination occurred during the torsion test, and the torsion characteristic was poor.

Further, from the results shown in the table, the samples of test numbers 1 to 19 and 26 of the examples of the present invention satisfy the desired range of the present invention in terms of chemical composition and also have the required tensile strength, and therefore, the wire drawing workability is good, and the torsional characteristics after the wire drawing are good, because the manufacturing conditions of the wire rod are also appropriate.

However, the sample No. 20 had a low C content, an excessively large ferrite area ratio, and insufficient steel wire strength.

In the sample of test No. 21, the content of C was high, and the steel was excessively hardened, so that the wire drawability was lowered, and wire breakage occurred during the wire drawing.

In the sample of test No. 22, since the Si content is low, ρ 1 is low, and delamination occurs in the torsion test.

In the sample of test No. 23, since the content of Mn was too high, and the non-ferrite and non-pearlite structures were large, wire breakage occurred during wire drawing.

The sample No. 24 had a low Si content and a low ρ 1 content, and was delaminated in the torsion test.

The sample No. 25 had a low Mn content and a low ρ 1 content, and was delaminated in the torsion test.

As is apparent from the results shown in the table, if a wire rod is obtained in which C, Si, Mn, N, P, and S are defined as the previously described desired ranges, and the main structure is pearlite, the ferrite structure is 45% or less, the non-ferrite and non-pearlite structures are 5% or less, the angle difference of crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °, the subgrain grain boundary density ρ 1 is 70/mm ≦ ρ 1 ≦ 600/mm, and the large angle grain boundary density ρ 2 in which the angle difference of ferrite crystal orientation is 15 ° or more in the entire structure is 200/mm or more, it is possible to provide a wire rod for wire drawing processing that can produce a steel wire that has high tensile strength after wire drawing processing and can be stably twisted without causing delamination in a torsion test after wire drawing processing, and that has good wire drawing processability. That is, it is possible to provide a wire rod for wire drawing which can suppress wire breakage during wire drawing and stably produce a steel wire having high strength and excellent torsional characteristics suitable as a raw material for a metal wire or the like.

Description of the reference numerals

1: a wire rod; 11: a surface layer portion; 12: a central portion; 21: an original gamma grain boundary;

22: a large angle grain boundary; 23: a subgrain boundary; 31: a lamellar cementite;

32: lamellar ferrite.

Claims (11)

1. A wire rod characterized by comprising a chemical composition containing, in mass%

C：0.30～0.75％、

Si：0.80～2.00％、

Mn：0.30～1.00％、

N: less than 0.0080 percent,

P: less than 0.030%,

S: less than 0.020%,

O: less than 0.0070 percent of,

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 to 0.0050%, and

Mg：0～0.0040％，

the balance of the Fe and impurities are contained,

in both a surface layer portion, which is a range of a depth of 150 to 400 [ mu ] m from the surface of the wire rod, and a central portion, which is a range of 1/10 of a diameter of the wire rod from the central axis of the wire rod, the main structure is a pearlite structure, the area ratio of the ferrite structure in a cross section perpendicular to the longitudinal direction of the wire rod is 45% or less, the area ratio of a non-pearlite and non-ferrite structure in the cross section is 5% or less, the density ρ 1 of a sub-grain boundary, in which the angular difference of crystal orientation of lamellar ferrite in the pearlite structure is 2 ° or more and less than 15 °, is 70/mm or more and ρ 1 or less and 600/mm, the density ρ 2 of a large-angle grain boundary, in which the angular difference of crystal orientation of ferrite in the entire structure is 15 ° or more, is 200/mm or more, and the density ρ 1 of the sub-grain boundary is the total of the lengths of the sub-grain boundaries per unit area of pearlite in the inspection visual field, the density ρ 2 of the high-angle grain boundary is a total of lengths of the high-angle grain boundaries per unit area of the inspection field.

2. The wire according to claim 1,

the chemical composition contains, in mass%, a chemical component selected from

Al：0.010～0.050％、

Cr：0.05～1.00％、

V：0.005～0.15％、

Ti：0.002～0.050％、

Nb：0.002～0.050％、

B：0.0001～0.0040％、

Ca: 0.0002 to 0.0050%, and

mg: 0.0002 to 0.0040% of any 1 or more than 2.

3. The wire according to claim 1 or 2,

the density ρ 1 of the subgrain boundary satisfies the following formula 1 at the surface layer portion and the central portion of the wire rod,

220 × (C) +100 < ρ 1 < 220 × (C) +300 … formula 1

The (C) in the formula 1 is a C content in mass% in the chemical composition of the wire rod.

4. The wire according to claim 1 or 2,

the diameter of the wire rod is 3.5-7.0 mm.

5. The wire according to claim 3,

the diameter of the wire rod is 3.5-7.0 mm.

6. The wire according to claim 1 or 2,

the steel wire is used as a material for steel wires.

7. The wire according to claim 3,

the steel wire is used as a material for steel wires.

8. The wire according to claim 4,

the steel wire is used as a material for steel wires.

9. The wire according to claim 5,

the steel wire is used as a material for steel wires.

10. A steel wire, characterized in that,

is produced by drawing the wire rod according to any one of claims 1 to 9,

the diameter is 0.5 to 1.5 mm.

11. A method for producing a steel wire, characterized in that,

comprising a step of obtaining a steel wire by drawing the wire rod according to any one of claims 1 to 9,

the diameter of the steel wire is 0.5-1.5 mm.

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