KR20190073456A - Wire rod and manufacturing method thereof - Google Patents

Abstract

The wire material according to the present embodiment has a predetermined chemical composition and has an area fraction of pearlite of 90.0% or more in the structure observed at the central portion of the cross section of the wire rod, an area fraction of the corner stone cinnitate of 1.00% The average thickness is 0.25 占 퐉 or less, the total length of the elementary cementite cementite per unit area is less than 40.0 mm / mm2, the tensile strength satisfies the formula (1), and the diameter is 3.0 to 5.5 mm.

Description

Wire rod and manufacturing method thereof

The present invention relates to a wire and a method of manufacturing the wire.

The present application claims priority based on Japanese Patent Application No. 2016-211590 filed on October 28, 2016, the contents of which are incorporated herein by reference.

High strength steel wires such as steel cords and sawing wires are usually produced by drawing a high carbon steel wire having a C content of about 0.7 to 0.9%. Since the high carbon steel has high strength, breakage is likely to occur during drawing processing. If the working deformation increases in the drawing process, since the drawing material has high strength and low ductility, breakage is particularly likely to occur. Disconnection at the time of drawing processing remarkably lowers the productivity. Therefore, there is a demand for a high-carbon steel wire rod (that is, a high-carbon steel wire rod with good drawability) that is not easily broken during wire drawing.

On the other hand, high strength is required for the steel wire. For example, the steel cord is required to have a high strength in order to reduce the weight of the tire and to improve the fuel efficiency of the automobile. Sawing wire is required to have high strength and small diameter in order to prevent disconnection and cutting margin at the time of cutting a silicon wafer. In order to comply with the demand for strength for these steel wires, high-carbon steels are used as steel steels, and in particular, quartz steels containing C in an amount of at least a vacancy steel.

Generally, in the quartz, quartzite cementite is precipitated on the hot-rolled wire, so that the drawing workability of the wire is remarkably lowered. For this reason, it has been desired to suppress the precipitation amount of crude stone cementite in the hot-rolled wire of the quartz. Here, in the present specification, the term "hot-rolled wire" means a hot-rolled wire in which hot-rolling is not performed after hot-rolling.

Patent Document 1 discloses that the drawing workability of the hot-rolled wire is improved by defining the pearlite lamella spacing of the hot-rolled wire. However, in Patent Document 1, the influence of the elementary stone cementite on the drawing workability is not studied. Further, in Patent Document 1, a cooling rate from winding to a predetermined temperature is set to 20 ° C / s or more, and thereafter, a step of heating is performed, complicating the manufacturing process. Further, there is a problem that the load of the cooling capacity after winding is large, and the manufacturing cost is increased.

Patent Document 2 aims at improving the drawing processability of hot-rolled wire rods by limiting the tensile strength, fracture shrinkage ratio and nodule diameter of the hot-rolled wire rods. However, as in Patent Document 1, Patent Document 2 does not study the influence of crude stone cementite on drafting workability. When a fracture shrinkage ratio and a nodule diameter defined in Patent Document 2 are realized in a wire having a high C content, a large amount of corner stone cementite is precipitated, resulting in deterioration of wire drawing workability.

Patent Document 3 improves the drawing workability of a wire rod by finely austenitizing the wire rod after hot rolling and further reducing the area fraction and aspect ratio of the quartz stone cementite after cooling to a predetermined range. The wire material disclosed in Patent Document 3 is expected to further reduce the tensile strength, thereby improving the drafting workability and reducing the manufacturing cost by reducing the load at the time of drawing.

Japanese Patent No. 5179331 Japanese Patent No. 4088220 Japanese Patent Laid-Open No. 2001-181789

The present invention has been made to solve the above problems. That is, the object of the present invention is to provide a wire having excellent drawability, which is obtained without containing a C amount of vacancy steel or more and heat treatment after hot rolling, and a method for producing the wire.

The inventors of the present invention fabricated high-carbon steel hot-rolled wire rods (hereinafter sometimes referred to as " wire rods ") in which the steel material having a C content of 0.90 to 1.15% was used to control the metal structure and tensile strength under various rolling conditions . The inventors of the present invention evaluated the wire drawing workability of these wire rods and studied in detail the effect of the wire rope's structure and tensile strength on the wire drawing workability. As a result, the present inventors have found that by controlling the tensile strength within a predetermined range, suppressing the area fraction and the thickness of cornerstone cementite and controlling the total length of cornerstone cementite per unit area in accordance with the C content and the Cr content, And the fresh workability was improved. In the present specification, the term " drawing processability " refers to a property that can be drawn without being broken. In the present specification, the drawing workability of the wire rod is evaluated by the true strain when the wire breakage occurs during drawing processing.

The present invention has been completed on the basis of the above findings, and its main points are as follows.

(1) A wire according to one aspect of the present invention includes, in mass%, 0.90 to 1.15% of C, 0.10 to 0.50% of Si, 0.10 to 0.80% of Mn, 0.10 to 0.50% of Cr, %, Co: 0 to 1.00%, Mo: 0 to 0.20% and B: 0 to 0.0030%, P: not more than 0.020% and S: not more than 0.010%, the balance being Fe and impurities, The area fraction of the pearlite is 90.0% or more and the area fraction of the basic stone cementite is 1.00% or more in the structure observed at the central portion within (1/5) R from the center of the transverse section of the wire rod, Wherein the average thickness of the cornerstone cementites is 0.25 m or less and the total length of the cornerstone cementites per unit area is less than 40.0 mm / mm < 2 > and the tensile strength satisfies the formula (1) And the diameter is 3.0 to 5.5 mm.

The total length (mm / mm < 2 >) of the basic stone cementites per unit area is the sum of the lengths of the basic stone cementites observed per unit area. The TS in the formula (1) represents the tensile strength of the wire when the unit is expressed in MPa. "C amount (%)" in the above formula (1) represents the content mass% of C in the wire rod and "Cr amount (%)" represents the content mass% of Cr in the wire rod.

(2) The wire according to the above item (1), which contains, as mass%, at least one of 0.10 to 0.50% of Ni, 0.10 to 1.00% of Co, 0.05 to 0.20% of Mo and 0.0002 to 0.0030% of B Or more.

(3) In the wire described in the above (1) or (2), the area fraction of the cornerstone cementite may be more than 0% to 1.00%.

(4) The wire according to any one of the above-mentioned (1) to (3) may include one or more of the elementary stone cementite, grain boundary ferrite and bainite in the structure observed at the center portion.

(5) In the wire rod manufacturing method according to another aspect of the present invention, the steel strip having the components described in the above (1) is rolled at a temperature of from 3.0 to 5.5 mm in diameter, rolled at 940 to 800 캜, ° C. is cooled at an average cooling rate of 6.0 to 15.0 ° C./s and 650 to 600 ° C. is cooled at an average cooling rate of 1.0 to 3.0 ° C./s and a temperature of 600 to 300 ° C. is maintained at an average cooling rate of 10.0 ° C./s or more Cool.

According to this aspect, it is possible to provide a wire having excellent wire drawing workability and a method of manufacturing the same, which can be obtained without carrying out a heat treatment for heating after hot rolling, containing C in an amount equal to or greater than the vacancy steel. Further, according to the above aspect, the wire drawing workability of the wire material having a bimetallic steel composition can be improved without requiring an extra facility cost. Further, according to the above-described aspect, it is possible to increase the cost of the steel cord and the sawing wire due to the increase in the cost of the steel wire, such as an increase in the monofilament ratio during the wire drawing process, a middle wire tent, an increase in wear of the die, Can be suppressed. Therefore, the wire material according to the above embodiment is useful as a material for a high strength steel wire such as a steel cord used as a reinforcing material for a tire and a hose, and a sawing wire used for cutting a silicon wafer or the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing the precipitation state of cornerstone cementite in the old austenite grain boundary. FIG.
2 is a view for explaining a method of measuring thickness and length of cornerstone cementite.
3 is a view for explaining a method of measuring thickness and length of cornerstone cementite.
4 is a view for explaining a method of measuring thickness and length of cornerstone cementite.

Hereinafter, the wire material according to the present embodiment will be described. Further, the present embodiment is to be described in detail in order to better understand the object of the present invention, and thus the present invention is not limited unless otherwise specified.

First, the steel composition of the wire rod according to the present embodiment will be described. Hereinafter, unless otherwise specified,% of the steel composition represents% by mass.

C: 0.90 to 1.15%

C is an essential element for securing the strength of the steel wire. When the C content is less than 0.90%, the strength of the steel wire is lowered. Therefore, the lower limit of the C content is set to 0.90%. The lower limit of the preferred C content is 0.96%, or 1.00%. On the other hand, if the C content exceeds 1.15%, a large amount of quartzite cementite is precipitated in the wire, and breakage is likely to occur. On the other hand, if the C content exceeds 1.15%, the strength of the wire and the steel wire becomes excessively high, so that the drawing workability of the wire and the steel wire is deteriorated. Therefore, the upper limit of the C content is set to 1.15%. The upper limit of the preferred C content is 1.10%, or 1.08%.

Si: 0.10 to 0.50%

Si has an effect of increasing the strength of ferrite in pearlite. In order to effectively exhibit this action, the lower limit of the Si content is set to 0.10%. The lower limit of the preferable Si content is 0.15% or 0.20%. However, if Si is excessively contained, SiO ₂ inclusions which are detrimental to the wire drawing processability may be generated. Therefore, the upper limit of the Si content is set to 0.50%. The upper limit of the preferable Si content is 0.40% or 0.35%.

Mn: 0.10 to 0.80%

Mn has an effect of delaying the transformation from austenite to crushed stone cementite and pro-eutectoid ferrite, and is a useful element for obtaining the texture of the pearlite-based material. In order to effectively exhibit this action, the lower limit of the Mn content is set to 0.10%. The lower limit of the preferable Mn content is set to 0.20% or 0.30%. However, even if Mn is excessively contained, the action is saturated. Mn also has an effect of improving the quenching of the steel. Therefore, in the case where the wire contains excess Mn, overcooled structures such as bainite and martensite are generated in the wire rod during the cooling process after hot rolling, and the strength of the wire rod excessively increases to deteriorate the drawability. Therefore, the upper limit of the Mn content is set to 0.80%. The upper limit of the preferable Mn content is 0.70%, 0.60%, or 0.50%.

Cr: 0.10 to 0.50%

Cr has an effect of increasing the work hardening rate of the pearlite of the steel. When the work hardening rate of pearlite is increased, a higher tensile strength can be obtained in a drawing process of low strain. Cr is also an element useful for obtaining a pearlite-based structure because it has an effect of delaying the transformation from austenite to corner stone cementite and pro-eutectoid ferrite. In order to effectively exhibit this action, the lower limit of the Cr content is set to 0.10%. The lower limit of the preferable Cr content is 0.15% or 0.20%. However, when the Cr content is more than 0.50%, the quenching of the wire becomes high, and undercooling such as bainite and martensite occurs in the cooling process after the hot rolling, and the wire rod is excessively intensified to have high strength and drawability is deteriorated. Therefore, the upper limit of the Cr content is set to 0.50%. The upper limit of the preferable Cr content is 0.40% or 0.35%.

Mn and Cr are both elements that improve the steel quenching and also have an effect of retarding the transformation to crushed stone cementite. It is preferable to control the total content of Mn and Cr in order to suppress the occurrence of non-pearlite structure (such as cobalt cementite, bainite and martensite) on the wire rods. The lower limit of the total content of Mn and Cr is preferably 0.40% or 0.45%. The upper limit of the total content of Mn and Cr is preferably 0.60% or 0.55%.

The wire material according to the present embodiment may further contain one or more of Ni, Co, Mo, and B as shown below in addition to the above-described basic elements. When these elements are not contained, the content of these elements is 0%.

Ni: 0 to 0.50%

Ni is an element which is useful for obtaining the texture of the pearlite-based body, because it has an action of delaying the transformation from austenite to cornerstone cementite and pro-eutectoid ferrite. Ni is also an element having an action of increasing the toughness of the drawing material. In order to obtain the above-mentioned action, it is preferable to set the lower limit of the Ni content to 0.10%. The lower limit of the Ni content is more preferably 0.15% or 0.20%. On the other hand, if Ni is contained excessively, the quenching becomes excessive, and undercooling such as bainite and martensite occurs in the wire rod during the cooling process after hot rolling, so that the wire drawing workability of the wire rod may be lowered. Therefore, it is preferable to set the upper limit of the Ni content to 0.50%. The upper limit of the Ni content is more preferably 0.30% or 0.25%.

Co: 0 to 1.00%

Co has an action of suppressing the precipitation of pro-eutectoid ferrite in the rolled material. Further, Co has an effect of improving the ductility of the fresh material. In order to effectively exhibit this action, the lower limit of the Co content is preferably 0.10%. The lower limit of the Co content is more preferably 0.20%, 0.30%, or 0.40%. On the other hand, even if Co is excessively contained, the action is saturated, and the cost is increased. Therefore, it is preferable to set the upper limit of the Co content to 1.00%. The upper limit of the Co content is more preferably 0.90% or 0.80%.

Mo: 0 to 0.20%

Mo has a function of delaying the transformation from austenite into corner stone cementite and pro-eutectoid ferrite, and is a useful element for obtaining the texture of the pearlite main body. In order to obtain the above action, the lower limit of the Mo content is preferably 0.05%. The lower limit of the Mo content is more preferably 0.08%. However, if the Mo content exceeds 0.20%, the quenching becomes excessive, and undercooling such as bainite and martensite may occur during the cooling process after the hot rolling, and the drawing workability of the wire rod may be deteriorated. Therefore, it is preferable to set the upper limit of the Mo content to 0.20%. The upper limit of the Mo content is more preferably 0.15% or 0.11%.

B: 0 to 0.0030%

B is concentrated in the grain boundary, and has an action of suppressing the precipitation of pro-eutectoid ferrite. In order to obtain the above action, the lower limit of the B content is preferably 0.0002%. More preferably, the lower limit of the B content is 0.0005%, 0.0007%, 0.0008%, or 0.0009%. On the other hand, when B is excessively contained, B sometimes forms a carbide such as Fe ₂₃ (CB) ₆ in austenite, thereby deteriorating the drawing workability of the wire. Therefore, the upper limit of the B content is preferably 0.0030%. More preferably, the upper limit of the B content is 0.0020%.

The wire material according to the present embodiment contains one or more of Ni, Co, Mo and B as essential elements and, if necessary, the remainder is substantially Fe and impurities. The wire material according to the present embodiment may contain P and S as impurities to be mixed in the production of molten steel.

P: not more than 0.020%

P is an element that deteriorates the drawing workability of the wire by being segregated at grain boundaries. Therefore, it is preferable to reduce the P content as much as possible. The upper limit of the P content is set to 0.020% in order to ensure the wire workability of the wire. The upper limit of the preferable P content is 0.014% or 0.010%. P may be incorporated as an impurity in the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit thereof is 0%. If the P content is excessively reduced, the solvent cost may increase, so the lower limit of the P content may be 0.003% or 0.005%.

S: not more than 0.010%

S is an element which deteriorates the drawing workability of a wire by forming a precipitate with Mn or the like. Therefore, it is preferable to reduce the S content as much as possible. The upper limit of the S content is set to 0.010% in order to ensure the drawing processability of the wire rod. The upper limit of the preferred S content is 0.008%, 0.007%, or 0.005%. S may be incorporated as an impurity in the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit thereof is 0%. If the S content is excessively reduced, the solvent cost may increase, so the lower limit of the S content may be 0.001% or 0.003%.

The wire material according to the present embodiment has pearlite as a main structure and the remaining structure is composed of one kind or two kinds or more of cornerstone cementite, grain boundary ferrite and bainite. The residual cobalt cementite, intergranular ferrite, and bainite, which are the residual structure, sometimes serve as propagation paths of fracture. As the area fraction of these remaining tis- sue increases, the wire drawing workability may be deteriorated. Therefore, in the wire according to the present embodiment, when the radius of the wire is R, the area fraction of the pearlite is 90.0% or more in the structure observed at the center within (1/5) R from the center of the cross- , And the area fraction of crude stone cementite is set to 1.00% or less. The preferred area fraction of pearlite is 93.0% or more, 95.0% or more, or 97.0% or more. Preferably, the area fraction of the cornerstone cementite is 0.50% or less, or 0.20% or less.

The area fraction of pearlite may be 100% in a structure observed at a central portion within (1/5) R from the center of the cross section of the wire. However, in the chemical composition of the wire material according to the present embodiment, It is difficult to completely inhibit the precipitation of the nitrate. If the area fraction of the pearlite is assumed to be 100% in a structure observed at a central portion within (1/5) R from the center of the transverse section of the wire rod, a very excellent cooling ability is required, There are cases where the drawability decreases due to an increase in tensile strength and the cost increases in the secondary working due to an increase in the load during drawing. Therefore, the area fraction of the pearlite may be less than 100% in a structure observed at a central portion within (1/5) R from the center of the cross section of the wire rod.

If the precipitation amount is small, the fresh formability of the wire rod does not deteriorate. On the other hand, in order to make the area fraction of the corner stone cementite 0% in the structure observed at the center within (1/5) R from the center of the cross section of the cross section of the wire, excellent cooling ability is required and facility cost may increase . Therefore, the area fraction of the cornerstone cementite may be set to be more than 0% in the structure observed at the central portion within (1/5) R from the center of the cross section of the wire rod.

It is preferable to reduce the area fraction of intergranular ferrite and bainite as much as possible in a structure observed at a central portion within (1/5) R from the center of the cross section of the wire rod. It is preferable that the total area fraction of grain boundary ferrite and bainite is 5.0% or less, or 4.5% or less. Setting the total area fraction of the intergranular ferrite and bainite to 0% may increase the manufacturing cost, and therefore, the total area fraction of intergranular ferrite and bainite may be set to exceed 0%.

Crushed stone cementite in the wire rod is a factor of disconnection in drawing processing. However, if the precipitation amount of the cornerstone cementite is small, particularly the relationship with the old austenite grain boundary can be appropriately adjusted, deterioration of the drawability can be suppressed. Concretely, by reducing the thickness of cornerstone cementite and by shortening the total length of cornerstone cementite per unit area, it is possible to suppress deterioration of the wire drawing workability.

The thickness and total length of the cornerstone cementite will be described with reference to Figs. 1 to 4. Fig. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a precipitation state of quartz cementite in the old austenite grain boundary. FIG. Fig. 2 is a view for explaining a method of measuring the thickness and length of the cornerstone cementite 10a of Fig. 1. Fig. Figs. 3 and 4 are views for explaining a method of measuring the thickness and length of the cornerstone cementites 10b and 10c of Fig. 1, respectively.

Crushed stone cementite precipitates in a shape along the old austenite grain boundaries. Specifically, as shown in Fig. 1, the corner stone cementites 10a to 10d are precipitated along the old austenite grain boundary 20. For each cornerstone cementite, the length is defined as the direction along the old austenite grain boundary, and the thickness is defined as the direction perpendicular to the old austenite grain boundary. With respect to the thickness of the corner stone cementite, the thickness is measured at three places at intervals obtained by dividing the length in the direction along the old austenite grain boundary, and the average of the measured values is defined as the thickness of the cornerstone cementite. Further, in the measurement of the thickness of cornerstone cementite, when it is judged that the measurement point is different from the normal point such as the bifurcation point or the end portion, the point is not included in the average. 2, the length of the corner stone cementite 10a is L1 and the thickness of the corner stone cementite 10a is the average of T1, T2 and T3. As with the corner stone cementite 10b of Fig. 1, the sum of the lengths of the branches is defined as the length of the corner stone cementite. In other words, in Fig. 3, the length of the corner stone cementite 10b is the sum of OA, OB, and OC. The thickness of the cornerstone cementite is measured at three intervals in the direction along the old austenite grain boundaries as described above at each quarter, and the average of these measured values is defined as the thickness of the cornerstone cementite. 3, the thickness of the corner stone cementite 10b is an average of TA1, TA2, TA3, TB1, TB2, TB3, TC1, TC2 and TC3. For a corner stone cementite having a shape bent along the old austenite grain boundary, like the cornerstone cementite 10c in Fig. 1, the length is measured along the old austenite grain boundary. That is, in Fig. 4, the length of the cornerstone cementite 10c is the sum of O'D and O'E. Further, the thickness is divided at the bent portions, and the portions are measured at three intervals at intervals of four equal lengths in the direction along the old austenite grain boundaries as described above, and the average of these measured values is defined as the thickness of the cornerstone cementite do. That is, in Fig. 4, the thickness of the corner stone cementite 10c is an average of TD1, TD2, TD3, TE1, TE2 and TE3. The total length of the corner stone cementites in Fig. 1 is the sum of the lengths of the corner stone cementites 10a to 10d.

The wire according to the present embodiment is characterized in that the average thickness of the cornerstone cementite is 0.25 탆 or less and the total length of the cornerstone cementite per unit area in the structure observed at the central portion within (⅕) R from the center of the cross- Is less than 40.0 mm / mm < 2 >. The average thickness of the preferred cornerstone cementite is 0.20 mu m or less. The total length of the elementary cementitious cementite per unit area is preferably not more than 30.0 mm / mm 2, not more than 20.0 mm / mm 2, or not more than 10.0 mm 2 / mm 2. If the average thickness of the cornerstone cementite exceeds 0.25 占 퐉 or the total length of the elementary cementite cementite per unit area is 40.0 mm / mm2 or more, defects during wire drawing of the wire become large, which may cause a disconnection.

In the wire according to the present embodiment, the degree of occupancy at the old austenite grain boundaries of the cornerstone cementite in the structure observed at the central portion within (1/5) R from the center of the cross section of the wire rod is reduced, The workability may be lowered further. The degree of occupancy of the corner stone cementite in the old austenite grain boundary is evaluated by the product of the total length of the cornerstone cementite per unit area and the grain size of the old austenite as shown on the left side of the following formula (A). The left side of the following formula (A) is preferably less than 1.2. More preferably, the left side of the following formula (A) is less than 1.0.

The tensile strength (MPa) of the wire according to the present embodiment is defined by the following formula (1) according to the C content (mass%) and the Cr content (mass%). If the tensile strength of the wire rod is lower than the lower limit value (left side) shown in the following formula (1), the roughness of the cornerstone cementite, the area fraction of the cornerstone cementite, or the thickness of the lamellar cementite is increased, There may be a case where it is lowered. On the other hand, if the tensile strength of the wire rod exceeds the upper limit value (right side) shown in the following formula (1), the work hardening rate at the time of drawing processing is increased, the tensile strength of the drawing material is increased and the ductility is lowered, . Further, if the tensile strength of the wire rod exceeds the upper limit value (right side) shown in the following formula (1), the load of the die and the drawing machine increases, and the manufacturing cost may increase.

The constant term of the right side of the preferred expression (1) is +150 (MPa). In other words, the tensile strength of the wire preferably satisfies the following formula (2). More preferably, the constant term of the left side of the equation (1) is +80 (MPa), and the more preferable right side term is +150 (MPa). In other words, it is more preferable that the tensile strength of the wire satisfies the following formula (3). More preferably, the constant term of the left side of the equation (1) is +90 (MPa), and the more preferable right side term is +140 (MPa). In other words, it is more preferable that the tensile strength of the wire satisfies the following formula (4). In addition, the TS in the following formulas (1) to (4) represents the tensile strength of the wire, "C amount (%)" represents the content mass% of C in the wire, "Cr amount (% Represents the content by mass of Cr in the wire.

The wire diameter of the wire affects the cooling rate after winding, and as a result influences the metal structure and the tensile strength of the wire. If the diameter of the wire rod exceeds 5.5 mm, the cooling rate of the wire rod center portion is slowed, and a large amount of cornerstone cementite may be generated in the wire rod. On the other hand, if the diameter of the wire rod is less than 3.0 mm, the production of the wire rod becomes difficult and the production efficiency is lowered, thereby increasing the cost of the wire rod. Therefore, the wire diameter of the wire according to the present embodiment is 3.0 to 5.5 mm.

The measurement of the area fraction of pearlite and basic stone cementite is carried out by the following method.

First, the wire rod is cut, and the wire rod is filled with resin so that the cross-section perpendicular to the longitudinal direction of the wire rod can be observed. The resin-embedded wire rod is polished with abrasive paper and alumina abrasive grains, and mirror-finished to obtain a sample. After the observation surface of the sample (that is, the cross section of the wire rod) is corroded with the solution for dissolution or the solution of the pigment, observation surface of the sample is observed using a scanning electron microscope (SEM).

The dissolution solution is a solution of nitric acid and ethyl alcohol. Corrosion of the observation surface of the sample is carried out by immersing the surface to be observed for a few seconds to 1 minute in a release solution having a concentration of 5% or less and a temperature of 15 to 30 캜 or so, And a method of wiping the observation surface with a cotton wool cloth moistened with water. The picual solution is a solution of picric acid and ethyl alcohol. Corrosion of the observation surface of the sample is performed by immersing the observation surface in a solution of a concentration of about 5% and a temperature of about 40 to 60 ° C for 30 seconds to 2 minutes. After the corrosion, the observation surface of the sample is immediately rinsed thoroughly, and then it is quickly dried by cold air or hot air.

Subsequently, the center of the sample (the radius of the wire is R and the area within (1/5) R from the center of the wire) is measured at a magnification of 2000 times or more and the total observation field area Is 0.08 mm 2 or more. Using these SEM photographs and image analysis software such as particle analysis software, the area fraction of pearlite and core stone cementite at the center of the wire rod is obtained.

The average thickness and length of the corner stone cementite are measured using the above SEM photograph. The average thickness of the cornerstone cementite is obtained by calculating the thickness of all the cornerstone cementites in the SEM photograph and calculating the average value thereof. The thickness of the corner stone cementite is obtained by measuring the thickness in the direction perpendicular to the old austenite grain boundary. In the case of the cementite 10a of FIG. 2, the thicknesses T1, T2 and T3 are measured, and the average thereof is taken as the thickness of the cornerstone cementite. In addition, the length (mm) of the cornerstone cementite is determined based on the shape of the cornerstone cementite in the SEM photograph, and the length is measured along the line. If the cementite does not have a specific curved shape like the cementite 10a in Fig. 2, a straight line imaginative of the old austenite grain boundary along the major axis direction is measured, and the length L1 is measured along the straight line. As in the case of the cementite 10c shown in Fig. 4, if a cornerstone cementite having a specific curved portion is used, a line imaginative of the old austenite grain boundaries is formed according to the shape thereof, and the cornerstone cementite length along the line is measured. As in the case of the cementite 10b of Fig. 3, if the corner stone cementite has branches, the length of each branch is totalized. The total length (mm / mm < 2 >) of the cornerstone cementite per unit area is the value obtained by dividing the sum of the lengths of the respective cornerstone cementites in the measured field of view by the viewing area. That is, the total length (mm / mm 2) of the elementary cementite cementite per unit area is the sum of the lengths of the elementary cementite cementites observed per unit area. Further, at the time of measurement, the average thickness and length of the cornerstone cementite may be measured by photographing a region containing cornerstone cementite at a higher magnification, if necessary.

The old austenite grain size is measured by using a wire material called water after cooling the water ring from the final end portion of the coil immediately after being wound while being hot rolled. The wire is cut and the resin is embedded into the wire so that the cross section can be observed. The resin-embedded wire rod is polished with abrasive paper and alumina, and mirror-finished to obtain a sample. The old austenitic grain boundaries are exposed by etching the observation surface of the sample (that is, the cross section of the wire rod) with an alkali solution of picric acid. Corrosion of the observation surface of the sample is performed by immersing the observation surface of the sample in the alkali solution of picric acid at a temperature of 75 to 90 캜 for about 10 to 20 minutes. After the corrosion, the observation surface of the sample is immediately rinsed well and dried quickly by cold air or hot air. The alkali solution of picric acid used for the corrosion of the observation surface is a mixed solution of picric acid 2, sodium hydroxide 5 and water 100 in a weight ratio.

After the observation surface was corroded, the center of the observation surface of the sample (the radius of the wire was R, the area within (1/5) R from the center of the wire) was observed at a magnification of 400 times or more using an optical microscope Multiple field of view is taken so that the area is 0.15 mm2 or more. Using the photographed photograph and the cutting method described in JIS G 0551: 2013, the old austenite grain size is measured. In the cutting method, ten or more straight lines having a length of 400 占 퐉 are drawn so as not to overlap each other at intervals of 100 占 퐉, and the number is determined by the number of trapping grains captured in a straight line of 4mm or more in total.

The tensile strength of the wire rod is measured by the following method. Three or more samples are taken from the front part, the middle part, and the tail part of the wire coil, respectively, except for the abnormal part. A tensile test is carried out in accordance with JIS Z 2241: 2011 using the collected samples. The tensile strength of the wire rod is obtained by calculating the average value of the tensile strengths of all the samples.

Next, a method of manufacturing the wire rod according to the present embodiment will be described. The manufacturing method described below is merely an example, and the present invention is not limited to the following procedure and method, and any method can be employed as long as it is a method capable of realizing the configuration of the wire according to the present embodiment.

The material to be provided to the hot rolling can be obtained under ordinary production conditions. For example, after a steel having the above-mentioned components is cast and a soaking treatment (heat treatment for reducing segregation caused by casting) in which the cast steel is held at about 1100 to 1200 DEG C for 10 to 20 hours is carried out, , Thereby obtaining a piece of a steel sheet having a size suitable for hot rolling (a billet called a billet before hot rolling).

Next, hot rolling is performed under the following conditions. First, the billet is heated to 900 to 1200 占 폚, and the starting temperature of the finish rolling is controlled to 750 to 950 占 폚. The temperature of the wire during hot rolling indicates the surface temperature of the wire. The temperature of the wire at the time of hot rolling may be measured using a radiation thermometer.

The temperature of the wire rod after finish rolling is higher than the start temperature of finish rolling by the heat generated by processing. In the present embodiment, the coiling temperature is controlled to 800 to 940 占 폚. If the coiling temperature is lower than 800 占 폚, the austenite grain size of the wire becomes finer, so that it becomes easy to precipitate outbreak cementite, intergranular ferrite and bainite, and the mechanical scale peeling property of the wire may be deteriorated. On the other hand, if the coiling temperature exceeds 940 占 폚, the austenite grain size of the wire becomes excessively large, so that the drawing workability of the wire may be deteriorated. The preferred coiling temperature is 830 to 920 占 폚. A more preferable coiling temperature is 850 to 900 占 폚.

As described above, it is preferable to control the starting temperature of the finish rolling and the coiling temperature so that the grain size of the old austenite of the wire is 15 to 60 탆. More preferably, the grain size of the old austenite is 20 to 45 占 퐉.

The austenite in the wire rod is transformed into pearlite during cooling after winding. Therefore, the cooling rate after winding is an important factor for controlling the texture and tensile strength of the wire rod. In the present embodiment, cooling after winding is divided into three temperature ranges, and the average cooling rate in each temperature range is controlled.

When the average cooling rate up to 650 占 폚 after winding is less than 6.0 占 폚 / s, it may be difficult to suppress precipitation of crushed stone cementite. On the other hand, if the average cooling rate up to 650 占 폚 after winding is more than 15.0 占 폚 / s, transformation from austenite to bainite, deterioration of drawing workability due to high strength, and deterioration of mechanical scaling- . Further, if the average cooling rate up to 650 占 폚 after winding is more than 15.0 占 폚 / s, a large-scale cooling facility is required, and the equipment cost may increase. Therefore, after winding, the average cooling rate up to 650 占 폚 is set to 6.0 to 15.0 占 폚 / s. After winding, the preferred average cooling rate up to 650 占 폚 is 7.0 to 10.0 占 폚 / s.

In the temperature range of 650 to 600 캜, the average cooling rate is controlled to 1.0 to 3.0 캜 / s in order to transform the austenite in the wire into pearlite. If the average cooling rate at 650 to 600 占 폚 is less than 1.0 占 폚 / s, the tensile strength of the wire rod may be lowered or the thickness of the cornerstone cementite may be increased, thereby deteriorating the wire drawing workability. On the other hand, when the average cooling rate at 650 to 600 占 폚 exceeds 3.0 占 폚 / s, the transformation from austenite to pearlite does not terminate to 600 占 폚, and the tensile strength of the wire rod increases, , And the lifetime of the fresh die may be lowered. The preferable average cooling rate at 650 to 600 占 폚 is 1.5 to 2.8 占 폚 / s.

At a temperature range of 600 占 폚 or less, the average cooling rate is set to 10.0 占 폚 / s or more and further cooled to 300 占 폚 or less. This is because, if the wire rod is maintained around the transformation temperature even after the austenite is transformed into pearlite, the tensile strength of the wire rod may be lowered. The preferable average cooling rate at 600 to 300 占 폚 is 15.0 占 폚 / s or more. If the average cooling rate at 600 to 300 占 폚 is set to exceed 50 占 폚 / s, an excellent cooling facility is required, thereby increasing facility costs. Therefore, the upper limit of the average cooling rate at 600 to 300 占 폚 may be 50 占 폚 / s or less.

The temperature of the wire at the time of cooling may be measured by a radiation thermometer. Generally, the cooling of the wire after the hot rolling is carried out in the form of a coil, followed by cooling. In the wire wound in a coil shape, there are a bundle portion where the wire material is overlapped with each other and a solder portion where there is little overlap between the wire materials. In the method of manufacturing a wire according to the present embodiment, the temperature of the wire after winding is measured at a portion where a plurality of wires overlap each other in a coil-like manner.

By having the above-mentioned composition and adjusting the production conditions as described above, the structure and tensile strength of the wire can be made within the scope of the present invention.

Example

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described more specifically by way of examples of the wire material according to the present invention. However, the present invention is not limited to the following examples, but can be carried out by appropriately changing the scope of the present invention, and they are all included in the technical scope of the present invention.

Table 1 shows the chemical composition of the steel and the hot rolling conditions. Table 2 shows the results of evaluating the structure of the wire rods, and the results of evaluating the tensile characteristics and the drawability. The cooling rates 1 to 3 in Table 1 are as follows. The average cooling rate was controlled by adjusting the wind amount of the wind. In Table 1 and Table 2, figures deviating from the scope of the present invention are underlined.

Cooling speed 1: Average cooling rate up to 650 ° C after winding

Cooling speed 2: Average cooling rate from 650 占 폚 to 600 占 폚

Cooling speed 3: Average cooling rate from 600 캜 to 300 캜

Numerals A1 to A22 in Table 1 are examples. In addition, No. B1 to B13 in Table 1 are comparative examples in which at least one of the component or the hot rolling condition is out of the appropriate range.

In all of the examples and comparative examples, the billet was heated to 1000 to 1200 占 폚 in a heating furnace, and the starting temperature of the finish rolling was set to 750 to 950 占 폚. The temperature of the wire roughened by the processing heat generated at the time of finish rolling was controlled, and the wire was wound in the form of a coil at the winding temperature shown in Table 1. The cooling after the coiling was carried out in such a manner that the average cooling rate (cooling rate 1 in Table 1) up to 650 ° C after winding, the average cooling rate (cooling rate 2 in Table 1) from 650 ° C to 600 ° C, The average cooling rate (cooling rate 3 in Table 1) was measured under the conditions shown in Table 1. By the above method, a wire rod having the wire diameter shown in Table 1 was obtained.

The area fraction of pearlite in the wire rod and the area fraction of the cemented stone cementite were measured by the following methods.

First, the wire rod was cut and the wire rod was filled with resin so as to observe a transverse section perpendicular to the longitudinal direction. The resin-embedded wire rod was polished with abrasive paper and alumina abrasive grains, and mirror-finished to obtain a sample. The observation surface of the sample (that is, the cross section of the wire) was corroded with a solution for dissolution or a solution of the pigment, and the observation surface of the sample was observed using a scanning electron microscope (SEM). The used dissolution solution was a mixture of nitric acid and ethyl alcohol. Corrosion of the observation surface of the sample is carried out by immersing the observation surface for a few seconds to 1 minute in the leaving solution at a concentration of 5% or less and a temperature of about 15 to 30 캜, And a method of wiping the observation surface with a cotton swab soaked with water. The used solution was a mixture of picric acid and ethyl alcohol. Corrosion of the observation surface of the sample was carried out by immersing the observation surface in a solution of the concentration of about 5% and the temperature of about 40 to 60 캜 for 30 seconds to 2 minutes. After the corrosion, the observation surface of the sample was immediately washed with water and then quickly dried by a cold wind or a hot wind.

Subsequently, the center of the sample (the radius of the wire is R and the area within (1/5) R from the center of the wire) is measured at a magnification of 2000 times or more and the total observation field area Were photographed in a plurality of fields so as to be 0.08 mm 2 or more. Using these SEM photographs and image analysis software such as particle analysis software, the area fraction of pearlite and core stone cementite at the center of the wire rod was obtained. Luzex (registered trademark, manufactured by Nippon Kayaku Co., Ltd.) was used as the image analysis software.

In both of the present invention and the comparative example, the metal structure observed at the center portion was a composite structure of at least one of pearlite, cobalt cementite, grain boundary ferrite, and bainite.

The average thickness and length of the corner stone cementite were measured using the above SEM photograph. The average thickness of the cornerstone cementites was obtained by measuring the thickness of all the cornerstone cementites in the SEM photographs and calculating the average value thereof. The thickness of the corner stone cementite was obtained by measuring the thickness in the direction perpendicular to the old austenite grain boundary. 2, the thicknesses T1, T2 and T3 were measured, and the average of them was defined as the thickness of the cornerstone cementite. The length of the cornerstone cementite was determined based on the shape of the cornerstone cementite in the SEM photograph, and the length of the cornerstone cementite was measured along the line. As in the case of the cementite 10a shown in Fig. 2, if the cementite does not have a specific curved shape, a straight line imaginative of the old austenite grain boundary along the major axis direction is measured, and the length L1 is measured along the straight line. As in the case of the cementite 10c shown in Fig. 4, if a cornerstone cementite having a specific curved portion was used, a line simulating the old austenitic grain boundary was formed in accordance with the shape and the cornerstone cementite length along the line was measured. As in the case of the cementite 10b of Fig. 3, if the corner stone cementite has branches, the length of each branch is totalized. The total length of cemented cementite per unit area was obtained by dividing the total length of each cemented cementite in the measured field of view by the area of view. That is, the total length (mm / mm < 2 >) of the basic stone cementites per unit area was the sum of the lengths of the basic stone cementites observed per unit area. At the time of measurement, the area including cornerstone cementite was photographed at a magnification of 3000 to 5000 times as required, and the average thickness and length of the cornerstone cementite were measured.

The old austenite grain size was measured by using a wire material called water ring after water-cooling from the final end of the coil immediately after being wound while being hot-rolled. Called resin was cut, the resin was embedded so as to observe the cross-section, and then polished with alumina to obtain a sample. Thereafter, the polished sample was corroded with an alkali solution of picric acid to expose the old austenite grain boundaries. Corrosion of the observation surface of the sample was carried out by immersing the observation surface of the sample for about 10 to 20 minutes in an alkali solution of picric acid at a temperature of 75 to 90 캜. After the corrosion, the observation surface of the sample was immediately rinsed well and then quickly dried by a cold wind or a hot wind. The alkali solution of picric acid used for the corrosion of the observation surface was a mixed solution of picric acid 2, sodium hydroxide 5 and water 100 in a weight ratio.

The observation surface of the sample was corroded by immersing the observation surface of the sample in the alkali solution of picric acid at a temperature of 75 to 90 캜 for about 10 to 20 minutes. After the corrosion, the observation surface of the sample was immediately rinsed well and quickly dried in a cold or hot air. Thereafter, an optical microscope was used to measure the center of the observation surface of the sample (the radius of the wire was R, the area within (1/5) R from the center of the wire) at a magnification of 400 times or more, And the photographs were taken at a plurality of fields. Using these SEM photographs and the cutting method described in JIS G0551: 2013, the old austenite grain size was measured. In the cutting method, 15 or more straight lines having a length of 400 m were laminated so as not to overlap each other at intervals of 100 占 퐉, and the number of grains caught by a straight line with a total length of 6 mm was evaluated.

The tensile strength was measured by the following method. Three rings are taken from the front part (place on the side of 50 ring end from the front end), middle part (in 100 ring from the middle of the tip in the coil and the center of the coil), and teep part A total of 72 samples were collected from each ring, with 8 samples at equal intervals. Using these samples, a tensile test was conducted in accordance with JIS Z 2241: 2011. The tensile strength of the wire rod was obtained by calculating an average value of the tensile strengths obtained from these 72 samples. The tensile test was carried out with a sample length of 400 mm, a crosshead speed of 10 mm / min and a jig distance of 200 mm.

The wire drawing workability was evaluated by the following method. Ten rings were taken from the wire rod, pickled and scaled off, and then subjected to lime coating treatment. Thereafter, drawing processing (dry drawing processing) was performed without performing the faceting processing. The reduction ratio per pass in the drawing process was set to 17 to 23%. When the drawing was performed and the true strain at the time of disconnection was 2.9 or more, it was judged to be acceptable because of excellent drawing workability. On the other hand, when the drawing was performed and the true strain at the time of disconnection was less than 2.9, it was judged that the drawing had failed because the drawing performance was poor. The true strain was obtained by calculating -2 占 n (line diameter of the drawn material / line diameter of the wire). "Ln" is a natural log.

All of No.A1 to A22 are examples of the present invention, and exhibit excellent drawing workability which enables drafting with a true strain of 2.9 or more without performing the putting treatment.

On the other hand, since No. B1 to B13 do not satisfy any of the requirements of the present invention, the true strain at the time of disconnection was less than 2.9, and the drawability was lower than that of the present invention.

No. B1 had a high C content, the area fraction of the elemental brick cementite was increased, the average thickness of the elemental brick cementite was increased, and the total length of the elemental cementite cementite per unit area was prolonged.

The No. B2 had a high Si content and the No. B3 had a high Mn content, all of which resulted in an increase in the tensile strength of the wire rod and a deterioration in drawability.

No. B4 had a high Cr content, the area fraction of pearlite was reduced, and the tensile strength was increased, so that the drawing workability of the wire rod was deteriorated.

No. B5 and No. B11 had a large average cooling rate (cooling rate 2) of 650 to 600 占 폚, the tensile strength was increased and the wire drawing workability was deteriorated.

In No. B6, since the average cooling rate (cooling rate 1) from after winding to 650 占 폚 was small, the average thickness of cornerstone cementite was increased, and the wire drawing workability was deteriorated.

In No. B7, since the average cooling rate (cooling rate 2) at 650 to 600 占 폚 was small, the tensile strength was lowered and the drawability of the wire rod was lowered.

In No. B8, since the average cooling rate (cooling rate 1) from after winding to 650 占 폚 was large, the wire material was excessively cooled, the tensile strength was increased, and the drawability was deteriorated.

No. B9 had a low coiling temperature and a small average cooling rate (cooling rate 1) from after winding to 650 DEG C, so that the grain size of old austenite became finer and a large amount of crude stone cementite precipitated, The total length of the cementite becomes longer, and the drawing workability of the wire rod is lowered.

In No. B10, the average cooling rate (cooling rate 3) at 600 to 300 占 폚 was small, so that the tensile strength of the wire rod was lowered and the drawability was deteriorated.

No. B12 had a high coiling temperature, the former austenite grain size became larger, and the total length of cementite cementite per unit area became longer, so that the drawing workability of the wire rod was lowered.

No. B13 had a small average cooling rate (cooling rate 1) from after winding to 650 DEG C, the area fraction of the corner stone cementite increased, the average thickness of the cornerstone cementite increased, and the total length of the cornerstone cementite And the drawing workability of the wire rod was deteriorated.

Claims (5)

In terms of% by mass,
C: 0.90 to 1.15%
Si: 0.10 to 0.50%,
Mn: 0.10 to 0.80%
Cr: 0.10 to 0.50%
Ni: 0 to 0.50%,
Co: 0 to 1.00%
Mo: 0 to 0.20% and
B: 0 to 0.0030%
≪ / RTI >
P: 0.020% or less and
S: not more than 0.010%
, The balance being Fe and impurities,
The area fraction of the pearlite is 90.0% or more and the area fraction of the basic stone cementite is 1.00% or more in the structure observed at the central portion within (1/5) R from the center of the transverse section of the wire rod, Or less,
In the center portion, the average thickness of the cornerstone cementite is 0.25 탆 or less,
In the center portion, the total length of the cornerstone cementites per unit area is less than 40.0 mm / mm < 2 &
When the tensile strength satisfies the formula (1)
A wire having a diameter of 3.0 to 5.5 mm.

The total length (mm / mm < 2 >) of the basic stone cementite per unit area is the sum of the lengths of the basic stone cementite observed per unit area. The TS in the formula (1) represents the tensile strength of the wire rod when the unit is expressed in MPa. The "C content (%)" in the above formula (1) represents the content by mass of C in the wire rod and the "Cr content (%)" represents the content by mass of Cr in the wire rod. The method according to claim 1,
In terms of% by mass,
Ni: 0.10 to 0.50%,
Co: 0.10 to 1.00%
Mo: 0.05 to 0.20% and
B: 0.0002 to 0.0030%
Or a mixture thereof. 3. The method according to claim 1 or 2,
Wherein the area fraction of the basic stone cementite is more than 0% to 1.00%. 4. The method according to any one of claims 1 to 3,
The wire material according to any one of claims 1 to 3, wherein the wire has one or more of cobblestone cementite, intergranular ferrite and bainite. A steel piece having the components described in claim 1 is rolled at a temperature of from 3.0 to 5.5 mm in diameter and then rolled at 940 to 800 캜 and cooled to an average cooling rate of 6.0 to 15.0 캜 / A process according to any one of claims 1 to 4, characterized in that 650 to 600 占 폚 is cooled at an average cooling rate of 1.0 to 3.0 占 폚 / s and 600 to 300 占 폚 is cooled at an average cooling rate of 10.0 占 폚 / Wherein the wire is produced by a method comprising the steps of:

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