JP2021195566A - High carbon steel wire - Google Patents

Abstract

To provide a steel wire material excellent in wire drawability.SOLUTION: A high carbon steel wire excellent in wire drawability has a predetermined chemical composition. A metallographic structure includes perlite of 97.0% or more in a surface fraction and the remainder consisting of bainite, pseudo perlite and pro-eutectoid ferrite; when the average of carbon concentration by mass% in a region from the center to 17% of the radius of a steel wire material on a C cross section is defined as C17, the average of carbon concentration by mass% in a region from the surface to 10% or less of the radius of the steel wire material is defined as CS10 and the average of carbon concentration by mass% in the entire C cross section is defined as C0, C17/C0 is 0.99 or less, and CS10/C0 is 0.98 or less; a wire diameter is 4.0-6.0 mm; and a tensile strength is 1000-1400 MPa.SELECTED DRAWING: Figure 2

Description

The present invention relates to high carbon steel wire rods. More specifically, the present invention relates to steel cords used as reinforcing materials for, for example, radial tires of automobiles, various industrial belts and hoses, and high carbon steel wire rods suitable for applications such as sewing wires.

Extra-fine steel wires for steel cords used as reinforcing materials for radial tires of automobiles, various belts and hoses, or ultra-fine steel wires for sewing wires are generally obtained by the following methods. A steel wire having a wire diameter (diameter) of 4 to 6 mm that has been adjusted and cooled after hot rolling is subjected to primary wire drawing to a wire diameter of 3 to 4 mm, then intermediate patterning is performed, and then secondary wire drawing is performed. Process to a wire diameter of 1 to 2 mm. After that, a final patterning treatment is performed, then brass plating is performed, and then a final wet wire drawing process is performed to obtain a wire diameter of 0.15 to 0.40 mm. A steel cord is manufactured by twisting a plurality of the ultrafine steel wires thus obtained by further twisting to form a twisted steel wire. Further, the sewing wire is manufactured by drawing a wire having a wire diameter of less than 0.15 mm.

In recent years, for the purpose of reducing the manufacturing cost, the above intermediate patenting is omitted, and the steel wire rod after hot rolling that has been adjusted and cooled is directly drawn to a wire diameter of 1 to 2 mm to be subjected to the final patenting treatment. Many things are done. For this reason, the steel wire after hot rolling that has been adjusted and cooled is required to have direct wire drawing characteristics from the steel wire, so-called raw pullability, and the steel wire is required to have high ductility and high workability. It is extremely large.

Furthermore, in recent years, there has been an increasing tendency to reduce the weight of steel cords and the like for various purposes. Therefore, high strength is required for the above-mentioned various products, and a carbon steel wire having a C content of less than 0.9% by mass cannot obtain the desired high strength. Steel wires with a C content of 9% by mass or more are often used. However, when the C content is increased, the wire drawing workability is lowered due to the formation of proeutectoid cementite, so that the frequency of wire breakage increases. Therefore, there is an extremely large demand for a steel wire rod having a high C content to ensure high strength in the steel wire rod and having excellent wire drawing workability.

In response to the above-mentioned demands from the industrial world in recent years, various manufacturing techniques for high carbon steel wire rods have been proposed. For example, Patent Document 1 describes "high-strength, high-toughness ultrafine steel wire, high-strength, high-toughness ultrafine steel wire, and the ultrafine steel", which are made of steel having a specific chemical composition and define the average area ratio of cementite contained in the initial analysis. A twisted product using a steel wire and a method for manufacturing the ultrafine steel wire are disclosed. However, since the wire rod proposed in Patent Document 1 contains one or more of expensive elements Ni and Co as essential components, the manufacturing cost may increase.

Patent Document 2 proposes a technique for suppressing the formation of proeutectoid cementite in high carbon steel having a C content of more than 1% by adding Al of 0.6% or more. However, Al is a strongly deoxidizing element, and since the amount of hard inclusions that cause disconnection in wire drawing increases, it may be difficult to apply it to steel wire rods for ultrafine steel wires such as steel cords.

In Patent Document 3, a high carbon wire rod is heated to an austenite temperature range, cooled to a temperature range of 823 to 1023 K, and subjected to plastic machining with a workability of 15 to 80% in this temperature range, and then has a temperature of 823 to 923 K. A technique for suppressing the precipitation of austenitic cementite by constant temperature transformation in the region has been proposed. However, a large amount of capital investment is required to perform a predetermined process in such a temperature range, which may cause an increase in manufacturing cost.

On the other hand, a technique has also been proposed in which the cooling rate is low during the patenting treatment of the steel wire after hot rolling, and the C segregation degree in the center of the steel wire in which proeutectoid cementite is easily generated is reduced. For example, in Patent Document 4, in continuous casting of molten steel, forging is performed in the vicinity of the crater end where the molten steel inside the slab completes solidification, and the C content in the slab shaft core is 0.8 to 1.05 on average. A technique for improving the workability of a high carbon steel wire rod by doubling the number and then hot rolling the slab is disclosed. However, this method is costly, and the region where the C segregation degree is reduced in the high carbon steel wire rod is not clear.

Patent Document 5 discloses a technique for reducing the degree of C segregation at the center of a slab by a non-solidification reduction method. However, the degree of C segregation in the center of segregation is at least about 0.97, and the initial cementite of the steel wire containing more than 0.9% C, especially the steel wire containing more than 1.0% C. Not enough to suppress.

Japanese Patent No. 2609387 Japanese Unexamined Patent Publication No. 2003-193129 Japanese Unexamined Patent Publication No. 8-283867 Japanese Patent No. 2927823 Japanese Patent No. 3119203

The present invention has been made in view of the above situation, and an object of the present invention is to provide a high carbon steel wire rod having excellent wire drawing workability.

The gist of the present invention is as follows.

[1] The high carbon steel wire rod according to one aspect of the present invention is in mass%.
C: 0.80-1.20%,
Si: 0.10% or more, less than 0.30%,
Mn: 0.10 to 1.00%,
N: 40 ppm or less,
O: 40 ppm or less,
P: 0.020% or less, and S: 0.020% or less,
The rest consists of Fe and impurities
The metallographic structure contains 97.0% or more of pearlite in area fraction, and the balance consists of bainite, pseudo-pearlite and proeutectoid ferrite.
In the C cross section, the average value of carbon concentration in mass% in the region from the center to within 17% of the radius of the steel wire (the region from the center of the steel wire to 17% of the radius) is defined as C ₁₇ , and the surface to the surface. The average value of the carbon concentration in the mass% by mass% of the carbon concentration in the region of 10% or less of the radius of the steel wire (the region of the depth from the surface of the steel wire to 10% of the radius) is defined as _CS10, and the carbon concentration of the entire C cross section is defined as CS10. When the average value in% by mass is C ₀ , C ₁₇ / C ₀ is 0.99 or _{less, and CS 10} / C ₀ is 0.98 or less.
The wire diameter is 4.0 to 6.0 mm, and the tensile strength is 1000 to 1400 MPa.
[2] The high carbon steel wire rod according to the above [1] is in mass%.
Al: Over 0%, 0.010% or less,
Ti: Over 0%, 0.010% or less,
Cr: Over 0%, 0.50% or less,
Ni: Over 0%, 0.50% or less,
Co: Over 0%, 0.50% or less,
V: Over 0%, 0.50% or less,
Cu: Over 0%, 0.20% or less,
Nb: Over 0%, 0.10% or less,
Mo: Over 0%, 0.20% or less,
It may contain at least one selected from the group consisting of W: more than 0% and 0.20% or less and B: more than 0 ppm and 30 ppm or less.
[3] The high carbon steel wire rod according to the above [1] or [2] has a natural number of 1 to N in the C cross section, and is (k-1) × 30 μm from the center to k × from the center. when the average value of the percentage by weight of carbon concentration in the 30μm region was ^{_C avek, C avek} / ^C ₀ may be 1.10 or less.

According to the above aspect according to the present invention, it is possible to provide a high carbon steel wire rod having excellent wire drawing workability. The high carbon steel wire rod according to the above aspect is suitable for applications such as steel cords and sewing wires because it is excellent in direct wire drawing characteristics, so-called raw pullability.

It is a figure explaining the method of measuring the carbon concentration in the C cross section. It is a figure which shows the relationship between the TS of a steel wire rod in an Example, and the true strain of the limit which breaks in wire drawing with a die of a 20 degree approach angle.

The present inventors have repeatedly investigated and studied the effects of the chemical composition and mechanical properties of high carbon steel wire rods (hereinafter, sometimes simply referred to as steel wire rods) on wire drawing workability. As a result, the present inventors have obtained the following findings and have completed the present invention.

(A) When a high carbon steel wire containing 0.80% by mass or more, particularly 0.90% by mass or more of carbon is rolled and then subjected to a stellmore treatment of cooling by an impulse, carbon segregates in the center of the steel wire. It is easy to make it easy, and it becomes easy to generate pregelatinized cementite in the segregated portion of this carbon. Initialized cementite becomes the starting point of crack generation during wire drawing and induces disconnection. Since most cracks occur in the region from the center of the steel wire to 17% of the radius of the steel wire, suppressing the segregation of carbon in this region and suppressing the formation of proeutectoid cementite suppresses the occurrence of disconnection. That is, it is effective in improving the wire drawing workability.

(B) By suppressing the carbon content in the region from the center of the steel wire rod to 17% of the steel wire rod radius to 99% or less of the average carbon content of the steel wire rod, the disconnection due to the proeutectoid cementite can be significantly suppressed. .. In order to suppress the carbon content in the above region to 99% or less of the average carbon content of the steel wire, a flow velocity of 20 cm / s or more is applied to the solidification interface in the mold, and the slab is reduced before solidification is completed. It is necessary to do.

(C) By reducing the amount of carbon in the surface layer of the steel wire rod, the surface hardness of the wire after the wire drawing process is reduced, and the occurrence of vertical cracks (delamination) during the twisting process is suppressed. In order to suppress the occurrence of vertical cracks during twisting, it is necessary to set the carbon content in the region from the surface of the steel wire to a depth of 10% of the radius to 98% or less of the average carbon content of the steel wire. Is. Examples of the method for reducing the amount of carbon in this region include decarburization of the steel wire in a heating furnace or application of a flow velocity at the solidification interface.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.
The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values marked "less than" or "greater than" do not fall within the numerical range. For the chemical composition,% indicates mass% and ppm indicates mass ppm.

[Chemical composition]
The high carbon steel wire rod according to the present embodiment has a chemical composition of mass%, C: 0.80 to 1.20%, Si: 0.10% or more, less than 0.30%, Mn: 0.10 to 10. It contains 1.00%, N: 40 ppm or less, O: 40 ppm or less, P: 0.020% or less, and S: 0.020% or less.

C: 0.80-1.20%
C is an element effective for increasing the strength of the steel wire rod. When the C content is less than 0.80%, it is difficult to stably impart the desired strength to the final product, and at the same time, the precipitation of proeutectoid ferrite is promoted at the austenite grain boundaries, and the desired pearlite is promoted. It becomes difficult to obtain the area fraction of. Therefore, the C content is set to 0.80% or more. The C content is preferably 0.90% or more. On the other hand, if the C content is too high, net-like ductile cementite is generated at the austenite grain boundaries, and not only is it easy for wire breakage to occur during wire drawing, but also the toughness of the ultrafine steel wire after final wire drawing and the toughness of the ultrafine steel wire. Significantly deteriorates ductility. Therefore, the content of C is set to 1.20% or less. The C content is preferably 1.10% or less.

Si: 0.10% or more and less than 0.30% Si is an element effective for increasing the strength of steel wire. Further, Si is an element useful as a deoxidizing agent and is an element necessary when Al is not contained in the steel wire rod. If the Si content is less than 0.10%, a sufficient deoxidizing action cannot be obtained. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.20% or more. On the other hand, if the Si content is too high, the precipitation of proeutectoid ferrite is promoted even in hypereutectoid steel, and the limit workability during wire drawing is lowered. Further, mechanical descaling (hereinafter abbreviated as MD) makes wire drawing difficult. Therefore, the Si content is set to less than 0.30%. That is, by setting the Si content to less than 0.30%, the lamellar ferrite is softened, the limit workability during wire drawing is set to 4 or more in true strain, and the tensile strength when made into steel wire is 4000 MPa or more. Can be. Further, the patenting can be performed at a lower temperature for a shorter time.

Mn: 0.10 to 1.00%
Like Si, Mn is a useful element as a deoxidizing agent. Further, Mn has an effect of improving hardenability and increasing the strength of the steel wire rod. Further, Mn has an action of fixing S in the steel as MnS and preventing hot brittleness of the steel wire rod. Since it is difficult to obtain the above effect when the Mn content is less than 0.10%, the Mn content is set to 0.10% or more. The Mn content is preferably 0.30% or more. On the other hand, Mn is an element that is easily segregated, and when the Mn content exceeds 1.00%, Mn segregates particularly in the center of the steel wire rod, and martensite and bainite are generated in the segregated portion. The wire drawing workability of the wire is reduced. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.80% or less.

N: 40 ppm or less N tends to form a hard nitride when the steel material contains Ti, and tends to be the starting point of disconnection during wire drawing. Further, the solid solution N may accelerate the aging during the wire drawing process. Therefore, the N content is set to 40 ppm or less. The N content is preferably 30 ppm or less.

O: 40 ppm or less O is an element that easily forms an oxide, and when it is present in the wire together with Al, it forms a hard _{oxide-based inclusion containing Al 2} O ₃ as a main component and reduces the wire drawing workability. It is also an element that makes you. In particular, when the O content exceeds 40 ppm, the oxide-based inclusions become coarse even if the Al content is limited to the above range, and wire breakage occurs frequently during wire drawing, resulting in deterioration of wire drawing workability. Becomes remarkable. Therefore, the O content is limited to 40 ppm or less. It is preferably 30 ppm or less.

P: 0.020% or less and S: 0.020% or less The impurities P and S are preferably 0.020% or less, respectively, from the viewpoint of ensuring ductility as in the case of conventional ultrafine steel wire. Since excessive reduction of the P content and the S content causes an increase in the refining cost, the P content and the S content may be 0.002% or more and 0.005% or more.

The balance of the chemical composition of the high carbon steel wire rod according to the present embodiment consists of Fe and impurities. In the present embodiment, the impurities are mixed from ore, scrap, manufacturing environment, etc. as a raw material when the high carbon steel wire rod is industrially manufactured, and the high carbon steel according to the present embodiment. It means that it is acceptable as long as it does not adversely affect the characteristics of the wire. The high carbon steel wire rod according to the present embodiment contains the above elements as basic components, but one of the group consisting of Al, Ti, Cr, Ni, Co, V, Cu, Nb, Mo, W and B or Two or more kinds may be positively contained. When positively contained, the lower limit of these elements may be more than 0%. When these elements are not contained, the lower limit is 0%.

Al: 0 to 0.010%
Al is _{an element that forms oxide-based inclusions containing Al 2} O ₃ as a main component and reduces the wire drawing processability of the wire rod. In particular, when the Al content exceeds 0.010%, the oxide-based inclusions are coarsened and disconnection occurs frequently during the wire drawing process, and the wire drawing workability is significantly deteriorated. Therefore, the Al content is limited to 0.010% or less. The Al content is preferably 0.002% or less.

Ti: 0 to 0.010%
Ti may or may not be contained in the high carbon steel wire rod according to the present embodiment. Therefore, the lower limit of the Ti content is 0%. The Ti content may be 0.001% or more in order to ensure the effect of deoxidation. On the other hand, if the Ti content is too high, hard non-deformable oxides are generated, which causes deterioration of ductility and wire drawing processability of the wire, so the Ti content is set to 0.010% or less.

Cr: 0 to 0.50%
Cr is an element effective for reducing the lamellar spacing of pearlite and improving the strength and wire drawing workability of steel wire rods. In order to ensure this effect, the Cr content is preferably 0.10% or more. On the other hand, if the Cr content is too high, the transformation completion time from austenite to the supercooled structure becomes long, and supercooled structures such as martensite and bainite may occur in the transformed steel wire, and mechanical descaling. The sex may also get worse. Therefore, the Cr content is preferably 0.50% or less.

Ni: 0 to 0.50%
Ni does not contribute much to the increase in the strength of the steel wire, but is an element that enhances the toughness of the steel wire. In order to ensure this effect, the Ni content is preferably 0.10% or more. On the other hand, if Ni is excessively contained, the transformation completion time from austenite to the supercooled structure becomes long, and supercooled structures such as martensite and bainite may occur in the hot-rolled wire, so the Ni content is high. It is preferably 0.50% or less.

Co: 0 to 0.50%
Co is an element effective in suppressing the precipitation of proeutectoid cementite in rolled steel wire rods. In order to ensure this effect, the Co content is preferably 0.10% or more. On the other hand, even if Co is excessively contained, the above effect is saturated and economically unfavorable. Therefore, the Co content is preferably 0.50% or less.

V: 0 to 0.50%
By forming fine carbonitrides in the ferrite, V prevents coarsening of austenite grains during heating and also contributes to an increase in strength after rolling. In order to ensure that these effects are exhibited, the V content is preferably 0.05% or more. However, if V is excessively contained, the amount of carbonitride formed becomes too large, the particle size of the carbonitride also increases, and the wire drawing limit may decrease. Therefore, the V content is preferably 0.50% or less.

Cu: 0 to 0.20%
Cu has the effect of increasing the corrosion resistance of ultrafine steel wire. In order to ensure this effect, the Cu content is preferably 0.10% or more. However, if Cu is excessively contained, Cu reacts with S and segregates as CuS in the grain boundaries, which may cause defects in the ingot or steel wire in the process of manufacturing the steel wire. In order to prevent such an adverse effect, the Cu content is preferably 0.20% or less.

Nb: 0 to 0.10%
Nb has the effect of increasing the corrosion resistance of ultrafine steel wire. In order to ensure this effect, the Nb content is preferably 0.05% or more. On the other hand, if Nb is excessively contained, the transformation completion time from austenite to the supercooled structure becomes long, and supercooled structures such as martensite and bainite may occur in the hot-rolled wire, so the Nb content is high. It is preferably 0.10% or less.

Mo: 0 to 0.20%
Mo segregates at the growth interface of pearlite and has the effect of suppressing the growth of pearlite by the so-called solution drug effect. By containing an appropriate amount of Mo, it is possible to suppress only the growth of pearlite in a high temperature region of 600 ° C. or higher, and it is possible to suppress the formation of course pearlite having a large lamella interval. In addition, Mo has an effect of suppressing ferrite formation and an effect of improving hardenability, and is an element effective in reducing non-pearlite structure. In order to ensure that these effects are exhibited, the Mo content is preferably 0.05% or more. On the other hand, when Mo is excessively contained, the growth of pearlite in the entire temperature range is suppressed, the patenting takes a long time, the productivity is lowered, and the coarse Mo ₂ C carbide is precipitated to form the steel wire rod. Wire drawing workability may decrease. Therefore, the Mo content is preferably 0.20% or less.

W: 0 to 0.20%
W segregates at the growth interface of pearlite and has the effect of suppressing the growth of pearlite by the so-called solution drug effect. By containing an appropriate amount of W, it is possible to suppress only the growth of pearlite in a high temperature region of 600 ° C. or higher, and it is possible to suppress the formation of course pearlite having a large lamella interval. Further, W has an effect of suppressing ferrite formation and an effect of improving hardenability, and is an element effective in reducing non-pearlite structure. In order to ensure that these effects are exhibited, the W content is preferably 0.05% or more. On the other hand, if excessively contained the W, pearlite growth is suppressed in the entire temperature range, a long time is required for patenting, with the productivity is reduced, coarse W _{2 C} carbides are precipitated, wire drawing Sex may be reduced. Therefore, the W content is preferably 0.20% or less.

B: 0 to 30 ppm
When B is present in austenite in a solid solution state, it concentrates at the grain boundaries and suppresses the formation of non-pearlite precipitates such as ferrite, pseudo-pearlite and bainite. In order to surely exert this effect, the content of the solid solution B needs to be 3 ppm or more, and in order to satisfy this, the B content is preferably 4 ppm or more. On the other hand, if B is contained too much, _{the precipitation of coarse Fe 3} (CB) ₆ carbides in austenite may be promoted, which may adversely affect the wire drawing workability of the steel wire rod. Therefore, the B content is preferably 30 ppm or less.

In this embodiment, C is measured according to JIS G 1213-1: 2018, S is measured according to JIS G 1215-4: 2018, N is measured according to JIS G 1228, and other elements are measured according to JIS G 1253: 2013. And measure.

The high carbon steel wire rod according to the present embodiment is stretched even if it contains elements such as Pb, Ca, Mg, Sb, Bi, As, Ta, Sn, In, Zr, Te, Se, and Zn as impurities. It does not affect the line workability and does not cause any particular problem.

[Metallic structure]
In the high carbon steel wire rod according to the present embodiment, the metal structure contains 97.0% or more of pearlite in terms of area fraction, and the balance structure is composed of bainite, pseudo-pearlite and proeutectoid ferrite. If the surface integral of pearlite is less than 97.0%, the desired wire drawing workability cannot be obtained. The surface integral of pearlite is preferably 98.0% or more and 99.0% or more. The surface integral of the remaining structure is 3.0% or less, preferably 2.0% or less or 1.0% or less. The pseudo-pearlite is a structure in which cementite is present in a dotted line in ferrite, and pearlite (a layered structure composed of ferrite and cementite) can be distinguished from pearlite by SEM observation.

[Measurement method of metal structure]
In the present embodiment, the surface integral of the metal structure is measured by the following method.
First, a sample with a length of 10 mm is cut out from a predetermined position of the steel wire, embedded with resin so that the C cross section (cross section perpendicular to the longitudinal direction of the steel wire) can be observed, then polished with alumina and corroded with saturated picral. do. Next, using a scanning electron microscope (SEM), for the metal structure in the region within 25% of the steel wire radius from the center, at least 10 visual fields of 100 μm × 100 μm at a magnification of 3000 times were observed, and pearlite, bainite, etc. The area fraction of each metal structure is obtained by obtaining the area fraction of pseudo-pearlite and bainite and calculating the average of them.

[C concentration in C cross section]
C ₁₇ / C ₀ : 0.99 or less The high carbon steel wire rod according to this embodiment has a C cross section (cross section perpendicular to the longitudinal direction of the steel wire rod) in a region within 17% of the steel wire rod radius from the center to the center. When the average value of carbon concentration in mass% is C _17, and the average value of carbon concentration in mass% of the entire C cross section is C ₀ , C ₁₇ / C ₀ is 0.99 or less.
In the manufacturing process of a steel wire having a C content of 0.8% or more, carbon segregates in the center of the steel wire during the stealmore treatment after hot rolling. Initially deposited cementite, which is the starting point of wire breakage during wire drawing, is likely to be generated in the segregated portion of carbon. _{Therefore, in the high carbon steel wire rod according to the present embodiment, the C 17} / C ₀ in the region from the center to the center within 17% of the steel wire rod radius is set to 0.99 or less in the C cross section, so that the central portion of the steel wire rod Suppresses the formation of pro-eutectoid cementite and improves wire drawing workability. _{If C 17} / C ₀ in the above region of the C cross section exceeds 0.99, the desired wire drawing workability cannot be obtained. _{C 17} / C ₀ in the above region of the C cross section is preferably 0.98 or less and 0.97 or less.

C _S10 / C ₀ : 0.98 or less The high carbon steel wire rod according to this embodiment has a C cross section, and the average value of the carbon concentration in the region from the surface to the surface to the steel wire rod radius of 10% or less is C. _S10 and the _{time, C} S10 / _{C 0} is 0.98 or less. C _{S10 /} C ₀ to be to 0.98 or less, can reduce the surface hardness of the wire after drawing, the occurrence of longitudinal crack during twisting (delamination) can be suppressed.

^Cable / C ₀ : 1.10 or less In the high carbon steel wire rod according to the present embodiment, k is a natural number of 1 to N in the C cross section, and is (k-1) × 30 μm from the center to k × 30 μm from the center. When the average value of the carbon concentration in the region in mass% is ^Cavek , ^Cavek / C ₀ may be 1.10 or less. By ^{setting Coverk} / C ₀ to 1.10 or less, the wire drawing workability of the steel wire can be further improved. C and ^Avek / _{C 0} is 1.10 or less, can in other words the value which the maximum value was divided by _{C 0} of ^{C ave1,} C ave2 ^{... C Aven} described below is 1.10 or less. N is a natural number, and the upper limit thereof depends on the radius of the steel wire. When k = 1, ( ^Cave 1) is the mass% of the carbon concentration in the region from the center to the center of the steel wire rod 30 μm.

[Measurement method of carbon concentration in C cross section]
Hereinafter, a method for measuring the carbon concentration (mass%) of the C cross section in the present embodiment will be described with reference to FIG. 1. FIG. 1 shows a C cross section of the high carbon steel wire rod 100 according to the present embodiment.
One ring of ring-shaped steel wire after winding is divided into eight equal parts, and a sample having a length of 10 mm is cut out from these eight steel wires. After embedding the C cross section of these samples with resin so that they can be observed, alumina polishing is performed, and the carbon concentration of the C cross section is analyzed using EPMA (Electron Probe Micro Analyzer).
In the carbon concentration analysis by EPMA, a line analysis is performed on an arbitrary line segment from the center O of the C cross section to the surface, for example, on the line segment 1 in FIG. 1, with a spot size of 1 μm and a pitch of 3 μm. Next, the same line analysis is performed on the line forming an angle of 45 ° with the line segment (line segment 2 or line segment 8 in FIG. 1). Such line analysis is performed on eight line segments (line segments 1 to 8 in FIG. 1) on the C cross section. As a result, the carbon concentration every 3 μm is obtained on eight line segments from the center O of the C cross section to the surface.

Next, from the center O of the C cross section, every 10 points line-analyzed on one line segment is defined as one set, and the average carbon concentration of each set on each line segment (average every 80 points in total) is calculated. doing, obtain (from (k-1) × 30 microns to the center from the center k × 30 [mu] m region, region 10 in FIG. 1) from the k-th set around the ^{C Avek} is the average of the concentration of carbon. For example, the first set of ^Cave1 from the center is the average of the carbon concentrations from the first point to the tenth point (the region from the center to 30 μm) on the line segments 1 to 8, and the second set of C from the center. ^ave2 is the average carbon concentration from the 11th point to the 20th point (region of 30 to 60 μm) on the line segments 1 to 8. Here, for example, when the radius of the steel wire rod is 2 mm (2000 μm), in the 67th set from the center, there are only 7 analysis points on each line segment (1980 μm position from the center to 1998 μm position from the center). In this case, ^Cave67 is obtained by calculating the average carbon concentration at a total of 56 points in the 67th set from the center on the line segments 1 to 8.
Note that k is a natural number of 1 to N, and the upper limit (N) of k when the radius of the steel wire is 2 mm (2000 μm) is 67.
^{By calculating the average value of Cave1} , ^Cave2 ... ^CaveN (N varies depending on the wire diameter) obtained by the above method, _{the average value C 0} of the carbon concentration of the entire C cross section is obtained.

Next, _{how to obtain C 17} and _{CS 10} will be described.
The calculation of the average value C ₁₇ in carbon concentration in a region within 17% of the steel wire rod radius from the center-center of C cross-section, using equation _{(r n = 30 × n)} . Continue to substitute the order natural number from 1 to n in the formula, the maximum n ₁₇ where r n _([mu] m) does not exceed 17% of the radial steel wire rod. From ^{C ave1, C} ave2 ^{... C aveN} obtained in the method described ^above, by calculating the average value of ^{C ave1, C ave2 ... C aven17} , obtain _{C 17.}

C _S10 is gradually substituting the order natural number from 1 to n in formula _{(r n = 30 × n)} , determining the minimum _{n 90 where r n} ([mu] m) is more than 90% of the radius steel wire rod. From ^{C ave1, C} ave2 ^{... C aveN} obtained by the above ^method, by calculating the average value of ^{C aven90, C aven91 ... C aveN} , obtain _{C S10.}

Wire diameter: 4.0-6.0 mm
If the wire diameter of the steel wire is made less than 4.0 mm, handling during hot rolling becomes difficult. Further, when the wire diameter of the steel wire is 6.0 mm, the processing rate at the time of wire drawing to manufacture the ultrafine steel wire becomes large, and it becomes difficult to manufacture the ultrafine steel wire. Therefore, the wire diameter of the steel wire rod according to the present embodiment is set to 4.0 to 6.0 mm.

Tensile strength: 1000 to 1400 MPa
If the tensile strength is less than 1000 MPa, the strength is too low and it becomes difficult to apply it to ultrafine steel wire. If the tensile strength is more than 1400 MPa, it becomes difficult to wire the wire into an ultrafine steel wire. Therefore, the tensile strength of the steel wire rod according to this embodiment is set to 1000 to 1400 MPa. In this embodiment, the tensile strength is measured according to JIS Z 2241: 2015.

[Production method]
In the method for producing a high carbon steel wire rod according to the present embodiment, when continuously casting steel having the above-mentioned chemical composition, a flow velocity of 20 cm / s or more is applied to the solidification interface in the mold, and before solidification is completed. It is necessary to reduce the slab. The solidification interface is in the range from the position of the mold meniscus to the depth of 300 mm.

The reason for imparting a flow rate of 20 cm / s or more to the solidification interface in the mold is to generate granular crystals by fusing the columnar crystals or promoting the formation of equiaxed crystals. Due to the solid-liquid distribution effect of carbon, the granular crystals have a lower carbon concentration than the surrounding molten steel and a higher density than the molten steel, so that they settle during solidification. Since the precipitated granular crystals are present near the center of the slab, C ₁₇ (the average value in mass% of the carbon concentration in the region within 17% of the radius of the steel wire from the center) can be suppressed as a result. As a result, C ₁₇ / C ₀ can be set to 0.99 or less, and the wire drawing workability of the steel wire can be improved. _{Also, C S10} (average value in mass% of carbon concentration from the surface - the surface at 10% or less of the area of the steel wire rod radius) can be _reduced, also the _C S10 / _{C 0} 0.98 or less can.

The flow velocity in the mold may be applied by electromagnetic stirring, the nozzle discharge flow at the time of casting may be used by devising the shape of the immersion nozzle, or an external force such as gas blowing may be used. The flow velocity at the solidification interface in the mold may be 20 cm / s or more, preferably 30 cm / s or more, and more preferably 50 cm / s or more. If the flow velocity at the solidification interface in the mold is less than 20 cm / s, the effect of fusing the columnar crystals is lost, a sufficient amount of granular crystals cannot be generated, and as a result, the amount of carbon in the above region cannot be reduced. Further, in order to promote the formation of granular crystals, it is more effective to control the degree of superheat of molten steel (difference between molten steel temperature and liquidus temperature) at the time of casting to 30 ° C. or lower.

In the method for producing a high carbon steel wire rod according to the present embodiment, the slab is reduced before solidification is completed. The reason why the slab is reduced before the solidification is completed is to discharge the carbon-concentrated molten steel at the final solidification portion and prevent the carbon concentration from increasing locally. Performing the reduction of the slab before the solidification is completed means that the reduction is performed in the range of the central solid phase ratio fs of 0.1 to 1.0. In particular, it is preferable to reduce the pressure in the range of 0.3 to 0.7 in the central solid phase ratio fs to match the solidification shrinkage.

The central solid phase ratio fs is fs = (TL-T) / (TL-Ts) from the liquidus temperature TL of the molten steel, the solid phase temperature Ts, and the center temperature T in the thickness direction of the slab. demand.
The center temperature T can be obtained by unsteady heat transfer analysis calculation considering the casting speed, surface cooling of the slab, physical properties of the cast steel grade, and the like.

The slabs obtained by the above method are block-rolled, if necessary, and then hot-rolled. A high carbon steel wire rod according to the present embodiment is obtained by rolling to a wire diameter of 4.0 to 6.0 mm by hot rolling, winding at 800 to 950 ° C., and then patenting by a stealmore treatment. In the method for producing a high carbon steel wire according to the present embodiment, carbon in the surface layer of the steel wire (a region having a depth of up to 10% of the radius from the surface of the steel wire) is further performed by further decarburizing the steel wire in the heating furnace. The amount may be further reduced.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as well as the present invention, and the present invention is appropriately modified to the extent that it can be adapted to the gist of the present invention. Of course, they are all within the technical scope of the present invention.

In continuous casting of steel with the chemical components shown in Table 1, the degree of superheat of molten steel is controlled to 10 to 25 ° C, and 60 cm / s is applied to the solidification interface in the range from the position of the mold meniscus to the depth of 300 mm using an in-mold electromagnetic stirrer. The central solid phase ratio fs was reduced by a roll in the range of 0.1 to 1.0. The slab thus obtained was block-rolled into 120-160 mm square billets. In addition, "-" in Table 1 means that it is equal to or less than the analysis limit of the element. In addition, No. in Table 2 A-3, No. A-4, No. B-3, No. B-4, No. C-3, No. C-4, No. D-3, No. D-4, No. E-3, No. E-4, No. F-3, No. F-4, No. H-3 and No. I-2 did not impart a flow rate to the solidification interface.

The value obtained by dividing the carbon content of the 1 mm × 1 mm region in the center of the billet width and thickness direction by the ladle value (analyzed value of molten steel component) of the billet, which is the average value analyzed by EPMA with a spot size of 1 μm and a step of 3 μm. Was calculated to obtain the degree of segregation of carbon in the center of the billet. Table 2 shows the degree of segregation of carbon in the center of the obtained billet.

After heating this billet, it was hot-rolled to obtain a steel wire having a wire diameter shown in Table 2, wound at the temperature shown in Table 2, and then subjected to a patenting treatment by a stealmore treatment. The wire diameter of the steel wire was measured by a laser wire diameter measuring device.

The metallographic structure of the steel wire was measured by the method described above. The surface integral of the obtained pearlite is shown in Table 2. The remaining structure other than pearlite was one or more of bainite, pseudo-pearlite and proeutectoid ferrite.

_{In addition, the carbon concentration (C 17} , C ₀ , ^Ave and _{CS 10} ) in the C cross section of the steel wire was measured by the above method. From the carbon concentration obtained by the measurement, C ₁₇ / C ₀ , ^Ave / C ₀ and _{CS 10} / C ₀ were calculated. The obtained C ₁₇ / C ₀ , ^Ave / C ₀ and _{CS 10} / C ₀ are shown in Table 2.

The tensile strength of the steel wire was measured according to JIS Z 2241: 2015. When the tensile strength was 1000 MPa to 1400 MPa, it was judged to be acceptable as satisfying the condition specified in the present invention.

For the wire drawing workability of the steel wire, after removing the scale by pickling, prepare a steel wire with a length of 10 m to which a zinc phosphate film is applied by bonding, and use a die with an approach angle of 20 degrees per pass. Single-headed wire drawing with a surface reduction rate of 16 to 20% is performed, and the critical true strain at which disconnection occurs (2 x ln (d ₀ / d), d: wire diameter at the time of disconnection, d ₀ : before wire drawing. The wire diameter and ln were evaluated by the natural logarithm).

In Table 1, steels A to F, H, I and L are steels satisfying the chemical composition specified in the present invention, and steels G, J, K, M and N do not satisfy the chemical composition specified in the present invention. It is steel.

In Table 2, No. A-1, No. A-2, No. B-1, No. B-2, No. C-1, No. C-2, No. D-1, No. D-2, No. E-1, No. E-2, No. F-1, No. F-2, No. H-1, No. H-2, No. I-1 and No. L-1 is an example of the present invention in which the chemical composition, the metal structure, and the carbon concentration of the C cross section satisfy the conditions specified in the present invention, and is an example in which excellent wire drawing processability is obtained. On the other hand, No. in Table 2 A-3, No. A-4, No. B-3, No. B-4, No. C-3, No. C-4, No. D-3, No. D-4, No. E-3, No. E-4, No. F-3, No. F-4, No. G-1, No. H-3, No. I-2, No. J-1, No. K-1, No. M-1 and No. N-1 is a comparative example in which any one or more of the chemical composition, the metal structure, and the carbon concentration of the C cross section does not meet the conditions specified in the present invention, and is an example in which the wire drawing processability is inferior. For the invention examples and comparative examples in Table 2, FIG. 2 shows the relationship between the tensile strength and the true strain at the limit of wire breakage in wire drawing with a die having a 20-degree approach angle. According to FIG. 2, it can be seen that the invention example has excellent wire drawing workability as compared with the comparative example having the same tensile strength (TS) as the invention example.

No. A-3 and No. In E-3, C ₁₇ / C ₀ was more than 0.99, ^CS ₁₀ / C ₀ was more than 0.98, and the maximum value of Cavek / C ₀ was more than 1.10. This is an example in which the true strain at the time of disconnection was low when compared with the invention example in which cementite was generated in a wide range in the central portion and had the same tensile strength.
No. B-3, No. C-3 and No. _{Since C 17} / C ₀ was more than 0.99 and CS ₁₀ / C ₀ was more than 0.98 in F-4, proeutectoid cementite was generated in a wide range in the center, and the same tensile strength was obtained. This is an example in which the true strain at the time of disconnection was low when compared with the example of the invention having the invention.
No. A-4, No. B-4, No. C-4, No. D-3, No. E-4, No. F-3, No. H-3 and No. In I-2, since C ₁₇ / C ₀ exceeded 0.99, proeutectoid cementite was generated, and when compared with the invention example having the same tensile strength, the true strain at the time of disconnection was low. be.

No. For G-1 was excessive C content, C _{S10 /} C ₀ is 0.98 ultra next can not suppress the generation of pro-eutectoid cementite in Suterumoa, a true strain was very low example when disconnection occurs be.
No. Since J-1 was excessive Si _{content, C} S10 / _{C 0} is 0.98 ultra next, also can not suppress the precipitation of non-pearlite structures (pro-eutectoid ferrite), area fraction of pearlite is insufficient, This is an example in which the true strain at the time of disconnection was low.
No. Since K-1 had an excessive Si content and Mn content, the formation of micromartensite could not be suppressed, the surface integral of pearlite was insufficient, and the true strain at the time of disconnection was low.
No. M-1 is an example in which the true strain at the time of disconnection was low because the O content was excessive.
No. N-1 is an example in which the true strain at the time of disconnection was low because the N content was excessive.

According to the above aspect according to the present invention, it is possible to provide a high carbon steel wire rod having excellent wire drawing workability. Since the high carbon steel wire rod according to the above aspect is excellent in direct wire drawing property, so-called raw pullability, it can be suitably applied to steel cords, sewing wires and the like.

10 Area from center (k-1) x 30 μm to center k x 30 μm 100 High carbon steel wire

Claims (3)

By mass%,
C: 0.80-1.20%,
Si: 0.10% or more, less than 0.30%,
Mn: 0.10 to 1.00%,
N: 40 ppm or less,
O: 40 ppm or less,
P: 0.020% or less, and S: 0.020% or less,
The rest consists of Fe and impurities
The metallographic structure contains 97.0% or more of pearlite in area fraction, and the balance consists of bainite, pseudo-pearlite and proeutectoid ferrite.
In the C cross section, the average value in mass% of the carbon concentration in the region from the center to within 17% of the radius of the steel wire is defined as C _17, and the carbon in the region from the surface to the surface to 10% or less of the radius of the steel wire. the average value of the mass percent concentration of _{C S10,} when the average value of the percentage by weight of carbon concentration of the entire C cross section and _{C 0,} C 17 / _{C 0} is 0.99 or _{less, C} S10 / _{C 0} Is 0.98 or less,
A high carbon steel wire having excellent wire drawing workability, characterized in that the wire diameter is 4.0 to 6.0 mm and the tensile strength is 1000 to 1400 MPa. By mass%,
Al: Over 0%, 0.010% or less,
Ti: Over 0%, 0.010% or less,
Cr: Over 0%, 0.50% or less,
Ni: Over 0%, 0.50% or less,
Co: Over 0%, 0.50% or less,
V: Over 0%, 0.50% or less,
Cu: Over 0%, 0.20% or less,
Nb: Over 0%, 0.10% or less,
Mo: Over 0%, 0.20% or less,
The excellent wire drawing workability according to claim 1, wherein it contains at least one selected from the group consisting of W: more than 0% and 0.20% or less and B: more than 0 ppm and 30 ppm or less. High carbon steel wire rod. In the C cross section, when k is a natural number of 1 to N and the average value in mass% of the carbon concentration in the region from (k-1) × 30 μm from the center to k × 30 μm from the center is ^Cavek . The high carbon steel wire rod having excellent wire drawing workability according to claim 1 or 2, wherein ^Cavek / C _{0 is 1.10 or less.}

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