Technical field
[0001]
　The present invention has a drawability superior in as-hot-rolled without performing spheroidizing heat treatment, and relates to a steel wire material for a bearing having a coil forming properties after excellent wire drawing.
　The present application, on October 20, 2014, claiming priority based on Japanese Patent Application No. 2014-213479, filed in Japan, the contents of which are incorporated here.
Background technique
[0002]
　Steel wire rod bearing is used as a bearing part of the material of the roller or the like of the steel ball and roller bearings of the ball bearing. In a general manufacturing method thereof bearing component, it is carried out such spheroidizing annealing before drawing. Also, some thin bearing parts, for disconnection Shin wire with work hardening by wire drawing even performed spheroidizing annealing occurs, it is performed further annealing during wire drawing.
[0003]
　Bearing steel as defined in JIS G 4805 is, C weight of eutectoid point or more over-eutectoid steel, and includes Cr. Therefore, usually in the steel wire rod has precipitated pro-eutectoid cementite and martensite, wire drawability of such steel wire rod are considerably lower. Therefore, at present, but before drawing performing spheroidizing annealing to enhance the drawability, the spheroidizing annealing, or exacerbate production efficiency, and or increasing the cost. Recently, the spheroidizing annealing is omitted in order to reduce costs, wire drawing excellent in steel wire rod bearing remains hot rolling has been required.
[0004]
　Also, drawing material remained hot rolling, the strength is high, it is difficult to process the product shape, it is necessary to perform heat treatment on drawn wire. Since this heat treatment, it is necessary to the drawn wire in the coil state, it is important to ensure the workability can be molded into a wire drawing after the coil.
[0005]
　The high carbon steel wire disclosed in Patent Document 1, an average particle size of the ferrite to 20μm or less, thereby improving the wire drawability by limiting the maximum particle diameter 120μm or less. However, Patent Document 1, not intended omission of spheroidizing annealing, in the region Cr addition amount is large, technical consideration is not. The study by the present inventors, sufficient drawability be limited to a maximum particle size 120μm or less was not obtained.
[0006]
　In Patent Document 2, the pearlite colonies finer, by increasing the amount of pro-eutectoid cementite, have been proposed to improve the wire drawability of the wire rod. However, in the study by the present inventors, even if fine pearlite colonies sufficient wire drawability could not be obtained. In Patent Document 2, as an essential requirement, and finely many disperse the pro-eutectoid cementite. However, in the study by the present inventors, the precipitation amount of pro-eutectoid cementite drawability is lowered to be excessive.
[0007]
　In Patent Document 3, by controlling the average diameter of the region surrounded by the pro-eutectoid cementite 20μm or less, thereby improving the wire drawability. However, in the study by the present inventors, even if refining area surrounded by pro-eutectoid cementite could not result obtained necessarily improves the drawability. Also, as with Patent Document 2, even Patent Reference 3 suggests a positive precipitation of pro-eutectoid cementite.
[0008]
　Further, Patent Document 4, the area ratio of the pro-eutectoid cementite of 3% or more, thereby improving the wire drawability by controlling the lamellar spacing to 0.15μm or less. However, the present inventors have studied, if excessively finer lamellar spacing, the strength of the wire rod becomes too high, the greater the burden on the device and die, die life was reduced.
[0009]
　Patent Document 5 and Patent Document 6, by rapid cooling after hot rolling to suppress the generation of pro-eutectoid cementite, thereby improving the wire drawability by refining the grain size of the pro-eutectoid cementite. Also investigated by the present inventors, by refining the eutectoid cementite to reduce the amount of pro-eutectoid cementite, drawability is improved. However, the present inventors have also to suppress the generation of pro-eutectoid cementite, the hardness of the surface layer region of the wire by a reduction of the transformation temperature rises by rapid cooling as disclosed in Patent Documents 5 and 6 breakage during coil forming after drawing was newly found problems such as occur in.
[0010]
　In Patent Document 7, while suppressing the generation of pro-eutectoid cementite, thereby improving the wire drawability by controlling the strength of the wire. However, the present inventors, suppressing generation of pro-eutectoid cementite at a constant cooling rate, as disclosed in Patent Document 7, the hardness of the surface layer region and the center hardness of the surface layer region of the wire is increased increased difference in the disconnection during coil forming is newly found problems such as occur.
[0011]
　Patent Document 8 discloses a manufacturing method of HRC30 following hardness wire such as can wire drawing remains hot rolling. However, Patent Document 8 does not disclose the components of the bearing steel. The chemical composition of JIS G 4805 bearing steel disclosed, it is difficult to obtain a pearlite structure having the following hardness HRC 30, sufficient elongation by the generation or the like even abnormal tissue as hardness was HRC 30 or less it was not possible to obtain a line workability.
[0012]
　Patent Document 9, the ferrite grain size is small, discloses a Cr amount is large wire in the carbide. In the wire disclosed in Patent Document 9, to promote spheroidizing of carbides during spheroidizing annealing and reduces the time required for spheroidizing annealing. Thus, wire disclosed in Patent Document 9, the spheroidizing annealing is an essential, without omitting the spheroidizing annealing, it was not possible to obtain a sufficient drawability.
CITATION
Patent Document
[0013]
Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-200039
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-100016
Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-129176
Patent Document 4: Japanese Patent Laid-Open 2003-171737 Publication No.
Patent Document 5: Japanese Laid-open Patent Publication No. 08-260046
Patent Document 6: Japanese Laid-open Patent Publication No. 2001-234286
Patent Document 7: international Publication No. 2013/108828 SUPER nn fu TorayッSuites
Patent Document 8: Japanese Patent Laid-open 2003-49226 Publication No.
Patent Document 9: Japanese Laid-open Patent Publication No. 2012-233254
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　The present invention has been made to solve the above problems, a bearing steel having a high drawability that can omit the annealing treatment before drawing, and a high coil-forming properties after drawing and to provide a wire.
Means for Solving the Problems
[0015]
　The present inventors have found that the microstructure and internal hardness of the steel wire material for a bearing were studied in detail the effects on the coil forming property after drawability and wire drawing. As a result, the excessive precipitation of pro-eutectoid cementite while reducing the wire drawability, the coil formability hardness of the surface layer region is increased in the wire when you try to excessively suppress the precipitation of pro-eutectoid cementite is reduced It was heading. Furthermore, the present inventors have found that it is possible to even a small amount of pro-eutectoid cementite precipitated, improving the wire drawability due finer pearlite blocks. As a result, in order to suppress the disconnection wire by an internal cracks during drawing is important suppression of precipitation of fine and pro-eutectoid cementite of pearlite blocks, when forming the wire after drawing the coil the addition to controlling the hardness of the surface layer region, the present inventors have the knowledge that it is important to reduce the amount of pro-eutectoid cementite of the difference and the surface area of ​​the hardness of the surface layer region and the central portion It is obtained, which resulted in the completion of the present invention.
[0016]
　The present invention has been completed based on the above findings and has as its gist is as follows.
[0017]
(1) steel wire rod according to one aspect of the present invention, in mass%, C: 0.95 ~ 1.10% , Si: 0.10 ~ 0.70%, Mn: 0.20 ~ 1.20% , Cr: 0.90 ~ 1.60%, Mo: 0 ~ 0.25%, B: 0 ~ 25ppm, P: 0 ~ 0.020%, S: 0 ~ 0.020%, O: 0 ~ 0 .0010%, N: 0 ~ 0.030 %, Al: 0.010 ~ 0.100%, the balance: Fe and impurities, in a cross section perpendicular to the longitudinal direction, from the surface of the circle-equivalent diameter of half 0 surface layer region is a region between .1 times away lines and said surface has a microstructure consisting of pearlite and pro-eutectoid cementite and the balance, in the surface layer region, Vickers hardness be HV300 ~ 420 , the area ratio of the pearlite is not less than 80%, the area ratio of the eutectoid cementite 2.0 Or less, the balance being ferrite, and a spherical cementite, one or more selected from the group consisting of bainite, in a cross section perpendicular to the longitudinal direction, 0.1 times away of half of the equivalent circle diameter from the surface inner region is a region including the center surrounded by a line, has a microstructure consisting of the eutectoid cementite and the balance with the perlite, in the interior region, the area ratio of the pearlite has 90% or more the area ratio of the eutectoid cementite is not more than 5.0%, the balance is not less ferrite spherical cementite, one or more selected from the group consisting of bainite, 40 [mu] m of the pearlite blocks present in said pearlite area ratio of pearlite blocks having a circle equivalent diameter of greater than is less 0.62%, in a cross section perpendicular to the longitudinal direction , A Vickers hardness of the central portion is a region including the center surrounded by 0.5 times away line of half of the equivalent circle diameter from the center, the difference between the Vickers hardness of the surface layer region, HV20.0 less.
(2) above (1) Steel wire according to the, Mo: 0.05 ~ 0.25%, B: 1 ~ 25ppm may further contain at least one selected from the group consisting of.
(3) In the steel wire material according to the above (1) or (2), the wire diameter may be a diameter 3.5 mm ~ 5.5 mm.
Effect of the Invention
[0018]
　Bearing steel wire rod according to the embodiment of the present invention, a high drawability that can omit the annealing process before drawing, since it has a high coil-forming properties after drawing, without reducing the yield the manufacturing process of the bearing member can be largely omitted, while significantly reducing the energy and cost favorable bearing member can be stably manufactured.
　Furthermore, a bearing steel wire rod according to the embodiment of the present invention has a sufficient hardenability for surface hardening of the bearing components, it is possible to produce a bearing member having an excellent surface hardness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
It is a schematic diagram of a tissue mainly pearlite in [1] over eutectoid steel.
Is a schematic view showing the FIG. 2A] surface region.
Is a schematic view showing the FIG. 2B] interior region.
It is a schematic diagram illustrating a FIG. 2C] center.
It is a diagram showing a C section of FIG. 2D] wire.
3 is a diagram showing the relationship between the area ratio and the drawing of eutectoid cementite in the surface layer region.
Is a diagram showing the relationship between the [4] The hardness of the surface layer region and the coil-forming of drawn wire.
Shows [5] and the difference between the hardness of the hardness and the center part of the surface layer region, the relationship between the coil forming of the drawn wire.
DESCRIPTION OF THE INVENTION
[0020]
　Hereinafter, embodiments will be described drawability and bearing steel wire rod having excellent coil forming property after drawing according to the present invention. In this embodiment, since the gist of the present invention are intended to illustrate better detail in order to understand, unless otherwise specified, the present invention according to this embodiment is not limited.
[0021]
　First, it will be described steel composition of the wire according to the present embodiment. Hereinafter, the unit of the amount of chemical elements,% and ppm means mass% and mass ppm.
[0022]
　C: 0.95 ~ 1.10%
　C is essential for imparting strength necessary for bearing steel. Therefore, it is necessary that the C content is 0.95% or more. For greater strength of the bearing components manufactured from bearing steel, C content is preferably 0.98% or more, more preferably 1.00 percent. On the other hand, when the C content exceeds 1.10%, and in the cooling process after hot rolling, it becomes difficult to suppress the precipitation of pro-eutectoid cementite, wire drawability and the coil molding is impaired. Therefore, it is necessary that the C content is less than 1.10%. To obtain a wire drawability and the coil-forming properties more stably, it is preferable that C content is not more than 1.08%, and more preferably less than 1.05%.
[0023]
　Si: 0.10 ~
　0.70% Si is useful as a deoxidizer, suppresses the precipitation of pro-eutectoid cementite without reducing the carbon content. Furthermore, Si increases the ferrite strength in pearlite. Therefore, it is necessary that the Si content is 0.10% or more. In order to impart stable strength and wire drawability by bearing steel component, Si content thereof is preferably 0.12% or more and 0.15% or more, further to be 0.20 percent preferable. However, when Si is excessively contained in steel, harmful SiO the product properties of drawability and the bearing component 2 presents inclusions is likely to occur, the strength is excessively increased coil forming property is lowered . Therefore, it is necessary that the upper limit of the Si amount is 0.70%. To increase further the wire drawability and the coil moldability, it is preferable that the Si content is less than 0.50%, more preferably 0.30% or less and 0.25% or less.
[0024]
　Mn: 0.20 ~
　1.20% Mn is not only useful for deoxidation and desulfurization, are useful in order to ensure the hardenability of steel. Therefore, it is necessary that the Mn amount is 0.20% or more. To enhance the hardenability, Mn amount is preferably 0.23% or more, more preferably 0.25 percent. However, if Mn is contained excessively in steel, in addition to economic waste caused by saturation of the effect of Mn, supercooled structure such as harmful martensite drawability in the cooling process after hot rolling There is likely to occur. Therefore, it is necessary that the upper limit of the Mn amount is 1.20%. Mn amount is preferably not more than 1.00%, more preferably less than 0.80% or less and 0.50%.
[0025]
　Cr: 0.90 ~
　1.60% Cr is to promote spheroidizing after the heat treatment of the drawn material improves the hardenability, carbide amount is increased. Furthermore, Cr is very effective in suppressing the coarsening of pearlite blocks during slow cooling after rolling. However, the Cr content is less than 0.90%, sufficient effect of Cr can not be obtained, product characteristics of the bearing components is reduced. Therefore, it is necessary that the Cr content is 0.90% or more. To obtain a higher hardenability, Cr amount is preferably 1.00 percent and 1.10% or more, more preferably 1.20% or more and 1.30% or more. On the other hand, the Cr content of 1.60 percent, hardenability becomes excessive, bainite in the cooling process after hot rolling, supercooled structures, such as martensite tends to occur. Therefore, it is necessary that the upper limit of Cr amount is 1.60%. To obtain a more stable wire drawability, Cr amount, preferably less than 1.50%, more preferably at most 1.40%.
[0026]
　P: 0 ～ 0.020 Pasento
　P is an impurity. When the P content exceeds 0.020%, there is a possibility that the P is deteriorating the drawability of segregation to wire the grain boundaries. Therefore, it is preferred to limit the P content to 0.020% or less. More preferably, to limit the P content to 0.015% or less. Further, since the P content is as small desired, the lower limit of the P content may be 0%. However, reducing the P content to 0% is not technically easy. Also, if stably reduce the P content to less than 0.001%, steel cost increases. Therefore, the lower limit of the P content may be 0.001%.
[0027]
　S: 0 ～ 0.020 Pasento
　S is an impurity. When S content exceeds 0.020%, there is a possibility that the coarse MnS is formed impair the wire drawing workability of the wire. Therefore, it is preferred to limit the S content to 0.020% or less. More preferably, to limit the S content to 0.015% or less. Further, since the S content is as small desired, the lower limit of the S content may be 0%. However, reducing the S content to 0% is not technically easy. Furthermore, stably reducing the S content to less than 0.001%, steel cost increases. Therefore, the lower limit of the S content may be 0.001%.
[0028]
　Mo: 0 ~ 0.25% Mo
　is very effective for improving the hardenability, it is preferred to include a steel Mo as a chemical element of any (optional). However, the Mo content of 0.25 percent, hardenability becomes excessive, bainite in the cooling process after hot rolling, supercooled structures, such as martensite tends to occur. Therefore, it is necessary that the upper limit of the Mo content is 0.25%. In the case that contains Mo in the steel, in order to obtain a wire drawability more stably, Mo amount may be less than 0.23% or less and 0.20%. On the other hand, the lower limit of the Mo content may be 0% in order to enhance the hardenability, Mo amount may be 0.05% or more.
[0029]
　B: 0 ~ 25 ppm (0 ~ 0.0025%)
　B suppresses the generation of pseudo-pearlite or bainite by enrichment of the grain boundaries of the solid solution B. However, when the B content in the steel is excessive, tissue (austenite at high temperatures, i.e., prior austenite) Fe in 23 (CB) 6 carbides such as forms, reducing the product properties of the bearing components. Therefore, it is necessary that the upper limit of the B content is 25 ppm. B is a chemical element of any (optional), the lower limit of the B amount is may be 0 ppm (0%). Suppressing the generation of pseudo-pearlite and bainite, and more in order to obtain a stable wire drawability and coil moldability, B amount is 1 ppm (0.0001%) or more and 2 ppm (0.0002%) than, 5 ppm ( it may be 0.0005%) or more.
[0030]
　O: 0 ～ 0.0010 Pasento
　O is an impurity. When O content exceeds 0.0010% are oxide inclusions are formed, the wire drawability and product properties of the bearing parts of the wire is reduced. Therefore, to limit the O content to 0.0010% or less. Since O content as small desired, it contains 0% to the limits. However, to the O content to 0% is not technically easy. Therefore, from the viewpoint of steelmaking cost, the lower limit of the O content may be 0.0001%. Considering the normal operating conditions, O content is preferably 0.0005% - 0.0010%.
[0031]
　N: 0 ～ 0.030 Pasento
　N is an impurity. When N content exceeds 0.030%, coarse inclusions are generated, product properties of drawability and bearing components of the wire is reduced. Therefore, the N content is 0.030%. N combines with Al or B forms nitrides and fine crystal grains the nitride functions as pinning particles. Therefore, if the small amount of N content, the steel may contain N. For example, the lower limit of the N content may be 0.003%. If further increase the effect of refining crystal grains, the lower limit of the N content may be 0.005%.
[0032]
　Al: 0.010 Pasento ～ 0.100
　Pasento Al is a deoxidizing element. If Al content is less than 0.010%, deoxidation is insufficient, by the oxide precipitates, wire drawability and product properties of the bearing parts of the wire is reduced. On the other hand, Al content be greater than 0.100%, AlO inclusions are generated, product properties of drawability and bearing components of the wire is reduced. Therefore, the Al content is set to 0.010% to 0.100%. To prevent more reliably decrease in wire drawability and product properties, Al content is preferably 0.015% 0.078%. More preferably, Al content is 0.018% to 0.050%.
[0033]
　In some cases, the chemical elements other than the above are contained as impurities, but the amount of such impurities are similar to that JIS G 4805. That limits the Cu content below 0.20%, limiting the amount of elements other than the listed elements above the 0.25% or less.
[0034]
　Steel according to one embodiment of the present invention includes a C, a Si, and Mn, and Cr, the balance being Fe and impurities. Further, the steel according to the present embodiment, Mo, may include at least one chemical element selected from the group of B. Therefore, the steel according to another embodiment of the present invention, and C, and the Si, and Mn, as chemical elements and Cr, optionally (optional), Mo, comprises at least one selected from B, and the balance consisting of Fe and impurities. Steel according to this embodiment is classified into over-eutectoid steel from the amount of essential elements, the impurities, P and S, O, N, and the like Al.
[0035]
　Next, a description will be given structure of the steel wire rod according to the present embodiment.
　In the present invention, as shown in FIG. 2A, C depth from the surface 100 of the steel wire rod in the cross section of 0.1 × r (mm): area up (r the radius of the steel wire rod (half circle equivalent diameter)) (hatched the part) is referred to as "surface area" 10. Then, as shown in FIG. 2B, a surface layer region 10 other regions In the inside of the surface layer region 10 (hatched portion) is referred to as "inner region" 11. That is, when the radius of the steel wire material (half circle equivalent diameter) was defined as r (mm), surface area 10, the surface 100 of the steel wire rod distance 0.1 × r by (mm) away plane (C section in line) and a region between the surface 100 of the steel wire rod, the internal region 11 is surrounded by the surface 100 from a distance 0.1 × r of steel wire rod (mm) lines in spaced apart surface (C cross section) center of the wire (the center line) is a region including 101. Further, as shown in Figure 2C, wire center (center line) 101 from the distance 0.5 × r (mm) spaced apart surface areas including the center 101 of the wire surrounded by (C circular in cross-section) (shaded the part) is referred to as a "center" 12. The central portion 12 is contained within region 11. Incidentally, as shown in FIG. 2D, C cross section is the cross section perpendicular to the longitudinal direction of the wire (hatched portion), the center line (center) 101 extends in the longitudinal direction of the wire.
[0036]
　First, a description will be given tissue of the internal region.
[0037]
　The over-eutectoid steel, as shown in FIG. 1, eutectoid cementite 2 is precipitated along the austenite grain boundaries 1, pearlite 1a is formed in a region excluding the pro-eutectoid cementite 2. Within this pearlite structure 1a, a region called perlite block 3, i.e., the crystal orientation of the ferrite (each ferrite between lamellar cementite in the pearlite) is formed the same region. Furthermore, this is pearlite block 3, the area called the pearlite colonies 4, i.e., a region lamellar cementite are aligned parallel to each other are formed. In FIG. 1, a portion of the pearlite block 3 is omitted.
[0038]
　If the martensite is present as or when supercooled structure tissue other than pearlite is 10% or more in the inner region, the amount of elongation of the tissue at the time of drawing varies depending on the position, uneven strain in drawn wire resulting wire rod is disconnected. Therefore, a primary tissue pearlite, the area ratio of pearlite is required to be 90% or more. To enhance the wire drawing workability, the area ratio of pearlite is at least 92% preferably. The upper limit of the area ratio of pearlite, may be 100%, in order to impart high flexibility production conditions of the wire may be 99% or 98%. Here, perlite, including the pseudo-pearlite. Further, pearlite equivalent circle diameter of all pearlite blocks is 40μm or less and more preferably is 90% or more. Pro-eutectoid cementite, as long as a small amount of precipitation, not specifically inhibit the drawability. However, if a large amount of pro-eutectoid cementite is precipitated so as to surround the prior austenite grains, the deformation of the prior austenite grains is inhibited at the time of wire drawing, wire drawability is reduced. Therefore, it is necessary to limit the area ratio of the pro-eutectoid cementite in an internal region below 5.0%. To obtain a more stable wire drawability, it is preferred to limit the area ratio of the pro-eutectoid cementite 3.0% or less, more it is limited to 3.0% or less than or 2.8% preferable. Perlite, tissues other than pro-eutectoid cementite (balance) is bainite, ferrite, is at least one selected from the group of spherical cementite, it is necessary to limit the area ratio of the remainder to 10% or less. To obtain a more stable wire drawability is the area ratio of the balance, it is preferable to limit to 8.0% or less, it is preferable to limit below 5.0% less than or 3.0%.
[0039]
　Thus, in this embodiment, the precipitation of a small amount of pro-eutectoid cementite is allowed, it is desirable to eutectoid cementite unlike Patent Document 2 described above does not precipitate.
[0040]
　Diameter of pearlite blocks (particle size), there is a very strong correlation with ductility, if finer pearlite block, thereby improving drawability. In particular, the particle size of pearlite blocks is coarse, wire internal cracks generated potential is disconnected higher during wire drawing. Therefore, it is important that the particle size of the pearlite blocks can be suppressed so as not too large. Therefore, to suppress the formation of internal cracks, in order to sufficiently improve the wire drawability, to limit the maximum grain size of pearlite blocks in 40μm or less. That is, it is necessary that the area ratio of pearlite blocks having a circle equivalent diameter of greater than 40μm is not more than 0.62%. It is more preferable to limit the maximum grain size of pearlite blocks in 35μm or less. That is, it is more preferable area ratio of pearlite blocks having a circle equivalent diameter of greater than 35μm is not more than 0.48%.
[0041]
　Next, a description will be given tissue surface area.
[0042]
　When forming the drawn wire into a coil shape, bending and twisting is imparted to the extension wire. Deformation amount given by the bending and twisting because the largest in the surface region, the tissue of the surface layer region (the amount of perlite, the amount of pro-eutectoid cementite, hardness, and the difference in hardness relative to the center) is an important control of the is there. For example, if the small pearlite amount, drawn wire is broken during coil forming. Further, for example, as shown in FIG. 3, the amount of pro-eutectoid cementite are many, the pro-eutectoid cementite is present in the network form, drawn wire is broken during coil forming. Therefore, in the surface region, and the area ratio of pearlite is 80% or more, it is necessary to ensure the coil forming of limiting the area ratio of the pro-eutectoid cementite 2.0% or less. For greater coil formability, the area ratio of pearlite in the surface layer region is preferable to be 85% or more and 90% or more, more preferably 95% or 97% or more. Again, perlite, including the pseudo-pearlite. Perlite, tissues other than pro-eutectoid cementite (balance) is bainite, ferrite, is at least one selected from the group of spherical cementite, it is necessary to limit the area ratio of the balance 20% or less. To obtain a more stable coil moldability, the balance of the area ratio, to be limited to 15% or less and 10% or less and preferably, limited to 5.0% or less than or 3.0% It is preferred.
[0043]
　Further, the amount of the pearlite, the amount of pro-eutectoid cementite, in addition to the tissue and the amount of the balance, for example, the amount of Si contained in the ferrite in the pearlite, the size of the lamellar spacing and pearlite blocks perlite (particle size), the amount of pseudo-pearlite in pearlite provides cementite forms, the amount of inclusions, the amount of grain boundary segregation chemical elements (solute), the particle size of prior austenite is also an effect on the coil moldability. For example, pseudo-pearlite lamellar cementite in pearlite and granulated is, coils moldability caused uneven strain due to a difference in elongation between the surrounding tissue may be reduced. However, the amount of pearlite, the amount of pro-eutectoid cementite, to define elements other than tissue and the amount of the balance, since it is difficult to or measured, microstructure summarizing the factors affecting coil moldability the elements according to, defined as the hardness of the surface layer region. When the hardness of the surface layer region is greater than HV420, it is wire broken during coil forming. Therefore, as shown in FIG. 4, the surface from a depth 0.1 × r of the material (mm): Hardness in the surface layer region extending in the (r the radius of the steel wire rod) is required to be HV420 or less. On the other hand, when the hardness in the surface layer region is less than HV300, another it is difficult to obtain a sufficient amount of pearlite structure, the particle size of prior austenite and pearlite blocks becomes large, wire drawability is reduced. Therefore, it is necessary that the lower limit of the hardness of the surface layer region is 300 or more HV (Vickers hardness). Therefore, hardness in the range of surface area is HV300 ~ HV420.
[0044]
　Further, reducing the difference even coil forming of the tissue between the surface region and the inner region. The difference between the tissues in position, for example, under the influence of the distribution of impacts and microscopic chemical elements of the cooling control after the chemical composition and hot-rolling, the largest between the center of the surface and the wire of the wire. Therefore, the difference in the tissue between the surface region and the interior region is defined as a hardness difference between the surface region and the center. When the hardness difference between the surface region and the central portion is more than 20.0 in HV, as shown in FIG. 5, the wire breaks during coil forming. Therefore, it is necessary to limit the hardness difference between the surface region and the central portion HV20.0 below. That is, the hardness range of the difference between the surface region and the center part is a HV0 ~ HV20.0.
[0045]
　Described method of measuring tissue described above.
　The area ratio of the pro-eutectoid cementite and pearlite were determined as follows. First, cut out specimen from an arbitrary position of the wire rod, after filling the specimen in a resin, C cross section of the wire (cross section perpendicular to the center line of the wire) is a rough polishing so that the surface (cut surface) do. Then, after polishing with alumina as finish polishing, corroded with 3% nital solution and picral. Then, observing the corroded surface by scanning electron microscopy to identify the phases and tissues (SEM). Further, SEM in the surface layer region and were taken 10 area at 2000 times for each of the inner area (1 per area observation field: 0.02 mm 2 ). Extracts the region and pearlite region of eutectoid cementite by using image analysis to calculate the area ratio of the pro-eutectoid cementite and pearlite from the area of the regions.
[0046]
　The size of the pearlite blocks was measured as follows. First, cut out specimen from an arbitrary position of the wire, the specimen after filling the resin, C cross section of the wire (cross section perpendicular to the center line of the wire) is lapped such that the surface (cut surface) . Then by sequentially finish polished with alumina and colloidal silica, to remove the distortion. Thereafter, using the electron backscatter diffraction apparatus (EBSD), comprehensive observation field 200000μm internal region 2 to analyze more. Incidentally, 200000M in one field of view 2 is not necessary to measure, may be divided visual field into a plurality. Boundary difference is more than 9 ° in the crystal orientation (angle) is defined as the grain boundaries of pearlite blocks, measuring the size of pearlite blocks (particle diameter). The size of the pearlite blocks is equivalent circle diameter, the size of the largest pearlite blocks in the resulting pearlite blocks (grain) a (diameter) is defined as the maximum diameter of pearlite blocks.
[0047]
　Surface area and the hardness of the central portion of the C cross-section, because determined by local tissue within (distribution of microstructure and chemical composition, etc.), yield strength of the wire can not be estimated from the tensile strength. Therefore, the hardness and the hardness of the central portion of the surface region was measured as follows. First, after successive three rings taken from wire wound in a ring shape, collecting 24 specimens having a length of about 10mm from respectively each ring 8 equally divided points. Four specimens arbitrarily selected from these specimens embedded in resin, C cross section of the wire (the center line in the cross section perpendicular wire) cuts the resin so that the surface (cut surface). After removal of the distortion of the surface was polished with alumina, to measure the hardness of the surface layer region and the center of the polishing surface at the hardness test using a Vickers hardness tester.
[0048]
　Hardness of the surface layer region is evaluated by averaging the results obtained by measuring 0.1 × r (mm) area of ​​the three or more points within the surface of the wire. For example, to select the area of ​​the four points so that at regular intervals (90 ° intervals) with each other from the surface layer region of the C cross-section of one specimen to evaluate the hardness of the area of ​​the four points. And also performs the evaluation for the remaining three specimens, the hardness of one region of the wire per meter 16 points were measured, the hardness of the surface layer region by averaging the hardness of these 16 points region to evaluate the.
[0049]
　Hardness of the central part, in the C section identical to the C section of the evaluation of the hardness of the surface layer region, the three points or more regions within the center of the specimen (the center line) from 0.5 × r (mm) the results obtained by measuring an average is evaluated. Hardness difference between the surface region and the center part are obtained by converting the hardness values ​​calculated by subtracting the central portion from the hardness of the surface layer region to an absolute value. For example, in the C section identical to the C section of the evaluation of the hardness of the surface layer region, select an area of ​​the three points from the center (area of ​​the total of 12 points), to evaluate the hardness of those areas. Then, to evaluate the hardness of the center by averaging the hardness of these 12 points. Hardness difference between the surface region and the central portion by subtracting the hardness of the central portion from the hardness of the aforementioned surface region can be obtained.
[0050]
　Incidentally, the distance between the measuring area of ​​hardness indentation formed in this region after measuring is so as not to affect the measurement of the remaining hardness hardness of the area are using the Vickers hardness meter indentation size release more than five times. Also, when measuring the hardness of the surface layer region, the measurement region selects a load or measuring area of ​​the Vickers hardness meter away more than three times the indentation size from the surface of the wire.
[0051]
　The size of the wire according to the present embodiment is not particularly limited, considering the bearing parts production of rollers such as steel balls or roller bearings productivity and ball bearings of the wire, wire diameter of the wire diameter 3 desirably When it is .5mm ~ 5.5 mm, more desirably When it is 4.0 mm ~ 5.5 mm. Incidentally, the wire diameter of the wire is evaluated by equivalent circle diameter.
[0052]
　Next, the manufacturing method will be described. The manufacturing method described below is one example of a method of manufacturing the wire drawability and coil forming excellent in steel wire rod bearing after wire drawing. Method for producing a steel wire material for a bearing according to the present invention is not limited to the following procedures and methods, as long as the method capable of producing the steel wire material for a bearing according to the present invention, any method As a method for producing the steel wire material for a bearing it is possible to employ.
[0053]
　The material to be subjected to hot rolling (wire rod rolling), normal manufacturing conditions (e.g., casting conditions and soaking conditions) the steel slab obtained by employing the can be used. For example, soaking treatment (for reducing segregation occurring cast such possibility that some temperature range at 10 ~ 20 hr of 1100 ~ 1200 ° C. against cast pieces obtained by casting steel having the chemical composition described above subjected to a heat treatment). It is produced by blooming from the cast piece after soaking size billet suitable for wire rolling (typically wire before rolling the steel pieces called billets). Incidentally, when the above-mentioned soaking treatment should be performed on the cast piece, it is advantageous to control the stably wire tissue as described above.
[0054]
　Then, after heating the steel slab to 900 ~ 1300 ° C., rolling while controlling the rolling temperature. In this rolling, to start finish rolling from a temperature range of 700 ° C. or higher 850 ° C. or less. In this case, the temperature rise due to the rolling, the temperature for ending the finish rolling is generally reached a temperature range of 800 ~ 1000 ° C.. The temperature of the rolled wire rod is measured by a radiation thermometer, strictly means a surface temperature of the steel material. Immediately after finish rolling temperature, i.e., the average cooling rate in a temperature range from the temperature immediately after hot rolling to 700 ° C. to cool the 5 ~ 20 ℃ / s hot rolled wire to be within the scope of. Thereafter, the average cooling rate is a hot-rolled wire rod is cooled to be a 0.1 ~ 1 ℃ / s, a temperature range of pearlite transformation is in the range of 650 ° C. ~ 700 ° C. in a temperature range from 700 ° C. to 650 ° C. adjusting the cooling rate so. The switching temperature of the cooling rate is not particularly limited, as long as the average cooling rate in the temperature range is maintained, may be switched to the cooling rate at 700 ° C. vicinity continuously to 650 ° C. after hot rolling (smooth to) may change the cooling rate. Also, the winding is also performed at the time of cooling, the coiling temperature is 700 ° C. or higher.
[0055]
　To initiate finish rolling from a temperature range of 850 ° C. or less, the austenite grains finer to increase the nucleation site of pearlite during transformation, in order to miniaturize the size of pearlite blocks. When finishing starts rolling from a temperature range over 850 ° C., pearlite blocks can not be sufficiently miniaturized. Therefore, the finish rolling is started from a temperature range of 850 ° C. or less. To finer pearlite block, more preferable to start the finish rolling at 800 ° C. or less. Meanwhile, in addition to equipment load during rolling and start the finish rolling from a temperature range below 700 ° C. increases, the surface area of ​​the wire is excessively cooled, cracks and abnormal tissue is generated in the surface layer region, wire drawability Ya there is a concern that the coil-forming property is lowered. Therefore, to start the finish rolling from a temperature range of not lower than 700 ° C.. In order to more stably control the tissue surface area of ​​the wire, and more preferable to start the finish rolling at 750 ° C. or higher.
[0056]
　The average rate of cooling is 5 ° C. / s or more at 700 ° C. or higher temperature range, in addition to being able to suppress the formation of precipitation or spherical cementite eutectoid cementite, finely divided by the finish rolling austenite grains finish rolling it can be inhibited from growing by working heat generation (temperature rise) when. When the austenite grains become coarse, other pearlite blocks becomes coarse, also increases the variations in hardness. Therefore, sufficiently reducing the amount of pro-eutectoid cementite in the surface layer region, and a fine pearlite blocks, in order to obtain a uniform hardness at C section more stably, the average cooling rate in a temperature range of not lower than 700 ° C. 5 ℃ / it is necessary that s or more. On the other hand, if the average cooling rate in a temperature range of not lower than 700 ° C. is at 20 ° C. / s or more, in addition to equipment cost manufacturing cost increases to increase the hardness of the surface layer region is coil-forming property is lowered to increase . Therefore, it is necessary that the upper limit of the average cooling rate is 20 ° C. / s. In order to further reduce the hardness of the surface layer region, the average cooling rate is preferably not more than 15 ° C. / s. Incidentally, when the wire wound in a ring shape below 700 ° C., since the possibility of flaws occurring on the wire surfaces is increased, winding the wire rod at 700 ° C. or higher.
[0057]
　When the hot-rolled wire rod at an average cooling rate of 5 ~ 20 ℃ / s to 700 ° C. cooling the hot-rolled wire rod after cooling to a temperature range of 700 ° C. or less, the austenite is transformed to pearlite. Therefore, the average cooling rate in the temperature range of 700 ° C. The following are factors to control the pearlite transformation temperature. The average cooling rate is 1.0 ° C. / s greater than pearlite transformation temperature is lowered to below 650 ° C., or an increase in the hardness of the surface layer region, since the hardness difference between the surface layer region and the central portion or increased , it leads to a decrease in the coil forming property after reduction and drawing the wire drawability. Therefore, the average cooling rate in a temperature range of 650 ° C. ~ 700 ° C. is required to be less 1.0 ° C. / s. Drawing workability and to further improve the coil moldability, the average cooling rate is preferably not more than 0.8 ° C. / s. Incidentally, the control of the cooling rate was up to 650 ° C. is because the long and the average cooling rate at coiling temperature 700 ° C. or higher 1.0 ° C. / s or less, transformation to pearlite has been completed is there. On the other hand, if excessively small average cooling rate, pro-eutectoid cementite large amount of precipitated network form on the old austenite grain boundaries, wire drawability is reduced. Therefore, in order to suppress the area ratio of the eutectoid cementite internal area (deposition amount) to less than 5%, the lower limit of the average cooling rate is required to be 0.1 ° C. / s or higher. To further reduce the amount of pro-eutectoid cementite in the internal region, the average cooling rate is preferably a 0.3 ° C. / s or higher.
[0058]
　By applying the manufacturing method described above with respect to materials having a chemical composition defined in the present embodiment, without performing the spheroidizing annealing against the hot-rolled wire rod after hot rolling, the steel wire material for a bearing according to the present invention it can be produced. It may be carried out patenting heat treatment on the hot-rolled wire rod after hot rolling.
[0059]
　As described above, in the manufacturing method of the wire in this embodiment, by mass%, C: 0.95 ~ 1.10%, Si: 0.10 ~ 0.70%, Mn: 0.20 ~ 1.20 % Cr: contains 0.90 to 1.60%, optionally, Mo: 0.25% or less, B: contains 25ppm or less, casting to cast steel the balance being Fe and unavoidable impurities get a piece. Obtaining a steel strip the slab slabbing to. The steel slabs were heated to 900 ~ 1300 ° C., hot rolling starting the finish rolling from a temperature range of 700 ~ 850 ° C. performed on steel strip, to obtain a hot-rolled wire rod. The average cooling rate is 5 ~ 20 ℃ / s in the temperature range from the finish temperature of hot rolling to 700 ° C., an average cooling rate of the temperature range of 650 ~ 700 ° C. is 0.1 ~ 1 ℃ / s, as it is the winding end temperature is 700 ~ 820 ° C., performing the cooling and coiling against hot rolled wire.
Example
[0060]
　Hereinafter, examples of drawability and drawing after coil forming excellent in steel wire rod bearing of the present invention, an example of the present invention in detail. However, the present invention is not limited to the following examples, it can be practiced with appropriate modifications to the embodiments within the scope adaptable to the purposes of the present invention. Such modifications are also included in the technical scope of the present invention.
[0061]
　Table 1 and Table 2 shows the amount of the chemical component in the wire, a tissue of the wire, the wire drawing workability, and a coil forming property after drawing.
[0062]
　In this example, samples were prepared for controlled from steel pearlite structure having the chemical components shown in Table 1 by the hot rolling and subsequent cooling.
[0063]
　The basic method of the wire according to the present embodiment is as follows, in some steel wires, by changing a part or all of the conditions of the basic method. After heating the billet in a heating furnace to 1000 ~ 1200 ° C., finish rolling was performed hot rolled to be started in a temperature range of 700 ~ 800 ° C.. Thereafter, the average cooling rate in the temperature range from the temperature of the hot rolling finished to 700 ° C. is 5 ~ 20 ℃ / s, the average cooling rate in a temperature range of 650 ~ 700 ° C. is 0.1 ~ 1 ℃ / s , and the pearlite transformation temperature was controlled stepwise cooling conditions such that the 650 ~ 700 ° C.. It should be noted that the diameter of the wire was φ3.6mm ~ 5.5mm.
[0064]
　No. In the wire rod of 15-21 it was partially modified conditions of the basic method as described below. In addition, No. The wires 22, rather than the basic production method, using the following method. That is, by controlling the hot rolling conditions, the austenite grain size is 9.5, the wire diameter was obtained from the billet hot-rolled wire is 3.0 mm. Then, the lamellar spacing of the pearlite to be 0.08 .mu.m, the resulting hot-rolled wire rod is cooled at a constant rate of 9 ° C. / sec to 650 ° C., 1.0 ° C. / sec from 650 ° C. to 400 ° C. It was cooled at a constant rate.
[0065]
　First, the surface area (depth from the surface of the wire is 0.1 × r (mm) (r: radius) within the region of the steel wire rod) area ratio of the pro-eutectoid cementite in an internal region (region other than the surface layer region) and to evaluate the area ratio of pearlite, and then evaluated the maximum diameter of pearlite blocks in the interior region.
[0066]
　The resulting wire embedded in a resin, C cross section of the wire is subjected to rough polishing so that the surface. The surface was polished finish with alumina, corroded with 3% nital and picral. Thereafter, the phase and tissues was identified by observation using SEM, it was determined the area ratio of the pro-eutectoid cementite and pearlite by imaging using SEM.
[0067]
　The area ratio of the pro-eutectoid cementite and pearlite were determined as follows. Surface area and (measuring total field per area: 0.02 mm taking 10 region at a magnification 2000 times for each of the inner region 2 ) was. The resulting images were extracted and the area and pearlite region of eutectoid cementite by image analysis, the area ratio of the pro-eutectoid cementite and pearlite from the area of the regions is calculated, eutectoid cementite in the surface layer region and the inner region and to obtain the area ratio of pearlite.
[0068]
　Maximum diameter of pearlite blocks was measured using a backscatter diffraction apparatus (EBSD). The resulting wire embedded in a resin, C cross section of the wire is subjected to rough polishing so that the surface. After removing the distortion of the surface by sequentially finish polished with alumina and colloidal silica, using EBSD pearlite blocks in the polishing surface, first region 50000 2 4 region (total observation field area: 200000Myuemu 2 ) was measured . Misorientation in the observation field a boundary is 9 ° or more was measured pearlite block size is regarded as grain boundaries of the pearlite blocks. Diameter of the obtained greatest pearlite blocks in the pearlite block size (particle) was determined in maximum diameter.
[0069]
　The hardness of the surface layer region was measured as follows. Resulting from wire 3 rings taken and further taken eight specimens of (in equal intervals) 10 mm in eight equal intervals from each ring. Any specimen from the 24 specimens four were selected. Selected specimens embedded in resin, C cross section of the wire is subjected to rough polishing so that the surface. Moreover, subjected to finish polishing with alumina, after removing the distortion from the polishing surface, to select the area of ​​the four points at equal intervals (90 ° intervals) with each other from the surface layer region of the C cross-section of one specimen, the hardness was measured in the region of its four points. In addition, we also for the remaining three specimens of this measurement to measure the hardness of the area of ​​one wire per meter 16 points, the surface area of ​​the average hardness of these 16 points regions wire to obtain a hardness of. In the hardness measurement of the surface layer region, the measurement region was controlled load and measurement region of Vickers hardness meter away 3 times the indentation size from the surface of the wire.
[0070]
　Further, the hardness difference between the surface layer region and the center part was evaluated by the measurement method similar to method of measuring the hardness of the surface layer region. In the C section identical to the C section of the evaluation of the hardness of the surface layer region, select an area of ​​the three points from the center (center from 0.5 × r (mm) region within), the hardness of those areas It was measured. The hardness of 12 points obtained were calculated the hardness of the central portion, on average. To obtain a hardness difference between the surface layer region and the central portion by subtracting the hardness of the central portion from the hardness of the aforementioned surface region.
[0071]
　Next, we described evaluation test drawability. Without performing the spheroidizing annealing to the line materials, and pickled wire obtained in order to remove scale, was subjected apply a bonderized Rising and lime coating against the wire to wire in order to form a lubricating coating. Thereafter, evaluation tests were carried out in the drawability of the wire rod. In this test, the wire was 25m collected by a dry single Atamashiki drawing machine, reduction of area of 20% per pass, and drawing the wire so that the wire drawing speed becomes 50 m / min, the the drawing wire rod was repeated until the disconnection. Disconnected true strain from wire diameter of wire drawing material when the (-2 × Ln (d / d 0 ), d: diameter of the drawn wire, d 0 : diameter of steel wire rod) was calculated. The true strain was measured 5 times was defined as the five true strain average wire breakage strain (wire drawing limit strain).
[0072]
　Next, described evaluation test coil moldability. This test is 1.8 or more wire drawing limit distortion in drawing evaluation test described above was performed on the obtained wire. Taken wire of 300 kg, without performing spheroidizing annealing, and pickling the wire in order to remove scale, was subjected apply a bonderized Rising and lime coating against the wire to wire in order to form a lubricating coating. Thereafter, in dry 貯線 type continuous wire drawing machine, reduction of area is 17 to 23% per pass, the total area reduction of 70% or more, as final drawing rate is 150 ~ 300m / min the wire was drawn, continuously, and shaping the resulting drawn wire into a coil shape. At that time, inspects breakage of the wire was evaluated coil formability by breaking number of times per 300 kg. The coil diameter was 600 mm.
[0073]
[Table 1]

[0074]
[Table 2]

[0075]
　The results are shown in Table 2. And underlined item departing from the scope of the present invention. In columns of tissue in Table 2, P pearlite, theta is pro-eutectoid cementite, M means martensite. Besides tissues listed in this column, ferrite, spherical cementite, bainite was observed. In Table 2, the maximum particle size, the maximum grain size of pearlite blocks, coarse grain area ratio indicates the area ratio of pearlite blocks having a circle equivalent diameter of greater than 40μm in the microstructure. Further, the coil-forming properties, the figures in the table 2, the breaking number of times, the symbol - indicates that not evaluated test.
[0076]
　No. All 1-9 wire of an invention example, even without disconnection by adding 2.8 or more true strain against the wire, had excellent wire drawability. In addition, No. All 1 1-9 wire rod, had excellent formability that can be processed into a coil shape without breaking even when drawing at a total reduction rate of 70% or more.
[0077]
　No. All 10-14 wire of a comparative example, their chemical composition is different from the range of the chemical composition of the wire according to the present invention. No. The 10 wires of, for the amount of C is large, the surface layer region and other regions, pro-eutectoid cementite is excessively precipitated, drawability and coil forming property deteriorate. No. The 11 wires of, for the amount of Si is large, the hardness of the surface layer region becomes excessively large, the coil moldability is lowered. No. In the wire rod of 12-14, since Mn, Cr, is any amount of Mo increases, the wire is contains martensite, drawability is lowered.
[0078]
　No. All 15 also wire rod to 21 are comparative examples, has the chemical composition of the wire according to the present invention differ in the wire and the tissue according to the present invention. No. The wire rod 15 and 19, the average cooling rate from finish rolling is completed to 700 ° C. since was less than 5 ° C. / s, pro-eutectoid cementite is excessively deposited in the surface layer region, the coil forming property deteriorate . No. The 16 wires of 650 to a result of the rapid cooling of the wire rod at an average cooling rate of 1.0 ° C. / s greater in a temperature range of 700 ° C., since the transformation temperature is lowered to below 650 ° C., excess hardness of the surface layer region become large, the coil moldability is lowered in. No. The wire 17, for began finish rolling at a temperature exceeding 850 ° C., pearlite block grain diameter increases, drawability is lowered. The No. The wires 17, the area ratio of pearlite blocks having a circle equivalent diameter of more than 40μm was more than 0.62%. No. The 18 wires of, for began finish rolling at a temperature below 700 ° C., in the surface region, and cementite spheroidization pseudo pearlite and in pearlite, the less area ratio of pearlite by the generation of the spherical cementite, coil moldability decreases did. No. The wire rod 20, but was rapidly cooled the wire to 700 ° C. After completion of the finish rolling, since the average cooling rate in a temperature range of 650 ~ 700 ° C. was less than 0.1 ° C. / s, in a region other than the surface layer region, first analysis cementite excessively precipitated, since pearlite area ratio is decreased, wire drawability is reduced. No. The wire rod 21, increases until the average cooling rate in a temperature range of 650 ~ 700 ° C. (constant speed) because it was 1.0 ° C. / s greater than the hardness difference between the surface layer region and the central portion is HV20 or more, the coil moldability is lowered. No. 22 wire rod is 0% the amount of pro-eutectoid cementite, lamellar spacing had a pearlitic single-phase structure is 0.08 .mu.m. However, this No. The wires 22, the hardness of the surface layer region becomes excessively large, the coil moldability is lowered.
Industrial Applicability
[0079]
　It is possible to provide a steel wire material for a bearing having omitted the spheroidizing annealing before drawing also excellent wire drawability and the coil forming properties after drawing.
DESCRIPTION OF SYMBOLS
[0080]
　1 prior austenite grain boundary
　1a pearlite
　2 pro-eutectoid cementite
　3 pearlite blocks
　4 pearlite colonies
　10 surface area
　11 inside the area
　12 center
　100 surface of the steel wire rod
　101 centerline (center-center axis)

CLAIMS
1. A steel wire rod comprising:
C: 0.95 to 1.10 mass%,
Si: 0.10 to 0.70 mass%,
Mn: 0.20 to 1.20 mass%,
Cr: 0.90 to 1.60 mass%,
Mo: 0 to 0.25 mass%,
B: 0 to 25 ppm,
P: 0 to 0.020 mass%,
S: 0 to 0.020 mass%,
0:0 to 0.0010 mass%,
N: 0 to 0.030 mass%,
AI: 0.010 to 0.100 mass%, and
a balance: Fe and impurities,
wherein a surface area is an area between a surface and a line 0.1 times a half of
. an equivalent circle diameter of the steel wire rod apart from the smface in a
cross-section perpendicular to a longitudinal direction, and has a microstructure
20 consisting of pearlite, pro-eutectoid cementite, and a balance,
wherein in the surface area, a Vickers hardness is HV 300 to 420, an area ratio of
the pearlite is 80% or more, an area ratio of the pro-eutectoid cementite is 2.0% or less,
and the balance is one or more selected from the group consisting of ferrite, spheroidal
cementite, and bainite,
25 wherein an inner area is an area enclosed by the line 0.1 times the half of the
40
equivalent circle diameter of the steel wire rod apart from the surface and includiljg a
center in the cro·ss-section perpendicular to the longitudinal direction, and has a
. microstructure.consisting of pearlite, pro-eutectoid cementite, and a balance,
wherein in the inner area, an area ratio of the pearlite is 90% or more, an area
5 ratio of the pro-eutectoid cementite is 5.0% or less, the balance is one or more selected
from the group consisting of fen·ite, spheroidal cementite, and bainite, and an area ratio
· .. 1. of pearlite blocks existing in the pearlite and having an equivalent circle diameter of
more than 40 f.im is 0.62% or less, and
wherein a center portion is an area enclosed by a line 0.5 times the half of the
.... . .... ,. J.O .equivalent circle diam~ter.