Steel bar or wire rod

Abstract

The present invention provides a steel bar or a wire rod, which has excellent resistance to cracking during hot working and excellent machinability. A steel bar or wire rod according to the present disclosure has a chemical composition that is set forth in the description, and Fn1 defined by formula (1) is 1.2 to 5.0. In addition, the number density of sulfides having an equivalent circle diameter of 1 μm or more is less than 660 sulfides / mm2 in a longitudinal cross-section parallel to the central axis of the steel bar or wire rod and including the central axis at a depth position of 0.16R from the surface of the steel bar or wire rod. In addition, the number density of sulfides having an equivalent circle diameter of 1 μm or more is 660 sulfides / mm2 or more in the longitudinal cross-section at a depth position of 0.65R.　In the formula (1): Fn1 = Mn / S, the content (mass%) of the corresponding element is substituted for each atomic symbol.

Description

Steel bars or wire rods

[0001] The present invention relates to a steel bar or wire rod, and more particularly to a steel bar or wire rod that can be used in general machinery, automobiles, etc.

[0002] General machinery, automobiles, and the like are manufactured by assembling a variety of parts. These parts are often manufactured by cutting processes from the viewpoint of the required dimensional accuracy and manufacturing efficiency. Therefore, the steel bars or wire rods used as the raw materials for these parts are required to have excellent machinability.

[0003] A technique for improving the machinability of steel bars or wire rods used in general machinery, automobiles, etc. is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2009-120955 (Patent Document 1).

[0004] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.005 to 0.2%, Si: 0.003 to 0.009%, Mn: 0.3 to 3.0%, P: 0.001 to 0.2%, S: 0.1 to 1.0%, total-N: 0.001 to 0.02%, total-O: 0.0005 to 0.035%, Al: 0.001 to 0.1%, and the balance consisting of Fe and unavoidable impurities. Furthermore, regarding MnS sampled by the extraction replica method and observed with a transmission electron microscope, the steel material has a density of 10,000 particles / mm2 with a circle equivalent diameter of 0.1 to 0.5 μm in a cross section parallel to the rolling direction of the steel material. ２ In this steel material, the amount of MnS having a circle equivalent diameter of 0.1 to 0.5 μm is adjusted to improve the machinability of the steel material.

[0005] Japanese Patent Application Laid-Open No. 2009-120955

[0006] Meanwhile, steel bars or wire rods are subjected to hot working such as rolling or forging during the manufacturing process of the steel bars or wire rods themselves and during the manufacturing process of parts made from the steel bars or wire rods, and therefore, the steel bars or wire rods are required to have excellent hot working cracking resistance.

[0007] Furthermore, as described above, steel bars or wire rods are subjected to cutting during the manufacturing process of parts. At this time, cutting may be performed on a region (internal region) deeper than the surface layer of the steel bar or wire rod. For example, in a steel bar or wire rod having a circular cross section perpendicular to the axial direction, cutting may be performed on the internal region including the central axis in the axial direction to form a hole. In this case, excellent machinability is required in the internal region of the steel bar or wire rod.

[0008] In the above-mentioned Patent Document 1, the machinability of the surface layer region of the steel material is examined, but the machinability of the inner region of the steel material is not examined, and furthermore, the hot work cracking resistance of the steel material is not examined.

[0009] An object of the present disclosure is to provide a steel bar or wire rod that has excellent hot work cracking resistance and excellent machinability in the inner region rather than the surface layer.

[0010] The steel bar or wire rod of the present disclosure is a steel bar or wire rod having a circular cross section perpendicular to the axial direction with a radius R, and has a chemical composition, in mass %, of C: 0.005 to 0.200%, Si: 0.001 to 0.500%, Mn: 0.30 to 3.00%, P: 0.001 to 0.200%, S: 0.250 to 0.750%, B: 0.0005 to 0.01 50%, N: 0.0020-0.0200%, O: 0.0030-0.0350%, Al: 0.010% or less, V: 0-0.200%, Nb: 0-0.200 %, Ti: 0-0.200%, Cr: 0-2.000%, Mo: 0-0.300%, W: 0-0.200%, Ni: 0-0.500%, Cu: 0-0.500 %, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0050%, rare earth elements: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, Te: 0 to 0.010%, Se: 0 to 0.010%, In: 0 to 0.010%, Pb: 0 to 0.090%, and Bi: 0 to 0.250%, with the balance being Fe and impurities, and Fn1 defined by formula (1) is 1.2 to 5.0, and the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 / mm at a depth of 0.16R from the surface of the steel bar or wire rod in a vertical cross section that is parallel to the central axis of the steel bar or wire rod and includes the central axis.２ In the longitudinal section, at a depth of 0.65R from the surface of the steel bar or wire rod, the number density of the sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ That is all. Fn1=Mn / S (1) Here, the content (mass %) of each element is substituted for each element symbol in the formula.

[0011] The steel bar or wire rod of the present disclosure has excellent hot work crack resistance and excellent machinability in the inner region rather than the surface layer.

[0012] FIG. 1 is a cross-sectional view perpendicular to the axial direction of a steel bar or wire rod according to this embodiment. FIG. 2 is a longitudinal cross-sectional view parallel to and including the central axis of the steel bar or wire rod. FIG. 3 is a schematic diagram illustrating the arrangement of a rectangular field of view at a depth of 0.16R in the longitudinal cross-sectional view of FIG. 2. FIG. 4A is a schematic diagram illustrating an example of the arrangement of black pixels observed in a binarized rectangular field of view. FIG. 4B is a schematic diagram illustrating another example of the arrangement of black pixels different from that of FIG. 4A. FIG. 4C is a schematic diagram illustrating another example of the arrangement of black pixels different from that of FIGS. 4A and 4B. FIG. 4D is a schematic diagram illustrating another example of the arrangement of black pixels different from that of FIGS. 4A to 4C. FIG. 5 is a schematic diagram illustrating the arrangement of a rectangular field of view at a depth of 0.65R in the longitudinal cross-sectional view of FIG. 2. FIG. 6 is a cross-sectional view perpendicular to the longitudinal direction of a slab 30 produced by a continuous casting method.

[0013] The present inventors first investigated the chemical composition of a steel bar or wire rod that would provide excellent machinability, and as a result, the present inventors found that the composition is, by mass%, C: 0.005 to 0.200%, Si: 0.001 to 0.500%, Mn: 0.30 to 3.00%, P: 0.001 to 0.200%, S: 0.250 to 0.750%, B: 0.0005 to 0.0150%, N: 0.0020 to 0.0200%, O: 0.0030 to 0.0350%, Al: 0.010% or less, V: 0 to 0.200%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Cr: 0 to 2.000%, Mo: 0 to 0.300%, W: 0 to 0.200%, Ni: 0.005 to 0.200%, It was thought that machinability would be improved if the steel bar or wire rod had a chemical composition containing the following elements: Cu: 0-0.500%, Cu: 0-0.500%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.0050%, rare earth elements: 0-0.0050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Te: 0-0.010%, Se: 0-0.010%, In: 0-0.010%, Pb: 0-0.090%, and Bi: 0-0.250%, with the balance being Fe and impurities.

[0014] Subsequently, the present inventors investigated means for further improving the machinability of steel bars or wire rods having the above-mentioned chemical composition. As a result, the present inventors concluded that machinability could be further improved by setting Fn1, defined by formula (1), to 1.2 to 5.0. Fn1=Mn / S (1) Here, the content (mass %) of the corresponding element in the above-mentioned chemical composition is substituted for each element symbol in the formula.

[0015] The present inventors further investigated means for obtaining excellent machinability in the internal region of a steel bar or wire rod and means for improving hot work cracking resistance, and as a result, the following findings were obtained.

[0016] Sulfides with a circle equivalent diameter of 1 μm or more are defined as coarse sulfides. Coarse sulfides improve the machinability of steel bars or wire rods. However, coarse sulfides become the starting points for cracks in the surface layer of steel bars or wire rods during hot working. Therefore, if the number density of coarse sulfides is increased, the machinability of steel bars or wire rods is improved, but the hot working crack resistance of steel bars or wire rods is reduced. Therefore, it is preferable that the number density of coarse sulfides is as low as possible in the surface layer region of steel bars or wire rods.

[0017] If the number density of coarse sulfides can be adjusted according to each region (surface region and internal region) of a steel bar or wire rod, it is possible to achieve both excellent hot work crack resistance and excellent machinability in the internal region. Specifically, hot work cracks are likely to occur in the surface region of the steel bar or wire rod. On the other hand, hot work cracks are unlikely to occur in the internal region of the steel bar or wire rod. Therefore, it is sufficient to ensure high machinability in the internal region of the steel bar or wire rod.

[0018] Based on the above technical concept, investigations and studies were carried out on the relationship between the number density of coarse sulfides in the surface and inner regions of a steel bar or wire rod and the hot work cracking resistance and machinability in the inner region of the steel bar or wire rod. As a result, it was found that in a steel bar or wire rod that satisfies the above chemical composition and has an Fn1 of 1.2 to 5.0, the number density of sulfides with a circle equivalent diameter of 1 μm or more at a position 0.16R depth from the surface of the steel bar or wire rod in a longitudinal cross section that is parallel to the central axis of the steel bar or wire rod and includes the central axis is 660 / mm ２ The number density of sulfides having a circle equivalent diameter of 1 μm or more at a depth of 0.65R from the surface of the steel bar or wire rod in the longitudinal section is less than 660 pieces / mm ２ It has been found that if the above conditions are met, excellent hot work crack resistance can be obtained, and excellent machinability can be obtained in the internal region.

[0019] The steel bar or wire rod according to this embodiment, which has been completed based on the above technical concept, has the following configuration.

[0020] The first configuration of the steel bar or wire rod is a steel bar or wire rod having a circular cross section perpendicular to the axial direction with a radius R, and has a chemical composition, in mass %, of C: 0.005 to 0.200%, Si: 0.001 to 0.500%, Mn: 0.30 to 3.00%, P: 0.001 to 0.200%, S: 0.250 to 0.750%, B: 0.0005 to 0.0 150%, N: 0.0020-0.0200%, O: 0.0030-0.0350%, Al: 0.010% or less, V: 0-0.200%, Nb: 0-0.20 0%, Ti: 0-0.200%, Cr: 0-2.000%, Mo: 0-0.300%, W: 0-0.200%, Ni: 0-0.500%, Cu: 0-0.50 0%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0050%, rare earth elements: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, Te: 0 to 0.010%, Se: 0 to 0.010%, In: 0 to 0.010%, Pb: 0 to 0.090%, and Bi: 0 to 0.250%, with the balance being Fe and impurities, and Fn1 defined by formula (1) is 1.2 to 5.0, and the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 / mm at a depth of 0.16R from the surface of the steel bar or wire rod in a vertical cross section that is parallel to the central axis of the steel bar or wire rod and includes the central axis. ２ In the longitudinal section, at a depth of 0.65R from the surface of the steel bar or wire rod, the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ That is all. Fn1=Mn / S (1) Here, the content (mass %) of each element is substituted for each element symbol in the formula.

[0021] The steel bar or wire rod of the second configuration is the steel bar or wire rod of the first configuration, and has a chemical composition, in mass%, of V: ​​0.001 to 0.200%, Nb: 0.001 to 0.200%, Ti: 0.001 to 0.200%, Cr: 0.001 to 2.000%, Mo: 0.001 to 0.300%, W: 0.001 to 0.200%, Ni: 0.001 to 0.500%, Cu: 0.001 to 0.500%, Ca: 0.0001 to 0.0050%, Mg: 0. 0.001 to 0.010%, Zr: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Te: 0.001 to 0.010%, Se: 0.001 to 0.010%, In: 0.001 to 0.010%, Pb: 0.001 to 0.090%, and Bi: 0.001 to 0.250%.

[0022] The steel bar or wire rod of the third configuration is the steel bar or wire rod of the first or second configuration, and further has a chemical composition containing, in mass%, Ca: 0.0001 to 0.0050%, and Fn2 defined by formula (2) is 4.0 × 10 －５ ~4.8 x 10 －４ In the longitudinal section, at a depth of 0.65R from the surface of the steel bar or wire rod, the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ Furthermore, the ratio of the number of sulfides containing Ca to the number of sulfides having an equivalent circle diameter of 1 μm or more is 0.50% or more. Fn2=Ca×S (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula.

[0023] The steel bar or wire rod of this embodiment will be described in detail below. "%" regarding elements means mass % unless otherwise specified.

[0024] [Features of Steel Bar or Wire Rod of the Present Embodiment] In the present embodiment, the steel bar or wire rod is defined as follows. As described in JIS G 0203 (2009) No. 2501, the steel bar is a steel material that is hot-rolled or forged into a rod shape and cut to a predetermined length, and the cross section perpendicular to the axial direction is, for example, circular. The axial direction corresponds to the longitudinal direction of the steel bar or wire rod. Furthermore, as described in JIS G 0203 (2009) No. 2512, the wire rod is a steel material that is rolled into a rod shape, has, for example, a circular cross section, and is wound into a coil, and is also called a wire rod.

[0025] The steel bar or wire rod of this embodiment is a steel bar or wire rod having a circular cross section perpendicular to the axial direction with a radius R, and satisfies the following Features 1 to 4. (Feature 1) The chemical composition is, in mass %, C: 0.005 to 0.200%, Si: 0.001 to 0.500%, Mn: 0.30 to 3.00%, P: 0.001 to 0.200%, S: 0.250 to 0.750%, B: 0.0005 to 0.0150%, N: 0.0020 to 0.0200%, O: 0.0030 to 0.0350%, Al: 0.010% or less, V: 0 to 0.200%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Cr: 0 to 2.000%, and Mo: 0 to 0.300%. %, W: 0-0.200%, Ni: 0-0.500%, Cu: 0-0.500%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.0050%, rare earth elements: 0-0.0050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Te: 0-0.010%, Se: 0-0.010%, In: 0-0.010%, Pb: 0-0.090%, Bi: 0-0.250%, and the balance consisting of Fe and impurities. (Feature 2) Fn1 defined in formula (1) is 1.2-5.0. Fn1=Mn / S (1) where the content (mass%) of the corresponding element is substituted for each element symbol in the formula. (Feature 3) In a longitudinal section parallel to the central axis of the steel bar or wire rod and including the central axis, the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２(Feature 4) In the longitudinal section, at a depth of 0.65R from the surface of the steel bar or wire rod, the number density of sulfides having a circle equivalent diameter of 1 μm or more is less than 660 pieces / mm ２ That's all.

[0026] Here, in this specification, "steel bar or wire rod" refers to a steel bar having a circular cross section perpendicular to the axial direction with a radius R. Both steel bars and wire rods have a circular cross section. Steel bars with a large radius R are classified as steel bars, and steel bars with a small radius R are classified as wire rods. Steel bars, for example, extend linearly, while wire rods are wound in a coil. Although steel bars and wire rods differ in the size of the radius R and the shape, such as linear or coiled, they have in common the fact that they are steel bars having a circular cross section perpendicular to the axial direction with a radius R. More specifically, as described in JIS G 0203 (2009) No. 2501, steel bars are steel bars that have been hot-rolled or forged into a rod shape and cut to a predetermined length, and the cross section perpendicular to the axial direction is circular. The axial direction corresponds to the longitudinal direction of the steel bar or wire rod. Further, as described in JIS G 0203 (2009) No. 2512, the wire rod is a steel material rolled into a rod shape, has a cross section that is, for example, circular, and is wound into a coil, and is also called a wire rod.

[0027] Features 1 to 4 will be explained below.

[0028] [(Feature 1) Chemical Composition] The chemical composition of the steel bar or wire rod of this embodiment contains the following elements.

[0029] C: 0.005 to 0.200% Carbon (C) increases the strength of steel bars or wire rods. When the C content is 0.005% or more, the above effect is sufficiently achieved. On the other hand, when the C content is 0.200% or less, the strength of the steel bars or wire rods is excessively high, which can prevent a decrease in machinability. Therefore, the C content is 0.005 to 0.200%. The preferred lower limit of the C content is 0.010%, more preferably 0.030%, even more preferably 0.050%, even more preferably 0.060%, and even more preferably 0.070%. The preferred upper limit of the C content is 0.180%, even more preferably 0.150%, even more preferably 0.130%, even more preferably 0.100%, even more preferably 0.090%, and even more preferably 0.080%.

[0030] Si: 0.001 to 0.500% Silicon (Si) deoxidizes steel during the steelmaking process. When the Si content is 0.001% or more, the above effect is sufficiently achieved. On the other hand, when the Si content is 0.500% or less, excessive increase in strength of the steel bar or wire rod can be suppressed, and as a result, the machinability of the steel bar or wire rod can be improved. Therefore, the Si content is 0.001 to 0.500%. The preferred lower limit of the Si content is 0.005%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.030%. The preferred upper limit of the Si content is 0.450%, even more preferably 0.400%, even more preferably 0.300%, even more preferably 0.250%, and even more preferably 0.200%.

[0031] Mn: 0.30 to 3.00% Manganese (Mn) combines with S to form sulfides, improving the machinability of steel bars or wire rods. When the Mn content is 0.30% or more, the above effect is sufficiently achieved. On the other hand, when the Mn content is 3.00% or less, excessive increase in the strength of the steel bars or wire rods can be prevented, resulting in improved machinability of the steel bars or wire rods. Therefore, the Mn content is 0.30 to 3.00%. The preferred lower limit of the Mn content is 0.50%, more preferably 0.70%, even more preferably 0.90%, even more preferably 0.92%, even more preferably 0.95%, even more preferably 1.03%, and even more preferably 1.15%. The upper limit of the Mn content is preferably 2.80%, more preferably 2.50%, even more preferably 2.30%, even more preferably 2.00%, even more preferably 1.50%, even more preferably 1.48%, even more preferably 1.43%, even more preferably 1.40%, even more preferably 1.35%, even more preferably 1.30%, even more preferably 1.25%, and even more preferably 1.20%.

[0032] P: 0.001 to 0.200% Phosphorus (P) embrittles steel bars or wire rods, improving machinability. If the P content is 0.001% or more, the above effect is sufficiently obtained. On the other hand, if the P content is 0.200% or less, grain boundary segregation of P can be sufficiently suppressed, improving hot work cracking resistance. Therefore, the P content is 0.001 to 0.200%. The preferred lower limit of the P content is 0.010%, more preferably 0.020%, even more preferably 0.030%, even more preferably 0.050%, even more preferably 0.052%, even more preferably 0.055%, and even more preferably 0.060%. The upper limit of the P content is preferably 0.180%, more preferably 0.150%, even more preferably 0.130%, even more preferably 0.100%, even more preferably 0.098%, even more preferably 0.095%, even more preferably 0.090%, and even more preferably 0.085%.

[0033] S: 0.250 to 0.750% Sulfur (S) forms sulfides and improves the machinability of steel bars or wire rods. If the S content is 0.250% or more, the above effect is sufficiently obtained. On the other hand, if the S content is 0.750% or less, cracks can be suppressed from occurring on the surface of the steel bars or wire rods during hot working, improving hot working crack resistance. Therefore, the S content is 0.250 to 0.750%. The preferred lower limit of the S content is 0.270%, more preferably 0.280%, even more preferably 0.282%, even more preferably 0.285%, even more preferably 0.295%, and even more preferably 0.300%. The upper limit of the S content is preferably 0.700%, more preferably 0.650%, even more preferably 0.600%, even more preferably 0.550%, even more preferably 0.500%, even more preferably 0.480%, even more preferably 0.478%, even more preferably 0.475%, even more preferably 0.460%, even more preferably 0.440%, even more preferably 0.410%, even more preferably 0.380%, and even more preferably 0.360%.

[0034] B: 0.0005 to 0.0150% Boron (B) improves the machinability of steel bars or wire rods. If the B content is 0.0005% or more, the above effect is sufficiently obtained. On the other hand, if the B content is 0.0150% or less, cracks on the surface of the steel bars or wire rods during hot working can be suppressed, and hot working crack resistance is improved. Therefore, the B content is 0.0005 to 0.0150%. The preferred lower limit of the B content is 0.0010%, more preferably 0.0020%, even more preferably 0.0030%, even more preferably 0.0040%, even more preferably 0.0050%, even more preferably 0.0055%, even more preferably 0.0065%, and even more preferably 0.0100%. The upper limit of the B content is preferably 0.0145%, more preferably 0.0140%, even more preferably 0.0130%, even more preferably 0.0120%, and still more preferably 0.0110%.

[0035] N: 0.0020 to 0.0200% Nitrogen (N) combines with B to form BN, improving the machinability of steel bars or wire rods. If the N content is 0.0020% or more, the above effect is sufficiently obtained. On the other hand, if the N content is 0.0200% or less, sufficient hot work cracking resistance is obtained in the steel bars or wire rods. Therefore, the N content is 0.0020 to 0.0200%. The preferred lower limit of the N content is 0.0030%, more preferably 0.0035%, even more preferably 0.0040%, even more preferably 0.0044%, and even more preferably 0.0046%. The upper limit of the N content is preferably 0.0180%, more preferably 0.0160%, even more preferably 0.0150%, even more preferably 0.0140%, even more preferably 0.0100%, even more preferably 0.0090%, even more preferably 0.0070%, and even more preferably 0.0050%.

[0036] O: 0.0030 to 0.0350% Oxygen (O) generates oxides, which act as nuclei for the formation of MnS, finely dispersing the MnS. As a result, coarse sulfides are reduced in the surface region of the steel bar or wire rod. If the O content is 0.0030% or more, the above effect can be sufficiently achieved. On the other hand, O tends to form bubbles and pinholes during casting, but this does not pose a problem if the O content is 0.0350% or less. Therefore, the O content is 0.0030 to 0.0350%. The preferred lower limit of the O content is 0.0040%, more preferably 0.0050%, even more preferably 0.0070%, even more preferably 0.0090%, even more preferably 0.0100%, even more preferably 0.0121%, even more preferably 0.0123%, even more preferably 0.0130%, and even more preferably 0.0150%. The upper limit of the O content is preferably 0.0345%, more preferably 0.0340%, even more preferably 0.0330%, even more preferably 0.0300%, even more preferably 0.0250%, and even more preferably 0.0200%.

[0037] Al: 0.010% or less Aluminum (Al) deoxidizes steel during the steelmaking process. On the other hand, Al forms coarse oxides. If the Al content is 0.010% or less, the generation of coarse oxides is suppressed, and sufficient machinability is obtained in the steel bar or wire rod. Therefore, the Al content is 0.010% or less. The preferred lower limit of the Al content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Al content is 0.009%, even more preferably 0.008%, even more preferably 0.006%, and even more preferably 0.005%.

[0038] The balance of the chemical composition of the steel bar or wire rod of this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the steel bar or wire rod, and are acceptable within a range that does not adversely affect the steel bar or wire rod of this embodiment.

[0039] [Regarding optional elements] The chemical composition of the steel bar or wire rod of this embodiment may further include, in place of a portion of Fe, V: 0 to 0.200%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Cr: 0 to 2.000%, Mo: 0 to 0.300%, W: 0 to 0.200%, Ni: 0 to 0.500%, Cu: 0 to 0.500%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, It may contain one or more elements selected from the group consisting of Zr: 0-0.0050%, rare earth elements: 0-0.0050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Te: 0-0.010%, Se: 0-0.010%, In: 0-0.010%, Pb: 0-0.090%, and Bi: 0-0.250%. All of these elements are optional elements. These optional elements will be described below.

[0040] [First Group: V, Nb, Ti, Cr, Mo, W, Ni, and Cu] The chemical composition of the steel bar or wire rod of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of V, Nb, Ti, Cr, Mo, W, Ni, and Cu. These elements are optional elements, and all of them increase the strength of the steel bar or wire rod. Each element will be described below.

[0041] V: 0 to 0.200% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When vanadium is contained, that is, when the V content exceeds 0%, V forms precipitates to increase the strength of the steel bar or wire rod. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content is 0.200% or less, excessive increase in the strength of the steel bar or wire rod can be suppressed. Therefore, sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the V content is 0 to 0.200%. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the V content is preferably 0.180%, more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0042] Nb: 0 to 0.200% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms precipitates to increase the strength of the steel bar or wire rod. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is 0.200% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the Nb content is 0 to 0.200%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.190%, more preferably 0.180%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0043] Ti: 0 to 0.200% Titanium (Ti) is an optional element and does not necessarily need to be included. In other words, the Ti content may be 0%. When included, that is, when the Ti content exceeds 0%, Ti forms precipitates to increase the strength of the steel bar or wire rod. Even if even a small amount of Ti is included, the above effect can be achieved to some extent. However, if the Ti content is 0.200% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be achieved in the steel bar or wire rod. Therefore, the Ti content is 0 to 0.200%, and when included, the Ti content is 0.200% or less. The preferred lower limit of the Ti content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Ti content is preferably 0.180%, more preferably 0.175%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0044] Cr: 0 to 2.000% Chromium (Cr) is an optional element and does not necessarily need to be contained. In other words, the Cr content may be 0%. When Cr is contained, that is, when the Cr content exceeds 0%, Cr improves the hardenability of the steel bar or wire rod, thereby increasing the strength of the steel bar or wire rod. Even if even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content is 2.000% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the Cr content is 0 to 2.000%. The preferred lower limit of the Cr content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Cr content is preferably 1.700%, more preferably 1.500%, even more preferably 1.000%, even more preferably 0.200%, even more preferably 0.180%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0045] Mo: 0 to 0.300% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. In other words, the Mo content may be 0%. When contained, that is, when the Mo content is greater than 0%, Mo increases the strength of the steel bar or wire rod. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content is 0.300% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the Mo content is 0 to 0.300%. The preferred lower limit of the Mo content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Mo content is preferably 0.280%, more preferably 0.250%, even more preferably 0.230%, even more preferably 0.200%, even more preferably 0.180%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0046] W: 0 to 0.200% Tungsten (W) is an optional element and does not necessarily need to be included. In other words, the W content may be 0%. When included, that is, when the W content exceeds 0%, W forms precipitates to increase the strength of the steel bar or wire rod. Even if even a small amount of W is included, the above effect can be achieved to some extent. However, if the W content is 0.200% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be achieved in the steel bar or wire rod. Therefore, the W content is 0 to 0.200%. The preferred lower limit of the W content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the W content is preferably 0.180%, more preferably 0.170%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0047] Ni: 0 to 0.500% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, that is, when the Ni content is greater than 0%, Ni increases the strength of the steel bar or wire rod. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content is 0.500% or less, excessive increase in the strength of the steel bar or wire rod can be prevented. Therefore, sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the Ni content is 0 to 0.500%. The preferred lower limit of the Ni content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Ni content is preferably 0.490%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, even more preferably 0.250%, even more preferably 0.200%, even more preferably 0.180%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0048] Cu: 0 to 0.500% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, that is, when the Cu content is greater than 0%, Cu increases the strength of the steel bar or wire rod. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content is 0.500% or less, excessive increase in the strength of the steel bar or wire rod can be prevented, and sufficient machinability can be obtained in the steel bar or wire rod. Therefore, the Cu content is 0 to 0.500%. The preferred lower limit of the Cu content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the Cu content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, even more preferably 0.300%, even more preferably 0.250%, even more preferably 0.200%, even more preferably 0.180%, even more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably less than 0.050%, and even more preferably 0.049%.

[0049] [Regarding Group 2: Ca, Mg, Zr, rare earth elements, Sn, Sb, As, Te, Se, In, Pb, and Bi] The chemical composition of the steel bar or wire rod of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, rare earth elements, Sn, Sb, As, Te, Se, In, Pb, and Bi. All of these elements are optional elements, and all of them improve the machinability of the steel bar or wire rod. Each element will be described below.

[0050] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca dissolves in sulfides to modify the morphology of the sulfides, improving the machinability of the steel bar or wire rod. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content is 0.0050% or less, the generation of coarse oxides is suppressed. As a result, sufficient machinability is obtained in the steel bar or wire rod. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0007%. The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, even more preferably 0.0012%, even more preferably 0.0010%, and even more preferably 0.0008%.

[0051] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg dissolves in sulfides, modifying the morphology of the sulfides and improving the machinability of the steel bar or wire rod. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is 0.0050% or less, the generation of coarse oxides is suppressed. As a result, sufficient machinability is obtained in the steel bar or wire rod. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0007%. The upper limit of the Mg content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, even more preferably 0.0012%, even more preferably 0.0010%, and even more preferably 0.0008%.

[0052] Zr: 0 to 0.0050% Zirconium (Zr) is an optional element and does not necessarily need to be contained. In other words, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr dissolves in sulfides to modify the sulfide morphology and improve the machinability of the steel bar or wire rod. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is 0.0050% or less, the formation of coarse oxides is suppressed. As a result, sufficient machinability is obtained in the steel bar or wire rod. Therefore, the Zr content is 0 to 0.0050%. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0007%. The upper limit of the Zr content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, even more preferably 0.0012%, even more preferably 0.0010%, and even more preferably 0.0008%.

[0053] Rare Earth Elements (REM): 0 to 0.0050% Rare earth elements (REM) are optional elements and do not necessarily need to be present. In other words, the REM content may be 0%. When present, i.e., when the REM content is greater than 0%, REM dissolves in sulfides, modifying the sulfide morphology and improving the machinability of the steel bar or wire rod. Even if even a small amount of REM is present, the above effect can be achieved to some extent. However, if the REM content is 0.0050% or less, the formation of coarse oxides is suppressed. As a result, sufficient machinability is achieved in the steel bar or wire rod. Therefore, the REM content is 0 to 0.0050%, and when present, the REM content is 0.0050% or less. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0007%. The upper limit of the REM content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, even more preferably 0.0012%, even more preferably 0.0010%, and even more preferably 0.0008%.

[0054] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In this specification, the REM content refers to the total content of these elements.

[0055] Sn: 0 to 0.100% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. If it is contained, that is, if the Sn content exceeds 0%, Sn segregates at the interface between the matrix and inclusions, embrittling the steel bar or wire rod. This embrittlement action improves the machinability of the steel bar or wire rod. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is 0.100% or less, excessive segregation of Sn is sufficiently suppressed. Therefore, sufficient hot work cracking resistance can be obtained in the steel bar or wire rod. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sn content is preferably 0.098%, more preferably 0.095%, even more preferably 0.090%, even more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, even more preferably 0.050%, and even more preferably 0.040%.

[0056] Sb: 0 to 0.050% Antimony (Sb) is an optional element and does not necessarily need to be contained. In other words, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb segregates at the interface between the matrix and inclusions, embrittling the steel bar or wire rod. This embrittlement action improves the machinability of the steel bar or wire rod. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content is 0.050% or less, excessive segregation of Sn is sufficiently suppressed. Therefore, sufficient hot work cracking resistance can be obtained in the steel bar or wire rod. Therefore, the Sb content is 0 to 0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%.

[0057] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be present. In other words, the As content may be 0%. When present, i.e., when the As content exceeds 0%, As segregates at the interface between the matrix and inclusions, embrittling the steel bar or wire rod. This embrittlement improves the machinability of the steel bar or wire rod. Even if even a small amount of As is present, the above effect can be achieved to some extent. However, if the As content is 0.050% or less, excessive segregation of As is sufficiently suppressed. Therefore, sufficient hot work cracking resistance is obtained in the steel bar or wire rod. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%.

[0058] Te: 0 to 0.010% Tellurium (Te) is an optional element and does not necessarily need to be contained. In other words, the Te content may be 0%. When contained, that is, when the Te content exceeds 0%, Te improves the machinability of the steel bar or wire rod. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content is 0.010% or less, excessive deterioration in the hot work cracking resistance of the steel bar or wire rod is suppressed. Therefore, the Te content is 0 to 0.010%. The preferred lower limit of the Te content is 0.001%, more preferably 0.002%. The preferred upper limit of the Te content is 0.009%, more preferably 0.008%, and even more preferably 0.004%.

[0059] Se: 0 to 0.010% Selenium (Se) is an optional element and does not necessarily need to be contained. That is, the Se content may be 0%. When contained, that is, when the Se content exceeds 0%, Se improves the machinability of the steel bar or wire rod. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, when the Se content is 0.010% or less, excessive deterioration in the hot work cracking resistance of the steel bar or wire rod is suppressed. Therefore, the Se content is 0 to 0.010%, and when contained, the Se content is 0.010% or less. The preferred lower limit of the Se content is 0.001%, more preferably 0.002%. The preferred upper limit of the Se content is 0.009%, more preferably 0.008%, and even more preferably 0.004%.

[0060] In: 0 to 0.010% Indium (In) is an optional element and does not necessarily need to be contained. That is, the In content may be 0%. When contained, that is, when the In content exceeds 0%, In improves the machinability of the steel bar or wire rod. Even if even a small amount of In is contained, the above effect can be obtained to some extent. However, if the In content is 0.010% or less, excessive deterioration in the hot work cracking resistance of the steel bar or wire rod is suppressed. Therefore, the In content is 0 to 0.010%. The preferred lower limit of the In content is 0.001%, and more preferably 0.002%. The preferred upper limit of the In content is 0.009%, more preferably 0.008%, and even more preferably 0.004%.

[0061] Pb: 0 to 0.090% Lead (Pb) is an optional element and does not necessarily need to be contained. In other words, the Pb content may be 0%. If it is contained, that is, if the Pb content exceeds 0%, Pb generates Pb particles in the matrix, embrittling the steel bar or wire rod. This embrittlement action improves the machinability of the steel bar or wire rod. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content is 0.090% or less, excessive generation of Pb particles can be suppressed. Therefore, sufficient hot work cracking resistance can be obtained in the steel bar or wire rod. Therefore, the Pb content is 0 to 0.090%. The preferred lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Pb content is preferably 0.080%, more preferably 0.050%, even more preferably 0.040%, even more preferably 0.030%, even more preferably 0.010%, even more preferably 0.009%, and even more preferably 0.005%.

[0062] Bi: 0 to 0.250% Bismuth (Bi) is an optional element and does not necessarily need to be contained. In other words, the Bi content may be 0%. When contained, that is, when the Bi content exceeds 0%, Bi generates Bi particles in the matrix, embrittling the steel bar or wire rod. This embrittlement action improves the machinability of the steel bar or wire rod. Even if even a small amount of Bi is contained, the above effect can be achieved to some extent. However, if the Bi content is 0.250% or less, the excessive generation of coarse Bi particles can be suppressed. Therefore, cracking during hot working can be suppressed, and sufficient hot working crack resistance can be obtained in the steel bar or wire rod. Therefore, the Bi content is 0 to 0.250%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.003%, even more preferably 0.008%, even more preferably 0.010%, and even more preferably 0.012%. The upper limit of the Bi content is preferably 0.240%, more preferably 0.220%, even more preferably 0.200%, even more preferably 0.150%, even more preferably 0.100%, even more preferably 0.090%, even more preferably 0.050%, even more preferably 0.030%, and even more preferably 0.015%.

[0063] [(Feature 2) Regarding Fn1] In the steel bar or wire rod of this embodiment, Fn1 defined by formula (1) is further 1.2 to 5.0. Fn1=Mn / S (1) Here, each element symbol in the formula is substituted with the content (mass%) of the corresponding element in the chemical composition of the steel bar or wire rod. Fn1 will be explained below.

[0064] [Regarding Fn1] Fn1 is an index for adjusting the number density of coarse sulfides to an appropriate amount. If Fn1 is 1.2 or more, excessive generation of FeS can be suppressed. As a result, hot work cracking resistance can be ensured. On the other hand, if Fn1 is 5.0 or less, excessive crystallization of coarse sulfides during solidification of the steel bar or wire rod can be suppressed. Therefore, the number density of coarse sulfides in the surface layer region can be suppressed to an appropriate amount. Therefore, Fn1 is 1.2 to 5.0.

[0065] The lower limit of Fn1 is preferably 1.5, more preferably 1.7, even more preferably 2.0, even more preferably 2.2, and even more preferably 2.3. The upper limit of Fn1 is preferably 4.5, more preferably 4.0, even more preferably 3.5, even more preferably 3.0, and even more preferably 2.7.

[0066] (Feature 3) 0.16R depth position D ０．１６Ｒ 1 is a cross-sectional view perpendicular to the axial direction of the steel bar or wire rod of this embodiment. As shown in FIG. 1, the cross-section 10 perpendicular to the axial direction of the steel bar or wire rod is circular. Here, the axial direction of the steel bar or wire rod means the longitudinal direction of the steel bar or wire rod. The radius of the cross-section 10 is defined as R (mm). The surface D of the steel bar or wire rod is ０．００Ｒ Starting from this, the position of 0.16R depth in the radial direction is set as "0.16R depth position D ０．１６Ｒ " is defined as

[0067] In the steel bar or wire rod of this embodiment, a 0.16R depth position D in a longitudinal section that is parallel to the central axis of the steel bar or wire rod and includes the central axis ０．１６Ｒ In this case, the number density of sulfides (coarse sulfides) having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ Here, the sulfide refers to particles having an S content of 10% by mass or more and an Mn content of 10% by mass or more, as a result of performing point analysis of all detectable elements using an energy dispersive X-ray analyzer (EDX: Energy Dispersive X-ray spectroscopy) and quantifying all elements in the obtained X-ray profile excluding Fe.

[0068] As shown in Figure 1, the surface D ０．００Ｒ From 0.16R depth position D ０．１６Ｒ The area ranging from 100 to 1500 is defined as the surface layer area SA. The surface layer area SA of the steel bar or wire rod is an area close to the surface that comes into contact with a processing tool such as a roll or a mold in a hot working step in the manufacturing process of the steel bar or wire rod, or in a hot working step in the manufacturing process of a part made from the steel bar or wire rod. Therefore, the surface layer area SA is required to have sufficient hot working cracking resistance.

[0069] During hot working, the surface layer region SA is subjected to a large external force from the hot working tool. Therefore, the coarse sulfides in the surface layer region SA become the starting points of hot working cracks. Therefore, it is preferable that the number density of the coarse sulfides in the surface layer region SA is as low as possible. 0.16R depth position D in the surface layer region SA ０．１６Ｒ In this case, the number density of coarse sulfides is 660 pieces / mm ２ If the number density of the coarse sulfides in the surface layer region SA is sufficiently small, the steel bar or wire rod can have sufficient hot work cracking resistance.

[0070] 0.16R depth position D ０．１６Ｒ The preferred upper limit of the number density of coarse sulfides is 650 pieces / mm ２ and more preferably 630 pieces / mm ２ and more preferably 610 pieces / mm ２ and more preferably 600 pieces / mm ２ and more preferably 580 pieces / mm ２ and more preferably 550 pieces / mm ２ and more preferably 530 pieces / mm ２ and more preferably 500 pieces / mm ２ and more preferably 480 pieces / mm ２ and more preferably 460 pieces / mm ２ and more preferably 430 pieces / mm ２ and more preferably 400 pieces / mm ２ 0.16R depth position D ０．１６Ｒ When the steel bar or wire rod satisfies Features 1, 2, and 4, the lower limit of the number density of coarse sulfides at the depth position D of 0.16R is not particularly limited. ０．１６Ｒ The lower limit of the number density of coarse sulfides is 0 pieces / mm ２ However, when industrial production is taken into consideration, the depth position D ０．１６Ｒ The preferred lower limit of the number density of coarse sulfides is 100 pieces / mm ２ and more preferably 200 pieces / mm ２ and more preferably 250 pieces / mm２ and more preferably 300 pieces / mm ２ is.

[0071] [0.16R depth position D ０．１６Ｒ Regarding the method for measuring the number density of coarse sulfides at a depth position D of 0.16R ０．１６Ｒ The number density of coarse sulfides at a depth of 0.16R can be measured by the following method. Fig. 2 is a longitudinal cross-sectional view parallel to and including the central axis of a steel bar or wire rod. The L direction in Fig. 2 means the axial direction (longitudinal direction) of the steel bar or wire rod. The R direction means the radial direction of the steel bar or wire rod. Referring to Fig. 2, in a longitudinal cross-section 20 parallel to the central axis C1 of the steel bar or wire rod and including the central axis C1, ０．１６Ｒ Of the test specimens thus obtained, a longitudinal cross section that is parallel to the central axis of the steel bar or wire rod and includes said central axis is used as the observation surface.

[0072] The observation surface is mirror-polished. The mirror-polished observation surface is observed at 500x magnification using a scanning electron microscope (SEM) with a tungsten filament. The acceleration voltage for SEM observation is 20 kV. Specifically, as shown in FIG. 3, rectangular fields RF1 to RF52, each 405 μm x 405 μm square, are arranged continuously in the axial direction L. In other words, 52 rectangular fields RF1 to RF52 are arranged at a pitch of 405 μm. At this time, a 0.16R depth position D is positioned at the center of each rectangular field RFn (n is a natural number from 1 to 52). ０．１６Ｒ The rectangular fields RFn are arranged so that each field is located at a position corresponding to the center of the image. Each rectangular field RFn is observed to obtain a backscattered electron image. The total area of ​​the observation fields (total area of ​​the rectangular fields RF1 to RF52) is 8.53 mm ２ is.

[0073] Based on the backscattered electron image of each rectangular field RFn obtained by SEM observation, the number density of coarse sulfides is examined using a well-known particle analysis method of image analysis. Specifically, the rectangular field RFn is divided into square pixels with sides of 0.1875 μm. In the backscattered electron image, the color tone of each pixel is set to 256 gradations. When acquiring the backscattered electron image, the contrast is set so that the steel structure (bright contrast) of the base steel (parent phase) and the inclusions (dark contrast) are sufficiently distinguished. The rectangular field RFn of the acquired backscattered electron image is binarized. Specifically, a histogram of pixel brightness values ​​is created for each rectangular field RFn. The histogram shows peaks in high-brightness areas (bright contrast) and low-brightness areas (dark contrast). The peak gradation PH at bright contrast and the peak gradation PL at dark contrast are identified. Using the identified peak gradations PH and PL, the intermediate gradation PM is calculated using the following equation: PM=(PH+PL) / 2

[0074] The obtained intermediate gradation PM is used as a threshold value. Based on the threshold value, the rectangular field of view RFn is binarized. At this time, pixels with a brightness higher than the threshold value are set to white (255 gradations), and pixels with a brightness lower than the threshold value are set to black (0 gradations).

[0075] In the binarized rectangular field of view RFn, black pixels indicate regions of inclusions or precipitates. As shown in Figures 4A to 4C, a region in which five or more consecutive black pixels PX are connected is considered to be a coarse particle with an equivalent circle diameter of 1 μm or more. Here, "connected" means that adjacent pixels PX are in line contact or point contact. Note that, as in Figure 4D, if only four or fewer consecutive black pixels PX are connected, the region is not considered to be a coarse particle with an equivalent circle diameter of 1 μm or more.

[0076] For the regions identified as coarse particles in each rectangular field of view RFn, all detectable elements are analyzed by point analysis using EDX equipped in the SEM. The EDX source is an electron beam source with a tungsten filament, and the acceleration voltage is 20 kV. This results in an EDX spatial resolution of approximately 2 μm. EDX obtains an X-ray profile of the coarse particles. After quantifying all elements obtained from the X-ray profile excluding Fe, particles with an S content of 10% by mass or more and an Mn content of 10% by mass or more are recognized as coarse sulfides.

[0077] The spatial resolution of EDX is larger than the spot size of the electron beam due to the spread of the electron beam within the analysis target. Therefore, to identify inclusions and the base steel (parent phase), EDX analysis is performed using a backscattered electron image with high spatial resolution. In EDX analysis, the spread of the electron beam mentioned above causes the characteristic X-rays of the base steel (parent phase) to be mixed in. Therefore, when analyzing the composition of inclusions, Fe is excluded from the X-ray profile and quantified, as mentioned above.

[0078] The total number of coarse sulfides identified in each rectangular field RFn and the total area of ​​the multiple fields (8.53 mm ２ ) and based on the 0.16R depth position D ０．１６Ｒ The number density of coarse sulfides (pieces / mm ２ ) is required.

[0079] (Feature 4) 0.65R depth position D ０．６５Ｒ Number density of coarse sulfides on the surface D of the steel bar or wire rod] ０．００Ｒ Starting from this, the position of 0.65R depth in the radial direction is set as "0.65R depth position D ０．６５Ｒ " Surface D of steel material ０．００Ｒ Starting from this, the position of 1.00R depth in the radial direction is designated as "1.00R depth position D １．００Ｒ " is defined as

[0080] In the steel bar or wire rod of this embodiment, the depth position D ０．６５Ｒ In this case, the number density of sulfides (coarse sulfides) having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ That's all.

[0081] As shown in FIG. 1, the 0.65R depth position D０．６５Ｒ From 1.00R depth position D １．００Ｒ The region ranging from the surface layer region SA to the inner region CA is defined as the inner region CA. The inner region CA of the steel bar or wire rod is less susceptible to external forces during hot working than the surface layer region SA. Therefore, the inner region CA does not require the same excellent hot work crack resistance as the surface layer region SA. On the other hand, the inner region CA may be subjected to cutting processing in the manufacturing process of parts made from the steel bar or wire rod. In this case, the inner region CA requires excellent machinability.

[0082] During cutting, the coarse sulfides become the starting points for chips to peel off from the main body of the steel bar or wire rod. Therefore, the coarse sulfides improve the machinability of the steel bar or wire rod. Therefore, it is preferable that the number density of the coarse sulfides is high in the inner region CA. 0.65R depth position D in the inner region CA ０．６５Ｒ In this case, the number density of coarse sulfides is 660 pieces / mm ２ If the above condition is satisfied, a sufficient amount of coarse sulfides are present in the inner region CA, and therefore, excellent machinability can be obtained in the inner region CA of the steel bar or wire rod.

[0083] 0.65R depth position D ０．６５Ｒ The preferred lower limit of the number density of coarse sulfides is 700 pieces / mm ２ and more preferably 720 pieces / mm ２ and more preferably 740 pieces / mm ２ and more preferably 760 pieces / mm ２ and more preferably 780 pieces / mm ２ and more preferably 800 pieces / mm ２ and more preferably 820 pieces / mm ２ and more preferably 840 pieces / mm ２ and more preferably 860 pieces / mm ２ and more preferably 880 pieces / mm ２ and more preferably 900 pieces / mm ２ 0.65R depth position D ０．６５Ｒ However, when the steel bar or wire rod satisfies the characteristics 1 to 3, the upper limit of the number density of the coarse sulfides at the depth position D of 0.65R is ０．６５ＲThe upper limit of the number density of coarse sulfides is, for example, 1500 pieces / mm ２ and more preferably 1200 pieces / mm ２ is.

[0084] [0.65R depth position D ０．６５Ｒ Regarding the method for measuring the number density of coarse sulfides at a depth position D of 0.65R ０．６５Ｒ The number density of coarse sulfides in the above can be measured by the following method.

[0085] Referring to FIG. 2, in a longitudinal section 20 parallel to the central axis C1 of the steel bar or wire rod and including the central axis C1, ０．６５Ｒ Of the test specimens thus obtained, a longitudinal cross section that is parallel to the central axis of the steel bar or wire rod and includes said central axis is used as the observation surface.

[0086] The observation surface is mirror-polished. The mirror-polished observation surface is observed at a magnification of 500x using an SEM with a tungsten filament. The acceleration voltage for SEM observation is 20 kV. Specifically, as shown in FIG. 5, rectangular fields RF1 to RF52, each 405 μm x 405 μm square, are arranged continuously in the axial direction L. In other words, 52 rectangular fields RF1 to RF52 are arranged at a pitch of 405 μm. At this time, a 0.16R depth position D is placed at the center position of each rectangular field RFn (n is a natural number from 1 to 52). ０．１６Ｒ The rectangular fields RFn are arranged so that each field is positioned at a different position. Each rectangular field is observed to obtain a backscattered electron image. The total area of ​​the observation fields (total area of ​​the rectangular fields RF1 to RF52) is 8.53 mm ２ is.

[0087] Based on the backscattered electron image of each rectangular field RFn obtained by SEM observation, the above-mentioned [0.16R depth position D ０．１６Ｒ The number of coarse sulfides in each visual field is counted using the same method as described in [Method for measuring the number density of coarse sulfides in each visual field]. The total number of coarse sulfides counted in each visual field and the total area (8.53 mm2) of the above multiple visual fields are counted. ２ ) Based on this, 0.65R depth position D ０．６５Ｒ The number density of coarse sulfides (pieces / mm ２ ) is required.

[0088] [Preferable number density ratio NDR] In the steel bar or wire rod of this embodiment, ０．１６Ｒ The number density of coarse sulfides at 0.65R depth position D ０．６５Ｒ The ratio of the number density of coarse sulfides at the depth position D ０．１６Ｒ 0.65R depth position D ０．６５Ｒ The number density ratio NDR (Number Density Ratio) of coarse sulfides at a temperature of 1000°C is defined as the number density ratio NDR (Number Density Ratio) of coarse sulfides at a temperature of 1000°C. In the steel bar or wire rod of this embodiment, the number density ratio NDR is not particularly limited. However, a higher number density ratio NDR is preferable.

[0089] 0.16R depth position D ０．１６Ｒ 0.65R depth position D ０．６５Ｒ The number density ratio NDR of the coarse sulfides in the surface layer region SA is preferably 1.40 or more. In this case, excellent hot work crack resistance can be more effectively obtained in the surface layer region SA, and excellent machinability can be more effectively obtained in the inner region CA.

[0090] 0.16R depth position D ０．１６Ｒ 0.65R depth position D ０．６５Ｒ A more preferable lower limit of the number density ratio NDR of coarse sulfides at is 1.42, more preferably 1.45, more preferably 1.50, more preferably 1.55, and more preferably 1.60.

[0091] 0.16R depth position D ０．１６Ｒ 0.65R depth position D ０．６５Ｒ However, when the steel bar or wire rod satisfies the characteristics 1 to 4, the upper limit of the number density ratio NDR of the coarse sulfides at the depth position D ０．１６Ｒ 0.65R depth position D ０．６５Ｒ The upper limit of the number density ratio NDR of coarse sulfides in this case is, for example, 3.00, and more preferably 2.00.

[0092] [Effects of the Steel Bar or Wire Rod of the Present Embodiment] As described above, the steel bar or wire rod of the present embodiment satisfies Features 1 to 4. Therefore, the steel bar or wire rod can achieve both excellent hot work cracking resistance and excellent machinability.

[0093] [Preferred Shapes and Applications of the Steel Bar or Wire Rod of the Present Embodiment] The steel bar or wire rod of the present embodiment can be widely applied, for example, as a material for parts for general machinery and automobiles. The steel bar or wire rod of the present embodiment is particularly suitable when hot working, such as hot rolling or hot forging, and cutting are performed in the manufacturing process of the steel bar or wire rod and parts. However, even when neither hot working nor cutting is performed, the steel bar or wire rod of the present embodiment can be applied as a material for parts.

[0094] [Regarding the microstructure of the steel material] The microstructure of the steel material of this embodiment is not particularly limited. As described above, the steel material of this embodiment is subjected to hot working and surface hardening heat treatment such as carburizing treatment and thermal refining treatment in the manufacturing process of parts for general machinery or automobiles using the steel material as a raw material. In such hot working and surface hardening treatment, the steel material is subjected to A ｃ３ Since the steel is heated to above this temperature, the microstructure is transformed. Therefore, the microstructure of the steel is not particularly limited.

[0095] [Preferred embodiment of the steel bar or wire rod of this embodiment] [(Feature 5) Preferred Fn2] Preferably, the steel bar or wire rod of this embodiment further contains, in mass%, Ca: 0.0001 to 0.0050%, and Fn2 defined by formula (2) is 4.0 × 10 －５ ~4.8 x 10 －４ Fn2=Ca×S (2) Here, the content (mass %) of each element is substituted for each element symbol in the formula. Fn2 will be explained below.

[0096] [Regarding Fn2] Fn2 is an index relating to the Ca content and S content required for controlling the morphology of sulfides. －５ If it is more than this, it is sulfides that improve the machinability of the steel bar or wire rod, that is, 0.65R depth position D ０．６５Ｒ Among the sulfides having an equivalent circle diameter of 1 μm or more (i.e., coarse sulfides), the number ratio of coarse sulfides containing Ca (hereinafter also referred to as Ca-containing coarse sulfides) can be made 0.50% or more.

[0097] On the other hand, Fn2 is 4.8 × 10－４ If Fn2 is 4.0 × 10 or less, excessive generation of Ca-containing coarse sulfides can be suppressed. As a result, cracking during hot working can be more sufficiently suppressed, and the hot working cracking resistance of the steel bar or wire rod can be further improved. Therefore, preferably, Fn2 is 4.0 × 10 －５ ~4.8 x 10 －４ is.

[0098] A more preferable lower limit of Fn2 is 5.0 × 10 －５ and more preferably 6.0 × 10 －５ and more preferably 7.0 × 10 －５ and more preferably 8.0 × 10 －５ and more preferably 1.0 × 10 －４ A more preferable upper limit of Fn2 is 4.5 × 10 －４ and more preferably 4.0 × 10 －４ and more preferably 3.0 × 10 －４ is.

[0099] [Preferable Number Ratio RA of Ca-Containing Coarse Sulfides] Here, the Ca-containing coarse sulfides are ０．６５Ｒ As a result of point analysis of regions recognized as coarse particles using EDX in [Method for measuring the number density of coarse sulfides in a sample], the term "coarse particles" refers to coarse particles having an S content of 10% by mass or more, an Mn content of 10% by mass or more, and a Ca content of 1% by mass or more. Note that, among coarse particles, coarse particles having an S content of 10% by mass or more, an Mn content of 10% by mass or more, and a Ca content of less than 1% by mass are also referred to as "Ca-free coarse sulfides."

[0100] The Ca coarse sulfides have a higher hardness and are more spheroidized than Ca-free coarse sulfides. The Ca-containing coarse sulfides are less likely to deform under external force than Ca-free coarse sulfides. Therefore, during cutting, the Ca-containing coarse sulfides are more likely to serve as starting points for peeling off chips from the main body of the steel bar or wire rod than the Ca-free coarse sulfides. If Feature 4 is satisfied and the number ratio of the Ca-containing coarse sulfides to the coarse sulfides is 0.50% or more, it is preferable to obtain excellent machinability in the internal region of the steel bar or wire rod.

[0101] A more preferable lower limit of the number ratio RA of Ca-containing coarse sulfides among the coarse sulfides is 0.55%, more preferably 0.60%, even more preferably 0.65%, even more preferably 0.70%, even more preferably 0.90%, and even more preferably 1.10%. The upper limit of the number ratio RA of Ca-containing coarse sulfides is not particularly limited. However, in a steel bar or wire rod having a chemical composition that satisfies Feature 1, the upper limit of the number ratio RA of Ca-containing coarse sulfides among the coarse sulfides is, for example, 20.00%, more preferably 15.00%, even more preferably 12.00%, even more preferably 10.00%, and even more preferably 5.00%.

[0102] [0.65R depth position D ０．６５Ｒ Regarding the method for measuring the number ratio RA of Ca-containing coarse sulfides at a depth position D of 0.65R ０．６５Ｒ The number ratio RA of Ca-containing coarse sulfides at the depth position D of the above-mentioned [0.65R] can be measured by the following method. ０．６５Ｒ In the method for measuring the number density of coarse sulfides in each field of view, EDX is used to perform point analysis of the areas identified as coarse particles in order to identify the coarse sulfides in each field of view. Among the coarse particles, particles having an S content of 10% by mass or more, an Mn content of 10% by mass or more, and a Ca content of 1% or more are identified as Ca-containing coarse sulfides. Using the total number of coarse sulfides identified in each rectangular field of view RFn and the total number of Ca-containing coarse sulfides, the number ratio RA (%) of Ca-containing coarse sulfides among the coarse sulfides is calculated using the following formula: RA = total number of Ca-containing coarse sulfides / total number of coarse sulfides × 100

[0103] [Manufacturing Method] An example of a method for manufacturing a steel bar or wire rod according to this embodiment will be described. A steel bar or wire rod satisfying Features 1 to 4 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel bar or wire rod according to this embodiment.

[0104] An example of the method for manufacturing a steel bar or wire rod according to this embodiment includes the following steps. Note that step 3 is an optional step and does not need to be performed. (Step 1) Refining step (Step 2) Casting step (Step 3) Hot working step Each step will be described below.

[0105] [(Step 1) Refining Step] In the refining step, molten steel is produced having a chemical composition that satisfies the above-mentioned Features 1 and 2. The refining step may be carried out by a known process.

[0106] [(Step 2) Casting Step] The casting step includes the following steps: (Step 21) Continuous Casting Step (Step 22) Immersion and Heating Step (Step 23) Slab Reduction Step Each step will be explained below. [(Step 21) Continuous Casting Step] In the casting step, first, a slab (bloom) is produced using a continuous casting device. In the continuous casting step, the cooling rate satisfies the following condition: (Condition 1) When the solidification cooling rate at a depth of 15 mm from the surface in a cross section perpendicular to the longitudinal direction of the slab is defined as the surface solidification cooling rate, the surface solidification cooling rate is set to 550°C / min or more.

[0107] [Condition 1: Surface Layer Solidification and Cooling Rate V] Sulfides crystallize or precipitate during the continuous casting process. If the solidification and cooling rate in the surface layer of the slab during the continuous casting process is sufficiently fast, the sulfides are less likely to coarsen in the surface layer of the slab. In this case, the number density of coarse sulfides in the surface region SA of the produced steel bar or wire rod can be sufficiently reduced. In this embodiment, the cooling rate of the slab in the mold of the continuous casting device is adjusted so that the solidification and cooling rate at a depth of 15 mm from the surface of the produced slab is 550°C / min or higher.

[0108] Here, the cooling rate from the liquidus temperature to the solidus temperature within the temperature range of the steel in the casting process is defined as the solidification cooling rate (°C / min). As described above, the solidification cooling rate at a depth of 15 mm from the surface of the slab is defined as the "surface solidification cooling rate V." The surface solidification cooling rate V is determined by the following method.

[0109] 6 is a cross-sectional view perpendicular to the longitudinal direction of a slab 30 produced by a continuous casting method. The direction W in FIG. 6 refers to the width direction of the slab 30. The direction T refers to the thickness direction of the slab 30. The direction L refers to the longitudinal direction of the slab 30.

[0110] Referring to FIG. 6, the thickness direction (T direction) of the cast 30 is measured at a depth D １５ｍｍ For example, when the cross section perpendicular to the longitudinal direction of the slab is a rectangle as shown in FIG. 6 (that is, when the surface LH of the slab 30 corresponds to the long side and the surface LV of the slab 30 corresponds to the short side in the cross section), a test piece including the following is taken from a position D at a depth of 15 mm from the width center position of the surface LH, which is the long side. １５ｍｍ In addition, when the cross section of the slab 30 is square, a test piece containing １５ｍｍ A test piece containing the above is taken. The surface of the test piece that corresponds to a cross section perpendicular to the longitudinal direction (L direction) of the slab 30 is used as the observation surface. Of the observation surface of the test piece, a 5 mm x 5 mm region centered at a depth of 15 mm from the surface of the slab is used as the observation region 40. The dendrite secondary arm spacing is measured at 10 locations in the observation region 40. The arithmetic mean value of the 10 dendrite secondary arm spacings is set to λ2 (μm). Using the obtained dendrite secondary arm spacing λ2 (μm), the surface solidification cooling rate V (°C / min) is calculated using the following equation: V = (λ2 / 770) －１／０．４１

[0111] The region of the slab 30 at a depth of 15 mm from the surface solidifies while passing through the mold of the continuous casting machine. Therefore, the surface solidification cooling rate is adjusted by the cooling mechanism in the mold. There is no particular upper limit to the surface solidification cooling rate. The preferred lower limit of the surface solidification cooling rate is 600°C / min or more, and more preferably 700°C / min or more.

[0112] [(Step 22) Immersion and Heating Step] The slab produced in the continuous casting step is cut with a torch. The temperature of the surface layer region of the slab cut with the torch and the temperature of the inner region below the surface layer region are both high, at 600°C or higher. In the immersion and reheating step, the slab cut with the torch in a high-temperature state is first immersed in a water tank for a predetermined time. That is, the slab cut with the torch is not allowed to cool to room temperature, but is instead immersed in a water tank in a high-temperature state. At this time, the surface layer region of the slab is rapidly cooled, and the surface temperature drops to 500°C or lower. On the other hand, the temperature in the inner region of the slab does not drop as much, and is maintained at a temperature higher than the surface temperature.

[0113] After immersion, the slab is heated in a heating furnace. During this process, a reverse transformation from ferrite to austenite occurs in the surface region, resulting in finer crystal grains. This suppresses the formation of coarse sulfides in the surface region. Meanwhile, the internal region of the slab is maintained at a high temperature even during immersion and is further heated in the heating furnace. This promotes the growth of sulfides in the internal region of the slab. Specifically, the immersion and heating processes satisfy the following conditions: (Condition 2) The surface temperature T0 of the slab immediately before the start of immersion is set to 600 to 850°C. (Condition 3) The immersion time t0 is set to 5 to 50 seconds. (Condition 4) The heating temperature T1 in the heating furnace is set to 1000 to 1200°C, and the holding time t1 at the heating temperature T1 is set to 30 to 120 minutes. Each condition is explained below.

[0114] [Condition 2: Regarding the surface temperature T0 of the slab immediately before the start of immersion] If the surface temperature T0 of the slab is less than 600°C, the temperature of the slab before immersion is too low. In this case, sulfides cannot grow sufficiently in the internal region. On the other hand, if the surface temperature T0 exceeds 850°C, the temperature of the slab before immersion is too high. In this case, even if immersion is performed, the surface layer temperature of the slab does not drop sufficiently. As a result, sulfides grow excessively coarse in the surface layer region of the slab. Therefore, the surface temperature T0 is set to 600 to 850°C.

[0115] [Condition 3: Immersion time t0] If the immersion time t0 is too short, the surface temperature of the slab will not drop sufficiently even after immersion. As a result, sulfides will grow excessively coarse in the surface region of the slab. On the other hand, if the immersion time t0 is too long, the temperature of the internal region of the slab will drop excessively. In this case, sulfides will not grow sufficiently in the internal region. Therefore, the immersion time t0 is 5 to 50 seconds.

[0116] [Condition 4: Heating time T1 and holding time t1 in the heating furnace] If the heating temperature T1 in the heating furnace after immersion is too low or the holding time t1 is too short, sulfides will not grow sufficiently in the inner region of the slab. On the other hand, if the heating temperature T1 is too high or the holding time t1 is too long, sulfides will grow excessively coarse in the surface region of the slab. Therefore, the heating temperature T1 is 1000 to 1200°C, and the holding time t1 is 30 to 120 minutes.

[0117] [(Step 23) Slab Reduction Step] The heated slab is hot worked to produce a billet. The cumulative reduction is not particularly limited, but is, for example, 30 to 60%. The cumulative reduction is calculated as follows: Cumulative reduction = Short side length of billet after reduction / Short side length of material (bloom) before reduction × 100. If the billet or bloom has a square cross section, the short side in the above formula is read as one side.

[0118] [(Step 3) Hot Working Step] The hot working step is an optional step and may not be performed. If performed, the hot working step involves hot working the billet produced in the casting step to produce a steel bar or a wire rod.

[0119] In the hot working process, for example, a heated billet is subjected to finish rolling using a known continuous rolling mill to produce a steel bar or a wire rod. The heating temperature in the rough rolling process is, for example, 1000 to 1300°C. The heating temperature in the finish rolling process is, for example, 1000 to 1300°C.

[0120] In the above-described hot working step, a steel bar or wire rod is produced by hot rolling. However, the steel bar or wire rod may be produced by hot working other than hot rolling. For example, instead of hot rolling, the steel bar or wire rod may be produced by hot forging. Furthermore, the steel bar or wire rod may be produced by hot forging after hot rolling. Even when hot forging is performed in the hot working step, the heating temperature is, for example, 1000 to 1300°C.

[0121] The steel bar or wire rod of this embodiment is manufactured by the above manufacturing process.

[0122] The effects of the steel bar or wire rod of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel bar or wire rod of this embodiment. Therefore, the steel bar or wire rod of this embodiment is not limited to this one example of conditions.

[0123] Steel bars having the chemical compositions shown in Table 1 (Tables 1A to 1F) were produced.

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130] Specifically, a 270-ton converter was used to produce molten steel through a well-known refining process. A slab was produced from the molten steel by a continuous casting method. Conditions 1 to 4 for each test number are shown in Table 2 (Table 2A and Table 2B). The cumulative reduction rate in the slab reduction step of the casting process was 30 to 60%. Through the above process, a billet was produced, the cross section of which perpendicular to the axial direction was rectangular and measured 162 mm x 162 mm.

[0131]

[0132]

[0133] The produced billet was subjected to hot working. Specifically, hot forging equivalent to finish rolling was performed on the billet to produce a steel bar with a diameter of 12 mm. The heating temperature of the billet during hot forging was 1250°C. Hot forging was performed using an air hammer (model 600HP) manufactured by Otani Machinery Manufacturing Co., Ltd. The billet was heated at 1250°C for 1 hour and then hot forged to produce a rectangular billet with a cross section perpendicular to the axial direction of 120 mm x 120 mm. The billet was then heated again at 1250°C for 1 hour. The heated billet was further hot forged using the above-mentioned air hammer (model 600HP) to produce a billet with an octagonal cross section perpendicular to the axial direction circumscribing a circle with a diameter of 100 mm. The billet was then heated again at 1250°C for 1 hour. The heated billet was subjected to hot forging using the above-mentioned air hammer (model 600HP) to form an intermediate round steel bar having a circular cross section perpendicular to the axial direction and a diameter of 60 mm.

[0134] The intermediate round steel was heated again at 1250°C for 1 hour. The heated intermediate round steel was hot forged using the above-mentioned air hammer (model 100HP) to produce an intermediate round steel with a diameter of 30 mm. Thereafter, the intermediate round steel was heated again at 1250°C for 1 hour. The heated intermediate round steel was hot forged using the above-mentioned air hammer (model 100HP) to produce a circular steel bar with a cross section perpendicular to the axial direction (longitudinal direction) and a diameter of 12 mm. Steel bars were produced using the above manufacturing process.

[0135] [Evaluation Tests] The following evaluation tests were carried out on the steel bars with each test number. (Test 1) 0.16R depth position D ０．１６Ｒ Measurement test of the number density of coarse sulfides at 0.65R depth position D (Test 2) ０．６５Ｒ Measurement of the number density of coarse sulfides at depth D (Test 3) ０．６５Ｒ RA measurement test for the proportion of the number of Ca-containing coarse sulfides in the test piece (Test 4) Hot work crack evaluation test (Test 5) Machinability evaluation test (drill life test) Each evaluation test will be described below.

[0136] [(Test 1) 0.16R depth position D ０．１６ＲFor each test number of steel bar, the above-mentioned [0.16R depth position D ０．１６Ｒ [Method for measuring the number density of coarse sulfides at a depth of 0.16R] ０．１６Ｒ The number density of coarse sulfides (pieces / mm ２ The results are shown in Table 3 (Table 3A and Table 3B).

[0137]

[0138]

[0139] [(Test 2) 0.65R depth position D ０．６５Ｒ For each test number of steel bar, the above-mentioned [0.65R depth position D ０．６５Ｒ [Method for measuring the number density of coarse sulfides at a depth of 0.65R] ０．６５Ｒ The number density of coarse sulfides (pieces / mm ２ The results are shown in Table 3. ０．１６Ｒ 0.65R depth position D ０．６５Ｒ Table 3 shows the number density ratio NDR of coarse sulfides at 1000 kJ / s.

[0140] [(Test 3) 0.65R depth position D ０．６５Ｒ For each test number of steel bars, the above-mentioned [0.65R depth position D ０．６５Ｒ [Method for measuring the number ratio RA of Ca-containing coarse sulfides at a depth of 0.65R] ０．６５Ｒ The percentage RA (%) of the number of Ca-containing coarse sulfides was calculated. The results are shown in Table 3.

[0141] [(Test 4) Hot Working Cracking Evaluation Test] The surface of the steel bar manufactured with each test number was observed. As a result of the observation, if four or more clear cracks were observed on the surface of the steel bar per 1 m in the longitudinal direction of the steel bar, it was determined that hot working cracking had occurred (represented by "B (Bad)" in the "Hot Working Cracking Resistance" column in Table 3). On the other hand, if three or fewer clear cracks were observed on the surface of the steel bar per 1 m in the longitudinal direction of the steel bar, it was determined that hot working cracking had been suppressed (represented by "G (Good)" in the "Hot Working Cracking Resistance" column in Table 3). Furthermore, if two or fewer clear cracks were observed, it was determined that hot working cracking had been significantly suppressed (represented by "E (Excellent)" in the "Hot Working Cracking Resistance" column in Table 3). Here, "clear cracks" means cracks of 3 mm or more in length observed with the naked eye or using a simple magnifying glass.

[0142] [(Test 5) Machinability Evaluation Test (Drill Life Test)] The steel bar of each test number was cut to a length of 21 mm, and test pieces with a diameter of 12 mm and a length of 21 mm were collected. A drill was used to drill a hole at any position in a region of the test piece corresponding to the internal region CA of a cross section perpendicular to the axial direction (longitudinal direction) of the steel bar. A drill with model number SD3.0 manufactured by Fujikoshi Corporation was used as the tool. The drilling conditions were a feed rate per revolution of 0.25 mm / rev and a drilling depth of 9 mm per hole. The lubricant was a water-soluble cutting oil.

[0143] Drilling was performed under the above drilling conditions to evaluate the machinability of the steel bars. The maximum cutting speed VL1000 (m / min) was used as an evaluation index. The maximum cutting speed VL1000 is the cutting speed of a drill capable of drilling a hole with a cumulative hole depth of 1000 mm.

[0144] Based on the maximum cutting speed VL1000, machinability was evaluated as follows: E: VL1000 was 120 m / min or more, and machinability was particularly excellent. G: VL1000 was 100 m / min or more and less than 120 m / min, and machinability was excellent. B: VL1000 was less than 100 m / min, and machinability was poor. The "Cutting ability" column in Table 3 shows the evaluation results for "E", "G", and "B".

[0145] [Test Results] The test results are shown in Tables 1 to 3. The steel bars of Test Nos. 1 to 39 satisfied Features 1 to 4. Therefore, these test numbers had excellent hot work cracking resistance, and further had superior machinability in the inner region rather than the surface layer.

[0146] The steel bars of test numbers 13, 26, 28, and 32 to 35 satisfied not only characteristics 1 to 4 but also characteristic 5. Therefore, with these test numbers, even better hot workability was obtained, and even better machinability was obtained.

[0147] On the other hand, in test numbers 40 and 41, Fn1 was too low, and therefore sufficient hot work cracking resistance was not obtained.

[0148] In test numbers 42 and 43, Fn1 was too high. Therefore, the depth position D of 0.16R was ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0149] In test numbers 44 and 45, the surface solidification cooling rate V was too slow. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0150] In test numbers 46 and 47, the surface temperature T0 of the slab just before the start of immersion was too low. ０．６５Ｒ The number density of coarse sulfides in the surface layer was too low, resulting in insufficient machinability in the inner region compared to the surface layer.

[0151] In test numbers 48 and 49, the surface temperature T0 of the slab just before the start of immersion was too high. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0152] In test numbers 50 and 51, the immersion time t0 was too short. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0153] In test numbers 52 and 53, the immersion time t0 was too long. ０．６５Ｒ The number density of coarse sulfides in the surface layer was too low, resulting in insufficient machinability in the inner region compared to the surface layer.

[0154] In test numbers 54 and 55, the heating temperature T1 in the heating furnace after immersion was too low. ０．６５Ｒ The number density of coarse sulfides in the surface layer was too low, resulting in insufficient machinability in the inner region compared to the surface layer.

[0155] In test numbers 56 and 57, the heating temperature T1 in the heating furnace after immersion was too high. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0156] In test numbers 58 and 59, the holding time t1 in the heating furnace after immersion was too short. ０．６５Ｒ The number density of coarse sulfides in the surface layer was too low, resulting in insufficient machinability in the inner region compared to the surface layer.

[0157] In test numbers 60 and 61, the holding time t1 in the heating furnace after immersion was too long. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0158] In test numbers 62 and 63, the immersion step was not performed. ０．１６Ｒ The number density of coarse sulfides was too high, resulting in insufficient hot work cracking resistance.

[0159] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. A steel bar or wire rod having a circular cross section perpendicular to the axial direction with a radius R, and the chemical composition, in mass%, is as follows: C: 0.005-0.200%, Si: 0.001-0.500%, Mn: 0.30-3.00%, P: 0.001-0.200%, S: 0.250-0.750%, B: 0.0005-0.0150%, N: 0.0020-0.0200%, O: 0.0030-0.0350%, Al: 0.010% or less, V: 0-0.200%, Nb: 0-0.200%, Ti: 0-0.200%, Cr: 0-2.000%, Mo: 0-0.300%, W: 0-0.200%, Ni: 0-0.500%, Cu: 0-0.500%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.0050%, rare earth elements: 0-0.0050%, Sn: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, Te: 0-0.010%, Se: 0-0.010%, In: 0-0.010%, Pb: 0-0.090%, and Bi: 0-0.250%, with the balance being Fe and impurities; Fn1 defined in formula (1) is 1.2-5.0; In a longitudinal section parallel to and including the central axis of the steel bar or wire, the number density of sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ In the longitudinal section, at a depth position of 0.65R from the surface of the steel bar or wire rod, the number density of the sulfides having a circle equivalent diameter of 1 μm or more is less than 660 pieces / mm ２ The steel bar or wire rod is as above. Fn1=Mn / S (1) Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula.

2. The steel bar or wire rod according to claim 1, wherein the chemical composition is, in mass%, V: 0.001-0.200%, Nb: 0.001-0.200%, Ti: 0.001-0.200%, Cr: 0.001-2.000%, Mo: 0.001-0.300%, W: 0.001-0.200%, Ni: 0.001-0.500%, Cu: 0.001-0.500%, Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Zr: 0.0001-0.0050%, rare earth elements: 0.0001-0.0050%, A steel bar or wire rod containing one or more elements selected from the group consisting of Sn: 0.001-0.100%, Sb: 0.001-0.050%, As: 0.001-0.050%, Te: 0.001-0.010%, Se: 0.001-0.010%, In: 0.001-0.010%, Pb: 0.001-0.090%, and Bi: 0.001-0.250%.

3. The steel bar or wire rod according to claim 1 or 2, further comprising: the chemical composition contains, in mass%, Ca: 0.0001 to 0.0050%; and Fn2 defined by formula (2) is 4.0 x 10 －５ ~4.8 x 10 －４ In the longitudinal section, at a depth position of 0.65R from the surface of the steel bar or wire rod, the number density of the sulfides having a circle equivalent diameter of 1 μm or more is 660 pieces / mm ２ The steel bar or wire rod, further comprising: a ratio by number of the sulfides containing Ca among the sulfides having an equivalent circle diameter of 1 μm or more of 0.50% or more. Fn2=Ca×S (2) where each element symbol in the formula is substituted with the content (mass%) of the corresponding element.

Images