Free-machining steel drawing material and manufacturing method therefor

Abstract

A free-machining steel drawing material including, by mass%: 0.03% to 0.09% C; 0.50% to 2.00% Mn; 0.020% to 0.100% P; 0.250% to 0.500% S; 0.05% to 1.50% Cr; 0.05% to 0.27% Cu; 0.0050% to 0.0150% N; 0.0300% to 0.1000% Pb; and 0.0100% to 0.0400% O, the remainder consisting of Fe and unavoidable impurities, wherein the free-machining steel drawing material has a component composition satisfying mathematical expression (1), has a steel structure in which Pb particles with an equivalent circle diameter of 0.05 μm to 0.5 μm are distributed in a quantity of 5000 / mm2 or greater, and satisfies mathematical expressions (2), (3), and (4).　(1): 1.50≤[Pb] / [Cu]+[Mn] / 2[S]≤3.00　(2): 210≤H0.1≤340　(3): 185≤H0.5≤280　(4): 150≤Hc≤240

Description

Free-cutting steel drawing material and its manufacturing method

[0001] The present invention relates to a free-cutting steel drawn material and a method for producing the same.

[0002] Low carbon sulfur-lead free-cutting steel, typified by JIS standard SUM24L, is a steel whose excellent machinability is improved by adding a large amount of lead (Pb) and sulfur (S) as free-cutting elements.

[0003] With the recent increase in environmental awareness, there has been a growing global movement to eliminate or restrict the use of environmentally hazardous substances. Lead (Pb) is one such example, and as exemplified by the RoHS Directive, there is a growing demand to limit the use of Pb, an environmentally hazardous substance, to 1000 ppm or less in target materials. However, Pb is an extremely important and useful element for industrial products, and is also used in steel materials as an element that significantly improves the machinability of materials, such as reducing tool wear during cutting and improving surface roughness and chip disposal, and is still used in many steel products manufactured by cutting processes.

[0004] Against this background, Patent Document 1 discloses a Pb-free free-cutting non-heat treated steel. Similarly, Patent Document 2 discloses a Pb-free free-cutting steel.

[0005] Patent Document 3 discloses a free-cutting steel that achieves excellent machinability by using CrS instead of MnS as the free-cutting component.

[0006] Japanese Patent Laid-Open No. 9-25539 Japanese Patent Laid-Open No. 2000-160284 Japanese Patent Laid-Open No. 57-63667

[0007] However, the technology described in Patent Document 1 has the problem that the steel type it targets is a hard, non-tempered steel containing 0.2 to 0.6 mass% C, and that it uses the special element Nd (neodymium), resulting in high manufacturing costs. Furthermore, the technology described in Patent Document 2 adds a large amount of S, resulting in low hot ductility, which causes cracks during continuous casting and hot rolling, resulting in problems in terms of surface quality.

[0008] On the other hand, in the technology described in Patent Document 3, the amount of Mn added is reduced and Cr and S are added as components. However, the amount of Cr added is as high as 3.5 to 5.9 mass %, which is disadvantageous in terms of cost reduction. In addition, the generation of a large amount of CrS poses a manufacturing problem because it is difficult to perform a material melting process in a steelmaking process.

[0009] The present invention aims to solve the above-mentioned problems and to provide a low-cost, high-quality free-cutting steel drawn material that has machinability equal to or better than that of low-carbon sulfur-lead composite free-cutting steel while significantly reducing the amount of Pb added, which is an element that improves machinability, from the amount added in the past.

[0010] As a result of extensive research conducted by the present inventors to achieve the above object, the present inventors have obtained the following findings.

[0011] (1) By adding a predetermined amount of Cu to steel, work hardening during drawing and cutting can be moderated, and the depth of cut during cutting can be stabilized, thereby making chip thickness uniform and improving machinability. However, an excessive Cu content results in excessive hardness and shortens the tool life. In addition, the inclusion of Cu is advantageous in that it promotes the introduction of strain, particularly during billet rolling, and in the subsequent process, supersaturated dissolved Mn combines with S to precipitate fine MnS. The fine MnS is effective as a nucleus for the formation of Pb, resulting in high lubricity and improved machinability.

[0012] (2) By adding a specified amount of Pb to the steel, the Pb is finely dispersed, which increases the lubricating effect between the tool and the workpiece during cutting, thereby suppressing the formation of built-up edge and significantly improving machinability, including tool wear, dimensional accuracy, surface roughness, and chip disposal.

[0013] (3) By controlling the hardness of the drawn material to fall within a predetermined range, the tool wear, dimensional accuracy, surface roughness, and chip disposal properties of the product are improved in combination with the effect of Cu described above.

[0014] The present invention was made based on the above findings and has the following features.

[0015] 1. A steel sheet containing, by mass%, C: 0.03% or more and 0.09% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or more and 0.100% or less, S: 0.250% or more and 0.500% or less, Cr: 0.05% or more and 1.50% or less, Cu: 0.05% or more and 0.27% or less, N: 0.0050% or more and 0.0150% or less, Pb: 0.0300% or more and 0.1000% or less, and O: more than 0.0100% and 0.0400% or less, with the balance consisting of Fe and unavoidable impurities, and having a component composition that satisfies the following formula (1), and having a density of Pb granules having an equivalent circle diameter of 0.05 μm or more and 0.5 μm or less of 5,000 particles / mm ２ A free-cutting drawn steel material having a steel structure in which Pb and Cu are distributed in a range of 1.50 to 3.00, and satisfying the following formulas (2), (3), and (4): 1.50≦[Pb] / [Cu]+[Mn] / 2[S]≦3.00 (1) 210≦H ０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) Here, [ ] indicates the content (unit mass%) of the element in [ ], H ０．１ is the Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material, and H ０．５is the Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material, and Hc is the Vickers hardness (unit: HV) at a position 1 / 2 the diameter of the drawn material to the surface of the drawn material. 2. The free-cutting steel drawn material of 1 above, wherein the chemical composition further contains at least one component selected from the following groups A to D. Group A: In mass %, one or more selected from Si: 0.20% or less, Al: 0.20% or less, and Ti: 0.15% or less. Group B: In mass %, one or more selected from Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less. Group C: In mass%, one or more selected from Sb: 0.020% or less, Sn: 0.020% or less, Ni: 1.0% or less, and Mo: 1.0% or less. Group D: In mass%, one or more selected from Nb: 0.050% or less, V: 0.050% or less, Zr: 0.050% or less, W: 0.050% or less, Ta: 0.050% or less, Y: 0.050% or less, Hf: 0.050% or less, and B: 0.050% or less. 3. In mass%, C: 0.03% or more and 0.09% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or more and 0.100% or less, S: 0.250% or more and 0.500% or less, Cr: 0.05% or more and 1.50% or less, Cu: 0.05% or more and 0.27% or less, N: 0.0050% or more and 0.0150% or less, Pb: 0.0300% or more and 0.1000% or less, and A method for producing a steel sheet comprising rolling a slab produced by continuous casting at a heating temperature of 900°C or more and 1050°C or less at an area reduction rate of 50% or more into a billet, rolling the billet at a heating temperature of 1100°C or more into a steel bar or wire rod, and then drawing the bar or wire rod at an area reduction rate of 1.0% or more, wherein the steel sheet contains 5000 Pb granules per mm2 having an equivalent circle diameter of 0.05 μm or more and 0.5 μm or less. ２ A method for producing a free-cutting drawn steel material having a steel structure with the above distribution and satisfying the following formulas (2), (3), and (4): 1.50≦[Pb] / [Cu]+[Mn] / 2[S]≦3.00 (1) 210≦H０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) Here, [ ] indicates the content (unit mass%) of the element in [ ], H ０．１ is the Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material, and H ０．５ is the Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material, and Hc is the Vickers hardness (unit: HV) at a position 1 / 2 the diameter of the drawn material to the surface of the drawn material. 4. The method for producing a free-cutting steel drawn material according to 3 above, wherein the chemical composition further contains at least one element selected from the following groups A to D. Group A: In mass %, one or more elements selected from Si: 0.20% or less, Al: 0.20% or less, and Ti: 0.15% or less. Group B: In mass %, one or more elements selected from Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less. Group C: By mass%, one or more selected from Sb: 0.020% or less, Sn: 0.020% or less, Ni: 1.0% or less, and Mo: 1.0% or less. Group D: By mass%, one or more selected from Nb: 0.050% or less, V: 0.050% or less, Zr: 0.050% or less, W: 0.050% or less, Ta: 0.050% or less, Y: 0.050% or less, Hf: 0.050% or less, and B: 0.050% or less.

[0016] According to the present invention, a free-cutting steel drawn material having machinability equal to or better than that of low-carbon sulfur-lead composite free-cutting steel can be provided at low cost and with high quality, while the amount of Pb added, a machinability-improving element, is significantly reduced from the amount added conventionally. The free-cutting steel drawn material of the present invention is particularly suitable for cutting. The present invention also provides a method for producing such a free-cutting steel drawn material.

[0017] The free-cutting steel drawn material of the present invention (hereinafter also referred to simply as "drawn material") will be described. Hereinafter, "%" regarding the composition of elements means "mass %" unless otherwise specified.

[0018] <C: 0.03% or more and 0.09% or less> C is an important element because it has a significant effect on the strength and machinability of steel. However, if the C content is less than 0.03%, not only does the steelmaking cost increase, but also insufficient hardness is not obtained, resulting in unstable cutting, which increases the burden on the tool and shortens tool life. As a result, the surface becomes rough, chips do not break, and processability decreases. On the other hand, if the C content exceeds 0.09%, a large amount of hard carbides precipitates, hardening the material and increasing tool wear. As with the case of a low C content, the surface becomes rough and chips become long, resulting in reduced processability. Therefore, the C content is set to 0.03% or more, preferably 0.04% or more, and 0.09% or less, preferably 0.08% or less.

[0019] <Mn: 0.50% or More and 2.00% or Less> Mn is an element that forms the sulfide MnS, which is important for machinability. However, if the Mn content is less than 0.50%, the amount of MnS is small, and the generation of fine Pb using MnS as nuclei is insufficient. As a result, the lubricating effect of Pb is insufficient, and tool wear, surface roughness, and chip disposability are reduced, resulting in the desired machinability being unobtainable. Furthermore, sufficient hardness cannot be obtained by drawing, and the cutting depth is unstable, which increases the load on the tool and shortens the tool life. As a result, the surface becomes rough, chips are not broken, and chip disposability is reduced. On the other hand, if the Mn content exceeds 2.00%, in addition to excessive hardening, MnS becomes coarse and elongated, resulting in insufficient generation of fine Pb, resulting in reduced tool life, surface roughness, and chip disposability, resulting in the desired machinability being unobtainable. Furthermore, adding an excessive amount increases the alloy cost, which is economically disadvantageous, and also deteriorates the mechanical properties. Therefore, the Mn content is set to 0.50% or more, preferably 0.70% or more, and is set to 2.00% or less, preferably 1.80% or less.

[0020] <P: 0.020% or More and 0.100% or Less> P has the effect of suppressing tool wear during cutting, reducing finished surface roughness, and improving chip disposability. However, if the P content is less than 0.020%, these effects are insufficient. Furthermore, the material becomes soft and the depth of cut becomes unstable, which increases tool wear and reduces surface roughness and chip disposability. On the other hand, if the P content exceeds 0.100%, the material becomes significantly hard and increases tool wear, resulting in poor surface roughness, unstable chip length, and poor processability. Furthermore, excessive P content degrades mechanical properties and hot workability. Therefore, the P content is set to 0.020% or more, preferably 0.040% or more, and 0.100% or less, preferably 0.080% or less.

[0021] <S: 0.250% or More and 0.500% or Less> S is an important element that improves machinability by bonding with Mn to form sulfides (e.g., MnS). However, if the S content is less than 0.250%, the amount of MnS is low, making the above effect insufficient. Furthermore, the amount of fine Pb formed around the MnS nuclei is reduced, resulting in insufficient lubrication between the tool and the material. As a result, machinability, such as tool wear, surface roughness, and chip disposability, deteriorates. On the other hand, if the S content exceeds 0.500%, the MnS becomes too coarse and elongates, reducing the amount of fine Pb formed around the MnS nuclei and degrading ductility, an important mechanical property. Therefore, the S content is set to 0.250% or more, preferably 0.280% or more, and 0.500% or less, preferably 0.450% or less.

[0022] <Cr: 0.05% or More and 1.50% or Less> Cr forms sulfides (Mn,Cr)S with Mn and has the effect of improving machinability through its lubricating action. Furthermore, by suppressing the elongation of sulfides, machinability can be improved, such as tool wear, surface roughness, and chip disposability. However, if the Cr content is less than 0.05%, the generation of (Mn,Cr)S is insufficient, and it tends to remain in an elongated form. As a result, it is difficult to generate fine Pb with (Mn,Cr)S as nuclei, the lubrication effect between the tool and the workpiece material is reduced, and machinability, such as tool wear, surface roughness, and chip disposability, deteriorates. On the other hand, if the Cr content exceeds 1.50%, the material itself hardens, (Mn,Cr)S becomes coarse, making it difficult to generate fine Pb, and the effect of suppressing the elongation of (Mn,Cr)S is reduced. This leads to deterioration of machinability, such as tool wear, surface roughness, and chip disposability. Furthermore, the addition of an excessive amount of Cr increases alloy costs, which is economically disadvantageous. Therefore, the Cr content is set to 0.05% or more, preferably 0.06% or more, and 1.50% or less, preferably 1.30% or less.

[0023] <Cu: 0.05% or More and 0.27% or Less> Addition of an appropriate amount of Cu provides moderate work hardening during drawing and stabilizes the depth of cut during cutting. This effect improves tool life and product diameter accuracy, reduces the burden on the tool, and ensures a consistent depth of cut, improving surface roughness and chip disposal. Furthermore, adding an appropriate amount of Cu optimizes the strain introduced, particularly during billet rolling. The strain then acts as a nucleus for supersaturated dissolved Mn to bond with S, effectively promoting the precipitation of fine MnS. Furthermore, the fine MnS serves as a nucleus for the formation of Pb, resulting in an increase in the amount of fine Pb. This lubricating effect is advantageous for improving machinability, such as tool life, surface roughness, and chip disposal. However, a Cu content of less than 0.05% results in softening and insufficient formation of the fine Pb described above, making it difficult to achieve the desired machinability (tool life, surface roughness, and chip disposal). Therefore, the lower limit of the Cu content is set to 0.05%. On the other hand, if the Cu content exceeds 0.27%, the hardening will shorten the tool life and the surface roughness and chip disposability will deteriorate. In addition, adding an excessive amount will increase the alloy cost, which is economically disadvantageous. Therefore, the Cu content is set to 0.05% or more, preferably 0.06% or more, and 0.27% or less, preferably 0.25% or less.

[0024] <N: 0.0050% or More and 0.0150% or Less> Like C, N is an important element that significantly affects the strength and machinability of steel. However, in free-cutting steels, a N content of less than 0.005% is ineffective in improving strength and softens the steel, resulting in unstable cutting, increased strain on the tool, and shortened tool life. As a result, the surface becomes rough, chips do not break, and processability deteriorates. In addition, denitrification is required in the steelmaking process, increasing costs. On the other hand, a N content exceeding 0.0150% hardens the material itself, and hard nitrides precipitate, increasing tool wear, resulting in poor surface roughness and poor chip processability. Therefore, the N content is set to 0.0050% or more, preferably 0.0060% or more, and 0.0150% or less, preferably 0.0140% or less.

[0025] <Pb: 0.0300% or More and 0.1000% or Less> Pb is an extremely important element for improving machinability, but in the present invention, the Pb content is limited to 0.1000% or less. If the Pb content exceeds 0.1000%, the Pb particles become coarse, and the lubricating effect of the fine Pb particles becomes insufficient. As a result, the formation of built-up edge cannot be suppressed. In addition, the melted lead adheres to the tool surface, softening the tool and increasing wear, which may ultimately lead to deterioration in surface roughness and chip disposability. By limiting the Pb content to 0.1000% or less, these problems can be easily avoided. The Pb content is preferably 0.0900% or less. On the other hand, by adding Pb and optimizing the hot rolling process, it is possible to finely disperse Pb, which not only increases the lubricating effect between the tool and the workpiece during cutting, but also suppresses the formation of built-up edge, thereby significantly improving machinability, including tool wear, dimensional accuracy, surface roughness, and chip disposal. If the Pb content is less than 0.0300%, the above effects are insufficient, so in the present invention, the Pb content is set to 0.0300% or more, and preferably 0.0400% or more.

[0026] <O: More than 0.0100% and 0.0400% or Less> O forms oxides and serves as nuclei for sulfide precipitation. O is also an effective element for suppressing the elongation of sulfides during hot working such as rolling. This action can improve machinability, such as tool wear, surface roughness, and chip disposability. However, when the O content is 0.0100% or less, the effect of suppressing the elongation of sulfides described above is insufficient, and elongated sulfides are formed, making it impossible to expect effects on machinability, such as tool wear, surface roughness, and chip disposability. Furthermore, elongated coarse sulfides are not effective as nuclei for the formation of fine Pb, and the sulfides become coarse, making it difficult to generate fine Pb. On the other hand, when the O content exceeds 0.0400%, not only is the effect of suppressing the elongation of sulfides saturated, but the sulfides also become coarse, making it difficult to generate fine Pb. Furthermore, the amount of oxide-based inclusions increases, hardening the material itself, and abrasive wear due to hard oxides becomes significant, shortening tool life. This significantly deteriorates machinability, such as surface roughness and chip disposability. Therefore, the O content is set to more than 0.0100%, preferably 0.0120% or more, and 0.0400% or less, preferably 0.0350% or less.

[0027] In addition to the basic components described above, the composition may contain the following optional components:

[0028] <One or more elements selected from Si: 0.20% or less, Al: 0.20% or less, and Ti: 0.15% or less (Group A)> Si, Al, and Ti are deoxidizing elements and, by combining with oxygen during cutting, form an oxide film called Bélag on the tool surface. Bélag enhances lubrication between the tool and workpiece and reduces friction, thereby suppressing tool wear. However, even if the content exceeds 0.20% Si, 0.20% Al, and 0.10% Ti, not only does the effect saturate, but the increased amount of oxide in the steel leads to significant abrasive wear and significantly shortens tool life. Therefore, when these elements are contained, the respective contents should be limited to 0.20% Si, 0.20% Al, and 0.15% Ti, and preferably 0.18% Si, 0.18% Al, and 0.10% Ti. The content of Ti is more preferably 0.08% or less. On the other hand, in order to fully obtain the effects of their inclusion, when these elements are contained, it is preferable that the content be Si: 0.001% or more, Al: 0.0001% or more, and Ti: 0.001% or more, respectively.

[0029] <One or more elements selected from Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less (Group B)> Bi, Ca, Se, and Te can all be optionally added when machinability is important. However, if the content of each element exceeds Bi: 0.50%, Ca: 0.02%, Se: 0.15%, or Te: 0.1%, the effect saturates and it is economically disadvantageous. Therefore, if these elements are added, the following limits are set: Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less, respectively. Ca is preferably 0.01% or less, and Se is preferably 0.1% or less. When these elements are contained, in order to fully obtain the effects of their inclusion, it is preferable that the contents of Bi be 0.01% or more, Ca be 0.001% or more, Se be 0.01% or more, and Te be 0.01% or more.

[0030] <One or more selected from Sb: 0.020% or less, Sn: 0.020% or less, Ni: 1.0% or less, and Mo: 1.0% or less (Group C)> Sb, Sn, Ni, and Mo can be added optionally when the scale properties and corrosion resistance after rolling are important. Sb and Sn have the effect of improving descaling properties in shot blasting and pickling before cold wire drawing, and can be added optionally. However, even if Sb and Sn are contained in an amount exceeding 0.020%, the effect of improving descaling properties saturates. Therefore, when these elements are contained, the Sb and Sn contents are set to 0.020% or less and 0.020% or less, respectively, preferably 0.010% or less and 0.010% or less, respectively, and more preferably 0.009% or less and 0.009% or less, respectively. Furthermore, when these elements are contained, in order to fully obtain the effects of their inclusion, it is preferable that the Sb content be 0.003% or more and the Sn content be 0.003% or more, respectively, and more preferably, Sb: 0.005% or more and Sn: 0.005% or more. Ni and Mo have the effect of improving the corrosion resistance of steel and can be added arbitrarily. However, excessive Ni and Mo contents lead to solid solution strengthening of the steel, which increases hardness and shortens tool life during cutting processing. Therefore, when these elements are contained, Ni: 1.0% or less and Mo: 1.0% or less, respectively. When these elements are contained, it is preferable that Ni: 0.001% or more and Mo: 0.001% or more, respectively.

[0031] <One or more elements selected from Nb: 0.050% or less, V: 0.050% or less, Zr: 0.050% or less, W: 0.050% or less, Ta: 0.050% or less, Y: 0.050% or less, Hf: 0.050% or less, and B: 0.050% or less (Group D)> Nb, V, Zr, W, Ta, Y, and Hf form fine precipitates and have the effect of improving the strength of steel. Furthermore, B segregates at grain boundaries to strengthen the grain boundaries, thereby increasing the strength of steel. In particular, in components with high load stress, adding one or more elements selected from the group consisting of Nb, V, Zr, W, Ta, Y, Hf, and B can improve fatigue strength. However, an excessive content of any of these elements reduces the hot workability of the steel, so when these elements are contained, the content of each of V, Zr, W, Ta, Y, Hf, and B is set to 0.050% or less. Furthermore, when these elements are contained, the content of each is preferably 0.0001% or more in order to fully obtain the effects of their inclusion.

[0032] The balance other than the above basic components and optional components is Fe and unavoidable impurities.

[0033] The component composition of the drawn material of the present invention satisfies the following formula (1): 1.50≦[Pb] / [Cu]+[Mn] / 2[S]≦3.00 (1) where [ ] represents the content (mass %) of the element in [ ].

[0034] As mentioned above, the inclusion of Cu in the composition provides moderate work hardening during cutting, which has an advantageous effect on machinability. Furthermore, steels containing an appropriate amount of Cu relative to Pb are prone to strain accumulation during billet rolling, and a relatively large [Pb] / [Cu] ratio is effective for the precipitation of fine MnS. On the other hand, if the [Pb] / [Cu] ratio is too large, strain is not easily introduced during hot rolling, resulting in fewer fine MnS particles and coarsening of Pb particles centered around MnS. Furthermore, while a larger [Mn] / [S] ratio is more advantageous for MnS precipitation, if this value is too large, the MnS particles coarsen and fail to provide a sufficient lubricating effect between the tool and the workpiece. On the other hand, if the [Mn] / [S] ratio is too small, the amount of MnS precipitated is insufficient. For these reasons, the drawn material of the present invention satisfies formula (1). Outside the above range, the number of fine Pb particles is small, which not only increases tool wear and surface roughness but also reduces chip disposability. The component composition preferably satisfies the following formula (1'): 1.70≦[Pb] / [Cu]+[Mn] / 2[S]≦2.70 (1'), where [ ] has the same meaning as above.

[0035] The drawn material of the present invention has a circular equivalent diameter of 0.05 μm to 0.5 μm, and the number of Pb particles is 5,000 / mm ２ With regard to machinability, finely dispersed sulfides and Pb not only ensure a high and uniform lubrication between the tool and the workpiece in peripheral turning and drilling, but also have the effect of suppressing the formation of built-up edges. As a result, tool wear is suppressed and surface roughness and chip disposal are improved. To achieve these effects, it is important that a considerable number of fine Pb particles are distributed, and from this point of view, the drawn material of the present invention has a distribution of Pb particles with a circle equivalent diameter of 0.05 μm to 0.5 μm at a density of 5,000 particles / mm ２ The steel composition is to have a distribution of 6,000 particles / mm2 or more. ２ The upper limit is not particularly limited, but it is preferable that the number of particles is 20,000 or more per mm. ２ It can be:

[0036] The distribution of Pb particles can be measured by the method described in the Examples.

[0037] The steel structure of the drawn material of the present invention is not particularly limited except for the distribution of Pb particulates, and can be any structure. Normally, the drawn material preferably has a ferrite structure that does not contain a large amount of hard carbides. Furthermore, a pearlite structure, in which ferrite and carbides are regularly stacked in layers, is advantageous in terms of machinability because stress and strain caused by cutting are concentrated at the edges of the carbides, making chip breakage more likely.

[0038] The drawn material of the present invention satisfies the formulas (2), (3) and (4). ０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) where, H ０．１ is the Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material, and H ０．５ is the Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material, and Hc is the Vickers hardness (unit: HV) at a position 1 / 2 of the diameter of the drawn material from the surface of the drawn material.

[0039] The positions 0.1 mm and 0.5 mm from the surface of the drawn material on the diameter of the drawn material are positions at depths of 0.1 mm and 0.5 mm, respectively, from the surface in the diameter direction toward the center, and the position 1 / 2 of the diameter of the drawn material from the surface of the drawn material is the center position of the drawn material. ０．１ , H ０．５ and Hc can be measured by the methods described in the Examples.

[0040] When processing drawn materials such as peripheral turning and drilling, the distribution of the Pb particles (Pb particles with a circular equivalent diameter of 0.05 μm to 0.5 μm at a density of 5,000 particles / mm) is a material-side factor that affects the machinability. ２In addition to the lubrication effect and built-up edge suppression effect of the above-mentioned factors, the hardness of the drawn material is also important. To improve the tool life, surface roughness, and chip disposability of the drawn material during cutting, it is necessary to stabilize the depth of cut. To stabilize the depth of cut, in addition to good lubrication between the tool and the workpiece, the drawn material must be moderately hard and have stable sharpness. If the hardness of the drawn material is too soft, the depth of cut will be excessively large, causing the tool to be repeatedly pushed back in reaction, resulting in increased tool wear. This not only shortens tool life, but also leads to inconsistent chip thickness, and deteriorates surface roughness and chip disposability. On the other hand, if the drawn material is too hard, tool wear will increase and the depth of cut will be excessively small, resulting in deteriorated surface roughness and chip disposability. Therefore, the drawn material of the present invention satisfies the above (2), (3), and (4).

[0041] <H ０．１ : Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material>H ０．１ The provision of H is intended to stabilize the depth of cut during peripheral cutting, and if the value is less than 210, the depth of cut becomes excessively large and unstable. ０．１ If the value is 340 or more, the surface is hard, so the depth of cut becomes excessively small, and the initial wear of the tool increases, shortening the tool life, and at the same time, the surface roughness and chip disposal ability deteriorate. ０．１ should satisfy the formula (2), and preferably the formula (2'). ０．１ ≦340 ... (2) 220≦H ０．１ ≦320 (2')

[0042] <H ０．５ : Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material>H ０．５ The provision of H relates to machinability from the initial stage of peripheral turning to the progress of cutting work, and if the value is less than 185, the depth of cut becomes excessively large, shortening the tool life, and at the same time, the surface roughness and chip disposal ability deteriorate. ０．５If the value is 280 or more, the cutting depth becomes excessively small due to the hardness, and in addition, the tool life becomes short, and at the same time, the surface roughness and chip disposability deteriorate. ０．５ satisfies the formula (3), and preferably satisfies the formula (3'). ０．５ ≦280 ... (3) 195≦H ０．５ ≦270 (3')

[0043] <Hc: Vickers hardness (unit: HV) at a position half the diameter of the drawn material from the surface> The Hc specification is intended to ensure stable cutting (e.g., drilling) of the drawn material, particularly the center portion. If the Hc value is less than 150, chips tend to elongate during cutting, resulting in poor chip disposal. In addition, chips tend to get caught between the cutting edge and the workpiece, causing fluctuations in the depth of cut. Furthermore, the chips rub against the surface, shortening tool life and increasing surface roughness. On the other hand, if the Hc value exceeds 240, the chisel edge at the tip of the drill wears during cutting, such as when drilling an internal diameter or a center hole, resulting in increased thrust force and increased core runout. As a result, the tool is more susceptible to wear, and surface roughness and chip disposal deteriorate. Furthermore, tool wear increases during cutting of the center portion, such as when cutting the center portion of end face cutting or cut-off, resulting in poor machinability. Therefore, Hc satisfies the formula (4), and preferably the formula (4'): 150≦Hc≦240 (4) 160≦Hc≦230 (4')

[0044] The cross-sectional shape of the drawn material of the present invention is not particularly limited and can be any shape. For example, the cross section perpendicular to the longitudinal direction may be circular, rectangular, or hexagonal. The size of the cross section of the drawn material of the present invention is not particularly limited, but the cross section perpendicular to the longitudinal direction of the drawn material preferably has a diameter of 45 mm or less if the drawn material is a round material, and a distance to the piece of 45 mm or less if the drawn material is a square material, and more preferably has a diameter or distance to the piece of 40 mm or less.

[0045] The method for producing a free-cutting steel drawn material of the present invention (hereinafter also simply referred to as "the method for producing a drawn material") will now be described. The method for producing a free-cutting steel drawn material of the present invention, particularly a free-cutting steel drawn material for cutting, can be produced by the method for producing a free-cutting steel drawn material of the present invention.

[0046] The method for producing a drawn material of the present invention includes rolling a slab at a heating temperature of 900°C or higher and 1050°C or lower at an area reduction rate of 50% or more to form a billet, rolling the billet at a heating temperature of 1100°C or higher to form a steel bar or wire rod, and then drawing the steel bar or wire rod at an area reduction rate of 1% or more.

[0047] <Cast Slab> The cast slab is not particularly limited as long as it has the component composition (including satisfying formula (1)) described in relation to the drawn material of the present invention and is produced by continuous casting.

[0048] The shape and size of the slab are not particularly limited, and may be, for example, a slab of a known shape and size.

[0049] <Rolling from slab to billet> (Heating temperature of slab: 900°C or more and 1050°C or less) The method for producing a drawn material of the present invention includes rolling a slab to form a billet at a heating temperature of 900°C or more and 1050°C or less. If the heating temperature of the slab is less than 900°C, diffusion of the supersaturated solid solution Mn is difficult to occur, so that fine MnS does not precipitate, and the number density of Pb granules with a circle equivalent diameter of 0.05 μm or more and 0.5 μm or less that are generated using these as nuclei is 5000 pieces / mm ２ Furthermore, if the heating temperature of the slab is not 1050°C or less, the strain introduced by rolling is recovered during rolling, and the number of MnS generated in the strain decreases, so that the fine dispersion of Pb generated with MnS as nuclei becomes insufficient, and the number density of Pb granules having a circle equivalent diameter of 0.05 μm to 0.5 μm does not exceed 5000 pieces / mm ２ Therefore, the heating temperature for rolling the slab is set to 900°C or higher, preferably 930°C or higher, and 1050°C or lower, preferably 1030°C or lower.

[0050] (Area reduction rate of 50% or more in rolling into billet) The method for producing a drawn material of the present invention includes rolling a slab at a heating temperature of 900°C to 1050°C with an area reduction rate of 50% or more to form a billet. If the area reduction rate in rolling from the slab to the billet is not 50% or more, the strain introduced by rolling is small and the supersaturated solid solution Mn is difficult to precipitate finely, resulting in insufficient fine dispersion of Pb that is generated using these as nuclei, and the number density of Pb granules having a circle equivalent diameter of 0.05 μm to 0.5 μm becomes 5000 pieces / mm ２ Therefore, the area reduction rate of the slab is set to 50% or more, and preferably 60% or more. The upper limit of the area reduction rate of the slab is not particularly limited, and can be set to, for example, 90% or less.

[0051] Here, the area reduction rate can be calculated as follows: (1 - (cross-sectional area of ​​billet / cross-sectional area of ​​cast piece)) x 100 (%)

[0052] The shape and size of the billet are not particularly limited, and may be, for example, a billet of a known shape and size.

[0053] <Rolling from billet to steel bar or wire rod> (Heating temperature of billet: 1100°C or higher) The manufacturing method of the present invention involves rolling a billet at a heating temperature of 1100°C or higher to form a steel bar or wire rod. If the heating temperature is not 1100°C or higher, diffusion of the supersaturated dissolved Mn in the hot rolling process is difficult to occur, and fine precipitation of Mn is difficult. As a result, the fine dispersion of Pb generated using these as nuclei becomes insufficient, and the number density of Pb particles having a circle-equivalent diameter of 0.05 μm to 0.5 μm does not reach 5000 particles / mm ２ Therefore, the heating temperature of the billet is set to 1100° C. or higher, and preferably 1120° C. or higher. The heating temperature of the billet can be set to 1250° C. or lower in order to suppress surface oxidation.

[0054] The rolling conditions are not particularly limited as long as they allow forming into a predetermined shape, and for example, known conditions can be adopted.

[0055] The cross-sectional shape of the steel bar is not particularly limited, and may be circular or polygonal, such as hexagonal or square. Normally, in the case of a straight bar, it is finished as a straight steel bar cut to a desired length, and in the case of a coil, it is finished in a shape wound into a coil in a predetermined weight unit. As for the shape of wire rod, the cross-sectional shape is the same as that of steel bar, but it is usually finished in a shape wound into a coil in a predetermined weight unit.

[0056] <Processing into Drawn Material> (Drawing Area Reduction Rate of 1.0% or More) The method for producing a drawn material of the present invention involves drawing a steel bar or wire rod at an area reduction rate of 1.0% or more. If the area reduction rate is less than 1.0%, the drawn material will not undergo sufficient work hardening and will be soft, resulting in unstable cutting during cutting. Therefore, the area reduction rate in drawing is set to 1.0% or more, preferably 5.0% or more. The upper limit of the area reduction rate is not particularly limited and can be, for example, 40.0% or less.

[0057] Here, the area reduction rate can be calculated as follows: (1 - (cross-sectional area of ​​drawn material / cross-sectional area of ​​steel bar or wire rod)) x 100 (%)

[0058] (Steel structure of drawn material) The drawn material obtained by drawing has 5000 Pb particles per mm with a circle equivalent diameter of 0.05 μm or more and 0.5 μm or less. ２ The steel structure has the above distribution. The examples and preferred ranges of the steel structure can be applied to the description of the free-cutting steel drawn material of the present invention.

[0059] (Hardness of Drawn Material) The drawn material obtained by drawing satisfies the following formulas (2), (3), and (4): 210≦H ０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) where H ０．１ , H ０．５ The meaning and preferred range of Hc can be applied to the description of the free-cutting steel drawn material of the present invention.

[0060] (Shape of Drawn Material) The shape and size of the drawn material obtained by drawing can be the same as those described for the free-cutting steel drawn material of the present invention.

[0061] The free-cutting steel drawn material of the present invention and the free-cutting steel drawn material obtained by the method for producing a free-cutting steel drawn material of the present invention are particularly suitable for cutting processes and can be advantageously used in products produced using cutting processes, such products including automobiles, electrical appliances, etc.

[0062] Next, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples in any way.

[0063]

[0064] Steels having the chemical compositions shown in Table 1 were cast in a continuous casting machine, and rolled into billets under the production conditions shown in Table 2, which were then rolled into steel bars. The rolled steel bars were drawn using a wire drawing machine. The drawn materials of the inventive and comparative examples were then subjected to the following tests.

[0065] (Number Density of Pb Granules) To investigate the circle-equivalent diameter and number density of Pb granules in the drawn material, test pieces were taken from a cross section (L cross section) parallel to the longitudinal direction (rolling direction) of the drawn material, and a position at a depth of 1 / 4 of the diameter from the surface of the cross section perpendicular to the longitudinal direction (1 / 4 position) was observed using a scanning electron microscope (SEM). Furthermore, Pb composition analysis was performed using energy dispersive X-ray spectrometry (EDX) at an incident voltage of 15 kV and a current of 30 mA. 50 fields of view were photographed at a magnification of 3000x over an area of ​​300 μm × 400 μm. The observed granules in the obtained SEM images were confirmed to be Pb using EDX, and then binarized by image analysis to determine the circle-equivalent diameter of the granules. The particulate matter having a circle-equivalent diameter of 0.05 μm or more was measured, and the number density of Pb particulate matter having a circle-equivalent diameter of 0.05 μm to 0.5 μm was determined.

[0066] (Hardness Measurement) For a cross section (L cross section) parallel to the longitudinal direction (rolling direction) of the drawn material, the Vickers hardness (unit: HV) was measured at 50 points 0.1 mm from the surface, 0.5 mm from the surface, and at a center 1 / 2 position under a load of 0.05 kgf, and the average value of the obtained Vickers hardness (unit: HV) was calculated and evaluated as the hardness at each depth. Note that in each measurement, the distance between two adjacent indentations was set to 0.5 mm or more.

[0067] (Machinability) Machinability was evaluated by a peripheral turning test and a drilling test. The test methods for each test are shown below.

[0068] (Peripheral turning test method) A BNC-34C5 manufactured by Citizen Machinery was used as the cutting machine, a carbide EX35 tool TNGG160404R-N manufactured by Hitachi Tool Engineering was used as the turning insert, and a DTGNR2020 manufactured by Kyocera was used as the holder. A 15-fold diluted emulsion of Yushiroken FGE283PR, a water-soluble cutting tool manufactured by Yushiro Chemical, was used as the lubricant. The cutting conditions were a cutting speed of 100 m / min, a feed rate of 0.05 mm / rev, a cutting depth of 2.0 mm, and a machining length of 10 m.

[0069] (Drilling test method) A Mori Seiki NV5000α1 cutting machine was used, and a NACHI SD5.0 drill with a diameter of 3 mm was used. A 25-fold diluted emulsion of Yushiroken FGR283PR, a water-soluble cutting material manufactured by Yushiro Chemical Co., Ltd., was used as the lubricant. The cutting conditions were a cutting speed of 60 m / min, a rotation speed of 3822 rpm, a feed rate of 0.2 mm / rev, a feed rate of 0.05 mm / rev, a feed rate of 750 mm / min, and a drilling depth of 20 mm per hole.

[0070] (Tool life in peripheral turning test) The tool life in the peripheral turning test was evaluated based on the flank wear Vb of the turning tip after cutting a length of 10 m. When Vb was 200 μm or less, it was rated as "good," and when it was more than 200 μm, it was rated as "poor." The results are shown in Table 2.

[0071] (Tool life in drilling test) The tool life in the drilling test was measured by the flank wear Vb of the drill after the 100th hole was drilled. When Vb was 30 μm or less after the drilling test, it was evaluated as "good," and when Vb was more than 30 μm, it was evaluated as "poor." The results are shown in Table 2.

[0072] (Surface roughness in peripheral turning test) The surface roughness after cutting in the peripheral turning test was evaluated based on the results of measuring the 10-point average roughness Rz (JIS B0601) using a stylus roughness tester over a 10 mm range immediately before the end of cutting after cutting the drawn material to a length of 2 m. The reference length in the measurement was 5 mm. Therefore, in Table 2, if the 10-point average roughness Rz was 20 μm or less, it was evaluated as "pass," and if Rz was more than 20 μm, it was evaluated as "fail." The results are shown in Table 2.

[0073] (Surface roughness in drill hole drilling test) The surface roughness after cutting in the drill hole drilling test was evaluated based on the results of measuring the 10-point average roughness Rz (JIS B0601) using a stylus roughness tester over a range of 10 mm deep immediately after drilling, after cutting up to 100 holes in the drawn material. The reference length in the measurement was 5 mm. Therefore, in Table 2, if the 10-point average roughness Rz was 50 μm or less, it was evaluated as "pass," and if Rz was more than 50 μm, it was evaluated as "fail." The results are shown in Table 2.

[0074] (Chip Disposability in Peripheral Turning Test) The chip disposability in the peripheral turning test was evaluated based on the shape of the chips in the section from 1.9 m to 2.0 m when cutting a drawn material over a length of 2 m. If the chips were finely broken up, it was judged that the chip disposability was excellent. Therefore, if the chip length was 2.0 mm or less, it was rated as "good," and if the chips exceeded 2.0 mm or had a curled shape of one or more turns, it was rated as "poor." The results are shown in Table 2.

[0075] (Chip Disposability in Drilling Test) The chip disposability in the drilling test was evaluated based on the shape of the chips when 100 holes were drilled into the drawn material. If the chips were finely broken, it was determined that the chip disposability was excellent. Therefore, if the chip length was 5.0 mm or less, it was rated as "good," and if the chips exceeded 5.0 mm or had a curled shape of one or more turns, it was rated as "poor." The results are shown in Table 2.

[0076]

[0077] Table 2 shows the evaluations of tool wear, surface roughness, and chip disposal in peripheral turning tests and drilling tests for the invention examples and comparative examples. The invention examples have better machinability than the comparative examples.

[0078] According to the present invention, a low-cost, high-quality free-cutting steel drawn material having machinability equal to or better than that of low-carbon sulfur-lead composite free-cutting steel can be provided together with a manufacturing method thereof, while the amount of Pb added, an element for improving machinability, is significantly reduced from the amount added conventionally added, and the present invention is highly industrially useful.

Claims

1. A steel sheet containing, by mass%, C: 0.03% or more and 0.09% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or more and 0.100% or less, S: 0.250% or more and 0.500% or less, Cr: 0.05% or more and 1.50% or less, Cu: 0.05% or more and 0.27% or less, N: 0.0050% or more and 0.0150% or less, Pb: 0.0300% or more and 0.1000% or less, and O: more than 0.0100% and 0.0400% or less, with the balance being Fe and unavoidable impurities, and having a component composition that satisfies the following formula (1), and having a circular equivalent diameter of 0.05 μm or more and 0.5 μm or less and a density of 5,000 particles / mm ２ A free-cutting drawn steel material having a steel structure in which Pb and Cu are distributed in a range of 1.50 to 3.00, and satisfying the following formulas (2), (3), and (4): 1.50≦[Pb] / [Cu]+[Mn] / 2[S]≦3.00 (1) 210≦H ０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) Here, [ ] indicates the content (unit mass%) of the element in [ ], H ０．１ is the Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material, and H ０．５ is the Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material, and Hc is the Vickers hardness (unit: HV) at a position 1 / 2 of the diameter of the drawn material from the surface of the drawn material.

2. The free-cutting steel drawn material according to claim 1, wherein the composition further contains at least one component selected from the following groups A to D: Group A: By mass%, one or more selected from Si: 0.20% or less, Al: 0.20% or less, and Ti: 0.15% or less. Group B: By mass%, one or more selected from Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less. Group C: By mass%, one or more selected from Sb: 0.020% or less, Sn: 0.020% or less, Ni: 1.0% or less, and Mo: 1.0% or less. Group D: In mass%, one or more selected from Nb: 0.050% or less, V: 0.050% or less, Zr: 0.050% or less, W: 0.050% or less, Ta: 0.050% or less, Y: 0.050% or less, Hf: 0.050% or less, and B: 0.050% or less.

3. In mass%, C: 0.03% or more and 0.09% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or more and 0.100% or less, S: 0.250% or more and 0.500% or less, Cr: 0.05% or more and 1.50% or less, Cu: 0.05% or more and 0.27% or less, N: 0.0050% or more and 0.0150% or less, Pb: 0.0300% or more and 0.1000% or less, and A method for producing a steel sheet comprising rolling a slab produced by continuous casting at a heating temperature of 900°C or more and 1050°C or less at an area reduction rate of 50% or more into a billet, rolling the billet at a heating temperature of 1100°C or more into a steel bar or wire rod, and then drawing the bar or wire rod at an area reduction rate of 1.0% or more, wherein the steel sheet contains 5000 Pb granules per mm2 having an equivalent circle diameter of 0.05 μm or more and 0.5 μm or less. ２ A method for producing a free-cutting drawn steel material having a steel structure with the above distribution and satisfying the following formulas (2), (3), and (4): 1.50≦[Pb] / [Cu]+[Mn] / 2[S]≦3.00 (1) 210≦H ０．１ ≦340 ... (2) 185≦H ０．５ ≦280 (3) 150≦Hc≦240 (4) Here, [ ] indicates the content (unit mass%) of the element in [ ], H ０．１ is the Vickers hardness (unit: HV) at a position 0.1 mm from the surface of the drawn material to the diameter of the drawn material, and H ０．５ is the Vickers hardness (unit: HV) at a position 0.5 mm from the surface of the drawn material to the diameter of the drawn material, and Hc is the Vickers hardness (unit: HV) at a position 1 / 2 of the diameter of the drawn material from the surface of the drawn material.

4. A method for producing a free-cutting steel drawn material according to claim 3, wherein the component composition further contains at least one component selected from the following groups A to D: Group A: By mass%, one or more selected from Si: 0.20% or less, Al: 0.20% or less, and Ti: 0.15% or less. Group B: By mass%, one or more selected from Bi: 0.50% or less, Ca: 0.02% or less, Se: 0.15% or less, and Te: 0.1% or less. Group C: By mass%, one or more selected from Sb: 0.020% or less, Sn: 0.020% or less, Ni: 1.0% or less, and Mo: 1.0% or less. Group D: In mass%, one or more selected from Nb: 0.050% or less, V: 0.050% or less, Zr: 0.050% or less, W: 0.050% or less, Ta: 0.050% or less, Y: 0.050% or less, Hf: 0.050% or less, and B: 0.050% or less.

Images