Cold-working structural steel wire for machinery and method for producing the same

Abstract

To provide a cold-working structural steel wire for machinery having both cold forgeability and machinability superior to those of conventional ones, and a method for producing the same.SOLUTION: A cold-working structural steel wire for machinery having a predetermined chemical composition, the steel structure comprising ferrite, carbide, and inclusions, wherein the average major-axis length of the inclusions is 30 μm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a steel wire for cold working machine structural use and a method for producing the same. [Background technology]

[0002] When manufacturing various mechanical structural parts such as automobile parts and construction machinery parts, spheroidizing annealing is usually performed on steel bars including hot-rolled wire rods in order to impart cold workability. The steel wire obtained by spheroidizing annealing is then subjected to cold working such as cold forging, and then mechanical working such as cutting. Therefore, the steel wire is required to have high cold workability such as cold forgeability and machinability. For example, Patent Document 1 describes a steel wire having a predetermined chemical composition, satisfying formulas (1) and (2) regarding sulfides, and containing sulfides with a circle equivalent diameter of 1.0 to 10.0 μm in the metal structure at a density of 1200 / mm 2 and wherein the average distance between the sulfides is less than 30.0 μm.

[0003] Patent Document 2 discloses a steel for cold forging that satisfies predetermined chemical compositions, satisfies formula (1) and formula (2) regarding sulfides, and has an average distance between sulfides in the steel of less than 30.0 μm. Patent Document 3 discloses a steel for cold forging that has a chemical composition that satisfies formula (1) that defines the range of Ca / S, and has a number density of sulfides with a circle equivalent diameter of less than 2 μm of 300 / mm in a cross section parallel to the rolling direction. 2 Furthermore, Patent Document 4 discloses a free-cutting steel for cold forging that satisfies predetermined chemical compositions and also satisfies formulas (1) to (3) regarding sulfide-based inclusions, Te, S, Ti, and N. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 061191 [Patent Document 2] Japanese Patent Application Publication No. 2018-035411 [Patent Document 3] Japanese Patent Application Publication No. 2017-193767 [Patent Document 4] Japanese Patent Application Laid-Open No. 2013-067833 Summary of the Invention [Problem to be solved by the invention]

[0005] As in Patent Document 1 and the like, cold forgeability and machinability have been conventionally studied, but in recent years, there has been a demand for a steel wire for cold-worked machine structures that exhibits both excellent machinability and higher cold forgeability. The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a steel wire for cold-worked machine structures that has both better cold forgeability and better machinability than conventional steel wires, and a manufacturing method thereof.

[0006] In this specification, "wire rod" and "steel bar" refer to steel material in the form of a line or a bar obtained by hot rolling, respectively, that has not been subjected to heat treatment such as spheroidizing annealing or wire drawing. Also, "steel wire" refers to wire rod or steel bar that has been subjected to at least one of heat treatment such as spheroidizing annealing and wire drawing. In this specification, the above-mentioned wire rod, steel bar, and steel wire are collectively referred to as "steel section." [Means for solving the problem]

[0007] Aspect 1 of the present invention is The chemical composition is C: 0.05% by mass to 0.50% by mass, Si: 0.005% by mass to 0.50% by mass, Mn: 0.30% by mass to 1.20% by mass, P: more than 0% by mass, 0.050% by mass or less, S: 0.010% by mass to 0.050% by mass, Al: 0.001% by mass to 0.10% by mass, Cr: more than 0 mass%, 1.50 mass% or less, Cu: 0% by mass to 0.25% by mass, Ni: 0% by mass to 0.25% by mass, Mo: 0% by mass to 0.50% by mass, B: 0% by mass to 0.01% by mass, Ti: 0% by mass to 0.20% by mass, Nb: 0% by mass to 0.20% by mass, V:0 mass%~0.50 mass%, N: 0% by mass to 0.010% by mass, Mg: 0% by mass ~ 0.020% by mass, Ca: 0% by mass to 0.050% by mass, Li: 0% by mass to 0.020% by mass, and REM: 0% by mass ~ 0.050% by mass and the balance consisting of Fe and inevitable impurities, The steel wire for cold working machine structural use has a steel structure containing ferrite, carbides, and inclusions, the average major axis diameter of which is 30 μm or less.

[0008] Aspect 2 of the present invention is A steel wire for cold working machine structural use according to aspect 1, wherein the carbides have an average aspect ratio of 3.0 or less, and a ratio expressed as (average circular equivalent diameter of carbides / average circular equivalent diameter of inclusions) is 0.46 or more.

[0009] Aspect 3 of the present invention is A method for producing a steel wire for cold working machine structural use according to aspect 1 or 2, A steel material having the chemical composition described in aspect 1 is hot worked at a strain of 2.5 or more and a temperature of 995°C or more; After the hot working, spheroidizing annealing is performed, which includes the following steps (1) to (3) in this order: The present invention relates to a method for producing a steel wire for cold working machine structural use, comprising the steps of: (1) Heat to a temperature T1 of (A1+5°C to A1+60°C) and maintain at that temperature T1 for 1 to 6 hours. (2) A cooling-heating step is carried out one or more times, in which the material is cooled from the temperature T1 to a temperature T2 (A1-10°C to A1-100°C) at an average cooling rate CR1 of 25°C / hour to 100°C / hour, and then reheated from the temperature T2 to the temperature T1. (3) Cool from temperature T1 to temperature T3 (A1+5°C to A1-10°C) at an average cooling rate CR2 of 25°C / hour or more, and then cool from temperature T3 to temperature T4 (A1-20°C to A1-80°C) at an average cooling rate CR3 of 15°C / hour or less. Here, A1 is calculated by the following formula (1). A1(℃)=723-10.7×(%Mn)-16.9×(%Ni)+29.1×(%Si)+16.9×(%Cr)+290×(%As)+6.38×(%W)...(1) Here, (% element) represents the content (mass%) of each element, and the content of elements that are not included is zero. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a steel wire for cold working machine structural use that has both better cold forgeability and better machinability than conventional steel wires, and a method for manufacturing the same. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating the spheroidizing annealing conditions (a) performed in the examples. [Figure 2] 1 is a diagram illustrating the spheroidizing annealing conditions (b) performed in the examples. [Figure 3] 1 is a diagram illustrating the spheroidizing annealing conditions (c) performed in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have conducted extensive research to realize a steel wire for cold-worked machine structural use (hereinafter, sometimes simply referred to as "steel wire") that combines better cold forgeability and machinability than conventional steel wires, and a manufacturing method thereof. Specifically, in the past, increasing the amount of S added to increase inclusions (mainly MnS, a manganese-based sulfide) has been used to ensure machinability, but this has the problem of easily deteriorating cold forgeability. Therefore, the present inventors conducted extensive research to ensure both machinability and excellent cold forgeability, focusing particularly on the steel wire microstructure and its manufacturing conditions. They found that controlling the shapes of both inclusions (mainly MnS, a manganese-based sulfide) and carbides in the steel wire is effective, and that controlling both the shapes of the inclusions and carbides is effective by controlling both the hot rolling conditions and the spheroidizing annealing conditions in the manufacturing method of a steel wire for cold-worked machine structural use. Hereinafter, the steel wire microstructure will first be described.

[0013] 1.Steel structure The steel structure of the steel wire of the present disclosure includes ferrite, carbides, and inclusions. The ferrite may account for, for example, 90 area % or more, further 95 area % or more, and even 99 area % or more. In addition to the ferrite, carbides, and inclusions, the steel structure of the steel wire of the present disclosure may include 1 area % or less of austenite.

[0014] (Average major axis diameter of inclusions: 30 μm or less) In the steel structure of the steel wire of the present disclosure, the average major axis diameter of the inclusions is 30 μm or less. By keeping the average major axis diameter of the inclusions at 30 μm or less, it is possible to suppress the generation of voids during cold forging and the subsequent propagation of cracks, thereby improving the critical compression ratio for cracking, i.e., enhancing cold forgeability. The average major axis diameter of the inclusions is preferably 28 μm or less, more preferably 25 μm or less. The average major axis diameter of the inclusions is determined by the method described in the Examples below.

[0015] In this specification, "inclusions" refers to non-metallic inclusions other than carbides, such as sulfides, oxides, nitrides, or composite inclusions thereof. The steel wire of the present disclosure has a relatively high S content, and therefore has many sulfides as inclusions, and furthermore, MnS may be the main inclusion.

[0016] (Average aspect ratio of carbide: 3.0 or less) In the steel wire of the present disclosure, the average aspect ratio of the carbides is preferably kept to 3.0 or less, thereby suppressing the generation of voids during cold forging and the subsequent propagation of cracks, and improving the critical compression ratio for cracking. The average aspect ratio of the carbides is more preferably 2.7 or less, and even more preferably 2.5 or less. The average aspect ratio of the carbides is determined by the method described in the examples below.

[0017] In this specification, carbides include cementite, Cr carbide, Ti carbide, Nb carbide, V carbide, and the like.

[0018] (ratio expressed as (average circular equivalent diameter of carbides / average circular equivalent diameter of inclusions): 0.46 or more) In the steel wire of the present disclosure, cracking can also be suppressed by increasing (approaching 1) the ratio (hereinafter sometimes referred to as (carbide size / inclusion size)) expressed as (average circular equivalent diameter of carbides / average circular equivalent diameter of inclusions). The assumed mechanism is that when inclusions are large and carbides are small, voids and stress concentration tend to occur early, originating from the inclusions, resulting in a lower cracking threshold. However, by making the sizes of the two closer together, it is thought that the void generation and stress concentration can be suppressed. The (carbide size / inclusion size) ratio is preferably 0.49 or more, more preferably 0.55 or more, and the larger (approaching 1), the better.

[0019] 2.Chemical composition The chemical composition of the steel wire according to the present disclosure will be described below. First, the basic elements C, Si, Mn, P, S, Al, and Cr will be described, and then elements that may be selectively added will be described.

[0020] C: 0.05% by mass ~ 0.50% by mass C is an element that governs the strength of steel materials, and the higher the content, the higher the strength after quenching and tempering. To effectively exert the above effects, the lower limit of the C content is set to 0.05 mass%. The C content is preferably 0.10 mass% or more, more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. However, if the C content is excessive, the number of spheroidized cementite in the structure after spheroidizing annealing will be excessive, increasing hardness and reducing cold workability. Therefore, the upper limit of the C content is set to 0.50 mass%.

[0021] Si:0.005 mass%~0.50 mass% Si is used as a deoxidizer during melting and also contributes to improving strength. To effectively exert this effect, the lower limit of the Si content is set to 0.005 mass%. The Si content is preferably 0.010 mass% or more, more preferably 0.050 mass% or more, and even more preferably 0.10 mass% or more. However, Si contributes to solid solution strengthening of ferrite and has the effect of significantly increasing strength after spheroidizing annealing. If the Si content is excessive, the above effect deteriorates cold workability, so the upper limit of the Si content is set to 0.50 mass%. The Si content is preferably 0.40 mass% or less, more preferably 0.35 mass% or less.

[0022] Mn:0.30 mass%~1.20 mass% Mn is an element that effectively acts as a deoxidizer and also contributes to improving hardenability. To fully exert this effect, the lower limit of the Mn content is set to 0.30 mass%. The Mn content is preferably 0.35 mass% or more, and more preferably 0.40 mass% or more. However, if the Mn content is excessive, segregation tends to occur and toughness decreases. Therefore, the upper limit of the Mn content is set to 1.20 mass%. The Mn content is preferably 1.10 mass% or less, and more preferably 1.00 mass% or less.

[0023] P: More than 0% by mass, 0.050% by mass or less P (phosphorus) is an inevitable impurity and a harmful element that causes grain boundary segregation in steel, adversely affecting forgeability and toughness. Therefore, the P content is set to 0.050% by mass or less. The P content is preferably 0.030% by mass or less, and more preferably 0.020% by mass or less. The lower the P content, the better, but it can usually be 0.001% by mass or more.

[0024] S: 0.010 mass% to 0.050 mass% or less S (sulfur) is an element that forms MnS in steel and contributes to improving machinability. Therefore, the S content must be 0.010% by mass or more. On the other hand, excessive S content leads to a decrease in cold forgeability as described above. Therefore, the S content is set to 0.050% by mass or less. The S content is preferably 0.040% by mass or less, and more preferably 0.030% by mass or less.

[0025] Al: 0.001 mass%~0.10 mass% Al is an element contained as a deoxidizer, and has the effect of reducing impurities as a result of deoxidation. To achieve this effect, the lower limit of the Al content is set to 0.001% by mass. The Al content is preferably 0.005% by mass or more, and more preferably 0.010% by mass or more. However, if the Al content is excessive, non-metallic inclusions increase and toughness decreases. Therefore, the upper limit of the Al content is set to 0.10% by mass. The Al content is preferably 0.080% by mass or less, and more preferably 0.050% by mass or less.

[0026] Cr: More than 0 mass%, 1.50 mass% or less Cr is an element that improves the hardenability and strength of steel and also promotes the spheroidization of cementite. Specifically, Cr dissolves in cementite and delays its dissolution during heating in spheroidizing annealing. When some cementite remains undissolved during heating, rod-shaped cementite with a large aspect ratio is less likely to form during cooling, making it easier to obtain a spheroidized structure. Therefore, the Cr content is set to more than 0 mass%, preferably 0.01 mass% or more. It may also be set to 0.05 mass% or more, or even 0.10 mass% or more. From the perspective of further promoting the spheroidization of cementite, it can be set to more than 0.30 mass% or even more than 0.50 mass%. An excessive Cr content retards the diffusion of carbon-containing elements, unnecessarily delaying the dissolution of cementite, making it difficult to obtain a spheroidized structure. As a result, the hardness reduction effect of the present disclosure may be reduced. Therefore, the Cr content is 1.50 mass% or less, preferably 1.40 mass% or less, and more preferably 1.25 mass% or less. From the viewpoint of accelerating the diffusion of elements, the Cr content can be further set to 1.00 mass% or less, further 0.80 mass% or less, or further 0.30 mass% or less.

[0027] The steel wire for cold working machine structural use (steel wire for machine structural parts) according to this embodiment only needs to contain the above elements in its chemical composition. Desired properties can be achieved even if the optional elements (selected elements) described below are not contained. By including these elements together with the above elements as needed, it is possible to more easily ensure hardenability and the like. The optional elements will be described below.

[0028] Cu: 0 mass% to 0.25 mass%, Ni: 0 mass% to 0.25 mass%, Mo: 0 mass% to 0.50 mass%, B: 0 mass% to 0.01 mass% Cu, Ni, Mo, and B are all elements effective in improving the hardenability of steel materials and thereby increasing the strength of final products. If necessary, one or more elements selected from the group consisting of Cu, Ni, Mo, and B may be contained. The effects of these elements become greater as their contents increase. To effectively exert the above effects, the lower limits of Cu, Ni, and Mo are preferably greater than 0 mass%, more preferably 0.02 mass% or more, and even more preferably 0.05 mass% or more for each of Cu, Ni, and Mo, and greater than 0 mass%, more preferably 0.0003 mass% or more, and even more preferably 0.0005 mass% or more for B.

[0029] On the other hand, excessive contents of these elements may result in excessively high strength and deteriorate cold workability, so the preferred upper limits for each element have been set as above. The contents of Cu and Ni are each preferably 0.22% by mass or less, and even more preferably 0.20% by mass or less, the content of Mo is more preferably 0.40% by mass or less, and even more preferably 0.35% by mass or less, and the content of B is more preferably 0.0070% by mass or less, and even more preferably 0.0050% by mass or less.

[0030] Ti: 0 mass% to 0.20 mass%, Nb: 0 mass% to 0.20 mass%, V: 0 mass% to 0.50 mass% Ti, Nb, and V form compounds with N, reducing the amount of solute N and thereby reducing deformation resistance. Therefore, if necessary, one or more elements selected from the group consisting of Ti, Nb, and V may be contained. The effects of these elements increase as their contents increase. For each element, the lower limit for effectively achieving the above effects is preferably greater than 0% by mass, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. However, if the contents of these elements are excessive, the compounds formed may increase deformation resistance, which may actually reduce cold workability. Therefore, the contents of Ti and Nb are preferably 0.20% by mass or less, and the V content is preferably 0.50% by mass or less. The contents of Ti and Nb are more preferably 0.18% by mass or less, even more preferably 0.15% by mass or less, and the V content is more preferably 0.45% by mass or less, even more preferably 0.40% by mass or less.

[0031] N: 0 mass% to 0.010 mass%, Mg: 0 mass% to 0.020 mass%, Ca: 0 mass% to 0.050 mass%, Li: 0 mass% to 0.020 mass%, and REM: 0 mass% to 0.050 mass% Mg, Ca, Li, and REM are elements effective in spheroidizing sulfide compound inclusions such as MnS and improving the deformability of steel. Therefore, if necessary, one or more elements selected from the group consisting of Mg, Ca, Li, and REM may be contained. This effect increases with increasing content. To effectively exert the above-mentioned effects, the contents of Mg, Ca, Li, and REM are each preferably greater than 0% by mass, more preferably 0.0001% by mass or more, and even more preferably 0.0005% by mass or more. However, even if they are contained in excess, the effect saturates and no effect commensurate with the content can be expected. Therefore, the contents of Mg and Li are each preferably 0.020% by mass or less, more preferably 0.018% by mass or less, and even more preferably 0.015% by mass or less. The contents of Ca and REM are each preferably 0.050% by mass or less, more preferably 0.045% by mass or less, and even more preferably 0.040% by mass or less. Furthermore, N is an element that is inevitably contained in steel, and the presence of solute N in steel leads to an increase in hardness and a decrease in ductility due to strain aging, thereby deteriorating cold workability. Therefore, the N content is preferably set to 0.010 mass% or less. The N content is more preferably 0.008 mass% or less, and even more preferably 0.005 mass% or less. The term REM includes lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0032] [Remainder: Fe and unavoidable impurities] In a preferred embodiment, the balance is Fe and unavoidable impurities. The unavoidable impurities include trace elements (e.g., As, Sb, Sn, etc.) that are introduced due to the conditions of raw materials, materials, manufacturing facilities, etc. Note that, for example, there are elements such as P, whose content is usually the lower the better, and therefore they are unavoidable impurities, but whose composition ranges are separately specified as above. In this specification, the "unavoidable impurities" that make up the balance of the [balance: Fe and unavoidable impurities] conceptually exclude the element (P) whose composition range is separately specified, as well as Cr and optional elements at the unavoidable impurity level described below. Cr<0.01 mass% Cu<0.02 mass% Ni<0.02 mass% Mo<0.02 mass% B<0.0003 mass% Ti<0.03 mass% Nb<0.03 mass% V<0.03 mass% N<0.0001 mass% Mg<0.0001 mass% Ca<0.0001 mass% Li<0.0001 mass% REM<0.0001 mass%

[0033] 3. Manufacturing method In order to obtain the metallographic structure of the steel wire for mechanical structural components of the present disclosure, it is preferable to appropriately control the hot rolling conditions and the spheroidizing annealing conditions for producing the wire rod or steel bar to be subjected to spheroidizing annealing as described below when producing the steel wire for mechanical structural components. The diameter of the wire rod, steel wire, or steel bar, which is the steel section to be subjected to spheroidizing annealing, is not particularly limited, and is, for example, 5.5 mm to 60 mm for wire rod and steel wire, and, for example, 18 mm to 105 mm for steel bar.

[0034] The method for producing a steel wire for cold working machine structural use according to the present disclosure includes: Hot working the steel material having the above chemical composition at a strain of 2.5 or more and a temperature of 995°C or more; After the hot working, spheroidizing annealing is performed, which includes the following steps (1) to (3) in this order: This includes. In order to shorten the major axis diameter of inclusions and to increase the carbide size / inclusion size ratio, it is effective to perform hot working under the following conditions and to perform spheroidizing annealing under the following conditions. Also, in order to shorten the aspect ratio of carbides, it is effective to perform spheroidizing annealing under the following conditions. Furthermore, as will be described later, in the cooling-heating step, cooling at a relatively fast cooling rate for spheroidizing annealing and reheating, particularly cooling at an average cooling rate of 25°C / hour to 100°C / hour, contributes to reducing the major axis diameter of inclusions and the aspect ratio of carbides. Each manufacturing condition will be explained below. (1) Heat to a temperature T1 of (A1+5°C to A1+60°C) and maintain at that temperature T1 for 1 to 6 hours. (2) A cooling-heating step is carried out one or more times, in which the material is cooled from the temperature T1 to a temperature T2 (A1-10°C to A1-100°C) at an average cooling rate CR1 of 25°C / hour to 100°C / hour, and then reheated from the temperature T2 to the temperature T1. (3) Cool from temperature T1 to temperature T3 (A1+5°C to A1-10°C) at an average cooling rate CR2 of 25°C / hour or more, and then cool from temperature T3 to temperature T4 (A1-20°C to A1-80°C) at an average cooling rate CR3 of 15°C / hour or less. Here, A1 is calculated by the following formula (1). A1(℃)=723-10.7×(%Mn)-16.9×(%Ni)+29.1×(%Si)+16.9×(%Cr)+290×(%As)+6.38×(%W)...(1) Here, (% element) represents the content (mass%) of each element, and the content of elements that are not included is zero.

[0035] [Steel material of the above chemical composition is hot worked at a strain of 2.5 or more and a temperature of 995°C or more] In the present disclosure, hot working is performed at a high temperature of 995°C or higher with a strain of 2.5 or more. This shortens the major axis diameter of inclusions and increases the carbide size / inclusion size ratio. Although the detailed mechanism is unclear, it is believed that applying a high strain at a high temperature promotes the fragmentation and spheroidization of inclusions. The strain is preferably 3.0 or more. While a higher strain is preferable, the effect saturates if it is too high, so the upper limit is approximately 10.0. The heating temperature for the hot working can further be 1000°C or higher. However, the effect saturates if it is too high, so the upper limit is approximately 1300°C. The above temperature can be, for example, the temperature at the inlet of the finish rolling mill. Examples of hot working include hot rolling and hot forging.

[0036] [(1) Heat to a temperature T1 of (A1+5°C to A1+60°C) and maintain at that temperature T1 for 1 to 6 hours] Heating to a temperature (T1) of (A1+5°C to A1+60°C) promotes the dissolution of rod-shaped cementite, which has a large aspect ratio and a large amount of interfacial strain and was formed during the rolling stage. If the temperature T1 is too low, the rod-shaped cementite, which has a large amount of interfacial strain, does not dissolve during heating and remains in the ferrite, increasing hardness and deteriorating cold workability. To obtain a sufficiently softened steel wire, the temperature T1 must be A1+5°C or higher. The temperature T1 is preferably A1+10°C or higher, and more preferably A1+20°C or higher. On the other hand, if the temperature T1 is too high, the crystal grains become too coarse, making it difficult for spheroidal cementite to precipitate at the ferrite grain boundaries during the cooling process in the subsequent step. This increases the amount of rod-shaped cementite, increasing hardness and deteriorating cold workability. For this reason, the temperature T1 is set to A1+60°C or lower. The temperature T1 is preferably A1+50°C or lower, and more preferably A1+40°C or lower.

[0037] Furthermore, if the heating holding time (t1a) at temperature T1 is too short, rod-shaped cementite with a large amount of interfacial strain remains within the ferrite grains, increasing hardness and reducing cold workability. To obtain a steel wire that is sufficiently softened, the heating holding time (t1a) must be 1 hour or more and 6 hours or less. A preferred heating holding time (t1a) is 1.5 hours or more, and more preferably 2.0 hours or more. If the heating holding time (t1a) is too long, the heat treatment time becomes longer and productivity decreases. Therefore, the heating holding time (t1a) is 6 hours or less, preferably 5 hours or less, and more preferably 4 hours or less. Note that the average heating rate during heating to a temperature (T1) of (A1 + 5°C) to (A1 + 60°C) does not affect the steel material properties, so the temperature may be raised at any rate. For example, the temperature may be raised at a rate of 30°C / hour to 100°C / hour.

[0038] The temperature at point A1 is calculated using the following formula (1) described in "Lectures on Modern Metallurgy, Materials, Vol. 4, Steel Materials" (edited and published by the Japan Institute of Metals, sold by Maruzen). A1(℃)=723-10.7×(%Mn)-16.9×(%Ni)+29.1×(%Si)+16.9×(%Cr)+290×(%As)+6.38×(%W)...(1) Here, (% element) represents the content (mass%) of each element, and the content of elements that are not included is zero.

[0039] [(2) cooling from the temperature T1 to a temperature T2 of (A1-10°C to A1-100°C) at an average cooling rate CR1 of 25°C / hour to 100°C / hour, and then heating again from the temperature T2 to the temperature T1, a cooling-heating step being carried out one or more times] Among the manufacturing methods disclosed herein, the cooling-heating step, in which the steel is cooled at a relatively high average cooling rate CR1 of 25°C / hr to 100°C / hr and then reheated, is believed to promote the decomposition of inclusions and carbides. While the details of this mechanism are unclear, it is believed that cooling at a relatively high rate introduces strain into the steel structure and refines the crystal grains, thereby promoting the decomposition of inclusions and carbides during reheating. To achieve this effect, the cooling-heating step is performed at least once. Repeated cooling-heating steps can further promote the decomposition of inclusions and carbides, so the cooling-heating step is preferably performed at least twice, more preferably at least five times. Performing multiple cooling-heating steps can also introduce strain over a wide area within the steel, which is believed to further promote the decomposition of the inclusions and carbides. From the perspective of saturation of the effect, the cooling-heating step is preferably performed 20 times or less.

[0040] The temperature T2 is set in the range of (A1-10°C to A1-100°C). To obtain the effect of introducing strain into the steel structure (thereby breaking up carbides and inclusions) by cooling at a relatively high average cooling rate CR1 of 25°C / hour or more, the temperature T2 must be set to A1-10°C or less. The temperature T2 is preferably A1-20°C or less. On the other hand, if the temperature T2 is too low, the above effect will saturate, so the temperature T2 is set to A1-100°C or more.

[0041] The average cooling rate CR1 is preferably 30°C / hour or more, more preferably 60°C / hour or more. After cooling from temperature T1 to temperature T2, the material may optionally be held at temperature T2. The holding time at temperature T2 may be set arbitrarily, and is preferably, for example, 1 hour or less. Furthermore, after reheating from temperature T2 to temperature T1, the material may optionally be held at temperature T1. The holding time (t1b) at temperature T1 may be set arbitrarily, and is, for example, 1 hour or more, and is preferably more than 1 hour and 6 hours or less. In this case, the holding time (t1b) at temperature T1 may be the same as or different from the holding time (t1a).

[0042] When the cooling-heating step is carried out one or more times, each of the multiple T1s, multiple T2s, and multiple CR2s may be the same or different within the specified range.

[0043] [(3) Cooling from the temperature T1 to a temperature T3 of (A1+5°C to A1-10°C) at an average cooling rate CR2 of 25°C / hour or more, and then cooling from the temperature T3 to a temperature T4 of (A1-20°C to A1-80°C) at an average cooling rate CR3 of 15°C / hour or less] Cooling at a relatively fast rate followed by further slow cooling is effective in reducing the aspect ratio of carbides. Although the details of the mechanism are unclear, it is believed that cooling at a relatively fast rate up to temperature T3, where carbides substantially precipitate, followed by slow cooling allows carbides to precipitate uniformly within the structure and then grow, thereby suppressing the formation of carbides with high aspect ratios. The average cooling rate CR2 is preferably 30°C / hour or more, with an upper limit of approximately 200°C / hour. The average cooling rate CR3 is preferably 12°C / hour or less, with a lower limit of approximately 5°C / hour. The temperature T4 is preferably A1-60°C or more, and preferably A1-30°C or less.

[0044] The manufacturing method of the present disclosure includes, after the hot working, performing spheroidizing annealing, which includes the above steps (1) to (3) in this order. In the spheroidizing annealing, in addition to the above steps (1) to (3), for example, additional steps such as heating and holding and cooling may be included. Furthermore, the manufacturing method of the present disclosure may simply perform spheroidizing annealing under the above conditions after hot working under the above conditions, and after hot working and before spheroidizing annealing, for example, for the purpose of homogenizing the structure, Heat to 1200°C, hold for 1 hour, then air cool to 100°C or less, then heat to 850°C, hold for 0.25 hours, then water cool, or Heat to 850°C, hold for 0.25 hours, then water cool. may be included.

[0045] The shape and the like of the cold-worked steel wire for machine structure use (steel wire for machine structure parts) according to this embodiment are not particularly limited, and examples thereof include those with a diameter of 5.5 mm to 60 mm. [Example]

[0046] The present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples, and can be implemented by making appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are included in the technical scope of the present disclosure.

[0047] 1. Sample Preparation Steels having the chemical compositions shown in Table 1 were obtained by melting steel Nos. 1 to 6 in Table 2 using a converter, and steel Nos. 7 and 8 in a vacuum induction melting furnace (VIF). The "unavoidable impurities" shown in Table 1 may also include any element at the level of the inevitable impurities described above.

[0048] The steel materials Nos. 1 to 6 were hot rolled, and Nos. 7 and 8 were hot forged under the conditions (strain amount, finish rolling entry temperature) shown in Table 2, to obtain wire rods and rods with diameters of 30 to 50 mm. Note that for Nos. 7 and 8, the heating temperature before hot forging is shown in parentheses, assuming that it corresponds to the heating temperature of the hot working (finish rolling entry temperature). Next, the wire rods and rods were subjected to spheroidizing annealing using one of the following spheroidizing annealing methods (a) to (e) to obtain steel wires. The manufacturing conditions, such as the temperature T1, are as shown in Table 2.

[0049] (Spheroidizing annealing conditions) (a) The temperature is increased from 600°C to 740°C at a rate of 80°C / hour, and held at 740°C for 5 hours. Then, the material is cooled from 740°C to 640°C at an average cooling rate of 10°C / hour, and air-cooled from 640°C to room temperature. Figure 1 shows a diagram explaining spheroidizing annealing condition (a). (b) The temperature is increased from 600°C to 740°C at a rate of 80°C / hour, and held at 740°C for two hours. This process is then repeated twice: cooling from 740°C to 660°C at an average cooling rate of 30°C / hour, then increasing the temperature from 660°C to 740°C at an average rate of 80°C / hour, and holding at 740°C for one hour. The material is then cooled from 740°C to 720°C at an average cooling rate of 30°C / hour, and then cooled from 720°C to 640°C at a rate of 10°C / hour, and air-cooled from 640°C to room temperature. Figure 2 shows a diagram illustrating spheroidizing annealing condition (b). (c) The temperature is increased from 600°C to 740°C at a rate of 80°C / hour, and held at 740°C for two hours. This process is then repeated nine times: cooling from 740°C to 660°C at an average cooling rate of 30°C / hour, then increasing the temperature from 660°C to 740°C at an average rate of 80°C / hour, and holding at 740°C for one hour. The material is then cooled from 740°C to 720°C at an average cooling rate of 30°C / hour, and then cooled from 720°C to 640°C at a rate of 10°C / hour, and air-cooled from 640°C to room temperature. Figure 3 shows a diagram illustrating spheroidizing annealing condition (c). (d) Heat to 1200°C, hold for 1 hour, then air cool to 100°C or less. Then heat to 850°C, hold for 0.25 hours, then water cool. Then, repeat (b) above. (e) Heat to 850°C, hold for 0.25 hours, then water cool. Then, proceed as in (b) above.

[0050] The steel wires thus obtained were used to observe the steel structure and evaluate the cold forgeability and machinability as described below.

[0051] [Observation of steel structure] (Measurement of the major axis diameter of an inclusion) A longitudinal cross section of the steel wire, including the central axis and parallel to the longitudinal direction of the steel wire, i.e., a cross section parallel to the longitudinal direction including the diameter D of the steel wire, was mirror-polished so that only the inclusions could be observed. Next, three photographs (pixel count: 1600 (width) × 1200 (height)) were taken at a position D / 4 of the cross section using an optical microscope at 200x magnification (field of view size: approximately 520 μm × approximately 740 μm), and image analysis was performed. A preliminary investigation of the inclusions contained in several steel wires manufactured using the method of this embodiment revealed that Al-based inclusions were also detected in some cases. However, in most steel wires, the inclusions were primarily MnS. Therefore, in this example, MnS was the only inclusion measured. Furthermore, in the image analysis, inclusions with a size of 1 pixel or less were excluded from the analysis as noise. The major axis lengths of all inclusions (targets of measurement) in the three photographs after noise removal were calculated, and the average of the top 10% of the calculated major axis lengths of the inclusions was calculated.

[0052] (carbide aspect ratio) A longitudinal cross section containing the central axis of the steel wire and parallel to the longitudinal direction of the steel wire, i.e., a cross section parallel to the longitudinal direction including the diameter D of the steel wire, was mirror-polished and then etched with picral to reveal carbides. Next, three photographs were taken at a position D / 4 of the cross section after the above-mentioned corrosion using an optical microscope at 1000x magnification (field of view size approximately 150 μm × approximately 100 μm), and image analysis was performed. Note that during image analysis, carbides recognized at 1 pixel or less were excluded from the analysis as noise. The aspect ratios of all carbides (to be measured) in the three photographs after noise removal were calculated, and the average of the top half was calculated.

[0053] When the structure was observed to determine the aspect ratio of the above-mentioned carbides, it was confirmed separately using an optical microscope at 200x or 1000x magnification that the main component was ferrite in addition to carbides and inclusions.

[0054] (Carbide size / Inclusion size) For each of the D / 2, D / 4, and surface layers of the steel wire, sample preparation and photographs of the inclusions (magnification: 200x) were performed in the same manner as described above (measurement of the major axis diameter of inclusions). Furthermore, for each of the D / 2, D / 4, and surface layers, sample preparation and photographs of the carbides (magnification: 1000x) were performed in the same manner as described above (measurement of the aspect ratio of carbides). The equivalent circle diameters of all carbides and inclusions (targets of measurement) were calculated after noise removal, and the average value of the top half (average equivalent circle diameter) was calculated. For example, if there were 100 carbides in total, the average equivalent circle diameter was calculated for the 50 carbides with the largest equivalent circle diameters. The ratio of the average equivalent circle diameter of carbides to the average equivalent circle diameter of inclusions was calculated from the obtained average equivalent circle diameters of carbides and inclusions. The results of the observation of the steel structure are shown in Table 3.

[0055] [Evaluation of cold forgeability] Compression test specimens with a diameter of 10 mm and a height of 15 mm were prepared around the D / 4 steel wire. Compression was applied with the compression ratio changed by 2.5% in increments, and the occurrence of cracks after compression was investigated. Tests were conducted five times (N5) for each compression ratio condition. In the test, samples that had been compressed at a certain compression ratio and showed no cracks were subjected to compression at a 2.5% increase in compression ratio, but rather new samples were used for compression at all compression ratios. This method, in which new samples are compressed, generally makes it easier for cracks to occur at lower compression ratios, and this method evaluates cold forgeability under strict test conditions.

[0056] For Samples Nos. 1 to 6 in Table 2, the standard evaluation criteria were used. The critical compression ratio was determined as the value just before cracks occurred in three or more of the five samples. For Samples Nos. 7 and 8 in Table 2, the critical compression ratio was determined as the value just before cracks occurred in one or more of the five samples. For Samples Nos. 7 and 8 in Table 2, the critical compression ratio was determined as the value just before cracks occurred in one or more of the five samples. The evaluation criteria were stricter as described above because the hot forging was performed at a particularly high temperature (1200°C) and an additional spheroidizing annealing process was performed. It is believed that these samples exhibited better properties. A critical compression ratio of 57.5% or greater was evaluated as excellent in cold forgeability, and a critical compression ratio of less than 57.5% was evaluated as poor in cold forgeability. The cold forgeability evaluation results are also shown in Table 3. In this example, the sample contained 0.010% or more by mass of sulfur (S), which resulted in a high machinability rating. The inclusion of 0.010% by mass or more of S provides a steel wire with high machinability, as described, for example, in "Lead-Free Free-Cutting Steel" by Somekawa et al., Kobe Steel Technical Report, Vol. 51, No. 1 (Apr. 2001).

[0057] [Table 1]

[0058] [Table 2]

[0059] [Table 3]

[0060] The following can be seen from Tables 1 to 3. Samples Nos. 1 to 3 were subjected to spheroidizing annealing under conditions that did not involve repeated cooling and heating steps, so the major axis diameter of the inclusions was large and the aspect ratio of the carbides was also large. In particular, sample No. 2 was hot-rolled at a low processing temperature, resulting in a small ratio of (carbide size / inclusion size). As a result, all of samples Nos. 1 to 3 had poor cold forgeability. Furthermore, sample No. 8 had a small amount of strain during hot rolling, resulting in a large major axis diameter of the inclusions and a small ratio of (carbide size / inclusion size), resulting in poor cold forgeability.

[0061] On the other hand, for Samples Nos. 4 to 7, the steel wires were manufactured under the manufacturing conditions specified in the present disclosure, and therefore the obtained steel wires had the desired steel structure and were excellent in cold forgeability. As described above, the cold forgeability evaluation test performed in this specification is stricter than the test conditions shown in Patent Document 1, etc., and therefore it can be said that the cold forgeability is superior to that of the prior art. [Industrial Applicability]

[0062] The steel wire for cold-worked mechanical structural parts according to the present disclosure can be used to manufacture various mechanical structural parts such as automobile parts, construction machinery parts, etc. by subjecting it to cold working such as cold forging, cold heading, cold rolling, etc. Specific examples of such mechanical structural parts include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, catch washers, tappets, saddles, bulges, inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, mouthpieces, valve lifters, spark plugs, pinion gears, steering shafts, common rails, and other mechanical parts and electrical parts.

Claims

1. The chemical composition is C: 0.05% by mass to 0.50% by mass, Si: 0.005% by mass to 0.50% by mass, Mn: 0.30% by mass to 1.20% by mass, P: more than 0 mass%, 0.050 mass% or less, S: 0.010% by mass to 0.050% by mass, Al: 0.001% by mass to 0.10% by mass, Cr: more than 0 mass%, 1.50 mass% or less, Cu: 0% by mass to 0.25% by mass, Ni: 0% by mass to 0.25% by mass, Mo: 0% by mass to 0.50% by mass, B: 0% by mass to 0.01% by mass, Ti: 0% by mass to 0.20% by mass, Nb: 0% by mass to 0.20% by mass, V: 0% by mass to 0.50% by mass, N: 0% by mass to 0.010% by mass, Mg: 0% by mass to 0.020% by mass, Ca: 0% by mass to 0.050% by mass, Li: 0% by mass to 0.020% by mass, and REM: 0% by mass to 0.050% by mass and the balance being Fe and inevitable impurities, A steel wire for cold working machine structural use, the steel structure of which has ferrite, carbides and inclusions, the average major axis diameter of which is 30 μm or less.

2. 2. The steel wire for cold working machine structural use according to claim 1, wherein the average aspect ratio of the carbides is 3.0 or less, and a ratio expressed as (average circular equivalent diameter of carbides / average circular equivalent diameter of inclusions) is 0.46 or more.

3. A method for producing a steel wire for cold working machine structure use according to claim 1 or 2, Hot working a steel material satisfying the chemical composition of claim 1 at a strain of 2.5 or more and a temperature of 995°C or more; After the hot working, spheroidizing annealing is performed, which includes the following steps (1) to (3) in this order: A method for producing a steel wire for cold working machine structure, comprising: (1) Heat to a temperature T1 of (A1+5°C to A1+60°C) and hold at that temperature T1 for 1 to 6 hours. (2) A cooling-heating step is carried out one or more times, in which the material is cooled from the temperature T1 to a temperature T2 of (A1-10°C to A1-100°C) at an average cooling rate CR1 of 25°C / hour to 100°C / hour, and then reheated from the temperature T2 to the temperature T1. (3) Cool from the temperature T1 to a temperature T3 of (A1+5°C to A1-10°C) at an average cooling rate CR2 of 25°C / hour or more, and then cool from the temperature T3 to a temperature T4 of (A1-20°C to A1-80°C) at an average cooling rate CR3 of 15°C / hour or less. Here, A1 is calculated by the following formula (1). A1 (°C) = 723-10.7×(%Mn)-16.9×(%Ni)+29.1×(%Si)+16.9×(%Cr)+290×(%As)+6.38×(%W)...(1) Here, (% element) represents the content (mass %) of each element, and the content of elements that are not included is set to zero.

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