Steel wire for machine structural parts and its manufacturing method

Abstract

To provide a steel wire for machine structural components excellent in cold workability and excellent in hardening, and a manufacturing method of the steel wire for machine structural components.SOLUTION: A steel wire for machine structural components comprises: C: 0.05 mass% to 0.60 mass%, Si: 0.005 mass% to 0.50 mass%, Mn: 0.30 mass% to 1.20 mass%, P: more than 0 mass% and 0.050 mass% or less, S: more than 0 mass% and 0.050 mass% or less, Al: 0.001 mass% to 0.10 mass%, Cr: more than 0 mass% and 1.5 mass% or less, and N: more than 0 mass% and 0.02 mass% or less, and the balance made of iron and inevitable impurities, and a half width of an X-ray diffraction peak on a (211) plane of the ferrite grain is 0.500° or less, an average equivalent circle diameter of all cementite is (1.863-2.13 [C]) μm or less when an amount of C (mass%) in the steel is represented by [C].SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a steel wire for machine structural parts and a method for manufacturing the same.

Background Art

[0002] When manufacturing various machine structural parts such as automobile parts and construction machine parts, spheroidizing annealing is usually performed on bar steel including hot-rolled wire rods for the purpose of imparting cold workability. Then, cold working is performed on the steel wire obtained by spheroidizing annealing, and then machining such as cutting is performed to form a predetermined part shape. Further, quenching and tempering are performed for final strength adjustment, and machine structural parts are manufactured.

[0003] In recent years, in the cold working process, in order to prevent cracking of steel materials and improve die life, steel wires that are softer than before have been desired.

[0004] As a method for obtaining a softened steel wire, for example, Patent Document 1 shows that, as a method for manufacturing medium-carbon steel excellent in cold forging properties, heating to an austenitizing temperature range two or more times is performed in the spheroidizing annealing treatment. According to the manufacturing method of Patent Document 1, it is shown that a steel for cold forging can be obtained in which the hardness after spheroidizing annealing is 83 HRB or less and the ratio of spherical carbides in the structure is 70% or more.

[0005] Patent Document 2 discloses a steel material having characteristics of low deformation resistance after spheroidizing annealing and excellent cold forging properties, and a method for manufacturing the same. As the manufacturing method, a steel satisfying a predetermined component composition is hot-worked, cooled to room temperature, then heated to a temperature range of A1 point to A1 point + 50°C, held in the temperature range of A1 point to A1 point + 50°C for 0 to 1 hour after heating, and then subjected to annealing treatment of cooling from the temperature range of A1 point to A1 point + 50°C to a temperature range of A1 point - 100°C to A1 point - 30°C at an average cooling rate of 10 to 200°C / hr two or more times. Then, when heating to a temperature range of A1 point to A1 point + 30°C and holding in the temperature range of A1 point to A1 point + 30°C and then cooling, when cooling after reaching the A1 point and holding in the temperature range of A1 point to A1 point + 30°C, the residence time in the temperature range of A1 point to A1 point + 30°C until reaching the A1 point is set to 10 minutes to 2 hours, and after cooling the cooling temperature range from the temperature range of A1 point to A1 point + 30°C to A1 point - 100°C to A1 point - 20°C at an average cooling rate of 10 to 100°C / hr, it is shown that after holding for 10 minutes to 5 hours in the cooling temperature range and then further cooling.

[0006] Patent Document 3 discloses a steel wire for mechanical structural parts having a predetermined component composition, which can reduce the deformation resistance during cold working, improve the crack resistance, and exhibit excellent cold workability. The metal structure of the steel is composed of ferrite and cementite, and the proportion of the number of cementite existing at the ferrite grain boundaries is 40% or more with respect to the total number of cementite. In Patent Document 3, the manufacturing conditions of the rolled wire rod to be subjected to spheroidizing annealing are finish-rolled at 800°C or higher and 1050°C or lower, and the first cooling with an average cooling rate of 7°C / sec or more, the second cooling with an average cooling rate of 1°C / sec or more and 5°C / sec or less, and the third cooling with an average cooling rate faster than the second cooling and 5°C / sec or more are performed in this order. It is shown that it is preferable to perform the end of the first cooling and the start of the second cooling within the range of 700 to 750°C, perform the end of the second cooling and the start of the third cooling within the range of 600 to 650°C, and make the end of the third cooling 400°C or lower.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the conventional technologies disclosed in Patent Documents 1 to 3, the hardness after spheroidizing annealing cannot be sufficiently reduced, and the workability in cold working performed after spheroidizing annealing is inferior, or the hardness cannot be sufficiently increased by the quenching treatment performed after cold working, that is, the hardenability may be inferior. That is, conventionally, there has been no technology focusing on improving both cold workability and hardenability.

[0009] The present invention has been made in view of such a situation, and an object thereof is to provide a steel wire for mechanical structure parts having a sufficiently low hardness, excellent cold workability, and excellent hardenability, that is, capable of obtaining a high hardness by quenching treatment, and a method for manufacturing a steel wire for mechanical structure parts that can manufacture the above steel wire for mechanical structure parts in a relatively short time.

[0010] In this specification, "wire rod" and "bar steel" each refer to linear or bar-shaped steel materials obtained by hot rolling, and are steel materials that have not been subjected to any heat treatment such as spheroidizing annealing or wire drawing. Further, "steel wire" refers to a wire rod or bar steel that has been subjected to at least one of heat treatment such as spheroidizing annealing and wire drawing. In this specification, the above wire rod, bar steel, and steel wire are collectively referred to as "long steel products".

Means for Solving the Problems

[0011] Aspect 1 of the present invention is C: 0.05% by mass to 0.60% 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: more than 0% by mass, 0.050% by mass or less, Al: 0.001% by mass to 0.10% by mass, Cr: more than 0 mass% and 1.5 mass% or less, and N: More than 0% by mass, 0.02% by mass or less, and the remainder being iron and unavoidable impurities, The half-width of the X-ray diffraction peak on the (211) plane of the ferrite grains is 0.500° or less, The steel wire for machine structural parts has an average circle-equivalent diameter of all cementite of (1.863-2.13[C]) μm or less, when the amount of C (mass%) in the steel is expressed as [C].

[0012] Aspect 2 of the present invention is Furthermore, Cu: more than 0 mass%, 0.25 mass% or less, Ni: more than 0 mass%, 0.25 mass% or less, Mo: more than 0 mass% and 0.50 mass% or less B: The steel wire for machine structural parts according to Aspect 1, containing one or more selected from the group consisting of more than 0 mass % and 0.01 mass % or less of B.

[0013] Aspect 3 of the present invention is Furthermore, Ti: more than 0 mass%, 0.2 mass% or less, Nb: more than 0 mass% and 0.2 mass% or less, and V: The steel wire for machine structural parts according to Aspect 1 or 2, containing one or more selected from the group consisting of more than 0 mass % and 0.5 mass % or less of V.

[0014] A fourth aspect of the present invention is Furthermore, Mg: more than 0% by mass, 0.02% by mass or less, Ca: more than 0% by mass, less than 0.05% by mass, Li: more than 0 mass% and 0.02 mass% or less, and The steel wire for machine structural parts according to any one of Aspects 1 to 3, further comprising one or more selected from the group consisting of more than 0 mass % and 0.05 mass % or less of REM.

[0015] A fifth aspect of the present invention is The steel wire for machine structural components according to any one of Aspects 1 to 4, wherein the average value of the ferrite grain size is 30 μm or less.

[0016] A sixth aspect of the present invention is A section steel satisfying the chemical composition according to any one of aspects 1 to 4, The method for producing a steel wire for a machine structural part according to any one of Aspects 1 to 5 includes a step of performing spheroidizing annealing, which includes the following steps (1) to (3): (1) After heating to a temperature T1 of (A1+8°C) to (A1+31°C), the mixture is heated and maintained at the temperature T1 for more than 1 hour and not more than 6 hours; (2) cooling to a temperature T2 exceeding 650°C and not exceeding (A1-17°C), and then heating to a heating temperature higher than temperature T2 and not exceeding (A1+60°C), a cooling-heating step is carried out a total of 2 to 6 times; (3) Cool to a temperature below (A1-30°C) at an average cooling rate of 5°C / hour to 20°C / hour. Here, A1 is calculated by the following formula (1). A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]...(1) Here, [element] represents the content (mass%) of each element, and the content of elements that are not included is zero.

[0017] A seventh aspect of the present invention is Aspect 7. The method for producing a steel wire for a machine structural part according to aspect 6, wherein the steel section is a steel wire obtained by drawing a wire rod at an area reduction rate of more than 5%. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a steel wire for machine structural parts that has excellent cold workability and hardenability, and a method for manufacturing the steel wire for machine structural parts.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0020] The inventors have earnestly studied from various angles in order to realize a steel wire for mechanical structure parts having excellent cold workability and hardenability.

[0021] When the amount of strain in ferrite after spheroidizing annealing is low, the hardness of the steel after spheroidizing annealing can be reduced, and the cold workability is improved. The inventors focused on the idea that the rod-like cementite generated during the cooling process in spheroidizing annealing contains a large amount of interfacial strain, and considered that reducing the rod-like cementite generated during the cooling process can reduce the amount of strain in ferrite. Furthermore, the inventors considered that by reducing the size of all cementite, the cementite that remains undissolved during the high-temperature holding in the quenching treatment process can be reduced, and the hardenability can be improved.

[0022] Therefore, as a result of further intensive studies by the present inventors, it has been found that, particularly in the metal structure, the half-value width of the X-ray diffraction peak on the (211) plane of ferrite grains, which is an index of the amount of strain in ferrite, should be set to a value equal to or less than a certain value, and the average circle equivalent diameter of all cementite should be set to a value equal to or less than a certain value according to the C content in the steel material. Further, the present inventors have found that, in order to realize the above metal structure, it is effective to set the chemical composition within a certain range and to perform spheroidizing annealing under particularly specified conditions in the method for manufacturing a steel wire for mechanical structure parts. Hereinafter, first, the steel wire for mechanical structure parts according to the present embodiment will be described in terms of the metal structure of the steel wire for mechanical structure parts.

[0023] 1. Metal structure [The half-value width of the X-ray diffraction peak on the (211) plane of ferrite grains is 0.500° or less] In the steel wire for mechanical structure parts of the present invention, the half-value width of the X-ray diffraction peak on the (211) plane of ferrite grains is 0.500° or less. The half-value width of the peak of ferrite in X-ray diffraction indicates the degree of introduction of strain related to the dislocation density. A small peak half-value width indicates that the strain in the steel is small. Therefore, the smaller the peak half-value width, the smaller the strain in the steel. As a result, the hardness of the steel decreases and the cold workability improves. Although substantially the same tendency is shown regardless of which crystal orientation of ferrite is measured, in the present invention, the peak half-value width of the (211) plane of ferrite, which can clearly grasp the tendency, is typically defined. The peak half-value width is preferably 0.495° or less, more preferably 0.493° or less. The lower limit of the peak half-value width is not particularly limited, but is approximately 0.100° considering the component composition and manufacturing conditions according to the embodiment of the present invention.

[0024] [The average circle equivalent diameter of all cementite is (1.863 - 2.13[C]) μm or less when the C content (mass%) in the steel material is represented by [C]] If the cementite grows too coarsely, during the high-temperature holding in the quenching process after cold working, the cementite will not dissolve sufficiently, and a sufficiently high hardness cannot be obtained by quenching. Therefore, in the present invention, when the C content (mass%) in the steel material is represented by [C], the average circle-equivalent diameter of all cementite is set to be (1.863 - 2.13[C]) μm or less. Preferably, it is (1.858 - 2.13[C]) μm or less.

[0025] On the other hand, when the amount of cementite in the steel is constant, the larger the size of the cementite, the lower the number density of the cementite, and the longer the distance between the cementites. The longer the distance between the cementites in the steel material, the more difficult it is to achieve precipitation strengthening, and as a result, the hardness after spheroidizing annealing can be further reduced. From these viewpoints, it is preferable that the average circle-equivalent diameter of all cementite is (1.668 - 2.13[C]) μm or more. More preferably, the average circle-equivalent diameter of all cementite is (1.669 - 2.13[C]) μm or more.

[0026] Regarding the above-mentioned all cementite, the morphology is not particularly limited, and in addition to spherical cementite, rod-shaped cementite with a large aspect ratio is included. The above-mentioned aspect ratio is the ratio (major axis / minor axis) of the major axis, which is the longest length of the cementite particle, and the minor axis, which is the longest length in the direction perpendicular to the major axis. In addition, the standard for the size of the cementite to be measured is not limited, but the size of the cementite that can be discriminated by the measurement method of the average circle-equivalent diameter of all cementite described later is the minimum size. Specifically, cementite particles with a circle-equivalent diameter of 0.3 μm or more are the measurement objects.

[0027] The metallographic structure of the steel wire for mechanical structure parts according to the present embodiment is a spheroidized structure having spheroidized cementite, and it can be obtained by subjecting a bar steel that satisfies the chemical composition described later to, for example, spheroidizing annealing described later.

[0028] The metallographic structure of the steel wire for mechanical structure parts of the present invention is substantially composed of ferrite and cementite. The above "substantially" means that the ferrite in the metallographic structure of the steel wire for mechanical structure parts of the present invention has an area ratio of 90% or more, and the rod-shaped cementite with an aspect ratio of 3 or more has an area ratio of 5% or less. If the adverse effect on cold workability is small, nitrides such as AlN and inclusions other than nitrides are allowed to have an area ratio of less than 3%. The area ratio of the ferrite may further be 95% or more.

[0029] In this specification, "ferrite" refers to a portion having a bcc crystal structure, and also includes the ferrite in pearlite, which is a layered structure of ferrite and cementite. Also, the "ferrite crystal grain" which is the measurement object of the "ferrite crystal grain size" includes the crystal grains containing rod-shaped cementite generated during spheroidizing annealing with insufficient spheroidization, but the crystal grains (pearlite crystal grains) containing rod-shaped cementite that can remain before spheroidizing annealing are excluded. Specifically, after etching using nital (2% by volume of nitric acid, 98% by volume of ethanol), the "crystal grains in which cementite does not exist inside the grains" and the "crystal grains in which cementite exists inside the grains and the shape of the cementite can be observed (that is, the boundary between cementite and ferrite can be clearly observed)" that can be confirmed when observed at 1000 times using an optical microscope are referred to. Crystal grains in which the shape of cementite cannot be observed at 1000 times using the above optical microscope (that is, the boundary between cementite and ferrite cannot be clearly observed) are not the judgment object in this embodiment and are not included in the "ferrite crystal grains".

[0030] [Average value of ferrite crystal grain size: 30 μm or less] In the steel wire for mechanical structural components according to this embodiment, the average ferrite grain size in the metal structure is preferably 30 μm or less. If the average ferrite grain size is 30 μm or less, the ductility of the steel wire for mechanical structural components can be improved and the occurrence of cracks during cold working can be further suppressed. The average ferrite grain size is more preferably 25 μm or less, and even more preferably 20 μm or less. The smaller the average ferrite grain size, the better, but considering possible manufacturing conditions, etc., the lower limit can be approximately 2 μm.

[0031] (characteristic) The steel wire for machine structural components according to this embodiment, which satisfies the following chemical composition and has the above-described metallographic structure, can achieve both low hardness that allows good cold working and high hardness after quenching. In this embodiment, when the C content (mass%), Cr content (mass%), and Mo content (mass%) in the steel are expressed as [C], [Cr], and [Mo], respectively (elements that are not contained are regarded as zero mass%), if the hardness, or the hardness after spheroidizing annealing in the examples described below, satisfies the following formula (2) and the hardness after quenching satisfies the following formula (3), it was determined that the hardness was sufficiently low, the cold workability was excellent, and high hardness was achieved after quenching, i.e., the hardness was excellent. Hardness (HV) (after spheroidizing annealing) < 91 ([C] + [Cr] / 9 + [Mo] / 2) + 91 (2) Hardness after quenching (HV)>380ln([C])+1010 ···(3)

[0032] 2.Chemical composition The chemical composition of the steel wire for machine structural components according to this embodiment will be described.

[0033] [C:0.05 mass%~0.60 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 becomes excessive, increasing hardness and reducing cold workability. Therefore, the upper limit of the C content is set to 0.60 mass%. The C content is preferably 0.55 mass% or less, more preferably 0.50 mass% or less.

[0034] [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, and more preferably 0.050 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, and more preferably 0.35 mass% or less.

[0035] [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 is more likely 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. From the viewpoint of further suppressing the decrease in toughness, it can be set to less than 0.50 mass%, or even 0.45 mass% or less.

[0036] [P: More than 0 mass%, 0.050 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 is usually 0.001% by mass or more.

[0037] [S: More than 0 mass%, 0.050 mass% or less] S (sulfur) is an inevitable impurity that forms MnS in steel, degrading ductility and therefore being harmful to cold workability. Therefore, the S content is set to 0.050% by mass or less. The S content is preferably 0.030% by mass or less, and more preferably 0.020% by mass or less. The lower the S content, the better, but it is usually 0.001% by mass or more.

[0038] [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 0.08% by mass or less, and more preferably 0.05% by mass or less.

[0039] [Cr: more than 0 mass%, 1.5 mass% or less] Cr is an element that has the effect of improving the hardenability of steel and increasing its strength, and also has the effect of promoting the spheroidization of cementite. Specifically, Cr dissolves in cementite and delays the dissolution of cementite during the heating of spheroidizing annealing. By having a part of the cementite remaining without dissolving during heating, it becomes difficult to form rod-shaped cementite with a large aspect ratio during cooling, and it becomes easier to obtain a spheroidized structure. Therefore, the Cr content is preferably more than 0 mass% and 0.01 mass% or more. It may be 0.05 mass% or more, and even more preferably 0.10 mass% or more. From the viewpoint of further promoting the spheroidization of cementite, it can be more than 0.30 mass%, and can also be more than 0.50 mass%. If the Cr content is excessive, the diffusion of elements containing carbon is delayed, and the dissolution of cementite is delayed more than necessary, making it difficult to obtain a spheroidized structure. As a result, the effect of reducing hardness according to the present invention 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 more, it can be further 1.00 mass% or less, further 0.80 mass% or less, and further 0.30 mass% or less.

[0040] [N: More than 0 mass% and 0.02 mass% or less], N is an impurity inevitably contained in steel. However, if a large amount of N is dissolved in the steel, it causes an increase in hardness and a decrease in ductility due to strain aging, and the cold workability deteriorates. Therefore, the N content is 0.02 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less.

[0041] [Balance] The balance consists of iron and inevitable impurities. As inevitable impurities, the inclusion of trace elements (such as As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. For example, elements such as P and S are usually more preferably contained in smaller amounts. Therefore, although they are inevitable impurities, there are elements whose composition ranges are separately defined as above. For this reason, in this specification, when referring to "inevitable impurities" constituting the balance, it is a concept excluding elements whose composition ranges are separately defined.

[0042] The steel wire for mechanical structure parts according to this embodiment only needs to contain the above elements in its chemical composition. The selected elements described below may not be contained, but by containing them as needed together with the above elements, it is possible to more easily achieve ensuring hardenability and the like. The selected elements will be described below.

[0043] [One or more selected from the group consisting of: Cu: more than 0 mass%, 0.25 mass% or less, Ni: more than 0 mass%, 0.25 mass% or less, Mo: more than 0 mass%, 0.50 mass% or less, and B: more than 0 mass%, 0.01 mass% or less] Cu, Ni, Mo, and B are all elements effective in increasing the strength of the final product by improving the hardenability of the steel material, and are contained alone or in combination of two or more as needed. The effects of these elements increase as their contents increase. The preferable lower limit for effectively exerting the above effects is more than 0 mass% for each of Cu, Ni, and Mo, more preferably 0.02 mass% or more, still more preferably 0.05 mass% or more, and for B it is more than 0 mass%, more preferably 0.0003 mass% or more, still more preferably 0.0005 mass% or more. On the other hand, if the content of these elements becomes excessive, the strength may become too high and the cold workability may deteriorate. Therefore, the preferable upper limits of each were determined as described above. More preferably, the content of each of Cu and Ni is 0.22% by mass or less, still more preferably 0.20% by mass or less. The content of Mo is more preferably 0.40% by mass or less, still more preferably 0.35% by mass or less. The B content is more preferably 0.007% by mass or less, still more preferably 0.005% by mass or less.

[0044] [One or more selected from the group consisting of Ti: more than 0% by mass and 0.2% by mass or less, Nb: more than 0% by mass and 0.2% by mass or less, and V: more than 0% by mass and 0.5% by mass or less] Ti, Nb, and V form compounds with N and reduce the dissolved N, thereby exerting the effect of reducing the flow stress. Therefore, they can be contained alone or in combination of two or more as necessary. The effect of these elements increases as the content increases. For any of these elements, the preferable lower limit for effectively exerting the above effect is more than 0% by mass, more preferably 0.03% by mass or more, still more preferably 0.05% by mass or more. However, if the content of these elements becomes excessive, the formed compounds may cause an increase in the flow stress, and conversely, the cold workability may deteriorate. Therefore, the content of each of Ti and Nb is preferably 0.2% by mass or less, and the content of V is preferably 0.5% by mass or less. The content of each of Ti and Nb is more preferably 0.18% by mass or less, still more preferably 0.15% by mass or less, and the V content is more preferably 0.45% by mass or less, still more preferably 0.40% by mass or less.

[0045] [One or more selected from the group consisting of Mg: more than 0% by mass and 0.02% by mass or less, Ca: more than 0% by mass and 0.05% by mass or less, Li: more than 0% by mass and 0.02% by mass or less, and rare earth metal (REM): more than 0% by mass and 0.05% by mass or less] Mg, Ca, Li, and REM are elements effective in spheroidizing sulfide compound inclusions such as MnS and improving the deformability of steel. This effect increases as their contents increase. To effectively exert the above 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 added 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 0.02% by mass or less, more preferably 0.018% by mass or less, and even more preferably 0.015% by mass or less, and the contents of Ca and REM are each preferably 0.05% by mass or less, more preferably 0.045% by mass or less, and even more preferably 0.040% by mass or less. Mg, Ca, Li, and REM may each be contained alone or in combination of two or more, and when two or more are contained, the content may be any content within the above range. The REM includes lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0046] The shape of the steel wire for machine structural components according to this embodiment is not particularly limited, and examples thereof include those with a diameter of 5.5 mm to 60 mm.

[0047] 3. Manufacturing method In order to obtain the metallographic structure of the steel wire for machine structural parts according to the embodiment of the present invention, it is preferable to appropriately control the spheroidizing annealing conditions as described below when producing the steel wire for machine structural parts. The hot rolling process for producing the wire rod or steel bar to be subjected to spheroidizing annealing is not particularly limited, and may be performed according to a conventional method. As described below, wire drawing may be performed before spheroidizing annealing. 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.

[0048] The spheroidizing annealing conditions in the manufacturing method of a steel wire for machine structural components according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 shows an example of a diagram illustrating the spheroidizing annealing conditions in the manufacturing method according to an embodiment of the present invention, and the number of repetitions of the cooling-heating step, etc. are not limited to those shown in Fig. 1.

[0049] The method for producing a steel wire for machine structural components according to an embodiment of the present invention includes a spheroidizing annealing step including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1+8°C) to (A1+31°C), the mixture is heated and maintained at the temperature T1 for more than 1 hour and not more than 6 hours; (2) Cooling to a temperature T2 exceeding 650 ° C. and not exceeding (A1 - 17 ° C.), and then heating to a heating temperature higher than temperature T2 and not exceeding (A1 + 60 ° C.), the cooling-heating process is carried out a total of 2 to 6 times; (3) Cool to a temperature below (A1-30°C) at an average cooling rate of 5°C / hour to 20°C / hour. Here, A1 is calculated by the following formula (1). A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]...(1) Here, [element] represents the content (mass%) of each element, and the content of elements that are not included is zero.

[0050] [(1) After heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), the sample is maintained at that temperature T1 for more than 1 hour and not more than 6 hours ([1] and [2] in Figure 1)] By heating to a temperature (T1) in the range of (A1 + 8°C) to (A1 + 31°C), the dissolution of rod-shaped cementite with a large aspect ratio and a large amount of interfacial strain generated during the rolling stage is promoted. If the temperature T1 is too low, the rod-shaped cementite with a large amount of interfacial strain will not be dissolved during the heat holding process and will continue to remain in the ferrite, increasing the hardness and reducing the cold workability. To obtain a sufficiently softened steel wire, the temperature T1 needs to be A1 + 8°C or higher. The temperature T1 is preferably A1 + 15°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 will become too coarse, making it difficult for spherical cementite to precipitate at the ferrite grain boundaries during the cooling process of the next step, increasing the rod-shaped cementite, increasing the hardness, and reducing the cold workability. Therefore, the temperature T1 is set to A1 + 31°C or lower. The temperature T1 is preferably A1 + 30°C or lower, and more preferably A1 + 29°C or lower.

[0051] Also, if the heat holding time (t1) at the temperature T1 is too short, the rod-shaped cementite with a large amount of interfacial strain will remain in the ferrite crystal grains, increasing the hardness and reducing the cold workability. To obtain a sufficiently softened steel wire, the heat holding time (t1) needs to be more than 1 hour and 6 hours or less. The preferred heat holding time (t1) is 1.5 hours or more, and more preferably 2.0 hours or more. If the heat holding time (t1) is too long, the heat treatment time will be long and the productivity will decrease. Therefore, the heat holding time (t1) is 6 hours or less, preferably 5 hours or less, and more preferably 4 hours or less. Note that since the average heating rate during heating to the temperature (T1) in the range of (A1 + 8°C) to (A1 + 31°C) ([1] in Fig. 1) does not affect the steel material properties, the temperature can be raised at any rate. For example, it can be raised at a rate of 30°C / hour to 100°C / hour.

[0052] Note that the temperature of the above A1 point is calculated by the following formula (1) described on page 273 of Leslie's Steel Materials Science (Maruzen). A1 (°C) = 723 + 29.1×[Si] - 10.7×[Mn] + 16.9×[Cr] - 16.9×[Ni] ··· (1) However, [element] represents the content (% by mass) of each element, and the content of elements not contained is taken as zero.

[0053] [(2) Cool to a temperature T2 above 650°C and below (A1 - 17°C), and heat to a heating temperature higher than temperature T2 and below (A1 + 60°C). The cooling-heating process is carried out a total of 2 to 6 times ([7] in Fig. 1)] Subsequently, cool to a temperature T2 above 650°C and up to (A1 - 17°C), and heat to a heating temperature higher than temperature T2 and below (A1 + 60°C). The cooling-heating process is carried out a total of 2 to 6 times. Each step of the cooling-heating process will be described in detail below.

[0054] <(2-i) Cool to a temperature T2 above 650°C and up to (A1 - 17°C) ([3] and [4] in Fig. 1)> Cooling is carried out to promote the precipitation of spherical cementite with less interfacial strain. Slowing down the average cooling rate from temperature T1 suppresses the excessive reprecipitation of rod-shaped cementite with a large amount of interfacial strain, reduces the amount of strain in ferrite, and can further improve the cold workability. Therefore, the average cooling rate is preferably 100°C / hour or less. More preferably, it is 90°C / hour or less, and even more preferably 80°C / hour or less. On the other hand, increasing the average cooling rate suppresses the excessive coarsening of cementite generated during cooling. As a result, the cementite is sufficiently dissolved during the high-temperature holding in the quenching process, and the hardness after quenching is further improved, that is, the hardenability can be further enhanced. Furthermore, the annealing time can be shortened, and the productivity is improved. Therefore, the average cooling rate is preferably 1°C / hour or more, more preferably 3°C / hour or more, and even more preferably 5°C / hour or more.

[0055] Also, if the cooling reach temperature (T2) is too low, it will lead to a longer annealing time. Therefore, the cooling reach temperature (T2) needs to be above 650°C. According to the manufacturing method according to this embodiment, even when the cooling reach temperature (T2) is above 650°C, cementite can be controlled to a desired form without performing long-time annealing. The cooling reach temperature (T2) is preferably 670°C or higher. On the other hand, if the cooling reach temperature (T2) is too high, rod-shaped cementite with a large amount of interfacial strain will precipitate excessively, the amount of strain in ferrite will increase, and the cold workability will decrease. Therefore, the cooling reach temperature (T2) needs to be a value that is a certain amount or more lower than the A1 point. Therefore, the upper limit of the cooling reach temperature (T2) is set to A1 - 17°C. The cooling reach temperature (T2) is preferably A1 - 18°C or lower. Also, if it is held after reaching the cooling reach temperature (T2), it will cause a longer heat treatment time. Therefore, it is better not to hold it from these viewpoints. However, in order to make the temperature variation in the furnace uniform, it may be held for a short time. The holding time at the cooling reach temperature T2 is preferably within 1 hour.

[0056] <(2-ii) Heating to a heating temperature higher than the temperature T2 and below (A1 + 60°C) (steps [5] and [6] in Fig. 1)> In order to redissolve the rod-shaped cementite with a large amount of interfacial strain precipitated in the above step (2-i), heating is performed from the above cooling reach temperature (T2). The heating reach temperature shown in [6] of Fig. 1, that is, the heating temperature (T3), may be any temperature within the temperature range higher than the temperature T2 and below (A1 + 60°C). From the viewpoint of sufficiently redissolving the rod-shaped cementite with a large amount of interfacial strain generated in the above step (2-i), the heating temperature is preferably A1°C or higher. Also, from the viewpoint of suppressing the redissolution of spherical cementite on the ferrite grain boundaries and suppressing the increase in hardness after spheroidizing annealing, the heating temperature (T3) is preferably A1 + 57°C or lower.

[0057] The average heating rate from the cooling temperature (T2) to the heating temperature (T3) as shown in [5] of FIG. 1 is not particularly limited either. For example, from the viewpoint of further sufficiently redissolving the rod-shaped cementite with a large amount of interfacial strain generated in the step (2-i) above and further suppressing the hardness after spheroidizing annealing, the average heating rate may be 200 ° C. / hour or less. Further, for example, from the viewpoint of sufficiently suppressing the coarsening of the cementite generated by this heating and further enhancing the hardenability, it can be set to 5 ° C. / hour or more.

[0058] After reaching the above heating temperature (T3), it does not matter whether or not to hold at the heating temperature. When holding at the heating temperature, for example, the holding time may be within 1 hour to suppress the redissolution of the spherical cementite generated in the step of cooling to the above temperature T2.

[0059] Note that the magnitude relationship between the above heating temperature (T3) and the above temperature T1 is not particularly limited. For example, the above heating temperature (T3) may be the same as the above temperature T1, or the above heating temperature (T3) may be higher than the above temperature T1.

[0060] In the manufacturing method according to the present embodiment, the cooling-heating steps of the above (2-i) cooling and the above (2-ii) heating are repeated a plurality of times. However, in each time, the temperature T2 and the temperature T3 need to satisfy the above range.

[0061] <(2-iii) The cooling-heating step is carried out a total of 2 to 6 times (in [7] of FIG. 1)> To suppress the precipitation of rod-shaped cementite with a large amount of interfacial strain in the step (2-i), after heating and holding at temperature T1 in the step (1), the cooling-heating steps (2-i) and (2-ii) must be performed a total of two to six times. If these cooling-heating steps are not repeated, the amount of strain in the ferrite increases, resulting in increased hardness after spheroidizing annealing. Therefore, the cooling-heating step is performed two or more times, preferably three or more times. While the hardness decreases as the number of steps increases, the effect saturates if the number of steps is too many. Furthermore, this leads to an increase in the annealing time, reducing productivity. Therefore, the number of times the cooling-heating step is performed is set to six or less. In the case of FIG. 1, the cooling step (2-i) and the heating step (2-ii) are performed four times. Furthermore, the temperature (T2) reached in each cooling step may differ within the respective specified ranges. Furthermore, the average cooling rate in the cooling-heating process refers to the average cooling rate from temperature T1 to the cooling temperature (T2) in the first cooling-heating process, and from the second cooling-heating process onwards, it refers to the average cooling rate from the heating temperature (T3) to the cooling temperature (T2).

[0062] [(3) Cooling to a temperature below (A1-30°C) at an average cooling rate of 5°C / hour to 20°C / hour ([8] and [9] in Figure 1)] Cooling is performed from the heating temperature (T3) of the final cooling-heating step. If the cooling temperature (T4) is (A1-30°C) or higher, rod-shaped cementite with a large amount of interfacial strain will reprecipitate, increasing strain in the ferrite and reducing cold workability. Therefore, the cooling temperature (T4) is set to be less than (A1-30°C). It is preferably (A1-35°C) or lower, more preferably (A1-40°C) or lower. From the viewpoint of shortening the annealing time, the cooling temperature (T4) is preferably (A1-250°C) or higher, more preferably (A1-200°C) or higher, and even more preferably (A1-150°C) or higher.

[0063] In order to suppress the re-precipitation of rod-shaped cementite with a large amount of interface strain and reduce the amount of strain in ferrite, the average cooling rate (R3) needs to be 20°C / hour or less. The average cooling rate (R3) is preferably 18°C / hour or less, more preferably 15°C / hour or less. If the average cooling rate (R3) is too slow, the cementite will coarsen excessively, the cementite will not be sufficiently dissolved during the high-temperature holding in the quenching process, and the hardness after quenching will decrease, that is, the hardenability will deteriorate. Furthermore, it will lead to an increase in the annealing time and a decrease in productivity. Therefore, the lower limit of the average cooling rate (R3) is determined to be 5°C / hour. The average cooling rate (R3) is preferably 10°C / hour or more.

[0064] In the temperature range below (A1 - 30°C), the precipitation of rod-shaped cementite with a large amount of interface strain does not occur. Therefore, the cooling arrival temperature (T4) may be any temperature as long as it is below (A1 - 30°C). The cooling below the arbitrary temperature is not particularly limited and may be, for example, air cooling.

[0065] The spheroidizing annealing (steps (1) to (3)) as described above may be repeated once or a plurality of times. From the viewpoints of suppressing excessive coarsening of cementite and ensuring productivity, for example, it is preferably 4 times or less, more preferably 3 times or less. When the spheroidizing annealing is repeated a plurality of times, within the above-specified range, it may be repeated under the same conditions or different conditions. Also, when the spheroidizing annealing is repeated a plurality of times, wire drawing may be applied between the spheroidizing annealings. For example, it can be carried out in the order of wire drawing before spheroidizing annealing described later → the first spheroidizing annealing → wire drawing → the second spheroidizing annealing.

[0066] In the method for manufacturing a steel wire for mechanical structure parts according to the present embodiment, the steps other than the spheroidizing annealing step are not particularly limited. For example, after spheroidizing annealing, a wire drawing step with a reduction ratio preferably of 15% or less may be included for the purpose of adjusting the dimensions. By setting the reduction ratio to 15% or less, an increase in hardness before cold working can be suppressed. The reduction ratio is more preferably 10% or less, still more preferably 8% or less, and even more preferably 5% or less.

[0067] In order to promote the formation of the microstructure of the present invention, it is preferable to provide a step of drawing the wire rod at an area reduction rate of more than 5% before spheroidizing annealing. By drawing the wire rod at this area reduction rate, cementite in the steel is destroyed, and the subsequent spheroidizing annealing can promote the aggregation of cementite, which makes it easier for cementite to coarsen without being excessive, and is effective for softening. The area reduction rate is more preferably 10% or more, even more preferably 15% or more, and even more preferably 20% or more. On the other hand, an excessively large area reduction rate may lead to a risk of wire breakage. Therefore, the area reduction rate is preferably 50% or less. When wiredrawing is performed multiple times, the number of times is not particularly limited and can be, for example, two times. Note that when wiredrawing is performed multiple times, the above-mentioned "area reduction rate during wiredrawing" refers to the area reduction rate from the steel material before wiredrawing to the steel material after multiple wiredrawings. [Example]

[0068] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can of course be practiced with appropriate modifications within the scope of the above and below-described aims, all of which are included in the technical scope of the present invention.

[0069] Test materials having the chemical compositions shown in Table 1 were melted in a converter, and then the resulting cast billets were hot-rolled to produce wire rods having diameters of 12 to 16 mm. In Table 2 described later, in cases where wire drawing before spheroidizing annealing was "yes," i.e., in the case of Samples Nos. 2 and 12 in Table 3, the above wire rods were subjected to wire drawing at an area reduction rate of 25%, and the resulting steel wires were then subjected to spheroidizing annealing.

[0070] Using the above wire or steel wire, annealing was carried out using a laboratory furnace. In the annealing, the wire or steel wire was heated to T1 shown in Table 2 and held for t1 hours. Then, at an average cooling rate of 5°C / hour to 100°C / hour, it was cooled to the temperature T2 in Table 2, and then heated to a heating temperature higher than the temperature T2 in Table 2 and below (A1 + 60°C) at an average heating rate of 5°C / hour to 100°C / hour. This cooling and heating process was carried out for the number of cooling-heating repetitions shown in Table 2. Then, from the heating temperature in the final cycle of the cooling-heating process, it was cooled to the temperature T4 at the average cooling rate R3 in Table 2. Then, it was air-cooled to obtain a sample. As shown in Table 2, the manufacturing conditions G1, G2, T, and U did not repeat the cooling-heating process. Specifically, for example, in the manufacturing condition G1, after heating to 740°C, it was held for 5 hours, and then cooled to 640°C at an average cooling rate of 10°C / hour, and then air-cooled.

[0071] Also, as a comparative example, in Sample No. 13 shown in Table 3, as the manufacturing condition H, heat treatment conditions satisfying the manufacturing conditions of Patent Document 3 were implemented. Specifically, the conditions shown as SA2 in the examples of Patent Document 3 were implemented, that is, the heat treatment process shown in FIG. 2 was repeated 5 times. In Sample No. 17 shown in Table 3, as the manufacturing condition L, heat treatment conditions satisfying the manufacturing conditions of Patent Document 1 were implemented. Specifically, the fifth spheroidizing annealing condition in No. 1 of Table 2 of Patent Document 1 was implemented, that is, the heat treatment process shown in FIG. 3 was repeated 3 times. Also, in Sample No. 18 shown in Table 3, as the manufacturing condition M, heat treatment conditions satisfying the manufacturing conditions of Patent Document 2 were implemented. Specifically, condition c in Table 2 of Patent Document 2 was used, that is, heat treatment in the pattern shown in FIG. 4 was performed. The annealing parameters described in Table 2 are the set temperatures of the heat treatment furnace. When a thermocouple was attached to the steel material and the deviation between the actual temperature of the steel material and the set temperature was tested, it was confirmed that the temperature of the steel material and the set temperature were comparable.

[0072] Using the sample obtained by the above annealing, as an evaluation of the metallographic structure, the average value of the ferrite crystal grain size, the average equivalent circle diameter of all cementite, and the half-width of the X-ray diffraction peak on the (211) plane of the ferrite grains were determined as follows. Also, as characteristics, the hardness after spheroidizing annealing and the hardness after quenching were measured and evaluated by the following methods.

[0073] [Metal structure evaluation] [Average ferrite grain size] First, the ferrite grain size was measured as follows. The test specimen was embedded in resin so that the cross section of the steel wire after spheroidizing annealing, i.e., the D / 4 position (D: diameter of the steel wire) of the cross section perpendicular to the axial direction of the steel wire, could be observed. The test specimen was then etched using an etching solution containing nital (2% by volume of nitric acid and 98% by volume of ethanol) to reveal the microstructure. The microstructure of the test specimen was then observed at 400x magnification using an optical microscope. A field of view within the evaluation surface was selected in which ferrite grains of average size representative of the microstructure of the entire steel wire could be observed, and a micrograph was taken. The ferrite grain size (G) value was then calculated from the photograph based on the comparative method of JIS G0551 (2020). Then, using the calculated value of ferrite grain size (G), the average ferrite grain size dn was calculated from the following formula (4), which shows the relationship between the ferrite grain size G(orN) and the average ferrite grain size dn in the relationship between the quantities related to grain size and grain size listed in Table 1 on page 32 of "Introductory Lecture on Technical Terms - Steel Materials - 3: Grain Size Number and Grain Size" by Umemoto Minoru, Ferum Vol. 2 (1997) No. 10, pp. 29-34. The results are shown in Table 3. In this example, all of Samples Nos. 1-10 in Table 3 had a ferrite area fraction of 90% or more. dn=0.254 / (2 (G-1) / 2 ) ···(4)

[0074] [FWHM of X-ray diffraction peak on the (211) plane of ferrite grains] The half-width of the X-ray diffraction peak of ferrite grains was measured at the D / 4 position (D: diameter) of the steel wire after spheroidizing annealing, measuring the half-width of the X-ray diffraction peak of the (211) plane of ferrite. Specifically, the test specimen was embedded in resin so that the cross section of the steel wire after spheroidizing annealing could be observed, and then polished with an Emily buff, diamond buffed, and electropolished to remove strain on the evaluation surface introduced during sample preparation. Then, the half-width of the peak near 148° to 165° of ferritic iron was determined using a Rigaku PSPC (Position-Sensitive Proportional Counter) micro-X-ray stress measurement device. The measured value was the average of two measurements. Other conditions for the X-ray diffraction were as follows: Target:Cr Acceleration voltage: 40kV ·Acceleration current: 40mA Collimator: φ0.5mm

[0075] [Average circular equivalent diameter of all cementite] To measure the average equivalent circle diameter of all cementite in the steel wire after spheroidizing annealing, the test specimen was embedded in resin so that the cross section could be observed, and the cut surface was mirror-polished using emery paper and a diamond buff. The cut surface was then etched for 30 seconds to 1 minute using nital (2% by volume of nitric acid, 98% by volume of ethanol) as an etching solution to reveal the ferrite grain boundaries and cementite at the D / 4 position (D: diameter of the steel wire). The test specimen with the revealed cementite and other features was then subjected to microstructural observation using a field-emission scanning electron microscope (FE-SEM), and three fields of view were photographed at a magnification of 2500x.

[0076] An OHP film was placed over the micrograph taken above, and all cementite in the micrograph was filled in from above the OHP film to obtain a projection image for analyzing all cementite. The projection image was binarized to a black-and-white photograph, and the circle-equivalent diameter of all cementite was calculated using image analysis software "Particle Analysis ver. 3.5" (Nippon Steel Technology Co., Ltd.). The average circle-equivalent diameter of all cementite listed in Table 3 is the average value of values calculated from three fields of view. The minimum size (circle-equivalent diameter) of cementite to be measured was 0.3 μm.

[0077] [Evaluation of characteristics] [Hardness measurement after spheroidizing annealing] To evaluate the cold workability, the hardness of each sample after spheroidizing annealing was measured as follows. A Vickers hardness test was conducted at the D / 4 position (D: steel wire diameter) of the cross section of the test piece in accordance with JIS Z 2244 (2009). The Vickers hardness obtained by calculating the average of three or more points was defined as the hardness after spheroidizing annealing. The measurement results are shown in Table 3. In Table 3, the hardness after spheroidizing annealing is shown as "spheroidized hardness." In this example, when the hardness after spheroidizing annealing satisfies the following formula (2), where the C content (mass%), Cr content (mass%), and Mo content (mass%) in the steel are expressed as [C], [Cr], and [Mo] (elements not contained are defined as zero mass%), the cold workability was evaluated as "OK" for excellent hardness, and when the hardness did not satisfy the following formula (2), the cold workability was evaluated as "NG" for poor hardness. Hardness after spheroidizing annealing (HV) < 91 ([C] + [Cr] / 9 + [Mo] / 2) + 91 (2)

[0078] [Measurement of hardness after quenching] To evaluate hardenability, the hardness of each sample after quenching was measured as follows. First, as a sample for quenching, each sample after spheroidizing annealing was processed to a thickness (t) of 5 mm, which is the length in the rolling direction, so that the sample would be sufficiently hardened by quenching. As a quenching treatment, the sample was held at a high temperature of A3+ (30 to 50°C) for 5 minutes, and then water-cooled after the high-temperature holding. The A3 is a value derived from the following formula (5). Here, the high-temperature holding time was defined as the time after the furnace temperature reached the set temperature. A3(°C) = 910 - 203×√([C]) - 14.2×[Ni] + 44.7×[Si] + 104×[V] + 31.5×[Mo] + 13.1×[W] - 30×[Mn] - 11×[Cr] - 20×[Cu] + 700×[P] + 400×[Al] + 120×[As] + 400×[Ti] ··· (5) Here, [element] represents the content (% by mass) of each element, and elements not contained are calculated as 0%.

[0079] Then, at the t / 2 position and D / 4 position (D: diameter of the steel wire, t: thickness of the sample) of the sample after the quenching treatment, a Vickers hardness test was carried out. The Vickers hardness obtained by calculating the average of three or more points was taken as the hardness after the quenching treatment. The measurement results are shown in Table 3. In Table 3, the hardness after the quenching treatment is indicated as "quenching hardness". In this example, when the hardness after the quenching treatment satisfies the following formula (3) when the C content (% by mass) in the steel is represented by [C], it is evaluated as "OK" as having excellent hardenability, and when it does not satisfy the following formula (3), it is evaluated as "NG" as having poor hardenability. Hardness after quenching treatment (HV) > 380ln([C]) + 1010 ··· (3)

[0080] In Table 3, when both the hardness after the spheroidizing annealing and the hardness after the quenching treatment are OK, the comprehensive judgment is "OK" as having both excellent cold workability and excellent hardenability, and when at least one of the hardness after the spheroidizing annealing and the hardness after the quenching treatment is NG, the comprehensive judgment is "NG" as not being able to have both excellent cold workability and excellent hardenability. In Tables 2 and 3, the underlined numerical values indicate that they are outside the range defined in the embodiments of the present invention or do not satisfy the desired characteristics.

[0081]

Table 1

[0082]

Table 2

[0083] [Table 3]

[0084] The results in the table will be considered. The following numbers indicate the sample numbers in Table 3. Nos. 1 to 10 are inventive examples that satisfy all of the component compositions, metal structures, and spheroidizing annealing conditions specified in the embodiments of the present invention.

[0085] For Nos. 11, 12, 19, and 21 to 26, the cooling-heating process was not performed or was performed only once. As a result, the amount of strain in the ferrite increased due to the rod-shaped cementite with a lot of interfacial strain generated in step [3] in Figure 1, and the half-width of the X-ray diffraction peak exceeded 0.500°. As a result, the hardness after spheroidizing annealing was higher than the standard value, resulting in poor cold workability.

[0086] No. 13 is an example in which annealing was performed under annealing conditions SA2 in Patent Document 3, which are manufacturing conditions H that satisfy the manufacturing conditions set forth in Patent Document 3. Under these manufacturing conditions, cementite was excessively coarsened by annealing, and the hardness after quenching was lower than the reference value, resulting in poor hardenability.

[0087] For Nos. 14 and 20, T1 was 730°C, which was below A1+8°C. Therefore, rod-shaped cementite with a lot of interfacial strain remained before annealing, increasing the amount of strain in the ferrite, and the half-width of the X-ray diffraction peak exceeded 0.500°. As a result, the hardness after spheroidizing annealing was higher than the standard value, resulting in poor cold workability.

[0088] For No. 15, the average cooling rate R3 was high at 21°C / hour, and therefore rod-shaped cementite with a lot of interfacial strain formed in step [8] in Figure 1 increased the amount of strain in the ferrite, causing the half-width of the X-ray diffraction peak to exceed 0.500°. As a result, the hardness after spheroidizing annealing was higher than the standard value, resulting in poor cold workability.

[0089] For No. 16, since T2 was as high as 710°C, the amount of strain in the ferrite increased due to the rod-shaped cementite with a large amount of interfacial strain generated in the process [3] of Fig. 1, and the half-value width of the X-ray diffraction peak exceeded 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0090] No. 17 is an example of annealing under manufacturing condition L that satisfies the manufacturing conditions shown in Patent Document 1. Under this manufacturing condition, due to the absence of holding of t1 and other reasons, the amount of strain in the ferrite increased due to the rod-shaped cementite with a large amount of interfacial strain remaining from before annealing, and the half-value width of the X-ray diffraction peak exceeded 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0091] No. 18 is an example of annealing under condition c of Patent Document 2 as manufacturing condition M that satisfies the manufacturing conditions shown in Patent Document 2. Due to the rod-shaped cementite with a large amount of interfacial strain remaining from before annealing, the amount of strain in the ferrite increased, and the half-value width of the X-ray diffraction peak exceeded 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

Industrial Applicability

[0092] The steel wire for machine structural parts according to the present embodiment has low deformation resistance at room temperature when various machine structural parts are manufactured, and can suppress wear and breakage of plastic processing tools such as dies. It also exhibits excellent cold workability, for example, by suppressing the occurrence of cracks during heading. Furthermore, since it has excellent hardenability, high hardness can be ensured by quenching treatment after cold working. For these reasons, the steel wire for machine structural parts according to the present embodiment is useful as a steel wire for machine structural parts to be cold worked. For example, the steel wire for machine structural parts according to the present embodiment can be used to manufacture various machine structural parts such as automobile parts and construction machinery parts by subjecting it to cold working such as cold forging, cold heading, and cold rolling. Specific examples of such mechanical structural parts include mechanical parts and electrical parts such as bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, washers, tappets, saddles, valves, inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, nozzles, valve lifters, spark plugs, pinion gears, steering shafts, and common rails.

Claims

1. C: 0.05% by mass to 0.60% 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 and 0.050% by mass or less, S: More than 0% by mass and 0.050% by mass or less, Al: 0.001% by mass to 0.10% by mass, Cr: More than 0% by mass and 1.5% by mass or less, and N: More than 0% by mass and 0.02% by mass or less, containing the balance consisting of iron and unavoidable impurities, the half-value width of the X-ray diffraction peak on the (211) plane of ferrite grains is 0.500° or less, when the minimum size (equivalent circle diameter) of the measured cementite is 0.3 μm, the average equivalent circle diameter of all cementite is (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less, where the C content (% by mass) in the steel is represented by [C], a steel wire for mechanical structural parts, wherein the average value of the ferrite crystal grain size is 30 μm or less.

2. Furthermore, Cu: More than 0% by mass and 0.25% by mass or less, Ni: More than 0% by mass and 0.25% by mass or less, Mo: More than 0% by mass and 0.50% by mass or less and B: containing one or more selected from the group consisting of more than 0% by mass and 0.01% by mass or less, the steel wire for mechanical structural parts according to Claim 1.

3. Furthermore, Ti: More than 0% by mass and 0.2% by mass or less, Nb: More than 0% by mass and 0.2% by mass or less, and V: containing one or more selected from the group consisting of more than 0% by mass and 0.5% by mass or less, the steel wire for mechanical structural parts according to Claim 1 or 2.

4. Furthermore, Mg: More than 0% by mass and 0.02% by mass or less, Ca: More than 0% by mass and 0.05% by mass or less, Li: More than 0% by mass and 0.02% by mass or less, and REM: containing one or more selected from the group consisting of more than 0% by mass and 0.05% by mass or less, the steel wire for mechanical structural parts according to any one of Claims 1 to 3.

5. A method for manufacturing a steel wire for mechanical structural parts according to any one of Claims 1 to 4, comprising subjecting a bar steel satisfying the chemical composition according to any one of Claims 1 to 4 to spheroidizing annealing including the following steps (1) to (3). The method for manufacturing a steel wire for mechanical structural parts according to any one of Claims 1 to 4, comprising subjecting the bar steel to spheroidizing annealing including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8°C) to (A1 + 31°C), holding at the temperature T1 for more than 1 hour and 6 hours or less, (2) Cooling to a temperature T2 above 650°C and below (A1 - 17°C), and then heating to a heating temperature higher than the temperature T2 and below (A1 + 60°C), and performing the cooling-heating process a total of 2 to 6 times. Cool it to a temperature below (A1 - 30°C) at an average cooling rate of 5°C / hour to 20°C / hour. Here, A1 is calculated by the following formula (1). A1 (°C) = 723 + 29.1×[Si] - 10.7×[Mn] + 16.9×[Cr] - 16.9×[Ni]... (1) However, [element] represents the content (mass%) of each element, and the content of elements not contained is taken as zero.

6. The method for manufacturing a steel wire for a mechanical structure part according to claim 5, wherein the bar steel is a steel wire obtained by subjecting a wire rod to wire drawing with a reduction ratio of more than 5%.

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