Steel wire for mechanical structure parts and method for manufacturing the same

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% and 0.02 mass% or less, and the balance made of iron and inevitable impurities, and an area of cementite present at the ferrite grain boundaries is 32% or more with respect to an area of all cementite, and an 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.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] When manufacturing various machine structure 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 to finally adjust the strength, and machine structure 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. When heating to a temperature range of A1 point to A1 point + 30°C and holding and then cooling in the temperature range of A1 point to A1 point + 30°C, when cooling after reaching the A1 point and holding in the temperature range of A1 point to A1 point + 30°C until reaching the A1 point, 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 and improve the crack resistance, and can exhibit excellent cold workability. The metal structure of the steel is composed of ferrite and cementite, and the proportion of the number of cementite present at the ferrite grain boundaries is 40% or more based on 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 higher, the second cooling with an average cooling rate of 1°C / sec or higher and 5°C / sec or lower, and the third cooling with an average cooling rate faster than the second cooling and 5°C / sec or higher are performed in this order. The end of the first cooling and the start of the second cooling are performed within the range of 700 to 750°C, the end of the second cooling and the start of the third cooling are performed within the range of 600 to 650°C, and it is shown that it is preferable to set the end of the third cooling to 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 circumstances, and an object thereof is to provide a steel wire for mechanical structure parts having a sufficiently low hardness, excellent cold workability, and capable of obtaining a high hardness by quenching treatment, that is, excellent hardenability, and a method for manufacturing a steel wire for mechanical structure parts that can manufacture the 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 mass% to 1.20 mass%, P: More than 0 mass%, 0.050 mass% or less, S: More than 0 mass%, 0.050 mass% or less, Al: 0.001 mass% to 0.10 mass%, Cr: More than 0 mass%, 1.5 mass% or less, and N: More than 0 mass%, 0.02 mass% or less containing, with the balance being iron and unavoidable impurities, the proportion of the area of cementite present at the ferrite grain boundaries being 32% or more with respect to the total area of cementite, and the average equivalent circle diameter of the total cementite being (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less when the C content (mass%) in the steel is represented by [C], is a steel wire for mechanical structural parts.

[0012] Aspect 2 of the present invention is further, 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, selected from the group consisting of one or more, is the steel wire for mechanical structural parts according to Aspect 1.

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

[0014] Aspect 4 of the present invention is further, Mg: More than 0 mass%, 0.02 mass% or less, Ca: More than 0 mass%, 0.05 mass% or less, Li: More than 0 mass %, 0.02 mass % or less, and REM: It contains one or more selected from the group consisting of more than 0 mass % and 0.05 mass % or less, and is the steel wire for mechanical structure parts according to any one of Aspects 1 to 3.

[0015] Aspect 5 of the present invention is the steel wire for mechanical structure parts according to any one of Aspects 1 to 4, wherein the average value of the ferrite crystal grain size is 30 μm or less.

[0016] Aspect 6 of the present invention is a method for manufacturing a steel wire for mechanical structure parts according to any one of Aspects 1 to 5, including a step of subjecting a bar steel satisfying the chemical composition according to any one of Aspects 1 to 4 to spheroidizing annealing including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8 °C) or higher, holding at the temperature T1 for more than 1 hour and 6 hours or less, (2) Cooling at an average cooling rate R1 of 10 °C / hour to 30 °C / hour to a temperature T2 of more than 650 °C and (A1 - 17 °C) or lower, and then heating to a heating temperature higher than the temperature T2 and (A1 + 60 °C) or lower, and performing the cooling-heating step a total of 2 to 6 times, (3) Cooling from the heating temperature of the final cooling-heating step. 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 an element not contained is zero.

[0017] Aspect 7 of the present invention is the method for manufacturing a steel wire for mechanical structure parts according to Aspect 6, 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%.

Advantages of the Invention

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

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0020] The inventors of the present invention have earnestly studied from various angles in order to realize a steel wire for mechanical structural parts having excellent cold workability and hardenability. As a result, particularly in the metal structure, the ratio of the area of cementite existing at the ferrite grain boundaries to the area of all cementite is set to be a certain value or more, and the average size of all cementite is set within a certain range according to the C content in the steel. Furthermore, in order to realize the above metal structure, it is found that the chemical composition should be within a certain range, and in the method for manufacturing a steel wire for mechanical structural parts, it is effective to perform spheroidizing annealing under particularly specified conditions. Hereinafter, first, the steel wire for mechanical structural parts according to the present embodiment will be described in terms of the metal structure of the steel wire for mechanical structural parts.

[0021] 1. Metal Structure [Ratio of the area of cementite existing at the ferrite grain boundaries to the area of all cementite: 32% or more] When the proportion of cementite present at the ferrite grain boundaries is reduced and the proportion of cementite within the ferrite grains increases relatively, the movement of dislocations introduced into the ferrite grains during cold working is hindered by the cementite within these ferrite grains. As a result, it causes an increase in dislocations, exhibits work hardening, and is inferior in cold workability. In the present embodiment, for the purpose of reducing the proportion of cementite within the ferrite grains and suppressing the hardness of the steel wire for mechanical structural parts, the area ratio of the cementite present at the ferrite grain boundaries is set to 32% or more with respect to the total area of all cementite. The "cementite present at the ferrite grain boundaries" includes both the cementite in contact with the ferrite grain boundaries and the cementite existing on the ferrite grain boundaries. Hereinafter, the "area ratio of the cementite present at the ferrite grain boundaries" may be referred to as the "grain boundary cementite ratio". The grain boundary cementite ratio is preferably 35% or more, more preferably 40% or more, and still more preferably 45% or more. On the other hand, since the higher the grain boundary cementite ratio, the more preferable it is, there is no particular upper limit provided, and it may be 100%.

[0022] 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. Note that 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 grain boundary cementite ratio described later is the minimum size. Specifically, cementite particles with an equivalent circle diameter of 0.3 μm or more are the measurement targets.

[0023] [When 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 [C] represents the C content (mass%) in the steel] When the amount of cementite in 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, the more difficult it is to achieve precipitation strengthening, and as a result, the hardness can be reduced. From these viewpoints, in the present invention, the average equivalent circle diameter of all the cementites is set to be (1.668 - 2.13[C]) μm or more when the C content (mass%) in the steel is represented by [C]. The average equivalent circle diameter of all the cementites is preferably (1.669 - 2.13[C]) μm or more. On the other hand, if the cementite becomes too coarse, during the high-temperature holding in the quenching process after cold working, the cementite cannot be sufficiently dissolved, and a sufficiently high hardness cannot be obtained by quenching. Therefore, in the present invention, the average equivalent circle diameter of all the cementites is set to be (1.863 - 2.13[C]) μm or less. Preferably, it is (1.858 - 2.13[C]) μm or less.

[0024] Patent Document 3 shows that the cementite existing at the ferrite grain boundaries has a smaller amount of strain received during cold working compared to the cementite existing within the ferrite grains, and thus can reduce the deformation resistance. However, in Patent Document 3, the average size of all the cementites is not controlled. As a result, the cementite cannot be sufficiently dissolved during the high-temperature holding in the quenching process, and the hardenability is poor. The present invention is a technology that focuses on both the grain boundary cementite ratio and the average size of all the cementites in order to realize a steel wire for mechanical structural parts having excellent cold workability and excellent hardenability.

[0025] The metallographic structure of the steel wire for mechanical structural parts according to the present embodiment is a spheroidized structure having spheroidized cementite, and can be obtained by subjecting a bar steel satisfying the chemical component composition described below to, for example, spheroidizing annealing described below.

[0026] The metallographic structure of the steel wire for machine structural 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 machine structural 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.

[0027] In this specification, "ferrite" refers to a portion having a bcc crystal structure, and also includes ferrite in pearlite, which is a layered structure of ferrite and cementite. In addition, the "ferrite crystal grain" which is the measurement object of the "ferrite crystal grain size" includes crystal grains containing rod-shaped cementite generated during spheroidizing annealing with insufficient spheroidization, but crystal grains (pearlite crystal grains) containing rod-shaped cementite that can remain before spheroidizing annealing are excluded. Specifically, after etching with nital (2% by volume of nitric acid, 98% by volume of ethanol), it refers to "crystal grains in which cementite does not exist inside the grains" and "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 magnification using an optical microscope. Crystal grains in which the shape of cementite cannot be observed (that is, the boundary between cementite and ferrite cannot be clearly observed) at 1000 times magnification using the above optical microscope are not the object of judgment in this embodiment and are not included in the "ferrite crystal grains".

[0028] [Average value of ferrite crystal grain size: 30 μm or less] The steel wire for mechanical structure parts according to this embodiment preferably has an average value of ferrite crystal grain size in the metal structure of 30 μm or less. If the average value of the ferrite crystal grain size is 30 μm or less, the ductility of the steel wire for mechanical structure parts can be improved, and the occurrence of cracks during cold working can be further suppressed. The average value of the ferrite crystal grain size is more preferably 25 μm or less, and still more preferably 20 μm or less. The smaller the average value of the ferrite crystal grain size, the better. However, considering possible manufacturing conditions and the like, the lower limit can be approximately 2 μm.

[0029] (Characteristic) The steel wire for mechanical structure parts according to this embodiment that satisfies the following chemical composition and has the above-described metal structure can achieve both a low hardness that enables good cold working and a high hardness after quenching treatment. In this embodiment, when the C content (mass %), Cr content (mass %), and Mo content (mass %) in the steel are represented by [C], [Cr], and [Mo] respectively (elements not included are considered to have a zero mass %), when the hardness, specifically the hardness after spheroidizing annealing in the examples described later, satisfies the following formula (2), and the hardness after quenching treatment satisfies the following formula (3), it is determined that the hardness is sufficiently low and the cold workability is excellent, and at the same time, the high hardness after quenching treatment is achieved, that is, the hardenability is excellent. Hardness (HV) after spheroidizing annealing < 91([C] + [Cr] / 9 + [Mo] / 2) + 91 ··· (2) Hardness (HV) after quenching treatment > 380ln([C]) + 1010 ··· (3)

[0030] 2. Chemical composition The chemical composition of the steel wire for mechanical structure parts according to this embodiment will be described.

[0031] [C: 0.05 mass % to 0.60 mass %] C is an element that controls the strength of steel, 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 was set at 0.05% by mass. The C content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and still more preferably 0.20% by mass or more. However, if the C content is excessive, the number of spherical cementite in the structure after spheroidizing annealing becomes excessive and the hardness increases, resulting in a decrease in cold workability. Therefore, the upper limit of the C content was determined to be 0.60% by mass. The C content is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

[0032] [Si: 0.005% by mass to 0.50% by mass] Si is used as a deoxidizer during melting and also contributes to the improvement of strength. To effectively exert the effect, the lower limit of the Si content was set at 0.005% by mass. The Si content is preferably 0.010% by mass or more, more preferably 0.050% by mass or more. However, Si contributes to the solid solution strengthening of ferrite and has the effect of significantly increasing the strength after spheroidizing annealing. If the Si content is excessive, the cold workability deteriorates due to the above effect, so the upper limit of the Si content was set at 0.50% by mass. The Si content is preferably 0.40% by mass or less, more preferably 0.35% by mass or less.

[0033] [Mn: 0.30% by mass to 1.20% by mass] Mn effectively acts as a deoxidizer and is an element that contributes to the improvement of hardenability. To fully exert the effect, the lower limit of the Mn content was set at 0.30% by mass. The Mn content is preferably 0.35% by mass or more, more preferably 0.40% by mass or more. However, if the Mn content is excessive, segregation is likely to occur and the toughness decreases. Therefore, the upper limit of the Mn content was set at 1.20% by mass. The Mn content is preferably 1.10% by mass or less, more preferably 1.00% by mass or less.

[0034] [P: More than 0% by mass and 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 was set at 0.050 mass% or less. The P content is preferably 0.030 mass% or less, more preferably 0.020 mass% or less. The lower the P content, the more preferable, but it usually contains 0.001 mass% or more.

[0035] [S: More than 0 mass%, 0.050 mass% or less] S (sulfur) is an inevitable impurity and a harmful element for cold workability because it forms MnS in steel and deteriorates ductility. Therefore, the S content was set at 0.050 mass% or less. The S content is preferably 0.030 mass% or less, more preferably 0.020 mass% or less. The lower the S content, the more preferable, but it usually contains 0.001 mass% or more.

[0036] [Al: 0.001 mass% - 0.10 mass%] Al is an element contained as a deoxidizer and has the effect of reducing impurities during deoxidation. To exert this effect, the lower limit of the Al content was set at 0.001 mass%. The Al content is preferably 0.005 mass% or more, more preferably 0.010 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 was set at 0.10 mass%. The Al content is preferably 0.08 mass% or less, more preferably 0.05 mass% or less.

[0037] [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 a spheroidized structure is more easily obtained. Therefore, the Cr content is preferably more than 0 mass% and 0.01 mass% or more. It may be further 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 further more than 0.30 mass%, and can also be further 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.

[0038] [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 dissolved N is contained 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.

[0039] [Remainder] The balance consists of iron and inevitable impurities. As inevitable impurities, the inclusion of trace elements (such as As, Sb, Sn, etc.) brought in according to the conditions of raw materials, materials, manufacturing equipment, etc. is allowed. For example, elements such as P and S are usually preferably present in smaller amounts, and thus are inevitable impurities, but there are elements whose composition ranges are separately defined as above. Therefore, in this specification, when referring to "inevitable impurities" constituting the balance, it is a concept excluding elements whose composition ranges are separately defined.

[0040] The steel wire for mechanical structure parts according to this embodiment only needs to contain the above elements in its chemical component 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.

[0041] [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 content increases. 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, and still more preferably 0.05 mass% or more, and for B it is more than 0 mass%, more preferably 0.0003 mass% or more, and still more preferably 0.0005 mass% or more.

[0042] 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 element are 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.

[0043] [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 deformation resistance. Therefore, they can be contained alone or in combination of two or more as necessary. The effect of these elements increases as their 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 deformation resistance, and conversely, the cold workability may decrease. 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.

[0044] [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-based inclusions such as MnS and improving the deformability of steel. Such effects increase as their contents increase. To effectively exhibit the above effects, the contents of Mg, Ca, Li, and REM are preferably each more than 0% by mass, more preferably 0.0001% by mass or more, and still more preferably 0.0005% by mass or more. However, even if contained in excess, the effects saturate and effects commensurate with the contents cannot be expected. Therefore, the contents of Mg and Li are preferably each 0.02% by mass or less, more preferably 0.018% by mass or less, and still more preferably 0.015% by mass or less. The contents of Ca and REM are preferably each 0.05% by mass or less, more preferably 0.045% by mass or less, and still more preferably 0.040% by mass or less. Note that Mg, Ca, Li, and REM may each be contained alone or in combination of two or more. When contained in combination of two or more, the content of each may be any content within the above range. The REM means including lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0045] The shape etc. of the steel wire for mechanical structural parts according to the present embodiment are not particularly limited. For example, those having a diameter of 5.5 mm to 60 mm can be mentioned.

[0046] 3. Manufacturing method To obtain the metal structure of the steel wire for mechanical structural parts according to the present invention embodiment, when manufacturing the steel wire for mechanical structural parts, it is preferable to appropriately control the spheroidizing annealing conditions as described below. The hot rolling process for manufacturing the wire rod or bar to be subjected to spheroidizing annealing is not particularly limited, and may follow a conventional method. As described later, wire drawing may be applied before spheroidizing annealing. The diameter of the wire rod, steel wire, or bar which is the bar steel to be subjected to spheroidizing annealing is not particularly limited. In the case of wire rod and steel wire, for example, it is 5.5 mm to 60 mm, and in the case of bar steel, for example, it is 18 mm to 105 mm.

[0047] While referring to FIG. 1, the spheroidizing annealing conditions in the method for manufacturing a steel wire for mechanical structural parts according to an embodiment of the present invention will be described. FIG. 1 shows an example of a diagram for explaining the spheroidizing annealing conditions in the manufacturing method according to the embodiment of the present invention, and the number of repetitions of the cooling-heating process and the like are not limited to FIG. 1.

[0048] The method for manufacturing a steel wire for mechanical structural parts according to an embodiment of the present invention includes a spheroidizing annealing process including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8°C) or higher, heat and hold at the temperature T1 for more than 1 hour and 6 hours or less. (2) Cool at an average cooling rate R1 of 10°C / hour to 30°C / hour to a temperature T2 of higher than 650°C and (A1 - 17°C) or lower, and then heat to a heating temperature higher than the temperature T2 and (A1 + 60°C) or lower. The cooling-heating process is carried out a total of 2 to 6 times. (3) Cool from the heating temperature of the final time of the cooling-heating process. 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 (% by mass) of each element, and the content of an element not contained is set to zero.

[0049] [(1) Heating and holding ( [2] in FIG. 1) after heating to a temperature T1 of (A1 + 8°C) or higher and then heating and holding at the temperature T1 for more than 1 hour and 6 hours or less] By heating to a temperature T1 above (A1 + 8°C), the dissolution of rod-shaped cementite with a large aspect ratio generated in the rolling stage is promoted. If the temperature T1 is too low, the rod-shaped cementite will not dissolve during the heat retention, and will continue to remain in the ferrite, increasing the hardness. 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, to sufficiently suppress excessive coarsening of the crystal grains, more easily precipitate spherical cementite at the ferrite crystal grain boundaries during the cooling process of the next step, and suppress the remaining amount of rod-shaped cementite to more easily reduce the hardness, it is preferable to set the temperature T1 to (A1 + 57°C) or lower.

[0050] Also, if the heat retention time (t1) is too short, the rod-shaped cementite will remain in the ferrite crystal grains, increasing the hardness. To obtain a sufficiently softened steel wire, the heat retention time (t1) needs to be more than 1 hour and 6 hours or less. The preferable heat retention time (t1) is 1.5 hours or more, and more preferably 2.0 hours or more. If the heat retention time (t1) is too long, the heat treatment time will be long and the productivity will decrease. Therefore, the heat retention 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 a temperature T1 above (A1 + 8°C) ([1] in Figure 1) does not affect the steel material properties, the heating can be performed at any rate. For example, the heating rate can be 30°C / hour to 100°C / hour.

[0051] Note that the temperature of the above A1 point is calculated by the following formula (1) described on page 273 of Leslie's Steel Material Science (Maruzen). 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 set to zero.

[0052] [(2) Cool at an average cooling rate R1 of 10°C / hour to 30°C / hour to a temperature T2 above 650°C and below (A1 - 17°C), and then heat to a heating temperature higher than temperature T2 and below (A1 + 60°C). Perform the cooling-heating process a total of 2 to 6 times ([3] to [7] in Fig. 1)]

[0053] (2-i) Cool at an average cooling rate R1 of 10°C / hour to 30°C / hour to a temperature T2 above 650°C to (A1 - 17°C) (Figs. 1 [3] and [4]) Cool to precipitate spherical cementite on the ferrite grain boundaries. If the average cooling rate R1 from temperature T1 is too fast, rod-shaped cementite will re-precipitate excessively, resulting in a decrease in cold workability. Therefore, the average cooling rate R1 should be 30°C / hour or less. The average cooling rate R1 is preferably 25°C / hour or less, and more preferably 20°C / hour or less. On the other hand, if the average cooling rate R1 is too slow, the cementite formed during cooling will coarsen excessively. As a result, the cementite will not be sufficiently dissolved during the high-temperature holding in the quenching process, leading to a decrease in the hardness after quenching, that is, deterioration of hardenability. Furthermore, it will lead to an increase in the annealing time and a decrease in productivity. Therefore, the average cooling rate R1 should be 10°C / hour or more, preferably 11°C / hour or more, and more preferably 12°C / hour or more.

[0054] Also, if the reaching temperature T2 during cooling at the average cooling rate R1 is too low, it will lead to an increase in the annealing time. Therefore, the reaching temperature T2 of cooling needs to exceed 650°C. According to the manufacturing method according to this embodiment, even when the reaching temperature T2 of cooling exceeds 650°C, cementite can be controlled to a desired form without performing long-time annealing. The reaching temperature T2 of cooling is preferably 670°C or higher. On the other hand, if the reaching temperature T2 of cooling is too high, rod-shaped cementite will excessively re-precipitate in the ferrite crystal grains, increasing the hardness and reducing the cold workability. Therefore, the upper limit of the reaching temperature T2 of cooling is set to A1 - 17°C. The reaching temperature T2 of cooling is preferably A1 - 18°C or lower. Also, if it is held after reaching the reaching temperature T2 of cooling, it will cause an increase in the 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 (t2) at the reaching temperature T2 of cooling is preferably within 1 hour.

[0055] (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 precipitated in the ferrite crystal grains in the step (2-i) above, heating is performed from the reaching temperature T2 of the above cooling. The reaching temperature of heating, that is, the heating temperature, as shown in [6] of FIG. 1, 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 generated in the step (2-i) above, the heating temperature is preferably A1°C or higher. Also, from the viewpoint of suppressing the redissolution of the spherical cementite on the ferrite crystal grain boundaries and suppressing the increase in hardness after spheroidizing annealing, the heating temperature is preferably (A1 + 57°C) or lower.

[0056] The average heating rate during the temperature rise from the cooling reach temperature T2 to the heating temperature, as shown in [5] of FIG. 1, is not particularly limited either. For example, from the viewpoint of more sufficiently redissolving the rod-shaped cementite in the ferrite crystal grains 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. Also, for example, from the viewpoint of sufficiently suppressing the coarsening of the cementite generated by this heating and enhancing the hardenability more, it can be set to 5°C / hour or more.

[0057] After reaching the above heating temperature, whether or not to hold at the heating temperature does not matter. When holding at the heating temperature, for example, the holding time can be within 1 hour to suppress the redissolution of the spherical cementite on the ferrite crystal grain boundaries.

[0058] In the manufacturing method according to this embodiment, the cooling-heating process of the above (2-i) cooling and this (2-ii) heating is repeated a plurality of times. However, in each time, the average cooling rate R1 and the temperature T2 need to satisfy the above ranges.

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

[0060] (2-iii) The cooling-heating process is carried out a total of 2 to 6 times (in [7] of FIG. 1) In order to increase the proportion of cementite present at the ferrite grain boundaries and promote the coarsening of the cementite present at the ferrite grain boundaries, after heating and holding at temperature T1 in the above (1), it is necessary to perform the cooling-heating steps of the above (2-i) and (2-ii) a total of 2 or more times. If this cooling-heating step is not repeated, the proportion of cementite present at the ferrite grain boundaries will be insufficient, or the coarsening of the cementite present at the ferrite grain boundaries will be insufficient, and the hardness after spheroidizing annealing will increase. Therefore, the above cooling-heating step is performed 2 or more times. Preferably, it is 3 or more times. The more the number of times of implementation, the lower the hardness, but even if the number of times of implementation is too large, the effect will saturate. Also, it will lead to a longer annealing time and reduce productivity. Therefore, the number of times of implementing the cooling-heating step is set to 6 or less. In the case of FIG. 1, the number of times of implementing the cooling-heating steps of the above (2-i) and (2-ii) is 4 times. Also, the reaching temperature T2 of each cooling and the average cooling rate R1 may be different within the respectively specified ranges. Also, the average cooling rate R1 refers to the average cooling rate from temperature T1 to the reaching temperature T2 of cooling in the first cooling-heating step, and from the heating temperature of [6] in FIG. 1 to the reaching temperature T2 of cooling in the second and subsequent steps.

[0061] [(Cooling from the heating temperature of the final time of the cooling-heating step ( [8] in FIG. 1))] Cool from the heating temperature in the final round of the cooling-heating process. The average cooling rate and the cooling reach temperature during this cooling are not particularly limited. From the perspective of further suppressing the reprecipitation of rod-like cementite, the average cooling rate may be, for example, 100 °C / hour or less. Also, from the perspective of further suppressing excessive coarsening of cementite, the average cooling rate may be 5 °C / hour or more. Also, the cooling reach temperature can be, for example, (A1 - 30 °C) or lower. For example, cooling to a temperature range of (A1 - 30 °C) or lower and (A1 - 100 °C) or higher at the above average cooling rate and then air-cooling can be mentioned. Or, for example, by setting it to less than (A1 - 100 °C), the reprecipitation of rod-like cementite can be further suppressed and the cold workability can be further improved. In this case, from the perspective of shortening the annealing time, the cooling reach temperature may be (A1 - 250 °C) or higher, further (A1 - 200 °C) or higher, and further (A1 - 150 °C) or higher.

[0062] The above spheroidizing annealing ((1) to (3) steps) may be repeated once or a plurality of times. From the perspective 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 above spheroidizing annealing is repeated a plurality of times, within the above specified range, it may be repeated under the same conditions or under different conditions. Also, when the above 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 the spheroidizing annealing described later → the first spheroidizing annealing → wire drawing → the second spheroidizing annealing.

[0063] In the method for manufacturing a steel wire for mechanical structure parts according to the present embodiment, the steps other than the above spheroidizing annealing step are not particularly limited. For example, after the 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.

[0064] In order to promote the generation of the structure of the present invention, it is preferable to provide a step of wire drawing the wire rod at a reduction ratio of more than 5% before spheroidizing annealing. By performing wire drawing at the above reduction ratio, the cementite in the steel is broken, and the aggregation of cementite can be promoted in the subsequent spheroidizing annealing, so that the cementite can be appropriately coarsened, which is effective for softening. The reduction ratio is more preferably 10% or more, still more preferably 15% or more, and even more preferably 20% or more. On the other hand, if the reduction ratio is excessively large, there is a possibility of causing a wire break risk. Therefore, the reduction ratio is preferably 50% or less. When wire drawing is performed multiple times, the number of wire drawing times is not particularly limited, and for example, it can be 2 times. In addition, when multiple wire drawing operations are performed, the "reduction ratio during wire drawing" means the reduction ratio from the steel material before wire drawing to the steel material after multiple wire drawing operations are performed.

Examples

[0065] Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and it is of course possible to appropriately modify and implement it within the range that can conform to the gist of the foregoing and the following, and all of them are included in the technical scope of the present invention.

[0066] After melting a test material having the chemical composition shown in Table 1 in a converter and casting it, hot rolling was performed on the obtained steel slab to produce a wire rod with a diameter of 12 to 16 mm. In Table 2 described later, in the case of "yes" for wire drawing before spheroidizing annealing, that is, in Sample No. 2 of Table 3 manufactured under manufacturing condition B, the wire rod was wire drawn at a reduction ratio of 25% to obtain a steel wire, which was subjected to spheroidizing annealing.

[0067] Using the above wire rod or steel wire, annealing was carried out using a laboratory furnace. In annealing, the wire rod or steel wire was heated to T1 shown in Table 2 and held for t1 hours. Then, it was cooled to the temperature T2 in Table 2 at the average cooling rate R1 in Table 2, and then heated to a heating temperature higher than the temperature T2 in Table 2 and below (A1 + 60°C). This cooling and heating process was carried out for the number of times of the cooling-heating process shown in Table 2. Then, it was cooled from the heating temperature in the final cycle of the cooling-heating process to obtain a sample.

[0068] As a comparative example, in Sample No. 14 shown in Table 3, as manufacturing condition J1, in the heat treatment process shown in FIG. 2, that is, the cooling-heating process, the heat treatment process was carried out 0 times. Incidentally, under this manufacturing condition J1, wire drawing was not performed at a reduction ratio of 25% before annealing. Further, in Sample No. 15 shown in Table 3, as manufacturing condition J2, using a steel wire obtained by wire drawing at a reduction ratio of 25% before annealing, the heat treatment process shown in FIG. 2, that is, the cooling-heating process, was carried out 0 times.

[0069] Furthermore, as a comparative example, in Sample No. 16 shown in Table 3, as manufacturing condition K, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 3 were used. Specifically, the conditions shown as SA2 in the examples of Patent Document 3 were implemented, that is, the heat treatment process shown in FIG. 3 was repeated 5 times. In Sample No. 20 shown in Table 3, as manufacturing condition O, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 1 were used. 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. 4 was repeated 3 times. Also, in Sample No. 21 shown in Table 3, as manufacturing condition P, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 2 were used. Specifically, the condition c in Table 2 of Patent Document 2, that is, the heat treatment of the pattern shown in FIG. 5 was carried out. T1 and T2, which are 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 about the same.

[0070] Using the samples obtained by the above annealing, as an evaluation of the metallographic structure, the average value of the ferrite crystal grain size, the average size of all cementites, and the grain boundary cementite ratio were determined as follows, respectively. Also, as characteristics, the hardness after spheroidizing annealing and the hardness after quenching were measured and evaluated by the following methods.

[0071] [Evaluation of Metallographic Structure] [Average Value of Ferrite Crystal Grain Size] First, the measurement of the ferrite crystal grain size was carried out as follows. The test piece was resin-embedded so that the cross-section of the steel wire after spheroidizing annealing, that is, 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. Using nitol (2% by volume of nitric acid, 98% by volume of ethanol) as the etching solution, the above test piece was etched to reveal the structure. Then, the structure of the test piece with the revealed structure was observed under an optical microscope at a magnification of 400 times, and one field of view where ferrite crystal grains of an average size representative of the entire structure of the steel wire could be observed within the evaluation plane was selected to obtain a micrograph. Next, the value of the ferrite crystal grain size (G) was calculated from the taken micrograph based on the comparison method of JIS G0551 (2020). Then, using the calculated value of the ferrite crystal grain size (G), in the relationship between various quantities regarding crystal grain size and particle diameter described in Table 1 on page 32 of "Introduction Course - Technical Terms - Steel Materials Edition - 3 Crystal Grain Number and Crystal Grain Diameter", Minoru Umemoto, Ferramu Vol.2 (1997) No.10, pp.29 - 34, the average value dn of the ferrite crystal grain diameter was obtained from the following formula (4) shown as the relationship between the ferrite crystal grain size G (or N) and the average value dn of the ferrite crystal grain diameter. The results are shown in Table 3. In this example, for all of Sample Nos. 1 to 13 in Table 3, the area ratio of ferrite was 90% or more. dn = 0.254 / (2 (G-1) / 2 ) ···(4)

[0072] [Average Size of Total Cementite and Cementite Ratio at Grain Boundaries] For the measurement of the average size of total cementite and the cementite ratio at grain boundaries of the steel wire after spheroidizing annealing, the test piece was resin-embedded so that the cross-section could be observed, and the cut surface was mirror-polished with emery paper and a diamond buff. Then, using nitol (2% by volume of nitric acid, 98% by volume of ethanol) as the etching solution, the cut surface was etched for 30 seconds to 1 minute to reveal the ferrite crystal grain boundaries and cementite at the D / 4 position (D: diameter of the steel wire). Then, using FE-SEM (Field-Emission Scanning Electron Microscope), the structure of the test piece with the revealed cementite, etc., was observed, and three fields of view were photographed at a magnification of 2500 times.

[0073] An OHP film was overlaid on the photographed micrograph, and the cementite present at the ferrite grain boundaries in the micrograph was painted over on the OHP film to obtain a first projection image for analyzing the grain boundary cementite. As described above, the "cementite present at the ferrite grain boundaries" includes both the cementite in contact with the ferrite grain boundaries and the cementite present on the ferrite grain boundaries.

[0074] Thereafter, on the above OHP film, the cementite in the ferrite grains was further painted over to obtain a second projection image for analyzing all the cementite.

[0075] The first projection image was binarized to obtain a black-and-white photograph, and the grain boundary cementite ratio was calculated using image analysis software "Particle Analysis Ver. 3.5" (manufactured by Nippon Steel Technology Co., Ltd.). Also, the second projection image was binarized to obtain a black-and-white photograph, and the equivalent circle diameter of all the cementite was calculated using the above image analysis software. Note that the average size and the grain boundary cementite ratio of all the cementite described in Table 3 are the average values of the values calculated from three fields of view.

[0076] The minimum size (equivalent circle diameter) of the cementite to be measured was set to 0.3 μm. Also, even if in contact with the ferrite grain boundaries, those cementite particles with an aspect ratio exceeding 3.0 are considered to extend not only to the ferrite grain boundaries but also into the ferrite grains and have the same influence as the cementite present in the ferrite grains, so they were judged to be "cementite in ferrite grains". In this specification, the aspect ratio is the ratio (major axis / minor axis) of the major axis, which is the longest length of the cementite particle, to the minor axis, which is the longest length in the direction perpendicular to the major axis.

[0077] 〔Evaluation of Properties〕 [Measurement of Hardness after Spheroidizing Annealing] To evaluate the cold workability, the hardness of each sample after spheroidizing annealing was measured as follows. At the D / 4 position (D: diameter of the steel wire) of the cross-section of the test piece, i.e., the section perpendicular to the rolling direction, a Vickers hardness test was carried out in accordance with JIS Z2244 (2009). The Vickers hardness obtained by calculating the average of three or more points was taken as the hardness after spheroidizing annealing. The measurement results are shown in Table 3. In Table 3, the hardness after spheroidizing annealing is indicated as "spheroidized hardness". In this example, when the hardness after spheroidizing annealing is expressed as [C], [Cr], and [Mo] for the C content (mass %), Cr content (mass %), and Mo content (mass %) in the steel, respectively (elements not included are taken as 0 mass %), the case where the following formula (2) is satisfied is evaluated as "OK" as having excellent cold workability, and the case where the following formula (2) is not satisfied is evaluated as "NG" as having poor cold workability. Hardness after spheroidizing annealing (HV) < 91([C] + [Cr] / 9 + [Mo] / 2) + 91 ···(2)

[0078] [Measurement of hardness after quenching treatment] To evaluate the hardenability, the hardness of each sample after quenching treatment was measured as follows. First, as a sample for quenching treatment, each sample after spheroidizing annealing was processed into a sample with a thickness (t) of 5 mm, which is the length in the rolling direction, so that sufficient hardening could be achieved by quenching treatment. For this sample, as the quenching treatment, high-temperature holding at A3 + (30 to 50 °C) for 5 minutes was carried out, and after the high-temperature holding, water cooling was performed. The A3 is the value derived from the following formula (5). Also, the time of high-temperature holding here was 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) However, [element] represents the content (mass %) of each element, and elements not included are calculated as 0%.

[0079] Then, a Vickers hardness test was carried out at the t / 2 position and the D / 4 position (D: diameter of the steel wire, t: thickness of the sample) of the sample after the quenching treatment. 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 (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 (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 scope of the present invention or do not satisfy the desired characteristics.

[0081]

Table 1

[0082]

Table 2

[0083]

Table 3

[0084] Consider the results in the table. The following No. indicates the sample No. in Table 3. Nos. 1 to 13 are inventive examples that satisfy all of the component compositions, metal structures, and spheroidizing annealing conditions defined in the embodiments of the present invention.

[0085] For No.14, since the cooling-heating process was carried out 0 times, the grain boundary cementite ratio was low, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0086] No.15 is an example where annealing was performed after wire drawing with a 25% area reduction. By wire drawing, the grain boundary cementite ratio could be increased. However, since the cooling-heating process was carried out 0 times, the average size of all cementite could not be made constant or more, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0087] No.16 is an example where annealing was performed under the annealing condition SA2, which is the production condition K satisfying the production conditions shown in Patent Document 3. Under this production condition, the cementite was excessively coarsened by annealing, the hardness after quenching treatment was lower than the reference value, resulting in poor hardenability.

[0088] For No.17 and No.24, since the temperature T1 was 730°C, which was lower than (A1 + 8°C), a large amount of small-sized rod-shaped cementite remained in the crystal grains, the average size of all cementite did not reach a certain value or more, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0089] For No.18, since the reaching temperature T2 of cooling at the average cooling rate R1 was set to 710°C, which was higher than A1 - 17°C, the coarsening of cementite during the above cooling was insufficient, the average size of all cementite did not reach a certain value or more, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

[0090] For No.19, since the average cooling rate R1 was as slow as 9°C / hour, the cementite was excessively coarsened, the average size of all cementite became high, the hardness after quenching treatment was lower than the reference value, resulting in poor hardenability.

[0091] No. 20 is an example of annealing under manufacturing condition O that satisfies the manufacturing conditions shown in Patent Document 1. Under this manufacturing condition, since the heating and holding time t1 at temperature T1 is particularly short at 0.5 hours, a large amount of small-sized rod-shaped cementite remains in the crystal grains, the average size of all cementite does not reach a certain level or more, the hardness after spheroidizing annealing is higher than the reference value, and the cold workability is poor as a result.

[0092] No. 21 is an example of annealing under condition c of Patent Document 2 as manufacturing condition P that satisfies the manufacturing conditions shown in Patent Document 2. Under this manufacturing condition, due to the lack of holding at temperature T1 and other reasons, a large amount of small-sized rod-shaped cementite remains in the crystal grains, the average size of all cementite does not reach a certain level or more, the hardness after spheroidizing annealing does not fall below the reference value, and the cold workability is poor as a result.

[0093] For No. 22, 23, and 25 to 27, since the cooling-heating process is not performed or not repeated, the coarsening of cementite is insufficient, the average size of all cementite does not reach a certain level or more, the hardness after spheroidizing annealing does not fall below the reference value, and the cold workability is poor as a result.

[0094] For No. 28 to 31, since the cooling-heating process is not performed or not repeated, the coarsening of cementite is insufficient, the average size of all cementite does not reach a certain level or more, the hardness after spheroidizing annealing does not fall below the reference value, and the cold workability is poor as a result.

Industrial Applicability

[0095] The steel wire for mechanical structure parts according to this embodiment has a low deformation resistance at room temperature when manufacturing various mechanical structure parts, can suppress wear and breakage of plastic working tools such as dies, and can also suppress the occurrence of cracks during, for example, forging. Furthermore, since it has excellent hardenability, high hardness can be ensured by quenching after cold working. From these facts, the steel wire for mechanical structure parts according to this embodiment is useful as a steel wire for cold working mechanical structure parts. For example, the steel wire for mechanical structure parts according to this embodiment is used in the manufacture of various mechanical structure parts such as automobile parts and construction machinery parts by being subjected to cold working such as cold forging, cold rolling, and cold swaging. As such mechanical structure parts, specifically, bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, bearing seats, tappets, saddles, bulges, inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, fittings, yokes, bases, valve lifters, spark plugs, pinion gears, steering shafts, common rails and other mechanical parts, electrical components, etc. can be mentioned.

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 are contained, the balance being composed of iron and unavoidable impurities, the ratio of the area of cementite present at the ferrite grain boundaries is 32% or more with respect to the total area of cementite, and when the minimum size (equivalent circle diameter) of the measured cementite is 0.3 μm, the average equivalent circle diameter of the total cementite is (1.668 - 2.13[C]) μm or more and (1.863 - 2.13[C]) μm or less, where [C] represents the C content (% by mass) in the steel, a steel wire for mechanical structure parts having an average value of the ferrite crystal grain size of 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 structure 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 structure 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 structure parts according to any one of claims 1 to 3.

5. For a bar steel satisfying the chemical composition according to any one of claims 1 to 4, a method for manufacturing a steel wire for mechanical structure parts, comprising a step of performing spheroidizing annealing including the following steps (1) to (3). (1) After heating to a temperature T1 of (A1 + 8°C) or higher, holding at the temperature T1 for more than 1 hour and 6 hours or less, (2) Cooling is carried out at an average cooling rate R1 of 10°C / hour to 30°C / hour to a temperature T2 above 650°C and below (A1 - 17°C), and then heating is carried out 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. (3) Cooling is carried out from the heating temperature in the final cycle of the cooling-heating process. 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 structural 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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