Steel wire for mechanical structural parts and its manufacturing method

Abstract

A steel wire for mechanical structural parts contains: C: 0.05% to 0.60% by mass, Si: 0.005% to 0.50% by mass, Mn: 0.30% to 1.20% by mass, P: more than 0% by mass and less than 0.050% by mass, S: more than 0% by mass and less than 0.050% by mass, Al: 0.001% to 0.10% by mass, Cr: more than 0% by mass and less than 1.5% by mass, and N: more than 0% by mass and less than 0.02% by mass, with the balance consisting of iron and unavoidable impurities. The area of ​​cementite present at the ferrite grain boundaries is more than 32% relative to the total area of ​​cementite, and when the amount of C in the steel is expressed as [C] (by mass%), the average equivalent circle diameter of all cementite is more than (1.668-2.13[C]) μm and less than (1.863-2.13[C]) μm.

Description

Technical Field

[0001] This invention relates to steel wire for mechanical structural parts and its manufacturing method. Background Technology

[0002] In manufacturing various mechanical structural parts, such as automotive parts and construction machinery parts, spheroidizing annealing is typically performed to impart cold workability to steel bars, including hot-rolled wire rods. The spheroidized annealed wire is then cold-worked, followed by machining processes such as cutting to form a specified part shape. Finally, quenching and tempering are performed for final strength adjustment to manufacture the mechanical structural parts.

[0003] In recent years, in order to prevent steel from cracking and to improve the life of molds in cold working processes, there has been a demand for a type of steel wire that is softer than before.

[0004] As a method for obtaining softened steel wire, for example, Patent Document 1 discloses a method for manufacturing medium carbon steel with excellent cold forgeability, which involves heating to the austenitizing temperature region more than twice during spheroidizing annealing. According to the manufacturing method in Patent Document 1, it is shown that cold-forging steel with a hardness of 83 HRB or less after spheroidizing annealing and a spheroidal carbide ratio of 70% or more in the microstructure can be obtained.

[0005] Patent Document 2 discloses a steel with low deformation resistance and excellent cold forging properties after spheroidizing annealing, and a method for manufacturing the same. The manufacturing method includes the following steps: After hot working of a steel meeting a specified composition, the steel is cooled to room temperature. Then, the temperature is raised to a temperature range of A1 point to A1 point + 50°C. After holding this temperature range for 0 to 1 hour, the steel undergoes at least two annealing treatments at an average cooling rate of 10 to 200°C / hr, cooling the steel from the A1 point to A1 point + 50°C to a temperature range of A1 point - 100°C to A1 point - 30°C. Afterward, the temperature is raised to the A1 point to A1 point + 30°C. When maintaining and then cooling within the temperature range of A1 point to A1 point +30°C, after heating to A1 point, maintaining and cooling within the temperature range of A1 point to A1 point +30°C until reaching A1 point again, the residence time within the temperature range of A1 point to A1 point +30°C is 10 minutes to 2 hours. The average cooling rate is 10 to 100°C / hr. After cooling from the temperature range of A1 point to A1 point +30°C to the cooling temperature range of A1 point -100°C to A1 point -20°C, the temperature is maintained in this cooling temperature range for 10 minutes to 5 hours before further cooling.

[0006] Patent Document 3 discloses a steel wire for mechanical structural parts that exhibits excellent cold workability, reducing deformation resistance during cold working and improving crack resistance. The wire has a specified composition, with the steel's microstructure consisting of ferrite and cementite, and the proportion of cementite present at the ferrite grain boundaries being 40% or more of the total cementite. Patent Document 3 shows that the preferred manufacturing conditions for the rolled wire for spheroidizing annealing are: finishing rolling at 800°C or higher and 1050°C or lower; sequentially performing a first cooling with an average cooling rate of 7°C / second or higher; a second cooling with an average cooling rate of 1°C / second or higher and 5°C / second or lower; and a third cooling with an average cooling rate faster than the second cooling and 5°C / second or higher. The end of the first cooling and the beginning of the second cooling are performed within the range of 700–750°C; the end of the second cooling and the beginning of the third cooling are performed within the range of 600–650°C; and the end of the third cooling is below 400°C.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2011-256456

[0010] Patent Document 2: Japanese Patent Application Publication No. 2012-140674

[0011] Patent Document 3: Japanese Patent Application Publication No. 2016-194100 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, the prior art disclosed in Patent Documents 1-3 cannot sufficiently reduce the hardness after spheroidizing annealing, resulting in poor machinability during cold working after spheroidizing annealing, or the hardness cannot be sufficiently improved through quenching treatment after cold working, i.e., there is poor hardenability. In other words, to date, there is no technology that addresses both cold workability and hardenability.

[0014] The present invention is made in view of the following circumstances, and its object is to provide a steel wire for mechanical structural parts with very low hardness, excellent cold workability, and high hardness that can be obtained by quenching treatment, i.e., excellent hardenability, and a method for manufacturing the steel wire for mechanical structural parts in a short time.

[0015] In this specification, "wire rod" and "bar steel" refer to hot-rolled wire and bar steel, respectively, which are steels that have not undergone heat treatment such as spheroidizing annealing or wire drawing. "Steel wire" refers to wire rod or bar steel that has undergone at least one heat treatment such as spheroidizing annealing or wire drawing. In this specification, the aforementioned wire rod, bar steel, and steel wire are collectively referred to as "bar steel."

[0016] Problem-solving methods

[0017] Embodiment 1 of the present invention is a steel wire for a mechanical structural part, wherein it comprises:

[0018] C: 0.05% by mass to 0.60% by mass

[0019] Si: 0.005% by mass to 0.50% by mass

[0020] Mn: 0.30%–1.20% by mass

[0021] P: Above 0% by mass and below 0.050% by mass

[0022] S: greater than 0% by mass and less than 0.050% by mass

[0023] Al: 0.001% by mass to 0.10% by mass

[0024] Cr: above 0% by mass and below 1.5% by mass, and

[0025] N: greater than 0% by mass and less than 0.02% by mass.

[0026] The balance consists of iron and unavoidable impurities.

[0027] The proportion of cementite area existing at ferrite grain boundaries is more than 32% relative to the total cementite area, and...

[0028] When [C] represents the amount of C in steel (mass%), the average equivalent circle diameter of all cementite is greater than (1.668-2.13[C]) μm and less than (1.863-2.13[C]) μm.

[0029] In embodiment 2 of the present invention, the steel wire for mechanical structural parts according to embodiment 1 further comprises one or more elements selected from the group consisting of:

[0030] Cu: above 0% by mass and below 0.25% by mass

[0031] Ni: above 0% by mass and below 0.25% by mass

[0032] Mo: above 0% by mass and below 0.50% by mass, and

[0033] B: Above 0% by mass and below 0.01% by mass.

[0034] In embodiment 3 of the present invention, the steel wire for mechanical structural parts according to embodiment 1 or 2 further comprises one or more elements selected from the group consisting of:

[0035] Ti: above 0% by mass and below 0.2% by mass

[0036] Nb: above 0% by mass and below 0.2% by mass, and

[0037] V: Above 0% by mass and below 0.5% by mass.

[0038] In embodiment 4 of the present invention, the steel wire for a mechanical structural part according to any one of embodiments 1 to 3 further comprises one or more elements selected from the group consisting of:

[0039] Mg: above 0% by mass and below 0.02% by mass

[0040] Ca: above 0% by mass and below 0.05% by mass

[0041] Li: above 0% by mass and below 0.02% by mass, and

[0042] REM: Above 0% by mass and below 0.05% by mass.

[0043] In embodiment 5 of the present invention, the steel wire for mechanical structural parts according to any one of embodiments 1 to 4, wherein the average value of the ferrite grain diameter is 30 μm or less.

[0044] Method 6 of the present invention is a method for manufacturing steel wire for mechanical structural parts according to any one of methods 1 to 5, wherein the method includes performing a spheroidizing annealing process including the following steps (1) to (3) on a bar steel that satisfies the chemical composition described in any one of methods 1 to 4:

[0045] (1) After heating to a temperature T1 above (A1+8℃), maintain the temperature T1 for more than 1 hour but less than 6 hours.

[0046] (2) Perform a total of 2 to 6 cooling-heating processes, cooling to a temperature T2 above 650°C and below (A1-17°C) at an average cooling rate R1 of 10°C / hour to 30°C / hour, and then heating to a temperature above T2 and below (A1+60°C).

[0047] (3) Cooling is performed from the final heating temperature of the cooling-heating process.

[0048] Here, A1 is calculated by the following formula (1).

[0049] A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1)

[0050] Wherein, [element] represents the content (mass%) of each element, and the content of elements not included is 0.

[0051] In embodiment 7 of the present invention, a method for manufacturing steel wire for mechanical structural parts according to embodiment 6 is provided, wherein the steel bar is a steel wire obtained by drawing wire with a reduction rate of more than 5%.

[0052] The effects of the invention

[0053] According to the present invention, it is possible to provide steel wire for mechanical structural parts with excellent cold workability and excellent hardenability, and a method for manufacturing the steel wire for mechanical structural parts. Attached Figure Description

[0054] Figure 1 This is a diagram illustrating the spheroidizing annealing conditions in the manufacturing method of steel wire for mechanical structural parts according to this embodiment.

[0055] Figure 2 This is a diagram illustrating the heat treatment process of the comparative example.

[0056] Figure 3 This is a diagram illustrating the heat treatment process in existing technology.

[0057] Figure 4 This is a diagram illustrating a heat treatment process in another prior art.

[0058] Figure 5 This is a diagram illustrating a heat treatment process in another prior art. Detailed Implementation

[0059] The inventors have conducted extensive research from various angles to achieve steel wire for mechanical structural parts that combines cold workability and hardenability. Their results show that, particularly in the metal structure, it is sufficient to ensure that the area of ​​cementite present at ferrite grain boundaries is at least a certain ratio to the total area of ​​cementite, and that the average size of all cementite is within a certain range depending on the carbon content in the steel. Furthermore, it has been found that to achieve the aforementioned metal structure, the chemical composition must be within a certain range, and that spheroidizing annealing under specified conditions is particularly effective in the manufacturing method of the steel wire for mechanical structural parts. Hereinafter, the metal structure of the steel wire for mechanical structural parts according to this embodiment will be described first.

[0060] 1. Metal structure

[0061] [The proportion of cementite area existing at ferrite grain boundaries to the total cementite area: 32% or more]

[0062] If the proportion of cementite present at ferrite grain boundaries decreases, the proportion of cementite within the ferrite grains relatively increases. This cementite within the ferrite grains can hinder the movement of dislocations introduced into the ferrite grains during cold working. As a result, dislocations increase, leading to work hardening and poor cold workability. In this embodiment, with the aim of reducing the proportion of cementite within the ferrite grains and suppressing the hardness of steel wire for mechanical structural parts, the area ratio of cementite present at ferrite grain boundaries is 32% or more relative to the total area of ​​cementite. "Cementite present at ferrite grain boundaries" includes both cementite adhering to ferrite grain boundaries and cementite present on the ferrite grain boundaries. Hereinafter, the "area ratio of cementite present at ferrite grain boundaries" will be referred to as "grain boundary cementite ratio." The grain boundary cementite ratio is preferably 35% or more, more preferably 40% or more, and even more preferably 45% or more. On the other hand, a higher grain boundary cementite content is preferred, so there is no specific upper limit set, and it can also be 100%.

[0063] Regarding all the aforementioned cementite, there are no particular limitations on its morphology; in addition to spherical cementite, rod-shaped cementite with a high aspect ratio is also included. Furthermore, there is no limitation on the size standard of the cementite used for measurement, but the smallest size is defined as the cementite that can be determined by the method for measuring the grain boundary cementite ratio described later. Specifically, cementite particles with an equivalent circle diameter of 0.3 μm or more are used as the measurement objects.

[0064] [When the amount of carbon (mass%) in steel is expressed as [C], the average equivalent circle diameter of all cementite is greater than (1.668–2.13 [C]) μm and less than (1.863–2.13 [C]) μm.]

[0065] When the amount of cementite in steel is constant, the larger the size of the cementite, the lower the number density of cementite, and the longer the distance between cementite particles. The longer the distance between cementite particles in steel, the more difficult it is for precipitation strengthening, resulting in a decrease in hardness. Based on these points, in this invention, when [C] represents the amount of carbon (mass%) in the steel, the average equivalent circle diameter of all cementite particles is (1.668-2.13 [C]) μm or more. Preferably, the average equivalent circle diameter of all cementite particles is (1.669-2.13 [C]) μm or more. On the other hand, if the cementite is excessively coarse, it cannot be fully melted during high-temperature holding in the quenching process after cold working, and sufficiently high hardness cannot be obtained through quenching. Therefore, in this invention, the average equivalent circle diameter of all cementite particles is (1.863-2.13 [C]) μm or less. Preferably, it is (1.858-2.13 [C]) μm or less.

[0066] Patent Document 3 discloses cementite present at ferrite grain boundaries, which, compared to cementite present within ferrite grains, results in less strain during cold working, thus reducing deformation resistance. However, Patent Document 3 does not control the average size of all cementite, leading to insufficient melting of the cementite during the high-temperature holding phase of the quenching process, resulting in poor hardenability. This invention aims to achieve steel wire for mechanical structural parts that combines excellent cold workability and excellent hardenability, focusing on both the cementite content at grain boundaries and the average size of all cementite.

[0067] The metal structure of the steel wire for mechanical structural parts in this embodiment is a spheroidized structure with spheroidized cementite, which can be obtained by performing spheroidizing annealing, for example, on bar steel that meets the chemical composition described later.

[0068] The metallic structure of the steel wire for mechanical structural parts of the present invention is substantially composed of ferrite and cementite. The term "substantially" means that the ferrite comprising the metallic structure of the steel wire for mechanical structural parts of the present invention is 90% or more in area, and the rod-shaped cementite with an aspect ratio of 3 or more is 5% or less in area. If the adverse effect on cold workability is minimal, inclusions such as AlN nitrides and other inclusions other than nitrides are permitted to be less than 3% in area. The area ratio of the ferrite may further be 95% or more.

[0069] In this specification, "ferrite" refers to the portion with a bcc crystal structure, and also includes ferrite in the layered structure of ferrite and cementite, i.e., pearlite.

[0070] Furthermore, the measurement object of the so-called "ferrite grain diameter" is "ferrite grain," which includes grains containing rod-shaped cementite formed during spheroidizing annealing due to insufficient spheroidization. However, grains containing rod-shaped cementite that may have remained before spheroidizing annealing (pearlite grains) are excluded from the evaluation. Specifically, this refers to grains that, when etched using nital (2 vol% nitric acid, 98 vol% ethanol) and observed under an optical microscope at 1000x magnification, are either "grains without cementite within the grain" or "grains containing cementite within the grain, and whose cementite shape can be observed (i.e., the boundary between cementite and ferrite is clearly distinguishable)." Grains whose cementite shape cannot be observed at 1000x magnification using the aforementioned optical microscope (i.e., the boundary between cementite and ferrite cannot be clearly observed) are excluded from the evaluation objects in this embodiment and are not included in "ferrite grains."

[0071] [Average ferrite grain diameter: below 30 μm]

[0072] The steel wire for mechanical structural parts in this embodiment preferably has an average ferrite grain diameter of 30 μm or less in its microstructure. If the average ferrite grain diameter is 30 μm or less, the ductility of the steel wire for mechanical structural parts can be improved, and the occurrence of cracks during cold working can be further suppressed. The average ferrite grain diameter is more preferably 25 μm or less, and even more preferably 20 μm or less. A smaller average ferrite grain diameter is more preferred, but considering feasible manufacturing conditions, the lower limit is approximately 2 μm.

[0073] (characteristic)

[0074] The steel wire for mechanical structural parts of this embodiment, which satisfies the following chemical composition and has the above-described metal structure, can achieve both low hardness that allows for good cold working and high hardness after quenching. In this embodiment, when [C], [Cr], and [Mo] represent the C content (mass%), Cr content (mass%), and Mo content (mass%) in the steel (elements not included are 0 mass%), the hardness, in the embodiments described later, is the hardness after spheroidizing annealing, satisfies the following formula (2), and the hardness after quenching satisfies the following formula (3). It is determined that the hardness is very low, the cold workability is excellent, and the high hardness after quenching can be achieved, that is, the quenchability is excellent.

[0075] (Hardness after spheroidizing annealing) < 91([C]+[Cr] / 9+[Mo] / 2)+91…(2)

[0076] The hardness (HV) after quenching is greater than 380 ln([C]) + 1010…(3)

[0077] 2. Chemical composition

[0078] The chemical composition of the steel wire used for the mechanical structural parts in this embodiment will be explained.

[0079] [C: 0.05% by mass to 0.60% by mass]

[0080] Carbon (C) is the element that governs the strength of steel; the higher the content, the higher the strength after quenching and tempering. To effectively achieve this effect, the lower limit of C content is 0.05% by mass. The C content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and even more preferably 0.20% by mass or more. However, if the C content is excessive, the microstructure after spheroidizing annealing will have an excessive number of spheroidized cementite, increasing hardness and thus reducing cold workability. Therefore, the upper limit of C content is 0.60% by mass. The C content is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

[0081] [Si: 0.005% by mass to 0.50% by mass]

[0082] Besides its use as a deoxidizer during smelting, silicon (Si) also contributes to strength improvement. To effectively achieve this effect, the lower limit for Si content is 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, significantly improving the strength after spheroidizing annealing. If the Si content is excessive, the aforementioned effects lead to deterioration in cold workability; therefore, the upper limit for Si content is 0.50% by mass. The Si content is preferably 0.40% by mass or less, more preferably 0.35% by mass or less.

[0083] [Mn: 0.30% by mass to 1.20% by mass]

[0084] Mn is an element that effectively functions as a deoxidizer and contributes to improved hardenability. To fully realize this effect, the lower limit of the Mn content is 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, resulting in reduced toughness. Therefore, the upper limit of the Mn content is 1.20% by mass. The Mn content is preferably 1.10% by mass or less, more preferably 1.00% by mass or less.

[0085] [P: above 0% by mass and below 0.050% by mass]

[0086] Phosphorus (P) is an unavoidable impurity that causes grain boundary segregation in steel and is a harmful element that adversely affects forgeability and toughness. Therefore, the P content is 0.050% by mass or less. The P content is preferably 0.030% by mass or less, more preferably 0.020% by mass or less. The lower the P content, the more preferred, but it is generally 0.001% by mass or more.

[0087] [S: Above 0% by mass and below 0.050% by mass]

[0088] Sulfur (S) is an unavoidable impurity that forms MnS in steel, deteriorating ductility and thus being detrimental to cold workability. Therefore, the S content is 0.050% by mass or less. The S content is preferably 0.030% by mass or less, more preferably 0.020% by mass or less. Lower S content is preferred, but it typically contains 0.001% by mass or more.

[0089] [Al: 0.001% by mass to 0.10% by mass]

[0090] Al is an element included as a deoxidizing material, which has the effect of reducing impurities along with deoxidation. To achieve this effect, the lower limit of the Al content is 0.001% by mass. The Al content is preferably 0.005% by mass or more, 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 0.10% by mass. The Al content is preferably 0.08% by mass or less, more preferably 0.05% by mass or less.

[0091] [Cr: greater than 0% by mass and less than 1.5% by mass]

[0092] Cr is an element that improves the hardenability of steel, increases its strength, and promotes the spheroidization of cementite. Specifically, Cr is dissolved in cementite and delays its melting during spheroidizing annealing. When heated, some cementite remains unmelted, making it difficult to form rod-shaped cementite with a large aspect ratio upon cooling, thus easily resulting in a spheroidized structure. Therefore, the Cr content is higher than 0% by mass, preferably 0.01% by mass or more. It can also be 0.05% by mass or more, and further, 0.10% by mass or more. From the viewpoint of further promoting cementite spheroidization, it can be higher than 0.30% by mass, and further higher than 0.50% by mass. If the Cr content is excessive, the diffusion of carbon-containing elements is delayed, causing the melting of cementite to be excessively delayed, making it difficult to obtain a spheroidized structure. As a result, the hardness reduction effect of this embodiment may decrease. Therefore, the Cr content is 1.50% by mass or less, preferably 1.40% by mass or less, and more preferably 1.25% by mass or less. From the perspective of further accelerating element diffusion, the Cr content can be further reduced to below 1.00% by mass, further reduced to below 0.80% by mass, and further reduced to below 0.30% by mass.

[0093] [N: greater than 0% by mass and less than 0.02% by mass]

[0094] Nitrogen (N) is an unavoidable impurity in steel. However, if a large amount of dissolved N is present in the steel, strain aging will cause an increase in hardness, leading to a decrease in ductility and a deterioration in cold workability. Therefore, the amount of N is 0.02% by mass or less, preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

[0095] [margin]

[0096] The balance consists of iron and unavoidable impurities. As unavoidable impurities, trace elements (e.g., As, Sb, Sn, etc.) are permitted to be introduced due to the conditions of raw materials, supplies, manufacturing equipment, etc. Furthermore, for example, elements like P and S are generally preferred in lower amounts; therefore, although they are unavoidable impurities, their composition range is defined separately as described above. Therefore, in this specification, the term "unavoidable impurities" constituting the balance refers to elements whose composition range is separately defined.

[0097] The steel wire for the mechanical structural parts in this embodiment only needs to contain the aforementioned elements in its chemical composition. The optional elements described below may not be included, but if necessary, including them together with the aforementioned elements makes it easier to ensure properties such as hardenability. The optional elements will be explained below.

[0098] [Select one or more from the group consisting of Cu: above 0% by mass and below 0.25% by mass, Ni: above 0% by mass and below 0.25% by mass, Mo: above 0% by mass and below 0.50% by mass, and B: above 0% by mass and below 0.01% by mass]

[0099] Cu, Ni, Mo, and B are all elements that improve the hardenability of steel, thereby effectively increasing the strength of the final product. They can be contained individually or in two or more forms as needed. The effect of these elements increases with their content. The preferred lower limit for effectively exerting the above effects is that Cu, Ni, and Mo are each above 0% by mass, more preferably above 0.02% by mass, and even more preferably above 0.05% by mass; and B is above 0% by mass, more preferably above 0.0003% by mass, and even more preferably above 0.0005% by mass.

[0100] On the other hand, if the content of these elements is excessive, the strength will be too high, and the cold workability may deteriorate. Therefore, the upper limits of each element are determined as described above. More preferably, the content of Cu and Ni is 0.22% by mass or less, more preferably 0.20% by mass or less, the content of Mo is more preferably 0.40% by mass or less, more preferably 0.35% by mass or less, and the content of B is more preferably 0.007% by mass or less, more preferably 0.005% by mass or less.

[0101] [Select one or more from the group consisting of Ti: above 0% by mass and below 0.2% by mass, Nb: above 0% by mass and below 0.2% by mass, and V: above 0% by mass and below 0.5% by mass]

[0102] Ti, Nb, and V form compounds with N, reducing the amount of N dissolved in solids and thus lowering the deformation resistance. Therefore, they can be included individually or in combination as needed. The effect of these elements increases with their content. For any single element, the preferred lower limit for effectively achieving the above-mentioned effect is higher than 0% by mass, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. However, if the content of these elements is excessive, the deformation resistance of the formed compound will increase, and the cold workability may decrease. Therefore, it is preferable that the content of Ti and Nb is 0.2% by mass or less each, and the content of V is 0.5% by mass or less each. More preferably, the content of Ti and Nb is 0.18% by mass or less each, even more preferably 0.15% by mass or less each, and the content of V is more preferably 0.45% by mass or less, even more preferably 0.40% by mass or less each.

[0103] [Select one or more elements from the group consisting of Mg: above 0% by mass and below 0.02% by mass, Ca: above 0% by mass and below 0.05% by mass, Li: above 0% by mass and below 0.02% by mass, and Rare Earth Elements (REM): above 0% by mass and below 0.05% by mass]

[0104] Mg, Ca, Li, and REM are elements that effectively spheroidize sulfide compound inclusions such as MnS, thereby improving the deformability of steel. This effect increases with increasing content. To effectively achieve the above effects, the contents of Mg, Ca, Li, and REM are preferably above 0% by mass, more preferably above 0.0001% by mass, and even more preferably above 0.0005% by mass. However, even with excessive content, the effect is saturated, and an effect matching the content cannot be expected. Therefore, the contents of Mg and Li are preferably below 0.02% by mass, more preferably below 0.018% by mass, and even more preferably below 0.015% by mass, and the contents of Ca and REM are preferably below 0.05% by mass, more preferably below 0.045% by mass, and even more preferably below 0.040% by mass. Furthermore, Mg, Ca, Li, and REM can be contained individually or in more than two types. When more than two types are contained, their contents can be any amount within the above-mentioned ranges. The term REM means that it includes the lanthanides (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0105] The shape of the steel wire used for the mechanical structural parts in this embodiment is not particularly limited. For example, wires with diameters ranging from 5.5 mm to 60 mm can be listed.

[0106] 3. Manufacturing method

[0107] To obtain the metallic structure of the steel wire for mechanical structural parts according to the present invention, the spheroidizing annealing conditions are preferably properly controlled during the manufacture of the steel wire for mechanical structural parts as described below. The hot rolling process for manufacturing the wire or bar steel for spheroidizing annealing is not particularly limited, and general methods can be followed. As described later, a wire drawing process may also be applied before spheroidizing annealing. The diameter of the wire, steel wire, or bar steel for spheroidizing annealing is not particularly limited; for wire and steel wire, it is, for example, 5.5 mm to 60 mm, and for bar steel, it is, for example, 18 mm to 105 mm.

[0108] Reference Figure 1 The spheroidizing annealing conditions in the manufacturing method of steel wire for mechanical structural parts according to embodiments of the present invention will be described. Figure 1 This is an example of a diagram illustrating the spheroidizing annealing conditions in the manufacturing method according to an embodiment of the present invention. The number of repetitions of the cooling-heating process, etc., are not affected by this. Figure 1 limited.

[0109] The method for manufacturing steel wire for mechanical structural parts according to embodiments of the present invention includes a spheroidizing annealing process comprising the following steps (1) to (3).

[0110] (1) After heating to a temperature T1 above (A1+8℃), maintain the temperature T1 for more than 1 hour but less than 6 hours.

[0111] (2) Perform the following cooling-heating process a total of 2 to 6 times, cooling to a temperature T2 above 650°C and below (A1-17°C) at an average cooling rate R1 of 10°C / hour to 30°C / hour, and then heating to a temperature above T2 and below (A1+60°C).

[0112] (3) Cooling is performed from the final heating temperature of the cooling-heating process.

[0113] Here, A1 is calculated by the following formula (1).

[0114] A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1)

[0115] Wherein, [element] represents the content (mass%) of each element, and the content of elements not included is 0.

[0116] [(1) Heating and holding, heating to a temperature T1 above (A1+8℃), and holding at that temperature T1 for more than 1 hour and less than 6 hours. Figure 1 [2])]

[0117] Heating to a temperature T1 of (A1+8°C) or higher promotes the melting of the high aspect ratio rod-shaped cementite generated during the rolling stage. If the temperature T1 is too low, the rod-shaped cementite will not melt during heating and will continue to remain in the ferrite, increasing the hardness. To obtain a sufficiently softened steel wire, the temperature T1 needs to reach (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, in order to sufficiently suppress excessive grain coarsening and to make it easier for spherical cementite to precipitate at the ferrite grain boundaries during the cooling process of the next process, thereby suppressing the residual amount of rod-shaped cementite and making it easier to reduce the hardness, the temperature T1 is preferably (A1+57°C) or lower.

[0118] Furthermore, if the heating holding time (t1) is too short, rod-shaped cementite remains within the ferrite grains, increasing hardness. To obtain sufficiently softened steel wire, the heating holding time (t1) needs to be more than 1 hour and less than 6 hours. A preferred heating holding time (t1) is 1.5 hours or more, more preferably 2.0 hours or more. If the heating holding time (t1) is too long, the heat treatment time becomes longer, reducing productivity. Therefore, the heating holding time (t1) is 6 hours or less, preferably 5 hours or less, more preferably 4 hours or less. Also, when heated to a temperature T1 of (A1+8°C) or higher (… Figure 1 The average heating rate of [1] does not affect the properties of the steel, so it can be heated at any rate. For example, heating at 30℃ / hour to 100℃ / hour is sufficient.

[0119] Furthermore, the temperature at point A1 above is calculated using the following formula (1) as described on page 273 of Leslie Steel Materials Science (Maruzen).

[0120] A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1)

[0121] Wherein, [element] represents the content (mass%) of each element, and the content of elements not included is 0.

[0122] (2) Perform a total of 2 to 6 cooling-heating processes, cooling to a temperature T2 above 650°C and below (A1-17°C) at an average cooling rate R1 of 10°C / hour to 30°C / hour, then heating to a temperature above T2 and below (A1+60°C). Figure 1 [3]~[7])]

[0123] (2-i) Cool to a temperature T2 above 650℃~(A1-17℃) at an average cooling rate R1 of 10℃ / hour~30℃ / hour. Figure 1 [3] and [4])

[0124] Cooling is performed to allow spherical cementite to precipitate at the ferrite grain boundaries. If the average cooling rate R1 from temperature T1 is too fast, excessive re-precipitation of rod-shaped cementite occurs, reducing cold workability. Therefore, the average cooling rate R1 is 30°C / hour or less. The average cooling rate R1 is preferably 25°C / hour or less, 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 becomes too coarse. As a result, the cementite cannot fully melt during the high-temperature holding period of the quenching process, leading to reduced hardness after quenching and thus deterioration of hardenability. Furthermore, it results in a longer annealing time, reducing productivity. Therefore, the average cooling rate R1 is 10°C / hour or more, preferably 11°C / hour or more, more preferably 12°C / hour or more.

[0125] Furthermore, if the cooling temperature T2 reached at the average cooling rate R1 is too low, it leads to a prolonged annealing time. Therefore, the cooling temperature T2 needs to be higher than 650°C. According to the manufacturing method of this embodiment, even if the cooling temperature T2 is higher than 650°C, the cementite can be controlled into the desired morphology without prolonged annealing. The cooling temperature T2 is preferably 670°C or higher. On the other hand, if the cooling temperature T2 is too high, rod-shaped cementite will excessively redeprecipitate within the ferrite grains, increasing hardness and reducing cold workability. Therefore, the upper limit of the cooling temperature T2 is A1-17°C. The cooling temperature T2 is preferably A1-18°C or lower. In addition, if the cooling temperature T2 is held after reaching it, it leads to a prolonged heat treatment time. Therefore, from these viewpoints, it is preferable not to hold it. However, in order to make the temperature fluctuation in the furnace uniform, it can be held for a short time. The holding time (t2) at the cooling temperature T2 is preferably within 1 hour.

[0126] (2-ii) Heating to a temperature higher than T2 but lower than (A1+60℃) Figure 1 [5] and [6])

[0127] In order to remelt the rod-shaped cementite precipitated in the ferrite grains in the above-mentioned (2-i) process, heating is performed from the above-mentioned cooling temperature T2. Figure 1The heating temperature shown in [6] can be any temperature within the range of above temperature T2 and below (A1+60°C). From the viewpoint of fully remelting the rod-shaped cementite generated in the above (2-i) process, the heating temperature is preferably above A1°C. In addition, from the viewpoint of suppressing the remelting of the spheroidal cementite at the ferrite grain boundaries and suppressing the increase in hardness after spheroidizing annealing, the heating temperature is preferably below (A1+57°C).

[0128] Figure 1 The average heating rate when the temperature T2 is raised to the heating temperature as shown in [5] is not particularly limited. For example, from the viewpoint of more fully remelting the rod-shaped cementite within the ferrite grains generated in the above-mentioned (2-i) process and further suppressing the hardness after spheroidizing annealing, the average heating rate can be 200°C / hour or less. In addition, from the viewpoint of fully suppressing the coarsening of cementite generated in this heating and further improving hardenability, the average heating rate can be 5°C / hour or more.

[0129] Once the aforementioned heating temperature is reached, it is irrelevant whether or not the temperature is maintained at that temperature. When maintaining this heating temperature, for example, keeping the holding time within 1 hour can suppress the remelting of spherical cementite at the ferrite grain boundaries.

[0130] In the manufacturing method of this embodiment, the cooling-heating process of (2-i) and (2-ii) described above is repeated multiple times, but in each iteration, the average cooling rate R1 and temperature T2 need to meet the above range.

[0131] Furthermore, the relationship between the heating temperature and the temperature T1 is not particularly limited. For example, the heating temperature can be equal to or higher than the temperature T1.

[0132] (2-iii) Perform a total of 2 to 6 cooling-heating processes. Figure 1 [7])

[0133] To increase the proportion of cementite present at ferrite grain boundaries and promote the coarsening of cementite present at ferrite grain boundaries, after heating and holding at temperature T1 in (1), the cooling-heating process of (2-i) and (2-ii) needs to be performed at least twice in total. If the cooling-heating process is not repeated, the proportion of cementite present at ferrite grain boundaries will be insufficient, or the coarsening of cementite present at ferrite grain boundaries will be inadequate, resulting in increased hardness after spheroidizing annealing. Therefore, the above-mentioned cooling-heating process is performed at least twice. Preferably, it is performed at least three times. The more times it is performed, the lower the hardness becomes, but if it is performed too many times, the effect will saturate. In addition, it will cause the annealing time to be longer, reducing productivity. Therefore, the number of cooling-heating processes is 6 times or less. Also, Figure 1 In this case, the cooling-heating process described in (2-i) and (2-ii) is performed four times. Furthermore, the cooling temperature T2 reached and the average cooling rate R1 for each iteration can vary within a specified range. The average cooling rate R1 refers to the average cooling rate from temperature T1 to the cooling temperature T2 during the first cooling-heating process; for subsequent iterations, it refers to the average cooling rate from temperature T1 to the cooling temperature T2. Figure 1 [6] The average cooling rate from the heating temperature to the cooling temperature T2.

[0134] (3) Cooling from the final heating temperature of the cooling-heating process ( Figure 1 [8])]

[0135] Cooling is performed from the final heating temperature of the cooling-heating process. The average cooling rate and the temperature reached during this cooling process are not particularly limited. From the viewpoint of further suppressing the re-precipitation of rod-shaped cementite, the average cooling rate can be, for example, 100°C / hour or less. Furthermore, from the viewpoint of further suppressing excessive coarsening of cementite, the average cooling rate can be 5°C / hour or more. Additionally, the temperature reached during cooling can, for example, be (A1-30°C) or less. For example, a temperature range of (A1-30°C) or less but (A1-100°C) can be described, where the temperature is cooled at the aforementioned average cooling rate, followed by air cooling. Alternatively, for example, by cooling to below (A1-100°C), the re-precipitation of rod-shaped cementite can be further suppressed, further improving cold workability. In this case, from the viewpoint of shortening the annealing time, the temperature reached during cooling can be (A1-250°C) or more, further (A1-200°C) or more, and even further (A1-150°C) or more.

[0136] The spheroidizing annealing process described above (steps (1) to (3)) can be repeated once or more. From the viewpoint of suppressing excessive coarsening of cementite and ensuring productivity, it is preferable to repeat it 4 times or less, and more preferably 3 times or less. When repeating the spheroidizing annealing process multiple times, it can be repeated under the same conditions or under different conditions within the range specified above. In addition, when repeating the spheroidizing annealing process multiple times, wire drawing can be applied between spheroidizing annealing processes. For example, it can be performed in the following order: wire drawing before spheroidizing annealing → first spheroidizing annealing → wire drawing → second spheroidizing annealing.

[0137] In the method for manufacturing steel wire for mechanical structural parts according to this embodiment, there are no particular limitations on the processes other than the spheroidizing annealing process described above. For example, after spheroidizing annealing, a wire drawing process may also be included, and for the purpose of adjusting dimensions, the wire drawing process is preferably performed with a reduction rate of 15% or less. By keeping the reduction rate at 15% or less, it is possible to suppress the increase in hardness before cold working. The reduction rate is more preferably 10% or less, further preferably 8% or less, and even more preferably 5% or less.

[0138] To promote the formation of the microstructure of the present invention, it is preferable to perform a wire drawing process with a reduction rate of more than 5% for the wire before spheroidizing annealing. By performing wire drawing with the aforementioned reduction rate, the cementite in the steel is destroyed, and the cementite can be promoted to agglomerate through subsequent spheroidizing annealing. Therefore, the cementite can be moderately coarsened, which is effective for softening. The reduction rate is more preferably 10% or more, further preferably 15% or more, and even more preferably 20% or more. On the other hand, if it is increased excessively, there is a possibility of wire breakage. Therefore, the reduction rate is preferably 50% or less. When multiple wire drawing processes are performed, the number of wire drawing processes is not particularly limited, for example, it can be 2 times. Also, when multiple wire drawing processes are performed, the aforementioned "reduction rate during wire drawing" means the reduction rate from the steel before wire drawing to the steel after multiple wire drawing processes.

[0139] Example

[0140] The present invention will now be described in more detail with reference to specific embodiments. The present invention is not limited to these embodiments; any modifications and implementations may be made that conform to the foregoing and following principles, and all such modifications and implementations are included within the scope of the present invention.

[0141] After the test materials with the chemical composition shown in Table 1 were smelted in a converter, steel sheets were cast, and then hot-rolled to manufacture wires with a diameter of 12-16 mm. Furthermore, in Table 2 (described later), sample No. 2 in Table 3, manufactured under manufacturing conditions B, was prepared by drawing the aforementioned wire with a 25% reduction in surface area, and then subjected to spheroidizing annealing.

[0142] Using the aforementioned wire or steel wire, annealing was performed in a laboratory furnace. During annealing, the wire or steel wire was heated to T1 as shown in Table 2 and held for t1 time. Then, it was cooled to the temperature T2 in Table 2 at the average cooling rate R1, and then heated to a temperature higher than T2 but below (A1+60°C). This cooling and heating process was performed the number of times shown in Table 2. Finally, the sample was cooled from the final heating temperature of the cooling-heating process to obtain the specimen.

[0143] As a comparative example, in sample No. 14 shown in Table 3, manufacturing condition J1 was implemented. Figure 2 The heat treatment process shown is a heat treatment process in which the cooling-heating process is repeated 0 times. Furthermore, under manufacturing condition J1, wire drawing with a 25% reduction in surface area was not performed before annealing. In contrast, in sample No. 15 shown in Table 3, under manufacturing condition J2, wire drawing with a 25% reduction in surface area was performed before annealing, and the resulting steel wire was used for… Figure 2 The heat treatment process shown is a heat treatment process in which the cooling-heating process is 0 times.

[0144] Furthermore, as a comparative example, in sample No. 16 shown in Table 3, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 3 were implemented as manufacturing condition K. Specifically, this is the condition represented as SA2 in the embodiment of Patent Document 3, which was repeated 5 times. Figure 3 The heat treatment process is shown. In sample No. 20 shown in Table 3, as manufacturing condition O, the heat treatment conditions that satisfy the manufacturing conditions of Patent Document 1 are implemented. Specifically, it is the fifth spheroidizing annealing condition in No. 1 of Table 2 of Patent Document 1, that is, repeated 3 times. Figure 4 The heat treatment process is shown. Furthermore, in sample No. 21 shown in Table 3, as manufacturing condition P, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 2 are performed; specifically, condition c in Table 2 of Patent Document 2, i.e. Figure 5 The heat treatment mode is shown. Table 2 shows T1 and T2 as annealing parameters, which are the set temperatures of the heat treatment furnace. When thermocouples are installed on the steel, tests are conducted to determine if the actual steel temperature deviates from the set temperature, confirming that the steel temperature and the set temperature are of the same degree.

[0145] Using the samples obtained after the above annealing process, the average ferrite grain diameter, the average size of all cementite, and the cementite ratio at grain boundaries were determined as follows for evaluating the metal microstructure. Furthermore, as a characteristic feature, the hardness after spheroidizing annealing and the hardness after quenching were measured and evaluated according to the following methods.

[0146] [Evaluation of Metal Structure]

[0147] [Average ferrite grain diameter]

[0148] First, the ferrite grain size was measured as follows: A test piece was embedded in resin at a cross-section (D / 4 position, orthogonal to the wire's axis) that allowed observation of the cross-section of the spheroidized annealed steel wire. Using nital (2% vol% nitric acid, 98% vol% ethanol) as the etching solution, the test piece was etched to reveal the microstructure. Then, the microstructure of the revealed test piece was observed using an optical microscope at 400x magnification. Within the evaluation plane, one field of view was selected that allowed observation of the ferrite grains representing the average size of the overall microstructure of the steel wire, and a micrograph was obtained. Next, based on the micrograph, the ferrite grain size (G) was calculated using the comparison method according to JIS G0551 (2020). Then, using the calculated value of ferrite grain size (G), the average ferrite grain diameter dn was obtained according to the following formula (4) which represents the relationship between ferrite grain size G (or N) and the average ferrite grain diameter dn, as described in Table 1 of “Introductory Lecture Terminology - Steel Materials - 3 Grain Size Numbers and Grain Diameters”, by Umemoto Minoru, Ferrum Vol. 2 (1997) No. 10, pp. 29-34 and 32. The results are shown in Table 3. Furthermore, in this embodiment, the ferrite area fraction of samples No. 1 to 13 in Table 3 is all above 90%.

[0149] dn=0.254 / (2 (G－1) / 2 (4)

[0150] [Average size of all cementite and cementite ratio at grain boundaries]

[0151] In measuring the average size of all cementite and the cementite content at grain boundaries in the spheroidizing annealed steel wire, the test pieces were resin-embedded to allow for cross-sectional observation, and the cut surfaces were mirror-polished using sandpaper and a diamond polishing wheel. Next, nital (2% nitric acid, 98% ethanol) was used as the etching solution to etch the cut surfaces for 30 seconds to 1 minute, revealing the ferrite grain boundaries and cementite at position D / 4 (D: wire diameter). Then, FE-SEM (Field-Emission Scanning Electron Microscope) was used to observe the microstructure of the exposed cementite, etc., capturing three fields of view at 2500x magnification.

[0152] An OHP film is superimposed on the aforementioned microscope photograph, and cementite present at the ferrite grain boundaries shown in the microscope photograph is coated onto the OHP film to obtain a first projection image for analyzing grain boundary cementite. The term "cementite present at ferrite grain boundaries," as mentioned above, includes both cementite in contact with ferrite grain boundaries and cementite present on ferrite grain boundaries.

[0153] Subsequently, cementite within the ferrite crystals was coated onto the aforementioned OHP film to obtain a second projection image for analyzing all cementite.

[0154] The first projected image was binarized into a black and white photograph, and the grain boundary cementite ratio was calculated using the image analysis software "Particle Analysis Ver. 3.5" (manufactured by Nippon Steel Technology Co., Ltd.). The second projected image was also binarized into a black and white photograph, and the equivalent circle diameter of all cementite was calculated using the same image analysis software. Furthermore, the average size of all cementite and the grain boundary cementite ratio described in Table 3 are averages calculated from the three fields of view.

[0155] The minimum measured size of cementite (equivalent circle diameter) is 0.3 μm. Furthermore, even when in contact with ferrite grain boundaries, cementite particles with an aspect ratio exceeding 3.0 are considered to have the same effect as cementite existing within ferrite grains, as the cementite particles not only touch the ferrite grain boundaries but also reach the ferrite grain interior. Therefore, they are classified as "ferrite-intragranular cementite." In this specification, the aspect ratio is the ratio of the longest diameter of the cementite particle to the longest minor diameter in the direction perpendicular to the long diameter (long diameter / minor diameter).

[0156] [Evaluation of characteristics]

[0157] [Measurement of hardness after spheroidizing annealing]

[0158] To evaluate cold workability, the hardness of each specimen after spheroidizing annealing was measured as follows. Vickers hardness testing was performed at the D / 4 position (D: wire diameter) of the cross-section of the test piece, i.e., the section perpendicular to the rolling direction. The Vickers hardness was calculated as the average of three or more points and taken as the hardness after spheroidizing annealing. The measurement results are shown in Table 3. In Table 3, the hardness after spheroidizing annealing is expressed as "spheroidized hardness". In this embodiment, regarding the hardness after spheroidizing annealing, when [C], [Cr], and [Mo] represent the C content (mass%), Cr content (mass%), and Mo content (mass%) in the steel (elements not included are 0 mass%), if the following formula (2) is satisfied, the cold workability is excellent and rated as "OK"; if the following formula (2) is not satisfied, the cold workability is poor and rated as "NG".

[0159] The hardness (HV) after spheroidizing annealing is < 91([C]+[Cr] / 9+[Mo] / 2)+91…(2)

[0160] [Measurement of hardness after quenching]

[0161] To evaluate hardenability, the hardness of each sample after quenching was measured as follows. First, as the quenching material, samples that had undergone spheroidizing annealing were prepared, such that the samples were fully quenched during the quenching process, and the length in the rolling direction, i.e., the thickness (t), was 5 mm. As the quenching process, the sample was held at a high temperature of A3+ (30~50℃) for 5 minutes, and then water-cooled after the high temperature holding. A3 is a value derived from the following formula (5). In addition, the high temperature holding time here is the time from when the furnace temperature reaches the set temperature.

[0162] A3(℃)=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)

[0163] Wherein, [element] represents the content (mass%) of each element, and elements not included are calculated as 0%.

[0164] Then, a Vickers hardness test was performed at the t / 2 and D / 4 positions (D: diameter of the steel wire, t: thickness of the sample) of the quenched sample. The Vickers hardness obtained by averaging more than 3 points was calculated as the hardness after quenching. The measurement results are shown in Table 3. In Table 3, the hardness after quenching is expressed as "quenching hardness". In this embodiment, regarding the hardness after quenching, when [C] represents the amount of C (mass%) in the steel, if the following formula (3) is satisfied, the hardenability is excellent and is evaluated as "OK"; if the following formula (3) is not satisfied, the hardenability is poor and is evaluated as "NG".

[0165] The hardness (HV) after quenching is greater than 380 ln([C]) + 1010…(3)

[0166] In Table 3, if both the hardness after spheroidizing annealing and the hardness after quenching are OK, the material exhibits excellent cold workability and excellent hardenability, and is therefore judged as "OK". If at least one of the hardness after spheroidizing annealing and the hardness after quenching is NG, the material does not exhibit both excellent cold workability and excellent hardenability, and is therefore judged as "NG". In Tables 2 and 3, underlined values ​​indicate values ​​that deviate from the scope of this invention or do not meet the desired characteristics.

[0167] Table 1

[0168]

[0169] Table 2

[0170]

[0171] Table 3

[0172]

[0173] The results in the table are examined. The No. below indicates the sample No. in Table 3. Nos. 1 to 13 are inventive examples in which the specified composition, metal structure, and spheroidizing annealing conditions are all satisfied in the embodiments of this invention.

[0174] No. 14 has zero cooling-heating processes, resulting in a low cementite content at grain boundaries and a higher hardness than the standard value after spheroidizing annealing, leading to poor cold workability.

[0175] Example No. 15 is an example of annealing after wire drawing with a 25% reduction in surface area. Although wire drawing can increase the cementite content at grain boundaries, because there are 0 cooling-heating processes, the average size of all cementite cannot reach a certain level. The hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0176] Example No. 16 is an example of annealing under annealing condition SA2 of Patent Document 3, which satisfies manufacturing condition K as shown in manufacturing condition K. Under this manufacturing condition, the cementite becomes excessively coarse due to annealing, resulting in a hardness lower than the standard value after quenching, thus causing poor hardenability.

[0177] For No.17 and No.24, the temperature T1 is 730℃, which is lower than (A1+8℃). Therefore, a large amount of small rod-shaped cementite remains in the crystal, and the average size of all cementite does not reach a certain level. The hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0178] No. 18 Because the average cooling rate R1 leads to a cooling temperature T2 of 710℃, which is higher than A1-17℃, the coarsening of the cementite during the above cooling is insufficient. The average size of all cementite does not reach a certain level, and the hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0179] No. 19 has a slow average cooling rate R1 of 9℃ / hour, resulting in excessively coarse cementite. The average size of all cementite is increased, and the hardness after quenching is lower than the standard value, resulting in poor hardenability.

[0180] No. 20 is an example of annealing under manufacturing condition O, which satisfies the manufacturing conditions shown in Patent Document 1. Under these manufacturing conditions, because the heating and holding time t1 at temperature T1 is short, specifically 0.5 hours, a large amount of small rod-shaped cementite remains in the grains, and the average size of all cementite does not reach a certain level. As a result, the hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0181] No. 21 is an example of annealing under condition c of Patent Document 2, which satisfies manufacturing condition P as shown in manufacturing condition P. Under this manufacturing condition, due to the lack of holding at temperature T1, a large amount of small rod-shaped cementite remains in the grain, the average size of all cementite does not reach a certain level, and the hardness after spheroidizing annealing is not lower than the standard value, resulting in poor cold workability.

[0182] Nos. 22, 23, and 25-27, because the cooling-heating process was not performed or was not repeated, the coarsening of the cementite was insufficient, the average size of all cementite did not reach a certain level, and the hardness after spheroidizing annealing was not lower than the standard value, resulting in poor cold workability.

[0183] No. 28 to 31 did not undergo a cooling-heating process, or did not repeat the process, resulting in insufficient coarsening of the cementite, the average size of all cementite did not reach a certain level, and the hardness after spheroidizing annealing was not lower than the standard value, resulting in poor cold workability.

[0184] This application is accompanied by a priority claim based on Japanese patent applications No. 2021-061572 and No. 2021-211498, which are incorporated herein by reference.

[0185] Industrial availability

[0186] The steel wire for mechanical structural parts of this embodiment exhibits low deformation resistance at room temperature when manufacturing various mechanical structural parts, which can suppress wear and damage to fixtures used for plastic processing such as molds. Furthermore, it demonstrates excellent cold workability, for example, by suppressing crack formation during upsetting. In addition, due to its excellent hardenability, high hardness can be ensured through quenching treatment after cold working. Therefore, the steel wire for mechanical structural parts of this embodiment is useful as a cold-working steel wire for mechanical structural parts. For example, the steel wire for mechanical structural parts of this embodiment can be cold-forged, cold-upset, and cold-rolled for cold working, and can be used in the manufacture of various mechanical structural parts such as automotive parts and construction machinery parts. Specifically, examples of such mechanical structural parts include: bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch housings, cages, housings, hubs, covers, shells, washers, tappets, saddles, valves, inner shells, clutches, sleeves, outer rings, sprockets, iron cores, stators, anvils, cross shafts, rocker arms, vehicle bodies, flanges, rollers, joints, connectors, pulleys, metal fittings, fork-shaped parts, bases, valve tappets, spark plugs, pinions, steering shafts, common rail mechanical parts, electrical components, etc.

Claims

1. A steel wire for a mechanical structural part, comprising: 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: Above 0% by mass and below 0.050% by mass S: Above 0% by mass and below 0.050% by mass Al: 0.001% by mass to 0.10% by mass Cr: above 0% by mass and below 1.5% by mass, and N: greater than 0% by mass and less than 0.02% by mass. The balance contains iron and unavoidable impurities. The proportion of cementite area existing at ferrite grain boundaries is more than 32% relative to the total cementite area, and When cementite particles with an equivalent circle diameter of 0.3 μm or more are used as the measurement object, and [C] represents the amount of carbon in the steel by mass%, 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. The average diameter of the ferrite grains is less than 30 μm.

2. The steel wire for mechanical structural parts according to claim 1, wherein, Satisfying one or more of the following conditions (a) to (c): (a) It also contains one or more of the following: Cu: greater than 0% by mass and less than 0.25% by mass, Ni: greater than 0% by mass and less than 0.25% by mass, Mo: greater than 0% by mass and less than 0.50% by mass, and B: greater than 0% by mass and less than 0.01% by mass; (b) It also contains one or more of the following: Ti: greater than 0% by mass and less than 0.2% by mass, Nb: greater than 0% by mass and less than 0.2% by mass, and V: greater than 0% by mass and less than 0.5% by mass; (c) It also contains one or more of the following: Mg: greater than 0% by mass and less than 0.02% by mass, Ca: greater than 0% by mass and less than 0.05% by mass, Li: greater than 0% by mass and less than 0.02% by mass, and REM: greater than 0% by mass and less than 0.05% by mass.

3. A method for manufacturing steel wire for mechanical structural parts, which is the method for manufacturing steel wire for mechanical structural parts as described in claim 1 or 2, wherein, This includes a spheroidizing annealing process for bar steel that meets the chemical composition described in claim 1 or 2, comprising the steps of (1) to (3) below: (1) After heating to a temperature T1 above (A1+8℃), maintain the temperature T1 for more than 1 hour and less than 6 hours; (2) Perform the following cooling-heating process a total of 2 to 6 times, cooling to a temperature T2 above 650°C and below (A1-17°C) at an average cooling rate R1 of 10°C / hour to 30°C / hour, and then heating to a temperature above T2 and below (A1+60°C); (3) Cooling is performed from the final heating temperature of the cooling-heating process. Here, A1 is calculated by the following formula (1), A1=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1) Wherein, [element] represents the content of each element in terms of mass%, the content of elements not included is 0, and the unit of A1 is ℃.

4. The method for manufacturing steel wire for mechanical structural parts according to claim 3, wherein, The aforementioned steel bar is a steel wire obtained by drawing wire with a reduction rate of more than 5%.

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