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

Abstract

A steel wire for mechanical structural parts, wherein C, Si, Mn, P, S, Al, Cr and N are contained in prescribed amounts, respectively, the balance being constituted by iron and unavoidable impurities, the total content (mass %) of Cr and Mn in cementite in the metal structure is represented by {Cr+Mn}, the total content (mass %) of Cr and Mn in the steel is represented by [Cr+Mn], and the amount (mass %) of C in the steel is represented by [C], the concentration ratio {Cr+Mn} / [Cr+Mn] is (0.5[C]+0.040) or more, and 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.

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. Furthermore, the spheroidized annealed wire is cold-worked, followed by machining processes such as cutting, thereby forming 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] Let the total content (mass%) of Cr and Mn in cementite in the metal microstructure be {Cr+Mn}, the total content (mass%) of Cr and Mn in the steel be [Cr+Mn], and when [C] represents the amount of C in the steel (mass%), the concentration ratio {Cr+Mn} / [Cr+Mn] is greater than or equal to (0.5[C]+0.040), 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, from...

[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: One or more from the group consisting of more than 0% by mass and less than 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, from...

[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: One or more selected from the group consisting of more than 0% mass and less than 0.5% 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, from...

[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: One or more selected from the group consisting of more than 0% by mass and less than 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] Embodiment 6 of the present invention is a method for manufacturing steel wire for mechanical structural parts according to any one of Embodiments 1 to 5, wherein the method includes a spheroidizing annealing process for bar steel that satisfies the chemical composition of any one of Embodiments 1 to 4, comprising the steps of (1) to (3) below:

[0045] (1) After heating to a temperature T1 of (A1+8℃)~(A1+31℃), maintain the temperature T1 for more than 1 hour and less than 6 hours;

[0046] (2) Perform the following cooling-heating process a total of 2 to 6 times to cool to a temperature T2 above 650°C and below (A1-17°C), and then heat to a temperature T3 of (A1+8°C) to (A1+31°C) at an average heating rate of 75°C / hour to 160°C / hour.

[0047] (3) Cooling is performed at the temperature T3 of the last step 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 conditions for spheroidizing annealing 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 conducted a detailed study to achieve steel wire for mechanical structural parts with excellent cold workability and hardenability. The results showed that it was sufficient to ensure that the total content of Mn and Cr in the cementite was at least a certain proportion relative to the total content of Mn and Cr in the steel, and that the average size of all cementite was within a certain range depending on the C content in the steel. Furthermore, it was found that to achieve the aforementioned metal structure, forming a metal structure with a chemical composition within a certain range, and particularly to perform spheroidizing annealing under specified conditions, was effective in the manufacturing method of the steel wire for mechanical structural parts. Hereinafter, the steel wire for mechanical structural parts according to this embodiment will be described, starting with the metal structure of the steel wire for mechanical structural parts.

[0060] 1. Metal structure

[0061] Traditionally, spheroidizing annealing of steel has been used to form a metallic structure consisting of ferrite and cementite to ensure cold workability. However, to achieve both superior cold workability and even better hardenability, simply forming the aforementioned metallic structure is insufficient. Therefore, the inventors have conducted intensive research from various angles to achieve steel wire for mechanical structural parts that possesses both excellent cold workability and excellent hardenability. First, the inventors focused on the Mn and Cr content in cementite. They discovered that, for example, if spheroidizing annealing is performed under the manufacturing conditions described later, and the average size of all cementite reaches a certain level, while increasing the Mn and Cr content in the cementite, the Mn and Cr content in the ferrite can be relatively reduced, suppressing hardening caused by solid solution strengthening, achieving lower hardness, and improving cold workability. Furthermore, they found that by suppressing the average size of all cementite to a certain level, the non-melting of cementite during the high-temperature holding period in the quenching process can be suppressed, thereby improving hardenability. To date, no simultaneous attention has been paid to both the Mn and Cr content in cementite and the average size of all cementite.

[0062] [Let the total content (mass%) of Cr and Mn in cementite be {Cr+Mn}, and let the total content (mass%) of Cr and Mn in steel be [Cr+Mn], when [C] represents the amount of C in steel (mass%), the concentration ratio {Cr+Mn} / [Cr+Mn] is greater than (0.5[C]+0.040)]

[0063] Cr and Mn are representative elements that readily dissolve in cementite. However, a portion of them dissolve in ferrite, and the greater the amount dissolved, the stronger the ferrite matrix phase becomes, resulting in increased hardness. Therefore, the higher the proportion of the total Cr and Mn content in cementite ({Cr+Mn}) to the total Cr and Mn content in steel ([Cr+Mn]), i.e., the higher the concentration ratio ({Cr+Mn} / [Cr+Mn]), the more it reduces the proportion of Cr and Mn in the ferrite phase outside cementite. As a result, the solid solution strengthening effect of Cr and Mn on ferrite decreases, leading to a reduction in hardness and an improvement in cold workability. The lower limit of the concentration ratio ({Cr+Mn} / [Cr+Mn]) is affected by the C content in the steel, so we assume that the C content (mass%) in the steel is [C], which is (0.5[C] + 0.040) or higher. The concentration ratio {Cr+Mn} / [Cr+Mn] is preferably (0.5[C]+0.042) or higher. On the other hand, considering feasible manufacturing conditions, the upper limit of the concentration ratio {Cr+Mn} / [Cr+Mn] is approximately 0.5[C]+0.500.

[0064] Regarding the aforementioned cementite, its morphology is not particularly limited; in addition to spherical cementite, it also includes rod-shaped cementite with a large aspect ratio. The aspect ratio is the ratio of the longest axis of the cementite particle to its longest minor axis in the direction perpendicular to the long axis (long axis / minor axis). Furthermore, the standard for the size of the cementite being measured is not limited, but as shown in the examples described later, the smallest size of cementite capable of measuring the total Cr and Mn content is used. Specifically, when measuring electrolytic extraction residue using the method shown in the examples described later, the cementite remaining on a filter with a pore size of 0.10 μm is used as the measurement object. Additionally, the so-called total Cr and Mn content in steel, as shown in the examples described later, is the sum of the average Cr content and the average Mn content in the steel. For example, when the metal structure is formed by ferrite and cementite, it refers to the total Cr and Mn content in the ferrite and cementite, expressed as a percentage by mass.

[0065] [When C is expressed as C (mass %) in steel, 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.]

[0066] 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 reduction in hardness. Furthermore, by ensuring that the size of the cementite reaches a certain level, the hardness-reducing effect of increasing the combined Cr and Mn content in the cementite can be more easily achieved. From these perspectives, in this invention, when [C] represents the amount of C in the steel (mass %), the average equivalent circle diameter of all cementite particles is (1.668–2.13 [C]) μm or more. The average equivalent circle diameter of all cementite particles is preferably (1.669–2.13 [C]) μm or more. On the other hand, if the cementite is too 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 is (1.863-2.13 [C]) μm or less. Preferably, it is (1.858-2.13 [C]) μm or less.

[0067] Patent Document 3 discloses that cementite present at ferrite grain boundaries experiences less strain during cold working compared to cementite present within ferrite grains, thus reducing deformation resistance. However, Patent Document 3 does not control the average size of all cementite, resulting in insufficient melting of the cementite during the high-temperature holding phase of the quenching process, leading to poor hardenability. This invention aims to achieve steel wire for mechanical structural parts that combines excellent cold workability and excellent hardenability, focusing on the ratio of the total Cr and Mn content in the cementite and the average size of all cementite.

[0068] 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.

[0069] The microstructure 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 microstructure of the steel wire for mechanical structural parts of the present invention is 90% or more in area ratio, and the rod-shaped cementite with an aspect ratio of 3 or more is 5% or less in area ratio. 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 ratio. The area ratio of the ferrite may further be 95% or more.

[0070] 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.

[0071] 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."

[0072] [Average ferrite grain diameter: below 30 μm]

[0073] 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.

[0074] (characteristic)

[0075] 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.

[0076] (Hardness after spheroidizing annealing) < 91([C]+[Cr] / 9+[Mo] / 2)+91…(2)

[0077] The hardness (HV) after quenching is greater than 380 ln([C]) + 1010…(3)

[0078] 2. Chemical composition

[0079] The chemical composition of the steel wire used for the mechanical structural parts in this embodiment will be explained.

[0080] [C: 0.05% by mass to 0.60% by mass]

[0081] 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.

[0082] [Si: 0.005% by mass to 0.50% by mass]

[0083] 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.

[0084] [Mn: 0.30% by mass to 1.20% by mass]

[0085] Mn functions effectively as a deoxidizer and is an element that 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.

[0086] [P: above 0% by mass and below 0.050% by mass]

[0087] Phosphorus (P) is an unavoidable impurity that causes grain boundary segregation in steel, negatively impacting forgeability and toughness. Therefore, the P content is 0.050% by mass or less. Preferably, the P content is 0.030% by mass or less, more preferably 0.020% by mass or less. Lower P content is preferred, but it typically contains 0.001% by mass or more.

[0088] [S: Above 0% by mass and below 0.050% by mass]

[0089] 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.

[0090] [Al: 0.001% by mass to 0.10% by mass]

[0091] 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.

[0092] [Cr: greater than 0% by mass and less than 1.5% by mass]

[0093] 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.

[0094] [N: greater than 0% by mass and less than 0.02% by mass]

[0095] 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.

[0096] [margin]

[0097] 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.

[0098] 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.

[0099] [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]

[0100] 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.

[0101] 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.

[0102] [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]

[0103] 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.

[0104] [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 Metals (REM): above 0% by mass and below 0.05% by mass]

[0105] 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).

[0106] The shape of the steel wire used for the mechanical structural parts in this embodiment is not particularly limited. For example, wires with a diameter of 5.5 mm to 60 mm can be listed.

[0107] 3. Manufacturing method

[0108] 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.

[0109] 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.

[0110] 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).

[0111] (1) After heating to a temperature T1 between (A1+8℃) and (A1+31℃), maintain this temperature T1 for more than 1 hour but less than 6 hours.

[0112] (2) Perform the following cooling-heating process a total of 2 to 6 times, cooling to a temperature T2 above 650°C but below (A1-17°C), and then heating to a temperature T3 of (A1+8°C) to (A1+31°C) at an average heating rate of 75°C / hour to 160°C / hour.

[0113] (3) Cooling is performed at the temperature T3 of the last step of the cooling-heating process.

[0114] Here, A1 is calculated by the following formula (1).

[0115] A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1)

[0116] Wherein, [element] represents the content (mass%) of each element, and the content of elements not included is 0.

[0117] [(1) After heating to a temperature T1 of (A1+8℃)~(A1+31℃), maintain this temperature T1 for more than 1 hour and less than 6 hours. Figure 1 [2])]

[0118] Heating to a temperature T1 between (A1+8°C) and (A1+31°C) 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 remain within the ferrite grains, increasing hardness. To obtain 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, more preferably (A1+20°C) or higher. On the other hand, if the temperature T1 is too high, the grains become too coarse, making it difficult for spherical cementite to precipitate at the ferrite grain boundaries during the cooling process in the next step, increasing the amount of rod-shaped cementite and thus increasing hardness. Therefore, the temperature T1 is (A1+31°C) or lower. The temperature T1 is preferably (A1+30°C) or lower, more preferably (A1+29°C) or lower.

[0119] Furthermore, if the heating holding time (t1) at temperature T1 is too short, rod-shaped cementite will remain 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 between (A1+8°C) and (A1+31°C)... 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, it can be heated at 30℃ / hour to 100℃ / hour.

[0120] 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).

[0121] A1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]…(1)

[0122] Wherein, [element] represents the content (mass%) of each element, and the content of elements not included is 0.

[0123] [(2) Perform the following cooling-heating process a total of 2 to 6 times, cooling to a temperature T2 above 650°C but below (A1-17°C), and then heating to a temperature T3 (A1+8°C) to (A1+31°C) at an average heating rate of 75°C / hour to 160°C / hour.] Figure 1 [3]~[7])]

[0124] (2-i) Cool to a temperature T2 above 650℃ and below (A1-17℃). Figure 1 [3])

[0125] After the heating and holding process described in (1) above, in order to promote the precipitation of cementite with high concentrations of Mn and Cr, the temperature is cooled to a temperature T2 above 650°C and below (A1-17°C). If the temperature T2 is too low, the annealing time will be prolonged. In addition, if the temperature T2 is too low, the cementite will be too fine, and cementite with low Cr and Mn content will be easily generated. Therefore, the cooling temperature T2 needs to be above 650°C. According to the manufacturing method of this embodiment, the cooling temperature T2 is above 650°C, and the desired cementite can be obtained without prolonged annealing. The temperature T2 is preferably 670°C or higher. On the other hand, if the temperature T2 is too high, the cementite cannot be fully precipitated. As a result, Cr and Mn are not sufficiently thickened in the cementite, the total content of Cr and Mn in the cementite is low, the hardness increases, and the cold workability decreases. Therefore, the upper limit of the temperature T2 is A1-17°C. The temperature T2 is preferably A1-18°C or lower. Furthermore, if the temperature is held after reaching T2, the heat treatment time will be prolonged. Therefore, from these perspectives, it is preferable not to hold the temperature. However, to ensure uniform temperature fluctuation within the furnace, holding the temperature for a short period is permissible. The holding time (t2) at the reached cooling temperature T2 is preferably less than 1 hour.

[0126] Also, during the cooling-heating process ( Figure 1 The average cooling rate of [3] is not particularly limited. From the viewpoint of further promoting the diffusion of Mn and Cr from the parent phase into the cementite, it is preferable that the average cooling rate from temperature T1 or temperature T3 to temperature T2 is 100°C / hour or less. In addition, from the viewpoint of further suppressing the excessive coarsening of the cementite generated in process (2), further improving hardenability, and further improving productivity, it is preferable that the above-mentioned average cooling rate is 5°C / hour or more.

[0127] (2-ii) Heat to a temperature T3 (A1+8℃)~(A1+31℃) at an average heating rate of 75℃ / hour~160℃ / hour. Figure 1 [5] and [6])

[0128] To increase the Cr and Mn content in the cementite precipitated during the cooling process described above (2-i), the cementite is heated from temperature T2 to a temperature T3 ranging from (A1+8°C) to (A1+31°C) at an average heating rate R of 75°C / hour to 160°C / hour. If the average heating rate R is too fast, the diffusion of Cr and Mn is insufficient, resulting in insufficient Cr and Mn content in the cementite formed after the heating and holding process, leading to increased hardness and reduced cold workability. Therefore, the average heating rate R is 160°C / hour or less. The average heating rate R is preferably 155°C / hour or less, more preferably 150°C / hour or less, further preferably 120°C / hour or less, and particularly preferably 100°C / hour or less. On the other hand, if the average heating rate R is too slow, the cementite melts excessively, resulting in a decrease in the total Cr and Mn content contained in the cementite. Furthermore, if the average heating rate R is too slow, the cementite formed during cooling from temperature T1 will become excessively coarse. As a result, the cementite will not fully melt during the high-temperature holding period of the quenching process, leading to a decrease in hardness after quenching and thus deterioration of hardenability. This also results in a longer annealing time, reducing productivity. Therefore, the average heating rate R should be 75°C / hour or higher, preferably 80°C / hour or higher.

[0129] Furthermore, in the cooling-heating process, if the temperature T3, which is the temperature at which heating is achieved, is too low, the diffusion of Cr and Mn will be insufficient. This results in insufficient Cr and Mn content in the cementite generated during the heating and holding process, increased hardness, and reduced cold workability. Therefore, the temperature T3 needs to be at or above (A1 + 8°C). Preferably, the temperature T3 is at or above (A1 + 15°C), more preferably at or above (A1 + 20°C). On the other hand, if the temperature T3, which is the temperature at which heating is achieved, is too high, the cementite will over-melt, resulting in a decrease in the total content of Cr and Mn contained in the cementite. Therefore, the temperature at which heating is achieved (T3) is below (A1 + 31°C). Preferably, the temperature T3 is below (A1 + 30°C), more preferably below (A1 + 29°C).

[0130] Alternatively, the temperature can be held at the heating threshold T3. However, if the holding time (t3) at T3 is too long, the spherical cementite formed during the heating and holding process at temperature T1 is prone to remelting, potentially increasing its hardness. Furthermore, if the holding time (t3) at T3 is too long, it results in a prolonged annealing time, which may reduce productivity. Therefore, it is preferable that the holding time (t3) at T3 be less than 1 hour.

[0131] In the manufacturing method of this embodiment, as described later, the cooling-heating process of (2-i) and (2-ii) described above is repeated multiple times, but in each iteration, the temperature T2, the average heating rate R, and the temperature T3, which are the cooling temperature reached, need to meet the above ranges.

[0132] Furthermore, the relationship between the aforementioned temperature T3 and the aforementioned temperature T1 is not particularly limited. For example, the aforementioned temperature T3 can be the same as the aforementioned temperature T1, or the aforementioned temperature T3 can be higher than the aforementioned temperature T1. Alternatively, from the viewpoint of ensuring that the rod-shaped cementite is fully dissolved in austenite, the aforementioned temperature T1 can be higher than the aforementioned temperature T3.

[0133] (2-iii) Perform a total of 2 to 6 cooling-heating processes. Figure 1 [7])

[0134] To increase the concentration of Mn and Cr in the cementite and promote its coarsening, the aforementioned cooling-heating process needs to be performed a total of 2 to 6 times. Without repeating the cooling-heating process, the concentration of Mn and Cr in the cementite is insufficient, or the coarsening of the cementite is inadequate. As a result, the hardness increases after spheroidizing annealing. Therefore, the cooling-heating process is performed at least twice, preferably at least three times. The more times the process is performed, the lower the hardness becomes, but too many times will saturate the effect. Furthermore, it results in a longer annealing time, reducing productivity. Therefore, the cooling-heating process is performed no more than 6 times. Also, Figure 1 In this case, the cooling-heating process is performed four times. The temperature T2, the average heating rate R, and the temperature T3, which are the cooling temperatures reached in each step, can differ within their respective specified ranges.

[0135] [(3) Cooling is performed from the final temperature T3 of the cooling-heating process ( Figure 1 [8])]

[0136] Cooling is performed from the final temperature T3 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 the cementite, the average cooling rate can be 5°C / hour or more. Additionally, the temperature reached during cooling can be, for example, (A1-30°C) or less. For example, a temperature range of (A1-30°C) or more can be achieved by cooling at the aforementioned average cooling rate, followed by air cooling. Alternatively, by setting the temperature reached during cooling to, for example, 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.

[0137] The spheroidizing annealing process described above (steps (1) to (3)) can be repeated once or multiple times. From the viewpoint of suppressing excessive coarsening of cementite and ensuring productivity, it is preferable to repeat it four times or less, and more preferably three 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.

[0138] 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, that is, a wire drawing process with a reduction rate preferably of 15% or less is performed for the purpose of adjusting dimensions. By keeping the reduction rate at 15% or less, the increase in hardness before cold working can be suppressed. The reduction rate is more preferably 10% or less, further preferably 8% or less, and even more preferably 5% or less.

[0139] 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 above-mentioned reduction rate, the cementite in the steel is destroyed, and the subsequent spheroidizing annealing can promote the agglomeration of cementite, thus enabling the cementite to be moderately coarsened, which is effective for softening. In addition, by performing wire drawing with the above-mentioned reduction rate, the movement of interfaces and the diffusion of elements are active, and the content of Cr and Mn in the cementite increases. The reduction rate is more preferably 10% or more, more preferably 15% or more, and even more preferably 20% or more. On the other hand, if the reduction rate is excessively increased, there is a risk 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 above-mentioned "reduction rate during wire drawing" means the reduction rate from the steel before wire drawing to the steel after multiple wire drawing processes.

[0140] Example

[0141] 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.

[0142] 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.

[0143] 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 temperature T2 in Table 2 at an average cooling rate of 5–100 °C / hour, and subsequently heated to temperature T3 at an average heating rate R as shown in Table 2. 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.

[0144] As a comparative example, in sample No. 12 shown in Table 3, manufacturing condition H1 was implemented. Figure 2 The heat treatment process shown is a heat treatment process with zero cooling-heating cycles. Furthermore, in manufacturing condition H1, wire drawing with a 25% reduction in surface area was not performed before annealing. In contrast, in sample No. 13 shown in Table 3, under manufacturing condition H2, 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.

[0145] Furthermore, as a comparative example, in sample No. 14 shown in Table 3, as manufacturing condition I, the heat treatment conditions that satisfy the manufacturing conditions of Patent Document 3 were implemented. Specifically, the conditions shown as SA2 in the embodiment of Patent Document 3 were implemented, that is, repeated 5 times. Figure 3 The heat treatment process is shown. In sample No. 18 shown in Table 3, as manufacturing condition M, the heat treatment conditions that satisfy the manufacturing conditions of Patent Document 1 are implemented. Specifically, the fifth spheroidizing annealing condition in No. 1 of Table 2 of Patent Document 1 is implemented, that is, repeated 3 times. Figure 4 The heat treatment process is shown. Furthermore, in sample No. 19 shown in Table 3, as manufacturing condition N, 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 Heat treatment as shown.

[0146] 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.

[0147] Using the samples obtained after the above annealing process, the average ferrite grain diameter, the average size of all cementite, and the total Cr and Mn content in the cementite were determined as follows for evaluating the metal structure. Furthermore, as a characteristic feature, the hardness after spheroidizing annealing and the hardness after quenching were measured and evaluated according to the following methods.

[0148] [Evaluation of Metal Structure]

[0149] [Average ferrite grain diameter]

[0150] 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 Number and Grain Diameter”, Umemoto, Ferrum Vol. 2 (1997) No. 10, pp. 29-34, p. 32, regarding the relationship between various quantities of grain size and grain diameter. The results are shown in Table 3. Furthermore, in this embodiment, the ferrite area fraction of samples No. 1 to 11 in Table 3 is all above 90%.

[0151] dn=0.254 / (2 (G－1) / 2 (4)

[0152] [Average size of all cementite]

[0153] In measuring the average size of all cementite in the spheroidized annealed steel wire, the test piece was resin-embedded to allow for cross-sectional observation, and the cut surface was 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 surface 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.

[0154] An OHP film was superimposed on the aforementioned microscopic photographs, and the entire cementite from the microscopic photographs was coated onto the OHP film to obtain an analytical projection image. This projection image was binarized into a black and white photograph, and the grain boundary cementite ratio was calculated using the image package software "Particle Analysis Ver. 3.5" (manufactured by Nippon Steel Technology Co., Ltd.). Additionally, the second projection image was 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 described in Table 3 is the average of values ​​calculated from three fields of view. The minimum measured cementite size (equivalent circle diameter) was 0.3 μm.

[0155] [Measurement of the total Cr and Mn content in cementite (measurement of electrolytic extraction residue)]

[0156] A test material was prepared by electrolytically cutting or grinding approximately 9g of a sample from a portion where the outer layer (less than 1mm) of the steel wire had been removed. This test material was then immersed in an electrolyte (10% acetylacetone – 1% tetramethylammonium chloride – methanol), and approximately 9g of the test material was electrolyzed under constant current. The electrolyzed electrolyte was then filtered through a 0.10μm filter (Advantec Toyo Co., Ltd. polycarbonate membrane filter), and the residue remaining on the filter was taken as cementite in the steel. Next, the residue was dissolved in an acid solution and analyzed using ICP-N spectroscopy to determine the Cr and Mn content in the cementite. The total value of these values ​​was taken as the total Cr and Mn content in the cementite, expressed as % by mass {Cr + Mn}.

[0157] In addition, the total content of Cr and Mn in the steel, expressed as a percentage by mass, was measured as follows: Approximately 4g of sample was extracted from the above sample, dissolved in an acid solution, and analyzed by ICP-N spectroscopy to determine the amount of Cr and Mn in the steel, obtaining their total value [Cr+Mn]. Then, the total content of Cr and Mn in the cementite, expressed as a percentage by mass {Cr+Mn}, was divided by the total content of Cr and Mn in the steel, expressed as a percentage by mass [Cr+Mn], to obtain the concentration ratio {Cr+Mn} / [Cr+Mn].

[0158] [Evaluation of characteristics]

[0159] [Measurement of hardness after spheroidizing annealing]

[0160] To evaluate cold workability, the hardness of each specimen after spheroidizing annealing was measured as follows. A Vickers hardness test was performed at position D / 4 of the cross-section of the test piece (D: diameter of the steel wire) according to JIS Z2244 (2009). The Vickers hardness obtained by averaging at 3 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 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 evaluated as "OK"; if the following formula (2) is not satisfied, the cold workability is poor and evaluated as "NG".

[0161] The hardness (HV) after spheroidizing annealing is < 91([C]+[Cr] / 9+[Mo] / 2)+91…(2)

[0162] [Measurement of hardness after quenching]

[0163] 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.

[0164] 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)

[0165] Wherein, [element] represents the content (mass%) of each element, and elements not included are calculated as 0%.

[0166] 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".

[0167] The hardness (HV) after quenching is greater than 380 ln([C]) + 1010…(3)

[0168] 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.

[0169] Table 1

[0170]

[0171] Table 2

[0172]

[0173] Table 3

[0174]

[0175] The results in the table are examined. The No. below indicates the sample No. in Table 3. Nos. 1 to 11 are inventive examples in which the specified composition, metal structure, and spheroidizing annealing conditions are all satisfied in the embodiments of this invention.

[0176] Nos. 12, 20, 22 and 23 have insufficient cooling-heating cycles, resulting in low total Cr and Mn content in the cementite, or insufficient coarsening of the cementite. The hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0177] Example No. 13 is an example of annealing after wire drawing with a 25% reduction in surface area. Wire drawing can increase the total content of Cr and Mn in cementite. However, because the cooling-heating process is 0 times, 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.

[0178] Example No. 14 is an example of annealing under annealing condition SA2 of Patent Document 3, which satisfies manufacturing condition I as shown in manufacturing condition I. Under this manufacturing condition, the cementite becomes excessively coarse due to annealing, and the hardness after quenching is lower than the standard value, resulting in poor hardenability.

[0179] No. 15 Because temperature T2 is 710℃, which is higher than A1-17℃, the coarsening of cementite is insufficient when cooling from temperature T1, and the total content of Cr and Mn in cementite becomes lower. The hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0180] No. 16 and 17 have lower average heating rates R from temperature T2, resulting in lower total Cr and Mn content in the cementite. Their hardness after spheroidizing annealing is not lower than the standard value, leading to poor cold workability. Alternatively, their hardness after quenching is lower than the standard value, resulting in poor hardenability.

[0181] No. 18 is an example of annealing performed under manufacturing condition M, which satisfies the manufacturing conditions shown in Patent Document 1. In this manufacturing condition, in particular, the heating and holding time at temperature T1 is short, at 0.5 hours. As a result, 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. The hardness after spheroidizing annealing is higher than the standard value, resulting in poor cold workability.

[0182] No. 19 is an example of annealing under condition c of Patent Document 2, which satisfies manufacturing condition N as shown in manufacturing condition N. Under this manufacturing condition, since holding at temperature T1 is not performed, a large amount of small rod-shaped cementite remains in the grain, and the average size of all cementite does not reach a certain level. In addition, the average heating rate R from temperature T2 is low, so the total content of Cr and Mn in the cementite is low, and the hardness after spheroidizing annealing is not lower than the standard value, resulting in poor cold workability.

[0183] No. 21 Because the temperature T3 is 730℃, which is lower than (A1+8℃), the total content of Cr and Mn in the cementite is lower, and the hardness after spheroidizing annealing is not lower than the standard value, resulting in poor cold workability.

[0184] No. 24 to 27 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.

[0185] This application is accompanied by a priority claim based on Japanese Patent Applications No. 22021-061575 and No. 22021-211501. Japanese Patent Applications No. 22021-061575 and No. 22021-211501 are incorporated herein by reference.

[0186] Industrial availability

[0187] 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. Let the total mass percentage (%) of Cr and Mn in the cementite of the metallic microstructure be {Cr + Mn}, and let the total mass percentage (%) of Cr and Mn in the steel be [Cr + Mn], and when [C] represents the mass percentage of C in the steel, the concentration ratio {Cr + Mn} / [Cr + Mn] is greater than or equal to (0.5[C] + 0.040). When the minimum measured size of cementite, expressed as the equivalent circle diameter, is 0.3 μm, and [C] represents the amount of carbon in the steel as a percentage by 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. 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 of (A1+8℃) to (A1+31℃), 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), and then heating to a temperature T3 of (A1+8°C) to (A1+31°C) at an average heating rate of 75°C / hour to 160°C / hour. (3) Cooling is performed from the final temperature T3 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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