JP2022158883A - Steel wire for machine structural component and its manufacturing method - Google Patents

Abstract

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

Description

TECHNICAL FIELD The present invention relates to a steel wire for machine structural parts and a method for producing the same.

BACKGROUND ART In the manufacture of various machine structural parts such as automobile parts and construction machine parts, spheroidizing annealing is usually performed for the purpose of imparting cold workability to bars including hot-rolled wire rods. Then, the steel wire obtained by the spheroidizing annealing is subjected to cold working, and then subjected to machining such as cutting to form a predetermined component shape. Further, quenching and tempering are performed to adjust the final strength, and the mechanical structural parts are manufactured.

In recent years, in order to prevent cracking of steel materials and improve die life in cold working processes, there has been a demand for steel wires that are softer than ever before.

As a method for obtaining a softened steel wire, for example, in Patent Document 1, as a method for producing a medium carbon steel with excellent cold forgeability, heating to an austenitizing temperature range two or more times in a spheroidizing annealing treatment is disclosed. shown to do. According to the manufacturing method of Patent Document 1, it is indicated that a steel for cold forging having a hardness of 83 HRB or less after spheroidizing annealing and a spheroidal carbide ratio in the structure of 70% or more can be obtained.

Patent Literature 2 discloses a steel material having low deformation resistance after spheroidizing annealing and excellent cold forgeability, and a manufacturing method thereof. As the manufacturing method, steel satisfying a predetermined chemical composition is hot-worked, cooled to room temperature, then heated to a temperature range of A1 point to A1 point + 50 ° C., and after the temperature rise, the A1 After holding for 0 to 1 hour in the temperature range from point to A1 point +50 ° C, the temperature range from the temperature range from A1 point to A1 point +50 ° C to A1 point -100 ° C to A1 point -30 ° C is 10 to 200 ° C. After performing the annealing treatment for cooling at an average cooling rate of /hr twice or more, the temperature is raised to the temperature range of A1 point to A1 point + 30 ° C. and held in the temperature range of A1 point to A1 point + 30 ° C. When cooling, after reaching the A1 point when the temperature is raised, when cooling after holding in the temperature range from the A1 point to the A1 point + 30 ° C., the temperature range from the A1 point to the A1 point + 30 ° C. until the A1 point is reached. The residence time is 10 minutes to 2 hours, and the cooling temperature range from the temperature range of A1 point to A1 point +30 ° C to A1 point -100 ° C to A1 point -20 ° C is an average cooling rate of 10 to 100 ° C / hr. After cooling at , it is shown to be held in the cooling temperature range for 10 minutes to 5 hours and then further cooled.

Patent Document 3 describes a steel wire for machine structural parts that can exhibit excellent cold workability by reducing deformation resistance during cold working and improving crack resistance. A steel wire for machine structural parts is disclosed, in which the metallographic structure of the steel is composed of ferrite and cementite, and the number ratio of cementite present in ferrite grain boundaries is 40% or more of the total number of cementite. In Patent Document 3, the manufacturing conditions for the rolled wire rod subjected to spheroidizing annealing are finish rolling at 800°C or higher and 1050°C or lower, first cooling at an average cooling rate of 7°C/sec or higher, and average cooling rate of 1°C. / sec or more and 5 ° C./sec or less, and third cooling whose average cooling rate is faster than the second cooling and 5 ° C./sec or more are performed in this order, and the first cooling is performed. The end and the start of the second cooling are performed within the range of 700 to 750 ° C., the end of the second cooling and the start of the third cooling are performed within the range of 600 to 650 ° C., and the end of the third cooling is performed. It is indicated that it is preferable to set the temperature to 400° C. or less.

JP 2011-256456 A JP 2012-140674 A Japanese Patent Application Laid-Open No. 2016-194100

However, in the conventional techniques disclosed in Patent Documents 1 to 3, the hardness after spheroidizing annealing cannot be sufficiently reduced, and the workability in cold working performed after spheroidizing annealing is poor, or cold working is performed. In some cases, the hardness cannot be sufficiently increased by the quenching treatment performed after working, that is, the quenchability is poor. That is, conventionally, there has been no technique focused on improving both cold workability and hardenability.

The present invention has been made in view of such circumstances, and its object is to provide a steel sheet having sufficiently low hardness and excellent cold workability, and to obtain high hardness by hardening treatment, that is, excellent hardenability. Another object of the present invention is to provide a steel wire for machine structural parts and a method for producing the steel wire for machine structural parts, which can produce the steel wire for machine structural parts in a relatively short period of time.

In this specification, the terms "wire rod" and "steel bar" refer to wire-shaped and rod-shaped steel materials obtained by hot rolling, and are not subjected to heat treatment such as spheroidizing annealing or wire drawing. Refers to steel. The term "steel wire" refers to a wire rod or steel bar subjected to at least one of heat treatment such as spheroidizing annealing and wire drawing. In this specification, the wire rod, steel bar and steel wire are collectively referred to as "long steel".

Aspect 1 of the present invention is
C: 0.05% by mass to 0.60% by mass,
Si: 0.005% by mass to 0.50% by mass,
Mn: 0.30% by mass to 1.20% by mass,
P: more than 0% by mass, 0.050% by mass or less,
S: more than 0% by mass, 0.050% by mass or less,
Al: 0.001% by mass to 0.10% by mass,
Cr: more than 0% by mass, 1.5% by mass or less, and N: more than 0% by mass, 0.02% by mass or less,
containing, the balance consisting of iron and inevitable impurities,
The half width of the X-ray diffraction peak on the (211) plane of the ferrite grain is 0.500° or less,
Steel for machine structural parts, wherein the average equivalent circle diameter of all cementite is (1.863-2.13 [C]) μm or less when the amount of C (% by mass) in the steel is represented by [C] is a line.

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

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

Aspect 4 of the present invention is
Furthermore,
Mg: more than 0% by mass, 0.02% by mass or less,
Ca: more than 0% by mass, 0.05% by mass or less,
Li: more than 0 mass%, 0.02 mass% or less, and REM: more than 0 mass%, containing one or more selected from the group consisting of 0.05 mass% or less, any one of aspects 1 to 3 1. The steel wire for machine structural parts according to 1.

Aspect 5 of the present invention is
5. The steel wire for machine structural parts according to any one of aspects 1 to 4, wherein the average ferrite grain size is 30 μm or less.

Aspect 6 of the present invention is
A long steel satisfying the chemical composition according to any one of aspects 1 to 4,
A method for producing a steel wire for machine structural parts according to any one of aspects 1 to 5, which includes a step of performing spheroidizing annealing including the following steps (1) to (3).
(1) After heating to a temperature T1 of (A1 + 8 ° C.) to (A1 + 31 ° C.), heating and holding at the temperature T1 for more than 1 hour and 6 hours or less,
(2) Cooling to a temperature T2 exceeding 650 ° C. and not exceeding (A1-17 ° C.), and then heating to a heating temperature higher than the temperature T2 and not exceeding (A1 + 60 ° C.), a total of 2 or more cooling-heating steps 6 times,
(3) Cool to a temperature below (A1-30°C) at an average cooling rate of 5°C/hr to 20°C/hr.
Here, A1 is calculated by the following formula (1).
A1 (° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni] (1)
However, [element] represents the content (% by mass) of each element, and the content of elements not included is zero.

Aspect 7 of the present invention is
Aspect 6. The method for producing a steel wire for machine structural parts according to aspect 6, wherein the steel bar is a steel wire obtained by drawing a wire with a reduction in area of more than 5%.

ADVANTAGE OF THE INVENTION According to this invention, the steel wire for machine structural parts which is excellent in hardenability as well as cold workability, and the manufacturing method of the steel wire for machine structural parts can be provided.

4 is a diagram illustrating conditions for spheroidizing annealing in the method for manufacturing a steel wire for machine structural parts according to the present embodiment. 1 is a diagram illustrating a heat treatment process in the prior art; Fig. 3 is a diagram illustrating a heat treatment process in another prior art; Fig. 3 is a diagram illustrating a heat treatment process in another prior art;

The inventors of the present invention have made intensive studies from various angles in order to realize a steel wire for machine structural parts having both excellent cold workability and hardenability.

If the amount of strain in the ferrite after spheroidizing annealing is low, the hardness of the steel after spheroidizing annealing can be reduced and the cold workability is improved. The present inventors focused on the idea that the rod-shaped cementite generated in the cooling process during spheroidizing contains a large amount of interfacial strain. We thought that it would be possible to reduce the Furthermore, the present inventors thought that by reducing the size of all cementite, it is possible to reduce the amount of cementite that remains undissolved during high-temperature holding in the quenching process, thereby improving the hardenability.

Therefore, as a result of further intensive studies, the present inventors have found that, particularly in the metal structure, the half-value width of the X-ray diffraction peak in the (211) plane of the ferrite grain, which is an indicator of the amount of strain in ferrite, is set to a certain value or less. In addition, the inventors found that the average equivalent circle diameter of all cementite should be set to a certain value or less according to the amount of C in the steel material. Further, in order to further realize the above metal structure, the present inventors set the chemical composition within a certain range, and performed spheroidizing annealing under particularly specified conditions in the method of manufacturing the steel wire for machine structural parts. I found that it is effective. Hereinafter, the metal structure of the steel wire for machine structural parts according to the present embodiment will be described first.

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

[The average equivalent circle diameter of all cementite is (1.863-2.13 [C]) μm or less when the amount of C (% by mass) in the steel material is represented by [C]]
If the cementite is excessively coarsened, the cementite will not be sufficiently dissolved during the high temperature hold in the quenching treatment step after cold working, and a sufficiently high hardness cannot be obtained by quenching. Therefore, in the present invention, when the amount of C (% by mass) in the steel material is represented by [C], the average circle equivalent diameter of all cementite is set to (1.863-2.13 [C]) μm or less. Preferably, it is (1.858-2.13 [C]) μm or less.

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

The form of all cementite is not particularly limited, and includes spherical cementite as well as rod-shaped cementite having a large aspect ratio. The aspect ratio is the ratio (major axis/minor axis) of the longest diameter of cementite particles to the longest diameter in the direction perpendicular to the long diameter. Although the standard for the size of cementite to be measured is not limited, the size of cementite that can be determined by the method of measuring the average circle equivalent diameter of all cementites, which will be described later, is the minimum size. Specifically, cementite particles having an equivalent circle diameter of 0.3 μm or more are to be measured.

The metal structure of the steel wire for machine structural parts according to the present embodiment is a spheroidized structure having spheroidized cementite, and is obtained by, for example, performing spheroidizing annealing, which will be described later, on a steel bar that satisfies the chemical composition described later. be able to.

The metal structure of the steel wire for machine structural parts of the present invention is substantially composed of ferrite and cementite. The above-mentioned "substantial" means that ferrite accounts for 90% or more in terms of area ratio in the metal structure of the steel wire for machine structural parts of the present invention, and rod-shaped cementite having an aspect ratio of 3 or more accounts for 5% or less in terms of area ratio. If the adverse effect on workability is small, it means that nitrides such as AlN and inclusions other than nitrides are permitted in an area ratio of less than 3%. Further, the area ratio of the ferrite may be 95% or more.

As used herein, the term “ferrite” refers to a portion whose crystal structure is the bcc structure, and includes ferrite in pearlite, which is a layered structure of ferrite and cementite.
In addition, the "ferrite grains" that are the object of measurement of the "ferrite grain size" include grains containing rod-shaped cementite that is insufficiently spheroidized and generated during spheroidizing annealing. Crystal grains containing rod-shaped cementite (pearlite grains) that may remain before annealing are excluded. Specifically, after etching using nital (nitric acid 2% by volume, ethanol 98% by volume), "crystal grains without cementite present in the grains" can be confirmed when observed at 1000 times using an optical microscope. It means "crystal grains in which cementite is present in the grains and the shape of the cementite can be observed (that is, the boundary between cementite and ferrite can be clearly observed)". Crystal grains in which the shape of cementite cannot be observed at a magnification of 1000 using the optical microscope (that is, the boundary between cementite and ferrite cannot be clearly observed) are not subject to judgment in the present embodiment, and "ferrite crystal grains" exclude.

[Average value of ferrite crystal grain size: 30 μm or less]
In the steel wire for machine structural parts according to the present embodiment, it is preferable that the average value of the ferrite crystal grain size in the metal structure is 30 μm or less. When the average ferrite grain size is 30 μm or less, the ductility of the steel wire for machine structural parts can be improved, and cracking during cold working can be further suppressed. The average ferrite crystal grain size is more preferably 25 μm or less, still more preferably 20 μm or less. The smaller the average ferrite crystal grain size, the better, but the lower limit can be about 2 μm, considering possible production conditions and the like.

(Characteristic)
The steel wire for machine structural parts according to the present embodiment, which satisfies the following chemical composition and has the metal structure described above, can achieve both a low hardness that enables good cold working and a high hardness after quenching treatment. . In this embodiment, when the amount of C (% by mass), the amount of Cr (% by mass), and the amount of Mo (% by mass) in the steel are represented by [C], [Cr], and [Mo], respectively (not included element is zero mass %), hardness, and in the examples described later, the hardness after spheroidizing annealing satisfies the following formula (2), and the hardness after quenching satisfies the following formula (3) Secondly, it was judged that the hardness is sufficiently low and the cold workability is excellent, and high hardness after quenching treatment is achieved, that is, the quenchability is excellent.
Hardness (HV) (after spheroidizing annealing) <91([C]+[Cr]/9+[Mo]/2)+91 (2)
Hardness after quenching treatment (HV) > 380 ln ([C]) + 1010 (3)

2. Chemical Component Composition The chemical component composition of the steel wire for machine structural parts according to the present embodiment will be described.

[C: 0.05% by mass to 0.60% by mass]
C is an element that controls the strength of steel materials, and the strength after quenching and tempering increases as the content increases. In order to effectively exhibit the above effects, the lower limit of the amount of C was set to 0.05% by mass. The amount of C is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and still more preferably 0.20% by mass or more. However, if the amount of C is excessive, the number of spheroidal cementites becomes excessive in the structure after spheroidizing annealing, and the hardness increases, resulting in deterioration of cold workability. Therefore, the upper limit of the amount of C was set at 0.60% by mass. The amount of C is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

[Si: 0.005% by mass to 0.50% by mass]
Si is used as a deoxidizer during smelting and contributes to strength improvement. In order to effectively exhibit this effect, the lower limit of the amount of Si was set to 0.005% by mass. The amount of Si is preferably 0.010% by mass or more, more preferably 0.050% by mass or more. However, Si contributes to solid-solution strengthening of ferrite and has the effect of considerably increasing the strength after spheroidizing annealing. If the Si content is excessive, the cold workability deteriorates due to the above effect, so the upper limit of the Si content was made 0.50% by mass. The amount of Si is preferably 0.40% by mass or less, more preferably 0.35% by mass or less.

[Mn: 0.30% by mass to 1.20% by mass]
Mn is an element that effectively acts as a deoxidizer and contributes to the improvement of hardenability. In order to sufficiently exhibit the effect, the lower limit of the amount of Mn was set to 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 amount of Mn is excessive, segregation tends to occur and the toughness decreases. Therefore, the upper limit of the amount of Mn was set to 1.20% by mass. The Mn content is preferably 1.10% by mass or less, more preferably 1.00% by mass or less. From the viewpoint of further suppressing a decrease in toughness, the content may be less than 0.50% by mass, and may be 0.45% by mass or less.

[P: more than 0% by mass, 0.050% by mass or less]
P (phosphorus) is an unavoidable impurity and a harmful element that causes grain boundary segregation in steel and adversely affects forgeability and toughness. Therefore, the amount of P was set to 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. Although the P content is preferably as small as possible, it is usually contained in an amount of 0.001% by mass or more.

[S: more than 0% by mass, 0.050% by mass or less]
S (sulfur) is an unavoidable impurity, forms MnS in steel, and deteriorates ductility, so it is an element harmful to cold workability. Therefore, the amount of S is set to 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. Although the amount of S is preferably as small as possible, it is usually contained in an amount of 0.001% by mass or more.

[Al: 0.001% by mass to 0.10% by mass]
Al is an element contained as a deoxidizing agent, and has the effect of reducing impurities accompanying deoxidizing. In order to exhibit this effect, the lower limit of the amount of Al was set to 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 amount of Al is excessive, nonmetallic inclusions increase and the toughness decreases. Therefore, the upper limit of the amount of Al was set to 0.10% by mass. The Al content is preferably 0.08% by mass or less, more preferably 0.05% by mass or less.

[Cr: more than 0% by mass, 1.5% by mass or less]
Cr is an element that has the effect of improving the hardenability of steel and increasing the strength, and also has the effect of promoting spheroidization of cementite. Specifically, Cr dissolves in cementite and delays the dissolution of cementite during heating for spheroidizing annealing. Since the cementite does not dissolve and partially remains during heating, rod-shaped cementite having a large aspect ratio is less likely to form during cooling, making it easier to obtain a spheroidized structure. Therefore, the Cr content is more than 0% by mass, preferably 0.01% by mass or more. Further, it may be 0.05% by mass or more, and even more preferably 0.10% by mass or more. From the viewpoint of further promoting cementite spheroidization, the content can be more than 0.30% by mass, and can also be more than 0.50% by mass. If the amount of Cr is excessive, the diffusion of elements including carbon is delayed, and the dissolution of cementite is delayed more than necessary, making it difficult to obtain a spheroidized structure. As a result, the hardness reduction effect of the present invention may be reduced. 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 viewpoint of accelerating the diffusion of the elements, the Cr content can be set to 1.00% by mass or less, 0.80% by mass or less, or 0.30% by mass or less.

[N: more than 0% by mass, 0.02% by mass or less],
N is an impurity that is inevitably contained in steel, but when a large amount of solid-solution N is contained in steel, it causes an increase in hardness and a decrease in ductility due to strain aging, and deteriorates cold workability. Therefore, the N content is 0.02% by mass or less, preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

[Remainder]
The balance is iron and unavoidable impurities. As unavoidable impurities, trace elements (for example, As, Sb, Sn, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed. For example, there are elements, such as P and S, whose content is generally preferably as low as possible and thus are unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, the term "inevitable impurities" constituting the balance is a concept excluding elements whose composition range is separately defined.

The steel wire for machine structural parts according to the present embodiment may contain the above elements in its chemical composition. The optional elements described below may not be contained, but by containing them together with the above elements as necessary, it is possible to more easily ensure hardenability and the like. The selected elements are described below.

[Cu: more than 0% by mass, 0.25% by mass or less, Ni: more than 0% by mass, 0.25% by mass or less, Mo: more than 0% by mass, 0.50% by mass or less, and B: more than 0% by mass , one or more selected from the group consisting of 0.01% by mass or less]
Cu, Ni, Mo and B are all effective elements for increasing the strength of the final product by improving the hardenability of the steel material, and may be contained alone or in combination of two or more. The effect of these elements increases as their content increases. A preferable lower limit for effectively exhibiting the above effects is more than 0% by mass, more preferably 0.02% by mass or more, and still more preferably 0.05% by mass or more for each of Cu, Ni, and Mo. It is more than 0% by mass, more preferably 0.0003% by mass or more, and still more preferably 0.0005% by mass or more.
On the other hand, if the content of these elements becomes excessive, the strength becomes too high and the cold workability may deteriorate. More preferably, the content of each of Cu and Ni is 0.22% by mass or less, more preferably 0.20% by mass or less, and the content of Mo is more preferably 0.40% by mass or less, and further preferably It is preferably 0.35% by mass or less, and the B content is more preferably 0.007% by mass or less, still more preferably 0.005% by mass or less.

[Ti: more than 0% by mass, 0.2% by mass or less, Nb: more than 0% by mass, 0.2% by mass or less, and V: more than 0% by mass, selected from the group consisting of 0.5% by mass or less 1 or more]
Ti, Nb and V form a compound with N and reduce solid solution N to exhibit the effect of reducing deformation resistance. The effect of these elements increases as their content increases. The preferable lower limit for effectively exhibiting the above effects for any element is more than 0% by mass, more preferably 0.03% by mass or more, and still more preferably 0.05% by mass or more. However, if the content of these elements becomes excessive, the compounds formed may lead to an increase in deformation resistance, which may rather deteriorate the cold workability. Hereinafter, the V content is preferably 0.5% by mass or less. The content of each of Ti and Nb is more preferably 0.18% by mass or less, more preferably 0.15% by mass or less, and the V content is more preferably 0.45% by mass or less, further preferably It is 0.40% by mass or less.

[Mg: more than 0% by mass, 0.02% by mass or less, Ca: more than 0% by mass, 0.05% by mass or less, Li: more than 0% by mass, 0.02% by mass or less, and rare earth elements (Rare Earth Metal :REM): One or more selected from the group consisting of more than 0% by mass and 0.05% by mass or less]
Mg, Ca, Li and REM are elements effective in spheroidizing sulfide-based inclusions such as MnS and improving the deformability of steel. These effects increase as the content increases. In order to effectively exhibit the above effects, the content of Mg, Ca, Li and REM is preferably more than 0% by mass, more preferably 0.0001% by mass or more, and still more preferably 0.0005% by mass or more. be. However, even if it is contained excessively, the effect is saturated, and the effect corresponding to the content cannot be expected. , more preferably 0.015% by mass or less, and the content of Ca and REM is preferably 0.05% by mass or less, more preferably 0.045% by mass or less, and still more preferably 0.040% by mass or less. is. Incidentally, each of Mg, Ca, Li and REM may be contained alone, or two or more kinds thereof may be contained. Content is fine. The REM is meant to include lanthanoid elements (15 elements from La to Lu), Sc (scandium) and Y (yttrium).

The shape and the like of the steel wire for machine structural parts according to the present embodiment are not particularly limited. For example, those having a diameter of 5.5 mm to 60 mm can be used.

3. Manufacturing Method In order to obtain the metal structure of the steel wire for machine structural parts according to the embodiment of the present invention, the spheroidizing annealing conditions should be appropriately controlled as described below in manufacturing the steel wire for machine structural parts. is preferred. The hot rolling process for producing a wire rod or steel bar to be subjected to spheroidizing annealing is not particularly limited, and a conventional method may be followed. As will be described later, wire drawing may be applied before spheroidizing annealing. The diameter of the wire rod, steel wire, and steel bar to be subjected to spheroidizing annealing is not particularly limited.

The spheroidizing annealing conditions in the method for manufacturing a steel wire for machine structural parts according to the embodiment of the present invention will be described with reference to FIG. FIG. 1 shows an example of a diagram explaining the conditions of spheroidizing annealing in the manufacturing method according to the embodiment of the present invention, and the number of repetitions of the cooling-heating process is not limited to this FIG.

A method for manufacturing a steel wire for machine structural parts according to an embodiment of the present invention includes a spheroidizing annealing step including steps (1) to (3) below.
(1) After heating to a temperature T1 of (A1 + 8 ° C.) to (A1 + 31 ° C.), heating and holding at the temperature T1 for more than 1 hour and 6 hours or less,
(2) Cooling to a temperature T2 exceeding 650 ° C. and not exceeding (A1-17 ° C.) and heating to a heating temperature higher than the temperature T2 and not exceeding (A1 + 60 ° C.), performing a total of 2 to 6 times of the cooling-heating process. death,
(3) Cool to a temperature below (A1-30°C) at an average cooling rate of 5°C/hr to 20°C/hr.
Here, A1 is calculated by the following formula (1).
A1 (° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni] (1)
However, [element] represents the content (% by mass) of each element, and the content of elements not included is zero.

[(1) After heating to a temperature T1 of (A1 + 8 ° C.) to (A1 + 31 ° C.), heating and holding at the temperature T1 for more than 1 hour and 6 hours or less ([1] and [2] in FIG. 1)]
Heating to a temperature (T1) of (A1+8° C.) to (A1+31° C.) accelerates the dissolution of the rod-shaped cementite having a large aspect ratio and a large interfacial strain generated in the rolling stage. If the temperature T1 is too low, the rod-shaped cementite, which has a large interfacial strain, is not dissolved during heating and holding, and remains in the ferrite, increasing the hardness and degrading the cold workability. To obtain a sufficiently softened steel wire, the temperature T1 must be 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 crystal grains become too coarse, making it difficult for spherical cementite to precipitate at the ferrite crystal grain boundaries in the cooling process of the next step, increasing rod-like cementite, increasing hardness, Cold workability deteriorates. Therefore, the temperature T1 was set to A1+31° C. or lower. The temperature T1 is preferably A1+30° C. or lower, more preferably A1+29° C. or lower.

On the other hand, if the heating and holding time (t1) at the temperature T1 is too short, rod-shaped cementite with a large interfacial strain remains in the ferrite crystal grains, increasing the hardness and lowering the cold workability. In order to obtain a sufficiently softened steel wire, the heating holding time (t1) should be more than 1 hour and 6 hours or less. The heating and holding time (t1) is preferably 1.5 hours or longer, more preferably 2.0 hours or longer. If the heating holding time (t1) is too long, the heat treatment time will be long and the productivity will be lowered. Therefore, the heating and holding time (t1) is 6 hours or less, preferably 5 hours or less, and more preferably 4 hours or less. Note that the average heating rate during heating ([1] in FIG. 1) to the temperature (T1) from (A1 + 8 ° C.) to (A1 + 31 ° C.) does not affect the steel material properties, so the temperature can be raised at an arbitrary rate. . For example, the temperature may be raised at a rate of 30° C./hour to 100° C./hour.

The temperature at point A1 is calculated by the following formula (1) described on page 273 of Leslie Iron and Steel Materials Science (Maruzen).
A1 (° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni] (1)
However, [element] represents the content (% by mass) of each element, and the content of elements not included is zero.

[(2) Cooling to a temperature T2 exceeding 650 ° C. and not exceeding (A1-17 ° C.) and heating to a heating temperature higher than the temperature T2 and not exceeding (A1 + 60 ° C.) Perform a total of 2 to 6 times of the cooling-heating process. ([7] in Fig. 1)]
Subsequently, the cooling-heating step of cooling to a temperature T2 above 650° C. to (A1-17° C.) and heating to a heating temperature higher than the temperature T2 (A1+60° C.) or less is performed 2 to 6 times in total. Each step of the cooling-heating process is described in detail below.

<(2-i) Cooling to temperature T2 from over 650° C. to (A1-17° C.) ([3] and [4] in FIG. 1)>
Cool to promote the precipitation of spherical cementite with less interfacial strain. When the average cooling rate from temperature T1 is slowed down, excessive reprecipitation of rod-shaped cementite with a large interfacial strain can be suppressed, the amount of strain in ferrite can be reduced, and cold workability can be further improved. Therefore, the average cooling rate is preferably 100° C./hour or less. The average cooling rate is more preferably 90° C./hour or less, still more preferably 80° C./hour or less. On the other hand, when the average cooling rate is increased, excessive coarsening of the cementite formed during cooling is suppressed, and as a result, the cementite is sufficiently dissolved during the high temperature holding of the quenching process, and the hardness after quenching is reduced. Further improvement, that is, the hardenability can be further enhanced. Furthermore, the annealing time can be shortened, improving productivity. Therefore, the average cooling rate is preferably 1° C./hour or more, more preferably 3° C./hour or more, and even more preferably 5° C./hour or more.

Further, if the ultimate cooling temperature (T2) is too low, the annealing time will be prolonged. Therefore, the ultimate temperature (T2) of cooling needs to exceed 650°C. According to the manufacturing method according to the present embodiment, even if the ultimate cooling temperature (T2) exceeds 650° C., the cementite can be controlled into a desired form without long-term annealing. The cooling temperature (T2) is preferably 670° C. or higher. On the other hand, if the ultimate cooling temperature (T2) is too high, excessive reprecipitation of rod-shaped cementite with a large interfacial strain will increase the amount of strain in the ferrite and lower the cold workability. Therefore, the ultimate temperature (T2) of cooling needs to be lower than the A1 point by a certain amount or more. Therefore, the upper limit of the cooling temperature (T2) was set to A1-17°C. The ultimate temperature (T2) of cooling is preferably A1-18° C. or lower. Further, if the temperature is maintained after reaching the ultimate cooling temperature (T2), the heat treatment time will be prolonged. Therefore, it is better not to hold from these points of view. However, in order to equalize temperature variations in the furnace, it may be held for a short period of time. The holding time at the cooling reaching temperature T2 is preferably within 1 hour.

<(2-ii) Heating to a heating temperature higher than temperature T2 (A1 + 60 ° C.) or less ([5] and [6] in FIG. 1)>
In order to re-dissolve the rod-shaped cementite having a large interfacial strain precipitated in the step (2-i), the steel is heated from the ultimate temperature (T2) of the cooling. The ultimate temperature of heating as shown in [6] in FIG. 1, that is, the heating temperature (T3) may be any temperature within the temperature range higher than the temperature T2 and not higher than (A1+60° C.). The heating temperature is preferably A1° C. or higher from the viewpoint of sufficiently remelting the rod-shaped cementite having a large interfacial strain generated in the step (2-i). Moreover, from the viewpoint of suppressing remelting of spherical cementite on ferrite grain boundaries and suppressing an increase in hardness after spheroidizing annealing, the heating temperature (T3) is preferably A1+57° C. or lower.

The average heating rate from the cooling ultimate temperature (T2) to the heating temperature (T3) as shown in [5] in FIG. 1 is not particularly limited either. The average temperature increase rate is, for example, 200° C./ It may be less than an hour. Further, for example, from the viewpoint of sufficiently suppressing coarsening of the cementite generated by this heating and further enhancing hardenability, the heating rate can be set to 5° C./hour or more.

After reaching the heating temperature (T3), it does not matter whether or not the heating temperature is maintained. In the case of holding at the heating temperature, for example, the holding time is set to 1 hour or less to suppress redissolution of spherical cementite generated in the step of cooling to the temperature T2.

The magnitude relationship between the heating temperature (T3) and the temperature T1 is not particularly limited. For example, the heating temperature (T3) may be the same temperature as the temperature T1, or the heating temperature (T3) may be It may be higher than the temperature T1.

In the manufacturing method according to the present embodiment, the cooling-heating step of (2-i) cooling and (2-ii) heating is repeated multiple times, but each time, the temperature T2 and the temperature T3 satisfy the above ranges. There is a need.

<(2-iii) Performing the cooling-heating step a total of 2 to 6 times ([7] in FIG. 1)>
In order to suppress the precipitation of rod-shaped cementite with a large interfacial strain that precipitates in the step (2-i), after heating and holding at the temperature T1 in the step (1), the above (2-i) and the above (2) It is necessary to perform the cooling-heating step of -ii) a total of 2 to 6 times. If this cooling-heating process is not repeated, the amount of strain in the ferrite increases, resulting in an increase in hardness after spheroidizing annealing. Therefore, the cooling-heating process is performed twice or more. It is preferably three times or more. The hardness is reduced as the number of times of implementation is increased, but the effect is saturated even if the number of times of implementation is too large. In addition, the annealing time is lengthened and the productivity is lowered. Therefore, the number of cooling-heating steps was set to 6 or less. In the case of FIG. 1, the number of times of the above cooling (2-i) and the above heating (2-ii) is four. Also, the temperature (T2) reached by each cooling may be different within a defined range. In addition, the average cooling rate in the cooling-heating process refers to the average cooling rate from the temperature T1 to the cooling target temperature (T2) in the first cooling-heating process, and from the heating temperature (T3) after the second time. It means the average cooling rate up to the cooling ultimate temperature (T2).

[(3) cooling to a temperature below (A1-30°C) at an average cooling rate of 5°C/h to 20°C/h ([8] and [9] in Figure 1)]
Cool from the final heating temperature (T3) of the cooling-heating process. When the ultimate cooling temperature (T4) is (A1-30° C.) or higher, rod-shaped cementite with a large interfacial strain reprecipitates, strain in ferrite increases, and cold workability deteriorates. Therefore, the ultimate cooling temperature (T4) was set to less than (A1-30°C). It is preferably (A1-35°C) or less, more preferably (A1-40°C) or less. The ultimate cooling temperature (T4) is preferably (A1-250°C) or higher, more preferably (A1-200°C) or higher, and even more preferably (A1-150°C) or higher, from the viewpoint of shortening the annealing time. be.

The average cooling rate (R3) must be 20° C./hour or less in order to suppress the reprecipitation of rod-shaped cementite with a large amount of interfacial strain and reduce the amount of strain in ferrite. The average cooling rate (R3) is preferably 18°C/hour or less, more preferably 15°C/hour or less. If the average cooling rate (R3) is too slow, the cementite is excessively coarsened, and the cementite is not sufficiently dissolved during the high temperature holding in the quenching process, resulting in a decrease in hardness after quenching, that is, deterioration of hardenability. . Further, the annealing time is lengthened, resulting in a decrease in productivity. Therefore, the lower limit of the average cooling rate (R3) was set at 5°C/hour. The average cooling rate (R3) is preferably 10°C/hour or more.

In a temperature range of less than (A1-30° C.), precipitation of highly strained rod-shaped cementite at the interface does not occur. Therefore, the ultimate cooling temperature (T4) may be any temperature as long as it is less than (A1-30° C.). Cooling below the arbitrary temperature is not particularly limited, and may be air cooling, for example.

The spheroidizing annealing (steps (1) to (3)) as described above may be repeated once or multiple times. From the viewpoint of suppressing excessive coarsening of cementite and securing productivity, for example, it is preferably 4 times or less, more preferably 3 times or less. When the spheroidizing annealing is repeated multiple times, it may be repeated under the same conditions or under different conditions within the above specified range. Further, when the spheroidizing annealing is repeated multiple times, wire drawing may be added between the spheroidizing annealing. For example, wire drawing before spheroidizing annealing to be described later→first spheroidizing annealing→wire drawing→second spheroidizing annealing can be performed in this order.

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

In order to promote the formation of the structure morphology of the present invention, it is preferable to provide a step of drawing the wire with a reduction in area of more than 5% before the spheroidizing annealing. By performing wire drawing with the above area reduction rate, the cementite in the steel is destroyed, and the subsequent spheroidizing annealing can promote the aggregation of cementite. is. The area reduction rate is more preferably 10% or more, still more preferably 15% or more, and even more preferably 20% or more. On the other hand, excessively increasing the area reduction rate may invite disconnection risk. Therefore, the area reduction rate is preferably 50% or less. When wire drawing is performed multiple times, the number of times of wire drawing is not particularly limited, and can be, for example, two times. In addition, when wire drawing is performed multiple times, the above "area reduction rate during wire drawing" is the reduction from the steel material before wire drawing to the steel material after wire drawing is performed multiple times. means area ratio.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and it is of course possible to make appropriate modifications within the scope that can be adapted to the gist of the above and below. Included in scope.

After smelting the test material having the chemical composition shown in Table 1 in a converter, the steel slab obtained by casting was subjected to hot rolling to produce a wire rod having a diameter of 12 to 16 mm. In Table 2, which will be described later, in the case of wire drawing "yes" before spheroidizing annealing, that is, sample No. In Nos. 2 and 12, the steel wires obtained by drawing the above wire rods at a rate of area reduction of 25% were subjected to spheroidizing annealing.

Annealing was performed using the above wire or steel wire using a laboratory furnace. In annealing, the wire or steel wire was heated to T1 shown in Table 2 and held for t1. Then, at an average cooling rate of 5 ° C. / hour to 100 ° C. / hour, after cooling to the temperature T2 in Table 2, 5 ° C. / hour to a heating temperature higher than the temperature T2 in Table 2 (A1 + 60 ° C.) or less Heating was performed at an average heating rate of 100°C/hour. This cooling and heating process was repeated for the number of cooling-heating cycles shown in Table 2. Next, from the heating temperature in the final cooling-heating process, the material was cooled to temperature T4 at an average cooling rate R3 in Table 2. Then, it was air-cooled to obtain a sample. As shown in Table 2, manufacturing conditions G1, G2, T and U do not repeat the cooling-heating process. Specifically, for example, under manufacturing conditions G1, the temperature was raised to 740° C. and held for 5 hours, then cooled to 640° C. at an average cooling rate of 10° C./hour, and then air-cooled.

Moreover, as a comparative example, sample No. shown in Table 3 was used. In 13, the heat treatment conditions that satisfy the manufacturing conditions of Patent Document 3, specifically, the conditions indicated as SA2 in the example of Patent Document 3 are performed as the manufacturing conditions H, that is, the heat treatment process shown in FIG. 2 is repeated five times. rice field. Sample No. shown in Table 3. 17, as the manufacturing condition L, the heat treatment conditions satisfying the manufacturing conditions of Patent Document 1; The fifth spheroidizing annealing condition in 1 was performed, ie the heat treatment step shown in FIG. 3 was repeated three times. In addition, sample No. shown in Table 3. In No. 18, as manufacturing conditions M, heat treatment conditions satisfying the manufacturing conditions of Patent Document 2, specifically condition c in Table 2 of Patent Document 2, that is, heat treatment in the pattern shown in FIG. 4 was performed. The annealing parameters listed in Table 2 are the set temperatures of the heat treatment furnace. A thermocouple was attached to the steel material to test the difference between the actual temperature of the steel material and the set temperature.

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

[Evaluation of metal structure]
[Average value of ferrite grain size]
First, the ferrite grain size was measured as follows. The cross section of the steel wire after spheroidizing annealing, that is, the test piece was embedded in resin so that the D/4 position (D: diameter of the steel wire) of the cross section perpendicular to the axial direction of the steel wire could be observed. The test piece was etched using nital (2% by volume of nitric acid, 98% by volume of ethanol) to expose the structure. Then, with an optical microscope, the structure of the test piece in which the above structure is exposed is observed at a magnification of 400 times, and ferrite grains of an average size representing the structure of the entire steel wire can be observed within the evaluation surface. One field was selected to obtain a photomicrograph. Then, the value of the ferrite grain size (G) was calculated from the photographed based on the comparison method of JIS G0551 (2020). Then, using the calculated value of the ferrite grain size (G), "Introductory Lecture Technical Terms-Iron and Steel Materials Edition-3 Grain Size Number and Grain Size", Minoru Umemoto, Ferrum Vol. 2 (1997) No. 10, p29 to 34, in the relationship between various amounts related to the grain size and grain size described in Table 1 on p32, the relationship between the ferrite grain size G (orN) and the average value dn of the ferrite grain size, The average value dn of the ferrite crystal grain size was obtained from the following formula (4). Table 3 shows the results. Note that sample No. in Table 3 was used in this example. All of Nos. 1 to 10 had a ferrite area ratio of 90% or more.
dn=0.254/(2 ^(G−1)/2 ) (4)

[Half width of X-ray diffraction peak in (211) plane of ferrite grain]
The half-value width of the X-ray diffraction peak in ferrite grains was obtained by measuring the half-value width of the X-ray diffraction peak in the (211) plane of ferrite at the D/4 position (D: diameter) of the steel wire after spheroidizing annealing. . Specifically, the test piece was embedded in resin so that the cross section of the steel wire after spheroidizing annealing could be observed, and was subjected to Emily polishing, diamond buffing, and electropolishing to remove strain on the evaluation surface introduced during sample preparation. provided. Then, a PSPC (Position-Sensitive Propotional Counter) manufactured by Rigaku Corporation was used to determine the peak half-value width around 148° to 165° of ferrite iron. The measured value was the average value of the values measured twice. Other conditions for X-ray diffraction are as follows.
・Target: Cr
・Acceleration voltage: 40 kV
・Acceleration current: 40mA
・Collimator: φ0.5mm

[Average circle equivalent diameter of all cementite]
For the measurement of the average equivalent circle diameter of all cementite in the steel wire after spheroidizing annealing, the test piece was embedded in resin so that the cross section could be observed, and the cut surface was mirror-polished with emery paper and a diamond buff. Next, the cut surface is etched for 30 seconds to 1 minute using nital (2% by volume of nitric acid, 98% by volume of ethanol) as an etchant to obtain ferrite at the D/4 position (D: diameter of steel wire). Grain boundaries and cementite were revealed. Then, using FE-SEM (Field-Emission Scanning Electron Microscope, field emission scanning electron microscope), the structure of the test piece in which the above cementite etc. is exposed is observed, and 3 fields of view are photographed at a magnification of 2500 times. did.

An OHP film was superimposed on the micrograph taken above, and all cementite in the micrograph was painted over from the OHP film to obtain a projection image for analyzing all cementite. The projected image was binarized into a black-and-white photograph, and the equivalent circle diameter of all cementite was calculated using image analysis software "particle analysis ver 3.5" (Nippon Steel Technology Co., Ltd.). The average circle equivalent diameter of all cementites shown in Table 3 is the average of values calculated from three fields of view. The minimum size (equivalent circle diameter) of cementite to be measured was 0.3 μm.

[Evaluation of characteristics]
[Measurement of hardness after spheroidizing annealing]
In order to evaluate cold workability, the hardness of each sample after spheroidizing annealing was measured as follows. A Vickers hardness test was performed in accordance with JISZ2244 (2009) at the D/4 position (D: diameter of steel wire) of the cross section of the test piece. The Vickers hardness obtained by calculating the average of 3 or more points was defined as the hardness after spheroidizing annealing. Table 3 shows the measurement results. In Table 3, the hardness after spheroidizing annealing is indicated as "spheroidizing hardness". In this example, the hardness after spheroidizing annealing is determined by the amount of C (% by mass), the amount of Cr (% by mass), and the amount of Mo (% by mass) in the steel [C], [Cr], and [Mo], respectively. (elements not included are zero mass%), the case where the following formula (2) is satisfied is evaluated as "OK" as being excellent in cold workability, and the following formula (2) is not satisfied The case was evaluated as "NG" as being inferior in cold workability.
Hardness after spheroidizing annealing (HV)<91([C]+[Cr]/9+[Mo]/2)+91 (2)

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

Then, a Vickers hardness test was performed on the quenched sample at the t/2 position and the D/4 position (D: diameter of steel wire, t: thickness of sample). The Vickers hardness obtained by calculating the average of 3 or more points was defined as the hardness after the quenching treatment. Table 3 shows the measurement results. In Table 3, the hardness after quenching treatment is shown as "quenching hardness". In this example, when the hardness after quenching satisfies the following formula (3) when the amount of C (% by mass) in the steel is represented by [C], it is considered to be excellent in hardenability. When the following formula (3) was not satisfied, the hardenability was evaluated as "NG".
Hardness after quenching treatment (HV) > 380 ln ([C]) + 1010 (3)

In Table 3, when both the hardness after the spheroidizing annealing and the hardness after the quenching treatment are OK, the overall judgment is "OK" as having both excellent cold workability and excellent hardenability. When at least one of the hardness after annealing and the hardness after quenching treatment was NG, it was judged that both excellent cold workability and excellent hardenability could not be achieved, and was judged as "NG". In Tables 2 and 3, the underlined values deviate from the ranges specified for the embodiments of the present invention or do not meet the desired properties.

Consider the results in the table. The following No. is sample No. in Table 3. indicates No. Nos. 1 to 10 are invention examples that satisfy all of the chemical composition, metallographic structure and spheroidizing annealing conditions specified in the embodiments of the present invention.

No. 11, 12, 19, 21 to 26 were not subjected to the cooling-heating process, or were performed only once. The amount of strain inside increased, and the half width of the X-ray diffraction peak exceeded 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

No. No. 13 is an example in which annealing is performed under the annealing conditions SA2 of Patent Document 3 as the manufacturing condition H that satisfies the manufacturing conditions shown in Patent Document 3. Under these manufacturing conditions, the cementite was excessively coarsened by annealing, and the hardness after quenching treatment was lower than the reference value, resulting in poor hardenability.

No. 14 and 20, T1 is 730 ° C., which is lower than A1 + 8 ° C., so the amount of strain in ferrite increases due to rod-shaped cementite with a large amount of interfacial strain remaining from before annealing, and the half-value width of the X-ray diffraction peak was over 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

No. In No. 15, the average cooling rate R3 is as high as 21 ° C./hour, so the rod-shaped cementite with a large interface strain generated in the step [8] in FIG. The price range exceeded 0.500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

No. In No. 16, T2 is as high as 710° C., so the amount of strain in the ferrite increases due to rod-shaped cementite with a large interface strain generated in the step [3] of FIG. over 500°. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

No. No. 17 is an example in which annealing is performed under manufacturing condition L, which satisfies the manufacturing conditions shown in Patent Document 1. Under these manufacturing conditions, t1 is not maintained, etc., and due to rod-shaped cementite with a large amount of interfacial strain remaining from before annealing, the amount of strain in the ferrite increases, and the half-value width of the X-ray diffraction peak exceeds 0.500°. rice field. Therefore, the hardness after spheroidizing annealing was higher than the reference value, resulting in poor cold workability.

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

The steel wire for machine structural parts according to the present embodiment has low deformation resistance at room temperature when manufacturing various machine structural parts, and can suppress abrasion and breakage of jigs and tools for plastic working such as molds. It exhibits excellent cold workability, such as suppressing the occurrence of cracks during forging. Furthermore, since it is excellent in hardenability, high hardness can be secured by hardening treatment after cold working. For these reasons, the steel wire for machine structural parts according to the present embodiment is useful as a steel wire for cold working machine structural parts. For example, the steel wire for machine structural parts according to the present embodiment can be subjected to cold working such as cold forging, cold heading, and cold rolling to obtain various machine structures such as parts for automobiles and parts for construction machinery. Used to manufacture parts. Specific examples of such mechanical structural parts include bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, receiving washers, tappets, saddles, bulks, Inner cases, clutches, sleeves, outer races, sprockets, cores, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, mouthpieces, valve lifters, spark plugs, pinion gears, steering Examples include mechanical parts such as shafts and common rails, electrical parts, and the like.

Claims (7)

C: 0.05% by mass to 0.60% by mass,
Si: 0.005% by mass to 0.50% by mass,
Mn: 0.30% by mass to 1.20% by mass,
P: more than 0% by mass, 0.050% by mass or less,
S: more than 0% by mass, 0.050% by mass or less,
Al: 0.001% by mass to 0.10% by mass,
Cr: more than 0% by mass, 1.5% by mass or less, and N: more than 0% by mass, 0.02% by mass or less,
containing, the balance consisting of iron and inevitable impurities,
The half width of the X-ray diffraction peak on the (211) plane of the ferrite grain is 0.500° or less,
Steel for machine structural parts, wherein the average equivalent circle diameter of all cementite is (1.863-2.13 [C]) μm or less when the amount of C (% by mass) in the steel is represented by [C] line. Furthermore,
Cu: more than 0% by mass, 0.25% by mass or less,
Ni: more than 0% by mass, 0.25% by mass or less,
Mo: more than 0 mass%, 0.50 mass% or less and B: more than 0 mass%, containing one or more selected from the group consisting of 0.01 mass% or less, mechanical structural component according to claim 1 steel wire. Furthermore,
Ti: more than 0% by mass, 0.2% by mass or less,
Nb: more than 0 mass%, 0.2 mass% or less, and V: more than 0 mass%, containing one or more selected from the group consisting of 0.5 mass% or less, according to claim 1 or 2 Steel wire for machine structural parts. Furthermore,
Mg: more than 0% by mass, 0.02% by mass or less,
Ca: more than 0% by mass, 0.05% by mass or less,
Any one of claims 1 to 3, containing one or more selected from the group consisting of Li: more than 0% by mass and 0.02% by mass or less, and REM: more than 0% by mass and 0.05% by mass or less. The steel wire for machine structural parts according to item 1. The steel wire for machine structural parts according to any one of claims 1 to 4, wherein the average ferrite grain size is 30 µm or less. A long steel satisfying the chemical composition according to any one of claims 1 to 4,
The method for producing a steel wire for machine structural parts according to any one of claims 1 to 5, comprising a step of applying spheroidizing annealing including the following steps (1) to (3).
(1) After heating to a temperature T1 of (A1 + 8 ° C.) to (A1 + 31 ° C.), heating and holding at the temperature T1 for more than 1 hour and 6 hours or less,
(2) Cooling to a temperature T2 exceeding 650 ° C. and not exceeding (A1-17 ° C.), and then heating to a heating temperature higher than the temperature T2 and not exceeding (A1 + 60 ° C.), a total of 2 or more cooling-heating steps 6 times,
(3) Cool to a temperature below (A1-30°C) at an average cooling rate of 5°C/hr to 20°C/hr.
Here, A1 is calculated by the following formula (1).
A1 (° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni] (1)
However, [element] represents the content (% by mass) of each element, and the content of elements not included is zero. 7. The method of manufacturing a steel wire for machine structural parts according to claim 6, wherein the bar steel is a steel wire obtained by drawing a wire with a reduction of area of more than 5%.

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