JP2017214621A - Manufacturing method of hyper-eutectoid steel wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hyper-eutectoid steel wire which is efficiently softened and excellent in cold workability even when conducting spheroidizing with shortened time of a heating treatment.SOLUTION: There is provided a manufacturing method of a hyper-eutectoid steel wire having C content of 0.8 to 1.3 mass%,, including a rolling process to obtain a rolling wire and a heat treatment process for spheroidizing, the rolling process includes a process for finish rolling a steel material at a finish rolling temperature of 750 to 800°C, a process for cooling the same from the finish rolling temperature to 650°C at average cooling rate of les than 1°C/sec., the heat treatment process includes a process for heating the rolled wire from room temperature to a heating temperature Tof 800 to 880°C at average heating temperature of 10°C/sec. or more, a process for holding at the heating temperature Tfor 0 to 30 sec., a process for cooling from the heating temperature Tto a slow cooling initiation temperature T(=880-0.2×T) at average cooling rate of 10°C/sec. or more and a process for cooling the slow cooling initiation temperature Tto a slow cooling finish temperature T(=T-40) at average cooling rate of 0.03 to 1°C/sec.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a hypereutectoid steel wire. More specifically, the present invention relates to a method for producing a hypereutectoid steel wire used as a material for bearing element parts.

  For hypereutectoid steel wires (eg SUJ2) used as materials for balls, rollers, needles and shafts that make up bearings, post-processing such as cutting and cold forging, and cold forming such as cutting are easy. In addition, for the purpose of improving wear resistance, spheroidizing annealing is applied to the rolled wire obtained by hot rolling. Conventionally, spheroidizing annealing requires about 10 to 25 hours after hot rolling.

For this reason, shortening of the spheroidizing annealing time is required from the viewpoint of energy saving. Various methods have been proposed as a method for shortening the spheroidizing annealing time.
For example, Patent Document 1 discloses a method in which a wire rod after hot rolling is rewound and subjected to a spheroidizing annealing process using an induction heating apparatus capable of rapid heating. According to this method, since the spheroidizing annealing process can be performed in-line, the time can be greatly shortened as compared with the offline heat treatment using a conventional batch furnace or the like.

JP 2001-123221 A

  However, the steel wire manufacturing methods proposed so far, including the steel wire manufacturing method described in Patent Document 1, have greatly reduced the spheroidizing annealing time, but the resulting steel wire has a high hardness. Therefore, there is a problem that sufficient cold workability may not be obtained.

  The present invention has been made under such circumstances, and the purpose thereof is sufficiently softened even when a spheroidizing annealing process in which the heat treatment time is shortened than usual is performed, To provide a hypereutectoid steel wire manufacturing method excellent in cold workability.

Aspect 1 of the present invention is a method for producing a hypereutectoid steel wire having a C content of 0.8 to 1.3% by mass, in which a rolling step of rolling a steel material to obtain a rolled wire material, and the rolling A heat treatment step for spheroidizing the wire, and the rolling step includes a step of finish rolling the steel material at a finish rolling temperature of 750 to 800 ° C, and after the finish rolling, from the finish rolling temperature to 650 ° C, Cooling at an average cooling rate of less than 1 ° C./second, and the heat treatment step comprises heating the rolled wire rod from room temperature to a heating temperature T _{1 of} 800 to 880 ° C. at an average heating of 10 ° C./second or more. The step of heating at a temperature, the step of holding at the heating temperature T ₁ for 0 to 30 seconds, and the annealing temperature T ₂ represented by the following formula (1) from the heating temperature T ₁ to 10 ° C./second or more Step of cooling at an average cooling rate of _And a step of cooling at an average cooling rate of 0.03 to 1 ° C./second from the cooling end temperature T ₃ represented by the following formula (2) to a slow cooling end temperature T _3. is there.
T ₂ (° C.) = 880−0.2 × T ₁ (1)
T ₃ (° C.) = T ₂ −40 (2)

  Aspect 2 of the present invention is the hyper-eutectoid according to aspect 1, wherein after the rolling step and before the heat treatment step, the wire is subjected to wire drawing with an area reduction ratio of more than 0% and 30% or less. It is a manufacturing method of a steel wire.

  Aspect 3 of the present invention is the method for producing a hypereutectoid steel wire according to aspect 1 or 2, wherein, in the heating step, the rolled wire rod is heated by a high-frequency heating method or an electric heating method.

Embodiment 4 of the present invention, in the step of the holding, the rolled wire rod, at a heating temperature T ₁ of hold 0 seconds than 30 seconds or less, the production method of the over-eutectoid steel wire according to any of embodiments 1 to 3 It is.

  According to the present invention, a method for producing a hypereutectoid steel wire that is sufficiently softened and excellent in cold workability even when subjected to spheroidizing annealing that shortens the heat treatment time than usual. Can be provided.

The inventors of the present invention are sufficiently softened even when subjected to spheroidizing annealing (hereinafter sometimes referred to as “short-time spheroidizing annealing”) in which the heat treatment time is shortened than usual. In order to realize a method for producing hypereutectoid steel wires with excellent cold workability, studies were made from various angles.
As a result, in order to obtain a hypereutectoid steel wire that is sufficiently softened and excellent in cold workability even when subjected to spheroidizing annealing for a short time, the structure before spheroidizing annealing, that is, hot The idea that it is important to increase the amount of grain boundary cementite and reduce the amount of layered cementite in pearlite in the rolled structure of the rolled material was obtained.

The rolled structure of hypereutectoid steel is composed of proeutectoid cementite (hereinafter, sometimes referred to as “grain boundary cementite”) precipitated at grain boundaries and pearlite. The rolled material of the over-eutectoid steel, when the spheroidizing annealing process for heating and maintaining the temperature range between _one point and the A _cm point A, cementite in pearlite of the rolled material during austenite matrix and grain boundary cementite in Some cementite particles that are not dissolved in the solid solution remain. When this is slowly cooled, austenite does not become pearlite when passing through point A ₁ (which means Ar ₁ ), and the cementite of the decomposition product is formed with cementite particles remaining at a high temperature as nuclei. A structure in which spherical cementite is dispersed in the ferrite structure is obtained.
Fine spherical cementite dispersed in ferrite suppresses the movement of dislocations moving in the crystal grains (that is, strengthens the dispersion of particles). Therefore, the hardness of the hypereutectoid steel wire after the spheroidization treatment is dominated by particle dispersion strengthening due to fine spherical cementite precipitated in the ferrite crystal grains.

The inventors of the present invention do not obtain a sufficiently spheroidized structure for the hypereutectoid steel wire obtained by performing conventional short-time spheroidizing annealing because the spheroidizing annealing time is short. We focused on the fact that fine spherical cementite that was not spheroidized contributed to the strengthening of particle dispersion, and increased the hardness of the steel wire after spheroidizing treatment.
That is, in order to obtain a hypereutectoid steel wire that is sufficiently softened and excellent in cold workability even when subjected to spheroidizing annealing for a short time, the fine spherical shape in the ferrite structure that contributes to particle dispersion strengthening is obtained. We thought it important to reduce the amount of cementite itself.
As a result of further intensive studies, the inventors of the present invention performed finish rolling at a lower temperature than in the past during hot rolling before spheroidizing annealing, and appropriately controlled the slow cooling rate after finish rolling to reduce cooling. It has been found that the pro-eutectoid cementite that sometimes precipitates can be made coarser than before, so that the amount of grain boundary cementite is increased and a rolled structure with a small amount of cementite in pearlite is obtained. In the case of a wire having such a rolled structure, when spheroidizing annealing is performed, the amount of fine spherical cementite in ferrite that contributes to particle dispersion strengthening can be reduced. Even so, it has been found that a hypereutectoid steel wire that is sufficiently softened and excellent in cold workability can be obtained.
Below, the detail of the manufacturing method of embodiment which concerns on this invention is shown.

  In addition, in this specification, a "wire" is used for the meaning of a rolled wire, and points out the linear steel material cooled to room temperature after hot rolling. The “steel wire” refers to a linear steel material obtained by subjecting a rolled wire material to a tempering treatment such as spheroidizing annealing. Moreover, in this specification, the "temperature" of the steel material containing a wire and a steel wire means the surface temperature of the linear steel material which can be measured with a radiation thermometer etc., for example. If the wire diameter of the steel material is 25 mm or less, the internal temperature of the steel material after finish rolling is uniform, and the surface temperature of the steel material can be regarded as the internal temperature of the steel material.

  In the method for producing a hypereutectoid steel wire according to an embodiment of the present invention, a steel material having a C content of 0.8 to 1.3% by mass is finish-rolled at an appropriate finish rolling temperature, and is appropriate after finish rolling. It includes a rolling step for obtaining a rolled wire by cooling at a cooling rate, and a heat treatment step for spheroidizing and annealing the obtained rolled wire at an appropriate temperature, holding time and heating / cooling rate.

  In addition, the hypereutectoid steel wire which the manufacturing method which concerns on embodiment of this invention makes object requires that C content is 0.8-1.3 mass% as a composition. The composition of chemical components other than C does not need to be specially limited, and may include other components within a range normally included in a hypereutectoid steel wire used as a material for bearing element parts.

Specifically, the chemical composition other than C of the hypereutectoid steel wire according to the embodiment of the present invention is, for example, mass%, Si: 0.15 to 0.35%, Mn: 0.5% or less, P: 0.025% or less, S: 0.025% or less, Cr: 1.30-1.60% is contained, and the balance consists of Fe and inevitable impurities.
Below, preferable content about elements other than C is illustrated. In addition,% is the mass%.

Si: 0.15-0.35%
Si is an effective element for deoxidation of steel, and is also an extremely effective element for improving rolling fatigue life. If Si is less than 0.15%, the above effect cannot be exhibited, and if it exceeds 0.35%, the matrix hardness is increased and the machinability is deteriorated. The preferable content of Si is 0.17 to 0.33%, more preferably 0.20 to 0.30%.

Mn: 0.5% or less Mn is used as a deoxidation and desulfurization agent. Further, it is an element that greatly contributes to improvement of hardenability and is very effective in improving the rolling fatigue life. However, since it impairs machinability, it is set to 0.5% or less. The preferable content of Mn is 0.48% or less, and more preferably 0.45% or less.

P: 0.025% or less When P is contained in a large amount, inclusions are formed and the impact value and rolling fatigue life are deteriorated. The preferable content of P is 0.023% or less, and more preferably 0.020% or less.

S: 0.025% or less S improves the machinability, but generates inclusions and becomes brittle, so 0.025% or less. The preferable content of S is 0.023% or less, and more preferably 0.020% or less.

Cr: 1.30 to 1.60%
Cr is an element that combines with C to form fine carbides, imparts wear resistance, and contributes to improving hardenability. In order to exert such an effect, the Cr content is set to 1.30% or more. However, if Cr is present excessively, coarse carbides are generated, and the rolling fatigue life is decreased. Therefore, the Cr content is 1.60% or less. The preferable content of Cr is 1.32 to 1.58%, more preferably 1.35 to 1.55%.

  Details of each requirement defined by the manufacturing method according to the embodiment of the present invention will be described below.

1. Rolling step In the rolling step, a steel material having a C content of 0.8 to 1.3 mass% is hot-worked to obtain a rolled wire. The rolling process according to the manufacturing method according to the embodiment of the present invention,
(A) a step of finish rolling the steel material at a finish rolling temperature of 750 to 800 ° C;
(B) after finish rolling, cooling from the finish rolling temperature to 650 ° C. at an average cooling rate of less than 1 ° C./second;
including. By performing finish rolling and subsequent cooling under such conditions, it is possible to obtain a rolled wire having a coarse and large grain boundary cementite and a small amount of layered cementite in the pearlite structure. And by performing spheroidizing annealing on the rolled wire having such a structure under the conditions described later, the amount of spherical cementite in ferrite contributing to particle dispersion strengthening can be sufficiently reduced and sufficiently softened. A hypereutectoid steel wire excellent in cold workability can be obtained.
The finish rolling and subsequent cooling will be described in detail below.

(A) Finish rolling process:
The finish rolling temperature at the time of hot rolling is 750 to 800 ° C.
By appropriately controlling the finish rolling temperature, precipitation of pro-eutectoid cementite can be promoted, and a rolled wire rod having a structure in which the grain boundary cementite is coarse and large and the amount of layered cementite in the pearlite structure is small is obtained. be able to. In order to promote precipitation of proeutectoid cementite, it is effective to lower the finish rolling temperature.
When the finish rolling temperature exceeds 800 ° C., the amount of pro-eutectoid cementite deposited becomes insufficient, the layered cementite in the pearlite structure increases, and the hardness of the steel wire finally obtained increases. Therefore, the upper limit of the finish rolling temperature is 800 ° C. or lower, preferably 795 ° C. or lower, more preferably 790 ° C. or lower.
On the other hand, when the finish rolling temperature is lower than 750 ° C., the load on the rolling roll increases and the productivity deteriorates. Therefore, the lower limit of the finish rolling temperature is 750 ° C. or higher, preferably 755 ° C. or higher, more preferably 760 ° C. or higher.

  The finish rolling temperature can be confirmed, for example, by measuring the temperature of the steel material immediately after finish rolling, that is, immediately after leaving the rolling roll, using a radiation thermometer or the like.

(B) Step of cooling after finish rolling:
After finish rolling, the steel is cooled from a finish rolling temperature of 750-800 ° C. to at least 650 ° C. at an average cooling rate of less than 1 ° C./second.
By gradually cooling the steel material that has been finish-rolled under the above-mentioned conditions of the finish rolling temperature to Ar ₁ point, precipitation of pro-eutectoid cementite can be promoted and coarsened, and the amount of cementite in the pearlite of the rolled structure can be reduced. Less. As a result, by performing spheroidizing annealing described later, a sufficiently soft steel wire can be obtained with a sufficiently small amount of spherical cementite in ferrite.
When the average cooling rate from the finish rolling temperature to 650 ° C. is 1 ° C./second or more, the pro-eutectoid cementite is not sufficiently coarsened, the amount of cementite in the pearlite is increased, and finally obtained by spheroidizing annealing. The hardness of the steel wire is increased. Therefore, the upper limit of the average cooling rate is less than 1 ° C./second, preferably 0.8 ° C./second or less, more preferably 0.5 ° C./second or less.
When the average cooling rate from the finish rolling temperature to 650 ° C. is less than 0.01 ° C./second, it takes a long time for cooling, and the productivity deteriorates. Therefore, the lower limit of the average cooling rate is preferably 0.01 ° C./second or more. More preferably, it is 0.03 degreeC / second or more, More preferably, it is 0.05 degreeC / second or more.

  Such an average cooling rate can be adjusted, for example, by controlling the conditions of the Stemmore cooling. For example, the average cooling rate can be reduced by reducing the air volume, slowing the wind speed, slowing the conveyor speed, reducing the winding density, or providing a heat insulating cover.

  The average cooling rate may be obtained by measuring the temperature of the rolled material with a radiation thermometer or the like. As a method for calculating the average cooling rate, the temperature of the wire is measured at any place on the conveyor that controls the cooling rate, at least 5 points in the area excluding 10 turns at both ends of the wire (coil). taking measurement. The area to be measured is a so-called dense part where the wires overlap each other. Based on the conveyor speed and the distance from the first measurement location, the time required for the coil to move from the first measurement location to each other measurement location is calculated, and the relationship between the wire temperature and the required time It represents as a linear function using a multiplication method, and calculates an average cooling rate from the inclination.

  After cooling from the finish rolling temperature to 650 ° C. at the above-described average cooling rate, cooling to room temperature may be performed by ordinary cooling such as cooling. Moreover, you may continue cooling to the temperature lower than 650 degreeC (for example, 500 degreeC) with the cooling rate equivalent to the average cooling rate from the finishing rolling temperature mentioned above to 650 degreeC.

2. Heat treatment process The structure of the rolled wire obtained in the rolling process described above has a large amount of grain boundary cementite and a small amount of layered cementite in pearlite. Therefore, when such a rolled wire is heat-treated (spheroidizing annealing) under appropriate conditions, the amount of fine spherical cementite in ferrite that contributes to the increase in hardness of the steel wire is small, and as a result, it becomes sufficiently soft And a steel wire excellent in cold workability can be obtained. The heat treatment step according to the manufacturing method according to the embodiment of the present invention includes:
(A) heating the rolled wire rod from room temperature to a heating temperature T _{1 of} 800 to 880 ° C. at an average heating temperature of 10 ° C./second or more;
(B) a step of holding at a heating temperature T ₁ for 0 to 30 seconds;
(C) cooling from the heating temperature T ₁ to the slow cooling start temperature T ₂ at an average cooling rate of 10 ° C./second or more;
(D) from the slow cooling starting temperature _{T 2,} until the slow cooling end temperature _{T 3,} and a step of cooling at an average cooling rate of 0.03 to 1 ° C. / sec, the.
Each step of the heat treatment will be described in detail below.

(A) a rolled wire rod is heated from room temperature to the heating temperature T ₁ of step:
A rolled wire (hereinafter also referred to as “material to be treated”) is heated from room temperature to a heating temperature T _{1 of} 800 to 880 ° C.
When the heating temperature T ₁ is below 800 ° C., the formation of a solid solution in the degradation and austenite layered cementite in pearlite is insufficient, no longer spheroidized structure is obtained, finally obtained steel wire becomes hard, cold Interworkability is reduced. Therefore, the lower limit of the heating temperature _{T 1} of is 800 ° C. or higher. Preferably it is 810 degreeC or more, More preferably, it is 820 degreeC or more.
On the other hand, if the heating temperature T ₁ is greater than 880 ° C., layered cementite in pearlite solved in excessive degradation and austenite, cementite particles as the nuclei of spheroidized is excessively reduced. Thereby, since the spheroidized structure cannot be sufficiently obtained by the cooling process, the obtained steel wire becomes hard and cold workability is deteriorated. Therefore, the upper limit of the heating temperature _{T 1} of is at 880 ° C. or less, preferably 870 ° C., more preferably at most 860 ° C..

Further, in order to shorten the processing time of the spheroidizing annealing, it is effective to increase the heating rate from room temperature to the heating temperature T _1. The average heating rate from room temperature to the heating temperature T ₁ is is less than 10 ° C. / sec, spheroidizing treatment time becomes long. Therefore, the lower limit of the average heating rate is 10 ° C./second or more, preferably 20 ° C./second or more, more preferably 30 ° C./second or more.
Heating from room temperature to the heating temperature T ₁ of the high-frequency heating method may be performed by energization heating method, other heating means. By heating with such a heating means, the material to be processed can be rapidly heated, and the spheroidizing time can be shortened. In addition, by heating with such a heating means, the material to be treated can be heated uniformly, the decomposition of the pearlite structure inside the material to be treated can be further promoted, and spheroidized in the subsequent cooling step Can be promoted.

  Such an average heating rate may be obtained by measuring the temperature of the material to be processed using a radiation thermometer or the like. As a method for calculating the average cooling rate, the temperature of the wire is measured at any place on the conveyor that controls the cooling rate, at least 5 points in the area excluding 10 turns at both ends of the wire (coil). taking measurement. The area to be measured is a so-called dense part where the wires overlap each other. Based on the conveyor speed and the distance from the first measurement location, the time required for the coil to move from the first measurement location to each other measurement location is calculated, and the relationship between the wire temperature and the required time It represents as a linear function using a multiplication method, and calculates an average cooling rate from the inclination.

(B) a step of holding at the heating temperature _{T 1:}
The material to be treated, held 0-30 sec at a heating temperature _{T 1} of the from 800 to 880 ° C.. By maintaining at a heating temperature of 800 to 880 ° C., decomposition of layered cementite in pearlite can be promoted, and solid solution in austenite can be promoted.
Holding time at the heating temperatures T ₁ is preferably 0 seconds, more preferably above 5 seconds or more, more preferably 10 seconds or more.
If the holding time at the heating temperature T ₁ is greater than 30 seconds, layered cementite in pearlite ends up excessively dissolved, cementite particles as the nuclei of spheroidized is excessively reduced. Thereby, since the spheroidized structure cannot be sufficiently obtained by the cooling process, the obtained steel wire becomes hard and cold workability is deteriorated. Therefore, the holding time at the heating temperature T ₁ of not more than 30 seconds, preferably 25 seconds or less, more preferably 20 seconds or less.

In this process, “holding the material to be processed at a heating temperature T ₁ of 800 to 880 ° C. for 0 to 30 seconds” means “the time during which the material to be processed is in the temperature range of 800 to 880 ° C. “30 seconds”, not only holding the material to be processed at a specific temperature in the temperature range of 800 to 880 ° C., but also being able to receive a temperature increase and / or temperature decrease treatment in the temperature range. means.
Further, the term "800-880 to" 0 seconds hold "at the heating temperature _{T 1} of the ° C.", after reaching the heating temperature _{T 1} of the 800 to 880 ° C., the cooling without maintaining the heating temperatures _{T 1} Thus, even in the case of “holding for 0 second”, the decomposition of the layered cementite in the pearlite can be promoted, and the solid solution in the austenite can be promoted.

(C) a step of cooling from the heating temperature _{T 1} of up to slow cooling starting temperature _{T 2:}
The material to be treated is cooled at an average cooling rate of 10 ° C./second or more from the heating temperature T ₁ to the slow cooling start temperature T ₂ calculated by the following formula (1).

T ₂ (° C.) = 880−0.2 × T ₁ (1)

When the average cooling rate up to T ₂ exceeds 10 ° C./second, not only the time for spheronization treatment cannot be shortened, but also cementite particles, which are the core of spheronization, are excessively dissolved in austenite. A spheroidized structure cannot be obtained sufficiently by the process (slow cooling), the resulting steel wire becomes hard, and cold workability is lowered. Therefore, cooling is performed at an average cooling rate of 10 ° C./second or more from the heating temperature T ₁ to the slow cooling start temperature T ₂ . The average cooling rate from the heating temperature T ₁ of up to slow cooling starting temperature T ₂ is preferably 20 ° C. / sec or more, more preferably 30 ° C. / sec or more.

  Such an average cooling rate may be obtained by measuring the temperature of the material to be processed using a radiation thermometer or the like. As a method for calculating the average cooling rate, the temperature of the wire is measured at any place on the conveyor that controls the cooling rate, at least 5 points in the area excluding 10 turns at both ends of the wire (coil). taking measurement. The area to be measured is a so-called dense part where the wires overlap each other. Based on the conveyor speed and the distance from the first measurement location, the time required for the coil to move from the first measurement location to each other measurement location is calculated, and the relationship between the wire temperature and the required time It represents as a linear function using a multiplication method, and calculates an average cooling rate from the inclination.

Step (d) cooling from annealing starting temperature _{T 2} to the slow cooling end temperature _(T 2 -40) ℃:
The treated material is cooled at 1 ° C. / sec of average cooling rate from the annealing starting temperature T ₂ to the slow cooling end temperature (T 2 _-40) ℃. By cooling at such an average cooling rate, spheroidization can be promoted and spherical cementite in ferrite can be coarsened. Thereby, the amount of fine cementite that deteriorates (that is, increases) the hardness of the steel wire can be reduced, and the cold workability of the obtained steel wire can be enhanced.
If the average cooling rate from the annealing starting temperature T ₂ to the slow cooling end temperature (T 2 _-40) ℃ exceeds 1 ° C. / sec, spheroidization is not sufficiently promoted, sufficiently coarse spherical cementite in the ferrite Can not be converted. Therefore, the amount of fine cementite that contributes to the increase in hardness is increased, and the hardness of the steel wire is increased. Therefore, the upper limit of the average cooling rate is 1 ° C./second or less, preferably 0.7 ° C./second or less, more preferably 0.5 ° C./second or less.
On the other hand, if the average cooling rate from the annealing starting temperature _{T 2} to the slow cooling end temperature _(T 2 -40) ℃ is below 0.03 ° C. / sec, spheroidizing treatment time becomes long. Therefore, the lower limit of the average cooling rate is 0.03 ° C./second or more, preferably 0.05 ° C./second or more, more preferably 0.07 ° C./second or more.

  Such an average cooling rate may be obtained by measuring the temperature of the material to be processed using a radiation thermometer or the like. As a method for calculating the average cooling rate, the temperature of the wire is measured at any place on the conveyor that controls the cooling rate, at least 5 points in the area excluding 10 turns at both ends of the wire (coil). taking measurement. The area to be measured is a so-called dense part where the wires overlap each other. Based on the conveyor speed and the distance from the first measurement location, the time required for the coil to move from the first measurement location to each other measurement location is calculated, and the relationship between the wire temperature and the required time It represents as a linear function using a multiplication method, and calculates an average cooling rate from the inclination.

3. Wire Drawing Process The manufacturing method according to the embodiment of the present invention is the above-described 1. You may perform a wire drawing process at room temperature with respect to the rolling wire obtained by the rolling process. By performing the wire drawing process on the rolled wire rod, carbides in the steel are destroyed. Agglomeration of carbides can be promoted in the spheroidizing annealing step, that is, spheroidization of cementite can be promoted, and a softer steel wire can be obtained.

If the area reduction ratio of the wire drawing process exceeds 30%, the hardness after spheroidizing annealing becomes high, and the cold workability may be deteriorated. Therefore, the area reduction rate of wire drawing is preferably 30% or less, more preferably 25% or less, and still more preferably 20% or less.
The lower limit of the area reduction rate is not particularly limited, but if it exceeds 0%, the above-described effects can be obtained. The area reduction rate is preferably 5% or more, and more preferably 10% or more.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with modifications within a range that can be adapted to the above-described gist, which are all included in the technical scope of the present invention. Is done.

  From a hot forged steel material having the chemical composition shown in Table 1 below, a test piece for machining formaster of φ20 mm × 30 mm was obtained by machining. Using the obtained test specimen for processing for master, the processing for master test machine was first held at 1000 ° C. for 1200 seconds, and then a heat treatment test was performed under the hot processing conditions shown in Table 2. Each test piece was hot-compressed from 30 mm to 14 mm at the processing temperature described in Table 2, then cooled from the processing temperature to 600 ° C. at the cooling rate described in Table 2, and then released to room temperature. Chilled. The hot working conditions listed in Table 2 simulate the hot rolling conditions in an actual machine. In Table 2, the processing temperature corresponds to the finish rolling temperature, and the cooling rate corresponds to the cooling rate from the finish rolling temperature to 650 ° C.

  A processed formaster test piece having a diameter of 5 mm × 7.5 mm was obtained by machining from the 1/4 position of the diameter D after the heat treatment and the center position of the height of the processed formaster specimen subjected to the heat treatment. A heat treatment test was carried out under the spheroidizing annealing conditions shown in Table 2 using a processing for master tester using the obtained test for master test piece. Cooling 1 indicates cooling from the heating temperature to the slow cooling start temperature, and cooling 2 indicates cooling from the slow cooling start temperature to the slow cooling end temperature. Test No. In order to simulate the wire drawing process, 3, 8, 9, and 14 were subjected to cold compression immediately before the heat treatment test under the spheroidizing annealing condition in order to introduce strain corresponding to the area reduction ratio shown in Table 2. did. (For example, when the drawing area reduction is 30%, 30% compression deformation was applied in the compression direction: 7.5 mm → 5.25 mm)

  About the test piece after implementing the heat processing test, the cold compression test, and the heat processing test on the conditions of Table 2, the hardness after spheroidizing annealing was measured in the following way. In any measurement, the resin was embedded so that the longitudinal section of the test piece after the heat treatment was observed, and the measurement was performed within a range of 1/4 position of the diameter D after the heat treatment and the height center within ± 1.5 mm. Table 2 shows the measured hardness after spheroidizing annealing. The standard hardness after spheroidizing annealing was 250 HV or less.

・ Measurement of hardness after spheroidizing annealing Measurement of hardness after spheroidizing annealing is performed by measuring a mirror-polished longitudinal section sample at 5 points with a load of 500 gf at a D / 4 position using a Vickers hardness meter. The average value (HV) was determined.

  From the results in Table 2, it can be considered as follows. No. in Table 2 1 to 14 are examples satisfying all the production conditions defined in the present invention, that is, finish rolling conditions and heat treatment conditions in spheroidizing annealing, and the hardness is good even in a short spheroidizing annealing process. Yes, softening can be achieved.

  On the other hand, No. 15 to 20 are examples lacking any of the requirements defined in the present invention, and the hardness after spheroidizing annealing does not reach the standard.

  No. No. 15 had a long holding time at the heating temperature under the spheroidizing annealing condition, so that pearlite was excessively dissolved, a large amount of regenerated pearlite was precipitated, and the hardness after spheroidizing annealing remained high.

  No. In Nos. 16 and 19, since the cooling rate of cooling 2 (slow cooling) was high, the spherical cementite was not coarsened, and the hardness after spheroidizing annealing remained high.

  No. Since No. 17 had a high processing temperature, the amount of pro-eutectoid cementite was small, and the hardness after spheroidizing annealing remained high.

  No. No. 18 had a high cooling rate after hot working, so the amount of pro-eutectoid cementite was small and the hardness after spheroidizing annealing remained high.

  No. In No. 20, since the heating temperature of spheroidizing annealing was low, the decomposition and solid solution of pearlite was insufficient, the spheroidized structure was not obtained, and the hardness remained high.

Claims (4)

A method for producing a hypereutectoid steel wire having a C content of 0.8 to 1.3% by mass,
A rolling step of rolling a steel material to obtain a rolled wire, and a heat treatment step of spheroidizing and annealing the rolled wire,
The rolling step
Finishing rolling the steel material at a finish rolling temperature of 750 to 800 ° C .;
And after the finish rolling, cooling from the finish rolling temperature to a temperature of 650 ° C. or lower at an average cooling rate of less than 1 ° C./second,
The heat treatment step includes
Heating the rolled wire rod from room temperature to a heating temperature T _{1 of} 800 to 880 ° C. at an average heating temperature of 10 ° C./second or more;
A step of holding 0-30 seconds at the heating temperature _{T 1,}
Wherein the heating temperatures T ₁ to the slow cooling starting temperature T ₂ represented by the following formula (1), a step of cooling at an average cooling rate of more than 10 ° C. / sec,
Cooling from the slow cooling start temperature T ₂ to the slow cooling end temperature T ₃ represented by the following formula (2) at an average cooling rate of 0.03 to 1 ° C./second,
Method for producing hypereutectoid steel wire.
T ₂ (° C.) = 880−0.2 × T ₁ (1)
T ₃ (° C.) = T ₂ −40 (2)   The method for producing a hypereutectoid steel wire according to claim 1, wherein after the rolling step and before the heat treatment step, the rolled wire is subjected to wire drawing with a reduction in area of more than 0% and not more than 30%. .   The method for producing a hypereutectoid steel wire according to claim 1 or 2, wherein in the heating step, the rolled wire rod is heated by a high-frequency heating method or an electric heating method. In the step of the holding, the rolled wire rod is held at a heating temperature T ₁ 0 seconds than 30 seconds or less, the production method of the over-eutectoid steel wire according to claim 1.