JP7383437B2 - Simplified spheroidizing annealing method for case hardened steel - Google Patents

Description

The present invention relates to a method for spheroidizing case hardening steel.

Examples of spheroidizing annealing methods include heating, pre-transformation cooling, controlled cooling, and post-transformation cooling to efficiently perform spheroidizing annealing when bearing steel needs to be made softer than usual. A method for heat treatment of bearing steel is disclosed, which comprises controlling the ratio of the high temperature holding time in the high temperature holding step and the controlled cooling time in the controlled cooling step when performing spheroidizing annealing consisting of each step. (See Patent Document 1.)

In addition, the spheroidizing annealing of steel materials for mechanical structures containing 0.15 to 1.10 mass% C: (1) (temperature at which austenite appears -50°C) during heating to (austenite appears (2) maximum heating temperature (temperature at which austenite becomes a single phase -30 °C) to (temperature at which austenite becomes a single phase) at a rate of 0.01 °C/s or less After heating to a temperature range of -5℃), immediately turn to cooling, and (3) during cooling, reduce the temperature range from (temperature at which ferrite appears +10℃) to (temperature at which ferrite appears -40℃) to 0. A method for short-time spheroidizing annealing of a steel material is disclosed, which is characterized by cooling at a rate of 0.005° C./s or less and then air cooling (see Patent Document 2). The aim is to shorten the high temperature holding time by controlling the temperature increase rate.

C: For case hardening steel of 0.3% or less, the structure before spheroidizing annealing has an area ratio of (ferrite + pearlite) of 75% or more, and the average grain size of ferrite is 40 μm or less, and the average grain of pearlite A case hardening steel for cold forging whose diameter is controlled to be 30 μm or less has been disclosed (see Patent Document 3). By controlling the structure before spheroidizing annealing, the carbide distribution after spheroidizing annealing is made uniform and cold forgeability is ensured.

However, in these patent documents, since the holding temperature during spheroidizing annealing reaches the two-phase region of austenite + ferrite, slow cooling must be performed for a long time in the subsequent cooling process.

Japanese Patent Application Publication No. 2009-242917 Japanese Patent Application Publication No. 2001-131631 Japanese Patent Application Publication No. 11-12684

By the way, for case-hardened steel, which is a low carbon steel, cold forging is sometimes selected from the viewpoint of yield and processing efficiency when molding parts. During cold forging, spheroidizing annealing, which generally takes about 20 hours in total, is performed to reduce deformation resistance.

That is, in the conventional spheroidizing annealing of case hardened steel, the following steps are taken.
(1) A: Raise the temperature to _one point or more to transform the previous structure of pearlite or bainite into austenite.
(2) A: Hold at _one point or more to create a two-phase structure of ferrite + austenite. At this time, carbide nuclei remain within the austenite grains.
(3) Thereafter, by slow cooling from just above the _A1 point to just below the _A1 point, spheroidal carbides are precipitated using the carbide nuclei within the austenite grains and the austenite/ferrite grain boundaries as precipitation sites.

At this time, if the slow cooling rate beyond the _A1 point is high, the formation of hard pearlite cannot be suppressed. In order to prevent this, the slow cooling rate is controlled at about 10° C./h, which takes a long time, which causes the total time to increase.

Furthermore, due to the step (2) of maintaining the spheroidized annealed structure in the two-phase region of ferrite + austenite, no spheroidal carbide can precipitate within the ferrite grains in the two-phase region. This inevitably causes non-uniform distribution of spherical carbides after slow cooling. Such non-uniform carbide distribution causes cracking during cold forging and coarsening of crystal grains in the subsequent carburizing process, so it is desirable to make the carbide distribution uniform.

To address these issues, the above-mentioned Patent Document 1 attempts to shorten the total processing time by devising the time distribution of high temperature holding time and controlled cooling time regarding the spheroidizing annealing conditions for bearing steel. . Further, Patent Document 2 attempts to shorten the high temperature holding time by controlling the temperature increase rate. Further, Patent Document 3 attempts to make the carbide distribution uniform after spheroidizing annealing by controlling the previous structure. However, the methods disclosed in these documents have the problem that the slow cooling step, which takes up most of the spheroidizing annealing process time, cannot be omitted, and that the non-uniformity of carbide distribution cannot be completely resolved.

Therefore, the problem to be solved by the invention of the present application is to provide a spheroidizing annealing method that can shorten the spheroidizing annealing time by omitting the slow cooling step and can obtain a structure in which carbides are uniformly dispersed. That's true.

These conventional techniques have a problem in that the slow cooling step, which occupies most of the time for the spheroidizing annealing process, cannot be omitted. Therefore, as a result of intensive studies, the inventors of the present application found that it is possible to shorten the spheroidizing annealing time by omitting the slow cooling process by adjusting the components of the sample material, and We have devised a method for spheroidizing case-hardened steel, which is characterized by the ability to obtain a structure in which carbon atoms are uniformly dispersed.

In other words, in order to realize spheroidizing annealing in which carbides are uniformly dispersed while shortening the time, by adjusting the composition of the sample material, (a) the temperature at point _A was made higher than that of normal case hardening steel, and ( b) Nano-order fine precipitates (Al nitride, Nb carbonitride, Fe carbide, Cr carbide, etc.) were dispersed in the ferrite of the structure before spheroidizing annealing.

Note that the temperature at point _A1 can be calculated as in [Formula 1].
[Formula 1]: A ₁ = 723°C - 14Mn [%] + 22Si [%] - 14.4Ni [%] + 23.3Cr [%]

Therefore, the first means for solving the problem of the present application is as follows: C: 0.15-0.26%, Si: 0.05-1.00%, Mn: 0.1-0. 9%, P: 0.030% or less, S: 0.030% or less, Cr: 1.30 to 2.50%, Al: 0.020 to 0.050%, N: 0.0040 to 0.0300 %, balance Fe and unavoidable impurities, A steel material with a temperature of 750 ° C or _higher ,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: 1 h or more,
This is a method of annealing steel to spheroidize it so that it satisfies the following conditions:

The second means includes, in addition to the ingredients described in the first means,
Contains at least one of Ni: 0.02 to 2.00% and Mo: 0.05 to 2.00%,
A steel material with a temperature of 750°C or higher _{at one} point, consisting of the balance Fe and unavoidable impurities,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: 1 h or more,
This is a method of annealing steel to spheroidize it so that it satisfies the following conditions:

The third means includes, in addition to the components described in the first means or the second means,
At least one of Nb: 0.02 to 0.10%, Ti: 0.020 to 0.200%, B: 0.0010 to 0.0050%, and V: 0.010 to 0.500%. Contains more than
A steel material with a temperature of 750°C or higher _{at one} point, consisting of the balance Fe and unavoidable impurities,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: 1 h or more,
This is a method of annealing steel to spheroidize it so that it satisfies the following conditions:

The steel made of the chemical composition of the present invention has a mixed phase structure of ferrite, pearlite, and bainite before spheroidizing annealing, and in the spheroidizing annealing treatment, the holding temperature T is 1 hour or more at A ₁ point or less. By holding, (a) spherical carbides are directly precipitated within the ferrite grains using fine precipitates as nuclei, and (b) spheroidization of the carbides that constitute pearlite and bainite is promoted. As a result, after cooling, a structure in which spherical carbides are uniformly dispersed within the ferrite grains is obtained, and the steel material has a hardness of 84 HRB or less.

In addition, when spheroidizing annealing is performed by heat treatment that maintains the _A1 point or lower, austenite is not involved in the formation of spheroidal carbides, so there is no need for a normally long slow cooling process. Therefore, the total time required for the spheroidizing annealing treatment can be significantly shortened.

In the present invention, the holding time itself can be shortened to nearly 1 hour by setting the temperature conditions for the spheroidizing annealing to be slightly higher. Therefore, in addition to shortening the slow cooling process, the holding time itself can be shortened, so that the total time required is further shortened.

Further, the steel material spheroidized and annealed by the means of the present invention has a structure in which the area ratio of ferrite grains in which no spheroidal carbide is precipitated is 3% or less, the area ratio of pearlite or bainite is 10% or less, and the area ratio of marten The occurrence of this site is not permitted. In addition, the material hardness is steel material with a hardness of 84HRB or less.

It is a schematic diagram for demonstrating the holding temperature T (degreeC) and holding time t (h) of spheroidizing annealing. It is a structure observation photograph by an optical microscope of an example of measurement of the area ratio of ferrite in which no spheroidized carbide is precipitated. The photo (left) shows an example of the present invention in which the area ratio of ferrite is 0%, and the photo (right) shows an area ratio of ferrite of 78.1%. It is a structure observation photograph by an optical microscope of an evaluation example of the area ratio of pearlite and bainite. The photo (left) is an example of the present invention, where the area ratio of pearlite and bainite is 0%, and the photo (right) is a comparative example, where the area ratio is 21.1%. This is a microstructure observation photograph taken using an optical microscope to observe the formation of martensite.

Prior to describing the mode for carrying out the invention of the present application, the reason for limiting the chemical composition of the invention steel to which the method of the present application is applied and the reason for limiting the characteristics when the steel material of the steel is carburized will be explained. In addition, % in a chemical component is mass %.

C: 0.15-0.26%
C is an element that increases the hardness of the material. However, if C is less than 0.15%, the core hardness after carburization decreases, resulting in insufficient strength. On the other hand, if C exceeds 0.26%, the material hardness increases and the workability decreases, resulting in poor machinability and cold workability. Therefore, C is set at 0.15 to 0.26%.

Si: 0.05-1.00%
Si is a deoxidizing agent and an element that increases the hardness of the material. However, if Si is less than 0.05%, the deoxidizing agent is insufficient and deoxidizing is not sufficient. On the other hand, if Si exceeds 1.00%, the material hardness will increase, resulting in a decrease in workability, and carburization will also be inhibited. Therefore, Si is set at 0.05 to 1.00%.

Mn: 0.1-0.9%
Mn is an element that contributes to hardening. If Mn is less than 0.1%, hardenability will not be sufficient and hardening will be insufficient. When Mn exceeds 0.9%, workability decreases. Therefore, Mn is set to 0.1 to 0.9%.

P: 0.030% or less P is an unavoidable impurity, but if it exceeds 0.030%, the toughness will decrease due to grain boundary segregation. Therefore, P is set to 0.030% or less.

S: 0.030% or less S is an unavoidable impurity, but if it exceeds 0.030%, toughness decreases due to the formation of MnS, and fatigue strength also decreases. Therefore, S is set to 0.030% or less.

Cr: 1.30-2.50%
Cr is an element that contributes to raising the _A1 point and hardenability, but if it is less than 1.30%, the raising of the _A1 point will be insufficient. Furthermore, hardenability is also insufficient. However, when Cr exceeds 2.50%, the material hardness increases and workability decreases. Therefore, Cr is set at 1.30 to 2.50%.

Al: 0.020-0.050%
Al is a deoxidizing agent and is an element that generates fine nitrides and suppresses crystal grain coarsening. If Al is less than 0.020%, there will be a shortage of deoxidizing material. Further, since fine nitrides become insufficient and the crystal grains become coarse, toughness and fatigue properties deteriorate. On the other hand, if Al exceeds 0.050%, coarse nitrides are formed, resulting in poor fatigue properties and workability. Therefore, Al is set at 0.020 to 0.050%.

N:0.0040~0.0300%
When N exceeds 0.0300%, coarse carbonitrides are formed, which deteriorates fatigue properties and workability. However, if N is less than 0.0040%, fine carbonitrides will be insufficient, so crystal grains will tend to become coarser, which will reduce toughness and fatigue properties.
Therefore, N is set to 0.0040 to 0.0300%.

Next, components that can be optionally added to the chemical components of these steels will be explained.
Ni: 0.02-2.00%
Ni is an element that improves hardenability and toughness. When Ni is less than 0.02%, the effect of improving hardenability is small, and the effect of improving toughness is also small. On the other hand, if Ni exceeds 2.00%, the cost will increase and the material hardness will increase, resulting in a decrease in workability. Therefore, when Ni is added, it should be 0.02 to 2.00%.

Mo: 0.05-2.00%
Mo is an element that improves hardenability. When Mo is less than 0.05%, the effect of improving hardenability is small. On the other hand, if Mo exceeds 2.00%, the cost will increase and the material hardness will also increase, resulting in a decrease in workability. Therefore, when Mo is added, it is added in an amount of 0.05 to 2.00%.

Nb: 0.02-0.10%
Nb is an element that suppresses crystal grain coarsening by generating fine carbonitrides. If Nb is less than 0.02%, fine carbonitrides are insufficient and the effect of suppressing crystal grain coarsening is small, resulting in insufficient toughness and fatigue strength. When Nb exceeds 0.10%, the amount of carbonitrides becomes excessive and workability deteriorates. Therefore, when adding Nb, it should be 0.02 to 0.10%.

Ti: 0.020-0.200%
Ti is an element that suppresses crystal grain coarsening by producing fine carbonitrides. If Ti is less than 0.020%, the amount of fine carbonitrides will be insufficient, and N will not be fixed, forming BN, resulting in a decrease in hardenability and a decrease in the effect of suppressing crystal grain coarsening. If Ti exceeds 0.200%, the amount of carbonitrides becomes excessive, resulting in poor workability. Therefore, when adding Ti, it should be 0.020 to 0.200%.

B: 0.0010-0.0050%
B is an element that improves hardenability and increases material hardness. If B is less than 0.0010%, the effect of improving hardenability is small. On the other hand, when B exceeds 0.0050%, workability decreases due to increase in material hardness. Therefore, when B is added, it is added in an amount of 0.0010 to 0.0050%.

V:0.010~0.500%
V is an element that generates fine carbonitrides and suppresses crystal grain coarsening. When V is less than 0.010%, fine carbonitrides are insufficient and the effect of suppressing crystal grain coarsening is small, resulting in insufficient toughness and fatigue strength. On the other hand, when V exceeds 0.500%, the amount of carbonitrides becomes excessive and workability deteriorates. Therefore, when V is added, it is added in an amount of 0.010 to 0.500%.

A ₁ point temperature: 750℃ or more A If the ₁ point temperature is less than 750℃, the holding temperature for spheroidizing annealing will be low, and the carbides that make up pearlite and bainite will not be sufficiently spheroidized, so softening will be delayed. The effect is insufficient. Therefore, the temperature at point _A1 should be 750°C or higher. Preferably, the temperature at point _A1 is 750°C to 800°C.

Holding temperature T (℃) for spheroidizing annealing: ( ₁ point A - 10℃) ≦T ≦ ( ₁ point A)
Holding time t (h) for spheroidizing annealing: 1 h or more First, regarding the holding temperature T and holding time t in the heat treatment for spheroidizing annealing, as schematically shown in Figure 1, the holding temperature is It refers to the temperature T at which the steel material is held after being heated for annealing, and the holding time refers to the time t during which the steel material is held at the temperature T.

Next, the reason why the holding temperature T (℃) for spheroidizing annealing is set from (A ₁ point - 10 ℃) to (A ₁ point) is that if the holding temperature is lower than A ₁ point - 10 ℃, pearlite This is because the carbides constituting bainite are not sufficiently spheroidized and the softening effect is insufficient.
On the other hand, if the holding temperature T (°C) for spheroidizing annealing is higher than ( _A1 point), the material temperature will exceed _A1 point due to temperature variations in the annealing furnace, so austenite will occur and the subsequent Pearlite or martensite is generated during air cooling or water cooling, which increases hardness and tends to reduce workability.
Therefore, the holding temperature T (°C) for spheroidizing annealing is set in the range of ( _A1 point - 10°C)≦T≦( _A1 point).

The reason why the holding time t(h) for holding at the holding temperature T for spheroidizing annealing is set to 1 h or more is because if the holding time is short, carbides constituting pearlite and bainite will not be spheroidized. This is because the softening effect becomes insufficient.

When a steel with a _single point temperature of 750°C or higher is annealed by a predetermined method, a structure in which spherical carbides are uniformly dispersed in ferrite is obtained, and the hardness of the material increases. A steel material having a value of 84HRB or less can be obtained.

That is, the steel material spheroidized and annealed by the means of the present invention has a structure in which (1) the area ratio of ferrite grains in which no spheroidal carbide is precipitated is 3% or less, and (2) the area ratio of pearlite or bainite is 3% or less. 10% or less, (3) generation of martensite is not observed.
Moreover, the material hardness is a steel material of 84HRB or less.

Now, Table 1 shows the chemical composition values of the invention steel components and comparative steel components used in the test materials, and the calculated A ₁ point as ₁ point. With reference to this value, steels with an _A1 point temperature of 750°C or higher can be selected.
Note that the calculated _A1 point is obtained by adding the [%] value of each chemical component to the formula _A1 = 723°C - 14Mn [%] + 22Si [%] - 14.4Ni [%] + 23.3Cr [%]. It can be found by substitution.

Next, Table 2 shows the heat treatment conditions (holding temperature, holding time, cooling method) for spheroidizing annealing using each of the prepared test materials, and the structure after annealing (martensite Indicates the presence or absence of pearlite, area ratio of pearlite and bainite), and hardness of the material.

<Manufacturing process of sample material>
After melting 100 kg of steel consisting of the chemical components listed in Table 1 and the balance Fe and unavoidable impurities into a steel ingot in a vacuum melting furnace, it was forged at 1250°C to a diameter of 32 mm, and then heated at 925°C for 1 hour. I performed normalizing. These were cut into 100 mm pieces to prepare test materials for spheroidizing annealing.

<Spheroidizing annealing process>
The produced sample material was subjected to spheroidizing annealing using a Kanthal furnace according to the following procedure. The prepared sample material was placed in a furnace set at the holding temperature for spheroidizing annealing listed in Table 2, and after securing 30 minutes as the temperature rise time for the sample material, the temperature was set at the specified holding temperature shown in Table 2. kept in the oven for an hour. Thereafter, as shown in Table 2, air cooling or water cooling was performed to obtain a spheroidizing annealing treatment. It also shows the total in-furnace time.

<Evaluation items>
In order to evaluate the properties of each sample material subjected to spheroidizing annealing treatment, (1) Observation of the microstructure (a. Area ratio of ferrite grains without intragranular precipitation of spheroidal carbide, b. (2) material hardness were measured, and the results for each sample material are shown in Table 2, respectively. The area ratio and hardness of ferrite grains (α grains) that are not used are listed and shown.

<Evaluation method>
(1) Regarding the microstructure To observe the structure of the sample material, first cut the sample material parallel to the rolling direction through the center, polish the cut surface, and polish the polished surface with nital liquid. Corroded. Thereafter, the D/4 position of the cut surface was observed using an optical microscope and evaluated as follows.
a. Evaluation of the area ratio of ferrite grains in which spherical carbides are not precipitated within the grains Photographed at the D/4 position with a field of view of ×400, and the area ratio of ferrite grains in which spherical carbides are not precipitated in the target area of the image is measured. do.
b. Evaluation of area ratio of pearlite and bainite A photograph is taken at the D/4 position with a field of view of ×400, and the area ratio of pearlite and bainite in the target area of the image is measured.
c. Evaluation of presence/absence of martensite formation Photographing was performed at the D/4 position with a field of view of x400, and the presence/absence of martensite tissue generation in the target area of the image was observed.

(2) Material hardness The sample material was cut in the direction perpendicular to the rolling direction, and the cut surface was surface ground, then a Rockwell hardness test was performed at the D/4 position, and the obtained hardness was calculated as the material hardness. It is described and shown in Table 2 as follows.

Figures 2 to 4 show images taken at 400x magnification at the D/4 position.
FIG. 2 is a microstructure observation photograph taken with an optical microscope of an example of measuring the area ratio of ferrite in which no spheroidized carbide is precipitated. The photo (left) shows the cross-sectional structure of the invention example test material, which was made by annealing C steel, which is the composition of the invention steel, at a holding temperature of 760°C and a holding time of 6 hours to form a spheroid. This is a cross-sectional structure of a sample material of a comparative example in which steel D, which is a component, is air-cooled at a holding temperature of 720° C. and a holding time of 2 hours. The photo (left) shows an example of the present invention in which the area ratio of ferrite is 0%, and the photo (right) shows an area ratio of ferrite of 78.1%.

FIG. 3 is a microstructure observation photograph using an optical microscope of an evaluation example of the area ratio of pearlite and bainite. The photo (left) shows the cross-sectional structure of the invention example test material obtained by annealing C steel, which is a component of the invention steel, at a holding temperature of 760°C and a holding time of 6 hours to form a spheroid. This is a cross-sectional structure of a sample material of a comparative example in which component C steel was air-cooled at a holding temperature of 730°C and a holding time of 1 hour. The photo (left) is an example of the present invention, where the area ratio of pearlite and bainite is 0%, and the photo (right) is a comparative example, where the area ratio is 21.1%.

FIG. 4 is an example of martensite generation. The photograph shows a cross-sectional structure of a comparative sample material in which steel C, which is an inventive steel component, was annealed to spheroidize by water cooling at a holding temperature of 780° C. for a holding time of 6 hours, and the occurrence of a martensitic structure was observed.

Table 2 shows examples of inventions in which the invented steel components were spheroidized by a method that satisfies the spheroidizing annealing conditions of the present invention, and examples of inventive steel components that were spheroidized using a method that did not satisfy the spheroidizing annealing conditions of the present invention. A comparative example is shown in which the steel components were heat-treated in the above manner, and a comparative example in which the comparative steel components were subjected to a spheroidizing annealing treatment.

In a steel that satisfies the spheroidizing annealing conditions of the present invention while using the invention steel components, the area ratio of ferrite grains in which no spheroidal carbide is precipitated is 3% or less, the area ratio of pearlite or bainite is 10% or less, and the area ratio of marten No sites were observed, and the hardness of the steel material was 84 HRB or less after spheroidizing annealing. Since the furnace time was also within 2.5 hours, the desired steel properties were obtained in an extremely short time.

On the other hand, even when using A steel, B steel, and C steel of the invention steel composition, in the case of comparative examples with different heat treatment conditions, for example, in the comparative example with a high holding temperature, a martensitic structure may occur or , material hardness is too hard, material has a high area ratio of pearlite or bainite, material has a high area ratio of ferrite grains without precipitated spherical carbide, etc., which do not satisfy the characteristics of the examples of the present invention. It became a thing.
In addition, in comparative examples in which steels A and C, which are inventive steel components, were normalized at a low holding temperature, the area ratios of pearlite and bainite were high.
In addition, in the comparative example where the holding time was insufficient, the area ratio of pearlite and bainite was high.
In addition, in the case of comparative steel D steel, the area ratio of ferrite grains without precipitated spheroidal carbides is high, and the area ratio of pearlite and bainite is also high, regardless of the holding temperature and holding time. , the material hardness has also become higher.

Claims (3)

In mass%, C: 0.15 to 0.26%, Si: 0.05 to 1.00%, Mn: 0.1 to 0.9%, P: 0.030% or less, S: 0.030 % or less, Cr: 1.30-2.50%, Ni: 0.02-2.00%, Al: 0.020-0.050%, N: 0.0040-0.0300%, balance Fe and A steel material consisting of unavoidable impurities and having a temperature of 750°C or higher at _one point,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: Holding for 1 h or more,
A method for spheroidizing steel material , characterized by cooling without passing through an annealing process when annealing to satisfy the following conditions. In addition to the ingredients according to claim 1,
Contains Mo: 0.05 to 2.00% ,
A steel material with a temperature of 750°C or higher _{at one} point, consisting of the balance Fe and unavoidable impurities,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: Holding for 1 h or more,
A method for spheroidizing steel material , characterized by cooling without passing through an annealing process when annealing to satisfy the following conditions. In addition to the ingredients according to claim 1 or claim 2,
At least one of Nb: 0.02 to 0.10%, Ti: 0.020 to 0.200%, B: 0.0010 to 0.0050%, and V: 0.010 to 0.500%. Contains more than
A steel material with a temperature of 750°C or higher _{at one} point, consisting of the balance Fe and unavoidable impurities,
Holding temperature T for spheroidizing annealing (°C): (A ₁ point - 10) ≦T ≦ (A ₁ point),
Holding time t (h) for spheroidizing annealing: Holding for 1 h or more,
A method for spheroidizing steel material , characterized by cooling without passing through an annealing process when annealing to satisfy the following conditions.

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