JPH09316540A - Manufacture of steel for machine structural use for contour induction hardening, excellent in cold forgeability, and manufacture of cold forged part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacture of a steel for machine structural use, having excellent cold forgeability enabling the omission of process annealing required in the course of manufacture of parts, such as thin-walled flat gear parts, accompanied by great deformation at the time of parts manufacture and capable of providing desired surface hardness and hardening depth after contour induction hardening. SOLUTION: A steel, which has a composition consisting of, by weight ratio, 0.28-0.47% C, 0.06-0.25% Si, 0.10-0.50% Mn, <=0.020% P, <=0.010% S, <=0.15% Cu, 0.10-0.30% Cr, <=0.03% Mo, 0.005-0.03% Ti, <=0.0100% N, and the balance Fe with impurity elements and satisfying 1.20<=Si+2.5Mn+4Cr+6Mo, is subjected to spheroidizing annealing treatment consisting of temp. holding in a temp. region between (AC1 point +20 deg.C) and (AC1 point +50 deg.C) for 100-300min and cooling down to <=650 deg.C at (0.1 to 1.0) deg.C/min cooling rate, by which the rate of spheroidizing of carbides and the circle-equivalent average diameter of spheroidal carbides are regulated to >=95% and <=0.5μm, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine structure suitable for use in a component which is subjected to contour induction hardening after cold forging with large deformation in order to manufacture a thin-walled flat gear component or the like. The present invention relates to a method for manufacturing steel and a method for manufacturing cold forged parts.

[0002]

2. Description of the Related Art Since many mechanical parts such as gears and clutches used for automobile transmissions are required to have excellent toughness and wear resistance, low alloy steels such as SCr420 and SCM420 are heat-treated. The one that has been forged for a while, carburized, quenched and tempered after cutting has been widely used.

However, in recent years, in order to reduce the cost, in the above gear parts, the hot forging and cutting processes are replaced with the cold forging to improve the productivity and the material yield, and the carburizing, quenching and tempering. A manufacturing method for improving productivity by replacing the above with induction hardening that can be processed online is being studied. In order to enable conversion to this new manufacturing process, the steel material used must have excellent cold forgeability and induction hardenability, but it is technically difficult to satisfy both requirements. For example, if a low carbon steel material is used to improve cold forgeability, sufficient induction hardening hardness cannot be obtained, and addition of an alloy to obtain the required induction hardenability requires cold forgeability. It deteriorates.

Recently, in order to solve the above problems, new steel grades have been actively developed, and new materials have been proposed. For example, JP-A-61-113744, JP-B-1-38847,
JP-A-2-129341, JP-A-2-145744, JP-A-5-59486
The invention described in Japanese Patent Application Laid-Open Publication No. H10-209,837 is disclosed. The steels described in these publications reduce Si, Mn and S, P, N and O, which are contained as impurities, which cause the increase in deformation resistance as much as possible to reduce the deformation resistance after annealing and improve the deformability. By improving the cold forgeability, Cr, B, Mo, etc. were added to secure the necessary hardenability.

However, when a thin flat gear part or the like is to be formed by cold forging, cold forging accompanied by large deformation is required, which requires not only reduction of deformation resistance but also excellent deformability of the steel material. To be done. In this case, in the invention steels described in the above publication, especially the deformability is often less than the requirement, intermediate annealing is required between cold forging steps, and a sufficient cost reduction effect compared with the case of using conventional steel is obtained. There was a problem that I could not get it.

[0006]

SUMMARY OF THE INVENTION The present invention eliminates the intermediate annealing that was performed in the intermediate step of cold forging for parts such as thin flat gear parts that require high deformability in the steel material used. And a method for producing a mechanical structural steel that has excellent cold forgeability that can be formed without heat treatment to the final shape, and that can secure the target surface hardness and hardening depth after contour induction hardening and An object is to provide a method for manufacturing a cold forged part.

[0007]

Means for Solving the Problems In view of the above-mentioned objects, the present inventor is excellent in cold forgeability, particularly in deformability, and has desired surface hardness and hardening depth after contour induction hardening. As a result of intensive studies to obtain the obtained steel for machine structural use, the present invention has been completed by obtaining the following knowledge.

First of all, it was found that when Ti is added, spherical carbides are formed with Ti carbonitrides finely precipitated in the steel as nuclei, so that the spherical carbides become fine and exhibit excellent deformability. It was Conventionally, for improving deformability,
It is known that it is useful to make spheroidal carbide fine, and for that purpose, alloying elements such as Mo and Cr are added, the structure before spheroidization is refined, A _C1 point or less or A _C1
The spheroidizing annealing treatment etc. was performed right above the point. However, even if the spherical carbide is refined by the above method, the deformability can be enhanced by the effect, but on the other hand, the increase in hardness is avoided due to the decrease in the spheroidization rate, the solid solution strengthening of the ferrite matrix phase, and the like. As a result, the deformation resistance becomes high, and excellent cold forgeability cannot be obtained. However, higher than the A _C1 point temperature (A _C1 point in case of adding Ti + 20 ° C. ~
Even after spheroidizing annealing at 50 ° C, a spheroidizing rate of 95% or more (in the present invention, spheroidizing means an aspect ratio of carbide (major axis /
It is assumed that the minor axis) is 2 or less. ) And the spherical equivalent average grain size of the spherical carbides is 0.5 μm or less, a fine structure can be obtained and the deformability can be improved by refining the spherical carbides without increasing the hardness as in the above method. Therefore, it has been found that excellent cold forgeability can be obtained.

Second, in order to secure the contour induction hardenability required for gear parts and the like, among the alloy elements in steel, the amount of Si, Mn, Cr, and Mo, which has a great influence on the hardenability, should be 1.20≤Si. It was found that it should be contained so as to satisfy the formula of + 2.5Mn + 4Cr + 6Mo. In other words, for normal induction hardening with an effective hardened layer depth of 1 mm or more, it is necessary to add some Cr, B, Mo, etc. to ensure hardenability, but for gear parts, etc. In the case of so-called contour induction hardening, which hardens only the surface layer, the amount of alloying elements in the steel is 1.20 ≦ Si + 2.5Mn + 4Cr
It was found that + 6Mo is sufficient. By suppressing the addition amount of the alloy as low as possible within the range satisfying this formula, the hardness after spheroidizing annealing can be suppressed low, and excellent cold forgeability can be obtained.

The first invention of the present invention obtained by taking the measures described above is C: 0.28 to 0.47% in weight ratio,
Si: 0.06 to 0.25%, Mn: 0.10 to 0.50%, P: 0.020% or less,
S: 0.010% or less, Cu: 0.15% or less, Cr: 0.10 to 0.30%, Mo:
Steel containing 0.03% or less, Ti: 0.005 to 0.030%, N: 0.0100% or less, 1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo, and the balance Fe and impurity elements, the A _C1 point + 20 ° C. A _{C 1} point +50
After heating to a temperature of ℃ for 100 min to 300 min and then 0.1 to
The spheroidizing rate of the carbide is 95% or more by performing the spheroidizing annealing process, which consists of cooling to 650 ° C or less at a cooling rate of 1.0 ℃ / min, and the average equivalent circle diameter of the spherical carbide is 0.5 μm or less. The method for producing a machine structural steel for contour induction hardening, which is excellent in cold forgeability, characterized by
The invention is the same steel as that of the first invention, but after hot rolling at 100 ° C / min
Rapidly cool to 300 ° C or less at the above cooling rate, or after hot rolling to room temperature, reheat to a temperature of AC ₃ points or more and then rapidly cool to 300 ° C or less at a cooling rate of 100 ° C / min or more. Then, the same spheroidizing annealing treatment as in the first invention is performed, whereby the spheroidizing rate of the carbide is 95% or more, and the circular equivalent average diameter of the spherical carbide is 0.4 μm or less. It is a method for producing a machine structural steel for contour induction hardening, which has excellent properties.

A third aspect of the present invention is a method for producing a cold forged part by using the steel material produced by the method of the first aspect or the second aspect of the present invention, in which the final shape of the part is not subjected to intermediate annealing. It is a method for manufacturing a cold forged part, which is characterized by the above. Next, the reason for limiting the range of the addition amount of each element of the steel used in the method for manufacturing the machine structural steel for contour induction hardening excellent in cold forging property according to the present invention will be described below.

C: 0.28 to 0.47% C is an element necessary to secure necessary strength and induction hardenability, and it is necessary to contain 0.28% or more. But,
If the content exceeds 0.47%, the hardness becomes too high and the amount of carbide increases, making it difficult to obtain excellent deformability by cold forging even if the above-mentioned measures for improving workability are taken. Therefore, the upper limit was set to 0.47%. Considering the points of strength and hardenability more strictly, the lower limit is preferably 0.33%, and considering the point of cold forgeability, the upper limit is 0.42%.
It is preferably set to%.

Si: 0.06 to 0.25% As the content of Si decreases, the cold forgeability improves. However, since Si is an element necessary for deoxidation, if it is extremely decreased, deoxidation treatment becomes difficult and the steelmaking cost Will increase. Therefore, it is necessary to contain at least 0.06% or more, more preferably 0.10% or more. Deoxidation treatment is sufficiently possible with the inclusion of 0.10 to 0.20% of Si.If the amount is further increased, the hardness after spheroidizing annealing increases, and the spheroidizing rate decreases and cold forgeability deteriorates. Therefore, the upper limit was set to 0.25%. The upper limit is preferably 0.20% in order to secure better cold forgeability.

Mn: 0.10 to 0.50% Mn improves the cold forging property when it is reduced like Si, but if it is too low, it becomes difficult to secure the necessary hardenability, and an expensive element other than Mn is added. Then, it becomes necessary to secure the necessary hardenability. Therefore, in the present invention, at least 0.10%,
It is preferable to contain 0.20% or more. But,
Even if added in a large amount, only the hardenability is improved more than necessary, and the hardness after spheroidizing annealing increases and it becomes difficult to obtain excellent cold forgeability, so the upper limit was made 0.50%. . More preferably, the upper limit is 0.40%.

P: 0.020% or less P is an unavoidable impurity in manufacturing, but even a small amount of P increases the ferrite hardness, increases the spheroidizing annealing hardness, and adversely affects the cold forgeability. It is an element that exerts. Therefore, it is preferable to reduce as much as possible if only the cold forgeability is taken into consideration. However, since an extreme reduction causes an increase in steelmaking cost, the upper limit was set to 0.020% in consideration of process capability. Preferably 0.01
It should be 5% or less.

S: 0.010% or less Since S is an element that adversely affects cold forgeability, it is preferable to reduce it as much as possible. However, reduction of S lowers machinability, so in the present invention, it was determined that it can be added in the range of 0.010% or less. If a part to be manufactured without cutting or a part with a relatively small amount of cutting is manufactured, it is more preferable to reduce it as much as possible.

Cu: 0.15% or less Cu, like P, is an element that increases the ferrite hardness due to the inclusion of a trace amount and adversely affects the cold forgeability.
It should be 15% or less, preferably 0.10% or less. Especially in the case of melting in an electric furnace, if it is contained in scrap, it is difficult to remove it by the subsequent refining process, so the scrap to be used is appropriately selected and kept within the prescribed amount range. It is necessary.

Cr: 0.10 to 0.30% Cr is an element that promotes the refinement of spherical carbides, improves the deformability, and is effective in improving the hardenability.
The content must be 0.10% or more, preferably 0.15% or more. However, addition of a large amount causes an increase in deformation resistance, and the solid solution of spherical carbides may become insufficient during heating during contour induction hardening, which may rather lower the quenching hardness, so the upper limit was made 0.30%. . Preferably, the upper limit is 0.25%.

Mo: 0.03% or less Mo is an element having an effect of improving hardenability like Mn and Cr. However, when added, the hardness after spheroidizing annealing becomes high, which causes an increase in deformation resistance. In the present invention, no positive addition is made, and the upper limit of the amount of impurities is 0.03%.
Therefore, in electric furnace melting, it is necessary to use scrap containing a small amount of Mo so that the content does not exceed 0.03%.

Ti: 0.005 to 0.030% Ti is present in the steel in the form of finely precipitated Ti carbonitride and serves as a nucleus of the spherical carbide during spheroidizing annealing. Therefore, the spherical carbide is refined to improve the deformability. Has the effect of improving. Further, Ti also has the effect of reducing the solid solution N 2 in the steel to reduce the hardness after spheroidizing annealing and suppressing work hardening to a small extent. To obtain the above effects, at least 0.005% or more,
It is necessary to add 0.007% or more. However, excessive addition causes an increase in cost, increases the amount of Ti carbonitride and coarsens it, and conversely decreases the deformability, so the upper limit was made 0.030%. Preferably, the upper limit is 0.020%.

N: 0.0100% or less If N is present as solid solution N, the hardness after spheroidizing annealing increases and the work hardening also increases, so it is preferable that the N content is less, but in order to reduce N content, it is special. The treatment increases the steelmaking cost and is not preferable. Therefore, in the present invention, Ti nitride is formed by adding Ti to reduce the solid solution N as much as possible. However, if a large amount of N is contained, the solid solution N is
The amount of Ti required for the reduction of the amount increases, which causes an increase in cost. Further, as described above, when the amount of Ti is increased, not only the Ti carbonitride increases, but also it coarsens, and as a result, the deformability decreases. Therefore, the upper limit of N content was set to 0.0100% so that this problem would not occur. Preferably, the upper limit is 0.0080%. It should be noted that if the upper limit is made extremely strict, a special treatment is required and the steelmaking cost increases, but if the regulation is about the upper limit value described above, it is possible to manufacture without causing a large increase in cost.

1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo Contour induction hardening with an effective hardened layer depth of 1 mm or less Inferior hardenability compared with normal induction hardening with an effective hardened layer depth of 1 mm or more However, surface hardening is possible. In the present invention, Si, Mn, Cr, which can secure the required hardenability, a relational expression of the amount of Mo is obtained, the addition amount of each element is suppressed within the range that satisfies this equation, and excellent cold forgeability is secured. ing. Therefore, it is necessary to satisfy 1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo in order to secure the necessary hardenability.

Next, the reason for limiting the heat treatment conditions in the present invention will be described. Generally globular carbides spheroidizing annealing temperature for the finely is directly above Ac ₁ point, i.e. Ac ₁ point to Ac ₁ point + 10 ° C. of about itself or Ac ₁ point directly below is good, in the steel of the present invention As a result of the addition of Ti, fine spherical carbides can be obtained even at relatively high temperatures. Further, if the spheroidizing annealing temperature is increased, the hardness can be lowered and the deformation resistance can be lowered. Therefore, the lower limit of the spheroidizing annealing temperature is set to Ac ₁ point + 20 ° C. However, if the spheroidizing annealing temperature is too high, it becomes difficult to obtain fine spherical carbides, and the deformability decreases, so the upper limit of the spheroidizing annealing temperature was set to Ac ₁ point + 50 ° C.

In the steel of the present invention, the heating time for spheroidizing annealing and the cooling rate after heating are the target spheroidizing rate of 95% or more, and the spherical equivalent average diameter of the spherical carbides is 0.5 μm or less. Heating time to get 100 ~ 30
The cooling rate up to 0 min and 650 ° C should be 0.1 to 1.0 ° C / min.

The heating time is set to 100 to 300 min because if it is less than 100 min, austenitization and solid solution of carbide become incomplete and carbide spheroidization becomes insufficient. If it exceeds 300 min, decarburization is performed. This is because the surface skin is deteriorated and the productivity is lowered.

Further, the cooling rate is set to 0.1 to 1.0 ° C./min. If the cooling rate is less than 0.1 ° C./min, decarburization causes a problem that the surface skin is deteriorated and productivity is lowered.
This is because if it exceeds 0 ° C./min, regenerated pearlite appears and the spheroidization of carbide becomes insufficient.

In addition, in the second invention, after hot rolling, 100
Rapidly cool down to 300 ° C or less at a cooling rate of ℃ / min or more, or let it cool to room temperature after hot rolling and then reheat it to a temperature of AC ₃ points or more and then cool to 100 ° C / min or more to 300 ° C or less The process of quenching is to make the spherical equivalent average diameter of spherical carbide 0.4
It is a process for making finer than 1 μm.
It is possible to obtain a more excellent deformability as compared with the invention.

The cooling rate of 100 ° C./min or more is that the cooling rate of less than 100 ° C./min does not result in a structure such as bainite, martensite or fine pearlite.
This is because there is a problem that the carbide after spheroidization does not become fine. The reason why we decided to quench below 300 ° C is
This is because if the material is rapidly cooled to 300 ° C, the subsequent cooling rate does not affect the tissue.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION The features of the manufacturing method of the present invention will be described below in comparison with comparative examples and conventional examples, and will be clarified by examples. Table 1 shows the chemical components of the test materials used in the examples.

[0030]

[Table 1]

Steels having the components shown in Table 1 were melted in an electric furnace and hot-rolled to produce a round bar having a diameter of 50 mm.
The test material was used. Among the steels shown in Table 1, steels 1 to 5 are steels within the composition range of the production method of the present invention (hereinafter referred to as the present invention steel). Steels 6 to 10 are comparative steels in which some elements do not satisfy the conditions of the present invention, and steels 11 and 12 are conventional steels S35C and S40C.

A round bar having a diameter of 50 mm and having the components shown in Table 1 was used for any of the test materials, from A _c1 + 20 ° C. to A _c1 + 40 ° C.
After heating for 240 min at 755 ° C, which is the temperature range of 650 ° C, and then spheroidizing annealing that gradually cools down to 650 ° C at a cooling rate of 0.3 ° C / min is used as the test material. The test was conducted.

The cold forgeability was evaluated by the hardness after spheroidizing annealing, the spheroidizing rate, the circle-equivalent average diameter of the spheroidal carbide, the deformation resistance obtained by the compression test, and the cracking limit upsetting rate. For the hardness after spheroidizing annealing, the cross section of a round bar having a diameter of 50 mm was measured at 10 points with a Vickers hardness meter, and the average value was used as the measured value. The spheroidization rate and the circle-equivalent average diameter of the spherical carbides were measured using scanning electron micrographs with magnifications of × 1000 and × 10000, respectively. In the compression test, a test piece with a diameter of 8 mm and a height of 12 mm was prepared from the above-mentioned test material, and the deformation resistance at an upsetting rate of 60% was based on the end face restraint compression test method of the Japan Plastic Working Society Cold Forging Subcommittee standard. And the cracking limit upsetting rate.

To evaluate the induction hardenability, a round bar with a diameter of 40 mm cut out from the test material was drawn to a diameter of 29 mm, and then an induction hardening machine with a maximum output of 250 kW was used.
The frequency was set to 400 kHz so that the depth of 50 to 0.60 mm was sufficiently heated, and the surface heating temperature was controlled to 1000 ° C. After heating, stationary quenching was performed, and the cross-sectional hardness distribution was measured using a Vickers hardness tester. The effective hardened layer depth (depth at which the hardness is Hv 450 or more) of each test material was obtained. Table 2 shows the performance evaluation results of each test material.

[0035]

[Table 2]

As is clear from Table 2, when the evaluation results of the comparative steels and the conventional steels 6 to 12 are compared with the evaluation results of the steels of the present invention, since 6 steel has a high Si content, the hardness after spheroidizing annealing is high. The spheroidization rate also decreased with the increase in crack resistance, the deformation resistance increased, and the crack upset limit upsetting rate decreased.
Due to the high Cr content, the deformation resistance increased, and the solid solution of carbide during heating during induction hardening became insufficient, resulting in a decrease in the effective hardened layer depth. Since 8 steel has a high Mo content, The hardness after spheroidizing was increased, the deformation resistance was increased, and the cracking limit upsetting ratio was decreased.
Since Ti content is low, spherical carbides become coarse, and the crack generation limit upsetting ratio is reduced.10 steels have individual elements within the scope of the present invention, but Si + 2.5Mn + 4Cr + 6Mo The value is 0.
It is 84, which is not satisfied with 1.20 or more, so that the effective hardened layer depth is lowered. In addition, conventional steel 11, 12
Steel has a low Ti content and a high Mn and S content (11 steel also has a high N content). Therefore, the spheroidizing annealing hardness and deformation resistance are high, and the cracking critical upsetting rate is low. The cold forgeability is inferior to that of the steel of the present invention.

On the other hand, the steels 1 to 5 of the present invention are Ti
By adding, to generate spherical carbides with Ti carbonitrides as nuclei, and to make the spherical carbides fine, excellent deformability, that is, a cracking limit upsetting ratio of 78% or more is obtained,
Deformation resistance is as low as 800 N / mm ² or less, and 1.20 ≦ Si + 2.5Mn + 4
It was confirmed that the required effective hardened layer depth can be obtained by using Cr + 6Mo.

Next, the effects obtained by the method for manufacturing a steel for machine structure according to the present invention will be clarified by another embodiment. For the three steels in Table 1 above, rolled to φ50 and allowed to cool,
Specimens in which the spheroidizing temperature was changed to 3 levels (A: Spheroidizing annealing was performed at 725 ° C (A _c1 + 2 ° C), B: Spheroidizing annealing was performed at 75)
Conducted at 5 ℃ (A _{c1 +32} ℃), C: Spheroidizing annealing at 780 ℃ (A _c1 + 32 ℃)
+ 57 ° C)), and after rolling to φ50 at 200 ° C / min 2
Cooled to below 00 ℃, spheroidized and annealed at 755 ℃ (D), rolled to φ50, allowed to cool, reheated to 950 ℃ and 1000 ℃
A sample (E) which had been cooled to 200 ° C. or less at / min and spheroidized and annealed at 755 ° C. was prepared as a test material. These test materials are
Test evaluation was performed in the same manner as the above method. The results are shown in Table 3.

[0039]

[Table 3]

As is clear from Table 3, the hardness after spheroidizing is high for A whose spheroidizing annealing temperature is lower than the range of the present invention, A _C1 point + 20 ° C to A _C1 point + 50 ° C. With respect to C whose deformation resistance is high and whose spheroidizing annealing temperature is higher than the range of the present invention, the spheroidal carbide is coarsened and the crack generation limit is lowered. On the other hand, it was confirmed that B having a spheroidizing annealing temperature within the range of the present invention has a low deformation resistance and a high crack generation limit upsetting rate. In addition, immediately after rolling before spheroidizing annealing, or after reheating after rolling to an A _C3 point or higher, D and E were subjected to a quenching treatment at 100 ° C / min or more, compared to B without no quenching treatment It has been confirmed that the particles can be made finer and the deformability can be improved.

Next, an example in which a cold forged product was actually manufactured using the steel of the present invention will be shown. As the test materials, 3 steels in Table 1 above and 9 steels and 12 steels for comparison were used. In the cold forging process as shown in Fig. 1, 800t hydraulic press without intermediate annealing was performed. Using, each of 20 trial forgings was performed. The results are shown in Table 4.

As is clear from Table 4, forging cracks were found in steps 3 and 4 when 9 steel was used. With respect to 12 steels, cracks were observed in step 3 and lack of wall was generated in step 3, so that it was not possible to carry out up to step 4. On the other hand, the steel of the present invention is 3
All of the 20 steels were able to be formed without cracking and generating a wall thickness.

This result means that if the parts shown in FIG. 1 are to be manufactured using 9 and 12 steels, it is necessary to perform intermediate annealing after the completion of step 2. On the other hand, the steel of the present invention can be processed to the final shape without performing the annealing treatment, and the productivity can be greatly improved.

[0044]

EFFECTS OF THE INVENTION The method for producing a steel for mechanical structure according to the present invention is excellent in cold forgeability by adding Ti, forming spherical carbide with Ti carbonitride as a nucleus, and making the spherical carbide fine. In addition, by suppressing the addition amounts of Si, Mn, Cr, and Mo in the range that satisfies 1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo, it is excellent while ensuring the effective hardening depth necessary for contour induction hardening. The cold forgeability is obtained. Therefore, it is possible to manufacture parts that were difficult to cold forge with conventional steel, such as thin-walled flat gear parts, or reduce the number of intermediate anneals for parts that require intermediate anneal on the way even though part production is possible. Or it can be omitted, and the efficiency of production can be greatly improved.

[Table 4]

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[Procedure amendment]

[Submission date] August 9, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Added

[Correction contents]

[Brief description of drawings]

FIG. 1 is a diagram illustrating a process of a cold forged product prototyped as an example of the present invention.

Claims (3)

[Claims] 1. A weight ratio of C: 0.28 to 0.47%, Si: 0.06.
~ 0.25%, Mn: 0.10 ~ 0.50%, P: 0.020% or less, S: 0.010%
Below, Cu: 0.15% or less, Cr: 0.10 to 0.30%, Mo: 0.03% or less, Ti: 0.005 to 0.030%, N: 0.0100% or less, and
A 1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo, the steel comprising the balance Fe and impurity elements, heated to a temperature of A _C1 point + 20 ° C. to A _C1 point + 50 ° C., after 100min~300min temperature hold, 0.1 to 1.0 ℃ / m
The spheroidization rate of the carbide is increased by performing the spheroidization annealing treatment that consists of cooling to 650 ° C or less at the cooling rate of in.
95% or more, the equivalent circle diameter of spherical carbide is 0.5μ
A method for producing a machine structural steel for contour induction hardening, which is excellent in cold forgeability and is characterized in that the thickness is m or less. 2. A weight ratio of C: 0.28 to 0.47%, Si: 0.06.
~ 0.25%, Mn: 0.10 ~ 0.50%, P: 0.020% or less, S: 0.010%
Below, Cu: 0.15% or less, Cr: 0.10 to 0.30%, Mo: 0.03% or less, Ti: 0.005 to 0.030%, N: 0.0100% or less, and
1.20 ≦ Si + 2.5Mn + 4Cr + 6Mo, the steel consisting of balance Fe and impurity elements is hot-rolled and then rapidly cooled to 300 ° C or less at a cooling rate of 100 ° C / min or more, or after hot-rolling at room temperature. After cooling down to 100 ° C, reheat to a temperature of _{AC 3} points or higher and 100
Rapidly cool down to 300 ℃ or less at a cooling rate of ℃ / min or more, and then heat to a temperature of AC ₁ point + 20 ℃ ~ AC ₁ point + 50 ℃ for 100 min
After holding the temperature for 300 min, 6 at the cooling rate of 0.1 to 1.0 ° C / min.
Cold forging characterized in that the spheroidizing rate of carbides is 95% or more and the circle-equivalent average diameter of the spherical carbides is 0.4 μm or less by performing spheroidizing annealing treatment including cooling to 50 ° C or less. A method for manufacturing machine structural steel for high-frequency induction hardening, which has excellent properties. 3. A method for manufacturing a cold forged part by using the steel material manufactured by the method according to claim 1, wherein the final part shape is not subjected to intermediate annealing. A method for manufacturing a cold forged part, comprising: