JP2015143377A - SPHEROIDIZING HEAT TREATMENT METHOD OF HIGH CARBON LOW Cr STEEL MATERIAL FOR COLD FORGING - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spheroidizing heat treatment method applicable for a high carbon low Cr steel material with a reduced amount of Cr for cold forging, the method enabling good cold forgeability to be secured despite being a short time process with reduced heat treatment costs.SOLUTION: A spheroidizing heat treatment method of a high carbon low Cr steel material for cold forging comprises the steps of: keeping the steel material in a temperature range of (A1+10°C) to (A1+50°C) for 5 min. or more and less than 60 min.; then cooling the steel material so that a cooling rate between A1 to (A1-35°C) or less may become 15°C/h or less; thereafter performing, once or twice or more repeatedly, a heat treatment of "keeping the steel material in a temperature range of (A1-20°C) to (A1+10°C) for 60 min. or more, and further keeping the steel material in a temperature range of (A1-100°C) to A1 for 60 min. or more"; and thereafter cooling the steel material.

Description

  The present invention relates to a spheroidizing heat treatment method for low Cr steel materials, and in particular, high structural steel for cold forging used mainly for manufacturing various parts such as automobile parts, bearings, and construction machine parts. The present invention relates to a spheroidizing heat treatment method for carbon low Cr steel.

  Various parts such as automobile parts, bearings, construction machine parts, etc. are usually cold forged from hot-rolled material, then cut into shapes, and then subjected to quenching and tempering. The final strength adjustment is performed.

  The cold forgeability of hot-rolled material is affected by the shape of carbides present in the steel material, and if bar-like carbides are present, they become starting points of cracking and the cold forgeability deteriorates. . In addition, it is generally known that when the granulation of carbide is insufficient, the hardness of the steel material is high and the die life during forging is reduced.

  Therefore, when cold forging the hot-rolled material, spheroidizing heat treatment (spheroidizing annealing) for spheroidizing the carbide is performed prior to the processing.

  As a spheroidizing annealing method, for example, techniques of Patent Documents 1 and 2 are known. Among these, in Patent Document 1, the following elementary processes (1) and (2) are carried out on a steel material either alone or in combination once or twice or more, thereby spheroidizing steel that has required 20 hours or more. A technique for shortening the annealing treatment to about 1 hour is disclosed.

Elementary process (1): After heating and heating at a temperature rising rate of 1 K / second or more in a temperature range of Ae ₁ point or more and Ae ₁ point +150 K or less and holding for 0 second or more and less than 600 seconds in the temperature range it holds the temperature range of Ae ₁ point + 50K~Ae ₁ point -150K to or the temperature range of the temperature is cooled below the cooling rate of 5K / sec; and fundamental process (2): Ae ₁ point + 80K or higher, ae ₁ point + 300K following was heated heated at a temperature region in 1K / sec or more heating rate, after maintaining the temperature for less than 120 seconds 0 seconds in the region, the temperature of ae ₁ point ～Ae ₁ point -150K Cool the area at a cooling rate of 5 K / sec or less, or keep the temperature within the temperature range.

  In Patent Document 1, in order to shorten the spheroidizing annealing time, the heating is performed at a heating rate of 1 K / second or more by temperature control using means for energizing and heating the material to be rolled from a power source connected between roll stands. Therefore, special equipment is required for implementation, and it is extremely difficult to carry out with a normal batch furnace.

In Patent Document 2, spheroidizing annealing is performed by heating and cooling a low alloy steel containing carbon: 0.15 to 1.50% by weight and chromium: 0.10 to 2.50% by weight. In this case, the low alloy steel is heated to a temperature in the range of (A ₁ point−30) to (A ₁ point−5) ° C. and held at that temperature for a certain period of time, and (A ₁ point + 30) to (A Disclosed is a method of spheroidizing annealing of a low alloy steel in which the temperature is raised to and maintained at a maximum temperature in the range of ₁ point + 50) ° C. and then cooled to a temperature of A ₁ point or less at a rate of 15 ° C./hour or less. Yes. In Patent Document 2, in steel for bearings having a low Cr content, the effect of stabilizing the spherical carbide by Cr is small, so the carbide is difficult to be spheroidized. It is described that carbide is formed and cold forgeability is lowered.

JP-A-8-246040 JP-A-9-87736

  By the way, as a material of the above-mentioned various parts, a high carbon Cr bearing steel material defined in JIS G4805 is usually used. This high carbon Cr bearing steel contains 0.90 to 1.20% Cr in SUJ1, SUJ3, and SUJ5, and 1.30 to 1.60% Cr in SUJ2 and SUJ4. However, since the price of Cr is rising in recent years, it is desired to reduce the amount of Cr used from the viewpoint of cost reduction. However, as pointed out in Patent Document 2, if the amount of Cr is reduced, the carbide may not be sufficiently spheroidized and cold forgeability is lowered.

  The present invention has been made paying attention to the circumstances as described above, and its object (problem) is a high carbon Cr bearing steel defined in JIS G4805, which is generally used as a material for various parts. In the spheroidizing heat treatment method for high-carbon low-Cr steel materials for cold forging with a reduced amount of Cr than steel materials, it is a method that suppresses heat treatment costs in a short time, but it can ensure good cold forgeability It is to provide a possible method.

The invention described in claim 1
C: 0.7 to 1.5% (meaning mass%. The same applies to the components below),
Cr: Less than 0.9% (including 0%),
Si: 0.001 to 0.7%,
Mn: 0.1 to 2.0%,
Al: 0.01 to 0.1%,
P: 0.05% or less (excluding 0%),
S: 0.001 to 0.05%,
N: 0.015% or less (excluding 0%),
O: 0.015% or less (excluding 0%)
In the spheroidizing heat treatment method for high carbon low Cr steel material for cold forging, the balance of which includes iron and inevitable impurities,
The steel material is held in a temperature range of (A1 + 10 ° C.) to (A1 + 50 ° C.) for 5 min or more and less than 60 min,
Next, the steel material is cooled so that the cooling rate from A1 to (A1-35 ° C) or less is 15 ° C / h or less,
Thereafter, a heat treatment of “holding the steel material in a temperature range of (A1-20 ° C.) to (A1 + 10 ° C.) for 60 min or more and further holding the steel material in a temperature range of (A1-100 ° C.) to A1 for 1 min” is performed. Repeatedly after 2 or more times, then cool down,
This is a spheroidizing heat treatment method for a high carbon low Cr steel material for cold forging.

The invention according to claim 2
The steel material is further
Cu: 0.25% or less (excluding 0%),
Ni: 0.25% or less (excluding 0%),
Mo: 0.25% or less (excluding 0%), and B: 0.01% or less (not including 0%),
It is the spheroidization heat processing method of the high carbon low Cr steel materials for cold forging of Claim 1 containing 1 or more types selected from the group which consists of.

The invention according to claim 3
The steel material is further
Ti: 0.2% or less (excluding 0%),
Nb: 0.2% or less (not including 0%), and V: 0.5% or less (not including 0%),
It is the spheroidizing heat processing method of the high carbon low Cr steel materials for cold forging of Claim 1 or 2 containing 1 or more types selected from the group which consists of.

Furthermore, the invention described in claim 4
The spheroidizing heat treatment method for a high carbon low Cr steel material according to any one of claims 1 to 3, wherein the spheroidizing heat treatment is performed in a batch furnace.

  According to the present invention, the amount of Cr is smaller than that of the high carbon Cr bearing steel, that is, even in the low Cr cold forging steel having a Cr amount of less than 0.9% by mass, it is excellent in processing in a relatively short time. It is possible to provide a spheroidizing heat treatment method for a high carbon low Cr steel material that has spheroidizing characteristics and can ensure good cold forgeability. Therefore, reduction of the Cr amount and shortening of the heat treatment time can reduce the material cost and the heat treatment cost and improve the productivity. In addition, since there is no need for rapid heating and temperature rise, no special temperature control, facilities and equipment for that purpose are required, and the heat treatment can be easily carried out in a normal batch furnace.

  Examples of the spheroidizing treatment method (spheroidizing annealing method) of the steel material include, for example, (a) a method of maintaining a temperature below the A1 point for a long time, (b) a method of gradually cooling after heating to the A1 point, and (c) near the A1 point. A method of repeatedly heating the upper and lower sides is generally known. (Refer to “Spherical annealing of steel” heat treatment Vol. 15, No. 4, p. 237).

  However, when applying the low Cr steel (Cr <0.9%) targeted in the present invention, rod-like carbides are likely to remain in the method (a), and in the methods (b) and (c). The rod-like carbide tends to reprecipitate during slow cooling, and the hardness is likely to be insufficiently reduced. Therefore, sufficient cold forgeability of the spheroidized heat treatment material cannot be ensured sufficiently.

  Even when the present inventors are directed to the above-mentioned low Cr steel material, this rod-like carbide remains, does not reprecipitate, further reduces the hardness of the spheroidizing heat treatment material, sufficiently softens this, As for the spheroidizing heat treatment method that can ensure good cold forgeability, as a result of earnest and research, the low Cr steel material is held in the temperature range of (A1 + 10 ° C.) to (A1 + 50 ° C.) for 5 min or more and less than 60 min, and then the steel material is Then, cooling is performed so that the cooling rate from Ac1 to (Ac1-35 ° C.) or less is 15 ° C./h or less, and then “the steel material is in the temperature range of (A1-20 ° C.) to (A1 + 10 ° C.) for 60 min or more. The above-mentioned object of the present invention is advantageously achieved by cooling after the heat treatment is repeated once or twice or more, and the steel material is further held in the temperature range of (A1-100 ° C.) to A1 for 60 min or more. Achievement It found Rukoto. The contents of the spheroidizing heat treatment method of the present invention and the reasons for defining the heat treatment conditions will be specifically described below.

In addition, A1 prescribed | regulated as temperature conditions in the following spheroidization heat processing of this invention means an A1 transformation point, and is specifically set as the temperature calculated | required by the following formula. The definitions of these formulas have already been clarified by books (Lecture, Modern Metallurgy Materials Volume 4 Steel Materials Editing and Publication Office: The Japan Institute of Metals Release: Maruzen).
A1 (℃)
= 723-10.7 × (% Mn) −16.9 × (% Ni) + 29.1 × (% Si) + 16.9 × (% Cr) + 290 × (% As) + 6.38 × (% W)

(Spheroidizing heat treatment of the present invention)
[Holding in the temperature range of (A1 + 10 ° C.) to (A1 + 50 ° C.) for 5 min or more and less than 60 min] ... Heat treatment 1
First, in the present invention, a high carbon low Cr steel material (components will be described later) after hot rolling is heated and heated, and is maintained in a temperature range of (A1 + 10 ° C.) to (A1 + 50 ° C.) for 5 min or more and less than 60 min. I do. By giving such a heat history to the raw material, the granulation of the rod-like carbide is sufficiently promoted when “the steel material is kept in the temperature range of (A1-20 ° C.) to (A1 + 10 ° C.) for 60 min or longer”. Is done.

  If the holding temperature is less than (A1 + 10 ° C.) or the holding time is less than 5 minutes, this effect is insufficient, and if the holding temperature exceeds (A1 + 50 ° C.) or the holding time is 60 minutes or more, the carbide dissolves and becomes prominent by subsequent cooling. The rod-like carbide tends to reprecipitate. The preferred holding temperature is (A1 + 15 ° C.) to (A1 + 45 ° C.), and the holding time is 10 min or more and less than 50 min.

  Although the rate of temperature increase to the holding temperature range is not particularly specified, it is preferably 20 ° C./h or more from the viewpoint of productivity and appropriate temperature management. There is no problem even if the rate of temperature increase is high, but when it is performed in a batch furnace, it is preferably performed at 1000 ° C./h or less because it is necessary to raise the temperature of the entire coil as uniformly as possible. More preferably, it is 40 ° C./h or more and 200 ° C./h or less.

[Cooling so that the cooling rate from A1 to (A1-35 ° C) or less is 15 ° C / h or less] ... Cooling treatment 1
Next, in the present invention, the steel material heated and held in the temperature range is cooled, but the cooling treatment is performed so that the cooling rate from A1 to (A1-35 ° C) or less is 15 ° C / h or less. . That is, an arbitrary cooling rate may be used from the holding temperature to A1, but the cooling rate from A1 to (A1-35 ° C.) or less needs to be cooled (slow cooling) at 15 ° C./h or less. That is, the cooling stop temperature at this cooling rate must be (A1-35 ° C.) or less. When the cooling rate is higher than 15 ° C./h, the rod-like carbide is liable to reprecipitate. A preferred cooling rate is 12 ° C./h.

  Further, if cooling at 15 ° C./h or less from A1 is not carried out to (A1-35 ° C.) or less, the precipitation of carbides is insufficient, and it takes a long time for the precipitation of spherical carbides in the subsequent heat treatment, and the rod-like carbides are formed. Easy to precipitate. It is preferable to cool at this cooling rate to (A1-40 ° C.) or less.

[Hold for 60 min or more in the temperature range of (A1-20 ° C) to (A1 + 10 ° C)]
... Heat treatment 2
And in this invention, the said steel material after the said cooling is heated again after that, and it heat-processes, and heat processing which hold | maintain for 60 minutes or more in the temperature range of (A1-20 degreeC)-(A1 + 10 degreeC) is performed. This treatment is carried out mainly to promote the granulation of carbides precipitated by the precooling treatment.

  When the holding temperature in this heat treatment is less than Ac1-20 ° C. or when the holding time is less than 60 min, the rod-shaped carbide is not sufficiently dissolved. When the holding temperature is higher than (A1 + 10 ° C.), the carbide is dissolved too much, and there arises a problem that the rod-like carbide tends to precipitate again in the final cooling process. In addition, it is recommended to heat and hold at (Ac1-10 ° C) to (Ac1 + 5 ° C) for 180-700 min.

[Hold for 60 min or more in the temperature range of (A1-100 ° C.) to A1]
... Heat treatment 3
In this invention, the heat processing which hold | maintains 60 minutes or more in the temperature range of (A1-100 degreeC)-A1 further performs the heat processing (heat processing 2). This treatment is mainly performed in order to suppress the reprecipitation of rod-like carbides.

  When the holding temperature in this heat treatment is lower than (A1-100 ° C.) or when the holding time is less than 60 min, the rod-shaped carbide is not sufficiently dissolved. When the holding temperature is higher than A1, the carbide is excessively dissolved, which causes a problem that rod-like carbide is liable to precipitate again in the final cooling process. Preferably, it is good to hold | maintain for 70 minutes or more in the temperature range of (A1-80 degreeC)-(A1-20 degreeC).

  Moreover, when the holding temperature in the said heat processing 2 is the temperature of (A1-20) -A1, of course, you may implement this heat processing at the same temperature continuously. The “holding (temperature)” referred to in the present invention means that the temperature range of (A1-100 ° C.) to A1 may be maintained for 60 min or longer. The temperature may be kept isothermal, kept at the same temperature after heating / cooling / cooling, or kept in the temperature change state of heating / cooling / cooling. is there.

  In the present invention, the high-carbon low-Cr steel material is sequentially subjected to the heat treatment 1 to heat treatment 3 described above, and the steel material subjected to the heat treatment 3 is finally cooled to be spheroidized. Complete the heat treatment.

  In addition, the above-described heat treatments 2 and 3 in the present invention can obtain sufficiently good characteristics even once, but in order to further improve the characteristics, each is repeated twice or more as necessary, It is also useful to employ a method of final cooling.

  However, it is desirable to repeat the heat treatments 2 and 3 repeatedly many times, because the heat treatment time increases, the heat treatment cost increases, and the productivity also decreases. .

  Next, C, Cr, Si, Mn, Al, P, S, N which are the main defining elements for the component range and the reason for the definition of the steel components of the high carbon low Cr steel material to be subjected to the spheroidizing heat treatment of the present invention And O will be described. The unit of the content of each element in the present invention is all mass%, and is simply abbreviated as% hereinafter.

(Steel component of the present invention)
[C: 0.7 to 1.5%]
C is an element necessary for ensuring the strength of the steel material (that is, the strength of the final product), and has an important influence on the cold forgeability. In addition, since carbide is generated, it must be taken into consideration when designing the spheroidizing annealing method. The present invention is directed to a steel material containing 0.7% or more of C. The amount of C is preferably 0.8% or more. However, when C is contained excessively, the strength becomes too high and the cold forgeability deteriorates. Therefore, the C amount is 1.5% or less. The amount of C is preferably 1.2% or less.

[Cr: less than 0.9 (including 0%)]
Cr is an element that affects the difficulty of spheroidization. The present invention designs and proposes a spheroidizing heat treatment method that can reliably spheroidize even when this Cr is reduced to less than 0.9%, and it is essential to specify the amount of Cr. According to the present invention, even if Cr is 0.8% or less, or 0.6% or less, and even 0.3% or less, spheroidization can be reliably performed, and the Cr amount is 0%. Also good. In addition, since Cr is an element which acts to improve the hardenability of the steel material and increase the strength of the final product, it may be contained in a large amount within a range satisfying less than 0.9%. For example, the Cr amount may be 0.01% or more, preferably 0.05% or more, and more preferably 0.1% or more.

[Si: 0.001 to 0.7%]
Si is an element that is preferably contained as a deoxidizing element and to increase the strength of the final product by solid solution hardening. The Si content is preferably 0.001% or more, more preferably 0.05% or more, still more preferably 0.1% or more, and particularly preferably 0.2% or more. However, when the amount of Si exceeds 0.7%, the strength is excessively increased and the cold forgeability may be deteriorated. Therefore, the Si content is preferably 0.7% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.

[Mn: 0.1 to 2.0%]
Mn is an element that effectively acts to improve hardenability and increase the strength of the final product. In order to effectively exhibit such an action, the content is preferably 0.1% or more, more preferably 0.3% or more, and still more preferably 0.50% or more. However, when it contains excessively, intensity | strength rises excessively and cold forgeability may deteriorate. Accordingly, the Mn content is preferably 2.0% or less, more preferably 1.5% or less, still more preferably 1.2% or less, and particularly preferably 1.0% or less.

[Al: 0.001 to 0.1%]
Al is an element that acts as a deoxidizing element, fixes solid solution N present in the steel as AlN, and improves cold forgeability. In order to effectively exert such effects, the Al content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.01% or more. However, when the amount of Al is excessive, Al ₂ O ₃ is excessively generated in the steel material, and cold forgeability may be deteriorated. Accordingly, the Al content is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.05% or less.

[P: 0.05% or less (excluding 0%)]
P is an element inevitably contained in the steel material, and when grain boundary segregation occurs, it causes ductile deterioration. Accordingly, the P content is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less.

[S: 0.001 to 0.05%]
S is an element inevitably contained in the steel material, but acts to improve the machinability of the steel material. Therefore, the S amount is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more. However, when it contains excessively, S exists as MnS in steel materials, and may deteriorate ductility and cold forgeability. Accordingly, the S amount is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less.

[N: 0.015% or less (excluding 0%)]
N is an element inevitably contained in the steel material. When N is present as a solid solution N in the steel material, the hardness increases due to strain aging and the ductility decreases, and the cold forgeability may be deteriorated. Therefore, the N content is preferably 0.015% or less, more preferably 0.013% or less, and still more preferably 0.01% or less.

[O: 0.015% or less (excluding 0%)]
O is an element inevitably contained in the steel material, and if it exists as an oxide in the steel material, cold forgeability may be deteriorated. Therefore, the O content is preferably 0.015% or less, more preferably 0.013% or less, and still more preferably 0.01% or less.

  The components of the target steel of the present invention are composed of the above main prescribed elements, and the balance is composed of iron and inevitable impurities. In addition to these essential elements, one or more elements selected from the group consisting of Cu, Ni, Mo and B and one or more elements selected from the group consisting of Ti, Nb and V are contained. This is also useful for further improving the properties of the steel material. Hereinafter, this selective element will be described.

[Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), Mo: 0.25% or less (not including 0%), and B: 0.0. One or more selected from the group consisting of 01% or less (excluding 0%)]
Cu, Ni, Mo, and B are all elements that effectively work to improve the hardenability of the steel material and increase the strength of the final product. These elements should be contained alone or in combination of two or more. Is preferred. In order to effectively exert such an action, it is preferable to contain Cu 0.01% or more, Ni 0.01% or more, Mo 0.01% or more, and B 0.001% or more, more preferably Cu is 0.03% or more, Ni is 0.03% or more, Mo is 0.03% or more, and B is 0.0015% or more. However, when it contains excessively, intensity | strength will become high too much and cold forgeability may deteriorate. Accordingly, Cu is preferably 0.25% or less, Ni is 0.25% or less, Mo is 0.25% or less, and B is 0.01% or less, more preferably, Cu is 0.15% or less, Ni is 0.15% or less, Mo is 0.2% or less, and B is 0.008% or less.

[From the group consisting of Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.5% or less (not including 0%) One or more selected]
Ti, Nb, and V are elements that combine with N present in the steel material to form a nitride, reduce the solid solution N, thereby reducing deformation resistance and improving cold forgeability, These elements are preferably used alone or in combination of two or more. In order to effectively exert such an effect, it is preferable that Ti is 0.02% or more, Nb is 0.02% or more, and V is 0.05% or more, more preferably, Ti is 0.04% or more. , Nb is 0.05% or more, and V is 0.08% or more. However, when it contains excessively, the nitride formed may raise deformation resistance and may deteriorate cold forgeability. Therefore, Ti is preferably 0.2% or less, Nb is 0.2% or less, and V is preferably 0.5% or less, more preferably, Ti is 0.1% or less, Nb is 0.1% or less, V is 0.25% or less.

(Example)
(1) Preliminary experiment The superior effect of the present invention will be demonstrated based on this example.
First, prior to the examples of the present invention (this experiment), representative spheroidization described in the above-mentioned reference ("Spheroidizing annealing of steel" heat treatment Vol. 15, No. 4, P.237). As a heat treatment (annealing) method, an experiment was performed using both (a) a method of holding at a temperature of A1 point or lower for a long time and (b) a method of gradually cooling after heating to A1 point.
In this preliminary experiment, the following heat treatment conditions were adopted and implemented as the heat treatment methods (a) and (b) above for the hot rolled steel material of steel type A in Table 1.
(A) A steel test piece was heated and heated from room temperature to 150 ° C./h in an experimental heat treatment furnace, held at 710 ° C. for 40 h, and then cooled to 680 ° C. at a cooling rate of 5 ° C./h ( Total processing time: 51h)
(B) A steel test piece was heated and heated from room temperature to 150 ° C./h in an experimental heat treatment furnace, held at 760 ° C. for 5 h, and then cooled to 600 ° C. at 4 ° C./h (total treatment time) : 51h)
The spheroidizing characteristics (hardness and structure) at the position of the transverse section D / 4 of the test piece taken out from the heat treatment furnace were measured and evaluated in the following manner.

[Evaluation of spheroidizing characteristics]
-Hardness evaluation Using a Vickers hardness meter, five points were measured with a load of 1 kgf (≈9.8 N), and the average value (HV) was obtained. Even if the spheroidization is sufficiently promoted, the hardness changes depending on the carbon content of the steel material, and therefore it cannot be evaluated only by its absolute value. Therefore, in the present invention, HV is below the following Y value as a criterion for the evaluation. In this case, it was judged that the steel material was softened, and it was determined to be acceptable (◯), and those having an HV greater than or equal to the Y value were determined to be unacceptable (x).
Y = (carbon amount of steel) [mass%] × 27.3 + 143.5

-Tissue evaluation 10 images were taken at a magnification of 4000 times by SEM observation. Usually, when a large amount of rod-like carbides are present, they become starting points of cracks and cause a decrease in cold forgeability. In this evaluation, the obtained SEM photograph was subjected to image analysis, and the aspect ratio (ratio of major axis / minor axis) of carbides in the photograph was calculated. If the ratio of carbides in the photo with an aspect ratio of 3.0 or less passes 90% or more, and all 10 sheets pass, it is finally passed (◯) as an excellent spheroidized structure. Except for the above, it was judged as rejected (x).

  As a result of the evaluation of the test pieces of the steel materials after the heat treatment of (a) and (b) by this preliminary experiment, the hardness exceeded the Y value at HV179 and HV171, respectively, and also failed in the structure evaluation. Did not meet. Thus, it was confirmed that spheroidization is difficult even when heat treatment is performed for a relatively long time (50 h, 51 h). Based on these facts, the following experiments were conducted.

(2) This experiment Each steel material of the component of Table 1 is hot-rolled by a conventional method, each is manufactured as a Φ15 mm wire coil, a test piece having a height of 5 mm is cut out from this wire, and sealed for vacuum. Spheroidizing heat treatment was performed in a heat treatment furnace under various heat treatment conditions. Table 2 shows the evaluation results of the heat treatment conditions and the spheroidizing characteristics of the steel material (test piece) after the spheroidizing heat treatment (corresponding to numbers 1 and 3 to 18 in Table 2). The evaluation of the spheroidization characteristics was performed according to the same criteria as the evaluation procedure described in the preliminary experiment.

  Further, in the additional number 2 in Table 2, the wire coil after hot rolling was subjected to spheroidizing heat treatment in a batch furnace of an actual machine, and the same test piece was cut out from the heat treated wire material for evaluation. The temperature of the wire coil here is based on the result of measuring the coil temperature by attaching a thermocouple to the coil to be evaluated, and the holding temperatures in Table 2 were obtained by this measurement during holding. Average temperatures are listed.

In Table 2, those having no shading in the serial number and heat treatment conditions are examples of the present invention (sequential numbers 1 to 10), and those having shading are the conditions defined by the present invention (steel components, heat treatment conditions). The comparative example (sequential numbers 11-18) which is not satisfied is shown.
As apparent from Table 2, it can be seen that the high-carbon low-Cr steel material subjected to the spheroidizing heat treatment according to the example of the present invention all passed both the hardness evaluation and the structure evaluation. That is, the HV hardness is low and sufficiently softened, the amount of rod-like carbides in the steel structure is suppressed to a very low level, and it has been found that it has excellent spheroidizing characteristics even with a low Cr content. Therefore, it can be easily understood that the steel material obtained by the present invention has a good cold forgeability.

  On the other hand, since all the steel materials according to the comparative examples are out of the conditions specified by the present invention, it is known that both the hardness evaluation and the structure evaluation are all rejected, compared with the present invention examples. It can be understood that the spheroidizing characteristics are inferior and, therefore, sufficient cold forgeability cannot be obtained in the comparative example.

Claims (4)

C: 0.7 to 1.5% (meaning mass%. The same applies to the components below),
Cr: Less than 0.9% (including 0%),
Si: 0.001 to 0.7%,
Mn: 0.1 to 2.0%,
Al: 0.01 to 0.1%,
P: 0.05% or less (excluding 0%),
S: 0.001 to 0.05%,
N: 0.015% or less (excluding 0%),
O: 0.015% or less (excluding 0%)
In the spheroidizing heat treatment method for high carbon low Cr steel material for cold forging, the balance of which includes iron and inevitable impurities,
The steel material is held in a temperature range of (A1 + 10 ° C.) to (A1 + 50 ° C.) for 5 min or more and less than 60 min,
Next, the steel material is cooled so that the cooling rate from A1 to (A1-35 ° C) or less is 15 ° C / h or less,
Thereafter, a heat treatment of “holding the steel material in a temperature range of (A1-20 ° C.) to (A1 + 10 ° C.) for 60 min or more and further holding the steel material in a temperature range of (A1-100 ° C.) to A1 for 1 min” is performed. A method of spheroidizing heat treatment of a high carbon low Cr steel material for cold forging, characterized by cooling after repeating once or twice or more. The steel material is further
Cu: 0.25% or less (excluding 0%),
Ni: 0.25% or less (excluding 0%),
Mo: 0.25% or less (excluding 0%), and B: 0.01% or less (not including 0%),
The spheroidizing heat treatment method for a high carbon low Cr steel material for cold forging according to claim 1, comprising at least one selected from the group consisting of: The steel material is further
Ti: 0.2% or less (excluding 0%),
Nb: 0.2% or less (not including 0%), and V: 0.5% or less (not including 0%),
The spheroidizing heat treatment method for a high carbon low Cr steel material for cold forging according to claim 1 or 2, comprising at least one selected from the group consisting of:   The spheroidizing heat treatment method for a high carbon low Cr steel material for cold forging according to any one of claims 1 to 3, wherein the spheroidizing heat treatment is performed in a batch furnace.