JP2014177690A - Manufacturing method of steel for cold forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of steels for cold forging excellent in cold forgeability, by heating a steel material with a Cr content lower than that of high carbon chromium bearing steels in general use as material for various parts as specified in JIS G4805 (specifically, steel material with a Cr content less than 0.9%) for spheroidizing annealing, capable of forming excellent spheroidized structure in any of a high-temperature portion and a low-temperature portion generated due to temperature distribution caused during heating for spheroidizing annealing.SOLUTION: The manufacturing method of steels for cold forging includes heating a steel material containing 0.7 to 1.5% of C and less than 0.9% (excluding 0%) of Cr, with an average lamellar spacing of pearlite structure of 130 nm or less, for spheroidizing annealing. A first spheroidizing annealing is performed such that a portion heated at the lowest temperature in the steel material due to temperature distribution caused during heating for spheroidizing annealing is subject to predetermined conditions for spheroidizing annealing, and a portion heated at the highest temperature is heated at a heating temperature of higher than point A+25°C. Subsequently a second spheroidizing annealing is performed such that the portion heated at the highest temperature in the first spheroidizing annealing is subject to predetermined conditions for spheroidizing anneal.

Description

  The present invention relates to a method of manufacturing cold forging steel used for manufacturing various parts such as automobile parts, bearings, and construction machine parts.

  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. . Therefore, when cold forging the hot-rolled material, heat treatment (spheroidizing annealing) for spheroidizing the carbide is performed in order to improve the cold forgeability of the hot-rolled material.

  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 the above Patent Document 1, spheroidizing annealing is performed by charging a steel material in a coil shape into an annealing furnace and performing a heat treatment in a batch manner, or by continuously moving the inside of the furnace of the annealing furnace while being in a coil shape. It is described that it is performed.

Patent Document 2 discloses a bearing steel material containing C: 0.40 to 0.80 wt% and Cr: 0.80 wt% or less, after the following treatments (1) to (3), (4 ) To (5) are disclosed one or more times, and then a spheroidizing annealing method in which cooling is performed is disclosed.
(1) A Rapid cooling is performed after heating and holding at ₃ points or more.
(2) A ₃ point + (5-30) reheating kept at a temperature in the range of ° C..
(3) A ₁ point-held at a temperature range of (5 to 30) ° C.
(4) Hold in a temperature range of (A ₁ point + 5) to (A ₃ point + 30) ° C.
(5) A ₁ point-held in a temperature range of (5 to 30) ° C.

  In Patent Document 2, in steel for bearings with a low Cr content, since the effect of stabilizing spherical carbides by Cr is small, the carbides are difficult to spheroidize, and if spheroidizing annealing conditions are inappropriate, rod-like or coarse carbides It is described that cold forgeability is reduced.

JP-A-8-246040 JP-A-4-362123

  By the way, as a material of the above-mentioned various parts, a high carbon chrome bearing steel material defined by JIS G4805 is usually used. This high carbon chromium bearing steel material 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.

  In particular, when spheroidizing annealing is performed in a batch manner, a plurality of coiled steel materials may be charged into an annealing furnace in order to increase productivity. However, when a steel material is heat-treated in large quantities simultaneously in a batch type furnace such as an STC furnace, a temperature distribution is generated in the furnace during spheroidizing annealing. A steel material with reduced Cr is difficult to spheroidize, and particularly when there is a temperature distribution in the furnace, it is extremely difficult to make all steel materials charged in the annealing furnace into a good spheroidized structure.

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is higher than that of the high carbon chromium bearing steel material defined in JIS G4805, which is generally used as a material for various parts. This is a case where steel for cold forging is manufactured by heating a steel material with reduced Cr content (specifically, a steel material with Cr reduced to less than 0.9%) and spheroidizing annealing. To provide a method capable of producing a steel for cold forging excellent in cold forgeability, capable of forming a good spheroidized structure in any part even if a temperature distribution sometimes occurs and a high temperature part and a low temperature part occur. It is in.

As a result of intensive studies to solve the above problems, in order to appropriately spheroidize a steel material in which Cr is reduced to less than 0.9%, the average lamella spacing of the pearlite structure of the steel material is set to 130 nm or less. in, was kept a ₁ point + 5 ° C. to a at a temperature range of ₁ point + 25 ° C. 60 to 360 minutes, cooled, average cooling rate of 10 temperature range from a ₁ point -10 ° C. until a ₁ point -30 ° C. It has been found that spheroidizing annealing may be performed under the condition of slow cooling at a temperature of not more than ° C / hour. Moreover, when performing this spheroidizing annealing, the part that becomes the lowest temperature heating (lowest temperature part) and the part that becomes the highest temperature heating (highest temperature part) due to the temperature distribution generated during the spheroidizing annealing heating Therefore, after annealing the lowest temperature part and the highest temperature part so that the highest temperature part satisfies the above conditions, the lowest temperature part and the highest temperature part are annealed so that the lowest temperature part satisfies the above conditions. As a result, the present invention has been completed.

That is, the manufacturing method of the steel for cold forging according to the present invention that has solved the above problems is C: 0.7 to 1.5% (meaning mass%, the same shall apply hereinafter), Cr: 0.00. A method for producing a steel for cold forging by heating a steel material containing less than 9% (not including 0%) and having an average lamella spacing of a pearlite structure of 130 nm or less to spheroidizing annealing. The temperature distribution at the time of annealing heating is such that the portion of the steel that is the lowest temperature heating meets the following spheroidizing annealing conditions, and the heating temperature of the portion that is the highest temperature heating is higher than A ₁ point + 25 ° C. After performing the first spheroidizing annealing, the second spheroidizing annealing is performed so that the portion that has been heated at the highest temperature in the first spheroidizing annealing meets the following spheroidizing annealing conditions Has a summary.

Specifically, the above-described problems include C: 0.7 to 1.5%, Cr: less than 0.9% (not including 0%), and a plurality of pearlite structures having an average lamella spacing of 130 nm or less. When producing steel for cold forging by heating the steel material in the same furnace, the steel material at the lowest heating temperature due to the temperature distribution in the furnace generated during spheroidizing annealing is as follows: After performing the first spheroidizing annealing so as to conform to the spheroidizing annealing condition and the heating temperature of the steel material at the highest heating temperature exceeds A ₁ point + 25 ° C., the first spheroidizing annealing is performed. This can be achieved by performing the second spheroidizing annealing so that the steel material at the highest temperature in the spheroidizing annealing meets the following spheroidizing annealing conditions.
Spheroidizing annealing conditions: After holding A ₁ point + 5 ° C. to A at a temperature range of ₁ point + 25 ° C. 60 to 360 minutes, cooled, a temperature range from A ₁ point -10 ° C. until A ₁ point -30 ° C. The average Slowly cool at a cooling rate of 10 ° C / hour or less.

The heating temperature when the portion that has been the lowest temperature heating in the first spheroidizing annealing is heated for the second spheroidizing annealing may be less than A ₁ point + 5 ° C.

  The steel material further contains Si: 0.001 to 0.7%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1%, the balance being iron and inevitable impurities Things can be used. Examples of the inevitable impurities include P: 0.05% or less (excluding 0%), S: 0.001 to 0.05%, N: 0.015% or less (not including 0%). Can be used.

The steel material is further
(1) 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: At least one element selected from the group consisting of 0.01% or less (not including 0%),
(2) A 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%) At least one element selected from
May be included.

  According to the present invention, after paying attention to the temperature distribution generated during the spheroidizing annealing, the first spheroidizing annealing is performed most in the first spheroidizing annealing after the temperature distribution in the steel material is subjected to the first spheroidizing annealing. This is the case where the steel material with Cr reduced to less than 0.9% is subjected to spheroidizing annealing because the portion that has been heated at a high temperature is subjected to the second spheroidizing annealing, and a temperature distribution occurs during annealing. However, the whole steel material can be made into a favorable spheroidized structure. As a result, a steel material excellent in cold forgeability can be provided.

  The present invention relates to a spheroidizing annealing method for a steel material containing 0.7 to 1.5% C and less than 0.9% Cr (not including 0%). The reason for designing the spheroidizing method for cold forging steel by paying attention to C and Cr is as follows.

  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 is an element that affects the difficulty of spheroidization. In the present invention, even when this Cr is reduced to less than 0.9%, a spheroidizing annealing method is designed for the purpose of surely spheroidizing, and it is essential to specify the amount of Cr. According to the present invention, even when Cr is 0.8% or less, 0.6% or less, and even 0.3% or less, spheroidization can be reliably performed. Note that Cr is an element that acts to improve the hardenability of the steel material and increase the strength of the final product, and therefore 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.

  The amount of C and Cr mentioned above should be considered in designing the annealing method of the present invention, but the steel material actually used in the annealing method of the present invention usually further contains Si, Mn, and Al in the following ranges. To do. The balance is not particularly limited, but may be iron and inevitable impurities. The addition amount of Si, Mn, and Al and the reason for the addition are as follows.

[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. Accordingly, 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.

  As said inevitable impurity, P, S, and N may be contained in the following range.

[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. Accordingly, the N content is preferably 0.015% or less, more preferably 0.013% or less, and still more preferably 0.01% or less.

  The steel material further includes (1) at least one element selected from the group consisting of Cu, Ni, Mo and B, and (2) at least one element selected from the group consisting of Ti, Nb and V. But you can.

[(1) 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: at least one selected from the group consisting of 0.01% 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 (2) 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%) At least one selected from the group consisting of]
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.

  The present invention designs an appropriate spheroidizing method for the above-mentioned low Cr steel material, which is difficult to spheroidize, and forms a good spheroidized structure in both the high temperature part and the low temperature part. It is to secure the sex. In applying the spheroidizing annealing method of the present invention, it is important to set the average lamella spacing of the pearlite structure of the steel material to 130 nm or less before spheroidizing annealing. When the average lamella spacing of the pearlite structure exceeds 130 nm, a lot of rod-like carbides are formed, and even if heat treatment is performed under the spheroidizing annealing conditions described later in the present invention, the rod-like carbides do not sufficiently decompose and dissolve. Even after spheroidizing annealing, it remains as a rod. The rod-shaped carbide becomes a starting point of cracking, and cold forgeability deteriorates. Therefore, in the present invention, the average lamella spacing of the pearlite structure is 130 nm or less. The average lamella spacing of the pearlite structure is preferably 120 nm or less. The lower limit of the average lamella interval is not particularly limited, and may be about 70 nm, for example.

  In the present invention, the lamella spacing of the pearlite structure means the center-to-center distance between adjacent carbides. For the lamella spacing of the pearlite structure, the sample after mirror polishing was etched with picral (5% picric acid + ethanol), and 10 photographs were taken at a magnification of 5000 times at the D / 8 position (D is the diameter of the sample) in the cross section. , Determine the finest lamella in each photo, draw a line segment perpendicular to this lamella, determine the lamella spacing from the length of the line segment and the number of lamellas crossing the line segment, totaling 10 line segments Was determined by averaging the measured lamella spacing.

  As a method of setting the average lamella spacing of the pearlite structure to 130 nm or less, for example, after hot rolling, the pearlite may be transformed by quenching from the austenite temperature region at an average cooling rate of 30 ° C./min or more. The average cooling rate is more preferably 40 to 300 ° C./min.

In the spheroidizing annealing method of the present invention, the steel material having the above-described component composition and pearlite structure is annealed a plurality of times. In the steel material in which the Cr content is reduced to less than 0.9%, it is difficult to spheroidize the carbide, and it is strongly influenced by the temperature distribution in the furnace, and there are parts that succeed in spheroidization and parts that fail. . The annealing is performed a plurality of times for each of the part that is the lowest temperature heating (lowest temperature part) and the part that is the highest temperature heating (highest temperature part) depending on the temperature distribution. In order to ensure that the spheroidization succeeds at least once, the spheroidization is performed under the following spheroidizing annealing conditions. Depending on the multiple annealing pattern, the part treated under the following spheroidizing annealing condition may be further heat-treated (annealed). In the subsequent heat treatment (annealing), it is important to make the temperature equal to or lower than the temperature specified by the spheroidizing annealing conditions.
Spheroidizing annealing conditions: After holding A ₁ point + 5 ° C. to A at a temperature range of ₁ point + 25 ° C. 60 to 360 minutes, cooled, a temperature range from A ₁ point -10 ° C. until A ₁ point -30 ° C. The average Slowly cool at a cooling rate of 10 ° C / hour or less.

The temperature of the A ₁ point is Ac ₁ point is calculated by the following equation according to the 273 pages of Leslie steel Metallurgical (Maruzen) (1). In the following formula (1), [] indicates the content (% by mass) of each element, and elements not contained may be calculated as 0%.
Ac ₁ point = 723 + 29.1 × [Si] −10.7 × [Mn] + 16.9 × [Cr] −16.9 × [Ni] + 290 × [As] + 6.38 × [W] (...) 1)

(1) Hold for 60 to 360 minutes in the temperature range of A ₁ point + 5 ° C. to A ₁ point + 25 ° C. Under the above spheroidizing annealing conditions, set the heating holding temperature to A ₁ point + 5 ° C. or higher, and set the holding time to 60 minutes or longer. Thus, the rod-like carbide can be sufficiently separated and dissolved, and the cold forgeability can be improved. The holding temperature is preferably A ₁ point + 8 ° C. or higher. The holding time is preferably 90 minutes or longer.

On the other hand, the heating and holding temperature was set to A ₁ point + 25 ° C. or less and the holding time was set to 360 minutes or less because the carbide was too solid and the spheroidization of the carbide became insufficient because the thermal conditions were too strong. Is to avoid. The holding temperature is preferably A ₁ point + 20 ° C. or lower. The holding time is preferably 330 minutes or less.

(2) Cooling conditions: A temperature range from A ₁ point −10 ° C. to A ₁ point −30 ° C. is gradually cooled at an average cooling rate of 10 ° C./hour or less. The average cooling rate is 10 ° C./hour in the spheroidizing annealing condition. The reason for the following is to prevent the precipitation of rod-like carbides and to make the carbides spherical. The average cooling rate in the temperature range is preferably 8 ° C./hour or less, more preferably 6 ° C./hour or less. The lower limit of the cooling rate may be, for example, about 0.5 ° C./hour, preferably about 1 ° C./hour.

  In order to anneal a steel material a plurality of times and treat the highest temperature part and the lowest temperature part at least once under the spheroidizing annealing condition, for example, the lowest temperature part first satisfies the spheroidizing annealing condition. The first spheroidizing annealing is performed, and after the first spheroidizing annealing, the second spheroidizing annealing is performed so that the portion that has become the highest temperature portion satisfies the spheroidizing annealing condition. Spheroidizing annealing may be performed.

In the first spheroidizing annealing, the highest temperature part is also heat-treated (annealing process) in conjunction with the heat pattern of the lowest temperature part. The heating temperature of the highest temperature part may be allowed to depend on the difference caused by the temperature distribution, and is higher than A ₁ point + 25 ° C.

Even in the second spheroidizing annealing, the lowest temperature part is also heat-treated in conjunction with the heat pattern of the highest temperature part. The heating temperature in the lowest temperature part may also depend on the difference caused by the temperature distribution. For example, the heating temperature equivalent to the temperature specified in the spheroidizing annealing condition (for example, A ₁ point + 5 ° C. to A ₁ point) + 25 ° C.), but it is preferable to make it less than A ₁ point + 5 ° C. In any case, the lowest temperature portion does not exceed A ₁ point + 25 ° C. during the second spheroidizing annealing, and it is possible to prevent the carbide from re-dissolving in the austenite and dissolving, and maintain a good spheroidized structure. .

  In addition, unless the first spheroidizing annealing and the second spheroidizing annealing are appropriately performed and strong annealing is performed so as to impair the effect of the spheroidizing annealing that satisfies the spheroidizing annealing conditions performed previously, The first and second spheroidizing annealing may be performed at any timing, and other heat treatment (including failure of the spheroidizing annealing) may be performed before and after these spheroidizing annealing. For example, after appropriately heating the low temperature part by the first spheroidizing annealing and then heating the high temperature part for the purpose of performing the second spheroidizing annealing, the heating temperature of the high temperature part is insufficient. When the cooling rate becomes inappropriate, the second spheroidizing annealing may be performed again.

  The spheroidizing annealing can be used in various production facilities as long as annealing is performed a plurality of times and the heat treatment conditions are controlled by dividing into the lowest temperature part and the highest temperature part. This is particularly effective when steel materials (such as wire coils) are charged and processed simultaneously. That is, a plurality of steel materials (wire coils, etc.) are charged into the heat treatment furnace, and the first spheroidizing annealing is performed on the steel material arranged at the lowest temperature due to the temperature distribution in the furnace generated during the spheroidizing annealing heating. Then, the second spheroidizing annealing may be performed on the steel material arranged at the highest temperature.

  Thus, when heating several steel materials in the same furnace, the heat history of the steel materials in the 1st spheroidizing annealing is measured, and the steel materials of the part which becomes the lowest temperature in the 1st spheroidizing annealing When the heat history of the above satisfies the spheroidizing annealing condition, the second spheroidizing annealing is performed, and the steel material of the portion that has become the highest heating temperature in the first spheroidizing annealing is described above. What is necessary is just to heat-process on spheroidizing annealing conditions. In the case where the thermal history of the steel material at the lowest heating temperature in the first spheroidizing annealing does not satisfy the spheroidizing annealing condition, the lowest temperature steel material satisfies the spheroidizing annealing condition. After the heat treatment as described above, the second spheroidizing annealing is performed, and the steel material at the highest heating temperature in the first spheroidizing annealing may be heat-treated under the spheroidizing annealing conditions.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

After hot rolling a steel material (steel types A to K) having a component composition shown in Table 1 (the balance is iron and inevitable impurities other than P, S, and N), the steel material is rapidly cooled from the austenite temperature range, and a wire coil having a diameter of 15 mm is obtained. 21 pieces were produced. Table 1 below shows the temperature at point A ₁ calculated based on the above formula (1). Table 2 below also shows the steel types used and the temperatures of A ₁ point, A ₁ point + 5 ° C., and A ₁ point + 25 ° C. in each steel type.

  First, the test piece for structure | tissue observation was cut out from the edge part of each obtained wire coil.

  Next, the obtained wire coil was put in a batch furnace and subjected to spheroidizing annealing. The wire coil is 1 ton per piece, the wire coils stacked in three layers are arranged and heated in seven places in the furnace (that is, 21 wire coils in total), and the first coil Spheroidizing annealing was performed. The first spheroidizing annealing was performed under the conditions of heating and holding 1 and cooling 1 shown in Table 2 below. At this time, a thermocouple was attached to each wire coil, and the temperature of the wire coil was measured. When heating holding 1 and cooling 1 were completed, the wire coil that became the highest temperature heating was determined as the highest temperature portion, and the wire coil that became the lowest temperature was determined as the lowest temperature portion. The holding temperature in the highest temperature part and the holding temperature in the lowest temperature part are shown in Table 2 below.

  Next, second spheroidizing annealing was performed under the conditions of heating and holding 2 and cooling 2 shown in Table 2 below. In addition, No. shown in Table 2 below. About 19-21, after 2nd spheroidizing annealing, 3rd spheroidizing annealing was performed on the conditions of the heating holding 3 and the cooling 3 which are shown in following Table 2.

  After the second spheroidizing annealing or the third spheroidizing annealing, the metal structure was observed by the following procedure, the shape of the carbide was examined, and whether or not the spheroidizing annealing was appropriately performed was evaluated.

  That is, a test piece was cut out from the end of the wire coil obtained by spheroidizing annealing, and the cross section at the D / 8 position (D is the diameter of the test piece) was scanned with a scanning electron microscope (SEM) at a magnification of 4000 times. Observed and taken 10 photos. Each photograph taken was subjected to image analysis, and the aspect ratio (ratio of major axis / minor axis) of the carbide observed in the photograph was calculated. The number ratio of carbides having an aspect ratio of 3.0 or less was calculated with respect to the number (total number) of carbides observed in the photograph. Of the 10 photographs taken, the spheroidizing pass was accepted when the ratio of the number of carbides having an aspect ratio of 3.0 or less in all the photographs was 90% or more (circle mark in Table 3 below). A case where the number ratio of carbides having a particle size of 3.0 or less was less than 90% was evaluated as a spheroidizing annealing failure (indicated by x in Table 3 below). The evaluation results are shown in Table 3 below.

  In addition, the lamella spacing of the pearlite structure is measured by the following procedure for the test piece collected in advance before spheroidizing annealing from the wire coil selected as the highest temperature part or the lowest temperature part in the first spheroidizing annealing. did. That is, the test piece was cut out in a cross section, embedded in resin, polished and etched, and the metal structure was observed with a scanning electron microscope (SEM). The lamella spacing of the pearlite structure was taken at a D / 8 position in the cross section (D is the diameter of the test piece) at a magnification of 5000, and the finest lamella was determined in each photograph, which was perpendicular to this lamella. A line segment was drawn as described above, and the lamella interval was obtained from the length of the line segment and the number of lamellae crossing the line segment. The average value was calculated by averaging the lamella spacing measured for a total of 10 line segments. The calculation results are shown in Table 3 below.

  Table 3 below also shows the average cooling rate from the austenite temperature range when the wire coil selected as the highest temperature part or the lowest temperature part is manufactured.

  The following Table 1 to Table 3 can be considered as follows. No. 1 to 10, 12, 15, 19, and 20 are examples that satisfy the requirements defined in the present invention.

  Of these, No. Reference numerals 1 to 10, 12 and 15 are examples in which the lowest temperature portion was subjected to spheroidizing annealing in heating and holding 1 and cooling 1, and then the highest temperature portion was subjected to spheroidizing annealing in heating and holding 2 and cooling 2. No. No. 19 tried to spheroidize and anneal the highest temperature part in heating and holding 2 and cooling 2 after spheroidizing annealing in the heating and holding 1 and cooling 1, but the average cooling rate after heating and holding was large. This is an example in which the highest temperature portion is subjected to spheroidizing annealing again in the heating hold 3 and the cooling 3 because it has passed. No. No. 20 spheroidizing annealing of the lowest temperature part in heating holding 1 and cooling 1, and then trying to spheroidize the highest temperature part in heating holding 2 and cooling 2, but the heating holding temperature was too low. In the holding 3 and the cooling 3, the highest temperature portion is an example of spheroidizing annealing.

  No. 1 to 10, 12, 15, 19 and 20 are both wire coils arranged in the highest temperature part and the lowest temperature part, although the Cr amount contained in the wire coil is less than 0.9%. , A spheroidized structure is obtained. Accordingly, both wire coils are considered to be excellent in cold forgeability.

  On the other hand, no. Nos. 11, 13, 14, 16-18, and 21 are examples that do not satisfy the requirements defined in the present invention, and a spheroidized structure is obtained in at least one of the highest temperature part and the lowest temperature part. Not. The details are as follows.

No. No. 11 is an example in which the Cr amount exceeds the range defined in the present invention. Under the conditions shown in Table 2, the spheroidized structure is not obtained in the highest temperature part and the lowest temperature part. That is, when heated above the temperature of point A ₁ (γ + θ region), as the heating temperature is increased or the heating time is lengthened, rod-like carbides are divided when the previous structure contains pearlite, and insoluble carbides Decrease. However, when the amount of Cr is increased, the fragmentation of the rod-like carbide is suppressed, so that the decrease in undissolved carbide is suppressed and the carbide is stabilized. Therefore, when the amount of Cr is increased, the carbide is stable even after being cooled for a long time and then cooled, so that perfect spheroidization is easily achieved (since the carbide is stabilized, the rod-shaped carbide is Difficult to re-deposit). However, even if the amount of Cr is increased, it is considered that under the conditions shown in Table 2, the splitting of the rod-like carbides is insufficient and a spheroidized structure cannot be obtained. Therefore, it is thought that cold forgeability has not been improved.

  No. No. 13, since the average cooling rate from the austenite temperature region after hot rolling is less than 30 ° C./min and is not rapidly cooled, the average lamella spacing of the pearlite structure of the steel before spheroidizing annealing is defined in the present invention. This is an example that does not satisfy the requirements, and since a rod-like carbide remains even after spheroidizing annealing, a spheroidized structure is not obtained. Therefore, it is thought that cold forgeability has not been improved.

  No. No. 14 could not obtain a spheroidized structure because the temperature of the wire coil arranged in the lowest temperature part could not be appropriately controlled. Therefore, it is thought that cold forgeability has not been improved.

  No. 16 is a condition of the cooling 1 condition after heating and holding the lowest temperature part, a temperature when heating and holding the highest temperature part, and a time when heating and holding the highest temperature part are defined in the present invention. Since it is not satisfied, a spheroidized structure is not obtained in the highest temperature part and the lowest temperature part. Therefore, it is thought that cold forgeability has not been improved.

  No. No. 17 is an example in which both the spheroidizing treatment condition of the lowest temperature part and the spheroidizing treatment condition of the highest temperature part are outside the requirements defined in the present invention. Therefore, a spheroidized structure is not obtained in both the lowest temperature part and the wire coil arranged in the highest temperature part. Therefore, it is thought that cold forgeability has not been improved.

  No. No. 18 is a wire rod arranged in the highest temperature part because the average cooling rate after heating and holding in order to spheroidize and anneal the wire coil arranged in the highest temperature part is out of the requirement defined in the present invention. The coil has no spherical structure. Therefore, it is thought that cold forgeability has not been improved.

  No. No. 21 is because the heating and cooling conditions for spheroidizing annealing of the wire coil disposed in the highest temperature part are outside the requirements defined in the present invention, the wire coil disposed in the highest temperature part is spherical. No chemical organization has been obtained. Therefore, it is thought that cold forgeability has not been improved.

  As described above, according to the present invention, even when a plurality of wire coils are heated in the same furnace and subjected to spheroidizing annealing, both the highest temperature part and the lowest temperature part generated by the temperature distribution in the furnace generated during spheroidizing annealing heating. Can produce a steel material with a spheroidized structure.

Claims (7)

C: 0.7 to 1.5% (meaning mass%, the same shall apply hereinafter),
Cr: less than 0.9% (not including 0%),
A steel material having an average lamella spacing of a pearlite structure of 130 nm or less is heated and spheroidized and annealed to produce a steel for cold forging,
Due to the temperature distribution that occurs during spheroidizing annealing heating, the part that becomes the lowest temperature heating in the steel material meets the following spheroidizing annealing conditions, and the heating temperature of the part that becomes the highest temperature heating is A ₁ point + 25 ° C or more After performing the first spheroidizing annealing,
A method for producing a steel for cold forging, characterized in that the second spheroidizing annealing is performed so that a portion that has been heated at the highest temperature in the first spheroidizing annealing meets the following spheroidizing annealing conditions: .
Spheroidizing annealing conditions: After holding A ₁ point + 5 ° C. to A at a temperature range of ₁ point + 25 ° C. 60 to 360 minutes, cooled, a temperature range from A ₁ point -10 ° C. until A ₁ point -30 ° C. The average Slowly cool at a cooling rate of 10 ° C / hour or less. C: 0.7 to 1.5%,
Cr: less than 0.9% (not including 0%),
A method of producing a steel for cold forging by heating a plurality of steel materials having an average lamella spacing of a pearlite structure of 130 nm or less in the same furnace and spheroidizing annealing,
Due to the temperature distribution in the furnace that occurs during spheroidizing annealing, the steel material at the lowest heating temperature conforms to the following spheroidizing annealing conditions, and the heating temperature of the steel material at the highest heating temperature is After performing the first spheroidizing annealing so that A ₁ point + 25 ° C. is exceeded,
In the steel for cold forging, the second spheroidizing annealing is performed so that the portion of the steel material that has been heated at the highest temperature in the first spheroidizing annealing conforms to the following spheroidizing annealing conditions. Production method.
Spheroidizing annealing conditions: After holding A ₁ point + 5 ° C. to A at a temperature range of ₁ point + 25 ° C. 60 to 360 minutes, cooled, a temperature range from A ₁ point -10 ° C. until A ₁ point -30 ° C. The average Slowly cool at a cooling rate of 10 ° C / hour or less. The heating temperature when the portion that has been heated at the lowest temperature in the first spheroidizing annealing is heated for the second spheroidizing annealing is less than A ₁ point + 5 ° C. The manufacturing method as described in.   The steel material further contains Si: 0.001 to 0.7%, Mn: 0.1 to 2.0%, Al: 0.001 to 0.1%, the balance being iron and inevitable impurities The manufacturing method in any one of Claims 1-3 using this.   The inevitable impurities include P: 0.05% or less (not including 0%), S: 0.001 to 0.05%, N: 0.015% or less (not including 0%). The manufacturing method in any one of -4. The steel material is further
Cu: 0.25% or less (excluding 0%),
Ni: 0.25% or less (excluding 0%),
The element according to any one of claims 1 to 5, comprising at least one selected from the group consisting of Mo: 0.25% or less (excluding 0%) and B: 0.01% or less (not including 0%). The manufacturing method as described. The steel material is further
Ti: 0.2% or less (excluding 0%),
Nb: 0.2% or less (not including 0%) and V: at least one selected from the group consisting of 0.5% or less (not including 0%) The manufacturing method as described.