JP6148994B2 - Forged parts for reduced-pressure high-temperature carburizing treatment and manufacturing method thereof - Google Patents

Description

  The present invention relates to a forged part suitable for reduced-pressure high-temperature carburizing performed at, for example, a carburizing temperature of over 1050 ° C. to about 1100 ° C. and a method of manufacturing the same.

  For example, various forged parts are used in automobiles, construction vehicles, construction equipment, and the like. These forged parts are often carburized to ensure wear resistance and high toughness. On the other hand, the conventional carburizing process is a process that requires a very long time. For this reason, various studies have been made on techniques that can shorten the processing time from the viewpoint of reducing processing costs. Among them, it has been found that a high-temperature carburizing process in which the carburizing temperature is higher than before is effective in shortening the processing time.

  However, the carburizing process at a higher carburizing temperature than the conventional one is effective for shortening the processing time, but may cause a problem in quality. That is, in a forged part subjected to high-temperature carburizing treatment, there is a problem that austenite crystal grains are coarsened during the carburizing treatment, and strength deterioration after quenching and shape deterioration due to distortion are likely to occur. Various proposals have been made as techniques for solving these problems (for example, Patent Documents 1 to 4).

Japanese Patent Laid-Open No. 10-121128 JP 2001-303174 A JP 2003-27135 A JP 2005-139523 A

  The technology of Patent Document 1 regulates the Nb amount and the N amount, coarsely and agglomerates the Nb carbonitride by the stabilization treatment before the carburizing treatment, and generates carbonitride particles that are only partially soluble even at high temperature carburizing. That's it. In the technique of Patent Document 2, the steel is heated at a temperature of 1150 ° C. or higher to perform hot forging, the final processing of hot forging is performed in the temperature range of 900 to 1100 ° C., and then the temperature range of 800 to 500 ° C. is performed. Slow cooling is performed at a cooling rate of 1 ° C./second or less.

  In the technique of Patent Document 3, the steel is heated to 1150 ° C or higher, hot worked at 1000 ° C or higher, cooled to 500 ° C at a rate of 25 ° C / min or higher, and then reheated to a temperature of 900 to 1000 ° C. Then, after heating and holding for 30 minutes or more, cooling to 500 ° C. is performed at a rate of 25 ° C./min or less. In the technique of Patent Document 4, the steel material is heated and held at a temperature of 1200 ° C. or higher for 30 minutes or more, and then finish-rolled at a temperature of 1100 ° C. or higher, and the temperature range up to 500 ° C. after rolling is 25 ° C./min or higher. The forging base material is manufactured by cooling at, and then forged at a specific temperature and subjected to a specific heat treatment.

  However, these prior arts are actually effective to some extent for the production of high-temperature carburizing members at a carburizing temperature of 1050 ° C. or less, but at carburizing temperatures exceeding 1050 ° C., for example, from about 1050 ° C. to about 1100 ° C. When the high temperature carburizing treatment is performed, a sufficient effect cannot be obtained. In particular, the reduced pressure carburizing process performed in an atmosphere reduced in pressure from atmospheric pressure has a problem that abnormal grain growth is more likely to occur, although the effect of shortening the processing time is greater than that of normal gas carburizing. That is, when the member manufactured by the above-described conventional technique is subjected to a reduced pressure carburizing process having a large processing time shortening effect at a carburizing temperature exceeding 1050 ° C., at least, it is difficult to sufficiently suppress grain coarsening. .

  The present invention has been made in view of such a background, and in order to make it possible to significantly reduce the carburizing time compared to the conventional case, the grain coarsening is performed even if a low-pressure high-temperature carburizing process at a carburizing temperature exceeding 1050 ° C. is performed. It is an object of the present invention to provide a forged part and a method for manufacturing the same.

One aspect of the present invention is a method for producing a forged part that is subjected to reduced pressure high temperature carburizing at a carburizing temperature exceeding 1050 ° C.
By mass ratio, C: 0.10 to 0.30%, Si: 0.03 to 1.50%, P: 0.035% or less, S: 0.035% or less, Mn: 0.30 to 1 .50%, Cr: 0.30 to 3.00%, Al: 0.030 to 0.100%, N: 0.0150 to 0.0250%, Nb: 0.08 to 0.12%, Mo: Preparing a forging base material containing 0-0.80%, the balance being Fe and inevitable impurities,
Heating the base material for forging to 1300 ° C. or higher,
Then, forging parts are produced by hot forging at a hot forging temperature of 1230 ° C. or higher without cooling the forging base material to less than 1230 ° C.,
Thereafter, the forged part is maintained at a temperature of 620 to 1000 ° C. for 30 minutes or more, and a precipitation heat treatment for promoting precipitation of carbonitrides is performed.

Another aspect of the present invention is a forged part that is subjected to reduced pressure high temperature carburizing at a carburizing temperature exceeding 1050 ° C.
By mass ratio, C: 0.10 to 0.30%, Si: 0.03 to 1.50%, P: 0.035% or less, S: 0.035% or less, Mn: 0.30 to 1 .50%, Cr: 0.30 to 3.00%, Al: 0.030 to 0.100%, N: 0.0150 to 0.0250%, Nb: 0.08 to 0.12%, Mo: Containing 0-0.80%, the balance consisting of Fe and inevitable impurities,
The forged part for low-pressure high-temperature carburizing treatment is characterized in that the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more in a metal structure is 20 pieces / μm ² or more.

  In the manufacturing method, the forging base material having the specific chemical component is heated to a high temperature of 1300 ° C. or higher before hot forging. By this high-temperature heating, an effect of solid solution of carbonitrides existing in the steel, that is, AlN and Nb carbonitrides, is obtained. Further, after this heat treatment, the forging base material is hot forged at a hot forging temperature of 1230 ° C. or higher without cooling the forging base material to less than 1230 ° C. to produce a forged part. By performing hot forging under such temperature conditions, the metal structure of the forged part is maintained in a state in which AlN, Nb carbonitride, and the like are dissolved in the matrix.

  That is, in this production method, first, AlN and Nb carbonitride in the forging base material are sufficiently dissolved in the parent phase by heating at 1300 ° C. or higher, and the subsequent hot forging is performed at a temperature exceeding 1230 ° C. By carrying out at high temperature, the solid solution state of Nb and Al is maintained during hot forging, and precipitation of AlN and Nb carbonitrides in the metal structure of the forged part is suppressed.

  Then, after that, the forged part is held at a temperature of 620 to 1000 ° C. for 30 minutes or more to perform precipitation heat treatment that promotes precipitation of carbonitride. By carrying out the precipitation heat treatment after securing the above-mentioned solid solution state, carbon nitrides such as AlN and Nb carbonitride having an appropriate size can be precipitated in a relatively large amount. That is, instead of suppressing the large growth of few carbonitrides, a larger number of carbonitrides having an appropriate size are deposited than before.

  As the number of AlN and Nb carbonitrides in the metal structure having an appropriate size is larger, the pinning effect for preventing the coarsening of crystal grains is higher. Therefore, when the forged parts produced by the above manufacturing method are subjected to reduced-pressure high-temperature carburization at a carburizing temperature exceeding 1050 ° C. due to the presence of a large amount of moderately sized AlN and Nb carbonitride in the metal structure. Also, it is possible to suppress the coarsening of the crystal grains.

  Thus, according to the said manufacturing method, the forged component which can suppress a crystal grain coarsening can be manufactured even if it performs the pressure reduction high temperature carburizing process in the carburizing temperature exceeding 1050 degreeC.

In particular, forged parts having the above-mentioned specific chemical components and the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more in the metal structure are 20 pieces / μm ² or more are AlN and Nb carbonitrides. The pinning effect by carbonitrides such as is sufficiently excellent even in the temperature range from 1050 ° C. to 1100 ° C.

FIG. 6 is an explanatory diagram showing the relationship between the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more and the average grain size number after carburizing treatment when carburizing at 1100 ° C. in a carburizing temperature in Example 4;

The reasons for limiting the chemical components of the ingot and forged parts will be described below.
C: 0.10 to 0.30%,
C (carbon) is an element necessary for ensuring the strength and internal hardness required for parts subjected to carburizing treatment. In order to obtain the effect, C (carbon) needs to be contained in an amount of 0.10% or more. It is. On the other hand, when C is excessively contained, internal toughness deterioration, machinability reduction, cold forgeability deterioration, and the like are caused, so the C content is 0.30% as an upper limit.

Si: 0.03 to 1.50%,
Si (silicon) is an element necessary for deoxidation during the production of steel, and in order to obtain the effect, the content of 0.03% or more is necessary. On the other hand, if Si is excessively contained, toughness deterioration and workability deterioration are caused, so the upper limit of Si content is 1.50%.

P: 0.035% or less,
P (phosphorus) is an impurity that cannot be avoided during production. P lowers the grain boundary strength and deteriorates fatigue characteristics, so the upper limit of the P content is 0.035%.

S: 0.035% or less,
Similar to P, S (sulfur) is an impurity that cannot be avoided during production. Since S exists as sulfide inclusions such as MnS, for example, and causes a decrease in fatigue strength, the upper limit of the S content is 0.035%.

Mn: 0.30 to 1.50%,
Mn (manganese) is an element necessary for improving the hardenability and securing the strength to the inside, and in order to obtain the effect, it is necessary to contain 0.30% or more. On the other hand, when a large amount of Mn is contained, retained austenite increases, causing a decrease in hardness, deterioration of internal toughness, a decrease in machinability, and the like, so the upper limit of the Mn content is 1.50%. .

Cr: 0.30 to 3.00%
Cr (chromium) is an element necessary for improving the hardenability and securing the strength to the inside, and in order to obtain the effect, it is necessary to contain 0.30% or more. On the other hand, if Cr is contained in a large amount, it causes toughness deterioration and machinability deterioration, so the upper limit of Cr content is 3.00%.

Al: 0.030 to 0.100%,
Al (aluminum) is an element necessary for deoxidation similarly to Si, and is an element effective for preventing abnormal grain growth after carburizing treatment by exhibiting a pinning effect when present as AlN. In order to secure the amount of AlN necessary for obtaining this effect, it is necessary to contain 0.030% or more of Al. On the other hand, if the Al content is increased to some extent, the pinning effect is saturated even if the Al content is increased further, and the effect of preventing abnormal grain growth is not improved. On the other hand, Al produced in the steel as the Al content increases. _{Since 2} O ₃ inclusions increase and adversely affect the strength and machinability, the upper limit of the Al content is 0.100%.

N: 0.0150-0.0250%,
N (nitrogen) is combined with Al and Nb by precipitation heat treatment to be described later after forging, becomes AlN and Nb carbonitride, precipitates in steel, and prevents abnormal grain growth after carburizing treatment. It is an effective element. In order to obtain this effect, it is necessary to contain 0.0150% or more of N. On the other hand, if the amount of N is too large, the effect of coarsening the crystal grains is saturated and the fatigue strength may be lowered. Therefore, the upper limit of the N content is 0.0250%.

Nb: 0.08 to 0.12%,
Nb (niobium) is an element having an effect of preventing abnormal growth of crystal grains in carburizing treatment at a higher temperature than Al by being present in the steel as carbonitride. When the amount of Nb added is small, the amount of Nb carbonitride that contributes to the pinning effect is insufficient and an effect of suppressing abnormal grain growth cannot be obtained sufficiently, so the lower limit of the Nb content is 0.08%. . On the other hand, if the Nb content is too large, it becomes difficult to sufficiently dissolve the Nb carbonitride (Nb (C, N)) in the forging base material even by heating at 1300 ° C. or higher. The upper limit is 0.12%.

Mo: 0 to 0.80%,
Mo (molybdenum) is an optional element and need not necessarily be contained. On the other hand, since Mo is effective for improving hardenability, it can be contained in order to ensure the necessary hardenability according to the size of the forged part. However, if the Mo content is too large, the toughness and machinability may be reduced, so the upper limit when Mo is contained is 0.80%.

Next, manufacturing conditions in the above manufacturing method will be described.
In the manufacturing method described above, a forging base material prepared by cutting a steel material having a cross-sectional dimension suitable for component manufacturing by rolling or the like into a predetermined dimension is heated to 1300 ° C. or higher. By heating this forging base material at a very high temperature, as described above, the effect of sufficiently dissolving AlN and Nb carbonitride existing in the steel having the specific chemical component in the matrix phase. Is obtained. That is, the steel material used for the base material for forging is Nb in comparison with Nb-added case-hardened steel, which is conventionally used in many cases, in order to reliably obtain the effect of suppressing abnormal grain growth when carburized at a higher temperature than before. Since the contents of Al and N are increased, when the heating temperature of the forging base material is less than 1300 ° C., it is difficult to sufficiently dissolve the precipitate.

  After heating the forging base material, the forging base material is manufactured by hot forging at a hot forging temperature of 1230 ° C. or higher without cooling the forging base material to less than 1230 ° C. In this way, by performing hot forging without cooling the forging base material to less than 1230 ° C., it is possible to obtain a forged product that maintains a state where Nb and Al are sufficiently dissolved in steel. it can. On the other hand, when hot forging is performed by cooling to less than 1230 ° C before or during hot forging, Al and Nb dissolved in the steel are precipitated as AlN and Nb carbonitride, Since this precipitate grows during the precipitation heat treatment, the number of precipitates decreases, and the number of AlN and Nb carbonitrides necessary to suppress the grain coarsening in the reduced pressure carburizing process exceeding 1050 ° C. is precipitated. It becomes difficult.

  The precipitation heat treatment is performed by first cooling the forged part after hot forging and then reheating (first precipitation heat treatment method), and by the cooling process after hot forging (second precipitation heat treatment). Method). Further, both of these two types of heat treatment methods can be combined.

  The specific precipitation heat treatment of the first precipitation heat treatment method is performed by cooling the forged part after hot forging to at least 500 ° C. or less and then reheating it and holding it at 850 ° C. to 1000 ° C. for 30 minutes or more. Can be carried out by cooling at a cooling rate of 25 ° C./min or less.

  In this case, the forged part after hot forging is cooled to 500 ° C. or lower before reheating, so that the state in which Al and Nb are dissolved can be maintained. The cooling rate in this case does not need to be controlled as long as it is cooled by a method that does not actively slow down the cooling rate, but in order to maintain the above solid solution state sufficiently, it is higher than the cooling rate obtained by so-called air cooling. It is preferable that

  After the cooling, the forged part is reheated and held at 850 ° C. to 1000 ° C. for 30 minutes or more. Thereby, many moderately sized AlN and Nb carbonitrides can be precipitated. When this reheating temperature is less than 850 ° C. and when the holding time is less than 30 minutes, the austenite of the metal structure becomes insufficient, resulting in poor hardness after processing, non-uniform microstructure, etc., and workability. There is a possibility that it may cause a decrease in carburization and deterioration of carburization distortion. In addition, when the reheating temperature exceeds 1000 ° C., AlN and Nb carbonitride grow excessively, while the number of them decreases, which may reduce the pinning effect.

  After the reheating, the forged part is cooled. In this case, the cooling rate is 25 ° C./min or less until at least 500 ° C. is reached. Thereby, the quenching effect is minimized and the workability can be maintained. On the other hand, when the cooling rate exceeds 25 ° C./min, the hardness of the forged part is increased, and the subsequent workability may be reduced.

  The specific precipitation heat treatment of the second precipitation heat treatment method can be performed by holding the forged part at a temperature of 620 to 700 ° C. for 30 minutes or more during the cooling of the forged part after hot forging. In this case, it is possible to precipitate a large number of moderately sized AlN and Nb carbonitrides by keeping the forged parts after hot forging at 620 ° C. to 700 ° C. without cooling them to 620 ° C. or lower. it can. When the holding temperature is less than 620 ° C. or the holding time is less than 30 minutes, the precipitation of AlN and Nb carbonitride may not proceed sufficiently. On the other hand, when the holding temperature exceeds 700 ° C., a bainite structure is likely to be generated after the processing, and machinability is deteriorated and crystal grain coarsening after the reduced-pressure high-temperature carburizing processing is likely to occur.

And it is preferable to set conditions for the said precipitation heat treatment so that the precipitation number of carbonitride having an equivalent circle diameter of 20 nm or more in the forged part is 20 pieces / μm ² or more by the precipitation heat treatment. If one or both of the above two specific types of precipitation heat treatment are employed, this precipitation state can be easily realized. In addition, if the forged parts after the hot forging are kept at a temperature of 620 to 1000 ° C. for 30 minutes or more, it is possible to obtain the same precipitation state even if conditions other than the above two kinds of precipitation heat treatment are adopted. It is possible to suppress crystal grain coarsening during reduced-pressure carburization at over 1050 ° C. On the other hand, in order to more reliably manufacture a forged part that satisfies other characteristics than the suppression of grain coarsening, it is preferable to employ the two types of specific precipitation heat treatments.

Even if a large amount of carbonitride having an equivalent circle diameter of less than 20 nm is present in the metal structure of the forged part, the pinning effect is not so much exhibited. Further, when the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more is less than 20 / μm ^2, a sufficient pinning effect cannot be obtained.

  The carbonitride is substantially composed of AlN and Nb carbonitride, but other types of carbonitride, for example, Ti carbonitride contained as impurities, are present. Including.

Example 1
Examples of the forged part for low-pressure high-temperature carburizing treatment and the manufacturing method thereof will be described. In this example, as shown in Table 1, a plurality of types of steel materials (samples 1 to 10) having different chemical components are prepared, a forged part is prepared by one type of manufacturing method, and grain coarsening by reduced-pressure high-temperature carburizing treatment is performed. The presence or absence was evaluated. In this example, the first precipitation heat treatment method described above was employed as the precipitation heat treatment method.

  Each sample was produced as follows. First, steel ingots having chemical components shown in Table 1 were prepared. Specifically, melting, refining and casting of the raw materials of each sample were performed using a VIM (Vacuum Induction Melting) to obtain a steel ingot. A forging base material having a diameter of 65 mm × 120 mm is prepared from the steel ingot by forging and the like, heated to 1330 ° C., and then cooled to less than 1230 ° C. at a hot forging temperature of 1280 ° C. Forged parts were produced by hot forging. The temperatures described are all surface temperatures (the same applies hereinafter).

  The forged parts after hot forging were cooled to room temperature under air cooling conditions of 100 ° C./min up to a cooling rate of about 600 ° C. and 20 ° C./min in a temperature range of 600 ° C. or lower. Thereafter, the forged part was reheated and held at 900 ° C. for 60 minutes, and then cooled to 500 ° C. at a cooling rate of about 30 ° C./min. The obtained forged parts were subjected to reduced-pressure high-temperature carburizing treatment at three carburizing temperatures of 1060 ° C., 1080 ° C., and 1100 ° C. And the metal structure before and behind carburizing treatment was observed and the quality was evaluated.

  The reduced pressure carburizing treatment was performed in a reduced pressure atmosphere in which the pressure in the furnace during the carburizing period was 150 Pa for a total of about 5 minutes including the carburizing period and the diffusion period. As the atmospheric gas, acetylene gas was used, and the carburizing process was performed by a pulse carburizing method (see, for example, JP-A-2004-300520). Moreover, after the carburizing treatment, the forged parts were quenched by gas cooling using nitrogen gas. The metal structure mentioned later was observed using the forged parts processed so far.

For the observation of the metal structure, the number of carbonitrides (pieces / μm ² ) before carburizing treatment of forged parts and the presence or absence of grain coarsening after carburizing treatment were evaluated. In the determination of the coarsening of the crystal grains, the measurement method described in JIS G0551 has an average of 6 or more and the case where there is no 3 or less portion is good (◯), the average is less than 6 or the 3 or less portion Is at least partially defective (Δ), the average is less than No. 6, and the case where the number 3 or less is at least partially present is determined as defective (×). The evaluation results are shown in Table 2.

The number of carbonitrides (precipitated particles) was measured as follows.
The sample for observation was adjusted by a commonly used replica method. Specifically, first, the steel material of each sample was mirror-polished and the polished surface was etched with nital (nitric acid 3% ethanol solution) for about 30 seconds. Next, a carbon film (replica film) having a film thickness of about 200 nm was deposited on the etching surface by a vacuum deposition apparatus. The sample provided with the replica film was immersed in nital for about 30 minutes, and the replica film was removed from the steel material by dissolving the interface between the steel material and the replica film. Thereafter, the replica membrane was washed three times with distilled water and dried to obtain a sample for observation.
This replica film was set on a transmission electron microscope, and 20 TEM images were taken for each execution condition under a condition of 200,000 times. All the photographs of the TEM images were binarized and subjected to image analysis, the equivalent circle diameter and number were calculated, and the number per unit area and the particle size distribution were determined to quantify the number of precipitated particles of 20 nm or more.

  As is known from Table 2, the samples (samples 1 to 6) with appropriate chemical components were good without any grain coarsening under any of the carburizing temperatures of 1060 ° C to 1100 ° C. On the other hand, the samples (samples 7 to 10) in which at least one component of Al, N, and Nb is below the lower limit value of the limited range, at least when the carburizing temperature is 1100 ° C., almost the entire surface of the observed metal structure is coarse. It was converted.

(Example 2)
In this example, a plurality of types of forging base materials (samples 1 to 10) having different chemical components shown in Table 1 of Example 1 are prepared, and a forged part is produced by one type of manufacturing method different from Example 1. Then, the presence or absence of crystal grain coarsening due to reduced-pressure high-temperature carburizing treatment was evaluated. In this example, the second precipitation heat treatment method described above was employed as the precipitation heat treatment method.

  In this example, each sample was produced as follows. First, steel ingots having chemical components shown in Table 1 were prepared. Specifically, melting, refining and casting of the raw materials of each sample were performed using a VIM (Vacuum Induction Melting) to obtain a steel ingot. A base material for forging is prepared from this steel ingot in the same manner as in Example 1, heated to 1330 ° C., and then forged by hot forging at a hot forging temperature of 1280 ° C. without cooling to less than 1230 ° C. Parts were produced. The steps so far are the same as in the first embodiment.

  The forged parts after hot forging are put into a 650 ° C. atmosphere heating furnace in the middle of cooling, held for 60 minutes in a state where the temperature is not lowered to that temperature, and then cooled by air at about 20 ° C./min. At room temperature. The obtained forged parts were subjected to reduced-pressure high-temperature carburizing treatment at three kinds of carburizing temperatures of 1060 ° C., 1080 ° C. and 1100 ° C. in the same manner as in Example 1. And the metal structure before and behind carburizing treatment was observed and the quality was evaluated. The evaluation method was the same as in Example 1. The evaluation results are shown in Table 3.

  As is known from Table 3, also in this example, for the samples (samples 1 to 6) with appropriate chemical components, no coarsening of crystal grains occurs in any of the carburizing temperatures of 1060 ° C to 1100 ° C. It was good. On the other hand, in the samples (samples 7 to 10) in which at least one component of Al, N and Nb is below the lower limit value of the limited range, at least when the carburizing temperature is 1100 ° C., almost the entire surface of the observed metal structure is coarse. It was converted.

(Example 3)
In this example, a plurality of one kind of samples (sample 2) in Table 1 of Example 1 are prepared, forged parts are produced by a plurality of production methods having different production conditions, and whether or not grain coarsening is caused by reduced-pressure high-temperature carburizing treatment. Evaluated.

  As shown in Table 4, the manufacturing conditions are as follows: the heating temperature of the forging base material before the hot forging step (a) the hot forging temperature (b) when performing hot forging, and the room temperature after hot forging. The cooling condition (c), the reheating temperature (d) in the case of reheating and the holding temperature (e), and the cooling condition (f) after the reheating in the case of reheating were variously changed. Evaluation similar to Example 1 was performed with respect to the obtained forged part. The evaluation results are shown in Table 5.

  As is known from Table 5, the heating temperature (a) is 1300 ° C. or higher, the hot forging temperature (b) of hot forging performed continuously thereafter is 1230 ° C. or higher, and the conditions for the precipitation heat treatment ( When (c) to (f)) have the above-described conditions of either the first precipitation heat treatment method or the second precipitation heat treatment method (conditions 1 to 11) and both conditions (condition 12) In the case of (1), the carburization temperature was good without any coarsening of the crystal grains under any conditions of 1060 ° C to 1100 ° C.

  On the other hand, at least when any of the above conditions (a) to (d) is inappropriate (conditions 13 to 17), at least when the carburizing temperature is 1100 ° C., almost the entire surface of the observed metal structure is coarse. It was converted.

Example 4
In the above-described Examples 1 to 3, forged parts subjected to high-pressure carburization under reduced pressure at a carburizing temperature of 1100 ° C., the relationship between the number of precipitated carbonitrides with an equivalent circle diameter of 20 nm or more and the average grain size number after carburizing was investigated. . The relationship between the two is shown in FIG. In the figure, the horizontal axis represents the number of precipitated carbonitrides with a circle equivalent diameter of 20 nm or more (pieces / μm ² ), and the vertical axis represents the average grain size number.

As is known from the figure, in the case of carrying out an ultra-high temperature carburizing treatment of 1100 ° C., at least the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more is 20 (pieces / μm ² ) or more. The average grain size number can be 6 or more. From this, when judged by at least the average grain size number, the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more is set to 20 (pieces / μm ² ) or more. It can be seen that this is effective in preventing grain coarsening in the forged parts.

Claims (5)

A method for producing a forged part that is subjected to high-pressure carburization under reduced pressure at a carburizing temperature exceeding 1050 ° C,
By mass ratio, C: 0.10 to 0.30%, Si: 0.03 to 1.50%, P: 0.035% or less, S: 0.035% or less, Mn: 0.30 to 1 .50%, Cr: 0.30 to 3.00%, Al: 0.030 to 0.100%, N: 0.0150 to 0.0250%, Nb: 0.08 to 0.12%, Mo: Preparing a forging base material containing 0-0.80%, the balance being Fe and inevitable impurities,
Heating the base material for forging to 1300 ° C. or higher,
Then, forging parts are produced by hot forging at a hot forging temperature of 1230 ° C. or higher without cooling the forging base material to less than 1230 ° C.,
Then, the forged part is maintained at a temperature of 620 to 1000 ° C. for 30 minutes or longer, and a precipitation heat treatment for promoting precipitation of carbonitride is performed.   In the precipitation heat treatment, the forged part after hot forging is cooled to at least 500 ° C. or less, reheated and held at 850 ° C. to 1000 ° C. for 30 minutes or more, and then cooled to 500 ° C. at 25 ° C./min or less. The method for producing a forged part for reduced-pressure high-temperature carburizing treatment according to claim 1, wherein the method is performed by cooling at a speed.   2. The reduced-pressure high-temperature carburization according to claim 1, wherein the precipitation heat treatment is performed by holding the forged part at a temperature of 620 to 700 ° C. for 30 minutes or more during the cooling of the forged part after hot forging. Manufacturing method of forged parts for processing. The reduced pressure and high temperature according to any one of claims 1 to 3, wherein the precipitation heat treatment sets the number of precipitation of carbonitride having an equivalent circle diameter of 20 nm or more in the forged part to 20 pieces / µm ² or more. A method for manufacturing forged parts for carburizing. A forged part that is subjected to high-pressure carburization under reduced pressure at a carburizing temperature exceeding 1050 ° C,
By mass ratio, C: 0.10 to 0.30%, Si: 0.03 to 1.50%, P: 0.035% or less, S: 0.035% or less, Mn: 0.30 to 1 .50%, Cr: 0.30 to 3.00%, Al: 0.030 to 0.100%, N: 0.0150 to 0.0250%, Nb: 0.08 to 0.12%, Mo: Containing 0-0.80%, the balance consisting of Fe and inevitable impurities,
A forged part for reduced-pressure high-temperature carburizing treatment, wherein the number of precipitated carbonitrides having an equivalent circle diameter of 20 nm or more in a metal structure is 20 pieces / μm ² or more.

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