JP2001303174A - Shaped material for high-temperature carburized parts having excellent crystal grain coarsening prevention properties and method for producing the same - Google Patents

Abstract

(57) [Problem] The present invention does not require normalizing treatment after hot forging, and can stably suppress generation of coarse grains in high-temperature carburizing treatment. Provided is a shaped member for a high-temperature carburized part and a method for producing the same. SOLUTION: Al: 0.015 to 0.045%, N
b: 0.005 to 0.050%, N: 0.01 to 0.0
2% and other specific components in a specific range, Nb after hot forging
The precipitation amount of (CN) is 0.005% or more, the precipitation amount of AlN is limited to 0.01% or less, and the bainite fraction in the structure after hot forging is limited to 10% or less. A shaped material for high-temperature carburized parts, wherein the pearlite fraction in the structure after forging is 75% or less and the ferrite crystal grain size number is 7 to 10. Further, the steel having the above composition was added to 1150
Hot forging by heating at a temperature of at least 100 ° C., the final working of hot forging is performed at a temperature in the range of 900 to 1100 ° C., and then the temperature in the range of 800 to 500 ° C. is gradually reduced at a cooling rate of 1 ° C./sec or less. A method for producing a shaped material for a high-temperature carburized part, characterized by cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast material for high-temperature carburized parts formed by hot forging, and more particularly, to an excellent property of preventing crystal grain coarsening which does not require normalizing after hot forging. And a method for producing the same.

[0002]

2. Description of the Related Art Gears, bearing parts, rolling parts, and shafts are usually made of, for example, JIS G 4052, JIS G
4104, JIS G 4105, JIS G 410
It is manufactured by using a medium carbon alloy steel for machine structural use specified in No. 6 or the like, working it into a predetermined shape by hot forging, normalizing, and cutting, and then carburizing and quenching. In the above-mentioned manufacturing process, the normalizing process is a process of heating to a temperature range of about 900 to 950 ° C. to once austenite, and then adjusting to a ferrite-pearlite structure by cooling.

[0003] Since hot forging is usually carried out in a high temperature range of 1100 to 1300 ° C, the structure as hot forged is a non-uniform and hard structure containing one or more coarse ferrite, pearlite, and bainite structures. Become an organization. Therefore, it is difficult to perform cutting work because it is hard in the state of hot forging.
Further, if the carburizing treatment is performed in the state of hot forging, since the original structure is coarse and non-uniform, "coarse grains" in which some crystal grains are coarsened during carburizing heating are generated. The coarsening of the crystal grains of the carburized part is a major cause of heat treatment distortion, and a large heat treatment distortion causes noise and vibration. Therefore, at present, the structure before carburizing is made into a relatively soft and homogenous ferrite-pearlite structure by normalizing after hot forging, the cutting workability is improved by softening, and the homogeneity during carburizing is improved. The crystal grains are prevented from becoming coarse. In recent years, from the viewpoint of energy saving and parts manufacturing cost reduction,
Although it is required to omit the normalizing step, at present, it is not possible to omit the normalizing step due to the above-described problems of machinability and coarsening of crystal grains.

On the other hand, among bearing parts and rolling parts to which high surface pressure is applied, deep carburization is performed. Since deep carburization usually requires a long time of about ten to several tens of hours, shortening the carburizing time is an important issue from the viewpoint of energy saving. To shorten the carburizing time, it is effective to increase the carburizing temperature. Normal carburizing temperature is 930
However, if high-temperature carburizing is performed in a temperature range of 1000 to 1050 ° C., the carburizing time can be shortened to about に 対 し て, so that there is a great need for raising the carburizing temperature. However, when high-temperature carburizing is performed, coarse particles are generated, and there is a problem that necessary material properties such as rolling fatigue properties cannot be obtained. The reason is that fine pinning particles (AlN
Etc.) are aggregated and coarsened, and the pinning effect is reduced due to a decrease in the number of pinned particles. As described above, if the structure before carburizing is adjusted to homogenous ferrite pearlite by normalizing, it is possible to prevent the generation of coarse grains even with conventional steel in the case of normal carburizing, but not in the case of high-temperature carburizing.

On the other hand, the present inventors have already published WO 99/05333 (Japanese Patent Application No. 11-5096).
In 60), the amount of precipitation of Nb (CN) and AlN after hot rolling or hot forging of steel containing specific amounts of Al, Nb and N is defined, and fine AlN and Nb as pinning particles are defined.
A technique for preventing the generation of coarse particles even in high-temperature carburizing by dispersing a large amount of (CN) during carburizing heating is shown. However, this technique is based on the premise that normalizing is performed after hot forging, and it is necessary to perform normalizing after hot forging from the viewpoint of cutting workability and restrictions on preventing generation of coarse grains. That is, this technique cannot omit normalization when a cutting process is performed after hot forging. Further, if normalizing is performed after hot forging, generation of coarse grains can be prevented even in high-temperature carburizing, but it cannot be prevented when high-temperature carburizing is performed in the state of normal hot forging.

As described above, there is still no technique that can prevent the generation of coarse grains during high-temperature carburizing and can omit normalization.

[0007]

In the disclosed method as described above, it is not possible to prevent the generation of coarse grains during high-temperature carburizing and to omit normalizing. The present invention solves such a problem and provides a cast material for high-temperature carburized parts and a method for producing the same, which does not require a normalizing process after hot forging and has excellent crystal grain coarsening prevention properties.

[0008]

The gist of the present invention is as follows.

(1) In mass%, C: 0.1 to 0.4
%, Si: 0.02 to 1.3% Mn: 0.3 to 1.8
%, S: 0.001 to 0.15%, Al: 0.015
0.045%, Nb: 0.005 to 0.05%, N:
0.01 to 0.02%, and Cr: 0.4
-1.8%, Mo: 0.02-1.0%, Ni: 0.1
３3.5%, V: 0.03 to 0.5%, one or more of them, P: 0.025% or less, Ti: 0.01%
Below, O: each is limited to 0.0025% or less, and the remainder is F
e and unavoidable impurities, and Nb (C
The precipitation amount of N) is 0.005% or more, the precipitation amount of AlN is limited to 0.01% or less, the bainite fraction in the structure after hot forging is limited to 10% or less, and hot forging is performed. The pearlite fraction in the subsequent structure is 75% or less, and the ferrite crystal grain size number is 7 to 10. Excellent in crystal grain coarsening prevention properties that do not require normalizing after hot forging. Shaped material for high-temperature carburized parts.

(2) The steel having the chemical composition described in the above (1) is heated at a temperature of 1150 ° C. or more to perform hot forging, and the final processing of the hot forging is performed at a temperature of 900 to 1100 ° C. And then increase the temperature range from 800 to 500 ° C to 1 ° C
A method for producing a cast material for high-temperature carburized parts having excellent crystal grain coarsening prevention characteristics that does not require normalizing after hot forging, characterized by gradually cooling at a cooling rate of not more than / sec.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted intensive studies on the factors governing the coarsening of crystal grains in order to achieve the above object, and have clarified the following points.

(1) Even with steel materials having the same chemical composition, the generation of coarse particles may or may not be suppressed.
Coarse grains cannot be prevented only by limiting the chemical composition. As a factor other than the chemical composition, the precipitation state of carbonitride of the steel material after hot forging is important.

(2) To prevent coarsening of crystal grains during carburization, fine AlN, Nb (CN)
The point is to disperse a large amount of carbon during carburizing heating.

(3) To stably exert the pinning effect of Nb (CN) during carburizing heating, a predetermined amount or more of Nb (CN) should be finely precipitated in advance in the steel material after hot forging. is necessary. Also, coarse AlN is precipitated on the steel material after hot forging, or TiN or A
When l ₂ O ₃ is present, it serves as a nucleus for the deposition of coarse Nb (CN), and prevents the fine precipitation of Nb (CN). for that reason,
It is necessary to limit the contents of Ti and O as impurities as much as possible.

(4) In order to stably exhibit the pinning effect of AlN during carburizing heating, contrary to the case of Nb (CN), the precipitation amount of AlN should be minimized in the state of the steel material after hot forging. It needs to be restricted. This is because Nb (C
This is an essential requirement for the fine precipitation of N). If TiN or Al ₂ O ₃ is present in the steel material after hot forging, it becomes a nucleus of AlN precipitation, and the amount of AlN deposited increases, so that the contents of Ti and O as impurities are also reduced. Must be restricted as much as possible.

(5) In the state of the steel material after hot forging, AlN
In order to limit the precipitation amount of as much as possible, it is necessary to set the heating temperature of the forging as high as possible.

(6) In order to pre-deposit a predetermined amount or more of Nb (CN) in the steel material after hot forging in advance, the heating temperature of the hot forging is set to be as high as possible, and Nb (CN) is once added to the matrix. Nb (CN) after hot forging
By gradually cooling the precipitation temperature range of Nb, a large amount of Nb (CN) can be finely dispersed.

(7) Even if the regulation of carbonitrides is satisfied as described above, if bainite structure is mixed in a steel material after hot forging in a certain amount or more, coarse steel grains may be generated during carburizing heating. However, the increase in hardness makes it difficult to perform cutting before carburizing.

(8) In order to prevent bainite from being mixed as much as possible in the state of the steel material after hot forging and to obtain a ferrite-pearlite structure equivalent to normalizing without going through a normalizing step, hot forging is performed. It is necessary to make the structure finer by lowering the final processing temperature than before, and to optimize the subsequent cooling conditions.

(9) Further, when the final working temperature of the hot forging is too low, the ferrite grains after the hot forging become excessively fine, so that the austenite grains in the surface layer during carburization also become excessively fine, and When the hardenability of the steel is reduced, a carburized abnormal structure such as bainite is easily generated in the surface layer. On the other hand, if the final processing temperature is too high, the amount of coarse pearlite increases. When such coarse pearlite increases, austenite grains during carburization become mixed grains, and coarse grains are generated. That is, there is a close relationship between the austenite grain size during carburization and the structure before carburizing, and it is necessary to optimize the structure before carburizing in order to obtain a desired structure after carburizing.

(10) As described above, in order to optimize the structure before carburizing, it is necessary to perform the final working temperature of hot forging within a certain optimum range and then optimize the cooling conditions.

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the components will be described.

C is an element effective for imparting necessary strength to steel, but if it is less than 0.1%, the required tensile strength cannot be secured, and if it exceeds 0.4%, it becomes hard. Since the cold workability deteriorates and the core toughness after carburization deteriorates, it is necessary to be within the range of 0.1 to 0.4%.

Si is an element effective for deoxidizing steel, and is also an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. The effect is inadequate. On the other hand, when the content exceeds 1.3%, hardness is increased, and workability is deteriorated. For the above reasons, the content needs to be within the range of 0.02 to 1.3%. Si is an element effective for increasing grain boundary strengthening,
Further, in bearing parts and rolling parts, it is an element effective for prolonging the life by suppressing the structural change and material deterioration in the rolling fatigue process. However, on the other hand, it is an element that promotes internal oxidation during carburization, and internal oxidation becomes the starting point of fatigue cracks,
In the case of parts used for heat-treated skin as it is carburized, it is desirable to lower it. Therefore, in the case of a part whose surface layer is removed by working after carburizing, it is desirable to set the range to 0.4 to 1.3% in order to increase the strength, and in the case of a part used for a heat-treated skin as it is carburized. Is 0.02-0.4
It is desirable to be within the range.

Mn is an element effective for deoxidizing steel and is an element effective for imparting necessary strength and hardenability to steel. However, if it is less than 0.3%, the effect is insufficient. .
If it exceeds 8%, the effect is not only saturated, but also the hardness is increased, and the workability is deteriorated.
Must be within the range. The preferred range is 0.5 to 1.2
%.

S forms MnS in the steel and is added for the purpose of improving the machinability. However, if it is less than 0.001%, the effect is insufficient. On the other hand, if the content exceeds 0.15%, the effect is saturated, and rather, grain boundary segregation is caused, which causes grain boundary embrittlement. For the above reasons, the content of S is set to 0.001.
Must be within the range of ~ 0.15%. In bearing parts and rolling parts, it is necessary to reduce S as much as possible because MnS deteriorates rolling fatigue life.
It is desirable to set it in the range of 0.01%.

Al is an element that forms AlN by combining with N in steel during carburizing heating and is effective in refining crystal grains and coarsening crystal grains. If it is less than 0.015%, the effect is insufficient. On the other hand, if it exceeds 0.045%, A
The precipitate of 1N becomes coarse and does not contribute to suppression of coarsening of crystal grains. For the above reasons, the content is 0.01
It must be within the range of 5 to 0.045%. The preferred range is 0.02 to 0.035%.

Nb combines with C and N in the steel to form Nb (CN) during carburizing heating, and is an element effective in miniaturizing crystal grains and suppressing coarsening of crystal grains. 0.005%
If less, the effect is insufficient. On the other hand, if the content exceeds 0.05%, the hardness of the material increases, the workability is deteriorated, and the precipitate of Nb (CN) becomes coarse, which does not contribute to suppressing the coarsening of the crystal grains. For the above reasons, the content needs to be in the range of 0.005 to 0.05%. A preferred range is 0.015 to 0.04%.

N is added for the purpose of refining the crystal grains during carburization by precipitation of AlN and Nb (CN) and suppressing the coarsening of the crystal grains, but if the content is less than 0.01%, the effect is insufficient. . On the other hand, if it exceeds 0.02%, the effect is saturated. Excessive N addition embrittles steel, so casting,
It causes cracks and scratches during rolling. For the above reasons, its content needs to be within the range of 0.01 to 0.02%. A preferred range is 0.013 to 0.02%.

Next, in the present invention, Cr, Mo, Ni, V
Or one or more of these.

[0032] Cr is an element effective for imparting strength and hardenability to steel. In addition, in bearing parts and rolling parts, it is effective for prolonging the life by suppressing the structural change and material deterioration in the rolling fatigue process. Element. If the content is less than 0.4%, the effect is insufficient. If the content exceeds 1.8%, the hardness is increased and the workability is deteriorated. For the above reasons, the content needs to be in the range of 0.4 to 1.8%. The preferred range is 0.7-1.6%.

Mo is also an effective element for imparting strength and hardenability to steel. Further, in bearing parts and rolling parts, Mo is effective for prolonging the life by suppressing the structural change and material deterioration in the rolling fatigue process. Element. If it is less than 0.02%, the effect is insufficient, and if it exceeds 1.0%, the hardness is increased and the workability is deteriorated. For the above reasons, the content needs to be within the range of 0.02 to 1.0%. The preferred range is 0.02 to 0.5%.

Ni is an effective element for imparting strength, hardenability and toughness to steel, but its effect is insufficient when it is less than 0.1%, and when it exceeds 3.5%, the hardness is lowered. As a result, the workability deteriorates. For the above reasons, the content needs to be within the range of 0.1 to 3.5%. The preferred range is 0.4-2.0%.

V is an element effective for imparting strength and hardenability to steel, but its effect is insufficient when it is less than 0.03%, and when it exceeds 0.5%, the hardness increases. Invited workability deteriorates. For the above reasons, the content is set to 0.
It must be in the range of 03-0.5%. The preferred range is 0.07-0.2%.

Since P deteriorates the fatigue strength by making the grain boundaries of the carburized parts brittle, it is desirable to reduce P as much as possible. Therefore, it is necessary to limit the content to 0.025% or less. A preferred range is 0.015% or less.

In high N steels such as the present invention, Ti combines with N in the steel to form TiN. The precipitate of TiN is coarse and does not contribute to the refinement of the crystal grains during carburization and the suppression of the coarsening of the crystal grains. Rather, if TiN is present, it becomes a precipitation site for AlN or Nb (CN), and AlN or Nb (CN) precipitates coarsely during hot rolling, making it impossible to suppress coarsening of crystal grains during carburizing. Therefore, it is desirable to reduce the amount of Ti as much as possible. For the above reasons, it is necessary to limit the content of Ti to 0.01% or less. In bearing components and rolling fatigue components, the presence of coarse TiN causes remarkable deterioration of rolling fatigue characteristics of final components. Therefore, when applied as a bearing component or a rolling component, it is desirable to limit the content of Ti to 0.0025% or less.

In a high Al steel as in the present invention, O forms oxide inclusions such as Al ₂ O _{3 in} the steel. If oxide-based inclusions are present in a large amount in steel, AlN and Nb
(CN) precipitation site, and AlN or N
b (CN) precipitates coarsely, making it impossible to suppress the coarsening of crystal grains during carburization. Therefore, it is desirable to reduce the amount of O as much as possible. For the above reasons, the content is set to 0.
It must be limited to 0025% or less. The preferred range is 0.
002% or less. In bearing components and rolling components, oxide inclusions become the starting point of rolling fatigue fracture, so that the lower the O content, the longer the rolling life. Therefore, in the bearing parts and the rolling parts, the O content is set to 0.0
It is desirable to limit it to 012% or less.

In the present invention, the precipitation amount of Nb (CN) after hot forging is 0.005% or more, and the precipitation amount of AlN is limited to 0.01% or less (including 0%). The reason for the limitation is described below.

In order to prevent the crystal grains from becoming coarse during carburization,
It is effective to disperse a large amount of fine AlN and Nb (CN) as pinning particles during carburization. Coarse AlN,
Nb (CN) is not only completely useless to prevent coarsening of crystal grains during carburization, but rather acts to reduce the number of pinned particles, and is therefore harmful to the prevention of coarsening. Here, Nb combines with C and N in steel to form NbC, NbN and Nb (CN) in which both are combined, and Nb (CN) referred to in the present invention is a generic term for these three types of precipitates. Used.

First, in order to stably exert the pinning effect of Nb (CN) during carburizing heating, a certain amount or more of Nb (CN) should be finely precipitated in advance in the steel material after hot forging. is necessary. In addition, AlN
In order to stably exhibit the pinning effect of Al, it is necessary to limit the precipitation amount of AlN as much as possible in the state of the steel material after hot forging. This is because AlN precipitated in the state of the steel material after hot forging is coarse and does not contribute as pinning particles, but rather serves as a nucleus of the coarse precipitation of Nb (CN), and the fineness of Nb (CN) Precipitation is hindered, which promotes coarsening of crystal grains. From the above, Nb (CN) after hot forging
It is necessary to limit the amount of precipitation to 0.005% or more and the amount of AlN to 0.01% or less. The preferred range is 0.01% or more of Nb (CN) deposited after hot forging,
The precipitation amount of N is 0.005% or less.

In the present invention, the bainite fraction in the structure after hot forging is limited to 10% or less (including 0%).
If bainite exceeds 10% in the structure, it causes not only coarse grains during carburizing heating but also increases the hardness after hot forging and makes cutting difficult. For the above reasons, it is necessary to limit the structure fraction of bainite after hot forging to 10% or less. A preferred range is 5% or less.

In the present invention, the pearlite fraction in the structure after hot forging is 75% or less, and the ferrite crystal grain size number is 7 to 10. If the pearlite fraction exceeds 75% or the ferrite crystal grain size number is 7 or less, austenite grains during carburization become mixed grains and coarse grains are generated.
On the other hand, when the ferrite crystal grain size is 10 or more, it becomes too fine, and coarse grains are likely to be generated during subsequent carburization, and even when no coarse grains are generated, the austenite crystal grain size during carburization becomes too fine, and the hardenability Decrease.

According to the second aspect of the present invention, the heating temperature of the hot forging is set to 1150 ° C. or higher. The reason for this limitation is that fine Al which is effective in preventing the crystal grains from being coarsened during carburization.
To disperse a large amount of N and Nb (CN), coarse Al
This is because N and Nb (CN) are once dissolved in the matrix. If the heating temperature is lower than 1150 ° C., AlN and Nb (CN) cannot be once dissolved in the matrix. For this reason, coarse AlN and Nb (CN) exist after hot forging, and the generation of coarse particles during carburization cannot be suppressed. Therefore, during hot forging, 115
It is necessary to heat to above 0 ° C. The preferred range is 1200 to
1300 ° C.

According to the second aspect of the present invention, the final working of the hot forging is performed in a temperature range of 900 to 1100 ° C. If the final processing temperature is higher than 1100 ° C., the structure of the forged material becomes coarse austenite, and is transformed into coarse ferrite bainite or coarse ferrite pearlite by subsequent cooling. In the case of a coarse ferrite-bainite structure, not only this causes the generation of coarse grains during carburizing heating, but also increases the hardness, so that cutting before carburizing becomes difficult. When coarse pearlite increases, austenite grains during carburization become mixed grains, and coarse grains are generated. On the other hand, when the final processing temperature is less than 900 ° C., the ferrite crystal grain size becomes too fine, and coarse grains are easily generated during subsequent carburization, and even when no coarse grains are generated, the austenite crystal grain size during carburization becomes fine. Too hard and the hardenability decreases. For the above reasons, the final working temperature of hot forging is set to 90
It needs to be 0 to 1100 ° C.

According to a second aspect of the present invention, after hot forging,
The temperature range of 0 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./sec or less. When the cooling condition exceeds 1 ° C./sec, Nb (CN)
Can be passed only for a short time in the precipitation temperature range, the amount of fine Nb (CN) deposited after hot forging becomes insufficient, and the structure fraction of bainite increases.
Therefore, not only coarse particles are likely to be generated, but also the hardness after hot forging increases, and cutting becomes difficult. Therefore, it is desirable to reduce the cooling rate as much as possible. A preferred range is 0.7 ° C./sec or less. As a method for reducing the cooling rate, there is a method in which an isothermal furnace or a cover for keeping the temperature is installed behind the hot forging line, and cooling is performed by an external heat source or self-heating.

[0047]

The present invention will be further described below with reference to examples.

(Example 1) Converter steel smelting steel having the composition shown in Table 1 was continuously cast and, if necessary, was subjected to a slab rolling process to obtain a rolled material of 162 mm square. Subsequently, a steel bar having a diameter of 19 to 120 mm was manufactured by hot rolling.

The steel bar manufactured in the above process was subjected to hot forging under the following conditions to prepare a shaped material for a carburized part. All hot forging conditions were performed within the range specified in the present invention.
Heating temperature 1150-1350 ° C, final processing temperature 900-
The cooling rate at 1100 ° C and 800 to 500 ° C after hot forging is in the range of 0.1 to 1.0 ° C / sec.

From the cast material after hot forging, AlN, Nb
The amount of (CN) deposited was determined by chemical analysis. In addition, the cut surface of the shaped material after hot forging is polished and corroded, and the bainite fraction, pearlite fraction,
The ferrite grain size number was measured. The measurement of the old ferrite particle size number was performed according to JIS G 0552. The Vickers hardness of the cast material after hot forging was measured and used as an index of machinability. Vickers hardness is HV2
Those exceeding 00 were judged to be inferior in machinability.

[0051] The hot-forged cast material under the above conditions was
Carburizing simulation was performed. The conditions of the carburizing simulation are heating to 910 to 1090 ° C. for 5 hours and water cooling. After that, the cut surface is polished and corroded, and the old austenite grain size is observed, and the coarse grain generation temperature (crystal grain coarsening temperature)
I asked. Since high-temperature carburization is usually performed in a temperature range of 1000 to 1050 ° C., those having a coarse grain generation temperature of 1000 ° C. or less were determined to be inferior in crystal grain coarsening characteristics. The measurement of the prior austenite grain size was carried out in accordance with JIS G 0551. Observation was performed at a magnification of about 400 for about 10 visual fields. If at least one coarse grain having a grain size number of 5 or less was present, it was determined that coarse grains were generated.

Table 2 summarizes the results of these investigations.
The grain coarsening temperature of the present invention example is 1010 ° C. or higher,
It turns out that it is excellent in the coarse grain prevention property. Further, the hardness after hot forging is low, and it can be seen that normalization after hot forging can be omitted.

On the other hand, in Comparative Example 11, the Al content was below the range specified in the present invention, and the coarsening characteristics were poor.
Comparative Examples 12 and 13 are cases where the Al content exceeds the range specified in the present invention, and the coarsening characteristics are inferior. This is because coarse AlN was present, which prevented the fine dispersion of AlN and Nb (CN). Comparative Examples 14 and 15 are cases where the content of Nb is below the range specified in the present invention, and the coarsening characteristics are inferior. Comparative Examples 16 and 17 are cases where the Nb content exceeds the range specified in the present invention, and the coarsening characteristics are inferior. This is because coarse Nb (CN) exists and AlN and Nb (C
This is because the fine dispersion of N) was hindered. Comparative Example 18 is a case where the content of N is lower than the range specified in the present invention,
Since the amount of nitride is insufficient, the coarsening characteristics are inferior. Comparative Example 19 is a case where the content of N exceeds the range specified in the present invention, the precipitate becomes coarse, and the coarsening characteristic is also inferior.
Comparative Examples 20 to 21 are cases in which the content of Ti and the content of O exceeded the range specified in the present invention, and all of them had poor coarsening characteristics.

[0054]

[Table 1]

[0055]

[Table 2]

(Example 2) The steel bar manufactured in Example 1 was hot forged under the following conditions to prepare a shaped material for a carburized part. To see the effect of the hot forging heating conditions,
The heating temperature of hot forging was performed in the range of 980 to 1350 ° C. All hot forging conditions other than the heating temperature were performed within the range specified in the present invention. The final processing temperature is 900 to 1100 ° C and the cooling rate at 800 to 500 ° C after hot forging is 0.1 to 1.0.
° C / sec.

For the shaped material after hot forging, the precipitation amounts of AlN and Nb (CN), the bainite fraction in the structure, the pearlite fraction, the ferrite grain size number, and the Vickers were determined in the same manner as in Example 1. The hardness was measured.

The hot-forged shaped material under the above conditions was
A carburizing simulation was performed in the same manner as in Example 1.

Table 3 summarizes the results of these investigations.
The grain coarsening temperature of the present invention example is 1010 ° C. or higher,
It turns out that it is excellent in the coarse grain prevention property. Further, the hardness after hot forging is low, and it can be seen that normalization after hot forging can be omitted.

On the other hand, in Comparative Examples 28 to 32, the heating temperature of hot forging was lower than the range specified in the present invention, and the coarsening characteristics were inferior. In this case, the amount of AlN after hot forging also exceeds the range specified in the present invention. This is because AlN and Nb are present due to the presence of coarse AlN remaining after hot forging.
This is because the fine dispersion of (CN) was hindered.

[0061]

[Table 3]

(Example 3) The steel bar manufactured in Example 1 was hot forged under the following conditions to prepare a shaped material for a carburized part. The hot forging conditions were all within the range specified in the present invention except for the final working temperature. Heating temperature 1150-1
The cooling rate at 350 ° C. and 800 to 500 ° C. after hot forging is in the range of 0.1 to 1.0 ° C./sec. In order to see the effect of the final processing temperature of hot forging, the final processing temperature was set to 850 to 11
The test was performed in the range of 75 ° C.

For the shaped material after hot forging, the amounts of AlN and Nb (CN) precipitated, the bainite fraction in the structure, the pearlite fraction, the ferrite crystal grain size number, and the Vickers, in the same manner as in Example 1. The hardness was measured.

[0064] The hot-forged shaped material under the above conditions
A carburizing simulation was performed in the same manner as in Example 1.

Table 4 summarizes the results of these investigations.
The grain coarsening temperature of the present invention example is 1010 ° C. or higher,
It turns out that it is excellent in the coarse grain prevention property. Further, the hardness after hot forging is low, and it can be seen that normalization after hot forging can be omitted.

On the other hand, in Comparative Examples 39 to 40 and 42 to 43, the final working temperature of hot forging exceeded the range specified in the present invention, and the coarsening characteristics were inferior. In this case, the bainite fraction or the pearlite fraction of the shaped material after hot forging exceeds the range specified in the present invention. Also, in Comparative Examples 39 and 40, the ferrite crystal grain size number is lower than the range specified in the present invention. This is because when the final working temperature of the hot forging is high, the austenite crystal grains become coarse and are inherited by the structure after transformation. Further, Comparative Examples 39, 42, and 4 in which the bainite fraction exceeded the range specified in the present invention.
3 has high hardness after hot forging and is inferior in machinability,
The normalizing process after hot forging cannot be omitted. In Comparative Example 41, the final working temperature of hot forging was lower than the range specified in the present invention, and the ferrite crystal grain size number was higher than the range specified in the present invention. In this case also, the coarsening property was inferior.

[0067]

[Table 4]

(Example 4) The steel bars produced in Example 1 were hot forged under the following conditions to prepare a shaped material for carburized parts. The hot forging conditions were all within the range specified in the present invention except for the cooling rate after hot forging. Heating temperature 11
The range is 50 to 1350 ° C, and the final processing temperature is 900 to 1100 ° C. To see the effect of the cooling rate after hot forging,
A cooling rate of 800 to 500 ° C is set to 0.56 to 2.01 ° C /
Performed in seconds.

For the shaped material after hot forging, the precipitation amounts of AlN and Nb (CN), the bainite fraction in the structure, the pearlite fraction, the ferrite grain size number, and the Vickers were determined in the same manner as in Example 1. The hardness was measured.

The hot forged material under the above conditions was
A carburizing simulation was performed in the same manner as in Example 1.

Table 5 summarizes the results of these investigations.
The grain coarsening temperature of the present invention example is 1010 ° C. or higher,
It turns out that it is excellent in the coarse grain prevention property. Further, the hardness after hot forging is low, and it can be seen that normalization after hot forging can be omitted.

On the other hand, in Comparative Examples 48 to 51, the cooling rate after hot forging exceeded the range specified in the present invention, and the bainite fraction of the cast material after hot forging was out of the range specified in the present invention. Since the hardness exceeds that, the hardness after hot forging is high and the cutting workability is poor, so that the normalizing process after hot forging cannot be omitted. In Comparative Example 51 having a particularly high cooling rate, Nb (C
The precipitation amount of N) and the ferrite crystal grain size number are also out of the range specified in the present invention, and the coarse grain generation temperature is low.

[0073]

[Table 5]

[0074]

EFFECTS OF THE INVENTION The shaped material for high-temperature carburized parts of the present invention which is excellent in the characteristics of preventing grain coarsening and the method of manufacturing the same. The high-temperature carburized parts which are excellent in the characteristics of coarsening of grains which do not require normalizing after hot forging. The use of the production method for cast steel materials not only saves energy and improves productivity by increasing the temperature and shortening the carburizing time,
Since it is possible to omit the normalizing process after hot forging, the industrial effect of the present invention is extremely remarkable.

Continued on the front page F term (reference) 4K032 AA01 AA05 AA11 AA12 AA16 AA19 AA21 AA22 AA23 AA24 AA26 AA27 AA29 AA31 AA32 AA35 AA36 BA02 CA02 CA03 CC04 CD01

Claims (2)

[Claims] C .: 0.1 to 0.4% by mass, S
i: 0.02 to 1.3% Mn: 0.3 to 1.8%, S:
0.001 to 0.15%, Al: 0.015 to 0.04
5%, Nb: 0.005 to 0.05%, N: 0.01 to
0.02%, and further, Cr: 0.4 to 1.8.
%, Mo: 0.02 to 1.0%, Ni: 0.1 to 3.5
%, V: contains one or more of 0.03 to 0.5%, P: 0.025% or less, Ti: 0.01% or less,
O: each is limited to 0.0025% or less, the balance consists of Fe and unavoidable impurities, the precipitation amount of Nb (CN) after hot forging is 0.005% or more, and the precipitation amount of AlN is 0. 01% or less, the bainite fraction in the structure after hot forging is limited to 10% or less, the pearlite fraction in the structure after hot forging is 75% or less, and the ferrite grain size number is 7 to 7. A shaped material for high-temperature carburized parts which is No. 10 and has excellent crystal grain coarsening prevention properties that do not require normalizing after hot forging. 2. The steel having the chemical composition according to claim 1 is heated at a temperature of 1150 ° C. or more to perform hot forging.
The hot forging final processing is performed in a temperature range of 900 to 1100 ° C., and thereafter, the temperature range of 800 to 500 ° C. is gradually cooled at a cooling rate of 1 ° C./second or less. A method for producing a shaped material for high-temperature carburized parts, which has an excellent property of preventing crystal grain coarsening that does not require semi-treatment.