JPH11310824A - Carburized and quenched steel member and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carburized and quenched steel member capable of suppressing a heat treatment strain while maintaining excellent strength characteristic and its manufacture. SOLUTION: After a stock in which 0.24-0.30 wt.% carbon is contained and the 1/2-in. depth quenched performance value obtained by the Jominy test is 33 to 50 points, is carburizing-treated, is quenched at a cooling rate R1 in the vicinity of the critical cooling rate R0 of the resultant carburized layer to undergo carburizing and quenching treatment. It is preferable to regulate the cooling rate to <=500 deg.C/sec.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburized and quenched steel member effective for suppressing heat treatment distortion and a method for producing the same.

[0002]

2. Description of the Related Art For example, as gear members for automobiles,
In order to improve wear resistance, fatigue resistance, and the like, carburizing and quenching is widely performed. Specifically, for example, JIS-SCR
420, SCM420 and other low carbon carburizing steels (approximately 0.2
% C) as a raw material, which is provided in a desired shape in advance, and thereafter carburizing treatment and carburizing quenching are performed. As is well known, there are various carburizing methods for forming a carburized layer having a high carbon concentration on the surface of a material. The carburizing and quenching treatment is a treatment for quenching the material after the above carburizing treatment. By performing this series of carburizing and carburizing and quenching, a steel member having high surface hardness and toughness can be obtained.

Conventionally, various methods have been proposed as the carburizing and quenching treatment. Generally, the material heated to the quenching heating temperature is rapidly cooled to a temperature below the Ms point (martensite transformation start temperature) of the carburized layer. This method is the most basic quenching method, but causes relatively large heat treatment distortion.

Therefore, conventionally, in order to reduce the dimensional variation due to heat treatment distortion, the type of coolant, liquid temperature,
Various methods have been proposed in which the viscosity, components, and the like are devised. Also, there are various two-stage quenching methods that apply a quenching method called a martemper, a marquench, or an Msquench, in which the material is put into a bath kept at a temperature just above or below the Ms point of the carburized layer, kept, and then gradually cooled. Is being developed.

[0005]

However, the manufacturing method using the conventional carburizing and quenching method has the following problems. In other words, although the measures against distortion such as improvement of the quenching bath and adoption of two-step quenching are effective to some extent, they are still insufficient for steel members such as gears with extremely strict dimensional accuracy, and the dimensional accuracy may not be maintained in some cases. .

Further, it is known that it is effective to reduce the cooling rate during rapid cooling in order to reduce the heat treatment distortion. However, simply reducing the cooling rate does not provide the required mechanical strength even if the distortion can be reduced.

The present invention has been made in view of the above-mentioned conventional problems, and provides a carburized and quenched steel member capable of suppressing heat treatment distortion while maintaining excellent strength characteristics, and a method of manufacturing the same. It is assumed that.

[0008]

The invention according to claim 1 contains 0.24 to 0.30% by weight of carbon and has a 1/2 inch depth quenching performance value obtained by a Jominy test of 33 to 50 points. The method for producing a carburized and quenched steel member is characterized in that after the material is carburized, the material is quenched by rapidly cooling the material at a cooling rate near the critical cooling rate of the carburized layer.

The most remarkable point in the present invention is to use a material having a carbon content and the above-mentioned quenching performance value within the above-mentioned specific ranges, and to perform quenching after carburizing at the above-mentioned cooling rate. .

The above material has a carbon (C) content of 0.2
It is in the range of 4 to 0.30% by weight. Carbon content of 0.
When the content is less than 24% by weight, there is a problem that the internal strength of the obtained carburized and hardened steel member is insufficient. on the other hand,
If it exceeds 0.30% by weight, there is a problem that it is difficult to process the material before carburizing into a desired shape.

The above material has a quenching performance value of 1/2 inch (hereinafter referred to as J value) obtained by a Jominy test of 33 to 50 points. This J value is calculated by JI
The value obtained in the Jominy test based on S, at a half inch from the hardened end of the test piece. Here, when the J value is less than 33 points, there is a problem that the internal strength of the obtained carburized and hardened steel member is insufficient. On the other hand, if it exceeds 50 points, there is a problem that the internal strength becomes too high and the toughness is reduced.

Further, the above material can be combined with various conventional known techniques. That is, the content of Si, Mn, and Cr can be reduced to reduce grain boundary oxidation, and Ni and Mo can be added to secure the hardenability of the carburized layer. Also, in order to increase the crack generation resistance, Al and N are added to refine the crystal grains, to reduce the Mn content and to increase the Mo content for strengthening the grain boundaries, and to increase the intragranular strength. N for rise
i, Mo can be added.

Next, the carburizing treatment of the above-mentioned material is a treatment in which only the surface layer is carbonized to form a carburized layer, and the inside thereof is made into an uncarburized layer having the composition of the material as it is. Law can be applied. For example, gas carburizing, solid carburizing, liquid carburizing, or a carbonitriding method that combines nitriding can also be applied.

Further, the carbon concentration of the carburized layer formed on the surface of the material by the carburizing treatment is preferably 0.7 to 0.9% by weight. When the carbon concentration of the carburized layer is less than 0.7% by weight, there is a problem that the effect of improving the hardness by the subsequent carburizing and quenching treatment is not sufficiently obtained. On the other hand, when the content exceeds 0.9% by weight, there is a problem that the effect of improving the hardness is saturated and the cementite becomes brittle due to precipitation of cementite. However, in the case of high-concentration carburization that actively uses cementite, the carbon concentration is 0.9%.
It does not matter if it exceeds the weight percentage.

[0015] Also in the above-mentioned carburizing treatment, conventional known techniques can be combined. That is, various known techniques such as a vacuum carburizing method for reducing grain boundary oxidation, a carburizing and nitriding method for ensuring the hardenability of the carburized layer, and a high-temperature carburizing method for shortening the carburizing time can be applied. .

Next, the carburizing and quenching treatment after the carburizing treatment is performed by rapidly cooling the raw material from the quenching heating temperature at a cooling rate near the critical cooling rate of the carburized layer.
Here, the cooling rate is an average cooling rate obtained by dividing a temperature difference when the carburized layer is cooled from a quenching heating temperature to a quenching temperature by a cooling time.

The critical cooling rate of the carburized layer means a lower limit cooling rate at which the carburized layer can be completely transformed into martensite when rapidly cooled from a uniform austenite structure. Therefore, in order to increase the hardness of the carburized layer, it is necessary to rapidly cool at a cooling rate higher than the critical cooling rate. On the other hand, the cooling rate near the critical cooling rate is
A cooling rate slightly higher than the critical cooling rate,
The rate at which the cooling rate is reduced as much as possible while ensuring martensitic transformation.

After the series of processes up to the above-mentioned carburizing and quenching, various known post-treatment techniques can be applied. For example, shot blasting, shot peening, various types of polishing (including electrolytic polishing, honing, etc.) for reducing grain boundary oxidation can be performed. Also, shot peening, fillet roll processing, and the like for imparting residual compressive stress can be performed.

Next, the operation of the present invention will be described. In the present invention, as described above, a material containing 0.24 to 0.30% by weight of carbon and having a J value in the above specific range is used. Therefore, about 0.2% by weight of the conventional
Compared with the case where the carburizing steel of C is carburized, the heat treatment distortion of the resulting carburized and quenched steel member can be largely suppressed, and sufficient strength characteristics can be maintained.

The reason for this is not necessarily clear, but can be considered as follows. That is, the material of the present invention is:
As described above, the carbon content is higher than that of the conventional carburizing steel (about 0.2% by weight C). Therefore, after the carburizing treatment, the carbon content of the carburized layer is controlled to the same level as that of the conventional case, but the carbon concentration of the uncarburized layer is maintained as it is in the material. As a result, the Ms point in the uncarburized layer inside the material becomes significantly lower than before, and the critical cooling rate of the uncarburized layer becomes lower than before.

Therefore, in the present invention, even if the quenching during carburizing and quenching is reduced to a cooling rate near the critical cooling rate of the carburized layer as described above, the quenching effect of the uncarburized layer can be maintained to some extent. it can. Further, since the J value of the uncarburized layer is within the above range, excellent strength characteristics can be obtained by the quenching effect.

On the other hand, the uncarburized layer in the conventional steel has its M
Since the s point is higher than that of the product of the present invention and its critical cooling rate is also faster, if the cooling rate is reduced to a cooling rate near the critical cooling rate of the carburized layer, the quenching effect of the internal structure (uncarburized layer) decreases. Generally, the J value of the uncarburized layer of the conventional steel is lower than that of the present invention. Therefore, the internal strength is insufficient.

As described above, in the present invention, a reduction in the cooling rate, which was impossible in the prior art from the viewpoint of strength, can be realized while maintaining high strength characteristics. In addition, due to the decrease in the cooling rate, the occurrence of thermal distortion due to the cooling rate can be significantly suppressed.

As described above, the Ms point of the uncarburized layer is lower than that of the conventional case, and approaches the Ms point of the carburized layer. This means that the transformation start point of the uncarburized layer and the transformation start point of the carburized layer at the time of rapid cooling during carburizing and quenching are closer than before. This makes it possible to approach two martensitic transformation start times at the time of quenching, thereby suppressing the occurrence of distortion due to the phase transformation as compared with the conventional case.

Further, since the uncarburized layer in the present invention has a higher carbon concentration than in the conventional case, its austenite transformation starting point is lower than in the conventional case. Therefore, the heating temperature immediately before the quenching can be lowered as compared with the conventional case, whereby the heat treatment distortion can be further reduced.

As described above, in this embodiment, the material is limited to those having the above specific carbon content and J value, and at the time of carburizing and quenching after the carburizing treatment, quenching is performed at a lower cooling rate than the conventional one as described above. By doing so, the heat treatment distortion of the resulting carburized and quenched steel member can be significantly suppressed, and sufficient strength characteristics can be maintained.

Further, as in the second aspect of the present invention, the cooling rate during the carburizing and quenching is preferably 500 ° C./sec or less. That is, it is preferable that the cooling rate is higher than the critical cooling rate of the carburized layer and 500 ° C./sec or less. By setting the cooling rate within this range, it is possible to reliably exert the effect of suppressing distortion while maintaining strength.

According to a third aspect of the present invention, the quenching during the carburizing and quenching is performed to a holding temperature near the Ms point of the carburized layer, and then the material is held at the holding temperature for a predetermined time. Thereafter, it is preferable to cool to room temperature.
That is, it is preferable to reduce the cooling rate as described above and perform so-called two-stage quenching. Thereby, a more stable distortion suppressing effect can be obtained. In addition,
The holding time after quenching is adjusted to a predetermined time according to the shape and size of the material.

According to a fourth aspect of the present invention, the material after the carburizing treatment is made of a material having a carburized layer and an uncarburized layer.
It is preferable that the difference between the s points is 200 ° C. or less. That is,
For example, when a conventional general carburizing steel is carburized to 0.8% by weight C, the difference in the Ms point between the carburized layer and the uncarburized layer exceeds 200 ° C. In this case, large heat treatment distortion occurs during carburizing and quenching. In contrast, the above Ms
By suppressing the point difference to 200 ° C. or less, the heat treatment distortion can be reduced as compared with the related art. The above Ms
It is preferable that the point difference approaches 0 ° C.

Further, as in the invention according to the fifth aspect, the material can be formed in a gear shape in advance. Also in this case, the function and effect of the above-described excellent manufacturing method can be sufficiently exhibited, and even a product such as a gear having strict dimensional accuracy can be finished with high quality.

Next, it is preferable that the cooling rate during the carburizing and quenching is 70 to 130% of the critical cooling rate of the uncarburized layer of the material after the carburizing treatment. If the cooling rate is less than 70% of the critical cooling rate of the uncarburized layer, there is a problem that the martensitic transformation rate of the uncarburized layer becomes too low and the strength is reduced. Has a problem that heat treatment distortion tends to occur.

Next, according to the invention as set forth in claim 6, the steel contains 0.24 to 0.30% by weight of carbon and has a quenching performance value of 33 to 1/2 inch obtained by the Jominy test. After carburizing 50 points of material,
There is a carburized and quenched steel member manufactured by performing a carburizing and quenching treatment by rapidly cooling the material at a cooling rate near a critical cooling rate of the carburized layer.

This carburized and hardened steel member is, as described above,
Since it is excellent in dimensional accuracy while maintaining excellent strength characteristics, it can be applied to various applications in which both strict requirements are made on both strength characteristics and dimensional accuracy, such as gears.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 A method for manufacturing a carburized and hardened steel member according to an embodiment of the present invention will be described with reference to FIGS. In this example, as shown in Table 1, carbon containing 0.24 to 0.30% by weight and 1
A material having a 1/2 inch depth quenching performance value (J value) of 33 to 50 points was used. Then, after carburizing the material, the material was rapidly cooled at a cooling rate R1 near the critical cooling rate R0 of the carburized layer to carry out carburizing and quenching to produce a product E1 of the present invention.

The carburizing treatment in this example was performed so that the carbon concentration of the carburized layer was 0.8% by weight. Then, as shown in FIG. 1, the cooling rate R1 during carburizing and quenching was reduced as close as possible to the critical cooling rate R0 of the carburized layer.
The cooling rate R1 was actually set to a rate lower than 500 ° C./sec.

For comparison, conventional steel of the same size (conventional product C
FIG. 1 also shows the cooling rate R9 when carburizing and quenching SCR420 or SCM420 (JIS) having a carbon concentration of about 0.2% by weight, which is 1). This cooling rate R9 is 50
This is a fast cooling rate exceeding 0 ° C./sec. At a rate lower than this, sufficient strength cannot be obtained. Further, as shown in Table 1, the above SCR420, SCM42
0 is a conventional carburizing steel widely used,
The values are 22-33 and 25-37, respectively.

In the product E1 of the present invention, as shown in FIG. 1, rapid cooling during carburizing and quenching is performed up to the holding temperature near the Ms point (Ms1) of the carburized layer in order to increase the distortion suppressing effect. After holding at the holding temperature for a predetermined time, the steel sheet was cooled to room temperature by two-stage quenching. The holding temperature in this example was set to a temperature of Ms1-10 ° C. Also, the conventional product C1 was subjected to two-step quenching. The holding temperature in this case is also the Ms point (Ms9) -1 of the carburized layer.
The test was performed at 0 ° C. In FIG. 1, Ms
1 and Ms9 are shown at the same position.

As a result of the carburizing and quenching treatment under such conditions, the distortion of the product E1 of the present invention was significantly reduced as compared with the case of the conventional product C1, and the mechanical strength was sufficient.
On the other hand, in the case of the conventional product C1, the mechanical strength was obtained, but the distortion was relatively large.

The reason will be discussed with reference to FIG. In the figure, time is plotted on the horizontal axis, and temperature is plotted on the vertical axis. C. T. Curve S1
And S9. FIG. 3 also shows the cooling speeds R12 and R92 of the uncarburized portion and the critical cooling points P1 and P9 of the uncarburized portion when the carburized layer is cooled at the cooling speeds R1 and R9. Here, the critical cooling point indicates an intersection between the critical cooling rate line and the Ms point.

As shown in the figure, the Ms point (Ms12) in the uncarburized layer of the material of the product E1 of the present invention is much lower than the Ms point (Ms92) in the uncarburized portion of the conventional product C1. The critical cooling rate of the carburized layer is lower than the critical cooling rate of the uncarburized layer of the conventional steel.

Therefore, in the case of the product E1 of the present invention, the quenching during carburizing and quenching is performed by the critical cooling rate R of the carburized layer as described above.
Even if the cooling rate is reduced to near zero and the cooling rate of the uncarburized portion is reduced to the cooling rate R12, the quenching effect of the uncarburized layer can be maintained to some extent. and again,
Since the material of the product E1 of the present invention has the J value within the above range, excellent strength characteristics can be obtained by the above quenching effect.

On the other hand, since the Ms point (Ms92) of the uncarburized layer of the conventional product C1 is higher than that of the product E1 of the present invention and its critical cooling speed is higher, the uncarburized layer has a cooling rate near the critical cooling rate R0 of the carburized layer. If the cooling rate is reduced to make the cooling rate of the uncarburized layer equal to that of R12, the quenching effect of the internal structure (uncarburized layer) is reduced, resulting in insufficient internal strength. Therefore, conventionally, the cooling rate cannot be reduced to near the critical cooling rate R0 of the carburized layer.

Therefore, in the case of the product E1 of the present invention, the cooling rate R1 can be reduced to a rate near the critical cooling rate R0 of the carburized layer while maintaining the strength characteristics. And, due to the decrease in the cooling rate, the occurrence of thermal distortion due to the cooling rate can be significantly suppressed.

[0044]

[Table 1]

Embodiment 2 In this embodiment, the cooling rate during carburizing and quenching in Embodiment 1 was variously changed, and the effects on the amount of distortion, surface hardness, and internal hardness were measured. In this example, the carbon content is 0.1.
A sample having 27% by weight and the J value of 40 points was used as a representative of the product of the present invention (product E2 of the present invention). For comparison, SCR420 (conventional product C2) was used. The conditions other than the cooling rate were the same as above, and the measurement was performed for each evaluation item as follows.

First, the relationship between the cooling rate and the amount of distortion is shown in FIG. In the figure, the horizontal axis shows the cooling rate (° C./sec) of the carburized layer during carburizing and quenching, and the vertical axis shows the amount of strain (μm). Here, the measurement of the distortion amount was performed by a dedicated precision measuring instrument. As can be seen from the figure, both the product E2 of the present invention and the conventional product C2 resulted in a decrease in the amount of distortion as the cooling rate was lower.

Next, the relationship between the cooling rate and the surface hardness is shown in FIG.
Shown in In the figure, the horizontal axis shows the cooling rate of the carburized layer (° C / sec)
And the vertical axis indicates the surface hardness (Hv). Here, the surface hardness was measured by applying a measurement load of 10 kgf to the surface with a Vickers (Hv) hardness meter. As can be seen from the figure, the cooling rate of the conventional steel C2 is 500 ° C.
/ Sec or less, the surface hardness was significantly reduced. On the other hand, the product E2 of the present invention was able to maintain high surface hardness even if the cooling rate was reduced to 350 ° C./sec or less.

Next, the relationship between the cooling rate and the internal hardness is shown in FIG.
Shown in In the figure, the horizontal axis shows the cooling rate of the carburized layer (° C / sec)
And the vertical axis represents the internal hardness (Hv). The internal hardness was measured with a Vickers (Hv) hardness meter by applying a measurement load of 20 kgf to the center of the uncarburized portion of the cut section of the sample. As can be seen from the figure, the product E2 of the present invention exhibited higher internal hardness than the comparative product C2 over the entire measurement range.

From the above results, it is excellent to carry out carburizing and quenching at a slow cooling rate near the critical cooling rate of the carburized layer as described above using a material in which the carbon concentration and the J value are regulated as described above. It can be seen that this is a very effective means for obtaining a steel member in which distortion is suppressed while maintaining strength characteristics.

[0050]

As described above, according to the present invention, it is possible to provide a carburized and quenched steel member capable of suppressing heat treatment distortion while maintaining excellent strength characteristics, and a method of manufacturing the same.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a cooling rate of a carburized layer during carburizing and quenching in a first embodiment.

FIG. 2 is an explanatory diagram showing a cooling rate of an uncarburized layer during carburizing and quenching in the first embodiment.

FIG. 3 is an explanatory diagram showing a relationship between a cooling rate and a distortion amount according to the second embodiment.

FIG. 4 is an explanatory diagram showing a relationship between a cooling rate and a surface hardness in the second embodiment.

FIG. 5 is an explanatory diagram showing a relationship between a cooling rate and an internal hardness in the second embodiment.

[Explanation of symbols]

 R0. . . Critical cooling rate, R1, R9. . . Cooling rate of carburized layer, R12, R92. . . Cooling rate of uncarburized layer, Ms1, Ms9. . . Ms point of carburized layer, Ms12, Ms92. . . Ms point of uncarburized layer,

Claims (6)

[Claims] A carburizing treatment is performed on a material containing 0.24 to 0.30% by weight of carbon and having a quenching performance value of 33 to 50 points obtained by a 1/2 inch depth obtained by a Jominy test. A method for producing a carburized and quenched steel member, wherein carburizing and quenching is performed by rapidly cooling a material at a cooling rate near a critical cooling rate of the carburized layer. 2. The method for producing a carburized and quenched steel member according to claim 1, wherein the cooling rate during the carburizing and quenching is 500 ° C./sec or less. 3. The quenching according to claim 1 or 2, wherein the quenching during the carburizing and quenching is performed to a holding temperature near the Ms point of the carburized layer, and then the material is held at the holding temperature for a predetermined time and then cooled to room temperature. A method for producing a carburized and quenched steel member, characterized by cooling. 4. The method according to claim 1, wherein:
A method for manufacturing a carburized and quenched steel member, wherein the difference between the Ms points of the carburized layer and the uncarburized layer of the material after the carburizing treatment is 200 ° C. or less. 5. The method according to claim 1, wherein:
A method for manufacturing a carburized and quenched steel member, wherein the material is formed in a gear shape in advance. 6. After carburizing a material containing 0.24 to 0.30% by weight of carbon and having a 1/2 inch depth quenching performance value of 33 to 50 points obtained by a Jominy test, A carburized and quenched steel member manufactured by performing a carburizing and quenching treatment by rapidly cooling a material at a cooling rate near a critical cooling rate of the carburized layer.