JP2839481B2 - Heat-treated steel part and method of manufacturing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-treated steel part and a method for producing the same. 2. Description of the Related Art In the case of gears and the like for power transmission components, when high fatigue strength is required, shot peening is performed after carburizing and quenching. This shot
This is intended to suppress the generation or propagation of fatigue cracks by forming a compressive residual stress on the steel surface by the thinning. As a preferable example of the above shot peening, as disclosed in Japanese Patent Application Laid-Open No. Sho 60-218422,
Low carbon alloy steel SCM415, that is, C: 0.13-
0.18%, Si: 0.15 to 0.35%, Mn: 0.1%
60-0.85%, P ≦ 0.03, S ≦ 0.03, C
r: 0.90 to 1.20%, Mo: 0.15 to 0.30
%, The alloy steel consisting of the balance Fe is carburized and quenched, and the shot particle projection speed for the alloy steel is 35 to 50 m / sec.
c, Production method of performing shot pinning under the conditions of shot hardness HRC 45 to 50 is known. According to this, the compressive residual stress can be left while the surface roughness is suppressed to about 1 μm or less, which is allowable, and a constant fatigue strength can be obtained. By the way, the higher the fatigue strength, the more
It is preferable for components requiring strength, and from this viewpoint, it is desirable that a fatigue strength equal to or higher than the fatigue strength obtained by the above-described manufacturing method be obtained. In such a case, it is necessary to further increase the compressive residual stress formed on the steel surface, and to increase the compressive residual stress, it is conceivable to increase shot hardness, shot particle projection speed, and the like as much as possible. . [0005] However, in order to improve the fatigue strength, shot peening is performed on the above alloy steel with a shot hardness, a shot particle projection speed, and the like higher than those in the above manufacturing method. Even if it is carried out, the surface roughness of the steel surface becomes rougher than the allowable range, and fatigue cracks occur due to the notch effect based on the roughness, or even the necessary compressive residual stress is not formed on the steel surface. Therefore, the fatigue strength is rather lowered, and it is quite difficult to increase the fatigue strength as compared with the above-described manufacturing method. [0006] Under such circumstances, the present inventor:
As a result of intensive studies on the above problems, they discovered that an incompletely quenched layer (abnormal surface layer) associated with grain boundary oxidation was involved, and found the following knowledge. High-speed shot particle projection speed using a high-hardness shot (compared to conventional shot peening conditions)
In Shottopi - Doing training, Shottopi - depth of training before the average surface abnormal layer and Shottopi - relationship between the average surface roughness after training is obtained as a characteristic curve shown in FIG. 1 (the characteristic line f ₁ Shottopi - training conditions: shot hardness HRC54, shot particles blasting speed 90m / sec, shot size 0.6 mm, the shot time 120 sec, the characteristic line f ₂ Shottopi - training conditions:. shot hardness HR
C50, shot particle projection speed 60 m / sec, shot diameter 0.6 mm, shot time 120 sec. , Shottopi characteristic line f ₃ - training conditions: shot Hardness HRC5
8, shot particle projection speed 120 m / sec, shot diameter 0.6 mm, shot time 120 sec. In this case, since the surface roughness after shot peening and the surface roughness after shot peening required from the viewpoint of abrasion resistance are not more than about 1 μm because fatigue cracks do not occur due to the notch effect. To reduce the surface roughness to about 1 μm or less, the depth of the abnormal surface layer before shot peening should be about 15 μm.
It must be less than μm. When shot peening is performed under the same conditions as in FIG. 1, the relationship between the depth of the average surface abnormal layer before shot peening and the surface compressive residual stress after shot peening is obtained as a characteristic line shown in FIG. In this case, the shot
If the depth of the average surface abnormal layer before the peening is 15 μm or less, the compressive residual stress formed on the steel surface after the shot peening can be particularly increased. The present inventors have completed the present invention based on the above findings. That is, a first object of the present invention is to provide a heat-treated steel part capable of dramatically improving fatigue strength while maintaining wear resistance. Also, the second
It is an object of the present invention to provide a method for manufacturing a heat-treated steel part that can easily manufacture the heat-treated steel part. Means for Solving the Problems In order to achieve the first object, in the first invention, C: 0.10 to 0.4
0%, Si: 0.06 to less than 0.15%, Mn: 0.3
0 to 1.00%, Cr: 0.90 to 1.20%, Mo:
A steel part obtained by carburizing or carbonitriding and quenching an alloy steel consisting of more than 0.30% and 0.50% or less, with the balance being Fe, wherein the depth of the incomplete surface quenched layer due to grain boundary oxidation is 15 μm or less. In this case, shot peening is applied to the steel sheet formed as described above. According to the above-described structure, high hard shot and high-speed shot particle projection are performed on a steel part in which the depth of an incompletely hardened surface layer (hereinafter referred to as an abnormal surface layer) to be present is 15 μm or less. Even if shot peening is performed at a speed or the like, the surface roughness does not exceed the allowable limit of about 1 μm due to the abnormal surface layer, and a fatigue crack occurs due to a notch effect based on the surface roughness, or A reduction in wear resistance due to the roughness is prevented. On the other hand, the kinetic energy of the shot particles is suppressed from being absorbed as the rough surface roughness of the abnormal surface layer, and the shot peening is performed at a high hard shot, a high shot particle projection speed, etc. The compression residual stress can be formed on the surface more than ever. [0011] Further, an embodiment of the first invention (Claim 1) is as described in Claim 2. Further, in order to achieve the second object, in the second invention, C: 0.10 to 0.40%, S
i: 0.06 to less than 0.15%, Mn: 0.30 to 1.
00%, Cr: 0.90 to 1.20%, Mo: 0.30
% Or more and 0.50% or less, and the alloy steel consisting of the balance Fe is carburized or carbonitrided and quenched. Then, the quenching reduces the depth of the incomplete surface quenched layer caused by grain boundary oxidation to 1%.
Shot peening is performed on an alloy steel formed to have a diameter of 5 μm or less to provide a method of manufacturing a heat-treated steel part. In the above-described structure, the amount of Mo which is effective for hardenability and does not cause grain boundary oxidation is set to be in the above range by increasing the amount of Mo as compared with the prior art, but remarkably promotes grain boundary oxidation and does not affect hardenability. Since the amount of Si was set lower than the conventional range and the above range was set, an abnormal surface layer was formed on the alloy steel after carburizing and quenching and nitrocarburizing due to the action of both, but the depth of the abnormal surface layer of the alloy steel was increased. Is 15
μm or less, as described above, even if high-hardness shots are performed on the alloy steel at a high shot particle projection speed or the like, fatigue cracks occur due to the notch effect based on the surface roughness, or Abrasion resistance is prevented from deteriorating due to surface roughness.
Such shot peening will allow the formation of compressive residual stress on the steel surface. Moreover, once the composition of the alloy steel is determined as described above, the depth of the abnormal surface layer of the alloy steel can be reduced to 15 μm or less by ordinary carburizing and quenching or carbonitriding and quenching. The critical significance of each component of the above alloy steel is as follows. C: C is a basic element necessary for imparting the strength of steel, and is required to be 0.1% or more in order to secure the strength of the core (inside) by carburizing and quenching. However, when the C content is 0.40
%, The toughness decreases and the material becomes brittle, and the machinability also deteriorates. Therefore, the content of C is 0.1
It is set in the range of 0 to 0.40%. Si: Si is an element having an extremely strong tendency to promote grain boundary oxidation which causes the formation of a surface abnormal layer, and the content of Si is 0.1%.
If it exceeds 5%, the adverse effect cannot be ignored. Moreover,
Si is an element that does not contribute to hardenability. Therefore, S
It is preferable to reduce the content of i as much as possible. But,
Si is used as a deoxidizing agent or used for lowering the melting point and reducing the melting energy. For this reason, the content of Si is required to be 0.06% or more in steelmaking. Therefore, the content of Si is 0.06 to 0.15%.
It is set in the range of less than. Mn: Mn is an element that promotes grain boundary oxidation that causes the formation of an abnormal surface layer, and the smaller the Mn content, the more desirable.
However, provided that the addition of Si is less than 0.15% and the addition of Mo is 0.30% or more, there is no adverse effect even if Mn is 1.00% or less. If the Mn content is less than 0.30%, the hardenability of the core portion becomes insufficient. For this reason, the content of Mn is set in the range of 0.30 to 1.00%. Cr: Cr is also an element that promotes grain boundary oxidation which causes the formation of an abnormal surface layer, and the smaller the content, the better. However, provided that the content of Si is less than 0.15% and the content of Mo is 0.30% or more, there is no adverse effect even if Cr is 1.20% or less. On the other hand, if the Cr content is less than 0.90%, the hardenability of the core portion and the carburizing property decrease. For this reason,
The Cr content is set in the range of 0.90 to 1.20%. Mo: Mo is an element that does not cause grain boundary oxidation and enhances hardenability, and is an element that contributes to the reduction of the abnormal surface layer. Mo
When the content exceeds 0.50%, the effect tends to be saturated, and when the content is less than 0.30%, the effect is low and the hardenability of the core portion becomes insufficient. For this reason, Mo
The content is set in the range of 0.30 to 0.50%. Mo also has the effect of toughening the metallographic structure itself, in addition to the effect of reducing the abnormal surface layer in the above range. The embodiments of the second invention (Claim 3) are as described in Claims 4 and 5. Embodiments of the present invention will be described below. FIG. 3 shows a process for producing a heat-treated steel part. In this manufacturing process, each step of cutting, forging, normalizing, machining, heat treatment, tempering, and shot peening of the material is performed in order, and in some cases, each of forging, normalizing, and tempering is performed. The steps are to be omitted. In the manufacturing method according to the present invention, of the above manufacturing steps, the heat treatment step and the shot peening step are deeply related, and these steps will be described in detail. Omitted. First, in the heat treatment step, the alloy steel is carburized and quenched or carbonitrided and quenched. The above alloy steel has a component of C: 0.30 to 0.30.
0.40%, Si: 0.06 to less than 0.15%, Mn:
0.30 to 1.00%, Cr: 0.90 to 1.20%,
Mo: More than 0.30% and 0.5% or less, with the balance being Fe. These components are determined from the viewpoint of reducing the depth of the surface abnormal layer of the alloy steel after carburizing and quenching to 15 μm or less, and the critical significance of each component is as described above. The above-described carburizing and quenching and quenching and quenching can be performed under general conditions. For example, for carburizing and quenching, the carburizing temperature is 900 to 950 ° C., and the carburizing time is 0.5 to 5.0 hours. This example has a preferred carburized quench hardening depth (Hv> 550) of 0.2 to 1.3 m.
m. The range of this cure depth is
If it is less than 0.2 mm, the surface pressure resistance of the component is insufficient,
If the thickness exceeds 1.3 mm, the internal oxidation of the alloy element is remarkable, so that it is set in the above range. By the ordinary carburizing and quenching and quenching and quenching, the depth of the abnormal surface layer formed on the alloy steel can be suppressed to 15 μm or less, and the surface can be treated without any special treatment or treatment. 15μm abnormal layer depth
It can be: In particular, when carbonitriding and quenching is performed on the above-mentioned alloy steel, the quenching property is improved by nitrogen, so that the depth of the abnormal surface layer can be further reduced as compared with the case of performing carburizing and quenching. As described above, the Si and Mo components of the alloy steel are greatly involved in suppressing the depth of the abnormal surface layer to 15 μm or less, as described above.
This can be confirmed by the characteristic lines shown in FIGS.
That is, FIG. 4 shows the influence of the Si content on the depth of the abnormal surface layer. According to this, it can be understood that the lower the Si content, the shallower the depth of the abnormal surface layer. In this case, each component other than Si is a steel type within a range considered to be substantially the same as shown in FIG. 10, carburizing is performed at 930 ° C. for 2 hours, and quenching is performed at 850 ° C. after carburizing. Tempering was performed at 180 ° C. for 1.5 hours. FIG. 5 shows the effect of the Mo content on the depth of the abnormal surface layer. According to this, it can be understood that the greater the Mo content, the shallower the depth of the abnormal surface layer. In this case, each component other than Mo is a steel type within a range considered to be substantially the same as shown in FIG.
As in case i, carburizing is performed at 930 ° C. for 2 hours,
Quenching is performed at 850 ° C after carburizing, and tempering is 18
Performed at 0 ° C. for 1.5 hours. FIG. 6 is a photomicrograph of 400 times showing the case where the depth of the abnormal surface layer is set to about 6 μm by the above method.
FIG. 7 shows that the depth of the surface abnormal layer is about 10 μm by the above method.
It is a microscope photograph figure of 400 times which shows the case where it was set to. 6 and 7, the lower side is the internal structure of the alloy steel,
The thin layer formed on the upper part of the alloy steel is the abnormal surface layer. On the other hand, FIG. 8 shows a conventional alloy steel (for example, SC
M415) shows the case where the depth of the abnormal surface layer is set to about 18 μm by performing normal carburizing and quenching.
FIG. 9 is a photomicrograph at × 100 magnification and FIG. 9 is a photomicrograph at × 400 magnification showing the case where the depth of the abnormal surface layer is set to about 25 μm by the same method as in FIG. 8 and 9, the lower side is the internal structure of the alloy steel, and the layer formed above the alloy steel is the abnormal surface layer. In the shot peening step, the shot hardness HRC 50 to 58, the shot particle projection speed 60 to 120 m / sec, the shot diameter 0.1 to 1.0 mm, the shot peening time 10 to 3 are preferable.
Shot peening is performed under the condition of 00 seconds. As described above, the shot hardness and the shot particle projection velocity are preferably as large as possible from the viewpoint of forming a compressive residual stress if the surface abnormal layer is 15 μm or less, as described above. However, shot hardness is HRC5
When it is lower than 0, the processing power of shot pinning becomes insufficient to improve the fatigue strength, and when it is higher than HRC 58, the shot pinning effect is saturated and the shot is easily cracked. When the shot particle projection speed is lower than 60 m / sec, the shot
The working force of the ning becomes insufficient to improve the fatigue strength, and when it is larger than 120 m / sec, the shot is liable to be broken and the economy is impaired. For this reason,
The shot hardness and shot particle projection speed are preferably set in the above ranges. As for the shot diameter, when the shot diameter is 0.
When it is smaller than 1 mm, the distribution layer of the compressive residual stress becomes thin, and when the shot diameter is larger than 1.0 mm, the thickness of the distribution layer of the compressive residual stress becomes sufficient, but the compressive residual stress value of the surface layer becomes low. Therefore, as described above,
The shot diameter is set within a range of 0.1 to 1.0 mm. For the shot peening time, 10
If less than seconds, the effect of shot peening is insufficient,
After 300 seconds, the effect of shot peening saturates,
The economy will be impaired. Therefore, the shot peening time is set within the range of 10 to 300 seconds as described above. As a result, a sufficient compressive residual stress can be formed on the surface of the steel part without roughening the surface roughness of the steel part, and the fatigue strength can be improved while maintaining the wear resistance. You can do it. EXAMPLE The effect based on the above embodiment can be supported by an experimental example shown in FIG. FIG. 12 shows the specific experimental conditions (chemical composition of alloy steel, etc.) and the results of each experimental example. The common experimental conditions are as follows. Common experimental conditions: Experimental parts: Automotive transmission gear (module
2.50) Carburizing: After carburizing at 930 ° C for 2 hours,
Quenching was performed at 850 ° C., and thereafter, tempering was performed at 180 ° C. for 1.5 hours. Shot-Pining Conditions: Shot Diameter 0.6 mm, Shot Time 150 Seconds (Experimental Example 1) In Experimental Example 1, when Si is near the lower limit and Mo is at the upper limit in the setting range of the present invention, This shows that if the alloy steel is carburized and quenched, the depth of the abnormal surface layer before shot peening is extremely small, that is, 6 μm. Also, in this experimental example 1, if shot peening was performed on such an alloy steel within the set range of the present invention, the depth of the abnormal surface layer was 15 μm or less, and the surface roughness was within the allowable range. Of 1.0 μm or less, the compressive residual stress becomes 50 kgf / mm ² required for improving the fatigue strength (FIG. 12).
The symbol “−” means that compressive residual stress remains in the alloy steel), and also indicates that fatigue strength can be improved. (Experimental Example 2) In Experimental Example 2, Si, M
When o is in the middle, it indicates that if the alloy steel is carburized and quenched, the depth of the abnormal surface layer before shot peening is within an allowable range of 10 μm. Experimental Example 2 also shows that, similarly to Experimental Example 1, the fatigue strength can be improved by shot peening. Experimental Example 3 In Experimental Example 3, the upper limit of Si was within the range of the present invention.
When Mo is near the lower limit, if the alloy steel is carburized and quenched, it indicates that the depth of the abnormal surface layer before shot peening becomes 14 μm near the allowable limit. Further, Experimental Example 3 also shows that the fatigue strength can be improved by shot peening as in Experimental Examples 1 and 2. (Experimental Examples 4, 5, 6) In Experimental Examples 4, 5, and 6, when the amount of Si is too large and the amount of Mo is too small with respect to the setting range of the present invention, the alloy steel is carburized and quenched. This also indicates that the depth of the abnormal surface layer before shot peening exceeds the allowable limit of 15 μm. Also, in Experimental Examples 4 and 5, when the depth of the abnormal surface layer exceeds the allowable limit as described above, even when the shot hardness and the shot particle projection speed are within the set ranges of the present invention, the shot peak Surface roughness after polishing exceeds the allowable limit of 1.0 μm or less, and the compressive residual stress does not become 50 kgf / mm ² or more required for improvement of fatigue strength.
This shows that improvement in fatigue strength cannot be expected. The reason why the surface roughness after the shot peening exceeds the allowable range is that the soft surface abnormal layer is relatively thicker than the other surface layers, so that the shot hardness and the shot particle projection speed increase. It is considered that the reason why the surface is easily roughened and that the compressive residual stress after the shot peening is not more than necessary is that the kinetic energy of the shot particles is consumed as the surface abnormal layer is roughened. In Experimental Example 6, when the depth of the abnormal surface layer exceeds the permissible limit and the shot peening conditions (shot hardness, projection speed) are out of the setting range of the present invention, the fatigue strength is improved. Is not expected. (Experimental Example 7) In Experimental Example 7, when both Si and Mo were within the setting range of the present invention, if the alloy steel was carburized and quenched, the depth of the surface abnormal layer before shot peening was reduced. However, when shot peening is performed on the alloy steel under the conditions of shot hardness HRC50 or less and shot particle projection speed of less than 60 m / sec, improvement in fatigue strength cannot be expected. It is shown that. Although the above embodiment describes carburizing and quenching, the present invention can also be applied to carbonitriding and quenching. In this case, the number of abnormal surface layers can be reduced, which is a preferable result. According to the first aspect, even when shot peening is performed in the presence of an abnormal surface layer, the occurrence of fatigue cracks due to the notch effect based on the surface roughness and the resistance due to the surface roughness. In addition to preventing wear reduction, increasing the kinetic energy of the shot particles can create more compressive residual force on the surface of heat-treated steel parts, so that fatigue resistance is maintained while maintaining wear resistance. A heat-treated steel part capable of dramatically improving strength can be provided. According to the second aspect of the present invention, not only can the above-described heat-treated steel part be obtained by the method, but once the components of the alloy steel have been determined as described above, they can be subjected to ordinary carburizing and quenching and quenching. Since the depth of the abnormal surface layer to be present can be made 15 μm or less, when obtaining the heat-treated steel part, in order to reduce the depth of the abnormal surface layer to be present to 15 μm or less,
It is not necessary to perform a special treatment / treatment, and it is possible to avoid a complicated treatment for reducing the depth of the abnormal surface layer to 15 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a characteristic diagram showing a relationship between a depth of an average surface abnormal layer before shot peening and an average surface roughness after shot peening. FIG. 2 is a characteristic diagram showing a relationship between a depth of an average surface abnormal layer before shot peening and a surface compressive residual stress after shot peening. FIG. 3 is a view showing a manufacturing process of a heat-treated steel part. FIG. 4 shows the depth and S of the abnormal surface layer before shot peening.
FIG. 4 is a characteristic diagram showing a relationship with an i content. FIG. 5: Depth of surface abnormal layer before shot peening and M
FIG. 4 is a characteristic diagram showing a relationship with o content. FIG. 6 is a 400 × photomicrograph showing a metallographic structure when the depth of the abnormal surface layer is about 6 μm. FIG. 7 is a 400 × photomicrograph showing a metallographic structure when the depth of the abnormal surface layer is about 10 μm. FIG. 8 is a photomicrograph of 400 times showing a metal structure in a case where the depth of the abnormal surface layer is about 18 μm. FIG. 9 is a photomicrograph at 400 × showing the metallographic structure when the depth of the abnormal surface layer is about 25 μm. FIG. 10 is a diagram showing the ratio of each component when obtaining the characteristic diagram of FIG. 4; FIG. 11 is a diagram showing the ratio of each component when obtaining the characteristic diagram of FIG. 5; FIG. 12 shows an experimental example.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Miwa Nohisa 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture Inside Mazda Co., Ltd. (56) References JP-A-60-21359 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) C22C 38/00 301 C21D 7/06 C23C 8/32

Claims (1)

(57) [Claims] C: 0.10 to 0.40%, Si: 0.06 to 0.
Less than 15%, Mn: 0.30-1.00%, Cr: 0.
90 to 1.20%, Mo: 0.30% to 0.50
% Or less, a steel part obtained by carburizing or carbonitriding and quenching an alloy steel consisting of the balance of Fe and having a surface incompletely quenched layer formed to a depth of 15 μm or less due to grain boundary oxidation. A heat treated steel part, which has been subjected to lining. 2. The steel part according to claim 1, wherein the hardening depth based on the carburizing quenching or the carbonitriding quenching is 0.2 mm to 1.3 mm in a region where the Vickers hardness is greater than 550. And heat treated steel parts. 3. C: 0.10 to 0.40%, Si: 0.06 to 0.
Less than 15%, Mn: 0.30-1.00%, Cr: 0.
90 to 1.20%, Mo: 0.30% to 0.50
% Or less, by carburizing or carbonitriding and quenching an alloy steel consisting of the balance of Fe, and then, for the alloy steel in which the depth of the surface imperfect quenching layer due to grain boundary oxidation is reduced to 15 μm or less by the quenching,
A method for producing a heat-treated steel part, comprising performing shot peening. 4. The method according to claim 3, wherein the quenching temperature is 900 ° C. to 950 ° C., and the quenching time is 0.5 to 5.0 hours. 5. The shot pinning according to claim 3, wherein the shot diameter is 0.1 mm or more.
1.0 mm, shot particle projection speed 60 to 120 m / s
A method for producing a heat-treated steel part, which is performed under the condition of ec.