JPH0582452B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method of manufacturing a maraging steel member having excellent wear resistance and fatigue strength. The present invention can be used, for example, to manufacture hoops for CVT belts. Here, the CVT belt hoop refers to an endless blocking member used to hold a plurality of torque transmission plates that transmit torque in a continuously variable transmission that has a pair of pulleys that can change the V-groove spacing.

[Prior art] Maraging steel has the best mechanical properties among age-hardened steels, and has advantages such as high strength, high toughness, and excellent cold workability, so it is used in various fields. It is used in

By the way, in order to improve the wear resistance and fatigue strength of maraging steel members, it is common to use members with a composition that is maraging steel.
A first step in which the member is heated to about 800 to 1000°C to form a solution and then cooled; a second step in which the member is heated to about 500°C and subjected to aging treatment to precipitate a precipitated phase;
A third step of nitriding the surface portion of the member is performed.

Furthermore, there is a demand in industry for the development of members made of maraging steel that have excellent wear resistance and fatigue strength.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a manufacturing method that can manufacture members with further improved wear resistance and fatigue strength. It is on offer.

[Means for Solving the Problems] As a result of intensive research on maraging steel, the present inventor has found that nitriding can be accelerated by applying compressive stress to the surface of the member as a pre-nitriding step. It has been discovered that this makes it possible to increase the surface hardness and increase the surface compressive residual stress, thereby further improving wear resistance and fatigue strength. The reason for this is not necessarily clear, but it is presumed that the crystal lattice at the surface is distorted, making it easier for nitrogen atoms with a small atomic radius to diffuse into the surface, making nitridation easier.

The manufacturing method according to the present invention includes a first step of solutionizing a member having a composition to become maraging steel and then cooling it, and applying 5Kg/mm ² to 60Kg/mm ² to at least a part of the surface of the solutionized member. The second step is to apply a compressive residual stress of A third step of performing aging treatment and then cooling to room temperature is performed in order.

The manufacturing method according to the present invention will be further explained below.

In the first step, a member having a composition that becomes maraging steel is solution-treated and then cooled. Here, the composition of the component is set appropriately depending on the use of the component, but in terms of weight percent, nickel is 17-26%, aluminum is 0.05-26%.
0.5%, molybdenum 4-6%, titanium 0.2-1.6%,
It can be 7-12% cobalt, the balance iron. The thickness of the member is appropriately set depending on the use of the member, and can be 0.1 to 1.0 mm. The heating temperature and heating time in the first step must be set at a temperature that allows solute atoms such as nickel, aluminum, titanium, etc. to sufficiently dissolve into austenite. Therefore, the heating temperature in the first step is usually about 800 to 850°C. Further, the heating time in the first step is usually several hours. Cooling in the first step is usually performed by air cooling. Note that the structure of the member subjected to the first step is martensite.

In the second step, compressive residual stress is applied to at least a portion of the surface portion of the solution-treated member. Examples of methods for applying compressive residual stress include methods of bending the member to a small radius of curvature while stretching it and applying compressive residual stress to the outside of the curved part, and methods of projecting shot, grit, etc. onto the surface of the member. Alternatively, a rotary forging method may be employed in which the casting mold is oscillated and pressed onto the surface of the member to upset it. The value of compressive residual stress is set appropriately depending on the required wear resistance and fatigue strength, but it is 5 to 60 kg/
^mm2 . Note that the portion to which compressive residual stress is applied may be the entire surface of the member, or may be only a portion thereof.

In the third step, by heating the member to approximately 400 to 480°C in a nitriding atmosphere, the surface portion of the member, including the portion to which compressive residual stress has been applied, is nitrided, and the inside of the member is aged. , then cool to room temperature. Here, while the normal ammonia gas nitriding temperature is 450 to 530°C, the nitriding temperature of the manufacturing method according to the present invention is about 400 to 480°C, which is a lower temperature range than the conventional normal ammonia gas nitriding temperature. It is. If the nitriding temperature exceeds 480℃,
This is because the compressive residual stress imparted to the surface in the second step is relaxed, making it impossible to perform nitriding while ensuring the distortion of the crystal lattice of the surface portion, which is the object of the present invention. Particularly, the present invention is capable of performing nitriding treatment even at a temperature lower than the usual treatment temperature of 450° C. by applying compressive residual stress to the surface of the member, thereby making it possible to minimize deformation of the member. or,
This is because if the temperature is less than 400°C, nitriding will not be performed sufficiently. The nitriding atmosphere is generally an atmosphere containing ammonia gas as a main component, and preferably does not contain carbon components, ie, RX gas, CO gas, and CO ₂ gas. The time for heating to 400 to 480° C. can be shorter than the normal nitriding time because nitriding is promoted by applying compressive residual stress to the surface portion. For example, it can be about 0.1 to 10 hours.

When the third step is carried out in this manner, the interior of the member is subjected to aging treatment, and a precipitated phase is precipitated into martensite that constitutes the matrix.

By the way, after carrying out the third step, 470 to 580
It is preferable to carry out a fourth step of heating the member to a temperature of 0.degree. C. to add an aging treatment. By performing the fourth step in this manner, the delayed fracture resistance inside the member can be further improved while ensuring the wear resistance and fatigue strength of the member. The time for heating to 470 to 580°C will vary depending on the type of material, but
Usually, it can be about 1 to 24 hours.

For maraging steel (particularly maraging steel containing 18% nickel), when aging treatment is performed at approximately 440°C in an atmosphere containing hydrogen ions,
The delayed fracture strength of the interior of the member tends to decrease significantly. In this regard, even if the nitriding atmosphere in the third step contains hydrogen ions, if the fourth step is performed after the third step, it is possible to add aging treatment to the inside of the member. It is possible to improve the delayed fracture resistance of.

[Effects of the Invention] According to the manufacturing method of the present invention, the surface hardness and surface compressive residual stress can be increased compared to conventional nitrided maraging steel members, and therefore wear resistance and fatigue resistance can be increased. A member made of maraging steel with excellent strength can be manufactured. Therefore, if the member obtained by the present invention is applied to the manufacture of a hoop for a CVT belt, a hoop that is durable and has a long life can be obtained.

[Example] An example of the manufacturing method according to the present invention will be described.

(First Example) In the first step, the member is charged into a vacuum furnace and
It was converted into a solution by heating at ℃, thereby solidly dissolving solute atoms such as nickel and titanium into austenite. The member was then cooled. Cooling was performed with nitrogen gas and argon gas.
Here, the shape of the member is an endless hoop, and its dimensions are 0.19 mm thick, 8.6 mm wide, and 224 mm in inner circumference length.
The composition of the component is 17.75% nickel, 0.080% aluminum, 4.76% molybdenum, and 0.48% titanium.
%, Cobalt 7.75%, C0.0054%, Si0.038%,
S0.0003%, the balance is iron.

In the second step, two tension rollers apply a tension of 50 kg/mm ² to the member, while a small diameter bending roll bends the member.
A compressive residual stress of Kg/mm ² was applied. The diameter of the tensioning roller is 70mm, and the diameter of the small diameter roll for bending is 20mm.
mm.

In the third step, the member is inserted into a nitriding retort furnace, and the temperature inside the retort furnace is raised to 435°C while flowing ammonia gas (NH ₃ ) with a purity of 99% or higher at a rate of 1.5 liters per minute into the retort furnace. The temperature was then maintained for 6 hours. After holding, it was cooled to room temperature.

In the parts that have undergone the first to third steps in this way, nitrogen decomposed from the ammonia gas diffuses into the surface of the part and nitrides it, so the hardness of the surface of the part is about Hv1030. The internal hardness of the member was approximately Hv520. The hardness was measured using a micro-Vickers machine under a load of 50 g. Furthermore, the compressive residual stress on the surface of the member increased to 160 kg/mm ² due to nitriding. Here, the compressive residual stress was measured by the X-ray side tilt method. The parts subjected to the first to third steps as described above have a hard surface and an increased value of compressive residual stress on the surface, so they have poor wear resistance and fatigue strength. Are better.

The hoop that underwent the first to third steps as described above was incorporated as a member of a CVT belt.
In addition, Figure 1 shows a schematic diagram of CVT, and Figure 2 shows a schematic diagram of CVT.
A portion of the CTV belt is shown. This CVT includes a pulley 1, a pulley 2, and a belt 3 stretched between the pulleys 1 and 2. As shown in FIG. 2, the hoop 4 is held in a groove of a metal torque transmission plate 5, and together with the torque transmission plate 5 constitutes the belt 3. The hoops incorporated into the CTV belt as described above were durable and had a long life.

By the way, in the above-mentioned example, the relationship between the time for heating the member for nitriding and aging treatment in the third step and the surface hardness of the hoop was tested. The heating time was 1.5, 3, 6, and 12 hours. The test results are shown in Figure 3 with black circles. As shown in Figure 3, when the heating time is 3 hours, the surface hardness is about Hv780, and when the heating time is 6 hours, the nitriding is sufficient, so the surface hardness is
It was hard, about Hv1030.

In addition, in the third step, the relationship between the heating time of the member for nitriding and aging treatment and the compressive residual stress on the surface of the hoop was tested. The heating time is
The duration was 1.5, 3, and 6 hours. The test results are shown in Figure 4 with black circles. As shown in Figure 4, the heating time is 3.
When the heating time was 6 hours, the residual stress value was about 108 Kg/mm ² , and when the heating time was 6 hours, the residual stress value increased dramatically to about 152 Kg/mm ² . Note that the residual stress was measured by the X-ray side tilt method, which is an X-ray stress measurement method.

On the other hand, as a comparative example, regarding the conventional manufacturing method,
The relationship between the heating time for nitriding and the surface hardness of the hoop, and the relationship between the heating time for nitriding and the value of compressive residual stress on the surface of the hoop were tested. Here, in the conventional manufacturing method, a member having the same composition as the maraging steel as in the first embodiment was carried out at the same time as the first embodiment, skipping the second step. The test results of the comparative example are shown as white circles in FIG. 3 and white circles in FIG. 4.

Now, when comparing the test results in the first example and the test results in the comparative example, in Fig. 3, there is not much difference in hardness until the heating time is about 1.5 hours, but after this, the difference in hardness between the two is found. The difference in hardness increases. In Fig. 4, in the comparative example, the increase in the value of compressive residual stress is almost linear as shown by the broken line in Fig. 4, but in the first example, the heating time is
When the time exceeds 1.5 hours, the value of compressive residual stress increases dramatically. From the above test results, it is understood that applying compressive residual stress before nitriding promotes nitriding.

In addition, the relationship between the temperature at which the member was heated for nitriding and aging treatment in the third step and the surface hardness of the hoop was tested. Heating temperature is 435, 450, 465,
The temperature was 480 and 495℃. Note that the heating time was 6 hours. The test results are shown in Figure 5. As shown in FIG. 5, the surface hardness decreases as the heating temperature increases. This is the opposite of the normal relationship between nitriding temperature and surface hardness. That is, in normal nitriding, the surface hardness increases as the nitriding temperature increases, but in this example, when the heating temperature was 450°C, the surface hardness was about Hv950, but when the heating temperature exceeded 480°C. and the surface hardness decreases to about Hv700. This result shows that it is better to set the heating temperature in the third step so as not to exceed 480°C.

In addition, the relationship between the temperature at which the member was heated for nitriding and aging treatment in the third step and the value of the compressive residual stress on the surface of the hoop, and the relationship between the temperature and the strain on the surface of the hoop were tested. The test results are shown in Figures 6 and 7.
As shown in the figure. As shown in FIG. 6, it can be seen that the higher the temperature, the lower the value of compressive residual stress. Here, the "front" shown in FIG. 6 refers to the outside of the endless hoop shape, and the "back" refers to the inside. Furthermore, as shown in FIG. 7, the higher the temperature, the smaller the half width, which indicates that the strain on the surface area becomes smaller.

(Second Example) In the manufacturing method according to the second example, the above-mentioned first
After carrying out the first to third steps of the example, the following fourth step was carried out. The fourth step is to introduce argon gas into the retort furnace containing the parts, replace the ammonia gas in the retort furnace with argon gas, and then heat the parts in the retort furnace at 500°C for 2 hours. I went. In this way, the aging treatment inside the member further progresses. Therefore, the hardness of the surface part is about Hv1010, and the hardness of the inside after the third process is as described above.
It was about Hv520, but it is about Hv600.
Also, the compressive residual stress in the surface area is 150Kg/mm ² . As is clear from the above measurement results, the internal hardness of the member is increasing. This result shows that the aging effect inside the member was promoted and the precipitation of the precipitated phase increased. When the aging effect is promoted in this way, the internal feed fracture resistance of the member is improved.

(Third Example) In the first step, having a maraging steel composition,
Two members with a width of 8 mm and a thickness of 0.165 mm were heated at 820° C. for 30 minutes to form a solution. The composition of the members here is the same as in the first embodiment. Next, in the second step, compressive residual stress was applied to the surface portions of the two members by roll processing. Then, the residual stress distribution in the thickness direction of the member subjected to the second step was measured by a strain meter method. Measurements were made by attaching a strain gauge to one surface of a 60 mm long member, corroding the other side with a mixed acid to reduce the thickness, and measuring the depth in the thickness direction from the output of the strain gauge at the time of reduction. The corresponding residual stress was calculated. The measurement results are curve A-1 in Figure 8.
Shown below. As shown in Figure 8, there are 20 to 30
Kg/mm ² (This test piece had a compressive stress of 50 Kg/mm ² in its hoop shape before cutting, but the surface compressive stress decreased slightly because it was cut to a length of 60 mm.)
A compressive residual stress of 100% was generated.

Next, as a third step, the remaining members are heated at 440°C for 4 hours in an ammonia gas atmosphere to nitride them, and the inside of the members is aged.
It was then cooled to room temperature. After completing the third step, the member was cut to a length of 60 mm, and the value of residual stress in the thickness direction was measured. The measurement method was the same as described above using the strain meter method. The measurement results are shown in curve A-2 in FIG.
As shown by curve A-2 in FIG. 9, it can be seen that the value of the compressive residual stress in the surface area is close to 100 Kg/mm ² and the value of the compressive residual stress in the surface area is significantly increased compared to before nitriding.

As a comparative example, the first to third steps were performed on members having the same composition and size, except that no compressive residual stress was applied, and the other conditions were the same as in the third example. In the comparative example, the compressive residual stress value of the member that completed the first step was almost 0, as shown by curve B-1 in FIG. 8, while the value of the compressive residual stress of the member that completed the third step was shown in FIG. As shown by curve B-2, the compressive residual stress in the surface portion was as small as about 20 kg/mm ² at most.

[Brief explanation of the drawing]

Figure 1 is a side view of the entire CUT belt, Figure 2 is a perspective view of the main parts of the belt, Figure 3 is a graph showing the relationship between heating time and hoop surface hardness, and Figure 4 is a graph showing the relationship between heating time and hoop surface hardness. A graph showing the relationship between the compressive residual stress of the surface portion, Figure 5 is a graph showing the relationship between the heating time and the surface hardness of the hoop, Figure 6 is a graph showing the relationship between the heating time and the compressive residual stress, and Figure 7 is a graph showing the relationship between the heating time and the compressive residual stress. The figure is a graph showing the relationship between heating time and half width. FIGS. 8 and 9 are graphs showing stress distribution. In the figure, 1 and 2 are pulleys, 3 is a belt, 4 is a hoop, and 5 is a torque transmission plate.

Claims (1)

[Claims] 1. A first step of solutionizing a member having a composition to become maraging steel and then cooling it;
The second one imparts a compressive residual stress of Kg/ ^mm2 to 60Kg/ ^mm2 .
step, by heating the member to 400 to 480°C in a nitriding atmosphere, the surface portion of the member, including the portion to which compressive residual stress has been applied, is nitrided, and the inside of the member is aged, and then A method for manufacturing a maraging steel member having excellent wear resistance and fatigue strength, characterized by carrying out the following in order: a third step of cooling to room temperature; and a third step of cooling to room temperature.