JPS62192528A - Manufacture of maraging steel member having superior wear resistance and fatigue strength - Google Patents

Abstract

PURPOSE:To provide superior wear resistance and fatigue strength to a maraging steel member by producing compressive stress in the surface part of the member before nitriding. CONSTITUTION:In the 1st stage, a member having the composition of a maraging steel is subjected to soln. heat treatment and cooled. In the 2nd stage, compressive residual stress is produced in at least a part of the member. In the 3rd stage, the member is heated to 400-480 deg.C in a nitriding atmosphere to age the interior of the member as well as to nitride the surface part including the part having the produced compressive residual stress and then the member is cooled to room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a maraging steel member having excellent RFL wear resistance and fatigue strength. INDUSTRIAL APPLICATION This invention can be utilized for [1 of the hoop for CVT belts, for example. 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 a member made of maraging steel, generally speaking, the first step is to heat the member having the composition of maraging steel to about 800 to 1000°C to form a solution, and then cool it. The second step is to heat the component to about 500'C and perform an aging treatment to precipitate the precipitated phase, and the third step is to perform a nitriding treatment to nitride the surface of the component. .

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 inventor of the present invention found that, as a pre-stage of nitriding treatment, compressive stress is applied to the surface of the member, and then the nitriding treatment is carried out. It has been discovered that this can increase the surface hardness and increase the surface compressive residual stress, thereby further improving the 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 a second step of applying compressive residual stress to at least a part of the surface of the solutionized member. 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 subjected to aging treatment. A third step of 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 member is appropriately set depending on the use of the member, but in weight%, nickel 17
~26%, aluminum 0°05-0.5%, molybdenum 4-6%, titanium 0°2-1.6%, cobalt 7-1
2%, the balance can be iron. The thickness of the member is appropriately set depending on the intended use of the member, and may be 0.1 to i, Qs+s. The heating temperature and heating temperature in the first step must be set at a temperature that allows solute atoms such as nickel, aluminum, and titanium to be sufficiently infiltrated into austenite. Therefore, the heating concentration in the first step is
Usually, the temperature is 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. Methods for applying compressive residual stress include, for example, stretching the member and bending it to a small radius of curvature and applying compressive residual stress to the outside of the curved part, or projecting shot or grit onto the surface of the member. Method, W! It is possible to employ a rotary forging method in which the mold is oscillated and pressed against the surface of the member to upset it.

The value of compressive residual stress is set appropriately depending on the required values of wear resistance and fatigue strength, but is usually 5 to 60 Kll.
l/Im'. Note that the portion to which the 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 subjected to aging treatment. , then cool LD to room temperature. Here, the normal ammonia gas nitriding temperature is 450 to 530°C, whereas the nitriding temperature of the manufacturing method according to the present invention is 40°C.
The temperature range is about 0 to 480°C, which is lower than the conventional normal ammonia gas nitriding temperature. If the nitriding temperature exceeds 480°C, the compressive residual stress imparted to the surface in the second step will be relaxed, making it impossible to perform nitriding while ensuring the distortion of the crystal lattice of the surface, which is the objective of the present invention. This is because Particularly, in the present invention, by applying compressive residual stress to the surface of the member, mononitriding treatment can be performed even at a temperature lower than the usual processing temperature of 450° C., and deformation of the member can be minimized. Also, if the temperature is less than 400°C,
This is because 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, that is, RX gas, CO gas, and COt 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 inside 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, it is preferable to carry out a fourth step of adding an aging treatment by heating the member to 470 to 580° C. in a reducing atmosphere either continuously or at intervals. 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. In addition, 470
The time for heating to ~580°C is changed as appropriate depending on the type of member, but can usually be about 1 to 24 hours.

For maraging steel (particularly maraging steel containing 18% nickel), when aging treatment is performed at around 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. Maraging steel members 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] Examples of the manufacturing method according to the present invention will be described.

(First Act 11 In the first step, the member was placed in a vacuum furnace and heated at 820°C to form a solution, thereby solidly dissolving solute atoms such as nickel and titanium into the austenite.Then, the member was Cooling was performed using nitrogen gas and argon gas.

Here, the shape of the member is an endless hoop, and its size is 0.19 m thick, 8.6 m wide, and 224 m long in inner circumference.
It is m. The composition of the part is nickel 17.75% by weight.
%, aluminum o, oso%, molybdenum 4゜76%
, Titanium 0.48%, Cobalt 7.75%, Go, 00
54%, Si0.038%, SO, O003%, and the balance is iron.

In the second step, two tension rollers apply 50
While applying a tension of KO/mm2, the part is bent with a small diameter bending roll, and the outer surface of the part is bent at 50K (+/m2).
A compressive residual stress of +a2 was applied.

The diameter of the tension roller is 70+ani, and the diameter of the small diameter bending roll is 20 mm.

In the third step, the member is inserted into a nitriding retort furnace,
Then, while flowing ammonia gas (NH3) with a purity of 99% or more into the retort furnace at a rate of 1.5 liters per minute, the temperature inside the retort furnace was raised to 435° C., and the temperature was maintained at that temperature for 6 hours. After holding, it was cooled to room temperature.

In the parts that have been subjected to steps 11 to 3 in this way, the nitrogen decomposed from the ammonia gas diffuses into the surface of the part and is nitrided, so that the hardness of the surface of the part is HV.
The hardness of the inside of the member was about HV520. The hardness was measured using a micro Vickers machine under a load of 50a. Furthermore, the compressive residual stress on the surface of the member increased to 160 KIJ/mm2 due to nitriding.

Here, the compressive residual stress was measured by the X-ray lateral 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, Fig. 1 shows a schematic diagram of CVT, and Fig. 2 shows a CTV.
Part of the 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 tool 74 is held in the groove of the metal torque transmission plate 5, and together with the torque transmission plate 5 constitutes the belt 3. Hoops incorporated into CTV belts 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 1,5,3,6,12 hours. The test results are shown in Figure 3 with black circles. As shown in Figure 3,
When the heating time was 3 hours, the surface hardness was about HV780, and when the heating time was 6 hours, nitriding was sufficient, so the surface hardness was about HV1030.

Furthermore, 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 was 1°5.3.6 hours. The test results are shown in Figure 4 with black circles. As shown in Figure 4, when the heating time was 3 hours, the residual stress value was 10.
It's about 8 KO/am', plus p! When time A was 6 hours, the value of residual stress increased dramatically to about 152 KIJ/a+m2.

Note that the residual stress was measured by the X-ray lateral method, which is an X-ray stress measurement method.

On the other hand, as a comparative example, we tested 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 using the conventional manufacturing method. did. Here, in the conventional manufacturing method, a member having the same composition as the maraging steel as in the first embodiment was removed from the second step and carried out simultaneously with the first embodiment.

The test results of the comparative example are shown by white circles in FIG. 3 and white circles in FIG. 4.

Now, when we compare the test results in the first example and the test results in the comparative example, we find that 3rd! ! In l, heating rum is 1
．． Up to about 5 hours, the hardness does not change much, but after this time the difference in hardness between the two becomes large. Fig. 4 shows that in the comparative example, the increase in the value of the compressive residual stress is almost linear as shown by the broken line in Fig. 4, but in the first example, when the heating time exceeds 1.5 hours, the compressive rA residual stress increases. The value of 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 435.450.465.480.495
℃. Note that the heating time was 6 hours. The test results are shown in the fifth factor. 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 is 450°C, the surface hardness is Hv950.
When the heating temperature exceeds 480°C, 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 FIGS. 6 and 7. As shown in Figure 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 rl[J refers to the inside. Furthermore, as shown in Figure 7, the higher the temperature, the smaller the half-value width.
It can be seen that the distortion in the surface area is reduced.

(Second Example) In the manufacturing method according to the second example, - after implementing the first to third steps of the first example described above, the following fourth step was performed. 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. went. If this J: is done, the aging treatment inside the member will further proceed. Therefore, the hardness of the surface part is about Hv 1010, and the hardness of the inside after the third step is Hv as mentioned above.
While it was about 520, Hv is about 600.

Moreover, the compressive residual stress of the surface portion is 150 Kg/Im''.

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 11!1 process, the maraging steel composition is used, and the width is 8.
-11 A member with a thickness of 0.165 m is used, and this member is
The solution was heated at 820° C. for 30 minutes. 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.

Measurement was carried out by pasting a strain gauge on one surface of a 60-length member, corroding the other side with mixed acid to reduce the thickness, and calculating the depth in the thickness direction from the output of the strain gauge at the time of reduction. This was done by calculating the residual stress corresponding to . The measurement results are shown in curve Δ-1 in FIG. As shown in Figure 8, there are 20
Compression of ~30K (1/am" (this test piece had a compressive stress of 50KIJ/a+a+ffi when it was hoop-shaped before cutting, but the surface compressive stress was slightly reduced because it was cut to a length of 60II11) Residual stress had occurred.

Next, as a third step, the remaining member was placed in an ammonia gas atmosphere and heated at 440° C. for 4 hours to nitridate, and the inside of the member was subjected to aging treatment, and then cooled to room temperature. After completing the third step, the member was cut into a length of 60111, 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.

Song 1 in Figure 9! As shown in A-2, the value of the compressive residual stress in the surface portion is close to 100 KQ/am'', which indicates that the value of the compressive residual stress in the surface portion 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 the same 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 after the first step was almost 0, as shown by curve B-1 in FIG.
On the other hand, the part that has undergone the 8113 process is shown in curve B-2 in Figure 9.
As shown in , the compressive residual stress on the surface is at most 20 kg/m.
It was small, about m'.

[Brief explanation of drawings]

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, and Figure 6 is a graph showing the relationship between the heating time and the compressive 3I residual stress. FIG. 7 is a graph showing the relationship between heating time and half width. FIGS. 8 and 9 are graphs showing stress distribution. Patent Applicant: Representative of ΦDA Central Research Institute Co., Ltd.
Patent Attorney Hirodo Okawa Patent Attorney Akio Maruyama Figure 1 Figure 2 Figure 3 Time (Hr, Zr) Figure 4 Figure 5 i degree (0C) Figure 6 Excess (°C) A Figure 7 Star Trap (°C) $8 Figure

Claims (7)

[Claims] (1) A first step of solutionizing a member having a composition to become maraging steel and then cooling it; a second step of applying compressive residual stress to at least a portion of the surface of the solutionized member; and a nitriding atmosphere. By heating the member to 400 to 480°C, 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 subjected to aging treatment, and then the third step is to cool the member to room temperature. A method for manufacturing a maraging steel member with excellent wear resistance and fatigue strength, characterized by carrying out the following steps in order. (2) After the third step, 470-580 in a reducing atmosphere
A method for manufacturing a maraging steel member with excellent wear resistance and fatigue strength, characterized by carrying out a fourth step of heating the member to a temperature of °C to add an aging treatment and increasing the delayed fracture resistance of the member. . (3) In the second step, the surface compressive residual stress imparted to the member is 10 Kg/mm^2 to 60 Kg/mm^2. A method for producing aged steel members. (4) The method for manufacturing a maraging steel member having excellent wear resistance and fatigue strength according to claim 1, wherein the nitriding atmosphere in the third step is a gas containing ammonia gas as a main component. (5) The composition of the member is 17 to 26% Ni, A
l0.05-0.5%, Mo4-6%, Ti0.2-1
．． 6% Co, 7 to 12% Co, and the balance is Fe. 2. The method of manufacturing a maraging steel member having excellent wear resistance and fatigue strength according to claim 1. (6) The method for manufacturing a maraging steel member with excellent wear resistance and fatigue strength according to claim 1, wherein the member has a thickness of 0.1 to 1.0 mm. (7) A method for manufacturing a maraging steel member having excellent wear resistance and fatigue strength as set forth in claim 1, wherein the member is used for a hoop for a CVT belt.