JPS61147814A - Heat treatment of endless belt made of maraging steel - Google Patents

Abstract

PURPOSE:To improve remarkably the fatigue strength of material quality, by gas nitriding treating, or gas soft nitriding treating the titled belt for a specified time, in the stage of aging treatment of said belt. CONSTITUTION:A steel belt used for stepless transmission of an automobile is produced with a maraging steel. At the stage of aging treatment, the treating temp. and the treating time (ta) are set to 450-550 deg.C and 20-1hr respectively, and the gas nitriding or the gas soft nitriding treatment is carried out for treating time (tn) before completing the aging treatment. In such a case, time is controlled so that the ratio of (tn)/(ta) becomes to 0.015-0.25, in the times (ta) and (tn). The steel belt having the high fatigue strength of about 10<5> repeated numbers at 165kg/mm<2> total stress can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for heat treating endless belts made of maraging steel.

(Conventional technology and its problems) Steel belts used in continuously variable transmissions such as automobiles are required to have fatigue strength characteristics that can withstand repeated bending deformation during use, so high-strength materials such as maraging steel are generally used. has been done. Various manufacturing methods have been proposed in order to improve lifespan, but they have been proposed to achieve a satisfactory fatigue strength (for example, a total stress of 16
5kg/Atsusada, the number of repetitions is about 10S). For example, JP-A No. 57-45035 states that solution treatment and aging treatment, which are the usual heat treatments for maraging steel, are carried out after ring rolling, but the target fatigue strength has not been achieved. . Also, JP-A-53-42172 and JP-A-59-10407
Publication No. 0 discloses that compressive residual stress is imparted to the belt surface through plastic deformation due to roll bending to improve fatigue strength, but sufficient fatigue strength has not yet been obtained.

Further, methods are known in which surface hardening treatments such as carburizing treatment, shot method, nitriding and soft nitriding treatments are used to improve fatigue strength and wear resistance. However, carburizing treatment cannot be applied to maraging steel because it becomes brittle due to the formation of carbides such as TiC, and the shot method treats narrow and thin steel belts, so deformation due to residual stress after treatment is a problem. Therefore, this is also not valid. On the other hand, although there are many treatment methods for nitriding and soft nitriding, gas nitriding or gas soft nitriding is presumed to be effective for narrow and thin steel belts.

However, the following problems can be considered.

(1) Since the generally used treatment temperature is a high temperature of 550° C. or higher, this results in an overaging condition that is higher than the peak aging temperature of maraging steel, resulting in a decrease in strength.

(2) In addition to the heat treatment equipment for solution treatment and aging treatment of maraging steel, heat treatment equipment for nitriding or soft nitriding is required, which significantly increases the cost of the steel belt.

Furthermore, Arch, Eisenhiittenwes'47
(1976) 697 shows the fatigue properties of 170 kg/n 1st class maraging steel after nitriding treatment. Although the details of this nitriding treatment condition are unknown, this result shows that the treatment time is long (430"c x 48hr, 430℃
×, 27 hr), and the nitrided layer is also thick, so it cannot be applied to mass production of thin materials such as steel belts (for example, 0.2 m thick).

In view of the problem of ridges, an object of the present invention is to provide a heat treatment method that significantly improves fatigue strength and does not increase costs when gas nitriding or gas nitrocarburizing an endless belt made of maraging steel. shall be.

(Means for solving problems) In order to achieve the above objectives, the following measures were taken.

That is, an endless belt made of maraging steel was aged at a temperature of 450 to 550°C and an aging time of tA20.
When performing aging treatment for ~1 hour, gas nitriding or gas soft nitriding treatment is performed before the aging treatment is completed, and the aging treatment time tA, l! : The ratio tN/tA to the nitriding treatment time tN in tA is set to tN/tA: 0.015 to 0.25.

(Example) Next, the present invention will be described in detail based on the heat treatment diagram shown in FIG.

In FIG. 1, S indicates a solution treatment step, and A indicates a subsequent aging treatment step. Further, before the aging treatment step A is completed, a gas nitriding or gas soft nitriding treatment step N is provided.

Solution treatment is a treatment for heating maraging steel to an austenite region and then quenching to obtain a martensitic structure, and the solution treatment temperature Ts is 800 to 850°C. The solution temperature Ts is appropriately selected depending on the holding time (1 minute to 60 minutes). In other words, the minimum temperature at which recrystallization occurs when solutionized after cold working is 800°C for 20 minutes (when the maximum cold working rate of steel belt is 75%, the minimum temperature is 800°C for 20 minutes)
20 minutes at 0°C) and does not recrystallize below 800°C. In addition, when the cold working rate is 30%, the minimum
℃ is required, and 1 minute may be sufficient at 850℃. Heating at 850° C. for 60 minutes or more causes coarsening of crystal grains.

Aging treatment is a treatment to strengthen the hardened structure by aging, and the main strengthening mechanism is the hardening phenomenon in the preparation stage for precipitation of Ni-Ti and (Fe, Ni + Co) Mo-based intermetallic compounds in the case of 18Ni. be. Figure 2 shows 18
%Ni maraging steel (Fe-0,005C-17
,9Ni -7,90Go -4,85Mo -0,4
2Ti-0,09412) is shown. As is clear from FIG. 2, the over-aged state is accompanied by a softening phenomenon due to the formation of reversely transformed austenite, so it is necessary to set the temperature and time until peak aging as the aging conditions. For this reason, the aging treatment temperature TA:
The temperature is 450 to 550°C and the aging treatment time tA is 20 to 1 hour.

In other words, if the temperature exceeds 550°C, peak aging will occur in a short period of time and an over-aged state will occur immediately, making it virtually difficult to set and control the nitriding conditions.On the other hand, if the temperature is below 450°C, a long heat treatment will be required, making production difficult. In addition to being inferior in properties, nitriding without sufficient aging (slow aging) causes brittle fracture at grain boundaries and conversely reduces fatigue strength. The aging treatment time is set to 20 to IHr in balance with the aging treatment temperature.

The processing time tN of the gas nitriding treatment or the gas soft nitriding treatment is set to tN/tA: 0.015 to 0.25.

Below 0.015, the effect of improving fatigue life is small, while above 0.25, the hard and brittle nitrogen compound ε phase (Fe
This is because 3N) is formed in the structure and the fatigue life is reduced.

Note that the nitriding treatment on the ridges is performed during the aging treatment.

That is, a heat treatment furnace is used in common, and when only aging treatment is performed, the atmosphere in the furnace is a vacuum or inert gas atmosphere, and when nitriding treatment is involved, a nitriding gas atmosphere is used. As the nitriding gas, NX (CO2, Co, H2, N2) or R
X (CO2, C01H2, CH4, N2) gas and NH
A mixture of three gases is usually used.

Next, specific examples and comparative examples will be listed and explained.

Mass, 18% Ni maraging steel belt aging temperature TA: 480°C1 aging treatment time tA: 5H
r, nitriding gas N2 + NH3 (aging treatment atmosphere Ar gas without nitriding treatment), heat treatment was performed by selecting various nitriding treatment times, and a fatigue test was conducted using the obtained steel belt.

FIG. 3 shows the results, and tN=20mi according to the present invention.
n (tN/tA = 0.067), the fatigue strength was significantly improved by nitriding, but tA = 9
At 0 min (tN/tA=0.3), the improvement effect is extremely small, and at tN=240 min (t
H/1A=0.8), it is actually worse than before the treatment.

FIG. 4 is a graph showing the hardness distribution in the thickness section of the belt used in the fatigue test. LN according to the present invention =
In the case of 20 min, an increase in hardness due to the formation of a nitrided layer is observed at 20 to 30 μm from the surface layer, but the center part of the plate thickness is hardly affected by nitriding.

Furthermore, the belt subjected to the fatigue test was subjected to X-ray diffraction to investigate the presence or absence of nitrogen compounds in the nitrided layer.

As a result, at tN = 90m1n, the nitrogen compound ε phase (F
e3 N) was recognized, but tN=20rrl
No ε phase was detected in the in. From this, it is preferable that the nitriding conditions for a steel belt for the purpose of improving fatigue life be such that the nitrided layer is thin and the formation of a compound layer is hardly observed. If nitriding becomes excessive, the fatigue life will actually decrease due to the appearance of a hard and brittle compound layer (ε phase).

Next, as shown in Table 1, various heat treatment conditions were changed to obtain tN.
/tA different 18% Ni maraging steel belt samples were prepared and subjected to rotational fatigue tests (total stress σt = 165
kg/uJ) and compared the lifespan.

The lifespan was evaluated by the lifespan improvement rate defined below.

Np-N.

However, NL: tN life after nitriding treatment (times) Np
: Peak life of nitrided material (times) No. Life of non-nitrided material (times) (next leaf) The test results of the samples in Table 1 are shown in Table 2, and the life improvement rate is calculated from Table 2. This is shown in Figure 5.

From Table 2, Figure 5, tN/tA=0. It was confirmed that the samples according to the present invention within the O15 to 0.25 range had an extremely excellent fatigue life improvement rate of 20% or more.

Note that tN/LA=0. O15 to 0.25 can be applied not only when aging and nitriding are performed sequentially, but also when each is performed individually.

(Effects of the Invention) As explained above, in the present invention, an endless belt made of maraging steel is aged at a temperature of 450 to 550°C and an aging time tA of 20 to 1 hour, so that aging treatment can be easily managed. It is easy and excellent in terms of productivity. In addition, since gas nitriding or gas nitrocarburizing treatment is performed before the aging treatment is completed, aging treatment and nitriding treatment can be performed simultaneously in a single heat treatment furnace, and the aging treatment time tA and the t
The ratio tN/LA to the nitriding time tN occupied in A is L
Since N/tA: 0.015 to 0.25, it is possible to significantly improve the fatigue life without generating the hard and brittle nitrogen compound ε phase (Fe3N). For example, when the total stress is 165 kg/mTl, the number of repetitions A steel heald with a high fatigue strength of about 105 can be easily manufactured. As described above, the present invention has great industrial practical value as a heat treatment method for maraging steel endless belts.

[Brief explanation of drawings]

Figure 1 is a heat treatment diagram according to the present invention, Figure 2 is a graph showing the relationship between aging treatment temperature, aging treatment time, and hardness, and Figure 3 is a graph showing the relationship between nitriding treatment time, number of repetitions, and total stress. FIG. 4 is a graph showing the relationship between depth from the surface and hardness with respect to nitriding treatment time, and FIG. 5 is a graph showing the relationship between tN/tA and fatigue life improvement rate.

Claims (1)

[Claims] 1. An endless belt made of maraging steel is aged at a temperature of 450 to 550°C and an aging time of t_A20 to 1.
When aging treatment is performed for a time, gas nitriding or gas soft nitriding treatment is performed before the aging treatment is completed, and the aging treatment time is t.
Ratio t_ of _A and nitriding treatment time t_N occupied in t_A
N/t_A, t_N/t_A: 0.015 to 0.25
A heat treatment method for an endless belt made of maraging steel, characterized by