JPH01142022A - Manufacture of seamless metallic belt - Google Patents

Abstract

PURPOSE:To manufacture a seamless metallic belt excellent in workability, material strength, fatigue strength, and wear resistance by cold-working a seamless steel pipe made of Ni-Co-Mo steel with a specific composition into a metallic belt and then subjecting the above belt to solution heat treatment, ageing treatment, and nitriding treatment. CONSTITUTION:An ingot of an alloy steel having a composition consisting of, by weight, <0.01% C, <0.05% Si, <0.05% Mn, <0.01% P, <0.01% S, 16-19% Ni, 8-15% Co, 3-6% Mo, 0.3-1.2% Ti, <0.15% Al, <0.0020% N, <0.0015% O, and the balance Fe is hot-extruded into a thick-walled seamless steel pipe, and this pipe is subjected to spinning working so as to be formed into a thin-walled tube stock, which is successively cut into a breadth necessary for a belt. This belt is subjected to solution heat treatment at 800-880 deg.C for 0.5-2hr and, if necessary, to ageing treatment at 420-520 deg.C for 1-6hr, and finally to nitriding treatment at the same temp. in an atmosphere of NH3 gas alone for 1-10hr.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of manufacturing a seamless metal belt for power transmission used in continuously variable transmissions of automobiles and the like.

[Conventional technology]

BACKGROUND ART In continuously variable transmissions of automobiles, belts for transmitting power are required to have excellent material strength, so metal belts are beginning to be used. In this case, since metal belts have low elasticity, it is premised that thin seamless belts are used by overlapping them according to the required torque. Therefore, such a seamless metal belt is required to be able to be processed thinly, to have high material strength and fatigue strength, and to have good wear resistance.

In response to these demands, 18% Ni-based marage steel has been used in the past because of its excellent workability, material strength, and fatigue strength. In addition, in terms of the manufacturing method, a processing method is adopted in which a cylindrical material made of marage steel is thinned by spinning processing etc. to the required wall thickness and circumference for the belt, and after being processed into a belt, it is nitrided to improve fatigue strength. It is customary to increase the

[Problem that the invention seeks to solve]

However, in continuously variable transmissions of automobiles, bell,
In addition to strong torque resistance, there is also a strong demand for smaller and lighter weight bearings, and extremely strict requirements are placed on them, such as ensuring that they do not break over a long period of time.

Seamless metal belts manufactured by conventional methods cannot be said to fully satisfy these requirements, and have problems with workability,
The current situation is that higher performance than ever before in terms of material strength, fatigue strength, and wear resistance is required.

In view of the current situation, the present invention provides a method for manufacturing a seamless metal belt that exceeds conventional levels in all of processability, material strength, fatigue strength, and wear resistance.

[Means for solving problems]

The present inventors investigated various seamless metal belts manufactured by conventional methods and found that there were problems mainly with the composition and the nitriding treatment after processing. It was found that fatigue strength and wear resistance were further improved.

0 Workability The seamless metal belt that is the object of the present invention needs to be able to be worked into a thin layer in order to ensure elasticity, as described above. Marage steel, which has traditionally been used as a material for this type of belt, can be processed to a certain extent, but when the wall thickness is less than 0.2n, cracks and surface defects (blisters, wrinkles) due to inclusions occur during processing. is more likely to occur.

By the way, the thickness of belts currently used in continuously variable transmissions of automobiles is 0.2t because the thinner the belt, the less bending stress.
It is said that less than m is preferable.

According to the research conducted by the present inventors, JISGO5
It is effective to suppress the C-based cleanliness specified in 55 to 0.02% or less, and for that purpose, P.

It has been found that S, N, and O must be kept sufficiently low. In particular, N forms hard TiN and promotes cracking and surface defects, so strict control is required.

○ Material strength and fatigue strength 3. When used in continuously variable transmissions of automobiles, etc., hardness of Hv500 or higher is required to withstand tensile strength during use. However, when Hv exceeds 650, fatigue strength decreases. Therefore, Ni, Co.

Belt hardness can be adjusted to HV500-650 by adjusting the amount of Mo and Ti.
It is necessary to manage the

In addition, the inclusions mentioned above not only cause cracks and surface defects during processing, but also cause fatigue fracture, so N,
P, S, and O require this double-sided limit, and it is also necessary to add a limit to C.

Among the inclusions, TiN is the main inclusion in maraging steel, so we thought that the influence of N was large, and as a result of repeated experiments, we found that N was reduced to 0. It has been found that by limiting OO to 2% or less, the C-based cleanliness specified by JIS G0555 can be rapidly improved, and cracks and surface defects during processing can be prevented and fatigue strength can be improved.

In Figure 1, the basic component is 0.005C-0,0ISj -0
,01Mn-0,005P-0,0OIS-18Ni-
8,5Co-5,0Mo'-0.5Ti-0.06Aj
! -C-based cleanliness (JISGO55
5) shows the transition. As is clear from the figure, the amount of N in the steel is dominant with respect to the C-based cleanliness, and the C-based cleanliness is improved when the N amount in the steel is 0.002% or less.

Note that TiN distributed in a dot array has a greater negative effect on fatigue than independent TiN, but when N is O, OO'
It has also been found that by limiting the content to 2% or less, inclusions in the form of dot arrays disappear and fatigue strength is significantly improved.

O Abrasion Resistance It is already known technology to improve fatigue strength by imparting abrasion resistance and imparting compressive residual stress to the surface. It is known that this effect can also be brought out by surface nitriding treatment of maraging steel. However, when a large bending strain is applied, such as the seamless metal belt that is the object of the present invention, conventional nitriding treatment (tufftride treatment...salt bath nitriding, ion nitriding, gas nitrocarburizing) has a negative effect on maraging steel. It was found that fatigue life was reduced. This is because conventional nitriding treatment inevitably forms a compound layer (!It! compound layer) on the belt surface.

The inventors believe that the hardness distribution in the cross section of the belt is important in order to improve the fatigue life and durability of the belt, and as a result of various experimental studies, the hardness distribution as shown in Figure 2 was determined. I learned that giving is effective.

In other words, if the surface hardness is less than 780 Hv, sufficient wear resistance and compressive residual stress cannot be obtained, and if it exceeds 860 Hv, a brittle layer will be formed and early failure will occur due to bending strain.
780-860 is required. Belt thickness is 0.2+n
If the nitride layer is about 20 to 40 μm (10 to 2 μm thick)
0%) thickness is required. If the nitrided layer is less than 20 μm, the nitrided layer will be insufficient, the wear resistance and compressive residual stress will be insufficient, and a bending strain exceeding 40 μm will cause early failure.

As for the center hardness, as mentioned above, Hv500~6
Set to 00.

When attempting to obtain such a cross-sectional hardness distribution by nitriding, mixing of RX gas to promote dissociation of N becomes an obstacle in conventional gas nitriding.

In addition, the processing temperature is 540 to 57 in conventional gas nitriding.
At 0° C., a brittle layer forms on the surface before the necessary hardness is achieved in the center. From these facts, it has been found that the preferred nitriding treatment is treatment at 420 to 520° C. using NH or gas alone.

The present invention was made based on this knowledge, and the weight %
C: 0.01% or less, Si: 0.05% or less, Mn:
0.05% or less, P: 0.01% or less, S:Q.

01% or less, Ni: 16-19%, Co: 8-15
%, Mo: 3-6%, Ti: 0.3-1.2%, A1:
O, 15% or less, N: 0.002% or less, O: 0°00
A seamless steel pipe containing 15% or less and the remainder substantially Fe is cold processed into a metal belt, and then heated at 800 to 880°C.
After performing solution treatment for 0.5 to 2 hours at 420 to 520°C, if necessary, aging treatment for 1 to 6 hours at 420 to 520°C.
The gist of the present invention is a method for manufacturing a seamless metal belt, which is characterized by performing nitriding treatment using I-gas alone.

[For production]

Hereinafter, the manufacturing method of the present invention will be explained in detail in the order of component composition and manufacturing method, and its effects will be clarified.

Component composition of the O material C: O. If it exceeds 01%, carbides are formed, the amount of precipitated intermetallic compounds is reduced, and fatigue strength is reduced.

From this, C is 0. (It should be 11% or less, preferably 0.005% or less.

Si, Mn: Both Si02. 0.0 because it forms inclusions such as MnO and MnS and reduces fatigue strength.
Limit to 5% or less. Si in terms of fatigue strength.

The lower the Mn content, the better.

P, S: Decreases fatigue strength due to grain boundary embrittlement and inclusion formation. Therefore, it should be 0.01% or less.

The lower the fatigue strength, the more advantageous it becomes, so the lower the fatigue strength, the more desirable.

Ni: If it is less than 16%, the strength and toughness of the material will decrease.
%, 100% martensite cannot be obtained and strength decreases. Therefore, Ni should be 16 to 19%.

Co: If it is less than 8%, the strength will decrease, and if it exceeds 15%, the toughness will decrease, so it is set at 8 to 15%.

If MO is less than 3%, the strength equivalent to Hv≧500 cannot be obtained, and if it exceeds 6%, the toughness is significantly reduced, so it is set to 3 to 6%.

Ti: If it is less than 0.3%, the minimum required Hv500 for this type of belt cannot be obtained, and if it exceeds 1.2%, the center hardness will be Hv500.
v650, and inclusions Ti(C).

N) increases, deteriorating durability. Therefore, Ti is set to 0.3 to 1.2%.

Aa: Effective in deoxidizing, but if it exceeds 0.15%, alumina-based oxides will increase and durability will decrease, so 0.1%
5% or less.

N: A harmful element that has a negative effect on fatigue strength, 0.002
% or less, and if it exceeds 0.002%, T i N will mainly increase rapidly and this will become a series of dots, resulting in a significant decrease in fatigue strength. Therefore, N is limited to 0.002% or less. Fatigue strength is N
The smaller the amount, the more advantageous it is, and when it is 0°001% or less, the durability is further improved.

O: Forms oxide-based (B, C-based) inclusions, 0.001
It is important to keep it as low as 5% or less, 0.0015%
If it exceeds this, the fatigue strength will drop significantly.

In terms of fatigue strength, the smaller the O content, the more advantageous it is, and by setting it to 0.001% or less, the durability is further improved.

O manufacturing method The manufacturing method basically consists of ingot making, processing, and heat treatment.

(2) Degassing treatment such as V or OD may be used to reduce the amount of agglomerated inclusions, but it is preferable to perform vacuum induction melting as much as possible. After melting, it is also effective to perform remelting using a high vacuum arc.

■ Steel ingots obtained by processing ingots are hot-forged or hot-extruded into thick seamless pipes, which are then formed directly or after solution treatment into raw pipes for metal belts by cold working. .

Two well-known cold working methods are spinning processing and belt rolling, and these are usually carried out singly or in combination. In spinning processing, the inner diameter of the raw tube does not change, only the wall thickness is reduced, and after processing, it is cut into the width required for the belt. In belt rolling, a raw pipe is cut into a belt shape in advance, and the wall thickness is reduced and the diameter is increased at the same time.

The form of cold working, degree of working, etc. are appropriately selected depending on the wall thickness, diameter, dimensional accuracy, etc. of the final product.

(2) Heat treatment (A) Solution treatment This treatment is performed after cold working to remove work hardening caused by cold working and to obtain a fine-grained martensitic structure.

If the heating time is lower than 800° C. and shorter than 0.5 hr, in any case, 4λ undissolved intermetallic compounds remain, resulting in a decrease in strength and toughness. On the other hand, if the temperature exceeds 880° C. and the time exceeds 2 hours, the crystal grains will become coarser, reducing the strength and toughness and increasing the deformation of the belt. Therefore, the solution treatment is carried out at 800-880℃.
The treatment time is 0.5 to 2 hours.

In addition, this treatment reduces the wall thickness by 80% due to cold working.
The following can be omitted. If this treatment is omitted, the nitriding conditions will change slightly, but even in that case, the treatment can be carried out under conditions within the scope of the present invention.

(B) Aging treatment If the aging treatment is less than 420° C. or less than 1hr, sufficient precipitation strengthening (Hv≧500) cannot be obtained in either case. On the contrary, 5
If the temperature exceeds 20° C. and the time exceeds 10 hr, overaging will occur in any case, and the strength and ductility will actually decrease. Therefore, the aging treatment is carried out at 420 to 520°C for 1 to 10 hours.

Note that if the nitriding treatment to be performed later is carried out under conditions that satisfy this aging treatment, this aging treatment can be omitted.

(C) Nitriding process Normal gas nitriding process uses NH to promote the dissociation of N.
, l gas mixed with about 50% RX gas. When a metal belt, which is the object of the present invention, is treated in such an atmosphere, the belt surface hardness exceeds H.860 and the brittle layer is formed. As a result of this formation, the fatigue life actually decreases. Therefore, in the present invention, the conversion process is substantially performed using NH3 gas alone.

In this case, even if RX gas is mixed up to about 10%, the formation of a brittle layer can be prevented by lowering the processing temperature and shortening the processing time. “Substantially” means R up to about 10%.
This means that X gas may be mixed.

If the treatment temperature is lower than 420° C., the decomposition of NHL will be insufficient, and a nitrided layer with the required surface hardness and depth will not be obtained. On the other hand, if the temperature exceeds 520°C, the decomposition of NH3 will proceed excessively, and the surface hardness will reach Hv860 before the necessary nitrided layer is formed.
This results in the formation of a brittle layer. Therefore 4
The temperature shall be 20-520°C.

If the treatment time is less than 1hr, the necessary nitrided layer will not be obtained, resulting in insufficient wear resistance and compressive residual stress. On the contrary, 1
If it exceeds 0 hr, the nitrided layer becomes too thick and cracks occur due to bending strain, and the surface hardness exceeds H.860, significantly reducing fatigue strength.

Therefore, it is set as 1~], Q h r.

〔Example〕

Next, examples will be explained using steels within the scope of the present invention shown in Table 1 with numbers A-H, and I
Steels outside the scope of the present invention, indicated by -0, were agglomerated by vacuum induction melting and high vacuum arc remelting, each weighing 500 kg. after that,
The obtained steel ingots are hot extruded into thick-walled seamless steel pipes,
After that, the wall thickness is 0.18~0.5mm by spinning process.
, a thin-walled raw tube for belts with an inner diameter of 100 to 250+n.

For those whose wall thickness did not reach 0.18 mm, the wall thickness was further reduced to 0.18 m by belt rolling.

Then, a belt with a width of 10n is cut out from each of the obtained raw tubes, subjected to solid solution treatment under the conditions shown in the left column of Table 2, and subjected to aging treatment if necessary. Nitriding treatment was performed.For comparison, some of the belts were investigated for the presence of surface cracks, cross-sectional hardness distribution, and fatigue strength after NH, gas + 5 notes treatment.The results are shown in the right column of Table 2. .

Surface cracking is an indicator of the presence or absence of a brittle layer and of workability. Judgment was made based on the presence or absence of cracks, and fatigue tests were omitted for those with cracks.

Regarding the hardness distribution, we measured the surface hardness, nitrided layer depth, and center hardness.The conditions necessary to ensure durability are as shown in Figure 2, when the surface hardness is Hv780-860 and the nitrided layer depth is 20~40μm, center hardness Hv500~65
Since it is 0, it was determined whether it falls within this range or not.

Fatigue strength is determined by placing the belt in a pulley, rotating it, and applying a constant bending stress (pulsation.2 to 50 fur f /1
1+1'') was repeatedly bent and evaluated by the limit number N of bending, and N≧107 was considered to be a pass.

In Table 2, columns 1 to 11 are examples of the present invention in which steels A-H having component compositions within the range of the present invention were heat-treated under conditions within the range of the present invention after being subjected to heat treatment.

In either case, no surface cracks occurred and no brittle layer was formed. Regarding the hardness distribution of the belt cross section, the surface hardness is HV780-860, and the nitrided layer depth is 20-40/7m.
, the center hardness is within the range of Hv500 to 650,
The target hardness distribution shown in FIG. 2 is satisfied. All fatigue strengths exceed the passing line N=IX]O'.

Nos. 12 to 17 are steels A whose compositions are within the range of the present invention.

This is a comparative example in which E was processed into a belt and then heat-treated under conditions outside the scope of the present invention.

In floor 12, the treatment temperature in the solution treatment is too high and the treatment temperature in the nitriding treatment is too high, so the surface hardness is excessive and surface cracks occur, and the fatigue strength is also insufficient. In point 13, the nitriding temperature is too low, resulting in insufficient surface hardness and nitrided layer depth, and insufficient fatigue strength. 1lkL
In No. 14, the nitriding treatment time was too long, resulting in a brittle layer. In ceramic 15, the aging treatment temperature is low;
Further, since the nitriding temperature is low, precipitation strengthening cannot be obtained, and the depth of the nitriding layer is also shallow, so sufficient fatigue strength cannot be obtained.

In No. 16, the aging treatment temperature and time were excessive, resulting in overaging, and the strength and ductility were actually reduced.

In 11kL17, the solution treatment temperature was too low, resulting in a decrease in strength and toughness, resulting in surface cracks and insufficient hardness in the center.

South Nos. 18 to 24 are other comparative examples in which steels 1 to 0, whose compositions are outside the range of the present invention, were processed into belts and then heat-treated under conditions within the range of the present invention.

In positive 1B (using steel T), Ni is insufficient and A2 is excessive, resulting in surface cracking and reduced fatigue strength. In No. 19 (using steel J), S and Ni are excessive, and Co
Due to the lack of hardness, the central part lacks hardness and fatigue strength is insufficient. In case of No. 20 (used for steel), due to excessive amounts of C and Co, hardening progresses, surface cracks occur, and fatigue strength is insufficient.

In case No. 21 (using steel), there is too much Si and not enough MO and Ti, so the center hardness is insufficient and the fatigue strength is also insufficient. P, M on floor 22 (using steel M)
Due to excessive amounts of o and ]i, curing progresses. positive 23
(Steel N used) has too much Mn and N,
The fatigue strength is insufficient, and 11k124 (using mO) has too much O, so the fatigue strength is also insufficient.

Box 25 is a general-purpose Marage M whose component composition is outside the scope of the present invention.
After processing N into a belt, we perform the normal gas nitriding process (using NH3 gas + 50% RX gas, 500℃ x 4 hours).
This is a conventional example implementing r).

Compared to the invention examples (11 & 11 to 11), surface cracks were generated, the hardness distribution was significantly off target, and the fatigue strength was also significantly low. Therefore, processability is low, material strength is low,
Fatigue strength and wear resistance are also significantly inferior.

〔Effect of the invention〕

As is clear from the above description, the manufacturing method of the present invention provides a seamless metal belt with high workability, material strength, fatigue strength, and wear resistance, and strict performance is required for these properties. For example, it can be applied to the production of continuously variable transmission belts for automobiles, and has great effects in improving the durability of this type of transmission, making it smaller and lighter, and so on.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the amount of N in steel and the C-based cleanliness.
FIG. 2 is a graph showing the belt cross-sectional hardness distribution necessary to ensure durability.

Claims (1)

[Claims] (1) C: 0.01% or less, Si: 0.05% by weight
Below, Mn: 0.05% or less, P: 0.01% or less, S
: 0.01% or less, Ni: 16-19%, Co: 8-1
5%, Mo: 3-6%, Ti: 0.3-1.2%, Al
: 0.15% or less, N: 0.0020% or less, O: 0.
A seamless steel pipe containing 0.015% or less and the remainder substantially Fe is cold processed into a metal belt, and then 800 to 88
After performing solution treatment at 0°C for 0.5 to 2 hours, if necessary, aging treatment at 420 to 520°C for 1 to 6 hours,
After that, substantially N
A method for manufacturing a seamless metal belt, characterized by performing nitriding treatment using H_3 gas alone.