JPH01142021A - 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 forming a metallic belt from a seamless steel pipe made of Ni-Co-Mo steel with a specific composition by means of cold working and subjecting this belt to solution heat treatment, to ageing treatment, and then to 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, <0.01% Ti, <0.15% Al, <0.003% N, <0.0015% O, and the balance Fe is formed into a thick-walled seamless steel pipe by means of hot extrusion, and this pipe is formed into a thin-walled tube stock by means of spinning working, and this tube stock is cut into a breadth needed 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 for manufacturing a seamless metal heald for power transmission used in continuously variable transmissions of automobiles and the like.

[Conventional technology]

BACKGROUND ART Metal belts have begun to be used in continuously variable transmissions for automobiles and the like, since excellent material strength is required for the healds used to transmit power. In this case, since metal belts have low elasticity, it is a prerequisite that thin seamless healds 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 a heald, and after being processed into a heald, 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, belts are required not only to withstand torque, but also to be small and lightweight, and extremely strict requirements are placed on the belts, such as not being damaged over a long period of time.

It is difficult for seamless metal belts produced by conventional methods to fully satisfy these requirements, and their workability and
Currently, even higher performance is required in all aspects of material strength, fatigue strength, and wear resistance.

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.

○ Processability As mentioned above, the seamless metal belt that is the object of the present invention needs to be able to be processed into a thin layer in order to ensure elasticity. Marage steel, which has traditionally been used as a material for this type of belt and has worn out, can be processed to a certain extent, but when the wall thickness becomes 0.2 m or less, cracks during processing due to inclusions and surface defects (blisters, wrinkles) occur. ) is more likely to occur.

By the way, the thinner the belts currently used in automobile continuously variable transmissions are, the lower the bending stress, so the thickness is 0.2.
1) The following are preferred.

According to the research conducted by the present inventors, J, I
The B and C system cleanliness specified in S G O555 is 0.
It is effective to suppress P and S to 0.2% or less.
It has been found that it is necessary to keep ,N,0 sufficiently low. In particular, N forms hard TiN together with Ti and promotes the occurrence of cracks and surface defects, so it is necessary to strictly control at least one of N and Ti.

○ Material strength and fatigue strength When used in continuously variable transmissions of automobiles, etc., hardness of Hv500 or higher is required to withstand tensile strength during use. However, when the HV exceeds 650, the fatigue strength decreases. In the marage net, Ni, Co, Mo, and Ti contribute to the tensile strength, but as mentioned above, Ti
This is a factor in the formation of N. If Ti is not used, there is a concern that the tensile strength will decrease, but the required tensile strength is HV5 in terms of hardness.
It has become clear from experiments by the present inventors that within the range of 00 to 600, the necessary tensile strength can be ensured without depending on Ti in particular. If the addition of Ti becomes unnecessary, not only will the component cost be reduced accordingly, but strict restrictions on N will no longer be necessary, making it possible to achieve significant cost reductions in terms of both component costs and steel manufacturing costs. .

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 restrictions on both sides,
It is also necessary to add restrictions to C.

Among inclusions, especially oxides, O≦0.0
It has been found that by increasing the content to 15%, the B+C system cleanliness specified in J r sco 555 is rapidly improved, cracks during processing and surface defects are prevented, and fatigue strength is improved.

In Figure 1, the basic components are 0.005C-0,0ISi-〇, 01Mn-0,005P-0,00l5-18Ni-
8,5Co-5,5Mo70.06Af-0,002N
This figure shows the change in B+C system cleanliness (J I 5G (1555)) when the amount of 0 in the steel is changed in maraging steel.As is clear from the figure, B+C
The amount of O in steel is dominant for system cleanliness, and when the amount of O in steel is 0.0015% or less, the cleanliness of the B+C system is significantly improved.

It should be noted that B-based inclusions distributed in a dot array have a greater negative impact on fatigue than independent C-based inclusions, but when O is 0.
．． It has also become clear that by limiting the content to 0.0015% or less, B-based inclusions are significantly reduced, and at 0.0010% or less, they completely disappear, resulting in a marked improvement in fatigue strength.

○ 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 large bending strains are applied, such as the seamless metal heald 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 (embrittlement layer) on the belt surface9. believed that the hardness distribution of the heald cross section was important, and as a result of various experimental studies, found that it was effective to provide a hardness distribution as shown in Figure 2.

That is, if the surface hardness is less than 780 Hv, sufficient wear resistance and compressive residual stress will be obtained, but if it exceeds ff'Hv s 60, a brittle layer will be formed and early breakage will occur due to bending strain, so the surface hardness is set to 780 to 860 Hv. Heald thickness is 0.2mm
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, HV500-60
Set to 0.

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: o, o 1% or less, Si: 0.05% or less, Mrz
o, 05% or less, P: 0.01% or less, S; 0°01%
Below, Ni: 16-19%, Co: 8-15%, Mo:
3 to 6%, Ti: 0.01% or less, A1: 0.15% or less, N: 0.003% or less, 0:0゜0015% or less, and the remainder substantially consists of Fe as a metal belt. cold processing, then at 800-880℃ for 0.5-2h
After performing solid solution treatment of r, 420 to 520
Aging treatment for 1 to 6 hours at ℃, then 420 to 5 hours
The gist of the present invention is a method for manufacturing a seamless metal belt, which is characterized by carrying out a nitriding treatment using substantially NH gas alone at 20° C. for 1 to 10 hours.

[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 C of the material: If it exceeds 0.01%, carbides are formed, the amount of precipitated intermetallic compounds is reduced, and fatigue strength is reduced.

For this reason, C should be 0.01% or less, preferably 0.005% or less.

Si, Mn: Both form inclusions such as Sio, MnO, MnS, etc. and reduce fatigue strength, so 0.0
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 0.010% 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 the strength will decrease. 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: TiNTiC in the amount naturally mixed as an impurity
Since it forms and reduces fatigue strength, it is limited to 0°01% or less.

A2: It is effective in deoxidizing, but if it exceeds 0.15%, the amount of alumina-based oxide increases and the durability decreases, so 0.1%
5% or less.

N: This is an element that forms TiN, but since it hardly contains Ti, there is no need to limit it extremely. But 0.00
If it exceeds 3%, nitrides are formed with elements other than Ti, reducing toughness, so it is limited to 0°003% or less.

0: Formation of 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 less 0 is, the more advantageous it is, 0.001%
By doing the following, there will be no B-based (dot array) inclusions, and the fatigue strength will be significantly improved.

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

(2) Degassing treatment such as VOD may be used to reduce agglomeration inclusions, but vacuum induction melting is preferably used. After dissolving,
It is also effective to perform remelting using a high vacuum arc.

■ The steel ingot obtained by processing ingots is hot-forged or hot-extruded into thick-walled seamless tubes, which are then formed directly or after solution treatment into raw tubes for metal healds 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 to the width required for the heald. In belt rolling, a raw pipe is cut into heddles 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. or 0.5 hr, undissolved intermetallic compounds remain in both cases, 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 800 to 880
The treatment time is 0.5 to 2 hours at ℃.

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

(C) Nitriding process Normal gas nitriding process uses NH to promote the dissociation of N.
, by mixing about 50% RX gas with the gas, but if the metal belt targeted by the present invention is treated in such an atmosphere, the heald surface hardness will exceed Hv860 and a brittle layer will be formed. As a result, 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.

Regarding the treatment temperature, if it is lower than 420° C., the decomposition of NH becomes insufficient, and a nitrided layer with the required surface hardness and depth cannot 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 Hv860, significantly reducing fatigue strength.

Therefore, it is set as 1~]Ohr.

〔Example〕

Next, an example will be described.

Steels within the scope of the present invention indicated by A-H in Table 1, and I-N
500 kg each of steels outside the scope of the present invention shown in Table 1 were formed into ingots by vacuum induction melting and high vacuum arc remelting. Thereafter, the obtained steel ingots are hot extruded into thick-walled seamless steel tubes, and then spun into thin-walled raw tubes for healds with a wall thickness of 0.18 to 0.51 mm and an inner diameter of 100 to 250 mm. And so. For those whose wall thickness did not reach 0.18 μl, the wall thickness was further reduced to 0.181 by heald rolling.

Then, a 10 mm wide helt was cut out from each of the obtained raw tubes, and it was subjected to solid solution treatment under the conditions shown in the left column of Table 2.
After performing aging treatment as necessary, nitriding treatment using NH and gas alone was performed. For comparison, some of the helds were subjected to normal gas nitriding treatment using NH1 gas-50% RX gas.

After the treatment, the belt was examined for surface cracks, cross-sectional hardness distribution, and fatigue strength. 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 workability, and D =]5t (D = diameter of bending rod, t: heald wall thickness)
A belt was wrapped around a round bar, and the presence or absence of surface cracks was determined when the belt was bent at 180°C, and the fatigue test was 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 measured by placing the heald in a Boogu, rotating it, and applying a constant bending stress (pulsating vibration, 2 to 50 ktr f/cm2).
), and evaluate it by the limit number of times N.
N≧107 was considered to be a pass.

In Table 2, floors 1 to 1) are examples of the present invention in which steels A to H having compositions within the range of the present invention were processed into belts and then heat-treated under conditions within the range of the present invention.

In either case, no surface cracks occurred and no brittle layer was formed. Regarding the hardness distribution of the heald cross section, the surface hardness is Hv780 to 860, the nitrided layer depth is 20 to 40 μm,
The center hardness is within the range of Hv500 to Hv650 and satisfies the target hardness distribution shown in FIG. All fatigue strengths exceed the passing line of N=IX107.

hh12 to 17 are steels A whose component 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 case 12, the treatment temperature in the solution treatment is too high and the treatment temperature in the nitriding treatment is too high, resulting in excessive surface hardness and surface cracking, and insufficient fatigue strength. positive 13
Since the nitriding temperature is too low, the surface hardness and depth of the nitrided layer are insufficient, and the fatigue strength is also insufficient. No,
At +4, the nitriding time was too long, resulting in a brittle layer. In No. 15, the aging treatment temperature is low and the nitriding treatment temperature is also low, so precipitation strengthening cannot be obtained, and the depth of the nitriding layer is also shallow, so sufficient fatigue strength cannot be obtained.

In floor 16, the aging treatment temperature and time were too high, resulting in overaging, which resulted in a decrease in strength and ductility. 1
In lh17, 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.

Grades 18 to 23 are other comparative examples in which steels 1 to 2 whose compositions are outside the range of the present invention were processed using a held and then heat-treated under conditions within the range of the present invention.

In positive 18 (using steel I), Ni was insufficient and A1 was 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 Th21 (using steel), Si and 0 are excessive,
Due to the lack of MO, the center hardness is insufficient and the fatigue strength is also insufficient. In case 22 (using steel M), P, Mo
Also, due to excessive Ti content, hardening progresses. Number 23 (
In steel (using N), fatigue strength is insufficient due to excess Mn and N.

1) h24 is a conventional example in which general-purpose marage steel N, whose composition is outside the scope of the present invention, is processed into a belt and then subjected to normal gas nitriding treatment (NH, gas + 5 notes used, 500°C x 4 hours). .

Compared to the invention examples (numbers 1 to 1)), surface cracks were generated, the hardness distribution was far off target, and the fatigue strength was also extremely low. Therefore, the workability is low, and the material strength, 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 manufacture of healds for continuously variable transmissions in automobiles, and has great effects in improving the durability and reducing the size and weight of this type of transmission.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the O content in steel and the B+C system cleanliness, and 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: 0.01% or less, Al:
0.15% or less, N: 0.003% 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℃, if necessary, aging treatment for 1 to 6 hours at 420 to 520℃, followed by substantially NH_
A method for producing a seamless metal belt, characterized by performing nitriding treatment using three gases alone.