JP2000303150A - Stainless steel for direct diffusion joining - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce stainless steel for direct diffusion joining capable of general use. SOLUTION: This steel has a compsn. contg., by weight, <=0.08% C, 0.01 to 2% Si, 0.05 to 1.5% Mn, <=0.05% P, <=0.01% S, 0.005 to 0.1% Al, 13 to 32% Cr, 0.01 to 4% Ni, 0.1 to 6% Mo, <=0.05% Ti, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a heat exchanger, a mechanical part, a fuel cell part, a home electric appliance part, a plant part, a decorative component, a building material, and a part where various stainless steels are used. Involved in stainless steel that can be diffusion bonded without.

[0002]

2. Description of the Related Art In many fields, stainless steel has been used by diffusion bonding. The joining method is roughly classified into two methods: an insert material insertion method that inserts some kind of insert material between the surfaces to be joined so that solid-phase diffusion or liquid-phase diffusion occurs without delay. Can be classified as law.

First, regarding the insert method, for example,
Japanese Patent Application Laid-Open No. 63-1191993 discloses a diffusion bonding method in which a duplex stainless steel is sandwiched between base materials and subjected to a predetermined temperature and pressure, and Ni and Au described in Japanese Patent Application Laid-Open No. Liquid phase diffusion bonding method of stainless steel performed by inserting a foil-like insert material having the same composition as that of the μm-plated workpiece between base materials, Si described in Japanese Patent Publication No. 57-4431: 0.5 to 11. Many techniques have been known as shown in an austenitic stainless steel member diffusion bonding insert material containing 5% by weight, Ni and unavoidable impurities. In general, solid-phase or liquid-phase diffusion bonding is performed using a nickel-based brazing material (JIS: BNi-1 to 7) as an insert material. These technologies have the advantage of relatively simple and reliable diffusion bonding.However, the need for an insert material increases the cost, and at the same time, the state of contact between dissimilar metals at the joints When exposed to a corrosive environment, the joints are preferentially corroded, resulting in poor long-term reliability.

On the other hand, the direct method has recently been developed for cost reduction. For example, the S content in steel disclosed in Japanese Patent Application Laid-Open No. 62-199277 is set to 0.1%.
A diffusion bonding method for stainless steel in which deformation of the material can be avoided by bonding at a predetermined temperature in a non-oxidizing atmosphere by setting it to 0.01% or less. Diffusion bonding using uneven stainless steel foil material, method for producing catalytic metal carrier for exhaust gas purifying device for automobile, Japanese Patent Laid-Open No. 7-21
Japanese Patent Application Laid-Open No. 9-279310 discloses a catalyst metal honeycomb using stainless steel in which the amount of Al is suppressed to an impurity level of 0.8% in order to suppress the formation of an alumina film as an inhibiting factor at the time of diffusion bonding described in Japanese Patent No. 3918. A stainless steel foil having excellent diffusion bonding properties established based on the finding that stainless steel deformed by cold working and having been subjected to a large amount of deformation due to cold working is easy to perform diffusion bonding, and a metal carrier using the same, JP-A-9-99218 In order to reduce the damage caused by the formation of chromium carbonitride in the diffusion bonding process described in 1 above, a strong bonding strength obtained by stacking and winding a foil material made of a ferritic stainless steel to which a predetermined amount of Ti and / or Nb is added is applied. Diffusion-bonded metal carriers for catalysts and their manufacturing methods, but each has its own strengths and weaknesses. There is no technology.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a stainless steel for direct diffusion bonding which can be used for general purposes.

[0006]

Means for Solving the Problems In performing the diffusion bonding of stainless steel by the direct method, the inventors have set a common rule for both ferritic stainless steel and austenitic stainless steel so as to perform more general and reliable bonding. Vacuum degree is 10-4 torr
Various kinds of stainless steels were subjected to diffusion bonding in a vacuum furnace of r or less.

[0007] Among them, analysis of the surface of the stainless steel in which the bonding failure occurred was performed by X-ray diffraction and glow discharge spectroscopy. The coating was Ti carbide or nitride, and the Ti content was low. Bonding failure does not occur in low Nb-added stainless steel, that is, stainless steel capable of direct diffusion bonding by keeping only Ti content low among Ti and Nb added to suppress chromium carbide formation. That we can provide
The present invention has been completed, and its gist is as follows. (1) By weight%, C: 0.08% or less, Si: 0.01 to 2%, Mn: 0.05 to 1.5%, P: 0.05% or less, S: 0.01% or less , Al: 0.005 to 0.1%, Cr: 13 to 32%, Ni: 0.01 to 4%, Mo: 0.1 to 6%, Ti: 0.05% or less. A ferritic stainless steel for direct diffusion bonding, comprising Fe and unavoidable impurities. (2) Further, in weight%, Nb: 5 × (C% + N%)
The ferritic stainless steel for direct diffusion bonding according to the above (1), which contains at least 1% or less. (3) By weight%, C: 0.08% or less, Si: 0.01 to 2%, Mn: 0.05 to 1.5%, P: 0.05% or less, S: 0.01% or less , Al: 0.005 to 0.1%, Cr: 13 to 25%, Ni: 7 to 15%, Si + Mo: 1.5% or more, Mo: 6% or less, Ti: 0.05% or less Austenitic stainless steel for direct diffusion bonding, the balance being Fe and unavoidable impurities. (4) Further, in weight%, Nb: 5 × (C% + N%)
The austenitic stainless steel for direct diffusion bonding according to the above (3), containing at least 1% or less.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more detail.
First, the findings that led to the present invention will be described. The most important point of the present invention is that it was possible to extract a diffusion bonding property inhibiting factor common to both ferritic stainless steel and austenitic stainless steel.
It has been found that the effect of direct diffusion bonding property starts to be exhibited by suppressing the i content to 0.05% by weight or less.
Based on this finding, the upper limit of titanium addition was set to 0.05%. In the stainless steel for direct diffusion bonding of the present invention,
Ti should be suppressed to 0.05% or less, but the lower limit is not particularly set, since it may be added in this range or reduced as an impurity.

The reasons for limiting the components other than Ti will be described. First, a ferritic stainless steel having excellent diffusion bonding properties will be described. When performing welding that cannot be avoided when using stainless steel, C forms a compound called Cr ₂₃ C _{6 at the} site affected by heat, takes in the surrounding chromium, forms a chromium-deficient layer, and causes corrosion resistance. It is known that deterioration and reduction in bonding strength occur. In that sense, the smaller the C content, the better. Also,
From the viewpoints of workability and toughness, the lower the content, the better, and the upper limit is set to 0.08%. However, taking into account that if the temperature is too low, the steel making time becomes longer and the cost increases.
The upper limit is preferably 002%.

Although Si has a deoxidizing effect, its effect cannot be expected if it is less than 0.01%. On the other hand, if it exceeds 2%, workability is impaired. Therefore, the upper limit is limited to 2%.

Since Mn has no special effect, it has a normal component range of 0.05 to maintain strength.
% And 1.5% or less, which does not adversely affect toughness.

Since P is an element which lowers the corrosion resistance of stainless steel, the smaller the better, the better the corrosion resistance is deteriorated when the content exceeds 0.05%.

Since S is also an element that deteriorates the corrosion resistance, the lower the better, the better. The upper limit was set to 0.01%.

The lower limit of Cr is set to 13% or more in order to secure sufficient corrosion resistance, and the upper limit is set to 32% from the limit in steel production.

Al is added as a deoxidizing agent in an amount of 0.005% or more, which is a value at which the effect starts to appear, but if it exceeds 0.1%, the toughness is reduced. Therefore, the upper limit is set to 0.1%.
If it exceeds 0.1%, it is not preferable in terms of manufacturing cost.

In the case of ferritic stainless steel, Ni and Mo for further improving corrosion resistance are contained. Ni is
Addition of 0.01% or more, preferably 0.05% or more is effective in improving the corrosion resistance. However, if added too much, it becomes martensite during processing and causes hydrogen embrittlement, so it is 4% or less, preferably up to 1%. Add within the range. Mo is
When added in an amount of 0.1% or more, it is an element which exerts a very large effect on the improvement of corrosion resistance, especially pitting corrosion resistance, but the upper limit is 6% from the viewpoint of cost. When importance is attached to cost, it is preferable to be 3% or less.

Next, components other than Ti of austenitic stainless steel having excellent direct diffusion bonding properties will be described.
C forms a compound called Cr ₂₃ C ₆ at a site affected by heat when performing welding which cannot be avoided due to the use of austenitic stainless steel, and depletes the surrounding chromium to reduce corrosion resistance and strength toughness of the joint. In particular, it deteriorates at the grain boundaries. In that sense, the lower the C, the better. Further, from the viewpoint of workability and toughness, the lower the content, the better, and the upper limit is set to 0.08%. However, if the temperature is set too low, the steelmaking time is prolonged and the cost is increased. Therefore, the upper limit is preferably set to 0.002%.

Although Si has a deoxidizing effect, its effect cannot be expected if it is less than 0.01%. On the other hand, if it exceeds 2%, workability is impaired. Therefore, the upper limit is limited to 2%.

Mn has no particular effect on properties such as corrosion resistance. The strength can be maintained by adding 0.05% or more, and if it is 1.5% or less, the toughness is not affected.

P is an element that degrades the corrosion resistance of stainless steel, and is particularly harmful to stress corrosion cracking.
Therefore, it is necessary to reduce the upper limit to 0.05%.

S is combined with Mn or the like to form M soluble in water.
It is an element that forms nS and deteriorates corrosion resistance. Therefore, the upper limit is set to 0.01%.

Al is added as a deoxidizing agent in an amount of 0.005% or more, which is a value at which the effect starts to be exhibited, but if it exceeds 0.1%, the toughness is reduced. Therefore, the upper limit is 0.1%.
If it exceeds 0.1%, it is not preferable in terms of manufacturing cost.

Cr is an essential element for suppressing the overall corrosion, and the higher the content, the more effective. The range in which the austenite phase can be stabilized without adding unnecessarily expensive Ni and the corrosion resistance can be maintained is 13
% To 25% or less.

Ni is an essential component for turning austenitic stainless steel into austenite, and is effective in improving workability. Although there is a balance with other alloy components, the effect is exhibited by adding at least about 7%. In the case of a high Cr material, it is necessary to add Ni as calculated from the Schaeffler diagram.
If added too much, it is disadvantageous in terms of cost.
% Or less.

Mo improves stress corrosion cracking resistance.
In this case, the effect is exhibited by adding the lower limit of Si% + Mo% to 1.5% or more. However, if the amount of Mo exceeds 6%, the effect is saturated and the cost is disadvantageous. In particular, when cost is important, it is preferable to set the range to 3% or less.

As is common to both ferritic stainless steel and austenitic stainless steel, if the degree of vacuum is increased from 10 -4 torr in a vacuum furnace, the black or yellow film formed during diffusion bonding becomes Nb.
Was not detected at all. That is, the method of fixing C and N in the steel material by adding Nb to prevent the formation of a chromium-deficient phase does not adversely affect the direct diffusion bonding property. Based on this finding, it is possible to selectively add Nb as 5 × (C% + N%) as the lower limit for exhibiting the performance of fixing C and N, and 1% as the upper limit at which the effect is saturated.

Cu is also added to both ferritic and austenitic alloys because addition of 0.5% or more can improve the corrosion resistance. However, too much Cu causes precipitation of the ε-Cu phase and adversely affects toughness. % May be added.

In the production of the stainless steel of the present invention, any production method conventionally used for ferritic or austenitic stainless steels may be applied, provided that attention is paid to the limitation of the amount of Ti added. In consideration of bondability, sufficient attention should be paid to the cleanliness of the surface such as contamination by oils and fats.

[0029]

An embodiment of the present invention will be described. A stainless steel having the components shown in Table 1 was prepared by laboratory melting, and hot-rolled, annealed, pickled, and cold-rolled to prepare a steel sheet having a thickness of 0.3 mm. As a specific application example, laminated heat exchange was performed. A vessel was created. Since the laminated heat exchanger has a relatively complicated shape and requires airtightness and corrosion resistance including a joint, direct diffusion bonding is preferably used. Diffusion bonding was performed by increasing the degree of vacuum in the vacuum furnace with a rotary pump until the pressure became 10 −4 torr or less, and then heating to 1000 ° C. Weight is 0.
It was set so that a compressive stress of about 03 to 0.10 kg / mm ² was applied to the joint surface. The degree of vacuum and the level of pressurization are levels that can be sufficiently realized in a vacuum furnace in actual operation.
The state of the test specimen after the joining treatment was observed, and the presence or absence of brazing failure due to the formation of a black or yellow film was inspected. At the same time, compressed air at 5 atm was sealed inside to evaluate the presence or absence of leakage.

[0030]

[Table 1]

The results are as shown in the right column of Table 1. In each case, if Ti is 0.05% or less, it can be seen that poor diffusion bonding has not occurred. It has also been clarified that adding Nb in an amount sufficient to fix C and N does not cause poor brazing. Although the laminated heat exchanger has been described in this embodiment, the specific application of the stainless steel of the present invention is not limited to the laminated heat exchanger.

[0032]

According to the present invention, in manufacturing a stainless steel part requiring corrosion resistance by diffusion bonding by a direct method, the Ti content is reduced to within the limited range of the present invention, so that an actual production facility can be obtained. It has become possible to provide stainless steel that can be diffusion-bonded without using an insert even at a vacuum degree. The present invention realizes direct diffusion bonding of stainless steel, which can eliminate the need for an insert material and can reduce the cost, and at the same time, can ensure a highly durable and reliable joint without lowering the reliability of the insert material. The above value can be said to be extremely high.

 ──────────────────────────────────────────────────の Continued on the front page (72) Osamu Ikegami, Inventor 2-6-3 Otemachi, Chiyoda-ku, Tokyo Nippon Steel Corporation Technology Development Division (72) Inventor Toshihiko Matsudaira 2-, Otemachi, Chiyoda-ku, Tokyo 6-3 Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] C: 0.08% or less, Si: 0.01 to 2%, Mn: 0.05 to 1.5%, P: 0.05% or less, S: 0.01% by weight %: Al: 0.005 to 0.1%, Cr: 13 to 32%, Ni: 0.01 to 4%, Mo: 0.1 to 6%, Ti: 0.05% or less, A ferritic stainless steel for direct diffusion bonding, the balance being Fe and unavoidable impurities. 2. The ferritic stainless steel for direct diffusion bonding according to claim 1, further comprising Nb: 5 × (C% + N%) or more and 1% or less by weight%. 3. In% by weight, C: 0.08% or less, Si: 0.01 to 2%, Mn: 0.05 to 1.5%, P: 0.05% or less, S: 0.01 %, Al: 0.005 to 0.1%, Cr: 13 to 25%, Ni: 7 to 15%, Si + Mo: 1.5% or more, Mo: 6% or less, Ti: 0.05% or less An austenitic stainless steel for direct diffusion bonding, wherein the austenitic stainless steel contains Fe and inevitable impurities. 4. The austenitic stainless steel for direct diffusion bonding according to claim 3, further comprising Nb: 5 × (C% + N%) or more and 1% or less by weight%.