JP2011157616A - Ferritic stainless steel for brazing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which has satisfactory Ni brazability and Cu brazability. <P>SOLUTION: The ferritic stainless steel for brazing has a composition comprising, by mass, ≤0.03% C, >0.1 to 1% Si, ≤2% Mn, ≤0.05% P, ≤0.03% S, 16 to 25% Cr, 0.15 to 0.8% Nb, ≤0.03% N, ≤0.03% Al, ≤0.03% Ti, and the balance Fe with inevitable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a ferritic stainless steel used as a member constituting a heat exchanger or the like, and particularly relates to a steel material assembled by brazing in a hydrogen atmosphere.

The shape of the heat exchanger includes a multi-tube type and a laminated plate type. When pipes and plates are joined in production, joining by a mechanical joint, welding, brazing, and the like are performed. Of these, Ni or Cu brazing is the most commonly used joining method for the manufacture of small heat exchangers. Brazing is performed while maintaining a temperature of 1000 ° C. or higher in a reducing atmosphere such as a vacuum or a hydrogen atmosphere.
By reducing the passive film of stainless steel under these conditions and melting Ni brazing, the brazing wets and spreads the surface of the stainless steel, and brazing can be performed.
However, ferritic stainless steel has poor wettability compared to austenitic stainless steel, and stable brazing may be difficult. On the other hand, a ferritic stainless steel is disclosed in which brazing properties are improved by making the amounts of Ti and Al appropriate.

JP 2009-174046 A

  It was expected that the brazing of ferritic stainless steel could be performed stably by utilizing the invention of Patent Document 1. However, it has been found that in the production by an industrial brazing furnace, the expected brazing property may not always be obtained. As a result of examining the cause, the following was found.

In the case of brazing in a hydrogen atmosphere, the passive film is reduced by hydrogen and at the same time, the hydrogen itself is oxidized to become water vapor. For this reason, the dew point is lowered, an Al oxide film is formed, and wettability is lowered.
In the case of brazing in vacuum, if the degree of vacuum is maintained at a high vacuum of 1 Pa or less, good wetting and spreading properties are exhibited. However, Ar and N ₂ are introduced to lower the degree of vacuum to evaporate the brazing material. If the degree of vacuum deteriorates for some reason, an Al oxide film is also formed and wettability decreases.

Ti also has the same action, but unless Ti is intentionally added, its content is 0.03% or less and does not form an oxide film. However, since Al is used as a deoxidizing material, it contains a certain amount of Al if not consciously adjusted. In order to carry out industrially stable brazing, the limitation on the amount of Al described in Patent Document 1 is not sufficient, and the amount of Al in the steel is made sufficiently low to form an Al oxide film. It is necessary to suppress it.
The above-mentioned problem becomes remarkable especially in a hydrogen atmosphere where oxidation due to water vapor is likely to occur. Therefore, ferritic stainless steel excellent in brazing property, which is suitable as a heat exchanger member used for Ni brazing and Cu brazing, has been demanded.

The above object is achieved by severely limiting the Ti and Al contents in stainless steel and preventing the formation of an oxide film that inhibits brazing.
Specifically, by mass%, C: 0.03% or less, Si: more than 0.1 to 1%, Mn: 2% or less, P: 0.05% or less, S: 0.03% or less, Cr : 16 to 25%, Nb: 0.15 to 0.8%, N: 0.03% or less, Ti: 0.03% or less, Al: 0.03% or less, and other, if necessary,
(1) A total of 4% or less of at least one of Mo, Ni, Cu, V, W and Co,
(2) REM (rare earth element) and one or more of Ca in a total range of 0.2% or less,
Each of which is selectively contained, and is achieved by a ferritic stainless steel comprising the balance Fe and inevitable impurities.

FIG. 1 shows the brazing properties and the Al content in the stainless steel when various brazing materials of BNi-5, 7 and BCu-1 are used. Brazing was performed in a hydrogen atmosphere with a dew point of −50 ° C. by raising the temperature to the brazing temperature in 20 minutes, and holding for 10 minutes. The brazing temperatures were 1150 ° C. for BNi-5, 1000 ° C. for BNi-7, and 1120 ° C. for BCu-1.
It can be seen that a steel material having an Al content of 0.03% or less exhibits good expansibility with a wet area of 100 mm ² or more using any brazing material.

  According to the present invention, a ferritic stainless steel having good Ni brazing and Cu brazing properties can be provided. By using this steel, a heat exchanger having a lower material cost than that of a conventional heat exchanger member using austenitic stainless steel as a member is realized.

FIG. 1 is a graph showing the relationship between the wettability of brazing and the Al content of steel for each brazing material in a hydrogen atmosphere.

  The reason for selecting the range for each alloy element constituting the ferritic stainless steel of the present invention will be described.

  C and N are elements inevitably contained in the stainless steel. When the C content and the N content are reduced, the formation of carbides and nitrides is reduced, and the weldability and the corrosion resistance of the welded portion are improved. However, since the refining time becomes longer for the reduction and the cost of the stainless steel production increases, it was decided to allow the content of C up to 0.020 mass% and N up to 0.020 mass%.

  Si is added as a deoxidizer for stainless steel. However, excessive Si content hardens the ferrite phase and causes deterioration of workability and toughness, so the upper limit is made 1% in the present invention.

  Mn combines with S contained as an impurity in stainless steel to form MnS, which is a chemically unstable sulfide, and lowers the corrosion resistance. Therefore, the lower the Mn content, the better. In the present invention, the upper limit is 2%.

  P is desirable to be low because it impairs the toughness of the base material and the brazed part. However, since dephosphorization by refining is difficult in the production of Cr-containing steel, excessively increasing the cost, such as careful selection of raw materials, is required to minimize the P content. Therefore, in the present invention, as in general ferritic stainless steel, P content up to 0.05% by mass is allowed.

  S is an element that forms MnS that tends to be the starting point of pitting corrosion and inhibits corrosion resistance. Further, since hot cracking of the brazed portion is likely to occur, the S content is specified to be 0.03% by mass or less.

  Cr is a main constituent element of the passive film, and improves local corrosion properties such as pitting corrosion resistance and crevice corrosion resistance. However, when the Cr content is increased, it is difficult to reduce C and N, which causes a deterioration in mechanical properties and toughness and an increase in cost. Therefore, in this invention, Cr content shall be 16-25 mass%.

  Nb is an element effective for fixing C and N and preventing intergranular corrosion, which is a problem in stainless steel, and needs to be added in an amount of 0.15% or more. However, if added excessively, the stainless steel is hardened, and in the present invention, the upper limit of the Nb content is 0.80% by mass.

  Ti, like Nb, fixes C and N and is an effective element for the purpose of preventing intergranular corrosion, which is a problem with stainless steel. However, since it is an element that is very easy to oxidize, a Ti oxide film is easily formed even in a high vacuum or hydrogen atmosphere with a low dew point, and the brazing property is lowered. Therefore, in this invention, Ti content mixed as an impurity is restrict | limited to 0.03 mass%.

  Al is used as a deoxidizing material. Like Ti, it is an element that is very easy to oxidize. Therefore, an Al oxide film is easily formed even in a high vacuum or hydrogen atmosphere with a low dew point, and the brazing property is lowered. Therefore, in the present invention, deoxidation is performed without using Al, or even in the case of Al deoxidation, the upper limit of the Al content remaining in the steel is 0.03% by mass.

Mo, Ni, Cu, V, W, and Co are effective elements for improving the corrosion resistance level together with Cr, and may be added in an appropriate amount depending on the intended use. Moreover, REM (rare earth element) and Ca can also be added in order to improve hot workability.
The upper limit in the case of adding is as follows, respectively.
Mo, Ni, Cu, V, W and Co: 4% or less in total REM (rare earth element), Ca: 0.2% or less in total

  Stainless steel having the chemical composition shown in Table 1 was melted, and a hot-rolled sheet having a thickness of 3 mm was produced by hot rolling. Thereafter, the plate thickness was 1.0 mm by cold rolling, finish annealing was performed at 1000 to 1070 ° C., and pickling was performed to obtain a test material. The comparative steel 17 is austenitic stainless steel SUS304L.

The obtained steel material was used to evaluate the wettability of the wax. On a 50 mm × 50 mm flat plate, 0.1 g of brazing was applied in a range of φ6 mm (area 28 mm ² ), and brazing was performed under the hydrogen atmosphere conditions shown in the table below.

After the brazing treatment, the area where the brazing wetted and spread was determined. For Ni brazing, wetting area passed if more than 100 mm @ 2 (○), and unacceptable (×) in the case of less than 100 mm @ 2, in the case of Cu brazing, passed when spread area of 70 mm ² or more wetting, 70 mm The case of less than ^{2 is} regarded as rejected (x) and shown in Table 1. Steel types within the scope of the present invention have good wettability for any brazing.

As described above, by using the present invention, a ferritic stainless steel having good Ni brazing and Cu brazing can be provided. By using this steel, a heat exchanger having a lower material cost than that of a conventional heat exchanger member using austenitic stainless steel as a member is realized.

Claims (4)

% By mass
C: 0.03% or less,
Si: more than 0.1 to 1%,
Mn: 2% or less,
P: 0.05% or less,
S: 0.03% or less,
Cr: 16 to 25%,
Nb: 0.15 to 0.8%,
N: 0.03% or less,
Al: 0.03% or less Ti: 0.03% or less Ferritic stainless steel for brazing composed of the balance Fe and inevitable impurities.   Furthermore, the ferritic stainless steel of Claim 1 which contains 1 or more types of Mo, Ni, Cu, V, W, and Co in the range of 4% or less in total.   The ferritic stainless steel according to claim 2, further comprising at least one of REM (rare earth element) and Ca in a range of 0.2% or less in total. The ferritic stainless steel for hydrogen brazing according to claim 1.

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