JPH0832941B2 - Sheath heater coated pipe material for cooking - Google Patents

Description

TECHNICAL FIELD The present invention relates to a sheath heater coated pipe material for cooking which is excellent in high temperature corrosion resistance and weldability.

Generally, in the case of a heater outer coating tube for cooking an electric stove, there are problems of high temperature corrosion due to chloride and weldability, and various researches and developments are currently underway. In particular, since this type of material is in the presence of high temperature chloride, when NaCl contacts the steel surface, Fe in the steel reacts with NaCl and has high volatility.
Since NaFeCl ₄ is generated and corrosion is promoted, it must be considered from a different viewpoint from the usual high temperature oxidation resistance.

[Conventional technology]

Soy sauce, salt, etc. are likely to adhere to the sheath heater for cooking, and if they are exposed to a high temperature atmosphere while they are adhered, they will be subject to high temperature corrosion (dry food) and the higher the temperature, the less the damage. It will be noticeable.

Conventionally, NCF800 (JIS G4901) materials, NCF600 materials, etc. have been used as sheath heater coated pipe materials that are subject to such high temperature corrosion. On the other hand, as VA material of NF800,
Heretofore, steels having a low Ni content have also been proposed.

For example, in Japanese Examined Patent Publication No. Sho 64-8695 (steel for high temperature dry environment in which chloride exists), Ni is in the range of 16 to 30 wt% (hereinafter, simply expressed as “%”), especially Mo and W. , V has been proposed to reduce Ni, and Japanese Patent Application Laid-Open No. 64-7305
No. 6 shows that Ni promotes internal erosion and is harmful, and S
i discloses that it is effective for oxidation resistance. further,
Japanese Unexamined Patent Publication No. 63-65058 proposes a steel excellent in high temperature corrosion resistance, which is characterized by increasing the amount of Si.

In addition, as an example of an alloy having excellent high temperature corrosion resistance, there is also an example of a Ni-based alloy containing Mo and high Si content as disclosed in Japanese Patent Publication No. 62-6623 (Japanese Patent Laid-Open No. 58-117847). .

As explained above, regarding the effects of alloying elements on high temperature corrosion in environments containing chlorides, etc., both effectiveness and harmful aspects have hitherto been reported in a mutually contradictory form, and they have not yet been reported. The reality is that it has not been finalized.

In addition, regarding the weldability of Fe-Ni alloys containing C and Si, "Summary of the National Meeting of the Welding Society" (Vol. 39, 1986 No. 2)
19.p128 to p129), it is reported that Mo improves weldability in the range of Si: 0.04 to 1.42%, but C has no effect, and if it exceeds 0.10%, it is harmful.

[Problems to be Solved by the Invention]

By the way, in recent years, 200V wiring to general households has been promoted, and along with this, demand for electric heaters is expected to expand. In particular, it is expected that the electric power of the above-mentioned sheath heater for cooking will also increase, and that the external coating material thereof will also use a high temperature material such as JIS NCF600 material. However, this NCF600 material has a number of problems (corrosion resistance in chloride-containing high temperature corrosive environment) that still need to be resolved in terms of cost and high temperature corrosiveness, and the emergence of alternative materials has been strongly desired these days. By the way.

However, all of the alloys proposed in Japanese Patent Publication No. 64-8695 and the like described above are inferior in corrosion resistance to the NCF600 material, and the target characteristics cannot be obtained. Moreover, for these alloys, high temperature corrosion resistance,
Add to improve nitric acid resistance and stress corrosion cracking resistance
In order for Si and Ni to form a low melting point eutectic of Ni silicide,
It has been pointed out that the solidification cracking susceptibility becomes high and the hot workability and weldability are significantly deteriorated, and the actual situation is that no solution has been found yet.

Further, the alloy according to Japanese Patent Publication No. 62-6623 contains a large amount of C (0.55 to 2.0%) and therefore contains a large amount of carbide precipitates. However, the precipitation of carbide in the alloy is rather harmful to the high temperature corrosion caused by sodium chloride, and the precipitate of this carbide is not only weldable but also
It also significantly deteriorates hot workability and cold workability.

On the other hand, the conventional technology described in the above-mentioned summary of lectures of the Japan Welding Society is low in Si% and inferior in high temperature corrosion resistance, so that it cannot be said that the alloy is basically aimed at the solution in the present invention.

The object of the present invention is to show sufficient corrosion resistance even when exposed to an atmosphere in which accelerated oxidation occurs due to the adhesion of chlorides, etc. in a high-temperature air atmosphere, and also for welding, which exhibits remarkably excellent weldability. An object of the present invention is to overcome the problems of the above-described prior arts by providing a sheath heater coated pipe material.

[Means for solving the problem]

For the above-mentioned purpose, the present inventors have obtained new findings on the influence of alloying elements on the corrosion resistance and weldability in a high temperature corrosive environment. That is, unlike the case of single addition, there is a case where a synergistic action is exerted between certain kinds of alloying elements to sometimes exhibit an unexpectedly excellent action and effect. That is, in the present invention, 10 to 25% Cr
Regarding the Fe-Cr-Ni-based alloys containing Fe, the following became clear from the mutual influence of each of these additive elements.

(1) Basically, in this ternary alloy, surprisingly, Ni
At high Ni of ≧ 50%, local erosion such as grain boundary erosion can be prevented without causing any other significant damage.

(2) Regarding the action of Si, there is an action of inducing local pit-like erosion in which the amount of Ni is small, but when the amount of Ni is increased to some extent, the high temperature corrosion resistance is significantly improved. And, when this Si is increased to more than 2.5%, if a certain amount of C is contained, the precipitation of Mo silicide is promoted, and thus the precipitation of Ni silicide that reduces the solidification crack susceptibility is suppressed, and as a result, To improve weldability. (3) As described above, addition of Si is effective for high temperature corrosion characteristics. However, in this Ni-based alloy, the solid solution amount of Si was originally small, and it was easy to form compound silicides of various forms, which was a cause of significantly deteriorating the weldability. On the other hand, if the morphology of the precipitate is controlled in advance to deposit a compound in a useful form in advance, it is prevented.

That is, the present invention is firstly characterized by focusing on the synergistic action of Ni and Si. That is, the amount of Ni is
Even if the amount is less than that of F600 material, the method of simultaneously adding controlled Ni and Si (higher than 2.5%) can conversely yield alloys with excellent high-temperature corrosion resistance and weldability. .

Secondly, we focused on molybdenum silicide as the compound silicide to be deposited in advance. This is effective in improving the weldability because it suppresses the formation of Ni silicide that is harmful to weldability and precipitates Mo silicide. Is.

Thirdly, in a high Ni-based alloy system containing a large amount of Si, the weldability is generally deteriorated due to the formation of the eutectic of Si and Ni. However, regarding the improvement of weldability, in the alloy system in which Mo silicide is precipitated, when a certain amount of C is contained, the high Si alloy is rather effective in improving the weldability. It was accompanied by the existence of a suitable quantitative relationship between Ni, Si and Mo for retention.

Based on such knowledge, the present invention has developed the following Fe-Ni based alloys.

That is, C: 0.015 to 0.030%, Si: over 2.5 to 6.0%, Mn ≦
2.0%, Ni: 50.0-80.0%, Cr: 10.0-26.0%, Mo: 1.0-9.0
% And Al ≦ 0.2% with the balance being Fe and impurities, and a sheath heater coating pipe material for cooking.

[Work]

According to the research conducted by the present inventors, Ni which is effective in high temperature corrosion resistance is effective.
In the Fe-Cr-Ni-based material containing 50% or more of
When contained over 2.5%, the synergistic effect with Ni can significantly improve beyond the expected range of usual Si addition, which was not obtained with conventional low Ni-Cr-Fe alloys. This was accompanied by the formation of an alloy with high high temperature corrosion resistance.

Hereinafter, the details of the component composition of the alloy of the present invention will be described together with the explanation of the reasons for limitation.

C: In this type of alloy, it is an element necessary for suppressing the precipitation of Ni silicide, which tends to occur during solidification, to promote the precipitation of molybdenum silicide, and for suppressing solidification cracking to obtain weldability and high temperature. However, if the amount of C is too large, the amount of precipitation of carbide increases, resulting in deterioration of corrosion resistance and workability. Further, this C is desirable to be low because it combines with Cr element at high temperature to precipitate Cr ₂₃ C ₆ at the grain boundary and forms a Cr deficient phase near the grain boundary to promote the progress of high temperature corrosion. Therefore, the upper limit of C is 0.030%.

Figure 1 shows the weld crack ratio: (total crack length / bead length)
It is a figure which shows the relationship between x100 and C%, and is 3% Si-60% Ni.
In the -3% Mo-remaining Fe alloy, it is clear that when C%: 0.015 to 0.10%, almost no cracking occurs.

Si: In the alloy of the present invention, it is the element that plays the most important role. ▲ Ni ⁸⁰ _A3 ▼ 50%, which has a significant effect on oxidation resistance, high temperature corrosion resistance, and nitric acid resistance due to its synergistic effect with Ni. Show. It is exerted far beyond the general effect of Si, which is said to have a corrosion resistance improving action in a high temperature environment where chloride is present. Therefore, under the condition that Ni ≧ 50%, the effect of addition occurs with a lower limit of 1.5%, but in the present invention, the content exceeds 2.5%. On the other hand, the added amount is 6.0%
If it exceeds, the weldability of high austenitic steel with high Ni will be impaired, and the ductility and toughness after long-term use at high temperature will be deteriorated in order to promote the precipitation of intermetallic compounds such as σ, so the Si content exceeds 2.5. It was set to ~ 6.0%.

Mn: It is an element necessary to maintain the hot workability of steel. However, if it exceeds 2.0%, the corrosion resistance and oxidation resistance in the high temperature environment in which chloride exists will deteriorate. Therefore, the Mn content was determined to be 2.0% or less. The Mn content is preferably adjusted to 0.1 to 0.5% if possible.

Ni: It is extremely effective in improving high temperature corrosion resistance and oxidation resistance in high temperature corrosive environments containing chlorides. Especially, when this Ni content is 50% or more, it is possible to dramatically improve the high temperature corrosion resistance of Si. Yes, below that, rather high such as precipitation of σ phase
It is not preferable because the disadvantage of using Ni is promoted. Therefore, Ni is 30% or more.

Further, the addition of Ni is also effective for improving the structural stability and weldability against precipitation of intermetallic compounds composed of Cr, Si, Mo, etc. In this sense, the more it is, the better, 80%.
Up to is effective. It is preferably in the range of 50 to 75%.

Cr: High temperature corrosion resistance in the presence of chloride and 900 ° C
It is effective for improving general oxidation resistance in the vicinity, but if the amount is less than 10%, scale peelability is large even in a high temperature corrosive environment due to chloride, and the desired effect cannot be obtained, so 10%
That is all. However, too much promotes internal erosion, so the upper limit is 28%. It is preferably in the range of 16.0 to 20.0%.

Mo: It is one of the very effective elements for improving the corrosion resistance and weldability in the presence of chloride in a high temperature environment, and is particularly effective for weldability. The effect of adding C together with Si on weldability is particularly effective. Has the effect of significantly increasing the value of at least 0.5%. However, if the addition amount of Mo is too large, the toughness and corrosion resistance are deteriorated and the scale peeling property is increased, so the content is made 9.0% or less. It is preferably 4% or less.

In the presence of Al: Cr, the effect of improving high temperature corrosion resistance and oxidation resistance can be expected, but it is harmful to weldability and
It is necessary to keep the percentage below.

W, V, Zr, Cu: Has the same effect as Mo on hot corrosion resistance in the presence of chloride. However, if any element is contained in excess of 3.0%, precipitation of intermetallic compounds is promoted to impair workability and increase scale releasability.

Note that FIG. 2 is a diagram showing the effect of Si% and Mo% on welding cracks. In the alloy of 0.03C-60Ni-16Cr-balance Fe, when Si ≧ 1.5%, especially when Si> 2.5. , ▲ Mo ⁸⁰ _A3
▼ It can be seen that cracks do not form under the condition of 1.0%.

〔Example〕

This example confirms the hot corrosion and weldability of the alloys of the present invention. The test was carried out by using alloys (No. 1 to No. 7) having the composition shown in Table 1 in a 10 kg ingot in an air induction furnace, hot forging and cold rolling, and 2.0 and 0.5.
A mm plate was used for the test. First, the high-temperature corrosion test piece was cut into a piece having a thickness of 0.5 mmt, a width of 20 mm and a length of 30 mm, and then subjected to atmospheric oxidation at 1050 ° C. for 30 minutes, and then subjected to the following high-temperature corrosion test.

The high temperature corrosion test was performed by immersing in saturated saline solution (5 minutes) → drying (10
Minutes) → repeated oxidation (800 ° C. × 30 minutes → air cooling 5 minutes 50 times) as one cycle. The results of 4 cycle tests are shown below.

On the other hand, the welding test is a restraint welding crack test when restraint butt TIG welding is performed at a current of 50 A and a speed of 900 mm / min.
The results are shown in Table 1.

As is clear from these test results, the alloy of the present invention (N
o.1 ~ No2) has a smaller maximum erosion depth than the comparative alloys (No3 ~ No.7) and no weld cracks occur.
The effectiveness of the alloy of the present invention was confirmed.

[Effects of the Invention] As described above, according to the present invention, it is possible to inexpensively provide a cooking sheath heater covered pipe material that exhibits excellent corrosion resistance in a high temperature corrosive environment containing chloride and is also excellent in welding.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of C content on the weld crackability of alloys containing 3% Si-60% Ni-3% Mo, and FIG. 2 is the effect of Si and Mo on the weld crackability. It is a graph shown.

Claims (1)

[Claims] 1. C: 0.015 to 0.030 wt%, Si: over 2.5 to 6.0 wt%, Mn ≦ 2.0 wt%, Ni: 50.0 to 80.0 wt%, Cr: 10.0 to 26.0 wt%, Mo: 1.0 to 9.0 wt %, Al ≤ 0.2 wt%, and the sheath sheath heater coated pipe material for cooking with the balance being Fe and impurities.