JPH0254741A - Austenitic stainless steel having excellent high temperature salt corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of the title steel at the time of repeated heating and cooling by incorporating specific trace amounts of Ca, rare earth elements or Ti, V, Nb, B, etc., to an austenitic stainless steel. CONSTITUTION:As an austenitic stainless steel material used for a member subjected to repeated heating and cooling in the environment where, e.g., corrosion caused by molten salt at a high temp. is a problem, a stainless steel having the compsn. contg., by weight, <0.06% C, 2 to 10% Si, <2% Mn, 10 to 20% Ni, 16 to 23% Cr and 0.5 to 4% Mo, or furthermore contg. total 0.01 to 1.0% of one or more kinds among Ti, V and Nb, or 0.001 to 0.1% of one or more kinds among rare earth elements and 0.001 to 0.05% Ca or furthermore contg. 0.0005 to 0.01% B is used. The deposition of the carbide and nitride of Cr into the grain boundaries is prevented, by which the austenitic stainless steel having less intergranular corrosion in the high temp. corrosive environment can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used in high-temperature corrosive environments such as automobile exhaust gas purification systems, waste (including garbage) incinerators, and sheath heaters. This invention relates to austenitic stainless steel that is used in applications where it is repeatedly heated and cooled in environments where corrosion by salts, such as corrosion by molten salts, is a problem.

[Conventional technology]

Materials used in automobile exhaust gas purification systems, waste incinerators, sheath heaters, etc. are exposed to high temperatures and high-temperature combustion atmospheres, as well as coming into contact with various oxides and chlorides, so they can suffer significant corrosion damage from molten or high-temperature salts. receive.

In other words, an automobile's exhaust gas purification system is not only repeatedly heated and cooled to high temperatures, but also comes into contact with various Φ oxides inside, and outside with NaC, which is sprayed on roads to melt snow.
1. Contact with chlorides such as CaC1□ and MgC1□. Waste incinerators are exposed not only to a high-temperature combustion atmosphere but also to hydrogen chloride produced by the combustion of polyvinyl chloride, which is often used as a packaging material. In some cases, sheath heaters must heat water that contains salt.

In such a severe high-temperature corrosive environment, heat-resistant austenitic stainless steel has been widely used in consideration of its workability and weldability in addition to its heat resistance. Existing steel is SOS 30
4. SO5302B, SUS XM15I], 5U
S321, SO53109, etc. Among these applications, let's look at automobile exhaust gas purification systems.

In recent years, in cold regions, as mentioned earlier, road surface antifreeze agents made of chlorides are sprayed, and this chloride adheres to the bodies of moving cars, causing significant corrosion damage due to high-temperature salt corrosion caused by chlorides. Problems have arisen. In particular, since the gas components of the exhaust gas purification system are heated to high temperatures,
When salt adheres, there are many cases of severe corrosion and damage occurring within a short period of time. Furthermore, as the output of automobile engines increases, the exhaust gas temperature tends to rise, and it is expected that corrosion conditions will become even more severe in the future.

In an environment where such severe corrosion occurs, existing rust is not sufficient for any type of rust, and the development of new materials is desired.

Hereinafter, both high temperature salt and molten salt will be referred to simply as salt corrosion.

The present invention is directed to JP-A No. 60-230966 (Sumitomo Metals, Nippon Stainless Steel), which is characterized in that it provides an austenitic stainless steel that exhibits excellent salt corrosion resistance under the above-mentioned severe corrosive environment. teeth.

c: o, os% or less Si: 0.1-2.0% Mn: 2.0% or less Cr: 18-26% Ni: 16-30%, further Mo: 0.5-4.0 %, W: 0.01-4
．． 00%, containing one or more of V2 0.01 to 4.0%,
If desired, further N: 0.02 to 0.25% and/or Ti and Nb
Steel for high-temperature corrosive environments containing one or more of: 1.5% or less is disclosed. This tlN
Although this method is effective against corrosion due to chlorides at high temperatures, the S1 content is relatively low, making it difficult to withstand long-term use in extremely severe corrosive environments such as this one.

JP-A No. 62-20858 (Sumitomo Metals) has C:
Contains 0.10% or less Si: 1.0-5.0% Mn: 3.0% or less Cr: 10-15% or less Ni: 5-30%, and further N: 0.02-0.20% One or more of Mo, W and V = 4% or less One or more of Ti, Nb and Zr: 1 or more of the group consisting of 1.5 or less A heat-resistant steel with biased high temperature corrosion resistance due to chlorides Disclosed. This steel is also resistant to corrosion caused by chlorides at high temperatures.

JP-A No. 63-65058 (Nissin Steel) has C: 0
．． 06% or less Si: More than 4% and 10% or less Mn: 2.0% or less Cr: 15-23% or less Ni: 12-23% or less as basic additive elements, if desired.

One or more types of Nb or Ti 0.1 to 1.0% (however,
In this case, Nb/(C+N)≧8, Ti/(C+N)≧4
or (Tx + 0, 5Nb) / (C+N
)≧4) Steel containing one or more of the following: Ca: O, OO 1-0.05% REM: 0.001-0.05%, and one or more of Nb or Ti 0.1% in addition to the above basic additive elements ~1.0% (however,
In this case Nb/(C+N)≧8, Ti/
(C+N)≧4 or (Ti+0.5Nb)/
(C+N)≧4) and B: 0.0005
~0.010% and Cu: 0.5-2.5% are disclosed. This steel is also intended for high-temperature salt corrosion, but it is not necessarily sufficient to withstand extremely severe corrosive environments such as those used in five-wheel applications, and long-term use such as materials for automobile exhaust gas purification systems. It could not be said that it was.

The present inventors have conducted research to solve the above problems and have arrived at the present invention.

(Structure of the Invention) The present invention has the following basic components: C: 0.06% or less, Si: 2% or more and 10% or less, Mn: 2% or less, Ni: 10% or more and 20% or less, Cr: 16% or more23 % or less, Mo: 0.5% or more and 4% or less, and if necessary, trace amounts of Ca, rare earth elements,
Further, a steel containing Ti, V, and Nb-B is provided.

In other words, in order to improve the high salt corrosion resistance properties that are the subject of the present invention, in addition to sufficiently adding Si, XO is added, and if necessary, rare earth elements and/or Ca are added.
is added to increase protection in high-temperature corrosive environments. In addition, the addition of appropriate amounts of Nb, V, and Ti prevents sensitization during use, improves both intergranular corrosion resistance due to intergranular high-temperature corrosion and cold wet corrosion, and improves high-temperature strength and workability. and has improved hot workability by adding B.

The reason for the composition limitation in the steel of the present invention will be explained below.

C: An essential element to obtain high-temperature strength,
Cr2. combines with Cr at high temperature and forms grain boundaries. Since it precipitates as C, a Cr-depleted layer is formed near the grain boundaries, which promotes high-temperature salt corrosion. Therefore, the lower the better, the upper limit is 0.06%.
The following shall apply.

Si: One of the most important elements for improving oxidation resistance and high mixed salt corrosion resistance, but it was previously thought that exceeding 2% would impair the weldability and hot workability of austenitic steel. However, according to research conducted by the present inventors, it has been found that Si significantly improves the high mixed salt corrosion resistance of steel. In order to exhibit this effect, it is necessary to add 2% or more.

If the stiffness exceeds 10%, the σ phase is less likely to precipitate at high temperatures, resulting in deterioration of toughness. Further, as the amount of δ ferrite increases, hot workability becomes a problem, and workability also decreases, so the upper limit was set at 10%.

Mn: Improves hot workability but deteriorates oxidation resistance, so Mn should be limited to 2% or less.

N1: A basic element contained in austenitic stainless steel, but effective for high salt corrosion resistance, and high SL and M in the steel of the present invention.

Since δ ferrite is easily generated due to the addition of
%. On the other hand, since adding a large amount increases the cost, the upper limit was set at 20%.

Cr: A basic element in stainless steel that provides corrosion resistance and oxidation resistance. It is the most essential element for basic fishing in order to maintain heat resistance under severe corrosive environments such as those used in this application. Addition of less than 16% will not produce a sufficient effect in a severe and highly resistant mixed salt corrosion environment, so addition of 16% or more is required. on the other hand.

When added in excess of 23%, high Si, M
In o-containing steel, a large amount of δ ferrite is likely to be produced, which impairs hot workability, so the upper limit was set at 23%.

Mo2 This is an important additive element in the present invention and significantly improves the high mixed salt corrosion resistance. The effect becomes noticeable when 0.5% or more is added, but even if it is added over 4%, the properties do not improve much and the cost of the steel increases.
The cost increases as the amount of Ni required to limit the amount of ferrite increases. Therefore, the maximum amount is 4%.

Nb, Ti, V: C is added to 0.06 to prevent grain boundary erosion type high-temperature corrosion and moisture corrosion due to condensed water during rest.
% or less, but if this is still insufficient, Nb
, Ti, and V are added to prevent the precipitation of carbon and nitrides of Cr at grain boundaries. In addition, since it is used at high temperatures in this application, high-temperature strength is also important, and it is a very effective element for this purpose as well. Furthermore, it is effective in making crystals finer and improving workability. This effect requires a minimum addition of 0.01%. - If added in excess of 1%, it will adversely affect the workability of the steel and increase the amount of δ ferrite, so it should be limited to a maximum of 1%.

Ca and Rare Earths 2 The main uses of the steel of the present invention, such as automobile exhaust gas purification systems, waste incinerators, garbage incinerators, and sheath heaters, are subject to intermittent heating and cooling, which causes scale to easily peel off. Accelerates corrosion. It has been found that addition of Ca or rare earth elements is effective against such problems. In this case, Ca is required to be at least 0.001%, and O and OS% are the upper limits that can be practically contained. Similarly, for rare earths, at least o, ooi% is required, and even if the content exceeds 0.1%, the high mixed salt corrosion resistance will not be improved and it is expensive, so the upper limit is set at 0.1%. .

B: B is effective in increasing grain boundary strength and improving hot workability, but if it is less than o,oos%, such an effect cannot be obtained, and if it is more than 0.05%, the B compound , and the hot workability deteriorates.
, ooos% to 0.05%.

As unavoidable impurities, P is up to 0.04% and S is 0.
．． Up to 0.02% is allowed. The amount of nitrogen and other substances that are mixed in during normal melting is allowed.

(Specific Disclosure of the Invention) The present invention will be specifically explained below. The first basic experiment
The steel shown in the table is vacuum melted and forged to a thickness of 30IDI.
There were 11 plates. The forged plate was extracted at 1200°C and hot-rolled to a thickness of 5 mm, and then conventionally cooled and annealed to produce the plate of 2).The entire surface of the test piece was polished to #400 and subjected to a high-temperature salt corrosion test. The high-temperature salt corrosion test was conducted by immersing the test material in 20°C saturated saline for 5 minutes, heating it at 650°C for 2 hours, and air cooling for 5 minutes, each cycle consisting of 10 cycles. Tests were taken out and scaled, and high mixed salt corrosion resistance was evaluated by corrosion weight loss. The test results are also shown in Table 1.

From this result, SO5304, SO5310S, 5tl
It can be seen that the corrosion weight loss of ingot tN Na 3 to 6 containing high Si and steel Nα 7 to 11 containing Sl and MO is significantly reduced compared to standard steel such as S321. Figure 1 shows the influence of Si on the high-temperature salt corrosivity of chu. It can be seen that when 2% or more of 31 is added, the corrosion loss is significantly reduced, and that adding 2% or more of Si is very effective in imparting high monosalt corrosion resistance. Moreover, when Mo is added to this, the high-temperature salt corrosion resistance of the liquor is further improved. Figure 2 shows the influence of Mo on the hot salt corrosion resistance of steel with 3.3 to 3.4% Si added.
It can be seen that by adding 0.5% or more of MO, the corrosion loss is significantly reduced, and the high temperature salt corrosiveness of the liquor is improved. Generally, the excellent heat resistance of austenitic stainless steel is due to Cr formed on the surface, 0. Although this film exhibits excellent protection against atmospheric oxidation, it cannot provide sufficient protection in the high-temperature salt corrosion environment in which the steel of the present invention is used, and it corrodes significantly. be done. To this, 2% or more of SL is added,
It is believed that by adding 0.5% or more of 'Ao to this, it became possible to form a film that exhibits excellent protection in a high-temperature salt corrosion environment.

〔Example〕

Next, the present invention will be explained with reference to Examples and Comparative Examples. Table 2 shows the composition of the steel used in the test.

These steels are melted in a vacuum melting furnace and forged to a
mm, extracted at 1200°C, hot-rolled to 30 mm, and then subjected to normal rolling and annealing processes to obtain test pieces with a thickness of 2 mm. Using this test steel, immerse it in saturated saline solution at 20°C for 5 minutes.
Table 2 also shows the results of 10 cycles of a high mixed salt corrosion test consisting of heating at 550°C or 650'C for 2 hours and air cooling for 5 minutes. The corrosivity of high-temperature salt in chu was evaluated by the corrosion weight loss after the test.

From this table, it can be seen that the steel of the present invention has a smaller corrosion loss than other steels and is excellent in high-temperature salt corrosion resistance.

〔Effect of the invention〕

As shown in the examples above, in the past, existing austenitic stainless steels did not have sufficient resistance to corrosion in a highly mixed salt corrosion environment, but the steel of the present invention has sufficient resistance to high mixed salt corrosion. , which has great technological and social effects.

[Brief explanation of the drawing]

Figure 1 shows the effect of Si addition on the high mixed salt corrosion resistance at 650°C. Figure 2 shows steel with 3.3-3.4% SL added at 650℃.
The influence of Mo addition on the high mixed salt corrosion resistance of

Claims (1)

[Claims] 1. C: 0.06% or less, Si: 2% or more and 10% or less, Mn: 2% or less, Ni: 10% or more and 20% or less, Cr: 16% or more and 23% An austenitic stainless steel having excellent high-temperature salt corrosion resistance, containing Mo: 0.5% or more and 4% or less, with the balance being Fe and inevitable impurities. 2. Furthermore, one or more of Ti, V, and Nb is 0.0 in total.
The austenitic stainless steel according to claim 1, containing 1% or less. 3. Furthermore, one or more rare earth elements: 0.001% or more 0
．． The austenitic stainless steel according to claim 1 or 2, containing Ca: 0.001% or more and 0.05% or less. 4. The austenitic stainless steel according to claim 1, 2 or 3, further containing B: 0.0005% or more and 0.01% or less.