JP2002069591A - High corrosion resistant stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material excellent in corrosion resistance, mechanical properties, and economy for an oxidative decomposition treatment plant or the like using a high-temperature and high-pressure fluid. SOLUTION: The chemical component is mass%, C: 0.02% or less, Si:
1.0% or less, Mn: 2.0% or less, Cr: 20 to 27%, Ni + Co: 17 to 27
%, Mo + 1 / 2W: 2-7%, N: 0.01-0.3%, Cu: 0.1-3%,
High corrosion-resistant stainless steel whose balance is substantially iron and satisfies the following formula: Cr + Ni + Co + 2Cu + 4.1 (Mo + 1 / 2W) + 24N ≧ 62 B: 0.01% or less, Zr: 0.5% or less, C
a: 0.02% or less, Al: 0.1% or less, La: 0.04% or less, Ce: 0.04% or less, Y: 0.1% or less, or Ti: 0.5% or less, Nb:
One or more of 0.8% or less, Ta: 1.6% or less, and V: 1% or less can be added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus material related to a technology for using high-temperature and high-pressure fluid in a supercritical water / subcritical water oxidation plant capable of treating toxic waste.

[0002]

2. Description of the Related Art With the growing interest in environmental conservation throughout the society, treatment of hardly decomposable toxic substances represented by chlorine-containing organic compounds has been highlighted as a major problem in the future. Recently, an oxidation treatment technique using a high-temperature and high-pressure fluid has attracted attention as a decisive technique for these decomposition treatments.

When a substance exceeds predetermined temperature and pressure conditions,
A supercritical fluid state where liquid and gas are united together,
It provides a very active reaction field for chemical reactions of substances, including the subcritical fluid state before it. In such a state, water is a very stable and safe substance, and various applications to waste treatment in supercritical and subcritical states have been attempted.Currently, as a medium for oxidative decomposition of harmful organic compounds, water is used as a medium. It is one of the most widely used.

The oxidative decomposition method using supercritical water is in a state where the basic technology itself has been almost established, but practically leaves a problem in the corrosion resistance of the material. That is, the equipment material itself in contact with a chemical reaction field having extremely high reactivity is currently severely damaged by corrosion.

[0005] Therefore, Ni-base alloys having high corrosion resistance are mainly used. In addition, ceramic materials such as titanium oxide and carbon / nitride, zirconium and aluminum oxide are generally studied in part because of their good corrosion resistance.

[0006] In addition, austenitic heat-resistant steel is generally used for boilers and the like that require creep strength, steam oxidation resistance, and high-temperature corrosion resistance. In particular, the operating environment of boilers is becoming severer, and heat-resistant austenitic alloys have been developed for the purpose of sufficient corrosion resistance and excellent high-temperature strength.

For example, JP-A-6-322488 discloses that
In, C: less than 0.02%, Si: 1.5% or less (0.47 to 0.50
%), Mn: 0.3-1.5%, P: 0.02% or less, S: 0.005% or less, Cr: 18-2
6%, Ni: 20 to 40%, W: 0.5 to 10.0%, Nb: 0.05 to 0.4% (0.18 to 0.23% in the example), Ti: 0.01 to 0.2%, B: 0.003 to 0.008%, N: 0 .
0.05-0.3% (0.008-0.147% in the examples), and further, if necessary, Mo: 0.5-2.0% and / or Mg: 0.001-0.
05%, Ca: 0.001-0.05%, Rare earth element: 0.001-0.15% 1
Austenitic heat-resistant alloys containing one or more species have been proposed.

[0008] JP-A-2000-129403 discloses that C: 0.01 to 0.2
0% (preferably 0.035 to 0.15%), Si: 3.0% or less (preferably 0.5 to 2.0%), Mn: 0.01 to 3.0%, Ni: 15.0 to 40.0%, Cr:
15.0-30.0%, Mo: 0.01-1.0%, W: 2.0-8.0%, Nb: 0.05-0.8
%, Ti: 0.2% or less, B: 0.006% or less, N: 0.05 to 0.25% (preferably 0.07 to 0.02%), W / Mo is 2 or more and Fe 20 to 55%
Having, in addition, Ta0.01-0.5%, Zr0.001-0.2%, Hf0.0
Austenitic heat-resistant alloys containing at least one of 01 to 0.2% and superheated tubes for boilers using the same have been proposed.

[0009]

However, these materials have the following problems. First, for Ni-based alloys, assuming expensive and large-scale practical plants,
Material costs have become enormous, and there is a major problem in balancing cost-effectiveness with waste treatment. At present, ceramic materials are generally easily damaged by thermal shock and lack practical reliability.

From the viewpoint of reliability against thermal shock and the like, it is conceivable to use the above-described austenitic heat-resistant steel for boilers. However, these materials are mainly designed for durability in high-temperature corrosive environments, and therefore, in oxidizing and corrosive environments where supercritical water and subcritical water transition, wet corrosion mainly causes corrosion. Has a problem in corrosion resistance under the governing conditions. That is, both the technology described in JP-A-6-322488 and the technology described in JP-A-2000-129403 have not sufficiently considered the optimization of the addition of elements such as Cu and Mo that can improve the corrosion resistance to wet corrosion. Or not added at all. That is, there is a problem from the viewpoint of general corrosion in a wet erosion region.

The present invention solves the above problems and has excellent corrosion resistance and high reliability in mechanical properties as an apparatus material in an oxidative decomposition treatment plant for toxic waste utilizing a high-temperature and high-pressure fluid, It is another object of the present invention to provide a highly corrosion-resistant stainless steel excellent in economy.

[0012]

The above object is achieved by the following invention. In the invention, the chemical component is mass%,
C: 0.02% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 20 to 27
%, Ni + Co: 17-27%, Mo + 1 / 2W: 2-7%, N: 0.01-0.3%, Cu:
High corrosion resistant stainless steel containing 0.1 to 3%, with the balance being substantially iron, satisfying the following equation:

Cr + Ni + Co + 2Cu + 4.1 (Mo + 1 / 2W) + 24N ≧ 62 The element symbols in the formula indicate mass% of each element.

Here, any of the following items may be added in mass%. B: 0.01% or less, Zr: 0.5% or less, at least one of Ca: 0.02% or less, Al: 0.1% or less, La: 0.04% or less, Ce: 0.
04% or less, Y: 0.1% or less, one or more of Ti: 0.5% or less, Nb: 0.8% or less, Ta: 1.6% or less, V: 1% or less One or more of the above They can be added together.

The present invention was made in the course of studying the use of iron-based stainless steel instead of an expensive Ni-based alloy as a material used in the oxidative decomposition method using supercritical water.
As a result of intensive studies on the combinations of various additive elements, we succeeded in securing the corrosion resistance in the oxidative decomposition treatment without greatly increasing the amount of Ni added.

Hereinafter, the reasons for limiting the chemical components in the present invention will be described.

C: 0.02% or less C is preferably combined with Cr to form carbides and reduce the effect of improving the high-temperature corrosion resistance of steel by Cr, so that its content is small. If the C content is 0.02% or less, the deterioration of corrosion resistance is slight, so the addition amount is set to 0.02% or less.

Si: 1.0% or less Si is effective as a deoxidizing agent, but if it exceeds 1.0%, the precipitation of intermetallic compounds is remarkably accelerated, and the hot workability is lowered. Therefore, the addition amount of Si is set to 1.0% or less.

Mn: 2.0% or less Mn is a deoxidizing agent, and is used in an amount of 2.0% to improve hot workability.
It may include: If it exceeds 2.0%, the corrosion resistance is reduced. Therefore, the addition amount of Mn is set to 2.0% or less.

Cr: 20-27% Cr has an important function to improve the high-temperature corrosion resistance of steel.
In a high-temperature and high-pressure oxidation process environment, the effect is not sufficient if added less than 20%. On the other hand, if Cr is added in excess of 27%, brittle intermetallic compounds tend to precipitate, and mechanical properties and workability at high temperatures deteriorate. Therefore, the added amount of Cr is set in the range of 20 to 27%.

Ni + Co: 17-27% Ni enters the protective coating on the steel surface generated at high temperatures and improves the corrosion resistance in the high temperature and high pressure oxidation process environment by improving the adhesion of the protective coating. Regarding this effect, since Co is equivalent to Ni, the total is treated as the addition amount of Ni + Co. If the addition amount of Ni + Co is less than 17%, the effect of improving corrosion resistance is not significant. On the other hand, if Ni + Co is added in excess of 27%, the effect is not only saturated, but also impairs economic efficiency. Therefore, the addition amount of Ni + Co is set in the range of 17 to 27%.

Mo + 1 / 2W: 2 to 7% Mo has a remarkable effect of improving the corrosion resistance of steel against pitting corrosion. The effect is not remarkable when less than 2% of Mo is added, while when more than 7% is added, the high-temperature oxidation resistance is deteriorated. W is 1/2 of mass%, that is, W
Since it is equivalent to Mo at 1/2 W, it is treated as the addition amount of Mo + 1/2 W in total. Therefore, the added amount of Mo + 1 / 2W is set in the range of 2 to 7%.

N: 0.01 to 0.3% N has the effect of improving the corrosion resistance of steel against pitting corrosion, and at the same time, stabilizes the austenite structure of steel and suppresses the precipitation of brittle intermetallic compounds. Having. To obtain these effects, addition of 0.01% or more is necessary. However, addition of more than 0.3% increases steelmaking cost and impairs economic efficiency. Therefore, the addition amount of N is 0.01-0.3%
Within the range.

Cu: 0.1-3% Although Cu improves the acid resistance of steel, its effect is not sufficient when added below 0.1%, and hot workability is deteriorated when added over 3%. Therefore, the addition amount of Cu is set in the range of 0.1 to 3%. The present invention is based on the above chemical components as basic components,
To further improve various properties, the following elements can be added.

B, Zr: one or more of B ≦ 0.01% and Zr ≦ 0.5% B is effective for improving the grain boundary strength by adding a small amount, but when added over 0.01%, welding hot cracking may occur. Make the tendency remarkable. Therefore, when B is added, the content is set to 0.01% or less. Zr is also effective in improving the grain boundary strength, but 0.5%
If added in excess of, the tendency of welding hot cracking will be marked.
Therefore, when Zr is added, the content is set to 0.5% or less.

Ca, Al, La, Ce, Y: Ca ≦ 0.02%, Al ≦ 0.1%, La ≦
At least one of 0.04%, Ce ≦ 0.04%, Y ≦ 0.1% Ca, Al, La, Ce, Y forms a dense oxide film on the surface or Incorporated into oxides to improve high temperature oxidation resistance. But,
0.02% for Ca, 0.1% for Al, 0.04% for La, 0.04% for Ce
%, And in Y exceeding 0.1%, the hot workability of the steel is degraded and surface flaws are likely to occur. Therefore, Ca, Al, La, C
When adding e and Y, Ca: 0.02% or less, Al: 0.1% or less, L
a: 0.04% or less, Ce: 0.04% or less, Y: 0.1% or less.

Ti, Nb, Ta, V: Ti ≦ 0.5%, Nb ≦ 0.8%, Ta ≦ 1.6
%, V ≦ 1% One or more of Ti, Nb, Ta, V combine with carbon in steel to form carbides and suppress the formation of Cr carbides, thereby reducing corrosion resistance deterioration . However, 0.5% for Ti and 0 for Nb.
Addition of more than 8%, 1.6% for Ta, and 1% for V facilitates precipitation of brittle intermetallic compounds. Therefore, when adding Ti, Nb, Ta, V, Ti: 0.5% or less, Nb: 0.8% or less, Ta: 1.6%
Hereinafter, one or more of V: 1% or less are selected and added.

Formula for defining chemical components: Cr + Ni + Co + 2Cu + 4.1 (Mo +
1 / 2W) + 24N ≧ 62 The effect of chemical components on the corrosion rate in a high-temperature, high-pressure hydroxylated environment can be evaluated by the corrosion resistance index R of the following equation (1).

R = Cr + Ni + Co + 2Cu + 4.1 (Mo + 1 / 2W) + 24N (1) Here, the element symbols indicate mass% of each element.

According to the value of the equation (1), the corrosion rate of the apparatus material in a high-temperature and high-pressure hydroxylation environment containing chlorine ions is well arranged as shown in FIG. From this figure, the value of equation (1) is 62
In the above, the corrosion rate is 0.5 g / m ² h or less, and the level of corrosion resistance that is permissible as the device material can be secured.
At the same time, fluctuations in the corrosion rate rapidly converge, and stable corrosion resistance is exhibited. From the above, the value of equation (1) is 6
Define chemical components to be 2 or more. This is represented by the following equation: Cr + Ni + Co + 2Cu + 4.1 (Mo + 1 / 2W) + 24N ≧ 62 (2)

In these means, "the balance is substantially iron" means that the substance containing other trace elements including unavoidable impurities is used in the present invention as long as the function and effect of the present invention are not lost. Is included in the range.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION In practicing the present invention, it can be manufactured in the same manner as ordinary stainless steel based on the above chemical components. That is, a slab obtained by continuous casting or the like is rolled as it is or after cooling to produce a steel plate or a steel pipe.

The chemical components of the present invention are based on studies on the corrosion and practicality of equipment materials in a supercritical water / subcritical water environment. Through investigations, it was clarified that not only the oxidizing property of the medium in the high-temperature and high-pressure water environment but also the local corrosion due to hydrochloric acid generated in the decomposition process is likely to accelerate the material damage.

It is considered that, in the first place, in an environment containing chlorine and containing a large amount of oxidizing power, which is the object of the present invention, corrosion mechanisms close to high-temperature corrosion and wet corrosion, especially local corrosion, are accelerating material damage. However, improvement of high-temperature corrosion resistance and improvement of corrosion resistance in a local corrosion environment are incompatible in many aspects from the viewpoint of alloy design, and it is difficult to achieve both.

Therefore, the present inventors have conducted intensive studies on a technique for improving corrosion resistance from a new point of view. In the process, the stability of the oxide film generated at a high temperature is determined not only by the conventional Cr content but also by the Ni content. Was found to be affected. As a result of the investigation, it was found that if the oxide film is mainly composed of Cr oxide and contains Ni, the adhesion of the film to the base material and the protection tend to be improved.

Considering this tendency, a slight increase in the ductility of the oxide film itself and a reduction in the difference in linear expansion coefficient between the oxide and the base material are related to the improvement in the adhesion. Further, it is considered that the improvement of the protection is related to a decrease in the diffusion rate of oxygen and the like in the Ni-Cr-based oxide as compared with the Fe-Cr-based oxide. Heretofore, this has been the reason why Ni-based alloys had to be applied to high-temperature and high-pressure water plant applications, albeit expensive and shunned.

Therefore, as long as the solid solution of Ni in the oxide film is effective for resistance to an environment in which high temperature corrosion and wet corrosion are superimposed, it is sufficient if there is a sufficient amount of Ni to improve the characteristics of the film. . The experiment was continued based on the concept that even if such a minimum amount of Ni was secured, sufficiently excellent corrosion resistance would be exhibited even with iron-based stainless steel.

In the experiment, an alloy having a composition which was prepared by changing the addition amounts of Cr, Ni, Fe and other elements was prepared. Through the investigation, it was found that even if Ni was added in a certain amount or more, the composition of the oxide film was saturated and the adhesion was saturated. It was clarified that the content of Cr should be 20% or more, and that the addition of approximately the same amount of Ni would bring about a dramatic improvement in the film properties. Further, it has been clarified that N, Mo, Cu, and the like are also useful elements for improving the stability of the oxide film.

In order to be useful as an actual device material, it is extremely important that the material has excellent workability, ductility, weldability, and the like. From that point of view, the additive element should be set on condition that the metal structure becomes an austenitic structure. Also, in order to avoid large dissimilar metal contact corrosion, etc., between stainless steel and steel materials that are expected to be used in large quantities in the surrounding area, take care to design materials as low as possible in the range of alloys. There is a need to.

The present invention provides a stainless steel having excellent corrosion resistance in a high-temperature and high-pressure water environment based on the above concept.
The chemical components are preferably as follows.

P: 0.002 to 0.02% P is an impurity, and the lower the content, the better. If the content exceeds 0.02%, the weldability is deteriorated. However, if the content is reduced to less than 0.002%, the cost of the de-P treatment increases. Therefore, P is 0.00
It is preferable to be in the range of 2 to 0.02%.

S: 0.01% or less S is an impurity, and the lower the better, the better. If the content exceeds 0.01%, the hot workability deteriorates. Therefore, it is preferable that S is set to 0.01% or less.

N: 0.15 to 0.3% N is preferably added in an amount of 0.15% or more to stabilize the austenite structure of the steel and more reliably obtain the effect of suppressing precipitation of brittle intermetallic compounds.

As the application form of the high corrosion resistant stainless steel of the present invention, it is most suitable to use it as a structure or a flow pipe for a reaction vessel of a plant facility. Furthermore, suitable corrosion resistance and mechanical properties can be exhibited even when used as the internal structure of a reaction vessel or a casing of a control device.

[0045]

EXAMPLE A stainless steel having the chemical components shown in Table 1 was melted by vacuum induction melting, cast, soaked at 1200 ° C., and hot rolled to produce a steel sheet having a thickness of 10 mm. Regarding the steel sheet after hot rolling, the state of edge cracking and the state of occurrence of surface flaws were evaluated. Subsequently, a test piece of 135 mm ^L × 5 mm ^w × 1 mm ^t was cut out and subjected to a corrosion test with a surface polished finish.

[0046]

[Table 1]

The corrosion test simulated the environment of a reaction vessel of a trichlorethylene decomposition plant, and was immersed in pure water to which 2% trichlorethylene, equimolar sodium hydroxide, and equimolar hydrogen peroxide were added. . Measurements were taken at a temperature of 550
The temperature was maintained at 35 ° C. and a pressure of 35 MPa for 1 hour, the amount of corrosion was measured, and the conversion was converted into a wall thinning rate and evaluated. Table 1 also shows the above evaluation results.

The hot workability of the steel of the present invention was satisfactory without any cracks or flaws (marked with ○ in the table). The corrosion rate in the simulated reaction vessel environment of the trichlorethylene decomposition treatment plant (corrosion rate column * in the table) is the target 0.5 g / m
A corrosion rate of less than ² h (= 0.55 mm / year) has been achieved.

If the target value of the corrosion rate (= 0.55 mm / year) can be maintained, the life of the apparatus is assumed to be 10 years, and the required sheet thickness for stopping the thickness reduction to 1/2 thickness is estimated to be 11 mm. It is. This is consistent with the thickness (10 to 15 mm) of the reaction vessel assumed in an industrial treatment plant of 1 ton / day, and it can be seen that an economical and limit design with respect to characteristics can be realized.

For comparative steels whose chemical components do not fall within the scope of the present invention, evaluation of ear cracks and surface flaws and evaluation of corrosion resistance were also carried out through the steps of vacuum melting, casting and hot rolling. Table 2 summarizes the chemical composition and evaluation results of the comparative steel.

[0051]

[Table 2]

Regarding the hot workability of the comparative steels, some of them have cracks and surface flaws. Those with cracks (x marks) are those with low ductile cracks at high temperatures, which are thought to be related to the precipitation of intermetallic compounds, or ear cracks, which are thought to be due to insufficient grain boundary strength. It is considered that the surface flaw (marked by △) is caused by the same cause. Regarding the corrosion resistance of the comparative steels, there are those in which the corrosion rate exceeds the above-mentioned target value of 0.5 g / m ² h.

As described above, in the invention steels shown in Table 1, both the hot workability and the corrosion resistance are good in all the examples by the proper combination of the additive elements. On the other hand, the comparative steels shown in Table 2 have difficulty in either hot workability or corrosion resistance, and cannot achieve both. From the above, it can be seen that the provisions of the present invention are effective for the uses of the present invention.

[0054]

The highly corrosion-resistant stainless steel of the present invention can be applied to a supercritical water / subcritical water oxidation plant by appropriately preparing chemical components. As a result, when applied to a reaction vessel structure, a flow pipe, an internal structure of a reaction vessel, etc., it is possible to exhibit good corrosion resistance and mechanical properties, and to overcome economical obstacles. Thus, it is possible to obtain prospects for the production of SCWO (supercritical water oxidation) treatment plants for industrial hazardous wastes, and to obtain extremely beneficial effects on the industry, as well as contributing to the creation of environment-related industries.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a corrosion resistance index R and a corrosion rate in a high-temperature and high-pressure hydroxylation environment.

Claims (4)

[Claims] The chemical component is mass%, C: 0.02% or less, Si:
1.0% or less, Mn: 2.0% or less, Cr: 20-27%, Ni + Co: 17-27
%, Mo + 1 / 2W: 2-7%, N: 0.01-0.3%, Cu: 0.1-3%,
High corrosion resistant stainless steel whose balance is substantially iron and the following formula is satisfied when the mass% of each element is represented by each element symbol. Cr + Ni + Co + 2Cu + 4.1 (Mo + 1 / 2W) + 24N ≧ 62 2. High corrosion resistance, characterized in that the chemical component further contains at least one of mass%, B: 0.01% or less and Zr: 0.5% or less in addition to the chemical component according to claim 1. Stainless steel. 3. The chemical component in addition to the chemical component according to claim 1 or 2, further, in mass%, Ca: 0.02% or less, Al: 0.1% or less, La: 0.04% or less, Ce: 0.04% % Or less, Y: 0.1
% High-corrosion-resistant stainless steel, characterized in that it contains at least one of the following: 4. The chemical component according to claim 1 or 3,
High corrosion resistance characterized by containing at least one of mass%, Ti: 0.5% or less, Nb: 0.8% or less, Ta: 1.6% or less, V: 1% or less in addition to the described chemical components. Stainless steel.