JP2002121655A - Stainless steel for crude phosphoric acid having excellent corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide stainless steel for handing crude phosphoric acid having excellent corrosion resistance, and by which an extended life, safety, the maintenance of good appearance or the like of equipment such as apparatus handling crude phosphoric acid, phosphoric acid production plants, tanks for storing phosphoric acid and pipes for transporting phosphoric acid can be secured over a log period. SOLUTION: As to the stainless steel for crude phosphoric acid having excellent corrosion resistance, in an alloy having a composition containing, by mass, 0.004 to 0.05% C, 0.01 to 1% Si, 0.1 to 2% Mn, <=0.03% P, <=0.01% S, 18 to 25% Cr, 1 to 4% Cu and 2 to 6% Mo and further containing Ni by an amount calculated by -11.385+1.357×([Cr]+[Mo]+1.5×[Si])±0.5, and the balance substantially Fe with inevitable impurities, based on the composite addition of each component in the following formula, for the purpose of securing the uniform corrosion resistance in the environment of crude phosphoric acid, the GI value (uniform corrosion resistance index) expressed by mass%=-[Cr]+0.94×[Ni]+4.61×[Mo]+7.74×[Cu] is 28 to 40%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a crude product having excellent corrosion resistance in an environment for producing, storing and transporting crude phosphoric acid, such as a phosphoric acid production plant, phosphoric acid storage tanks, and phosphoric acid transport pipes. It relates to stainless steel for phosphoric acid.

[0002]

2. Description of the Related Art Conventionally, equipment for handling crude phosphoric acid, such as a phosphoric acid production plant, a phosphoric acid storage tank, a phosphoric acid transport pipe, and the like, include phosphoric acid concentration, temperature conditions, fluoride ions and chloride ions, and the like. Depending on the concentration of corrosive impurities such as sulfuric acid, carbon steel, acid-resistant brick, lead, high Cr-Mo containing steel, Ni
At present, base alloys, rubber lining materials, and the like are properly used. In particular, there are many problems of corrosion of crude phosphoric acid produced by a wet method of extracting phosphate ore by dissolving it in sulfuric acid or the like and concentrating the resulting diluted phosphoric acid to obtain phosphoric acid of a predetermined concentration. Phosphoric acid corrosion is strongly influenced by coexisting substances, similar to sulfuric acid. Therefore, corrosion of wet phosphoric acid containing many impurities is complicated, and a technique for preventing corrosion in phosphoric acid has not been sufficiently studied from the viewpoint of material technology.

In a sulfuric acid environment, for example, Japanese Patent Application Laid-Open No. 2-170
No. 946, a general corrosion resistance index (Gene
ral Corrosion Resistance Index: GI value = -Cr +
(3.6Ni + 4.7Mo + 11.5Cu) is obtained, and there is a material selection guideline that stainless steel having this value exceeding 60 can secure excellent corrosion resistance. At present, no knowledge on the selection of materials has been obtained.

[0004]

The crude phosphoric acid is used as a fertilizer,
Although it is an essential chemical for the production of detergents, feeds, pharmaceuticals, etc., Japan has almost no phosphorus resources and is imported from the Middle East, North Africa, the South Sea Islands (Nauru), etc. in chemical tankers. Conventionally, stainless steels such as SUS304 steel and SUS316L steel, which have been conventionally used as materials for tanks, are corroded by crude phosphoric acid, and the inner surface of the tank is often discolored black, which has been a problem.

The present invention enables equipment for handling crude phosphoric acid, such as a phosphoric acid production plant, phosphoric acid storage tanks, phosphoric acid transport pipes, etc., to prolong the life of equipment and to ensure safety over a long period of time. And a crude stainless steel for phosphoric acid having excellent corrosion resistance.

[0006]

Means for Solving the Problems From the above viewpoints, the present inventors first investigated the components of crude phosphoric acid, which is a corrosive environment to which materials are exposed, and conducted a corrosion test of various stainless steels in crude phosphoric acid. The test was performed separately for the gas phase and the liquid phase. Then, the effects of the amounts of alloying elements such as Cr, Ni, Mo, and Cu on the corrosion resistance and discoloration state of various stainless steels were examined, and as a result of intensive studies, the present invention was completed.

That is, the gist of the present invention is as follows. In mass%, C: 0.004 to 0.05%, Si: 0.01 to 1%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.01% or less, Cr : 18 to 25% or less, Cu: 1 to 4% or less, Mo: 2 to 6%, and Ni in an amount of −11.385 + 1.357 × ([Cr] + [M
o] + 1.5 × [Si]), and the balance of the phosphoric acid environment is based on the composite addition of each component of the following formula in the alloy substantially consisting of Fe and unavoidable impurities. GI expressed in mass% to ensure overall corrosion resistance
Value (overall corrosion resistance index) =-[Cr] + 0.94 x [N
i] + 4.61 × [Mo] + 7.74 × [Cu] is 28
% Of stainless steel for phosphoric acid having excellent corrosion resistance.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, the inventors analyzed crude phosphoric acid, a corrosive environment to which the material was exposed. As a result, it was determined that crude phosphoric acid usually contained iron, aluminum, magnesium, calcium, fluorine, chlorine and other gas components and sulfuric acid used in the wet process, which are considered to be derived from phosphate rock. . It was also found that gas components such as silicofluoride gas and sulfurous acid gas were dissolved in the crude phosphoric acid.

Next, the corrosion rate and discoloration state of various stainless steels immersed in crude phosphoric acid (liquid phase) and exposed to the upper part of the crude phosphoric acid solution (gas phase) were examined. Was. As a result, in any of the stainless steels, the surface of the material immersed in the liquid phase portion has a metallic luster and is not corroded at all. In contrast, the surface exposed to the gas phase was discolored because its surface was covered with a black film. The film formed in the gas phase was removed, and the surface of the metal was observed using an optical microscope. As a result, it was found that any stainless steel exhibited slight general corrosion.

That is, it has been newly found that corrosion of crude phosphoric acid does not occur in the liquid phase portion but occurs in the gas phase portion. Although the level of the corrosion rate generated in the gas phase portion is not a problem in practical use (commonly, if the corrosion rate is 0.1 mm / y or less, there is no problem in use), but the appearance of the black film is beautiful. It was concluded that loss of quality was a problem. This black film is a compound of Fe, Cr, Ni, Mo and F, O, which are components of stainless steel, and is essentially a corrosion product generated by a reaction between a metal and a silicon fluoride gas volatilized from crude phosphoric acid. The substance was confirmed by Auger electron spectroscopy.

Subsequently, a corrosion resistance test was performed in a medium phase portion of crude phosphoric acid using stainless steel in which the amounts of various alloying elements were systematically changed. Specifically, 18% Cr-12%
Ni-2.5% Mo-1% Cu with Cr as the central component
Using a stainless steel to which 15 to 28% of Ni, 4 to 28% of Ni, 0 to 5% of Mo and 0 to 4% of Cu are added in a single correlation, respectively. The phase was exposed for 260 h. As a result, it was found that the corrosion rate in the gas phase decreased with an increase in the overall corrosion resistance index GI, which is a function of the stainless steel component. The adhesion of the black film by visual observation was 0.004 m.
It was also found that only those materials exhibiting a corrosion rate of m / y or more were observed.

[0012] From this, from the content mass% of each component-[C
r] +0.94 [Ni] +4.61 [Mo] +7.74
The value of the general corrosion resistance index GI represented by [Cu] is 28%
It became clear that the black film did not adhere to the above-mentioned materials, and that the beauty of the surface was not impaired. As described above, the present invention provides a stainless steel having a corrosion rate of a level which does not impair the beauty from the viewpoint of aesthetic beauty, although the stainless steel can withstand use if the corrosion rate is 0.1 mm / y or less. The elements to be contained and the amounts thereof are limited.

The reasons for limiting the constituent elements of the present invention will be described below. [C content: 0.004% or more and 0.05% or less] C is harmful to the corrosion resistance of stainless steel, but a certain amount of C is necessary from the viewpoint of strength. Very low C of less than 0.004%
Amount increases manufacturing costs. On the other hand, if it exceeds 0.05%, the corrosion resistance is significantly deteriorated, so that the content is made 0.004% or more and 0.05% or less.

[Si content: 0.01% or more and 1% or less] As a component range of ordinary stainless steel that can be hot-rolled within a range that does not affect corrosion resistance, the Si content is set to 1% or less.
Further, if the amount of Si is less than 0.01%, the production cost increases, so that the content is made 0.01% or more.

[Mn content: 0.1% or more and 2% or less] Mn
Is an austenite-stabilizing element and can be added as an alternative to expensive Ni. However, the corrosion resistance in crude phosphoric acid, which is the object of the present invention, is ineffective at more than 2% and affects the corrosion resistance. The upper limit of the amount of Mn that does not affect
% Or less. Further, when the Mn content is less than 0.1%, the production cost is increased, so that the content is set to 0.1% or more.

[P content: 0.03% or less] It is desirable that P is small from the viewpoint of corrosion resistance and hot workability. 0.
If it exceeds 03%, the hot workability is extremely deteriorated. Therefore, the P content is set to 0.03% or less.

[S content: 0.01% or less] S is an element that has a greater effect on hot workability than on corrosion resistance. The lower the content, the better. Therefore, the S content is set to 0.01% (100 ppm) or less.

[Cr content: 18% or more and 25% or less] Cr
Is added in a form coexisting with Ni, Mo, and Cu. To obtain good corrosion resistance in a crude phosphoric acid gas phase, Cr must be reduced as much as possible, but at least 18% or more must be added to maintain the form of stainless steel. Also, N
Good corrosion resistance can be ensured by the coexistence of i, Mo, and Cu. If it exceeds 25%, the manufacturability becomes somewhat difficult, and it becomes economically expensive. Therefore, the range of the appropriate amount of Cr is set to 18% or more and 25% or less.

[Ni content: -11.385 + 1.357]
(Cr + Mo + 1.5Si) ± 0.5] Ni is Mo,
It is a basic component of the stainless steel of the present invention together with Cu. In order to facilitate the production of a thick plate of stainless steel and to ensure cold workability and weldability, it is necessary to change the metal structure to an austenitic phase. The minimum amount of Ni for turning the steel of the present invention into the austenitic phase is -11.385 + 1.357.
(Cr + Mo + 1.5Si) ± 0.5%.
The amount of Ni which is economically inexpensive and maintains the austenitic phase of stainless steel is -11.385 + 1.357 (Cr +
(Mo + 1.5Si) ± 0.5.

[Mo amount: 2% or more and 6% or less] Mo is C
Along with r, Ni and Cu, it is a basic component of the stainless steel of the present invention. It is an essential element for obtaining high corrosion resistance in a crude phosphoric acid gas phase. In the range of 2 to 6%, it is effective in coexistence with Ni and Cu. If it is less than 2%, the overall corrosion resistance becomes insufficient and discoloration becomes remarkable. On the other hand, if it exceeds 6%, manufacturability becomes difficult and it becomes expensive. Therefore, the range of the appropriate Mo amount is limited to 2% or more and 6% or less.

[Cu content: 1% or more and 4% or less] Cu is C
It is a basic component of the present invention together with r, Ni, and Mo, and is an element necessary for securing high corrosion resistance and beautiful surface in a crude phosphoric acid gas phase. At less than 1% addition, discoloration is significant. On the other hand, if it exceeds 4%, the manufacturability becomes difficult. Therefore, the appropriate range of the amount of Cu is set to 1% or more and 4% or less.

[Overall corrosion resistance index GI =-[Cr] +
0.94 [Ni] +4.61 [Mo] +7.74 [C
u]: 28% or more and 40% or less] Overall corrosion resistance index GI
Is an index of the corrosion resistance of stainless steel made of various alloying elements by component. The higher the GI value, the better the overall corrosion resistance of stainless steel is, but in order to keep the metal surface beautiful in a crude phosphoric acid gas phase,
The material must have a GI value of at least 28%. Also,
If the GI value exceeds 40%, the effect is saturated and the economy is disadvantageous. Therefore, the GI value is set to 28% or more and 40% or less.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments.
Table 1 shows the chemical composition of the steel of the present invention and the comparative steel, and the results of evaluation of the overall corrosion rate and the discoloration state of the surface.
Each was melted by an electric furnace vacuum melting method and cast into a mold to produce an ingot. Then, 1150-125
After soaking treatment at 0 ° C for 0.5 to 1 hour, after surface treatment, it is heated again to 1250 ° C, hot-rolled to a thickness of 6 mm, heated at 1100 ° C for 30 minutes, and then solidified by water quenching. Perform heat treatment, dimensions 25mm width x 25mm length x
A 4 mm-thick corrosion test piece was sampled and subjected to a corrosion test in a crude phosphoric acid gas phase.

The corrosion test was carried out by first polishing the entire surface of the corrosion test piece by wet # 400 polishing, and then exposing the corrosion test piece to a crude phosphoric acid gas phase at 40 ° C. for 200 hours. As for the discoloration state of the material, the evaluation was performed on the assumption that the mark “○” was not discolored (the metallic luster was maintained) and the mark “X” was discolored. From the results in Table 1, it can be seen that the steel of the present invention is a material that shows extremely excellent overall corrosion resistance and does not discolor as compared with the comparative steel.

[0025]

[Table 1]

[0026]

As described above, according to the steel of the present invention, the overall corrosion resistance in the crude phosphoric acid gas phase is greatly improved, and the surface can be kept beautiful.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazuhiro Suetsugu 2-6-3 Otemachi, Chiyoda-ku, Tokyo Nippon Steel Corporation (72) Inventor Masaki Umeno 1-1 Tobata-cho, Tobata-ku, Kitakyushu-shi New Japan Inside Yawata Steel Works, Ltd.

Claims (1)

[Claims] C: 0.004 to 0.05%, Si: 0.01 to 1%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0. 01% or less, Cr: 18 to 25% or less, Cu: 1 to 4% or less, Mo: 2 to 6%, and further, Ni is −11.385 + 1.357 × ([Cr] + [M
o] + 1.5 × [Si]) In an alloy containing an amount calculated by ± 0.5, with the balance substantially consisting of Fe and unavoidable impurities, crude phosphorus In order to ensure the overall corrosion resistance in an acid environment, a GI value (General Corrosion Resistance Index: total corrosion resistance index) expressed in mass% =-[Cr] + 0.94 x [Ni] +
A crude stainless steel for phosphoric acid having excellent corrosion resistance, wherein 4.61 × [Mo] + 7.74 × [Cu] is 28% or more and 40% or less.