JP4466211B2 - Stainless steel for sulfuric acid at low temperature and low concentration - Google Patents

Description

  The present invention relates to a low temperature, low concentration sulfuric acid stainless steel. Specifically, the present invention relates to a stainless steel having corrosion resistance to low temperature and low concentration sulfuric acid such as about 50 ° C. or less and about 25% by weight or less.

Stainless steel such as SUS304 and SUS316, depending on the type and conditions, has a low concentration of about 5 to 10% by weight and a high concentration of about 80 to 90% by weight at a temperature of about 70 to 80 ° C. or less. This sulfuric acid has corrosion resistance.
The concentration range with corrosion resistance is expanded by increasing the concentration of Si, Mo and Cu, and the main component containing Mo and Cu at high Ni is 18Cr-18Ni-2Mo-2Cu (the numerical value represents weight%, the balance is Fe and Stainless steel, which is an unavoidable impurity (hereinafter the same), has corrosion resistance in a wide temperature range and concentration range (see, for example, Non-Patent Document 1).

  Examples of such materials include warsight (22Cr-22Ni-2.2Mo-1.6Cu), carpenter 20 (20Cr-29Ni-2.3Mo-3.5Cu), and iridium R (22Cr-64Ni-4Mo-2.5Cu). However, these are expensive, and DIN4577 (25Cr-25Ni-2Mo-0.3Ti), which is relatively inexpensive, can be used for low temperature, low concentration sulfuric acid such as about 50 ° C or less and about 25% by weight or less. is there. DIN4577 is a symbol according to German standards.

However, when DIN4577 is actually used for a long period of time in low-temperature sulfuric acid at a low temperature of about 50 ° C or less and about 25% by weight or less, the reason is not clear, but it is a boundary that has corrosion resistance due to fluctuations in use conditions Corrosion may have occurred in the base metal or the welded part, probably because it is close to the concentration or temperature.
“Handbook of Steel Materials” Maruzen Co., Ltd., May 30, 1990, p.618-619

  Specifically, the object of the present invention is to provide an inexpensive stainless steel having corrosion resistance to low temperature and low concentration sulfuric acid such as about 50 ° C. or less and about 25% by weight or less.

  In order to solve such problems, the present inventors have intensively studied the corrosion resistance of DIN4577 against sulfuric acid at a low temperature and a low concentration such as about 50 ° C. or less and about 25 wt% or less. To find that Ti affects corrosion, C: 0.03% or less, B: 0.0008% or less, and Ti: 10 × C% or more, it is found that corrosion can be suppressed, to complete the present invention It came.

That is, in the present invention, by weight, C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.030% or less, Cr : 25.0 to 26.0%, Ni: 25.0 to 26.0%, Mo: 2.0 to 2.5%, B: 0.0008% or less, Ti: 10 × C% to 20 × C % containing the balance 50 ° C. or less, characterized in that it consists of Fe and unavoidable impurities, 25 wt% or less of a low temperature, a stainless steel for low concentration sulfuric acid.

  The stainless steel of the present invention has stable corrosion resistance against low temperature, low concentration sulfuric acid such as about 50 ° C. or less and about 25% by weight or less.

  The composition of DIN4577 is% by weight, C: 0.05% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.030% or less, Cr: 25.0-26.0%, Ni: 25.0- It contains 26.0%, Mo: 2.0 to 2.5%, Ti: 5 × C% or more, and the balance consists of Fe and inevitable impurities.

  1 to 3 show the results of the corrosion rate with respect to the C concentration, B concentration, and Ti / C of the steel materials in the examples. In the figure, ■ indicates the steel material as it is, ● indicates the result of the sulfuric acid / ferric sulfate test for the sensitized heat-treated material, and the circled circle is the actual machine (sulfuric acid: about 18 wt%, temperature: about 40 ° C). When used, steel materials with significant corrosion of the base metal, and those enclosed with □ are steel materials with significant corrosion at the heat affected zone when used in actual equipment.

C combines with Cr to form Cr carbide, and thus forms a Cr-depleted layer. The higher the C concentration, the higher the corrosion sensitivity. Therefore, the C concentration is often defined as 0.05% or less. As is clear from the above, 0.05% or less is insufficient and 0.03% or less is necessary.
As is apparent from FIG. 2, the corrosion rate increases as the B concentration increases. In steel materials having a B concentration of 9 ppm or more, grain boundary precipitates are observed in the steel materials as they are. Therefore, the B concentration needs to be 8 ppm or less.
As is apparent from FIG. 3, the corrosion rate is decreased by increasing Ti with respect to C. It is considered that Ti forms TiC to stabilize C and suppress precipitation of Cr carbide at the grain boundary. By setting the Ti concentration to 10 times the C concentration or more, the corrosion rate can be suppressed to a level with no problem.

  EXAMPLES Hereinafter, although an Example is shown and this invention is shown concretely, this invention is not restrict | limited to the following Example.

  Stainless steels A to D (unit:% by weight, where Ti / C is dimensionless, the same applies hereinafter) shown in the table below are used in a reactor for producing hydroxylamine (sulfuric acid: about 18% by weight, temperature: about 40 ° C.). After 5 to 15 years of use on the parts, the corrosion situation was observed.

  In Steel A, the base metal was significantly corroded, and the weld heat affected zone was hardly corroded. In the steel material B, the base metal showed almost no corrosion, but significant corrosion was observed in the weld heat affected zone. In Steel C, significant corrosion was observed in the base material, and almost no corrosion was observed in the weld heat affected zone. In the steel material D, almost no corrosion was observed in the base material and the weld heat affected zone.

For the stainless steels A to D and the stainless steels E to J shown in the following table, the base material and the sensitized heat treated material heat treated at 750 ° C. for 2 hours are sulfuric acid / second sulfuric acid specified by JIS. An accelerated corrosion test using an aqueous iron solution was performed.
After immersing a steel material of 25 mm × 20 mm × 2 mmt for 140 hours in a boiling solution obtained by adding 25 g of ferric sulfate to 600 ml of an aqueous solution containing 50% by weight of sulfuric acid, the amount of corrosion was measured to determine the corrosion rate. The results are shown in Table 3.

These results are shown in FIGS. In the figure, ■ is the steel material as it is, ● is the result for the sensitized heat treated material, and as described above, the one surrounded by ○ is the mother when used in the actual machine (sulfuric acid: about 18 wt%, temperature: about 40 ° C) Steel materials that are markedly corroded, and those surrounded by □ are steel materials that are severely corroded by the heat affected zone when used in actual equipment.
By setting C: 0.03% or less, B: 0.0008% or less, and Ti: 10 × C% or more, corrosion is suppressed. In FIG. 2, steels B and F show corrosion rates of 10 and 0.5 g / m ² hr when B is 0.0008% or less, respectively, and the C concentration exceeds 0.03% by weight. The corrosion rate seems to increase due to the influence. In FIG. 3, the steel material F shows a corrosion rate of 0.5 g / m ² hr when Ti is 10 × C% or more.

It is a figure which shows the result of the corrosion rate with respect to C density | concentration of the steel materials in an Example. It is a figure which shows the result of the corrosion rate with respect to B density | concentration of the steel materials in an Example. It is a figure which shows the result of the corrosion rate with respect to Ti / C of the steel materials in an Example.

Claims (1)

C: 0.03% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.030% or less, Cr: 25.0 to 26.0%, Ni: 25.0 to 26.0%, Mo: 2.0 to 2.5%, B: 0.0008% or less, Ti: 10 × C% to 20 × C% , Stainless steel for low-temperature, low-concentration sulfuric acid at a low temperature of 50 ° C. or less and 25% by weight or less, characterized in that the balance consists of Fe and inevitable impurities. "

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