JP6526436B2 - Ferritic stainless steel plant components for plant applications with excellent corrosion resistance in environments containing sulfate and chloride ions - Google Patents

Description

  The present invention relates to a ferritic stainless steel plant component having excellent corrosion resistance in a plant component environment containing chloride ions and sulfide ions.

  In a coal-fired thermal power plant or the like, sulfide and chloride are removed by a flue gas desulfurization apparatus, but about 1000 ppm of sulfate ion and chloride ion carry over into the exhaust gas. Since condensed water containing these ions is formed in the chimney or in the gas reheater and its piping, the material is required to have pitting resistance and sulfuric acid resistance. Therefore, conventionally, as described in Patent Document 1, SUS304 and SUS316L have been widely used.

  However, austenitic stainless steels such as SUS304 are concerned about the increase in material cost due to the recent increase of the price of Ni raw material. In addition, although many constructions by welding are used at the time of plant construction, stress corrosion cracking often occurs due to sensitization of materials caused by the thermal effects of welding and maintenance in a corrosive environment in the presence of welding distortion. Become.

  It is known that the above problem can be solved by using a ferritic stainless steel to which a stabilizing element such as Ti or Nb is added after reducing the C content, but the ferritic stainless steel has a viewpoint of corrosion resistance Application is often difficult.

  It is known that the oxide scale on the weld surface has low corrosion resistance, and Fe ions eluted from the oxide scale promote the corrosion of the material. Therefore, in plant applications, after welding the material, it is generally used by grinding with a brush and a grinder to remove the oxide scale. However, grinding with a grinder or the like, in addition to degenerating the passive film, produces fine irregularities serving as a starting point of corrosion on the material surface. Therefore, the corrosion resistance of the material may be reduced.

  From the above reasons, plant members are required to have sufficient corrosion resistance in a plant environment that produces condensed water containing chloride and sulfide even if the surface condition of the material changes due to welding or grinding. Therefore, it has been difficult to apply a ferritic stainless steel generally inferior in corrosion resistance to austenitic stainless steel.

Unexamined-Japanese-Patent No. 2007-46890

The present invention has been made in view of the above circumstances, and is excellent in workability because welding corrosion resistance is not reduced and stress corrosion cracking does not occur, and corrosion resistance is excellent in a plant member environment including sulfate ion and chloride ion. An object of the present invention is to provide a ferritic stainless steel plant member excellent in cost merit.

The above object is, C: and 0.005 to 0.025 wt%, Cr: and 18.00 to 23.00 wt%, Mo: from 0.05 to 2.50 wt% and, N: 0.009 ~0 and .03 wt%, Cu: and 0.05 to 0.50 wt%, Nb: from 0.15 to .80 wt% and Ti: 1 and one or two 0.05 to 0.80 wt%, Si: and from 0.21 to 0.51 wt%, Mn: and 0.20 to 1.01 wt%, Ni: 0.10 to 0.20 mass% and, meet the lower SL (1) And a weld joint which has a composition of the balance Fe and an unavoidable impurity, and which has been subjected to dry grinding to have a roughness Ra of 0.1 μm or more, and the coating composition of the weld joint is Cr mass% / ( In a plant environment containing sulfate ion and chloride ion , having a passive film satisfying Cr mass% + Fe mass%) ≧ 0.30 This is achieved by a ferritic stainless steel plant component having excellent corrosion resistance .
Cr + 5 × Cu + 3 × Mo-30 × N ≧ 20 (1)

  The inventors have extensively studied the effects of alloy components and passive film of stainless steel on the corrosion resistance of stainless steel in aqueous solution environment, and control mechanical film of ferritic stainless steel to achieve mechanical grinding. We grasped the alloy component system in which the corrosion resistance is hard to change due to etc., and reached the present invention.

  According to the present invention, there is no sensitization and stress corrosion cracking, and even if the surface condition of the material changes due to welding or grinding, it has excellent corrosion resistance in a plant environment that produces condensed water containing chloride and sulfide, and A ferritic stainless steel excellent in cost merit is provided.

The reasons for limitation of each component in the present invention are as follows.
C: 0.005 to 0.025 mass%
C is an element unavoidably contained in stainless steel. When the C content is reduced, the formation of carbides is reduced, and the weldability and the corrosion resistance and sensitization characteristics of the weld are improved. However, in order to reduce the C content, the refining time becomes long and the production cost is increased, so the C content is in the range of 0.005 to 0.025 mass%. Preferably, it is in the range of 0.005 to 0.015% by mass.

Cr: 18.00-23.00 mass%
Cr is a major alloying element that forms a passive film on the surface of stainless steel, and improves the resistance to pitting corrosion, crevice corrosion and general corrosion. As a result of studies by the inventors, it was found that a Cr content of more than 18% by mass should be ensured to provide the corrosion resistance required in a condensed water environment. However, when the Cr content is increased, mechanical properties and toughness are impaired, which further increases the cost. Therefore, in the present invention, the upper limit is 23.00% by mass.

Mo: 0.05 to 2.50 mass%
Mo, like Cr, is a basic component for securing stable corrosion resistance. Mo is an effective element for improving the corrosion resistance level together with Cr. However, when the Mo content is increased, the hot workability is reduced. Therefore, the Mo content is in the range of 0.05 to 2.50% by mass.

Cu: 0.05 to 0.50 mass%
Cu, like Mo, is an important element in improving the corrosion resistance, particularly the pitting resistance. However, if a large amount of Cu is added, it hardens and reduces the processability. Therefore, the Cu content is in the range of 0.05 to 0.50 mass%.

N: 0.009 to 0.03 mass%
Since N forms a nitride with Ti and Nb, an increase in the N content inhibits the sensitization-suppressing effect of Ti and Nb. Furthermore, nitrides are likely to be the base point of corrosion, and thus reduce the corrosion resistance, particularly the pitting resistance, of ferritic stainless steels. Therefore, N content makes an upper limit 0.03 mass%. However, since the refining time is prolonged to reduce the N content and the production cost is increased, the lower limit of the N content is 0.009 % by mass.

One or two of Nb: 0.15 to 0.80% by mass and Ti: 0.05 to 0.80% by mass Nb and Ti are elements having strong affinity with C and N, and therefore, when added, carbonitrides It is an element having the effect of reducing the solid solution C and N by forming By the reduction of the solid solution C and N, the precipitation of Cr carbides in a high temperature environment can be suppressed, and the sensitization characteristic can be enhanced. Therefore, it is necessary to add 0.15 mass% or more for Nb and 0.05 mass% or more for Ti. However, excessive addition of Nb and Ti saturates the effect of reducing solid solution C and N, and the effects of Nb and Ti itself reduce cold workability and hot workability, so Nb and Ti addition The upper limit of the amount was 0.80% by mass.

In addition to the definition of the above-mentioned component composition, Cr, Mo, Cu, and N which are main elements which affect corrosion resistance were specified in the range of the following (1) types.
Cr + 5 × Cu + 3 × Mo-30 × N ≧ 20 (1)

  In addition, since the plant member which concerns on this invention is what is assembled by welding construction, it has a weld joint part. This joint is dry ground after welding and has a roughness Ra of 0.1 μm or more.

  Thus, in order to prevent the corrosion resistance of the material from decreasing even after the change of the surface state by grinding etc., in addition to the above-mentioned definition of the component composition, the Cr content contained in the passive film is Cr / (Cr + Fe) ≧ It was defined as 0.30.

Method of Analyzing Cr Content in Passive Film The passive film is analyzed by GDS at a sputtering rate of 0.5 μm / min, and among the obtained GDS peaks, Fe mass% and Cr mass at the point where oxygen intensity is the highest. The value of Cr mass% / (Cr mass% + Fe mass%) with respect to the value of% is taken as the Cr content in the passivation film.

  A stainless steel having the chemical components shown in Table 1 was melted, and a hot-rolled sheet having a thickness of 3.0 mm was produced by hot rolling. The hot-rolled sheet was cold-rolled to a plate thickness of 1.0 mm, subjected to finish annealing at 1030 ° C. for 1 minute, and then immersed in a hydrofluoric-nitric acid aqueous solution to remove oxide scale, and subjected to a test.

Surface roughness measurement method The roughness of the surface 3 mm range of the test piece was measured using a contact-type two-dimensional surface roughness meter, and the average value measured at any 10 locations was taken as the surface roughness Ra (μm). .

Corrosion resistance evaluation method Corrosion resistance was evaluated from pitting potential in a solution containing sulfate ion and chloride ion simulating a plant environment and corrosion loss in a sulfuric acid solution.

Pitting potential measuring method The test piece cut out each stainless steel of 1.0 mm of board thickness by 20 mm x 20 mm by cutting. The test surface was dry ground by a wire brush so that Ra ≧ 1.0 μm, or was wet-polished to No. 600 with a water-resistant abrasive paper so that Ra <0.1 μm. The conducting wire was spot-welded to one end of the test piece and coated with a silicone resin except for the exposed test surface of 10 mm × 10 mm. The test solution was prepared with reference to an analysis example of condensed water formed on a real plant material. Table 2 shows the composition of the test solution. The temperature of the test solution is 60 ° C., and the test surface is completely immersed in the test solution in the air atmosphere, and left for 10 minutes, then an anodic current density of 500 μA is determined by potentiostatic potential 20mV / min potentiostatic potential It went until it reached / cm ² . The pitting potential was the most noble value among the potentials corresponding to 100 μA / cm ² in the anodic polarization curve.

Sulfuric acid immersion test method test pieces were cut out by cutting each stainless steel with a plate thickness of 1.0 mm to 40 mm × 40 mm. The test surface was wet-polished to No. 600 with a water-resistant abrasive paper so that Ra <0.1 μm. The test piece was completely immersed in a sulfuric acid aqueous solution at a temperature of 80 ° C. and pH 1 in an air atmosphere, and left for 24 hours. The weight of the test piece was measured before and after immersion, and the difference in weight was taken as corrosion loss. The results are shown in Table 3.

  As can be seen from the results in Table 3, it was confirmed that the steel of the present invention has excellent acid resistance because the corrosion loss in a sulfuric acid environment of pH = 1 is extremely small. Furthermore, in an environment containing sulfate ions and chloride ions, the steel of the present invention has a pitting potential difference of 150 mV or less when Ra ≧ 1.0 μm and Ra <0.1 μm, and has excellent pitting resistance. Was confirmed.

The plant member made of a ferritic stainless steel according to the present invention can be applied to a plant member because it has excellent corrosion resistance in a plant environment that generates condensed water containing sulfate ions and chloride ions. Moreover, since the corrosion resistance which was excellent after welding and grinding is maintained, a publicly known construction method is adopted.

Claims (1)

C: and 0.005 to 0.025 wt%, Cr: and 18.00 to 23.00 wt%, Mo: 0.05-2.50 mass% and, N: 0.009 ~ 0.03 wt% When, Cu: and 0.05 to 0.50 wt%, Nb: 0.15-.80 wt% and Ti: 0.05 to 0.80 mass% of one or, Si: 0. and 21 to 0.51 wt%, Mn: and 0.20 to 1.01 wt%, Ni: 0.10 to 0.20 mass% and, meets the following expression (1), the balance Fe and It has a welded joint having a composition that is an unavoidable impurity and subjected to dry grinding to have a roughness Ra of 0.1 μm or more, and the film composition of the welded joint is Cr mass% / (Cr mass% + Fe mass % Excellent in plant environments containing sulfate and chloride ions with a passive film that meets Ferritic stainless steel plant component with corrosion resistance.
Cr + 5 × Cu + 3 × Mo-30 × N ≧ 20 (1)