JP2011105976A - Drain pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel drain pipe having excellent weld zone corrosion resistance which uses a stainless steel stock having reduced deterioration in corrosion resistance by TIG welding as a drain pipe, satisfies corrosion resistance to chloride ions and hydrogen sulfide required in a drain pipe environment as-welded, and achieves the improvement of a working environment caused by the emission of an oxide scale removal operation after welding and the reduction of the cost required therefor. <P>SOLUTION: As the stock for a drain pipe, a ferritic stainless steel is used. The ferritic stainless steel has a composition comprising, by weight, ≤0.02% C, 0.2 to 1% Si, ≤0.4% Mn, ≤0.04% P, ≤0.005% S, 17 to 26% Cr, 0.4 to <2% Mo, 0.1 to 0.5% Nb, 0.2 to 0.4% Ti, ≤0.025% N and 0.04 to 0.3% Al, and further comprising 0.4 to 2% Ni, and in which S value is ≥25.0, and the balance Fe with the other inevitable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a drain pipe made of ferritic stainless steel.

There are many types of drainage pipes such as rainwater, toilets, kitchen waste, and laundry wastewater, and the flow is indefinite, and the environment is a mixture of water, sediment, and air. The necessary corrosive environmental conditions and the necessary corrosion resistance level cannot be specified.
In addition, the diffusion of oxygen is blocked under the sediment, and anaerobic sulfate-reducing bacteria can grow depending on the conditions, and corrosion or cracking of metal drainage pipes may occur due to the generation of highly corrosive hydrogen sulfide. is there. As the pipe type, cast iron pipes of 30 to 100 A, resin-coated pipes, resin pipes and the like are used, but the use of stainless steel pipes that are expected to have good hydrogen sulfide resistance is extremely small.

  Cast iron pipes and resin-coated pipes that are widely used at present are heavy because they are thick-walled pipes with a thickness of 3 mm or more, and the construction of drain pipes is performed in a limited narrow space, so there is a problem of poor workability. . In cast iron pipes and resin-coated pipes, the effective inner diameter becomes narrow due to the generation and adhesion of iron rust with the years of use. Therefore, in addition to the problem of workability, the space for installing the pipe also becomes large, and there is a problem that the effective floor area becomes small in an apartment house or the like.

  JIS G 3448 “Stainless steel pipe for general piping” defines a stainless steel pipe used for piping for water supply, hot water supply, drainage, and cold / hot water. The tube types are SUS304TPD, SUS316TPD, SUS315JlTPD and SUS315J2TPD austenitic stainless steel. Stainless steel pipes for general piping are superior in corrosion resistance compared to the above pipe types. If the diameter is the same, the pipe thickness is almost ½, so it is lightweight and has excellent workability, and there is no iron rust adhesion. The diameter can be reduced, which is advantageous from the viewpoint of material cost, and because it requires less space for installation, there is an advantage that the floor area can be effectively used in an apartment house.

However, there are also problems in the application of stainless steel pipes. That is, welded joints are frequently used as joints in stainless steel pipes. Welding of stainless steel requires a certain level of welding skills, and welding at the construction site involves welding in a limited and confined space. Moreover, if it uses it in a corrosive environment with welding construction, it will become easy to produce pitting corrosion and crevice corrosion in a welding part, and it may lead to a malfunction. Since the deterioration in corrosion resistance due to welding is caused by the generation of a weld scale, it is essential to remove the oxide scale generated by heating the weld.
Therefore, when a stainless steel pipe is used, welding is not performed at the construction site, and welding and scale removal at the member processing manufacturer are essential.
Since removal of scale uses polishing and acid, there are environmental and construction problems such as work for removing acid and detoxification used in addition to deterioration of working environment.

  Patent Document 1 discloses a flare joint that is a stainless steel pipe and does not require welding. Although there is an advantage that corrosion resistance does not deteriorate due to welding by not performing welding, crevice corrosion on the flange surface occurs in the case of an austenitic stainless steel pipe because the residual stress due to swallowing remains on the flange surface because the tube end is subjected to swallowing. There is concern about the occurrence of stress corrosion cracking starting from. Therefore, the use of stainless steel having corrosion resistance excellent in stress corrosion cracking is unavoidable, which is disadvantageous in terms of cost. Further, when a ferritic stainless steel pipe is used for the purpose of avoiding stress corrosion cracking, a problem remains in the workability.

JP-A-9-279314

As described above, if a stainless steel pipe is used for the drain pipe, it is lighter than the current pipe type, and also has excellent corrosion resistance, and it is not necessary to consider the decrease in the effective inner diameter of the pipe due to the adhesion of iron rust. There is an advantage that the workability and workability are overwhelmingly superior.
On the other hand, when welded joints are prefabricated at the factory, it is necessary to remove the weld oxide scale by acid or polishing after welding. Invite. Furthermore, if welding cannot be performed at the site of piping construction, there is a problem that the degree of freedom in construction is impaired, and there is a risk that construction will be hindered.
In view of such a current situation, the present invention uses a stainless steel material having a small deterioration in corrosion resistance due to TIG welding as a drain pipe, and satisfies the corrosion resistance to chloride ions and hydrogen sulfide required in the drain pipe environment in a welded state. It is an object of the present invention to develop and provide a stainless steel pipe drain pipe excellent in corrosion resistance of a welded portion that can improve the working environment by omitting the oxide scale removal work after welding and reduce the cost required for this.

As a result of detailed studies to achieve the above object, the inventors have newly found the following.
(I) In welding construction of a drain pipe of a stainless steel pipe, the outer surface of the tube is a welding torch surface, and the portion requiring corrosion resistance is the inner surface of the tube. Although it is necessary to make the pipe inner surface into an Ar gas atmosphere and suppress the generation of the welding scale, it is difficult to seal the pipe inner surface with Ar gas at the piping construction site. The decrease in the corrosion resistance of the weld is caused by the occurrence of a weld scale. The welded portion includes from the deposited metal to the heat affected zone, and the composition and thickness of the weld scale differ depending on the position from the weld bond. Pitting corrosion at the weld occurs and grows at a site heated to 400 to 600 ° C.
The occurrence of pitting corrosion in the weld is closely related to the corrosion resistance of the scale itself. Oxide scale containing a certain amount or more of Cr oxide is excellent in corrosion resistance, and the oxide scale mainly composed of Fe oxide is easy to dissolve. Is likely to occur.

(Ii) By ensuring the Cr content of the ferritic stainless steel to which Ti is added is 21% by mass or more and adding an appropriate amount of Ni, it is heated to 400 to 600 ° C. where pitting corrosion is likely to occur in the TIG welding heat affected zone. By increasing the Cr oxide concentration in the oxide scale at the site, the corrosion resistance of the oxide scale is improved, and it is extremely effective in improving the pitting corrosion resistance of the TIG weld-affected zone in a chloride environment. As a result, it is possible to omit the welding scale removal work after welding, which has been performed so far in order to improve the corrosion resistance of the welded pipe portion and the welded joint.

(Iii) In the drain pipe, an H ₂ S environment is formed by sulfate-reducing bacteria, and the concentration reaches a maximum of 10 ppm. Addition of Ti, Al, Mo and Ni is effective for the H ₂ S corrosion resistance of stainless steel. The effect of improving the corrosion resistance of these elements increases as the Cr content increases. As an index of H ₂ S corrosion resistance of stainless steel, the S value shown in the following formula can be set depending on the Cr amount. When the H ₂ S concentration is 10 ppm, the S value defined below is set to 28.0 or more. Corrosion resistance is achieved. Here, each term in the formula is the content (mass%) of the alloy element.
When 17% ≦ Cr <21% S value = Cr + 6Mo + 9 (Ti−0.1) +5 (Al−0.03) + 0.2Ni (1)
When 21% ≦ Cr <26% S value = Cr + 7Mo + 9 (Ti−0.1) +5 (Al−0.03) + 0.2Ni (2)
The coefficient of each element of these two formulas was obtained from the H ₂ S gas corrosion test, and is represented by the ratio of the effect per unit mass, where the effect of Cr is 1. The effect of improving the corrosion resistance of Mo and Ti is determined by the Cr content, and increases as the Cr content increases.

(Iv) The drainage pipe needs to be drainable to drain solids together with water. When the tube is clogged, mechanical cleaning using a brush or the like is performed. Therefore, drainage pipes are required to have drainability and repassivation ability against the destruction of the passive film by mechanical cleaning.
It was found that drainage is effective when the surface roughness of the stainless steel pipe is 2 μm or less in terms of Rz, and repassivation is effective by increasing the Cr content. The present invention provides a drain pipe using ferritic stainless steel whose components are designed based on such knowledge.

That is, the mass of the material of the drain pipe is C: 0.02% or less, Si: 0.2 to 1%, Mn: 0.4% or less, P: 0.04% or less, S: 0.005% Hereinafter, Cr: 17 to 26%, Mo: 0.4 to less than 2%, Nb: 0.1 to 0.5%, Ti: 0.2 to 0.4%, N: 0.025% or less, Al : 0.04 to 0.3%, and in some cases, Ni: 0.4 to 2% is included, and the S value shown in the following formula is 25.0 or more, and the balance is Fe and other inevitable impurities. A drainage pipe characterized by using ferritic stainless steel.
17% ≦ C r <21% S value = Cr + 6Mo + 7 (Ti−0.1) +5 (Al−0.03) + 0.2Ni
When 21% ≦ C r <26% S value = Cr + 7Mo + 9 (Ti−0.1) +5 (A1−0.03) + 0.2Ni
Here, each term in a formula is content (mass%) of an alloy element, and substitutes zero when not containing the element.
Moreover, when the surface roughness of the steel pipe is 2 μm or less in terms of Rz, a hydrophilic steel pipe having a contact angle with water of 60 degrees or less is obtained. Furthermore, by performing a wet skin pass with an elongation of 1% or less, smoothing of the rough surface by polishing and pickling in the cold rolling process can be achieved, and a hydrophilic surface can be obtained by the action of a surfactant in the skin pass oil. Obtained stably.

  The drainage pipe of the present invention has significantly improved corrosion resistance of the welded part in a hydrogen sulfide environment, and has excellent pitting corrosion resistance for a long period of time even when it is used by exposing it to drainage without maintaining the oxidized scale generation surface on the backside of the weld. Is maintained. That is, when manufacturing a welded pipe or a welded joint by welding, high reliability can be obtained even if the welding scale removal operation is omitted, and the cost for welding joint construction can be reduced. In addition, the size of the existing pipe can be reduced, and the construction cost can be reduced by reducing the weight.

The component elements constituting the ferritic stainless steel of the present invention will be described.
C and N are elements inevitably contained in the steel. When the content of C and N is reduced, the steel becomes soft and the workability is improved, and the formation of carbides and nitrides is reduced, and the weldability and the corrosion resistance of the welded portion are improved. Therefore, in the present invention, it is better that the contents of both C and N are small, and C is allowed to be contained up to 0.02% by mass and N is allowed to be contained up to 0.025% by mass.

  In TIG welding using Ar gas as a sealing gas, Si effectively improves the corrosion resistance of the oxide scale on the welding torch surface and effectively improves the pitting corrosion resistance of the weld. Further, since Si contributes to the hardening of the base material of the ferritic steel and the welded portion, the addition of Si is advantageous when used as the drainage pipe material of the present invention. As a result of various studies, it is desired to secure a Si content of 0.2% by mass or more in order to fully enjoy the strength improvement effect of Si. However, if it is added in a large amount exceeding 1%, the growth of pitting corrosion is promoted conversely. Therefore, in the present invention, the Si content is controlled in the range of 0.2 to 1.0 mass%.

  Mn is used as a deoxidizer for stainless steel. However, since Mn lowers the Cr concentration in the passive film and causes a decrease in corrosion resistance, the Mn content is preferably low, and is defined as a content of 0.4% by mass or less. Since some amount of Mn is unavoidable in the stainless steel made from scrap, it is necessary to manage it so that it is not excessively contained.

  P is desirable to be low because it impairs the toughness of the base metal and the weld. However, since dephosphorization by refining is difficult in the production of Cr-containing steel, excessively increasing the cost, such as careful selection of raw materials, is required to minimize the P content. Therefore, in the present invention, the P content up to 0.04% by mass is allowed as in the case of general ferritic stainless steel.

  It is known that S becomes a starting point of pitting corrosion by forming sulfides such as MnS and CaS that are likely to start pitting corrosion. Since an appropriate amount of Ti is added to the drainage pipe material of the present invention, formation of MnS can be avoided. That is, since Ti has a strong affinity for S and forms a chemically stable sulfide, the generation of MnS that causes a decrease in corrosion resistance is sufficiently suppressed. However, since Al is used as a deoxidizing material, formation of CaS may not be avoided. In order not to impair the pitting corrosion resistance of the drain pipe, the upper limit of the S amount is specified to be 0.005 mass% or less.

Cr is a main constituent element of the passive film, and improves local corrosion properties such as pitting corrosion resistance and crevice corrosion resistance. In addition, since the resistance to hydrogen sulfide corrosion and the corrosion resistance of the welded portion required for the drain pipe depend greatly on the Cr content, Cr is an important element also in the present invention.
As a result of the examination by the present inventors, in order to obtain the corrosion resistance required for the corrosion resistance of hydrogen sulfide and the corrosion resistance of the welded portion, it is necessary to add Mo, Ti, etc., but the effect of improving the corrosion resistance of these elements is effective. In order to exert it, a Cr content exceeding 17% by mass should be secured.
The effect of improving corrosion resistance increases as the Cr content increases, but if added over 26% by mass, the reduction of C and N becomes difficult and the mechanical properties and toughness are impaired, and the surface flaw of the cold-rolled annealed plate increases. This increases the manufacturing cost. Therefore, in the present invention, the Cr content is 17 to 26% by mass, preferably 21 to 26%.

  Mo is an effective element for improving the corrosion resistance level together with Cr, and the effect of improving the corrosion resistance increases as the Cr content increases. According to detailed examinations by the inventors, a strong corrosion resistance improving effect is exhibited in a reducing hydrogen sulfide environment. On the other hand, the corrosion resistance improving effect brought about by Mo is not so great for the welded portion in which the oxide scale as formed by TIG welding is formed. The drain pipe, which is the main application of the present invention, requires weld corrosion resistance in a hydrogen sulfide atmosphere. However, it is not desirable from the viewpoint of resource saving to increase the amount of Mo addition. In the present invention, since the hydrogen sulfide corrosion resistance is supplemented by Ti, Al, and Ni, 0.4 mass% or more of Mo is contained. The upper limit is less than 2% by mass, which is necessary and sufficient for use in a drainage pipe environment.

    Nb has a strong affinity for C and N like Ti, and is an effective element for preventing intergranular corrosion, which is a problem in ferritic stainless steel. In order to sufficiently exhibit the effect, it is desirable to secure an Nb content of 0.1% by mass or more. However, when it adds excessively, a hot cracking will arise and a weld part toughness will also fall. The drain pipe is often used with a plate thickness of 1.5 to 2 mm, and a welded portion toughness of a certain level or more is required. The upper limit of the Nb content is 0.5% by mass.

Ti is an element that contributes to improving the corrosion resistance of the weld in TIG welding. In particular, the weld gap is shielded from the atmosphere because Cr is not oxidized, and the corrosion resistance of the scale is hindered. In addition, the corrosion resistance of the oxide scale is improved by forming a stable oxide film and, in addition, suppressing the formation of Fe ₂ O ₃ . Ti also improves the resistance to hydrogen sulfide corrosion. This is thought to be because the oxide of Ti is less susceptible to reduction and the passive film can be maintained even in a reducing environment.
The effect of improving the hydrogen sulfide corrosion resistance of Ti increases as the Cr content increases. In order to fully enjoy the effect, it is desirable to secure a Ti content of 0.2% by mass or more. However, as the Ti content increases, the surface quality of the material deteriorates, and oxide formation (slag spots) in the weld bead increases and the corrosion resistance of the weld bead decreases, so the upper limit of the Ti content is 0.4 mass. %.

Al is preferentially oxidized together with Ti on the steel surface of the weld metal group and the heat affected zone in TIG welding by composite addition with Ti, thereby suppressing the formation of Fe ₂ O ₃ and enhancing the corrosion resistance of the oxide scale.
Moreover, the hydrogen sulfide corrosion resistance is improved similarly to Ti. This is probably because Al has a strong affinity for oxygen and its oxides are less susceptible to reduction, so that a passive film can be maintained even in a reducing environment. In order to sufficiently obtain the effects of improving the corrosion resistance of welds and the corrosion resistance of hydrogen sulfide, it is necessary to secure an Al content of 0.04% by mass or more. On the other hand, excessive Al content causes deterioration of the surface quality and weldability of the material, so the Al content is set to 0.3% by mass or less.

Ni increases the Cr concentration in the weld scale in TIG welding in which the Ar gas seal is omitted, and increases the amount of chemically stable Cr ₂ O ₃ to improve the corrosion resistance of the scale. Furthermore, the corrosion resistance of the TIG welded part is improved by suppressing the progress of pitting corrosion at the weld metal part (bead part) and the ripening affected part. This effect is greater as the Cr content is higher. As a means for improving the Cr ratio by the metal element ratio in the oxide scale, it is effective not to produce an Fe-based oxide. The effect of Ni is different from that of Ti and Al, and is effective in suppressing the oxidation of Fe in the matrix and consequently increasing the Cr ratio in the oxide scale.
Moreover, although the effect is small, it has the effect | action which improves hydrogen sulfide corrosion resistance. As a result of preliminary studies, Ni needs to be 0.4% by mass or more in order to obtain the above-mentioned corrosion resistance improvement effect. However, a large amount of Ni is contained in a range of 2% by mass or less because it makes the steel hard and impairs workability.

It is desirable that the surface of the stainless steel pipe is hydrophilic so that dirt can easily adhere to the drainage pipe from its use and the attached dirt can be easily removed by washing with water. As for hydrophilicity, adjustment of surface irregularities, adjustment of the surface film composition, and application of a hydrophilic film to the surface of stainless steel, etc. can be considered, but the passive film is mainly composed of oxides of Fe and Cr and changes this. It ’s difficult. In addition, in consideration of erosion due to wastewater, it is required to maintain the effect over a long period of time in applying the hydrophilic film, and in the present invention, drainage is controlled by adjusting the surface roughness. By setting the surface roughness of the steel pipe to 2 μm or less in terms of Rz, a hydrophilic steel pipe having a contact angle with water of 60 degrees or less is obtained.
In addition, by performing wet skin pass with an elongation rate of 1% or less, smoothing of the rough surface by polishing and pickling in the cold rolling process can be achieved, and a hydrophilic surface is obtained by the action of a surfactant in the skin pass oil. Obtained stably.

Stainless steel having the chemical composition shown in Table 1 was melted, and a hot-rolled sheet having a thickness of 4 mm was produced by hot rolling. Thereafter, the plate thickness was 2.0 mm by cold rolling, and finish annealing at 1000 to 1070 ° C., pickling, polishing, and pickling were performed to obtain test materials.
No. Steel Nos. 1 to 10 are steel materials for the drainage pipe of the present invention. 1-5 steel is 17-21 mass% Cr steel, No.1. 6-10 steel is 22-26 mass% Cr. No. Steel Nos. 11 to 17 are comparative steels. No. 16 steel is SUS304, no. 17 steel is SUS316.

  Cut out a 20 mm x 15 mm x 2 mmt strip from the cold rolled annealed plate of each test material, connect the lead wire to the end of the plate, cover it with silicon resin leaving a 1 mm square measurement surface, and pitting potential A test specimen for measurement was prepared.

The H ₂ S corrosion resistance of the test steel was evaluated by measuring the pitting potential in a 1000 ppm Cl ⁻ +10 ppm H ₂ S aqueous solution at 30 ° C. The H ₂ S concentration of 10 ppm is the upper limit value of the concentration generated in the drain pipe. The measuring method was performed according to JISG577 “Method for measuring pitting corrosion potential of stainless steel”. Corrosion, which is a problem in the drainage pipe environment, is caused by H ₂ S generated by sulfate-reducing bacteria under the sediment in the pipe, and is absorbed again in the condensed gas in the gas phase of the pipe. The part is subject to corrosion. The reference value of the pitting potential was set to be equal to or higher than the natural potential reached by the stainless steel base material in the drying process of the condensed water. That is, when the pitting corrosion potential of steel is 0.4 V or more than SCE, pitting corrosion does not occur under this environmental condition.

As can be seen from Table 1, all of the examples of the present invention showing the chemical composition and S value specified in the present invention were judged to be acceptable in the corrosion resistance evaluation at the above pitting potential. That is, it was confirmed that it has excellent corrosion resistance in the H ₂ S environment assumed for the drain pipe. Basically, an increase in Cr content and addition of Mo are effective for H ₂ S corrosion resistance. For example, comparative steel No. Although 16 is SUS304, since it does not contain Mo, corrosion resistance in a drainage pipe environment cannot be obtained. No. The amount of Cr in Steel No. 13 is as high as 23% by mass, but Mo is not added as in 304, so that the corrosion resistance in an H ₂ S environment is insufficient.
No. No. 1 steel (18Cr-1.2Mo-0.1Ni-0.3Ti-0.2Al, S value 28.1) and No. 1 steel. No. 11 steel (18Cr-1.2Mo-0.1Ni-0.01Ti-0.2Al, S value 25.7) and No. 11 steel. From the comparison of 12 steel (18Cr-1.2Mo-0.4Ni-0.3Ti-0.02Al, S value 27.7), 0.2 mass% or more of Ti and 0.04 mass% or more of Al are added. It is apparent that the pitting corrosion resistance in an H ₂ S environment containing chloride ions is improved.
No. No. 1 steel No. No. 2 steel (18Cr-1.5Mo-1.1Ni-0.3Ti-0.2Al, S value 29.7) and No. 2 steel. From the comparison of 4 steels (19Cr-1.2Mo-1.8Ni-0.3Ti-0.1Al, S value 28.7), it can be seen that the addition of Ni is effective.
The effect of Mo is great. Steel No. 5 (18Cr-1.9Mo-0.1Ni-0.2Ti-0.05Al, S value 30.6) shows an S value exceeding 30 and a high pitting corrosion potential. However, the addition of Mo leads to an increase in the cost of the tube material, so the addition amount should be as small as possible.
No. No. 1 steel, No. 3 steel (21Cr-1Mo-0.1-Ni-0.2Ti-0.2Al, S value 28.7), No. 1 steel. No. 6 steel (22Cr-0.7Mo-0.1Ni-0.2Ti-0.1Al, S value 28.7), No. 6 No. 8 steel (24Cr-0.5Mo-0.1Ni-0.2Ti-0.1Al, S value 28.9), No. 8 From the comparison of 10 steel (26Cr-0.4Mo-0.1Ni-0.2Ti-0.1Al, S value 30.3), the amount of Mo can be reduced by increasing the amount of Cr. It has the required good H ₂ S corrosion resistance.

As a welding test piece for evaluating the corrosion resistance of the welded portion, a TIG welding test piece shown in FIG. The welding conditions were such that the penetration (welded metal part) reached the back surface and a “back bead” having a width of about 4 mm was formed on the back surface. The Ar back seal gas on the back of the weld was omitted. In the case of this condition, the weld maturation-affected zone (HAZ) is in the range of about 10 mm from the bead center at the center of the plate thickness. The shape of the test piece was 40 mm × 60 mm × 2.0 mm, and the test piece was cut out from the welded material so that the welded portion was at the center of the test piece. A lead wire was attached to the end of the test piece by spot welding. The spot weld was covered with silicon resin.
In the test, an immersion test using a Pt auxiliary cathode shown in FIG. 2 was performed. The test solution was a 6000 ppm Cl-water solution adjusted with the reagent NaCl, the test temperature was 60 ° C., and the test period was 30 days. 400 mL of test liquid was poured into a cylindrical glass container (test container) having a capacity of 500 mL, a TIG welding test piece was immersed, and connected to a Pt auxiliary cathode via a non-resistance ammeter. During the test, the corrosion current was monitored. After the test, the corrosion products were removed, and the pitting depth was measured by a depth of focus method using an optical microscope. The corrosion resistance evaluation surface was the TIG welding back surface.
Corrosion resistance was judged to be acceptable when the corrosion current was 1 μA or less and repassivation was observed in the test period of 30 days, and the maximum pitting depth was 0.1 mm or less.

The immersion test results are shown in Table 1. As can be seen from the table, no. Except for 17 steel (SUS316), all of the examples of the present invention having the chemical composition defined in the present invention passed the corrosion resistance evaluation in the above immersion test. That is, it was confirmed that the welded portion where the oxide scale was formed by performing TIG welding without an Ar back gas seal had excellent corrosion resistance in a relatively high concentration chloride ion environment. In this test, the comparative steel No. No. 16 steel (SUS304) and no. In No. 17 steel (SUS316), stress corrosion cracking occurred in a direction perpendicular to the weld bead.
No. No. 1 steel (18Cr-1.2Mo-0.1Ti-0.3Ti-0.2Al, S value 28.1) and No. of comparative steel. No. 11 steel (18Cr-1.2Mo-0.1N-0.01Ti-0.2Al, S value 25.7) and No. 11 steel. From the comparison of 12 steels (18Cr-1.2Mo-04Ni-0.31Ti-0.02Al, S value 27.7), 0.2 mass% or more of Ti and 0.04 mass% or more of Al should be added. Thus, it is clear that the corrosion resistance of the welded portion in which the Ar back gas seal in the chloride ion environment is omitted is remarkably improved.
No. No. 8 steel (24Cr-0.5Mo-0.1Ni-0.2Ti-0.1Al, S value 28.9) and No. 8 steel. From the comparison of 9 steels (24Cr-0.5Mo-1Ni-0.2Ti-0.1Al, S value 29.1), the addition of Ni improves the repassivation ability in the weld zone. No. Ten steels (26Cr-0.4Mo-0.1Ni-0.2Ti-0.1Al, S value 30.3) also had a corrosion current of 1 μA or less within 7 days after the start of the test. It is effective in improving the ability of repassivation.
However, the comparative steel No. 14 steel (24Cr-0.5Mo-0.1Ni-0.2Ti-0.02Al, S value 28.6) and No. 14 steel. 15 steel (24Cr-1Mo-0.02Ni-0.1Ti-0.1Al, S value 31.7) is 24 mass% Cr steel and S value also satisfies the condition of 28 mass% or more. The amount of addition does not satisfy the conditions of the steel of the present invention, and the corrosion resistance of the welded portion is inferior while the oxide scale is generated.

[Drainage]
The contact angle with a 0.1 ml droplet of deionized water was measured by the sessile drop method, and those having a contact angle of 60 degrees or less were evaluated as having hydrophilicity.

  In Table 2, steel No. 8 and the results using cast iron pipe. Test No. Nos. 1 and 2 are comparative rituals. 3 and 4 are comparative examples. As can be seen from the table, after surface pickling or mixed pickling, the surface roughness is 0.8 and 1.2 μm and the contact angle is 60 degrees or less by applying skin pass with elongation of 0.8%. As a result, better drainage was obtained compared to the current cast iron pipe.

Diagram showing TIG welding specimen Diagram showing immersion test method

1 Pt auxiliary cathode 2 Welding specimen 3 Test solution 4 Aeration nozzle 5 Saturated permeation reference electrode

Claims (5)

The mass of the tube material
C: 0.02% or less,
Si: 0.2-1%,
Mn: 0.4% or less,
P: 0.04% or less,
S: 0.005% or less,
Cr: 17 to 25%,
Mo: 0.4 to less than 2%,
Nb: 0.1 to 0.5%
Ti: 0.2 to 0.4%,
N: 0.025% or less,
Al: 0.04 to 0.3%, and S value defined in the following formula (1) or (2) is 28.0 or more, and ferritic stainless steel made of remaining Fe and other inevitable impurities is used. A drainage pipe characterized by that.
When 16% ≦ Cr <21% S value = Cr + 6Mo + 9 (Ti−0.1) +5 (Al−0.03) (1)
When 21% ≦ Cr ≦ 26% S value = Cr + 7Mo + 9 (Ti−0.1) +5 (Al−0.03) (2)
However, each term in a formula is content (mass%) of an alloy element. The mass of the tube material
C: 0.02% or less,
Si: 0.2-1%,
Mn: 0.4% or less,
P: 0.04% or less,
S: 0.005% or less,
Cr: 17 to 26%,
Mo: 0.4 to less than 2%,
Nb: 0.1 to 0.5%
Ti: 0.2 to 0.4%,
N: 0.025% or less,
Al: 0.04 to 0.3%, and
Ni: 0.4-2%, and the S value defined in the following formula (3) or (4) is 28.0 or more, and ferritic stainless steel made of the remaining Fe and other inevitable impurities is used. Characteristic drain pipe.
When 17% ≦ Cr <21% S value = Cr + 6Mo + 9 (Ti−0.1) +5 (Al−0.03) + 0.2Ni (3)
When 21% ≦ Cr ≦ 26%, S value = Cr + 7Mo + 9 (Ti−0.1) +5 (Al−0.03) + 0.2Ni (4)
However, each term in a formula is content (mass%) of an alloy element.   The water pipe according to claim 1 or 2, wherein the steel pipe has a surface roughness Rz of 2 µm or less.   A method for producing a drain pipe according to claim 1, wherein a skin pass having an elongation of 1% or less is performed after annealing and pickling, and the surface roughness is 2 μm or less in terms of Rz.   3. A method of manufacturing a drain pipe according to claim 1, wherein annealing after pipe forming and removal of weld scale in the pipe forming are omitted.

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