JP2017061748A - Ferritic stainless steel for tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel for a tube having good corrosion resistance even when omitting conventional post welding heat treatment for suppressing corrosion sensitization for welded stainless steel.SOLUTION: A ferritic stainless steel for a tube contains, by mass%, C:0.030% or less, Si:1.00% or less, Mn:1.00% or less, Cr:16 to 20%, Ni:0.6% or less, N:0.025% or less and further one or more kind of Nb:8(C+N) to 0.8% and Ti:8(C+N) to 0.8% and the balance Fe with inevitable impurities, and is excellent in corrosion resistance capable of omitting post welding heat treatment.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a ferritic stainless steel for tubes.

  Exhaust gas treatment with a built-in device for denitration and desulfurization of exhaust gas in boilers for thermal power plants so that nitrogen oxides (NOx), sulfur oxides (SOx) and soot in the combustion gas are not released to the atmosphere The system is installed. An example of an exhaust gas treatment system from a coal-fired or heavy oil-fired thermal power plant boiler is shown in FIG. 3 (see Patent Document 1).

  The NOx is removed from the exhaust gas emitted from the boiler 1 by the denitration device 2. Next, after passing through the boiler combustion air preheater 3, it is cooled by a gas / gas heater (hereinafter abbreviated as “GGH”) 4 in order to improve the dust collection efficiency and desulfurization performance. SOx is removed from the cooled exhaust gas by the desulfurizer 7. The exhaust gas after the desulfurization treatment is heated by the GGH 5 on the reheating side in order to remove the mist of the absorbing solution by the mist eliminator 8 and then discharge the detoxified exhaust gas to the atmosphere in a dry state. The exhaust gas is heated by the reheating-side GGH 5 by the amount of heat recovered in the cooling-side GGH 4.

  FIG. 4 shows a typical GGH structure of the hot water circulation system. In the cooling side GGH4, the boiler exhaust gas 12 having a temperature of 120 to 150 ° C. is usually heat-exchanged with the hot water flowing in the heat transfer tube 10a, and the exhaust gas 13 is obtained by cooling to 80 to 120 ° C. In the reheating side GGH5, the dehydrated 50 to 60 ° C. exhaust gas 14 is heated to 80 to 100 ° C. with the amount of hot water flowing in the heat transfer tube 10b heated to 100 to 130 ° C. 15 is obtained. In the heat transfer tube, generally, a tube having a finned tube structure is sometimes referred to as a “fin tube”.

  The heat transfer tube 10a of the cooling side GGH4 is in contact with the boiler exhaust gas 12 to transfer heat. The boiler exhaust gas contains high concentrations of SOx and NOx, and the heat transfer tube 10a is exposed to a severe corrosive environment such as chloride or sulfide. The heating side GGH5 is exposed to an environment in which adhesion and evaporation of chloride-containing droplets and mist are repeated. Therefore, stainless steel having corrosion resistance, for example, austenitic stainless steel (SUS304) and ferritic stainless steel (SUS430) are generally used for the heat transfer tubes 10a and 10b.

  The hot water flowing through the heat transfer tube 10a on the GGH device cooling side 4 is heat recovered and then sent to the heat transfer tube 10b on the GGH device heating side 5 to be used for heating the desulfurized exhaust gas 14 and again on the GGH device cooling side. Circulating to return to 4. The heat transfer tube 10a of the cooling side GGH4 and the heat transfer tube 10b of the heating side GGH5 are connected using a pipe (tube) made of a material other than stainless steel (for example, ordinary steel). Both tubes are usually connected by welding.

JP 2001-248826 A JP-A-6-248350

  Ferritic stainless steel is generally cheaper than austenitic stainless steel and is excellent in stress corrosion cracking properties, so it is used for GGH heat transfer tubes. Welding is performed to connect a ferritic stainless steel (SUS430) tube and a normal steel tube. In that case, intergranular corrosion due to sensitization (precipitation of Cr carbide at the crystal grain boundary) occurring in the weld affected zone (HAZ) after welding, reduction in toughness due to crystal grain coarsening and residual stress. Arise. Usually, post-weld heat treatment (PWHT: Post Weld Heat Treatment) is applied after welding in order to eliminate sensitization and toughness deterioration in the HAZ. The heat treatment is performed for 30 minutes to 1 hour at a recrystallization temperature or lower, and then rapidly cooled to decompose Cr carbide precipitated at the grain boundaries to solidify Cr into the grains, and to improve the structure and residual stress. Mitigation takes place.

  A high frequency heating method is used for the heat treatment after the tube is welded. An induction heating coil is wound around the outer periphery of the welded portion, and the welded portion is heated by high-frequency induction heating (for example, Patent Document 2). When such welding is performed at a construction site, it is necessary to bring a heating power source into the site and perform high-frequency heating for post-weld heat treatment.

  In manufacturing GGH, a normal steel tube and a ferritic stainless steel (SUS430) tube may be connected, and conventionally, SUS430 having a carbon content of about 0.1 mass% is used. After welding this SUS430 tube, in order to improve the deterioration of corrosion resistance and toughness in the weld-affected zone (HAZ) after welding, the heat treatment at 700 to 800 ° C. for 30 minutes to 1 hour is performed as the post-welding heat treatment described above. Applied.

  However, the above heat treatment work needs to be performed in the entire region of the welded portion, and it takes a lot of time to install the induction heating device and prepare the power source. In addition, there is a problem that a space for installing the power supply and the heating device on the site is required, and costs are increased. Therefore, it has been desired to simplify the welding work.

  The present invention has been devised in order to solve the above-mentioned problems. Even if the post-weld heat treatment is omitted, the ferrite system for a tube having good corrosion resistance in which a decrease in corrosion resistance due to sensitization is suppressed. It aims to provide stainless steel. Another object of the present invention is to provide a heat exchange device in which a tube made of ferritic stainless steel is welded to a pipe made of a material different from that of the stainless steel. Moreover, when welding said tube, it aims at providing the construction method which abbreviate | omits a post-welding process and performs welding.

  The present inventors diligently studied the possibility of simplifying the welding work. By adopting stainless steel, which is less susceptible to sensitization, as a tube material, the present invention finds that corrosion resistance equivalent to or higher than that of conventional heat transfer tubes can be imparted even if post-welding heat treatment is omitted when welding is applied. It came to complete. Specifically, the present invention provides the following.

  (1) The present invention is mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, Cr: 16-20%, Ni: 0.6% or less, N: not more than 0.025%, further including at least one of Nb: 8 (C + N) to 0.8%, Ti: 8 (C + N) to 0.8%, the balance being Fe and inevitable impurities Therefore, it is a ferritic stainless steel for tubes having excellent corrosion resistance that can omit heat treatment after welding.

  (2) The present invention is the ferritic stainless steel for a tube according to the above (1), further comprising Cu: 0.3 to 0.8% by mass.

  (3) The present invention is the ferritic stainless steel for a tube according to the above (1) or (2), further comprising, by mass%, Mo: 0.45 to 2.5%.

  (4) The present invention includes a piping structure in which a tube made of the ferritic stainless steel according to any one of (1) to (3) is welded to a tube made of a material different from the stainless steel. It is a heat exchange device.

  (5) The present invention includes a step of welding and joining a tube made of the ferritic stainless steel according to any one of the above (1) to (3) to a tube made of a material different from the stainless steel, And the said welding is a welding construction method which does not give a heat treatment after welding.

  Since the present invention uses a ferritic stainless steel in which one or more of Nb or Ti are added to reduce the C and N contents to a predetermined range or less, even if the post-weld heat treatment is omitted, the ferritic stainless steel The tube having the corrosion resistance equivalent to or higher than that of the conventional SUS430 can be provided. Thereby, the number of welding processes is reduced, and a space provided for a heat treatment apparatus for performing post-weld heat treatment is not required, so that the workability of welding work is improved and the cost is reduced.

It is a figure for demonstrating the test piece in an Example. (A) is a figure which shows the test material before butt welding, (b) is a figure which shows the test body after performing butt welding. It is a schematic diagram which shows the cross-sectional shape of a test piece. It is a schematic diagram which shows the structure regarding the exhaust gas processing apparatus in a thermal power plant. It is a schematic diagram which shows the structure regarding a GGH (gas / gas heater) apparatus.

  Embodiments of the present invention will be described below, but these do not limit the present invention.

  The sensitization in the HAZ (welding affected zone) is, in short, a property that Cr in the parent phase precipitates as carbides at the grain boundaries due to heating during welding, and the Cr concentration near the grain boundaries decreases, which increases the susceptibility to corrosion. Yes, if the sensitization is increased, the corrosion resistance is reduced.

  In the present invention, in order to suppress chromium carbides precipitated at the grain boundaries that are the main cause of such sensitization, the carbon content is reduced, and further ferrite containing Nb and Ti that precipitates carbides in the grains is added. Stainless steel is used as the tube material. Therefore, since the sensitization resulting from the heating at the time of welding can be suppressed, it is suitable for the purpose of welding without the post-weld heat treatment (PWHT).

(Steel composition)
Next, the component composition of the ferritic stainless steel of the present invention will be described. Hereinafter, unless otherwise specified, mass% is expressed in “%”.

  Since C lowers intergranular corrosion resistance, its content needs to be reduced. Therefore, the content of C is preferably 0.025% or less.

  Si is an element effective for deoxidation, and is also effective for improving corrosion resistance. If added excessively, workability will be reduced. Therefore, the Si content is preferably 1.00% or less.

  Mn is an element effective for deoxidation, but if added excessively, a Mn compound is formed and the corrosion resistance is lowered. Therefore, the Mn content is preferably 1.00% or less.

  Cr is an element that forms a passive film and imparts corrosion resistance. Furthermore, it has the effect | action which stabilizes a ferrite. If added excessively, workability will be reduced. Therefore, the content of Cr is preferably 16 to 20%.

  Ni is an element having an action of improving the corrosion resistance in a reducing acid environment. If it is added excessively, the workability is lowered and the ferrite phase becomes unstable. Therefore, the Ni content is preferably 0.6% or less.

  Cu is effective in improving the corrosion resistance and has an action of generating a ferrite phase. If added excessively, workability will be reduced. Therefore, the Cu content is preferably 0.3 to 0.8%.

  N, like C, reduces intergranular corrosion resistance, so its content needs to be reduced. Therefore, the N content is preferably 0.025% or less.

  Nb and Ti are elements that precipitate and form carbides and nitrides in crystal grains and fix and stabilize carbon and nitrogen, and suppress the sensitization of welds and improve intergranular corrosion resistance. Have Moreover, since the precipitate suppresses the coarsening of a crystal grain, it is preferable also at the point which improves toughness. If added excessively, workability will be reduced. Therefore, it is preferable to contain one or more of Nb or Ti, and the content of Nb and Ti is preferably 8 (C + N) or more and 0.8% or less, respectively. The “8 (C + N)” means 8 times the total content of C and N.

  Mo is an element having an effect of repairing the passive film and having an effect of improving the corrosion resistance, and is preferably added. If added excessively, the workability is lowered, so the Mo content is preferably 0.45 to 2.5%.

For example, ferritic stainless steel having the following composition is preferable.
In mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, Cr: 16-20%, Ni: 0.6% or less, Nb: 8 (C + N) It contains 0.8%, Cu: 0.3-0.8%, N: 0.025% or less, and the balance is Fe and inevitable impurities.

  In mass%, C: 0.025% or less, Si: 1.00% or less, Mn: 1.00% or less, Cr: 16-19%, Ni: 0.6% or less, Mo: 0.50-1 .50%, Ti: 8 (C + N) to 0.8%, N: 0.025% or less, with the balance being Fe and inevitable impurities.

  In mass%, C: 0.020 (0.025)% or less, Si: 1.00% or less, Mn: 1.00% or less, Cr: 17 to 20%, Ni: 0.6% or less, Mo: 1.75 to 2.50%, Nb: 10 (8) (C + N) to 0.8%, N: 0.025% or less, with the balance being Fe and inaccessible impurities.

(tube)
The ferritic stainless steel according to the present invention can be applied to a tube material constituting a piping structure. The structure, shape, dimensions, etc. of the tube are not limited. Applicable to ordinary steel pipe products. Moreover, the said tube may be a fin tube provided with the fin in the tube outer surface in order to improve heat exchange capability. The structure and distribution form of fins in the fin tube are not limited. The fin tube can be used in any type, in which the outer periphery of the tube is rolled and the fin is integrally formed, or the fin is welded to the outer surface of the tube.

(Heat exchange device)
The ferritic stainless steel according to the present invention can be used as a tube material in a piping structure of a heat exchange device. It is preferable to apply to a fin tube material in an exhaust gas treatment system from a power plant boiler such as GGH. You may apply to the piping structure in heat exchange apparatuses other than GGH.

(Welding method)
As the welding method, known welding means such as TIG welding, MIG welding, carbon dioxide arc welding can be applied. As the welding material, a high purity austenitic stainless steel welding rod can be used. For example, welding by TIG welding using YS309L (Fe-24Cr-13Ni-1.75Mn-low C) defined in JIS Z3321 is preferable. In the present invention, the heat treatment (PWHT) after welding can be omitted.

  The ferritic stainless steel according to the present invention manufactures a tube having a predetermined shape using a steel plate produced by each process of normal melting, hot rolling, hot rolled sheet annealing, cold rolling, and cold rolling annealing. be able to. In the case of a fin tube, the outer periphery of the tube can be rolled to form a fin, or a fin member can be welded to the outer periphery of the tube.

  Examples of the present invention will be described below. The present invention is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the gist of the invention.

(Preparation of test specimen)
A ferritic stainless steel having the components (mass%) shown in Table 1 (the remainder of Table 1 is Fe and inevitable impurities) is melted by vacuum melting, and the resulting steel ingot is a 30 mm thick plate After forging, hot rolling was performed to obtain a hot-rolled sheet having a thickness of 4 mm. Next, annealing, pickling and cold rolling were performed to obtain a 2.6 mm thick cold rolled sheet. Then, the cold-rolled sheet was subjected to an annealing treatment at 1050 ° C. to produce a cold-rolled sheet. Processing was performed with a shearing machine using a cold-rolled annealed plate to obtain a tube material having an outer diameter of 38.1 mm and a length of 1000 mm. This was used as a test material. Invention Example 1 to Invention Example 6 of the test specimen are ferritic stainless steels included in the composition range of the present invention, and the comparative example of the test specimen is a ferritic stainless steel corresponding to conventional SUS430.

(Welding conditions)
As shown in FIG. 1 (a), two specimens 21 (outer diameter 38.1 mm and length 1000 mm) were prepared. Thereafter, as shown in FIG. 1B, butt welding of the specimen 21 was performed, and a test body 22 made of a welded joint in which end faces were joined to each other was produced. The butt welding was TIG welding with a feed rate of 1 m / min and a voltage of 20V. The welding current value was 85 to 100 A in order to control the bead width. TG309L φ2.4 was used as the welding material. The welding posture was performed downward, and the back shield was performed with argon gas at a flow rate of 2 L / min.

  Thereafter, some of the test bodies were subjected to post-weld heat treatment (PWHT). The heat treatment was carried out by heating at 780 ° C. for 1 hour in a furnace in an air atmosphere, and then taking out from the furnace and cooling with air.

(Sensitization evaluation test)
In order to evaluate the degree of sensitization, a grain boundary corrosion test was conducted. As shown in FIG. 1B, the test piece 24 subjected to the intergranular corrosion test has a length of 70 mm and a width of 20 mm from which the weld line (butt weld 23) is located at the center. Cut in size and collected. The cut end face was finished by buffing after emery wet polishing # 2400. As shown in FIG. 2, the cut surface 27 has a shape in which the butt weld 23 is located substantially at the center. The surrounding structure of the butt weld 23 on the outer surface 25 and the cut surface 27 of the test piece was observed with a 500 times magnifier. When cracks due to intergranular corrosion could be confirmed with a size of 10 μm or more, it was evaluated that cracks due to intergranular corrosion had occurred.

  The specimen from the butt welded specimen is referred to as an “as welded material”, and the specimen from the specimen that has been subjected to the post-weld heat treatment is sometimes referred to as a “PWHT material”. . The intergranular corrosion test was conducted in the following two types using each test piece of As welded material and PWHT material.

(1) Nitric acid / hydrofluoric acid corrosion test method (weld decay method)
This test method is performed according to JIS G0574. Since this JIS has been deleted at the present time, in the examples, the JIS was used as a reference. As the test solution, an aqueous solution containing 10% nitric acid and 3% hydrofluoric acid was used. The test piece was taken out of the test solution after being immersed for 2 hours while maintaining the test solution at 70 ± 0.5 ° C. in a thermostatic bath. After the test piece was washed and dried, the structure of the outer surface and the cross section was observed.

(2) Sulfuric acid / copper sulfate corrosion test method (Strauss method)
This test method is performed according to JIS G0575. As the test solution, an aqueous solution containing 45.5% copper sulfate and 15.7% sulfuric acid was used. The test piece was immersed in the boiled test solution for 24 hours and then removed from the test solution. After the test piece was washed and dried, the structure of the outer surface and the cross section was observed.

(Test results)
As shown in FIG. 2, the test piece had a weld heat affected zone (HAZ) on the base metal side adjacent to the butt weld. The results of evaluating the corrosion status in the HAZ structure are as shown in Table 2. When cracks due to intergranular corrosion occurred, “x” was indicated, and when no intergranular cracks occurred, “o” was indicated.

(1) Test results by nitric acid / hydrofluoric acid corrosion test method (weld decay method) The present invention example 1 to present invention example 6 are caused by sensitization to the weld heat-affected zone in both the as welded material and the PWHT material. No discoloration or grain boundary cracking was observed. In the comparative example, intergranular corrosion was not confirmed for the PWHT material, but in the As Welded material, the weld heat affected zone was changed to black, causing intergranular cracking, which was sensitized.

(2) Test results by sulfuric acid / copper sulfate corrosion test method (Strauss method) Invention Example 1 to Invention Example 6 showed that intergranular cracking was confirmed in the weld heat affected zone in both As Welded material and PWHT material There wasn't. In the comparative example, intergranular corrosion was not confirmed for the PWHT material, but intergranular cracking occurred in the weld heat affected zone in the As Welded material. The crack depth of the As Welded material reached about half of the plate thickness.

  From the above, it was confirmed that the ferritic stainless steel according to the present invention was not sensitized by the weld heat affected zone. That is, it was found that even when the post-weld heat treatment is omitted, intergranular corrosion cracking due to sensitization does not occur.

  According to the above test results, the ferritic stainless steel according to the present invention, when applied to the material of the tube to be welded, has good corrosion resistance without performing post-weld heat treatment, which has conventionally been essential. It is possible to construct a piping structure having Therefore, the present invention can perform welding without omitting the heat treatment after welding, can simplify the welding process related to the tube, and has confirmed that it is highly effective in improving workability and reducing costs.

1 boiler 2 denitration equipment 3 preheater 4 gas / gas heater GGH (heat recovery side)
5 Gas / gas heater GGH (reheating side)
6 Electric dust collector (EP)
7 Desulfurizer 8 Mist separator 9 Chimney 10a, 10b GGH heat transfer tube 11 Pump 12, 13, 14, 15 Exhaust gas 21 Specimen 22 Specimen 23 Butt weld 24 Specimen 25 Specimen outer surface 26 Specimen inner surface 27 Cutting surface

Claims (5)

  In mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, Cr: 16 to 20%, Ni: 0.6% or less, N: 0.025% or less And Nb: 8 (C + N) to 0.8%, Ti: 8 (C + N) to 0.8%, and the balance is made of Fe and inevitable impurities, eliminating post-weld heat treatment. Ferritic stainless steel for tubes with excellent corrosion resistance.   Furthermore, the ferritic stainless steel for tubes of Claim 1 which contains Cu: 0.3-0.8% by mass%.   Furthermore, the ferritic stainless steel for tubes of Claim 1 or 2 which contains Mo: 0.45-2.5% by mass%.   The heat exchange apparatus provided with the piping structure formed by welding the tube which uses the ferritic stainless steel in any one of Claims 1-3 as the raw material to the tube of the material different from the said stainless steel.   A process comprising welding a tube made of the ferritic stainless steel according to any one of claims 1 to 3 to a tube made of a material different from the stainless steel, and the welding includes a post-weld heat treatment. Welding method not applied.

Images