WO2013035775A1

WO2013035775A1 - Ferritic stainless steel of exceptional corrosion resistance and processability

Info

Publication number: WO2013035775A1
Application number: PCT/JP2012/072709
Authority: WO
Inventors: 佑一田村; 透松橋
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2011-09-06
Filing date: 2012-09-06
Publication date: 2013-03-14
Also published as: TWI503422B; JP6097693B2; TW201323628A; JPWO2013035775A1

Abstract

This ferritic stainless steel contains 0.030% or less of C, 0.030% or less of N, 0.60% or less of Si, 0.01-0.5% of Mn, 0.05% or less of P, 0.01% or less of S, 13-22.5% of Cr, less than 0.35% of Ni, 0.05-0.30% of Ti, 0.01-0.2% of Al, 0.5% or less of Cu, and less than 0.30% of Mo (percentages given with respect to weight); Fe and inevitable impurities constitute the balance. The stainless steel complies with formulae (A) and (B). (A): Cr+10Ti+10Al≥15; (B): Si+Cu≤1.1 (Cr, Ti, Al, Si, and Cu refer to amounts of the corresponding element contained in the steel (wt%)).

Description

Ferritic stainless steel with excellent corrosion resistance and workability

The present invention relates to a ferritic stainless steel excellent in corrosion resistance and workability that can be suitably used for a member exposed to an environment where chloride exists at a low pH such as a secondary heat exchanger.
This application claims priority based on Japanese Patent Application No. 2011-193915 filed in Japan on September 6, 2011, the contents of which are incorporated herein by reference.

The exhaust gas generated by the combustion of various fossil fuels includes SOx and NOx derived from fuel and air. These components react with oxygen and water vapor in the atmosphere to produce H ₂ SO ₄ , HNO _3, etc., and are wet deposited in cloud water or dry deposited in the atmosphere as an aerosol. This is considered to be the mechanism of acid rain. Acid rain is rain having a pH of about 5.6 or less, and contains about 10 ppm of SO ₄ ²⁻ ions and NO ₃ ⁻ ions, respectively. In particular, in industrial zones and areas where volcanic gas is generated, acid rain has a lower pH and contains more SO ₄ ²⁻ ions and NO ₃ ⁻ ions. Furthermore, there is a possibility that chloride derived from sea salt or the like is contained in an amount of several ppm to 100 ppm. If rainwater of such a component adheres, the pH is lowered and the concentration is increased by drying, resulting in a severe corrosive environment. However, such an acid rain environment is a mild corrosive environment as compared with an environment where a heat exchanger in which exhaust gas is directly condensed is exposed.

A heat exchanger is a device that supplies heat, which is obtained by burning various fuels, to a medium mainly composed of water, and is used in various fields from a steam generator for nuclear power generation to a hot water heater for general households. Among these, a heat exchanger is built in a gas or oil water heater in a general household in order to turn the water into hot water by the combustion heat. Conventionally, this heat exchanger has been made of copper that can be easily processed into a fin structure and has excellent thermal conductivity in order to increase thermal efficiency. However, due to recent environmental problems, CO ₂ reduction is also required for water heaters. For this reason, a latent heat recovery type water heater that further utilizes the heat of the conventional exhaust gas has been developed for the purpose of further improving the thermal efficiency. This water heater further uses the heat of the exhaust gas which burned the gas after passing through the conventional heat exchanger (primary heat exchanger) and oil. For this reason, it has another heat exchanger (secondary heat exchanger). The exhaust gas after passing through the primary heat exchanger is about 150-200 ° C. and contains a large amount of water vapor. For this reason, in the secondary heat exchanger, the total heat efficiency is improved to 95% or more by recovering not only direct heat but also condensation heat in which water vapor becomes water droplets, that is, latent heat. An example of the structure of this latent heat recovery type hot water heater is disclosed in Patent Document 1, for example.

Here, the condensed water generated in the secondary heat exchanger is generated from exhaust gas obtained by burning hydrocarbon-based raw materials such as city gas, LPG, and petroleum. For this reason, the condensed water is a weakly acidic aqueous solution having a pH of about 3 or less, including nitrate ions and sulfate ions derived from its exhaust gas components and the like. In a solution having a low pH, conventionally used copper (corrosion at a pH of 6.5 or less) cannot be used. Other ordinary steels (corrosion when the pH is about 7 or less) and aluminum (corrosion when the pH is about 3 or less) corrode in the environment exposed to the condensed water. Therefore, a stainless steel having excellent corrosion resistance in a weakly acidic region is currently selected as a secondary heat exchanger material. Among general-purpose stainless steels, SUS316L (18Cr-10Ni-2Mo), which is an austenitic stainless steel having excellent corrosion resistance, is selected. ) Is mainly adopted. SUS316L satisfies the corrosion resistance necessary for a secondary heat exchanger member applied to a latent heat recovery type water heater. However, the raw material contains a large amount of Ni and Mo which are expensive and have very unstable price stability. The latent heat recovery type water heater is expected to be widely spread to the general public as a trump card for CO ₂ reduction. In order to realize this, further cost reduction of materials and good workability are strongly demanded. In SUS316L, which is a secondary heat exchanger material, it is naturally expected to propose a lower cost alternative material. Also, although there is no problem of corrosion resistance in a general usage environment, SUS316L can also corrode in areas such as the coast where sea salt particles are likely to fly, which is one of the factors that hinder the corrosion resistance of stainless steel. Sex cannot be denied. In this case, in SUS316L, stress corrosion cracking, which is one of the weak points of austenitic stainless steel, may occur.

In order to solve such problems that occur when austenitic stainless steel is applied, attempts to apply ferritic stainless steel to secondary heat exchanger members have been made in recent years (Patent Documents 2 to 6). .
Patent Document 2 proposes ferritic stainless steel as a heat exchanger material having corrosion resistance against sulfur-containing gas. In this ferritic stainless steel, Mo is added together with Nb or Ti, thereby suppressing a decrease in corrosion resistance and improving high-temperature strength. In addition, brazing and moldability are improved by reducing the contents of Si and Al.

Patent Document 3 discloses a ferritic stainless steel that exhibits durability in a high temperature steam environment in an environment to which a heat exchanger is exposed. In this ferritic stainless steel, the components are adjusted so as to satisfy a specific relational expression derived from the contents of Cr, Mo, Si, and Al and the intended use temperature. However, since the amount of Al added is large, there is a problem that the material becomes very hard and brittle. Further, the assumed temperature is 300 to 1000 ° C., and a material to be used in an environment much higher than the latent heat recovery type hot water heater targeted by the present invention is defined.

Patent Document 4 discloses a ferritic stainless steel excellent in brazeability, characterized in that the contents of Ti and Al are reduced.
Patent Document 5 discloses ferritic stainless steel suitable as a heat exchanger member used for brazing. In this ferritic stainless steel, the components are adjusted so that the A value calculated from the contents of Nb, C, and N is 0.1 or more. However, although Nb is an essential element in order to prevent the crystal grains from becoming coarse during the heat treatment in brazing, there is no indication for improving the corrosion resistance.

Patent Document 6 discloses a ferritic stainless steel material for brazing and a heat exchanger member. In order to prevent coarsening during high temperature brazing, the area ratio occupied by recrystallized grains is specified, but there is no indication for improving corrosion resistance.
Further, the heat exchange pipe of the secondary heat exchanger needs to be bent, and a flexible pipe is also used in some products. For this reason, favorable workability is required for the heat exchanger member. Although conventionally used austenitic stainless steel has sufficient workability, ferritic stainless steel is inferior in workability compared to austenitic stainless steel. For this reason, a material excellent in workability is particularly demanded.

According to conventional knowledge, there is no prior art that mentions both corrosion resistance and workability in an environment where chloride exists at low pH, and ferritic stainless steel that can be suitably used in such an environment is sufficiently disclosed. It was a situation that could not be said.

JP 2002-106970 A JP 7-292446 A JP 2003-328088 A JP 2009-174046 A JP 2009-299182 A JP 2010-285683 A

In view of such circumstances, an object of the present invention is to provide a ferritic stainless steel that is inexpensive, excellent in corrosion resistance, has good workability, and can be suitably used in the above environment.

In order to solve the above problems, the present inventors evaluated the corrosion resistance of various stainless steels in such an environment. As a result, it has been clarified that the corrosion resistance is particularly excellent when the contents of Cr, Al, and Ti are large and particularly when they are concentrated on the surface of the passive film.
Moreover, the following matters were found from the evaluation of the generated corrosion starting point.
(A) By reducing the contents of Cu and Si, the corrosion resistance in the target environment is improved.
(B) Furthermore, the workability is improved by reducing the contents of Mo and Ni.
In the present invention, for the inside of such a secondary heat exchanger, studies have been made on materials having excellent corrosion resistance in a corrosive environment where chlorides exist at low pH. As a result, we have developed a ferritic stainless steel with excellent corrosion resistance and workability in the target environment.
That is, the present invention is a ferritic stainless steel having the following characteristics, excellent corrosion resistance even in an environment where chloride is present at low pH, and good workability.

(1) By mass%, C: 0.030% or less, N: 0.030% or less, Si: 0.60% or less, Mn: 0.01 to 0.5%, P: 0.05% or less, S: 0.01% or less, Cr: 13 to 22.5%, Ni: less than 0.35%, Ti: 0.05 to 0.30%, Al: 0.01 to 0.2%, Cu: 0 .5% or less, and Mo: less than 0.30%, the balance being Fe and inevitable impurities, and satisfying the following formulas (A) and (B): It is a ferritic stainless steel with excellent resistance.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
(However, Cr, Ti, Al, Si, and Cu in the formula mean the content (mass%) of each element.)
(2) The ferritic stainless steel having excellent corrosion resistance and workability as described in (1) above, further comprising Nb: 0.05 to 0.50%.
(3) The ferritic stainless steel having excellent corrosion resistance and workability as described in (1) or (2) above, further comprising Sn: 0.005 to 1.0%.
(4) Further, any one or both of B: 0.0001 to 0.003% and V: 0.03 to 1.0% are contained, (1) to (3 ) Ferritic stainless steel having excellent corrosion resistance and workability.
(5) The ferritic stainless steel having excellent corrosion resistance and workability according to any one of (1) to (4), wherein the following formula (B ′) is satisfied instead of the formula (B):
(B ′) Formula: Si + Cu ≦ 0.5
(However, Si and Cu in the formula mean the content (mass%) of each element.)
(6) The corrosion resistance according to any one of (1) to (5) above, wherein an average corrosion weight loss measured by a corrosion test using a simulated acid rain solution is 0.4 mg / cm ² or less. Ferritic stainless steel with excellent workability.
In the corrosion test, an aqueous solution containing a pH of 4.5 and containing 10 ppm nitrate ions, 10 ppm sulfate ions, and 5 ppm chloride ions is used as the simulated solution, and a gap imparting test piece is used in the aqueous solution. Is subjected to 10 cycles of the wet and dry repeated test that is held at 50 ° C. for 24 hours, and the amount of decrease in mass after the dry and wet repeated test is measured to obtain the average corrosion weight loss.
(7) The average corrosion weight loss measured by a corrosion test using a simulated liquid of condensed water of combustion exhaust gas is 1.0 mg / cm ² or less. Excellent ferritic stainless steel.
In the corrosion test, an aqueous solution having a pH of 2.5 and containing 100 ppm nitrate ions, 10 ppm sulfate ions, and 100 ppm chloride ions is used as the simulated solution, and a gap imparting test piece is placed in the aqueous solution. 10 cycles of the wet and dry repeated test that is semi-immersed and held at 80 ° C. for 24 hours, the amount of decrease in mass after the dry and wet repeated test is measured, and the average corrosion weight loss is obtained.

According to the embodiment of the present invention, it is possible to provide a ferritic stainless steel having excellent corrosion resistance and good workability in an environment where chloride exists at low pH such as acid rain. Further, unlike austenitic stainless steel, this ferritic stainless steel does not contain a large amount of expensive Ni or Mo. In particular, according to a preferred embodiment of the present invention, it exhibits excellent corrosion resistance as a material for equipment exposed to a condensed water environment of combustion gas using hydrocarbons such as LNG and petroleum as fuel, such as a water heater. It becomes possible.

It is a figure explaining the evaluation method of corrosion resistance, (a) shows the installation condition of a sample, (b) shows the shape of the sample used for the test, (c) shows CC sectional view taken on the line. It is a figure which shows the relationship between the average corrosion weight loss measured by the corrosion test by the acid rain simulated liquid, and a component element. It is a figure which shows the relationship between the average corrosion weight loss measured by the corrosion test with the simulation liquid of the condensed water of combustion exhaust gas, and a component element. In the result of an Example and a comparative example, it is a figure which shows the relationship between the average corrosion weight loss measured by the corrosion test by the simulated liquid of acid rain, and a component element. In the result of an Example and a comparative example, it is a figure which shows the relationship between the average corrosion weight loss measured by the corrosion test with the simulation liquid of the condensed water of combustion exhaust gas, and a component element.

Hereinafter, unless otherwise specified, “%” means “mass%”.
The inventors have made extensive developments in order to provide a ferritic stainless steel that exhibits excellent corrosion resistance in an environment where chlorides are present at low pH and has good workability. As a result, the following matters were discovered.

(1) A wet and dry repeated test simulating an environment in which acid rain falls was performed, and the average corrosion weight loss of the gap imparted test piece was measured. As a result, in the ferritic stainless steel satisfying the following formulas (A) and (B), the average corrosion weight loss was 0.4 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
(2) Comparing the results of ferritic stainless steel and austenitic stainless steel with the same value on the left side of equation (A), the average corrosion weight loss of ferritic stainless steel is less than that of austenitic stainless steel in an acid rain environment. It became smaller than average corrosion weight loss.
(3) A dry and wet repeated test simulating the environment of the condensed water generated by the combustion exhaust gas was performed, and the average corrosion weight loss of the gap imparted test piece was measured. As a result, in the ferritic stainless steel satisfying the following formulas (A) and (B ′), the average corrosion weight loss was 1.0 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B ′) Formula: Si + Cu ≦ 0.5
(4) Similarly to the above (2), when comparing the results of ferritic stainless steel and austenitic stainless steel having the same value on the left side of the formula (A), the ferritic stainless steel can be used in the condensed water environment of the combustion exhaust gas. The average corrosion weight loss was smaller than the average corrosion weight loss of austenitic stainless steel.
(5) In order to evaluate workability, the elongation value was measured by a tensile test. As a result, when Cr, Si, Cu, Mo, and Ni were contained, the elongation value decreased and the workability deteriorated.

First, a corrosion test (corrosion resistance evaluation method) simulating an acid rain environment will be described.
A test solution containing pH = 4.5, nitrate ion 10 ppm, sulfate ion 10 ppm, and chloride ion 5 ppm was prepared using a reagent as a simulated acid rain solution.
For each of the various stainless steels (test materials), three test pieces 1 having a size of 25 × 50 mm were prepared as shown in FIG. In this test, in order to evaluate crevice corrosion, a glass / metal gap was given to the test piece 1 as follows. A hole 9 having a diameter of 6 mm was formed in a substantially central portion of the test piece 1. Before starting the test, the entire surface of the test piece 1 was wet-polished with # 400 emery paper, and immediately using a Teflon (registered trademark) bolt 2, a Teflon (registered trademark) nut 3, and a titanium washer 5, Test piece 1 was sandwiched between two glass plates 4. Thus, a glass / metal gap was imparted to the test piece 1.

The test piece 1 was placed in a beaker 7 as shown in FIG. 1A, 50 ml of acid rain simulated liquid 8 was filled, and the test piece 1 was semi-immersed. The beaker 7 was placed in a 50 ° C. warm bath and held for 24 hours. During this time, the simulation liquid 8 was dried and concentrated. Next, a stainless steel sample (test piece 1) was taken out from the dried and concentrated simulated liquid 8, and lightly washed with distilled water. Then, the newly washed beaker 7 was filled again with the test solution (acid rain simulated solution 8). Subsequently, the stainless steel sample (test piece 1) was again semi-immersed and held at 50 ° C. for 24 hours. The test piece 1 was submerged in the simulated solution 8 and then held at 50 ° C. for 24 hours was repeated 10 times (wet and dry repetition test).
Note that the set temperature of 50 ° C. simulates a temperature that is considered to be relatively high in a corrosion target object installed outdoors where acid rain falls.

After 10 cycles, the rust of the test piece 1 was removed, and the mass was measured with an electronic balance. The weight loss of the corrosion was determined by subtracting the mass of the test piece 1 after the test from the mass of the test piece 1 before the test measured in advance.
Each of the three test pieces 1 was subjected to the same wet and dry repeated test to determine the corrosion weight loss. And the average value (average corrosion weight loss) of corrosion weight loss was calculated | required.

As the test material, 20 steel types having the composition shown in Table 1 were used. In the composition shown in Table 1, the balance is iron and inevitable impurities. Moreover, the code | symbol * of Table 1 shows that it is austenitic stainless steel (steel No. B9, B10).
The results of this test are shown in Table 2 and FIG. In stainless steel having an average corrosion weight loss of more than 0.4 mg / cm ² , it was judged that rust flowed out of the gap and the appearance was impaired. This stainless steel was judged to be inferior in corrosion resistance, and plotted in FIG. 2 with black circles (●). Further, stainless steel having an average corrosion weight loss of 0.4 mg / cm ² or less was determined to be excellent in corrosion resistance, and plotted with white circles (◯) in FIG.

It has been found that when the content of Cr, Al, or Ti is increased based on a ferritic stainless steel containing Cr, the average corrosion weight loss is improved in any case. And when the effect by having increased Cr content was set to 1, the effect by having increased content of Al and Ti was each about 10 (about 10 times the effect by Cr).
It has also been found that both Si and Cu increase the average corrosion weight loss of ferritic stainless steel. It has also been found that the contribution ratios of Si and Cu are almost equal.

Therefore, an evaluation was made as to how the average corrosion weight loss of the gap-applied test piece is affected by the two parameters Cr + 10Ti + 10Al and Si + Cu.
As a result, as shown in Table 2 and FIG. 2, in the ferritic stainless steel satisfying the following formulas (A) and (B), the average weight loss was 0.4 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
Even if the formula (A) was satisfied, the average corrosion weight loss exceeded 0.4 mg / cm ² when the formula (B) was not satisfied.
On the other hand, Steel No. As in B9 and B10, in the austenitic stainless steel, the average corrosion weight loss exceeded 0.4 mg / cm ² even when the formulas (A) and (B) were satisfied. This is presumably because the passive film of austenitic stainless steel is more unstable than the passive film of ferritic stainless steel.

In this way, in an environment where chloride ions are present at a predetermined ratio or higher at a low pH, in an environment where drying and wetting are repeated, ferritic stainless steel satisfying the following formulas (A) and (B) has excellent corrosion resistance. It became clear to have.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
Here, Cr, Ti, Al, Si, and Cu in the formula mean the content (% by mass) of each element.

When Cr + 10Ti + 10Al ≧ 15 (formula (A)) is satisfied, the reason why the average corrosion weight loss of the gap-applied test piece under the test conditions is considered as follows.
In the test, the passive film after the test was analyzed by AES for samples having an average weight loss of 0.4 mg / cm ² or less. As a result, it was confirmed that Al and Ti were concentrated together with Cr on the surface film. In the wet and dry test, it is presumed that Al and Ti are concentrated and oxidized in the surface film due to the reduction reaction of nitrate ions, thereby improving the corrosion resistance. From this, it is considered that the average corrosion weight loss in the test environment was expressed as an index indicated by Cr + 10Ti + 10Al as a result.

The reason why the average corrosion weight loss decreases when satisfying Si + Cu ≦ 1.1 (formula (B)) is considered as follows.
Cu is an element that usually increases the corrosion resistance by reducing the active dissolution rate. However, once corrosion occurs, Cu in the steel is eluted. In particular, in an environment where there are many nitrate ions serving as an oxidizing agent, such as in the test environment, the eluted Cu ions become Cu ²⁺ . It is presumed that this Cu ²⁺ becomes an oxidizing agent to promote the cathode reaction, thereby increasing the corrosion rate and increasing the corrosion depth.
In the test liquid (simulated liquid), when the wet and dry repeated test was performed with the gap-applied test piece, the Si-containing test material was confirmed to deposit Si oxide mainly at the gas-liquid interface. It was also confirmed that corrosion occurred in the vicinity of the Si oxide precipitate. The reason why the corrosion occurred is considered to be that the clearance generated between the precipitate and the specimen was the starting point of the corrosion and the clearance corrosion was promoted. At this time, it is presumed that the corrosion is further accelerated by the presence of Cu ^{2+ in the} environment.

As described above, in the austenitic stainless steel, the average corrosion weight loss exceeded 0.4 mg / cm ² even when the formulas (A) and (B) were satisfied. This is considered to be because the passive film is unstable in the general-purpose austenitic stainless steel compared to the ferritic stainless steel.
Further, austenitic stainless steel has a larger amount of water-soluble inclusions such as MnS than ferritic stainless steel. For this reason, the dissolution rate of acid rain in the simulated liquid is large, and this is also presumed to be one of the causes of large corrosion weight loss.

The value of Cr + 10Ti + 10Al on the left side of the formula (A) is more desirably 17 or more, and further desirably 18 or more. In addition, the value of Si + Cu on the left side of the formula (B) is more preferably 0.90 or less, and even more preferably 0.70 or less.

Next, the corrosion resistance in the condensed water produced from combustion exhaust gas which is a more severe corrosive environment was evaluated.
As described above, condensed water produced from general LNG and petroleum combustion exhaust gas contains nitrate ions and sulfate ions, and exhibits an acidity of pH = 3 or less. The secondary heat exchanger is fed with 150 to 200 ° C. exhaust gas from the primary heat exchanger when in use, and returns to room temperature when stopped. In this way, the environment of 150 to 200 ° C. and the environment of room temperature are repeated.
Further, compared to the acid rain environment, the concentration of nitrate ions NO ₃ ⁻ and sulfate ions SO ₄ ²⁻ is higher because the exhaust gas is exposed to condensed water generated by direct condensation. Therefore, a test solution containing pH = 2.5, nitrate ion 100 ppm, sulfate ion 10 ppm, and chloride ion (Cl ⁻ ) 100 ppm was prepared using a reagent as a simulated condensed water solution.
The composition of the condensed water simulated liquid simulates condensed water generated from LNG combustion exhaust gas. For Cl ^- ions, the actual chloride ion concentration in the condensed water is a few ppm. However, the chloride ion concentration was set high, assuming operating conditions in a highly corrosive environment such as the beach environment.

For each of various stainless steels (test materials), three test pieces 1 having a size of 25 × 50 mm were prepared as shown in FIG. Since the secondary heat exchanger has a complicated structure, crevice corrosion is a particular concern. Therefore, in this test, in order to cause crevice corrosion intentionally, a glass / metal gap was given to the test piece 1 as follows. A hole 9 having a diameter of 6 mm was formed in a substantially central portion of the test piece 1. Before starting the test, the entire surface of the test piece 1 was wet-polished with # 400 emery paper, and immediately using a Teflon (registered trademark) bolt 2, a Teflon (registered trademark) nut 3, and a titanium washer 5, Test piece 1 was sandwiched between two glass plates 4. Thus, a glass / metal gap was imparted to the test piece 1.

The test piece 1 was placed in a beaker 7 as shown in FIG. 1A, filled with 50 ml of simulated liquid 8 of condensed water of combustion exhaust gas, and the test piece 1 was semi-immersed. This beaker 7 was placed in a warm bath at 80 ° C. and held for 24 hours. During this time, the simulated liquid 8 was concentrated by drying and completely dried. Next, a completely dried stainless steel sample (test piece 1) was taken out and gently washed with distilled water. Then, the newly cleaned beaker 7 was filled again with the test solution (combustion exhaust gas condensed water simulation solution 8). Subsequently, the stainless steel sample (test piece 1) was again half-immersed and held at 80 ° C. for 24 hours. The test piece 1 was half-immersed in the simulated solution 8 and then held at 80 ° C. for 24 hours was repeated 10 times (wet and dry repetition test).
The reason for setting the temperature to 80 ° C. is shown below. The temperature of the exhaust gas is 150 to 200 ° C. However, the temperature decreases due to the generation of condensed water. Further, it is considered that the actual member temperature is further lowered by contact with the generated condensed water. For this reason, it was set to 80 degreeC aiming at comparatively high temperature in order to accelerate corrosion and lower than 100 degreeC.

As the test material, 20 steel types having the composition shown in Table 1 were used.
The results of this test are shown in Table 2 and FIG. In stainless steel having an average corrosion weight loss of more than 1.0 mg / cm ² , it was judged that it reached the perforation at the clearance in the long term. This stainless steel was judged to be inferior in corrosion resistance, and plotted in FIG. 3 with black circles (●). Further, stainless steel having an average corrosion weight loss of 1.0 mg / cm ² or less was determined to be excellent in corrosion resistance, and plotted in FIG. 3 with white circles (◯).

Therefore, an evaluation was made as to how the average corrosion weight loss of the gap-applied test piece is affected by the two parameters Cr + 10Ti + 10Al and Si + Cu.
As a result, as shown in Table 2 and FIG. 3, in the ferritic stainless steel satisfying the formulas (A) and (B ′), the average corrosion weight loss was 1.0 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B ′) Formula: Si + Cu ≦ 0.5
Even if the formula (A) was satisfied, the average corrosion weight loss exceeded 1.0 mg / cm ² when the formula (B ′) was not satisfied.
On the other hand, general-purpose austenitic stainless steel satisfies the formula (A) but does not satisfy the formula (B ′). Therefore, the average corrosion weight loss exceeded 1.0 mg / cm ² .

Thus, in an environment where nitrate and sulfate ions are present at a predetermined ratio or more at a low pH, in an environment where drying and wetting are repeated, a ferritic stainless steel satisfying the following expressions (A) and (B ′) is obtained. It was revealed that it has excellent corrosion resistance.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B ′) Formula: Si + Cu ≦ 0.5
Here, Cr, Ti, Al, Si, and Cu in the formula mean the content (% by mass) of each element.

When Cr + 10Ti + 10Al ≧ 15 (formula (A)) is satisfied, the reason why the average corrosion weight loss of the gap-applied test piece under the test conditions is considered as follows.
In this test, the passive film after the test was analyzed by AES for samples having an average weight loss of 1.0 mg / cm ² or less. As a result, it was confirmed that Al and Ti were concentrated together with Cr on the surface film. In the wet and dry repeated test, it is estimated that Al and Ti were concentrated and oxidized in the surface film due to the reduction reaction of nitrate ions, thereby improving the corrosion resistance. From this, it is considered that the average corrosion weight loss in the test environment was expressed as an index indicated by Cr + 10Ti + 10Al as a result.

The reason why the average corrosion weight loss decreases when Si + Cu ≦ 0.5 (formula (B ′)) is considered as follows.
Cu is an element that usually increases the corrosion resistance by reducing the active dissolution rate. However, once corrosion occurs, Cu in the steel is eluted. In particular, in an environment where there are many nitrate ions serving as an oxidizing agent, such as in the test environment, the eluted Cu ions become Cu ²⁺ . It is presumed that this Cu ²⁺ becomes an oxidizing agent to promote the cathode reaction, thereby increasing the corrosion rate and increasing the corrosion depth.
In the test liquid (simulated liquid), when the wet and dry repeated test was performed with the gap-applied test piece, the Si-containing test material was confirmed to deposit Si oxide mainly at the gas-liquid interface. It was also confirmed that corrosion occurred in the vicinity of the Si oxide precipitate. The reason why the corrosion occurred is considered to be that the clearance generated between the precipitate and the specimen was the starting point of the corrosion and the clearance corrosion was promoted. At this time, it is presumed that the corrosion is further accelerated by the presence of Cu ^{2+ in the} environment.

As described above, even when austenitic stainless steel satisfies Cr + 10Ti + 10Al (formula (A)), the average corrosion weight loss exceeded 1.0 mg / cm ² . In general-purpose austenitic stainless steel, the contents of Si and Cu are inevitably high due to the steelmaking conditions, and most of the values of Si + Cu do not become 0.5 or less. For this reason, it is thought that average corrosion weight loss becomes large.
Further, austenitic stainless steel has a larger amount of water-soluble inclusions such as MnS than ferritic stainless steel. For this reason, the dissolution rate of the condensate in the simulated liquid is large, and this is also presumed to be one of the causes of large corrosion weight loss.

The value of Cr + 10Ti + 10Al on the left side of the formula (A) is more desirably 17 or more, and further desirably 18 or more. Further, the value of Si + Cu on the left side of the formula (B ′) is more preferably less than 0.35, and still more preferably less than 0.20.

Moreover, in order to cope with applications where the shape of a heat exchanger pipe or the like is complicated, a higher elongation value is desirable. Therefore, a material whose surface was polished after cold rolling, annealing, and pickling was processed into a shape specified in JIS No. 13B to prepare a test piece. Using this test piece, a tensile test was performed.
In the results of this test, stainless steel having an elongation value of 32% or more was determined to have good workability. The obtained results are shown in Table 2. Cu and Si also need to have a value of Si + Cu of 1.1 or less in order to increase the tensile strength of stainless steel. Furthermore, since Ni and Mo also raise the tensile strength, it is desirable that the contents of Mo and Ni are also low. The elongation value is more desirably 34% or more.

The detailed definition of the composition will be described below.
Cr is the most important element in securing the corrosion resistance of stainless steel. At least 13% Cr is required to maintain passivity. Increasing the Cr content improves corrosion resistance, but decreases workability and manufacturability. For this reason, the upper limit of the Cr amount is set to 22.5%. The Cr content is desirably 14.5 to 22.0%, and more desirably 16.0 to 20.0%.

Ti generally fixes C and N in a welded portion of ferritic stainless steel, thereby suppressing intergranular corrosion and improving workability. For this reason, Ti is a very important element. Furthermore, in the corrosive environment targeted in this embodiment, Ti is an important element in terms of corrosion resistance. Ti has a very strong affinity for oxygen. Therefore, the present inventor has found the following matters.
Ti forms a stainless steel surface film together with Cr in the corrosive environment targeted in the present embodiment containing nitrate ions. For this reason, Ti is very effective in suppressing the occurrence of pitting corrosion.
In order to form a film or to fix C and N using Ti as a stabilizing element, an amount of Ti more than four times the total amount of C and N (C + N) is required. However, since excessive addition causes surface flaws during production, the range of Ti content is 0.05 to 0.3%, and more preferably 0.08 to 0.2%.

Al is important as a deoxidizing element, and also has an effect of controlling the composition of nonmetallic inclusions and refining the structure. Furthermore, in the corrosive environment targeted in this embodiment, Al is an important element in terms of corrosion resistance. Al, like Ti, has a very strong affinity for oxygen. However, the present inventor has found the following matters.
Al forms a stainless steel surface film together with Cr in the corrosive environment targeted in the present embodiment containing nitrate ions. For this reason, Al is very effective in suppressing the occurrence of pitting corrosion.
However, excessive addition leads to coarsening of non-metallic inclusions, which may be a starting point for product wrinkling. Therefore, the lower limit value of the Al amount is 0.01%, and the upper limit value of the Al amount is 0.20%. The amount of Al is more desirably 0.03% to 0.10%.

Cu is included as an inevitable impurity from the raw material in an amount of 0.01% or more. However, in the environment targeted by this embodiment, Cu is not desirable because it promotes corrosion. The reason is presumed that, as described above, once corrosion starts, the eluted Cu ions promote the cathode reaction. Therefore, the smaller the amount of Cu, the better. Therefore, the range of the Cu amount is set to 0.5% or less. The amount of Cu is more desirably 0.25% or less.

Si is an element inevitably mixed as a deoxidizer. In general, Si is also effective for corrosion resistance and oxidation resistance. However, in the environment targeted by this embodiment, Si has an action of promoting the progress of corrosion. Furthermore, excessive addition reduces workability and manufacturability. Therefore, the upper limit of Si content is 0.60%. The amount of Si is more desirably less than 0.2%. Further, since extremely reducing causes an increase in cost, the Si amount is desirably 0.05% or more.

Ni is not essential, but suppresses the active dissolution rate. However, excessive addition reduces workability and not only makes the ferrite structure unstable, but also worsens the cost. For this reason, the amount of Ni is made less than 0.35%. The amount of Ni is desirably 0.05% or more and less than 0.25%.

Although Mo is not essential, it is contained in an amount of 0.01% or more as an inevitable impurity from the raw material. In general, Mo is said to enhance the recovery effect of the passive film. However, in the environment targeted by this embodiment, the contribution to the corrosion resistance improvement is small. On the other hand, Mo reduces workability and costs. Therefore, since it is desirable that the amount of Mo is small, the range of the amount of Mo is set to less than 0.30%. The amount of Mo is desirably 0.20% or less, and even more desirably 0.10% or less.

Further, other chemical components defined in this embodiment will be described in detail below.
C has an effect of suppressing strength increase and coarsening of crystal grains due to a combination with a stabilizing element. However, C decreases the intergranular corrosion resistance and workability of the weld. In high-purity ferritic stainless steel, it is necessary to reduce the C content, so the upper limit of the C content is 0.030%. Since excessive reduction deteriorates the refining cost, the C amount is more preferably 0.002 to 0.020%.

N, like C, reduces the intergranular corrosion resistance and workability, so it is necessary to reduce the N content. For this reason, the upper limit of the N amount is set to 0.030%. However, since excessive reduction deteriorates the refining cost, the N amount is more preferably 0.002 to 0.020%.

Mn is an important element as a deoxidizing element. However, when Mn is added excessively, it becomes easy to produce MnS which becomes a starting point of corrosion, and the ferrite structure is destabilized. Therefore, the Mn content is set to 0.01 to 0.5%. The amount of Mn is more desirably 0.05 to 0.3%.

Ｐ P not only deteriorates weldability and workability but also easily causes intergranular corrosion. For this reason, it is necessary to keep the amount of P low. Therefore, the P content is 0.05% or less. The amount of P is more preferably 0.001 to 0.04%.

S needs to reduce the amount of S in order to produce the water-soluble inclusions that are the starting points of corrosion such as CaS and MnS described above. Therefore, the S content is 0.01% or less. However, excessive reduction leads to cost deterioration, so the S amount is more preferably 0.0001 to 0.006%.

Nb fixes C and N similarly to Ti, suppresses intergranular corrosion of the welded portion, and improves workability. For this reason, Nb is a very important element. For this purpose, it is desirable that the Nb amount be 8 times or more the total amount of C and N (C + N). However, excessive addition reduces workability, so when Nb is added, the Nb content is preferably 0.05 to 0.5%. The amount of Nb is more preferably 0.1 to 0.3%.

Sn can be added as necessary to ensure flow rust resistance. Sn is an important element for suppressing the corrosion rate. However, excessive addition deteriorates manufacturability and cost, so the Sn content range is 0.005 to 1.0%. The amount of Sn is more desirably 0.05 to 0.5%.

B is a grain boundary strengthening element effective for improving secondary work brittleness, and can be added as necessary. However, excessive addition causes solid solution strengthening of ferrite and causes a decrease in ductility. For this reason, the lower limit of the B amount is 0.0001%, and the upper limit of the B amount is 0.003%. The amount of B is more preferably 0.0002 to 0.0020%.

V improves weather resistance and crevice corrosion resistance. If V is added instead of Cr and Mo, excellent workability can be obtained, so V can be added as necessary. However, excessive addition of V reduces workability and also saturates the effect of improving corrosion resistance. For this reason, the lower limit of the V amount is 0.03%, and the upper limit of the V amount is 1.0%. The amount of V is more desirably 0.05 to 0.50%.

Steels having the chemical compositions shown in Tables 3 and 4 were manufactured by a normal high purity ferritic stainless steel manufacturing method. The balance of the chemical compositions in Tables 3 and 4 is iron and inevitable impurities. Moreover, the code | symbol * of Table 4 shows that it is austenitic stainless steel (steel No. B9, B10).
Specifically, first, vacuum melting was performed, and then an ingot having a thickness of 40 mm was manufactured. This ingot was rolled to a thickness of 4 mm by hot rolling. Thereafter, heat treatment was performed for 1 minute at a temperature of 900 to 1000 ° C. based on each recrystallization behavior. The scale was then ground away. Furthermore, a 1.0 mm thick steel plate was manufactured by cold rolling. This steel plate was subjected to heat treatment (final annealing) for 1 minute at a temperature of 900 to 1000 ° C. based on each recrystallization behavior.
In addition, when manufacturing austenitic stainless steel, the heat processing temperature was 1100 degreeC.

Repeated wet and dry tests were performed under the same conditions as described above.
A test solution (simulated acid rain solution) 8 simulating a relatively mild corrosive environment contained 10 ppm nitrate ions, 10 ppm sulfate ions, and 5 ppm chloride ions, and had a pH of 4.5.
For each of the various stainless steels (test materials), three test pieces 1 having a size of 25 × 50 mm were prepared as shown in FIG. In this test, in order to evaluate crevice corrosion, a glass / metal gap was given to the test piece 1 as follows. A hole 9 having a diameter of 6 mm was formed in a substantially central portion of the test piece 1. Before starting the test, the entire surface of the test piece 1 was wet-polished with # 400 emery paper, and immediately using a Teflon (registered trademark) bolt 2, a Teflon (registered trademark) nut 3, and a titanium washer 5, Test piece 1 was sandwiched between two glass plates 4. Thus, a glass / metal gap was imparted to the test piece 1.

The test piece 1 was placed in a beaker 7 as shown in FIG. 1A, filled with 50 ml of acid rain simulation solution 8 and semi-immersed. The beaker 7 was placed in a 50 ° C. warm bath and held for 24 hours. Next, a stainless steel sample (test piece 1) was taken out from the dried and concentrated simulated liquid, and lightly washed with distilled water. Then, the newly washed beaker 7 was filled again with the test solution (acid rain simulated solution 8). Subsequently, the stainless steel sample (test piece 1) was again semi-immersed and held at 50 ° C. for 24 hours. The test piece 1 was half-immersed in the simulated solution and then held at 50 ° C. for 24 hours was repeated 10 times (wet and dry repetition test).

After 10 cycles, the rust of the test piece 1 was removed, and the mass was measured with an electronic balance. The weight loss of the corrosion was determined by subtracting the mass of the test piece 1 after the test from the mass of the test piece 1 before the test measured in advance.
Each of the three test pieces 1 was subjected to the same wet and dry repeated test to determine the corrosion weight loss. And the average value (average corrosion weight loss) of corrosion weight loss was calculated | required. The average weight loss obtained was listed in the “Corrosion weight loss 1” column of Tables 5 and 6.

Test solution (simulated liquid of combustion exhaust gas condensate) 8 that simulates a severe corrosive environment contains 100 ppm nitrate ions (NO ³⁻ ), 10 ppm sulfate ions (SO ₄ ²⁻ ), and 100 ppm chloride ions (Cl ⁻ ). The pH was adjusted to 2.5.
For each of the various stainless steels (test materials), three test pieces 1 having a size of 25 × 50 mm were prepared as shown in FIG.
A corrosion test was conducted in the same manner as the test using the acid rain simulated liquid except that the condensed water simulated liquid was used and the holding temperature was 80 ° C., and the average corrosion weight loss was determined. The average weight loss obtained was listed in the “Corrosion weight loss 2” column of Tables 5 and 6.

A tensile test was performed under the same conditions as described above. A material whose surface was polished after cold rolling, annealing, and pickling was processed into a shape defined in IS13B, and a test piece was produced. Using this test piece, a tensile test was performed.
The test results obtained are shown in Tables 5 and 6.

No. in Tables 3-5. A1 to A25 are examples of the present invention. B1 to B13 are comparative examples. Numerical values that deviate from the range defined in the present embodiment are highlighted with an underline.
Tables 5 and 6 and FIG. 4 show the results of the wet and dry repeated test using the simulated acid rain solution on the test piece 1 provided with the clearance.
No. A1 to A25 contain components in amounts within the ranges defined in this embodiment, and satisfy the following formulas (A) and (B). In all cases, the average corrosion weight loss was 0.4 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
In the example in which the value of the left side of the formula (A) is 17 or more and the value of the left side of the formula (B) is 0.70 or less, the average corrosion weight loss was reduced to 0.30 mg / cm ² . .
On the other hand, in an example where the formula (A) or (B) is not satisfied, or in an example where the element content is outside the range defined in the present embodiment, the average corrosion weight loss exceeds 0.40 mg / cm ^2. became.

Tables 5 and 6 and FIG. 5 show the results of repeated wet and dry tests on the test piece 1 provided with the clearance using a simulated liquid of condensed water generated from the combustion exhaust gas.
No. A1 to A20 contain components in amounts within the ranges defined in the present embodiment, and satisfy the following formulas (A) and (B ′). In all cases, the average corrosion weight loss was 1.0 mg / cm ² or less.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B ′) Formula: Si + Cu ≦ 0.5
In the example where the value on the left side of the formula (A) is 17 or more and the value on the left side of the formula (B ′) is less than 0.35, the average corrosion weight loss is as small as 0.7 mg / cm ² .
Further, in the example where the value of the left side of the formula (A) is 18 or more and the value of the left side of the formula (B ′) is less than 0.20, the average corrosion weight loss is 0.5 mg / cm ² or less, and the corrosion resistance The results are very good.
On the other hand, in the examples where one or both of the formulas (A) and (B ′) were not satisfied, the average corrosion weight loss exceeded 1.0 mg / cm ² .

Moreover, as a result of the tensile test, all examples satisfying the requirements defined in the present embodiment achieved an elongation of 32% or more. Further, in the example where the value (Si + Cu) on the left side of the formula (B) is 0.25 or less, and Mo is not contained and the Ni amount is less than 0.35%, the elongation value is 34% or more, The result was very good workability.

From the above results, it was clarified that the ferritic stainless steel having excellent corrosion resistance and good workability can be provided by the present embodiment. The corrosion resistance means corrosion resistance in a corrosive environment caused by acid rain or a corrosive environment caused by condensed water generated from combustion exhaust gas in the secondary heat exchanger.

This embodiment can be applied to materials used outdoors in areas where there is a lot of damage due to acid rain. Specifically, various heat exchangers, outdoor exterior materials in acid rain environments, building materials, roofing materials, outdoor equipment, water and hot water storage tanks, home appliances, bathtubs, kitchen equipment, and other general outdoor and indoor applications It is applicable to. Moreover, it is applicable to the material for heat exchangers, especially the material for secondary heat exchangers of the latent heat recovery type water heater. Specifically, it can be applied not only to cases and partition plates but also to materials that require workability such as heat exchange pipes. Further, the secondary heat exchanger material is exposed not only to the combustion exhaust gas of hydrocarbon fuel but also to a low pH solution containing a large amount of nitrate ions and sulfate ions. In this state, drying and drying are repeated. This embodiment can also be applied to materials exposed to such an environment.

1 Test specimen 2 Teflon (registered trademark) bolt 3 Teflon (registered trademark) nut 4 Glass plate 5 Titanium washer 7 Beaker 8 Simulated liquid (simulated liquid of acid rain or condensed water of combustion exhaust gas)
9 holes

Claims

In mass%, C: 0.030% or less, N: 0.030% or less, Si: 0.60% or less, Mn: 0.01 to 0.5%, P: 0.05% or less, S: 0 0.01% or less, Cr: 13 to 22.5%, Ni: less than 0.35%, Ti: 0.05 to 0.30%, Al: 0.01 to 0.2%, Cu: 0.5% Ferrite with excellent corrosion resistance and workability characterized in that it contains less than 0.30% and Mo: the balance is Fe and inevitable impurities, and satisfies the following formulas (A) and (B): Stainless steel.
(A) Formula: Cr + 10Ti + 10Al ≧ 15
(B) Formula: Si + Cu ≦ 1.1
(However, Cr, Ti, Al, Si, and Cu in the formula mean the content (mass%) of each element.)
The ferritic stainless steel excellent in corrosion resistance and workability according to claim 1, further comprising Nb: 0.05 to 0.50%.
The ferritic stainless steel having excellent corrosion resistance and workability according to claim 1 or 2, further comprising Sn: 0.005 to 1.0%.
4. The method according to claim 1, further comprising any one or both of B: 0.0001 to 0.003% and V: 0.03 to 1.0%. Ferritic stainless steel with excellent corrosion resistance and workability.
The ferritic stainless steel excellent in corrosion resistance and workability according to any one of claims 1 to 4, wherein the following formula (B ') is satisfied instead of the formula (B):
(B ′) Formula: Si + Cu ≦ 0.5
(However, Si and Cu in the formula mean the content (mass%) of each element.)
The average corrosion weight loss measured by a corrosion test using a simulated acid rain solution is 0.4 mg / cm 2 or less,
In the corrosion test, an aqueous solution containing a pH of 4.5 and containing 10 ppm nitrate ions, 10 ppm sulfate ions, and 5 ppm chloride ions is used as the simulated solution, and a gap imparting test piece is used in the aqueous solution. The wet and dry repeated test is performed for 10 cycles, and the weight loss after the dry and wet repeated test is measured to obtain the average corrosion weight loss. 4. Ferritic stainless steel having excellent corrosion resistance and workability according to any one of 4 above.
The average corrosion weight loss measured by a corrosion test using a simulated liquid of condensed water of combustion exhaust gas is 1.0 mg / cm 2 or less,
In the corrosion test, an aqueous solution having a pH of 2.5 and containing 100 ppm nitrate ions, 10 ppm sulfate ions, and 100 ppm chloride ions is used as the simulated solution, and a gap imparting test piece is placed in the aqueous solution. The wet and dry repeated test that is semi-immersed and held at 80 ° C. for 24 hours is carried out 10 times, the amount of decrease in mass after the dry and wet repeated test is measured, and the average corrosion weight loss is obtained. Ferritic stainless steel with excellent corrosion resistance and workability.