TWI503422B - Ferritic stainless steel excellent in corrosion resistance and workability - Google Patents

Description

Fermented iron-based stainless steel with excellent corrosion resistance and processability Field of invention

The present invention relates to a ferrite-based iron-based stainless steel having excellent corrosion resistance and workability, which can be suitably used for a member exposed to a low pH and in the presence of a chloride, such as a secondary heat exchanger.

The present application claims priority based on Japanese Patent Application No. 2011-193915, filed on Sep. 6, 2011.

Background of the invention

The exhaust gas generated by burning various fossil fuels contains SOx and NOx from fuel/atmosphere. These components react with oxygen or water vapor in the atmosphere to produce H ₂ SO ₄ , HNO _{3 ,} etc., or wet settle in cloud water, or dry settle as an aerosol in the atmosphere. This is seen as a mechanism for acid rain. The acid rain has a pH of about 5.6 or less, and contains about 10 ppm of SO ₄ ^2- ions and NO ₃ ^- ions, respectively. Especially in industrial areas and areas where volcanic gases are produced, acid rain has a lower pH and contains many SO ₄ ^2- ions and NO ₃ ^- ions. Further, there is a possibility that the chloride derived from sea salt or the like is contained in an amount of several ppm to 100 ppm. If rainwater adhered to such a component is dried, the pH is lowered and the concentration progresses, and it becomes a severe corrosive environment. However, such an acid rain environment is a mild corrosive environment as compared with an environment in which the exhaust gas exposed to the heat exchanger directly condenses.

The heat exchanger is a device that supplies heat generated by combustion of various fuels to a water-based medium, and is used in various fields, from a steam generator for nuclear power generation to a water heater for a general household. Among them, in general household gas water heaters or oil water heaters, in order to use the combustion heat thereof to make water into hot water, it is built in the heat exchanger. Conventionally, in order to improve the heat efficiency, this heat exchanger has been used for copper which is easy to process into a fin structure and has excellent heat transfer properties. However, due to environmental problems in recent years, the reduction of CO ₂ is also required in water heaters. For this reason, in order to further improve the heat efficiency, a latent heat recovery type water heater that further utilizes the heat of the conventional exhaust gas has been developed. The water heater further utilizes heat of exhaust gas generated by combustion of gas and petroleum after a conventional heat exchanger (primary heat exchanger). For this reason, there is another heat exchanger (secondary heat exchanger). The exhaust gas after passing through the primary heat exchanger is about 150 to 200 ° C and contains a large amount of water vapor. For this reason, in the case of the secondary heat exchanger, not only the direct heat but also the condensation heat of the water droplets is the latent heat of the water droplets, so that the total heat efficiency is increased to 95% or more. An example of the structure of the latent heat recovery type water heater is disclosed, for example, in Patent Document 1.

Here, the condensed water generated in the secondary heat exchanger is generated from an exhaust gas which is produced by burning a municipal gas fuel, a hydrocarbon-based raw material such as LPG or petroleum. For this reason, the condensed water contains nitrate ions and sulfate ions derived from the exhaust gas component and the like, and becomes a weakly acidic aqueous solution having a pH of about 3 or less. For this low pH solution, the copper used (which corrodes below pH 6.5) cannot be used. Even other ordinary steels (corrosive at pH values below about 7) and aluminum (corrosive at pH values below about 3) can corrode in environments exposed to the above condensed water. Therefore, stainless steel which is excellent in corrosion resistance in the weakly acidic range is selected as the material for the secondary heat exchanger. Among the general-purpose stainless steels, the SUS316L (18Cr-10Ni- of the Osfield iron-based stainless steel which is more excellent in corrosion resistance is also used. 2Mo). SUS316L meets the corrosion resistance necessary for secondary heat exchanger components used in latent heat recovery water heaters. However, this raw material contains a large amount of Ni and Mo which are expensive and have a very stable price stability. A latent heat recovery type water heater is expected to be widely used in general, and as a countermeasure for CO ₂ reduction, in order to achieve this, further cost reduction and good processability of materials are urgently desired. As for the SUS316L of the secondary heat exchanger material, of course, a proposal for a lower cost alternative material is expected. In addition, in the general use environment, it is considered that there is no problem of corrosion resistance. However, in areas such as the coast where sea salt particles, which are one of the main causes of corrosion resistance of stainless steel, are likely to fly, even SUS316L cannot be denied. The possibility of corrosion. In this case, in the case of SUS316L, there is a possibility that stress corrosion cracking occurs, and the stress corrosion cracking is one of the weaknesses of the Osbane iron-based stainless steel.

In recent years, attempts have been made to apply ferrite-grained stainless steel to secondary heat exchanger components (Patent Documents 2 to 6) in order to solve the problem of the application of the Oswego iron-based stainless steel.

Patent Document 2 proposes a ferrite-based iron-based stainless steel as a heat exchanger material having corrosion resistance to a sulfur-containing gas. In the ferrite-based iron-based stainless steel, Mo is added together with Nb or Ti, whereby the deterioration of corrosion resistance is suppressed and the high-temperature strength can be improved. Further, by reducing the contents of Si and Al, it is possible to improve weldability and formability.

Patent Document 3 discloses an environment in which a ferrite-based iron-based stainless steel exhibiting durability in a high-temperature steam environment and a high-temperature water vapor-based heat exchanger is exposed. In the case of the ferrite-based stainless steel, the composition is adjusted to satisfy a specific relationship derived from the contents of Cr, Mo, Si, and Al and the use of a predetermined temperature. However, due to the large amount of Al added, there is a problem that the material becomes very hard and brittle. In addition, the temperature of 300 to 1000 ° C is assumed to be a material used in an environment of a very high temperature which is higher than that of the latent heat recovery type water heater which is the object of the present invention.

Patent Document 4 discloses a ferrite-based iron-based stainless steel having excellent weldability, which is characterized in that the content of Ti and Al is lowered.

Patent Document 5 discloses a ferrite-based iron-based stainless steel which is suitable as a heat exchanger member for supplying to welding. In the ferrite-based iron-based stainless steel, the composition is adjusted such that the A value calculated from the contents of Nb, C, and N is 0.1 or more. However, in order to prevent coarsening of crystal grains during heat treatment in welding, Nb is regarded as an essential element, but there is no indication of how to improve corrosion resistance.

Patent Document 6 discloses a ferrite-grained stainless steel material for welding and a heat exchanger member. In order to prevent coarse graining during high-temperature welding, the area ratio of recrystallized grains is specified, but there is no indication of how to improve corrosion resistance.

Further, the heat exchange tubes of the secondary heat exchanger are required to be bent, and a part of the products are also made of a flexible tube. For this reason, good processability is required for the heat exchanger components. Although the Osbane-based stainless steel used in the prior art has sufficient workability, the ferrite-based stainless steel is inferior to the Osbane-based stainless steel in processing. For this reason, materials having excellent workability are particularly required.

As far as the prior knowledge is concerned, there is no mention of the prior art of corrosion resistance and processability in a low pH and chloride-containing environment, and it cannot be said that it is sufficiently disclosed that it can be suitably used in such an environment. The condition of the ferrite iron stainless steel.

Advanced technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-106970

Patent Document 2: Japanese Patent Laid-Open No. Hei 7-292446

Patent Document 3: Japanese Patent Laid-Open Publication No. 2003-328088

Patent Document 4: Japanese Laid-Open Patent Publication No. 2009-174046

Patent Document 5: Japanese Laid-Open Patent Publication No. 2009-299182

Patent Document 6: Japanese Laid-Open Patent Publication No. 2010-285683.

Summary of invention

In view of the above, the present invention has an object of providing a ferrite-based iron-based stainless steel which is inexpensive and has excellent corrosion resistance and has good processability and can be suitably used in the above environment.

In order to solve the above problems, the inventors of the present invention evaluated the corrosion resistance of various stainless steels in such an environment. As a result, it has been revealed that the contents of Cr, Al, and Ti are large, and particularly when such a system is concentrated on the surface of the passivation film, the corrosion resistance is particularly excellent.

Also, from the evaluation of the origin of corrosion generated, the following matters were recognized.

(a) By reducing the content of Cu and Si, the corrosion resistance in the target environment is improved.

(b) Further, workability is improved by lowering the contents of Mo and Ni.

The present invention is directed to the inside of such a secondary heat exchanger, and is specialized in conducting a review of a material excellent in corrosion resistance in a corrosive environment having a low pH and a chloride. As a result, a ferrite-based iron-based stainless steel having excellent corrosion resistance and workability in a target environment has been developed.

That is, the present invention has the following characteristics, and is a ferrite-based iron-based stainless steel which has excellent corrosion resistance and good processability in an environment of low pH and presence of chloride.

(1) A ferrite-based iron-based stainless steel having excellent corrosion resistance and workability, characterized by containing C: 0.030% or less, N: 0.030% or less, Si: 0.60% or less, and Mn: 0.01% by mass%. 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%, and the remainder is Fe and unavoidable impurities, and satisfy the following formulas (A) and (B): (A): Cr+10Ti+10Al≧15; (B) :Si+Cu≦1.1; (However, Cr, Ti, Al, Si, and Cu in the formula means the content (% by mass) of each element).

(2) The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as in the above item (1), further containing Nb: 0.05 to 0.50%.

(3) The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as in the above item (1) or (2), further containing Sn: 0.005 to 1.0%.

(4) The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability according to any one of the above items (1) to (3), further comprising B: 0.0001 to 0.003% and V: 0.03 to 1.0% Either or both elements.

(5) The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability according to any one of the above items (1) to (4), which satisfies the following formula (B') in place of the above formula (B): (B') Formula: Si + Cu ≦ 0.5; (However, Si and Cu in the formula means the content (% by mass) of each element).

(6) A ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as described in any one of the above items (1) to (5), which is subjected to an average corrosion measured by a corrosion test using a simulated liquid using acid rain The weight loss is 0.4 mg/cm ² or less; in the above corrosion test, an aqueous solution having a pH of 4.5 and containing 10 ppm of nitrate ions, 10 ppm of sulfate ions, and 5 ppm of chloride ions is used as the above-mentioned simulated liquid, and dry and wet repeating is performed. The above-mentioned average corrosion loss was obtained by measuring 10 times of the cycle and measuring the mass reduction after the dry-wet test, and the dry-wet test was performed by semi-immersing the test piece with the gap in the aqueous solution at 50 ° C. Hold for 24 hours.

(7) The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as described in the above item (6), which has an average corrosion weight loss of 1.0 mg as measured by a corrosion test using a simulated liquid using condensed water for burning exhaust gas. /cm ² or less; in the above corrosion test, an aqueous solution having a pH of 2.5 and containing 100 ppm of nitrate ions, 10 ppm of sulfate ions, and 100 ppm of chloride ions was used as the above-mentioned simulated liquid, and the dry-wet repeated test was carried out for 10 cycles. And determining the above-mentioned average corrosion loss by the amount of reduction after the dry-wet test, wherein the dry-wet test is semi-immersed in the aqueous solution and maintained at 80 ° C for 24 hours. .

According to the aspect of the present invention, it is possible to provide a ferrite-based iron-based stainless steel having excellent corrosion resistance and good processability in an environment such as acid rain-like low pH and presence of chloride. Also, unlike the Osbane iron-based stainless steel, the ferrite-based iron-based stainless steel does not contain a large amount of expensive Ni and Mo. In particular, according to a preferred aspect of the present invention, a mechanical material in an environment exposed to a condensed water of a combustion gas which uses a hydrocarbon such as LNG or petroleum as a fuel, such as a water heater, can exhibit excellent corrosion resistance.

Simple illustration

Fig. 1 is a view for explaining a method for evaluating corrosion resistance, wherein (a) shows the setting state of the sample, (b) shows the shape of the sample for the test, and (c) shows a sectional view of the direction of the CC arrow. .

Fig. 2 is a graph showing the relationship between the average corrosion weight loss measured by the corrosion test by the acid rain simulation liquid and the constituent elements.

Fig. 3 is a graph showing the relationship between the average corrosion loss and the constituent elements measured by a corrosion test using a simulated liquid of condensed water for burning exhaust gas.

Fig. 4 is a graph showing the relationship between the average corrosion loss and the constituent elements measured by a corrosion test using a simulated liquid of acid rain in the results of the examples and the comparative examples.

Figure 5 shows the use of combustion waste in the results of the examples and comparative examples. The graph of the relationship between the average corrosion weight loss measured by the corrosion test conducted by the simulated liquid of the gas condensate and the constituent elements.

Form for implementing the invention

Hereinafter, "%" means "% by mass" unless otherwise specified.

The inventors have conducted specialization research and development in order to provide a ferrite-based iron-based stainless steel which exhibits excellent corrosion resistance in an environment of low pH and presence of chloride and has good processability. As a result, the following matters were recognized.

(1) A dry-wet test was carried out to simulate an environment of acid rain, and the average corrosion weight loss of the test piece to which the gap was applied was measured. In the case of the ferrite-based iron-based stainless steel which satisfies the following formulas (A) and (B), the average corrosion weight loss is 0.4 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B) Formula: Si+Cu≦1.1

(2) Comparing the results of the ferrite-based stainless steel with the same value on the left side of the formula (A) and the Osfield iron-based stainless steel, the average corrosion weight loss of the ferrite-based stainless steel in the acid rain environment becomes smaller than that of the Oswald iron system. The average corrosion weight loss of stainless steel.

(3) A dry-wet test was conducted to simulate an environment in which condensed water was generated by combustion exhaust gas, and the average corrosion weight loss of the test piece to which the gap was applied was measured. In the case of the ferrite-based iron-based stainless steel which satisfies the following formulas (A) and (B'), the average corrosion weight loss is 1.0 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B') formula: Si+Cu≦0.5

(4) In the same manner as in the above (2), the result of comparing the fat iron-based stainless steel having the same value on the left side of the formula (A) with the Aostian iron-based stainless steel is even in the environment of the condensed water of the combustion exhaust gas. The average corrosion weight loss of the ferrite-based stainless steel becomes smaller than the average corrosion weight loss of the Osbane iron-based stainless steel.

(5) In order to evaluate the workability, the elongation value was measured by a tensile test. When Cr, Si, Cu, Mo, and Ni were contained, it was found that the elongation value was decreased and the workability was lowered.

First, a corrosion test (method for evaluating corrosion resistance) simulating an acid rain environment will be described.

A test solution having a pH of 4.5 and containing 10 ppm of nitrate ions, 10 ppm of sulfate ions, and 5 ppm of chloride ions was prepared using a reagent as a simulated liquid of acid rain.

For each of various stainless steels (sample materials), three test pieces 1 having a size of 25 × 50 mm as shown in Fig. 1 (b) were prepared. In this test, in order to evaluate the crevice corrosion, a gap of glass/metal was imparted to the test piece 1 as follows. Digging in the center of the test piece 1 6mm hole 9. Before the start of the test, the test piece 1 was wet-polished with #400 sandpaper, and the Teflon (registered trademark) bolt 2, the Teflon (registered trademark) nut 3, and the titanium gasket 5 were quickly used. The test piece 1 was sandwiched between two glass plates 4. The glass/metal gap was imparted to the test piece 1 by the above.

This test piece 1 was placed in a beaker 7 as shown in Fig. 1 (a), and 50 ml of acid rain simulated liquid 8 was filled, and the test piece 1 was semi-impregnated. The beaker 7 was placed in a warm bath at 50 ° C for 24 hours. During this period, the simulated liquid 8 was dried and concentrated. Shrink. Next, a stainless steel sample (test piece 1) was taken out from the dried and concentrated simulation liquid 8, and gently washed with distilled water. Next, the test solution (acid rain simulating solution 8) was again filled into the freshly washed beaker 7. Next, the stainless steel sample (test piece 1) was further semi-impregnated and kept at 50 ° C for 24 hours. The test piece 1 was half-immersed in the dummy liquid 8, and then kept at 50 ° C for 24 hours, and this was repeated 10 times (dry and dry repeated test).

Further, the set temperature of 50 ° C is a simulation of a higher temperature in the object to be corroded, and the object to be corroded is placed outside the acid rain.

After 10 cycles, the rust of the test piece 1 was removed, and the mass was measured using an electronic balance. The corrosion weight loss was obtained by subtracting the mass of the test piece 1 after the test from the mass of the test piece 1 before the test which was previously measured.

The same dry-wet test was performed on each of the three test pieces 1 to obtain corrosion loss. Then, the average value of the corrosion weight loss (average corrosion weight loss) was obtained.

Twenty kinds of steels having the compositions shown in Table 1 were used as the sample materials. Further, in the composition described in Table 1, the remaining portion was iron and unavoidable impurities. In addition, the symbol * shown in Table 1 is indicated by the Osbane iron-based stainless steel (steel No. B9 and B10).

The results of this test are shown in Table 2 and Figure 2. In the case of stainless steel having an average corrosion loss of more than 0.4 mg/cm ² , it was judged that rust was formed outside the gap and the appearance was impaired. This stainless steel was judged to have poor corrosion resistance and was drawn in a solid circle (•) in Fig. 2 . Further, stainless steel having an average corrosion loss of 0.4 mg/cm ² or less was judged to be excellent in corrosion resistance, and was drawn in a hollow circle (○) in Fig. 2 .

It has been found that when the iron-based stainless steel containing Cr is used as a base material and the content of Cr, Al or Ti is increased, the average corrosion weight loss is improved in either case. Further, it is understood that when the effect of increasing the Cr content is set to 1, the effect of increasing the content of Al and Ti is about 10 (about 10 times that of Cr).

Further, it was found that both Si and Cu increased the average corrosion weight loss of the ferrite-based iron-based stainless steel. It is also clear that the contribution rate of this Si and Cu is almost equal.

Therefore, the influence of the average corrosion loss on the test piece to which the gap is applied is evaluated by the two parameters of Cr+10Ti+10Al and Si+Cu.

As shown in Table 2 and Fig. 2, the ferrite-based iron-based stainless steel which satisfies the following formulas (A) and (B) has an average corrosion weight loss of 0.4 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B) Formula: Si+Cu≦1.1

Further, when the condition of the formula (A) was satisfied but the formula (B) was not satisfied, the result was that the average corrosion loss was more than 0.4 mg/cm ² .

On the other hand, in the Osbane-based stainless steel such as Steel No. B9 and B10, even if the formulas (A) and (B) are satisfied, the average corrosion loss is more than 0.4 mg/cm ² . This is considered to be because the passivation film of the Osbane iron stainless steel is more unstable than the passivation film of the ferrite iron stainless steel.

In this way, it has been revealed that in a solution having a low pH and a chloride ion having a specified ratio or more, in the dry-wet environment, the following ferrite-based stainless steels satisfying the following formulas (A) and (B) are excellent. Corrosion resistance.

(A): Cr+10Ti+10Al≧15

(B) Formula: Si+Cu≦1.1

Here, the Cr, Ti, Al, Si, and Cu systems 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 loss of the test piece having a gap under the test conditions is small is considered to be as follows.

In this test, the passivation film after the test was analyzed by AES for a sample having an average corrosion 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. It is presumed that in the dry-wet repeat test, Al and Ti are concentrated/oxidized in the surface film by the reduction reaction of nitrate ions, and the corrosion resistance is improved. From this point of view, from the results, it can be considered that the average corrosion weight loss in this test environment is expressed by the index indicated by Cr+10Ti+10Al.

When Si+Cu≦1.1 ((B)) is satisfied, the reason why the average corrosion weight loss is small is considered to be as follows.

Cu is usually an element which lowers the active dissolution rate and improves the corrosion resistance, but when corrosion occurs, Cu in the steel is eluted. In particular, in an environment where the amount of nitrate ions of the oxidant is large in the test environment, the eluted Cu ions become Cu ²⁺ . It is presumed that this Cu ^{2+ acts} as an oxidant to promote the cathode reaction, so the corrosion rate increases and the corrosion depth becomes deep.

In the case where the dry-and-wet test was carried out in the test piece (simulation liquid) in which the gap was applied, it was confirmed that the sample material containing Si had a precipitation of Si oxide around the gas-liquid interface. . Further, it was confirmed that corrosion occurred in the vicinity of the precipitate of the Si oxide. The reason why corrosion occurs is considered to be that the gap generated between the precipitate and the sample material becomes the starting point of corrosion and promotes crevice corrosion. It is further presumed that since Cu ²⁺ is present in the environment at this time, corrosion is more accelerated.

Further, as described above, in the case of the Osbane iron-based stainless steel, even if the formulas (A) and (B) are satisfied, the average corrosion weight loss is more than 0.4 mg/cm ² . This is considered to be because the general Aostian iron-based stainless steel passivation film is unstable compared to the ferrite-based iron-based stainless steel.

In addition, Osbane iron-based stainless steel, MnS and other water-soluble mediators are more than the ferrite-based iron-based stainless steel. For this reason, the dissolution rate in the simulated solution of acid rain is large, and this matter is also presumed to be one of the causes of the major corrosion loss.

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

Secondly, the corrosion resistance in the more severe corrosive environment, that is, in the condensed water produced by the combustion exhaust gas, was evaluated.

As described earlier, the condensed water generated from the combustion gas of general LNG or petroleum contains nitrate ions and sulfate ions and exhibits acidity of pH=3 or less. Further, in the case of the secondary heat exchanger, at the time of use, the exhaust gas of 150 to 200 ° C is fed from the primary heat exchanger, and returns to room temperature when it is stopped. This is repeated in an environment of 150 to 200 ° C and a room temperature environment.

Further, compared with the acid rain environment, the concentration of the nitrate ion NO ₃ ^- and the sulfate ion SO ₄ ^2- is high because it is exposed to the condensed water generated by the direct condensation of the exhaust gas. For this reason, a reagent containing pH of 2.5 and containing 100 ppm of nitrate ions, 10 ppm of sulfate ions, and 100 ppm of chloride ions (Cl ^- ) was used as a simulating solution of condensed water.

The composition of the condensate simulating liquid simulates the condensate produced by the combustion exhaust gas of LNG. In the case of Cl ^- ions, the concentration of chloride ions in the actual condensed water is several ppm. However, it is assumed that the chlorine ion concentration is set to be high in an environment in a corrosive environment such as a seashore environment.

For each of various stainless steels (sample materials), three test pieces 1 having a size of 25 × 50 mm as shown in Fig. 1 (b) were prepared. Because of the complexity of the secondary heat exchanger structure, there is a particular concern about crevice corrosion. Therefore, in this test, in order to cause crevice corrosion intentionally, a glass/metal gap was imparted to the test piece 1 as follows. Digging in the center of the test piece 1 6mm hole 9. Before the start of the test, the test piece 1 was wet-polished with #400 sandpaper, and the Teflon (registered trademark) bolt 2, the Teflon (registered trademark) nut 3, and the titanium gasket 5 were quickly used. The test piece 1 was sandwiched between two glass plates 4. The glass/metal gap was imparted to the test piece 1 by the above.

This test piece 1 was placed in a beaker 7 as shown in Fig. 1 (a), and 50 ml of the simulated liquid 8 of the condensed water of the combustion exhaust gas was filled, and the test piece 1 was semi-impregnated. The beaker 7 was placed in a warm bath at 80 ° C for 24 hours. During this time, the simulated liquid 8 was dried and concentrated, and completely dried. Next, the completely dried stainless steel sample (test piece 1) was taken out and gently washed with distilled water. Next, the test solution (condensed water simulating liquid 8 of the combustion exhaust gas) was again filled into the freshly washed beaker 7. Next, the stainless steel sample (test piece 1) was further semi-impregnated and kept at 80 ° C for 24 hours. The test piece 1 was half-immersed in the simulation liquid 8, and then kept at 80 ° C for 24 hours, and this was repeated for 10 cycles (dry and dry repeated test).

Further, the reason why the temperature is set to 80 ° C is shown below. Exhaust gas The temperature is 150~200 °C. However, the temperature is lowered due to the generation of condensed water. Further, it is conceivable that the actual component temperature becomes lower due to contact with the generated condensed water. For this reason, it is set at 80 ° C in order to be slower than 100 ° C and to accelerate the corrosion while aiming at a relatively high temperature.

After 10 cycles, the rust of the test piece 1 was removed, and the mass was measured using an electronic balance. The corrosion weight loss was obtained by subtracting the mass of the test piece 1 after the test from the mass of the test piece 1 before the test which was previously measured.

The same dry-wet test was performed on each of the three test pieces 1 to obtain corrosion loss. Then, the average value of the corrosion weight loss (average corrosion weight loss) was obtained.

As the sample material, 20 kinds of steels having the compositions shown in Table 1 were used.

The results of this test are shown in Table 2 and Figure 3. In the case of stainless steel having an average corrosion loss of more than 1.0 mg/cm ² , it was judged that perforation was reached in the gap portion in the long term. This stainless steel was judged to have poor corrosion resistance and was drawn in a solid circle (•) in Fig. 3 . Further, stainless steel having an average corrosion weight loss of 1.0 mg/cm ² or less was judged to be excellent in corrosion resistance, and was drawn in a hollow circle (○) in Fig. 3 . It has been found that when the iron-based stainless steel containing Cr is used as a substrate and the Cr, Al or Ti content is increased, the average corrosion weight loss is improved in either case. Further, it is understood that when the effect of increasing the Cr content is 1, the effect of increasing the content of Al and Ti is about 10 (about 10 times that of Cr).

Further, it was found that both Si and Cu increased the average corrosion weight loss of the ferrite-based iron-based stainless steel. It is also clear that the contribution rate of this Si and Cu is almost equal.

Therefore, through the two parameters of Cr+10Ti+10Al and Si+Cu, what is the effect on the average corrosion weight loss of the test piece with gaps? Evaluation.

As shown in Table 2 and FIG. 3, the ferrite-based iron-based stainless steel satisfying the formulas (A) and (B') showed an average corrosion weight loss of 1.0 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B') formula: Si+Cu≦0.5

Further, when the condition of the formula (A) was satisfied but the formula (B') was not satisfied, the result was that the average corrosion weight loss exceeded 1.0 mg/cm ² .

On the other hand, the general Osbane iron-based stainless steel satisfies the (A) formula but does not satisfy the (B') formula. For this reason, the result becomes an average corrosion loss of more than 1.0 mg/cm ² .

In this way, it is revealed that in the solution of nitrate ions and sulfate ions having a low pH value and a predetermined ratio or more, the following formula (A) and (B') are satisfied in the dry and wet environment. Granular iron stainless steel has excellent corrosion resistance.

(A): Cr+10Ti+10Al≧15

(B') formula: Si+Cu≦0.5

Here, the Cr, Ti, Al, Si, and Cu systems in the formula mean the content (% by mass) of each element.

When Cr+10Ti+10Al≧15 ((A) formula) is satisfied, the reason why the average corrosion weight loss of the test piece with a gap under the test conditions is small is considered to be as follows.

In this test, the passivation film after the test was analyzed by AES for a sample having an average corrosion weight loss of 1.0 mg/cm ² or less. As a result, it was confirmed in the surface film that Al and Ti were concentrated together with Cr. It is presumed that in the dry-wet repeat test, Al and Ti are concentrated/oxidized in the surface film by the reduction reaction of nitrate ions, and the corrosion resistance is improved. From this point of view, from the results, it can be considered that the average corrosion weight loss in this test environment is expressed by the index indicated by Cr+10Ti+10Al.

When Si+Cu≦0.5 ((B')) is satisfied, the reason why the average corrosion weight loss is small is considered to be as follows.

Cu is usually an element which lowers the active dissolution rate and improves the corrosion resistance, but in the case where corrosion occurs, Cu in the steel is eluted. In particular, in an environment where the amount of nitrate ions of the oxidant is large in the test environment, the eluted Cu ions become Cu ²⁺ . It is presumed that this Cu ^{2+ acts} as an oxidant to promote the cathode reaction, so the corrosion rate increases and the corrosion depth becomes deep.

In the case where the dry-and-wet test was carried out in the test piece (simulation liquid) in which the gap was applied, it was confirmed that the sample material containing Si had a precipitation of Si oxide around the gas-liquid interface. . Further, it was confirmed that corrosion occurred in the vicinity of the precipitate of the Si oxide. The reason why corrosion occurs is considered to be that the gap generated between the precipitate and the sample material becomes the starting point of corrosion and promotes crevice corrosion. Further, it is presumed that corrosion is more accelerated at this time because of the presence of Cu ²⁺ in the environment.

Further, as described above, in the case of the Osbane iron-based stainless steel, even if Cr+10Ti+10Al ((A) formula) is satisfied, the average corrosion weight loss is more than 1.0 mg/cm ² . In the general Osbane iron-based stainless steel, the content of Si and Cu is inevitably high due to the steelmaking conditions, and the value of Si+Cu is generally not less than 0.5. For this reason, it is conceivable that the average corrosion weight loss will become larger.

Further, the amount of water-soluble intermediaries such as Osbane iron-based stainless steel MnS is more than that of ferrite-based iron-based stainless steel. For this reason, in the simulated liquid of the condensate The dissolution rate is large, and this matter is also presumed to be one of the causes of the loss of corrosion.

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

Further, in order to cope with a complicated shape such as a tube of a heat exchanger, it is preferable to have a high elongation value. Therefore, after cold rolling, annealing, and pickling, the surface-polished material was processed into a shape defined by JIS No. 13 B to prepare a test piece. This test piece was used to carry out a tensile test.

As a result of the test, the stainless steel having an elongation value of 32% or more was judged to have good workability. The results obtained are shown in Table 2. For both Cu and Si, in order to increase the tensile strength of stainless steel, the value of Si+Cu must be 1.1 or less. Further, both Ni and Mo are preferable in order to increase the tensile strength, and the contents of Mo and Ni are both low. Further, the elongation value is preferably 34% or more.

The detailed regulations of the above composition will be described below.

Cr is the most important element in ensuring the corrosion resistance of stainless steel. In order to maintain a passive state, at least 13% of Cr is required. If the amount of Cr is increased, the corrosion resistance is improved, but the workability and manufacturability are lowered. For this reason, the upper limit of the amount of Cr is 22.5%. The ideal amount of Cr is 14.5-22.0%, and the ideal system is 16.0~20.0%.

In general, Ti is used in the welded portion of the ferrite-based iron-based stainless steel to suppress the grain boundary corrosion by fixing C and N, and to improve the workability. For this reason, Ti is a very important element. Further, in the corrosive environment which is the object of the present embodiment, Ti is an element which is important in corrosion resistance. Ti has a very strong affinity with oxygen. Thus, the inventors recognized the following matters.

In the corrosive environment which is the object of the present embodiment containing nitrate ions, Ti forms a surface coating of stainless steel together with Cr. For this reason, Ti is very effective in suppressing pitting corrosion.

In order to form a film and to fix C and N by using Ti as a stabilizing element, it is necessary to have four times or more of the total amount (C+N) of the Ti amount system C and N. However, since excessive addition may cause surface flaws during production, the amount of Ti ranges from 0.05 to 0.3%, and is preferably from 0.08 to 0.2%.

Al is important as a deoxidizing element system, and it also has an effect of controlling the composition of the non-metal intervening substance and making the structure fine. Further, in the corrosive environment which is the object of the present embodiment, Al is an important element in terms of corrosion resistance. Like Ti, Al has a very strong affinity for oxygen. However, the inventors recognized the following matters.

In the corrosive environment which is the object of the present embodiment including the nitrate ion, the Al system forms a surface coating of stainless steel together with Cr. For this reason, Al is very effective in suppressing pitting corrosion.

However, if it is added in excess, it will lead to the roughening of the non-metallic media, and it will become the starting point for the production of the product. Therefore, the lower limit of the amount of Al is 0.01% and the upper limit of the amount of Al is 0.20%. The amount of Al is preferably 0.03% to 0.10%.

The Cu system is contained in an amount of 0.01% or more as an unavoidable impurity derived from a raw material. However, in the environment which is the object of the present embodiment, it is not preferable because Cu promotes corrosion. The reason for this is presumed to be that, as described above, when the corrosion starts, the eluted Cu ions promote the cathode reaction. Therefore, the smaller the amount of Cu, the more ideal, so the amount of Cu is in the range of 0.5%. under. The amount of Cu is preferably less than 0.25%.

The Si system is an element that is inevitably mixed as a deoxidizer. In general, Si is also effective for corrosion resistance and oxidation resistance. However, in the environment which is the object of the present embodiment, the Si system has an effect of promoting corrosion. Further, excessive addition causes a decrease in workability and manufacturability. Therefore, the upper limit of the amount of Si is made 0.60%. The amount of Si is less than 0.2%. Further, the extreme reduction causes an increase in cost, and therefore the amount of Si is preferably 0.05% or more.

Ni is not essential but inhibits the rate of active dissolution. However, excessive addition causes a decrease in workability, and the cost is not only caused by the instability of the ferrite and iron structure. For this reason, the amount of Ni is less than 0.35%. The amount of Ni is preferably 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 unavoidable impurity derived from a raw material. In general, it is said that Mo improves the recovery effect of the passivation film. However, in the environment which is the object of the present embodiment, the contribution to the improvement of corrosion resistance is small. On the other hand, Mo is degraded in workability and the cost is also deteriorated. Therefore, the amount of Mo is preferably as small as possible, so that the amount of Mo is less than 0.30%. The amount of Mo is preferably 0.20% or less, and more preferably 0.10% or less.

Further, other chemical components defined in the present embodiment will be described in detail below.

The C system has an effect of increasing the strength and suppressing coarsening of crystal grains by a combination with a stabilizer element. However, C deteriorates the intergranular corrosion resistance and workability of the welded portion. In the case of high-purity ferrite-based stainless steel, since it is necessary to lower the content of C, the upper limit of the amount of C is 0.030%. Refining due to excessive reduction This deterioration, the amount of C is ideally 0.002~0.020%.

In the same manner as N, the intergranular corrosion resistance and the workability are deteriorated, and it is necessary to lower the content of N. For this reason, the upper limit of the amount of N is 0.030%. However, excessive reduction causes the refining cost to deteriorate, so the amount of N is preferably 0.002 to 0.020%.

Mn is an important element of the deoxidizing element system. However, if Mn is excessively added, it becomes easy to generate MnS which will become a corrosion initiation point, and the ferrite iron structure will be unstable. For this reason, the content of Mn is 0.01 to 0.5%. The amount of Mn is 0.05 to 0.3% of the ideal system.

P not only reduces weldability and workability, but also causes grain boundary corrosion to occur easily. For this reason, it is necessary to suppress the amount of P to be low. Therefore, the P content is made 0.05% or less. The amount of P is preferably 0.001 to 0.04%.

Since the S system causes the above-described CaS and MnS to form a water-soluble intervening substance at the corrosion initiation point, it is necessary to lower the amount of S. Therefore, the amount of S is made 0.01% or less. However, excessive reduction will lead to cost deterioration, so the amount of S is more than 0.0001 to 0.006%.

Similarly to Ti, the Nb system fixes C and N and suppresses grain boundary corrosion of the welded portion to improve workability. For this reason, Nb is a very important element. Therefore, it is preferable to make the amount of Nb 8 times or more of the total amount of C and N (C+N). However, excessive addition causes a decrease in workability, so when the state of addition is made, it is preferable to make the amount of Nb 0.05 to 0.5%. The amount of Nb is preferably 0.1 to 0.3%.

To ensure rust resistance, Sn can be added as needed. In order to suppress the corrosion rate, Sn is an important element. However, excessive addition causes deterioration in manufacturability and cost, so the amount of Sn is in the range of 0.005 to 1.0%. The ideal amount of Sn 0.05~0.5%.

B can be added as needed because it is a grain boundary strengthening element effective for improving secondary processing brittleness. However, excessive addition will solidify and strengthen the ferrite and cause the ductility to decrease. Therefore, the lower limit of the amount of B is 0.0001% and the upper limit of the amount of B is 0.003%. The amount of B is more than 0.0002~0.0020%.

V will improve rust resistance and crevice corrosion resistance. Excellent workability can be obtained by adding V instead of Cr and Mo, and V can be added as needed. However, excessive addition of V causes a decrease in workability and an effect of improving corrosion resistance. For this reason, the lower limit of the amount of V is 0.03% and the upper limit of the amount of V is 1.0%. The ideal amount of V is 0.05~0.50%.

[Examples]

Steel having the chemical compositions shown in Tables 3 and 4 was produced by a method of generally producing high-purity ferrite-based stainless steel. Also, the remainder of the chemical composition of Tables 3 and 4 is iron and unavoidable impurities. Moreover, the symbol * shown in Table 4 is indicated by the Osbane iron-based stainless steel (steel No. B9 and B10).

In detail, vacuum melting was first performed, followed by production of an ingot having a thickness of 40 mm. The 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. Next, the scale is removed by grinding. Further, a steel plate having a thickness of 1.0 mm was produced by cold rolling. For this steel sheet, heat treatment (final annealing) was performed for 1 minute at a temperature of 900 to 1000 ° C based on each recrystallization behavior.

Further, when the condition of the Osbane iron-based stainless steel was produced, the heat treatment temperature was 1,100 °C.

The dry-wet test was carried out under the same conditions as described above.

The test solution (acid rain simulating solution) 8 which simulates a relatively mild corrosive environment contains 10 ppm of nitrate ions, 10 ppm of sulfate ions, and 5 ppm of chloride ions, and has a pH of 4.5.

For each of various stainless steels (sample materials), three test pieces 1 having a size of 25 × 50 mm as shown in Fig. 1 (b) were prepared. For the purpose of this test, in order to evaluate the crevice corrosion, a glass/metal gap was imparted to the test piece 1 as follows. Digging in the approximate central portion of the test piece 1 6mm hole 9. The entire surface of the test piece 1 was wet-polished by #400 sandpaper before the start of the test, and the bolts of the Teflon (registered trademark), the nut 3 of the Teflon (registered trademark), and the titanium were quickly used. The gasket 5 is used to sandwich the test piece 1 between the two glass plates 4. The glass/metal gap was imparted to the test piece 1 by the above.

This test piece 1 was placed in the beaker 7 shown in Fig. 1 (a), and 50 ml of acid rain simulated liquid 8 was filled to be semi-impregnated. The beaker 7 was placed in a warm bath at 50 ° C for 24 hours. Next, a stainless steel sample (test piece 1) was taken out from the dried and concentrated simulation liquid, and gently washed with distilled water. Next, the test solution (acid rain simulating solution 8) was again filled into the freshly washed beaker 7. Next, the stainless steel sample (test piece 1) was further semi-impregnated and kept at 50 ° C for 24 hours. The test piece 1 was half-immersed in a simulating liquid, followed by maintaining at 50 ° C for 24 hours, and this was repeated for 10 cycles (dry and dry repeated test).

After 10 cycles, the rust of the test piece 1 was taken out, and the mass was measured by an electronic balance. The corrosion weight loss was determined from the mass of the test piece 1 before the test which was measured in advance and the mass of the test piece 1 after the test.

The same dry and wet repeated test was performed on each of the three test pieces 1. Get the weight loss of corrosion. Then, the average value of the corrosion weight loss (average corrosion weight loss) was obtained. The average corrosion loss obtained is described in the "corrosion loss 1" column of Tables 5 and 6.

The test solution for simulating a harsh corrosive environment (simulation of condensate for combustion exhaust gas) 8 series contains nitrate ions (NO ^3- ) 100 ppm, sulfate ions (SO ₄ ^2- ) 10 ppm, and chloride ions (Cl ^- ) 100 ppm. And let the pH of 2.5.

For each of various stainless steels (sample materials), three test pieces 1 having a size of 25 × 50 mm as shown in Fig. 1 (b) were prepared.

The average corrosion loss was obtained by performing a corrosion test in the same manner as the test using the acid rain simulation liquid except that the simulation liquid using the condensed water was used and the holding temperature was maintained at 80 °C. The average corrosion loss obtained is shown in the "corrosion loss 2" column of Tables 5 and 6.

The tensile test was carried out under the same conditions as described above. After the cold rolling, annealing, and pickling, the surface-polished material was processed into a shape specified in IS13B to prepare a test piece. This test piece was used to carry out a tensile test.

The test results obtained are shown in Tables 5 and 6.

Nos. A1 to A25 of Tables 3 to 5 are examples of the present invention, and Nos. B1 to B13 are comparative examples. The numerical values that do not fall within the range defined by the present embodiment are emphasized by the bottom line.

The test results of the test piece 1 to which the gap was applied were subjected to a dry-wet test using a simulated liquid using acid rain are shown in Tables 5, 6 and 4.

No. A1 to A25 contain the components in the range specified in the present embodiment and satisfy the following formulas (A) and (B). The average corrosion weight loss of either of them was 0.4 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B) Formula: Si+Cu≦1.1

In the case 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 0.70 or less, the average corrosion weight loss is reduced to 0.30 mg/cm ² .

On the other hand, in the case where the formula (A) or the formula (B) is not satisfied, and the element content is not in the range specified in the embodiment, the average corrosion loss is more than 0.40 mg/cm ² .

The test piece 1 to which the gap was applied was subjected to a dry-wet test using a simulated liquid using condensed water generated from the combustion exhaust gas, and the results are shown in Tables 5, 6 and 5.

No. A1 to A20 contain the components in the range specified in the present embodiment and satisfy the following formulas (A) and (B'). The average corrosion weight loss of either of them was 1.0 mg/cm ² or less.

(A): Cr+10Ti+10Al≧15

(B') formula: Si+Cu≦0.5

In the case where the value on the left side of the formula (A) is 17 or more and the value on the left side of the (B') formula is less than 0.35, the average corrosion weight loss is 0.7 mg/cm ^{2 which} is small.

Further, in the case where the value on the left side of the formula (A) is 18 or more and the value on the left side of the (B') formula is less than 0.20, the average corrosion loss is 0.5 mg/cm ² or less, and the corrosion resistance is extremely excellent.

On the other hand, in the case where one of the formulas (A) and (B') or both were not satisfied, the average corrosion loss of either of the results exceeded 1.0 mg/cm ² .

Further, as a result of the tensile test, an example of the requirements specified in the present embodiment was satisfied, and either of them achieved an elongation of 32% or more. Further, the value (Si+Cu) on the left side of the formula (B) is 0.25 or less, and in the case where Mo is not contained and the amount of Ni is less than 0.35%, the elongation value is 34% or more, and as a result, the workability is extremely excellent.

From the above results, it has been revealed that the ferrite-based iron-based stainless steel having excellent corrosion resistance and good processability can be provided by the present embodiment. Further, the term "corrosion resistance" means a corrosion environment caused by acid rain or corrosion in a corrosive environment caused by condensed water generated from a combustion exhaust gas in a secondary heat exchanger.

Industrial availability

This embodiment can be applied to materials used outdoors in areas where there is much damage due to acid rain. Specifically, it can be applied to exterior heat and exterior materials in various heat exchangers, acid rain environments, building materials, roofing materials, outdoor equipment, water storage/storage sinks, home appliances, bathtubs, kitchen appliances, and other outdoor/households. General purpose. Further, the material for the heat exchanger is particularly applicable to a material for a secondary heat exchanger of a latent heat recovery type water heater. Specifically, it is not only a case and a separator, but also a material which requires workability like a heat exchange tube. Further, the material for the secondary heat exchanger is not only exposed to hydrocarbons The combustion exhaust gas of the compound fuel is also exposed to a low pH solution containing a large amount of nitrate ions and sulfate ions. In this state, wet and dry will be repeated. This embodiment can also be applied to materials exposed to such an environment.

1‧‧‧Test piece

2‧‧‧Teflon (registered trademark) bolts

3‧‧‧Teflon (registered trademark) nuts

4‧‧‧ glass plate

5‧‧‧Titanium washers

7‧‧‧ beaker

8‧‧‧Simulation fluid (simulation of acid rain or condensate of combustion exhaust gas)

9‧‧‧ hole

Fig. 1 is a view for explaining a method for evaluating corrosion resistance, wherein (a) shows the setting state of the sample, (b) shows the shape of the sample for the test, and (c) shows a sectional view of the direction of the CC arrow. .

Fig. 2 is a graph showing the relationship between the average corrosion weight loss measured by the corrosion test by the acid rain simulation liquid and the constituent elements.

Fig. 3 is a graph showing the relationship between the average corrosion loss and the constituent elements measured by a corrosion test using a simulated liquid of condensed water for burning exhaust gas.

Fig. 4 is a graph showing the relationship between the average corrosion loss and the constituent elements measured by a corrosion test using a simulated liquid of acid rain in the results of the examples and the comparative examples.

Fig. 5 is a graph showing the relationship between the average corrosion loss and the constituent elements measured by a corrosion test using a simulated liquid of condensed water for burning exhaust gas in the results of the examples and the comparative examples.

Claims (5)

A ferrite-based iron-based stainless steel having excellent corrosion resistance and workability, characterized by containing C: 0.030% or less, N: 0.030% or less, Si: 0.60% or less, and Mn: 0.01 to 0.5% by mass%; 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, Mo: lower than 0.30%, and Nb: 0.05~0.50%, the rest are Fe and unavoidable impurities, and satisfy the following formulas (A) and (B): (A): Cr+10Ti+10Al≧15; B) Formula: Si+Cu≦1.1; (However, Cr, Ti, Al, Si, and Cu in the formula means the content (% by mass) of each element). The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as in the first aspect of the patent application, further containing at least one of Sn: 0.005 to 1.0%, B: 0.0001 to 0.003%, and V: 0.03 to 1.0%. . A ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as claimed in claim 1 or 2, which satisfies the following formula (B') in place of the above formula (B): (B'): Si+Cu ≦0.5; (However, Si and Cu in the formula mean the content (% by mass) of each element). The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability according to claim 1 or 2, wherein the average corrosion weight loss measured by a corrosion test using a simulated liquid using acid rain is 0.4 mg/cm ² or less; In the above corrosion test, an aqueous solution having a pH of 4.5 and containing 10 ppm of nitrate ions, 10 ppm of sulfate ions, and 5 ppm of chloride ions was used as the above simulated liquid, and a dry-wet repeated test was performed for 10 cycles, and the above-mentioned wet and dry was measured. The above-described average corrosion loss was obtained by repeating the amount of reduction in mass after the test; the dry-wet test was carried out by semi-immersing the test piece having the gap in the aqueous solution at 50 ° C for 24 hours. The ferrite-based iron-based stainless steel having excellent corrosion resistance and workability as in the third paragraph of the patent application has an average corrosion weight loss of 1.0 mg/cm ² as measured by a corrosion test using a simulated liquid using condensed water for burning exhaust gas. In the above corrosion test, an aqueous solution having a pH of 2.5 and containing 100 ppm of nitrate ions, 10 ppm of sulfate ions, and 100 ppm of chloride ions was used as the simulated liquid, and a dry-wet repeated test was performed for 10 cycles, and the foregoing was measured. The above-mentioned average corrosion loss was obtained by reducing the mass after the dry-wet test, and the dry-wet test was performed by semi-immersing the test piece having the gap in the aqueous solution at 80 ° C for 24 hours.