JPS642660B2 - - Google Patents

Description

[Detailed description of the invention]

In the present invention, C: 0.08% or less, Si: 1.0~
2.0%, Mn: 1.0% or less, Ni: 8.0-16.0%, Cr:
16.0-21.0%, Mo: less than 1.0%, Cu: 4.0% or less,
N: 0.2% or less, the balance being Fe, and relates to austenitic stainless steel that has excellent corrosion resistance in environments where seawater is used for cooling in various industries such as steel mills and power plants, and in seawater desalination equipment. . Seawater is currently widely used as cooling water in various industries because its quantity is almost inexhaustible, but as desalination of seawater becomes even more extensive in the future, stainless steel will be used in these devices. It is clear that steel will be used in large quantities. In the above applications, seawater becomes hot and contains chlorides, making it a harsh environment for stainless steel as it tends to cause localized corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking. . Austenitic stainless steel can exhibit sufficient corrosion resistance if a management system is established to prevent local corrosion that occurs in seawater usage environments, and in practical terms, SUS316 is seawater resistant due to its excellent strength, weldability, and workability. It is widely used as stainless steel.
However, since SUS316 is originally an acid-resistant stainless steel, it is not necessarily the best in neutral environments such as seawater, and it is expensive. The present invention is made of austenitic stainless steel, which has excellent corrosion resistance in seawater, and also has acid resistance in consideration of corrosion resistance to cooled liquid when used in heat exchangers, and is the most suitable material at a low price. The composition was discovered. In the stainless steel according to the present invention, the ingredients, reasons for limiting the composition, and effects are as follows. Carbon: Austenitic stainless steel generally has poor corrosion resistance in the heat-affected zone during welding, and pitting corrosion is likely to occur in this area, especially in seawater, and pitting corrosion often induces stress corrosion cracking. This tendency becomes more sensitive as the amount of C in the alloy increases, so in consideration of this point, the upper limit of C in the steel of the present invention was set at 0.08.
%. Silicon: Addition of Si is effective for pitting corrosion resistance, hydrochloric acid resistance, and stress corrosion cracking resistance, so it is necessary to add 1.0% or more. However, if the amount is increased to 2.0% or more, non-metallic inclusions in the steel will increase, deteriorating the surface quality of the material and causing problems in weldability, so the upper limit is set at 2.0%. Mitsuru: Mn should be 1.0% or less considering the internal properties of the material. Nickel: One of the drawbacks of austenitic stainless steel is stress corrosion cracking, and it is a well-known fact that the susceptibility to this decreases as the Ni content increases. In consideration of this point, the lower limit of Ni content in the steel of the present invention is set to 8.0%, and the upper limit is set to 16.0% due to the hot workability and economic efficiency of the material. Chromium: Cr is the most effective element in increasing the corrosion resistance of stainless steel, especially when considering the corrosion resistance of austenitic stainless steel, it accounts for 16.0%.
is required at least. However, an increase in the amount of Cr increases the ferrite phase, disrupts the balance of the metal structure in the cast state, and worsens hot workability, so the upper limit is set at 21.0%. Molybdenum: Mo is the most effective additive element for improving the pitting corrosion resistance, crevice corrosion resistance, and acid resistance of austenitic stainless steel. However, increasing the amount of Mo added is not a good idea from the point of view of resource conservation and product price. One of the advantages of the present invention is that the amount of Mo
The ingredients and composition have been kept to less than 1.0%, much lower than SUS316, and do not impair the above characteristics. Copper: Cu is an effective element to improve acid resistance and reduce stress corrosion cracking susceptibility. However, if the amount added is increased, it will precipitate at grain boundaries, causing high-temperature embrittlement. If the Cu amount exceeds 4.0%, hot workability will be inhibited, so the upper limit is set at 4.0%. Nitrogen: N is an effective element for improving pitting corrosion resistance and crevice corrosion resistance, but adding a large amount increases stress corrosion cracking susceptibility and has the effect of increasing strength, as seen in high-strength stainless steel. There is also. However, excessive strengthening requires high loads during material manufacturing and secondary processing, so the upper limit is set at 0.2% in consideration of the balance between corrosion resistance and strength, as well as weldability. Note that in order to reduce sensitivity to intergranular corrosion, it is preferable to further add Ti and Nb. Next, examples of the present invention will be described. In order to confirm the effects of the present invention, inventive steels of sample numbers 1 to 3 shown in Table 1 were manufactured, and commercially available SUS304 and SUS316 were prepared for comparison. The results of investigating the corrosion resistance of these materials are shown in the second
As shown in the table. The steel of the present invention has a Mo content of less than 1% in consideration of corrosion resistance and product price, but as is clear from Table 2,
The pitting potential, which shows pitting corrosion resistance in an environment equivalent to the NaCl concentration in seawater, and the stress corrosion cracking susceptibility in 20% NaCl are particularly excellent, and the overall corrosion in sulfuric acid and hydrochloric acid is equivalent to or higher than SUS316. It exhibits high corrosion resistance and is ideal as the austenitic stainless steel required for this application. Sample 1 shows superior corrosion resistance to SUS316 by resisting pitting corrosion, stress corrosion cracking susceptibility, and localized corrosion in a neutral environment such as crevice corrosion without causing much general corrosion. Samples 2 and 3 are ideal materials for stainless steel pipes for heat exchangers that use seawater as cooling water, and can be applied when the corrosive environments inside and outside the pipes are different. In other words, Samples 2 and 3 are resistant to both local and general corrosion, but Sample 2 is better against local corrosion and Sample 3 is better against general corrosion than SUS316. This shows that cheaper materials can be selected depending on the usage environment. As described above, it has been proven that the austenitic stainless steel according to the present invention exhibits corrosion resistance superior to SUS316 in a neutral environment, which is generally required as a seawater-resistant stainless steel. The steel of the present invention exhibits excellent corrosion resistance, especially in environments where seawater is used, even though the amount of expensive molybdenum is greatly reduced, and is an excellent product with great advantages from the standpoint of resource conservation.

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【table】

Claims (1)

[Claims] 1% by weight: C: 0.08% or less, Si: 1.0-2.0%,
Mn: 1.0% or less, Ni: 8.0~16.0%, Cr: 16.0~
21.0%, Mo: less than 1.0%, Cu: 4.0% or less, N:
Austenitic stainless steel consisting of 0.2% or less, the balance being Fe, and characterized by excellent corrosion resistance in seawater environments.