JPH0132290B2 - - Google Patents

Description

[Detailed description of the invention]

(Background of the Invention) The present invention relates to a method for producing austenitic stainless steel with excellent nitric acid resistance, particularly in a corrosive environment of a medium to high concentration nitric acid solution at high temperatures or a nitric acid solution containing an oxidizing agent such as Cr ⁶⁺ ions. The present invention relates to a method for manufacturing an austenitic stainless steel having a fine grain structure in which carbides are uniformly dispersed and exhibiting excellent corrosion resistance even under conditions of corrosion. With the development of the chemical industry and the advancement of technology in recent years, the environment in which metal materials are used has become increasingly severe, and the development of materials corresponding to these conditions has become increasingly important. For example, there are many opportunities for metal materials to be used on an industrial scale in corrosive environments of nitric acid solutions. Conventionally, 25%Cr-20%
Ni-based material (eg URANUS65...product name) is used. This is because the chromium component is thought to be passivated in nitric acid, resulting in good corrosion resistance. In addition, as a countermeasure against nitric acid corrosion, austenitic stainless steel that has been subjected to solution treatment (eg, held at 1100°C for 30 minutes and then water cooled) is used. However, as mentioned above, in the corrosive environment of medium- to high-concentration nitric acid solutions at high temperatures, as well as nitric acid solutions containing oxidizing agents such as Cr ⁶⁺ ions, corrosion protection by passivation of Cr in conventional stainless steel cannot be achieved. ,During·
Even in the case of a high concentration nitric acid solution, some Cr ⁶⁺ is dissolved from the steel surface and exists in the nitric acid solution.
Since the ions increase the surface potential of the stainless steel and cause hyperpassive dissolution, the dissolution of the steel surface progresses at an accelerated rate. Further, even if solution treatment is performed, the intergranular corrosion phenomenon that is accelerated at an accelerated pace when an oxidizing agent such as Cr ⁶⁺ ions as described above is present cannot be prevented, and corrosion resistance deteriorates. Furthermore, heating during welding sensitizes grain boundaries, which causes sensitization that deteriorates corrosion resistance and mechanical properties, and grain boundary corrosion due to grain boundary precipitation of carbides or grain boundary segregation of impurity elements such as P. It becomes remarkable. Under these circumstances, the following properties are required for materials used in the corrosive environment of nitric acid solution. In other words, not only is the corrosion resistance against nitric acid satisfied, but also the corrosion rate increases and accelerates as the corrosion potential rises due to the generation and mixing of oxidizing agents such as Cr ⁶⁺ ions into the solution. It must also have sufficient resistance to intergranular corrosion. Furthermore, since welding is performed in the assembly of devices or members, intergranular corrosion will be even greater if sensitization during welding is taken into account, so resistance to such sensitization must also be demonstrated. Therefore, new materials are required to have excellent resistance to all these corrosion conditions. (Summary of the Invention) Thus, the object of the present invention is to provide excellent corrosion resistance even under the corrosive environment of medium to high concentration nitric acid solutions at high temperatures or nitric acid solutions containing oxidizing agents such as Cr ⁶⁺ ions. It is an object of the present invention to provide a method for producing a fine grained austenitic stainless steel exhibiting the following properties. Here, the present inventors have found that resistance to intergranular corrosion can be improved by uniformizing the structure by uniformly dispersing carbides and refining crystal grains, and by lowering the grain boundary segregation concentration of impurity elements. Focusing on this, they found that by precipitating carbides after cold working and then recrystallizing them, a microcrystalline structure in which carbides were uniformly dispersed was obtained.As a result of further research, they completed the present invention. It is something that Therefore, the present invention provides, in weight percent, C: 0.04% or less, Si: 0.4% or less, Mn: 2.0% or less, Cr: 18-30%, Ni: 7-28%, P: 0.02% or less, and If desired, an austenitic stainless steel having a composition consisting of at least one of Nb, Ti, and Ta in a total of 1.0% or less, and the remainder Fe and incidental impurities is subjected to cold working at a working degree of 40% or more, and then the obtained The cold-worked material is held in a temperature range below the recrystallization temperature and in which carbides precipitate, and then in a temperature range above the recrystallization temperature and below the recrystallization temperature + 50°C (hereinafter referred to as the temperature range immediately above the recrystallization temperature). )
This is a method for producing a nitric acid-resistant austenitic stainless steel, which is characterized by maintaining a fine grain structure in which carbides are uniformly dispersed. In the present invention, the reason why the steel composition is limited as described above is as follows. C: C promotes sensitization, so if the C content is too high, intergranular corrosion resistance, that is, nitric acid resistance will deteriorate. Therefore, in the present invention, the C content is set to 0.04% or less. Si: Contains 0.4% or less as a deoxidizing agent. Mn: Contains 2.0% or less as a deoxidizing agent. Cr: 18% or more is required to ensure general corrosion resistance and corrosion resistance in environments containing only nitric acid. In addition, in order to create an austenitic structure,
Increasing the amount of Cr also requires an increase in the amount of Ni,
On the other hand, since increasing the Ni content will lead to deterioration of workability and increased costs, the upper limit of the Cr content is 30
%. Therefore, in the present invention, the Cr content is 18 to 30%. Ni: The Ni content is set to 7 to 28% as the amount necessary to form an austenitic structure. Nb, Ti, Ta: These elements are added as stabilizing elements for C as necessary. Particularly when the content of C is 0.02% or less, it is not necessarily necessary to add it. In consideration of weldability, the amount of addition of at least one of the above elements is 1.0% or less in total. P: In order to improve intergranular corrosion resistance, it is desirable that the P content is low, and in the present invention, it is 0.02%.
The following shall apply. In the present invention, as already mentioned, the austenitic stainless steel having the above steel composition is used.
Cold working, carbide precipitation treatment and recrystallization treatment are performed to impart the desired properties. That is, first, cold working is applied to a working degree of 40% or more to introduce a sufficient amount of working strain, and then the temperature is lower than the recrystallization temperature and in the temperature range where carbide precipitation occurs, generally 550~ Carbide is predistributed and precipitated at grain boundaries by maintaining the temperature in a temperature range of 750° C. for preferably 1 hour or more. In the examples described below, this will be referred to as a first heat treatment. Next, the cold-worked material is maintained in a temperature range immediately above the recrystallization temperature, that is, a temperature range above the recrystallization temperature and below the recrystallization temperature +50°C, generally in a temperature range of 750 to 1000°C, preferably for one hour or more. Similarly, this is called second heat treatment. This second heat treatment causes recrystallization and disperses the carbides precipitated at the prior austenite grain boundaries and slip bands within the grains.Moreover, since the temperature is maintained just above the recrystallization temperature, the recrystallized grains are also fine. becomes. Therefore, the second heat treatment temperature is preferably as close as possible to the recrystallization temperature, that is, as low as possible within the temperature range immediately above the recrystallization temperature. Note that the amount of segregation of impurity elements per unit grain boundary area becomes smaller as the crystal grain size becomes smaller, resulting in a more uniform structure. In other words, as the crystal grains become finer and more uniform, grain boundary segregation of impurities is reduced accordingly, and intergranular corrosion resistance is improved. Thus, by the above-described component composition and heat treatment according to the present invention, selective corrosion at grain boundaries can be significantly reduced. Next, the present invention will be explained in more detail with reference to Examples, but these are illustrative of the present invention and are not intended to limit the present invention in any way. In this specification, "%" means "% by weight" unless otherwise specified. Example Table 1 shows the steel composition of each sample material used in this example. The test materials A steel to E steel show examples of the present invention, and F steel and G steel show comparative examples. The recrystallization temperature is 775℃ for A steel, B steel, F steel, C steel, D steel, E steel,
G steel was heated to 900℃. Table 2 shows the heat treatment conditions conducted on each sample material and the results of the corrosion resistance test conducted thereafter. The corrosion resistance test used a nitric acid solution containing 0.25 g of Cr ⁶⁺ ions in 8N- _HNO3 , and after immersing the test material in a boiling solution of this nitric acid solution for 48 hours, the corrosion rate and intergranular corrosion depth were measured. This was done by measuring. Note that the working degree was determined by the amount of reduction in plate thickness.

【table】

[Table] Note) *〓Quick cooling in water The following can be seen from Tables 1 and 2. First, regarding the steel composition, steel F and steel G, which have a high C content, that is, many sensitizing components, have high values for both corrosion rate and intergranular corrosion depth (see test Nos. 27 and 28). In particular, for G steel, no improvement in corrosion resistance, especially intergranular corrosion resistance, was observed even if the treatment of the present invention was performed (Test No. 28). A steel, B
Comparison of steel with C steel, D steel, and E steel shows that steel A and steel B, which have relatively low Cr and Ni contents (within the scope of the present invention), have a faster corrosion rate even when subjected to the same treatment. However, there is no difference in intergranular corrosion depth (see Test Nos. 1 to 4). A comparison between conventional solution treatment and the method of the present invention shows that the method of the present invention improves corrosivity in both corrosion rate and intergranular corrosion depth, and the improvement in intergranular corrosion depth is particularly remarkable (Test No.
15, 18, 20 and 24). In addition, when the second heat treatment is performed at a temperature 50℃ or more higher than the recrystallization temperature (Test No. 19, 21, 25, 26) and the degree of cold work is
In all cases where it is less than 40% (Test Nos. 14, 22, and 23), the corrosion resistance is inferior to that of the examples of the present invention. If the holding time of the first and second heat treatments is too short, it is not preferable to achieve the initial purpose (Test No. 29, 30
reference). As is clear from the above explanation, by performing the treatment according to the present invention on austenitic stainless steel having the composition specified in the present invention, medium to high concentration nitric acid solution or oxidizing agent such as Cr ⁶⁺ is introduced at high temperature. This yields an austenitic stainless steel with excellent corrosion resistance in the corrosive environment of nitric acid solution.

Claims (1)

[Claims] 1% by weight: C: 0.04% or less, Si: 0.4% or less, Mn: 2.0% or less, Cr: 18-30%, Ni: 7-28%, P: 0.02% or less, balance Austenitic stainless steel with a composition consisting of Fe and accompanying impurities is subjected to cold working to a working degree of 40% or more, and the resulting cold worked material is then held at a temperature below the recrystallization temperature and in the temperature range where carbides precipitate. A method for producing a nitric acid-resistant austenitic stainless steel characterized by forming a fine grain structure in which carbides are uniformly dispersed by further maintaining the temperature in a temperature range above the recrystallization temperature and below the recrystallization temperature +50°C. . 2% by weight, C: 0.04% or less, Si: 0.4% or less, Mn: 2.0% or less, Cr: 18-30%, Ni: 7-28%, P: 0.02% or less, among Nb, Ti, and Ta. Austenitic stainless steel having a composition consisting of at least one type in a total of 1.0% or less and the balance Fe and incidental impurities is subjected to cold working at a working degree of 40% or more, and then the obtained cold worked material is heated to a recrystallization temperature. It is characterized by a fine grain structure in which carbides are uniformly dispersed by holding it in a temperature range below the recrystallization temperature and at which carbides precipitate, and then holding it in a temperature range above the recrystallization temperature and below the recrystallization temperature + 50°C. A method for producing nitric acid-resistant austenitic stainless steel.