JPS60100629A - Production of austenite stainless steel - Google Patents

Abstract

PURPOSE:To obtain a steel having high corrosion resistance to a nitric acid soln. having middle and high concn. or contg. an oxidizing agent such as Cr<6+> ion at a high temp. by subjecting a titled steel having a specific compsn. to cold working and heat treatment under limited conditions to form the fine crystal structure in which carbide is uniformly dispersed. CONSTITUTION:An austenite stainless steel consisting, by weight, of <=0.04% C, <=0.4% Si, <=2.0% Mn, 18-30% Cr, 7-28% Ni, <=0.02% P and if desired, <=1.0 total of >=1 kind among Nb, Ti and Ta and the balance Fe is subjected to cold working at >=40% reduction ratio. The resultant cold worked material is held at the temp. region below the recrystallization temp. and at which carbide is deposited and is thereafter held further in the temp. region of the recrystallization temp. or above - the recrystallization temp. +50 deg.C. The fine structure in which the carbide is once precipitated after cold working and is then uniformly dispersed by the recrystallization is thus obtd. and the steel having high resistance to nitric acid is obtd.

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 medium to high concentration nitric acid solution at high temperature or containing an oxidizing agent such as Cr6+ ion. The present invention relates to a method for producing austenitic stainless steel having a fine grain structure in which carbides are uniformly dispersed and exhibiting excellent corrosion resistance even in a corrosive environment of nitric acid solution.

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 compatible with this has become increasingly important.

For example, metal materials are increasingly being used on an industrial scale in corrosive environments such as nitric acid solutions. Conventionally, 25% Cr-20% Ni based materials (Example 2 UR
ANIJS 65...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 treatment measure 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, and furthermore in the presence of oxidizing agents such as Cr6+ ions, Cr in conventional stainless steel
Corrosion prevention through passivation is achieved even in the case of medium- to high-concentration nitric acid solutions. Cr6+ ions, which are partially dissolved from the steel surface and present in the nitric acid solution, increase the surface potential of the stainless steel and cause excessive corrosion. In order to generate passive 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 Cr6+ 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 due to the increase in corrosion potential due to the generation and mixing of oxidizing agents such as Cr6+ ions into the solution, and the grain boundaries are accelerated at an accelerated rate. It must also have sufficient resistance to corrosion. Furthermore, since welding is performed when assembling equipment or components, intergranular corrosion will become even greater if we take into account the increased sensitivity during welding.
It must also exhibit resistance to such sensitization. Therefore, new materials are required to have excellent resistance to all of these corrosive conditions.

(Summary of the Invention) The object of the present invention is to
An object of the present invention is to provide a method for producing a fine-grained austenitic stainless steel that exhibits excellent corrosion resistance even in a corrosive environment of a highly concentrated nitric acid solution or a nitric acid solution containing an oxidizing agent such as Cr6+ ions.

Here, the present inventors have attempted to homogenize the structure by uniformly dispersing carbides and refining crystal grains, and by reducing the grain boundary segregation concentration of impurity elements, it is possible to improve resistance to intergranular corrosion. Focusing on this, they discovered that after cold working, by precipitating carbides and then recrystallizing them, a fine grain structure with uniformly dispersed carbides could be obtained.
As a result of further research, the present invention was completed.

Therefore, the present invention provides the following in weight%: C: 0.04% or less, Si: 0.4% or less, Mn
: 2.0 or less, Cr: 18-30%, total 1.0
% or less, austenitic stainless steel with a composition consisting of the balance Fe and incidental impurities,
Cold working is performed to a working degree of 40% or more, and then the obtained cold worked material is held in a temperature range where carbide precipitates below the recrystallization temperature, and then further heated to a temperature above the recrystallization temperature. Manufacture of nitric acid-resistant austenitic stainless steel characterized in that it has a fine grain structure in which carbides are uniformly dispersed by maintaining it in a temperature range of +50°C or lower (hereinafter also referred to as the temperature range immediately above the recrystallization temperature). It's a method.

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.

18% or more is necessary to ensure corrosion resistance of Cr in general 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 increasing the amount of Ni. On the other hand, increasing the amount of Ni leads to deterioration of workability and cost increase, so the upper limit of the amount of Cr is 30%. shall be. Therefore, in the present invention, the Cr content is 18 to 30%.

The Ni content is set to 7 to 28% as the amount necessary to form a Ni niostenite structure.

Nb, Ti, Ta: These elements are added as necessary as stabilizing elements for C. Especially when the C content is 0
．． If it is less than 0.02%, 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 be low, and in the present invention, it is set to 0.02% or less.

In the present invention, cold working, carbide precipitation treatment, and recrystallization treatment are performed in order to impart the desired properties to the austenitic stainless steel having the above-mentioned steel composition.

That is, first, cold working with a working degree of 40% or more is applied,
After introducing a sufficient amount of processing strain, the process is then maintained at a temperature below the recrystallization temperature and in a temperature range where carbide precipitation occurs, generally in the temperature range of 550 to 750°C, preferably for 1 hour or more. , carbide is predispersed and precipitated at grain boundaries. In the examples described below, this will be referred to as a first heat treatment.

Next, the temperature range immediately above the recrystallization temperature, the temperature range above the recrystallization temperature and below the recrystallization temperature +50℃, generally 750 to 1
The cold-worked material is preferably kept in a temperature range of 1,000° C. for one hour or more. Similarly, this is called second heat treatment. This second heat treatment causes recrystallization, and the former austenite I
- Carbides precipitated at grain boundaries and slip bands are dispersed within the grains, and the recrystallized grains are also fine because the temperature is maintained in the temperature range just above the recrystallization temperature. Therefore, it is preferable that the second heat treatment temperature be as close as possible to the recrystallization temperature, that is, in the temperature range immediately above the recrystallization temperature and as low as possible.

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. That is, 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 detail with reference to examples, but these are merely 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.

Table 1 shows the steel composition of each sample material used in this example. The test materials A steel to E steel are examples of the present invention, F steel,
G steel shows a comparative example. The recrystallization temperature was 775°C for A steel, D steel, and F steel, and 900°C for C steel, D steel, E steel, and G steel.

Table 2 shows the heat treatment conditions conducted on each sample material and the results of the corrosion resistance test conducted thereafter. Corrosion resistance test was conducted using 0.25g of Cr6+ ions in 8N-11NOa.
The corrosion rate and intergranular corrosion depth were measured after immersing the test material in the boiling nitric acid solution for 48 hours. The degree of processing was determined by the amount of reduction in plate thickness.

Table 1 -7° Table 2 (gamma book) (To) Continuation of Table 2) Φ＊：Water towel quenching The following can be seen from Tables 1 and 2.

First of all, regarding the steel composition, F steel has a high C content, that is, it has many sensitization promoting components (Jl has high values for both corrosion rate and intergranular corrosion depth (Test N1127).
．． (Refer to 28), especially for G steel, even if the treatment of the present invention is applied, the corrosion resistance, especially the intergranular corrosion resistance, is improved (Renai).
Test No. 28). Awi, B 61 To C steel, D steel,
Compared with Steel E, Steel A and Steel B have relatively low Cr and Ni contents (within the scope of the present invention) even after the same treatment.
Although the corrosion rate of steel is faster, there is no difference in intergranular corrosion depth (test positive, see 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 corrosion resistance in both corrosion rate and intergranular corrosion depth.
In particular, the improvement in intergranular corrosion depth is remarkable (Test No. 15.1
8.20 and 24). Also, 50% higher than the recrystallization temperature.
When the second heat treatment is performed at a temperature higher than ℃ (v, test NO.
, 19, 21, 25, 26) Narahini cold working strength: 40
% (Test No. 14, 22, 23), the corrosion resistance is inferior to that of the invention examples. If the holding times of the first and second heat treatments are too short, it is not preferable to achieve the initial objective (see Test 29.30).

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, it is possible to produce a medium-to-high concentration nitric acid solution or nitric acid containing an oxidizing agent such as Cr6+ at a high temperature. This provides an austenitic stainless steel with excellent corrosion resistance in a corrosive solution environment.

Applicant: Sumitomo Metal Industries, Ltd. Agent Patent Attorney Akira Hirose -

Claims (1)

[Claims] In weight %, C: 0.04% or less, St: 0.4% or less, Mn +
2.0 or less, Cr: 18-30%, Ni mini full 28, P: 0.02% or less, further N if desired
b, a total of 1 of at least one of TI and Ta;
．． Austenitic stainless steel with a composition of 0% or less, the balance consisting of Fe and incidental impurities,
Cold working is performed at a working degree of 40% or more, and then the obtained cold worked material is held at a temperature below the recrystallization temperature and in a temperature range where carbides precipitate, and then further heated at a temperature above the recrystallization temperature at a recrystallization temperature +50. A method for producing a nitric acid-resistant austenitic stainless steel, which is characterized by forming a fine grain structure in which carbides are uniformly dispersed by maintaining the steel in a temperature range of °C or lower.