JPH0357181B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention is applicable to stress corrosion cracking (hereinafter abbreviated as "SCC") in a high-temperature sulfide environment, particularly in a high-temperature and humid environment where hydrogen sulfide and chloride ions are present.
The present invention relates to an austenitic stainless steel that has excellent corrosion resistance, such as extremely high resistance to intermittent corrosion, and also has good hot workability. In recent years, the demand for stainless steel, which is a stable material with excellent corrosion and oxidation resistance, has permeated not only in fields such as heavy and chemical industries, but also in the field of various daily necessities, and is showing a tendency to further increase. . On the other hand, given the remarkable development of seawater utilization industries and the growing interest in the utilization of seabed resources, there is a strong demand for the development of steel materials with superior corrosion and oxidation resistance than conventional stainless steel. It was also done. <Conventional technology> By the way, when using stainless steel in an environment where chlorine ions (Cl ^- ) exist,
Localized corrosion problems such as "intermittent corrosion" and "pitting corrosion" cannot be avoided, but the effectiveness of adding Mo, N, etc. as a means of improving corrosion resistance including preventing this type of corrosion has become known. After that, for example, in a corrosive environment where there is contact with seawater, high
Austenitic stainless steel containing high Cr and Mo has been adopted and has achieved great results.
Austenitic stainless steels have been proposed (for example, Japanese Patent Publication No. 40331/1983,
135812, JP-A-52-95524). <Problems to be solved by the invention> However, in consideration of the current situation where it is becoming necessary to use steel materials in even harsher environments, the invention was made assuming various practical environments in which stainless steel is used. The results of experiments and studies conducted by the present inventors indicate that the conventional N-containing, high-Cr, high-Mo austenitic stainless steel is a highly corrosion-resistant material in a neutral Cl ^- -O ₂ environment, and that it is highly corrosion resistant in a high-temperature H ₂ S environment. -Cl ^- ``Because corrosion resistance in the environment was not taken into consideration, SCC occurs in high-temperature environments containing H ₂ S.'' Such problems with conventional N-containing, high Cr, high Mo austenitic stainless steels have caused concerns in the design of piping and coal liquefaction equipment on submarine resource drilling rigs, where demand is expected to increase in the future. . This is because the inner surface of the piping materials on a resource drilling rig is exposed to a "CO ₂ − _{H 2} S−Cl ^- " environment, and the outer surface of the pipe is exposed to a "seawater mist" environment.
Even if the seawater resistance is good, if the SCC resistance on the inner surface of the pipe is poor, it is judged that the pipe cannot withstand use.On the other hand, in coal liquefaction equipment, high-temperature H ₂ S
This is because corrosive corrosion, especially SCC, has recently received attention as a problem. On the other hand, conventional N-containing, high-Cr, high-Mo austenitic stainless steels have poor hot workability, and have been a material that has been pointed out to pose problems in the manufacture of plate materials. <Means for Solving the Problems> From the above-mentioned viewpoints, the present inventors have solved the above-mentioned problems observed in conventional N-containing, high-Cr, high-Mo austenitic stainless steels, and improved corrosion resistance, especially "high-temperature" stainless steel. H ₂ S−Cl ^- “As a result of repeated research to provide a steel material that has excellent SCC resistance in the environment and has sufficient strength and hot workability, we have developed (a) High Cr, high Mo content stainless steel. “H ₂ S−Cl ^- ”
The SCC resistance in the environment is highly dependent on the Ni content,
Furthermore, the corrosion resistance can be significantly improved by adding an appropriate amount of W or Cu. (c) By adjusting the content of Cr (%) + 3Mo (%) + 10N (%) ≧38.0, it is possible to stably secure basic corrosion resistance performance that is fully satisfactory; (c) N-containing high Cr Adding an appropriate amount of Al to high-Mo austenitic stainless steel suppresses the deterioration of hot workability that occurs due to the increase in Mo and N. However, by adding an appropriate amount of Al and reducing S and O as much as possible, the hot workability of N-containing, high-Cr, high-Mo austenitic stainless steel is further improved. ) B to the above austenitic stainless steel,
Adding a small amount of Mg, La or Ce will further improve its hot workability. In addition to adjusting, W or Cu is added to this,
Furthermore, by extremely reducing S and O and containing an appropriate amount of Al, a steel material with greatly improved corrosion resistance including SCC resistance and hot workability in an "H ₂ S-Cl ^- " environment can be realized. This led us to the findings shown in (a) to (f) above. This invention was completed based on the above knowledge, and the austenitic stainless steel has C: 0.03% or less (hereinafter, % is based on weight), Si: 1.5% or less, Mn: 2.0% or less, Cr: 19.0% or more and less than 22.5%, Ni: 21.0% or more and less than 30.0%, Mo: more than 6.0 to 10.0%, one or more of W or Cu: 0.3 to 2.0%, Al: 0.01 to 0.15%, N: 0.05 to 0.30% , P: 0.03% or less, S: 0.002% or less, O: 0.005% or less, or further contains one or more of B, Mg, La, or Ce: 0.001 to 0.100.
%, the balance consists of Fe and other unavoidable impurities, and by configuring the composition to satisfy the formula Cr (%) + 3Mo (%) + 10N (%) ≧ 38.0, sufficient strength can be achieved. Of course, it has excellent corrosion resistance (especially
It is characterized by having good hot workability (including SCC resistance in a high-temperature H ₂ S-Cl ^- environment) and good hot workability. Next, in the austenitic stainless steel of the present invention, the reason why the content ratio of each composition component is numerically limited as described above will be explained. (a) C C is an austenite forming element, but 0.03
If the C content exceeds 0.03%, the intergranular corrosion resistance of the steel decreases, so the C content was set at 0.03% or less. Note that the lower the C content, the lower the sensitization sensitivity, so it is preferable to suppress the C content as much as possible. (b) Si The Si component is an effective element for improving acid resistance and pitting corrosion resistance, but if it is contained in excess of 1.5%, it tends to impede the weldability and hot workability of steel. Therefore, the Si content was determined to be 1.5% or less. Note that even if the amount of Si is small, a certain effect of improving acid resistance and pitting corrosion resistance is recognized, but it is desirable to secure a content of 0.05% or more if possible. (c) Mn The Mn component has the effect of increasing the solid solution amount of N, which has the effect of improving corrosion resistance, and stabilizing it, but if it is contained in excess of 2.0%, sulfide ( MnS) production increases,
Not only “H ₂ S−Cl ⁻ ” environment, but especially “Cl ⁻ −O ₂ ”
The Mn content was set at 2.0% or less because it degrades pitting corrosion resistance in the environment and also causes deterioration of workability. In addition, even if the amount of Mn is small, a certain amount of N
Stabilizing effect is exerted, preferably 0.1%
It is preferable to contain Mn in the above amount. (d) Cr The Cr component has the effect of improving acid resistance, pitting corrosion resistance, inter-corrosion resistance, and other general corrosion resistance, and is an extremely important element, but if its content is less than 19.0%, “ H ₂ S−Cl ^- “Corrosion resistance in the environment becomes insufficient. On the other hand, in a high Mo alloy system such as the present invention, if the Cr content is 22.5% or more, precipitation of the σ phase is promoted, embrittlement is likely to occur, and complete austenite is difficult to obtain. Therefore, the Cr content was set at 19.0% or more and less than 22.5%. (e) Ni Ni is a major austenite-forming element and plays an important role in maintaining the structure as a single austenite phase by balancing with ferrite-forming elements such as Cr. Although it has the effect of improving SCC resistance in high-temperature solution environments, especially in "H ₂ S-Cl ^- " environments, it also has the effect of improving pitting corrosion and crevice corrosion resistance in "Cl ^- -O ₂ " environments, especially in seawater environments. It has a big impact. If the content is less than 21.0, the SCC resistance improvement in the above-mentioned "H ₂ S-Cl ^- " environment will not be sufficient, while if the content exceeds 30.0%, the depassivation pH will increase. Therefore, _the Ni ^content is
It is set at 21.0% or more and less than 30.0%. (f) Mo Mo component, like Cr, is an essential element for improving the pitting corrosion resistance and interstitial corrosion resistance of steel, but if its content is less than 6.0%, the Cl ^- concentration, such as seawater, Interstitial corrosion resistance deteriorates in high environments,
In addition, pitting corrosion resistance in a chloride environment containing H ₂ S also deteriorates. Corrosion resistance improves as the Mo content increases, but if it exceeds 10.0%, the precipitation of the σ phase will be promoted and the steel will become brittle, resulting in extremely poor hot workability. ,
The Mo content was set at over 6.0% to 10.0%. (g) W and Cu These components are both effective in improving pitting corrosion resistance and interlocking corrosion resistance in chloride solutions, and are particularly effective when added in combination with Ni. Since it has an even effect of significantly improving SCC resistance in the "H ₂ S-Cl ^- " environment, one or both of them are added at the same time, but the content If the content of W and Cu is less than 0.3%, the desired effect cannot be obtained, while if the content exceeds 2.0%, the hot workability of the steel will deteriorate. Or, it was set at 0.3 to 2.0% in combination. (h) Al The Al component is an indispensable element because it has the effect of improving the hot workability of high-Mo high-N steel, but if its content is less than 0.01%, the desired effect cannot be achieved. No effect, while 0.15%
The Al content was set at 0.01% to 0.15% since the corrosion resistance of steel deteriorates if the Al content exceeds 0.01% to 0.15%. (i) N Like Cr, Ni and Mo, the N component is effective in improving the corrosion resistance (especially pitting corrosion resistance and interlocking corrosion resistance) of steel, but if its content is less than 0.05%, it is insufficient. The effect of improving corrosion resistance was not obtained; on the other hand,
If the N content exceeds 0.30%, bubbles tend to occur in the steel ingot, so the N content should be 0.05~
It was set at 0.30%. (j) PP Although P is an impurity element that inevitably mixes into steel, it is preferable to reduce it as much as possible since it causes a decrease in weldability and hot workability. If the P content exceeds 0.03%, the tendency for deterioration of weldability and hot workability becomes noticeable, so the P content was set at 0.03% or less. (k) S S, like P, is an impurity element that inevitably exists in steel, and has an adverse effect on pitting corrosion resistance and hot workability. In particular, if the S content exceeds 0.002%, the deterioration of hot workability becomes significant, so the allowable amount was limited to 0.002% or less. In other words, in a high-Mo, high-N content alloy like the one of the present invention, the deformation resistance at high temperatures is extremely high, so deformation concentrates at the grain boundaries during hot working, and cracks are likely to occur from that location. , S is an element that segregates at grain boundaries and exhibits behavior that promotes embrittlement. In the steel of the present invention, the allowable upper limit of the S content was set at 0.002% in order to ensure the effect of improving hot workability by reducing the S content.
It is best to limit the amount of S to 0.001% or less. By the way, Figure 1 shows (19.5-21.6)%Cr-
(23.4~26.2)%Ni−(5.0~6.7)%Mo−(0.12
This is a graph showing the results of investigating the influence of S on hot workability for ~0.22)%N steel. The workability is considerably improved, and at 0.001% or less, an even more remarkable hot workability improvement effect can be obtained. (l) O O, like S, is an impurity element that reduces the high temperature deformation of steel and significantly deteriorates hot workability, but if its content is suppressed to 0.005% or less, the hot workability described above decreases. The O content was set at 0.005% or less because the deterioration effect would be less noticeable. (m) B, Mg, La, and Ce All of these components contain high Mo, high N, along with "resistance of S and O" and "addition of Al".
Since they have a uniform effect of improving the hot workability of steel, one or more types are added especially when it is necessary to further improve the hot workability.
If the content is less than 0.001%, no significant effect will be obtained on the above action, while if the content exceeds 0.100%, defects will occur in the steel block. or in combination
It was set at 0.001-0.100%. Note that Cr, Mo, and N are all essential components for improving the pitting and interlocking corrosion resistance of steel in a chloride solution environment, but they are closely related to each other and are It has the effect of improving pitting corrosion resistance and intermittent corrosion resistance, such as Cr (%) + 3Mo (%) + 10N (%) ≧ 38.0. In order to further stabilize the performance, it was decided that the Cr, Mo and N contents should be further adjusted comprehensively so as to satisfy the above formula. In addition, normally, in order to fix C in steel, Ti
However, in the steel of this invention, since N is added, no stabilizing elements are added. This is because, for example, when adding Ti,
This is because the Ti combines with N in the steel to produce TiN, reducing the effective amount of N. As mentioned above, the austenitic stainless steel of the present invention is a material that exhibits excellent corrosion resistance in the "H ₂ S-Cl ^- " environment, which is more corrosive than the "Cl ^- -O ₂ " environment. It goes without saying that it exhibits sufficiently excellent corrosion resistance even in a "Cl ^- -O ₂ " environment. Next, the present invention will be explained using examples and in comparison with comparative examples, but it goes without saying that the technical scope of the present invention is not limited by the examples. <Example> First, steels 1 to 30 having the compositions shown in Table 1 were melted by a vacuum melting method. Next, this was hot rolled into a hot-rolled steel sheet in the usual manner, then subjected to solution heat treatment (held at 1100°C for 30 minutes, then water cooled), followed by TIG welding (no weld filler). , SCC test piece (75 mml × 10 mm ^w ×
2 pieces with dimensions of 2 mm ^t ) and 30
One with dimensions of mm ^l x 20mm ^w x 3mm ^t and one with dimensions of 30mm ^l x 15mm ^w
A piece with a size of 3mm x ^t ) was cut out and subjected to an SCC test and an interstitial corrosion test. In the SCC test, as shown in Fig. 2, the two test pieces 1, 1 are stacked and bent into a U-shape, and then inserted through fluorocarbon resin bushes 2, 2.
Restrained with Ti bolt 3 and Ti nut 4

【table】

【table】

[Table] ``Double U'' is a method of immersing a substance in a test liquid for a certain period of time.
"Bent test" (5 mm restraint) was adopted. In addition, the reference numeral 5 in Fig. 1 indicates the welded part.
Moreover, the test conditions were as follows. Test solution: Saline solution with a H ₂ S partial pressure of 10 atm (room temperature) (Cl ^- concentration of 10 ⁵ ppm), solution temperature: 150°C, test time: 720 hours. The SCC resistance was evaluated by examining the presence or absence of cracks through magnifying glass observation or cross-sectional microscopic observation after the test was completed. On the other hand, for the interstitial corrosion test, as shown in Fig. 3, two large and small test pieces 6, 6 are overlapped.
After forming a gap, the central part is strongly tightened with a Ti bolt 7 and a Ti nut 8 (at this time, fluororesin washers 9, 9 were inserted), and this is immersed in the test liquid for a certain period of time. I adopted the method of saying. Note that the reference numeral 10 in FIG. 2 indicates a welded portion, and the test conditions were as follows. Test solution: Saline solution with H ₂ S partial pressure of 10 atm (room temperature) (Cl ^- concentration of 10 ⁴ ppm), solution temperature: 150°C, test time: 720 hours. The interstitial corrosion resistance was evaluated by examining the presence or absence of interstitial corrosion by visual observation and corrosion weight loss after the above test. Furthermore, 20 mml x 50 mmw x 3 was obtained from a hot rolled steel sheet that had been subjected to solution heat treatment under the same conditions as above after hot rolling.
A pitting corrosion test piece of mmt was cut out and a pitting corrosion test was also conducted. The test was conducted as a rapid test under the following conditions, simulating seawater. Test solution: 10% FeCl ₃ +1/20NHCl solution, temperature: 60°C, test time: 48 hours. The pitting corrosion resistance was evaluated by visually observing the presence or absence of pitting corrosion after the above test was completed. Separately, we also conducted a hot workability investigation. To investigate hot workability, a tensile test piece is cut out by mechanical processing from the above material after hot rolling and solution treatment, and a high temperature twisting test is performed at 1200°C to calculate the number of twists until the test piece breaks. It was carried out using the method required. The above test results are shown in Table 2. The following is clear from the results shown in Table 2. That is, regarding SCC resistance, inventive steels 1 to 18 did not cause any SCC, whereas comparative steels 19 to 21, which had a Ni content of 21.0% or less,
and comparative steels 22 to 23 with a W or Cu content of less than 0.3%.
Occurrence of SCC is observed. Interpolation corrosion resistance Inventive steels 1 to 18 showed no occurrence of interpolation corrosion, whereas [Cr (%) + 3Mo (%) + 10N]
(%)] is less than 38.0, and comparison steels 29 and 30 with a Mn content of more than 2% show that interstitial corrosion occurs. It was also confirmed that the steel of the present invention has good crevice corrosion resistance even in a "Cl ^- -O ₂ " environment (for example, seawater). Pitting corrosion resistance Invention steels 1 to 18 and low Ni and [Cr (%)
Comparative steel with a value of +3Mo (%) + 10N (%)] of 38.0 or more
No pitting corrosion was observed in samples 19 and 20, but the value of [Cr (%) + 3Mo (%) + 10N (%)] was 38.0.
Comparative steels less than 21, 22, 24~28, [Cr (%) + 3Mo
(%) + 10N (%)] is 38.0 or more, but it can be seen that pitting corrosion occurs in 23 with a high S content and 29 and 30 with a Mn content of 3% or more. Hot workability Inventive steels 1 to 18 show extremely excellent hot workability, with the number of twists until fracture being 20 or more, but the compositional conditions of the steels are outside the scope of the present invention. It can be seen that comparative steels 19 to 27, 29, and 30 were all twisted less than 20 times. Note that although Comparative Steel 30 has a low Mo content and exhibits excellent hot workability, it has poor corrosion resistance as described above. As is clear from these examples, the austenitic stainless steel of the present invention has high Cr,
Mainly based on the combined effects of high Mo, high N, and high Ni, we have improved SCC resistance and crevice corrosion resistance in a "high-temperature _H2S -Cl- ^" aqueous environment, as well as Cl ^-- _O2 environment. Improved pitting corrosion resistance, extremely low S, extremely low O, and
It has improved hot workability mainly due to the addition of Al. <Overall Effects> As explained above, according to the present invention, not only excellent corrosion resistance in a neutral high concentration chloride environment such as seawater (O ₂ -Cl ^- environment), but also excellent corrosion resistance in H ₂ Excellent resistance in chloride environments containing S (H ₂ S−Cl ^- environments)
We can provide austenitic stainless steel that has SCC properties and is perfect in terms of hot workability and strength, and can further improve the performance of various equipment and equipment used in harsh corrosive environments. This brings about extremely useful effects industrially.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of S on the hot workability of high-Mo, high-N content austenitic stainless steel. Figure 2 shows the double U-bend specimen used in the stress corrosion cracking (SCC) test. Schematic diagram, FIG. 3 is a schematic diagram showing the test piece used in the interstitial corrosion test. 1, 6... Test piece, 2... Fluorine resin sleeve, 3, 7... Ti bolt, 4, 8... Ti nut, 5, 10... Welded (TIG lick weld) part, 9
... Fluorine resin washers.

Claims (1)

[Claims] 1. In terms of weight percentage, C: 0.03% or less, Si: 1.5% or less, Mn: 2.0% or less, Cr: 19.0% or more and less than 22.5%, Ni: 21.0% or more and less than 30.0%, Mo: More than 6.0 to 10.0%, One or more of W or Cu: 0.3 to 2.0%, Al: 0.01 to 0.15%, N: 0.05 to 0.30%, P: 0.03% or less, S: 0.002% or less, O: 0.005% or less , the remainder: Fe and other unavoidable impurities, and the composition satisfies the formula Cr (%) + 3Mo (%) + 10N (%) ≧ 38.0, and has excellent corrosion resistance and hot workability. Superior austenitic stainless steel. 2 In terms of weight percentage, C: 0.03% or less, Si: 1.5% or less, Mn: 2.0% or less, Cr: 19.0% or more and less than 22.5%, Ni: 21.0% or more and less than 30.0%, Mo: more than 6.0 to 10.0%, One or more of W or Cu: 0.3~2.0%, Al: 0.01~
0.15%, N: 0.05-0.30%, one or more of B, Mg, La or Ce: 0.001-0.100%, P: 0.03% or less, S: 0.002% or less, O: 0.005% or less, balance: Fe and others An austenitic stainless steel with excellent corrosion resistance and hot workability, which is characterized by a composition that satisfies the formula Cr (%) + 3Mo (%) + 10N (%) ≧ 38.0. .