JPS5929106B2 - High strength austenitic stainless steel - Google Patents

Description

[Detailed Description of the Invention] The present invention is applicable to various plants such as chemical, seawater, nuclear, etc.
This relates to high-strength austenitic stainless steel with excellent corrosion resistance and workability used in machinery, ship machinery, etc.

Austenitic stainless steel is widely used because of its excellent corrosion resistance, heat resistance, workability, and mechanical properties.

Recently, as machines and structures have become larger, there has been a strong demand for improved strength of structural stainless steel. To meet this requirement, the austenitic stainless steel with improved strength is SUS304N1, which is SUS304 with approximately 0.615% N added, and SUS304N1 with Nb added to it.
SUS304N2 with approximately 0.001% addition, and 22C
r-13Ni-5Mn-O, 3N steel, 20Cr-6Ni
-9Mn-O, 3N steel is known. The strength of these steels after solution heat treatment is higher than that of the former two SUS304N1 and S
The yield strength of US304N2 is around 35Kg/MA and the tensile strength is around 75Kg/7m4, which does not necessarily have sufficient strength, and the latter two steels have a yield strength of around 45Kg/MA.
It has a tensile strength of about 80 kg/1, which is almost satisfactory. However, since the latter steel contains a large amount of N (0.3%), it is less susceptible to stress corrosion cracking than SUS3O.
This is a major drawback as a high-strength structural austenitic stainless steel. Furthermore, since the Mn content is high, hot workability is poor. In addition, although it is not directly aimed at high strength, 25Cr-13Ni-0.3N-IMO is a steel with high strength.
Steel, 17Cr-8Ni-8Mn-48i-0.15N
Steel is known.

However, since the former steel is a high Cr steel and the latter steel is a high Si steel, both have the disadvantage of poor hot workability. The present invention overcomes these drawbacks of conventional steels, and as a result of various studies conducted by the present inventors, it has been found that simply increasing the amount of N to increase the strength of austenitic stainless steel results in a decrease in hot workability. The disadvantage is that the susceptibility to stress corrosion cracking is increased.

Therefore, in addition to N, the amount of Si was further increased, and the hot workability of a steel that can obtain a tensile strength of about 80 kg/1 due to the combination of Si and N was investigated. The results show that as the Si content increases, the torsion value increases as shown in Figure 1.
．． 2%, it shows an almost satisfactory value, and Sil. It is almost constant at 5%, and it was found that the workability is improved when Si is used in combination with N. Furthermore, the stress corrosion cracking resistance of steel with the same composition as above was investigated. The results are shown in FIG. 2, and it was found that this improved as the amount of Si increased and the amount of N decreased.

Based on the above results, we have developed a new type of high-strength austenitic stainless steel that utilizes the solid solution strengthening effect of Si and N in addition to N. It was discovered that the workability is improved while maintaining the same strength, and it is possible to suppress the increase in stress corrosion cracking susceptibility.

However, when the amount of Si is increased, the solid solubility limit of N is lowered, and the risk of bubble generation during solidification of the steel ingot increases.

Therefore, as a result of further research, the present inventors found that an appropriate amount of M.
It has been found that this problem can be solved by incorporating n and adjusting the amounts of Cr and Ni so that the amount of segregated delta ferrite during solidification is 2% or less. The steel of the present invention was developed based on these findings.
, is a high-strength austenitic stainless steel that utilizes the solid solution strengthening effect of Si.

By containing MO and Cu in the steel of the present invention, the corrosion resistance can be further improved, and
Addition of small amounts of AI, Ca, Mg, B, and Ce can further improve hot workability.

Furthermore, addition of small amounts of S, Se, and Te improves the stiffness. It is something that can be improved. As described above, the steel of the present invention is a high-strength austenitic stainless steel with excellent corrosion resistance and workability, and is suitable for strength members of various large-scale plants used in harsh environments such as seawater chemistry and nuclear power plants. .

The steel of the present invention will be explained in detail below.

The first invention steel has a weight ratio of CO. Os% or less, Sil
．． 2-3.0%, Mn2.3-5.5%, Ni8.
4-15%, Crl6-22%, NO. 23-0.40
%, and the second invention steel has MOO of the first invention steel.
．． 2-3.0%, CuO. 2w3. The third invention steel further improves the corrosion resistance of the first invention steel by containing one or two of AIO. O4% or less, CaO. O2% or less, MgO. Ol% or less, 80.
01% or less, CeO. The fourth invention steel is one in which the hot workability of the first invention steel is further improved by containing one or more of O5% or less. 1% or less, SeO. 1% or less TeO. The machinability of the first invention steel is improved by containing one or more of AIO. O4% or less,
CaO. O2% or less, MgO. Ol% or less, 80.01
% or less, CeO. The hot workability of the second invention steel is further improved by containing one or more of O5% or less.

The reasons for limiting the composition of the steel of the present invention will be explained below.

Like N, C is an important element that strengthens the base material.

On the other hand, C is an element that significantly impairs corrosion resistance, and 0.08%
If the content exceeds this amount, corrosion resistance and hot workability will be impaired, so the upper limit was set at 0.08%. N is the most important strengthening element in the steel of the present invention, and is a strong austenite-forming element that significantly improves pitting corrosion resistance. In order to exhibit these properties, it must be contained in an amount of 0.23% or more. The lower limit was set at 0.23%, but on the other hand, N is an element that increases stress corrosion cracking susceptibility, and if the content exceeds 0.40%, hot workability will be significantly impaired, so the upper limit was set at 0.4%.
It was set to 0%.

Si is one of the main elements that dissolves in geology and strengthens it, and its Sil% content is NO. It has a strengthening ability equivalent to 8% O content.

Furthermore, Si is an element that improves corrosion resistance, stress corrosion cracking resistance, and susceptibility to cracking. In order to obtain sufficient strength and excellent hot workability, the button must contain 1.2% or more of Si, and the lower limit is 1.2%. %. However, if the content exceeds 3.0%, the solid solubility limit of N will be significantly lowered and the austenite balance will be impaired, so the upper limit was set at 3.0%. Cr is a basic element of stainless steel, and in order to obtain excellent corrosion resistance, it must be contained at least 16% or more.

However, as the amount of Cr increases, the amount of ferrite increases.
If the content exceeds 2%, the austenite balance will be impaired, so the upper limit was set at 22%. Ni is a basic element of austenitic stainless steel, and must be contained in an amount of 8.4% or more in order to obtain excellent corrosion resistance and an austenitic structure.

However, if the content exceeds 15%, hot workability will be impaired.
In addition, since it lowers the solid solubility limit of N, the upper limit was set at 15%. Mn is a main element contained in the steel of the present invention in order to increase the solid solubility limit of N depending on the amounts of Si, Cr, and Ni.

The N and Si contents of the steel of the present invention must be at least 2.3%.

However, if the content exceeds 5.5%, hot workability will be impaired, so the upper limit was set at 5.5%. Both MO and Cu are elements that further improve the corrosion resistance of the steel of the present invention, and in order to exhibit their effects, MO should be contained in an amount of 0.2% or more. , Cu is 0.2%
The above content is necessary.

However, if MO is contained in excess of 3.0%, the austenite balance will be impaired, and if Cu is contained in excess of 3.0%, the strength will be greatly reduced, so the upper limit is MO3. O
%, Cu3. It was set as 0%. B, Ca, AI, Ce, M
Each of g is an element that further improves the hot workability of the steel of the present invention. However, if it is contained in a large amount, it impairs the cleanliness of the steel and even impairs hot workability, so the upper limit is set as A.
IO. O4%, CaO. O2%, MgO. Ol%,BO
．． Ol%, CeO. O was set at 5%. S, Se, and Te are all elements that improve the rigidity of the steel of the present invention, but adding large amounts impairs hot workability, so the upper limit for each is set to 0.
．． It was set at 1%.

Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 1 shows the chemical composition of these test steels.

In Table 1, A1 to A6 are conventional steels, and A1 is SUS
3O4, A2 is SUS3O4Ni, A3 is SUS3O4
N2 and A4 are 22Cr-13Ni-5Mn-0.
3N- 2M0- 0.2V steel, A5 is 17Cr-8
Ni- 8Mn-48i- 0.2N steel, A6 is 25C
It is r-13Ni-0.3N-IMO steel.

B1 to B8 are steels of the present invention, B1 to B3 are steels of the first invention, and B4
~B5 is the second invention steel, B6 is the third invention steel, B7 is the fourth invention steel, and B8 is the fifth invention steel. Table 2 shows the strength, hot working method, and corrosion resistance of the A1 to A6 steels and B1 to B8 steels subjected to the solid solution heat treatment shown in Table 1. Regarding strength, a JI No. 84 test piece was used to measure proof stress, tensile strength, and elongation.

Workability was evaluated by an 80% hot upsetting test, with 10 pieces being tested, and the cracking incidence rate is shown.

Regarding corrosion resistance, pitting corrosion resistance, stress corrosion cracking resistance, and sulfuric acid resistance were evaluated based on JIS test methods.

Pitting corrosion resistance indicates the corrosion loss when immersed in a 4% FeCl solution at 40°C for 24 hours, and stress corrosion cracking resistance indicates the corrosion loss when immersed in a boiling 42% MgCI solution.
It shows the time (minutes) until rupture under a stress of 25k19/1, and the sulfuric acid resistance shows the corrosion loss when immersed in a boiling 5% H2SO4 solution for 6 hours. As is known from Table 2, the conventional steel ゜A1 steel has excellent corrosion resistance and hot workability, but its strength is 24 kg/M4 in yield strength and 61 k9/Mt in tensile strength.
It is as low as lt.

A2 and A3 steels, which are made by adding N or N and Nb to Al steel, have excellent corrosion resistance and hot workability like Al steel.
As for strength, the yield strength is 31k9/M4, the tensile strength is 71k9/MJ, 75kg7m4, which is a considerable improvement compared to Al steel, but the strength is still insufficient as a high-strength austenitic stainless steel. Also,
For A4 steel, yield strength is 48 kg/m/t, tensile strength is 8
It has a satisfactory strength of 5k9/1 and good corrosion resistance, but regarding hot workability, 20% cracking occurred in an 80% hot upsetting test.

For A5 and A6 steel, the strength is 38 to 41 kg and 7 m.
4. Tensile strength is currently 77-78 kg/1, which is insufficient, and hot workability is 10-10 in 80% hot upsetting test.
20% cracking occurred, and the hot workability was also poor.

Furthermore, A4 steel is inferior to Al steel, which is SUS3O4, in terms of stress corrosion resistance, and A4 to A6 steels are still unsatisfactory as high-strength corrosion-resistant structural members. On the other hand, the B1 to B8 steels of the present invention have N, Si, Mn
By appropriately containing Cr and Ni and adjusting the amounts of Cr and Ni, the strength can be satisfied as a high-strength austenitic stainless steel with a yield strength of 40 kg/1 or more and a tensile strength of about 80 kg/1, and hot workability. The corrosion resistance is comparable to that of A1 to A3 steel, and its corrosion resistance is even better than that of A1 to A3 steel. This shows that the steel of the present invention is excellent not only in strength but also in hot workability and corrosion resistance. As mentioned above, the steel of the present invention contains N,
By appropriately containing Si and Mn and adjusting the amounts of Cr and Ni, we have succeeded in obtaining high strength without impairing the hot workability and corrosion resistance of SUS3O4, and it can be used in harsh environments such as seawater and chemical nuclear power. It has extremely high practicality as a high-strength, corrosion-resistant structural member for various large-scale plants used in various operating environments.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of Si and N on hot workability, and FIG. 2 is a diagram showing the influence of Si and N on stress corrosion cracking resistance.

Claims (1)

[Claims] 1. C 0.08% or less, Si 1.2-3.
0%, Mn2.3-5.5%, Ni8.4-15%, C
A high-strength austenitic stainless steel containing 16 to 22% r, 0.23 to 0.40% N, and the remainder consisting of Fe and impurity elements. 2 C0.08% or less in weight ratio, Si1.2-3.
0%, Mn2.3-5.5%, Ni8.4-15%, C
Contains r16-22%, N0.23-0.40%, further contains one or two of Mo0.2-3.0%, Cu0.2-3.0%, and the balance is Fe and High-strength austenitic stainless steel characterized by being composed of impurity elements. 3 C 0.08% or less, Si 1.2-3.
0%, Mn2.3-5.5%, Ni8.4-15%, C
Contains r16-22%, N0.23-0.40%, furthermore, B0.01% or less, Ca0.02% or less, Al0
．． A high-strength austenitic stainless steel characterized by containing one or more of 0.03% or less, 0.05% or less of Ce, and 0.01% or less of Mg, with the remainder consisting of Fe and impurity elements. 4 Weight ratio C: 0.08% or less, Si: 1.2-3.
0%, Mn2.3-5.5%, Ni8.4-15%, C
Contains r16-22%, N0.23-0.40%, and further contains S0.1% or less, Se0.1% or less, Te0.1
A high-strength austenitic stainless steel characterized by containing one or more of the following: % or less, with the remainder consisting of Fe and impurity elements. 5 C 0.08% or less, Si 1.2-3.
0%, Mn2.3-5.5%, Ni8.4-15%, C
Contains r16-22%, N0.23-0.40%, and further contains one or two of Mo0.2-3.0%, Cu0.2-3.0%, and B0.01% or less. , Ca0.0
2% or less, Al 0.04% or less, Ce 0.05% or less,
A high-strength austenitic stainless steel characterized by containing one or more of Mg 0.01% or less, with the remainder consisting of Fe and impurity elements.