JP2011195880A - Austenitic stainless steel - Google Patents

Abstract

An austenitic stainless steel having excellent creep strength at a high temperature is provided.
[Solution]
The austenitic stainless steel according to the present embodiment is in mass% and contains Cr: 17-19%, Ni: 30-32%, Nb: 3.0-3.6%, with the balance being Fe and impurities. . In austenitic stainless steel according to this embodiment, the aging treatment at a high temperature range of not lower than 700 ° C., _Ni 3 Nb or _Fe 2 NB 40 are precipitated in the grain of the crystal grains 30, _Fe 2 NB 20 is deposited on the grain boundaries 10 . These intermetallic compounds (Ni ₃ Nb and Fe ₂ Nb) improve the creep strength in a high temperature range of 700 ° C. or higher.
[Selection] Figure 2

Description

  The present invention relates to an austenitic stainless steel, and more particularly to an austenitic stainless steel used for high-temperature pressure vessels, boilers, power generation turbine blades, chemical industrial plant equipment, and the like.

  Chemical industrial plant equipment, high-temperature pressure vessels, boilers, power generation turbines, etc. handle high-temperature and high-pressure fluids. The structural members of these facilities are made of heat-resistant steel. The heat-resistant steel material is, for example, a steel pipe, a steel plate, a steel bar, a forged steel product, a cast steel product, or the like.

  Conventionally, 18-8 austenitic stainless steel has been used as a heat resistant steel material. Examples of the 18-8 austenitic stainless steel include SUS304H, SUS316H, SUS321H, and SUS347H in JIS standards.

  Heat-resistant steel materials for chemical industrial plant equipment, high-temperature pressure vessels, boilers, and turbines are required to improve high-temperature strength, and in particular, to improve creep strength at a high temperature of 700 ° C. or higher.

  JP-A-62-133048 (Patent Document 1), JP-A-7-138708 (Patent Document 2), JP-A-8-13102 (Patent Document 3), JP-A 2004-323937 (Patent Document) 4) and Japanese Patent Application Laid-Open No. 2004-323937 (Patent Document 5) disclose austenitic stainless steel for the purpose of improving high-temperature strength. The austenitic stainless steel disclosed in these documents contains Nb, C, and N. These elements form carbides, nitrides and carbonitrides and improve the high temperature strength of the steel. The austenitic stainless steel disclosed in these documents further contains Cu. At high temperatures, Cu precipitates in the austenite matrix and improves the high temperature strength of the steel.

JP-A-62-133048 JP 7-138708 A JP-A-8-13102 JP 2004-323937 A JP 2004-3000 A JP 2005-2929 A

Naoki Takada, Takashi Matsuo, Masao Takeyama, “Structure of Fe2Nb Laves Phase and Its Role as Strengthening Phase of Austenitic Heat-Resistant Steel”, Japan Society for the Promotion of Science Refractory Metal Materials 123rd Committee Report Vol. 50 No. 3 (2009), p. 389-400

  Recently, in order to improve the power generation efficiency of a thermal power plant, there is a demand for a heat resistant steel material with further improved creep strength at high temperatures.

  An object of the present invention is to provide an austenitic stainless steel having excellent creep strength at high temperatures.

Means for Solving the Problems and Effects of the Invention

  The austenitic stainless steel according to the present invention contains, in mass%, Cr: 17 to 19%, Ni: 30 to 32%, Nb: 3.0 to 3.6%, and the balance consists of Fe and impurities.

  The austenitic stainless steel described above has excellent creep strength at a high temperature of 700 ° C. or higher.

  Further, in the austenitic stainless steel, one or more of C, Si, Mn, P, S, Mo, W, and N as impurities are contained in mass%, and C: less than 0.01%. Si: less than 0.1%, Mn: less than 0.1%, P: less than 0.010%, S: less than 0.003%, Mo: less than 0.01%, W: less than 0.01%, and N: You may contain less than 0.01%.

  Further, the austenitic stainless steel may contain B: 0.0001 to 0.0030% in mass% instead of part of Fe.

  Moreover, the austenitic stainless steel is replaced by a part of Fe, and in mass%, Al: 0.001 to 0.10%, Mg: 0.0001 to 0.010%, and Ca: 0.0001 to You may contain 1 type, or 2 or more types selected from 0.010%.

In the chemical components of the austenitic stainless steel is a diagram showing a Nb content, the relationship between the deposition amount of _Fe 2 Nb and _Ni 3 Nb. (A) is the SEM image of the austenitic stainless steel by this Embodiment, (B) is the schematic diagram. It is a schematic diagram for explaining the grain boundary coverage of Fe 2 _Nb. (A) is an SEM image of austenitic stainless steel according to the present embodiment containing boron, and (B) is a schematic diagram thereof. It is a SEM image of several austenitic stainless steel manufactured in the Example. And aging time of the plurality of austenitic stainless steels shown in FIG. 5 is a diagram showing a grain boundary coverage of Fe 2 _Nb. Of austenitic stainless steel in the embodiment, a diagram showing the relationship between the grain boundary coverage and creep rate of the Fe 2 _Nb. It is a stress-creep rupture time diagram in 700 degreeC of the austenitic stainless steel in an Example. It is a figure which shows the breaking elongation and fracture | rupture drawing of austenitic stainless steel obtained by the creep test. It is a figure which shows the crack length after hot processing of the austenitic stainless steel in an Example.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. Moreover, in the following description,% regarding an element means the mass%.

  As a result of research, the present inventors have obtained the following knowledge.

  (A) Conventional austenitic stainless steel used for heat-resistant steel materials improves creep strength by carbides, nitrides typified by NbCrN composite nitrides, and carbonitrides. However, carbides, nitrides, and carbonitrides are difficult to improve long-term creep strength at high temperatures. More specifically, carbides, nitrides, and carbonitrides are unlikely to contribute to the improvement of the 100,000 hour creep rupture strength at 700 ° C. or higher.

  (B) The conventional austenitic stainless steel for heat-resistant steel further improves the strength due to the co-precipitation Cu phase. The Cu phase can improve high temperature strength in a short time. However, it is difficult to contribute to the improvement of the long-term creep strength at high temperatures.

(C) In conventional austenitic stainless steel, an Fe ₂ W-type Laves phase is mainly precipitated. Fe ₂ W improves the short-time creep strength at high temperatures. However, when heated at a high temperature for a long time, Fe ₂ W aggregates and becomes coarse. Therefore, Fe ₂ W is unlikely to contribute to improvement of long-term creep strength at high temperatures.

(D) If Ni ₃ Nb and Fe ₂ Nb-type Laves phases precipitate in the steel instead of carbide, nitride and carbonitride, the long-term creep strength at high temperature is improved. More specifically, Ni ₃ Nb precipitates in the crystal grains and improves the strength in the grains. On the other hand, Fe ₂ Nb precipitates at the grain boundary and improves the strength of the grain boundary.

  (E) Al, Mg, and Ca improve the hot workability of austenitic stainless steel.

  The austenitic stainless steel according to the present embodiment is based on the above knowledge. Hereinafter, details of the austenitic stainless steel according to the present embodiment will be described.

[Chemical composition]
The austenitic stainless steel according to the present embodiment has the following chemical composition.

Cr: 17-19%
Chromium (Cr) improves the oxidation resistance, steam oxidation resistance, and hot corrosion resistance of steel. If the Cr content is too small, effective oxidation resistance and high temperature corrosion resistance cannot be obtained in a high temperature range of 700 ° C. or higher. On the other hand, if there is too much Cr content, ductility will fall. Therefore, the Cr content is 17 to 19%.

Ni: 30-32%
Nickel (Ni) stabilizes the austenite phase. That is, Ni is an austenite generating element. On the other hand, Cr and Nb are ferrite forming elements. The Ni content is defined corresponding to the Cr content and the Nb content so that the austenite phase is stabilized.
Furthermore, the precipitation amount of Ni ₃ Nb and Fe ₂ Nb is related to the Ni content. If the Ni content is too small, the amount of Ni ₃ Nb deposited is small. On the other hand, if the Ni content is too large, the amount of Ni ₃ Nb deposited becomes excessive, and the amount of Fe ₂ Nb deposited decreases. If the precipitation amount of Fe ₂ Nb decreases, the long-term creep strength at high temperatures decreases. Therefore, the Ni content is 30 to 32%.

Nb: 3.0-3.6%
Niobium (Nb) combines with Ni to form Ni ₃ Nb. Niobium further combines with Fe to form Fe ₂ Nb. These intermetallic compounds (Ni ₃ Nb and Fe ₂ Nb) depend on the Nb content.
As described above, Ni ₃ Nb precipitates in the crystal grains and strengthens the grains, and Fe ₂ Nb preferentially precipitates at the grain boundaries and strengthens the grain boundaries. Therefore, in order to strengthen the crystal grains and the grain boundaries in the high temperature region of 700 ° C. or higher, Ni ₃ Nb is precipitated in the steel and Fe ₂ Nb is precipitated in the high temperature region. preferable. More preferably, in a high temperature range of 700 ° C. or higher, the amount of Fe ₂ Nb deposited is preferably substantially equal to or greater than the amount of Ni ₃ Nb deposited.

If the Nb content is 3.0% or more, the precipitation amount of Fe ₂ Nb becomes substantially equal to or more than the precipitation amount of Ni ₃ Nb in the temperature range of 700 ° C. or more.

FIG. 1 is a diagram showing the amounts of Ni ₃ Nb and Fe ₂ Nb deposited at 700 ° C. when the Nb content is changed in the chemical composition of the austenitic stainless steel according to the present embodiment. Referring to FIG. 1, the horizontal axis represents the Nb content (% by mass). The vertical axis indicates the amount of precipitated _Ni 3 Nb and _Fe 2 Nb (volume%). The solid line L1 in the figure indicates the amount of Fe ₂ Nb deposited. A solid line L2 in the figure indicates the amount of Ni ₃ Nb deposited.

Referring to FIG. 1, Ni ₃ Nb (solid line L2) increases rapidly as the Nb content increases from 0%. However, when the Nb content exceeds 1.5%, even if the Nb content increases, Ni ₃ Nb does not precipitate so much. On the other hand, Fe ₂ Nb (solid line L1) does not precipitate until the Nb content exceeds 1.5%. When the Nb content exceeds 1.5%, with the increase of the Nb content, also increases the amount of precipitated _Fe 2 Nb.

If the Nb content is 3.0% or more, the precipitation amount of Fe ₂ Nb with respect to the total precipitation amount of Fe ₂ Nb and Ni ₃ Nb (hereinafter referred to as Fe ₂ Nb phase fraction) is 45% or more. Become. That is, the precipitation amount of Fe ₂ Nb becomes substantially equal to or more than the precipitation amount of Ni ₃ Nb.

On the other hand, if Nb is contained excessively, undissolved precipitates after solution treatment increase, and the toughness of steel decreases. Therefore, the Nb content is 3.0 to 3.6%. If the Nb content is 3.0 to 3.6%, as shown in FIG. 1, the phase fraction of Fe ₂ Nb is 45 to 60%, and the precipitation amount of Fe ₂ Nb is Ni ₃ Nb. It becomes substantially equivalent to the amount of precipitation.

  The balance is Fe and impurities. Impurities are mixed into steel due to various factors in the manufacturing process, including raw materials such as ore or scrap, when industrially producing stainless steel.

  The austenitic stainless steel according to the present embodiment may contain one or more of a plurality of elements (C, Si, Mn, P, S, Mo, W, N) as impurities. However, it is preferable that the content of impurities is small. Therefore, the content of the above-mentioned impurities is preferably limited as follows.

C: Less than 0.01% In the present embodiment, carbon (C) is an impurity. C combines with Ni, Nb, and Cr to form a carbide. When steel is heated at a high temperature for a long time, the carbides become agglomerated and coarsened. The strength of the crystal grains and the grain boundaries is reduced by the coarsened carbides.
C further inhibits the production of Ni ₃ Nb and Fe ₂ Nb by binding to Ni and Nb. In particular, if the C content is large, the generation of Fe ₂ Nb is inhibited, and the high-temperature strength of the crystal grain boundaries is lowered.

  From the above, C decreases the long-term creep strength at high temperatures. Therefore, in the austenitic stainless steel in the present embodiment, it is preferable that the C content is small. The C content is less than 0.01%. The preferred C content is less than 0.005%.

Si: less than 0.1% In the present embodiment, Si is an impurity. Si combines with Ni to form silicide. Silicide is a very brittle intermetallic compound. Therefore, silicide embrittles the steel. Therefore, the austenitic stainless steel in the present embodiment preferably has a lower Si content. Si content is less than 0.1%. The preferred Si content is less than 0.05%.

Mn: less than 0.1% In the present embodiment, manganese (Mn) is an impurity. Mn promotes the generation of the σ phase. Since the σ phase is a brittle intermetallic compound, it embrittles steel. Therefore, the austenitic stainless steel in the present embodiment preferably has a smaller Mn content. The Mn content is less than 0.1%. The preferred Mn content is less than 0.05%.

P: Less than 0.01% Phosphorus (P) is an impurity. P decreases hot workability and ductility. Therefore, it is preferable that the P content is small. The P content is less than 0.01%.

S: Less than 0.003% Sulfur (S) is an impurity. S decreases hot workability. Therefore, it is preferable that the S content is small. The S content is less than 0.003%.

Mo: less than 0.01% In the present embodiment, molybdenum (Mo) is an impurity. Mo forms an Fe ₂ Mo type Laves phase and inhibits the formation of Fe ₂ Nb. Therefore, in the austenitic stainless steel in the present embodiment, it is preferable that the Mo content is smaller. Mo content is less than 0.01%.

W: Less than 0.01% In the present embodiment, tungsten (W) is an impurity. As described above, W forms an Fe ₂ W type Laves phase. When steel is heated at a high temperature for a long time, Fe ₂ W becomes coarse and the creep strength is lowered. W further inhibits the production of Fe ₂ Nb. Therefore, in the austenitic stainless steel in the present embodiment, it is preferable that the W content is small. W content is less than 0.01%.

N: Less than 0.01% Nitrogen (N) combines with Nb to form Nb nitride. When steel is heated at a high temperature for a long time, the nitrides become agglomerated and coarsened. Coarse nitrides reduce the creep strength of steel. N further binds to Nb and thus inhibits the generation of Ni ₃ Nb and Fe ₂ Nb. Therefore, in the austenitic stainless steel in the present embodiment, it is preferable that the N content is small. The N content is less than 0.01%, more preferably less than 0.005%.
Another impurity is oxygen (O). O reduces hot workability. Therefore, it is preferable that the O content is as small as possible.

  The austenitic stainless steel according to the present embodiment may further contain B instead of a part of Fe.

B: 0.030% or less Boron (B) promotes the formation of Fe ₂ Nb at the grain boundaries and increases the amount of Fe ₂ Nb deposited. Therefore, B improves long-term creep strength at high temperatures. On the other hand, if there is too much boron content, the toughness of steel will fall. When boron is contained excessively, the solution temperature of boride generated during the manufacturing process is increased. Therefore, undissolved boride remains after the solution treatment. Undissolved boride reduces the toughness of the steel. Therefore, the boron content is 0.030% or less. If the boron content is 0.0001% or more, the above effect can be obtained particularly effectively.

  The austenitic stainless steel according to the present embodiment further contains one or more selected from Al, Mg, and Ca in place of part of Fe. Al, Mg and Ca improve the hot workability of steel.

sol. Al (acid-soluble Al): 0.10% or less Aluminum (Al) deoxidizes steel. Al further fixes O and S to improve the hot workability of steel. On the other hand, if there is too much Al content, toughness will fall. Therefore, Sol. The Al content is 0.10% or less. Sol. If the Al content is 0.0001% or more, the above effect can be obtained particularly effectively.

Mg: 0.010% or less Magnesium (Mg), like Al, deoxidizes steel and improves hot workability of steel. If there is too much Mg content, the toughness of steel will fall. Therefore, the Mg content is 0.010% or less. If the Mg content is 0.0001% or more, the above effect can be obtained particularly effectively.

Ca: 0.010% or less Calcium (Ca), like Al and Mg, deoxidizes steel and improves hot workability of steel. On the other hand, if there is too much content of these elements, the toughness of steel will fall. Therefore, the Ca content is 0.010% or less. If the Ca content is 0.0001% or more, the above effect can be obtained particularly effectively.

[Production method]
An example of the manufacturing method of the austenitic stainless steel by this Embodiment is demonstrated.
Molten steel having the above chemical composition is produced by melting in a blast furnace or electric furnace. The manufactured molten steel is subjected to a known degassing treatment as necessary.

  Next, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, a slab, bloom or billet. Alternatively, molten steel is made into an ingot by the ingot-making method. A continuously cast material or an ingot is hot-worked by a known method to obtain an austenitic stainless steel material. Examples of the austenitic stainless steel material include a steel pipe, a steel plate, a steel bar, and a forged steel.

Solution treatment is performed on the manufactured austenitic stainless steel material. The solution treatment is performed by a known method. The solution treatment temperature (solution treatment temperature) is, for example, 1000 to 1300 ° C. The solution treatment time is, for example, 0.1 to 2 hours.
A well-known aging treatment may be applied to the solution-treated austenitic stainless steel. Moreover, a cast steel product is manufactured by casting molten steel using a desired mold.

  Through the above steps, the austenitic stainless steel according to the present embodiment is manufactured.

[Organization]
When the austenitic stainless steel according to the present embodiment is heated at a high temperature of 700 ° C. or higher for a predetermined time, Fe ₂ Nb precipitates at the grain boundaries in the steel, and Ni ₃ Nb precipitates within the crystal grains.

FIG. 2A is an SEM image (reflected electron image) of an austenitic stainless steel (not containing B) according to the present embodiment that was aged at 800 ° C. for 240 hours. FIG. 2B is a schematic diagram of FIG. Referring to FIGS. 2A and 2B, a plurality of Fe ₂ Nb 20 is generated on the grain boundary 10. Fe ₂ Nb20 covers a part of the grain boundary. Further, a plurality of Ni ₃ Nb or Fe ₂ Nb 40 is generated in the crystal grain 30.
As described above, in the austenitic stainless steel according to the present embodiment, precipitation of carbide, carbonitride and nitride is suppressed, and Fe ₂ Nb and Ni ₃ Nb are precipitated. The grain boundary is strengthened by Fe ₂ Nb, and the interior of the crystal grains is strengthened mainly by Ni ₃ Nb. Therefore, the austenitic stainless steel according to the present embodiment improves the long-term creep strength at a high temperature.

Here, the ratio of the length of the grain boundary covered with Fe ₂ Nb to the length of the crystal grain boundary is defined as the grain boundary coverage (%). The grain boundary coverage is calculated by the following method.

Samples are taken from any location of austenitic stainless steel. From the collected sample, an area of 30 μm ² is observed in five visual fields. A scanning electron microscope (SEM) is used for observation. FIG. 3 is a schematic diagram of the observed region. The total length L of the crystal grain boundary in the region is measured. Then, measuring the length A1~An of each grain boundary part covered by the _Fe 2 Nb (total n pieces).

  Based on the obtained L and A1 to An, the grain boundary coverage (%) in each region (five in total) is obtained based on the following formula (1).

Grain boundary coverage = (A1 + A2 + A3 +... + An) / L (1)
The average of the obtained five grain boundary coverages is defined as the grain boundary coverage ρ (%) of Fe ₂ Nb in the present embodiment.

When heated at 700 ° C. or higher for 500 hours or longer, the grain boundary coverage of Fe ₂ Nb in the steel is 50% or higher. As described above, a large amount of Fe ₂ Nb is formed at the grain boundaries by suppressing the formation of carbides, nitrides, and carbonitrides. Therefore, the strength of the crystal grain boundary is improved, and the long-term creep strength at high temperature is improved. More specifically, the 100,000 hour creep rupture strength at 700 ° C. is 75 MPa or more.

If B is contained, the grain boundary coverage of Fe ₂ Nb further increases. 4A is an SEM image (reflected electron image) of austenitic stainless steel containing B, and FIG. 4B is a schematic diagram of FIG. 4A. As shown in FIGS. 4A and 4B, the inclusion of B causes more Fe ₂ Nb20 to precipitate on the grain boundaries 10 than in FIG. 2A. When heated at 700 ° C. or higher for 500 hours or longer, the grain boundary coverage of Fe ₂ Nb in the austenitic stainless steel containing B according to the present embodiment is 60% or higher.

The higher the grain boundary coverage of Fe ₂ Nb, the longer the creep strength at high temperatures. Therefore, preferably, the austenitic stainless steel according to the present embodiment contains B in the above range.

Fe ₂ Nb formed at the grain boundaries further improves the ductility of the steel. Similar to the long-term creep strength at high temperature, the higher the grain boundary coverage of Fe ₂ Nb, the better the ductility.

A plurality of austenitic stainless steels having the chemical composition shown in Table 1 were produced. The grain boundary coverage of Fe ₂ Nb in the manufactured austenitic stainless steel was investigated.
Subsequently, the relationship between grain boundary coverage and long-term creep strength at high temperatures was investigated. Furthermore, the relationship between grain boundary coverage and hot workability was investigated.

  Referring to Table 1, the chemical composition of the marks A to C, T1 and T2 steel was within the scope of the present invention. On the other hand, the chemical compositions of marks D, E and T3 were outside the scope of the present invention. Specifically, the C content of marks D and E exceeded the upper limit of the present invention. The mark T3 had a chemical composition of steel corresponding to JIS standard SUS347H. The C content, Si content, Mn content and P content of the mark T3 exceeded the upper limit of the present invention. Further, the Ni content and the Nb content of the mark T3 were less than the lower limit of the present invention.

[Investigation method]
The steel of each mark was manufactured by the following method. Specifically, the steel of each mark was melted by a high-frequency vacuum melting furnace to produce a cylindrical ingot. The outer diameter of each mark ingot was 180 mm, and the weight was 50 kg.

  Each ingot was soaked at 1100 ° C. for 2 hours. The soaked ingot was hot-worked to produce a plate material having a thickness of 50 mm and a width of 100 mm. The obtained plate was heated at 1100 ° C. The heated plate was hot rolled to produce a steel plate having a thickness of 15 mm. The obtained steel plate was solution treated. Specifically, the obtained steel plate was soaked at 1250 ° C. for 2 hours and then cooled with water. The steel plate manufactured by the above process substantially had an austenite single phase structure.

A sample was taken from the solution treated steel sheet. An aging treatment was performed on the collected samples. The aging temperature was 800 ° C., and the aging time was 1200 hours. The sample after the aging treatment was observed with an SEM. Then, the grain boundary coverage ρ of Fe ₂ Nb was obtained by the method described above. Moreover, the creep rupture test was implemented with respect to the manufactured steel plate. The creep rupture test was performed based on JIS Z2271. The test temperature was 700 ° C. and the load was 120 MPa.

[Investigation result]
FIG. 5 is an SEM image of steel of marks A to E after aging treatment. “A” in FIG. 5 is an SEM image of the steel of the mark A. Similarly, in the figure, “b” is the mark B, “c” is the mark C, “d” is the mark D, and “e” is the SEM image of the mark E. FIG. 6 is a diagram showing the relationship between the aging time at 800 ° C. and the grain boundary coverage of Fe ₂ Nb. A curve MarkA in FIG. 6 indicates the test result of the mark A. Similarly, the curves MarkB to MarkE indicate the test results of the marks B to E, respectively.

5 and 6, the chemical compositions of the steels of marks A to C were within the scope of the present invention. Therefore, the grain boundary coverage when the aging time is 500 hours or more was 50% or more. On the other hand, the steels of marks D and E contained C in excess of the present invention. Therefore, the grain boundary coverage when the aging time is 500 hours or more was less than 50%. As shown in Table 1, the C content of the mark D was 0.042%, and the C content of the mark E was 0.11%. Therefore, the grain boundary coverage decreased as the C content increased.
Although not shown in the figure, the grain boundary coverage of mark T3 containing C in excess of the present invention at an aging time of 500 hours or more was 25.7%.

  With reference to marks A to C, the grain boundary coverage of marks B and C was higher than that of mark A. Further, the grain boundary coverage of the mark C was higher than that of the mark B. From the above, the grain boundary coverage of the steel having the chemical composition of the present invention was higher as the B content was higher.

FIG. 7 is a diagram showing the relationship between the grain boundary coverage of steel of marks T1 and T2 and the creep speed. The creep rate was obtained by a creep rupture test. The grain boundary coverage shown on the horizontal axis of FIG. 7 is the grain boundary coverage of the marks T1 and T2 when subjected to aging treatment at 700 ° C. for 500 hours (a test is performed separately from the creep rupture test).
Point P2 _T1 is the creep speed of the mark T1 at a test time of 2000 hours in the creep rupture test. Point P3 _T1 is the creep speed of the mark T1 at the test time of 3000 hours. Point P4 _T1 is the creep speed of the mark T1 at the test time of 4000 hours.
Similarly, the point P2 _T2 is the creep speed of the mark T2 at the test time of 2000 hours. Point P3 _T2 is the creep speed of the mark T2 at the test time of 3000 hours. Point P4 _T2 is the creep speed of the mark T2 at the test time of 4000 hours.
The grain boundary coverage of the mark T1 was 55%. Therefore, in FIG. 7, the point P2 _T1 , the point P3 _T1, and the point P4 _T1 are plotted on the grain boundary coverage = 55%. The grain boundary coverage of the mark T2 was 75%. Therefore, in FIG. 7, the point P2 _T2 , the point P3 _T2, and the point P4 _T2 are plotted on the grain boundary coverage = 75%.

As described above, the grain boundary coverages of the steels of the marks T1 and T2 were both 50% or more. Furthermore, the grain boundary coverage of the mark T2 was higher than that of the mark T1. It is considered that the mark T2 had a higher B content than the mark T1.
Further, referring to FIG. 7, the creep rate of mark T2 was lower than that of mark T1 when the test time was 2000 hours, 3000 hours, or 4000 hours. That is, the mark T2 has a higher creep resistance than the mark T1.

  FIG. 8 is a stress-creep rupture time diagram of marks T1 to T3. A line segment CT1 in the figure is a test result of the mark T1. The line segment CT2 is the test result of the mark T2, and the line segment CT3 is the test result of the mark T3. Referring to FIG. 8, line segments CT1 and CT2 were higher than line segment CT3. Therefore, the creep rupture strength of the marks T1 and T2 at the test time of 100,000 hours was higher than that of the mark T3. Furthermore, the line segment CT2 was higher than the line segment CT1. Therefore, from FIGS. 7 and 8, it was found that the creep rupture strength of the mark T2 is higher than that of the mark T1.

  FIG. 9 is a diagram illustrating elongation at break and drawing at break in a creep rupture test of marks T1 and T2. Referring to FIG. 9, both the breaking elongation and the breaking drawing of mark T2 were higher than those of mark T1. That is, the mark T2 had a ductility superior to that of the mark T1.

The following results were derived based on FIGS. In the austenitic stainless steel, when the C content is small, the grain boundary coverage of Fe ₂ Nb is high (see FIGS. 5 and 6). And if it was in the range of this invention, the grain-boundary coverage was 50% or more when it heated at 700 degreeC or more for 500 hours (refer FIG. 6). Furthermore, the larger the B content, the higher the grain boundary coverage (see FIGS. 6 and 7). Furthermore, the higher the grain boundary coverage, the higher the long-term creep strength at high temperatures (see FIGS. 6 to 8). When the grain boundary coverage was 50% or more, the 100,000 hour creep strength at 700 ° C. was higher than that of the conventional austenitic stainless steel (see FIG. 8). Specifically, the 100,000 hour creep rupture strength of the austenitic stainless steel of the present invention (marks T1 and T2) was 75 MPa or more.

  Furthermore, in the steel having a chemical composition within the scope of the present invention, the higher the B content, the better the ductility was obtained (see FIG. 9).

  Using steel having the chemical composition of the present invention, the influence of Al and Ca on hot workability was investigated.

Steels with marks T10 to T12 shown in Table 2 were prepared.

  Referring to Table 2, all the chemical compositions of the marks T10 to T12 were within the scope of the present invention. The mark T10 did not contain Al and Ca. The mark T11 contained 0.04% Al. Moreover, the mark T12 contained 0.0009% of Ca.

[Investigation method]
Steels with marks T10 to T12 were melted in a high frequency vacuum melting furnace to produce an ingot. The shape of each mark ingot was as follows. The outer diameter of the bottom of the ingot was 140 mm, and the outer diameter of the top of the ingot was 180 mm. The height was 440 mm.
Each ingot was soaked at 1100 ° C. for 2 hours. Hot forging the soaked ingot,
A round bar with an outer diameter of 100 mm was produced.
The surface of the manufactured round bar was visually observed. And the length of the circumferential direction of the crack which was able to be confirmed visually was measured. For each mark, the total length of the observed cracks was determined.

[Investigation result]
FIG. 10 is a diagram showing the total crack length (mm) of the marks T10 to T12. Referring to FIG. 10, the total crack length of marks T11 and T12 was significantly smaller than that of mark T10. Since the marks T11 and T12 contain Al or Ca, the hot workability was superior to the mark T10.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

10 Grain boundary 20 Fe ₂ Nb type Laves phase 30 Crystal grain 40 Ni ₃ Nb or Fe ₂ Nb

Claims (4)

% By mass
Cr: 17 to 19%,
Ni: 30 to 32%,
Nb: 3.0 to 3.6%,
The balance consists of Fe and impurities,
Austenitic stainless steel with excellent creep strength. The austenitic stainless steel according to claim 1,
One or more of C, Si, Mn, P, S, Mo, W, and N as impurities are expressed by mass%, C: less than 0.01%, Si: less than 0.1%, Mn: less than 0.1%, P: less than 0.010%, S: less than 0.003%, Mo: less than 0.01%, W: less than 0.01%, and N: less than 0.01% Austenitic stainless steel. The austenitic stainless steel according to claim 1 or 2,
Instead of a part of the Fe, in mass%,
B: Austenitic stainless steel containing 0.0001 to 0.0030%. The austenitic stainless steel according to any one of claims 1 to 3,
Instead of a part of the Fe, in mass%,
Al: 0.001 to 0.10%,
An austenitic stainless steel containing one or more selected from Mg: 0.0001 to 0.010% and Ca: 0.0001 to 0.010%.