JP7167707B2 - Austenitic heat resistant steel - Google Patents

Description

The present invention relates to austenitic heat resistant steel.

18-8 series austenitic stainless steels such as SUS304H, SUS316H, SUS321H and SUS347H have been used as equipment materials in boilers for thermal power generation, chemical plants, etc., which are used in high-temperature environments.

However, in recent years, construction of new ultra-supercritical pressure boilers in which the temperature and pressure of steam are increased for higher efficiency is underway all over the world. The operating conditions of devices under such high-temperature environments have become extremely severe, and along with this, the performance requirements for the materials used have become stricter. The 18-8 series austenitic stainless steels that have been used in the past are remarkably deficient in high-temperature strength, particularly creep rupture strength, steam oxidation resistance, and high-temperature corrosion resistance, in addition to corrosion resistance.

Therefore, austenitic stainless steels containing about 20% or more of Cr have been developed with improved creep rupture strength by adding optimum amounts of various elements. Recently, however, in the field of boilers for thermal power generation, for example, plans to increase the steam temperature to 700° C. or higher have been promoted. In this case, the temperature of the members used will far exceed 700°C. Therefore, even newly improved austenitic stainless steels have insufficient creep rupture strength and corrosion resistance.

In order to solve the above problems, various studies have been made so far. For example, Patent Document 1 discloses an austenitic stainless steel containing more than 20% and less than 28% Cr. The document describes that the creep rupture strength, creep rupture ductility, and hot workability were improved by adding Cu, Nb, and N in combination and controlling P and O according to the Cu content. It is

In Patent Document 2, by limiting the content of P, S, Sn, Sb, Pb, Zn and As in an austenitic stainless steel containing 15 to 30% Cr, embrittlement resistance in the weld heat affected zone is improved. It is disclosed that the cracking resistance is improved. In Patent Document 3, in an austenitic stainless steel containing more than 22% and less than 30% of Cr, by limiting the content of P, S, Sn, Sb, Pb, Zn and As, processing of aged material improved performance.

Patent Document 4 discloses an austenitic stainless steel containing 20 to 27% Cr and having excellent creep rupture strength, steam oxidation resistance, and the like. Patent Document 5 discloses that austenitic stainless steel containing 18.0 to 26.0% Cr can obtain high creep rupture strength by adding Mo, W and N in combination. .

Patent Document 6 discloses that in a heat-resistant austenitic steel containing 18 to 23% Cr, excellent high-temperature strength and post-aging toughness can be obtained by stipulating the austenite balance. Patent Document 7 discloses that an excellent creep rupture strength can be obtained by optimizing the added amounts of Mo, W, and Nb in an austenitic heat-resistant steel containing 18 to 23% Cr. there is

JP-A-2004-323937 Japanese Patent No. 4258678 JP 2009-084606 A Japanese Patent Publication No. 2002-537486 JP 2012-001749 A JP 2013-044013 A JP 2013-067843 A

However, it has been clarified that in steels in which the improvement of creep rupture strength is emphasized, the creep rupture ductility on the high temperature long time side may be rather low. Therefore, it has been found that the prior art may not be sufficient from the viewpoint of high creep rupture strength and creep rupture ductility. There is also room for improvement in weld crack resistance, which is essential when used as a structure.

It is an object of the present invention to solve the above problems and to provide an austenitic heat-resistant steel having excellent long-term structural stability, high creep rupture strength and creep rupture ductility, and good resistance to weld cracking. aim.

The present invention has been made to solve the above problems, and the gist thereof is the heat-resistant austenitic steel described below.

(1) in mass %,
C: 0.02 to 0.12%,
Si: 0.01 to 0.40%,
Mn: 0.10-1.50%,
P: 0.040% or less,
S: 0.020% or less,
Cr: 20.0 to 24.0%,
Ni: more than 26.0% and 35.0% or less,
W: 2.5 to 7.0%,
Nb: 0.01 to 0.50%,
V: 0.01 to 1.0%,
REM: 0.005 to 0.060%,
B: 0.0005 to 0.0050%,
Al: 0.015-0.30%,
N: more than 0.13% and 0.35% or less,
Mo: 0.30% or less,
Cu: 0.30% or less,
Ti: less than 0.010%,
Co: 0 to 10.0%,
Mg: 0-0.050%,
Ca: 0-0.050%,
Zr: 0 to 0.10%,
Hf: 0-1.0%,
Ta: 0 to 1.0%,
Re: 0 to 5.0%,
balance: Fe and impurities,
satisfying the following formulas (i) and (ii),
Austenitic heat resistant steel.
6.0≤4V+2Nb+W+15N-6Si≤9.0 (i)
0.030≦15B+REM≦0.075 (ii)
However, the element symbol in the above formula represents the content (% by mass) of each element.

(2) the chemical composition, in mass %,
Mg: 0.0005-0.050%,
Ca: 0.0005 to 0.050%,
Zr: 0.005 to 0.10%,
Hf: 0.005 to 1.0%,
Ta: 0.01 to 1.0%, and
Re: 0.01 to 5.0%,
containing one or more selected from
The austenitic heat-resistant steel according to (1) above.

According to the present invention, it is possible to obtain an austenitic heat-resistant steel that has excellent long-term structural stability, high creep rupture strength and creep rupture ductility, and excellent weld crack resistance.

Each requirement of the present invention will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.02-0.12%
C is an essential element for forming carbides and maintaining the high-temperature tensile strength and creep rupture strength required for an austenitic heat-resistant alloy. However, if the Cr content is excessive, not only undissolved carbides are generated, but also Cr carbides increase, degrading mechanical properties such as ductility and toughness and weld crack resistance. Therefore, the C content should be 0.02 to 0.12%. The C content is preferably 0.05% or more and preferably 0.10% or less.

Si: 0.01-0.40%
Si is contained as a deoxidizing element. Moreover, Si is an effective element for improving oxidation resistance, steam oxidation resistance, and the like. However, if the content is excessive, the precipitation amount and precipitation form of Laves phases or nitrides are affected, and the creep rupture strength is lowered. Therefore, the Si content should be 0.01 to 0.40%. The Si content is preferably 0.35% or less, more preferably 0.30% or less.

Mn: 0.10-1.50%
Like Si, Mn has a deoxidizing effect on molten steel, and also fixes S, which is unavoidably contained in steel, as sulfides to improve ductility at high temperatures. However, if its content is excessive, it promotes the precipitation of intermetallic compound phases such as σ phases, and deteriorates structural stability, high-temperature strength, and mechanical properties. Therefore, the Mn content should be 0.10 to 1.50%. The Mn content is preferably 0.20% or more. Also, the Mn content is preferably 1.20% or less, more preferably 1.00% or less.

P: 0.040% or less P is contained in steel as an unavoidable impurity, and significantly reduces weld crack resistance and high-temperature ductility. Therefore, the P content should be 0.040% or less. The P content is preferably as low as possible, preferably 0.030% or less, more preferably 0.020% or less.

S: 0.020% or less S, like P, is contained in steel as an unavoidable impurity, and significantly reduces weld crack resistance and high-temperature ductility. Therefore, the S content should be 0.020% or less. When emphasizing hot workability, the S content is preferably 0.010% or less, more preferably 0.003% or less.

Cr: 20.0-24.0%
Cr is an important element that exhibits excellent effects in improving corrosion resistance such as oxidation resistance, steam oxidation resistance, and high-temperature corrosion resistance, and also forms Cr carbonitrides and contributes to creep rupture strength. However, if the content is excessive, the structure becomes unstable due to the precipitation of the σ phase, etc., and the weld crack resistance deteriorates. Therefore, the Cr content should be 20.0 to 24.0%. The Cr content is preferably 21.0% or more and preferably 23.5% or less.

Ni: More than 26.0% and 35.0% or less Ni is an element that stabilizes the austenite structure and is an element that is also effective in ensuring corrosion resistance. From the above balance with the Cr content, when the Ni content is insufficient, the σ phase precipitates on the high temperature long time side, and the creep rupture strength, creep rupture ductility and toughness are significantly reduced. However, if its content is excessive, it impairs weld crack resistance and economy. Therefore, the Ni content should be more than 26.0% and not more than 35.0%. The Ni content is preferably 26.5% or more and preferably 33.0% or less. Also, it is preferable to adjust the Ni content in consideration of the content of ferrite stabilizing elements such as W.

W: 2.5-7.0%
W dissolves in the matrix and contributes to the improvement of creep strength as a solid-solution strengthening element, and also precipitates intermetallic compound phases such as the Fe ₂ W type Laves phase, and nitrides to strengthen the creep strength significantly. It is an element that improves the However, if the content is excessive, precipitates become excessive, resulting in deterioration of creep rupture ductility and toughness. Therefore, the W content should be 2.5 to 7.0%. The W content is preferably 3.0% or more. Also, the W content is preferably 6.0% or less, more preferably 5.5% or less.

Nb: 0.01-0.50%
Nb is an element that forms fine carbonitrides together with V and greatly contributes to the improvement of creep rupture strength. Furthermore, it suppresses the precipitation of Cr carbonitrides at grain boundaries and contributes to the improvement of stress corrosion cracking resistance. However, if the content is excessive, the creep rupture ductility and toughness are lowered, and the weld crack resistance and hot workability are also deteriorated. Therefore, the Nb content should be 0.01 to 0.50%. The Nb content is preferably 0.05% or more, more preferably 0.10% or more. Also, the Nb content is preferably 0.45% or less.

V: 0.01-1.0%
V is an element that forms fine carbonitrides together with Nb and greatly contributes to improvement of creep rupture strength. However, when the content is excessive, the creep rupture ductility and toughness are lowered, and the hot corrosion resistance is also deteriorated. Therefore, the V content should be 0.01 to 1.0%. The V content is preferably 0.05% or more, more preferably 0.08% or more. Also, the V content is preferably 0.80% or less, more preferably 0.70% or less.

REM: 0.005-0.060%
REM is an element that fixes S in the grain boundary as a sulfide and improves the creep rupture ductility especially on the high temperature long time side. Furthermore, REM has the effect of improving the adhesion of the Cr ₂ O ₃ protective film on the steel surface, and in particular, improving the oxidation resistance during repeated oxidation. However, if the content is excessive, the melting point of the grain boundary is lowered, thereby impairing the weld crack resistance. Therefore, the REM content should be 0.005 to 0.060%. The REM content is preferably 0.050% or less, more preferably 0.040% or less.

In addition, REM indicates a total of 17 elements of Sc, Y and lanthanoids, and the content of REM means the total content of these elements.

B: 0.0005 to 0.0050%
B is an element that exists in carbides or in the matrix phase, and not only promotes refinement of precipitated carbides and Laves phases, but also strengthens grain boundaries to improve creep rupture strength and rupture ductility. However, if the content is excessive, the ductility at high temperatures is lowered and the melting point is also lowered. Therefore, the B content should be 0.0005 to 0.0050%. The B content is preferably 0.0010% or more, more preferably 0.0015% or more. Also, the B content is preferably 0.0045% or less.

Al: 0.015-0.30%
Al is an element contained as a deoxidizing agent for molten steel, and has the effect of maximizing the effect of the above REM. However, if its content is excessive, a large amount of non-metallic inclusions precipitate, deteriorating ductility, toughness, workability, and the like. Therefore, the Al content should be 0.015 to 0.30%. The Al content is preferably 0.25% or less, more preferably 0.20% or less.

N: More than 0.13% and 0.35% or less N is an element that forms a nitride together with V or Nb and improves the creep rupture strength. In addition, it has the effect of improving the tensile strength by solid-solution strengthening. Furthermore, it is also an element that has the effect of stabilizing the austenite structure. However, if its content is excessive, it not only causes a decrease in ductility and toughness due to excessive nitride precipitation, but also forms blowhole defects in the steel. Therefore, the N content should exceed 0.13% and be 0.35% or less. The N content is preferably 0.15% or more and preferably 0.30% or less.

Mo: 0.30% or Less Conventionally, Mo has been thought to be an element that dissolves in the matrix and contributes to improving the creep strength as a solid-solution strengthening element, and has an effect equivalent to that of W. However, in the present invention, it has been found that when Mo is contained, the σ phase precipitates on the long-time side and the creep rupture strength, ductility and toughness are lowered. In particular, since the Cr content is 20.0% or more in the present invention, the Mo content should be as low as possible. Therefore, Mo content shall be 0.30% or less. Mo content is preferably 0.20% or less.

Cu: 0.30% or less In ordinary austenitic steel, Cu precipitates as a fine Cu phase and improves the creep rupture strength, but the nitrides of V and Nb and the Laves phase are sufficiently high up to the high temperature side of 700 ° C. or higher. It was also found that in the steel of the present invention, in which creep rupture ductility and toughness are emphasized in addition to ensuring creep rupture strength, REM inhibits the effect of improving ductility. Therefore, the content of Cu that is unavoidably mixed from the molten raw material or the like is limited to 0.30% or less.

Ti: less than 0.010% In the steel of the present invention containing more than 0.13% of N, Ti forms coarse Ti nitrides that do not contribute to the creep rupture strength and consumes N. Reduces the strength-enhancing effect. Therefore, in the present invention, the Ti content is limited to less than 0.010%.

Co: 0-10.0%
Like Ni, Co stabilizes the austenitic structure and contributes to the improvement of creep strength, so it may be contained as necessary. However, if the content is excessive, the above effects will be saturated and the economy will only decrease. Therefore, when Co is contained, the Co content should be 10.0% or less. The Co content is preferably 8.0% or less, more preferably 7.0% or less. In order to obtain the above effect, the Co content is preferably 0.5% or more, more preferably over 1.0%.

Mg: 0-0.050%
Mg has the effect of fixing S, which inhibits ductility at high temperatures, as a sulfide to improve high-temperature ductility, so it may be contained as necessary. However, if the content is excessive, the detergency is lowered, and rather the hot ductility is impaired. Therefore, when Mg is contained, the Mg content should be 0.050% or less. The Mg content is preferably 0.020% or less, more preferably 0.010% or less. To obtain the above effects, the Mg content is preferably 0.0005% or more, more preferably 0.001% or more.

Ca: 0-0.050%
Ca has the effect of fixing S, which inhibits ductility at high temperatures, as a sulfide to improve high-temperature ductility, so it may be contained as necessary. However, if the content is excessive, the detergency is lowered, and rather the hot ductility is impaired. Therefore, the content of Ca when it is contained is set to 0.050% or less. The Ca content is preferably 0.020% or less, more preferably 0.010% or less. To obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more.

Zr: 0-0.10%
Zr is an element that promotes the refinement of carbonitrides and improves the creep rupture strength as a grain boundary strengthening element, so it may be contained as necessary. However, if its content is excessive, the ductility at high temperatures decreases. Therefore, the Zr content when it is contained is set to 0.10% or less. The Zr content is preferably 0.06% or less, more preferably 0.05% or less. To obtain the above effect, the Zr content is preferably 0.005% or more, more preferably 0.01% or more.

Hf: 0-1.0%
Hf contributes to precipitation strengthening as a carbonitride and has the effect of improving the creep rupture strength, so it may be contained as necessary. However, if its content is excessive, workability and weld crack resistance are impaired. Therefore, when Hf is contained, the Hf content is set to 1.0% or less. The Hf content is preferably 0.80% or less, more preferably 0.50% or less. To obtain the above effects, the Hf content is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.02% or more.

Ta: 0-1.0%
Ta forms carbonitrides and acts as a solid-solution strengthening element to improve high-temperature strength and creep rupture strength, so it may be contained as necessary. However, excessive content impairs processability and mechanical properties. Therefore, the content of Ta when it is contained is set to 1.0% or less. The Ta content is preferably 0.70% or less, more preferably 0.60% or less. To obtain the above effects, the Ta content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

Re: 0-5.0%
Since Re mainly acts as a solid-solution strengthening element to improve high-temperature strength and creep rupture strength, it may be contained as necessary. However, excessive content impairs processability and mechanical properties. Therefore, if Re is included, the Re content should be 5.0% or less. The Re content is preferably 4.0% or less, more preferably 3.0%. To obtain the above effects, the Re content is preferably 0.01% or more, more preferably 0.10% or more, and even more preferably 0.50% or more.

The chemical composition of the austenitic heat resistant steel according to the present invention has the content of each element within the range described above and satisfies the following formulas (i) and (ii). I will explain each.

In the present invention, in order to ensure sufficient creep rupture strength and rupture ductility at high temperatures for a long time and toughness, V, Nb and N which mainly form nitrides, W which mainly forms an intermetallic compound phase, and , the amount of precipitation and the content of Si, which affects the form of precipitation, must be adjusted so as to satisfy the following formula (i).

By setting the value in the formula (i) to 6.0 or more, a sufficient intragranular precipitation strengthening effect can be obtained. On the other hand, by setting the median value of the formula (i) to 9.0 or less, it is possible to suppress deterioration in fracture ductility and toughness due to excessive strengthening of grain interiors.
6.0≤4V+2Nb+W+15N-6Si≤9.0 (i)
However, the element symbol in the above formula represents the content (% by mass) of each element.

Both B and REM must be contained in order to ensure sufficient creep rupture ductility on the high temperature long time side. By containing these elements in a composite manner, the free S segregated at the grain boundary is fixed as REM sulfide, and the grain boundary strengthening effect of B can be effectively exerted. (ii) By setting the value in the formula to 0.030 or more, a sufficient effect of improving the creep rupture ductility can be obtained. On the other hand, since B and REM may adversely affect weld crack resistance, sufficient weld crack resistance is ensured by setting the median value of formula (ii) to 0.075 or less.
0.030≦15B+REM≦0.075 (ii)
However, the element symbol in the above formula represents the content (% by mass) of each element.

In the chemical composition of the austenitic heat resistant steel of the present invention, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially, within a range that does not adversely affect the present invention. means acceptable.

2. Manufacturing Method The method for manufacturing the heat-resistant austenitic steel of the present invention is not particularly limited, but it can be manufactured, for example, by subjecting a steel ingot or slab having the chemical composition described above to hot working. Moreover, after the hot working, if necessary, hot working by a different method such as hot extrusion may be further performed. Furthermore, after performing annealing as needed, you may perform cold working.

After the above steps, in order to suppress variations in the metal structure and mechanical properties of each part and maintain high creep rupture strength, a final heat treatment may be performed by heating to a temperature range of 1100 to 1250 ° C. and holding. good. Water cooling is desirable after heating and holding.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

Steels (Steels 1 to 6, A to E) having chemical compositions shown in Table 1 were melted in a high-frequency vacuum melting furnace to obtain ingots of 30 kg.

The obtained ingot was heated to 1200° C. and hot forged at a finishing temperature of 1000° C. to obtain a plate material having a thickness of 15 mm. Using this 15 mm thick plate material, it was subjected to a softening heat treatment at 1100° C., then cold rolled to 10 mm, held at 1180° C. for 30 minutes and water cooled to obtain a test material.

A round bar tensile test piece with a diameter of 6 mm and a gauge length of 30 mm described in JIS Z 2241 (2011), parallel to the longitudinal direction, from the center in the thickness direction using a part of each of the above test materials. was fabricated by machining and subjected to a creep rupture test. A creep rupture test was performed at 750° C. and 130 MPa to measure the rupture time and rupture elongation. The creep rupture strength was judged to be excellent when the rupture time was 1200 hours or more, and the creep rupture ductility was judged to be excellent when the creep rupture elongation was 35% or more.

Next, a plate-shaped test piece having a thickness of 10 mm, a width of 50 mm, and a length of 100 mm was obtained by machining from a part of each test material. The test pieces were grooved along the longitudinal direction, the grooved test pieces were butted against each other, and two joints were butt-welded according to the following procedure to produce welded joints.

First, by gas tungsten arc welding, only the first layer was welded with a heat input of 5 kJ/cm without using a filler material, then placed on a commercially available carbon steel plate and restraint welded at the four corners. After that, using a TIG wire corresponding to SNi6625 specified in JIS Z 3334, lamination welding was performed in the groove by TIG welding at a heat input of 10 to 15 kJ/cm to produce a welded joint.

Samples were taken from each of the five welded joints so that the observation surface was the cross section of the joint (the cross section perpendicular to the weld bead). After polishing and corroding the collected samples, the presence or absence of cracks in the weld heat affected zone was examined by optical microscope observation. When cracks were not observed in all of the five samples or in four samples, the resistance to weld cracking was judged to be excellent and rated as "good", and the others were rated as "bad".

These results are summarized in Table 2.

As shown in Table 2, Steels 1 to 6 satisfying the requirements of the present invention had a creep rupture time of 1200 hours or more, a creep rupture elongation of 35% or more, and good weld crack resistance. In contrast, Steels A to E, which are comparative examples, were inferior in any one of creep rupture strength, creep rupture elongation, and weld crack resistance.

According to the present invention, it is possible to obtain an austenitic heat-resistant steel that has excellent long-term structural stability, high creep rupture strength and creep rupture ductility, and excellent weld crack resistance. Therefore, the austenitic heat-resistant steel of the present invention can be suitably used as large-scale structural members such as boilers for thermal power generation and chemical plants used in high-temperature environments.

Claims (2)

C: 0.02 to 0.12%,
Si: 0.01 to 0.40%,
Mn: 0.10-1.50%,
P: 0.040% or less,
S: 0.020% or less,
Cr: 20.0 to 24.0%,
Ni: more than 26.0% and 35.0% or less,
W: 2.5 to 7.0%,
Nb: 0.01 to 0.50%,
V: 0.01 to 1.0%,
REM: 0.005 to 0.060%,
B: 0.0005 to 0.0050%,
Al: 0.015-0.30%,
N: more than 0.13% and 0.35% or less,
Mo: 0.30% or less,
Cu: 0.30% or less,
Ti: less than 0.010%,
Co: 0 to 10.0%,
Mg: 0-0.050%,
Ca: 0-0.050%,
Zr: 0 to 0.10%,
Hf: 0-1.0%,
Ta: 0 to 1.0%,
Re: 0 to 5.0%,
balance: Fe and impurities,
satisfying the following formulas (i) and (ii),
Austenitic heat resistant steel.
6.0≤4V+2Nb+W+15N-6Si≤9.0 (i)
0.030≦15B+REM≦0.075 (ii)
However, the element symbol in the above formula represents the content (% by mass) of each element. The chemical composition, in mass %,
Mg: 0.0005-0.050%,
Ca: 0.0005 to 0.050%,
Zr: 0.005 to 0.10%,
Hf: 0.005 to 1.0%,
Ta: 0.01 to 1.0%, and
Re: 0.01 to 5.0%,
containing one or more selected from
The austenitic heat-resistant steel according to claim 1.