JPH06271989A - Ferritic heat resistant steel for nuclear fusion furnace and its production - Google Patents

Abstract

PURPOSE:To produce a heat resistant steel for nuclear fusion reactor structure, excellent in high temp. creep strength. CONSTITUTION:This steel is a heat resistant steel for nuclear fusion reactor which has a composition consisting of, by weight, 0.05-0.15% C, 0.02-0.25% Si, 0.01-0.50% Mn, <=0.01% P, <=0.01% S, 5.0-13.0% Cr, 0.3-3.0% W, 0.05-0.40% V, 0.002-0.08% N, 0.3-2.5% tantalum oxide of <=1mum grain size, 0.25-3.0% total Ta, and the balance iron with inevitable impurities and containing, if necessary, one or <=2 elements among Ti, B, Mg, Y, Zr, La, Ce, and Ca. After the addition of Ta of 0.007/O to 0.0013/O Ta to a molten steel containing 0.002-0.08% dissolved oxygen, tantalum oxide is added.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant steel sheet which is excellent in low induction activation, high-temperature creep strength and toughness and is used for the first wall of a fusion reactor, etc.

[0002]

2. Description of the Related Art A nuclear fusion reactor has already achieved critical plasma conditions in an experimental reactor. Currently, SUS316 is used as the structural material for the first wall. However, SUS316 cannot be used at the neutron irradiation dose expected in a commercial reactor due to the problem of void swelling. SUS3 instead
JPCA steel in which Ti is added to 16 and HT-9 steel (12Cr-1Mo-W-V) which is a ferrite system have been developed. In addition, Journal of Nuclear
Materials 133 & 134 (1985) 14
As described in 9-155, it is economically remarkably advantageous to reduce radioactive contamination at the time of repair during use and at the time of disposal after shutdown. Therefore, low radiation with little induced activation is used as the first wall material. Material is required, and it is a ferritic steel with less void swelling and harmful elements such as Ni, Cu, Mo, Nb, Hf, Al, Co and Ag.
Nowadays, steels that have reduced the above are required.

As a low activation steel, a Cr-W type steel ("Al
loy Development For Irradi
ation Performance / Semiann
ual Progress Report For P
eriod Ending September 30,
1985; U. S. Department of Ene
rgy, p117-123 ", and" heat resistant material No. 123 "
Committee Research Report Vol. 27, No.1, pp105-1
17 ") and Japanese Patent Publication No. 3-61749, 8Cr-2W-Ta-V steel and the like have been developed. By adding W, V, Ta, etc. to these steels,
It is intended to make the high temperature creep strength equal to that of HT-9 steel.

As a material excellent in high temperature creep strength, steel in which oxide is dispersed has been developed. JP-A-1
Steel obtained by dispersing Y ₂ O ₃ and TiO ₂ at a mechanical alloying method, as described in -272746 discloses or 2~12Cr-W, as described in Japanese Patent Application No. Hei 4-45820 A steel in which Ta ₂ O ₅ is dispersed has been studied as a structural material for the first wall.

[0005]

However, Cr-
In W-based steels, W precipitates as coarse Fe ₂ W during high temperature holding and the amount of W contributing to solid solution strengthening decreases, and V carbonitrides and Ta carbonitrides become coarser and the dispersion strengthening effect decreases. Therefore, the high temperature creep rupture strength is not sufficient. Also, 8Cr-2W-Ta, V described in Japanese Patent Publication No. 3-61749 is disclosed.
Since steel has Ni added thereto from the viewpoint of ensuring toughness, it cannot be said to be a sufficiently low activation material, and the high temperature creep rupture strength is not sufficient for the same reason as Cr-W steel. Therefore, it is necessary to increase the high temperature creep rupture strength by dispersing in the steel particles that do not coarsen even at high temperatures.

On the other hand, the steel described in JP-A-1-272746 in which Y ₂ O ₃ and TiO ₂ are dispersed has sufficient high temperature creep strength, but it is expensive due to the powder metallurgical manufacturing method and is used for commercial furnaces. Does not fit into the structural materials of. The steel described in Japanese Patent Application No. 4-45820 has a relatively low cost because the oxide is dispersed by the conventional melting method, but Ta ₂
O ₅ may agglomerate during steel making and the creep strength may decrease.

An object of the present invention is to provide a ferritic heat-resistant steel which has improved high-temperature creep rupture strength and at the same time has low emission, and which is of low cost and mass-produced.

[0008]

DISCLOSURE OF THE INVENTION The present invention is to disperse an oxide of Ta, which is an oxide which is stable at high temperature and long time in steel and which is a low activation element, and by dispersion strengthening, high temperature creep rupture strength. It also realizes low emission at the same time,
Further, it is characterized in that tantalum oxide is dispersed by a dissolution method, and the gist of the invention is as follows.

(1) C: 0.05 to 0.15%, Si:
0.02-0.25%, Mn: 0.01-0.50%,
P: 0.01% or less, S: 0.01% or less, Cr: 5.
0 to 13.0%, W: 0.3 to 3.0%, V: 0.05
-0.40%, N: 0.002-0.08%, tantalum oxide: 0.3-2.5%, total Ta amount including Ta existing as tantalum oxide: 0.25-3.0% And a balance of Fe and unavoidable impurities, and an average particle diameter of tantalum oxide is 1 μm or less.

(2) Ti: 0.00 by weight
2 to 0.05%, B: 0.0001 to 0.01%, M
g: 0.001 to 0.05%, Y: 0.001 to 0.0
5%, Zr: 0.001 to 0.05%, La: 0.00
1 to 0.05%, Ce: 0.001 to 0.05%, C
a: 0.001 to 0.05% of 1 type or 2 types or more, The ferritic heat resistant steel for fusion reactors of the preceding clause 1 characterized by the above-mentioned.

(3) C: 0.05 to 0.15 by weight
%, Si: 0.02 to 0.25%, Mn: 0.01 to
0.50%, P: 0.01% or less, S: 0.01% or less, Cr: 5.0-13.0%, W: 0.3-3.0
%, V: 0.05 to 0.40%, N: 0.002 to 0.
08%, dissolved oxygen ( O 2 ): 0.002 to 0.08%, the balance of molten steel consisting of Fe and unavoidable impurities, T
a: 0.0007 / O to 0.0013 / O % is added, and then tantalum oxide: 0.3 to 2.7% is added, the method for producing a ferritic heat-resistant steel for a fusion reactor.

(4) Molten steel is on a weight basis and Ti:
0.002-0.05%, B: 0.0001-0.01
%, Mg: 0.001-0.05%, Y: 0.001-
0.05%, Zr: 0.001 to 0.05%, La:
0.001-0.05%, Ce: 0.001-0.05
%, Ca: 0.001-0.05% of 1 type or 2 types or more, The manufacturing method of the ferritic heat resistant steel for fusion reactors of the preceding clause 3 characterized by the above-mentioned.

[0013]

The present invention will be described in detail below. The feature of the present invention is that Ni, Cu, Mo, which are elements harmful to low emission,
This is because tantalum oxide was contained as dispersed particles for reducing Nb, Hf, Al, Co, Ag, etc. as much as possible and improving the strength by the dispersion strengthening mechanism. Tantalum oxide, which has a specific gravity of about 8 and is slightly heavier than iron, does not float as slag even when added to molten steel, has low radiation, and is stable in steel at high temperature for a long time. Actually, the tantalum oxide can be dispersed in the molten steel by a method of adding the tantalum oxide-containing core wire to the tundish or the ladle, or a method of placing the tantalum oxide-containing core wire in the mold and injecting the molten steel.

Next, in order to achieve the desired characteristics of the present invention, it is necessary to limit each constituent element to an appropriate range as described below. C functions as a solid solution strengthening element and also forms carbides to improve creep rupture strength, and the effect is remarkable at 0.05% or more. On the other hand, if it exceeds 0.15%, the weldability decreases, so
It was limited to 0.15%.

Si is 0.2 because it reduces tantalum oxide.
It should be 5% or less. On the other hand, if it is less than 0.02%, the viscosity of the molten steel increases, casting becomes difficult, and welding defects increase, so it was limited to 0.02 to 0.25%. M
Although n is small, it has a function of improving creep rupture strength by solid solution strengthening. However, if it exceeds 0.50%, the toughness decreases, so the content is limited to 0.01 to 0.5%.

Since P causes grain boundary embrittlement, it is limited to 0.01% or less. P It is desirable that it is as low as possible. Since S causes a decrease in toughness, it is limited to 0.01% or less. It is desirable that S be as low as possible. Cr is an essential element for heat resistant steels that improve corrosion resistance and oxidation resistance at high temperatures. The effect is remarkable at 5.0% or more. On the other hand, if it exceeds 13.0%, δ ferrite is formed and the toughness is lowered, so the content was limited to 5.0 to 13.0%.

W remarkably improves the high temperature creep rupture strength by solid solution strengthening. The effect becomes remarkable at 0.3% or more. On the other hand, if it exceeds 3.0%, a coarse intermetallic compound is formed and the toughness is remarkably reduced, so the content is limited to 0.3 to 3.0%. V enhances high temperature creep strength by solid solution strengthening and precipitation strengthening. Although the effect becomes remarkable at 0.05% or more, addition of more than 0.40% causes a decrease in toughness due to the formation of δ ferrite, so the content was limited to 0.05 to 0.40%.

N increases the high temperature creep strength by solid solution strengthening and carbonitride precipitation strengthening. The effect is 0.002
%, The content becomes remarkable, but addition of more than 0.08% causes a decrease in toughness, so the content was limited to 0.002 to 0.08%.
Most of Ta exists as tantalum oxide, but it also exists as solid solution Ta or TaC. These Ta sources are due to Ta added before the addition of tantalum oxide in molten steel, or a part of tantalum oxide decomposed. When tantalum oxide is 0.3%, the total Ta amount is 0.25% or more. Further, if the total Ta amount exceeds 3.0%, the toughness is deteriorated, so 0.25 to
It was limited to 3.0%.

Tantalum oxide (chemical type is mainly Ta ₂ O ₅ )
Acts as a dispersion strengthening source and significantly improves the high temperature creep rupture strength. The effect is remarkable at 0.3% or more, but 2.
Addition of more than 5% causes a decrease in creep rupture strength due to a decrease in creep ductility and also a decrease in toughness. Further, tantalum oxide is relatively expensive and it is not preferable to add it in a large amount. Therefore, it is limited to 0.3 to 2.5%. Tantalum oxide is Ta ₂ O ₅ , Ta ₆ O, TaO, Ta
₂ O, TaO ₂ , Ta _0.83 O ₂ , Ta _0.97 O ₂ , Ta
_0.8 O ₂ , Fe ₄ Ta ₂ O ₉ , FeTaO ₄ , FeTa
It exists in the form of O ₆ , Ta ₂ CrO ₆ , CrTaO _{4, and the} like. Nb, Si, and A are inevitable impurities in Ta.
1, Ca, Mn, Ti, etc. are included.

The average particle size of tantalum oxide is set to 1 μm or less, because when the average particle size exceeds 1 μm, the number of tantalum oxide per unit area is small and the high temperature creep rupture strength This is because tantalum oxide is apt to be the starting point of brittle fracture and the toughness is significantly reduced. The smaller the thickness is 1 μm or less, the higher the high temperature creep rupture strength is.

The average particle size of tantalum oxide is measured as follows. Total 30 tantalum oxide on any 3 cross sections
Measure 0 or more. When it looks like a circle, its diameter is taken as the particle size. In the case of an ellipse, a polygon or an irregular shape, the particle size is the geometric average of the shortest distance and the longest distance between the particles. The arithmetic average of these particle sizes is defined as the average particle size.

Next, the reasons for limiting the manufacturing conditions will be described. O is important in controlling the dispersion of tantalum oxide. If O is less than 0.002%, the yield of tantalum oxide to be added later is significantly reduced. 0.08% again
Exceeds 0.002, tantalum oxide becomes coarse, so 0.002
Limited to ~ 0.08%.

Ta is measured by measuring O (unit: wt%),
When added in the range of 0.0007 / O 2 to 0.0013 / O %, the yield of tantalum oxide is high and tantalum oxide is finely dispersed. If it is less than 0.0007 / O 2, tantalum oxide to be added later is decomposed. Also 0.001
If it exceeds 3 / O 2 %, the toughness decreases due to the coarsening of tantalum oxide or the excessive solid solution Ta.

If the balance of tantalum oxide added to molten steel: 0.3 to 2.7% O 2 and Ta is within the range of the present invention, the yield of tantalum oxide (chemical type is mainly Ta ₂ O ₅ ) is about. 90
%, About 0.3% to about 2.5%, and in order to disperse 0.3 to 2.5% of tantalum oxide, 0.3 to 2.7 of tantalum oxide is contained in molten steel.
It is necessary to add in the range of%.

The molten steel temperature at the time of addition is 1570
-1680 degreeC is preferable. Ti, B, Mg, Y, Zr,
By adding La, Ce and Ca, the toughness is improved without lowering the creep strength. Especially when the plate thickness is large, those effects are great. When Ti is added in an amount of 0.002% or more, the precipitate makes the crystal grains finer and improves the toughness. However, if over 0.05% is added, the Ti precipitates become coarse and conversely the toughness decreases, so 0.002 to 0.05
Limited to%.

When B is added in an amount of 0.0001% or more, the hardenability is enhanced and the toughness is improved. This effect is remarkable in the case of an extremely thick steel plate. If added in excess of 0.01%, coarse BN precipitates and reduces toughness, so the content was limited to 0.0001 to 0.01%. Mg, Y, Zr, La, Ce, and Ca each add 0.001% or more to increase the cleanliness of steel and reduce segregation to improve toughness. On the other hand, 0.
Since the addition of more than 05% reduces tantalum oxide and reduces the creep strength, the addition amount of each of these elements was limited to 0.001 to 0.05%. It is desirable to add these elements before adding tantalum oxide.

The preferable heat treatment conditions for the steel of the present invention are 900
Normalizing at or above ℃ and tempering at or above 660 ° C. or normalizing after rolling is omitted, and tempering at or above 700 ° C. is performed. By tempering, V carbonitrides are deposited and the high temperature creep rupture strength is further improved.

[0028]

Example: After adjusting the components in 50 kg vacuum melting, molten steel (1600 to 1650 ° C.) was poured into a tundish, and at the same time, an iron wire (wall thickness 0.5 t, outer diameter 7φ) containing tantalum oxide was added to the tundish. Added in dish. After casting into an ingot, let cool to room temperature 120
After heating to 0 ℃, finish temperature is 1000 ℃ and thickness is 25mm.
Rolled, air-cooled, and then quenched at 930 to 1080 ° C. to 70
A sample steel was prepared by tempering at 0 to 790 ° C. A 6φ × 30GL creep test piece was machined from the test steel and
The creep rupture strength (CRS) of 10000 h at 0 ° C. was investigated. Further, a 2 mmV notch Charpy test piece was processed and the impact absorption energy (vE0) at 0 ° C. was also investigated.

The results are shown in Table 1, Table 2 (continued from Table 1),
The results are shown in Table 3 and Table 4 (continued from Table 3). As can be seen from these tables, the steel of the present invention No. 1 to 13 have high creep rupture strength, while comparative steel No. 14-2
9 has a low creep rupture strength. Invention Steel No. 1, 2,
Comparative steel Nos. 3 and 4 and comparative steel No. The effect of tantalum oxide can be known from the six steel types 14 and 15. Comparative steel No. No. 14 had a low dissolved oxygen content, so that the yield of tantalum oxide decreased, and the tantalum oxide content was lower than the lower limit (0.3
%). As a result, the dispersion strengthening was not sufficient and the strength was low.

Inventive Steel No. Creep rupture strengths of 1, 2, 3, and 4 gradually increase in the order of tantalum oxide content, but all are at high levels. Comparative steel No. 15
Is the upper limit of the tantalum oxide content of the present invention range (2.5%)
The toughness is remarkably deteriorated because it exceeds. Comparative steel No. Although 15 has a sufficiently high creep rupture strength, it contains 2.62% tantalum oxide,
Inventive Steel No. 1 containing 1.88% tantalum oxide. It is lower than 4.

Comparative steel No. No. 16 had insufficient C, so solid solution strengthening and precipitation strengthening by carbide were not sufficient, and the creep rupture strength decreased. Comparative steel No. Since No. 17 had an excessive amount of Si, tantalum oxide was decomposed and the creep rupture strength was lowered. Comparative steel No. No. 18 had too much Mn to reduce ductility, and rupture occurred before sufficient plastic deformation, so the creep rupture strength was rather low. In addition, some cracks occurred during rolling.

Comparative steel No. No. 19 had a structure in which 35% of δ-ferrite was present due to too much Cr and W, and this relatively soft δ-ferrite phase led to creep rupture with low stress, resulting in low creep rupture strength. Also, the toughness was significantly reduced. Comparative steel No. Since No. 20 had insufficient W, the solid solution strengthening was insufficient and the creep rupture strength decreased.

Comparative Steel No. In No. 21, creep rupture strength decreased because solid solution strengthening and precipitation strengthening were insufficient due to lack of V. Comparative steel No. In No. 22, since V was excessive, δ ferrite was precipitated and the creep rupture strength and toughness were lowered. Comparative steel No. In No. 23, since N is excessive, the creep rupture strength is high but the toughness is reduced.

Comparative Steel No. 24 is Ta added to the molten steel
However, the particle size of tantalum oxide exceeds 1 μm and the creep rupture strength and toughness are both reduced. Comparative steel N
o. In No. 25, since the total Ta is excessive, the creep rupture strength is high, but the toughness is lowered. Comparative steel No. In No. 26, since Ti was excessive, coarse TiN was precipitated and the toughness was lowered.

Comparative Steel No. In No. 27, since B was excessive, coarse BN was precipitated and the toughness was lowered. Comparative steel No. In No. 28, since tantalum oxide was reduced because Mg was excessive, the amount of tantalum oxide was less than 0.25%, and the creep rupture strength decreased. Comparative steel No. In No. 29, since LaCe and Ca were excessive, tantalum oxide was reduced, the amount of tantalum oxide was less than 0.25%, and the creep rupture strength was lowered.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

As described above, according to the ferritic steel for a fusion reactor excellent in high temperature strength of the present invention, for example, the high temperature strength, especially the high temperature creep rupture strength as compared with the conventional fusion reactor steel. Is significantly improved, and the toughness and workability are excellent, so that it is extremely effective as a first wall material for a fusion reactor.

Claims (4)

[Claims] 1. C: 0.05 to 0.15%, Si: 0.02 to 0.25%, Mn: 0.01 to 0.50%, P: 0.01% or less, S on a weight basis. : 0.01% or less, Cr: 5.0 to 13.0%, W: 0.3 to 3.0%, V: 0.05 to 0.40%, N: 0.002 to 0.08% Tantalum oxide: 0.3 to 2.5% Total Ta amount including Ta existing as tantalum oxide:
A ferritic heat-resistant steel for a fusion reactor, comprising 0.25 to 3.0%, balance Fe and unavoidable impurities, and having an average particle size of tantalum oxide of 1 μm or less. 2. On a weight basis, Ti: 0.002 to 0.05%, B: 0.0001 to 0.01%, Mg: 0.001 to 0.05%, Y: 0.001 to 0. 0.05%, Zr: 0.001 to 0.05%, La: 0.001 to 0.05%, Ce: 0.001 to 0.05%, Ca: 0.001 to 0.05%, one kind Alternatively, the ferritic heat-resistant steel for a fusion reactor according to claim 1, containing two or more kinds. 3. C: 0.05 to 0.15%, Si: 0.02 to 0.25%, Mn: 0.01 to 0.50%, P: 0.01% or less, S on a weight basis. : 0.01% or less, Cr: 5.0 to 13.0%, W: 0.3 to 3.0%, V: 0.05 to 0.40%, N: 0.002 to 0.08% , dissolved oxygen (O): comprises 0.002 to 0.08%, the molten steel and the balance Fe and unavoidable impurities, Ta: 0.0007 / O ~0.0013 / O% after addition of tantalum oxide : 0.3-2.7% is added, The manufacturing method of the ferritic heat resistant steel for fusion reactors characterized by the above-mentioned. 4. Molten steel is based on weight, and further Ti: 0.002-0.05%, B: 0.0001-0.01%, Mg: 0.001-0.05%, Y: 0.001. -0.05%, Zr: 0.001-0.05%, La: 0.001-0.05%, Ce: 0.001-0.05%, Ca: 0.001-0.05% The method for producing a ferritic heat-resistant steel for a fusion reactor according to claim 3, characterized in that it contains one kind or two or more kinds.