JPS60155652A - Heat resistant steel - Google Patents

Abstract

PURPOSE:To obtain a heat resistant steel having high strength, maintaining its ductility in a state with a strain applied by cold working, and usable at high temp. by adding V, Nb and other element to a heat resistant Cr-Ni steel. CONSTITUTION:An alloy steel consisting of 0.02-0.2% C, <1.5% Si, <3% Mn, 10-25% Cr, 8-25% Ni, 0.1-3% Mo, at least one between 0.02-0.5% Nb and 0.05-0.5% V, and the balance >=50% Fe is used as a heat resistant steel having a plastically cold worked state and suitable for use as the material of a fuel cladding pipe for a fast breeder or as a material for a tower for synthesizing a styrene monomer. The heat resistant steel may further contain at least one among 0.5-5% Cu, 0.005-0.02% B, 0.01-0.2% Ti, 0.005-0.2% Al and 0.005- 0.2% Zr.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a new heat-resistant steel, and particularly to a heat-resistant steel suitable for fuel cladding tubes for fast breeder reactors and styrene monomer synthesis towers.

[Background of the invention]

SUS304.31 is a material for high-temperature equipment such as nuclear power plants, power generation plants, and chemical plants.
Austenitic stainless steels such as 6,321,347 are widely used. Japanese Patent Publication No. 58-19742 discloses a material containing MO and Ti. Due to their structure, these high-temperature devices include parts that are manufactured into a shape with curvature by cold working and parts that are used after being intentionally cold worked. For example, the former is a curved part of a piping system.

There are bend parts of heat exchanger tubes, expansions that absorb elongation due to thermal expansion, parts with curvature of various containers and casings, etc., and for the latter, cold processing of about 10 to 20% is required considering the usage environment. Examples include fuel cladding tubes for fast reactors that are used after being given a

The materials used for such parts are cold-processed into a predetermined shape or subjected to a predetermined processing strain.
Incorporated into equipment. A material used in such a state is subjected to a processing strain of 3 to 30 inches by cold working, and the material is hardened by this processing strain. Although conventionally hardened materials have high strength, they have extremely low ductility, which causes cracks to occur during high-temperature use. As a means of preventing cracks from occurring due to a decrease in ductility in materials that have been subjected to processing strain, some heat treatment methods have been used to remove the strain. Due to limitations and the high costs involved, it is often used with some distortion caused by processing.

To deal with this current situation, it is necessary to provide a material that has high strength and little decrease in ductility even if it is used for a long time at high temperature under cold working strain.
This is important from the viewpoint of equipment reliability and economic efficiency.

[Purpose of the invention]

An object of the present invention is to provide a heat-resistant steel that undergoes cold working and has high strength, with little decrease in ductility during use, even in a strained state.

[Summary of the invention]

The present invention has a weight ratio of C0.02 to 0.15%, Si1.
5% or less, Mn 25 or less, Cr1O~25%.

Ni 8-25%, Mo 0.1-3.0%, Nb
o, 02 to 0.5% and VO, 05 to 0.5% at least once, the remainder consisting of pe of 50% or more,
Or Cu0.5-596. B0.001~0.0
296°AtO, 01-0.2%, Ti0.01-0.
2%, Zr0.005 to 0.2%, and the remainder is 50% or more of pe, preferably 3
The characteristics of the present invention, which consists of heat-resistant steel that has been subjected to ~30% cold plastic working strain, are: (1) The material has high strength before cold working due to the solid solution strengthening effect in IJ lux. What we do, Cu, V, Nb, Ti, Zr.

Among At and B additions, in particular, one or more of Nb and V, or at least one of cul'r+, zr, Az, and B is appropriately added to prevent a decrease in ductility due to cold working.

In cold-worked steel, there are multiple dislocations formed within the crystal grains, and these dislocations interact elastically with each other, restraining the free movement of the dislocations.

Furthermore, when used at high temperatures in this state, dislocations become trapped in the diffusion of interstitial atoms (C, N) or in the atmosphere of diffusion and precipitation, making them even more immobile, causing hardening of the crystals and a decrease in ductility. Become. Although cold working cannot prevent the occurrence of dislocations, it can reduce the decrease in ductility during high temperature use by preventing the diffusion of C and N into dislocations.

The feature of the present invention is that stable V carbides, nitrides, or carbonitrides are formed by adding V, which has a strong affinity with C and N. The purpose is to suppress the precipitation of carbides.

Based on the above phenomena, we propose a cold-worked heat-resistant steel with excellent high-temperature strength and ductility by adding appropriate amounts of CU and V.

The reasons for limiting each component are shown below.

C is important for improving high temperature strength, but 0.2
% or more, workability and weldability are significantly reduced. Ma*
If it is less than 0.02%, sufficient strength cannot be obtained. In particular, 0.03 to 0.15% is preferable, and 0.04 to 0.15% is preferable.
0.1 inch is more preferable.

Si is added as a deoxidizing agent, but if it is too large, weldability and ductility deteriorate, so the upper limit is set at 1.5%.

Preferably it is 0.4 to 1.0%.

Mn is also added as a deoxidizing agent, but if it is too large, corrosion resistance and oxidation resistance will be lowered, so the upper limit is set at 3.0%. It is preferably 2% or less, more preferably 1 to 2%.

Cr is an important element for obtaining high temperature strength, oxidation resistance, and corrosion resistance, and 10 to 25% is sufficient to obtain these effects. Preferably it is 14 to 20%.

Ni is an important element for creating an austenite structure and obtaining high temperature strength. If it is less than 8%, the austenite structure becomes unstable and becomes brittle if used at high temperatures for a long time. On the other hand, if the amount increases by more than 25%, the processability decreases. Preferably it is 10-15%.

Mo strengthens the austenite matrix, some of which precipitates as carbides, and increases the high temperature strength by adding 0.1
% or more is required. When the amount exceeds 3.0%, workability and oxidation resistance decrease, and sigma phase tends to precipitate. Preferably it is 2 to 3%.

Cu dissolves in the austenite matrix and improves high-temperature strength, so 5% or more of O is required, but if the content exceeds 5%, it reduces weldability and embrittles grain boundaries.

B is 0.0 to particularly improve long-term creep strength.
0.1% or more is required. On the other hand, when the content exceeds o and oi%, workability and weldability are reduced. Preferably 0.001
~0.008%.

At fixes nitrogen and improves high-temperature strength and ductility by solid solution in the matrix. If there is too much, solid solution KL
increases, promoting the growth of carbides.

It's a hill, if too little, the effect of p1 and other additive elements with a large amount of nitrogen will be halved. The amount of At added is 0.005~
0.2% is required. Preferably 0.01 to 0.05
≠.

Addition of (2) improves strength and corrosion resistance, but too much of it reduces workability, weldability, and oxidation resistance.

Furthermore, the effect of addition is that by forming stable carbides, nitrides, or carbonitrides, it prevents the diffusion and precipitation of C and N into dislocations caused by cold working, and improves ductility. The goal is to suppress the decline in In particular, the correlation between the amount of addition of (■) and the amount of C added is important. If V/C is small in terms of weight ratio, the effect of addition (1) will be small, and 2 or more is preferable for significant improvement in strength. On the other hand, if it is too large, the amount of carbide itself is greater than the effect of suppressing the diffusion of C, resulting in a decrease in ductility, so it is preferably 7 or less. The amount of ■ added should be 0.05 to 0.5%, 2≦■/C≦7
is suitable. Preferably it is 0.1 to 0.3%.

Nb also has the same effect as ■, and by adding appropriate amounts of each, strength and ductility can be improved by adding 0.0.
0.02% or more, and 0.5% or less in order to reduce the decrease in ductility after cold working. It is preferably 0.05 to 0.2%, and furthermore, the weight ratio of Nb/
C is preferably 1 to 5.

Ti, Zr, B, and At all similarly improve strength. In particular, they improve both strength and ductility when added in combination with Nb and ■. After cold plastic working, the crystal grains are refined when heated at high temperatures. Ti0.01-0.2%, AtO,005-0.
2%, z r 0.005-0.5 and 130.0
01-0.02% of each is required.

Both Tit At and zr are 0.2% and B is 0.0
If the ratio exceeds 2, the ductility decreases and should be avoided.

Preferably Ti0.03-0.15%, At0.01-0.01%
0.05%, Z r 0.01-0.1% and BO,
OO1 to 0.006%. In addition, the weight ratio (Nb+
T i )/C is preferably 1 to 5.

The fuel cladding tube for fast breeder reactors made of the steel of the present invention is melted, forged, hot rolled, annealed and cold plastic worked repeatedly,
It is manufactured into a pipe with an outer diameter of 6 to 7 mm and a wall thickness of 0.4 to 0.5 m, and is used with a final cold plastic working rate of 3 to 20%. A preferred composition is C0.05-0.07% by weight;
SiO, 4-0.7%.

Mn 1.5-1.8%, Ni 12-15%, Mo1
．． 8-2.5%, Cu2-2.5%, Vo, 25-0.
45% (V104~6.5>, NbO, 1~0.3%,
Ti0.08-0.12%, 80.003-0.005
%, At0901-0.04%. Processing rate is 10~
20 inches is preferable.

The steel of the present invention can be used for at least one of the head plate and the expansion plate of a styrene monomer synthesis tower. The mirror plate is 3~
10% and expansions are cold plastic worked at a cold plastic working rate of 3 to 20 inches, butt welded to other parts as they are, and used as welded. A composition similar to that of the cladding described above is preferred.

[Embodiments of the invention]

(Example 1) A creep rupture test was conducted using steel materials having the compositions shown in Table 1 as test materials. The tests were conducted on the compositions shown in Table 1, which had been subjected to solution treatment at 1100C for 1 hour (solution treatment materials), and the materials with the compositions shown in Table 1 that had been subjected to tensile processing after solution treatment.
The test was carried out on a material that had been subjected to 0° plastic processing (cold plastic processing material). The creep test was conducted at a temperature of 6501:l' and a stress of 26 kg/2 cods. Table 2 shows each creep rupture time, elongation rate and shrinkage rate. The invention steel is A2-11, and A1 and 12 are comparison steels. As shown in the table, it is clear that the cold-worked steel of the present invention has excellent strength, elongation, and drawing ratio.

FIGS. 1 and 2 are diagrams showing the relationship between the creep rupture time and the elongation rate of the solution-treated material and the cold-plastically worked material. The numbers in the diagram are steel. As shown in the figure, it is clear that A2-12, which is a cold-worked steel of the present invention, has higher strength and elongation than A1, which contains only MO.

Figures 3 and 4 show the creep rupture time, V and V/, respectively.
It is a diagram showing the relationship with C ratio. The numbers in the diagram are steel. As is clear from both figures, the addition of ■ and Cu
Creep rupture strength is significantly improved by adding . V
When it is 0.4% or more, it is saturated.

Figure 5 shows the elongation at break, reduction ratio and V after the creep rupture test.
It is a diagram showing the relationship of /C. The elongation rate and squeezing rate of the boiled steel 12 in which the V amount exceeds 0.5% or the V/C ratio exceeds 8 rapidly decrease. The steel of the present invention exhibits a higher value than the conventional material A1 and the comparative material A12. In particular, A2, 3, 4.5,
6, 7. Compared to 10, it contains Cu and has a v/C of 2 or less.
A8 and 9 containing A8 and A9 have high strength both in the as-solution treated state and in the 10% processed state. On the other hand, the ductility of AIo, 11.12, which contains There is extremely little decrease in ductility after machining. In this way, it contains Cu and V/C is 2.
It can be seen that the materials of the present invention have high strength after cold working, and have little decrease in ductility, although the values are in the range of 7 to 7.

(Example 2) Using a commercially available 5US30'4 material and the steel shown in Table 3 (wt%), cold plastic working was performed at room temperature, and a tensile test at 650C was conducted. The steels in Table 3 were subjected to solution treatment by heating at 110 (l) for 1 hour.

As shown in Table 3, the 0.2% yield strength shows a large improvement in strength when the cold working rate is 3% or more.

The ductility does not change at a cold working rate of 3% or less, and is not affected by cold working. On the other hand, when the cold working rate increases by more than 30%, the decrease in ductility increases. When this ductility is less than 15 to 20%, there is a very high probability that cracks will occur during use due to residual strain (residual stress) imparted by cold working.

(Example 3) FIG. 8 is a cross-sectional diagram of a fuel rod for a fast breeder reactor showing an example of the use of the steel of the present invention. The fission product 2 is inserted into the cladding tube 1 and fixed by a spring 4, with the upper end 3 and the lower end 3
Each is sealed by The cladding tube 1 is made of alloy A8 in Table 1, with an outer diameter of 6 to 7 mm and a wall thickness of 0.4 mm.
A pipe of ~0.5 wm was manufactured by vacuum melting, forging, hot extrusion, annealing, and repeated cold plastic working and annealing in a pilger mill. The final cold plastic working rate is 15%.

As shown in the figure, nuclear fuel material is inserted into the manufactured cladding tube, inert gas is sealed, and end plugs are welded to both ends of the cladding tube. Electron beam welding was used for welding.

Annealing after extrusion was performed at 1,100 to 1.150C.

(Example 4) FIG. 9(a) is a front view of a styrene monomer synthesis tower showing another example of use of the steel of the present invention. This synthesis tower has head plates 5 and 6 at the upper and lower parts, respectively, and the body parts 7 and 9 are butt-welded to each other via an expansion 8. FIG. 9(b) is an enlarged view of the expansion 8. FIG. The alloy A8 in Table 1 was used as the end plates 5 and 6 and the expansion 8, and vacuum melted.

After forging and hot rolling to the desired thickness, 1.1
It was annealed at 00 to 1,150°C to give the desired surface finish.

Mirror plate 5-6 Thickness 20 tra, expansion 8
is 10 strips thick. Both of these are processed into the shape shown in the figure at room temperature, and the former has a processing rate of 7% and the latter 15%. These are butt welded to each other with matching metal wire as shown in the figure and used as they are.

〔Effect of the invention〕

According to the present invention, a heat-resistant steel with high high-temperature strength and little decrease in ductility during high-temperature use can be obtained, and a remarkable effect is exhibited in improving the life of high-temperature equipment.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing the relationship between the creep rupture time and elongation of cold-plastically worked materials and solution-treated materials, respectively.
Figures and Figure 4 show the time of creep rupture, the amount and V/C, respectively.
Figure 5 is a diagram showing the relationship between creep rupture elongation rate and reduction ratio and V/C ratio, Figures 6 and 7 are based on cold plastic working rate and tensile test. A diagram showing the relationship between tensile strength, yield strength, and elongation rate, FIG. 8 is a sectional view of a fuel rod for a fast breeder reactor, and FIG. 9 is a front view of a styrene monomer synthesis tower. 1... Cladding tube, 2... Nuclear fuel material, 3... End plug,
4... Spring, 5, 6... End plate, 7, 9... Body part,
8... Expansion. Agent Patent Attorney Akita Takahashi'tiyr Wall D '200 400 400 800 7θ00m;
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Claims (1)

[Claims] 1. By weight, C0.02 to 0.2%, Si 1.5 or less. Mn 3% or less, Cr10-25%, Ni8-25%
, Mo0.1-3%, Nb0.02-0.5% and V
1. A heat-resistant steel characterized by containing at least one type of 0.05 to 0.5 mm, the remainder being 50% or more of PE, and having a cold plastic working state. 2. By weight, C0102~0.2%, Si 1.5% or less. Mn 3% or less, Cr1O~25%, Ni8~25%, M
o0.1-3%, Nb0.02-0.5% and Vo,
05-0.5% and CuO, 5-5%
, Bo, 0005~0.02%, Ti0.01~0.2
%, At0.005~0.2% and Zr01OO5~0
．． 2% of at least one species, and the remaining 50% or more of p
A heat-resistant steel characterized by having a cold plastic working state. 3. By weight, CO, 02-0.2%, Si 1.5% or less. Mn 3% or less, Cr1O~25%, Ni8~25%, M
o0.1-3%, NbO,02-0.5% and V
O, 05 to 0.5%, and the balance is 50%.
A fuel cladding tube for a fast breeder reactor, characterized in that the fuel cladding tube has a PE of at least % and is in a cold plastic working state. 4. By weight, CO, 02%, Si 1.5% or less, Mn3
% or less, 0rlO~25%, Ni8~25%. Mo0.1-3%, Nb0.02-0.5% and VO
, 05 to 0.5%, and the remainder is Fe of 50 to 0.5%, and has a cold plastic working state, and the member constitutes at least one of the end plate and the expansion. A styrene synthesis tower characterized by