KR102411794B1 - Ti-RAFM having good mechanical properties, and manufacturing method thereof - Google Patents

Abstract

Provided are a Ti-containing low-emission steel having excellent mechanical properties and a method for manufacturing the same.
Low-emission steel of the present invention, by weight%, C; 0.05 to 0.2%, Cr: 7 to 10%, W: 1 to 3%, V: 0.05 to 0.3%, Ti: 0.01 to 0.15%, Mn: 0.1 ~1%, Si: 0.5% or less, Al: 0.1% or less, P: 0.02% or less, S: 0.01% or less, contains residual Fe and unavoidable impurities, does not contain Ta, and its matrix structure is tempered martensite It is structured, and nano-sized fine (Ti,W)C carbides and fine M23C6 carbides are precipitated in the martensitic structure.

Description

Ti-containing low-radiation steel with excellent mechanical properties and a manufacturing method thereof {Ti-RAFM having good mechanical properties, and manufacturing method thereof}

The present invention relates to the production of low-emissivity steel materials by fusion reactor containing Ti having excellent mechanical properties. The present invention relates to a Ti-containing fusion furnace low-emission steel material having excellent strength and toughness and a method for manufacturing the same.

Conventionally, austenitic 316 stainless alloy has been widely used as the fuel clad pipe and wrapper pipe of the high-speed breeder furnace and the first furnace wall material of the fusion reactor. However, since the austenitic alloy has low swelling resistance, stress corrosion cracking (SCC) resistance, and high embrittlement susceptibility to sodium and helium, 8 to 13 having excellent resistance to the above phenomenon to improve these problems A ferritic alloy containing % chromium can be used as a candidate material.

However, ferritic steels containing 8 to 13% chromium have many superior properties compared to austenitic steels, but have low impact toughness, embrittlement when used for a long time at high temperatures, and weldability. This disadvantage needs to be improved in order to replace the 316 stainless alloy because it is not good.

On the other hand, high-chromium ferritic/martensitic low-emission steel is manufactured by dissolving raw materials in a vacuum induction method, and then sequentially performing hot rolling, normalizing, tempering, cold rolling, and final heat treatment processes. Here, normalizing and tempering are performed for 1 hour in a temperature environment of 1050° C. and 750° C., respectively, and then the rolling reduction during cold rolling is approximately 75%. And the final heat treatment is made for 30 minutes at a temperature of 700 ℃. And the high chromium ferritic/martensitic steel manufactured in this way has mechanical properties similar to that of tempering heat treatment at a temperature of 730~800℃. Therefore, there is a limit to improving strength by changing manufacturing parameters such as tempering temperature, cold rolling, and final heat treatment during the manufacturing of general high chromium ferritic/martensitic steel. In particular, there is a problem that the yield strength and tensile strength are insufficient in a high temperature environment of 600° C. or more.

Therefore, in the low-emission material, molybdenum and niobium, which are important strengthening elements of the conventional 8-13% chromium ferritic alloy, are replaced with tungsten and tantalum, respectively, and a part of nickel is replaced with manganese, cobalt, etc. Although alloys that can satisfy both impact toughness and creep strength have been reported, there is a continuing demand for the development of low-emission steels with excellent tensile strength and impact toughness as well as guaranteeing thermal stability at high temperatures. the current situation.

Therefore, the present invention has been devised to solve the problems of the prior art, and by precipitating fine (Ti,W)C nano-carbides in the matrix tissue, it contains Ti, which has excellent thermal stability at high temperature as well as excellent tensile strength and toughness. An object of the present invention is to provide a low-emission steel material for a nuclear fusion reactor.

Another object of the present invention is to provide a method for manufacturing the low-emission steel.

In addition, the problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.

The present invention for achieving the above object,

In wt%, C:0.05~0.2%, Cr:7~10%, W:1~3%, V:0.05~0.3%, Ti:0.01~0.15%, Mn:0.1~1%, Si:0.5% or less, Al: 0.1% or less, P: 0.02% or less, S: 0.01% or less, residual Fe and unavoidable impurities, but not Ta,

The matrix structure is a tempered martensitic structure, and nano-sized fine (Ti,W)C carbide and fine M23C6 carbide are precipitated in the martensitic structure. Ti containing excellent strength and toughness It relates to low-radiation steel.

Also, the present invention

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In wt%, C:0.05~0.2%, Cr:7~10%, W:1~3%, V:0.05~0.3%, Ti:0.01~0.15%, Mn:0.1~1%, Si:0.5% Below, Al: 0.1% or less, P: 0.02% or less, S: 0.01% or less, a process of casting after melting a steel material containing residual Fe and unavoidable impurities and not including Ta;

hot rolling the cast steel to a predetermined thickness;

Normalizing the hot-rolled steel in a temperature range of 900 to 1100° C. for 20 minutes to 2 hours, and then rapidly cooling to room temperature to make the microstructure into a martensitic structure; and

The process of precipitating nano-sized fine (Ti,W)C carbides and fine M23C6 carbides in the martensitic structure by heating the steel material having the structure to a temperature range of 500 to 750 ° C, and then tempering for 1 to 3 hours; It relates to a method for manufacturing a Ti-containing low-emission steel having excellent strength and toughness, including:

The present invention having the configuration as described above has excellent tensile strength and impact toughness while having low radiation characteristics, so that it can be effectively used in high-temperature and high-pressure areas of nuclear power plants, high-speed breeder reactors, and fusion reactors.

In addition, as a steel material with excellent high-temperature creep strength, it has the advantage of being able to weld and form a fuel pipe, a pressure pipe, and a pressure-resistant plate material, and it also has the effect of improving the performance, lifespan and economic feasibility of equipment.

1 is a SEM photograph showing the microstructure of the invention steel (Ti-RAFM) in comparison with that of the comparative steel (Eurofer97) in this example.
FIG. 2 is a TEM microstructure photograph of the invention steel of FIG. 1. FIG.
3 is a diagram showing the tensile properties of the invention steel (Ti-RAFM) and the comparative steel (Eurofer97) in this example.
4 is a diagram showing the Charpy impact characteristics of the invention steel (Ti-RAFM) and the comparative steel (Eurofer97) in this example.
5 is a diagram showing the high-temperature tensile strength (TS) and yield strength (YS) characteristics of the invention steel and the comparative steel in this Example.
6 is a diagram showing that the invention steel in this embodiment has improved strength characteristics without compromising the toughness compared to the comparative steel.

Hereinafter, the present invention will be described.

Ti-containing low-emission steel of the present invention, by weight, C; 0.05 to 0.2%, Cr: 7 to 10%, W: 1 to 3%, V: 0.05 to 0.3%, Ti: 0.01 to 0.15%, Mn :0.1~1%, Si:0.5% or less, Al:0.1% or less, P:0.02% or less, S:0.01% or less, contains residual Fe and unavoidable impurities, and the matrix structure becomes a tempered martensitic structure And, nano-sized fine (Ti,W)C carbide and fine M23C6 carbide are precipitated in the martensitic structure.

First, the steel composition component of the low-emission steel of the present invention and the reason for its limitation will be described.

・C: 0.05 to 0.2%

As an austenite stabilizing element, carbon is supersaturated in steel and combines with elements such as chromium, vanadium, and tungsten during quenching, tempering, or use to form precipitates to improve the strength of steel. In addition, carbon greatly affects room temperature and high temperature strength, weldability, formability, and the like.

If the carbon content is less than 0.05%, the mechanical strength may be insufficient at room temperature, and if the carbon content exceeds 0.2%, the weldability and formability are deteriorated, which causes the toughness of the steel to be deteriorated. Therefore, in the present invention, it is preferable to add carbon in the range of 0.05 to 0.2%.

·Cr: 7~10%

Chromium, as a ferrite stabilizing element, is an essential element in materials used for high-temperature and high-pressure areas that require oxidation resistance, corrosion resistance and creep strength at the same time.

If the chromium content is less than 7%, the oxidation resistance and corrosion resistance of the steel may deteriorate, and if it exceeds 10%, the strength and toughness of the steel may deteriorate due to delta ferrite formation. Therefore, in the present invention, it is preferable to add chromium in the range of 7 to 10%.

・W:1~3%

Since tungsten is dissolved in the iron matrix and interferes with the diffusion of iron atoms, it has the effect of delaying the recovery and recrystallization rate of the dislocation structure. effective on However, since tungsten is a strong ferrite stabilizing element, it is difficult to suppress the formation of delta ferrite by greatly increasing the chromium equivalent when the amount of tungsten is increased. In addition, it generates the Laves phase (Fe2W) and M6C phase with fast growth rates to increase the creep strength for a long time. lower and induce creep brittleness. In the present invention, at least 1% is added to obtain the effect of adding tungsten, and it is limited to 3% or less for toughness and long-term creep strength.

・V: 0.05 to 0.3%

Vanadium is a ferrite stabilizing element and has a very strong tendency to form carbides, so it combines with solid solution carbon and nitrogen in steel to form V(C,N) or V4(C,N)3-type carbonitrides, thus greatly increasing creep strength. do. However, if the amount added is large, the coarse carbonitride produced loses the coherence with the matrix structure, thereby reducing the toughness of the steel material. If the amount added is small, the desired strength of the steel material cannot be obtained.

In consideration of this, in the present invention, it is preferable to add vanadium in the range of 0.05 to 0.3%.

Ti: 0.01~0.15%

Titanium, as the most essential element in the present invention, serves to improve the tensile strength and toughness of steel by forming nano-sized fine Ti-based carbides such as (Ti,W)C. However, if the added amount is excessive, the toughness of the steel may deteriorate due to the formation of coarse carbonitrides, and if it is too small, the desired strength of the steel may not be secured.

Therefore, in the present invention, titanium is preferably added in the range of 0.01 to 0.15%.

·Mn: 0.1~1%

Manganese is an austenite stabilizing element and has a solid solution strengthening effect.

In the present invention, it is preferable to limit the addition amount of manganese in the range of 0.1 to 1%. If it is less than 0.1%, the desired strength of the steel cannot be obtained, and if it exceeds 1%, the weldability of the steel may deteriorate.

Si: 0.5% or less

Silicon is a strong ferrite stabilizing element and can be used together with aluminum as a deoxidizer. Silicon not only increases the tendency to form delta ferrite, but also increases the precipitation of the Labes phase (Fe2W) and carbide, and promotes cohesive coarsening, thereby causing creep brittleness. In the present invention, the silicon residual content is limited to a maximum of 0.5% or less from the viewpoint of deoxidation and toughness.

・Al: 0.1% or less

Aluminum is a ferrite stabilizing element and is used as a deoxidizer in the present invention. Aluminum does not significantly affect ductility, but if it remains above an appropriate content, it consumes all of the dissolved nitrogen in the steel to generate aluminum nitride (AIN), making it difficult to form carbonitrides such as V(C,N), thereby lowering the creep strength for a long time. . Therefore, in the present invention, the aluminum residual content is limited to a maximum of 0.1% or less.

·

P: 0.02% or less, S: 0.01% or less

Phosphorus and sulfur are inevitably present in iron, and if their content is high, they cause grain boundary drop and are harmful to toughness and creep strength. Therefore, it is limited to 0.02% and 0.01% or less, respectively.

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Next, a method for manufacturing a low-radiation steel material of the present invention will be described.

The present invention is a process of casting after melting the steel material composed as described above; hot rolling the cast steel to a predetermined thickness; Normalizing the hot-rolled steel in a temperature range of 900 to 1100° C. for 20 minutes to 2 hours, and then rapidly cooling to room temperature to make the microstructure into a martensitic structure; and tempering the steel having the structure at a temperature range of 500 to 750° C. for 1 to 3 hours.

First, in the present invention, after melting the steel material composed as described above, it is cast. The present invention is not limited to the above melting and casting methods, and various methods can be used. For example, a vacuum induction melting method may be used as an example of the melting process.

Next, in the present invention, the cast steel is hot-rolled to a predetermined thickness. The present invention is also not limited to this specific hot-rolling method, and well-known and conventional hot-rolling methods can be used. And the hot-rolled steel product can be made to a desired final thickness.

And in the present invention, the hot-rolled steel is normalized for 20 minutes to 2 hours at a temperature range of 900 to 1100 ℃. Through this normalizing treatment, the microstructure is made into austenite and, after subsequent cooling, a martensitic structure can be effectively obtained. In addition, it is possible to achieve fine precipitation in the subsequent tempering process by re-dissolving carbonitrides by maintaining high temperature.

In the present invention, it is preferable to set the normalizing treatment temperature in the range of 900 to 1100 ° C. If it is less than 900 ° C, not only the structure may not be fully austenitized, but also the re-dissolution of carbonitride may be insufficient, and 1100 ° C. If it is exceeded, austenite grains may become coarse and toughness may deteriorate, and delta ferrite may be generated to deteriorate strength and toughness.

In addition, it is preferable to limit the normalizing time to 20 minutes to 2 hours, but if it is less than 20 minutes, not only may the austenitization of the structure be insufficient, but also the dissolution of carbonitride may be insufficient, and if it exceeds 2 hours, the grains become coarse because it can be

Next, in the present invention, by rapidly cooling the normalized hot-rolled steel material to room temperature, a steel material having a martensitic structure as the microstructure is manufactured.

Finally, in the present invention, after heating the steel material having the martensitic structure to a temperature range of 500 to 750 ° C., by tempering for 1 to 3 hours, nano-sized (Ti,W)C carbides and fine M23C6 carbide precipitates.

This tempering process makes it possible to secure the toughness of the steel by forming a tempered martensitic structure through tempering the martensitic structure formed by the normalizing. In addition, by forming M23C6 carbide in the matrix structure, the creep property of steel is improved, and the strength of steel can be secured by precipitating nano-sized fine (Ti,W)C carbide.

In the present invention, it is preferable to limit the tempering treatment temperature to a range of 500 to 750 ° C. If the temperature is less than 500 ° C, (Ti, W) C does not precipitate and the strength may be lowered, and the tempering effect is insignificant. As a result, there may be a problem in securing toughness, and when it exceeds 750°C, M23C6 precipitates may be coarsened, and reverse transformation may occur due to austenitization.

In addition, the tempering time is preferably limited to 1 to 3 hours, if less than 1 hour, the precipitation of precipitates may be insufficient, and the tempering effect may be insufficient, which may cause a problem in toughness. On the other hand, if it exceeds 3 hours, the precipitate may become coarse, and this is because a decrease in strength may occur due to over-tempering.

The low-emission steel of the present invention manufactured through the manufacturing process as described above is a steel having excellent strength and toughness by precipitating nano-sized fine (Ti,W)C carbide and fine M23C6 carbide in the tempered martensite structure, which is a matrix structure. can be obtained effectively.

Hereinafter, the present invention will be described in detail through examples.

(Example)

C Si Mn Cr W V Ta Ti Comparative steel (Eurofer97) 0.10 0.11 0.40 9.3 0.93 0.22 0.094 - Invention Steel (Ti-RAFM) 0.10 0.13 0.41 9.3 0.95 0.23 - 0.064

* In Table 1, the unit is % by weight.

Steel materials having steel composition components as shown in Table 1 were prepared, respectively. The steel materials prepared as described above were each melted in a vacuum induction furnace, and then cast in a conventional manner to prepare steel materials. Then, a hot-rolled material having a thickness of 7t was prepared by hot-rolling the manufactured steel material.

Each of the hot-rolled materials prepared as described above was subjected to normalizing treatment at 1000° C. for 1 hour, and then rapidly cooled to room temperature to obtain a martensitic microstructure of the steel. And among the steel materials, the invention steel was tempered at 650°C for 2 hours, and the comparative steel was tempered at 750°C for 2 hours and then cooled to room temperature to prepare a final product.

1 is an SEM photograph showing the microstructure of the invention steel (Ti-RAFM) prepared as described above in comparison with the comparative steel (Eurofer97). As shown in FIG. 1 , it can be seen that the inventive steel has an internal structure similar to that of the comparative steel.

Meanwhile, FIG. 2 is a photograph of the TEM microstructure of the inventive steel (Ti-RAFM), in which fine M23C6 precipitates in micro (㎛) units are precipitated in the matrix structure, and also fine (Ti, It can be seen that W)C carbides are precipitated and distributed.

3 is a diagram showing the tensile properties of the inventive steel (Ti-RAFM) and the comparative steel (Eurofer97), and FIG. 4 is a comparison showing the Charpy impact properties of the inventive steel (Ti-RAFM) and the comparative steel (Eurofer97). It's a picture. As shown in FIGS. 3-4 , it can be seen that the inventive steel has superior tensile properties than the comparative steel, and also has impact toughness comparable to that of the comparative steel. Table 2 below numerically compares the tensile properties and Charpy impact properties of the inventive steel (Ti-RAFM) and the comparative steel (Eurofer97).

Tensile properties (MPa, %) Charpy characteristics (℃, J/cm ² ) YS ts EL DBTT USE Comparative steel (Eurofer97) 538 657 17 -68 266 Invention Steel (Ti-RAFM) 573 698 18 -69 253

* In Table 2, YS is the yield strength, TS is the tensile strength, EL is the elongation, DBTT is the ductility-brittle transition temperature, and USE is the maximum absorbed energy.

Meanwhile, Figure 5 shows the high-temperature tensile strength (TS) and yield strength (YS) characteristics of the inventive steel (Ti-RAFM) and the comparative steel (Eurofer97). It can be seen that this is excellent.

And FIG. 6 is a diagram showing that the strength characteristics are improved without compromising the toughness compared to the inventive steel (Ti-RAFM) and the comparative steel (Eurofer97).

In the above, the embodiments of the present invention have been described, but those of ordinary skill in the art to which the present invention pertains will understand that the present invention may be implemented in other specific forms without changing the technical spirit or essential features. . Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (4)

By weight%, C:0.05~0.2%, Cr:7~10%, W:1~3%, V:0.05~0.3%, Ti:0.01~0.15%, Mn:0.1~1%, Si:0.5% or less, Al: 0.1% or less, P: 0.02% or less, S: 0.01% or less, residual Fe and unavoidable impurities, but not Ta,
The matrix structure is a tempered martensitic structure, and nano-sized fine (Ti,W)C carbide and fine M23C6 carbide are precipitated in the martensitic structure. Ti containing excellent strength and toughness Low-emission steel.
delete By weight%, C:0.05~0.2%, Cr:7~10%, W:1~3%, V:0.05~0.3%, Ti:0.01~0.15%, Mn:0.1~1%, Si:0.5% Below, Al: 0.1% or less, P: 0.02% or less, S: 0.01% or less, a process of casting after melting a steel material containing residual Fe and unavoidable impurities and not including Ta;
hot rolling the cast steel to a predetermined thickness;
Normalizing the hot-rolled steel in a temperature range of 900 to 1100° C. for 20 minutes to 2 hours, and then rapidly cooling to room temperature to make the microstructure into a martensitic structure; and
The process of precipitating nano-sized fine (Ti,W)C carbides and fine M23C6 carbides in the martensitic structure by heating the steel material having the structure to a temperature range of 500 to 750 ° C, and then tempering for 1 to 3 hours; A method for producing a Ti-containing low-emission steel with excellent strength and toughness.
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