JP2002339035A - Ferritic heat resistant steel and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferritic heat resistant steel which has improved creep strength at a cost equal to that for the conventional one by suppressing the generation of an MC pair, and enabling the steel to effectively exhibit the effect of strengthening solid solution. SOLUTION: The ferritic heat resistant steel has a composition containing, 0 to 0.01% C, 0.4 to 4.0% Cr, 0.05 to 0.8% V, 0 to 2.0% Mo, 0 to 4% W, 0.2 to 2% (Mo+W/2) and 0.005 to 0.2% N by weight, and the balance Fe with inevitable impurities. Alternatively, the ferritic heat resistant steel has a composition containing 0 to 0.01% C, 12 to 27% Cr, 0.05 to 2.0% V, 0 to 2.0% Mo, 0 to 4% W, 0.2 to 2% (Mo+W/2) and 0.005 to 0.5% N by weight, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant ferritic steel requiring high creep strength, such as a boiler tube, a heating tube, and a turbine rotor, and a method for producing the same.

[0002]

2. Description of the Related Art Hitherto, in order to obtain a high creep strength by strengthening a ferritic heat-resistant steel, a method is mainly used in which dislocations in the steel are hardly moved. There are various strengthening methods such as precipitation strengthening by usually precipitating carbide in steel and solid solution strengthening by adding molybdenum (Mo) or tungsten (W) to form a solid solution. Have been applied.

[0003] Here, Mo and W dissolved in steel have a high chemical affinity with C, and therefore, are paired with C in steel not precipitated as carbide (hereinafter referred to as MC pair). Easy to form. When Mo or W forms a Cottrell atmosphere around the edge dislocation, it is considered that an MC pair is formed. It seems that there is a strain that expands the atomic lattice around this MC pair, so there are atomic vacancies that have an affinity interaction with this strain around the MC pair. Probability is high. If the atomic vacancy exists around M of MC, the diffusion coefficient of M becomes large and the mobility of the MC pair itself becomes large, so that it is presumed that the dislocation is likely to move. Therefore, C in steel due to carbide formation
It was found that when C was added, this C might inhibit the effect of solid solution strengthening by Mo and W.

[0004]

Therefore, in the present invention,
Taking into account the relationship between solid solution strengthening and precipitation strengthening, the creep strength is improved at the same cost as conventional ferritic heat-resistant steel by suppressing the occurrence of MC pairs and effectively exerting the effect of solid solution strengthening. It is an object of the present invention to provide a heat-resistant ferritic steel.

[0005]

In order to achieve the above object, the ferritic heat-resistant steel according to the present invention has a C content of 0% by weight.
~ 0.01%, Cr 0.4 ~ 4.0%, V 0.05 ~ 0.8%, Mo 0 ~
2.0%, W is 0-4%, (Mo + W / 2) is 0.2-2%, N is 0.005--0.
2%, with the balance substantially consisting of Fe and unavoidable impurities. The content ranges of C, Mo, and W include "0%", which is a state in which C is not added at all. However, for Mo and W, the Mo equivalent (Mo + W / 2) should be 0.2-2%.
The amount of each addition is determined. Since the amount of C added is much smaller than that of the conventional heat-resistant ferritic steel, the amount of so-called MC pairs is reduced, and the solid solution strengthening by Mo, W, or the like can be maximized.

[0006] In one embodiment of the heat-resistant ferritic steel according to the present invention, 0.0001 to 0.02% of B, 0.01 to 1.0% by weight.
% Si, 0.01-1.5% Mn, 0.01-1.0% Ni, 0.01%
It further contains at least one element from the group consisting of Co and less than 0.1%. B, Ni, Co
Is effective in improving hardenability and the like, and Si and Mn have an effect as a deoxidizing agent.

Further, in another embodiment of the heat-resistant ferritic steel according to the present invention, at least one of elements selected from the group consisting of Nb, Ta, Ti, Hf and Zr is further contained in an amount of 0.01 to 0.2% by weight. . The Nb, Ta, Ti, Hf, and Zr are N
To form a so-called MX type nitride, thereby strengthening the heat-resistant steel by precipitation strengthening. Then, the method for producing a ferritic heat-resistant steel according to the present invention is a method of cooling a steel having the above composition at a cooling rate of 0.01 to 100 ° C./sec after keeping the steel having a temperature of 870 to 1250 ° C. for 5 to 1200 minutes. Performing a chemical conversion treatment, and at a temperature of 500 to 850 ° C.
Performing a tempering treatment of cooling at a cooling rate of 0.01 to 10 ° C./sec after holding for 20002000 minutes. The above-mentioned manufacturing method is a method based only on heat treatment of solution treatment and tempering. By this heat treatment, the structure can be controlled efficiently.

Further, the ferritic heat-resistant steel according to the present invention has a C content of 0 to 0.01%, a Cr content of 12 to 27%, and a V content of 0.05% by weight.
~ 2.0%, Mo 0 ~ 2.0%, W 0 ~ 4%, (Mo + W / 2) 0.2 ~ 2
%, N 0.005 to 0.5%, and the balance substantially consists of Fe and unavoidable impurities. Since this heat-resistant steel also has a very small C content, the amount of so-called MC pairs can be suppressed, and the solid solution strengthening by Mo, W, etc. can be maximized. Further, in one embodiment of the heat-resistant ferritic steel according to the present invention, 0.0001 to 0.04% of B, 0.01 to
It further contains at least one element from the group consisting of 1.0% Si, 0.01 to 1.5% Mn, and 0.01% or more and less than 0.1% Co.

[0009] In another embodiment of the heat-resistant ferritic steel according to the present invention, at least one of elements selected from the group consisting of Nb, Ta, Ti, Hf and Zr is contained in an amount of 0.01 to 0.5% by weight. Incidentally, the method for producing a ferritic heat-resistant steel according to the present invention, the steel having the above composition in a temperature range of 900 to 1300 ° C. for 5 to 600 minutes, and then cooled at a cooling rate of 0.1 to 30 ° C./sec. Performing a conversion process and 500
Performing plastic working at a temperature of 401300 ° C. and a working rate of 40% or more. The plastic working includes various processes such as rolling and drawing, and the working ratio means, for example, in the case of rolling, the ratio of the thickness of the sheet after processing to the thickness of the sheet before processing. .

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a heat-resistant ferritic steel according to an embodiment of the present invention and a method for producing the same will be described in detail. Conventional ferritic heat-resistant steels have been developed to increase the creep strength by adding C to precipitate carbides, and by adding solid solution strengthening elements such as Mo and W into the steel to form a solid solution. A strengthening method such as solid solution strengthening is applied. The precipitation strengthening by the carbide is
This is a strengthening method that reduces the speed of creep deformation by inhibiting the movement of dislocations that cause creep deformation by carbides.

On the other hand, solid solution strengthening by adding Mo, W, etc. occurs because Mo, W, etc. are unevenly distributed around the edge dislocations so as to relax the strain field existing around the edge dislocations. This uneven distribution is called a Cottrell atmosphere. When the energy of the edge dislocation is relaxed by the Cottrell atmosphere, the speed of the recovery that occurs during creep is reduced, and the speed of the creep deformation is reduced.

However, in a high-temperature environment in which Mo, W, and the like can diffuse and move in accordance with the movement of the edge dislocations, the disappearance of the dislocations due to the recovery is higher than the relaxation of the energy of the edge dislocations due to the Cottrell atmosphere. Since the dislocation is energetically stable, the speed of the recovery is high. That is, when the temperature increases and a solid solution strengthening element such as Mo or W can easily accompany the dislocation movement during recovery, the effect of solid solution strengthening is significantly reduced. When the combination of precipitation strengthening by the precipitation of carbide and solid solution strengthening by adding a solid solution element to form a solid solution is performed, the solid solution strengthening element dissolved in steel is chemically Because of its high affinity, it is easy to form C and MC pairs (M is a solid solution strengthening element such as Mo and W) in steel that did not precipitate as carbides. Specifically, even when M moves by diffusion, M moves close to C as if it were dragging or guided by C. And
When M forms a Cottrell atmosphere around the edge dislocations, it is considered that an MC pair is formed.

Here, the structure of the MC pair will be briefly described. Solid solution strengthening elements such as Mo and W, which are equivalent to M, are substitutional elements in ferritic steel, and are BCC (body-centered cubic lattice).
It exists at the position of a lattice point in an atomic lattice in ferritic steel having a structure. Since Mo and W have a larger atomic radius than Fe, which is the main component of the ferritic steel's parent phase, the presence of Mo and W distorts the surrounding atomic lattice so as to expand. On the other hand, since the atomic radius of C is small and exists between atomic lattices, a distortion occurs that expands the surrounding atomic lattice. When M and C are adjacent or close to each other, it is thought that there is a strain around the periphery that causes the atomic lattice to expand. Therefore, there is a high possibility that the vacancies having an affinity interaction with the strain that expands the atomic lattice accompany the periphery of the MC pair in order to alleviate the strain. The atomic vacancies are point defects in which no atoms exist at lattice points. If an atomic vacancy exists around M in MC, M
It is estimated that the mobility of the MC pair itself, which is considered to be the rate limiting factor for M, is also large.

That is, when a certain amount or more of C is added to steel to form carbides, MC pairs are formed, and dislocations in the steel are easily moved by the entrainment of atomic vacancies by the MC pairs. The effect of reinforcement is reduced. Therefore, MC
If solid solution strengthening by M alone can be realized without forming pairs around the edge dislocations, creep rupture strength will be significantly improved.

As described above, the inventors of the present application have made intensive studies and found that the amount of C added is extremely low, and if possible, 0 wt%, so that the solid solution strengthening of Mo and W can be maximized. In addition, Nb, V, Ti, Ta, etc. are added to N to form an MX-type nitride (hereinafter, referred to as MN) to exert the effect of precipitation strengthening, thereby making ferritic heat-resistant steel They have come to realize that the creep strength can be greatly improved.

Hereinafter, the amount of each element added and the method for producing a heat-resistant ferritic steel will be described. Note that the addition amount of some elements differs between the case where the structure control is performed only by heat treatment and the case where the structure control is performed by working heat treatment in which heat treatment is combined with plastic working. In the following, all units of% are% by weight.

[Addition amount of each element] Carbon (C) is desirably as small as possible, preferably 0%, in order to reduce the formation of MC pairs. Therefore, in order to remarkably produce the effect of reducing the formation of MC pairs, it is necessary to set the addition amount to 0 to 0.01%. Preferably, it is 0 to 0.008%. Although the required amount of chromium (Cr) varies depending on the use temperature and the use environment, it is indispensable to impart corrosion resistance and oxidation resistance to the heat-resistant ferritic steel, and it is desirable that the addition amount is large. Also,
As described above, the range of the appropriate amount of addition differs between the case where the structure of the heat-resistant steel is controlled only by heat treatment and the case where the structure is controlled by thermomechanical treatment. In the case where the structure is controlled only by the heat treatment, the appropriate amount of Cr is 0.4 to 4.0%. Preferably, it is 0.5 to 3.0%. On the other hand, when thermomechanical treatment is used to control the structure, Cr has the effect of imparting high oxidation resistance and corrosion resistance.
In view of the fact that the steel is easily embrittled when the addition amount is too large, the Cr content is suitably 12 to 27%. Preferably, it is 13.5 to 26.0%.

Vanadium (V) combines with N to form a nitride and contributes to precipitation strengthening. In addition, C contained in steel
It is believed that the formation of carbides is associated with the formation of the carbides, so that the concentration of C in the parent phase is reduced, which has the effect of inhibiting the formation of MC pairs. However, if added excessively, it becomes impossible to control the structure by quenching, and if added too much, the steel causes embrittlement. Therefore, when controlling the structure only by heat treatment (when the Cr content is 0.4 to 4.0%), the appropriate amount of V is 0.0
It is 5 to 0.8%, preferably 0.1 to 0.6%. When microstructure is controlled by thermomechanical treatment (Cr content is 12 to 27%)
Should be set at 0.05 to prevent steel from embrittlement.
To 2.0%, preferably 0.2 to 1.2%.

Molybdenum (Mo) is one of the solid solution strengthening elements. However, if added in a large amount, the steel causes embrittlement. Therefore, the addition amount is suitably 0 to 2.0%. Preferably, it is 0 to 1.7%. Tungsten (W) is also one of the solid solution strengthening elements, but if added in a large amount, the steel causes embrittlement, so that the addition amount is suitably 0 to 4.0%. Preferably, it is 0 to 3.4%. However, the above Mo and
Since W functions as a solid solution strengthening element by adding each alone or in combination, it is necessary to have a so-called Mo equivalent (Mo + W / 2) of 0.2% or more to exhibit its effect. . However, if Mo and W are added excessively, the steel becomes brittle, so that (Mo + W / 2) is suitably 2.0% or less. Preferably,
0.3-1.7%.

Nitrogen (N) has an important effect that it combines with V or Nb, Ti, Ta, Hf, Zr, etc. to form a nitride (MN) and contributes to precipitation strengthening. In the case of controlling the structure by heat treatment, if the amount of N is too small, quenching becomes difficult. However, if added in excess, heat treatment
Since MN cannot be sufficiently dissolved in the matrix and MN cannot be finely dispersed and precipitated, the contribution of MN to precipitation strengthening decreases. Taking these into consideration, the amount of N to be added should be set to 0.3 when the structure is controlled by heat treatment (when the Cr content is 0.4 to 4.0%).
005-0.2%, preferably 0.006-0.05%. On the other hand, when controlling the structure by thermomechanical treatment (Cr content is 12 to 27%
In the case of), 0.005 to 0.5% is appropriate. Preferably, 0.
01 to 0.15%.

Boron (B) has the effect of increasing the hardenability and the effect of increasing the grain boundary strength.
The steel becomes brittle. Therefore, the amount of B added is 0.001 when the structure is controlled only by heat treatment (when the Cr content is 0.4 to 4.0%).
0.00.02%, preferably 0.001 to 0.006%. on the other hand,
When the structure is controlled by the thermomechanical treatment (when the Cr content is 12 to 27%), 0.001 to 0.04% is appropriate, and preferably 0.001 to 0.04%.
~ 0.012%.

Silicon (Si) plays a role as a deoxidizing agent in the steel making process and also has an effect of contributing to the improvement of the oxidation resistance of steel. However, if added excessively, it becomes brittle. Therefore, 0.01 to 1.0% is appropriate as the addition amount of Si. Preferably, it is 0.1 to 0.9%. However, the addition of Si is not always necessary when a deoxidizing step using no Si is performed and the oxidation resistance is sufficient for the use environment. Manganese (Mn) plays a role as a deoxidizing agent in a steel making process, and has an effect of improving hardenability and improving hydrogen resistance. However, if added in excess, it has a bad effect on creep strength, so the amount of Mn added is 0.
The appropriate amount is from 01 to 1.5%, preferably from 0.2 to 1.2%. However, Mn is not necessarily required when a deoxidizing step using no Mn is performed, a heat treatment for obtaining a sufficient cooling rate is performed, and sufficient hydrogen resistance to the use environment is obtained.

Nickel (Ni) is effective in improving hardenability, but has an adverse effect on creep strength. Therefore, the appropriate amount of Ni added is 0.01 to 1.0%. Preferably, 0.01 to
0.04%. However, it is not necessarily required when the device is manufactured under conditions that provide sufficient hardenability. Cobalt (Co)
Is effective in improving the hardenability and also in improving the creep strength, but is very expensive, so that excessive addition of it increases the material cost of steel. Therefore, an appropriate amount of Co is 0.01% or more and less than 0.1%. Preferably, it is at least 0.04% and less than 0.1%. However, when the material cost needs to be reduced and sufficient creep strength is obtained, and when the hardenability of steel is sufficiently obtained, Co is not necessarily required.

Niobium (Nb), tantalum (Ta), titanium (Ti), hafnium (Hf), or zirconium (Zr)
Are added alone or in combination of two or more,
By forming a nitride called MN in combination with N, it contributes to precipitation strengthening. In addition, it is considered that since carbides are formed by bonding with C contained in the steel, the concentration of C in the parent phase is reduced, which has an effect of inhibiting the formation of MC pairs. However, when excessively added, MN cannot be sufficiently dissolved in the parent phase by heat treatment, and MN cannot be finely dispersed and precipitated, so that the contribution of MN to precipitation strengthening decreases.

Considering these, Nb, Ta, Ti, Hf, Zr
The amount of Cr added is controlled when the structure is controlled only by heat treatment.
In the case of 4 to 4.0%), 0.01 to 0.2% is appropriate, and preferably 0.02 to 0.07%. On the other hand, when controlling the structure by thermomechanical treatment (when the Cr content is 12 to 27%), 0.01 to 0.5
% Is appropriate, and preferably 0.02 to 0.12. The unavoidable impurities are P, S, Cu, Sb, Sn, As, Al, O, and the like, and are elements that may remain in steel in the current steelmaking technology. However, when microstructure control is performed by thermomechanical treatment (when the Cr content is 12 to 27%), the presence of Ni has an adverse effect on creep strength, but it may remain in the steel in the current steelmaking technology. is there.

[Production method of heat-resistant ferritic steel] Heat-resistant ferritic steel is manufactured by using a steel having the above-mentioned composition and treating it under the following conditions. When controlling the structure only by heat treatment, that is, solution treatment and tempering (Cr content is
In the case of 0.4 to 4.0%), first, a solution treatment is performed in which the temperature is kept at 870 to 1250 ° C. for 5 to 1200 minutes and then cooled at a cooling rate of 0.01 to 100 ° C./sec. After this, the temperature of 500 ~ 850 ℃
After holding for 5 to 2000 minutes, a tempering treatment is performed to cool at a cooling rate of 0.01 to 10 ° C./sec. On the other hand, when thermomechanical treatment is used for microstructure control (Cr content is 12 to 27%), first,
After holding at 900 ~ 1300 ℃ for 5 ~ 600 minutes, cooling rate 0.1 ~
Solution treatment for cooling at 30 ° C / sec is performed. After this, 500
A thermomechanical heat treatment of applying plastic working at a working rate of 40% or more at ~ 1300 ° C.

[0027]

Next, the present invention will be described specifically with reference to examples. Table 1 shows the compositions of the heat-resistant ferritic steels of the present invention. First, the materials A1 to A10 of the present invention having the compositions shown in Table 1 were prepared.
The ferrite heat-resistant steel was prepared by melting and forging the B1 to B4 materials and the comparative materials. In Table 1, A1 to A6 are Cr
The amount is 0.4 to 4.0%, and the A7 to A10 materials have a Cr amount of 12 to 27%.

[0028]

[Table 1]

Next, Table 2 shows the contents of the heat treatment and the thermomechanical treatment of the ferritic heat-resistant steel of the present invention. Under the conditions shown in Table 2, the above-mentioned A1 to A10 and B1 to B4 The material was subjected to heat treatment and thermomechanical treatment. Of these, A1
A6 to B6 and B1, B2 only heat treatment, A7 to A10 and B
3, B4 material has been subjected to thermomechanical treatment. The comparative materials B1 to B4 are ferritic heat-resistant steels to which C is added in an amount of 0.04% or more in view of the effect of precipitation strengthening by carbide based on the idea of the prior art.

[0030]

[Table 2]

Then, the obtained inventive materials A1 to A10 and comparative materials B1 to B4 were subjected to a creep rupture test at a test temperature of 600 ° C. Table 3 shows the results of the creep rupture test.

[0032]

[Table 3]

As shown in Table 3, the material of the present invention was remarkably superior in creep rupture strength to the comparative material.

[0034]

The heat-resistant ferritic steel according to the present invention is remarkably superior in creep rupture strength to the conventional heat-resistant steel utilizing the effect of precipitation strengthening by carbide. By applying the steel of the present invention to various members used for conventional heat-resistant ferritic steels, the material cost of the conventional steel hardly changes, and the service life of each member can be significantly extended, or the life of each member can be significantly extended. The service temperature can be raised. Therefore, a product incorporating a member using the steel of the present invention is expected to greatly improve performance.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 8/00 C21D 8/00 D C22C 38/24 C22C 38/24 38/54 38/54 F01D 5/02 F01D 5/02 5/28 5/28 (72) Inventor Tatsuo Morimoto 1-8-1 Koura, Kanazawa-ku, Yokohama-shi, Kanagawa Pref. AA02 AA04 AA09 AA13 AA16 AA19 AA20 AA21 AA22 AA31 AA33 AA35 AA36 AA37 AA39 CB01 CB02 CF01 CF02 CF03

Claims (8)

[Claims] C. 0 to 0.01%, Cr: 0.4 to 4.0% by weight.
%, V 0.05-0.8%, Mo 0-2.0%, W 0-4%, (Mo + W
/ 2) is 0.2 to 2%, N is 0.005 to 0.2%, and the balance is Fe and inevitable impurities. 2. 0.0001-0.02% B, 0.01-0.0% by weight.
1.0% Si, 0.01-1.5% Mn, 0.01-1.0% Ni, 0.01
2. The composition according to claim 1, further comprising at least one element selected from the group consisting of Co of not less than 0.1% and less than 0.1%.
Ferritic heat-resistant steel described in 1. 3. A group consisting of Nb, Ta, Ti, Hf, and Zr.
0.01 to 0.2% by weight of at least one of each element
The ferritic heat-resistant steel according to claim 1, further comprising: 4. A steel having the composition described in any one of claims 1 to 3 is kept in a temperature range of 870 to 1250 ° C. for 5 to 1200 minutes, and then cooled at a cooling rate of 0.01 to 100 ° C./sec. Performing a solution treatment, at a temperature of 500 to 850 ° C.
Performing a tempering process of cooling at a cooling rate of 0.01 to 10 ° C./sec after holding for 5 to 2000 minutes. 5% by weight of C, 0 to 0.01% and Cr of 12 to 27%.
%, V 0.05-2.0%, Mo 0-2.0%, W 0-4%, (Mo + W
/ 2) is 0.2 to 2%, N is 0.005 to 0.5%, and the balance is Fe and inevitable impurities. 6. 0.0001 to 0.04% B by weight, 0.01 to 0.01% by weight.
The ferritic heat-resistant steel according to claim 5, further comprising at least one element selected from the group consisting of 1.0% Si, 0.01 to 1.5% Mn, and 0.01% or more and less than 0.1% Co. . 7. A group consisting of Nb, Ta, Ti, Hf, and Zr.
0.01 to 0.5% by weight of at least one of each element
The ferritic heat-resistant steel according to claim 5, wherein the heat-resistant ferritic steel is contained. 8. A steel having the composition described in any one of claims 5 to 7 is kept in a temperature range of 900 to 1300 ° C. for 5 to 600 minutes, and then cooled at a cooling rate of 0.1 to 30 ° C./sec. A method for producing a heat-resistant ferritic steel, comprising a step of performing a solution treatment and a step of performing plastic working at a temperature of 500 to 1300 ° C. with a working ratio of 40% or more.