JP3475927B2 - Low Cr ferritic heat-resistant steel and its manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has excellent high temperature strength suitable for use as heat exchangers, steel pipes for piping, heat resistant valves, connecting joints, etc. used in the fields of boilers, chemical industry, nuclear power, etc. The present invention relates to a low Cr ferritic heat resistant steel, and more particularly to a low Cr ferritic heat resistant steel excellent in creep strength, creep ductility and creep embrittlement resistance at high temperatures of 400 ° C. or higher, and a method for producing the same.

[0002]

2. Description of the Related Art A heat-resistant steel used at a high temperature of 400 ° C. or higher has an austenitic stainless steel and a Cr content of 9
-12 mass% of high Cr ferritic steel, Cr content is several mass% of low Cr ferritic steel, and carbon steel.
Then, each of the above steel types is appropriately selected and used in consideration of the use environment (temperature, pressure, etc.) and economical efficiency.

Of the above steel types, low Cr ferritic steel is
Since it contains Cr, it has excellent oxidation resistance, high temperature corrosion resistance and high temperature strength as compared with carbon steel. Moreover, although it is inferior in high temperature strength to austenitic stainless steel, it is significantly cheaper and has a small thermal expansion coefficient. Furthermore, high Cr
It is cheaper than ferritic steel and has excellent toughness, weldability and thermal conductivity.

High temperature strength is extremely important in designing a pressure resistant member, and it is desirable that the strength be high regardless of the operating temperature. In particular, in heat resistant and pressure resistant steel pipes used for boilers, chemical industry, nuclear power, etc., the wall thickness of the pipe is determined according to the high temperature strength of the material.

Improvement of the high temperature strength of low and medium Cr ferritic steels is generally carried out by adding proper amounts of C, Cr, Mo and W, but when used at high temperature for a long time, coarsening of carbides and Precipitation of intermetallic compounds occurs and the creep strength at high temperature for a long time decreases. A possible method is to increase the amount of solid solution element added to increase the strength and enhance the effect of solid solution strengthening.However, addition above the solid solution limit promotes precipitation of these elements, rather toughness and workability. , Deteriorates weldability.

Improvement of high temperature strength by precipitation strengthening is performed by adding precipitation strengthening elements such as V, Nb and Ti. Mo is also an element that contributes to precipitation strengthening. As such a low Cr ferritic steel, STBA20 (0.5Cr-0.5Mo), S specified in JIS standard is used.
TBA22 (1Cr-0.5Mo), STBA23
(1.25Cr-0.5Mo), STBA24 (2.2
5Cr-1Mo) and the steels disclosed in JP-A-6-100929, JP-A-8-134584 and JP-A-8-158022.

Further, as precipitation strengthening type low Cr ferritic steel, 1Cr-1Mo-0.
2.25Cr-, which is a structural material for 25V steel and fast breeder reactors
1Mo-Nb steel and the like are well known.

All of the above precipitation-strengthened steels are normalized at the A _C3 transformation point or higher and then cooled to the Ms point (martensite transformation start temperature) or the Bs point (bainite transformation start temperature) or lower. A martensite or bainite structure is obtained, and then carbonitrides are precipitated in the process of tempering below the A _C1 transformation point. However, in the case of such a tempered martensite structure or a tempered bainite structure, the following problems may occur.

Unused materials and materials used for a short time have high strength, but when exposed to a high temperature for 10,000 hours or more, the lath structure of martensite and bainite collapses as the dislocations recover, and the creep strength increases. May be significantly reduced. Further, carbonitrides precipitated in the tempering process tend to aggregate and coarsen during long-term use at high temperature, and the precipitation strengthening ability may be lost. Further, in a steel having a martensite structure or a bainite structure, segregation easily occurs in the former austenite grain boundaries, and coarse carbides may be formed to cause embrittlement during use.

Further, from the viewpoint of the manufacturing process,
Performing tempering after normalizing and cooling lengthens the lead time and increases the manufacturing cost.

Therefore, if a low-cost manufacturing process capable of further improving the high temperature strength and preventing creep embrittlement can be proposed for the low Cr ferritic steel, the following advantages can be obtained.

Conventionally, high Cr ferritic steel has been used to secure high temperature strength even in a use environment where high temperature corrosion resistance is not so strict. However, the use of low Cr ferritic steel has a great advantage for economic efficiency. Further, even in the conventional application, it is possible to reduce the wall thickness, thereby improving the thermal conductivity and improving the thermal efficiency of the plant itself, as well as the thermal fatigue load due to the start and stop of the plant. Can be reduced. In addition, preventing creep embrittlement enhances plant safety.

[0013]

The object of the present invention is 40
High creep strength at high temperatures of 0-530 ° C,
Another object of the present invention is to provide a low Cr ferritic heat-resisting steel that exhibits stable strength and ductility even when used for a long time in such a temperature range, and a method for producing the same.

[0014]

The gist of the present invention resides in a ferritic heat-resistant steel of the following (1) and a ferritic heat-resistant steel of the following (2) and (3). (1) Mass%, C: 0.05 to 0.15%, Cr:
0.1-3%, V: 0.02-0.5%, Nb: 0.0
05-0.2%, including N: 0.001-0.01%,
The dispersion σ _MX defined by the following equation (1) of MX-type precipitates in the crystal grains is 20% or less, and the (001) plane of the ferrite phase which is the matrix and the MX-type precipitates (0
01) Low-Cr ferritic heat-resistant steel with a single ferrite phase having an angle difference of 5 degrees or less.

Σ _MX = {(L _MAX −L _MIN ) / 2L _MEAN } × 100 (%) (1) Here, L _MAX is the thin film sample subjected to the electrolytic polishing treatment at an acceleration voltage of 200 kV. Observed at 40,000 times with a transmission electron microscope, the maximum value and L _MIN are the maximum values when 100 closest distances L (μm) between MX type precipitates identified by analysis of electron beam diffraction patterns are measured. , L _MEAN are the same average value. (2) Mass%, C: 0.05 to 0.15%, Cr:
0.1-3%, V: 0.02-0.5%, Nb: 0.0
From 05 to 0.2% N: a low Cr ferritic heat-resistant steel material containing 0.001% to 0.01%, after heating to A _C3 transformation point or higher, A _C1 transformation point of the cooling process ~ (A _C1 Transformation point-10
Hold the isothermal condition in the temperature range of 0) ℃ to complete the ferrite transformation.
Ferrite single phase characterized by being hardened and not tempered
Manufacturing method of low Cr ferritic heat resistant steel of. (3) C: 0.05 to 0.15% by mass%, Cr:
0.1-3%, V: 0.02-0.5%, Nb: 0.0
From 05 to 0.2% N: a low Cr ferritic heat-resistant steel material containing 0.001% to 0.01%, after heating to A _C3 transformation point or higher, A _C1 transformation point of the cooling process ~ (A _C1 Transformation point-10
Special feature is that the temperature range of 0) ° C is cooled at a cooling rate of 500 ° C / h or less to complete the ferrite transformation and no tempering is performed.
A method for producing a single-phase low-Cr ferritic heat-resisting steel of ferrite.

The above ferritic heat-resistant steel of the present invention contains, in addition to the above alloy components, one or more groups selected from the following eight groups, if necessary. The content of P and S contained as impurities may be 0.03% or less by mass%.
It is preferably 0.015% or less.

B: mass%, Ti: 0.001 to 0.1
%, Ta: 0.002 to 0.2%, one or more. B:% by mass, Mo: 0.01 to 2.5%, W: 0.0
2 to 5% of one or more. C:% by mass, B: 0.0001 to 0.01%. D; in mass%, Co: 0.01 to 0.5%, Ni: 0.
01-0.5%, Cu: 01-0.5%, 1 or more types. E; mass% Ca: 0.0001 to 0.005%, M
g: one or more of 0.0001 to 0.005%. F: Mass%, Al: 0.001 to 0.05%. G; in mass%, Si: 0.01 to 0.7. H; mass%, Mn: 0.01 to 1%.

The inventors of the present invention repeated various tests on a production method for uniformly dispersing stable precipitates even at high temperatures in order to improve the high-temperature strength of low Cr ferritic heat-resistant steel, especially the creep strength at 400 ° C. or higher. It was As a result, the following knowledge was obtained and the present invention was completed.

In the low Cr steel containing V, Nb, C, N, immediately after the solution heat treatment of carbonitride is carried out at a temperature above the A _C3 transformation point, that is, in the austenite region, immediately below the A _C1 transformation point. When cooled and held at that temperature, the transformation from the austenite phase to the ferrite phase proceeds with the precipitation of fine MX type precipitates at the austenite / ferrite phase interface.

The MX type precipitate obtained by this austenite / ferrite phase interfacial precipitation is extremely stable at high temperatures, and is unlikely to aggregate and coarsen. Furthermore, the separation distance between MX type precipitates (hereinafter sometimes referred to as the distance between precipitates or the distance between particles) is determined depending on the interfacial movement speed associated with the phase transformation, and the distance between the particles is almost equal. To deposit. That is, the control of the transformation rate determines the precipitation density and distribution of MX type precipitates.

The crystal form of this MX-type precipitate is a face-centered cubic lattice, and the constituent elements of M are V, Nb, Ti, and T.
Metal elements such as a, Mo and W, and interstitial elements such as C and N can be contained in X, but at least MX is V, Nb, C and N.
In the case of being composed of, the matrix has a specific azimuth relationship, and the compatibility with the ferrite phase of the matrix becomes high.

If the MX-type precipitate and the matrix have high compatibility, a matching strain is generated around the MX-type precipitate to increase the deformation resistance, and the room temperature strength and the creep strength are remarkably improved. In addition, as described above, after the solution heat treatment of carbonitrides is performed in the austenite region, if it is not cooled to the Ms point or the Bs point or lower and continuously maintained at the _AC1 transformation point or lower, the matrix is completely ferritic. Turn into.

Therefore, since the dislocation density is low and the residual stress is low even without performing the tempering treatment, the decrease in creep strength due to the dislocation recovery, which is a problem in the tempered martensite structure and the tempered bainite structure, does not occur. Further, since there is no prior austenite grain boundary such as tempered martensite structure and tempered bainite structure, problems of temper embrittlement and creep embrittlement due to segregation and grain boundary precipitation do not occur.

As described above, the tempering can be performed by applying the manufacturing method in which after the solution heat treatment of the carbonitride in the austenite region is continuously cooled to the A _C1 transformation point or less without being cooled to the Ms point or the Bs point or less. The cost reduction effect is large due to the omission of processing.

The effect of precipitation strengthening by the MX type precipitate is that the inter-particle distance satisfies the above expression (1), and the (001) plane of the matrix ferrite phase and the (001) plane of the MX type precipitate are Is exhibited only when the angle difference of is less than 5 degrees, and such a structure is obtained by heating a material steel containing C, Cr, V, Nb and N in the above ranges to a temperature above the A _C3 transformation point. , the a _C1 transformation point of the cooling process ~ (a _C1 transformation point -100) ° C. or isothermal holding in a temperature range of, or the cooling rate 500 the temperature range
This is ensured by cooling at or below ° C / h to complete the ferrite transformation.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reason why the ferritic heat-resistant steel of the present invention and the method for producing the same are limited as described above will be described in detail. In the following, "%" means "mass%" unless otherwise specified.

First, the chemical composition of steel will be described.

C: 0.05 to 0.15% C is an essential element for forming MX type precipitates. Further, it is important for controlling the phase balance between the austenite phase and the ferrite phase. C content is 0.05%
If the amount is less than the range, the amount of MX type precipitates is insufficient and does not contribute to the increase in strength. On the other hand, when it exceeds 0.15%, carbides other than MX-type precipitates, for example _{M 23 C 6, M 7 C} 3, M 6 C , etc. are coarsened, significantly impair toughness, creep ductility, and the like. Therefore, the C content is set to 0.05 to 0.15%. A preferred range is 0.07 to 0.13%, and a more preferred range is 0.09 to 0.12%.

Cr: 0.1 to 3% Cr is an essential element for improving the oxidation resistance and high temperature corrosion resistance. Furthermore, it also has an effect of finely precipitating MX type precipitates by suppressing the moving speed of the austenite / ferrite interface accompanying the phase transformation. If the Cr content is less than 0.1%, these effects cannot be obtained. On the other hand, its content is 3%
When it exceeds, the MX type precipitates are not sufficiently precipitated, the economical efficiency is lowered, and the advantages of the low Cr ferritic heat resistant steel are reduced. Therefore, the Cr content is set to 0.1% to 3%. A preferred range is 0.5 to 2.5%, and a more preferred range is 1 to 1.5%.

V: 0.02-0.5% V is an important element that forms MX type precipitates. That is, V is C at the austenite / ferrite interface.
And N to form MX type precipitates, which are uniformly distributed in the ferrite phase of the mother phase as the austenite / ferrite interface moves. However, V content is 0.02%
If it is less than the above, the amount of MX-type precipitates is small, and it does not contribute to the improvement of room temperature strength and creep strength. On the other hand, when the content exceeds 0.5%, MX type precipitates are excessively precipitated,
Toughness reduction and creep embrittlement are likely to occur. Therefore, the V content is set to 0.02 to 0.5%. A preferable range is 0.05 to 0.25%, and a more preferable range is 0.1 to 0.15%.

Nb: 0.005 to 0.2% Nb is an important element that forms MX type precipitates, as in the case of V described above. That is, Nb combines with C and N at the austenite / ferrite interface to form MX type precipitates, and is uniformly distributed in the ferrite phase of the parent phase as the austenite / ferrite interface moves. Also,
This MX type precipitate is more finely dispersed and precipitated when V and Nb are compositely precipitated, such as (V, Nb) (C, N), than V (C, N) containing V as a main component, The contribution to strengthening becomes large. However, if the Nb content is less than 0.005, this effect cannot be obtained. On the other hand, if it exceeds 0.2%, A
_It does not dissolve even if subjected to solution heat treatment above the _C3 transformation point, leaving undissolved coarse precipitates, adversely affecting toughness and strength. Therefore, the Nb content is 0.005-0.2%
And A preferred range is 0.01 to 0.1%, and a more preferred range is 0.03 to 0.05%.

N: 0.001 to 0.01% N, like C, is an essential element for forming MX type precipitates. Furthermore, it is an important element for controlling the balance between the austenite phase and the ferrite phase.
MX-type precipitates are better than (V, Nb) C carbides.
Carbonitrides such as (V, Nb) (C, N) have a lattice constant closer to that of the matrix and MX type precipitates, and have higher compatibility with the matrix, which contributes to higher strength. Get higher However, if the N content is less than 0.001%, this effect cannot be obtained. On the other hand, if it exceeds 0.01%,
The MX-type precipitates do not form a solid solution even if a solution heat treatment is performed at a temperature above the A _C3 transformation point, and undissolved coarse MX-type precipitates remain.
It adversely affects toughness and strength. Therefore, the content of N is set to 0.001 to 0.01%. The preferred range is 0.
002 to 0.008%, more preferably 0.004
Is about 0.0075%.

Precipitation interval of MX type precipitates: In the ferritic heat-resistant steel of the present invention, variation σ _{MX of} precipitation intervals defined by the following formula (1) of MX type precipitates in the crystal grains is 20% or less. There is a need.

Σ _MX = {(L _MAX −L _MIN ) / 2L _MEAN } × 100 (%) (1) where L _MAX is the transmission of the thin film sample subjected to electrolytic polishing treatment at an acceleration voltage of 200 kV. Observed at 40,000 times with an electron microscope, the maximum value, L _MIN , when 100 closest gaps L (μm) between adjacent MX type precipitates identified by analysis of electron diffraction patterns were measured The minimum value and L _MEAN are the same average value.

The closest distance L is the shortest distance L between adjacent MX type precipitates M as shown in FIG.

That is, MX defined by the above equation (1)
When the variation σ _MX of the precipitation intervals of the mold precipitates exceeds 20%, the strength is reduced due to the local concentration of strain,
The desired creep strength cannot be secured. This is also clear from the results of Examples described later. The preferable upper limit of σ _MX is 15%, and the more preferable upper limit thereof is 10%.

Relationship between the ferrite phase of the mother phase and the crystallographic orientation of the MX type precipitate: The ferritic heat-resistant steel of the present invention has a (001) plane of the ferrite phase as the mother phase and (00) of the MX type precipitate.
1) The angle difference with the plane must be 5 degrees or less. That is, when the angle difference between the (001) plane of the ferrite phase of the mother phase and the (001) plane of the MX-type precipitate exceeds 5 degrees, the matching relationship between the two collapses and the strength is not improved. Creep strength cannot be secured. This is clear from the results of Examples described later as well as the deposition interval of the MX type deposits. The preferable upper limit of the deviation angle α is 2 °,
A more preferable upper limit is 1 °.

Note that the above-mentioned angle difference means, as shown in FIG. 2, (001) of the matrix ferrite phase by electron beam diffraction.
The deviation angle α (°) between the diffraction patterns which are the reflection patterns on the surface and the (001) surface of the MX type precipitate.

The ferritic heat-resistant steel of the present invention is a ferritic heat-resistant steel containing the above-mentioned amounts of C, Cr, V, Nb and N, and the precipitation interval of MX type precipitates satisfies the above formula (1),
It is sufficient if the relationship between the ferrite phase of the mother phase and the crystal orientation of the MX-type precipitate satisfies the above relationship,
Regarding the chemical composition of steel, the following elements can be selectively contained in addition to the above-mentioned components, if necessary.

Ti, Ta: These elements may not be added, but if they are added, they are combined with C and N (V, Nb,
It forms MX type precipitates of Ti, Ta) (C, N) and contributes to improvement of creep strength. Also, the crystal grains are refined,
It improves weldability and toughness, and is also effective in preventing softening of the weld heat affected zone. These effects are 0.001% for Ti
As described above, Ta is obtained at 0.002% or more. But,
If Ti is contained in excess of 0.1% and Ta is contained in excess of 0.2%, the steel is significantly hardened in any case and the toughness, workability and weldability are impaired. Therefore, the content of these elements when added is 0.001 to 0.
1% and Ta is preferably 0.002 to 0.2%. The preferable range of Ti is 0.002-0.05, T
The preferable range of a is 0.005 to 0.1%. In addition, any one of these elements may be added alone or both of them may be added in combination.

Mo, W: These elements do not have to be added, but if added, both elements have the effect of solid solution strengthening. Also, by combining with C (V, Nb, Mo, W)
It forms MX type precipitates of (C, N) and contributes to improvement of creep strength. These effects are obtained when Mo is 0.01% or more and W is 0.02% or more. However, when Mo is contained in excess of 2.5% and W in excess of 5%, the effect is saturated and rather the weldability and toughness are impaired. Therefore, the content of these elements when added is preferably 0.01 to 2.5% for Mo and 0.02 to 5% for W. The preferable range of Mo is 0.05 to 1%, and the more preferable range is 0.1 to 0.7%. The preferable range of W is 0.1 to 2%, and the more preferable range is 0.2 to 1.4%.
Is. In addition, any one of these elements may be added alone or both of them may be added in combination.

B: B does not have to be added, but if it is added, it is an element effective for increasing the grain boundary strength and obtaining stable strength at high temperature and for a long time. This effect is 0.0001%
The above is obtained. However, if the content exceeds 0.01%, the carbides are agglomerated and coarsened, which causes a reduction in strength and a reduction in toughness. Therefore, the B content when added is preferably 0.0001 to 0.01%. A preferred range is 0.0025 to 0.005%, and a more preferred range is 0.003 to 0.004%.

Co, Ni, Cu: It is not necessary to add these elements, but if added, all elements have a solid solution strengthening action, so that the creep strength is improved and the creep strength on the long time side is improved. It is effective in preventing deterioration. In addition, Ni
Has the effect of improving the toughness, and Cu has the effect of improving the thermal conductivity. Each of these effects is 0.01
It is obtained in%. However, if any element is contained in an amount exceeding 0.5%, the high temperature creep strength is lowered. Also, from the viewpoint of economy, excessive addition is not preferable. Therefore,
When added, the content of each of these elements is preferably 0.01 to 0.5%. In addition, any one of these elements may be added alone or both of them may be added in combination.

Ca, Mg: These elements may not be added, but if added, they are effective in reducing inclusions and improve castability. It also fixes S and contributes to improving toughness and preventing creep embrittlement. However, each element
If the content is less than 0.0001%, the above effect cannot be obtained. On the other hand, if any element is contained in an amount of more than 0.005%, oxides and sulfides are increased, and the toughness and strength are rather deteriorated. Therefore, the content of these elements when added is 0.0001 to 0.
It is better to set it to 005%. In addition, any one of these elements may be added alone or both of them may be added in combination.

Al: Al does not have to be added, but if added, it is an effective element as a deoxidizer. This effect is obtained at 0.001% or more. However, if the content exceeds 0.05%, the creep strength and workability are impaired. Therefore, the Al content when added is 0.001 to 0.
It is good to set it to 05%. Preferably, the upper limit is 0.02%
Is. The Al referred to in the present invention means acid-soluble Al (s
ol. Al).

Si: Si may not be added, but if added, it is effective as a deoxidizing agent and is an element that enhances the steam oxidation resistance of steel. These effects are obtained at 0.01% or more. However, if the content exceeds 0.7%,
Toughness is significantly reduced, and it is also harmful to creep strength. Therefore, the Si content when added is 0.01
It is good to be set to 0.7%. The preferred range is 0.05-
It is 0.5%.

Mn: Mn may not be added, but if it is added, hot workability is improved by the desulfurization and deoxidation effects. These effects are obtained at 0.01% or more. However, if the content exceeds 1%, carbides other than MX-type precipitates are likely to coarsen, which adversely affects the creep strength. Therefore, the Mn content when added is 0.
It is preferable to set it to 01 to 1%. The preferred range is 0.05-
It is 0.5%.

P, S: These elements are contained as impurities in steel and are harmful to toughness, workability and weldability. Therefore, the smaller the content, the better.
Is preferably 0.03% or less and S is 0.015% or less.

Next, the manufacturing method of the present invention will be described.

In the production method of the present invention, a low Cr ferritic steel, which is a material satisfying the above chemical composition, is melted and cast in accordance with a conventional method, as-cast, or hot-worked with an appropriate working degree. molded product having a predetermined shape by performing a and cold working, etc., after heating to a temperature above a _C3 transformation point (annealing), a temperature of the a _C1 transformation point of the cooling process ~ (a _C1 transformation point -100) ° C. Or keep the temperature isothermal to complete the ferrite transformation,
Alternatively, it is a method of cooling the temperature range from A _C1 transformation point to (A _C1 transformation point −100) ° C. at a cooling rate of 500 ° C./h or less to complete the ferrite transformation, and it is not necessary to perform tempering treatment unlike the conventional method. Is the way.

That is, A_C3Heating to a temperature above the transformation point
Later, the cooling process A_C1Transformation point ~ (A _C1Transformation point-100
Austenite → ferrite transformation is completed in the temperature range of ℃)
If it is used, the austenite / ferrite phase interface
MX type precipitates are deposited, austenite / ferrite phase
MX type precipitates are uniformly distributed within the crystal grains as the interface moves.
As it is spread, its precipitation orientation is
It is close to the crystal orientation. As a result, MX-type precipitation
The strengthening effect is remarkable, and the creep strength at high temperature is
Not only improved, but stable strength even after long-term use
A low Cr ferritic heat resistant steel exhibiting ductility is obtained.

Here, the reason why the heating temperature is set to the A _C3 transformation point or higher is that the structure needs to be once made into an austenite single-phase structure, and the upper limit of the heating temperature does not have to be specified, but carbonitride From the viewpoint of completely forming a solid solution, it is preferable to make it as high as possible.

After the above heating, A _{C1 in the} cooling process
In the temperature range from transformation point to (A _C1 transformation point −100) ° C.,
The reason why the ferrite transformation is completed by maintaining the same temperature or the temperature range is cooled at a cooling rate of 500 ° C./h or less to complete the ferrite transformation is as follows.

In the manufacturing method of the present invention, the structure is transformed from the austenite phase to the ferrite single phase.
When the upper limit temperature of the isothermal holding temperature exceeds the A _C1 transformation point,
A two-phase mixed structure of a ferrite phase and a fresh martensite phase or a ferrite phase and a fresh bainite phase is obtained, and a ferrite single phase structure cannot be obtained. On the other hand, when the lower limit temperature of the isothermal holding temperature is less than (A _C1 transformation point −100) ° C., the diffusion of solid solution atoms is slowed down, so that precipitation of MX-type precipitates is less likely to occur and MX-type precipitates are precipitated. However, the distribution becomes non-uniform and the high temperature strength is not improved.
Therefore, the temperature range for isothermal holding is A _C1 transformation point ~
(A _C1 transformation point −100) ° C.

The isothermal holding time need not be specified in particular, but if it is too short, the transformation from the austenite phase to the ferrite phase will not be completed, so it is desirable to hold it for 5 minutes or longer. In order to obtain a ferrite single-phase structure, the longer the holding time is, the better, but if it is too long, the productivity is lowered and the manufacturing cost is increased. Therefore, the holding time is preferably at most 3 hours. .

As described above, in the manufacturing method of the present invention,
For A_C1Transformation point ~ (A_C1Transformation point -100) ° C temperature range
Even when cooling at a cooling rate of 500 ° C / h or less,
Of A _C1Transformation point ~ (A_C1Transformation temperature -100), etc. in the temperature range
The same structure as when maintaining the temperature to complete the ferrite transformation
A low Cr ferritic heat resistant steel with
If the speed exceeds 500 ℃ / h, austenite → Fe
Light transformation is less likely to occur and the ferrite phase and
Schumatensite phase, or ferrite phase and flexi
It has a two-phase mixed structure with the au bainite phase and has a single ferrite structure.
A phase organization cannot be obtained. Therefore, the cooling rate is 500 ℃
/ H or less.

[0057]

EXAMPLES 21 kinds of steels having the chemical compositions shown in Tables 1 and 2 were melted using a high frequency vacuum melting furnace having a capacity of 150 kg, and the obtained ingots of the respective steels were 1000 to 1200 ° C.
After being heated to 0, it was hot forged and hot rolled into a plate having a thickness of 30 mm.

[0058]

[Table 1]

[Table 2] After measuring the A _C1 transformation point and the A _C3 transformation point from the volume change rate using a thermal expansion meter for each of the obtained plate materials, a normalizing treatment of holding for 1 hour at various heating temperatures was performed, and this normalizing was performed. or isothermal process for a predetermined period of time at various temperatures in the course of cooling after treatment, or a _C1 transformation point ~ (a _C1 transformation point -100) after cooling process temperature range of ℃ at various cooling rates, room temperature Cooled down.

A thin piece was taken from each plate after the heat treatment, and this sample was subjected to electrolytic polishing treatment to form a thin film sample. The thin film sample was subjected to an accelerating voltage of 200 kV and a magnification of 40,000 times using a transmission electron microscope. And observed. Then, after identifying the precipitates by specifying the MX-type precipitates by analyzing the electron diffraction pattern, the shortest distance L between the adjacent MX-type precipitates M is set to 100 by the method shown in FIG. The individual pieces were measured, and the dispersion σ _MX (%) of the precipitation intervals was investigated based on the following formula (1).

Σ _MX = {(L _MAX −L _MIN ) / 2L _MEAN } × 100 (%) (1) where L _MAX is the maximum value in the measured L, and L _MIN is the same minimum value. L _MEAN is the same average value.

Further, the azimuth relationship between the parent phase and the MX type precipitate was measured by electron diffraction, and the deviation angle α (°) between the (001) planes of the two was examined.

In the creep test, a test piece having a diameter of 6 mm and a parallel portion length of 30 mm was prepared, and the temperature was 500 ° C. and the load was 3
Condition of 00 MPa (condition 1), temperature of 550 ° C., load of 20
Tested under two conditions of 0 MPa (condition 2),
The creep rupture time (h) and the drawing ratio at rupture (%) were examined.

[0063] The above results, normalizing the heating temperature, an isothermal treatment temperature and the retention time, A _C1 transformation point ~ (A _C1 transformation point -10
It is shown in Table 3 together with the cooling rate in the temperature range of 0) ° C.

[0064]

[Table 3] As can be seen from the results shown in Table 3, steel Nos. A to 1 satisfying the conditions specified in the present invention were heat-treated according to the manufacturing method specified in the present invention to obtain low C of trial Nos. 1 to 12.
In all of the r-ferrite heat-resistant steels, the variation in the precipitation interval of MX-type precipitates is 20% or less, and the orientation difference from the parent phase is 5%.
It is less than °. As a result, the breaking time in the creep test under the condition 1 was 3800 hours or more and the drawing ratio was 60% or more, and the breaking time in the creep test under the condition 2 was 5300 hours or more and the drawing ratio was 60% or more. Even under the conditions, it showed a high creep life and a good creep ductility value.

On the other hand, although the manufacturing method satisfies the conditions specified in the present invention, the low C of trial Nos. 13 to 21 using steel Nos. M to u whose chemical composition is out of the range specified in the present invention.
In each of the r-ferrite heat-resistant steels, the variation in the precipitation interval of MX type precipitates exceeds 20%, and the orientation difference from the parent phase exceeds 5 °. As a result, the breaking time in the creep test under the condition 1 was 2764 hours at the maximum, the drawing ratio was 53% at the maximum, and the breaking time in the creep test under the condition 2 was 288 at the maximum.
The draw ratio was 55% at maximum for 1 hour, and the creep life and creep ductility were poor under any conditions.

Although the chemical composition of the steel is within the range defined by the present invention, the low Cr ferritic heat-resistant steels of trial Nos. 22 to 27 whose manufacturing method is outside the range defined by the present invention are all MX type. The variation in precipitation interval is more than 20%, and the orientation difference from the parent phase is more than 5 °. As a result, the breaking time in the creep test under the condition 1 was 2730 hours at the maximum and the drawing ratio was 51% at the maximum, and the breaking time in the creep test under the condition 2 was 3191 hours at the maximum and the drawing ratio was 55% at the maximum.
The creep life and creep ductility were poor under any of the conditions.

[0067]

The ferritic heat resistant steel of the present invention is 400
The creep rupture strength is stable and high even when it is used at a high temperature of ℃ or higher for a long time, and the creep ductility is excellent. For this reason, it can be used in applications where only expensive high-Cr ferritic steels could be used conventionally, and its economical effect is great. Furthermore, since the manufacturing method of the present invention does not require tempering, the manufacturing cost can be reduced and an inexpensive product can be provided.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a method for measuring a distance between MX type precipitates.

FIG. 2 is a diagram for explaining a method of measuring the consistency of a matrix and MX-type precipitates.

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00

Claims (3)

(57) [Claims] 1. C: 0.05 to 0.15% by mass%, C
r: 0.1-3%, V: 0.02-0.5%, Nb:
0.005 to 0.2%, N: 0.001 to 0.01%, and variation of precipitation interval σ _MX defined by the following formula (1) of MX type precipitates in crystal grains is 20% or less. , and the and ferrite angular difference between the (001) plane of the ferrite phase as a matrix phase and (001) plane the MX type precipitates is not more than 5 degrees
Single phase low Cr ferritic heat resistant steel. σ _MX = {(L _MAX −L _MIN ) / 2L _MEAN } × 100 (%) (1) where L _MAX is a transmission electron microscope with an accelerating voltage of 200 kV for a thin film sample subjected to electrolytic polishing treatment. The maximum value, L _MIN, is the minimum value, and L _MEAN is the maximum value when 100 closest distances L (μm) between MX type precipitates identified by electron diffraction pattern analysis are observed. Is the same average value. 2. C: 0.05 to 0.15% by mass%, C
r: 0.1-3%, V: 0.02-0.5%, Nb:
0.005 to 0.2% N: a low Cr ferritic heat-resistant steel material containing 0.001% to 0.01%, after heating to A _C3 transformation point or higher, A _C1 transformation point of the cooling process - ( A _C1 transformation point Ferrite characterized by holding ferrite in the temperature range of -100) ° C to complete ferrite transformation and not tempering
A method for producing a single-phase low Cr ferritic heat resistant steel. 3. In mass%, C: 0.05 to 0.15%, C
r: 0.1-3%, V: 0.02-0.5%, Nb:
0.005 to 0.2% N: a low Cr ferritic heat-resistant steel material containing 0.001% to 0.01%, after heating to A _C3 transformation point or higher, A _C1 transformation point of the cooling process - ( A _C1 transformation point -100) ° C temperature range should be cooled at a cooling rate of 500 ° C / h or less to complete the ferrite transformation and not perform tempering.
A method for producing a single-phase ferrite low-Cr ferritic heat-resistant steel , characterized by :