JP3214068B2 - Method for producing high Cr ferritic steel with excellent creep rupture strength and ductility - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention has excellent creep rupture strength and creep rupture ductility, and is used as a heat and pressure resistant member (steel pipe, steel plate, forged material, etc.) in the fields of boilers, nuclear power, chemical industry and the like. A method for producing high Cr ferritic steel.

[0002]

2. Description of the Related Art Heat-resistant steel used as a heat-resistant and pressure-resistant member for boilers, nuclear power, and chemical industries requires high-temperature strength, corrosion / oxidation resistance, and toughness. It must also be excellent and inexpensive.

[0003] Materials used in the above applications include austenitic stainless steel, ferritic low alloy steel, and 9-12Cr high Cr ferritic steel. Especially high
Cr ferritic steel has strength and corrosion resistance in the temperature range of 500 to 650 ° C.
Oxidation resistance is superior to low alloy steel, and it has the advantage of higher thermal conductivity and lower coefficient of thermal expansion than austenitic steel.
It is used in various fields, up to thick materials.

[0004] Representative high Cr ferritic steels include 9Cr-1Mo steel (STBA26) and modified 9Cr-1Mo steel (ASTM SA).
213 T91) and 12Cr-1Mo steel (DIN X20CrMoV121). Further, for the purpose of increasing the high-temperature strength, a steel to which W, V, Nb, N, etc. are added in combination is disclosed in Japanese Patent Publication No. 62-8502,
There are steels disclosed in respective publications such as 310340. In addition, the present inventors have proposed high-strength, high-chromium steels to which Cu has been added in order to increase the oxidation resistance at a high temperature of 600 ° C. or higher (JP-A-2-232345, JP-A-3-9732). .

[0005] By adding Cu, in addition to improving oxidation resistance, the formation of δ-ferrite, which is harmful to toughness, is suppressed.
It can save expensive Ni addition, and A
c ₁ High temperature tempering is possible because the transformation point is not significantly reduced. Heretofore, it has been pointed out that a decrease in ductility during processing due to the addition of a large amount of Cu has been pointed out. However, the present inventors have found that by adding a small amount of Mg in combination, as described in Japanese Patent Application Laid-Open No.
No. 45) and a structure containing a predetermined amount of δ ferrite (Japanese Patent No. 3-97832) can solve the problem of reduced ductility.

FIG. 1 shows a heat pattern at the time of final processing and heat treatment in the production of a conventional high Cr ferritic steel. In the conventional final processing, the material is heated in a relatively high temperature range of 1100 to 1250 ° C. of _three or more Ac, then hot-worked by forging or rolling, and then cooled. Since the final processing is usually for the purpose of finishing, it is often performed with a light draft of 10% or less. In addition, the final processing end temperature is often 1000 ° C. or lower in many cases. Cooling is allowed to cool,
In the case of a thick material, the temperature is gradually cooled at less than 1 ° C./min.

[0007] In the subsequent normalizing, the material is heated to a predetermined temperature (usually 950 to 1100 ° C) of _three or more Ac, kept at that temperature, and then allowed to cool. Also in this case, the cooling time becomes longer with a thick material. The final tempering is usually performed at 750 to 800 ° C, and then air-cooled.

[0008] However, it has been pointed out that the high Cr ferritic steel with increased high-temperature strength has a problem that the creep rupture ductility and the creep rupture strength on the long-time side are reduced. This is due to the inhomogeneity of the structure (coarse grain growth, abnormal grain growth), coagulation of coarse carbonitrides during processing, etc., and high strength with the addition of V, Nb, N, Mo, W, etc. This occurs in steel, and particularly in a high Cr ferritic steel to which a large amount of Cu has been added, the creep rupture ductility (fracture drawn, elongation) of the material is greatly reduced. Conventionally, there has been little report on a method for improving the creep rupture ductility of steel.

As a countermeasure, it is conceivable to suppress the amount of alloying elements such as V, Nb, W, N, C, and Cu, which cause a decrease in creep rupture ductility. However, the inclusion of these elements is for the purpose of improving strength, corrosion resistance, oxidation resistance, and the like. Therefore, reducing these elements impairs the basic properties of steel. It is also conceivable to increase the creep rupture ductility by generating a large amount of δ-ferrite.However, this method reduces toughness, and particularly in the case of thick materials, the structure, toughness and material anisotropy of strength are problematic. Become. Further, it is necessary to add an alloying element for producing δ-ferrite, which may not be preferable in terms of strength. In some cases, normalization is repeated twice to homogenize the structure and size the crystal grains, but this increases the cost and reduces the process efficiency.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a conventional
W, V, Nb, N and the like are added to improve the high-temperature strength of the high-Cr ferritic steel described above, namely, improve the creep rupture ductility and the decrease in creep rupture strength, and further improve the creep rupture strength. Producing high Cr ferritic steels, especially Cu
It is an object of the present invention to provide a method for producing a high Cr ferritic steel to which Cr is added. The target for Cu-added steel is the heat resistance temperature of high Cr ferritic steel due to the above-mentioned effects of Cu addition (improvement of oxidation resistance, suppression of formation of δ-ferrite harmful to toughness, saving of Ni addition). This is because it is expected that the use will be expanded, such as application to inexpensive thick-walled materials.

[0011]

In order to solve the above-mentioned problems, various tests were conducted, and as a result, the following new knowledge was obtained.

(1) Conventional high Cr with increased high-temperature strength
Compared to ferritic steels, it is necessary to add Cu in order to be superior in high-temperature strength and to increase corrosion resistance and oxidation resistance.

(2) In order to improve the creep rupture ductility, it is effective to suppress the agglomeration of carbonitride during hot working. Therefore, the final processing temperature should be 1000 ° C or higher, and the processing end temperature should be 9
It is preferable that the cooling rate is not less than 00 ° C. and the working degree is not less than 15%, and after the working, the cooling rate is not less than 1 ° C./min.

(3) From the viewpoint of suppressing grain boundary segregation and precipitation of Cu during normalization and regulating the size of crystal grains by fine precipitation of carbonitride, normalization is performed at 700 to 850 ° C. After heating, it is preferable to maintain the temperature in the range of 1000 to 1150 ° C. and rapidly cool it at a cooling rate of 1 ° C./min or more.

[0015] Thereafter, tempering is performed at least once in a temperature range of 650 to 850 ° C to prevent a decrease in the creep rupture ductility of the high Cr ferritic steel having an increased high temperature strength, and to further improve the creep rupture ductility. Can be.

The present invention has been made based on the above findings, and the gist of the invention resides in the following method for producing a high Cr ferritic steel.

By weight%, C: 0.02 to 0.20%, Si: 1% or less, Mn: 1.5% or less, Ni: 1% or less, Cr: 8 to 14%, M
o: 0.01-3%, V: 0.1-0.3%, Nb: 0.01-0.2
%, N: 0.001 to 0.1%, Al: 0.001 to 0.05%, Cu: 0.0
Final heat of high Cr ferritic steel containing 1-3.5%, the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, S is 0.015% or less, and O (oxygen) is 0.015% or less. In the cold working, after heating and holding at 1100-1250 ° C for 1 minute or more, processing at 15% or more at 1000 ° C or more and 1 ° C
After cooling at a cooling rate of
~ Preheat to 850 ° C and hold for more than 1 minute, then 1000 ~
Heating and holding at 1150 ° C for 1 minute or more, then cooling at a cooling rate of 1 ° C / min or more.
A method for producing a high Cr ferritic steel having excellent creep rupture strength and ductility, which is performed at a temperature of 50 ° C.

In addition to the above components, 0.01 to 3% by weight
W, 0.0001 to 0.02% by weight of B, or a high Cr ferritic steel containing both, may be subjected to the above-described processing and heat treatment.

[0019]

The reasons why the chemical composition of the high Cr ferritic steel produced by the method of the present invention and the final processing and heat treatment conditions in the production are determined as described above.

[Chemical Composition] C: C forms carbides, contributes to high-temperature strength, and stabilizes the martensite structure itself as an austenite stabilizing element. If it is less than 0.02% by weight (hereinafter, "%" of the alloy element means "% by weight"), the amount of precipitated carbide is small, and a large amount of δ-ferrite is formed, and the strength and toughness are reduced. On the other hand, if it exceeds 0.20%, the steel is remarkably hardened, and the ductility, weldability and workability are reduced. Therefore, the content of C is 0.02 to 0.20%, preferably 0.06 to 0.13%.
%.

Cr: Cr is an element indispensable for ensuring the oxidation resistance and corrosion resistance of steel. If it is less than 8%, sufficient oxidation resistance and corrosion resistance as a high Cr ferrite steel cannot be obtained, while if it exceeds 14%, strength, workability and toughness are impaired due to an increase in the amount of δ-ferrite. Therefore, the Cr content is set to 8 to 14%, preferably 8.5 to 12.5%.

Si: Si is added as a deoxidizing agent and is an element that enhances the resistance to steam oxidation, but if it exceeds 1%, the toughness is significantly reduced, and is harmful to the creep rupture strength. In particular, in the case of a thick material, it is better to keep it low from the viewpoint of suppressing the formation of δ-ferrite and suppressing long-time heating embrittlement. If toughness is important, the content is preferably 0.1% or less. If importance is given to steam oxidation resistance, the content is preferably 0.2 to 0.4%.

Mn: Mn improves hot workability and is effective for stabilizing the structure. However, if it exceeds 1.5%, it hardens steel and impairs workability, weldability and creep rupture strength. Therefore, the content of Mn should be 1.5% or less. Preferably 0.4 to
0.7%.

Ni: Ni suppresses the formation of δ-ferrite as an austenite stabilizing element and stabilizes the martensite structure. However, if the content exceeds 1%, the coarsening of carbides is promoted, and the creep rupture strength is reduced. In addition, the Ac ₁ transformation point is lowered so that sufficient tempering cannot be performed, and it is economically disadvantageous. Therefore, Ni
The content is 1% or less, preferably 0.1 to 0.8%. More preferably, in order to improve the hot workability of the Cu-added steel, Ni corresponding to the Cu content, specifically, Cu / Ni =
Ni that satisfies 2.5 to 4.5 (weight% ratio) may be contained.

Mo: Mo is an element effective for improving the creep rupture strength as an element for solid solution strengthening and precipitation strengthening of fine carbides. However, if the content is less than 0.01%, the above effect cannot be obtained. On the other hand, if the content exceeds 3%, δ-ferrite is generated in a large amount and the steel is hardened, and the toughness, ductility and workability are reduced.
Therefore, the Mo content is 0.01 to 3%, preferably 0.8 to 2.2%.
%.

It is more effective to add Mo and W in combination. In this case, the content of Mo may be 0.1-1.2%, and Mo + 1 / 2W = 1.2-1.8%.

V: V combines with C and N to form a fine precipitate of V (C, N), and greatly contributes to improvement in creep rupture strength at high temperature and for a long time. If it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 0.3%, the solid solution V increases and the strength is impaired. Therefore, the V content is
0.1 to 0.3%.

Nb: Like V, Nb is bonded to C and N to form Nb
It forms fine precipitates of (C, N) and greatly contributes to improvement in creep rupture strength. Further, it is effective for making crystal grains finer and improving toughness. If it is less than 0.01%, the above effects cannot be obtained, and if it exceeds 0.2%, undissolved NbC
And the strength, ductility and weldability are impaired. Therefore, the Nb content is 0.01 to 0.2%, preferably 0.02 to 0.08%
And

N: N combines with V and Nb to form a carbonitride and contributes to an improvement in creep rupture strength. However, if less than 0.001%, the effect is not recognized. On the other hand, if it exceeds 0.1%, creep rupture ductility, weldability and workability are significantly impaired. Therefore, the N content is set to 0.001 to 0.1%, preferably 0.02 to 0.06%.

Al: Al (aluminum) is added as a deoxidizing agent, but if its content is less than 0.001%, deoxidizing is not sufficiently performed, and toughness, ductility and strength are impaired. Also, 0.05
%, The creep rupture strength decreases, so the Al content is set to 0.001 to 0.05%.

Cu: Cu is one of the basic components of the high Cr ferritic steel produced by the method of the present invention as described above, and has the following effects. That is, (a) the formation of δ-ferrite is suppressed and the toughness is improved. (b) Increase oxidation resistance at 600 ° C or higher. (c) The formation of a softened layer at the weld is suppressed, and the creep rupture strength of the welded joint is improved. However, if the content is less than 0.01%, the above effects cannot be obtained. If the content exceeds 3.5%, not only the hot workability is remarkably deteriorated, but also the creep rupture ductility is reduced, and the creep rupture strength is greatly reduced. Therefore, the content of Cu is set to 0.01 to 3.5%. Preferably 0.3
~ 2.2%.

In addition to the above components, one or both of W and B can be added as needed.

W: Like W, W is effective for improving the creep rupture strength as a solid solution strengthening and fine carbide precipitation strengthening element. In particular, it is preferable to add a large amount of W to improve the creep rupture strength on the high temperature side. Normally, when adding Mo in place of part or most of Mo, it is necessary to add Mo twice as much (by weight%). However, when the content exceeds 3%, the creep rupture ductility and the toughness are significantly reduced. On the other hand, if it is less than 0.01%, the above effects cannot be obtained. Therefore, the W content is
0.01 to 3%. If the strength is emphasized, it is preferable that the content is 0.8 to 2.2% and that Mo + 1 / 2W is 1.2 to 1.8%.

B: B has the effect of dispersing and stabilizing carbides by adding a small amount. If it is less than 0.0001%, the above effect is small, and if it exceeds 0.02%, the creep rupture ductility, workability and weldability are significantly impaired. Therefore, when B is added, the content is made 0.0001 to 0.02%. Preferably 0.001 to 0.004
%.

In addition to the above-mentioned components, La, Ce, Ca, Zr, Ti, Y, Ta, and Mg are added at 0.2% each as a trace addition element.
May be contained. These elements are P, S,
It has the function of stabilizing these by binding to impurity elements such as O and improving ductility, toughness and strength.

The high Cr ferritic steel produced by the method of the present invention is a steel comprising the above components and the balance of Fe and inevitable impurities. Typical impurities are P, S
And O (oxygen). These are creep rupture ductility,
It is desirable to reduce as much as possible in terms of toughness, workability, and weldability. P is 0.025% or less, S is 0.015% or less,
O is set to 0.015% or less.

The method of the present invention is characterized in that the following final processing and heat treatment are performed on a steel containing the above alloy components in the ranges specified respectively.

Processing prior to final processing may be performed by a conventional method, and there is no limitation. That is, the ingot or slab is made into a material having a predetermined shape and size by hot working such as ingot rolling or forging, and this material is, for example,
Hot working means such as the punching tube method represented by the Erhard Pushbench method, the extrusion tube method represented by the Eugene Sejournet method, and the oblique roll perforated rolling method represented by the Mannesmann plug mill method The intermediate product may be subjected to final processing described below, for example, by finishing into a raw tube having a predetermined size.

[Processing and heat treatment conditions] In order to improve the creep rupture ductility, the structure must be uniform, the growth of crystal grains must be suppressed, and the grain size must be regulated, and the precipitation and segregation of Cu must be prevented. . For this purpose, it is necessary to perform a relatively high-pressure working at a temperature higher than before in the final hot working to promote the diffusion of alloy elements and to suppress the coarsening of carbonitrides. Heating at a temperature lower than 1100 ° C. causes insufficient solidification segregation of alloy elements and solid solution of coarse precipitates, which causes a reduction in strength and ductility. On the other hand, when heated in a temperature range exceeding 1250 ° C., a large amount of δ-ferrite is formed, and toughness and strength are reduced. Therefore, the heating temperature before final hot working is 1100-
Set to 1250 ° C.

In order to sufficiently diffuse the alloying elements and homogenize the structure, the rolling reduction at the time of processing should be set to 15% under a relatively high pressure.
Above. Under light pressure of less than 15%, sufficient strength and ductility cannot be obtained. The processing end temperature is 1000 ° C. or higher.
If the processing is performed at a temperature lower than 1000 ° C., a deformed structure remains, and precipitation of carbonitride occurs, thereby impairing strength and ductility.

If the cooling rate after processing is less than 1 ° C./min, carbonitrides and Cu phases are precipitated during cooling and ductility is significantly impaired. The cooling rate is the average cooling rate when the center of the material is cooled from 800 ° C to 500 ° C.

In the heat treatment after processing, normalization is performed by two-stage heating, and then tempering is performed.

The normalization is performed by first preheating to 700 to 850 ° C. to suppress the precipitation of the Cu phase, and to cause fine precipitation of V and Nb carbonitrides, followed by heating at 1000 to 1150 ° C. In addition, creep rupture ductility and creep rupture strength can be improved by suppressing the growth of abnormal crystal grains and sizing the grains.

If the preheating is lower than 700 ° C., the above effects cannot be obtained due to insufficient precipitation of carbonitride,
Creep rupture ductility decreases due to precipitation of the phase. While 8
If the temperature exceeds 50 ° C., the precipitation of carbonitrides is reduced, and the effect cannot be expected. The holding time at the time of preheating is 1 minute or more. If the time is less than 1 minute, the precipitation is insufficient, and the above effects cannot be obtained.

If the normalizing temperature is lower than 1000 ° C., the carbonitride precipitated by preheating becomes coarse, and the amount of finely dispersed precipitates during the next tempering decreases, resulting in a significant loss of strength. It is. On the other hand, if it exceeds 1250 ° C., toughness and ductility are impaired due to an increase in the amount of δ-ferrite and coarsening of crystal grains. Therefore, the heating temperature during normalizing is set to 1000 to 1150 ° C. The holding time during heating is 1 minute or more. Preferably, for example, for 0.5 mm for a plate thickness of 25 mm.

The cooling rate after normalizing is set to 1 ° C./min or more in order to suppress the precipitation of carbonitride and Cu phase during cooling.

The final tempering may be performed under normal processing conditions, and the temperature is 650 to 850 ° C. If the temperature is lower than 650 ° C., the steel is not sufficiently tempered and causes a decrease in creep rupture strength on a long-time side. On the other hand, the upper limit should be ₁ point or less of Ac. If it exceeds 850 ° C, fine carbonitrides will no longer precipitate. In order to suppress the precipitation of the Cu phase, the temperature is preferably set to 770 to 820 ° C.

[0048]

EXAMPLES High Cr ferritic steels A with the chemical composition shown in Table 1
G is melted in a 150kg vacuum melting furnace, and the ingot is heated from 1150 to 9
It was hot forged at 50 ° C and processed into a rolled block having a thickness of 60 mm, a width of 100 mm and a length of 500 mm. A to F steels are steels subject to the present invention, G
Steel is a comparative steel.

For each of the test steels A to G, FIG.
(D) Hot rolling, normalizing and tempering treatments were performed according to the heat patterns exemplified in (d). (A) is a conventional method, (b) and (c) are examples of the present invention, and are referred to as Method 1 of the present invention and Method 2 of the present invention, respectively. (D) is a comparative method.

A tensile test piece of 6 mmφ × 30 mmGL was sampled in the longitudinal direction from the center of the thickness of the plate material processed and heat-treated by these manufacturing methods, and subjected to a creep rupture test. Creep rupture test conditions are 600 ℃ × 16kgf / mm ² , 650 ℃ × 7kg
In f / mm ^2, it carried out a long time test that exceeds the maximum 30000 hours,
Creep rupture ductility (elongation, drawing) and strength (rupture time) were measured.

Table 2 summarizes the test results. 3 and 4 show the creep rupture time and the rupture draw under each creep condition by method and steel type.

Although the creep rupture times (strengths) of the A to F steels cannot be compared due to their different compositions, when the same steel is used, the improvement of the strength is apparent when the method of the present invention is applied as compared with the conventional method and the comparative method. It is. In particular, when the method 2 of the present invention shown in FIG. 2 is applied, the strength is high. This is considered to be due to the effects of increasing the final hot working temperature and working ratio (reduction ratio) and rapidly cooling. Further, the G steels not covered by the present invention originally have low strength, and the effect of the present invention is not recognized.

On the other hand, the creep rupture ductility was
Low values are shown for materials with a high Cu addition. Also 600 ℃
At 650 ° C, the drawing is smaller and the elongation is lower. But,
For each of A to F steels, by applying the method of the present invention,
The ductility, especially the drawing, is improved.

On the other hand, when the comparative method (d) shown in FIG. 2 is applied, although the ductility is slightly improved as compared with the conventional method,
The improvement is not as great as with the method according to the invention. This is because precipitation of carbonitrides is insufficient because the preheating temperature for normalizing is as low as 650 ° C. Further, in the case of the comparative G steel, no improvement in ductility is recognized by the method of the present invention.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

According to the method of the present invention when producing a high Cr ferritic steel, it is possible to improve the creep rupture ductility and the decrease in creep rupture strength. In particular, with respect to steel containing Cu, it is possible to significantly improve long-time high-temperature creep rupture ductility and strength without adding or increasing or decreasing alloy components.

The high Cr ferritic steel obtained by this method is suitable for heat and pressure resistant members (steel pipes, plates, forgings, etc.) for boilers, nuclear power and chemical industries, and greatly improves the durability and reliability of these members. Can contribute.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a heat pattern during processing and heat treatment in a conventional method for producing a high Cr ferritic steel.

FIG. 2 is a view showing an example of a heat pattern at the time of working and heat treatment in the method of manufacturing a high Cr ferritic steel used in Examples, (A) is a conventional method, (B) is a method 1 of the present invention,
(C) is the case of the method 2 of the present invention, and (d) is the case of the comparative method.

[Fig. 3 (1)] Creep rupture test (600 ° C, 16kgf / m
It is a comparison diagram of the creep rupture time by m ^2).

[Fig. 3 (2)] Creep rupture test (600 ° C, 16kgf / m
is a comparison diagram of the throttle creep rupture by m ^2).

[Fig. 4 (1)] Creep rupture test (650 ° C, 7kgf / m
It is a comparison diagram of the creep rupture time by m ^2).

[Fig. 4 (2)] Creep rupture test (650 ° C, 7kgf / m
is a comparison diagram of the throttle creep rupture by m ^2).

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-247531 (JP, A) JP-A-2-232345 (JP, A) JP-A-3-97832 (JP, A) JP-A-62-1987 238330 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (4)

(57) [Claims] C. 0.02 to 0.20%, Si: 1% or less, Mn: 1.5% or less, Ni: 1% or less, Cr: 8 to 14%, M:
o: 0.01-3%, V: 0.1-0.3%, Nb: 0.01-0.2
%, N: 0.001 to 0.1%, Al: 0.001 to 0.05%, Cu: 0.0
Final heat of high Cr ferritic steel containing 1-3.5%, the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, S is 0.015% or less, and O (oxygen) is 0.015% or less. In the cold working, after heating and holding at 1100-1250 ° C for 1 minute or more, processing at 15% or more at 1000 ° C or more and 1 ° C
After cooling at a cooling rate of
~ Preheat to 850 ° C and hold for more than 1 minute, then 1000 ~
Heating and holding at 1150 ° C for 1 minute or more, then cooling at a cooling rate of 1 ° C / min or more.
A method for producing a high Cr ferritic steel having excellent creep rupture strength and ductility, which is performed at a temperature of 50 ° C. 2. In the final hot working of high Cr ferritic steel containing 0.01 to 3% by weight of W in addition to the component of claim 1, after heating and holding at 1100 to 1250 ° C. for 1 minute or more,
After processing at 15 ° C or higher at 15 ° C or higher and cooling at a cooling rate of 1 ° C / min or higher, normalize, preheat to 700 to 850 ° C and hold for 1 minute or longer, and then 1000 to 1150 ° C High-Cr with excellent creep rupture strength and ductility characterized in that it is heated and held for at least 1 minute, then cooled at a cooling rate of 1 ° C / min or more, and the final tempering is performed at a temperature of 650 to 850 ° C.
Manufacturing method of ferritic steel. 3. The composition according to claim 1, further comprising 0.0001 to 0.2.
In the final hot working of high Cr ferritic steel containing 02% by weight of B, after heating and holding at 1100-1250 ° C for 1 minute or more,
After processing 15% or more at 1000 ° C or more and cooling at a cooling rate of 1 ° C / min or more, normalize, preheat to 700 to 850 ° C and hold for 1 minute or more. Heating and holding at 1 ° C for 1 minute or more, followed by cooling at a cooling rate of 1 ° C / min or more, and final tempering at a temperature of 650 to 850 ° C, characterized by excellent creep rupture strength and ductility. Manufacturing method of Cr ferritic steel. 4. A high chromium containing 0.01 to 3% by weight of W and 0.0001 to 0.02% by weight of B in addition to the component of claim 1.
In the final hot working of ferritic steel, after heating and holding at 1100-1250 ° C for 1 minute or more, working at 15% or more at 1000 ° C or more, cooling at a cooling rate of 1 ° C / min or more, then normalizing Is preheated to 700-850 ° C and held for 1 minute or more,
Subsequently, after heating and holding at 1000-1150 ° C for 1 minute or more, 1 ° C / min
A method for producing a high-Cr ferritic steel having excellent creep rupture strength and ductility, wherein cooling is performed at the above cooling rate, and final tempering is performed at a temperature of 650 to 850 ° C.