JPH0610041A - Production of high cr ferritic steel excellent in creep rupture strength and ductility - Google Patents

Abstract

PURPOSE:To improve creep ructure strength and ductility by subjecting a high Cr ferritic steel, having a specific composition where the amounts of impurities are controlled, to specific final hot working, to specific cooling, and to respectively specified normalizing and final tempering. CONSTITUTION:At the time of applying final hot working to the steel having a composition which contains, by weight, 0.02-0.20% C, <=1% Si, <=1.5% Mn, <=1% Ni, 8-14% Cr, 0.01-3% Mo, 0.1-0.3% V, 0.01-0.2% Nb, 0.001-0.1% N, 0.001-0.05% Al, and 0.01-3.5% Cu and further contains, if necessary, 0.01-3% W and/or 0.0001-0.02% B and where the contents of P, S, and O among impurities are regulated to <=0.025%, <=0.015%, and <=0.015%, respectively, this steel is held at 1100-1250 deg.C for >=1min and worked at >=1000 deg.C at >=15% draft. This steel is cooled at >=1 deg.C/min cooling rate, held at 700-850 deg.C for >=1min and successively at 1000-1150 deg.C for >=1min, and cooled at >=1 deg.C/min cooling rate to undergo normalizing. Further, the final tempering is done at 650-850 deg.C.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY 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 boiler, nuclear power, chemical industry and the like. And a method for producing high Cr ferritic steel.

[0002]

2. Description of the Related Art Heat-resistant steels used as heat-resistant pressure-resistant members for boilers, nuclear power plants, and chemical industries are required to have high-temperature strength, corrosion / oxidation resistance, and toughness. It is also required to be excellent and inexpensive.

Materials used for the above applications include austenitic stainless steels, ferritic low alloy steels, and 9-12Cr high Cr ferritic steels. Above all
Cr ferritic steel has strength and corrosion resistance in the temperature range of 500 to 650 ℃.
It has the advantages of superior oxidation resistance to low alloy steel, higher thermal conductivity than austenitic steel, and a small coefficient of thermal expansion.
It is used in many fields, including thick materials.

Typical of high Cr ferritic steels are 9Cr-1Mo steel (STBA26) and improved 9Cr-1Mo steel (ASTM SA
213 T91), 12Cr-1Mo steel (DIN X20CrMoV121), etc. Further, as a steel to which W, V, Nb, N, etc. are added in combination for the purpose of increasing the high temperature strength, Japanese Patent Publication No. Sho 62-8502, JP-A-2-
There is steel disclosed in each publication such as 310340. In addition, the present inventors have proposed a high-strength and high-chromium steel to which Cu is added in order to enhance the oxidation resistance at high temperatures of 600 ° C. or higher (JP-A-2-232345, JP-A-3-97832). .

The addition of Cu not only improves the oxidation resistance but also suppresses the formation of δ-ferrite, which is harmful to the toughness,
It saves the expensive addition of Ni, and compared with C and Ni, A
Since the c ₁ transformation point is not significantly lowered, high temperature tempering is possible. Although it has been conventionally pointed out that the ductility is decreased during processing due to the addition of a large amount of Cu, the present inventors have found that by adding a small amount of Mg in combination (see the above-mentioned JP-A-2-2323).
It was also found that the problem of reduced ductility can be solved by using a structure containing a predetermined amount of δ ferrite (Japanese Patent Laid-Open No. 3-97832).

FIG. 1 shows a heat pattern at the time of final working and heat treatment in the production of a conventional high Cr ferritic steel. In the conventional final processing, the material is heated in a temperature range of 1100 to 1250 ° C., which is a relatively high temperature of Ac ₃ or higher, and then hot working finish is performed by forging or rolling, and then cooled. Since the final processing is usually intended for finishing, it is often performed at a slight reduction rate of 10% or less. In addition, the final processing end temperature is often 1000 ° C or lower. Cooling is cold,
Thick materials are gradually cooled at less than 1 ° C / min.

In the subsequent normalizing, the material is heated to a predetermined temperature of Ac ₃ or higher (usually 950 to 1100 ° C.), held at that temperature, and then allowed to cool. Also in this case, the cooling time becomes long with the thick material. The final tempering is usually done at 750-800 ° C, then air cool.

However, it has been pointed out that the long-term creep rupture ductility and the decrease in creep rupture strength are problems of the high Cr ferritic steel having improved high temperature strength. This is due to inhomogeneity of the structure (coarsening of crystal grains, abnormal grain growth), agglomeration and coarsening of coarse carbonitrides during processing, and high strength with addition of V, Nb, N, Mo, W, etc. This occurs in steel, and particularly in high Cr ferritic steel containing a large amount of Cu added, the creep rupture ductility (breaking reduction, elongation) of the material is greatly reduced. There have been almost no reports on methods for improving the creep rupture ductility of such steels.

As a countermeasure against this, it is conceivable to suppress the amount of alloying elements such as V, Nb, W, N, C and Cu, which cause the reduction of creep rupture ductility. However, since the inclusion of these elements is for improving the strength, corrosion resistance and oxidation resistance, reducing these elements impairs the basic properties of steel. It is also possible to increase the creep rupture ductility by producing a large amount of δ-ferrite, but this method reduces toughness, and especially in thick-walled materials, the structure, toughness, and material anisotropy of strength are problems. Become. Further, it is necessary to add an alloying element for forming δ-ferrite, which may be unfavorable in terms of strength. A method of homogenizing the structure by repeating the normalization twice to adjust the grain size may be adopted, but the cost increases and the process efficiency decreases.

[0010]

SUMMARY OF THE INVENTION The present invention is based on the conventional
Improves the above-mentioned problems in high Cr ferritic steels that have been added with W, V, Nb, N, etc. to increase the high temperature strength, namely the creep rupture ductility and the decrease in creep rupture strength, and further improve the creep rupture strength. To produce high-Cr ferritic steels, especially Cu
An object of the present invention is to provide a method for producing a high Cr ferritic steel to which is added. The target of Cu-added steel is the heat resistance temperature of high Cr ferritic steel due to the effects of the above-mentioned addition of Cu (improvement in oxidation resistance, suppression of δ-ferrite formation that is harmful to toughness, saving of Ni addition, etc.). This is because it is expected that the application will be expanded, such as by increasing the cost and applying it to inexpensive thick materials.

[0011]

[Means for Solving the Problems] In order to solve the above problems, various tests were conducted, and the following new findings were obtained.

(1) Conventional high Cr with high strength at high temperature
Compared with ferritic steel, Cu is indispensable in order to excel in high temperature strength and to improve corrosion and oxidation resistance.

(2) In order to improve the creep rupture ductility, it is effective to suppress the agglomeration and coarsening of carbonitrides during hot working. For this reason, the final processing temperature is 1000 ° C or higher and the processing end temperature is 9
It is recommended that the temperature be 00 ° C or higher and the processing rate be 15% or higher, and that after processing, the cooling rate be 1 ° C / min or higher.

(3) From the viewpoint of suppressing grain boundary segregation and precipitation of Cu during normalizing and sizing the crystal grains by fine precipitation of carbonitride, normalizing should be performed at 700 to 850 ° C. After heating, it is preferable that the temperature is maintained in the temperature range of 1000 to 1150 ° C. and the material is rapidly cooled at a cooling rate of 1 ° C./min or more.

Thereafter, tempering is performed at least once in the temperature range of 650 to 850 ° C. to prevent the deterioration of the creep rupture ductility of the high Cr ferritic steel having the high temperature strength and further to increase it. You can

The present invention was made on the basis of the above findings, and the gist thereof lies 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 to 3%, V: 0.1 to 0.3%, Nb: 0.01 to 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 to 3.5%, balance of Fe and unavoidable impurities, P content of 0.025% or less, S content of 0.015% or less, and O (oxygen) content of 0.015% or less. In hot working, after heating and holding at 1100 to 1250 ℃ for 1 minute or more, then processing at 15 ℃ or more at 1000 ℃ or more, 1 ℃
After cooling at a cooling rate of / min or more, normalize 700
~ Preheat to 850 ℃ and hold for 1 minute or more, then 1000 ~
After heating and holding at 1150 ℃ for 1 minute or more, and then cooling at a cooling rate of 1 ℃ / min or more, the final tempering is 650 to 8
A method for producing a high Cr ferritic steel excellent in creep rupture strength and ductility, which is characterized in that it is carried out 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 of them may be subjected to the above-mentioned working and heat treatment.

[0019]

The reason 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 during production are determined as described above will be explained below.

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

Cr: Cr is an essential element for ensuring the oxidation resistance and corrosion resistance of steel. If it is less than 8%, sufficient oxidation resistance and corrosion resistance as high Cr ferritic steel cannot be obtained, while if it exceeds 14%, the 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 an element which is added as a deoxidizer and enhances steam oxidation resistance, but if it exceeds 1%, the toughness is remarkably lowered and it is harmful to the creep rupture strength. In particular, in the case of thick-walled materials, it is better to keep it low from the viewpoint of suppressing the formation of δ-ferrite and suppressing long-term heat embrittlement. If toughness is important, 0.1% or less is preferable, and if steam oxidation resistance is important, 0.2 to 0.4% is preferable.

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

Ni: Ni suppresses the formation of δ-ferrite as an austenite stabilizing element and stabilizes the martensite structure. However, if its content exceeds 1%, the growth coarsening of carbides is promoted and the creep rupture strength is reduced. In addition, the Ac ₁ transformation point is lowered to prevent sufficient tempering treatment, 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 =
It is preferable to contain Ni that satisfies 2.5 to 4.5 (weight% ratio).

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

Further, it is more effective to add Mo and W in combination, but in this case, the Mo content may be 0.1 to 1.2%, and Mo + 1 / 2W = 1.2 to 1.8%.

V: V combines with C and N to form a fine precipitate of V (C, N), which greatly contributes to the improvement of 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 rather deteriorated. Therefore, the V content is
0.1 to 0.3%

Nb: Nb, like V, is bonded to C and N to form Nb.
It forms fine precipitates of (C, N) and greatly contributes to the improvement of creep rupture strength. Further, it is effective for improving the toughness by making the crystal grains finer. If it is less than 0.01%, the above effect cannot be obtained, and if it exceeds 0.2%, undissolved NbC is obtained by normalizing.
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 the improvement of creep rupture strength, but if it is less than 0.001%, its effect is not recognized. On the other hand, if it exceeds 0.1%, the 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 carried out and toughness, ductility and strength are impaired. Also, 0.05
%, The creep rupture strength decreases, so the Al content should be 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) Improves oxidation resistance at 600 ° C or higher. (c) The formation of a softened layer in the welded part is suppressed, and the creep rupture strength of the welded joint is improved. However, if it is less than 0.01%, the above effect cannot be obtained, and if it exceeds 3.5%, not only the hot workability is significantly deteriorated, but also the creep rupture ductility is deteriorated and the creep rupture strength is greatly decreased. Therefore, the Cu content is 0.01 to 3.5%. Preferably 0.3
~ 2.2%.

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

W: W, like Mo, is effective as a solid solution strengthening and fine carbide precipitation strengthening element for improving creep rupture strength, and it is particularly preferable to add a large amount of W for improving creep rupture strength at high temperatures. Usually, when replacing with a part or most of Mo, it is necessary to add twice as much as Mo (ratio by weight). However, if its content exceeds 3%, the creep rupture ductility and toughness are markedly reduced. If it is less than 0.01%, the above effect cannot be obtained. Therefore, the W content is
0.01 to 3%. If strength is important, it is preferable to contain 0.8 to 2.2% and Mo + 1 / 2W = 1.2 to 1.8%.

B: B has the effect of dispersing and stabilizing the carbide by adding a trace 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, so when B is added, its content is made 0.0001 to 0.02%. Preferably 0.001-0.004
%.

In addition to the above components, 0.2% of La, Ce, Ca, Zr, Ti, Y, Ta and Mg are added as trace elements.
May be included. These elements are P, S, and
It has a function to combine with an impurity element such as O to stabilize them and improve ductility, toughness and strength.

The high Cr ferritic steel produced by the method of the present invention is a steel which, in addition to the above-mentioned components, has the balance Fe and unavoidable impurities. Typical impurities are P and S
And O (oxygen). These are creep rupture ductility,
From the viewpoint of toughness, workability, and weldability, it is desirable to reduce it as much as possible. P is 0.025% or less, S is 0.015% or less,
O should be 0.015% or less.

The method of the present invention is characterized by performing the following final processing and heat treatment on steels containing the above alloy components in the respective ranges specified.

The processing before the final processing may be carried out by an ordinary method without any limitation. That is, an ingot or a slab is made into a material having a predetermined shape and dimension by hot working such as slab rolling or forging, and this material is, for example,
Hot working means such as punching pipe making method typified by Erhard Pushbench method, extruding pipe making method typified by Eugene Sejournet method, oblique roll piercing rolling pipe making method typified by Mannesmann plug mill method, etc. The intermediate product may be processed into a predetermined intermediate product such as finishing into a raw tube having a predetermined size, and this intermediate product may be subjected to the final processing described below.

[Working and heat treatment conditions] In order to improve the creep rupture ductility, it is necessary to make the structure uniform, suppress the growth of crystal grains and regulate the grain size, and at the same time, prevent precipitation and segregation of Cu. . For that purpose, in the final hot working, it is necessary to carry out comparatively strong reduction working at a temperature higher than that in the past to promote diffusion of alloying elements and suppress coarsening of carbonitrides. If heated below 1100 ° C, solidification segregation of alloy elements and solid solution of coarse precipitates are not sufficient, which causes deterioration of strength and ductility. On the other hand, when it is heated in a temperature range exceeding 1250 ° C, a large amount of δ-ferrite is formed, and the toughness and strength are reduced. Therefore, the heating temperature before final hot working is 1100 ~
Set to 1250 ° C.

The rolling reduction at the time of processing is 15%, which is a relatively high rolling reduction in order to sufficiently diffuse the alloy elements and homogenize the structure.
That is all. Under a light pressure of less than 15%, sufficient strength and ductility cannot be obtained. The processing end temperature is 1000 ° C or higher.
This is because if the working is performed at less than 1000 ° C, the deformed structure remains, and carbonitride precipitates, which deteriorates the strength and ductility.

When the cooling rate after processing is less than 1 ° C./min, carbonitrides and Cu phases are precipitated during cooling and ductility is significantly impaired, so the cooling rate is set to 1 ° C./min or more. The cooling rate is the average cooling rate when the material thickness center is cooled from 800 ℃ to 500 ℃.

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

Normalizing is carried out by first preheating to 700 to 850 ° C. to suppress the precipitation of Cu phase and causing fine precipitation of V and Nb carbonitrides, and then heating at 1000 to 1150 ° C. By suppressing the growth of abnormal crystal grains and controlling the grain size, creep rupture ductility and creep rupture strength can be improved.

If the preheating is less than 700 ° C., not only the above effects cannot be obtained due to insufficient precipitation of carbonitrides, but also Cu
Creep rupture ductility decreases due to phase precipitation. On the other hand, 8
If the temperature exceeds 50 ° C, the precipitation of carbonitrides will decrease and the effect cannot be expected. Hold time during preheating is 1 minute or more. If it is less than 1 minute, the precipitation is insufficient and the above effect cannot be obtained.

If the normalizing temperature is less than 1000 ° C., the carbonitrides precipitated by preheating become coarse, and the amount of finely dispersed precipitates during the subsequent tempering decreases, resulting in a significant loss of strength. Be done. On the other hand, when the temperature exceeds 1250 ° C, the amount of δ-ferrite increases and the crystal grains become coarse, so that the toughness and ductility are impaired. Therefore, the heating temperature during normalizing is 1000 to 1150 ° C. Hold time during heating is 1 minute or more. Preferably, it is 0.5 hours for a plate thickness of 25 mm, for example.

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

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

[0048]

Examples High Cr ferritic steels A having chemical compositions shown in Table 1
G is melted in a vacuum melting furnace of 150 kg and the ingot is heated to 1150 to 9
Hot forging was performed at 50 ° C to form a rolling block having a thickness of 60 mm, a width of 100 mm and a length of 500 mm. Steels A to F are steels of the present invention, G
Steel is a comparative steel.

For each of the test steels A to G, (a) in FIG.
Hot rolling, normalizing and tempering treatments were carried out with the heat patterns exemplified in (d) to (d). (A) is a conventional method, and (B) and (C) are examples of the present invention, which are the present invention method 1 and the present invention method 2, respectively. (D) is the comparison method.

Tensile test pieces of 6 mmφ × 30 mm GL were sampled in the longitudinal direction from the center of the wall thickness of the plate processed and heat-treated by these manufacturing methods, and subjected to the creep rupture test. Creep rupture test conditions are 600 ℃ × 16kgf / mm ² , 650 ℃ × 7kg
At f / mm ² , a long-term test of more than 30,000 hours was performed.
Creep rupture ductility (elongation, reduction) and strength (rupture time) were measured.

The test results are summarized in Table 2. The creep rupture time and the rupture reduction under each creep condition are shown in FIGS. 3 and 4 in comparison with each method and steel type.

The creep rupture times (strengths) of the steels A to F cannot be compared because the compositions are different. However, when viewed in the same steel, the strength is clearly improved when the method of the present invention is applied as compared with the conventional method and the comparative method. Is. In particular, when the method 2 of the present invention in FIG. 2 is applied, the strength is high. It is considered that this is due to the effect of increasing the final hot working temperature and working degree (reduction rate) and quenching. Further, G steel, which is not the object of the present invention, originally has low strength, and the effect of the present invention cannot be recognized.

On the other hand, in the case of the conventional method, the creep rupture ductility is
The values are low for materials with high Cu addition. Also 600 ℃
Compared with the above, the value is lower at 650 ℃ and the elongation is lower. But,
By applying the method of the present invention to any of A to F steels,
Ductility, especially drawing is improved.

On the other hand, when the comparison 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 of the invention. This is because the preheating temperature for normalizing is as low as 650 ° C, and carbonitride precipitation is insufficient. Further, in the case of the comparative G steel, the improvement of ductility is not recognized even by the method of the present invention.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

When the method of the present invention is applied to the production of high Cr ferritic steel, the decrease in creep rupture ductility and creep rupture strength can be improved. In particular, regarding steel containing Cu, it is possible to remarkably 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 resistant and pressure resistant members (steel pipes, plates, forged products, etc.) for boilers, nuclear power, and the chemical industry, and greatly improves the durability and reliability of these members. Can contribute.

[Brief description of drawings]

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

FIG. 2 is a diagram showing an example of a heat pattern at the time of processing and heat treatment in the method for producing a high Cr ferritic steel used in Examples, where (a) is the conventional method, (b) is the method 1 of the present invention,
(C) is the method 2 of the present invention, and (d) is the comparison method.

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

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

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

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

Claims (4)

[Claims] 1. In 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 to 3%, V: 0.1 to 0.3%, Nb: 0.01 to 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 to 3.5%, balance of Fe and unavoidable impurities, P content of 0.025% or less, S content of 0.015% or less, and O (oxygen) content of 0.015% or less. In hot working, after heating and holding at 1100 to 1250 ℃ for 1 minute or more, then processing at 15 ℃ or more at 1000 ℃ or more, 1 ℃
After cooling at a cooling rate of / min or more, normalize 700
~ Preheat to 850 ℃ and hold for 1 minute or more, then 1000 ~
After heating and holding at 1150 ℃ for 1 minute or more, and then cooling at a cooling rate of 1 ℃ / min or more, the final tempering is 650 to 8
A method for producing a high Cr ferritic steel excellent in creep rupture strength and ductility, which is characterized in that it is carried out 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 components of claim 1, after heating and holding at 1100 to 1250 ° C. for 1 minute or more, 1000
After processing 15% or more at ℃ or more and cooling at a cooling rate of 1 ℃ / min or more, normalize preheat to 700 to 850 ℃ and hold for 1 minute or more, then 1000 to 1150 ℃ High-Cr with excellent creep rupture strength and ductility, characterized in that it is heated and held for 1 minute or more and 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.
Ferrite steel manufacturing method. 3. In addition to the components of claim 1, 0.0001-0.
In the final hot working of high Cr ferritic steel containing 02 wt% B, after heating and holding at 1100 to 1250 ° C for 1 minute or more,
After processing 15% or more at 1000 ℃ or more and cooling at a cooling rate of 1 ℃ / min or more, normalize and preheat to 700 to 850 ℃ and hold for 1 minute or more, then 1000 to 1150 It has a high creep rupture strength and high ductility, characterized by being heated and held at ℃ for 1 minute or more and then cooled at a cooling rate of 1 ℃ / min or more, and the final tempering is performed at a temperature of 650 to 850 ℃. Cr ferritic steel manufacturing method. 4. A high Cr content which, in addition to the components of claim 1, further contains 0.01 to 3% by weight of W and 0.0001 to 0.02% by weight of B.
In the final hot working of ferritic steel, after heating and holding at 1100 to 1250 ° C for 1 minute or more, processing at 15% or more at 1000 ° C or more, cooling at a cooling rate of 1 ° C / min or more, and then normalizing Is preheated to 700-850 ° C and held for 1 minute or more,
Then, heat and hold at 1000 to 1150 ℃ for 1 minute or more, then 1 ℃ / min
A method for producing a high Cr ferritic steel excellent in creep rupture strength and ductility, which comprises performing cooling at the above cooling rate and performing final tempering at a temperature of 650 to 850 ° C.