JP5835079B2 - Method for producing ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a method for producing a ferritic heat resistant steel, and more particularly, to a method for producing a ferritic heat resistant steel having an excellent high temperature strength and a Cr content of 8 to 10%. The “ferritic heat-resistant steel” in the present invention is a heat-resistant steel whose metal structure is mainly a so-called “tempered martensite structure”, that is, “tempered martensite” is 80% or more of the metal structure in area ratio. Point to. In the present invention, the same applies to the metal structure in the case of “ferritic steel”.

  In recent years, in thermal power plants, etc., there has been an increase in operation with higher steam temperature and pressure. Due to the higher temperature and pressure of such steam, even heat-resistant steel, which is a constituent material of boiler tubes, is more High performance is required.

  As heat-resistant steel used for boiler tubes, for example, low alloy steel such as 2.25% Cr-1% Mo steel, ferritic heat-resistant steel having 5 to 13% Cr content and austenitic heat-resistant steel are known. It has been.

  Among the above, 2.25% Cr-1% Mo steel has lower high-temperature strength and creep rupture strength than ferritic heat-resistant steel and austenitic heat-resistant steel having a Cr content of 5 to 13%, and steam oxidation resistance. The characteristics are also insufficient in an environment exceeding 550 ° C.

  Austenitic heat-resistant steel has excellent high-temperature strength and creep rupture strength even at a high temperature of 600 ° C. or higher, and relatively good steam oxidation resistance. However, in addition to the fact that the coefficient of thermal expansion is large and stress corrosion cracking may occur, it is also expensive.

  On the other hand, ferritic heat-resistant steel having a Cr content of 5 to 13% (hereinafter sometimes simply referred to as “Cr-based ferritic steel”) ensures high-temperature characteristics using solid solution strengthening or precipitation strengthening. For example, 9% Cr-1% Mo steel, improved 9% Cr-1% Mo steel and 12% Cr steel have high temperature strength and creep rupture strength comparable to austenitic heat resistant steel even at temperatures of 600 ° C. or higher. It is known to have, and it is used also for the super supercritical pressure boiler whose steam conditions are around 600 ° C.

  However, due to global environmental problems, there is a demand for heat resistant steel that can be used in higher temperature and higher pressure environments. There is also a movement to improve the Cr-based ferritic steel so that it can be applied to a super supercritical pressure boiler having a steam temperature exceeding 600 ° C. Furthermore, the use of Cr-based ferritic steels as a heat-resistant material not only in boilers but also in nuclear power plants and chemical industrial plants is highly expected.

  Known techniques for improving the heat resistance of Cr-based ferritic steels include those that focus on alloy compositions such as the addition of B, W, and Mo, or the combined addition of Nb and V, or those that focus on the crystal grain size. For example, Patent Document 1 discloses a ferritic heat resistant steel containing W, Mo, V, Nb and B. W and Mo have a solid solution strengthening action, Nb and V have a precipitation strengthening action, and B has a creep strength improving action.

  On the other hand, work heat treatment of Cr-based ferritic steel has also been studied. For example, Patent Document 2 and Patent Document 3 specify a steel material at 900 to 1300 ° C. in the final hot working of steel containing 5 to 13% Cr. A thermomechanical processing method including a step of hot working and rapid cooling after holding for a time is disclosed. However, the thermomechanical technology disclosed in these patent documents is only intended to streamline the process and reduce the cost of the method of performing normalization by reheating after hot working, which is the “conventional technology” at that time. . Patent Document 4 also discloses a technique having an object similar to that of Patent Document 2 and Patent Document 3.

  On the other hand, with respect to austenitic stainless steel or austenitic alloy, conventionally, improvement of properties such as strength has been performed by using thermomechanical processing.

  For example, Patent Document 5 discloses a technology for obtaining austenitic stainless steel having a fine crystal structure and high yield strength by subjecting a low C 18Cr-8Ni type austenitic stainless steel containing N and Nb to a heat treatment. .

  Patent Document 6 discloses that austenite is subjected to finishing in a temperature range of 50 ° C. or more and 600 ° C. or less after hot working or solution heat treatment for stabilizing the austenitic structure and high temperature strength of austenitic stainless steel. A stainless steel thermomechanical process is disclosed.

  Furthermore, with respect to austenitic stainless steel or austenitic alloy, a technique has been proposed in which grain boundary structure is controlled by thermomechanical processing to achieve more desirable characteristics.

  For example, Patent Document 7 discloses that a structure having a twinning frequency (the ratio of crystal grains in which deformation twins are generated) of 70% or more is excellent in resistance to stress corrosion cracking and resistance to hydrogen embrittlement. A technique for obtaining a high Ni-based alloy is disclosed.

  Further, Patent Document 8 discloses that a “special” grain boundary fraction is obtained by repeating a cold working process with a working degree of 5 to 30% and an annealing (annealing) process at a temperature of 900 to 1050 ° C. for 2 to 10 minutes. A technique is disclosed in which an austenitic stainless alloy is provided with good grain boundary degradation resistance by increasing (the “special” grain boundary ratio) to 60% or more.

JP-A-4-354856 JP-A 64-39323 Japanese Patent Laid-Open No. 1-139717 Japanese Patent Laid-Open No. 2-138417 JP 54-51923 A Japanese Patent Laid-Open No. Sho 61-270332 Japanese Patent Laid-Open No. 7-3368 International Publication No. 94/14986

  As described above, in the case of austenitic stainless steel or austenitic alloy, it has been proposed to actively apply thermomechanical treatment with the intention of obtaining better properties, including control of the grain boundary structure. Yes.

  On the other hand, in the case of Cr-based ferritic steel, the purpose of the thermomechanical treatment is simply streamlining the process and reducing the cost, and does not actively apply the thermomechanical treatment in order to obtain better properties.

  The present invention has been made in view of the above situation, and its purpose is to provide a Cr-based ferritic steel, in particular, a ferritic heat-resistant steel having a Cr content of 8 to 10%, with higher temperature strength. It is to provide a manufacturing method that can be used.

  Ferritic steel transforms to austenite at high temperatures. Therefore, the present inventors have worked in various conditions by combining heat treatment and hot working in the austenite temperature region in order to provide excellent high temperature strength to the ferritic heat resistant steel having a Cr content of 8 to 10%. Heat treatment was performed. As a result, the following findings (a) to (c) were obtained.

(A) When a series of steps described below are continuously performed on a ferritic heat resistant steel having a Cr content of 8 to 10% before tempering, the high temperature strength is improved as compared with a conventional product.
-Heat the ferritic heat-resistant steel to the austenite temperature range.
-Hot working is performed at a specific range of reduction in the austenite temperature range.
-Immediately after the above hot working, perform appropriate heat treatment and cooling again.

  (B) When the reduction ratio of the hot working is 15 to 30%, the increase in high-temperature strength is most remarkable.

  (C) In the ferritic heat resistant steel having a Cr content of 8 to 10%, it is estimated that the driving force for moving the grain boundary is accumulated when the hot working of (b) is performed. . And, after the hot working of (b), when heating immediately and then cooling, the frequency of low-angle grain boundaries that are estimated to be more stable than the corresponding grain boundaries has increased significantly, This is considered to contribute to the improvement of the high temperature strength.

  This invention is completed based on said knowledge, The summary exists in the manufacturing method of the ferritic heat resistant steel shown to following (1)-(3).

(1) By mass%, C: 0.05 to 0.35%, Si: 0.05 to 0.60%, Mn: 0.10 to 0.80%, Cr: 8 to 10%, Mo: 0 .60 to 1.50%, W: 0.01 to 2.5%, V: 0.10 to 0.30%, Nb: 0.01 to 0.12%, Al: 0.02% or less, N : 0.015-0.080% and Cu: 0.20% or less, with the balance being Fe and impurities, P, S and Ni in the impurities are respectively P: 0.020% or less, S: A ferritic heat-resistant steel having 0.010% or less and Ni: 0.40% or less is heated to 950 to 1200 ° C. and hot-worked at a reduction rate of 1 to 30 % in the austenite temperature range, and immediately thereafter 900 and held at 1200 temperature range of ° C. 20 minutes or more to cool, then, to perform the tempering at a temperature of Ac ₁ point Wherein, manufacturing method of ferritic heat resistant steels.

  (2) The method for producing a ferritic heat resistant steel according to the above (1), including a step of holding the steel in a temperature range of 950 to 1200 ° C. before hot working in the austenite temperature range.

  (3) The method for producing a ferritic heat resistant steel according to the above (1) or (2), wherein the rolling reduction in hot working is in the range of 15 to 30%.

  According to the production method of the present invention, it is possible to obtain a ferritic heat resistant steel having an excellent high temperature strength with a Cr content of 8 to 10%.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass%”.

(A) Chemical composition of steel:
Cr: 8 to 10%
In the production method of the present invention, it is necessary to use ferritic steel containing 8 to 10% of Cr as material steel for a series of thermomechanical treatments performed before tempering. That is, Cr is an indispensable element for ensuring corrosion resistance and oxidation resistance at high temperatures, in particular, steam oxidation resistance. Furthermore, Cr contributes to the improvement of creep strength by forming carbides. These effects are stably obtained when the Cr content is 8% or more. However, when the Cr content is excessive, δ ferrite is formed to reduce toughness, and precipitation and coarsening of M ₂₃ C ₆ type carbide may cause a decrease in creep strength. For this reason, an upper limit is set so that the Cr content is 8 to 10%. A preferable Cr content is 8.3 to 9.7%, and more preferably 8.6 to 9.5%.

  The steel used as the material steel in the production method of the present invention only needs to be ferritic steel containing 8 to 10% of Cr. That is, the chemical composition of the steel used as the material steel in the production method of the present invention is not necessarily limited except that it is a ferritic steel having a Cr content of 8 to 10%.

  In addition, as preferable ferritic steel when heat-resistant steel applications such as boilers are assumed, in addition to the above amount of Cr, for example, C: 0.05 to 0.35%, Si: 0.05 to 0.60 %, Mn: 0.10 to 0.80%, Mo: 0.60 to 1.50%, W: 0.01 to 2.5%, V: 0.10 to 0.30%, Nb: 0.0. 01 to 0.12%, Al: 0.02% or less, N: 0.015 to 0.080% and Cu: 0.20% or less, with the balance being Fe and impurities, Examples include ferritic steels in which S and Ni are P: 0.020% or less, S: 0.010% or less, and Ni: 0.40% or less, respectively.

  Hereinafter, the elements from C to Ni in a preferable ferritic steel when heat-resistant steel applications such as boilers are assumed will be described.

C: 0.05 to 0.35%
C has the effect of improving hardenability and ensuring strength, and the above effect can be stably obtained when the content is 0.05% or more. On the other hand, when the C content exceeds 0.35%, the toughness tends to decrease. Therefore, the C content is preferably 0.05 to 0.35%. When emphasizing weldability, the C content is preferably 0.20% or less. The more preferable content of C is 0.07 to 0.20%, and further preferably 0.08 to 0.13%.

Si: 0.05-0.60%
Si is an element having a deoxidizing effect, and the above effect can be stably obtained when its content is 0.05% or more. On the other hand, if the Si content becomes excessive and exceeds 0.60, the toughness may be reduced. Therefore, from the viewpoint of ensuring deoxidation and toughness, the Si content is preferably 0.05 to 0.60%. A more preferable Si content is 0.15 to 0.50%.

Mn: 0.10 to 0.80%
Mn has an action of ensuring deoxidation action and hardenability, and the above action can be stably obtained when the content is 0.10% or more. On the other hand, if the content of Mn exceeds 0.80%, the toughness, creep strength and the like may be lowered. Therefore, the Mn content is preferably 0.10 to 0.80%. The content of Mn is more preferably 0.15 to 0.75%, and further preferably 0.25 to 0.65%.

Mo: 0.60 to 1.50%
Mo contributes to the improvement of creep strength as a solid solution strengthening element and partly dissolves in the Cr carbide, thereby suppressing the aggregation and coarsening of the carbide and contributing to the improvement of the creep strength. Furthermore, Mo also has an effect of preventing temper embrittlement. These effects are stably obtained when the Mo content is 0.60% or more. On the other hand, if the Mo content exceeds 1.50%, δ ferrite may be generated and the creep strength may be lowered. Therefore, the Mo content is preferably 0.60 to 1.50%. The content of Mo is more preferably 0.75 to 1.20%, and further preferably 0.85 to 1.15%.

W: 0.01 to 2.5%
W, like Mo, has the effect of improving high-temperature strength and preventing temper embrittlement. These effects are stably obtained when the W content is 0.01% or more. On the other hand, when a large amount of W exceeding 2.5% is contained, the toughness may be lowered or the creep strength may be lowered due to the formation of δ ferrite. Therefore, the W content is preferably 0.01 to 2.5%.

  In order to suppress the formation of δ ferrite, the contents of Mo and W are preferably “Mo + 0.5 W” and 1.5% or less.

V: 0.10 to 0.30%
V is an element effective for precipitating fine carbides in the crystal grains in the tempering process and ensuring high-temperature strength and toughness. can get. On the other hand, even if it contains V exceeding 0.30%, the effect may be saturated and the cost may increase. Therefore, the V content is preferably 0.10 to 0.30%. The content of V is more preferably 0.15 to 0.28%, and further preferably 0.18 to 0.27%.

Nb: 0.01 to 0.12%
Nb is an element that precipitates as carbonitride and is effective in increasing the strength, and the above action can be stably obtained when the content is 0.01% or more. On the other hand, when Nb is contained in an amount exceeding 0.12%, the formation of δ ferrite is promoted, and the creep strength may be lowered for a long time. Therefore, the Nb content is preferably 0.01 to 0.12%. A more preferable Nb content is 0.03 to 0.08%.

Al: 0.02% or less Al has a deoxidizing action, but if its content exceeds 0.02%, the creep strength may be easily lowered. Therefore, the Al content is preferably 0.02% or less. If the Al content is 0.005% or more, the deoxidation effect can be obtained stably, so the Al content is more preferably 0.005 to 0.02%. In addition, said Al content in this invention is sol. This is the amount of Al (acid-soluble Al).

N: 0.015-0.080%
N is effective in securing creep rupture strength and preventing the formation of δ ferrite, and the above effect can be stably obtained when the content is 0.015% or more. However, if the N content exceeds 0.080%, the toughness or creep rupture strength may be reduced. Therefore, the N content is preferably 0.015 to 0.080%. The N content is more preferably 0.020 to 0.070%, still more preferably 0.025 to 0.060%.

Cu: 0.20% or less
Cu has the effect of improving the oxidation resistance, but if its content exceeds 0.20%, the toughness and hot workability may be impaired. Therefore, the Cu content is preferably 0.20% or less. If the Cu content is 0.05% or more, the effect of improving the oxidation resistance can be obtained stably, so the Cu content is more preferably 0.005 to 0.20%.

P: 0.020% or less P is an unavoidable impurity, and its content is 0.02% or less from the viewpoint of securing good hot workability, weldability, creep strength, creep fatigue strength, and the like. It is preferable to do. The content of P in the impurities is more preferably 0.015% or less.

S: 0.010% or less S is also an impurity, and from the viewpoint of ensuring good hot workability, weldability, creep strength, creep fatigue strength, etc., its content should be 0.010% or less. preferable. The content of S in the impurities is more preferably 0.005% or less.

Ni: 0.4% or less
Ni is also an inevitable impurity from the melting raw material, and its content is preferably 0.4% or less from the viewpoint of ensuring good creep strength. The content of Ni in the impurities is more preferably 0.2% or less, and further preferably 0.1% or less.

(B) Production conditions In the production method of the present invention, the ferritic steel containing 8 to 10% of Cr described in the item (A) is heated to 950 to 1200 ° C, and the rolling reduction is 30% or less in the austenite temperature range. Immediately after hot working, it is necessary to hold and cool in the temperature range of 900 to 1200 ° C. for 20 minutes or more, and then temper. The “rolling rate” refers to the cross-sectional reduction rate.

  Hereinafter, for convenience, in the above-described production method of the present invention, heating before hot working in the austenite temperature range is “primary heating”, and is performed immediately after hot working at 900 to 1200 ° C. for 20 minutes or more. This holding may be referred to as “secondary heating”.

  In the production method of the present invention, the present inventors continuously perform a series of steps of primary heating, hot working, and secondary heating before tempering, thereby causing grain boundary movement. In particular, it is estimated that the frequency of low-angle grain boundaries increases.

  Here, the “low angle grain boundary” is a term indicating the character of the grain boundary together with the “corresponding grain boundary” described later.

  The crystal grain boundary is referred to as a “low angle grain boundary (also referred to as“ small angle grain boundary ”) having an angle of 15 ° or less due to a difference in crystal orientation between two adjacent crystal grains across the grain boundary. It is divided into “large-angle grain boundaries” larger than 15 °.

  Also, in order to show the regularity of atomic arrangement at the grain boundary, the “Σ value” defined by the reciprocal of the density of the corresponding lattice point formed by the crystal lattice of two crystal grains adjacent across the grain boundary is used. It has been. In general, it is considered that the smaller the Σ value, the smaller the disorder of regularity at the grain boundary and the lower the grain boundary energy. In general, among the large-angle grain boundaries, the above-described grain boundary having a Σ value of 29 or less is referred to as a “corresponding grain boundary”. The grain boundaries excluding the “corresponding grain boundaries” and the “low angle grain boundaries” among the large angle grain boundaries are referred to as “random grain boundaries”.

  The ferritic steel used as the material steel in the production method of the present invention is not particularly limited as long as it is prepared by an ordinary method. For example, the ingot or slab after melting can be made into a predetermined shape by hot working such as ingot rolling or forging, and this can be used.

  In the present invention, the primary heating of the ferritic steel also has the significance of removing the strain due to the solid solution treatment in which carbides and the like are dissolved and the previous process history in the raw steel. Therefore, there is no particular limitation regarding the history of the ferritic steel manufacturing process before the primary heating.

  However, when the primary heating temperature is lower than 950 ° C., solid solution of carbides does not proceed sufficiently. On the other hand, when the primary heating temperature exceeds 1200 ° C., δ ferrite is generated and the mechanical properties between the heats are lowered. Therefore, it was decided to heat the ferritic steel to 950 to 1200 ° C. before hot working. In addition, it is preferable that the primary heating temperature before hot processing is 1000-1150 degreeC.

  Furthermore, in order to stably achieve the purpose of the solid solution treatment of carbide or the like, it is desirable to maintain the temperature range of 950 to 1200 ° C. in the primary heating before hot working to be heated to 950 to 1200. . The holding temperature range is more preferably 1000 to 1150 ° C.

  In particular, when the heating rate is high, it is desirable to ensure a holding time with a constant temperature range of 950 to 1200 ° C. The holding time in the temperature range of 950 to 1200 ° C. is desirably 20 to 60 minutes.

  As described above, the ferritic steel heated to 950 to 1200 ° C. and preferably held at 950 to 1200 ° C. is hot-worked in the austenite temperature range, but the reduction ratio at that time is 30% or less. There is a need.

  This is because if the rolling reduction ratio of hot working exceeds 30%, recrystallization tends to occur and the high-temperature strength is not improved. This is presumably because the frequency of low-angle grain boundaries decreases when the reduction ratio of hot working exceeds 30%. The rolling reduction is preferably 1% or more, and more preferably 15 to 25%.

  In the production method of the present invention, the ferritic steel heated to 950 to 1200 ° C., preferably held at 950 to 1200 ° C. and hot-worked at a reduction rate of 30% or less in the austenite temperature range, In order to increase the frequency of low-angle grain boundaries, secondary heating is performed immediately after the hot working, that is, holding for 20 minutes or more in a temperature range of 900 to 1200 ° C.

  Even if the temperature of the secondary heating is below 900 ° C. or above 1200 ° C., the frequency of low-angle grain boundaries does not increase, so the high-temperature strength is not improved. Moreover, even if the temperature of secondary heating is 900-1200 degreeC, since the frequency of a low angle grain boundary will not increase if holding time is less than 20 minutes, high temperature intensity | strength does not improve.

The ferritic steel after the secondary heating is cooled once and then tempered . After the secondary heating and cooling, the metallic structure mainly composed of martensite, in particular, to tissue martensite is 80% or more in area ratio, 8 ° C. The range of 800 to 500 ° C. It is desirable to carry out at a cooling rate of at least 1 minute. When the cooling rate is low, ferrite may be generated and the high-temperature strength may be reduced.

Note that martensite is tempered because it has a high dislocation density and lacks structural stability at high temperatures. When the tempering temperature exceeds the Ac ₁ point, reverse transformation to austenite occurs, and therefore tempering is performed at a temperature of the Ac ₁ point or lower.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

After melting steel having the chemical composition shown in Table 1, hot forging was performed by a normal method, and then a block of 100 mm × 100 mm × 200 mm was taken from a steel plate subjected to solution treatment at 1000 ° C. for 45 minutes. Used as test material. The Ac ₁ point of the steel is about 820 ° C., and the temperature at which the reverse transformation to austenite is completed (Ac ₃ point) is about 920 ° C.

  After the block of 100 mm × 100 mm × 200 mm was heated at the heating temperature shown in Table 2 for 45 minutes, it was immediately subjected to hot rolling at a reduction rate of 1-35% in the austenite temperature range, followed by secondary A heat treatment comprising a series of steps of holding for 20 to 60 minutes at the temperature shown in Table 2 was performed as heating, and tempering was further performed at 770 ° C. for 60 minutes to obtain steel plates (test numbers 2 to 19).

  In addition, as a standard for characteristic investigation, a block tempered at 770 ° C. without subjecting it to a heat treatment of the above size was used (Test No. 1).

  A test piece having a diameter of 6 mm and a distance between gauge points of 60 mm was taken from the longitudinal direction of each steel plate and block thus obtained, and strain rates of 0.001 / second and 0.00001 / second were 2. Under the conditions, a tensile test at 600 ° C. was conducted to investigate the high temperature strength characteristics.

  In addition, since it is considered that the grain boundary structure of the test material does not change by tempering, test number 1 in which the heat treatment was not performed and test numbers 2, 4, 7, 10, For a total of seven conditions of 15 and 18, electron beam backscatter diffraction (hereinafter referred to as “EBSD”) was performed with a scanning electron microscope (hereinafter referred to as “SEM”) in the state before tempering at 770 ° C. It was used to analyze the grain boundary structure.

  Specifically, it was carried out using Hitachi FEG-SEM (S-4200) under step conditions of an acceleration voltage of 30 kV, a beam current of 8 μA, and 0.25 μm.

  A sample for EBSD observation was electropolished using a solution in which acetic acid and perchloric acid were mixed at a ratio of 77:23 to remove distortion on the sample surface in advance.

  For the analysis, crystal analysis software “TSL OIM analysis 5.2” was used.

Table 2 also shows the results of the above tensile test. In Table 2, the high-temperature strength characteristics are obtained by dividing the numerical value obtained by dividing the 0.2% proof stress (σ _0.2 ) by the Young's modulus (hereinafter referred to as “E”) at 600 ° C. (strain rate is 0.001). / B) (when the strain rate is 0.00001 / second). In addition, numerical values obtained by dividing [a] and [b] for each test number by [a] and [b] of test number 1 are also shown. Here, for E at 600 ° C., “1400 GPa”, which is a value obtained by extrapolation from measurements at room temperature to 550 ° C., was used.

  Table 3 shows the analysis result of the grain boundary structure together with the conditions for the heat treatment and tempering.

  From Table 2, in the case of test numbers 17 to 19 that deviate from the conditions defined by the production method of the present invention, no improvement in high strength is observed as compared with the standard test number 1 that was tempered without performing the heat treatment.

  On the other hand, when the test numbers 2 to 16 satisfy the conditions defined by the production method of the present invention, it is apparent that excellent high temperature strength can be obtained. In the case of the present invention example, when the strain rate is small, a further remarkable improvement in high-temperature strength is recognized.

  According to the production method of the present invention, it is possible to obtain a ferritic heat resistant steel having an excellent high temperature strength with a Cr content of 8 to 10%.

Claims (3)

In mass%, C: 0.05 to 0.35%, Si: 0.05 to 0.60%, Mn: 0.10 to 0.80%, Cr: 8 to 10%, Mo: 0.60 1.50%, W: 0.01 to 2.5%, V: 0.10 to 0.30%, Nb: 0.01 to 0.12%, Al: 0.02% or less, N: 0.00. 015 to 0.080% and Cu: 0.20% or less, the balance is made of Fe and impurities, and P, S and Ni in the impurities are respectively P: 0.020% or less, S: 0.010 % And Ni: 0.40% or less ferritic heat-resisting steel is heated to 950 to 1200 ° C. and hot-worked at a reduction rate of 1 to 30 % in the austenite temperature range, and then immediately 900 to 1200 ° C. in a temperature range and held to cool 20 minutes or more, then, characterized by performing tempering at a temperature of Ac ₁ point To method of ferritic heat resistant steels.   The manufacturing method of the ferritic heat-resistant steel of Claim 1 including the process of hold | maintaining steel in the temperature range of 950-1200 degreeC before hot working in an austenite temperature range.   The method for producing a ferritic heat resistant steel according to claim 1 or 2, wherein the rolling reduction in hot working is in a range of 15 to 30%.