JP5097017B2 - Manufacturing method of high Cr ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a method for producing a high Cr ferritic heat resistant steel material. Specifically, in the production of high Cr ferritic heat-resistant steel materials with excellent high-temperature and long-term creep strength used for heat exchange and piping used in high-temperature and high-pressure environments in boilers, nuclear power generation facilities, and chemical industrial facilities. Regarding the method. In particular, it is a suitable manufacturing method when manufacturing a pipe material made of high Cr ferritic heat resistant steel such as a boiler tube.

  Heat resistant steel materials used in high-temperature and high-pressure environments in boilers, nuclear power generation facilities, chemical industrial facilities, etc. are generally high-temperature creep strength and creep fatigue strength regardless of their shape (plate material, pipe material, bar material, etc.) and size. Corrosion resistance, oxidation resistance, etc. are required.

  When used at a temperature of 500 to 650 ° C., the high Cr ferritic steel material is superior to the low alloy steel in terms of strength and corrosion resistance. In addition, high Cr ferritic steel has high heat conductivity and a low coefficient of thermal expansion. Therefore, the high Cr ferritic steel has excellent heat-resistant fatigue properties and is inexpensive compared to austenitic stainless steel. In addition, there are a number of advantages such as less scale peeling and no stress corrosion cracking.

  Conventionally, as a high Cr ferritic heat resistant steel, 9Cr-1Mo steel indicated by “Tue STBA26”, 9Cr-2Mo steel indicated by “Tue STBA27”, and the like have been used.

  From the late 1980s to the 1990s, higher strength high Cr ferritic heat resistant steels were proposed. As disclosed in Patent Documents 1 to 8, this contains W or Mo, and is further reinforced with a composite carbonitride of Nb and V. For example, disclosed in Patent Documents 1 to 8 Yes.

  And, high strength high Cr ferritic heat resistant steel containing W or Mo and further reinforced with Nb and V composite carbonitride is ASME P91 / T91 steel indicated by “Tue STBA28” and “Tue STPA28”, ASME P92 / T92 steel indicated by “Tue STBA29” and “Tue STPA29”, ASME P122 / T122 steel indicated by “Tue SUS410J3TB” and “Tue SUS410J3TP” have been put into practical use, and supercritical boilers with a steam temperature of 566 ° C or higher. Has been used. All of these materials ensure a long-term creep strength by finely depositing MX type carbonitrides (M = Nb, V; X = C, N) composed of a composite carbonitride of Nb and V. Yes.

  And the method of further improving creep strength is proposed by patent documents 9-12. That is, Patent Document 9 discloses a method of performing hot working by heating to 1100 to 1130 ° C. and then performing tempering heat treatment at 730 to 830 ° C., and Patent Documents 10 and 11 disclose two steps after hot working. A method of performing normalizing treatment is disclosed, and Patent Document 12 discloses a method of performing two-stage tempering after normalizing treatment.

  Such high strength high Cr ferritic heat resistant steels containing W or Mo and further reinforced with Nb and V composite carbonitrides are used in boilers, nuclear power generation facilities and chemical industrial facilities in high temperature and high pressure environments. As a heat-resistant steel material to be used, its practical use has been attempted. For example, it is applied to an ultra-supercritical boiler with a steam temperature of about 600 ° C.

However, in recent years, there has been a demand for CO ₂ emission reduction due to global environmental problems, and there is a demand for heat-resistant steel materials that can be used in higher temperature and pressure environments. For example, there is a need for a heat-resistant steel material that can be applied to a super-supercritical boiler with a high temperature range of about 630 ° C.

JP 61-69948 A JP-A-61-231139 JP-A-62-297436 Japanese Unexamined Patent Publication No. 7-286246 JP-A-9-71846, JP 2000-26940 A Japanese Patent Laid-Open No. 2001-192781 JP 2002-363709 JP Japanese Patent Laid-Open No. 3-20410 JP-A-4-350118 JP-A-8-225832 JP-A-8-225833

  However, when manufacturing a heat-resistant steel material used in a high-temperature and high-pressure environment using a high-strength, high-Cr ferritic heat-resistant steel containing W or Mo and further strengthened with a composite carbonitride of Nb and V The long-term creep strength does not depend on the shape (plate material, pipe material, bar material, etc.), but a tendency to depend on the size of the size was found. Such a tendency is not observed in conventional high Cr ferritic steels such as 9Cr-1Mo steel and 9Cr-2Mo steel.

  For example, with regard to boiler tubes, the allowable tensile stress in the “Interpretation of Technical Standards for Thermal Power Facilities for Power Generation” issued on July 10, 2007 shows that the allowable tensile strength of large-diameter pipes is 625 ° C in comparison with fire STPA28 and fire STBA28. The stress is high, and the allowable tensile stress of the large-diameter pipe is high at 600 ° C and 625 ° C in comparison with fire STPA29 and fire STBA29. And even in comparison between fire SUS410J3TP and fire SUS410J3TB, the allowable tensile stress of the large-diameter pipe is high at 600 ° C.

  That is, high strength high Cr ferritic heat resistant steel materials containing W or Mo and further reinforced with Nb and V composite carbonitrides tend to have a long-term creep strength that tends to be lower in small diameter tubes than in large diameter tubes. I found out that Since the long-term creep strength does not depend on the shape of a steel material such as a plate material, pipe material, or bar material, it is determined that the long-term creep strength tends to depend on the size of the steel material.

  An object of the present invention is to solve the problem of size dependence peculiar to a high strength high Cr ferritic heat resistant steel material containing W or Mo and further reinforced with a composite carbonitride of Nb and V. Thus, an object of the present invention is to provide a method for producing a high Cr ferritic heat resistant steel material having excellent high temperature long time creep strength.

  In order to produce a high Cr ferritic heat-resistant steel material having excellent high-temperature and long-term creep strength, the present inventors focused on the prior austenite grain size of the metal structure of the steel material as shown below.

  ASME P91 steel, ASME P92 steel, ASME P122 steel, and other high strength high Cr ferritic heat resistant steels containing W or Mo and reinforced with Nb and V composite carbonitrides are all tempered martensite single phase Or a two-phase structure containing at most 10% of a δ ferrite phase. The metal structure of these steel materials does not change substantially depending on the type of steel and the size of the product (steel plate, steel pipe, etc.).

  However, looking at the metal structure in more detail, the prior austenite particle size of the metal structure of the steel material is irrelevant to the shapes of the plate material, pipe material, bar material, and the like, and is highly dependent on the size of the product. In particular, in a seamless steel pipe, its particle size varies significantly depending on the outer diameter of the pipe. For example, a small-diameter tube having an outer diameter of 150 mm or less used as a boiler tube is a fine particle having an ASTM particle size number of 7.5 or more, whereas a large-diameter tube having an outer diameter of 150 mm or more is a coarse particle having an ASTM particle size number of less than 7.5. .

  Therefore, the present inventors conducted various experiments as shown in the following (i) to (v), the influence of the prior austenite grain size on the long-term creep test results, and the manufacturing conditions in the processing and heat treatment steps. We obtained new knowledge about the influence on the prior austenite grain size.

  (i) First, the present inventors changed the hot working conditions for the plate material made of steel equivalent to “Fire STBA29” (ASME T92) having the chemical composition shown in Table 1, and changed the following (A) and Two types of experiments (B) were conducted.

  In the first experiment (A), a 30 mm thick plate was heated to 1000 ° C, rolled to a thickness of 15 mm by hot working at an end temperature of 900 ° C, and then maintained at 1050 ° C for 10 minutes. Thereafter, tempering treatment was performed by holding at 780 ° C. for 1 hour. As a result, a fine steel material having a prior austenite grain size of ASTM grain size number 8 was obtained. When this steel material was subjected to a creep test for a long time at a tensile strength of 90 MPa at 650 ° C., the rupture time for the creep test was about 8000 hours.

  In the second experiment (B), a 30 mm thick plate was heated to 1000 ° C, rolled to a thickness of 15 mm by hot working at an end temperature of 900 ° C, then air cooled and soaked at 1120 ° C for 1 hr. After tempering treatment held at 1050 ° C. for 10 minutes, tempering treatment was carried out at 780 ° C. for 1 hour. As a result, a coarse steel material having a prior austenite grain size of ASTM grain size number 5 was obtained. When this steel material was subjected to a creep test for a long time at a tensile strength of 90 MPa at 650 ° C., the creep test rupture time was about 14000 hours.

  Table 2 compares the above two hot working processes, the prior austenite grain size (ASTM grain size number) of the steel, and the creep test rupture time at a tensile strength of 90 MPa at 650 ° C.

  From these experimental results, it was found that the size of the prior austenite grain size greatly affects the long-term creep test results. This is because by increasing the old austenite grain size before normalization by soaking, the number of nucleation sites during normalization decreases, so the final structure becomes coarse, and as a result, It is thought that the creep test rupture time was greatly increased.

  However, it is difficult to apply this method to a product manufacturing process. In other words, a thick oxide scale adheres to the steel surface by soaking at 1120 ° C for 1 hr, which takes time to remove the scale and greatly reduces the production capacity of the product. There is.

  (ii) Next, experiments and examinations were conducted to confirm that the knowledge about the prior austenite grain size is not affected by the difference in the shape of the steel material and can be applied to, for example, pipe materials.

  In general, a steel pipe is manufactured in the order of hot working, normalizing treatment, and tempering treatment. Usually, after shaping | molding by hot processing, the normalization process which cools to room temperature after hold | maintaining in the temperature range of 1050-1100 degreeC, and the tempering process hold | maintained at about 750-800 degreeC are used. For example, hot processing of steel pipes is performed by the Mannesmann-Mandrel mill method, the Eugene Sejurune method, the Erhard push bench method, etc., but generally, the smaller the diameter of the tube, the greater the degree of processing and the lower the temperature. The particle size of the metal structure at the stage of completion of hot working tends to be fine. The final product particle size is determined in the normalization process.

  The present inventors made three types of sizes from ASME P92 steel having the chemical composition shown in Table 1 with an outer diameter of 60 mm (wall thickness of 8 mm), an outer diameter of 350 mm (wall thickness of 35 mm), and an outer diameter of 500 mm (wall thickness of 70 mm). The steel pipe was manufactured.

  At the end of hot working at the time of steel pipe production and at the end of 30 min normalization at 1070 ° C, samples were taken and tempered at 780 ° C for 1 hour, and subjected to old austenite grains. The diameter was measured. Incidentally, the final hot working end temperatures of each size were 950 ° C., 1050 ° C., and 1130 ° C., respectively. Separately, after normalizing the material as it was hot-worked at the time of steel pipe production at 1070 ° C. for 30 min, a tempering treatment was performed for 1 hour at 780 ° C., and the prior austenite grain size was measured.

That is, the above three types of thick steel pipes were tested under the following three types of processing and heat treatment conditions shown in (C) to (E).
(C) As hot working (D) Hot working → Factory normalizing process → Laboratory tempering process (E) Hot working → Laboratory normalizing process → Laboratory tempering process The measurement result of each prior austenite particle size (ASTM particle size number) is shown.

  In this way, the particle size after the normalizing treatment in the pipe material also depends on the particle size at the stage of completion of hot working, and the larger the outer diameter of the pipe material, that is, the higher the end temperature of hot working. It became clear that it became a coarse grain. Also, in the laboratory normalization process, steel with the same particle size number as the factory normalization process is obtained, and the normalization process in the laboratory is equivalent to the normalization process in the factory. It was confirmed that the process was reproduced.

  Here, the dominating factors of the crystal grain size are nucleation and grain growth. In general, the most prominent nucleation sites are crystal grain boundaries, and it is considered that finer materials are more likely to become fine grains because they have more nucleation sites as they are hot-worked. Regarding grain growth, the presence of pinning particles that inhibit grain growth is important.

  In the normalization treatment of a steel pipe made of ASME P92 steel having the chemical composition shown in Table 1, the pinning particles that inhibit grain growth are V and Nb carbonitrides. This steel pipe contains 0.06% Nb and 0.2% V in mass%, but some of the carbonitrides are precipitated in the state of completion of hot working, which inhibits grain growth during normalization. To do. When the amount of Nb carbonitride deposited in the hot-worked state of this steel pipe was investigated, it was 0.039% for a steel pipe with an outer diameter of 60 mm, 0.035% for a steel pipe with an outer diameter of 350 mm, and 0.032% for a steel pipe with an outer diameter of 500 mm. However, each was deposited. It is considered that the amount of Nb carbonitride deposited in the small-diameter tube is larger in the small-diameter tube, and the number of pinning particles that inhibit grain growth during the normalizing treatment is larger in the small-diameter tube. However, in large-diameter pipes, the number of nucleation sites is considered to be the most important factor governing the grain size after normalization, since there are corresponding pinning particles and they are coarse. It is done.

  On the other hand, in a small-diameter pipe that has a high degree of hot processing and is processed to a low temperature, it is considered that the number of nucleation sites is very large at the stage of completion of hot processing and is likely to become fine particles. That is, in a small-diameter tube, in addition to being finely grained by low-temperature strong processing, a large amount of dislocations introduced by the martensitic transformation that occurs during processing and cooling after processing can also become nucleation sites. Therefore, the driving force for nucleation is considered to be very high.

(iii) Next, the present inventors have found that, as a method of reducing the driving force of the nucleation of hot working Mom material was examined whether the softening treatment in the vicinity A ₁ transformation point. The purpose of this softening treatment is to reduce the dislocations introduced in the low temperature strong working and martensitic transformation, and to cause recrystallization to increase the grain size.

  For this reason, the softening treatment was performed after the hot working and before the normalizing and tempering treatment (hereinafter, the working and heat treatment conditions at this time are referred to as (F)). That is, after heating a plate material with a thickness of 30 mm to 1000 ° C., after rolling to a thickness of 15 mm by hot working with an end temperature of 900 ° C., the metal structure was observed with an optical microscope after softening treatment at 800 ° C. for 30 min. .

  The structure of the softened material was a tempered martensite structure in which the structure as hot worked was tempered, and recrystallization as expected was not caused. However, the Vickers hardness of the steel after the softening treatment is 240, which is softer than the Vickers hardness 430 after hot working, and the dislocations introduced by hot working and martensitic transformation are reduced. The driving force for nucleation is expected to be slightly smaller. Then, the softening material was subjected to a normalizing treatment for 10 minutes at 1050 ° C. and a tempering treatment for 1 hour at 780 ° C., and the metal structure was investigated. The prior austenite grain size after the normalizing and tempering treatment was equivalent to the case of normalizing the material as it was hot worked with ASTM grain size number 7.5, and the expected coarsening did not occur. This is presumably because the softening treatment at 800 ° C x 30 min did not cause recrystallization, and there were still many dislocations, and the grain size as hot worked was maintained, so there were still many nucleation sites. The

  Table 4 shows the measurement results of the prior austenite grain size (ASTM grain size number) of the steel manufactured under the processing and heat treatment conditions of (F) in comparison with those of the steel manufactured under the conditions of (A) described above. .

The reason did not produce recrystallization by softening treatment with (iv) A ₁ transformation point near, the distribution of dislocations introduced in the martensitic transformation after hot working and the like to be very uniform. The ferrite temperature region at a low temperature of A ₁ transformation point near the driving force is small nucleation, rather the distribution of dislocations heterogeneous, the possibility prone to Write some dislocations rearranged regions recrystallization In view of this, we investigated the forced introduction of dislocations by cold working followed by softening.

  For this reason, after the hot working, the softening treatment was performed, and after the cold working, the softening treatment was performed again (hereinafter, the working and heat treatment conditions at this time are referred to as (G)). That is, after heating a plate material with a thickness of 30 mm to 1000 ° C, rolling it to a thickness of 15 mm by hot working at an end temperature of 900 ° C, then performing a softening treatment at 800 ° C for 30 minutes, and further colding to a thickness of 10.5 mm After processing, it was again softened at 800 ° C. for 30 minutes. FIG. 1 shows an optical microstructure of a material that has been softened again at 800 ° C. for 30 minutes after cold working. The softened material after cold working had a Vickers hardness of 165, indicating that a recrystallized ferrite structure was formed.

  By the softening treatment after cold working, the recrystallized material was further subjected to a normalizing treatment for 10 minutes at 1050 ° C. and a tempering treatment for 1 hour at 780 ° C., and the metal structure was observed with an optical microscope.

  Table 4 shows the measurement results of the prior austenite particle size (ASTM particle size number) of the steel material produced under the processing and heat treatment conditions of (G).

  As a result, the steel material manufactured under the processing and heat treatment conditions of (G), that is, the structure of the steel material forcibly introduced with dislocation by cold working after the softening treatment and then further softening treatment is the old austenite grains It is a coarse particle having a diameter of ASTM particle size number 6.0 and is obviously coarse. This is presumably because the number of nucleation sites was reduced by recrystallization.

  (v) By the way, a process that simulates the cold work that is partly performed in a normal boiler tube, performs the softening process after hot working, and further cold-workes, but does not perform the softening process again. The experiment was carried out (hereinafter, the processing and heat treatment conditions at this time are referred to as (H)).

  In other words, after heating a plate with a thickness of 30 mm to 1000 ° C, rolling it to a thickness of 15 mm by hot working with an end temperature of 900 ° C, then softening it to 800 ° C x 30 min, and then colding to a thickness of 10.5 mm After the processing, after a normalizing treatment held at 1050 ° C. for 10 minutes, a tempering treatment was carried out by holding at 780 ° C. for 1 hour.

  Table 4 shows the measurement results of the prior austenite particle size (ASTM particle size number) of the steel material manufactured under the processing and heat treatment conditions of (H).

  As a result, the structure of the steel material manufactured under the processing and heat treatment conditions of (H), that is, the steel material when the dislocation is forcibly introduced by cold working after the softening treatment, the prior austenite grain size is ASTM grain size number 8.0. It is a fine grain. This is presumably because the heat treatment after cold working is performed at a high temperature (austenite region) where nucleation is easy, and as a result, there are many nucleation sites.

  The present invention has been made on the basis of the above-mentioned knowledge, and the gist thereof resides in a method for producing a high Cr ferritic heat resistant steel material shown in the following (1) to (4).

(1) By mass%, C: 0.05 to 0.12%, Si: 0.2 to 0.5%, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.005% or less, Cr: less than 8.0 to 12%, V: 0.15 to 0.25%, Nb: 0.03 to 0.08%, N: 0.005 to 0.07%, sol.Al: 0.015% or less, Ni: 0.5% or less, Mo: 0.1 to 1.1% and W: 1.5 to 3.5% A high Cr ferrite characterized in that a steel having a composition containing one or two of them, the balance being Fe and impurities, is processed and heat-treated by the following steps (1) to (5) Of manufacturing heat-resistant steel.
(1) A hot working process in which the final finishing temperature is 1000 ° C. or lower,
(2) Softening process of 20 min to 2 hr in the range of 750 to 820 ° C,
(3) Cold working process with a cross-section reduction rate of 15% or more,
(4) Softening process of 20 min to 2 hr in the range of 750 to 820 ° C,
(5) Normalizing and tempering process.

(2) by mass%, further containing at least one component selected from at least one of the following first group to third group: (1) A method for producing a high Cr ferritic heat resistant steel material.
First group:% by mass, B: 0.015% or less,
Second group:% by mass, Cu: 1.5% or less and Co: 5% or less,
Third group:% by mass, Ti: 0.05% or less, Ta: 0.05% or less, Nd: 0.05% or less, and Ca: 0.01% or less.

  (3) In the method for producing a high Cr ferritic heat-resistant steel material according to (1) or (2) above, between the step (1) and the step (4), the step (2) and the step (3) Is repeated a plurality of times, and a method for producing a high Cr ferritic heat resistant steel material.

  (4) The method for producing a high Cr ferritic heat resistant steel material according to any one of (1) to (3) above, wherein the high Cr ferritic heat resistant steel material is a boiler tube.

  According to the present invention, in boilers, nuclear power generation facilities, chemical industrial facilities, etc., it is used for heat exchange and piping used in a high temperature environment of 600 to 650 ° C. A Cr ferritic heat resistant steel material is obtained. In particular, it is suitable for obtaining a tube material made of a high Cr ferritic heat resistant steel such as a boiler tube.

  Below, the reason for prescription | regulation of the component which comprises the steel used in the manufacturing method of the high Cr ferritic heat-resistant steel material which concerns on this invention, and the prescription | regulation reason of a process and a heat processing process are demonstrated. In addition, "%" regarding content means "mass%".

A. Reasons for prescribing the components constituting steel C: 0.05 to 0.12%
C stabilizes the structure as an austenite stabilizing element. Further, MC (M is an alloying element) carbide or M (C, N) carbonitride is formed, which contributes to improvement of creep strength. However, if it is 0.05% or less, the above effects cannot be obtained sufficiently, and the amount of δ ferrite increases and the strength may be lowered. On the other hand, if the content exceeds 0.12%, the workability and weldability are deteriorated, and the agglomeration and coarsening of carbides occur from the beginning of use, leading to a decrease in creep strength for a long time. Therefore, the content of C is set to 0.05 to 0.12%.

Si: 0.2-0.5%
Si is an element that has an effect on deoxidation of steel and has an effect of improving the steam oxidation resistance. In order not to impair the steam oxidation performance, it is necessary to contain 0.2% or more. However, when the content exceeds 0.5%, the creep strength is remarkably lowered. Therefore, the Si content is set to 0.2 to 0.5%.

Mn: 0.3 to 0.6%
Mn is effective in deoxidizing steel and contributes as an austenite stabilizing element. Further, MnS is formed and S is fixed. In order to obtain those effects, 0.3% or more is necessary. However, if it exceeds 0.6%, the creep strength is reduced. Therefore, the Mn content is set to 0.3 to 0.6%.

P: 0.02% or less P is preferably as low as possible from the viewpoints of hot workability, weldability, creep strength, creep fatigue strength, and the like. However, the remarkable cleaning of the steel causes a significant cost increase, so the upper limit was made 0.02%.

S: 0.005% or less S is preferably as low as possible from the viewpoints of hot workability, weldability, creep strength, creep fatigue strength, and the like. However, the remarkable cleaning of the steel causes a significant cost increase, so the upper limit was made 0.005%.

Cr: 8.0% or more and less than 12% Cr is an indispensable element for ensuring corrosion resistance and oxidation resistance at high temperatures, particularly steam oxidation resistance. Furthermore, a carbide is formed to improve the creep strength. In order to acquire those effects, it is necessary to set it as 8.0% or more. However, if contained in a large amount, the creep strength is lowered for a long time, so the content was made less than 12%.

V: 0.15-0.25%
V is an element that contributes to the improvement of creep strength by forming solid solution strengthening and fine carbonitride. In order to exert the effect, it is necessary to contain 0.15% or more. However, if the content exceeds 0.25%, the formation of δ ferrite is promoted and the creep strength is lowered for a long time. Therefore, the content of V is set to 0.15 to 0.25%.

Nb: 0.03-0.08%
Nb is an element that contributes to the improvement of creep strength for a long time by forming fine carbonitrides. In order to exert the effect, it is necessary to contain 0.03% or more. However, if it exceeds 0.08%, the formation of δ ferrite is promoted and the creep strength is lowered for a long time. Therefore, the Nb content is set to 0.03 to 0.08%.

N: 0.005-0.07%
N, like C, is effective as an austenite stabilizing element. N also precipitates nitrides or carbonitrides to increase the high temperature strength. In order to exhibit the effect, it is necessary to contain 0.005% or more. However, if it is contained in a large amount, blow holes may be generated at the time of melting, causing welding defects and reducing the creep strength due to the coarsening of nitrides and carbonitrides, so the upper limit was made 0.07% .

solu.Al: 0.015% or less Al is used as a deoxidizer for molten steel, but if it is contained in a large amount exceeding 0.015%, the creep strength is reduced, so the upper limit of the solu.Al content is 0.015%. did.

Ni: 0.5% or less Ni is an element that reduces creep even when contained in a trace amount. However, a very small amount of Ni is unavoidably mixed from the melting raw material. If it is 0.5% or less, the effect on the creep strength is small, so the upper limit of the allowable amount is set to 0.5%.

One or two of Mo: 0.1 to 1.1% and W: 1.5 to 3.5% Mo and W each contribute to improvement of creep strength as a solid solution strengthening element. Further, it partially dissolves in the Cr carbide, thereby suppressing the agglomeration and coarsening of the carbide and contributing to the improvement of the creep strength. When added alone, if the Mo content is less than 0.1% and the W content is less than 1.5%, the effect of improving the creep strength is small. On the other hand, when Mo is contained exceeding 1.1%, the formation of δ ferrite is promoted and the creep strength is lowered. Moreover, when W is contained exceeding 3.5%, the formation of δ ferrite is promoted, and the creep strength is lowered. In addition, when combining and containing 2 types of Mo and W, it is desirable from the same viewpoint that the upper limit of Mo + 0.5W is 1.5%.

  As a component constituting the steel used in the method for producing a high Cr ferritic heat-resistant steel material according to the present invention, in addition to the above components, at least one of the following first group to third group in mass% You may contain at least 1 sort (s) of the components selected from a group.

First group:
B: 0.015% or less B is an optional additive element and can be contained as necessary. When B is contained, it enhances hardenability and plays an important role in securing high temperature strength. However, if it exceeds 0.015%, weldability and long-term creep strength are lowered. Therefore, when it contains, the upper limit of the content shall be 0.015%. In addition, in order to acquire the said effect reliably, it is preferable to make it contain 0.003% or more.

Second group:
One or two of Cu: 1.5% or less and Co: 5% or less Cu is an optional additive element and can be contained as required. When Cu is contained, it acts as an austenite stabilizing element. However, if the content exceeds 1.5%, the creep strength is reduced. Therefore, when it makes it contain, the upper limit of the content shall be 1.5%. In addition, in order to acquire said effect reliably, it is preferable to make it contain 0.3% or more.

  Co is an optional additive element and can be contained as necessary. When Co is contained, it acts as an austenite stabilizing element. However, if the content exceeds 5%, the creep strength is reduced. Therefore, when it contains, the upper limit of the content shall be 5%. In order to reliably obtain the above action, it is preferable to contain 0.5% or more.

Third group:
One or more of Ti: 0.05% or less, Ta: 0.05% or less, Nd: 0.05 or less, and Ca: 0.01 or less These elements are optional addition elements, and can be contained as necessary. When these elements are contained, fine carbonitrides are formed, which is effective in improving creep strength. However, even if Ti, Ta, and Nd are contained in excess of 0.05%, the effect is saturated, and on the contrary, toughness and creep strength are deteriorated. Moreover, even if Ca is contained in excess of 0.01%, the effect is saturated, and on the contrary, toughness and creep strength are deteriorated. Therefore, when these elements are contained, the upper limit of the content is 0.05% for Ti, Ta, and Nd, and 0.01% for Ca. In order to reliably obtain the effect of improving the creep strength, Ti, Ta, and Nd are preferably contained in an amount of 0.005% or more, and Ca is preferably contained in an amount of 0.0005% or more.

B. Reasons for defining processing and heat treatment processes B-1. About the hot working process which makes final processing end temperature 1000 degrees C or less There exist a board | plate material, a pipe material, a bar, etc. in the shape of steel materials. Of these, the plate material is dimensioned by rolling. The pipe material is piped by hot working such as Mannesmann-Mandrel Mill method, Eugene Sejurnee method or Erhard push bench method. Alternatively or later, the dimensions can be adjusted with a stretch reducer or sizer. And the bar is dimensioned by rolling.

  Here, the `` final processing end temperature '' in the step (1), that is, the hot processing step in which the final processing end temperature is 1000 ° C. or less is, in the case of a plate material and a bar material, by hot rolling. Refers to the finishing temperature when the dimensions are adjusted. In the case of pipes, if the pipe finished by the above-mentioned Mannesmann-Mandrel Mill method, Eugene Sejurne method or Erhard push bench method is the final hot working, it indicates the processing end temperature, and Further, when the dimension is adjusted with a stretch reducer or a sizer thereafter, it indicates the end temperature when the dimension is adjusted.

  The reason why the “final processing end temperature” in the step (1), that is, the hot processing step in which the final processing end temperature is 1000 ° C. or lower, is 1000 ° C. or lower is as follows. In other words, in the hot working of steel materials with a small degree of work like a large diameter pipe, the hot working can be finished at a temperature exceeding 1000 ° C, so as long as the hot working is finished at a temperature exceeding 1000 ° C. The prior austenite grains become coarse grains. In this case, a coarse grain structure can be obtained by a normal normalizing and tempering treatment without applying the steps (2) to (4) of the present invention. However, in hot working of steel materials with a high degree of work such as small diameter pipes, hot working cannot be completed at a temperature exceeding 1000 ° C, and it is processed to a low temperature of 1000 ° C or less. The organization becomes finer. Further, even in the case of hot working with a small degree of work, when the work is performed to a low temperature of 1000 ° C. or less, the structure at the stage of completion of the hot work becomes fine.

  In the present invention, not only hot working of a steel material having a high workability such as a small diameter pipe but also hot working of a steel material having a low workability such as a large diameter pipe, the processing end temperature is set to 1000 ° C. or less. In order to change the hot-worked material in which the prior austenite grains become fine grains by hot working into coarse grains, the step (1) is applied. There is no particular lower limit for the processing end temperature, but 800 ° C. or higher is preferable to ensure mechanical properties such as workability and toughness during hot working.

B-2. Softening treatment step of 20 min or more and 2 hr or less in the range of 750 to 820 ° C. The material as hot worked is usually air-cooled, and as a result, martensite transforms and becomes hard. Therefore, it cannot be cold worked as it is. Therefore, when performing cold working, the step (2), that is, a softening treatment step of 20 min or more and 2 hr or less in the range of 750 to 820 ° C. is required. Note that the temperature and time of the softening treatment step are determined so as to be within a range in which the material is softened and does not hinder manufacturing.

B-3. About cold working process with a cross-section reduction rate of 15% or more The purpose of cold work is to produce a recrystallized ferrite structure in the next softening process, but when the cross-section reduction rate during cold work is less than 15%. However, sufficient strain that can be a driving force for recrystallization in the ferrite temperature range is not added. In cold working, it is generally difficult to uniformly perform light machining with a cross-section reduction rate of 15% or less.

  Therefore, the step (3), that is, a cold working step in which the cross-sectional reduction rate is 15% or more is required. Although there is no particular upper limit on the degree of cold work, in general, in the case of ferritic heat-resisting steel, it is often used by finishing by hot working to a size where the cross-section reduction rate in cold working is 40% or less. Here, when it is necessary to adjust the size in the cold working, cold rolling capable of working at a higher workability may be performed, or the steps (2) and (3) may be repeated. In addition, although cold work refers to cold rolling, cold drawing, etc., in the case of a pipe material, cold drawing is often applied.

B-4. About softening treatment process of 20min or more and 2hr or less in the range of 750 to 820 ℃ Normally, the normalization and tempering treatment is performed after making the product dimensions in the cold working process of (3) above. A softening treatment is performed before the normalizing and tempering treatment.

In this softening treatment step, the strain introduced by cold working is recrystallized in the ferrite temperature range using the driving force as a driving force to form a recrystallized ferrite structure having a Vickers hardness of 200 or less. Therefore, it is necessary to perform the softening treatment step (4), that is, a time of 20 min or more and 2 hours or less in a temperature range of 750 to 820 ° C. Without recrystallization Namaze at temperatures below 750 ° C., the other hand, since more than when heat-treated at a temperature of A ₁ transformation point in excess of 820 ° C., the martensite structure after transformation to the fine austenite grains. Therefore, the softening temperature range was set to 750 to 820 ° C. Moreover, in order to recrystallize, the softening processing time of 20 minutes or more is required. In terms of characteristics, the upper limit of the softening treatment time is not particularly required, but even if the softening treatment for over 2 hours is performed, there is no significant difference in the metal structure, but the disadvantages in terms of economy and productivity are avoided. Therefore, the upper limit of the softening treatment time is set to 2 hours.

B-5. About the normalizing and tempering treatment step The steel material after the softening treatment needs to be subjected to the step (5), that is, the normalizing and tempering treatment step. The normalizing treatment may be performed for a period of about 10 min to 1 hr in a temperature range of 1040 to 1100 ° C. as in the case of ordinary high Cr ferritic heat resistant steel. Further, the tempering treatment may be performed for about 1 to 6 hours in a temperature range of 760 to 780 ° C., similarly to the ordinary high Cr ferritic heat resistant steel.

B-6. Regulations specific to boiler tubes Boiler tubes are often manufactured from heat-resistant steel into a predetermined shape through hot working and cold working. Especially in the hot processing of small-diameter pipes, the degree of processing becomes large, the hot processing cannot be completed at temperatures exceeding 1000 ° C, and the processing is performed to a low temperature of 1000 ° C or less, so the structure at the stage of completion of hot processing Becomes finer. Further, even in the case of hot working with a small degree of work such as a large-diameter pipe, when the work is performed to a low temperature of 1000 ° C. or less, the structure at the stage of completion of hot work becomes fine. Therefore, not only hot working of small diameter pipes, but also hot working of large diameter pipes, hot working materials in which old austenite grains became fine grains by hot working with a finishing temperature of 1000 ° C or less are It is necessary to change to coarse grains. In addition, the tube size of small diameter pipes that are processed to a low temperature of 1000 ° C or less cannot be finished at a temperature exceeding 1000 ° C because the degree of processing becomes large, but the hot processing is not specified. The boiler tube manufactured through cold working usually has an outer diameter of 150 mm or less.

C. Others The steel used for the manufacturing method of the high Cr ferritic heat-resistant steel material according to the present invention described above can be manufactured using a manufacturing facility and a manufacturing process that are usually used industrially. That is, the components may be adjusted by adding deoxidizing elements and alloy elements to molten steel refined by a furnace such as an electric furnace or a converter. In particular, when strict component adjustment is required, a method of appropriately performing a treatment such as a vacuum treatment may be adopted before the molten steel contains the alloy element.

  Molten steel adjusted to a prescribed chemical composition is cast into slabs, billets, or steel ingots by continuous casting or ingot forming methods, and plates, pipes, and bars can be produced from these slabs and steel ingots. it can.

  In the case of pipes, the Mannesmann mandrel mill method or the Eugene Sejurne method is used for hot working of small diameter tubes. In addition, the dimensions may be adjusted with a stretch reducer or sizer hot after pipe making. Further, the cold working is performed by a cold drawing method or a cold rolling method. The steel pipe after the heat treatment may be subjected to a surface treatment such as shot peening or pickling as necessary.

  In a vacuum induction melting furnace, an alloy having the chemical composition shown in Table 5 was melted to produce a 50 kg ingot having a diameter of 144 mm. Steel Nos. 1 to 5 are steels included in the range defined by the present invention, and Steel Nos. 6 to 9 are comparative materials whose components are out of the range of the present invention. These ingots were made into 30 mm thick plates by hot forging and used as the base material for investigation. Steel No. 2 is “Tue STBA29” (ASME T92) equivalent steel, which is the same as shown in Table 1.

  In order to examine the influence of the manufacturing process on various manufacturing methods, using steel No. 2, as shown in Table 6, the presence or absence of recrystallization and the oldness under nine types of processing and heat treatment conditions (marks 2A to 2I) The creep rupture test was performed by measuring the ASTM particle size number of the austenite particle size and applying a load of 90 MPa at 650 ° C.

  Here, the presence or absence of recrystallization in the softening treatment after cold working was determined by measuring the Vickers hardness (load 9.8 N). If the Vickers hardness is softened to 190 or less, it is judged that it is recrystallized, and the mark in the table is marked with ○, and the one whose Vickers hardness exceeds 190 is judged not to be recrystallized, A cross was marked in the table. The ASTM grain size was obtained by embedding the tempered material in a resin, corroding it with a Virella reagent (1 g of picric acid, 5 ml of hydrochloric acid, 100 ml of ethanol), and taking 10 fields of view with an optical microscope at a magnification of 100 times. The photographed photograph was used to determine the prior austenite particle size compared to ASTM E112 Plate I. Coarse particles having an ASTM particle size number of 7.5 or less with an average of 10 fields of view were evaluated as ◯, and fine particles exceeding 7.5 were evaluated as ×.

  Furthermore, from the heat-treated plate, a test piece with a diameter of 6.0 mm was collected so that the length direction of the test piece was the rolling direction, and creeped under the conditions of a distance between gauge points of 30 mm, a test temperature of 650 ° C., and a load stress of 90 MPa. A rupture test was performed, and the time (hr) until rupture was measured.

  The mark 2A simulates a manufacturing method of a large-diameter pipe (when the end temperature of hot working is high). Since the heating temperature is high and the processing is finished at a high temperature, the crystal grain size after the hot processing is large. Under the influence of the structure of the material as it is hot-worked, it remains coarse after normalizing and tempering. The creep rupture time of this material is good at over 14000hr.

  The marks 2B and 2C simulate the small diameter pipe manufacturing method (when the end temperature of hot working is low). Both have a fine-grained structure, and the rupture time in the creep test is significantly inferior to that of the mark 2A.

  The mark 2D is obtained by performing a softening process after hot working and then performing a normalizing and tempering process. As described above, even if the material is softened while being hot worked, the martensite structure generated after hot working is only tempered, and no recrystallized ferrite structure is formed. The particle size after application remains fine. Further, the rupture time in the creep test is significantly inferior to that of the mark 2A.

  The marks 2E to 2I are intended to perform a normalizing and tempering treatment after making the metal structure coarse ferrite by cold working and subsequent softening treatment. In the mark 2E that satisfies the various conditions defined in the present invention, the structure after cold working is recrystallized ferrite. As a result, the particle size after the normalizing and tempering treatment is also a coarse particle having a particle size number of 7.5 or less, and the rupture time in the creep test is at the same level as that of the mark 2A. On the other hand, the marks 2F to 2I in which any one of the cold work degree, the temperature of the softening process, and the time of the softening process is outside the range defined in the present invention does not cause recrystallization after the softening process. Moreover, the structure | tissue after normalizing and tempering processing is a fine grain, and the fracture | rupture time in a creep test is also significantly inferior compared with the mark 2A.

  As described above, the phenomenon that the creep rupture strength of the small-diameter pipe when the end temperature of hot working is low is inferior to the creep rupture strength of the large-diameter pipe when the end temperature of hot working is high is W or Mo. This phenomenon is manifested in a high-strength Cr ferritic heat-resistant steel material that is contained and further reinforced with a composite carbonitride of V and Nb.

  Therefore, in order to clarify the influence of V and Nb, as shown in Table 7, using the steel Nos. 1 and 3 to 9, various processing and heat treatment conditions (marks 1A, 1B, 1E, 3A, 3B) 3E, 4A, 4B, 4E, 5A, 5B, 5E, 6A, 6B, 6E, 7A, 7B, 7E, 8A, 8B, 8E, 9A, 9B, and 9E), and the presence of old austenite grains The ASTM particle size number of the diameter was measured and a creep rupture test was performed at 650 ° C. with a load of 90 MPa.

Here, among the marks, the numbers indicate the steel No., and the alphabets indicate the processing and heat treatment conditions set for the following purposes.
Condition A: Simulating a manufacturing method of a large-diameter pipe (when the end temperature of hot working is high). Since the heating temperature is high and the processing is finished at a high temperature, the crystal grain size after the hot processing is large. Under the influence of the structure of the material as it is hot-worked, it remains coarse after normalizing and tempering.
Condition B: Simulating the manufacturing method of a small diameter pipe (when the end temperature of hot working is low). Both have a fine grain structure.
Condition E: Aimed at performing normalizing and tempering treatment after making the metal structure coarse ferrite by cold working and subsequent softening treatment.

  As a result, steels No. 1 and 3 to 5 containing V and Nb had the same tendency as steel No. 2. That is, the steel material produced on condition B became fine grain, and the creep strength for a long time was low as compared with the steel material produced on condition A. Moreover, the steel material produced on condition E became a coarse grain, and had the creep strength of the same level as the steel material produced on condition A.

  On the other hand, in the comparative steel (steel No. 6-9) which does not contain both or one of V and Nb, even if it was the steel material produced on the conditions B, it was a coarse grain with a particle size number of 7.5 or less. This is because if the contents of V and Nb are small, extreme fine grains cannot be obtained. This also revealed that the composite carbonitride of V and Nb contributed to the formation of a fine grain structure when the steel material having the chemical composition of the present invention was produced under Condition B. In addition, the long-term creep strength of the comparative steel is low in any of the conditions A to C, and it has become clear that it is necessary to contain both V and Nb in order to improve the long-term creep strength. .

  According to the present invention, in boilers, nuclear power generation facilities, chemical industrial facilities, etc., it is used for heat exchange and piping used in a high temperature environment of 600 to 650 ° C. A Cr ferritic heat resistant steel material is obtained. In particular, it is suitable for obtaining a tube material made of a high Cr ferritic heat resistant steel such as a boiler tube.

It is the optical microscope structure of the material which performed the softening process of 800 degreeC x 30 min again after cold working.

Claims (4)

In mass%, C: 0.05 to 0.12%, Si: 0.2 to 0.5%, Mn: 0.3 to 0.6%, P: 0.02% or less, S: 0.005% or less, Cr: 8.0 to less than 12%, V: 0.15 to 0.25 %, Nb: 0.03-0.08%, N: 0.005-0.07%, sol.Al: 0.015% or less, Ni: 0.5% or less, Mo: 0.1-1.1%, and W: 1.5-3.5% A high Cr ferritic heat-resistant steel material characterized by subjecting a steel having a composition comprising seeds or two kinds, the balance of Fe and impurities to be processed and heat-treated by the following steps (1) to (5) Manufacturing method.
(1) A hot working process in which the final finishing temperature is 1000 ° C. or lower,
(2) Softening process of 20 min to 2 hr in the range of 750 to 820 ° C,
(3) Cold working process with a cross-section reduction rate of 15% or more,
(4) Softening process of 20 min to 2 hr in the range of 750 to 820 ° C,
(5) Normalizing and tempering process. 2. The composition according to claim 1, further comprising at least one component selected from the group consisting of at least one of the following first group to third group: The manufacturing method of the high Cr ferritic heat-resistant steel material of description.
First group:% by mass, B: 0.015% or less,
Second group:% by mass, Cu: 1.5% or less and Co: 5% or less,
Third group:% by mass, Ti: 0.05% or less, Ta: 0.05% or less, Nd: 0.05% or less, and Ca: 0.01% or less.   3. The method for producing a high Cr ferritic heat resistant steel material according to claim 1 or 2, wherein the steps (2) and (3) are repeated a plurality of times between the steps (1) and (4). A method for producing a high Cr ferritic heat resistant steel material.   4. The method for producing a high Cr ferritic heat resistant steel material according to claim 1, wherein the high Cr ferritic heat resistant steel material is a boiler tube.

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