JPH0543986A - High chromium ferritic heat resisting steel reduced in deterioration in strength in weld heat-affected zone - Google Patents

Abstract

PURPOSE:To prevent the occurrence of the softening and deterioration in strength in the weld heat-affected zone of a high Cr ferritic heat resisting steel. CONSTITUTION:The steel is a high chromium ferritic heat resisting steel which consists of 0.03-0.15% C, <=0.7% Si, 0.1-1.5% Mn, 0.05-1.0% 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, 0.4-3.5% Cu, and the balance Fe with inevitable impurities and where solid-solution oxygen as impurity is regulated to <=0.015% and deterioration in a weld heat-affected zone is reduced. As alloy components, other than the above-mentioned components, one or more components selected from one or more groups among the following groups (a), (b), (c), (d) can further be incorporated : group (a)... 0.0001-0.02wt.% B; group (b)... <0.5wt.% W; group (c)... 0.0001-0.1% Mg; (d)... 0.01-0.2wt.% each of La, Ce, Ca, Zr, Ti, Y, and Ta.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has high strength at high temperature, excellent oxidation resistance and weldability, and is used as a high temperature pressure resistant member in the fields of boiler, nuclear power, chemical industry and the like, for example, for steel pipes and steel plate forgings. The present invention relates to a high-chromium ferritic heat-resisting steel having a small strength reduction particularly in the heat-affected zone of a welded joint.

[0002]

2. Description of the Related Art Heat-resistant steel used for high-temperature heat-resistant pressure-resistant members for boilers, nuclear power plants, chemical industries, etc. has excellent workability, weldability, and economic efficiency as well as high-temperature strength, corrosion resistance, oxidation resistance and toughness. Is required. Conventionally, as materials used for the above-mentioned applications, SUS321H, SUS347
Austenitic stainless steel such as H steel, 2/1/4 C
There are low alloy steels such as r-1Mo steel and high Cr ferritic steels of 9-12Cr series. Among them, high Cr ferritic steel is 500 ~
It is superior in strength and corrosion resistance to low alloy steels at a temperature of 650 ° C, has a higher thermal conductivity than austenitic stainless steels, and has a low coefficient of thermal expansion, resulting in excellent thermal fatigue resistance and scale delamination. The advantages are that it is difficult and stress corrosion cracking does not occur.

As high Cr ferritic steels, 9Cr-1Mo steel (STBA26), improved 9Cr-1Mo steel (ASTM SA213 T91), 12Cr
-1Mo steel (DIN X20CrMoV121) etc. are well known. Further, as a steel to which Mo, W, V, Nb, N, etc. are added in combination for the purpose of improving high temperature strength, Japanese Patent Publication Nos. 628502 and 62-1230.
There are steels disclosed in JP-A Nos. 62-297435 and 2-310340.

The present inventors have conducted extensive research on the composition of heat-resistant materials for the purpose of improving high-temperature oxidation resistance at 600 ° C. or higher. The result is that as a Cu-added steel
This is as proposed in the publications of 232345 and 3-97832. This Cu addition improves the oxidation resistance above 600 ℃ and
Cr is effective in suppressing δ-ferrite, which is easily generated in ferritic steel, and saves expensive Ni. It is also characterized in that it is difficult to lower the Ac ₁ transformation temperature as compared with C and Ni, which is advantageous in selecting heat treatment conditions during manufacturing. Note that Cu-added steel is also proposed in Japanese Patent Laid-Open No. 2-294452, and in that publication, Cu is added.
It is described that the addition of is effective for maintaining the toughness of a welded joint, particularly the toughness of a welded part after heating for a long time.

Attempts have been made to add Cu to high Cr steels. For example, Japanese Patent Publication No. 46-15210 discloses 1
It has been shown that 4% of Cu is effective in improving hardenability and toughness, and 1% is also disclosed in JP-A-60-155649.
The following Cu has been shown to be effective in improving toughness. Further, the above-mentioned Japanese Patent Publication No. 12304/1987 and Japanese Patent Laid-Open No. 59/1985.
-211553 also discloses a high Cr steel containing Cu.

However, the high Cr content proposed so far
Ferrite steel has a common problem that the heat-affected zone (hereinafter referred to as HAZ) of the welded joint is largely softened and the creep strength is lowered. As described above, the conventional high Cr ferritic steel emphasizes the improvement of the high temperature creep strength of the base metal, but on the other hand, almost no study has been conducted on the problem that the strength of the welded joint is much lower than that of the base metal. Not not. The inventions disclosed in the above-mentioned many patent publications are all inventions in which the composition of the base material is emphasized, and no consideration is given to the structural change of the HAZ of the welded joint and the accompanying reduction in the strength of the HAZ.

In summary, the decrease in creep strength due to the softened layer of the welded joint of high strength and high Cr ferritic steel has been a major obstacle in using this type of steel as a high temperature pressure resistant member.

[0008]

The decrease in strength of the welded joint in high Cr ferritic heat resistant steel is considered to be due to the unstable softened layer formed in the HAZ. There is no report. For example, in the existing welded joint of improved 9Cr-1Mo steel, the creep strength at 600 to 650 ° C is 15 to 20% lower than that of the base metal on the long side of 10 ⁴ h or more. Such decrease in creep strength is due to the high
The same applies to Cr ferritic steel.

The object of the present invention is that the basic properties such as high temperature strength, workability and weldability are equal to or higher than those of conventional steels, especially the softening of the welded joint is small, the high temperature creep strength is not lowered, and the cost is low. To provide a high Cr ferritic heat resistant steel.

[0010]

[Means for Solving the Problems] The decrease in creep strength of welded joints is characterized by low ductility shear fracture (called Type IV cracking) caused by the formation of a softened layer in the HAZ portion. .. On the other hand, in the steel containing a large amount of W from the viewpoint of increasing the strength, the decrease in creep strength of the joint portion is extremely larger than the strength of the base metal. Further, addition of a large amount of W has a drawback of increasing material cost.

The present invention is based on the following findings obtained from many research results on high Cr ferritic steel.

The addition of Cu enhances the HAZ structure stability and prevents the softening of the welded joint. This action is
-It is different from the previously known actions such as suppression of ferrite and improvement of toughness, and is obtained only in the specific chemical composition range described later.

In the high Cr ferritic steel containing a large amount of W, the strength of the base material is increased, but the joint strength is greatly reduced. W
Even if the strength of the base material is slightly reduced by adding no or adding a minimum amount of Cu, the addition of Cu improves the creep strength of the joint so that the member can be used under high stress. Become.

To obtain the above effects and
It is essential to add Al for sufficient deoxidation. If the amount of Al added is small, the joint strength will drop significantly. On the other hand, Al
Excessive addition of Al reduces the strength including the base metal.

The present invention was mainly made on the basis of the above findings, and its gist resides in the following high Cr ferritic heat resistant steel.

(1) C: 0.03 to 0.15% by weight, Si: 0.
7% or less, Mn: 0.1 to 1.5%, Ni: 0.05 to 1.0%, Cr:
8-14%, Mo: 0.01-3%, V: 0.1-0.3%, Nb: 0.
01 to 0.2%, N: 0.001 to 0.1%, Al: 0.001 to 0.05
%, Cu: 0.4 to 3.5%, the balance consisting of iron and unavoidable impurities, and O (solid solution oxygen) as impurities of 0.01
High chrome ferritic heat resistant steel with less than 5% strength reduction in the weld heat affected zone.

(2) A high chromium content containing, as an alloy component, one or more components selected from one or more of the following (a) to (d) groups in addition to the components described in (1) above. Ferrite heat resistant steel.

Group (a): 0.0001 to 0.02% by weight of B (b) group: less than 0.5% by weight of W (c) group: 0.0001 to 0.1% of Mg (d) group: 0.01-0.2 wt% La, Ce, Ca, Z respectively
r, Ti, Y and Ta The steels of the present invention described above are excellent in overall characteristics as heat resistant steels by containing a large number of alloy components in proper amounts in a well-balanced manner as described later. In particular, the steel of the present invention is characterized by being sufficiently deoxidized by adding an appropriate amount of Al, reducing the W amount or adding Cu without adding W, and preventing Type IV cracks due to HAZ softening.

High Cr with a small amount of W or no W added
Sufficient creep rupture strength can be obtained even with ferritic steel.

[0020]

The function and effect of the alloy components of the steel of the present invention and the reason for limiting the content thereof will be described below. In addition, all% of the content of alloy components are weight%. C: C combines with Cr, Fe, W, V and Nb to form a carbide, contributes to high temperature strength and stabilizes itself by austenite. Stabilizes the structure as an element. If it is less than 0.03%, the precipitation amount of carbides is small and a large amount of δ-ferrite is produced, resulting in a decrease in strength and toughness. On the other hand, if it exceeds 0.15%, the steel is significantly hardened and the weldability and workability are deteriorated. That is, the proper content of C is 0.03 to 0.15%. A more preferable range is 0.06 to 0.13%.

Si: Si is an element that acts as a deoxidizer and enhances the steam oxidation resistance of steel. However, Si is 0.7%
If it exceeds 1.0, the toughness of the steel is remarkably reduced and it is harmful to the creep strength. Especially for thick materials, it is desirable to keep it low to avoid embrittlement due to long-term heating, so the upper limit is made 0.7%.

Mn: Mn improves hot workability and is effective in stabilizing the structure, but if it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 1.5%, it hardens the steel to improve workability and welding. Impair sex. Therefore, the Mn content is 0.1 to 1.5%.

Ni: Ni suppresses the formation of δ-ferrite as an austenite stabilizing element and stabilizes the tempered martensite structure. Also, from the viewpoint of improving the hot workability of Cu-added steel, it is necessary to add an appropriate amount of Ni in proportion to the Cu content. If the Ni content is less than 0.05%, the above effect cannot be obtained.
If it exceeds 0%, the creep strength will decrease and the economy will be impaired. Therefore, the Ni content is 0.05 to 1.0%. A desirable range is 0.1 to 0.8%. More preferably, the weight ratio is Cu / Ni = 2.5 to 4.5. This Cu
The / Ni ratio is a condition as a measure to prevent cracks during hot working, which is peculiar to Cu-containing steel.

Cr: Cr is an indispensable element for ensuring the oxidation resistance and high temperature corrosion resistance of the steel, and if the content of Cr is less than 8%, the oxidation resistance is sufficient for the above-mentioned applications as a high Cr steel. , High temperature corrosion resistance cannot be obtained. In the steel containing Cr less than 8%, a large amount of bainite is formed by normalizing, and the material is different from the tempered martensite-type material targeted by the present invention. On the other hand, when Cr exceeds 14%, the amount of δ-ferrite increases, and the strength, workability and toughness are impaired. Therefore, the proper amount of Cr is 8 to 14%, and the preferable range is
It is 8.5 to 12%.

Mo: Mo is one of the important basic components of the steel of the present invention, and is effective for improving the creep strength as a solid solution strengthening and precipitation strengthening element for fine carbonitrides. Although Mo easily forms δ-ferrite, it has an advantage that it is cheaper than W. Although the contribution of the base material to the creep strength is slightly smaller than that of W, the decrease in creep strength of the welded joint is small in the Mo-containing steel. For these reasons, the positive use of Mo rather than W is one of the major features of the present invention.

It has been found that the structural stability of the softened layer of the joint HAZ of the Mo-containing steel during creep is significantly improved by the appropriate Al deoxidation and Cu addition described later. This is joint H
This also results in suppressing the concentration of creep strain in the softened layer of AZ. However, such effect is 0.01%
Not available with less than Mo. On the other hand, when Mo exceeds 3%, δ
-A large amount of ferrite is generated and the steel is hardened to impair toughness and workability. Therefore, the Mo content is 0.01
It is preferable to be in the range of 3%. A more preferable range is
0.8 to 2.2%.

V: V combines with C and N to form V (C, N) fine precipitates. These precipitates are stable even at high temperature for a long time and contribute greatly to the improvement of creep strength for a long time.

If the V content is less than 0.1%, the above effect cannot be sufficiently 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
~ 0.3%

Nb: Nb is the same as V and is bonded to C and N to form Nb.
It forms fine precipitates of (C, N) and contributes to the improvement of creep strength. This precipitate is effective for improving short-time creep strength, and is also effective for improving the toughness by refining the crystal grains. If the Nb content is less than 0.01%, the above effect cannot be obtained.
On the other hand, if it exceeds 0.2%, undissolved Nb is
C increases, and strength and weldability are impaired. Therefore, the Nb content is 0.01 to 0.2%.

N: N combines with V and Nb to form a carbonitride and contributes to the improvement of creep strength, but if it is less than 0.001%, it has no effect. On the other hand, if it exceeds 0.1%, the weldability and workability are impaired. Therefore, the N content is 0.001 to 0.1
%. 0.02 to 0.07% is preferable.

Al: Al is added as a deoxidizing agent, but as described below, deoxidation by Al must be sufficiently performed in order to reduce the solid solution oxygen and exert the effect of Cu. If the Al content is less than 0.001%, a sufficient deoxidizing effect cannot be obtained and the toughness and strength are impaired. On the other hand, if it exceeds 0.05%, the creep strength is impaired, so the Al content is 0.001
~ 0.05%.

Cu: Cu is one of the important additive elements of the steel of the present invention, and has the following effects when the steel of the composition system of the present invention is targeted. Suppresses the recovery and recrystallization of dislocations during the welding and use of the structure of the HAZ softening layer formed in the weld (mainly tempered martensite structure and sometimes containing δ-ferrite). Therefore, even if a softened layer is formed, the change during use is small and the structure is stable.

Solid solution strengthening of the HAZ softening layer. Therefore, the strain concentration in the low strength portion (softening layer) during creep is reduced, and the creep resistance is increased.

Local deformation failure in the softening layer during creep is called Type IV cracking, and in connection with this local deformation is reduced. Type IV cracking is a low ductility fracture of the joint, but the addition of Cu also contributes to the ductility improvement of the joint and prevents this fracture.

These effects of Cu are newly found by the present inventor, and in addition to that, the δ-
It is also effective in suppressing ferrite, improving toughness, improving hardenability, and improving oxidation resistance. However, if Cu is added alone, it tends to become brittle due to the precipitation of Cu phase, and the hot workability and creep ductility are impaired. Therefore, if the Cu content is less than 0.4%, the above effect is not sufficient, while if it exceeds 3.5%, not only the workability and ductility are impaired, but also the retained austenite phase increases and the creep strength is impaired. Therefore, a Cu content of 0.4 to 3.5% is appropriate. The preferable content is 0.8 to 2.2%.

O (solid solution oxygen): O is an impurity. In the present invention, the amount of solid solution oxygen remaining is defined as 0.015% or less as a guide for performing sufficient deoxidation. If the O content exceeds 0.015%, the long-term creep strength and the toughness of the welded joint are impaired. In particular, in the HAZ of the joint, the reduction of dissolved oxygen is important for suppressing the recovery and softening of the structure during use. Further, the reduction of oxygen is also effective in utilizing the effect of Cu for solid solution strengthening.

The steel of the present invention, in addition to the above components,
The components of the groups (A) to (D) can be contained as necessary.

(A) Group (B): B has the effect of dispersing and stabilizing the carbide by adding a trace amount. If it is less than 0.0001%, its effect is small, and if it exceeds 0.02%, the weldability and workability are impaired, so when B is added, its content is 0.0001-0.
It is good to set it in the range of 02%.

Group (B) (W): W, like Mo, is effective as a solid solution strengthening and fine carbide precipitation strengthening element for improving creep strength. Usually, addition of twice Mo (weight ratio) is effective for creep strength on the high temperature side, but the strength of the base metal increases, but softening of the welded joint and strength reduction relatively increase. Further, W is also in terms of creep ductility and toughness.
It has a disadvantage over Mo. Since the purpose of the present invention is to reduce the decrease in strength of the welded joint, W is not used, or the content thereof is kept low. If the W content is 0.5% or more, HAZ softening becomes large and the creep strength of the joint is impaired. Therefore, even if W is added, its content should be kept below 0.5%.

(C) Group (Mg): Mg is effective in improving the hot workability of steel, and has a high bonding strength with impurities S and O, and also contributes to the elimination of the damage of these elements. .. If it is less than 0.0001%, the above effect cannot be obtained, and if it exceeds 0.1%, the strength and toughness are significantly impaired.
The content is 0.0001 to 0.1%.

Group D (La, Ce, Ca, Zr, Ti, Y, Ta):
These elements are added as necessary in order to combine with impurities such as P, S, and O in the steel and suppress the morphology of these precipitates (inclusions). By adding at least one of these elements in an amount of 0.01% or more for each element, the impurity element can be fixed as a stable and harmless precipitate, and the strength and toughness can be improved.
However, if the content of each exceeds 0.2%, inclusions increase and the strength and toughness are impaired.
~ 0.2%

The steel of the present invention comprises, in addition to the above components, the balance of Fe and inevitable impurities. Typical impurities of steel are P and S. P should be 0.025% or less and S should be 0.015% or less. These are all toughness, workability,
Since it is an element that is detrimental to weldability, it should be as small as possible even below the above allowable upper limit.

The steel of the present invention is used as a tempered martensite structure. Δ in this tempered martensite structure
-The amount of ferrite is preferably as low as possible in order to improve the toughness of the welded joint. In order to improve the ductility of HAZ, δ-ferrite should be kept below 30%, toughness,
If importance is attached to creep strength, the amount of δ-ferrite is preferably less than 5%.

The standard heat treatment for the steels of the present invention is the normalizing-tempering process. The normalizing treatment temperature is set to an Ac ₃ transformation point or higher in order to sufficiently dissolve the coarse precipitates generated in the previous working and to homogenize the segregation of the solid solution alloy elements due to casting segregation or the like. The upper limit is 1150 ° C to prevent large precipitation of δ-ferrite. The preferred temperature range is 1000-1100
℃.

After normalizing, a tempering process is performed. This tempering treatment is performed in a temperature range of 150 to 200 ° C. higher than the operating temperature and lower than the Ac ₁ transformation point because it is necessary to lower the dislocation density in martensite in order to stabilize the high temperature creep strength. Specifically, 750 to 830 ° C is suitable. In addition, if tempering is insufficient, a significant decrease in strength may occur at high temperature for a long time. Since this is particularly remarkable at the welded joint, it is necessary to take care to perform sufficient tempering.

[0046]

[Example] Steel having the chemical composition shown in Table 1 was melted in a 150 kg vacuum melting furnace, and an ingot was forged at 1150 to 950 ° C to obtain a thickness of 20.
mm plate.

The steel with the code A1 is STBA26, and the steel with the code A2 is
(Tue) STBA27 (Thermal and Nuclear Power Association Standard), Steel with code A3 is ASTM SA213 T91, Steel with code A4 is DIN X20CrMoWV12
1 is a typical existing high Cr ferritic steel. Steels A5 to A10 are comparative steels in which Cu is added to the base of A3 steel, and steels A11 and A12 are combined additions of excess Mo and W.
It is a comparative steel containing Cu. The steels denoted by B1 to B25 are the steels of the present invention.

The steels A1 and A2 were subjected to normal heat treatment at 950 ° C. × 1 hour → air cooling and then at 750 ° C. × 1 hour → air cooling. The steels with symbols A3 to A12 and symbols B1 to B25 are 10
50 ℃ × 1 hour → air-cooled normalization and 780 ℃ × 3 hours → air-cooled tempering.

The welded joint was prepared by placing TIG weld beads so as to almost penetrate a plate having a thickness of 20 mm, and then air-cooling at 740 ° C. for 5 hours, followed by heat treatment.

All creep test pieces were sampled from the center of the build-up perpendicular to this bead. The normal temperature tensile test piece of the base material is φ6 mm × GL 30 mm, and the joint creep test is the same φ6 mm
A test piece of × GL 30 mm was used so that the molten metal, the heat-affected zone, and the base metal would enter the parallel part. Creep test is 650 ℃
The maximum time was about 10,000 hours.

Regarding the longest creep rupture material at 650 ° C.,
A test piece that was sheared from the interface between the heat-affected zone and the base metal and had an elongation of 3% or less was considered to have Type IV cracking. The hardness was measured with a Vickers load of 10 kgf on the base material and the post-heat-treated welded joint cross section.

Table 2 collectively shows the measurement results. In addition,
The softening tendency of the welding heat affected zone was evaluated by the difference between the hardness of the softest part of the welding heat affected zone and the hardness of the base material.

With respect to the normal temperature tensile properties of the base material, as shown in Table 2, the steel of the present invention has an elongation of 20% or more, which is slightly higher than that of the comparative steel.

The difference between the softest part of the welded joint and the hardness of the base material was as large as Hv45 for the A3 steel, and all of the A5 to A12 steels containing Cu showed a large decrease in hardness of Hv50 or more. In contrast, all of the steels of the present invention, B1 to B25, have small Hv40 or less.

That is, in the steel of the present invention, the softening of the weld heat affected zone is remarkably suppressed.

Looking at the creep rupture strength of the welded joint of Table 2 at 650 ° C. × 10 ⁴ hours, the steels of the present invention having the symbols B1 to B25 are 8.2.
It has excellent creep rupture strength of up to 9.5 kgf / mm ² , whereas the comparative steels with codes A3 to A12 are all 5.2 kgf / mm ²
Low as below. The reference steels A1 and A2 of the comparative steels do not contain V and Nb, have low base metal strength and creep rupture strength.
It is as low as 3.3 kgf / mm ² and 3.6 kgf / mm ² .

Further investigation of creep rupture materials revealed that none of the inventive steels produced Type IV cracking.

[0058]

[Table 1 (1)]

[0059]

[Table 1 (2)]

[0060]

[Table 1 (3)]

[0061]

[Table 2 (1)]

[0062]

[Table 2 (2)]

[0063]

[Table 2 (3)]

[0064]

EFFECTS OF THE INVENTION The high Cr ferritic steel of the present invention causes less decrease in strength of the heat-affected zone of the welded joint, has better creep rupture strength of the joint than conventional steel, and does not cause Type IV cracking. That is, the steel of the present invention, as a high-Cr ferritic steel having a small strength reduction in the heat-affected zone of the welded joint, which is a drawback of conventional high-Cr ferritic heat-resistant steel, as a heat-resistant pressure-resistant member in the fields of boilers, chemical industry, nuclear power, etc. It can be widely used for sheet, plate, forging, rolled material, etc.

[0065]

Claims (5)

[Claims] 1. C: 0.03 to 0.15% by weight, Si: 0.7% by weight
Below, Mn: 0.1-1.5%, Ni: 0.05-1.0%, Cr: 8-
14%, Mo: 0.01-3%, V: 0.1-0.3%, Nb: 0.01-
0.2%, N: 0.001-0.1%, Al: 0.001-0.05%, C
u: 0.4-3.5%, balance is iron and unavoidable impurities, and O (solid solution oxygen) as impurities is 0.015%
The high-chromium ferritic heat-resistant steel with less strength reduction in the weld heat-affected zone as follows. 2. Further, as an alloy component, 0.0001 to 0.02% by weight
The high chromium ferritic heat-resistant steel according to claim 1, which contains B. 3. Further, as an alloy component, W of less than 0.5% by weight.
The high chromium ferritic heat resistant steel according to claim 1 or 2, containing. 4. Further, as an alloy component, 0.0001 to 0.1% by weight
The high-strength, high-chromium ferritic heat-resistant steel according to claim 1, 2 or 3, which contains Mg. 5. Further, as an alloy component, 0.01 to 0.2, respectively.
The high chromium ferrite heat-resistant steel according to claim 1, 2, 3 or 4, which contains at least one selected from the group consisting of La, Ce, Ca, Zr, Ti, Y and Ta in a weight percentage.