JP3237137B2 - High chromium ferritic heat-resistant steel with small decrease in strength of weld heat affected zone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to, for example, steel pipes and steel sheet forgings which have high strength at high temperatures, have excellent oxidation resistance and weldability, and are used as high-temperature pressure-resistant members in fields such as boilers, nuclear power and chemical industries. High-chromium ferritic heat-resistant steel having a small decrease in strength 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 and pressure-resistant members for boilers, nuclear power, chemical industry, etc. has excellent workability, weldability, and economic efficiency, as well as high-temperature strength, corrosion resistance, oxidation resistance, and toughness. Is required. Conventionally, materials used for the above applications include 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 ferrite steels of 9-12Cr series. Among them, high-Cr ferritic steel is 500-
At a temperature of 650 ° C, it is superior to low alloy steel in strength and corrosion resistance, and has higher thermal conductivity and lower thermal expansion coefficient than austenitic stainless steel. It is advantageous that it is difficult to cause stress corrosion cracking.

As high Cr ferritic steels, 9Cr-1Mo steel (STBA26), improved 9Cr-1Mo steel (ASTM SA213 T91), 12Cr
-1Mo steel (DIN X20CrMoV121) and the like are famous. In order to further improve the high-temperature strength, steels to which Mo, W, V, Nb, N, etc. are added in combination are disclosed in JP-B-62-8502 and JP-B-6-1230.
No. 4, JP-A-62-297435, and JP-A-2-310340.

[0004] The present inventors have repeatedly studied the composition of heat-resistant materials for the purpose of increasing the high-temperature oxidation resistance at 600 ° C or higher. The result is that as disclosed in
As proposed in the gazettes of 232345 and 3-97832. This addition of Cu improves oxidation resistance above 600 ° C and
This is effective in suppressing δ-ferrite, which is easily formed in Cr ferritic steel, and saves expensive Ni. Further, it is 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 production. Incidentally, Cu-added steel is also proposed in Japanese Patent Application Laid-Open No. 2-294452.
It is described that the addition of is effective in maintaining the toughness of a welded joint, particularly the toughness of a weld after long-time heating.

[0005] Attempts have been made to add Cu to high Cr steels. For example, Japanese Patent Publication No. 46-15210 discloses
It has been shown that 4% of Cu is effective in improving hardenability and toughness, and 1% of Cu is also disclosed in JP-A-60-155649.
The following Cu has been shown to be effective in improving toughness. Further, Japanese Patent Publication No. Sho 62-12304 described above and
JP-A-211553 also discloses a high Cr steel to which Cu is added.

[0006] However, the high Cr
Ferrite steels have a common problem in that the heat-affected zone (hereinafter, referred to as HAZ) of the welded joint is greatly softened and creep strength is reduced. While the conventional high-Cr ferritic steels mentioned above emphasize improvement in the high-temperature creep strength of the base metal, studies on the problem that the strength of the welded joint is greatly reduced compared to the base metal have been made. Not. The inventions disclosed in the above-mentioned many patent publications are all inventions of compositions in which the characteristics of the base material are emphasized, and no consideration is given to the change in the HAZ structure of the welded joint and the accompanying decrease in the strength of the HAZ.

[0007] In short, the decrease in creep strength caused by the softened layer of the welded joint of high-strength high-Cr ferritic steel has been a major obstacle in using this kind of steel as a high-temperature pressure-resistant member.

[0008]

The decrease in the strength of the welded joint in the high Cr ferritic heat-resistant steel is considered to be caused by an unstable softened layer generated in the HAZ (this is called a HAZ softened layer). There are no reports of fundamental measures to prevent this. For example, 600 to 650 ° C. creep strength in the welded joint of the steel existing improved 9Cr-1Mo drops 15-20% compared with the base material at 10 ^{4 h} or more long side. Such a decrease in creep strength is the same even in a high W ferritic steel of high W type. The structure of HAZ is generally from the side of the weld metal.
It is classified into a coarse grain area, a mixed grain area, and a fine grain area.
The softened layer corresponds to the fine grain area and has the lowest hardness.
And therefore the region with the lowest intensity. High Cr ferrai
As in heat-resistant steel, the base material is a tempered martensite
In the case of weaving, the fine grain area of the HAZ is heated just above the transformation point
This is a part, and the hardness decreases significantly as the tissue recovers and softens
Area.

An object of the present invention is to provide basic properties such as high-temperature strength, workability, weldability, etc., which are equal to or higher than those of conventional steel, and in particular, softening of a welded joint is small, there is no decrease in high-temperature creep strength, and the cost is low. It is to provide a high-Cr ferritic heat-resistant steel.

[0010]

The creep strength of the welded joint is reduced by the formation of a softened layer in the HAZ, which is accompanied by low ductile shear fracture (referred to as Type IV cracking). . On the other hand, in the case of steel to which a large amount of W is added from the viewpoint of increasing the strength, the creep strength of the joint is greatly reduced compared to the base metal. Further, there is also a problem that addition of a large amount of W increases material cost.

The present invention has been made based on the following findings obtained from the results of numerous studies on high Cr ferritic steels.

[0012] The addition of Cu increases the structural stability of the HAZ and can prevent softening of the weld joint. This effect is δ
-It is different from previously known effects such as suppression of ferrite and improvement of toughness, and can be obtained only in a specific chemical composition range described later.

[0013] The high Cr ferritic steel to which a large amount of W is added increases the strength of the base material, but greatly reduces the joint strength. W
Even if the strength of the base material is slightly reduced without adding or minimizing the amount of addition, the addition of Cu improves the creep strength of the joint, allowing the member to be used under high stress. Become.

In order to obtain the above effects, the steel
It is essential to add Al to perform sufficient deoxidation. If the addition amount of Al is small, the joint strength is greatly reduced. On the other hand, Al
Excessive addition of iron causes a decrease in strength including the base material.

The present invention has been made mainly on the basis of the above findings, and its gist lies in the following high Cr ferrite heat-resistant steel.

(1) By weight%, C: 0.03 to 0.15%, Si: 0.
7% or less, 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%, Cu:
0.4-3.5%, Cu / Ni is 4.5 or less, and the balance consists of iron and unavoidable impurities.
A high chromium ferrite heat-resistant steel with a small decrease in the strength of the weld heat-affected zone having a solid solution oxygen content of 0.015% or less.

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

Group (a): 0.0001 to 0.02% by weight of B (B) Group: W of less than 0.5% by weight, provided that the W content contains Mo.
Less than Yuryou (c) group ... from 0.0001 to 0.1 percent of Mg (d) group ... respectively 0.01 to 0.2 wt% of La, Ce, Ca, Z
r, Ti, Y and Ta The steel of the present invention has excellent overall properties as a heat-resistant steel by containing a number of alloy components in appropriate amounts in a well-balanced manner as described later. In particular, the present invention steel, sufficiently deoxidized by addition a suitable amount of Al, Cu is added to W amount as reduced or no addition, a significant feature that prevents the TypeIV cracks due HAZ softening.

High Cr with low W content or no W added
Sufficient creep rupture strength can be obtained even with ferritic steel.

[0020]

The effects of the alloy components of the steel of the present invention and the reasons for limiting the contents will be described below. All the percentages of the contents of the alloy components are% by weight. C: C combines with Cr, Fe, W, V and Nb to form carbides and contributes to high-temperature strength and stabilizes austenite itself. Stabilizes the structure as an element. If it is less than 0.03%, the amount of carbide precipitation is small, and a large amount of δ-ferrite is formed, and strength and toughness are reduced. If it exceeds 0.15%, the steel is extremely hardened, and the weldability and workability deteriorate. That is, the appropriate content of C is 0.03 to 0.15%. A more preferred range is from 0.06 to 0.13%.

Si: Si acts as a deoxidizing agent and is an element that enhances the steam oxidation resistance of steel. However, Si is 0.7%
If it exceeds, the toughness of the steel is significantly reduced, and is harmful to the creep strength. Especially for thick materials, it is desirable to keep the thickness low to avoid embrittlement due to prolonged heating, so the upper limit is set to 0.7%.

[0022] Mn: Mn improves the hot workability, but Ru Oh effective in stabilizing the tissue, sufficient effect can not be obtained is less than 0.1%, to cure the steel exceeds 1.5%, workability Impairs weldability. Therefore
The content of Mn 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 the Cu-added steel, it is necessary to add an appropriate amount of Ni corresponding to the Cu content. If the Ni content is less than 0.05%, the above effects cannot be obtained, and 1.
If it exceeds 0%, the creep strength is reduced and the economy is impaired. Therefore, the content of Ni is set to 0.05 to 1.0%. The preferred range is 0.1-0.8%. More preferably, Cu / Ni = 2.5-4.5 in weight% ratio. This Cu
The / Ni ratio is a condition as a measure to prevent cracking during hot working that is specific to Cu-containing steel.

Cr: Cr is an element indispensable for securing the oxidation resistance and high-temperature corrosion resistance of the steel. If its content 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 steel with less than 8% Cr, much bainite is formed by normalizing, which is a material different from the tempered martensitic type material targeted by the present invention. On the other hand, when Cr exceeds 14%, the strength, workability, and toughness are impaired due to the increase in the amount of δ-ferrite. Therefore, the Cr content is in a proper range of 8 to 14%, and is preferably
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 element and a precipitation strengthening element for fine carbonitrides. Mo easily produces δ-ferrite, but has the advantage of being cheaper than W. Further, although the contribution of the base metal to the creep strength is slightly smaller than that of W, the decrease in the creep strength of the welded joint of the Mo-containing steel is small. For these reasons, the active use of Mo over W is one of the major features of the present invention.

In particular, it has been found that the microstructure stability during creep of the softened layer of the joint HAZ of the Mo-containing steel is significantly improved by appropriate Al deoxidation and Cu addition described later. This is the joint H
This also results in suppressing the concentration of creep strain on the softened layer of AZ. However, such an effect is 0.01%
It cannot be obtained with less than Mo. On the other hand, when Mo exceeds 3%, δ
-A large amount of ferrite is formed and the steel is hardened, resulting in impaired toughness and workability. Therefore, the content of Mo is 0.01
It is better to be in the range of 3%. A more preferred range is
0.8 to 2.2%.

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

If the V content is less than 0.1%, the above effects cannot be sufficiently obtained. If the V content is more than 0.3%, the solid solution V increases and the strength is impaired. Therefore, the content of V is 0.1
To 0.3%.

Nb: Nb is bonded to C and N in the same manner as V to form Nb
A fine precipitate of (C, N) is formed, which contributes to improvement in creep strength. These precipitates are effective for improving short-time creep strength, and are also effective for refining crystal grains and improving toughness. If the Nb content is less than 0.01%, the above effects cannot be obtained.
On the other hand, if it exceeds 0.2%, the undissolved Nb
C is increased and strength and weldability are impaired. Therefore, the content of Nb is set to 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 less than 0.001% has no effect. On the other hand, if it exceeds 0.1%, weldability and workability are impaired. Therefore, the content of N is 0.001-0.1
%. Preferred is 0.02-0.07%.

Al: Al is added as a deoxidizing agent, but as described below, in order to reduce the dissolved oxygen and exert the effect of Cu, deoxidation with Al must be sufficiently performed. 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%.
To 0.05%.

Cu: Cu is one of the important addition elements of the steel of the present invention. When the steel of the component system of the present invention is targeted, it has the following effects. Suppresses recovery of dislocation and recrystallization during welding and use of the structure of the HAZ softening layer (mainly tempered martensite structure which may contain δ-ferrite) formed in the welded portion. Therefore, even when a softened layer is formed, the change during use is small and the structure is stable.

The HAZ softening layer is solid-solution strengthened. Therefore, strain concentration on a low strength portion (softened layer) during creep is reduced, and creep resistance is increased.

[0034] Local deformation fracture in the softened layer during creep is called Type IV cracking, but in conjunction therewith, this local deformation is reduced. Type IV cracking is a low ductility fracture of the joint, but the addition of Cu also contributes to improving the ductility of the joint and prevents this fracture.

These effects of Cu are newly found by the present inventor.
It is also effective in suppressing ferrite, improving toughness, improving hardenability, and improving oxidation resistance. However, when Cu alone is added, it tends to become brittle due to the precipitation of the Cu phase and impairs hot workability and creep ductility. Therefore, it is used in combination with Ni as described above. Therefore, if the Cu content is less than 0.4%, the above effect is not sufficient. On the other hand, if it exceeds 3.5%, not only the workability and ductility are impaired, but also the retained austenite phase is increased and the creep strength is impaired. Therefore, the proper content of Cu is 0.4 to 3.5%. The preferred content is 0.8 to 2.2%.

O (Solute O): O is an impurity. In the present invention, the amount of remaining solid-dissolved oxygen is specified 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. Particularly in the HAZ of the joint portion, the reduction of dissolved oxygen is important for suppressing the recovery and softening of the tissue during use. Further, the reduction of oxygen is also effective in making use of the effect of solid solution strengthening of Cu.

The steel of the present invention has the above-mentioned components and
Components (a) to (d) can be contained as necessary.

(A) Group (B): B has an effect of dispersing and stabilizing carbides by adding a small amount. If the content is less than 0.0001%, the effect is small, and if it exceeds 0.02%, the weldability and workability are impaired. Therefore, when B is added, the content is 0.0001 to 0.1%.
It is better to be in the range of 02%.

(B) Group (W): W, like Mo, is effective for improving the creep strength as a solid solution strengthening element and a fine carbide precipitation strengthening element. Normally, the addition of twice the weight (molar ratio) of Mo is effective for the creep strength on the high temperature side. W also has a creep ductility and toughness.
Disadvantage than Mo. Since the object of the present invention is to reduce the decrease in the strength of the welded joint, W is not used, or the content thereof is suppressed to a low level. When the content of W is 0.5% or more, HAZ softening is large and the creep strength of the joint is impaired. Therefore, even when W is added, its content should be kept below 0.5%.

(C) Group (Mg): Mg is effective for improving the hot workability of steel, and has a high bonding force with S and O as impurities, and contributes to eliminating harm of these elements. . If the content is less than 0.0001%, the above effects cannot be obtained. If the content exceeds 0.1%, the strength and toughness are significantly impaired.
Its content is 0.0001-0.1%.

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

The steel of the present invention contains Fe and inevitable impurities in addition to the above-mentioned components. Representative examples of steel impurities are P and S. P should be kept below 0.025% and S should be kept below 0.015%. These are all toughness, workability,
Since it is an element harmful to weldability, it is preferable that the amount is as small as possible even at or below the allowable upper limit.

The steel of the present invention is used as a tempered martensite structure. Δ in this tempered martensitic structure
-The ferrite content is preferably as low as possible in order to improve the toughness of the welded joint. To improve the ductility of HAZ, δ-ferrite should be suppressed to less than 30%.
If emphasis is placed on creep strength, the amount of δ-ferrite is preferably less than 5%.

The standard heat treatment of the steel according to the invention is a normalizing-tempering treatment. The normalizing treatment temperature is set to the Ac ₃ transformation point or higher for the purpose of sufficiently dissolving the coarse precipitates generated in the previous processing and uniformizing the segregation of the solid solution alloy element by casting segregation or the like. The upper limit is set to 1150 ° C to suppress the precipitation of a large amount of δ-ferrite. Desirable temperature range is 1000-1100
° C.

After the normalizing, a tempering process is performed. This tempering treatment is performed in a temperature range higher than the working temperature by 150 to 200 ° C. and lower than the Ac ₁ transformation point because the dislocation density in martensite needs to be lowered to stabilize the high temperature creep strength. Specifically, 750 to 830 ° C is appropriate. If the tempering is insufficient, the strength may be remarkably reduced at a high temperature for a long time. Since this is particularly remarkable in a welded joint portion, care must be taken to perform sufficient tempering.

[0046]

EXAMPLE A steel having a 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 a thickness of 20 kg.
mm plate.

The steel with reference number A1 is STBA26, and the steel with reference number A2 is
(Tue) STBA27 (Thermal Atomic Power Association standard), A3 steel is ASTM SA213 T91, A4 steel is DIN X20CrMoWV12
1 Both are existing representative high Cr ferritic steels. A5 to A10 steels are comparative steels based on A3 steel with Cu added, and A11 and A12 steels are Mo and W combined with excess Mo.
This is a comparative steel containing Cu. Steels denoted by reference numerals B1 to B25 are steels of the present invention.

The steels A1 and A2 were subjected to ordinary heat treatment at 950 ° C. × 1 hour → air cooling and then at 750 ° C. × 1 hour → air cooling. A3 to A12 and B1 to B25 steel are 10
50 ° C x 1 hour → air cooling normalization and 780 ° C x 3 hours → air cooling tempering.

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

All creep test pieces were taken from the center of the build-up perpendicular to this bead. Room temperature tensile test specimen of base material is φ6mm × GL30mm, joint creep test is same φ6mm
× GL Using a 30 mm test piece, samples were taken so that the molten metal, the heat-affected zone, and the base metal entered the parallel portion. 650 ° C for creep test
At a maximum of about 10,000 hours.

For the longest creep rupture material at 650 ° C.,
In a test piece which was shear-destructed from the interface between the weld heat-affected zone and the base material and had an elongation of 3% or less, it was determined that Type IV cracking had occurred. The hardness was measured with a Vickers 10 kgf load on the base metal and the post-heat-treated cross section of the welded joint.

Table 2 summarizes the results of each measurement. In addition,
The evaluation of the softening tendency of the weld heat affected zone was made based on the difference between the hardness of the softened portion of the weld heat affected zone and the base metal hardness.

With respect to the room-temperature tensile properties of the base material, as shown in Table 2, the steel of the present invention has a slightly higher elongation of 20% or more than the comparative steel.

The difference between the softened portion of the welded joint and the hardness of the base metal is as large as Hv45 in the A3 steel, and all of the A5 to A12 steels added with Cu show a large decrease in hardness of Hv50 or more. On the other hand, the reference numerals B1 to B25 of the steels of the present invention are all as small as Hv40 or less.

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

Looking at the creep rupture strength at 650 ° C. × 10 ⁴ hours of the welded joints in Table 2, the steels of the present invention with reference numerals B1 to B25 showed 8.2 strength.
While the creep rupture strength is excellent at ~ 9.5 kgf / mm ² , the comparative steels A3 to A12 have 5.2 kgf / mm ^{2 at all.}
Below and low. Reference symbols A1 and A2 of the comparative steels do not contain V and Nb, and have low base material strength and low creep rupture strength, respectively.
It is as low as 3.3 kgf / mm ² and 3.6 kgf / mm ² .

Further, examination of the creep rupture material revealed that Type IV cracking did not occur in any of the steels of the present invention.

[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]

The high Cr ferritic steel of the present invention has a small decrease in the 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 is a high Cr ferritic steel having a small strength decrease in the heat-affected zone of a welded joint, which is a drawback of the conventional high Cr ferritic heat resistant steel. , Plate, forging, rolled material, etc.

[0065]

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-201445 (JP, A) JP-A-2-310340 (JP, A) JP-A-62-156256 (JP, A) JP-A-2- 232345 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] (1) C: 0.03 to 0.15%, Si: 0.7% by weight
Hereinafter, Mn: 0.1 to 1.5%, Ni: 0.05 to 1.0%, Cr: 8 to 14
%, Mo: 0.01 to 3%, V: 0.1 to 0.3%, Nb: 0.01 to 0.2
%, N: 0.001 to 0.1%, Al: 0.001 to 0.05%, Cu: 0.4 to
It contains 3.5%, Cu / Ni is 4.5 or less, the balance consists of iron and unavoidable impurities, and O (solid solution oxygen) as impurities is 0.015% or less. Chrome ferrite heat resistant steel. (2) 0.0001 to 0.02% by weight as an alloy component.
2. The high chromium ferrite heat-resistant steel according to claim 1, which contains B. 3. The alloy according to claim 1, further comprising less than 0.5% by weight of W.
The high chromium ferrite heat-resistant steel according to claim 1 or 2, wherein the content of W is lower than the content of Mo. (4) 0.0001 to 0.1% by weight as an alloy component
The high-strength, high-chromium ferrite heat-resistant steel according to claim 1, 2 or 3, which contains Mg. 5. The alloy composition further comprises 0.01 to 0.2 each.
5. The high chromium ferritic heat resistant steel according to claim 1, wherein the steel contains at least one selected from the group consisting of La, Ce, Ca, Zr, Ti, Y and Ta.