JPH0871784A - High-strength austenitic steel welding material and production thereof - Google Patents

Abstract

PURPOSE: To provide an austenitic steel welding material which can impart the excellent high temperature strength onto a welding material without sacrificing the weld metal high temperature crack resistance. CONSTITUTION: The composition of this welding material contains, by weight %, 0.03 to 0.13% C, 0.1 to 0.5% Si, 0.6 to 6% Mn, 20 to 24% Cr, 6 to 21% Ni, 0.01 to 0.7% Nb, 0.5 to 1.5% Mo, 1 to 3% W, 1 to 4% Cu, 0.2 to 0.4% N, <=0.5% Al, 0.002 to 0.02% Ca, <=0.02% Mg, <=0.01% O, <=0.01% P, <=0.02% P+S, and the balance Fe with inevitable impurities. Further, -8<=AP<=2. and AP=Ni+0.5Mn+30(C+N)-1.1(Cr+1.5Si+Mo+0.5W) are satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welding material for welding austenitic steel used for applications requiring high temperature strength such as boilers for power generation and chemical reactors, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, 18-8 type austenitic stainless steel has been mainly used for power generation boilers, chemical reactors and the like used at high temperatures. However, for example, in boilers and the like, operating conditions have become extremely severe due to the recent increasing demand for improvement in energy efficiency.
Materials having higher high-temperature strength than 8 type austenitic stainless steel have been required.

To meet such requirements, the amount of Cr and Ni
Many new materials have been developed for boilers in higher quantities. However, although they have no problem in terms of performance, they are economically very expensive as compared with 18-8 austenitic stainless steel. Therefore, based on 18-8 austenitic stainless steel, Cu, N
An austenitic steel improved in high temperature strength by adding b, N and B is disclosed in JP-A-62-133048.
It is presented by the official gazette.

This high-strength austenitic steel based on 18-8 has high economic strength because it has high strength at high temperature irrespective of the increase of Cr and Ni. It is expected to exert a great effect on the reduction of manufacturing cost.

On the other hand, as a welding material for welding such an austenitic steel, it is usually considered to use the base material as it is or use a welding material for high Ni alloys. However, welding materials for high Ni alloys are expensive. Further, as will be described later in detail, even if austenitic steel having high strength at high temperature is used as a welding material as it is, it is impossible to give the weld metal the same performance as the base metal.

Therefore, for example, V, N is added to austenitic steel.
A relatively low-cost welding material for obtaining a weld metal having high creep strength by co-existing Co., and further adding Mo, W, Ti, Ni, V, and B in combination is disclosed in JP-A-63-3093.
No. 92 is disclosed.

Considering that the welding material for high Ni alloy is expensive and the performance of the weld metal is inferior to that of the base material when the base material is used as it is, Nb and N are added to the austenitic steel. JP-A-6-142980 discloses a welding material that imparts high-temperature strength to the as-solidified weld metal by refining the solidified structure by precipitating NbC and NbCrN.

[0008]

Problems to be Solved by the Invention JP-A-62-1330
The high temperature and high strength austenitic steel as shown in Japanese Patent Publication No. 48 is cheaper than the high Ni alloy, but if this is used as it is as a welding material, the base material is melted and then the structure is adjusted by rolling heat treatment. Therefore, while the high temperature strength is ensured, the weld metal is almost always used in the as-solidified structure, so that it is essentially difficult to increase the high temperature strength of the weld metal.

[0009] Further, in the austenite structure, welding high temperature cracks are essentially liable to occur, and in the weld metal, solidification cracking which particularly occurs during solidification poses a problem. Moreover, some of the components added to increase the high temperature strength of this type of steel material adversely affect welding hot cracking, making it more difficult to prevent it.

For example, B which is effective for improving the high temperature strength of the base material
Enhances the susceptibility of weld metal to hot cracking. Further, it is effective to reduce the Ni content and crystallize δ ferrite by several% in order to reduce the hot susceptibility to welding, but this δ ferrite transforms into a sigma phase during use at high temperature and becomes brittle. Therefore, the crystallization of δ-ferrite must be suppressed.

In the welding material disclosed in Japanese Unexamined Patent Publication No. 63-309392, Ti and Nb, and V and Nb are added in combination in connection with the component design for ensuring the creep strength. Since the combined addition of Ti and Nb and V and Nb increases the hot susceptibility to welding, welding hot cracking becomes a problem in this welding material. Regarding the creep strength as well, considering the long-term range, it is not always possible to obtain a high creep strength in the temperature range of 600 to 700 ° C.

In the welding material disclosed in Japanese Unexamined Patent Publication No. 6-142980, since Ti and V, especially V are not used, the weld metal has relatively low susceptibility to hot cracking. The strength at high temperature is also excellent, but the 10 ⁵ hour creep strength at 650 ° C has a limit of about 10 kgf / mm ² , and when it is used as a base material having higher high temperature strength than this, the creep strength is insufficient at the welded part. . Therefore, it is necessary to secure higher high temperature strength while suppressing the weld hot cracking susceptibility.

Further, in any of the welding materials, Cu is added to increase the creep strength,
The hot workability is poor, it is not easy to process the wire rod to be used as a welding material, and it is possible that the wire rod cannot be processed into a wire rod due to cracking due to hot working.

An object of the present invention is to provide a high-strength austenitic steel welding material which imparts a high-temperature strength to the weld metal without increasing the hot-cracking susceptibility of the weld metal and has good hot workability.

Another object of the present invention is to provide a method for producing a welding material which can produce the welding material by hot working without causing cracks.

[0016]

Means for Solving the Problems JP-A-62-13304
In the welding of austenitic steel excellent in high temperature strength as disclosed in Japanese Patent Publication No. 8 publication, it is necessary to increase the high temperature strength of the weld metal without increasing the susceptibility to high temperature hot cracking. As a means for this, the present inventors have made high N, added Cu,
In addition to the addition of W, it has been found from various experiments that it is effective to design the alloy so that the intermetallic compound such as Laves phase is finely dispersed when used at high temperature.

The high N content enhances the high temperature strength of the weld metal by solid solution strengthening and strengthening by fine precipitation as NbCrN. Cu addition enhances the high temperature strength of the weld metal by precipitation strengthening. The addition of W enhances the high temperature strength by solid solution strengthening, and becomes a source of generation of Laves phase and the like. The synergies of these strengthening and the strengthening by finely dispersing the intermetallic compounds such as Laves phase at the time of use at high temperature enable welding without adding Ti and V, which cause the deterioration of hot crack resistance of welding. It is possible to remarkably improve the creep strength of the metal.

Further, in order to finely disperse the intermetallic compound at the time of use at an important high temperature and effectively contribute to the creep strength improvement, it is necessary to appropriately control the austenite formation potential of the metal structure. Was also found. Specifically, this is realized by regulating the alloy components, mainly the amount of Ni by the following formula. -8 ≦ A _P ≦ 2 A _P = Ni + 0.5Mn + 30 (C + N) -1.1 (Cr +
1.5Si + Mo + 0.5W)

The welding material which realizes these is excellent in weld hot crack resistance and high temperature strength of the weld metal. Regarding the high temperature strength of the weld metal, the 10 ⁵ hour creep strength at 650 ° C. exceeds 10 kgf / mm ^2. It is also possible and surpasses the welding material described in JP-A-6-142980. The reason is that by adding W, a Laves phase is generated and then a fine Laves phase containing W is precipitated by optimizing the A _P value, and the precipitation strengthening effectively contributes to the improvement of creep strength. Can be considered.

Regarding the improvement of hot workability required for producing a welding material, the following facts have been found. Cu added as an essential element to secure creep strength
Combines with O in the steel to form a low melting point phase. Since N is an essential element together with Cu, hot deformation resistance increases. As a result, cracking occurs in the process of hot working the steel ingot into round bar steel and rough wire. As a countermeasure against this, the present inventors have found that it is effective to reduce the amount of O and add Ca to suppress the formation of a low melting point phase due to Cu.

Further, appropriate hot working conditions for producing such a welding material have also been revealed by the investigation by the present inventors.

The welding material of the present invention contains C: 0.03% by weight.
~ 0.13%, Si: 0.1-0.5%, Mn: 0.6-6%,
Cr: 20-24%, Ni: 6-21%, Nb: 0.01
~ 0.7%, Mo: 0.5-1.5%, W: 1-3%, Cu1
~ 4%, N: 0.2-0.4%, Al: 0.5% or less, Ca:
0.002-0.02%, Mg: 0.02% or less, O: 0.01
% Or less, P: 0.01% or less, P + S: 0.02% or less,
If necessary, B: 0.01% or less is included, and −8 ≦ A _P ≦ 2 A _P = Ni + 0.5Mn + 30 (C + N) −1.1 (Cr +
1.5 Si + Mo + 0.5 W) and the balance is Fe and inevitable impurities.

In the method for producing the welding material of the present invention, the material having the above-described composition is forged at 1100 ° C to 1250 ° C, and thereafter 10
Hot rolling at 50 to 1250 ° C. into a coarse wire having an outer diameter of 6 mm or more, further cold working at 40% or less and 1000 to 1250.
It is characterized in that it is processed into a wire by repeating heat treatment at ℃.

[0024]

The reason for limiting the conditions in the present invention will be described below.

[Components of Welding Material] C: Contributes to improvement of tensile strength and creep strength at high temperature. However, excessive addition causes precipitation as carbonitrides, leading to a decrease in strength. Therefore, it is set to 0.03 to 0.13%. Preferably 0.04 to 0.12%, more preferably 0.05 to 0.
It is 10%.

Si: added as a deoxidizer, but excessive addition increases solidification cracking susceptibility, so the content was made 0.1 to 0.5%.

Mn: It is added as a deoxidizing agent, but since it fixes S, it also contributes to the reduction of the hot susceptibility to welding.
However, excessive addition causes embrittlement. Therefore, 0.6-6%
And

Cr: An essential element for ensuring high temperature strength, oxidation resistance and corrosion resistance, but excessive addition impairs hot workability. Therefore, it is set to 20 to 24%.

Ni: Indispensable for stabilizing the austenite structure and ensuring high temperature strength, and is set to 6 to 21%.
If it is less than 6%, a ferrite phase is generated in the weld metal and promotes precipitation of harmful sigma phase during use, resulting in a decrease in creep strength. On the other hand, if it exceeds 21%, the susceptibility to hot cracking of the weld is increased.

Then, after setting the amount of Ni to 6 to 21%,
As A _P becomes -8～2 mainly adjusts the amount of Ni.
This is because adjusting the amount of Ni is an important point in improving the high temperature strength.

The intermetallic compound in use at high temperatures by the -8～2 the A _P (gamma phase) is finely precipitated, thereby improving the creep strength. If A _P is less than −8, coarse intermetallic compounds are precipitated and grown during use at high temperature, resulting in a decrease in creep strength. On the other hand, when it exceeds 2, the austenite phase becomes stable at high temperature, and improvement in creep strength due to fine precipitation of intermetallic compounds cannot be expected. The lower limit of A _P is preferably -6 or more, more preferably -4 or more. The upper limit is preferably 1 or less, more preferably 0 or less.

Nb: Fine precipitation of carbonitride contributes to improvement of high temperature strength. However, excessive addition lowers the weld hot cracking susceptibility, and particularly under the condition that δ ferrite is not crystallized, this cracking susceptibility is greatly affected. Therefore, it is set to 0.01 to 0.7%.

W: Solid solution strengthening and precipitation of fine intermetallic compound (Γ phase) during use at high temperature contribute to improvement of creep strength. However, excessive addition promotes coarsening of the intermetallic compound, and rather reduces the creep strength. Therefore, it is set to 1 to 3%.

Al: Used as a deoxidizer, but excessive addition thereof remains as inclusions in the weld metal, leading to a decrease in creep ductility, so the content was made 0.03% or less.

Mg: Effective for reducing the sensitivity to hot cracking in welding and improving hot workability during wire processing. However, excessive addition is not preferable because it increases the inclusions in the weld metal. Therefore, it is set to 0.02% or less.

N: Indispensable for ensuring the high temperature strength of the weld metal. That is, N solid-dissolves in the matrix of the solidified structure to strengthen it, and partly precipitates as a nitride to strengthen the precipitation. However, excessive addition causes a large amount of carbonitride to precipitate during use at high temperature, which causes embrittlement. Therefore, it is set to 0.2 to 0.4%.

Mo: A high temperature strength can be enhanced by forming a solid solution in the matrix of the as-solidified structure. But 0.5
If it is less than%, the effect is small, and if it exceeds 1.5%, the effect is alleviated and the corrosion resistance is deteriorated. Therefore, when Mo is added, the amount is set to 0.5 to 1.5%.

Cu: Effectively contributes to high temperature tensile strength and creep strength. This is because fine precipitation occurs as a Cu-rich phase. In order to fully exert the effect, it is 1% or more. Excessive addition causes a decrease in ductility, so the content is made 4% or less.

B: It segregates at the grain boundaries to increase the strength of the grain boundaries, which is effective for improving the creep strength. However, in order to promote welding high temperature cracking, if added, the content should be 0.01% or less.

P, S: Since a eutectic having a low melting point is formed during solidification and hot welding cracks are generated, P <0.01% and P + S <0.02%. As a result, it is possible to suppress the weld hot cracking susceptibility associated with the addition of Nb and B.
Even with b and B added steel, sufficient weldability is secured and N
It becomes easy to secure high-temperature strength by adding b and B.

O: Since Cu is indispensable, Cu
It forms a low-melting-point eutectic by combining with and causes cracking during hot working. Therefore, as 0.007% or less,
Ensure the workability from billet to wire.

Ca: O is fixed without impairing the creep strength to prevent cracks generated during hot working. However, excessive addition induces instability of the welding arc during welding, and as a result, welding defects are likely to occur. Therefore, 0.002-
It was set to 0.02%.

[Conditions for Manufacturing Welding Material] In the manufacturing process, first, the ingot is made into a round bar by hot forging, and then this is made into a coarse wire rod by hot rolling.

The forging temperature is 1100 to 1250 ° C.
If the temperature is lower than 1100 ° C, the deformation resistance in the crystal grains exceeds the strength of the crystal grain boundaries, causing grain boundary cracking. On the other hand, if it exceeds 1250 ° C, a part of the grain boundary is liquefied and cracks occur.

The hot rolling temperature is 1050 ° C to 1250 ° C. The reason is the same as that at the forging temperature, but the lower limit is 50 ° C. lower because grain boundary cracking is less likely to occur in rolling.

The outer diameter after hot rolling is 6 mm or more. 6 mm
If it is less than this, cooling of the outer surface becomes faster, and it becomes difficult to maintain the above-mentioned lower limit temperature.

After hot rolling, the outer diameter is reduced by cold rolling to obtain a wire having a predetermined outer diameter, but the workability is 40% or less in order to prevent breakage due to work hardening. The work-hardened wire is softened by heat treatment.
If the temperature is lower than 0 ° C, the effect is small, and if the temperature exceeds 1250 ° C, the surface is significantly oxidized.
The temperature is 250 ° C.

[0048]

EXAMPLES Examples of the present invention will be shown below, and the effects of the present invention will be clarified by comparison with Comparative Examples.

Using austenitic steel having the chemical composition shown in Table 1 as a base metal, the performance comparison of the welding materials was carried out. This base material is 22Cr and contains Cu, Mo, W, Nb, B
And its creep strength at 700 ° C. for 100,000 hours (10 ⁵ hours) is 9 kgf / mm ² . The welding materials were six kinds shown in Tables 2 and 3, and all were manufactured by vacuum melting in a laboratory and then processing into a wire having an outer diameter of 2 mm.

In the comparative test, first, as shown in FIG.
The pipe base materials 1 and 1 provided with the predetermined groove 4 were restrained and welded to the restraint rod 2 made of steel. 3 is a restraint welded portion. Then
Multi-layer welding was performed on the groove 4 with the test welding material. The restrained welding causes thermal stress during welding to the groove, which easily causes cracking. The TIG method was used for welding.

After welding, a side bending test piece shown in FIG. 2 (A) was sampled by bending, and this was bent 180 degrees with a bending radius (10 mm) twice the plate thickness, and welding hot cracking in weld metal 5 was performed. Was checked for. Further, a high temperature tensile test and a creep test were conducted using the test piece shown in FIG.

In the creep test, the fracture time of the weld metal 5 was examined by conducting a test under the conditions of 700 ° C. and 16 kgf / mm ² at which the fracture time in the base metal was about 3000 hours. Then, 80% of the base material rupture time was used as a criterion for determining the quality, and those that did not reach this were determined to have insufficient creep strength. The results are shown in Table 3. Welding hot cracking ○ indicates no cracking, × indicates cracking, creep strength ○ indicates exceeding the above-mentioned criteria, and × indicates insufficient strength.

A1 to A7 satisfy the component conditions of the present invention. These welding materials showed good weld hot crack resistance and creep strength. On the other hand, when the component conditions of the present invention were not satisfied, weld cracking occurred like B3 and B4, and creep strength was insufficient like B1, B2 and B5 to B7. In addition, there are cases where it cannot be processed into a wire rod like B8 and B9.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

Steels A1, A2 and B shown in Tables 2 and 3
For No. 7 and B8, an ingot having a weight of 150 kg and an outer diameter of about 300 mm was vacuum-melted, held at a predetermined temperature for 4 hours in a heating furnace, and then hot forged into a round bar having an outer diameter of 60 mm. Further, the obtained round bar was held in a heating furnace for 2 hours and then hot-rolled into a wire having a predetermined outer diameter. The presence or absence of cracks was visually examined in each step. The results are shown in Table 4.

[0058]

[Table 4] * Indicates outside the scope of the present invention

As shown in WB1 to WB3, cracking occurred under forging and rolling conditions outside the scope of the present invention. WB4
As shown in No. 5, even within the forging and rolling conditions of the present invention, cracking occurs if the chemical composition is outside the range of the invention. On the other hand, like WA to WA4, those satisfying the chemical composition, forging conditions, and rolling conditions within the scope of the present invention were able to be satisfactorily processed into a wire without cracking.

[0060]

As described above, the welding material of the present invention imparts particularly excellent high temperature strength to the weld metal without sacrificing the weld hot cracking resistance. Moreover, it has excellent hot workability, which tends to be lacking in this kind of welding material.

In addition, the welding material manufacturing method of the present invention eliminates obstacles in commercialization by preventing cracking during hot working for processing into a wire rod, and puts high-performance products on the market. Can be provided.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of restraint welding used in a welding test.

FIG. 2 is an explanatory diagram of a test piece taken from a joint.

Claims (3)

[Claims] 1. C: 0.03 to 0.13% by weight, Si:
0.1-0.5%, Mn: 0.6-6%, Cr: 20-24
%, Ni: 6 to 21%, Nb: 0.01 to 0.7%, Mo:
0.5-1.5%, W: 1-3%, Cu 1-4%, N: 0.2
~ 0.4%, Al: 0.5% or less, Ca: 0.002 to 0.02
%, Mg: 0.02% or less, O: 0.01% or less, P: 0.0
1% or less, including P + S: 0.02% or less, -8 ≦ A _P ≦ 2 A _P = Ni + 0.5Mn + 30 (C + N) -1.1 (Cr +
1.5Si + Mo + 0.5W), and the balance is Fe and inevitable impurities, and is a high-strength austenitic steel welding material. 2. A high-strength austenitic steel welding material, which further contains B: 0.01% or less in addition to the components described in claim 1. 3. A method for producing the high-strength austenitic steel welding material according to claim 1 or 2, wherein the raw material is 1
After forging at 100 ° C to 1250 ° C, 1050 to 1250
Hot rolled into a coarse wire with an outer diameter of 6 mm or more at ℃
% Or less of cold working and heat treatment at 1000 to 1250 ° C. are repeatedly processed into a wire rod.