JP7002197B2 - High Ni alloy with excellent intergranular corrosion resistance and pitting corrosion resistance - Google Patents

Description

This application relates to a high Ni-based alloy having excellent intergranular corrosion resistance and pitting corrosion resistance, which is used for chemical plant members, pipes, heat exchangers using seawater, and the like.

Intergranular corrosion is corrosion that progresses along the grain boundaries of a metallic material, and this occurs due to local corrosion caused by impurities or segregation of alloying elements into the grain boundaries. Taking austenitic stainless steel as an example, intergranular corrosion often occurs because chromium carbides are generated at the grain boundaries due to improper reheating or the operating environment temperature, which creates a chromium-deficient area around them. That is, it is considered that if the grain boundary coverage of the carbide is large, the surrounding chromium-deficient region increases and the corrosion resistance is greatly reduced.

Previous patents have proposed alloys having excellent corrosion resistance in various corrosive environments such as chemical plant members, pipes, and heat exchangers using seawater (see, for example, Patent Document 1). This patent focuses specifically on hot and cold processability in addition to corrosion resistance. However, Patent Document 1 does not describe the effect of the grain boundary coverage of carbides on corrosion resistance.

Further, a patent relating to a metal tube made of an austenitic alloy and a method for producing the same has been proposed (see, for example, Patent Document 2). This patent adjusts the ratio of tensile yield strength and compressive yield strength in the axial and circumferential directions of the metal pipe, so that austenite can withstand different stress distributions depending on the usage environment of the metal pipe. It relates to a metal tube made of an alloy. However, this Patent Document 2 does not describe the influence of the grain boundary coverage of carbides on the corrosion resistance, as in Patent Document 1.

Japanese Patent No. 5748216 Japanese Patent No. 5137048

For applications that require high corrosion resistance, such as chemical plants and heat exchangers, excellent intergranular corrosion resistance and pitting corrosion resistance are required. Although no reference has been made to the mechanism for improving intergranular corrosion resistance in the literature on the prior art, the present invention was inspired by the finding that the intergranular corrosion resistance of carbides affects the intergranular corrosion resistance. I got it.

Therefore, an object to be solved by the present invention is to provide a high nickel alloy having excellent intergranular corrosion resistance and corrosion resistance in alloys in applications such as chemical plants and heat exchangers.

The means for solving the problem of the present invention is, in the first means, by mass%, C: 0.001 to 0.100%, Si: 0.01 to 1.50%, Mn: 0.01. ~ 1.50%, Ni: 30.00-50.00%, Cr: 18.50 ~ 25.00%, Mo: 2.00 ~ 10.00%, Cu: 0.10 ~ 5.00%, It is a Ni alloy containing Al: 0.010 to 2.500%, Ti: 0.010 to 2.500%, Fe: ≧ 20.00%, and 100% including other unavoidable impurities. Intergranular corrosion index IRE value: Equation (1) = 1.08 ([% Cr] + [% Ti])-4.34 [% C]: ≧ 22, Porous corrosion resistance index PRE value: Equation (2) = [% Cr] +3.3 [% Mo] +16 [% N]: ≧ 32, intergranular corrosion resistance: ≦ 13%, which is a high Ni alloy having excellent intergranular corrosion resistance and pore corrosion resistance. be. In addition, all of the above [% element] are mass%.

In the second means, in addition to the constituent requirements of the first means, N: 0.0001 to 0.0500% is contained in mass%, and B: 0.0001 to 0.0250%, Ca: 0. Excellent intergranular corrosion resistance and pore corrosion resistance, characterized by containing one or more of one or more elements of .0001 to 0.0250% and Mg: 0.0001 to 0.0250%. It is a high Ni alloy. In addition, all of the above [% element] are mass%.

The third means has intergranular corrosion resistance and resistance to intergranular corrosion, which is characterized by having S: ≤ 0.0150% in% by mass, in addition to the constituent requirements of the first means or the second means. A high Ni alloy with excellent pitting corrosion properties. In addition, all of the above [% element] are mass%.

By using the above means, it is possible to obtain a high nickel alloy having excellent corrosion resistance, intergranular corrosion resistance and pore corrosion resistance in corrosion resistant alloys for applications such as chemical plants and heat exchangers.

Prior to the description of the embodiment for carrying out the invention, each chemical component of the high Ni alloy of the first invention, the second invention and the third invention of the present application, and intergranular corrosion resistance, pore corrosion resistance, and grain boundary. Each property of coverage is described below.

C: 0.001 to 0.100%
C is an element that affects the strength, hot workability and cold workability, intergranular corrosion resistance index, pitting corrosion resistance index, and grain boundary coverage of Ni alloys. If C is less than 0.001%, the strength of the Ni alloy is insufficient. On the other hand, when C exceeds 0.100%, the hot workability and cold workability of the Ni alloy are lowered, and the intergranular corrosion resistance index, the pitting corrosion resistance index, and the grain boundary coverage are further lowered. .. Therefore, C is set to 0.001 to 0.100%.

Si: 0.01-1.50%
Si is an element that acts as a deoxidizing agent and affects hot workability and cold workability. If Si is less than 0.01%, it is insufficient as a deoxidizing agent. On the other hand, when Si exceeds 1.50%, the hot workability and cold workability of the alloy material deteriorate. Therefore, Si is set to 0.01 to 1.50%.

Mn: 0.01 to 1.50%
Mn is an element that affects hot workability and further an element that affects the austenite phase. When Mn is less than 0.01%, the hot workability of the alloy material is lowered and the austenite phase becomes unstable. On the other hand, if Mn exceeds 1.50%, the hot workability deteriorates. Therefore, Mn is set to 0.01 to 1.50%.

Cr: 18.50 to 25.00%
Cr is an element that affects the intergranular corrosion resistance index and pitting corrosion resistance, and is an element that affects hot workability and cold workability. If Cr is less than 18.50%, the intergranular corrosion resistance index and pitting corrosion resistance are lowered. On the other hand, when Cr exceeds 25.00%, hot workability and cold workability deteriorate. Therefore, Cr is set to 18.50 to 25.00%.

Mo: 2.00 to 10.00%
Mo is an element that affects pitting corrosion resistance, and is an element that affects hot workability and cold workability. If Mo is less than 2.00%, the corrosion resistance is lowered. On the other hand, when Mo exceeds 10.00%, the hot workability and the cold workability are lowered, and the cost is increased. Therefore, Mo is set to 2.00 to 10.00%.

Cu: 0.10 to 5.00%
Cu is an element that affects the corrosion resistance and further an element that affects the austenite phase. Further, Cu is an element that affects hot workability. If Cu is less than 0.10%, the corrosion resistance is lowered and the austenite phase becomes unstable. On the other hand, when Cu exceeds 5.00%, the hot workability deteriorates. Therefore, Cu is set to 0.10 to 5.00%.

Al: 0.010 to 2.500%
Al is an element that acts as a deoxidizing agent and is effective for strength and toughness. If Al is less than 0.010%, it is insufficient as a deoxidizing agent, and the strength and toughness of the high Ni alloy are insufficient. On the other hand, when Al is more than 2.500%, the hot workability and the cold workability are lowered. Therefore, Al is set to 0.010 to 2.500%.

Ti: 0.010 to 2.500%
Ti is an element that affects the intergranular corrosion resistance index, and is an element that affects hot workability and cold workability. If Ti is less than 0.010%, the intergranular corrosion resistance index decreases. On the other hand, when Ti exceeds 2.500%, hot workability and cold workability deteriorate. Therefore, Ti is set to 0.010 to 2.500%.

Fe: ≧ 20.00%
Fe is an element that affects hot workability and cold workability. When Fe is less than 20.00%, hot workability and cold workability are deteriorated. Therefore, Fe is set to 20.00% or more.

Ni: 30.00-50.00%
Ni is the most abundant element in this high Ni alloy as the remainder of each chemical component of the above high Ni alloy, and is a metal contained as 30.00 to 50.00% in the total alloy. .. It is a high Ni alloy having 100% of the above alloy elements other than Ni and other unavoidable impurities.

Intergranular corrosion Resistance Equivalent: ≧ 22
If the intergranular corrosion resistance index is less than 22, intergranular corrosion progresses. Therefore, the IRE is set to 22 or more.

Pitting Resistance Equivalent: ≧ 32
If the pitting corrosion resistance index is less than 32, pitting corrosion will occur. Therefore, PRE is set to 32 or more.

Grain boundary coverage: ≤13%
If the grain boundary coverage is greater than 13%, intergranular corrosion progresses. Therefore, the grain boundary coverage is set to 13% or less.

The above are the constituent requirements of the first means of the present invention.
The above-mentioned intergranular corrosion resistance index: formula (1) and pitting corrosion resistance index: formula (2) have IRE values = 1.08 × ([% Cr] + [% Ti]) 4.34 ×, respectively. [% C] ... (1)
PRE value = [% Cr] +3.3 x [% Mo] +16 x [% N] ... (2)
Is.
[% Element] in the above formulas (1) and (2) indicates the amount of contained elements represented by mass%.

The second means contains N: 0.0001 to 0.0500%, B: 0.0001 to 0.0250%, and Ca: 0.0001 to 0, in addition to the constituent requirements of the first means. .0250%, Mg: 0.0001 to 0.0250%, which is a high Ni alloy having excellent intergranular corrosion resistance and pore corrosion resistance, which is characterized by containing one or more elements. be.

N: 0.0001 to 0.0500%
N is an element that acts on the maintenance of the austenite phase of the high Ni alloy and contributes to corrosion resistance and cold workability. By the way, when N is less than 0.0001%, the austenite phase of the high Ni alloy becomes unstable. On the other hand, if N is more than 0.0500%, the corrosion resistance of the high Ni alloy is lowered, and the cold workability is further lowered. Therefore, N is set to 0.0001 to 0.0500%.

One or more elements of B, Ca, Mg: 0.0001 to 0.0250%
When any one or more elements of B, Ca, and Mg are selectively contained, it contributes to the hot workability of the high Ni alloy. By the way, if the element of any one or more of B, Ca and Mg is less than 0.0001%, it does not contribute to the hot workability of the high Ni alloy. On the other hand, when the amount of any one or more of B, Ca and Mg is more than 0.0250%, the hot workability of the high Ni alloy is lowered. Therefore, the element of any one or more of B, Ca, and Mg is set to 0.0001 to 0.0250%.

In the third means, in addition to the constituent requirements of the first means or the constituent requirements of the second means described above, intergranular corrosion resistance is characterized by containing S: ≦ 0.0150% in% by mass. A high Ni alloy with excellent properties and corrosion resistance.

S: ≦ 0.0150%
S is an element that contributes to machinability and has the effect of facilitating cutting such as mechanization. However, if S is contained in an amount of more than 0.0150%, the hot workability is deteriorated. Therefore, S is set to 0.0150% or less.

The high Ni alloy of the developed steel 1 having the chemical components shown in Table 1, the high Ni alloys of the developed steel 2 and the developed steel 3 having the chemical components shown in Table 2, and the Ni of the comparative steel 1 having the chemical components shown in Table 3. The alloy, the Ni alloy of Comparative Steel 2 and the high Ni alloy of Comparative Steel 3 were melted in 100 kg each with VIM, and they were cast into an ingot. Then, they were forged to a diameter of 20 mm, held at 940 ° C. for 120 minutes, and then subjected to a water-cooled heat treatment.

The chemical composition values of the high Ni alloys of the developed steel 1, the developed steel 2 and the developed steel 3 and the high Ni alloys of the comparative steel 1, the comparative steel 2 and the comparative steel 3 were investigated by fluorescent X-ray and wet analysis, and the formula (1). The IRE (Intergranular corrosion Resistance Equivalent) value, which is the intergranular corrosion resistance index represented by), and the PRE (Pitting Resistance Equivalent) value, which is the pore corrosion resistance index represented by the formula (2), were calculated. Regarding the test results, Table 1 shows the high Ni alloy of the developed steel 1 , Table 2 shows the high Ni alloy of the developed steel 2 and the developed steel 3 , and Table 3 shows the comparative steel 1, the comparative steel 2 and the comparative steel 3. The values of high Ni alloys are shown.
Here, the IRE value of the formula (1) and the PRE value of the formula (2) are shown.
PRI value = 1.08 x ([% Cr] + [% Ti])-4.34 x [% C] ... (1)
PRE value = [% Cr] +3.3 x [% Mo] +16 x [% N]
However, [% M] is a numerical value of mass%.

Further, Table 4 shows the characteristics of the high Ni alloy of the developed steel 1 , and Table 5 shows the characteristics of the high Ni alloy of the developed steel 2 and the developed steel 3 . On the other hand, Table 6 shows the characteristics of the comparative steel 1, which is a comparative example corresponding to claim 1, the comparative steel 2 and the comparative steel 3 corresponding to claim 2, respectively. The hot workability in Tables 4, 5 and 6 was evaluated by judging from the state of occurrence of cracks during forging, and those without cracks were designated as GOOD and those with cracks were designated as NG.

For the above forged materials, the cold workability, the grain boundary coverage of carbides, the degree of erosion according to the ASTM A262-C method, and the degree of corrosion according to the ASTM G48-A method are evaluated by the following evaluation methods. It was evaluated and listed in Tables 4, 5 and 6 as the properties of the developed steel and the properties of the comparative steel.

The grain boundary coverage in Tables 4, 5 and 6 is determined by measuring the grain boundary coverage of carbides, which is the microstructure of the longitudinal cross section after adjusting the forged material with a diameter of 20 mm to a length of 15 mm. It is a value obtained by carrying out the observation of. That is, the grain boundary coverage is a value obtained by dividing the total length of precipitates on the large angle grain boundaries by the total length of the large angle grain boundaries, and 100% of the cases where the grains are completely covered are completely covered. It is a parameter to judge 0% when it is not. First, the particles deposited on the large-angle grain boundary by observation with a 5000x electron microscope are subjected to energy dispersive X-ray spectroscopic analysis (EDS) or transmission electron diffraction pattern analysis with a 5000x thin film transmission electron microscope. Precipitates that can be determined to be M ₂₃ C ₆ type charcoal or M ₇ C ₃ type charcoal shall be identified. Furthermore, the length of the particles covering the large-angle grain boundary is measured, and the measurement is performed with at least 10 visual fields per sample and 5 or more samples per alloy, and in-situ observation with a total of 50 visual fields or more. , Or the grain boundary coverage of the charcoal was obtained by electron microscopic analysis.

The degree of corrosion according to the ASTM A262-C method is adjusted to a size of 12 mm in diameter and 21 mm in length after holding it at 675 ° C for 60 minutes and then performing an air-cooling sensitizing heat treatment. -It was obtained by conducting an intergranular corrosion test according to the C method. Specifically, after measuring the initial mass and surface area of the test piece, the test piece was immersed in 65% boiling nitric acid for 48 hours, washed after the test, and the mass measurement was performed again. The degree of erosion (mm / year) was calculated from the following formula (A). This operation was repeated 5 times, and the degree of erosion each time and the total degree of erosion derived from the initial mass and the final mass were calculated to evaluate the intergranular corrosion resistance.
Formula (A): Degree of erosion (mm / year) = (12 × 7290 × W) / (A × t × d)
The symbols indicate the following values.
W: Mass reduction (g), A: Surface area (cm ² ), t: Immersion time (h), d: Density (g / cm ³ )

For the degree of corrosion according to the ASTM G48-A method, the test material was adjusted to a size of 12 mm in diameter and 21 mm in length, and a pitting corrosion test was carried out according to the ASTM G48-A method. Specifically, after measuring the initial mass and surface area of the test piece, the test piece was immersed in 6% ferric chloride at 22 ° C. for 72 hours, washed after the test, and mass measurement was performed again. The degree of corrosion (g / cm ² ) was calculated from the obtained mass loss to evaluate the pitting corrosion resistance.

For cold workability, the test material was adjusted to a size of 14 mm in diameter and 21 mm in length, and a cold installation test was carried out. The installation was performed up to a processing rate of 60%, and the cold workability was evaluated by setting GOOD for those without cracks and NG for those with cracks.

Claims (2)

By mass%, C: 0.001 to 0.100%, Si: 0.01 to 1.50%, Mn: 0.01 to 1.50%, Ni: 30.00 to 50.00%, Cr: 18.50 to 25.00%, Mo: 2.00 to 10.00%, Cu: 0.10 to 5.00%, Al: 0.010 to 2.500%, Ti: 0.010 to 2. It contains 500%, Fe: ≧ 20.00% , N: 0.0052 to 0.0500%, and further B: 0.0001 to 0.0250%, Ca: 0.0001 to 0.0250%, Mg: It is a Ni alloy containing one or more elements from 0.0001 to 0.0250% and 100% including other unavoidable impurities. Intergranular corrosion resistance index IRE value: formula (1) = 1.08 ([% Cr] + [% Ti])-4.34 [% C]: ≧ 22, Intergranular corrosion resistance index PRE value: Equation (2) = [% Cr] +3.3 [ % Mo] + 16 [% N]: ≧ 32, grain boundary coverage: ≦ 13%, a high Ni alloy having excellent intergranular corrosion resistance and pitting resistance.
In addition, all of the above [% element] are mass%. A high Ni alloy having excellent intergranular corrosion resistance and pitting corrosion resistance, which is characterized by having S: ≦ 0.0150% in mass% in addition to the constituent requirements of claim 1.