JP6895787B2 - Austenitic stainless steel, brazed structures, brazed structural parts and exhaust gas heat exchange parts - Google Patents

Description

The present invention relates to austenitic stainless steel, brazed structures, brazed structural parts and exhaust gas heat exchange parts.

Since austenitic stainless steel containing a large amount of Cr, Ni, Mo, etc. has excellent corrosion resistance, brazing structures such as marine structures and flue gas desulfurization equipment and brazing structural parts such as automobile exhaust system parts are extremely difficult. Used in highly corrosive environments.

When the above-mentioned austenitic stainless steel is used for marine structures, there is a concern that it will be corroded by seawater having a high ^{Cl − concentration.} In particular, since there is a concern that the structure may be perforated or the life may be shortened due to corrosion due to ^{Cl −, the material is required to reduce the starting point of pitting corrosion.}

In addition, when used in a flue gas desulfurization device, when the exhaust gas temperature drops in the pipe, SO _x reacts with the moisture in the exhaust gas to form sulfuric acid, which condenses on the inner surface of the pipe, resulting in a severe corrosive environment. Be exposed to. Furthermore, in the actual environment, Cl ⁻ may be mixed, and the water content containing _{SO x} ^{and Cl − may be concentrated due to repeated drying and wetting.} Therefore, when austenitic stainless steel is used in a flue gas desulfurization apparatus, high corrosion resistance is also required.

Further, in recent years, in the automobile field, various regulations have been tightened because each component such _{as CO 2} , NO _x , and SO _{x contained in exhaust gas causes air pollution and environmental pollution.} Therefore, for the _{purpose of reducing CO 2} emissions and improving fuel efficiency of automobiles, not only improvement of engine efficiency by high-efficiency combustion and idling stop, weight reduction by material replacement, but also hybrid electric vehicle (HEV), biofuel, hydrogen / fuel Improvements are needed due to energy diversification of battery vehicles (FCVs), electric vehicles (EVs), and the like.

Among them, efforts are being made to improve fuel efficiency by installing a heat exchanger that recovers exhaust heat, a so-called exhaust heat recovery device. The exhaust heat recovery device transfers the heat of the exhaust gas to the cooling water by heat exchange, recovers and reuses the heat energy to raise the cooling water temperature, thereby improving the heating performance in the passenger compartment and the engine warm-up time. It is a system that shortens and improves fuel efficiency, and is also called an exhaust heat recirculation system.

Efforts are also being made to install an exhaust gas recirculation device that recirculates the exhaust gas. As an exhaust gas recirculation device, for example, there is an EGR cooler. The EGR cooler is a device that lowers the combustion temperature by cooling the exhaust gas of the engine with engine cooling water or air and then returning it to the intake side to reburn it, thereby reducing the amount of _{NO x, which is a harmful gas.}

The heat exchange section of such an EGR cooler or exhaust heat recovery device is required to have good thermal efficiency, good thermal conductivity, and excellent corrosion resistance to exhaust gas condensed water because it is in contact with exhaust gas. To. In particular, since engine cooling water flows through these parts, there is a risk of serious accidents if holes are created due to corrosion, and the materials used are thick to improve heat exchange efficiency. Since it is thin, a material having better corrosion resistance than the downstream member of the exhaust system is required.

The composition of the exhaust gas condensed water changes depending on the quality of the fuel. If purification of the fuel is large amount of S insufficient fuel, SO _x concentration in the exhaust gas becomes high, _SO ⁴ 2-exhaust gas condensed water, _SO ^{3 2-concentration} becomes higher. As a result, the pH of the exhaust gas condensed water becomes high, resulting in a severe corrosive environment. In addition, Cl ⁻ may be contained in the fuel. Cl ⁻ is contained in the exhaust gas condensed water via the exhaust gas and may cause pitting corrosion.
As described above, the corrosiveness of the exhaust gas condensed water greatly depends on the quality of the fuel. In particular, ^Cl exhaust gas condensed water _-, ^SO 4 2-, _{regions SO} ^{3 2-concentration} using a higher fuel are called inferior fuel region.

Conventionally, among the exhaust system downstream members mainly composed of mufflers, ferritic stainless steels containing 17% or more of Cr such as SUS430LX, SUS436J1L, and SUS436L have been used for the parts where corrosion resistance is particularly required. Corrosion resistance higher than these is required for the material of the heat recovery device and EGR cooler, and highly corrosion-resistant ferritic stainless steel such as SUS444 is used. Further, corrosion resistance is required for the material of the exhaust heat recovery device and the EGR cooler used in the poor fuel area, and high alloy austenitic stainless steel containing a large amount of Ni, Cr, Mo and the like is used.

Here, the EGR cooler and the exhaust heat recovery device are generally assembled by brazing, but the high alloy austenitic stainless steel is subjected to vacuum heat treatment such as brazing to reduce the corrosion resistance of the material. May be done. The cause is the precipitation of Cr and Mo-rich intermetallic compounds called the σ phase. It is considered that when the σ phase is precipitated by brazing, the Cr and Mo concentrations in the base material in the vicinity of the σ phase decrease and become the starting point of corrosion.

Therefore, not only after manufacturing, but also when brazing is applied to various structures and plants, and when manufactured in automobile parts such as EGR coolers and exhaust heat recovery devices, the σ phase is unlikely to precipitate and the σ phase is hard to precipitate. Austenitic stainless steel with a small decrease in corrosion resistance is required even if

In Patent Document 1, in% by weight, C: ≤0.03, Si: ≤0.5, Mn: ≤6.0, Cr: 28 to 30, Ni: 21 to 24, (Mo + W / 2): 4 ~ 6, of which W: ≤0.7, N: 0.5 to 1.1, Cu: ≤1.0, and the balance is composed of Fe and impurities of normal content originating from the production of steel. The steel is disclosed, which is characterized by having a PRE value of at least 60 or more, which is represented by PRE = Cr + 3.3Mo + 1.65W + 30N. We have proposed a stainless steel that suppresses σ phase formation by suppressing the Mo content that promotes σ phase formation and containing N instead of Mo, and has high corrosion resistance. However, if the Mo content is kept low, the effect of suppressing the dissolution rate when pitting corrosion occurs in the exhaust gas condensed water environment is reduced. , When pitting corrosion occurs, there is a risk of piercing through in a short period of time.

Patent Document 2 describes C: 0.001 to 0.030% by mass, Si: 0.10 to 0.70% by mass, Mn: 0.10 to 1.00% by mass, P: 0.005 to 0. 045% by mass, S: 0.003% by mass or less, Ni: 18.0 to 40.00% by mass, Cr: 20.00 to 30.00% by mass, Cu: 2.00% by mass or less, Mo: 3. It has a composition of 00 to 8.00% by mass, N: 0.05 to 0.30% by mass, Al: 0.13% by mass or less, Cr + 2Mo + 0.5Ni ≧ 40, and the balance is substantially Fe. , Austenite-based stainless steel for exhaust gas flow path members is disclosed. The result of evaluation of the above steel by the boiling dew condensation test is shown, but the corrosion resistance after the σ phase is precipitated by brazing is not mentioned.

Patent Document 3 describes C: 0.015 mass% or less, Si: 0.01 to 0.20 mass%, Mn: 0.01 to 2.0 mass%, P: 0.020 mass% or less, S: 0.010 mass%. Hereinafter, Ni: 10 to 30 mass%, Cr: 16 to 30 mass%, N: 0.20 mass% or less are contained, and the formula (1); 20 × Si + 100 × P ≦ 4.5 ... (1) ( However, Si and P in the above formula indicate an austenite-based stainless steel containing Si and P satisfying the content (mass%) of each component, and the balance is Fe and unavoidable impurities. ing. Although the intergranular corrosion resistance is improved by lowering the P and Si, there is no mention of the corrosion resistance after the σ phase is precipitated by brazing.

Patent Document 4 describes C: 0.030% by mass or less, Si: 0.10 to 0.70% by mass, Mn: 0.10 to 2.00% by mass, Ni: 10.00 to 40.00% by mass. , Cr: 17.00 to 30.00% by mass, P: 0.005 to 0.40% by mass, S: 0.0005 to 0.003% by mass, Cu: 0.01 to 0.5% by mass, Mo : 1.00 to 6.00% by mass, Al: 0.1% by mass or less, N: 0.005 to 0.050% by mass, Nb: 2.00% by mass or less, and Nb / (C + N) ≧ For an exhaust gas flow path member having a chemical composition of the balance Fe and unavoidable impurities satisfying 20, having a solid solution C amount of 0.005% by mass or less, no loves phase precipitation, and excellent corrosion resistance. Austenite-based stainless steel materials and methods for producing the same are disclosed. Although the intergranular corrosion resistance is improved by controlling the heat treatment time, there is no mention of the corrosion resistance after the σ phase is precipitated by brazing.

Patent Document 5 describes in terms of mass%, C: 0.08% or less, Si: 4.0% or less, Mn: 1.5% or less, P: 0.05% or less, S: 0.005% or less, Cr: 20 to 30%, Ni: 20 to 35%, Mo: 3 to 8%, N: 0.02 to 0.3%, Al: 0 (additive-free) to 4.0%, Cu: 0 (none) (Addition) ~ 4.0%, La + Ce: 0 (no addition) ~ 0.3% or less, B: 0 (no addition) ~ 0.05%, the balance consists of Fe and unavoidable impurities, and the σ phase is 1. Austenite-based stainless steel sheets having a volume of 0.0% by volume or less are disclosed. By controlling the hot rolling conditions, σ-phase precipitation is suppressed to improve ductility and workability, but σ-phase precipitation after brazing and its effect on corrosion resistance are not mentioned.

Patent Document 6 describes, in terms of mass%, C: 0.1% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.1% or less, S: 0.005% or less, Ni: 20.0 to 37.0%, Cr: 18.0 to 28.0%, Mo: 4.0 to 7.0%, Cu: 0.5 to 2.5%, N: 0.10 to 0 It contains 0.40%, Al: 0.001 to 0.10%, and consists of the balance Fe and unavoidable impurities. The PI value defined by the following equation (1) is 44.0 to 50.0, and the following ( The GI value defined by the formula 2) is 120.0 to 150.0, the Md value defined by the formula (3) below is less than 0.920, and the Mdc value defined by the formula (4) below is 0.40. A high alloy austenite-based stainless steel plate having less than 1.0% and an area ratio of less than 1.0% in the σ phase at the center of the plate thickness is disclosed.
PI = Cr + 3.3Mo + 16N ... (1)
GI = -Cr + 4Ni + 5Mo + 18Cu ... (2)
Md = (1.142Cr + 0.717Ni + 1.550Mo + 0.957Mn + 0.6515Cu + 1.33Si + 1.9Al + 0.858Fe-1.4N-0.43C) / 100 ... (3)
Mdc = (Md-0.91) × (Cr + 2Mo)… (4)
However, the element symbol in the formulas (1), (2) and (4) is the content rate (mass%) of each element, and the element symbol in the formula (3) is the content rate (atomic%) of each element. Is.
By increasing the PI value, the occurrence of corrosion is suppressed, and by increasing the GI value, the dissolution rate when corrosion occurs is slowed down. Further, by lowering the Md value and Mdc value, the precipitation of the σ phase at the center of the plate thickness is suppressed, but the precipitation of the σ phase after brazing and the depth from the surface are adjusted by adjusting the cooling conditions. There is no mention of reducing the proportion of sigma phase precipitation area in the range of 0.1 μm and further increasing corrosion resistance.

Special Table 2008-525643 Japanese Unexamined Patent Publication No. 2013-199661 Japanese Unexamined Patent Publication No. 2014-34694 JP 2015-189990 JP-A-2002-322545 International Publication No. 2016/076254

The present invention is used in brazed structures such as marine structures and flue gas desulfurization equipment, and brazed structural parts such as exhaust gas heat exchange parts such as exhaust heat recovery devices and EGR coolers which are automobile parts. , It is an object of the present invention to provide an austenitic stainless steel showing excellent corrosion resistance.

In order to solve the above-mentioned problems, the present inventors have produced austenitic stainless steel containing various additive elements, examined the corrosion resistance after various heat treatments, and adjusted the chemical composition of the steel. Then, by satisfying Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 50.000 and adjusting the cooling rate, the σ phase precipitation area in the range of 0.1 μm from the surface. It has been found that the ratio can be set to 1.00% or less, and a steel having good corrosion resistance can be obtained.

The gist of the present invention for solving the above problems is as follows.
[1] By mass%
C: 0.001 to 0.012%,
Si: 0.37 to 0.79%,
Mn: 0.01 to 0.89 %,
P: 0.050% or less,
S: 0.0050% or less,
Ni: 17.000 to 35.00%,
Cr: 18.0 to 30.00%,
Mo: 4.00 to 7.49%,
Cu: 0.10 to 3.00%,
N: 0.100 to 0.400% and Al: 0.001 to 0.100%
An austenitic stainless steel containing ferritic stainless steel, the balance of which is composed of Fe and unavoidable impurities, and which satisfies the following formula (1).
Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 50.000 ・ ・ ・ (1)
However, the element symbol in the above formula (1) means the content mass% of the element in steel.
[2] Further, in% by mass,
B: 0.0001 to 0.0050%,
Ti: 0.001 to 0.300%,
Nb: 0.001 to 0.300%,
W: 0.01 to 1.00%,
V: 0.01-0.50%,
Sn: 0.001 to 0.500%,
Sb: 0.005 to 0.500% and Co: 0.010 to 0.500%
The austenitic stainless steel according to [1], which contains any one or more of the above.
[3] Further, in% by mass,
Mg: 0.0001 to 0.0050%,
Zr: 0.001 to 0.300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050% and REM: 0.001 to 0.100%
The austenitic stainless steel according to [1] or [2], which contains one or more of the above.
[4] The austenitic stainless steel according to any one of [1] to [3], wherein the ratio of the σ-phase precipitation area in the range of 0.1 μm from the surface is 1.00% or less. ..
[5] The austenitic stainless steel according to any one of [1] to [4] used for exhaust gas heat exchange parts.
[6] The austenitic stainless steel according to any one of [1] to [4] used for brazed structural parts.
[7] Any one of [1] to [3], wherein the brazed steel has a σ-phase precipitation area ratio of 1.00% or less in a depth range of 0.1 μm from the surface. Austenitic stainless steel having the steel components described in.
[8] The brazed structure made of austenitic stainless steel according to [7].
[9] The brazed structural component made of austenitic stainless steel according to [7].
[10] The exhaust gas heat exchange component made of austenitic stainless steel according to [7].

According to the present invention, it is used for brazing structures such as marine structures and flue gas desulfurization equipment, and brazing structural parts such as exhaust gas heat exchange parts such as exhaust heat recovery devices and EGR coolers which are automobile exhaust system parts. Austenitic stainless steels that exhibit excellent corrosion resistance can be provided.

The figure which shows the relationship between the amount of Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu in steel and the σ phase precipitation ratio, and the result of the condensed water corrosion test after brazing.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the following description, as an example, an exhaust gas heat exchange component that is required to have better corrosion resistance due to a particularly severe usage environment among the above-mentioned application examples will be described.
Exhaust gas heat exchange parts such as an exhaust heat recovery device and an EGR cooler are exposed to the exhaust gas condensed water environment after brazing and joining, so that the usage environment is particularly severe among the above-mentioned application examples. The present inventors prepared steel sheets having various compositions, prepared test pieces subjected to heat treatment corresponding to brazing, and conducted a condensed water corrosion test. In addition, the ratio of σ-emerged area output in the range of 0.1 μm from the surface was investigated.

FIG. 1 shows the relationship between the amount of Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu in steel and the σ phase precipitation ratio, and the results of the condensed water corrosion test after brazing. Here, the experimental points in FIG. 1 are extracted from Table 1 (steel types No. A1 to No. 8, No. A11, No. A12, steel types No. B5 to No. B8) of Examples described later. The criteria for the condensed water corrosion test were as follows: those having a maximum pitting corrosion depth of less than 100 μm were marked with ◯, and those having a maximum pitting depth of 100 μm or more were marked with x.

From FIG. 1, it can be seen that the steel grades that satisfy the following formula (1) and are within the scope of the present invention have excellent corrosion resistance after brazing. On the other hand, it can be seen that the steel grades that do not satisfy the following formula (1) and are outside the scope of the present invention are inferior in corrosion resistance after brazing.
The element symbol in the following formula means the mass% of the element contained in the steel. The Fe content in the following formula (1) was determined as the total amount of the balance excluding the active ingredient of the present invention.

Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 50.000 ・ ・ ・ (1)

From FIG. 1, it can be seen that the steel grades that do not satisfy the above formula (1) have a high σ phase precipitation area ratio. From this, it is considered that the corrosiveness is inferior because the region where the Cr and Mo concentrations in the vicinity of the σ phase precipitated in a large amount is the starting point of corrosion.

The above-mentioned defined formula (1) is more preferably Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 45.000, and further preferably Fe + 1.2 × Cr + 1.5 × Mo. -2 x Ni-5 x Cu ≦ 40.000. That is, it is more preferable to satisfy the following formula (2), and it is further preferable to satisfy the following formula (3).

Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 45.000 ・ ・ ・ (2)
Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 40.000 ・ ・ ・ (3)

The chemical composition of the steel specified in the present invention will be described in more detail below. In addition,% means mass%.

C: 0.001 to 0.100%
Since C lowers intergranular corrosion resistance and workability, it is necessary to keep its content low. Therefore, the upper limit of the C content is set to 0.100% or less. However, excessive reduction promotes grain coarsening and sensitization during brazing, σ phase precipitation, and raises the refining cost. Therefore, the lower limit of the C content is set to 0.001% or more. The preferred C content is 0.003 to 0.050%, more preferably 0.005 to 0.020%.

Si: 0.01-1.00%
Si is useful as a deoxidizing element, but if it is contained in excess, it promotes the precipitation of the σ phase, so the content thereof is set to 0.01 to 1.00%. The preferred Si content is 0.02 to 0.80%, more preferably 0.03 to 0.70%.

Mn: 0.01 to 1.00%
Mn is useful as a deoxidizing element, but if it is contained in excess, the corrosion resistance is deteriorated, so the content thereof is set to 0.01 to 1.00%. The preferred Mn content is 0.02 to 0.80%, more preferably 0.03 to 0.70%.

P: 0.050% or less P is an element that deteriorates workability, weldability, and corrosion resistance, and its content needs to be limited. Therefore, the upper limit of the P content is set to 0.050% or less. The upper limit of the preferable P content is 0.040% or less, and more preferably 0.030% or less.

S: 0.0050% or less Since S is an element that deteriorates corrosion resistance, it is necessary to limit its content. Therefore, the upper limit of the S content is set to 0.0050% or less. The upper limit of the preferable S content is 0.0030% or less, and more preferably 0.0010% or less.

Ni: 17.000 to 35.00%
The content of Ni is required to be 17.00% or more in order to secure corrosion resistance in an exhaust gas condensed water environment in a poor fuel area, improve brazing property, and suppress σ phase precipitation. Therefore, the lower limit of the Ni content is set to 17.00% or more. However, since the alloy cost increases if it is contained in excess, the upper limit is set to 35.00% or less. The preferred Ni content is 17.20 to 28.00%, more preferably 17.50 to 26.00%.

Cr: 18.0 to 30.00%
Since Cr is required to have at least 18.00% or more in order to secure corrosion resistance in the exhaust gas condensed water environment of the poor fuel area, the lower limit of the Cr content is set to 18.00% or more. Corrosion resistance improves as the content is increased, but in order to reduce processability and manufacturability, the upper limit of Cr content is set to 30.00% or less. The preferred Cr content is 18.50 to 27.00%, more preferably 19.0 to 24.50%.

Mo: 4.00 to 8.00%
Since Mo is required to have at least 4.00% or more in order to secure corrosion resistance in the exhaust gas condensed water environment of the poor fuel area, the lower limit of the Mo content is set to 4.00% or more. However, since increasing the content promotes the precipitation of the σ phase, the upper limit of the Mo content is set to 8.00% or less. The preferred Mo content is 4.50 to 7.50%, more preferably 5.00 to 7.00%.

Cu: 0.10 to 3.00%
Cu is an important element for improving corrosion resistance and suppressing precipitation of the σ phase, and is required to be at least 0.10% or more. Therefore, the lower limit of the Cu content is set to 0.10% or more. However, if it is excessively contained, cracks will occur during hot working, so the upper limit of the Cu content is set to 3.00% or less. The Cu content is preferably 0.30 to 2.00%, more preferably 0.55 to 1.50%.

N: 0.1000 to 0.400%
Since N is required to be at least 0.100% or more in order to improve the corrosion resistance and further suppress the precipitation of the σ phase, the lower limit of the N content is set to 0.100% or more. However, if it is excessively contained, the sensitivity to generate bubbles during casting is increased, so the upper limit of the N content is set to 0.400% or less. The preferred N content is 0.120 to 0.350%, more preferably 0.150 to 0.300%.

Al: 0.001 to 0.100%
Al is an element useful for refining because it has a deoxidizing effect and the like, and at least 0.001% or more is required. Therefore, the lower limit of the Al content is set to 0.001% or more. However, if it is excessively contained, Al is preferentially oxidized and concentrated on the surface, which lowers the brazing property. Therefore, the upper limit of the Al content is set to 0.100% or less. The Al content is preferably 0.005 to 0.080%, more preferably 0.010 to 0.070%.

The above is the basic chemical composition of the austenitic stainless steel of the present invention, and the balance is Fe and unavoidable impurities. The austenitic stainless steel according to the present invention may or may not contain the following elements as necessary. When not contained, the lower limit of the content of each element is 0% or more.

B: 0.0001 to 0.0050%
B is an element useful for improving hot workability, and may be contained in an amount of 0.0001% or more if necessary. However, if it is excessively contained, the corrosion resistance is lowered, so the upper limit of the B content is set to 0.0050% or less. The preferred B content is 0.0005 to 0.0040%, more preferably 0.0010 to 0.0030%.

Ti: 0.001 to 0.300%
Ti is an element useful for improving corrosion resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if it is excessively contained, Ti is preferentially oxidized and concentrated on the surface, which lowers the brazing property. Therefore, the upper limit of the Ti content is set to 0.300% or less. The Ti content is preferably 0.002 to 0.200%, more preferably 0.003 to 0.100%.

Nb: 0.001 to 0.300%
Nb is an element useful for improving corrosion resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are lowered, so the upper limit of the Nb content is set to 0.300% or less. The preferred Nb content is 0.005 to 0.200%, more preferably 0.010 to 0.100%.

W: 0.01 to 1.00%
W is a very useful element for improving corrosion resistance while suppressing σ-phase precipitation, and is preferably contained in an amount of 0.01% or more. However, if it is excessively contained, the processability and manufacturability are deteriorated, so the upper limit of the W content is set to 1.00% or less. A more preferable W content is 0.03 to 0.70%, and even more preferably 0.05 to 0.50%.

V: 0.01 to 0.50%
V is an element useful for improving corrosion resistance, and may be contained in an amount of 0.01% or more if necessary. However, if it is excessively contained, the processability and manufacturability are lowered, so the upper limit of the V content is set to 0.50% or less. The preferred V content is 0.03 to 0.40%, more preferably 0.05 to 0.30%.

Sn: 0.001 to 0.500%
Sn is an element useful for improving corrosion resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are deteriorated, so the upper limit of the Sn content is set to 0.500% or less. The Sn content is preferably 0.005 to 0.400%, more preferably 0.010 to 0.300%.

Sb: 0.005 to 0.500%
Sb is an element useful for improving corrosion resistance, and may be contained in an amount of 0.005% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are lowered, so the upper limit of the Sb content is set to 0.500% or less. The preferred Sb content is 0.008 to 0.400%, more preferably 0.010 to 0.300%.

Co: 0.010 to 0.500%
Co is an element that improves secondary processability and toughness, and may be contained in an amount of 0.010% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are lowered, so the upper limit of the Co content is set to 0.500% or less. The preferred Co content is 0.020 to 0.400%, more preferably 0.030 to 0.300%.

The total content of one or more of B, Ti, Nb, W, V, Sn, Sb, and Co described above is preferably 6% or less from the viewpoint of suppressing an increase in alloy cost.

Ca: 0.0001 to 0.0050%
Ca is an element that improves desulfurization and hot workability, and may be contained in an amount of 0.0001% or more if necessary. However, if it is excessively contained, CaS, which is a water-soluble inclusion, is generated and the corrosion resistance is lowered. Therefore, the upper limit of the Ca content is set to 0.0050% or less. The preferred Ca content is 0.0002 to 0.0045%, more preferably 0.0003 to 0.0040%.

Mg: 0.0001 to 0.0050%
Mg is an element that refines the structure and improves processability and toughness, and may be contained in an amount of 0.0001% or more, if necessary. However, if it is excessively contained, the hot workability is lowered, so the upper limit of the Mg content is set to 0.0050% or less. The preferred Mg content is 0.0003 to 0.0040%, more preferably 0.0005 to 0.0030%.

Zr: 0.001 to 0.300%
Zr is an element that improves corrosion resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are deteriorated, so the upper limit of the Zr content is set to 0.300% or less. The preferred Zr content is 0.005 to 0.200%, more preferably 0.010 to 0.100%.

Ga: 0.0001 to 0.0100%
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and may be contained in an amount of 0.0001% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are lowered, so the upper limit of the Ga content is set to 0.0100% or less. The preferred Ga content is 0.0005 to 0.0080%, more preferably 0.0010 to 0.0050%.

Ta: 0.001 to 0.050%
Ta is an element that improves corrosion resistance, and may be contained in an amount of 0.001% or more, if necessary. However, if it is excessively contained, the processability and manufacturability are deteriorated, so the upper limit of the Ta content is set to 0.050% or less. The preferred Ta content is 0.005 to 0.040%, more preferably 0.010 to 0.030%.

REM: 0.001 to 0.100%
REM is an element useful for refining because it has a deoxidizing effect and the like, and may be contained in an amount of 0.001% or more if necessary. However, if it is excessively contained, the processability and manufacturability are deteriorated, so the upper limit of the REM content is set to 0.100% or less. The preferred REM content is 0.005 to 0.080%, more preferably 0.010 to 0.050%.
REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These elements may be contained alone or may be a mixture.

The austenitic stainless steel according to the present invention described above has the above-mentioned chemical components and satisfies Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 50.000, and is therefore after brazing. In this steel, the ratio of the σ-phase precipitation area in the range of 0.1 μm from the surface is 1.00% or less, and the austenitic stainless steel exhibits excellent corrosion resistance. Since the austenitic stainless steel according to the present invention has excellent corrosion resistance even after brazing, it can be used as a member of brazed structures such as marine structures and flue gas desulfurization devices, and brazed structural parts such as exhaust gas heat exchange parts. It can be preferably used.

Next, a method for producing an austenitic stainless steel according to the present invention will be described.
The austenitic stainless steel according to the present invention may be basically produced by a general method for producing an austenitic stainless steel. For example, the following manufacturing method can be mentioned. First, as molten steel having the above chemical composition in a converter or an electric furnace, it is refined in an AOD (Argon Oxygen Decarburization) furnace, a VOD (Vacuum Arc Degassing) furnace, or the like. Then, after making steel pieces by a continuous casting method or an ingot method, the steel pieces are manufactured through the steps of hot rolling-hot rolling sheet annealing-pickling-cold rolling-finish annealing-pickling. If necessary, hot-rolled sheet annealing may be omitted, or cold rolling-finish annealing-pickling may be repeated. Further, between finish annealing and pickling, mechanical treatment such as shot blasting or grinding brush, or chemical treatment such as molten salt treatment or neutral salt electrolysis treatment may be performed.

Further, when brazing the austenitic stainless steel according to the present invention to manufacture brazed structural parts such as brazed structures and automobile exhaust system parts, it is most suitable for suppressing σ phase precipitation. Brazing is required. The vacuum atmosphere in brazing is preferably 100 Pa or less, and the brazing time is preferably 5 to 20 minutes. The brazing temperature is preferably in the range of 1000 to 1200 ° C. More preferably, in order to suppress σ-phase precipitation on the surface after brazing, N ₂ is sprayed on the surface immediately after brazing to rapidly cool only the outermost surface. The cooling rate of the outermost surface is preferably 5 ° C./min or higher, and more preferably 10 ° C./min or higher. The temperature range in which the cooling rate is required is 800 to 1200 ° C, more preferably 500 to 1200 ° C.

By brazing the austenitic stainless steel according to the present invention by the method described above, the ratio of the σ phase precipitation area in the range of 0.1 μm from the surface becomes 1.00% or less even after brazing. An austenitic stainless steel having excellent corrosion resistance can be obtained.

The present invention will be described in more detail based on examples.
First, the steels having the compositions shown in Table 1 were melted in a converter, smelted in an AOD furnace, cast by a continuous casting machine, heated to 1200 ° C., and then hot forged. Then, it was hot-rolled to a thickness of 6 mm, annealed at 1150 ° C. for 10 minutes, and then pickled. Then, it was cold-rolled to a thickness of 1 mm and annealed at 1150 ° C. for 10 minutes to prepare a cold-rolled annealed plate. Then, after immersing in salt, pickling was performed under the conditions of HF: 20 g / L, HNO ₃ : 50 g / L, and 50 ° C. to prepare a steel sheet having a plate thickness of 1.0 mm.
This steel sheet was heat-treated at 1150 ° C. for 10 minutes in a vacuum atmosphere of 50 Pa containing _{N 2.} The above heat treatment is hereinafter referred to as brazing. Immediately after the brazing was completed, N ₂ was sprayed on the surface to rapidly cool only the outermost surface. The cooling rate in the outermost surface temperature range of 500 to 1200 ° C. was 8 ° C./min.
The Fe content in Table 1 was determined as the total amount of the balance excluding the active ingredient of the present invention.

A test piece having a width of 25 mm and a length of 100 mm was cut out from the brazed steel sheet, and the corrosiveness was evaluated by a semi-immersion test.
As the simulated condensed water used in the semi-immersion test, hydrochloric acid, ammonium chloride, sulfuric acid, and ammonium sulfite were used as reagents, and the mixture was adjusted to ^{100,000 ppmCl −} + 1000 ppmSO ₄ ^{2 to} + 1000 _{ppm SO 3} ^2-. After adding the reagent, the simulated condensed water was adjusted to pH 2.0 using aqueous ammonia. The test piece was half-immersed in this simulated condensed water heated to 80 ° C. using a jig adjusted so that the test piece was approximately half-immersed at about 55 °. The test was carried out for 168 hours, and the simulated condensed water in which the test piece was semi-immersed was replaced with a newly prepared one every 24 hours.

Corrosiveness was evaluated by the maximum pitting depth. After the completion of the semi-immersion test, the corrosion product was removed using an aqueous solution of ammonium dihydrogen dihydrogen citrate, and the depth of the deepest corroded portion of the test piece was determined by the depth of focus method using a microscope. The criterion for the semi-immersion test was 100 μm, where the growth of pitting corrosion was remarkable. Those having a maximum pitting depth of less than 100 μm were evaluated as “◯”, and those having a maximum pitting depth of 100 μm or more were evaluated as “x”. The results are shown in Table 1.

Further, a test piece having a width of 20 mm and a length of 30 mm was cut out from the brazed steel sheet, a resin-filled sample was prepared so that the cross section of the steel sheet along the rolling direction could be observed, and mirror polishing was performed. After the σ phase was revealed by KOH electrolytic etching, the area of the σ phase observed in a field of view of × 1000 was measured by image analysis. LUZEX (registered trademark) SE manufactured by Nireco Corporation was used for image analysis. The σ phase having a diameter of 0.1 μm or more was used as the measurement target. The number of fields of view was 200. The measurement was performed from the steel surface to a depth of 0.1 μm. By the above method, the ratio of the σ phase precipitation area in the range of 0.1 μm from the surface was determined. The results are shown in Table 1.

From Table 1, the steel types (No. A1 to No. A12) whose chemical composition and the value of the formula (1) are within the range of the present invention are the ratio of the σ phase precipitation area in the range of 0.1 μm from the surface (hereinafter, simply Σ phase precipitation area ratio) is 1.00% or less, and it can be seen that the corrosion resistance after brazing is excellent. On the other hand, it can be seen that the steel grades (No. B1 to No. B8) whose chemical composition and any one of the values of the formula (1) are outside the scope of the present invention are inferior in corrosion resistance after brazing. Steel grades (No. B5 to No. B8) whose chemical composition is within the range of the present invention but whose value in the formula (1) is outside the range of the present invention have a σ-phase precipitation area ratio in the range of 0.1 μm from the surface. It becomes high and it can be seen that the corrosion resistance is inferior. It is considered that this is because the low Cr and Mo concentration regions near the σ phase deposited in large quantities serve as the starting point of corrosion, leading to an increase in the maximum pitting corrosion depth and deterioration of corrosion resistance. Hereinafter, each comparative example will be described.

Steel type No. B1 is an example in which the corrosion resistance is inferior because the Ni content is outside the range of the present invention. Steel type No. B2 is an example in which the corrosion resistance is inferior because the Cr content is outside the range of the present invention. Steel type No. B3 is an example in which the corrosion resistance is inferior because the Mo content is outside the range of the present invention. Steel type No. B4 is an example in which the corrosion resistance is inferior because the Cu content is outside the range of the present invention.

Steel type No. B5-No. Each of B8 has a chemical composition within the range of the present invention, but since the value of the formula (1) is outside the range of the present invention, the ratio of the σ phase precipitation area is large and the corrosion resistance is inferior.

Since the austenitic stainless steel having excellent corrosion resistance according to the present invention can suppress the precipitation of the σ phase even after brazing, it is a brazed structure such as an offshore structure or a flue gas desulfurization device, and exhaust gas that is an automobile exhaust system component. It is suitable as a member used for brazed structural parts such as exhaust gas heat exchange parts such as heat recovery devices and EGR coolers.

Claims (10)

By mass%
C: 0.001 to 0.012%,
Si: 0.37 to 0.79%,
Mn: 0.01 to 0.89 %,
P: 0.050% or less,
S: 0.0050% or less,
Ni: 17.000 to 35.00%,
Cr: 18.0 to 30.00%,
Mo: 4.00 to 7.49%,
Cu: 0.10 to 3.00%,
N: 0.100 to 0.400% and Al: 0.001 to 0.100%
An austenitic stainless steel containing ferritic stainless steel, the balance of which is composed of Fe and unavoidable impurities, and which satisfies the following formula (1).
Fe + 1.2 × Cr + 1.5 × Mo-2 × Ni-5 × Cu ≦ 50.000 ・ ・ ・ (1)
However, the element symbol in the above formula (1) means the content mass% of the element in steel. In addition, by mass%,
B: 0.0001 to 0.0050%,
Ti: 0.001 to 0.300%,
Nb: 0.001 to 0.300%,
W: 0.01 to 1.00%,
V: 0.01-0.50%,
Sn: 0.001 to 0.500%,
Sb: 0.005 to 0.500% and Co: 0.010 to 0.500%
The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel contains any one or more of them. In addition, by mass%,
Mg: 0.0001 to 0.0050%,
Zr: 0.001 to 0.300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.001 to 0.050% and REM: 0.001 to 0.100%
The austenitic stainless steel according to claim 1 or 2, wherein the austenitic stainless steel contains one or more of the above. The austenitic stainless steel according to any one of claims 1 to 3, wherein the ratio of the σ-phase precipitation area in the range of 0.1 μm from the surface is 1.00% or less. The austenitic stainless steel according to any one of claims 1 to 4, which is used for an exhaust gas heat exchange component. The austenitic stainless steel according to any one of claims 1 to 4, which is used for brazed structural parts. The invention according to any one of claims 1 to 3, wherein the brazed steel has a σ-phase precipitation area ratio of 1.00% or less in a depth range of 0.1 μm from the surface. Austenitic stainless steel with a steel component. The brazed structure made of austenitic stainless steel according to claim 7. The brazed structural component made of austenitic stainless steel according to claim 7. The exhaust gas heat exchange component made of austenitic stainless steel according to claim 7.

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