JP2018031028A - Fe-Ni-Cr-Mo alloy and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Fe-Ni-Cr-Mo alloy having both excellent intergranular corrosion resistance and excellent surface properties. SOLUTION: C: 0.005 to 0.03 mass%, Si: 0.02 to 0.25 mass%, Mn: 0.03 to 0.40 mass%, P: 0.040 mass% or less, S: 0.003 mass. % Or less, Ni: 32.0 to 38.0 mass%, Cr: 21.0 to 25.0 mass%, Mo: 6.0 to 8.0 mass%, N: 0.20 to 0.30 mass%, Cu: 0 It contains 0.01 to 0.40 mass%, Al: 0.012 to 0.1 mass%, B: 0.0005 to 0.005 mass%, and further contains V: 0.005 to 0.250 mass% or Nb: 0.005. An Fe-Ni-Cr-Mo alloy containing 1 or 2 types of ~ 0.250 mass%, the balance of which is composed of Fe and unavoidable impurities, and satisfying the following formula (1). 0.005 ≤ [mass% V + mass% Nb] ≤ 0.250 ... (1) [Selection diagram] Fig. 1

Description

  The present invention relates to an Fe—Ni—Cr—Mo alloy having excellent surface properties used in an environment where extremely excellent intergranular corrosion resistance is required, such as a chemical plant, and a method for producing the same.

  Fe-Ni-Cr-Mo alloys are used in various fields because of their good corrosion resistance, but in environments containing corrosive substances, such as seawater environments, flue gas desulfurization equipment, oil wells, food plants, chemical plants And used in nuclear power plants. When a general-purpose alloy such as carbon steel, SUS304, or SUS316 is used in the above environment, pitting corrosion, crevice corrosion, stress corrosion cracking, or intergranular corrosion is likely to occur, and the range of use is greatly restricted.

  Therefore, as a technique for improving the corrosion resistance of the Fe—Ni—Cr—Mo alloy, attempts have been made to improve the corrosion resistance by adding a large amount of Cr, Mo or N in the alloy components. For example, an austenitic stainless steel having a maximum Cr content of 35% proposed in Patent Document 1 and an austenitic stainless steel having a maximum Mo content of 8.0% proposed in Patent Document 2 have been developed. Further, Patent Document 3 proposes an austenitic stainless steel in which the N content is increased to a maximum of 0.50 mass%, and can be suitably used in the above highly corrosive environment.

  As an issue in manufacturing the alloys of the above-mentioned literature, since elements such as S, Si, and P segregate to the grain boundary, ear cracks and surface cracks occur in manufacturing, particularly in the hot rolling process, and the yield significantly decreases. In addition, there is a problem that wrinkles remain on the surface of the product plate or coil, causing corrosion starting from the surface and cracking during processing. In order to avoid this problem, in the alloys described in the above documents, hot workability is improved by adding a small amount of B or Mg of several tens of ppm, and generation of wrinkles on the surface of the plate or the coil is prevented.

  However, the technique disclosed in the above document aims to improve pitting corrosion resistance and crevice corrosion resistance, and no investigation has been made on intergranular corrosion resistance.

  On the other hand, in recent years, the demand for a high corrosion resistance alloy in a high potential region represented by a nuclear power plant and a fertilizer plant has further increased. In the above plant, it is known that intergranular corrosion of the alloy used in the plant occurs because it is exposed to a high potential region, but development of an alloy having far higher intergranular corrosion resistance than SUS304 and SUS316 is desired. ing.

  As described above, B is positively added as an element that improves hot workability, but is also known as an element that lowers intergranular corrosion resistance. As shown in Patent Document 4, Patent Document 5 and Patent Document 6, an austenitic stainless steel and a duplex stainless steel also have an adverse effect on B intergranular corrosion in a high potential environment. In Patent Document 4, it is proposed to contain B at 3 ppm or less for the purpose of improving the intergranular corrosion resistance, but no investigation has been made on its surface properties. In addition, since B is also a kind of element that is inevitably mixed from raw materials such as scrap in the production of Fe—Ni—Cr—Mo alloy, it is considered that not less than several ppm is mixed at the industrial level. Therefore, in order to control the B amount to 3 ppm or less, an increase in raw material cost can be considered.

  In Patent Document 5, the relationship between the amount of B and the austenite grain size is controlled to B (ppm) × d (μm) ≦ 700, and the intergranular corrosion sensitivity is evaluated for the electrochemical reactivation rate based on JIS G 0580. It has been proposed that the B content is 30 ppm or less. However, the alloys being studied are general-purpose austenitic stainless steels such as SUS304, SUS316, and SUS317. These studied alloys are low grade alloys with low Ni, Cr and Mo contents and low corrosion resistance, and are unsuitable for recent severe intergranular corrosion environments. Furthermore, no investigation has been made on the surface properties.

  In Patent Document 6, for the duplex stainless steel having good intergranular corrosion resistance, the difference in average Cr concentration between the α phase and γ phase interfaces is controlled by variously defining heat treatment and cooling conditions, and further, the B amount is reduced to 0. A technique for improving the intergranular corrosion resistance by specifying 0.0001 to 0.001 mass% has been proposed. However, no investigation has been made on the surface properties.

JP-A-5-247597 Japanese Patent Laid-Open No. 10-060603 JP 2010-31313 A JP 2009-197316 A JP-A-7-113146 JP 2006-53213 A

  From the above prior art, it is considered that B is an element which is essential to be added from the viewpoint of surface properties, improving the hot workability, improving the yield of the product, preventing ear cracks and surface cracks. However, B is known as an element that lowers intergranular corrosion resistance and has been studied, but in the prior art, a relatively low grade alloy or a two-phase alloy having a low Cr, Mo and Ni content in the alloy. Since it is intended for stainless steel, the content of Cr and Ni effective for intergranular corrosion resistance is low, and intergranular corrosion resistance in a recent highly corrosive environment is not sufficient.

  As described above, in the prior art, the correlation between the effect of the amount of B on the intergranular corrosion resistance and the surface properties of Fe-Ni-Cr-Mo alloys containing high Ni and Cr and having improved corrosion resistance is as follows. Absent.

  The present invention has been made in view of the above circumstances, and an object thereof is high Fe-Ni, which contains Ni and Cr at a high level and has excellent intergranular corrosion resistance in a high corrosion environment in a high potential region. -Cr-Mo alloy and its manufacturing method are proposed.

  The inventors have intensively studied to solve the above problems. As a result, the intergranular corrosion resistance is improved by increasing the concentration of Ni and Cr in the alloy, and one or two of V or Nb are added in a certain amount range based on JIS G 0511. The crystal grain size is controlled to 5.0 to 7.0, the adverse effect on the intergranular corrosion property of B is reduced, the surface property is ensured, and the excellent intergranular corrosion resistance is obtained. The headline, the present invention has been reached.

That is, the Fe-Ni-Cr-Mo alloy of the present invention has C: 0.005-0.03 mass%, Si: 0.02-0.25 mass%, Mn: 0.03-0.40 mass%, P: 0.040 mass% or less, S: 0.003 mass% or less, Ni: 32.0 to 38.0 mass%, Cr: 21.0 to 25.0 mass%, Mo: 6.0 to 8.0 mass%,
N: 0.20 to 0.30 mass%, Cu: 0.01 to 0.40 mass%, Al: 0.012 to 0.1 mass%, B: 0.0005 to 0.005 mass%, and V: One or two of 0.005 to 0.250 mass% or Nb: 0.005 to 0.250 mass% is contained, the balance is composed of Fe and inevitable impurities, and satisfies the following formula (1) It is characterized by.
0.005 ≦ [mass% V + mass% Nb] ≦ 0.250 (1)

Moreover, the manufacturing method of the Fe-Ni-Cr-Mo alloy of this invention is C: 0.005-0.03 mass%, Si: 0.02-0.25 mass%, Mn: 0.03-0.40 mass% , P: 0.040 mass% or less, S: 0.003 mass% or less, Ni: 32.0-38.0 mass%, Cr: 21.0-25.0 mass%, Mo: 6.0-8.0 mass%, N: 0.20 to 0.30 mass%, Cu: 0.01 to 0.40 mass%, Al: 0.012 to 0.1 mass%, B: 0.0005 to 0.005 mass%, and V: F containing 0.005 to 0.250 mass% or Nb: 0.005 to 0.250 mass%, one or two of the remainder, the balance being Fe and inevitable impurities, and satisfying the following formula (1) To produce a hot-rolled steel sheet or cold-rolled steel sheet -Ni-Cr-Mo alloy, characterized by annealing the hot-rolled steel sheet or cold-rolled steel sheet at a temperature of 1100 to 1,180 ° C..
0.005 ≦ [mass% V + mass% Nb] ≦ 0.250 (1)

  The present invention is characterized by having a grain size of 5.0 to 7.0 based on JIS G 0511.

In the present invention, the following expression (2) is satisfied.
100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.90 (2)

  According to the present invention, it is possible to provide a Fe—Ni—Cr—Mo alloy having improved surface properties by adding B and improved intergranular corrosion resistance by adding V and / or Nb. It can be suitably used as a corrosion-resistant material used in an environment where the occurrence of intergranular corrosion is a concern, such as a chemical plant.

It is a figure which shows the relationship between mass% B and mass% V + mass% Nb in a preliminary experiment. It is a figure which shows the relationship between the corrosion rate and annealing temperature in a preliminary experiment, and a crystal grain size and annealing temperature. It is a figure which shows the relationship between mass% B in an Example, and mass% V + mass% Nb.

Conventionally, the intergranular corrosion resistance is evaluated by the so-called Strecher test or Huey test defined in ASTM G28 Method A using sulfuric acid-ferric sulfate solution or ASTM A262 Practice C using boiling 65 mass% nitric acid. In order to improve the intergranular corrosion resistance in a solution in which 0.5 g / L of Cr ⁶⁺ is added to a boiling 70 mass% nitric acid solution, the inventors have assumed a high intergranular corrosion environment that has recently become increasingly severe. Focusing on B segregating or precipitating at the boundary, the influence on intergranular corrosion resistance and the effect of improving the surface properties were studied together. In addition, it is considered that the adverse effect of B on the crystal grain boundary per unit area can be reduced by controlling the crystal grain size by using the pinning effect of V and Nb and reducing the crystal grain size. The effect on intergranular corrosion was investigated.

<Experiment 1>
Steel having Fe-23 mass% Cr-35 mass% Ni-7.5 mass% Mo-0.25 mass% N as a basic component was melted in a high-frequency induction furnace for testing with a capacity of 20 kg. The molten steel was then cast into a mold to form a steel ingot, and then hot forged to obtain a forged plate having a thickness of 8 mm. Thereafter, annealing and pickling were performed, and further cold rolling to a thickness of 2 mm was performed, annealing and pickling were performed, and a cold-rolled sheet was produced. A corrosion test piece having a width of 20 mm, a length of 25 mm, and a thickness of 2 mm was collected from the cold-rolled sheet. In this dissolution, as shown in Table 1, the component contents of B, V and Nb were variously changed.

The above-mentioned corrosion test piece was used for a grain boundary corrosion test using a solution in which 0.5 g / L of Cr ⁶⁺ was added to a boiling 70 mass% nitric acid solution. In the above corrosion test, 5 batches of immersion test were performed with 48 batches as one batch while renewing the corrosive solution, the corrosion weight loss was measured to calculate the corrosion rate, and the intergranular corrosion resistance was evaluated. The corrosion test piece was not subjected to sensitization heat treatment, and the test piece was subjected to wet polishing with No. 80 water-resistant polishing paper for the test.

The evaluation of the intergranular corrosion resistance is that if the average corrosion rate of 5 batches is 0.25 g / m ² · hr or less, it can be judged that the intergranular corrosion resistance is good, so 0.25 g / m ² · hr is determined. When exceeding, it was determined that the intergranular corrosion resistance was inferior (x), and 0.25 g / m ² · hr or less was judged to be excellent (◯).

  In addition, the corrosion resistance of stainless steel and Fe—Ni—Cr—Mo alloy is a value calculated by mass% Cr + 3.3 × mass% Mo + 16 × mass% N, which is generally called Pitting Resistance Equivalent (PRE). The higher the value, the better the corrosion resistance. In this environment, the pitting corrosion index value may be 45 ≦ PRE ≦ 55. When PRE is less than 45, the corrosion resistance of the alloy is low, and sufficient intergranular corrosion resistance cannot be obtained. On the other hand, when PRE exceeds 55, the σ phase or nitride is likely to be precipitated, so the intergranular corrosion resistance is lowered.

  Next, a round bar test piece having a diameter: 8 mmφ × length: 70 mm was sampled by machining from the ingot having the same component composition, and subjected to a high temperature tensile test using a hot workability reproduction test device (Cermec Master Z). The surface properties were evaluated.

  Furthermore, the crystal grain size of the cold-rolled plate used for the corrosion test piece was measured based on JIS G 0511.

The test results are shown in Table 1. FIG. 1 shows the range in which intergranular corrosion rates of 0.25 g / m ² · hr or less are obtained and good surface properties are obtained in relation to the B content and the total content of V and Nb. . As described above, B is an element that lowers the intergranular corrosion resistance. From FIG. 1, when B exceeds 0.0050 mass%, any steel has a grain boundary of 0.25 g / m ² · hr or less. The corrosion rate was not satisfied. Therefore, the amount of B needs to be 0.0050 mass% or less. If the amount of B exceeds 0.0050 mass%, even if the amounts of V and Nb are increased, the effect of B on intergranular corrosion resistance is significant, and good grain resistance of 0.25 g / m ² · hr or less It was confirmed that no inter-corrosion was obtained. It was also found that good surface properties could not be obtained when the B content was 0.0004 mass% or less. For this reason, the B content needs to be contained in the range of 0.0005 mass% or more and 0.0050 mass% or less.

It can be seen that when the total amount of V and Nb is 0.005% or more, the degree of corrosion is 0.25 g / m ² · hr or less. That is, paying attention to steel l and steel m in Table 1, the B content and the crystal grain size of both are the same, but steel m with a large total amount of V and Nb of 0.010 mass% has a lower corrosion rate. I understand that. From this, it was found that the intergranular corrosion resistance is improved by adding a small amount of V and Nb to the Fe—Ni—Cr—Mo alloy. The inventors presume that the dissolution rate was suppressed by the addition of V or Nb. Furthermore, the addition of V and Nb precipitates these nitrides, and even when annealed at a high temperature, the growth of crystal grain size is suppressed by the so-called pinning effect, resulting in segregation at the grain boundaries per unit area. It is considered that the amount of B decreased, the adverse effect of B on intergranular corrosion resistance was reduced, and good intergranular corrosion resistance was obtained. However, if the total amount of V and Nb is less than 0.005 mass%, a sufficient pinning effect cannot be obtained and the crystal grains are coarsened, and intergranular corrosion resistance of 0.25 g / m ² · hr or less is obtained. There wasn't. On the other hand, when the total amount of V and Nb exceeds 0.250 mass%, the intergranular corrosion rate increases and does not satisfy 0.25 g / m ² · hr. This is because V and Nb promote the precipitation of the σ phase, and it was recognized that the addition of V and Nb more than necessary causes a decrease in intergranular corrosion resistance. Therefore, it is understood that good intergranular corrosion resistance can be obtained if V and Nb satisfy the relationship of 0.005 ≦ [mass% V + mass% Nb] ≦ 0.250.

  Further, in the range where the amount of B is large and the amount of addition of V and Nb is large, for example, No. The steel of k has a B addition amount as high as 0.0045 mass%, and the total content of V and Nb is 0.254 mass%, which exceeds the above range. It is considered that the intergranular corrosion resistance has deteriorated due to both factors of precipitation. It has been found that if the above range satisfies the relationship of 100 × mass% B + 2.0 (mass% V + mass% Nb) ≦ 0.90, good intergranular corrosion resistance can be obtained.

As a result of measuring the crystal grain size of the steel having a corrosion rate of 0.25 g / m ² · hr or less at this time based on JIS G 0511, all have a crystal grain size of 5.0 to 7.0. It was recognized that In addition, when the experiment was further advanced, it was also found that when the amount of Al added is small, AlN does not precipitate, and the intergranular corrosion resistance decreases. This was considered to be affected by the pinning effect of AlN.

  From the results of Experiment 1 above, if the B content, V and Nb content are limited to appropriate ranges, and the crystal grain size based on JIS G 0511 is controlled to 5.0 to 7.0, the surface properties are excellent. It was also found that an Fe—Ni—Cr—Mo alloy having excellent intergranular corrosion resistance can be obtained.

<Experiment 2>
By the way, in the component system in which intermetallic compounds such as σ phase and χ phase having high Cr and high Mo content are likely to precipitate, it is desirable that the annealing temperature be sufficiently high in order to obtain a solid solution austenite phase structure. However, if the annealing temperature is too high, the nitrides of V and Nb are solidified so that the pinning effect cannot be obtained, and the crystal grain size cannot be controlled. Therefore, steel containing Fe-23 mass% Cr-35 mass% Ni-7.5 mass% Mo-0.25 mass% N as a basic component is 0.0048 mass% for B, 0.105 mass% for V, and 0.104 mass% for Nb. The steel that has been added and melted is cold-rolled, and the cold-rolled sheet is annealed by changing the annealing temperature to 1080 ° C, 1100 ° C, 1120 ° C, 1140 ° C, 1160 ° C, 1180 ° C, 1200 ° C. After the test, the cold-rolled sheet produced by immediately water cooling was subjected to the intergranular corrosion test. The annealing time was 1 minute in all cases. The test results are shown in Table 2. FIG. 2 shows the influence of the annealing temperature on the intergranular corrosion rate. From this result, it can be seen that if the annealing temperature is 1100 ° C. to 1180 ° C., good intergranular corrosion resistance with a grain boundary corrosion rate of 0.25 g / m ² · hr or less can be obtained.

  As a result of measuring the grain size, it was confirmed that the steel annealed in the temperature range of 1100 ° C. to 1180 ° C. had a grain size based on JIS G 0511 of 5.0 to 7.0. As described above, the addition of V and Nb causes their nitrides to precipitate, and even if annealed, the growth of crystal grain size is suppressed by the pinning effect, and fine crystal grains having a crystal grain size of 5.0 to 7.0 The diameter can be obtained. As a result, the amount of segregation at the grain boundaries per unit area of B is reduced, the adverse effect on B intergranular corrosion resistance is reduced, and good intergranular corrosion resistance is considered to be obtained. Since the steel annealed at 1200 ° C. was annealed at a high temperature, the nitrides of V and Nb were dissolved, and as a result, the pinning effect was not obtained and the crystal grain size was coarsened. For this reason, it was considered that the amount of B segregated at the crystal grain boundaries per unit area increased, and the intergranular corrosion resistance deteriorated. The crystal grain size was 2.0.

  The crystal grain size of steel annealed at 1080 ° C was as fine as 8.0, but the annealing temperature was too low, so the σ phase remained and the intergranular corrosion resistance deteriorated. it was thought.

Next, the composition components that the Fe—Ni—Cr—Mo alloy of the present invention should have will be described.
C: 0.005 to 0.03 mass% or less C is an austenite phase stabilizing element. However, when added in a large amount, it combines with Cr and Mo to form carbides to form carbides, reducing the amount of solid solution Cr and solid solution Mo in the base material, and reducing corrosion resistance. On the other hand, the lower limit of C is set to 0.005 mass% from the viewpoint of preventing the strength from being lowered. Therefore, C is limited to 0.005 to 0.03 mass%. Preferably it is 0.005-0.025 mass%, More preferably, it is 0.005-0.02 mass%.

Si: 0.02-0.25 mass%
Si is an element added as a deoxidizer. Further, since Si is an element that improves the fluidity of the molten steel and improves the weldability, addition of 0.02 mass% or more is desirable. However, since Si is an element that promotes precipitation of intermetallic compounds such as the σ phase and increases the intergranular corrosion sensitivity, the upper limit is set to 0.25 mass%. Preferably it is 0.24 mass% or less, More preferably, it is 0.23 mass% or less.

Mn: 0.03 to 0.40 mass%
Since Mn is an element having a deoxidizing action, at least 0.03 mass% or more is necessary to obtain the effect. However, since Mn also causes precipitation of intermetallic compounds such as σ phase and χ phase like Si, addition of more than necessary is not preferable. Therefore, it is necessary to make it 0.40 mass% or less. Preferably it is 0.30 mass% or less, More preferably, it is 0.20 mass% or less.

P: 0.040 mass% or less P is an element inevitably mixed in the steel as an impurity, and precipitates at the grain boundary as a phosphide, which harms intergranular corrosion resistance and hot workability. It is desirable to reduce. However, extremely reducing the P content causes an increase in manufacturing cost. Therefore, in the present invention, P is limited to 0.040 mass% or less. Preferably it is 0.030 mass% or less, More preferably, it is 0.020 mass% or less.

S: 0.003 mass% or less S, like P, is an element that is inevitably mixed as an impurity, is easily segregated at grain boundaries, and is harmful to corrosion resistance and hot workability. In particular, when the content exceeds 0.003%, the harmful effect appears remarkably, so it is necessary to make it 0.003 mass% or less. Preferably it is 0.002 mass% or less, More preferably, it is 0.001 mass% or less.

Ni: 32.0-38.0 mass%
Ni is an important element that suppresses precipitation of intermetallic compounds such as σ phase and χ phase and improves intergranular corrosion resistance and overall corrosion resistance. When the content is less than 32.0 mass%, precipitation of intermetallic compounds is promoted. On the other hand, when the content is more than 38.0 mass%, hot workability is deteriorated and hot deformation resistance is increased. Therefore, the Ni content is 32.0 to 38.0 mass%. Preferably it is 33.0 to 37.0 mass%, more preferably 34.0 to 36.0 mass%.

Cr: 21.0-25.0 mass%
Cr is an important element that improves not only intergranular corrosion resistance but also pitting corrosion resistance and crevice corrosion resistance. In order to obtain the effect sufficiently, it is necessary to contain 21.0 mass% or more. However, if the content exceeds 25.0 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted and the corrosion resistance is deteriorated. Therefore, the content is set to 21.0 to 25.0 mass%. Preferably it is 22.0-25.0 mass%, More preferably, it is 23.0-25.0 mass%.

Mo: 6.0-8.0 mass%
Mo is an element useful for improving the general corrosion resistance, pitting corrosion resistance and crevice corrosion resistance, and therefore needs to be contained in an amount of 6.0 mass% or more. However, excessive addition of Mo promotes precipitation of intermetallic compounds such as σ phase and χ phase, and reduces intergranular corrosion resistance. Therefore, Mo is set to a range of 6.0 to 8.0 mass%. Preferably it is 7.0-8.0 mass%, More preferably, it is 7.5-8.0 mass%.

N: 0.20 to 0.30 mass%
N is an element useful for improving the general corrosion resistance, pitting corrosion resistance, and crevice corrosion resistance similarly to Cr and Mo. In order to obtain the effect, addition of 0.20 mass% or more is necessary. In addition, addition of 0.20 mass or more is necessary to precipitate V or Nb nitride for controlling the crystal grain size. However, when N is contained in excess of 0.30 mass%, the hot deformation resistance is extremely increased and the hot workability is hindered, so the N content is set to 0.20 to 0.30 mass%. Preferably it is 0.21 mass%-0.28 mass%, More preferably, it is 0.21-0.27 mass%.

Cu: 0.01-0.40 mass%
Cu is effective in improving general corrosion resistance, but in order to obtain the effect, it is necessary to contain 0.01% by mass or more. However, in an environment of a high potential region, it is an element that promotes corrosion, so 0.30 mass% or less is preferable. Therefore, the content was set to 0.01 to 0.40 mass%. Preferably it is 0.05-0.35 mass%, More preferably, it is 0.08-0.30 mass%.

Al: 0.012-0.1 mass%
Al has the effect of promoting desulfurization by deoxidation to reduce S and improving hot workability, so it is necessary to add it actively, but if Al exceeds 0.1 mass%, precipitation of intermetallic compounds In addition, since Al oxide precipitates and dissolves in a high potential region, the intergranular corrosion resistance decreases. Moreover, since AlN has a pinning effect, it is effective in suppressing the coarsening of the crystal grain size and improves the intergranular corrosion resistance. However, when the content is less than 0.012 mass%, the effect cannot be obtained. Therefore, the content of Al is set to 0.012 to 0.1 mass%. Preferably it is 0.012-0.075 mass%, More preferably, it is 0.012-0.050 mass%.

B: 0.0005 to 0.005 mass%
B segregates preferentially at the grain boundaries over S which lowers the hot workability, and has the effect of improving the hot workability, but if it is less than 0.0005 mass%, the effect cannot be obtained. On the other hand, since it segregates at the grain boundary, if it exceeds 0.005 mass%, the intergranular corrosion resistance is remarkably deteriorated. Therefore, the content of B is set to 0.0005 to 0.005 mass%. Preferably it is 0.0005-0.0045 mass%, More preferably, it is 0.0005-0.004 mass%.

V: 0.005-0.250 mass%
V is a component that plays an important role in the present invention from the viewpoint of improving the intergranular corrosion resistance by addition of a small amount and further suppressing the effect of reducing the intergranular corrosion resistance of B. That is, V precipitates as a nitride of V and prevents the crystal grain size from becoming coarse during annealing at a high temperature, thereby making it possible to control the crystal grain size. The intergranular corrosion resistance can be improved by reducing the grain size and reducing the amount of segregation at the grain boundaries per unit area of B. However, if it is less than 0.005 mass%, the effect cannot be sufficiently obtained. On the other hand, if it exceeds 0.250 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted and intergranular corrosion resistance is deteriorated. Therefore, the content of V is set to 0.005 to 0.250 mass%. Preferably it is 0.010-0.240 mass%, More preferably, it is 0.015-0.230 mass%.

Nb: 0.005 to 0.250 mass%
Nb, like V, is a component that plays an important role in the present invention from the viewpoint of improving the intergranular corrosion resistance by addition of a small amount and further suppressing the action of reducing the intergranular corrosion resistance of B. . The mechanism is the same as that of V, which precipitates as Nb nitride, prevents coarsening of the crystal grain size during annealing at high temperature, and controls the crystal grain size. However, if it is less than 0.005 mass%, the effect cannot be sufficiently obtained. On the other hand, if it exceeds 0.250 mass%, precipitation of intermetallic compounds such as σ phase and χ phase is promoted and intergranular corrosion resistance is deteriorated. Therefore, the Nb content is set to 0.005 to 0.250 mass%. Preferably it is 0.010-0.240 mass%, More preferably, it is 0.015-0.230 mass%.

  In addition to satisfying the above composition components, the Fe—Ni—Cr—Mo alloy of the present invention has a grain size of 5.0 to 7.0 based on JIS G 0511, and further satisfies the following formula (1) It is necessary to contain.

0.005 ≦ [mass% V + mass% Nb] ≦ 0.250 (1)
As described above, V and Nb need to be added in order to improve the intergranular corrosion resistance of the Fe—Ni—Cr—Mo alloy even with a slight addition. Further, even when nitrides of V and Nb are precipitated and annealed at a high temperature, the growth of the crystal grain size is suppressed by a so-called pinning effect. The adverse effect of B on intergranular corrosion resistance is reduced, and good intergranular corrosion resistance is obtained. In order to obtain the effect sufficiently, as shown in FIG. 1, the total amount of the quantity elements of V and Nb needs to be 0.005 mass% or more. On the other hand, when the total amount of V and Nb exceeds 0.250 mass%, precipitation of the σ phase is promoted, and instead the intergranular corrosion resistance is lowered. Therefore, in order to obtain good intergranular corrosion resistance, V and Nb need to satisfy 0.005 ≦ [mass% V + mass% Nb] ≦ 0.250. Preferably, 0.010 ≦ [mass% V + mass% Nb] ≦ 0.249, and more preferably 0.015 ≦ [mass% V + mass% Nb] ≦ 0.248.

100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.90 (2)
B is an element that must be added in order to produce a steel sheet or coil with good surface properties, while it segregates at the grain boundaries, thus reducing the intergranular corrosion resistance. V and Nb improve the intergranular corrosion resistance by adding a small amount, and precipitate nitrides, and improve the intergranular corrosion resistance by controlling the crystal grain size by its pinning effect. Excessive addition of V and Nb promotes precipitation of the σ phase, leading to a decrease in intergranular corrosion resistance. That is, in the range where the amount of B is large and the amounts of V and Nb added are large, the intergranular corrosion resistance deteriorates due to both the adverse effects of B on intergranular corrosion resistance and the σ phase precipitation. Therefore, in order to obtain good intergranular corrosion resistance and good surface properties, 100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.90 must be satisfied. is necessary. Preferably 100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.88, more preferably 100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.86. .

Crystal grain size of 5.0 to 7.0 based on JIS G 0511 Since the intergranular corrosion rate is affected by the amount of B segregated at the crystal grain boundary per unit area, it is important to control the crystal grain size. When the crystal grain size based on JIS G 0577 is less than 5.0, the amount of B segregated at the crystal grain boundary per unit area increases, and the adverse effect of B on the grain boundary corrosion becomes remarkable. On the other hand, when the crystal grain size exceeds 7.0, the total area of the crystal grain size increases, and the adverse effect on B intergranular corrosion is reduced, so intergranular corrosion resistance is ensured, but the hardness is too high. Therefore, processing becomes difficult. Therefore, the crystal grain size based on JIS G 0511 needs to be 5.0 to 7.0.

Annealing at a temperature of 1100 to 1180 ° C. By the way, in a high Cr and high Mo steel as in the present invention, intermetallic compounds such as σ phase and χ phase are likely to remain, so that a solid austenite phase structure is obtained. Is preferably annealed at a high temperature. However, as described above, when the crystal grain size increases, the amount of B segregated at the crystal grain boundary per unit area increases, and the adverse effect of B on the grain boundary corrosion becomes remarkable. As shown in FIG. 2, when annealing is performed at a high temperature of 1200 ° C., the nitride of V or Nb is solidified, the crystal grain size is coarsened to 2.0, and good intergranular corrosion resistance cannot be obtained. . Conversely, when annealed at 1080 ° C., a fine crystal grain size of 8.0 was obtained, but the σ phase remained undissolved due to the low temperature, and good intergranular corrosion resistance. Was not obtained. Therefore, when producing the Fe—Ni—Cr—Mo alloy of the present invention, the hot-rolled steel sheet or cold-rolled steel sheet produced according to a conventional method needs to be annealed at a temperature of 1100 to 1180 ° C.

Next, the manufacturing method of the Fe-Ni-Cr-Mo alloy of this invention is demonstrated.
In the Fe-Ni-Cr-Mo alloy of the present invention, raw materials such as iron scrap, stainless steel scrap, ferronickel and ferrochrome are melted in an electric furnace, and a mixed gas of oxygen and rare gas is blown in an AOD furnace or a VOD furnace. smelting and then decarburization refining, burnt lime, Fe-Si alloy, after the addition of Al or the like reduction treatment of Cr oxides in the slag, the addition of fluorite _{CaO-SiO 2 -Al 2 O 3} -MgO -F-type slag is formed, deoxidized and desulfurized, and further Ca and Mg are added, and then a steel slab is formed by a continuous casting method or ingot-bundling rolling method, and then the steel slab is hot-rolled. Alternatively, it is preferable to further cold-roll to form various steel materials such as a thin steel plate, a thick steel plate, a shape steel, a bar steel, and a wire rod.

  Raw materials prepared by adjusting iron scrap, ferrochrome, ferronickel, stainless steel scrap, etc. to a predetermined ratio are melted in an electric furnace and secondarily refined in an AOD (Argon Oxygen Decarburization) furnace or VOD (Vacuum Oxygen Decarburization) furnace. After adjusting to the various component compositions shown in Table 3, it was continuously cast into steel slabs. In addition, the composition of C and S shown in Table 3 is a carbon / sulfur simultaneous analyzer (combustion in an oxygen stream-infrared absorption method), and the composition of N is an oxygen / nitrogen simultaneous analyzer (inert gas). -Impulse heating melting method), and the composition other than the above is a value analyzed using fluorescent X-ray analysis.

Subsequently, the slab was hot-rolled, and cold rolling, heat treatment and pickling were repeated to obtain a cold-rolled coil having a thickness of 2 to 3 mm. At this time, the surface and the back surface of the steel sheet after the cold rolling were visually observed over the entire length of the coil, and the coil in which surface cracks having a length of 10 mm or more occurred at 6 or more locations per 60 m ² had poor surface properties ( X) The coil having 5 or less locations was judged to have good surface properties (◯), and the coil having no surface cracks was judged to have excellent surface properties (◎).

Next, a corrosion test piece having a width of 20 mm, a length of 25 mm, and a plate thickness of 2 to 3 mm was taken from the cold-rolled coil, and Cr ⁶⁺ was added at 0.5 g / L in a boiling 70 mass% nitric acid solution without performing sensitizing heat treatment. Using the added solution, 5 batches were immersed in 48 hours for 48 batches while renewing the corrosive solution, the corrosion weight loss was measured, the corrosion rate was determined, and the intergranular corrosion resistance was evaluated. In the evaluation of intergranular corrosion resistance, if it exceeds 0.25 g / m ² · hr, the intergranular corrosion resistance is inferior (x), and the intergranular corrosion resistance is 0.25 g / m ² · hr or less. It was determined to be excellent (◯).

Furthermore, the crystal grain size of the steel sheet was measured based on JIS G 0511.
The surface property evaluation results, intergranular corrosion resistance evaluation results, and crystal grain size measurement results are also shown in Table 3.

  No. shown in Table 3. Steel plates 1 to 29 are invention examples that satisfy the conditions of the present invention, and have excellent intergranular corrosion resistance and surface properties. Among these, the example in which the B amount is 0.0015 mass% or less is within the acceptable range in the determination, but the hot workability is slightly lowered, so the evaluation of the surface property is from (（) to (○). It was.

On the other hand, no. Steel plates from 30 to 42 are comparative examples.
No. Steel No. 30 satisfies the formulas (1) and (2), but the amount of B is as high as 0.0058 mass% and is inferior in intergranular corrosion resistance.
No. Steel No. 31 does not satisfy the formula (1) and is inferior in intergranular corrosion resistance.
No. Since steel No. 32 has a large V content of 0.261 mass%, a σ phase is precipitated, the formula (1) is not satisfied, and the intergranular corrosion resistance is poor.
No. Steel No. 33 has a large Nb content of 0.268 mass%, so a sigma phase is precipitated, the equation (1) is not satisfied, and the intergranular corrosion resistance is poor.
No. Steel No. 34 does not satisfy the formula (1) because the total amount of V and Nb added is small, so that the pinning effect cannot be obtained, the crystal grain size becomes coarse, and the intergranular corrosion resistance is poor.
No. Steel No. 35 is No. Since the total addition amount of V and Nb is small as in the case of steel No. 34, the annealing temperature is lowered and annealing is performed, but the pinning effect cannot be obtained, the crystal grain size becomes coarse, and the intergranular corrosion resistance is poor.
No. Steel No. 36 had a low B content of 0.0001 mass% and good intergranular corrosion resistance, but surface cracks occurred and the surface properties were inferior.
No. In Steel No. 37, the amount of B is as low as 0.0002 mass%, so surface cracks occur, the surface properties are inferior, and the σ phase is generated because the formula (1) is not satisfied, and the intergranular corrosion resistance is also inferior.
No. No. 38 has a very high B content of 0.055 mass%, does not satisfy the formulas (1) and (2), and is inferior in intergranular corrosion resistance.
No. In the steel No. 39, Al is as low as 0.005 mass%, the pinning effect by AlN is not obtained, and the intergranular corrosion resistance is inferior.
No. In the steel No. 40, Al is as high as 0.158 mass%, and the oxide of Al is dissolved in a high-potential intergranular corrosion test, so that the intergranular corrosion resistance is inferior.
No. Steel No. 41 has an annealing temperature as low as 1090 ° C., and a σ phase is precipitated, which is inferior in intergranular corrosion resistance.
No. Steel No. 42 has an annealing temperature as high as 1190 ° C., the nitrides of V and Nb are solidified, the crystal grain size becomes coarse, and the intergranular corrosion resistance is inferior.

Since the Fe—Ni—Cr—Mo alloy of the present invention has both excellent intergranular corrosion resistance and excellent surface properties, it can be suitably used for chemical plants where intergranular corrosion causes corrosion.

Claims (6)

C: 0.005-0.03 mass%,
Si: 0.02-0.25 mass%,
Mn: 0.03 to 0.40 mass%,
P: 0.040 mass% or less,
S: 0.003 mass% or less,
Ni: 32.0-38.0 mass%,
Cr: 21.0-25.0 mass%,
Mo: 6.0-8.0 mass%,
N: 0.20-0.30 mass%,
Cu: 0.01-0.40 mass%,
Al: 0.012-0.1 mass%,
B: 0.0005 to 0.005 mass%
And further V: 0.005 to 0.250 mass%
Or Nb: 0.005-0.250 mass%
1 type or 2 types, and the balance consists of Fe and inevitable impurities,
And the Fe-Ni-Cr-Mo alloy characterized by satisfying the following formula (1).
0.005 ≦ [mass% V + mass% Nb] ≦ 0.250 (1)   2. The Fe—Ni—Cr—Mo alloy according to claim 1, having a grain size of 5.0 to 7.0 based on JIS G 0511. 3. The Fe-Ni-Cr-Mo alloy according to claim 1 or 2, wherein the following formula (2) is satisfied.
100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.90 (2) C: 0.005-0.03 mass%,
Si: 0.02-0.25 mass%,
Mn: 0.03 to 0.40 mass%,
P: 0.040 mass% or less,
S: 0.003 mass% or less,
Ni: 32.0-38.0 mass%,
Cr: 21.0-25.0 mass%,
Mo: 6.0-8.0 mass%,
N: 0.20-0.30 mass%,
Cu: 0.01-0.40 mass%,
Al: 0.012-0.1 mass%,
B: 0.0005 to 0.005 mass%
And further V: 0.005 to 0.250 mass%
Or Nb: 0.005-0.250 mass%
1 type or 2 types, and the balance consists of Fe and inevitable impurities,
And manufacturing the hot-rolled steel sheet or cold-rolled steel sheet of the Fe-Ni-Cr-Mo alloy which satisfies the following (1) formula,
A method for producing an Fe—Ni—Cr—Mo alloy, comprising annealing the hot-rolled steel plate or the cold-rolled steel plate at a temperature of 1100 to 1180 ° C.
0.005 ≦ [mass% V + mass% Nb] ≦ 0.250 (1)   5. The method for producing an Fe—Ni—Cr—Mo alloy according to claim 4, having a grain size of 5.0 to 7.0 based on JIS G 0511. 6. The method for producing an Fe-Ni-Cr-Mo alloy according to claim 4 or 5, wherein the following formula (2) is satisfied.
100 [mass% B] + 2.0 × [mass% V + mass% Nb] ≦ 0.90 (2)

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