KR20200080312A - Method for manufacturing two-phase stainless steel and two-phase stainless steel - Google Patents

Abstract

Provided is a two-phase stainless steel with reduced generation of formula. The two-phase stainless steel according to the present disclosure, in mass%, Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to 0.10% Less than, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, and the balance is composed of Fe and impurities, and the chemical composition satisfying formula (1), The ferrite phase and the remainder of 35 to 65 volume% contain a microstructure composed of austenite phase, and the area percentage of Cu precipitated in the ferrite phase is 0.5% or less.

Description

Method for manufacturing two-phase stainless steel and two-phase stainless steel

The present invention relates to a two-phase stainless steel and a method for manufacturing the two-phase stainless steel.

It is known that a two-phase stainless steel having a two-phase structure of ferrite phase and austenite phase has excellent corrosion resistance. The two-phase stainless steel is particularly excellent in corrosion resistance (hereinafter referred to as "pitting resistance") against crevice and/or crevice corrosion, which is problematic in aqueous solutions containing chloride. Therefore, two-phase stainless steel is widely used in a wet environment containing chlorides such as seawater. In a wet environment containing chloride, two-phase stainless steel is used, for example, in flow line pipes, umbilical tubes and heat exchangers.

In recent years, the corrosion conditions in the use environment of a two-phase stainless steel have become increasingly strict. Therefore, more excellent pitting resistance is required for the two-phase stainless steel. To further improve the pitting resistance of two-phase stainless steel, various techniques have been proposed.

International Publication No. 2013/191208 (Patent Document 1) contains, in mass%, Ni: 3 to 8%, Cr: 20 to 35%, Mo: 0.01 to 4.0%, N: 0.05 to 0.60%, and Re Disclosed is a two-phase stainless steel characterized by further containing at least one selected from: 2.0% or less, Ga: 2.0% or less, and Ge: 2.0% or less. In Patent Document 1, by containing Re, Ga, or Ge in a two-phase stainless steel, the critical potential (formal potential) at which the formula is generated is raised to increase the pitting resistance and crevice corrosion resistance.

International Publication No. 2010/082395 (Patent Document 2) is mass%, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to A method for producing a two-phase stainless steel pipe by cold rolling after producing a small pipe for cold working by hot-working or further heat-treating heat treatment of a two-phase stainless steel material containing 3% and N: 0.15 to 0.60%. It starts. The manufacturing method of the two-phase stainless steel pipe of patent document 2 is the workability Rd(=exp[{In(MYS)-In(14.5×Cr+48.3×Mo+20.7×W+) at the section reduction rate in the final cold rolling process. 6.9×N)}/0.195]) is cold rolled within a range of 10 to 80%, and is characterized in that it is a method for producing a two-phase stainless steel pipe having the lowest yield strength of 758.3 to 965.2 MPa. Thereby, it is described in patent document 2 that a biphasic stainless steel pipe which can be used in, for example, an oil well or a gas well, exhibits excellent corrosion resistance in a carbonic acid gas corrosion environment or a stress corrosion environment, and also has high strength, is obtained.

Japanese Patent Publication No. 2007-84837 (Patent Document 3) is mass%, Cr: 20 to 30%, Ni: 1 to 11%, Cu: 0.05 to 3.0%, Nd: 0.005 to 0.5%, N: Two-phase stainless steel containing 0.1 to 0.5% and one or both of Mo: 0.5 to 6 and W: 1 to 10 is disclosed. In Patent Document 3, the hot workability of the two-phase stainless steel is improved by containing Nd.

Japanese Patent Publication No. 2005-520934 (Patent Document 4) is, by weight, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5% , N: 0.2% to 0.7%, Ba: 0.0001 to 0.6%, and the official resistance equivalent index PREW satisfies 40≤PREW≤67. By this, it is possible to obtain a super two-phase stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability, in which the formation of intermetallic phases such as fragile sigma (σ) phase and chi (χ) phase is suppressed. It is described in.

International Publication No. 2013/191208 International Publication No. 2010/082395 Japanese Patent Publication No. 2007-84837 Japanese Patent Publication No. 2005-520934

As described above, in recent years, two-phase stainless steels having better pitting resistance have been required. Therefore, by means other than the techniques described in Patent Documents 1 to 4, a two-phase stainless steel exhibiting excellent pitting resistance may be obtained.

An object of the present disclosure is to provide a two-phase stainless steel having excellent pitting resistance and a method for manufacturing the two-phase stainless steel.

The two-phase stainless steel according to the present disclosure is, in mass%, Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to 0.10% Less than, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, B: 0 to 0.0040% And the remainder is composed of Fe and impurities, and contains a chemical composition that satisfies Formula (1), and a microstructure consisting of 35 to 65% by volume of the ferrite phase and the remainder of the austenite phase. In the two-phase stainless steel according to the present disclosure, the area ratio of Cu precipitated in the ferrite phase is 0.5% or less.

Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).

The method for manufacturing a two-phase stainless steel according to the present disclosure includes a preparation step, a hot working step, a cooling step, and a solution heat treatment step. In the preparation step, a material having the above-described chemical composition is prepared. In the hot working step, the material is hot worked at 850°C or higher. In the cooling step, the material after hot working is cooled to 5°C/sec or more. In the solution heat treatment step, the cooled material is subjected to solution heat treatment at 1070°C or higher.

The two-phase stainless steel according to the present disclosure has excellent pitting resistance. The method for producing a two-phase stainless steel according to the present disclosure can produce the two-phase stainless steel described above.

The present inventors investigated and examined the method of improving the pitting resistance of a two-phase stainless steel. As a result, the following knowledge was obtained.

It is known that Cr, Mo and Cu are effective for improving the pitting resistance of a two-phase stainless steel. Among Cr, Mo and Cu, the mechanism by which Cr and Mo improve the pitting resistance of a two-phase stainless steel is considered as follows. Cr is an oxide and is a main component of the passivation film on the surface of the two-phase stainless steel. The passivation film prevents the corrosion factor from contacting the surface of the two-phase stainless steel. As a result, the two-phase stainless steel with the passivation film formed on the surface has high pitting resistance. Mo is contained in the passivation film to further improve the pitting resistance of the passivation film.

On the other hand, among Cr, Mo and Cu, the mechanism by which Cu increases the pitting resistance of a two-phase stainless steel is considered as follows. It is said that the next two steps exist until the formula is created. The first step is the occurrence of the formula (initial step). The next step is the official progress (progress step). Conventionally, Cu has been considered to have an effect of suppressing the progress of the formula. In particular, in an acidic solution, an active site having a high dissolution rate is formed on the surface of a two-phase stainless steel. Cu covers the active site and suppresses dissolution of the two-phase stainless steel. Thereby, it has been thought that Cu suppresses the progress of the formula of a two-phase stainless steel.

Based on the above mechanism, in two-phase stainless steel, Cr, Mo and Cu have been considered to be effective elements for improving pitting resistance. Therefore, in the conventional two-phase stainless steel, Cr, Mo, and Cu have been actively contained for the purpose of improving pitting resistance. However, as a result of examination by the present inventors, the following knowledge, which has not been conventionally known, was obtained. Specifically, the present inventors have found that, among Cr, Mo, and Cu, Cu may lower the pitting resistance in the generation of the formula (initial stage).

Table 1 is a table showing the chemical composition of the test pieces of Test Nos. 2 and 5 and the official potential as an index of pitting resistance in Examples described later. The chemical composition of Table 1 is excerpted and divided into two stages for the chemical compositions of Steel Types B and E corresponding to Test Nos. 2 and 5 from Table 3 described later. The chemical composition in Table 1 is described in mass%, and the balance is Fe and impurities. The formal dislocation of Table 1 is obtained by extracting the official dislocation of the corresponding test number from Table 4 to be described later.

With reference to Table 1, the Cu content of the test piece of Test No. 2 was higher than the Cu content of the test piece of Test No. 5. Moreover, the test piece Cr and Mo content of test number 2 was high compared with the test piece Cr and Mo content of test number 5. Therefore, based on the conventional knowledge, it can be expected that the test piece of Test No. 2 with a high content of Cr, Mo and Cu has better pitting resistance than the test piece of Test No. 5. However, the official potential, which is an index of test piece pitting resistance of Test No. 2, was 71 mVvs.SCE, and was lower than the official potential of test specimen 5, 346 mVvs.SCE.

That is, from the conventional knowledge, the test piece of Test No. 2, which is expected to have superior pitting resistance than that of Test No. 5, had lower pitting resistance than that of Test No. 5. Therefore, the present inventors focused on the microstructures of test pieces 2 and 5 and investigated in more detail. As a result, it was found that in the test piece of Test No. 2, the area ratio of Cu deposited in the ferrite phase (referred to as the Cu area rate in the ferrite phase) was higher than that of the test piece of Test No. 5.

Therefore, the present inventors also investigated and examined in detail the effect of Cu precipitated in the ferrite phase on the pitting resistance of the two-phase stainless steel. Table 2 is a table showing the chemical composition of the test pieces of Test Nos. 3 and 6, the Cu area ratio in the ferrite phase, and the official potential as an index of pitting resistance in Examples described later. The chemical composition of Table 2 is excerpted and divided into two stages for the chemical composition of Steel Type C corresponding to Test Nos. 3 and 6 from Table 3 described later. The chemical composition in Table 2 is described in mass%, the balance being Fe and impurities. The Cu area ratio in the ferrite phase of Table 2 is obtained by extracting the Cu area ratio in the ferrite phase of the corresponding test number from Table 4 to be described later. The official potential of Table 2 is the excerpt from Table 4 to be described later, with the official potential of the corresponding test number.

With reference to Table 2, the chemical composition of the test piece of Test No. 3 and the test piece of Test No. 6 was the same. On the other hand, in the test piece of Test No. 6, the Cu area ratio in the ferrite phase of Test No. 3 was lower than the Cu area rate in the Ferrite phase of Test No. 3. As a result, the test specimen formula potential of Test No. 6 was 204 mVvs.SCE, and was higher than that of Test No. 3 specimen formula potential -12 mVvs.SCE. That is, in the test piece of Test No. 6, precipitation of Cu in the ferrite phase was reduced, and as a result, it had better pitting resistance than the test piece of Test No. 3.

As described above, it has been conventionally thought that when the content of Cr, Mo, and Cu is increased, the pitting resistance increases. However, among the Cr, Mo, and Cu, the present inventors have found for the first time that Cu has a possibility of lowering the pitting resistance. The present inventors have also obtained a knowledge that is not known at all in the past, that by reducing the amount of Cu precipitated in the ferrite phase, the pitting resistance can be improved.

The detailed reason why Cu precipitated in the ferrite phase degrades the pitting resistance of the two-phase stainless steel is unknown. However, the present inventors think as follows. Cu precipitated in the ferrite phase may possibly inhibit the uniform formation of the passivation film. Therefore, when the amount of Cu precipitated in the ferrite phase is large, there is a possibility that the effect of suppressing the contact between the corrosion factor and the surface of the two-phase stainless steel by the passivation film may be reduced. As a result, it is thought that the formula is generated on the surface of the two-phase stainless steel.

The two-phase stainless steel according to the present embodiment, completed on the basis of the above findings, in mass%, Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00 %, Cu: 0.01 to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, B: 0 to 0.0040% And the remainder is composed of Fe and impurities, and contains a chemical composition that satisfies Formula (1), and a microstructure consisting of 35 to 65% by volume of the ferrite phase and the remainder of the austenite phase. In the two-phase stainless steel according to the present embodiment, the area ratio of Cu precipitated in the ferrite phase is 0.5% or less.

Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).

The two-phase stainless steel according to the present embodiment has the above-described chemical composition and microstructure described above, and the area ratio of Cu in the ferrite phase is 0.5% or less. As a result, the two-phase stainless steel according to the present embodiment has excellent pitting resistance.

Preferably, the chemical composition is 1% or more selected from the group consisting of Ca: 0.0001 to 0.0040%, Mg: 0.0001 to 0.0040% and B: 0.0001 to 0.0040% in mass%.

In this case, hot workability of the two-phase stainless steel according to the present embodiment is improved.

The manufacturing method of the two-phase stainless steel which concerns on this embodiment is equipped with a preparation process, a hot working process, a cooling process, and a solution heat treatment process. In the preparation step, a material having the above-described chemical composition is prepared. In the hot working step, the material is hot worked at 850°C or higher. In the cooling step, the material after hot working is cooled at 5°C/sec or higher. In the solution heat treatment step, the cooled material is subjected to solution heat treatment at 1070°C or higher.

The two-phase stainless steel produced by the production method according to the present embodiment has the above-described chemical composition and microstructure described above, and the area ratio of Cu in the ferrite phase is 0.5% or less. As a result, the two-phase stainless steel produced by the manufacturing method according to the present embodiment has excellent pitting resistance.

Hereinafter, the two-phase stainless steel according to the present embodiment will be described in detail.

[Chemical composition]

The chemical composition of the two-phase stainless steel according to the present embodiment contains the following elements. In addition, unless otherwise specified,% with respect to the element means mass%.

[About essential elements]

The chemical composition of the two-phase stainless steel according to the present embodiment essentially contains the following elements.

Cr: more than 27.00% to 29.00%

Chromium (Cr) is an oxide, and forms a passivation film on the surface of a two-phase stainless steel. The passivation film prevents the corrosion factor from contacting the surface of the two-phase stainless steel. As a result, the occurrence of the formula of the two-phase stainless steel is suppressed. Cr is also an element necessary to obtain a ferrite structure of two-phase stainless steel. By obtaining a sufficient ferrite structure, stable pitting resistance is obtained. If the Cr content is too low, this effect is not obtained. On the other hand, if the Cr content is too high, the hot workability of the two-phase stainless steel decreases. Therefore, the Cr content is more than 27.00% to 29.00%. The preferable lower limit of the Cr content is 27.50%, and more preferably 28.00%. The preferable upper limit of the Cr content is 28.50%.

Mo: 2.50 to 3.50%

Molybdenum (Mo) is contained in the passivation film, further enhancing the corrosion resistance of the passivation film. As a result, the pitting resistance of the two-phase stainless steel is enhanced. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, workability in the case of assembling a steel pipe containing two-phase stainless steel is deteriorated. Therefore, the Mo content is 2.50 to 3.50%. The preferable lower limit of the Mo content is 2.80%, and more preferably 3.00%. The preferable upper limit of the Mo content is 3.30%.

Ni: 5.00 to 8.00%

Nickel (Ni) is an austenite stabilizing element and is an element necessary for obtaining a two-phase structure of ferrite and austenite. If the Ni content is too low, this effect cannot be obtained. On the other hand, when the Ni content is too high, the balance of the ferrite phase and the austenite phase is not obtained. In this case, two-phase stainless steel is not stably obtained. Therefore, the Ni content is 5.00 to 8.00%. The preferable lower limit of the Ni content is 5.50%, and more preferably 6.00%. The preferable upper limit of the Ni content is 7.50%.

W: 4.00 to 6.00%

Tungsten (W), like Mo, is contained in the passivation film, further enhancing the corrosion resistance of the passivation film. As a result, the occurrence of the formula of the two-phase stainless steel is suppressed. If the W content is too low, this effect cannot be obtained. On the other hand, when the W content is too high, the σ phase tends to precipitate, and toughness decreases. Therefore, the W content is 4.00 to 6.00%. The preferable lower limit of the W content is 4.50%. The preferable upper limit of the W content is 5.50%.

Cu: 0.01 to less than 0.10%

Copper (Cu) is an element effective for suppressing the evolution (progression stage) of the formula. If the Cu content is too low, this effect cannot be obtained. On the other hand, among Cr, Mo and Cu, Cu decreases pitting resistance in the generation of the formula (initial stage). Therefore, the two-phase stainless steel of this embodiment reduces Cu content compared with the conventional two-phase stainless steel. As a result, the precipitation of Cu in the ferrite phase is suppressed, and the generation (initial step) of the formula of the two-phase stainless steel is suppressed. When the Cu content is too high, the Cu area ratio in the ferrite phase becomes too high. In this case, the pitting resistance of the two-phase stainless steel decreases. Therefore, the Cu content is less than 0.01 to 0.10%. The upper limit with preferable Cu content is 0.07 %, More preferably, it is 0.05 %.

N: more than 0.400% to 0.600%

Nitrogen (N) is an austenite stabilizing element and is an element necessary for obtaining a two-phase structure of ferrite and austenite. N also increases the pitting resistance of the two-phase stainless steel. If the N content is too low, these effects are not obtained. On the other hand, if the N content is too high, the toughness and hot workability of the two-phase stainless steel decreases. Therefore, the N content is more than 0.400% to 0.600%. The preferable lower limit of the N content is 0.420%. The preferable upper limit of the N content is 0.500%.

C: 0.030% or less

Carbon (C) is inevitably contained. That is, the C content is more than 0%. C forms Cr carbide at the grain boundaries and increases the corrosion susceptibility at grain boundaries. Therefore, the C content is 0.030% or less. The upper limit with preferable C content is 0.025 %, More preferably, it is 0.020 %. The C content is preferably as low as possible. However, the extreme reduction of the C content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the preferable lower limit of the C content is 0.001%, and more preferably 0.005%.

Si: 1.00% or less

Silicon (Si) deoxidizes the steel. When Si is used as a deoxidizer, the Si content is more than 0%. On the other hand, when the Si content is too high, the hot workability of the two-phase stainless steel decreases. Therefore, the Si content is 1.00% or less. The upper limit with preferable Si content is 0.80%, More preferably, it is 0.70%. Although the lower limit of the Si content is not particularly limited, it is, for example, 0.20%.

Mn: 1.00% or less

Manganese (Mn) deoxidizes the river. When Mn is used as a deoxidizer, the Mn content is more than 0%. On the other hand, when the Mn content is too high, the hot workability of the two-phase stainless steel decreases. Therefore, the Mn content is 1.00% or less. The upper limit with preferable Mn content is 0.80%, More preferably, it is 0.70%. The lower limit of the Mn content is not particularly limited, but is, for example, 0.20%.

sol. Al: 0.040% or less

Aluminum (Al) deoxidizes the steel. When Al is used as a deoxidizer, the Al content is more than 0%. On the other hand, when the Al content is too high, the hot workability of the two-phase stainless steel decreases. Therefore, the Al content is 0.040% or less. The upper limit with preferable Al content is 0.030%, More preferably, it is 0.025%. Although the lower limit of the Al content is not particularly limited, it is, for example, 0.005%. In the present embodiment, the Al content refers to an acid-soluble Al (sol. Al) content.

V: 0.50% or less

Vanadium (V) is inevitably contained. That is, the V content is more than 0%. When the V content is too high, the ferrite phase excessively increases, and the toughness and corrosion resistance of the two-phase stainless steel may occur. Therefore, the V content is 0.50% or less. The preferable upper limit of the V content is 0.40%, and more preferably 0.30%. The lower limit of the V content is not particularly limited, but is, for example, 0.05%.

O: 0.010% or less

Oxygen (O) is an impurity. That is, the O content is more than 0%. O lowers the hot workability of the two-phase stainless steel. Therefore, the O content is 0.010% or less. The upper limit with preferable O content is 0.007%, More preferably, it is 0.005%. It is preferable that the O content is as low as possible. However, the extreme reduction of the O content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the preferable lower limit of the O content is 0.0001%, and more preferably 0.0005%.

P: 0.030% or less

Phosphorus (P) is an impurity. That is, the P content is more than 0%. P lowers the pitting resistance and toughness of the two-phase stainless steel. Therefore, the P content is 0.030% or less. The upper limit with preferable P content is 0.025 %, More preferably, it is 0.020 %. The P content is preferably as low as possible. However, the extreme reduction of the P content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the preferable lower limit of the P content is 0.001%, and more preferably 0.005%.

S: 0.020% or less

Sulfur (S) is an impurity. That is, the S content is more than 0%. S degrades the hot workability of two-phase stainless steel. Therefore, the S content is 0.020% or less. The upper limit with preferable S content is 0.010 %, More preferably, it is 0.005 %, More preferably, it is 0.003 %. The S content is preferably as low as possible. However, the extreme reduction of the S content greatly increases the manufacturing cost. Therefore, when industrial production is considered, the preferable lower limit of the S content is 0.0001%, and more preferably 0.0005%.

The balance of the chemical composition of the two-phase stainless steel of the present embodiment is made of Fe and impurities. Here, the impurities in the chemical composition are mixed from ores as raw materials, scrap, or a manufacturing environment when industrially producing two-phase stainless steel, and do not adversely affect the two-phase stainless steel according to the present embodiment. It means that it is allowed within the scope.

[About arbitrary elements]

The chemical composition of the two-phase stainless steel according to the present embodiment may optionally contain the following elements.

Ca: 0 to 0.0040%

Calcium (Ca) is an arbitrary element and need not be contained. That is, the Ca content may be 0%. When contained, Ca increases the hot workability of the two-phase stainless steel. If Ca is contained even finely, this effect is obtained to some extent. On the other hand, if the Ca content is too high, coarse oxides are generated, and the hot workability of the two-phase stainless steel decreases. Therefore, the Ca content is 0 to 0.0040%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0030%.

Mg: 0 to 0.0040%

Magnesium (Mg) is an arbitrary element and need not be contained. That is, the Mg content may be 0%. When contained, Mg, like Ca, enhances the hot workability of the two-phase stainless steel. If Mg is contained even finely, this effect is obtained to some extent. On the other hand, if the Mg content is too high, coarse oxides are generated, and the hot workability of the two-phase stainless steel decreases. Therefore, the Mg content is 0 to 0.0040%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

B: 0 to 0.0040%

Boron (B) is an arbitrary element and need not be contained. That is, the B content may be 0%. When contained, B, like Ca and Mg, enhances the hot workability of the two-phase stainless steel. If B is contained even at a fine level, this effect is obtained to some extent. On the other hand, if the B content is too high, the toughness of the two-phase stainless steel decreases. Therefore, the B content is 0 to 0.0040%. The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

[About equation (1)]

The chemical composition of the two-phase stainless steel according to the present embodiment satisfies the content of each element, and satisfies the following formula (1).

Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).

It is defined as F1=Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu). F1 is an index showing pitting resistance. When F1 is less than 65.2, the pitting resistance of the two-phase stainless steel decreases. Therefore, F1≥65.2. The lower limit of F1 is preferably 68.0, more preferably 69.0, and even more preferably 70.0. The upper limit of F1 is not particularly limited, but is, for example, 90.0.

[About micro organization]

The microstructure of the two-phase stainless steel according to the present embodiment is made of ferrite and austenite. Specifically, the microstructure of the two-phase stainless steel according to the present embodiment is composed of an austenitic phase and a ferrite phase of 35 to 65% by volume. When the volume fraction of the ferrite phase (hereinafter also referred to as ferrite fraction) is less than 35%, the possibility of occurrence of stress corrosion cracking increases depending on the use environment. On the other hand, when the volume ratio of the ferrite phase exceeds 65%, the possibility that the toughness of the two-phase stainless steel decreases increases. Therefore, the microstructure of the two-phase stainless steel of the present embodiment contains a ferrite phase and a balance of 35 to 65% by volume of the austenite phase.

[Measurement method of ferrite fraction]

In the present embodiment, the ferrite fraction of two-phase stainless steel can be determined by the following method. First, a microstructure observation test piece is taken from a two-phase stainless steel. If the two-phase stainless steel is a steel sheet, a section perpendicular to the plate width direction of the steel sheet (hereinafter referred to as an observation surface) is polished. When the two-phase stainless steel is a steel pipe, a cross section (observation surface) including the axial direction and the thickness direction of the steel pipe is polished. If the two-phase stainless steel is a steel bar or a wire rod, the cross section (observation surface) including the axial direction of the steel bar or wire rod is polished. Next, the observation surface after polishing is etched using a mixture of aqua regia and glycerin.

The etched observation surface is observed with an optical microscope for 10 fields of view. The viewing area is, for example, 2000 µm ² (magnification 500 times). In each viewing field, ferrite and other phases can be distinguished from contrast. Therefore, ferrite in each observation is specified from the contrast. The area ratio of the specified ferrite is measured by a dot method based on JIS G0555 (2003). The measured area ratio is equal to the volume fraction, and this is defined as the ferrite fraction (volume%).

[About Cu area ratio in ferrite phase]

The area percentage of Cu deposited in the ferrite phase of the two-phase stainless steel according to the present embodiment is 0.5% or less. As mentioned above, Cu contained in two-phase stainless steel is considered to suppress the progress of the formula of two-phase stainless steel. Therefore, in the two-phase stainless steel according to the present embodiment, Cu is contained in an amount of 0.01 to less than 0.10%. On the other hand, in two-phase stainless steels containing less than 0.01 to 0.10% of Cu, metal Cu may precipitate in the ferrite phase. As described above, it has been found that Cu precipitated in the ferrite phase lowers the effect of suppressing the generation of the formula due to the passivation film. That is, the metal Cu precipitated in the ferrite phase deteriorates the pitting resistance of the two-phase stainless steel.

Thus, the two-phase stainless steel according to the present embodiment reduces the Cu area ratio in the ferrite phase to 0.5% or less. Therefore, the occurrence of the formula of the two-phase stainless steel is suppressed. The lower the Cu area ratio in the ferrite phase, the more preferable. The upper limit of the Cu area ratio in the ferrite phase is preferably 0.3%, and more preferably 0.1%. The lower limit of the Cu area ratio in the ferrite phase is 0.0%.

[Measurement method of Cu area ratio in ferrite phase]

In this specification, the Cu area ratio in the ferrite phase means the area ratio of the Cu precipitated in the ferrite phase to the ferrite phase in the microstructure of the two-phase stainless steel. In the present embodiment, the Cu area ratio in the ferrite phase can be measured by the following method. A thin film sample for transmission electron microscope (TEM) observation is prepared by FIB-micro sampling. A focused ion beam processing apparatus (manufactured by Hitachi High-Tech Science, MI4050) is used for the production of thin film samples. A thin film sample for TEM observation is produced from an arbitrary portion of a two-phase stainless steel. For the production of a thin film sample, a carbon depot film is used as a mesh made of Mo and a surface protective film.

For TEM observation, an electrolytic emission type transmission electron microscope (JEM-2100F manufactured by Nippon Denshi Co., Ltd.) is used. The observation magnification is 10000 times, and TEM observation is performed. The contrast between the ferrite phase and the austenite phase in the field of view is different. Therefore, a grain boundary is specified based on contrast. The image of the region surrounded by each grain boundary is specified by X-ray diffraction (XRD). Of the regions surrounded by the grain boundaries, the area of the ferrite phase and the specified region can be determined by image analysis.

Elemental analysis by energy dispersive X-ray spectrometry (EDS) is performed on the observation field of view, and an elemental map is generated. In addition, a precipitate can be specified from contrast. Therefore, it can be specified by EDS that the specific precipitate is metal Cu based on the contrast in the ferrite phase specified by XRD.

The area of Cu precipitated in the specified ferrite phase can be determined by image analysis. The total area of Cu precipitated in the ferrite phase is divided by the total area of the ferrite phase. In this way, the Cu area ratio (%) in the ferrite phase is measured.

The two-phase stainless steel according to the present embodiment satisfies all of the above-described chemical composition including formula (1) and the microstructure containing Cu area fraction in the ferrite phase. Therefore, the two-phase stainless steel according to the present embodiment has excellent pitting resistance.

[Yield strength]

The yield strength of the two-phase stainless steel according to the present embodiment is not particularly limited. However, if the yield strength is 750 MPa or less, cold working can be omitted in the manufacturing process. In this case, manufacturing cost can be reduced. Therefore, the yield strength is preferably 750 MPa or less. More preferably, the yield strength is 720 MPa or less. The lower limit of the yield strength is not particularly limited, but is, for example, 300 MPa.

[Method for measuring yield strength]

Yield strength in the present specification means 0.2% yield strength obtained by a method in accordance with JIS Z2241 (2011).

[Shape of 2-phase stainless steel]

The shape of the two-phase stainless steel according to the present embodiment is not particularly limited. The two-phase stainless steel may be, for example, a steel pipe, a steel plate, a steel bar, or a wire rod.

[Manufacturing method]

The two-phase stainless steel of the present embodiment can be produced, for example, by the following method. The manufacturing method includes a preparation step, a hot working step, a cooling step, and a solution heat treatment step.

[Preparation process]

In the preparation step, a material having the above-described chemical composition is prepared. The material may be a cast steel produced by a continuous casting method (including round CC), or a steel cast manufactured from the cast steel. Moreover, the steel piece manufactured by hot-processing the ingot manufactured by the ingot process may be sufficient.

[Hot processing process]

The prepared material is charged into a heating furnace or a cracking furnace, and heated to, for example, 1150 to 1300°C. Next, the heated material is hot worked. The hot working may be hot forging, for example, hot extrusion using the Eugene-Sejournet method or the Elhard push bench method, or hot rolling. The hot working may be performed once or multiple times.

The heated material is hot processed to 850°C or higher. More specifically, the surface temperature of the steel at the end of hot working is 850°C or higher. When the surface temperature of the steel at the end of the hot working is less than 850°C, a large amount of Cu precipitates in the ferrite phase. As a result, the Cu area ratio in the ferrite phase may not be sufficiently reduced even by the solution treatment described later. In this case, the pitting resistance of the two-phase stainless steel decreases. Therefore, the surface temperature of the steel at the end of hot working is 850°C or higher. In the case where the hot working is performed multiple times, the surface temperature of the steel material at the end of the final hot working is 850°C or higher. Thereby, it can suppress that Cu precipitates in a ferrite phase at the time of completion of hot working. The upper limit of the surface temperature of the steel at the end of the hot working is not particularly limited, but is, for example, 1300°C. In addition, the end time of hot working means within 3 seconds after the end of hot working.

[Cooling process]

Next, the material after hot working is cooled at 5°C/sec or higher. At around 850°C, Cu starts to precipitate in the ferrite phase. Therefore, if the cooling rate after hot working is too low, a large amount of Cu precipitates in the ferrite phase. As a result, the Cu area ratio in the ferrite phase may not be sufficiently reduced even by the solution treatment described later. In this case, the pitting resistance of the two-phase stainless steel decreases. Therefore, the cooling rate after hot working is 5°C/sec or more. Here, when performing hot working multiple times, after hot working means the last hot working. That is, in the present embodiment, the material after the final hot working is cooled at 5°C/or higher. The upper limit of the cooling rate is not particularly limited. The cooling method is, for example, air cooling, water cooling, or oil cooling.

[Solution heat treatment process]

Subsequently, the cooled material is subjected to solution heat treatment at 1070°C or higher. Cu precipitated in the ferrite phase is dissolved by solution heat treatment. The Cu area ratio in the ferrite phase can be set to 0.5% or less by performing a solution heat treatment at 1070°C or higher for a material in which the precipitation of Cu in the ferrite phase is sufficiently suppressed at the end of the hot working and after cooling. The upper limit of the solution heat treatment temperature is not particularly limited, but is, for example, 1150°C. The treatment time of the solution heat treatment is not particularly limited. The treatment time of the solution heat treatment is, for example, 1 to 30 minutes.

Through the above steps, the two-phase stainless steel according to the present embodiment can be produced. Moreover, in this embodiment, it is preferable that cold working is not performed because manufacturing cost becomes high.

Example

The alloy having the chemical composition shown in Table 3 was melted in a vacuum melting furnace of 50 kg, and the obtained ingot was heated to 1200°C, hot-forged and hot-rolled, and processed into a steel plate having a thickness of 10 mm. The temperature at the end of rolling, shown in Table 4, is the surface temperature of the steel sheet at the end of hot rolling. The cooling rate after rolling shown in Table 4 is the cooling rate after hot rolling. In addition, the steel sheet was subjected to solution treatment at a solution temperature (°C) shown in Table 4 to obtain test pieces of each test number.

[Ferrite Fraction Measurement Test]

About the test piece of each test number, the ferrite fraction (volume %) was measured by the method mentioned above. Table 4 shows the results. In addition, the microstructure remainder of the test piece of each test number was austenite.

[Measurement test of Cu area ratio in ferrite phase]

About the test piece of each test number, the Cu area ratio (%) in a ferrite phase was measured by the method mentioned above. Table 4 shows the results.

[Official potential measurement test]

The test piece formula potential of each test number after solution treatment was measured. First, the test piece was machined to obtain a test piece having a diameter of 15 mm and a thickness of 2 mm. Using the obtained test piece, 80°C, 25% NaClaq. Among them, the official potential was measured. Conditions other than the test temperature and NaCl concentration were performed according to JIS G0577 (2014). Table 4 shows the measurement results of the test piece formula potential Vc' ₁₀₀ for each test number.

[Tensile test]

About the test piece of each test number, 0.2% yield strength was calculated|required by the method based on JISZ2241 (2011). Table 4 shows the results.

[Evaluation results]

With reference to Tables 3 and 4, the test pieces of Test Nos. 5 to 8 had a suitable chemical composition, and manufacturing conditions were appropriate. Therefore, the test pieces of Test Nos. 5 to 8 had a ferrite fraction of 35 to 65% by volume and the remainder was a two-phase stainless steel composed of an austenite phase, and the Cu area ratio in the ferrite phase was 0.5% or less. As a result, the steel plate formula potentials (mVvs.SCE) of Test Nos. 5 to 8 were 100 or more, showing excellent pitting resistance.

On the other hand, in the test piece of Test No. 1, the Cu content was too high. In the test piece of Test No. 1, F1 was also 59.8, and Expression (1) was not satisfied. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 1 was 0.8%. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 1 was -60, and did not show excellent pitting resistance.

In the test piece of Test No. 2, the Cu content was too high. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 2 was 0.6%. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 2 was 71, and did not show excellent pitting resistance.

In the test piece of Test No. 3, the solution temperature was 1050°C and was too low. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 3 was 0.7%. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 3 was -12, and did not show excellent pitting resistance.

In the test piece of Test No. 4, the content of each element was appropriate, but F1 was 65.1 and did not satisfy Formula (1). As a result, the test specimen formula potential (mVvs.SCE) of Test No. 4 was 85, and did not show excellent pitting resistance.

In the test piece of Test No. 9, the W content was too low. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 9 was 70, and did not show excellent pitting resistance.

In the test piece of Test No. 10, the Mo content was too low. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 10 was 76, and did not show excellent pitting resistance.

In the test piece of Test No. 11, the Cr content was too low. As a result, the test piece formula potential (mVvs.SCE) of Test No. 11 was 81, and did not show excellent pitting resistance.

In the test piece of Test No. 12, the temperature at the end of hot rolling was 840°C, and it was too low. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 12 was 1.1%. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 12 was -150, and did not show excellent pitting resistance.

The test piece of Test No. 13 had a cooling rate of 3°C/sec after the completion of hot rolling and was too slow. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 13 was 1.6%. As a result, the test specimen formula potential (mVvs.SCE) of Test No. 13 was -71, and did not show excellent pitting resistance.

The embodiments of the present invention have been described above. However, the above-described embodiment is only an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be carried out by appropriately changing the above-described embodiment without departing from the spirit.

Claims (3)

In mass%,
Cr: more than 27.00% to 29.00%,
Mo: 2.50 to 3.50%,
Ni: 5.00 to 8.00%,
W: 4.00 to 6.00%,
Cu: 0.01 to less than 0.10%,
N: more than 0.400% to 0.600%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
sol. Al: 0.040% or less,
V: 0.50% or less,
O: 0.010% or less,
P: 0.030% or less,
S: 0.020% or less,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
B: 0 to 0.0040% and
The balance is made of Fe and impurities, and the chemical composition that satisfies Formula (1),
35 to 65% by volume of the ferrite phase and the balance contains a microstructure composed of austenite phase,
Two-phase stainless steel, in which the area ratio of Cu precipitated in the ferrite phase is 0.5% or less.

Here, the content (mass%) of each element is substituted into each element symbol in the formula (1). The method according to claim 1, which is a two-phase stainless steel, the chemical composition in mass%,
Ca: 0.0001 to 0.0040%,
Mg: 0.0001 to 0.0040% and
B: Two-phase stainless steel containing one or two or more selected from the group consisting of 0.0001 to 0.0040%. In mass%,
Cr: more than 27.00% to 29.00%,
Mo: 2.50 to 3.50%,
Ni: 5.00 to 8.00%,
W: 4.00 to 6.00%,
Cu: 0.01 to less than 0.10%,
N: more than 0.400% to 0.600%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
sol. Al: 0.040% or less,
V: 0.50% or less,
O: 0.010% or less,
P: 0.030% or less,
S: 0.020% or less,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
B: 0 to 0.0040% and
The remainder is made of Fe and impurities, and the process of preparing a material having a chemical composition satisfying formula (1),
A process of hot-working the material at 850°C or higher,
A step of cooling the material after the hot working at 5°C/sec or more,
A method for producing a two-phase stainless steel, comprising a step of heat-treating the cooled material at a temperature of 1070°C or higher.

Here, the content (mass%) of each element is substituted into each element symbol in the formula (1).