KR102067033B1 - Lean duplex stainless steel with excellent pitting corrosion resistance - Google Patents

Abstract

A low alloy duplex stainless steel excellent in pitting resistance and a method for producing the same are disclosed.
Duplex stainless steel according to the present invention by weight%, Cr: 24 to 26%, Mn: 5.5 to 7% by weight, Ni: 3.0 to 4.0% by weight, Mo + 0.5W: 2.5 to 3.5% (W> 0, Mo ≥0), C: 0.08 to 0.15%, N: 0.32 to 0.45%, consisting of the remaining Fe and inevitable impurities, has a microstructure containing austenite and ferrite, the ferrite fraction is 40 ~ 60vol% It is characterized by. In addition, the duplex stainless steel according to the present invention may further include one or more of Nb: 0.25% by weight or less and Cu: 0.6% by weight or less.

Description

Low alloy type duplex stainless steel with excellent formula resistance and its manufacturing method {LEAN DUPLEX STAINLESS STEEL WITH EXCELLENT PITTING CORROSION RESISTANCE}

The present invention relates to a stainless steel manufacturing technology, more specifically, low alloy type duplex excellent in formula resistance It relates to stainless steel and a method for producing the same.

Stainless steel is classified into austenitic stainless steel, ferritic stainless steel, and duplex stainless steel depending on the microstructure.

Of these, duplex stainless steel is a stainless steel that ensures resistance to stress corrosion cracking and mechanical strength at the same time by containing about 50% of ferrite and austenite, respectively. Such a duplex stainless steel has economical and excellent corrosion resistance and mechanical properties compared to the conventional austenitic stainless steel, and has advantages such as reduced maintenance costs when applying structural materials, for example, a large amount of piping, valves, etc. are required. It is used in many fields, including offshore plants that require high strength and high corrosion resistance.

Duplex stainless steel usually contains a large amount of corrosion resistance improving elements such as nickel (Ni), chromium (Cr), and molybdenum (Mo), which may cause problems in manufacturing cost and environmental pollution.

Background art related to the present invention is a super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability is suppressed the formation of the intermetallic phase disclosed in Republic of Korea Patent Publication No. 10-2003-0077239 (published on October 1, 2003) There is a river.

An object of the present invention is to provide a low-alloy type duplex stainless steel having a low nickel alloy content such as nickel and excellent mechanical properties including pitting resistance.

The present invention also provides a method for producing the above duplex stainless steel.

Duplex stainless steel according to an embodiment of the present invention for achieving the above object by weight, Cr: 24 to 26%, Mn: 5.5 to 7% by weight, Ni: 3.0 to 4.0% by weight, Mo + 0.5W: 2.5 ~ 3.5% (W> 0, Mo≥0), C: 0.08 ~ 0.15%, N: 0.32 ~ 0.45%, consisting of the remaining Fe and inevitable impurities, and has a microstructure containing austenite and ferrite. , Characterized in that the ferrite fraction is 40 ~ 60vol%.

In this case, the duplex stainless steel may further include one or more of Nb: 0.25% by weight or less and Cu: 0.6% by weight or less.

In addition, the W may include 2.5 to 4.5% by weight.

In addition, the Mo may include 0.15 to 1.5% by weight.

In addition, the total content of Ni and Mo may be 4.5% by weight or less.

In addition, the combined content of N and C may be 0.43 to 0.57% by weight.

In addition, 1.5 ≦ Cr _eq / Ni _eq ≦ 1.65 may be satisfied.

Cr _eq = [Cr] +1.5 [Mo] +0.75 [W]

Ni _eq = [Ni] +0.5 [Mn] +0.3 [Cu] +25 ([N] -0.152 × 0.66 × [Nb]) + 30 ([C] -0.131 × 0.34 × [Nb])

(In the formula, [alloy element] means the weight% of the alloy element.)

Duplex stainless steel production method according to an embodiment of the present invention for achieving the other object by weight, Cr: 24 to 26%, Mn: 5.5 to 7% by weight, Ni: 3.0 to 4.0% by weight, Mo + 0.5 Preparing a semi-finished steel comprising W of 2.5 to 3.5% (W> 0, Mo ≧ 0), C of 0.08 to 0.15%, and N of 0.32 to 0.45% and consisting of the remaining Fe and unavoidable impurities; And solidifying heat treatment of the semi-finished steel at 1050 to 1200 ° C.

The duplex stainless steel according to the present invention may have a duplex structure in which ferrite and austenite fractions are 40 to 60 vol%, respectively, through addition of C, N, and W even though the content of expensive elements such as Ni and Mo is lowered, CPT (Critical Pitting Temperature), which is the officially generated temperature, is 30 ° C or higher, and has excellent pitting resistance. In addition, the duplex stainless steel according to the present invention may exhibit excellent mechanical properties of yield strength of 600MPa or more, tensile strength of 900MPa or more, and elongation of 40% or more.

In addition, according to the method for manufacturing a duplex stainless steel according to the present invention, by having a solid solution heat treatment temperature of 1200 ℃ or less has the effect of preventing the increase of heat treatment cost, preventing mechanical properties and corrosion resistance through grain growth inhibition, easy to adjust the phase ratio, etc. Can be.

Therefore, in the case of the duplex stainless steel produced by the method according to the invention, it can be applied to various fields such as offshore plants, ships.

1 is a state diagram showing a phase fraction according to the temperature of steel specimens according to Comparative Example 10.
Figure 2 is a state diagram showing the phase fraction according to the temperature of the steel specimen according to Comparative Example 12.
Figure 3 is a state diagram showing the phase fraction according to the temperature of the steel specimen according to Comparative Example 14.
Figure 4 shows the CPT of the example specimens and the comparative example specimens according to Mo + 0.5W.
Figure 5 shows the CPT of the example specimens and the comparative specimens according to C + N.
Figure 6 shows the CPT of the example specimens and comparative specimens according to Cr _eq / Ni _eq .

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, only the present embodiments to make the disclosure of the present invention complete, and common knowledge in the technical field to which the present invention belongs. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims.

Hereinafter, with reference to the accompanying drawings, it will be described in detail with respect to the low-alloy-type duplex stainless steel excellent in pitting resistance according to the present invention and its manufacturing method.

Low alloy type duplex stainless steel according to the present invention is by weight%, Cr: 24 to 26%, Mn: 5.5 to 7% by weight, Ni: 3.0 to 4.0% by weight, Mo + 0.5W: 2.5 to 3.5% (W> 0, Mo ≧ 0), C: 0.08 to 0.15%, N: 0.32 to 0.45%, and the remaining Fe and inevitable impurities.

Hereinafter, the role and content of each component included in the low alloyed duplex stainless steel according to the present invention will be described.

Chrome (Cr)

Chromium (Cr) is an element that forms a stable passivation film on the steel surface and plays an essential role in improving pitting resistance. For this purpose, Cr needs to be included at least 24% by weight of the total weight of the steel. However, Cr is a ferrite-forming element, when excessively added in excess of 26% by weight, the heat treatment temperature range for the duplex structure is narrowed, and the formation of δ-ferrite may lower the austenite phase stability and hard phase sigma It may form a Cr-rich phase.

Thus, in the present invention, the content of Cr is 24 to 26% by weight of the total weight of the steel.

Manganese (Mn)

Manganese (Mn) is an essential element for stabilizing austenite, replacing Ni, and implementing two-phase tissue, and is an effective element for increasing nitrogen solubility. In addition, manganese is an element that is advantageous for securing melt flow rate and is an element that can contribute to improving hot workability. For this purpose, Mn needs to be added at least 5.5% by weight of the total weight of the steel. However, when the manganese is excessively added in excess of 7% by weight, deterioration of passivation protection may result in deterioration of pitting resistance, and mechanical properties due to Mn oxide and sulfide formation may be deteriorated. In addition, in order to suppress the formation of Mn-based oxides and sulfides, O and S contents should be managed extremely low.

For this reason, in the present invention, the content of Mn is set to 5.5 to 7% by weight of the total weight of the steel.

Nickel (Ni)

Nickel (Ni) is an austenite stabilizing element, which makes the austenite phase noble, which is essential for suppressing galvanic corrosion between austenite and ferrite. Nickel is also essential for improving the toughness of steel. For this purpose Ni needs to be added at least 3.0% by weight of the total weight of the steel. However, even if Ni exceeds 4.0% by weight, the effect improvement compared to the cost increase due to the increase in content is insignificant.

For this reason, in this invention, Ni content was made into 3.0 to 4.0 weight% of the total weight of steel.

Tungsten (W), Molybdenum (Mo)

Tungsten (W) and molybdenum (Mo) are ferrite stabilizing elements and contribute to the improvement of pitting resistance through the promotion of passivation, and also to the improvement of general corrosion resistance.

In particular, in the case of W, there is an advantage of less risk of sigma precipitation than Mo.

This effect can be remarkably exerted when Mo + 0.5W is 2.5% by weight or more. At this time, W is essentially included and Mo may not be included. However, when Mo + 0.5W exceeds 3.5% by weight, the stability of the austenite phase can be reduced by the formation of δ-ferrite.

More specifically, W may be included in 2.5 to 4.5% by weight of the total weight of the steel. When the content of W is 2.5% by weight or more, the above W addition effect can be sufficiently exhibited. However, when the amount of W added exceeds 4.5% by weight, alloying may be difficult due to the high melting temperature of W and there is a risk of formation of δ-ferrite.

In addition, Mo may not be included depending on the W content, more preferably may be included in 0.5 to 1.5% by weight of the total weight of the steel. If the Mo content is 0.5% by weight or more, the effect of adding Mo can be sufficiently exhibited. However, when the Mo content is more than 1.5% by weight, the corrosion resistance and impact resistance due to the sigma phase formation may be lowered, there is a risk of the formation of δ-ferrite.

On the other hand, the total content of Ni and Mo may be less than 4.5% by weight. This can be achieved through the complex addition of austenite stabilizing elements Mn, C, N, and ferrite stabilizing elements W.

Carbon (C)

Carbon (C) is a strong austenite stabilizing element and, as a low cost element, contributes to the reduction of the manufacturing cost of steel. In the case of C, on the other hand, it is possible to prevent formation of δ-ferrite during solidification of molten metal or N solid solution, thereby preventing the loss of N during ingot cooling. It can be secured by In addition, C exhibits an effect of suppressing elongation decrease along with an increase in strength due to solid solution strengthening. In addition, C improves formula resistance when kept in employment. C also controls galvanic corrosion between the two phases by noble of the austenite phase with Ni. In order to achieve this effect, C needs to be added at least 0.08% by weight of the total weight of the steel. However, when the C content is excessively greater than 0.15% by weight, it may cause corrosion resistance and mechanical properties deterioration due to Cr-C formation, and may cause grain coarsening problem due to an increase in the heat treatment heat treatment temperature.

In view of this point, in the present invention, the content of C is made 0.08 to 0.15% by weight of the total weight of the steel.

Nitrogen (N)

Nitrogen (N), together with C, is a low cost element and a strong austenite stabilizing element. In addition, N contributes to the effect of increasing strength and reducing elongation due to solid solution strengthening, and exhibits an effect of better pitting resistance than C when maintained in solid solution. In order to fully exhibit this effect, N needs to be added at least 0.32% by weight of the total weight of the steel. However, when N is excessively added in excess of 0.45% by weight, problems of grain coarsening may occur due to deterioration of corrosion resistance and mechanical properties due to Cr-N formation and an increase in solid solution heat treatment temperature. Special manufacturing processes such as pressure manufacturing process and powder metallurgy are required.

In this regard, in the present invention, the content of N was limited to 0.32 to 0.45% by weight of the total weight of the steel.

On the other hand, the carbon and nitrogen content is preferably C + N: 0.43 ~ 0.57% by weight. When C + N is less than 0.43% by weight, the austenite forming effect may be insufficient in steels in which Ni is not sufficiently added, and sufficient physical properties of mechanical properties and pitting resistance are difficult to secure. On the other hand, when C + N exceeds 0.57% by weight, it is possible to form a large amount of Cr (C, N) during the heat treatment, it may reduce the pitting resistance.

Niobium (Nb)

Niobium (Nb) is an element that may be further included in place of Fe in the alloying components shown above. Nb is a strong carbide (MX type) forming element, can contribute to the improvement of room temperature and high temperature strength, thermal fatigue, and in particular can contribute to the grain refining effect.

Nb is a ferrite forming element, but in the duplex stainless steel according to the present invention, the content of N and C is sufficiently high, so that Nb in solid solution does not have to be considered, and all the added Nb may be consumed to form carbonitride.

However, when such Nb exceeds 0.25% by weight, coarse Nb (C, N) (Nb-based carbide, nitride, carbonitride) is formed and its volume fraction exceeds 2 vol%, resulting in deterioration of corrosion resistance and mechanical properties. Can be.

Therefore, in the present invention, when Nb is added, the content is 0.25% or less of the total weight of the steel, the volume fraction of Nb (C, N) was 2vol% or less of the total volume of the steel.

Copper (Cu)

Copper (Cu) is an element that may be further included in place of Fe in the alloying components shown above. Cu is an austenite stabilizing element and can exhibit an antibacterial effect. In addition, copper may noble the austenite phase in terms of corrosion resistance, thereby contributing to galvanic corrosion control between the two phases. However, when copper is added in excess of 0.6% by weight, due to the Cu clustering can reduce the pitting resistance, hot workability may be deteriorated. In addition, it was found through this study that there is a problem that the carbide and nitride formation temperature increases when Cu is excessively added.

 Thus, in the present invention, when Cu is added, the content is set to 0.6% by weight or less of the total weight of the steel.

Other

Duplex stainless steel according to the present invention is an unavoidable impurity, and may include phosphorus (P): 0.01 wt% or less, sulfur (S): 0.01% or less, silicon (Si): 0.4 wt% or less.

Phase fraction control

On the other hand, it is necessary to adjust the amount of use of ferrite forming elements (Cr _eq ) and the amount of use of austenite forming elements (Ni _eq ), that is, to control the phase fraction between austenite and ferrite two-phase.

Cr _eq related to the amount of the ferrite forming elements Cr, Mo, and W may satisfy Equation 1.

[Equation 1]

28≤Cr _eq ≤32,

Cr _eq = [Cr] + 1.5 [Mo] + 0.75 [W] (where [] is the weight percentage of the component)

In addition, Ni _eq related to the amount of austenite forming elements Ni, Mn, Cu, N, and C may satisfy Equation 2.

[Equation 2]

16≤Ni _eq ≤21

Ni _eq = [Ni] +0.5 [Mn] +0.3 [Cu] +25 ([N] -0.152 × 0.66 × [Nb]) + 30 ([C] -0.131 × 0.34 × [Nb])

At this time, Cr _eq in the equation 1 Mo + W content has a dominant influence, in the case of Ni _eq , C + N dominant influence, N, C consumption by Nb was considered.

It is preferable that ratio of Cr _eq and Ni _eq satisfy | _fills Formula 3.

[Equation 3]

1.5≤Cr _eq / Ni _eq ≤1.65

When the amount of Cr _eq relative to Ni _eq is relatively large, the ferrite is stable. When the amount of Cr _eq relative to Ni _eq is relatively small, austenite is stabilized. When the ferrite is stabilized, the solid solution heat treatment temperature can be generally lowered and the strength can be improved, but there is a fear that pitting resistance and elongation are lowered. On the contrary, when austenite is stabilized, since the solubilization heat treatment temperature is high, it is uneconomical and the pitting resistance may be improved, but the strength may be deteriorated due to grain growth.

Therefore, the ratio of Cr _eq and Ni _eq needs to be appropriately controlled, and the inventors of the present invention satisfy the yield strength of 600 MPa or more, with excellent pitting resistance of CPT 30 ° C. or higher when 1.5 ≦ Cr _eq / Ni _eq ≦ 1.65. It was confirmed that excellent mechanical properties of more than 900MPa strength, 40% or more elongation.

The duplex stainless steel according to the present invention having the alloying component has a microstructure including austenite and ferrite in conjunction with the production method described later. At this time, the ferrite fraction is 40-60 vol%. When the ferrite fraction was out of this, a decrease in corrosion resistance was observed.

In addition, the duplex stainless steel according to the present invention has a feature of excellent pitting resistance as CPT is 30 ℃ or more, it can exhibit excellent mechanical properties of yield strength 600MPa or more, tensile strength 900MPa or more, elongation 40% or more.

The duplex stainless steel according to the present invention may be manufactured by a method including preparing a semi-finished steel and a solid solution heat treatment step.

Preparing the semifinished steel produces a semifinished steel having the alloy composition described above. The semifinished steel can be in the form of slabs, ingots, billets and the like, and these semifinished steels can be produced in a variety of ways, for example vacuum melting or casting methods can be presented.

After that, the semi-finished steel is heat-treated at 1050 to 1200 ° C. At this time, the reason for limiting the solute heat treatment temperature to 1050 ~ 1200 ℃ is as follows. If the solid solution heat treatment temperature is less than 1050 ℃ Cr ₂ N, Cr ₂₃ C ₆ , the sigma phase may be formed. In addition, when the heat-treatment heat treatment temperature is excessively high, more than 1200 ℃, there is a problem such as increase in cost of the manufacturing process, mechanical properties and corrosion resistance due to grain growth, deepening two-phase composition imbalance due to alloy element distribution.

After the heat treatment heat treatment of the semi-finished steel, a cooling process such as air cooling and water cooling may be performed, and a process such as rolling may be included between the solid solution heat treatment and cooling.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. However, this is presented as a preferred example of the present invention and in no sense can be construed as limiting the present invention.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Fabrication of Steel Specimen

Steel specimens having the composition shown in Table 1 and consisting of the remaining iron and inevitable impurities were heat-treated at the solid solution heat treatment temperature shown in Table 2, and then cooled to room temperature at a cooling rate of about 50 ° C./s.

[Table 1] (Unit: weight%)

TABLE 2

In relation to Cr _eq / Ni _{eq in} Table 2, when Cr _eq / Ni _eq shows a relatively high value of more than 1.65, there are many ferrite phases, and the CPT of Table 3 shows a relatively low value. On the contrary, when Cr _eq / Ni _eq shows a relatively low value of less than 1.5, there are many austenite phases, and the solute heat treatment temperature becomes high.

Physical properties evaluation results are shown in Table 3.

In Table 3, CPT was measured according to the corrosion resistance measurement method specified in ASTM G48. In addition, the tensile test was measured based on ASTM E8 / E8M test.

TABLE 3

Referring to Table 3, in the case of the specimens according to Examples 1 to 9 satisfying the alloy components presented in the present invention, the ferrite fraction is 40 ~ 60vol%, the CPT was 30 ℃ or more, the yield strength 600MPa or more, Tensile strength of 900MPa or more and elongation of 40% or more were shown.

On the other hand, the specimen according to Comparative Example 1, the N + C content was relatively high. Accordingly, Cr _eq / Ni _eq showed a relatively small value. This is difficult to control the phase ratio 1: 1 by increasing the heat treatment heat solubility and Cr ₂ N can be easily formed can lead to a decrease in corrosion resistance. In the case of the specimen according to Comparative Example 2, the C content was insufficient, and thus stable employment of N was difficult. In addition, in the case of the specimen according to Comparative Example 2, since the C + N is low, the problem of strength decreases, and as the Cr _eq / Ni _eq exhibits a large value, there is a problem that the corrosion resistance is lowered.

In the case of the specimen according to Comparative Example 3, in Example 2 standards, as a case of out of phase ratio (austenite excess), the pitting resistance was reduced. In addition, in the case of the specimen according to Comparative Example 4, the elongation was lowered and the pitting resistance was further reduced as the case of the phase ratio out of (ferrite excess) in Example 2 criteria.

In the case of the specimen according to Comparative Example 5, Mo + 0.5W is 2.5% by weight or less, poor pitting resistance. In the case of the specimen according to Comparative Example 6, Cr _eq / Ni _eq showed a relatively large value, the N content was too small. Accordingly, in the case of the specimen according to Comparative Example 6, the elongation was low, the CPT was low, and the pitting resistance was not good.

In the case of the specimen according to Comparative Example 7, stable employment of N is difficult due to the insufficient C content, which leads to a decrease in pitting resistance. In addition, in the case of the specimen according to Comparative Example 8, Mo + 0.5W was less than 2.5% by weight, leading to a decrease in pitting resistance. In the case of the specimens according to Comparative Example 9, the Mn content exceeded 7% by weight. As a result, the pitting resistance was reduced.

In the case of the specimen according to Comparative Example 10, as a result of the C content exceeds 0.15% by weight, it showed a problem that both Cr ₂ N, Cr ₂₃ C _{6 is} formed at the ferrite: austenite 5: 5 phase fraction formation temperature (FIG. 1). Reference).

In the case of the specimen according to Comparative Example 11, the Mo + 0.5W content was not satisfied, the yield strength was low as 369.8 MPa, and the corrosion resistance was not good. In addition, in the case of the specimen according to Comparative Example 12, Mo + 0.5 is not satisfied, the elongation is low, the ferrite fraction did not meet 40 ~ 60vol% (see Figure 2). In addition, in the specimen according to Comparative Example 13, Mo + 0.5W was not satisfied, the elongation was relatively low. In addition, in the case of the specimen according to Comparative Example 14, the Cr content was not good corrosion resistance was low, because the ferrite stabilizing element is less, solubilization heat treatment temperature may be too high (see Figure 3).

In the case of the specimen according to Comparative Example 15, as a commercial DSS2205 steel, in the steel specimens according to Examples 1 to 9, both strength and elongation were superior to those of Comparative Example 15, and the pitting resistance was also equal or higher.

1 to 3 are state diagrams showing phase ratios according to temperatures of steel specimens according to Comparative Examples 10, 12, and 14;

Referring to Figure 1, it can be seen that the ferrite (α): austenite (γ) of the steel specimen according to Comparative Example 10 to form Cr ₂₃ C ₆ and Cr ₂ N at a temperature of 1: 1. In addition, referring to Figure 2, in the case of steel specimens according to Comparative Example 12, the ferrite (α): austenite (γ) = 1: 1 temperature does not appear, Cr ₂₃ C ₆ and Cr ₂ N at the nearest temperature It can be seen that forming. In addition, referring to Figure 3, in the case of steel specimens according to Comparative Example 14 it can be seen that the ferrite (α): austenite (γ) = 1: 1 temperature is higher than 1200 ℃.

Figure 4 shows the CPT of the example specimens and the comparative example specimens according to Mo + 0.5W. Referring to Figure 4, in the case of the test specimen in the implementation, it can be seen that Mo + 0.5W is in the range of 2.5 to 3.5% by weight, CPT indicates 30 ℃ or more. In the case of comparative specimens, Mo + 0.5W is generally outside the range of 2.5 to 3.5% by weight, and a large number of cases where the CPT is less than 30 ℃ is confirmed.

Figure 5 shows the CPT of the example specimens and the comparative specimens according to C + N. Referring to Figure 5, in the case of the test specimen in the implementation, it can be seen that the C + N is in the range of 0.43 ~ 0.57% by weight, CPT indicates 30 ℃ or more. In the case of comparative specimens, C + N is generally outside the range of 0.43 to 0.57% by weight, and a large number of cases where the CPT is less than 30 ℃ is confirmed.

Figure 6 shows the CPT of the example specimens and comparative specimens according to Cr _eq / Ni _eq . Referring to Figure 6, in the case of the test specimens in the implementation, Cr _eq / Ni _eq is in the range of 1.50 ~ 1.65, it can be seen that the CPT is 30 ℃ or more. In the case of comparative specimens, Cr _eq / Ni _eq is generally outside the range 1.50 ~ 1.65, and a large number of cases where the CPT is less than 30 ℃.

Although the present invention has been described with reference to the embodiments, these are merely exemplary, and those skilled in the art will appreciate that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention will be defined by the claims.

Claims (8)

By weight%, Cr: 24 ~ 26%, Mn: 5.5 ~ 7% by weight, Ni: 3.0 ~ 4.0% by weight, Mo: 0.15 ~ 1.5%, Mo + 0.5W: 2.5 ~ 3.5% (W> 0), C : 0.08 ~ 0.15%, N: 0.32 ~ 0.45%, N + C: 0.43 ~ 0.57%, consisting of the remaining Fe and inevitable impurities,
A duplex stainless steel having a microstructure including austenite and ferrite, but having a ferrite fraction of 40 to 60 vol%.
The method of claim 1,
The duplex stainless steel further comprises at least one of Nb: 0.25 wt% or less and Cu: 0.6 wt% or less.
The method according to claim 1 or 2,
Duplex stainless steel, characterized in that containing W 2.5 to 4.5% by weight.
delete The method according to claim 1 or 2,
Duplex stainless steel, characterized in that the combined content of Ni and Mo is 4.5% by weight or less.
delete The method according to claim 1 or 2,
Duplex stainless steel which is characterized by satisfying the 1.5≤Cr _eq / Ni _eq ≤1.65.
Cr _eq = [Cr] +1.5 [Mo] +0.75 [W]
Ni _eq = [Ni] +0.5 [Mn] +0.3 [Cu] +25 ([N] -0.152 × 0.66 × [Nb]) + 30 ([C] -0.131 × 0.34 × [Nb])
By weight%, Cr: 24 ~ 26%, Mn: 5.5 ~ 7% by weight, Ni: 3.0 ~ 4.0% by weight, Mo: 0.15 ~ 1.5%, Mo + 0.5W: 2.5 ~ 3.5% (W> 0), C Preparing a semi-finished steel comprising: 0.08 ~ 0.15%, N: 0.32 ~ 0.45%, N + C: 0.43 ~ 0.57%, consisting of the remaining Fe and inevitable impurities; And
Duplex stainless steel manufacturing method comprising the step of heat-treating the semi-finished steel at 1050 ~ 1200 ℃.

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