WO2005014872A1

WO2005014872A1 - Duplex stainless steel and method for production thereof

Info

Publication number: WO2005014872A1
Application number: PCT/JP2004/011070
Authority: WO
Inventors: Tomohiko Omura; Satoshi Matsumoto
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2003-08-07
Filing date: 2004-08-03
Publication date: 2005-02-17
Also published as: NO336117B1; EP1561834B1; EP1561834A1; BRPI0406423A; AU2004262702B2; CN100427627C; JPWO2005014872A1; EP1561834A4; JP4155300B2; NO20052266L; BRPI0406423B1; KR20060024316A; KR100661328B1; NO20052266D0; AU2004262702A1; CN1701126A

Abstract

A duplex stainless steel which has a chemical composition, in mass %, that C: 0.03 % or less, Si: 0.01 to 2 %, Mn: 0.1 to 2 %, P: 0.05 % or less, S: 0.001 % or less, Al: 0.003 to 0.05 %, Ni: 4 to 12 %, Cr: 18 to 32 %, Mo: 0.2 to 5 %, N (nitrogen): 0.05 to 0.4 %, O (oxygen): 0.01 % or less, Ca: 0.0005 to 0.005 %, Mg: 0.0001 to 0.005 %, Cu: 0 to 2 %, B: 0 to 0.01 %, W: 0 to 4 %, and the balance: Fe and impurities, characterized in that it contains oxide type inclusions having a total content of Ca and Mg of 20 to 40 mass % and having a longer diameter of 7 μm or more in an amount of 10 pieces or less per 1 mm2 of the cross section perpendicular to the working direction, or further in that it contains oxide type inclusions having a content of S of 15 mass % or more and having a longer diameter of 1 μm or more in an amount of 10 pieces or less per 0.1 mm2 of the cross section perpendicular to the working direction. In particular, it is desirable that contents of Cu, B and W are 0.2 to 2 %, 0.001 to 0.01 % and 0.1 to 4 %, respectively.

Description

Specification

Duplex stainless steel and manufacturing method thereof

Technical field

The present invention relates to a duplex stainless steel having excellent corrosion resistance in seawater. This steel may be steel pipes or steel plates, such as heat exchange piping, piping or structures for chemical plants, line pipes, casings or tubing for oil or gas wells, or umbilical tubes (piping for controlling offshore oil wells). Used for etc.

Background art

[0002] Conventionally, crude oil and natural gas mined from offshore oil wells and the like have been shunned due to the harsh working environment. However, due to the recent tightening of the energy situation, these crude oil and natural gas cannot be used. The situation is getting worse. For this reason, there is an increasing demand for stainless steel, particularly duplex stainless steel, having excellent pitting corrosion resistance as a material for steel pipes or structures used in seawater.

[0003] Patent Document 1 discloses that in general, the content of Cr, Mo and N (nitrogen) effective for improving the pitting corrosion resistance of a duplex stainless steel is adjusted, and that W is contained to improve the pitting corrosion resistance. An enhanced, so-called super duplex stainless steel is disclosed. In this document, as an index indicating the pitting corrosion resistance of a duplex stainless steel, the pitting corrosion resistance index PRE (Pitting Resistance Equivalent) of the following generally known formula (A), PREW of equation (B) has been proposed.

[0004] In a normal duplex stainless steel, the pitting resistance index PRE or PREW is adjusted to be 35 or more, and further, in a super duplex stainless steel, it is adjusted to be 40 or more. Conventional techniques for improving pitting resistance have been based on the ability to increase the pitting resistance index PRE or PREW.

[0005] PRE = Cr + 3.3Mo + 16N (nitrogen) (A)

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (nitrogen) (B)

Each element symbol in the above formulas (A) and (B) indicates the content (mass%) of each element. [0006] In duplex stainless steel, the effect of nonmetallic inclusions on pitting corrosion resistance has not been studied. However, regarding the pitting corrosion resistance of austenitic stainless steel, as described in Non-Patent Document 1, it is known that Mn sulfide is the most harmful to pitting corrosion resistance and oxides are harmless. I have.

[0007] Oxide-based inclusions contained in stainless steel are generally A1 oxide (Al〇), Si

A composite oxide composed of oxides such as 23 oxide (SiO 2) and Cr oxide (Cr 2 O 3). They are

2 2 3

It has been considered that it does not affect pitting corrosion because it has a property of being insoluble in aqueous solution, so-called insolubility. On the other hand, Ca and Mg, and S, which are impurity elements in steel materials, may be contained in oxides.However, the effect of these elements on pitting corrosion resistance was investigated in an example. Not before.

Patent Document 1: JP-A-5-132741

[0009] Non-patent document 1: J.E.Castle, et al., "Studies by Auger Spectroscopy of Pitlnitiation at the site of Inclusions in Stainless Steel ,,, Corrosion Science ^ Volume 30, No. 4/5, p. 409

Disclosure of the invention

Problems to be solved by the invention

[0010] In recent years, the application of duplex stainless steel to severe corrosive environments such as high-temperature seawater environments has been increasing. Corrosion tests that simulate such harsh conditions, such as the ferric chloride test at 80 ° C, do not always provide sufficient pitting resistance, even with super duplex stainless steel. . Further, the pitting corrosion resistance may not be sufficiently improved only by adjusting the contents of Cr, Mo, N (nitrogen), and W. In addition, similar to the austenitic stainless steel, the pitting corrosion resistance can be improved to some extent by reducing the Mn sulfide in the steel, but the pitting corrosion can be completely prevented. is not.

[0011] The present invention has been made to solve these problems, and an object of the present invention is to provide a duplex stainless steel capable of stably obtaining good pitting corrosion resistance and a method for producing the same. Means to solve

The present inventors have investigated metallurgical factors affecting pitting resistance of a duplex stainless steel in detail. As a result, even if the oxide inclusions formed during the melting process only by the conventional causes of pitting corrosion described above, those containing Ca and Mg and those containing S, Has a significant effect on The findings obtained by the study of the present inventors are as follows.

[0013] When the Ca content is less than 0.0005% by mass or when the Mg content is less than 0.0001% by mass, oxide-based inclusions formed in the steel mainly contain insoluble Al 2 O 3. so

twenty three

Yes, pitting does not occur. When the content of Ca or Mg exceeds 0.005% by mass, oxide-based inclusions formed in steel are mainly composed of (Ca, Mg) 0, and such oxides are , It is hard to be the starting point of pitting.

[0014] When the steel contains 0.0005-0.005% by mass of Ca and 0.0001 0.005% by mass of Mg, the oxide-based inclusions formed in the steel include Al O and (Ca, Mg) 0 and coexistence

twenty three

When these oxide-based inclusions are formed adjacent to each other, they easily become pitting corrosion starting points.

[0015] Therefore, the present inventors have found that 0.0005-0.005% by mass of Ca and 0.0001-

As a result of repeated studies on the causes of pitting corrosion in duplex stainless steel containing 0.005 mass%, pitting corrosion may or may not occur depending on the size and number of oxide-based inclusions formed in the steel I found that.

[0016] S is an element inevitably present in steel, and its content cannot be made completely zero by current steelmaking technology. When S is contained in a large amount in oxide-based inclusions formed in steel, S deteriorates pitting corrosion resistance. It has been found that the occurrence of pitting can be suppressed by adjusting the size and the number.

[0017] It is not possible to produce a desired two-phase stainless steel in the state of oxide-based inclusions by smelting or thermomechanical treatment by a conventional method. As a result of various studies, the present inventors have found that the optimal combination of (h) slag basicity during reduction treatment, (/ 3) killing temperature and time in a ladle, and (γ) 錡 total working ratio after fabrication It has been found that the desired oxide-based inclusion state can be obtained by controlling the amount of oxides, and that an unprecedented high-purity steel can be manufactured.

As described above, the present invention provides a chemical composition of a steel material capable of securing performance as a duplex stainless steel, It was completed based on the state of oxide inclusions that greatly improve pitting corrosion resistance and the manufacturing conditions for achieving high cleanliness.

The gist of the present invention is a method for producing a duplex stainless steel shown in the following (a) and (b) and a duplex stainless steel shown in the following (c).

[0020] (a) In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1 2%, P: 0.05% or less, S:

0.001% or less, A1: 0.003 to 0.05%, Ni: 4 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, 〇 (oxygen): 0.01% or less , Ca: 0.0005-0.005%, Mg: 0.0001 0.005%, Cu: 0-2%, B: 0-0.01% and W: 4%, with the balance being Fe and impurities. Among the inclusions contained therein, oxide inclusions having a total content of Ca and Mg of 2040% by mass and a major axis of not less than 7 μm have a cross section lmm ² perpendicular to the processing direction. A duplex stainless steel characterized in that there are no more than 10 per steel.

[0021] (b) In mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1-2%, P: 0.05% or less, S:

0.001% or less, A1: 0.003 to 0.05%, Ni: 4 to 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, 〇 (oxygen): 0.01% The following two phases contain Ca: 0.0005—0.005%, Mg: 0.0001—0.005%, Cu: 0—2%, B: 0—0.01%, and W: 0—4%, with the balance being Fe and impurities. Oxide inclusions that are stainless steel and have a total content of Ca and Mg of 20-40% by mass and a major axis of 7 μm or more in the inclusions contained therein Vertical cross section Not more than 10 pieces per lmm ² , and oxide inclusions with an S content of 15% by mass or more and a major axis of 1 μm or more are perpendicular to the processing direction. Duplex stainless steel characterized in that there are no more than 10 per ² .

[0022] The steel described in (a) or (b) above has a Cu, B and W content of 0.2-2%, 0.001 0.01% and 0.1-4% in mass%, respectively. Desired Les ,. Further, it is desirable that the pitting resistance index PREW represented by the following formula (1) is 40 or more. However, each element symbol in the formula (1) means the content (% by mass) of each element.

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)

[0023] (c) The slag basicity represented by the following equation (2) is reduced under the condition of 0.5-3.0, and the molten steel that has been tapped is subjected to killing for 5 minutes or more at a temperature of 1500 ° C or more. After implementation, it was manufactured and the obtained piece Characterized in that a total working ratio R represented by the following formula (3) is 10 or more: The method for producing a duplex stainless steel according to the above (a) or (b). However, (2) the compound of formula means a slag concentration of each compound (by mass ^_0/0). In Equation (3), AO denotes the cross-section 変形 before deformation in the plastic deformation process, and A denotes the cross-sectional area after deformation in the plastic deformation process, and each subscript n (l, 2, ί) means the order of each stand in the plastic deformation process.

[Slag basicity :! = (CaO + lVlgO) / (Al 0 + SiO) (2) (3)

The invention's effect

According to the present invention, a duplex stainless steel having good porosity resistance can be stably obtained.

For this purpose, for example, pipes or structures for heat exchange, pipes or structures for chemical plants, line pipes, casings or tubing for oil wells or gas wells, or steel pipes or steel plates such as umbilical tubes (pipes for controlling submarine oil wells) Thus, it is possible to provide a duplex stainless steel that is optimal for such purposes.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] 1. Chemical composition

The chemical composition of the steel material must be in the following range to ensure sufficient pitting resistance as duplex stainless steel. In the following description, “% of the content” means I ”mass ⁰ / oj.

[0026] C: 0.03% or less

C is inevitably present in steel. If the content exceeds 0.03%, carbides are liable to precipitate and pitting corrosion resistance is reduced. Therefore, the content of C is set to 0.03% or less.

[0027] Si: 0.01 to 2 ^o / _o

Si is an element effective for deoxidizing steel and must be contained at 0.01% or more. However, if its content exceeds 2%, it promotes the formation of intermetallic compounds and lowers pitting resistance. Therefore, the content of ^_Si 0.01~2 ο / ο.

[0028] Μη: 0.1 to 2%

Μη is effective in stabilizing the austenite phase like Ni, and contains 0.1% or more. Need to be done. On the other hand, if Mn exceeds 2%, pitting corrosion resistance is reduced. Therefore, the content of Mn was set to 0.1-2%.

[0029] P: 0.05% or less

P is inevitably present in steel as an impurity, and dissolves actively to lower pitting corrosion resistance. If the content exceeds 0.05%, this effect becomes remarkable, so it is necessary to reduce the content to 0.05% or less. It is desirable that the content of P be as low as possible.

[0030] S: 0.001% or less

S, like P, is inevitably present in steel and forms sulfides that are easily dissolved, thereby reducing pitting corrosion resistance. When the content power exceeds .001%, this effect becomes remarkable. Further, as described later, even if the content is 0.001% or less, the content of s is desirably as low as possible in this range in order to promote the occurrence of pitting corrosion when contained in the oxide-based inclusions.

[0031] A1: 0.003 0.05%

A1 is an element necessary for deoxidation of steel and must be contained at 0.003% or more. On the other hand, when contained excessively, A1 nitride precipitates and absorbs N (nitrogen), which is an element effective in improving pitting corrosion resistance, and lowers pitting corrosion resistance. Therefore, the content of A1 was set to 0.003-0.05%. A1 means "sol.Al (acid soluble A1)".

[0032] Ni: 4-12%

M is an element that stabilizes the austenitic phase, and its effect is insufficient if less than 4%. On the other hand, if it exceeds 12%, the austenite phase becomes excessive and the mechanical properties of the duplex stainless steel deteriorate. Therefore, the content of M was set to 412%.

[0033] Cr: 18-32%

Cr is effective in improving pitting corrosion resistance, and if its content is less than 18%, pitting corrosion resistance becomes insufficient. On the other hand, if the content exceeds 32%, the ferrite phase becomes excessive and the mechanical properties of the duplex stainless steel are impaired. Therefore, the content of Cr was set to 1832%.

[0034] Mo: 0.2-5%

Mo is also an element that enhances pitting corrosion resistance like Cr, and its effect is not sufficient if it is less than 0.2%. On the other hand, if it exceeds 5%, an intermetallic compound is precipitated, and on the contrary, pitting corrosion resistance is reduced. Therefore, the content of Mo is set to 0.25%. [0035] N (nitrogen): 0.05—0.4%

N (nitrogen), like Ni, is an element that has the effect of stabilizing the austenite phase. N (nitrogen) is also an element having an effect of improving pitting corrosion resistance, like Cr and Mo. However, if the content is less than 0.05%, these effects are insufficient. On the other hand, if the content exceeds 0.4%, the hot workability decreases. Therefore, the content of N (nitrogen) was set to 0.05 0.4%.

[0036] 0 (oxygen): 0.01% or less

o (Oxygen) is inevitably present in steel, as in S, and exists as oxide inclusions. As described later, depending on the composition, the oxide becomes a starting point of pitting corrosion and lowers pitting resistance. In particular, when the content exceeds 0.01%, coarse oxides increase and this tendency becomes remarkable. Therefore, o (oxygen) must be limited to 0.01% or less. It is desirable that the content of 〇 (oxygen) be as low as possible.

[0037] Ca: 0.0005—0.005%, Mg: 0.0001—0.005%

Ca and Mg are elements that have the effect of improving the hot workability of steel by fixing S as sulfide. As described above, in a duplex stainless steel containing Ca: 0.0005-0.005% and Mg: 0.0001-0.005%, A1〇 and (Ca, Mg) 0

twenty three

Coexist, and when they are formed adjacently, adversely affects pitting corrosion resistance. Therefore,

The contents of Ca and Mg are specified as 0.0005-0.005% and 0.0001-0.005%, respectively, in which the pitting corrosion resistance tends to deteriorate. The pitting corrosion resistance of the duplex stainless steel of the present invention is improved by regulating the state of the oxide inclusions, as described later.

[0038] The duplex stainless steel of the present invention is a steel having the above chemical composition, with the balance being Fe and impurities. Further, the duplex stainless steel of the present invention may contain one or more of Cu, B and W as optional additional elements.

[0039] Cu: 0— 2%

Cu, like Ni, stabilizes the austenitic phase. It also stabilizes the sulfide film in a hydrogen sulfide environment and improves pitting resistance. Therefore, Cu may be included if necessary. To obtain the above effects, it is desirable to contain 0.2% or more, but if it exceeds 2%, the hot workability decreases. Therefore, when Cu is contained, the content is desirably 0.2 to 2%. [0040] B: 0-0.01%

Since B is an element effective for improving hot workability, it may be contained as necessary. In order to obtain this effect, it is desirable that the content is 0.001% or more, but if the content exceeds 0.01%, the effect is saturated. Therefore, when B is contained, its content is preferably set to 0.001 to 0.01%.

[0041] W: 0— 4%

W is an element that is effective in improving pitting corrosion resistance, like Cr and Mo, and therefore may be included as necessary. These effects become remarkable when the content is 0.1% or more. However, if the content exceeds 4%, an intermetallic compound is precipitated and the pitting corrosion resistance is rather lowered. Therefore, when W is contained, its content is desirably 0.1-4%.

[0042] 2. Pitting resistance index

The duplex stainless steel of the present invention is desirably a super duplex stainless steel having the above chemical composition and having a pitting resistance index of 40 or more as defined below. However, each element symbol in the formula (1) means the content (% by mass) of each element.

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)

[0043] 3. Conditions for oxide-based inclusions

The present inventors investigated the effect of oxide-based inclusions on pitting corrosion resistance by the following method.

[0044] Molten steel having the chemical composition shown in Tables 3 and 4 described below was processed under various conditions to produce a duplex stainless steel pipe having a wall thickness of 1.4 to 16 (mm). After flattening these steel pipes, test pieces having a pipe wall thickness of X10 mm X 10 mm were cut out. After the resin was loaded in a cross section perpendicular to the processing direction of the test piece (the “observation surface” shown in FIG. 1), the cross section was mirror-polished. The polished surface was observed using a scanning electron microscope (SEM), and the major diameter and chemical composition of the oxide-based inclusions were measured.

[0045] The major axis of the oxide-based inclusion is, as shown in Fig. 2, the length of the longest straight line (al or al) among two straight lines connecting two different points on the interface between the base material and the inclusion. means a2). The composition of the oxide-based inclusions is EDX (energy dispersive X-ray analysis) near the center of the inclusion (bl or b2 in the example shown in Fig. 2), ie, near the center of gravity of the cross-sectional shape of the inclusion. Using The content of alloying elements other than o (oxygen) was determined.

After observing the oxide-based inclusions, they were immersed in a 6% ferric chloride aqueous solution at 80 ° C. for 6 hours, and the corrosion state around the oxide-based inclusions was observed. Pitting corrosion originating from oxide inclusions was observed. The oxide inclusions that have caused pitting are Al O and (Ca,

twenty three

(Ca, Mg) 0 was eluted preferentially, forming a gap with the base material, from which pitting progressed.

[0047] Therefore, each of the generated oxide-based inclusions was observed by SEM, and the relationship between the oxide inclusions and the presence or absence of pitting corrosion was investigated.

FIG. 3 is a diagram showing the relationship between the major axis of the oxide-based inclusions and the total content of Ca and Mg. Note that “X” in FIG. 3 indicates an oxide-based inclusion that started pitting, and “〇” indicates an oxide-based inclusion that did not start pitting.

[0049] As shown in Fig. 3, the total content of Ca and Mg was 20 40%, and the major axis was 7%.

Oxides of not less than / im were the starting point of pitting corrosion. However, oxides having a total content of Ca and Mg of less than 20% were mainly composed of A1 oxides, and eluted as a starting point of pitting corrosion. Oxides with a total content of Ca and Mg exceeding 40% were completely eluted, but did not progress to pitting, where the effect of forming gaps with the base material was small. Furthermore, even for oxide-based inclusions with a total Ca and Mg content of 20-40%, if the major axis is less than 7 μm, the gaps will not be sufficiently large and oxide will elute. Did not progress to pitting

[0050] For this reason, the pitting corrosion temperature was investigated by focusing on oxide-based inclusions having a total content of Ca and Mg of 20 to 40% and a major axis of 7 µm or more. The critical pitting temperature is the maximum temperature at which pitting did not occur, after immersing for 24 hours in a 6% ferric salt aqueous solution of 35 ° C-80 ° C with the temperature changed every 5 ° C for 24 hours. Means As a result, the number of oxide-based inclusions with a total Ca and Mg content of 20-40% and a major axis of 7 μm or more was reduced to 10 per lmm ^{2 in} a cross section perpendicular to the processing direction. If it exceeds, the critical pitting temperature is remarkably lowered, and it has been found that the corrosion resistance in the above severe corrosive environment becomes insufficient.

[0051] Therefore, the content of Ca and Mg is 2040% in total, and the number of oxide-based inclusions having a major axis of 7 µm or more is 10 or less per lmm ^{2 in} a cross section perpendicular to the processing direction. The Cases. In addition, the tendency of pitting to occur for various oxide-based inclusions was arranged in the same way as for Ca and Mg.

FIG. 4 is a diagram showing the relationship between the major axis of the oxide-based inclusions and the S content. Figure 4

The meanings of “X” and “〇” in FIG.

As shown in FIG. 4, oxide inclusions having an S content of 15% or more and a major axis of 1 μm or more became pitting initiation points. The oxide-based inclusions containing S are minute and completely eluted after the pitting test, but the hydrogen sulfide generated after the elution accelerated the corrosion and progressed to pitting. On the other hand, oxide inclusions having a major axis of less than 1 zm and oxide inclusions having an S content of less than 15% did not become pitting corrosion starting points.

[0054] For this reason, the same critical pitting temperature was investigated by focusing on oxide-based inclusions having an S content of 15% or more and a major axis of 1 µm or more. It was found that the pitting corrosion resistance was improved when the number of the inclusions was 10 or less per 0.1 mm ^{2 in} a cross section perpendicular to the processing direction.

[0055] Therefore, is not less than 15% content of S, and major axis that 1 mu oxide inclusions is greater than or equal to m is more than 10 pieces per ^square cross section perpendicular 0.1mm in processing direction desired.

4. Method for Producing Duplex Stainless Steel of the Present Invention

The production method for controlling the composition of oxide-based inclusions in duplex stainless steel was studied in detail. As a result, in particular, by optimizing the respective manufacturing processes of (c reduction treatment, () killing, and (γ) 铸 post-fabrication processing), it has been possible to obtain a zero-clean, high-cleanliness duplex stainless steel. The respective manufacturing steps will be described below.

[0057] (a) Reduction treatment

The reduction treatment is performed under the condition that the slag basicity represented by the following equation (2) is 0.5-3.0. However, each compound in the formula (2) means the concentration of each compound in slag (% by mass).

[Slag basicity] = (CaO + MgO) / (Al O + SiO) (2)

2 3 2

[0058] In a crude stainless steel obtained by melting a raw material in an electric furnace or the like, oxygen is blown into the molten steel in a secondary refining furnace such as AOD or VOD to decarbonize the steel, and then chromium oxidized during the decarburization is removed. For recovery, a deoxidizing material such as metallic aluminum and a desulfurizing material such as limestone are charged, and a process called reduction is performed. During this reduction phase, the oxygen and sulfur associated with them are converted to Al 2 O 3, CaS It is removed from the molten steel by moving into the slag as such.

[0059] In order to achieve low oxygen and low sulfur which are features of the present invention, the slag basicity represented by the above formula (2) needs to be 0.5 or more. In particular, in order to minimize the S content in the oxide-based inclusions, it is desirable that the slag basicity be 1.0 or more. On the other hand, if the slag basicity is too high, the fluidity becomes poor as the melting point rises, and oxide inclusions with a total content of Ca and Mg of 2040% tend to remain in the steel. As a result, the pitting corrosion resistance of the steel decreases. From this viewpoint, the upper limit must be 3.0. In order to sufficiently reduce the Ca content and the Mg content in the oxide-based inclusions, the slag basicity is desirably 2.5 or less.

[0060] Further, the above-mentioned reduction treatment with slag basicity is usually performed only once, but in order to further reduce oxygen and sulfur, it is desirable to repeat this reduction period two or more times. At this time, the slag generated in the first reduction treatment is discharged out of the furnace before performing the second reduction by tilting the secondary refining furnace body and extracting it with an appropriate jig. This is important for removing desulfurized slag generated in the first reduction phase, thereby increasing the desulfurization capacity in the second reduction phase.

[0061] () Killing

Killing after the reduction treatment is performed at a temperature of 1500 ° C or more for 5 minutes or more.

[0062] After the reduction treatment shown in the above (α), the molten steel that has been subjected to secondary refining by fine adjustment to a predetermined component is tapped into a ladle and manufactured. Until the molten steel is tapped, the molten steel is floated on the molten steel, moved to a place where it is mixed with the slag again, or moved to a place where it is filled. This treatment is called killing, and during the killing, a part of the oxide suspended in the molten steel floats due to the difference in specific gravity and is absorbed and separated in the slag. In order to give the desired state of oxide-based inclusions to the duplex stainless steel, it is necessary to separate large oxides by flotation. To this end, the killing temperature is set to 1500 ° C or higher, and the killing time is reduced. You need to wait at least 5 minutes. To further promote the floating removal of these oxides, it is desirable to set the killing temperature to 1550 ° C or more and the killing time to 10 minutes or more.

[0063] (γ) Processing after fabrication

加工 Processing after fabrication is performed under the condition that the total working ratio R expressed by the following equation (3) is 10 or more. However AO in equation (3) is the cross-sectional area before deformation in the plastic deformation process, and A is the change in the plastic deformation process.

The subscript ii (l, 2, -ί) means the order of each stand in the plastic deformation process.

⁽³⁾

[0064] The produced piece is made by hot rolling and 1 / 緞 hot rolling or cold rolling! After being processed, it is molded to the specified product dimensions. At this time, as the material is deformed in the processing direction by the processing, the oxide-based inclusion is crushed and miniaturized. In order to give the desired state of oxide inclusions to the duplex stainless steel, it is necessary to make the total processing ratio R from the single strength to the final product 10 or more.

[0065] Note that the plastic deformation step does not include a cutting step or any other processing step that does not involve elongation. Therefore, even when a cutting step is included between the plastic deformation steps, the calculation of the above equation (3) is performed without considering the change in the cross section による due to the cutting step.

Example 1

[0066] Duplex stainless steel having the composition shown in Table 1 (super duplex stainless steel with a pitting resistance index PREW of 40 or more) was melted in a 500 kg induction melting furnace, then transferred to an AOD furnace, and subjected to secondary » I got it. At this time, the slag basicity in the reduction phase was set to 2.0. Slag and molten steel were sampled after the end of the reduction period, respectively. Further, after was allowed measured immediately mono- Mokappuru molten steel is tapped into a ladle, to measure the time until鏺inclusive initiation _D

[0067] At this time, the ladle was allowed to stand still at a certain position and the vibration was not applied to perform the killing until the ladle was lifted with a ladle crane to start the loading. The killing conditions at this time are as shown in Table 2.

[0068] [Table 1]

用用鉞鉞. (Rule 91) K

table 1

*: PREW = Cr + 3.3 (Mo + 0.5W) + 16N

2]

cm

"籙 ¾ © ί¾¾Glue ^ i¾i5κκS-Hι0n ： ί.εεε ...

[0070] The molten steel was formed into a steel ingot having an average size of 160 mm on one side by a lower casting method or into a round piece having an outer diameter of 180 mm by a continuous forming method.鋼 The forged steel slab is subjected to forging, hot extrusion, and cold rolling to various degrees of processing to form a seamless steel pipe with an outer diameter of 16-280 mm and a wall thickness of 1.4--16 mm. After holding at C for 3 minutes, a water-cooled solution heat treatment was performed.

After cutting and flattening the above-mentioned tube material, two test pieces each having dimensions of a tube thickness X 10 mm X 10 mm were cut out. After that, the resin was embedded in the tube cross-sectional direction, and the cross-section was mirror-polished. After that, SEM observation was performed on the oxide-based inclusions with a major axis of 7 μm or more at 5 times each with a magnification of 50 times, and on the oxide-based inclusions with a major diameter of 1 μm or more at 5 times each with 200 times magnification. Was.

[0072] The major axis of the oxide-based inclusion was measured according to the definition in Fig. 2, and the composition analysis of the vicinity of the center of the oxide-based inclusion (bl or b2 in Fig. 2) by EDX (energy dispersive X-ray analysis) was performed. did. At the time of analysis, the measured value of 〇 (oxygen) had low reliability in accuracy, so the mass ratio of Al, Ca, Mg, S, and Mn excluding O (oxygen) was measured.

[0073] Further, the tube was cut into 10 mm lengths, and the cut end face was polished with No. 600 emery paper and subjected to a pitting corrosion test. It was immersed for 24 hours in a 6% ferric chloride aqueous solution of 35 ° C. to 80 ° C., the temperature of which was changed every 5 ° C., and the maximum temperature at which no pitting occurred was measured. The measurement was performed using five test pieces per test tube, and the lowest value was defined as the critical pitting temperature, which was used as a measure of pitting resistance.

As shown in Table 2, even steel having the same composition has different pitting corrosion resistance depending on the killing conditions.

That is, in Example 13 of the present invention, since the starting temperature of the killing was 1500 ° C. or more and the temperature was maintained for 5 minutes or more, the total content of Ca and Mg among the inclusions was 20 to 40% and the major axis was long. Good pitting corrosion resistance was obtained when the number of oxide-based inclusions having a diameter of 7 μm or more was 10 or less per lmm ² of the cross section perpendicular to the processing direction. In particular, in Examples 1 and 2 of the present invention, the content of S was 15% or more, and the oxide-based inclusions having a long diameter of 1 μm or more were 10 pieces or less per 0.1 mm ^{2 in} a cross section perpendicular to the processing direction. As a result, the critical pitting temperature was 80 ° C, showing extremely good pitting resistance.

On the other hand, in Comparative Examples 13 and 13 in which one or both of the killing temperature and the holding time were out of the ranges specified in the present invention, the number of coarse oxide-based inclusions was increased and the pitting corrosion resistance was poor. It has become.

Example 2

[0076] Duplex stainless steels having the compositions shown in Tables 3 and 4 were melted in a 500 kg induction melting furnace, transferred to an AOD furnace, and subjected to secondary refining. At this time, the slag basicity in the reduction phase was varied. Slag and molten steel were sampled after the completion of the reduction phase and immediately after the fine adjustment of the components, respectively, and their composition was analyzed by chemical analysis. Also, go out to the ladle Immediately after the temperature of the steel melted was measured with a thermo cup, the time until the start of filling was measured.

[Table 3]

Table 3

Steel Chemical lake formation (% by mass, balance: Fe and impurities)

No. C Si Mn P S Ni Cr Mo Al N 0 Ca MQ Cu B W ③

1 0.020 0.35 0.72 0.021 0.0004 4.54 22.50 3.18 0.007 0.145 0.0031 0.0022 0.0011---35.31

2 0.022 0.33 1.41 0.028 0.0002 4.89 23.12 3.22 0.012 0.158 0.0029 0.0005 0.0021 0.21--36.27

3 0.017 0.19 0.51 0.024 0.0004 4.58 22.89 3.17 0.009 0.175 0.0030 0.0011 0.0018 one 0.0027-36.15

4 0.018 0.39 0.56 0.005 0.0003 4.73 22.94 3.10 0.016 0.182 0.0028 0.0020 0.0004 0.52 0.0020-36.08

5 0.021 0.09 0.22 0.033 0.0006 4.31 22.85 2.86 0.023 0.151 0.0036 0.0019 0.0009---34.70

6 0.019 0.33 0.43 0.018 0.0006 4.61 22.64 3.08 0.026 0.120 0.0042 0.0018 0.0002 0.48--34.72

7 0.020 0.34 0.51 0.021 0.0007 4.69 22.81 3.12 0.039 0.138 0.0032 0.0005 0.0008-0.0019-35.31

8 0.018 0.41 0.55 0.039 0.0009 4.52 23.01 3.11 0.046 0.164 0.0031 0.0048 0.0004 0.51 0.0025-35.90

9 0.019 0.03 0.51 0.019 0.0003 6.66 27.11 3.14 0.022 0.381 0.0030 0.0019 0.0019 0.24 0.0019-43.57

10 0.022 0.31 0.53 0.021 0.0003 4.56 22.71 3.11 0.015 0.165 0.0031 0.0021 0.0017 0.20 0,0021 0.40 36.27

11 0.008 0.31 0.21 0.023 0.0002 6.71 25.31 3.08 0.018 0.311 0.0051 0.0022 0.0001--2.00 43.75

12 0.025 0.27 0.53 0.021 0.0003 6.60 25.50 3.18 0.007 0.291 0.0049 0.0021 0.0013 0.49-2.02 43.98

13 0.022 0.31 0.44 0.018 0.0003 6.67 25.81 2.86 0.014 0.281 0.0029 0.0019 0.0018-0.0018 2.23 43.42

14 0.018 0.43 0.21 0.014 0.0004 6.51 26.10 3.12 0.034 0.301 0.0028 0.0005 0.0011 0.82 0.0022 2.18 44.81

15 0.017 0.49 0.64 0.047 0.0009 6.97 25.51 3.12 0.038 0.321 0.0036 0.0022 0.0022-1 2.51 45.08

16 0.022 0.44 0.23 0.022 0.0008 6.78 25.11 3.09 0.022 0.313 0.0041 0.0023 0.0026 0.22-2.53

17 0.020 0.23 0.31 0.019 0.0007 6.85 25.36 3.46 0.009 0.324 0.0029 0.0019 0.0021-0.0017 2.31 45.77

18 0.009 0.29 0.34 0,009 0.0008 6.69 25.12 3.07 0.022 0.298 0.0030 0.0031 0.0023 0.51 0.0015 2.18 43.62

19 0.017 0.45 1.01 0.017 0.0003 10.71 30.51 3.01 0.017 0.321 0.0030 0.0022 0.0004--2.21 49.23

20 0.021 0.31 0.99 0.021 0.0004 11.98 31.94 3.13 0.022 0.318 0.0045 0.0021 0.0005 One-2.31 51.17

21 0.011 0.28 0.41 0.022 0.0003 4.61 22.81 3.06 0.019 0.151 0.0045 0.0043 0.0019 One--35.32

22 0.008 0.37 0.75 0.012 0.0004 4.63 23.01 2.51 0.018 0.132 0.0039 0.0045 0.0017 0.53-33.41

23 0.022 0.43 1.09 0.011 0.0003 4.45 23.11 3.21 0.023 0.148 0.0032 0.0027 0.0045 one 0.0020 one 36.07

24 0.013 0.91 0.29 0.022 0.0004 4.84 22.51 3.09 0.031 0.144 0.0041 0.0031 0.0048 0,51 0.0021 ― 35.01

25 0.023 0.37 0.44 0.002 0.0002 6.69 23.50 2.96 0.019 0.344 0.0088 0.0022 0.0013-11 3877

③: PREW (= Gr + 3.3 (Mo + 0.5W) + 16N)

[0078] [Table 4]

[0079] At this time, the ladle was placed blue at a fixed position so as not to vibrate until the ladle was lifted by a ladle crane to start loading. The molten steel was formed into a steel ingot with an average size of 160 mm on one side by the sub-casting method or a round piece with an outer diameter of 180 mm by the continuous manufacturing method. The forged, hot extruded, and cold-rolled steel slabs are added to various degrees of work to obtain an outer diameter of 16 After being formed into a seamless steel pipe having a size of 280 mm and a wall thickness of 1.4 to 16 mm, the solution was kept at 1100 ° C. for 3 minutes, and then subjected to a water-cooled solution heat treatment. Tables 5 and 6 show the slag basicity during the reduction phase, the killing conditions and the total processing ratio.

[0080] After cutting and flattening the above-mentioned tube material, two test pieces each having a tube thickness of X10mm X 10mm were cut out, resin was embedded in the tube cross-sectional direction, and the cross-section was mirror-polished. After that, SEM observation was performed on the oxide inclusions with a major axis of 7 μm or more at 5 times each with a magnification of 50 times, and on the oxide inclusions with a major diameter of 1 μm or more at 5 times each with a magnification of 200 times. Was. The major axis of the oxide-based inclusion was measured according to the definition in Fig. 2, and the vicinity of the center of the oxide-based inclusion (bl or b2 in Fig. 2) was analyzed by EDX (energy dispersive X-ray analysis). At the time of analysis, the measured value of 〇 (oxygen) was unreliable in accuracy, so the mass ratio of Al, Ca, MgS, and Mn excluding 0 (oxygen) was measured. The results are shown in Tables 5 and 6.

Further, the tube was cut into 10 mm lengths, and the cut end faces were polished with No. 600 emery paper and subjected to a pitting corrosion test. The specimens were immersed in a 6% ferric salt aqueous solution of 35 ° C-80 ° C for 24 hours with the temperature changed every 5 ° C for 24 hours, and the maximum temperature at which no pitting occurred was measured. The measurement was performed using five test pieces per test tube, and the lowest value was defined as the critical pitting temperature, which was used as a measure of pitting resistance.

[0082] The target value of pitting corrosion resistance is a normal duplex stainless steel having a pitting resistance index PRE (or PREW) of less than 40 (steel Nos. 1-8, 10, and 21 shown in Tables 3 and 4). — For 27, 42, 43 and 46), a super duplex stainless steel with a critical pitting temperature of 35 ° C and a pitting resistance index PRE (or PREW) of 40 or more (Steel No. 9 in Tables 3 and 4) , 11-12, 28-41, 44, 45, 47 and 48), the critical pitting temperature was 70 ° C. The results are shown in Tables 5 and 6.

[Table 5] Table 5

S0084

①: the total content of Ga and Mg is meant a 20-40% der Li, and the cross section perpendicular 1 mm ² number per the working direction of the oxide inclusions is major axis 7 U m or more .

②: Oxide-based inclusions whose S content is 15% or more and whose major axis is 1 m or more

Means the number per 0.1 mm ² of the cross section perpendicular to the processing direction.

*: It means that the number of ① is out of the range specified in the present invention.

**: Meaning that the number of ② is out of the range defined in the present invention.

[0085] In the present invention example 423, the chemical composition and the total number of oxide-based inclusions having a total content of Ca and Mg of 20 to 40% and a major axis of 7 µm or more are defined by the present invention. It was within the specified range. For this reason, excellent pitting corrosion resistance exceeding the above-mentioned target values was obtained for both ordinary stainless steel and super stainless steel. In particular, the present invention example 4 7 12 13 in which the content of S was 15% or more and the number of oxide-based inclusions having a major axis of 1 m or more per 10 mm ^{2 in} a cross section perpendicular to the processing direction was 10 or less. In 15 18 22 and 23, even better pitting corrosion resistance was obtained with ordinary stainless steel and super stainless steel.

[0086] On the other hand, Comparative Example 2031 in which the chemical composition was out of the range specified in the present invention could not secure sufficient corrosion resistance as a two-phase stainless steel. Further, the chemical group defined in the present invention In Comparative Example 419, in which the production conditions are not suitable, the pitting corrosion resistance was poor because a large amount of oxide inclusions harmful to pitting remained.

Industrial applicability

[0087] According to the present invention, a duplex stainless steel having good pitting corrosion resistance can be stably obtained.

For this purpose, for example, pipes or structures for heat exchange, pipes or structures for chemical plants, line pipes, casings or tubing for oil wells or gas wells, or steel pipes or steel plates such as umbilical tubes (pipes for controlling submarine oil wells) Thus, it is possible to provide the most suitable duplex stainless steel.

Brief Description of Drawings

FIG. 1 is a view showing an observation surface of an oxide-based inclusion.

FIG. 2 is a diagram that defines the measurement points of the major axis and the composition of oxide-based inclusions.

FIG. 3 is a graph showing the relationship between the major axis of oxide-based inclusions and the total content of Ca and Mg.

FIG. 4 is a graph showing the relationship between the major axis of oxide-based inclusions and the S content.

Explanation of symbols

[0089] 1 · Steel plate (or steel pipe)

Claims

The scope of the claims

By mass%, C: 0.03% or less, Si: 0.01-2%, Mn: 0.1 2%, P: 0.05% or less, S: 0.001% or less, Al: 0.003 0.05%, Ni: 4 12%, Cr: 18 — 32%, Mo: 0.2—5%, N (nitrogen): 0.05—0.4%, 〇 (oxygen): 0.01% or less, Ca: 0.0005—0.005%, Mg: 0.0001 0.005%, Cu: 0—2%, This is a duplex stainless steel containing B: 0-0.01% and W: 0-4%, with the balance being Fe and impurities, and the total content of Ca and Mg in the inclusions contained therein A duplex stainless steel comprising 20 to 40% by mass and having 10 or less oxide-based inclusions having a major axis of 7 µm or more per lmm ^{2 in} a cross section perpendicular to the application direction. In mass%, C: 0.03% or less, Si: 0.01-2%, Μη: 0.1-2%, Ρ: 0.05% or less, S: 0.001% or less, Α1: 0-003—0.05%, Ni : 4—12%, Cr: 18—32%, Mo: 0.2—5%, N (nitrogen): 0.05—0.4%, 〇 (oxygen): 0.01% or less, Ca: 0.0005—0.005%, Mg: 0.0001— Duplex stainless steel containing 0.005%, Cu: 0-2%, B: 0-0.01%, and W: 0-4%, with the balance being Fe and impurities. Of which, the total content of Ca and Mg is 20-40% by mass, and the number of oxide-based inclusions having a major axis of 7 μm or more is 10 or less per lmm ^{2 in} a section perpendicular to the application direction. S is a content of 15 mass% or more of, and biphasic major axis and wherein the oxide inclusions is l xm above is 10 or less the cross section perpendicular 0. lmm ² per the working direction Stainless steel.

0.1% by mass

3. The two-phase stainless steel oka according to claim 1, comprising 22% Cu.

The duplex stainless steel according to any one of claims 1 to 3, comprising 0.001 0.01% B by mass%.

The duplex stainless steel according to any one of claims 1 to 4, comprising 0.14% W in mass%.

The duplex stainless steel according to any one of claims 1 to 5, wherein the pitting resistance index PREW represented by the following formula (1) is 40 or more.

PREW = Cr + 3.3 (Mo + 0.5W) + 16N (1)

However, each element symbol in the formula (1) means the content (% by mass) of each element. The molten steel that has been reduced under the condition that the slag basicity expressed by the following formula (2) is 0.5-3.0 To 1500. It is manufactured after performing the killing for 5 minutes or more at the temperature of C or more, and processing the obtained stalk under the condition that the total processing ratio R expressed by the following formula (3) is 10 or more. The method for producing a duplex stainless steel according to any one of claims 1 to 6.

[Slag basicity] = (CaO + gO) / (Al 0 + SiO) (2)

° 3 2

(3)

However, each compound in the formula (2) means the concentration of the compound in slag (% by mass). In equation (3), AO is the cross-sectional area before deformation in the plastic deformation process, and A is

n n

The subscript n (l, 2, '') means the order of each stand in the plastic deformation process.

Revised JE Form (Rule 91)