NO336117B1 - Duplex stainless steel and its manufacturing method - Google Patents

Description

The present invention relates to duplex stainless steel which has excellent corrosion resistance in seawater. This steel is used for steel pipes, steel plates or the like, such as pipelines for heat exchange, pipelines or structures for a chemical plant, conduit pipes, oil well or gas well lining or piping, and connecting pipes (control pipelines for an undersea oil field).

BACKGROUND OF THE INVENTION

Conventionally, although crude oil and natural gas drilled from undersea oil fields and the like have been avoided due to harsh working environments, the new strict energy conditions bring with them a situation in which the crude oil and natural gas must be utilized. Therefore, the demand for stainless steel, which has excellent pitting resistance, especially duplex stainless steel, as a material for steel pipes or other structures used in seawater.

A so-called super duplex stainless steel which has improved pitting resistance because it contains W in addition to the adjustment of the contents of Cr, Mo and N (nitrogen), which are generally effective in improving the pitting resistance of duplex stainless steel, is shown in Patent Document 1. It proposes an index, showing the pitting resistance of duplex stainless steel, PREW according to the following equation (B) containing W, in addition to PRE (pitting resistance equivalent) according to the following equation (A).

The pitting resistance index PRE or PREW is adjusted to not less than 35 in the general duplex stainless steel and to not less than 40 in the super duplex stainless steel. Conventional techniques for improving pitting resistance were carried out based on how much the pitting resistance index PRE or PREW can be increased.

In equations (A) and (B), each chemical symbol indicates the content of each element (wt%).

The influence on pitting resistance of non-metallic inclusions has not been investigated in the duplex stainless steel. However, with respect to the pitting resistance of austenitic stainless steel, it is known that Mn sulfides are most detrimental to the pitting resistance, and oxides thereof are harmless as described in Non-Patent Document 1.

Oxide-based inclusions contained in stainless steel types are generally composite oxides composed of oxides such as Al-oxide (AI2O3), Si-oxide (SiC>2), Cr-oxide (Cr2O3). These oxides were assumed to have no effect on pitting because they hardly dissolve in aqueous solutions or so-called insolubility. On the other hand, although Ca and Mg, and furthermore S which are impurity elements in steel product, may be contained in the oxides, the influence of these elements on the pitting corrosion resistance has never been investigated.

WO 03044237 describes a duplex stainless steel with high resistance to stress corrosion cracking and which is suitable for use in plants for the production of foodstuffs.

EP 545753 A describes a duplex stainless steel with a PREW value of at least 40, where PREW is defined as:

PREW=[%Cr]+3.3[%Mo]+0.5[%W]+[%N]

EP 757112 A describes a duplex steel having a pitting corrosion resistance index, PREW, greater than 40

[Patent Document 1] Japanese Patent Publication No. H05-132741

[Non-Patent Document 1] J. E. Castle et al., "Studies by Auger Spectro-scopy of Pit Initiation at the site of Inclusions in Stainless Steel", Corrosion Science, Volume 30, No. 4/5, p. 409

SUMMARY OF THE INVENTION

In recent years, the application of duplex stainless steel to strictly corrosive environments such as a high temperature seawater environment has increased. For example, in a corrosion test that simulates such a severe condition, an 80°C trivalent iron chloride test, sufficient pitting resistance cannot necessarily be achieved even in the case of super duplex stainless. Only the adaptation of the contents of Cr, Mo and N (nitrogen) and further W or the like is often insufficient for the improvement in pitting resistance. Furthermore, although the pitting resistance can be somewhat improved by reducing the Mn sulfides in the steel, even in duplex stainless steels similar to the austenitic stainless steel, the pitting cannot be absolutely prevented.

The present invention solves these problems, and its purpose is to provide a duplex stainless steel capable of stably achieving satisfactory pitting resistance, and a method for producing this.

As a result of detailed investigations for metallurgical factors affecting the pitting resistance of duplex stainless steel, the present inventors found that, in addition to the above conventional factor contributing to pitting, even the oxide-based inclusions generated in the steelmaking process can significantly affect the pitting resistance, if they contain Ca and Mg, and also if they contain S. The knowledge obtained by the studies of the present inventors is as follows.

Oxide-based inclusions formed in steels with a Ca content of less than 0.0005 wt% or a Mg content of less than 0.0001 wt% are mainly composed of insoluble AI2O3, and never cause pitting. Oxide-based inclusions formed with a Ca or Mg content exceeding 0.005% by weight are mainly composed of (Ca,Mg)0, and pitting almost does not begin in such oxides.

However, oxide-based inclusions formed in steels with a Ca content of 0.0005 to 0.005 wt% and a Mg content of 0.0001 to 0.005 wt% produce a condition where Al 2 O 3 and (Ca,Mg) 0 coexist, and when these oxide-based inclusions are formed adjacent, pitting corrosion is apt to start in such oxides.

As a result of various studies to clarify the causes of pitting in duplex stainless steel containing 0.0005 to 0.005 wt% of Ca and 0.0001 to 0.005 wt% of Mg, the present inventors found that the occurrence of pitting depends on the size and number of oxide inclusions formed in the steel.

S is an element that is unavoidably present in steel, and it is impossible to completely remove its content in current steelmaking techniques. Although S deteriorates the pitting resistance when it is contained in the oxide-based inclusions formed in steel in large amounts, it was made clear by the studies of the present inventors that the pitting can be suppressed, even in such oxide-based inclusions, by adjusting the size and number thereof.

Duplex stainless steel, of a desired oxide-based inclusion state, cannot be produced by steelmaking or heat treatment using conventional methods. As a result of various investigations, the present inventors found that (a) the slag base degree in reduction, (p) the deoxidation temperature and time in the ladle, and (y) the overall processing conditions after casting are controlled to an optimum combination, whereby a desired oxide-based inclusion state can be achieved, which enables the production of unconventional high-purity steel.

The present invention has been completed based on the chemical composition of a steel product capable of ensuring the performance of a duplex stainless steel; an oxide-based inclusion state capable of significantly improving pitting resistance, and a manufacturing process to achieve increased purity.

The present invention involves duplex stainless steel types shown in the following description (a), and a method for producing duplex stainless steel shown in the following description (b). (a) A duplex stainless steel containing, by weight %, C: not more than 0.03%, Si: 0.01 to 2%, Mn: 0.1 to 2%, P: not more than 0, 05%, S: not more than 0.001%, 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): not more than 0.01%, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0 .01%, and W: 0 to 4%, and the balance is Fe and impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40 wt% and also have a long diameter of not less than 7u.m, is not more than 10 per 1 mm<2> of the cross-section perpendicular to the processing direction, in which the number of oxide-based inclusions, which have a content of S of not less than 15% by weight and also have a long diameter of not less than 1 u.m, is not more than 10 per 0.1 mm2 of the cross section perpendicular to the machining direction and where the pitting resistance index PREW, represented by the following equation (1), is not less than 40. In equation one (1), each chemical symbol shows the content of each element (% by weight).

In the steels described in (a) above, the contents of Cu, B and W are desirably 0.2 to 2%, 0.001 to 0.01% and 0.1 to 4% by weight, respectively.

(b) A process for producing a duplex stainless steel, according to the above (a), characterized by reducing the assumption that a slag base degree, represented by the following equation (2) is 0.5 to 3.0, deoxidation to tapped molten steel at the temperature not lower than 1500°C for not less than 5 minutes followed by casting, and forming the resulting rolling block on the condition that the total processing ratio R represented by the following equation (3) is not less than 10. In equation ( 2), each compound represents the concentration in slag (wt%) of each compound. In equation (3), A0 and An respectively represent a cross-sectional area before the deformation in the plastic deformation

the mering process and a cross-sectional area after the deformation in the plastic deformation process, and each suffix n (1, 2, ... i) represents each position order in the plastic deformation process.

According to the present invention, duplex stainless steel having good pitting resistance can be stably obtained. Therefore, duplex stainless steel can be provided which is most suitable for steel pipes, steel plates or the like, such as heat exchange pipelines, chemical plant pipelines or structures, pipeline, oil well or gas well lining or piping, or connecting pipes (control pipelines for submarine oil fields).

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a section showing an observation surface for oxide-based inclusions; Figure 2 is a section to define the long diameter and measurement position of the composition of oxide-based inclusions; Figure 3 is a section showing the relationship between long diameter and total content of Ca and Mg in oxide-based inclusions; and Figure 4 is a section showing the relationship between long diameter and a content of S in oxide-based inclusions.

(Explanation of number)

1: steel plate (or steel tube)

DETAILED DESCRIPTION OF THE INVENTION

1. Chemical composition

It is required to organize the chemical composition of the steel product within the following range to ensure sufficient pitting resistance in a duplex stainless steel. In the following descriptions, "%" for content means "% by weight".

C: Not more than 0.03%

C is inevitably present in steel. When the content exceeds 0.03%, carbides tend to precipitate, resulting in deterioration of pitting resistance. Accordingly, the content of C is set to no more than 0.03%.

Say: 0.01 to 2%

Si is an element that is effective for the deoxidation of steel, and a content of no less than 0.01% is therefore required. However, a content exceeding 2% promotes the formation of intermetallic compounds, resulting in the deterioration of pitting resistance. Accordingly, the content of Si is set at 0.01 to 2%.

Mn: 0.1 to 2%

Mn is effective for stabilizing austenitic phases similar to Ni, and a content of no less than 0.1% is therefore required. On the other hand, a content exceeding 2% leads to deterioration of pitting resistance. Accordingly, the content of Mn is set at 0.1 to 2%.

P: Not more than 0.05%

P is inevitably present in steel as contaminants, and actively dissolves to weaken pitting resistance. Since a content exceeding 0.05% makes this effect noticeable, the content must be set to no more than 0.05%. The content of P is desirable as low as possible.

S: Not more than 0.001%

S is inevitably present in steel in a similar way to P, and degrades pitting resistance by forming sulphides which easily dissolve. A content that exceeds 0.001% makes this effect noticeable. Since even a content of no more than 0.001% can support pitting when contained in oxide-based inclusions, as described later, the content of S is desirably as low as possible within this range.

Al: 0.003 to 0.05%

Al is an element that is necessary for the deoxidation of steel, and a content of no less than 0.003% is therefore required. On the other hand, an unreasonably large content causes deterioration of pitting resistance due to precipitation of Al nitrides, which absorb N (nitrogen) which is an element effective in improving pitting resistance. Accordingly, the content of Al is set at 0.003 to 0.05%. Al means "sol. Al (acid-soluble Al)".

Nine: 4 to 12%

Ni is an element that stabilizes austenitic phases, and its effect is insufficient within a content of less than 4%. On the other hand, a content exceeding 12% causes excessive austenitic phases, resulting in a loss of mechanical properties in duplex stainless steel. Accordingly, the content is set at 4 to 12%.

Cr: 18 to 32%

Cr is effective in improving pitting resistance, and a content of less than 18% results in insufficient pitting resistance. On the other hand, a content exceeding 32% causes excessive ferritic phases, resulting in a loss of mechanical properties in duplex stainless steel. Accordingly, the content of Cr is set at 18 to 32%.

Mo: 0.2 to 5%

Mo is also an element that can improve pitting resistance in a similar way to Cr, and the effect is not sufficient with a content of less than 0.2%. On the other hand, a content exceeding 5% causes precipitation of intermetallic compounds, and inversely results in deterioration of pitting resistance. Accordingly, the content of Mo is set at 0.2 to 5%.

N (Nitrogen): 0.05 to 0.4%

N (Nitrogen) is an element that affects the stabilization of austenitic phases in a similar way to Ni. N (nitrogen) also has the effect of improving pitting resistance in a similar way to Cr and Mo. However, these effects are insufficient with a content of less than 0.05%. On the other hand, a content exceeding 0.4% causes deterioration of hot workability. Accordingly, the content of N (nitrogen) is set at 0.05 to 0.4%.

0 (Oxygen): Not more than 0.01%

O (Oxygen) is inevitably present in steel in a similar way to S; it is present in an oxide-based inclusion state. These oxides weaken pitting resistance depending on their compositions, because these oxides are the source of pitting. Especially when the content exceeds 0.01%, coarse oxides increase, which makes this tendency striking. Accordingly, O (oxygen) must be limited to no more than 0.01%. The content of O (oxygen) is desirable as low as possible.

Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%

Ca and Mg are elements that have the effect of improving hot workability of steel by controlling S as sulphides. However, as described above, in duplex stainless steel containing Ca: 0.0005 to 0.005% and Mg: 0.0001 to 0.005%, when Al2O3 and (Ca,Mg)0 coexist and form close to each other, the pitting resistance is adversely affected. Consequently, the contents of Ca and Mg are limited to ranges of 0.0005 to 0.005% and 0.0001 to 0.005%, respectively, where the pitting resistance tends to be weakened. The pitting resistance of the duplex stainless steel according to the present invention can be improved by limiting the oxide-based inclusion state as described later.

The duplex stainless steel according to the present invention has the above-mentioned chemical composition, where the balance is Fe and impurities. The duplex stainless steel according to the present invention can include one or more of Cu, B and W as possible additive elements.

Cu: 0 to 2%

Cu stabilizes the austenitic phase in a similar way to Ni. It also stabilizes sulphide coatings in a hydrogen sulphide environment which improves pitting resistance. Therefore, Cu can be added as needed. Although a content of no less than 0.2% is desirable to achieve the effect above, a content exceeding 2% impairs hot workability. Accordingly, when Cu is added, the content is desirably set to 0.2 to 2%.

B: Less than 0.01%

B may be added as needed since it is an element effective in improving hot workability. Although the content is preferably set to no less than 0.001% to achieve this effect, the effect is satisfied even if the content exceeds 0.01%. Accordingly, when B is added, the content is desirably set at 0.001 to 0.01%.

W: 0 to 4%

W can be added as needed since it is an element effective in improving pitting resistance similar to Cr and Mo. This effect becomes noticeable when the content is not less than 0.1%. However, a content exceeding 4% causes precipitation of intermetallic compounds, which somewhat degrades the pitting resistance. Accordingly, when W is added, the content is desirably set to 0.1 to 4%.

2. Pitting corrosion resistance index

The duplex stainless steel of the present invention is a super duplex stainless steel, which has the above chemical composition and the pitting corrosion resistance index, which is defined as follows, is not less than 40. In the equation (1), each chemical symbol represents the content (% by weight) of each element.

3. Condition of oxide-based inclusions

The present inventors investigated the influence of oxide-based inclusions on pitting resistance by the following methods.

Molten steels having chemical compositions shown in Tables 3 and 4 were processed under various conditions to produce duplex stainless steel tubes 1.4 by 16 (mm) thick. After these steel tubes were printed flat, test pieces of tube thickness x 10 mm x 10 mm were cut out from them. The specimens were mounted in a resin to cross-section ("observation surface" shown in Fig. 1) direction perpendicular to the machining direction of each specimen, and this cross-section was completed by polishing. The polished-finished surface was observed by a scanning microscope (SEM) to measure a long diameter and the chemical composition of oxide-based inclusions.

The long diameter of oxide-based inclusions means the length (a1 or a2) of the longest straight line of the lines connecting two different points on the interface between a base metal and each inclusion as shown in Fig. 2. For the composition of the oxide-based inclusion, the proximity of the center part of the inclusion (b1 or b2 in the example shown in Fig. 2) or the vicinity of the gravity center part of the cross-sectional shape of the inclusion measured by EDX (energy dispersive X-ray spectroscopy) to determine the contents of alloying elements other than 0 (oxygen).

After the observation of the oxide-based inclusions, the test pieces were immersed in a 6% aqueous solution of trivalent ferric chloride at 80°C for 6 hours, and the corrosive state of the periphery of the oxide-based inclusions was then observed. As a result, pitting initiated at the oxide-based inclusions was observed in parts of the specimens. The oxide-based inclusions which caused the pitting are composite oxides of Al2O3 and (Ca,Mg)0, in which the proportion of (Ca,Mg)0 is preferentially eluted to form fractures with the base metal, and the fractures developed into pitting.

Each of the generated oxide-based inclusions was observed by SEM to examine the relationships of the oxide-based inclusions with the presence/absence of pitting.

The relationship between the long diameter and the total content of Ca and Mg in oxide-based inclusions is shown in Fig. 3, where "x" indicates an oxide-based inclusion at which pitting started, and "o" indicates an oxide-based inclusion at which no pitting begins.

As shown in Fig. 3, the pitting began when the oxides had a total content of Ca and Mg of 20 to 40% and a long diameter of not less than 7 µm. However, the pitting did not start when the oxides had a total content of Ca and Mg of less than 20% because the oxides are mainly composed of Al oxides, which were difficult to elute. Although oxides with a total content of Ca and Mg exceeding 40% are definitely eluted, the fractures did not develop into pitting because the effect of formation of the fractures, with the base metal, is low. In oxide-based inclusions, with a total content of Ca and Mg of 20 to 40%, but a long diameter less than 7 µm, the fractures did not develop into pitting even with elution of the oxides because the size of the fractures was not sufficient.

Therefore, when observing oxide-based inclusions having a total content of Ca and Mg of 20 to 40% and a long diameter of not less than 7 µm, the pitting resistance temperature was investigated. The critical pitting temperature means the highest temperature at which no pitting is caused by immersion in a 6% aqueous solution of trivalent ferric chloride at 35 to 80°C with a change in temperature of 5°C for 24 hours. It was found that, when the number of oxide-based inclusions, having a total content of Ca and Mg of 20 to 40% and a long diameter of not less than 7 µm exceeds 10 per 1 mm<2> of the cross-section perpendicular to the machining direction, the critical the pitting temperature is noticeably reduced resulting in the corrosion resistance, in the above-mentioned harsh corrosive environment, becoming insufficient.

Accordingly, the number of oxide-based inclusions, which have a total content of Ca and Mg of 20-40% and a long diameter of not less than 7 µm, is set to not more than 10 per 1 mm<2> of the cross-section perpendicular to the processing direction. For different oxide-based inclusions, the occurrence tendency of pitting corrosion was organized in a similar way as the case of Ca and Mg.

The relationships between long diameter and a content of S in the oxide-based inclusions are shown in Fig. 4, where "x" and "o" mean the same as they do in Fig. 3.

As shown in Fig. 4, pitting began with oxide-based inclusions that have an S content of not less than 15% and a long diameter of not

less than 1 u.m. Although the oxide-based inclusions containing S eluted perfectly after the pitting test, due to very small size, the hydrogen sulfide generated after elution promoted corrosion and developed into pitting. On it

on the other hand, oxide-based inclusions with a long diameter of less than 1 µm and oxide-based inclusions with a content of S of less than 15% do not cause pitting.

Therefore, when observing oxide-based inclusions having a content of S of not less than 15% and a long diameter of not less than 1 µm, the same critical pitting temperature as above was investigated. As a result, it was found that when the number of these inclusions is not more than 10 per 0.1 mm<2> of the cross section perpendicular to the machining direction, the pitting resistance is improved.

The number of the oxide-based inclusions which have an S content of not less than 15% and a long diameter of not less than 1 µm is therefore desirably set to not more than 10 per 0.1 mm<2> of the cross-section perpendicular to the processing direction.

4. Method for producing duplex stainless steel according to the present invention

The manufacturing process to control the composition of oxide-based inclusions in duplex stainless steel was investigated in detail. As a result, it was found that an outstanding high-purity duplex stainless steel can be obtained, in particular, by optimizing respective production processes of (a) reductive treatment, (P) deoxidation, and (y) post-casting processing. The respective production processes are described as follows.

(a) Reductive treatment

The reductive treatment is carried out under a condition which provides a slag base degree, represented by the following equation (2), of 0.5 to 3.0. In equation (2), each compound represents the concentration in slag (wt%) of each compound.

Stainless raw molten steel, obtained by melting a raw material in an electric furnace or the like, is decarburized while blowing oxygen to the molten steel in a secondary refining furnace such as AOD or VOD, and is subjected to a treatment called reduction in which a deoxidizer is applied, such as metallic aluminum and a desulphurisation agent such as limestone to recover chromium oxidized in the decarburisation. In this reductive step, the oxygen and sulfur bound to these agents are removed from the molten steel by transfer as Al 2 O 3 , CaS or the like to the slag.

In order to achieve the low oxygen and low sulfur characteristics of the present invention, the slag base degree represented by equation (2) must be set to no less than 0.5. In particular, to minimize the content of S in oxide-based inclusions, the slag base degree is set to not less than 1.0. On the other hand, an excessively high slag base degree makes it easy for oxide-based inclusions with a total content of Ca and Mg of 20 to 40% to be left in the steel, resulting in the deterioration of pitting resistance of the steel product, and in addition to that, the flow property becomes deficient, according to a rise of the melting point. From this point of view, it is required to set the upper limit value to 3.0. In order to sufficiently reduce the Ca content and Mg content in the oxide-based inclusions, the slag base degree is desirably set to no more than 2.5.

The reductive treatment at the above slag base grade is generally performed once. To further reduce the oxygen and sulfur contents, the reductive step is repeated two or more times. At this point, the slag generated by the first reductive treatment is discharged into the secondary refining furnace before performing the second reduction by tilting the furnace and scraping it out of the furnace using the appropriate tool. This operation is important to improve the desulfurization performance of the second reductive step by removing the slag containing a large amount of sulfur generated in the first reductive step.

(P) Deoxidation

The deoxidation after reductive treatment is carried out at a temperature not lower than 1500°C for 5 minutes or more.

After the reductive treatment shown in (a), the molten steel, which has completed secondary refining by a slight adjustment to a predetermined composition, is tapped into a ladle and cast. The tapped molten steel is kept stationary or moved to a casting site so as not to mix again with the slag floating on the molten steel before casting. This treatment is called deoxidation. During the deoxidation, part of the oxides suspended in the molten steel are lifted by the difference in specific gravity and separately absorbed into the slag. To give a desired oxide-based inclusion state to the duplex stainless steel, it is required to raise and separate coarse oxides. It is therefore important to ensure a deoxidation temperature that is not lower than 1500°C and a deoxidation time that is not less than 5 minutes. To further promote the flotation of the oxides, a deoxidation temperature not lower than 1550°C and a deoxidation time not less than 10 minutes is desired,

(y) Processing after casting

The processing after casting is carried out in a condition that provides a total processing ratio R, represented by the following equation (3), of not less than 10. In equation (3), A0 and An respectively represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, and each suffix n (1, 2, ... i) represents each position order in the plastic deformation process.

The cast ingots are subjected to a hot working such as forging or hot rolling or a cold working such as cold rolling and then formed into a predetermined product dimension. At this point, the oxide-based inclusions are crushed and cleaned, according to the working direction deformation of the material during processing. To give a desired oxide-based inclusion state to the duplex stainless steel, the total machining ratio R from roll block to finished product must be set to no less than 10.

The plastic deformation process does not include the cutting process and other machining processes that do not involve rolling and drawing. Accordingly, even if a cutting process is contained in the plastic deformation process, the calculation according to equation (3) is performed without taking into account the change in the cross-sectional area by this cutting process.

EXAMPLES

[Reference Example 1]

Each duplex stainless steel having a composition shown in Table 1 (super duplex stainless steel with a pitting resistance index PREW of not less than 40) in which 500 kg was melted in an induction melting furnace, transferred to an AOD furnace, and then refined again therein. At this time, the slag base degree of the reductive step was set to 2.0. A sample was taken of the slag and the molten steel, respectively, after the completion of the reductive step. The temperature of the molten steel poured into a ladle was immediately measured by a thermocouple, and the total elapsed time until the start of casting was measured.

At this point, the ladle is stationary and deoxidized in a given position without producing vibration until it is lifted up by a ladle crane to start casting. The deoxidation condition is shown in Table 2.

The molten steel was cast into a steel ingot, 160 mm on a side by average dimension, by bottom casting or into a round roll block 180 mm in outer diameter by continuous casting. The resulting rolled block was variously processed by forging, hot extrusion, or cold rolling and formed into a fully drawn steel tube 16-280 mm in outer diameter and 1.4 to 16 mm in thickness. The steel pipe was held at 1100°C for 3 minutes, and then subjected to "solution heat treatment" with water cooling.

After the pipe material above was cut and pressed flat, two test pieces, having a dimension of pipe thickness x 10 mm x 10 mm each, were cut out. The test pieces were mounted in a resin to the cross-sectional direction of the pipe, and this cross-section was then completed by polishing. Subsequently, the oxide-based inclusions of no less than 7 nm in diameter were observed by SEM in 5 fields of view, each at x50 magnification, and the oxide-based inclusions of no less than 1 nm in diameter in 5 fields of view, each at x200 magnification.

The long diameters of the oxide-based inclusions were measured according to the definition according to Fig. 2, and the vicinity of the center part of each oxide-based inclusion (b1 or b2 in Fig. 2) was compositionally analyzed by EDX (energy dispersive X-ray spectrometry). In the analysis, mass ratios of Al, Ca, Mg, S and Mn except 0 (oxygen) were measured because the measurement value of 0 (oxygen) has low precision reliability.

The pipe material was sectionally cut to a length of 10 mm, the cut end surface was polished with a No. 600 emery paper, and provided for a pitting test. The cut part was immersed in a 6% aqueous solution of trivalent ferric chloride at 35 to 80°C, changed in temperature by 5°C for 24 hours, and the highest temperature at which no pitting is generated was measured. The measurement was carried out using five specimens for one test pipe, and the lowest value of them was taken as the critical pitting temperature and used as an indication of the pitting resistance.

As shown in Table 2, in similar steel types having the same composition, the pitting resistance is varied depending on the deoxidation condition. Namely, in inventive examples 1 to 3 with a deoxidation starting temperature of 1500°C and a residence time of not less than 5 minutes, the number of oxide-based inclusions with a total content of Ca and Mg of 20 to 40% and a long diameter of not less than 7 nm no more than 10 per 1 mm<2> of the cross-section perpendicular to the machining direction, and satisfactory pitting resistance could be achieved. In particular, in inventive examples 1 and 2, extremely satisfactory pitting resistance was observed at a critical pitting temperature of 80°C, since the condition that the number of oxide-based inclusions with a content of S of not less than 15% and a long diameter of not less than 1 nm was 10 per 1 mm<2> of the cross-section perpendicular to the processing direction is also satisfied.

On the other hand, in Comparative Examples 1 to 3 where one or both of the deoxidation temperatures and residence time are outside the ranges limited by the present invention, the number of coarse oxide-based inclusions was increased to weaken the pitting resistance.

[Example 2]

Each duplex stainless steel having a composition shown in Tables 3 and 4 was melted in a 500 kg induction melting furnace, transferred to an AOD furnace, and secondarily refined therein. At this point, the slag base grade in the reductive step was changed in various ways. A sample was taken of the slag and the molten steel, respectively, after the end of the reductive step and just after the minor adjustment of the composition after reduction, and analyzed for composition by chemical analysis. The temperature of the molten steel tapped into a ladle was immediately measured by a thermocouple, and the time to start of casting was then measured.

At this point, the ladle is kept stationary and deoxidized in a given position without producing vibration until it is lifted up by a ladle crane to start casting. The molten steel was cast into a steel block 160 mm on a side by average dimension, by bottom casting or into a round roll block 180 mm in outer diameter, by continuous casting. The resulting billet was processed in various ways by forging, hot extrusion, or cold rolling and formed into a fully drawn steel tube 16-280 mm in outer diameter and 1.4 to 16 mm in thickness. The resulting tube was maintained at 1100°C for 3 minutes, and subjected to "solution heat treatment" with water cooling. The slag base degree of the reductive step, the deoxidation condition and the total processing ratio are shown in Tables 5 and 6.

After the above pipe material was cut and pressed flat, two test pieces, having a dimension of pipe thickness x 10 mm x 10 mm each, were cut out. The test pieces were mounted in a resin to the pipe's cross-sectional direction, and this cross-section was completed by polishing. Then, the oxide-based inclusions of no less than 7 nm in diameter were observed by SEM in 5 fields of view each at x50 magnification, and the oxide-based inclusions of no less than 1 nm in diameter in 5 fields of view each at x200 magnification. The long diameter of the oxide-based inclusions was measured according to the definition of Fig. 2, and the vicinity of the center part of each oxide-based inclusion (b1 or b2 in Fig. 2) was compositionally analyzed by EDX (energy dispersive X-ray spectrometry). In the analysis, the mass ratio of Al, Ca, Mg, S and Mn except 0 (oxygen) was measured because the measurement value of 0 (oxygen) has low precision reliability. The result is also shown in tables 5 and 6.

The pipe material was cut in sections to a length of 10 mm, the cut end surface was polished with a No. 600 emery paper and subjected to a pitting test. The cut part was immersed in a 6% aqueous solution of trivalent ferric chloride at 35 to 80°C, changed in temperature by 5°C for 24 hours, and the highest temperature at which no pitting was generated was measured. The measurement was carried out using five test pieces for one test pipe, and the lowest value of them was taken as the critical pitting temperature and used as an indication of pitting resistance.

As the target pitting resistance value, a critical pitting temperature of 35°C is taken for general duplex stainless steel (steel No. 1 to 8, 10, 21 to 27, 42, 43 and 46 shown in Tables 3 and 4) with a pitting resistance index PRE

(or PREW) of less than 40, and a critical pitting temperature of 70°C for super duplex stainless steel (steel Nos. 9, 11 to 20, 28 to 41, 44, 45, 47 and 48 shown in Tables 3 and 4) with a pitting resistance index PRE (or PREW) of not less than 40. The result is also shown in tables 5 and 6.

In Comparative Examples 4a to 11a, 13a, 14a and 19a-21a and Inventive Examples 12, 15 to 18, 22 and 23, the chemical composition and number of oxide-based inclusions with a total content of Ca and Mg of 20 to 40% and a long diameter of not less than 7 nm within the areas limited by the present invention. Therefore, excellent pitting resistance equal to or more than the above target value can be obtained in both the general stainless steel types (comparative examples) and the super stainless steel types (inventive examples). In particular, in Comparative Examples 4a to 7a and 13a and Inventive Examples 12, 15 to 18, 22 and 23 where the number of oxide-based inclusions with a content of S of not less than 15% and a long diameter of not less than 1 nm was not more than 10 per 0.1 mm<2> of the cross-section perpendicular to the machining direction, excellent pitting resistance was achieved in both the general stainless steel types (comparative examples) and the super stainless steel types ((inventive examples).

On the other hand, in Comparative Examples 20 to 31 where the chemical composition was outside the range limited by the present invention, sufficient anti-corrosion performance as duplex stainless steel could not be ensured. In Comparative Examples 4 to 19 where steel types have chemical compositions within the range limited by the present invention, but manufacturing conditions are not suitable, pitting resistance is not good because a large amount of oxide-based inclusions harmful to pitting remained.

According to the present invention, duplex stainless steel, which has satisfactory pitting resistance, can be stably obtained. Therefore, duplex stainless steel can be provided, most suitable for steel pipes, steel plates or the like such as pipelines for heat exchange, pipelines or structures for chemical plants, pipelines, oil well or gas well casing or piping, or connecting pipes (control pipelines of submarine oil field).

Claims (5)

1. Duplex stainless steel containing, by weight %, C: not more than 0.03%, Si: 0.01 to 2%, Mn: 0.1 to 2%, P: not more than 0.05% , S: not more than 0.001%, 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%, 0 (oxygen): not more than 0.01%, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01 %, and W: 0 to 4%, and the balance is Fe and impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40 wt% and also have a long diameter of not less than 7 nm , is not more than 10 per 1 mm<2> of the cross-section perpendicular to the processing direction, and a number of oxide-based inclusions, which have an S content of not less than 15% by weight and also have a long diameter of not less than 1 nm, is not more than 10 per 0.1 mm<2> of the cross-section perpendicular to the processing direction and in which a pitting corrosion resistance index PREW represented by the following equation (1) is not less than 40; in which each chemical symbol represents the content of each element (wt%). 2 Duplex stainless steel according to claim 1, which further contains 0.2 to 2% by weight Cu. 3. Duplex stainless steel according to any one of claims 1 or 2, further containing 0.001 to 0.01 wt% B. 4. Duplex stainless steel according to any one of claims 1 to 3, further containing 0.1 to 4 wt% W. 5. Process for producing a duplex stainless steel, according to any one of claims 1 to 4, characterized by reducing twice or more under the condition that a slag base degree, represented by the following equation (2), is 1.0 to 3.0, deoxidation to tapped molten steel at the temperature not lower than 1500°C for not less than 5 minutes followed by casting, and forming the resulting roll block under the condition that the total processing ratio R, represented by the following equation (3), is not less than 10; in which each compound in equation (2) represents the concentration in slag of each compound (wt%), A0 and An in equation (3) respectively represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, and each suffix n (1,2, ... i) represents each position order in the plastic deformation process.