KR20180081582A - Two-phase stainless steel material and two-phase stainless steel pipe - Google Patents

Abstract

The present invention relates to a two-phase stainless steel material comprising a ferrite phase and an austenite phase. The present invention is characterized in that at least one kind of metal selected from the group consisting of V: 0.01-0.50 mass%, Ti: 0.0001-0.0500 mass%, Nb: 0.0005-0.0500 mass%, and Ta: 0.01-0.50 mass% X group element, and the steel has a composite inclusion, a composite inclusion, and inclusions, and the inclusions include at least one of an oxide, a sulfide, and an oxysulfide, and the inclusion contains the inclusions as nuclei, And an outer periphery of a carbide or a nitride containing at least one of the above X group elements, wherein the ratio of the number of the complex inclusions to the number of the inclusions is 30% or more of the total number of the inclusions .

Description

Two-phase stainless steel material and two-phase stainless steel pipe

The present invention relates to a two-phase stainless steel material and a two-phase stainless steel pipe.

The stainless steel material is a material that naturally forms a stable surface coating mainly composed of an oxide of Cr called a passive film in a corrosive environment and exhibits corrosion resistance. Particularly, the two-phase stainless steel material composed of the ferrite phase and the austenite phase is superior in strength characteristics to austenitic stainless steel or ferritic stainless steel, and has excellent pitting corrosion resistance and stress corrosion cracking resistance. Because of these characteristics, the two-phase stainless steel is widely used as a structural material for marine environments such as an umbilical tube, a seawater desalination plant, a LNG (Liquefied Natural Gas) vaporizer, as well as a structural material for a fluid pipe and various chemical plants have.

However, when a corrosive substance such as chloride, hydrogen sulfide or carbon dioxide gas is contained in a large amount in the use environment, the corrosion of the two-phase stainless steel material, the so-called formula May occur. In addition, corrosive substances such as chloride ions are concentrated in the gaps in a part where structural gaps are formed in piping or flanges of two-phase stainless steel materials, resulting in a more severe corrosive environment. Further, an oxygen-enriched cell is formed between the outside and the inside of the gap to further promote local corrosion in the inside of the gap, and so-called crevice corrosion may occur. In addition, local corrosion such as corrosion or crevice corrosion may be a starting point of stress corrosion cracking.

As a countermeasure for such a problem, for example, Patent Document 1 discloses a two-phase stainless steel in which corrosion resistance is improved by controlling the content of Cr, Mo, N and W to have a PREW value of 40 or more. Patent Document 2 discloses a two-phase stainless steel excellent in corrosion resistance and hot workability by controlling the contents of B, T and the like in addition to controlling the contents of Cr, Mo, W, and N.

Also, in Non-Patent Document 1, it is experimentally shown that MnS in inclusions in steel becomes a starting point of local corrosion (formula) in stainless steel. In the technique described in Patent Document 3, CaO crucible and CaO-CaF ₂ -Al ₂ O ₃ -based slag are used in a vacuum melting furnace in order to reduce sulfide inclusions in steel which adversely affect hot workability and corrosion resistance, S amount is reduced to 3 ppm or less.

Patent Document 4 discloses an austenitic-ferritic stainless steel which has a ferritic phase shape controlled by finely dispersing a composite nonmetallic inclusion of a Ti-based nitride and a Mg-based oxide in an arbitrary section from a surface layer of a cast steel to a depth of 10 mm, A method for producing a stainless steel casting slab having a ferrite biaxial phase is disclosed.

Patent Document 5 discloses a method of finely dispersing a composite non-metallic inclusion in which a Ti-based oxide or nitride is produced using Mg-based inclusions as a nucleus to prevent the inclusion starting point formula and to further refine the structure of the cast steel, Based ferritic stainless steel.

Patent Document 6 discloses a method of finely dispersing an inclusions having an outer shell of a carbonitride of Ti and / or Nb around the nucleus of an Al-Ca-based oxysulfide to prevent the inclusion-based formula , A method of producing a low alloy steel excellent in pitting resistance without inducing stress cracking of sulfide starting from the formula is disclosed.

Patent Document 7 discloses that a sulfur-oxide composite inclusion containing Ta having a major diameter of 1 占 퐉 or more is 500 or less per 1 mm ² section perpendicular to the processing direction and the Ta content in the sulfur-oxide inclusion complex is 5 By mass or more, or more by atomic% or more, so that sulfide inclusions contained in ordinary stainless steels are modified with sulfur-oxide inclusion inclusions containing Ta to provide a two-phase stainless steel excellent in local corrosion resistance.

However, in the techniques reported in Patent Documents 1 to 7, the continuity of the passive film at the interface between the ferrite phase and the austenite phase, and at the interface between the inclusions such as oxides and sulfides unavoidably formed in the steel, The passive film tends to become unstable. As a result, there remains room for reviewing the inclusions that cause local corrosion.

Japanese Unexamined Patent Application Publication No. 5-132741 Japanese Patent Laid-Open No. 8-170153 Japanese Patent Application Laid-Open No. 3-291358 Japanese Patent Application Laid-Open No. 2002-69592 Japanese Patent Application Laid-Open No. 2000-212704 Japanese Patent No. 3864921 Japanese Patent Application Laid-Open No. 2015-110828

 Muto Izumi and others, Ferum Vol. 17 (2012), No. 12, pp. 858-863

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its main object is to provide a two-phase stainless steel material and a two-phase stainless steel pipe exhibiting excellent corrosion resistance.

Outline of representative examples of the invention disclosed in the present specification will be briefly described below. That is, a two-phase stainless steel material according to one aspect of the present invention is a two-phase stainless steel material composed of a ferrite phase and an austenite phase, wherein the steel has a composition of metal of V: 0.01-0.50 mass%, Ti: 0.0001-0.0500 At least one X group element selected from the group consisting of Nb: 0.0005 to 0.0500 mass%, and Ta: 0.01 to 0.50 mass%, wherein the steel has a composite inclusion, a composite inclusion and an inclusion, , A sulfide and an oxysulfide, and the complex inclusion has the inclusions as its nuclei and the outer periphery of the carbide or nitride containing Cr and at least one kind of the X group element around the nucleus And the ratio of the number of the composite inclusions to the total number of the inclusions is 30% or more.

Further, the two-phase stainless steel pipe according to another aspect of the present invention is characterized by being made of the above-mentioned two-phase stainless steel material.

According to the two-phase stainless steel material and the two-phase stainless steel pipe according to the present invention, excellent corrosion resistance can be exhibited.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view schematically showing composite inclusions and inclusions of a two-phase stainless steel material according to the present invention on an enlarged scale. FIG.
Fig. 2 is an explanatory view schematically showing a composite inclusion in the region A of Fig. 1 on an enlarged scale in a two-phase stainless steel material according to the present invention. Fig.
Fig. 3 is an explanatory diagram schematically showing an inclusion in the region B in Fig. 1 in a two-phase stainless steel material according to the present invention.
4 is an explanatory view schematically showing an enlarged form of a composite inclusion of a two-phase stainless steel material according to the present invention.
Fig. 5 is an explanatory diagram showing the difference in the heat treatment process of the method for producing a preferable two-phase stainless steel material in the present invention and the heat treatment process in the conventional method for manufacturing a two-phase stainless steel material.
6 is a graph showing the relationship between the inclusion coverage rate at which the formal potential and the inclusion are covered by the outer angle.
FIG. 7 is a graph showing the relationship between the formula potential and the Ta content in the metal component of the external angle, by dividing the judgment into two according to the PRE value for the two-phase stainless steel material according to the present invention.
FIG. 8 is a graph showing the relationship between the formula potential and the Ta / Ti ratio by dividing judgment into two according to the value of PRE with respect to the two-phase stainless steel material according to the present invention.

<Two Phase Stainless Steel>

The two-phase stainless steel material and the two-phase stainless steel pipe according to the embodiment of the present invention will be described. On the other hand, the complex inclusions 20 and the core inclusions 10 will be described with reference to Figs. 1 to 3 as appropriate.

The two-phase stainless steel material according to the present embodiment is a two-phase stainless steel material composed of a ferrite phase and an austenite phase. This two-phase stainless steel material contains a predetermined amount of at least one element selected from Cr, V, Ti, Nb and Ta, as well as a basic metal component composition. On the other hand, in the present embodiment, V, Ti, Nb and Ta are collectively referred to as &quot; X group element &quot;. Further, the two-phase stainless steel material is made of a material having an inclusion 10 made of at least one of oxide, sulfide and oxysulfide as its nucleus and containing at least one of Cr and at least one carbide or nitride of the X group element Is controlled to be 30% or more of the total number of inclusions (10) containing oxides, sulfides or oxysulfides, based on the total number of inclusions (20).

On the other hand, when the two-phase stainless steel contains a predetermined amount of Ta among the X group elements, it is preferable that the Ta content in the metal component of the carbide or nitride of the outer angle 15 is 5 atomic% or more. When Ta and Ti are contained, it is preferable that Ta is at least 25% by mass of Ti. As an example of the basic metal composition of the two-phase stainless steel material, it is preferable that a predetermined amount of C, Si, Mn, S, Ni, Mo, N, Cr and O are contained and the balance of Fe and inevitable impurities desirable. The two-phase stainless steel material preferably further contains one or two of Mg and Al in a predetermined amount. Further, in the two-phase stainless steel material, it is preferable that the above-mentioned metal component composition additionally contains Ca in a predetermined amount, and that the above-mentioned metal component composition additionally contains one or both of Co and Cu in a predetermined amount, The metal composition may further contain B in a predetermined amount.

Each constitution of the two-phase stainless steel material will be described below.

(Steel structure)

The two-phase stainless steel material according to the present embodiment is composed of two phases of a ferrite phase and an austenite phase. In a two-phase stainless steel material composed of a ferrite phase and an austenite phase, ferrite phase stabilizing elements such as Cr and Mo are concentrated on the ferrite phase, and austenite phase stabilizing elements such as Ni and N tend to be concentrated in the austenite phase have. At this time, when the area ratio of the ferrite phase is less than 30% or exceeds 70%, the difference in concentration between the ferrite phase and the austenite phase of elements contributing to corrosion resistance such as Cr, Mo, Ni and N becomes excessively large. Therefore, in the two-phase stainless steel material, unless the area ratio of the ferrite phase is within the above-mentioned range, the side of the ferrite phase and the austenite phase, which are poor in corrosion resistance, is selectively corroded and the corrosion resistance tends to deteriorate. Therefore, it is recommended to optimize the ratio of the ferrite phase to the austenite phase in the two-phase stainless steel material, and the area ratio of the ferrite phase is preferably 30 to 70%, more preferably 40 to 60% from the viewpoint of corrosion resistance. The area ratio of the ferrite phase can be optimized by adjusting the contents of the ferrite phase stabilizing element and the austenite phase stabilizing element.

Further, in the two-phase stainless steel material according to the present embodiment, an abnormal phase such as a sigma phase other than the ferrite phase and the austenite phase can be allowed to such an extent as not to impair various characteristics such as corrosion resistance and mechanical characteristics. The total area ratio of the ferrite phase and the austenite phase is preferably 95% or more, more preferably 97% or more, with respect to the total phase of the steel material.

(Inclusions in steel)

The duplex stainless steel material of the present embodiment contains a predetermined amount of Cr and at least one X group element selected from the group X consisting of V, Ti, Nb and Ta in addition to the basic metal composition. The two-phase stainless steel material has an outer periphery 15 of Cr and at least one carbide or nitride of an X group element around a core inclusion 10 made of at least one of oxide, sulfide and oxysulfide The ratio of the number of composite inclusions 20 is controlled to 30% or more of the total number of inclusions 10 including oxides, sulfides or oxysulfides. Therefore, even when a formula is generated in the oxide, sulfide and oxysulfide, in the complex inclusions 20 described above, by the outer angle 15 of the carbide or nitride consisting of Cr and X group elements excellent in corrosion resistance, Progress is suppressed. In order to make the ratio of the number of the composite inclusions 20 to the total number of the inclusions 10 or more in the two-phase stainless steel material, that is, [composite inclusions 20] / [inclusions 10] The ingot can be adjusted by hot working (hot forging) at a predetermined temperature for a predetermined time and by heat treatment for solidification after the ingot having the metal component composition adjusted to the constitution shown in the present embodiment as described later have.

(Oxide · sulfide)

In the duplex stainless steels of the present embodiment, it is preferable to control the oxides to the Al-Mg-based oxides as much as possible from the viewpoints of corrosion resistance and coating rate of the inclusions 10 by the external angle 15. [ If the addition amount of Mg is insufficient, it becomes an Al-Ca oxide which is likely to be dissolved in an oxidizing atmosphere. On the other hand, if the Mg addition amount is excessive, the Mg oxide which is poor in corrosion resistance is precipitated in a large amount, so that the corrosion resistance of the steel is lowered. By controlling the amount of Mg to be within the range specified by the present invention, it is possible to control the Al-Mg oxide having excellent corrosion resistance and the outer angle 15 of the nitride to reduce the formula based on the inclusion 10 .

When the sulfide of Mn is precipitated as the inclusions 10, the sulfide of Mn is deformed at the time of processing, so that the outer angle 15 of the nitride is destroyed and the improvement of the corrosion resistance by the outer angle 15 of the nitride can be expected none. Therefore, it is preferable to control Ca with sulfides as much as possible by adding Ca.

Next, the numerical range of the metal component composition, which is an example of the two-phase stainless steel material, and the reason for its limitation will be described. First, the X group element and Cr are described, and then the basic metal composition is described.

(X group element: V: 0.01 to 0.50 mass%, Ti: 0.0001 to 0.0500 mass%, Nb: 0.0005 to 0.0500 mass%, Ta: 0.01 to 0.50 mass%

The X group element, which is at least one kind selected from V, Ti, Nb and Ta, is an element (10) composed of at least one of oxides, sulfides and oxysulfides combined with C and N, , And the composite inclusions 20 having the outer angle 15 of the carbide or nitride of the X group element are formed. Of the X-group elements, Ta can cover the oxide, sulfide or oxysulfide inclusions 10 adversely affecting the corrosion resistance with a carbonitride layer containing Ta, that is, an outer shell 15, which has higher corrosion resistance . Therefore, Ta is an element improving the corrosion resistance.

For the above reason, the upper and lower limits of the X group element are set as follows. That is, the V content is 0.01 mass% or more, preferably 0.05 mass% or more. The Ti content is 0.0001 mass% or more, preferably 0.0003 mass% or more, and more preferably 0.0010 mass% or more. The Nb content is 0.0005 mass% or more, preferably 0.0010 mass% or more. The Ta content is 0.01 mass% or more, preferably 0.02 mass% or more, and more preferably 0.03 mass% or more.

The V content and the Ta content are set to 0.50 mass% or less, preferably 0.40 mass% or less, and more preferably 0.30 mass% or less, respectively. The Ti content and the Nb content are 0.0500 mass% or less, preferably 0.0400 mass% or less, more preferably 0.0300 mass% or less, further preferably 0.0200 mass% or less, still more preferably 0.0050 mass% or less, Or less. The total content of V, Ti, Nb and Ta is preferably 0.02 to 1.00 mass% in consideration of corrosion resistance and hot workability.

When the Ta group is contained in the range of 0.01 to 0.50 mass% as the X group element, it may be constituted so as to contain Ta in an amount satisfying the following expression (1) in the outer angle 15. [

([Ta] / [M] total) x 100? 5 (1)

[Ta] represents the number of Ta atoms included in the outer angle 15, and [M] total represents the sum of the number of atoms of the metal element contained in the outer angle 15, respectively.

When the Ta content (atomic%) satisfies the expression (1), the corrosion resistance of the outer shell 15 of the carbide or nitride is improved, so that the effect of suppressing the formal progress is enhanced and the corrosion resistance of the steel is improved. On the other hand, in order to satisfy the expression (1) in the content of Ta in the shell 15, it is necessary to add Ta in the range of 0.01 to 0.50 mass% to satisfy the predetermined production conditions described later.

In the present embodiment, it is more preferable that the inclusions 10 are controlled by Mg-Al-based oxides having excellent corrosion resistance.

In the case where the X group element contains 0.01 to 0.50 mass% of Ta and 0.0001 to 0.0500 mass% of Ti, the mass% of Ta / mass% of Ti that satisfies the following expression (2) may be used.

Ta mass% / Ti mass%? 25 (2)

When the ratio of the Ta mass% / Ti mass% satisfies the expression (2), the effect of suppressing the formal progress is enhanced since the carbide or nitride containing Ta, which is the outer angle 15, is easily covered with the inclusions 10 , The corrosion resistance of the steel material is improved.

Next, an example of the basic metal composition in the steel will be described. On the other hand, it is preferable to set the basic metal composition in the following range in consideration of the performance such as workability required to become a product, the conditions for making the metal structure into a two-phase structure, and the manufacturing cost.

(C: 0.100 mass% or less)

If C is excessive, the hot workability is deteriorated. In addition, by setting C to a predetermined value or less, unnecessary carbides are not formed, and the effect of suppressing deterioration of corrosion resistance is suppressed. Therefore, the C content is set to 0.100 mass% or less. On the other hand, the C content is preferably as small as possible, and therefore it is preferably 0.080 mass% or less, and more preferably 0.060 mass% or less. Further, C may be not contained in the steel material, that is, it may be 0 mass%.

(Si: 0.10 to 2.00 mass%)

Si is an element necessary for deoxidation and stabilization of the ferrite phase. In order to obtain such an effect, the Si content is 0.10 mass% or more, preferably 0.15 mass% or more, and more preferably 0.20 mass% or more. However, the Si content is 2.00% by mass or less, preferably 1.50% by mass or less, and more preferably 1.00% by mass or less, because the workability is deteriorated by adding Si excessively.

(Mn: 0.10 to 3.00 mass%)

Mn, like Si, has a deoxidizing effect and is an element necessary for securing strength. In order to obtain such an effect, the Mn content is 0.10% by mass or more, preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. However, if Mn is contained excessively, the Mn content is 3.00 mass% or less, preferably 2.50 mass% or less, and more preferably 2.00 mass% or less, since it causes deterioration of toughness and deterioration of corrosion resistance.

(S: 0.0100 mass% or less)

S is an element that is inevitably incorporated as an impurity in the same way as P, and forms an inclusions 10 of sulfide by bonding with Mn or the like to deteriorate corrosion resistance and hot workability. If S is excessively contained, coating with the outer shell 15 including the X group element to the inclusion 10 of the sulfide becomes insufficient and the corrosion resistance is lowered. Therefore, the S content is 0.0100 mass% or less, preferably 0.0050 mass% or less, and more preferably 0.0030 mass% or less. On the other hand, as described in the background art, S is preferable as the content is low, and it may be not contained in the steel, that is, it may be 0 mass%. However, an excessive reduction in the S content, for example, a reduction of 3 ppm or less causes an increase in the production cost, so that the lower limit is generally about 0.0004 mass%.

(Ni: 1.0 to 10.0 mass%)

Ni is an element necessary for improving the corrosion resistance, and is particularly effective in inhibiting local corrosion in a chloride environment. Further, Ni is effective for improving low-temperature toughness and is also an element necessary for stabilizing the austenite phase. In order to obtain such an effect, the Ni content is set to 1.0 mass% or more, preferably 2.0 mass% or more, and more preferably 3.0 mass% or more. However, if Ni is added excessively, the austenite phase is excessively increased, and the strength is lowered. Therefore, the Ni content is 10.0 mass% or less, preferably 9.5 mass% or less, and more preferably 9.0 mass% or less.

(Mo: 0.05 to 6.00 mass%)

Mo produces molybdenum acid at the time of dissolution and exhibits an effect of improving local corrosion resistance by inhibitor action, thereby improving the corrosion resistance. Mo is an element for stabilizing the ferrite phase and has an effect of improving the pitting resistance and crack resistance of the steel material. In order to obtain such an effect, the Mo content is 0.05 mass% or more, preferably 0.50 mass% or more, and more preferably 1.00 mass% or more. However, if Mo is added in excess, the generation of intermetallic compounds such as sigma phase is promoted and the corrosion resistance and hot workability are lowered. Therefore, the Mo content is 6.00 mass% or less, preferably 5.50 mass% or less, Is set to 5.00 mass% or less.

(N: 0.10 to 0.50 mass%)

N is an element that stabilizes a strong austenite phase and has the effect of improving the corrosion resistance without increasing the susceptibility to formation of the? Phase. Further, N is an element effective for increasing the strength of steel, and therefore, N is positively utilized in the present embodiment. On the other hand, if the content of N is small, the N may not play an important role in stabilizing the austenite phase, and corrosion resistance may be deteriorated. In order to obtain such an effect, the N content is set to 0.10 mass% or more, preferably 0.15 mass% or more, and more preferably 0.20 mass% or more. However, if N is added excessively, a coarse nitride is formed and the toughness is lowered. Further, if N is added in excess, the hot workability is deteriorated, and cracks and surface defects are generated at the time of forging and rolling. Therefore, the N content is 0.50 mass% or less, preferably 0.45 mass% or less, and more preferably 0.40 mass% or less.

(Cr: 20.0 to 28.0% by mass)

Cr is bonded to C and N, and X group element composed of at least one of V, Ti, Nb and Ta described above around the nucleus which is the inclusion (10) composed of at least one of oxide, sulfide and oxysulfide The composite inclusions 20 having the outer angle 15 in the inclusions 10 are formed by forming the outer shells 15 of carbide or nitride together. Therefore, Cr has an effect of improving pitting resistance. Cr is a main component of the passive film and is a basic element of the corrosion resistance of the stainless steel. Cr is an element stabilizing the ferrite phase. In order to obtain such effects, the Cr content is 20.0 mass% or more, preferably 21.0 mass% or more, and more preferably 21.5 mass% or more. However, when Cr is excessively contained, coarse carbides or nitrides are formed to deteriorate toughness. Therefore, the Cr content is 28.0 mass% or less, preferably 27.5 mass% or less, and more preferably 27.0 mass% or less.

In addition to the basic metal composition, the metal composition may be optionally selected as follows, and an example will be described below.

(Al: 0.001 to 0.050 mass%)

Al is a deoxidizing element, and is an element necessary for reducing the amount of O and S in the solvent. Among the inclusions of the oxides, Mg is an element necessary for the formation of Mg-Al oxide which is relatively excellent in corrosion resistance. In order to obtain such an effect, the Al content is 0.001 mass% or more, preferably 0.003 mass% or more, and more preferably 0.005 mass% or more. However, when Al is excessively added, the Al content is 0.050 mass% or less, preferably 0.040 mass% or less, and more preferably 0.030 mass% or less, since the inclusions 10 of coarse oxide are generated to adversely affect the pitting resistance. Or less.

(Mg: 0.0001 to 0.0200 mass%)

Mg is an element necessary for forming Mg-Al oxide, which is relatively excellent in corrosion resistance, together with Al among inclusions of oxides. Mg is an element which bonds with S or O in the steel to inhibit segregation of these inclusions in the grain boundaries to improve hot workability. In order to obtain such an effect, the Mg content is set to 0.0001 mass% or more, preferably 0.0003 mass% or more. However, if Mg is excessively contained, the Mg-based oxide having poor corrosion resistance is stabilized and the corrosion resistance is deteriorated. Therefore, the Mg content is 0.0200 mass% or less, preferably 0.0150 mass% or less, and more preferably 0.0100 mass% or less.

(Ca: 0.0001 to 0.0200% by mass)

Ca is an element that combines with S contained as an impurity in the steel to inhibit the formation of MnS which is likely to become a starting point of local corrosion, thereby forming CaS and improving local corrosion resistance. Further, Ca is an element which bonds with S or O in the steel to suppress segregation of these inclusions in grain boundaries and improve hot workability. In order to obtain such an effect, the Ca content is set to 0.0001 mass% or more, preferably 0.0003 mass% or more. However, if Ca is excessively contained, inclusion of oxide is increased, and corrosion resistance and workability are deteriorated. Therefore, the Ca content is 0.0200 mass% or less, preferably 0.0100 mass% or less.

(Co: 0.1 to 2.0 mass%, Cu: 0.1 to 2.0 mass%),

Co and Cu are elements that improve the corrosion resistance and stabilize the austenite phase. In order to obtain such an effect, when Co or Cu is contained, the Co content and the Cu content are each set to 0.1 mass% or more, preferably 0.2 mass% or more. However, if these elements are contained in excess, the Cu content and the Co content are respectively 2.0 mass% or less, preferably 1.5 mass% or less, in order to deteriorate hot workability. On the other hand, Co and Cu exhibit an effect of improving the corrosion resistance and stabilizing the austenite phase by containing one or both of them in the above-mentioned predetermined range.

(B: 0.0005 to 0.0100% by mass)

B is effective for improving the corrosion resistance and hot workability. In order to obtain such an effect, when B is contained, the B content is set to 0.0005 mass% or more, preferably 0.0010 mass% or more. Since B is an optional component, it may be 0 mass%. On the other hand, if B is excessively contained, cracking occurs during hot working, or BN is formed by binding with N in the steel, thereby lowering the concentration of N contributing to corrosion resistance and reducing the corrosion resistance have. Therefore, the B content is 0.0100 mass% or less, preferably 0.0050 mass% or less, and more preferably 0.0020 mass% or less.

(Fe and inevitable impurities)

The basic components of the constituents of the two-phase stainless steel material are as described above, and the remainder are Fe and inevitable impurities. On the other hand, as the inevitable impurities, for example, P and O can be mentioned.

(P: not more than 0.05% by mass)

P is an element that is inevitably incorporated as an impurity and harmful to corrosion resistance, and also deteriorates weldability and workability. Therefore, the P content is set to 0.05 mass% or less. On the other hand, the P content is preferably as small as possible, preferably 0.04 mass% or less, and more preferably 0.03 mass% or less. On the other hand, although P is not contained in the steel material, that is, it may be 0 mass%, excessive reduction of the P content leads to an increase in the production cost, so that the lower limit of the P content is practically 0.01 mass%.

(O: 0.030 mass% or less)

O is an impurity introduced into a solvent and is an element that precipitates as oxides in the steel by binding with a deoxidizing element such as Si or Al to lower the workability and corrosion resistance of the two-phase stainless steel material. If O is contained excessively, a large amount of oxides and oxysulfides are precipitated, so that corrosion resistance and hot workability are deteriorated. Therefore, the O content is 0.030 mass% or less, preferably 0.028 mass% or less, and more preferably 0.024 mass% or less. On the other hand, the lower the content of O is, the better, but the lowering of O in excess leads to an increase in cost, so the lower limit is generally about 0.0005 mass%.

Unavoidable impurities are impurities that are inevitably incorporated in the solvent, and are contained within a range that does not impair various properties of the steel. Further, the composition of the steel material may contain other elements in addition to the above-mentioned components in a range that does not adversely affect the effect of the steel material according to the present invention. On the other hand, since the two-phase stainless steel material according to the present invention does not need to add W compared with the conventional one, it is economically advantageous to suppress an increase in cost.

The foregoing has described an example of a basic metal composition suitable for a super duplex stainless steel material having a calculation result of PRE = [Cr] +3.3 [Mo] +16 [N] of 40 or more and a standard duplex stainless steel material of 30 or more and less than 40 will be.

An example of a basic metal composition suitable for a lean two-phase stainless steel having a PRE of less than 30 is as follows.

That is, the lean two-phase stainless steel material has a metal component composition of 0.100 mass% or less of C, 0.10 to 2.00 mass% of Si, 0.10 to 3.0 mass% of Mn, 0.0100 mass% or less of Si, 3.0 to 7.0 mass% of Ni 0.05 to 1.00% by mass of Mo, 0.05 to 0.20% by mass of N, 20.0 to 25.0% by mass of Cr and 0.030% by mass of O and the balance of Fe and inevitable impurities.

In the case of the lean two-phase stainless steel material, the super duplex stainless steel material and the super duplex stainless steel material are used in view of the fact that 3.0 to 7.0 mass% of Ni, 0.05 to 1.00 mass% of Mo, 0.05 to 0.20 mass% of N and 20.0 to 25.0 mass% Phase stainless steel, but the meaning of defining the effect or content of each component is the same as that of the standard two-phase stainless steel, so that the description thereof will be omitted. On the other hand, in the case of the lean two-phase stainless steel material, the Ni content is preferably 3.5 mass% or more, and more preferably 6.5 mass% or less. The Mo content is preferably 0.10% by mass or more, and more preferably 0.95% by mass or less. The N content is preferably 0.10% by mass or more, and more preferably 0.15% by mass or less. The Cr content is preferably 20.5 mass% or more, and more preferably 24.5 mass% or less.

In the two-phase stainless steel material having the above-described structure, that is, the super duplex stainless steel material, the standard two-phase stainless steel material, and the lean two-phase stainless steel material, as shown in FIGS. 1 to 4, It is possible to prevent the formula from progressing because the composite inclusion 20 in which the inclusion 15 is formed is 30% or more of the number of the inclusions 10. As shown in Fig. 4, a state in which a hole tries to advance in a part of the inclusion 10 can be understood as a state in which the hole 15 is prevented from forming due to the formation of the outer angle 15. Fig. That is, in the two-phase stainless steel material, as the number of the complex inclusions 20 having the outer angle 15 around the inclusions 10 is increased, the progress of the holes can be prevented and excellent corrosion resistance can be exhibited even in a severe corrosive environment have. On the other hand, it is most preferable that the outer periphery 15 completely covers the entire circumference of the inclusion 10 as a nucleus. However, it is sufficient that the outer periphery 15 is formed in a range of 30% or more, preferably 40% or more, , More preferably 80% or more, and most preferably 100%. The outer angle 15 may be a single layer or a plurality of layers.

&Lt; Process for producing two-phase stainless steel material >

Next, a method of manufacturing a two-phase stainless steel material according to the present embodiment will be described.

The two-phase stainless steel material according to the present embodiment can be produced by a manufacturing facility and a manufacturing method used for mass production of ordinary stainless steel. In order to reduce O as an impurity in steel, deoxidation is performed by adding a large amount of elements having a large affinity to O, such as Si or Al, and the secondary refining time such as vacuum degassing or argon gas stirring is prolonged, The oxide inclusions 10 are removed by pivoting or the like.

For example, molten steel dissolved in a converter or an electric furnace is refined by an AOD (Argon Oxygen Decarburization) method or a VOD (Vacuum Oxygen Decarburization) method to adjust the components, and then the molten steel is subjected to a casting method such as a continuous casting method, do. The obtained ingot can be subjected to hot working at a temperature range of about 900 to 1300 占 폚, followed by cold working to obtain a desired dimensional shape. In addition, the total machining ratio of the cross-sectional area of the original steel ingot / the cross-sectional area after machining is usually about 10 to 50.

In this embodiment, the total number of the inclusions 20 and the inclusions 10 is adjusted by controlling the temperature in the hot working and the total heat treatment time of the heat treatment just before the hot working and the hot working at the hot working . That is, in the two-phase stainless steel material, in order that the ratio of the number of the composite inclusions 20 having the outer angle 15 around the inclusions 10 is 30% or more of the total number of the inclusions 10, The heat treatment at a temperature range of about 1300 deg. C is performed for 3 hours or more, preferably 5 hours or more, and more preferably 10 hours or more. On the other hand, the longer the heat treatment time, the better. However, if the heat treatment time is excessively long, the productivity tends to deteriorate. Therefore, it is preferably 100 hours or less.

On the other hand, in the production of the conventional two-phase stainless steel material, the time for the heat treatment immediately before the hot working and the hot working is controlled to be about 1 hour or less because productivity is short. The ratio of the number of the inclusions 20 to the number of the inclusions 10 is set to 30% or more of the total number of the inclusions 10 by deliberately setting the heat treatment time within the above- Or more exist in a state of being present.

Fig. 5 is a graph showing the superiority of the heat treatment process of the method for producing a preferable two-phase stainless steel material in the present embodiment and the heat treatment process for the conventional method for producing a two-phase stainless steel material. As shown in Fig. 5, in the heat treatment process of the method for producing a two-phase stainless steel material, which is preferable in the present embodiment, hot forging is performed for 3 hours or more, The hot forging is performed for one hour.

In the duplex stainless steel material of the present embodiment, it is preferable to perform quenching by performing a heat treatment for solidification as necessary in order to eliminate precipitates harmful to the mechanical properties. The lower limit of the temperature for the heat treatment for solidification is preferably 1000 deg. C or higher, more preferably 1200 deg. C or lower, more preferably 1100 deg. C or lower, and most preferably 1080 deg. The holding time is preferably 1 to 30 minutes. The ratio of the number of the composite inclusions 20 having the outer angle 15 around the inclusions 10 is 30% or more of the total number of the inclusions 10 by setting the heat treatment temperature at 1000 占 폚 to 1200 占 폚. When the temperature is 1000 ° C or more and 1080 ° C or less, the above ratio becomes 70% or more.

The quenching is preferably performed at a cooling rate of 10 ° C / second or more. In addition, pickling can be carried out for surface adjustment such as scale removal, if necessary.

<Two Phase Stainless Steel Pipe>

An embodiment of a two-phase stainless steel pipe according to the present embodiment will be described. The two-phase stainless steel pipe is made of the above-mentioned two-phase stainless steel material. Therefore, as described above, the two-phase stainless steel pipe according to the present embodiment exhibits excellent corrosion resistance. Therefore, the two-phase stainless steel pipe according to the present embodiment can be used as a structural material for seawater environments such as an umbilical tube, a seawater desalination plant, and an LNG vaporizer as well as a structural material for a fluid pipe and various chemical plants. The two-phase stainless steel pipe according to the present embodiment can be manufactured by a manufacturing facility and a manufacturing method used for mass production of a normal stainless steel pipe. For example, a desired dimension can be obtained by an extruded pipe made of a round bar, a Mannesmann pipe, a welded pipe for welding seams after molding using a plate material, and the like. The two-phase stainless steel pipe can be set to a suitable size according to the fluid pipe, chemical plant, umbrella tube, and the like. Also, the two-phase stainless steel pipe can be set to a suitable dimension as a structural material of a seawater desalination plant, an LNG vaporizer, and the like.

On the other hand, in the case of manufacturing a welded pipe, or in the case of joining two or more two-phase stainless steel pipes by welding, a method commonly used for stainless steels, for example, TIG, MIG, SAW, Various methods such as arc welding, electron beam welding, laser welding, electric resistance welding, etc. may be used.

Although the present disclosure discloses various aspects of the techniques as described above, the main techniques among them are summarized below.

A two-phase stainless steel material according to one aspect of the present invention is a two-phase stainless steel material comprising a ferrite phase and an austenite phase, wherein the steel has a composition of V: 0.01 to 0.50 mass%, Ti: 0.0001 to 0.0500 mass% , Nb: 0.0005 to 0.0500 mass%, and Ta: 0.01 to 0.50 mass%, wherein the steel has a composite inclusion, a composite inclusion, and inclusions, and the inclusions are oxides, sulfides And at least one of oxides and oxysulfides. The composite inclusion has the inclusions as its nuclei, and surrounds the nucleus with an outer angle of a carbide or nitride containing Cr and at least one X group element , And the ratio of the number of the composite inclusions to the total number of the inclusions is 30% or more.

According to the above-described constitution, the two-phase stainless steel material contains at least one selected from the group consisting of V, Ti, Nb and Ta and a predetermined amount of Cr in the outer periphery formed around the inclusions and Cr, . V, Ti, Nb and Ta are combined with C and N to form carbides or nitrides at the interface between the inclusions of oxides, sulfides or oxysulfides and the steel. In addition, Ta has an effect of forming a coating layer of carbonitride having better corrosion resistance.

Further, in the above two-phase stainless steel material, it may be constituted such that it contains 0.01 to 0.50% by mass of Ta as the metal component composition and Ta in an amount satisfying the following expression (1) in the outer angle.

([Ta] / [M] total) x 100? 5 (1)

[Ta] represents the number of Ta atoms included in the outer angle, and [M] total represents the sum of the number of atoms of the metal element contained in the outer angle.

According to the above-described construction, the two-phase stainless steel material is improved in the corrosion resistance of the outer periphery of the carbide or nitride, so that the effect of suppressing the formal progress is enhanced and the corrosion resistance of the steel material is improved.

The two-phase stainless steel material according to claim 1, wherein the steel has a composition of metal of 0.01 to 0.50 mass% of Ta, 0.0001 to 0.0500 mass% of Ti, and a mass% of Ta / Ti satisfying the following formula (2) % By weight.

Ta mass% / Ti mass%? 25 (2)

According to the above-described configuration, in the two-phase stainless steel material, since the inclusions are easily covered with carbide or nitride containing Ta, the effect of suppressing the formal progress is enhanced, and the corrosion resistance of the steel material is improved.

Further, in the above two-phase stainless steel material, it is preferable that the steel contains 0.0001 to 0.0200 mass% of Mg and 0.001 to 0.050 mass% of Al as the metal component composition, and the core of the composite inclusion contains an oxide of Mg and Al May be included.

According to the above-described configuration, in the two-phase stainless steel material, the cores and inclusions of the composite inclusion contain Mg and Al which influence the corrosion resistance, so that the corrosion resistance is further improved.

0.1 to 3.0% by mass of Si, 0.1 to 3.0% by mass of S, 0.0100% by mass or less of Si, 0.10 to 3.0% by mass of Si, And the balance of Fe and inevitable impurities is preferably 1.0 to 10.0 mass%, Mo: 0.05 to 6.00 mass%, N: 0.10 to 0.50 mass%, Cr: 20.0 to 28.0 mass%, and O: 0.030 mass% or less.

0.1 to 3.0% by mass of Si, 0.1 to 3.0% by mass of Mn, 0.0100% by mass or less of Si, and 3.0 to 3.0% by mass of Ni, 0.05 to 0.20 mass% of N, 0.05 to 0.20 mass% of N, 20 to 25 mass% of Cr and 0.030 mass% of O to 0.0 mass% of Mo, and Fe and inevitable impurities.

By these constitutions, the two-phase stainless steel material contains a predetermined amount of a metal component such as C, Si, Mn, S, Ni, Mo, N, Cr and O to form a two- You can get an organization. When C is set to a predetermined value or less, unnecessary carbide is not formed and the effect of suppressing deterioration of corrosion resistance is obtained. Si and Mn are effective for deoxidation. S is set to be not more than a predetermined value, thereby reducing deterioration of corrosion resistance and hot workability. In addition, S is a cause of formation of MnS which damages corrosion resistance and toughness. Cr, Mo, and N are effective in improving the pitting resistance. Ni is effective in improving the corrosion resistance and stabilizing the austenite phase. Further, since O is set to a predetermined value or less, the corrosion resistance and hot workability are hardly deteriorated.

The two-phase stainless steel material according to any one of claims 1 to 3, wherein the steel composition further contains 0.0001 to 0.0200 mass% of Ca, and the steel composition further contains 0.1 to 2.0 mass% of Co, : 0.1 to 2.0 mass%, or the steel composition may further contain B: 0.0005 to 0.0100 mass%.

According to the above-described construction, the two-phase stainless steel material further contains any of Ca, Co, Cu and B in a predetermined amount, thereby further improving the corrosion resistance. Ca binds with S or O in the steel to inhibit segregation in grain boundaries, thereby improving corrosion resistance and hot workability. Co and Cu are effective in improving the corrosion resistance and stabilizing the austenite phase. In the two-phase stainless steel material, B is effective for improving hot workability.

The two-phase stainless steel pipe according to another aspect of the present invention comprises the above-mentioned two-phase stainless steel material.

According to the above-described constitution, in the two-phase stainless steel pipe, the steel pipe is made of the two-phase stainless steel material, the inclusions serving as a starting point of local corrosion are modified to improve the corrosion resistance and the stress corrosion cracking resistance .

Example

Hereinafter, the two-phase stainless steel material according to the present invention will be described in more detail with reference to Examples. The two-phase stainless steel material according to the present invention is not limited to the following examples. In the following examples, the metal composition of the two-phase stainless steel material and the evaluation results are shown in Tables 1 to 6.

(Production of super two-phase stainless steel and two-phase stainless steel)

A steel having the composition shown in Tables 2 and 4 was cast by a small melting furnace having a capacity of 53 kg and cast using each mold having a size of about 120 mm × about 350 mm. On the other hand, in the component composition column of Table 2 and Table 4, the blank (-) indicates that the corresponding component is not contained, and the balance is Fe and inevitable impurities. Quot; - &quot; for &quot; Ta / Ti &quot; in Table 2 and Table 4 indicate that it was not appropriate to calculate in this embodiment because at least one of Ta and Ti is not contained. Table 1 and Table 3 show the calculation results of PRE = [Cr] +3.3 [Mo] +16 [N] for each steel. The blank (-) in Tables 1 and 3 indicates that the element to be analyzed was not detected as a result of EDX analysis of the outer angle of the composite inclusion.

As the steel, here, the solidified steel ingot was heated to 900 to 1300 占 폚, subjected to hot forging at the same temperature, and then cut. Heat treatment at a temperature range of about 900 to 1300 占 폚 was carried out for three hours or more immediately before hot forging and hot forging. Next, a solid solution heat treatment at 1100 占 폚 for 3 minutes was carried out, followed by cooling at a cooling rate of 12 占 폚 / sec followed by cutting into a steel material of 300 占 120 占 50 mm. The steel material produced as described above is referred to as steel material No. 1. A1 to 6, A9 to 20, B2 and B5 to 8, respectively.

Further, the cast steel is solidified, and the solidified steel ingot is heated to 900 to 1300 占 폚 and hot forging is conducted at the same temperature, and at the temperature just before the hot forging and the hot forging, Was performed for 3 hours or more. Next, solidification treatment at 1050 占 폚 for 3 minutes was carried out, followed by water cooling at a cooling rate of 12 占 폚 / sec, followed by cutting and finishing with a steel material of 300 占 120 占 50 mm. The steel material produced as described above is referred to as steel material No. 1. A7, A8, A21, and A22.

Further, the cast steel is solidified, and the solidified steel ingot is heated to 900 to 1300 占 폚 and hot forging is conducted at the same temperature, and at the temperature just before the hot forging and the hot forging, Was performed for one hour. Next, a solid solution heat treatment at 1100 占 폚 for 3 minutes was carried out, followed by cooling at a cooling rate of 12 占 폚 / sec followed by cutting into a steel material of 300 占 120 占 50 mm. As described above, B1, B3 and B4.

(Production of lean two-phase stainless steel material)

The steel having the composition shown in Table 6 was melted by means of a small melting furnace (capacity 20 kg / 1 ch) and cast using a circumferential mold (main body: φ110 × about 200 mm) (Steel No. A23 to A27, B11). On the other hand, in the component composition column of Table 6, the blank (-) indicates that the corresponding component is not contained, and the balance is Fe and inevitable impurities. Table 5 shows the calculation results of PRE = [Cr] +3.3 [Mo] +16 [N] for each steel. The blank (-) in Table 5 indicates that the element to be analyzed was not detected as a result of EDX analysis of the outer angle of the composite inclusion.

On the other hand, the volume of the melting furnace and the shape and dimensions of the casting are different from that of the lean two-phase stainless steel material, the super duplex stainless steel material and the standard two-phase stainless steel material.

Here, as the steel material, the solidified steel ingot was heated to 1300 占 폚, subjected to hot forging (forging temperature: 1000 to 1300 占 폚) at the same temperature, and then cut. At this time, heat treatment at a temperature range of about 1000 to 1300 占 폚 was performed for 3 hours or more. Subsequently, the steel sheet was subjected to a solidification treatment at a temperature of 1100 占 폚 for 30 minutes, cut at a cold speed of 12 占 폚 / sec and then cut into a steel sheet having a size of 20 占 50 占 150 mm and subjected to cold rolling to remove deformation and precipitates (Steel materials No. A23 to A27, B10 and B11) at 1100 占 폚 for 5 minutes and water-cooled at a cooling rate of 12 占 폚 / sec.

Further, the solidified steel ingot was heated to 1300 占 폚, subjected to hot forging (forging temperature: 1000 to 1300 占 폚) at the same temperature, and then cut. A long time heat treatment is not carried out but a long time heat treatment is carried out at 1100 占 폚 for 30 minutes to cool the steel sheet at a cold speed of 12 占 폚 per second and then cut into a steel sheet having a size of 20 占 50 占 150 mm, ° C for 5 minutes, and then subjected to a heat treatment for finishing by water cooling at a cooling rate of 12 ° C / second to obtain an evaluation sample (Steel No. B9).

(Sample collection and tissue observation)

Subsequently, inclusions and pitting resistance were evaluated in the following procedure using samples of 20 mm x 30 mm x 2 mm t taken from the steel material in parallel to the working direction. The results are shown in Tables 1, 3 and 5.

Further, the sample was buried in a resin, processed such that a cross section perpendicular to the processing direction of the sample was exposed, and the mirror surface was polished. Then, A 1 to A 22 and B 1 to B 8 were subjected to electrolytic etching in an oxalic acid aqueous solution, followed by optical microscopic observation at a magnification of 100 times, and the texture of each sample was observed. Steel No. A23 to A27 and B9 to B11 were subjected to electrolytic etching in a KOH aqueous solution, and then observed under an optical microscope at a magnification of 100 times to observe the texture of each sample. As a result, all of the samples were composed of two phases of a ferrite phase and an austenite phase.

(Evaluation of inclusions and complex inclusions)

(Evaluation of inclusion of core)

The sample was buried in a resin, processed such that a cross section perpendicular to the processing direction of the sample was exposed, and the mirror surface was polished. The surface subjected to the mirror polishing was observed using an electron probe micro-probe Micro-analyzer (EPMA, trade name &quot; JXA-8500F &quot;) manufactured by Nippon Electronics Datum Co., The composition was quantitatively analyzed.

At this time, For A1 to A22 and B1 to B8, the observation area was set to 100 mm ² , and the composition of the components at the center of the inclusion of the oxide was quantitatively analyzed by the wavelength dispersive spectroscopy of the characteristic X-rays.

Further, A23~A27, the observation area for B9~B11 to 600mm ^2, followed by quantitative analysis by the component composition of characteristic X-ray wavelength dispersion spectroscopic at the central portion of the oxide inclusions.

The relationship between the X-ray intensity of each element and the element concentration was determined in advance as a calibration curve using a known substance as Ca, Al, Si, Ti, Mg, Mn, Na, K, Cr, S and O as the elements to be analyzed. Then, the amount of the elements contained in each sample was determined from the X-ray intensity obtained from the inclusion of the oxide to be analyzed and the calibration curve, and the results were arithmetically averaged to obtain an average composition of the inclusions.

Of the quantitative results thus obtained, the inclusions having an O content of 5 mass% or more were judged to be "oxides" and the inclusions having an S content of 0.5 mass% or more were judged to be "sulfides". Furthermore, inclusions having an S content of 0.5% by mass or more and an inclusion having a Ca content of 1.0% by mass or more were judged to be &quot; Ca-based sulfide &quot;.

At this time, when a plurality of elements were observed from one inclusion, the composition of the oxides was calculated in terms of a single oxide of each element from the ratio of X-ray intensity indicating the presence of the elements. In the present embodiment, the average oxide composition was obtained by averaging the mass of the oxide as a single oxide.

With respect to the average composition of the oxide obtained as described above, MgO content of 20 mass% or more inclusions, the "Mg-containing oxide" in, and the inclusions each content of MgO and Al ₂ O ₃ is not less than 20% by mass of "Mg, Al-containing oxide Quot ;, and the inclusions each containing Al ₂ O ₃ and CaO in an amount of 20 mass% or more were defined as &quot; Ca, Al-based oxide &quot;. On the other hand, "Ca-based sulphide" is also referred to as "Ca, Al-based sulphide" in the case of "Ca, Al-based oxide".

In addition, the average composition of the oxide obtained as described above, O content is 5 mass% or more, S content is more than 0.5 mass%, Ca content is more than 1.0% by mass and the respective contents of MgO and Al ₂ O ₃ 20% by weight Mg, Ca, Al-based oxysulfides &quot;.

The inclusions having an average composition of the oxides as described above having an O content of 5 mass% or more, an S content of 0.5 mass% or more, a Ca content of 1.0 mass% or more and a MgO content of 20 mass% Ca-based oxysulfide &quot;.

(Evaluation of composite inclusions)

Steel No. For A1 to A22 and B1 to B8, five oxides, sulfides or oxysulfides (that is, inclusions) existing in an observation area of 100 mm ² were selected in order from the largest one.

Further, A23 to A27 and B9 to B11 were selected from among the oxide, sulfide or oxysulfide (that is, inclusion) existing in 600 mm ^{2 in} order from the largest size, and analyzed.

Although not all inclusions are analyzed, it is considered that inclusions having a smaller size than the above-mentioned inclusions are generated by the same mechanism as inclusions having a selected size. Further, it is confirmed that these inclusions have similar compositions and shapes .

In other words, it is considered that inclusions having a predetermined composition and shape distribution are generated in oxides, sulfides or oxysulfides of all sizes. On the other hand, the sizes of the oxides, sulfides or oxysulfides were compared in terms of the area of the oxides, sulfides or oxysulfides on the observation surface.

Thereafter, oxide, sulfide or oxysulfide of the object was thinned to a thickness at which TEM observation was possible by the FIB method (Focused Ion Beam, focused ion beam processing method). The apparatus used was a focused ion beam processing observation apparatus FB2000A manufactured by Hitachi, Ltd., and an acceleration voltage of 30 kV and Ga was used as an ion source.

Thereafter, the sliced inclusions were observed by TEM. The device was subjected to EDX analysis at the interface between oxide, sulfide or oxysulfide and steel using the field emission TEM (JEM-2010F) made by Nihon Electronics Co., Ltd. and Vantage of Noran EDX (Energy Dispersive X-ray spectrometry) did.

The element to be analyzed in the EDX analysis is Ca, Al, Ti, V, Mg, Mn, Na, K, Cr, Nb, Ta, S, O and at least one of Cr, V, Ti, Nb, And the total concentration of Cr, V, Ti, Nb, and Ta was 30 mass% or more.

Then, the selected phase was subjected to identification analysis by electron beam diffraction, and a sample showing a cubic crystal structure was determined to be carbide or nitride. At this time, when carbides or nitrides are generated at an interface of an object oxide, sulfide or oxysulfide with steel, that is, in a range of 60% or more of the peripheral circumference of the circumference, inclusions of oxides, sulfides or oxysulfides are used as nuclei, It was judged that it was a composite inclusion having an outer periphery of carbide or nitride.

Then, the ratio of the number of oxides, sulfides or oxysulfides in the carbides or nitrides in the five oxides, sulfides or oxysulfides measured was measured.

The results are shown in Tables 1, 3 and 5 as inclusion coverage (%).

(Evaluation of corrosion resistance)

The evaluation of the corrosion resistance was carried out with reference to the method described in JIS G 0577: 2014 &quot; Method for measuring the formal potential of stainless steel &quot;. After the surface of the sample was wet-polished with a SiC # 600 abrasive paper and subjected to ultrasonic cleaning, the lead wire was placed on the sample by spot welding, and the portion other than the test surface (10 mm x 10 mm) of the surface of the sample was covered with an epoxy resin or a silicone resin .

For the super duplex stainless steel material having a PRE value of 40 or more, the sample was immersed in a 20% NaCl aqueous solution maintained at 80 DEG C for 10 minutes, and then subjected to anode polarization at a sweep speed of 20 mV / min to obtain a current density of 0.1 mA / cm The electric potential at the time point exceeding ² was regarded as the formal potential (VC '100) (Steel Nos. A9 to A22, B3 to B8).

For a standard two-phase stainless steel material having a PRE value of 30 or more and less than 40 and a lean two-phase stainless steel material having a PRE value of less than 30, the sample was immersed in an artificial seawater solution maintained at 80 DEG C for 10 minutes, (Steel materials No. A1 to A8, A23 to A27, B1, B2, and B9 to B11) at the time when the current density exceeded 0.1 mA / cm &lt; ² & .

The evaluation of the pitting corrosion resistance was expressed as AAAA for a super duplex stainless steel material and a standard two-phase stainless steel material having an official dislocation of 900 mV (vs. SCE (saturated carbomel electrode)) or more. In addition, AAA indicates that the formal potential is greater than 750 mV (vs. SCE) and less than 900 mV (vs. SCE). Also, those with a voltage of 600 mV (vs. SCE) and less than 750 mV (vs. SCE) are indicated by AA. A, where the formal potential is greater than 550 mV (vs. SCE) and less than 600 mV (vs. SCE). In addition, those having an official potential of less than 550 mV (vs. SCE) are indicated by B.

For a lean two-phase stainless steel material, those with an official dislocation of 250 mV (vs SCE) or more were designated as A In addition, those having an average potential of less than 250 mV (VSCE) were designated as B.

The results are shown in Tables 1, 3 and 5 together with the formula potential.

On the other hand, in each evaluation, since the evaluation standard is different between the super duplex stainless steel material having the PRE value of 40 or more, the standard two-phase stainless steel material having the PRE value of 30 or more and less than 40, and the lean- Respectively. Therefore, Tables 1 and 2 are made of a standard two-phase stainless steel having a PRE of not less than 30 and less than 40, and Tables 3 and 4 are super duplex stainless steels having a PRE of not less than 40, Phase two-phase stainless steel material.

6 is a graph in which the abscissa represents the ratio (%) of the number of inclusions to the sum of the inclusions covered by the inclusion, that is, the number of inclusions, and the ordinate represents the ordinate (mV) . 7 shows a graph in which the Ta content in the metal component in the external angle, that is, the Ta content (atomic%) in the coating inclusion is plotted on the abscissa and the formula potential (mV) is plotted on the ordinate. 8 shows a graph in which the Ta mass% / Ti mass% is plotted on the abscissa and the formula potential (mV) is plotted on the ordinate. On the other hand, "mV" in FIGS. 6 to 8 is "mV (vs. SCE)". In Fig. 6 to Fig. 8, &quot; super &quot; represents a super duplex stainless steel material, and &quot; Std. &Quot; and &quot; standard &quot; represents a standard two-phase stainless steel material.

As shown in FIG. 6, even if the value of PRE is 40 or more, the value of the formal potential does not exceed 550 mV when the inclusion coverage rate of the two-phase stainless steel material is less than 30%.

As shown in Fig. 7, when the Ta group element is contained in the coating inclusions in the above-mentioned formula (1) in an amount of 5 atomic% or more, the super duplex stainless steel material having a PRE value of 40 or more has a , And the standard two-phase stainless steel material having a PRE value of 30 or more and less than 40 is found to be more than 700 mV.

As shown in Fig. 8, when the content of Ta and Ti is 25 or more in the above-mentioned formula (2), the super duplex stainless steel material having a PRE value of 40 or more is more than 900 mV and the value of PRE is not less than 30 And the standard two-phase stainless steel material of less than 40 is more than 700 mV.

From the results of Tables 1 and 2, Tables 3 and 4, and Tables 5 and 6, it can be seen that the embodiment satisfying the requirements of the present invention, that is, A1 to A8, A9 to A22, and A23 to A27 all have excellent airtightness.

On the other hand, in the comparative example which does not satisfy the requirements of the present invention, that is, B1, B2, B3 to B8, and B9 to B11 have the following problems.

In the comparative examples (steel materials No. B1, B3, B4 and B9), heat treatment at a temperature range of about 1000 to 1300 占 폚 was not performed for 3 hours or more. Therefore, the outer angle of the carbide or nitride of at least one X group element is set around the nucleus which is an inclusion containing Cr and at least one kind of X group element and composed of at least one of oxide, sulfide and oxysulfide (The &quot; inclusion coverage ratio &quot; in Tables 1, 3 and 5) was less than 30%, which did not satisfy the criteria for corrosion resistance. Further, Since the value of Nb exceeds the predetermined range in B1, the value of the formula potential is also the same as that of the steel No. 1. B3 and B4.

In the comparative examples (steel materials No. B5, B6 and B11), the content of the X group elements of Ta, V, Nb and Ti is less than the lower limit of the present invention. Further, Comparative Examples (Steel Nos. B5, B6, and B11) were prepared by adding at least Cr, at least one kind of X group element, and at least one element selected from the group consisting of oxides, sulfides, The ratio (inclusion coverage ratio) of the number of composite inclusions having an outer shell of carbide or nitride of one kind of X group element was less than 30%, so the criteria of corrosion resistance were not satisfied.

The comparative example (steel No. B2) is a steel having a low content of N, at least one element selected from the group consisting of Cr, at least one kind of X group element, and at least one element selected from the group consisting of oxides, sulfides, The ratio (inclusion coverage ratio) of the number of composite inclusions having an outer shell of carbide or nitride of one kind of X group element was less than 30%, so that the criteria of corrosion resistance were not satisfied.

A comparative example (steel No. B7) was prepared by adding at least a mixture of at least one element selected from the group consisting of Cr, at least one X group element, and at least one of oxides, sulfides and oxysulfides, The ratio (inclusion coverage ratio) of the number of composite inclusions having an outer shell of carbide or nitride of one kind of X group element was less than 30%, so that the criteria of corrosion resistance were not satisfied.

The comparative example (steel No. B8) is a steel having a high content of O, at least one element selected from the group consisting of Cr, at least one kind of X group element, The ratio (inclusion coverage ratio) of the number of composite inclusions having an outer shell of carbide or nitride of one kind of X group element was less than 30%, so that the criteria of corrosion resistance were not satisfied.

The comparative example (steel No. B10) is a steel having a low content of N, at least one element selected from the group consisting of Cr, at least one kind of X group element, at least one kind of inclusions of an oxide, a sulfide and an oxysulfide, The ratio (inclusion coverage ratio) of the number of composite inclusions having an outer shell of carbide or nitride of one kind of X group element was less than 30%, so the criteria of corrosion resistance were not satisfied.

This application is based on Japanese Patent Application No. 2015-225214 filed on November 17, 2015 and Japanese Patent Application No. 2016-170298 filed on August 31, 2016, the content of which is incorporated herein by reference .

In order to express the present invention, although the present invention has been appropriately and fully described through the embodiments with reference to the drawings and the like in the foregoing description, those skilled in the art should recognize that it is easy to change and / or improve the above- do. It is therefore to be understood that the type of modification or mode of modification encompassed by the scope of the claims is not intended to be limited by the scope of the claims as amended by the person skilled in the art.

INDUSTRIAL APPLICABILITY The present invention has wide industrial applicability in the technical field of two-phase stainless steel material and two-phase stainless steel pipe.

Claims (10)

A two-phase stainless steel material comprising a ferrite phase and an austenite phase,
At least one X group element selected from the group consisting of V: 0.01 to 0.50 mass%, Ti: 0.0001 to 0.0500 mass%, Nb: 0.0005 to 0.0500 mass%, and Ta: 0.01 to 0.50 mass% &Lt; / RTI &
The composite steel sheet according to claim 1,
Wherein the inclusions comprise at least one of oxide, sulfide and oxysulfide,
Wherein the inclusion contains the inclusions as nuclei and surrounds the nucleus with an outer angle of a carbide or nitride containing Cr and at least one of the X group elements,
Wherein the ratio of the number of the inclusions to the total number of the inclusions is 30% or more of the total number of the inclusions. The method according to claim 1,
The two-phase stainless steel material according to claim 1, wherein the steel contains 0.01 to 0.5% by mass of Ta, and Ta in an amount satisfying the following formula (1).
([Ta] / [M] total) x 100? 5 (1)
[Ta] represents the number of Ta atoms included in the outer angle, and [M] total represents the sum of the number of atoms of the metal element contained in the outer angle. 3. The method according to claim 1 or 2,
Wherein the steel material has a metal composition of Ta: 0.01 to 0.50 mass%, Ti: 0.0001 to 0.0500 mass%, and a Ta mass% / Ti mass% ratio satisfying the following formula (2) Steel.
Ta mass% / Ti mass%? 25 (2) The method according to claim 1,
Characterized in that the metal composition of the steel contains 0.0001 to 0.0200 mass% of Mg and 0.001 to 0.050 mass% of Al, and the core of the composite inclusion includes an oxide of Mg and Al . The method according to claim 1,
Wherein the steel material has a metal component composition,
C: 0.100 mass% or less,
Si: 0.10 to 2.00 mass%
Mn: 0.10 to 3.00 mass%
S: 0.0100 mass% or less,
Ni: 1.0 to 10.0 mass%
Mo: 0.05 to 6.00 mass%
N: 0.10 to 0.50% by mass,
Cr: 20.0 to 28.0% by mass,
O: 0.030 mass% or less,
And the remainder being Fe and inevitable impurities. The method according to claim 1,
Wherein the steel material has a metal component composition,
C: 0.100 mass% or less,
Si: 0.10 to 2.00 mass%
Mn: 0.10 to 3.00 mass%
S: 0.0100 mass% or less,
Ni: 3.0 to 7.0 mass%
Mo: 0.05 to 1.00 mass%
N: 0.05 to 0.20% by mass,
Cr: 20.0 to 25.0% by mass,
O: 0.030 mass% or less,
And the remainder being Fe and inevitable impurities. The method according to claim 5 or 6,
Wherein the steel material further contains 0.0001 to 0.0200 mass% of Ca in terms of the metal component. The method according to claim 5 or 6,
The two-phase stainless steel material according to claim 1, wherein the steel composition further contains at least one selected from the group consisting of Co: 0.1 to 2.0 mass% and Cu: 0.1 to 2.0 mass%. The method according to claim 5 or 6,
And the metal composition of the steel material further contains B: 0.0005 to 0.0100 mass%. A two-phase stainless steel pipe comprising the two-phase stainless steel material according to claim 1.

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