JP2015078429A - Duplex stainless steel material and duplex stainless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a duplex stainless steel material and a duplex stainless steel tube exhibiting good corrosion resistance and hot workability in an environment containing corrosive substances such as chloride, hydrogen sulfide and carbon dioxide gas.SOLUTION: The duplex stainless steel material containing a ferrite phase and an austenite phase and has a constitution composition containing, C:0.10 mass% or less, Si:0.1 to 2.0 mass%, Mn:0.1 to 2.0 mass%, P:0.05 mass% or less, S:0.01 mass% or less, Al:0.001 to 0.050 mass%, Ni:1.0 to 10.0 mass%, Cr:22.0 to 28.0 mass%, Mo:2.0 to 6.0 mass%, N:0.2 to 0.5 mass%, Ta:0.01 to 0.50 mass% and O:0.030 mass% or less and the balance Fe with inevitable impurities and a ratio of Ta/O of 2.5 or more by mass%.

Description

The present invention relates to a duplex stainless steel material used in an environment containing a corrosive substance such as chloride, hydrogen sulfide (H ₂ S), carbon dioxide (CO ₂ ) (hereinafter sometimes referred to as a corrosive environment), and It relates to a duplex stainless steel pipe.

  Stainless steel is a material that naturally forms a stable surface film mainly composed of a Cr oxide called a passive film in a corrosive environment and exhibits corrosion resistance. In particular, a duplex stainless steel material composed of a ferrite phase and an austenite phase is superior in strength characteristics to austenitic stainless steel and ferritic stainless steel, and has good pitting corrosion resistance and stress corrosion cracking resistance. Due to these characteristics, duplex stainless steel materials are used for structural materials in highly corrosive environments such as oil well pipes and various chemical plants, including structural materials for seawater environments such as umbilicals, seawater desalination plants, and LNG vaporizers. It is used as

  However, if the usage environment contains a large amount of corrosive substances such as chloride (chloride ions), the duplex stainless steel material starts from inclusions in the duplex stainless steel material or defects in the passive film. In some cases, local corrosion, so-called pitting corrosion, may occur. In addition, in the gap portion of duplex stainless steel material, corrosive substances such as chloride ions are concentrated inside the gap, resulting in a more severe corrosive environment, and an oxygen concentration cell is formed between the outside and inside of the gap, Local corrosion inside the crevice is further promoted, and so-called crevice corrosion may occur. Furthermore, local corrosion such as pitting corrosion and crevice corrosion often becomes the starting point of stress corrosion cracking (SCC), and further improvement in corrosion resistance, particularly local corrosion resistance, is required from the viewpoint of safety.

  In particular, in oil well pipe materials used for oil and natural gas drilling, development of deeper oil wells and gas wells has been progressing in recent years, and at higher temperatures than conventional ones, hydrogen sulfide, carbon dioxide, chlorides. Since there are many cases where it is exposed to an environment containing a large amount of corrosive substances such as the above, further superior corrosion resistance is required.

  The pitting corrosion resistance of stainless steel is [Cr] +3 when the Cr amount (% by mass) is [Cr], the Mo amount (% by mass) is [Mo], and the N amount (% by mass) is [N]. It is represented by a pitting corrosion index PRE (Pitting Resistance Equivalent) calculated by 3 [Mo] +16 [N], and when W is included, the amount of W (mass%) is [W], and [Cr] +3.3 ( It is represented by a pitting corrosion index PREW calculated by [Mo] +0.5 [W]) + 16 [N], and it is known that if the content of Cr, Mo, N is increased, excellent pitting corrosion resistance can be obtained. ing. The contents of Cr, Mo, N, and W are adjusted so that PRE (or PREW) is 35 or more in normal duplex stainless steel, and 40 or more in super duplex stainless steel. Further, it is known that an increase in the content of Cr, Mo, and N contributes to an improvement in crevice corrosion resistance.

  For example, Patent Document 1 discloses a duplex stainless steel excellent in corrosion resistance having a PREW of 40 or more by controlling the contents of Cr, Mo, N, and W. Patent Document 2 discloses a duplex stainless steel having excellent corrosion resistance and hot workability by controlling the contents of B and Ta in addition to controlling the contents of Cr, Mo, W, and N. Yes. In Patent Document 3, in addition to controlling the contents of Cr, Mo, W, and N, the contents of Ti, V, Nb, Ta, Zr, B, and the like are controlled to provide excellent corrosion resistance and hot workability. Phase stainless steel is disclosed.

JP-A-5-132741 JP-A-8-170153 JP 61-157626 A

  The duplex stainless steel material is excellent in strength characteristics, but on the other hand, processing such as rolling and drawing is often more difficult than ordinary stainless steel material. Further, in order to apply the duplex stainless steel material in a severe corrosive environment containing hydrogen sulfide, carbon dioxide gas, and chloride ions, further improvement in corrosion resistance is necessary. However, the improvement of corrosion resistance may be insufficient only by adjusting the contents of Cr, Mo, N, and W. Furthermore, since sigma phase precipitation is promoted by an increase in Cr and Mo added for the purpose of improving corrosion resistance, there is a concern that toughness and hot workability are deteriorated.

  And in patent document 1, although the corrosion resistance (pitting corrosion resistance) of steel materials is evaluated by the pitting corrosion potential in 80 degreeC and 20% -NaCl, when PREW = 42, it is about 300 mV and is calculated | required these days. In a severe corrosive environment, it cannot always be said that sufficient corrosion resistance can be secured.

  Moreover, in patent document 2, although B is added in steel, there exists a possibility that B may combine with N in steel and produces | generates BN, and may reduce N density | concentration which contributes to corrosion resistance. Moreover, in patent document 2, W addition amount is as high as 5-10 mass%, and raises cost, and is economically disadvantageous.

  In Patent Document 3, Nb, V, Ti, and Zr are added to the steel. These elements combine with N in the steel to generate nitrides, thereby contributing to the N concentration contributing to corrosion resistance. May be reduced. Moreover, when the produced | generated nitride is coarse, toughness will be reduced.

  The present invention has been made in view of such a situation, and its problem is to develop good corrosion resistance and hot workability in an environment containing corrosive substances such as chloride, hydrogen sulfide, and carbon dioxide gas. An object of the present invention is to provide a duplex stainless steel material and a duplex stainless steel pipe.

  As described above, a stainless steel material is a material that exhibits corrosion resistance by a passive film mainly composed of Cr oxide. Since the duplex stainless steel material is composed of a ferrite phase and an austenite phase, it has a discontinuity at the interface between these different phases. Therefore, the continuity of the passive film is reduced at the interface between the ferrite phase and the austenite phase, or at the interface between the inclusion (oxide, sulfide) inevitably formed in the steel and the base metal. , Passive film tends to be unstable. As a result, the passive film is easily destroyed by chloride ions, and local corrosion is likely to occur.

  In order to solve the above-mentioned problems, the inventors focused on enhancing the stability and protective property of the passive film of the duplex stainless steel material within the range not impairing the production surface and various characteristics, and improving the corrosion resistance. The technology was examined.

  As described above, since the stainless steel material is a material that exhibits corrosion resistance by a passive film mainly composed of Cr oxide, the present inventors have studied from the viewpoint of improving the effective Cr concentration in the steel. If unnecessary Cr-based inclusions are formed in the steel, the effective Cr concentration in the steel decreases. Therefore, in order to improve the corrosion resistance of the stainless steel material, it has been found that a method for suppressing the precipitation of unnecessary Cr inclusions is effective.

In general, carbides and oxides are examples of Cr-based inclusions in stainless steel materials. Therefore, it is important to fix C and O in the steel that causes the formation of these inclusions with other elements. Here, with regard to carbide inclusions, especially Cr-based carbides (Cr ₂₃ C ₆ ) and σ phases (Fe—Cr intermetallic compounds) that cause a Cr-deficient layer that greatly reduces corrosion resistance in steel are formed. Therefore, methods for reducing the amount of C in steel and fixing C with carbides other than Cr (for example, TiC, NbC, VC, etc.) are known.

Therefore, the present inventors paid attention to oxide inclusions in order to further improve the corrosion resistance. In general, O is fixed by Si or Al added for deoxidation, or Ca or Mg. When oxides in stainless steel are analyzed, Al oxide (Al ₂ O ₃ ), Si oxide (SiO ₂ ), and Cr oxide (Cr ₂ O ₃ ) caused by Cr having a large content in the steel, etc. A complex oxide consisting of the oxides of the above is observed. Here, when Ta which easily forms an oxide rather than Cr is appropriately added to actively precipitate Ta-based oxide (Ta ₂ O ₅ ) in steel, unnecessary Cr-based oxide (Cr It has been found that the precipitation of ₂ O ₃ ) can be suppressed and the effective concentration of Cr in the steel can be increased. As a result, the stability of the passive film can be increased and the corrosion resistance can be improved. Furthermore, it has also been found that hot workability can be improved by suppressing the precipitation of oxide inclusions such as carbide inclusions and Cr oxides, and suppressing the formation of σ phase. .
The present invention has been completed by obtaining new findings from the above examination.

  The duplex stainless steel material according to the present invention is a duplex stainless steel material composed of a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is C: 0.10% by mass or less, Si: 0.1 -2.0 mass%, Mn: 0.1-2.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.001-0.050 mass%, Ni : 1.0-10.0 mass%, Cr: 22.0-28.0 mass%, Mo: 2.0-6.0 mass%, N: 0.2-0.5 mass%, Ta: 0 0.01 to 0.50% by mass, O: 0.030% by mass or less, the balance being Fe and inevitable impurities, and the Ta / O ratio by mass% is 2.5 or more Yes.

  As described above, the duplex stainless steel material contains a predetermined amount of C, Si, Mn, P, S, Al, Ni, Cr, Mo, N, Ta, O, and by adding a predetermined amount of Ta, While suppressing the precipitation of Cr-based oxides, the corrosion resistance is improved and the decrease in hot workability is suppressed.

  In the duplex stainless steel material according to the present invention, the component composition further includes Co: 0.1 to 2.0 mass%, Cu: 0.1 to 2.0 mass%, and V: 0.01 to 0. It is preferable to contain at least one selected from the group consisting of .50% by mass, Ti: 0.01 to 0.50% by mass, and Nb: 0.01 to 0.50% by mass.

  As described above, the duplex stainless steel material further improves corrosion resistance by further containing at least one selected from the group consisting of Co, Cu, V, Ti, and Nb in a predetermined amount. Further, Co and Cu contribute to stabilization of the austenite phase, and V, Ti and Nb contribute to improvement of strength characteristics and hot workability.

  In addition, the duplex stainless steel material according to the present invention further includes one or two of the above component compositions of Mg: 0.0005 to 0.0200 mass% and Ca: 0.0005 to 0.0200 mass%. It is preferable to do.

  As described above, the duplex stainless steel material further contains a predetermined amount of Mg, Ca, or two kinds of coarse MnS or the like which becomes a passive film deficient portion that tends to start local corrosion. Formation of inclusions is suppressed and local corrosion resistance is improved. Moreover, hot workability improves by the production | generation of inclusions, such as coarse MnS, being suppressed.

Furthermore, the duplex stainless steel pipe according to the present invention is characterized by comprising the duplex stainless steel material described above.
As mentioned above, the duplex stainless steel pipe is made of a duplex stainless steel material, which increases the stability of the passive film formed on the surface of the steel pipe, so that local corrosion can be greatly suppressed and corrosion resistance is improved. To do.

  The duplex stainless steel material of the present invention exhibits good corrosion resistance and hot workability in an environment containing corrosive substances such as chloride, hydrogen sulfide, and carbon dioxide gas. Moreover, according to the duplex stainless steel pipe of the present invention, good corrosion resistance is exhibited in an environment containing corrosive substances such as chloride, hydrogen sulfide, carbon dioxide gas and the like. As a result, duplex stainless steel pipes can be used not only for structural materials in seawater environments such as umbilicals, seawater desalination plants, and LNG vaporizers, but also for structural materials in highly corrosive environments such as oil well pipes and various chemical plants. It becomes possible.

<Duplex stainless steel>
An embodiment of the duplex stainless steel material according to the present invention will be described in detail.
The duplex stainless steel material of the present invention is a duplex stainless steel material composed of a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is C, Si, Mn, P, S, Al, Ni, Cr , Mo, N, and Ta are contained in a predetermined amount, with the balance being Fe and inevitable impurities. Each configuration will be described below.

(Steel structure)
The duplex stainless steel material of the present invention is composed of two phases of a ferrite phase and an austenite phase. In a duplex stainless steel material composed of a ferrite phase and an austenite phase, ferrite phase stabilizing elements such as Cr and Mo tend to concentrate in the ferrite phase, and austenite phase stabilizing elements such as Ni and N tend to concentrate in the austenite phase. . At this time, when the area ratio of the ferrite phase to the austenite phase is less than 30% or more than 70%, the concentration difference between the ferrite phase and the austenite phase of elements contributing to the corrosion resistance such as Cr, Mo, Ni, and N becomes large. Too much, either the ferrite phase or the austenite phase, which is inferior in corrosion resistance, is selectively corroded, and the tendency of the corrosion resistance to deteriorate increases. Therefore, it is recommended to optimize the area ratio between the ferrite phase and the austenite phase. The area ratio of the ferrite phase to the austenite phase is preferably 30 to 70%, more preferably 40 to 60% from the viewpoint of corrosion resistance. Such an area ratio between the ferrite phase and the austenite phase can be optimized by adjusting the contents of the ferrite phase stabilizing element and the austenite phase stabilizing element.

  In addition, the duplex stainless steel material of the present invention can tolerate other phases such as the σ phase and Cr carbonitride in addition to the ferrite phase and austenite phase to such an extent that they do not impair various properties such as corrosion resistance and mechanical properties. The ratio of the total area of the ferrite phase and austenite phase to the total area is preferably 95% or more, and more preferably 97% or more.

The reason for limiting the numerical range of the component composition of the duplex stainless steel material will be described.
(C: 0.10 mass% or less)
C forms a carbide with Cr or the like in the steel material to lower the corrosion resistance. Therefore, the content of C is preferably as small as possible, and is 0.10% by mass or less. The content of C is preferably 0.08% by mass or less, and more preferably 0.06% by mass or less. C may not be contained in the steel material, that is, 0% by mass.

(Si: 0.1 to 2.0% by 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 0.1% by mass or more. The Si content is preferably 0.15% by mass or more, and more preferably 0.2% by mass or more. However, if Si is excessively contained, the hot workability deteriorates, so the Si content is set to 2.0 mass% or less. The content of Si is preferably 1.5% by mass or less, and more preferably 1.0% by mass or less.

(Mn: 0.1 to 2.0% by mass)
Mn has a deoxidizing effect like Si, and is an element necessary for ensuring strength. In order to obtain such an effect, the Mn content is 0.1% by mass or more. The Mn content is preferably 0.15% by mass or more, more preferably 0.2% by mass or more. However, if Mn is excessively contained, coarse MnS is formed and the corrosion resistance is deteriorated. Therefore, the Mn content is set to 2.0% by mass or less. The Mn content is preferably 1.5% by mass or less, and more preferably 1.0% by mass or less.

(P: 0.05% by mass or less)
P is an element that deteriorates corrosion resistance, weldability, and workability. Therefore, the content of P is preferably as small as possible, and is 0.05% by mass or less. The content of P is preferably 0.04% by mass or less, more preferably 0.03% by mass or less. Further, P may not be contained in the steel material, that is, 0% by mass. However, excessive reduction of the P content causes an increase in production cost, so the lower limit of the P content in actual operation is 0.01% by mass.

(S: 0.01% by mass or less)
S, like P, is an impurity mixed during melting, and is an element that combines with Mn or the like to form sulfide inclusions and degrades corrosion resistance and hot workability. Therefore, the content of S is preferably as small as possible, and is 0.01% by mass or less. The S content is preferably 0.003% by mass or less.

(Al: 0.001 to 0.050 mass%)
Al, like Si and Mn, has a deoxidizing effect and is an element necessary for reducing the amount of oxygen during melting. In order to obtain such an effect, the Al content is set to 0.001% by mass or more. However, if Al is excessively contained, oxide inclusions are generated and adversely affect pitting corrosion resistance. Therefore, the Al content is set to 0.050% by mass or less. The content of Al is preferably 0.02% by mass or less.

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

(Cr: 22.0-28.0 mass%)
Cr is a main component of the passive film, and is a basic element for developing the corrosion resistance of the stainless steel material. Cr is also an element that stabilizes the ferrite phase. In order to maintain the two-phase structure of the ferrite phase and the austenite phase and achieve both corrosion resistance and strength, the Cr content is set to 22.0 mass% or more. The content of Cr is preferably 23.0% by mass or more, and more preferably 24.0% by mass or more. However, when Cr is excessively contained, hot workability deteriorates, so the Cr content is set to 28.0% by mass or less. The content of Cr is preferably 27.5% by mass or less, and more preferably 27.0% by mass or less.

(Mo: 2.0 to 6.0 mass%)
Mo is an element that generates molybdic acid at the time of dissolution and exhibits an effect of improving local corrosion resistance by an inhibitor action, thereby improving the corrosion resistance. Mo is also an element that stabilizes the ferrite phase and is an element that improves the pitting corrosion resistance and crack resistance of the steel material. In order to obtain such an effect, the Mo content is set to 2.0% by mass or more. The Mo content is preferably 2.2% by mass or more, and more preferably 2.5% by mass or more. However, if Mo is excessively contained, the formation of intermetallic compounds such as the σ phase is promoted, and the corrosion resistance and hot workability deteriorate. Therefore, the Mo content is set to 6.0% by mass or less. The content of Mo is preferably 5.5% by mass or less, and more preferably 5.0% by mass or less.

(N: 0.2-0.5% by mass)
N is an element that stabilizes a strong austenite phase, and has an effect of improving corrosion resistance without increasing the formation sensitivity of the σ phase. Furthermore, since it is an effective element for increasing the strength of steel, it is actively used in the present invention. In order to obtain such an effect, the N content is 0.2% by mass or more. N content becomes like this. Preferably it is 0.22 mass% or more, More preferably, it is 0.25 mass% or more. However, when N is contained excessively, nitrides are formed, and toughness and corrosion resistance are lowered. In addition, hot workability is deteriorated, and ear cracks and surface defects are generated during forging and rolling. Therefore, the N content is 0.5% by mass or less. The N content is preferably 0.45% by mass or less, and more preferably 0.4% by mass or less.

(Ta: 0.01 to 0.50 mass%)
Ta is an element that suppresses the formation of Cr-based oxides by combining with O and has the effect of suppressing the precipitation of the σ phase, which affects the deterioration of toughness and corrosion resistance. Moreover, there exists an effect which contributes to the substantial Cr density | concentration improvement of steel materials. In order to obtain such an effect, the content of Ta is set to 0.01% by mass or more. The content of Ta is preferably 0.02% by mass or more, and more preferably 0.03% by mass or more. However, excessive addition of Ta results in precipitation as nitrides due to bonding with N in the steel, thereby reducing toughness and hot workability. Further, the effective concentration of N is reduced by the precipitation of nitride. Therefore, the Ta content is 0.50% by mass or less. The content of Ta is preferably 0.40% by mass or less, and more preferably 0.30% by mass or less.

(O: 0.030% by mass or less)
O combines with a deoxidizing element such as Si or Al to precipitate as an oxide in the steel, thereby reducing the workability and corrosion resistance of the duplex stainless steel. Therefore, the content of O as an impurity is preferably as low as possible, and is set to 0.030% by mass or less. The content of O is preferably 0.028% by mass or less, more preferably 0.025% by mass or less, and further preferably 0.024% by mass or less. In addition, although O content is so preferable that it is low, since reducing O excessively leads to a cost increase, the minimum is about 0.0005 mass%.

(Ta / O ratio is 2.5 or more)
In order to efficiently produce a Ta-based oxide, the ratio of Ta and O is important. For that purpose, it is necessary to set the ratio of Ta / O by mass% to 2.5 or more. Preferably it is 2.7 or more, more preferably 2.9 or more. In addition, corrosion resistance and workability are improved only by adding Ta, but Ta-O is generated more efficiently than by simply adding Ta by appropriately controlling Ta / O. be able to. Thereby, corrosion resistance and workability can be further improved. Even when the amount of Ta added is relatively small, the desired corrosion resistance can be ensured by reducing the O content.

  Further, the duplex stainless steel material of the present invention may further contain other elements as long as the effects of the present invention are not adversely affected. For example, in the duplex stainless steel material of the present invention, the component composition preferably further contains one or more kinds from a group consisting of a predetermined amount of Co, Cu, V, Ti, and Nb.

(Co: 0.1-2.0 mass%, Cu: 0.1-2.0 mass%, V: 0.01-0.50 mass%, Ti: 0.01-0.50 mass%, Nb : One or more selected from the group consisting of 0.01 to 0.50% by mass)
Co and Cu are elements that improve corrosion resistance and stabilize the austenite phase. In order to obtain such an effect, when Co or Cu is contained, the content of these elements is 0.1% by mass or more. However, if Co or Cu is excessively contained, the hot workability deteriorates, so the content of these elements is 2.0% by mass or less. The contents of these elements are each preferably 0.2% by mass or more and 1.5% by mass or less.

  V, Ti, and Nb are elements that improve corrosion resistance and improve strength characteristics and hot workability. In order to obtain such an effect, when V, Ti or Nb is contained, the content of these elements is 0.01% by mass or more. However, if V, Ti, or Nb is excessively contained, coarse carbides and nitrides are formed and the toughness is deteriorated. Therefore, the content of these elements is set to 0.50% by mass or less. The contents of these elements are each preferably 0.05% by mass or more and 0.40% by mass or less.

  Moreover, as for the duplex stainless steel material of this invention, it is preferable that the said component composition contains 1 type or 2 types of Mg and Ca of a predetermined amount further.

(Mg: 0.0005 to 0.0200 mass%, Ca: 0.0005 to 0.0200 mass%, one or two)
Mg and Ca are elements that combine with S contained as an impurity in steel to suppress the formation of MnS, which tends to be a starting point of local corrosion, and improve local corrosion resistance. Moreover, it combines with S and O in steel, and suppresses that these inclusions segregate at a grain boundary, and has the effect of improving hot workability. In order to obtain such an effect, when Mg or Ca is contained, the content of these elements is 0.0005% by mass or more. However, if Mg or Ca is excessively contained, the oxide inclusions increase, and the corrosion resistance and workability deteriorate. Therefore, the content of these elements is 0.0200% by mass or less. The content of these elements is preferably 0.0020 to 0.0200% by mass.

(Remainder Fe, inevitable impurities)
The basic components of the duplex stainless steel used in the present invention are as described above, and the remaining component is substantially Fe, but it is of course acceptable to mix inevitable impurities. Inevitable impurities can be contained to such an extent that the various properties of the duplex stainless steel material of the present invention are not harmed.

(PRE)
In the duplex stainless steel material according to the present invention, the component composition is such that the Cr content (% by mass) is [Cr], the Mo content (% by mass) is [Mo], and the N content (% by mass). When [N] is set, it is preferable that PRE = [Cr] +3.3 [Mo] +16 [N] ≧ 40.
[Cr] +3.3 [Mo] +16 [N] is a pitting corrosion resistance index (PRE) that is conventionally known as an index representing the corrosion resistance of steel materials. By setting PRE ≧ 40, the balance of Cr content, Mo content, and N content in the structure becomes appropriate, and the corrosion resistance and strength of the steel material can be further improved.

(Method for producing duplex stainless steel)
The duplex stainless steel material of the present invention can be produced by a production facility and a production method used for mass production of ordinary stainless steel materials. In order to reduce O as an impurity in steel, deoxidation is performed by adding a large amount of elements having high affinity with O, such as Si and Al, and secondary refining such as vacuum degassing and argon gas stirring. Inclusions are removed by increasing the time of the process or by performing the process a plurality of times. For example, the molten steel melted in a converter or electric furnace is refined by an AOD method, a VOD method, or the like to adjust the components, and then formed into a steel ingot by a casting method such as a continuous casting method or an ingot-making method. The obtained steel ingot can be hot-worked in a temperature range of about 1000 to 1200 ° C., and then cold-worked to obtain a desired dimensional shape.

  In the present invention, in order to eliminate precipitates detrimental to mechanical properties, it is preferable to quench by applying a solution heat treatment as necessary. The temperature of the solution heat treatment is preferably 1000 to 1100 ° C., the holding time is preferably 1 to 30 minutes, and the rapid cooling is preferably performed at a cooling rate of 10 ° C./second or more. Moreover, the pickling for surface adjustments, such as scale removal, can be performed as needed.

<Duplex stainless steel pipe>
An embodiment of a duplex stainless steel pipe according to the present invention will be described.
The duplex stainless steel pipe of the present invention is made of the duplex stainless steel material and can be produced by a production facility and a production method used for mass production of ordinary stainless steel pipes. For example, a desired size can be obtained by an extruded pipe made of a round bar, a Mannesmann pipe, a welded pipe made by welding a seam after forming a plate material. Moreover, the dimension of a duplex stainless steel pipe can be suitably set according to the oil well pipe, chemical plant, umbilical tube, etc. in which the steel pipe is used. The duplex stainless steel pipe of the present invention can also be used for seawater desalination plants, LNG vaporizers, and the like.

  Examples of the duplex stainless steel material according to the present invention will be described below. The following examples are not intended to limit the present invention, and can be implemented with appropriate modifications within a range that can be adapted to the gist of the preceding and following descriptions, all of which fall within the technical scope of the present invention. included.

(Sample preparation)
By using molten steel processing equipment equipped with an electrode arc heating function, steels having the component compositions shown in Table 1 (steel symbols A1 to A28, B1 to B8) are melted, and using a round mold (main body: about φ140 mm × 320 mm), A 50 kg steel ingot was cast. In Table 1, underlined numerical values indicate that the present invention deviates from the configuration of the present invention. Moreover, the column of “-” indicates that the corresponding component is not contained.

  The solidified steel ingot is heated to 1200 ° C, hot forged at the same temperature, then cut, held at 1100 ° C for 30 minutes, subjected to solution heat treatment, water-cooled, length 600 mm x width 120 mm x A forged steel product (test material Nos. 1-36) having a thickness of 60 mm was finished. For steel symbols A10 to A16, the deoxidation step was performed stronger than usual in order to reduce the O content.

  Table 1 also shows the results of calculating Ta / O and PRE = [Cr] +3.3 [Mo] +16 [N] for each steel composition. Further, the finished forged steel product was embedded in a cross section parallel to the machining direction, mirror-polished, electrolytically etched in an oxalic acid aqueous solution, and then observed with an optical microscope at a magnification of 100 times to confirm the structure of each forged steel product. . As a result, each forged steel product was composed of two phases of a ferrite phase and an austenite phase.

(Collecting samples for evaluation)
Next, pitting corrosion resistance and hot workability were evaluated by the following procedure using a sample (length 20 mm × width 30 mm × thickness 2 mm) collected from the forged steel product in parallel to the processing direction.

(Evaluation of pitting corrosion resistance)
Pitting corrosion resistance was evaluated with reference to the method described in JIS G0577. The sample surface was wet-polished with SiC # 600 abrasive paper, subjected to ultrasonic cleaning, and then a lead wire was attached to the sample by spot welding. The part other than the test surface (test area: 10 mm × 10 mm) of the sample surface was coated with an epoxy resin. The sample was immersed in a 20% NaCl aqueous solution maintained at 80 ° C. for 10 minutes. Thereafter, anodic polarization was performed at a sweep rate of 20 mV / min, and the potential when the current density exceeded 0.1 mA / cm ² was defined as the pitting potential (V _C '100). The evaluation of pitting corrosion resistance is excellent when the pitting potential exceeds 500 mV (vs. SCE) (◎), better than 300 mV but not higher than 500 mV (vs. SCE) (◯), exceeds 100 mV And those with 300 mV or less (vs. SCE) were evaluated as slightly defective (Δ) and those with 100 mV or less (vs. SCE) were evaluated as defective (x). Here, SCE is a saturated calomel electrode. The evaluation results of pitting corrosion potential and pitting corrosion resistance are shown in Table 2.

(Evaluation of hot workability)
The surface of the forged steel product was visually observed, and the presence or absence of surface defects was observed. Also, hot workability is poor (×) when cracks occur, hot workability is slightly poor (△) when surface defects occur frequently, and hot work is performed when surface defects are slight. Good (◯) and no surface defects were evaluated as being excellent in hot workability (）). The results are shown in Table 2.

  From the results shown in Table 2, test materials No. 1 prepared using steels (steel symbols A1 to A28) satisfying the component composition and Ta / O ratio as requirements of the present invention. 1 to 28 (Examples) were all found to have excellent pitting corrosion resistance and hot workability.

In contrast, test material No. 29-36 (comparative example) had the following problems.
Test material No. No. 29 was slightly inferior in pitting corrosion resistance because the content of each component satisfied the requirements of the present invention but did not satisfy the Ta / O ratio. Test material No. No. 30 was inferior in pitting corrosion resistance and hot workability because Ta was excessive and a large amount of coarse nitride was formed. Test material No. No. 31 was not added with Ta, and many unnecessary Cr-based oxides and σ phases were formed, resulting in poor pitting corrosion resistance and hot workability. Test material No. No. 32 was inferior in pitting corrosion resistance because O was excessive and a large amount of Cr-based oxide was formed to reduce the effective concentration of Cr in the steel. Also, a large amount of Al-based oxide and Si-based oxide were formed, and the hot workability was poor.

  Test material No. No. 33 lacked Cr and was inferior in pitting corrosion resistance. Test material No. Since Mn was excessive in No. 34, many inclusions were precipitated, and the pitting corrosion resistance and hot workability were poor. Test material No. No. 35 was inferior in pitting corrosion resistance and hot workability because S was excessive and a large amount of coarse sulfide was formed. Test material No. No. 36 lacked Ni and was inferior in pitting corrosion resistance.

Claims (4)

It is a duplex stainless steel material composed of a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is:
C: 0.10 mass% or less,
Si: 0.1 to 2.0% by mass,
Mn: 0.1 to 2.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
Al: 0.001 to 0.050 mass%,
Ni: 1.0-10.0 mass%,
Cr: 22.0-28.0 mass%,
Mo: 2.0 to 6.0 mass%,
N: 0.2-0.5% by mass
Ta: 0.01 to 0.50 mass%,
O: 0.030% by mass or less, with the balance being Fe and inevitable impurities,
A duplex stainless steel material having a Ta / O ratio by mass% of 2.5 or more.   The component composition is further Co: 0.1-2.0 mass%, Cu: 0.1-2.0 mass%, V: 0.01-0.50 mass%, Ti: 0.01-0. The duplex stainless steel material according to claim 1, comprising at least one selected from the group consisting of 50 mass% and Nb: 0.01 to 0.50 mass%.   The component composition further contains one or two of Mg: 0.0005 to 0.0200 mass% and Ca: 0.0005 to 0.0200 mass%. The duplex stainless steel material described in 1.   A duplex stainless steel pipe comprising the duplex stainless steel material according to any one of claims 1 to 3.