WO2014112445A1 - Duplex stainless steel material and duplex stainless steel pipe - Google Patents

Abstract

This duplex stainless steel material is composed of a ferrite phase and an austenite phase, and has a component composition that contains 0.100% by mass or less of C, 0.10-2.00% by mass of Si, 0.10-2.00% by mass of Mn, 0.050% by mass or less of P, 0.0100% by mass or less of S, 0.001-0.050% by mass of Al, 1.0-10.0% by mass of Ni, 22.0-28.0% by mass of Cr, 2.0-6.0% by mass of Mo and 0.20-0.50% by mass of N, while containing 0.01-0.50% by mass of Ta and/or 0.1-1.0% by mass of Ge, with the balance made up of Fe and unavoidable impurities.

Description

Duplex stainless steel and duplex stainless steel pipe

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

Stainless steel is a material that naturally forms a stable surface film mainly composed of 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 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 and defects in the passive film. In some cases, 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 as follows: Cr amount (% by mass) is [Cr], Mo amount (% by mass) is [Mo], W amount (% by mass) is [W], and N amount (% by mass) is [N]. ], The pitting corrosion index PRE (Pitting Resistance Equivalent) calculated by [Cr] +3.3 [Mo] +16 [N], or [C] +3.3 ([Mo] +0. 5 [W]) + 16 [N]. It is known that if the content of Cr, Mo, N is increased, excellent pitting corrosion resistance can be obtained. The addition amounts 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 that controls the contents of B and Ta in addition to controlling the contents of Cr, Mo, W, and N, and is excellent in corrosion resistance and hot workability. . In Patent Document 3, in addition to controlling the contents of Cr, Mo, W, and N, the contents of Ti, V, Nb, Ta, Zr, and B are controlled, and two phases that are excellent in corrosion resistance and hot workability Stainless steel is disclosed.

In Patent Document 4, assuming that Cr and N are particularly effective in improving crevice corrosion resistance, a duplex stainless steel excellent in crevice corrosion resistance and overhang formability is obtained while saving Ni which increases the cost. Disclosure. Patent Document 5 discloses a duplex stainless steel in which Cu and Al are added and the amounts of O, S, and Ca are controlled to improve crevice corrosion resistance.

In Patent Document 6, in order to reduce sulfide inclusions in steel that adversely affect hot workability and corrosion resistance, a CaO crucible and a CaO—CaF ₂ —Al ₂ O ₃ slag are used in a vacuum melting furnace. The amount of S is reduced to 3 ppm or less.

In Patent Document 7, as a technique for controlling oxide inclusions as a starting point of pitting corrosion, the total content of Ca and Mg in the oxide inclusions, the S content is controlled, and the inclusion form And duplex stainless steels with adjusted density are disclosed. And in Patent Document 7, since insoluble Al oxide containing Ca, Mg, S more than a certain amount becomes a local corrosion starting point, slag basicity at the time of reduction treatment, killing temperature and time in the ladle, A duplex stainless steel is disclosed in which the size and number of the inclusions are controlled by optimally combining the total processing ratio after casting and the occurrence of local corrosion is suppressed.

Japanese Patent Laid-Open No. 5-132741 Japanese Laid-Open Patent Publication No. 8-170153 Japanese Unexamined Patent Publication No. 61-157626 Japanese Unexamined Patent Publication No. 2006-200035 Japanese Laid-Open Patent Publication No. 63-157838 Japanese Patent Laid-Open No. 3-291358 International Publication No. 2005/014872

In order to apply the duplex stainless steel material in a severe corrosive environment containing hydrogen sulfide, carbon dioxide, and chloride ions, it is necessary to improve the corrosion resistance. However, the improvement of corrosion resistance may be insufficient only by adjusting the contents of Cr, Mo, N, and W.

In Patent Document 1, the corrosion resistance (pitting corrosion resistance) of a steel material is evaluated by a pitting potential in 80 ° C., 20% -NaCl. However, when PREW = 42, it is about 300 mV, which is required recently. In a severe corrosive environment, it cannot always be said that sufficient corrosion resistance can be secured.

Further, in Patent Document 2, B is added to the steel, but B combines with N in the steel to produce BN, which may reduce the N concentration contributing to corrosion resistance. Further, in Patent Document 2, the added amount of W is as high as 5 to 10% by mass, which causes an increase in cost and is economically disadvantageous.

In Patent Document 3, Nb, Ti, and Zr are added to the steel, but these elements combine with N in the steel to form nitrides, thereby reducing the N concentration that contributes to corrosion resistance. There is a risk of letting you. Moreover, when the produced | generated nitride is coarse, toughness will be reduced.

The duplex stainless steel disclosed in Patent Document 4 assumes the use of automobile materials, and is insufficient in crevice corrosion resistance in severe corrosive environments such as oil wells. In addition, the duplex stainless steel disclosed in Patent Document 5 has been evaluated for crevice corrosion resistance in artificial seawater at 30 ° C., and crevice corrosion resistance is insufficient in severe corrosive environments such as oil wells.

Further, in Patent Document 6, it is evaluated that S is 3 ppm or less because the industrial load is large and the cost is high, and that the critical pitting corrosion temperature is 35 ° C. or more is excellent in corrosion resistance. This is considered insufficient for use in severe corrosive environments.

In Patent Document 7, even if Ca and Mg are added and inclusions are controlled, there is a concern that these may aggregate to cause local corrosion and crack initiation, and the present invention basically uses conventional pores. It is a direction to reduce the inclusions that serve as the starting point of the erosion, and excessively reducing O and S that are the source of formation is industrially expensive and expensive.

On the other hand, duplex stainless steel materials are excellent in strength characteristics, but are often more difficult to process such as rolling and drawing than ordinary stainless steel materials. 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 hot workability may be insufficient depending on the application.

The present invention has been made in view of such a situation, and the problem is that a duplex stainless steel material that exhibits good corrosion resistance in an environment containing corrosive substances such as chloride, hydrogen sulfide, and carbon dioxide gas. To provide a duplex stainless steel material that also exhibits good hot workability, and to provide a duplex stainless steel tube that exhibits good corrosion resistance by using such a duplex stainless steel material There is to do.

As described above, the stainless steel material is a material that exhibits corrosion resistance by a passive film mainly composed of Cr oxide. Since duplex stainless steel is generally composed of a ferrite phase and an austenite phase, there is discontinuity at the interface between these different phases, and the passive film becomes unstable at the interface between the ferrite phase and the austenite phase. Therefore, it is easy to receive a passive film destruction action of chloride ions, and local corrosion is likely to occur. In order to solve the above-mentioned problems, the present inventors pay attention to enhancing the stability and protective property of the passive film of the duplex stainless steel material within a range that does not impair the manufacturing surface and various characteristics, and improve the corrosion resistance. Technical study was conducted.

As described above, since the stainless steel material is a material that exhibits corrosion resistance by the passive film mainly composed of Cr oxide, the present inventors have studied from the viewpoint of improving the effective Cr concentration in the steel. As a result, the formation of unnecessary Cr inclusions in the steel results in a decrease in the effective Cr concentration in the steel, and it has been found that a method for suppressing the precipitation of unnecessary Cr inclusions is effective. .

Since carbides and oxides are generally used as inclusions in stainless steel materials, it is important to fix C and O in the steel that causes these inclusions to be formed with other elements. Here, O can be fixed with Si or Al added for deoxidation, or Ca or Mg, and therefore, examination was performed from the viewpoint of fixing unnecessary C in steel. Further, as described above, the pitting corrosion resistance of the stainless steel material is expressed by the pitting corrosion index PRE (W) including the N amount ([N]), and therefore the effective N concentration in the steel also affects the improvement of the corrosion resistance. Therefore, examination was also performed from the viewpoint of suppressing the precipitation of unnecessary N-based inclusions.

And, the effective concentration of Cr and N in steel is increased by moderately adding Ta as an element that has a high ability to fix unnecessary C in steel and that is difficult to fix N necessary for ensuring corrosion resistance. As a result, it has been found that the stability of the passive film is increased and the corrosion resistance is improved.

In addition to Cr, Mo is known as an additive element for improving corrosion resistance. However, when local corrosion occurs and Mo becomes an acidic environment in the pit, it elutes as ions and repairs the passive film (repassivation). Effect). Therefore, the inventors focused on this effect and extracted an element that elutes ions similar to Mo in an acidic environment. As a result, it was found that Ge has an electrochemical property in the acidic region close to Mo, and has a function of enhancing the re-passivation ability of stainless steel and improving local corrosion resistance by adding moderately. .

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. The component composition of the duplex stainless steel material is C: 0.100% by mass or less, Si: 0.10 To 2.00% by mass, Mn: 0.10 to 2.00% by mass, P: 0.050% by mass or less, S: 0.0100% by mass or less, Al: 0.001 to 0.050% by mass, Ni : 1.0 to 10.0% by mass, Cr: 22.0 to 28.0% by mass, Mo: 2.0 to 6.0% by mass, N: 0.20 to 0.50% by mass, Furthermore, it contains at least one selected from Ta: 0.01 to 0.50 mass% and Ge: 0.1 to 1.0 mass%, with the balance being Fe and inevitable impurities. And

As described above, the duplex stainless steel material includes a predetermined amount of C, Si, Mn, P, S, Al, Ni, Cr, Mo, N, and Ta and / or Ge, thereby improving the corrosion resistance. . Moreover, when Ta is selected as the contained element, a decrease in hot workability is also suppressed.

The duplex stainless steel material according to the present invention has a Cr content (mass%) of [Cr], an Mo content (mass%) of [Mo], and an N content (mass%) of [N]. Furthermore, when the PRE value represented by the following formula is 40 or more, the corrosion resistance and strength of the steel material are preferably improved.
PRE = [Cr] +3.3 [Mo] +16 [N]

Furthermore, the duplex stainless steel material according to the present invention contains the Ta, limits the impurity O to 0.01% by mass or less, and, among the inclusions of the duplex stainless steel material, has a long diameter. The number of sulfur / oxide composite inclusions containing Ta that is 1 μm or more is 500 or less per 1 mm ^{2 in} cross section perpendicular to the processing direction, and the Ta content of the sulfur / oxide composite inclusions is 5 atomic%. It is preferable to make it as described above. By doing so, the corrosion resistance is further improved.

In the duplex stainless steel material according to the present invention, the component composition is further Co: 0.10 to 2.00% by mass, Cu: 0.10 to 2.00% by mass, V: 0.01 to 0.50% by mass. %, Ti: 0.01 to 0.50 mass%, and Nb: 0.01 to 0.50 mass%.

As described above, the duplex stainless steel material further improves the corrosion resistance by further containing at least one selected from the group consisting of Co, Cu, V, Ti, and Nb. 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.

Further, the duplex stainless steel material according to the present invention further includes one or two of the above-mentioned 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 described 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 in an environment containing corrosive substances such as chloride, hydrogen sulfide and carbon dioxide. Moreover, when it contains Ta, favorable hot workability is also expressed. Furthermore, the duplex stainless steel pipe of the present invention exhibits good corrosion resistance in an environment containing corrosive substances such as chloride, hydrogen sulfide and carbon dioxide. 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 and N are contained in a predetermined amount, and Ta and / or Ge 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 that the area ratio of the ferrite phase and the austenite phase is also optimized, and the area ratio of the ferrite phase is preferably 30 to 70% and 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.

Further, in the duplex stainless steel material of the present invention, other phases such as σ phase and Cr carbonitride as well as ferrite phase and austenite phase can be tolerated to such an extent that various properties such as corrosion resistance and mechanical properties are not harmed. The total area of the ferrite phase and the austenite phase 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: 0.100 mass% or less)
C is a harmful element because it forms a carbide with Cr or the like in the steel material to lower the corrosion resistance and hot workability. Therefore, the C content is 0.100% by mass or less. In addition, since it is better that the C content is as small as possible, it is preferably 0.080% by mass or less, more preferably 0.060% by mass or less. Note that C may not be contained in the steel material, that is, 0% by mass.

(Si: 0.10 to 2.00% by mass)
Si is an element necessary for deoxidation and stabilization of the ferrite phase. In order to acquire such an effect, Si content shall be 0.10 mass% or more. However, if Si is excessively contained, hot workability deteriorates, so the Si content is 2.00% by mass or less. The preferable lower limit of the Si content is 0.15% by mass, and more preferably 0.20% by mass. Moreover, the upper limit with preferable Si content is 1.50 mass%, and a more preferable upper limit is 1.00 mass%.

(Mn: 0.10 to 2.00% by mass)
Mn has a deoxidizing effect like Si, and is an element necessary for ensuring strength. In order to acquire such an effect, Mn content shall be 0.10 mass% or more. However, if Mn is excessively contained, coarse MnS is formed and the corrosion resistance and hot workability deteriorate, so the Mn content is 2.00% by mass or less. The lower limit with preferable Mn content is 0.15 mass%, More preferably, it is 0.20 mass%. Moreover, the upper limit with preferable Mn content is 1.50 mass%, More preferably, it is 1.00 mass%.

(P: 0.050 mass% or less)
P is an impurity mixed during melting, an element harmful to corrosion resistance, and an element that deteriorates weldability and workability. Therefore, the P content is 0.050 mass% or less. In addition, since it is better that the P content is as small as possible, it is preferably 0.040% by mass or less, and more preferably 0.030% by mass or less. Further, P may not be contained in the steel material, that is, it may be 0% by mass. However, excessive reduction of the P content causes an increase in production cost, so the lower limit of P content in actual operation. The value is 0.010% by mass.

(S: 0.0100 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 S content is 0.0100% by mass or less. Since the S content is preferably as small as possible, it is preferably 0.0030% by mass or less. Further, S is not contained in the steel material, that is, it may be 0% by mass. However, excessive reduction of the S content causes an increase in manufacturing cost, so the lower limit value in actual operation of the S content. Is 0.0001 mass%.

(Al: 0.001 to 0.050 mass%)
Al, like Si and Mn, has an effect of deoxidation, and is an element necessary for reducing the amount of oxygen during melting. In order to acquire such an effect, Al content shall be 0.001 mass% or more. However, if Al is excessively contained, oxide inclusions are generated and the pitting corrosion resistance is adversely affected. Therefore, the Al content is set to 0.050% by mass or less. A preferred range for the Al content is 0.010 to 0.020 mass%.

(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 acquire such an effect, Ni content shall be 1.0 mass% or more. However, if Ni is excessively contained, the austenite phase is excessively increased and the strength is lowered, and an intermetallic compound (σ phase) is easily generated and hot workability is deteriorated. 0.0 mass% or less. The lower limit with preferable Ni content is 2.0 mass%, More preferably, it is 3.0 mass%. Moreover, the upper limit with preferable Ni content is 9.5 mass%, More preferably, it is 9.0 mass%.

(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 to achieve both corrosion resistance and strength, the Cr content is set to 22.0% by mass or more. However, when Cr is excessively contained, an intermetallic compound (σ phase) is easily generated and hot workability is deteriorated, so the Cr content is set to 28.0% by mass or less. The lower limit with preferable Cr content is 23.0 mass%, More preferably, it is 24.0 mass%. Moreover, the upper limit with preferable Cr content is 27.5 mass%, More preferably, it is 27.0 mass%.

(Mo: 2.0 to 6.0% by 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 acquire such an effect, Mo content shall be 2.0 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 are deteriorated. Therefore, the Mo content is set to 6.0% by mass or less. The lower limit with preferable Mo content is 2.2 mass%, More preferably, it is 2.5 mass%. Moreover, the upper limit with preferable Mo content is 5.5 mass%, More preferably, it is 5.0 mass%.

(N: 0.20 to 0.50 mass%)
N is an element that stabilizes a strong austenite phase, has an effect of improving corrosion resistance without increasing the formation sensitivity of the σ phase, and is also an element effective for increasing the strength of a steel material. In order to acquire such an effect, N content shall be 0.20 mass% or more. However, if N is excessively contained, nitrides are formed and the toughness and corrosion resistance are deteriorated, and hot workability is deteriorated, and ear cracks and surface defects are generated during forging and rolling. Therefore, the N content is 0. .50% by mass or less. The lower limit with preferable N content is 0.22 mass%, More preferably, it is 0.25 mass%. Moreover, the upper limit with preferable N content is 0.45 mass%, More preferably, it is 0.40 mass%.

(Ta: 0.01 to 0.50 mass%)
Ta is an element having the effect of suppressing the formation of Cr-based carbides by combining with C, and the effect of suppressing the precipitation of σ phase, which affects the deterioration of toughness and corrosion resistance, and contributes to the substantial improvement of Cr concentration in steel materials. There is. In order to acquire such an effect, Ta content shall be 0.01 mass% or more. However, excessive Ta addition 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, and the corrosion resistance is lowered by the precipitation of the σ phase. Therefore, the Ta content is 0.50% by mass or less. A preferable lower limit of the Ta content is 0.02% by mass, and more preferably 0.03% by mass. Moreover, the upper limit with preferable Ta content is 0.30 mass%, More preferably, it is 0.25 mass%.

(Ge: 0.1 to 1.0% by mass)
Ge has the effect of improving local corrosion resistance by increasing and stabilizing the Cr concentration in the passive film. In order to acquire such an effect, 0.1 mass% or more, Preferably 0.2 mass% or more is added. On the other hand, excessive addition degrades hot workability and also increases costs, so the upper limit is made 1.0% by mass or less, preferably 0.9% by mass or less.

In order to improve the corrosion resistance, either Ta or Ge may be contained. However, when it is desired to improve the hot workability, it is preferable to select Ta.

(Inevitable impurities)
Inevitable impurities can be contained to the extent that they do not harm the properties of the duplex stainless steel material. For example, if it is O, the content is 0.1 mass% or less, preferably 0.05 mass% or less. Moreover, although mentioned later for details, when it contains Ta, it is more preferable to make O amount into 0.01 mass% or less. Thereby, the corrosion resistance manifesting effect of the present invention can be maximized.

Moreover, 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: 0.10 to 2.00% by mass, Cu: 0.10 to 2.00% by mass, V: 0.01 to 0.50% by mass, Ti: 0.01 to 0.50% by mass, Nb : One or more from the group consisting of 0.01 to 0.50 mass%)
Co and Cu are elements that improve the corrosion resistance and stabilize the austenite phase. In order to obtain such effects, the content of these elements is 0.10% by mass or more. However, since hot workability deteriorates when Co and Cu are excessively contained, the content of these elements is 2.00% by mass or less. A preferable lower limit of the content of these elements is 0.20% by mass. Moreover, the preferable upper limit of content of these elements is 1.50 mass%.

V, Ti and Nb are elements that improve the corrosion resistance and improve the strength characteristics and hot workability. In order to obtain such an effect, the content of these elements is 0.01% by mass or more. However, if V, Ti, and Nb are 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. A preferable lower limit of the content of these elements is 0.05% by mass. Moreover, the preferable upper limit of content of these elements is 0.40 mass%.

Further, in the duplex stainless steel material of the present invention, the component composition preferably further contains one or two kinds of Mg and Ca in predetermined amounts.

(Mg: 0.0005 to 0.020 mass%, Ca: 0.0005 to 0.020 mass%, one or two)
Mg and Ca are combined with S or O contained in the steel as impurities, and suppress the formation of inclusions such as MnS and Al ₂ O ₃ , thereby improving the hot workability. In order to obtain such an effect, the content of these elements is 0.0005% by mass or more. However, excessive inclusion of Mg and Ca leads to an increase in oxide inclusions, and these inclusions serve as starting points for pitting corrosion and cracking, so that corrosion resistance and hot workability deteriorate. The element content is 0.020% by mass or less. A preferable content of these elements is 0.002 to 0.020 mass%.

In addition, the duplex stainless steel material according to the present invention has a composition of [Cr] +3.3 [Mo] when the Cr content is [Cr], the Mo content is [Mo], and the N content is [N]. ] +16 [N] ≧ 40 is preferable.

[Cr] +3.3 [Mo] +16 [N] is a pitting corrosion resistance index (PRE: Pitting Resistance Equivalent) which 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 are improved.

(Sulfur / oxide composite inclusions)
In the duplex stainless steel material according to the present invention, by containing Ta and controlling the amount of O to a predetermined amount (0.01% by mass or less), the sulfur-oxide composite inclusions in the steel are modified, Corrosion resistance can be further improved.

Specifically, by adding and refining Ta, sulfide inclusions (MnS) contained in normal stainless steel are modified into sulfur / oxide composite inclusions containing Ta. Then, the local corrosion resistance is improved by the sulfur / oxide composite inclusions containing Ta.

For that purpose, the Ta content of the sulfur-oxide composite inclusions containing Ta is 5 atomic% or more, preferably 7 atomic% or more, more preferably 10 atomic% or more. In addition, although the upper limit of Ta content is not specifically defined, it is about 50 atomic%.

Even if the inclusions are modified by the addition of Ta, if a large number of coarse inclusions are present in the steel, the hot workability is deteriorated. The number of oxide-based composite oxides is 500 or less, preferably 450 or less, more preferably 400 or less per 1 mm ^{2 in} cross section perpendicular to the processing direction. The lower limit of the number density of the sulfur / oxide composite inclusions containing Ta is not particularly defined, but is about 20 per 1 mm ² . Fine inclusions whose major axis is less than 1 μm are excluded from the target because they have a low degree of adverse effect on local corrosion resistance.

In addition, the Ta content and number density of such sulfur / oxide composite inclusions control the Ta content and O content of the duplex stainless steel, and also control the thermal processing conditions during steel production. Is achieved by doing

(Method for producing duplex stainless steel)
When the duplex stainless steel material according to the present invention is manufactured, if the above-described control of the sulfur / oxide composite inclusion is not performed, it is manufactured by a manufacturing facility and a manufacturing method used for mass production of a normal stainless steel material. can do. 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 ° C. 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 if necessary. The solution heat treatment temperature is preferably 1000 to 1100 ° C., the holding time is preferably 10 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.

Further, when the duplex stainless steel material of the present invention is manufactured, when the above-described control of the sulfur / oxide composite inclusion is performed, it is manufactured as follows.

First, in order to reduce O as an impurity in steel, deoxidation is performed by adding a large amount of elements having a high affinity with O, such as Si and Al, and further, vacuum degassing, argon gas stirring, etc. Oxide inclusions are removed by increasing the time of secondary refining or by performing it multiple times.

After that, after adjusting the components by refining the molten steel melted in the converter or electric furnace by the AOD method or the VOD method, as described above, the steel ingot is obtained by a casting method such as a continuous casting method or an ingot forming method. To do. 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 size and shape. Here, the total working ratio during hot working (the cross-sectional area of the original steel ingot / the cross-sectional area after working) is about 10 to 50 as usual, but a sulfur / oxide-based composite containing a desired Ta In order to make an object exist, the processing ratio (cross-sectional area before processing / cross-sectional area after processing) in the temperature range of 1100 to 1200 ° C. during hot processing is 50% of the total processing ratio. It is preferable to perform hot working so that the working ratio exceeds.

As described above, in the production of ordinary duplex stainless steel materials, the steel ingot is hot-worked in a temperature range of about 1000 to 1200 ° C. Due to the influence of the temperature drop at the time, the processing ratio in the temperature range of 1000 to 1100 ° C. is higher than the processing ratio in the temperature range of 1100 to 1200 ° C. As a result, in the conventional manufacturing, the processing ratio in the temperature range of 1100 to 1200 ° C. is 50% or less of the total processing ratio. In the present invention, when the above-described control of the sulfur / oxide composite inclusions is performed, a desired ratio of sulfur / oxide composite containing Ta is obtained by deliberately increasing the processing ratio in the temperature range of 1100 to 1200 ° C. The presence of inclusions is obtained.

<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. The dimensions of the duplex stainless steel pipe can be appropriately set according to the umbilical, the seawater desalination plant, the LNG vaporizer, the oil well pipe, various chemical plants, etc. in which the steel pipe is used.

Examples of the duplex stainless steel material according to the present invention will be described below.

[Example 1: Example of Ta-containing steel]
(Sample preparation)
Steels (steel symbols A to Z) having the composition shown in Table 1 were melted by a molten steel processing facility having an electrode arc heating function, and cast using a 50 kg round mold (main body: about φ140 × 320 mm). The solidified steel ingot is heated to 1200 ° C., hot forged at the same temperature, then cut, subjected to a solution heat treatment held at 1100 ° C. for 30 minutes, water cooled, and a forged steel product of 600 × 120 × 60 mm (sample No. .1 to 26).

Also, PRE = [Cr] +3.3 [Mo] +16 [N] for each steel was calculated, and the results are also shown in Table 1. Furthermore, the finished forged steel product was embedded in a cross section parallel to the processing 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.

(Sample collection)
Next, pitting corrosion resistance and hot workability were evaluated by the following procedure using a sample (20 mm × 30 mm × 2 mm) collected from the forged steel product in parallel with the processing direction.

(Evaluation of pitting corrosion resistance)
The sample surface was wet-polished with SiC # 600 abrasive paper, subjected to ultrasonic cleaning, and then immersed in 30% nitric acid at 50 ° C. for 1 hour for passivation treatment. Next, a lead wire was attached to the sample by spot welding, and the test part (test area: 10 mm × 10 mm) was left and covered with an epoxy resin. The sample was immersed in a 20% NaCl aqueous solution maintained at 80 ° C. for 10 minutes, then held at +600 mV (vs. SCE: saturated calomel electrode) for 1 minute, and the maximum pitting depth of the test part was measured with a laser microscope. did. And when the maximum pitting depth exceeds 40 μm, the pitting corrosion resistance is poor (×), and when the maximum pitting depth is 40 μm or less and exceeds 20 μm, the pitting corrosion resistance is good (◯), and the maximum pitting corrosion depth. Of 20 μm or less were evaluated as having excellent pitting resistance (食). The results are shown in Table 2.

(Evaluation of hot workability)
The surface of the forged steel product was visually observed to observe the presence or absence of surface defects. 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 in Table 2, sample Nos. Produced using steels (steel symbols A to R) satisfying the requirements of the present invention. 1 to 18 (Examples) were confirmed to have good or excellent pitting corrosion resistance and good or excellent hot workability.

On the other hand, Sample No. produced using steel (steel symbols S to Z) that does not satisfy the requirements of the present invention. 19 to 26 (comparative examples) were confirmed to have the following problems.

Sample No. In No. 19, since Ta was excessive, a large amount of coarse nitride was formed, and the hot workability was poor. Also, a σ phase was formed, and the pitting corrosion resistance was poor. Sample No. In No. 20, Ta was not added, so many σ phases were formed, and the pitting corrosion resistance and hot workability were inferior. Sample No. In No. 21, since Mn was excessive, a large number of inclusions (MnS) were precipitated, resulting in poor pitting corrosion resistance and hot workability. Sample No. In No. 22, since S was excessive, a large amount of coarse sulfide was formed, and pitting corrosion resistance and hot workability were inferior. Sample No. No. 23 was inferior in pitting corrosion resistance and hot workability due to lack of Cr. Sample No. No. 24 was inferior in pitting corrosion resistance and hot workability due to lack of Ni. Sample No. No. 25 was excessive in C, so a large amount of carbide was formed, and the pitting corrosion resistance and hot workability were inferior. Sample No. In No. 26, Mo was excessive, so a large amount of σ phase was formed, and the pitting corrosion resistance and hot workability were inferior.

[Example 2: Example of Ge-containing steel]
(Production of test materials No. 1 to 17)
Stainless steel with the component composition shown in Table 3 (the remainder is Fe and inevitable impurities) are melted by molten steel processing equipment equipped with an electrode arc heating function, and a 50 kg square mold (main body: about □ 120 x 450 mm) is used. And cast. Table 3 also shows the result of calculating the PRE value for each steel structure. In Table 3, a blank indicates that the corresponding component is not contained. The solidified steel ingot was heated to 1200 ° C. and hot forged at the same temperature to finish a forged steel product of 600 × 120 × 60 mm. Then, it cut | disconnected and hold | maintained at 1100 degreeC for 30 minutes as heat processing, and water-cooled.

(Collecting test materials Nos. 1 to 17)
Next, crevice corrosion resistance was evaluated by the following procedure using a sample (20 mm × 30 mm × 2 mmt) collected from the forged steel product in parallel with the processing direction.

(Evaluation of crevice corrosion resistance)
The evaluation of crevice corrosion resistance was performed by immersing a test piece provided with a gap in 6% FeCl ₃ + 0.05N HCl for 24 hours in accordance with ASTM G48 Method F, and measuring the maximum crevice corrosion depth after the test. evaluated. The test temperature was 60 ° C. For evaluation of crevice corrosion resistance, the maximum crevice corrosion depth was judged to be excellent when it was less than 200 μm, then good when it was 200 μm or more and less than 400 μm, and poor when it was 400 μm or more. The results are shown in Table 3.

(Component composition)
The component composition was measured by the following method. C, S; infrared absorption method, Si, Mn, P, Ni, Cr; fluorescent X-ray analysis method; Mo, Sn, Ge, Ta; ICP analysis method, S, N; inert gas melting method. The measurement site | part of a test material will not be specifically limited if a measurement is possible. In Table 3, compositions that do not satisfy the provisions of the present invention are indicated by underlining the numerical values.

It can be seen that all of the stainless steel materials (test materials No. 1 to 12) having composition symbols A1 to A12 that satisfy the component composition that is a requirement of the present invention have good crevice corrosion resistance.

On the other hand, composition symbols B1 to B5 (test materials No. 13 to 17) have the following problems.

In B1, a large amount of Ge was added, the σ phase increased, and the crevice corrosion resistance deteriorated. In B2, since no Ge was added, the passive film was unstable, and the crevice corrosion resistance was deteriorated. Since B3 and B4 each contain a large amount of S and Mn, a large number of Mn sulfides were precipitated, and the crevice corrosion resistance was deteriorated. B5 had a small amount of N, and the crevice corrosion resistance was deteriorated.

[Example 3: Example in which Ta-containing steel is subjected to control of sulfur-oxide composite inclusions]
(Production of steel)
Steel with the composition shown in Table 4 (steel symbols: A1 to A16, B1 to B9) is melted by molten steel processing equipment equipped with an electrode arc heating function, and a 50 kg round mold (main body: about φ140 × 320 mm) is prepared. Used to cast. Table 4 also shows the calculation results of PRE = [Cr] +3.3 [Mo] +16 [N] for each steel. In the component composition column of Table 4, a blank indicates that the corresponding component is not contained, and the balance is Fe and inevitable impurities.

The solidified steel ingot was heated to 1200 ° C., subjected to hot forging (forging temperature: 1000 to 1200 ° C.) at the same temperature, and then cut. Next, cold rolling and a solution heat treatment at 1100 ° C. for 30 minutes were performed, and after cooling with water at a cooling rate of 12 ° C./second, the steel was cut into 300 × 120 × 10 mm steel materials (No. 1 to 25).

For steel (steel symbols: A11 to A16), the deoxidation process at the time of melting was stronger than usual in order to reduce the amount of O. For steel materials (Nos. 1 to 16, 18 to 25), hot forging at 1100 to 1200 ° C. was performed at a processing ratio exceeding 50% of the total processing ratio of hot forging. For steel (No. 17), hot forging at 1100 to 1200 ° C. was performed at a processing ratio of 50% or less of the total processing ratio of hot forging.

(Sample collection)
Next, using a sample (20 mm × 30 mm × 2 mmt) taken from the steel material in parallel with the processing direction, the number density and Ta content of the sulfur / oxide composite inclusions are measured by the following procedure. The pitting corrosion resistance and hot workability were evaluated. The results are shown in Table 5.

Further, the sample was embedded in a cross section perpendicular to the processing direction, mirror-polished, subjected to electrolytic etching in an aqueous oxalic acid solution, and then observed with an optical microscope at a magnification of 100 to observe the structure of each sample. As a result, all samples consisted of two phases of a ferrite phase and an austenite phase.

(Measurement of number density and Ta content of sulfur / oxide composite inclusions)
The major axis (equivalent circle diameter), number density and Ta content of inclusions can be measured by the following procedure. That is, with respect to the sample used for the tissue observation, the surface of the sample was subjected to SEM-EPMA (scanning electron microscope-electron probe microanalyzer, “JXA-8900RL”, “XM-Z0043T”, “XM-Z0043T”, “ XM-87562 "), and the composition of the observed inclusions is analyzed by EDX (energy dispersive X-ray detector). The component composition analysis by EDX may be performed for inclusions having a major axis of 1 μm or more, and the center of gravity of the inclusions may be automatically analyzed in about 10 seconds per point. Inclusions whose major axis is less than 1 μm have a low adverse effect on local corrosion resistance. Therefore, in the present invention, in order to improve the measurement efficiency, inclusions whose major axis is less than 1 μm are excluded from the measurement target.

For the measurement of the number density and Ta content of the sulfur / oxide composite inclusions, the sulfide inclusions having a major axis of 1 μm or more observed in an automatic EPMA according to the above procedure and a measurement area of 3 mm ² and For the oxide inclusions, the number density and the Ta content of each inclusion were measured and determined as the average value.

(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 and subjected to ultrasonic cleaning, and then a lead wire was attached to the sample by spot welding, and the portion other than the test surface (10 mm × 10 mm) portion of the sample surface was coated with an epoxy resin. After immersing the sample in a 20% NaCl aqueous solution maintained at 80 ° C. for 10 minutes, 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 pitting corrosion. A potential (V _C ' ₁₀₀ ) was used. Pitting corrosion resistance was evaluated as good (◯) when the pitting potential exceeded 500 mV (vs. SCE (saturated calomel electrode)), slightly poor (Δ), 100 mV (100 mV (vs. SCE)). (vs. SCE) was evaluated as defective (x).

(Evaluation of hot workability)
The surface of the sample was visually observed, and the presence or absence of surface defects (◎: no defects, ○: slight defects, Δ: frequent defects, x: occurrence of cracks) was observed. The results are shown in Table 5.

From the results of Tables 4 and 5, it can be seen that the examples (steel materials Nos. 1 to 16) satisfying the requirements of the present invention have excellent pitting corrosion resistance and hot workability.

In contrast, steel No. About 17-25, it was the following situation.

In the reference example (steel material No. 17), the component composition satisfies the requirements of the present invention, but the processing ratio of hot forging at 1100 to 1200 ° C. is 50% or less of the total processing ratio. The Ta content of the oxide-based composite inclusion is less than the lower limit, and is superior in pitting corrosion resistance as compared with conventional steels that do not contain Ta. Compared with 1 to 16, the properties were slightly inferior because no sulfur / oxide composite inclusions were formed.

The comparative example (steel material No. 18) was inferior in pitting corrosion resistance because Ta was not contained, and the sulfur / oxide composite inclusions were not modified. In the comparative example (steel material No. 19), since the Ta content exceeded the upper limit, the sulfur / oxide composite inclusion was modified, but the number density exceeded the upper limit, and at the same time coarse nitrides were precipitated. The pitting corrosion resistance and hot workability were inferior.

In the reference example (steel material No. 20), since the O content exceeds 0.01% by mass, the Ta content of the sulfur / oxide composite inclusion is less than the lower limit, and the number density also exceeds the upper limit. At the same time, a large number of Cr-based oxides were deposited, so that the pitting corrosion resistance is superior to conventional steels not containing Ta. Compared with 1 to 16, the properties were slightly inferior because no sulfur / oxide composite inclusions were formed.

Comparative Example (steel material No. 21) was inferior in pitting corrosion resistance because the Cr content was less than the lower limit.

In the comparative example (steel material No. 22), since the S content exceeds the upper limit, the Ta content of the sulfur / oxide composite inclusion is less than the lower limit, and at the same time, a large number of sulfide inclusions (MnS) are precipitated. The pitting corrosion resistance and hot workability were inferior. In the comparative example (steel material No. 23), since the Mn content exceeded the upper limit, the precipitation suppression of MnS was insufficient, and the pitting corrosion resistance and hot workability were inferior.

Comparative Example (steel material No. 24) was inferior in pitting corrosion resistance because the Mo content was less than the lower limit. The comparative example (steel material No. 25) was inferior in pitting corrosion resistance because the N content was less than the lower limit.

As described above, the duplex stainless steel material and duplex stainless steel pipe of the present invention have been described. However, the present invention is not limited by the embodiments and examples, and is appropriately within a range that can meet the spirit of the present invention. It is also possible to carry out with modification, and they are all included in the technical scope of the present invention.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is a Japanese patent application filed on January 15, 2013 (Japanese Patent Application No. 2013-004891), a Japanese patent application filed on March 5, 2013 (Japanese Patent Application No. 2013-043250), and an application filed on November 5, 2013. This is based on a Japanese patent application (Japanese Patent Application No. 2013-229754), the contents of which are incorporated herein by reference.

The duplex stainless steel material of the present invention is useful as a structural material for 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. .

Claims (5)

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: 0.100% by mass or less,
Si: 0.10 to 2.00% by mass,
Mn: 0.10 to 2.00% by mass,
P: 0.050 mass% or less,
S: 0.0100% 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% by mass,
N: 0.20 to 0.50 mass%,
Furthermore, it contains at least one selected from Ta: 0.01 to 0.50 mass% and Ge: 0.1 to 1.0 mass%, with the balance being Fe and inevitable impurities. Duplex stainless steel material. When the Cr content (% by mass) is [Cr], the Mo content (% by mass) is [Mo], and the N content (% by mass) is [N], the PRE value represented by the following formula is 40 The duplex stainless steel material according to claim 1, which is as described above.
PRE = [Cr] +3.3 [Mo] +16 [N] Sulfur / oxide system containing Ta, containing O as an impurity, limited to 0.01% by mass or less, and containing Ta having a major axis of 1 μm or more among inclusions of the duplex stainless steel material ^2. The duplex stainless steel material according to claim 1, wherein the composite inclusions are 500 or less per 1 mm ^{2 in} cross section perpendicular to the processing direction, and the Ta content of the sulfur / oxide composite inclusions is 5 atomic% or more. . The component composition is further Co: 0.10 to 2.00% by mass, Cu: 0.10 to 2.00% by mass, V: 0.01 to 0.50% by mass, Ti: 0.01 to 0.00%. Contains one or more selected from the group consisting of 50% by mass, Nb: 0.01 to 0.50% by mass, Mg: 0.0005 to 0.020% by mass, and Ca: 0.0005 to 0.020% by mass The duplex stainless steel material according to any one of claims 1 to 3, wherein: A duplex stainless steel pipe comprising the duplex stainless steel material according to any one of claims 1 to 3.