JP2015110828A - Duplex stainless steel material and duplex stainless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a duplex stainless steel material and duplex stainless steel tube, having excellent corrosion resistance and also hot workability in an environment including corrosive substances, such as chloride, hydrogen sulfide and carbon dioxide.SOLUTION: The duplex stainless steel material comprises a ferrite phase and an austenite phase. The component composition of the duplex stainless steel material contains predetermined amounts of C, Si, Mn, P, S, Al, Cr, Ni, Mo, N, Ta, O and the remainder consisting of Fe and inevitable impurities. Sulfide/oxide based composite inclusions including Ta and having a major axis of 1 μm or more out of the inclusions of the duplex stainless steel material are 500 or less pieces per 1 mmof a cross section vertical to a processing direction. The Ta content of the sulfide/oxide based composite inclusions is 5 atom% or more.

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 in structural materials for seawater environments such as umbilicals, seawater desalination plants, and LNG vaporizers, as well as in highly corrosive environments such as oil well pipes and various chemical plants. It is used as

  However, when a corrosive substance such as chloride (chloride ion) is contained in a large amount in the usage environment, 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 parts that form structural gaps such as pipes and flanges of duplex stainless steel materials, corrosive substances such as chloride ions are concentrated inside the gap, resulting in a more severe corrosive environment. In some cases, an oxygen concentration cell is formed between them, and local corrosion inside the crevice is further promoted, so-called crevice corrosion may occur. Furthermore, local corrosion such as pitting corrosion and crevice corrosion is often 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 addition, in oil well pipe materials used for drilling of oil and natural gas, development of deeper oil wells and gas wells has been progressing in recent years, and at a higher temperature than before, hydrogen sulfide, carbon dioxide, chlorides. In many cases, it is exposed to an environment containing a large amount of corrosive substances such as the above, and therefore, 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]. ], A pitting corrosion index PRE (Pitting Resistance Equivalent) calculated by “[Cr] +3.3 [Mo] +16 [N]” or “[Cr] +3.3 ([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. The content of Cr, Mo, N (or W) is 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 value 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, Ta, etc. in addition to controlling the contents of Cr, Mo, W, and N. ing.

Further, Non-Patent Document 1 experimentally shows that MnS, which is an inclusion in steel, is the starting point of local corrosion (pitting corrosion) in stainless steel. Further, in Patent Document 3, 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 added in a vacuum melting furnace. The amount of S is reduced to 3 ppm or less.

  In Patent Document 4, 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, S content is controlled, and further interposed A duplex stainless steel having an adjusted form and density is disclosed. And in patent document 4, 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 a 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.

JP-A-5-132741 JP-A-8-170153 JP-A-3-291358 International Publication No. 2005/014872 Pamphlet

Muto Izumi et al, Ferum Vol. 17 (2012), no. 12,858-

  While duplex stainless steel is excellent in strength characteristics, it is often more difficult to process such as rolling and drawing than ordinary stainless steel. In addition, in order to apply duplex stainless steel in severe corrosive environments containing a large amount of hydrogen sulfide, carbon dioxide and chloride ions, such as deep oil wells that have been developed in recent years, it is necessary to improve corrosion resistance. 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 workability are deteriorated. In order to suppress local corrosion starting from inclusions, it is necessary to control the amount of S and O in the steel, but there is a limit from the industrial viewpoint in reducing these.

  And in patent document 1, it cannot necessarily be said that sufficient corrosion resistance is always securable in the severe corrosive environment calculated | required nowadays. 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.

  Further, in Patent Document 3, 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 occurrence temperature is 35 ° C. or more is excellent in corrosion resistance. This is considered insufficient for use in severe corrosive environments.

  Further, in Patent Document 4, 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 invention described in Patent Document 4 Is basically a direction to reduce the inclusions that become the conventional pitting corrosion starting point, and excessively reducing O and S that are the source of formation thereof is industrially expensive and expensive.

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

  As described above, the stainless steel material is a material that exhibits corrosion resistance by a passive film mainly composed of Cr oxide on the surface of the steel material. Since duplex stainless steel is generally composed of a ferrite phase and an austenite phase, it has a discontinuity at the interface between these different phases and is inevitably formed in the interface between the ferrite phase and the austenite phase. Since the continuity of the passive film decreases at the interface between the inclusions (oxides, sulfides) and the base metal, the tendency to become unstable is strong, so the chloride ion has a destructive effect on the passive film. Susceptible to local corrosion. The inventors of the present invention have intensively studied the inclusions that cause local corrosion.

  In particular, MnS is a typical example of inclusions in steel that adversely affect the properties and corrosion resistance of steel materials. MnS is more soluble in water than other oxide inclusions as described in Non-Patent Document 1. It is known that it is likely to become a starting point of local corrosion because it is high and easily eluted. However, since Mn is an austenite forming element and has an effect of increasing the strength of the steel material, it must be contained in a certain amount. Further, since S is contained as an impurity element in steel, the content is preferably as low as possible. However, as described above, the reduction has an industrial limit. For this reason, the present inventors have conceived of detoxifying inclusions (oxides and sulfides) that become the starting point of local corrosion without reducing Mn and S in the steel. As a result, the corrosion resistance is improved by modifying the inclusions by appropriately controlling the addition of Ta and the amount of O in the steel, and taking appropriate production conditions (heating temperature, cooling rate, rolling conditions, etc.). I found that it could be improved.

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: 0.10% 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%, Cr : 22.0 to 28.0 mass%, Ni: 1.0 to 10.0 mass%, Mo: 2.0 to 6.0 mass%, N: 0.2 to 0.5 mass%, Ta: 0 0.01 to 0.50% by mass, O: 0.030% by mass or less, with the balance being Fe and inevitable impurities, and among the inclusions of the duplex stainless steel material, Ta having a major axis of 1 μm or more vulcanization-oxide composite inclusions containing found 500 cross section perpendicular 1 mm ² per the working direction A lower, wherein the Ta content of the vulcanization-oxide composite inclusions is 5 atomic% or more.

  As described above, the duplex stainless steel material contains a predetermined amount of C, Si, Mn, P, S, Al, Cr, Ni, Mo, N, Ta, and O, thereby improving corrosion resistance and heat. A decrease in hot workability is suppressed. By setting C to a predetermined value or less, unnecessary carbides are not formed, and there is an effect of suppressing a decrease in corrosion resistance. Si, Mn, and Al are effective for deoxidation. By making P and S below a predetermined value, there exists an effect which suppresses the fall of corrosion resistance and hot workability. In particular, S causes formation of MnS that impairs corrosion resistance and toughness. Cr, Mo, and N are effective in improving pitting corrosion resistance. Ni is effective in improving the corrosion resistance and stabilizing the austenite phase. Ta has the effect of modifying the sulfide inclusions that are the starting point of pitting corrosion into Ta-containing sulfur / oxide composite inclusions. And by defining the major axis, number density and Ta content of the sulfur / oxide composite inclusions containing Ta, local corrosion resistance and hot workability are improved.

  In the duplex stainless steel material according to the present invention, the component composition is further Cu: 0.1 to 2.0 mass%, Co: 0.1 to 2.0 mass%, V: 0.01 to 0.5 mass%. %, Ti: 0.01 to 0.5 mass%, Nb: It is preferable to contain one or more selected from the group consisting of 0.01 to 0.5 mass%.

  As described above, the duplex stainless steel material further improves the corrosion resistance by further containing one or more selected from the group consisting of a predetermined amount of Cu, Co, V, Ti, and Nb. Cu and Co are effective in improving the corrosion resistance and stabilizing the austenite phase. V, Ti, and Nb are effective in improving corrosion resistance, strength characteristics, and workability.

  In the duplex stainless steel material according to the present invention, the component composition may further contain one or two of Mg: 0.0005 to 0.020 mass% and Ca: 0.0005 to 0.020 mass%. preferable.

  As described above, the duplex stainless steel material further improves the hot workability by further containing a predetermined amount of one or two of Mg and Ca. Ca and Mg are effective in improving hot workability by suppressing S and O contained as impurities in steel and segregating at the grain boundaries.

The duplex stainless steel pipe according to the present invention is made of the duplex stainless steel material described above.
As described above, in the duplex stainless steel pipe, by forming the steel plate with a duplex stainless steel material, inclusions that are the starting point of local corrosion are modified, and the corrosion resistance is improved.

  The duplex stainless steel material of the present invention exhibits excellent corrosion resistance and excellent hot workability in an environment containing corrosive substances such as chloride, hydrogen sulfide, and carbon dioxide. In addition, according to the duplex stainless steel pipe of the present invention, since excellent corrosion resistance is expressed, structural materials for seawater environments such as umbilicals, seawater desalination plants, LNG vaporizers, oil wells, various chemical plants, etc. It can be used as a structural material for corrosive environments.

<Duplex stainless steel>
An embodiment of the duplex stainless steel material according to the present invention will be described.
The duplex stainless steel material of the present invention is a duplex stainless steel material comprising a ferrite phase and an austenite phase, and the component composition of the duplex stainless steel material is a predetermined amount of C, Si, Mn, P, S, Al, Cr. , Ni, Mo, N, Ta, and O, with the balance being Fe and inevitable impurities.
Moreover, it is preferable that a duplex stainless steel material further contains 1 or more types by which a component composition is chosen from the group which consists of predetermined amount Cu, Co, V, Ti, and Nb.
Furthermore, it is preferable that the duplex stainless steel material further contains one or two kinds of Mg and Ca having a predetermined composition.
The duplex stainless steel material is characterized in that, among the inclusions in the steel material, the sulfur / oxide composite inclusions having a predetermined Ta content and a major axis have a predetermined number density.
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 concentrate in the ferrite phase, and austenite phase stabilizing elements such as Ni and N tend to concentrate in the austenite phase. is there. 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 ratio of the ferrite phase to the austenite phase, and 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 of 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 ratio of the ferrite phase and the austenite phase is preferably 95% or more, and more preferably 97% or more with respect to the total phase (total structure) of the steel material.

The numerical range of the component composition of the duplex stainless steel material and the reason for the limitation will be described.
(C: 0.10 mass% or less)
C is a harmful element that reduces the corrosion resistance by forming carbides with Cr or the like in the steel material. Therefore, C content shall be 0.10 mass% or less. In addition, since it is better that the C content is as small as possible, it is preferably 0.08% by mass or less, 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, preferably 0.15% by mass or more, more preferably 0.2% by mass or more. However, since the workability deteriorates when Si is excessively contained, the Si content is 2.0% by mass or less, preferably 1.5% by mass or less, 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, preferably 0.15% by mass or more, and more preferably 0.20% by mass or more. However, if Mn is excessively contained, coarse MnS is formed and the corrosion resistance is deteriorated. Further, when Mn is excessively contained, the formation of oxide inclusions is promoted and hot workability is deteriorated. Therefore, the Mn content is 2.0% by mass or less, preferably 1.5% by mass or less. More preferably, the content is 1.0% by mass or less.

(P: 0.05% by mass or less)
P is an element that is inevitably mixed in as an impurity, is harmful to corrosion resistance, and deteriorates weldability and workability. Therefore, the P content is 0.05% by mass or less. The P content is preferably as small as possible, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less. In addition, although P may not be contained in the steel material, that is, 0% by mass, excessive reduction of the P content causes an increase in manufacturing cost. The lower limit is about 0.010% by mass.

(S: 0.01% by mass or less)
S, like P, is inevitably mixed as an impurity, and combines with Mn to form sulfide inclusions (MnS), thereby deteriorating corrosion resistance and hot workability. And when S is contained excessively, the modification | reformation by Ta of a sulfur and oxide type composite inclusion will become inadequate, and corrosion resistance will fall. Therefore, the S content is 0.01% by mass or less, preferably 0.005% by mass or less, more preferably 0.003% by mass or less. In addition, as described in the background art, S is preferably as low as possible, and S is not contained in the steel material, that is, it may be 0% by mass, but the S content is excessively reduced. Causes an increase in production cost, so that there is no problem even if the S content exceeds 0.001% by mass with appropriate control of the Ta content and the O content.

(Al: 0.001 to 0.05 mass%)
Al is a deoxidizing element 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, so the Al content is 0.05% by mass or less, preferably 0.02% 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 an element that stabilizes the ferrite phase. Therefore, 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 22.0% by mass or more, preferably 23.0% by mass or more, more preferably It shall be 24.0 mass% or more. However, if Cr is excessively contained, workability is deteriorated, so the Cr content is 28.0% by mass or less, preferably 27.5% by mass or less, more preferably 27.0% 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, preferably 2.0% by mass or more, 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, preferably 9.5% by mass or less, more preferably 9%. 0.0 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 an element that stabilizes the ferrite phase, and has the effect of improving 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 mass% or more, preferably 2.2 mass% or more, more preferably 2.5 mass% or more. However, if Mo is excessively contained, the formation of intermetallic compounds such as σ phase is promoted, and the corrosion resistance and hot workability are reduced. Therefore, the Mo content is 6.0 mass% or less, preferably 5. It is 5 mass% or less, More preferably, it is 5.0 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 N is an element effective 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, preferably 0.22% by mass or more, more preferably 0.25% by mass or more. However, if 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, preferably 0.45% by mass or less, more preferably 0.40% by mass or less.

(Ta: 0.01 to 0.50 mass%)
Ta is an element that improves corrosion resistance by modifying sulfide-based inclusions (MnS) that adversely affect corrosion resistance to sulfur-oxide-based complex inclusions containing Ta. Ta is an element that suppresses the formation of Cr-based oxides by combining with O, and has an effect of contributing to a substantial improvement in Cr concentration of the steel material. In order to obtain such an effect, the Ta content is set to 0.01% by mass or more, preferably 0.02% by mass or more, more preferably 0.03% by mass or more. However, if Ta is excessively contained, it will precipitate as a nitride by combining with N in the steel, reducing toughness and hot workability, reducing the effective concentration of N, and reducing the corrosion resistance. To do. In addition, a large number of sulfur-oxide composite inclusions modified with Ta are deposited, which deteriorates hot workability. Therefore, the Ta content is 0.50% by mass or less, preferably 0.40% by mass or less, more preferably 0.30% by mass or less.

(O: 0.030% by mass or less)
O is an impurity mixed during melting, and is an element that precipitates as an oxide in the steel by combining with a deoxidizing element such as Si or Al, and lowers the workability and corrosion resistance of the duplex stainless steel. And if O is contained excessively, the modification of the sulfur / oxide composite inclusions with Ta becomes insufficient, and a large number of sulfur / oxide composite inclusions precipitate, so that the corrosion resistance and hot workability are increased. Decreases. Therefore, the O content is 0.030% by mass or less, preferably 0.028% by mass or less, more preferably 0.025% by mass or less, and further preferably 0.024% by mass or less. The lower the O content, the better. However, excessive reduction of O leads to an increase in cost, so the lower limit is about 0.0005% by mass.

(Cu: 0.1-2.0 mass%, Co: 0.1-2.0 mass%, V: 0.01-0.5 mass%, Ti: 0.01-0.5 mass%, Nb : One or more selected from the group consisting of 0.01 to 0.5% by mass)
Cu and Co are elements that improve the corrosion resistance and stabilize the austenite phase. Therefore, the Cu content and the Co content are each 0.1% by mass or more, preferably 0.2% by mass or more. However, when these elements are contained excessively, the hot workability deteriorates, so the Cu content and the Co content are each 2.0% by mass or less, preferably 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, the V content, the Ti content, and the Nb content are each 0.01% by mass or more, preferably 0.05% by mass or more. However, when these elements are contained excessively, coarse carbides and nitrides are formed and the toughness is deteriorated. Therefore, the V content, the Ti content, and the Nb content are each 0.5% by mass or less, preferably 0.4% by mass or less. Further, the total content of Cu, Co, V, Ti, and Nb is preferably 0.02 to 1.00% by mass in consideration of corrosion resistance and hot workability.

(Mg: 0.0005 to 0.020 mass%, Ca: 0.0005 to 0.020 mass%, one or two)
Mg and Ca are elements that improve the local corrosion resistance by suppressing the formation of MnS that is likely to be a starting point of local corrosion by combining with S contained as an impurity in steel. Mg and Ca are elements that combine with S and O in steel to suppress the segregation of these inclusions at grain boundaries and improve hot workability. In order to obtain such an effect, the Mg content and the Ca content are set to 0.0005 mass% or more, preferably 0.0020 mass% or more. However, when these elements are excessively contained, the oxide inclusions increase, and the corrosion resistance and workability deteriorate. Therefore, Mg content and Ca content shall be 0.020 mass% or less. In addition, the total content of Mg and Ca is preferably 0.001 to 0.02% by mass in consideration of corrosion resistance and hot workability.

(Fe and inevitable impurities)
The basic components of the component composition constituting the duplex stainless steel material are as described above, and the remaining components are Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed during melting, and are contained within a range that does not impair various properties of the steel material.
Moreover, in addition to the said component, you may actively contain another element in the component composition of steel materials in the range which does not have a bad influence on the effect of the steel materials of this invention.

(Sulfur / oxide composite inclusions)
In the present invention, by adding and refining Ta, sulfide inclusions (MnS) contained in ordinary stainless steel are modified to sulfur / oxide composite inclusions containing Ta. Further, the local corrosion resistance is improved by the sulfur / oxide composite inclusions containing Ta.

  For this 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%.

Further, even if the inclusions are modified by 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 . And the fine inclusion whose major axis is less than 1 μm is excluded from the object because it has 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

In the duplex stainless steel material according to the present invention, the component composition is such that the Cr content (Cr content) is [Cr], the Mo content (Mo content) is [Mo], and the N content (N content) is [N]. It is preferable that [Cr] +3.3 [Mo] +16 [N] ≧ 40.
[Cr] +3.3 [Mo] +16 [N] is a pitting resistance index (PRE) that is conventionally known as an index representing the corrosion resistance of steel. In the present invention, 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.

<Method for producing duplex stainless steel>
The duplex stainless steel material of the present invention can be manufactured by a manufacturing facility and a manufacturing method used for mass production of ordinary stainless steel. In order to reduce O as an impurity in steel, deoxidation is performed by adding a large amount of elements having high affinity with O, such as Si and Al, and further, secondary degassing such as vacuum degassing and argon gas stirring is performed. Oxide inclusions are removed by increasing the refining time or by performing the refining time a plurality of times.

  For example, a molten steel melted in a converter or an 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. Moreover, the total processing ratio (cross-sectional area of the original steel ingot / cross-sectional area after processing) at the time of hot working is set to about 10 to 50 as usual. Here, in order to obtain a desired state of sulfur-oxide composite inclusions containing Ta, during hot working, the working ratio in the temperature range of 1100 to 1200 ° C. (cross-sectional area before working / It is preferable to perform hot working so that the cross-sectional area after working) becomes a working ratio exceeding 50% of the total working ratio.

  In the production of the conventional duplex stainless steel material, the steel ingot is hot-worked in a temperature range of about 1000 to 1200 ° C., but no particular control is performed. For this reason, 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. due to the influence of the temperature drop during processing. 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, as described above, the existence state of the desired sulfur / oxide composite inclusion containing Ta can be obtained by deliberately increasing the processing ratio in the temperature range of 1100 to 1200 ° C.

  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 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.

<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 is made of the duplex stainless steel material, and can be manufactured by a manufacturing facility and a manufacturing method used for mass production of a normal stainless steel pipe. For example, the desired dimensions can be obtained by an extruded pipe or Mannesmann pipe made of a round bar, or a weld 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 can also be used for seawater desalination plants, LNG vaporizers, and the like.

Hereinafter, the present invention will be described in more detail with reference to examples.
(Production of steel)
By using molten steel processing equipment equipped with an electrode arc heating function, steels having the composition shown in Table 1 (steel symbols: A1 to A23, B1 to B11) are respectively melted, and a 50 kg round mold (main body: about φ140 × 320 mm) is prepared. Used to cast. Table 1 also shows the calculation results of PRE = [Cr] +3.3 [Mo] +16 [N] for each steel. In the component composition column of Table 1, 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 held 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 34).

  In addition, about steel (steel symbol: A11-A23), in order to make it low O amount, the deoxidation process at the time of melting was performed more strongly than usual. Moreover, about steel materials (No. 1-23, 25-34), the hot forging in 1100-1200 degreeC was performed by the processing ratio exceeding 50% of the total processing ratio of hot forging. For steel (No. 24), 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 2.

  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”, “JM Corporation”, “ XM-87562 "), and the composition of the observed inclusions is analyzed with an 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 measurement efficiency, inclusions whose major axis is less than 1 μm are excluded from the measurement target.

Regarding 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 observed in 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)
The pitting corrosion resistance was evaluated with reference to the method described in JIS G 0577: 2005. 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. The evaluation of pitting corrosion resistance is good (◯) when the pitting potential exceeds 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 2.

From the results of Tables 1 and 2, it can be seen that Examples (Steel Nos. 1 to 23) satisfying the requirements of the present invention have excellent pitting corrosion resistance and hot workability.
On the other hand, the comparative example (steel materials No. 24-34) which does not satisfy the requirements of the present invention has the following problems.

  In the comparative example (steel No. 24), 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 composite inclusions was less than the lower limit, and the pitting corrosion resistance was poor.

Since the comparative examples (steel materials No. 25 and 26) did not contain Ta, the sulfur / oxide composite inclusions were not modified, and the pitting corrosion resistance was poor.
In the comparative example (steel material No. 27), since the Ta content exceeds the upper limit, the sulfur / oxide composite inclusions were 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.

The comparative example (steel material No. 28) had poor pitting corrosion resistance because the Cr content was less than the lower limit.
In the comparative example (steel material No. 29), 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. 30), 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.

In the comparative example (steel material No. 31), since the Mo content exceeded the upper limit, many σ phases were precipitated in the steel, and the pitting corrosion resistance and hot workability were inferior.
Since comparative example (steel material No. 32) had Mo content less than a minimum, it was inferior to pitting corrosion resistance.
Since the N content was less than the lower limit, the comparative example (steel material No. 33) was inferior in pitting corrosion resistance.
Since the O content exceeded the upper limit, the comparative example (steel material No. 34) was inferior in pitting corrosion resistance and hot workability.

  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.

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% by 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%,
Cr: 22.0-28.0 mass%,
Ni: 1.0-10.0 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,
Among the inclusions of the duplex stainless steel material, the sulfur / oxide composite inclusions containing Ta having a major axis of 1 μm or more is 500 or less per 1 mm ^{2 in} cross section perpendicular to the processing direction. A duplex stainless steel material characterized in that the Ta content of a physical composite inclusion is 5 atomic% or more.   The component composition is further Cu: 0.1-2.0% by mass, Co: 0.1-2.0% by mass, V: 0.01-0.5% by mass, Ti: 0.01-0. The duplex stainless steel material according to claim 1, comprising at least one selected from the group consisting of 5% by mass and Nb: 0.01 to 0.5% by mass.   The said component composition contains 1 type or 2 types of Mg: 0.0005-0.020 mass% and Ca: 0.0005-0.020 mass% further, The Claim 1 or Claim 2 characterized by the above-mentioned. 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.