JP6513495B2 - Duplex stainless steel and duplex stainless steel pipe - Google Patents

Description

  The present invention relates to a duplex stainless steel material and a duplex stainless steel pipe.

  Stainless steel is a material that naturally forms a stable surface film mainly composed of an oxide of Cr called a passive film in a corrosive environment to develop corrosion resistance. In particular, duplex stainless steels composed of a ferrite phase and an austenite phase are used as chemical plant members, heat exchangers, etc. as high strength and corrosion resistant materials. Furthermore, since duplex stainless steel is a component system of low Ni compared to austenitic stainless steel, it is a material that is low in cost and less susceptible to price fluctuations of metal raw materials.

  When the content of Cr is [Cr], the content of Mo is [Mo], the content of N is [N], and the content of W is [W], the pitting resistance of stainless steel is “[[ Cr] + 3.3 [Mo] + 16 [N] ”calculated pitting index PRE: Pitting Resistance Equivalent or, if including W,“ [Cr] + 3.3 ([Mo] + 0.5 [W] The pitting index PREW is calculated by: +16 [N] ”. It has started to be used as a substitute for SUS304 and SUS316L because it has advantages of high strength and low cost compared to austenitic stainless steel. In addition, all content of each above-mentioned element shows mass%, and what is shown as content by the following description shows mass%.

  For example, Patent Document 1 discloses a duplex stainless steel which is excellent in cost by reducing the addition amounts of Ni and Mo in an attempt to substitute SUS304.

  In addition, Non-Patent Document 1 experimentally shows that in stainless steel, MnS of inclusions in the steel is a starting point of local corrosion.

  Further, Patent Document 2 discloses a duplex stainless steel in which the ratio of the amount of ferrite to the amount of austenite is adjusted by increasing the amount of addition of Mn.

  Further, Patent Document 3 discloses a duplex stainless steel in which V nitride is precipitated by addition of V to improve HAZ toughness. However, since V acts as an α-phase stabilizing element, it is assumed that the α fraction of the welded part is increased by promoting the precipitation of the σ phase, which lowers the corrosion resistance and the toughness.

JP, 2009-35782, A JP, 2006-193823, A WO2009 / 119885 Izumi Muto et al., Vol. 17 (2012), no. 12, pp. 858-863

  While duplex stainless steels are excellent in strength characteristics, they are often more difficult to process such as rolling and drawing than ordinary stainless steels. Moreover, in order to suppress local corrosion originating from inclusions, it is necessary to control the amount of S and O in the steel, but there is a limit from the industrial point of view to reduce these.

Moreover, when welding with respect to duplex stainless steel materials, corrosion resistance of a welding part may fall rather than a base material by sensitization. In addition, with sensitization, elements such as Cr and Mo contained in the steel due to heat history form precipitates such as Cr ₂ N in the weld heat affected zone, and the Cr concentration in the base material is lowered, which leads to corrosion resistance. Is a phenomenon that decreases.

  The present invention has been made to solve the above-mentioned conventional problems, and its main object is, for example, a duplex stainless steel material which exhibits excellent corrosion resistance in an environment containing a corrosive substance such as chloride and a two-phase stainless steel. It is to provide a duplex stainless steel pipe.

  Another object of the present invention is to provide a duplex stainless steel material and a duplex stainless steel pipe which exhibit excellent corrosion resistance in a welding heat affected zone when welding is performed.

  Another object of the present invention is to provide a duplex stainless steel material and duplex stainless steel pipe which exhibit corrosion resistance with excellent workability.

  Since a duplex stainless steel is generally composed of a ferrite phase and an austenite phase, it has discontinuities at the interface of these different phases. Therefore, the continuity of the passive film is reduced at the interface between the ferrite phase and the austenite phase or at the interface between inclusions such as oxides and sulfides inevitably formed in the steel and the base metal. And passive films tend to be unstable. As a result, it becomes susceptible to the destructive action of the passive film by chloride ions, and local corrosion tends to occur.

  Therefore, the present inventors focused attention on enhancing the stability and protection of the passive film of duplex stainless steel within the range that does not impair the manufacturing aspect and various properties, and the intervention that causes these local corrosion I made a thorough study of things.

  In particular, sulfide inclusions such as MnS can be mentioned as typical ones that adversely affect the properties of steel materials and corrosion resistance as inclusions in steel. For example, as described in Non-Patent Document 1, it is known that MnS is likely to be a starting point of local corrosion because it has high water solubility and is easily eluted as compared with other oxide inclusions. However, since Mn is an austenite-forming element and has the effect of enhancing the strength of the steel material, it must be contained in a certain amount. Moreover, since S is contained as an impurity element in steel, it is preferable that the content be as low as possible, but as described above, there is an industrial limit in the reduction of S. Therefore, the inventors of the present invention have conceived of a method of rendering harmless inclusions such as oxides and sulfides which become a starting point of local corrosion without reducing Mn and S in steel.

  As a result, by adding at least one of Ta, REM or Zr, and appropriately controlling the amount added, MnS is reformed or dispersed into highly resistant acid sulfide, The inventors have found that corrosion resistance can be improved by reducing the influence of MnS on corrosion resistance, and the present invention has been completed.

  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 in mass%, C: 0 to 0.05%, Si: 0.1 to 2.0%, Mn : 0.1 to 2.0%, P: more than 0% 0.05% or less, S: more than 0% 0.004% or less, Al: more than 0% 0.05% or less, Ni: 2.0 to 6 .0%, Cr: 19.0 to 25.0%, Mo: 0% to 1.0% or less, N: 0.01 to 0.20%, V: 0 to 0.01% Ta: 0.01 to 0.30%, REM: 0.0005 to 0.07%, Zr: at least one of 0.01 to 0.10%, and the balance is iron and unavoidable impurities It is characterized by

  As described above, the duplex stainless steel material according to the present invention contains predetermined amounts of C, Si, Mn, P, S, Al, Ni, Cr, Mo, N, V, and at the same time, Ta, REM, Zr While containing any at least 1 sort (s), while corrosion resistance improves, the fall of 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 drop in corrosion resistance. Si, Mn and Al have an effect of enhancing hot working by deoxidation. However, if the value of Mn is large, precipitation of MnS which promotes corrosion resistance and workability is promoted. By setting P to a predetermined value or less, there is an effect of improving processability and corrosion resistance. S has an effect of suppressing the decrease in corrosion resistance and hot workability by setting it to a predetermined value or less, and in particular, by suppressing the value of S, it is possible to reduce the formation of MnS that impairs corrosion resistance and toughness. it can. Cr, Mo and N are effective in improving the pitting resistance. Ni is effective in improving the corrosion resistance and stabilizing the austenitic phase. Ta and REM have the effect of reforming sulfide-based inclusions, which are the starting point of pitting corrosion, into oxysulfide-based composite inclusions that are less likely to be the starting point of pitting corrosion. In addition, it is preferable not to contain V as much as possible.

  The duplex stainless steel material of the present invention preferably contains, by mass%, Ta: 0.01 to 0.30%, and [Ta] / [Mn] is 0.02 or more. However, in each above-mentioned formula, [] shows mass%.

  As described above, by controlling [Ta] / [Mn], the sulfide-based inclusions that become the starting point of pitting corrosion are reformed into the Ta-containing oxysulfide-based composite inclusions that are less likely to be the starting point of pitting corrosion. The effect can be enhanced.

  Further, the duplex stainless steel material of the present invention preferably further contains at least one of Co: 0.1 to 1.0% and Cu: 0.1 to 2.0% by mass%.

  As described above, the duplex stainless steel material further improves the corrosion resistance by further containing at least one of Co and Cu in a predetermined amount. That is, Co and Cu are effective in improving the corrosion resistance and stabilizing the austenite phase.

  In addition, the duplex stainless steel material of the present invention further comprises, in mass%, B: 0.0005 to 0.010%, Mg: 0.0005 to 0.020%, Ca: 0.0005 to 0.020% It is preferable to contain at least one kind.

  As described above, in the duplex stainless steel material, the hot workability is further improved by further containing at least one of B, Mg, and Ca in a predetermined amount. B, Mg, and Ca are combined with S and O contained as impurities in the steel to suppress segregation at grain boundaries, and are effective in improving hot workability.

  The duplex stainless steel pipe of the present invention is characterized by being made of the duplex stainless steel described above.

  As described above, in the duplex stainless steel pipe, inclusions that are the starting point of local corrosion are reformed by forming the steel pipe from duplex stainless steel, and the corrosion resistance is improved.

  The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows.

  According to one embodiment of the present invention, a duplex stainless steel excellent in corrosion resistance can be provided. Moreover, according to one embodiment of the present invention, it is possible to provide a duplex stainless steel pipe that exhibits excellent corrosion resistance in an environment where local corrosion by chloride ions such as seawater heat exchangers is a problem.

It is a graph which shows the heat pattern of the heat processing which reproduced the thermal cycle of the welding part in TIG welding.

<Duplex stainless steel>
First, an embodiment of a duplex stainless steel material according to the present invention will be described. The duplex stainless steel is a duplex stainless steel composed of a ferrite phase and an austenite phase. The component composition of the duplex stainless steel contains predetermined amounts of C, Si, Mn, P, S, Al, Ni, Cr, Mo, N, and V, and at least one of Ta, REM, and Zr. And the balance is composed of Fe and unavoidable impurities. Furthermore, when the content of Ta is [Ta] and the content of Mn is [Mn], [Ta] / [Mn] is controlled within a predetermined range. Furthermore, the duplex stainless steel material preferably contains at least one of predetermined amounts of Co and Cu, and preferably further contains at least one of predetermined amounts of B, Mg, and Ca. Hereinafter, the duplex stainless steel material of the present invention will be described in the order of steel material structure, component composition, and manufacturing method.

(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 consisting 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 corrosion resistance such as Cr, Mo, Ni, N becomes large. Too much, either the ferrite phase or the austenite phase, which is inferior in corrosion resistance, is selectively corroded and the corrosion resistance tends to be deteriorated. Therefore, it is recommended to optimize the ratio between the ferrite phase and the austenite phase, and the area ratio of the ferrite phase to the austenite phase is preferably 30 to 70% from the viewpoint of corrosion resistance. The more preferable lower limit of the area ratio is 40%, and the preferable upper limit is 60%. The area ratio of the ferrite phase to the austenite phase can be optimized by adjusting the content of the ferrite phase stabilizing element and the austenite phase stabilizing element.

  Further, in the duplex stainless steel material of the present invention, in addition to the ferrite phase and the austenite phase, different phases such as σ phase and carbonitride of Cr can be tolerated to the extent that various properties such as corrosion resistance and mechanical properties are not impaired. 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 entire phase of the steel material, that is, the entire structure.

(Component composition)
Next, the numerical range of the component composition of the duplex stainless steel material of the present invention and the reason for limitation will be described in detail. In addition,% which is a display unit of a component composition means mass% altogether.

C: 0 to 0.05%
C is a harmful element which forms a carbide with Cr or the like in the steel to reduce the corrosion resistance. Therefore, the content of C is set to 0.05% or less. The content of C is preferably as small as 0.04% or less, more preferably 0.03% or less, because the content of C is preferably as small as possible. C may not be contained in the steel material, that is, it may be 0%.

Si: 0.1 to 2.0%
Si is an element necessary for deoxidation and stabilization of the ferrite phase. In order to obtain such an effect, the content of Si is 0.1% or more, preferably 0.15% or more, more preferably 0.2% or more. However, when the Si is excessively contained, the processability is degraded, so the content of Si is 2.0% or less, preferably 1.5% or less, more preferably 1.0% or less.

-Mn: 0.1 to 2.0%
Mn is an element necessary for securing the strength and stabilizing the austenitic phase. When the content of Mn is insufficient, the amount of austenitic phase in the weld heat affected zone is insufficient, N which can not be dissolved in the ferrite phase precipitates as Cr ₂ N, and the corrosion resistance is lowered. Therefore, the content of Mn is 0.1% or more, preferably 0.15% or more, and more preferably 0.2% or more. However, excessive inclusion of Mn promotes the formation of coarse MnS and promotes the formation of composite inclusions of Mn, Cr, O, and S, and suppresses the formation of Ta-containing oxysulfide, and the base material And, since the corrosion resistance and cold workability of the weld heat affected zone are deteriorated, the content of Mn is 2.0% or less, preferably 1.7% or less, more preferably 1.5% or less.

P: more than 0% and 0.05% or less P is an element which is unavoidably mixed as an impurity, is an element harmful to corrosion resistance, and is also an element which degrades weldability and processability. Therefore, the content of P is set to 0.05% or less. In addition, the content of P is preferably as small as possible, preferably 0.04% or less, and more preferably 0.03% or less. It is considered that P may not be contained in the steel material and may be 0% as long as it is acceptable, but P is also an element which is inevitably mixed as described above. In addition, since the excessive reduction of the content of P leads to an increase in manufacturing cost, the practical lower limit of the content of P is about 0.01%.

S: more than 0% and 0.004% or less S, as well as P, is inevitably mixed as an impurity, and combined with Mn etc. to form sulfide-based inclusions such as MnS, etc., corrosion resistance and hot working It is an element that degrades the And when S is contained excessively, modification | reformation by Ta addition to acid sulfide type composite inclusion becomes inadequate, and corrosion resistance falls. Furthermore, the hot workability is also reduced. Therefore, the content of S is set to 0.004% or less, preferably 0.003% or less, more preferably 0.001% or less. As described in the background art, S is preferably as low as possible, and may not be contained in the steel material, and it may be 0% if possible. As mentioned above, it is also an element which is inevitably mixed. Therefore, the practical lower limit of the S content is about 0.0004%.

Al: more than 0% and 0.05% or less Al is a deoxidizing element and is an element necessary to reduce the content of O and the content of S at the time of melting. However, excessive content of Al causes coarse oxide inclusions to adversely affect pitting resistance, so the content of Al is made 0.05% or less. Preferably it is 0.04% or less, More preferably, it is 0.03% or less. In the present invention, the lower limit of the content of Al is not particularly limited, but the lower limit is preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%.

Ni: 2.0 to 6.0%
Ni is an element necessary for improving the corrosion resistance, and is particularly effective in suppressing local corrosion in a chloride environment. Ni is also effective in improving low temperature toughness, and is also an element necessary for stabilizing the austenite phase. When the content of Ni is insufficient, a sufficient amount of austenite phase does not precipitate in the weld heat affected zone, so N which can not form a solid solution in the ferrite phase precipitates as Cr ₂ N, which lowers the corrosion resistance. Therefore, the content of Ni is set to 2.0% or more, preferably 2.5% or more, and more preferably 3.0% or more. However, since the addition of excessive Ni causes an increase in cost, the content of Ni is made 6.0% or less, preferably 5.0% or less, more preferably 4.0% or less.

Cr: 19.0 to 25.0%
Cr is a main component of the passive film, and is a basic element of corrosion resistance of stainless steel materials. Also, Cr is an element that stabilizes the ferrite phase. Therefore, in order to maintain the two-phase structure of ferrite and austenite and achieve both corrosion resistance and strength, the content of Cr is 19.0% or more, preferably 20.0% or more, more preferably 21.0%. And above. If the content of Cr is less than the lower limit value, a homogeneous passive film can not be formed, and the corrosion resistance of the base material and the heat affected zone of the weld greatly decreases. However, when Cr is excessively contained, the amount of ferrite phase in the welding heat affected zone increases, and N remaining in the ferrite phase during cooling precipitates as Cr ₂ N to lower the corrosion resistance. Therefore, the content of Cr is 25.0% or less, preferably 24.5% or less, more preferably 24.0% or less.

Mo: More than 0% and 1.0% or less Mo is an element that produces molybdic acid during dissolution, exhibits an effect of improving local corrosion resistance by an inhibitor action, and dramatically improves corrosion resistance. Mo is an element that stabilizes the ferrite phase, and has an effect of improving the pitting resistance and cracking resistance of the steel material. On the other hand, it is also an element that is very expensive and has poor price stability. From this, in the present invention, the content of Mo is 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less. In the present invention, the lower limit of the content of Mo is not particularly limited, but the lower limit is preferably 0.1%, more preferably 0.2%, and still more preferably 0.25%.

・ N: 0.01 to 0.20%
N is an element that stabilizes the strong austenite phase, and has the effect of improving the corrosion resistance without increasing the formation sensitivity of the σ phase. Furthermore, N is an element effective also to high strengthening of steel. When the content of N is too small, a sufficient amount of austenite phase does not precipitate in the weld heat affected zone, so N which can not form a solid solution in the ferrite phase precipitates as Cr ₂ N, which lowers the corrosion resistance. Therefore, the content of N is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. However, when N is excessively contained, N which can not be diffused into the austenite phase precipitates as Cr ₂ N in the ferrite phase, which lowers the toughness and the corrosion resistance. It also degrades hot workability and causes edge cracking and surface defects during forging and rolling. Therefore, the upper limit of the content of N is 0.20% or less, preferably 0.19% or less, and more preferably 0.17% or less.

V: 0 to 0.01%
Although V is an impurity element, its inclusion in an extremely small amount adversely affects the properties of duplex stainless steel. Further, V is likely to be bonded to N and lowers the amount of solid solution N, thereby reducing the corrosion resistance. In addition, it is an element that acts as a strong ferrite forming element. In the present invention in which the amount of Mn, which is an austenite stabilizing element, is limited for inclusion control, the addition of V breaks the balance of the α / γ phase ratio, promotes the precipitation of the σ phase, and improves the corrosion resistance and toughness. descend. Therefore, it is preferable not to contain it as much as possible, and it is most preferable that it is 0%, but the upper limit of the content of V is 0.01%.

  Ta, REM and Zr are all elements useful for improving the corrosion resistance by modifying or dispersing sulfide inclusions such as MnS, and in the present invention, those elements It contains at least one of them.

Ta: 0.01 to 0.30%
Ta is an element that improves the corrosion resistance by reforming a sulfide-based inclusion such as MnS that adversely affects the corrosion resistance into an acid-sulfide-based composite inclusion containing Ta. Further, Ta is an element that suppresses the formation of a Cr-based oxide by being bonded to O, and has an effect of contributing to the substantial improvement of the Cr concentration of the steel material. In order to obtain such an effect, the content of Ta is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.04% or more. However, in addition to being a very expensive element, if Ta is contained in excess, by combining with N in the steel, it precipitates as a nitride, and the toughness, the hot workability and the effectiveness of the N It reduces the concentration. Therefore, the corrosion resistance is reduced. In addition, a large number of oxysulfide-based composite inclusions modified with Ta are precipitated, which reduces the hot workability. Therefore, the Ta content is 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

REM: 0.0005 to 0.07%
Like REM, REM is an element that improves the corrosion resistance by reforming sulfide-based inclusions such as MnS that adversely affect the corrosion resistance to REM-containing oxysulfide-based composite inclusions, Instead of Ta, REM may be used. In order to obtain such an effect, the lower limit of the content of REM is 0.0005% or more, preferably 0.001% or more, and more preferably 0.002% or more. However, if REM is contained excessively, REM segregates at grain boundaries and hot workability deteriorates, so the upper limit of the content of REM is 0.07% or less, preferably 0.06% or less, more preferably Is less than 0.05%. In the present invention, REM means lanthanoid elements, that is, 15 elements from La to Lu, and Sc and Y. Among these elements, at least one element selected from the group consisting of La, Ce and Y is preferably contained, and it is more preferable to contain at least one element of La or Ce.

・ Zr: 0.01 to 0.10%
Zr is an element acting as a strong deoxidizing agent, and the corrosion resistance is improved by uniformly dispersing the oxide or nitride containing Zr as a core of sulfide-based inclusions such as MnS which adversely affects the corrosion resistance. Have an effect. Further, the oxides or nitrides of Zr function as pinning of ferrite crystal grains in the welding heat affected zone and as a formation nucleus of the austenite phase, and the structure of the welding heat affected zone can be refined. By refining the structure of the heat-affected zone, the formation of precipitates of Cr and Mo by welding can be suppressed, and the corrosion resistance can be prevented from decreasing. In order to obtain such an effect, the content of Zr is made 0.01% or more, preferably 0.015% or more, more preferably 0.02% or more. However, excessive addition forms a large amount of coarse Zr nitride and reduces hot workability. Therefore, the upper limit of the Zr content is 0.10% or less, preferably 0.07% or less, and more preferably 0.05% or less.

[Ta] / [Mn] is 0.02 or more As described above, in order to improve the corrosion resistance, it is important to suppress MnS which is a starting point of local corrosion by inclusions in the steel. Ta can reform sulfides such as MnS that adversely affect corrosion resistance into Ta-containing acid sulfide composite inclusions.

  When Mn is contained in excess and the ratio of [Ta] / [Mn] is small, an acid sulfide complex inclusion containing Ta is not formed, and an acid sulfide complex inclusion containing a large amount of Mn is formed. The inclusions have lower corrosion resistance than those containing Ta, and can be a starting point of pitting depending on the environment. By increasing the ratio of [Ta] / [Mn], an acid sulfide complex inclusion containing Ta can be formed, and the larger the ratio of the Ta-containing acid sulfide complex inclusion in the inclusion, the more corrosion resistance improves. In order to obtain such an effect, it is preferable to set [Ta] / [Mn] to 0.02 or more. More preferably, it is 0.03 or more, more preferably 0.05 or more. The higher the ratio of [Ta] / [Mn], the better, but in view of the cost of the raw material and hot workability, excessive addition should be avoided, and Mn is an element important as an austenite stabilizing element Therefore, the upper limit is about 0.5.

  The component composition of the duplex stainless steel material of the present invention is as described above, and the balance is iron and unavoidable impurities. Unavoidable impurities are impurities which are inevitably mixed at the time of melting and are contained in the range which does not injure various characteristics of steel materials. As unavoidable impurities, O, Sb, Sn, As, and Zn can be exemplified, and the content of these elements is preferably 0.1% or less in total.

  Moreover, corrosion resistance, hot workability, etc. can be further improved by adding Co, Cu, B, Mg, and Ca in the following amounts in addition to the above elements. Moreover, in addition to the said component, you may contain another element in the range which does not have a bad influence on the effect of the duplex stainless steel of this invention. Hereinafter, these elements will be described.

-Co: 0.1 to 1.0%, Cu: 0.1 to 2.0% One or two kinds of Co Co is an element improving the corrosion resistance and stabilizing the austenite phase. In order to obtain such an effect, when Co is contained, the content is made 0.1% or more, preferably 0.2% or more. However, since Co is a very expensive element, from the viewpoint of raw material cost, when Co is contained, it is 1.0% or less, preferably 0.7% or less.

  Cu is an element that improves the corrosion resistance and stabilizes the austenite phase. In order to obtain such an effect, when Cu is contained, the content is made 0.1% or more, preferably 0.2% or more. However, if Cu is excessively contained, ε-Cu precipitates, and not only the effect of austenite phase stabilization can not be obtained, but also the corrosion resistance and the hot workability deteriorate, so the content of Cu is 2.0 % Or less, preferably 1.5% or less.

・ B: 0.0005 to 0.010%, Mg: 0.0005 to 0.020%, Ca: 0.0005 to 0.020% One or more of B By segregating at grain boundaries , An element that improves the hot workability. In order to obtain this effect, it is necessary to contain 0.0005% or more, preferably 0.0006% or more, more preferably 0.0008% or more, and still more preferably 0.001% or more. However, when incorporated in excess, borides are formed and hot workability and corrosion resistance are degraded. Therefore, the preferable content is 0.010% or less, more preferably 0.005% or less.

  Mg and Ca are elements that combine with S and O contained as impurities in the steel to suppress segregation of these inclusions at grain boundaries to improve hot workability. Moreover, it is an element which suppresses formation of MnS which tends to become a starting point of local corrosion by bonding with S, and improves local corrosion resistance. In order to obtain such effects, when Mg and Ca are contained, the lower limits of the content of Mg and the content of Ca are each set to 0.0005%, preferably 0.0010%. However, when these elements are contained excessively, oxide inclusions increase, and the corrosion resistance and the processability deteriorate. Therefore, the content of Mg and the content of Ca are each set to 0.020% or less, preferably 0.010% or less. In addition, the total content of Mg and Ca is preferably 0.001 to 0.020% in consideration of corrosion resistance and hot workability.

・ PRE
In the duplex stainless steel material according to the present invention, when the content of Cr is [Cr], the content of Mo is [Mo], and the content of N is [N], [Cr] +3.3 [Mo] The target is a low-alloy duplex stainless steel of about 20 to 30 in which PRE, which is a value of +16 [N]. The higher the value of PRE, the higher the re-passivating ability of stainless steel, and the effect of inclusion dissolution can be ignored, but the content of expensive alloys increases, so in the present invention the content of these alloys is It limits, and the upper limit is set to 30.

(Production method)
The duplex stainless steel material of the present invention can be manufactured by the manufacturing equipment and manufacturing method used for mass production of ordinary stainless steel. In order to reduce O as an impurity in steel, deoxidization is performed by adding a large amount of an element having a high affinity to O, such as Si or Al, and further, a secondary such as vacuum degassing or argon gas stirring. Oxide inclusions can be removed by extending the time for refining or by performing the process multiple times.

  Here, in order to suppress sulfide-based inclusions such as MnS which is inferior in corrosion resistance to obtain a desired reformed oxysulfide-based composite inclusion which is excellent in corrosion resistance, or in a dispersed state, Depending on the S concentration in the molten steel, a Ta source, an REM source or a Zr source may be added. The timing of these additions is not particularly limited, but addition after deoxidation and desulfurization has a high yield and is desirable. Also, metals, alloys, oxides, fluorides, metal lumps and powders can be used as the Ta source, REM source or Zr source, but there is no particular limitation.

  In the production of steel ingots, for example, molten steel melted in a converter or an electric furnace is subjected to refining by the AOD method or VOD method to adjust the components, and then the casting method such as the continuous casting method or the ingot method It is a steel ingot. The obtained steel ingot may be hot-worked in a temperature range of about 1000 to 1200 ° C., and then cold-worked to form a desired shape. Further, the total working ratio at the time of hot working = the cross-sectional area of the base steel ingot / the cross-sectional area after working is set to about 10 to 50 as usual.

  In the present invention, in order to reduce precipitates harmful to mechanical properties, it is preferable to carry out solution heat treatment and quench as necessary. The temperature of solution heat treatment is preferably 1000 to 1100 ° C., the holding time is preferably 10 to 30 minutes, and quenching is preferably performed at a cooling rate of 10 ° C./second or more. In addition, it is possible to carry out pickling for surface conditioning such as scale removal, if necessary.

  The duplex stainless steel manufactured by the above manufacturing method exhibits excellent corrosion resistance in an environment containing a corrosive substance, and is also excellent in hot workability and weld zone corrosion resistance.

<Duplex stainless steel pipe>
Next, 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, and can be manufactured by a production facility and a manufacturing method used for mass production of normal stainless steel pipes. For example, it can be made into a desired size by an extrusion tube made of a round bar, a Mannesman tube, a welded tube made by welding a joint after forming a plate material, or the like. Further, the dimensions of the duplex stainless steel pipe can be appropriately set according to the seawater desalination plant, the LNG vaporizer, the fuel injection pipe, etc. in which the steel pipe is used.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples. However, the present invention is of course not limited by the following examples, and the present invention is implemented by adding appropriate changes as long as they can conform to the spirit of the present invention. It is also possible that they are all included in the technical scope of the present invention.

(Production of steel products)
Small melting furnace: A steel having the composition shown in Tables 1 and 2 was melted with a capacity of 20 kg / 1 ch and cast using a cylindrical mold: main body: φ 110 × about 200 mm. Table 2 also shows the calculation results of PRE = [Cr] +3.3 [Mo] +16 [N] for each steel. In the component composition column of Tables 1 and 2, the blank indicates that the corresponding component is not contained, and the balance is Fe and unavoidable impurities. The solidified steel ingot was heated to 1200 ° C. and subjected to hot forging: forging temperature: 1000 to 1200 ° C. at the same temperature, and then cut. Next, solution heat treatment was carried out at 1100 ° C. for 30 minutes, cooled with water at a cooling rate of 12 ° C./sec and cut to obtain a steel of 40 × 100 × 400 mm.

(Sample collection)
Next, corrosion resistance such as pitting resistance, hot workability, etc. were evaluated according to the procedure shown below using a sample: 20 mm × 30 mm × 2 mmt collected parallel to the processing direction from the steel material.

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

(Evaluation of corrosion resistance of base material)
The corrosion resistance of the base material was evaluated in reference to the method described in JIS G 0577. The sample surface was wet-polished with SiC # 600 abrasive paper, ultrasonic cleaned, and then the lead wire was attached to the sample by spot welding, and the test surface of the sample surface: 10 mm × 10 mm except for the portion was coated with epoxy resin. The sample was immersed in 3.5% artificial seawater maintained at 80 ° C. for 10 minutes. Thereafter, anodic polarization was performed at a sweep rate of 20 mV / min, and a potential at the time when the current density exceeded 0.1 mA / cm ² was defined as pitting potential: VC'100. The corrosion resistance was evaluated as 130 mV vs. pitting potential. Based on SCE, 150 mV vs. Those exceeding the SCE were evaluated as good. The results are shown in Tables 3 and 4. SCE is a saturated calomel electrode.

(Evaluation of corrosion resistance of weld heat affected zone)
The evaluation of the corrosion resistance of the weld heat affected zone is, as shown in FIG. 1, after heating to 1300 ° C. for 15 seconds, holding for 5 seconds, cooling from 1300 ° C. to 500 ° C. at 7 ° C./sec, and further from 500 ° C. After giving the steel material a heat treatment that reproduces the thermal cycle of the welded portion in TIG welding to cool at room temperature at 3 ° C./sec, the evaluation was performed under the same conditions as the evaluation of the corrosion resistance of the base material described above. The corrosion resistance was evaluated as 130 mV vs. pitting potential. Based on SCE, 150 mV vs. Those exceeding the SCE were evaluated as good. The results are shown in Tables 3 and 4.

(Evaluation of hot workability)
The hot workability was evaluated by visually observing surface defects generated when the melted material was subjected to 30% hot forging. The hot workability was evaluated as surface defects of 1 cm or more with less than 0.05 pieces per 1 cm ^{2 of the} sample with no defects and 0.05 pieces or more as defects at a location excluding 1 cm from the end face . The results are shown in Tables 3 and 4.

  The test result shown in Table 3 is a test result performed using the duplex stainless steel containing Ta shown in Table 1. In Table 3 and Table 4 described later, the pitting potential is 130 mV vs. The comprehensive judgment is indicated by A as a steel material which is SCE and which is free from defects in hot workability as a steel material meeting the criteria of corrosion resistance and workability. Furthermore, the pitting potential is 150 mV vs. The steel which is SCE and which is free from defects due to hot workability is indicated as AA by the general judgment as a steel material having further excellent corrosion resistance.

  Steel No. which satisfies the requirements of the present invention. It is understood that all of the examples A1 to A17 satisfy the criteria of corrosion resistance and workability. Furthermore, steel No. In the examples of A1 to A16, since the ratio of Ta to Mn is appropriately controlled, it is understood that they have excellent corrosion resistance. On the other hand, steel No. which does not satisfy the requirements of the present invention. The comparative examples of B1 to B8 have the following problems.

  Steel No. Since the content of Mn in the B1 exceeds the upper limit of the present invention, MnS precipitates, and the corrosion resistance and the hot workability in the weld heat affected zone did not meet the criteria. Steel No. Since the content of S exceeds the upper limit of the present invention, B2 precipitates MnS, and the corrosion resistance and hot workability in the base material and the weld heat affected zone did not meet the criteria. Steel No. B3, steel No. In B4, the content of Ni and N was below the lower limit of the present invention, so the amount of ferrite phase was excessive, and the corrosion resistance and hot workability in the base material and the weld heat affected zone did not meet the standard. Steel No. In B5, the Cr content was below the lower limit of the present invention, so the corrosion resistance in the base metal and the weld heat affected zone did not meet the standard. Steel No. B6, steel No. B7 is such that the content of Ta deviates from the upper limit or the lower limit of the present invention. Below the lower limit, the corrosion resistance in the weld heat affected zone did not meet the standard, and when the upper limit was exceeded, coarse Ta nitride was formed, and the hot workability did not meet the standard. Steel No. In B8, the V content exceeded the upper limit, and the corrosion resistance of the weld heat affected zone did not meet the criteria due to the reduction of the effective nitrogen concentration due to the formation of nitride and the formation of a ferrite phase in the weld heat affected zone.

  The test result shown in Table 4 is a test result performed using the duplex stainless steel containing Zr shown in Table 2. Steel No. which satisfies the requirements of the present invention. It is understood that all of the examples of C1 to C15 have excellent corrosion resistance. On the other hand, steel No. which does not satisfy the requirements of the present invention. The comparative examples D1 to D7 have the following problems.

  Steel No. As D1 had a content of Mn exceeding the upper limit of the present invention, coarse MnS was precipitated, and the corrosion resistance in the base material and the weld heat affected zone did not meet the standard. Steel No. Since D2 content of Zr exceeded the upper limit of the present invention, the hot workability did not meet the standard due to the precipitation of coarse Zr nitride. Steel No. As D3 had a very small amount of Zr added, coarse MnS was precipitated, and the corrosion resistance in the base metal and the weld heat affected zone did not meet the standard. Steel No. In D4, since the content of S exceeds the upper limit of the present invention, coarse MnS precipitates, and the corrosion resistance and hot workability in the base material and the weld heat affected zone did not meet the criteria. Steel No. In the case of D5, the content of Ni was below the lower limit of the present invention, so the amount of ferrite phase was excessive, and the corrosion resistance in the base material and the weld heat affected zone did not meet the standard. Steel No. In the case of D6, the Cr content was below the lower limit of the present invention, so the corrosion resistance and hot workability in the base metal and the weld heat affected zone did not meet the criteria. Steel No. In D7, the V content exceeds the upper limit of the present invention, corrosion resistance of the weld heat-affected zone, hot working, due to the reduction of the effective nitrogen concentration due to nitride formation and the formation of a ferrite phase in the weld heat-affected zone. Sex did not meet the criteria.

Claims (5)

A duplex stainless steel consisting of a ferrite phase and an austenite phase,
C: 0 to 0.05%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, P: more than 0% 0.05% or less, S: 0% by mass% More than 0.004% or less, Al: more than 0% and 0.05% or less, Ni: 2.0 to 6.0%, Cr: 19.0 to 25.0%, Mo: 0.1% or more and 1.0 %, N: 0.01 to 0.20%, V: 0 to 0.01%, Ta: 0.01 to 0.30%, REM: 0.0005 to 0.07%, Zr The duplex stainless steel material characterized in that it contains at least one of 0.01 to 0.10% and the balance is iron and unavoidable impurities. Ta: 0.01 to 0.30% is contained by mass%,
The duplex stainless steel material according to claim 1, wherein [Ta] / [Mn] is 0.02 or more.
However, in each above-mentioned formula, [] shows mass%.   The duplex stainless steel material according to claim 1, further comprising at least one of Co: 0.1 to 1.0%, Cu: 0.1 to 2.0% by mass%.   The composition according to claim 1, further comprising at least one of B: 0.0005 to 0.010%, Mg: 0.0005 to 0.020%, and Ca: 0.0005 to 0.020% by mass. The duplex stainless steel according to any one of the above.   A duplex stainless steel pipe made of the duplex stainless steel according to any one of claims 1 to 4.