JP2017002352A - Duplex stainless steel material and duplex stainless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a duplex stainless steel material and a duplex stainless steel pipe exhibiting excellent corrosion resistance under environment containing corrosive materials such as chloride.SOLUTION: There is provided the duplex stainless steel composed of a ferrite phase and an austenite phase and containing C:0 to 0.05%, Si:0.1 to 2.0%, Mn:0.1 to 2.0%, P:over 0% to 0.05%, S:over 0% to 0.004%, Al:over 0% to 0.05%, Ni:2.0 to 6.0%, Cr:19.0 to 25.0%, Mo:over 0% to 1.0%, N:0.01 to 0.20%, V:0 to 0.01% as well as at least one of Ta:0.01 to 0.30%, REM:0.0005 to 0.07%, Zr:0.01 to 0.10%.SELECTED DRAWING: None

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 a Cr oxide called a passive film in a corrosive environment and exhibits corrosion resistance. In particular, a duplex stainless steel material comprising a ferrite phase and an austenite phase is used as a high-strength, high-corrosion-resistant material for chemical plant members, heat exchangers, and the like. Furthermore, since duplex stainless steel is a low Ni component system compared to austenitic stainless steel, it is a material that is low in cost and less susceptible to price fluctuations of metal raw materials.

  The pitting corrosion resistance of stainless steel is “[Cr]” for the Cr content, [Mo] for the Mo content, [N] for the N content and [W] for the W content [W]. When the pitting corrosion index PRE calculated by Cr] +3.3 [Mo] +16 [N] ”or containing W or W is included,“ [Cr] +3.3 ([Mo] +0.5 [W] ” ) +16 [N] "and is expressed by the pitting corrosion index PREW. Since it has the advantages of higher strength and lower cost compared with austenitic stainless steel, it has begun to be used as an alternative steel type for SUS304 and SUS316L. In addition, all content of each above-mentioned element shows the mass%, and what shows with content in the following description shows all the mass%.

  For example, Patent Document 1 discloses a duplex stainless steel that is excellent in cost by reducing the addition amount of Ni and Mo with the aim of replacing SUS304.

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

  Patent Document 2 discloses a duplex stainless steel in which the ratio of the amount of ferrite and the amount of austenite is adjusted by increasing the amount of Mn added.

  Patent Document 3 discloses a duplex stainless steel in which V nitride is precipitated by addition of V and HAZ toughness is improved. However, since V acts as an α-phase stabilizing element, it is assumed that the precipitation of the σ phase is promoted, so that the α fraction of the welded portion is increased and the corrosion resistance and toughness are reduced.

JP 2009-35782 A JP 2006-193823 A WO2009 / 119895 Muto Izumi et al, Ferum Vol. 17 (2012), no. 12, pp. 858-863

  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. 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.

Moreover, when welding with respect to a duplex stainless steel material, the corrosion resistance of a welding part may fall from a base material by sensitization. Note that sensitization means that the elements such as Cr and Mo contained in the steel due to the thermal history form precipitates such as Cr ₂ N in the weld heat affected zone, and the Cr concentration of the base material decreases, thereby reducing the corrosion resistance. Is a phenomenon that decreases.

  The present invention has been made as a solution to the above-described conventional problems, and its main purpose is, for example, a duplex stainless steel material and a duplex stainless steel material that exhibit excellent corrosion resistance in an environment containing a corrosive substance such as chloride. Is to provide phase stainless steel pipe.

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

  Another object of the present invention is to provide a duplex stainless steel material and a duplex stainless steel tube that exhibit corrosion resistance excellent in workability.

  Since the duplex stainless steel material is generally composed of a ferrite phase and an austenite phase, the duplex stainless steel material has discontinuities at these heterophase interfaces. For this reason, the continuity of the passive film is reduced at the interface between the ferrite phase and the austenite phase, or the interface between the base metal and an oxide or sulfide inclusion inevitably formed in the steel. , Passive film tends to be unstable. As a result, the passive film is easily destroyed by chloride ions, and local corrosion is likely to occur.

  Therefore, the present inventors focused on enhancing the stability and protective properties of the passive film of the duplex stainless steel material within a range that does not impede the production surface and various characteristics, and these intervening causes local corrosion. We proceeded with intensive studies on things.

  In particular, sulfide inclusions such as MnS are typical examples of inclusions in steel that adversely affect the properties and corrosion resistance of steel materials. 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 is highly soluble in water and easily eluted as compared with other oxide inclusions. 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 the steel, the content is preferably as low as possible. However, as described above, there is an industrial limit to the reduction of S. For this reason, the present inventors have conceived a method of detoxifying inclusions such as oxides and sulfides that are the starting point of local corrosion without reducing Mn and S in the steel.

  As a result, by adding at least one of Ta, REM, or Zr and appropriately controlling the amount added, MnS can be modified into a highly corrosion-resistant oxysulfide or dispersed. It has been found that the corrosion resistance can be improved by reducing the influence of MnS on the 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% to 0.05% or less, S: more than 0% to 0.004% or less, Al: more than 0% to 0.05% or less, Ni: 2.0 to 6 0.0%, Cr: 19.0 to 25.0%, Mo: more than 0% and 1.0% or less, N: 0.01 to 0.20%, V: 0 to 0.01%, Contains at least one of Ta: 0.01-0.30%, REM: 0.0005-0.07%, Zr: 0.01-0.10%, the balance being iron and inevitable impurities It is characterized by being.

  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, and V, and Ta, REM, and Zr. By containing at least one of these, the corrosion resistance is improved, and the 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 have an effect of enhancing hot working by deoxidation. However, if the value of Mn is large, the precipitation of MnS that reduces the corrosion resistance and workability is promoted. By making P below a predetermined value, there is an effect of improving workability and corrosion resistance. S has an effect of suppressing deterioration of corrosion resistance and hot workability by being not more than a predetermined value. In particular, by suppressing the value of S, S can reduce the formation of MnS that impairs corrosion resistance and toughness. it can. 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 and REM have the effect of modifying the sulfide inclusions that are the starting point of pitting corrosion into oxysulfide composite inclusions that are less likely to start pitting corrosion. V is preferably not contained as much as possible.

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

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

  Moreover, it is preferable that the duplex stainless steel material of this invention contains the at least 1 sort (s) of Co: 0.1-1.0% and Cu: 0.1-2.0% further by the mass%.

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

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

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

  The duplex stainless steel pipe of the present invention is characterized by comprising the aforementioned duplex stainless steel material.

  As described above, in the duplex stainless steel pipe, by constituting the steel pipe with a duplex stainless steel material, inclusions that are the starting point of local corrosion are modified, and the corrosion resistance is improved.

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

  According to one embodiment of the present invention, a duplex stainless steel material 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 due to chloride ions is a problem, such as a seawater heat exchanger.

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 material is a duplex stainless steel material composed of a ferrite phase and an austenite phase. The component composition of the duplex stainless steel material contains a predetermined amount of C, Si, Mn, P, S, Al, Ni, Cr, Mo, N, and V, and at least one of Ta, REM, and Zr The balance is composed of Fe and inevitable impurities. Furthermore, when the content of Ta is [Ta] and the content of Mn is [Mn], [Ta] / [Mn] is controlled within a predetermined range. Further, the duplex stainless steel material preferably contains a predetermined amount of at least one of Co and Cu, and preferably further contains a predetermined amount of at least one of B, Mg and Ca. Hereinafter, the duplex stainless steel material of the present invention will be described in the order of the steel material structure, the component composition, and the production 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 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. Accordingly, 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% 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 and the austenite phase can be optimized by adjusting the contents of the ferrite phase stabilizing element and the austenite phase stabilizing element.

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

・ C: 0 to 0.05%
C is a harmful element that reduces the corrosion resistance by forming carbides with Cr or the like in the steel material. Therefore, the C content is 0.05% or less. In addition, since it is better that the content of C is as small as possible, it is preferably 0.04% or less, more preferably 0.03% or less. C may not be contained in the steel material, that is, 0%.

・ Si: 0.1-2.0%
Si is an element necessary for deoxidation and stabilization of the ferrite phase. In order to obtain such an effect, the Si content is set to 0.1% or more, preferably 0.15% or more, and more preferably 0.2% or more. However, if Si is excessively contained, workability deteriorates, so the Si content 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 strength and stabilizing the austenite phase. When the content of Mn is insufficient, the amount of austenite phase in the weld heat-affected zone is insufficient, and N that cannot be dissolved in the ferrite phase precipitates as Cr ₂ N, thereby reducing the corrosion resistance. Therefore, the Mn content is 0.1% or more, preferably 0.15% or more, more preferably 0.2% or more. However, when Mn is excessively contained, the formation of coarse MnS is promoted, and the formation of complex inclusions of Mn, Cr, O, and S is promoted, and the formation of Ta-containing oxysulfides is suppressed, and the base material In addition, since the corrosion resistance and cold workability of the weld heat affected zone deteriorate, the Mn content is set to 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 that is inevitably mixed as an impurity, is harmful to corrosion resistance, and deteriorates weldability and workability. Therefore, the P content is 0.05% or less. Further, the P content is preferably as small as possible, preferably 0.04% or less, and more preferably 0.03% or less. P may not be contained in the steel material and may be 0% if possible, but P is an element inevitably mixed as described above. Moreover, since excessive reduction of P content brings about an increase in manufacturing cost, the lower limit in actual operation of P content is about 0.01%.

・ S: More than 0% and 0.004% or less S, like P, is inevitably mixed as an impurity, and combines with Mn to form sulfide inclusions such as MnS, thereby providing corrosion resistance and hot working. It is an element that deteriorates properties. And when S is contained excessively, the modification | reformation by Ta addition to an oxysulfide type complex inclusion will become inadequate, and corrosion resistance will fall. Furthermore, hot workability is also reduced. Therefore, the S content is 0.004% or less, preferably 0.003% or less, and more preferably 0.001% or less. In addition, as described in the background art, the lower the content of S, the better it may not be contained in the steel material, and it may be 0% if possible. As described above, it is also an element inevitably mixed in. Therefore, the lower limit in actual operation 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 for reducing the O content and the S content during melting. However, if Al is excessively contained, coarse oxide inclusions are generated, which adversely affects pitting corrosion resistance. Therefore, the Al content is set to 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 Al content is not particularly limited, but a preferable lower limit is 0.001%, a more preferable lower limit is 0.003%, and a further preferable lower limit is 0.005%.

・ Ni: 2.0-6.0%
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. When the content of Ni is insufficient, a sufficient amount of austenite phase does not precipitate in the weld heat affected zone, so that N that cannot be dissolved in the ferrite phase precipitates as Cr ₂ N and lowers the corrosion resistance. Accordingly, the Ni content is set to 2.0% or more, preferably 2.5% or more, more preferably 3.0% or more. However, addition of excessive Ni causes an increase in cost, so the Ni content is 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 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 ferrite and austenite and achieve both corrosion resistance and strength, the Cr content is 19.0% or more, preferably 20.0% or more, more preferably 21.0%. That's it. If the Cr content is less than the lower limit value, a homogeneous passive film cannot be formed, and the corrosion resistance of the base material and the weld heat affected zone is greatly reduced. However, if Cr is excessively contained, the amount of ferrite phase in the weld heat affected zone increases, and N remaining in the ferrite phase during cooling precipitates as Cr ₂ N, thereby reducing the corrosion resistance. Therefore, the Cr content is 25.0% or less, preferably 24.5% or less, more preferably 24.0% or less.

Mo: more than 0% and not more than 1.0% Mo is an element that produces molybdic acid when dissolved and exhibits an effect of improving local corrosion resistance by an inhibitor action, thereby dramatically improving 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. On the other hand, it is also an element with very high cost and poor price stability. Therefore, in the present invention, the Mo content 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 Mo content is not particularly limited, but a preferable lower limit is 0.1%, a more preferable lower limit is 0.2%, and a more preferable lower limit is 0.25%.

-N: 0.01-0.20%
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, N is an element effective for increasing the strength 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. Therefore, N that cannot be dissolved in the ferrite phase precipitates as Cr ₂ N, thereby reducing the corrosion resistance. Therefore, the N content is set to 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. However, when N is contained excessively, N that could not be diffused into the austenite phase is precipitated as Cr ₂ N in the ferrite phase, thereby reducing toughness and corrosion resistance. In addition, hot workability is deteriorated, and ear cracks and surface defects are generated during forging and rolling. Therefore, the upper limit of the N content is 0.20% or less, preferably 0.19% or less, and more preferably 0.17% or less.

・ V: 0 to 0.01%
V is an impurity element, but if contained in an extremely small amount, it adversely affects the properties of the duplex stainless steel. Moreover, since V is easy to combine with N and lowers the amount of dissolved N, the corrosion resistance is lowered. 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 α / γ phase ratio, promotes precipitation of σ phase, and improves corrosion resistance and toughness. descend. Therefore, it is preferable not to contain as much as possible, and it is most preferable that it is 0%, but the upper limit of acceptable V content is 0.01%.

  Ta, REM and Zr are all useful elements for improving the corrosion resistance by modifying or dispersing sulfide inclusions such as MnS. In the present invention, these elements Among them, at least one kind is contained.

-Ta: 0.01-0.30%
Ta is an element that improves corrosion resistance by modifying sulfide inclusions such as MnS that adversely affect corrosion resistance into oxysulfide composite 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 content of Ta is set to 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more. However, in addition to Ta being a very expensive element, if Ta is excessively contained, it precipitates as a nitride by bonding with N in the steel, resulting in toughness, hot workability, and effective N It will reduce the density. Therefore, the corrosion resistance decreases. In addition, a large number of oxysulfide-based composite inclusions modified with Ta are precipitated, which deteriorates hot workability. Therefore, the Ta content is set to 0.30% or less, preferably 0.25% or less, more preferably 0.20% or less.

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

-Zr: 0.01-0.10%
Zr is an element that acts as a strong deoxidizer and improves corrosion resistance by uniformly dispersing oxides and nitrides containing Zr, which is the core of sulfide inclusions such as MnS, which adversely affect corrosion resistance. Has the effect of Also, Zr oxides and nitrides function as pinning of ferrite crystal grains in the welding heat-affected zone and as nuclei for forming austenite phases, and the microstructure of the welding heat-affected zone can be refined. By miniaturizing the structure of the weld heat affected zone, formation of Cr and Mo precipitates and the like can be suppressed by welding, and deterioration of corrosion resistance can be prevented. In order to obtain such an effect, the content of Zr is set to 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, which decreases hot workability. Therefore, the upper limit of the Zr content is 0.10% or less, preferably 0.07% or less, 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 that becomes an origin of local corrosion by inclusions in steel. Ta can modify a sulfide such as MnS that adversely affects corrosion resistance into an oxysulfide composite inclusion containing Ta.

  When Mn is contained excessively and the ratio of [Ta] / [Mn] is small, an oxysulfide composite inclusion containing a large amount of Mn is formed without forming an oxysulfide composite inclusion containing Ta. This inclusion has lower corrosion resistance than that containing Ta, and can be a starting point for pitting corrosion depending on the environment. By increasing the ratio of [Ta] / [Mn], it is possible to form Ta-containing oxysulfide composite inclusions, and as the ratio of Ta-containing oxysulfide composite inclusions in the inclusions increases, the corrosion resistance increases. improves. In order to obtain such an effect, [Ta] / [Mn] is preferably set to 0.02 or more. More preferably 0.03 or more, still more preferably 0.05 or more. The higher the ratio of [Ta] / [Mn] is, the more preferable, but Ta should be avoided in consideration of the cost of raw materials and hot workability, and Mn is an important element 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, with the balance being iron 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. Examples of unavoidable impurities include O, Sb, Sn, As, and Zn, and the total content of these elements is preferably 0.1% or less.

  In addition to the above elements, the following amounts of Co, Cu, B, Mg, and Ca can be further added to further improve the corrosion resistance and hot workability. Further, in addition to the above components, other elements may be further contained within a range that does not adversely affect the effect of the duplex stainless steel material of the present invention. Hereinafter, these elements will be described.

One or two types of Co: 0.1 to 1.0% and Cu: 0.1 to 2.0% Co is an element that improves the corrosion resistance and stabilizes the austenite phase. In order to obtain such an effect, when Co is contained, its content is set to 0.1% or more, preferably 0.2% or more. However, since Co is a very expensive element, from the viewpoint of raw material costs, when Co is contained, the content is made 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, its content is set to 0.1% or more, preferably 0.2% or more. However, when Cu is excessively contained, ε-Cu is precipitated, and not only the effect of stabilizing the austenite phase is obtained, but also the corrosion resistance and hot workability are deteriorated. % Or less, preferably 1.5% or less.

-B: 0.0005-0.010%, Mg: 0.0005-0.020%, Ca: 0.0005-0.020% 1 type or 2 types or more B is segregating at a grain boundary. An element that improves hot workability. In order to acquire this effect, it is necessary to contain 0.0005% or more, Preferably it is 0.0006% or more, More preferably, it is 0.0008% or more, More preferably, it is 0.001% or more. However, if contained excessively, a boride is formed, and hot workability and corrosion resistance are deteriorated. Therefore, the preferable content is 0.010% or less, and 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 and improve hot workability. Moreover, it is an element which suppresses formation of MnS which is likely to be a starting point of local corrosion and improves local corrosion resistance by combining with S. In order to obtain such effects, when Mg and Ca are contained, the lower limit of the Mg content and the Ca content is 0.0005%, preferably 0.0010%, respectively. However, when these elements are excessively contained, the oxide inclusions increase, and the corrosion resistance and workability deteriorate. Therefore, the Mg content and the Ca content are each 0.020% or less, preferably 0.010% or less. Further, the total content of Mg and Ca is preferably 0.001 to 0.020% in consideration of corrosion resistance and hot workability.

・ PRE
The duplex stainless steel material according to the present invention has [Cr] +3.3 [Mo] when the Cr content is [Cr], the Mo content is [Mo], and the N content is [N]. It is intended for an alloy-saving duplex stainless steel having a value of +16 [N] of about 20-30. The higher the PRE, the higher the repassivation ability of the stainless steel, and the influence of inclusion dissolution can be ignored. However, the content of expensive alloys increases, so in the present invention the content of these alloys is increased. The upper limit is set to 30.

(Production method)
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 can be removed by increasing the refining time or performing the refining time a plurality of times.

  Here, in order to suppress sulfide inclusions such as MnS that are inferior in corrosion resistance and to obtain a desired modified oxysulfide composite inclusion that is excellent in corrosion resistance, or in a dispersed state, A Ta source, a REM source, or a Zr source may be added according to the S concentration in the molten steel. These addition timings are not particularly limited, but it is preferable to add them after the deoxidation and desulfurization treatment because the yield is high. Further, as the Ta source, REM source, or Zr source, metals, alloys, oxides, fluorides, metal lumps / powder, and the like can be used, but are not particularly limited.

  For example, steel ingots can be produced by, for example, refining the molten steel melted in a converter or electric furnace by the AOD method or the VOD method to adjust the components, and then using a casting method such as a continuous casting method or an ingot forming method. Use a steel ingot. 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 at the time of hot processing = the cross-sectional area of the original steel ingot / the cross-sectional area after processing is about 10 to 50 as usual.

  In the present invention, in order to reduce 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.

  The duplex stainless steel material produced by the above production method exhibits excellent corrosion resistance in an environment containing a corrosive substance, and is also excellent in hot workability and welded portion corrosion resistance.

<Duplex stainless steel pipe>
Next, an embodiment of the 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 seawater desalination plant in which a steel pipe is used, an LNG vaporizer, a fuel injection pipe, etc.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

(Production of steel)
A small melting furnace: steel having the composition shown in Tables 1 and 2 was melted in a capacity of 20 kg / 1 ch, and cast using a cylindrical mold: main body: φ110 × 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 columns of Tables 1 and 2, the 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, it was subjected to a solution heat treatment held at 1100 ° C. for 30 minutes, and after water cooling at a cooling rate of 12 ° C./second, it was cut into a steel material of 40 × 100 × 400 mm.

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

  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.

(Evaluation of corrosion resistance of base material)
The corrosion resistance of the base material was evaluated with reference to the method described in JISG0577. 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 test surface of the sample surface was covered with an epoxy resin except for a portion of 10 mm × 10 mm. 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 the potential at the time when the current density exceeded 0.1 mA / cm ² was defined as the pitting potential: VC′100. Corrosion resistance was evaluated using a pitting potential of 130 mV vs. Based on SCE, 150 mV vs. Those exceeding 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)
As shown in FIG. 1, the corrosion resistance of the weld heat-affected zone was heated to 1300 ° C. for 15 seconds, held for 5 seconds, cooled from 1300 ° C. to 500 ° C. at 7 ° C./sec, and further from 500 ° C. The steel material was subjected to heat treatment that reproduced the thermal cycle of the welded part in TIG welding, which was cooled to room temperature at 3 ° C./sec, and then evaluated under the same conditions as those for the evaluation of the corrosion resistance of the base material. Corrosion resistance was evaluated using a pitting potential of 130 mV vs. Based on SCE, 150 mV vs. Those exceeding SCE were evaluated as good. The results are shown in Tables 3 and 4.

(Evaluation of hot workability)
Evaluation of hot workability was performed by visually observing surface defects generated when the melted material was hot forged by 30%. In the place excluding 1 cm from the end face, the hot workability was evaluated assuming that surface defects with a length of 1 cm or more were less than 0.05 per 1 cm ^{2 of the} sample without defects and 0.05 or more with defects. . The results are shown in Tables 3 and 4.

  The test results shown in Table 3 are test results obtained using the duplex stainless steel materials containing Ta shown in Table 1. In Table 3 and Table 4 described later, the pitting potential is 130 mV vs. A steel sheet that was SCE and had no defects due to hot workability was indicated by A as a comprehensive judgment as a steel material that satisfies the standards of corrosion resistance and workability. Furthermore, the pitting potential is 150 mV vs. The comprehensive judgment was indicated by AA as a steel material that was SCE and had no defects due to hot workability and was further excellent in corrosion resistance.

  Steel No. 1 satisfying the requirements of the present invention. It can be seen that the examples of A1 to A17 all satisfy the standards of corrosion resistance and workability. Furthermore, the steel material No. About the Example of A1-A16, since the ratio of Ta and Mn is controlled appropriately, it turns out that it has the outstanding corrosion resistance. On the other hand, steel No. which does not satisfy the requirements of the present invention. About the comparative example of B1-B8, it has the following malfunctions.

  Steel No. In B1, since the Mn content exceeded the upper limit of the present invention, MnS was precipitated, and the corrosion resistance and hot workability at the weld heat affected zone did not satisfy the standards. Steel No. In B2, since the S content exceeded the upper limit of the present invention, MnS was precipitated, and the corrosion resistance and hot workability in the base metal and the weld heat affected zone did not satisfy the standards. Steel No. B3, steel material No. In B4, since the contents of Ni and N were below the lower limit of the present invention, the ferrite phase amount was excessive, and the corrosion resistance and hot workability at the base metal and the weld heat affected zone did not meet the standards. Steel No. In B5, since the Cr content is lower than the lower limit of the present invention, the corrosion resistance in the base material and the weld heat affected zone did not satisfy the standard. Steel No. B6, Steel No. B7 is one in which 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 content of V exceeded the upper limit, and the corrosion resistance of the weld heat affected zone did not satisfy the standard due to the decrease in effective nitrogen concentration due to nitride formation and the formation of the ferrite phase in the weld heat affected zone.

  The test results shown in Table 4 are test results obtained using the duplex stainless steel materials containing Zr shown in Table 2. Steel No. 1 satisfying the requirements of the present invention. About the Example of C1-C15, it turns out that all have the outstanding corrosion resistance. On the other hand, steel No. which does not satisfy the requirements of the present invention. About the comparative example of D1-D7, it has the following malfunctions.

  Steel No. In D1, since the Mn content exceeds the upper limit of the present invention, coarse MnS was precipitated, and the corrosion resistance in the base metal and the weld heat affected zone did not satisfy the standard. Steel No. D2 has a Zr content exceeding the upper limit of the present invention, and therefore, hot workability did not satisfy the standard due to the precipitation of coarse Zr nitride. Steel No. In D3, since the amount of Zr added was extremely small, coarse MnS was precipitated, and the corrosion resistance in the base metal and the weld heat affected zone did not satisfy the standard. Steel No. In D4, since the S content exceeds the upper limit of the present invention, coarse MnS was precipitated, and the corrosion resistance and hot workability in the base material and the weld heat affected zone did not satisfy the standards. Steel No. In D5, since the Ni content was below the lower limit of the present invention, the ferrite phase amount was excessive, and the corrosion resistance at the base metal and the weld heat affected zone did not satisfy the standard. Steel No. In D6, since the Cr content was lower than the lower limit of the present invention, the corrosion resistance and hot workability in the base metal and the weld heat affected zone did not satisfy the standards. Steel No. In D7, the content of V exceeds the upper limit of the present invention, and the corrosion resistance of the weld heat affected zone, hot working due to the decrease in effective nitrogen concentration due to nitride formation and the formation of ferrite phase in the weld heat affected zone Sex did not meet the standards.

Claims (5)

A duplex stainless steel material comprising a ferrite phase and an austenite phase,
In mass%, C: 0 to 0.05%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, P: more than 0% and 0.05% or less, S: 0% More than 0.004% or less, Al: more than 0% to 0.05%, Ni: 2.0 to 6.0%, Cr: 19.0 to 25.0%, Mo: more than 0% to 1.0% or less N: 0.01-0.20%, V: 0-0.01%, Ta: 0.01-0.30%, REM: 0.0005-0.07%, Zr: 0 A duplex stainless steel material comprising at least one of .01 to 0.10%, the balance being iron and inevitable impurities. Containing Ta: 0.01-0.30% 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 the mass%.   Furthermore, the duplex stainless steel material of Claim 1 or 2 which contains at least 1 sort (s) of Co: 0.1-1.0% and Cu: 0.1-2.0% by mass%.   Furthermore, it contains 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 material according to any one of the above.   A duplex stainless steel pipe comprising the duplex stainless steel material according to any one of claims 1 to 4.