Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K9/00—Arc welding or cutting
  • B23K9/23—Arc welding or cutting taking account of the properties of the materials to be welded
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a welded joint using duplex stainless steel suitable for a corrosive environment due to brackish water and a method for manufacturing the same.
• Two-phase stainless steel is a stainless steel that contains approximately equal amounts of ferrite phase and austenite phase. In addition to corrosion resistance, it has higher strength than other stainless steels and carbon steels and can be made thinner and lighter, resulting in an increase in weight due to the increase in scale of the structure. The big merit that can be reduced fits and it has become widely used.
• SUS821L1 is a steel grade developed as a substitute for SUS304 and SUS323L is a steel grade developed as a substitute for SUS316L, and SUS329J3L, SUS329J4L, and SUS327L are highly corrosion-resistant steel grades having corrosion resistance in a harsher environment.
• duplex stainless steel In the case of duplex stainless steel, it is necessary to consider the decrease in toughness and corrosion resistance of the welded part. N added to duplex stainless steel is precipitated as Cr nitride by heating and cooling during welding. This nitride lowers the toughness by promoting the propagation of cracks, and also lowers the corrosion resistance by consuming Cr by precipitation and forming a so-called Cr-deficient layer.
• the two-phase stainless steel is once solidified with a single ferrite phase, and during subsequent cooling, the austenite phase is precipitated and finally becomes a two-phase structure of a ferrite phase and an austenite phase.
• the austenite phase is insufficiently precipitated due to the high cooling rate, resulting in a structure in which the ferrite phase is excessive as compared with the base metal.
• the solid solubility of C and N in the austenite phase is high, whereas the solid solubility of C and N in the ferrite phase is extremely low, so that excess C and N are precipitated as chromium carbonitride.
• Chromium-deficient phase is formed to reduce corrosion resistance.
• the corrosion resistance of the base metal of the above-mentioned SUS323L is equal to or higher than that of the SUS316L, but it may be lower than the corrosion resistance level of the SUS316L depending on the welding conditions.
• SUS821L1 is a component system capable of suppressing a decrease in corrosion resistance of a welded portion, but is not suitable for this application because it is a SUS304 substitute steel.
• SUS329J3L, SUS329J4L, and SUS327L have very excellent corrosion resistance, but need to contain 3% or more of expensive Mo.
• SUS329J1 has higher corrosion resistance than SUS323L and has a low Mo content, so that it is suitable for this application, but there is a problem that the corrosion resistance of the welded portion deteriorates more severely than other steel types.
• Patent Document 2 describes SUS329J1 improved duplex stainless steel in which the corrosion resistance of the welded portion is improved by adding an appropriate N in relation to Ni.
• more than 2% of Mo is added to the steel in the examples.
• Patent Document 3 defines an appropriate amount of N 2 gas according to the amount of heat input when welding duplex stainless steel without a filler material, and by mixing more N 2 gas, corrosion resistance is achieved. Welding methods for obtaining good duplex stainless steel welded joints are disclosed. However, if the amount of N 2 gas to be mixed is too large, blowhole defects may occur in the weld metal. Further, compared with the Ar shield gas, the N 2 mixed gas requires more time and effort to be mixed.
• Patent Document 4 by using a welded member in which the calculated value of the precipitation start temperature of Cr nitride is 1250 ° C. or less, the precipitation of Cr nitride can be suppressed, and the weld metal portion, the weld heat affected zone, and the base metal An alloy-saving duplex stainless steel laser welded member in which the difference in characteristics of the above is suppressed is disclosed. It is disclosed that the welded member has corrosion resistance used in an air environment, a water environment, and the like. Patent Document 4 describes that since the elements other than N in the steel material are not easily released at the time of melting, the contents in the steel material and the weld metal can be regarded as the same.
• the present invention optimizes the components of a duplex stainless steel base material and the welding conditions of a duplex stainless steel base material, and optimizes the components of the weld metal of a welded joint that does not use a filler metal, thereby in a steam environment.
• An object of the present invention is to provide a welded joint having excellent corrosion resistance and excellent toughness and a method for manufacturing the same.
• the pitting corrosion index (PREN) is often expressed by the formula Cr + 3.3Mo + 16N for duplex stainless steel.
• the present inventors estimated a method for improving the corrosion resistance of the welded portion of SUS329J1 by including N in the composition range of SUS329J1 by simulation calculation, and confirmed it by an experiment.
• the value of the PREN (formula (1) below) is 28.0 or more, even considering the decrease in corrosion resistance due to the precipitation of Cr nitride in the weld heat affected zone, and the formula (2) below. ) Is 170.0 or more, the required corrosion resistance is equivalent to or more than the SUS316L welded joint, specifically, the pitting corrosion potential by the JIS G0577 A method measured at 50 ° C is 0.27 V vs SSE or more.
• the PREN value is 29.0 or more and the PREN value is 29.0 or more, even if the local corrosion resistance is reduced due to component segregation, as described later.
• the amount of austenite in the weld heat affected zone and the weld metal is measured by the area ratio. Each should be 8% or more.
• the present inventors need to satisfy the following formula (3) in the N amount of the steel base material and the weld metal used. I found that there is. N ⁇ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.4 ⁇ 0.08 ... (3)
• the N amount required to make the austenite amount of the weld heat affected zone and the weld metal 8% or more in area ratio in the non-consumable electrode type welding without using a filler metal is mainly used. It is an equation estimated from the contents of Cr, Ni, and Mo which are elements.
• the components of the weld metal are composed of the components of the duplex stainless steel base material except for C, N and O. By mass%, it contains C: 0.001 to 0.060%, N: 0.100 to 0.250%, O: 0.0100% or less. Moreover, the PREN value defined by the following formula [1] is 29.0 or more, and The amount of austenite in the two-phase stainless steel base material is 30 to 70 area%, and the amount of austenite in the weld metal and the heat-affected zone is 8 to 70 area%, respectively.
• a welded joint characterized in that the duplex stainless steel base material and the weld metal have an NI value of 170 or more defined by the following formula [2] and satisfy the following formula [3].
• the duplex stainless steel base material and the weld metal further contain Nb: 0.150% or less in mass%.
• Ti 0.020% or less, Ta: 0.200% or less, Zr: 0.050% or less, Hf: 0.080% or less, Sn: 0.100% or less, W: 1.00% or less, Co: 1.00% or less, Cu: 3.00% or less, V: 0.300% or less, B: 0.0050% or less, Ca: 0.0050% or less, Described in any one of (1) to (3), which contains one or more of the group consisting of Mg: 0.0050% or less and REM: 0.050% or less.
• the filler metal is added by controlling the amount of nitrogen Ngas in the shield gas and the amount of heat input Q for welding so that the value of Nweld defined by the following formula [5] is 0.100 to 0.250.
• Q [Welding current (A)] x [Welding voltage (V)] ⁇ [Welding speed (cm / s)] ...
• Nweld Nbase ⁇ ⁇ 0.978-0.0029 ⁇ ⁇ (Q) +0.28 ⁇ ⁇ (Ngas) +0.00074 ⁇ ⁇ (Q) ⁇ ⁇ (Ngas) ⁇ ⁇ ⁇ ⁇ [5]
• Nbase indicates the amount of nitrogen (mass%) of the duplex stainless steel base material
• Ngas indicates the amount of nitrogen in the shield gas (vol%)
• Q indicates the amount of heat input to welding (J / cm).
• the welded joint obtained by the present invention has sufficient corrosion resistance equal to or higher than that of the SUS316L welded joint in a brackish water environment such as a floodgate near the mouth of a river, and can be further reduced in weight due to high toughness.
• the contribution to the environment is extremely large.
• the welded metal of the present invention is obtained from the weld metal described below and the duplex stainless steel base material.
• the welded joint of the present invention has a weld metal of the joint portion and two base materials made of a duplex stainless steel steel plate sandwiching the weld metal.
• the specific shape of the welded joint of the present invention is not particularly limited.
• the present invention is a method of manufacturing a joint in which two two-phase stainless steel steel plates are grooved and then butt welded, and two pipes are connected to each other. It can be applied to a method for manufacturing a pipe joint to be butt welded.
• composition of duplex stainless steel base material The reasons for limiting the composition and structure of the duplex stainless steel base material constituting the welded joint of the present invention will be described below. Unless otherwise specified in the present specification,% for a component represents mass%.
• C is limited to a content of 0.050% or less in order to ensure the corrosion resistance of stainless steel. If it is contained in excess of 0.050%, Cr carbide is generated during hot rolling, and corrosion resistance and toughness are deteriorated. It is preferably 0.030% or less, and more preferably 0.025% or less. On the other hand, 0.001% is set as the lower limit from the viewpoint of reducing the amount of C in stainless steel.
• Si is contained in the duplex stainless steel base material and the welded portion in an amount of 0.05% or more for deoxidation.
• the Si content is preferably 0.10% or more, more preferably 0.20% or more.
• the toughness deteriorates. Therefore, it should be 0.80% or less. It is preferably 0.50% or less, more preferably 0.40% or less.
• Mn has the effect of increasing the austenite phase and improving toughness.
• 0.10% or more is contained in the base metal and the welded part. It is preferably 0.30% or more, more preferably 0.50% or more.
• Mn is an element that lowers the corrosion resistance of stainless steel, so Mn should be 2.00% or less. It is preferably 1.80% or less, more preferably 1.50% or less.
• Cr is contained in the base metal and the welded portion in an amount of 23.000% or more in order to ensure the basic corrosion resistance of the steel of the present invention. It is preferably 23.20% or more, and more preferably 23.40% or more. On the other hand, when Cr is contained in an amount of more than 26.00%, the ferrite phase fraction increases and Cr nitride precipitates, which impairs toughness and corrosion resistance of the welded portion. Therefore, the Cr content was set to 26.00% or less. It is preferably 25.00% or less, more preferably 24.50% or less.
• Ni is contained in the base metal and the welded portion in an amount of 4.50% or more in order to stabilize the austenite structure, improve the corrosion resistance to various acids, and improve the toughness.
• Ni content it is possible to increase the NI value of the base metal and the weld metal and reduce the N amount that needs to be contained in the base metal and the weld metal. It is preferably 4.80% or more, more preferably 5.00% or more.
• Ni is an expensive alloy, and the content of the steel of the present invention, which is aimed at resource-saving duplex stainless steel, is limited to 6.00% or less from an economical point of view.
• Mo is a very effective element that enhances the corrosion resistance of stainless steel, and needs to be contained in an amount of 1.00% or more in order to impart corrosion resistance of SUS316L or more. It is preferably 1.10% or more, more preferably 1.20% or more. On the other hand, since Mo is expensive, it is preferable that the Mo content is low from an economic point of view, so the content is set to 2.00% or less. It is preferably less than 2.00%, more preferably 1.80% or less, and more preferably 1.60% or less.
• N is a strong austenite-forming element, and in particular, it has the effect of greatly increasing the amount of austenite in the welded part, and is a very effective element that dissolves in the austenite phase to enhance the corrosion resistance of the two-phase stainless steel.
• the solid solution limit increases according to the Cr content, but in the steel of the present invention, if the content exceeds 0.250%, Cr nitride is precipitated and the toughness and corrosion resistance are impaired. Therefore, the N content was set to 0.250% or less. It is preferably 0.230% or less, more preferably 0.200% or less.
• Al is an important element for deoxidation of steel, and is contained in the base metal and welds in order to control the composition of inclusions in the steel. Al may be contained together with Si in order to reduce oxygen in the steel. Al is contained in an amount of 0.003% or more in order to control the composition of inclusions and enhance pitting corrosion resistance. It is preferably 0.005% or more.
• Al is an element having a relatively large affinity for N, and when it is added in excess, a nitride of Al is generated and the toughness of stainless steel is inhibited. The degree depends on the N content, but if Al exceeds 0.050%, the toughness is significantly reduced, so the content should be 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.
• O oxygen
• the O content is limited to 0.006% or less. Further, since a very large cost for refining is required to extremely reduce oxygen, the amount of oxygen may be 0.001% or more in consideration of economic efficiency.
• P is an element that is inevitably mixed from the raw material, and since it deteriorates hot workability and toughness, it should be as small as possible and limited to 0.05% or less. Preferably, it is 0.04% or less. In order to reduce P to an extremely low amount, the cost at the time of refining becomes high. Therefore, from an economic point of view, the lower limit of the amount of P may be set to 0.01%.
• S is an element that is inevitably mixed from the raw material, and it also deteriorates hot workability, toughness, and corrosion resistance. Therefore, it should be as small as possible, and the upper limit is limited to 0.0050% or less. It is preferably 0.0020% or less, more preferably 0.0010% or less. In order to reduce S to an extremely low amount, the cost at the time of refining becomes high. Therefore, from an economic point of view, the lower limit of the amount of S may be 0.0001%.
• the amount of austenite in two-phase stainless steel is close to the amount of ferrite.
• the amount of ferrite is excessive, the toughness is lowered and Cr nitrides are likely to be precipitated.
• austenite is excessive, stress corrosion cracking and ear cracking during hot rolling are likely to occur. Further, in either case, the component difference between the ferrite phase and the austenite phase becomes large, and the corrosion resistance decreases in either phase.
• the amount of austenite in which the above-mentioned problems are unlikely to occur in the component system of the present invention is defined as 30 area% or more and 70 area% or less.
• the austenite content of the two-phase stainless steel base material is 30 to 70 area%, and the following formula (1) of the two-phase stainless steel base material is used.
• the PREN value defined in is required to be 28.0 or more and 35.0 or less.
• the preferable range of the amount of austenite in the duplex stainless steel base material is 40 to 60 area%.
• NI (Cr + Mo + 2 ⁇ Ni) / N ... (2)
• the element symbol in the above formula (2) indicates the content (mass%) of each element.
• This NI value is an index of the level at which chromium nitride precipitation is delayed in the relationship between Cr, Mo, Ni and N. Even in the case of welding in which this value is 170 or more and no filler metal is used as in the present invention, the precipitation is limited to a level such that the corrosion resistance does not fall below the SUS316L welded joint. It is preferably 185 or more.
• the amount of austenite in the present invention is obtained by collecting a cross section parallel to the rolling direction of the thick steel plate from a position corresponding to t / 4 (t is the plate thickness) of the base steel plate and resin.
• the ferrite fraction (area%) is measured by performing image analysis by observing with an optical microscope after performing electropolishing in a KOH aqueous solution by embedding in a mirror surface, and determining the amount of austenite in the remaining portion.
• the amount of austenite in the weld metal and the weld heat-affected portion is determined by collecting a test piece so as to include the weld portion (weld metal and the weld heat-affected portion) and the base metal in the vicinity thereof, and rolling the two-phase stainless steel base metal. By performing etching treatment, observation with an optical microscope, and image analysis in the same manner as for the two-phase stainless steel base metal using a mirror-polished directional cross section, each of the weld metal and the weld heat affected part The amount of austenite in the metallographic structure of the metal structure is measured.
• the weld metal constituting the welded joint of the present invention is derived from a melt-solidified base metal, which is a duplex stainless steel base material. It has almost the same components as the components. However, as described above, the gas components C, N, and O cause a difference in component from the base material due to volatile evaporation or mixing when the metal is melted.
• C may be mixed into the weld metal due to adhering oil, etc. during welding, but it is harmful to corrosion resistance. If the content exceeds 0.060%, C becomes Cr in the state of welding and when reheated. Cr carbides are precipitated by bonding, and the intergranular corrosion resistance and pitting corrosion resistance are significantly deteriorated. On the other hand, the lower limit is 0.001% from the viewpoint of reducing the amount of C, so the content is 0.001 to 0. Limited to .060%.
• N is a strong austenite-forming element, which improves pitting corrosion resistance in a chloride environment.
• the pitting corrosion resistance and crevice corrosion resistance of the weld metal are improved at 0.100% or more, and the larger the content, the greater the effect.
• the N content of the weld metal is limited to 0.150 to 0.250%. It is preferably 0.120% or more, more preferably 0.130%, and 0.200% or less.
• O produces oxides, and excessive content significantly reduces toughness, so the upper limit of the content was set to 0.0100%.
• the C, N, and O contents of the weld metal are measured by cutting out the weld metal portion and performing an analysis in accordance with JIS G121-13, G1228-e) and G1239, respectively.
• the amount of austenite is close to the amount of ferrite, but the amount of austenite phase produced in the weld metal tends to be small, especially in the case of welding without a filler metal as in the present invention. Yes, we will try to increase the amount of austenite phase as much as possible. Further, in order to suppress the precipitation of Cr nitride from the ferrite phase as much as possible, welding is performed by controlling the amount of N at the time of welding so that the NI value defined by the above formula (2) is 170.0 or more. Improve the metal composition. In addition, the amount of austenite that does not cause a problem that the corrosion resistance is lowered as compared with SUS316L is defined as 8 area% or more and 70 area% or less.
• the PREN value which is an index of pitting corrosion resistance
• the amount of austenite in the weld metal is 8 area% or more and 70 area% or less
• 316 L and the base metal of the steel are used.
• the corrosion resistance of the weld metal is lower than that of the 316L welded joint in a steam water environment due to the local deterioration of the corrosion resistance due to the segregation of components and the deterioration of the corrosion resistance due to the increase in the amount of ferrite from the base metal.
• the amount of austenite in the weld metal needs to be 8 area% or more and 70 area% or less, and the PREN value of the weld metal needs to be 29.0 or more.
• the preferable lower limit of the amount of austenite in the weld metal is 10.0 area%, and the more preferable lower limit is 60.0 area%.
• the preferred upper limit of the amount of austenite in the weld metal is 65.0 area%, and the more preferable upper limit is 60.0 area%.
• the weld heat affected zone in order to secure the corrosion resistance of the welded portion of the welded joint of the present invention, it is necessary for the weld heat affected zone to have an austenite amount of 8 area% or more and 70 area% as in the case of the weld metal.
• duplex stainless steel base material and the weld metal constituting the welded joint of the present invention may contain one or more of the following optional additive elements. These optional additive elements may not be added to the duplex stainless steel base material, and the content in the duplex stainless steel base material may be 0%.
• Nb is an element that has a strong affinity for N and has an effect of further reducing the precipitation rate of chromium nitride. Therefore, the steel of the present invention contains 0.005% or more as necessary. It is preferably 0.010% or more, more preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when Nb is contained in an amount of more than 0.150%, a large amount of nitride of Nb is precipitated and the toughness is inhibited. Therefore, the content is set to 0.150% or less. It is preferably 0.090% or less, more preferably 0.070% or less, and more preferably 0.050% or less. Although Nb is an expensive element, the cost of the stainless steel melting raw material can be reduced by positively using Nb contained in low-grade scrap. It is preferable to reduce the melting cost of the Nb-containing steel by such a method.
• Ti has a very strong affinity with N and may be contained because it forms a nitride of Ti in steel. Therefore, when Ti is contained, it is necessary to use a very small amount. If the content exceeds 0.020%, the toughness will be inhibited by the nitride of Ti. Therefore, the content thereof is 0.020% or less, preferably 0.015% or less, more preferably 0.010%. It should be as follows. When Ti is contained, the content is preferably 0.003% or more, preferably 0.005% or more, and more preferably 0.006% or more in order to obtain the effect.
• Ta is an element that improves corrosion resistance by modifying inclusions, and may be contained if necessary. Since the effect is exhibited by the content of Ta of 0.005% or more, the lower limit of the amount of Ta is set to 0.005% or more. When the Ta amount exceeds 0.200%, the toughness is lowered. Therefore, the upper limit of the Ta amount is preferably 0.200% or less, more preferably 0.100% or less. When the effect is exhibited with a small amount of Ta, the amount of Ta is preferably 0.050% or less.
• W is an element that improves the corrosion resistance of stainless steel like Mo, and may be contained. It may be contained in the steel of the present invention for the purpose of enhancing corrosion resistance. However, since it is an expensive element, it should be 1.00% or less. It is preferably 0.70% or less, more preferably 0.50% or less. When W is added, the W content is preferably 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more in order to obtain the effect.
• V is an element that has an affinity for N and has the effect of reducing the precipitation rate of chromium nitride. Therefore, it may be contained. However, if the content exceeds 0.300%, a large amount of V nitride is precipitated and the toughness is inhibited. Therefore, the V content is 0.300% or less, preferably 0.250% or less. , More preferably 0.200% or less. When V is contained, the content may be 0.010% or more, preferably 0.030% or more, and more preferably 0.080% or more in order to obtain the effect.
• Ca and Mg are added to control the composition of inclusions in the steel of the present invention and to enhance the pitting corrosion resistance and hot workability of the steel of the present invention.
• the Ca content is reduced to 0 by adding it together with Al of 0.0030% or more and 0.0500% or less using a dissolving raw material, or by adjusting the content through deoxidation and desulfurization operations.
• the Mg content is controlled to 0.0005% or more and 0.0005% or more. It is preferable that Ca is 0.0010% or more, Mg is 0.0010% or more, and more preferably Ca is 0.0015% or more and Mg is 0.0015% or more.
• the contents of Ca and Mg should be controlled to 0.0050% or less for Ca and 0.0050% or less for Mg because excessive addition of both Ca and Mg deteriorates hot workability and toughness.
• Ca is preferably 0.0040% or less, Mg is 0.0025% or less, and more preferably Ca is 0.0035% or less and Mg is 0.0020% or less.
• Co is an element effective for enhancing the toughness and corrosion resistance of steel, and may be contained. Co is an expensive element, and even if it is contained in excess of 1.00%, the effect commensurate with the cost will not be exhibited. Therefore, it is preferable to contain Co in an amount of 1.00% or less. It is preferably contained in an amount of 0.70% or less, more preferably 0.50% or less. When Co is contained, the content may be 0.01% or more, preferably 0.03% or more, and more preferably 0.10% or more in order to obtain the effect.
• Cu may be contained because it is an element that additionally enhances the corrosion resistance of stainless steel to acid and has an action of improving toughness. If Cu is contained in an amount of more than 3.00%, ⁇ Cu is precipitated and embrittled in excess of the solid solubility during cooling after hot rolling. Therefore, it is preferable to contain Cu in an amount of 3.00% or less. It is preferably contained in an amount of 1.70% or less, more preferably 1.50% or less. When Cu is contained, it is preferable to contain it in an amount of 0.01% or more, preferably 0.33% or more, and more preferably 0.45% or more.
• B is an element that improves the hot workability of steel, and may be contained if necessary. Further, it is an element having a very strong affinity for N, and when it is contained in a large amount, the nitride of B is precipitated and the toughness is inhibited. Therefore, the content may be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When B is contained, the B content may be 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0014% or more in order to obtain the effect.
• REM is an element that improves the hot workability of steel, and for that purpose, it may be added so that its content is 0.005% or more. It is preferably contained in an amount of 0.010% or more, more preferably 0.020% or more. On the other hand, excessive addition lowers hot workability and toughness, so it is preferable to contain 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.
• REM is the sum of the contents of lanthanoid rare earth elements such as La and Ce.
• Zr, Hf, and Sn segregate at the grain boundaries and suppress the coarsening of crystal grains during welding. Further, Zr and Hf are conventionally effective elements for improving hot workability, cleanliness of steel, and improving oxidation resistance. Sn is concentrated near the surface and suppresses the oxidation of Cr.
• the base metal and the weld metal contain at least one element of the Zr, Hf, and Sn element groups instead of the Ni, Cu, Mo, and W element groups. It may be contained in a range.
• the contents of Zr, Hf, and Sn are Zr: 0.050% or less, Hf: 0.080% or less, and Sn. : Must be 0.100% or less.
• N amount In the present invention, the N amount of the duplex stainless steel base material and the weld metal satisfies the following formula (3). N ⁇ (0.08Cr + 0.08Mo-0.06Ni-1.21) /0.4 ⁇ 0.08 ... (3) However, the element symbol in the formula (3) indicates the content (mass%) of each element.
• Formula (3) is required for two-phase stainless steel base metal and weld metal for making the amount of austenite in the weld heat-affected zone and weld metal 8% or more, respectively, in non-consumable electrode type welding that does not use filler metal.
• This is a formula for estimating the amount of N from the contents of Cr, Ni, and Mo, which are the main elements.
• the component formula for estimating the amount of austenite in the two-phase stainless steel is, for example, Ni-bal described in Patent Document 1. However, all of these estimate the amount of austenite in the solution-heat-treated steel material. In this case, Cr and Mo are distributed and concentrated in the ferrite phase, and Ni and N are distributed and concentrated in the austenite phase to form each phase.
• a ferrite single phase is once formed during heating, and then an austenite phase is formed during cooling, but at that time, Cr, Ni, and Mo are hardly concentrated, and only almost N is present.
• the austenite phase is formed by concentrating on the austenite phase.
• the amount of N concentrated in the austenite phase varies depending on the amount of Cr, Ni, and Mo, and when the austenite-forming element Ni is high, the amount of N is small, and the opposite is true for Cr and Mo.
• the austenite phase can be produced with a smaller amount of N.
• the duplex stainless steel base material and the weld metal satisfy the above-mentioned composition range
• the welded joint manufactured by the above-mentioned manufacturing method is the JIS G0577 A method measured at 50 ° C. for the welded portion including the weld metal and the heat-affected zone.
• the pitting potential is 0.27 V vs. SSE or higher.
• the welded joint manufactured by the above-mentioned manufacturing method has a pitting potential of 0.27 V vs SSE or more according to the JIS G0577 A method measured at 50 ° C. for the steel base material.
• the welded joint of the present invention has corrosion resistance equal to or higher than that of the SUS316L welded joint.
• the welded joint of the present invention preferably has an impact value of 100 J / cm 2 or more when the Charpy impact test is carried out at a test temperature of ⁇ 20 ° C. by the test method specified in JIS Z 2242.
• the plate thickness of the duplex stainless steel base material is not limited.
• the test piece of the duplex stainless steel base material used for measuring the impact value is 10 mm from the portion not including the weld to the plate thickness center. It is created by cutting out a thick member and processing a V notch in the direction perpendicular to rolling with respect to the member. Further, for the test piece of the welded portion used for measuring the impact value, a 10 mm thick member including the welded metal and the heat-affected zone is cut out with the welded line as the center by removing the surplus of the welded metal of the welded joint. , Created by machining a V notch in the direction perpendicular to the weld line of the member. If the plate thickness of the base material is less than 10.0 mm, collect the test piece with the base material plate thickness as it is.
• the welding joint of the present invention is formed by welding the duplex stainless steel base metal by a non-consumable electrode type welding method without using a filler metal.
• a welding method tungsten arc welding, plasma welding, laser welding and the like can be exemplified.
• the N content in the weld metal is affected by the welding conditions of the duplex stainless steel base metal, particularly the N content contained in the shield gas and the amount of heat input during welding.
• the present inventors consider the amount of nitrogen in the shield gas Ngas (vol%) and the amount of heat input Q (J / cm) in the weld metal. It was found that the N content (mass%) can be estimated within the range of ⁇ 10.0% from the value of Nweld defined by the following formula (5).
• Q [Welding current (A)] x [Welding voltage (V)] ⁇ [Welding speed (cm / s)] ...
• Nweld Nbase x ⁇ 0.978-0.0029 x ⁇ (Q) + 0.28 x ⁇ (Ngas) + 0.00074 x ⁇ (Q) x ⁇ (Ngas) ⁇ ... (5)
• Q indicates the amount of heat input to welding (J / cm).
• the range of the welding heat input is the range in which the weld metal melts and does not melt down.
• the upper limit of the amount of heat input for welding is preferably 50,000 (J / cm) in order to suppress the precipitation of nitrides.
• the welding speed is 0.2 to 7.0 cm / s and the shield gas flow rate is 5 to 50 liters / min at the position of the melting pool.
• the shield gas preferably has a composition consisting of a pure Ar gas: 90.0 Vol% or more, a pure N 2 gas of 10.0 vol% or less, and an impurity gas having a balance of less than 0.1 vol%.
• the present inventor can estimate that the value of Nweld in the predetermined welding heat input Q (J / cm) calculated using the above equations (4) and (5) is the nitrogen content (mass%) of the weld metal. Found.
• the value of Nweld defined by the above formula (5) becomes 0.100 or more and 0.250 or less.
• the manufacturing method of the present invention can be applied not only to the manufacturing of welded joints but also to the repair welding of those structures.
• a welded joint made of a duplex stainless steel base material and a weld metal having a defined component content as described above is manufactured, welding is performed under the above-mentioned welding conditions to obtain excellent low temperature toughness.
• a welded joint having a weld metal whose corrosion resistance is ensured in a steam water environment can be stably obtained.
• Duplex stainless steels whose components are shown in Tables 1-1 and 1-2 were melted in an MgO crucible in a 50 kg vacuum induction furnace in a laboratory and cast into a flat steel ingot having a thickness of about 100 mm.
• the material for hot rolling is processed from the main body of the ingot, heated to a temperature of 1180 ° C for 1 to 2 hours, rolled so that the rolling reduction ratio of 1050 ° C or less is 35%, and heat of 12 mm thickness x about 700 mm length.
• An inter-rolled thick steel plate was obtained.
• the steel material temperature immediately after rolling was spray-cooled from 800 ° C. or higher to 200 ° C. or lower.
• the final solution heat treatment was carried out under the conditions of 1050 ° C.
• a groove with a groove angle of 60 ° and a root surface of 1 mm was prepared.
• the welding current is 180 to 250 A
• the arc voltage is 11 to 14 V
• the welding speed is 10 to 25 cm / min
• the shield gas flow rate is 15 liters / min.
• Tables 2-1 and 2-2 show the base metal used, the welding method, and the amount of heat input for welding.
• TIG is a welding method by TIG welding
• plasma is a welding method by plasma welding
• laser is a welding method by "laser welding”. Indicates that.
• any of the above welding methods in this embodiment was performed without using either the filler rod or the filler metal.
• the N content (mass%) and O content (mass%) of the weld metal formed under the conditions of Table 2-1 or Table 2-2 are shown in the above formula. Estimated from the PREN value of the weld metal obtained from (1), the NI value of the weld metal obtained by the formula (2), the amount of N (mass%) obtained from the formula (3), and the formula (5). The N content (mass%) of the weld metal is shown.
• the PREN value of the weld metal was calculated using the components of the base metal.
• the surfaces of the welded pitting test piece and the base metal pitting test piece are polished with a polishing particle size of # 600, and the pitting potential is in a 3.5% NaCl solution at 50 ° C. on a surface 1 mm below the surface.
• the results are shown in Table 3.
• the underline in Table 3 indicates that it is outside the scope of the present invention.
• a welded part test piece was cut out in the same manner as the welded part pitting test piece, and a standard test piece conforming to JIS Z 2242 was cut out from this test piece.
• a V-notch test piece is prepared by processing a V-notch in a direction perpendicular to the welding line of this standard test piece, and the test temperature is ⁇ 20 ° C. using the V-notch test piece in accordance with JIS Z 2242.
• the Charpy impact test was conducted at. The results are shown in Table 3.
• the welded joints of the examples of the present invention had a pitting potential of 0.27 V vs SSE or more at 50 ° C. This indicates that it has good corrosion resistance equal to or higher than that of the SUS316L welded joint.
• the N amounts of the base metal and the weld metal do not satisfy the relationship of the above formula (3).
• the N amount of the weld metal does not satisfy the relationship of the above formula (3).
• the N amount of the weld metal is out of the range of the present invention and does not satisfy the relationship of the above formula (3).
• the welding joints of 64 and 65 are described in Invention Example No. Although it is a comparative example corresponding to 44 and 45, it is considered that the N 2 gas content in the shield gas was too small according to the relationship of the above formula (5).
• the present invention in a brackish water environment such as a sluice near the mouth of a river, it has sufficient corrosion resistance equal to or higher than that of a SUS316L welded joint, and it is possible to reduce the weight due to high toughness, which is suitable for industrial and environmental aspects. The contribution is extremely large.

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