WO2020138490A1 - 溶接構造物及びその製造方法 - Google Patents
溶接構造物及びその製造方法 Download PDFInfo
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- WO2020138490A1 WO2020138490A1 PCT/JP2019/051604 JP2019051604W WO2020138490A1 WO 2020138490 A1 WO2020138490 A1 WO 2020138490A1 JP 2019051604 W JP2019051604 W JP 2019051604W WO 2020138490 A1 WO2020138490 A1 WO 2020138490A1
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- 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
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- 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
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
-
- 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
- B23K9/16—Arc welding or cutting making use of shielding gas
- B23K9/167—Arc welding or cutting making use of shielding gas and of a non-consumable electrode
-
- 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
- B23K9/16—Arc welding or cutting making use of shielding gas
- B23K9/173—Arc welding or cutting making use of shielding gas and of a consumable electrode
-
- 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
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- 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
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- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- 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
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- 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
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- 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 structure using duplex stainless steel and a manufacturing method thereof.
- Duplex stainless steel has higher strength than other stainless steels and carbon steel in addition to corrosion resistance, and can be made thinner and lighter, which has the great advantage of reducing the weight increase due to the increase in the size of the structure, and has become widely used. It was
- SUS821L1 is a steel type developed as a substitute for SUS304
- SUS323L is developed as a substitute for SUS316L.
- duplex stainless steel In the case of duplex stainless steel, it is necessary to consider the toughness of the weld and the deterioration of corrosion resistance. N added to the duplex stainless steel precipitates as Cr nitride by heating and cooling during welding. This nitride reduces the toughness of the welded portion by promoting the propagation of cracks, and also consumes Cr by precipitation to form a so-called Cr-deficient layer, thereby reducing corrosion resistance.
- SUS323L has a corrosion resistance of the base metal equal to or higher than that of SUS316L, but may fall below the corrosion resistance level of SUS316L depending on the welding conditions.
- SUS821L1 is a component system that can suppress the deterioration of the corrosion resistance of the welded portion as shown in Patent Document 1, but since it is a SUS304 alternative steel, it is not suitable for use in a welded structure in a brackish water environment.
- SUS329J3L, SUS329J4L, and SUS327L have very excellent corrosion resistance, but contain expensive Mo in an amount of 3% or more and are very expensive.
- Patent Document 2 describes an improved duplex stainless steel of SUS329J1 in which corrosion resistance of a welded portion is improved by adding appropriate N in relation to Ni.
- the steel is designed based on the premise that there is no filler metal by TIG welding, and to what extent the welded structure manufactured by welding the duplex stainless steel has toughness in the base material and the welded portion. It is not disclosed in Reference 2.
- Patent Document 3 describes a welding method in which nitrogen is mixed in the weld metal by using a filler material coated with a coating material containing nitrogen, but a special filler material is required and the base metal No improvement is made to the weld heat affected zone.
- Patent Document 4 discloses a duplex stainless steel welded structure having corrosion resistance in an ozone-containing water environment by optimizing elemental components. However, there is no mention of the heat affected zone of the base metal. Further, Patent Document 5 discloses a duplex stainless steel in which deterioration of corrosion resistance due to precipitation of nitrides in the HAZ portion is suppressed by optimizing elemental components. However, neither invention is premised on use in a brackish environment.
- the present invention provides a welded structure having excellent corrosion resistance of a welded portion in a brackish water environment and excellent toughness as a structure using a duplex stainless steel having a Mo content of less than 3%.
- the purpose is to do.
- the present inventors have conducted detailed research on the components of steel materials, the components of weld metals, the manufacturing conditions of steel materials, and the welding conditions from the viewpoint of improving corrosion resistance and toughness in a brackish water environment.
- the pitting corrosion index (PREN) is often expressed by the formula Cr+3.3Mo+16N in duplex stainless steel.
- the present inventors used this formula to estimate by simulation calculation a method for increasing the corrosion resistance of the welded portion of SUS329J1 by incorporating N in the composition range of SUS329J1, and confirmed by experiments.
- the value of PREN (the following formula (1)) is 28 or more, the required corrosion resistance is satisfied even if the corrosion resistance decrease due to the precipitation of Cr nitride in the heat affected zone is taken into consideration.
- the amount of austenite is secured as described below, and the PREN value is 30.0 or more and Mo is appropriately set. It has been clarified that by increasing the amount, a duplex stainless steel having corrosion resistance equal to or higher than SUS316L can be obtained.
- PREN Cr+3.3Mo+16N (1)
- the corrosion resistance equal to or higher than that of SUS316L means that "the pitting potential measured by JIS G0577 A method at 50°C is 0.30 V vs SSE or more".
- the amount of austenite in the heat-affected zone of the weld is less than that of the base metal, the toughness is reduced due to excess ferrite, and the corrosion resistance in the austenite phase is likely to be reduced. Since the weld metal has a particularly high cooling rate, not only the time during which the austenite phase can reprecipitate is limited, but also it is necessary to consider the local decrease in the components, and from the viewpoint of securing toughness, the amount of Ni should be increased appropriately. The present inventors have made earnest studies from the viewpoint that it is necessary to do so.
- the composition is adjusted so that the N amounts of the steel material and the weld metal satisfy the following formula (3).
- the present inventors have found that it effectively acts to improve the strength and corrosion resistance of duplex stainless steel. N ⁇ (0.08Cr+0.08Mo ⁇ 0.06Ni ⁇ 1.21)/0.6 ⁇ 0.15 (3)
- the formula (3) represents the amount of N effectively acting to improve the strength and corrosion resistance of the duplex stainless steel when the lower limits of the austenite amount of the weld heat affected zone and the weld metal in the present invention are 15%, respectively. It is an equation estimated from the contents of Cr, Ni and Mo which are main elements. The present invention has been made based on these findings, and the gist thereof is as follows.
- PREN Cr+3.3Mo+16N (1)
- the element symbol in the formula (1) indicates the content (mass %) of each element, and 0 is substituted when not containing.
- the composition of the duplex stainless steel base material satisfies the expression (2), and the N amounts of the duplex stainless steel base material and the weld metal satisfy the expression (3), Further, when the duplex stainless steel base material contains Nb, the chromium nitride precipitation temperature TN of the duplex stainless steel base material is 1010° C.
- a chromium nitride precipitation temperature TN of the duplex stainless steel base material is 980° C. or lower.
- T ⁇ 1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N ⁇ 1330...(2)
- the element symbols in the formulas (2) and (3) indicate the contents (mass %) of the respective elements, and 0 is substituted when they are not contained.
- Thickness of material for hot rolling/thickness of duplex stainless steel base material (6) (Thickness when reaching 1050°C or lower-thickness of duplex stainless steel base material)/thickness when reaching 1050°C or lower ⁇ 100 (7)
- the weld metal is formed by gas shield arc welding or tungsten arc welding using a filler rod, and the welding heat input Q defined by the following formula (8) is 5,000 J/cm or more 50,
- the method for producing a welded structure according to (5) which is formed under welding conditions of 000 J/cm or less and a base material dilution ratio D defined by the following formula (9) of 50% or less.
- the welded structure obtained by the present invention has sufficient corrosion resistance equal to or higher than that of SUS316L in a brackish water environment such as a floodgate near the mouth of a river, and further achieves weight reduction due to high strength, resulting in significant cost reduction. It can contribute to higher efficiency, and has a great deal of contribution to industrial and environmental aspects.
- FIG. 51 is a partially enlarged cross-sectional view of welded portions 51 to 88.
- composition of Duplex Stainless Steel Base Material First, the reasons for limiting the composition and structure of the duplex stainless steel base material forming the welded structure of the present invention will be described below. In this specification, unless otherwise specified,% relating to components represents% by mass.
- [Essential element] C is limited to 0.050% or less in order to secure the corrosion resistance of stainless steel. If the content exceeds 0.050%, Cr carbide is generated during hot rolling, and corrosion resistance and toughness deteriorate. It is preferably 0.030% or less, and more preferably 0.025% or less. On the other hand, the lower limit is 0.001% from the viewpoint of the cost of reducing the C content of stainless steel.
- Si is contained in 0.05% or more for deoxidation. It is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, if the content exceeds 0.80%, the toughness deteriorates. Therefore, it is 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 the toughness. It also has the effect of lowering the nitride precipitation temperature TN. Due to the toughness of the base material and welded portion, 0.10% or more is contained. It is preferably 0.30% or more, more preferably 0.50% or more. On the other hand, Mn is an element that reduces 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 an amount of 21.50% or more in order to secure the basic corrosion resistance of the steel of the present invention. It is preferably 22.00% or more, and more preferably 23.00% or more. On the other hand, if Cr is contained in an amount of more than 26.00%, the ferrite phase fraction increases and the toughness and the corrosion resistance of the welded portion are impaired. Therefore, the content of Cr is set to 26.00% or less. It is preferably 25.00% or less, more preferably 24.50% or less.
- Ni is contained in an amount of 3.00% or more in order to stabilize the austenite structure, improve corrosion resistance to various acids, and further improve toughness. By increasing the Ni content, it becomes possible to lower the nitride precipitation temperature. It is preferably 4.00% or more, and more preferably 5.00% or more. On the other hand, Ni is an expensive alloy, and in the steel of the present invention intended for alloy-saving duplex stainless steel, the content is limited to 7.00% or less from the viewpoint of cost. It is preferably 6.50% or less, more preferably 6.00% or less.
- Mo is a very effective element that enhances the corrosion resistance of stainless steel, and is contained in an amount of 0.50% or more in order to impart corrosion resistance of SUS316 or more. It is preferably 0.80% or more, and more preferably 1.00% or more. On the other hand, Mo is an element that promotes the precipitation of intermetallic compounds while being expensive. In the steel of the present invention, it is preferable that the Mo content is small from the viewpoint of suppressing precipitation during hot rolling and from the economical viewpoint. 50% or less. It is preferably less than 2.00%, more preferably 1.80% or less, and further preferably 1.50% or less.
- N is an effective element that forms a solid solution in the austenite phase and enhances the strength and corrosion resistance of the duplex stainless steel, so N is contained by 0.100% or more. It is preferably 0.120% or more, and more preferably 0.150% or more.
- the solid solution limit increases depending on the Cr content, but in the steel of the present invention, if the content exceeds 0.250%, Cr nitrides are precipitated and toughness and corrosion resistance are impaired. Therefore, the N content is 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 deoxidizing steel, and is contained together with Ca and Mg 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 with N, and when added in excess, it forms a nitride of Al and impairs the toughness of stainless steel. The degree depends on the N content, but if Al exceeds 0.050%, the toughness is significantly deteriorated, so the content is preferably 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.
- the balance is Fe and impurities.
- the impurities are those that are mixed from the ore as a raw material, scrap, or the manufacturing environment when the steel base material is industrially manufactured, and are allowed within a range that does not adversely affect the steel. Means what is done.
- the main impurities include P, S, and O, but are not limited thereto, and other elements may be contained as impurities.
- O oxygen
- the O content is limited to 0.006% or less. Further, since extremely high cost is required for refining to extremely reduce oxygen, the oxygen amount may be 0.001% or more in consideration of economical efficiency.
- ⁇ P is an element that is inevitably mixed from the raw material and deteriorates the hot workability and toughness. Preferably, it is 0.040% or less. To reduce P to an extremely low amount, the cost for refining becomes high. Therefore, the lower limit of the amount of P should be set to 0.010% from the viewpoint of cost.
- S is an element that is inevitably mixed from the raw material and deteriorates hot workability, toughness, and corrosion resistance, so it is preferable that it is as small as possible, and the upper limit is limited to 0.0050% or less. It is preferably 0.0020% or less, and more preferably 0.0010% or less. To reduce S to an extremely low amount, the cost for refining becomes high. Therefore, the lower limit of the amount of S may be set to 0.0001% in consideration of cost.
- the austenite content is preferably close to the ferrite content.
- the toughness is lowered and the precipitation of Cr nitride is likely to occur.
- the amount of austenite is excessive, stress corrosion cracking and edge cracking during hot rolling tend to occur. Further, in both cases, the component difference between the ferrite phase and the austenite phase becomes large, and the corrosion resistance decreases in either phase.
- the lower limit of the amount of austenite in which the above problems are less likely to occur in the component system of the present invention is defined as 30 area% and the upper limit is defined as 70 area %.
- the duplex stainless steel base material for a welded structure according to the present invention has an austenite content of 30.0 to 70.0 area% and the above formula.
- the PREN value defined in (1) is 28.0 or more.
- the preferable lower limit of the PREN value of the duplex stainless steel base material is 30.0.
- the PREN value of the duplex stainless steel is preferably 35.0 or less.
- the preferable lower limit of the amount of austenite in the duplex stainless steel base material is 40.0 area %, and the preferable upper limit thereof is 60.0 area %.
- the amount of austenite in the present invention is, in the case of a duplex stainless steel base metal, a cross section parallel to the rolling direction of the thick steel plate is sampled from a position corresponding to t/4 (t is the plate thickness) of the base metal plate and embedded in resin. After mirror polishing and electrolytic etching in a KOH aqueous solution, the ferrite fraction (area %) is measured by performing image analysis by observation with an optical microscope, and the remaining portion is determined as the amount of austenite.
- the amount of austenite in the weld metal and the heat-affected zone of the weld is measured by collecting a test piece so that the weld metal (weld metal and the heat-affected zone of the weld) and the base metal in the vicinity thereof are collected, Using a mirror-polished directional cross-section, in the same way as in the case of duplex stainless steel base material, by performing etching treatment, observation with an optical microscope and image analysis, each of the weld metal and the weld heat affected zone The amount of austenite in the metallic structure of is measured.
- [Essential element] C is detrimental to corrosion resistance, but is preferably contained to some extent from the viewpoint of strength, so the C content is 0.001% or more. Further, if its content exceeds 0.060%, C is combined with Cr to precipitate Cr carbides when it is left in the as-welded state and reheated, and the intergranular corrosion resistance and pitting corrosion resistance are significantly deteriorated. Since the toughness and ductility of the weld metal are remarkably reduced, the content thereof is limited to 0.001 to 0.060%.
- Si is added as a deoxidizing element, but if its content is less than 0.05%, its effect is not sufficient. On the other hand, if its content exceeds 0.80%, the ductility is reduced and the toughness is significantly reduced, and at the time of welding. The melt penetration of is also reduced, which is a problem in practical welding. Therefore, its content is limited to 0.05 to 0.80%.
- Mn is added as a deoxidizing element and as an element that increases the solubility of N, but if its content is less than 0.10%, the effect is not sufficient, while if it exceeds 3.00%, the ductility decreases. Therefore, the lower limit of the content is set to 0.10 and the upper limit is limited to 3.00%.
- the Mn content is preferably 2.00% or less.
- Cr forms a passive film as a main element of stainless steel and contributes to the improvement of corrosion resistance. To obtain excellent corrosion resistance in brackish water environment, 21.50% or more is contained. On the other hand, as the Cr content increases, the pitting corrosion resistance in a brackish water environment improves, but the brittle intermetallic compound such as the sigma phase ( ⁇ phase) easily precipitates, and the toughness decreases. Further, since Cr is a ferrite-forming element, it is necessary to increase the amounts of Ni, Cu, and N in order to secure the austenite phase, which reduces the manufacturability of the wire used for welding and increases the manufacturing cost. The upper limit of the content is set to 28.00%. It is preferable to set it to 26.00% or less.
- Ni gives remarkable resistance to corrosion in a neutral chloride environment and strengthens the passive film, the higher the Ni content, the more effective the corrosion resistance. Further, Ni is an austenite forming element and forms and stabilizes the austenite phase. As described above, in the weld metal, the cooling rate is particularly high, the time during which the austenite phase can be reprecipitated is limited, it is necessary to consider the local decrease in the composition, and the toughness is further secured. It is desirable to increase the amount. In the present invention, in order to secure sufficient austenite formation in the weld metal, from the viewpoint of the phase balance when the weld metal contains 21.50 to 28.00% of Cr, which is a ferrite forming element, Ni is more preferable than the steel base metal.
- the lower limit is 4.00% and the upper limit is 10.00%.
- the reason why the upper limit of the Ni content is 10.00% is that the manufacturing cost of the wire used for welding becomes high. It is preferably 6.00% or more.
- Mo is an extremely effective element for stabilizing the passive film and obtaining high corrosion resistance, and the improvement of pitting corrosion resistance is particularly remarkable in a chloride environment.
- the weld metal in addition to the above, it is necessary to consider the local corrosion resistance decrease due to the occurrence of component segregation. As a result of the experiment, it was found that if it is less than 1.00%, the effect of improving the corrosion resistance is insufficient. Further, in order to compensate for the decrease in austenite in the weld metal, it is preferable to increase the Mo content in the weld metal higher than that of the steel base material.
- the lower limit is set to 1.00 and the upper limit is limited to 3.50%.
- it is 2.00% or more and 3.00% or less.
- N is a strong austenite forming element and improves pitting corrosion resistance in chloride environment. If it is 0.080% or more, the pitting corrosion resistance and the crevice corrosion resistance are improved, and the larger the content, the greater the effect. On the other hand, if the N content is increased, especially if it exceeds 0.250%, blowholes are likely to occur during welding. Therefore, the lower limit of the N content is limited to 0.080% and the upper limit is limited to 0.250%. It is preferably 0.100% or more and 0.200% or less.
- Al is added as a deoxidizing element and as an element for improving the droplet transfer phenomenon, but if it is less than 0.001%, its effect is not sufficient, while excessive addition thereof reacts with N to react with AlN.
- the balance is Fe and impurities.
- the impurities are those that are mixed from the ore as a raw material, scrap, or the manufacturing environment when the steel base material is industrially manufactured, and are allowed within a range that does not adversely affect the steel. Means what is done.
- the main impurities include P, S, and O, but are not limited thereto, and other elements may be contained as impurities.
- ⁇ O, P, and S are unavoidable components in weld metal, and are limited to a small amount for the following reasons.
- O forms an oxide, and excessive content significantly reduces toughness, so the upper limit of its content was made 0.150%.
- the content be small, and the upper limit of its content was made 0.050%.
- the presence of a large amount of S also lowers the hot crack resistance, ductility and corrosion resistance, so it is preferable that the content be small, and 0.0200% was made the upper limit.
- the amount of austenite in the weld metal is close to the amount of ferrite.
- the weld heat-affected zone and the weld metal tend to have a small amount of austenite phase formation, and in addition to increasing the austenite phase as much as possible, for the weld metal, the austenite amount further decreases from the weld heat-affected zone.
- the composition is improved by a filler rod such as a steel welding wire.
- the amount of austenite that does not cause the problem of deterioration in corrosion resistance as compared with SUS316L is specified to be 15 area% or more and 70 area% or less.
- PREN is an index of pitting corrosion resistance
- the amount of austenite is 15 area% or more and 70 area% or less, and the weld metal PREN is 30.0 or more.
- the preferable lower limit of the amount of austenite in the weld metal is 18.0 area %, and the further preferable lower limit is 20.0 area %.
- the preferable upper limit of the amount of austenite in the weld metal is 60.0 area %, and the further preferable upper limit is 50.0 area %.
- the PREN value of the weld metal is preferably higher than the PREN value of the duplex stainless steel base material.
- the PREN value of the weld metal is preferably 35.0 or less.
- the weld heat affected zone in order to secure the corrosion resistance of the welded portion of the welded structure of the present invention, also has an austenite amount of 15 area% or more and 70 area or more, similar to the weld metal. %.
- duplex stainless steel base metal and weld metal constituting the welded structure of the present invention is one of the following elements. 0% or more can be contained if necessary. However, the object of the present invention can be achieved without containing any of these elements.
- Nb is an element having a strong affinity with N and having an action of further reducing the precipitation rate of chromium nitride. Therefore, the base metal and weld metal of the welded structure of the present invention may contain 0.005% as a lower limit. It is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more. On the other hand, when Nb exceeds 0.150%, a large amount of Nb nitride precipitates, which impairs toughness, so the content was set to 0.150% or less. It is preferably 0.090% or less, more preferably 0.070% or less, and further preferably 0.050% or less. Although Nb is an expensive element, the cost of raw material for melting stainless steel can be reduced by positively using Nb contained in low-grade scrap. It is preferable to reduce the melting cost of Nb-containing steel by such a method.
- Ti has a very strong affinity with N and forms a nitride of Ti in steel, so it is desirable to use a very small amount when Ti is contained. If the content exceeds 0.020%, the nitride of Ti will impair the toughness, so the content is 0.020% or less, preferably 0.015% or less, more preferably 0.010%. The following is recommended. When Ti is contained, in order to obtain the effect, it is preferable to contain 0.003% or more, preferably 0.005% or more, and more preferably 0.006% or more.
- Ta is an element that improves corrosion resistance by modifying inclusions, and may be contained if necessary. Since the effect is exhibited by the inclusion of 0.005% or more of Ta, the lower limit of the Ta content may be 0.005% or more. When the Ta amount exceeds 0.200%, the room temperature ductility and the toughness decrease, so the upper limit of the Ta amount is preferably 0.200% or less, and 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 and may be included. 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 is preferable to set it to 1.00% or less. It is preferably 0.70% or less, more preferably 0.50% or less. When added, it is preferable that the content be 0.05 or more. When W is contained, in order to obtain the effect, the W content is preferably 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.
- V is an element that has an affinity with N and has the action of reducing the precipitation rate of chromium nitride. Therefore, it may be contained. However, if the content of V exceeds 0.300%, a large amount of nitride of V precipitates and the toughness is impaired. Therefore, the content of V is 0.300% or less, preferably 0.250% or less. , And more preferably 0.200% or less. When V is contained, in order to obtain the effect, the V content may be 0.010% or more, preferably 0.030% or more, and more preferably 0.080% or more.
- Ca and Mg are added to control the composition of inclusions of the steel of the present invention and to enhance the pitting corrosion resistance and hot workability of the steel of the present invention.
- the steel containing Ca and Mg it is added together with 0.0030% or more and 0.0500% or less of Al by using a melting raw material, or the content thereof is adjusted through deoxidation and desulfurization operation, and the content of Ca is set to 0.
- the content of Mg is controlled to 0.0005% or more and the content of Mg to 0.0001% or more.
- Ca is 0.0010% or more
- Mg is 0.0003% or more
- Ca is 0.0015% or more and Mg is 0.0005% or more.
- Ca and Mg excessively decrease the hot workability and toughness, so that the content should be controlled to 0.0050% or less for Ca and 0.0050% or less for Mg. ..
- Ca is 0.0040% or less
- Mg is 0.0025% or less
- more preferably Ca is 0.0035% or less and Mg is 0.0020% or less.
- Co is an element effective for improving the toughness and corrosion resistance of steel, and may be contained. Since Co is an expensive element even if it is contained in excess of 1.00%, the effect commensurate with the cost will not be exhibited, so Co is preferably contained in 1.00% or less. The content is preferably 0.70% or less, more preferably 0.50% or less. When Co is contained, the Co 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 against acid and has the effect of improving toughness. If Cu is contained in an amount of more than 3.00%, it exceeds the solid solubility at the time of cooling after hot rolling and ⁇ Cu precipitates and becomes brittle, so it is preferable to contain 3.00% or less. The content is preferably 1.70% or less, more preferably 1.50% or less. When Cu is contained, 0.01% or more, preferably 0.33% or more, and more preferably 0.45% or more may be contained.
- B is an element that improves the hot workability of steel, and may be contained if necessary. In addition, it is an element having a very strong affinity with N, and when contained in a large amount, a nitride of B is precipitated, which impairs toughness. Therefore, the content thereof may be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less. When B is contained, in order to obtain the effect, the B content may be 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0014% or more.
- REM is an element that improves the hot workability of steel, and for that purpose 0.005% or more may be contained.
- the content is preferably 0.010% or more, and more preferably 0.020% or more.
- REM should be contained at 0.050% or less. It is preferably 0.040% or less, more preferably 0.030% or less.
- REM is the total content of lanthanoid rare earth elements such as La and Ce.
- Zr, Hf, and Sn segregate at the grain boundaries to suppress the coarsening of crystal grains during welding. Further, Zr and Hf are elements which have been conventionally effective for improving hot workability, cleanliness of steel and improving oxidation resistance. Sn is concentrated near the surface and suppresses the oxidation of Cr.
- the weld metal portion has, in place of the element group of Ni, Cu, Mo, and W, at least one element of the element groups of Zr, Hf, and Sn in the above-described content. You may contain in the range.
- the components of the duplex stainless steel base material preferably satisfy the following formula (2).
- T ⁇ 1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N ⁇ 1330...(2)
- T ⁇ is a component formula for estimating the temperature at which austenite disappears and becomes a ferrite single phase (hereinafter referred to as “ferrite single phase conversion temperature”, the unit is °C) when the duplex stainless steel base material is heated. .. If the ferrite single-phase temperature is low, the ferrite is exposed to the ferrite single-phase region for a long time during welding, the coarsening of the ferrite phase is promoted, and the toughness of the heat-affected zone of welding is reduced.
- the temperature is preferably 1330°C or higher. More preferably, it is 1340°C or higher.
- This formula was obtained by equilibrium calculation using thermodynamic calculation software "Thermo-Calc" (registered trademark) of Thermocalc Co., and corrected by experiments.
- N [Chromium nitride precipitation temperature and N content]
- the amounts of N in the duplex stainless steel base material and the weld metal satisfy the following expression (3).
- the element symbol in the formula (3) indicates the content (mass %) of each element, and 0 is substituted when not containing.
- Formula (3) is a solid solution of the austenite phase of the weld heat affected zone and the weld metal when the lower limits of the austenite amount of the weld heat affected zone and the weld metal in the present invention are set to 15%, respectively. This is an equation for estimating the amount that effectively acts to improve strength and corrosion resistance from the contents of Cr, Ni, and Mo which are main elements.
- the constituent formula for estimating the amount of austenite in duplex stainless steel is, for example, Ni-bal. There are many, etc., but all of them are for estimating the amount of austenite in the solution-annealed steel. In this case, Cr and Mo are distributed and concentrated in the ferrite phase and Ni and N are formed in the austenite phase to form the respective phases.
- the precipitate that mainly affects the material of the duplex stainless steel base material that constitutes the welded structure of the present invention is chromium nitride.
- Chromium nitride is a precipitate in which Cr and N are combined, and in duplex stainless steel, cubic CrN or hexagonal Cr 2 N often precipitates in ferrite grains or at ferrite grain boundaries.
- cubic CrN or hexagonal Cr 2 N often precipitates in ferrite grains or at ferrite grain boundaries.
- the chromium nitride precipitation temperature TN which is an index for precipitation of such chromium nitride during hot rolling, is a characteristic value experimentally obtained by the following procedure. (1) After heat-rolling a 10 mm-thick test steel, it is heat-treated once at 1050° C. for 20 minutes, then soaked at an arbitrary temperature of 800 to 1100° C. for 20 minutes, and then water-cooled within 5 seconds. .. (2) The surface of the sample steel after cooling is polished with #500.
- the duplex stainless steel base material constituting the welded structure of the present invention has a chromium nitride precipitation temperature TN of 1010° C. or lower when it contains Nb, and a chromium nitride precipitation temperature TN when it does not contain Nb. Is preferably 980° C. or lower.
- the chromium nitride precipitation temperature TN described above may be estimated using the following formula (4) or formula (5).
- 8Cr-20Ni+30Mo+50Si-10Mn+550N+730 when the duplex stainless steel base material contains Nb
- 8Cr-20Ni+30Mo+50Si-10Mn+550N+700 when the duplex stainless steel base material does not contain Nb
- the element symbols in the formulas (4) and (5) indicate the content (mass %) of each element, and 0 is substituted when they do not contain.
- the steel material used for the floodgate is often thick, for example, 20 mm or 50 mm.
- the impact value of the base material is lowered, and as a result, the toughness of the heat-affected zone is further lowered, which may be a problem.
- the hot rolling material having the composition of the duplex stainless steel base material described above is used in which the reduction ratio shown by the following formula (6) is 3.0 or more and the following formula (7) is used. It is effective to add an appropriate strain to form a fine structure by hot rolling so that the rolling reduction is 1050° C. or less and 30% or more.
- Thickness of material for hot rolling/thickness of duplex stainless steel base material of welded structure of the present invention (6) (Thickness when reaching 1050° C. or lower-thickness of duplex stainless steel base material of welded structure of the present invention)/thickness when reaching 1050° C. or lower ⁇ 100 (7)
- the "thickness when reaching 1050°C or less" is obtained by sequentially measuring the surface temperature of the hot rolling material during hot rolling and measuring the thickness when reaching 1050°C or less. ..
- hot-rolled steel sheets are heat-treated for 5 minutes or more at a chromium nitride precipitation temperature (TN) + 20°C or more and 1100°C or less. If the heat treatment temperature is less than TN+20° C. or the heat treatment time is less than 5 minutes, the chromium nitride precipitated by hot rolling does not form a solid solution and the toughness and corrosion resistance are impaired. If the heat treatment temperature is higher than 1100°C, the ferrite content may be excessive. This heat treatment may be performed continuously from the hot rolling step, or may be performed by cooling the hot rolled steel sheet and then reheating the cooled steel sheet.
- TN chromium nitride precipitation temperature
- the weld metal of the present invention can be formed by any method of gas shield arc welding or tungsten arc welding, but it is preferable to define the welding heat input Q and the base metal dilution rate D for the following reasons.
- the precipitation of intermetallic compounds such as sigma phase and Cr nitride is suppressed, and the two phases excellent in toughness and corrosion resistance are provided.
- a welded structure composed of a stainless steel base material and weld metal is obtained.
- gas shielded arc welding or tungsten arc welding if the welding heat input Q exceeds 50,000 J/cm and is excessively large, the base material dilution rate will be high and the cooling rate will be low, and the temperature of 900°C to 600°C will decrease.
- the manufacturing conditions of the welded structure that is, the welding heat input during welding to 50,000 J/cm or less.
- the welding heat input Q (J/cm) is defined by the following equation (8).
- Q (J/cm) [welding current (A)] ⁇ [welding voltage (V)] ⁇ [welding speed (cm/s)] (8)
- the weld metal in order to secure the corrosion resistance and the amount of austenite of the weld metal, the weld metal preferably has a high Mo content, a Ni content, and at least one of PREN with respect to the duplex stainless steel base material.
- the welded structure of the present invention can also be manufactured by submerged arc welding, plasma welding, etc. on the premise that proper welding rod and welding heat input control is performed. Further, the manufacturing method is applicable not only to manufacturing of welded structures, but also to repair welding or padding of those structures.
- the pitting potential of the welded portion measured by the JIS G0577 A method measured at 50° C. is 0.30 V vs SSE or more].
- the welded structure of the present invention has a pitting potential of 0.30 V vs SSE or more measured by a JIS G0577 A method at a welded portion including a weld metal and a heat-affected zone at 50°C.
- the welded structure of the present invention has corrosion resistance equal to or higher than that of SUS316L in a brackish water environment.
- the Duplex stainless steel base material forming the welded structure of the present invention has a Charpy impact value of 100 J/cm 2 or more at ⁇ 20° C. measured by the Charpy impact test method defined in JIS Z 2202. Further, the welding heat-affected zone and the weld metal of the welded structure of the present invention have Charpy impact values measured by the Charpy impact test method defined in JIS Z 2202, which are both not less than 50 J/cm 2 at ⁇ 20° C. is there.
- the present invention will be described below with reference to examples.
- the present invention example will be described based on a butt joint made of the same steel base material, but the welded structure according to the present invention is limited to the illustrated structure.
- the welded structure according to the present invention can have not only a butt joint but also a structure of a general welded joint such as a T joint, a cross joint, and a lap joint, and has a structure in which different types of welded joints are combined. You can do it.
- the welded structure according to the present invention may have a structure in which steel base materials different in at least one of steel composition and metal structure are welded.
- duplex stainless steel having the components shown in Tables 1-1 and 1-2 was melted in a MgO crucible in a laboratory 50 kg vacuum induction furnace and cast into a flat steel ingot.
- the flat steel ingot was ground so that the surface of the flat steel ingot was smooth, and a material for hot rolling of about 100 mm was prepared.
- the material for hot rolling was heated to a temperature of 1180° C. for 1 to 2 hours and then rolled so that the rolling reduction at 1050° C. or less was 35% to obtain a hot rolled thick steel plate having a plate thickness of 12 mm ⁇ about 700 mm.
- spray cooling was performed from a state in which the temperature immediately after hot rolling was 800°C or higher to 200°C or lower.
- the cooled steel plate was heat-treated so that it was soaked at 1050° C. for 20 minutes, and the steel plate was water-cooled after the heat treatment.
- T ⁇ (° C.) in Tables 1-1 and 1-2 is a temperature value defined by the above formula (2), and “value of the formula (3)” is defined by the above formula (3).
- the amount of N and the “TN estimated value (° C.)” is the value of the temperature defined by the formula (4) or the formula (5).
- the measured TN values shown in Tables 1-1 and 1-2 are the measured values of the chromium nitride precipitation temperature of each steel base material. 10 mm thick sample steels were cut out from each steel base material other than 24, and the cut-out sample steels were subjected to soaking treatment and extraction of precipitates from the soaked test steels by the procedure described above. It was measured by determining the lowest temperature of the soaking temperature at which the chromium content in the precipitate was 0.03% or less.
- the steel Nos. 1 to 8 are the welded structure Nos. It is a duplex stainless steel base material constituting 51 to 61.
- Steel Nos. 9 to 25 are welded structure Nos. 62-73, 81, 84-88 are duplex stainless steel base materials.
- Steel No. Nos. 9 to 17, 20, 21, and 24 are steel base materials that do not satisfy the requirements for the composition of the steel base material of the welded structure of the present invention.
- Nos. 13, 15, and 21 are duplex stainless steel base materials whose PREN values do not meet the requirements for the steel base material of the welded structure of the present invention.
- Steel No. 18 is a duplex stainless steel base material in which the amount of N does not satisfy the formula (3).
- Steel No. No. 19 is a duplex stainless steel base material having an excessive amount of ferrite and an insufficient amount of austenite (welded structure No. 73 in Tables 3 and 5).
- Steel No. No. 22 is a duplex stainless steel base material having an excessive amount of austenite (welded structure No. 85 in Tables 3 and 5).
- Steel No. 24 is a 12 mm thick ⁇ 700 mm long stainless steel base material
- steel Nos. in Table 1-1 and Table 1-2 Using Nos. 1 to 25 as steel base materials 11a and 11b, as shown in FIG. 1, grooves having a groove angle of 90° on one side and 35° on one side and a root interval of 4 mm were prepared.
- the steel base materials 11a and 11b have the same steel No. It is a steel base material.
- Table 2-1 and Table 2-2 show the welded structure No. No. 51 of the steel welding wire used to manufacture Nos. 51-88.
- the composition of components 31 to 43 is shown below.
- the wire diameter is 1.2 mm ⁇ .
- the welded structure No. Nos. 51 to 88 are the butt-type welded joints 1 shown in FIG. 1, and using these welding wires, steel Nos. 1 and 2 in Tables 1-1 and 1-2 were used.
- the steel base materials 1 to 25 were manufactured by abutting and welding the backing metal 2 on the back surface of the steel base material.
- the welding conditions are as shown in Table 3.
- GMAW gas shield arc welding
- GTAW tungsten arc welding
- Table 3 the welded structure No.
- the combination of the steel base material and the welding wire used for manufacturing 51 to 88, the welding method, and the welding heat input are shown below.
- GMAW indicates gas shield arc welding
- GTAW indicates tungsten arc welding.
- Tables 4-1 to 4-3 show the composition of the weld metal 12 formed under the conditions of Table 3, the dilution ratio of the base metal, PREN and the N content (mass %) defined by the above formula (3) ("equation “Value of (3)”), and the temperature estimated from the equation (4) or the equation (5) (item “TN estimated value (° C.)” in Table 4).
- Welded structure No. 62 was manufactured using the same steel base material and steel welding wire as the welded structure No. 61 of the present invention example, but since the oil and the like were mixed into the welded portion during welding, the carbon content of the weld metal became excessive. became.
- the welded structure No. shown in Table 3 was used. From 51 to 88, a pitting corrosion test piece was collected from the steel base material in the vicinity of the weld heat affected zone and the weld metal so as to include all the weld heat affected zone and the weld metal, and the sample was placed in a 3.5% NaCl solution at 50°C. The pitting potential was measured in accordance with the method specified in JIS G0577.
- a V-notch test piece was sampled in the direction perpendicular to the rolling direction based on the Charpy impact test method specified in JIS Z 2202 so as to correspond to the notch of the test piece.
- a Charpy impact test was performed on each of these V-notch test pieces at a test temperature of -20°C. The results of the pitting potential and the Charpy impact test are shown in Table 5.
- the welded structure No. The amounts of austenite contained in the respective metallographic structures of the duplex stainless steel base materials of Nos. 51 to 86 and No. 88, the weld metal and the weld heat affected zone were measured by the method described above. The results are shown in Table 5.
- the invention examples Nos. 51 to 61 have sufficient corrosion resistance equal to or higher than that of SUS316L. Further, in the invention examples Nos. 51 to 61, the Charpy impact value of the duplex stainless steel base material is 100 J/cm 2 or more at ⁇ 20° C., and the Charpy impact value of the welding heat affected zone and the weld metal is ⁇ It is 50 J/cm 2 or more at 20°C. As described above, it is understood that the invention examples Nos. 51 to 61 have excellent toughness in addition to excellent corrosion resistance.
- the welded structure No. 86 of the comparative example does not reach SUS316L in corrosion resistance because the PREN value of the weld metal was less than 30.0.
- Manufacturing conditions other than the manufacturing conditions shown in Table 6-1 were the same as the manufacturing conditions for the duplex stainless steel base materials such as Steel Nos. 1 to 8 in Tables 1-1 and 1-2. Then, using the steel base materials Nos. 1, 3, and 5, steel welding wire Nos. Using No. 31, welded structures No. 101 to 107 were manufactured under the conditions shown in Table 6-1.
- the manufacturing conditions of the welded structure Nos. 101 to 107 are the same as those of the welded structure Nos. 101 to 107 except the manufacturing conditions of Table 6-1. 51 to 86, the same as No. 88.
- the present invention in a brackish water environment such as a floodgate near the mouth of a river, it has sufficient corrosion resistance equal to or higher than that of SUS316L, and can further reduce weight due to high strength, resulting in significant cost reduction and high efficiency. It is possible to contribute, and the place that contributes to the industrial side and the environment side is extremely large.
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Abstract
Description
PREN=Cr+3.3Mo+16N・・・(1)
Tα=1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N≧1330・・・(2)
溶接金属は特に冷却速度が大きいため、オーステナイト相が再析出し得る時間が限られるだけでなく、局所的な成分低下を考慮する必要があり、更に靭性を確保する点から、Ni量を適宜増量する必要があるという観点から、本発明者ら鋭意検討を行った。
その結果、溶接熱影響部及び溶接金属のオーステナイト量の下限をそれぞれ15%とした場合であっても、鋼材及び溶接金属のN量が下記式(3)を満たすように組成を調整することによって、二相ステンレス鋼の強度、耐食性の向上に有効に作用することを、本発明者らは見出した。
N≧(0.08Cr+0.08Mo-0.06Ni-1.21)/0.6×0.15…(3)
C:0.001~0.050%、
Si:0.05~0.80%、
Mn:0.10%~2.00%、
Cr:21.50~26.00%、
Ni:3.00~7.00%、
Mo:0.50~2.50%、
N:0.100~0.250%、
Al:0.003~0.050%、
を含有し、
Oは0.0060%以下、
Pは0.050%以下、
Sは0.0050%以下に制限し、
かつ下記式(1)で定義されるPREN値が28.0以上で、
残部がFe及び不純物からなる二相ステンレス鋼母材と、
溶接金属及び熱影響部とを含む溶接部とを備える溶接構造物であって、
前記溶接金属は、
質量%で、
C:0.001~0.060%、
Si:0.05~0.80%、
Mn:0.10%~3.00%、
Cr:21.50~28.00%、
Ni:4.00~10.00%、
Mo:1.00~3.50%、
N:0.080~0.250%、
Al:0.001~0.100%、
を含有し、
Oは0.150%以下、
Pは0.050%以下、
Sは0.0200%以下に制限し、
かつ下記式(1)で定義されるPREN値が30.0以上で、
残部がFe及び不純物からなり、
前記二相ステンレス鋼母材のオーステナイト量は30~70面積%、前記溶接金属及び溶接熱影響部のオーステナイト量はそれぞれ15~70面積%であって、
前記溶接部及び前記二相ステンレス鋼母材を含む孔食試験片の50℃で測定したJIS G0577 A法による孔食電位が0.30V vs SSE以上であることを特徴とする溶接構造物。
PREN=Cr+3.3Mo+16N・・・(1)
ただし、式(1)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
(2)前記二相ステンレス鋼母材の成分が式(2)を満たし、且つ前記二相ステンレス鋼母材及び前記溶接金属のN量が式(3)を満足し、
更に前記二相ステンレス鋼母材がNbを含有する場合、前記二相ステンレス鋼母材のクロム窒化物析出温度TNが1010℃以下であり、前記二相ステンレス鋼母材がNbを含有しない場合、前記二相ステンレス鋼母材のクロム窒化物析出温度TNが980℃以下であることを特徴とする(1)に記載の溶接構造物。
Tα=1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N≧1330・・・(2)
N≧(0.08Cr+0.08Mo-0.06Ni-1.21)/0.6×0.15・・・(3)
ただし、式(2)、(3)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
(3)クロム窒化物析出温度TNは、下記推定式(4)又は式(5)であることを特徴とする、(2)に記載の溶接構造物。
8Cr-20Ni+30Mo+50Si-10Mn+550N+730(前記二相ステンレス鋼母材がNbを含有する場合)・・・(4)
8Cr-20Ni+30Mo+50Si-10Mn+550N+700(前記二相ステンレス鋼母材がNbを含有しない場合)・・・(5)
ただし、式(4)、(5)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
(4)前記二相ステンレス鋼母材及び前記溶接金属のうち少なくとも1つは、更に
Nb:0.005~0.150%
Ti:0.003~0.020%
Ta:0.005~0.200%、
Zr:0.001~0.050%
Hf:0.001~0.080%
Sn:0.005~0.100%、
W:0.01~1.00%
Co:0.01~1.00%
Cu:0.01~3.00%
V:0.010~0.300%
B:0.0001~0.0050%
Ca:0.0005~0.0050%
Mg:0.0005~0.0050%
REM:0.005~0.050%
のうち1種または2種以上を含有していることを特徴とする(1)乃至(3)のうちいずれかに記載の溶接構造物。
(5)前記二相ステンレス鋼母材の組成を有する熱延用素材を、下記式(6)で示す圧減比が3.0以上、かつ下記式(7)で示す1050℃以下の圧下率が30%以上となるように熱間圧延し、TN+20℃以上1100℃以下で5分以上熱処理して、前記二相ステンレス鋼母材を製造することを特徴とする、(1)乃至(4)のうちいずれかに記載の溶接構造物の製造方法。
熱延用素材の厚さ/二相ステンレス鋼母材の厚さ・・・(6)
(1050℃以下に到達した時の厚さ-二相ステンレス鋼母材の厚さ)/1050℃以下に到達した時の厚さ×100・・・(7)
(6)前記溶接金属は、溶加棒を使用するガスシールドアーク溶接またはタングステンアーク溶接を用いて形成され、下記式(8)で定義される溶接入熱量Qが5,000J/cm以上50,000J/cm以下、下記式(9)で定義される母材希釈率Dが50%以下の溶接条件で形成されたことを特徴とする(5)に記載の溶接構造物の製造方法。
Q=[溶接電流(A)]×[溶接電圧(V)]÷[溶接速度(cm/s)]・・・(8)
D=[二相ステンレス鋼母材の溶融体積]/[全溶接金属体積]×100・・・(9)
以下に、まず本発明の溶接構造物を構成する二相ステンレス鋼母材の組成及び組織の限定理由について説明する。なお本明細書において特に断りのない限り成分に関する%は質量%を表す。
Cは、ステンレス鋼の耐食性を確保するために、0.050%以下の含有量に制限する。0.050%を越えて含有させると熱間圧延時にCr炭化物が生成して、耐食性、靱性が劣化する。好ましくは、0.030%以下であり、さらに好ましくは0.025%以下にするとよい。
一方、ステンレス鋼のC量を低減するコストの観点から0.001%を下限とする。
一方、0.80%を超えて含有させると靱性が劣化する。そのため、0.80%以下にする。好ましくは0.50%以下、さらに好ましくは0.40%以下にするとよい。
一方、Mnはステンレス鋼の耐食性を低下する元素であるので、Mnを2.00%以下にするとよい。好ましくは1.80%以下、さらに好ましくは1.50%以下にするとよい。
一方で、Crを、26.00%を超えて含有させるとフェライト相分率が増加し靭性及び溶接部の耐食性を阻害する。このためCrの含有量を26.00%以下とした。好ましくは25.00%以下、さらに好ましくは24.50%以下にするとよい。
一方、Niは高価な合金であり、省合金型二相ステンレス鋼を対象とした本発明鋼ではコストの観点より7.00%以下の含有量に制限する。好ましくは6.50%以下、さらに好ましくは6.00%以下にするとよい。
一方、Moは高価であるとともに、金属間化合物析出を促進する元素であり、本発明鋼では熱間圧延時の析出を抑制する観点と経済的観点からMo含有量は少ない方が好ましいので2.50%以下とする。好ましくは2.00%未満、さらに好ましくは1.80%以下、より好ましくは1.50%以下にするとよい。
一方、固溶限度はCr含有量に応じて高くなるが、本発明鋼においては0.250%超含有させるとCr窒化物を析出して靭性及び耐食性を阻害するようになる。そのため、N含有量を0.250%以下とした。好ましくは0.230%以下、さらに好ましくは0.200%以下にするとよい。
一方、AlはNとの親和力が比較的大きな元素であり、過剰に添加するとAlの窒化物を生じてステンレス鋼の靭性を阻害する。その程度はN含有量にも依存するが、Alが0.050%を超えると靭性低下が著しくなるためその含有量を0.050%以下にするとよい。好ましくは0.040%以下、より好ましくは0.030%以下にするとよい。
本発明の溶接構造物を構成する二相ステンレス鋼母材の化学組成において、残部は、Fe及び不純物である。ここで、不純物とは、前記鋼母材を工業的に製造する際に、原料としての鉱石、スクラップ、又は製造環境などから混入されるものであって、当該鋼に悪影響を与えない範囲で許容されるものを意味する。主な不純物としては、P、S、Oが挙げられるが、これに限定されず、他の元素も不純物として含有されうる。
河川の淡水、汽水等の自然水の環境下では、微生物の活動により二相ステンレス鋼の自然電位が高くなる。自然電位が高い環境下ではCr濃度の僅かな低下であっても耐食性に大きな影響を及ぼす。このため、本発明鋼が適用される環境下では、二相ステンレス鋼を溶接してCr窒化物が析出した場合、Cr窒化物周囲のCr欠乏層が孔食の起点となる。
PREN=Cr+3.3Mo+16N・・・(1)
ただし、式(1)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
次に、本発明における溶接構造物に形成される溶接金属の成分組成の限定理由を以下に説明する。なお、以下に示す「%」は、特に説明がない限り「質量%」を意味するものとする。
Cは耐食性に有害であるが、強度の観点からある程度の含有が好ましいため、C含有量は0.001%以上である。また、その含有量が0.060%超では溶接のままの状態及び再熱を受けるとCはCrと結合してCr炭化物を析出し、耐粒界腐食性及び耐孔食性が著しく劣化するとともに、溶接金属の靱性、延性が著しく低下するため、その含有量を0.001~0.060%に限定した。
但し、その含有量が3.50%を越えるとシグマ相など脆い金属間化合物を生成して溶接金属の靱性が低下するため、下限を1.00とし、上限を3.50%に制限する。好ましくは2.00%以上で、3.00%以下にするとよい。
本発明の溶接構造物に形成される溶接金属の化学組成において、残部は、Fe及び不純物である。ここで、不純物とは、前記鋼母材を工業的に製造する際に、原料としての鉱石、スクラップ、又は製造環境などから混入されるものであって、当該鋼に悪影響を与えない範囲で許容されるものを意味する。主な不純物としては、P、S、Oが挙げられるが、これに限定されず、他の元素も不純物として含有されうる。
河川の淡水、汽水等の自然水の環境下では、微生物の活動により二相ステンレス鋼の自然電位が高くなる。自然電位が高い環境下ではCr濃度の僅かな低下であっても耐食性に大きな影響を及ぼす。このため、本発明鋼が適用される環境下では、二相ステンレス鋼を溶接してCr窒化物が析出した場合、Cr窒化物周囲のCr欠乏層が孔食の起点となる。本発明者らは、溶接構造物の二相ステンレス鋼溶接部のオーステナイト量が15面積%未満となるか、70面積%超となる場合、SUS316Lを下回る耐食性となることを明らかにした。
さらに、本発明の溶接構造物を構成する二相ステンレス鋼母材及び溶接金属(以下、単に「本発明の溶接構造物の母材及び溶接金属」ともいう。)は、以下の元素のうち1種または2種以上を必要に応じて0%以上含有することができる。もっとも、これらの元素をいずれも含有しなくとも本発明の目的は達成できる。
一方、Nbが0.150%を越えて含有させるとNbの窒化物が多量に析出し、靱性を阻害するようになることから、その含有量を0.150%以下と定めた。好ましくは0.090%以下、さらに好ましくは0.070%以下、より好ましくは0.050%以下にするとよい。
なお、Nbは高価な元素であるが、品位の低いスクラップに含有されるNbを積極的に利用することで、ステンレス溶解原料コストを安価にすることができる。このような方法により、Nb含有鋼の溶解コストの低減を図ることが好ましい。
ここでREMはLaやCe等のランタノイド系希土類元素の含有量の総和とする。
本発明において、二相ステンレス鋼母材の成分は以下の式(2)を満たすことが好ましい。
Tα=1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N≧1330・・・(2)
Tαは二相ステンレス鋼母材を加熱した際に、オーステナイトが消失しフェライト単相となる温度(以下、「フェライト単相化温度」という。単位は℃である。)を推定する成分式である。このフェライト単相化温度が低いと、溶接時に長時間フェライト単相域に晒されることになり、フェライト相の粗大化が助長され、溶接熱影響部の靭性が低下する。実験の結果、Tαが1320℃を下回ると極端に熱影響部の靭性が低下することを見出したため、1330℃以上とした方が好ましい。より好ましくは1340℃以上である。
この式は、サーモカルク社の熱力学計算ソフト「Thermo-Calc 」(登録商標) を用いた平衡計算により求め、実験により修正した。
本発明において、二相ステンレス鋼母材及び溶接金属のN量は、以下の式(3)を満たすことが好ましい。
N≧(0.08Cr+0.08Mo-0.06Ni-1.21)/0.6×0.15・・・(3)
ただし、式(3)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
クロム窒化物は、CrとNが結合した析出物であり、二相ステンレス鋼においては立方晶のCrNまたは六方晶のCr2Nがフェライト粒内もしくはフェライト粒界に析出することが多い。これらのクロム窒化物が生成すると、衝撃特性を低下させるとともに、析出にともなって生成するクロム欠乏層により耐食性が低下する。
(1) 10mm厚の供試鋼を、熱延後一旦1050℃×20分の熱処理を行ったのち800~1100℃の任意の温度で20分間均熱処理を行い、その後5秒以内に水冷を行う。
(2) 冷却後の供試鋼表層を#500研磨する。
(3) 3g試料を分取し、室温の非水溶液(3%マレイン酸と、1%テトラメチルアンモニウムクロライドとを含み、残部がメタノール)中で電解(100mV定電圧)してマトリックスを溶解する。
(4) 0.2μm穴径のフィルターで残渣(すなわち、析出物)を濾過し、析出物を抽出する。
(5) ICPを用いて、残渣の化学組成を分析し、前記残渣に含有されるクロム含有量(質量%)を求める。この残渣中のクロム含有量をクロム窒化物の析出量の指標とする。
(6) (1)の均熱処理温度を種々変化させ、残渣中のクロム含有量が0.03%以下となる均熱処理温度のうちの最低温度をTNとする。
8Cr-20Ni+30Mo+50Si-10Mn+550N+730(二相ステンレス鋼母材がNbを含有する場合)・・・(4)
8Cr-20Ni+30Mo+50Si-10Mn+550N+700(二相ステンレス鋼母材がNbを含有しない場合)・・・(5)
ただし、式(4)、(5)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。
次に、本発明の溶接構造物の製造方法について説明する。
[二相ステンレス鋼母材の製造方法]
熱延用素材の厚さ/本発明の溶接構造物の二相ステンレス鋼母材の厚さ・・・(6)
(1050℃以下に到達した時の厚さ-本発明の溶接構造物の二相ステンレス鋼母材の厚さ)/1050℃以下に到達した時の厚さ×100・・・(7)
なお、「1050℃以下に到達した時の厚さ」とは、熱間圧延中に前記熱延用素材の表面温度を逐次測定し、1050℃以下に到達した際の厚さを測定して求める。
本発明では、優れた靱性と海水環境下での耐食性を有する溶接部を形成するために溶接金属を形成する際の溶接条件について以下のように限定するのが好ましい。
Cr、Moを含有する二相ステンレス鋼は、約700℃~900℃の温度域に保持されると、靭性に有害なシグマ相などの脆い金属間化合物が析出し、耐食性、靱性が著しく低下する。また、同様に耐食性、靭性に有害なCr窒化物は、約600℃から800℃の温度域で析出する。溶接金属は、凝固後の冷却過程において900℃~600℃を通過する時間が長くなると、シグマ相もしくはCr窒化物が多量に析出する。また、多層パス溶接により形成された溶接金属では、前層パスが後続パスによる熱サイクルを受け、600℃~900℃の温度域となる時間が長くなる場合も同様である。
Q(J/cm)=[溶接電流(A)]×[溶接電圧(V)]÷[溶接速度(cm/s)]・・・(8)
本発明では、溶接金属の耐食性及びオーステナイト量を確保するために、溶接金属は、二相ステンレス鋼母材に対して、Mo含有量及びNi含有量、PRENの少なくとも1種が高いことが好ましい。しかしながら、母材による希釈率が高すぎると、適正な溶加棒を使用しても母材の混合が大きく、狙いの成分を得難くなる。つまり溶接条件として、溶接時の母材希釈率は、50%以下に限定するのが好ましい。母材希釈率Dは、以下の式で定義される。
D=[二相ステンレス鋼母材の溶融体積]/[全溶接金属体積]×100・・・(9)
本発明の溶接構造物は、溶接金属及び熱影響部とを含む溶接部について50℃で測定したJIS G0577 A法による孔食電位が0.30V vs SSE以上になる。このように、本発明の溶接構造物は、汽水環境においてSUS316Lと同等以上の耐食性を有する。
本発明の溶接構造物を構成する二相ステンレス鋼母材は、JIS Z 2202に規定されたシャルピー衝撃試験方法により測定されたシャルピー衝撃値が、-20℃で100J/cm2以上である。
また、本発明の溶接構造物の溶接熱影響部及び溶接金属は、JIS Z 2202に規定されたシャルピー衝撃試験方法により測定されたシャルピー衝撃値が、いずれも-20℃で50J/cm2以上である。
11a 鋼母材
11b 鋼母材
12 溶接金属12
Claims (6)
- 質量%で、
C:0.001~0.050%、
Si:0.05~0.80%、
Mn:0.10%~2.00%、
Cr:21.50~26.00%、
Ni:3.00~7.00%、
Mo:0.50~2.50%、
N:0.100~0.250%、
Al:0.003~0.050%、
を含有し、
Oは0.0060%以下、
Pは0.050%以下、
Sは0.0050%以下に制限し、
かつ下記式(1)で定義されるPREN値が28.0以上で、
残部がFeおよび不純物からなる二相ステンレス鋼母材と、
溶接金属及び熱影響部とを含む溶接部とを備える溶接構造物であって、
前記溶接金属は、
質量%で、
C:0.001~0.060%、
Si:0.05~0.80%、
Mn:0.10%~3.00%、
Cr:21.50~28.00%、
Ni:4.00~10.00%、
Mo:1.00~3.50%、
N:0.080~0.250%、
Al:0.001~0.100%、
を含有し、
Oは0.150%以下、
Pは0.050%以下、
Sは0.0200%以下に制限し、
かつ下記式(1)で定義されるPREN値が30.0以上で、
残部がFeおよび不純物からなり、
前記二相ステンレス鋼母材のオーステナイト量は30~70面積%、前記溶接金属及び溶接熱影響部のオーステナイト量はそれぞれ15~70面積%であって、
前記溶接部及び前記二相ステンレス鋼母材を含む孔食試験片の50℃で測定したJIS G0577 A法による孔食電位が0.30V vs SSE以上であることを特徴とする溶接構造物。
PREN=Cr+3.3Mo+16N・・・(1)
ただし、式(1)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。 - 前記二相ステンレス鋼母材の成分が式(2)を満たし、且つ前記二相ステンレス鋼母材及び前記溶接金属のN量が式(3)を満足し、
更に前記二相ステンレス鋼母材がNbを含有する場合、前記二相ステンレス鋼母材のクロム窒化物析出温度TNが1010℃以下であり、前記二相ステンレス鋼母材がNbを含有しない場合、前記二相ステンレス鋼母材のクロム窒化物析出温度TNが980℃以下であることを特徴とする、請求項1に記載の溶接構造物。
Tα=1455-13.6Cr+22.7Ni-11.2Mo+2.1Mn+781.8N≧1330・・・(2)
N≧(0.08Cr+0.08Mo-0.06Ni-1.21)/0.6×0.15・・・(3)
ただし、式(2)、(3)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。 - クロム窒化物析出温度TNは、下記推定式(4)又は式(5)であることを特徴とする、請求項2に記載の溶接構造物。
8Cr-20Ni+30Mo+50Si-10Mn+550N+730(前記二相ステンレス鋼母材がNbを含有する場合)・・・(4)
8Cr-20Ni+30Mo+50Si-10Mn+550N+700(前記二相ステンレス鋼母材がNbを含有しない場合)・・・(5)
ただし、式(4)、(5)中における元素記号は、それぞれの元素の含有量(質量%)を示し、含有していない場合は0を代入する。 - 前記二相ステンレス鋼母材及び前記溶接金属のうち少なくとも1つは、更に
Nb:0.005~0.150%
Ti:0.003~0.020%
Ta:0.005~0.200%、
Zr:0.001~0.050%
Hf:0.001~0.080%
Sn:0.005~0.100%、
W:0.01~1.00%
Co:0.01~1.00%
Cu:0.01~3.00%
V:0.010~0.300%
B:0.0001~0.0050%
Ca:0.0005~0.0050%
Mg:0.0005~0.0050%
REM:0.005~0.050%
のうち1種または2種以上を含有していることを特徴とする請求項1乃至3のうちいずれか1項に記載の溶接構造物。 - 前記二相ステンレス鋼母材の組成を有する熱延用素材を、下記式(6)で示す圧減比が3.0以上、かつ下記式(7)で示す1050℃以下の圧下率が30%以上となるように熱間圧延し、TN+20℃以上1100℃以下で5分以上熱処理して、前記二相ステンレス鋼母材を製造することを特徴とする、請求項1乃至4のうちいずれか1項に記載の溶接構造物の製造方法。
熱延用素材の厚さ/二相ステンレス鋼母材の厚さ・・・(6)
(1050℃以下に到達した時の厚さ-二相ステンレス鋼母材の厚さ)/1050℃以下に到達した時の厚さ×100・・・(7) - 前記溶接金属は、溶加棒を使用するガスシールドアーク溶接またはタングステンアーク溶接を用いて形成され、下記式(8)で定義される溶接入熱量Qが5,000J/cm以上50,000J/cm以下、下記式(9)で定義される母材希釈率Dが50%以下の溶接条件で形成されたことを特徴とする、請求項5に記載の溶接構造物の製造方法。
Q=[溶接電流(A)]×[溶接電圧(V)]÷[溶接速度(cm/s)]・・・(8)
D=[二相ステンレス鋼母材の溶融体積]/[全溶接金属体積]×100・・・(9)
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JPWO2020138490A1 (ja) | 2021-10-14 |
CN113227409B (zh) | 2023-07-25 |
KR102520119B1 (ko) | 2023-04-10 |
KR20210069097A (ko) | 2021-06-10 |
CN113227409A (zh) | 2021-08-06 |
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