WO2017145766A1 - 強度-低温靱性バランスに優れたCu含有低合金鋼およびその製造方法 - Google Patents
強度-低温靱性バランスに優れたCu含有低合金鋼およびその製造方法 Download PDFInfo
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- 229910052802 copper Inorganic materials 0.000 title claims abstract description 7
- 238000004519 manufacturing process Methods 0.000 title claims description 21
- 229910045601 alloy Inorganic materials 0.000 title abstract description 7
- 239000000956 alloy Substances 0.000 title abstract description 7
- 239000010949 copper Substances 0.000 title abstract 9
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 title abstract 4
- 238000009863 impact test Methods 0.000 claims abstract description 15
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- 229910052758 niobium Inorganic materials 0.000 claims abstract description 4
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 3
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 3
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/78—Combined heat-treatments not provided for above
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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
- C21D6/00—Heat treatment of ferrous alloys
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- 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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/008—Heat treatment of ferrous alloys containing Si
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- 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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
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- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C—ALLOYS
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- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
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- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Definitions
- the present invention relates to a Cu-containing low alloy steel having an excellent balance between strength and low temperature toughness, which is used in applications requiring low temperature toughness, and a method for producing the same.
- Oil and natural gas are widely used as energy centers. In recent years, these developments are shifting from land to ocean, and in particular, ocean resource development is mainly mining at a deeper depth than the continental shelf. From the viewpoint of ensuring safety, the marine structural steel used for the development of this ultra-large water depth is required to have high yield strength in addition to excellent low temperature toughness.
- steel for offshore structures steel containing 1.0 to 1.3% by mass of Cu as defined in ASTM A710 is used for steel, and forged steel is made of ASTM, for example. A steel containing 0.43% by mass or less of Cu defined by A707 is known.
- Non-Patent Document 1 describes the results of improving the component system based on ASTM A707 Grade L5, performing quenching and tempering, and evaluating the mechanical properties.
- FATT is ⁇ 60 ° C., and it is necessary to further improve the low temperature toughness from the viewpoint of ensuring safety.
- Patent Document 1 defines an M * value composed of C, Si, Al, N, and B in order to produce a high-strength steel plate excellent in CTOD (crack tip opening displacement) characteristics, and after rolling. Proposes a manufacturing method for direct quenching.
- Patent Document 2 a steel plate containing 0.7 to 1.5% by mass of Cu: 0.7 to 1.5% is subjected to Cu precipitation at 900 ° C. or lower and 700 ° C. or higher and 30% or higher, and then Cu is precipitated in a range of 500 to 650 ° C.
- a method for producing a low C—Cu precipitation hardening type high strength steel excellent in low temperature toughness and weldability has been proposed. Research results on improvement of material properties by two-phase quenching have also been reported.
- Patent Document 3 as a two-phase region quenching method for B-added steel, a high-tensile steel having a low yield ratio is stabilized by defining B, N, and Ti addition amounts and by defining a two-phase region quenching temperature. It is proposed to manufacture. Patent Document 4 proposes that a Ni-containing steel sheet having excellent low-temperature toughness and strength-toughness balance is produced by two-phase quenching.
- Japanese Unexamined Patent Publication No. 2001-81529 Japanese Unexamined Patent Publication No. 61-149430 Japanese Laid-Open Patent Publication No. 5-171263 Japanese Unexamined Patent Publication No. 2008-81776
- Patent Documents 1 and 2 require temper rolling in the manufacturing process, and is not applicable when rolling is not performed or when the plate thickness is large and rolling is difficult.
- a plate thickness of 120 mm is the maximum. Therefore, this manufacturing method cannot be applied to a manufacturing method in which rolling is not performed or a large structure including a thick flange portion of 150 mm or more.
- Patent Documents 3 and 4 there is no definition of the Cu content, and it is clear that a manufacturing method for obtaining a steel having an excellent strength-toughness balance in a Cu-containing low alloy steel whose strength changes by aging treatment. is not.
- the present invention has been made against the background of the above circumstances, and aims to provide a Cu-containing low alloy steel having an excellent balance between strength and low temperature toughness.
- the first form is mass%, C: 0.01 to 0.08%, Si: 0.10 to 0.00. 40%, Mn: 0.80 to 1.80%, Ni: 0.80 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0 20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.030 to 0.080%, N: 0.005 to 0.020%, the balance being Fe and inevitable impurities
- the 0.2% proof stress is 525 MPa or more
- the ductile brittle fracture surface transition temperature (FATT) measured by a V-notch Charpy impact test is ⁇ 70 ° C. or less. .
- Another aspect of the invention of the Cu-containing low alloy steel excellent in the strength-low temperature toughness balance is characterized in that the chemical composition further contains Ca: 0.010% or less by mass%.
- the invention of the Cu-containing low alloy steel having an excellent balance between strength and low temperature toughness in another form is characterized in that, in the form of the invention, the absorbed energy of the 2 mm V notch Charpy impact test at ⁇ 80 ° C. is 130 J or more. To do.
- the invention of the Cu-containing low alloy steel having an excellent balance between strength and low temperature toughness in another form is the average EBSD (Electron BackScatter Diffraction) grain size when the boundary angle after tempering is 15 ° or more in the present invention of the above form.
- the diameter is 10 ⁇ m or less, and the maximum EBSD particle size is 120 ⁇ m or less.
- the first aspect is the first aspect of the invention for the Cu-containing low alloy steel excellent in strength-low temperature toughness balance.
- An invention of a method for producing a Cu-containing low alloy steel excellent in strength-low temperature toughness balance in another form is the large-scale structure in which the tempering treatment has a thick part having a thickness of 150 mm to 500 mm in the present invention of the form described above. It is characterized by being applied to steel for industrial use.
- the invention of a method for producing a Cu-containing low alloy steel having an excellent balance between strength and low temperature toughness in another form is characterized in that, in the present invention in the form, produced by hot forging and then the tempering treatment is performed.
- C 0.01 to 0.08% From the viewpoint of ensuring strength, C is a necessary additive element, so 0.01% is made the lower limit. However, if the content exceeds 0.08%, the toughness decreases due to the increase in strength, the hard phase precipitates during quenching in the two-phase region, and the weldability decreases, so 0.08% is made the upper limit. For the same reason, it is desirable that the lower limit is 0.02% and the upper limit is 0.05%.
- Si 0.10 to 0.40% Si is used as a deoxidizing element when melting and refining the alloy. Moreover, since it is an element necessary for ensuring strength, 0.10% is made the lower limit. However, excessive content causes a reduction in toughness and weldability, so 0.40% is made the upper limit. For the same reason, it is desirable that the lower limit is 0.20% and the upper limit is 0.35%.
- Mn 0.80 to 1.80% Mn is an element useful as a deoxidizing element like Si, and contributes to improvement of hardenability. In order to exhibit the effect, a content of 0.80% or more is necessary. However, excessive content causes a decrease in toughness, so the upper limit is made 1.80%. For the same reason, it is desirable to set the lower limit to 1.00% and the upper limit to 1.50%. More preferably, the lower limit is 1.20% and the upper limit is 1.45%.
- Ni 0.80 to 2.50%
- Ni is an element necessary for securing strength by improving hardenability and securing low temperature toughness, so 0.80% is made the lower limit.
- the upper limit is made 2.50%.
- the lower limit is 0.50%.
- the upper limit is made 1.00%.
- Cu 0.80 to 1.50% Cu precipitates during the aging treatment and improves the strength of the steel. In low carbon steel, it is very important to secure strength by Cu precipitates. Further, since it is an important element for improving the corrosion resistance, 0.80% is made the lower limit. However, excessive inclusion causes a decrease in toughness and a decrease in hot workability, so 1.50% is made the upper limit. For the same reason, it is desirable to set the lower limit to 1.10% and the upper limit to 1.30%. More preferably, the lower limit is 1.20% and the upper limit is 1.25%.
- Mo 0.20 to 0.60% Mo contributes to improving hardenability and is an important element for securing strength and toughness, so 0.20% is made the lower limit. However, an excessive content causes a decrease in toughness and weldability, so 0.60% is made the upper limit. More preferably, the lower limit is 0.30% and the upper limit is 0.50%. More preferably, the lower limit is 0.40% and the upper limit is 0.45%.
- Al 0.010 to 0.050% Al combines with N to become AlN and suppresses crystal grain growth. Refinement of the crystal grain size is essential to improve toughness, and the lower limit of the Al content is 0.010%. However, excessive content causes a decrease in toughness due to coarse AlN, so 0.050% is made the upper limit. For the same reason, it is desirable that the lower limit is 0.010% and the upper limit is 0.030%. More preferably, the lower limit is 0.020% and the upper limit is 0.030%.
- Nb 0.030 to 0.080% Nb, as carbonitride, suppresses crystal grain growth and is an important element for refining the crystal grain size, so 0.030% is made the lower limit.
- 0.080% is made the upper limit.
- the lower limit is 0.04% and the upper limit is 0.060%. More preferably, the lower limit is 0.040% and the upper limit is 0.050%.
- N 0.005 to 0.020% N is contained as AlN and carbonitride because it is an important element for suppressing crystal grain growth and miniaturizing the crystal grain size.
- the lower limit is made 0.005%.
- the upper limit is made 0.020%. More preferably, the lower limit is 0.005% and the upper limit is 0.011%.
- Ca 0.010% or less Since Ca forms oxides and sulfides, it is used as desired as a deoxidizing and desulfurizing element. However, excessive addition causes a decrease in toughness, so the content is made 0.010% or less. For the same reason, it is desirable to further limit the upper limit to 0.005%. In addition, in order to acquire the said effect
- EBSD particle diameter 10 ⁇ m or less on average and 120 ⁇ m or less at maximum EBSD (electron beam backscatter diffraction method) is a method for measuring the orientation of each crystal.
- EBSD grain size crystal grain size
- the crystal grain size (EBSD grain size) surrounded by a large angle boundary of 15 ° or more has a correlation with toughness.
- the finer the EBSD grain size the better the low temperature toughness of the steel.
- the average EBSD particle size is 10 ⁇ m or less and the maximum EBSD particle size is 120 ⁇ m or less, a Cu-containing low alloy steel having a better balance between strength and low temperature toughness can be obtained.
- the average EBSD particle size exceeds 10 ⁇ m or the maximum EBSD particle size exceeds 120 ⁇ m, the low-temperature toughness characteristics deteriorate. More preferably, the average EBSD particle size is 10 ⁇ m or less and the maximum EBSD particle size is 110 ⁇ m or less.
- the heating means and cooling means at the time of quenching are not particularly limited as the present invention, and means capable of obtaining desired heating ability and cooling ability can be appropriately selected.
- Steel that has been subjected to quenching treatment is then (A C3 transformation point -80 ° C.) or higher, after being heated in the temperature range of (A C3 transformation point -10 ° C.) or less, two-phase region quenching process for cooling is applied
- the two-phase region quenching a heat treatment method of cooling after heating the steel to a temperature (two-phase region temperature) between the A C1 point and A C3 point two phases of ⁇ -phase and ⁇ -phase is present.
- the heating means and the cooling means in the two-phase region quenching are not particularly limited in the present invention, and a means for obtaining desired heating ability and cooling ability can be appropriately selected. This heat treatment is the most important in the present invention.
- the heating temperature in the two-phase quenching heat treatment as described above, (A C3 transformation point -80 ° C.) or higher, defined in a temperature range of (A C3 transformation point -10 ° C.).
- the heating temperature is less than ( AC3 transformation point ⁇ 80 ° C.)
- the transformation amount to the ⁇ phase is insufficient
- the ⁇ phase that undergoes high-temperature tempering is large
- the Cu precipitate is coarsened.
- the proof stress cannot be secured. Further, the subsequent crystal grain size is not reduced, and it is difficult to ensure low temperature toughness.
- the two-phase quenching is specified in a temperature range of ( AC3 transformation point ⁇ 80 ° C.) or more and ( AC3 transformation point ⁇ 10 ° C.) or less.
- tempering treatment is performed in a temperature range of 560 ° C. or more and 660 ° C. or less. If heating temperature is less than 560 degreeC, 0.2% yield strength will increase by the aging effect of Cu precipitate, and the fall of toughness will be caused. In addition, at a tempering temperature of less than 560 ° C., internal stress during tempering cannot be relaxed, causing damage during service. On the other hand, if it exceeds 660 ° C., it becomes over-aged and 0.2% proof stress cannot be secured. Therefore, the temperature range of the tempering treatment is set to 560 ° C. or more and 660 ° C. or less.
- Thick wall portion The present invention can be applied to the production of a material having a thick wall portion.
- the maximum thickness of the thick part is 150 mm or more and 500 mm or less.
- the thickness exceeds 500 mm, the cooling rate is lowered in the cooling process of quenching and two-phase quenching, and the strength is lowered.
- a ductile brittle fracture surface transition temperature (FATT) measured in a V-notch Charpy impact test is good at ⁇ 70 ° C. or lower.
- FATT ductile brittle fracture surface transition temperature
- the ductile brittle fracture surface transition temperature is a temperature at which a material that has undergone ductile fracture with a decrease in temperature transitions to brittle fracture. It shows that it has toughness to low temperature, so that a ductile brittle fracture surface transition temperature is low.
- the ductile brittle fracture surface transition temperature is more preferably ⁇ 80 ° C. or lower. Therefore, it is possible to provide a Cu-containing low alloy steel excellent in strength-low temperature toughness balance.
- the steel having the chemical composition defined in the present invention can be melted by a conventional method if the target is the composition, and the method is not particularly limited as the present invention.
- the smelted steel ingot is hot forged into an arbitrary shape, and then subjected to a tempering treatment including quenching (Q), two-phase region quenching (L), and tempering (T) treatment.
- Q quenching
- L two-phase region quenching
- T tempering
- the content and method of hot training are not particularly limited, and the training ratio is not particularly limited.
- the hot tempered material can be thick, for example, a material having a thick portion with a plate thickness of 150 mm to 500 mm.
- the Cu-containing low alloy steel is heated to a temperature range of 850 to 950 ° C. to perform a quenching treatment. Thereafter, a two-phase quenching process is performed in a temperature range of ( AC3 transformation point ⁇ 80 ° C.) or more and ( AC3 transformation point ⁇ 10 ° C.) or less, and further a tempering process is performed at 560 to 660 ° C.
- heat processing such as normalization (N) can also be performed between hot forging and tempering treatment.
- normalization conditions for example, heating conditions of 950 to 1000 ° C. can be shown.
- composition range and the manufacturing method thereof it is suitable as a steel for offshore structures used in mooring equipment, risers, flow lines, etc., and has excellent low-temperature toughness, particularly excellent strength-low-temperature toughness balance. It is possible to produce meat Cu-containing low alloy forged steel.
- the Cu-containing low alloy steel obtained above has the characteristics that the 0.2% proof stress is 525 MPa or more and the ductile brittle fracture surface transition temperature (FATT) measured by the 2 mmV notch Charpy impact test is ⁇ 70 ° C. or less. Have. Further, the absorbed energy at ⁇ 80 ° C. in the 2 mmV notch Charpy impact test is 130 J or more.
- the absorbed energy is preferably 140 J or more.
- the boundary angle after tempering is 15 ° or more, the average EBSD particle size is 10 ⁇ m or less, and the maximum EBSD particle size is 120 ⁇ m or less. It is preferable that the average EBSD particle size is 10 ⁇ m or less and the maximum EBSD particle size is 110 ⁇ m or less.
- the 0.2% proof stress is 525 MPa or more and the tensile strength is 600 MPa or more.
- the ductile brittle fracture surface transition temperature (FATT) measured by a 2 mmV notch Charpy impact test is ⁇ 80 ° C. or lower.
- a specimen having the composition shown in Table 1 was melted into a 50 kg steel ingot using a vacuum induction melting furnace. Each ingot was melted by hot forging at 1250 ° C. to a thickness of 45 mm ⁇ width of 130 mm (forging ratio: 3.1 s or more), further subjected to normalization (960 ° C.), and then tempered as shown in Table 2. Tempering was performed under the conditions (Q treatment, L treatment, T treatment). The Q treatment (quenching) temperatures in the examples are all performed at 900 ° C. However, there is no particular limitation as long as the quenching temperature is in the range of 850 to 950 ° C. for the reason described above.
- the cooling in the Q treatment and the L treatment was performed at a cooling rate (10 ° C./min) simulating water cooling equivalent to a plate thickness of 450 mm.
- Table 2 shows the conditions for T treatment (tempering) of each specimen.
- test piece was collected and subjected to a tensile test and a Charpy impact test to evaluate strength and low temperature toughness.
- the test method was as follows. Tensile test: A round bar tensile test piece (parallel part diameter: 12.5 mm, GL: 50 mm) was sampled from the obtained test material and pulled at room temperature in accordance with the provisions of JIS Z 2241: 2005. The test was carried out to determine 0.2% yield strength (YS) and tensile strength (TS).
- YS yield strength
- TS tensile strength
- Impact test A 2 mmV notch Charpy impact test piece was sampled from the obtained test material and subjected to a Charpy impact test in accordance with the provisions of JIS Z 2242: 2005.
- the test piece has a square cross section with a length of 55 mm and a side of 10 mm.
- a V-groove having a notch angle of 45 °, a notch depth of 2 mm, and a notch bottom radius of 0.25 mm is provided at the center in the length direction of the test piece.
- a Charpy impact test was conducted at -80 ° C. Three tests were performed for each, and the obtained absorbed energy was arithmetically averaged, and the average value was taken as the absorbed energy value of the steel material.
- FATT performed the Charpy impact test at arbitrary test temperatures, and collected FATT from the transition curve.
- EBSD TSL (TexSEM Laboratories, Inc.) OIM (Orientation Imaging Microscopy)
- Steel No. 1 and steel no. Steel type C was used as the test material of 2.
- Steel No. 1 and 2 are comparative examples using a QT process which is a general manufacturing process.
- Steel No. 3 (Example of the present invention) and Steel No. No. 4 (Example of the present invention) is a steel No. 4 sample.
- the same steel type as No. 1 is used, and the manufacturing process is the QLT process. In either case, good results were obtained for both 0.2% proof stress and low temperature toughness.
- FIG. 1 shows the microstructure of 3 (QLT process)
- FIG. 3 shows the large-angle boundary map obtained from the EBSD measurement result with a boundary angle of 15 ° or more. From the microstructural observation results and the large-angle boundary map, the microstructure was complicated by performing the L process, and meandering of the large-angle boundary was recognized. Steel No. In No. 3, fine crystal grains are also observed in the grains. The meander boundary meandering and fine grain dispersion contribute to the improvement of low temperature toughness.
- Steel No. Steel samples A are used for the test materials 5 to 7 (examples of the present invention), and excellent strength and toughness are obtained by applying the heat treatment process of the present invention.
- Steel No. Steel grade B was used for 8-19 specimens.
- Steel type no. In 8 (comparative example), the L process is performed, but the T process is not performed. In the case of this comparative example, the aging effect due to the Cu precipitates cannot be obtained sufficiently, so that a 0.2% decrease in yield strength is observed.
- Steel No. In No. 9 (Comparative Example), the L temperature is ( AC 3 -80 ° C.) or lower, the transformation amount to the ⁇ phase is insufficient, and the EBSD particle size is not reduced. As a result, the low temperature toughness is insufficient.
- Steel No. In No. 19 (Comparative Example), the T temperature becomes excessive, resulting in overaging, and a 0.2% decrease in yield strength is observed.
- Steel No. 22 (comparative example) is the result of using steel type E as a comparative material.
- steel type E even when the QLT process recommended in the present invention is applied, the 0.2% proof stress is 525 MPa or less. Since this steel type ensures strength by the aging effect due to Cu precipitates, the effect cannot be sufficiently obtained when the amount of Cu is small.
- the present invention is suitable as steel for offshore structures used for mooring equipment, risers, flow lines, and the like.
- the intended use is not limited to these.
Abstract
Description
優れた強度-靱性バランスを確保するために、海洋構造物用鋼として、鋼板では例えばASTM A710で規定された1.0~1.3質量%のCuを含有する鋼が、鍛鋼材では例えばASTM A707で規定された0.43質量%以下のCuを含む鋼が知られている。
前記の鋼は、時効処理でCuを析出させることによって、低炭素かつ低炭素当量の成分系で強度を確保し、強度と低温靱性を両立させたものである。
非特許文献1ではASTM A707 Grade L5を基に成分系を改良し、焼入れおよび焼戻しを実施し、機械的特性を評価した結果について解説がある。この非特許文献1ではFATTが-60℃であり、安全性を確保する観点から、更なる低温靱性の改善が必要となる。
特許文献1では、M※値として、
M※=5C(%)+2Si(%)+20Al(%)+70N(%)+1400B(%)が示されている。
また、二相域焼入れによる材料特性の改善に関する研究成果も報告されている。例えば特許文献3では、B添加鋼の二相域焼入れ方法として、B、NおよびTi添加量を規定し、かつ二相域焼入れ温度を規定することによって、低降伏比を有する高張力鋼を安定して製造することを提案している。
また、特許文献4では、低温靱性および強度-靱性バランスに優れたNi含有鋼板を二相域焼入れによって製造することを提案している。
また、特許文献1、2のいずれも製造工程に調質圧延が必要であり、圧延を行わない場合や、板厚が厚くて圧延が困難な場合には適用できない。特許文献1では板厚120mmが最大である。したがって、圧延を行わない製造方法や、150mm以上の厚肉のフランジ部など含む大型構造体には本製造方法は適用できない。
さらに、特許文献3、4では、Cuの含有量の規定はなく、時効処理によって強度が変化するCu含有低合金鋼において、強度-靱性バランスに優れた鋼を得るための製造方法については明らかとなっていない。
強度を確保するという観点からはCは必要な添加元素であるため0.01%を下限とする。しかし、0.08%を超える含有は強度の増加による靱性の低下、二相域焼入れ時の硬質相の析出、溶接性の低下が生じることから、0.08%を上限とする。なお、同様の理由で下限を0.02%、上限を0.05%とするのが望ましい。
Siは合金の溶解・精錬を行う際に脱酸元素として使用される。また、強度確保のために必要な元素であるため0.10%を下限とする。しかし、過剰な含有は靱性の低下や溶接性の低下を招くので0.40%を上限とする。なお、同様の理由で下限を0.20%、上限を0.35%とするのが望ましい。
MnはSiと同様に脱酸元素として有用な元素であり、焼入れ性の向上にも寄与する。その効果を発揮するためには、0.80%以上の含有量が必要である。しかし、過剰な含有は靱性の低下を招くので1.80%を上限とする。なお、同様の理由で下限を1.00%、上限を1.50%とするのが望ましい。より好ましくは、下限を1.20%、上限を1.45%とする。
Niは焼入れ性の向上による強度の確保、低温靱性の確保のために必要な元素であるため0.80%を下限とする。しかし、過剰な含有は残留γを安定化し、靱性の低下を招くので2.50%を上限とする。なお、同様の理由で下限を1.50%、上限を2.30%とするのが望ましい。より好ましくは、下限を2.00%、上限を2.20%とする。さらに好ましくは、下限を2.10%、上限を2.15%とする。
Crは焼入れ性を確保し、強度と靭性を確保する上で重要な元素であるため、0.50%を下限とする。しかし、過剰の含有は焼入れ性を高め、靱性の低下、溶接割れ感受性が高くなることから、1.00%を上限とする。なお、同様の理由で下限を0.60%、上限を0.80%とするのが望ましい。より好ましくは、下限を0.70%、上限を0.75%とする。
Cuは時効処理の際に析出し、鋼の強度を向上させる。低炭素鋼においてはCu析出物による強度の確保は非常に重要である。また、耐食性を向上する上でも重要な元素であるため、0.80%を下限とする。しかし、過剰な含有は靱性の低下、熱間加工性の低下を招くため1.50%を上限とする。なお、同様の理由で下限を1.10%、上限を1.30%とするのが望ましい。より好ましくは、下限を1.20%、上限を1.25%とする。
Moは焼入れ性の向上に寄与し、強度と靱性を確保する上で重要な元素であるため、0.20%を下限とする。しかし、過剰な含有は靱性の低下、溶接性の低下を招くため0.60%を上限とする。より好ましくは、下限を0.30%、上限を0.50%とする。さらに好ましくは、下限を0.40%、上限を0.45%とする。
AlはNと結合してAlNとなり、結晶粒成長を抑制する。結晶粒径の微細化は靱性を向上させるために必須であり、Alの含有量は0.010%を下限とする。しかし、過剰な含有は粗大なAlNによる靱性の低下を招くため0.050%を上限とする。なお、同様の理由で下限を0.010%、上限を0.030%とするのが望ましい。より好ましくは、下限を0.020%、上限を0.030%とする。
Nbは炭窒化物として結晶粒成長を抑制し、結晶粒径の微細化のために重要な元素であるため、0.030%を下限とする。しかし、過剰な添加は炭窒化物の凝集粗大化を促進し、靱性の低下を招くため0.080%を上限とする。なお、同様の理由で下限を0.04%、上限を0.060%とするのが望ましい。より好ましくは、下限を0.040%、上限を0.050%とする。
NはAlNおよび炭窒化物として、結晶粒成長を抑制し、結晶粒径の微細化のために重要な元素であるため含有される。その作用を十分に得るため0.005%を下限とする。しかし、過剰な添加は多量のAlNや炭窒化物の析出および凝集粗大化を促進し、靱性の低下を招くため0.020%を上限とする。より好ましくは、下限を0.005%、上限を0.011%とする。
Caは酸化物や硫化物を形成するため、脱酸、脱硫元素として所望により使用される。しかし、過剰な添加は靱性の低下を招くため、0.010%以下とする。なお、同様の理由で上限をさらに0.005%とするのが望ましい。なお、上記作用を得るために、Caを0.0005%以上含有するのが望ましい。Caを積極的に添加しない場合、0.0005%未満でCaを不可避不純物として含むものであってもよい。
EBSD(電子線後方散乱回折法)は各結晶の方位を測定する方法である。一般的に鋼の場合は15°以上の大角境界で囲まれた結晶粒径(EBSD粒径)が靱性と相関を持つことが報告されている。このEBSD粒径が細かいほど鋼の低温靱性が良好な結果となる。平均EBSD粒径が10μm以下でかつ最大EBSD粒径が120μm以下であることにより、強度-低温靱性バランスがより優れたCu含有低合金鋼が得られる。一方で、平均EBSD粒径が10μmを超えるか、最大EBSD粒径が120μmを超える場合は、低温靱性の特性が低下する。より好ましくは、平均EBSD粒径は10μm以下でかつ最大EBSD粒径は110μm以下とする。
焼入れ処理の場合には少なくともAC3変態点(オーステナイト変態する温度)以上の温度に加熱する必要がある。また、焼入れ処理の加熱温度がAC3変態点以上であっても、温度が低い場合には焼入れ性が確保できないため、下限温度を850℃とする。しかし、焼入れ処理温度の高温化は加熱時にγ粒径が粗大化し、その後の靱性の低下を招くため、上限を950℃とする。
なお、この焼入れ処理は、必要に応じて複数回繰り返すことができる。また、該焼入れに際しての加熱手段や冷却手段は、本発明としては特に限定されるものではなく、所望の加熱能および冷却能が得られる手段を適宜選択することが出来る。
本願発明では、厚肉部を有する材料の製造に適用することができる。例えば、厚肉部の最大肉厚が150mm以上で、500mm以下のものが示される。
肉厚が150mm以上の材料では、調質圧延が難しく、本願発明による効果を顕著に得ることができる。一方、肉厚が500mmを超えると、焼入れおよび二相域焼入れの冷却過程で、冷却速度が低下し、強度の低下を招く。
(1)525MPa以上の0.2%耐力を確保し、かつ
(2)Vノッチシャルピー衝撃試験にて測定された延性脆性破面遷移温度(FATT:fracture appearance transition temperature)が-70℃以下で良好な低温靱性を有している。延性脆性破面遷移温度とは、温度の低下に伴って延性破壊していたものが脆性破壊に遷移するときの温度である。延性脆性破面遷移温度が低いほど低温まで靱性を有することを示す。なお、延性脆性破面遷移温度が-80℃以下であるとより好ましい。
したがって、強度-低温靱性バランスに優れたCu含有低合金鋼を提供することができる。
溶製された鋼塊は熱間鍛錬を行って、任意の形状にした後、焼入れ(Q)、二相域焼入れ(L)、焼戻し(T)処理を有する調質処理が施される。
なお、熱間鍛錬の内容、方法は特に限定されるものではなく、鍛錬比なども特に限定されない。熱間鍛錬された材料は、厚肉のものとすることができ、例えば板厚150mm~500mmの厚肉部を有する材料とすることができる。
なお、熱間鍛錬と調質処理の間に、焼準(N)等の熱処理を行うこともできる。上記焼準条件としては、例えば950~1000℃の加熱条件を示すことができる。
上記で得られたCu含有低合金鋼は、0.2%耐力が525MPa以上で、かつ2mmVノッチシャルピー衝撃試験にて測定された延性脆性破面遷移温度(FATT)が-70℃以下の特性を有している。
さらには、2mmVノッチシャルピー衝撃試験における-80℃での吸収エネルギーが130J以上を有している。該吸収エネルギーは140J以上であることが好ましい。
調質後の境界角度を15°以上とした際の平均EBSD粒径が10μm以下であり、かつ最大EBSD粒径が120μm以下である。平均EBSD粒径が10μm以下であり、かつ最大EBSD粒径が110μm以下であることが好ましい。
なお、強度について、0.2%耐力が525MPa以上でかつ引張強さが600MPa以上であることが好ましい。低温靱性について、2mmVノッチシャルピー衝撃試験にて測定された延性脆性破面遷移温度(FATT)が-80℃以下であることが好ましい。
表1に示す組成を有する供試材を真空誘導溶解炉により、50kg鋼塊に溶製した。溶製した各鋼塊は1250℃で熱間鍛造により、厚さ45mm×幅130mm(鍛造比:3.1s以上)とし、さらに焼準(960℃)を施した後、表2に示す調質条件(Q処理、L処理、T処理)で調質を施した。なお、実施例のQ処理(焼入れ)温度は全て900℃で実施しているが、上述した理由により、焼入れ温度が850~950℃の範囲であれば、特に限定するものではない。また、Q処理およびL処理(二相域焼入れ処理)の冷却は、板厚450mmの水冷相当を模擬した冷却速度(10℃/min)とした。各供試材のT処理(焼戻)の条件は表2に示した。
引張試験:得られた試験材から、丸棒引張試験片(平行部径:12.5mm、G.L.:50mm)を採取し、JIS Z 2241:2005の規定に準拠して、室温で引張試験を実施し、0.2%耐力(Y.S.)と引張強さ(T.S.)を求めた。
衝撃試験:得られた試験材から、2mmVノッチシャルピー衝撃試験片を採取し、JIS Z 2242:2005の規定に準拠してシャルピー衝撃試験を行った。試験片は、長さ55mm、1辺が10mmの正方形断面をもつものとする。試験片の長さ方向の中央には、ノッチ角度45°、ノッチ深さ2mm及びノッチ底半径0.25mmのV溝が設けられている。-80℃における吸収エネルギーvE-80℃(J)を求めるため、-80℃でシャルピー衝撃試験を実施した。なお、試験は各3本行い、得られた吸収エネルギーを算術平均して、その平均値をその鋼材の吸収エネルギー値とした。
さらに、FATTは任意の試験温度でシャルピー衝撃試験を実施し、その遷移曲線からFATTを採取した。
また、これらの試験材からサンプルを採取し、EBSD(TSL(TexSEM Laboratories, Inc.)社製OIM(Orientation Imaging Microscopy))測定も実施した。EBSDによる試料の評価は以下のようなものである。試料表面の一点に電子線を照射してその後方散乱(backscatter diffraction)を測定すると、その点における結晶の方位角を知ることができる。電子線を照射する位置を微小にずらしながら(測定ピッチを0.3μmとした)、300μm×400μmの視野角の範囲内を走査すると、該範囲内の結晶の方位角のマップを得ることができる。隣り合う測定点の方位角が15°以上異なる領域の間に境界線を引くと、図3に示したような境界角度を15°以上としたときのマップを得ることができる。この境界線は結晶界面とみなすことができ、この境界線で囲まれた領域を一つの結晶粒とみなすことができる。そこで、この境界線で囲まれた領域の面積を算出し、この面積に等しい円の直径を算出してこれを結晶粒の直径(EBSD粒径)とした。一つの試験材について、任意に5つの異なる300μm×400μmの視野角を選択し、各々の視野角内でEBSD粒径を算出した。その平均の値を平均EBSD粒径とした。また、最も大きな値を最大EBSD粒径とした。
各試験によって得られた結果を表3に示す。
鋼No.8~19の供試材については鋼種Bを用いた。鋼種No.8(比較例)ではL処理は実施しているが、T処理が施されていない。本比較例の場合、Cu析出物による時効効果が十分に得られないため、0.2%耐力の低下が認められる。
鋼No.9(比較例)ではL温度が(AC3-80℃)以下となっており、γ相への変態量が不十分であり、EBSD粒径の細粒化が得られていない。結果として、低温靱性が不十分となっている。
鋼No.16(比較例)では550℃でT処理を行っている。本比較例の場合、T温度の低下による時効効果により、0.2%耐力が増加している。その結果、低温靱性が低下してしまう。
鋼No.22(比較例)は比較材である鋼種Eを用いた結果である。鋼種Eを用いた場合、本発明で推奨しているQLTプロセスを適用した場合であっても0.2%耐力が525MPa以下となっている。本鋼種はCu析出物による時効効果によって強度を確保しているため、Cu量が少ない場合では、その効果を十分に得ることはできない。
Claims (7)
- 質量%で、C:0.01~0.08%、Si:0.10~0.40%、Mn:0.80~1.80%、Ni:0.80~2.50%、Cr:0.50~1.00%、Cu:0.80~1.50%、Mo:0.20~0.60%、Al:0.010~0.050%、Nb:0.030~0.080%、N:0.005~0.020%を含有し、残部がFe及び不可避不純物からなる化学組成を有し、0.2%耐力が525MPa以上で、かつ2mmVノッチシャルピー衝撃試験にて測定された延性脆性破面遷移温度(FATT)が-70℃以下を有することを特徴とする、強度-低温靱性バランスに優れたCu含有低合金鋼。
- 前記化学組成に、さらに、質量%で、Ca:0.010%以下を含有することを特徴とする請求項1に記載の強度-低温靱性バランスに優れたCu含有低合金鋼。
- -80℃での2mmVノッチシャルピー衝撃試験の吸収エネルギーが130J以上を有することを特徴とする請求項1または2に記載の強度-低温靱性バランスに優れたCu含有低合金鋼。
- 調質後の境界角度を15°以上とした際の平均EBSD粒径が10μm以下であり、かつ最大EBSD粒径が120μm以下であることを特徴とする請求項1~3のいずれか1項に記載の強度-低温靱性バランスに優れたCu含有低合金鋼。
- 請求項1~4のいずれか一項に記載したCu含有低合金鋼を製造する方法であって、850~950℃の温度域に加熱して焼入れ処理を行い、その後、(AC3変態点-80℃)以上、(AC3変態点-10℃)以下の温度範囲に加熱して二相域焼入れ処理を行い、さらに560~660℃にて焼戻し処理を行う調質処理を有することを特徴とする強度-低温靱性バランスに優れたCu含有低合金鋼の製造方法。
- 前記調質処理が、板厚150mm~500mmの厚肉部を有する大型構造用の鋼に適用されることを特徴とする請求項5に記載の強度-低温靱性バランスに優れたCu含有低合金鋼の製造方法。
- 熱間鍛錬によって製造され、その後、前記調質処理が行われることを特徴とする請求項5または6に記載の強度-低温靱性バランスに優れたCu含有低合金鋼の製造方法。
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KR1020187023597A KR20180118117A (ko) | 2016-02-25 | 2017-02-08 | 강도-저온 인성 밸런스가 우수한 Cu 함유 저합금강 및 그 제조 방법 |
US16/079,769 US20190055620A1 (en) | 2016-02-25 | 2017-02-08 | Cu-containing low-alloy steel having excellent balance between strength and low-temperature toughness and method for producing same |
EP17756207.1A EP3421630B1 (en) | 2016-02-25 | 2017-02-08 | Method for producing cu-containing low alloy steel having excellent balance between strength and low-temperature toughness |
BR112018067794-9A BR112018067794B1 (pt) | 2016-02-25 | 2017-02-08 | Processo para produzir aço baixa liga contendo cobre |
KR1020247005834A KR20240027879A (ko) | 2016-02-25 | 2017-02-08 | 강도-저온 인성 밸런스가 우수한 Cu 함유 저합금강 및 그 제조 방법 |
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JP7402055B2 (ja) | 2020-01-07 | 2023-12-20 | 日本製鋼所M&E株式会社 | 溶接熱影響部の靱性が優れたCu含有低合金鋼およびその製造方法 |
CN114134404B (zh) * | 2021-05-20 | 2022-07-29 | 江阴兴澄特种钢铁有限公司 | 一种经济型破冰船用fh36钢板及其制备方法 |
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JPS61149430A (ja) | 1984-12-25 | 1986-07-08 | Kawasaki Steel Corp | 低温じん性および溶接性の優れた低C−Cu析出型高張力鋼の製造方法 |
JPH05171263A (ja) | 1991-12-24 | 1993-07-09 | Kawasaki Steel Corp | B添加鋼の2相域焼入れ方法 |
JPH07207334A (ja) * | 1994-01-12 | 1995-08-08 | Nippon Steel Corp | 溶接性および塑性変形能に優れた高張力鋼の製造法 |
JP3262972B2 (ja) * | 1995-07-31 | 2002-03-04 | 新日本製鐵株式会社 | 低降伏比を有する低温靭性に優れた溶接性高強度鋼 |
JP3541746B2 (ja) | 1999-09-13 | 2004-07-14 | 住友金属工業株式会社 | Ctod特性に優れた高強度厚鋼板及びその製造方法 |
JP3968011B2 (ja) * | 2002-05-27 | 2007-08-29 | 新日本製鐵株式会社 | 低温靱性および溶接熱影響部靱性に優れた高強度鋼とその製造方法および高強度鋼管の製造方法 |
JP5076423B2 (ja) | 2006-09-27 | 2012-11-21 | Jfeスチール株式会社 | Ni含有鋼板の製造方法 |
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JPS62177120A (ja) * | 1986-01-30 | 1987-08-04 | Nippon Chiyuutankou Kk | 低温靭性のすぐれた鋳鋼の製造法 |
JPH01219121A (ja) * | 1988-02-26 | 1989-09-01 | Nippon Steel Corp | 低温靭性の優れた極厚調質高張力鋼板の製造方法 |
JPH06220577A (ja) * | 1993-01-26 | 1994-08-09 | Kawasaki Steel Corp | 耐hic特性に優れた高張力鋼及びその製造方法 |
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JP2017150041A (ja) | 2017-08-31 |
EP3421630B1 (en) | 2021-03-24 |
BR112018067794A2 (pt) | 2019-02-12 |
US20190055620A1 (en) | 2019-02-21 |
KR20180118117A (ko) | 2018-10-30 |
JP6242415B2 (ja) | 2017-12-06 |
EP3421630A1 (en) | 2019-01-02 |
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